Benzodiazepine receptors in the human spinal cord: a detailed anatomical and pharmacological study

The anatomical distribution and pharmacological characteristics of benzodiazepine receptors in the human spinal cord were examined in four cases aged 20-41 years using in vitro autoradiography and biochemical assays of [3H]flunitrazepam binding. In all cases, the autoradiograms demonstrated that benzodiazepine receptors were distributed in a consistently similar fashion in the gray matter of the cervical, thoracic, lumbar and sacral regions of the human spinal cord. At all levels, the highest densities of benzodiazepine receptors were found to be localized within lamina II of the dorsal horn as defined on cytoarchitectonic, myeloarchitectonic and substance P immunocytochemical criteria. Within this lamina the receptors were concentrated mainly in its deeper, inner portion which lies immediately adjacent to lamina III, with some overlap dorsally into the outer segment of lamina II and ventrally into the adjacent region of lamina III. The lowest density of receptors was found in regions of laminae I, IV, VII and X; in particular, in lamina VII the lowest concentration of receptors was found in the dorsal nucleus of Clarke and the sacral parasympathetic nucleus. The remaining laminae of the spinal gray (laminae, V, VI, VIII and IX) showed a moderate density of receptors. Biochemical assays of membranes prepared from the lumbosacral cord indicated that these [3H]flunitrazepam binding sites have high affinity and have the pharmacological characteristics of the "central" Type II benzodiazepine receptor. These results show a high concentration of Type II benzodiazepine receptors in the substantia gelatinosa of the human spinal cord and suggest a possible role for these receptors in spinal sensory functions.