EFFECT OF ATROPINE INJECTED INTO THE NUCLEUS-TRACTUS-SOLITARIUS ON THE REGULATION OF BLOOD-PRESSURE

Previous experiments have demonstrated that stimulation of muscarinic cholinergic receptors in the nucleus tractus solitarius (NTS) of the r at decreases arterial blood pressure and heart rate. The present studi es were designed to examine the role of cholinergic mechanisms in the NTS in the tonic regulation of arterial pressure and the baroreceptor reflex. Atropine injected into the NTS of chloralose-anesthetized rats produced a dose-dependent inhibition of cardiovascular responses elic ited by injection of acetylcholine into the same site; 240 pmol atropi ne eliminated acetylcholine-evoked responses. Atropine also increased arterial brood pressure but only at higher doses. Even larger doses of atropine were required to alter cardiovascular responses elicited by electrical stimulation of the aortic depressor nerve. Methylatropine i njected into the NTS also blocked acetylcholine-evoked responses but, in contrast to the actions of atropine, did not increase arterial pres sure in the dose range required to block acetylcholine-evoked response s. Furthermore, a dose of methylatropine (1 nmol) capable of blocking acetylcholine-evoked cardiovascular responses did not alter aortic dep ressor nerve-evoked cardiovascular responses. This lack of an effect o f methylatropine on arterial pressure and aortic depressor nerve-evoke d responses was not due to limited diffusion of the drug within the NT S since 1 nmol methylatropine completely blocked acetylcholine-evoked responses even when injected 0.5 mm distant from the site of acetylcho line injection. These results suggest that cholinergic mechanisms in t he NTS are not involved in the tonic regulation of cardiovascular func tion or the baroreceptor reflex. Furthermore, these results highlight the importance of characterizing doses of drugs used in microinjection studies.