In vitro activity: Ramelteon inhibits forskolin-stimulated cAMP production with IC50 of 21.2 pM in CHO cells. Ramelteon has high affinity with recombinant human MT1 and MT2 receptors with pKi of 10.05 and 9.70, respectively. Ramelteon inhibits Xenopus laevis melanophore pigment granule aggregation with pEC50 of 11.48. Ramelteon (1 nM) increases ERK1/2 phosphorylation not only in MT1/MT2 cerebellar granule cells but also in cerebellar granule cells expressing only one of the two melatonin receptors. 4P-PDOT blocks the stimulatory action of Ramelteon (1 nM) in MT1 KO cerebellar granule cells, while luzindole attenuates the action of Ramelteon (1 nM) in MT2 KO cerebellar granule cells. Ramelteon (100 μM) induces any pigment dispersion while melatonin completely disperses aggregated melanophores at 10 μM.

Kinase Assay: cDNA encoding the human MT1 gene is introduced into CHO cells. Cells are harvested at confluence in Ca2+ and Mg2+ free Hanks’ balanced salt solution containing 5 mM EDTA and collected by centrifugation. Cells are homogenized in ice-cold 50 mM Tris–HCl buffer, washed twice, pelleted, and stored at -30°C until the binding assays are conducted. Test compound and 40 pM 2-[ 125I]melatonin are mixed with the thawed homogenate in a total volume of 1 mL and incubated at 25°C for 150 min. The reaction is terminated by addition of 3 mL of icecold buffer followed by vacuum filtration on a Whatman GF/B. The filter is washed twice and radioactivity is counted by a g-counter.

Cell Assay: Ramelteon shows very high affinity for human melatonin1 and melatonin2 receptors (expressed in CHO cells), and chick forebrain melatonin receptors (consisting of melatonin1 and melatonin2 receptors) with Ki values of 14.0, 112, and 23.1 pM, respectively. The affinity of ramelteon for hamster brain melatonin3 binding sites is extremely weak (Ki: 2.65 μM) compared to melatonin's affinity for the melatonin3 binding site Ki: 24.1 nM). In addition, ramelteon shows no measurable affinity for a large number of ligand binding sites (including benzodiazepine receptors, dopamine receptors, opiate receptors, ion channels, and transporters) and no effect on the activity of various enzymes. Ramelteon inhibits forskolin-stimulated cAMP production in the CHO cells that express the human melatonin1 and melatonin2 receptors.