Pharmacological profile and diurnal rhythmicity of 2-[125I]-iodomelatonin binding sites in murine mammary tissue

Abstract

Recent studies demonstrated that melatonin treatment decreased the growth of mammary glands in pubertal and pregnant mice. In vitro, melatonin inhibited murine mammary gland growth at microM concentrations and increased it at pM concentrations. Melatonin-induced changes of cyclic nucleotide synthesis was also demonstrated in mammary gland slices in vitro. The objective of the present study was to assess the possible existence of specific binding sites for melatonin in murine mammary gland by using 2-[125I]-iodomelatonin as a probe. The specific binding of 2-[125I]-iodomelatonin to murine mammary gland membranes was rapid, saturable, and reversible, showed an affinity in the low nM range, and displayed time, temperature, and pH dependence. Scatchard analysis indicated the existence of a single class of binding sites that exhibited a diurnal rhythmicity in affinity (Kd) and receptor density (Bmax). A maximum in Bmax (267 +/- 42 fmol/mg protein) was found at the light period, while affinity was maximal during darkness (Kd = 1.33 +/- 0.22 nM). In competition studies dopamine and dopamine-related agents, as well as 6-hydroxymelatonin and serotonin, but not melatonin, effectively displaced 2-[125I]-iodomelatonin from mammary binding sites. The results demonstrated a specific binding of 2-[125I]-iodomelatonin to murine mammary glands, with affinity in the low nM range, and a pharmacological profile that differed from that reported for 2-[125I]-iodomelatonin acceptor sites in other tissues.