Central activation of the sympathetic nervous system including the adrenals in anaesthetized guinea pigs by the muscarinic agonist talsaclidine.

Talsaclidine, a novel M1-receptor selective muscarinic agonist for cholinergic substitution therapy of Alzheimer's disease, activates the sympathetic nervous system in guinea pigs and dogs at the orthosympathic ganglia and the paraganglionic adrenals. Results from guinea pigs provide indirect evidence for an additional central site of action. The present investigation in anaesthetized and vagotomized guinea pigs intended to demonstrate central activation of the sympathetic nervous system directly by comparing the blood pressure effects of intracerebroventricular and intravenous injections of small doses of talsaclidine. Increasing doses of 0.2 and 0.6 mg/kg talsaclidine were injected alternately into the third cerebral ventricle and intravenously in 6 guinea pigs before and after blockade of peripheral muscarinic receptors with 1 mg/kg ipratropium bromide i.v. In another group of 6 animals the injections were given into the cisterna cerebellomedullaris using the same protocol. In both groups central administration of talsaclidine caused dose-related hypertension while intravenous injections were hypotensive. Ipratropium bromide, a peripheral antimuscarinic drug, reversed this hypotensive action of intravenous talsaclidine into hypertension, but did not inhibit the effects of central administration. In contrast, atropine, an antimuscarinic drug which passes the blood-brain barrier, abolished the effect of 0.6 mg/kg talsaclidine injected into the cisterna cerebellomedullaris of 8 guinea pigs. The hypertensive effect of a first injection of 0.6 mg/kg talsaclidine into the cisterna cerebellomedullaris of 6 guinea pigs was approximately twice as large as that of a second given 90 min after bilateral adrenalectomy. Sham operation in another 6 animals was not inhibitory. The results demonstrate that talsaclidine, a selective muscarinic M1-receptor agonist, activates central parts of the sympathetic nervous system, including central projections of the adrenals by an action mediated by central muscarinic receptors.