Dexamethasone Rapidly Increases GABA Release in the Dorsal Motor Nucleus of the Vagus via Retrograde Messenger-Mediated Enhancement of TRPV1 Activity

Abstract

Glucocorticoids influence vagal parasympathetic output to the viscera via mechanisms that include modulation of neural circuitry in the dorsal vagal complex, a principal autonomic regulatory center. Glucocorticoids can modulate synaptic neurotransmitter release elsewhere in the brain by inducing release of retrograde signalling molecules. We tested the hypothesis that the glucocorticoid agonist dexamethasone (DEX) modulates GABA release in the rat dorsal motor nucleus of the vagus (DMV). Whole-cell patch-clamp recordings revealed that DEX (1-10 µM) rapidly (i.e. within three minutes) increased the frequency of tetrodotoxin-resistant, miniature IPSCs (mIPSCs) in 67% of DMV neurons recorded in acutely prepared slices. Glutamate-mediated mEPSCs were also enhanced by DEX (10 µM), and blockade of ionotropic glutamate receptors reduced the DEX effect on mIPSC frequency. Antagonists of type I or II corticosteroid receptors blocked the effect of DEX on mIPSCs. The effect was mimicked by application of the membrane-impermeant BSA-conjugated DEX, and intracellular blockade of G protein function with GDP βS in the recorded cell prevented the effect of DEX. The enhancement of GABA release was blocked by the TRPV1 antagonists, 5’-iodoresiniferatoxin or capsazepine, but was not altered by the cannabinoid type 1 receptor antagonist AM251. The DEX effect was prevented by blocking fatty acid amide hydrolysis or by inhibiting anandamide transport, implicating involvement of the endocannabinoid system in the response. These findings indicate that DEX induces an enhancement of GABA release in the DMV, which is mediated by activation of TRPV1 receptors on afferent terminals. The effect is likely induced by anandamide or other ‘endovanilloid’, suggesting activation of a local retrograde signal originating from DMV neurons to enhance synaptic inhibition locally in response to glucocorticoids.

Presynaptic ionotropic glutamate receptors modulate GABA release in the mouse dorsal motor nucleus of the vagus

“…This is consistent with previous findings indicating that the TRPV1 agonist, capsaicin increases the release of glutamate, which binds preNMDAR and enhances GABA release in DMV neurons (Derbenev et al, 2006, Derbenev and Smith, 2013). Similarly, blockade of AMPA/KA receptors with CNQX, but not selective AMPAR blockade with GYKI-52466, suppressed the capsaicin-induced increase in mIPSC frequency.…”

“…Previous experiments in rats and mice indicated that TRPV1 activation with capsaicin enhanced mIPSC frequency in the DMV, and that a significant proportion of this effect was due to TRPV1-mediated release of glutamate, which binds iGluRs located on the synaptic terminals of afferent GABAergic neurons (Derbenev et al, 2006, Derbenev and Smith, 2013). To establish that glutamate could act at presynaptic terminal receptors to increase GABA release in the mouse DMV, we determined the effect on mIPSCs of L-glutamate (50 μM) in nine DMV neurons, voltage-clamped at 0 mV to reduce direct effects of the agonist on the recorded neuronal membrane.…”

“…Here, we identified functional preKAR and preNMDAR that participate in increasing synaptic GABA release in the DMV, with preKAR located on GABAergic terminals contributing most directly to the effect. The circumstances under which these receptors are recruited into circuit modulation are not known, but since they mediate effects of TRPV1 receptor activation on GABA release, it is reasonable to postulate that conditions leading to the release of the endocannabinoid, anandamide or other ‘endovanilloid’ substances, which result in TRPV1-mediated modulation of GABA release in the DMV and elsewhere (Boychuk et al, 2013, Derbenev and Smith, 2013), might recruit this heterosynaptic mechanism of GABA release modulation during periods of excessive excitatory synaptic drive.…”

“…In the DMV, presynaptic N-methyl-D-aspartate (NMDA) receptors (preNMDAR) on glutamatergic terminals contribute to ongoing glutamate release tonically (Bach and Smith, 2012, Bach et al, 2015), and preliminary evidence has been presented to support the hypothesis that functional presynaptic iGluRs on terminals of inhibitory neurons may enhance GABA release (Derbenev et al, 2006). Activation of transient receptor potential vanilloid type 1 (TRPV1) receptors increases both glutamate and GABA release in the DMV; at least a component of GABA release modulation occurs subsequent to TRPV1-induced, glutamate-mediated heterosynaptic activation of iGluRs on GABAergic terminals (Derbenev et al, 2006, Derbenev and Smith, 2013). Since GABA plays a prominent role in regulating DMV motor neuron activity, rapid modulation of GABA release by presynaptic iGluR activation could potently affect DMV neuron activity and, consequently, parasympathetic output to the viscera.…”

Dexamethasone Regimens Alter Spatial Memory and Anxiety Levels in Mice

“…Glutamate GABA imbalance has been implicated in anxiety disorders and the response seen here could be the result of dexamethasone eliciting an increase in brain glutamate levels enough to alter the glutamate GABA balance. Increases in glutamate efflux in brain regions such as the prefrontal cortex and hippocampus was observed after stress; also a lack of GAD65 enzyme which is responsible for converting glutamate into GABA leads to exhibition of anxiety in mice [21] [22]. Actions of corticosteroids in the limbic region of the brain are strongly linked to their observed anxiogenic response and a disruption of their forebrain/limbic circuits is probably involved in the development of affective and anxiety disorders [23].…”