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1. Extracellular action potentials were recorded from forty antidromically identified single units in the supraoptic nucleus of lactating, urethane-anaesthetized female rats. The activity was monitored both during reflex milk ejection and during an increase of 10-15 m-osmole/kg in plasma osmotic pressure induced by intraperitoneal injection of 1 ml. of 1.5 M-NaCl solution.
2. About half (eighteen) the cells showed a burst of activity before reflex milk ejection and were dubbed oxytocin cells. Oxytocin cells responded to a hypertonic injection with a smooth sustained threefold increase in firing rate.
3. The remainder (twenty-two) showed no burst of activity before reflex milk ejection and were dubbed vasopressin cells. Vasopressin cells doubled their firing rate as plasma osmotic pressure increased. Neither cell type increased its firing rate after injections of isotonic NaCl.
4. A phasic firing pattern was rarely seen in slow firing vasopressin cells (< 2 spikes/sec) but was seen in almost all vasopressin cells (twelve out of fourteen) firing between 3 and 8 spikes/sec. Above 8 spikes/sec, some vasopressin cells fired continuously. Phasic firing was only once encountered in an oxytocin cell.
5. The firing rate of both oxytocin and vasopressin cells decreased when plasma osmotic pressure was reduced 10-15 m-osmole/kg by an intragastric water load of 10 ml.
6. Hypothalamic cells lying just outside the supraoptic nucleus did not show a consistent response to injection of hypertonic NaCl.
7. Clearly, both oxytocin and vasopressin cells are osmoresponsive, but phasic firing is characteristic of stimulated vasopressin cells. Thus, osmotic activation allows discrimination between oxytocin- and vasopressin-secreting neurones.
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