Lack of endogenous opioid inhibitory tone on LH secretion in early puberty

Role of Kisspeptin and Neurokinin B in Puberty in Female Non-Human Primates

In human patients, loss-of-function mutations in the genes encoding kisspeptin (KISS1) and neurokinin B (NKB) and their receptors (KISS1R and NK3R, respectively) result in an abnormal timing of puberty or the absence of puberty. To understand the neuroendocrine mechanism of puberty, we investigated the contribution of kisspeptin and NKB signaling to the pubertal increase in GnRH release using rhesus monkeys as a model. Direct measurements of GnRH and kisspeptin in the median eminence of the hypothalamus with infusion of agonists and antagonists for kisspeptin and NKB reveal that kisspeptin and NKB signaling stimulate GnRH release independently or collaboratively by forming kisspeptin and NKB neuronal networks depending on the developmental age. For example, while in prepubertal females, kisspeptin and NKB signaling independently stimulate GnRH release, in pubertal females, the formation of a collaborative kisspeptin and NKB network further accelerates the pubertal increase in GnRH release. It is speculated that the collaborative mechanism between kisspeptin and NKB signaling to GnRH neurons is necessary for the complex reproductive function in females.

Effects of estrogen and opioid blockade on blood pressure reactivity to stress in postmenopausal women

Estrogen may influence coronary heart disease risk in women through the effects of endogenous opioids on autonomic control of blood pressure. In a randomized, placebo-controlled trial, we examined the combined effects of estrogen and the opioid antagonist, naltrexone, on blood pressure responses to psychological stress in 42 postmenopausal women. After 3 months of estrogen or estrogen plus progestin (hormone replacement therapy; n = 27) or placebo replacement, participants completed a mental arithmetic task after administration of .7 mg/kg oral naltrexone or placebo. Systolic blood pressure (SBP), diastolic blood pressure, mean arterial pressure and heart rate (HR) were measured at rest and during the arithmetic stressor. Stress produced significant increases in circulatory measures regardless of estrogen condition or opioid blockade (p's < .001). Interestingly, there was an estrogen by naltrexone interaction on SBP reactivity scores [F(1,38) = 4.36, p < .05], where women on estrogen with intact opioid receptors showed the largest SBP responses to stress, compared with all other conditions. This is consistent with some studies of premenopausal women, suggesting that estrogens may alter opioid function during stress. The interaction between estrogen and endogenous opioids may explain sex differences in opioid effects on stress reactivity in younger premenopausal women.

Effect of opioid antagonists on sex hormone secretion

BACKGROUND

Endogenous opioids have roles in various functions in different parts of the body, including intestinal motility, suppression of pain, reinforcement of behavior, and regulation of the hypothalamic-pituitary-gonadal axis. The endogenous opioid system is also recognized to be involved in the negative-feedback regulation of the release of LH and testosterone.

AIM

The reviewed articles herein show the development of the current model of this regulation, the evidence supporting it, and also the observed effects of opioid antagonist (naloxone, naltrexone, and nalmefene) on the system.

MATERIALS AND METHODS

Review of the studies published during the years 1979-1996 (no significant studies made after that). Search from databases Pubmed, SciFinder, and Medline with search words opioid antagonists, hormones, LH, testosterone, and GnRH, in different combinations.

RESULTS/CONCLUSIONS

Opioid antagonists seem to increase the secretion of GnRH in the hypothalamus which then causes a pulsatile release of LH in the pituitary and secretion of testosterone. According to the experiments, the frequency of pulses and concentration of LH and testosterone in plasma seem to increase. These effects are seen in both men and women (at early follicular phase). More research is needed to investigate the consequences of these effects in general.

The effects of opioids and opioid analogs on animal and human endocrine systems

Opioid abuse has increased in the last decade, primarily as a result of increased access to prescription opioids. Physicians are also increasingly administering opioid analgesics for noncancer chronic pain. Thus, knowledge of the long-term consequences of opioid use/abuse has important implications for fully evaluating the clinical usefulness of opioid medications. Many studies have examined the effect of opioids on the endocrine system; however, a systematic review of the endocrine actions of opioids in both humans and animals has, to our knowledge, not been published since 1984. Thus, we reviewed the literature on the effect of opioids on the endocrine system. We included both acute and chronic effects of opioids, with the majority of the studies done on the acute effects although chronic effects are more physiologically relevant. In humans and laboratory animals, opioids generally increase GH and prolactin and decrease LH, testosterone, estradiol, and oxytocin. In humans, opioids increase TSH, whereas in rodents, TSH is decreased. In both rodents and humans, the reports of effects of opioids on arginine vasopressin and ACTH are conflicting. Opioids act preferentially at different receptor sites leading to stimulatory or inhibitory effects on hormone release. Increasing opioid abuse primarily leads to hypogonadism but may also affect the secretion of other pituitary hormones. The potential consequences of hypogonadism include decreased libido and erectile dysfunction in men, oligomenorrhea or amenorrhea in women, and bone loss or infertility in both sexes. Opioids may increase or decrease food intake, depending on the type of opioid and the duration of action. Additionally, opioids may act through the sympathetic nervous system to cause hyperglycemia and impaired insulin secretion. In this review, recent information regarding endocrine disorders among opioid abusers is presented.

Few oligo-amenorrheic athletes have vasomotor symptoms

OBJECTIVE

To assess whether women with athletic oligo-amenorrhea have vasomotor symptoms.

MATERIAL AND METHODS

A mailed questionnaire was sent to 252 female athletes about vasomotor symptoms. Identical questions were also mailed to 1523 peri- and postmenopausal women.

RESULTS

The prevalence of vasomotor symptoms was low in female athletes with oligo- and amenorrhea and similar to that found in athletes with regular menstruations. The prevalence was significantly lower than in menopausal women. Although more than a third of the menopausal women had hormone replacement therapy, 30% of them still had vasomotor symptoms at least every week compared with only 2% of the oligo-amenorrheic athletes.

CONCLUSION

Vasomotor symptoms are very uncommon in oligo-amenorrheic athletes, although many of them are hypoestrogenic. It was suggested that one factor contributing to these symptoms around menopause is low hypothalamic activity of beta-endorphins, which makes the thermoregulatory centre labile. On the other hand, supraphysiological activity in hypothalamic beta-endorphins may cause the oligo-amenorrhea in athletes, but may stabilise the thermoregulatory centre and thus prevent hot flushes.

Role of the central endogenous opiate system in patients with syndrome X

BACKGROUND

To evaluate the role of the endogenous opioid system (EOS) in abnormal pain perception in patients with syndrome X, we used a neuroendocrine approach, evaluating plasmatic luteinizing hormone (LH) changes after naloxone, a competitive antagonist of opioid receptors able to unblock tonic EOS inhibition on gonadotropin release. Thus LH response to naloxone test indicates the central EOS activity on hypothalamic luteinizing hormone-releasing hormone (LH-RH) inhibitory opioid receptors.

METHODS

Ten patients with syndrome X, 10 age-matched male patients with coronary artery disease (CAD), and 10 normal subjects were analyzed. Naloxone tests were performed between 8 and 9 am. Basal beta-endorphin and LH levels were determined on 4 blood samples at 20-minute intervals; after naloxone (0.1 mg/kg intravenously in 4 minutes), LH was measured on 8 samples at 15-minute intervals. In all patients the test was also performed after LH-RH administration. Anginal pain on exercise testing was subjectively scored on a 1 to 10 analogic scale and wall motion abnormalities were quantified by a wall motion score index.

RESULTS

Significant differences were found in LH release after naloxone (CAD 260.3 +/- 42.6 vs syndrome X 151.6 +/- 48.5 mIU/mL, P <.05), angina score (CAD 5.5 +/- 1.3 vs syndrome X 7.2 +/- 1.7, P <.05), and wall motion abnormalities (CAD 3.6 +/- 1. 2 vs syndrome X 2.8 +/- 1.9, P <.05).

CONCLUSIONS

The reduced LH release after naloxone in syndrome X, with a normal LH-RH response, suggests a lower central EOS activity, which may be related to the higher anginal pain perception.

Effect of chronic naloxone and morphine treatments on testicular, body weight, and plumage pigmentation cycle of lal munia, Estrilda amandava

At Imphal (24 degrees 44' N) testes of lal munia, Estrilda amandava, began in June/July, peaked in September/ October, and thereafter declined to a minimum in December/January. Daily im treatments of 2.5-10 mg/kg/ bird/30 days of naloxone during progressive phase suppressed testicular growth, but without effects during quiescent, peak, and regression phases. Daily morphine (5 mg/kg/bird) during progressive and peak phases stimulated testicular growth, but without effects during quiescent and regression phases. Daily morphine (10 mg/kg/bird) during progressive phase stimulated the testes, an effect reversed by daily im treatments of an equivalent dose of naloxone. Seasonal changes in body weight closely correlated with testicular size. Daily im naloxone (5 and 10 mg/kg/bird/30 days) during progressive phase inhibited the increased body weight, but had no effects during quiescent, peak, and regression phases. Morphine (5 mg/kg/bird/day) during progressive and peak phases increased body weight, but had no effects during quiescent and regression phases. Morphine (10 mg/kg/bird/day) during progressive phase increased body weight, an effect which was reversed by equivalent dose of naloxone. Plumage color increased progressively between May and August/September, was maintained during October, and thereafter declined to reach dull-brown henny feathers by December. Daily im naloxone (2.5-10 mg/kg/bird/30 days) regardless of the reproductive states did not affect plumage color cycle. Morphine (5 mg/kg/ bird/day) accelerated plumage pigmentation between June and August, but had no effect during progressive or peak phases. Postnuptial decline in plumage color was inhibited by morphine (5 and 10 mg/kg/bird/day) and naloxone failed to reverse this effect. It is concluded that in the lal munia, endogenous opioid peptides are important constituents of the neuroendocrine mechanisms that influence development of the testes and body weight.

Endometrial corticotropin-releasing hormone: expression, regulation, and potential physiological implications

Our findings show that human and rat uterus express the CRH gene. Epithelial cells of both species are the main source of endometrial CRH, while stroma does not seem to express it, unless it differentiates to decidua. Immunoreactive CRH, produced by endometrial cells, has the chromatographic characteristics of authentic hypothalamic CRH, while the size of its mRNA in both human and rat uterus is similar to or identical with its counterpart, present in placenta and hypothalamus (1.3 kb). Estrogens and glucocorticoids inhibit and prostaglandin E2 stimulates the promoter of human CRH gene in transfected human endometrial cells, suggesting that endometrial CRH gene expression is under the control of these agents. Moreover, in rats, endometrial CRH expression is significantly higher at implantation sites, compared to that at interimplantation uterine regions. Given the proinflammatory/vasoregulatory properties of CRH, we hypothesize that endometrial CRH may participate in the regulation of intrauterine phenomena, such as blastocyst implantation, endometrial vascularization, and myometrial contractility.

Endogenous opioid system modulation in anginal pain: demonstration of its central activity

Plasma beta-endorphin levels provide controversial results on the role of endogenous opioid system in modulation of anginal pain. As an alternative, the action of plasmatic luteinizing hormone after administration of naloxone was investigated: naloxone blocks the tonic endogenous opioid system inhibition of gonadotropin release; thus, the level of luteinizing hormone after naloxone administration is an index of central endogenous opioid system activity. Twenty patients with coronary artery disease and positive results of stress tests were selected: 10 had angina (group I) and 10 did not (group II). Ten healthy subjects were also studied as a control group (group III). In all patients basal plasma beta-endorphin levels, basal luteinizing hormone plasma levels (every 15 minutes for 1 hour) and luteinizing hormone plasma levels after administration of 0.1 mg/kg naloxone over 4 minutes (every 15 minutes for 2 hours) were determined. In 15 patients the test was performed after luteinizing hormone releasing hormone was given. The integral concentration time of luteinizing hormone plasma level during baseline (LHiB) and after administration of naloxone (LHiN) or luteinizing hormone releasing hormone (LHiRH), the ratio (LHiN:LHiB and LHiRH:LHiB) and the differences (LHiN-LHiB and LHiRH-LHiB) between the postinfusion period and baseline were calculated. No difference was found in beta-endorphin plasma levels and luteinizing hormone response after luteinizing hormone releasing hormone infusion.(ABSTRACT TRUNCATED AT 250 WORDS)

The brain as a target organ of gonadal steroids

Gonadal steroids have many effects in the central nervous system. Through a feedback mechanism, they influence the synthesis and release of hypothalamic gonadotropin-releasing hormone (GnRH) and/or pituitary gonadotropic hormones (luteinizing hormone, LH, and follicle stimulating hormone, FSH). Endogenous opioid peptides (EOPs) represent one of the key factors modulating the activity of sex steroids on the hypothalamus-pituitary-gonadal (HPG) axis. In particular, these peptides control the secretion of LH by inhibiting the activity of the hypothalamic neurons which produce GnRH. The EOP effect is dependent on the steroid hormone milieu, as shown by different responses to naloxone administration, both in animals and in humans. For the naloxone-induced increase in LH secretion to occur, relatively high levels of sex steroids are required. In humans, LH release is absent before sexual maturation. In fertile women, naloxone administration increases LH levels in the luteal phase but not in the follicular phase. In the postmenopausal period, naloxone has no effect on LH release; estrogen/progestin therapy does restore the LH response.

Long-Term Treatments with Morphine and Naloxone have Sex-Differentiated Effects on Luteinizing Hormone Secretion in Chronically Catheterized Fetal Pigs

Short- (one bolus injection) and long-term (repeated injections over a period of at least 7 days) effects of drugs on pituitary function in pig fetuses were studied to investigate the influence of morphine and naloxone on luteinizing hormone secretion in the chronically catheterized pig fetus between days 102 and 110 of gestation (term: 113±1 SD day). Both substances were intravenously administered in two doses: 0.1 mg and 1 mg/fetus. Repeated injections at 2-day intervals enabled us to study short- as well as long-term effects. Morphine acutely inhibited luteinizing hormone secretion both in male and female fetuses. Long-term treatment with morphine at both doses caused an inhibition of basal luteinizing hormone (levels before treatment on each day) in females (0.1 mg: r=-0.60, P<0.001; 1 mg: r=-0.45, P<0.05) while male fetuses were unaffected. Naloxone did not have any short-term effects, either in females or in males. In the long-term study, however, naloxone at 1 mg dose decreased basal luteinizing hormone levels in male fetuses (r=-0.55, P<0.01), whereas in females no effect was evident. Co-administration of 0.1 mg naloxone +0.1 mg morphine abolished the long-term inhibitory effect observed in females when 0.1 mg morphine alone was given. These results indicate that the link between opioids and the luteinizing hormone system is functional in the pig from at least 2 weeks before birth. Furthermore, there is a sex difference in the long-term effects of both morphine and naloxone. The origin of the apparently paradoxical long-term effect of naloxone in male fetuses remains unclear.

Luteinizing hormone sensitivity to naloxone in maturing male chimpanzees

We have systematically investigated the involvement of endogenous opioids in gonadotropin secretion during primate sexual maturation by examining LH/FSH responses to gonadotropin-releasing hormone (GnRH) and changes in LH secretion during infusions of saline or naloxone, an opiate antagonist, in ten male chimpanzees between one and nine years of age. Animals were anesthetized with ketamine (10 mg/kg) and injected or infused IV with GnRH, naloxone or saline. Circulating levels of serum LH were elevated to the same extent (approximately 400%) in response to GnRH (100 micrograms) in animals 1-5 years old (juvenile) and in animals 6-9 years old (pubertal). No differences were noted between the two groups in GnRH-stimulated levels of serum FSH. During treatment with naloxone (0.14 mg/kg bolus followed by 0.2 mg/kg/h maintenance infusion for 3 h), serum LH levels in pubertal animals were significantly (p less than 0.05) elevated by as much as 95% over LH levels found during treatment with saline. Juvenile animals, on the other hand, failed to demonstrate significant increases in serum LH following naloxone at the doses tested. A strong correlation (r = .84) was found between circulating testosterone and serum LH levels during naloxone treatment. These data indicate that opioid inhibition of LH secretion can be reversed by naloxone only when puberty is reached in chimpanzees and suggest an alteration in opioid regulation of GnRH near the time of puberty. The strong correlation between testosterone levels and LH responses to naloxone suggests that steroids may participate in the maturation of opioid control of LH during puberty of nonhuman primates.

Absence of an effect of naloxone, an opioid antagonist, on luteinizing hormone release in vivo and luteinizing hormone-releasing hormone I release in vitro in intact, castrated, and food restricted cockerels

The possibility that the tonic secretion of luteinizing hormone (LH) and chicken luteinizing hormone-releasing hormone I (LHRH-I) is regulated by an inhibitory action of endogenous opioid peptides was investigated in cockerels using the opiate receptor antagonist, naloxone. Baseline concentrations of plasma LH in the experimental cockerels were increased by surgical castration or reduced by limiting food intake. Baseline and K(+)-induced releases of LHRH-I from perifused mediobasal-preoptic hypothalami from castrated cockerels were higher than those from hypothalami from intact cockerels. Similarly, baseline and K(+)-induced releases of LHRH-I from perifused mediobasal hypothalami from fully fed cockerels were higher than those from the hypothalami from fasting cockerels. Intravenous injections of 0.1, 1, or 10 mg naloxone/kg body weight failed to increase the concentration of plasma LH in castrated, intact, fully fed, or fasted cockerels. Perifusion of mediobasal-preoptic hypothalami from castrated or intact cockerels with 200 microM naloxone or mediobasal hypothalami from fully fed or fasted cockerels with 10 microM naloxone failed to stimulate the release of LHRH-I. These observations suggest in the cockerel that endogenous opioid peptides may not play an obligatory role in the inhibitory control of the tonic secretion of luteinizing hormone.

Relationship between cerebrospinal fluid beta-endorphin and plasma pituitary-gonadal hormone levels in women

Brain beta-endorphin (beta-EP) plays an important role in regulating the hypothalamus-pituitary-gonadal axis activity. Cerebrospinal fluid (CSF) beta-EP levels seem to reflect the central rather than pituitary secretion. With the aim to correlate the changes of plasma estradiol (E2), progesterone, luteinizing hormone (LH) and follicle-stimulating hormone with brain beta-EP, CSF levels of beta-EP were measured in 15 normally cycling and 15 postmenopausal women. CSF beta-EP levels in post-menopausal women were lower than in fertile women. A positive correlation between plasma E2 and CSF beta-EP level was found in all women. In fertile women CSF beta-EP levels were inversely correlated to plasma gonadotropin levels. These results showed that CSF beta-EP levels differ between fertile and postmenopausal women and are correlated with plasma LH and E2, suggesting a strong linkage between central beta-EP levels and pituitary-gonadal axis hormones.

Opioids and the developing organism: a comprehensive bibliography, 1984-1988

A comprehensive bibliography of the literature concerned with opioids and the developing organism for 1984-1988 is presented. Utilized with companion papers (Neurosci. Biobehav. Rev. 6:439-479; 1982; 8:387-403; 1984), these articles cover the clinical and laboratory references beginning in 1875. For the years 1984, 1985, 1986, 1987, and 1988, a total of 877 citations were recorded. A series of indexes accompanies the citations in order to make the literature more accessible. These indexes are divided into clinical and laboratory topics, and subdivided into such topics as the type of opioid explored and the general area of biological interest (e.g., physiology).

Opioid control of luteinizing hormone secretion in humans

The observation that heroin-addicted subjects are amenorrheic and/or hypogonadic suggested a possible role of endogenous opioid peptides (EOP) in the regulation of the hypothalamus-pituitary-gonodal axis. Because EOP are localized all through the axis, they may influence the reproductive function acting at various levels. The injection of morphine decreases plasma LH levels, abolishing the pulsatile pattern of secretion. The evidence that naloxone, an opiate receptor blockade, increases LH levels suggests a tonic inhibitory action of EOP. The naloxone-induced LH increase is not observed before pubertal maturation, in both sexes, and is absent in children with delayed or precocious puberty and in those with Klinefelter's or Turner's syndrome, suggesting a role of gonadal function on the opioid control of LH secretion. In adult women this LH response to naloxone is present during the periovulatory and the luteal phase of the menstrual cycle, suggesting a permissive role of estradiol and mainly of progesterone on the action of EOP on the LH secretion. Indeed, in amenorrheic subjects naloxone lacks to stimulate plasma LH levels and the treatment for the induction of ovulation restores this activity. An increased inhibitory action of EOP on GnRH release may explain the inefficacy of naloxone to stimulate LH secretion in hypogonadotropinic patients, while a decreased action has been hypothesized in postmenopausal women. The clinical implications of EOP in reproductive medicine appear to be promising.

Neuroendocrine control of male reproductive function. The opioid system as a model of control at multiple sites

It is known that the same peptide can be identified in different secretory tissues and in the central nervous system (CNS). We now provide evidence that the same peptides can be found in different organs related to the control of a single function, and speculate on the possibility that this reflects a common neuroendocrine programming. Endogenous opioid peptides (EOP) inhibit the reproductive function acting via the CNS. EOP inhibit gonadotropin secretion in rodents and humans via inhibition of GnRH release and have direct inhibitory actions at the pituitary level via specific binding sites on the gonadotrophs. However, EOP can also be synthesized in the testis and in different compartments of the male genital tract. Several findings indicate that EOP of the reproductive tract have a local, paracrine role. These include: (1) the detection of significant beta-endorphin (beta-EP) production by rat Leydig cells (Lc) in cultures; (2) the hormonal regulation of Lc beta-EP production by positive (gonadotropins) and negative (steroids, glucocorticoids, GnRH) factors; (3) the presence of opioid binding sites (Kd in the nanomolar range) in tubular homogenates and Sertoli cells (Sc) in culture of adult and immature rat testes; (4) the inhibition of basal and FSH-stimulated ABP production by Sc in culture when chronically exposed to beta-EP treatment; (5) the detection of high levels of beta-EP and met-enkephalin in human semen with values 6-12 times higher than in plasma; (6) the evidence for inhibitory functions of seminal opioids on sperm motility, vas deferens muscle contraction and partner immune system. Thus the same peptides, i.e. EOP, may control the reproductive function at multiple sites, operating as a multimessenger system in which the central and peripheral level are unified by the common chemical and inhibitory nature of the message.

Neuroendocrine evaluation of central opiate activity in primary headache disorders

The evaluation of central opiate activity could be of clinical value in the diagnosis and treatment of pain syndromes. The current approach via direct measurement of endogenous opioid peptides in cerebrospinal fluid (CSF) is not devoid of side effects and cannot be used in every-day practice. As an alternative to this method, we have studied the neuroendocrine response of plasma LH to an i.v. naloxone injection in 39 headache sufferers from different diagnostic subgroups, and in 12 age- and sex-matched healthy volunteers. Patients (19 females and 20 males) were affected by common migraine (CM, 11 cases), migraine with interparoxysmal headache (MIH, 9), classical migraine (CIM, 9), and chronic cluster headache (CH, 10). Headache lasted 3-36 years. Prior to naloxone challenge (4 mg i.v.), LH pulsatility was evaluated for 1 h. The next morning, the pituitary response to LH-RH (10 micrograms i.v.) was tested in 20 patients. Plasma LH was measured by RIA in every sample. The response to the tests was evaluated as secretion area of plasma LH minus the mean basal value. Controls (497.5 +/- 85.5 mIU/ml x 120 min), ClM (357.8 +/- 78.9) and CH (450.5 +/- 70.4) patients showed similar results, while in cases of CM (155.3 +/- 71.7, P less than 0.05) and MIH (104.1 +/- 53.7, P less than 0.01) the LH secretion after naloxone injection was significantly blunted. On the contrary, the response of LH to LH-RH was similar in controls and patient groups, thus excluding pituitary dysfunctions in this response.(ABSTRACT TRUNCATED AT 250 WORDS)

Changes in the responsiveness of luteinizing hormone secretion to infusion of the opioid antagonist naloxone throughout male sexual maturation

The present study investigated the time of male sexual maturation during which hypothalamic inhibitory opioid activity can be detected. Normal prepubertal (Tanner stage G 1 (Ts-G1) (n = 4], early pubertal (Ts-G2 (n = 5], pubertal (Ts-G3 (n = 4), and Ts-G4 (n = 2] and adult subjects (Ts-G5 (n = 4] receives a rapid infusion of the selective opiate antagonist nalocone (NAL) (20 mg over 10 min). LH secretion was assessed by frequent (every 10 min for 2 h) venous sampling before and after administration of the opiate blocker, as well as by the LH response to exogenous GnRH. All but one (a Ts-G2 subject) pubertal boys showed aprompt and sustained increase in serum LH concentrations after NAL administration, as disclosed by the areas under the LH curve (aLHc) calculated from samples obtained before and after NAL infusion (aLHc in four Ts-G2 responders, 162 +/- 20 (mean +/- SEM) vs 314 +/- 56 mIU/ml/min before and after NAL respectively, P less than 0.025; Ts-G3, 227 +/- 35 vs 362 +/- 56 mIU/ml/min, P less than 0.025; Ts-G4 and Ts-G5, 432 +/- 77 vs 687 +/- 91 mIU/ml/min, P less than 0.05). In contrast, none of the prepubertal children had significant changes in LH secretion after the NAL challenge (154 +/- 17 vs 154 +/- 9 mIU/ml/min). Although all NAL responders exhibited serum testosterone (T) levels above 5 nmol/l, a positive correlation between individual T values and magnitude of LH responses to NAL was not found. All subjects had significant serum LH increments after GnRH administration. In a second series of studies, additional groups of Ts-G1 subjects were primed during 5 days either with GnRH alone or with GnRH plus sex steroids (ethinyl oestradiol 12.5 micrograms/12 h or testosterone enanthate 1.8 mg/kg body weight (single dose], before NAL administration, to investigate whether hypothalamic opioid activity might be unmasked by additional sex steroids. None of the priming schemes significantly modified the pituitary LH responses to NAL infusion (GnRH-primed group, 145 +/- 48 vs 139 +/- 43 mIU/ml/min before and after NAL, respectively; GnRH plus ethinyl oestradiol-primed group, 124 +/- 42 vs 107 +/- 34 mIU/ml/min; GnRH plus testosterone enanthate-primed group, 64 +/- 10 vs 57 +/- 24 mIU/ml/min). This study suggests that the development and/or maturation of the opioid control of LH secretion is temporally related with the onset of puberty.(ABSTRACT TRUNCATED AT 400 WORDS)

Hormonal regulation of beta-endorphin in the testis

It is well established that beta-endorphin has a regulatory influence on the reproductive function at the level of the hypothalamic-pituitary axis. However, recent immunohistochemical evidence demonstrated that beta-endorphin is also present in the Leydig cells of fetal, neonatal and adult mice and hamsters. In addition, beta-endorphin synthesis was localized in the Leydig cells of adult rats, leading to the hypothesis of a direct function of the peptide in the reproductive organs. Our interest was to investigate the role of beta-endorphin at testicular level. We have demonstrated the presence of high-affinity opioid binding sites (Kd in the nanomolar range) in tubular homogenates and Sertoli cells in culture of adult (50 days) and immature (18 days post-natal) rat testes. Also, chronic beta-endorphin treatment of the Sertoli cells significantly inhibited basal and FSH-stimulated androgen-binding protein production, this effect being prevented by the universal opiate antagonist naloxone. No opiate binding was observed on Leydig cell cultures. Furthermore, we have demonstrated that acute or chronic beta-endorphin treatment does not affect testosterone production by Leydig cells in vitro, consistent with the absence of receptors on these cells. On the other hand, fetal Leydig cells (21 days fetal life) in culture produced considerable amounts of beta-endorphin. Also, fetal Leydig cells represented a preferred in vitro system to study beta-endorphin release since in adult cell culture a marked degradation of the peptide was detected (greater than 50%). beta-endorphin accumulation for 3 and 5 days was markedly increased by inhibitors of steroid biosynthesis (1.5-fold); a significant reduction by GnRH at both days (by 50-30%) was observed, while by dexamethasone the reduction was only noted after 5 days of treatment (by 50%). Acute stimulation (3 h) of control cells with hCG enhanced by 10-12-fold the beta-endorphin secretion. The hormone stimulation of beta-endorphin production was not mediated by testosterone. On the contrary, inhibition of Leydig cells steroid biosynthesis markedly increased basal and hCG-stimulated beta-endorphin production (150-200%), suggesting autocrine negative modulation of Leydig cell beta-endorphin by androgen and/or its metabolites. In contrast, dexamethasone reduced basal and hCG-stimulated beta-endorphin production (by 50%). Altogether these findings indicate that beta-endorphin produced within the Leydig cells may behave as a paracrine inhibitor of the Sertoli cell function and demonstrate that the peptide production is under direct control by gonadotropins and is modulated by steroids.

The effect of long-term opiate antagonist administration to pubertal boys

To test further the hypothesis that opiatergic pathways controlling gonadotropin production may be functional during early to mid adolescence, nine pubertal boys with bone ages ranging from 10 to 15 were given the long-acting opiate antagonist, naltrexone, for up to 4 weeks. Urinary gonadotropin measurements were assessed before, during, and after drug administration. In three early to mid-pubertal boys who received naltrexone for 3 to 4 weeks, LRH testing was also performed. No evidence of a stimulatory FSH or LH response to naltrexone was found in any of the patients evaluated. The data do not support the operation of an opiate-mediated mechanism in the control of pubertal onset in man.

The effect of androgen blockade on pulsatile gonadotrophin release and LH response to naloxone

In order to clarify the effects of androgen blockade on the hypothalamic-pituitary-testicular axis in man, four patients with advanced prostate cancer, not previously treated, were given oral flutamide, 250 mg three times daily for 9 days. Before, and 7, 8 and 9 days after starting flutamide treatment, on separate days, the following tests were performed: a gonadotrophin pulsatility study, with 20 min interval blood sampling for 12 h, a naloxone test and a GnRH test. Flutamide induced a significant increase in both LH and FSH pulse frequency, while pulse amplitudes and plasma integrated concentrations (IC) of LH and FSH were unaffected. Plasma integrated concentrations of testosterone and oestradiol rose significantly, while that of prolactin was unaffected. The increase in plasma LH concentration induced by naloxone injection was abolished by flutamide treatment. On the other hand, the small FSH response to naloxone was unaffected by flutamide treatment. Response to GnRH was unaffected by flutamide. These results suggest that flutamide exerts effective androgen blockade at the hypothalamic level, since, despite increased plasma testosterone concentrations, gonadotrophin pulse frequency increased and the LH response to naloxone was abolished.