Electrophysiological properties of neurones with CGRP-like immunoreactivity in rat dorsal root ganglia

Intracellular voltage recordings and fluorescent dye injections were made in vitro in 107 neurons in lumbar dorsal root ganglia (DRGs) of 6- to 8- week-old rats. Calcitonin gene-related, peptide-like immunoreactivity (CGRP- LI) was examined in these neurones, which were divided into C-, Aδ-, and Aα/β-fibre neurones on the basis of their conduction velocities (CVs). A- fibre neurones with CGRP-LI had significantly longer mean action potential (AP) and afterhyperpolarisation (AHP) durations than those without CGRP-LI. Aδ neurones with CGRP-LI had significantly longer AHP durations, slower CVs, and slower maximal fibre following frequencies than those without CGRP-LI. They also had longer AP durations (not significant). The largest Aδ neurones were CGRP-LI negative, whereas the smaller cells were either positive or negative. Aα/β neurones with CGRP-LI also had longer mean APs (not significant) and AHPs (significant) than those without CGRP-LI, and the cell size distributions were similar for positive and negative neurones. Most A- fibre neurones with CGRP-LI had inflections on the falling phase of the somatic AP. Of the A-fibre neurones with such inflections (A(i) neurones), those with CGRP-LI had longer AP durations (not significant) and longer AHP durations (significant) than A(i) neurones without CGRP-LI, pointing to a functionally distinct subgroup of A(i) neurones. There were no significant differences in electrophysiological properties or cell size measurements between C-fibre neurones with and without detectable CGRP-LI. The patterns of electrophysiological properties of Aδ neuronal somata with CGRP-LI and of most, but not all, Aα/β neuronal somata with CGRP-LI are similar to those reported for cutaneous nociceptors with A fibres in rat (Ritter and Mendell [1992] J. Neurophysiol. 68:2033-2041). Because rat DRG neurones that express CGRP normally also express trkA (Averill et al. [1995] Eur. J. Neurosci. 7:1484-1494), the properties described here of neurones with CGRP-LI are probably the same as those of DRG neurones with trkA.