Combined gonadal and photic influences on 2-[125I] iodomelatonin-binding level changes in some brain areas of the quail

Quantitative receptor autoradiography was used to study the binding of 2‐[125I] iodomelatonin in the brain of the castrated and gonadally intact male Japanese quail Coturnix japonica exposed to both long‐ and short‐day photoperiod cycles. The distribution study displayed that these conditions were responsible for a heterogeneous binding pattern as shown by elevated receptor levels being located in visual brain centers, such as the stratum opticum, nucleus pretectalis, and nucleus geniculatus lateralis, pars ventralis, while lower values were found in the nucleus lateralis hypothalami and nucleus isthmi pars magnocellularis. Closer examination of the 2‐[125I] iodomelatonin–binding pattern following the different gonadal and photic influences showed that combination of the gonadally intact condition and a 16L:8D (long‐day) photoperiod cycle was required for the greater binding changes. These differences occurred in brain sites such as the area preoptica, paleostriatum primitivum, and nucleus ectomamillaris. Saturation binding studies, which were carried out only in some of the above areas, revealed that the combined gonadal and photic‐induced changes are basically due to the modifications of total number of binding sites. The importance of a gonadal steroid modulatory role in the photic‐dependent melatonin binding activity suggests that other types of neuronal mechanisms might be involved in the regulation of neuroendocrine and socio–sexual behaviors in nonmammalian vertebrates. © 1994 Wiley‐Liss, Inc.