Diurnal rhythms of tryptophan hydroxylase activity in Xenopus laevis retina: opposing phases in photoreceptors and inner retinal neurons

Circadian clock organization in the retina: From clock components to rod and cone pathways and visual function

Circadian (24-h) clocks are cell-autonomous biological oscillators that orchestrate many aspects of our physiology on a daily basis. Numerous circadian rhythms in mammalian and non-mammalian retinas have been observed and the presence of an endogenous circadian clock has been demonstrated. However, how the clock and associated rhythms assemble into pathways that support and control retina function remains largely unknown. Our goal here is to review the current status of our knowledge and evaluate recent advances. We describe many previously-observed retinal rhythms, including circadian rhythms of morphology, biochemistry, physiology, and gene expression. We evaluate evidence concerning the location and molecular machinery of the retinal circadian clock, as well as consider findings that suggest the presence of multiple clocks. Our primary focus though is to describe in depth circadian rhythms in the light responses of retinal neurons with an emphasis on clock control of rod and cone pathways. We examine evidence that specific biochemical mechanisms produce these daily light response changes. We also discuss evidence for the presence of multiple circadian retinal pathways involving rhythms in neurotransmitter activity, transmitter receptors, metabolism, and pH. We focus on distinct actions of two dopamine receptor systems in the outer retina, a dopamine D4 receptor system that mediates circadian control of rod/cone gap junction coupling and a dopamine D1 receptor system that mediates non-circadian, light/dark adaptive regulation of gap junction coupling between horizontal cells. Finally, we evaluate the role of circadian rhythmicity in retinal degeneration and suggest future directions for the field of retinal circadian biology.

Circadian regulation in the retina: From molecules to network

The mammalian retina is the most unique tissue among those that display robust circadian/diurnal oscillations. The retina is not only a light sensing tissue that relays light information to the brain, it has its own circadian "system" independent from any influence from other circadian oscillators. While all retinal cells and retinal pigment epithelium (RPE) possess circadian oscillators, these oscillators integrate by means of neural synapses, electrical coupling (gap junctions), and released neurochemicals (such as dopamine, melatonin, adenosine, and ATP), so the whole retina functions as an integrated circadian system. Dysregulation of retinal clocks not only causes retinal or ocular diseases, it also impacts the circadian rhythm of the whole body, as the light information transmitted from the retina entrains the brain clock that governs the body circadian rhythms. In this review, how circadian oscillations in various retinal cells are integrated, and how retinal diseases affect daily rhythms.

Melatonin: an underappreciated player in retinal physiology and pathophysiology

In the vertebrate retina, melatonin is synthesized by the photoreceptors with high levels of melatonin at night and lower levels during the day. Melatonin exerts its influence by interacting with a family of G-protein-coupled receptors that are negatively coupled with adenylyl cyclase. Melatonin receptors belonging to the subtypes MT(1) and MT(2) have been identified in the mammalian retina. MT(1) and MT(2) receptors are found in all layers of the neural retina and in the retinal pigmented epithelium. Melatonin in the eye is believed to be involved in the modulation of many important retinal functions; it can modulate the electroretinogram (ERG), and administration of exogenous melatonin increases light-induced photoreceptor degeneration. Melatonin may also have protective effects on retinal pigment epithelial cells, photoreceptors and ganglion cells. A series of studies have implicated melatonin in the pathogenesis of age-related macular degeneration, and melatonin administration may represent a useful approach to prevent and treat glaucoma. Melatonin is used by millions of people around the world to retard aging, improve sleep performance, mitigate jet lag symptoms, and treat depression. Administration of exogenous melatonin at night may also be beneficial for ocular health, but additional investigation is needed to establish its potential.

Diurnal rhythms of tryptophan hydroxylase 1 and 2 mRNA expression in the rat retina

Tryptophan hydroxylase is the first of four enzymes in the melatonin biosynthetic pathway. Recent studies have shown that there are two genes, Tph1 and Tph2, that encode tryptophan hydroxylase in mammals. In this study, we investigated which of the two genes is expressed in the rat retina. To that end, we measured Tph1 (classical Tph) and Tph2 mRNA levels using real-time quantitative RT-PCR in the retina. Our data demonstrate that Tph1 mRNA is the prevalent form expressed in the retina; Tph2 mRNA is also present but the level is very low. We also measured Tph1 expression levels in the outer nuclear layer, inner nuclear layer, and ganglion cell layer by combining laser capture microdissection and real-time RT-PCR. Tph1 mRNA is more abundant in the photoreceptors of the outer nuclear layer than in the inner nuclear layer or ganglion cell layer. Tph1 and Tph2 transcripts showed robust diurnal rhythms of abundance, with highest levels at night. Our results support the hypothesis that Tph1 is involved in melatonin synthesis in retinal photoreceptor cells.

14-3-3 proteins in pineal photoneuroendocrine transduction: how many roles?

Recent studies suggest that a common theme links the diverse elements of pineal photoneuroendocrine transduction--regulation via binding to 14-3-3 proteins. The elements include photoreception, neurotransmission, signal transduction and the synthesis of melatonin from tryptophan. We review general aspects of 14-3-3 proteins and their biological function as binding partners, and also focus on their roles in pineal photoneuroendocrine transduction.

Serotonin synthesis and its light-dark variation in the rat retina

Retinal circadian rhythms are driven by an intrinsic oscillator, using chemical signals such as melatonin, secreted by photoreceptor cells. The purpose of the present work was to identify the origin of serotonin, the precursor of melatonin, in the retina of adult rat, where no immunoreactivity for serotonin or tryptophan hydroxylase had ever been detected. To demonstrate local synthesis of serotonin in the rat retina, substrates of tryptophan hydroxylase, the first limiting enzyme in the serotonin pathway, have been used. Tryptophan, in the presence of an inhibitor of aromatic amino acid decarboxylase, enhanced 5-hydroxytryptophan levels, whereas alpha-methyltryptophan, a competitive substrate inhibitor, was hydroxylated into alpha-methyl-5-hydroxytryptophan. Tryptophan hydroxylase substrate concentration was higher in the dark period than in the light period, and formation of hydroxylated compounds was increased. The presence of tryptophan hydroxylase mRNA in the rat retina was confirmed by RT-PCR. Taken together, the results support the local synthesis of serotonin by tryptophan hydroxylation, this metabolic pathway being required more critically when 5-HT is used for melatonin synthesis.

Regulation of tryptophan hydroxylase activity in Xenopus laevis photoreceptor cells by cyclic AMP

The aim of this study was to investigate the role of cyclic AMP in the regulation of tryptophan hydroxylase activity localized in retinal photoreceptor cells of Xenopus laevis, where the enzyme plays a key role in circadian melatonin biosynthesis. In photoreceptor-enriched retinas that lack serotonergic neurons, tryptophan hydroxylase activity is markedly stimulated by treatments that increase intracellular levels of cyclic AMP or activate cyclic AMP-dependent protein kinase, including forskolin, phosphodiesterase inhibitors, and cyclic AMP analogues. In contrast, cyclic AMP has no effect on tryptophan hydroxylase mRNA abundance. Experiments using cycloheximide and actinomycin D demonstrate that cyclic AMP exerts its regulatory effect via posttranslational mechanisms mediated by cyclic AMP-dependent protein kinase. The effect of cyclic AMP is independent of the phase of the photoperiod, suggesting that the nucleotide is not a mediator of the circadian rhythm of tryptophan hydroxylase. Cyclic AMP accumulation is higher in darkness than in light, as is tryptophan hydroxylase activity. Furthermore, the stimulatory effect of forskolin and that of darkness are inhibited by H89, an inhibitor of cyclic AMP-dependent protein kinase. In conclusion, cyclic AMP may mediate the acute effects of light and darkness on tryptophan hydroxylase activity of retinal photoreceptor cells.