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Dorsal medullary 5‐HT3 receptors and sympathetic premotor neurones 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 region of the nucleus tractus solitarii (NTS). Experiments were performed in pentobarbitone‐anaesthetized rats, artificially ventilated and paralysed with pancuronium bromide. 2 Using extracellular recordings, different types of RVLM neurones were characterized: cardiovascular (CV), ventilation‐related and baroreflex‐insensitive (unidentified) neurones. 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 μg kg−1 i.v.) of clonidine. 4 Microinjections into the region of the commissural NTS of 1‐(m‐chlorophenyl)‐biguanide (CPBG, 2 nmol), a selective 5‐HT3 receptor agonist, elicited an increase in both lumbar sympathetic nerve discharge (SND) and arterial pressure. In addition, this treatment produced a marked excitation of CV‐RVLM neurones of the A and B categories, without affecting those of the C type, as well as ventilation‐related and unidentified RVLM cells. 5 The activity of the CV neurones in the caudo‐ventrolateral part of the medulla oblongata (CV‐CVLM) was not modified by 5‐HT3 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 saline saturated with CO2 (chemoreceptor activation) elicited both an increase in the SND and an excitation of the clonidine‐insensitive CV‐RVLM neurones of the C type, without affecting A and B neurones. 8 In conclusion, the sympathoexcitation elicited following 5‐HT3 receptor stimulation in the region of the commissural NTS of pentobarbitone‐anaesthetized rats seems to result from the excitation of two different pools of clonidine‐sensitive CV‐RVLM neurones. These neurones are apparently not involved in the sympathetic component of the chemoreceptor reflex.