Endogenous opioid peptides and the control of gonadotrophin secretion

Ovulation of the first dominant follicle arising after day 21post partumin suckling beef cows

The effects of body condition score (BCS) of 2·0 or 3·0 units at calving (low v. moderate), post-partum energy intake at 0·6 or 1·0 MJ metabolizable energy (ME) per day per kg M0·75 (low v. high) and unrestricted or restricted (once daily) suckling on the ability of cows to ovulate were studied in a 2 ✕ 2 ✕ 2 factorial design with each treatment replicated eight times. Calf isolation and restricted suckling were imposed shortly after selection of the first dominant follicle (DF) to emerge after day 21 post partum. The episodic release of LH (sampled at 15-min intervals for 10 h) was determined 48 h before and 48 h after the day calf isolation and restricted suckling commenced. Additional blood samples were collected weekly for plasma insulin determination. The mean interval from calving to first ovulation was shorter for cows in moderate than low BCS at calving (47·8 v. 57·1 days, s.e.d. = 4·50, P < 0·05), and for cows suckling once daily than for those with unrestricted suckling (42·9 v. 62·0 days, s.e.d. = 4·50, P < 0·001). Post-partum nutrition did not affect this interval. Mean LH pulse frequency prior to the start of restricted suckling was higher for cows of moderate than low BCS at calving (3·2 v. 1·6 pulses per 10 h, s.e.d. = 0·60, P < 0·05). Subsequently, LH pulse frequency was higher for cows suckling once daily than for those with unrestricted suckling (4·0 v. 2·2 pulses per 10 h, s.e.d. = 0·82, P < 0·05). More cows in moderate than low BCS ovulated the first DF to emerge after day 21 post partum (within 4 to 6 days) in response to restricted suckling (69 v. 25%, P < 0·05). LH pulse frequency prior to restricted suckling increased (P < 0·05) with plasma insulin concentration (categorized as low, < 5; moderate, 5 to 8; and high, >8 mIU per l). There were indications of interactions between suckling treatment and BCS (P < 0·08), and suckling treatment and plasma insulin concentration (P < 0·06), on LH pulse frequency, which suggested that calf restriction could alleviate the suppressive effects of under nutrition on episodic LH release. Amongst cows suckling once daily, the non-ovulating animals had fewer LH pulses prior to restricted suckling and smaller, slower growing DF, indicating an inability of the DF to respond to increased LH pulse frequency following calf restriction. Cows of moderate BCS, particularly those with moderate to high levels of plasma insulin (³ 5 mIU per l), responded favourably to restricted suckling. In contrast, excessively thin cows with low plasma insulin concentrations (<5 mIU per l), that had most to gain from restricted suckling, responded poorly.

Naloxone evokes a nutritionally dependent LH response in post partum beef cows but not in mid-luteal phase maiden heifers

Data from two experiments are reported which test the hypothesis that nutrient and/or dry-matter intake and body condition may interact to modify hypothalamic opioidergic activity and thus influence the pulsatile release of LH during the early post-partum period and during the oestrous cycle. Experiment 1 involved 16 multiparous, twin-suckling beef cows, and was a 2 × 2 × 2 factorial design in which the factors were level of post-partum energy intake (80 v. 130 M) metabolizable energy (ME) per day), the digestible undegradable protein (DUP) content of the post-partum diet (14 v. 31 g/kg dry matter), and treatment with either 200 mg or 400 mg naloxone hydrochloride. Blood samples were collected at 15-min intervals for 4h at weeks 4 and 7 post partum. Naloxone was administered intravenously after the eighth sample. Experiment 2 involved 16 cyclic maiden heifers and was also arranged in a factorial manner, with two levels of body condition at the start of the experimental period (2·50 and 3·16 units) and two levels of energy intake thereafter (40 and 80 MJ ME per day). Seven blood samples were collected at 15-min intervals on 4 days consecutively during the mid-luteal phase of the oestrous cycle. On the first 2 of these 4 days naloxone was administered, whilst on the last 2 days a gonadotropin-releasing hormone agonist (buserelin; GnRH) was administered, both after the fourth sample. Plasma from both experiments was assayed for LH and prolactin (Prl).

In experiment 1, cows on 130 MJ ME per day returned to oestrus and ovulated earlier than cows on 80 MJ ME per day (44·5 v. 55·0 days; s.e.d. = 3·93; P < 0·05). At week 4 post partum the proportional increase in plasma LH following naloxone challenge was greater for cows on 130 MJ ME per day than cows on 80 MJ ME per day (1·38 v. 1·12; P < 0·05), but the converse was true at week 7 (1·15 v. 1·68; P < 0·05). Cows on the high DUP diet required a higher dose of naloxone to elicit an LH response. Few heifers in experiment 2 exhibited an LH response to naloxone. In contrast, there were significant dietary treatment effects on the LH response to GnRH (P < 0·01). Relatively thin heifers on 40 MJ ME per day exhibited the lowest proportional increases in plasma LH to GnRH challenge, whereas heifers on 80 MJ ME per day and given the higher dose of GnRH produced the greatest plasma LH responses. Mean Prl concentrations before and after feeding in experiment 2 were respectively 13·2 and 10·2 ng/l (P < 0·01).

Suckled cows given a high energy diet during the early post-partum period can overcome the opioid mediated block on LH release and resume oestrous cycles earlier than cows given a low energy diet. LH would appear to be inhibited by a non-opioid mechanism in mid-luteal phase heifers. Total pituitary reserves ofLH may be influenced by the animals nutritional status.

Reproductive suppression in subordinate female highveld mole-rats (Cryptomys hottentotus pretoriae): No role for endogenous opioid peptides

The influence of endogenous opioid peptides (EOPs) on plasma luteinizing hormone (LH) levels in subordinate female highveld mole-rats (Cryptomys hottentotus pretoriae) was investigated to elucidate the physiological mechanisms responsible for inhibiting their gonadotropin-releasing hormone (GnRH) and/or LH release. The opioid antagonist naloxone was administered either alone or with GnRH. A single injection of naloxone failed to alter plasma LH levels in dominant reproductive females or in subordinate non-reproductive females in the presence or absence of their ovaries. Pituitary sensitivity to a GnRH challenge was not influenced by naloxone administered acutely or according to longer-term regimens in any of the treatment groups. The results suggest no role for EOPs at the level of the pituitary or hypothalamus in the socially induced infertility evident in non-reproductive female highveld mole-rats.

Distribution of preprodynorphin mRNA and dynorphin-a immunoreactivity in the sheep preoptic area and hypothalamus

Endogenous opioid peptides (EOP) are important modulators in a variety of neuroendocrine systems, including those mediating reproduction, energy balance, lactation, and stress. Recent work in the ewe has implicated the EOP, dynorphin (DYN), in the inhibitory effects of progesterone on pulsatile gonadotropin releasing hormone secretion. Although DYN is involved in a number of hypothalamic functions in the sheep, little is known regarding the localization of preprodynorphin (PPD) expression and its major product DYN A (1-17). In this study, we determined the distribution of PPD mRNA and DYN A-containing cell bodies in the brains of ovary-intact, luteal ewes. To detect PPD mRNA, an ovine PPD mRNA was subcloned by reverse transcription-polymerase chain reaction from sheep hypothalamus and used to create a (35)S-labeled riboprobe for in situ hybridization. Neurons that expressed PPD mRNA and DYN A immunoreactivity were widely distributed in the ovine preoptic area and hypothalamus. PPD mRNA-expressing cells were seen in the supraoptic nucleus, paraventricular nucleus, preoptic area, anterior hypothalamus area, bed nucleus of the stria terminalis, ventromedial nucleus (VMN), dorsomedial nucleus of the hypothalamus, and the arcuate nucleus. All of these regions also contained DYN A-positive cell bodies except for the VMN, raising the possibility that PPD is preferentially processed into other peptide products in the VMN. In summary, based on the expression of both mRNA and peptide, DYN cells are located in a number of key hypothalamic regions involved in the neuroendocrine control of homeostasis in sheep.

Social suppression in nonreproductive female Damaraland mole-rats, Cryptomys damarensis: no apparent role for endogenous opioid peptides

The role of endogenous opioid peptides (EOPs) on LH secretion was examined to investigate the neuronal mechanisms responsible for the inhibition of GnRH and the resultant infertility in nonreproductive female Damaraland mole-rats, Cryptomys damarensis. The endorphin antagonist naloxone was administered to five groups of females to determine its effect on plasma LH levels: Grouping was determined by social status, social environment, and whether the females were ovariectomized. A single injection of naloxone had no significant effect on LH secretion in either intact or hystero-ovariectomized females. Multiple injections with naloxone failed to affect basal LH concentrations but did result in a decrease in GnRH-stimulated LH secretion in ovariectomized nonreproductive and reproductive females. A significant response to a single naloxone injection following GnRH priming was obtained in both nonreproductive females and in nonreproductive females housed in the absence of the reproductive pair. These results suggest EOPs play a role in sexual function but that socially induced infertility is unlikely to be mediated through the EOP system.

The effect of low doses of naloxone on the preovulatory surge of LH and on the onset and duration of oestrus in the ewe with induced oestrus during the non-breeding season

The objective of this work was to study the effect of the endogenous opiate peptide (EOP) antagonist, naloxone, on the preovulatory LH surge and on the time of onset and duration of oestrus in the ewe with induced oestrus during the non-breeding season. Forty Suffolk X Hampshire ewes 2-3-years-old and 50+/- 4kg were studied, ewes were divided at random in two groups of 20, housed in open paddocks under natural photoperiod (19 degrees latitude N); were fed with hay and commercial pellets, and provided water ad libitum. Group one received an intravaginal sponge with 45mg of medroxiprogesterone acetate for 14 days, and upon sponge withdrawal 250IU of eCG was administered i.m. Group two received the same treatment as group 1 but in addition they received two i.m. injections of 0.5mg of naloxone, one given on sponge withdrawal and the second 24h later (total dose 1.0mg). Oestrus in naloxone-treated ewes was present 32+/- 2h and in control ewes in 35+/- 3h after sponge withdrawal. Duration of oestrus in control ewes was shorter (27+/- 2.5h), than naloxone-treated ewes (39+/- 6h); (P<0.0001). The LH surge in naloxone-treated ewes was initiated 5h after onset of oestrus, and 8h after onset of oestrus in control ewes, and the difference was significative (P<0.0006). It was concluded that EOP are important modulators of reproductive function in the ewe.

Plasma beta-endorphin around parturition and its relationship to cortisol level and resumption of pituitary and ovarian functions in dairy cows

The objectives were to evaluate the concentrations of beta-endorphin in peripheral circulation around parturition and to investigate their relationship to the concentrations of cortisol and postpartum resumption of pituitary and ovarian functions in dairy cows. Subjects were 21 Holstein-Friesian cows in late pregnancy. Blood samples were collected from these animals from day 270 in pregnancy until the first ovulation after calving. Average immunoreactive (IR) beta-endorphin concentrations in cows with dystocia (n = 8) in periparturient period (from day 270 of pregnancy until 24 h after calving) were slightly higher than those in cows with normal calving (n = 13) in the same period although the difference was not significant. During the periparturient period, the peak level of IR beta-endorphin was seen at the time of rupture in cows with normal calving and it was observed at the time of parturition in cows with dystocia. The trend of IR beta-endorphin secretion appeared to be concomitant with cortisol secretion in the periparturient period but not in postpartum period (from 24 h after calving until the first ovulation). Average IR beta-endorphin concentrations in cows with dystocia in the postpartum period were not significantly different from those in cows with normal calving. There was no significant association between average IR beta-endorphin concentrations in early postpartum period (from 24 h until 7 days after calving) and the responsiveness of luteinising hormone (LH) to exogenous GnRH administered on day 7 postpartum. However, a negative correlation (r = -0.593; n = 21; P = 0.004) was observed between average IR beta-endorphin concentrations and LH concentrations during the period from day 1 until the first ovulation in the 21 cows. In addition, a positive correlation (r = 0.498; n = 21; P = 0.020) was seen between the beta-endorphin concentrations and time to the first ovulation after calving. The results from this study suggest that beta-endorphin may release into peripheral blood differently from cortisol and that it may be involved in regulating LH secretion and thus the resumption of ovarian cyclicity in postpartum dairy cows.

Effects of naloxone and animal temperament on serum luteinizing hormone and cortisol concentrations in seasonally anestrous Brahman heifers

The effect of endogenous opioid peptides (EOP) and individual animal temperament on serum luteinizing hormone (LH) were investigated in seasonally anestrous Brahman heifers (n = 24). Animals that had shown behavioral estrus in previous months but that had not returned to estrus for at least 30 d were selected. The heifers were ranked by temperament (tame = 1, normal = 2, wild = 3) and randomly allotted into three groups. Blood was collected from one heifer of each group per day. Blood samples were taken via jugular cannula every 15 min for 6 h and every 30 min for another 4 h. After the first hour of sampling, the heifers received intravenous saline (SAL, n = 8); naloxone (LN, 0.5 mg/kg i.v., n = 8); or naloxone (HN, 1.0 mg/kg i.v., n = 8). Three hours after naloxone treatment, each heifer was given gonadotropin releasing hormone (GnRH, 100 microg i.m.). All samples were processed to yield serum and were assayed for LH by radioimmunoassay (RIA). Hourly samples were assayed for cortisol by RIA. The area under the LH curve 60 min postnaloxone treatment was higher in LN and HN than in SAL (57.0 and 40.8 vs 6.1 units; P<0.01); and the area under the 180 min postnaloxone curve remained higher in LN than in SAL (106.2 vs 35.1 units; P<0.05). Cortisol concentrations 60 min postnaloxone administration were above prenaloxone levels(38.2 vs 26.7 ng/ml; P<0.0002). Temperament scores of heifers were positively correlated with cortisol release. The area under the cortisol curve had a negative correlation with mean LH. Serum LH concentrations appear to be suppressed by EOP in seasonally anestrous Brahman heifers, and EOP appear to reduce serum cortisol concentrations. Excitable heifers had higher concentrations of serum cortisol, which negatively affected serum LH concentrations.

Attenuation by an opioid agonist of the oestradiol-induced LH surge in anoestrous ewes and its reversal by naloxone

The effects of a potent opioid peptide agonist [D-ala2-Phe4, Met(0)ol5-enkephalin (FK 33-824) on the magnitude of the oestradiol-induced LH surge and on basal plasma LH concentrations were examined in intact and chronically-ovariectomized ewes during the late-anoestrous period. In intact ewes, treatment with FK 33-824 (0.5 mg i.v. every 3 hr) for a 24 hr period commencing at the time of oestradiol-17 beta administration (25 micrograms i.m. bolus) was associated with non-significant 65% reduction in the peak plasma LH level observed and a significant (P less than 0.05) 58% reduction in the total amount of LH released during the surge (calculated from the area under the curve). Concurrent treatment with the opioid antagonist naloxone (10 mg i.v. every 3 hr) partially reversed this suppressive effect on the magnitude of the LH-surge. In ovariectomized ewes no significant effects on the oestradiol-induced LH surge of either FK 38-824 alone or FK 33-824 in combination with naloxone were observed. Administration of FK 33-824 at a 6-fold higher dose rate (0.5 mg every 30 min) failed to modify basal plasma LH concentration in intact ewes. In ovariectomized ewes, however, a significant (P less than 0.05) 25% fall in basal plasma LH was observed, an effect which was completely reversed by combined treatment with naloxone (10 mg every 30 min). These results support the conclusion that endogenous opioid peptides may contribute to the neuroendocrine mechanism through which oestradiol promotes a preovulatory-like surge in the anoestrous ewe.

Opioid modulation of tonic luteinizing hormone release in ovariectomized dairy cows

A possible role of endogenous opioid peptides (EOP) in regulating the release of luteinizing hormone (LH) in the absence of ovarian influence was investigated. Experiments were conducted on three lactating Holstein-Friesian dairy cows, 20-27 days after ovariectomy. The cows were bled before and after a single intravenous (i.v.) injection of either 250 mg of naloxone (EOP antagonist) or 300 mg of morphine (EOP agonist) or a combination of the two in Experiments 1, 2 and 3, respectively. The mean and basal LH concentrations and the LH pulse frequency and amplitude were compared before and after each treatment in each cow. Naloxone induced an immediate rise in LH concentration by 60-300% above the preceding baseline values. This rise lasted for 15-30 min in each cow, after which the normal rhythmic LH release continued. One cow (A) suffered discomfort and respiratory distress 15-25 min after naloxone administration and the mean and basal LH concentration dropped significantly. Morphine significantly reduced the mean LH concentration by decreasing the number and amplitude of LH pulses and the basal LH values in two cows, although the decrease in one was not significant. The mean LH concentration in each cow remained unaffected by the combined treatment of morphine and naloxone. In conclusion, the elevation of LH concentration by naloxone, the suppression of LH release by morphine and the reversal by morphine and naloxone of each other's effects suggest that EOP could be involved in the control of LH release in cows in the absence of ovarian influence.

Effects of CRF, AVP and opioid peptides on pituitary-adrenal responses in sheep

Intravenous administration of equimolar doses of CRF (30 micrograms) and AVP (6 micrograms) to mature female sheep resulted in elevated plasma concentrations of ACTH and cortisol. Simultaneous administration of equimolar amounts of CRF and AVP resulted in a greater ACTH response compared with the sum of the responses to CRF or AVP given independently. Intravenous bolus administration of the endogenous opioid, Met-enkephalin (2.5 mg), and its potent and long-acting analogue, [D-Ala2,N-Phe4,Met(O)ol5]-enkephalin [FK33-824 (250 micrograms)], did not alter ACTH or cortisol secretion. Furthermore, naloxone, an opioid receptor antagonist given alone or concurrently with Met-enkephalin or FK33-824, was without effect. Pituitary-adrenal responses to CRF were unaltered by simultaneous administration of Met-enkephalin, FK33-824 or naloxone. These results suggest that in the sheep, opioid involvement in the tonic regulation of pituitary-adrenal function is absent. However, CRF and AVP may act alone or in synergy to control the release of biologically active ACTH from the sheep pituitary gland.

Evaluation of beta-endorphin and naloxone on bovine uterine motility

Effects of beta-endorphin and naloxone on bovine uterine motility were tested both in vivo and in vitro. Six cyclic Holstein cows were used to study in vivo effects of beta-endorphin and naloxone on uterine motility during estrus and diestrus. Intrauterine pressure changes were recorded by a microtip pressure transducer before and after treatment. Blood samples were taken every 10 min during the recording periods for beta-endorphin assay. The results revealed that beta-endorphin anc naloxone had no effect on intrauterine pressure in vivo. The effects of beta-endorphin and naloxone on myometrial contractility were also examined in vitro. Beta-endorphin and naloxone were added to tissue baths containing estrous and diestrous uterine strips. The results showed no significant effect of beta-endorphin and naloxone on bovine myometrial contractility. The role of beta-endorphin in bovine reproductive physiology is still not clearly understood, and additional studies are needed.

Naloxone does not Affect the Luteinizing Hormone-Releasing Hormone-Induced Inhibition of Luteinizing Hormone Secretion in Sheep

Abstract Injection of luteinizing hormone-releasing hormone (21 pmol) into the third cerebral ventricle of long-term ovariectomized ewes caused a marked inhibition of luteinizing hormone secretion. Mean luteinizing hormone levels and luteinizing hormone pulse frequency were reduced significantly when compared with the control responses to saline (50 mul). A notable characteristic of the response was the delayed and sustained nature of the luteinizing hormone-releasing hormone-induced inhibition. In the presence of the opioid antagonist naloxone (4 +/- 25 mg iv), the central administration of luteinizing hormone-releasing hormone still produced a marked inhibition of luteinizing hormone secretion. Again, mean luteinizing hormone levels and luteinizing hormone pulse frequency were reduced significantly. When naloxone was injected iv, there was a significant rise in mean luteinizing hormone levels as a consequence of an increase in pulse frequency (in four out of five ewes) and a significant increase in luteinizing hormone pulse amplitude. In conclusion, these data suggest that central opioid pathways sensitive to blockade by naloxone are not involved in the luteinizing hormone-releasing hormone-induced inhibition of luteinizing hormone release. Furthermore, in the long-term ovariectomized ewe, endogenous opioid peptides exert a tonic inhibitory influence on luteinizing hormone-releasing hormone/luteinizing hormone secretion.