Plasticity of &mgr; and delta Opioid Receptors in the Superficial Dorsal Horn of the Adult Rat Spinal Cord Following Dorsal Rhizotomies: A Quantitative Autoradiographic Study

Abstract

The aim was to study the regulation of &mgr; and delta opioid binding sites in the superficial layers (laminae I - II) of the dorsal horn of the adult rat spinal cord 1, 2, 4 and 12 weeks after unilateral dorsal rhizotomies of various extents. Using quantitative autoradiography and highly selective tritiated opioid ligands, we have shown that the decrease in [3H]Tyr*-d-Ala-Gly-NMe-Phe-Gly-ol ([3H]DAMGO) (&mgr; sites) and [3H]Tyr*-d-Thr-Gly-Phe-Leu-Thr ([3H]DTLET) (delta sites) binding in the side ipsilateral to the lesion as compared to the intact side is related to the number of dorsal roots cut. In the segment central to the lesion, 1 week after the lesion, ipsilateral/contralateral side binding ratios for [3H]DAMGO were 0.70, 0.49, 0.36 and 0.25 when 1, 3, 5 and 7 roots respectively were sectioned. For [3H]DTLET, the ratios were 0.71, 0.54, 0.42 and 0.39. The time-related analysis of binding ratios showed that, in partially deafferented spinal segments after long-term deafferentation (12 weeks postlesion) there were greater numbers of &mgr; and delta binding sites than in cases of short-term deafferentation (1 - 2 weeks). By contrast, in spinal segments considered as completely deafferented, there was no difference in the remaining &mgr; and delta binding sites at 12 weeks as compared to 1 week postlesion. Consequently, it is deduced that the partial recovery of &mgr; and delta binding observed after long-term partial deafferentation could be associated with neuronal plasticity (probably collateral sprouting) of fine diameter primary afferent fibres arising from intact dorsal roots.