Pituitary receptors for GnRH and estradiol, and pituitary content of gonadotropins in beef cows. II. Changes during the postpartum period

Pregnant beef heifers (n = 24) were assigned randomly to four groups and slaughtered at day 1, 15, 30 or 45 postpartum. The day prior to slaughter blood samples were taken from each cow every 15 min for 8 hr. The anterior pituitary gland, preoptic area (POA) and medial basal hypothalamus (HYP) were collected from each cow. Contents of gonadotropin-releasing hormone (GnRH) in extracts of POA and HYP, and luteinizing hormone (LH) and follicle-stimulating hormone (FSH) in extracts of anterior pituitary were quantified by radioimmunoassay. In the anterior pituitary gland, membrane receptors for GnRH were quantified by a standard curve technique and cytosolic receptors for estradiol were quantified by saturation analysis. Concentrations of LH, FSH and prolactin in serum were quantified by radioimmunoassay. Only one cow of eight had a pulse of LH during the 8 hr bleeding period on day 1 postpartum. This increased to 8 pulses in 6 cows on day 30 postpartum. Contents of GnRH in POA (15.0 +/- 3.2 ng) and HYP (14.0 +/- 2.0 ng) did not change significantly during the postpartum period. Pituitary content of LH was low following parturition (.2 +/- .1 mg/pituitary) and increased significantly through day 30 postpartum (1.2 +/- .1 mg/pituitary). Pituitary content of FSH did not change over the postpartum period. Receptors for both GnRH (.9 +/- .2 pmoles/pituitary) and estradiol (5.0 +/- .9/moles/pituitary) were elevated on day 15 postpartum, possibly increasing the sensitivity of the anterior pituitary gland to these hormones and leading to an increased rate of synthesis of LH that restored pituitary content to normal by day 30 postpartum.     

Pituitary effects of steroid hormones on secretion of follicle-stimulating hormone and luteinizing hormone

Steroid hormones have a profound influence on the secretion of the gonadotropins, follicle-stimulating hormone (FSH) and luteinizing hormone (LH). These effects can occur as a result of steroid hormones modifying the secretion of gonadotropin-releasing hormone (GnRH) from the hypothalamus, or a direct effect of steroid hormones on gonadotropin secreting cells in the anterior pituitary gland. With respect to the latter, we have shown that estradiol increases pituitary sensitivity to GnRH by stimulating an increase in expression of the gene encoding the GnRH receptor. Since an estrogen response element (ERE) has not been identified in the GnRH receptor gene, this effect appears to be mediated by estradiol stimulating production of a yet to be identified factor that in turn enhances expression of the GnRH receptor gene. However, the importance of estradiol for enhancing pituitary sensitivity to GnRH during the periovulatory period is questioned because an increase in mRNA for the GnRH receptor precedes the pre-ovulatory rise in circulating concentrations of estradiol. In fact, it appears that the enhanced pituitary sensitivity during the periovulatory period may occur as a result of a decrease in concentrations of progesterone rather than due to an increase in concentrations of estradiol. Estradiol also is capable of altering secretion of FSH and LH in the absence of GnRH. In a recent study utilizing cultured pituitary cells from anestrous ewes, we demonstrated that estradiol induced a dose-dependent increase in secretion of LH, but resulted in a dose-dependent decrease in the secretion of FSH. We hypothesized that the discordant effects on secretion of LH and FSH might arise from estradiol altering the production of some of the intrapituitary factors involved in synthesis and secretion of FSH. To examine this hypothesis, we measured amounts of mRNA for activin B (a factor known to stimulate synthesis of FSH) and follistatin (an activin-binding protein). We found no change in the mRNA for follistatin after treatment of pituitary cells with estradiol, but noted a decrease in the amount of mRNA for activin B. Thus, the inhibitory effect of estradiol on secretion of FSH appears to be mediated by its ability to suppress the expression of the gene encoding activin.     

Effects of active immunization against GnRH on LH, FSH and prolactin storage, secretion and response to their secretagogues in pony geldings

Six pony geldings were actively immunized against GnRH conjugated to bovine serum albumin (BSA) to study 1) the relative dependency of LH and FSH storage, secretion and response to GnRH analog on GnRH bioavailability and 2) the effects of reduced GnRH bioavailability on GnRH storage in the hypothalamus. Five geldings were immunized against BSA. Geldings were immunized in December and 4, 8, 14, 20, 26 and 32 wk later. Ponies immunized against GnRH had increased (P less than .01) GnRH binding in plasma within 6 wk. By June, plasma concentrations of LH and FSH in ponies immunized against GnRH had decreased (P less than .02) by 86 and 59%, respectively, relative to ponies immunized against BSA. The LH response to an injection of GnRH analog, which did not bind to anti-GnRH antibodies, was reduced (P less than .005) by 90% in ponies immunized against GnRH relative to ponies immunized against BSA. In contrast, the FSH response to GnRH analog was similar (P greater than .1) for both groups. Immunization against GnRH reduced (P less than .05) weight of the anterior pituitary (AP) by 31%, LH content in AP by 91% and FSH content in AP by 55% relative to ponies immunized against BSA. There was no effect of GnRH immunization on prolactin characteristics or on GnRH concentrations in the median eminence, preoptic area or body of the hypothalamus.(ABSTRACT TRUNCATED AT 250 WORDS)     

Pituitary responsiveness to GnRH, hypothalamic content of GnRH and pituitary LH and FSH concentrations immediately preceding puberty in gilts

To determine whether pituitary concentrations of luteinizing hormone (LH), follicle-stimulating hormone (FSH) or hypothalamic content of gonadotropin releasing hormone (GnRH) change before puberty, 40 prepubertal gilts averaging 7 mo of age were slaughtered before or on the second, third or fourth day after relocation and boar exposure. Some gilts responded to relocation and boar exposure as indicated by swollen vulvae, turgid uteri and enlarged ovarian follicles at the time of slaughter. Pituitary concentrations of LH and FSH and hypothalamic content of GnRH were similar between gilts that responded to relocation and boar exposure and gilts that did not respond. In addition, boar exposure and relocation had no effect on pituitary concentrations of LH and FSH or on hypothalamic content of GnRH. To determine whether pituitary responsiveness to GnRH changes before puberty, a third experiment was conducted in which 72 gilts were injected with 400 micrograms of GnRH either before or on the second, third or fourth day after relocation and boar exposure. In gilts that subsequently responded (i.e., ovulated) as a result of relocation and boar exposure, pituitary responsiveness to GnRH was reduced as compared with gilts that failed to ovulate after relocation and boar exposure. Peak concentrations of serum LH after GnRH injection were 4.6 +/- 1.3 vs 9.8 +/- .8 ng/ml for responders vs nonresponders. Peak serum FSH after GnRH injection was also lower for responders than for nonresponders (29.5 +/- 4.2 vs 41.2 +/- 2.4 ng/ml). When compared with controls, relocation and boar exposure did not significantly affect GnRH-induced release of LH and FSH.(ABSTRACT TRUNCATED AT 250 WORDS)     

The effect of pulsatile gonadotropin-releasing hormone and estradiol administration on luteinizing hormone and follicle-stimulating hormone concentrations in pituitary stalk-sectioned ovariectomized pony mares

Hourly pulses of gonadotropin-releasing hormone (GnRH) or bi-daily injections of estradiol (E2) can increase luteinizing hormone (LH) secretion in ovariectomized, anestrous pony mares. However, the site (pituitary versus hypothalamus) of positive feedback of estradiol on gonadotropin secretion has not been described in mares. Thus, one of our objectives involved investigating the feedback of estradiol on the pituitary. The second objective consisted of determining if hourly pulses of GnRH could re-establish physiological LH and FSH concentrations after pituitary stalk-section (PSS), and the third objective was to describe the declining time trends of LH and FSH secretion after PSS. During summer months, ovariectomized pony mares were divided into three groups: Group 1 (control, n = 2), Group 2 (pulsatile GnRH (25 μg/hr), n = 3), and Group 3 (estradiol (5 mg/12 hr), n = 3). All mares were stalk-sectioned and treatment begun immediately after stalk-section. Blood samples were collected every 30 min for 8 h on the day before surgery (DO) and 5 d post surgery (D5) to facilitate the comparison of gonadotropin levels before and after pituitary stalk-section. Additionally, jugular blood samples were collected every 12 hr beginning the evening of surgery, allowing for evaluation of the gonadotropin secretory time trends over the 10 d of treatment. On Day 10, animals were euthanized to confirm pituitary stalk-section and to submit tissue for messenger RNA analysis (parallel study). Plasma samples were assayed for LH and FSH by RIA. Mean LH secretion decreased from Day 0 to Day 5 in Groups 1 and 3, whereas LH secretion tended (P < 0.08) to decrease in Group 2 mares. On Day 5, LH was higher (P < 0.01) in Group 2 (17.26 ± 3.68 ng/ml; LSMEANS ± SEM), than either Group 1 (2.65 ± 4.64 ng/ml) or group 3 (4.28 ± 3.68 ng/ml). Group 1 did not differ from Group 3 on Day 5 (P < 0.40). Similarly, mean FSH levels decreased in all groups after surgery, yet Group 2 mares had significantly (P < 0.001) higher FSH concentrations (17.66 ± 1.53 ng/ml) than Group 1 or Group 3 (8.34 ± 1.84 and 7.69 ± 1. 63 ng/ml, respectively). Regression analysis of bi-daily LH and FSH levels indicated that the time trends were not parallel. These findings indicate: 1) Pituitary stalk-section lowered LH and FSH to undetectable levels within 5 d after surgery, 2) pulsatile administration of GnRH (25 μg/hr) maintained LH and FSH secretion, although concentrations tended to be lower than on Day 0, and 3) E2 did not stimulate LH or FSH secretion.     

Distribution of orexin B and its relationship with GnRH in the pig hypothalamus

Increasing evidence suggests that orexins--hypothalamic neuropeptides--act as neurotransmitters or neuromediators in the brain, regulating autonomic and neuroendocrine functions. Orexins are closely associated with gonadotropin-releasing hormone (GnRH) neurons in the preoptic area and alter luteinizing hormone (LH) release, suggesting that they regulate reproduction. Here, we investigated the distribution of orexin B (immunohistochemical technique) and the relationship between orexin B and GnRH containing fibres and neurons in the pig hypothalamus using double immunofluorescence and laser-scanning confocal microscopy. Orexin B immunoreactive neurons were mainly localized in the perifornical area (PeF), dorsomedial hypothalamic nucleus (DMH), zona incerta (ZI) and the posterior hypothalamic area (PH), with a sparser distribution in the preoptic and anterior hypothalamic area. Immunoreactive fibres were distributed throughout the central nervous system. Approximately 30% GnRH neurons were in close contact with orexin B immunoreactive fibres, among these approximately 6% of GnRH neurons co-localized with orexin B perikarya in the region between the caudal preoptic area and the anterior hypothalamic area. Orexin B may regulate reproduction by altering LH release in the hypothalamus.     

Effect of CD14/TLR4 antagonist on GnRH/LH secretion in ewe during central inflammation induced by intracerebroventricular administration of LPS

Background: Immune stress induced by lipopolysaccharide(LPS) influences the gonadotropin-releasing hormone(GnRH)/luteinizing hormone(LH) secretion. Presence of LPS interacting Toll-like receptor(TLR) 4 in the hypothalamus may enable the direct action of LPS on the GnRH/LH secretion. So, the aim of the study was to investigate the influence of intracerebroventricular(icv) injection of TLR4 antagonist on GnRH/LH secretion in anestrous ewes during LPS-induced central inflammation. Animals were divided into three groups icv-treated with: Ringer-Locke solution, LPS and TLR4 antagonist followed by LPS.Results: It was demonstrated that TLR4 antagonist reduced LPS-dependent suppression of GnRH gene expression in the preoptic area and in the medial basal hypothalamus, and suppression of receptor for GnRH gene expression in the anterior pituitary gland. It was also shown that TLR4 antagonist reduced suppression of LH release caused by icv injection of LPS. Central administration of LPS stimulated TLR4 gene expression in the medial basal hypothalamus.Conclusions: It was indicated that blockade of TLR4 prevents the inhibitory effect of centrally acting LPS on the GnRH/LH secretion. This suggests that some negative effects of bacterial infection on the hypothalamic-pituitary-gonadal axis activity at the hypothalamic level may be caused by central action of LPS acting through TLR4.     

Alterations in the ability of the bovine pituitary gland to secrete gonadotropins in vitro during the first follicle-stimulating hormone increase of the estrous cycle and in response to exogenous steroids

The objective was to determine if the endocrine status of the animal dictates the responsiveness of gonadotrophs to estradiol, activin, inhibin and follistatin; hormones implicated in the differential release of luteinizing hormone (LH) and follicle-stimulating hormone (FSH). Bovine pituitaries were obtained at 13 (n=8), 30 (n=24) and 66 (n=8) h after the onset of estrus, corresponding to before, during and the end of the first FSH increase of the estrous cycle which follows the pre-ovulatory gonadotropin surge in heifers. Heifers slaughtered at 30 h received no treatment, or were treated with progesterone with or without estradiol before slaughter to suppress the first transient FSH increase. Secretion of FSH from cultured pituitary cells, reflecting the prior in vivo status, was greater (P<0.01) at 30 h than 13 or 66 h, whereas, LH secretion was less (P<0.01) at 13 h compared with 30 h. Treatment with exogenous steroids decreased (P<0.05) the pituitary gland's ability to subsequently secrete FSH and LH. Inhibin and, to a greater extent, estradiol decreased (P<0.01) mean FSH secretion but increased (P<0.05) mean LH secretion. These findings suggest that estradiol and inhibin both have the ability to differentially modulate basal gonadotropin secretion during the first FSH increase of the bovine estrous cycle. Differential regulation of LH and FSH is mediated via an alteration in gonadotropin biosynthesis and basal secretion. Furthermore, the secretory capability of cultured pituitary cells and basal gonadotropin secretion reflect the prior endocrine status of the animal from which pituitaries were obtained.     

Nutritionally induced anovulation in beef heifers: ovarian and endocrine function preceding cessation of ovulation.

Angus x Hereford heifers were used to determine endocrine and ovarian function preceding nutritionally induced anovulation. Six heifers were fed to maintain body condition score (M), and 12 heifers were fed a restricted diet (R) until they became anovulatory. Starting on d 13 of an estrous cycle, heifers were given PGF2alpha every 16 d thereafter to synchronize and maintain 16 d estrous cycles. Ovarian structures of M and R heifers were monitored by ultrasonography daily from d 8 to ovulation (d 1 of the subsequent cycle) until R heifers became anovulatory. Concentrations of LH and FSH were quantified in serum samples collected every 10 min for 8 h on d 2 and 15 (48 h after PGF2alpha), and estradiol and IGF-I were quantified in daily plasma samples from d 8 to 16 during the last ovulatory cycle (Cycle -2) and the subsequent anovulatory cycle (Cycle -1). During the last two cycles before anovulation, M heifers had 50% larger (P < .0001) ovulatory follicles than R heifers and 61% greater (P < .0001) growth rate of the ovulatory follicles. There was a treatment x cycle x day effect (P < .001) for concentrations of estradiol. The preovulatory increase in estradiol occurred in the R and M heifers during Cycle -2 but only in M heifers during Cycle -1. A treatment x cycle x day effect (P < .05) influenced LH concentrations. During Cycle -2, LH concentrations were similar for M and R heifers, but during Cycle -1, M heifers had greater LH concentrations than did R heifers. Concentrations of FSH were greater (P < .05) in R than M heifers after induced luteolysis when R heifers failed to ovulate. There was a treatment x cycle interaction (P < .05) for IGF-I concentrations, and M heifers had 4.7- and 8.6-fold greater IGF-I concentrations than did R heifers during Cycle -2 and -1, respectively. We conclude that growth rate and diameter of the ovulatory follicle, and concentrations of LH, estradiol, and IGF-I are reduced before the onset of nutritionally induced anovulation in beef heifers.     

Postpartum reproduction in protein restricted beef cows: effect on the hypothalamic-pituitary-ovarian axis

The influence of dietary CP on circulating LH and anterior pituitary and hypothalamic function was examined. In Exp. 1, 28 cows were randomly assigned to four treatment groups: adequate CP (ADQ; .96 kg/d) or deficient CP (DEF; .32 kg/d) beginning at 90, 60 and 30 d before parturition and continued at a 33% increase in feed consumption after parturition. Cows were bled at 15-min intervals for 8 h on d 20, 40 and 60 after parturition. Pituitaries were collected on d 62 to analyze GnRH receptor numbers and gonadotropin content. Frequency of pulsatile LH release increased (P less than .05) from 20 to 60 d in ADQ cows. Basal and mean LH were not affected (P greater than .10) by CP restriction or by days after parturition. Crude protein did not affect pituitary GnRH receptors (P greater than .10), but it did affect pituitary LH content, FSH content and FSH concentration (P less than .05). In Exp. 2, 28 cows were assigned to treatment groups as in Exp. 1. All cows were challenged with GnRH (.22 micrograms/kg BW) at 20, 40 and 60 d after parturition and were bled every 30 min for 6 h. Responsiveness to GnRH increased with increased time after parturition (P less than .07). Deficient CP decreased GnRH-induced LH release (P less than .05). In Exp. 3, 12 cows were randomly assigned to ADQ or DEF CP beginning 120 d before parturition. All cows received 1 mg estradiol-17 beta (E2) on d 19, 39 and 59 after parturition and were bled every 30 min for 14 h beginning 14 h following E2. Response to E2 was unaffected by CP restriction (P greater than .10), whereas time to E2-induced LH peak decreased as time after parturition increased in ADQ cows (P less than .05). Results suggest that delayed return to estrus in CP-deficient postpartum beef cows might be due to reduced gonadotropin release from the anterior pituitary and decreased anterior pituitary responsiveness to GnRH.     

Hypothalamic deafferentation in prepuberal beef heifers: Effects of gonadotropin-releasing hormone and estradiol benzoate on luteinizing hormone secretion

Hypothalamic control of luteinizing hormone (LH) secretion was investigated in crossbred beef heifer calves by comparing anterior (AHD), posterior (PHD), and complete (CHD) hypothalamic deafferentation with sham operated controls (SOC). Heifers (n = 16) were fitted with an indwelling jugular catheter for 6 days before cranial surgery, and assigned randomly to treatments. Blood for radioimmunoassay of LH was collected sequentially at 15-min intervals during an 8-h period on days ? 1 before and day 6 after hypothalamic deafferentation or sham operation. On the day of surgery, blood samples were collected sequentially at 15-min intervals 2 h before induction of anesthesia and throughout surgery and early recovery. Seven months after hypothalamic deafferentation, all experimental and sham operated heifers were ovariectomized and treated with vegetable oil (i.m.) plus saline (i.v.), vegetable oil plus gonadotropin releasing hormone (GnRH), estradiol benzoate (EB, 1 mg) in vegetable oil. After ovariectomy basal plasma concentrations of LH increased (P < 0.01) compared with the low circulating hormone levels before ovariectomy. The amplitude of LH response to GnRH was greater (P < 0.01) in CHD and PHD when compared with SOC and AHD heifers. Injection of EB failed to induce a LH surge in CHD and PHD 900–1100 min later when compared with the robust response seen in SOC and AHD heifers. Injection of EB plus GnRH elicited LH release in all deafferentated and sham operated heifers. These results indicate a transient change in LH secretion after AHD or CHD in prepuberal heifers with intact ovaries. After OVX, the integrity of the neural connection of the posterior hypothalamus is required for EB-induced LH release in beef heifers.     

Endocrine responses and ovarian changes in inseminated dairy heifers after an injection of a GnRH agonist 11 to 13 days after estrus.

Our objective in this study was to determine endocrine responses and changes in ovarian structures after a single injection of a GnRH agonist in Holstein dairy heifers (n = 38). Heifers were inseminated and received (i.m.) either saline or 200 micrograms of fertirelin acetate once on d 11, 12, or 13 after estrus (d 0). Blood was collected at 15- to 30-min intervals for 6 h after the injection to determine concentrations of LH, FSH, estradiol (E), and progesterone (P) in serum and once daily for 8 to 12 d after the injection to determine concentrations of E and P. Pregnancy rates were 58% (11 of 19) in both treatment groups. Diameter of the corpus luteum and numbers and appearance of ovarian follicles were determined by real-time ultrasonography on d-1 through 5 after injection. No treatment-induced ovulations or changes in the number of ovarian follicles were observed after the injection of the GnRH agonist. More (P less than .05) of the largest follicles within heifers receiving fertirelin acetate showed changes in their appearance on at least the 1st d after injection (6 of 10 vs 1 of 9 control heifers). Fertirelin acetate induced release of LH and FSH from the pituitary within 15 min of injection; both hormones reached peak concentrations at 120 min and then returned to pretreatment concentrations by 300 to 360 min after injection.(ABSTRACT TRUNCATED AT 250 WORDS)     

Central injection of exogenous IL-1β in the control activities of hypothalamic-pituitary-gonadal axis in anestrous ewes

This study was performed to determine the effect of intracerebroventricular (icv) injection of interleukin (IL)-1β on the gene expression, translation and release of gonadotropin-releasing hormone (GnRH) and the GnRH receptor (GnRHR) gene expression in the hypothalamus of anestrous ewes. In the anterior pituitary gland (AP), the expression of genes encoding: GnRHR, β subunits of luteinizing hormone (LH) and folliculotropic hormone (FSH) was determined as well as the effect of IL-1β on pituitary gonadotropins release. The relative mRNA level was determined by real-time PCR, GnRH concentration in the cerebrospinal fluid (CSF) was assayed by ELISA and the plasma concentration of LH and FSH were determined by radioimmunoassay. Our results showed that icv injection of IL-1β (10 or 50 μg/animal) decreased the GnRH mRNA level in the pre-optic area (POA) (35% and 40% respectively; p ≤ 0.01) and median eminence (ME) (75% and 70% respectively; p ≤ 0.01) and GnRHR gene expression in ME (55% and 50% respectively; p ≤ 0.01). A significant decrease in GnRHR mRNA level in the AP in the group treated with the 50 μg (60%; p ≤ 0.01) but not with the 10 μg dose was observed. The centrally administrated IL-1β lowered also GnRH concentration in the CSF (60%; p ≤ 0.01) and reduced the intensity of GnRH translation in the POA (p ≤ 0.01). It was not found any effect of icv IL-1β injection upon the release of LH and FSH. However, the central injection of IL-1β strongly decreased the LHβ mRNA level (41% and 50%; p ≤ 0.01; respectively) and FSHβ mRNA in the case of the 50 μg dose (49%; p ≤ 0.01) in the pituitary of anestrous ewes. These results demonstrate that the central IL-1β is an important modulator of the GnRH biosynthesis and release during immune/inflammatory challenge.     

Relationships among LH, FSH and prolactin secretion, storage and response to secretagogue and hypothalamic GnRH content in ovariectomized pony mares administered testosterone, dihydrotestosterone, estradiol, progesterone, dexamethasone or follicular fluid

Thirty-five ovariectomized pony mares were used to study the relationships among luteinizing hormone (LH), follicle stimulating hormone (FSH) and prolactin (PRL) concentrations in blood (secretion), in pituitary (storage) and in blood after secretagogue administration, as well as the content of gonadotropin releasing hormone (GnRH) in hypothalamic areas, under various conditions of steroidal and nonsteroidal treatment. Five mares each were treated daily for 21 d with vegetable shortening (controls), testosterone (T; 150 micrograms/kg of body weight, BW), dihydrotestosterone (DHT; 150 micrograms/kg BW), estradiol (E2; 35 micrograms/kg BW), progesterone (P4; 500 micrograms/kg BW), dexamethasone (DEX; 125 micrograms/kg BW) or charcoal-stripped equine follicular fluid (FF; 10 ml). Secretagogue injections (GnRH and thyrotropin releasing hormone, TRH, at 1 and 4 micrograms/kg of BW, respectively) were given one d prior to treatment and again after 15 d of treatment. Relative to controls, treatment with T, DHT and DEX reduced (P less than .05) LH secretion, storage and response to exogenous GnRH, whereas treatment with E2 increased (P less than .05) these same characteristics. Treatment with P4 reduced (P less than .05) only LH secretion. Treatment with T, DHT, E2 and DEX reduced (P less than .05) FSH secretion, whereas treatment with P4 increased (P less than .05) it and FF had no effect (P greater than .1). All treatments increased (P less than .05) FSH storage, whereas only treatment with T and DHT increased (P less than .05) the FSH response to exogenous GnRH. Other than a brief increase (P less than .05) in PRL secretion in mares treated with E2, secretion of PRL did not differ (P greater than .1) among groups. Only treatment with E2 increased (P less than .01) PRL storage, yet treatment with T or DHT (but not E2) increased (P less than .05) the PRL response to exogenous TRH. Content of GnRH in the body and pre-optic area of the hypothalamus was not affected (P greater than .1) by treatment, whereas treatment with T, E2 and DEX increased (P less than .1) GnRH content in the median eminence. For LH, secretion, storage and response to exogenous GnRH were all highly correlated (r greater than or equal to .77; P less than .01). For FSH, only storage and response to exogenous GnRH were related (r = .62; P less than .01). PRL characteristics were not significantly related to one another. Moreover, the amount of GnRH in the median eminence was not related (P greater than .1) to any LH or FSH characteristic.     

Changes in the concentration of hypothalamic and hypophyseal receptors for estradiol in pregnant and postpartum ewes

The sensitivity of the hypothalamic-pituitary axis to estradiol was evaluated by monitoring changes in the concentration of receptors for estradiol during gestation and the postpartum period of the ewe. Ewes that had been bred during anestrus so that lambing would occur during the breeding season were killed on d 50 and 140 of gestation or on d 2, 13, 22 and 35 postpartum. Total cytosolic receptors for estradiol were quantified in the anterior and posterior hypothalamus, median eminence, anterior pituitary and cerebral cortex. Concentration and total number of receptors for estradiol in the anterior and posterior hypothalamus and anterior pituitary gland during gestation through d 13 postpartum were similar but increased (P less than .05) by d 22 and 35 postpartum. Changes in the concentration of receptors for estradiol were tissue-specific because stage of reproduction did not influence the number of receptors for estradiol in the median eminence or cerebral cortex. Increases in the concentration of receptors for estradiol in hypothalamic and pituitary tissues on d 22 postpartum immediately preceded the onset of normal ovarian cyclicity in ewes. From these data we conclude that resumption of reproductive cycles in postpartum ewes occurs about the time concentrations of receptors for estradiol in the anterior pituitary gland and hypothalamus increase.     

山羊GnRH和促性腺激素的释放特点

通过外科手术分别连续收集活体山羊中黄体期及早卵泡期的垂体门脉血样和外周血样，经放射免疫测定，山羊中黄体期和早卵泡期的促性腺激素释放激素（ＧｎＲＨ）、促黄体生成素（ＬＨ）和促卵泡素（ＦＳＨ）均呈波动式释放。在早卵泡期，ＦＳＨ单位时间内波动次数和血浆平均水平显著高于中黄体期；ＧｎＲＨ与ＬＨ的波动型基本一致，ＦＳＨ的变化不太规则。表明山羊垂体促性腺激素的释放受丘脑下部ＧｎＲＨ的调节，但ＦＳＨ似乎还存在其他调节机理。     

哺乳动物下丘脑-垂体-卵巢轴的研究进展

哺乳动物的下丘脑、垂体和卵巢分泌的激素在功能上相互作用,构成一个完整的神经内分泌生殖调节体系,即下丘脑垂体卵巢轴,它在生殖活动中起着主要的调节作用。下丘脑中分布的GnRH神经元可以分泌GnRH,GnRH调节垂体中促性腺激素细胞分泌促性腺激素FSH和LH,促性腺激素作用于卵巢受体,引起雌激素和孕酮分泌并影响生殖活动。从组织学角度上研究,下丘脑垂体卵巢轴中的结构,如GnRH神经元、促性腺激素细胞、卵泡随周期性变化而呈现出不同的形态结构和分泌特点。因此,对以上各种细胞的研究是探讨其所分泌激素的基础,而下丘脑垂体卵巢轴中的各种激素的研究则是了解和控制动物繁殖机能的关键。     

Androgen and estradiol effects on gonadotropin secretion and response to GnRH in ovariectomized pony mares

In Exp. 1, 16 long-term ovariectomized pony mares were used to determine the effects of treatment with estradiol benzoate (EB) and dihydrotestosterone (DHT) benzoate alone, and in combination, on secretion of follicle stimulating hormone (FSH) and luteinizing hormone (LH) in daily blood samples and after three consecutive injections of gonadotropin releasing hormone (GnRH). Administration of EB alone, or in combination with DHT, every other day for 11 d reduced (P less than .05) concentrations of FSH and increased (P less than .05) concentrations of LH in daily blood samples, and increased (P less than .05) the secretion of both gonadotropins after administration of GnRH. Treatment with DHT alone had no effect (P greater than .10) on LH or FSH concentrations in daily blood samples and no effect on the LH response to exogenous GnRH. There was no interaction (P greater than .10) between DHT and EB treatment for any hormonal characteristic. In Exp. 2, the control mares and mares treated with DHT in Exp. 1 were equally allotted to treatment with vehicle or testosterone propionate (TP) every other day for six injections, and then GnRH was administered as in Exp. 1. Treatment with TP had no effect (P greater than .10) on LH or FSH concentrations in daily blood samples but increased (P less than .05) the FSH response to exogenous GnRH, confirming our findings in previous experiments. It is concluded that the TP-induced stimulation of FSH secretion after exogenous GnRH in ovariectomized mares may involve estrogens produced from aromatization of the injected androgen.(ABSTRACT TRUNCATED AT 250 WORDS)     

Concentrations of ovarian and pituitary hormones following prostaglandin F2 alpha-induced luteal regression in ewes varies with day of the estrous cycle at treatment

Prostaglandin F2 alpha (PGF2 alpha) was injected on d 5, 8 or 11 postestrus in ewes to determine how stage of the estrous cycle would affect PGF2 alpha-induced changes in concentrations of ovarian and pituitary hormones and intervals to the onset of estrus and the preovulatory surge of luteinizing hormone (LH). Initial concentrations of progesterone and average values during the 12 h after PGF2 alpha were related positively to the day of cycle on which PGF2 alpha was administered. Patterns of decline in progesterone after injection of PGF2 alpha were similar among the 3 d. Concentrations of LH in plasma increased in a similar manner from 0 to 12 h in all ewes. After 12 h LH continued to increase, plateaued or declined in ewes treated on d 5, 8 or 11, respectively. Initial concentrations of follicle stimulating hormone (FSH) in plasma were related positively to day of treatment. After treatment with PGF2 alpha, FSH increased within 2 h on d 5 but declined by that time on d 8 or 11. Concentrations of estradiol following treatment did not vary with day. The onset of estrus and the preovulatory surge of LH occurred at 36 and 35, 40 and 45, and 48 and greater than 48 h in ewes treated on d 5, 8 or 11, respectively. It is concluded that: 1) the initial increase in LH is dependent on a decrease in plasma progesterone and 2) differences in patterns of secretion of gonadotropins before the preovulatory surge of LH might be caused by differences in progesterone or progesterone:-estradiol ratio when luteal regression is induced on different days of the estrous cycle.     

Influence of estradiol,progesterone, and nutrition on concentrations of gonadotropins and GnRH receptors,and abundance of mRNA for GnRH receptors and gonadotropin subunits in pituitary glands of beef cows

Nutritionally induced anovulatory cows (n = 28) were used to determine the effect of steroids on regulation of synthesis and secretion of gonadotropins. Anovulatory cows were ovariectomized and received intravaginal inserts containing estradiol (E2), progesterone (P4), E2 and P4 (E2P4), or a sham intravaginal insert (C) for 7 d. Concentrations of luteinizing hormone (LH) and follicle-stimulating hormone (FSH) were quantified in serum and E2 and P4 were quantified in plasma. Cows were exsanguinated within 1 to 2 h after removal of intravaginal inserts and pituitary glands were collected and stored at -80 degrees C until messenger ribonucleic acid (mRNA) for gonadotropin-releasing hormone receptor (GnRH-R) and gonadotropin subunits, pituitary content of GnRH-R, and LH and FSH were quantified. Pituitary glands from five proestrous cows were harvested to compare gonadotropin characteristics between ovariectomized, anovulatory cows and intact cows. Plasma concentrations of E2 were greater (P < 0.05) in E2-treated cows than in sham-treated cows. Concentrations of P4 were greater (P < 0.05) in cows treated with P4 than in sham-treated cows. Mean serum concentrations of LH and FSH were not significantly influenced by steroid treatments. However, frequency of LH pulses of ovariectomized, nutritionally induced anovulatory cows was increased (P < 0.05) by treatment with E2 and amplitude of LH pulses was greater (P < 0.05) in cows treated with E2 or P4 than in cows treated with E2P4 or sham-treated. Quantity of mRNA for LHbeta in the pituitary gland was greater when cows were treated with P4. Concentrations of LH in the pituitary gland were not affected by steroid treatments; however, pituitary concentrations of FSH were less (P < 0.1) in E2 cows than in sham-treated cows. The number of GnRH-R was increased (P < 0.05) in cows treated with E2, but P4 treatment did not influence the number of GnRH-R. Abundance of mRNA for GnRH-R, common alpha-subunit, and FSHbeta were not affected by treatments. Pituitary concentrations of LH were greater (P < 0.05) and concentrations of FSH were less (P < 0.05) in proestrous cows than in ovariectomized, anovulatory cows treated with or without steroids. Abundance of mRNA for GnRH-R, common alpha-subunit, LHbeta and FSHbeta were similar for proestrous and anovulatory cows. We conclude that treatment of nutritionally induced anovulatory cows with progesterone and estradiol may cause pulsatile secretion of LH.