CARDIOVASCULAR EFFECTS OF MICROINJECTION OF LOW-DOSES OF SEROTONIN INTO THE NTS OF UNANESTHETIZED RATS

In the present study, we analyzed in conscious rats the effects of mic roinjections of serotonin (5-HT; pmol range) into the nucleus of the s olitary tract (NTS) on basal mean arterial pressure (MAP) and heart ra te (HR) and also on the reflex bradycardia induced by the activation o f the baro- and chemoreflex evaluated 1 min after 5-HT microinjection into the NTS. The data show that unilateral microinjection of 5-HT in the picomolar range into the NTS of unanesthetized rats produced a dos e-dependent decrease in MAP and HR, which was blocked by previous micr oinjection of ketanserin (250 pmol/50 nl) into the NTS. The changes in MAP and HR induced by 5-HT were of very short duration, with a return to baseline values a few seconds later. The cardiovascular responses to bare- or chemoreflex activation 1 min after 5-HT microinjection int o the NTS did not differ from the control, indicating that low doses o f 5-HT produced no effect on the cardiovascular reflexes tested at tha t time. The present data show that, as also observed in anesthetized r ats, the microinjection of picomolar doses of 5-HT into the NTS elicit s the typical cardiovascular responses to baroreceptor activation. The se effects, hypotension and bradycardia, seem to be mediated by 5-HT2 receptors because both were blocked by a selective 5-HT2 receptor anta gonist. However, since microinjection of 5-HT (1 pmol) into the NTS pr oduced no changes in the cardiovascular responses to the bare- and che moreflex activated 1 min later, the role of 5-HT2 receptors in the pro cessing of the cardiovascular afferent messages in the NTS remains to be elucidated.