RT Journal Article SR Electronic T1 GR196429: A Nonindolic Agonist at High-Affinity Melatonin Receptors JF Journal of Pharmacology and Experimental Therapeutics JO J Pharmacol Exp Ther FD American Society for Pharmacology and Experimental Therapeutics SP 1239 OP 1245 VO 285 IS 3 A1 Isabel J. M. Beresford A1 Christopher Browning A1 Sarah J. Starkey A1 Jason Brown A1 Steven M. Foord A1 Josephine Coughlan A1 Peter C. North A1 Margarita L. Dubocovich A1 Russell M. Hagan YR 1998 UL http://jpet.aspetjournals.org/content/285/3/1239.abstract AB 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