Interactions of tricyclic antipsychotic and antidepressant medications with a novel binding site in GABAA receptors

Abstract

Background and Purpose: Many psychotherapeutic drugs including clozapine have a polypharmacological profile and act on GABA receptors, where subtype-specific information is often lacking. Patients with schizophrenia show alterations in function, structure and molecular composition of the hippocampus, and a recent study demonstrated aberrant levels of hippocampal α5 subunit containing GABA receptors. Experimental Approach: Functional studies of GABA modulatory effects by antipsychotic and antidepressant medications were performed in several GABA receptor subtypes by two‐electrode voltage‐clamp electrophysiology using Xenopus laevis oocytes. Computational structural analysis was employed to design mutated constructs of the α5 subunit, probing a novel binding site. Computational ligand analysis complemented the functional and mutational data. Key Results: We show that the antipsychotic drugs clozapine and chlorpromazine have negative modulatory effects on multiple GABA receptor subtypes, including α5-containing. On the latter we show negative modulatory effects for five additional antipsychotic and antidepressant drugs. Based on a chlorpromazine binding site observed in a GABA-gated bacterial homologue, we identified a novel site in α5 GABA receptor subunits. Conclusion and Implications: Our findings support previous studies suggesting a link between some of the therapeutic effects of clozapine and its negative modulatory action on certain GABA receptor subtypes. The novel site we describe in this study is a new potential target for optimizing antipsychotic medications with beneficial polypharmacology.

Probes for the heterogeneity of muscimol binding sites in rat brain

“…Here we now describe differential muscimol displacement by three tricyclic ligands clozapine, loxapine and chlorpromazine, compared to bicuculline. Those compounds are well known frequently prescribed antipsychotics, which elicit their effect mostly by targeting dopamine as well as serotonin receptors (Mauri et al, 2014), and have been suggested to also mediate at least part of their action by targeting GABA A receptors Saederup, 1997, 1998;Bampali et al, 2022). Our results suggest that those three compounds not only interact with GABA A receptors but can also distinguish between different receptors containing different muscimol sites.…”

“…In Figure 1A we show 3H-muscimol displacement in cerebellar membranes by BIC, LOX, CLZ and CPZ with the protocol as or study by Korpi et al (1995), displacement by CLZ is less in cerebellar membranes compared to hippocampal membranes. In contrast, CPZ which did not displace 3H-muscimol from hippocampal membranes (Bampali et al, 2022), displaced in the cerebellar membranes by 14%. Intrigued by these differences between hippocampal membranes and cerebellar membranes, we investigated thalamus and striatum in addition.…”

“…Inspired by our previous findings of incomplete 3H-muscimol displacement by several antipsychotic drugs in hippocampal membranes (Bampali et al, 2022), we studied cerebellar membranes, which have been investigated previously for CLZ (Korpi et al, 1995). In Figure 1A we show 3H-muscimol displacement in cerebellar membranes by BIC, LOX, CLZ and CPZ with the protocol as or study by Korpi et al (1995), displacement by CLZ is less in cerebellar membranes compared to hippocampal membranes.…”

“…In a previous study from our lab (Bampali et al, 2022) we analyzed several different, clinically frequently used antipsychotic and antidepressant medications for their effects on binding to and modulating several GABA A receptor subtypes. We used 3Hmuscimol displacement experiments to complement our twoelectrode voltage-clamp electrophysiological analysis on Xenopus laevis oocytes (see Figures 6e and S11 of Bampali et al, 2022).…”

“…In a previous study from our lab (Bampali et al, 2022) we analyzed several different, clinically frequently used antipsychotic and antidepressant medications for their effects on binding to and modulating several GABA A receptor subtypes. We used 3Hmuscimol displacement experiments to complement our twoelectrode voltage-clamp electrophysiological analysis on Xenopus laevis oocytes (see Figures 6e and S11 of Bampali et al, 2022). While all studied antipsychotic compounds exerted functional inhibition on multiple GABA A receptor subtypes, our radioligand displacement data indicated that from our selection of compounds only clozapine (CLZ), loxapine (LOX) and bicuculline (BIC) interacted directly with the orthosteric GABA binding site.…”