GABA(B) RECEPTORS MODULATE A TETANUS-INDUCED SUSTAINED POTENTIATION OF MONOSYNAPTIC INHIBITORY TRANSMISSION IN THE RAT NUCLEUS-TRACTUS-SOLITARII IN-VITRO

Whole-cell recordings were made in the nucleus tractus solitarii (NTS) in transverse brainstem slices from rats. Monosynaptic GABA(A)-recept or-mediated inhibitory postsynaptic currents (IPSCs) or potentials (IP SPs) were evoked (0.1-0.2 Hz) by electrical stimulation within and med ial to the tractus solitarius in the presence of the ionotropic glutam ate receptor antagonists 6-cyano-7-nitroquinoxaline-2,3-dione (CNQX; 1 0 mu M) or 6,7-dinitroquinoxaline-2,3-dione (DNQX; 10 mu M) and D-amin o-5-phosphonopentanoic acid (APV; 50 mu M). A brief period of tetanic stimulation (20 Hz, 2 s) resulted in posttetanic (< 5 min, 69 of 73 re cordings) and sustained potentiation (>15 min, 31 of 73 recordings) of the IPSP/Cs. Sustained potentiation was not due to alterations in the reversal potential of IPSP/Cs. Both pre- and post-tetanus IPSP/Cs wer e completely blocked by the GABA(A) antagonist bicuculline (10 mu M). Postsynaptic responses to pressure ejection of the GABA(A)-receptor ag onist muscimol were unaltered in cells displaying sustained potentiati on. Sustained potentiation of IPSP/Cs could be induced by tetanus in t he presence of either metabotropic glutamate receptor antagonists or b icuculline. However, sustained potentiation could not be induced in th e presence of the GABA(B)-receptor antagonists 2-OH-saclofen (400 mu M ) or CGP35348 (3-amino-propyl-(diethoxymethyl)phosphinic acid, 100 mu M), although a subsequent tetanus following washout induced sustained potentiation. Posttetanic potentiation was unaffected by GABA(B)-recep tor antagonists. These data suggest that neuronal or terminal excitabi lity of GABAergic interneurons in the NTS is enhanced following brief periods of increased frequency of activation in vitro. This novel phen omenon within the rat medulla may be involved in the temporal modulati on of autonomic reflex sensitivity observed during certain behavioral states, such as the defense reaction.