Extrasynaptic localization is essential for α5GABA _A receptor modulation of dopamine system function

Dopamine system dysfunction, observed in animal models with psychosis-like symptomatology, can be restored by targeting Gamma-Aminobutyric Acid type A receptors (GABA _A R) containing the α5, but not α1, subunit in the ventral hippocampus (vHipp). The reason for this discrepancy in efficacy remains elusive; however, one key difference is that α1GABA _A Rs are primarily located in the synapse, whereas α5GABA _A Rs 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 α5GABA _A Rs in the extrasynaptic space. We then administered GL-II-73, a positive allosteric modulator of α5GABA _A Rs (α5-PAM) known to reverse shock-induced deficits in dopamine system function, to determine if shifting α5GABA _A Rs from the extrasynaptic space to the synapse would prevent the effects of α5-PAM on dopamine system function. As expected, knockdown of radixin significantly decreased radixin-associated α5GABA _A Rs and increased the proportion of synaptic α5GABA _A Rs, without changing the overall expression of α5GABA _A Rs. Importantly, GL-II-73 was no longer able to modulate dopamine neuron activity in radixin-knockdown rats, indicating that the extrasynaptic localization of α5GABA _A Rs 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 α5GABA _A Rs, thus efficacy may be diminished in conditions where aberrant hippocampal activity is present.

Dopamine 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 α5GABA _A Rs (α5-PAM). Interestingly, previous research has shown that a positive allosteric modulator of α1GABA _A Rs (α1-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 α1GABA _A Rs are primarily located in the synapse, whereas α5GABA _A Rs are mostly extrasynaptic. Determining the mechanism of this differential efficacy could lead to the refinement of antipsychotic treatment and improve patient outcomes overall.