Oxytocin release following osmotic activation of oxytocin neurones in the paraventricular and supraoptic nuclei

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Abstract
Recordings were made from a total of 35 antidromically identified neurons in the paraventricular (PV) and supraoptic (SO) nuclei of urethane-anesthetized lactating rats. During recording plasma osmotic pressure was raised by 12 m-osmole/kg by injection of hypertonic solutions of NaCl, LiCl, or mannitol. Nine PV neurons (mean firing rate 4.2 .+-. 1.0 (SE) spikes/s) were classified as oxytocin cells because they gave a burst of activity before reflex milk ejections. None of these showed a bursting (phasic) firing pattern. Ten PV neurons (mean firing rate 1.8 .+-. 0.2 spikes/s) fired phasically either before or after injection of hypertonic NaCl and were classified as vasopressin cells. The remaining 6 PV cells (mean firing rate 1.6 .+-. 0.9 spikes/s) showed no bursts of firing related to milk ejection and did not fire phasically. Increasing plasma osmotic pressure by injection of hypertonic NaCl increased the mean firing rate of PV oxytocin cells to 7.0 .+-. 1.0 spikes/s. Vasopressin cells in the PV nucleus were much less responsive and the mean firing rate after injection was 2.9 .+-. 0.4 spikes/s. The 3rd group of PV neurons was unresponsive. Plasma oxytocin concentration (determined by radioimmunoassay) increased from 2.1 .+-. 0.3 .mu.U[microunits]/ml in the control period to 10.9 .+-. 2.8 .mu.U/ml. Thirty minutes after i.p. injection of 1 ml 1.5 M NaCl and to 14.8 .+-. 2.8 .mu.U/ml following injection of a 2nd 1 ml 1.5 M NaCl. The responses of oxytocin and vasopressin neurons in the SO nucleus to an increase in plasma osmotic pressure following injections of hypertonic solutions of LiCl or mannitol were similar to those observed when plasma osmotic pressure was raised by NaCl. Both oxytocin and vasopressin cells in the neurohypophyseal system are responsive to the osmotic pressure of the blood plasma rather than to Na+ or Cl- concentration. Osmotic activation of oxytocin cells releases sufficient oxytocin to increase its plasma concentration. There may be a functional difference between the SO and PV nuclei.