Influence of amygdala stimulation on the activity of identified tuberoinfundibular neurones in the rat hypothalamus.

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Abstract
Extracellular action potentials were recorded from 1246 neurons in the mediobasal hypothalamus of pentobarbitone or urethane anesthetized male rats. Antidromic invasion from the surface of the median eminence identified 165 cells, located in the arcuate and ventromedial neuclei and the periventricular area, as tuberoinfundibular neurons. The majority (65%) of these cells displayed no spontaneous activity. Latencies for antidromic invasion from median eminence ranged from 0.5 to 14.0 ms (mean 4.3 .+-. 2.9 ms, SD). Conduction velocities for axons of tuberoinfundibular neurons were under 1.0 m/s, and were slowest (under 0.2 m/s) for those tuberoinfundibular neurons located in the arcuate nucleus. Single 1 Hz stimulation of amygdala evoked short latency (mean 18.8 .+-. 7.0 ms; n = 30) excitation of tuberoinfundibular neurons in the ventromedial nucleus. Stria terminalis stimulation evoked simular responses at a shorter latency (mean 10.2 .+-. 3.5 ms; n = 12) from other ventromedial tuberoinfundibular neurons. Three of these neurons were also excited by amygdala stimulation at comparably longer latencies. In spontaneously active tuberoinfundibular cells, the initial excitation was followed by a decrease in excitability lasting 70-150 ms. Tuberoinfundibular neurons seldom followed orthodromic activation at frequencies beyond 30 Hz. An initial decrease in activity at latencies of 18-40 ms (mean 29.2 .+-. 10.2 ms) characterized the amygdala evoked responses from 9 tuberoinfundibular neurons. A similar response from 1 other tuberoinfundibular neuron followed stria terminalis stimulation at a latency of 11 ms. Most of these tuberoinfundibular neurons were located in the dorsal part of the ventromedial nueleus. Two ventromedial tuberoinfundibular neurons also displayed antidromic invasion from the amygdala; interaction studies suggested an axon collateral pathway that originated close to the origin of the axon. Tuberoinfundibular neurons unresponsive to amygdala stimulation were usually located in the arcuate nucleus or periventricular area. This is evidence for a direct influence of the amygdala on the activity of tuberoinfundibular neurons in the ventromedial hypothalamic nucleus. Some ventromedial tuberoinfundibular neurons apparently have axon collaterals that return to the amygdala. These reciprocal connections between the amygdala and ventromedial tuberoinfundibular neurons may indicate neural circuits important for extrahypothalamic modulation of adenohypophyseal secretion.