Rostral ventrolateral medulla suppresses reflex bradycardia by the releaseof gamma-aminobutyric acid in nucleus tractus solitarii of the rat

We investigated the role of gamma-aminobutyric acid (GABA) in the nucleus t ractus solitarii (NTS), the principal recipient of baroreceptor afferent fi bers in the medulla oblongata, in the suppression of cardiac baroreceptor r eflex (BRR) response by the rostral ventrolateral medulla (RVLM). Direct mi croinfusion via reverse microdialysis of L-glutamate (50 muM) into the RVLM promoted an inhibition of the ERR response, alongside an increase in the c oncentration of GABA in the dialysate collected from the ipsilateral NTS. S uch an increase in GABA concentration in the NTS to RVLM activation was sit e-specific, as microinfusion of L-glutamate into areas outside the confines of RVLM resulted in no discernible change in GABA concentration in the dia lysate of the NTS and minimal effect on the cardiac ERR response. The RVLM- induced ERR suppression of cardiac ERR response to microinjection into the bilateral RVLM of L-glutamate (1 nmol) was antagonized by administration in to the bilateral NTS of the GABA(A) receptor antagonist, bicuculline methio dide (1 or 5 pmol), or the GABA(B) receptor antagonist, 2-hydroxy-saclofen (100 or 500 pmol). These results suggest that GABA released in the NTS may participate in cardiac ERR suppression induced by glutamatergic activation of the RVLM, via an action on both GABA(A) and GABA(B) receptor subtypes. ( C) 2001 Wiley-Liss, Inc.