Pre- and postnatal development of opiate receptor subtypes in rat spinal cord

We have studied the developmental expression of opiate binding sites in the rat spinal cord at various prenatal and postnatal stages. For each developmental stage, we have compared the expression pattern of κ receptors with that of μ and δ receptor subtypes. Both μ and κ receptors appear relatively early during spinal cord ontogeny (from the 15th prenatal day), while δ sites are expressed later at the postnatal period (starting at the 1st postnatal day). The number of κ sites predominates throughout the development (55-80% of total opiate sites) with two peaks of binding activity: one at the 20th gestational day, and the other around the 7th postnatal day. μ sites represent 20-38% of the total opiate receptor population with one peak of binding activity appearing at the 1st postnatal day. The densities of μ and κ receptors at the adult stage are lower by 40-50% than the peak values observed at the early postnatal periods. The relative amounts of δ sites remain low throughout the ontogeny (4-8% of the total opiate sites). The binding properties of neonatal (1 day after birth) κ sites (ligand binding affinities, regulation of agonist binding by guanosine triphosphate and various cations) are similar to those displayed by κ receptors in adult spinal cord.