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SUMMARY
The responsiveness of the anterior pituitary gland to synthetic luteinizing hormone releasing factor (LH-RF) was tested in rats exposed to constant light. At a dosage of 50 ng LH-RF/ 100 g body wt the mean maximal increments in plasma LH and FSH were similar to those at 10.00 h of pro-oestrus. The increments in the plasma gonadotrophins at dosages of 500 and 1000 ng LH-RF/100 g body wt did not differ significantly from those at 250 ng LH-RF/ 100 g body wt. These findings suggest that, in contrast to rats which exhibit regular oestrous cycles, the preovulatory (post-coital) release of LH in rats exposed to constant light may depend almost entirely on the release of a relatively large amount of LH-RF into hypophysial portal vessel blood. Whereas in pro-oestrous animals a relatively small fraction of the readily releasable pool of LH is released during the spontaneous preovulatory surge, in rats exposed to constant light most releasable LH appears to be discharged during the reflex preovulatory surge of this hormone. The concentrations of radioimmunoassayable FSH in blood samples withdrawn before the injection of LH-RF support the view that FSH secretion in the rat is increased by constant exposure to light.
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Department of Human Anatomy, South Parks Road, Oxford, 0X1 3QX
(Received 1 January 1976)
Under certain conditions, luteinizing hormone releasing factor (LH-RF) exerts a priming effect on pituitary gonadotrophs. Thus, in terms of luteinizing hormone (LH) secretion, the response to the second of two exposures to the factor (in vivo and in vitro) is significantly greater than that to the first (Aiyer, Chiappa & Fink, 1974; Pickering & Fink, 1976), and, if the pituitary is exposed to constant increased levels of LH-RF, the secretion of LH increases exponentially after 45–60 min (Fink, Chiappa & Aiyer, 1976). The full development of the priming effect can be blocked by inhibitors of protein synthesis (Fink & Pickering, 1975; Pickering & Fink, 1976), suggesting that the effect depends upon a newly synthesized protein. This protein may be new LH, or some compound acting on the LH-RF receptor or LH release mechanisms. Samli & Geschwind
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Dispersed pituitary cells obtained from female rats with regular oestrous cycles were suspended in Bio-Gel columns and perfused with pulses of luteinizing hormone releasing hormone (LH-RH). There was a close relationship between the amount of LH released and the concentration of LH-RH in the perfusate. It was not possible to elicit the priming effect of LH-RH, but the LH-response changed markedly during the oestrous cycle in a manner similar to that seen in vivo; i.e. the responses of cells prepared from rats killed at pro-oestrus were much greater than the responses of cells prepared from rats killed on other days of the cycle. A similar change in responsiveness was obtained when the columns were perfused with 60 mmol K+/1, suggesting that at least part of the increase in pituitary responsiveness that occurs at pro-oestrus is not dependent upon changes in specific receptors for LH-RH.
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SUMMARY
The release of LH and FSH after the application of an electrical stimulus to the anterior diencephalon of male rats has been studied. The stimulus was applied through either platinum or steel electrodes implanted stereotaxically in animals anaesthetized with urethane. The efficacy of various parameters of stimulation by means of a current consisting of balanced biphasic square waves, was tested by systematically changing the frequency, amplitude and duration of the pulses. The effect of direct current (d.c.) stimulation on hormone release was also examined. The concentrations of the hormones in blood withdrawn from the femoral vein before and at frequent intervals up to 80 min after application of the stimulus were determined by radioimmunoassay.
The optimal parameters for the release of LH by square wave stimulation of the medial preoptic area were: frequency, 60 Hz; pulse amplitude, 0·50 mA; pulse duration, 1·00 ms. This stimulus was more effective when applied through steel than through platinum electrodes. Direct current stimulation (15 μA for 10 s) through steel electrodes was most effective of all. When applied through platinum electrodes to the medial preoptic and anterior hypothalamic areas, the optimal square wave stimulus produced significant increases in the concentration of LH after 5 and 10 min respectively. The concentration of plasma FSH in these animals also increased, but the increments were much less than the increments in LH. The magnitude of the respective increases of the gonadotrophins after stimulation of the two brain areas did not differ significantly. Measurement of the milk ejection response to stimulation of the hypothalamo-hypophysial tract in a lactating rat indicated that the spread of the square wave stimulus was no more than 1·5 mm from the electrode tip.
The significance of these findings is discussed with respect to the importance of the medial preoptic area in the male rat, the neurones which may be involved in the regulation of gonadotrophin secretion, and the parameters of stimulation used in studying the hypothalamo-hypophysial system.
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SUMMARY
Blood was collected from the cut pituitary stalk of male and female rats before and during the application of an electrical stimulus to the medial preoptic area. The plasma was assayed for immunoreactive LH releasing factor (RF) by a double antibody radioimmunoassay using a specific antiserum raised in rabbits against the free acid derivative of the decapeptide LH-RF conjugated to bovine serum albumin. The decapeptide (used as a standard) and pituitary stalk plasma cross-reacted in a similar manner with the antiserum. Stimulation of the preoptic area increased significantly the amount of LH-RF in pituitary stalk plasma in both male and female rats. The increase in LH-RF was linearly related to the strength of the stimulating current, and the amount of LH-RF liberated diminished on cessation of the stimulus. The concentration of LH-RF in pituitary stalk plasma from female rats was significantly greater than that in jugular venous plasma.
The magnitudes of the mean increments of LH-RF in pituitary stalk plasma (stimulation minus pre-stimulation values) at various times of the oestrous cycle in female rats suggests that between 18.00 h of dioestrus and 13.00 h of pro-oestrus there is an increase in sensitivity of the LH-RF secretory mechanism to electrical stimulation. However, the increments decreased in magnitude between 13.00 and 18.00 h of pro-oestrus, indicating that the marked increase in responsiveness of the hypothalamo-hypophysial system to electrical stimulation which occurs during this period is due mainly to a change in sensitivity of the pituitary gonadotrophs to LH-RF.
The LH-RF in pituitary stalk plasma collected before application of the stimulus was higher at some of the times examined during pro-oestrus than at other times of the oestrous cycle. A higher level of the secretion of the factor may be important for the full development of the priming effect of LH-RF and, consequently, the marked increase in responsiveness of the pituitary gland which occurs during the afternoon of pro-oestrus.
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We have investigated the effects of constant light on the patterns of LH release in long-term ovariectomized rats. Some animals were implanted with a silicone elastomer capsule containing oestradiol-17β. Plasma samples in anaesthetized animals were taken from the external jugular vein and in conscious animals from an indwelling intra-atrial catheter. The pulsatile release of LH that occurred in animals not treated with oestrogen was unaffected by constant light or the steroid anaesthetic alphaxalone plus alphadolone acetate (Althesin), but was abolished by sodium pentobarbitone. However, the diurnal release of LH produced by increasing the concentrations of plasma oestradiol in conscious animals was blocked by constant light. Thus, these two rhythms of LH release are controlled by different neural pathways; the one concerned with diurnal LH release being suppressed by exposure to constant light.
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ABSTRACT
Our aim was to determine whether oestrogen administered to ovariectomized rats in a manner which stimulates the release of LHRH also stimulates the synthesis of LHRH mRNA. Adult female Wistar rats were ovariectomized under halothane anaesthesia between 09.00 and 11.00 h of dioestrus, immediately injected with either oil or 10 μg oestradiol benzoate (OB) and then killed by cervical dislocation between 16.00 and 17.00 h of the next day (presumptive pro-oestrus). In-situ hybridization carried out with a 30 mer, 32P-labelled, oligonucleotide probe complementary to LHRH mRNA showed that the concentrations of LHRH mRNA in perikarya in the medial preoptic area, diagonal band and medial septum were significantly greater in OB-compared with oil-treated rats. Plasma LH concentrations were significantly increased in three out of four of the OB-treated rats. These results show for the first time that as well as stimulating LHRH and LH release, oestrogen positive feedback also stimulates the synthesis of LHRH mRNA.
Journal of Endocrinology (1990) 124, 285–289
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ABSTRACT
To investigate the feedback effects of corticosterone on the secretion of corticotrophin-releasing factor-41 (CRF-41), oxytocin and arginine vasopressin (AVP), hypophysial portal vessel blood was collected from control (intact) and long-term (6–8 weeks) hypophysectomized rats. In preliminary experiments in rats anaesthetized with urethane, long-term hypophysectomy resulted in a significant increase in the secretion of oxytocin and AVP; the hypothalamic contents of oxytocin and AVP were also increased in comparison with pituitary-intact rats. In long-term hypophysectomized rats anaesthetized with sodium pentobarbitone, but not with urethane, the output of CRF-41 into portal blood was increased twofold in comparison with that in control rats. In long-term hypophysectomized rats anaesthetized with pentobarbitone, the i.v. infusion of corticosterone (7·2 nmol/min) for a 2 h period of portal blood collection did not alter the secretion of CRF-41, oxytocin or AVP into portal blood; however, the secretion of CRF-41 and, to a lesser extent, AVP was significantly reduced in hypophysectomized rats by continuous corticosterone replacement, by a pellet of corticosterone implanted s.c. for 5 days before portal blood collection. These results confirm that the secretion of CRF-41 is differently affected by the anaesthetics urethane and pentobarbitone, and in long-term hypophysectomized rats show (i) that there were no apparent feedback effects of corticosterone infusion over a 2 h period on the secretion of any of the peptides studied, (ii) that late delayed feedback effects of continuous administration of corticosterone are mediated by a reduction in CRF-41 and AVP output, and (iii) that corticosterone has no effects on oxytocin secretion into portal blood.
Journal of Endocrinology (1991) 129, 91–98
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ABSTRACT
The effects of catechol oestradiol and catechol oestrone on the release of LH and prolactin were investigated in immature male and female Wistar rats. In male rats both catechol oestradiol and catechol oestrone significantly increased the plasma concentration of LH, and catechol oestradiol but not catechol oestrone significantly increased the plasma concentration of prolactin and decreased the pituitary concentration of LH. The parent oestrogens, oestradiol-17β and oestrone, had no effect on plasma LH concentrations, but both increased significantly the plasma concentration of prolactin, and oestrone but not oestradiol-17β increased the pituitary concentration of LH. In immature female rats, catechol oestradiol inhibited the surge of LH and the increase in uterine weight induced by injecting pregnant mare serum gonadotrophin (PMSG). The injection of oestrone induced an increase in the plasma concentration of LH which was about nine times greater than that produced by oestradiol-17β. There were no significant differences in the effects of these steroids on plasma prolactin concentration. These results (i) confirm that in the immature male rat catechol oestrogens can stimulate LH release and show that catechol oestradiol can increase prolactin release, (ii) show that catechol oestradiol can inhibit the stimulatory effects of PMSG on LH release and uterine weight in the immature female rat, and (iii) demonstrate that oestrone can stimulate LH release in the immature female rat.
J. Endocr. (1984) 103, 317-325
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ABSTRACT
The possible involvement of protein synthesis in the priming effect of LH-releasing hormone (LHRH) has been investigated in vitro using hemipituitary glands from pro-oestrous rats. Cycloheximide (7·1 μmol/l) blocked the priming effect of LHRH (elicited by 8·5 nmol LHRH/1) and protein synthesis (assessed by gel electrophoresis of 35S-labelled pituitary proteins). Pituitary glands were also incubated at 0–1 °C followed by incubation at 37 °C. While incubation at 0 °C for either 1 or 2 h reduced LH release and blocked protein synthesis, the LH response to LHRH in a subsequent 1-h incubation at 37 °C was similar to that during the corresponding period in pituitary glands incubated throughout at 37 °C. Incubation with medium alone at 0 °C during the first hour followed by incubation with LHRH at 37 °C during the second hour resulted in an LH response to LHRH which was similar to that in glands incubated with LHRH for 2 successive hours at 37 °C. Two-dimensional gel electrophoresis showed that LHRH priming was associated with the synthesis of a new protein of approximately 69000 molecular weight and with changes in the isoelectric point of two other high molecular weight proteins. These results suggest that the priming effect of LHRH involves the synthesis of a new protein as well as post-translational changes (possibly phosphorylation) in two other proteins and that exposure to cold may prime the pituitary gland to LHRH possibly by stimulating intracellular Ca2+ release and/or protein phosphorylation.
J. Endocr. (1985) 105, 163–168
