Melatonin binding sites in brain of the 2-day-old chicken: an autoradiographic localisation

Melatonin, released in a circadian manner from the avian pineal gland into the circulation, is concentrated in distinct brain areas, possibly by receptor-mediated uptake mechanisms. For anatomical localisation of putative melatonin receptors in birds, cryostat-cut, coronal 15 microns brain sections of 2-day-old chicken were investigated by means of in vitro receptorautoradiography, using 125I-melatonin as a ligand. Binding occurred in brain structures receiving or mediating visual or auditory sensory input. Binding characteristics were investigated in homogenates of a brain region labelled by 125I-melatonin and showed to be specific and saturable (Kd: 87.2 pM; Bmax: 16.1 fmol/mg protein). Results confirm that a wealth of structures in chicken brain are equipped with putative melatonin receptors, including the visual suprachiasmatic nucleus. However, as results show that fewer structures were labelled in the brain of 2-day-old chicken as compared to a similar investigation conducted with 12-day-old chicken, developmental changes in distribution and/or density of putative melatonin receptors might be present.