Neuropeptide Y in the rostral ventrolateral medulla blocks somatosympathetic reflexes in anesthetized rats

The rostral ventrolateral medulla (RVLM) is a major integrative center of cardiovascular reflexes that modulate vasomotor tone. The functions of Neuropeptide Y (NPY) in the RVLM on cardiorespiratory responses remain unknown. Arterial blood pressure (AP), heart rate (HR), splanchnic sympathetic (sSNA) and phrenic nerve activities, and responsiveness to baro-, somatosympathetic, and chemoreflex stimulation were recorded before and after bilateral NPY injection (100 pmol, 200 nl/side) in the RVLM of vagotomized urethane-anesthetized rats (n=7). Responses were characterized by an initial increase in AP followed by prolonged hypotension (P<0.01). A similar biphasic effect was exerted on HR (P<0.01). NPY caused a large increase of sSNA (P<0.01), that gradually recovered towards baseline. Somatosympathetic responses evoked by tibial nerve stimulation were largely abolished following NPY microinjection (P<0.01), but sympathoexcitatory responses evoked by acute hypoxia or sympathoinhibition evoked by aortic depressor nerve stimulation were unchanged following NPY. There was no effect of NPY on phrenic nerve amplitude or frequency. We conclude that NPY exerts excitatory effects on sympathetic tone, but inhibits responses evoked by somatic inputs. We speculate that this apparent contradiction may be due to differential expression of NPY receptor subtypes on the soma of sympathetic premotor neurons in the RVLM and on the presynaptic terminals of neurons that comprise excitatory afferent pathways.