Role of median preoptic area in vasopressin-mediated bradycardia

To determine whether neural traffic through the median preoptic nucleus (MnPO) is involved in arginine vasopressin (AVP)-mediated bradycardia and sympathoinhibition, we recorded reflex decreases in heart rate (HR) and lumbar sympathetic nerve activity, in response to increases in arterial pressure induced either by intravenous phenylephrine (PE) or AVP before, during, and after local administration of lidocaine (200 nl, 2%) in the MnPO of chloralose-anesthetized rabbits. Base-line blood pressure and HR did not change in response to administration of lidocaine into the MnPO. Blockade of neural traffic (by lidocaine) in the MnPO produced an attenuation of AVP-mediated bradycardia but not the baroreflex-mediated bradycardia caused by PE. Lidocaine in the MnPO did not alter the sympathoinhibition produced with AVP. These results indicate that part of the bradycardia produced by AVP is mediated via forebrain structures such as the MnPO and is selective for bradycardia. Additionally, this response was mimicked by administration of yohimbine, an α₂-antagonist, into the MnPO, which suggests that noradrenergic mechanisms are involved in the baroreflex-mediated facilitation of bradycardia by AVP at the level of the MnPO.