ELECTROPHYSIOLOGICAL PROPERTIES OF NEURONS WITH CGRP-LIKE IMMUNOREACTIVITY IN RAT DORSAL-ROOT GANGLIA

Intracellular voltage recordings and fluorescent dye injections were m ade in vitro in 107 neurons in lumbar dorsal root ganglia (DRGs) of 6- to 8-week-old rats. Calcitonin gene-related, peptide-like immunoreact ivity (CGRP-LI) was examined in these neurones, which were divided int o C-, A delta-, and A alpha/beta-fibre neurones on the basis of their conduction velocities (CVs). A-fibre neurones with CGRP-LI had signifi cantly longer mean action potential (AP) and afterhyperpolarisation (A HP) durations than those without CGRP-LI. A delta neurones with CGRP-L I had significantly longer AHP durations, slower CVs, and slower maxim al fibre following frequencies than those without CGRP-LI. They also h ad longer AP durations (not significant). The largest A delta neurones were CGRP-LI negative, whereas the smaller cells were either positive or negative. A alpha/beta neurones with CGRP-LI also had longer mean APs (not significant) and AHPs (significant) than those without CGRP-L I, and the cell size distributions were similar for positive and negat ive neurones. Most A-fibre neurones with CGRP-LI had inflections on th e falling phase of the somatic AP. Of the A-fibre neurones with such i nflections (A(i) neurones), those with CGRP-LI had longer AP durations (not significant) and longer AHP durations (significant) than Ai neur ones without CGRP-LI, pointing to a functionally distinct subgroup of A(i) neurones. There were no significant differences in electrophysiol ogical properties or cell size measurements between C-fibre neurones w ith and without detectable CGRP-LI. The patterns of electrophysiologic al properties of A delta neuronal somata with CGRP-LI and of most, but not all, A alpha/beta neuronal somata with CGRP-LI are similar to tho se reported for cutaneous nociceptors with A fibres in rat (Ritter and Mendell [1992] J. Neurophysiol. 68:2033-2041). Because rat DRG neuron es that express CGRP normally also express trkA. (Averill et al. [1995 ] fur. J. Neurosci. 7:1484-1494), the properties described here of neu rones with CGRP-LI are probably the same as those of DRG neurones with trkA. (C) 1996 Wiley-Liss, Inc.