CARDIOVASCULAR NEURONS IN CAT CAUDAL VENTROLATERAL MEDULLA - LOCATIONAND CHARACTERIZATION OF GABAERGIC INPUT

The purposes of the present study were to: (1) characterize the GABAer gic input to vasodepressor neurons in the caudal ventrolateral medulla of the cat, and (2) define more precisely the anatomical localization of these neurons in this species. This was done by microinjecting GAB A receptor antagonists and agonists, and a negative allosteric modulat or of the GABA receptor, namely, ethyl-beta-carboline-3-carboxylate, i nto the caudal ventrolateral medulla of alpha-chloralose-anesthetized animals while monitoring arterial blood pressure and heart rate. Local ization studies were performed relating injection sites in the caudal ventrolateral medulla where cardiovascular responses were elicited, to neurons exhibiting immunoreactivity to tyrosine hydroxylase (TH) and phenethyl-N-methyl-transferase (PNMT). Microinjection of 1 and 10 ng o f bicuculline into the caudal ventrolateral medulla produced decreases in mean blood pressure and heart rate of -34 +/- 6.4 and -49 +/- 9.2 mmHg, and -22 +/- 3.3 and -35 +/- 8.2 beats/min, respectively. Hypoten sion and bradycardia were also observed with picrotoxin microinjection (120 ng). Microinjection of muscimol (100-200 ng) and GABA(12 mu g) i nto the caudal ventrolateral medulla counteracted the cardiovascular e ffects of the GABA antagonists. Microinjection of muscimol per se had no effect on mean blood pressure and heart rate. Microinjection of eth yl-beta-carboline-3-carboxylate also decreased mean blood pressure (-3 9 +/- 7.0 mmHg). The location of the micropipette tip after bicucullin e microinjection in relation to TH and PNMT immunoreactive cells was a s follows: (1) TH-immunoreactive cells of the A1 cell group were visib le in the same relative location as the micropipette tip, and (2) no P NMT-positive cells were noted at the sites where bicuculline elicited hypotension. These results indicate that there is a tonic GABAergic in put to neurons in the caudal ventrolateral medulla. The location of th ese neurons overlaps with the A1 cells.