TY - JOUR T1 - GR196429: A Nonindolic Agonist at High-Affinity Melatonin Receptors JF - Journal of Pharmacology and Experimental Therapeutics JO - J Pharmacol Exp Ther SP - 1239 LP - 1245 VL - 285 IS - 3 AU - Isabel J. M. Beresford AU - Christopher Browning AU - Sarah J. Starkey AU - Jason Brown AU - Steven M. Foord AU - Josephine Coughlan AU - Peter C. North AU - Margarita L. Dubocovich AU - Russell M. Hagan Y1 - 1998/06/01 UR - http://jpet.aspetjournals.org/content/285/3/1239.abstract N2 - N-[2-[2,3,7,8-tetrahydro-1H-furo(2,3-g)indol-1-yl]ethyl]acetamide (GR196429) is a novel, nonindolic melatonin receptor agonist. GR196429 had high affinity for human mt1 (pKi 9.9) and MT2 (pKi 9.8) receptors expressed in Chinese hamster ovary cells and for 2-[125I]-iodomelatonin binding sites in human cerebellum, guinea pig superior colliculus and hypothalamus and chicken retina and tectum (pKi 8.8–9.5). GR196429 was inactive at a wide range of other hormone and neurotransmitter receptors. In Chinese hamster ovary cells expressing human mt1 or MT2 receptors, both melatonin and GR196429 dose-dependently inhibited forskolin-stimulated cAMP accumulation. In rabbit isolated retina, GR196429 inhibited calcium-dependent [3H]-dopamine release with potency (IC50 30 pM) and maximum effect (76 ± 5% at 1 nM) similar to those of melatonin. The response was antagonized by the melatonin receptor antagonist luzindole (1 μM). In slices of rat brain suprachiasmatic nucleus, perfusion (1 h) with GR196429 at zeitgeber time 10 phase advanced the circadian peak in neuronal activity measured on the following day, with a maximum phase advance of 2.7 ± 0.3 h at 10 pM and an EC50 of 0.6 pM, results that indicated a melatonin-like action on the phase of the circadian clock. CNS penetration and duration of receptor occupancy was determined in anex vivo radioligand binding assay. In membranes of guinea pig superior colliculus prepared 30 min after administration of GR196429 (s.c.), 2-[125I]-iodomelatonin binding was inhibited with an ED50 of 0.04 mg/kg. After a dose of 1 mg/kg, binding was significantly inhibited for at least 3 h. Thus GR196429 is a potent and selective agonist at high-affinity melatonin receptors, which modulates circadian rhythms in an in vitromodel of the circadian clock and which readily penetrates the CNS. The American Society for Pharmacology and Experimental Therapeutics ER -