Expression of nanomolar-affinity binding sites for melatonin in Syrian hamster RPMI 1846 melanoma cells

Pharmacological studies indicate that Syrian hamster melanoma (RPMI 1846) cells posses a melatonin binding site similar to that found in normal hamster cells. A high correlation was observed for a series of compounds between the K i in hamster hypothalamic membranes vs. RPMI 1846 membranes ( r = 0.94, slope = 0.93, P < 0.01, n = 14). Scatchard analysis of saturation binding of 2-[ 125I]-iodomelatonin to membranes (at 0°C) indicated: K d = 0.89 ± 0.08 nM, B max = 6.2 ± 2.9 fmol/mg protein ( n = 3). Melatonin did not alter basal or forskolin-stimulated adenylate cyclase activity in RPMI 1846 membranes or intact cells. Therefore, in contrast to the picomolar-affinity receptor for melatonin in the mammalian hypothalamus and pars tuberalis, the putative nanomolar-affinity receptor is not coupled to adenylate cyclase. The RPMI 1846 cell line provides a useful model system for further studies of signal transduction via the nanomolar-affinity site for melatonin.