Dopamine and melatonin interactions in the intact chicken eye. Electrooculographic and biochemical study

A systematic, updated review on the antidepressant agomelatine focusing on its melatonergic modulation

Objective:

To present an updated, comprehensive review on clinical and pre-clinical studies on agomelatine.

Method:

A MEDLINE, Psycinfo and Web of Science search (1966-May 2009) was performed using the following keywords: agomelatine, melatonin, S20098, efficacy, safety, adverse effect, pharmacokinetic, pharmacodynamic, major depressive disorder, bipolar disorder, Seasonal Affective Disorder (SAD), Alzheimer, ADHD, Generalized Anxiety Disorder (GAD), Panic Disorder (PD), Obsessive-Compulsive Disorder (OCD), anxiety disorders and mood disorder.

Study collection and data extraction:

All articles in English identified by the data sources were evaluated. Randomized, controlled clinical trials involving humans were prioritized in the review. The physiological bases of melatonergic transmission were also examined to deepen the clinical comprehension of agomelatine’ melatonergic modulation.

Data synthesis:

Agomelatine, a melatonergic analogue drug acting as MT₁/MT₂ agonist and 5-HT_2C antagonist, has been reported to be an effective antidepressant therapy.

Conclusions:

Although a bias in properly assessing the “sleep core” of depression may still exist with current screening instruments, therefore making difficult to compare agomelatine’ efficacy to other antidepressant ones, comparative studies showed agomelatine to be an intriguing option for depression and, potentially, for other therapeutic targets as well.

Daily Oscillation in Melatonin Synthesis in The Turkey Pineal Gland and Retina: Diurnal and Circadian Rhythms

The aim of the present study was to examine arylalkylamine N-acetyltransferase (AANAT) activity and melatonin content in the pineal gland and retina as well as the melatonin concentration in plasma of the turkey (Meleagris gallopavo), an avian species in which several physiological processes, including reproduction, are controlled by day length. In order to investigate whether the analyzed parameters display diurnal or circadian rhythmicity, we measured these variables in tissues isolated at regular time intervals from birds kept either under a regular light-dark (LD) cycle or under constant darkness (DD). The pineal gland and retina of the turkey rhythmically produced melatonin. In birds kept under a daily LD cycle, melatonin levels in the pineal gland and retina were high during the dark phase and low during the light phase. Rhythmic oscillations in melatonin, with high night-time concentrations, were also found in the plasma. The pineal and retinal melatonin rhythms mirrored oscillations in the activity of AANAT, the penultimate enzyme in the melatonin biosynthetic pathway. Rhythmic oscillations in AANAT activity in the turkey pineal gland and retina were circadian in nature, as they persisted under conditions of constant darkness (DD). Transferring birds from LD into DD, however, resulted in a potent decline in the amplitude of the AANAT rhythm from the first day of DD. On the sixth day of DD, pineal AANAT activity was still markedly higher during the subjective dark than during the subjective light phase; whereas, AANAT activity in the retina did not exhibit significant oscillations. The results indicate that melatonin rhythmicity in the turkey pineal gland and retina is regulated both by light and the endogenous circadian clock. The findings suggest that environmental light may be of primary importance in the maintenance of the high-amplitude melatonin rhythms in the turkey.

Dopamine in the Turkey retina-an impact of environmental light, circadian clock, and melatonin

Substantial evidence suggests that dopamine and melatonin are mutually inhibitory factors that act in the retina as chemical analogs of day and night. Here, we show an impact of environmental light, biological clock, and melatonin on retinal levels of dopamine and its major metabolite 3,4-dihydroxyphenylacetic acid (DOPAC) in the turkey. In turkeys held under different light (L) to dark (D) cycles (16L:8D, 12L:12D, 8L:16D), retinal levels of dopamine and DOPAC fluctuated with daily rhythms. High levels of dopamine and DOPAC were observed during light hours and low during dark hours. Under the three photoperiodic regimes, rhythms of dopamine and DOPAC were out of phase with daily oscillation in retinal melatonin content. In constant darkness, dopamine and DOPAC levels oscillated in circadian rhythms. Light deprivation resulted, however, in a significant decline in amplitudes of both rhythms. Injections of melatonin (0.1-1 mumol/eye) during daytime significantly reduced retinal levels of DOPAC. This suppressive effect of melatonin was more pronounced in the dark-adapted than light-exposed turkeys. Quinpirole (a D(2)/D(4)-dopamine receptor agonist; 0.1-10 nmol/eye) injected to dark-adapted turkeys significantly decreased retinal melatonin. Our results indicate that in the turkey retina: (1) environmental light is the major factor regulating dopamine synthesis and metabolism; (2) dopaminergic neurones are controlled, in part, by intrinsic circadian clock; and (3) dopamine and melatonin are components of the mutually inhibitory loop.

Photoperiod-Dependent Changes in Melatonin Synthesis in the Turkey Pineal Gland and Retina

The effect of photoperiod on melatonin content and the activity of the melatonin-synthesizing enzymes, namely, serotonin N-acetyltransferase (AANAT) and hydroxyindole-O-methyltransferase, were investigated in the pineal gland and retina of turkeys. The birds were adapted to 3 different lighting conditions: 16L:8D (long photoperiod), 12L:12D (regular photoperiod), and 8L:16D (short photoperiod). Pineal, retinal, and plasma melatonin concentrations oscillated with a robust diurnal rhythm, with high values during darkness. The duration of elevated nocturnal melatonin levels in the turkey pineal gland, retina, and plasma changed markedly in response to the length of the dark phase, being longest during the short photoperiod with 16 h of darkness. These photoperiodic variations in melatonin synthesis appear to be driven by AANAT, because changes in the activity of this enzyme were closely correlated with changes in melatonin. By contrast, pineal and retinal hydroxyindole-O-methyltransferase activities failed to exhibit any significant 24-h variation in the different photoperiods. A marked effect of photoperiod on the level of melatonin production was also observed. Peak values of melatonin and AANAT activity in the pineal gland (but not in the retina) were highest during the long photoperiod. During the light phase, mean melatonin concentrations in the pineal gland and retina of turkeys kept under the long photoperiod were significantly higher compared with those from birds maintained under the regular and short photoperiods. In addition, mean circulating melatonin levels were lowest in the short photoperiod. Finally, the magnitude of the light-evoked suppression of nighttime pineal AANAT activity was also influenced by photoperiod, with suppression being smallest under the long photoperiod. These findings show that in the turkey, photoperiod plays an important role in regulating the melatonin signal.

Diurnal and circadian rhythms in melatonin synthesis in the turkey pineal gland and retina

The pineal gland and retina of the turkey rhythmically produce melatonin. In birds kept under a daily light-dark (LD) illumination cycle melatonin concentrations in the pineal gland and retina were low during the light phase and high during the dark phase. A similar melatonin rhythm with high night-time values was also observed in the plasma. The pineal and retinal melatonin rhythms mirror oscillations in the activity of serotonin N-acetyltransferase (AANAT; the penultimate enzyme in the melatonin biosynthetic pathway). In contrast, in both the pineal gland and retina the activity of the enzyme hydroxyindole-O-methyltransferase (HIOMT) did not exhibit significant changes throughout the 24-h period. Acute exposure of turkeys to light at night dramatically decreased melatonin levels in the pineal gland, retina and plasma. The rhythms in AANAT activity and melatonin concentrations in the turkey pineal gland and retina were circadian in nature as they persisted under conditions of constant darkness (DD). Under DD, however, the amplitudes of AANAT and melatonin rhythms were significantly lower (by 50-80%) than those found under the LD cycle. The findings indicate that melatonin rhythmicity in the turkey pineal gland and retina is regulated both by light and the endogenous circadian clock. The rapid dampening of the rhythms under DD suggests that of these two regulatory factors, environmental light may be the primary stimulus in the maintenance of the high amplitude melatonin rhythms in the turkey.

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.

The relationship between melatonin and dopamine rhythms in the duck retina

In the retina of duck, levels of dopamine (DA) and its main metabolite, 3,4-dihydroxyphenylacetic acid (DOPAC), fluctuate throughout the day, with high values during the light phase. The rhythmic changes in DA content and metabolism are out of phase with the daily oscillations in melatonin (MEL) and serotonin N-acetyltransferase (AA-NAT; the penultimate and key regulatory enzyme in MEL biosynthesis) activity. Acute exposure of ducks to light at night potently increased levels of DA and DOPAC, and decreased AA-NAT activity and MEL content in the retina. Intraocular administration of MEL to light-adapted ducks produced a significant decline in retinal DA and DOPAC concentrations. On the other hand, quinpirole, a D(2)/D(4)-DA receptor agonist, administered intraocularly, markedly suppressed the night-time retinal AA-NAT activity and MEL. These findings provide, for the first time, evidence for an inverse relationship between the DA system and MEL in the duck retina.

Daily variation in the concentration of 5-methoxytryptophol and melatonin in the duck pineal gland and plasma

The duck pineal gland rhythmically produces two 5-methoxyindole compounds, i.e. 5-methoxytryptophol and melatonin. 5-Methoxytryptophol levels are low at night and high during the day, while melatonin concentrations are high at night and low during the day. The melatonin rhythm reflects oscillations in the activity of serotonin N-acetyltransferase (AA-NAT; a penultimate and key regulatory enzyme in the melatonin biosynthetic pathway). The activity of hydroxyindole-O-methyltransferase (HIOMT; an enzyme involved in the synthesis of both 5-methoxytryptophol and melatonin) does not exhibit any significant rhythmic changes throughout the 24-hr period. Plasma levels of melatonin exhibited daily changes that were parallel to fluctuations in pineal melatonin content. Although plasma concentrations of 5-methoxytryptophol were low in ducks, they showed daily variations. The mean 5-methoxytryptophol concentration between zeitgeber time 9 (ZT9) and ZT15 was 2.4-times higher than the mean value for samples collected between ZT18 and ZT3. These findings indicate that in the duck the pineal production of 5-methoxytryptophol and melatonin may be inversely correlated.

Effects of cycloheximide and aminophylline on 5-methoxytryptophol and melatonin contents in the chick pineal gland

The chick pineal gland rhythmically synthesizes two 5-methoxyindoles, melatonin and 5-methoxytryptophol. These rhythms are circadian in nature and have opposite phases. The aim of this study was to determine the effects of cycloheximide, a protein synthesis inhibitor, and aminophylline, an inhibitor of phosphodiesterase, on 5-methoxytryptophol content in the chick pineal gland and to compare this with the drugs' action on pineal melatonin production. Inhibition of melatonin biosynthesis by cycloheximide (1 mg/kg, i.p. ), revealed by a marked reduction in the nighttime activity of serotonin N-acetyltransferase (AA-NAT; a key regulatory enzyme in melatonin synthesis) and melatonin concentrations, was accompanied by a significant increase in 5-methoxytryptophol content. In contrast, administration of aminophylline (100 mg/kg, i.p.) to light-exposed chicks significantly increased pineal AA-NAT activity and melatonin levels and decreased 5-methoxytryptophol concentrations. It is concluded that in the chick the production of pineal 5-methoxytryptophol and melatonin is inversely correlated.

Two secreted retinal factors regulate different stages of development of the outer blood-retinal barrier

The retinal pigment epithelium (RPE) lies at the interface between the neural retina and the choriocapillaries where it forms a blood-retinal barrier. Like endothelial regions of the blood-brain barrier, the development of the RPE barrier is a gradual, multistep process. A culture model of chick RPE was used to study this development. The permeability of the tight junctions that limit diffusion between neighboring RPE cells was measured as the transepithelial electrical resistance (TER). Embryonic day 14 (E14) retinas were used to make a conditioned medium that lowered the permeability of cultured RPE. The TER of cultures prepared from E14 RPE was twice that of E7 RPE. In each culture, retinal conditioned medium increases the TER 2-2.5 fold. The active factors of conditioned medium that affected each culture had different physical properties. The factor that affected E7 was protease-resistant with a Mr<10 kDa, but the factor that affected E14 appeared to be a protein of approximately 49 kDa. Unlike the effect of astrocyte conditioned medium on endothelia, retinal conditioned medium did not act synergistically with cAMP. These data indicate that the chick retina, which lacks astrocytes, uses different diffusible factors to regulate different stages of tight junction development.

Pharmacological modifications in dopaminergic neurotransmission affect the quinpirole-evoked suppression of serotonin N-acetyltransferase activity in chick retina: an impact on dopamine D4-like receptors

Dopamine (DA) plays an important role in the regulation of melatonin biosynthesis in retinas of several vertebrate species. In the retina of chick, the DA receptor controlling melatonin production represents a D4-like subtype. Stimulation of this receptor by quinpirole (QNP) results in a dose-dependent decline of the nighttime activity of serotonin N-acetyltransferase (NAT; a key regulatory enzyme in melatonin biosynthesis) and melatonin level of chick retina. The present study was undertaken to determine whether long-term treatment with antipsychotic drugs (clozapine-30 mg/kg, i.m.; sulpiride-100 mg/kg, i.m.; and raclopride-10 mg/kg, i.p., once daily for 21 days) and L-DOPA (80 mg/kg, i.p., once daily for 7 days) affects the response of the melatonin generating system of chick retina to the suppressive effect of QNP. Chronic administration to chicks of clozapine and sulpiride, but not raclopride, resulted in a markedly increased response of retinal NAT activity to the action of QNP. ED50 values for QNP were 3-times (clozapine) and 4-times (sulpiride) lower than those in the respective vehicle-treated control groups. On the other hand, QNP was significantly less potent in retinas of birds treated with L-DOPA than in control animals; the ED50 value for QNP was 3-times higher in birds injected with L-DOPA than in the vehicle-treated group. These results indicate that long-term treatment with clozapine, sulpiride and L-DOPA may modify the reactivity of D4-like DA receptors regulating NAT activity of chick retina. A possibility of modifications of circadian and electrophysiological processes within the eye following prolonged administration of DA-ergic drugs is discussed.

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)

Studies on the role of the retinal dopamine/melatonin system in experimental refractive errors in chickens

We have found that development of both deprivation-induced and lens-induced refractive errors in chickens implicates changes of the diurnal growth rhythms in the eye (Fig. 1). Because the major diurnal oscillator in the eye is expressed by the retinal dopamine/melatonin system, effects of drugs were studied that change retinal dopamine and/or serotonin levels. Vehicle-injected and drug-injected eyes treated with either translucent occluders or lenses were compared to focus on visual growth mechanisms. Retinal biogenic amine levels were measured at the end of each experiment by HPLC with electrochemical detection. For reserpine (which was most extensively studied) electroretinograms were recorded to test retinal function [Fig. 3 (C)] and catecholaminergic and serotonergic retinal neurons were observed by immunohistochemical labelling [Fig. 3(D)]. Deprivation myopia was readily altered by a single intravitreal injection of drugs that affected retinal dopamine or serotonin levels; reserpine which depleted both serotonin and dopamine stores blocked deprivation myopia very efficiently [Fig. 3(A)], whereas 5,7-dihydroxy-tryptamine (5,7-DHT), sulpiride, melatonin and Sch23390 could enhance deprivation myopia (Table 1, Fig. 5). In contrast to other procedures that were previously employed to block deprivation myopia (6-OHDA injections or continuous light) and which had no significant effect on lens-induced refractive errors, reserpine also affected lens-induced changes in eye growth. At lower doses, the effect was selective for negative lenses (Fig. 4). We found that the individual retinal dopamine levels were very variable among individuals but were correlated in both eyes of an animal; a similar variability was previously found with regard to deprivation myopia. To test a hypothesis raised by Li, Schaeffel, Kohler and Zrenner [(1992) Visual Neuroscience, 9, 483-492] that individual dopamine levels might determine the susceptibility to deprivation myopia, refractive errors were correlated with dopamine levels in occluded and untreated eyes of monocularly deprived chickens (Fig. 6). The hypothesis was rejected. Although it has been previously found that the static retinal tissue levels of dopamine are not altered by lens treatment, subtle changes in the ratio of DOPAC to dopamine were detected in the present study. The result indicates that retinal dopamine might be implicated also in lens-induced growth changes. Surprisingly, the changes were in the opposite direction for deprivation and negative lenses although both produce myopia. Currently, there is evidence that deprivation-induced and lens-induced refractive errors in chicks are produced by different mechanisms. However, findings (1), (3) and (5) suggest that there may also be common features. Although it has not yet been resolved how both mechanisms merge to produce the appropriate axial eye growth rates, we propose a scheme (Fig. 7).

Melatonin receptors: localization, molecular pharmacology and physiological significance

A pre-requisite to understanding the physiological mechanisms of action of melatonin is the identification of the target sites where the hormone acts. The radioligand 2-[125I]iodo-melatonin has been used extensively to localize binding sites in both the brain and peripheral tissues. In general these binding sites have been found to be high affinity, with Kd in the low picomolar range, and selective for structural analogues of melatonin. Also the affinity of these sites can generally be modulated by guanine nucleotides, consistent with the notion that they are putative G-protein coupled receptors. However, only a few studies have demonstrated that these putative receptors mediate biochemical and cellular responses. In the pars tuberalis (PT) and pars distalis (PD) of the pituitary, the amphibian melanophore and vertebrate retina, evidence indicates that melatonin acts to inhibit intracellular cyclic AMP through a G-protein coupled mechanism, demonstrating that this is a common signal transduction pathway for many melatonin receptors. However in the pars distalis the inhibition of calcium influx and membrane potential are also important mediators of melatonin effects. How many different forms or states of the melatonin receptor exist is unknown, but clearly the identification of the structure of the melatonin receptor(s) and its ability to interact with different G-proteins and signal transduction pathways are quintessential to our understanding of the physiological mechanisms of action of melatonin. In parallel the recent development of new melatonin analogues will greatly aid our understanding of the pharmacology of the melatonin receptor both in terms of the development of potent melatonin receptor antagonists and for the definition of receptor sub-types. The wide species and phylogenic diversity of melatonin binding sites in the brain has probably generated more questions than answers. Nevertheless the localization of melatonin receptors to the suprachiasmatic nucleus of the hypothalamus is at least consistent with circadian effects within the foetus and the adult. In contrast the PT of the pituitary presents an enigma in relation to the seasonal effects of melatonin. A model of how melatonin might mediate the timing of the circannual events through the PT is proposed.

Acute blockade of dopamine receptors with haloperidol: a retinal model to study impairments of dopaminergic transmission

A pulse of dopamine produces a transient dose-correlated increase in the transepithelial potential (TEP) of the chicken eye, mimicking the light-induced response, the light peak (LP). Acute blockade of retinal dopaminergic transmission with haloperidol, a mixed antagonist, produced a dose-correlated TEP voltage decrease which was rapidly reversed by intravitreal injection of dopamine. The LP recorded thereafter was strongly reduced. These data confirm the hypothesis that dopamine released by light from amacrine cells triggers light-induced changes in the TEP of the intact chicken eye, and that these potentials could well provide an electrophysiological tool to evaluate retinal dopaminergic deficiency.