Melatonin synthesis in the greenfrog retina in culture: I. Modulation by the light/dark cycle, forskolin and inhibitors of protein synthesis

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.

Interactions between dopamine and melatonin organize circadian rhythmicity in the retina of the green iguana

Circadian physiology in the vertebrate retina is regulated by several neurotransmitters. In the lateral eyes of the green iguana the circadian rhythm of melatonin content peaks during the night while the rhythm of dopamine peaks during the day. In the present work, the authors explore the interaction of these 2 neurotransmitters during the circadian cycle. They depleted retinal dopamine with intravitreal injections of 6-hydroxydopamine (6-OHDA) and measured ocular melatonin content in vivo throughout 1 circadian cycle. The circadian rhythm of ocular melatonin not only persisted but increased 10-fold in amplitude. This increase was substantially reduced by the intraocular administration of dopamine. 6-OHDA-treated retinas, unlike those from untreated animals, did not express a circadian rhythm of melatonin synthesis in vitro. To deplete retinal melatonin, the authors pinealectomized iguanas and blocked retinal melatonin synthesis by depleting serotonin with intraocular injections of 5,6-dihydroxytryptamine. In animals so treated, they found that the circadian rhythm of retinal dopamine content was abolished, the levels of dopamine were lowered, and the levels of dopamine metabolites were greatly increased. The data suggest that in iguanas, the amplitude of the circadian rhythm of melatonin synthesis in the eye is suppressed by dopamine while the rhythm of dopamine depends, at least in part, on the presence of melatonin.

Retinal, pineal and diencephalic expression of frog arylalkylamine N-acetyltransferase-1

The arylalkylamine N-acetyltransferase (AANAT) is a key enzyme in the rhythmic production of melatonin. Two Aanats are expressed in Teleost fish (Aanat1 in the retina and Aanat2 in the pineal organ) but only Aanat1 is found in tetrapods. This study reports the cloning of Aanat1 from R. perezi. Transcripts were mainly expressed in the retina, diencephalon, intestine and testis. In the retina and pineal organ, Aanat1 expression was in the photoreceptor cells. Expression was also seen in ependymal cells of the 3rd ventricle and discrete cells of the suprachiasmatic area. The expression of Aanat1 in both the retina and pineal organ, and the absence of Aanat2 suggests that green frog resembles more to birds and mammals than to Teleost fish, as far as Aanat is concerned. The significance of Aanat1 in extra-pineal and extra-retinal tissues remains to be elucidated; in the diencephalon, it might be associated to the so-called deep brain photoreceptor cells.

Modulation by melatonin of glutamatergic synaptic transmission in the carp retina

Melatonin is involved in a variety of physiological functions through activating specific receptors coupled to GTP-binding protein. Melatonin and its receptors are abundant in the retina. Here we show for the first time that melatonin modulates glutamatergic synaptic transmission from cones to horizontal cells (HCs) in carp retina. Immunocytochemical data revealed the expression of the MT1 receptor on carp HCs. Whole-cell recordings further showed that melatonin of physiological concentrations potentiated glutamate-induced currents from isolated cone-driven HCs (H1 cells) in a dose-dependent manner, by increasing the efficacy and apparent affinity of the glutamate receptor. The effects of melatonin were reversed by luzindole, but not by K 185, indicating the involvement of the MT1 receptor. Like melatonin, methylene blue (MB), a guanylate cyclase inhibitor, also potentiated the glutamate currents, but internal infusion of cGMP suppressed them. The effects of melatonin were not observed in cGMP-filled and MB-incubated HCs. These results suggest that the melatonin effects may be mediated by decreasing the intracellular concentration of cGMP. Consistent with these observations, melatonin depolarized the membrane potential of H1 cells and reduced their light responses, which could also be blocked by luzindole. These effects of melatonin persisted in the presence of the antagonists of receptors for dopamine, GABA and glycine, indicating a direct action of melatonin on H1 cells. Such modulation by melatonin of glutamatergic transmission from cones to HCs is thought to be in part responsible for circadian changes in light responsiveness of cone HCs in teleost retina.

Circadian rhythm of iguana electroretinogram: the role of dopamine and melatonin

The amplitude of the b-wave of the electroretinogram (ERG) varies with a circadian rhythm in the green iguana; the amplitude is high during the day(or subjective day) and low during the night (or subjective night). Dopamine and melatonin contents in the eye are robustly rhythmic under constant conditions; dopamine levels are high during the subjective day, and melatonin levels are high during the subjective night. Dopamine and melatonin affect the amplitude of the b-wave in an antagonistic and phase-dependent manner: dopamine D2-receptor agonists injected intraocularly during the subjective night produce high-amplitude b-waves characteristic of the subjective day, whereas melatonin injected intraocularly during the subjective day reduces b-wave amplitude. Sectioning the optic nerve abolishes the circadian rhythms of b-wave amplitude and of dopamine content. The results of this study suggest that in iguana, a negative feedback loop involving dopamine and melatonin regulates the circadian rhythm of the ERG b-wave amplitude that is at least in part generated in the brain.

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.