Sympathetic-parasympathetic interactions at the heart, possibly involving neuropeptide Y, in anaesthetized dogs

1. Stimulation of cardiac sympathetic nerves caused prolonged inhibition of vagal effects on heart rate, an effect which has been proposed on the basis of previous studies to be due to neuropeptide Y or a neuropeptide Y-like substance, released from the sympathetic nerves. 2. This prolonged vagal inhibitory effect was attenuated or abolished when the sympathetic stimulation responsible was given together with continuous vagal stimulation. 3. Continuous vagal stimulation alone did not modify the ability of administered neuropeptide Y to cause inhibition of cardiac vagal action. 4. The results are consistent with the cardiac vagal nerves releasing a transmitter (probably acetylcholine) which acts on cardiac sympathetic nerve endings, inhibiting them from releasing neuropeptide Y or a neuropeptide Y-like substance.