Extrasynaptic localization is essential for α5GABAA receptor modulation of dopamine system function

Abstract

Dopamine system dysfunction, observed in animal models with psychosis-like symptomatology, can be restored by targeting Gamma-Aminobutyric Acid type A receptors (GABAAR) containing the 5, but not 1, subunit in the ventral hippocampus (vHipp). The reason for this discrepancy in efficacy remains elusive; however, one key difference is that 1GABAARs are primarily located in the synapse, whereas 5GABAARs are mostly extrasynaptic. To test whether receptor location is responsible for this difference in efficacy, we injected a small interfering ribonucleic acid (siRNA) into the vHipp to knock down radixin, a scaffolding protein that holds 5GABAARs in the extrasynaptic space. We then administered GL-II-73, a positive allosteric modulator of 5GABAARs (5-PAM) known to reverse shock-induced deficits in dopamine system function, to determine if shifting 5GABAARs from the extrasynaptic space to the synapse would prevent the effects of 5-PAM on dopamine system function. As expected, knockdown of radixin significantly decreased radixin-associated 5GABAARs and increased the proportion of synaptic 5GABAARs, without changing the overall expression of 5GABAARs. Importantly, GL-II-73 was no longer able to modulate dopamine neuron activity in radixin-knockdown rats, indicating that the extrasynaptic localization of 5GABAARs is critical for hippocampal modulation of the dopamine system. These results may have important implications for clinical use of GL-II-73, as periods of high hippocampal activity appear to favor synaptic 5GABAARs, thus efficacy may be diminished in conditions where aberrant hippocampal activity is present. Significance StatementDopamine activity is known to be altered in both psychosis patients and in animal models, with promising new antipsychotics restoring normal dopamine system function. One such drug is GL-II-73, a positive allosteric modulator of 5GABAARs (5-PAM). Interestingly, previous research has shown that a positive allosteric modulator of 1GABAARs (1-PAM) does not share this ability, even when directly given to the ventral hippocampus, a region known to modulate dopamine activity. One potential explanation for this difference we examined in this study is that 1GABAARs are primarily located in the synapse, whereas 5GABAARs are mostly extrasynaptic. Determining the mechanism of this differential efficacy could lead to the refinement of antipsychotic treatment and improve patient outcomes overall.