GABA_A receptor currents in the dorsal motor nucleus of the vagus in females: influence of ovarian cycle and 5α-reductase inhibition

Abstract

The dorsal motor nucleus of the vagus (DMV) contains the preganglionic motor neurons important in the regulation of glucose homeostasis and gastrointestinal function. Despite the role of sex in the regulation of these processes, few studies examine the role of sex and/or ovarian cycle in the regulation of synaptic neurotransmission to the DMV. Since GABAergic neurotransmission is critical to normal DMV function, the present study used in vitro whole cell patch-clamping to investigate whether sex differences exist in GABAergic neurotransmission to DMV neurons. It additionally investigated whether the ovarian cycle plays a role in those sex differences. The frequency of phasic GABAA receptor-mediated inhibitory postsynaptic currents in DMV neurons from females was lower compared with males, and this effect was TTX sensitive and abolished by ovariectomy (OVX). Amplitudes of GABAergic currents (both phasic and tonic) were not different. However, females demonstrated significantly more variability in the amplitude of both phasic and tonic GABAA receptor currents. This difference was eliminated by OVX in females, suggesting that these differences were related to reproductive hormone levels. This was confirmed for GABAergic tonic currents by comparing females in two ovarian stages, estrus versus diestrus. Female mice in diestrus had larger tonic current amplitudes compared with those in estrus, and this increase was abolished after administration of a 5α-reductase inhibitor but not modulation of estrogen. Taken together, these findings demonstrate that DMV neurons undergo GABAA receptor activity plasticity as a function of sex and/or sex steroids. NEW & NOTEWORTHY Results show that GABAergic signaling in dorsal vagal motor neurons (DMV) demonstrates sex differences and fluctuates across the ovarian cycle in females. These findings are the first to demonstrate that female GABAA receptor activity in this brain region is modulated by 5α-reductase-dependent hormones. Since DMV activity is critical to both glucose and gastrointestinal homeostasis, these results suggest that sex hormones, including those synthesized by 5α-reductase, contribute to visceral, autonomic function related to these physiological processes.

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

“…This spatial specificity of Allo’s actions across neuronal compartments was surprising given the relatively large conductance of tonic currents in DMV neurons ( Ferreira et al, 2001 ; Gao and Smith, 2010a ). Additionally, blocking the production of neurosteroids in female mice abolished estrous-cycle dependent changes in GABA A receptor tonic, but not phasic, currents in DMV neurons ( Littlejohn et al, 2019 ). Although sex differences do exist in Allo concentrations in the brainstem ( Sze et al, 2018 ), Allo was specifically chosen here since previous reports demonstrated minimal sex differences compared to other neurosteroids ( Wilson and Biscardi, 1997 ).…”

“…To determine GABA A receptor-mediated tonic current, BIC was bath applied until a steady state was reached (7-10 min) as previously described (Littlejohn et al, 2019).…”

“…The average time required for 90–37% of IPSC current to decay is reported as decay time. To determine GABA A receptor-mediated tonic current, BIC was bath applied until a steady state was reached (7–10 min) as previously described ( Littlejohn et al, 2019 ).…”

“…Neurosteroids readily cross the blood brain barrier and recently were shown to concentrate in the brainstem ( Sze et al, 2018 ). Although no studies to date directly assess the action of neurosteroids on DMV neurotransmission and activity, inhibition of 5α-reductase activity does restrain DMV inhibitory GABAergic currents ( Littlejohn et al, 2019 ). Additionally, in the nucleus tractus solaritus (NTS), a nucleus with significant projections to the DMV, neurosteroids influence neurotransmission enough to decrease neuronal activation to excitatory stimuli ( Kim et al, 2018 ).…”

Molecular Organization and Patterning of the Medulla Oblongata in Health and Disease

“…Although Olig1 and Olig2 are mainly linked to the development of oligodendrocytes, this domain is known to first generate somatic and visceral Phox2b -expressing motor neurons (E9–E10.5) and later oligodendrocytes (E11.5–E13.5) [ 91 , 92 ]. Neurons derived from this progenitor domain are characterized by the expression of ChAT and make up the hypoglossal nucleus (XII) (r9–r11) that co-expresses Trh and Adam19 [ 6 , 93 ], involved in the control of tongue muscles and respiratory motor control [ 94 ] ( Figure 3 and Figure 5 ); the dorsal motor nucleus of the vagus (DMX) (r8–r11) [ 6 ]; involved in the regulation of glucose homeostasis and gastrointestinal function [ 95 ] ( Figure 3 and Figure 5 ); and the nucleus ambiguus (Amb) (r7) [ 6 ], co-expressing Phox2b [ 91 ] and involved in the control of respiratory reflexes, swallowing, and cardiovascular regulation [ 96 ] ( Figure 3 ).…”