Discrete hypothalamic distribution of luteinizing hormone-releasing hormone (LHRH) content and of LHRH-degrading activity in laying and nonlaying hens

Hypothalamic enzymatic luteinizing hormone-releasing hormone (LHRH)-degrading activity (LHRH-DA) may play a physiologic role in the neuroendocrine control of LHRH in mammals. The present study analyzes the existence and possible physiologic role of LHRH-DA in birds. The LHRH content in discrete hypothalamic samples of laying and nonlaying hens was correlated to their LHRH-DA. Degrading activity was assessed by high-performance liquid chromatography (HPLC) of chicken LHRH and of its degradation fragments. Luteinizing hormone-releasing hormone content was estimated by radioimmunoassay. Luteinizing hormone-releasing hormone content of discrete medial preoptic, infundibulum, and arcuate samples, as well as serum LH and progesterone levels, were higher (P less than 0.05) in laying than in nonlaying hens. The LHRH content of these hypothalamic areas was also higher (P less than 0.05) than those of immediately adjacent areas, in both animal groups. Luteinizing hormone-releasing hormone-degrading activity, which generates LHRH1-5 as the main degradation fragment, was higher (P less than 0.01) in the infundibulum of laying than in nonlaying hens. It was also higher (P less than 0.01) in samples from the infundibulum and medial preoptic area than in immediately adjacent lateral samples. Finally, LHRH-DA, showing a similar HPLC profile of degradation fragments, was also present in areas of low or undetectable LHRH content.(ABSTRACT TRUNCATED AT 250 WORDS)     

Hypothalamic contents of LHRH and catecholamines during the ovulatory cycle of the hen (Gallus domesticus)

Concentrations of LHRH, dopamine, noradrenaline and adrenaline in the anterior hypothalamus-preoptic region (AH-POR) and posterior hypothalamus-median eminence (PH-me) were determined in hens killed at different times in relation to the first ovulation of a sequence. The occurrence of a preovulatory rise in plasma LH concentration 4-6 h before the expected time of ovulation was confirmed. This rise in plasma LH was accompanied by a significant (P less than 0.01) 50% reduction in the LHRH content of the AH-POR and PH-me while the subsequent fall in plasma LH was accompanied by a restoration of the LHRH content of both regions to their former levels. Although no significant fluctuations in the hypothalamic content of either dopamine, noradrenaline or adrenaline were detected during the ovulatory cycle, significant correlations between LHRH content and catecholamine content were observed in the AH-POR (P less than 0.05) and PH-me (P less than 0.01). Thus mean levels of each amine followed the same temporal pattern as LHRH content with minimum values being observed shortly before the peak of the preovulatory surge of LH. These findings support the conclusion that an enhanced secretion of LHRH from the median eminence, possibly associated with an increased activity of catecholaminergic neurones, is a prerequisite for the preovulatory release of LH in the hen.     

Frequency of luteinizing hormone surges and egg production rate in turkey hens

Whether the interval between preovulatory surges of LH was different between lines of turkey hens with either poor (RBC3 line, peak at 55%) or excellent rate of egg production (Egg line, peak at 85%) was examined. Laying hens were cannulated and bled hourly for 10 days at peak of production. A constant light photoschedule was used to avoid diurnal masking of innate circadian rhythms. The mean interval between LH surges in the RBC3 line was longer than in the Egg line and had a higher coefficient of variation. A few longer LH surge intervals (72 h) were found in some RBC3 line hens (2 of 7 hens), but none were found in Egg line hens (0 of 11 hens). All progesterone (P4) surges were coupled with LH surges, but not all LH-P4 surges were coupled with ovipositions (blind LH-P(4) surges). The percentage of blind LH-P4 surges was not different between lines. The baseline concentration of LH was higher in Egg line than RBC3 line hens, but LH surge amplitude, and surge duration were not different. The baseline and surge amplitude concentrations of P4 were not different between lines, nor was the concentration of estradiol-17beta. The longer interval between LH surges was the major factor tested that was associated with the poorer egg production rate in RBC3 line hens in comparison to Egg line hens. A higher incidence of blind LH surges further contributed to lower egg production in RBC3 line turkey hens.     

Gonadotropin-releasing hormone at estrus: luteinizing hormone, estradiol, and progesterone during the periestrual and postinsemination periods in dairy cattle

Administering gonadotropin-releasing hormone (GnRH) improved conception rates in our previous studies. Our objective was to determine if the effect of GnRH was mediated through serum luteinizing hormone (LH) and/or by altered secretion of serum progesterone (P) and estradiol-17 beta (E) during the periestrual and post-insemination periods. Cattle were given either GnRH (n = 54) or saline (n = 55) at 72 h and inseminated artificially (AI) 80 h after the second of two injections of either prostaglandin F2 alpha or its analog, cloprostenol. Progesterone and E were measured in blood serum collected during 3 wk after AI (estrus) from 60 females. Blood was collected for LH determinations via indwelling jugular cannulae from 14 cows and 11 heifers. Collections were taken every 4 h from 32 to 108 h after the second PGF injection (PGF-2) (periestrual period) and at more frequent intervals during 240 min after administration of GnRH (n = 18) or saline (n = 7). Ten females had a spontaneous preovulatory LH surge before GnRH treatment (GnRH-spontaneous), whereas GnRH induced the preovulatory LH surge in six females. A spontaneous LH surge appeared to be initiated in two heifers at or near the time of GnRH treatment (spontaneous and/or induced). The remaining seven cows had spontaneous LH surges with no subsequent change in LH after saline treatment. Serum P during the 21 days after estrus was lower (p less than 0.05) in both pregnant and nonpregnant (open) cattle treated previously with GnRH compared with saline. Serum P during the first week after estrus was greater (p less than 0.01) and increased (p less than 0.05) more rapidly in saline controls and in GnRH-spontaneous cattle than in those exhibiting GnRH-induced or GnRH-spontaneous and/or-induced surges of LH. Conception rate of cattle receiving GnRH was higher (p = 0.06) than that of saline-treated controls. These data suggest that GnRH treatment at insemination initiated the preovulatory LH surge in some cattle, but serum P in both pregnant and open cows was compromised during the luteal phase after GnRH treatment. Improved fertility may be associated with delayed or slowly rising concentrations of serum progesterone after ovulation.     

Pituitary luteinizing hormone-releasing hormone receptors in ovariectomized cows after challenge with ovarian steroids

Acute changes of bovine pituitary luteinizing hormone-releasing hormone (LHRH) receptors in response to steroid challenges have not been documented. To investigate these changes 96 ovariectomized (OVX) cows were randomly allotted to one of the following treatments: 1) 1 mg estriol (E3); 2) 1 mg 17 beta-estradiol (E2); or 3) 25 mg progesterone (P) twice daily for 7 days before 1 mg E2 and continuing to the end of the experiment. Serum was collected at hourly intervals from 4 animals in each group for 28 h following estrogen treatment. Four animals from each treatment were killed at 4-h intervals from 0 to 28 h after estrogen injection to recover pituitaries and hypothalami. Treatment with E3 or E2 decreased serum luteinizing hormone (LH) within 3 h and was followed by surges of LH that were temporally and quantitatively similar (P greater than 0.05). Progesterone did not block the decline in serum LH, but did prevent (P less than 0.05) the E2-induced surge of LH. Serum follicle-stimulating hormone (FSH) was unaffected (P less than 0.05) by treatment. Pituitary concentrations of LH and FSH were maximal (P less than 0.001) at 16 h for E3 and 20 h for E2, whereas P prevented (P greater than 0.05) the pituitary gonadotropin increase. Concentrations of LHRH in the hypothalamus were similar (P greater than 0.05) among treatments. Pituitary concentrations of receptors for LHRH were maximal (P less than 0.005) 12 h after estrogen injection (approximately 8 h before the LH surge), even in the presence of P. This study demonstrated that in the OVX cow: 1) E2 and E3 increased the concentration of receptors for LHRH and this increase occurred before the surge of LH; and 2) P did not block the E2-induced increase in pituitary receptors for LHRH but did prevent the surge of LH.     

Enhanced luteinizing hormone release by luteinizing hormone-releasing hormone in incubating female turkeys (Meleagris gallopavo)

To determine what role pituitary responsiveness plays in the suppression of gonadotropin level during incubation in the turkey, the ability of the pituitary to release luteinizing hormone (LH) in response to luteinizing hormone-releasing hormone (LHRH) was compared in incubating, laying, and photorefractory birds. In all three groups, the i.m. injection of LHRH (4 micrograms/kg) increased serum LH levels; however, the LH response was markedly enhanced in the incubating turkeys as compared with the laying (6.6-fold increase over preinjection levels vs. 1.9-fold; p less than 0.05) or the photorefractory birds (9.7-fold vs. 3.1-fold; p less than 0.05). The LHRH-induced LH release was also determined in turkeys as they shifted from the laying to the incubating phase of the reproductive cycle. This response increased (p less than 0.05) in magnitude as the birds started to incubate. The high prolactin level of incubating turkeys does not have a depressing effect on LHRH-stimulated LH release; thus, impaired LH response to LHRH is not a mechanism involved in the diminished gonadotropin secretion of incubating turkeys.     

Interval between preovulatory surges of luteinizing hormone increases late in the reproductive period in turkey hens

In turkey hens, the egg production rate is relatively high early during a reproductive period, but declines as the period progresses. Among lines with different egg production potential, the interval between preovulatory surges of LH is the primary determinant of the egg production rate. The main objective of this study was to determine whether the decline in egg production rate late during an egg production period is also associated with a difference in the interval between LH preovulatory surges. A group of photosensitive turkey hens (Early) were photostimulated with continuous light (24L:0D) at 40 wk of age to induce egg laying, and serial blood samples were collected after about 3 wk of egg production. A second group of hens (Late) were housed in floor pens and photostimulated with 14L:10D at 40 wk of age for a normal 36-wk reproduction period and were then switched to 24L:0D lighting for 2 wk before collection of serial blood samples. Continuous light photostimulation was used for at least 2 wk before and during serial blood sampling to avoid potential masking effects of diurnal lighting on the interval between LH surges. The Early (n = 12) and Late (n = 16) hens were cannulated 3 days before being serially bled hourly for 10 days. The mean interval between preovulatory surges of LH was shorter in the Early hens than in the Late hens (26.1 +/- 2.5 h and 34.7 +/- 3.9 h, respectively). The intra-hen LH surge interval coefficient of variation was lower in the Early hens than in the Late hens (7.2% and 18.6%, respectively). The inter-hen LH surge interval coefficient of variation was similar in the Early and Late hens (9.5% and 11.2%, respectively). The incidence of blind surges of LH (those not retrospectively associated with ovipositions) was not different between Early and Late laying hens (8.4% +/- 15.2% and 7.3% +/- 14.6%, respectively). In conclusion, in turkey hens, longer intervals and greater intra-hen variation between LH surges were associated with a poorer rate of egg production late in the reproductive period relative to early in the reproductive period.     

Corpus luteum development and function in cattle with episodic release of luteinizing hormone pulses inhibited in the follicular and early luteal phases of the estrous cycle.

The influence of episodic LH pulses before and subsequent to ovulation on size and function of the corpus luteum (CL) in cattle was examined. Treatments were 1) control; 2) LHRH antagonist starting 2 days before the preovulatory LH surge (Antagonist [Ant] -2); 3) LHRH antagonist at initiation of the preovulatory LH surge (Ant 0); and 4) LHRH antagonist starting 2 days after the preovulatory LH surge (Ant 2). Treatments with an LHRH antagonist were continued until 7 days after the preovulatory surge. Diameter of the CL and concentrations of progesterone were monitored during the luteal phase that ensued after treatment. Maximum average diameters of CL were 9.5, 17.5, 21.6, and 28.8 mm for females from the Ant -2, Ant 0, Ant 2, and control groups, respectively (P < 0. 01). Compared with those in control animals, concentrations of progesterone in plasma were less (P < 0.01) in animals in which release of LH pulses was inhibited by treatment with antagonist. Arbitrary units under the curve for concentrations of progesterone during the luteal phase of the estrous cycle for Ant -2, Ant 0, Ant 2, and control groups were 19.6, 41.6, 43.6, and 142.2, respectively. There was no difference in circulating concentrations of progesterone (P > 0.1) among antagonist-treated groups. In conclusion, episodic release of LH pulses before, during, and after the time of the preovulatory surge of LH may stimulate development and function of the CL in cattle.     

Alteration of gonadotrophin and steroid hormone release, and of ovarian function by a GnRH antagonist in gilts

This study examined the impact of the gonadotrophin-releasing hormone (GnRH) antagonist Antarelix on LH, FSH, ovarian steroid hormone secretion, follicular development and pituitary response to LHRH in cycling gilts. Oestrous cycle of 24 Landrace gilts was synchronised with Regumate (for 15 days) followed by 800 IU PMSG 24h later. In experiment 1, Antarelix (n=6 gilts) was injected i.v. (0.5mg per injection) twice daily on four consecutive days from day 3 to 6 (day 0=last day of Regumate feeding). Control gilts (n=6) received saline. Blood was sampled daily, and every 20 min for 6h on days 2, 4, 6, 8 and 10. In experiment 2, gilts (n=12) were assigned to the following treatments: Antarelix; Antarelix + 50 microg LHRH on day 4; Antarelix + 150 microg LHRH on day 4 or control, 50 microg LHRH only on day 4. Blood samples were collected daily and every 20 min for 6h on days 2, 4 and 6 to assess LH pulsatility. Ovarian follicular development was evaluated at slaughter.Antarelix suppressed (P<0.05) serum LH concentrations. The amount of LH released on days 4-9 (experiment 1) was 8.80 versus 36.54 ngml(-1) (S.E.M.=6.54). The pattern of FSH, and the preovulatory oestradiol rise was not affected by GnRH antagonist. Suppression of LH resulted in a failure (P<0.05) of postovulatory progesterone secretion. Exogenous LHRH (experiment 2) induced a preovulatory-like LH peak, however in Antarelix treated gilts the LH surge started earlier and its duration was less compared to controls (P<0.01). Furthermore, the amount of LH released from day 4 to 5 was lower (P<0.01) in Antarelix, Antarelix + 50 and Antarelix + 150 treated animals compared to controls. No differences were estimated in the number of LH pulses between days and treatment. Pulsatile FSH was not affected by treatment. Mean basal LH levels were lower (P<0.05) after antagonist treatment compared to controls. Antarelix blocked the preovulatory LH surge and ovulation, but the effects of Antarelix were reduced by exogenous LHRH treatment. The development of follicles larger than 4mm was suppressed (P<0.05) by antagonist treatment.In conclusion, Antarelix treatment during the follicular phase blocked preovulatory LH surge, while FSH and oestradiol secretion were not affected. Antarelix failed to alter pulsatile LH and FSH secretor or pituitary responsiveness to LHRH during the preovulatory period.     

Serum levels of luteinizing hormone, prolactin, estradiol and progesterone in laying and nonlaying mallards (Anas platyrhynchos)

Temporal changes of circulating serum hormones were measured to compare the reproductive endocrinology of laying and nonlaying mallards. In this study all sixteen control mallards left with their mates laid eggs, while only one of sixteen mallards stressed by daily movement into new pens, laid eggs. Serum levels of luteinizing hormone (LH), prolactin, estradiol, and progesterone were significantly lower (P less than 0.05) in stressed nonlaying mallards than in laying mallards over the 7-week period. Within 1 week of the rotation treatment, LH concentrations in stressed mallards averaged (means +/- SEM) 2.72 +/- 0.19 ng/ml and were significantly lower (P less than 0.05) than LH levels in the controls (3.62 +/- 0.18 ng/ml). After 7 weeks, injections of luteinizing hormone releasing hormone (LHRH) induced a greater change in circulating LH levels in stressed mallards (2.1 +/- 0.3 ng/ml) than in breeding control mallards (0.9 +/- 0.2 ng/ml). These data demonstrate that the lack of reproduction in stressed mallards was associated with LHRH-sensitive pituitary pools of LH, despite their low concentrations of serum LH. These data suggest that the block in reproduction is a failure of the hypothalamus to produce or release releasing hormones. The serum hormone levels of the control mallards varied temporally with stages in the nesting cycle. LH levels increased with the onset of nesting activity, and showed marked fluctuations during the laying period. LH levels fell at the onset of incubation but increased after loss of clutch. Estradiol levels were highest prior to the laying of the first egg and their peak coincided with the initial nest building behavior of the females. Progesterone levels increased sharply with the laying of the 2nd-4th eggs, decreased sharply with the laying of the 6th egg, and then increased slightly at the end of the nesting cycle. Prolactin levels were initially low but gradually increased with laying and incubation activity, declined with loss of clutch, and increased again with renesting activity. Prolactin levels in the stressed mallards also increased (P less than 0.01) over the 7-week period, but significantly less (P less than 0.05) than in layers.     

Circadian rhythm of the preovulatory surge of luteinizing hormone and its relationships to rhythms of body temperature and locomotor activity in turkey hens

Simultaneous measurements of plasma LH, body temperature, and locomotor activity were made in laying turkey hens and are reported. Blood samples were remotely collected using a jugular cannula system, and body temperature and locomotor activity were remotely monitored using a radiotelemetry system in freely moving laying turkeys. Under a photoschedule of 14L:10D, the period for preovulatory surges of LH was 25.7 +/- 0.4 h while the periods for peak body temperature and onset of sustained locomotor activity were 24.9 +/- 0.4 and 25.7 +/- 0.5 h, respectively. During exposure to constant light, the periods for preovulatory surges of LH, peak body temperature, and onset of sustained locomotor activity increased to 27.9 +/- 0.9, 26.7 +/- 0.7, and 27.4 +/- 0.7 h, respectively. With the 14L:10D photoschedule, initiation of LH surges was restricted to the scotophase, but after 8 days of constant light, initiation of LH surges had dispersed throughout the 24-h subjective day and night. With constant light, the amplitude of the peak body temperature rhythm decreased, while the duration of the locomotor activity rhythm became broadened and, in some birds, disorganized. Peak body temperature and onset of locomotor activity rhythms and LH surges did not coincide, even though peak body temperature, onset of locomotor activity, and LH surges had similar periods. It is concluded that 1) the photoschedule influences the periods of the LH surge, peak body temperature, and onset of locomotor activity; and 2) a specific or direct relationship between the rhythms of LH surge, body temperature, and locomotor activity remains to be determined in laying turkey hens.     

Ovulatory response, and plasma concentrations of luteinizing hormone and progesterone following administration of synthetic mammalian or chicken luteinizing hormone-releasing hormone relative to the first or second ovulation in the sequence of the domestic hen

Experiments were conducted to investigate hypophyseal and follicular competency at two distinct stages of the hen's egg laying sequence: 1) 14 h prior to the first (C1) ovulation of a sequence (27 h following the previous ovulation); and 2) 14 h prior to the second (C2) ovulation of a sequence (13 h following the previous ovulation). When a single dose of mammalian luteinizing hormone-releasing hormone (mLHRH) or chicken luteinizing hormone-releasing hormone (cLHRH) was injected 14 h prior to a C1 ovulation, premature ovulation was induced in 19 of 20 hens. In contrast, ovulation was premature in only 1 of 20 hens when mLHRH or cLHRH was injected 14 h prior to a C2 ovulation. There was no difference between the two stages of the sequence in the amount of luteinizing hormone (LH) released for up to 60 min following a single i.v. injection of 20 micrograms mLHRH. However, only prior to a C1 ovulation did LH levels further increase to reach preovulatory concentrations. By contrast, progesterone (P4) concentrations were increased within the first 60 min to a lesser extent in hens injected prior to a C2 ovulation compared to a C1 ovulation. In C2-injected birds, P4 fell to levels that were not different from vehicle-injected controls by 45 to 60 min following injection, whereas P4 secretion was maintained in hens injected prior to a C1 ovulation. We suggest that the lack of sustained LH secretion following treatment with either species of LHRH 14 h prior to a C2 ovulation is related to follicular immaturity with respect to ability to produce and secrete P4. At the dosage administered, there was no difference in the ability of mLHRH compared to cLHRH to release LH at either stage of the sequence. Finally, two successive injections of mLHRH at 14 and 13 h prior to a C2 ovulation induced premature ovulation in 6 of 11 hens. It is suggested that LH, and possibly P4, exerts a priming effect on the largest preovulatory follicle to initiate fully potentiated P4 production and secretion.     

Effects of pulsatile infusion of luteinizing hormone-releasing hormone on luteinizing hormone secretion and ovarian function in hypophysial stalk-transected beef heifers

Hypothalamic regulation of luteinizing hormone (LH) secretion and ovarian function were investigated in beef heifers by infusing LH-releasing hormone (LHRH) in a pulsatile manner (1 microgram/ml; 1 ml during 1 min every h) into the external jugular vein of 10 hypophysial stalk-transected (HST) animals. The heifers were HST approximately 30 mo earlier. All heifers had increased ovarian size during the LHRH infusion. The maximum ovarian size (16 +/- 2.7 cm3) was greater (P less than 0.01) than the initial ovarian size (8 +/- 1.4 cm3). Ovarian follicular growth occurred in 4 of 10 HST heifers in response to pulsatile LHRH infusion. In 2 heifers, an ovarian follicle developed to preovulatory size, but ovulation occurred in only 1 animal after the frequency of LHRH was increased (1 microgram every 20 min during 8 h). In blood samples obtained at 20-min intervals every 5th day, LH concentrations in peripheral serum remained consistently low (0.9 ng/ml) and nonepisodic in the 10 HST heifers during infusion of vehicle on the day before beginning LHRH. In 7 of 10 HST animals, episodic LH secretion occurred in response to pulsatile infusion of LHRH. In 3 of these long-term HST heifers, however, serum LH remained at basal levels and the isolated pituitary seemingly was unresponsive to pulsatile infusion of LHRH as indicated by sequential patterns of gonadotropin secretion obtained at 5-day intervals. These results indicate that pulsatile infusion of LHRH induces LH release in HST beef heifers.     

Effects of third ventricular injection of beta-endorphin on luteinizing hormone surges in female rat: sites and mechanisms of opioid actions in the brain

The effects of third ventricular injection of beta-endorphin (beta-EP) on spontaneous, brain stimulation-induced and estrogen-induced LH surges were studied in the adult female rat. It was found that beta-EP blocked the preovulatory surge of LH release and ovulation, while it did not affect LH release in response to LH-RH injection. The site of the beta-EP blockade of ovulation was proved to be in the brain. Beta-EP completely blocked ovulatory LH release induced by the electrochemical stimulation of the medial amygdaloid nucleus and medial septum-diagonal band of Broca, but failed to block ovulation due to the stimulation of the medial preoptic area (MPO) or median eminence, though serum LH levels after the MPO stimulation were inhibited by beta-EP. In the spayed rats treated with estradiol benzoate (EB) on Day 1 and 4 of experiment, beta-EP given on Day 5 blocked the LH surge that normally occurred on that day and led to a compensatory surge of LH on the following day. Moreover, the LH surge on Day 5 was inhibited by beta-EP given either on Day 1 or Day 4. Present data suggest that beta-EP may act in inhibiting the preovulatory LH surges not only by suppressing the preoptic-tuberal LH-RH activities but also by affecting the initiation and development of stimulatory feedback of estrogen in the central nervous system.     

Thermal stress reduces serum luteinizing hormone and bioassayable hypothalamic content of luteinizing hormone-releasing hormone in hens

Studies were conducted to evaluate the effects of acute (24 h) thermal stress on anterior pituitary function in hens. Circulating levels of luteinizing hormone (LH) were measured and the ability of the pituitary to respond to luteinizing hormone-releasing hormone (LHRH) challenge was determined. Moreover, bioassayable hypothalamic LHRH content was assessed by using dispersed anterior pituitary cells. In two separate experiments, circulating levels of LH were reduced in hens exposed to acute thermal stress (35 degrees C). Injection of LHRH did not result in significant differences in release of LH between normothermic and hyperthermic hens. However, the hypothalamic content of bioassayable hypothalamic releasing activity from hyperthermic hens were significantly reduced compared with normothermic hens. Taken together, these data suggest that the reproductive decline in the acutely heat-stressed hen is mediated by reduced LH releasing ability of the hypothalamus.     

A potential code of luteinizing hormone-releasing hormone-induced calcium ion responses in the regulation of luteinizing hormone secretion among individual gonadotropes

Luteinizing hormone-releasing hormone (LHRH) induces two Ca2+ responses in single gonadotropes: a Ca2+ spike/plateau or oscillation. Similar receptor-mediated Ca2+ signals have been reported in many cell types but their functional significance is obscure. Accordingly, we have determined the concentration-response properties of LHRH-induced luteinizing hormone (LH) release at the single cell level. We demonstrate a critical single cell LHRH threshold for LH release. Each gonadotrope had a particular LHRH threshold value and a range of different single cell thresholds was distributed in the gonadotrope population. The physiological significance of the threshold was demonstrated by a striking reduction (delta ED50 = 153 nM) of the LHRH threshold immediately before the preovulatory surge of LH release. The metestrous phenotype of secretion resembled a quantal process in contrast with the graded process of the proestrous phenotype. That is, the quantity of hormone secreted per metestrous gonadotrope was independent of LHRH concentration and more all-or-none than graded. The LHRH threshold and the quantal secretion process of metestrous gonadotropes was further studied by measuring cytosolic Ca2+ using fura-2 and digital imaging microscopy. We provide evidence suggesting that the Ca2+ spike/plateau and oscillation are the respective responses to subthreshold and suprathreshold concentrations of LHRH. It is proposed therefore that the Ca2+ oscillation and spike/plateau response form a binary intracellular signaling code that functions as an on-off switch. It is further proposed that this potential code unraveled here for the regulation of hormone secretion may also regulate other gonadotrope functions. Thus, while the Ca2+ spike/plateau response is strongly associated with LH release, it may be associated with reduced levels of LH-beta mRNA, and reduced numbers of LHRH receptors. Conversely, while the Ca2+ oscillation appears to be unrelated to LH release, it may be associated with increased levels of LH-beta mRNA, and increased numbers of LHRH receptors. This model may explain in molecular terms the long-standing observation that an invariant, albeit pulsatile, pattern of LHRH release is sufficient to support the preovulatory surge of LH release.     

Pituitary luteinizing hormone and prolactin messenger ribonucleic acid levels are inversely related in laying and incubating turkey hens.

The relationships of prolactin (PRL) and LH messenger (m) RNA to serum and pituitary content were determined for turkey hens at different phases of the reproductive cycle. In the nonphotostimulated, reproductively inactive hen, serum and pituitary PRL content and pituitary PRL mRNA levels were low. All three PRL values rose after photostimulation and peaked during the incubation phase. Relative to nonphotostimulated hens, hyperprolactinemic incubating hens showed 220-, 11-, and 57-fold increases in serum PRL, pituitary PRL content, and pituitary PRL mRNA levels, respectively. These peak levels declined 80-, 3-, and 6-fold, respectively, in photorefractory hens. In contrast to PRL levels, serum LH, pituitary LH, and pituitary LH beta-subunit mRNA levels did not change as dramatically. Serum LH showed no significant changes for the different reproductive phases. Pituitary LH peaked after photostimulation and declined to its lowest level in incubating hens. Pituitary LH-beta mRNA abundance was highest in photostimulated and laying hens and lowest in incubating and photorefractory hens. These results demonstrate that the abundance of LH-beta and PRL mRNA shows an inverse relationship in photostimulated/laying and incubating turkey hens.     

Serum concentrations of prolactin, oestrogen and LH during the perioestrous period in prepubertal gilts induced to ovulate and mature gilts

The temporal relationships of serum prolactin, oestrogen and LH concentrations during the perioestrous period were compared in prepubertal gilts induced to ovulate by PMSG and hCG and in mature gilts. In Exp. 1, 2 sustained prolactin surges, beginning 4 days and 1 day before the preovulatory LH surge, occurred in all mature gilts. A single preovulatory prolactin surge occurred in 3 prepubertal gilts, starting just before the preovulatory LH surge, but 4 prepubertal gilts had neither a prolactin nor an LH surge. A status (prepubertal or mature) versus time interaction (P less than 0.01) was detected for serum prolactin concentrations. A preovulatory oestrogen surge occurred in all gilts but was of lesser magnitude (P less than 0.01) and duration (P less than 0.05) in the prepubertal gilts without prolactin and LH surges compared to mature gilts and of lesser magnitude (P less than 0.01) compared to prepubertal gilts with prolactin and LH surges. The relative timing of the oestrogen surge in prepubertal gilts corresponded with that of mature gilts when adjusted to the LH surge (if present) but was delayed (P less than 0.01) in all prepubertal gilts if standardized to the hCG injection. In Exp. 2, mature gilts were examined to determine whether 2 perioestrous prolactin surges were characteristic of all cycling gilts. Of 9 gilts, 8 exhibited an initial prolactin surge 4-5 days before oestrus and 5/9 gilts exhibited a periovulatory prolactin surge. The presence of 2 perioestrous serum prolactin surges was not a requirement for subsequent pregnancy maintenance.(ABSTRACT TRUNCATED AT 250 WORDS)     

Hypothalamic deafferentation and luteinizing hormone-releasing hormone effects on secretion of luteinizing hormone in prepubertal pigs

The control of luteinizing hormone (LH) secretion was investigated in ovariectomized, prepubertal Yorkshire pigs by comparing the effects of anterior (AHD), complete (CHD), and posterior (PHD) hypothalamic deafferentation to sham-operated controls (SOC). Gilts (n = 16) were assigned randomly to treatments, fitted with an indwelling jugular catheter, and ovariectomized 2 days before deafferentation or sham-operation (Day 0). Blood for radioimmunoassay (RIA) of LH was collected sequentially at 20-min intervals for a period of 2 h before and 24, 48, 72, and 96 h after hypothalamic deafferentation or SOC. Episodic LH release after AHD or CHD was abolished (p less than 0.01), but not after PHD or SOC. Concentrations of serum LH in AHD and CHD dropped (p less than 0.01) at 24 and 48 h after surgery. Levels of LH before and after surgery in PHD and SOC were similar (p greater than 0.05). Infusion of 25 micrograms LH-releasing hormone (LHRH) i.v. at 72 and 96 h after hypothalamic deafferentation and SOC increased (p less than 0.01) serum LH to peak levels within 15 min. after infusion; LH returned to basal levels 60-80 min later. By 96 h after surgery, LH response to LH-releasing hormone (LHRH) was less in AHD and CHD as compared with the response at 72 h postinjection. Concentrations of LH in PHD and SOC were similar (p greater than 0.05) at 72 and 96 h, respectively. The results from this study clearly indicate that neural stimuli originating or traversing the neural areas rostral to the median eminence are required for secretion of LH in the pig.(ABSTRACT TRUNCATED AT 250 WORDS)