Cell types and axonal sizes of calcitonin gene-related peptide-containing primary sensory neurons of the rat.

Neurons with immunoreactivity (IR) for calcitonin gene-related peptide (CGRP) in the rat dorsal root ganglia were examined by immunoelectron microscopy. About one-half of the neurons of the L5 dorsal root ganglia of animals treated with colchicine had CGRP-IR, and 40% of these were large neurons of type A. The proximal parts of their peripheral axons were, however, unmyelinated (91%) or thinly myelinated (9%). Thickly myelinated axons observed in the same sections were always devoid of CGRP-IR. The CGRP-IR neurons were various subtypes of type A and B neurons. No specific morphological characteristics were associated with CGRP-IR.