Responses of rat brainstem neurons to carotid occlusion

The distribution and features of cardiovascular neurons (CVN) were studied in the brainstem of chloralose-anesthetized rats. Only neurons yielding an immediate response to carotid occulsion (CO) were considered as CVN. The criterion was deduced from the response patterns of cardiovascular afferent (carotid sinus nerve) and efferents (sympathetic and vagal nerves) during CO. Seventy-two CVN were identified and were observed to be mostly in the medullarly reticular formation and to some extent in the nucleus tractus solitarius and nucleus ambiguus. Fifty-five CVN (type A) decreased their firing rates in response to CO where 17 CVN (type B) increased their rates. Analyses of the time courses of their responses to CO revealed the convergence of the chemoreceptor and baroreceptor inputs onto some type A neurons. Electrical stimulation of the spinal cord evoked orthodromic responses in some neurons in both types. The evoked responses were blocked by CO in type A neurons whereas they were facilitated in those of type B. These results suggest that input from arterial baroreceptors, arterial chemoreceptors, and spinal ascending afferents is integrated within CVN.