Calcitonin gene-related peptide containing primary afferent fibers synapse on primate spinothalamic tract cells.

Spinothalamic tract (STT) cells were identified by intracellular injection or retrograde labeling with horseradish peroxidase (HRP) in the primate spinal cord. Using immunohistochemical techniques, a population of primary afferents containing calcitonin gene-related peptide (CGRP) is demonstrated in synaptic contact with these neurons. Large glomerular type CGRP terminals with morphology considered typical of primary afferent fibers are occasionally observed in contact with STT profiles in laminae I and IIo; however, this morphological type targets STT profiles chiefly in the deeper dorsal horn (laminae III, IV and V). In contrast, the majority of CGRP terminals contacting STT profiles in the superficial dorsal horn (laminae I and IIo), are small, round or oblong shaped terminals. Thus, evidenced by these data, the absence of large, glomerular type terminals does not rule out primary afferent input.