CARDIOVASCULAR-RESPONSES EVOKED BY CARBACHOL MICROINJECTION INTO THE POSTERIOR HYPOTHALAMUS INVOLVES GANGLIONIC NICOTINIC AND MUSCARINIC MECHANISM

1 Microinjection of the cholinergic agonist carbachol (3.3, 5.5 and 13 .2 nmol) into the posterior hypothalamic nucleus of conscious rats evo kes a dose-dependent increase in blood pressure. The presser response evoked by the lower doses of carbachol was attenuated by pretreatment with the ganglionic nicotinic receptor antagonist pentolinium (10 mg k g(-1), i.v.) while blockade of V-1-vasopressin receptors with [d(CH2)( 5)Tyr(Me)]AVP (20 mu g kg(-1), iv) reduced the presser response evoked by the highest dose. 2 The combination of pentolinium and the muscari nic receptor antagonist methylatropine (2 mg kg(-1), i.v.) completely blocked the response evoked by the lower doses while the addition of [ d(CH2)(5)Tyr(Me)]AVP to these two antagonists was required for further inhibition of the presser response to the highest dose of carbachol. Bilateral adrenal demedullation did not affect the presser response ev oked by 5.5 or 13.2 nmol of carbachol. 3 Treatment of intact and adren al demedullated rats with pentolinium after the presser response to 13 .2 nmol of carbachol was underway reversed the presser response, but n ot to the same degree as that provided by the combination of pentolini um and methylatropine, or pentolinium and [d(CH,),Tyr(Me)]AVP. 4 Methy latropine or [d(CH2)(5)Tyr(Me)]AVP caused a slight reversal of the car bachol-induced presser response once it was underway in intact rats. M ethylatropine given before or after pentolinium worked with the pentol inium to completely reverse the response. Methylatropine given alone r eversed the bradycardia evoked by carbachol to a tachycardia which its elf was antagonized by subsequent treatment with pentolinium. 5 These results suggest that the presser response evoked by carbachol microinj ection into the posterior hypothalamic nucleus of conscious rats invol ves sympathoexcitation and vasopressin release. The sympathoexcitation involves nicotinic and muscarinic receptors in autonomic ganglia.