Organization of calcitonin gene‐related peptide‐immunoreactive terminals in the primate dorsal horn

The present paper is concerned with the arrangement of axons and synaptic terminals immunostained for calcitonin gene‐related peptide (CGRP), a primary afferent marker, in the primate (Macaca fascicularis) dorsal horn. The CGRP axons and terminals are uniformly distributed in laminae I and II outer (o) but they are concentrated laterally and distributed intermittently in the reticulated region of lamina V. A prominent bundle of labeled axons is seen in the sacral cord dorsal to the central canal. Emphasis is given to the relation of CGRP‐immunoreactive terminals to other terminals, both labeled and unlabeled, in laminae I and IIo. In this regard, adjacent CGRP‐immunoreactive terminals are often united by puncta adhaerentia. Of particular interest is the observation that CGRP‐immunoreactive terminals can be found presynaptic to other terminals which sometimes resemble central primary afferent endings. In addition CGRP‐immunoreactive terminals end on other CGRP terminals. Both findings suggest that primary afferent terminals interact synaptically with other primary afferent terminals.