Pharmacological studies reveal novel aspects of the versatility of GABA_A receptors

When focusing on specific tasks of GABAergic transmission, it is not very common to realize an extraordinary multi-functionality of the GABAergic system. In the present article, I would like to emphasize some aspects of the amazing versatility of GABA_A receptors (GABA_ARs) and, on this background, to highlight novel findings by Mortensen et al. (2010) in a recent issue of The Journal of Physiology relating to distinct kinetics and pharmacology of synaptic and extrasynaptic GABA_ARs.

In early postnatal life, GABA is depolarizing and this mode of action coincides with intense growth of neurons and synaptogenesis, suggesting involvement in trophic mechanisms. Although glutamatergic cells outnumber the GABAergic interneurons, the latter show the largest morphological and functional diversity (e.g. in CA1 hippocampal region, 21 interneurons have been identified, Klausberger and Somogyi, 2008). GABA_ARs are highly heterogeneous and as many as 19 subunits have been cloned (α1–6, β1–3, γ1–3, δ, ɛ, π, θ, ρ1–3) and, most importantly, distinct GABA_AR types show different kinetics and pharmacology. The kinetics of GABAergic inhibitory postsynaptic currents (IPSCs) depends both on the interneuron type and on the postsynaptic cell identity, and IPSC decay may vary from 1 ms (e.g. parvalbumin positive basket cells) up to several tens of milliseconds (e.g. interneurons in thalamic reticular nucleus or neurogliaform cells, NGCs). Such a wide range of IPSC kinetics reflects differential expression of postsynaptic GABA_ARs with specific subunit compositions. However, IPSC kinetics and pharmacology strongly depend also on the spatiotemporal profile of the synaptic agonist (Mozrzymas, 2004). Several lines of evidence indicate that the synaptic GABA transient is extremely fast, enabling thus a high temporal precision of IPSCs. In a recent report, however, Oláh et al. (2009) have proposed that NGCs in the cortex are responsible for a volume release of GABA. Thus, besides mediating a precise point-to-point action of ‘classical’ IPSCs, GABA may be released from NGCs to mediate a paracrine modulatory action, mainly by increasing tonic inhibition. Interestingly, discovery of tonic GABAergic inhibition has preceded that of GABA volume release by more than one decade. The classical description of tonic inhibition is based on the observation that ambient GABA concentration is sufficient to induce tonic current that, typically, carries more electric charge than the phasic (synaptic) inhibition. From the kinetic point of view, phasic and tonic currents appear to represent two extreme ‘poles’ of GABAergic multi-functionality, yet they may be tightly coupled at the functional level, e.g. in synchronizing interneurons and controlling gamma frequency in hippocampal networks (Mann & Mody, 2010). Phasic and tonic inhibition and their functional interplay are thus a spectacular example of the versatility of GABA_A receptors. Tonic currents are mediated mainly by high-affinity δ-subunit containing GABA_ARs (GABA_ARs-δ) that are exclusively extrasynaptic. Moreover, while typical synaptic receptors show rapid and profound desensitization, GABA_ARs-δ are thought to only weakly desensitize. Another peculiar feature of GABA_ARs-δ is a weak coupling between binding of GABA and receptor gating that is manifested by a low efficacy. Interestingly, modulators enhancing the function of these receptors (e.g. neurosteroids) up-regulate the receptor efficacy, being ineffective on the agonist affinity. Similarly, THIP, a super-agonist of GABA_ARs-δ, elicits a markedly larger response than GABA due to a higher efficacy. The major features of GABA_ARs-δ emerging from current knowledge are thus high affinity, low efficacy and weak desensitization.

Mortensen et al. (2010) have investigated the activation properties of three recombinant GABA_ARs with subunit composition typical for a synaptic (α1β3γ2), an extrasynaptic (α4β3δ) and an intermediary receptor (α4β3γ2), using three agonists: GABA, muscimol and THIP. One of their major goals was to further explore the mechanisms of THIP super-agonism at the GABA_ARs-δ in comparison to GABA and muscimol. They confirmed that at α4β3δ receptors, THIP showed a super-agonist activity and GABA acted as a partial agonist. Interestingly, the opposite was observed for THIP and GABA at α1β3γ2 and α4β3γ2 receptors. An interesting, although indirect, finding is that the impact of α4β3δ receptor desensitization may be substantial, and in the continuous presence of low GABA concentration (0.3 μm), there might be a considerable proportion of desensitized receptors. Thus, the view of the α4β3δ receptor as nearly non-desensitizing requires revision. While the data of Mortensen et al. (2010) largely confirm that super-agonist activity of THIP at α4β3δ relies mainly on increased efficacy, curiously, THIP induces at this receptor a stronger desensitization than GABA. Particularly interesting is the proposal that the super-agonist action of musicmol at α4β3δ is not related to a larger efficacy (that is smaller than in the case of GABA) but results from a smaller extent of desensitization. This finding indicates that desensitization may be regarded as an additional ‘degree of freedom’ in the super-agonist action. It is worth noting that the model applied by Mortensen et al. (2010) predicts that in response to saturating agonist, a fraction of bound receptors desensitize before opening, and reduction of receptor entry into the desensitized state would enhance the current amplitude due to preferential transitions from the bound closed into the bound open state. This points to a simple realization of the super-agonist action due to a dynamic redistribution of receptors between open and desensitized conformations. Interestingly, it has been previously reported that agents reducing GABA_AR desensitization (e.g. low pH) may strongly enhance the amplitude of GABAergic currents elicited by saturating agonist due to the above-mentioned preferential entry into the open state (Mozrzymas et al., 2003). In conclusion, the work of Mortensen et al. (2010) provides a thorough description of synaptic and extrasynaptic GABA_AR activation mechanisms and proposes a novel mechanism underlying the super-agonist action of muscimol at the extrasynaptic receptors.