Increase in sensitivity of the baroreceptor reflex following microinjection of carbachol into the posterior hypothalamic nucleus of awake rats

1. In a rat model, the baroreceptor reflex can be assessed by graded infusions of either phenylephrine or sodium nitroprusside with continuous hemodynamic monitoring.
2. Microinjection of the cholinergic agonist carbachol (CCh) into the posterior hypothalamic nucleus (PHN) evokes an increase in mean arterial pressure and a change in heart rate. Lower doses of CCh evoke only tachycardia, whereas middle and higher doses evoke a biphasic change in heart rate of tachycardia followed by bradycardia.
3. The bradycardia following the microinjection of CCh into the PHN can be attenuated by the previous administration of the vasopressin V₁ receptor antagonist [d(CH₂)₅Tyr(Me)] arginine vasopressin (AVPX).
4. Circulating arginine vasopressin (AVP) has been shown to increase the sensitivity of the baroreceptor reflex by stimulating vasopressin V₁ receptors in the area postrema.
5. The attenuation by AVPX of the bradycardia that results following the high doses of CCh suggests that AVP is released into the circulation following stimulation of cholinergic systems within the PHN.
6. Thus, microinjection of a high dose of CCh (11 nmol) into the PHN alters the sensitivity of the baroreceptor reflex by increasing peripheral levels of AVP.