Midbrain central gray: influence on medullary sympathoexcitatory neurons and the baroreflex in rats.

The influence of the central gray (CG) of the midbrain on the activity of 19 barosensitive sympathoexcitatory neurons of the rostral ventrolateral medulla (RVLM) and on the sympathetic vasomotor baroreflex was studied in halothane-anesthetized rats. Eighteen RVLM barosensitive units were readily activated by train stimulation of the CG, although twin-pulse stimulation was less effective (10 of 19 neurons responded). Inhibition of neurons within the RVLM by bilateral microinjection of the GABA-mimetic drug muscimol abolished the pressor responses to CG stimulation, while the accompanying lumbar nerve sympathoexcitation was converted to sympathoinhibition. In baroreceptor-denervated vagotomized animals, unilateral microinjection of muscimol into the RVLM ipsilateral or contralateral to the site of CG stimulation resulted in approximately equal attenuation of the CG sympathoexcitatory and pressor responses. In contrast, the sympathoexcitatory response to electrical stimulation of the sciatic nerve was reduced more effectively by inhibition of the RVLM contralateral to the site of stimulation. Electrical stimulation of the CG lateral and ventrolateral to the aqueduct produced sympathoexcitation [increased discharge of the greater splanchnic and lumbar sympathetic nerves (SSN and LSN)] with an increase in mean arterial blood pressure. Activation of the SSN by CG stimulation was greater than that observed for the LSN (n = 5 rats). This differential influence of the CG on the sympathetic outflow was not a result of a differential influence of the baroreflex. Electrical stimulation of the CG produced elevations of the gain and the cut-off pressure of the baroreflex for both the SSN and LSN.(ABSTRACT TRUNCATED AT 250 WORDS)