DORSAL MEDULLARY 5-HT3 RECEPTORS AND SYMPATHETIC PREMOTOR NEURONS IN THE RAT

1. Our aim was to determine whether the cardiovascular neurones in the rostro-ventrolateral medulla (CV-RVLM neurones) were involved in the sympathoexcitation induced by stimulation of 5-HT3 receptors in the re gion of the nucleus tractus solitarii (NTS). Experiments ivere perform ed in pentobarbitone-anaesthetized rats, artificially ventilated and p aralysed with pancuronium bromide. 2. Using extracellular recordings, different types of RVLM neurones were characterized: cardiovascular (C V), ventilation-related and baroreflex-insensitive (unidentified) neur ones. The CV-RVLM cells were further subdivided into three populations according to their axonal conduction velocities: A (1.2 +/- 0.1 m s(- 1)), B (2.5 +/- 0.2 m s(-1)) and C (6.8 +/- 1.1. m s(-1)). 3. Only the CV-RVLM neurones of the A and B categories were partially inhibited ( -30 %) by a hypotensive dose (2.5 mu g kg(-1) I.V.) of clonidine. 4. M icroinjections into the region of the commissural NTS of 1-(m-chloroph enyl)-biguanide (CPBG, 2 nmol), a selective 5-HT3 receptor agonist, el icited an increase in both lumbar sympathetic nerve discharge (SND) an d arterial pressure. In addition, this treatment produced a marked exc itation of CV-RVLM neurones of the a and B categories, without affecti ng those of the C type, as well as ventilation-related and unidentifie d RVLM cells. 5. The activity of the CV neurones in the caudo-ventrola teral part of the medulla oblongata (CV-CVLM) was not modified by 5-HT 3 receptor stimulation in the NTS. 6. Prior intra-NTS microinjections of ondansetron (300 pmol, a selective 5-HT3, receptor antagonist) into the region of the commissural NTS prevented the excitation of A and B CV-RVLM neurones induced by CPBG. 7. Intracarotid administration of s aline saturated with CO2 (chemoreceptor activation) elicited both an i ncrease in the SND and an excitation of the clonidine-insensitive CV-R VLM neurones of the C type, without affecting A and B neurones. 8. In conclusion, the sympathoexcitation elicited following 5-HT3 receptor s timulation in the region of the commissural NTS of pentobarbitone-anae sthetized rats seems to result from the excitation of two different po ols of clonidine-sensitive CV-RVLM neurones. These neurones are appare ntly not involved in the sympathetic component of the chemoreceptor re flex.