Melatonin Receptor Subtypes Research Articles

Testicular atrophy and reproductive quiescence in photorefractory and scotosensitive quail: Involvement of hypothalamic deep brain photoreceptors and GnRH-GnIH system

Birds time their daily and seasonal activities in synchronization with circadian and annual periodicities in the environment, which is mainly provided by changes in photoperiod/day length conditions. Photoperiod appears to act at the level of eye, pineal and encephalic/deep brain photoperception and thus entrain the hypothalamic clock as well as reproductive circuitry in different avian species. In this article our focus of study is to elucidate out the underlying molecular mechanism of modulation of the hypothalamic reproductive circuitry following the photoperception through the hypothalamic photoreceptor cells and the subsequent alteration in the reproductive responses in quail, kept under different simulated photoperiodic conditions. Present study investigated the different simulated photoperiodic conditions induced hypothalamic DBP-GnRH-GnIH system mediated translation of photoperiodic information and subsequent exhibition of differential photosexual responses (scoto-/photo-sensitivity and refractoriness) in Japanese quail, Coturnix coturnix japonica. Paired testes weight and paired testicular volume increased 15.9 and 22.6-fold respectively in scotorefractory quail compare to that of scotosensitive phase and 12.8 and 24.3-fold in photosensitive quail compare to that of photorefractory phase. The pineal/eye melatonin (through melatonin receptor subtype Mel1cR) and hypothalamic deep brain photoreceptor (DBPs) cells directly modulate the hypothalamic GnRH-I/II and GnIH system and thus exhibit testicular stimulation or regression in response to different photoperiodic conditions (PS, PR, SS and SR). The hypothalamic alteration of DBP(s) and GnRH-GnIH system thus may induce the testicular stimulation in PS and SR quail and testicular regression in SS and PR quail.

Melatonin modulates monochromatic light-induced melatonin receptor expression in the hypothalamus of chicks

To study the mechanism of the effect of monochromatic light on physiological function in chicken, a total of 192 newly hatched chicks were randomly divided into intact, sham-operated and pinealectomy groups then exposed to white light (WL), red light (RL), green light (GL) and blue light (BL) using a light-emitting diode (LED) system for two weeks. At P14, the hypothalami were immediately collected for immunohistochemical staining of melatonin receptor subtypes (Mel1a and Mel1b) and detection of Mel1a and Mel1b expressions using RT-PCR and western blot. Immunohistochemical staining of the hypothalamus showed that the Mel1a-ir cells were distributed in the preoptic area (POA), nucleus preopticus periventricularis (POP) and suprachiasmatic nuclei (SCN), and the Mel1b-ir cells were presented in the POA and SCN. Analysis of RT-PCR and western blot showed that the mRNA and protein levels of Mel1a and Mel1b in the hypothalamus of chick exposed to GL were increased by 10.7–29.3%, 9.18–35.9% and 8.97–27.3% compared to those in the chicks exposed to WL (P=0.029–0.002), RL (P=0.027–0.001) and BL (P=0.038–0.007) in the intact group, respectively. After pinealectomy, however, these parameters decreased and there were no significant differences among the WL, RL, GL and BL groups. These findings suggested that melatonin plays a critical role in GL illumination-enhanced Mel1a and Mel1b expressions in the hypothalamus of chicks.

Physiological crosstalk between the AC/PKA and PLC/PKC pathways modulates melatonin-mediated, monochromatic-light-induced proliferation of T-lymphocytes in chickens.

Previous study has demonstrated that melatonin plays a critical role in monochromatic-light-induced lymphocyte proliferation in response to T cell mitogen concanavalin A (ConA). However, its intracellular mechanism is still unclear. In this study, we investigate the intracellular signal pathways of melatonin receptor-mediated T-lymphocyte proliferation in the spleens of chicks exposed to different light wavelengths. Results showed that green light enhanced T-lymphocyte proliferation by 2.46-6.83% and increased splenic mRNA and protein expressions of melatonin receptor subtypes (Mel1a, Mel1b and Mel1c) by 16.05-40.43% compared with the white, red and blue light groups. However, pinealectomy resulted in a decrease in T-lymphocyte proliferation and melatonin receptor expression with no statistically significant differences between the different light groups. In vitro experiments showed that the Mel1b selective antagonist 4P-PDOT, the Mel1c selective antagonist prazosin and the mitogen-activated protein kinase kinase-1 (MEK-1) inhibitor PD98059 suppressed both melatonin-induced lymphocyte proliferation in response to ConA and melatonin- and ConA-stimulated extracellular signal-regulated kinase 1/2 (ERK1/2) activity but that the Mel1a/Mel1b non-selective antagonist luzindole did not. In addition, pretreatment with forskolin (FSK, the adenylyl cyclase activator), H89 (the PKA inhibitor), U73122 (the PLC inhibitor) or Go6983 (the broad spectrum PKC inhibitor) markedly attenuated melatonin- and ConA-stimulated T-lymphocyte proliferation and ERK1/2 activity. These results demonstrate that melatonin mediates green-light-induced T-lymphocyte proliferation via the Mel1b and Mel1c receptors by triggering crosstalk between the cAMP/PKA and PLC/PKC signal pathways followed by ERK1/2 activation.

Green light inhibits GnRH-I expression by stimulating the melatonin-GnIH pathway in the chick brain.

To study the mechanism by which monochromatic light affects gonadotrophin-releasing hormone (GnRH) expression in chicken hypothalamus, a total of 192 newly-hatched chicks were divided into intact, sham-operated and pinealectomy groups and exposed to white (WL), red (RL), green (GL) and blue (BL) lights using a light-emitting diode system for 2weeks. In the GL intact group, the mRNA and protein levels of GnRH-I in the hypothalamus, the mean cell area and mean cell optical density (OD) of GnRH-I-immunoreactive (-ir) cells of the nucleus commissurae pallii were decreased by 13.2%-34.5%, 5.7%-39.1% and 9.9%-17.3% compared to those in the chicks exposed to the WL, RL and BL, respectively. GL decreased these factors related to GnRH-I expression and the effect of GL was not observed in pinealectomised birds. However, the mRNA and protein levels of hypothalamic gonadotrophin-inhibitory hormone (GnIH) and GnIH receptor (GnIHR), the mean cell area and mean cell OD of the GnIH-ir cells of the paraventricularis magnocellularis, and the plasma melatonin concentration in the chicks exposed to GL were increased by 18.6%-49.2%, 21.1%-60.0% and 8.6%-30.6% compared to the WL, RL and BL intact groups, respectively. The plasma melatonin concentration showed a negative correlation with GnRH-I protein and a positive correlation with GnIH and GnIHR proteins. Protein expression of both GnRH-I and GnIHR showed a negative correlation in the hypothalamus. After pinealectomy, GnRH-I expression increased, whereas plasma melatonin concentration, GnIH and GnIHR expression decreased, and there were no significant differences among the WL, RL, GL and BL groups. Double-labelled immunofluorescence showed that GnIH axon terminals were near GnRH-I neurones, some GnRH-I neurones coexpressed with GnIHR and GnIH neurones coexpressed with melatonin receptor subtype quinone reductase 2. These results demonstrate that green light inhibits GnRH-I expression by increasing melatonin secretion and stimulating melatonin receptor-GnIH-GnIH receptor pathway in the chick brain.

Melatonin and its Receptors

Background: Melatonin (5-methoxy-N-acetyltryptamine) is a hormone that has numerous physiological functions. Synthesized and released during nighttime, melatonin exerts its physiological effects in a circadian manner. Melatonin acts by binding to its different types of receptors. The purpose of this systematic review is to summarize recent findings about melatonin, its receptors, and the differential functionalities of the most characterized melatonin receptors MT1 and MT2. Methods: We searched PubMed and Google Scholar for studies that reported melatonin receptor subtypes, their differential functionalities, biochemical structures, signal transductions, and various functions of melatonin such as pain, sleep, temperature, and antioxidative effects. We chose seventy articles for this systematic review. Summary: These studies highlight melatonin’s range of physiological functions and the differential functionalities of the melatonin receptors MT1 and MT2; they characterize the receptors’ signal transduction cascades and their biochemical structures. More studies assessing melatonin receptors’ functions would help patients with disorders in sleep, pain, and circadian rhythm.

Brain injury results in lower levels of melatonin receptors subtypes MT1 and MT2

BackgroundTraumatic brain injury (TBI) is a devastating and costly acquired condition that affects individuals of all ages, races, and geographies via a number of mechanisms. The effects of TBI on melatonin receptors remain unknown. PurposeThe purpose of this study is to explore whether endogenous changes in two melatonin receptor subtypes (MT1 and MT2) occur after experimental TBI. SampleA total of 25 adult male Sprague Dawley rats were used with 6 or 7 rats per group. MethodsRats were randomly assigned to receive either TBI modeled using controlled cortical impact or sham surgery and to be sacrificed at either 6- or 24-h post-operatively. Brains were harvested, dissected, and flash frozen until whole cell lysates were prepared, and the supernatant fluid aliquoted and used for western blotting. Primary antibodies were used to probe for melatonin receptors (MT1 and MT2), and beta actin, used for a loading control. ImageJ and Image Lab software were used to quantify the data which was analyzed using t-tests to compare means. ResultsMelatonin receptor levels were reduced in a brain region- and time point- dependent manner. Both MT1 and MT2 were reduced in the frontal cortex at 24h and in the hippocampus at both 6h and 24h. DiscussionMT1 and MT2 are less abundant after injury, which may alter response to MEL therapy. Studies characterizing MT1 and MT2 after TBI are needed, including exploration of the time course and regional patterns, replication in diverse samples, and use of additional variables, especially sleep-related outcomes. ConclusionTBI in rats resulted in lower levels of MT1 and MT2; replication of these findings is necessary as is evaluation of the consequences of lower receptor levels.

Expression of Melatonin Receptor Subtype Genes and its Impact on Reproductive Traits in Japanese Quail in Different Lighting Systems

Melatonin receptors are G-protein coupled receptors which mediate the action of melatonin hormone. In this study, we investigated the effects of two different lighting systems, 23 h L: 1 h D (group I) and 12 h L: 12 h D (group II), on reproductive traits in Japanese Quail ( Coturnix coturnix japonica). As well as the expression pattern of three melatonin receptor subtypes, mRNA in ovaries and testes were analysed using quantitative real time PCR. The results revealed the expression of all three subtypes of melatonin receptors (MNTR1A, MNTR1B and MNTR1C) in ovaries and MNTR1B, MNTR1C in testes in both groups. Further, the expression of melatonin receptors was significantly higher in group II relative to group I. The fold change level of MNTR1C was 6.7 in ovaries and 3.1 in testes in short day light. Altered expression pattern of these genes associated with changes in Japanese Quail reproductive traits, while, Japanese Quail exposed to long day light started egg laying earlier than those in short day light. Egg weight and egg percentage increased significantly in group I compared to group II. The average of relative ovary and testis weight at six weeks of age were declined by 1.72 and 1.64 g, respectively, in group II. It was concluded that, different lighting system induced different expression levels of melatonin receptor genes accompanied with changes in reproductive traits in both sex of Japanese Quail.

Update on melatonin receptors: IUPHAR Review 20.

Melatonin receptors are seven transmembrane-spanning proteins belonging to the GPCR superfamily. In mammals, two melatonin receptor subtypes exist - MT1 and MT2 - encoded by the MTNR1A and MTNR1B genes respectively. The current review provides an update on melatonin receptors by the corresponding subcommittee of the International Union of Basic and Clinical Pharmacology. We will highlight recent developments of melatonin receptor ligands, including radioligands, and give an update on the latest phenotyping results of melatonin receptor knockout mice. The current status and perspectives of the structure of melatonin receptor will be summarized. The physiological importance of melatonin receptor dimers and biologically important and type 2 diabetes-associated genetic variants of melatonin receptors will be discussed. The role of melatonin receptors in physiology and disease will be further exemplified by their functions in the immune system and the CNS. Finally, antioxidant and free radical scavenger properties of melatonin and its relation to melatonin receptors will be critically addressed.

Polymorphism of the melatonin receptor 1A (MNTR1A) gene and association with seasonality of reproductive activity in a local Greek sheep breed.

BackgroundSheep’s reproductive physiology in temperate latitudes (such as Greece), is characterized by seasonality and is also regulated by photoperiodic exposure. Melatonin is the key hormone involved in this regulation. However, the melatonin secretion and therefore the ewes reproductive activity underlies variation, proposed to be linked with the melatonin receptor subtype 1A (MNTR1A) gene structure. This study was designed to investigate the polymorphism of the MNTR1A gene in a local Greek sheep breed and to determine its potential association with reproductive seasonality.ResultsTwo groups of farmed ewes, each consisted of 30 individuals, were chosen. Males were introduced in both groups in spring (April). The first group consisted of ewes that showed reproductive activity in spring (May), while the second of ewes that showed reproductive activity 3 months later, in summer. The PCR–RFLP methodology was carried out on a 824-bp DNA fragment of the MTNR1A exon 2 using the RsaI restriction endonuclease. The electrophoretic procedure revealed three genotypes, C/C, C/T and T/T. Specifically, 44 animals showed the C/C genotype (28 from the first group and 16 from the second), 14 the C/T genotype (2 from the first and 12 from the second) and 2 animals had the T/T genotype (both from the second group).ConclusionsStatistical analysis indicated a positive correlation between genotype and reproductive seasonality, with C/C genotype playing a crucial role in out-of-season reproduction activity.

Melatonin receptor deficiency decreases and temporally shifts ecto-5'-nucleotidase mRNA levels in mouse prosencephalon.

Ecto-5'-nucleotidase (eN) is the major extracellular adenosine-producing ecto-enzyme in mouse brain. Via the production of adenosine, eN participates in many physiological and pathological processes, such as wakefulness, inflammation, nociception and neuroprotection. The mechanisms regulating the expression of eN are therefore of considerable neurobiological and clinical interest. Having previously described a modulatory effect of melatonin in the regulation of eN mRNA levels, we decided to analyze the melatonin receptor subtype involved in the regulation of eN mRNA levels by comparing eN mRNA patterns in melatonin-proficient transgenic mice lacking either the melatonin receptor subtype 1 (MT1 KO) or both melatonin receptor subtypes (MT1 and MT2; MT1/2 KO) with the corresponding melatonin-proficient wild-type (WT) controls. By means of radioactive in situ hybridization, eN mRNA levels were found to be diminished in both MT1 and MT1/2 KO mice compared with WT controls suggesting stimulatory impacts of melatonin receptors on eN mRNA levels. Whereas eN mRNA levels increased during the day and peaked at night in WT and MT1 KO mice, eN mRNA levels at night were reduced and the peak was shifted toward day-time in double MT1/2 KO mice. These data suggest that the MT2 receptor subtype may play a role in the temporal regulation of eN mRNA availability. Notably, day-time locomotor activity was significantly higher in MT1/2 KO compared with WT mice. Our results suggest melatoninergic signaling as an interface between the purinergic system and the circadian system.

Melatonergic ligands: Design, synthesis and pharmacological evaluation of novel series of naphthofuranic derivatives

Following our research for new melatonergic ligands, herein we report the design, synthesis and biological evaluation of new series of naphthofuranic derivatives as MT1 and MT2 ligands. Binding affinity results of the prepared compounds revealed good binding affinities at both melatonin receptor subtypes. Particularly, compound 6a behaved as an MT1 partial agonist and MT2 full agonist and exhibited an excellent binding affinity at MT2 (Ki = 0.09 nM). In addition, lateral chain displacement from position 1 to 2 of the furan core had no effect on the binding affinity at both MT1 and MT2, while elongation of this side chain, led to decreased melatonergic binding affinities.

Characterization of MTNR1A gene in terms of genetic variability in a panel of subtemperate and subtropical Indian sheep breeds.

Seasonality of animals is an important adaptive trait for successful survival and production during limited food availability and extreme environmental conditions. Photoperiodic changes in day length are utilized by these seasonal animals as an important environmental cue for regulating their annual rhythms of reproduction cycles. Melatonin is an important hormone which is secreted by the pineal gland in proportion to darkness and its effect is mediated by melatonin receptor subtypes, principally MTNR1A. In the present study, polymorphism in the coding sequence at two important SNPs (C606T and G612A),known to be markers for out of season breeding in sheep were studied by PCR-RFLP in a panel of four breeds of sheep from subtemperate and subtropical arid conditions, respectively. The frequencies of 'G' and 'A' alleles with reference to G612A SNP did not differ considerably among all the breeds of sheep. Frequency of 'T' allele of the C606T SNP was found to be dom inantly higher in subtemperate sheep breeds in comparison to subtropical sheep breeds. Identified SNPs in the coding region were mostly synonymous and did not lead to any change in conformation of the MTNR1A receptor protein.

Developmental changes of melatonin receptor expression in the spleen of the chicken, Gallus domesticus

Melatonin plays an essential role in development and immunoregulation of the avian spleen through its receptors; however, the variations in the expression of the melatonin receptor subtypes in the developing avian spleen are still unclear. The objective of the present study was not only to investigate the distribution patterns and development changes of the expression of the melatonin receptor subtypes (Mel1a, Mel1b and Mel1c) in the chicken spleen but also to identify the correlation between the plasma melatonin concentration and the expression of the melatonin receptor subtypes. The immunohistochemical results indicated that Mel1a was mainly distributed in the red pulp and capillaries, Mel1b was predominantly distributed in the periarterial lymphatic sheath (PALS) and splenic nodule, and Mel1c was widely located in the red pulp, PALS and splenic nodule. From P0 to P21, the mRNA and protein expressions of Mel1a, Mel1b and Mel1c in the spleen were increased (P<0.05); however, a slight increase in the expression of the three melatonin receptor subtypes was observed after P21 (P>0.05). Furthermore, the mRNA levels of Mel1b and Mel1c between P0 and P14 raised more quickly than Mel1a. The plasma melatonin concentration increased in an age-dependent manner in the chicken from P0 to P42 (P<0.05), and this increasing change was linear after P14 (P<0.05). The melatonin level in the plasma is strongly correlated with the protein expressions of Mel1a (r=0.938, P=0.005), Mel1b (r=0.912, P=0.011), and Mel1c (r=0.906, P=0.012) in the chicken spleen. These results suggest the existence of age-related and region-specific changes in the expression of the melatonin receptor subtypes within the spleen of the chicken, and this characteristic pattern may be involved in the development and functional maturation of the avian spleen.

Co-expression of two subtypes of melatonin receptor on rat M1-type intrinsically photosensitive retinal ganglion cells.

Intrinsically photosensitive retinal ganglion cells (ipRGCs) are involved in circadian and other non-image forming visual responses. An open question is whether the activity of these neurons may also be under the regulation mediated by the neurohormone melatonin. In the present work, by double-staining immunohistochemical technique, we studied the expression of MT1 and MT2, two known subtypes of mammalian melatonin receptors, in rat ipRGCs. A single subset of retinal ganglion cells labeled by the specific antibody against melanopsin exhibited the morphology typical of M1-type ipRGCs. Immunoreactivity for both MT1 and MT2 receptors was clearly seen in the cytoplasm of all labeled ipRGCs, indicating that these two receptors were co-expressed in each of these neurons. Furthermore, labeling for both the receptors were found in neonatal M1 cells as early as the day of birth. It is therefore highly plausible that retinal melatonin may directly modulate the activity of ipRGCs, thus regulating non-image forming visual functions.

Ultradian oscillation in expression of four melatonin receptor subtype genes in the pineal gland of the grass puffer, a semilunar-synchronized spawner, under constant darkness.

Melatonin receptor gene expression as well as melatonin synthesis and secretion activities were examined in the pineal gland of the grass puffer, which exhibits unique lunar/tidal cycle-synchronized mass spawing: spawning occurs before high tide on the day of spring tide during spawing season. Melatonin synthesizing activity was assessed by the abundance of arylalkylamine N-acetyltransferase 2 (AANAT2) mRNA. The amount of aanat2 mRNA was low during light phase and initiated to increase after the light was turned off. The secretion of melatonin from primary pineal organ culture was stimulated after the light was turned off and ceased immediately after the light was turned on. The expression levels of four melatonin receptor subtype genes (mel1a1.4, mel1a1.7, mel1b, and mel1c) showed synchronous variations, and the levels tended to be high during the dark phase under light/dark conditions. These results suggest that the action of melatonin on the pineal gland is highly dependent on light and photoperiod, possibly with stronger action during night time. Under constant darkness, the expression of four melatonin receptor subtype genes showed unique ultradian oscillations with the period of 14.0–15.4 h, suggesting the presence of a circatidal oscillator in the pineal gland. The present results indicate that melatonin may serve local chronobiological functions in the pineal gland. These cyclic expressions of melatonin receptor genes in the pineal gland may be important in the control of the lunar/tidal cycle-synchronized mass spawning in the grass puffer.

Valproic Acid Influences MTNR1A Intracellular Trafficking and Signaling in a β-Arrestin 2-Dependent Manner.

Valproate exposure is associated with increased risks of autism spectrum disorder. To date, the mechanistic details of disturbance of melatonin receptor subtype 1 (MTNR1A) internalization upon valproate exposure remain elusive. By expressing epitope-tagged receptors (MTNR1A-EGFP) in HEK-293 and Neuro-2a cells, we recorded the dynamic changes of MTNR1A intracellular trafficking after melatonin treatment. Using time-lapse confocal microscopy, we showed in living cells that valproic acid interfered with the internalization kinetics of MTNR1A in the presence of melatonin. This attenuating effect was associated with a decrease in the phosphorylation of PKA (Thr197) and ERK (Thr202/Tyr204). VPA treatment did not alter the whole-cell currents of cells with or without melatonin. Furthermore, fluorescence resonance energy transfer imaging data demonstrated that valproic acid reduced the melatonin-initiated association between YFP-labeled β-arrestin 2 and CFP-labeled MTNR1A. Together, we suggest that valproic acid influences MTNR1A intracellular trafficking and signaling in a β-arrestin 2-dependent manner.

The melatonin receptor in the bactrian camel pineal: cloning experiments and distribution studies

The aim of this study was to identify the high-affinity melatonin-receptor subtypes expressed in pineal of camel. To achieve this, reverse transcription-polymerase chain reaction (RT-PCR), sequencing and immunohistochemistry were used. By using RT-PCR, mRNA of melatonin receptors MT1/MT2 were detected in the pineal of bactrian camel. The highest identity was 87.2% and 97.6%, respectivly for MT1 and MT2 compatively with white yak. The pridict protein of the deduce amino acid sequence showed a tendency towards forming the 7-TM motif profile of G-protein. The immunohistochemistry studies showed that melatonin receptors are expressed in pinealocytes and glial cells of pineal. Overall, these results showed that melatonin receptors are expressed in the bactrian camel pinealocytes, and melatonin, possible as intracrine or autocrine, can influence pinealocytes and pineal physiological function.

Synthesis and pharmacological evaluation of dual ligands for melatonin (MT1/MT2) and serotonin 5-HT2C receptor subtypes (II).

In this paper we report the investigation of C-3 and β-acetamide positions of agomelatine analogues. Concomitant insertion of a hydroxymethyl in the β-acetamide position and aliphatic groups in C-3 position produced a positive effect on both melatonin (MT1, MT2) and serotonin (5-HT2C) binding affinities. In particular, the allyl 6b and ethyl 15a represented the more interesting compounds of this series. Furthermore, the introduction of methyl cycloalkyl groups (compounds 11a, 12a) exhibited no change in both MT2 and 5-HT2C binding affinities while a decrease of MT1 binding affinity occurred leading to an MT2 selectivity. Finally, the acetamide modulation has led to methyl thiourea 11h, with a weak MT2 selectivity.

Differential Expression of Melatonin Receptor Subtypes MelIa, MelIb and MelIc in Relation to Melatonin Binding in the Male Songbird Brain

Previous autoradiography studies illustrated that several areas of the avian brain can bind the pineal hormone melatonin. In birds, there are three melatonin receptor (MelR) subtypes: MelIa, MelIb and MelIc. To date, their brain distribution has not been studied in any passerine bird. Therefore, we investigated mRNA distribution of MelR subtypes in adjacent sections of the brain of two songbirds, the blackcap and the zebra finch, in parallel with that of 2-[¹²⁵I]-iodomelatonin (IMEL) binding sites in the same brains. The general pattern of receptor expression shown by in situ hybridization of species-specific probes matched well with that of IMEL binding. However, the expression of the three subtypes was area specific with similar patterns in the two species. Some brain areas expressed only one receptor subtype, most brain regions co-expressed either MelIa with MelIb or MelIa with MelIc, whereas few areas expressed MelIb and MelIc or all three receptor subtypes. Since many sensory areas, most thalamic areas and subareas of the neopallium, a cortex analogue, express MelR, it is likely that most sensory motor integration functions are melatonin sensitive. Further, the area-specific expression patterns suggest that the regulatory role of melatonin differs among different brain areas. Since subareas of well-defined neural circuits, such as the visual system or the song control system, are equipped with different receptor types, we hypothesize a diversity of functions for melatonin in the control of sensory integration and behavior.

Nycthemeral variation in melatonin receptor expression in the lymphoid organs of a tropical seasonal breeder Funambulus pennanti.

Seasonal variations in immune functions point toward the involvement of melatonin in its regulation. These chronobiotic effects are exerted by receptors present on the immunocompetent cells. The present study investigated daily/nycthemeral variation in expression of melatonin receptor subtypes MT1/MT2 in the lymphoid organs (spleen/thymus) of a tropical squirrel, Funambulus pennanti. The receptor expression was noted every 4 h interval over 24 h under natural light-dark cycle, during two seasons and was correlated with peripheral level of melatonin. The MT1/MT2 receptor expression displayed higher levels at the time of dusk (light-dark transition; 1800 hours), while plasma melatonin was still low compared to the preceding time point. The receptors were downregulated during the nighttime with a minimum expression at 0200 hours. Thymus, during the long day length, showed a tissue-specific pattern of receptor expression with a minimum expression at 0600 hours. Results suggest that photoperiod by modulation of melatonin level inversely regulates the receptor expression. The observations imply that there exists a temporal window of sensitivity in the target organs to the melatonin signal that is regulated by modulation of melatonin receptor expression which might be involved in mediating the photoperiodic effects of melatonin in the control of seasonal immune physiology.