Density Of Melatonin Receptors Research Articles

Photic regulation of mt1 melatonin receptors in the Siberian hamster pars tuberalis and suprachiasmatic nuclei: involvement of the circadian clock and intergeniculate leaflet.

In the Siberian hamster suprachiasmatic nuclei and pars tuberalis of the pituitary, high affinity mt1 melatonin receptors are present. We have previously shown that night applied light pulse induced an increase in mt1 mRNA expression in the suprachiasmatic nuclei of this species, independently of the endogenous melatonin. Here, we report the photic regulation of melatonin receptor density and mRNA expression in the suprachiasmatic nuclei and pars tuberalis of pinealectomized Siberian hamsters and the implication in this control of either the circadian clock or the intergeniculate leaflet. The results show that: (1) A 1-h light pulse, delivered during the night, induces a transitory increase in mt1 mRNA expression in the suprachiasmatic nuclei and pars tuberalis. After 3 h this increase has totally disappeared (suprachiasmatic nuclei) or is greatly reduced (pars tuberalis). (2) The melatonin receptor density, in the suprachiasmatic nuclei, is not affected by 1 or 3 h of light, while it is strongly increased in the pars tuberalis. (3) In hamsters kept in constant darkness, the mt1 mRNA rise is gated to the subjective night in the suprachiasmatic nuclei and pars tuberalis. In contrast, the light-induced increase in melatonin binding is also observed in the subjective day in the pars tuberalis. (4) intergeniculate leaflet lesion totally inhibits the mt1 mRNA expression rise in the suprachiasmatic nuclei, while it has no effect on the light-induced increase in mt1 mRNA in the pars tuberalis. However, the light-induced increase in melatonin receptor density is totally prevented by the intergeniculate leaflet lesion in the pars tuberalis. These results show that: (1) the photic regulations of mt1 mRNA expression and receptor density are independent of each other in both the suprachiasmatic nuclei and pars tuberalis; and (2) the circadian clock and the intergeniculate leaflet are implicated in the photic regulation of melatonin receptors but their level of action differs totally between the suprachiasmatic nuclei and pars tuberalis.

Differences in hypothalamic 2-[ [formula omitted]]iodomelatonin binding in photoresponsive and non-photoresponsive white-footed mice, Peromyscus leucopus

Photoperiod is an environmental cue used by many temperate-zone species to regulate their reproductive timing. Within species, the degree of reproductive photoresponsiveness can vary widely both among and within populations. The neuroendocrine mechanisms causing this individual variation in photoresponsiveness are unknown. Using selected lines from a population of white-footed mice known to vary genetically in reproductive photoresponsiveness, we tested the hypothesis that variation in the number and/or location of melatonin receptors is the basis for individual differences in reproductive photoresponsiveness. The brains and pars tuberalis of the pituitary from sixteen mice, (eight mice from each of two lines selected for two generations to respond strongly or weakly to photoperiod), were processed for autoradiography using the radioligand 2-[ 125 I ]-iodomelatonin (IMEL). We found significantly higher specific IMEL binding in the medial preoptic area and the bed nucleus of the stria terminalis of non-responsive mice than responsive mice. There were no differences between groups in specific IMEL binding in the suprachiasmatic and dorsomedial nuclei of the hypothalamus, pars tuberalis, or paraventricular nucleus of the thalamus. These results provide support for the hypothesis that individual variation in photoresponsiveness is due in part to differences in the density or affinity of melatonin receptors.

Interstrain differences in activity pattern, pineal function, and SCN melatonin receptor density of rats.

We investigated the possibility that strain-dependent differences in the diurnal pattern of wheel running activity rhythms are also reflected in the melatonin profiles. The inbred rat strains ACI/Ztm, BH/Ztm, and LEW/Ztm. LEW were examined for diurnal [12:12-h light-dark (LD)] wheel running activity, urinary 6-sulphatoxymelatonin (aMT6s) excretion, melatonin concentrations of plasma and pineal glands, and melatonin receptor density in the suprachiasmatic nuclei (SCN). ACI rats displayed unimodal activity patterns with a high level of activity, whereas BH and LEW rats showed multimodal activity patterns with ultradian components and reduced activity levels. In contrast, the individual daily profiles of aMT6s excretion and mean melatonin synthesis followed a unimodal time pattern in all three strains, suggesting that different output pathways of the SCN are responsible for the temporal organization of locomotor activity and pineal melatonin synthesis. In addition, melatonin synthesis at night and SCN melatonin receptor density at day were significantly higher in BH and LEW rats than in ACI rats. These results support the hypothesis of a long-term stimulating effect of melatonin on its own receptor density in the SCN.

The putative melatonin receptor antagonist GR128107 is a partial agonist on Xenopus laevis melanophores.

1. GR128107 (3-(1-acetyl-3-methyl-piperidine)-5-methoxyindole) has previously been reported to be a competitive melatonin receptor antagonist in blocking melatonin inhibition of [3H]-dopamine release from rabbit retina, a response mediated by the MT2 receptor subtype. 2. GR128107, like melatonin, induced a rapid (maximum response in 60-90 min) pigment aggregation in a clonal line of Xenopus laevis melanophores. GR128107 behaved as a partial agonist (pEC50 8.58+/-0.03, n=3) with an Emax of 0.83 (relative to melatonin, pEC50 10.09+/-0.03, n=3). 3. The concentration-response curve for pigment granule aggregation to both melatonin and GR128107 was displaced in a parallel, rightward manner by melatonin receptor antagonists with very similar potencies; estimated pKB RJ252 (against melatonin 4.60/against GR128107 4.54) < GR135533 (6.40/6.14) < Luzindole (6.45/6.49) < S20929 (6.58/6.65) < 4-P-PDOT (6.73/6.85). 4. Both melatonin- and GR128107-induced pigment granule aggregation was prevented by pretreatment of melanophores with pertussis toxin (10-1000 ng ml(-1)). 5. Prolonged pre-treatment of melanophores with melatonin desensitized the pigment aggregation response to GR128107. In desensitized cells, the maximal aggregation produced by GR128107 was only 0.27+/-0.01 (n=4) and the pEC50 was reduced (vehicle 8.57+/-0.12; melatonin pre-treated 7.84+/-0.09, n=4). The maximal response to melatonin in desensitized melanophores was unchanged but the pEC50 was reduced (vehicle 10.49+/-0.03; melatonin pre-treated 9.83+/-0.04, n=4). 6. These results demonstrate that GR128107 induces pigment granule aggregation in Xenopus melanophores by activating a cell membrane melatonin receptor coupled via a pertussis toxin-sensitive G-protein. 7. The partial agonist activity of GR128107 in melanophores may be apparent because of the very high density of melatonin receptors in these cells (Bmax 1223 fmol mg protein(-1)) compared to the low density of sites in rabbit retina (Bmax 3.1 fmol mg protein(-1)). This suggestion is supported by the finding that GR128107, like melatonin, acted as a full agonist and inhibited forskolin-stimulation of cyclic AMP accumulation in NIH-3T3 cells expressing a high density of human mt1 or MT2 receptors.

Effects of two melatonin analogues, S-20098 and S-20928, on melatonin receptors in the pars tuberalis of the rat.

By using quantitative autoradiography, we studied the effects of two drugs related to melatonin on the 2-(125)I-melatonin binding in the pars tuberalis (PT) of rats. The drugs tested were two naphthalenic analogues of melatonin, S-20098 (N-[2-(7-methoxy-1-naphthyl) ethyl] acetamide), an agonist, and S-20928 (N-[2-(1-naphthyl) ethyl] cyclobutyl carboxamide), a putative antagonist. Melatonin (s.c. and i.p.), S-20098 (s.c.), and S-20928 (i.p.) were injected 4 hr before sacrifice. Acute administration of both melatonin and S-20098 decreased melatonin receptor density. In contrast, the putative antagonist S-20928, at a low dose (1 mg/kg), was ineffective on melatonin receptors. It neither affected the 2-(125)I-melatonin specific binding observed in the control group nor did it prevent the decrease in binding induced by melatonin when injected 5 min before the hormone. At a high dose (10 mg/kg), S-20928 totally blocked the effect of melatonin on melatonin receptor density and induced a decrease in binding capacity as melatonin did when injected alone. These results indicate that in the rat pars tuberalis, the melatonin agonist, S-20098, is able to down-regulate melatonin receptors, whereas S-20928 seems to behave as a partial agonist.

Cloning experiments and developmental expression of both melatonin receptor Mel 1A mRNA and melatonin binding sites in the Syrian hamster suprachiasmatic nuclei

The suprachiasmatic nuclei (SCN) are implicated in the control of circadian biological rhythms, and especially the melatonin nocturnal synthesis. In numerous rodents, melatonin has been shown to feed back on the SCN activity through high affinity receptors. In contrast, Syrian hamster SCN activity is unresponsive to melatonin injections. As this lack of effect could be linked to a developmental loss of SCN melatonin receptors, the goals of the present study were 1) to report in Syrian hamster SCN, and pars tuberalis (PT) as a control, a complete pattern of the postnatal (PN) development of the melatonin receptor density and 2) to investigate whether the regulation of the Mel 1a mRNA expression could be implicated in the post natal variations of the melatonin binding capacities. We first subcloned by PCR a partial cDNA encoding the Mel 1a receptor from Syrian hamster SCN. Subsequent quantification of Mel 1a mRNA expression and melatonin receptor density revealed that in the PT and SCN, both Mel 1a mRNA expression and melatonin binding capacities declined abruptly between PN 0 and PN 8. Afterwards, in the PT, both parameters went up until they got stabilized in adulthood. Therefore, in the PT, post natal melatonin receptor density variations were highly correlated with post natal variations of the Mel 1a mRNA expression. In the SCN, after PN 8, the melatonin receptor density followed its drop and then declined by more than 92% between post natal day 0 (PN 0) and PN 60 (12.11±0.27 vs. 0.94±0.08 fmol/mg protein at PN 0 and PN 60 respectively). In contrast, Mel 1a mRNA expression only slightly went down after PN 8 and got stabilized in adult age at 42% of the birth day expression level. These results show that Syrian hamster SCN undergo a dramatic post natal loss of their melatonin receptors that could explain the lack of effect of melatonin injections on SCN circadian activity. Furthermore, this SCN binding capacities decline could not be attributed to an inhibition of the mRNA expression, but rather to a post transcriptional blockade of the Mel 1a receptor expression.

Melatonin, immunity and cost of reproductive state in male European starlings.

The effects of reproductive condition and exogenous melatonin on immune function were investigated in castrated European starlings, Sturnus vulgaris. Photorefractory and photostimulated starlings exposed to long days were implanted with melatonin or with blank capsules. Photostimulated starlings with blank capsules exhibited reduced splenocyte proliferation in response to the T-cell mitogen, concanavalin A, compared with the other long-day birds. Exogenous melatonin prevented the suppression of immune function by photostimulation. Photorefractory starlings, with or without melatonin implants, exhibited enhanced immune function compared with photostimulated starlings implanted with blanks. This enhancement was not mediated by endogenous melatonin, but appeared to be related to changes in reproductive state. In addition to the traditional costs of reproduction in birds (e.g. raising of young), there may be a cost of the reproductive state of starlings (i.e. whether they are photorefractory or photostimulated). These data are, we believe, the first to indicate a direct effect of reproductive state on immune function that is independent of both photoperiod (i.e., changes in the duration of melatonin secretion) and gonadal steroids.

Regulation of melatonin receptors in the pars tuberalis of Syrian hamsters transferred from long to short photoperiod: implication of melatonin and testosterone.

The exposure of long day seasonal breeders to a short photoperiod (SP) induces both sexual quiescence and a decrease in pars tuberalis (PT) melatonin receptor density. Therefore, we studied the respective roles of melatonin and testosterone on the regulation of PT melatonin receptors in Syrian hamsters transferred from long photoperiod (LP) to SP. Compared with intact sexually active animals in LP, the density of melatonin receptors was not affected by the absence of melatonin after removal of the pineal gland from animals kept in either SP or LP. In contrast, the presence of a long melatonin peak in the blood which induces gonadal atrophy induced a significant decrease in binding capacity. The SP-induced decrease in PT melatonin receptor density was also observed in castrated animals showing that it was directly regulated by melatonin, independently of circulating testosterone concentrations. However, the absence of testosterone induced an increased binding in LP, while increasing blood testosterone concentration after implantation of one testosterone-filled silastic tube resulted in a decrease in binding both in LP-and in SP-animals. These results indicate that testosterone induces a photoperiod-independent decrease in PT melatonin receptor density. In summary, these results show that both melatonin and testosterone have negative regulatory effects on the density of PT melatonin receptors.

Daily and photoperiodic 2-125I-melatonin binding changes in the pars tuberalis of the Syrian hamster (Mesocricetus auratus): effect of constant light exposure and pinealectomy.

Using quantitative autoradiography, 2-125I-melatonin binding was investigated throughout the light/dark cycle in the pars tuberalis (PT) of the pituitary of adult Syrian hamsters kept for 8 weeks either in long or short photoperiod (LP or SP, respectively). Melatonin receptor density in the PT displayed photoperiod dependent daily variations (maximal values in LP). Indeed, in LP, melatonin receptor density underwent strong daily variations with maximal values during the first half of the light period and minimal values at the end of the night. These variations depended on changes in the maximal binding (Bmax) without differences in the dissociation constant (Kd). In contrast, PT melatonin receptor density was constant and at a very low level throughout the light:dark cycle in SP exposed animals. Daily PT melatonin receptor density variations of LP exposed animals were abolished by pinealectomy or continuous light exposure. These results show clearly that both at the daily and at the seasonal level the regulation of PT melatonin receptors is strongly dependent on circulating melatonin concentrations in the Syrian hamster, but that other regulatory factors, yet unclarified, might also play a role.

Localization and quantification of melatonin receptors in the diencephalon and posterior telencephalon of the sheep brain.

In an attempt to better understand the mechanisms by which melatonin controls neuroendocrine activity, we tried to define with accuracy the brain areas where the density of melatonin receptors is the highest in sheep and to establish their characteristics. The specific labelling of 125I-melatonin was first revealed by autoradiography on brain sections of the posterior telencephalon and diencephalon in three ewes. The extent and position of the five structures where the binding was found to be the highest (i.e., the pars verticalis and pars horizontalis of the nucleus tractus diagonalis, the septal area, the bed nucleus of the stria terminalis, and the ventromedial hypothalamic area) were then accurately defined by image analysis. In comparison to the landmarks given by image analysis, photographs of coronal sections of another ewe permitted the accurate definition of the limits of the structures to be punched in a second step. In six ewes, each of the five structures previously identified were punched from frozen coronal sections and binding of 125I-melatonin to membrane preparations was studied individually by Scatchard analysis. The correlation coefficient between the B/F ratio and binding (B) was in the range of 0.96-0.98, indicating that a precise quantification was possible in these different structures. The Bmax was the highest in the bed nucleus of the stria terminalis, the septal area, and the ventromedial hypothalamic area (1.38, 1.25, and 0.95 fmol/mg protein, respectively). All Kd values were less than 10 pM and the Hill coefficient was close to 1, indicating the presence of a single class of receptor to 125I-melatonin. These results indicate the reliability of a method used to measure with accuracy low concentrations of melatonin receptors in brain structures. In addition, the ventromedial hypothalamic area was found to be rich in melatonin receptors. This region is known to be involved in the central gonadotrope control in sheep.

Characterization, Guanosine 5′-O-(3-Thiotriphosphate) Modulation, Daily Variation, and Localization of Melatonin-Binding Sites in the Catfish (Silurus asotus) Brain

Characteristics, guanosine 5′-O-(3-thiotriphosphate) (GTPγS) modulation, daily variation, and localization of melatonin-binding sites in the brain of a nocturnal teleost, the catfishSilurus asotus,were studied by radioreceptor assay using 2-[125I]iodomelatonin as the radioligand. The specific binding was rapid, stable, saturable, and reversible. The radioligand binds to a single class of receptor site with an affinity (Kd) of 30.7 ± 7.3 pMand total binding capacity (Bmax) of 9.76 ± 0.79 fmol/mg protein (mean±SE,n=5). The binding sites were highly specific for 2-iodomelatonin and melatonin. The specificity was almost identical to that of functional melatonin receptors in the dermal and epidermal melanophores in this species and that of ML-1 subtype melatonin receptors in vertebrates, including melatonin-binding sites in the goldfish brain. GTPγS treatment altered both theKdandBmaxvalues, indicating that melatonin-binding sites in the catfish brain are coupled to G protein. TheBmaxvalues exhibited no daily variation under light–dark cycles of 12 hr light:12 hr dark whereas plasma melatonin levels andKdfluctuated in a rhythmic fashion. The density of melatonin-binding sites in discrete brain areas was determined to be highest in optic tectum–thalamus and hypothalamus, intermediate in telencephalon, cerebellum, and medulla oblongata, and lowest in olfactory bulbs. These results suggest that melatonin secreted from the pineal organ and/or retina plays neuromodulatory roles in the catfish brain via G protein-coupled melatonin receptors. Characteristics of melatonin receptors seem to be highly conserved during evolution, although the density of melatonin receptors is not regulated by melatonin itself in this species.

Effect of a light pulse on melatonin receptor density and mRNA expression in Siberian hamster suprachiasmatic nuclei

Melatonin receptors located in the suprachiasmatic nuclei (SCN) have been cloned in mammals hypothalamus. We previously reported a SCN melatonin receptor photic regulation in the rat. The present study's aim was to investigate whether a photic regulation of both melatonin receptor density and mRNA expression takes place in the SCN of a highly photoperiodic species, the Siberian hamster. To achieve this goal, we first set up an in situ hybridization protocol without RNase treatment to allow a highly sensitive and reproducible quantitative analysis of melatonin receptor mRNA expression. Results showed that a 1 h light pulse delivered during night-time did not affect SCN melatonin binding capacities while it induced a strong increase of the melatonin receptor mRNA expression in the SCN of pinealectomized animals.

Luteinising hormone secretion from the perifused ovine pars tuberalis and pars distalis: effects of gonadotropin-releasing hormone and melatonin.

It is not known where melatonin acts to influence the neuroendocrine axis of seasonally breeding mammals. However, since the pars tuberalis (PT) contains the highest density of melatonin receptors, this adenohypophyseal subdivision is a potential target. Gonadotropes are the only immunocytochemically detectable adenohypophyseal cell type of abundance in the PT. This study investigated whether melatonin could modulate basal and/or gonadotropin-releasing hormone (GnRH)-stimulated luteinising hormone (LH) secretion from the ovine PT and pars distalis (PD) in vitro. Tissue fragments from both pituitary areas were placed in separate chambers in a constant-environment perifusion system (37 degrees C; 100 microl/min) and 10-min effluent fractions were collected, frozen and later assayed for LH. Sixty minutes prior to a GnRH challenge (10 min; 10 nM), melatonin (1 microM or 100 nM) was added to the perifusate of half the tissue fragments. GnRH increased (p < 0.01) LH output from both pituitary subdivisions. Melatonin attenuated (p < 0.05) the GnRH-induced increase in LH output from the PT but not from the PD. The physiological importance of this melatonin-attenuated PT LH is unknown but it may play a role in modulating the neuroendocrine reproductive axis.

Daily and photoperiodic melatonin binding changes in the suprachiasmatic nuclei, paraventricular thalamic nuclei, and pars tuberalis of the female Siberian hamster (Phodopus sungorus).

Using quantitative autoradiography, 2-(125)I-melatonin binding was investigated throughout the light:dark cycle in the suprachiasmatic nuclei (SCN), paraventricular nuclei (PVT), and pars tuberalis (PT) of adult female Siberian hamsters kept for 10 weeks in either long or short photoperiods (LP or SP, respectively). Plasma melatonin concentrations were measured by radioimmunoassay, and the sexual status of the animals was established by visual inspection of vaginal smears and by weighing uteri after sacrifice. The SCN displayed neither daily nor photoperiod-dependent variations in specific binding. Melatonin receptors in these nuclei would be regulated neither by plasma melatonin nor by the light:dark cycle or sexual steroids. By contrast, melatonin receptor density in the PT displayed both strong daily (maximal values during the first half of the light period and minimal values during the night) and photoperiod-dependent (maximal values in LP) variations. These variations dependent on changes in the maximal binding (Bmax) without differences in the dissociation constant (Kd). Daily melatonin receptor densities in the PT of LP- and SP-exposed animals might be regulated by the dark:light transition but not by melatonin. Daily profiles of 2-(125)I-melatonin-specific binding in the PT were independent of photoperiod. Factors underlying the photoperiod-related variations presently are unknown. Concerning the PVT, weak variations in specific binding were detected in SP only when time points were grouped according to the light or dark periods. It is not yet possible to conclude whether they have any physiological relevance. These results show clearly that the regulation of melatonin receptors varies among structures (SCN, PVT, and PT) in the Siberian hamster and is also totally different from that found in the rat.

Serotonergic modulation of photically induced increase in melatonin receptor density and Fos immunoreactivity in the suprachiasmatic nuclei of the rat.

The mammalian suprachiasmatic nuclei (SCN) contain a circadian clock which is regulated by neuronal photic and non-photic afferences. Among these, the serotonergic input originating from the dorsal raphe nucleus (DRN) is extremely important. In rats, a light pulse administered during the dark period is known to induce the expression of the immediate early gene c-fos and to increase melatonin receptor density in the SCN. The aim of this study was to assess whether, in rats, these two phenomena were regulated by serotonin, acting via 5-HT1A receptors. Three days after pinealectomy, 4 groups of rats were injected i.p. 90 min before sacrifice with respectively: (1) vehicle, (2) the 5-HT1A-agonist 8-OH-DPAT (5 mg/kg), (3) the 5-HT1A-antagonist NAN-190 (10 mg/kg) or (4) NAN-190 and the 8-OH-DPAT. Half of the animals from each group were exposed to light for 60 min before sacrifice and the other half remained in darkness. Sacrifice took place 5 to 6 h after lights off. Our results show that the antagonist NAN-190: (1) completely blocked the photically-induced increase of melatonin receptor density in the SCN, with an IC50 = 0.352 +/- 0.103 mg/kg, and (2) partially blocked (30%) the photic induction of Fos (the protein product of c-fos) in the ventrolateral subdivision of the SCN. The agonist 8-OH-DPAT enhanced the photically-induced increase of melatonin receptors by 10% and decreased the photically-induced increase in Fos by 18%. Both drugs were devoid of any effect in non-light exposed animals. From these results we may suggest that, in rats, there is a serotonergic control of the neuronal path driving photic information to the SCN. This regulation seems to occur through 5-HT1A or 5-HT1a-like receptors.

Melatonin receptors in the suprachiasmatic nuclei and pars tuberalis of testosterone induced photoresponsive rats

The sexual axis of rats can be rendered photoresponsive by testosterone implants. We have studied in these conditions whether rat pars tuberalis (PT) melatonin receptor density would be decreased after exposure to short photoperiod (SP) like as observed in long day seasonal breeders. The answer is no, but we observed that testosterone induced a photoperiod independent decrease in PT melatonin receptor density. These results show that testosterone-induced photosensitivity in rat is not linked to an SP-induced decrease in PT melatonin receptor density. However, testosterone regulates PT melatonin receptors independently of the photoperiod.

Melatonin receptors in the suprachiasmatic nuclei and pars tuberalis of testosterone induced photoresponsive rats

The sexual axis of rats can be rendered photoresponsive by testosterone implants. We have studied in these conditions whether rat pars tuberalis (PT) melatonin receptor density would be decreased after exposure to short photoperiod (SP) like as observed in long day seasonal breeders. The answer is no, but we observed that testosterone induced a photoperiod independent decrease in PT melatonin receptor density. These results show that testosterone-induced photosensitivity in rat is not linked to an SP-induced decrease in PT melatonin receptor density. However, testosterone regulates PT melatonin receptors independently of the photoperiod.

Brain and Pituitary Melatonin Receptors in Male Rat during Post‐Natal and Pubertal Development and the Effect of Pinealectomy and Testosterone Manipulation

Using quantitative autoradiography, melatonin receptors have been studied during post-natal and pubertal development of the rat in 2 brain and 2 pituitary structures. In the pars distalis of anterior pituitary, melatonin receptors decrease gradually in density after birth and disappear in 30 day-old animals. In contrast melatonin binding is only expressed in the paraventricular nuclei of the thalamus at the age of 21-23 days and is always present in adult animals. In the suprachiasmatic nuclei and in the pars tuberalis of the pituitary, melatonin receptor density decreases after birth, remains stable for approximately 1 month and increases again at puberty to reach the birth values in the adult. This increase was absent in pinealectomized and in castrated animals but present in castrated animals receiving testosterone suggesting that it depends upon circulating testosterone and melatonin levels. These results show that melatonin receptors are differentially regulated during post-natal development in each of the 4 structures studied, and that melatonin and testosterone are 2 factors which could be involved in the regulation of melatonin receptor density in the suprachiasmatic nuclei and pars tuberalis.

Short photoperiod-dependent down-regulation of thyrotropin-alpha and - beta in hamster pars tuberalis-specific cells is prevented by pinealectomy

Hamster hypophyseal pars tuberalis (PT)-specific cells are characterized by the expression of common alpha-chain and TSH beta. Immunoreactivity for these subunits and the morphology of these cells are known to exhibit remarkable seasonal changes. The high density of melatonin (Mel) receptors on PT-specific cells leads to the supposition that fluctuations in circulating Mel levels induced by photoperiodic signals are a crucial factor for the morphological alterations. To more closely investigate transcriptional and translational activities in PT-specific cells, we cloned and sequenced hamster alpha and TSH beta complementary DNA fragments and assessed messenger RNA/protein formation by in situ hybridization and immunocytochemistry under short and long photoperiod and in pinealectomized animals kept in short photoperiod. Hamster common alpha-chain and TSH beta exhibited high sequence homology with the corresponding rat hormones [94% (alpha-chain) and 90% (TSH beta) on the nucleotide level and 100% (alpha-chain) and 96% (TSH beta) on the amino acid level]. Immunocytochemical staining with antibodies directed against the common alpha-chain and TSH beta revealed a reduced immunoreactivity of PT-specific cells under short photoperiod, but this was not altered in pinealectomized animals exposed to short photoperiod. In situ hybridization against both hormonal subunits paralleled these changes, with a dramatic decrease in hormonal messenger RNA in short photoperiod. This regulatory influence was also blocked in pinealectomy. Taken together, these results demonstrate that transcription and translation of hormonal subunits are regulated by photoperiod in hamster PT-specific cells, whereas expression remained unchanged in short photoperiod if pinealectomy was performed. We, therefore, conclude that in hamsters, the Mel Signal not the light regimen per se, is a direct or indirect Zeitgeber for the transduction of photoperiodic information to the secretory activity in this pituitary cell type.

Short photoperiod-dependent down-regulation of thyrotropin-alpha and -beta in hamster pars tuberalis-specific cells is prevented by pinealectomy.

Hamster hypophyseal pars tuberalis (PT)-specific cells are characterized by the expression of common alpha-chain and TSH beta. Immunoreactivity for these subunits and the morphology of these cells are known to exhibit remarkable seasonal changes. The high density of melatonin (Mel) receptors on PT-specific cells leads to the supposition that fluctuations in circulating Mel levels induced by photoperiodic signals are a crucial factor for the morphological alterations. To more closely investigate transcriptional and translational activities in PT-specific cells, we cloned and sequenced hamster alpha and TSH beta complementary DNA fragments and assessed messenger RNA/protein formation by in situ hybridization and immunocytochemistry under short and long photoperiod and in pinealectomized animals kept in short photoperiod. Hamster common alpha-chain and TSH beta exhibited high sequence homology with the corresponding rat hormones [94% (alpha-chain) and 90% (TSH beta) on the nucleotide level and 100% (alpha-chain) and 96% (TSH beta) on the amino acid level]. Immunocytochemical staining with antibodies directed against the common alpha-chain and TSH beta revealed a reduced immunoreactivity of PT-specific cells under short photoperiod, but this was not altered in pinealectomized animals exposed to short photoperiod. In situ hybridization against both hormonal subunits paralleled these changes, with a dramatic decrease in hormonal messenger RNA in short photoperiod. This regulatory influence was also blocked in pinealectomy. Taken together, these results demonstrate that transcription and translation of hormonal subunits are regulated by photoperiod in hamster PT-specific cells, whereas expression remained unchanged in short photoperiod if pinealectomy was performed. We, therefore, conclude that in hamsters, the Mel Signal not the light regimen per se, is a direct or indirect Zeitgeber for the transduction of photoperiodic information to the secretory activity in this pituitary cell type.