RESPONSES OF ELECTROPHYSIOLOGICALLY IDENTIFIED RAT PARAVENTRICULAR NEURONS TO CHOLECYSTOKININ AND OTHER STIMULI

Extracellular recordings were carried out in the paraventricular nucle us of halothane-anesthetized male rats. Responses of neurons identifie d by antidromic criteria with projections to the nucleus tractus solit arius or to the ventral lateral medulla were compared to those of neur ohypophysial neurons following alterations in blood pressure and osmol arity, hemorrhage and after intravenous injection of cholecystokinin. Neurohypophysial neurons displayed the well-described responses to blo od pressure for putative vasopressin neurons and increases in excitabi lity after cholecystokinin for putative oxytocin neurons. Twenty per c ent of the ventral lateral medulla-projecting neurons were responsive to cardiovascular perturbations, with these displaying reduced activit y after either decreases or increases in blood pressure. None of nine neurons projecting to the ventral lateral medulla responded to i.v. ch olecystokinin. Two of 20 nucleus tractus solitarius-projecting neurons showed reduced activity after cholecystokinin and none increased thei r firing rate. Nitroprusside-induced hypotension was associated with r educed activity in 10% of this population. Three neurons displayed axo n projections to both pituitary and medulla; two of these which projec ted to the nucleus tractus solitarius were activated by cholecystokini n. We conclude that some of the paraventricular nucleus neurons projec ting to the medulla respond to recognized cardiovascular stimuli for n eurohypophysial neurons, but neurons in these populations are generall y unresponsive to cholecystokinin. The former group of neurons may act to coordinate autonomic and endocrine responses to cardiovascular per turbation; however, there may be other stimuli; such as cholecystokini n, which act only on one of the populations of paraventricular nucleus neurons. Furthermore, many neurons in the descending pathways may res pond to stimuli not presently associated with activation of magnocellu lar neurons.