Evidence for a sympathoexcitatory pathway from the nucleus tractus solitarii to the ventrolateral medullary pressor area.

The role of the ventrolateral medullary pressor (VLPA) and depressor (VLDA) areas in mediating cardiovascular responses evoked from the nucleus tractus solitarius (NTS) was investigated. Male Wistar rats, anesthetized with pentobarbital or urethane, were artificially ventilated and blood pressure (BP) and heart rate (HR) were monitored. The VLPA, VLDA, and the NTS were identified bilaterally with microinjections of L-glutamate. Unilateral microinjections of muscimol or lidocaine into the VLPA or the VLDA blocked the decrease in BP produced by microinjections of L-glutamate (1.77 nmol) into the NTS. These findings indicate that both areas are essential for mediating depressor responses elicited from the NTS. When neuronal activity in the VLDA was depressed unilaterally (leaving the ipsilateral VLPA intact), with the microinjection of muscimol or lidocaine, microinjection of a larger dose (5.0 nmol) of L-glutamate into the ipsilateral NTS elicited a pressor response. This response was blocked by depressing neuronal activity in the ipsilateral VLPA by microinjection of muscimol into this site. This pressor response evoked from the NTS was not due to non-specific effects of L-glutamate since repeated microinjections of L-glutamate (5.0 nmol/site) into the NTS consistently produced decreases in BP and HR. The stimulation of the contralateral NTS by glutamate continued to elicit the usual decreases in BP and HR. Microinjections of either dose (1.77 or 5 nmol) of L-glutamate into the areas adjacent to the NTS (e.g. 1.0 mm rostral or lateral to the NTS, the gracile or cuneate nuclei and area postrema) failed to evoke any cardiovascular responses indicating that the responses were mediated by neurons localized within the intermediate one-third of the NTS. These results indicate that: (1) the depressor responses elicited from the NTS involve the pathways from the NTS to the VLDA and VLDA to VLPA and (2) there may be a pathway from the NTS to the VLPA which is sympathoexcitatory and is unmasked when neuronal activity in the VLDA is depressed.