Effects of progesterone and weaning on LH and FSH responses to naloxone in postpartum beef cows

Intrinsic and Regulated Gonadotropin-Releasing Hormone Receptor Gene Transcription in Mammalian Pituitary Gonadotrophs

The hypothalamic decapeptide gonadotropin-releasing hormone (GnRH), acting via its receptors (GnRHRs) expressed in pituitary gonadotrophs, represents a critical molecule in control of reproductive functions in all vertebrate species. GnRH-activated receptors regulate synthesis of gonadotropins in a frequency-dependent manner. The number of GnRHRs on the plasma membrane determines the responsiveness of gonadotrophs to GnRH and varies in relation to age, sex, and physiological status. This is achieved by a complex control that operates at transcriptional, translational, and posttranslational levels. This review aims to overview the mechanisms of GnRHR gene (Gnrhr) transcription in mammalian gonadotrophs. In general, Gnrhr exhibits basal and regulated transcription activities. Basal Gnrhr transcription appears to be an intrinsic property of native and immortalized gonadotrophs that secures the presence of a sufficient number GnRHRs to preserve their functionality independently of the status of regulated transcription. On the other hand, regulated transcription modulates GnRHR expression during development, reproductive cycle, and aging. GnRH is crucial for regulated Gnrhr transcription in native gonadotrophs but is ineffective in immortalized gonadotrophs. In rat and mouse, both basal and GnRH-induced Gnrhr transcription rely primarily on the protein kinase C signaling pathway, with subsequent activation of mitogen-activated protein kinases. Continuous GnRH application, after a transient stimulation, shuts off regulated but not basal transcription, suggesting that different branches of this signaling pathway control transcription. Pituitary adenylate cyclase-activating polypeptide, but not activins, contributes to the regulated transcription utilizing the protein kinase A signaling pathway, whereas a mechanisms by which steroid hormones modulate Gnrhr transcription has not been well characterized.

Luteinizing hormone and growth hormone secretion in early lactating Spanish beef cows

The episodic release of luteinizing hormone (LH) and growth hormones (GH) was studied in three suckling regimens and two breeds of Spanish suckled cows. Parda de Montaña (PA) cows (n = 21) were assigned to once-daily, twice-daily or ad libitum (ADLIB) suckling. Pirenaica (PI) cows (n = 7) were used to evaluate the breed effect in twice-daily suckling. Coccygeal blood samples were collected twice weekly during lactation to determine the interval from calving to first ovulation through peripheral progesterone. On day 32 ± 3 post-partum, jugular blood samples were drawn at 15 min intervals during 8 h to analyse circulating LH and GH. The interval to first ovulation was greater in PA cows suckling ADLIB than in restricted suckling treatment (RESTR1), whereas in RESTR2 it did not differ from the other two treatments. There were no differences between PA and PI cows in the interval to first ovulation. RESTR1 cows showed a tendency to have shorter LH peak widths than ADLIB cows. PA cows showed a tendency to have longer LH peak widths than their PI counterparts. There were no differences across treatments or breeds in any of the GH measures of secretion. The LH release was more affected by breed than by suckling frequency, whereas that of GH was not influenced by any of these parameters. The variables that best allowed discrimination between ADLIB and restricted nursing systems were the interval to post-partum first ovulation, LH peak number and the mean GH concentration.

Postpartum acyclicity in suckled beef cows: a review

Prolonged postpartum acyclicity in suckled beef cows is a source of economic loss to beef cattle producers. Duration of postpartum acyclicity is influenced by suckling status, nutritional status, calving season, age, and several other factors. Although uterine involution begins and ovarian follicular waves resume soon after parturition, dominant follicles of these waves fail to ovulate, due to a failure to undergo terminal maturation. As a result, postpartum anovulatory dominant follicles are smaller than the ovulatory follicles in cyclic cows. Failure of postpartum dominant follicles to undergo terminal maturation is due to absence of appropriate LH pulses, a prerequisite for follicular terminal maturation prior to ovulation. Absence of LH pulses early post partum is primarily due to depletion of anterior pituitary LH stores, although GnRH pulses are also absent during this period due to suckling. Following replenishment of LH stores between Days 15 and 30 post partum, absence of LH pulses is due to continued sensitivity of the hypothalamic GnRH pulse-generator to the negative feedback effect of ovarian estradiol-17beta, which results in absence of GnRH pulses. This negative feedback effect of estradiol-17beta is modulated by suckling which stimulates release of endogenous opioid peptides from the hypothalamus. As the postpartum interval increases, sensitivity of the GnRH pulse-generator to the negative feedback effect of ovarian estradiol-17beta decreases. This is followed by an increasing frequency of GnRH discharges and LH pulses, terminal follicular maturation, ovulation, and continued cyclicity. The first ovulation post partum is usually followed by a short cycle due to premature luteolysis because of premature release of PGF2alpha from the uterine endometrium, which is possibly intensified by the suckling-induced oxytocin release from the posterior pituitary. A model for the postpartum ovulatory acyclicity and for the resumption of cyclicity is presented.

Effect of a long-lasting opioid receptor antagonist (naltrexone) on pulsatile LH release in early postpartum Holstein dairy cows

The present trial was the first one to investigate the effect of an intravenous injection of naltrexone, an opioid receptor antagonist that has a longer duration of action than that of naloxone, on the LH pulse in early postpartum Holstein dairy cows. On Day 10 postpartum, blood samples were collected from cows at 10-min intervals for a period of 4 h before (pre-injection period) and a period of 5 h after (post-injection period) an intravenous injection of 10 mL of saline (Control Group, n=5) or 300 mg of naltrexone in 10 mL of saline (Naltrexone Group, n=5). The plasma LH level was assayed by double antibody radioimmunoassay. The number of LH peaks per 1 h, the mean LH level, and the amplitude of LH peaks were analyzed utilizing the Pulsar algorithm, and data were compared by repeated measures ANOVA. No differences were observed in the parameters of LH pulse in the pre-injection period between the Control and the Naltrexone Groups (P>0.10). In the Naltrexone Group, the number of LH peaks per 1 h and the mean LH level were significantly higher in the post-injection period than in the pre-injection period (0.85 +/- 0.29 vs. 1.24 +/- 0.17, P<0.05, and 1.81 +/- 0.70 vs. 2.47 +/- 0.92 ng/ml, P<0.05, respectively), but there was no significant increase in the amplitude of LH peaks (1.48 +/- 0.64 vs. 1.83 +/- 0.82 ng/ml, P>0.10). In contrast, all of the parameters of LH pulse remained unchanged in the Control Group (P>0.10). These results suggested that an intravenous injection of naltrexone activates the LH pulse.

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.

Effect of naloxone on serum luteinizing hormone concentration in anovulatory Holstein cows during the early postpartum period

This experiment was conducted to investigate the effect of naloxone (NAL), an opioid receptor antagonist, on pituitary LH secretion in anovulatory Holstein cows during the early postpartum period when cows were expected to be in negative net energy balance. Twenty-three cows (11 primiparous) received either saline (n = 12) or 1 mg/kg BW NAL i.v. (n = 11) on Day 14 or 15 postpartum. Jugular blood samples were collected at 15-min intervals for 2 hr before and 2.5 hr after NAL or saline. All cows received 3 ug gonadotropin releasing hormone (GnRH) at 2.5 hr post-NAL or -saline and blood collection was continued for 1 hr. Mean serum progesterone concentration was 0.33 +/- 0.2 ng/ml. Mean net energy balance for all cows was -5.5 +/- 0.6 Mcal/day. Naloxone caused a transient increase (P < 0.05) in serum LH concentrations in both primi- and multiparous cows within 45 min after administration. In contrast, serum LH concentrations remained unchanged in saline-treated cows. GnRH increased (P < 0.05) LH and there was no effect of treatment. These results suggest that modulation of LH secretion, at least in part, may be mediated via endogenous opioids in dairy cows before first postpartum ovulation.

Calcium/calmodulin-dependent protein kinase II involvement in release of gonadotropin-releasing hormone

Involvement of calcium/calmodulin-dependent protein kinase II (CaM kinase II) in regulation of GnRH release was tested by determining the effect of CaM kinase II antagonists (KN-62 or KN-93) on GnRH release from rat or cattle infundibular (stalk median eminence) explants. Preincubation of male rat infundibular explants for 30 min with KN-62 (0.5, 1, 5 or 10 microM) 1.5 h prior to the addition of 59.3 mM (high) K+ resulted in a dose-dependent suppression of GnRH release. A longer pretreatment period (2 h) of rat infundibular explants with KN-62 (1 or 10 microM) appeared to enhance the suppressive effect of the CaM kinase II antagonist. Exposure (2 h) of rat infundibular explants to 10 microM, but not 0.1 microM KN-93, resulted in a complete inhibition of high K+-induced GnRH release. Exposure of steer infundibular explant halves to KN-62 (50 or 100 microM) or KN-93 (50 microM) inhibited high K+-induced GnRH release. Likewise, treatment of heifer infundibular explant halves with KN-93 (50 microM) abolished high K+-induced GnRH release. The period of exposure required for KN-62 to elicit its effect was relatively short since exposure of KN-62 (100 microM) for only 91-150 min of incubation was sufficient to block high K+-induced GnRH release from steer infundibular explant halves. In conclusion, these results: (1) support the hypothesis that CaM kinase II is involved in GnRH release from the rat and cattle infundibulum, (2) demonstrate that the effect of CaM kinase II on GnRH release from cattle infundibula is independent of reproductive state, (3) confirm previous reports supporting Ca2+ and CaM involvement in GnRH release from rat and cattle infundibula and (4) establish that infundibular explants incubated in vitro are useful for studying selected mechanisms regulating hypothalamic neurohormone release from neuron terminals.

Serum luteinizing hormone, 13,14-dihydro-15-keto-prostaglandin F2α 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.

Hypothalamic proopiomelanocortin mRNA levels in suckled or nonsuckled beef cows: A preliminary study

This study was conducted to determine proopiomelanocortin (POMC) mRNA levels in the preoptic and hypothalamic brain regions of postpartum anestrous cows. An additional objective was to determine if calf suckling influences POMC mRNA concentration in these regions. Twenty cows were randomly assigned to suckled and nonsuckled treatment groups and slaughtered between 30 and 36 days postpartum. Serum luteinizing hormone (LH) concentrations were determined from blood collected every 15 minutes for 8 hours, starting 20 hours prior to slaughter. POMC mRNA levels in brain tissues were determined by dot blots. Serum LH concentrations between nonsuckled and suckled cows were 1.3 +/- 0.2 and 0.9 +/- 0.1 ng.ml(-1) (mean +/- SEM; P = 0.19), respectively. The POMC gene is expressed in the hypothalamus of postpartum anestrus cows with POMC mRNA levels higher (P<0.05) in the hypothalamus than in the preoptic region. Hypothalamic POMC mRNA levels tended (P = 0.12) to be lower in nonsuckled (14.9 +/- 3.8 ADU) than in suckled cows (23.5 +/- 3.6 ADU). Covariate analysis indicated (P = 0.10) that as mean serum LH concentrations increased, hypothalamic POMC mRNA levels decreased.

Hormonal interrelationships in postpartum suckled dairy cows

Three cross-bred cows calved in March and April and were followed until day 62 after parturition. Each animal was suckled by 2 calves ad libitum. All calves were removed from the cows on day 55 after parturition. Blood was collected 3 times per day from the jugular vein by venipuncture. On 4 occasions after parturition--i.e. days 7-8, 21-22, 35-36 and 49-50, the cows were bled through a jugular venous catheter every 30 min during the 24 h. The plasma samples were analyzed for the content of 15-keto-13,14-dihydro-PGF2 alpha (main PGF2 alpha metabolite), LH, prolactin, cortisol and progesterone by radioimmunoassay methods. The concentration of PGF2 alpha increased from 280 to 730 pmol/l within the last 4 days before parturition. The highest geometric mean was 3106 pmol/l on the day of parturition. Thereafter a steady decrease of PGF2 alpha metabolite concentration was seen until day 21 when it reached plateau at 148 pmol/l. In all cows plasma LH concentrations increased significantly (P < 0.05) from about 1.6 micrograms/l on days 7-8 to 2.4 micrograms/l on days 21-22 post partum. The frequency of LH pulses showed no tendency to increase as the postpartum period progressed and averaged 6.5 pulses/24 h. Mean plasma LH concentrations increased from 2.1 micrograms/l 2 days before weaning to 3.2 micrograms/l 2 days after weaning (P < 0.05). LH peaks occurred less frequently in association with prolactin and cortisol peaks than in their absence. A partial positive correlation between PGF2 alpha metabolite and cortisol (r = 0.30) was found on days 7-8 post partum. Correlation between prolactin and cortisol on days 7-8 and 21-22 post partum was also positive (r = 0.20 and r = 0.27, respectively). There was a negative correlation between LH and cortisol on days 7-8 (r = -0.27) and days 49-50 (r = -0.21) post partum. The first and short progesterone increase observed after weaning was terminated in conjunction with PGF2 alpha metabolite peaks.

Plasma progesterone concentrations during pregnancy and pseudopregnancy and onset of ovarian activity post partum in indigenous goats in Zimbabwe

Eight pregnant does were housed individually and fed a hay and concentrate diet throughout pregnancy and lactation. The mean gestation period was 146.7 +/- 3.0 days, with a twinning rate of 75 per cent. Mean body condition scores improved from 2.4 +/- 0.2 to 2.8 +/- 0.2 over the first 80 days of gestation and were maintained at 2.8 until 45 days before kidding. From then until kidding, mean scores fell to 2.2 +/- 0.2. Plasma progesterone concentrations during pregnancy rose significantly from 3.91 +/- 0.51 ng/ml on day 40 to 5.96 +/- 0.51 ng/ml on day 60 (P < 0.05) and remained high until 5 days before kidding. Three pseudopregnant does had similar progesterone profiles to pregnant does over the first 80 days, but the rise around day 35 to 40 was not significant and progesterone concentrations returned gradually to basal levels after day 100. The same 8 does, together with an additional 4 does which had been brought inside 60 to 70 days before kidding, were used to study onset of ovarian activity post partum. The twinning percentage was 83 per cent. Mean body condition score at parturition was 2.2 +/- 0.1. By day 35 post partum, mean condition scores had fallen to 1.9 +/- 0.1, and mean weights from 36.9 +/- 1.9 kg at kidding to 32.1 +/- 2.0 kg. Ovarian cyclicity was resumed just before mean scores and weights started to improve. The mean interval from kidding to onset of oestrous cycles was 97.3 +/- 9.5 days. This coincided with mean time to weaning which was 99.5 +/- 5.5 days.(ABSTRACT TRUNCATED AT 250 WORDS)

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.

Superovulation of beef heifers with Folltropin: A new FSH preparation containing reduced LH activity

The optimum superovulatory dose of Folltropin was determined and compared with a standard 28 mg dose of FSH-P in beef heifers. In Experiment 1, mean numbers of corpora lutea (CL) did not differ among the groups treated with 10, 20, 30 or 40 mg Folltropin or FSH-P, and the mean CL number was reduced (P<0.05) only in the 5 mg Folltropin group. Mean numbers of ova/embryos recovered, fertilized and transferable were greater (P<0.05) for the 10, 20 and 30 mg Folltropin groups than for the 5 mg group. The 40 mg Folltropin group and the FSH-P group were intermediate. The percentage of fertilized and transferable embryos did not differ over the dosages used in this experiment. In Experiment 2, mean numbers of CL were greater for the 9, 18 and 36 mg Folltropin groups than for the 4.5 mg group, with the 9 mg group being lower than the 36 mg group (P<0.05). The 18 mg group was intermediate and did not differ. Mean numbers of ova/embryos recovered and fertilized ova were greater for the 9, 18 and 36 mg groups (P<0.05) than for the 4.5 mg group. The percent of fertilized and mean number and percentage of transferable embryos did not differ among treatments. We conclude that Folltropin may be a satisfactory superovulatory replacement for FSH-P and that a dose of 18 to 20 mg Folltropin may be within the optimum superovulatory dosage range for beef heifers. Dosages of Folltropin of more than twice the optimum did not result in deterioration of ova/embryo quality.

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.