Electrophysiological analysis of pathways connecting the medial preoptic area with the mesencephalic central grey matter in rats.

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Abstract
An electrophysiological study of ascending and descending connections between the dorsal raphe region of the mesencephalic periaqueductal gray matter and the medial preoptic area was performed in dioestrous female rats anesthetized with urethane. Extracellular action potentials recorded from 208 neurons in the medial preoptic area were analyzed for a change in excitability following stimulation of the periaqueductal gray matter. Neurons [174] were also tested for changes in excitability following stimulation of the mediobasal hypothalamus. Stimulation of the periaqueductal gray matter at 1 Hz was rarely effective, but short trains of pulses (3 at 100 Hz) usually caused an initial inhibition (62.5% of 208) of both projection identified and adjacent neurons of the medial preoptic area, at latencies of 5-90 ms (mean 34.1 .+-. 1.4 ms). Inhibition following stimulation of the mediobasal hypothalamus occurred less frequently (34%) and at shorter latency (mean 12.0 .+-. 1.8 ms, n = 48). Less frequently (10.6%) periaqueductal gray matter stimulation caused an initial excitation of preoptic neurons at latencies of 15-180 ms, (mean 35.3 .+-. 7.2). Initial excitation following mediobasal hypothalamus stimulation was stronger, occurred more frequently (29%) and at shorter latencies (range 3-60 ms, mean 13.1 .+-. 1.5). Neurons (24) displayed antidromic invasion following periaqueductal gray matter stimulation. Latencies for invasion ranged from 13-50 ms (mean 25.5 .+-. 2.0 ms) and are suggestive of an unmyelinated projection. Antidromic invasion from mediobasal hypothalamus was characterized by a shorter latency (mean 12.5 .+-. 0.7 ms; n = 43). A period of reduced excitability lasting 40-100 ms followed antidromic invasion from either site. Antidromic responses to paired mediobasal hypothalamic or periaqueductal gray matter stimuli at 5 ms intervals revealed an increased latency of invasion of the 2nd response, due to the partial refractory period of the neuron. Cells [5] showed a decreased latency of invasion at stimulus separations of 10-150 ms, interpreted as evidence of a supranormal period. Evidence is provided for direct, reciprocal connections between the midbrain central gray and the medial preoptic area. These circuits may play a role in controlling neuroendocrine and behavioral aspects of reproductive functions.