Effects of testosterone implants and hypothalamic lesions on luteinizing hormone regulation in the castrated male rat.

The effects of intrahypothalamic and subcutaneous implants of testosterone (T) and those of hypothalamic lesions on resting levels of circulating LH and pituitary responsiveness to exogenous LHRH were studied in castrated male rats to elucidate hypothalamic and pituitary regulation of LH secretion. Two hundred mug implants of testosterone propionate (TP) in the median eminence region suppressed plasma LH titers before evidence of direct inhibition of pituitary function (as indicated by testing with LHRH) was found. Such implants release appreciable amounts of T into the peripheral circulation in the immediate post-operative period, and SC Silastic (constant release) capsules containing T have similar effects. The findings suggest that, regardless of the site of implant, the initial negative feedback inhibition of LH by T is not dependent on direct action at the pituitary levels but rather appears to be a hypothalamic effect. In the days following exposure to hypothalamic or peripheral implantation of T, however, a progressively developing decline in the response to exogenous LHRH was observed. In order to determine whether this effect results from suppression of endogenous LHRH release, the median eminence-arcuate region was destroyed to remove the source of LHRH. In these animals, the suppression of plasma LH was evident on the first day after the lesion, but pituitary responsiveness to LHRH was unaffected until after one week. When Sialastic capsules were implanted SC into lesioned animals, a more rapid (less than 1 week) inhibition of pituitary responsivity ensued. Suprachiasmatic lesions did not affect basal LH secretion or pituitary responses to LHRH. The data provide evidence for a dual feedback action of T on LH in castrated male rats: an initial inhibitory effect presumably due to hypothalamic inhibition (commencing at around 6h after hypothalamic of SC implantation of T), and a subsequent suppression of pituitary responisveness (after one day) presumably due to direct action of T on the pituitary. In addition to these phenomena, findings in rats bearing median eminence-arcurate lesions suggest that the removal of endogenous LHRH by itself leads to an eventual decline in pituitary responsiveness (greater than one week postoperatively).     

Atrial natriuretic factor inhibits luteinizing hormone secretion in the rat: evidence for a hypothalamic site of action

The presence of atrial natriuretic factor (ANF) immunoreactivity and receptors for ANF in the median eminence, hypothalamus, and anterior pituitary gland suggests a role for the peptide in the hypothalamic control of anterior pituitary function. In conscious ovariectomized female rats, transient elevation of plasma levels of ANF by volume loading, a stimulus known to release endogenous ANF from the heart, or by bolus iv injection of 0.1, 1.0, or 10 micrograms synthetic ANF failed to result in altered circulating levels of LH or GH. Constant iv infusion of ANF for 30 min, such that 2- to 3-fold elevations in plasma ANF were detected by RIA resulted, however, in significant inhibition of LH release in ovariectomized female rats (0.05 and 0.1 micrograms ANF/kg.min) and orchidectomized male rats (0.1 microgram ANF/kg.min). It was unlikely that this effect was exerted at the level of the anterior pituitary, since ANF failed to alter basal or LHRH-stimulated LH release from cultured anterior pituitary cells in vitro and since iv infusion of 0.1 microgram ANF/kg.min failed to alter pituitary responsiveness in vivo to a 10-ng bolus injection of LHRH. Significant inhibition of LH secretion was also observed after third cerebroventricular injection of 1.0 or 2.0 nmol ANF. As with iv infusion, central administration of ANF failed to significantly alter GH secretion. LHRH release from median eminence explants incubated in vitro in the presence of dopamine (60 or 120 microM) was inhibited by 10(-7) M ANF, suggesting a median eminence site of action of the peptide. Finally, an opiate involvement in the mechanism of ANF's action was suggested, since naloxone (0.5 mg, iv, followed by a 60-min infusion of an additional 1 mg) completely blocked the ability of ANF (0.1 or 0.5 microgram/kg.min, infused over the last 30 min of naloxone administration) to inhibit LH release. These data suggest that ANF can act centrally to alter the hypothalamic control of gonadotropin secretion, possibly by interacting with central dopaminergic and peptidergic systems. They further suggest actions of ANF within the brain unrelated to its previously described effects on fluid and electrolyte homeostasis.     

Pulsatile luteinizing hormone release, and the inhibitory effect of estradiol-17 beta in gonadectomized male and female rats: effects of neonatal androgen or exposure to constant light

The characteristics of pulsatile LH release and the acute inhibition of LH release by estradiol-17 beta (E2) were studied in long term (21 days) gonadectomized female and male Wistar rats. Three groups of female rats were examined; animals exposed either to summer lighting (14-h on; 10-h off; LD) or continuous illumination (LL) and animals treated neonatally with testosterone propionate (TP) and exposed to LD. The mean plasma LH concentrations and interpulse intervals were similar in both male and LD female rats. However, treatment of female rats with TP or exposure to LL reduced the mean plasma LH concentration in female rats and increased the interpulse interval when compared with LD female or male rats. The amplitude of the LH pulses was significantly greater in the LD female rats compared with those in the male rats; since pituitary responsiveness to a single iv injection of 50 ng LHRH/100 g BW was similar in the two groups, this suggests that the amount of LHRH released per pulse of LH is greater in the female than in the male. The greater amplitude but similar frequency of LH pulses in the LD female compared with the LD male suggest that the MCR of LH may be greater in the female. The pulse amplitude in the TP-treated rats was similar to that in the rats exposed to LL and since pituitary responsiveness to LHRH was significantly greater in the TP rats, there was probably more LHRH released per pulse of LH in the LL-treated rats. Pituitary responsiveness to LHRH was significantly lower in the LL- and TP-treated rats compared with males and LD females. The timing of the inhibition of LH secretion by E2 was similar in all four groups of animals, and in LD females was not affected by a 92% depletion of serotonin, 100% depletion of 5-hydroxyindoleacetic acid, or a 33% depletion of dopamine in the hypothalamus produced by the administration of parachlorophenylalanine.(ABSTRACT TRUNCATED AT 400 WORDS)     

Activation of luteinizing hormone-releasing hormone release advances the onset of female puberty

The juvenile-peripubertal transition period in the female rat is associated with an ovarian-independent afternoon increase in the amplitude of plasma luteinizing hormone (LH) pulses. To determine if the immature pituitary could be activated to cause precocious puberty juvenile female rats were subjected for 4 days to a microprocessor-driven pulsatile intravenous administration of LH-releasing hormone (LHRH) at a dose that produced a peripubertal pattern of LH release. To determine if the LHRH neurons themselves could be prematurely activated to induce such a pattern of plasma LH, and hence lead to precocious puberty, the neuroexcitatory amino acid analog N-methyl-DL-aspartic acid (NMA) was similarly administered. The time of puberty (vaginal opening and first ovulation) was advanced by both the LHRH and NMA treatments, by 5 and 7 days, respectively. Ovarian weight and incidence of corpora lutea at first diestrus were similar in all animals regardless of treatment, but a juvenile body weight was retained by the animals that underwent precocious puberty. Therefore, just as the adenohypophysis can be driven by exogenous LHRH to initiate puberty, the LHRH neuronal system can be precociously activated by the episodic administration of an excitatory amino acid analog that is known to interact with specific brain receptors. It is likely, therefore, that sexual maturation is limited by factors that lie further upstream in the hypothalamo-pituitary axis (e.g., the neuronal circuits that impinge upon LHRH-producing neurons).     

Hyperprolactinemia decreases the luteinizing hormone-releasing hormone concentration in pituitary portal plasma: a possible role for beta-endorphin as a mediator

Hyperprolactinemia can reduce the LH secretion in rats, but the mechanism of the effect of PRL is not clear. We have investigated the actions of PRL on the secretion of LHRH and LH and the interaction among PRL, beta-endorphin (beta-EP), and LHRH. The effects of PRL on LHRH and LH secretion were studied in ovariectomized female rats after transplanting four anterior pituitaries to the right kidney capsule of each ovariectomized rat for 2-3 weeks. The level of PRL in rats with pituitary transplants was approximately 5 times higher than that in control rats. The concentration of LHRH in pituitary portal plasma of hyperprolactinemic rats was approximately 4 times lower than that in control rats. Hyperprolactinemic animals also showed lower plasma LH levels than the controls. Since beta-EP inhibits the secretion of LHRH, we have tested whether the reduced secretion of LHRH in hyperprolactinemic ovariectomized rats is associated with an increase in beta-EP activity. This was studied by measuring the concentration of beta-EP in pituitary portal plasma and the response of LHRH and LH to the opiate antagonist naloxone. The level of beta-EP-like immunoreactivity in pituitary portal plasma was significantly higher in hyperprolactinemic rats than in control animals. Naloxone (10 mg/kg, sc) increased both LHRH and LH concentrations in hyperprolactinemic rats, but not in control rats. The present results demonstrate that hyperprolactinemia can reduce LHRH release and suggest a possible involvement of beta-EP in the PRL inhibitory action on LHRH.     

Effects of long- and short-term gonadectomy on the hypothalamo-hypophysial (LH-releasing hormone-LH) system in oestrogen-treated male and female rats

Considerable differences have previously been found in the hypothalamo-hypophysial responsiveness to oestrogen, depending upon the time between gonad removal and exposure to oestrogen. In the present study a detailed analysis was made of some of the differences which may exist in pituitary LH-releasing hormone (LHRH) receptors and the amount of LH released in response to electrochemical depolarization of the medial preoptic area after 2 or 7 days of oestradiol treatment of long- and short-term gonadectomized male and female rats. The pituitary glands of long-term gonadectomized males and females secreted more LH in response to two pulse injections of LHRH than did short-term gonadectomized rats. The amount of LH released on day 2, however, was equivalent to that secreted after 7 days of oestradiol treatment. Moreover, long-term gonadectomized males and females had equivalent LHRH receptor concentrations, which were greater than those of short-term gonadectomized animals. Peak serum LH concentrations observed after preoptic stimulation were equivalent in short- and long-term castrated rats after 2 days of oestrogen exposure. Serum LH concentrations following preoptic stimulation in short-term gonadectomized males and females were significantly greater on day 7 than on day 2 of oestradiol treatment, whereas in long-term gonadectomized animals the stimulated release of LH was equivalent both in magnitude and time of peak release on both days.(ABSTRACT TRUNCATED AT 250 WORDS)     

Naloxone stimulation of in vivo LHRH release is not diminished following ovariectomy

The effects of an opiate receptor antagonist, naloxone, on the in vivo release of luteinizing hormone releasing hormone (LHRH) in ovariectomized and intact female rats were examined using push-pull perfusion. Hypothalamic LHRH release rates were determined before and after naloxone (2.5 mg/kg s.c.) or saline vehicle injection in rats during metestrus or 4 or 8 days following ovariectomy. Naloxone was found to be equally effective in significantly (p less than 0.05) stimulating LHRH release in all three treatment conditions. In animals that were fitted with atrial catheters instead of push-pull cannulae, naloxone administration led to significant increases in peripheral LH levels in metestrous and ovariectomized rats. Peripheral follicle-stimulating hormone levels were unaffected by naloxone. These results suggest that opioid inhibitory tone is not diminished during pituitary escape from ovarian negative feedback in adult female rats. Our findings are not consistent with the hypothesis that ovarian negative feedback regulation of LH secretion is serially mediated by endogenous opioid peptide neurons in these animals. Rather, it is proposed that opioid inhibition of LHRH release and ovarian negative feedback may operate in parallel to control LH secretion in the female rat.     

Alterations in luteinizing hormone-releasing hormone release from the mediobasal hypothalamus of ovariectomized, steroid-primed middle-aged rats as measured by push-pull perfusion

The spontaneous luteinizing hormone (LH) surge on proestrus as well as the steroid-induced LH surge and hypersecretion of LH following ovariectomy are attenuated in middle-aged female rats when compared to their young counterparts. It is generally assumed that the lower titers of serum LH measured in aging animals result, in part, from age-related alterations in luteinizing hormone-releasing hormone (LHRH) neurosecretion, yet no direct measurements of LHRH release from the hypothalamus of aging females are currently available. The present study utilized the push-pull perfusion technique to characterize and compare in vivo LHRH output from the mediobasal hypothalamus of ovariectomized middle-aged and young females during a steroid-induced LH surge. Twelve-minute perfusates were collected for a period of 6 h from middle-aged and young animals outfitted with push-pull cannulae resting in the mediobasal hypothalamus in close proximity to the median eminence. The overall pattern of LHRH release differed in the two age groups. Mean levels of LHRH measured in brain perfusates from middle-aged females were significantly lower than mean levels measured in young animals. Moreover, mean levels of LHRH detected in perfusates during the 1- and 2-hour intervals prior to and the 1-hour interval after the time of maximal LHRH output were lower in middle-aged animals, and a larger percentage of samples from middle-aged females contained levels of LHRH below the detectability of the assay. The mean amplitude of identified LHRH pulses and maximal LHRH output, however, did not differ in the two age groups.(ABSTRACT TRUNCATED AT 250 WORDS)     

Influence of the gonad on the degree of suppression induced by an LHRH agonist implant in the marmoset monkey.

In a previous study pituitary-testicular function was shown to be maintained in a New World primate after the administration of an LHRH agonist implant. In the present study the mechanism of action of the same LHRH agonist (buserelin) on pituitary-gonadal function in the marmoset was investigated and a comparison made between the effects of treatment in three intact males, six adult cyclic females with regular ovulatory cycles, and six long-term ovariectomized animals. These were injected s.c. with an LHRH agonist implant (1.5 mg buserelin in a rod 0.5 cm long). In both the males and intact females, basal plasma LH concentrations were maintained within the normal range throughout the expected duration of agonist action (at least 3 months). Despite this, an absence of response to an LHRH challenge indicated that pituitary desensitization had occurred. In the intact females, ovulation was inhibited in five of six animals, plasma progesterone concentrations initially being maintained but subsequently remaining suppressed until 136 +/- 18 (S.E.M.) days after treatment. Responsiveness to administered LHRH returned prior to onset of return to ovarian cycles. In contrast, in ovariectomized marmosets, plasma LH was markedly suppressed to concentrations which were at or below the limit of detection of the assay and were therefore less than those observed in the buserelin-implanted intact animals. These results show that apparently normal pituitary-gonadal function in this species disguises an underlying pituitary desensitization to LHRH. This allows continuation of testosterone secretion in the male, but in the female ovulation is prevented, presumably as a result of failure of the desensitized pituitary to produce an LH surge.(ABSTRACT TRUNCATED AT 250 WORDS)     

Neonatal handling and reproductive function in female rats

Neonatal handling induces anovulatory estrous cycles and decreases sexual receptivity in female rats. The synchronous secretion of hormones from the gonads (estradiol (E2) and progesterone (P)), pituitary (luteinizing (LH) and follicle-stimulating (FSH) hormones) and hypothalamus (LH-releasing hormone (LHRH)) are essential for the reproductive functions in female rats. The present study aimed to describe the plasma levels of E2 and P throughout the estrous cycle and LH, FSH and prolactin (PRL) in the afternoon of the proestrus, and the LHRH content in the medial preoptic area (MPOA), median eminence (ME) and medial septal area (MSA) in the proestrus, in the neonatal handled rats. Wistar pup rats were handled for 1 min during the first 10 days after delivery (neonatal handled group) or left undisturbed (nonhandled group). When they reached adulthood, blood samples were collected through a jugular cannula and the MPOA, ME and MSA were microdissected. Plasma levels of the hormones and the content of LHRH were determined by RIA. The number of oocytes counted in the morning of the estrus day in the handled rats was significantly lower than in the nonhandled ones. Neonatal handling reduces E2 levels only on the proestrus day while P levels decreased in metestrus and estrus. Handled females also showed reduced plasma levels of LH, FSH and PRL in the afternoon of the proestrus. The LHRH content in the MPOA was significantly higher than in the nonhandled group. The reduced secretion of E2, LH, FSH and LHRH on the proestrus day may explain the anovulatory estrous cycle in neonatal handled rats. The reduced secretion of PRL in the proestrus may be related to the decreased sexual receptiveness in handled females. In conclusion, early-life environmental stimulation can induce long-lasting effects on the hypothalamus-pituitary-gonad axis.     

Mediation of the short-loop negative feedback of luteinizing hormone (LH) on LH-releasing hormone release by melatonin-induced inhibition of LH release from the pars tuberalis.

The pineal hormone melatonin is thought to mediate the effects of the pineal gland on seasonal reproduction by altering the release of gonadotropins. The mechanism by which melatonin controls gonadotropin secretion has been obscure. Recently, labeled 2-iodomelatonin was used to localize melatonin receptors in brain by radioautography. The highest concentration of melatonin receptors was found in the pars tuberalis of the pituitary gland of mammals. Pituitary hormones, in particular luteinizing hormone (LH), have been localized in cells of the pars tuberalis. Consequently, we hypothesized that melatonin might act on its receptors in the pars tuberalis to alter the release of LH. It would then be possible for this LH to diffuse into the overlying median eminence, there to alter the release of LH-releasing hormone (LHRH) from the axons of the LHRH neurons. To evaluate this hypothesis, we incubated median eminence-pars tuberalis tissue from male rats in vitro. After preincubation in Krebs-Ringer bicarbonate buffer for 30 min, test substances were added to fresh medium and the incubation was continued for 30 min. LHRH or LH released into the medium was measured by radioimmunoassay. Melatonin induced a dose-related release of LHRH with the maximum response at the greatest concentration tested (1 microM). This concentration of melatonin also significantly reduced the release of LH into the medium. The increased release of LHRH induced by melatonin (10 microM) was completely blocked by the addition of LH (50 ng/ml), which by itself had no significant effect on LHRH release. Rat LH antiserum (final dilution, 1:1800) significantly elevated LHRH output, whereas normal rabbit serum at a similar dilution had no effect. Finally, LHRH (0.1 microM) induced a significant release of LH from median eminence-pars tuberalis tissue that was completely blocked by melatonin (10 microM). The results support the hypothesis that LH released from the pars tuberalis diffuses to the LHRH terminals in the median eminence to suppress LHRH release. Melatonin acts on its receptors in the pars tuberalis to inhibit LH release, thereby stimulating the release of LHRH from its terminals in the median eminence. The negative short-loop feedback of LH inhibits basal LHRH release in vitro since antiserum against LH increased LHRH release. The results suggest a concept concerning the mechanism by which melatonin can affect the release of pituitary hormones from the pars tuberalis. It is likely that these pituitary hormones diffuse into the median eminence to modify the release of hypothalamic releasing and inhibiting peptides, thereby altering plasma pituitary hormone concentrations.     

Pituitary and gonadal response to LH releasing hormone administration in the female and male cheetah

Luteinizing hormone releasing hormone (LHRH, 50 micrograms) or saline was administered (i.m.) to adult female and male cheetahs under anaesthesia to evaluate pituitary and gonadal response. Serum LH levels did not fluctuate over a 120-min sampling period in saline-treated animals. Serum LH concentrations were raised (P less than 0.05) in both female and male cheetahs after LHRH injection, the temporal response being similar to previously reported results in unanaesthetized, domestic carnivores. The magnitude of the LHRH-induced LH response was sex-dependent. Over a 120-min post-injection period both saline control and LHRH-induced LH levels were about twofold greater in males than females. Although LHRH had no acute influence on ovarian oestradiol-17 beta production in the female, serum testosterone levels were raised (P less than 0.05) in male cheetahs by 60 min after treatment. This study (1) provides introductory endocrine information on the cheetah, an endangered species, and (2) indicates that exogenous LHRH is effective in acutely altering pituitary (female) and pituitary/gonadal (male) function in an anaesthetized, non-domestic felid.     

Effect of infundibular lesions on GnRH and LH release in the frog, Rana temporaria, during hibernation.

The effects of lesions and deafferentations on accelerated hibernatory ovulation and on the concentration of GnRH in blood collected from the hypothalamo-pituitary area along with the blood and pituitary levels of luteinizing hormone (LH) in hibernating female frogs, Rana temporaria, were studied. The lesions to the caudal portion of the nucleus infundibularis ventralis resulted in: (1) an elevation in GnRH concentration in blood collected from the hypothalamo-pituitary area (x +/- SEM = 51 +/- 9.5 and 100 +/- 15 pg/ml in control and lesioned females, respectively), (2) an increase in plasma LH (from 11 +/- 1.3 to 14.7 +/- 2.5 ng/ml in controls to 73.1 +/- 12.0 and 74.2 +/- 15.8 ng/ml in lesioned females at 3 and 7 days), and (3) accelerated ovulations whose onset advanced as the hibernation season progressed. Histological and immunohistochemical analyses of the brains showed that GnRH seems to be stored during hibernation in the median eminence and suggested that the complete disruption of projections to the median eminence is prerequisite for accelerated ovulation. The influence of the hypothalamic inhibitory action on the release of GnRH and the way in which GnRH is transferred to the pituitary gland are discussed.     

Expression of neuropeptide Y precursor-immunoreactivity in the hypothalamic dopaminergic tubero-infundibular system during lactation in rodents.

Evidence from physiological studies in rats shows that neuropeptide Y (NPY) has marked neuroendocrine effects on anterior pituitary function, and especially on LHRH and LH secretions. However, previous immunohistochemical studies in rats have revealed only scarce NPY-axons of medullary origin in the external zone of the hypothalamic median eminence, the common termination site of neuroendocrine adenohypophysiotropic systems. In view of this apparent contradiction, we used light microscopic immunohistochemistry to reassess the distribution of NPY in the hypothalamus of rodents of both sexes under physiological (estrous cycle in rats, pregnancy in rats, and lactation in both rats and mice) and experimental (gonadectomy in rats and adrenalectomy in both rats and mice) conditions with alterations of reproductive functions. We reasoned that such manipulations could induce changes in immunoreactivity in the NPY system involved in neuroendocrine regulation and would thus make it apparent to us. We show here that immunoreactivity for NPY and its carboxyterminal precursor-associated peptide are dramatically increased in the external median eminence of lactating female animals when compared to the other animal groups. This NYP-precursor-immunoreactivity is present, throughout lactation, in the tyrosine hydroxylase-immunoreactive (and therefore possibly dopaminergic) tubero-infundibular system. This immunoreactivity disappears rapidly from the median eminence after pup-removal. These observations suggest a role for NPY-precursor-derived peptides in the control of the suckling-induced PRL secretion and also demonstrate the chemical plasticity of the median eminence during a normal physiological event. Since in nonlactating animals and especially in normal cycling females NPY-precursor-immunoreactivity was detected in the system of medullary origin only, we conclude that, by exclusion, this system might be the one responsible for modulating gonadotropic secretion at the median eminence and/or pituitary levels.     

Gonadal steroid modulation of naloxone-induced LH secretion in the rat

The effect of acute administration of the opioid receptor antagonist naloxone hydrochloride (5 mg/kg, s.c.) on plasma LH levels was evaluated in female and male rats 24, 36 and 48 h and 1, 3 and 5 weeks after gonadectomy and in 5-week gonadectomized rats after acute or chronic (2 weeks) administration of oestradiol benzoate (OB, 10 micrograms/rat per day, s.c.), testosterone propionate (TP, 150 micrograms/rat, s.c.) or dihydrotestosterone propionate (DHT, 150 micrograms/rat, s.c.) respectively. Concurrent evaluation of plasma LH after administration of LH releasing hormone (LHRH, 1 microgram/kg, i.p.) was performed in the same experimental groups. In rats of both sexes, a significant rise in plasma LH after naloxone was observed in sham-operated and recently gonadectomized rats (24-48 h); in female rats 36 and 48 h after gonadectomy the rise was higher than in controls. One, 3 and 5 weeks after gonadectomy, naloxone failed to stimulate LH release in both female and male rats. In gonadectomized rats undergoing steroid replacement therapy, OB administered 72 h before testing, TP (16 and 72 h) and DHT (16 h) were the most effective in reinstituting the LH response to naloxone. Chronic administration of gonadal steroids did not restore normal LH responsiveness to naloxone. In most experimental groups, LH responses after naloxone were clearly unrelated to pituitary LH responsiveness to LHRH, which indicates that the opioid antagonist was acting via the central nervous system.(ABSTRACT TRUNCATED AT 250 WORDS)     

Altered sensitivity to an opiate antagonist, naloxone, in hyperprolactinemic male rats

An increase in endogenous opiatergic tone has been suggested as a mechanism for suppression of gonadotropin release in hyperprolactinemia (hyperPRL). In an attempt to evaluate this possibility in hyperprolactinemic males, we have examined the effects of a specific opiate antagonist, naloxone, on plasma LH levels in adult male rats rendered hyperprolactinemic by transplanting two pituitary glands underneath the kidney capsule. Naloxone was administered intravenously (i.v.) at different dose levels (0.2,2, or 20 mg/kg body weight) and blood samples were collected at intervals ranging from 15 to 60 min after naloxone administration. In all experiments, plasma LH levels in the sham-operated controls were significantly increased after naloxone administration, while those in the pituitary-grafted group remained unaltered. Injecting naloxone daily for 3 or for 10 days failed to alter plasma LH levels in both sham-operated and pituitary-grafted animals. Administration of LHRH induced a 7-fold increase in plasma LH levels in both the pituitary-grafted animals and the sham-operated controls. These studies demonstrate that hyperPRL interferes with the naloxone-induced rise in circulating LH levels in the male rat.     

Examination of a possible mechanism by which anisomycin suppresses pulsatile luteinizing hormone release in ovariectomized rats

An initial study was performed to ascertain the effects of anisomycin, a reversible inhibitor of protein synthesis, on pulsatile luteinizing hormone (LH) release in adult, ovarietomized (OVX) rats. For this experiment, rats OVX 3-4 weeks earlier were fitted with indwelling atrial cannulae. On the next day (approximately 13.00 h), the rats received a subcutaneous injection of either 100 mg/kg body weight (BW) anisomycin or its saline vehicle. Administration of anisomycin significantly suppressed mean plasma LH levels, mean trough values, and both LH pulse frequency (saline: 6.3 pulses/3 h vs. anisomycin: 2.7 pulses/3 h) and amplitude. To determine whether anisomycin affected anterior pituitary LH responses to LH-releasing hormone (LHRH), a second experiment was performed in which saline- and anisomycin-treated OVX rats were given an intravenous injection of 10 ng/100 g BW LHRH 1.5 h later (14.30 h). Rats then were sacrificed and the anterior pituitary and brain removed. Whereas preinjection plasma LH levels were significantly lower in anisomycin-treated rats, they were significantly higher in anisomycin-treated rats 20 min after LHRH. Consequently, mean maximal increments and percent increments were significantly higher in anisomycin-treated rats. AP LH content and content of LHRH in the medial preoptic and suprachiasmatic nuclei were not influenced by anisomycin treatment. However, median eminence (ME) LHRH concentrations in anisomycin-treated rats were almost double the LHRH levels measured in control rats. A third study was conducted to assess the effects of anisomycin on basal and potassium (K+)-stimulated LHRH release from superfused ME explants.(ABSTRACT TRUNCATED AT 250 WORDS)     

Sexual differences in the distribution of neurons coexpressing galanin and luteinizing hormone-releasing hormone in the rat brain.

We have recently reported that a subpopulation of galanin (GAL)-immunoreactive perikarya in the preoptic area near the organum vasculosum of the lamina terminalis (OVLT) has morphological characteristics similar to those of LHRH-containing neurons. In fact, both peptides are colocalized in those neurons in the male rat brain. In these studies we describe sexual differences in the incidence of neurons colocalizing GAL and LHRH in this region. In male rats, about 20% of LHRH-immunoreactive cells in the diagonal band of Broca and the medial preoptic area are also immunoreactive for GAL. In contrast, in female rats, about 65% of LHRH-containing perikarya near the OVLT are immunostained for GAL. In addition, GAL and LHRH levels were measured in tissue extracts containing either the OVLT and surrounding areas or the median eminence. The data indicate that the preoptic area, including the OVLT, contains a significantly higher amount of GAL in females killed in proestrus than in those killed in estrus. The amount of GAL measured in males is lower than that in the proestrous females, but somewhat greater than that present in the estrous females. There were no significant differences among the three groups in LHRH content in this region. The GAL content of the median eminence was the highest in proestrous females, followed by estrous females and males. The LHRH content in the median eminence was not significantly different among the three groups. In conclusion, these observations indicate that the coexpression of GAL with LHRH in a discrete subpopulation of LHRH-producing neurons is a sex-related phenomenon. The fact that changes in GAL levels in specific regions can be seen at different times during the estrous cycle suggests that gonadal steroids, possibly estrogens, may physiologically regulate the expression of GAL in this subpopulation of LHRH neurons.     

Ovarian steroid modulation of gonadotropin secretion and pituitary responsiveness to luteinizing hormone-releasing hormone in the female hamster.

The significance of ovarian estradiol (E2) and progesterone secretion in the regulation of pituitary LH and FSH secretion and pituitary responses to LHRH was investigated in the hamster. Cycling females showed increased LH and FSH responses to LHRH on the morning of proestrus as compared to the responses observed on diestrus day 2. Pituitary responsiveness to LHRH declined on the evening of proestrus, after the preovulatory LH/FSH release. The secondary increase in serum FSH concentration on the morning of estrus was accompanied by a selective increase in the pituitary FHS response to exogenous LHRH. Hamsters ovariectomized (ovx) on diestrus day 2 exhibited daily afternoon LH surges but not FSH surges for at least 10 days after ovx. The magnitude of the LH surges in ovx hamsters was approximately 30-50% of that observed in proestrous females. The pituitary LH response to exogenous LHRH in ovx animals was about 25% as great as in proestrus hamsters. Serum FSH concentrations in ovx females increased by only 30% after LHRH injection, while similar treatment with LHRH resulted in 3- to 4-fold increments in serum FSH in proestrous hamsters. Implantation of E2 capsules in ovx hamsters resulted in increased gonadotropin responses to exogenous LHRH. Serum LH concentrations in the E2-implanted, LHRH-injected animals were as great as those observed after LHRH injection in proestrous females. Administration of LHRH, LH, or progesterone on the morning of proestrus failed to detectably alter the timing or magnitude of the proestrus afternoon FSH surge. The present results suggest that the increasing serum titers of estrogen on diestrus and early proestrus result in increased pituitary sensitivity to LHRH, and this increased sensitivity probably contributes to the magnitude of the preovulatory LH surge. The increases in LH and progesterone which occur during the afternoon do not seem to be responsible for triggering the proestrous FSH surge.     

Effects of progesterone on the pituitary responsiveness to, and priming effect of luteinizing hormone releasing hormone in female rats exposed to constant light

We have investigated the role of progesterone in the mating-induced release of luteinizing hormone (LH) and ovulation in female rats exposed to a 60-day period of constant light (LL). Plasma LH and progesterone concentrations were increased after mating; plasma estradiol concentrations, although not increased after mating, were increased compared with the concentrations in female rats on light-dark (LD) exposure during diestrus, proestrus evening and estrus. Progesterone induced ovulation in about half the number of female rats exposed to long-term LL, and in these animals, there was a significant increase in pituitary responsiveness to luteinizing hormone releasing hormone (LHRH) 5 h after progesterone injection. The magnitude of the priming effect of LHRH was markedly increased 2 h after progesterone treatment. Treatment with sodium pentobarbitone (SP) 15 min before an injection of progesterone, blocked the increase in pituitary responsiveness to LHRH 5 h later, but treatment with SP 4 h before progesterone injection did not block the increase in the magnitude of the priming effect of LHRH. These results suggest that progesterone acts both at the brain and pituitary to facilitate LH release, and that the increase in plasma progesterone produced by mating is at least partly responsible for the LH surge induced by mating in LL rats.