[125I]iodomelatonin binding sites in the chicken spinal cord: binding characteristics and diurnal variation.

[125I]Iodomelatonin ([125I]MEL) binding in chicken spinal cords was characterized. The specific binding of [125I]MEL to the membrane preparation of spinal cord was rapid, stable, saturable, reversible, specific and of high affinity. Scatchard analysis of the specific binding data indicated an equilibrium dissociation constant (Kd) of 30.7 +/- 3.43 pM and a maximum number of binding sites (Bmax) of 1.69 +/- 0.14 fmol/mg protein in spinal cords collected at mid-light. The diurnal variation study showed that the Bmax was 32% higher (P < 0.05) at mid-light compared to mid-dark. The pharmacological characteristics demonstrated that [125I]MEL binding sites was highly specific for melatonin. Our results suggest that melatonin may exert a direct action on the spinal cord.