Expression of mt(1) melatonin receptor subtype mRNA in the entrained rat suprachiasmatic nucleus: a quantitative RT-PCR study across the diurnal cycle

Melatonin, Melatonin Receptors and Sleep: Moving Beyond Traditional Views

Sleep, constituting approximately one-third of the human lifespan, is a crucial physiological process essential for physical and mental well-being. Normal sleep consists of an orderly progression through wakefulness, non-rapid eye movement (NREM) sleep, and rapid eye movement (REM) sleep, all of which are tightly regulated. Melatonin, often referred to as the "hormone of sleep," plays a pivotal role as a regulator of the sleep/wake cycle and exerts its effects through high-affinity G-protein coupled receptors known as MT1 and MT2. Selective modulation of these receptors presents a promising therapeutic avenue for sleep disorders. This review examines research on the multifaceted role of melatonin in sleep regulation, focusing on selective ligands targeting MT1 and MT2 receptors, as well as studies involving MT1 and MT2 knockout mice. Contrary to common beliefs, growing evidence suggests that melatonin, through MT1 and MT2 receptors, might not only influence circadian aspects of sleep but likely, also modulate the homeostatic process of sleep and sleep architecture, or could be the molecule linking the homeostatic and circadian regulation of sleep. Furthermore, the distinct brain localization of MT1 and MT2 receptors, with MT1 receptors primarily regulating REM sleep and MT2 receptors regulating NREM sleep, is discussed. Collectively, sleep regulation extends beyond the circulating levels and circadian peak of melatonin; it also critically involves the expression, molecular activation, and regulatory functions of MT1 and MT2 receptors across various brain regions and nuclei involved in the regulation of sleep. This research underscores the importance of ongoing investigation into the selective roles of MT1 and MT2 receptors in sleep. Such research efforts are expected to pave the way for the development of targeted MT1 or MT2 receptors ligands, thereby optimizing therapeutic interventions for sleep disorders.

Hypnotic effects of melatonin depend on the environmental lighting conditions in the rat

Acute effects of exogenous melatonin have been widely reported to promote sleep or induce drowsiness in human. However, testing of the hypnotic effects of melatonin in nocturnal rodents has yielded contradictory results. The latter may be associated with differences in concentration, lighting conditions, time of administration of melatonin, and possibly the type of analysis. In this study, electroencephalogram (EEG) and electromyogram were recorded in pigmented male Brown Norway rats under both light-dark (LD) and constant dark (DD) conditions. Melatonin was administered intraperitoneally at a moderate dose of 3 mg/kg, at either 1 h after lights on under LD condition or 1 h after the activity offset under DD condition. The dosage is known to be able to entrain nocturnal rodents in DD conditions, but does not change sleep in rodents in LD. Only the rats under DD conditions showed a significant reduction in nonrapid eye movement (NREM) sleep latency, while the NREM sleep power spectrum remained unaffected. Under LD condition, melatonin did not alter NREM and rapid eye movement (REM) sleep latency, and had only minor effects on the NREM sleep EEG. Regardless of lighting conditions, melatonin administration resulted in less, but longer episodes for all vigilance states suggesting increased vigilance state consolidation. In the discussion, we compare our results with a summary of previously published data concerning the hypnotic effects of melatonin in polysomnographic/EEG-confirmed sleep in humans and nocturnal rodents. In conclusion, the hypnotic effect of exogenous melatonin in nocturnal rodents not only depends on the time of day, and concentration, but is also influenced by the lighting conditions. Regardless of inducing sleep or not, melatonin may consolidate sleep and through that enhance sleep quality.

Empowering Melatonin Therapeutics with Drosophila Models

Melatonin functions as a central regulator of cell and organismal function as well as a neurohormone involved in several processes, e.g., the regulation of the circadian rhythm, sleep, aging, oxidative response, and more. As such, it holds immense pharmacological potential. Receptor-mediated melatonin function mainly occurs through MT1 and MT2, conserved amongst mammals. Other melatonin-binding proteins exist. Non-receptor-mediated activities involve regulating the mitochondrial function and antioxidant cascade, which are frequently affected by normal aging as well as disease. Several pathologies display diseased or dysfunctional mitochondria, suggesting melatonin may be used therapeutically. Drosophila models have extensively been employed to study disease pathogenesis and discover new drugs. Here, we review the multiple functions of melatonin through the lens of functional conservation and model organism research to empower potential melatonin therapeutics to treat neurodegenerative and renal diseases.

Differential Function of Melatonin MT1 and MT2 Receptors in REM and NREM Sleep

The pathophysiological function of the G-protein coupled melatonin MT1 and MT2 receptors has not yet been well-clarified. Recent advancements using selective MT1/ MT2 receptor ligands and MT1/MT2 receptor knockout mice have suggested that the activation of the MT1 receptors are mainly implicated in the regulation of rapid eye movement (REM) sleep, whereas the MT2 receptors selectively increase non-REM (NREM) sleep. Studies in mutant mice show that MT1 knockout mice have an increase in NREM sleep and a decrease in REM sleep, while MT2 knockout mice a decrease in NREM sleep. The localization of MT1 receptors is also distinct from MT2 receptors; for example, MT2 receptors are located in the reticular thalamus (NREM area), while the MT1 receptors in the Locus Coeruleus and lateral hypothalamus (REM areas). Altogether, these findings suggest that these two receptors not only have a very specialized function in sleep, but that they may also modulate opposing effects. These data also suggest that mixed MT1-MT2 receptors ligands are not clinically recommended given their opposite roles in physiological functions, confirmed by the modest effects of melatonin or MT1/MT2 non-selective agonists when used in both preclinical and clinical studies as hypnotic drugs. In sum, MT1 and MT2 receptors have specific roles in the modulation of sleep, and consequently, selective ligands with agonist, antagonist, or partial agonist properties could have therapeutic potential for sleep; while the MT2 agonists or partial agonists might be indicated for NREM-related sleep and/or anxiety disorders, the MT1 agonists or partial agonists might be so for REM-related sleep disorders. Furthermore, MT1 but not MT2 receptors seem involved in the regulation of the circadian rhythm. Future research will help further develop MT1 and/or MT2 receptors as targets for neuropsychopharmacology drug development.

Melatonin influences the sonic hedgehog signaling pathway in porcine cumulus oocyte complexes

Melatonin, which is synthesized in the pineal gland and peripheral reproductive organs, has antioxidant properties and regulates physiological processes. It is well known that melatonin affects in vitro maturation (IVM) of oocytes and embryonic development in many species. However, beneficial effects of melatonin on IVM have been explained mainly by indirect antioxidant effects and little information is available on the underlying mechanism by which melatonin directly acts on porcine cumulus oocyte complexes (COCs). Sonic hedgehog (Shh) signaling is important for follicle development, oocyte maturation, and embryo development, and there may be a relationship between melatonin and Shh signaling. To examine this, we designed three groups: (i) control, (ii) melatonin (10^-9  mol/L), and (iii) melatonin with cyclopamine (2 μmol/L; Shh signaling inhibitor). The aim of this study was to investigate the effects of these agents on cumulus expansion, oocyte maturation, embryo development after parthenogenetic activation (PA), gene expression in cumulus cells, oocytes and blastocysts, and protein expression in COCs. Melatonin significantly increased the proportion of COCs exhibiting complete cumulus expansion (degree 4), PA blastocyst formation rates, and total cell numbers, which were inhibited by addition of cyclopamine. Simultaneously, the expression of cumulus expansion-related genes (Ptgs1, Ptgs2, and Has2) and Shh signaling-related genes (Shh, Pthc1, Smo, and Gli1) and proteins (Ptch1, Smo, and Gli1) in cumulus cells was upregulated in the melatonin-treated group, and these effects were also inhibited by cyclopamine. In conclusion, our results suggest that Shh signaling mediates effects of melatonin to improve porcine cumulus expansion and subsequent embryo development.

Circadian Pattern of Melatonin MT1 and MT2 Receptor Localization in the Rat Suprachiasmatic Nucleus

The suprachiasmatic nucleus (SCN) is the master circadian pacemaker. The pineal hormone melatonin is involved in the regulation of circadian phase. As a part of the circadian system, its synthesis and secretion is under SCN control. On the other hand, melatonin feeds back on the SCN to regulate its function. Melatonin has two specific windows of time at which it regulates SCN function, namely dusk and dawn. It has been suggested that melatonin exerts its effect on the SCN during that specific window of time via one or both of its specific receptors, MT1 or MT2. The hypothesis that the density of these receptors varies across the circadian cycle was tested. Using immunohistochemistry with receptor-specific antibodies, the localization and distribution of melatonin receptors MT1 and MT2 was studied in the SCN at different Zeitgeber times (ZT): ZT 11–13 (dusk), 23–01 (dawn), 5–7 (mid-day), and 17–19 (midnight). Our results show that MT1 receptor density significantly increased at dusk relative to dawn and midnight (p<0.01 and p<0.001 respectively). Although MT1 receptors were widespread in the SCN and parts of the optic chiasm at dusk, they were restricted to the SCN during the mid-day period. MT2 receptors were not detected in the SCN. Thus, we find that melatonin receptor MT1 density and distribution varies with circadian time. This creates a time window during which melatonin can affect the operation of the SCN. We also find that melatonin regulates SCN function via MT1 receptors with a minimal role for MT2.

Orofacial inflammatory pain affects the expression of MT1 and NADPH-d in rat caudal spinal trigeminal nucleus and trigeminal ganglion

Very little is known about the role of melatonin in the trigeminal system, including the function of melatonin receptor 1. In the present study, adult rats were injected with formaldehyde into the right vibrissae pad to establish a model of orofacial inflammatory pain. The distribution of melatonin receptor 1 and nicotinamide adenine dinucleotide phosphate diaphorase in the caudal spinal trigeminal nucleus and trigeminal ganglion was determined with immunohistochemistry and histochemistry. The results show that there are significant differences in melatonin receptor 1 expression and nicotinamide adenine dinucleotide phosphate diaphorase expression in the trigeminal ganglia and caudal spinal nucleus during the early stage of orofacial inflammatory pain. Our findings suggest that when melatonin receptor 1 expression in the caudal spinal nucleus is significantly reduced, melatonin's regulatory effect on pain is attenuated.

Direct and specific effect of sevoflurane anesthesia on rat Per2 expression in the suprachiasmatic nucleus

Background

Our previous studies revealed that application of the inhalation anesthetic, sevoflurane, reversibly repressed the expression of Per2 in the mouse suprachiasmatic nucleus (SCN). We aimed to examine whether sevoflurane directly affects the SCN.

Methods

We performed in vivo and in vitro experiments to investigate rat Per2 expression under sevoflurane-treatment. The in vivo effects of sevoflurane on rPer2 expression were examined by quantitative in situ hybridization with a radioactively-labeled cRNA probe. Additionally, we examined the effect of sevoflurane anesthesia on rest/activity rhythms in the rat. In the in vitro experiments, we applied sevoflurane to SCN explant cultures from Per2-dLuc transgenic rats, and monitored luciferase bioluminescence, representing Per2 promoter activity. Bioluminescence from two peripheral organs, the kidney cortex and the anterior pituitary gland, were also analyzed.

Results

Application of sevoflurane in rats significantly suppressed Per2 expression in the SCN compared with untreated animals. We observed no sevoflurane-induced phase-shift in the rest/activity rhythms. In the in vitro experiments, the intermittent application of sevoflurane repressed the increase of Per2-dLuc luminescence and led to a phase delay in the Per2-dLuc luminescence rhythm. Sevoflurane treatment did not suppress bioluminescence in the kidney cortex or the anterior pituitary gland.

Conclusion

The suppression of Per2-dLuc luminescence by sevoflurane in in vitro SCN cultures isolated from peripheral inputs and other nuclei suggest a direct action of sevoflurane on the SCN itself. That sevoflurane has no such effect on peripheral organs suggests that this action might be mediated through a neuron-specific cellular mechanism or a regulation of the signal transduction between neurons.

Electrophysiological effects of melatonin on mouse Per1 and non-Per1 suprachiasmatic nuclei neurones in vitro

The master circadian pacemaker in the suprachiasmatic nuclei (SCN) regulates the nocturnal secretion of the pineal hormone melatonin. Melatonin, in turn, has feedback effects on SCN neuronal activity rhythms via high affinity G protein-coupled receptors (MT(1) and MT(2) ). However, the precise effects of melatonin on the electrical properties of individual SCN neurones are unclear. In the present study, we investigated the acute effects of exogenous melatonin on SCN neurones using whole-cell patch-clamp recordings in brain slices prepared from Per1::d2EGFP-expressing transgenic mice. In current-clamp mode, bath applied melatonin, at near-physiological concentrations (1 nM), hyperpolarised the majority (63.7%) of SCN neurones tested at all times of the projected light/dark cycle. In addition, melatonin depolarised a small proportion of cells (11.0%). No differences were observed for the effects of melatonin between Per1::GFP or non-Per1::GFP SCN neurones. Melatonin-induced effects were blocked by the MT(1)/MT(2) antagonist, luzindole (1 μM) and the proportion of SCN neurones responsive to melatonin was greatly reduced in the presence of either tetrodotoxin (200 or 500 nM) or gabazine (20 μM). In voltage-clamp recordings, 1 nM melatonin increased the frequency of GABA-mediated currents. These findings indicate, for the first time, that exogenous melatonin can alter neuronal excitability in the majority of SCN neurones, regardless of whether or not they overtly express the core clock gene Per1. The results also suggest that melatonin acts mainly by modulating inhibitory GABAergic transmission within the SCN. This may explain why exogenous application of melatonin has heterogenous effects on individual SCN neurones.

The expression of MT1 and MT2 melatonin receptor mRNA in several rat tissues

The mechanisms that mediate the various effects of melatonin in mammalian tissues are not always known. Therefore, the aim of this study was to investigate whether MT(1) and MT(2) melatonin receptors are expressed in certain tissues of the rat. The expression of MT(1) and MT(2) melatonin receptor mRNA was determined using a real-time quantitative RT-PCR method. In addition, we examined whether mRNA for either subtype of receptor shows any difference in the expression between midnight and noon, similar to the changes in melatonin concentrations in plasma and tissue samples. MT(1) and MT(2) melatonin receptor mRNAs were found in the rat hypothalamus, retina and small intestine. We also showed a low expression of MT(2) mRNA in the rat liver and heart SA node. In the heart apex and the Harderian gland, no appearance of either of the receptor mRNAs was detectable. A significant difference in the expression of MT(1) mRNA between day and night was found in the hypothalamus. In conclusion, our findings suggest that at least some effects of melatonin are mediated through membrane MT(1) and MT(2) receptors in the hypothalamus, the retina and the small intestine. Down-regulation of receptors might be one reason for the difference in the hypothalamic MT(1) melatonin receptor mRNA expression between midnight and noon. In the liver and the heart SA node, the physiological significance of possible MT(2) receptors remains unclear. According to our negative midnight and noon results in the Harderian gland and heart apex melatonin may exert its effect on these tissues by a non-receptor mechanism.

The bidirectional phase-shifting effects of melatonin on the arginine vasopressin secretion rhythm in rat suprachiasmatic nuclei in vitro

In vivo melatonin serves as a feedback signal to the circadian pacemaker located in the suprachiasmatic nuclei (SCN) and in vitro it phase advances the circadian rhythm of electrical activity in pacemaker cells. However, the occurrence and nature of phase shifting in secretion by cultured SCN neurons has not yet been established. Here we studied the effects of melatonin on the pattern of spontaneous arginine vasopressin (AVP) release in organotypic SCN slices. This culture mimicked the in vivo circadian AVP secretory rhythm, with low release during the subjective night and with peaks in secretion during the middle of subjective day. The endogenous period of the AVP secretory rhythm in organotypic culture ranged between 23 and 26 h, with the mean period of 24.1 +/- 0.3 h. Melatonin (10 nM) had variable effects on the pattern of AVP secretion depending on time of its application directly to the medium with organotypic SCN slices. When introduced at circadian time 22, 2 and 6 (the times corresponding to the late night and early day), melatonin delayed the AVP secretory rhythm by 1-4 h. When applied at circadian time 10 (late day), however, melatonin advanced the AVP secretory rhythm by about 2 h. At other circadian times, melatonin was ineffective. These results indicate that melatonin exhibits the bidirectional phase-shifting effects on circadian secretory rhythm clock, which depends on the time-window of its application.

Identification of PAC1 receptor isoform mRNAs by real-time PCR in rat suprachiasmatic nucleus

The pituitary adenylate cyclase-activating polypeptide (PACAP) has been implicated in the photic resetting of the rodent circadian clock in the suprachiasmatic nucleus (SCN). PACAP can exert its effects via VPAC1, VPAC2 and PAC1 G-protein coupled receptors. PAC1 and VPAC2, but not VPAC1, mRNA is expressed in rat SCN. A variety of PAC1 receptor splice variants have been described showing differences in ligand binding affinity and selectivity, G-protein coupling and ability to activate signal transduction pathways. The present experiments used PCR with isoform specific primers to determine which PAC1 variants are expressed in rat SCN. The PAC1(null) isoform and a variant containing a single 28-amino acid insert in the third intracellular (IC3) loop (hop1/2) were detected. No other IC3 variants (hip, hip-hop), N-terminal variants (PAC1(short), PAC1(very short) and PAC1(3a)) or the variant differing in transmembrane II and IV (PAC1TM4) were detected in SCN obtained at any time of day. A quantitative real-time PCR assay was established which measured combined expression of the PAC1(null/hop) variants in rat SCN during a 12:12-h light:dark (L:D) cycle. There was no significant variation of PAC1 mRNA expression (PAC1(null)+PAC1(hop)) with time of day. Nor was there a significant difference in the proportion of these two variants with time of day. These results indicate that the phase-dependency of the actions of PACAP on SCN firing and circadian behaviour are not mediated by changes in the level of expression of PAC1 receptor mRNA, nor by phase-dependent expression of PAC1 receptor variants with altered ligand binding, G-protein coupling or signalling characteristics.

MT1 melatonin receptor mRNA expression exhibits a circadian variation in the rat suprachiasmatic nuclei

The aim of the present study was to investigate the daily regulation of both MT1 and MT2 melatonin receptor subtype mRNA expression in the rat SCN in order to clarify their role in the daily variation of SCN melatonin receptors. Existing MT1 and MT2 partial clones were extended by PCR to 982 and 522 bp, respectively. However, while the MT1 clone allowed us to set up a highly sensitive in situ hybridization (ISH) method, we could not detect MT2 expression within the SCN. Therefore, our results suggest that only MT1 mRNA can be correlated with 2-iodo-melatonin binding sites in the rat SCN. Investigation of MT1 mRNA expression throughout the 24 h light/dark cycle or in constant darkness clearly showed that in the two conditions, mRNA expression showed a robust rhythm with two peaks, one after the day/night and one after the night/day transitions in LD, and at the beginning of the subjective night and day in DD, respectively. Furthermore, these variations were not linked to the daily changes in melatonin receptor density. Thus, the transcriptional regulation of MT1 receptors does not appear to play a role in the daily regulation of melatonin binding sites availability.

Immortalized suprachiasmatic nucleus cells express components of multiple circadian regulatory pathways

We undertook an extensive antigenic characterization of the SCN 2.2 cell line in order to further evaluate whether the line expresses components of circadian regulatory pathways common to the hypothalamic suprachiasmatic nucleus (SCN), the central circadian clock in mammals. We found that differentiated SCN 2.2 cultures expressed a broad range of putative clock genes, as well as components of daytime, nighttime, and crepuscular circadian regulatory pathways found within the SCN in vivo. The line also exhibits several antigens that are highly expressed in a circadian pattern and/or differentially localized in the SCN relative to other hypothalamic regions. Expression of a broad complement of circadian regulatory proteins and putative clock genes further support growing evidence in recent reports that the SCN 2.2 cell line is an appropriate model for investigating the regulation of central mammalian pacemaker.

Melatonin generates an outward potassium current in rat suprachiasmatic nucleus neurones in vitro independent of their circadian rhythm

The present study investigated the membrane mechanisms underlying the inhibitory influence of melatonin on suprachiasmatic nucleus (SCN) neurones in a hypothalamic slice preparation. Perforated-patch recordings were performed to prevent the rapid rundown of spontaneous firing rate as observed during whole cell recordings and to preserve circadian rhythmicity in SCN neurones. In current-clamp mode melatonin (1 microM or 1 nM) application, in the presence of agents that block action potential generation and fast synaptic transmission, resulted in a membrane hyperpolarisation accompanied with a decrease in input resistance in the majority of SCN neurones (71-86%). The amplitude of the hyperpolarisation was not found to be significantly different between circadian time 5-12 and 14-21. In voltage-clamp mode melatonin (1 microM or 1 nM) induced an outward current accompanied with an increase in membrane conductance. The current was found to be mainly potassium driven with voltage kinetics resembling those of an open rectifying potassium conductance. Investigations into the signal transduction mechanism revealed melatonin-induced inhibition of SCN neurones to be sensitive to pertussis toxin but independent of intracellular cAMP levels and phospholipase C activity. The present study shows that melatonin, at night-time physiological concentrations, reduces the neuronal excitability of the majority of SCN neurones independent of the time of application in the circadian cycle. Thus in vivo melatonin may be important for circadian time-keeping by amplifying the circadian rhythm in SCN neurones, by lowering their sensitivity to phase-shifting stimuli occurring at night.

Activation of MT(2) melatonin receptors in rat suprachiasmatic nucleus phase advances the circadian clock

The aim of this study was to identify the melatonin receptor type(s) (MT(1) or MT(2)) mediating circadian clock resetting by melatonin in the mammalian suprachiasmatic nucleus (SCN). Quantitative receptor autoradiography with 2-[(125)I]iodomelatonin and in situ hybridization histochemistry, with either (33)P- or digoxigenin-labeled antisense MT(1) and MT(2) melatonin receptor mRNA oligonucleotide probes, revealed specific expression of both melatonin receptor types in the SCN of inbred Long-Evans rats. The melatonin receptor type mediating phase advances of the circadian rhythm of neuronal firing rate in the SCN slice was assessed using competitive melatonin receptor antagonists, the MT(1)/MT(2) nonselective luzindole and the MT(2)-selective 4-phenyl-2-propionamidotetraline (4P-PDOT). Luzindole and 4P-PDOT (1 nM-1 microM) did not affect circadian phase on their own; however, they blocked both the phase advances (approximately 4 h) in the neuronal firing rate induced by melatonin (3 pM) at temporally distinct times of day [i.e., subjective dusk, circadian time (CT) 10; and dawn, CT 23], as well as the associated increases in protein kinase C activity. We conclude that melatonin mediates phase advances of the SCN circadian clock at both dusk and dawn via activation of MT(2) melatonin receptor signaling.

Gastrin-releasing peptide phase-shifts suprachiasmatic nuclei neuronal rhythms in vitro

The main mammalian circadian pacemaker is located in the suprachiasmatic nuclei (SCN) of the hypothalamus. Gastrin-releasing peptide (GRP) and its receptor (BB(2)) are synthesized by rodent SCN neurons, but the role of GRP in circadian rhythm processes is unknown. In this study, we examined the phase-resetting actions of GRP on the electrical activity rhythms of hamster and rat SCN neurons in vitro. In both rat and hamster SCN slices, GRP treatment during the day did not alter the time of peak SCN firing. In contrast, GRP application early in the subjective night phase-delayed, whereas similar treatment later in the subjective night phase-advanced the firing rate rhythm in rat and hamster SCN slices. These phase shifts were completely blocked by the selective BB(2) receptor antagonist, [d-Phe(6), Des-Met(14)]-bombesin 6-14 ethylamide. We also investigated the temporal changes in the expression of genes for the BB(1) and BB(2) receptors in the rat SCN using a quantitative competitive RT-PCR protocol. The expression of the genes for both receptors was easily detected, but their expression did not vary over the diurnal cycle. These data show that GRP phase-dependently phase resets the rodent SCN circadian pacemaker in vitro apparently via the BB(2) receptor. Because this pattern of phase shifting resembles that of light on rodent behavioral rhythms, these results support the contention that GRP participates in the photic entrainment of the rodent SCN circadian pacemaker.