Adenylyl Cyclases

Panic Away

Panic Away Program

Get Instant Access

Nicole Mons and Jean-Louis Guillou Abstract

Although a number of signal transduction pathways have been implicated in short- and long-term adaptative changes in neuronal plasticity and memory formation, there is increasing evidence that cross-talk between the cAMP- and Ca2+-regulatory pathways may play a pivotal role in learning and memory processes. The fact that adenylyl cyclases (AC), in both invertebrates and mammals, are potentially subject to a wide range of influences has given rise to the notion that they can act as molecular coincidence detectors which are able of yielding a unique integrated response when simultaneously exposed to multiple stimuli. In this review, we discuss the role of AC in the molecular mechanisms underlying the induction and/ or expression of memory in various organisms that perform different behavioral tasks, ranging from studies of implicit memory for the acquisition of fear, such as behavioral sensitization in Aplysia or classical conditioning in Drosophila, to explicit forms of long-term memory (LTM) and synaptic plasticity in the rodent brain.

Introduction

Intracellular adenosine 3',5'-cyclic monophosphate (cAMP) is generated from ATP in a reaction catalyzed by adenylyl cyclase (AC) in response to a variety of extracellular signals, such as hormones, neurotransmitters, and other regulatory molecules, via the activation of specific receptors. The cAMP then propagates the hormone signal either by stimulating cAMP-dependent protein kinase (PKA) or directly by inducing protein-protein interactions independently of any phosphorylation (for a review see ref. 74). The subsequent activation of PKA controls multiple cell functions, including metabolism, cell growth, differentiation, ion channel activity, synaptic transmission, gene transcription and memory formation (for a review see refs. 61,118). A major surprise to emerge from the cloning and expression of the mammalian AC family is that most, if not all, ACs are potentially subject to dynamic control by multiple regulatory influences and that distinct coincident signals can be translated into a unique integrated response. In addition to their capability to respond via either Gia or Gsa subunits, the activity of particular AC can also be regulated either directly or indirectly by a variety of signals, including cytosolic calcium ions ([Ca2+];), protein kinase C (PKC) and Py subunits of G proteins (for a review see refs. 37,79,131). The fact that ACs are subject to this wide range of influences has given rise to the notion that they can act as «coincident signal detectors», which are capable of yielding a unique response when simultaneously exposed to multiple regulatory cues. In addition to interacting with various signalling pathways, increasing evidence indicates that the various ACs show distinct cellular distributions and subcellular compartmentation. Indeed, the role and specificity of the cAMP/PKA-signaling pathway may be critical in the regulation of synaptic functions by virtue of this restricted synaptic compartmentation. In particular, the selective targeting of both AC and PKA to discrete subcellular localizations via interaction with specific anchoring proteins, in juxtaposition with other Ca2+-regulated signaling molecules (mainly

From Messengers to Molecules: Memories Are Made of These, edited by Gernot Riedel and Bettina Platt. ©2004 Eurekah.com and Kluwer Academic / Plenum Publishers.

kinases and phosphatases) optimizes the reception and propagation of the signal carried by cAMP, such as those required for the establishment of learning and memory.

In this review, we will first outline the essential evidence implicating the cAMP/AC/PKA pathway in short- and long-term sensitization of the siphon and gill withdrawal reflex (GSWR) in Aplysia. We will examine the associative learning defects in the fruit fly, Drosophila melanogaster, caused by mutational perturbations of the cAMP cascade, focusing on the genes rutabaga (rut) and dunce (dnc), which encode a Ca2+/calmodulin (CaM) -responsive AC and a cAMP-specific phosphodiesterase, respectively. We will then briefly survey current evidence that mammalian AC isoforms are uniquely regulated by a variety of influences and are spatially organized for integrating coincident cellular signals and thus, modulate local regulatory components subserving early and late memory processes. We will then proceed to outline recent data implicating mammalian Ca2+/CaM-sensitive and -insensitive ACs as molecular coincidence detectors subserving synaptic function and memory formation. Particular attention will be given on the recent genetic studies demonstrating that Ca2+/CaM-stimulated ACs may have a crucial role in the hippocampus-dependent LTP and memory.

Adenylyl Cyclases and Memory Formation in Invertebrates

Molecular Mechanisms Underlying Memory in Aplysia californica

In the marine snail Aplysia californica, the role of the cAMP-dependent signaling pathway in short- and long-term memory (LTM) comes from studies on sensitization of the GSWR, which is a nonassociative form of learning.55 A weak stimulus (conditioned stimulus, CS) to the siphon leads to the animal's defensive response amplitude and duration of defensive withdrawal reflexes become enhanced when stimulation of the siphon is coupled to strong noxious stimulus (unconditioned stimulus, US), which is usually a shock to the tail. Whereas one single stimulation to the tail produces short-term sensitization (few minutes to hours), repeated spaced stimulations produce a long-term sensitization that lasts from days to is controlled by sensory (SN) and motor (MN) neurons27 and cellular analyses of the SN-MN synapses demonstrated that the site of induction and expression for sensitization is the presynaptic SN. Both short- and long-term facilitation induced by sensitizing stimuli (US presented alone or unpaired with CS) activate serotonergic (5-hydroxytryptamine; 5-HT) receptors and lead to increased cAMP levels, activation of PKA and thus, modulation of membrane channels and other effector proteins that contribute to enhanced transmitter release (for a review see refs 23,82). Five spaced 5-HT pulses can cause long-term facilitation by inducing a prolonged activation of PKA and translocation of its catalytic units into the nucleus of SN where it activates transcription factors belonging to the cAMP-response element binding protein (CREB) family.11,39 Manipulation of the signaling cascades in the presynaptic SN, such as intracellular injection of cAMP, induces long-term changes which can be blocked by anisomycin, an inhibitor of protein synthesis.28,112,117 Thus, it is likely that the transient 5HT-induced elevation of cAMP can lead to long-term facilitation in Aplysia. Anisomycin is not effective, however, when applied 12-15 hr after the cAMP injection, suggesting that transient cAMP elevation induced a signaling cascade of enduring process, such as protein synthesis whose products continue to be synthesized for several hours after cAMP levels have returned to baseline.104 All these findings support the hypothesis that the specific temporal activation of the cAMP cascade, dependent on distinct stimulation parameters, may be critical for the induction of long-lasting neuronal and behavioral changes in Aplysia.

Although direct evidence is lacking, cellular studies suggested that a dually-regulated AC serves as a coincidence detector for detection of US-CS contingencies (with 5-HT release and Ca2+ influx, respectively).2,4,75 By injecting a peptide inhibitor of PKA into the SN, Bao et al10 revealed that activation of the cAMP cascade is crucial for both associative and nonassociative facilitations. In contrast, associative, but not nonassociative, facilitation of SN-MN synapse is attenuated by either by presynaptic injection of Ca2+ chelators or a postsynaptic injection of an N-methyl-D-aspartate (NMDA)-receptor antagonist or a strong postsynaptic hyperpolariza-tion, suggesting that associative facilitation requires activation of a Ca2+-sensitive AC in the presynaptic SN and coincident elevation in [Ca2+] in both post- and presynaptic SN-MN. It was proposed that a postsynaptic site of detection involving Ca2+ influx through NMDA receptor-gated channels might serve for presynaptic glutamate release and postsynaptic depolarization to initiate induction of associative resulting rise in postsynaptic Ca2+ might induce the release of a retrograde signal which acts presynaptically by activation of Ca2+/CaM-stimulated AC. The Aplysia AC has not been cloned yet but it is clearly distinct from mammalian types 1 and 8 which do not require sequential application of Ca2+/CaM and Gsa to be synergistically stimulated.38 In addition to activation of the cAMP/PKA cascade, 5HT acting on different receptor subtypes can also activate other kinases, including PKC23,122 and the mitogen-activated protein kinase (MAPK).89 Recent studies indicated that prolonged activation of PKC is involved in the long-term facilitatory actions of 5-HT that are mediated primarily by the cAMP/PKA cascade, suggesting that AC activity can be modulated via cross-talk between different signal transduction pathways in the Aplysia SN.12-121

The Drosophila System

The cAMP signaling cascade has a crucial role in LTM of associative olfactory learning in which the fruit fly Drosophila melanogaster is presented with two novel odors, and then trained to avoid a particular odor by pairing that odor with an electric shock.138 Repeated, temporally-spaced training trials induced a stable, long-lasting memory that requires protein synthesis. 37 This memory can be dissectable into a medium-term memory (lasting few hours) which requires activation of PKC activity and a LTM (over 1 day) which requires a PKA- and nitric oxide-dependent processes.45,91 In the mushroom bodies, which mediate olfactory learning, multiple conditioning trials induced temporal dynamics of PKA activation which depend both on the sequence of CS (which triggers odor-specific Ca2+-mediated process) and US stimulation and also on the number of conditioning trials.46 Mutational analyses of associative learning behavior have identified genes that are required for olfactory associative memory and their molecular characterization indicating that they all affect, albeit in different ways, the cAMP signaling cascade.44,45,47,84,137 Gene disruptions of G-protein a subunit (dGsa), AC (Rutabaga), cAMP phosphodiesterase (dunce), catalytic (DCO) and regulatory subunits (dPKA-RT) of cAMP-dependent protein kinase (PKA) and cAMP-response element binding (CREB) (dCREB2) impair olfactory learning and/or memory formation in flies (for a review see refs. 111,136). Interestingly, both rutabaga and dunce are severely affected in initial memory acquisition and subsequent consolidation whereas relatively intact learning scores immediately after training are observed in dPKA-RT, DCO and dCREB2 mutations. A neuronal model involving the cAMP cascade has been proposed for olfactory associative learning.40,83 In this model, the rutabaga AC acts as a molecular coincidence integrator of associative learning cues responding synergistically to activated Gsa and Ca2+ signals.4 Interestingly, rutabaga AC shows high similarity to mammalian Ca2+/CaM-stimulable AC isoforms (types proposed that integration of sensory inputs from olfactory cues (increased [Ca2+ ]i) and footshock (activation of dGsa) in mushroom body neurons may lead to activation of AC and produce a synergistic increase of cAMP levels which then, may act as the primary mediator of downstream events that are responsible for long-term functional and structural changes. Zars et al157 have recently reported that a cell type-specific gene targeting the rutabaga gene in Kenyon cells (the primary afferents of which convey olfactory inputs via the antenno-glomerular tract) restores olfactory learning, and indicates that mushroom bodies are a critical locus for the signal-integrating properties of rutabaga AC.

A Specific Role for Mammalian Adenylyl Cyclases in Learning and Memory Processes: Heterogeneity of Mammalian Adenylyl Cyclases

Since the original cloning of the first mammalian AC isoform by Kuprinski et al80 nine isoforms have now been identified and characterized in brain, revealing variable sensitivities to regulators such as G proteins, Ca2+, CaM and protein kinases.37,74,79,131 Hyd ropathy analysis predicted that all isoforms are large (1080-1248 amino acids) polypeptides consisting of a short and variable cytoplasmic N-terminal region, followed by a double six-transmembrane spanning motif (M1 and M2) and two 40 kDa cytoplasmic domains (C1 and C2). Whereas the transmembrane domains are not highly conserved among ACs, two subregions of the cytosolic domains (termed C1a and C2a) are well conserved within a particular AC isoform, they also share homology with the cytoplasmic domains of Drosophila rutabaga AC, bacterial and yeast AC and even with the catalytic domains of membrane-bound guanylyl cyclases, suggesting that both eukaryotic and prokaryotic AC share the same ancestral origin.123,126 These homologies among the C1a and C2a domains from the same or different mammalian ACs suggest that the cytosolic domains constitute the site for cAMP synthesis. Indeed, molecular studies showed that a soluble chimeric construct consisting of C1a from type 1 and C2a from type 2 contains all of the catalytic apparatus of the wild type AC and is responsive to Fsk, Gsa-and GPy subunits.126

Based on their similarities in sequences and their distinct regulation by Ca2+ and G-protein signaling pathways, mammalian AC isoforms have been divided into distinct subfamilies as (1) Ca2+-stimulated AC types 1, 8 and 3 (types 1 and 8 act as coincidence detectors for positive cross-talk between Ca2+/CaM and Gsa whereas stimulation of type 3 by Ca2+/CaM is strictly conditional and requires concomitant activation by Gsa or

ACs (types 5 and 6); (3) Ca2+-insensitive ACs (types 2, 4 and 7 are insensitive to [Ca2+J, but stimulated by Gsa and Py under coincidental activation

CaM-dependent protein phosphatase (calcineurin)-inhibited type 9 (for a review see refs. 37,38,123).

Diversity in the Regulation of Mammalian Adenylyl Cyclases by G Proteins Ca2+ Signals and Phosphorylation

In light of their varied and complex modes of regulation by G-proteins, kinases (PKA, PKC, MAPK and CaMkinase), phosphatases (calcineurin), Ca2+ and Ca2+/CaM, mammalian ACs have been proposed to serve as critical « coincidence » detectors i.e., they could respond synergistically to multiple signals that arrive from independent transductional pathways to efficiently increase cAMP production6,20,95 (see Fig. 1). All of the ACs are regulated in type-specific patterns, and their mechanisms of regulation are often highly synergistic or conditional.

Regulation by G-proteins

Although the different isoforms differ greatly in their pattern of regulation, all ACs share the capacity to be stimulated by the plant diterpene Fsk and Gsa in vitro (except type 9). However, the Ca2+/CaM-stimulated isoforms (types 1 and 8) are insensitive to Gs in vivo.67,145 The different mammalian ACs exhibit different susceptibilities for activation by Fsk, Gsa or both. Coincident stimulation by both Fsk and Gsa results in synergistic, non competitive, stimulation of enzymatic activity for Ca2+-insensitive ACs (types type 5 whereas the two activators act independently for type 1.124,125,130

Although stimulation through Gsa is the principal mechanism whereby ACs are activated, the activity of certain isoforms is also regulated by the family of Gi-related proteins (Gi, Go, Gz) that can be activated by diverse hormones and neurotransmitters (i.e., adenosine, epinephrine and cannabinoids). Inhibition of catalytic activity by Gia is selective and variable degrees

Figure 1. Complex regulatory patterns of hippocampal AC by various G protein subunits, Ca2+/CaM, kinase (PKC), phosphatase (calcineurin). The different Ca2+-sensitive and insensitive AC act as molecular coincidence detectors i.e., they could respond to multiple signals that arrive from independent pathways to efficiently increase cAMP level and activate PKA activity. The AC/cAMP/PKA pathway, in addition to participate to early biochemical events, also interacts with other kinases (CaMKII, ERK/MAPK) to regulate transcriptional and translational events required for the establishment of late biochemical events. Stimulatory signals are shown as arrows and inhibitory signals as plungers. Abbreviations are described in the text. (adapted from ref. 74).

Figure 1. Complex regulatory patterns of hippocampal AC by various G protein subunits, Ca2+/CaM, kinase (PKC), phosphatase (calcineurin). The different Ca2+-sensitive and insensitive AC act as molecular coincidence detectors i.e., they could respond to multiple signals that arrive from independent pathways to efficiently increase cAMP level and activate PKA activity. The AC/cAMP/PKA pathway, in addition to participate to early biochemical events, also interacts with other kinases (CaMKII, ERK/MAPK) to regulate transcriptional and translational events required for the establishment of late biochemical events. Stimulatory signals are shown as arrows and inhibitory signals as plungers. Abbreviations are described in the text. (adapted from ref. 74).

of inhibition have been reported.129 The ability of AC to integrate multiple regulatory inputs from the a and the Py subunits released from Gi is isoform-specific.127, 30 Reconstitution or transfection studies demonstrated that activated Gi selectively inhibits types 3, 5 and 6.43,133 whereas types 1, 2, 7, 8 and 9 are less sensitive to Gia.109,129, The inhibition is noncompetitive with Gsa, arguing that Gsa and Gia bind to separate nonoverlapping sites on the AC protein. Type 1 is slightly inhibited by Gia, but in the presence of Py subunits released from hormonal activation of Gi, the CaM (or Fsk or Gsa)-stimulated activity of type 1 is inhibited by GpY subunits. Interestingly, Nielsen et al103 have shown that type 1, but not type 8, is inhibited by activation of Gi-coupled receptors in vivo.

Coincidence regulation has also been proposed for Ca2+-insensitive isoforms (types 2, 4 and 7) which are only weakly inhibited by activated Gia, but are synergistically stimulated by Py subunits in the presence of Gsa.130,131 In addition to in vitro regulation of Py subunits, types 2 and 4 also act as coincidence detectors of paired Gi and Gs inputs with Py potentiation in vivo.52'86'140 The cotransfection of HEK-293 cells with the Gi-coupled serotoninergic receptor (5-HT1A or 5-HT1B), type 2 and Gia greatly stimulates AC activity and this activation is blocked by pertussis toxin and a Gp^ antagonist.4 Similarly, activation of the 5-HT1A receptor in tissues in which type 2 is highly expressed (e.g., hippocampus) potentiates actions of Gs-coupled receptors (e.g., P-adrenergic receptor in CA1 neurons) by GPy-mediated activation of type 2 ACs.5'24

Regulation by Ca?+

Stimulation by Ca2+. Profound physiological significance derives from the regulation of mammalian AC by Ca2+, which provides a means of integrating the activities of the two crucial cAMP- and Ca2+-regulated signalling pathways.38 Submicromolar concentrations of Ca2+ elicit a prominent stimulation of type 1 and 8 ACs, in the presence of CaM.25^80 In vitro stimulation of type 3 by Ca2+/CaM requires low micromolar [Ca2+] and is seen only in the presence of activated Gas or Fsk.33

Inhibition by Ca2+. All AC activities are inhibited by high, nonphysiological submillimolar levels of [Ca2+]i, possibly by competition with magnesium which is required for catalysis.59 Submicromolar [Ca2+] directly inhibits the activity of types 5 and 6, independently of CaM.38 This inhibition by [Ca2+] is additive to that elicited by receptors acting via Gia.130 Interestingly, both types 5 and 6 are weakly expressed in regions associated with learning and memory, including the hippocampus and cortex, suggesting that a direct inhibitory control of AC by Ca2+ is not critical for memory processes.

Inhibition by Ca2+/Calcineurin. Calcineurin-dependent dephosphorylation represents another mode of regulating cAMP production by which Ca2+ signals may exert an indirect negative control on AC. In HEK-293 or COS7 cells transfected with AC9 or in AtT20 cells that express predominantly endogenous AC9, the inhibition of cAMP synthesis by a rise in [Ca2+]; is alleviated by specific inhibitors of calcineurin (FK506 or cyclosporin A).7

In vivo regulation ofAC by Ca2+. When Ca2+-sensitive ACs are directly regulated by changes in [Ca2+]i, studies in non excitable cells demonstrated that the positive or negative regulation of AC activity is strictly dependent on capacitative Ca2+ entry (CCE), activated secondary to the emptying of intracellular Ca2+ pool.49,50 In contrast, Ca2+ release from internal stores or non specific Ca2+ entry via ionophore is unable to regulate Ca2+-sensitive ACs.32 In neuronal cells in which the CCE plays a modest role, both CCE and prominent voltage-gated Ca2+ entry appear equally efficacious at regulating Ca2+-sensitive ACs, indicating that Ca +-sensitive AC is closer to the CCE channel than the voltage gated Ca2+ channel.48

Regulation by Protein Kinases

Serine/threonine phosphorylation of specific isoforms of ACs by protein kinases (PKC, PKA, CaMK) is a very important regulatory mechanism allowing a direct and efficient control of cAMP production within the cell (see also relevant chapters in this book).

PKC. The PKC-mediated phosphorylation of AC isoforms positively regulates types 1-5 and 7 but inhibits type 6. 69 Intriguingly the GpY potentiation of the Gsa-stimulated activities for types 2 and 4 is abolished by the PKC-mediated phosphorylation, indicating that PKC can exert an inhibitory effect on activated Ca2+-insensitive types 2 and 4.128 PKC synergistically increases the activity of type 2 evoked by Gsa or Gp^ whereas it inhibits Gsa-activated activity of type 4.71,86,154 These findings strongly suggest that activation of PKC pathway greatly reduces the ability of type 2 to integrate coincident signals from Gia- and Gsa-coupled receptors. Thus, the role of type 2 (or type 4) to mediate cross-modulation of synaptic plasticity between Gia and Gsa-coupled receptors in hippocampal neurons might be affected upon activation of PKC.4,5

PKA. Both Fsk- and Gsa-stimulation of Ca2+-inhibitable types 5 and 6 are inhibited by PKA-mediated phosphorylation,30,70 suggesting that both types 5 and 6 are under feedback inhibition by cAMP cascade. This effect is isoform-specific since types 1 and 2 are not susceptible to PKA-mediated loss of Gsa stimulation.30

CaMkinase. The best example of rapid desensitization of AC by CaM kinase phosphorylation is provided by the negative effect exerts by CaMKII on type 3 in olfactory signaling.14 ,147 Wayman et al146also reported that CaMKIV functions as a negative feedback regulator of Ca +-stimulation of type 1 activity, without affecting basal and Fsk-stimulated activity in vivo. Since type 1, but not type 8, is subject to inhibition by both CaM kinase and Gi-coupled receptors, it is suggested that the two Ca2+-stimulated ACs may have very distinct regulatory properties and thus, the presence of both types 1 and 8 in a particular neuron is not redundant.

Potential Targets of cAMP

cAMP-binding proteins. In addition to PKA activation, cAMP also regulates the activity of specific guanine nucleotide exchange factors (cAMP-GEF). Two genes have been identified for cAMP-GEF also called Epac (exchange protein directly activated by cAMP). They exhibit both a cAMP-binding site and a domain that is homologous to domains GEF for Ras and Ras-like GTPase (Rap1).41,42,77 Recent studies reveal complex regulation of Rap1 by cAMP including PKA-independent activation and PKA-dependent negative feedback regulation.139 As one Epac isoform (Epac 2) is strongly expressed in restricted brain areas, including the hippocampus (mainly CA3 and DG), the cortex and the cerebellum,77 a PKA-independent activation of Rap1 by Epac 2 may provide a direct mechanism for cAMP to activate the Rap1-MAPK/ERK cascade and thus, to stimulate the gene transcription in a PKA-independent manner. Furthermore, the restricted expression of Epac 2 could contribute to region- and cell type-specific cAMP-mediated neuronal functions.

Cyclic nucleotide gated ion channels (CNGC). As the CNGC conduct Ca2+ entry under the control of cAMP and cGMP,153 Fagan et al 51 proposed that they could also participate in the Ca2+ feedback regulation of Ca2+-sensitive AC, independently of voltage-operated Ca2+ channels and Ca2+ stores. Regulation ofAC by Ca2+-dependent CNGC modulation is particularly important in the context of short-term adaptation and desensitization in olfactory cilia, because Ca2+ transients present in the olfactory cilia following cAMP-mediated gating of CNGC inhibits the activity of AC3 via phosphorylation by CaMKII and also via a down-regulation of CNGC affinity to cAMP 159

The Specific Distribution and Expression Levels of Mammalian Adenylyl Cyclases in Brain

Although all AC isoforms are present in the brain, the various ACs are distributed in quite distinct patterns throughout the different regions. In situ hybridization studies showed that (1) only four AC isoforms are highly expressed in the brain (e.g., types 1, 2, 5 and 9); (2) many brain areas express multiple AC isoforms and (3) Ca2+-sensitive ACs are expressed in specific regions (e.g., type 3 in olfactory cilia, type 5 in basal ganglia; type 1 in areas implicated in memory formation)whereas others are widely distributed (e.g., types 2, 6, 7, 9) (for a review see refs. 61,93,94).

In the hippocampus, at least six AC isoforms (types 1,2,4,7,8,9) are expressed in the CA1-CA3 pyramidal layers and the dentate gyrus (DG). The pattern of expression of type 1 in the hippocampus provides a good example of cell-type specific expression of an individual AC isoform.9 2 Type 1 is expressed predominantly in the CA1-CA2 fields and the DG whereas it is barely expressed above background in CA3 field. Compared to type 1, the level of expression of type 8 in the hippocampus is weaker25,98 Since most forms of hippocampal LTP require increased [Ca2+]i which markedly elevates cAMP levels,76,87,102 the presence of types 1 and 8 in hippocampal subfields strongly suggests that Ca2+-mediated increased cAMP level depends upon these two Ca2+-stimulated ACs. In addition to types 1 and 8, high levels of mRNA encoding for Ca2+-insensitive, PKC-stimulated type 2 and Ca2+/calcineurin-inhibited type 9 are also expressed in all hippocampal subfields. Specific isoform-antibodies against types 2 and 9 have been developed to examine the distribution of the protein in the brain. Labeling for type 2 is found in the dendritic subfields of the CA1-CA3 pyramidal and the granular cells and type

2 colocalizes with the dendritic marker (MAP2), suggesting that type 2 plays an important role for the generation of the cAMP signal in the postsynaptic compartment.9 Type 9 also appears implicated in postsynaptic mechanisms underlying synaptic plasticity since it is also present in the dendritic fields in both hippocampus and neocortex and it colocalizes with calcineurin in synaptic structures of most cerebral neurons.8'119

Ultrastructural analysis using anti-AC antibodies that recognize a domain common to all mammalian AC confirmed that AC immunoreactivity is highly distributed near postsynaptic densities in dendritic spines of hippocampal CA1 region.96 Dendritic spines are areas of high concentrations of Ca2+ channels and well as may thus be precisely where they are most efficacious in the integration and propagation of Ca2+ signals. We might expect that cAMP would need to diffuse only a short distance before activating the anchored PKA, thereby greatly facilitating the local downstream phosphorylation steps that are responsible for short-term modifications.

Adenylyl Cyclase and Long-Term Potentiation

LTP is a robust and persistent modification of synaptic transmission in response to transient stimuli and is thought to be a candidate cellular mechanism for mediating some forms of explicit hippocampus-dependent memory. LTP requires stimulation of NMDA receptors, postsynaptic depolarization and Ca2+ influx into the postsynaptic cell in the Schaffer collateral/ commissural synapses in area CA1 and the perforant path/DG synapses31'102 whereas LTP in the mossy fibers is initiated presynaptically through voltage-sensitive Ca2+ channels.63'102'148

In contrast to the general agreement that the late phase of LTP (L-LTP) requires activation of AC and cAMP-dependent PKA, the issue of whether early phase of LTP (E-LTP) depends on a rise in cAMP level is not clear. Several pharmacological and genetic studies showed that interfering with the cAMP signal does not recent studies demonstrated that inhibition of the cAMP/PKA pathway indeed decreases E-LTP19'106'155 Blitzer et al18'19 proposed a postsynaptic mechanism by which the cAMP pathway may act as keeping the «gate open» for the induction of LTP by controlling the activity of protein phosphatases' such as calcineurin (see Fig. 2). They proposed that the gating mechanism comprises two opposite PKA and calcineurin pathways' which converge on the regulatory protein inhibitor-1 (I-1)' a specific blocker of protein phosphatase-1 (PP1). The cAMP pathway' through activation of I-1 and inhibition of PP1' enables the autophosphorylation of CaMKII to occur and thereby' enhances CaMKII activity.18 As calcineurin could mediate the decrease in synaptic strength through dephosphorylation of I-1 and thus' activation of PP1'99 the interactions between the two cAMP and Ca + signals at this point may play a key role in the modulation of LTP. 18'19'22'134 As shown by Raman et al108 in cultured hippocampal CA1 neurons' inhibition of PKA prevented recovery of NMDA receptors from calcineurin-mediated dephosphoryla-tion induced by synaptic activity whereas elevation of PKA activity by Fsk' cAMP analogs or P-adrenergic receptor agonists can antagonize the effects of calcineurin. Moreover' Malleret et al88 showed that the enhancement of E-LTP in area CA1 after altering calcineurin activity could be prevented by blocking PKA. Taken together' the findings suggest that a PKA/ calcineurin gate represents a major activator/suppressor mechanism for regulating E-LTP. Blitzer et al19 proposed that the direct mechanism for coupling increases when Ca2+ influx leading to rises in cAMP levels and this might be through activation of types 1 and 8. Interestingly' type 9 which is under inhibitory control by calcineurin' is inhibited by the same range stimu lates type 1.7 Thus' it is possible that cAMP generated by type 9 also provides a critical link in the balance between phosphorylation/dephosphorylation cascades that controls LTP.

In addition to the Ca2+ signal' cAMP-induced synaptic plasticity can also be modulated by neurotransmitter receptors acting on Gsa' Gia or Py subunits of G proteins. Thus' by acting as a molecular coincidence detector to integrate signals from PKC- and Gs/Gi-protein-regulated pathways' it is possible that the cAMP cascade arising from activation of Ca +-insensitive type 2 also participates in the molecular events that trigger LTP (See Fig. 1). In particular' electro-

Figure 2. Postulated interactions between Ca2+/CaM-stimulated types 1 and 8 and Ca2+-regulated pathways in the early and late biochemical events underlying LTP and memory formation. Increased [Ca2+]i arising from NMDA-R or VGCC induces elevation of intracellular cAMP via activation of type 1 or type 8. The resulting activation of the cAMP/PKA pathway, through phosphorylation of I-1 and inhibition of PP1, acts as keeping the «gate open» for Ca2+-dependent biochemical events by inhibiting calcineurin and thus, maintaining CaMKII activity. Abbreviations are described in the text. (adapted from refs. 18,19,74).

Figure 2. Postulated interactions between Ca2+/CaM-stimulated types 1 and 8 and Ca2+-regulated pathways in the early and late biochemical events underlying LTP and memory formation. Increased [Ca2+]i arising from NMDA-R or VGCC induces elevation of intracellular cAMP via activation of type 1 or type 8. The resulting activation of the cAMP/PKA pathway, through phosphorylation of I-1 and inhibition of PP1, acts as keeping the «gate open» for Ca2+-dependent biochemical events by inhibiting calcineurin and thus, maintaining CaMKII activity. Abbreviations are described in the text. (adapted from refs. 18,19,74).

physiological studies reported that, in hippocampal CA1 neurons, agonist stimulation ofGi-coupled 5-HT1A, GABA-B and a-adrenergic receptors leads to liberation of Py complex and potentiates Gsa-mediated actions of P-adrenergic receptor via activation of type 2 AC (or type 4).4,5

Studies using pharmacological inhibitors or genetic manipulation have implicated the cAMP cascade in the late phase of LTP (L-LTP) in all hippocampal pathways.54,62,66,73,148,150 There is increasing evidence that cross-talk between the Ca2+, cAMP and mitogen-activated protein ki-nase (MAPK) pathways is critical for the stimulation of CREB and thus, the expression of genes required for the formation of LTP and LTM (see Fig. 2).68,89 In the hippocampus, a rise in intracellular cAMP activates the Erk/MAPK cascade, much as it does in lateral amygdala, and coactivation of the cAMP and MAPK pathways by Ca2+ is essential for phosphorylation of CREB and L-LTP formation.62,65,110 In this context, the induction of arg3.1/arc mRNA in primary culture hippocampal neurons is strictly dependent on the coactivation of PKA and Erk/MAPK pathways.1 3 In neuronal cells, the effect of cAMP has been proposed to involve the sequential activation of Ca2+/CaM-sensitive ACs (types 1,8) and the phosphorylation and activation by PKA of Rap-1, then the coupling of Rap-1 to B-Raf results in the activation of ERK/MAPK pathway.56,115,142 Although PKA plays a crucial role in the activation of CREB, activation of Rap1 by cAMP-GEFII may also provide another mechanism by which cAMP can stimulate the Erk/MAPK pathway and thus, can induce gene transcription in a PKA-independent manner.42,77

To investigate the role of Ca2+/CaM-stimulated ACs in LTP, mice lacking either type 1 or type 8 (AC1 or AC8KO) or both ACs (DKO) were analyzed for several forms of LTIP120'141'150 Surprinsingly, LTP at the Schaffer collateral/CA1 pyramidal cell synapse was not affected in the KO mice whereas it was impaired in the DKO mice.150,151 Moreover, hippocampal Ca2+-stimulated AC activity was partially reduced in KO mice whereas response to Ca2+ was totally abolished in DKO. These observations suggest that the two Ca2+-stimulated AC1 and AC8 can, at least in part, substitute to each other for cAMP production in hippocampal CA1 region. In contrast to hippocampal CA1 LTP, AC1KO mice exhibit impaired mossy fiber/CA3 and cerebellar parallel fiber L-LTP, suggesting that presynaptic forms of LTP strictly depend upon AC1.120,141 In addition , since administration of Fsk (a nonselective stimulator of ACs) to DKO mice in hippocampal CA1, (or to AC1KO in mossy fiber) can restore L-LTP, it thus appears that postsynaptic activation of hippocampal ACs, other than types 1 and 8, could also modulate L-LTP.

Are Ca2+-Stimulated Adenylyl Cyclases Critical for Memory

Behavioural studies have provided evidence that AC activity is critical for learning and memory functions in mammals. A first study in our laboratory reported that AC activity was altered in mouse hippocampus following learning tasks. After acquisition of a spatial discrimination task performed in a 8-arm radial maze (a hippocampus-dependent task), Fsk-stimulated AC activity was down-regulated in the hippocampus and negatively correlated with the response accuracy attained by the subjects.57 In contrast, AC activity was increased following acquisition of a bar-pressing task, which is an hippocampal-independent task.72 Arguments based on phylogenetic adaptation supported our proposal that these opposite learning-induced alterations of AC activity might reflect an interaction between two (or more) competing memory systems at the hippocampal level, in which ACs might have a critical role. Meanwhile, Wu et al reported that AC1KO could acquire normally, as compared to controls, a task where they are required to find a hidden platform in the standard water maze task. Moreover, AC 1KO did not keep searching the quadrant where the platform had been previously located. This observation was interpreted as a spatial memory deficit although no argument excluded the possibility that these animals might be more flexible (i.e., search for the platform elsewhere). Whatever the case, the deficit was marginal and could be explained by the fact that 50-60 % of the Ca2+-stimulated AC activity remained in the hippocampus of AC1KO, suggesting that up-regulation of AC8 might have compensated the absence of AC1 function. To test this hypothesis, behavioural responses of AC8KO, AC1KO and DKO mice were analyzed.150 The results showed that the single mutants had normal LTM for contextual and passive avoidance learning whereas the DKO mice displayed a lower inhibitory response than controls after 30 minutes, but not 5 minutes, following acquisition of a single trial step-through passive avoidance paradigm. Also, DKO mice expressed a lower level of conditioned-fear when exposed, after 8 days (but not 24 h), to the context in which they had previously received an electric shock. Thus, it was hypothetized that hippocampal Ca2+-stimulated AC activity may be required for LTM, but not for short-term memory. This conclusion is in agreement with the idea that a cAMP cascade in the hippocampus is involved in the late, but not the early, phase of a memory consolidation process occurring after inhibitory learning in rats. Bernabeu et al13,14 showed that rats submitted to step-down passive avoidance learning displayed a time-dependent increase in hippocampal cAMP levels with a peak at 3-6 hr after training. This was supported by findings that intrahippocampal infusion of 8-Br-cAMP (a stable analogue of cAMP) or Fsk enhanced memory retrieval when given 3 or 6 hr (but not earlier than 3 hr) after the acquisi-tion.13,14,15,16,17 Moreover, activation of dopamine D1, P-noradrenergic or 5-HT1A receptors also modulates cAMP levels at 3-6 hr after training, and an increase in cAMP level is coincident in time with increases in PKA activity, and in phosphorylated CREB and c-fos immunoreac-tivities in the hippocampus after training. As emphasised by Wong et al150 the memory deficits of the DKO lacking AC1 and AC8 resembled those previously described in CREB deficient mutants in fear-conditioning experiments.21 They hypothesized that Ca2+ activation of type 1 and type 8 ACs play a crucial role in LTM because they can generate the critical cAMP signal required for Ca2+ stimulation of the CREB/CRE-mediated transcription (see Fig. 2). The use of similar fear-conditioning methods in both studies supported this conclusion. However, the interpretations of these experiments have relied on the assumption that this task is sensitive to hippocampal lesions in mice. Several years later, authors of the study of the CREB mutants reported behavioural findings, which were crucial for the interpretation of transgenic experiments with the widely used fear-conditioning paradigms. They demonstrated that hippocampal-lesioned mice are impaired in spatial versions of the Morris water maze task but can show contextual fear conditioning34 suggesting that, at least in some conditions (such as those used in the DKO study), the hippocampus may not be necessary for task acquisition. A second issue to consider is that AC8KO mice do not show normal increases in behavioural markers of anxiety when subjected to repeated stress, such as repetitive testing in the plus-maze or restraint preceding plus-maze testing, suggesting a role for type 8 in the modulation of anxiety.114 This observation is of significance because anxiolytic-like effects could have interfered with the estimation of retention performance of the DKO mice in tasks such as passive avoidance or fear-conditioning.

All these recent results gained from genetic strategies strengthened the hypothesis for a role of type 1 and/or type 8 in memory formation, which initially, was based only on brain locations and functional considerations related to their regulatory properties (see above). However, the conclusions remain still elusive and controversial. Considering that selective pharmacological tools are not available yet, further characterizations of the behavioural phenotypes of these genetically modified animals appear indispensable and should help to detail what is the exact nature of the memory processes in which the Ca2+-stimulated ACs have a role.

Among the pharmacological strategies, inhibitors of PKA activity have been commonly used to inhibit the cAMP signaling cascade and were shown to impair memory performance in a variety of tasks (including spatial learning) in correlation with impaired LTP in the hippocampus (for a review see ref. 92 and Vianna and Izquierdo in this book). Conversely, stimulation of PKA activity was used to demonstrate a role of PKA in the maintenance of LTP.

Pharmacological approaches supporting the view that an elevation in cAMP in the hippocampus is important for memory are based on the following data obtained using the passive avoidance paradigm: 1) Post-trial injections of Fsk or 8Br-cAMP in the hippocampus improved memory retrieval in the step-down passive avoidance13 and (2) in DKO mice, unilateral administration of Fsk to the CA1 subfield immediately before training was shown to restore LTM of passive avoidance.150 Recent studies in our laboratory have shown that increased hippocampal cAMP levels produced by local infusions of Fsk improve memory in a similar kind of task but impair spatial learning in water-maze tasks (unpublished data). The latter result is not isolated since Taylor et al132 also reported that injection of Sp-cAMP into the prefrontal cortex impair working memory in a delayed alternation task performed in a T-maze, suggesting that activation of PKA activity produces deleterious effects in spatial memory tasks. These findings greatly contrast with an extensive body of literature indicating that enhancement of the PKA pathway improves memory formation. Indeed, increased cAMP levels can oppositely alter mechanisms subserving different memory systems, suggesting mechanisms leading to "cognitive enhancement" are not universal (see ref. 132 for further discussion).

Adenylyl Cyclases Up or Down Depending on Task Demands

Even though Ca2+-stimulated AC might have a crucial role in the memory function of the hippocampus, these AC isoforms probably constitute only one part of a complex molecular system in which, interactions between diverse sources of cAMP (including from Ca2+-insensitive isoforms), would optimise the hippocampal functioning depending on the learning situation. Since the insight of Tolman135 that animals can learn about a particular experience in more than one way, it is now widely accepted that there exist multiple forms of memory and that the underlying neural substrates are distributed throughout the brain.113 An important implica-

Figure 3. Opposite regulations of Fsk-stimulated and Ca2+-stimulated AC activity occurs following spatial learning in the hippocampus. (A), In an 8-arms radial maze, mice were trained to discriminate 3 arms which were constantly baited. The top of the figure shows a representative track recorded at the end of learning and illustrates searching patterns occurring selectively into the 3 baited arms of the maze. The graph bellow shows changes in hippocampal AC activity in mice who had learned this task as compared to naive animals (controls). Fsk-stimulated AC activity was reduced after learning. (B) summarizes results obtained in mice who learned to locate a hidden platform in a circular water maze. In the hippocampus, in response to stimulation by Fsk, the AC activity was dose-dependently reduced after learning whereas, in sharp contrast, the AC responses were increased as function of the Ca2+

Figure 3. Opposite regulations of Fsk-stimulated and Ca2+-stimulated AC activity occurs following spatial learning in the hippocampus. (A), In an 8-arms radial maze, mice were trained to discriminate 3 arms which were constantly baited. The top of the figure shows a representative track recorded at the end of learning and illustrates searching patterns occurring selectively into the 3 baited arms of the maze. The graph bellow shows changes in hippocampal AC activity in mice who had learned this task as compared to naive animals (controls). Fsk-stimulated AC activity was reduced after learning. (B) summarizes results obtained in mice who learned to locate a hidden platform in a circular water maze. In the hippocampus, in response to stimulation by Fsk, the AC activity was dose-dependently reduced after learning whereas, in sharp contrast, the AC responses were increased as function of the Ca2+

concentration.

tion of this notion is that these different memory systems interact synergistically or competitively to produce behaviour.90 One consequence of this is that an animal may use different strategies in order to deal with a learning situation. Moreover, recent data have shown that hippocampal lesions facilitate the use of alternative learning strategies80-107 that are normally overridden by hippocampal-dependent memory processing. Jaffard and Meunier72 have reported data showing neurochemical or electrophysiological alterations in the hippocampus following the acquisition of tasks, which are not dependent on the hippocampal formation. Further, more neurobiological changes can be opposite to those observed following acquisition of hippocampal-dependent tasks and furthermore, one pharmacological treatment (like a lesion) can produce differential memory effects (no effect, facilitation or impairment) as a function of task demands.36 In the context of these findings, opposite alterations in hippocampal AC activity following acquisition ofhippocampal-dependent or hippocampal-independent learning have been reported.57,58,60 Increased Fsk-stimulated AC activity was observed after acquisition of a bar-pressing task (hippocampal-independent task) whereas a decrease occurred after acquisition of place learning in an 8-arm radial maze (see Fig. 3). Moreover, we showed that cysteamine-induced depletion of somatostatin produced an increase in AC activity in the hippocampus and improved acquisition of the bar-pressing task whereas place learning was impaired. Changes in AC activity were also studied following spatial learning in the water maze. Again, responses to Fsk were dose-dependently decreased in the hippocampus. However, in sharp contrast, responses to Ca2+ were enhanced. In other words, nonselective stimulation of hippocampal ACs was reduced whereas selective stimulation by Ca2+ was selectively increased.60

Figure 4. This model proposes the existence of two memory systems, a system I (coding for stimulus-response associations) and a second memory system (system II) coding for stimulus-stimulus associations, i.e., relational associations). When a novel learning situation occurs, both memory systems are a priori activated by incoming stimuli (S), process and emitted responses. In simple learning conditions, the responses emitted by system 1 (R1) can be sufficiently adapted to deal with the problem. In this condition, the inhibition (negative feedback) of the hippocampal functioning, blocking nonuseful information processing (SS-R), would speed-up acquisition. In contrast, when learning conditions are complex and required relational encoding between the stimuli, the adaptation of the responses emitted by system II (R2) would trigger a positive feedback to strengthen the hippocampus functioning. The bi-directional regulations of hippocam-pal AC activity, as observed following learning in tasks respectively involving each of these two kinds of information processing, might reflect a modulation of the hippocampal functioning. Further, we hypothesized that signals involving type 2 might be involved in this regulatory process. Beyond this, Ca 2+-sensitive ACs (types 1, 8) could be necessary to compute specific hippocampal functions such as the establishment of relational representations and/or spatial mapping.

Figure 4. This model proposes the existence of two memory systems, a system I (coding for stimulus-response associations) and a second memory system (system II) coding for stimulus-stimulus associations, i.e., relational associations). When a novel learning situation occurs, both memory systems are a priori activated by incoming stimuli (S), process and emitted responses. In simple learning conditions, the responses emitted by system 1 (R1) can be sufficiently adapted to deal with the problem. In this condition, the inhibition (negative feedback) of the hippocampal functioning, blocking nonuseful information processing (SS-R), would speed-up acquisition. In contrast, when learning conditions are complex and required relational encoding between the stimuli, the adaptation of the responses emitted by system II (R2) would trigger a positive feedback to strengthen the hippocampus functioning. The bi-directional regulations of hippocam-pal AC activity, as observed following learning in tasks respectively involving each of these two kinds of information processing, might reflect a modulation of the hippocampal functioning. Further, we hypothesized that signals involving type 2 might be involved in this regulatory process. Beyond this, Ca 2+-sensitive ACs (types 1, 8) could be necessary to compute specific hippocampal functions such as the establishment of relational representations and/or spatial mapping.

This suggests that an up-regulation of Ca2+-stimulated ACs associated with a down-regulation of other AC isoforms might be as critical for spatial learning (see Fig. 3). Because the Ca2+-insensitive type 2 AC is highly expressed within the hippocampal subfields, it was hypothesized that cAMP-signaling occurring at synapses expressing type 2 AC could also modulate hippocampus functioning as a function of the task demand. Based on widely accepted memory theories, which postulated the existence of at least two memory systems,113 a model was proposed to explain why bi-directional regulations might be relevant.72,95 This model confers a modulatory role on hippocampal functioning to signaling pathways involving the Ca2+-insensitive type 2 AC, (see Fig. 4). As a function of the task demand, activation of type 2 AC would block the information processing in hippocampus. Conversely, decreased cAMP

levels at synapses involving type 2 would be permissive for such a function to occur. In agreement with the extensive literature on the role of a Ca2+-stimulated ACs in memory formation, this model also integrates the idea that type 1 (and/or type 8) could be a critical component of an information processing system underlying the establishment of relational representation or spatial mapping.

Summary and Conclusions

Studies over the past few years have firmly established that members of the AC family play a key role in the complex intracellular network underlying synaptic plasticity and memory formation. As ACs are regulated by diverse extracellular stimuli through multiple signaling cascades, they could act as coincidence detectors to generate a unique cAMP response which then makes cross-talk with other signalling pathways to enable specific cellular functions. Overall, more detailed insight into the targeting of the different mammalian ACs in the neuronal compartments and the identification of complex mechanisms by which cAMP regulates other signaling systems, such as the Rap1-ERK pathway, as well as the knowledge of specific crosstalk between ACs and other cellular components, will be critical for a richer understanding of how the different ACs participate in the regulation of synaptic efficacy and memory formation.

References

1. Abel T, Nguyen PV, Barad M et al. Genetic demonstration of a role for PKA in the late phase of LTP and in hippocampus-based long-term memory. Cell 1997; 88:615-626.

2. Abrams TW, Castellucci VF, Camardo JS et al. Two endogenous neuropeptides modulate the gill and siphon withdrawal reflex in Aplysia by presynaptic facilitation involving cAMP-dependent closure of a serotonin-sensitive potassium channel. Proc Natl Acad Sci USA 1984; 81:7956-7960.

3. Abrams TW, Karl KA, Kandel ER. Biochemical studies of stimulus convergence during classical conditioning in Aplysia: dual regulation of adenylate cyclase by Ca2+/calmodulin and transmitter. J Neurosci 1991; 11:2655-2666

4. Albert PR, Sajedi N, Lemonde S et al. Constitutive G(i2)-dependent activation of adenylyl cyclase type II by the 5-HT1A receptor. Inhibition by anxiolytic partial agonists. J Biol Chem 1999; 274:35469-35474.

5. Andrade R. Enhancement of ^-adrenergic receptors by Gi linked receptors in rat hippocampus. Neuron 1993; 10:83-88

6. Anholt RR. Signal transduction in the nervous system: Adenylyl cyclases as molecular coincidence detectors. Trends Neurosci 1994; 17:37-41.

7. Antoni FA, Palkovits M, Simpson J et al. Ca2+/calcineurin-inhibited adenylyl cyclase, highly abundant in forebrain regions, is important for learning and memory. J Neurosci 1998; 18:9650-9661.

8. Antoni FA, Simpson J, Sosunov A. Calcineurin regulated adenylyl cyclase in the brain. In: Gold B, Fisher G, Hergegen T, eds. Immunophilins in the Brain-FKBP-Ligands: Novel Strategies treatments of Neurodegenerative disorders. Barcelona: Prous Sci, 2000.

9. Baker LP, Nielsen MD, Impey S et al. Regulation and immunohistochemical localization of betagamma-stimulated adenylyl cyclases in mouse hippocampus. J Neurosci 1999; 19:180-192.

10. Bao JX, Kandel ER, Hawkins RD. Involvement of presynaptic and postsynaptic mechanisms in a cellular analog of classical conditioning at Aplysia sensory-motor neuron synapses in isolated cell culture. J Neurosci 1998; 18:458-466.

11. Bartsch D, Casadio A, Karl KA et al. CREB1 encodes a nuclear activator, a repressor, and a cytoplas-mic modulator that form a regulatory unit critical for long-term facilitation. Cell 1998; 95:211-223.

12. Baxter DA, Canavier CC, Clark Jr JW et al. Computational model of the serotonergic modulation of sensory neurons in Aplysia. J Neurophysiol 1999; 82:2914-2935.

13. Bernabeu R, Bevilaqua L, Ardenghi P et al. Involvement of hippocampal cAMP/cAMP-dependent protein kinase signaling pathways in a late memory consolidation phase of aversively motivated learning in rats. Proc Natl Acad Sci USA 1997; 94:7041-7046

14. Bernabeu R, Cammarota M, Izquierdo I et al. Involvement of hippocampal AMPA glutamate receptor changes and the cAMP/protein kinase A/CREB-P signalling pathway in memory consolidation of an avoidance task in rats. Braz J Med Biol Res 1997; 30:961-965.

15. Bernabeu R, Schmitz P, Faillace MP et al. Hippocampal cGMP and cAMP are differentially involved in memory processing of inhibitory avoidance learning. Neuro Report 1996; 31:585-5888.

16. Bevilaqua L, Ardenghi P, Schroder N et al. Drugs acting upon the cyclic adenosine monophosphate/protein kinase A signalling pathway modulate memory consolidation when given late after training into rat hippocampus but not amygdala. Behav Pharmacol 1997; 8:331-338.

17. Bevilaqua L, Ardenghi P, Schroder N et al. Agents that affect cAMP levels or protein kinase A activity modulate memory consolidation when injected into rat hippocampus but not amygdala. Braz J Med Biol Res 1997; 30:967-970.

18. Blitzer RD, Connor JH, Brown GP et al. Gating of CaMKII by cAMP-regulated protein phosphatase activity during LTP. Science 1998; 280:1940-1942.

19. Blitzer RD, Wong T, Nouranifar R et al. Postsynaptic cAMP pathway gates early LTP in hippocampal CA1 region. Neuron 1995; 15:1403-1414.

20. Bourne HR, Nicoll R. Molecular machines integrate coincident synaptic signals. Cell 72/ Neuron 1993; 10:65-75.

21. Bourtchuladze R, Frenguelli B, Blendy J et al. Deficient long-term memory in mice with a targeted mutation of the cAMP-responsive element-binding protein. Cell 1994; 79:59-68.

22. Brown GP, Blitzer RD, Connor JH et al. Long-term potentiation induced by theta frequency stimulation is regulated by a protein phosphatase-1-operated gate. J Neurosci 2000; 20:7880-7887.

23. Byrne JH, Kandel ER. Presynaptic facilitation revisited: state and time dependence. J Neurosci 1996; 16:425-435.

24. Cadogan AK, Kendall DA, Marsden CA. Serotonin 5-HT1A receptor activation increases cyclic AMP formation in the rat hippocampus in vivo. J Neurochem 1994; 62:1816-1821.

25. Cali JJ, Zwaagstra JC, Mons N et al. Type VIII adenylyl cyclase. A Ca2+/calmodulin-stimulated enzyme expressed in discrete regions of rat brain. J Biol Chem 1994; 269:12190-12195.

26. Cann MJ, Levin LR. Genetic characterization of adenylyl cyclase function. Adv Second Messenger Phosphoprotein Res 1998; 32:121-135.

27. Carew TJ, Hawkins RD, Kandel ER. Differential classical conditioning of a defensive withdrawal reflex in Aplysia californica. Science 1983; 219:397-400.

28. Castellucci VF, Blumenfeld H, Goelet P et al. Inhibitor of protein synthesis blocks long-term behavioral sensitization in the isolated gill-withdrawal reflex of Aplysia. J Neurosci 1989; 20:1-9.

29. Castellucci VF , Pinsker H , Kupfermann I et al. Neuronal mechanisms of habituation and dishabituation of the gill-withdrawal reflex in Aplysia. Science 1970; 167:1745-1748.

30. Chen Y, Harry A, Li J et al. Adenylyl cyclase 6 is selectively regulated by protein kinase A phosphorylation in a region involved in Galphas stimulation. Proc Natl Acad Sci U S A 1997; 94:14100-14104.

31. Chetkovich DM, Gray R, Johnston D et al. N-methyl-D-aspartate receptor activation increases cAMP levels and voltage-gated Ca2+ channel activity in area CA1 of hippocampus. Proc Natl Acad Sci USA 1991; 88:6467-6471.

32. Chiono M, Mahey R, Tate G et al. Capacitative Ca2+ entry exclusively inhibits cAMP synthesis in C6-2B glioma cells. Evidence that physiologically evoked Ca2+ entry regulates Ca(2+)-inhibitable adenylyl cyclase in nonexcitable cells. J Biol Chem 1995; 270:1149-1155.

33. Choi EJ, Xia Z, Storm DR. Stimulation of the type III olfactory adenylyl cyclase by calcium and calmodulin. Biochemistry 1992; 31:6492-6498.

34. Cho YH, Friedman E, Silva AJ. Ibotenate lesions of the hippocampus impair spatial learning but not contextual fear conditioning in mice. Behavioural Brain Research 1999; 98:77-87.

35. Cleary LJ, Lee WL, Byrne JH. Cellular correlates of long-term sensitization in Aplysia. J Neurosci 1998; 18:5988-5998.

36. Cohen NJ, Eichenbaum H. Memory, amnesia and the hippocampal system. Bradford Book, The MIT Press.

37. Cooper DMF, Karpen JW, Fagan K et al. Ca2+-sensitive adenylyl cyclases. Adv Second Messenger Phosphoprotein Res 1998; 32:23-53.

38. Cooper DMF, Mons N, Karpen JW. Adenylyl cyclases and the interaction between calcium and cAMP signalling. Nature 1995; 374:421-424.

39. Dash PK, Hochner B, Kandel ER. Injection of the cAMP-responsive element into the nucleus of Aplysia sensory neurons blocks long-term facilitation. Nature 1990; 345:718-721

40. Davis RL, Cherry J, Dauwalder B et al. The cyclic AMP system and Drosophila learning. Mol Cell Biochem 1995; 149-150:271-278.

41. De Rooij J, Rehmann H, van Triest M et al. Mechanism of regulation of the Epac family of cAMP-dependent RapGEFs. J Biol Chem 2000; 275:20829-20836.

42. De Rooij J, Zwartkruis FJ, Verheijen MH et al. Epac is a Rap1 guanine-nucleotide-exchange factor directly activated by cyclic AMP. Nature 1998; 396:474-477.

43. Dessauer CW, Tesmer JJ, Sprang SR et al. Identification of a Gi alpha binding site on type V adenylyl cyclase. J Biol Chem 1998; 273:25831-25839.

44. DeZazzo J, Tully T. Dissection of memory formation: from behavioral pharmacology to molecular genetics. Trends Neurosci 1995; 18:212-218.

45. Dubnau J, Tully T. Gene discovery in Drosophila: new insights for learning and memory. Annu Rev Neurosci 1998; 21:407-444.

46. Dubnau J, Tully T. Functional anatomy: from molecule to memory. Curr Biol 2001; 11:R240-243

47. Dudai Y, Jan Y-N, Byers D et al. Dunce, a mutant of Drosophila deficient in learning. Proc Natl Acad Sci USA 1976; 73:1684-1688.

48. Fagan KA, Graf RA, Tolman S et al. Regulation of a Ca2+-sensitive adenylyl cy

Was this article helpful?

0 0
Eliminating Stress and Anxiety From Your Life

Eliminating Stress and Anxiety From Your Life

It seems like you hear it all the time from nearly every one you know I'm SO stressed out!? Pressures abound in this world today. Those pressures cause stress and anxiety, and often we are ill-equipped to deal with those stressors that trigger anxiety and other feelings that can make us sick. Literally, sick.

Get My Free Ebook


Post a comment