The action of melatonin on single amphibian pigment cells in tissue culture

Evidence for the presence of novel β-melatonin receptors along with classical α-melatonin receptors in the fish Rasbora daniconius (Ham.)

The effects of melatonin (MT) were examined on the isolated scale melanophores from dorso-lateral (D-L) and band regions of a tropical fish Rasbora daniconius. Our study primarily aimed for further depiction of the signaling receptors involved in MT mediated pigment translocations in the fish. Melanophore Size Index (MSI) was employed as a recording parameter for the responses of melanophores to MT and various antagonists. MT has induced aggregation as well as dispersion in D-L region and aggregation in band region melanophores during summer season. During winter, MT-induced responses were only of aggregatory type in D-L region, while in the band region there was an increase in the sensitivity. The responses of the melanophores to MT were reversible. The aggregation of innervated melanophores induced by MT on the D-L and band regions was partially mediated through the neurotransmitters released under the influence of MT and partially by the specific MT receptors. Luzindole and K185 have completely blocked the aggregatory responses of D-L and band region melanophores. Aggregatory receptors may be of the conventional α-MT type. Dispersion of D-L and band region melanophores induced by MT in the presence of various antagonists and on denervated band region could be the result of activation of β-MT receptors of dispersive nature. Presence of α and β MT receptors is thus indicated in this fish melanophores.

Prostanoids-induced dispersion in the melanophores of a carp Labeo rohita (Ham.)

Effects of three prostaglandins (i.e., prostanoids) and one of its precursors, arachidonic acid, were examined on the melanophores of the fish Labeo rohita (Ham.). PGE₁, PGE₂, PGF(2α) and arachidonic acid elicit a concentration-related dispersion in the fish melanophores. In vitro analysis of melanophores was performed through incubation of the isolated fish scales in different agonists and antagonists solutions. Dispersal effect of prostanoids may be mediated directly through the typical receptors or indirectly through release of neurotransmitter substance(s) from the melanophore nerve endings. Denervation of fish melanophores rendered them insensitive to prostanoid (PGF(2α)). Propranolol and verapamil completely inhibited the dispersal effects of PGF(2α); theophylline and indomethacine blocked the effects of higher concentrations of PGF(2α). During dispersing influence of PGF(2α), a free flux of Ca²⁺ ions was required and the indirectly released substance(s) from melanophore nerve endings would be the catecholamines of adrenergic and purinergic in nature.

Melatonergic regulation of hemolymph sugar levels in the freshwater edible crab, Oziotelphusa senex senex

In this study, the hyperglycemic effect of melatonin in the freshwater edible crab, Oziotelphusa senex senex, is investigated. Injection of melatonin induced hyperglycemia in a dose-dependent manner. Administration of melatonin produced hyperglycemia in both intact and eyestalk-ablated crabs. Bilateral eyestalk ablation resulted in significant increase in the total carbohydrates and glycogen levels with a significant decrease in phosphorylase activity in the hepatopancreas and muscle of the crabs. Injection of melatonin resulted in significant decrease in the total carbohydrate and glycogen levels, with an increase in phosphorylase activity in hepatopancreas and muscle of both intact and eyestalk-ablated crabs. From the results, it is hypothesized that melatonin-induced hyperglycemia in the crab, O. senex senex, is not mediated by eyestalk hyperglycemic hormone.

Products of tryptophan catabolism induce Ca2+ release and modulate the cell cycle of Plasmodium falciparum malaria parasites

Intraerythrocytic malaria parasites develop in a highly synchronous manner. We have previously shown that the host hormone melatonin regulates the circadian rhythm of the rodent malaria parasite, Plasmodium chabaudi, through a Ca2+-based mechanism. Here we show that melatonin and other molecules derived from tryptophan, i.e. N-acetylserotonin, serotonin and tryptamine, also modulate the cell cycle of human malaria parasite P. falciparum by inducing an increase in cytosolic free Ca2+. This occurs independently of the extracellular Ca2+ concentration, indicating that these molecules induce Ca2+ mobilization from intracellular stores in the trophozoite. This in turn leads to an increase in the proportion of schizonts. The effects of the indolamines in increasing cytosolic free Ca2+ and modulating the parasite cell cycle are both abrogated by an antagonist of the melatonin receptor, luzindole, and by the phospholipase inhibitor, U73122.

Cellular mechanisms of melatonin action

The pineal hormone melatonin is involved in photic regulations of various kinds, including adaptation to light intensity, daily changes of light and darkness, and seasonal changes of photoperiod lengths. The melatonin effects are mediated by the specific high-affinity receptors localized on plasma membrane and coupled to GTP-binding protein. Two different G proteins coupled to the melatonin receptors have been described, one sensitive to pertussis toxin and the other sensitive to cholera toxin. On the basis of the molecular structure, three subtypes of the melatonin receptors have been described: Mel1A, Mel1B, and Mel1C. The first two subtypes are found in mammals and may be distinguished pharmacologically using selective antagonists. Melatonin receptor regulates several second messengers: cAMP, cGMP, diacylglycerol, inositol trisphosphate, arachidonic acid, and intracellular Ca2+ concentration ([Ca2+]i). In many cases, its effect is inhibitory and requires previous activation of the cell by a stimulatory agent. Melatonin inhibits cAMP accumulation in most of the cells examined, but the indole effects on other messengers have been often observed only in one type of the cells or tissue, until now. Melatonin also regulates the transcription factors, namely, phosphorylation of cAMP-responsive element binding protein and expression of c-Fos. Molecular mechanisms of the melatonin effects are not clear but may involve at least two parallel transduction pathways, one inhibiting adenylyl cyclase and the other regulating phospholipide metabolism and [Ca2+]i.

Pigment cell signalling for physiological color change

The cellular signalling pathways participating in physiological color change are reviewed, particularly in crustaceans, teleosts, amphibians, and reptiles. This review is an attempt to summarize what is known and to raise some hypotheses about basic questions still to be elucidated. The first picture that emerges from the literature is that the transduction pathways are identical in the various types of chromatophores of a single species, except for the iridophore. The cAMP-dependent pathway has been well conserved throughout evolution: cAMP increase is the pigment dispersion signal whereas the nucleotide decrease leads to granule aggregation. On the other hand, the Ca(-2)-dependent pathways evoke pigment aggregation in teleosts and crustaceans, and dispersion in amphibians and probably reptiles as well. Another interesting point is the ultimate convergence of the signalling pathways of different agonists inducing the same response in one chromatophore type. A hypothesis is raised about why different chromatophores behave differently in the absence of agonists, that is, why some are punctate, whereas others are stellate.

Novel isoforms of Mel1c melatonin receptors modulating intracellular cyclic guanosine 3',5'-monophosphate levels

Two cDNAs encoding novel isoforms of Xenopus laevis melatonin receptors were cloned using PCR primers specific for the X. laevis-melanophore Mel1c melatonin receptor described in a recent publication. The novel isoforms were highly homologous to the described frog Mel1c cDNA, although the C-terminal tail of both was shorter by 65 amino acid residues. Nucleotide sequences of these novel isoforms, called Mel1c(alpha) and Mel1c(beta), differed from each other by only 35 nucleotides and six amino acid residues. Studies on several animals of various Xenopus species indicate that Mel1c(alpha) and Mel1c(beta) receptors may correspond to allelic variants of the same locus. Studies on cells transfected with both receptor cDNAs showed the expression of high-affinity 2-[125I]iodomelatonin binding sites. Agonist stimulation of Mel1c(alpha) receptor was associated with the inhibition of cAMP accumulation stimulated by forskolin (IC50 approximately 10(-10) M) in HeLa, Ltk-, and human embryonic kidney 293 (HEK 293) cells. Mel1c(beta) receptor modulated cAMP in HeLa and HEK 293 cells but not in Ltk- cells. Both receptors inhibited, in a dose-dependent manner, cGMP accumulation in all three cell lines incubated with a phosphodiesterase inhibitor. This effect was localized upstream of soluble guanylyl cyclase and was blocked by pertussis toxin treatment. However, IC50 values (approximately 10(-10) M for Mel1c(beta) and 10(-9) to 10(-7) M for Mel1c(alpha)) and maximal inhibition levels showed that Mel1c(alpha) receptors are much less efficiently coupled to the cGMP pathway. Coupling differences may be explained by the fact that five of the six amino acid substitutions between Mel1c(alpha) and Mel1c(beta) receptors are located within cytoplasmic regions potentially involved in signal transduction. The existence of coupling differences is in agreement with the observation that expression of both receptors is evolutionally conserved in native tissue. In conclusion, two novel, potentially allelic, isoforms of Xenopus Mel1c melatonin receptors display identical ligand-binding characteristics, but different potencies in modulating cAMP and cGMP levels through G(i)/G(o)-dependent pathways. Furthermore, to our knowledge, this study provides the first data on the modulation of intracellular cGMP levels by cloned melatonin receptors.

Calcium ion dependency and the role of inositol phosphates in melatonin-induced encystment of dinoflagellates

The unicellular eukaryotic dinoflagellates shed their flagella and form a new pellicle cyst wall in response to environmental stress. This encystment process can also be induced by indoleamines such as melatonin and 5-methoxytryptamine. To decipher the complex signaling events which lead to encystment, we have investigated the functional roles of Ca2+ and inositol phosphates in indoleamine-induced encystment of the dinoflagellates Alexandrium catenella and Crypthecodinium cohnii. Pretreatment with EGTA, but not with EDTA, effectively blocked the indoleamine-induced encystment of A. catenella in a dose-dependent manner. Conversely, agents that facilitate the influx of Ca2+ (Bay K 8644, A23187 and ionomycin) dose-dependently induced encystment of A. catenella. Endoplasmic Ca2+-ATPase inhibitors such as thapsigargin and the peptide toxin melittin also induced encystment of A. catenella. These results suggest that an elevation of intracellular [Ca2+] may be involved in the encystment response. In terms of the regulation of phospholipase C, melatonin dose- and time-dependently stimulated the formation of inositol phosphates in C. cohnii. The rank order of potency for several indoleamines to stimulate inositol phosphates formation was 2-iodomelatonin > 5-methoxytryptamine > or = melatonin >> N-acetylserotonin > 5-hydroxytryptamine. This rank order was the same as for the indoleamine-induced encystment of C. cohnii as previously reported. Our results indicate that indoleamine-induced activation of phospholipase C and elevation of intracellular [Ca2+] may be proximal steps in the signal transduction pathway leading to encystment in dinoflagellates. Moreover, this is the first demonstration of the possible involvement of Ca2+ and inositol phosphates as second messengers in dinoflagellates.

Melatonin agonists induce phosphoinositide hydrolysis in Xenopus laevis melanophores

Melatonin, the principal hormone of the vertebrate pineal gland, has been implicated in a variety of neurobiological processes such as circadian rhythmicity and reproductive function. One of the earliest described actions of melatonin was its ability to cause pigment translocation in the dermal melanophores of amphibians. Melatonin binding sites have been identified in the brain of many species and in pigmented tumour cell lines; however, the dermal melanophores of the frog Xenopus Laevis possess the highest known density of melatonin binding sites. These cells are the source from which a melatonin receptor has been cloned and provide an excellent model to study melatonin-mediated signal transduction in an isolated cell system. In Xenopus melanophores, melatonin induces a rapid perinuclear aggregation of intracellular pigment which is associated with a pertussis toxin-sensitive inhibition of cAMP. We have previously demonstrated that a subtype of melatonin binding sites found in selected regions of the pigeon brain and in Syrian Hamster RPMI 1846 melatonin cells are functionally coupled to phosphoinositide hydrolysis as a second messenger. Here we now present evidence to suggest that Xenopus Laevis melanophores also possess melatonin binding sites which are functionally linked to phosphoinositide hydrolysis. Melatonin agonists induced phosphoinositide hydrolysis in melanophores in a concentration-dependent manner with a rank order of potency of 2-iodomelatonin > 6-chloromelatonin > N-acetylserotonin > melatonin. Stimulatory response of 2-iodomelatonin was blocked by the melatonin antagonist N-acetyltryptamine and the alpha-adrenergic antagonist prazosin, which has been shown to have high affinity for melatonin binding sites. Phosphoinositide hydrolysis induced by melatonin agonists was not blocked by the serotonin antagonist ketanserin or by phentolamine, an alpha-adrenergic antagonist, indicating that the response observed was not due to stimulation of 5-HT2a/2c receptors or alpha-adrenergic receptors. Furthermore, incubation of melanophores with the non-hydrolyzable G-protein source GTP-gamma-S attenuated the phosphoinositide dose response induced by 2-iodomelatonin, and pre-incubation of the cells with pertussis toxin had no effect on 2-iodomelatonin-induced phosphoinositide hydrolysis. The present data suggest that Xenopus Laevis Melanophores possess G-protein linked pertussis toxin-insensitive melatonin binding sites which are functionally coupled to phosphoinositide hydrolysis as a signal transduction mechanism.

Analogues of diverse structure are unable to differentiate native melatonin receptors in the chicken retina, sheep pars tuberalis and Xenopus melanophores

1. The pineal hormone melatonin exerts its biological effects through specific, high affinity G-protein coupled receptors. Recently, three melatonin receptor subtypes (Mel1a, Mel1b and Mel1c) have been cloned. Neither the cloned subtypes, nor the native receptors have yet been compared in a detailed pharmacological analysis. 2. The present study examined the structure-activity relationships of a series of 21 melatonin analogues, by comparing their potency on the pigment aggregation response in Xenopus laevis melanophores with their affinity in radioligand binding competition studies in chicken retina and sheep pars tuberalis (PT), two tissues in which melatonin is known to mediate a biological response. 3. All but four of the analogues were full melatonin receptor agonists producing a concentration-related redistribution of pigment granules in cultured Xenopus melanophores. The remaining analogues produced little pigment aggregation at 10 microM. 4. Saturation studies with 2-[125I]-iodomelatonin identified a single binding site in the chicken retina and sheep PT membranes, with a KD of 36.6 +/- 2.8 and 37.3 +/- 4.3 pM, and a maximal number of binding sites (Bmax) of 16.6 +/- 0.5, and 40.1 +/- 1.7 fmol mg-1 protein, respectively. 5. Comparison of the potency/affinity of the analogues for the binding sites gave a highly significant correlation in each case, retina/melanophore, r = 0.97 (P < 0.001, n = 17), PT/melanophore, r = 0.97 (P < 0.001, n = 17) and PT/retina, r = 0.98 (P < 0.001, n = 21). 6. Despite their large range in affinity and structural diversity these melatonin agonists were unable to distinguish between melatonin receptors in the chicken retina, sheep pars tuberalis and Xenopus melanophores.

A melatonin binding site modulates the alpha 2-adrenoceptor

The melanophores of the cuckoo wrasse (Labrus ossifagus L., a teleost fish) can aggregate and disperse their pigment granules. This migration is controlled by sympathetic nerves and a postsynaptic alpha 2-adrenoceptor. Melatonin was discovered because of its ability to aggregate pigment granules, hence we used the cuckoo wrasse melanophore as a model for studying the effect of melatonin at a cellular level. We found that melatonin had no aggregating effect; instead the hormone enhanced the actions of several alpha 2-agonists, such as noradrenaline, medetomedine and clonidine. When the melanophores were pre-aggregated in vitro by use of the alpha 2-agonist B-HT 920, the aggregation was not augmented after the addition of melatonin. Instead the pre-aggregated granules were dispersed. This suggests that melatonin has affinity for an alpha 2-adrenoceptor site that can modulate the effect of alpha 2-adrenoceptor agonists.

Radioligand binding affinity and biological activity of the enantiomers of a chiral melatonin analogue

Melatonin, a hormone secreted by the pineal gland, can act on the central circadian oscillator in the suprachiasmatic nucleus of the hypothalamus. It has been proposed that melatonin or its analogues may be useful in restoring disturbed circadian rhythms in jet-lag, shift-work and some blind subjects, and as sleep-promoting agents. In the present study, the (-)- and (+)-enantiomers of N-acetyl-4-aminomethyl-6-methoxy-9-methyl-1,2,3,4-tetrahydrocarbazole (AMMTC) were separated and tested. The affinity of the enantiomers at the specific 2-[125I]iodomelatonin binding site in chick brain membranes was compared in competition assays, and their biological activity in a specific melatonin receptor bioassay, aggregation of pigment granules in Xenopus laevis melanophores. The (-)-enantiomer of AMMTC was 130-fold and 230-fold more potent than the (+)-enantiomer in competition radioligand binding assays and melanophores, respectively. Both enantiomers are melatonin receptor agonists; (-)-AMMTC is slightly more potent than melatonin itself. As the tetrahydrocarbazole nucleus holds the C-3 amido side-chain of AMMTC in a restricted conformation, the analogues will be useful in modelling the melatonin receptor binding site.

Melatonin desensitizing effects on the in vitro responses to MCH, alpha-MSH, isoproterenol and melatonin in pigment cells of a fish (S. marmoratus), a toad (B. ictericus), a frog (R. pipiens), and a lizard (A. carolinensis), exposed to varying photoperiodic regimens

Melatonin is a weak dose-independent lightening agonist in fish skin, a moderate dose-dependent lightening agonist in toad skin and a potent lightening agent in frog and lizard skins (reversing in a dose-dependent manner the darkening caused by alpha-melanocyte-stimulating hormone). In frog skins, previous exposure to melatonin reduced further lightening actions of the indoleamine, and in toad skins, increasing concentrations of melatonin elicited decreasing lightening responses, suggesting an autodesensitizing action of the hormone. Various concentrations of melatonin diminished the responses to the lightening agonist melanin-concentrating hormone (MCH) in fish skins and to the darkening agonists alpha-MSH in toad, frog and lizard skins and isoproterenol in frog skins. In vitro inhibitory actions of melatonin are mimicked in the absence of the hormone in skin preparations from toads kept in continuous darkness for 48 hr. The lipophylic nature of the indoleamine associated with the results herein described suggests intracellular actions of melatonin on vertebrate pigment cells.

Effects of melatonin on microtubule assembly depend on hormone concentration: role of melatonin as a calmodulin antagonist

Melatonin may play a key role in cytoskeletal rearrangements through its calmodulin antagonism. In the present work, we tested this hypothesis by studying melatonin effects on both microtubule polymerization in vitro and cytoskeletons in situ. Microtubule assembly is a dynamic process inhibited by Ca2+/calmodulin. Calmodulin antagonists prevent the inhibition by binding to Ca(2+)-activated calmodulin, thus causing microtubule enlargement. In the presence of calmodulin (5 microM) and CaCl2 (1 mM), polymerization at equilibrium was inhibited by 40%. Complete reversal of the Ca2+/calmodulin effect on microtubules was observed with 10(-9) M melatonin or with 10(-5) M trifluoperazine or 1 microgram/ml of compound 48/80. In the absence of Ca2+/calmodulin, melatonin at 10(-5) M inhibited tubulin polymerization like 10(-4) M trifluoperazine does. Melatonin effects on microtubule assembly at both nanomolar and micromolar ranges were corroborated in cytoskeletons in situ. Therefore, it is suggested that at a low concentration (10(-9) M), cytoskeletal melatonin effects are mediated by its antagonism to Ca2+/calmodulin. At a higher concentration (10(-5) M), non-specific binding of melatonin to tubulin occurs, thus overcoming the melatonin antagonism to Ca2+/calmodulin. The results support the hypothesis that under physiological conditions, melatonin synchronizes different body rhythms through cytoskeletal rearrangements mediated by its calmodulin antagonism.

N-acyl-3-amino-5-methoxychromans: a new series of non-indolic melatonin analogues

A novel series of melatonin analogues is described which are based on the chroman nucleus. These N-acyl-3-amino-5-methoxychromans competitively inhibit [125I]2-iodomelatonin binding to chicken brain membranes although with reduced affinity compared to melatonin. The slope of the competition curves suggests the interaction of the chromans with a single binding site. On cultured Xenopus laevis melanophores, the chroman analogues produce different responses; N-chloroacetyl-3-amino-5-methoxychroman (ClaMCh), like melatonin, is a full agonist at the melanophore receptor and produces a complete aggregation of pigment granules. In contrast, N-acetyl- and N-cyclopropyl-3-amino-5-methoxychroman have no agonist activity, while N-propionyl- and N-butanoyl-3-amino-5-methoxychroman produce only partial aggregation of pigment. ClaMCh is 40-fold weaker at inducing pigment aggregation in melanophores (EC50 = 15 microM) than in inhibiting [125I]2-iodomelatonin binding in chicken brain membranes (Ki = 0.38 microM) suggesting that this analogue may discriminate between melanophore and chicken brain melatonin receptors. Chroman-based melatonin analogues may be useful tools for characterizing potential melatonin receptor subtypes.

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.

Calmodulin mediates melatonin cytoskeletal effects

In this article, we review the data concerning melatonin interactions with calmodulin. The kinetics of melatonin-calmodulin binding suggest that the hormone modulates cell activity through intracellular binding to the protein at physiological concentration ranges. Melatonin interaction with calmodulin may allow the hormone to modulate rhythmically many cellular functions. Melatonin's effect on tubulin polymerization, and cytoskeletal changes in MDCK and N1E-115 cells cultured with melatonin, suggest that at low concentrations (10(-9) M) cytoskeletal effects are mediated by its antagonism to Ca2+-calmodulin. At higher concentrations (10(-5)M) non-specific binding of melatonin to tubulin occurs thus overcoming the specific melatonin antagonism to Ca2+-calmodulin. Since the structures of melatonin and calmodulin are phylogenetically well preserved, calmodulin-melatonin interaction probably represents a major mechanism for regulation and synchronization of cell physiology.

Protein kinase C activation antagonizes melatonin-induced pigment aggregation in Xenopus laevis melanophores

The pineal hormone, melatonin (5-methoxy N-acetyltryptamine) induces a rapid aggregation of melanin-containing pigment granules in isolated melanophores of Xenopus laevis. Treatment of melanophores with activators of protein kinase C (PKC), including phorbol esters, mezerein and a synthetic diacylglycerol, did not affect pigment granule distribution but did prevent and reverse melatonin-induced pigment aggregation. This effect was blocked by an inhibitor of PKC, Ro 31-8220. The inhibitory effect was not a direct effect on melatonin receptors, per se, as the slow aggregation induced by a high concentration of an inhibitor of cyclic AMP-dependent protein kinase (PKA), adenosine 3',5'-cyclic monophosphothioate, Rp-diastereomer (Rp-cAMPS), was also reversed by PKC activation. Presumably activation of PKC, like PKA activation, stimulates the intracellular machinery involved in the centrifugal translocation of pigment granules along microtubules. alpha-Melanocyte stimulating hormone (alpha-MSH), like PKC activators, overcame melatonin-induced aggregation but this response was not blocked by the PKC inhibitor, Ro 31-8220. This data indicates that centrifugal translocation (dispersion) of pigment granules in Xenopus melanophores can be triggered by activation of either PKA, as occurs after alpha-MSH treatment, or PKC. The very slow aggregation in response to inhibition of PKA with high concentrations of Rp-cAMPS, suggests that the rapid aggregation in response to melatonin may involve multiple intracellular signals in addition to the documented Gi-mediated inhibition of adenylate cyclase.

The protein-phosphatase inhibitor okadaic acid mimics MSH-induced and melatonin-reversible melanosome dispersion in Xenopus laevis melanophores

The present study describes the ability of 315 nM okadaic acid to induce melanosome dispersion in cultured Xenopus laevis melanophores. This effect of okadaic acid is similar to that of a-melanocyte stimulating hormone (MSH) and can be reversed by melatonin treatment; it indicates that a member of the protein-phosphatase 1 or 2A families must be active for maintenance of the aggregated state. Higher concentrations of okadaic acid (1 microM) attenuate the response of Xenopus melanophores to melatonin leading to the hypothesis that melatonin action is mediated by the calcium/calmodulin activated phosphatase 2B. This hypothesis seems unlikely, however, since the calcium/calmodulin inhibitors TFP and W7 do not prevent melatonin-induced pigment aggregation, but instead induce aggregation on their own.

Serotonin synthesis and accumulation by neurons of the anuran retina

Serotonin-synthesizing and serotonin-accumulating neurons were studied in the retinas of Xenopus laevis and Bufo marinus. All previously identified cell types exhibiting serotonin-like immunoreactivity (SLI) were labeled by intravitreal injection of 5,7-dihydroxytryptamine (5,7-DHT). They included two amacrine cell types (large and small) in both species, and one bipolar cell type in Xenopus. Incubation of retinas in culture medium in the ambient light reduced SLI in amacrine cells and enhanced the labeling in bipolar cells. After incubation, some photoreceptor cell bodies and large numbers of outer segments also displayed SLI in both species. Incubation with the serotonin-uptake inhibitor, fluoxetine, reduced immunolabeling in bipolar cells and outer segments to the level in the untreated retinas. Both large SLI and 5,7-DHT-accumulating amacrine cells in Xenopus and Bufo were labeled with an antibody raised against phenylalanine hydroxylase (PH), which binds to tryptophan 5-hydroxylase, one of the synthesizing enzymes for serotonin. Small SLI and 5,7-DHT-accumulating amacrine cells in both species represented two populations, one with and the other without PH-like immunoreactivity (PH-LI). The anti-PH antibody failed to label any SLI or 5,7-DHT-accumulating bipolar cells in Xenopus. These observations indicate that all large and some small SLI amacrine cells in the retinas of Xenopus and Bufo synthesize serotonin, while other small SLI amacrine, bipolar and photoreceptor cell bodies, and outer segments only accumulate serotonin.

Effect of putative melatonin receptor antagonists on melatonin-induced pigment aggregation in isolated Xenopus laevis melanophores

The ability of putative melatonin receptor antagonists to antagonise melatonin-induced aggregation of pigment granules in cultured neural crest Xenopus laevis melanophores was examined. Neither ML 23 (N-(2,4-dinitrophenyl)-5-methoxytryptamine) nor 6-methoxy-2-benzoxazolinone showed agonist or antagonist activity. N-Acetyltryptamine and N-butanoyltryptamine were partial agonists; both compounds aggregated pigment granules in some cells, but also reversed melatonin-induced pigment agreggation in a fraction of the cells tested. In contrast, 2-benzyl N-acetyltryptamine (luzindole) did not show agonist activity (upto 10 microM) but did reverse the aggregating action of melatonin at 1 and 10 microM. Pretreatment of melanophores with luzindole shifted the melatonin concentration-response curve to the right.

Aggregation of pigment granules in single cultured Xenopus laevis melanophores by melatonin analogues

1. Isolated melanophores were differentiated from aggregates of neural crest obtained from neurula stage Xenopus laevis embryos after 2 days in culture. 2. Condensation of pigment granules in these cells by melatonin (5-methoxy N-acetyltryptamine, aMT) and various novel analogues was monitored with an image analysis system to quantitate the area occupied by pigment in individual cells. 3. Melanophores exposed to vehicle (a maximum of 0.1% MeOH) showed little (less than 5%) change in pigment area. aMT produced a dramatic condensation of pigment granules (EC50 = the concentration producing a half maximal condensation, 9 pM). The response was rapid, reached a maximum (approximately 80% decrease in pigmented area) by 10 min, and was reversible after removal of aMT from the culture medium. 4. Aggregation to aMT was blocked by treating melanophores with pertussis toxin (1 microgram ml-1, 7 h) indicating a role for a guanosine 5' triphosphate (GTP)-binding protein in transducing the aMT receptor signal. 5. Structure-activity studies indicated that analogues of aMT lacking a side-chain N-acyl substituent (5-methoxytryptamine, MT) or a group at the 5-position of the indole ring (N-acetyltryptamine, aT) were unable to induce pigment aggregation (EC50 greater than 10 microM). 6. Lengthening the side-chain N-acyl group (N-propionyl, N-butanoyl) was tolerated to some degree but eventually (N-valeroyl and larger) activity diminished. Of the 5-position analogues tested 5-methoxy (aMT) was by far the most potent. 7. Halogen substitution in the 6-position of the indole ring led to some loss of activity as did a 6-OH substitution. The 6-OCH3 compound was inactive.8. These studies demonstrate the utility of this model in investigations of structure-activity relationships at the aMT receptor and suggest that it may be a valuable system for determining the transduction mechanisms coupled to the aMT receptor.

Central melatonin receptors: implications for a mode of action

The influence of melatonin on circadian and photoperiodic functions in numerous species is well documented. It is known that the effect of melatonin on circadian rhythmicity is mediated via the suprachiasmatic nucleus (SCN), the biological clock of the brain. It is not known however where the photoperiodic effects of melatonin are mediated. Evidence from brain lesioning and melatonin implant studies point to a site in or near the medial hypothalamus. In contrast to these studies, melatonin receptors have been reported in widespread areas of the brain, the pituitary and in peripheral tissues. The characteristics of the reported melatonin receptors vary widely between studies and consequently no definitive description of a physiologically relevant melatonin receptor has received universal recognition. This review marshals recent evidence for the localization and characterization of the melatonin receptor and discusses these findings in the context of the known effects of the hormone in different species.

Antigonadal activity of the melatonin analogs 2-iodomelatonin and 2-chloromelatonin in the juvenile Djungarian hamster, Phodopus sungorus campbelli

The antigonadal effects of daily (20 micrograms, s.c.) injection of melatonin and two analogs, 2-iodomelatonin and 2-chloromelatonin, were compared in juvenile Djungarian hamsters housed under long photoperiod (L:D 16:8). Melatonin, 2-iodomelatonin, and 2-chloromelatonin injected 3 h before lights off for 16 days (17-34 days of age) significantly inhibited testis growth compared to vehicle-injected hamsters. In addition, melatonin and both analogs significantly reduced body weight gain. These 2-substituted analogs appear to be melatonin agonists with a potency in vivo similar to the parent compound, melatonin.

Melatonin: parallels in pineal gland and retina

It is apparent that several relationships exist between the pineal gland and retina. The similarities in development and morphology have been obvious for many years. A recent resurgence of interest in this field has led to a further understanding of many functional similarities between these two organs. A notable feature of the pineal gland and retina is their common ability to synthesize the indolamine hormone, melatonin. Many investigators suspect that the cyclic rhythm of retinal melatonin synthesis may be related to other cyclic events which normally occur in the retina.

Methoxyindole synthesis in the retina of the frog (Rana esculenta) during a diurnal period

In the present study, the synthesis of methoxyindoles in the neural part and in the pigment epithelial layer of the retina of the frog eye was investigated on the basis of naturally occurring substrate at regular intervals during a 24 hour period. Melatonin, 5-methoxytryptophol and 5-methoxyindole acetic acid were synthesized by the neural part of the retina only, while 5-methoxytryptamine and 5-methoxytryptophan were produced by both, the neural part of the retina and the pigment epithelium. The synthesis of melatonin and of 5-methoxytryptamine showed a diurnal rhythmicity. The results obtained clearly indicate that another cell type, i.e. pigment cells, is involved in indole metabolism besides photoreceptor elements. A possible functional relationship between different methoxyindoles and different retino-motor processes in the amphibian eye is discussed.

Natural and synthetic analogues of melatonin and related compounds. I. Effects on the reproductive system of the male Syrian hamster

The potential antigonadotrophic properties of a number of synthetic and natural melatonin analogues were examined. Adult male hamsters received daily subcutaneous injections (25 micrograms/animal/day) each afternoon for 7 or 10 weeks. The only analogue to possess antigonadotrophic activity similar to melatonin wa 6-chloromelatonin; melatonin and 6-chloromelatonin produced a significant reduction in both testicular and accessory sex organ weights, plasma LH and pituitary prolactin. These results suggest that, in the Syrian hamster, 6-chloromelatonin acts as a melatonin agonist and may be of use to elucidate further the physiological role of melatonin.

Tetanus toxin: a marker of amphibian neuronal differentiation in vitro

Tetanus toxin may be used to specifically mark neurones in cultures of mammalian and chick nervous tissue. We have prepared cultures of neural ectoderm and underlying mesoderm from late neurula Xenopus laevis embryos, and shown that in this Amphibian species, neurones are labelled by tetanus toxin. Aggregate cultures of ectoderm and mesoderm from late blastula embryos were than examined to determine whether tetanus toxin might be used to mark neuronal precursor cells. Our observations suggest that this is not possible; neurones are seen to have acquired tetanus toxin receptors only after they have become morphologically differentiated.

The function of the sodium pump during differentiation of amphibian embryonic neurones

1. A method has been developed for studying the differentiation in tissue culture of ectoderm and mesoderm derivatives, dissected from amphibian embryos which have just completed neurulation. 2. Neurones, striated muscle cells and pigment cells, together with other unidentifiable cell types, differentiated as a monolayer with approximately the same time course as in the whole embryo. The proportion of different cell types in the cultures was measured quantitatively by cell counting. 3. Treatment of embryos during neurulation with the cardiac glycoside strophanthidin reduced the number of neurones which subsequently differentiated in culture. Other cell types were not affected. 4. The relationship between inhibition of neural differentiation and strophanthidin concentration was sigmoid, with maximum inhibition at 10(-5) M-strophanthidin and the mid-point at 5 X 10(-7) M-strophanthidin. 35% of neurones differentiating in culture were not affected by glycoside treatment. 5. The glycoside hexahydroscillaren A had no effect on neural differentiation. 6. Increasing extracellular potassium to 100 nM during strophanthidin treatment completely protected differentiating neurones from the inhibitory effect of strophanthidin. 7. Treatment of embryos with 100 mM-KCl during neurulation had no effect on the subsequent differentiation of neurones. 8. Treatment of cultures with an antibody to mouse salivary gland Nerve Growth Factor reduced the number of neurones by 30%. 9. Exposure to strophanthidin while the embryo moved from the early neural fold stage to the late neural fold stage was as effective in reducing subsequent neural differentiation as treatment throughout neurulation. 10. The proportion of nerve cells in the cultures was not affected if strophanthidin treatment ended before the early neural fold stage or did not begin until the late neural fold stage. 11. Embryos treated with strophanthidin during neurulation and then allowed to grow into tadpoles developed abnormal nervous systems. 10(-6) M-strophanthidin had little effect on the volume of grey matter, but reduced the white matter by 50%. 12. The results are consistent with the view that strophanthidin achieves its effect on neural differentiation by inhibiting the sodium pump. They are discussed in the light of the suggestion that activation of the sodium pump is an essential part of nerual differentiation.