Rat supraoptic neurones: the effects of locally applied hypertonic saline

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Abstract
Extracellular action potentials were recorded from supraoptic neurons in lactating, urethane-anesthetized 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 neighborhood of these units over about 1 min. Of 27 supraoptic neurons, 25 were excited by this local osmotic stimulus. Individual unit response was reversible and repeatable. Microtap applications of isotonic saline to supraoptic neurons were without observed effect. Continuously firing supraoptic neurons responded to hypertonic saline with smooth acceleration in firing rate. Phasic neurons showed an increase in the overall activity level and a prolongation of the active phases. Slow, irregularly firing cells responded with a smooth acceleration in firing rate or with phasic behavior. The response to local hypertonic saline appears to be reasonably specific to the supraoptic nucleus. Of 35 neurons recorded close to the supraoptic nucleus, but which were not antidromically activated from neural stalk stimulation, 9 responded to the local application of hypertonic saline. Similarities between the manner of response of supraoptic neurons to local application of hypertonic saline and the manner of their response to systemic increases in the blood plasma osmotic pressure support the hypothesis that supraoptic neurons are osmosensitive.