Callera, JC; Sevoz, C; Laguzzi, R; Machado, BH*

Journal of the Autonomic Nervous System [J. AUTON. NERV. SYST.], vol. 63, no. 3, pp. 127-136, Apr 1997

In the present study we investigated the effects of microinjection into the commissural nucleus tractus solitarius (NTS) of unanesthetized rats of 2-methylserotonin (2-methyl-5-HT), a 5-HT sub(3) receptor agonist, on the cardiac component of the baro- and chemoreflexes. The study was performed in conscious freely moving rats in order to avoid the possible effects of anesthetics on the cardiovascular responses to microinjection of neuroactive substances into the NTS. The baroreflex (phenylephrine, 0.5-2.0 mu g /kg, i.v.) and the chemoreflex (potassium cyanide, 40 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. Microinjections of 2-methyl-5-HT (5 nmol/50 nl) into the NTS produced a significant increase in basal mean arterial pressure (101 plus or minus 3 versus 125 plus or minus 8 mmHg), no changes in basal HR and a significant reduction in the reflex bradycardio triggered by baroreflex activation at 3 (-28 plus or minus 7 bpm), 10 (-35 plus or minus 4 bpm) and 20 min (-34 plus or minus 53 bpm) in comparison with the control value (-68 plus or minus 9 bpm). A similar reduction in the bradycardic response to chemoreflex activation was observed at 3 (-94 plus or minus 35 bpm), 10 (-98 plus or minus 38 bpm) and 20 min (-110 plus or minus 29 bpm) after 2-methyl-5-HT in comparison with the control value (-178 plus or minus 19 bpm). The effect of 2-methyl-5-HT on the basal mean arterial pressure and on the bradycardia evoked by stimulation of the baro- and chemoreflexes was blocked by pretreatment with granisetron bilaterally microinjected (500 pmol/50 nl) into the NTS. The data show that the stimulation of 5-HT sub(3) receptors in the NTS of unanesthetized rats elicits a significant increase in basal mean arterial pressure and decreases the bradycardic response to baro- or chemoreflex activation.