Identification and characterization of melatonin binding sites in the gastrointestinal tract of ducks

Utilizing 2-[125I]iodomelatonin as the radioligand, melatonin binding sites were identified and characterized in the jejunum of ducks. These sites were found to be reversible, saturable, specific and exhibited high affinity for melatonin. Scatchard analyses have established the equilibrium dissociation constant (Kd) for tissues collected during mid-photophase to be 40.9 +/- 7 pM and the maximum quantity of binding sites (Bmax) to be 2.0 +/- 0.4 fmol/mg protein while Kd of samples collected during mid-scotophase was found to be 54.1 +/- 10 pM with a corresponding Bmax of 1.5 +/- 0.3 fmol/mg protein. These Kd values are in good proximity to the kinetically derived equilibrium dissociation constant of 47.3 +/- 20 pM. No significant difference in Kd or Bmax was detected between the mid-light and mid-dark samples. Pharmacological profile of these binding sites, developed by their interactions with other indoles and compounds, indicated that these binding sites are highly specific for melatonin. Subcellularly, different densities of binding sites were localized to various fractions in the following order: nuclear greater than microsomal greater than mitochondrial greater than cytosolic. These binding sites in the jejunum might be the receptors accountable for promoting paracrine activities for the locally synthesized gastrointestinal melatonin and/or responsible for eliciting hormonal actions via interactions with melatonin of pineal origin.