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.