Responses of antidromically identified preoptic neurons in the rat to neurotransmitters and to estrogen

Abstract

One hundred and twenty neurons whose cell bodies lie in the preoptic area and whose axons project to the median eminence region of the basal hypothalamus were identified by antidromic stimulation in ovariectomized rats. Only 42% of these neurons were spontaneously active, their frequency of discharge ranging from 1 spike/10 sec to 15 spikes/sec. Of 39 identified neurons, 27 were inhibited by iontophoretically applied dopamine (DA) and 29 of 30 by norepinephrine (NE); acetylcholine (ACh) was without effect on 15 units similarly tested. The action of estrogen on the sensitivity of identified preoptic neurons to iontophoretically applied catecholamines was tested by comparing the slope of the dose-response curves to DA or NE before, and 30 min after, an intravenous (i.v.) injection of the steroid hormone. No significant differences in the slopes of these curves were observed. The effect of an i.v. injection of estrogens on the spontaneous activity was investigated in 10 identified preoptic units; in 6 of these, an inhibition was seen after a latency of 5–30 min; 3 responded to the injection with a rapid and short-lived excitation, and in one no effect was noted. We conclude that preoptic neurons which project to the basal hypothalamus are sensitive to DA and to NE, but not to ACh. They may change their spontaneous rate of discharge in response to an acute injection of estrogens, but no evidence for a modulatory effect of these steroids on the responsiveness of preoptic units to neurotransmitters were obtained in short-term experiments. These results could be explained by assuming a presynaptic site of action of estrogen.