In-Vivo Efficacy of Environmental Chemical & Novel Type Preferring Ligands at SCN Melatonin Receptors in Models of Chronobiological Behaviors
MetadataShow full item record
The goal of the present research was to investigate efficacy of environmental chemical and novel type preferring melatonin receptor ligands utilizing in-vivo rodent models of chronobiological behaviors. Environmental chemical and novel type preferring compounds tested were previously identified using in-silico molecular modeling and screening strategies using ultra large libraries for docking predictions based on receptor models as well as crystal structures of MT1 & MT2 melatonin receptors. Compounds analyzed were also previously tested in-vitro for affinity and canonical signaling through melatonin receptors to predict in-vivo responses in circadian models of behavior sensitive to melatonin receptor compounds. Environmental chemicals (carbamate insecticides, phthalate plasticizers) acting on melatonin receptors have recently been discovered and described by our team. Epidemiological data regarding exposure to these compounds suggests that melatonin receptors may be relevant to unexplained disease pathologies linked to consequences of circadian disturbances. Therefore the ability for several molecules from two classes of environmental compounds shown to interact with melatonin receptors (carbamate insecticides, phthalate plasticizers) were tested for their ability to compete for endogenous melatonin receptors in the suprachiasmatic nucleus of the hypothalamus (SCN), paraventricular nucleus of the thalamus (PVNT), and the pars tuberalis (PT). Results indicate that several environmental chemicals from both classes identified (carbaryl, fenobucarb, bendiocarb, benzylbutyl phthalate, diphenyl phthalate, dibutyl phthalate, diethyl phthalate) compete for melatonin receptor binding in brain slices containing the SCN, PVNT, and PT from C3H/HeN mice. Top candidate from the carbamate insecticide class, carbaryl, further demonstrated brain penetration as well as specificity at brain melatonin receptors in the SCN. Carbaryl displayed the same efficacy and time of day sensitivity as melatonin acting as an agonist in-vivo to phase shift circadian activity rhythms at circadian time (CT) 2 (delay) & 10 (advance). These actions were via actions at MT1 and MT2 melatonin receptors. Further in support of agonist action in-vivo, carbaryl and melatonin co-treatment was able to reduce phase shifts induced by light pulses given at CT 14. Lastly, carbaryl was able to decelerate the rate of re-entrainment to a 6 hour shift of the light-dark cycle in a jet-lag paradigm. Therefore carbaryl appears to have unique time of day & paradigm specific efficacies for melatonin receptor mediated in-vivo effects as agonist for phase shift (CT 10 & 2) and as an inverse agonist for re-entrainment in the jet lag paradigm. The efficacy of novel type preferring compounds designed to target MT1 (inverse agonist) vs. MT2 (agonist) melatonin receptors were also tested in circadian phase shift and re-entrainment behavioral paradigms. Results revealed that two type preferring MT1 inverse agonists displayed unique efficacy profiles across behavioral tests as agonist/inverse agonists for CT 10 phase shift and re-entrainment of circadian running wheel activity respectively. In contrast to melatonin and carbaryl, a type preferring MT1 compound was not able to phase delay circadian RW activity at CT 2 suggesting possible antagonist efficacy. Lastly, a type preferring MT2 agonist was not able to phase shift circadian activity rhythms when given at CT10 reinforcing earlier work showing that the behavioral response requires MT1. Efficacy for both environmental chemical and novel melatonin receptor compounds displayed a diverse, full range of efficacies from agonist to inverse agonist to antagonist across behavioral paradigms of melatonin receptor mediated effects on chronobiological behaviors. Diverse efficacies of carbaryl and MT1 type preferring compounds suggests additional non-canonical signaling pathways affected in the utilized behavioral paradigms that depend on time of administration. These results reveal that in-vivo behavioral efficacy of melatonin receptor ligands is complex and that future screening efforts may benefit from screening compounds for time of day specific signaling pathway efficacy beyond canonical signaling for melatonin receptors in native tissue systems.