Release of oxytocin within the supraoptic nucleus during the milk ejection reflex in rats

The release of endogenous oxytocin and vasopressin by rat paraventricular and supraoptic nuclei in vitro during a 10-min period, 30 min after beginning the incubation, was measured radioimmunologically. Mean basal hormone release per 10 min and per pair of nuclei was: 128.4 +/- 12.4 (S.E.M.) pg vasopressin (n = 15) and 39.0 +/- 3.0 pg oxytocin (n = 66) for supraoptic nuclei from male rats; 273.9 +/- 42.6 pg vasopressin (n = 11) and 34.2 +/- 3.5 pg oxytocin (n = 15) for supraoptic nuclei from lactating rats; 70.0 +/- 8.6 pg vasopressin (n = 52) and 21.8 +/- 1.3 pg oxytocin (n = 68) for paraventricular nuclei from male rats; 59.1 +/- 8.6 pg vasopressin (n = 10) and 27.0 +/- 4.6 pg oxytocin (n = 16) for paraventricular nuclei from lactating rats. In male and lactating rats, both nuclei contained and released more vasopressin than oxytocin. For oxytocin alone, the paraventricular nucleus of male rats contained and released significantly less hormone than the supraoptic nucleus. This difference was not apparent in lactating rats. For vasopressin alone, the paraventricular nucleus contained and released significantly less hormone than the supraoptic nucleus in both male and lactating rats. When the hormone released was calculated as a percentage of the total tissue content the release was about 0.9% for oxytocin from both nuclei in male and lactating rats and also for vasopressin in lactating rats, but was only about 0.5% for vasopressin from both nuclei in male rats. The influence of oxytocin and analogues of oxytocin (including one antagonist) upon the release of oxytocin and vasopressin was studied. Adding oxytocin to the incubation medium (0.4-4 nmol/1 solution) induced a dose-dependent rise in oxytocin release from both nuclei of male or lactating rats. A 4 nmol/l solution of isotocin had a similar effect to a 0.4 nmol/l solution of oxytocin, but arginine-vasopressin never affected basal release of oxytocin. In no case was vasopressin release modified. An oxytocin antagonist (1 mumol/l solution) significantly reduced basal oxytocin release and blocked the stimulatory effect normally induced by exogenous oxytocin, as did gallopamil hydrochloride (D600, 10 mumol/l solution), a Ca2+ channel blocker, or incubation in a Ca2+-free medium. These findings are discussed in relation to the literature on the central effects of neurohypophysial peptides. It may be concluded that the regulatory role of endogenous oxytocin in the hypothalamus on the milk-ejection reflex could result from its local release in the extracellular spaces of magnocellular nuclei.

Oxytocin cells in the paraventricular (p.v.) and contralateral supraoptic (s.o.) nuclei were pair-recorded (with two micro-electrodes) in suckled rats after being anaesthetized with urethane (1.2 g/kg), to study the synchronization of their neurosecretory bursts, the importance of cell recruitment and their firing characteristics. The synchronization of paired bursts was determined by measuring the onset time-lag (time in milliseconds between the onset of two corresponding bursts) and the maximum firing time-lag (time in milliseconds between the two shortest interspike intervals for the corresponding bursts). For each cell, the characteristics studied were: the background activity and the frequency and amplitude (total number of spikes) of the neurosecretory bursts. All paired p.v.-s.o. cells recorded were activated simultaneously 12-18 s before each milk ejection. The onset of a burst could vary either way, up to 680 ms, in relation to the other (mean onset time-lag was 206 +/- 18 ms; n = 85) but the maximum activation periods fitted more closely, the mean maximum firing time-lag being 122 +/- 14 ms (n = 64). Both parameters varied randomly, in duration and order from one pair of cells to another, from one pair of bursts to another for successive bursts of a given pair of cells and independently, whether the cells were in the p.v. or the s.o. nucleus. However, in most cases, the neurosecretory burst with the highest amplitude began and reached its peak firing rate before the corresponding burst from the other cell. Cell recruitment was observed when the milk ejection reflex began, for both the p.v. and the s.o. cells. The bursts of the non-responsive cells developed progressively with the reflex, but, as soon as a cell was recruited, all its successive bursts were simultaneous with those of the first-recruited oxytocin cells. During a regular pattern of milk ejections, the mean background activity of sixty p.v. cells (3.1 +/- 0.2 spikes/s) was significantly higher than that of their s.o. counterparts (1.9 +/- 0.2 spikes/s). Nevertheless, the mean amplitude of the neurosecretory bursts of the sixty p.v. cells (49 +/- 3 spikes) did not differ significantly from that of their s.o. counterparts (55 +/- 4 spikes).(ABSTRACT TRUNCATED AT 400 WORDS)

It has been hypothesized that chemical interactions between neurons in the central nervous system can occur in the absence of well defined synaptic complexes, but morphological correlates have been difficult to find. The present study demonstrates exocytotic release from large (70-130 nm) dense cored vesicles at structurally nonspecialized areas along the plasmalemma of structurally different categories of terminals and occasionally from dendrites and axons within the neuropil of the trigeminal subnucleus caudalis. In rats, the marginal (lamina I) and substantia gelatinosa (lamina II) layers contain the central terminals of primary afferent fibers from the infraorbital nerve that supply the skin and whiskers (vibrissae). Different types of interneurons are also present and may modify the input being relayed to higher centers. While exocytotic profiles were present in control animals, they increased significantly (P less than 0.01) on the ipsilateral side 1-24 h after a unilateral skin lesion in the vibrissae area. A second increase (P less than 0.001) occurred 14-15 days after the lesion. Virtually all examples of large vesicle exocytosis were observed at structurally nonspecialized sites while those at the active synaptic zones involved small clear vesicles. Substance P-like immunofluorescence, present in controls and on the ipsilateral side during the first 6 days, subsequently declined until 4 weeks after surgery when some recovery was noted. The increase in large vesicle exocytosis and the decrease in substance P are interpreted to reflect functional adjustments of different neurons in response to the lesion. The exocytosis involving large dense cored vesicles may serve to deliver transmitters and/or neuropeptide modulators to appropriate receptors in a wider area than release into a specialized synaptic cleft would allow.