LH Secretion In Rats Research Articles

Patterns of secretion of GnRH, LH and FSH during the postovulatory period in mares: mechanisms prolonging the LH surge.

To study the mechanisms responsible for the unusually slow decline of the ovulatory LH surge in mares, secretion patterns of GnRH, LH and FSH were monitored in pituitary venous blood collected every 2 or 5 min for 10.5-18.0 h from five mares on the third (n = 4) or fifth day after ovulation (first sampling period). To determine the effectiveness of progesterone negative feedback, mares were then given a luteolytic dose of a prostaglandin analogue (PGF2 alpha) and pituitary venous sampling (every 2 or 5 min for 16 h) recommenced 20-22 h later (second sampling period). During the declining arm of the LH surge, large peaks (detected by the Cluster algorithm) of concurrent LH and FSH secretion occurred infrequently, with four peaks being detected in a combined sampling period of 75 h. Outside the peaks, LH or FSH secretion continued (as assessed by a pituitary to jugular-venous concentration ratio > or = 1.25) during 46% +/- 13 or 40% +/- 10, respectively, of the sampling period. GnRH immunoactivity was detected during each spontaneous gonadotrophin peak, but at other times was generally at assay sensitivity. After PGF2 alpha, plasma progesterone fell (ng ml-1, mean +/- SEM; first sampling period: 8.6 +/- 0.8; second; 2.0 +/- 0.3; P = 0.001) and the frequency of LH (P < 0.05) and FSH (P < 0.02) peaks rose, with 28 peaks detected for each hormone in a total of 80 h sampling. Peaks in LH were smaller during the second period, with decreases observed in maximum (P = 0.027) and mean (P = 0.025) secretion rates. Maximum GnRH secretion rate during peaks also declined (P = 0.010); however, the decrement (-30 +/- 6%) was less than that in maximum LH secretion rate (-82 +/- 5%; P = 0.040), suggesting that other factors contribute to the reduced LH peak amplitude. In summary, gonadotrophin peak frequency during the downswing of the surge in mares is slow, as in the midluteal phase, and the slow rate of decline in peripheral gonadotrophin concentrations is due, at least in part, to continued secretion between pulses. Moreover, progesterone negative feedback is highly effective in early dioestrus, in that lessening it without complete removal markedly accelerates gonadotrophin pulse frequency.

Patterns of gonadotropin secretion in cyclic Finn ewes selected for and against high ovulation rate

AbstractA comparison of the patterns of LH and FSH secretion was undertaken in lines of Finn sheep selected for and against high ovulation rate. Mean ovulation rate was significantly higher in the high line ewes (mean 4·1) compared with both the control (mean 2·5) and low line (mean 2·7) ewes (P &lt;0·01). The pre-ovulatory LH peak occurred significantly earlier in the high line ewes (mean 52·1 h) compared with both the control (mean 65·0 h) and low line (mean 59·0 h) ewes (P &lt; 0·05). While mean LH pulse frequency and overall mean LH concentrations were both significantly higher during the follicular compared with the luteal phase of the cycle (P &lt; 0·05), there were no consistent relationships between patterns of pulsatile LH secretion and ovulation rate among the three selection lines. Plasma FSH concentrations remained significantly higher over the entire follicular phase of the oestrous cycle in the high line ewes compared with both the control and low line ewes (P &lt; 0·05). It is suggested that the ovulation rate achieved by high line ewes may be causally related to their higher follicular phase FSH concentrations.

Combined deficits in the somatotropic and gonadotropic axes in healthy aging men: An appraisal of neuroendocrine mechanisms by deconvolution analysis

We have employed deconvolution analysis of 24-h serum GH, LH, and FSH concentration profiles to evaluate the nature of age-associated changes in GH, LH, and FSH secretion and disappearance in healthy older individuals. Our findings indicate that healthy aging results in a diminished frequency of detectable GH secretory episodes and an apparent decrease in estimated GH half life in men. These combined effects lead to a significant reduction in mean serum GH concentrations over 24 h. In contrast, we find that LH secretory burst frequency tends to increase with a reciprocal decrease in amplitude as serum total and free testosterone concentrations decline during healthy aging. There is also a prolongation of LH secretory burst duration, and a putative increase in basal LH secretion rates. The changes in LH secretory dynamics are specific, because the half life of immunoradiometric LH and the mass of hormone secreted per burst are not altered. In contrast, the apparent half life of FSH increases with age, and the secretory burst duration may decrease. This spectrum of distinct neuroendocrine sequelae of aging has interesting implications to both investigative and therapeutic strategies for modifying the somatotropic and gonadotropic axes in aging individuals.

Effects of a non-steroidal pure antioestrogen, ZM 189,154, on oestrogen target organs of the rat including bones.

ZM 189,154 ([1RS,2RS]-2-(4-hydroxyphenyl)-2-methyl- 1-[9-(4,4,5,5,5-penta-fluoropentyl)sulphinylnonyl]-1,2,3,4- tetrahydronaphth-6-ol) is a non-steroidal pure antioestrogen. It has a high relative affinity for the oestrogen receptor, completely blocks the trophic action of oestradiol (OE2) on the uterus in immature and ovariectomized (OVX) adult rats and, in the latter, also completely blocks the trophic action of OE2 on vagina, bone and growth rate. ZM 189,154 displays no intrinsic oestrogen-agonist activity on uterus, vagina, bone, LH secretion or growth rate in OVX rats. Differential sensitivity of OE2-regulated processes was more apparent in intact rats. Daily doses of 0.6 mg/kg per day of ZM 189,154 blocked ovulation; 2 mg/kg per day achieved maximal uterine atrophy but did not affect bone density or growth rate; 10 mg/kg per day produced a broader spectrum of effects (reduced bone density, increased basal LH, slightly increased growth rate), but the magnitude of these was smaller than after ovariectomy; the 10 mg/kg dose also produced multiple ovarian follicular cysts. The failure of ZM 189,154 to achieve complete ovariectomy-like effects in intact rats may be due to the action of ovarian factors other than OE2, or to the circulating OE2 levels resulting from the disturbance to ovarian function posing too strong a challenge to the antagonist.

Hypothalamic neuropeptide-Y gene expression increases before the onset of the ovarian steroid-induced luteinizing hormone surge

Neuropeptide-Y (NPY), a hypothalamic peptide, is involved in stimulation of LHRH and LH surges during proestrus and those induced by ovarian steroids in ovariectomized (ovx) rats. The NPY neurons that reside in the arcuate nucleus of the medial basal hypothalamus (MBH) and accumulate 17 beta-estradiol participate in the initiation of LHRH and LH surges. To determine whether NPY synthesis is altered in conjunction with the LH surge, we studied the dynamic changes in prepro-NPY mRNA levels in the MBH in association with the LH surge elicited by estradiol benzoate (EB) alone or by progesterone (P) in EB-primed ovx rats. Five days after ovariectomy, rats received oil or EB (30 micrograms/rat) at 1000 h on day 0. On day 2, these rats were injected with either oil or P (2 mg/rat) at 1000 h. Rats were killed before (1000 h) and at 2-h intervals after oil or P injection. The MBHs were dissected out and processed for determination of prepro-NPY mRNA levels by solution hybridization/RNase protection assay using a cRNA probe. Although in control ovx rats, prepro-NPY mRNA levels remained unchanged between 1000-1600 h, prepro-NPY mRNA levels showed dynamic changes in steroid-primed rats. In the EB-primed rats, prepro-NPY mRNA levels rose significantly (100%) at 1200 and 1400 h before the LH rise at 1600 h, and the levels remained elevated up to 1800 h. After P injection to the EB-primed rats, this response was further augmented, with a slightly different temporal pattern. Prepro-NPY mRNA levels rose at 1400 h (600%) before the onset of the LH rise at 1600 h and declined steadily to significantly lower values at 1800 h, coincident with the highest rate of LH secretion. These studies demonstrate dynamic shifts in hypothalamic NPY gene expression in association with the LH (LHRH) surge, and that maximal increases occur before the onset of the LH rise, but thereafter, NPY gene expression diverged in the two ovarian-steroid treatment models. These findings along with previous evidence of similar antecedent increases in NPY content in the median eminence, followed by release, suggest that augmented NPY synthesis and release are two temporally dissociable neural events for the LHRH and LH surges.

Hypothalamic neuropeptide-Y gene expression increases before the onset of the ovarian steroid-induced luteinizing hormone surge.

Neuropeptide-Y (NPY), a hypothalamic peptide, is involved in stimulation of LHRH and LH surges during proestrus and those induced by ovarian steroids in ovariectomized (ovx) rats. The NPY neurons that reside in the arcuate nucleus of the medial basal hypothalamus (MBH) and accumulate 17 beta-estradiol participate in the initiation of LHRH and LH surges. To determine whether NPY synthesis is altered in conjunction with the LH surge, we studied the dynamic changes in prepro-NPY mRNA levels in the MBH in association with the LH surge elicited by estradiol benzoate (EB) alone or by progesterone (P) in EB-primed ovx rats. Five days after ovariectomy, rats received oil or EB (30 micrograms/rat) at 1000 h on day 0. On day 2, these rats were injected with either oil or P (2 mg/rat) at 1000 h. Rats were killed before (1000 h) and at 2-h intervals after oil or P injection. The MBHs were dissected out and processed for determination of prepro-NPY mRNA levels by solution hybridization/RNase protection assay using a cRNA probe. Although in control ovx rats, prepro-NPY mRNA levels remained unchanged between 1000-1600 h, prepro-NPY mRNA levels showed dynamic changes in steroid-primed rats. In the EB-primed rats, prepro-NPY mRNA levels rose significantly (100%) at 1200 and 1400 h before the LH rise at 1600 h, and the levels remained elevated up to 1800 h. After P injection to the EB-primed rats, this response was further augmented, with a slightly different temporal pattern. Prepro-NPY mRNA levels rose at 1400 h (600%) before the onset of the LH rise at 1600 h and declined steadily to significantly lower values at 1800 h, coincident with the highest rate of LH secretion. These studies demonstrate dynamic shifts in hypothalamic NPY gene expression in association with the LH (LHRH) surge, and that maximal increases occur before the onset of the LH rise, but thereafter, NPY gene expression diverged in the two ovarian-steroid treatment models. These findings along with previous evidence of similar antecedent increases in NPY content in the median eminence, followed by release, suggest that augmented NPY synthesis and release are two temporally dissociable neural events for the LHRH and LH surges.

Androgen-receptor blockade enhances pulsatile luteinizing hormone production in late pubertal males: evidence for a hypothalamic site of physiologic androgen feedback action.

To determine potential mechanisms by which androgens alter gonadotropin secretion and elimination in the pubertal male, we administered the potent nonsteroidal androgen receptor blocking agent, flutamide, to eight males with Tanner IV or V genital development. Venous blood samples were obtained every 10 min for 24 h and assayed for LH by a sensitive and high-precision fluorimmunoassay. Subjects were studied before and after the administration of flutamide. Deconvolution analysis was used to assess specific pulsatile LH secretory characteristics and estimate LH production and metabolic clearance rates quantitatively. After antagonism of endogenous androgen action, mean 24-h serum LH concentrations increased significantly. An increased mean 24-h LH production rate, without evident changes in serum LH half-life, accounted for the increase in average serum LH levels. The increased daily secretion rate of LH was in turn due to both an augmented mass of LH released per secretory episode and increased frequency of secretory events. There was no demonstrable change in the maximal rate of LH secretion attained within each secretory event. Serum concentrations of total testosterone, free-testosterone, and 17 beta-estradiol all increased during blockade of androgen action. Administration of the antiandrogen had no measurable effect on the pituitary response to a single maximally effective dose of exogenous gonadotropin releasing hormone (GnRH). These results indicate that, in the late pubertal male, endogenous androgen exerts negative feedback control of gonadotropin secretion primarily at a hypothalamic site reflected by regulation of the frequency of pulsatile LH secretion.(ABSTRACT TRUNCATED AT 250 WORDS)

Role of neuropeptide-Y in episodic luteinizing hormone release in ovariectomized rats: an excitatory component and opioid involvement.

We tested the hypothesis that hypothalamic neuropeptide-Y (NPY) is an excitatory signal in the episodic secretion of LH in ovariectomized (ovx) rats and that the suppression of LH secretion that consistently follows intracerebroventricular administration of NPY is due to concurrent release of opioids or CRH, both previously shown to readily inhibit LH release. In the first experiment, ovx rats received continuous intraventricular infusion of either serum containing NPY antibodies (NPY-Ab) or normal rabbit serum (control) at dilutions of 1:5 or 1:1. NPY-Ab infusion at a 1:5 dilution significantly decreased mean plasma LH levels and LH pulse amplitude without affecting LH pulse frequency over a 3-h period of observation. However, infusion of relatively more concentrated NPY-Ab (1:1) markedly decreased not only mean plasma LH levels and LH pulse amplitude, but also the frequency of LH episodes. In the next experiment, we observed that intraventricular administration of NPY (0.2 nmol) suppressed LH release for 60 min. However, blockade of opiate receptors with iv infusion of naloxone (2 mg/h) before and after NPY injection completely counteracted the NPY-induced inhibition of LH release. On the other hand, prior blockade of the CRH receptors with alpha-helical CRH-(9-41) (25 or 100 micrograms/rat) was ineffective in reversing the inhibitory LH response of NPY (0.125 nmol). These results together with our previous demonstration of morphological communication between NPY and beta-endorphin neurons, show that suppression of LH by exogenous NPY in ovx rats may result from concurrent stimulation of opioids, primarily beta-endorphin. However, diminution of all parameters of episodic LH secretion by NPY-Ab affirms the notion that the NPY network is a physiologically important excitatory component of the hypothalamic pulse generator circuitry that regulates episodic LH secretion in rats.

Involvement of Central Corticotropin‐Releasing Factor (CRF) in Suckling‐Induced Inhibition of Luteinizing Hormone Secretion in Lactating Rats

The lack of ovulation and the inhibition of reproductive functions observed in many species during lactation is closely related to the intensity of the suckling stimulus. However, the mechanisms by which suckling inhibits hypothalamic GnRH and pituitary LH secretion in rats are still unclear. Since we recently demonstrated that suckling is a persistent stimulus to the adrenococortical system of the rat, we tested the hypothesis that suckling-induced activation of central CRF release may mediate the associated inhibition of GnRH secretion. Lactating females were ovariectomized (OVX) on day 2 of lactation, and equipped with icv guide cannula on day 2 and indwelling jugular catheters on day 5 before testing on day 7. Lactating females were separated from their pups for 24 h prior to the suckling test with the following pretreatments: 1) icv injection with artificial CSF (aCSF) or a specific CRF antagonist, alpha-helical CRF (9-41), (25 micrograms/rat, CRF-AX) 15 min prior to pup reunion or 2) iv injection of normal sheep serum (NSS) or CRF antiserum (CRF-AB) 4 h prior to pup reunion. Plasma ACTH, LH and PRL concentrations were determined prior to and at various intervals after pup reunion. After 3 h of suckling, LH and PRL responses to a bolus injection of GnRH (10 ng/rat) were measured; a bolus injection of Angiotensin II (AII, 5 micrograms/rat) was administered after 4 h to test for ACTH responses. Non-lactating females injected with GnRH and AII were used as controls.(ABSTRACT TRUNCATED AT 250 WORDS)

Role of neuropeptide-Y in episodic luteinizing hormone release in ovariectomized rats: an excitatory component and opioid involvement

We tested the hypothesis that hypothalamic neuropeptide-Y (NPY) is an excitatory signal in the episodic secretion of LH in ovariectomized (ovx) rats and that the suppression of LH secretion that consistently follows intracerebroventricular administration of NPY is due to concurrent release of opioids or CRH, both previously shown to readily inhibit LH release. In the first experiment, ovx rats received continuous intraventricular infusion of either serum containing NPY antibodies (NPY-Ab) or normal rabbit serum (control) at dilutions of 1:5 or 1:1. NPY-Ab infusion at a 1:5 dilution significantly decreased mean plasma LH levels and LH pulse amplitude without affecting LH pulse frequency over a 3-h period of observation. However, infusion of relatively more concentrated NPY-Ab (1:1) markedly decreased not only mean plasma LH levels and LH pulse amplitude, but also the frequency of LH episodes. In the next experiment, we observed that intraventricular administration of NPY (0.2 nmol) suppressed LH release for 60 min. However, blockade of opiate receptors with iv infusion of naloxone (2 mg/h) before and after NPY injection completely counteracted the NPY-induced inhibition of LH release. On the other hand, prior blockade of the CRH receptors with alpha-helical CRH-(9-41) (25 or 100 micrograms/rat) was ineffective in reversing the inhibitory LH response of NPY (0.125 nmol). These results together with our previous demonstration of morphological communication between NPY and beta-endorphin neurons, show that suppression of LH by exogenous NPY in ovx rats may result from concurrent stimulation of opioids, primarily beta-endorphin. However, diminution of all parameters of episodic LH secretion by NPY-Ab affirms the notion that the NPY network is a physiologically important excitatory component of the hypothalamic pulse generator circuitry that regulates episodic LH secretion in rats.

Dynamic changes in hypothalamic angiotensin II levels and release in association with progesterone-induced luteinizing hormone surge.

To test the hypothesis that brain angiotensin II (Ang II) may be involved in the preovulatory release of LH on proestrus, we evaluated the pattern of changes in hypothalamic Ang II levels and release in ovariectomized (ovx) rats treated sequentially with estrogen and progesterone. This is an experimental paradigm that reliably evokes dynamic changes in hypothalamic LHRH levels in association with LH hypersecretion, simulating the LH surge on proestrus. Rats were ovx and after 4 weeks received estradiol benzoate followed by progesterone 2 days later at 1000 h. We observed that in these progesterone-treated rats, serum LH levels were low until 1400 h, but thereafter, the rate of LH secretion increased and remained elevated at 1600 h when the experiment was terminated. In these rats, hypothalamic Ang II levels increased abruptly at 1330 h and returned rapidly to baseline levels before the onset of LH surge. Also, a similar pattern in hypothalamic Ang II levels occurred at 1500 h with the rise and peak serum LH levels in the late afternoon. In the second experiment, Ang II levels in the cerebrospinal fluid (CSF) of rats similarly pretreated with ovarian steroids were evaluated. Again, CSF Ang II levels rose abruptly to a peak at 1330 h and returned to baseline range preceding the expected rise in serum LH. Thereafter, no further change in CSF Ang II levels was detected during the period of LH hypersecretion. In the third experiment, perfusates were collected from a push-pull cannula aimed at the paraventricular nucleus in ovx rats similarly treated with ovarian steroids. A peak of Ang II was observed at 1330 h and a later peak at 1430 h. A comparison with LH profiles indicated that these peaks in Ang II levels were evident before and during the LH surge. Thus, in three separate experiments, the results showed that rapid dynamic changes in hypothalamic Ang II levels and release occur in association with the progesterone-induced LH surge in estrogen-primed ovx rats. These findings support the previous observations that Ang II can stimulate LHRH and LH release. Since similar, temporally correlated changes occur in hypothalamic neuropeptide Y and LHRH, the peptides involved in the induction of LH surge, these results are in agreement with the hypothesis that Ang II-expressing neurons may play an important role in the hypothalamic circuitry responsible for stimulation of LH surge in ovarian steroid-treated ovx rats.

Dynamic changes in hypothalamic angiotensin II levels and release in association with progesterone-induced luteinizing hormone surge

To test the hypothesis that brain angiotensin II (Ang II) may be involved in the preovulatory release of LH on proestrus, we evaluated the pattern of changes in hypothalamic Ang II levels and release in ovariectomized (ovx) rats treated sequentially with estrogen and progesterone. This is an experimental paradigm that reliably evokes dynamic changes in hypothalamic LHRH levels in association with LH hypersecretion, simulating the LH surge on proestrus. Rats were ovx and after 4 weeks received estradiol benzoate followed by progesterone 2 days later at 1000 h. We observed that in these progesterone-treated rats, serum LH levels were low until 1400 h, but thereafter, the rate of LH secretion increased and remained elevated at 1600 h when the experiment was terminated. In these rats, hypothalamic Ang II levels increased abruptly at 1330 h and returned rapidly to baseline levels before the onset of LH surge. Also, a similar pattern in hypothalamic Ang II levels occurred at 1500 h with the rise and peak serum LH levels in the late afternoon. In the second experiment, Ang II levels in the cerebrospinal fluid (CSF) of rats similarly pretreated with ovarian steroids were evaluated. Again, CSF Ang II levels rose abruptly to a peak at 1330 h and returned to baseline range preceding the expected rise in serum LH. Thereafter, no further change in CSF Ang II levels was detected during the period of LH hypersecretion. In the third experiment, perfusates were collected from a push-pull cannula aimed at the paraventricular nucleus in ovx rats similarly treated with ovarian steroids. A peak of Ang II was observed at 1330 h and a later peak at 1430 h. A comparison with LH profiles indicated that these peaks in Ang II levels were evident before and during the LH surge. Thus, in three separate experiments, the results showed that rapid dynamic changes in hypothalamic Ang II levels and release occur in association with the progesterone-induced LH surge in estrogen-primed ovx rats. These findings support the previous observations that Ang II can stimulate LHRH and LH release. Since similar, temporally correlated changes occur in hypothalamic neuropeptide Y and LHRH, the peptides involved in the induction of LH surge, these results are in agreement with the hypothesis that Ang II-expressing neurons may play an important role in the hypothalamic circuitry responsible for stimulation of LH surge in ovarian steroid-treated ovx rats.

The hypophyseogonadal system of male rats with diabetes: an experimental study

The hypophyseogonadal system of male rats with streptozotocin-induced diabetes was studied. The hypophyseal sensitivity to LH releasing hormone was analyzed in vitro and concentrations of sex hormone nuclear receptors in the adenohypophysis, participating in gonadotropin secretion regulation according to a negative feedback mechanism, measured. Streptozotocin injection reduced blood testosterone concentration and levels of androgen nuclear receptors in the rat hypophysis. Blood LH and FSH levels in the rats with diabetes were virtually the same as in intact animals. In vitro experiments have demonstrated that diabetes development in rats did not influence the level of LH basal secretion by the hypophysis. The maximal response to LH releasing hormone was recorded in the control males in 3h incubation, whereas the rate of LH secretion in the experimental animals did not differ from the normal one. The authors suggest that changed mechanism of the hypothalamo-hypophyseo-gonadal system regulation in experimental diabetes is related to the hypophyseal disorders, involving reduction of the LH-RH-stimulated gonadotropin release and of the testosterone receptor levels, this resulting in poor reproductive function control according to the negative feedback principle.

Advancement of Postnatal Pulsatile Luteinizing Hormone Secretion in the Bull Calf by Pulsatile Administration of Gonadotropin-Releasing Hormone during Infantile Development1

The transition from infantile to prepubertal development in bull calves coincides with the onset of postnatal LH secretion following differentiation of pituitary gonadotropes. Increased pulsatile secretion of hypothalamic GnRH may contribute to maturational development of the hypothalamic-pituitary axis. To evaluate the role of GnRH pulse frequency in mediating changes in pituitary response that may lead to altered LH release, serum LH concentrations in Holstein bull calves treated with exogenous GnRH were examined. Nine calves received 200 ng GnRH for 5 min every hour from 1-6 wk of age (GnRH-T), while 5 animals served as controls. Weekly through 6 wk of age, jugular venous blood was collected every 10 min for 8 h. Blood samples from GnRH-T animals were taken during 2 exogenous GnRH pulses followed by 6 h of sampling without exogenous GnRH. Mean serum LH concentrations were greater (p less than or equal to 0.01) in GnRH-T calves than in controls at 2 through 6 wk of age. LH pulse height did not change in either group during the study period. However, LH pulse frequency in GnRH-T calves increased with age (P less than or equal to 0.01), and at 4-6 wk of age, the rate of pulsatile LH secretion was greater (p less than or equal to 0.01) in GnRH-T calves than in controls. Pituitary LH content increased 2.4-fold after GnRH infusion (p less than or equal to 0.01); this increase was associated with a doubling of the pituitary LH-beta mRNA concentration found in controls.(ABSTRACT TRUNCATED AT 250 WORDS)

Prolactin does not Mediate the Suppressive Effect of the Suckling Stimulus on Luteinizing Hormone Secretion in Ovariectomized Lactating Rats.

The plasma LH concentration in ovariectomized lactating rats is low for 14 days postpartum, while the prolactin concentration is high during this period. We examined the effect of the inhibition of increased prolactin secretion with bromocriptine (CB-154) on the LH secretion in lactating rats ovariectomized on day 2 (day 0 = day of parturition). Blood samples were collected through an indwelling atrial cannula every day. LH levels were kept low until day 9 in lactating rats injected daily with CB-154 (0.6 mg/day, s.c.). The duration of the period during which LH secretion was suppressed was shorter in lactating rats treated with CB-154 than in saline-injected controls. The replacement with ovine prolactin by means of a mini-osmotic pump (0.3 mg/day, s.c.) in CB-154-treated lactating rats restored the duration of LH suppression. In rats deprived of their pups on day 2, the LH concentration rose immediately after removal of the pups and the LH level was not significantly different between rats treated with CB-154, ovine prolactin and saline, indicating that neither the CB-154 treatment nor the high level of prolactin alone has any effect on LH secretion in rats deprived of their pups. The present results clearly demonstrate that prolactin does not mediate the suppressing effect of the suckling stimulus on LH secretion in early lactation and support our theory that the suckling stimulus controls the LH and prolactin secretion independently at the hypothalamic level.

The implication of opiates in the glucocorticoid-mediated inhibition of LH secretion in rats.

The present study was designed to test the effect of acute administration of dexamethasone on the postcastration and gonadotrophin-releasing hormone (GnRH)-induced rise of LH, and to examine whether the inhibitory action of glucocorticoids on LH secretion was mediated by the opioid system. Rats were castrated and injected 10-10.5 h later in a first set of experiments with saline, dexamethasone (250 micrograms/rat), nalmefene (2 mg/kg body weight) or nalmefene plus dexamethasone. The response to GnRH was tested 11 h after castration (time 0) in all groups. Dexamethasone caused a significant (P less than 0.005) decrease in basal serum concentrations of LH, while saline and nalmefene did not induce any change. The administration of dexamethasone preceded by nalmefene increased basal concentrations of LH (1.6 times), with an effect significantly greater than that of dexamethasone (P less than 0.001), nalmefene (P less than 0.05) or saline (P less than 0.005) alone. GnRH induced a significant (P less than 0.001) increase in serum concentrations of LH in all groups. In a second set of experiments, administration of naloxone (2 mg/kg body weight) increased LH levels (P less than 0.05) and similarly reversed the inhibitory effect of dexamethasone on LH.(ABSTRACT TRUNCATED AT 250 WORDS)

Effects of Gonadectomy on the in Vitro and in Vivo Gonadotropin Responses to Gonadotropin-Releasing Hormone in Male and Female Rats*

Pituitary gonadotropin responses to GnRH were measured using both in vitro and in vivo methods to investigate the contribution of increased pituitary responsiveness to GnRH in generating the rise in serum gonadotropin levels after gonadectomy. We compared in vitro GnRH-stimulated secretion rates of LH and FSH of perifused pituitaries obtained from intact female (metestrous) and male rats, and rats gonadectomized 2 or 6 days earlier. GnRH pulses (peak amplitude, 50, 500, or 5000 ng/ml; frequency, one per h) caused significant dose-dependent increases in gonadotropin secretion rates. However, gonadectomy resulted in decreased secretion rates of LH and FSH. Similar findings were observed for in vivo serum gonadotropin responses to a single iv injection of GnRH (males received 250 or 1000 ng; females received 1000 or 4000 ng). These results indicate that increases in serum LH and FSH levels 2 or 6 days after gonadectomy are not mediated by increased responses of the rat anterior pituitary to GnRH. We have also shown that perifused pituitaries from proestrous and diestrous rats exhibit significantly higher GnRH-stimulated gonadotropin secretion rates than pituitaries from metestrous and estrous rats. Therefore, we tested the effect of in vivo pretreatment with 17 beta-estradiol (E2) or testosterone (T) in both female and male rats on the in vitro secretion of LH and FSH. Rats were gonadectomized and received a sc Silastic implant containing E2, T, or no steroid as a control 6 days before perifusion. Perifused pituitaries received pulses of GnRH (peak amplitude, 50 ng/ml; frequency, one per h). In vivo pretreatment with E2, but not T, caused significant increases of in vitro LH and FSH secretion rates for pituitaries of both sexes. Overall, our data demonstrate that gonadectomy does not cause increases in LH and FSH secretory responses to GnRH, and that prior exposure to E2 in vivo has a major stimulatory influence on the in vitro secretion of both gonadotropins regardless of sex.

Estriol affects prolactin and LH secretion in rats

The effects of estriol on serum prolactin (PRL) and LH levels, on the pituitary response to TRH and LHRH and on the synthesis and release of PRL from the anterior pituitary gland were investigated in female rats. The increase of serum PRL levels after estradiol administration was found to be associated with an increase of glutamic acid decarboxylase (GAD) and GABA-transaminase (GABA-T) in the hypothalamus. Thus, a study was carried out on the effects of estradiol and estriol on PRL secretion and on GAD, GABA-T and gamma-amino butyric acid (GABA) in the hypothalamus and the anterior pituitary. Under basal and TRH-stimulated conditions, estriol increased serum PRL levels, decreased basal serum LH levels, and increased the response to LHRH, in terms of LH release. Estradiol and estriol increased the synthesis and release of 3H-PRL from hemipituitary glands in incubations of pretreated animals. Both estrogens induced hyperprolactinemia, concomitantly with an increase of hypothalamic GAD and GABA-T activity. Estriol increased hypothalamic GABA concentration, but did not modify GABA concentration in the pituitary glands. Our results show that estriol, at relatively high doses, seems to be active in increasing PRL synthesis and release and in decreasing serum LH levels; it can also modify pituitary response to TRH and LHRH stimulation.

Suppression of Basal and GnRH-Stimulated Gonadotropin Secretion Rate in vitro by GnRH Antagonist: Differential Effects on Metestrous and Proestrous Pituitaries

LH and FSH basal secretion rates (BSR) in vitro of pituitary fragments from female rats at metestrus or following ovariectomy parallel changes seen in serum gonadotropins. These findings led us to suspect that in vitro BSR of fragments depended on prior exposure to GnRH or lingering GnRH in the culture. We compared pituitaries removed from rats in two cycle stages, proestrus (PRO) and metestrus (MET). Rats were given a single injection of a potent GnRH antagonist, Antag-WY 45760-[Ac-beta(2)-D-Nal1,4-F-D-Phe2,D-Trp3,D-Arg6]-LHRH, or oil prior to sacrifice at 09.00 h on PRO or MET. Pituitary fragments were perifused for 8 h. To test the effects of Antag in vitro some pituitaries from oil-injected rats were perifused with medium containing 1 microM Antag for the first 4 h. Starting at 5 h all chambers received hourly 10-min pulses of 1 microM GnRH. In PRO rats Antag injection in vivo lowered LH BSR to 50% and FSH to 70% of oil-treated controls. Surprisingly, Antag administered in vitro suppressed LH BSR to 30% and FSH to 55% of controls. In pituitaries from MET rats, BSR or LH and FSH were 20 and 70% of control PRO values and were unaffected by Antag treatment in vivo or in vitro. In PRO pituitaries LH and FSH responses to the first GnRH pulse were blunted by Antag in vivo but responses to subsequent pulses were unaffected; MET responses were not different from controls. In vitro Antag suppressed GnRH-stimulated release to 10-20% of controls at both cycle stages.(ABSTRACT TRUNCATED AT 250 WORDS)

Corpus luteum progesterone secretion in rats undergoing pineal-mediated reproductive regression.

In nonseasonal breeders, such as the laboratory rat, the potentiation of the antigonadal action of light deprivation by olfactory bulbectomy is manifested, among other things, by a diminished secretion of testosterone in males and estradiol in females. Little is known about the actions of the pineal gland on progesterone secretion. Since secretion of the luteotrophic hormones (LH and PRL) decreases in blind-anosmic rats and because the rat corpus luteum exhibits different degrees of LH and/or PRL dependency, the purpose of these experiments was to study the corpus luteum progesterone secretion in both pseudopregnant and pregnant blinded-bulbectomized rats. Female rats blinded and bulbectomized at the age of 25 d showed at adulthood (75 d old) (1) estrous cycles longer than the regular 4-5 d in length exhibited by intact rats: 82.3 vs. 9.0% (P less than .01); (2) a decreased paired ovarian weight, 24.1 +/- 3.7 (nine) vs. 37.3 +/- 2.2 (ten) mg/100 gm BW (P less than .05), and lower plasma levels of LH on metestrus: 15.9 +/- 1.7 vs. 26.2 +/- 3.4 ng/ml (P less than .01). Pinealectomy, as previously reported, completely reversed the effects of dual sensory deprivation. In contrast to the clear antigonadal action of the pineal gland, which seems to affect the follicular component of the ovary through modifications in the rate of LH secretion, the production of progesterone by the corpus luteum remained almost unaffected. Although the duration of the diestrous phase in both pseudopregnancy and pregnancy was significantly longer in blind-anosmic rats than in intact ones (P less than .01), the serum levels of progesterone throughout both corpus luteum phases were identical.(ABSTRACT TRUNCATED AT 250 WORDS)