Peptidergic modulation of efferent sympathetic neurons in intrathoracic ganglia regulating the canine heart.

When either substance P or vasoactive intestinal peptide was injected into an acutely decentralized intrathoracic sympathetic ganglion, short-lasting augmentation of cardiac chronotropism and inotropism was induced. These augmentations were induced before the fall in systemic arterial pressure occurred which was a consequence of these peptides leaking into the systemic circulation in enough quantity to alter peripheral vascular resistance directly. When similar volumes of normal saline were injected into an intrathoracic ganglion, no significant cardiac changes were induced. When substance P or vasoactive intestinal peptide was administered into an intrathoracic ganglion, similar cardiac augmentations were induced either before or after the intravenous administration of hexamethonium. In contrast, when these peptides were injected into an intrathoracic ganglion in which the beta-adrenergic blocking agent timolol (0.1 mg/0.1 ml of normal saline) had been administered no cardiac augmentation occurred. These data imply that in the presence of beta-adrenergic blockade intraganglionic administration of substance P or vasoactive intestinal peptide does not modify enough intrathoracic neurons to alter cardiac chronotropism and inotropism detectably. When neuropeptide Y was injected into an intrathoracic ganglion, no cardiac changes occurred. However, when cardiac augmentations were induced by sympathetic preganglionic axon stimulation these were enhanced following the intraganglionic administration of neuropeptide Y. As this effect occurred after timolol was administered into the ipsilateral ganglia, but not after intravenous administration of hexamethonium, it is proposed that the effects of neuropeptide Y are dependent upon functioning intrathoracic ganglionic nicotinic cholinergic synaptic mechanisms. Intravenous administration of either morphine or [D-ala2,D-leu5]enkephalin acetate did not alter the capacity of the preganglionic sympathetic axons to augment the heart when stimulated. Following the intravenous administration of naloxone, the positive inotropic cardiac responses induced by efferent preganglionic sympathetic axonal stimulation were enhanced minimally in control states and significantly following hexamethonium administration. Thus, it appears that enkephalins are involved in the modulation of intrathoracic ganglion neurons regulating the heart, perhaps via modification of beta-adrenergic receptors. Taken together these data indicate that substance P, vasoactive intestinal peptide, neuropeptide Y, or enkephalins modify intrathoracic ganglionic neurons which are involved in efferent sympathetic cardiac regulation.