Dorsal medullary 5-HT 3 receptors and sympathetic premotor neurones in the rat

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. 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). 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. 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. 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. 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. 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. 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. The rostro-ventrolateral medulla (RVLM) and nucleus tractus solitarii (NTS) are critically involved in the reflex control of sympathetic activity (Guyenet, Filtz & Donaldson, 1987; Sun & Guyenet, 1987; Spyer, 1994). The RVLM contains neurones that receive a number of inputs both peripheral and central in origin that influence sympathetic nerve activity (Sun & Guyenet, 1987; Spyer, 1994). Previous reports have described two populations of cardiovascular (CV)-RVLM neurones that project to the thoracic spinal cord (Brown & Guyenet, 1985; Sun & Guyenet, 1985). The first population consists of clonidine-sensitive cells with slow-conducting axons. The second population corresponds to cells that do not respond to hypotensive doses of clonidine, and are characterized by a much higher conduction velocity (Sun & Guyenet, 1986). The NTS is the site of termination of afferent fibres arising from arterial baroreceptors (baroreflex), cardiopulmonary chemoreceptors (Bezold-Jarisch reflex) and carotid chemoreceptors (chemoreflex) (Palkovits & Zaborsky, 1977; Kalia & Mesulam, 1980; Jordan & Spyer, 1986). For baro- and Bezold-Jarisch reflexes, which seem to share the same integrating mechanisms and pathways in the brain (Verberne & Guyenet, 1992), second-order neurones project from the NTS to the caudal ventrolateral part of the medulla (CVLM) (Gordon 1987; Guyenet et al. 1987; Verberne & Guyenet, 1992). These neurones exert an excitatory action on the GABAergic CVLM neurones that project to the RVLM where they inhibit the CV-RVLM neurones of this pressor area (Brown & Guyenet, 1985; Sun & Guyenet, 1985; Jeske, Reis & Milner, 1995). Some of the CV-RVLM neurones additionally constitute an efferent link in the sympathetic component of the chemoreflex (Guyenet & Brown, 1986; Sun & Reis, 1995). Indeed, it has been demonstrated that some NTS chemosensitive neurones have axonal projections to the RVLM (Koshiya & Guyenet, 1996). Within the NTS, serotonin (5-hydroxytryptamine, 5-HT) seems to be involved in the reflex control of blood pressure. Studies in both anaesthetized and conscious rats have shown that 5-HT2 receptor stimulation in the NTS elicits the typical CV responses of baroreceptor activation (Merahi, Orer & Laguzzi, 1992a; Callera, Bonagamba, Sevoz, Laguzzi & Machado, 1997a). On the other hand, stimulation of NTS 5-HT3 receptors elicits a chemoreceptor-like increase in arterial pressure and lumbar sympathetic activity (Merahi, Orer, Laporte, Gozlan, Hamon & Laguzzi, 1992b; Callera, Sevoz, Laguzzi & Machado, 1997b). In experiments aimed at analysing the mechanism (s) responsible for this sympatho-excitatory effect, we observed that stimulation of 5-HT3 receptors in the NTS did not inhibit the sympathetic component of the baroreflex (Nosjean, Franc & Laguzzi, 1995). Accordingly, it can be inferred that 5-HT3 receptor-mediated sympathoexcitation is not the consequence of the disruption of tonic baroreceptor sympathoinhibitory messages. However, our finding is compatible with the idea that CV-RVLM neurones may be involved in the sympathoexcitatory effect of 5-HT3 receptor stimulation in the NTS. In other experiments, we also observed that prior microinjections of pressor (nanomolar) doses of 5-HT3 receptor agonists into the NTS did not increase the sympathetic chemoreflex response (Sevoz, Callera, Machado, Hamon & Laguzzi, 1997). However, this observation does not rule out the possibility that under some physiological conditions, 5-HT released in the NTS may excite the NTS-RVLM sympathetic chemoreflex pathway (Koshiya & Guyenet, 1996), and the CV-RVLM neurones involved in this reflex. Indeed, as previously observed with the pressor chemoreflex response (Sun & Reis, 1995), we recently found that the microinjection of kynurenic acid, a glutamate receptor antagonist, into the RVLM blocked the pressor effects elicited by 5-HT3 receptor stimulation in this area (Sevoz, Hamon & Laguzzi, 1996b). In order to elucidate the possible role of CV-RVLM neurones in the sympathetic response to 5-HT3 receptor stimulation in the NTS, we have analysed the effects of intra-NTS microinjections of a potent and selective 5-HT3 receptor agonist, 1-(m-chlorophenyl)-biguanide (CPBG), as well as the effects of chemoreflex activation, on the activity of the different categories of RVLM neurones. In addition, the possible effects of intra-NTS administration of CPBG on the CV-CVLM neurones were also investigated.