Time-related decreases in μ and δ opioid receptors in the superficial dorsal horn of the rat spinal cord following a large unilateral dorsal rhizotomy

Abstract

The aim of the present study was to measure the time-related modifications of μ and δ opioid binding sites in the superficial layers of the dorsal horn of the rat spinal cord after a C₄−T₂ unilateral dorsal rhizotomy. Using specific ligands, namely [³H]DAMGO for μ sites and [³H]DTLET for δ sites, and a quantitative autoradiographic analysis, we have observed: (a) a decrease in binding on the ipsilateral side to the lesion as early as the first day postrhizotomy, the maximal loss being attained at 8 days postlesion, (b) after 8 days postlesion, the residual binding remains stable over the period of analysis (90 days), (c) the loss of μ receptors (71–74%) is significantly more pronounced than the loss of δ receptors (57–62%) and (d) affinities of postsynaptic μ and δ receptors are similar to those of the total receptor population in the superficial layers of the dorsal horn. Comparison of these results with the degeneration of primary afferent fibers reported in literature favors the localization of the majority of μ and δ opioid binding sites on fine diameter primary afferent fibers.