Pregnenolone sulfate analogues differentially modulate GABAA receptor closed/desensitised states

Abstract

Background and purpose: GABA_A receptors are regulated by numerous classes of allosteric modulators. However, regulation of receptor macroscopic desensitisation remains largely unexplored and may offer new therapeutic opportunities. Here, we report the emerging potential for modulating desensitisation with analogues of the endogenous inhibitory neurosteroid, pregnenolone sulfate.

Experimental approach: New pregnenolone sulfate analogues were synthesised incorporating various heterocyclic substitutions located at the C-21 position on ring D. The pharmacological profiles of these compounds were assessed using electrophysiology and recombinant GABA_A receptors together with mutagenesis, molecular dynamics simulations, structural modelling and kinetic simulations.

Key results: All seven analogues retained a negative allosteric modulatory capability whilst exhibiting diverse potencies. Interestingly, we observed differential effects on GABA current decay by compounds incorporating either a six- (compound 5) or five-membered heterocyclic ring (compound 6) on C-21, which was independent of their potencies as inhibitors. We propose that differences in molecular charges, and the targeted binding of analogues to specific states of the GABA_A receptor, are the most likely cause of the distinctive functional profiles.

Conclusions and implications: Our findings reveal that heterocyclic addition to inhibitory neurosteroids not only affected their potency and macroscopic efficacy but also affected innate receptor mechanisms that underlie desensitisation. Acute modulation of macroscopic desensitisation will determine the degree and duration of GABA inhibition, which are vital for the integration of neural circuit activity. Discovery of this form of modulation could present an opportunity for next-generation GABA_A receptor drug design and development.

Keywords: GABA; GABAA receptor; chemical analogues; electrophysiology; human embryonic kidney cells; kinetic modelling; molecular dynamics simulations; organic chemistry; pregnenolone sulfate; recombinant expression; synthesis.