Benzodiazepines And Benzodiazepine Receptors

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MOLECULAR TARGETS FOR THE GABAa RECEPTOR

The first clues to the mechanism of action of benzodiazepines came from landmark experiments (Squires and Braestrup 1977; Moehler and Okada 1977) which showed that

Response

Loss of consciousness Muscle relaxation Hypnosis Sedation Ataxia Anti-epileptic Anti-anxiety

Dose of benzodiazepine Low M-► High

Figure 19.4 The activity spectrum of the benzodiazepines. Motor impairment and CNS depression increases with drug dose. (Based on data for chlordiazepoxide (Sternbach, Randall and Gustafson 1964))

[3H]diazepam binds to a specific site in the brain. Studies of solubilised receptors confirmed that this binding site was a component of the GABAa receptor which incorporates a Cl_ channel. GABA did not compete with [3H]benzodiazepine for binding to this receptor and so it was clear that their binding domains were not the same. It was soon realised that there is an allosteric interaction between them such that binding of [3H]benzodiazepines is increased by GABA (Fig. 19.5). This is thought to be due to an interaction between the GABA recognition site on a ¿-subunit of the GABAA receptor and the benzodiazepine recognition site on an a-subunit (see Chapter 11 and Doble and Martin 1996). The overall effect of benzodiazepines is to augment the increase in Cl_ conductance caused by GABA and thereby potentiate its inhibitory actions; they achieve this by increasing the probability (and as a consequence, the frequency) of Cl_ channel opening. This action is thought to play a crucial role in the anti-anxiety effects of these drugs. The progressive increase in CNS depression, as drug dose is increased, is attributed to an increase in receptor occupancy, although the extent to which binding in different brain regions contributes to these different actions is not known.

Barbiturates bind non-competitively to yet another, functionally distinct, domain on the receptor which is thought to be directly associated with the Cl_ channel itself. Although there is an allosteric interaction with GABA, as with the benzodiazepines, barbiturates also directly increase Cl_ conductance by increasing the duration of channel opening. Thus, by contrast with the benzodiazepines, the barbiturates activate this receptor even in the absence of GABA. This explains why antagonists of the GABA binding site, such as bicuculline, block the actions of benzodiazepines, but not those of the barbiturates, and probably accounts for the greater toxicity of the barbiturates in overdose. In contrast, so-called 'cage convulsants' (e.g. picrotoxinin, pentylenetetrazol and z-butylbicycloorthobenzoate ('TBOB')) are thought to bind directly to another site on the Cl_ channel and to reduce Cl_ conductance. In recent years, the range of

Dose of benzodiazepine Low M-► High

Figure 19.4 The activity spectrum of the benzodiazepines. Motor impairment and CNS depression increases with drug dose. (Based on data for chlordiazepoxide (Sternbach, Randall and Gustafson 1964))

Extreme Cases Animal Abuse
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Figure 19.5 A schematic diagram of the GABAa receptor. Binding of GABA to its domain on the receptor opens a Cl— channel. This action of GABA is augmented (+) by agonist benzodiazepines that bind to their own domain (BDZr) on the GABAA receptor and trigger an allosteric interaction with the GABA binding site. Other compounds that act in this way include the neurosteroids (e.g. allopregnanolone) and barbiturates but these compounds also bind to the Cl— channel directly and, at high concentrations, increase Cl— conductance in the absence of GABA. Some compounds, such as BDZr inverse agonists, have the opposite effect: i.e. they bind to the BDZr site but reduce Cl— channel opening through a negative allosteric interaction (—) with the GABA binding site. The so-called 'cage convulsants' (e.g. picrotoxinin, pentylenetetrazol and i-butylbicycloorthobenzoate ('TBOB'), which also reduce Cl— conductance, are thought to bind directly to a site on the Cl— channel compounds shown to have binding domains on the GABAA receptor has steadily increased and it is thought that there could be even more.

BENZODIAZEPINE RECEPTOR SUBTYPES

The discovery of the benzodiazepine receptor was quickly followed by their subdivision as evidenced by (see also Chapter 11):

(1) The biphasic dissociation of [3H]flunitrazepam binding (Chiu, Dryden and Rosenberg 1982).

(2) Competition binding studies showing that when using compounds like ¿-CCE (ethyl-^-carboline-3-carboxylate), which bind to the benzodiazepine receptor, the displacement curve for [3H]flunitrazepam was shallow in the hippocampus and cortex. In contrast, in the cerebellum, the curve was steep with a Hill coefficient of 1 (Duggan and Stephenson 1988). (3) Photoaffinity labelling confirming that there was more than one type of benzodiazepine receptor, rather than multiple sites on the same receptor. In this technique [3H]flunitrazepam binding is carried out under ultra-violet light which renders most of the ligand binding irreversible. Purification of the radioligand/receptor complex revealed a 51 kDa protein in the cerebellum (now known to be the a1 subunit of the 'BZ1 receptor') but quite different protein(s) in the hippocampus (now known to be the a2 and a3 subunits of the 'BZ2 receptor') (Sieghart and Karobath 1980).

The discovery of these receptor subtypes kindled the hope that it would be possible to develop subtype-selective drugs with specific clinical actions: i.e. that it would at last be feasible to produce an anti-anxiety agent that was genuinely non-sedative. It is now known that the GABAa receptor comprises different combinations of subunits, probably in a pentameric complex (see Chapter 11), and so it might even be possible to develop drugs that target different subunits, thereby increasing their functional specificity. One such compound has already been developed, the imidazopyridine zolpidem (Fig. 19.6), which was initially regarded as a BZj receptor ligand but is now classified as a high-affinity ligand for the a1 subunit of GABAA receptors (in contrast to a2 and a3 subunits which produce the so-called BZ2 receptor). Notwithstanding this selectivity, zolpidem turns out to be a potent hypnotic agent. Zopiclone is another hypnotic, available in the clinic; this drug displaces benzodiazepines from the GABAA receptor but lacks subunit selectivity and does not seem to target precisely the same binding domain as the benzodiazepines.

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