Chemosensitive ventrolateral medulla in the cat: the fine structure and GABA-induced cardiovascular effects.

These studies were designed to provide information on both GABA influences on the neuronal structures within the rostral ventrolateral medulla (RVLM) involved in the cardiovascular control, and fine structure of the GABA-sensitive RVLM in cats. By electron microscopy, the S-type asymmetric junctions (believed to mediate excitation) as well as F-type symmetric junctions (thought to provide inhibition) were identified within the RVLM area examined. The axonal terminals thick with spherical electron-transparent synaptic vesicles, measuring about 50 nm in diameter and forming S-type synapses, were distributed within the entire area. Large numbers of axonal terminals forming the F-type synapses and filled with flattened synaptic vesicles with a longitudinal axis of around 60-80 nm were found at the site located 5-6 mm caudal to the trapezoid bodies. Both spherical electron-transparent and flattened synaptic vesicles could be found together with electron-dense vesicles averaging 80-160 nm in diameter. No axonal terminals containing only the latter type of vesicles were found within the area studied. Some axo-axo-dendritic F-S-type synapses were identified within the RVLM. Unilateral injection of GABA into the RVLM site located 2-5 mm caudal to the trapezoid bodies induced a dose-related fall in the systemic arterial pressure and inhibition of the renal nerve sympathetic activity, the most marked responses being found at the site 4-5 mm caudal to the trapezoid bodies. It also resulted in decrease of heart rate and myocardial contractility when injected into a distinct site, located 0-1.5 mm rostral to the outflow of the 12th cranial nerve roots. There was some laterality in GABA effects on heart rate and myocardial contractility following its injections into the RVLM on the left and right side. Injections of bicuculline into the GABA-sensitive RVLM site resulted in an increase in cardio-haemodynamic responses together with the enhancement of renal nerve sympathetic activity in a dose-related fashion. These results suggest that, in cats, GABA-sensitive sympathoexcitatory neuronal structures in the RVLM alter peripheral sympathetic vasomotor and cardiac nerve background activities through GABA action on the bicuculline-sensitive GABAa-receptors.