C-FOS IDENTIFIES GABA-SYNTHESIZING BAROSENSITIVE NEURONS IN CAUDAL VENTROLATERAL MEDULLA

HYPERTENSION in the conscious rat, elicited by i.v. infusion of phenyl ephrine, evoked expression of the immediate early gene c-fos in discre te groups of brain stem neurons. Fos-immunoreactive neurons were locat ed in the caudal ventrolateral medulla (CVLM); others were located in the nucleus of the tractus solitarius (NTS). Because of their sensitiv ity to alterations in arterial pressure, these neurons are likely to s ubserve the arterial baroreceptor reflex. The aim of this study was to identify the brain stem projections and the neurotransmitter content of the barosensitive CVLM neurons using neuronal tracing and immunohis tochemistry. Some of the barosensitive CVLM neurons projected directly to the rostral ventrolateral medulla (RVLM), and many contained the G ABA synthesizing enzyme, glutamic acid decarboxylase (GAD). Other CVLM neurons, containing markers of glutamate or catecholamine synthesis, were insensitive to baroreceptor stimulation. This study delineates ne uronal pathways acting in the arterial baroreceptor reflex and identif ies precisely GABA-synthesizing CVLM neurons as the source of inhibito ry input to the RVLM.