Opiate receptors in the human spinal cord: A detailed anatomical study comparing the autoradiographic localization of [ 3H]diprenorphine binding sites with the laminar pattern of substance P, myelin and nissl staining

Abstract

The anatomical localization of opiate receptors in the human spinal cord has been examined in six cases aged 7–41 years using quantitative autoradiographic methods following the incubation of fresh, unfixed cryostat sections with [ 3H]diprenorphine. In order to precisely localize the distribution of receptors in the spinal cord, the laminar anatomy of the spinal grey was demonstrated at each spinal level examined using 50-μm sections stained for myelin, Nissl substance and substance P. In all cases, autoradiograms demonstrated that opiate receptors were distributed in a similar fashion in the grey matter of the cervical, thoracic, lumbar, sacral and coccygeal regions of the human spinal cord. At all 25 spinal levels examined, opiate receptors were mainly localized within the upper laminae of the dorsal horn (laminae I–III) and within the tract of Lissauer. The highest density of opiate receptors was localized within the inner segment of lamina II where the receptors formed a very dense band lying immediately dorsal to lamina III. The density of receptors in this inner region of lamina II (33 ± 2fmol/mg) was more than two-and-one-half times greater than that in the remaining upper laminae which showed moderate receptor densities: lamina 1(12 ± 4fmol/mg) and outer lamina II (13 ± 3fmol/mg) both showed similar receptor densities which were higher than those in lamina III (10 ± 3fmol/mg). The tract of Lissauer (11±2fmol/mg) also showed a moderate density of opiate receptors which was intermediate between the densities in laminae I/II o and the density of lamina III. The density of receptors in the remaining laminae of the spinal cord varied from moderately low to virtually zero. Moderately low densities of receptors were found in laminae V, VI, VIII, IX and X with very low levels within laminae IV and VII. In particular, in lamina VII opiate receptors were unable to be detected above normal background levels in the dorsal nucleus of Clarke. These results show that, as in other mammalian species, opiate receptors in the human spinal cord are mainly concentrated in the upper laminae of the dorsal horn and in the tract of Lissauer. The possible role of these receptors in modulating spinal nociceptive information is discussed with respect to previous findings on the relationship of opiate receptors to primary afferent fibres in the spinal cord.