Cardiovascular significance of neuropeptide Y in the caudal ventrolateral medulla of the rat.

The cardiovascular effects of intracisternal and intraparenchymal injection of neuropeptide Y (NPY) into the caudal ventrolateral medulla (CVLM) were investigated in anaesthetized normotensive rats. Intracisternal administration of 1.25 nmol NPY gave rise to a significant fall in blood pressure and heart rate, maximal 1 h postinjection. More localized microinjections of NPY into the CVLM similarly produced dose-dependent falls in blood pressure and heart rate. The bradycardia was significantly reduced or abolished by cardiac vagal blockade induced by: (1) pretreatment with the peripheral muscarinic antagonist methylatropine; or (2) ipsilateral vagotomy. These procedures also gave rise to a small reduction in the hypotensive response to NPY but the remaining component of the response was still significantly different from control, saline responses which were without significant haemodynamic effects. The results of this study provide further evidence for a central role for NPY in cardiovascular control. In the CVLM NPY responses appear to involve at least two different mechanisms: the fall in heart rate results from activation of cardiac vagal neurons in the nucleus ambiguus. The hypotensive response is more complex. The fall in blood pressure may be due in part to an activation of A1 noradrenergic neurons resulting in reduced sympathetic outflow but a small component of the response is a result of the profound slowing of the heart.