Oxytocin modulation of intrathoracic sympathetic ganglionic neurons regulating the canine heart

Substance P and vasoactive intestinal peptide are known to activate intrathoracic sympathetic neurons which regulate the heart. In the present series of experiments, when 1 I.U. of oxytocin in 0.1 cc of normal saline was administered into the cranial poles of stellate or the middle of middle cervical ganglia cardiac rate and force were augmented. The locations of ganglionic loci which, when injected, resulted in cardiac changes varied between animals. Twenty active sites were identified in the 12 dogs investigated. When the vehicle (0.1 cc of normal saline) was injected into these active sites minimal cardiac responses were induced in one instance. When 1 or 2 I.U. of oxytocin was administered into the superior vena cave of seven animals slight systemic hypotension occurred in two of these animals. Cardiac responses were induced when oxytocin was reinjected into active intrathoracic ganglionic sites after whole body administration of hexamethonium (10 mg/kg IV), but not after local administration of timolol into the ganglia. Thus, it appears that oxytocin can activate intrathoracic ganglionic neurons involved in efferent sympathetic cardiac regulation. That such responses persist in the presence of nicotinic blockade, but not following beta-adrenergic blockade of ganglionic neurons, indicates that oxytocin modifies beta-adrenergic and not nicotinic receptors on neurons in these ganglia.