A detailed secretory profile of oxytocin during suckling and parturition was determined in unanaesthetized freely moving rats. Ten pups were reunited with their mothers after 12-15 h of separation. Unless the milk-ejection reflex occurred, there was no difference in serum oxytocin levels before separation and during the suckling of either four or five, or nine or ten pups. Serum oxytocin levels increased abruptly by 50.1 +/- 4.2 (S.E.M.) pmol/l (n = 9) at milk ejection, and declined rapidly with a half-life of about 1.5 min. The peak concentration of blood oxytocin at each milk ejection was independent of the previous suckling period; values from the first three milk-ejection reflexes following the introduction of the pups and those observed 3-5 h after introduction were similar. The process of parturition was monitored by recording intra-uterine pressure with a balloon implanted in the uterus. On day 22 or 23 of pregnancy, continuous and rhythmical contractions of the uterus occurred (onset of parturition), but serum levels of oxytocin (21.1 +/- 1.9 pmol/l; n = 13) did not alter until the expulsive phase. During the expulsive phase, fetuses were delivered after fetus-expulsion reflexes which were recorded as sudden large increases in intra-uterine pressure. Basal levels of oxytocin in the blood increased during this phase (32.5 +/- 4.4 pmol/l; n = 13) and, in addition, rose by about 15 pmol/l and declined slowly after fetus-expulsion reflexes. The increase, however, was quite different from that seen at milk-ejection reflexes.

In order to determine whether oxytocin release is controlled by an osmoreceptor mechanism identical with that for vasopressin release, the plasma oxytocin concentration and plasma osmolality were measured during intraatrial infusion and after intraventricular injection of various osmotic solutions in unanesthetized rats. Intraatrial infusion of 0.6 M NaCI Locke solution (LS.) or 1.2 M mannitol L.S. elevated plasma oxytocin significantly, while 1.2 M urea L.S. caused only a small increase and isotonic L.S. did not change in plasma oxytocin. All hypertonic solutions produced significant and similar increases in the plasma osmolality. Plasma oxytocin was positively correlated with plasma osmolality in the animals infused with hypertonic NaCI or mannitol but not in the animals infused with hypertonic urea. The injection of 2 tl of 0.6 M NaCI artificial cerebrospinal fluid (CSF) or 1.2 M mannitol CSF into the third ventricle caused a significant increase in plasma oxytocin immediately (5 min after injection) without changing plasma osmolality, while the intraventricular injection of 1.2 M urea CSF or isotonic CSF produced no significant change in plasma oxytocin. These results indicate that oxytocin release is controlled by osmoreceptors rather than Na receptors, that the adequate stimulus for the osmoreceptors is one which produces cellular dehydration and that the osmoreceptors are located in the brain region which is accessible to osmotic agents from both the outside and inside of the bloodbrain barrier. Since the organum vasculosum of the lamina terminalis (OVLT) lacks a blood-brain barrier and is known to be involved in osmotic control of vasopressin release, a lesion was made in the anteroventral region of the third ventricle which encompasses the OVLT and the effect of hypertonic NaCI infusion on oxytocin release was examined. No significant increase in plasma oxytocin was observed after intraatrial infusion of 0.6 M NaCI L.S. in the lesioned rats. All of these findings lead to the conclusion that oxytocin release is under the control of osomoreceptors identical to those for vasopressin release.

Pulsatile secretion of luteinizing hormone (LH) in ovariectomized rats was evaluated before and after local implantation of crystalline estradiol benzoate (EB) into various regions within the forebrain. Serum concentrations of LH were measured by radioimmunoassay in samples collected at 6-min intervals through indwelling cardiac catheters. In rats with EB implanted in the preoptic suprachiasmatic area (POSC), and in the nucleus of the tractus diagonalis and the diagonal band of Broca (DBB) to a lesser extent, the mean LH concentration and LH pulse frequency decreased rapidly while the pulse amplitude did not change for 3 h after implantation. Rats with the EB implant in the bed nucleus of stria terminalis, the medial preoptic area, the medial septal nucleus, the anterior hypothalamic area or the third ventricle showed unchanged frequencies of LH secretory pulses. Implantation of progesterone into the POSC of ovariectomized rats produced no significant change in LH secretory profiles. It was suggested that the sites of action of estrogen in decreasing the LH pulse frequency are not widespread but rather restricted within a small part of the brain, including the POSC and DBB, in the ovariectomized rat.

Intracranial actions of estradiol benzoate (EB) in modulating the characteristics of pulsatile LH secretion were examined in ovariectomized rats. A minute amount of crystalline EB was implanted into the medial basal hypothalamus (MBH) or the preoptic suprachiasmatic area (POSC) and blood samples were collected at 6-min intervals for radioimmunoassay of serum LH concentrations. In rats with EB implanted in the MBH, the LH pulse amplitude decreased while the frequency did not change for 3 h after implantation. In rats bearing EB implants in the POSC, in contrast, the LH pulse frequency decreased without a significant variation in the pulse amplitude. Control animals with empty cannulae inserted into the MBH or POSC exhibited unchanged amplitude and frequency of LH secretory pulses. These results indicate that estrogen can regulate the amplitude and frequency of pulsatile LH secretion via mechanisms independent of each other. It appears that MBH and/or the pituitary is a site of the action of estrogen in decreasing LH pulse amplitude, while the POSC is a site of the steroid action in decreasing the pulse frequency.

Extracellular action potentials of paraventricular neurosecretory cells were obtained from urethan-anesthetized rats during microinjection of a hyper- or hypotonic solution into the anteroventral third ventricle (AV3V), third ventricle, or the vicinity of the recording site. The injection of a 0.2-microliter NaCl solution with a concentration range from 0.16 to 0.8 M into the AV3V excited most of the neurons tested. However, the magnitude of the response was dose related. In most cases, arterial blood pressure recorded simultaneously with the excitatory response of the neuron did not change after the injection. Distilled water (0.2 microliter) was injected into the AV3V after the intraperitoneal injection of a 1.5 M NaCl solution (1 ml), which decreased the firing rate in 67% of the neurons tested. The small volume (0.2 microliter) of hypertonic saline, which was effective when injected into the AV3V, excited none of the neurons tested when injected into the third ventricle or the vicinity of the recording site. These results strongly suggest that the AV3V is sensitive to the osmotic stimulus within a physiological range and that the region plays an important role as an osmoreceptor site for the osmoregulation of paraventricular neurosecretory cells.