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

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

Androgenic regulation of luteinizing hormone secretion: relationship to androgen binding in sheep pituitary

Castrated ram lambs (wethers) were investigated for sensitivity to androgen feedback and to determine whether this feedback inhibition of luteinizing hormone (LH) was associated with changes in pituitary androgen receptors. Administration of Silastic capsules containing either dihydrotestosterone or testosterone was found to produce dose-dependent inhibitory effects on serum LH levels in wethers. Physiological dosages of these androgens (i.e., those that produce serum levels of dihydrotestosterone [0.24 ng/ml] or testosterone [2.1 ng/ml] similar to those of intact rams) resulted in differential inhibition of serum LH and LH content of the anterior pituitary. Whereas the inhibitory effect of dihydrotestosterone on pituitary LH content was much more dramatic than that seen with testosterone, the high dosage of testosterone also produced a substantial decrease in pituitary LH content. Responses of the pituitary to changes in serum androgen were compared to responses of the seminal vesicle, which served as a control androgen target organ. Androgen levels were positively correlated with seminal vesicle weights, but pituitary weights were unaffected by castration and/or androgen replacement. Treatments with dihydrotestosterone were associated with decreased cytosol androgen binding activity (i.e., receptors) in pituitary and seminal vesicle, suggesting that both of these tissues were sites of androgen action. Although testosterone inhibited serum LH levels, pituitary cytosol androgen receptors were not affected by changes in serum testosterone. We conclude from these data that dihydrotestosterone is a physiological regulator of pituitary LH secretion in the ram and that further study is needed to investigate the complex actions of testosterone and its metabolites on pituitary function.     

Direct opioid regulation of pituitary release of bovine luteinizing hormone

Although endogenous opioid peptides (EOP) are thought to alter pituitary release of luteinizing hormone (LH) by modifying the release of gonadotropin-releasing hormone (GnRH) from the brain, EOP may also directly affect the release of LH from pituitary cells. This hypothesis was tested using dispersed cells from the bovine anterior pituitary gland. Pituitaries were enzymatically dissociated, preincubated for 18 h and then cultured for either 2 or 24 h with GnRH, naloxone, methionine-enkephalin (Met-enk) or their combinations. Basal release of LH into media was 18.2 and 38.4 ng/100,000 cells after culture for 2 or 24 h, respectively. When cultured for 2 or 24 h with 10 nM GnRH, LH release was 296% and 131% of the basal release for each culture period. Cellular viability (75% vs 68%) and total (cells + medium) LH (128 vs 134 ng/100,000 cells) did not differ (P greater than .05) between cells cultured for 2 or 24 h. Naloxone (1 microM) increased (P less than .01) basal release of LH by 57% after 2 h of culture but not after 24 h of culture. Naloxone did not augment the amount of LH released in response to 10 nM GnRH. Addition of Met-enk (1 nM to 1 microM) suppressed (P less than .05) basal release of LH (23% to 62%) after 2 h of culture. Similar suppressive effects (8% to 49%) occurred in a dose-dependent manner (0.1 nM to 1 microM) after 24 h of culture. Met-enk (1 and 100 nM) antagonized (P less than .05) the stimulatory effect of naloxone and reduced (P less than .05) the amount of LH released in response to GnRH after 2 h of culture. In summary, the stimulatory effect of naloxone on the basal release of LH suggests that EOP may directly regulate pituitary cell function; the inhibitory effect of physiological concentrations of Met-enk on the basal in vitro release of LH suggests that EOP may directly affect the release of LH in vivo; the antagonism between the stimulatory effect of naloxone and the inhibitory effect of Met-enk is consistent with effects exerted through opioid receptors; and the stimulatory effect of GnRH may be partially reduced by Met-enk. These results are consistent with the hypothesis that opioids may directly modulate the release of LH at the pituitary level.     

Catecholamine inhibition of luteinizing hormone secretion in isolated pig pituitary cells

This study investigated the direct effect of catecholamines, epinephrine (EPI), and norepinephrine (NE) on basal and gonadotropin-releasing hormone (GnRH)-stimulated secretion of luteinizing hormone (LH) from dispersed pig pituitary cells in vitro. Pig pituitaries were dispersed into cells with collagenase and DNAase and then cultured in McCoy's 5a medium containing horse serum (10%) and fetal calf serum (2.5%) pretreated with dextran-coated charcoal for 3 days. EPI and NE did not affect basal LH secretion after 4 h of incubation. When pituitary cells were incubated with EPI or NE (1 microgram/ml) for longer than 30 min, GnRH-stimulated LH secretion was reduced. The degree of this reduction was dependent on EPI and NE, and a concentration of EPI and NE higher than 1 ng/ml and 100 ng/ml, respectively, was needed. L-isoproterenol, a nonselective beta-agonist, also inhibited the LH response to GnRH. Propranolol, a beta-antagonist, blocked the inhibitory effect of EPI, whereas phentolamine, an alpha-antagonist, had no effect. These results suggest that catecholamines, acting by a beta 2-adrenergic receptor, may play a role in the control of the porcine pituitary gonadotrope's response to GnRH.     

Opioid regulation of luteinizing hormone secretion in the male rat

Current evidence suggests that endogenous opioid peptides (EOPs) tonically inhibit secretion of luteinizing hormone (LH) by modulating the release of gonadotropin-releasing hormone (GnRH). Because of their apparent inhibitory actions, EOPs have been assumed to alter both pulse frequency and amplitude of LH in the rat; and it has been hypothesized that EOP pathways mediate the negative feedback actions of steroids on secretion of GnRH. In order to better delineate the role of EOPs in regulating secretion of LH in the male rat, we assessed the effects of a sustained blockade of opiate receptors by naloxone on pulsatile LH release in four groups: intact male rats, acutely castrated male rats implanted for 20 h with a 30-mm capsule made from Silastic and filled with testosterone, acutely castrated male rats implanted for 20 h with an osmotic minipump dispensing 10 mg morphine/24 h, and male rats castrated approximately 20 h before treatment with naloxone. We hypothesized that if EOPs tonically inhibited pulsatile LH secretion, a sustained blockade of opiate receptors should result in a sustained increase in LH release. We found that treatment with naloxone resulted in an immediate but transient increase in LH levels in intact males compared to controls treated with saline. Even though mean levels of LH increased from 0.15 +/- 0.04 to a high of 0.57 +/- 0.14 ng/ml, no significant difference was observed between the groups in either frequency or amplitude of LH pulses across the 4-h treatment period. The transient increase in LH did result in a 3- to 4-fold elevation in levels of plasma testosterone over baseline. This increase in testosterone appeared to correspond with the waning of the LH response to naloxone. The LH response to naloxone was eliminated in acutely castrated rats implanted with testosterone. Likewise, acutely castrated rats treated with morphine also failed to respond to naloxone with an increase in LH. These observations suggest that chronic morphine and chronic testosterone may act through the same mechanism to modulate secretion of LH, or once shut down, the GnRH pulse-generating system becomes refractory to stimulation by naloxone. In acutely castrated male rats, levels of LH were significantly increased above baseline throughout the period of naloxone treatment; this finding supports the hypothesis that the acute elevation in testosterone acting through mechanism independent of opioid is responsible for the transient response of LH to naloxone in the intact rat.     

Neuroendocrine regulation of pulsatile luteinizing hormone secretion in elderly men

Leydig cell function is driven by LH, secreted in a pulsatile manner by the anterior pituitary in response to episodic discharge of hypothalamic LHRH into the pituitary portal circulation, under control of a yet to be defined neural mechanism, the "hypothalamic LHRH pulse generator". The normal aging process in elderly men is accompanied by a decline in Leydig cell function. Whereas primary testicular factors undoubtedly play an important role in the decrease of circulating (free) testosterone levels with age, recent studies demonstrated that aging also affects the central compartment of the neuroendocrine cascade. Hypothalamic alterations comprise changes in the regulation of the frequency of the LHRH pulse generator with an inappropriately low frequency relative to the prevailing androgen impregnation and opioid tone, and with an increased sensitivity to retardation of the LHRH pulse generator by androgens. As observed by some authors in basal conditions and by others after endocrine manipulations. LH pulse amplitude seems also to be reduced in elderly men as compared to young subjects. This is most probably the consequence of a reduction in the amount of LHRH released by the hypothalamus. Indeed, challenge of the gonadotropes with low, close to physiological doses of LHRH in young and elderly men reveals no alterations in pituitary responsiveness when looking at either the response for immunoreactive LH or bioactive LH. Deconvolution analysis on data obtained after low-dose LHRH suggests a markedly prolonged plasma half-life of LH in elderly men, a finding which may explain the paradoxical increase of mean LH levels in face of the reduced or unchanged frequency and amplitude of LH pulses.     

Testosterone-dependent effects of galanin on pituitary luteinizing hormone secretion in male rats

Galanin is a 29-amino-acid peptide that colocalizes with GnRH in hypothalamic neurons. High concentrations of galanin are present in portal vessel blood of both male and female rats, and galanin receptors are present on gonadotropes in both sexes. Results from studies of female rats indicate that galanin acts at the level of the pituitary to directly stimulate LH secretion and also to enhance GnRH-stimulated LH secretion. The effects of galanin on pituitary LH secretion in male rats are relatively uncharacterized; thus, the present in vivo study was conducted 1). to examine the ability of galanin to affect basal or GnRH-stimulated LH secretion in male rats and 2). to determine whether the effects of galanin on LH secretion in male rats are testosterone-dependent. All three doses of galanin used (1, 5, and 10 micro g/pulse) significantly enhanced GnRH-stimulated LH secretion in intact male rats. Only the highest dose of galanin directly stimulated LH secretion (without GnRH coadministration) in intact males. Galanin did not directly stimulate LH secretion or enhance GnRH-stimulated LH secretion in castrated male rats. In fact, the highest dose of galanin inhibited GnRH-stimulated LH secretion in castrated males. Upon testosterone replacement, the ability of galanin to directly stimulate LH secretion and to enhance GnRH-stimulated LH secretion was restored in castrated males. These results suggest a role for galanin in the regulation of LH release in male rats and demonstrate that testosterone upregulates the ability of the pituitary to respond to the stimulatory effects of galanin.     

Hypothalamic luteinizing hormone-releasing hormone (LHRH) and LH secretion in aging female rats

Age-related changes in hypothalamic luteinizing hormone-releasing hormone (LHRH) and luteinizing hormone (LH) secretion were studied in young (6 months), middle-aged (12 months) and old (18 months) female rats. The LHRH levels in the mid-hypothalamic area were higher in intact middle-aged and old females than in young ones. Additionally, there was no age difference in the hypothalamic LHRH levels in male rats. In order to clarify the significance of this age-related increase in female rats, we examined the effects of progesterone treatment in estrogen-primed ovariectomized young and old rats on the LHRH levels in the median eminence (ME) and on plasma LH levels. We found phasic changes in ME-LHRH and plasma LH levels in estrogen-primed rats following progesterone treatment in rats of both ages, but the progesterone-induced change in ME-LHRH levels tended to be delayed in old rats compared with young females. This delay may correspond to the delayed onset, slow and low magnitude of plasma LH increase in old females. The ME-LHRH levels were generally higher in old rats than in young rats. Nevertheless, we found that the increase in plasma LH in response to progesterone treatment in estrogen-primed ovariectomized females was smaller in old rats than young rats. These results suggest that the LHRH secretory mechanism changes with age in female rats. Such alterations may result in the accumulation of LHRH in the mid-hypothalamic area and an increase in ME-LHRH.     

Effect of synthetic luteinizing hormone releasing hormone on prolactin secretion from clonal pituitary cells.

The influence of LHRH, an analog of LHRH (hydroxy-PRO¹) and inulin on prolactin (PRL) secretion was studied using a clonal strain of pituitary cells. At low concentrations, 0.08 ng to 8 ng/ml, LHRH stimulated PRL release while at higher concentrations the opposite effect was obtained. The analog of LHRH inhibited PRL secretion at all concentrations studied. No effect was measured with inulin.     

Progesterone directly inhibits pituitary luteinizing hormone secretion in an estradiol-dependent manner.

We recently demonstrated that progesterone and estradiol inhibit pituitary LH secretion in a synergistic fashion. This study examines the direct feedback of progesterone on the estradiol-primed pituitary. Nine ovariectomized (OVX) ewes underwent hypothalamic-pituitary disconnection (HPD) and were infused with 400 ng GnRH every 2 h throughout the experiment. After 7 days of infusion, estradiol was implanted s.c. Four days later, estradiol implants were exchanged for blank implants in 4 ewes and for progesterone implants in 5 ewes. These implants remained in place for another 4 days. Blood samples were collected around exogenous GnRH pulses before and 0.5 to 96 h after implant insertion and exchange. Serum LH and progesterone concentrations were determined through RIA. One month later, 4 of the HPD-OVX ewes previously implanted with steroids were reinfused with GnRH and the implantation protocol was repeated using blank implants only. In estradiol-primed ewes, progesterone significantly lowered LH secretion after 12 h of implantation and LH secretion remained inhibited while progesterone implants were in place (p less than 0.05). Removing estradiol transiently lowered LH secretion, and this effect was significant only 24 h after estradiol withdrawal (p less than 0.05). These data suggest that progesterone has a direct, estradiol-dependent inhibitory effect on pituitary LH release and that estradiol may sustain pituitary gonadotrope response to GnRH.     

Contemporary insights into the regulation of luteinizing hormone secretion in man

Recent advances in clinical investigative techniques have now permitted the delineation of a spectrum of distinct pathophysiological disorders of gonadotropin secretion in the human, and have suggested important therapeutic avenues for the treatment of clinical hypogonadism in men and women. Advances have occurred in part in the arena of methodology with a broader assessment of the full physiological spectrum of pulsatile gonadotropin secretion, and in part in the use of more refined and selective pharmacological tools to investigate the neuroendocrine facets of gonadotropin dynamics.     

Effect of luteinizing hormone releasing hormone pulse characteristics on comparative luteinizing hormone and follicle stimulating hormone secretion from superfused rat anterior pituitary cell cultures

We have shown that 4 ng luteinizing hormone releasing hormone (LHRH) pulses induced significantly greater luteinizing hormone (LH) release from proestrous rat superfused anterior pituitary cells with no cycle related differences in follicle stimulating hormone (FSH). Current studies gave 8 ng LHRH in various pulse regimens to study amplitude, duration and frequency effects on LH and FSH secretion from estrous 0800, proestrous 1500 and proestrous 1900 cells. Regimen 1 gave 8 ng LHRH as a single bolus once/h; regimen 2 divided the 8 ng into 3 equal 'minipulses' given at 4 min intervals to extend duration; regimen 3 gave the 3 'minipulses' at 10 min intervals, thereby further extending duration: regimen 4 was the same as regimen 2, except that the 3 'minipulses' were given at a pulse frequency of 2 h rather than 1 h. In experiment 1, all four regimens were employed at proestrus 1900. FSH was significantly elevated by all 8 ng regimens as compared to 4 ng pulses; further, 8 ng divided into 3 equal 'minipulses' separated by 4 min at 1 and 3 h frequencies (regimens 2 and 4) resulted in FSH secretion that was significantly greater than with either a single 8 ng bolus (regimen 1) or when the 'minipulses' were separated by 10 min (regimen 3). In experiment 2, at proestrus 1500, FSH response to the second pulse of regimen 4 was significantly greater than in regimen 2; LH release was significantly suppressed at pulse 2 compared to regimen 2 accentuating divergent FSH secretion. At estrus 0800, FSH response to the second pulse of regimen 4 was significantly stimulated FSH at proestrus 1900, 1500 and estrus 0800, FSH divergence was most marked at proestrus 1500. These data indicate a potential role for hypothalamic LHRH secretory pattern in inducing divergent gonadotropin secretion in the rat.     

Insulin-like growth factor-I feedback regulation of growth hormone and luteinizing hormone secretion in the pig: evidence for a pituitary site of action

Three experiments (EXP) were conducted to determine the role of insulin-like growth factor-I (IGF-I) in the control of growth hormone (GH) and LH secretion. In EXP I, prepuberal gilts, 65 ± 6 kg body weight and 140 days of age received intracerebroventricular (ICV) injections of saline (n = 4), 25 μg (n = 4) or 75 μg (n = 4) IGF-I and jugular blood samples were collected. In EXP II, anterior pituitary cells in culture collected from 150-day-old prepuberal gilts (n = 6) were challenged with 0.1, 10 or 1000 nM [Ala15]-h growth hormone-releasing hormone-(1-29)NH2 (GHRH), or 0.01, 0.1, 1, 10, 30 nM IGF-I individually or in combinations with 1000 nM GHRH. Secreted GH was measured at 4 and 24 h after treatment. In EXP III, anterior pituitary cells in culture collected from 150-day-old barrows (n = 5) were challenged with 10, 100 or 1000 nM gonadotropin-releasing hormone (GnRH) or 0.01, 0.1, 1, 10, 30 nM IGF-I individually or in combinations with 100 nM GnRH. Secreted LH was measured at 4 h after treatment. In EXP I, serum GH and LH concentrations were unaffected by ICV IGF-I treatment. In EXP II, relative to control all doses of GHRH increased (P < 0.01) GH secretion. Only 1, 10, 30 nM IGF-I enhanced (P < 0.02) basal GH secretion at 4 h, whereas by 24 h all doses except for 30 nM IGF-I suppressed (P < 0.02) basal GH secretion compared to control wells. All doses of IGF-I in combination with 1000 nM GHRH increased (P < 0.04) the GH response to GHRH compared to GHRH alone at 4 h, whereas by 24 h all doses of IGF-I suppressed (P < 0.04) the GH response to GHRH. In EXP III, all doses of IGF-I increased (P < 0.01) basal LH levels while the LH response to GnRH was unaffected by IGF-I (P > 0.1). In conclusion, under these experimental conditions the results suggest that the pituitary is the putative site for IGF-I modulation of GH and LH secretion. Further examination of the role of IGF-I on GH and LH secretion is needed to understand the inhibitory and stimulatory action of IGF-I on GH and LH secretion.     

Effect of cortisol or adrenocorticotropic hormone on luteinizing hormone secretion by pig pituitary cells in vitro

The effect of cortisol or adrenocorticotropic hormone (ACTH) on basal and gonadotropin-releasing hormone (GnRH)-induced secretion of luteinizing hormone (LH) was studied in vitro using dispersed pig pituitary cells. Pig pituitary cells were dispersed with collagenase and DNAase and then grown in McCoy's 5a medium containing 10% dextran charcoal-pretreated horse serum and 2.5% fetal calf serum for 3 days. Cells were preincubated with cortisol or ACTH before GnRH was added. When pituitary cells were incubated with 400 micrograms cortisol/ml medium for 6 h or longer, increase basal secretion of LH was observed. However, GnRH-induced LH release was reduced by cortisol. The degree of this reduction was dependent on cortisol, and a concentration of cortisol higher than 100 micrograms/ml was needed. Cortisol also inhibited the 17 beta-estradiol-induced increase in GnRH response. ACTH-(1-24), ACTH-(1-39), or porcine ACTH had no influence on GnRH-induced LH secretion. Our results show that cortisol can act directly on pig pituitary to inhibit both normal and estradiol-sensitized LH responsiveness to GnRH.     

Second messenger pathways mediating chicken luteinizing hormone secretion from dispersed pituitary cells.

A series of studies was conducted to evaluate the ability of several second messengers/second messenger systems to stimulate LH secretion from dispersed chicken pituitary cells. [Gln8]-LHRH-(cLHRH) stimulated LH secretion in a dose-dependent fashion; this effect was potentiated in the presence of the phosphodiesterase inhibitor, 3-isobutyl-1-methylxanthine, and was mimicked by the cAMP analog, 8-bromo-cAMP. These data indicate that the production of cAMP in response to cLHRH can stimulate LH secretion, but do not necessarily provide evidence that such production is prerequisite. The tumor-promoting phorbol ester, phorbol 12-myristate 13-acetate (PMA), and diacylglycerol analogs, 1-oleoyl-2-acetylglycerol (OAG) and 1,2-dioctanoyl-sn-glycerol (DOG), also stimulated LH release; however, only PMA (and not cLHRH or DOG) promoted an accumulation of cAMP. The putative protein kinase C inhibitor, staurosporine, completely blocked LH release stimulated by PMA, but failed to block cLHRH-induced LH secretion. Such results indicate that protein kinase C activation can promote LH secretion, but also suggest that additional second messengers may exist to fully mediate the effects of cLHRH. Both the calcium ionophore, A23187, and the intracellular calcium mobilizing agent, thapsigargin, caused a dose-dependent increase in LH secretion; furthermore, thapsigargin augmented the stimulatory effects of PMA. These data are consistent with a role for calcium in the regulation of LH release, and indicate that the mobilization of intracellular calcium alone can affect such an action.(ABSTRACT TRUNCATED AT 250 WORDS)     

Role of central epinephrine in regulation of anterior pituitary hormone secretion

Hypothalamic regulation of anterior pituitary hormones is thought to be mediated by the release of stimulatory and/or inhibitory peptides that are, in turn, regulated by catecholaminergic neurons. The recent development of selective epinephrine (EPI) synthesis inhibitors has made it possible to disrupt central EPI neurotransmission without affecting norepinephrine or dopamine. These compounds were used in the present investigation to assess the involvement of brain EPI systems in regulation of GH, LH, and prolactin (PRL) in male and ovariectomized female rats. Inhibition of central EPI synthesis (1) inhibited episodic and morphine-, but not clonidine-induced GH release, and (2) blocked the LH surge induced by estrogen and progesterone, but did not affect episodic LH release in hormonally untreated rats. Inhibition of peripheral (adrenal) EPI synthesis had no effect on these hormones. Results of these studies suggest an excitatory role for EPI in regulation of GH and LH secretion, mediated by stimulation of GH-releasing hormone and LHRH, respectively. EPI does not appear to have a major function in regulation of PRL secretion.     

Effects of sequential acute stress exposure on stress-induced pituitary luteinizing hormone and prolactin secretion

The present study was carried out to determine the effects of repetitive acute stress exposure on pituitary secretion of both luteinizing hormone (LH) and prolactin (PRL). Adult male rats were exposed to sequential episodes of acute novel environment stress separated by intervals of either 60 or 120 minutes. Serial blood samples were obtained from animals before, during and after each stress episode via indwelling intra-cardiac cannulas. The imposition of 10 minute episodes of novel environment stress on an hourly basis eventually rendered the hypothalamic-hypophyseal LH axis refractory to the stimulatory effect of stress. If sequential stress was imposed at 120 minute intervals, LH release was significantly enhanced during each exposure. A different pattern of PRL release was observed during the same sequential stress schedule. After an initial increase in hormone release in response to the first hourly stress episode, PRL levels were unaltered during the second and third hourly stress exposures. Thereafter, plasma PRL levels showed a trend toward a progressive increase in release during each successive episode, and were significantly elevated above preceding baseline levels during the fourth and fifth hourly stress exposures. In rats exposed to stress every two hours, a significant increase in PRL levels occurred following the first, but not the second stress episode. Hormone release was again enhanced in response to the third exposure to novel environment. The present results demonstrate that the repetitive exposure to acute novel environment stress results in differential alterations in pituitary LH and PRL secretion over time, and that the timing of repeated episodes is an important determinant of continued responsiveness to stress, particularly with regard to LH release. These findings suggest that the LH and PRL hormonal responses to at least this specific stressor are mediated by independent neural mechanisms.