Opioid regulation of luteinising hormone and prolactin release in the horse—identical or independent endocrine pathways?

In order to determine if endogenous opioids regulate luteinising hormone (LH) and prolactin secretion via a common, gonadotropin-releasing hormone (GnRH) dependent pathway in the horse, effects of the opioid antagonist naloxone (300 mg) and the GnRH agonist buserelin (20 μg) on prolactin and LH secretion were investigated in stallions (n = 22), long-term castrated geldings (n = 15) and non-lactating mares during the luteal phase of the oestrous cycle (n = 16). Blood samples for determination of LH and prolactin concentrations were withdrawn at 15 min intervals for 120 min. After 60 min of sampling, animals were treated with either naloxone, buserelin or saline. In stallions, naloxone significantly increased LH as well as prolactin release (P < 0.05), indicating an opioid inhibition of both hormones, whereas in mares, naloxone stimulated only LH secretion (P < 0.05). No changes in plasma LH or prolactin concentrations after injection of naloxone were found in geldings. In all animal groups, buserelin induced a significant release of LH (P < 0.05) without affecting prolactin. We conclude that endogenous opioids inhibit LH and prolactin release in the horse but the regulation of these two hormones involves independent opioid pathways. These are activated differentially in stallions, geldings and mares. The opioid regulation of prolactin secretion is not mediated via GnRH.     

Naloxone evokes large-amplitude GnRH pulses in luteal-phase ewes

Ewes were sampled during the mid-late luteal phase of the oestrous cycle. Hypophysial portal and jugular venous blood samples were collected at 5-10 min intervals for a minimum of 3 h, before i.v. infusions of saline (12 ml/h; N = 6) or naloxone (40 mg/h; N = 6) for 2 h. During the 2-h saline infusion 2/6 sheep exhibited a GnRH/LH pulse; 3/6 saline infused ewes did not show a pulse during the 6-8-h portal blood sampling period. In contrast, large amplitude GnRH/LH pulses were observed during naloxone treatment in 5/6 ewes. The mean (+/- s.e.m.) amplitude of the LH secretory episodes during the naloxone infusion (1.07 +/- 0.11 ng/ml) was significantly (P less than 0.05) greater than that before the infusion in the same sheep (0.54 +/- 0.15 ng/ml). Naloxone significantly (P less than 0.005) increased the mean GnRH pulse amplitude in the 5/6 responding ewes from a pre-infusion value of 0.99 +/- 0.22 pg/min to 4.39 +/- 1.10 pg/min during infusion. This episodic GnRH secretory rate during naloxone treatment was also significantly (P less than 0.05) greater than in the saline-infused sheep (1.53 +/- 0.28 pg/min). Plasma FSH and prolactin concentrations did not change in response to the opiate antagonist. Perturbation of the endogenous opioid peptide system in the ewe by naloxone therefore increases the secretion of hypothalamic GnRH into the hypophysial portal vasculature. The response is characterized by a large-amplitude GnRH pulse which, in turn, causes a large-amplitude pulse of LH to be released by the pituitary gland.     

The importance of prolactin for initiation of lactation in the pregnant ewe

A single injection of ergocryptine (0.5 mg/kg liveweight) given to ewes 0.5-20 days prepartum or two injections (0.5 mg/kg liveweight per injection) given c. 30 and 10 days prepartum reduced concentrations of plasma prolactin to negligible (less than 5 ng/ml) values for 4 weeks after parturition, but did not affect concentrations of growth hormone and placental lactogen. Milking of treated ewes had no effect on concentrations of plasma prolactin during the first 4 weeks of lactation, but concentrations of growth hormone were increased during the 10-20 min period after milking. The half-life of prolactin in plasma was estimated as 21 min. In spite of the dramatic effect of ergocryptine on plasma prolactin all treated ewes secreted copious quantities of milk of normal composition. Mean daily yields of ewes treated with ergocryptine were not significantly different (P greater than 0.05) from those of untreated control ewes, but the mean +/- s.e.m. of total milk production over the first 3 weeks of lactation for ergocryptine-treated ewes was significantly lower (P less than 0.05) than that of control ewes (9.5 +/- 1.11 v. 14.1 +/- 1.20 kg milk). The results suggest that prolactin is not an essential component of the lactogenic and galactopoietic complexes of hormones in the ewe.     

Opioid inhibition of luteinizing hormone secretion in the postpartum lactating sow

Twelve lactating sows were used at 22.4 +/- 0.8 days postpartum to determine whether endogenous opioid peptides (EOP) are involved in the suckling-induced inhibition of luteinizing hormone (LH) secretion. Four sows each received either 1, 2, or 4 mg/kg body weight of naloxone (NAL), an opiate antagonist, in saline i.v. Blood was collected at 15-min intervals for 2 h before and 4 h after NAL treatment. All sows were then given 100 micrograms gonadotropin-releasing hormone (GnRH) in saline i.v., and blood samples were collected for an additional h. Pigs were weaned after blood sampling. At 40 h after weaning, sows were treated and blood samples collected as during suckling. Serum concentrations of LH after treatment with NAL were similar for all doses; therefore, the data were pooled across doses. During suckling, serum concentrations of LH were 0.41 +/- 0.04 ng/ml before NAL treatment, increased to 0.65 +/- 0.08 ng/ml at 30 min after NAL treatment, and remained elevated above pretreatment concentrations for 120 min (p less than 0.05). Naloxone failed to alter serum concentrations of LH after weaning. These data indicate that EOP may be involved in the suckling-induced suppression of LH secretion and that weaning may either decrease opioid inhibition of LH secretion or decrease pituitary LH responsiveness to endogenous GnRH released by NAL.     

Does prolactin influence the hypothalamo-pituitary GnRH-LH system in preovulatory-phase ewes?

The effects of prolonged infusions of prolactin (PRL) into the third ventricle of the brain of cycling ewes on the secretory activity of hypothalamic GnRH neurons and pituitary LH cells in the pars distalis during the proestrous day were studied. Mature Blackhead ewes were infused with vehicle (control, n=5) or with prolactin (200 mug/day, n=5) during 4 consecutive days prior to the next spontaneous ovulation. The dose of PRL was infused each day in 4 series of 50 mug/100 mul/h at 30-min. intervals, from 8.30 to 14.00 h. The animals were slaughtered on the 16th (proestrous) day of the estrous cycle immediately after the last infusion and their brains were fixed in situ. Plasma samples were collected for 6 h at 10 min. intervals, on days 12 (before the infusions) and 16 of the cycle. The distribution pattern, number and morphology of GnRH neurons in vehicle- and PRL-infused ewes were found to be similar and typical for the proestrous phase of the cycle. The immunoreactive (ir) GnRH stores in the median eminence were high and similar in both groups. There were no differences between control and PRL-treated ewes in the number or features of irLH cells. The area fraction and optical density for irLH cells and mRNA LHbeta-expressing cells did not differ between control and experimental groups. Irrespective of the kind of infusion, changes in LH secretion during the estrous cycle were similar in control and PRL-infused ewes. Mean plasma LH concentrations were higher (p<0.001) on day 16 compared to day 12 of the cycle. There were no differences in plasma LH concentrations or in the parameters of pulsatile LH secretion between groups. In conclusion, repeated, several-hour-long infusions of PRL into the CNS prior to the next spontaneous ovulation in ewes has no direct effect on the secretory activity of GnRH neurons, and/or the synthesis, accumulation, or tonic release of LH from the pituitary gonadotrophs.     

Effect of an acute ergotamine challenge on reproductive hormones in follicular phase heifers and progestin-treated cows

The objective of this research was to determine if ergotamine, an ergopeptine alkaloid isolated from Neotyphodium-infected grasses and associated with toxicoses in livestock, altered plasma concentrations of reproductive hormones in follicular phase heifers and in cows given a progestin implant. In Experiment 1, blood was sampled for 8h from four cycling heifers 2 days after synchronized luteolysis. Heifers were treated with ergotamine tartrate (19microg/kg) i.v. or saline vehicle in a simple cross-over design after 1h of pre-treatment blood sampling. Heifers received oxytocin (100USP units) i.v. 4h after ergotamine or saline treatment. Ergotamine reduced (P<0.01) prolactin concentrations from 1 to 4h post-treatment and increased (P<0.01) 13,14-dihydro-15-keto prostaglandin F2alpha (PGFM) concentrations from 2 to 5h post-treatment. A PGFM response to oxytocin was not detected. In Experiment 2, blood was sampled for 8h from six cycling cows 10 days after receiving a s.c. norgestomet implant. Cows were treated i.v. with ergotamine (20microg/kg) or saline in a simple cross-over design after 1h of pre-treatment blood sampling. Cows received gonadorelin (GnRH, 100microg) i.v. 1h after ergotamine or saline. Cows received oxytocin (100USP units) i.v. 4h after ergotamine or saline treatment. Ergotamine reduced (P<0.01) serum prolactin concentrations by 120min after treatment, with prolactin returning to pre-treatment concentrations by 200min after treatment. Saline-treated cows had lower (P<0.01) prolactin by 280min after treatment. Ergotamine-treated cows had higher (P<0.01) PGFM concentrations compared to saline-treated cows 120-240min after treatments, but the groups exhibited similar increases in PGFM after oxytocin. Plasma LH and FSH concentrations increased to peaks 100-120min after GnRH for both groups. However, the LH response to GnRH was greater (P<0.01) for ergotamine-treated cows. In summary, ergotamine lowered prolactin and elevated PGFM concentrations in follicular phase heifers and cows on norgestomet therapy. Ergotamine increased the LH response to exogenous GnRH in cows with norgestomet implants. These data highlight the potential of ergopeptine alkaloids to affect reproduction through altered endocrine function.     

Opioid modulation of LH secretion in the ewe

Administration of opioid agonists and antagonists and measurement of resulting hormone changes were used to study the possible effects of opioids on reproductive function in the ewe. Intravenous administration of the long-acting methionine-enkephalin analogue FK33-824 (250 micrograms/h for 12 h) to 3 ewes during the follicular phase of the oestrous cycle depressed episodic LH secretion. This effect was reversed by administration of the opiate antagonist naloxone (25 mg/h) in combination with the FK33-824 treatment; in fact LH secretion was enhanced by the combined regimen. Naloxone (25 mg/h for 12 h) administered alone to 3 ewes in the follicular phase also enhanced LH secretion. In 3 animals treated with FK33-824 during the follicular phase, progesterone remained basal for 14 days after treatment, suggesting that ovulation was blocked. Jugular venous infusion of naloxone (25, 50 or 100 mg/h for 8h) into 5 ewes during the early and mid-luteal phase of the cycle resulted overall in a significant increase in mean plasma LH concentrations and LH episode frequency. To investigate whether endogenous opioids suppress LH release in seasonally anoestrous sheep, naloxone was infused intravenously into mature (25, 50 or 100 mg/h for 8 h) and yearling ewes (12 . 5, 25 or 50 mg/h for 8 h) during early, mid- and late anoestrus and plasma LH concentrations were measured. In the mature ewes, there was a trend for naloxone to increase LH values during the early anoestrous period but naloxone was without effect during mid- and late anoestrus. In the yearlings, naloxone infusion consistently increased plasma LH concentrations as a result of a significant increase in LH episode frequency. These experiments indicate that endogenous opioid peptides probably modulate gonadotrophin secretion during both the follicular and luteal phases of the oestrous cycle. However, the follicular phase of the sheep cycle is of short duration, and there may be residual effects of luteal-phase progesterone during this period. Secondly, there may be an age-dependent effect of naloxone on LH secretion during seasonal anoestrus in the ewe, with opioids playing a part in the suppression of LH in young but not in mature animals.     

Effect of hysterectomy on the short life-cycle corpus luteum produced after GnRH-induced ovulation in the anoestrous ewe

Anoestrous Romney Marsh ewes were treated with small-dose (250 ng) multiple injections of GnRH. Ewes in Groups 1 and 3 were hysterectomized 2 weeks before treatment, while those in Groups 2 and 4 were intact controls. Groups 1 and 2 were primed with progesterone (+P) and treated with 2 h injections of GnRH (250 ng) for 36 h, while Groups 3 and 4 were not pretreated (-P) but were given 2 h injections of GnRH (250 ng) for 18 h. Both treatment regimens were terminated with a bolus injection of GnRH (125 micrograms), given to synchronize the timing of the LH surge and subsequent luteal progesterone production. The plasma progesterone profiles of 5/5 animals in Group 2 (+P controls) and 2/5 animals in Group 4 (-P controls) were indicative of normal luteal function, while the remaining 3/5 animals in Group 4 produced plasma progesterone profiles typical of abnormal luteal function. However, in all the hysterectomized animals (Groups 1 and 3) peripheral plasma progesterone concentrations rose to reach a mean peak value of 1.3 ng/ml plasma on Day 8 which was maintained in all animals irrespective of progesterone pretreatment. The absence of a fall in progesterone concentrations precluded the identification of any animal in Group 4 showing abnormal luteal function. It was also noted that, after hysterectomy, although the corpus luteum was maintained, it was with reduced secretory capacity. The prevention of the expected proportion (70%) of -P animals from displaying a decline in plasma progesterone concentration after hysterectomy provides firm evidence that the uterus is involved in the premature regression of the short-cycle corpus luteum.     

Suppression of plasma FSH concentrations with bovine follicular fluid blocks ovulation in GnRH-treated seasonally anoestrous ewes

The specific requirement for FSH in the final stages of preovulatory follicle development was assessed in seasonally anoestrous ewes given 2-h injections of GnRH (250 ng/injection), with (N = 10) or without (N = 10) concurrent treatment with bovine follicular fluid (bFF: 2 ml given i.v. at 8-h intervals). Treatment with bFF significantly (P less than 0.01) suppressed plasma FSH concentrations, but, at least for the first 30 h of treatment, did not influence the magnitude of GnRH-induced LH episodes (mean max. conc. 3.00 +/- 0.39 and 3.63 +/- 0.51 ng/ml for bFF-treated and control ewes, respectively). Of 10 animals treated with GnRH for 72 h, 5/5 control ewes showed oestrus and ovulated whereas 0/5 bFF-treated ewes showed oestrus or ovulated in response to GnRH treatment. There was, however, a transient (13.2 +/- 1.0 h) increase in plasma LH concentrations in the ewes given bFF (mean max. conc. 4.64 +/- 1.57 ng/ml), which was coincident with the preovulatory LH surge recorded in animals given GnRH alone. In 10 GnRH-treated ewes slaughtered after 32 h of treatment, the mean diameter of the largest antral follicle was significantly (P less than 0.001) greater in control ewes (5.92 +/- 0.17 mm) than in animals that were also given bFF (3.94 +/- 0.14 mm). In addition, the incidence of atresia in the 3 largest antral follicles present at this time was greater in bFF-treated ewes. These results show that, when plasma FSH concentrations are suppressed by administration of bFF, although the magnitude of GnRH-induced LH episodes is unchanged, preovulatory follicular development is impaired and ovulation does not occur.(ABSTRACT TRUNCATED AT 250 WORDS)     

Function of abnormal corpora lutea in vivo after GnRH-induced ovulation in the anoestrous ewe

Anoestrous Romney Marsh ewes with and without progesterone treatment (+P, -P) were treated with small-dose (250 ng) multiple injections of GnRH at 2-h intervals for 48 h. Animals were slaughtered on Days 4, 5, 7 and 11 after the end of GnRH treatment and luteal function was assessed by the measurement of daily plasma progesterone concentrations. In all animals which ovulated (29/32, 91%) peripheral progesterone concentrations rose to 0.5-1.0 ng/ml within 3 days of the end of GnRH treatment. In 7/7 (100%) +P animals and 5/22 (23%) -P animals, progesterone concentrations continued to rise and were maintained at levels greater than 1.5 ng/ml until slaughter. In the remaining -P animals, plasma progesterone concentrations declined to reach basal levels by Day 5. Corpora lutea recovered from these animals showed signs of premature regression on Day 5 and were fully regressed by Day 7. Progesterone priming delayed the occurrence of the LH surge which occurred 39.1 +/- 3.6 h after the end of GnRH treatment in the +P animals compared to 20.2 +/- 1.74 h (P less than 0.001) in the -P animals in which luteal function was abnormal and 22.4 +/- 4.35 h in the -P animals in which luteal function was normal. These results show that abnormal luteal function occurs in the majority of GnRH-treated ewes in the absence of progesterone pretreatment.(ABSTRACT TRUNCATED AT 250 WORDS)     

Opioidergic and nutritional involvement in the control of luteinizing hormone secretion of postpartum Rasa Aragonesa ewes lambing in the mid-breeding season

The role of endogenous opioids and nutrition on the inhibition of luteinizing hormone (LH) secretion during the postpartum period was investigated in a Spanish breed of sheep lambing in the mid-late breeding season. Two groups of adult Rasa Aragonesa ewes housed in individual pens and lambing on 30 December were fed during the suckling period to provide maintenance requirements and the production of 1.1 (M; n=8) or 0.55 (L; n=8) kg of milk per day. On days 10, 20 and 30 after lambing, the effect of a treatment with the opiate receptor antagonist naloxone (1 mg/kg at four hourly intervals) on LH secretion was assessed in half of the ewes of each group, the remaining females receiving four saline injections. After weaning, animals were fed to provide requirements for maintenance of liveweight. Blood samples were collected twice a week from day 20 postpartum until the end of March, and assayed for progesterone and prolactin. Although underfed ewes showed significantly lower mean plasma concentrations during the control period on day 20 postpartum, nutrition did not seem to modify LH secretion before naloxone or saline injections. Moreover, no differences between nutritional groups in the response to naloxone injections on pattern of LH secretion were found. In fact, naloxone treatment induced an increase of mean LH concentrations on days 10, 20 and 30 postpartum (at least, P<0.05), of LH pulse frequency on days 20 and 30 (P<0.05), and of LH pulse amplitude on days 10 and 20 (P<0.05). Underfed ewes during the postpartum period showed a slower decline in plasma prolactin levels, with significant differences on days 29, 36 and 39 after lambing (P<0.05). Only 3 M ewes ovulated before the onset of the seasonal anoestrus period. It is concluded that endogenous opioids are involved in the inhibition of LH secretion during the early suckling period of a reduced seasonality breed of sheep without any influence of nutrition on the response to naloxone treatment; however, ewes underfed before weaning failed to reactivate their cyclicity prior to the onset of the seasonal anoestrus.     

Early pituitary LH response to injections of GnRH in ewes

In the deep anoestrous period (June), five intact ewes and five ovariectomized ewes received 50 ug synthetic gonadotrophin-releasing hormone (GnRH). In the mid-breeding season (October), the GnRH administrations were repeated in five intact and four ovariectomized ewes; the former were in the luteal phase of the cycle. Blood samples were collected every 30 sec for 15 min, then at 15-min intervals. Release of luteinizing hormone (LH) occurred as soon as the second minute after injection in all ewes. This early response was earlier and more abrupt in the ovariectomized ewes than in the intact animals. In a second experiment three intact ewes that were in deep anoestrus received 50 ug GnRH followed 5 h 20 min later by a second identical injection. Another three intact ewes in deep anoestrus received two injections of 1 ug GnRH. Blood samples were taken every 15 sec for 15 min, then every 20 min until the next injection, and for a further 5 h after the second injection. This regimen was repeated in mid-breeding season during the luteal phase. There was again a very early release of LH; the magnitude of response was similar after the first injection of either 50 ug or 1 ug GnRH to intact ewes either in the breeding season or during deep anoestrus. However, a greater early release of LH was obtained at the lower dose only after the second injection of GnRH. Apart from this exception, the similar early release of LH occurred in spite of different amounts of LH released thereafter in response to the two doses of GnRH. It is suggested that the early response to GnRH consists of LH stored in a "readily releasable" pool in the pituitary, whereas the main release of LH may be a result of increased synthesis and/or release of a more stable pool.     

Effect of naloxone on plasma concentrations of prolactin and LH in lactating sows

Six lactating sows were injected through an indwelling vena cava cannula with naloxone (2.5 mg/kg body weight) on Day 15 post partum. Blood samples were collected through the cannulas at 10-min intervals for 8 h before and 10 h after naloxone administration. Plasma prolactin and LH concentrations were measured by radioimmunoassay. Naloxone caused a marked suppression of plasma prolactin concentrations lasting 4-6 h. LH concentrations were also affected by naloxone: LH rose to reach maximum values 20-50 min after naloxone treatment. Pretreatment values were recorded 200-300 min after the treatment. These results indicate that endogenous opioids are involved in causing the endocrine patterns occurring during lactation, i.e. high prolactin and low LH concentrations.     

Progesterone blockade of the positive feedback effects of 17β oestradiol in the ewe

Ovariectomized ewes (n = 24) were treated with implants that resulted in circulating concentrations of progesterone and 17β-oestradiol similar to those seen in intact ewes in the luteal phase of an oestrous cycle. Progesterone implants were left in for 10 days, and 17β-oestradiol implants for 14 days. Twelve of these ewes received daily injections of 17β-oestradiol in oil (i.m.) at doses sufficient to cause a surge release of luteinizing hormone (LH) in the absence of progesterone. The other 12 ewes were treated daily with vehicle (oil). Following progesterone withdrawal on Day 10, each group of 12 ewes was divided into three subgroups. Ewes in each subgroup of the groups treated daily with 17β-oestradiol or vehicle, received an injection of either 17β-oestradiol (oil i.m.), gonadotrophin-releasing hormone (GnRH) (saline, i.v.) or vehicle, 24 h after progesterone withdrawal. Following progesterone withdrawal, no LH surge was detected in ewes treated with vehicle. Surge secretion of LH was detected in ewes of all other groups. The data suggested that in progesterone-treated ewes, daily exposure to stimulatory doses of 17β-oestradiol did not desensitize the hypothalamic pituitary axis to the positive feedback effects of 17β-oestradiol. Daily exposure to 17β-oestradiol did not suppress pituitary responsiveness to GnRH. It was concluded that circulating concentrations of progesterone, similar to those seen during the luteal phase of an oestrous cycle in intact ewes, may prevent all necessary components of the LH surge secretory mechanism from responding to 17β-oestradiol.     

Estrogen and progesterone receptor content in the pituitary gland and uterus of progesterone-primed and gonadotropin releasing hormone-treated anestrous ewes

The objective of this work was to investigate the effect of progesterone (P) and gonadotropin-releasing hormone (GnRH) treatment on estrogen receptor (ER) and P receptor (PR) concentrations in the pituitary gland and uterus of anestrous ewes. Ewes were either not treated (group C, n = 4); were treated with 0.33 g P-controlled internal drug release (P-CIDR) for 10 days (group P, n = 4), with GnRH, 6.7 ng i.v. injections every 2 h for 18 h followed by a 4 microg bolus administration of Receptal at 20 h (group GnRH, n = 4), or with a combination of the P and GnRH treatment (group P + GnRH, n = 3). Ewes were humanely killed either at the beginning of the experiment (group C), when the CIDR was removed (group P), or 24 h after the GnRH bolus treatment (groups GnRH and P + GnRH). Progesterone treatment increased serum P concentrations, indicating that the treatment was effective. All GnRH treated ewes had similar luteinizing hormone (LH) surges, which lasted 8 h. At slaughter, estradiol (E2) concentrations in the GnRH group were higher than in groups C, P, and P + GnRH. Treatment with GnRH increased more than 10-fold the content of ER and PR in the pituitary gland without altering steroid receptor concentrations in the uterus. When GnRH was combined with P the uterine receptor contents were higher than with P treatment alone. The treatment with P decreased ER and PR content in the uterus, but had no effect on the pituitary gland. The results show that regulation by P and GnRH of ER and PR content in anestrous ewes is tissue-specific.     

Early pituitary follicle stimulating hormone response to gonadotrophin releasing hormone in ewes

The object of our experiments was to characterize the response of plasma follicle stimulating hormone (FSH) within minutes of an i.v. injection of high or low doses of gonadotrophin releasing hormone (GnRH), especially in relation to contemporary changes in luteinizing hormone (LH) concentrations. In the deep anoestrous period (June), three intact ewes and two ovariectomized ewes were injected with 1 mug synthetic GnRH followed 2 h later by a second identical injection. A week later, the same regimen was repeated with the same sheep but with 50 mug GnRH after an interval of 5 h 20 min. Blood samples were collected every 15 sec for 15 min after each injection (early release), then at longer intervals (main release) till the next treatment, followed by sampling for a further 6-h period after the second treatment. FSH was released as soon as the second minute after GnRH injection in all ewes. The mean pituitary FSH response, during this early release, in intact and ovariectomized ewes was similar after either 1 or 50 mug GnRH. However, the main release was less pronounced in the ovariectomized sheep and was not stimulated after the second treatment in all sheep. Three other ewes were injected with 40 mug GnRH and sampled every 15 sec for seven, 6-min periods during the period of release to compare FSH and LH secretion. The profiles reflected a similarity in sensitivity and responsiveness to GnRH, especially soon after GnRH injection. Increases in both hormones were formed by several grouped associated spikes. It is suggested that a readily releasable pool of FSH exists in the ewe. There are probably differences in the mechanisms of synthesis and/or release between pituitary FSH and LH.     

Ovarian steroid involvement in endogenous opioid modulation of LH secretion in seasonally anoestrous mature ewes

In June, 16 mature ewes were ovariectomized and allocated to four groups: 1, saline; 2, naloxone; 3, progesterone implant plus naloxone; 4, oestrogen implant plus naloxone. Steroids were implanted at the time of ovariectomy. At 5 days after ovariectomy, the animals were intravenously infused with saline for 8 h and naloxone (50 mg/h) in saline for 8 h the following day. Three intact ewes were given naloxone in a similar way. During infusions and for 8 h on the day after naloxone, jugular venous blood samples were taken every 15 min and assayed for LH. Naloxone resulted in significant increases in mean LH concentration (P less than 0.01), LH episode frequency and episode height (P less than 0.05) in Group 3 ewes, but was without effect in any other group. These results provide evidence that the progesterone status of the ewe affects its response to naloxone, that progesterone negative feedback on LH release may be mediated by an opioid system, and that increased oestradiol negative feedback during seasonal anoestrus is unlikely to work via increased opioid inhibition of LH.     

Effects of naloxone or transient weaning on secretion of LH and prolactin in lactating sows

Sows (N = 16) were infused intravenously for 8 h with saline or naloxone (200 mg/h) or their litters were transiently weaned for 8 h. Before infusion, 200 mg naloxone were administered to elevate quickly concentrations of naloxone. Blood samples were collected from sows at 15 min intervals for 24 h, beginning 8 h before and continuing until 8 h after imposition of treatments during the middle 8-h segment. Frequency of episodic release of LH and concentrations of prolactin were similar before, during and after infusion of saline. Average concentration of LH was greater during the last than during the middle 8-h segment when sows were given saline. Frequency of episodic release of LH increased and concentrations of prolactin decreased during infusion of naloxone or transient weaning; however, average concentration of LH increased during transient weaning, but not during infusion of naloxone. After transient weaning or infusion of naloxone, frequency of release of LH decreased, returning to pretreatment values in sows infused with naloxone but remaining above pretreatment values in sows subjected to transient weaning. At the resumption of suckling by litters in sows subjected to transient weaning, prolactin increased to levels not different from those observed during the 8-h pretreatment segment. Prolactin did not increase until 4-5 h after cessation of naloxone infusion. We conclude that continuous infusion of naloxone altered secretory patterns of LH and prolactin. Collectively these results provide evidence that the immediate effects of weaning on LH and prolactin in sows are mediated in part through a mechanism involving endogenous opioid peptides.     

Effects of hormonal manipulation on the resumption of postpartum reproductive activity in Texel ewes

Three experiments were conducted on Texel ewes to study the influence of prostaglandin F(2alpha) (PGF(2alpha)), prolactin (PRL), estradiol (E(2)), and gonadotrophin releasing hormone (GnRH) on postpartum reproductive activity. In Experiment 1, oral administration of indomethacin (25 to 50 mg/day/ewe) from Day 3 post partum to the first detected estrus inhibited plasma 13, 14-dihydro-15-keto, PGF(2alpha) (PGFM) concentrations (P < 0.0001). This treatment resulted in an earlier rise in the frequency and amplitude of luteinizing hormone (LH) pulses and a resumption of estrous behavior (P < 0.05), while ovarian activity estimated by progesterone (P(4)) concentrations resumed to the same extent in treated ewes and controls. Bromocriptine treatment (2.5 mg/day/ewe) reduced plasma PRL levels (P < 0.0001) but had no effect on ovarian activity as evidenced by P(4) and resumption of estrus or on either the frequency or amplitude of the LH pulse. In Experiment 2, a single injection of GnRH agonist (42 mcg of buserelin/ewe) on Day 16 post partum resulted in an abrupt elevation of plasma LH concentrations; mean LH values were 18 to 27 times higher when compared with those of the control ewes. Two days after this treatment, ovulations occurred in 5 of the treated ewes and in 2 of the control ewes. This induced ovarian activity was not associated with estrous behavior; however, after an adequate subsequent luteal phase all the treated ewes displayed estrus, the resumption of estrus thus being earlier in treated than in control ewes (P < 0.01). In Experiment 3, E(2) supplementation from Day 16 to Day 28 post partum increased the number of LH pulses per 6 hours in suckling ewes (P < 0.05) and induced earlier resumption of estrus in dry ewes but not in suckling ewes (P < 0.01). Luteal function was detected about 5 and 8 days after the insertion of E(2) implants in 4 dry ewes and in 2 suckling ewes, respectively.     

GnRH-induced gonadotrophin secretion in ovariectomized Booroola ewes with hypothalamic-pituitary disconnection

To test whether the F gene-specific differences in the plasma concentrations of FSH and LH are due to differences in the pituitary responsiveness to exogenous GnRH, ovariectomized Booroola ewes with hypothalamic-pituitary disconnection (HPD-ovx) were treated with GnRH (250 ng i.v.) once every 2 h for up to 5 weeks. In Exp. 1, jugular venous blood was collected once weekly from 13 FF and 14 ++ HPD-ovx ewes for 6 weeks before GnRH treatment and every 2nd, 3rd or 6th day for 5 weeks during treatment. In Exp. 2, jugular venous blood was collected from another 8 FF and 7 ++ HPD-ovx ewes at 5- or 10-min intervals over 4 GnRH pulses (250 ng i.v. once every 2 h) on 3 separate occasions after the animals had been subjected to the GnRH pulse regimen for approximately 7 days beforehand. Also in Exp. 2, the animals were extensively sampled around a larger (10 micrograms) i.v. injection of GnRH and the pituitary FSH and LH contents assessed after the animals had been re-exposed to the once every 2 h GnRH (250 ng i.v.) pulse regimen for several days following the larger GnRH bolus. In Exp. 3 the distributions of mean plasma concentrations of FSH and LH in individual GnRH-treated HPD-ovx ewes were compared with those in ovariectomized and ovary-intact FF and ++ ewes. During the 6 weeks before GnRH treatment (Exp. 1), the plasma concentrations of FSH (approximately 1 ng/ml) and LH (less than or equal to 0.8 ng/ml) were not different between the genotypes. After GnRH treatment both the mean FSH and LH concentrations increased significantly (P less than 0.01) above basal values after 2 days with F gene-specific differences being noted for FSH but not LH (FSH; FF greater than ++; P less than 0.05). Thereafter, the mean FSH but not LH concentrations increased at a faster rate in FF than in ++ ewes with the overall mean FSH concentrations between the genotypes being significantly different (P less than 0.05). In Exp. 2 considerable between-animal variation in the pulsatile pattern of FSH but not LH concentrations was seen in ewes of both genotypes during GnRH treatment. The overall mean FSH concentrations were higher in FF than in ++ ewes (P less than 0.05) and the mean FSH response to each GnRH pulse was significantly higher in FF than in ++ ewes (P less than 0.05).(ABSTRACT TRUNCATED AT 400 WORDS)