PHARMACOLOGICAL PROFILE AND DIURNAL RHYTHMICITY OF 2-[I-125]-IODOMELATONIN BINDING-SITES IN MURINE MAMMARY TISSUE

Recent studies demonstrated that melatonin treatment decreased the gro wth of mammary glands in pubertal and pregnant mice. In vitro, melaton in inhibited murine mammary gland growth at mu M concentrations and in creased it at pM concentrations. Melatonin-induced changes of cyclic n ucleotide synthesis was also demonstrated in mammary gland slices in v itro. The objective of the present study was to assess the possible ex istence of specific binding sites for melatonin in murine mammary glan d by using 2-[I-125]-iodomelatonin as a probe. The specific binding of 2-[I-125]-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 in dicated the existence of a single class of binding sites that exhibite d a diurnal rhythmicity in affinity (K-d) and receptor density (B-max) . A maximum in B-max (267 +/- 42 fmol/mg protein) was found at the lig ht period, while affinity was maximal during darkness (K-d = 1.33 +/- 0.22 nM). In competition studies dopamine and dopamine-related agents, as well as 6-hydroxymelatonin and serotonin, but not melatonin, effec tively displaced 2-[I-125]-iodomelatonin from mammary binding sites. T he results demonstrated a specific binding of 2-[I-125]-iodomelatonin to murine mammary glands, with affinity in the low nM range, and a pha rmacological profile that differed from that reported for 2-[I-125]-io domelatonin acceptor sites in other tissues.