Distribution and characterization of the melatonin receptors in the hypothalamus and pituitary gland of three domestic ungulates

With some exceptions, in most of the mammals the pituitary pars tuberalis and the hypothalamic suprachiasmatic nuclei are reportedly the main targets for the pineal hormone melatonin. However, it is not known if the conspicuous diversity in the distribution pattern of melatonin binding sites in these areas depicts differences in reproductive behavior observed in the seasonally breeding species in the temperate zones. We explored the distribution and the characteristics of melatonin binding sites in the hypothalamus and pituitary of three species (bovine, horse, and donkey) different in terms of seasonal reproductive competence. The topographical localization, investigated by in vitro autoradiography, revealed 2-[I-125]iodomelatonin binding sites only in the pituitary gland in all three species, primarily in the pars tuberalis (PT), but also in the pars distalis (PD) and pars intermedia (PI). Kinetic, inhibition, and saturation studies, performed by means of in vitro binding, revealed presence of a single class high affinity binding sites. The K-d values, melatonin, and 2-iodomelatonin K-i values were in the low picomolar range. Coincubation with GTP gamma S inhibited 2-[I-125]iodomelatonin binding, demonstrating that these putative receptors are linked to a G protein in their signal-transduction pathway. The hypothalamus was devoid of specific binding. In conclusion, the results suggest that in these species, the hypophysis may be a principal target for the melatonin action on the reproductive system.