Protein Kinase C-Dependent Effects of Neurosteroids on Synaptic GABAA Receptor Inhibition Require the δ-Subunit.

Erica L. Littlejohn,Carie R. Boychuk

doi:10.3389/fphys.2021.742838

Abstract

The dorsal motor nucleus of the vagus (DMV) contains preganglionic motor neurons important for interpreting sensory input from the periphery, integrating that information, and coding the appropriate parasympathetic (vagal) output to target organs. Despite the critical role of hormonal regulation of vagal motor output, few studies examine the role of neurosteroids in the regulation of the DMV. Of the few examinations, no studies have investigated the potential impact of allopregnanolone (Allo), a neuroactive progesterone-derivative, in the regulation of neurotransmission on the DMV. Since DMV neuronal function is tightly regulated by GABAA receptor activity and Allo is an endogenous GABAA receptor ligand, the present study used in vitro whole cell patch clamp to investigate whether Allo alters GABAergic neurotransmission to DMV neurons. Although Allo did not influence GABAergic neurotransmission during initial application (5–20 min), a TTX-insensitive prolongment of decay time and increase in frequency of GABAergic currents was established after Allo was removed from the bath for at least 30 min (LtAllo). Inhibition of protein kinase C (PKC) abolished these effects, suggesting that PKC is largely required to mediate Allo-induced inhibition of the DMV. Using mice that lack the δ-subunit of the GABAA receptor, we further confirmed that PKC-dependent activity of LtAllo required this subunit. Allo also potentiated GABAA receptor activity after a repeated application of δ-subunit agonist, suggesting that the presence of Allo encodes stronger δ-subunit-mediated inhibition over time. Using current clamp recording, we demonstrated that LtAllo-induced inhibition is sufficient to decrease action potential firing and excitability within DMV neurons. We conclude that the effects of LtAllo on GABAergic inhibition are dependent on δ-subunit and PKC activation. Taken together, DMV neurons can undergo long lasting Allo-dependent GABAA receptor plasticity.

Highlights

The efferent motor limb of the parasympathetic nervous system to subdiaphragmatic viscera originates from the brainstem nucleus known as the dorsal motor nucleus of the vagus (DMV)
Decay Time of Inhibitory Postsynaptic Currents The most consistent effect of Allo across multiple brain regions is an increase in the time for GABAA receptor currents to decay (Fancsik et al, 2000; Maguire and Mody, 2009; Ferando and Mody, 2012)
Throughout the central nervous system, Allo induces longer lasting changes often associated with its removal (Maguire and Mody, 2007; Abramian et al, 2014; Carver et al, 2014)

Summary

INTRODUCTION

The efferent motor limb of the parasympathetic nervous system to subdiaphragmatic viscera originates from the brainstem nucleus known as the dorsal motor nucleus of the vagus (DMV) These DMV motor neurons send direct axonal innervations to pancreas, liver, intestine and stomach through the vagus nerve (Berthoud et al, 1991; Browning et al, 1999; Babic et al, 2012). Little data exist on the role of non-GABA modulators of GABAA receptors and their effects on DMV neurons. One such class of GABA modulators are neurosteroids that result from the catalyzation of either cortisol or progesterone by 5α-reductase. Understanding how neurosteroids alter inhibitory GABAA receptor activity in the DMV is critical to understanding the brain’s role in maintaining physiological homeostasis, as it relates to energy homeostatic regulation. It was hypothesized that Allo induces an increase in inhibitory neurotransmission to DMV neurons, thereby allowing future inhibitory transmission to encode larger inhibition within the brainstem homeostatic regulatory network and decreasing DMV excitability

MATERIALS AND METHODS

RESULTS

DISCUSSION

ETHICS STATEMENT