ABSTRACT
Melatonin modulates many important functions within the eye by interacting with a family of G-protein-coupled receptors that are negatively coupled with adenylate cyclase. In the mouse, Melatonin Receptors type 1 (MT(1)) mRNAs have been localized to photoreceptors, inner retinal neurons, and ganglion cells, thus suggesting that MT(1) receptors may play an important role in retinal physiology. Indeed, we have recently reported that absence of the MT(1) receptors has a dramatic effect on the regulation of the daily rhythm in visual processing, and on retinal cell viability during aging. We have also shown that removal of MT(1) receptors leads to a small (3-4 mmHg) increase in the level of the intraocular pressure during the night and to a significant loss (25-30%) in the number of cells within the retinal ganglion cell layer during aging. In the present study we investigated the cellular distribution in the C3H/f(+/+) mouse retina of MT(1) receptors using a newly developed MT(1) receptor antibody, and then we determined the role that MT(1) signaling plays in the circadian regulation of the mouse electroretinogram, and in the retinal dopaminergic system. Our data indicate that MT(1) receptor immunoreactivity is present in many retinal cell types, and in particular, on rod and cone photoreceptors and on intrinsically photosensitive ganglion cells (ipRGCs). MT(1) signaling is necessary for the circadian rhythm in the photopic ERG, but not for the circadian rhythm in the retinal dopaminergic system. Finally our data suggest that the circadian regulation of dopamine turnover does not drive the photopic ERG rhythm.
Subject(s)
Circadian Rhythm/physiology , Dopamine/metabolism , Receptor, Melatonin, MT1/metabolism , Retina/metabolism , 3,4-Dihydroxyphenylacetic Acid/metabolism , Animals , Blotting, Western , Chromatography, High Pressure Liquid , Circadian Rhythm/genetics , Electroretinography , Immunohistochemistry , MiceABSTRACT
In mammals a subpopulation of retinal ganglion cells are intrinsically photosensitive (ipRGCs), express the photopigment melanopsin, and play an important role in the regulation of the nonimage-forming visual system. We have recently reported that melanopsin mRNA and protein levels in the rat retina are under photic and circadian control. The aim of the present work was to investigate the mechanisms that control melanopsin expression in the rat retina. We discovered that dopamine (DA) is involved in the regulation of melanopsin mRNA, possibly via dopamine D2 receptors that are located on these ipRGCs. Interestingly, we also discovered that pituitary adenylate cyclase-activating peptide (PACAP) mRNA levels are affected by DA. Dopamine synthesis and release in the retina are regulated by the rod and the cone photoreceptors via retinal circuitry; our new data indicate that DA controls melanopsin expression, indicating that classical photoreceptors may modulate the transcription of this new photopigment. Our study also suggests that DA may have an important role in mediating the light signals that are used for circadian entrainment and for other responses that are mediated by the nonimage-forming visual system.
Subject(s)
Dopamine/pharmacology , Gene Expression Regulation/drug effects , Retina/cytology , Retinal Ganglion Cells/drug effects , Rod Opsins/metabolism , 2,3,4,5-Tetrahydro-7,8-dihydroxy-1-phenyl-1H-3-benzazepine/pharmacology , 3,4-Dihydroxyphenylacetic Acid/metabolism , Animals , Blotting, Western/methods , Circadian Rhythm/drug effects , Circadian Rhythm/physiology , Dopamine/metabolism , Dopamine Agonists/pharmacology , Dose-Response Relationship, Drug , Drug Interactions , Excitatory Amino Acid Agonists/pharmacology , Gene Expression Regulation/physiology , Immunohistochemistry/methods , In Situ Hybridization/methods , Kainic Acid/pharmacology , Male , Quinpirole/pharmacology , RNA, Messenger/metabolism , Rats , Rats, Inbred F344 , Receptors, Dopamine D2/genetics , Receptors, Dopamine D2/metabolism , Reverse Transcriptase Polymerase Chain Reaction/methods , Rod Opsins/genetics , Time FactorsABSTRACT
The product of melatonin oxidation, N1-acetyl-N2-formyl-5-methoxykynuramine (AFMK), was synthesized and a method for its determination in biological samples was developed. High performance liquid chromatography (HPLC) with fluorescence detection provided good sensitivity and selectivity. Wavelengths of 350 and 480 nm were used for excitation and emission, respectively. Serum and retinal homogenates were extracted with chloroform prior to analysis by HPLC. Endogenous AFMK was detected in the retina of rats but the serum concentration of this melatonin metabolite was below the detection limit of the method for measurement. Retinal AFMK concentration was higher during the dark phase of the light/dark cycle, when the retinal melatonin content is maximal. Intraperitoneal administration of melatonin significantly increased serum and retinal AFMK levels. Formation of AFMK from melatonin was also confirmed by in vivo microdialysis with the probe implanted into the brain lateral ventricle. The study shows that AFMK is indeed a product of melatonin oxidation in vivo. The possible physiological significance of melatonin oxidation metabolic pathway is discussed.
