Effect of pinealectomy and the light/dark cycle on 2-[¹²⁵I]iodomelatonin binding in the chick optic tectum

1. These studies investigated the regulation of melatonin receptor sites within the chick brain by the light/dark cycle and by endogenous melatonin using quantitative autoradiography. Saturation analyses performed in coronal brain sections of the optic tectum revealed a single class of high-affinity binding sites. 2. For diurnal rhythm studies, chicks were maintained on a 14:10 L:D cycle (light on 0400) and sacrificed at 4-h intervals. No significant variation was observed in either the affinity (K_D = 67.8-76.4 p M) or the density (B_MAX = 88.9-118.9 fmol/mg protein) of 2-[¹²⁵I]iodomelatonin binding sites as a function of time of day. 3. In pinealectomized or age-matched sham-operated chicks, no changes were found in either the affinity (K_D = 66.7-89.1 p M) or the density (B_MAX = 112.8-180.4 fmol/mg protein) of 2-[¹²⁵I]iodomelatonin binding sites at either 1, 7, or 14 days following surgery. These data suggest that endogenous pineal melatonin may not play a role in the regulation of melatonin receptor site affinity or density in the chick optic tectum.