Effects of Neuropeptide Y on the Pressor Responses to Phenylephrine and to Activation of the Sympathetic Nervous System in Anaesthetized Rats

1. The effects of neuropeptide Y (NPY) on the pressor responses to intravenous injections of phenylephrine and to reflex activation of the sympathetic nervous system by stimulation of the sciatic nerve were examined in anaesthetized rats. 2. NPY (10-20 micrograms/kg) always potentiated the pressor response to exogenous phenylephrine (by a mean of 28.1 +/- 5.0%). The effect of the same dose of NPY on the pressor response to sciatic nerve stimulation was variable (sometimes inhibition, sometimes potentiation). 3. NPY appears to act by potentiating post-synaptic alpha-adrenoceptor-mediated vasoconstrictor effects. It may also inhibit noradrenaline release by a presynaptic action. Thus the net effect of NPY on sympathetic activation in vivo may depend on the balance between these two opposing actions.