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Neurophysin and the octapeptide hormones oxytocin and vasopressin are synthesized in the hypothalamus and stored in the posterior lobe of the pituitary gland. It has recently been shown that the release of both oxytocin and vasopressin or of vasopressin alone, in response to potent stimuli, is accompanied by a simultaneous release of neurophysin into the circulation (Burton, Forsling & Martin, 1971; McNeilly, Legros & Forsling, 1972). However, it has yet to be shown that neurophysin can be released at the same time as a specific release of oxytocin. This situation occurs in animals during both parturition (Folley & Knaggs, 1965) and lactation (Folley & Knaggs, 1966; McNeilly, 1972). The present report describes the simultaneous release of oxytocin and neurophysin during parturition in the goat.
Serial blood samples (approx. 10 ml each) were taken from an indwelling jugular cannula during the whole of labour in two pedigree British Saanen goats. Samples
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Abstract
Unilateral knife cuts were performed in the midbrain of lactating rats and the activities of oxytocin neurones were recorded extracellularly from the supraoptic nuclei (SON) in order to investigate the location of the neural mechanism responsible for the synchronization of milk-ejection bursts of oxytocin neurones in different magnocellular nuclei of the hypothalamus. The lesions involved the mesencephalic lateral tegmentum, the intermedial tegmentum and the central grey. Ninety-six SON neurones were antidromically activated by neurohypophyseal stimulation and were also identified as oxytocin neurones, which included 17 pair-recorded neurones. First, the response of oxytocin neurones recorded from the unilateral SON to bilateral or unilateral suckling was tested. During bilateral suckling, not only the oxytocin neurones recorded from the SON on the intact side (n=34) but also those recorded from the SON on the lesioned side (n=58) displayed milk-ejection bursts. When only the nipples ipsilateral to the lesion were suckled (ipsilateral suckling), bursts were induced in most of the oxytocin neurones on the intact (83·3%, n=12) and lesioned side (88·9%, n=27). In contrast, none of the oxytocin neurones (n=37) produced bursts and none of the rats tested (n=23) showed milk ejections during contralateral suckling. Secondly, some characteristics of the bursts of pair-recorded neurones during bilateral suckling and their response to different modes of suckling were investigated. When oxytocin neurones on both sides displayed milk-ejection bursts, they were always well synchronized but the mean burst amplitude of the neurones on the lesioned side (55·6 ±4·9 spikes, n=43) was significantly (P<0·05) lower than that of the neurones on the intact side (65·7 ±5·6 spikes, n=43). Late-recruited neurones were observed in 6 pairs of oxytocin neurones, and these mainly occurred in the neurones on the lesioned side (5/6). In 5 pair-recorded oxytocin neurones, bursts could also be induced synchronously by ipsilateral suckling but not by contralateral suckling. Thus it is very likely that the major mechanism synchronizing the milk-ejection bursts of oxytocin neurones in the bilateral SON is located in the region rostral to the midbrain.
Journal of Endocrinology (1995) 144, 463–470
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SUMMARY
Administration of methallibure (50 mg/kg body weight, daily) to male rabbits resulted in a 45% reduction in sperm number in ejaculates obtained during the treatment period. Recovery occurred within 48 h after the last dose of methallibure. This decrease in sperm number did not occur when oxytocin (0·2 i.u./kg body weight) was administered simultaneously with methallibure. This suggests that methallibure prevents the release of oxytocin during ejaculation.
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For full fertility in the female ovulation is necessary, which is dependent on the production of a surge of LH. An understanding of the processes which result in the high levels of LH requires delineation of the activities of individual component cells. In this study the responses of gonadotrophs to two signalling hypothalamic peptides, GnRH and oxytocin, were investigated. A cell immunoblot method was used to identify and distinguish between cells which secrete LH and those which contain LH but do not secrete the glycohormone. Rats were killed on the morning of pro-oestrus, the pituitary collected and the cells dispersed onto a protein-binding membrane for study. Cells were then incubated with GnRH and oxytocin, after which the membranes including the attached cells were stained by immunocytochemistry for LH. GnRH increased the total number of immunopositive cells which were present in a concentration-dependent manner. The most prominent change after 2 h incubation was in the number of secreting cells, whereas after 4 h there was also a marked increase in numbers of nonsecreting cells. Oxytocin also increased the total number of immunopositive cells in a concentration-responsive manner, however the profile of action of oxytocin was different from that observed for GnRH. Oxytocin had a relatively greater effect on numbers of immunopositive nonsecreting cells. Thus, the results reveal the potential for gonadotrophs to be flexibly and appropriately modulated by selected hypothalamic peptides. When cells were preincubated with oxytocin prior to GnRH there was not an additive increase in the numbers of immunopositive cells, suggesting that the two agonists act, in a nonidentical manner, on similar cells. The increase in the total number of immunopositive cells implies that there was a production of LH or post-translational processing, induced by exposure to GnRH or oxytocin. The results confirmed the heterogeneity of gonadotrophs and the existence of functionally distinguishable subpopulations, and revealed a difference between the effects of GnRH and oxytocin on expression and secretion of LH.
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ABSTRACT
Pregnant rats were ovariectomized (or shamovariectomized) on days 17, 18 or 21 of pregnancy and oestradiol-17β and progesterone were replaced. Prepartum oxytocin concentrations were significantly lower in ovariectomized steroid-treated rats than in intact controls, and on day 21 of pregnancy injection of relaxin into acutely ovariectomized rats significantly increased plasma oxytocin concentrations. During parturition, injection of the opioid antagonist naloxone induced significant increases in plasma oxytocin concentration compared with salineinjected rats. The naloxone-induced increase was significantly less in ovariectomized steroid-treated rats than in rats with intact ovaries, indicating that endogenous opioid activity is less in ovariectomized rats than in intact rats. The progress of parturition in the ovariectomized steroid-treated rats was severely disrupted compared with sham-ovariectomized rats despite similar plasma oxytocin levels at the birth of pup number 2; this disruption was not overcome by injection of naloxone or by the consequent increase in oxytocin secretion, indicating deficient preparation of the uterus and birth canal in the absence of relaxin. We conclude that the decreased oxytocin concentrations prepartum, the prolongation of parturition and the decrease in opioid tone in ovariectomized steroid-treated rats may be partly due to a lack of relaxin produced by the ovary.
Journal of Endocrinology (1993) 138, 13–22
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ABSTRACT
The regulation of both arginine vasopressin (AVP) and oxytocin secretion was studied during rapid and prolonged osmotic stimuli in normal adult volunteers. In five subjects given an intravenous infusion of 0·85 mol NaCl/l at 0·05 ml/kg per min over 2 h there was a significant (P<0·05) rise only in plasma AVP, with no significant change in plasma levels of oxytocin. In six further subjects 5 days of restriction to 500 ml fluid daily resulted in significant increases of both plasma and 24-h urinary AVP, whereas there was no change in corresponding oxytocin levels. During another 5-day period in which the same subjects were given an additional 200 mmol sodium as well as having their fluid intake restricted to 1000 ml daily, there were again significant rises in plasma and 24 h urinary AVP with no change in corresponding oxytocin levels. We conclude that, in man, AVP is selectively secreted in response to both dehydration and high sodium intake, whilst even after the stimulus of rapidly increasing plasma osmolality during intravenous infusion of hypertonic saline the rise in oxytocin is not statistically significant. It therefore appears unlikely that oxytocin has a significant role in the physiological control of fluid balance in man.
J. Endocr. (1986) 108, 163–168
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ABSTRACT
We have examined the ability of granulosa cells, from carefully selected preovulatory bovine follicles, to secrete oxytocin in vitro. Although cells from 83% of follicles underwent functional luteinisation (greatly increased progesterone secretion) in serum-supplemented culture, only 69% had cells capable of oxytocin secretion. Secretion followed a similar time course in all cultures, with the peak appearing on day 3. Oxytocin, but not progesterone, output could be consistently increased by addition of pieces of theca interna tissue, or theca conditioned medium, to the cultures. The effect could be achieved by exposure to theca tissue at any time prior to peak output without altering the time course of secretion. Oxytocin could not be detected in follicular fluid from any of the selected follicles, nor in medium from theca cultured alone. We conclude that the potential for oxytocin secretion is a feature of follicular maturation which is lost during atresia and that the stimulus to secretion is associated with luteinisation but not with progesterone output. Finally, the intermixing of follicular cells during corpus luteum formation may provide a mechanism for the enhancement of oxytocin secretion within a predetermined time frame.
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SUMMARY
Plasma oxytocin concentrations were measured during late pregnancy, parturition and lactation in the miniature pig. Measurements were made of plasma oestradiol, oestrone and progesterone to determine whether there was any relationship between the concentrations of oxytocin and these steroids in the circulation.
Plasma oxytocin concentrations were low or undetectable in late pregnancy. Rises of up to 68·8 μu./ml were seen at the time of delivery of the foetuses and at the expulsion of the placenta. The only steroid that seemed to relate to oxytocin release was progesterone. Oxytocin release was consistently seen when progesterone concentrations had fallen to below 10 ng/ml but no increase in concentration was observed while oestrone and oestradiol increased to their maximum concentrations of 3·86–11·6 and 0·43–0·70 ng/ml respectively.
During lactation, when both oestrogen and progesterone concentrations were low, suckling caused the levels of oxytocin to increase to 7·4 μu./ml. These increases were greater during the first 2 weeks of lactation than later.
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The release of oxytocin and neurophysin during suckling has been studied in conscious unrestrained guinea-pigs. After prior separation, mothers and litters were allowed to suckle for a period of 10 min, and the weight gain of the litter recorded as an index of milk transfer. Maternal blood samples were obtained without disturbance through previously implanted intravenous cannulae and neurophysin and oxytocin determined on unextracted plasma by specific and sensitive radioimmunoassays. In 82 out of 118 experiments the young gained weight during suckling (1·6±0·1 (s.e.m.) g/pup) and this was associated with large rises in both oxytocin and neurophysin concentrations in plasma (mean concentrations: oxytocin 65·4fmol/ml, neurophysin 360fmol/ml). Where serial samples were taken, oxytocin and neurophysin showed a rapid rise and fall in concentration closely associated with the occurrence of milk ejection as judged by the behaviour of the litter. The present results provide the first direct evidence of a spurt release of both oxytocin and neurophysin measured simultaneously during milk ejection. The conscious lactating guinea-pig thus provides a useful laboratory model in which to study hormone release during milk ejection.
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Abstract
Although GnRH is believed to be the primary secretagogue for LH, oxytocin has also been shown to stimulate LH release from the anterior pituitary. We investigated the possibility that the two secretagogues interact in the modulation of LH release. Anterior pituitaries were removed from adult female rats at pro-oestrus, and tissue pieces were pre-incubated in oxytocin for 3 h prior to being stimulated with 15 min pulses of GnRH. LH output over the 1 h period from the beginning of the GnRH pulse was determined. Control incubations were carried out in the absence of oxytocin, and background secretory activities without GnRH stimulation were also determined.
Tissue which was pre-exposed to oxytocin (0·012–1·25 μm) had an increased LH response to GnRH (1·25 nm). The increase was larger than the sum of the LH outputs obtained with oxytocin and GnRH separately, revealing that oxytocin synergistically enhanced LH secretion elicited by GnRH (P<0·05; ANOVA). If stimulation by GnRH was delayed for 2 h after incubation with oxytocin, an increase in LH secretion was still observed, indicating that oxytocin-induced modulation did not rapidly disappear. Oxytocin pre-incubation was observed to result in an increase of maximal GnRH-induced LH output (P<0·001; t-test), as well as an increase of intermediate responses.
The LH response of the anterior pituitary to subsequent pulses of GnRH was modified by the self-priming process. The effect of oxytocin pretreatment on the response of primed tissue to GnRH was also investigated. It was found that pre-incubation in oxytocin also enhanced the LH response of primed tissue to GnRH.
The study has revealed that oxytocin increases the LH output of anterior pituitary tissue in response to GnRH. The effect occurs on both GnRH-primed and unprimed tissues. The results suggest that oxytocin has the potential to regulate the dynamics of the pro-oestrous LH surge.
Journal of Endocrinology (1995) 145, 113–119
