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Intracerebroventricular choline reverses hypotension induced by acute chemical sympathectomy.

1. The effect of centrally administered choline on blood pressure was investigated in rats made hypotensive by chemical sympathectomy. Chemical sympathectomy was produced by intravenous (i.v.) injection of 50 mg kg-1 of 6-hydroxydopamine (6-OHDA). Intracerebroventricular (i.c.v.) administration of choline (50-150 micrograms) 2 h after 6-OHDA treatment increased blood pressure and reversed the hypotension in a dose-dependent manner without affecting the heart rate. The pressor response was associated with an increase in plasma vasopressin levels. 2. Pretreatment of rats with the nicotinic receptor antagonist, mecamylamine (50 micrograms, i.c.v.), but not the muscarinic receptor antagonist atropine (10 micrograms, i.c.v.), blocked both the pressor and vasopressin responses to choline (150 micrograms). Pretreatment of rats with hemicholinium-3 (HC-3), a high affinity choline uptake inhibitor, greatly attenuated the pressor response to i.c.v. choline (150 micrograms). 3. The vasopressin V1 receptor antagonist, beta-mercapto-beta,beta-cyclopentamethylenepropionyl-O-Me-Try,Arg) - vasopressin (10 micrograms kg-1; i.v.) given 5 min after i.c.v. choline, decreased the blood pressure but failed to return it to the pre-choline levels. Prazosine (0.5 mg kg-1; i.p.), an antagonist of alpha-adrenoceptors, also decreased blood pressure. Administration of both antagonists together eliminated the pressor response to choline, and the blood pressure was reduced further to below the pre-choline levels. 4. It is concluded that i.c.v. choline can increase blood pressure in rats made hypotensive by acute chemical sympathectomy through the activation of central nicotinic receptors by presynaptic mechanisms. An elevation in plasma levels of both vasopressin and catecholamines (possibly released from the adrenal medulla) is involved in the pressor response to choline.