Ion Channels Affecting The Pattern And Frequency Of Action Potential Discharges

The opening of Na+ ion channels for the initiation of neuronal depolarisation and action potential generation, as described above, can be induced by excitatory neuro-transmitters acting on receptors that are directly linked to cation channels. These include glutamate AMPA receptors (Chapters 3 and 10) and ACh nicotinic receptors (Chapter 6). The inhibitory neurotransmitter GABA has an opposing effect through receptors (GABAa) that are directly linked to the opening of chloride channels, inducing an influx of Cl — ions and subsequent hyperpolarisation (Chapters 1 and 11). There are, however, a number of other ion channels, generally for K+ or Ca2+, that have a more subtle controlling effect on neuronal activity. Their opening may be initiated by (or dependent on) preceding changes in membrane potential and ion flux, but they can be affected indirectly by various neurotransmitters, e.g. monoamines and peptides, acting on receptors linked to second messenger systems or more directly by various chemicals, some of which have clinical use. The role of these channels in controlling the overall activity of neurons is clearly important and needs to be considered.

'SLOW' K+ CHANNELS AND ADAPTATION

The K+ channels responsible for action potential repolarisation close fairly soon after repolarisation (usually within 5-10 ms). However, most nerve cells possess other K+ channels which are opened during nerve cell discharges but which stay open much longer. These do not contribute much to the repolarisation of individual action potentials but instead affect the excitability of the neuron over periods of hundreds of milliseconds or even seconds.

Two principal types of channel having this effect have been identified and their properties are summarised in Table 2.2. The first type (Ca2+-activated K+ channels or KCa channels) are opened ('gated'), not by membrane voltage but by a rise in intra-cellular Ca2+ ion concentration. This means that they are activated by the Ca2+ influx through voltage-gated Ca2+ channels when these are opened during a somatic or dendritic action potential, or during trains of action potentials. They then close slowly as the intracellular Ca2+ concentration recovers, so producing a long-lasting afterhyperpolarisation (AHP) following an action potential or after trains of action potentials. The particular KCa channels thought to be responsible for the long AHP have a low conductance (~ 10 pS in symmetrical high [K+] solution) so are called 'SK' ('small-conductance K') channels: three variants of SK channel have now been cloned, SK1, 2 and 3. These are resistant to normal K+ channel blocking agents such as tetraethylammonium or 4-aminopyridine, but can be selectively blocked (with varying affinities) by the bee-venom apamin or by certain quaternary ammonium compounds such as tubocurarine and derivatives therefrom.

The second type (M-channels) are voltage-gated, like delayed rectifier channels, but have a lower threshold (around — 60 mV) and open 10-100 times more slowly when the

Table 2.2 'Slow' potassium channels

Present in

Type

Descriptor Gene products Activated by Threshold Blocked by Inhibited by

Function

Ca-activated

SKCa

SK1-3

Intracellular Ca2+ > 100 nM [Ca2+]in Apamin (SK2 > SK3 > SK1) Acetylcholine1 Glutamate2 Noradrenaline3 5-Hydroxytryptamine4 Autonomic neurons Cortical pyramidal cells Hippocampal pyramidal cells Spike frequency adaptation

M-type km

KCNQ2/3

Voltage

Linopirdine

Acetylcholine1

Glutamate2

Peptides5

Sympathetic neurons Cortical pyramidal cells Hippocampal pyramidal cells Spike frequency adaptation Membrane potential stabilisation m

1Via ml, m3 muscarinic receptors.

2Via mGluR1,5 'metabotropic' glutamate receptors.

3Via ¿-adrenoceptors.

4Via 5-HT2 receptors.

5Including bradykinin, angiotensin, substance P.

membrane is depolarised. They were originally called M-channels because they were inhibited by activating Muscarinic acetylcholine receptors. (This turns out not to be a very good definition since other channels can be inhibited by these receptors but the name has stuck.) It is now known that M-channels are composed of protein products of members of the KCNQ family of K+ channel genes, mutations of which can give rise to certain forms of inherited epilepsy or deafness (depending where the proteins are expressed). M-channels, like SK channels, are generally resistant to common K+ channel-blocking agents, but are selectively blocked by the 'cognition-enhancer' linopirdine and congeners thereof.

In spite of their different structure and gating mechanisms, these channels have quite a lot in common in functional terms. First, they both open and close slowly. (The SKCa channels open slowly because of the time taken to build up the required concentration of Ca2+ in the cell, and close slowly because it takes hundreds of milliseconds or seconds for the Ca2+ to be extruded. M-channels open and close slowly because of their slow intrinsic gating.) Second, although there are differences in their distribution among different types of neuron (e.g. M-channels are abundant in sympathetic neurons whereas SKCa channels are more important in the enteric neurons in the intestine), they also co-exist in many neurons (such as hippocampal and cortical pyramidal cells — including human cells). Third, they have rather similar effects when they open (see below). Fourth, they are both closed by some important neurotransmitters. Thus, acetylcholine (acting via muscarinic receptors) and glutamate (acting via metabotropic glutamate receptors) close both types of channel, but noradrenaline (acting via ¿6-adrenoceptors) closes only the KCa channels. This effect makes an important contribution to the postsynaptic action of these transmitters, and is discussed further below.

Figures 2.5 and 2.6 shows some experimental records illustrating the function of these channels. Figure 2.5 illustrates the function of SKCa channels in a hippocampal pyramidal neuron. In the record marked 'control' in Fig. 2.5(b) the neuron was depolarised by injecting a 1-s long depolarising current. This makes it fire action

(a) Control Muscarine

(a) Control Muscarine

Figure 2.5 Effects of inhibiting SKCa Ca2+-activated K+ channels by stimulating muscarinic acetylcholine receptors (mAChRs) in rat hippocampal pyramidal neurons. (Micro-electrode recordings.) (a) Records showing SKCa current (I) and intracellular [Ca2+ ] transient (Ca) following a 50ms depolarisation (V). The depolarisation opens voltage-gated Ca2+ channels. The resultant Ca2+ influx leads to a rise in intracellular [Ca2+ ] that (after a delay) activates the KCa current. The mAChR agonist muscarine (10 p,M) does not affect the Ca2+ rise but inhibits the subsequent opening of the SKCa channels. (Adapted from Fig. 3 in Knopfel, T et al. (1990) Proc. Natl. Acad. Sci. USA 87: 4083-4087. Reproduced with permission). (b) Records showing the effect of inhibiting the SKCa current on the firing properties of a hippocampal neuron. Under normal circumstances (control) the development of the SKCa current arrests action potential firing during tonic depolarisation induced by injecting 1-s depolarising current ('spike frequency-adaptation'). When the SKCa current is inhibited with acetylcholine (ACh, 200 p,M) (see (a)) spike frequency-adaptation is reduced. This effect is reversed by adding 0.5 atropine, to block the mAChRs. Reprinted (adapted from Fig. 1) with permission, from Acetylcholine Mediates a Slow Synaptic Potential in Hippocampal Pyramidal Cells, Cole, AE and Nicoll, RA (1983) Science 221: 1299-1301). American Association for the Advance of Science

Figure 2.6 Effect of inhibiting M-type K+ channels in rat superior cervical sympathetic neurons with the muscarinic acetylcholine-receptor (mAChR) stimulant, muscarine. Micro-electrode recordings from different neurons. (a). Current responses to +10mV voltage steps from —50mV holding potential. (b). Voltage responses to injecting depolarising and hyperpolarising currents from an initial resting potential of around —47mV. Under control conditions, depolarisation produces a slow activation of the voltage-gated K+ current, /K(M) ('/M' in (a)); this raises the threshold for action potential generation so that the imposed depolarisation in (b) produces only a single action potential (i.e. this neuron, like that in Fig. 2.5, shows strong 'spike frequency-adaptation'). Muscarine strongly reduces /K(M); removal of this braking current now allows the neuron to fire a train of action potentials during the depolarising current injection. (Records in (a) A Constanti and DA Brown, unpublished; records in (b) adapted from Fig. 7 in Intracellular Observations on the Effects of Muscarinic Agonists on Rat Sympathetic Neurones by Brown DA and Constanti, A (1980) Br. J. Pharmacol. 70: 593-608.) Reproduced by permission of Nature Publishing Group

Figure 2.6 Effect of inhibiting M-type K+ channels in rat superior cervical sympathetic neurons with the muscarinic acetylcholine-receptor (mAChR) stimulant, muscarine. Micro-electrode recordings from different neurons. (a). Current responses to +10mV voltage steps from —50mV holding potential. (b). Voltage responses to injecting depolarising and hyperpolarising currents from an initial resting potential of around —47mV. Under control conditions, depolarisation produces a slow activation of the voltage-gated K+ current, /K(M) ('/M' in (a)); this raises the threshold for action potential generation so that the imposed depolarisation in (b) produces only a single action potential (i.e. this neuron, like that in Fig. 2.5, shows strong 'spike frequency-adaptation'). Muscarine strongly reduces /K(M); removal of this braking current now allows the neuron to fire a train of action potentials during the depolarising current injection. (Records in (a) A Constanti and DA Brown, unpublished; records in (b) adapted from Fig. 7 in Intracellular Observations on the Effects of Muscarinic Agonists on Rat Sympathetic Neurones by Brown DA and Constanti, A (1980) Br. J. Pharmacol. 70: 593-608.) Reproduced by permission of Nature Publishing Group potentials. However, the action potentials open Ca channels, so intracellular Ca2+ gradually rises as shown in Fig. 2.5(a), and this in turn opens SKCa channels to produce an outward (hyperpolarising) current. This current partly repolarises the cell and raises the threshold for action potential generation, so the action potential train in Fig. 2.5(b) dies out. The KCa channels were then inhibited with acetylcholine (or an analogue, muscarine). Now the SKCa channels cannot open, even though intracellular [Ca2+] still rises (Fig. 2.5(a)). This allows the action potential discharge to continue throughout the length of the depolarising current injection (Fig. 2.5(b)). Thus, the SKCa channels induce an adaptation of the action potential discharge to a maintained stimulus: this adaptation is lost when the SKCa channels are prevented from opening.

Figure 2.6 shows the effect of the M-channels on the action potential discharges of a rat sympathetic neuron during an equivalent (1-s) injection of depolarising current. (Hyperpolarising currents were also injected in this experiment, giving the downward voltage response.) This cell shows even stronger adaptation under normal circumstances ('control'), because the depolarisation itself is sufficient to open extra M-channels, even without the action potentials (Fig. 2.6(a)). When the opening of M-channels is inhibited by muscarine, this adaptation is again lost. Also note that muscarine has actually depolarised the cell — the level of membrane potential before injecting the current pulse has changed. This is because a few M-channels are open at the resting potential and actually contribute to the resting potential.

As mentioned above, M-channels and KCa channels co-exist in many neurons. This may seem odd, since Figs 2.5 and 2.6 suggest that they have the same effect. However, in practice, their effects are slightly different, depending on the pattern of stimulation, and in fact the two currents act synergistically — i.e. the effect of inhibiting both currents is far greater than the sum of inhibiting each individually. Their inhibition (separately or together) by neurotransmitters such as acetylcholine and noradrenaline removes a 'brake' on neural discharges and thereby induces a sustained increase in excitability. This is the prime mechanism underlying the arousal and attention-directing function of the ascending cholinergic and aminergic systems innervating the pyramidal cells of the cerebral cortex and hippocampus; the failure of this function, due to inadequate transmitter release, is thought to contribute to the cognitive deficits in such diseases as Alzheimer's disease.

SKCa and M channels are not the only K+ channels regulated by transmitters. As noted above, transmitters can also close, or open, other K+ channels that do not directly regulate excitability but instead determine the resting potential of the neuron, and hence depolarise or hyperpolarise the neuron.

Ca2+ CHANNELS: PLATEAU POTENTIALS AND PACEMAKING

As pointed out above, although the principal function of voltage-gated Ca2+ channels is to provide the charge of Ca2+ necessary for transmitter release, Ca2+ channels are also present on the somata and dendrites of most neurons. These include two classes of Ca2+ channel not involved in transmitter release — dihydropyridine-sensitive high-threshold L-type channels, homologous to the cardiac Ca2+ channels responsible for ventricular contraction and some pacemaking activity; and low-threshold, rapidly-inactivating T-type Ca2+ channels. These have multiple functions.

First, their opening during somato-dendritic action potentials provides the source of the increased intracellular [Ca2+] required to open Ca2+-activated K+ channels — BK channels, to accelerate spike repolarisation, and SK channels, to induce spike-train adaptation and limit repetitive firing. The BK channels are activated (primarily) following entry of Ca2+ through L-type channels; the source of Ca2+ for SK channel activation varies with different neurons, and may be either through L-type or N-type channels.

Second, as in the ventricular muscle fibres of the heart, opening of L-type channels can generate sustained plateau potentials following the initial Na2+-mediated action potential — for example, in the rhythmically firing neurons of the inferior olive (Fig. 2.7).

The T-type channels have a special 'pacemaking' function. This is well illustrated in thalamic relay neurons (Fig. 2.8). At resting potentials ^ — 60 mV, these channels are inactivated and hence non-conducting (a voltage-sensitive closure process resembling Na+ channel inactivation). Under these conditions, the relay neurons show sustained rhythmic firing when tonically depolarised. However, if the neurons are first hyper-polarised, T-channel inactivation is removed. Then, when the cells are depolarised, the T-channels open and generate a depolarising 'Ca2+ spike'. This in turn induces a rapid

Guinea Pig Ion Current

Figure 2.7 Oscillatory behaviour of guinea-pig inferior olivary neurons, The initial action potential, induced by the opening of conventional voltage-gated Na+ channels, in turn opens voltage-gated L-type Ca2+ channels to produce a 'plateau potential', The Ca2+ entry activates KCa channels, to produce a long-lasting (several hundred ms) after-hyperpolarisation, This de-inactivates the transient (T-type) Ca2+ channels (see Fig, 2,8), Hence, as the Ca2+ is extruded and the KCa current declines, the low-threshold T-type Ca2+ channels open, and the cell depolarises to reach the threshold for the Na+ channel, giving a new action potential, and so on, The interval between the action potentials is 650ms, (Adapted from Fig, 7 in Llinas, R and Yarom, Y (1981) J. Physiol, 315: 569-584, Reproduced by permission of the Physiological Society)

Figure 2.7 Oscillatory behaviour of guinea-pig inferior olivary neurons, The initial action potential, induced by the opening of conventional voltage-gated Na+ channels, in turn opens voltage-gated L-type Ca2+ channels to produce a 'plateau potential', The Ca2+ entry activates KCa channels, to produce a long-lasting (several hundred ms) after-hyperpolarisation, This de-inactivates the transient (T-type) Ca2+ channels (see Fig, 2,8), Hence, as the Ca2+ is extruded and the KCa current declines, the low-threshold T-type Ca2+ channels open, and the cell depolarises to reach the threshold for the Na+ channel, giving a new action potential, and so on, The interval between the action potentials is 650ms, (Adapted from Fig, 7 in Llinas, R and Yarom, Y (1981) J. Physiol, 315: 569-584, Reproduced by permission of the Physiological Society)

'burst' of Na+ action potentials, The burst is arrested first because the Na+ channels inactivate, and then because the T-type Ca2+ channels inactivate, Both inactivation processes are removed when the cell hyperpolarises back again, so becoming available for another burst, As a result, the cells change their firing pattern from tonic firing to burst-firing simply dependent on membrane potential, This is thought to explain the switch between tonic firing in awake animals to burst-firing during slow-wave sleep, In the awake state, the neurons are maintained in a tonic state of depolarisation due to the release of neurotransmitters such as histamine and acetylcholine, which inhibit K+ currents (see above), but hyperpolarise during slow-wave sleep when transmitter release diminishes — or when the receptors for the transmitters are blocked by anti-histamines or anti-cholinergic drugs, However, it should be emphasised that T-channels are quite widely distributed and their burst-inducing properties may also be important in some forms of epilepsy since they can be blocked by certain anti-epileptic drugs, such as ethosuximide,

Finally, entry of Ca2+ through somatic and dendritic Ca2+ channels activates calmodulin-dependent protein kinases to modulate transcription, and thereby plays a crucial role in certain components of neural development and plasticity,

Neither L nor T channels appear susceptible to the form of G-protein-mediated inhibition characteristic of N or P/Q channels, However, as in the heart cells, L-type Ca2+

channels in the nervous system are susceptible to more indirect forms of modulation (both enhancement and inhibition) through receptor-mediated phosphorylation.

ANOTHER PACEMAKER CHANNEL: HYPERPOLARISATION-ACTIVATED CATION CHANNELS ('h-CHANNELS')

Another analogy with heart cells is the presence in many neurons of the cardiac pacemaker current Ih. (The neural current is sometimes dubbed Iq.) This is a mixed cation current carried by channels that are permeant to both Na+ and K+ and which are opened by hyperpolarising the membrane — i.e. at potentials negative to the normal resting potential. It serves the same function as in the heart, to act as a pacemaker current. The way this works is illustrated in Fig. 2.9. As the membrane hyperpolarises (e.g. after an action potential, when K+ currents are active), h-channels open to give an inward (depolarising) current (Fig. 2.9(a)). This leads to a slow depolarisation until the threshold for the T-type Ca2+ channels open, leading to a rapid depolarisation and spiking (Fig. 2.9(b)). The h-channels then switch off (because the cell is depolarised) and reopen during the subsequent hyperpolarisation. In this way sustained oscillations of membrane potential, leading to a steady rhythmic action potential discharge, can be maintained. The h-channels are blocked by low concentrations of Cs+ ions, or by agents which block the cardiac current and slow the heart: such agents inhibit the neural membrane potential oscillations and discharges. Also like the cardiac pacemaker, the neural h-current is regulated by transmitters that activate adenylate cyclase, such as noradrenaline and 5-hydroxytryptamine: the cyclic AMP shifts the activation curve to more positive membrane potentials (by a direct action on the channels, not through phosphorylation), so accelerating the depolarisation and increasing the neural rhythm. Conversely, transmitters or mediators that inhibit adenylate cyclase, like enkephalins and adenosine, shift the activation curve to more negative potentials and slow rhythmic discharges.

Was this article helpful?

0 0

Post a comment