Cardiovascular and sympathetic effects of injecting serotonin into the nucleus tractus solitarius in rats.

Microinjections of serotonin (5-HT) delivered into the nucleus tractus solitarius (NTS) elicit opposite cardiovascular effects depending on the doses used; blood pressure and heart rate are decreased by low doses but increased by high doses. To examine this apparent contradiction, we compared cardiovascular and sympathetic nerve responses elicited in anesthetized rats by using glass micropipettes to inject 5-HT, the vehicle alone or various 5-HT agonists or antagonists into the NTS. Low (0.2 nmol) 5-HT doses lowered mean arterial pressure, heart rate and renal nerve activity consistently. By contrast, high (2 nmol) doses were relatively ineffective as were similar injections of the vehicle alone or of 5-HT1A or 5-HT2A agonists. Systemic cholinergic blockade with methylatropine abolished the bradycardia produced by low 5-HT doses with little or no effect on attendant depressor and sympathoinhibitory responses. Local blockade induced by injecting the 5-HT1A antagonist, WB4101, into the NTS abolished all 5-HT responses, but that induced with the 5-HT2 antagonist, ketanserin, inhibited the same responses only partially. These results suggest that low 5-HT doses injected into the NTS act mainly on 5-HT1A receptors to cause bradycardia through parasympathetic stimulation and lower blood pressure through sympathetic inhibition.