Quantitative autoradiographic distribution of calcitonin gene‐related peptide (hCGRPα) binding sites in the rat and monkey spinal cord

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Abstract
Calcitonin gene‐related peptide (CGRP) has been implicated in various spinal functions on the basis of its presence in the substantia gelatinosa and motoneurons and the biological effects induced by intrathecal CGRP injections. We investigated here the comparative distribution of [125I]hCGRPα binding sites in various segments of the rat and monkey spinal cord. The immunocytochemical localization of CGRP‐like material in rat spinal cord was also evaluated for comparison. In the rat spinal cord, high densities of [125I]hCGRPα binding sites were observed in lamina I, in a U‐shaped band that included lamina X and the medial parts of laminae III‐IV and in the intermediolateral and intermediomedial nuclei. The substantia gelatinosa (lamina II) contained relatively lower, but still significant, densities of [125I]hCGRPα binding sites, while the ventral horn showed low amounts of specific labeling. CGRP‐like immunoreactive fibers, on the other hand, were heavily concentrated in laminae I–II and in the reticulated portion of lamina V of the dorsal horn. Immunoreactivity to CGRP antiserum was also noted in fibers around the central canal and in a number of motoneurons of the ventral horn. In the monkey spinal cord, [125I]hCGRPα binding sites were present in lamina I in a U‐shaped band that included lamina X and the medial portions of laminae V–VI. Relatively low levels of [125I]hCGRPα binding were detected in laminae II to IV of the dorsal horn, while the ventral horn was more enriched with specific [125I]hCGRPα binding sites. Thus, it appears that the autoradiographic distribution of [125I]hCGRPα sites is species dependent in the spinal cord. Additionally, some differences are observed between the localization of [125I]hCGRPα binding sites and immunoreactive material in the rat spinal cord. These differences may be relevant to the purported roles of CGRP‐like peptides in spinal functions such as nociception, control of sympathetic output, and motor control.