MICROINJECTION OF A SEROTONIN, RECEPTOR AGONIST INTO THE NTS OF UNANESTHETIZED RATS INHIBITS THE BRADYCARDIA EVOKED BY ACTIVATION OF THE BAROREFLEXES AND CHEMOREFLEXES

In the present study we investigated the effects of microinjection int o the commissural nucleus tractus solitarius (NTS) of unanesthetized r ats of 2-methylserotonin (2-methyl-5-HT), a 5-HT3 receptor agonist, on the cardiac component of the baro- and chemoreflexes. The study was p erformed in conscious freely moving rats in order to avoid the possibl e effects of anesthetics on the cardiovascular responses to microinjec tion of neuroactive substances into the NTS. The baroreflex (phenyleph rine, 0.5-2.0 mu g/kg, i.v.) and the chemoreflex (potassium cyanide, 4 0 mu g/rat, i.v) were activated in different groups of rats before and after bilateral microinjection of 2-methyl-5-HT into the NTS. Microin jections of 2-methyl-5-HT (5 nmol/50 nl) into the NTS produced a signi ficant increase in basal mean arterial pressure (101 +/- 3 versus 125 +/- 8 mmHg), no changes in basal HR and a significant reduction in the reflex bradycardia triggered by baroreflex activation at 3 (-28 +/- 7 bpm), 10 (-35 +/- 4 bpm) and 20 min (-34 +/- 5 bpm) in comparison wit h the control value (-68 +/- 9 bpm). A similar reduction in the bradyc ardic response to chemoreflex activation was observed at 3 (-94 +/- 35 bpm), 10 (-98 +/- 38 bpm) and 20 min (-110 +/- 29 bpm) after 2-methyl -5-HT in comparison with the control value (-178 +/- 19 bpm). The effe ct of 2-methyl-5 HT on the basal mean arterial pressure and on the bra dycardia evoked by stimulation of the baro- and chemoreflexes was bloc ked by pretreatment with granisetron bilaterally microinjected (500 pm ol/50 nl) into the NTS. The data show that the stimulation of 5-HT, re ceptors in the NTS of unanesthetized rats elicits a significant increa se in basal mean arterial pressure and decreases the bradycardic respo nse to baro- or chemoreflex activation.