Raphe neurons with sympathetic-related activity: baroreceptor responses and spinal connections

We have previously used spike-triggered averaging to identify cat medullary raphe neurons with activity locked to the 2- to 6-cycles/s rhythm in sympathetic nerve discharge (SND) [Am. J. Physiol. 243 (Regulatory Integrative Comp. Physiol. 12): R49-R59, 1982]. In the present study, we classified such cat raphe neurons on the bases of their spinal connections and responses to baroreceptor reflex activation. Type I neurons, comprising 139 of 190 raphe units with activity related to inferior cardiac SND, were excited when carotid sinus pressure was elevated. The majority of these neurons were located in nucleus raphe pallidus. A time-controlled collision test for antidromic activation revealed that the axons of approximately one-third of type I neurons terminated in the sympathetic intermediolateral nucleus (IML) after coursing through the dorsolateral funiculus (DLF) of the spinal cord. The axons of the remaining type I neurons did not project to the spinal cord. It is suggested that type I raphe neurons are involved in mediating sympathoinhibition at spinal and supraspinal levels. Importantly, type I unit discharge remained locked to SND when the phase relationship between baroreceptor afferent and sympathetic efferent activities was disrupted. Thus type I neurons are not simple interneurons in the afferent limb of the baroreceptor reflex arc. Apparently they also receive input from the generator of the 2- to 6-cycles/s rhythm in SND. Type II raphe neurons (n = 51) were inhibited when carotid sinus pressure was elevated. Although the axons of these neurons coursed through the spinal DLF, they did not terminate in IML. Whether type II neurons subserve a sympathoexcitatory function remains to be determined.