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

Melatonin receptor structure and signaling

Melatonin (5-methoxy-N-acetyltryptamine) binds with high affinity and specificity to membrane receptors. Several receptor subtypes exist in different species, of which the mammalian MT1 and MT2 receptors are the best-characterized. They are members of the G protein-coupled receptor superfamily, preferentially coupling to Gi/o proteins but also to other G proteins in a cell-context-depending manner. In this review, experts on melatonin receptors will summarize the current state of the field. We briefly report on the discovery and classification of melatonin receptors, then focus on the molecular structure of human MT1 and MT2 receptors and highlight the importance of molecular simulations to identify new ligands and to understand the structural dynamics of these receptors. We then describe the state-of-the-art of the intracellular signaling pathways activated by melatonin receptors and their complexes. Brief statements on the molecular toolbox available for melatonin receptor studies and future perspectives will round-up this review.

Thirty-seven years of MT1 and MT2 melatonin receptor localization in the brain: Past and future challenges

Identifying the target cells of a hormone is a key step in understanding its function. Once the molecular nature of the receptors for a hormone has been established, researchers can use several techniques to detect these receptors. Here I will review the different tools used over the years to localize melatonin receptors and the problems associated with each of these techniques. The radioligand 2-[125I] iodomelatonin was the first tool to allow localization of melatonin receptors on tissue sections. Once the MT1 and MT2 receptors were cloned, in situ hybridization could be used to detect the messenger RNA for these receptors. The deduced amino acid sequences for MT1 and MT2 receptors allowed the production of peptide immunogens to generate antibodies against the MT1 and MT2 receptors. Finally, transgenic reporters driven by the promoter elements of the MT1 and MT2 genes have been used to map the expression of MT1 and MT2 in the brain and the retina. Several issues have complicated the localization of melatonin receptors and the characterization of melatonin target cells over the last three decades. Melatonin receptors are expressed at low levels, leading to sensitivity issues for their detection. The second problem are specificity issues with antibodies directed against the MT1 and MT2 melatonin receptors. These receptors are G protein-coupled receptors and many antibodies directed against such receptors have been shown to present similar problems concerning their specificity. Despite these specificity problems which start to be seriously addressed by recent studies, antibodies will be important tools in the future to identify and phenotype melatonin target cells. However, we will have to be more stringent than previously when establishing their specificity. The results obtained by these antibodies will have to be confronted and be coherent with results obtained by other techniques.

Recent Developments of Gramine: Chemistry and Biological Activity

The natural alkaloid gramine has attracted significant attention in both academic and industrial circles because of its potential and diverse biological activities, including antiviral, antibacterial, antifungal, anti-inflammatory and antitumor activities; application in therapy for Alzheimer's disease; serotonin-receptor-related activity; insecticidal activity; and application as an algicide. In this review, we focus on the research advances that have been made for gramine-based molecules since their discovery, providing key information on their extraction and separation, chemical synthesis and diverse biological activities. Data regarding their mechanisms of action are also presented. This comprehensive and critical review will serve as a guide for developing more drug candidates based on gramine skeletons.

Localization of neuropeptide receptor NPY4R in rat retina

Neuropeptide Y (NPY) is a significant neuromodulator implicated in a multitude of physiological functions via activating NPY receptors which belong to seven transmembrane G-protein-coupled receptors (GPCRs). However, the detailed cellular expression of NPY receptors in retina has been scarcely investigated. In this study, the expression of the special NPY4R receptor in rat retina was assessed using Western blot analysis and immunofluorescent staining. The detailed cellular localization of NPY4R receptor was studied using double immunofluorescent staining and laser-scanning confocal microscopy. Our data demonstrated that NPY4R receptor was weakly expressed in the inner segment of outer photoreceptors and extensively expressed in the outer segment of S-opsin-positive blue cones, L/M-opsin-positive red/green cones and in the somata of CB-positive horizontal cells, GAD65-positive GABAnergic amacrine cells, ChAT-positive cholinergic amacrine cells, TH-positive dopaminergic CA1 amacrine cells and CA2 amacrine cells, PV-positive AII amacrine cells, Brn3a-positive conventional ganglion cells and melanopsin-containing ipRGCs. In addition, NPY4R receptor was diffusely distributed throughout the full thickness of the inner plexiform layer and outer plexiform layer. However, the outer segment of Rho4D2-positive rods, the somata of ChX10-positive bipolar cells and CRALBP-positive Müller glial cells seemed to lack immunoreactivity of NPY4R receptor. The new finding that multiple types of retinal cell express NPY4R receptor provides new neurobiological basis for the participation of NPY in the regulation of retinal functions through activating NPY4R receptor.

Expression of Melatonin Receptor 1 in Rat Mesenteric Artery and Perivascular Adipose Tissue and Vasoactive Action of Melatonin

Melatonin is released by the pineal gland and can modulate cardiovascular system function via the G protein-coupled melatonin receptors MT1 and MT2. Most vessels are surrounded by perivascular adipose tissue (PVAT), which affects their contractility. The aim of our study was to evaluate mRNA and protein expression of MT1 and MT2 in the mesenteric artery (MA) and associated PVAT of male rats by RT-PCR and Western blot. Receptor localization was further studied by immunofluorescence microscopy. Effects of melatonin on neurogenic contractions were explored in isolated superior MA ex vivo by measurement of isometric contractile tension. MT1, but not MT2, was present in MA, and MT1 was localized mainly in vascular smooth muscle. Moreover, we proved the presence of MT1, but not MT2 receptors, in MA-associated PVAT. In isolated superior MA with intact PVAT, neuro-adrenergic contractile responses were significantly smaller when compared to arteries with removed PVAT. Pre-treatment with melatonin of PVAT-stripped arterial rings enhanced neurogenic contractions, while the potentiating effect of melatonin was not detected in preparations with preserved PVAT. We hypothesize that melatonin can stimulate the release of PVAT-derived relaxing factor(s) via MT1, which can override the direct pro-contractile effect of melatonin on vascular smooth muscle. Our results suggest that melatonin is involved in the control of vascular tone in a complex way, which is vessel specific and can reflect a sum of action on different layers of the vessel wall and surrounding PVAT.

Melatonin Prevents Non-image-Forming Visual System Alterations Induced by Experimental Glaucoma in Rats

Glaucoma is a blindness-causing disease that involves selective damage to retinal ganglion cells (RGCs) and their axons. A subset of RGCs expressing the photopigment melanopsin regulates non-image-forming visual system functions, such as pupillary light reflex and circadian rhythms. We analyzed the effect of melatonin on the non-image-forming visual system alterations induced by experimental glaucoma. For this purpose, male Wistar rats were weekly injected with vehicle or chondroitin sulfate into the eye anterior chamber. The non-image-forming visual system was analyzed in terms of (1) melanopsin-expressing RGC number, (2) anterograde transport from the retina to the olivary pretectal nucleus and the suprachiasmatic nuclei, (3) blue- and white light-induced pupillary light reflex, (4) light-induced c-Fos expression in the suprachiasmatic nuclei, (5) daily rhythm of locomotor activity, and (6) mitochondria in melanopsin-expressing RGC cells. Melatonin prevented the effect of experimental glaucoma on melanopsin-expressing RGC number, blue- and white light-induced pupil constriction, retina-olivary pretectal nucleus, and retina- suprachiasmatic nuclei communication, light-induced c-Fos expression in the suprachiasmatic nuclei, and alterations in the locomotor activity daily rhythm. In addition, melatonin prevented the effect of glaucoma on melanopsin-expressing RGC mitochondrial alterations. These results support that melatonin protected the non-image-forming visual system against glaucoma, probably through a mitochondrial protective mechanism.

Immunohistological Localization of Mel1a Melatonin Receptor in Pigeon Retina

BACKGROUND

Melatonin (N-acetyl-5-methoxytryptamine), a significant indoleamine neuromodulator implicated in circadian rhythms and sleep patterns, regulates diverse rhythmic functions via activating its high-affinity G-protein-coupled receptors. However, the detailed cellular expression of the Mel1a receptor in the retina is still a research gap.

METHODS

The expression of the Mel1a receptor in pigeon retina was assessed using Western blot analysis and immunofluorescent staining. The cellular localization of the Mel1a receptor was studied using double immunofluorescent staining and laser-scanning confocal microscopy.

RESULTS

Our data suggested that the Mel1a receptor was extensively expressed in the outer segment of Rho4D2-labeled rod and L/M-opsin-labeled red/green cone and in the somata of the CB-labeled horizontal cell, TH-labeled dopaminergic amacrine cell, ChAT-labeled cholinergic amacrine cell, PV-labeled AII amacrine cell, Brn3a-labeled conventional ganglion cell, melanopsin-containing ganglion cell and CRALBP-labeled Müller glial cell. In addition, the Mel1a receptor was diffusely distributed throughout the full thickness of the inner plexiform layer. However, the outer segment of S-opsin-labeled blue cone, the somata of ChX-10-labeled bipolar cell and outer plexiform layer seemed to lack immunoreactivity of the Mel1a receptor.

CONCLUSION

The finding that multiple types of retinal cells express the Mel1a receptor provides a new neurobiological basis for the participation of melatonin in the regulation of retinal functions through activating the Mel1a receptor.

Cellular localization of melatonin receptor Mel1b in pigeon retina

Melatonin, an important neuromodulator involved in circadian rhythms, modulates a series of physiological processes via activating its specific receptors, namely Mel1a (MT₁), Mel1b (MT₂) and Mel1c receptors. In this work, the localization of Mel1b receptor was studied in pigeon retina using double immunohistochemistry staining and confocal scanning microscopy. Our results showed that Mel1b receptor widely existed in the outer segment of photoreceptors and in the somata of dopaminergic amacrine cells, cholinergic amacrine cells, glycinergic AII amacrine cells, conventional ganglion cells and intrinsically photosensitive retinal ganglion cells, while horizontal cells, bipolar cells and Müller glial cells seemed to lack immunoreactivity of Mel1b receptor. That multiple types of retinal cells expressing Mel1b receptor suggests melatonin may directly modulate the activities of retina via activating Mel1b receptor.

Transcriptomic Signatures of Postnatal and Adult Intrinsically Photosensitive Ganglion Cells

Intrinsically photosensitive retinal ganglion cells (ipRGCs) are rare mammalian photoreceptors essential for non-image-forming vision functions, such as circadian photoentrainment and the pupillary light reflex. They comprise multiple subtypes distinguishable by morphology, physiology, projections, and levels of expression of melanopsin (Opn4), their photopigment. The molecular programs that distinguish ipRGCs from other ganglion cells and ipRGC subtypes from one another remain elusive. Here, we present comprehensive gene expression profiles of early postnatal and adult mouse ipRGCs purified from two lines of reporter mice that mark different sets of ipRGC subtypes. We find dozens of novel genes highly enriched in ipRGCs. We reveal that Rasgrp1 and Tbx20 are selectively expressed in subsets of ipRGCs, though these molecularly defined groups imperfectly match established ipRGC subtypes. We demonstrate that the ipRGCs regulating circadian photoentrainment are diverse at the molecular level. Our findings reveal unexpected complexity in gene expression patterns across mammalian ipRGC subtypes.

Bioactivity-guided synthesis of gramine derivatives as new MT1 and 5-HT1A receptors agonists

Twenty-four gramine derivatives were synthesized and evaluated on MT₁ and 5-HT_1A receptors in vitro. Among them, seven derivatives (7, 8, 16, 19, 20, 21, and 24) exhibited higher agonisting activities on MT₁ or 5-HT_1A receptors. Compared with gramine, derivatives 7, 8, 16, 19, 20, 21, and 24 displayed 1.6-3.5-fold increase in agonistic rates on 5-HT_1A receptor. Particularly, derivatives 7, 19, and 21 exhibited significant agonistic activities on MT₁ and 5-HT_1A receptors with EC₅₀ values of 0.51, 0.39, 0.50 mΜ and 0.28, 0.46, 0.23 mΜ, respectively. The preliminary structure-activity relationships of gramine derivatives were summarized for further investigation on MT₁ and 5-HT_1A receptors as new potential agonists.

Melatonin modulates M4-type ganglion-cell photoreceptors

In the retina, melatonin is secreted at night by rod/cone photoreceptors and serves as a dark-adaptive signal. Melatonin receptors have been found in many retinal neurons including melanopsin-containing intrinsically photosensitive retinal ganglion cells (ipRGCs), suggesting it could modulate the physiology of these inner retinal photoreceptors. Here, we investigated whether melatonin modulates the alpha-like M4-type ipRGCs, which are believed to mediate image-forming vision as well as non-image-forming photoresponses. Applying melatonin during daytime (when endogenous melatonin secretion is low) caused whole-cell-recorded M4 cells' rod/cone-driven depolarizing photoresponses to become broader and larger, whereas the associated elevation in spike rate was reduced. Melanopsin-based light responses were not affected significantly. Nighttime application of the melatonin receptor antagonist luzindole also altered M4 cells' rod/cone-driven light responses but in the opposite ways: the duration and amplitude of the graded depolarization were reduced, whereas the accompanying spiking increase was enhanced. These luzindole-induced changes confirmed that M4 cells are modulated by endogenous melatonin. Melatonin could induce the above effects by acting directly on M4 cells because immunohistochemistry detected MT1 receptors in these cells, although it could also act presynaptically. Interestingly, the daytime and nighttime recordings showed significant differences in resting membrane potential, spontaneous spike rate and rod/cone-driven light responses, suggesting that M4 cells are under circadian control. This is the first report of a circadian variation in ipRGCs' resting properties and synaptic input, and of melatoninergic modulation of ipRGCs.