Effects of leptin on gonadotropin-releasing hormone release from hypothalamic–infundibular explants and gonadotropin release from adenohypophyseal primary cell cultures: further evidence that fully nourished cattle are resistant to leptin

In rodents and pigs, leptin stimulates the release of gonadotropin-releasing hormone (GnRH) from hypothalamus, gonadotropins from adenohypophyseal (AP) explants and cells, and luteinizing hormone (LH) from full-fed animals. In the current studies, we investigated whether leptin could stimulate the release of GnRH from bovine hypothalamic-infundibular (HYP) explants and gonadotropins from bovine adenohypophyseal cells. In Experiment 1A, HYP explants collected from 17 bulls and seven steers were incubated with Krebs-Ringer bicarbonate buffer (KRB) containing 0, 10, 100, or 1000 ng/ml recombinant ovine leptin (oleptin) for 30 min after a 3-h period of equilibration. None of the doses of leptin affected (P > 0.05) GnRH release into the media. In Experiment 1B, HYP explants collected from six steers were incubated with KRB containing 0 or 1000 ng/ml oleptin for two consecutive 30-min periods and challenged with 60 mM K(+) afterwards. Leptin did not affect (P > 0.05) basal or K(+)-stimulated release of GnRH. In Experiment 2, adenohypophyses from steers were collected at slaughter and cells dispersed and cultured for 4 days. On day 5, cells were treated with media alone (control) or media containing 10(-11), 10(-10), 10(-9), and 10(-8)M oleptin. Three independent replications were performed. None of the doses of leptin stimulated (P > 0.05) the release of LH. Although leptin at 10(-11), 10(-10), and 10(-9)M increased (P < 0.03) slightly the release of FSH compared to control-treated cells in one replicate, this effect was not confirmed in the other two replicates. Results support the hypothesis that leptin has limited effects on the release of GnRH and gonadotropins in full-fed cattle and reiterate important species differences in responsiveness to leptin.

Leptin acts at the bovine adenohypophysis to enhance basal and gonadotropin-releasing hormone-mediated release of luteinizing hormone: differential effects are dependent upon nutritional history

Recombinant ovine leptin (oleptin) stimulates an acute increase in the secretion of LH in fasted, but not in normal-fed, cows through an augmentation of the magnitude of individual pulses of LH. Herein, we tested the hypothesis that this effect could be accounted for by functional changes at the adenohypophyseal (AP) level. Eleven ovariectomized, estradiol-implanted cows were assigned to one of two dietary groups: normal-fed (n = 6) and fasted (fasted for 72 h; n = 5). After the animals were killed, the adenohypophyses were collected and AP explants were perifused with Krebs-Ringer bicarbonate buffer (KRB) for a total of 6.5 h, including a 2-h treatment at 2.5 h with KRB or increasing doses of oleptin and a challenge at 4.5 h with 50 ng of GnRH. To test for effects of leptin at the hypothalamic level, explants encompassing the medial basal hypothalamus-infundibular complex (HYP) were incubated in KRB alone (control) or in KRB containing 1000 ng of oleptin. Basal release of LH from AP explants treated with leptin was greater (P < 0.02) than that from control-treated explants in fasted, but not in normal-fed, cows. To the contrary, leptin-treated explants from normal-fed, but not from fasted, cows released more (P < 0.001) LH in response to GnRH than control-treated tissues. Neither fasting nor leptin affected (P > 0.1) the secretion of GnRH from HYP explants. These observations support the hypothesis that leptin modulates the secretion of LH in mature cows, to a large extent, by its direct actions at the AP. Differential manifestations of these effects are dependent upon nutritional history.

Effects of leptin on basal and GHRH-stimulated GH secretion from the bovine adenohypophysis are dependent upon nutritional status

We have shown recently that leptin modulates at least two aspects of anterior pituitary LH release in ruminants: basal and GnRH-mediated release. To test the hypothesis that leptin directly affects basal and GHRH-mediated GH secretion from the adenohypophysis, we examined the effects of various doses of recombinant ovine leptin (oleptin) on perifused adenohypophyseal (AP) explants and compared responses of tIssues from control and fasted cows. Ten mature, ovariectomized and estradiol-implanted cows were assigned to one of two dietary groups: (1) normal-fed (n=5) and (2) fasted for 72 h (n=5). At the end of the fasting period, cows were euthanized and pituitaries were collected. Adenohypophyseal explants were perifused for a total of 6.5 h, including a 2-h treatment at 2.5 h with Krebs-Ringer bicarbonate buffer containing 0, 5, 10, 50, or 100 ng/ml oleptin, and a challenge with GHRH at 4.5 h. All doses of oleptin greater than 5 ng/ml decreased (P<0.01) basal GH secretion compared with controls in tIssues collected from normal-fed cows. In contrast, GH release from AP explants from fasted cows treated with the lowest dose of oleptin was 28% (P<0.002) higher than control explants, but larger doses had no effect. Leptin caused an inversely related, dose-dependent increase in GHRH-mediated GH release in tIssues from normal-fed cows. Marked increases (P<0.01-P<0.001) in GH release were observed for the 5 and 10 ng/ml oleptin, with lesser (P<0.08) and no effects observed at the 50 and 100 ng/ml doses respectively. In fasted cows, oleptin had no stimulatory effect on GHRH-induced GH release. Results show that leptin can act directly at the anterior pituitary level to modulate GH release, and this effect is dependent upon nutritional history.

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.

Effects of exogenous recombinant ovine interferon tau on circulating concentrations of progesterone, cortisol, luteinizing hormone, and antiviral activity; interestrous interval; rectal temperature; and uterine response to oxytocin in cyclic ewes

Interferon tau (IFN tau) is the conceptus-produced antiluteolytic signal in ruminants. Three experiments examined the effects of s.c. administration of recombinant ovine (ro)IFN tau on interestrous interval (IEI), oxytocin (OT)-induced uterine prostaglandin F2alpha metabolite (PGFM) production, rectal temperature (RT), respiration rate (RR), and plasma concentrations of progesterone, cortisol, LH, and antiviral activity (AVA) in plasma and uterine flushings. In experiment I, 20 ewes were treated s.c. with either 0, 1, 2, or 4 mg/day roIFN tau (0.7 x 10(8) U/mg; 5 ewes/dosage) from Days 11 to 15 of the estrous cycle (estrus = Day 0) and were challenged with OT (30 IU) on Day 15. Jugular blood samples were collected at -10, 0, 10, 20, 30, 40, 50, and 60 min relative to the OT challenge and assayed for PGFM. Recombinant oIFN tau increased IEI (16.7, 18.7, and 22.6 +/- 0.6 days for 0, 2, and 4 mg roIFN tau, respectively, p < 0.01). Recombinant oIFN tau did not affect peak PGFM response to OT (2309 +/- 172 pg/ml; p > 0.1). However, the 4 mg/day dosage delayed the time to peak PGFM (32.4 vs. 47.5 +/- 3.4 min; p < 0.01, 0 vs. 4 mg) and resulted in approximately 200% higher concentrations of PGFM at 60 min post-OT (0 vs. 4 mg/day, p < 0.07). Experiment II was similar to experiment I, except that only the 0- and 4-mg/day dosages of roIFN tau were administered. Ewes were hysterectomized on Day 16, and assay of uterine flushes detected no AVA from ewes treated with either 0 or 4 mg/day roIFN tau. In experiment III, 20 ewes were treated s.c. with either 0, 2, 4, or 6 mg roIFN tau on Day 12. Blood samples, RT, and RR were obtained at frequent intervals for 24 h, and plasma was assayed for progesterone, cortisol, LH, and AVA. Plasma AVA, which increased in a dose-dependent manner, was detectable within 60 min and remained elevated at 24 h compared to control values. RT (elevated 0.5-1.0 degrees C), RR, and cortisol increased in response to all dosages of roIFN tau, with peak values occurring 150-180 min postinjection. For all dosages of roIFN tau, plasma progesterone declined from 120 to 360 min posttreatment and then returned to pretreatment values by 24 h (p < 0.01) as compared to controls. Overall, exogenous roIFN tau altered uterine PGFM response to OT from a pulse to a gradual and sustained elevation and extended IEI with only a transient decline in progesterone and mild hyperthermia, effects that are not expected to compromise pregnancy.

Temporal aspects of ovarian follicular growth and steroidogenesis following exogenous follicle-stimulating hormone in Angus heifers

Ultrasonography and endocrine assay techniques were used to monitor structural and hormonal alterations made by the ovary in response to the biological actions of pituitary-derived follicle-stimulating hormone (FSH-P). Angus heifers (n = 36) were allotted to receive injections (twice per day) of either FSH-P (up to a total of 28 mg over a maximum of 4 days beginning on Day 10 of a synchronized estrous cycle) or saline in order to quantify temporal relationships among follicle growth and steroid hormone profiles. Transrectal ultrasonography was utilized at 12-h intervals to monitor and record follicle growth. Plasma was collected every 12 h for the first 48 h of the experiment and then every 6 h for the remainder of the experiment. At 48 and 60 h after the onset of treatments, prostaglandin F2 alpha (PGF2 alpha; 25 mg) was administered (i.m.). FSH-treated heifers (n = 6 at each time) were terminated at 24, 48, 72 and 96 h following the onset of treatment. Saline-treated heifers were terminated at 24 and 96 h (n = 6 at each time). After ovaries were obtained, follicular number and size were recorded and follicular fluid (FF) was collected. Plasma concentration of progesterone (P) and estradiol (E2) and FF concentration of P, E2, estrone, testosterone and androstenedione were determined by radioimmunoassays. Plasma concentration of E2 increased (P < 0.05) within 36 h of initiation of FSH treatment. Plasma P decreased (P < 0.0001) by 12 h post-PGF2 alpha. Ultrasonographic examination revealed a significant decrease in the number of small follicles by 48 h, whereas the number of medium follicles increased (P < 0.05) by 60 h after the initiation of FSH treatment. The number of large follicles (LF > or = 10 mm diameter) increased (P < 0.01) over the course of the experiment. The total number of ovarian follicles (TF) 24 h after the start of FSH treatment was correlated (r = 0.99; P < 0.0001) with the number of small follicles (SF < or = 5 mm). At 72 h after the onset of FSH treatment, the number of medium follicles (i.e. 6-9 mm) was correlated with TF (r = 0.97; P < 0.0001). Estradiol was the predominant FF steroid. Follicular fluid E2 was greatest in follicles at 72 h after FSH treatment. Follicular fluid E2 and plasma E2 were positively correlated (r = 0.66; P < 0.001). Follicular aromatase activity was estimated by evaluating the ratio of FF estrogens (E) to androgens (A). Elevated aromatase activity (E:A ratio > 1.0) was detected in 196 of 206 follicles. The estrogen to progesterone ratio was used as an estimate of follicle viability. Eighty-five percent of the follicles were estimated to be viable (E:P ratio > 1.0). The peak E:A ratio in LF preceded by 24 h the peak concentration in FF E2 and plasma E2. In MF and SF the E:A ratio increased by 72 h. Enhancement of ovarian follicular growth (i.e. increased number and size of follicles; increased steroidogenesis) by exogenous, pituitary-derived FSH is characterized by (1) increased activity of aromatase, and (2) accumulation of FF E2, events which temporally preceded the increase in plasma concentration of E2. These observations will aid efforts to incorporate recombinant bovine FSH and somatotropin in an effort to develop more predictable superstimulation and ovulation induction protocols.

Calcium involvement in luteinizing hormone-releasing hormone release from the bovine infundibulum

Bovine infundibular (stalk median eminence) explants were incubated in vitro to test the hypothesis that calcium (Ca) is involved in the release of luteinizing hormone-releasing hormone (LHRH) from LHRH neuron terminals in cattle. Right and left infundibular halves from individual heifers and/or steers were randomly assigned to either control or treated (EGTA [a Ca chelator] or verapamil [an L-type Ca channel antagonist]) groups. Each half was incubated in 600 microliters of Krebs-Ringer bicarbonate medium (KRB) in the presence or absence of a treatment agent for 180 min. At 30-min intervals, 500-microliters samples were removed from each incubated and replaced with fresh media. Spontaneous (basal) and depolarization-induced (60 mM potassium) LHRH release was evaluated by radioimmunoassay of the LHRH content in the media incubated from 91 to 120 and 121 to 150 min of culture, respectively. The effect of treatment on depolarization-induced LHRH release was analyzed by comparing the differences between spontaneous and depolarization-induced LHRH release in control and treated groups. Spontaneous LHRH release was not different between control and 1.25 mM EGTA- or 100 microM verapamil-treated halves from steers. In contrast, steer infundibular halves incubated with EGTA (replacing Ca in KRB and chelating any Ca in the media) released less LHRH during depolarization than did control halves. In addition, verapamil-treated (to block Ca uptake by the terminal) infundibular halves from steers or heifers released less LHRH in response to depolarization than did control halves.(ABSTRACT TRUNCATED AT 250 WORDS)

Regulation of adrenocorticotropin secretion in vitro by anterior pituitary corticotrophs from fallow deer (Dama dama)

The actions of corticotropin-releasing hormone (CRH), vasopressin (VP), the synthetic glucocorticoid dexamethasone (DEX), and mifepristone (RU 486), a glucocorticoid antagonist, on the secretion of adrenocorticotropin (ACTH) by cultured fallow deer corticotrophs were studied in vitro. On Day 5 of primary culture, corticotrophs were challenged for up to 4 hr with medium alone (Control), CRH, VP, DEX, forskolin (FSK), phorbol ester (TPA), cyclic AMP (cAMP), and/or RU 486 at various concentrations and combinations. CRH, VP, FSK and TPA each stimulated (P < 0.01) the secretion of ACTH in dose- and time-related manners. Relative to Control, CRH at 0.001 and 0.1 microM and VP at 0.01 and 1 microM increased (P < 0.01) medium concentration of ACTH by 7.3-, 13.5-, 3.7- and 9.0-fold, respectively. There was a treatment x incubation time interaction (P < 0.01) such that at 30-min posttreatment, CRH-induced ACTH secretion tended (P < 0.10) to be less than that obtained via VP treatment, whereas at 1, 3, and 4 hr posttreatment, medium concentration of ACTH from cells treated with 0.1 microM CRH was greater (P < 0.05) than that in cells treated with 1 microM VP. At equimolar doses of 0.01 and 0.1 microM, CRH was 3.4- and 3.0-fold more potent (treatment x dose, P < 0.05) than VP. Cotreatment with 1 microM DEX reduced (P < 0.001) the stimulatory effects of CRH (0.1 microM), VP (1 microM), FSK (10 microMs), TPA (0.1 microM), and cAMP (0.001 M). However, the coaddition of RU 486 (1 microM) to the CRH plus DEX- and the FSK plus DEX-treated wells partially negated the inhibitory effects of DEX. RU 486 completely negated the inhibitory effects of DEX on the VP-, TPA-, and cAMP-stimulated secretion of ACTH. These data indicate that CRH is a more potent stimulator of ACTH secretion than is VP in primary culture of fallow deer pituitary cells. This study also demonstrates the utility of an in vitro culture system to investigate stress-related hormonal interactions in cervids.

Luteinizing hormone-releasing hormone gene expression in the bovine brain: anatomical localization and regulation by ovarian state

Experiments were conducted to identify neurons in the bovine brain that express the LHRH gene and to determine whether LHRH mRNA levels are influenced by the ovaries. Two groups of postpubertal heifers were utilized: heifers killed during the mid-luteal phase of the estrous cycle (LUTEAL, n = 5) and heifers killed 14-16 wk following ovariectomy (OVX, n = 5). In situ hybridization was performed through use of a 32P-end-labeled deoxyoligonucleotide (59 mer) complementary to the human LHRH mRNA sequence. LHRH-expressing neurons were identified in the diagonal band of Broca, the preoptic area, and the anterior hypothalamus in a manner consistent with immunocytochemical localization. Reduced silver grains, proportional to LHRH mRNA content, were quantified (in pixels, 45x objective) with an image analysis system. Expected serum hormone concentration differences between endocrine states were confirmed by radioimmunoassay for progesterone (LUTEAL > OVX, p < 0.01) and for LH (OVX > LUTEAL, p < 0.01). Compared to the OVX group, LUTEAL heifers had 34% fewer LHRH-expressing neurons (p < 0.05); on the average, these neurons possessed 28% fewer pixels/cell (p < 0.01), indicating fewer copies of LHRH mRNA per cell. When the numbers of pixels in all labeled cells were totalled, LUTEAL animals had 57% fewer pixels (p < 0.05) than did the OVX females--probably reflecting a decrease in LHRH synthetic capacity in the LUTEAL animals. Therefore, during the mid-luteal phase of the bovine estrous cycle, ovarian steroid (i.e., luteal progesterone) suppression of LHRH release (as reflected by serum LH) is coincident with decreased LHRH mRNA in the brain.(ABSTRACT TRUNCATED AT 250 WORDS)

A possible dual mechanism involved in the inhibitory influence of suckling on luteinizing hormone release in the ovariectomized rat

A study was conducted to further understand involvement of the endogenous opioid peptides in suckling-induced inhibition of LH release in ovariectomized rats. The first experiment was designed to determine the effect of an opioid antagonist, naloxone (NAL, 1.0 mg. kg-1h-1), on the increase in peripheral LH concentration 18 h after pup removal and on the decrease in LH concentration 18 h after pup return. Infusion of NAL during the 18 h after pup removal or during the 18 h after pup return neither accentuated nor attenuated serum LH concentrations. The second experiment was designed to determine the effect of NAL on peripheral LH concentrations in continuously suckled rats. Serum LH increased (p less than 0.10 and p less than 0.005, respectively) in response to 18 and 36 h of NAL infusion. The third experiment was designed to determine the effect of pup removal during NAL infusion on serum LH. Peripheral LH concentrations were not different in the rats treated with 36 h of NAL infusion whether they were suckled for the duration of the infusion or nonsuckled for the last 18 h of infusion. These results suggest that suckling may inhibit LH release through two mechanisms. The first may be an opioid-independent or enhanced opioid tone mechanism important for the initiation of the inhibitory effect of suckling on LH release, while the second may be an opioid-dependent mechanism important for the sustained inhibitory effect of suckling on LH release.

GnRH in the infundibular stalk-median eminence is related to percentage body fat in carcasses of beef cows

Mature Hereford cows (n = 28) were used to determine the effect of percentage body fat on secretion of LH and content of GnRH in the infundibular stalk-median eminence (ISME). Cows were fed to maintain, lose, or gain weight to achieve body condition scores (BCS; 1 = emaciated; 9 = obese) of 3 to 7. Then cows were fed to maintain weight and body condition. Before slaughter, estrus was synchronized using two injections of prostaglandin F2 alpha(PGF) 11 d apart. Five d after the second PGF injection, cows were given 100 micrograms of GnRH (im) and serum samples were obtained. LH was quantified using RIA. The anterior pituitary and ISME were obtained within 45 min of death. Anterior pituitary weight and LH concentration, total GnRH in the ISME, total carcass fat, and percentage carcass fat were determined. BCS of cows at the time of slaughter influenced percentage carcass fat (P less than .001), total GnRH in the ISME (P less than .02), and maximum LH after GnRH treatment (P less than .09), but did not influence pituitary weight or concentration of LH in the pituitary. Content of GnRH in the ISME averaged 76 +/- 12, 32 +/- 14, 27 +/- 13, and 24 +/- 13 ng for cows with BCS of 3, 5, 6, and 7, respectively. BCS was correlated (P less than .001) with percentage carcass fat (r = .94) and total fat in the carcass (r = .92). Total GnRH in the ISME was negatively correlated (P less than .005) with BCS (r = -.54), percentage carcass fat (r = -.55), and total carcass fat (r = -.49).(ABSTRACT TRUNCATED AT 250 WORDS)

A priming effect of naloxone on in vitro LHRH release from the hypothalamus of mid-luteal ewes

The possible involvement of endogenous opioid peptides (EOPs) in LHRH release from hypothalami of ewes during the breeding season was investigated using an in vitro perifusion system. Hypothalami were procured in December from ovariectomized (OVX; 62-65 days before the experiment; n = 6) and mid-luteal (ML; n = 7) Western White-Face ewes. Hypothalami were mid-sagitally sectioned into halves containing the preoptic area, mediobasal hypothalamus, and infundibulum (median eminence). The left half (treated) received two 30-min challenges (beginning at 130 and 250 min, respectively, after onset of perifusion) of 500 microM naloxone (NAL) followed by a 30-min 60-mM potassium (K) challenge (at 370 min after onset of perifusion). The right half served as the control, receiving only K at the same time as the treated tissue. Both NAL challenges elicited (p less than 0.05) LHRH release from tissues of both ML and OVX ewes. Release of LHRH by hypothalami from ML, but not from OVX, ewes was greater (p less than 0.01) after the second than after the first NAL challenge. These results are consistent with the view that an inhibitory opioid influence exists on LHRH release from ovine hypothalami. The release of LHRH in response to NAL was dependent on the ovarian status in vivo since the priming effect of NAL on subsequent NAL-induced LHRH release occurred only from the hypothalami of ML ewes. We suggest from these results that EOPs may modulate LHRH release from ovine hypothalami in an ovarian steroid-dependent and independent manner.

Influence of dietary energy intake on prepubertal development of Brahman bulls

Twelve Brahman bulls (paired by sire, weight and age) were assigned randomly and limit fed to gain either .10 to .25 (moderate gain; MG) or .75 to 1.0 (high gain: HG) kg.hd-1.d-1 to examine the effect of dietary energy on onset of puberty. Hip height (HH), scrotal circumference (SC) and serum samples (20 min for 6 h) were obtained at four times (AGE): 0, 56 and 112 d on feed and after appearance of first motile spermatozoa (FS) in the ejaculate of HG bull of the pair. At FS both bulls of a pair were slaughtered, reproductive tissues were collected and in vitro GnRH release from the median eminence (ME) was measured. Increases in BW, HH and SC were greater (P less than .05) in HG bulls. Basal ME GnRH secretion was greater (P less than .05) in HG bulls. Serum LH concentrations were unchanged by energy level (P greater than .10) but increased (P less than .01) with increasing AGE. AGE and energy level increased (P less than .01) basal, mean and total serum testosterone (T) and these two factors acted synergistically (P less than .01). Height and amplitude of T pulses were increased by energy level (P less than .003) and AGE (P less than .002). Testicular T (P less than .08) and development (P less than .05) were increased in HG bulls. Growth hormone peak height and amplitude concentrations following feeding increased with AGE (P less than .06) but were not altered (P greater than .10) by energy level. Serum triglycerides (P less than .03) and BUN (P less than .003) increased with increasing AGE (P greater than .01). These data indicate that dietary energy level influences onset of puberty most directly at the testicular level.

Suckling inhibits release of luteinizing hormone-releasing hormone from the bovine median eminence following ovariectomy

The effect of suckling on depletion of hypothalamic LHRH from the median eminence (ME) following ovariectomy (OVX) was determined in cattle. Multiparous, postpartum Holstein cows were assigned randomly to three groups: intact, nonsuckled (INT, n = 4); ovariectomized (3 to 5 d after parturition), nonsuckled (OVX, n = 4); and ovariectomized (3 to 5 d after parturition) and suckled by three calves (OVX-S, n = 5). Blood samples were collected at three periods (1 to 7 d before parturition and 3 to 5 d and 31 to 37 d after parturition) to determine plasma LH concentration. At 31 to 37 d after parturition, all cows were slaughtered and each ME was collected and mid-sagitally sectioned. The left half of each ME was used to determine content and concentration of LHRH. Concentrations of LH and LHRH were determined by RIA. Plasma LH concentration was similar among the three groups at 1 to 7 d before parturition and 3 to 5 d after parturition; however, at 31 to 37 d after parturition, OVX cows had a greater (P less than .05) concentration of LH (2.25 +/- .64 ng/ml) than either INT (.47 +/- .10 ng/ml) or OVX-S (.92 +/- .14 ng/ml) cows. Content of LHRH in the ME of INT (80.12 +/- 15.0 ng) and OVX-S 109.8 +/- 16.4 ng) cows was similar but was greater (P less than .05) than that in OVX cows (48.95 +/- 5.9 ng).(ABSTRACT TRUNCATED AT 250 WORDS)

Postpartum reproductive performance in the multiparous mare fed to obesity

Twenty multiparous Quarter Horse mares were assigned to one of two treatment groups at 40 to 75 d of pregnancy. Group 1 was the control group and the mares were fed to maintain a moderate degree of body fat (condition score 5.5 to 7). Group 2 was the obese group and the mares were fed to achieve (prepartum) and then maintain (post partum) an extremely high degree of body fat (condition score 9). Estrous intensity was evaluated using subjective teasing scores ranging from 0 (rejection) to 4 (maximum receptivity). Mares were artificially inseminated beginning with the second postpartum ovulatory cycle; the study was terminated after 63 d of pregnancy. Duration of estrus, maximum teasing score and the number of mares exhibiting overt estrus (teasing score > 2) did not differ between treatment groups during the first and second postpartum ovulatory cycles. The intervals from foaling to first cycle ovulation, foaling to second cycle ovulation, and first to second cycle ovulation were also similar between treatment groups. All mares in both treatment groups conceived and maintained pregnancy. The first cycle conception rate and the number of cycles per conception did not differ between treatment groups. A high degree of body fat produced by overfeeding during gestation did not adversely affect postpartum reproductive performance in the multiparous mare.

Biological activity of LH during the peripartum period and the estrous cycle of the ewe

Biological activity of luteinizing hormone (LH) is related to the degree of glycosylation of the glycoprotein hormone. The objectives of this study were to determine changes in biologically (BLH) and immunologically (ILH) active LH concentrations in plasma (in vitro bioassay and radioimmunoassay, respectively) and in the ratio of BLH to ILH (B:I) during the peripartum period and during the estrous cycle of the ewe. Blood samples were collected daily 4 days before through 4 days after parturition and during one estrous cycle. Also, samples were collected at 15-min intervals for 6 hr on Days 3 and 12 of the estrous cycle to quantify the influence of an elevated plasma concentration of progesterone (P) on the episodic secretion profiles of BLH. Progesterone concentration was determined on the 4th days pre- and post-partum, on each day of the estrous cycle and at hourly intervals on Days 3 and 12 of the cycle to investigate the hypothesis of an inverse relationship between P and BLH. The BLH and ILH concentrations were low during the peripartum period, and the B:I ratio did not increase by the 4th day postpartum. Mean ILH concentration was greater (P less than .05) in the postpartum than during the prepartum period. During the estrous cycle, mean daily B:I ratio was consistently above unity except for the day of estrus. The pre-ovulatory LH surge (BLH and ILH) was associated with a decrease (P less than .05) in the mean B:I ratio to 0.0065. Mean concentrations of BLH and ILH in plasma samples collected every 15 min on Day 12 were similar to Day 3 of the cycle.(ABSTRACT TRUNCATED AT 250 WORDS)

LHRH and LH in peripubertal female rats following prenatal and/or postnatal ethanol exposure

The effects of pre- and postnatal exposure to ethanol (ETOH) on LHRH and LH were investigated. Pregnant and/or lactating dams were fed ETOH during: 1) gestation, 2) lactation, or 3) gestation-lactation. Female offspring were decapitated at 30 or 40 days-of-age; trunk blood was collected for plasma LH RIA; and hypothalamic tissues were collected for LHRH RIA. Hypothalamic LHRH content of all ETOH-exposed groups was less than that of non-ETOH-fed controls at 30 and 40 days-of-age (p less than 0.05). Plasma LH concentrations of all ETOH-exposed groups were less than those of non-ETOH-fed controls at 30 and 40 days-of-age (p less than 0.05). Also, at 30 and 40 days-of-age, the plasma LH concentrations of the animals exposed to ETOH during lactation and gestation-lactation were less than those of the animals exposed to ETOH during gestation (p less than 0.05). These data suggest that ETOH exposure during gestation and/or lactation negatively affects hypothalamic LHRH content of female rat offspring. Decreased hypothalamic LHRH content with corresponding lowered plasma LH concentration suggests that ETOH influences development or maturation of hypothalamic LHRH neurons by possibly decreasing their number or synthesizing capability.

Ovarian inhibition of peripheral plasma concentration of follicle stimulating hormone in prepuberal Holstein heifers

The objective of this study was to determine effects of age and castration on follicle stimulating hormone (FSH) secretion in prepuberal heifers. In experiment 1, twelve heifers were bilaterally ovariectomized at 3, 6, or 9 months of age (n = 4/group). Blood was collected at 10 min intervals for 8 hr at 1 week before ovariectomy and 1 and 4 weeks after ovariectomy. Frequency, amplitude and duration of FSH pulses were calculated. Mean plasma concentration of FSH (ng/ml), and frequency (pulses/8 hr), amplitude (ng/ml), and duration (min/pulse) of FSH pulses were not altered by age. Mean concentration of FSH increased (P less than .01) from 1 week before to 1 week and 4 weeks after ovariectomy, respectively, in all age groups. Pulse frequency increased (P less than .05) from 1 week before ovariectomy to 4 weeks after ovariectomy in 3 month old heifers, from 1 week before to 4 weeks after ovariectomy in 6 month old heifers, and from 1 week before to 1 week and 4 weeks after ovariectomy in 9 month old heifers. In experiment 2, twelve heifers were bilaterally ovariectomized at 3, 6 or 9 weeks of age (n = 4/group). Sample collection and measurement of mean concentration of FSH were the same as in experiment 1. Mean concentration of FSH increased (P less than .01) from 1 week before to 1 and 4 weeks after ovariectomy in heifers ovariectomized at 6 and 9 weeks of age.(ABSTRACT TRUNCATED AT 250 WORDS)

Biological activity of luteinizing hormone in the peripartum cow: least activity at parturition with an increase throughout the postpartum interval

The biological (B) and immunological (I) activities of luteinizing hormone (LH) were determined during four peripartum periods in the cow. The sampling periods were PRE-5 (approximately 5 days prior to parturition), POST-3 (Day 2 or 3 postpartum), POST-12 (Day 11 or 12 postpartum) and LUTEAL (Day 11 or 12 following the first observed postpartum estrus). Blood samples were collected at 10-min intervals for either 8 h (PRE-5, POST-3, and POST-12) or 6 h (LUTEAL). Four hours prior to the end of each sampling period, 100 micrograms of luteinizing hormone-releasing hormone (LHRH) was injected i.v. Plasma concentrations of LH were analyzed by a validated bioassay (rat interstitial cell testosterone) and a radioimmunoassay. Area under the LHRH-induced LH release curve was calculated by the trapezoidal rule, and endogenous LH was determined by averaging the pre-LHRH treatment values. Paired t-analyses were used to compare LH release between periods. The periods of lowest (p less than 0.05) bioactive LH (BLH) were PRE-5 and POST-3. During POST-12, BLH was maximal. Endogenous immunoactive LH (ILH) was maximal during the POST-12 period, with PRE-5, POST-3, and LUTEAL being the least (p less than 0.05). The B:I ratio was lowest (p less than 0.05) around the time of parturition (PRE-5 and POST-3), then increased (p less than 0.05) by Day 12 postpartum. The ratio then increased (p less than 0.05) again by the LUTEAL period. These data suggest that the biological quality of LH changes throughout the puerperium. We infer from these data that the low biological activity of LH (subquality LH) may contribute to the postpartum infertility in the cow.

Corpus luteum function following spontaneous or prostaglandin-induced estrus in Brahman cows and heifers

Corpus luteum (CL) function following spontaneous or prostaglandin F2 alpha-induced estrus was studied in 27 Brahman cows and 16 Brahman heifers. Females received one injection of 0, .38, .75 or 2.25 mg alfaprostol (PGF)/100 kg body weight at 12 +/- .1 d post-estrus. Serum progesterone (P4) during CL formation, following the subsequent estrus, increased over time (P less than .001, 1 to 13 d post-estrus) and was influenced by PGF (P less than .06) and a PGF X time interaction (P less than .02). Low serum P4 concentrations were observed during CL formation (d 3, 4, 10, and 12) in cattle that received 2.25 mg PGF. On d 13 post-estrus, CL were removed. No differences in weight, P4 content or number of luteinizing hormone (LH) receptors was evident between CL derived 13 d following spontaneous vs PGF-induced estrus. The CL formed following PGF-induced estrus had fewer large (P less than .002) and small (P less than .09) luteal cells in heifers, a lower (P less than .008) percentage composition of histological type I and II luteal cells in cows and a lower (P less than .0001) in vitro P4 response to LH in both cows and heifers when compared with the CL formed following spontaneous estrus. Cows had heavier (P less than .002) CL with a higher (P less than .05) number of large cells, a higher (P less than .09) percentage composition of histological cell type I and II luteal cells, and a higher (P less than .03) in vitro basal and stimulated P4 secretion. It is concluded that 1) the events associated with artificially shortening the estrous cycle with PGF altered subsequent CL function in Brahman females; 2) cows had heavier CL, which were composed of more steroidogenically functional luteal cells, than heifers and 3) age of animal interacted with PGF to alter subsequent CL function in Brahman females.

Temporal relationship of the pulsatile fluctuation of luteinizing hormone and progesterone in cattle: a time series cross-correlation analysis

The temporal relationship between the pulsatile patterns of plasma luteinizing hormone (LH) and progesterone (P4) was studied in mid-luteal (ML) and early-pregnant (EP) dairy cows. Blood samples were collected (via external jugular vein cannulae) at 10-min intervals for 16 h in 5 ML cows (d 10 to 12 of the cycle) and for 10 h in 5 EP cows (d 52 to 56 of gestation). Concentrations of LH and P4 were determined by radioimmunoassays and a time series cross-correlation analysis was utilized to evaluate the temporal relationship between them. A pulsatile pattern was found for both hormones in both groups, and in all animals LH peaks were uniformly followed by P4 peaks. In 80% of the cows in both groups the highest cross-correlation occurred between samples LH(n) and P4 (n + 1) (n = sample number), suggesting that a lag time of about 10 min is necessary for luteal stimulation. Results from both groups demonstrate that P4 is released from the corpus luteum in a pulsatile manner and that its release is at least partially dependent upon the pulsatile pattern of plasma LH. Correlation coefficients between LH and P4 mean level, basal levels, peak frequencies and peak amplitudes obtained in both groups indicate that the conceptus alters the hypothalamic-pituitary-ovarian relationship found in the ML cows, suggesting the existence of another factor(s) acting along with LH to release P4 in EP cows.

Growth and estrous activity of rats fed adequate and deficient levels of phosphorus

Female Sprague-Dawley rats were fed two levels of phosphorus ad libitum and in a paired feeding arrangement to yield the following three treatments: adequate phosphorus (0.40%) ad libitum intake, low phosphorus (0.04%) ad libitum intake and adequate phosphorus pair-fed with the low phosphorus group. Feed intake, growth rate and feed conversion in rats fed 0.04% phosphorus ad libitum and in pair-fed control rats were lower (P less than 0.01) than in rats fed 0.40% phosphorus ad libitum. Seventy-five percent of the rats exhibited normal estrous cycles, as indicated by vaginal smears, throughout the 105-d period regardless of dietary phosphorus level. Breeding efficiency and number of pups per litter were not altered by dietary treatment. However, compared to either phosphorus-adequate group, dams fed 0.04% phosphorus weaned fewer (P less than 0.05) pups and pup survival to 45 d of age was lower (P less than 0.05). Average daily gain postweaning was 67% lower for pups in the phosphorus-deficient group than for those in either phosphorus-adequate group. Phosphorus deficiency in rats does not alter estrous cycles but reduces pup survivability postnatally.

Ovarian Activity of Holstein and Jersey Cows of Diverse Transmitting Abilities for Milk

The relationship between ovarian activity and milk yield was studied in 35 daughters of 24 Holstein sires and 17 daughters of 14 Jersey sires in the same herd. Ovulations and length of estrous cycles were determined by progesterone concentration in postmilking strippings three times per week, by weekly palpation per rectum, and by twice daily estrus detection. Transmitting abilities were for 4% fat-corrected milk of cows and their sires. Yields of 4% fat-corrected milk were estimated for 60, 90, 120, and 305 days in lactation. Postpartum intervals to first ovulation averaged 22 and 20 days for Holsteins and Jerseys, but the interval to first standing estrus was shorter for Jersey than for Holstein. Postpartum intervals to each of the first three ovulations and length of estrous cycles were unrelated to actual yield or transmitting ability for yield of 4% fat-corrected milk in either breed. The percentage of cows observed in standing estrus at each of the first three ovulations increased from 23 to 43%.

Visualization of pronuclei in living bovine zygotes

Bovine embryos were surgically collected from the oviducts of superovulated crossbred heifers 48 h postonset of estrus. The one-celled ova were treated with 4'-6'-diamidino-2-phenylindole (DAPI) and observed under ultraviolet light by fluorescence microscopy. Both male and female pronuclei were visualized, identified and subjected to micromanipulation.

Effects of ethanol on hypothalamic luteinizing hormone-releasing hormone (LHRH) in the male rat. An immunocytochemical study

These studies were designed to determine if the acute alcohol-induced decreases in luteinizing hormone (LH) seen in previous studies using rats could be due to an inhibitory effect of ethanol (ETOH) on hypothalamic LHRH release. Thus, effects of multiple injections of ETOH on the relative amount of immunoreactive LHRH fibers in the hypothalamus and median eminence (ME) of castrate and intact male rats were determined immunocytochemically. Brains were removed following cardiac perfusion of 10% phosphate-buffered formalin. A block containing the hypothalamus with the ME was isolated from each brain, then postfixed in Bouin's solution. Paraffin sections were rehydrated and stained for LHRH with the peroxidase-antiperoxidase technique using an antiserum to synthetic LHRH conjugated to bovine serum albumen. Differences visualized immunocytochemically between saline-treated intact and castrate rats indicated that the LHRH content of the ME was markedly depleted after castration. Conversely, castrate rats treated with ETOH showed only a slight reduction in immunoreactive LHRH fibers. In ETOH-treated intact animals, the LHRH fiber content of both the hypothalamus and ME appeared to be slightly greater than the saline-treated intact controls. Thus, these data support the hypothesis that ETOH diminishes LHRH release, and hence provides an explanation for the depressed plasma LH levels observed in ETOH-treated intact and castrate rats.

Effects of ethanol on rat hypothalamic luteinizing hormone releasing hormone. A study utilizing radioimmunoassay

To further understand the mechanism of action by which ethanol (ETOH) decreases plasma luteinizing hormone (LH) levels, the effects of multiple i.p. injections of EOH (1.0--1.5 g/kg) or saline on hypothalamic luteinizing hormone releasing hormone (LHRH) and plasma LH concentrations were evaluated in intact and castrate male rats. After injections, animals were decapitated, brains rapidly removed, and blocks containing the hypothalamus [with median eminence (ME)] were isolated. Hypothalami were subjected to acetic acid extraction and LHRH content quantitated via radioimmunoassay (RIA). Hypothalamic LHRH was found to be inversely correlated with plasma LH. In response to castration, both saline and ETOH-treated rats showed a decrease in hypothalamic LHRH content with a concomitant increase in plasma LH; however, the ETOH-treated animals retained significantly greater concentrations of LHRH and showed significantly lower plasma LH levels when compared to saline-treated controls. Likewise, ETOH-treated intact animals showed significant increases in LHRH content, with LH levels remaining significantly lower than the saline-treated intact controls. Thus, these data from both intact and castrate rats provide evidence to support the hypothesis that alcohol-induced decreases in LH levels are due to a diminished release rate of hypothalamic LHRH.

Evidence for an inherent rhythm in pulsatile LH release in ovariectomized cows

Luteinizing hormone levels were measured in blood samples collected at 5 minute (min) intervals for 3 hours (hr) during the a.m. and p.m. of 3 consecutive days from long-term ovariectomized cows. Levels of LH fluctuated in a pulsatile manner in all animals. During the pulses, LH levels increased rapidly (2.5 to 6.0 ng/ml). Following the rapid increase, a more gradual exponential decline was observed. The interval between pulses was consistent both within and between days of blood sample collection within cows. From the results we suggest that each cow may have an inherent consistent rhythmic pattern of LH release in the absence of an ovarian source of hormones.

Influence of photoperiod on the pregnant and postpartum mare

Over a 2-year period, mares (n = 32) of predominantly Quarter Horse breeding and their foals were used to study the effects of photoperiod on reproduction during the periparturient period. Specific objectives of the study were (i) to evaluate the effects of an extended photoperiod regimen (16 hours' light) on gestation length, foal development, and postpartum reproductive performance, and (ii) to measure changes in plasma progesterone concentrations during the last trimester of gestation and plasma luteinizing hormone (LH) concentrations after foaling until ovulation. Mares in the extended daylength treatment group had shorter (10 days) mean gestation length (P less than 0.01) than did control mares (exposed to normal daylight hours). Although foals of mares exposed to long photoperiod were carried for a shorter term, their birth weights tended to be heavier, but not significantly, than those of foals from control mares. Foal size, as determined by body measurements, was not affected by the photoperiod. Neither interval from parturition to 1st ovulation nor onset of estrus to ovulation were significantly affected by the extended photoperiod. Plasma progestogen concentrations increased from basal levels at prepartum weeks 4 and 5 to peak values a week before foaling in both mare groups. The LH profiles from parturition to ovulation were similar for both mare groups. However, mean plasma LH values during the 7 days before the 1st ovulation were higher (P less than 0.05) in mares exposed to extended daylength than in control mares.

Effect of steroids and/or 48 HR calf removal on serum luteinizing hormone concentrations in anestrous beef cows

Serum luteinizing hormone (LH) concentrations were quantified in 27 thin, anestrous cows with suckling calves in each of three treatment groups: Syncro-Mate-B (SMB), 48 hr calf removal (CR), and SMB plus CR (SMB + CR). The SMB treatment consisted of a 9 day ear implant containing 6 mg norgestomet and an intramuscular injection containing 3 mg norgestomet and 6 mg estradiol valerate given at the time of implant placement. In the SMB + CR group, CR began at the time of implant removal (0 hr). Blood samples were collected every 4 hr via puncture of a tail vessel beginning 12 hr prior to implant and/or CR and continued for 72 hr thereafter. Before implant and/or CR (-12 to 0 hr), LH concentrations were higher (P<.01) in the CR (1.1 ng/ml) group than in the SMB (.6 ng/ml) and SMB + CR (.8 ng/ml) groups. Following implant and/or CR (4 to 48 hr), LH concentrations increased (P<.01) in the CR (1.8 ng/ml) and SMB + CR (1.3 ng/ml) groups, but remained unchanged in the SMB (.7 ng/ml) group. Furthermore, LH concentrations were higher (P<.05) in the CR group compared to the SMB + CR group. Circulating concentrations of LH declined (P<.01) to 1.2 ng/ml in the CR group following calf return, but remained unchanged in the other two groups. Although more (P<.01) cows in the SMB and SMB + CR groups were detected in estrus than in the CR group, there was no difference (P.10) in the number of cows ovulating between the three treatment groups. These results suggest that CR will increase circulating LH concentrations by 24 hr post CR and that SMB may partially suppress the CR induced LH release following implant and calf removal.

Induction of parturition in the mare with prostaglandin F2 alpha

Thirty-one mares of Quarter Horse and Thoroughbred breeding were utilized in two experiments to evaluate the efficacy of prostaglandin F2 alpha (PGF2 alpha) for induction of equine parturition and to monitor the effects of this treatment on viability of the resulting foals. Three of five mares given 5 mg PGF2 alpha (im) on day 338 of gestation foaled 19.6+/-8.2 hr postinjection. In the second experiment immediately following 3 daily injections of 10 mg estradiol cypionate (ECP) given on days 326, 327 and 328 of gestation, seven mares were infused (iv) with PGF2 alpha at the rate of 1.3 mg/hr for 24 hr or until parturition occurred. Four of the seven mares foaled in 8.8+/-1.8 hr after the start of infusion. Side effects including sweating, hypothermia, increased respiration rate and diarrhea were evident in both injected and infused mares, but effects were transient. Neither the injection, nor infusion route of administration of PGF2 alpha adversely affected the viability of foals. However, some mares induced to foal 12 days prior to expected parturition had foals with slightly weaker pasterns than those of control mares.

Inhibition of the postovariectomy depletion of hypothalamic luteinizing hormone releasing hormone (LHRH) by suckling

A study was undertaken to further the understanding of the mechanism by which suckling inhibits the release of pituitary LH and depresses the postovariectomy rise of plasma LH in lactating mammals. To that end, the effect of suckling (10 pups/animal) for 1 or 3 weeks on the LHRH content of the hypothalamus and preoptic area (POA) in ovariectomized and intact rats was examined. Controls consisted of intact and 1 or 3 week ovariectomized, nonlactating animals. Following decapitation, the brains were rapidly removed and blocks containing the POA and the hypothalamus (with median eminence) were isolated. Tissue was extracted with acetic acid and LHRH quantitated via validated RIAs utilizing 2 antisera specific for different portions of the LHRH molecule. Ovariectomy of nonlactating, diestrous animals resulted in a significant decline in hypothalamic LHRH, reaching 50% of control levels by 3 weeks. During the same intervals, plasma LH increased dramatically to 20- and 60-fold over intact controls by 1 and 3 weeks, respectively. In contrast, LHRH levels were not decreased at 1 or 3 weeks in ovariectomized rats which were suckled, at which time plasma LH was greatly depressed. When intact animals were evaluated, the suckling stimulus failed to induce a detectable change in LHRH content of the hypothalamus of LHRH in the POA between any of the treatment or control groups. These data from ovariectomized rats suggest that suckling inhibits LHRH release from the hypothalamus and hence provides an explanation for the depression of plasma LH observed in suckled ovariectomized and intact animals.

Immunocytochemical evidence that suckling inhibits the postovariectomy depletion of median eminence luteinizing hormone releasing hormone

Suckling has been demonstrated to impair the release of pituitary luteinizing hormone (LH) and to prevent the dramatic increase in plasma LH observed following ovariectomy. In the present study, the effect of suckling (10 pups/animal) for either 1 or 3 weeks on the relative amount of luteinizing hormone releasing hormone (LHRH) present in the hypothalamus and preoptic area of ovariectomized and intact rats was examined using immunocytochemical methodology. Controls consisted of nonlactating animals which were either intact (diestrous) or ovariectomized for 1 or 3 weeks. Brains were removed following transcardial perfusion of phosphate-buffered formalin and Bouin's fixative. After dehydration, clearing and paraffin embedding, the brains were sectioned and LHRH localized by an indirect immunoperoxidase technique. A positive reaction denoting the presence of immunoreactive LHRH was observed over axons and termini throughout the rostral to caudal extent of the median eminence (ME) and surrounding the organum vasculosum of the lamina terminalis (OVLT) in the preoptic area. Ovariectomy resulted in a progressive decline in the concentration of LHRH within the ME as evidenced by a reduction in the intensity of the staining reaction and in the number of axons over which the reaction was observed. In contrast, brains from ovariectomized rats which had been suckled appeared to have concentrations of LHRH in the ME equal to or greater than that of the diestrous controls. Similarly, the concentrations of LHRH In the ME of intact, suckled rats did not differ significantly from that of the diestrous controls. Neither ovariectomy nor suckling produced any observable change in the relative concentration of LHRH located near the OVLT. These data demonstrate that suckling prevents the depletion of LHRH from the ME following ovariectomy and provide evidence for mechanism by which the suckling stimulus may suppress plasma LH.

Pattern of plasma luteinizing hormone in the cyclic cow: dependence upon the period of the cycle

Docile cows were used to characterize the pattern of plasma LH in systemic circulation during three periods [day 3 (early luteal), day 10 or 11 (midluteal) and day 18-19] of the estrous cycle. Blood samples were collected at 10-min intervals for 24 continuous h during each period. Concentration of LH, as measured by RIA, was analyzed by time domain and frequency domain methods of time series analysis. During all periods of the cycle studied, LH fluctuated in a pulsatile manner. During the early luteal period, pulses were classified as low amplitude (delta LH, 0.3-1.8 ng) and high frequency (20-30 pulses/24 h), with each cow exhibiting an inherent rhythmic pattern. However, during the midluteal period, pulses were classified as high amplitude (delta LH, 1.2-7.0 ng) and low frequency (6-8 pulses/24 h), without an inherent rhythmic pattern. Even though patterns differed between the early and midluteal periods in all cows, mean LH concentrations were different (P less than 0.05) in only one cow. On day 18 or 19, two cows exhibited a preovulatory surge of LH. Throughout the surge, LH fluctuated in a pulsatile manner, with a frequency more like the early luteal than the midluteal period. Amplitude of the pulses was greater during the ascending than during the descending portion of the surge. These results suggest that LH fluctuates in a pulsatile manner in the plasma of the cyclic cow, and that the pattern is dependent upon the period of the estrous cycle and is probably modulated by ovarian steroids.

Time-dependent loss of radioimmunoassayable levels of progesterone following ambient temperature incubation of heparinized bovine blood

Plasma progesterone levels in heparinized blood collected at 10 min intervals for 8 continuous hours from four nulliparous Holstein cows on day 3 (early luteal), day 10 or 11 (mid-luteal) and day 18 or 19 of the estrous cycle were found to decline over time when blood was incubated at ambient temperature. The loss was more obvious during the mid-luteal collection period than either the day 3 or day 18 or 19 periods in all cows. This appeared to be associated more with high progesterone levels on day 10 or 11 rather than with differences in the period of the estrous cycle. There was an average decrease in progesterone levels of 3.4, 1.0 and 1.5 ng/ml between samples having the shortest and longest incubation periods on day 10 or 11, day 3 and day 18 or 19, respectively. This apparent decrease in levels of progesterone from bovine blood indicates need in the future for careful consideration concerning the handling of bovine blood collected for subsequent radioimmunoassay (RIA) of progesterone. Further work to elucidate the mechanism which is responsible for the apparent loss is also warranted.

Evidence for central nervous system (CNS) involvement in inhibition of luteinizing hormone (LH) release by dopamine receptor stimulation

A pharmacologic study using 2-Br-alpha-ergocryptine (CB-154), a dopamine (DA) receptor agonist, the pimozide, a specific DA receptor antagonist, was undertaken to evaluate effects of manipulation of the DA system on plasma LH levels and on the ability of the pituitary to release LH following LHRH stimulation. Intraveneous administration of CB-154 lowered plasma LH levels in ovariectomized rats. Prior treatment with pimozide blocked the effect of CB-154, supporting dopaminergic system mediation in the observed response. Inhibition of LH release by pharmacologic stimulation of DA receptors did not appear to occur at the level of the pituitary, as pituitary release of LH following LHRH was not altered in CB-154 treated rats.

Prolactin levels in ovariectomized Brahman, Brahman × Hereford and Hereford cows following a 20 mg dose of estradiol-17β

Six Brahman (B), six Brahman x Hereford (BxH) and six Hereford (H) chronically ovariectomized cows were injected intramuscularly with 20 mg of estradiol-17beta (E2). The cows were bled via coccygeal vessel puncture immediately before E2 injection, every 2 hr from 0 to 8 hr post-injection, every hr from 9 to 24 hr post-injection and every 2 hr from 26 to 36 hr post-injection. Serum prolactin (PRL) concentrations were quantitated by a validated radioimmunoassay. All cows exhibited a PRL surge following the E2 injection. A PRL surge was defined as a sustained elevation in PRL of at least one standard deviation above the level of PRL before the rise. Nadir levels of PRL prior to the surge did not differ significantly between breeds. Time (hr) to the onset of the E2-induced PRL surge was 5.0, 5.0 and 6.2 in B, BxH and H, respectively (P<.10). Elapsed time (hr) from E2 injection to the PRL peak level varied (P<.01) between B (10.8) and H (17.8) and BxH (11.8) and H. Peak PRL levels (ng/ml) varied (P<.10) between breeds (B, 70.6; BxH, 123.9; H, 49.4). Area under the PRL curve (sq cm) varied (P<.05) between BxH (45.2) and H (24.7) but not between BxH and B (34.3; P>.10) or B and H (P>.10). Duration (hr) of the PRL surge did not differ significantly between breeds (B, 19.3; BxH, 20.5; H, 21.2). Overall, bleeding period effects (P<.01), breed effects (P<.10), and breed x period interactions (P<.01) were found.

Serum levels of progesterone, 5 alpha-dihydroprogesterone and hydroxy-5 alpha-pregnanones in the prepartum and postpartum equine

Blood samples were collected from eight mares for ten days before and two days following parturition. Progesterone, 5 alpha-dihydroprogesterone and hydroxy-5 alpha-pregnanones were extracted from serum, separated by Sephadex LH-20 column chromatography and quantitated by radioimmunoassay. Progesterone levels ranged from 8.5 to 4.1 ng/ml among mares with normal parturition and decreased to 1.2 ng/ml after parturition. One mare with a retained placenta maintained the prepartal levels of progesterone into the postpartum period. 5 alpha-dihydroprogesterone increased to a level of 133 ng/ml at 80 hrs before parturition and began decreasing and reached a level of 9.2 ng/ml in the postpartum period. Levels of hydroxy-5 alpha-pregnanones were variable with a high of 1690 ng/ml and declined to a postpartum level of 60 ng/ml. The timing of the decline varied among the mares. The possible source and significance of these progestins remain speculative.

Isolation, identification and quantitation of serum 5alpha-pregnane-3,20-dione and its relationship to progesterone in the pregnant mare

5alpha-pregnane-3,20-dione was isolated from pooled pregnant mare serum using Sephadex LH-20 column chromatography and identified by the use of radioimmunoassay, gas-liquid chromatography and gas-liquid chromatography-mass spectrometry analyses. 5beta-pregnane-3,20-dione was not cross-reactive with the radioimmunoassay system and was not detected by gas-liquid chromatography. Peripheral blood levels of progesterone and 5alphs-pregnane-3,20-dione were determined by radioimmunoassay in four Quarter Horse mares for the first 150 days of gestation. Progesterone and 5alpha-pregnane-3,20-dione declined from a range of 6 to 15 ng/ml at day 10 to a range of 2 to 8 ng/ml at day 40. After day 40 there was an increase in progesterone and 5alpha-pregnane-3,20-dione concentration. Toward the end of 150 days of gestation progesterone tended to decline; whereas, 5alpha-pregnane-3,20-dione levels were maintained or increased to levels as high as 35 ng/ml. Neither source nor function of 5alpha-pregnane-3,20-dione is known.

Failure to monoaminergic and cholinergic receptor blockers to prevent prostaglandin E2-induced luteinizing hormone release

Receptor blocking drugs were used to determine whether adrenergic, dopaminergic, serotoninergic, or cholinergic synapses are involved in mediating the LH release induced by intraventricularly injected PGE2. Prostaglandin E2 (5mug) was injected into the 3rd ventricle (3rd V) of ovariectomized rats, and plasma LH concentrations before and after treatment were determined by radioimmunoassay. Phentolamine, 20 or 30 mug, or pronethalol, 20 mug (alpha and beta adrenergic receptor blockers, respectively) injected into the 3rd V failed to alter the elevation of plasma LH evoked by PGE2 injected into the ventricle 10 min later. Likewise, LH release following PGE2 was not changed when a dopaminergic blocker, pimozide (0.63 mg/kg, SC), was injected 2 h prior to PGE2. Two antagonists of serotonin, methysergide maleate (3 mg/kg ip) or cinanserin HC1 (1 mg/kg iv) given 2 h or 45 min before PGE2, respectively, failed to alter the action of PGE2. Atropine (100 or 250 mug) injected into the 3rd V 10 min prior to PGE2 was also ineffective in blocking the increase in plasma LH following PGE2. The results of this study indicate that the effect of PGE2 on LH release is not mediated by adrenergic, dopaminergic, serotoninergic, or cholinergic receptors. They also suggest that PGE2 is not acting trans-synaptically but probably directly on the LHRH neuron to induce the discharge of LHRH into the hypophysial portal vessels which then evokes release of LH from the adenohypophysis.

Effect of inhibitors of prostaglandin synthesis on gonadotropin release in the rat

To study the effect of blockade of prostaglandin (PG) synthesis on gonadotropin release in the rat, inhibitors of PG synthesis were injected by various routes in various experimental conditions. The injection of 5-, 8-, 11-, 14-eicosatetraynoic acid (TYA) into the third ventricle (3rd V) significantly decreased plasma LH of ovariectomized (OVX) rats 1, 2, and 4 h following its injection; however, TYA failed to alter plasma LH in OVX rats when administered as a single sc injection and also failed to prevent the post-castration rise in plasma LH when administered sc once daily for 4 days to short-term OVX rats. None of these treatments altered plasma FSH concentrations. Indomethacin (Id) injected into the 3rd V or implanted into the medial basal hypothalamus (MBH) of OVX rats depressed plasma LH 1--6 h later. This effect was no longer observed 24--72 h following its implantation in the MBH. When different doses of Id were administered as single sc injections to OVX rats, plasma LH titers were depressed 24--32 h later, whereas plasma FSH remained either unaltered or was slightly increased. Similarly, the post-castration rise of plasma LH but not that of FSH in male rats was suppressed by a single sc injection of Id given 6 h before orchidectomy. Id administered acutely iv failed to modify the pulsatile release of LH in OVX rats, but it effectively inhibited this release when injected sc 20--30 h before the initiation of blood collection. Moreover, Id blocked the progesterone-induced LH and FSH release in OVX estrogen-primed rats when given sc 24 h before progesterone, but not when it was injected either sc or iv shortly (2 h) before or shortly after (1--3 h) progesterone treatment. Rats treated with Id showed a decrease in BW 24--32 h afters its sc injection. However, the effects of Id on LH release could not be explained by lack of food intake since fasted controls showed LH titers similar to fed rats. Id did not significantly inhibit the LH release in response to synthetic LH-releasing hormone (LHRH) in OVX rats, but partially blocked the response in OVX estrogen, progesterone-treated (OEP) rats. Surprisingly, in OEP rats, Id appeared to potentiate the FSH release in response to LHRH. The results of this study indicate that inhibitors of PG synthesis administered at high doses can inhibit LH release in the rat and that this effect is mainly due to a direct effect of the drug or drugs on the central nervous systen. Consequently, the results of this study give further support to the hypothesis that PGs play a physiological role in the control of gonadotropin secretion.

Modification of stress-induced prolactin release by dexamethasone or adrenalectomy

A study was conducted to determine the influence of conditions known to alter ACTH secretion on prolactin release by the pituitary. Dexamethasone or 0.9% NaCl was injected subcutaneously into intact male rats 4 hr before a 2-min ether stress. Other rats were submitted to the ether stress 2 weeks following adrenalectomy. Five min following exposure to the ether stress, the rats were decapitated and trunk blood was collected. Dexamethasone (31, 125 or 500 mug/kg) blocked the stress-induced prolactin release in a dosage dependent manner with the highest dose decreasing plasma prolactin to levels similar to those observed in rats not exposed to the ether stress. Conversely, adrenalectomy was found to potentiate prolactin release due to ether stress. The results indicate that the adrenal may play a regulatory role in stress-induced prolactin release. The are also consistent with the suggestion that a common pathway or mechanism may be involved in pituitary release.