MELATONIN RECEPTORS IN RED DEER FETUSES (CERVUS-ELAPHUS)

Red deer (Cervus elaphus) exhibit highly seasonal rhythms in physiolog y and behaviour. The influence of photoperiod on the timing of these c hanges begins in utero where the fetus receives photoperiodic informat ion via the diurnal pattern of maternal melatonin secretion. The poten tial sensitivity of deer fetuses to melatonin was ascertained by mappi ng specific receptors and characterizing them using 2-[I-125]iodomelat onin and quantitative autoradiography in vitro. Specific binding occur red from day 31 of gestation onwards (term = 233 days) over the spinal nerves and respiratory system. At later stages of gestation binding o ccurred over the brain, particularly the brainstem, the pituitary glan d, thyroid gland, gastrointestinal tract including the pancreas, metan ephros, cochlea of the ear, spinal cord, and spinal and cranial nerves . Binding was abolished in the presence of 10(-7) mol mol melatonin l( -1) and diminished in the presence of 10(-4) mol GTP gamma S l(-1) (gu anosine-5-O-(3-thiotriphosphate)), confirming that binding represented functional G-protein-coupled melatonin receptors. Characterization st udies, carried out on fetal lung, revealed that binding was time-depen dent, reaching equilibrium at about 3 h at room temperature (22 degree s C), and saturable with a dissociation constant (K-d) of 104 pmol l(- 1). This study demonstrates the presence of G-protein-coupled melatoni n receptors over a wide range of tissues in red deer fetuses from earl y in gestation, indicating that in addition to its role in the communi cation of photoperiodic information to the fetus in this species, mela tonin may be involved in fetal growth and development.