Localisation of melanin-concentrating hormone receptor 1 in rat brain and evidence that sleep parameters are not altered despite high central receptor occupancy

The present study describes the optimisation of an autoradiography assay that provides a means to measure the in vitro potency of melanin-concentrating hormone receptor 1 (MCH1) antagonists in native tissues and their ex vivo receptor occupancy. Initial localisation studies demonstrated that the MCH1 receptor radioligand [125I]-S36057 bound to rat caudate putamen with specific binding of consistently > 60%. In vitro, the MCH1 receptor antagonists GW3430, SNAP-94847 and 4′-{[1-(cyclopropylmethyl)piperidin-4-ylidene] [5-fluoro-6-(trifluoromethyl)-1H-benzimidazol-2-yl]methyl}biphenyl-3-carbonitrile (referred to as Compound A) exhibited concentration dependent inhibition of the specific binding of [125I]-S36057, with a rank order of affinity of SNAP-94847 > Compound A > GW3430. In an ex vivo occupancy assay, Compound A dosed orally to rats caused a concentration dependent inhibition of the specific binding of [125I]-S36057 to rat caudate putamen. The occupancy reached 87 ± 11% at 30 mg/kg and the estimated ED50 was 9.3 mg/kg, which was equivalent to a free plasma concentration of 40 nM. As MCH has been reported to play a role in the regulation of the sleep cycle, the effect of Compound A on sleep parameters was investigated. However Compound A, at exposures that achieved near maximal receptor occupancy, failed to demonstrate any effects on the sleep/wake pattern in telemetered rats. We conclude that our ex vivo receptor occupancy assay is suitable for selecting centrally penetrant MCH1 receptor antagonists and that, despite high levels of receptor occupancy, the selective MCH1 receptor antagonist Compound A failed to elicit any changes in sleep electroencephalogram (EEG) parameters.