Melatonin inhibits ACh release from rabbit retina

Cloning and retinal expression of melatonin receptors in the European sea bass, Dicentrarchus labrax

Melatonin contributes to synchronizing behaviors and physiological functions to daily and seasonal rhythm in fish. However, no coherent vision emerges because the effects vary with the species, sex, age, moment of the year or sexual cycle. And, scarce information is available concerning the melatonin receptors, which is crucial to our understanding of the role melatonin plays. We report here the full length cloning of three different melatonin receptor subtypes in the sea bass Dicentrarchus labrax, belonging, respectively, to the MT1, MT2 and Mel1c subtypes. MT1, the most abundantly expressed, was detected in the central nervous system, retina, and gills. MT2 was detected in the pituitary gland, blood cells and, to a lesser extend, in the optic tectum, diencephalon, liver and retina. Mel1c was mainly expressed in the skin; traces were found in the retina. The cellular sites of MT1 and MT2 expressions were investigated by in situ hybridization in the retina of pigmented and albino fish. The strongest signals were obtained with the MT1 riboprobes. Expression was seen in cells also known to express the enzymes of the melatonin biosynthesis, i.e., in the photoreceptor, inner nuclear and ganglion cell layers. MT1 receptor mRNAs were also abundant in the retinal pigment epithelium. The results are consistent with the idea that melatonin is an autocrine (neural retina) and paracrine (retinal pigment epithelium) regulator of retinal function. The molecular tools provided here will be of valuable interest to further investigate the targets and role of melatonin in nervous and peripheral tissues of fish.

Influence of photoperiodic history on clock genes and the circadian pacemaker in the rat retina

The influence of seasonal lighting conditions on expression of clock genes and the circadian pacemaker was investigated in the rat retina. For this purpose, the 24-h profiles of nine clock genes (bmal1, clock, per1, per2, per3, dec1, dec2, cry1 and cry 2) and the arylalkylamine N-acetyltransferase gene as an indicator of the circadian pacemaker output were compared between light-dark periods of 8 : 16 and 16 : 8 h. The photoperiod influenced the daily patterns of the amount of transcript for per1, per3, dec2 and arylalkylamine N-acetyltransferase. This indicates that photoperiodic information modulates clock gene expression in addition to the circadian pacemaker of the retina. Under constant darkness, photoperiod-dependent changes in the daily profile of the level of transcript persisted for the arylalkylamine N-acetyltransferase gene but not for any of the clock genes. Hence, quantitative expression of each clock gene is influenced by the photoperiod only under the acute light-dark cycle, whereas the pacemaker is capable of storing photoperiodic information from past cycles.

Role of melatonin in Alzheimer-like neurodegeneration

Alzheimer disease (AD), an age-related neurodegenerative disorder with progressive loss of memory and deterioration of comprehensive cognition, is characterized by extracellular senile plaques of aggregated beta-amyloid (Abeta), and intracellular neurofibrillary tangles that contain hyperphosphorylated tau protein. Recent studies showed that melatonin, an indoleamine secreted by the pineal gland, may play an important role in aging and AD as an antioxidant and neuroprotector. Melatonin decreases during aging and patients with AD have a more profound reduction in this hormone. Data from clinical trials indicate that melatonin supplementation improves sleep, ameliorates sundowning, and slows down the progression of cognitive impairment in Alzheimer patients. Melatonin efficiently protects neuronal cells from Abeta-mediated toxicity via antioxidant and anti-amyloid properties: it not only inhibits Abeta generation, but also arrests the formation of amyloid fibrils by a structure-dependent interaction with Abeta. Our recent studies have demonstrated that melatonin efficiently attenuates Alzheimer-like tau hyperphosphorylation. Although the exact mechanism is still not fully understood, a direct regulatory influence of melatonin on the activities of protein kinases and protein phosphatases is proposed. Additionally, melatonin also plays a role in protecting cholinergic neurons and in anti-inflammation. Here, the neuroprotective effects of melatonin and the underlying mechanisms by which it exerts its effects are reviewed. The capacity of melatonin to prevent or ameliorate tau and Abeta pathology further enhances its potential in the prevention or treatment of AD.

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.

Rabbit retinal ganglion cell responses to nicotine can be mediated by beta2-containing nicotinic acetylcholine receptors

Acetylcholine (ACh) affects the response properties of many retinal ganglion cells (GCs) through the activation of nicotinic acetylcholine receptors (nAChRs). To date there have been few studies directly correlating the expression of specific nAChR subtypes with the physiological and morphological characteristics of specific retinal GCs. This study was designed to correlate responses to nicotine application with immunohistochemical evidence of nAChR expression in physiologically and morphologically identified ganglion cells. Extracellular recordings were used to physiologically identify rabbit retinal GCs, based on responses to light stimulation. Cells were then tested for responses to nicotine application and/or for expression of nAChRs, as judged by immunoreactivity to mAb210, an nAChR antibody. The morphologies of many physiologically identified cells were also determined by dye injection. More than three-fourths of ganglion cells tested responded to nicotine application under cobalt-induced synaptic blockade. The nicotine sensitivity was consistent with nAChR immunoreactivity and was also correlated with specific morphological subgroups of GCs. Overall, approximately two-thirds of all physiologically identified GCs that were processed for immunohistochemistry displayed immunoreactivity. In total, 18 of 22 physiologically identified cells demonstrated both sensitivity to nicotine application under synaptic blockade and mAb210 immunoreactivity (mAb210-IR). Thus, mAb210-IR is likely to represent functional nAChRs that can modulate retinal information processing and visual functioning via direct excitation of a number of GC classes.

AMPA receptors mediate acetylcholine release from starburst amacrine cells in the rabbit retina

The light response of starburst amacrine cells is initiated by glutamate released from bipolar cells. To identify the receptors that mediate this response, we used a combination of anatomical and physiological techniques. An in vivo, rabbit eyecup was preloaded with [(3)H]-choline, and the [(3)H]-acetylcholine (ACh) released into the superfusate was monitored. A photopic, 3 Hz flashing light increased ACh release, and the selective AMPA receptor antagonist, GYKI 53655, blocked this light-evoked response. Nonselective AMPA/kainate agonists increased the release of ACh, but the specific kainate receptor agonist, SYM 2081, did not increase ACh release. Selective AMPA receptor antagonists, GYKI 53655 or GYKI 52466, also blocked the responses to agonists. We conclude that the predominant excitatory input to starburst amacrine cells is mediated by AMPA receptors. We also labeled lightly fixed rabbit retinas with antisera to choline acetyltransferase (ChAT), AMPA receptor subunits GluR1, GluR2/3, or GluR4, and kainate receptor subunits GluR6/7 and KA2. Labeled puncta were observed in the inner plexiform layer with each of these antisera to glutamate receptors, but only GluR2/3-IR puncta and GluR4-IR puncta were found on the ChAT-IR processes. The same was true of starburst cells injected intracellularly with Neurobiotin, and these AMPA receptor subunits were localized to two populations of puncta. The AMPA receptors are expected to desensitize rapidly, enhancing the sensitivity of starburst amacrine cells to moving or other rapidly changing stimuli.

Photic regulation of melatonin in rat retina and the role of proteasomal proteolysis

Several investigations have shown that illumination at night reduces melatonin level in the mammalian pineal, but the effect of night illumination on the retina is not known. In this study retinas were cultured in a flow-through apparatus and then were exposed to light at ZT 18. Light exposure reduced melatonin levels to the daytime level within 30 min. The reduction of melatonin levels was due to a rapid decrease in the activity of the enzyme AA-NAT; AA-NAT mRNA levels were not affected by illumination. Pre-incubation with lactacystin (25 microM) prevented light-induced reduction of AA-NAT activity and melatonin levels. These results demonstrate that melatonin levels in the mammalian retina are affected by light exposure at night, via proteosomal proteolysis of AA-NAT.

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.

Melatonin acts on the nucleus accumbens to increase acetylcholine release and modify the motor activity pattern of rats

Brain microdialysis coupled to high performance liquid chromatography with electrochemical detection (HPLC-ED) was used to evaluate the influence of melatonin on extracellular concentration of acetylcholine (ACh) in the nucleus accumbens (NAc) of rats. Motor activity was simultaneously monitored during the dialysis sessions with an activity meter. Melatonin and prazosin were administered locally through the dialysis probe. It was found that melatonin dose-dependently increased accumbens ACh. Melatonin (3 microM) decreased horizontal activity and increased vertical activity, while another dose (100 microM) enhanced both horizontal and vertical activity. Prazosin, a putative melatonin antagonist, blocked the effects of melatonin on both motor activity and ACh release when given 20 min before melatonin. Overall, these results suggest that melatonin modulates the release of ACh in the NAc and the pattern of motor activity in the rat.

Expression of mt1 melatonin receptor in rat retina: evidence for multiple cell targets for melatonin

Melatonin is synthesized in the retina at night and acts as a local modulator within this tissue by mediating the effects of darkness. We investigated the expression and localization of the mt1 (Mel1a) melatonin receptor in rat retina in order to disclose the cellular and molecular bases of melatonin's action in the mammalian retina. Western blotting of the mt1 receptor in rat retina exhibited a single immunoreactive band of approximately 37,000 mol. wt, which corresponds to the predicted molecular size of the receptor. The mt1 receptor was immunocytochemically localized to both the inner and outer plexiform layers. During postnatal development, retina from two-week-old rats showed the highest mt1 immunoreactivity; the outer plexiform layer and horizontal cell bodies were strongly immunolabeled, with weaker labeling in the inner plexiform layer. Expression of mt1 receptor messenger RNA in the rat retina was demonstrated by reverse transcription-polymerase chain reaction and in situ hybridization. mt1 receptor transcripts were localized to ganglion cells, amacrine cells and horizontal cells. These results suggest that melatonin influences retinal physiology by acting on multiple retinal cell types, including ganglion, amacrine and horizontal cells, via the mt1 receptor expressed in their processes.

Melatonin biosynthesis in photoreceptor-enriched chick retinal cell cultures: role of cyclic AMP in the K(+)-evoked, Ca(2+)-dependent induction of serotonin N-acetyltransferase activity

The roles of cyclic AMP and calcium in the regulation of serotonin N-acetyltransferase (NAT) activity were studied in low density monolayer cultures of chick retinal photoreceptors and neurons. Photoreceptor-enriched retinal cell cultures were prepared from embryonic day 6 retinas and cultured for 6 days. NAT activity in these cultures could be induced by treatment with cyclic AMP protagonists, 8Br-cyclic AMP, forskolin, and 3-isobutyl-1-methylxanthine (IBMX), or by treatment with depolarizing concentrations of extracellular K+. The stimulatory effect of K+, which involves Ca2+ influx through dihydropyridine-sensitive channels, was mediated at least in part by cyclic AMP, as indicated by the following observations. Depolarizing concentrations of K+ stimulated the formation of cyclic AMP, and the stimulatory effects of K+ on both cyclic AMP formation and on NAT activity were synergistically potentiated by the cyclic nucleotide phosphodiesterase inhibitor 3-isobutyl-1-methylxanthine (IBMX). MDL 12,330A, a putative adenylate cyclase inhibitor, inhibited K(+)-evoked cyclic AMP accumulation and induction of NAT activity over the identical concentration range. In contrast, MDL 12,300A failed to inhibit the induction of NAT elicited by 8Br-cyclic AMP. H-89, an inhibitor of cyclic AMP-dependent protein kinase, antagonized the induction of NAT activity by either forskolin or K+ with equal potency for both stimuli. These results suggest that cyclic AMP plays an essential role in the induction of NAT activity that occurs as a consequence of membrane depolarization. Cyclic AMP and Ca2+ may also interact at a step distal to adenylate cyclase.(ABSTRACT TRUNCATED AT 250 WORDS)

Dopamine and serotonin turnover rate in the retina of rabbit, rat, goldfish, and Eugerres plumieri: light effects in goldfish and rat

The concentration of dopamine, and its metabolites 3,4-dihydroxyphenylacetic and homovanillic acids, as well as serotonin and its metabolite 5-hydroxyindoleacetic acid, were determined in the retina of two teleosts, C. auratus (goldfish) and E. plumieri (mojarra), and two mammals, R. norvegicus (rat) and O. cuniculus (rabbit). The turnover rate of these monoamines were investigated in the four species by the calculation of the ratio monoamine/metabolite as an indirect index, and in goldfish and rat by the inhibition of the synthesis with alpha-methyl-p-tyrosine or p-chlorophenylalanine, by the increase in dopamine or serotonin by the corresponding precursors, 3,4-dihydroxyphenylalanine or 5-hydroxytryptophan, and by inhibition of monoaminooxidase with pargyline. The modulation by light and dark stimulation was studied in the goldfish and the rat. Differences in the concentration and turnover rate were observed among the species. Serotonin concentration was higher in the teleosts. The administration of inhibitors of dopamine and serotonin synthesis differentially decreased the levels of the monoamines in the retina of goldfish and rat. The rate of formation of dopamine and serotonin by the corresponding precursors was much higher in the goldfish than in the rat. Pargyline administration decreased 3,4-dihydroxyphenylacetic and 5-hydroxyindoleacetic acids at different rates and time dependency in the retina of goldfish and rat. Dopamine and serotonin concentration did not exhibit high modifications by the inhibitor, suggesting the function of regulatory mechanisms or additional effect of pargyline at other sites different from monoaminooxidase.(ABSTRACT TRUNCATED AT 250 WORDS)

Regulation of endogenous dopamine release in amphibian retina by melatonin: the role of GABA

In the retina of the African clawed frog (Xenopus laevis), endogenous dopamine release increases in light and decreases in darkness. Exogenous melatonin and several chemical analogs of melatonin suppressed light-evoked dopamine release from frog retina in a concentration-dependent manner. The rank order of potency for inhibition of light-evoked dopamine release was melatonin >> 5-methoxytryptamine > or = N-acetylserotonin > 5-methoxytryptophol >>> serotonin. Melatonin did not suppress dopamine release below levels seen in darkness. The putative melatonin receptor antagonist luzindole inhibited the effect of melatonin. Luzindole enhanced dopamine release in darkness but had little effect in light. These data suggest a role for endogenous melatonin in dark-induced suppression of retinal dopamine. Picrotoxin and bicuculline, GABA-A receptor antagonists, blocked melatonin-induced suppression of dopamine release. In the presence of melatonin, bicuculline was significantly less potent in stimulating dopamine release. These results suggest that melatonin enhances GABAergic inhibition of light-evoked dopamine release. This mechanism may underlie the light/dark difference in dopamine release in vertebrate retina.

Statistical quantification of 24-hour and monthly variabilities of spontaneous otoacoustic emission frequency in humans

Previous evidence has suggested a relationship between spontaneous otoacoustic emissions (SOAEs) and established, biological cycles, although detailed statistical quantifications of the suggested relationships do not exist in the literature. In an attempt to statistically quantify the purported circadian and monthly influences on this phenomenon, two experiments were undertaken. The first experiment was conducted over eight weeks, investigating 31 SOAEs recorded from eight women and two men. Time series statistical analysis examined whether daily, weekly, and/or monthly cycles characterized SOAE frequency variability. Results yielded a significant monthly cycle for the majority of SOAEs recorded from the women but for none of the SOAEs recorded from the men. These results suggest the possibility that SOAE frequency fluctuation in women may be entrained to the monthly menstrual cycle. In the second experiment, hourly SOAE frequency stability was examined over a 24-h period to ascertain the nature of the daily frequency variation as precisely as possible. Four SOAEs from two subjects were examined, and time series analysis of these data included (1) modelling the autocorrelation structure of the measurements, (2) resolving each 24-h series of measurements into cyclical components of various periodicities, and (3) testing the statistical significance of given cycles within the spectrum of each series. Findings included a significant 24-h variability of frequency for each SOAE, suggesting the possibility of a circadian influence on frequency fluctuation. Results from the two experiments provide quantitative evidence supporting a hypothetical relationship between SOAEs and established, biological cycles.

Co-localization of serotonin and GABA in neurons of the Xenopus laevis retina

Serotonin-synthesizing neurons in the retina of Xenopus laevis have been identified using anti-phenylalanine hydroxylase (PH) antibody which recognizes tryptophan 5-hydroxylase, the rate-limiting enzyme for serotonin synthesis. Double-labelling experiments, using anti-PH antibody and anti-serotonin antibody/5,7-dihydroxytryptamine (5,7-DHT) uptake, have shown that some serotonin-like immunoreactive/5,7-DHT-labelled neurons exhibit PH-like immunoreactivity (PH-LI) (serotonin-synthesizing neurons), but the others do not (serotonin-accumulating neurons). In the present study, triple-labelling experiments were performed using 5,7-DHT uptake and antibodies raised against GABA and PH, to determine the possible co-localization of y-aminobutyric acid (GABA) in serotonin-synthesizing and/or -accumulating neurons in the Xenopus retina. All 5,7-DHT-labelled bipolar cells lacked PH-LI; all of them were immunoreactive to GABA. In contrast, all 5,7-DHT-labelled large amacrine cells exhibited PH-LI, but none of them expressed GABA-LI. Small amacrine cells labelled with 5,7-DHT but not PH-LI exhibited GABA-LI, whilst the small amacrine cells with PH-LI lacked GABA-LI. These observations indicate that GABA is co-localized in serotonin-accumulating amacrine and bipolar cells, whereas serotonin-synthesizing large and small amacrine cells do not contain GABA-LI.

Whole-cell currents activated at nicotinic acetylcholine receptors on ganglion cells isolated from goldfish retina

We have recorded whole-cell membrane currents in response to exogenously applied acetylcholine (ACh), nicotine, and 1,1 dimethyl-4-phenyl piperazinium iodide on retinal ganglion cells enzymatically dissociated from goldfish retina. Agonist applications induced nicotinic-type responses in a majority of cells when cells were isolated under optimal conditions. Currents were reminiscent of nicotinic-type ganglionic responses. Dose-response measurements of ACh-induced currents indicated an EC50 of 52 microM and a Hill coefficient of 0.6. Currents were selective for Na+ over Cl- and were highly inwardly rectifying. Responses were blocked reversibly by d-tubocurarine, hexamethonium chloride, and N-methyl-D-glucamine. In 50% of the cases, alpha-bungarotoxin reversibly blocked the current induced by ACh application. The blocking action of mecamylamine was irreversible and independent of the presence of agonist but was more effective in the presence of ACh. We conclude that functional nicotinic ACh receptors exist on most goldfish retinal ganglion cells.