Altered GABAergic neurotransmission at the nucleus tractus solitarii following Hindlimb Unloading

Abstract

Hindlimb Unloading (HU) in rats induces dysautonomia and cardiovascular deconditioning (CVD) similar to that seen in bedridden individuals. The autonomic alterations in CVD and HU are related to plasticity within the central nervous system. The nucleus tractus solitarii (nTS) is a major integrative brainstem region for autonomic modulation and processing of cardiovascular reflexes. GABA and glutamate are the main inhibitory and excitatory neurotransmitters within this nucleus. HU increases nTS glutamatergic neurotransmission yet decreases neuronal excitability. In this study, we investigated the effects of HU on the GABAergic neurotransmission in the nTS. We hypothesized that HU increases GABA synaptic modulation of nTS neurons, which contributes to impaired autonomic and cardiovascular control. HU was induced by placing stainless wire rings in the tail of male Sprague‐Dawley rats (3 weeks old), and after one week of recovery suspending rats at an angle of 30–35° for two weeks with free access to food and water. Control animals were maintained in normal postural conditions. After HU or the control period, coronal brainstem slices were generated and whole‐cell patch clamp was performed to evaluate GABAergic neurotransmission in the nTS region. The magnitude of GABA release was evaluated via an intensity‐based GABA sensing fluorescence reporter (iGABASnFR). Block of GABAA receptors (Gabazine, 20µM and Bicuculline, 20µM) depolarized membrane potential in HU but not in Control, suggesting greater tonic GABA inhibition in HU. In response to GABA interneuron stimulation the evoked IPSC amplitude and area, as well as iGABASnFR fluorescence, was greater in HU than Control. HU also elevated the frequency of spontaneous miniature IPSCs but not their amplitude. Puffer application of GABA (100µM, 15ms) produced similar postsynaptic current responses in nTS neurons of HU and Control. These results indicate that HU increases GABAergic signaling in the nTS likely via augmented release of GABA from presynaptic terminals. This synaptic plasticity may be a component of the mechanisms underlying the autonomic and cardiovascular alterations following CVD.