Cardiovascular effects of cholinergic agents in the ventral-lateral midbrain periaqueductal gray of the rat.

Local injection of the cholinergic agonist carbachol (1-5 nmol) into the ventral lateral periaqueductal gray region of the midbrain produced a dose-dependent bradycardia with minimal effects on blood pressure in the urethane-anesthetized rat. The decrease in heart rate was maximal in the region of the nuclei of the third and fourth cranial nerves. Intravenous injection of carbachol (5 nmol) had little effect on the heart rate. Injection of (-)-nicotine (5 nmol) into this region of the brain produced a slight, but significant, increase in heart rate. Pretreatment with an intracranial injection of atropine greatly reduced the bradycardia response, whereas local injection of mecamylamine or hexamethonium had no effect. The bradycardia resulting from cholinergic stimulation in this area of the brain appears to result from an enhanced vagal influence on the heart, as pretreatment with intravenous atropine methyl nitrate (1 mg/kg) prevented the carbachol-induced bradycardia. The possible physiological significance of this response is discussed.