hydroxyindole-O-methyltransferase Activity Research Articles

Melatonin and its synthesizing enzymes (arylalkylamine N‐acetyltransferase‐like and hydroxyindole‐O‐methyltransferase) in avian eggs and early embryos

The presence of melatonin and the enzymes (transcripts and activities) involved in its synthesis, i.e. arylalkylamine N-acetyltransferase (AA-NAT) and hydroxyindole-O-methyltransferase (HIOMT), was investigated in the eggs and early embryos of Japanese quail at Hamburger-Hamilton stages 1-10. Melatonin was present in the egg yolk (approximately 70 pg/g) and albumen (approximately 20 pg/g). The average content of melatonin was approximately 416 pg/egg. AA-NAT and HIOMT transcripts were present in the oocytes, blastoderms, and ovarian follicles. AA-NAT-like and HIOMT activities were detected in quail egg yolk. The activity of AA-NAT in yolk was comparable with that found in the pineal gland when calculated per milligram of yolk or pineal gland, but was significantly lower when re-calculated per milligram of protein in the yolk or pineal gland. AA-NAT-like activity was also identified in the ovarian follicles. Low HIOMT activity was detected in yolk, but not in the ovarian follicle. Both enzymes were essentially absent from early embryos although some residual activities, probably of yolk origin, were present in the stage 1 embryo. Melatonin and all the constituents needed for its synthesis (serotonin, AA-NAT and HIOMT activities) are contained within the avian yolk and could be used by the embryo from the very beginning of its development. The role of extrapineal melatonin in early avian development may be in protecting the embryo from the action of free radicals formed during intensive embryonic metabolism and/or it may participate (together with serotonin) in a 'diffuse neuroendocrine system' acting at early developmental stages, before differentiation of the nervous system.

Evidence for melatonin synthesis in the rat brain during development

Melatonin production is not restricted to the pineal gland. Several extrapineal sources of this indole such as retina, Harderian gland, and immune system are well documented. Melatonin of pineal origin is not present in the rat at early stages of development. To assess the potential capacity of local melatonin synthesis by the immature brain and to gain insight into the relationship between melatonin production by the brain (without the pineal gland) and pineal gland during rat development, the melatonin content as well as the expression and activity of the melatonin-synthesizing enzymes, N-acetyltransferase (NAT) and hydroxyindole-O-methyltransferase (HIOMT), were studied at fetal and postnatal stages. Moreover, melatonin-membrane receptor (MT(1)) expression was also analyzed. Both, the expression and activity of NAT and HIOMT were found in the brain with significant day/night differences in enzymes activities. Additionally, melatonin content was detected in all stages showing day/night differences depending on the stage of development. The brain nocturnal melatonin content was higher than diurnal content on postnatal day 16 and in adult rats which is in accordance with the pineal melatonin synthesis. To investigate the origin of this brain melatonin, pinealectomized rats were used and we found that the developing brain produced its own melatonin. Also, MT(1) expression was detected in brain during development. These results demonstrate that, when the pineal is not yet producing melatonin, there is melatonin synthesis by the brain that could be used as protection from free radical damage and/or could exert some actions through MT(1) receptors.

Characterization of Human Melatonin Synthesis Using Autoptic Pineal Tissue

The mammalian pineal gland synthesizes rhythmically the hormone melatonin, which provides the body with a signal coding the duration of the night period. The ultimate enzymatic step in melatonin synthesis is achieved by the hydroxyindole O-methyltransferase (HIOMT); the rate-limiting enzyme is, however, the arylalkylamine N-acetyltransferase (AA-NAT). In contrast to the central importance of a transcriptional regulation of the Aa-nat gene for rodent melatonin synthesis, mechanisms in the human pineal gland are elusive. Therefore, pineal tissue, taken from regular autopsies (n = 69; postmortem intervals ranging from 9 to 147 h) was analyzed simultaneously for Aa-nat and Hiomt mRNA levels by PCR, AA-NAT activity using (14)C-acetyl-coenzyme A, HIOMT activity using S-adenosyl-l-[(14)C]-methionine, and melatonin content using an ELISA. Results were allocated to asserted time-of-death groups (day, 1,000 to 1,630 h; dusk, 1,630 to 2,200 h; night, 2,200 to 0730 h; dawn, 0730 to 1,000 h). RNA degradation rates of genes of interest ran in parallel, and, therefore, data normalization could be established, regardless of postmortem delay in tissue sampling. Aa-nat and Hiomt mRNA and HIOMT activity showed no diurnal rhythm. In contrast, a significant rhythm was found for the correlation between time of death and both AA-NAT activity and melatonin content, with elevated values during dusk and night. Presented data demonstrate that postmortem brain tissue can be used to detect the remnant of premortem adaptive changes in neuronal activity. In particular, our results give strong experimental support for the idea that transcriptional mechanisms are not dominant for the generation of rhythmic melatonin synthesis in the human pineal gland.

Melatonin synthesis and melatonin‐membrane receptor (MT1) expression during rat thymus development: role of the pineal gland

To gain insight into the relationship between thymus and pineal gland during rat development, the melatonin content as well as the activity and expression of the two key enzymes for melatonin biosynthesis, i.e. N-acetyltransferase (NAT) and hydroxyindole-O-methyltransferase (HIOMT), were studied in the thymus at fetal and postnatal stages. Moreover, melatonin-membrane receptor (MT1) expression was also analyzed. We found both the expression and activity of thymic NAT and HIOMT at 18 days of fetal life. Additionally, there is production of melatonin in the thymus as well as MT1 expression at this fetal age. These results show values higher in day-time than at night-time. The pineal gland begins to produce significant levels of melatonin around postnatal day 16, and this synthesis shows a circadian rhythm with high values during the dark period; therefore the nocturnal serum melatonin may inhibit thymic melatonin production. To document this, we report an increased melatonin content of the thymus in pinealectomized rats compared with sham-pinealectomized. In conclusion, these results show, for the first time, the presence of the biosynthetic machinery of melatonin and melatonin production in developing rat thymus and that the pineal gland may regulate this process.

Environmental control and adrenergic regulation of pineal activity in the diurnal tropical rodent, Arvicanthis ansorgei

Like nocturnal rodents, the diurnal tropical rodent Arvicanthis ansorgei shows a daily rhythm in pineal melatonin content. Seasonal and photoperiodic variations in the biosynthetic activity of the pineal gland: arylalkylamine-N-acetyltransferase (AA-NAT), hydroxyindole-O-methyltransferase (HIOMT) activities and melatonin content were measured in male and female A. ansorgei captured near Samaya, Mali, and kept either under artificial laboratory photoperiods [light-dark (LD) cycles: LD 14:10, LD 12:12 or LD 10:14 or caught in the field in Mali and killed at four different times of the year (January, April, June and November). Under artificial photoperiod, the duration of the nocturnal peak of AA-NAT activity and melatonin content increased with the duration of the dark period while the amplitude did not significantly change. In the field, annual variations in the amplitude of the nocturnal melatonin peak were observed with a maximum in April (highest temperature, low humidity and no grass availability, only seeds) and a minimum in November (high humidity, maximum green grass availability). The variations in the amplitude of the melatonin peak were not correlated with changes in AA-NAT HIOMT activities, suggesting that seasonal variations in the amplitude of the melatonin peak are not driven by these enzymes. Daytime injections of the beta-adrenergic agonist, isoproterenol, stimulated melatonin synthesis in January, April and June, but not in November. The annual differences in the amplitude of the melatonin peak as well as the seasonal differences in the response to an adrenergic stimulation suggest that environmental factors other than photoperiod, such as temperature, humidity and consequent food availability, could be important in the regulation of the annual variations in the pineal biosynthetic activity in this species.

Hydroxyindole-O-methyltransferase, a Season-coding Enzyme for Melatonin Synthesis in the Pineal Gland of Rodents

The time giver hormone melatonin is secreted with both daily and seasonal rhythms in the mammalian pineal gland. We report here that the two main melatoninsynthesizing enzymes N-acetyltransferase (NAT) and hydroxyindole-O-methyltransferase (HIOMT) are differently involved in the regulation of the daily and photoperiodic rhythms of melatonin synthesis. The activity of the unstable NAT enzyme is stimulated acutely at night by norepinephrine via cAMP-dependent transcriptional mechanisms, indicating that it triggers the nocturnal increase in melatonin synthesis and drives its duration according to the night length. In contrast, the activity of the stable HIOMT enzyme is regulated chronically by a norepinephrine-induced, 2-fold increase in HIOMT mRNA that occurs every night. Such regulation leads to a photoperiodic variation in HIOMT activity which is positively related to duration of the night. Whether in vivo or in vitro, at high NAT activity, HIOMT activity appears to limit the amount of melatonin release. This suggests that HIOMT activity may tune the amplitude of the nocturnal increase in pineal melatonin secretion with photoperiodic variations.

Serotoninergic and melatoninergic systems are fully expressed in human skin.

We investigated the cutaneous expression of genes and enzymes responsible for the multistep conversion of tryptophan to serotonin and further to melatonin. Samples tested were human skin, normal and pathologic (basal cell carcinoma and melanoma), cultured normal epidermal and follicular melanocytes, melanoma cell lines, normal neonatal and adult epidermal and follicular keratinocytes, squamous cell carcinoma cells, and fibroblasts from dermis and follicular papilla. The majority of the samples showed simultaneous expression of the genes for tryptophan hydroxylase, arylalkylamine N-acetyltransferase (AANAT), and hydroxyindole-O-methyltransferase (HIOMT). The products of AANAT activity were identified by RP-HPLC with fluorimetric detection in human skin and in cultured normal and malignant melanocytes and immortalized keratinocytes; HIOMT activity was detected in human skin, keratinocytes, and melanoma cells. N-acetylserotonin (NAS) was detected by RP-HPLC in human skin extracts. NAS identity was confirmed further by LC/MS in keratinocytes. In conclusion, we provide evidence that the human skin expresses intrinsic serotonin and melatonin biosynthetic pathways.

Long-term daily melatonin infusion induces a large increase in N-acetyltransferase activity, hydroxyindole-O-methyltransferase activity, and melatonin content in the Harderian gland and eye of pinealectomized male Siberian hamsters (Phodopus sungorus).

The effects of long-term daily melatonin infusions on the melatonin synthetic pathway in the Harderian glands and eyes of male Siberian hamsters were studied. Hamsters were pinealectomized (PX) and infused daily for 8 hr with either melatonin (6 microg/hr) or vehicle for 7 days in short photoperiod (SP, 10L:14D), followed by 14 wk in either SP (SP group) or in constant darkness (DD group). After the infusion period (15 wk), the infusion was stopped and animals were transferred into SP for 3 wk. The hamsters were then killed at midday or midnight. Exogenous melatonin infusion caused an increase in the Harderian gland weight, which was still evident 3 wk after the end of the treatment. In addition, exogenous melatonin increased endogenous melatonin concentrations (4-fold) and hydroxyindole-O-methyltransferase (HIOMT) activity (2-fold). N-acetyltransferase (NAT) activity, however, was not increased, and no day/night difference in melatonin content and HIOMT activity was observed in the Harderian glands. In the eye, melatonin infusions significantly increased day and night-time melatonin levels (up to 3-fold) and both NAT and HIOMT activities (up to 3.5-fold). This effect of melatonin treatment was observed in both SP and DD groups. These observations demonstrate that exogenously-infused melatonin at relatively high doses activates the synthesis of endogenous melatonin in the Harderian gland and eye of the Siberian hamster. Circulating levels of melatonin were also markedly increased, indicating that in these conditions melatonin may be released from extra-pineal sites.

HIOMT drives the photoperiodic changes in the amplitude of the melatonin peak of the Siberian hamster.

In the pineal, melatonin (Mel) is synthesized from serotonin by arylalkylamine-N-acetyltransferase (AA-NAT) and hydroxyindole-O-methyltransferase (HIOMT). Although it is clear that AA-NAT drives the daily rhythm in Mel synthesis, the mechanisms involved in the photoperiodic changes of the amplitude of the Mel peak, as observed in the Siberian hamster, remain to be determined. We investigated the characteristics of AA-NAT and HIOMT in Siberian hamsters kept either under a short (SP) or a long photoperiod (LP). The amplitude of the nocturnal peak of Mel was about two times higher under SP than under LP, whereas AA-NAT activity was about two times smaller under SP. In contrast, a twofold increase of HIOMT activity was observed under SP compared with LP. No change in the affinity of the enzymes for their substrates was observed between the two photoperiods. Our data strongly suggest that the photoperiodic variations in the amplitude of the nocturnal peak of Mel are driven by HIOMT, thereby promoting an important physiological role for this enzyme in the seasonal regulation of Mel production.

Genetic variability in plasma melatonin in sheep is due to pineal weight, not to variations in enzyme activities.

This study was conducted to determine the origin of the high variability in the mean nocturnal plasma melatonin concentration (MC) in sheep. Two extreme groups of 25 lambs each [low (L) and high (H)] were obtained by calculating their genetic value on the basis of the MC of their parents. The MC of lambs was significantly higher in the H group than in the L group (L: 189.7 +/- 24.4 vs. H: 344.1 +/- 33.0 pg/ml, P < 0.001). Within each group, 13 lambs were slaughtered during the day (D) and 12 lambs during the night (N). Pineal weight was significantly higher in the H group than in the L group (L: 83.5 +/- 6.7 vs. H: 119.1 +/- 9.2 mg, P < 0.01) but did not differ between D and N. The amount of melatonin released in vitro per milligram of pineal gland, the arylalkylamine N-acetyltransferase (AANAT) activity, the AANAT protein content, and the level of AANAT mRNA differed significantly between D and N but not with genetic group. Hydroxyindole O-methyltransferase activity did not differ significantly between D and N or between genetic groups. Therefore, the genetic difference in MC between the two groups of lambs was attributed to a difference in pineal size, not in enzymatic activity of the pinealocytes.

Regulation of AA-NAT and HIOMT gene expression by butyrate and cyclic AMP in Y79 human retinoblastoma cells.

Two key enzymes involved in the synthesis of melatonin, hydroxyindole-O-methyltransferase (HIOMT) and arylalkylamine N-acetyltransferase (AA-NAT), are present in Y79 human retinoblastoma cells. Under certain conditions these cells produce melatonin and secrete it into the culture medium. In a previous study, it was observed that melatonin levels increase dramatically over control levels after the addition of dibutyryl cyclic AMP (dbcAMP), whereas after treatment with butyrate melatonin levels decreased. The changes in melatonin levels appeared to be the result of increases in AA-NAT activity or decreases in HIOMT activity, following dbcAMP or butyrate treatment. In this study, mechanisms by which these agents influence HIOMT and AA-NAT gene expression were examined. Levels of AA-NAT and HIOMT RNA expression in response to treatment of Y79 cultures with 4 mM dbcAMP or 2 mM butyrate were measured by semi-quantitative reverse-transcription/polymerase chain reaction. Butyrate and dbcAMP showed no effect on AA-NAT gene expression, whereas HIOMT gene expression was reduced by treatment with these agents. Levels of beta-actin RNA were increased following dbcAMP or butyrate treatment. This analysis suggests that the reduction in HIOMT activity caused by dbcAMP or butyrate treatment is the result of a decrease in HIOMT RNA synthesis or accumulation. Conversely, since AA-NAT RNA levels were unaffected by dbcAMP or butyrate treatment, the increase in AA-NAT activity previously observed may be the result of changes in the activational state of the AA-NAT protein.

Changes in nocturnal pineal melatonin synthesis during the perimenopausal period: relation to estrogen levels in female rats.

To evaluate changes in melatonin synthesis during the perimenopausal period in the female rat and to determine the effects of estrogen on melatonin synthesis, pineal levels of tryptophan, melatonin and norepinephrine and activities of N-acetyltransferase (NAT) and hydroxyindole-O-methyltransferase (HIOMT) were determined. Homogenates for assay were prepared from the pineal glands of female virgin Sprague-Dawley rats between 4 and 24 months of age in the middle of the dark period of a daily light/dark cycle. Serum 17 beta-estradiol (E2) concentrations were also determined. Pineal melatonin levels significantly decreased from month 4 12 and significantly increased from month 12 16, decreasing thereafter. Serum E2 concentrations significantly decreased from month 12-16, and remained low thereafter. No significant changes in tryptophan or norepinephrine were seen. NAT activities paralleled the time course of changes in melatonin. HIOMT activities decreased gradually from month 4 24. Subcutaneous implantation of an E2 capsule between months 12 and 16 resulted in significant decreases in levels of melatonin and NAT activity at month 16. Ovariectomy at month 4 or 12 led to significant increases in the levels of melatonin and NAT activity. These findings represent a temporal increase in pineal melatonin synthesis during the perimenopausal period, and suggest that the increase in melatonin synthesis activity at that time might result from decreasing levels of endogenous estrogen. The effect of estrogen on melatonin synthesis appeared to involve modulation of NAT activity.

Age-related differences in serum melatonin and pineal NAT activity and in the response of rat pineal to a 50-HZ magnetic field

In a previous study we have shown that exposure to a 50-Hz sinusoidal magnetic field decreased serum melatonin concentration and pineal enzyme activities in young rats (9 weeks). In the present study we looked for the effect of a magnetic field of 100 μT on serum melatonin and pineal NAT activity in aged rats and compared them to young rats. We hypothesized that aging may change sensitivity of rats to a magnetic field. Two groups of Wistar male rats [aged rats (23 months) and young rats (9 weeks)] were exposed to 50-Hz magnetic fields of 100 μT for one week (18h/day). The animals were kept under a standard 12:12 light: dark cycle with a temperature of 25° C and a relative humidity of 45 to 50%. Control (sham-exposed) animals were kept in a similar environment but without exposure to a magnetic field. The animals were sacrificed under red dim light. Serum melatonin concentration and pineal N-acetyltransferase (NAT) and hydroxyindole-O-methyltransferase (HIOMT) activities were studied. Our results showed that sinusoidal magnetic fields altered the production of melatonin (28% decrease; P < 0.05) through an inhibition of pineal NAT activity (52% decrease; P < 0.05) in the young rats whereas no effect was observed in aged ones. On the other hand, when comparing data from control animals between young and aged rats, we observed that serum melatonin level and NAT activity, but not HIOMT activity, decreased in aged rats (decrease by about 38% and 36% respectively). Our data strongly suggest that old rats are insensitive to the magnetic field.

Photoperiodic control of the rat pineal arylalkylamine-N-acetyltransferase and hydroxyindole-O-methyltransferase gene expression and its effect on melatonin synthesis.

Photoperiodic changes of pineal melatonin (MEL) profile are accompanied by parallel changes of arylalkylamine-N-acetyltransferase (AA-NAT) activity. In the present study, the authors investigated, for the first time, whether two other important variables of pineal metabolism, AA-NAT and hydroxyindole-O-methyltransferase (HIOMT) gene expression, also may be affected by the photoperiod. Evening rises in AA-NAT and HIOMT mRNA and in circulating MEL occurred concomitantly with an increased delay from dark onset as scotophase shortened. On the opposite, the morning declines of all three variables occurred with different kinetics but were locked to light onset. These observations demonstrate that the daily rhythms in AA-NAT and HIOMT gene expression are modulated by the photoperiod and bring further evidence in favor of nor adrenaline as the possible link between the endogenous clock and MEL. Interestingly, the duration of the nocturnal peak in HIOMT mRNA was positively correlated with HIOMT activity. In conclusion, this study adds two important links to the chain of mechanisms involved in the photoperiodic control of pineal metabolism. First, photoperiodic modulation of the MEL rhythm primarily results from changes in the AA-NAT gene expression. Second, the photoperiodic regulation of HIOMT activity occurs at the transcriptional level.

Photoneural Regulation of Rat Pineal Hydroxyindole-O-Methyltransferase (HIOMT) Messenger Ribonucleic Acid Expression: An Analysis of Its Complex Relationship with HIOMT Activity

Photoneural Regulation of Rat Pineal Hydroxyindole-O-Methyltransferase (HIOMT) Messenger Ribonucleic Acid Expression: An Analysis of Its Complex Relationship with HIOMT Activity

Detection of Melatonin, Its Precursors and Related Enzyme Activities in Rabbit Lens

Melatonin (N-acetyl-5-methoxytryptamine) was detected in extracts of albino rabbit lens using radioimmunoassay. Furthermore, melatonin precursors, tryptophan and serotonin (5-hydroxytryptamine), were found in the lens extracts by high-performance liquid chromatography coupled with fluorometric detection. Also, activities of two melatonin-synthesizing enzymes, serotoninN-acetyltransferase (NAT) and hydroxyindole-O-methyltransferase (HIOMT), were found in the lens. The apparent Michaelis constants (Km) for substrates of NAT in the lens were similar to those reported for the pineal gland, although the apparentKm values for substrates of HIOMT in the lens were 10-fold higher than those in the pineal gland. When the rabbits were entrained to a 14-hr light: 10-hr dark cycle, melatonin levels and NAT activity in the lens showed significant day/night changes with high levels during the dark period, but HIOMT activity did not show these changes. These findings strongly suggest that the rabbit lens may synthesize melatonin from serotonin by the sequential action of NAT and HIOMT, and that the melatonin synthesis may fluctuate in a diurnal and/or circadian manner.

Ontogenesis of hydroxyindole- O-methyltransferase gene expression and activity in the rat pineal gland

Postnatal development of hydroxyindole- O-methyltransferase (HIOMT) mRNA expression, HIOMT activity and melatonin content was investigated in the rat pineal gland from birth to adulthood (62-day old). For each age, animals were sacrificed at two different time-points: midday and midnight. HIOMT mRNA was first detectable one day after birth and maximal diurnal levels were reached at the end of the first week. A 2-fold nocturnal increase appeared significantly 11 days after birth. HIOMT activity was detectable from 5 days of life and significant day/night variations did not appear before 21 days after birth. Appearance of melatonin synthesis and rhythm followed that of HIOMT mRNA. The 10-day delay observed between the appearance of the nocturnal increase in HIOMT activity and expression is discussed in term of differential regulation of both HIOMT mRNA expression and HIOMT activity.

Possible involvement of neuropeptide Y in the seasonal control of hydroxyindole- O-methyltransferase activity in the pineal gland of the European hamster ( Cricetus cricetus)

Hydroxyindole- O-methyltransferase (HIOMT) catalyses the last step of all the 5-methoxyindoles synthesized in the pineal gland. The synthetic activity of this neuroendocrine structure is driven not only by noradrenaline but also by various neuropeptides. Recently we have established (1) that one of these neuropeptides, neuropeptide Y (NPY), stimulates specifically HIOMT activity in rat pinealocytes and (2) that the density of the NPY-immunoreactive (NPY-IR) fibers innervating the pineal gland of the European hamster ( Cricetus cricetus) displays seasonal variations with a large increase in the late autumn. These findings have led us to evaluate a possible seasonal control of NPY on the European hamster pineal gland. We thus compared the nycthemeral patterns of pineal HIOMT activity and 5-methoxytryptophol (5-ML) content and of circulating MEL levels in European hamsters when NPYergic innervation is low (end of October) and when it is the highest (mid-December). We report in this study that HIOMT activity is significantly increased by 80% in mid-December compared with end of October. This increase is correlated with the appearance of a nycthemeral rhythm of pineal 5-ML levels (with a fourfold increase occurring in early dawn and decreasing slowly towards the end of the day). These observations suggest that NPY could be an important neurotransmitter involved in the seasonal control of the biochemistry of the European hamster pineal gland via a stimulatory effect on HIOMT activity.

Estrogen modulates the nocturnal synthesis of melatonin in peripubertal female rats.

Our objective was to evaluate the effects of estrogen deficit and of estrogen stimulation on the synthesis of pineal melatonin in female rats during the peripubertal period. The levels of melatonin and N-acetylserotonin (NAS) and the activities of N-acetyltransferase (NAT) and hydroxyindole-O-methyltransferase (HIOMT) were determined in homogenates of pineal glands obtained from peripubertal female Sprague-Dawley rats 4 to 12 weeks of age in the mid-dark during the daily light/dark cycle. Animals were ovariectomized at 4 weeks of age; daily administration of estradiol benzoate (E2B, 1.0 microg/d, s.c.) was initiated at 4 weeks of age. A peak in the pineal levels of melatonin and NAS and in NAT activity was observed in untreated (control) rats with intact ovaries at 6 weeks. HIOMT activity increased from Week 4 to 6 and remained unchanged thereafter. Ovariectomy at Week 4 led to significant increases in the levels of melatonin and of NAS and NAT in activity at Week 8. NAT activity Week 10 resembled that of control animals, but levels of melatonin and NAS were slightly elevated. Ovariectomy did not affect HIOMT activity. Subcutaneous injection of E2B significantly decreased the levels of melatonin and NAS and of NAT activity at Week 4, as compared with those in control rats. E2B suppressed the ovariectomy-induced elevation of levels of melatonin and NAS and of NAT activity, similar to the effect in control animals. E2B did not affect HIOMT activity. Our results suggest that estrogen modulates the nocturnal synthesis of melatonin in the pineal gland in peripubertal female rats. The effects of estrogen on melatonin synthesis appeared to be mediated by the modulation of NAT activity.

Effect of estrogen on melatonin synthesis in female peripubertal rats.

Our objective was to evaluate the effect of estrogen on the synthesis of melatonin in female rats during the peripubertal period. The level of melatonin and of N-acetyl serotonin (NAS) and the activity of N-acetyltransferase (NAT) and of hydroxy-indole-O-methyltransferase (HIOMT) were determined in homogenates of pineal glands from peripubertal female Sprague-Dawley rats in the mid-dark during the daily light/dark cycle between 4 and 10 weeks of age. Ovariectomy was performed and daily administration of estradiol benzoate (E2B) was initiated at 6 weeks of age. A peak in the pineal level of melatonin and NAS and in NAT activity was observed in untreated (control) rats with intact ovaries at 6 weeks. Thereafter, HIOMT activity increased and remained unchanged. Ovariectomy at week 6 led to significant increases in the level of melatonin and of NAS and in NAT activity at week 8. At week 10, NAT activity was similar to that of control animals, but melatonin and NAS levels were slightly elevated. Ovariectomy did not affect HIOMT activity. The subcutaneous injection of a low dose (0.1 microg/day) of E2B suppressed the ovariectomy-induced elevation of levels of melatonin and NAS and of NAT activity, similar to that seen in rats with intact ovaries. A higher dose of E2B (1.0 microg/day) reduced the activity of NAT and HIOMT to values significantly below the control values. Results suggest that estrogen modulates the nocturnal synthesis of melatonin by the pineal gland in peripubertal female rats. The decline in melatonin synthesis during puberty may be related to an increase in the estrogen level. The inhibitory effect of estrogen in melatonin synthesis appeared to be mediated by the modulation of NAT activity.