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I. Functional cell types and their anatomical distribution in the supraoptic nucleus and the internuclear zone

1. We recorded with tungsten micro-electrodes the spontaneous and evoked activity of single cells in the supraoptic nucleus (n.s.o.) and internuclear zone (i.n.z.) of trained, unanaesthetized monkeys who accepted experimental restraints and pituitary gland stimulation without anxiety.

2. Of the 125 hypothalamic neurones analysed, 109 (87%) we classified as magnocellular neuroendocrine cells on the basis of the anatomical location of the cells, the pattern of spontaneous discharge, the effect of pituitary gland stimulation and the response to arousing sensory stimuli.

3. Sixteen (13%) of the cells studied under identical conditions we labelled as non-neuroendocrine cells. Located in the i.n.z. and supraoptic nucleus—optic tract junction (n.s.o.—t.o.), these cells responded to arousing sensory stimuli, showed `continuously active' (i.n.z.) and `high-frequency burster' (n.s.o.—t.o.) patterns of discharge and were not driven by pituitary stimuli.

4. Thirty-seven (30%) of the magnocellular neuroendocrine cells we describe as `identified' because with pituitary gland stimulation we evoked antidromic potentials, mean latency 8·1 msec and mean conduction velocity 0·8 m/sec, with collision between orthodromic and antidromic potentials. Antidromic excitation of neuroendocrine cells was followed by inhibition of spontaneous discharge of cells for 80-125 msec suggesting the presence of recurrent collaterals in primate neuroendocrine cells. Seventy-two (57%) of the magnocellular neuroendocrine cells we designated as `non-identified' because of lack of response, failure of collision or non-tested to pituitary gland stimulation. None of these mangocellular neuroendocrine cells, `identified' or `non-identified', responded to non-noxious arousing sensory stimuli.

5. We find seventy-nine (63%) magnocellular neuroendocrine cells in the supraoptic nucleus (n.s.o.) and thirty (24%) magnocellular neuroendocrine cells in the internuclear zone (i.n.z.) with three basic patterns of spontaneous activity: `silent' cells (s., four cells, 3%); `low-frequency burster' cells (l.f.b., twenty-six cells, 21%); and `continuously active' cells (c.a., seventy-nine cells, 63%).

6. Twenty-six (21%) l.f.b. magnocellular neuroendocrine cells exhibited regular, repetitive, periodic firing patterns with a cycle of 17 sec, involving 5 sec of discharge at 5 spikes/sec and 12 sec of silence. The 26 spikes/burst showed no consistent pattern of serial interspike interval distribution. The burst pattern of discharge was interrupted by intracarotid osmotic stimuli but not by mildly arousing sensory stimuli or sleep—waking behaviour. Mean antidromic latencies of 7·98 msec and the anatomical distributions were the similarities exhibited by the l.f.b. and the c.a. cells.

7. We conclude that the three functional types, s., c.a. and l.f.b., of the magnocellular neuroendocrine cells, are randomly distributed in the supraoptic nucleus (n.s.o.) and in the internuclear zone (i.n.z.) with each type receiving `specific' input connexions. We suggest that these three characteristic firing patterns of magnocellular neuroendocrine cells may be related to the `cellular' secretion of `specific' neurohypophysial hormones and neurophysins.