Osmosensitive single neurones in the hypothalamus of unanaesthetized monkeys

1. We recorded with tungsten micro‐electrodes the activity of single neurones in the supraoptic nucleus (NSO) and adjacent regions of the hypothalamus while repeatedly injecting solutions of varying tonicity into the common carotid artery of trained, unanaesthetized monkeys who accepted the experimental restraints without anxiety. 2. Intracarotid injections of mildly hypertonic solutions of sodium chloride produced a characteristic behavioural response during and immediately after injection: e.e.g. ‘arousal,’ lip and tongue smacking, chewing, irregular sniffing respiration and associated mildly increased movement of face, eyes and body. 3. Of the 130 cells analysed during hypertonic intracarotid injections, 105 (81%) were osmosensitive. Twenty‐five (19%) of the cells studied during similar injections were non‐osmosensitive. On the basis of the anatomical location of the cells, the pattern of discharge to intracarotid osmotic stimuli and the response to arousing sensory stimuli, we divided the osmosensitive cells into two major groups, ‘specific’ and ‘non‐specific’ osmosensitive cells. 4. Fifty‐two (50%) of the osmosensitive cells we labelled ‘specific’ because they responded to an intracarotid injection of hypertonic sodium chloride, generally did not respond to non‐noxious arousing sensory stimuli and were located in or near the supraoptic nucleus. We found two subtypes of these ‘specific’ osmosensitive cells: ( a ) twenty‐one (20%) NSO cells with ‘biphasic’ responses, that is, acceleration followed by inhibition; ( b ) thirty‐one (30%) cells in the immediate perinuclear zone of the NSO with ‘monophasic’ responses, subdivided into twenty‐one (20%) cells that accelerated and ten (10%) that were inhibited. 5. Fifty‐three osmosensitive cells (50%), located diffusely in the anterolateral hypothalamus, were ‘non‐specific’, responding both to intracarotid injections of hypertonic sodium chloride and also to sensory stimuli that were mildly arousing. Two groups of ‘non‐specific’ osmosensitive cells showed monophasic responses; thirty‐five (34%) cells accelerated and seventeen (16%) of them were inhibited. 6. The ‘monophasic’ specific osmosensitive neurones lying in the immediate perinuclear zone of the supraoptic nucleus in the primate could conceivably be the ‘osmoreceptors’ of Verney. The ‘biphasic’ specific osmosensitive neurones in the NSO may well represent the secretory cells of this system. From our data, the ‘non‐specific’ osmosensitive neurones, scattered diffusely in the anterolateral hypothalamus, have little to do with osmoregulation. Some of these cells located in the perinuclear zone of the NSO could act as interneurones, however, conveying afferent input to the osmoreceptor‐secretory complex of the supraoptic nucleus.