c-Fos induction in the rostroventrolateral medulla by 5-HT3 receptor activation in the nucleus tractus solitarius.

The nucleus tractus solitarius (NTS) plays a crucial role in the reflex control of sympathetic tone through direct projections to the caudal and rostral regions of the ventrolateral medulla (CVLM and RVLM respectively). 5-HT3 receptor stimulation in the NTS increases both blood pressure and sympathetic tone. In order to investigate whether the ventrolateral medulla participates in these effects, the expression of c-Fos protein, a marker of neuronal activation, was examined at this level after intra-NTS microinjections of 1-(m-chlorophenyl)-biguanide, a selective 5-HT3 receptor agonist. A marked increase (93%) in the density of c-Fos-immunoreactive neurons was restricted to the most rostral part of the RVLM after this treatment. This effect could be prevented by prior intra-NTS microinjection of ondansetron, a selective 5-HT3 antagonist. These results suggest the participation of sympathoexcitatory neurons in the rostral RVLM in the pressor response to 5-HT3 receptor stimulation in the NTS.