Central integration of the Bezold-Jarish reflex in the cat

The medullary structures involved in the central Bezold-Jarisch reflex pathway were studied by recording unit activity of sympatho-excitatory (SE) or inhibitory (SI) cardiovascular neurons in pentobarbital-anesthetized cats. The neurons were selected based upon their spontaneous activity and upon their sensitivity to baroreceptor reflex activation by L-phenylephrine. The Bezold-Jarisch reflex was induced by i.v. injection of chlorophenylbiguanide 10 micrograms/kg which produced a short-lasting decrease in blood pressure, heart rate and renal nerve activity. 76 neurons were studied. In 10 out of 12 SE neurons of the rostral ventrolateral medulla the activity was inhibited by chlorophenylbiguanide whereas in 10 out of 11 SI neurons and 6 out of 6 baroreflex-insensitive cells of the caudal ventrolateral medulla it was activated. The others cells were insensitive. Three types of neurons: excitatory, inhibitory or non-barosensitive, were recorded in the lateral tegmental field (27 cells) and the medullary raphe (20 cells). These neurons were either activated, inhibited or insensitive to Bezold-Jarisch reflex activation. Microinjection of the glutamate receptor antagonist kynurenic acid (2.5 nmol/site) or the GABAergic agonist muscimol (1 nmol/site) into the nucleus tractus solitarii abolished the effects of both L-phenylephrine and chlorophenyl-biguanide on heart rate and renal nerve activity. These results indicate that the cardiovascular neurons (sympatho-excitatory and sympatho-inhibitory) located in the medullary areas, involved in cardiovascular and baroreflex mechanisms, are implicated in the central Bezold-Jarish reflex pathway.