Evidence for opioidergic inhibition of oxytocin release in periparturient mares

In the horse mare, the onset of parturition is associated with an increase in oxytocin secretion, and it has been suggested that the onset of parturition may be triggered by endogenous oxytocin release. To test the hypothesis that oxytocin secretion is regulated by endogenous opioids in the periparturient period, we have 1) characterized oxytocin secretion in response to vaginocervical stimulation and 2) determined the effect of naloxone, an opioid antagonist, on oxytocin secretion induced by vaginocervical stimulation in prepartum mares and in postpartum mares at estrus and diestrus. During the last 2 months of pregnancy, the first diestrus and subsequent estrus post partum, a total of 66 vaginocervical stimulations were performed. Mares were pretreated with naloxone (0.5 mg/kg i.v.) or saline, administered 20 min before vaginocervical stimulation on subsequent days, using a randomized switchback design in which mares served as their own controls. Plasma was collected from 30 min before until 30 min after stimulation and was analyzed for oxytocin concentrations. Vaginocervical stimulation resulted in a significant increase in oxytocin secretion in all mares. Between Days 30 and 20 prepartum, the total amount of oxytocin secreted (calculated as area under the curve for 0 to 10 min after vaginocervical stimulation) was significantly greater in naloxone-treated than in saline-treated mares. From Day 20 prepartum until parturition, the differences between naloxone and saline-treated mares tended to decrease with approaching parturition, and were no longer statistically different. Peak plasma oxytocin concentrations were greater in naloxone-treated mares than in saline-treated mares during the entire prepartum period. During the postpartum period, total amount of oxytocin secreted following vaginocervical stimulation tended to be greater than during the prepartum period, and stimulated oxytocin secretion was significantly greater in naloxone-treated mares than in saline-treated mares. In conclusion, these data suggest that endogenous opioids suppress oxytocin secretion pre and post partum. It appears that opioid inhibition is not limited to the prepartum period, tends to decrease gradually towards parturition and is reinstated after foaling.     

The timing of parturition in the pig is altered by intravenous naloxone

This experiment tested the hypothesis that opioid antagonists could influence the timing of the onset and progress of parturition in the pig. Primiparous pigs (gilts) received a jugular catheter on Days 104 to 106 of pregnancy. At 1400 h on Day 112 the gilts received 10 mg PGF2alpha, i.m. to induce parturition. At 1000 h on Day 113 (i.e., 20 h later) gilts received either saline (n=6), 1 mg/kg, i.v. naltrexone (n=4) or 1 mg/kg, i.v. naloxone (n=5). Blood samples were taken daily from Days 108 to 116. On Day 113, blood samples were taken hourly from 0500 to 0900 h and then every 30 min until 2400 h, or until the birth of the last piglet (BLP) (whichever was sooner) and assayed for progesterone, oxytocin (OT), cortisol and PRL. Additional blood samples for OT and cortisol assay were taken every minute from 0930 to 1100 h on Day 113 and for 30 min during parturition. Naloxone, but not naltrexone, delayed the onset of parturition relative to saline controls (by 14 h 21 min; P<0.05). Duration of parturition and rate of births were not significantly affected by treatment. Mean plasma OT increased in the 4 h following naloxone but not saline treatment, during which time OT plasma pulse amplitude was reduced in naloxone and naltrexone-treated animals relative to saline treated controls. The PRL secretion rose following treatment in saline treated animals, consistent with approaching parturition, but failed to rise in opioid antagonist treated animals. Progesterone concentrations remained elevated in naloxone-treated animals for longer than in the other groups. These data suggest that a rapid change in overall effect of parenteral administration of naloxone to parturient pigs occurs from delaying its onset when administered as in these experiments, to facilitating its progress when given during parturition (earlier experiments). The delay of onset of parturition may be mediated by interference with hypothalamic control of OT or PRL release.     

Seasonal variation and opioidergic regulation of growth hormone release in cyclic, ovariectomized, and pregnant pony mares

Modulation of reproductive functions is one of the multiple effects of growth hormone (GH). To investigate effects of reproductive functions on GH release in the horse, plasma GH concentrations in ovary-intact (n = 7) and ovariectomized (n = 8) mares during the anovulatory and breeding seasons and in pregnant mares (n = 6) at various stages of gestation were determined. To analyze an opioidergic regulation of GH release, repeated blood samples were taken over 3 h, and mares were injected with the opioid antagonist naloxone (0.5 mg/kg i.v.) or saline. GH was determined by RIA with an antiserum raised against porcine GH and equine GH as standard. In ovariectomized and ovary-intact, cyclic mares, GH concentrations were low and not different between the two groups in November and December. GH concentrations increased significantly (P < 0.05) in cyclic mares during May and June but were not affected by stage of the cycle and were low in ovariectomized mares. In pregnant mares, plasma GH concentrations remained high throughout pregnancy and did not decrease during winter but increased significantly (P < 0.05) postpartum. Naloxone induced a significant GH release in ovary-intact mares; this response was most pronounced (P < 0.05) during the breeding season. Naloxone did not affect GH in ovariectomized mares. During pregnancy, naloxone induced a significant release of GH around Day 280 (P < 0.05) but not at other times of pregnancy. In conclusion, GH release is influenced by season. The seasonal changes depend on ovarian factors, are absent in ovariectomized mares, and can be modulated by pregnancy. GH release is regulated at least in part by opioidergic pathways.     

Effect of progesterone on the activation of neurones of the supraoptic nucleus during parturition

Parturition is driven by a pulsatile pattern of oxytocin secretion, resulting from burst firing activity of supraoptic oxytocin neurones and reflected by induction of Fos expression. Rats were injected with progesterone on day 20 of pregnancy to investigate the role of the decreasing progesterone:ratio oestrogen ratio, which precedes delivery, in the activation of supraoptic neurones. Progesterone delayed the onset of birth by 28 h compared with vehicle (control) and prolonged the duration of delivery, which was overcome by pulsatile injections of oxytocin, indicating that the slow delivery may reflect impaired oxytocin secretion. Parturient rats pretreated with progesterone had fewer Fos immunoreactive nuclei in the supraoptic nucleus than did parturient rats pretreated with vehicle. The number of Fos immunoreactive nuclei was not restored after oxytocin injection, indicating that appropriate activation of oxytocin neurones is impaired by progesterone and also that there is a lack of stimulatory afferent drive. Fos expression increased in the nucleus of the tractus solitarius during parturition in rats pretreated with either vehicle or progesterone, but not in rats that had been pretreated with progesterone and induced with oxytocin, indicating that this input was inhibited. Endogenous opioids inhibit oxytocin neurones in late pregnancy and the opioid antagonist, naloxone, increases Fos expression in supraoptic nuclei by preventing inhibition. However, progesterone attenuated naloxone-induced Fos expression in the supraoptic nucleus in late pregnancy and naloxone administered during parturition did not accelerate the duration of births delayed by progesterone administration, indicating that progesterone does not act by hyperactivation of endogenous opioid tone. RU486, a progesterone receptor antagonist, enhanced supraoptic neurone Fos expression in late pregnancy, indicating progesterone receptor-mediated actions. Thus, progesterone withdrawal is necessary for appropriate activation of supraoptic and tractus solitarius neurones during parturition.     

Rapid Estradiol-17β Modulation of Opioid Actions on the Electrical and Secretory Activity of Rat Oxytocin Neurons In vivo

During pregnancy, emergence of endogenous opioid inhibition of oxytocin neurons is revealed by increased oxytocin secretion after administration of the opioid receptor antagonist, naloxone. Here we show that prolonged estradiol-17β and progesterone treatment (mimicking pregnancy levels) potentiates naloxone-induced oxytocin secretion in urethane-anesthetized virgin female rats. We further show that estradiol-17β alone rapidly modifies opioid interactions with oxytocin neurons, by recording their firing rate in anesthetized rats sensitized to naloxone by morphine dependence. Naloxone-induced morphine withdrawal strongly increased the firing rate of oxytocin neurons in morphine dependent rats. Estradiol-17β did not alter basal oxytocin neuron firing rate over 30 min, but amplified naloxone-induced increases in firing rate. Firing pattern analysis indicated that acute estradiol-17β increased oxytocin secretion in dependent rats by increasing action potential clustering without an overall increase in firing rate. Hence, rapid estradiol-17β actions might underpin enhanced oxytocin neuron responses to naloxone in pregnancy. Special issue article in honor of George Fink.     

Reproductive experience and opioid regulation of luteinizing hormone release in female rats

The objective of the present study was to determine whether reproductive experience that produces shifts in opioid regulation of prolactin secretion and behavioural functions also alters opioid regulation of LH during the oestrous cycle or lactation. In Expt 1 the effect of naloxone administration (i.v.) on LH was compared between age-matched, nulliparous and primiparous, catheterized female rats on dioestrus II. In Expt 2, the effects of multiple reproductive experiences on opiate control of LH were investigated using cyclic, nulliparous and multiparous (three litters) rats. In both experiments, no differences in naloxone-stimulated LH release were found between groups even though multiple reproductive experiences resulted in the prolongation of oestrous cyclicity. In Expt 3, day 8 lactating primiparous rats were administered 2, 5, 10 or 25 mg naloxone kg-1 i.v. The three lowest naloxone doses, but not the 25 mg kg-1 dose, significantly increased LH concentrations. The possible effects of prior reproductive experience on opioid control of LH during lactation were then investigated. Naloxone at 0.5 mg kg-1, but not at 2 mg kg-1 or 10 mg kg-1, stimulated a significantly greater rise in LH in multiparous (two litters) than in primiparous females. Overall, these data indicate that while modest differences were found in naloxone-induced LH responses between multiparous and primiparous animals during lactation, reproductive experience did not significantly alter opioid regulation of LH during subsequent oestrous cycles at the naloxone doses examined. Hence, the effects of reproductive experience on opioid regulation of LH are less pronounced than those previously found for opioid regulation of prolactin and behaviour.     

Changing characteristics of the milk-ejection reflex during pregnancy, lactation and after weaning in the rat

The pattern of reflex milk ejection during suckling was investigated in anaesthetized Wistar rats at various stages of pregnancy, lactation and after weaning. Milk-ejection responses were measured using intramammary pressure recordings, and the amount of oxytocin released was estimated from log dose-response lines compiled from the mammary responses to exogenous oxytocin. The number of rats showing intramammary pressure responses to oxytocin increased on Day 22 of pregnancy (the day of parturition) and decreased at 8 days after weaning. The dose-response lines from pregnant animals were shallow, but steepened and shifted to the left during lactation and after weaning. Reflex milk-ejection responses during suckling were detectable in primigravid animals, indicating that birth of the litter and previous suckling experience are unnecessary for the immediate functioning of the reflex. Reflex milk-ejection responses improved during early lactation (such that the frequency and the amount of oxytocin released at each response were maximal at Day 10 of lactation), and subsequently declined in late lactation. Although the frequency of responses in animals 2 and 4 days after weaning was similar to that in late lactating animals, the amount of oxytocin released at each response had risen again to mid-lactation values. In animals undergoing a second pregnancy and lactation the pattern of change in the milk-ejection responses was similar to that of primiparous animals.     

Naloxone inhibits the plasma epinephrine response to ACTH but not to 2-deoxy-D-glucose in rats

Intravenous injection of (1-24) ACTH and 2-deoxy-d-glucose (2DG) stimulated the plasma epinephrine and norepinephrine levels in pentobarbital-anesthetized male rats. Naloxone, a specific opiate antagonist, inhibited the plasma epinephrine response to ACTH but not to 2DG. Norepinephrine release induced by ACTH or 2DG was not affected by naloxone. These results suggest that the opioid peptidergic synapse might be involved in the ACTH- but not in the 2DG-induced epinephrine release.     

Adrenalectomy affects pain behavior of rats after formalin injection

Stressful stimuli can activate the hypothalamo-pituitary-adrenal-axis and the endogenous opioid system. In addition, corticosterone and opioid release might cause analgesia. This rat study used adrenalectomy for corticosterone withdrawal and naloxone administration for opioid antagonism in order to study pain behavior and hypophyseal hormone release in the plasma after a formalin test. Twelve days before the formalin testing, male Sprague Dawley rats underwent adrenalectomy or sham-adrenalectomy, and non-operated rats were used as reference. The number of flinches and the duration of licking or biting behavior were measured during the early and late phase. In reference and sham-operated rats, injection of formalin 5% resulted in a marked pain behavior in the early and late phase with significant increases in ACTH and corticosterone plasma levels. In adrenalectomized rats, pain behavior was decreased during both phases. Naloxone, administered before the late phase, did not alter pain behavior in sham or reference rats, whereas in adrenalectomized rats pain reactivity returned to those levels observed in reference rats. Beta-endorphin plasma levels above the detection limit were more frequently found in adrenalectomized rats. Thyrotropin and prolactin levels were not different between studied groups. We speculate that the observed reduced pain behavior in adrenalectomized rats after formalin, is the result of an increased production of pro-opiomelanocortin, the pro-drug of both adrenocorticotrophic hormone and beta-endorphin.     

Oxytocin and V1 vasopressin receptors in rabbit endometrium during pregnancy

Neurohypophysial hormone receptors were identified and characterized in rabbit endometrium and decidua by radioligand binding methods. The results strongly support the presence of a heterogeneity of sites in the decidua of parturient rabbits. The oxytocin site (R1) binds oxytocin and oxytocin analogues ([Thr4, Gly7]oxytocin and OTA) with high affinity, whereas the AVP site (R2) was selective for the V1 AVP analogues, [Phe2, Orn8]VT and d(CH2)5TyrMeAVP. The concentration of oxytocin receptors was low (50-100 fmol/mg protein) at oestrus (Day 0) and on Day 29 of pregnancy, but increased significantly (about 8-fold, P less than 0.05) during parturition. Conversely, V1 AVP receptors were more concentrated than the oxytocin sites at the end of pregnancy (150 fmol/mg protein) but did not change during parturition. These results indicate that neurohypophysial hormones have specific receptors not only in the myometrium but also in the uterine mucosa and we suggest that these receptors may participate in the regulation of uterine activity during pregnancy.     

Sexual differentiation of the luteinizing hormone response of neonatal rats to the narcotic antagonist naloxone: critical role of estrogen receptors

Administration of the narcotic antagonist naloxone results in an elevation of serum luteinizing hormone (LH) levels in 10-day-old female, but not male, rats. Previous studies from this laboratory indicated a role for neonatal gonadal steroids in the development of this sex-specific response. In this study, the estrogen receptor antagonist OH-Tamoxifen or the androgen-receptor antagonist flutamide were injected on Days 1 or 9 of life, and the LH responses of male and female pups to naloxone were assessed on Day 10. Flutamide did not produce a response different from that seen in vehicle-treated pups, discounting a role for androgen receptors. OH-Tamoxifen on Day 9 caused an increase in basal levels of LH; neither sex showed a response to naloxone. However, OH-Tamoxifen treatment of 1-day-old males resulted in an enhanced release of LH upon challenge with naloxone on Day 10 of life; similar treatment of 1-day-old females resulted in a normal female-type response to the opioid antagonist. These results show that blockade of estrogen receptors in males during the "critical period" of sexual differentiation results in a female phenotypic response to naloxone. Therefore, estrogen receptors play a critical role in the sexual differentiation of the LH response to naloxone in neonatal male rats.     

Effects of beta-endorphin on spontaneous uterine contractions. Prostaglandins production and 45Ca2+ uptake in uterine strips from ovariectomized rats.

The effects of beta-endorphin, Met-enkephalin, dynorphin and SKF 10047 on the constancy of the isometric developed tension (IDT) of the spontaneous contractions of uterine strips isolated from ovariectomized rats were explored. beta-endorphin (10(-6) M) was the only opioid that depressed significantly uterine constancy of IDT in a concentration dependent fashion. Naloxone, neither at 10(-8) M nor at 10(-6) M, altered the negative inotropic influence of beta-endorphin. Moreover, the basal synthesis and outputs of some prostaglandins (PGE1, PGE2 and PGF2 alpha) from rat uteri and the effect of beta-endorphin (10(-6) M), were determined. It was found that the basal synthesis and release of PGs in uteri were significantly inhibited by this endogenous opioid. The effects of beta-endorphin (10(-8), 10(-6) and 10(-5) M) on the basal; and oxytocin or A23187, induced 45Ca2+ uptake, as well as the influence of naloxone were also studied. beta-endorphin at three of the concentrations tested decreased basal uterine 45Ca2+ uptake and this action was not prevented by naloxone (10(-8) M). The presence of oxytocin and of A23187 augmented significantly 45Ca2+ uptake, an effect that was antagonized by beta-endorphin (10(-6) M). The possible role of beta-endorphin in uterine functioning via the modulation of uterine PG synthesis and Ca2+ uptake is discussed.     

The effect of indomethacin on uterine contractility and luteal regression in pregnant rats at term.

Treatment of pregnant rats with 1 mg indomethacin/kg twice daily i.m. beginning on Day 20 delayed the onset of parturition by about 21 hr and prolonged the duration of spontaneous parturition by 4 hr. Plasma progesterone and oestradiol levels were determined in daily samples of peripheral blood, and uterine contractions were recorded before and during parturition by means of small, chronically implanted intrauterine balloons which were connected to pressure transducers via fluid-filled catheters. Indomethacin treatment did not inhibit or suppress spontaneous or oxytocin-induced contractions, which were of the same intensity in indomethacin-treated as in control rats. Parturition was induced with oxytocin in the same proportion of treated and control rats, but its induction was not successful in treated rats until 1 day later than in control rats, but its induction was not successful in treated rats until 1 day later than in controls. The onset of parturition was always related to the plasma progesterone level, which declined at a slower rate in indomethacin-treated than in control rats, reaching baseline values approximately 1 day later in the treated animals. The appearance of 20alpha-hydroxysteroid dehydrogenase in the CL of pregnant rats normally occurs on Day 21 of gestation, but activity was not observed until about 1 (0-3) day later in the indomethacin-treated rats, indicating that luteolysis was retarded. Prostaglandin F-2alpha infusions given on Day 21 reversed the effects of indomethacin treatment on plasma progesterone, luteal 20alpha-hydroxysteroid dehydrogenase activity and the timing and duration of parturition, and reduced the high perinatal mortality associated with indomethacin treatment, suggesting that the effects of indomethacin were related to its inhibitory action on prostaglandin synthetase activity. It is concluded that, in rats, indomethacin exerts its effects on parturition through inhibition of luteal regression which was significantly retarded but not prevented, and that indomethacin does not have a direct effect on myometrial contractility.     

Plasma oxytocin concentrations during late pregnancy and parturition in the dog

While oxytocin is widely used in the treatment of dystocia in dogs, there is little information about its secretion before and during normal unassisted whelping. We therefore measured plasma oxytocin concentrations during late pregnancy and the expulsive stage of parturition. Blood samples were collected from eight dogs at 3-min intervals during a 42-min period between the 2nd and 14th day before whelping and during parturition after the birth of 1-3 pups. The litters consisted of 5-15 pups and the progression of the expulsive stage was linear and nearly parallel in the eight bitches. The overall mean (+/-S.D.) plasma oxytocin concentration during late pregnancy was 3.6+/-2.1pg/ml. Mean values in individual dogs ranged from 1.2 to 7.4 pg/ml, but the intra-animal variation was rather small. During the expulsive stage the overall mean (+/-S.D.) plasma oxytocin concentration was 12.9+/-13.9 pg/ml, with mean values in individual dogs ranging from 3.5 to 46 pg/ml. The mean area under the oxytocin curve for parturient dogs was significantly higher (P<0.05) than for pregnant dogs. During the expulsive stage, the peak plasma oxytocin level in individual dogs ranged between 10 and 117 pg/ml. In six of the eight dogs a pup was born during blood collection and in five of these animals the plasma oxytocin concentration increased temporarily during periods of abdominal straining and expulsion. However, straining efforts and expulsion were not consistently associated with a rise in the circulating oxytocin level. It is concluded that in the dog plasma oxytocin levels are higher and more variable during the expulsive stage of parturition than during late pregnancy. Interrelationships between the secretion pattern of oxytocin, the level of uterine contractility, and the progress of fetal expulsion in dogs need further exploration.     

Lack of stimulation of relaxin secretion in lactating sows by suckling in vivo or by oxytocin in vitro.

After parturition, eight sows were zero weaned by removing all piglets 6 h after birth; a further 18 sows suckled at least ten piglets each. Blood samples were collected on Day 4 after zero weaning or on Days 4, 14 and 21 of lactation and the sampling frequency increased during suckling bouts. Ovaries were recovered from sows on these days and corpora lutea were either extracted for estimation of relaxin and progesterone concentration, fixed for immunohistochemical analysis or incubated in vitro in the presence or absence of luteinizing hormone (LH) or oxytocin. Luteal weight and progesterone were higher in the zero-weaned sows than in lactating sows (P less than 0.05 and less than 0.001, respectively); relaxin content was below detection by Day 14. This was supported by immunohistochemical staining for relaxin, which showed limited immunostaining in zero-weaned and Day 4 sows, but none in the tissue recovered on Days 14 and 21, which showed typical signs of regression. Secretion of progesterone and relaxin by luteal tissue in vitro was highest in zero-weaned sows (P less than 0.05), decreased as lactation progressed and neither LH nor oxytocin had any significant effect. Concentrations of plasma relaxin were all less than 0.2 ng/ml in three of the four zero-weaned and Day-4-suckled sows assayed; there was no detectable increase during suckling bouts. It was concluded that during lactation the old corpus luteum of pregnancy is not able to release relaxin in response to suckling in vivo or to oxytocin treatment in vitro.     

Suppression of appetitive behavior in the rat by naloxone: lack of effect of prior morphine dependence

Naloxone (0.3–10 mg/kg) produced a dose-related suppression of eating and drinking in rats that had been deprived of food for 48 hr or water for 24 hr. The suppression of water intake by naloxone was unaltered in rats that had been physically dependent upon morphine one week earlier and which were tolerant to the analgesic effect of morphine at the time naloxone was tested. These results confirm the ability of naloxone to suppress appetitive behavior in the rat but do not resolve the issue of whether or not this effect of naloxone is the consequence of an interaction with an endogenous opioid system.     

Oxytocin does not contribute to the effects of cervical dilation on progesterone secretion and embryonic development in mares

The aim of the present study was, to investigate the effects of oxytocin administration on Day 7 post-ovulation on progesterone secretion, pregnancy rate and embryonic growth in mares. Endogenous stimulation of oxytocin release was compared to the administration of native oxytocin or the long-acting oxytocin analogue carbetocin. At Day 7 after ovulation, mares had to undergo four treatments in a crossover design: (a) control, (b) oxytocin (10 IU i.v.), (c) carbetocin (280 microg i.m.) and (d) cervical dilation. On Day 13, all mares (8 of 8 mares) were pregnant on groups control, oxytocin and carbetocin and only 6 of 8 mares on group dilation. In one mare uterine fluid accumulation and uterine edema from Day 6 to 13 and early embryonic death by Day 11 occurred during dilation treatment. Another mare, which did not become pregnant during dilation treatment, developed uterine fluid accumulation and uterine edema from Day 10 to 14. Mean growth rates of the conceptuses did not differ among treatment groups and individual growth rates varied in a wide range from -0.1 to 0.8 cm per day. At Day 13, mean diameters of conceptuses yielded 1.4+/-0.1 cm in control group, 1.5+/-0.1 in oxytocin and carbetocin group and 1.3+/-0.2 cm in dilation group. Secretion of progesterone was not affected by treatments. Administration of oxytocin and carbetocin caused similar maximum plasma concentrations of oxytocin, but onset and duration of peaks differed. Maximum concentrations after intramuscular application of carbetocin were obtained almost 20 min later when compared to intravenous administration of oxytocin. Duration of peaks after injection of the long-acting oxytocin analogue was more than three-fold longer than after administration of native oxytocin. In conclusion, the present study showed that single administration of oxytocin or its long-acting analogue carbetocin at Day 7 after ovulation did not affect progesterone secretion, pregnancy rate and embryonic growth. Two possible scenarios concerning the effects of cervical dilation were observed: In the majority of mares, dilation of the caudal half to two-third of the cervical lumen up to a diameter of 4.5 cm had no negative consequences on progesterone secretion and pregnancy outcome. However, cervical dilation caused uterine inflammation and subsequent luteolysis in two mares and early embryonic death in one of them. Thus, manipulation of the cervix itself seems not to have negative impact on success rates of transcervical transfer of embryos in the mare.     

Serum luteinizing hormone, 13,14-dihydro-15-keto-prostaglandin F2alpha and cortisol profiles during postpartum anestrus in Brahman and Angus cows

Pluriparous suckled Brahman and Angus cows were utilized to evaluate the effect of breed, day after calving and endogenous opioid peptides (EOP) on hormonal profiles during postpartum anestrus. On Days 17 and 34 after calving, blood samples with and without heparin were collected at 15- and 30-min intervals, respectively, for a 7-h period via jugular cannula. Two hours after the start of blood sampling, cows of each breed were administered either 1 mg/kg iv naloxone or saline. Three hours later, all animals received 10 ng/kg iv GnRH. On Day 34 after calving cows received 0.2 IU/kg iv ACTH. Mean LH, basal LH and area under the LH curve increased (P < 0.01) from Day 17 to Day 34 after calving. Height of LH pulses increased (P < 0.05) by Day 34 after calving. Brahman cows had higher (P < 0.05) mean LH, basal LH, LH pulse frequency and area under the LH curve than Angus cows. Naloxone increased postchallenge area under the LH curve in treated cows above that of control cows (P < 0.06). Naloxone also increased the postchallenge area under the LH curve above that of the prechallenge level (P < 0.01). No breed differences in the response to the naloxone challenge were observed. The LH response to naloxone challenge occurred earlier on Day 34 than on Day 17 after calving but the amount of LH released was similar between days. The GnRH-induced LH release was greater in Brahman than in Angus cows (P < 0.04). Mean cortisol concentrations and area under the cortisol curve decreased (P < 0.05) between Day 17 and Day 34 after calving. Mean cortisol concentrations and area under the cortisol curve were lower (P < 0.01) in Brahman than in Angus cows. Cortisol secretion after ACTH treatment was similar between Brahman and Angus cows. The cortisol response after ACTH challenge was positively correlated (r=0.68; P < 0.001) to the prechallenge area under the cortisol curve. Under optimal environmental conditions Brahman cows have a greater LH release and their anterior hypophysis is more sensitive to GnRH challenge than the Angus cows.     

Effects of growth hormone and hypophysectomy of pregnant rats on serum concentrations of pregnancy-associated murine protein-1

Continuous infusion of bovine GH to hypophysectomized non-pregnant rats increased serum concentrations of pregnancy-associated murine protein-1 (PAMP-1) to the levels of adult female rats and pregnant rats. Serum concentrations of PAMP-1 were followed from Day 16 of gestation until 3 days after parturition in hypophysectomized (on Day 14 of gestation) and intact pregnant rats. In the intact pregnant rat there was a decrease in PAMP-1 values from Day 16 until delivery. The serum concentrations of PAMP-1 in hypophysectomized pregnant rats were similar to those in intact pregnant rats before parturition, but PAMP-1 concentrations decreased markedly after parturition in the hypophysectomized rats. We suggest that the serum concentrations of PAMP-1 can be maintained without pituitary GH in late pregnancy, while serum values of PAMP-1 in non-pregnant rats is dependent upon a continuous secretion of pituitary GH.     

Changes and localization of ovarian carbonyl reductase during pseudopregnancy and pregnancy in rats

The present study investigates changes in the activity and enzyme content of ovarian carbonyl reductase (CR), which catalyzes the reduction of 9-keto and 15-ketoprostaglandins in rats during pseudopregnancy and pregnancy. The activity of ovarian CR decreased from the onset of pseudopregnancy and pregnancy, reaching 20-30% of the Day 1 value by Day 12 of pseudopregnancy and 50-60% of the Day 1 value by Day 14 of pregnancy. In the case of pregnant rats, the enzyme activity maintained a minimal level between Day 14 of pregnancy and Day 22 of parturition. An acute increase of the enzyme activity was found on the morning after parturition. The CR content in the ovary maintained a constant level from Day 1 to Day 12 of pseudopregnancy and to Day 18 of pregnancy. In pregnant rats, there was a gradual decrease after 18 days and then a surge during parturition. CR was primarily localized in interstitial gland cells and in theca interna cells but was not found in corpora lutea cells in the ovary during the estrous cycle. Additional immunostaining was also observed in corpora lutea cells during pseudopregnancy and pregnancy. The changes in ovarian CR activity, i.e. the rapid decrease with progressing pseudopregnancy and pregnancy, correlated with the increase in progesterone and the decrease in LH. These results indicate that rat ovarian CR may be regulated via the hypothalamo-pituitary-ovarian axis and may also be involved in luteal function.