Characterisation of a low-affinity melatonin binding site in Syrian hamster brain

Abstract

<p>The pharmacology of 2-(¹²⁵I) iodomelatonin ((¹²⁵I) MEL) binding was investigated in Syrian hamster brain regions. Both kinetic and saturation binding analysis revealed a site with an affinity of ≅2 nM in the hypothalamus. Correlation of the pharmacological profiles in different brain regions indicated this site also to be present in the cerebral cortex and hippocampus. This binding site appeared to be present throughout the CNS and peripheral tissues of the hamster and is distinct from the picomolar-affinity melatonin receptor. Using the technique of radiation inactivation, the molecular sizes of the chick retinal picomolar-affinity melatonin receptor (Mᵣ = 44 kDa) and hamster hypothalamic low (nanomolar)-affinity binding site (Mᵣ = 30 kDa) were found to be significantly (p<0.04) different. This, along with the pharmacological differences, supports the notion of two separate melatonin binding sites. Binding studies indicated that the continuous cell line RPMI 1846 (Syrian hamster melanoma) possesses a melatonin binding site similar to that found in the hamster CNS. A high correlation was observed for a series of compounds between the Kᵢ in hamster hypothalamic membranes vs RPMI 1846 membranes. Scatchard-Rosenthal analysis of saturation binding of (¹²⁵I) MEL to membranes indicated a site of similar affinity to that in brain. This cell line was used to study potential signal transduction mechanisms for the site. No effect of melatonin was seen on basal or forskolin-stimulated membrane adenylate cyclase activity. Using (³H) adenine and (³H) myo-inositol prelabelling of cells, melatonin was found to be without effect on in situ basal or stimulated cAMP or basal IP₃ levels. In ⁴⁵Ca-flux experiments, melatonin was without effect on basal or K⁺-stimulated uptake in hamster brain synaptosomes or basal uptake in RPMI 1846 cells. This nanomolar-affinity melatonin binding site therefore does not seem to be coupled to the adenylate cyclase or inositol phosphate second messenger systems and its biochemical and physiological function(s) remain(s) unknown.</p>