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II. Osmosensitivity of functional cell types in the supraoptic nucleus and the internuclear zone

1. We studied magnocellular neuroendocrine cells and non-neuroendocrine cells in the supraoptic nucleus (n.s.o.) and internuclear zone (i.n.z.) in the hypothalamus of unanaesthetized, chronically prepared monkeys. After antidromic identification, functional cell typing and sensory testing we injected solutions of varying tonicity into an implanted carotid cannula to determine osmosensitivity.

2. On the basis of the anatomical location of the cells, the pattern of discharge in response to osmotic stimuli, the effect of posterior pituitary gland stimulation and the response to non-noxious arousing sensory stimuli, we divided the 101 osmosensitive cells studied into two major groups: eighty-nine (88%) `specific' biphasic osmosensitive magnocellular neuroendocrine cells and twelve (12%) `non-specific' monophasic osmosensitive non-neuroendocrine cells.

3. `Non-specific' non-neuroendocrine osmosensitive cells included nine (9%) cells lying in the internuclear zone (i.n.z.) and showing monophasic exictatory or inhibitory responses to both osmotic and to mildly arousing sensory stimuli. Three (3%) `high-frequency burster' (h.f.b.) osmosensitive cells were located in the n.s.o.—t.o. (supraoptic nucleus—optic tract) junction, showed accelerated discharge to auditory, but not to light or touch stimuli, and were monophasically inhibited by osmotic stimuli.

4. Thirty-three (33%) antidromically `identified' magnocellular neuroendocrine cells in the n.s.o. and i.n.z. we classify as `specific' biphasic osmosensitive cells on the basis of the excitatory-inhibitory response to intracarotid hypertonic sodium chloride and the lack of response to non-noxious arousing sensory stimuli. Fifty-six (55%) of the `non-identified' magnocellular neuroendocrine cells in the n.s.o. and i.n.z. we also found to be `specific' biphasic osmosensitive cells.

5. Magnocellular neuroendocrine cells in n.s.o. and i.n.z. exhibited three functional types, `silent' (s.), `continuously active' (c.a.), and `low frequency burster' (l.f.b.), all of which were osmosensitive with a `specific' biphasic response. With osmotic loading some of the `s.' cells tended to shift transiently to `c.a.' firing, an occasional `c.a.' cell burst briefly and a few `l.f.b.' cells increased bursting for a short time (60 sec).

6. We conclude that magnocellular neuroendocrine cells in n.s.o. and i.n.z. respond `specifically' to intracarotid hypertonic sodium chloride with biphasic, excitatory-inhibitory, discharges and do not respond to mildly arousing sensory stimuli. We suggest that, under the conditions of our experiments, the three functional types of magnocellular neuroendocrine cells (s., c.a., l.f.b.), arising from a common primitive neuroendocrine cell, are randomly dispersed in n.s.o. and i.n.z., are each `specialized' for secretion of a `specific' neurohypophysial hormone, and are sensitive to `specific' input connexions. Under this `specific hormonal state' hypothesis we explain the paradoxical and uniform osmosensitivity of these three different functional cell types on the basis of a suprathreshold osmotic stimulus and a residual `primitive' osmosensitivity. At present we are unable to say which functional magnocellular neuroendocrine cell types secrete a particular neurohypophysial hormone.