Rat supraoptic neurones: the effects of locally applied hypertonic saline

1. Extracellular action potentials were recorded from supraoptic neurones in lactating, urethane‐anaesthetized rats. A microtap was used to apply a very small volume (about 10 −7 ml.) of hypertonic saline (1‐4 M ‐NaCl) to the immediate neighbourhood of these units over about 1 min. 2. Twenty‐five of twenty‐seven supraoptic neurones were excited by this local osmotic stimulus. The response of individual units was reversible and repeatable. Microtap applications of isotonic saline to supraoptic neurones were without observed effect. 3. Continuously firing supraoptic neurones responded to hypertonic saline with a smooth acceleration in firing rate. Phasic neurones showed an increase in the over‐all level of activity, and in particular, a prolongation of the active phases. Slow, irregularly firing cells responded either with a smooth acceleration in firing rate, or with phasic behaviour. 4. The response to local hypertonic saline appears to be reasonably specific to the supraoptic nucleus. Of thirty‐five neurones recorded close to the supraoptic nucleus but which were not antidromically activated from stimulation of the neural stalk, only nine responded to the local application of hypertonic saline. 5. Similarities between the manner of response of supraoptic neurones to local application of hypertonic saline and the manner of their response to systemic increases in the osmotic pressure of blood plasma support the hypothesis that supraoptic neurones are osmosensitive.