Calcitonin gene-related peptide (CGRP) in the human spinal cord: a light and electron microscopic analysis

The distribution of CGRP immunoreactivity in the cervical, thoracic, lumbar, and sacral levels of the human spinal cord was mapped at the light microscopic level with the aid of a rabbit-generated antiserum against human calcitonin gene-related peptide (CGRP). CGRP-positive fibers formed a dense plexus in lamina I, II, the reticulated region of lamina V, and the tract of Lissauer at all spinal cord levels. The distribution of fibers showed some variations dependent on the cord level analyzed. At the light microscopic level, intervaricose fiber diameters consistently measured 1.0 micron or less, and two types of CGRP varicosities were observed: a small (1 to 2 microns in diameter), relatively round profile and a larger, (3 to 4 microns in diameter) oval or oblong profile. At the electron microscopic level, immunostained varicosities contained a mixture of round clear vesicles and vesicles that contained dense cores. The CGRP immunoreaction product was often associated with vesicles containing dense cores. The reaction product was also seen associated with clear round vesicles or in the cytoplasmic matrix. Postsynaptic elements included dendritic spines, small and large diameter dendritic shafts and vesicle containing profiles. The presence of CGRP in the superficial dorsal horn of human spinal cord is highly suggestive of a role in primary afferent transmission as postulated in lower vertebrates. This study establishes the distribution of CGRP at four different spinal levels in human cord and will serve as a basis for future studies related to the pathologic conditions affecting sensory systems.