Extracellular action potentials were recorded from 1246 neurones in the mediobasal hypothalamus of pentobarbitone or urethane anaesthetized male rats. Antidromic invasion from the surface of the median eminence identified 165 cells, located in the arcuate and ventromedial nuclei and the periventricular area, as tuberoinfundibular neurones. The majority (65%) of these cells displayed no spontaneous activity. 2. Latencies for antidromic invasion from median eminence ranged from 0-5 to 14-0 msec (mean 4-3 +/- 2-9 msec, S.D.). Conduction velocities for axons of tuberoinfundibular neurones were under 1-0 m/sec, and were slowest (under 0-2 m/sec) for those tuberoinfundibular neurones located in the arcuate nucleus. 3. Single 1 HZ stimulation of amygdala evoked short latency (mean 18-8 +/- 7-0 msec; n = 30) excitation of tuberoinfundibular neurones in the ventromedial nucleus. Stria terminalis stimulation evoked similar responses at a shorter latency (mean 10-2 +/- 3-5 msec; n = 12) from other ventromedial tuberoinfundibular neurones. Three of these neurones 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 msec. Tuberoinfundibular neurones soldom followed orthodromic activation at frequencies beyond 30 HZ. 4. An initial decrease in activity at latencies of 18-40 msec (mean 29-2 +/- 10-2 msec) characterized the amygdala evoked responses from nine tuberoinfundibular neurones. A similar response from one other tuberoinfundibular neurone followed stria terminalis stimulation at a latency of 11 msec. Most of these tuberoinfundibular neurones were located in the dorsal part of the ventromedial nucleus. 5. Two ventromedial tuberoinfundibular neurones also displayed antidromic invasion from the amygdala; interaction studies suggested an axon collateral pathway that originated close to the origin of the axon. 6. Tuberoinfundibular neurones unresponsive to amygdala stimulation were usually located in the arcuate nucleus or periventricular area. 7. These results provide electrophysiological evidence for a direct influence of the amygdala on the activity of tuberoinfundibular neurones in the ventromedial hypothalamic nucleus. There are also data to indicate that some ventromedial tuberoinfundibular neurones have axon collaterals that return to the amygdala. These reciprocal connexions between the amygdala and ventromedial tuberoinfundibular neurones may indicate neural circuits important for extrahypothalamic modulation of adenohypophyseal secretion.
