Different effects of aging on the opioid mechanisms controlling gonadotropin and cortisol secretion in man

The present study was undertaken in order to assess the influence of aging on the endogenous opioid control of gonadotropin and adrenocorticotropin/cortisol secretion in man. For this purpose, the capability of the opioid antagonist naloxone to increase circulating levels of luteinizing hormone (LH), follicle-stimulating hormone (FSH) and cortisol was tested in male subjects of different ages. Thirty normal men were randomly chosen and divided into 3 groups by age: group I = 22-40 years (n = 10); group II = 41-59 years (n = 10); group III = 62-80 years (n = 10). Since the men of group III showed higher basal serum gonadotropin concentrations than the subjects of group I and group II, we selected from a large population a fourth group of elderly men with normal basal LH and FSH levels: group IV = 61-82 years (n = 7). All subjects were tested for 120 min during the intravenous administration of naloxone (4 mg given in an intravenous bolus at time 0, plus 10 mg infused for 2 h). Control tests with normal saline instead of naloxone were performed in all groups. All subjects had similar blood testosterone and cortisol levels, whereas LH and FSH concentrations were significantly higher in group III than in groups I, II and IV. Naloxone increased plasma cortisol concentrations by 50% in all groups. The cortisol secretory response followed a similar pattern regardless of age.(ABSTRACT TRUNCATED AT 250 WORDS)     

Gonadotropin secretion in azotemic male rats--effect of opioid blockade

The role of endogenous opioids in the control of gonadotropin secretion in uremic male rats was investigated using the narcotic antagonist, naloxone. In order to eliminate the effect of weight loss due to uremia-induced anorexia as a cause of previously described altered gonadotropin secretion in uremia, we also studied a group of normal pair-fed control animals who exhibited a weight loss comparable to that of the uremic animals. Naloxone administration had no effect on the basal or LRH-stimulated peak concentrations of LH and FSH in the normal or the uremic rats. Basal and LRH-stimulated gonadotropin responses in the pair-fed rats were comparable to those seen in the normal rats. Similarly, opioid blockade produced no change in the basal or LRH-stimulated gonadotropin responses in the pair-fed animals. Testosterone concentrations were significantly lower in the uremic and pair-fed animals compared to the normal rats. The data suggest that experimental renal failure is not associated with altered opioidergic tone, as it relates to gonadotropin secretion, or to diminished sensitivity of the gonadotroph to LRH stimulation. The decreased testosterone concentration seen in the uremic and pair-fed rats may reflect abnormalities in gonadal hormone secretion due to primary pathology occurring at the level of the gonad. These abnormalities may be reflected as diminished Leydig cell sensitivity to LH. The inappropriately low concentrations of LH in the presence of low testosterone together with normal gonadotropin response to exogenous LRH also suggest an abnormal secretion of endogenous LRH. It is not clear whether this presumed abnormality in LRH secretion is a primary event or is related to decreased testosterone production by the testes in the uremic and pair-fed rats.     

Opioidergic control of gonadotrophin secretion in the prepubertal gilt during restricted feeding and realimentation

Prepubertal gilts, having undergone a 7-day period of feed restriction to a maintenance ration, were allocated to one of 4 treatments; restricted feeding at 09:00 and 17:00 h for an 8th day both with (Group RN) and without (Group R) administration of the opioid antagonist naloxone hydrochloride (1 mg.kg-1 at 09:30 h followed by 0.5 mg.kg-1 at hourly intervals for 7 h), or feed to appetite with (Group ALN) and without (Group AL) naloxone administration. Gilts were bled at 10-min intervals on Day 8 from morning to evening feed and plasma LH, FSH and prolactin concentrations were measured by radioimmunoassay. Compared with Group R gilts, Group AL gilts exhibited significantly (P less than or equal to 0.05) higher mean and maximum LH concentrations and pulsatility, lower prolactin concentrations (P less than 0.05) but no significant difference in FSH secretion. Naloxone significantly depressed the increase in LH after re-feeding (Group ALN) (P less than 0.05). Once again there were no significant effects on FSH secretion. Naloxone also significantly depressed prolactin secretion in feed-restricted gilts (P less than 0.05). These results confirm that re-feeding of feed-restricted prepubertal gilts stimulates an immediate increase in LH secretion and that this elevation is not mediated via a suppression of inhibitory endogenous opioidergic tone. Rather, naloxone treatment appeared to expose a latent inhibition of LH secretion. The control of LH secretion is distinct from that of FSH in this model.     

Unexpected effects of nalmefene, a new opiate antagonist, on the hypothalamic-pituitary-gonadal axis in the male rat

In order to gain additional information on the role of brain opioid peptides in the regulation of the hypothalamic-pituitary-gonadal axis, we studied the effects of nalmefene, a new opiate antagonist, on gonadotropin and testosterone secretion in male rats. The results were compared with those obtained with naloxone, a well-studied antagonist. Acute injections of either nalmefene or naloxone (2 mg/kg) produced 4-fold increases in LH and testosterone secretion. In castrated male rats treated with testosterone propionate (TP), nalmefene (10 mg/kg) reversed the androgen negative feedback on LH secretion; surprisingly, when higher doses (25 and 50 mg/kg) were injected, the compound lost its ability to antagonize the testosterone-induced inhibition of LH levels. In contrast, naloxone was able to increase LH levels in TP-treated castrated rats even at the highest dose tested (50 mg/kg). Chronic administration of these antagonists resulted in suppression of the acute release of LH and T secretion in nalmefene-treated but not in naloxone-injected animals. These data are consistent with previous observations suggesting that opioid peptides a) exert a tonic inhibitory effect on LH and testosterone production and b) participate in the negative androgen-induced feedback control of LH secretion. Our results also show that the antagonistic action of nalmefene, but not naloxone, is reversed when higher doses are used or following chronic administration.     

Modulation of luteinizing hormone and follicle-stimulating hormone in circulation by interactions between endogenous opioids and oestradiol during the peripubertal period of heifers.

The aim of this study was to determine whether the decline in oestradiol inhibition of circulating luteinizing hormone (LH) and follicle-stimulating hormone (FSH) during the peripubertal period of heifers is associated with a change in opioid modulation of LH and FSH secretion. Opioid inhibition of LH secretion was determined by response to administration of the opioid antagonist naloxone. Prepubertal heifers (403 days old) were left as intact controls, ovariectomized or ovariectomized and chronically administered oestradiol. Control heifers were used to determine time of puberty. Three weeks after ovariectomy, four doses of naloxone (0.13-0.75 mg kg-1 body weight) or saline were administered to heifers in the treatment groups in a latin square design (one dose per day). Blood samples were collected at intervals of 10 min for 2 h before and 2 h after administration of naloxone. This procedure was repeated four times at intervals of 3 weeks during the time intact control heifers were attaining puberty. All doses of naloxone induced a similar increase in concentration of serum LH within a bleeding period. During the initial bleeding period (before puberty in control heifers), administration of naloxone induced an increase in LH concentration, but the response was greater for heifers in the ovariectomized and oestradiol treated than in the ovariectomized group. At the end of the study when control heifers had attained puberty (high concentrations of progesterone indicated corpus luteum function), only heifers in the ovariectomized and oestradiol treated group responded to naloxone. Opioid inhibition of LH appeared to decline in heifers during the time control heifers were attaining puberty. Heifers in the ovariectomized group responded to naloxone at the time of administration with an increase in FSH, but FSH did not respond to naloxone at any other time. Administration of naloxone did not alter secretion of FSH in ovariectomized heifers. These results suggest that opioid neuropeptides and oestradiol are involved in regulating circulating concentrations of LH and possibly FSH during the peripubertal period. Opioid inhibition of gonadotrophin secretion appeared to decline during the peripubertal period but was still present in ovariectomized heifers treated with oestradiol after the time when age-matched control heifers had attained puberty. We conclude that opioid inhibition is important in regulating LH and FSH in circulation in heifers during the peripubertal period. However, opioids continue to be involved in regulation of circulating concentrations of LH after puberty.     

The hypothalamic-pituitary axis in diabetes mellitus

The hormonal response to LHRH and TRH was evaluated in three groups of male diaetics. Five patients were receiving therapy with the hypoglycemic agent glibenclamide, five were on NPH insulin and five were on dietary therapy alone. When compared to controls, the latter two groups had intact gonadotropin responses to LHRH. Despite normal basal gonadotropin levels, however, the group receiving glibenclamide therapy showed significantly exaggerated LH and FSH responses to LHRH. Both basal PRL and TSH levels, as well as the responses to TRH were normal in all three groups. These results indicate that LH, FSH, TSH and PRL secretion is intact in uncomplicated diabetes mellitus. The exaggerated LH and FSH responses to LHRH in the glibenclamide treated subjects are probably related to primary gonadal involvement; alternatively, there may be augmented pituitary gonadotropin secretion in this group.     

The dopaminergic inhibition of LH secretion during the menstrual cycle

To assess the potential inhibitory role of hypothalamic dopaminergic input on the LRF-LH system, the gonadotropin response to a dopamine receptor antagonist, metoclopramid (MCP, 10 mg iv bolus) was examined during different phases of the menstrual cycle in 12 women. In addition, the role of dopamine infusion on naloxone (opiate receptor antagonist) induced LH increments was examined. MCP induced an abrupt increase in circulating LH levels in the mid-luteal phases but not in the early and late follicular phase subjects. No significant changes in serum FSH levels were observed. Dopamine, when infused concomitantly with naloxone, completely suppressed the naloxone induced pulsatile increments of LH in mid-luteal subjects. These findings support the contention that an increased dopaminergic inhibition of LRF-LH system occurs during the high estrogen-progesterone phase of the menstrual cycle, and provide preliminary evidence that the inhibitory role of endogenous opioids on LRF release may involve the dopaminergic system.     

Naloxone does not interfere with the dopamine-induced decrease in gonadotropin secretion in women with polycystic ovarian disease

Dopamine infusion 4 micrograms/kg/min over 4 h, administered to six subjects with diagnosis of polycystic ovarian disease laparoscopically confirmed, produced a significant decrease in serum LH, FSH and PRL, suggesting a reduced dopamine activity in these subjects. The addition of naloxone 4 mg iv bolus plus 4 mg/h over 2 h, a specific opiate antagonist, does not interfere with the well-established dopaminergic inhibitory influence on LH, FSH and PRL secretion. This suggests that opiatergic pathways are not directly involved in the dopamine-induced suppressive effect on LH secretion in subjects with LH-dependent polycystic ovarian disease.     

Steroidal modulation of in vitro gonadotropin secretion in prepubertal and adult male Siberian hamsters

The effects of exogenous gonadal steroids, testosterone (T), and 17beta-estradiol (E(2)) upon the hypothalamo-pituitary-gonadal axis were reported to be different between prepubertal and adult Siberian hamsters. Utilizing an in vitro static culture system, we investigated if age-related differences in steroid responsiveness occurs at the pituitary. Prepubertal (20 days old) or adult (140 days old) male Siberian hamsters were implanted with 1 mm silastic capsules containing undiluted T, E(2) or cholesterol (Ch, control). After 15 days, pituitaries were removed, incubated in vitro, and subjected to the following treatments: two baseline measurements, one challenge with 10ng/ml of D-Lys(6)-gonadotropin-releasing hormone (GnRH), and three post-challenge washes. Fractions were collected every 30 minutes and measured for follicle-stimulating hormone (FSH) and luteinizing hormone (LH). T and E(2 )reduced basal secretion of LH and FSH in juveniles but not adults. In juveniles, E(2) increased GnRH-induced FSH and LH secretion, while T augmented GnRH-induced FSH secretion but attenuated GnRH-induced LH secretion. Steroid treatment had no effect on GnRH-stimulated LH or FSH release in adults. The only effect of steroid hormones upon adult pituitaries was the more rapid return of gonadotropin secretion to baseline levels following a GnRH challenge. These data suggest both basal and GnRH-induced gonadotropin secretion are more sensitive to steroid treatment in juvenile hamsters than adults. Further, differential steroidal regulation of FSH and LH at the level of the pituitary in juveniles might be a mechanism for the change in sensitivity to the negative effects of steroid hormones that occurs during the pubertal transition.     

Maturational changes in opioidergic control of luteinizing hormone and follicle-stimulating hormone in ram lambs

Stimulation by naloxone, an opioid antagonist, of luteinizing hormone (LH) and follicle-stimulating hormone (FSH) secretion was examined in spring-born crossbred ram lambs raised under natural photoperiod. Vehicle (n = 6) or 1 mg naloxone/kg vehicle (n = 6) was injected (i.m.) 3 times at 2-h intervals at 5, 10 and 15 weeks of age and 4 times at 2-h intervals at 20, 25, 30 and 35 weeks of age. Blood samples were taken every 12 min for 6 h at 5, 10 and 15 weeks of age and for 8 h at 20, 25, 30 and 35 weeks of age. Naloxone had no effect on age at sexual maturity (controls 239 +/- 23 days; naloxone 232 +/- 33 days). The only significant (P less than 0.05) effect of naloxone on FSH was a greater pulse amplitude in 10-week-old treated lambs than in control lambs. Naloxone treatment resulted in greater LH pulse amplitude at 5 and 10 weeks of age (P less than 0.05), lower basal serum concentration of LH at 10 weeks of age (P less than 0.05), greater LH pulse frequency at 25 weeks of age (P less than 0.05), and greater mean serum concentrations of LH, basal LH and LH pulse amplitude at 35 weeks of age (P less than 0.01) than in the controls. In both groups of lambs, mean and basal FSH, and LH and FSH pulse amplitude were highest at 5 weeks of age and fell with age. LH pulse amplitude was lowest at 35 weeks of age (P less than 0.05).(ABSTRACT TRUNCATED AT 250 WORDS)     

Opioidergic control of gonadotropin secretion in the female rabbit: divergent effects of morphine on secretion of follicle-stimulating hormone and luteinizing hormone

The nature of secretion of luteinizing hormone (LH) and follicle-stimulating hormone (FSH) was followed in female rabbits on a daily basis from age 36 to 60 days by sequential 5-min blood sampling over 1- to 2-h periods each day. Both LH and FSH were found to be secreted in a pulsatile manner. The mean LH pulse amplitude over the 25 days was 0.95 +/- 0.32 ng/mL and for FSH it was 10.15 +/- 1.11 ng/mL. Mean plasma LH levels were significantly increased from 1.46 +/- 0.08 ng/mL in 36 to 42-day-old rabbits to 1.89 +/- 0.12 ng/mL in 43 to 50-day-old rabbits and remained elevated from 50 to 60 days. FSH levels during the same periods also rose significantly from 14.93 +/- 0.79 to 19.57 +/- 2.05 ng/mL. To examine the influence of endogenous opioid peptides on the release of LH and FSH in 36 to 60-day-old female rabbits, morphine sulfate at 0.2, 0.5, 2.0, and 5.0 mg/kg was administered subcutaneously after 30 min baseline sampling, and blood was taken for another 60-120 min. Morphine at all doses and at all ages inhibited the amplitude and frequency of LH pulses but had no effect on FSH secretion. To determine whether the effects of morphine on LH secretion could be reversed with naloxone, females aged 82-114 days were used. Naloxone administered 1 h after morphine reversed the inhibitory effects of morphine, whereas the simultaneous administration of naloxone with morphine had variable effects but seemed to delay the LH increase.(ABSTRACT TRUNCATED AT 250 WORDS)     

Predominant involvement of mu-rather than delta- or kappa-opiate receptors in LH secretion

Opiate alkaloids and opioid peptides have been shown to suppress plasma LH and FSH levels via a naloxone sensitive mechanism in several species including man. Three subtypes of opiate receptors have been characterized: mu, delta and kappa. The present study was designed to investigate their role in gonadotropin release. Three highly selective opioid ligands, DAGO, MRZ and DTE12 (a dimeric tetrapeptide enkephalin), were injected intraventricularly into chronically ovariectomized rats. Injection of the mu-agonist at doses of 1 and 10 nmol produced a significant suppression of LH secretion, while the delta- and kappa-agonists had no significant effect. Thus, the mu-receptor seems to be the primary opiate receptor involved in the regulation of LH secretion. None of the opiate agonists employed had an effect on FSH secretion.     

Catecholamines and pituitary-function. V. Effect of low-dose dopamine infusion on basal and gonadotropin-releasing hormone stimulated gonadotropin release in normal cycling women and patients with hyperprolactinemic amenorrhea

To verify the role of dopaminergic mechanisms in the control of gonadotropin secretion in normal and hyperprolactinemic women, we examined the gonadotropin response to GnRH (100 micrograms i.v.) administration in both basal conditions and during low-dose dopamine (DA, 0.1 microgram/kg/min) infusion. Hyperprolactinemic women, either with microadenoma or without radiological signs of pituitary tumor, showed significantly enhanced LH and FSH responses to GnRH in comparison with normal cycling women. 0.1 microgram/kg/min DA infusion did not result in any appreciable suppression of serum gonadotropin levels but significantly reduced the LH and FSH responses to GnRH in both normal and amenorrheic hyperprolactinemic women. Although both LH and FSH levels remained higher in hyperprolactinemic patients than in normal women after GnRH, the gonadotroph's sensitivity to DA inhibition was normal in the hyperprolactinemic group, as both control subjects and patients with hyperprolactinemic showed similar per cent suppression of GnRH-stimulated gonadotropin release during DA. These data confirm that hypothalamic DA modulates the gonadotroph's responsiveness to GnRH. The increased LH and FSH responses to GnRH in hyperprolactinemic patients and their reduction during low-dose DA infusion seem to indicate that endogenous DA inhibition of pituitary gonadotropin release is reduced rather than enhanced in women with pathological hyperprolactinemia.     

The effect of low-dose naloxone infusion on plasma ACTH and LH in patients with Cushing's and Addison's diseases

The ACTH, cortisol and LH responses to low dose (0.8 mg/h) naloxone 90 min infusion were investigated in seven patients with untreated Cushing's disease, six patients with Addison's disease and four control subjects. Naloxone had no effects on ACTH hypersecretion or normal ACTH levels. These data confirm that naloxone cannot provide additional diagnostic or therapeutic approaches in ACTH hypersecretion syndromes, mainly in Cushing's disease. The mean percentage LH levels did not significantly change during low dose naloxone in controls or patients with Cushing's and Addison's diseases. This suggests that increased endogenous opioid peptides in these diseases may not modify the LH responses to low dose of naloxone. However, since three of five adults with Cushing's disease had increased LH levels during naloxone, further studies may be indicated.     

Cysteamine reduces serum gonadotropin concentrations in adult male rats

We have examined the effects of cysteamine on the hypothalamic-pituitary-gonadal axis of the adult male rat. A single subcutaneous injection of cysteamine (300 mg/kg) reduces significantly (p less than or equal to 0.05 serum concentrations of LH, FSH and T. Cysteamine blocked LH secretion induced by castration and administration of naloxone and LHRH. Neither acute nor chronic treatment (7 days) altered the hypothalamic LHRH content. These results suggest that cysteamine acts to reduce pituitary responsiveness to LHRH, resulting in lower mean serum gonadotropin and testosterone concentrations. It is possible, however, that cysteamine acts also at the hypothalamus to reduce LHRH secretion and/or at the testes to reduce testosterone release.     

LH- and FSH-secreting pituitary adenoma in a postmenopausal woman.

Gonadotropin secreting pituitary adenomas have been reported with increasing frequency in men, but they are still rarely recognized in women. We report a 52-year-old postmenopausal woman with LH- and FSH-secreting pituitary adenoma. She had increased LH (37.0 +/- 13.7 IU/l) (mean +/- SD) and FSH (109.9 +/- 26.7 IU/l) but these concentrations were within normal ranges in 80 postmenopausal women (LH: 29.7 +/- 18.3 IU/l, FSH: 104.0 +/- 43.9 IU/l). The administration of GnRH and conjugated estrogen resulted in normal response of LH and FSH. No abnormal response of gonadotropin to TRH and bromocriptine was observed. After transsphenoidal adenomectomy both LH and FSH decreased (LH: 11.1 +/- 4.2 IU/l, FSH: 37.0 +/- 9.6 IU/l). An immunocytochemical study revealed that the adenoma cells synthesize both LH and FSH. The rarity of gonadotropin secreting pituitary adenomas in women could be the result of greater difficulty in recognition due to an increase in serum gonadotropin in postmenopausal women.     

Reproductive experience and opioid regulation of luteinizing hormone release in female rats

The objective of the present study was to determine whether reproductive experience that produces shifts in opioid regulation of prolactin secretion and behavioural functions also alters opioid regulation of LH during the oestrous cycle or lactation. In Expt 1 the effect of naloxone administration (i.v.) on LH was compared between age-matched, nulliparous and primiparous, catheterized female rats on dioestrus II. In Expt 2, the effects of multiple reproductive experiences on opiate control of LH were investigated using cyclic, nulliparous and multiparous (three litters) rats. In both experiments, no differences in naloxone-stimulated LH release were found between groups even though multiple reproductive experiences resulted in the prolongation of oestrous cyclicity. In Expt 3, day 8 lactating primiparous rats were administered 2, 5, 10 or 25 mg naloxone kg-1 i.v. The three lowest naloxone doses, but not the 25 mg kg-1 dose, significantly increased LH concentrations. The possible effects of prior reproductive experience on opioid control of LH during lactation were then investigated. Naloxone at 0.5 mg kg-1, but not at 2 mg kg-1 or 10 mg kg-1, stimulated a significantly greater rise in LH in multiparous (two litters) than in primiparous females. Overall, these data indicate that while modest differences were found in naloxone-induced LH responses between multiparous and primiparous animals during lactation, reproductive experience did not significantly alter opioid regulation of LH during subsequent oestrous cycles at the naloxone doses examined. Hence, the effects of reproductive experience on opioid regulation of LH are less pronounced than those previously found for opioid regulation of prolactin and behaviour.     

The effect of opioid peptides on ovine pituitary gonadotropin secretion in vitro

Although a hypothalamic site of action has been firmly established for opiate-mediated gonadotropin regulation, there have been several reports which indicate the possibility of a direct influence on the pituitary gland. The objective of this study was to further investigate this possibility in an in vitro pituitary perifusion system utilizing ovine tissue. Treatment with gamma-endorphin (GE) or human beta-endorphin (hBE) resulted in elevated basal LH release (p less than 0.05), followed by an inhibition in the response to a subsequent GnRH challenge (p less than 0.05). The stimulatory effect of hBE was found to be dose-responsive (p less than 0.01). PRL secretion was not similarly stimulated. Ovine beta-endorphin (oBE) had no effect on LH secretion, even though it differs from hBE by only 2 amino acids and contains the active GE sequence. Met-enkephalin also did not influence gonadotropin secretion. Naloxone pretreatment did not reverse the effects of hBE on gonadotropin release. It was found, however, that [D-pGlu1, D-Phe2, D-Trp3,6]-GnRH, a specific GnRH receptor antagonist, did reduce hBE-induced LH and FSH release (p less than 0.05). Naloxone pretreatment alone suppressed the response to GnRH (p less than 0.05). These data indicate that certain opioid peptides can influence ovine gonadotropin secretion in vitro by activating the GnRH receptor. Furthermore, a facilitory role is suggested for endogenous opiates in the local regulation of pituitary gonadotropin secretion.     

Opioidergic control of luteinizing hormone secretion in the female rabbit: influence of age on the response to naloxone

To examine the role of opioid neurons on luteinizing hormone (LH) secretion in the female rabbit, we determined LH release at timed intervals after naloxone administration to rabbits aged 25-150 days. The LH response to naloxone (10 mg/kg) was not significantly elevated until day 43 when LH rose 76-113% above basal levels at 40-80 min. In 56-day-old females the corresponding increase was 160% at 15 min and in 65- to 67-day-olds it was 154%. From 70 to 80 days of age the LH response was blunted and no significant elevations could be elicited. By contrast, naloxone-induced LH increases were again evident when rabbits were older than 100 days. At all ages no significant change in FSH concentrations was observed. In the adult females, naloxone at 2.5, 5, and 10 mg/kg caused increases in LH secretion which occasionally were high enough to induce ovulation as exemplified by elevated serum progesterone 4 days later. These data suggest that opioid peptides may be involved in the prepubertal rise in LH and in the normal inhibition of adult secretion in the female rabbit.     

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.