High resolution radioautographic localization of [125I]FK-33-824-labelled mu opioid receptors in the spinal cord of normal and deafferented rats.

Recent data have shown that [125I]D-Ala2, MePhe4, Met(o)ol5-enkephalin (FK-33-824) is a highly selective and specific mu opioid receptor ligand [Moyse et al. (1986) Peptides 7, 351-355]. This probe was used here to investigate the detailed radioautographic distribution of mu sites at various levels of the spinal cord. [125I]FK-33-824 binding sites were localized by both tritium-sensitive film and liquid emulsion radioautography in the spinal cord of naive and deafferented rats. In naive animals, high densities of mu sites were apparent within laminae I-II at all levels of the dorsal horn, with higher levels of labelling seen in layer IIi as compared to IIo in the lumbar segment. Laminae III-IV contained about half the quantities of binding observed in superficial layers. Relatively high densities of sites were also seen over lamina VI in the upper cervical cord and throughout Clarke's column. Within the latter, [125I]FK-33-824 binding clearly spared the large perikarya of the spinocerebellar neurons. In the ventral horn, [125I]FK-33-824 binding was mainly concentrated in layer IX, at the level of cervical and lumbar enlargements. Labelled sites were confined to the neuropil, mostly sparing the soma of motoneurons. Significant decreases in [125I]FK-33-824 binding in laminae I-II (55%) and III-IV (28%) were detected four days following cervical (C3-C7) or lumbar (L1-L6) rhizotomies. These decrements were most evident at seven days post-lesion at C3-C7 levels (93 and 76% in laminae I-II and III-IV, respectively) and recovered slightly thereafter up to 28 days post-lesion. In contrast, dorsal rhizotomies did not influence mu labelling in either the ventral horn or Clarke's column. These results confirm the association of mu opioid binding sites with dorsal primary afferent fibres and demonstrate the presence of mu sites in Clarke's column and lamina IX of the ventral horn. These findings suggest that endogenous opioids in the spinal cord play a role in sensory motor integration as well as in the modulation of primary nociceptive inputs.