INCREASED SYMPATHETIC-NERVE DISCHARGE WITHOUT ALTERATION IN THE SYMPATHETIC BAROREFLEX RESPONSE BY SEROTONIN(3) RECEPTOR STIMULATION IN THE NUCLEUS-TRACTUS-SOLITARIUS OF THE RAT

Previous studies have shown that serotonin(3) receptor activation in the nucleus tractus solitarius (NTS) increased mean arterial pressure (MAP) and inhibited the cardiac component of the baroreceptor reflex [9]. We have examined the effects of such stimulation upon spontaneous and evoked sympathetic nerve activity. Microinjection of serotonin (10 nmol) into the NTS of halothane-anaesthetized, paralyzed and artificially ventilated rats produced an increase in MAP and lumbar sympathetic nerve discharge which could be completely prevented by prior local microinjection of zacopride, a potent serotonin(3) antagonist (200 pmol). In addition, 1-(m-chlorophenyl)-biguanide, a selective serotonin(3) receptor agonist, mimicked the sympathoexcitatory effect of serotonin. Since the gain of the sympathetic component of the baroreflex was unaltered after intra-NTS microinjection of serotonin, it could be concluded that serotonin(3) receptors activation in the NTS induces a sympathetic activation which is not mediated through an inhibition of the sympathetic baroreceptor reflex arc.