Microinjection of a 5-HT3 receptor agonist into the NTS of awake rats inhibits the bradycardic response to activation of the von Bezold-Jarisch reflex

In the present study we investigated the effects of bilateral microinjectio n into the lateral commissural nucleus tractus solitarius (NTS) of 2-methyl -5-HT, a 5-HT3 receptor agonist, on the bradycardic response of the von Bez old-Jarisch reflex of awake rats. We evaluated mainly the bradycardic respo nse because in previous studies we documented that the hypotensive response of the von-Bezold-Jarisch reflex in awake rats is secondary to the intense bradycardic response. The Bezold-Jarisch reflex was activated by intraveno us injection of serotonin (8 mug/kg) in awake rats before and 1, 3, 10, 20 and 60 min after bilateral microinjection of 2-methyl-5-HT (5 nmol/50 nl, n = 8) into the NTS. Microinjections of 2-methyl-5-HT into the NTS produced a significant increase in basal mean arterial pressure [(MAP), 97 +/- 4 vs. 114 +/- 4 mmHg), no changes in basal heart rate and a significant reductio n in bradycardic (-78 +/- 19; -94 +/- 24 and -107 +/- 21 bpm) and hypotensi ve (-16 +/- 4; -10 +/- 5 and -17 +/- 4 mmHg) responses to activation of the von Bezold-Jarisch reflex at 3, 10 and 20 min, respectively, when compared with the control value (-231 +/- 13 bpm and -43 +/- 4 mmHg). The data of t he present study suggest that serotonin acting on 5-HT3 receptors in the NT S may play an important inhibitory neuromodulatory role in the bradycardic response to activation of the von Bezold-Jarisch reflex. (C) 2001 Elsevier Science Inc.