Melatonin receptor-mediated inhibition of cyclic AMP accumulation in chick retinal cell cultures

Dose-Dependent Effect of Hyperbaric Oxygen Treatment on Burn-Induced Neuropathic Pain in Rats

Hyperbaric oxygen treatment (HBOT) has been used to reduce neuropathic pain. Melatonin and opioid receptors are involved in neuropathic pain, but it is not known if HBOT works through these pathways to achieve its antinociceptive effect. We divided anesthetized rats into two treatment and three sham groups. The two treatment groups received third-degree burns on their right hind paws, one treated in a hyperbaric chamber for a week and the other for two weeks. We evaluated the mechanical paw-withdrawal threshold (MWT) and expression of melatonin receptor 1 (MT1), melatonin receptor 2 (MT2), μ (MOR) and κ (KOR) opioid receptor, brain-derived neurotrophic factor (BDNF), Substance P, and calcitonin gene-related peptide (CGRP) in cuneate nucleus, dorsal horn, and hind paw skin by immunohistochemical, immunofluorescence assays and real-time quantitative polymerase chain reaction (RT-PCR). The group receiving one-week HBOT had increased expressions of MT1, MT2, MOR and KOR and decreased expressions of BDNF, Substance P, and CGRP. Their mechanically measured pain levels returned to normal within a week and lasted three weeks. This anti-allodynia effect lasted twice as long in those treated for two weeks. Our findings suggest that increasing the duration of HBOT can reduce burn-induced mechanical allodynia for an extended period of time in rats. The upregulation of melatonin and opioid receptors observed after one week of HBOT suggests they may be partly involved in attenuation of the mechanical allodynia. Downregulation of BDNF, substance P and CGRP may have also contributed to the overall beneficial effect of HBOT.

Role of melatonin and its receptors in the vertebrate retina

Melatonin is a chemical signal of darkness that is produced by retinal photoreceptors and pinealocytes. In the retina, melatonin diffuses from the photoreceptors to bind to specific receptors on a variety of inner retinal neurons to modify their activity. Potential target cells for melatonin in the inner retina are amacrine cells, bipolar cells, horizontal cells, and ganglion cells. Melatonin inhibits the release of dopamine from amacrine cells and increases the light sensitivity of horizontal cells. Melatonin receptor subtypes show differential, cell-specific patterns of expression that are likely to underlie differential functional modulation of specific retinal pathways. Melatonin potentiates rod signals to ON-type bipolar cells, via activation of the melatonin MT2 (Mel1b) receptor, suggesting that melatonin modulates the function of specific retinal circuits based on the differential distribution of its receptors. The selective and differential expression of melatonin receptor subtypes in cone circuits suggest a conserved function for melatonin in enhancing transmission from rods to second-order neurons and thus promote dark adaptation.

Visual photoreceptor subtypes in the chicken retina: melatonin-synthesizing activity and in vitro differentiation

The chicken retina contains five visual photoreceptor subtypes, based on the specific opsin gene they express. In addition to the central role they play in vision, some or all of these photoreceptors translate photoperiodic information into a day-night rhythm of melatonin production. This indolic hormone plays an important role in the photoperiodic regulation of retinal physiology. Previous studies have stopped short of establishing whether melatonin synthesis takes place in all the photoreceptor spectral subtypes. Another issue that has been left unsettled by previous studies is when during development are retinal precursor cells committed to a specific photoreceptor subtype and to a melatoninergic phenotype? To address the first question, in situ hybridization of the five opsins was combined with immunofluorescent detection of the melatonin-synthesizing enzyme hydroxyindole O-methyltransferase (HIOMT, EC.2.1.1.4). Confocal microscopy clearly indicated that all photoreceptor spectral subtypes are involved in melatonin synthesis. To tackle the second question, retinal precursor cells were dissociated between embryonic day 6 (E6) and E13 and cultured in serum-free medium for 4 days to examine their ability to autonomously activate the expression of opsins and HIOMT. Real-time PCR on cultured precursors indicated that red-, green- and violet-sensitive cones are committed at E6, rods at E10 and blue-sensitive cones at E12. HIOMT gene expression was programmed at E6, probably reflecting the differentiation of early cones. The present study provides a better characterization of photoreceptor subtypes in the chicken retina and describes a combination of serum-free culture and real-time PCR that should facilitate further developmental studies.

An unexpected effect of 5-MCA-NAT in chick retinal development

Luzindole is an unselective antagonist of the melatonin receptors and melatonin's other binding sites, although some exceptions have been observed in chick retinal neurodevelopment, where this unselective antagonist does not block melatonin's inhibitory effect on the adenylate cyclase enzyme, probably due to the presence of some other melatonin receptor(s) or binding site(s). The present study investigated the modulation of cyclic adenosine 3'-5'-monophosphate (cAMP) levels via MT3 melatonin-binding sites, located within the QR2 (dihydronicotinamide riboside: quinone oxidoreductase 2) enzyme, by observing the response to luzindole. Embryonic and post-hatch retinas, incubated with a selective agonist for the MT3 melatonin-binding site 5-methoxycarbonylamino-N-acetyltryptamine (5-MCA-NAT, 10 or 100 nM), had an increase in cAMP accumulation relative to control retinas. Luzindole (5microM) inhibited the 5-MCA-NAT stimulatory effect at all ages tested. The agonist 5-MCA-NAT enhanced the melatonin inhibitory effect on cAMP levels stimulated by forskolin (5microM), but not the stimulatory forskolin effect. The results suggest that MT3 melatonin-binding sites are present in embryonic and post-hatch chick retinas and that luzindole more selectively blocks the 5-MCA-NAT effect on cAMP accumulation than it blocks the melatonin inhibitory effect via G protein-coupled receptors in chick retinal neurodevelopment.

Melatonin receptor-mediated protection against myocardial ischaemia/reperfusion injury: role of its anti-adrenergic actions

Melatonin has potent cardioprotective properties. These actions have been attributed to its free radical scavenging and anti-oxidant actions, but may also be receptor mediated. Melatonin also exerts powerful anti-adrenergic actions based on its effects on contractility of isolated papillary muscles. The aims of this study were to determine whether melatonin also has anti-adrenergic effects on the isolated perfused rat heart, to determine the mechanism thereof and to establish whether these actions contribute to protection of the heart during ischaemia/reperfusion. The results showed that melatonin (50 microM) caused a significant reduction in both isoproterenol (10(-7) M) and forskolin (10(-6) M) induced cAMP production and that both these responses were melatonin receptor dependent, since the blocker, luzindole (5 x 10(-6) M) abolished this effect. Nitric oxide (NO), as well as guanylyl cyclase are involved, as L-NAME (50 microM), an NO synthase inhibitor and ODQ (20 microM), a guanylyl cyclase inhibitor, significantly counteracted the effects of melatonin. Protein kinase C (PKC), as indicated by the use of the inhibitor bisindolylmaleimide (50 microM), also play a role in melatonin's anti-adrenergic actions. These actions of melatonin are involved in its cardioprotection: simultaneous administration of L-NAME or ODQ with melatonin, before and after 35 min regional ischaemia, completely abolished its cardioprotection. PKC, on the other hand, had no effect on the melatonin-induced reduction in infarct size. Cardioprotection by melatonin was associated with a significant activation of PKB/Akt and attenuated activation of the pro-apoptotic kinase, p38MAPK during early reperfusion. In summary, the results show that melatonin-induced cardioprotection may be receptor dependent, and that its anti-adrenergic actions, mediated by NOS and guanylyl cyclase activation, are important contributors.

Melatonin inhibitory effect on cAMP accumulation in the chick retina development

During vertebrate neurodevelopment, neuritogenesis and synaptogenesis are modulated by intracellular cAMP rises. Melatonin, which is implicated in neuronal differentiation, mainly inhibits this pathway. Here, an investigation about the profile of this effect during the vertebrate neurodevelopment is reported. In the embryonic chick retinas at days 8, 12, 14, 16 and at 2 days post-hatched (E8, E12, E14, E16 and PH), those control embryonic retinas incubated only with the phosphodiesterase inhibitor at days corresponding to commencement of neuronal differentiation (E8, E12) and PH, presented cAMP levels inhibited by melatonin. While the cAMP accumulation stimulated by forskolin was inhibited in the embryonic retinas at all testing days. Neither the unselective antagonist N-acetyl-2-benziltryptamine (luzindole) nor the selective Mel(1b) antagonist 4-phenyl-2-propionamidotetralin (4-P-PDOT) blocked the melatonin concentration-dependent inhibitory effect on cAMP accumulation in the retinas initiating differentiation (E7-E9), suggesting a tight binding between melatonin and their receptors. However, 4-P-PDOT competitively reverted the melatonin effect on cAMP stimulated by forskolin during synaptogenesis stages. Together, the melatonin effect on cAMP levels in chick retina, which is mainly through melatonin receptors, is depending on the developmental period observed, probably taking part in the mechanisms surrounding the melatonin action on neuronal differentiation.

Melatonin reduces formalin-induced nociception and tactile allodynia in diabetic rats

The purpose of this study was to assess the antinociceptive and antiallodynic effect of melatonin as well as its possible mechanism of action in diabetic rats. Streptozotocin (50 mg/kg) injection caused hyperglycemia within 1 week. Formalin-evoked flinching was increased in diabetic rats as compared to non-diabetic rats. Oral administration of melatonin (10-300 mg/kg) dose-dependently reduced flinching behavior in diabetic rats. In addition, K-185 (a melatonin MT(2) receptor antagonist, 0.2-2 mg/kg, s.c.) completely blocked the melatonin-induced antinociception in diabetic rats, whereas that naltrexone (a non-selective opioid receptor antagonist, 1 mg/kg, s.c.) and naltrindole (a selective delta opioid receptor antagonist, 0.5 mg/kg, s.c.), but not 5'-guanidinonaltrindole (a selective kappa opioid receptor antagonist, 1 mg/kg, s.c.), partially reduced the antinociceptive effect of melatonin. Given alone K-185, naltrexone, naltrindole or 5'-guanidinonaltrindole did not modify formalin-induced nociception in diabetic rats. Four to 8 weeks after diabetes induction, tactile allodynia was observed in the streptozotocin-injected rats. On this condition, oral administration of melatonin (75-300 mg/kg) dose-dependently reduced tactile allodynia in diabetic rats. Both antinociceptive and antiallodynic effects were not related to motor changes as melatonin did not modify number of falls in the rotarod test. Results indicate that melatonin is able to reduce formalin-induced nociception and tactile allodynia in streptozotocin-injected rats. In addition, data suggest that melatonin MT(2) and delta opioid receptors may play an important role in these effects.

Oral and spinal melatonin reduces tactile allodynia in rats via activation of MT2 and opioid receptors

The antiallodynic effect of melatonin after intrathecal (it) and oral administration as well as the possible participation of MT(2) and opioid receptors in melatonin-induced antiallodynia in neuropathic rats were assessed. Ligation of the L5/L6 spinal nerves produced a clear-cut tactile allodynia in the rats. Intrathecal (3-100 microg) and oral (37.5-300 mg/kg) administration of melatonin decreased tactile allodynia induced by spinal nerve ligation. Intrathecal administration of the preferential MT(2) receptor antagonist luzindole (1-100 microg), but not vehicle, significantly diminished in a dose-dependent manner the antiallodynic effect induced by melatonin (100 microg, it). Oral (0.01-1mg/kg) or intrathecal (0.1-10 microg) administration of the highly selective MT(2) receptor antagonist 4P-PDOT diminished the antiallodynic activity induced by oral (150 mg/kg) or intrathecal (100 microg) administration of melatonin, respectively. Subcutaneous (1mg/kg) or intrathecal (0.5-50 microg) treatment with naltrexone, but not vehicle, significantly diminished the antiallodynic effect induced by oral (150 mg/kg) or intrathecal (100 microg) administration of melatonin. Oral melatonin (150 mg/kg)-induced antiallodynia was partially reduced by the spinal administration of 4P-PDOT (10 microg). Moreover, the spinal effect of melatonin (100 microg) was significantly reduced by the combination 4P-PDOT (0.1 microg)-naltrexone (0.5 microg). At the greatest tested doses, the antagonist drugs did not modify tactile allodynia in neuropathic rats. Melatonin (100 microg or 300 mg/kg) did not affect motor co-ordination in the rotarod test. Results indicate that melatonin reduces tactile allodynia in neuropathic rats after intrathecal and oral administration. Moreover, data suggest the participation of spinal MT(2) and opioid receptors in the melatonin-induced antiallodynic effect in this model.

Short- and long-term effects of melatonin on myocardial post-ischemic recovery

Melatonin, the chief secretory product of the pineal gland, has been shown to protect the heart against ischemia-reperfusion injury. This was attributed to its free radical scavenging and broad-spectrum antioxidant properties. The possibility that melatonin may act via its receptor and intracellular signaling, has not yet been addressed in this regard. In all previous studies, only the acute effects of melatonin on the heart, were evaluated. The aims of the present study were to: (i) compare the acute and long-term effects of melatonin on infarct size and functional recovery of the ischemic heart, and (ii) evaluate the role of the melatonin receptor in cardioprotection. For evaluation of the short-term effects of melatonin on contractile recovery and infarct size, the isolated perfused working rat heart was subjected to 20 min global ischemia or 35 min regional ischemia respectively, and melatonin (25-50 microm) administered either before and during reperfusion, or before ischemia or during reperfusion after ischemia. The melatonin receptor was manipulated using luzindole and N-acetyltryptamine. The long-term effects of melatonin were evaluated 24 hr after melatonin administration (2.5 or 5.0 mg/kg, i.p.) or after oral administration for 7 days (20 or 40 microg/mL). Infarct size and mechanical recovery during reperfusion of the working heart were used as endpoints. Melatonin (50 microm), when administered either before and during reperfusion after ischemia or during reperfusion only, significantly improved cardiac output and work performance and reduced infarct size compared with untreated controls. Luzindole (5 microm), a melatonin receptor antagonist, abolished these cardioprotective effects. Long-term administration of melatonin (i.p. or orally for 7 days) caused a significant reduction in infarct size of hearts subjected to 35 min regional ischemia. The cardioprotection persisted for 2-4 days after discontinuation of treatment. In summary, the results obtained suggest that melatonin induces short- as well as long-term protection and that the melatonin receptor is also involved in its cardioprotective actions.

Ontogeny of central melatonin receptors in tadpoles of the anuran Rana perezi: modulation of dopamine release

The objective of this work was to study melatonin receptors in the eye and the brain and their possible functionality in the ontogeny of Rana perezi. The binding of 2-[(125)I]melatonin increases throughout embryonic larval development in both tissues. The most pronounced increase takes place at the end of premetamorphosis and during early prometamorphosis. This rise coincides temporarily with the appearance of the rhythmic melatonin-synthesizing capacity in the retina. In the three studied developmental stages (32G, 40G and 49-50G), melatonin-binding sites are coupled to G proteins and become functional. Moreover, melatonin inhibits dopamine (DA) release by the eyecups and brain of R. perezi tadpoles in vitro (stage 40G). Thus, the modulation of DA release could be one mechanism by which melatonin interacts with hormones, like prolactin and thyroxine that are involved in the regulation of anuran development and metamorphosis. Finally, we show that melatonin decreases K(+)-evoked cAMP content in the frog retina in vitro, suggesting that the effect of melatonin on DA release in the frog retina is mediated by the inhibition of this intracellular messenger.

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.

Neural stem cells express melatonin receptors and neurotrophic factors: colocalization of the MT1 receptor with neuronal and glial markers

Background

In order to optimize the potential benefits of neural stem cell (NSC) transplantation for the treatment of neurodegenerative disorders, it is necessary to understand their biological characteristics. Although neurotrophin transduction strategies are promising, alternative approaches such as the modulation of intrinsic neurotrophin expression by NSCs, could also be beneficial. Therefore, utilizing the C17.2 neural stem cell line, we have examined the expression of selected neurotrophic factors under different in vitro conditions. In view of recent evidence suggesting a role for the pineal hormone melatonin in vertebrate development, it was also of interest to determine whether its G protein-coupled MT₁ and MT₂ receptors are expressed in NSCs.

Results

RT-PCR analysis revealed robust expression of glial cell-line derived neurotrophic factor (GDNF), brain-derived neurotrophic factor (BDNF) and nerve growth factor (NGF) in undifferentiated cells maintained for two days in culture. After one week, differentiating cells continued to exhibit high expression of BDNF and NGF, but GDNF expression was lower or absent, depending on the culture conditions utilized. Melatonin MT₁ receptor mRNA was detected in NSCs maintained for two days in culture, but the MT₂ receptor was not seen. An immature MT₁ receptor of about 30 kDa was detected by western blotting in NSCs cultured for two days, whereas a mature receptor of about 40 – 45 kDa was present in cells maintained for longer periods. Immunocytochemical studies demonstrated that the MT₁ receptor is expressed in both neural (β-tubulin III positive) and glial (GFAP positive) progenitor cells. An examination of the effects of melatonin on neurotrophin expression revealed that low physiological concentrations of this hormone caused a significant induction of GDNF mRNA expression in NSCs following treatment for 24 hours.

Conclusions

The phenotypic characteristics of C17.2 cells suggest that they are a heterogeneous population of NSCs including both neural and glial progenitors, as observed under the cell culture conditions used in this study. These NSCs have an intrinsic ability to express neurotrophic factors, with an apparent suppression of GDNF expression after several days in culture. The detection of melatonin receptors in neural stem/progenitor cells suggests involvement of this pleiotropic hormone in mammalian neurodevelopment. Moreover, the ability of melatonin to induce GDNF expression in C17.2 cells supports a functional role for the MT₁ receptor expressed in these NSCs. In view of the potency of GDNF in promoting the survival of dopaminergic neurons, these novel findings have implications for the utilization of melatonin in neuroprotective strategies, especially in Parkinson's disease.

Localization of Mel1b melatonin receptor-like immunoreactivity in ocular tissues of Xenopus laevis

The circadian signaling molecule, melatonin, is produced by pinealocytes and retinal photoreceptors. In the retina, melatonin is thought to diffuse into the inner retina to act as a paracrine signal of darkness by binding to specific receptors in retinal neurons. The retinal cell locations of the Mel1a and Mel1c melatonin receptor types have been reported, but the localization of the Mel1b receptor, which is the most highly expressed melatonin receptor type in the retina, is unknown. To determine the cellular distribution of Mel1b melatonin receptor protein in the Xenopus laevis retina and other ocular tissues, polyclonal antibodies were raised against a peptide fragment of the X. laevis Mel1b receptor. Western blot analysis of several ocular tissues revealed the presence of one or more immunoreactive bands in the sclera, cornea, lens, retinal pigment epithelium (RPE)/choroid, and neural retina. In the neural retina, the major immunoreactive bands displayed electrophoretic mobilities corresponding to approximately 35, 42, 45, and 80 Kd. Sections of X. laevis eyes were analyzed by immunocytochemistry and confocal microscopy, in combination with antibodies against the Mel1a melatonin receptor, a rod photoreceptor-specific protein, opsin, and two amacrine cell-specific markers, tyrosine hydroxylase (TOH; dopaminergic cells) and glutamic acid decarboxylase (GAD; GABA-ergic cells). Mel1b immunoreactivity was localized to the apical membranes of RPE cells, and punctate Mel1b immunoreactivity was observed in both rod and cone photoreceptor inner segments. Presumptive horizontal cells that ramify in the outer plexiform layer (OPL) were immunoreactive for Mel1b, and were exclusive of the Mel1a immunoreactivity present in the OPL. Neither TOH nor GAD co-localized with the Mel1b immunoreactivity that was present in the inner plexiform layer (IPL), suggesting that Mel1b is not expressed in dopaminergic or GABA-ergic amacrine cells. Mel1b immunoreactivity was observed in ganglion cells of the retina, a population of cells covering the outer surface of the outer fibrous layer of the sclera, and in lens fibers located in the outer regions of the lens. These results suggest that melatonin may influence retinal function by binding to receptors on RPE and photoreceptor cells, and by acting on neurons of the inner retina that do not use dopamine or GABA as a neurotransmitter. Furthermore, melatonin may bind to receptors on cells located in the sclera and lens, perhaps to modify the growth or function of these ocular tissues.

Melatonin-induced inhibition of spinal cord synaptic potentiation in rats is MT2 receptor-dependent

Systemically administered melatonin has been reported to produce antinociception and to inhibit spinal nociceptive transmission in rats. The present study was designed to investigate in anesthetized rats (i) whether intrathecally administered melatonin can depress synaptic potentiation (wind-up) in the spinal cord, and (ii) whether this effect is prevented by intrathecal (i.t.) administration of the MT2 receptor antagonist luzindole. Results showed that melatonin i.t. (10, 30 and 90 microg) induced dose-dependent inhibition of wind-up activity (ED50=52.06 microg i.t.), an effect that was prevented by 100 microg i.t. of luzindole. Since wind-up is dependent on NMDA receptor activation, the results suggest that melatonin can interfere with the NMDA-mediated glutamatergic component of pain transmission in rat spinal cord by acting on MT2 receptors.

Melatonin induces alterations in protein expression in the Xenopus laevis retina

The hormone melatonin is synthesized by pinealocytes and retinal photoreceptors with a diurnal rhythm. Melatonin produced in the retina at night is thought to exert local modulatory effects by binding to specific receptors in several different retinal cell types. The mechanisms by which melatonin influences circadian activity in retinal cells is poorly understood. Suppression of cyclic AMP synthesis appears to be a major signaling pathway in response to melatonin receptor binding in many tissues. A potential downstream consequence of melatonin-induced changes in cyclic AMP concentrations and protein phosphorylation is the up- or down-regulation of expression of specific genes. In this report, we examined the changes in expression levels of specific proteins in the neural retina and retinal pigment epithelium (RPE) in response to melatonin treatment, because both of these tissues express melatonin receptors. Neural retina and RPE isolated from the eyes of Xenopus laevis were treated with or without 1 microM melatonin for 6 hr, then the rapidly synthesized tissue proteins were radiolabeled by a 15 min incubation with 35S-methionine, and the proteins were subsequently analyzed by two-dimensional gel electrophoresis and autoradiography. In both the neural retina and RPE, the densities of some specific proteins were altered in response to melatonin treatment, and the few protein spots that were altered were distinct between the two tissues. These results support the concept that one function of melatonin may be to regulate the expression of specific genes and the consequent protein levels, and that the target genes may differ according to the cell or tissue type.

The effect of melatonin on the secretion of progesterone in sheep and on the development of ovine embryos in vitro

Two experiments were carried out in order to determine whether melatonin can improve secretion of progesterone in vivo, and its effect on embryonic development in vitro. In the first experiment, blood samples were collected from 5 ewes at 15 min intervals for 2 h at 7 and 10 days after withdrawal of progestagen pessaries. The first hour constituted a control period, which ended with an intravenous administration of 3 microg/(kg bw)(0.75) melatonin. All the ewes on day 7 and three of the ewes on day 10 showed a progesterone response to melatonin challenge, defined as an increase in the plasma progesterone concentration in at least two consecutive samples during the post-treatment period above the mean+2SD of the values in the pre-treatment period. A paired t-test revealed a significant effect of melatonin on the overall plasma progesterone concentrations before and after the challenge, both on day 7 (pre, 0.61 +/- 0.11; post, 0.73 +/- 0.13 ng/ml; p<0.01) and day 10 (pre, 1.16 +/- 0.19; post, 1.30 +/- 0.20 ng/ml; p<0.05). Ninety-one thawed embryos (46 morulae and 45 blastocysts) were used in the second experiment, being cultured with or without 1 microg/ml melatonin. If the embryos were blastocysts when the culture started. melatonin increased the percentage that had hatched after 24 h of culture (p<0.01), and there was a lower percentage of degenerated embryos at the end of the incubation period (p<0.05). It may be concluded that melatonin treatment in sheep can increase both fertility and prolificacy by improving luteal function and embryonic survival.

Melatonin-induced suppression of PC12 cell growth is mediated by its Gi coupled transmembrane receptors

The effects of pertussis toxin, an uncoupler of Gi protein from adenylate cyclase, and luzindole, a competitive inhibitor of melatonin receptor binding, were examined for their ability to inhibit melatonin-induced suppression of PC12 cell growth. Both agents inhibited the melatonin response suggesting that melatonin may be acting through one of its Gi coupled cell surface receptors. This is confirmed by Western blots demonstrating the presence of MT1 receptors in PC12 cells. Coupling of the Gi protein to these receptors is demonstrated by failure of melatonin to suppress cell growth in PKA deficient A126-1B2-1 mutant PC12 cells. Similarly, melatonin failed to prevent cell proliferation when cells were incubated in the presence of the PKA inhibitor, Rp-cAMP. Retinoic acid and dexamethasone, agents known to effect PC12 cell growth and/or differentiation, displayed differential effects on the actions of melatonin. In the presence of melatonin and low concentrations of retinoic acid (100 nM), PC12 cell proliferation was stimulated compared to that seen with either agent alone, whereas no increase in cell proliferation was observed when higher concentrations of retinoic acid (100 microM) were used. The effects of dexamethasone on suppression of PC12 cell growth were additive with that of melatonin whereas, 1,25-dihydroxyvitamin D(3) (IC(50)=10 nM), which by itself had no effect on PC12 cell growth, was found to inhibit the melatonin response. This study demonstrates that inhibition of PC12 cell growth, at physiological concentrations of melatonin, is mediated by cAMP-dependent cell surface receptors and this response is altered by other growth factors known to effect PC12 cell proliferation and differentiation.

Presence and effects of melatonin in Trypanosoma cruzi

The unicellular organism Trypanosoma cruzi is an eukaryote whose cell cycle mainly occurs under darkness in the insect gut. The unique external phase corresponds to the metacyclic forms, the forms that are able to infect humans, which appear within the insect deyections. Thus, light may be a powerful stressor in this unicell. Epimastigote forms (the parasite forms that grow and transform to metacyclic forms in the insect gut) of Trypanosoma cruzi grow normally when cultured in a LD cycle of 0:24 hr, reaching exponential growth by the 7th day. A pulse of 2 hr of light (LD 2:22) was enough to block the growth of the epimastigotes, an effect that was correlated with the expression of heat-shock proteins during the first 120 min of light exposure. Thereafter, protein synthesis decreased. Light exposure of metacyclic forms also inhibits the parasitization ability. It is known that light regulates the production of melatonin in most animal species studied, including other unicells such as dinoflagellates. T. cruzi contains and synthesizes melatonin and, thus, light-mediated events on the parasite biological cycle could be mediated by light-induced changes in melatonin produced by this unicell. Epimastigotes cultured under continuous darkness produce melatonin over the 24 hr period in a biphasic manner. Coinciding with the melatonin peaks, there was high melatonin efflux from the parasite into the medium. Epimastigotes cultured for 7 days under a LD cycle of 2:22 hr showed a 55% reduction in melatonin content, although this reduction seems not to be related with the growth delay. In fact, incubation of epimastigotes with exogenous melatonin (1 pM) did not affect parasite growth, but significantly reduced their transformation into metacyclic forms by the 7-8th day of treatment. Thus, the light-dependent decrease in melatonin production by the unicell may be responsible, at least partially, for the light-induce parasitization inhibition. Moreover, melatonin production is highest in the metacyclic forms. These data support a link between light, melatonin production and parasitization ability of T. cruzi and suggest the participation of the indoleamine in its biological cycle.

Biphasic effects of melatonin on the propagation of excitation waves in the chicken retina

The retinal spreading depression (SD) is a propagating wave in an excitable medium, the neuronal tissue of the retina. Its velocity is about 3 mm/min and it is accompanied by a variety of changes in the tissue, including electrical and optical events. The pronounced intrinsic optical signal (IOS) of the retinal SD makes it an extremely versatile tool for the investigation of the action of drugs on neuronal tissue and more specific on propagating excitation waves in neuronal tissue. Furthermore, in the last decade increasing evidence has been collected, which shows that SD waves are the basic mechanism of the aura in classical migraine. We have investigated the influence of melatonin on the propagation of retinal SD waves as it has been postulated to have protective effects on neuronal tissue. The results demonstrate that melatonin indeed slows down the retinal SD, however, only in a defined concentration range. Additionally, it changes the IOS of the wave.

PACAP-induced formation of cyclic AMP in the chicken brain: regional variations and the effect of melatonin

We have studied the effects of pituitary adenylate cyclase-activating polypeptide (PACAP27 and PACAP38) on cyclic AMP formation in chick brain, and the action of melatonin upon the PACAP-evoked effects. PACAP stimulated cyclic AMP production in the hypothalamus>cerebral cortex>pineal gland>optic lobes. In the hypothalamus and cerebral cortex, the rank-order of both PACAP forms and VIP in evoking the cyclic AMP response was: PACAP38 approximately PACAP27>>VIP, suggesting the presence in the tested tissues of PAC1 receptors. Melatonin suppressed (IC50=19.8 nM) the PACAP27 (0.1 microM)-induced cyclic AMP response in the hypothalamus, but not in the cerebral cortex. Melatonin also suppressed the hypothalamal cyclic AMP synthesis stimulated by forskolin, but not that evoked by histamine or isoprenaline. Our observations show that PACAP is capable of potently stimulating cyclic AMP formation in some regions of the chick brain, and suggest that the hypothalamus may be a site for a functional interaction between PACAP and the pineal hormone melatonin.

Characterization of membrane melatonin receptor in mouse peritoneal macrophages: inhibition of adenylyl cyclase by a pertussis toxin-sensitive G protein

Melatonin binding sites were characterized in mouse peritoneal macrophages. Binding of 2-[125I]melatonin by macrophages fulfills all criteria for binding to a receptor site. Thus, binding was dependent on time, temperature and cell concentration, stable, reversible, saturable and specific. Stoichiometric studies showed a high-affinity binding site with a Kd of 0.58-0.71 nM. These data are in close agreement with data obtained from kinetic studies (Kd = 0.29 nM). The affinity of these binding sites suggests that they may recognize the physiological concentrations of melatonin in serum. Moreover, binding experiments using macrophage crude membranes showed that melatonin bound specifically to the membranes. Additionally, in competition studies we observed a low-affinity binding site (Kd = 2.02 microM). Melatonin inhibited significantly forskolin-stimulated cyclic AMP accumulation in a dose-dependent manner. This effect was blocked by luzindole, an antagonist of the melatonin membrane receptor. Pretreatment of macrophages with pertussis toxin blocked the inhibitory effect of melatonin. Pertussis toxin ADP-rybosilation and Western blot experiments demonstrated both alpha(i1/2) and alpha(i3/o) G protein subunits expression in mouse peritoneal macrophages membranes. Our results demonstrate the existence of melatonin receptors in mouse peritoneal macrophages, and a pertussis toxin-sensitive melatonin signal transduction pathway that involves the inhibition of adenylyl cyclase.

Mechanisms of melatonin-induced vasoconstriction in the rat tail artery: a paradigm of weak vasoconstriction

1. Vasoconstrictor effects of melatonin were examined in isolated rat tail arteries mounted either in an isometric myograph or as cannulated pressurized segments. Melatonin failed by itself to mediate observable responses but preactivation of the arteries with vasopressin (AVP) reliably uncovered vasoconstriction responses to melatonin with maxima about 50% of maximum contraction. Further experiments were conducted with AVP preactivation to 5-10% of the maximum contraction. 2. Responses to melatonin consisted of steady contractions with superimposed oscillations which were large and irregular in isometric but small in isobaric preparations. Nifedipine (0.3 microM) reduced the responses and abolished the oscillations. Charybdotoxin (30 nM) increased the magnitude of the oscillations with no change in the maximum response. 3. Forskolin (0.6 microM) pretreatment increased the responses to melatonin compared to control and sodium nitroprusside (1 microM) treated tissues. The AVP concentration required for preactivation was 10 fold higher than control in both the forskolin and nitroprusside treated groups. 4. In isometrically-mounted arteries treated with nifedipine, melatonin receptor agonists had the potency order 2-iodomelatonin > melatonin > S20098 > GR196429, and the MT2-selective antagonist luzindole antagonized the effects of melatonin with a low pK(B) of 6.1+/-0.1. 5. It is concluded that melatonin elicits contraction of the rat tail artery via an mt1 or mt1-like receptor that couples via inhibition of adenylate cyclase and opening of L-type calcium channels. Calcium channels and charybdotoxin-sensitive K channels may be recruited into the responses via myogenic activation rather than being coupled directly to the melatonin receptors. 6. It is proposed that the requirement of preactivation for overt vasoconstrictor responses to melatonin results from the low effector reserve of the melatonin receptors together with the tail artery having threshold inertia. Potentiative interactions between melatonin and other vasoconstrictor stimuli probably also result from the threshold inertia. A simple model is presented and a general framework for consideration of interactions between weak vasoconstrictor agonists and other vasoconstrictor stimuli is discussed.

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.

Melatonin suppression of PC12 cell growth and death

Melatonin has previously been reported to influence cell differentiation and growth in a number of cell culture systems in vitro. In this paper, we describe the effects of high pharmacological and low physiological concentrations of melatonin on cell growth in rat pheochromocytoma cells (PC12 cells). Melatonin produced a biphasic response with respect to cell growth in PC12 cells. At low concentrations (1-10 nM) melatonin suppressed PC12 cell growth whereas at higher concentration (10 microM) it prevented cell death. Cultures treated with high concentrations of melatonin displayed an increase in cell number and a decreased release of lactic acid dehydrogenase (LDH) into the culture media, indicating that melatonin was enhancing cell survival as opposed to stimulating cell proliferation. Inhibition of cell death by high concentrations of melatonin was both time and concentration-dependent and did not require the continued presence of melatonin throughout the entire time of incubation. These studies suggest melatonin is preventing either apoptosis or programmed cell death. In contrast, concentrations of melatonin (1-10 nM) at or near the binding affinity for the nuclear receptor, RZRbeta, suppressed PC12 cell growth. At these concentrations, melatonin failed to inhibit forskolin-induced cAMP formation and process outgrowth as well as prevent forskolin suppression of cell growth. These data indicate that PC12 cells probably lack functionally active cell surface receptors for melatonin and suggest the interaction of melatonin with the nuclear receptor may be responsible for suppression of PC12 cell growth.

Effects of melatonin in isolated rat papillary muscle

Melatonin (N-acetyl-5-methoxytryptamine), the principal hormone of the vertebral pineal gland, elicits several neurobiological effects. However, the effects of melatonin on cardiac muscle are still unknown. The first goal of the study was to investigate the role of melatonin on myocardial contractility in isolated rat papillary muscle using dose-response curves to melatonin, to isoproterenol and calcium either in the presence or in the absence of melatonin (0.3 nM). Response curves to isoproterenol were additionally performed in the presence of melatonin plus the specific receptor antagonist N-acetyltryptamine (10 microM); the adenylate-cyclase stimulator forskolin (10 microM) was also used. Melatonin has no direct inotropic effect in isolated rat papillary muscle but counteracts isoproterenol but not [Ca2+] effects. In fact, the EC50 for isoproterenol was significantly higher in the presence than in the absence of melatonin (p < 0.001). This anti-adrenergic action occurs through an interaction to a specific cardiac receptor. Forskolin-stimulated adenylate cyclase induced an increase of contractile force (+118 +/- 25%) which was reduced in the presence of melatonin (+26 +/- 10%; p < 0.01). In conclusion, we found that melatonin possess anti-adrenergic effect in isolated rat papillary muscle. This phenomenon was abolished in the presence of its receptor antagonist N-acetyl-tryptamine demonstrating that melatonin operates through a specific cardiac receptor. The reduction of contractility increase, induced by forskolin-stimulated adenylate cyclase, shows that melatonin may act through a reduction of cyclic AMP accumulation.

Melatonin induces membrane conductance changes in isolated retinal rod receptor cells

Experiments were conducted to verify whether the neurohormone melatonin influences the membrane conductance of photoreceptors isolated from the frog retina. It has been found that 20 microM melatonin decreases membrane conductances both in the linear and non linear ranges by <0.4 nS. These actions are estimated to produce in dark adapted photoreceptors an increase of the response to a dim light induced change of the dark current of about 21%, i.e. from 1.3 to 1.62 mV/pA.

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.

Melatonin receptors are for the birds: molecular analysis of two receptor subtypes differentially expressed in chick brain

Two receptors (CKA and CKB) of the G protein-coupled melatonin receptor family were cloned from chick brain. CKA encodes a protein that is 80% identical at the amino acid level to the human Mel1a melatonin receptor and is thus designated the chick Mel1a melatonin receptor. CKB encodes a protein that is 80% identical to the Xenopus melatonin receptor and defines a new receptor subtype, the Mel1c melatonin receptor, which is distinct from the Mel1a and Mel1b melatonin receptor subtypes. A melatonin receptor family consisting of three subtypes is supported by PCR cloning of distinct melatonin receptor fragments from Xenopus and zebrafish. Expression of CKA and CKB results in similar ligand binding and functional characteristics. The widespread distribution of CKA and CKB mRNA in brain provides a molecular substrate for the profound actions of melatonin in birds.

Molecular and functional characterization of a partial cDNA encoding a novel chicken brain melatonin receptor

An approach based on homology probing was used to clone a partial cDNA encoding a novel melatonin (ML) receptor (MLR) from chicken (Gallus domesticus) brain. Based on available deduced amino-acid sequence, the chicken MLR (cMLR) displayed greater sequence homology to the frog (Xenopus) MLR than cloned human/mammalian receptors, with overall identities of 73% and 66%, respectively. In order to gain functional expression, a chimeric frog/chicken (flc)MLR was constructed in which the 5' end of the cMLR, including the N-terminus, TM1 and part of the first intracellular loop was substituted by fMLR sequence. [125I]Iodo-ML bound with high affinity (Kd of approximately 35 pM) to COS-7 cells transiently expressing the flcMLR in a saturable and guanine nucleotide-sensitive manner with the following rank order of potency: 2-iodo-ML > ML > 6-Cl-ML > S20750 > 6-OH-ML > S20642 > S20753 > N-acetyl-5HT >> 5-HT. Estimated Ki values for these compounds at the flcMLR correlated well to those obtained in native chicken brain membranes. In line with the observed structural similarity to the fMLR, the flcMLR exhibited affinities for ML, 6-Cl-ML and 6-OH-ML approximately 10-fold lower than mammalian receptors. Functionally, opposing interactions between ML and dopamine receptor signal transduction pathways were observed with ML potently inhibiting dopamine D1A-receptor-mediated cAMP accumulation in cells (HEK-293) transiently co-expressing these receptors. cMLR mRNAs were found expressed in chicken brain and kidney with trace levels observed in the lung. The availability of cloned vertebrate MLRs distinct at both the amino acid and pharmacological level from their mammalian counterparts may now allow for the identification of those amino-acid residues and structural motifs that regulate ML-binding specificity and affinity.

Melatonin receptors mediate potentiation of contractile responses to adrenergic nerve stimulation in rat caudal artery

The hormone melatonin potentiated contractile responses to adrenergic nerve stimulation in isolated ring segments of rat caudal artery. This effect was inhibited by the melatonin receptor antagonist luzindole but not by the serotonin 5-HT2 receptor antagonist ketanserin. Melatonin had no direct effects on vascular tone. Melatonin agonists potentiated contractile responses with a relative order of potency (2-iodomelatonin, EC50 = 0.6 nM; melatonin, EC50 = 4.7 nM; N-acetylserotonin, EC50 = 1.5 microM) that is consistent with the melatonin ML1 receptor subtype. Melatonin also potentiated contractions elicited by exogenous norepinephrine and produced its effects in the absence of an intact endothelium. These data suggest that melatonin acts on receptors in the smooth muscle. The caudal artery provides a useful functional assay for pharmacological analysis of melatonin receptors. Physiologically, melatonin may activate its receptors at night to influence thermoregulation in the rat by enhancing the effects of sympathetic input to the caudal artery.

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.