Connections of medial preoptic neurons with the median eminence and amygdala. An electrophysiological study in the rat.

An electrophysiological investigation was conducted on the connection of medial preoptic neurons with the median eminence and the basal and corticomedial amygdala in pentobarbital anesthesized male Sprague-Dawley rats. Of 411 medial preoptic neurons tested with median eminence stimuli, antidromic responses (mean latency 11.4 +/- SEM 0.4 ms) identified 177 cells as tuberoinfundibular neurons. Most were located in the periventricular area; 21 cells were in the area of the suprachiasmatic nucleus. Axon conduction velocities ranged between 0.01 and 0.9 m/s. 16 neurons displayed two distinct antidromic latencies when the median eminence stimulus was increased above threshold, indicating that their axons may branch in the region of the median eminence. Following suprathreshold stimulation, only 10% of spontaneously active neurons displayed features to suggest activation of a recurrent inhibitory pathway. In response to amygdala stimulation, 23% of tuberoinfundibular neurons displayed an orthodromic response; most (80%) responded with an increase in excitability after a latency of 21.6 +/- 2.4 ms; the remainder displayed a decrease in excitability at latencies of 20-28 ms lasting in excess of 100 ms. These data suggest an additional neural pathway whereby the amygdala can influence neuroendocrine regulation, i.e. through its actions on the excitability of medial preoptic tuberoinfundibular neurons.