Elevation of melatonin in chicken retina by 5-hydroxytryptophan: differential light/dark responses

Physiology and pharmacology of melatonin in relation to biological rhythms

Melatonin is an evolutionarily conserved molecule that serves a time-keeping function in various species. In vertebrates, melatonin is produced predominantly by the pineal gland with a marked circadian rhythm that is governed by the central circadian pacemaker (biological clock) in the suprachiasmatic nuclei of the hypothalamus. High levels of melatonin are normally found at night, and low levels are seen during daylight hours. As a consequence, melatonin has been called the "darkness hormone". This review surveys the current state of knowledge regarding the regulation of melatonin synthesis, receptor expression, and function. In particular, it addresses the physiological, pathological, and therapeutic aspects of melatonin in humans, with an emphasis on biological rhythms.

Role of melatonin in the eye and ocular dysfunctions

Melatonin is a ubiquitous molecule and widely distributed in nature, with functional activity occurring in unicellular organisms, plants, fungi, and animals. Several studies have indicated that melatonin synthesis occurs in the retina of most vertebrates, including mammals. The retinal biosynthesis of melatonin and the mechanisms involved in the regulation of this process have been extensively studied. Circadian clocks located in the photoreceptors and retinal neurons regulate melatonin synthesis in the eye. Photoreceptors, dopaminergic amacrine neurons, and horizontal cells of the retina, corneal epithelium, stroma endothelium, and the sclera all have melatonin receptors, indicating a widespread ocular function for melatonin. In addition, melatonin is an effective antioxidant which scavenges free radicals and up-regulates several antioxidant enzymes. It also has a strong antiapoptotic signaling function, an effect that it exerts even during ischemia. Melatonin cytoprotective properties may have practical implications in the treatment of ocular diseases, like glaucoma and age-related macular degeneration.

A new gain-of-function allele in chimpanzee tryptophan hydroxylase 2 and the comparison of its enzyme activity with that in humans and rats

Tryptophan hydroxylase 2 (TPH2) is a rate-limiting enzyme of neuronal serotonin biosynthesis. Recently, two single nucleotide polymorphisms (SNPs) at the exon 11 coding region that resulted in amino acid substitutions in the C-terminal domain have been reported to affect enzyme activity in humans and mice. We determined 175 base-pair sequences of the exon 11 region in nine primate species from all recognized lineages. All nucleotide sequence substitutions were synonymous, with the exception of one adenine (A) to guanine (G) substitution at the 1404th position in the open reading frame (ORF). This substitution leads to a glutamine (Q) to arginine (R) amino acid substitution at the 468th position within chimpanzee sequences. The frequency of the G allele was 0.24 among 66 chimpanzees. Therefore, it is a novel SNP observed in chimpanzees, and we have named these two alleles as ch468Q and ch468R, respectively. When expressed in HeLa cells, ch468R caused an approximate 20% increase in enzyme function during L-5-hydroxytryptophan (5HTP) production (P<0.001). We also surveyed the interspecies difference in enzyme activity among human, chimpanzee, and rat. Although the rat showed an identical amino acid sequence at the C-terminal region as those of human and ch468Q, the rat enzyme was more active than those of human or chimpanzee (P<0.001), indicating the importance of substitutions in other regions. Our findings on the chimpanzee SNP will be a useful genetic marker in understanding the individual difference in the serotonin-related behavior.

Opposite effects of tryptophan intake on motor activity in ring doves (diurnal) and rats (nocturnal)

The role of l-tryptophan as precursor of serotonin and melatonin synthesis on activity-rest rhythm was studied in ring doves, Streptopelia risoria, as a representative of diurnal animals and rats, Rattus norvegicus, as a typical nocturnal one. The animals were housed in cages equipped for horizontal activity recording in a thermostatized chamber and submitted to a 12/12h light/dark photoperiod (lights on at 08:00 h). After acclimatization, the animals received vehicle (methylcellulose) and l-tryptophan (240 mg/kg) by esophagic cannula 2h before the onset of either light or dark phase. Also, oral melatonin (2.5mg/kg) was tested for comparative purposes. After nocturnal l-tryptophan administration, rats showed increased activity (149%), while the opposite occurred in ring doves (39% decrease). No significant changes were found after diurnal l-tryptophan intake in either species. Melatonin produced effects similar to those of l-tryptophan. These results suggest that the effects of l-tryptophan administration are dependent on the nocturnal/diurnal habits of the studied species and, most probably, are mediated by increased melatonin synthesis.

Circadian clocks, clock networks, arylalkylamine N-acetyltransferase, and melatonin in the retina

Circadian clocks are self-sustaining genetically based molecular machines that impose approximately 24h rhythmicity on physiology and behavior that synchronize these functions with the solar day-night cycle. Circadian clocks in the vertebrate retina optimize retinal function by driving rhythms in gene expression, photoreceptor outer segment membrane turnover, and visual sensitivity. This review focuses on recent progress in understanding how clocks and light control arylalkylamine N-acetyltransferase (AANAT), which is thought to drive the daily rhythm in melatonin production in those retinas that synthesize the neurohormone; AANAT is also thought to detoxify arylalkylamines through N-acetylation. The review will cover evidence that cAMP is a major output of the circadian clock in photoreceptor cells; and recent advances indicating that clocks and clock networks occur in multiple cell types of the retina.

Identification of substrate orienting and phosphorylation sites within tryptophan hydroxylase using homology-based molecular modeling

Tryptophan hydroxylase (TPH) is the initial and rate-limiting enzyme in the biosynthesis of serotonin. The inherent instability of TPH has prevented a crystallographic structure from being resolved. For this reason, multiple sequence alignment-based molecular modeling was utilized to generate a full-length model of human TPH. Previously determined crystal coordinates of two highly homologous proteins, phenylalanine hydroxylase and tyrosine hydroxylase, were used as templates. Analysis of the model aided rational mutagenesis studies to further dissect the regulation and catalysis of TPH. Using rational site-directed mutagenesis, it was determined that Tyr235 (Y235), within the active site of TPH, appears to be involved as a tryptophan substrate orienting residue. The mutants Y235A and Y235L displayed reduced specific activity compared to wild-type TPH ( approximately 5 % residual activity). The K(m) of tryptophan for the Y235A (564 microM) and Y235L (96 microM) mutant was significantly increased compared to wild-type TPH (42 microM). In addition, kinetic analyses were performed on wild-type TPH and a deletion construct that lacks the amino terminal autoregulatory sequence (TPH NDelta15). This sequence in phenylalanine hydroxylase (residues 19 to 33) has previously been proposed to act as a steric regulator of substrate accessibility to the active site. Changes in the steady-state kinetics for tetrahydrobiopterin (BH(4)) and tryptophan for TPH NDelta15 were not observed. Finally, it was demonstrated that both Ser58 and Ser260 are substrates for Ca(2+)/calmodulin-dependent protein kinase II. Additional analysis of this model will aid in deciphering the regulation and substrate specificity of TPH, as well as providing a basis to understand as yet to be identified polymorphisms.

Melatonin circadian rhythm in the retina of mammals

Melatonin has been traditionally considered to be derived principally from the pineal gland. However, several investigations have now demonstrated that melatonin synthesis occurs also in the retina (and in other organs as well) of several vertebrate classes, including mammals. As in the pineal, melatonin synthesis in the retina is elevated at night and reduced during the day. Since melatonin receptors are present in the retina and retinal melatonin does not contribute to the circulating levels, retinal melatonin probably acts locally as a neuromodulator. Melatonin synthesis in the retinas of mammals is under control of a circadian oscillator located within the retina itself, and circadian rhythms in melatonin synthesis and/or release have been described for several species of rodents. These rhythms are present in vivo, persist in vitro, are entrained by light, and are temperature compensated. The recent cloning of the gene responsible for the synthesis of the enzyme arylalkylamine N-acetyltransferase (the only enzyme unique to the melatonin synthetic pathway) will facilitate localizing the cellular site of melatonin synthesis in the retina and investigating the molecular mechanism responsible for the generation of retinal melatonin rhythmicity. Melatonin has been implicated in many retinal functions, and the levels of melatonin and dopamine appear to regulate several aspects of retinal physiology that relate to light and dark adaptation. In conclusion, it seems that retinal melatonin is involved in several functions, but its precise role is yet to be understood.

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.

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

Tryptophan hydroxylase (TPH) is the first enzyme in the biosynthetic pathways of melatonin in photoreceptor cells and of serotonin in amacrine cells. To assess the regulation of TPH activity in photoreceptor cells, we pretreated retinas with kainic acid. The neurotoxin selectively killed inner retinal neurons while sparing photoreceptors. TPH activity in both control and kainate-treated retinas undergoes a day-night rhythm. The rhythms in both preparations fit sinusoidal functions. However, the rhythm in intact retinas peaks at midday while that in kainate-lesioned retinas does so at midnight. The daily rhythm of tryptophan hydroxylase activity in photoreceptors parallels that of melatonin release. Comparing the mean level of activity in rhythms of intact and lesioned retinas, we calculate that the TPH activity in photoreceptors represents 24% of the total activity. Therefore, the TPH activity measured in intact retinas reflects mainly the enzymatic activity in serotonergic neurons, masking that from photoreceptors. In contrast, the levels and diurnal variation of TPH mRNA did not differ in intact and kainate-lesioned retinas indicating that measurements of TPH mRNA content reflect primarily that in photoreceptor cells. Thus, TPH mRNA levels and enzyme activity are differentially regulated in amacrine neurons and photoreceptor cells. This differential regulation markedly impacts the patterns of daily rhythms observed in the intact retina.