Sexual Maturation In Male Rats Research Articles

The influence of psychological stress in early life on sexual maturation and sexual behavior in male and female rats.

PurposeWe studied the influence of psychological stress during the early neonatal period on sexual maturation and sexual behavior in rats.MethodsNeonatal male and female rats were divided into control (C) and maternal separation (MS) groups (n = 20‐24 per group). The pups in the MS groups were placed in isolation cages for 240 minutes/d from postnatal days 2‐11. Vaginal opening (VO) in females and preputial separation (PS) in males (indicators of sexual maturation) were monitored, as was the estrous cycle in females. Thereafter, sexual behavior was monitored twice at 13 and 15 weeks of age.ResultsAs for sexual maturation, the onset of PS occurred significantly earlier in the MS group than in the C group, whereas the onset of VO did not differ between the groups. The length of the estrous cycle did not differ between the groups. The frequencies of sexual behaviors did not differ between the groups in either sex.ConclusionsIn conclusion, early‐life psychological stress induced by MS advanced sexual maturation in male rats, whereas it did not affect sexual maturation in female rats. On the other hand, early‐life psychological stress might not affect sexual behavior in adulthood in either sex.

Prenatal stress alters the developmental pattern of behavioral indices of sexual maturation and copulation in male rats

Gestation and pre-puberty are critical periods during which several environmental factors can drastically affect the adequate development of subjects. Considering that stress is one of the most common factors to which subjects may be exposed during gestation, the present study evaluated the effects of prenatal stress on the behavioral indices of sexual maturation in male rats, including genital grooming (GG), preputial separation (PS), and spontaneous penile erections (SPE) during puberty, and on copulatory parameters during adulthood. Stress was exerted by immobilizing the female rats once per day for 2h from days 14–21 of pregnancy. The young rats born to the dams in the stressed group (SG) later presented a delayed occurrence of PS with a delayed onset and lower frequency and duration of GG compared to a control group (CG). Less than half of the subjects in SG presented SPE, and those that did showed delayed onset and lower frequency and duration. In adulthood, fewer subjects in SG showed sexual behavior responses (intromission and ejaculation), and their mount and intromission latencies on the first day they ejaculated were longer than those of the CG rats. Findings from this study provide additional evidence that stress caused by immobilization during the third period of pregnancy exerts a negative effect in the short-term (i.e., around puberty) by altering the typical development of GG and SPE and the occurrence of PS, while also demonstrating that this effect persists in the long-term, when it affects the performance of copulatory behavior in mature male rats.

Effects of exposure to Di-(2-ethylhexyl) phthalate (DEHP) during lactation and puberty on sexual maturation and glycemic homeostasis in males rats

This study aimed to evaluate the effects of Di-(2-ethylhexyl) phthalate (DEHP) exposure during lactation or puberty on the glycemic homeostasis and the sexual maturation of male rats. Thus, in a first experiment, Wistar rats were exposed to DEHP (7.5mg/kg/day and 75mg/kg/day) from the 1st to the 21st day of lactation. The dams and their male offspring were evaluated regarding weight gain, food ingestion, serum concentrations of glucose and lipids, and insulin tolerance test (ITT). In addition, the male offspring was submitted to the quantification of insulin secretion in pancreatic islets isolated invitro, concentration of fecal androgen metabolites and determination of the age of prepucial separation. In a second round of experiments, peripuberal male rats were exposed to the same DEHP doses for 30 days (22nd to 52nd day of life). These animals were evaluated regarding weight gain and food ingestion, concentration of fecal androgen metabolites, and prepucial separation along the treatment period. The ITT and analysis of serum concentrations of glucose and lipids were carried out at the end of the treatment. The male offspring presented higher vulnerability to DEHP exposure, revealing changes in the glycemic homeostasis in adulthood, characterized by the increase in fasting glycemia, decrease in insulin sensitivity and lower insulin secretion in isolated pancreatic islets. The concentrations of cholesterol and triglycerides were reduced in the offspring exposed during lactation. The animals exposed throughout the pubertal period also presented alterations in the fasting glycemia (hyperglycemia). The DEHP doses used in this study did not induce any alteration in the androgenic status of rats that were exposed either during lactation or puberty. Overall, our results suggest that DEHP exposure during lactation or puberty can induce metabolic changes at doses that do not induce classical anti-androgen alterations.

Associations between early alcohol and tobacco use and prolonged time to puberty in boys.

Previous research has demonstrated a relationship between prepubertal alcohol and tobacco use and delayed pubertal characteristics in girls. Although, laboratory research indicates that alcohol and tobacco use inhibits sexual maturation in male rats, human research in this area is lacking. To address this question among boys, we conducted a study to explore the association between early use of alcohol and tobacco and time to development of secondary sexual characteristics. The study population included 3199 boys interviewed between the ages of 11 and 21. Participants reported the ages at which they first experienced body hair growth, deepening of the voice and facial hair growth. Early alcohol and tobacco use were defined as first use preceding the age of pubertal development among those reporting regular consumption patterns. Hazard ratios (HR) and 95% confidence intervals (CI) were calculated using Cox proportional hazard models. Early alcohol use was associated with longer time to body hair growth (HR 0.77; 95% CI 0.69-0.87), voice changes (HR 0.72; 95% CI 0.64-0.82) and facial hair growth (HR 0.77; 95% CI 0.68-0.86), after adjusting for tobacco use and age at interview. Tobacco use was not independently associated with the puberty indicators after controlling for alcohol use and age at interview. Our findings are consistent with the hypothesis that alcohol may inhibit puberty onset in boys, an association that has been previously observed among young girls. Thus, alcohol may be an exposure deserving more scrutiny as a disruptor to normal pubertal development.

Testosterone Levels and Development of the Penile Spines and Testicular Tissue during the Postnatal Growth in Wistar Rats

Aim of Study: Gonadal hormones exert a profound influence on the development, structure and function of the sexual organs. The testosterone is one of the androgens that plays an essential role in the development of sexual organs in male mammals. Therefore, the present study was undertaken to evaluate the testosterone levels and developmental pattern of the penile spines and seminiferous tubules during early postnatal life of Wistar rats. Methods and Materials: At 7, 14, 21, 28, 35, 42, 49, 56, 63 and 70 days after birth, penile and testicular tissues of male rats were dissected out and fixed for histological study and plasma testosterone levels were determined using high resolution chromatography. Results: An increase in the number of penile follicles, primarily in the distal region of the penis, was observed from postnatal days 14 to 42, followed by a gradual decrease. Penile spines were absent from birth until the first growth peak, which was observed at 42 postnatal days. Both testicular weight and the area of seminiferous tubules showed gradual increases before achieving their highest values at 42 postnatal days. Similarly, a gradual increase in testosterone levels was detected from day 28, with a peak at 42 postnatal days. Conclusions: These data show a temporal association between the development of the penile spines and testicular tissue with gradual increases in testosterone levels. These results may contribute to a better understanding of the behavioral, hormonal and morphological changes underlying sexual maturation in male rats.

Effect of Garcinia cambogia Administration on Female Reproductive Organs in Rats

We investigated the effect of administering Garcinia cambogia (GA), which contains (−)-hydroxycitric acid ((−)-HCA), to female rats, focusing especially on changes in sexual hormones and its safety in terms of histopathological changes. A diet containing a high level of (−)-HCA (154 mmol/kg diet) was given to seven-week-old female rats with matched estrous cycles for two or four weeks. There were no significant differences in any of the serum hormones tested (follicle-stimulating hormone, luteinizing hormone, estradiol and progesterone) between the control group and the GA group during the observation period. Also, there were no abnormal morphological findings in the follicle and corpus luteum, nor were there any significant differences in the ovarian 4,4-dimethyl-5α-cholesta 8,14,24-triene-3β-ol (follicular fluid meiosis-activating sterol; FF-MAS) and 4,4-dimethyl-5α-cholesta-8,24-diene-3β-ol (testis meiosis-activating sterol; T-MAS) concentrations between the control and the GA groups. On the other hand, the final body weight of the GA group was statistically lower than that of the control group. The weight of abdominal fat in the GA group was markedly lower than that in the control group even after two weeks of feeding. Therefore, although we had confirmed in our previous study that GA had a marked effect on sexual maturation in male rats, no such deleterious influence was observed in the female rats.

A maternal low protein diet during pregnancy and lactation in the rat impairs male reproductive development

Nutrient restriction during pregnancy and lactation impairs growth and development. Recent studies demonstrate long-term programming of function of specific organ systems resulting from suboptimal environments during fetal life and development up to weaning. We determined effects of maternal protein restriction (50% control protein intake) during fetal development and/or lactation in rats on the reproductive system of male progeny. Rats were fed either a control 20% casein diet (C) or a restricted diet (R) of 10% casein during pregnancy. After delivery mothers received either C or R diet until weaning to provide four groups: CC, RR, CR and RC. We report findings in male offspring only. Maternal protein restriction increased maternal serum corticosterone, oestradiol and testosterone (T) concentrations at 19 days gestation. Pup birth weight was unchanged but ano-genital distance was increased by maternal protein restriction (P < 0.05). Testicular descent was delayed 4.4 days in RR, 2.1 days in CR and 2.2 days in RC and was not related to body weight. Body weight and testis weight were reduced in RR and CR groups at all ages with the exception of CR testis weight at 270 days postnatal age (PN). At 70 days PN luteinizing hormone and T concentrations were reduced in RR, CR and RC. mRNA for P450 side chain cleavage (P450scc) was reduced in RR and CR at 21 days PN but was unchanged at 70 days PN. Fertility rate was reduced at 270 days PN in RC and sperm count in RR and RC. We conclude that maternal protein delays sexual maturation in male rats and that some effects only emerge in later life.

Effects of alcohol on behavioral and morphologic indices of sexual maturation in male rats

Effects of alcohol on behavioral and morphologic indices of sexual maturation in male rats

Effects of alcohol on behavioral and morphologic indices of sexual maturation in male rats

Results of studies support the suggestion that alcohol treatment after weaning produces a delay in the onset of puberty and a decrease in the weight of sex organs of male rats. The effect of alcohol on other morphologic and behavioral indices related to sexual maturation, however, has not been evaluated. In the current study, chronic consumption of alcohol (6% ethanol) in male rats, from weaning to 47 days of age, advanced the occurrence of increased values of genital grooming and spontaneous penile erections and of preputial separation, but had no effect on sex organ weight or penile spine number. Findings from the current study reflect that the effects of alcohol on the morphologic and behavioral indices of sexual maturation are distinct and emphasize the importance of studying critical peripubertal ages after chronic exposure to alcohol.

Gonadotropin-releasing hormone receptor gene expression during pubertal development of male rats.

Appropriate expression of the GnRH receptor (GnRH-R) in gonadotropes is critical for GnRH signaling and hence for gonadotropin secretion and sexual development. In the present work, we have studied the ontogeny of the steady-state GnRH-R mRNA levels in pituitaries of male rats from Day 5 to Day 55, when sexual maturity is attained. Developmental changes of gonadotropin subunit (alpha, FSHbeta, and LHbeta) mRNA levels were also assessed. In addition, the role of the endogenous GnRH on the maturational changes of GnRH-R and gonadotropin subunit gene expression was investigated. Messenger RNA levels were determined by Northern blot analysis of total RNA from anterior pituitaries. Amounts of the most abundant (5.0 kb) GnRH-R mRNA increased slowly from Day 5 through the infantile and the juvenile periods, to peak at Day 35 (12-fold increase vs. Day 5). Thereafter, the levels of the GnRH-R mRNA decline slightly until Day 55 (33% decrease vs. Day 35). Parallel changes were observed on the 4.5-kb transcript of the GnRH-R gene. Alpha subunit mRNA was easily detected at Day 5, and its levels increased progressively through the infantile period (2.5-fold increase) and peaked at Day 25 (3.3-fold increase vs. Day 5) with a smooth nonstatistically significant increment until Day 35; then it decreased by 41.5% at Day 55. FSHbeta and LHbeta mRNA levels rose slowly until Day 25. A sharp rise occurred thereafter to reach maximum levels at Day 35 (5.8-fold for FSHbeta and 3.8-fold for LHbeta vs. Day 25). Thereafter, the levels of both mRNAs fell until Day 55 (44.1% decrease for FSHbeta and 37.1% decrease for LHbeta vs. Day 35). To ascertain whether developmental activation of the GnRH-R and gonadotropin subunit gene expression is GnRH dependent, we have studied the effect of blocking the endogenous GnRH action by treating developing male rats with the specific GnRH antagonist cetrorelix (1.5 mg/kg body weight/week, s.c.) through the infantile (Days 5-20) and the juvenile periods (Days 20-35). Cetrorelix completely blocked the rise of levels of the two most abundant species, 5.0 kb and 4.5 kb, of the GnRH-R mRNA, during both the infantile and the juvenile periods. Cetrorelix also abolished the developmental rise of the gonadotropin beta subunit mRNAs during the two periods of the study. In contrast, the alpha subunit gene expression was not altered by cetrorelix treatment during any of the two periods. These data demonstrate that sexual maturation of male rats is accompanied by a progressive and concerted induction of GnRH-R and gonadotropin subunit gene expression. Developmental activation of GnRH-R and gonadotropin beta subunit genes is GnRH dependent. The apparent GnRH-independent regulation of the alpha-glycoprotein subunit mRNA levels may be due to the contribution of thyrotropes and perhaps to the presence of exclusive regulatory signals for this gene.

Increase in the number of detectable preoptic glutamic acid decarboxylase 67-immunoreactive cells in immature male rats.

Gonadotropin-releasing hormone (GnRH) neurons are mainly located in the anterior preoptic area (aPOA) and gamma-aminobutyric acid (GABA) is known as a potent regulator of the GnRH neurons. To examine the development of the GABAergic system in the aPOA, immunocytochemistry of glutamic acid decarboxylase 67 (GAD(67)) was performed in immature (postnatal d16, d25 and d30) and mature (postnatal 10 weeks) male rats. All immunocytochemical procedures were simultaneously performed. In the lateral part of the aPOA, the detectable number of GAD(67)-immunoreactive cells was small in the d16 group, but significantly increased in the d25, d30 and mature groups, up to 2.7, 4.8 and 5.7 times the number in the d16 group, respectively. In the diagonal band of Broca (DBB), the number was also small in the d16 group, and significantly increased in the d25, d30 and mature groups upto 1.8, 2.2 and 2.8 times the number in the d16 group, respectively. However, in the cingulate cortex, no significant developmental change was observed. These results suggest that the development of the GABAergic system in the lateral aPOA and the DBB occurs before sexual maturation of male rats.

Inhibin functions in male rats.

The role of inhibin in modulating FSH secretion at sexual maturation in male rats was studied using antibodies against inhibin alpha subunit. An immunohistochemical study was performed by immunogold-silver staining; modulation of FSH secretion in sexual maturation was studied by immunoneutralization. The results show that immunoreactive inhibin alpha subunit is localized in the Sertoli cell, especially at young ages. There were no significant differences in testes weights and histology between the antibody-administered group and the control group. Marked FSH increase was observed in the antibody-administered group from 5 to 30 days. No significant difference was detected in the over 50 days group. It would appear that inhibin plays a role in suppressing FSH secretion in infantile male rats, especially before puberty. However, inhibin's role might not be particularly important, as shown by the lack of significant changes in the gonads of antibody-administered group.

Acute fasting is ineffective in suppressing pituitary-gonadal function of pubertal male rats

Effects of short-term fasting (3-4 days) on pituitary-testicular functions were studied during sexual maturation in male rats at 25, 35, 45, 55, and 65 days of age. Among the main findings, testicular testosterone decreased by 41-68% at all ages (P < 0.01-0.05). The pituitary steady-state mRNA levels of the common alpha-subunit (28-55%) and follicle-stimulating hormone (FSH) beta-subunit (25-50%) decreased (P < 0.01-0.05) at 25, 55, and 65 days of age but not at 35 and 45 days; the luteinizing hormone (LH) beta-subunit did not respond at any age. Fasting decreased serum LH (P < 0.01) at 25, 55, and 65 days of age but not at 35 and 45 days. Likewise, fasting decreased pituitary and/or serum FSH only in the 25- and 65-day-old rats (P < 0.01-0.05). In conclusion, LH and FSH secretion, and the gene expression of common alpha- and FSH beta-subunits, decreased consistently during short-term fasting only in prepubertal (25 days) and adult (65 days) but not in peripubertal animals (35 and 45 days). Hence, the pubertal rise in gonadotropins represents such a strong positive induction that it largely overrides the antigonadotropic effect of fasting.

Serum prolactin and luteinizing hormone levels and the activities of hypothalamic monoamine oxidase A and B and phenylethanolamine-N-methyl transferase are changed during sexual maturation in male rats treated neonatally with melatonin.

Male rat pups were given a single dose of melatonin on day 5 of age. On days 30, 45, and 60, prolactin (PRL), luteinizing hormone (LH), follicle-stimulating hormone (FSH), and testosterone (T) were measured in serum and monoamine oxidase A (MAO A) activity, monoamine oxidase B (MAO B), and phenylethanolamine-N-methyl transferase (PNMT) activity were measured in the hypothalamus. Melatonin administration increased serum PRL levels at all ages studied. Serum LH levels were decreased in the melatonin treated group on day 30, but levels were elevated on days 45 and 60 of age as compared to controls. LH response to luteinizing hormone-releasing hormone (LHRH) only increased in melatonin treated animals at 30 days of age. Serum T levels decreased with melatonin treatment at 30 days of age, but increased on days 45 and 60 of age. T response to human chorionic gonadotropin (hCG) was blunted by melatonin treatment at 30 days of life. Hypothalamic MAO A activity increased, after neonatal melatonin administration, at 30 and 45 days of age and decreased at 60 days of life. Early neonatal melatonin administration increased MAO B and PNMT activities on day 45. These findings suggest that neonatal melatonin administration induces an earlier sexual maturation in male rats, possibly related to PRL, LH, MAO, and PNMT increases.

Inhibition of Sexual Maturation in Male Rats by Melatonin: Evidence Linking the Mechanism of Action to Changes in the Regulation of Hypothalamic Neuropeptide Y

Activation of gonadotrophin-releasing hormone (GnRHJ pathways is a pivotal event in the process of sexual maturation, however the regulatory influences that precipitate this change and lead to the onset of puberty remain poorly understood. Recent studies indicate that neuropeptide Y (NPY) may participate in the regulation of luteinizing hormone secretion by modulating the pattern of GnRH secretion and by directly altering the pituitary responsiveness to GnRH stimulation. To determine whether NPY plays a role in puberty-associated changes in hypothalamic function, levels of NPY-like immunoreactivity (NPY-IR) were measured in a fragment of the hypothalamus encompassing the median eminence and medial portion of the arcuate nucleus (ME-AN), and also in the remainder of the hypothalamus from male rats of different ages. To identify changes in hypothaiamic NPY linked to the process of sexual development, the effect of delaying sexual maturation by daily afternoon administration of 100 μg melatonin (MT) from 20 to 40 days was investigated. In the hypothalamus and ME-AN, total NPY content increased progressively with age. Expressed as a concentration (fmol/μg extracted protein), peak values for the ME-AN (55.4 ± 7.0) were observed at 30 days of age followed by a decline to lower levels (30.2 ± 1.9) at 40 days. Daily afternoon administration of MT from 20 days of age resulted in significant increases (P<0.01) in the levels of NPY-IR in the ME-AN compared to control values at 30 and 40 days of age. MT was without effect on NPY-IR levels in the remainder of the hypothalamus. When MT was administered in the early morning, a procedure which does not delay sexual maturation, NPY-IR values for the ME-AN region were not different from control rats indicating that the MT-induced changes in NPY were related to the effects on sexual maturation. Using pituitary luteinizing hormone content and seminal vesicle weight as indices of sexual development, significant inverse correlation coefficients (P<0.001) between these parameters and the NPY concentration in the ME-AN were observed (r =-0.79 and -0.70, respectively). From published data it is not possible to conclude whether the main effects of NPY are exerted at the hypothalamic or pituitary level. However, the changes in the NPY content of the ME-AN observed during the onset of puberty, and the influence of MT on these changes, support assertions that NPY is involved in the regulation of sexual maturation.

The changing profiles of L-ornithine decarboxylase and S-adenosyl-L-methionine decarboxylase activities in testicular cell types during sexual maturation of male rats.

To understand the involvement of polyamines in testicular maturation and spermatogenesis, the activity of two enzymes involved in the polyamine synthetic pathway, L-ornithine decarboxylase (ODC) and S-adenosyl-L-methionine decarboxylase (AdoMetDC) were determined in whole testis and isolated testicular cell types during the sexual maturation of male rats. ODC activities per mg protein in whole testis and cultured Sertoli cells from 12- to 14-day-old rats were very high but declined as the age of the animals increased. ODC activities in both germ cells and interstitial cells increased as the age of the rats increased, reached a maximum at 22 days and then declined rapidly. On the other hand, AdoMetDC activity in whole testis, Sertoli cells, germ cells and interstitial cells was highest from 27 to 35 days, 18 to 22 days, 22 to 27 or 35 days, and 27 days of age, respectively, then decreased and remained at a constant lower level after 45 days. After 35 or 45 days of age, cellular AdoMetDC activities were relatively higher than ODC activities in the whole testis and germ cells, while the activity of both enzymes was comparable in the Sertoli cells. Correlation of these data with the development of germ cells during rat testicular maturation suggests that the high ODC activity in both Sertoli and germ cells is associated mainly with cell proliferation while the increase in AdoMetDC activity is most likely associated with the maturation of Sertoli cells and the meiosis of germ cells, particularly the formation of pachytene spermatocytes and the meiotic cell division of spermatocytes into spermatids.

Involvement of brain serotonin in regulation of sexual maturity in male rats

We have studied the effect of a reduction in the serotonin level at different times in the prepubertal period on the neuroendocrine and morphometric characteristics of sexual maturation of male rats. We established that injection of p-chlorophenylalanine, a blocker of serotonin synthesis, into male rats on the 30th and 34th day of life leads to activation of the endocrine function of the testes in 60-day-old animals. Injection of the preparation at later times, 40th and 44th day, produced a marked inhibiting effect on sexual maturation, expressed as inhibition of both the spermatogenic and the endocrine function of the testes of the animals investigated. The data obtained allow an assessment of the dual role of serotonin in the regulation of the rate of sexual maturation of male rats at different prepubertal periods.

Sexual maturation of male rats treated postnatally with a gonadotrophin-releasing hormone antagonist.

Postnatal secretion of gonadotrophin by male rats was inhibited by a potent gonadotrophin-releasing hormone (GnRH) antagonist analogue (N-Ac-4-Cl-D-Phe1,4-Cl-D-Phe2,D-Trp3,D-Phe6,des-Gly10-GnRH-D-al anylamide; Org 30039; 2 mg/kg s.c. twice daily) on days 1-5, 6-10, 11-15 or 16-20 of life. The onset of puberty was determined by monitoring the separation of the preputium from the glans penis, i.e. balano-preputial separation (BPS). Rats treated on days 1-5 matured normally, whereas all treatments between days 6 and 20 delayed BPS (P less than 0.01). In adult rats (between 110 and 160 days of age), testis weights were reduced by 21-35% (P less than 0.01) in groups treated between days 1 and 15, although weights of the accessory sex glands were normal. Testicular FSH receptors were decreased by 31-47% (P less than 0.01) in all treatment groups, whereas the LH receptor content was decreased only in rats treated between days 1 and 5 (18%; P less than 0.05) and prolactin receptor content decreased only in rats treated up to day 10 (31-33%; P less than 0.01). Concentrations of serum testosterone, LH and FSH, and pituitary contents of LH and FSH were unaffected by neonatal treatment with Org 30039. Animals treated with Org 30039 had reduced fertility which was most pronounced (88%; P less than 0.01) in rats treated between days 1 and 5. However, motile sperm were detectable in the cauda epididymis of the infertile rats.(ABSTRACT TRUNCATED AT 250 WORDS)

Short-term and long-term effects of melatonin on GnRH-stimulated gonadotropin secretion in pituitaries of sexually maturing rats.

Melatonin administration has been shown to delay sexual maturation in male rats, through an action which involves decreased binding of gonadotropin-releasing hormone (GnRH) in the pituitary and lower pituitary gonadotropin contents. It has been suggested that melatonin must act at a level higher than the pituitary to provoke these effects, but a direct action of melatonin on the pituitary has not been excluded. Using a cell culture system, the gonadotroph responsiveness to GnRH was studied. Pituitaries were obtained either from rats chronically treated with melatonin and showing delayed sexual maturation, or from control rats. In vitro luteinizing hormone and follicle-stimulating hormone response to GnRH was significantly lower when pituitaries were obtained from melatonin-treated rats. However, this diminished response was directly proportional to the amount of gonadotropin contents in cells, so that relative responsiveness, calculated as the amount of gonadotropins released in relation to the gonadotropin content was similar in cells from control and melatonin-treated rats. It is concluded that the effect of melatonin on the pituitary of male rats results from a decrease of gonadotroph growth or gonadotropin synthesis, as a consequence of a change located at the hypothalamic rather than at the pituitary level. This conclusion is further supported by results showing that melatonin added directly in culture medium prior to GnRH does not modify the pituitary responsiveness to GnRH.

Inhibitory action of exogenous melatonin, 5-methoxytryptamine, and 6-hydroxymelatonin on sexual maturation of male rats: activity of 5-methoxytryptamine might be due to its conversion to melatonin.

The effect on sexual maturation of 6 different pineal indoles, including melatonin, and of the metabolite 6-hydroxymelatonin was studied in the male rat after daily injections from 20 to 40 days of age. Only 5-methoxytryptamine (5MT) and 6-hydroxymelatonin (6M), in addition to melatonin, inhibited the neuroendocrine-reproductive axis during sexual maturation. Their potencies when injected in the afternoon were in the range of one-twentieth to one-fifth that of melatonin. Like melatonin these two indoles had no effect when injected in the morning. N-acetylserotonin, serotonin, 5-hydroxytryptophol and 5-methoxytryptophol did not influence sexual maturation either when injected in the morning or in the afternoon. Chromatographic separation was performed on plasma extracts from rats injected daily with the biologically active indoles and killed 10-120 min after the last injection. This procedure confirmed that 6M injections did not increase plasma melatonin levels. In contrast, plasma melatonin levels in 5MT-treated rats were increased 1 h after the 5MT injection. These results suggest that 5MT or part of it might be acetylated to melatonin; thus inhibition of sexual maturation might be mainly due to melatonin. These results indirectly support the contention that melatonin is the principal pineal indoleamine playing a role during sexual maturation.