Differential regulation of melatonin receptors in sheep, chicken and lizard brains by cholera and pertussis toxins and guanine nucleotides

Melatonin receptors in pancreatic islets: good morning to a novel type 2 diabetes gene

Melatonin is a circulating hormone that is primarily released from the pineal gland. It is best known as a regulator of seasonal and circadian rhythms; its levels are high during the night and low during the day. Interestingly, insulin levels also exhibit a nocturnal drop, which has previously been suggested to be controlled, at least in part, by melatonin. This regulation can be explained by the proposed inhibitory action of melatonin on insulin release. Indeed, both melatonin receptor 1A (MTNR1A) and MTNR1B are expressed in pancreatic islets. The role of melatonin in the regulation of glucose homeostasis has been highlighted by three independent publications based on genome-wide association studies of traits connected with type 2 diabetes, such as elevated fasting glucose, and, subsequently, of the disease itself. The studies demonstrate a link between variations in the MTNR1B gene, hyperglycaemia, impaired early phase insulin secretion and beta cell function. The risk genotype predicts the future development of type 2 diabetes. Carriers of the risk genotype exhibit increased expression of MTNR1B in islets. This suggests that these individuals may be more sensitive to the actions of melatonin, leading to impaired insulin secretion. Blocking the inhibition of insulin secretion by melatonin may be a novel therapeutic avenue for type 2 diabetes.

Social stress and corticosterone regionally upregulate limbic N-methyl-d-aspartatereceptor (NR) subunit type NR2A and NR2B in the lizard anolis carolinensis

Social aggression in the lizard Anolis carolinensis produces dominant and subordinate relationships while elevating corticosterone levels and monoaminergic transmitter activity in hippocampus (medial and mediodorsal cortex). Adaptive social behavior for dominant and subordinate male A. carolinensis is learned during aggressive interaction and therefore was hypothesized to involve hippocampus and regulation of N-methyl-d-aspartate (NMDA) receptors. To test the effects of social stress and corticosterone on NMDA receptor subunits (NR), male lizards were either paired or given two injections of corticosterone 1 day apart. Paired males were allowed to form dominant-subordinate relationships and were killed 1 day later. Groups included isolated controls, dominant males, subordinate males and males injected with corticosterone. Brains were processed for glutamate receptor subunit immunohistochemistry and fluorescence was analyzed by image analysis for NR(2A) and NR(2B) in the small and large cell divisions of the medial and mediodorsal cortex. In the small granule cell division there were no significant differences in NR(2A) or NR(2B) immunoreactivity among all groups. In contrast, there was a significant upregulation of NR(2A) and NR(2B) subunits in the large pyramidal cell division in all three experimental groups as compared with controls. The results revealed significantly increased NR(2A) and NR(2B) subunits in behaving animals, whereas animals simply injected with corticosterone showed less of an effect, although they were significantly increased over control. Upregulation of NR(2) subunits occurs during stressful social interactions and is likely to be regulated in part by glucocorticoids. The data also suggest that learning social roles during stressful aggressive interactions may involve NMDA receptor-mediated mechanisms.

Differential coupling of the extreme C-terminus of G protein alpha subunits to the G protein-coupled melatonin receptors

Melatonin receptors interact with pertussis toxin-sensitive G proteins to inhibit adenylate cyclase. However, the G protein coupling profiles of melatonin receptor subtypes have not been fully characterised and alternative G protein coupling is evident. The five C-terminal residues of Galpha subunits confer coupling specificity to G protein-coupled receptors. This report outlines the use of Galphas chimaeras to alter the signal output of human melatonin receptors and investigate their interaction with the C-termini of Galpha subunits. The Galphas portion of the chimaeras confers the ability to activate adenylate cyclase leading to cyclic AMP production. Co-transfection of HEK293 cells expressing MT(1) or MT(2) melatonin receptors with Galphas chimaeras and a cyclic AMP activated luciferase construct provided a convenient and sensitive assay system for identification of receptor recognition of Galpha C-termini. Luciferase assay sensitivity was compared with measurement of cyclic AMP elevations by radioimmunoassay. Differential interactions of the melatonin receptor subtypes with Galpha chimaeras were observed. Temporal and kinetic parameters of cyclic AMP responses measured by cyclic AMP radioimmunoassay varied depending on the Galphas chimaeras coupled. Recognition of the C-terminal five amino acids of the Galpha subunit is a requisite for coupling to a receptor, but it is not the sole determinant.

Melatonin modulation of prolactin and gonadotrophin secretion. Systems ancient and modern

Recent studies in sheep indicate that the pineal melatonin signal which transduces effects of photoperiod acts at separate sites in the pituitary gland and brain to regulate seasonality in prolactin (PRL) and gonadotrophin secretion. The pituitary gland is the proposed site for control of PRL based on the observation that hypothalamo-pituitary disconnected (HPD) rams continue to show normal patterns of PRL secretion in response to changes in photoperiod or treatment with melatonin. Lactotrophs do not express melatonin receptors, thus this pituitary effect is assumed to be mediated by cells in the pars tuberalis via "tuberalin". The mediobasal hypothalamus (MBH) is the putative target for gonadotrophin control since: i) gonadotrophin secretion is dependent on pulsatile GnRH secretion from the MBH, ii) local administration of melatonin in the MBH, but not in other areas of the brain and pituitary gland, readily reactivates GnRH-induced LH and FSH secretion in photo-inhibited rams; and iii) treatment of HPD rams with a chronic pulsatile infusion of GnRH stimulates gonadotrophin secretion irrespective of photoperiod. Complementary studies conducted by others in the Syrian hamster, have shown that lesions in the MBH block the action of melatonin on gonadotrophin but not on prolactin secretion; this supports the "dual-site hypothesis". Since all photoperiodic mammals are essentially similar in hyper-secreting PRL under long days, the pituitary control mechanism for PRL is regarded as conserved (ancient) with the pleiotrophic actions of PRL inducing a summer physiology (e.g. growth of summer pelage). In contrast, the variation between species in the timing of the gonadal cycle indicates that evolution has independently modified the melatonin-sensitive neural circuits in the MBH to permit the species-specific timing of the mating season.

Tight association of the human Mel(1a)-melatonin receptor and G(i): precoupling and constitutive activity

If stably expressed in human embryonic kidney (HEK)293 cells, the human Mel(1a)-melatonin receptor activates G(i)-dependent, pertussis toxin-sensitive signaling pathways, i.e., inhibition of adenylyl cyclase and stimulation of phospholipase Cbeta; the latter on condition that G(q) is coactivated. The antagonist luzindole blocks the effects of melatonin and acts as an inverse agonist at the Mel(1a) receptor in both intact cells and isolated membranes. This suggests that the Mel(1a) receptor is endowed with constitutive activity, a finding confirmed on reconstitution of the Mel(1a) receptor with G(i). Because the receptor density is in the physiological range, constitutive activity is not an artifact arising from overexpression of the receptor. In addition, the following findings indicate that the Mel(1a) receptor forms a very tight complex with G(i) which can be observed both in the presence and absence of an agonist. 1) In intact cells and in membranes, high-affinity agonist binding is resistant to the destabilizing effect of guanine nucleotides. 2) The ability to bind an agonist with high affinity is preserved even after exposure of the cells to pertussis toxin, because a fraction of G(i) is inaccessible to the toxin in cells expressing Mel(1a) receptors (but not the A(1)-adenosine receptor, another G(i)-coupled receptor). 3) An antiserum directed against the Mel(1a) receptor coprecipitates G(i) even in the absence of an agonist. We therefore conclude that the Mel(1a) receptor is tightly precoupled and that its constitutive activity may play a role in pacing the biological clock, an action known to involve the melatonin receptors in the suprachiasmatic nucleus.

Differential signaling of human Mel1a and Mel1b melatonin receptors through the cyclic guanosine 3'-5'-monophosphate pathway

Cyclic guanosine 3'-5'-monophosphate (cGMP) has recently been shown to constitute a second messenger for Xenopus laevis melatonin Mel1c receptors. To verify whether cGMP levels are also modulated by mammalian melatonin receptors, we cloned the genes encoding the human Mel1a and Mel1b receptor subtypes and expressed them in human embryonic kidney cells. Pharmacological profiles and inhibition of forskolin-stimulated adenosine 3'-5'-cyclic monophosphate levels by melatonin confirmed functional expression of high-affinity melatonin receptors. Mel1b receptor-transfected cells modulated cGMP levels in a dose-dependent manner via the soluble guanylyl cyclase pathway. In contrast, Mel1a receptors had no effect on cGMP levels. These results demonstrate that mammalian melatonin receptors modulate cGMP levels and reveal for the first time differences in signaling between melatonin receptor subtypes, which may explain the necessity to express different receptor subtypes.

Melatonin receptors in neonatal pig brain and pituitary gland

Summer infertility remains a major problem in domestic pigs. It has been proposed that sows which display this trait are inherent seasonal breeders. The influence of photoperiod on domestic pigs has been difficult to ascertain as significant diurnal fluctuations in blood levels of the pineal hormone, melatonin, which provide a direct neuroendocrine transduction of the ambient photoperiod in other species, remain questionable in adult pigs. To investigate whether the pig is potentially receptive to melatonin, central sites of action for this hormone were localized and characterized within the brain and pituitary of the neonatal pig by in vitro autoradiography using 2-((125)I)iodomelatonin. Specific binding was distributed over a number of discrete regions of the brain including the cerebral cortex, hypothalamus, thalamus, brainstem, and cerebellum. The choroid plexus, and the pars tuberalis and pars distalis of the pituitary were also specifically labeled. Specific binding was completely abolished in the presence of 10(-7) M melatonin, and inhibited in the presence of 10(-4) M GTPgammaS (guanosine-5-0-(3-thiotriphosphate)), a non-hydrolysable analogue of GTP, in all regions examined, indicating that binding is representative of a G-protein coupled receptor. Characterization studies showed that 2-((125)I)iodomelatonin binding was saturable with a dissociation constant (Kd) in the low picomolar range (approximately 30 pM). Competition studies with iodomelatonin, melatonin, N-acetylserotonin and serotonin (5-HT) gave IC50 values similar to those previously characterized for the melatonin receptor in the ovine pars tuberalis.

Cloning and functional analysis of a polymorphic variant of the ovine Mel 1a melatonin receptor

We have isolated a novel variant of the Mel 1a melatonin receptor from an ovine PT cDNA library. Relative to the reported sequence for the Mel 1a melatonin receptor there are 8 changes in the DNA sequence. Only 3 of these result in amino acid substitutions, one in extracellular loop 3 and two in the carboxy-terminal tail. We have designated the novel variant of the sheep Mel 1a receptor Mel 1a(beta), and correspondingly the previously reported variant Mel 1a(alpha). As minor changes in the primary amino acid sequence of G-protein-coupled receptors can influence their functional characteristics we have accordingly characterized this novel variant of the Mel 1a melatonin receptor. This melatonin receptor displays high affinity binding and inhibits the cAMP second messenger pathway in transfected L-cells demonstrating that this receptor is fully functional. PCR analysis shows Mel 1a(beta) is present in several breeds of sheep and suggests that the Mel 1a(beta) receptor was established early in the evolution of the sheep species.