Calcitonin gene-related peptide-immunoreactivity in functionally identified primary afferent neurones in the rat

In anaesthetized rats, intracellular recordings were made from the somata of lumbar (L4 and L5) dorsal root ganglion cells. The impaled afferent units were first functionally classified by testing the peripheral receptive endings with mechanical stimuli and then iontophoretically injected with a fluorescent dye. Serial sections of the dorsal root ganglion containing the injected soma were incubated with an antibody solution against calcitonin gene-related peptide (CGRP). Somata displaying calcitonin gene-related peptide-immunoreactivity (CGRP-IR) possessed receptive endings in the skin and deep somatic tissues (muscle, fascia, tendon, joint). The majority of calcitonin gene-related peptide-immunoreactive (CGRP-ir) neurones had conduction velocities below 2.5 m/s; only a few neurones conducted faster than 10 m/s. The immunostained somata were small to medium-sized (cross-sectional area < 1200 microns 2). With one exception, CGRP-IR was found in all types of ending studied, but the proportion of CGRP-ir neurones differed. Immunostained somata were rare among cutaneous and deep low-threshold mechanosensitive units (e.g. hair follicle and muscle spindle units). CGRP-ir somata were most frequent among high-threshold mechanosensitive (presumably nociceptive) afferent neurones (four out of six cells). The data suggest that CGRP can be expressed not only in nociceptive but also in many other types of primary afferent neurone, the condition being that the conduction velocity is slow and/or the cell soma small.