Differing action potential shapes in rat dorsal root ganglion neurones related to their substance P and calcitonin gene-related peptide immunoreactivity

Intracellular electrophysiological recordings were made in vitro at 36.5°C from lumbar (L4 to L6) dorsal root ganglion neurones of 6-8-week-old female rats. Electrophysiological properties were recorded prior to intracellular injection with fluorescent dye. The following showed substance P-like immunoreactivity (SP-LI): 8/19 C-fibre cells, 6/26 Aδ cells, and 0/52 Aα/β cells. In C-fibre neurones, there were no significant differences in action potential (AP) characteristics between those with SP-LI and those without. In contrast, the Aδ neurones with SP-LI had significantly deeper and longer afterhyperpolarisations (AHPs), but their AP durations did not differ from those without SP-LI. Both SP-LI and calcitonin gene-related peptide-LI (CGRP-LI) were examined on 18 Aδ and 11 C cells. Most (7/8) neurones with SP-LI also showed CGRP-LI, but only 7/21 neurones without SP- LI showed CGRP-LI. One C cell showed SP-LI but no CGRP-LI. Neurones with neither peptide (-/-), with only CGRP (-/CGRP) or with both peptides (SP/CGRP) were compared (n = 28). The numbers in each group were, respectively, 5, 2, and 3 with C-fibres and 9, 5, and 4 with Aδ-fibres. The peptide content and AP shape were related in Aδ neurones. Most -/- Aδ neurones had short APs and short AHPs; most -/CGRP neurones had long APs and long AHPs, and SP/CGRP neurones had short APs with deep, long AHPs. There was a positive correlation between log₁₀ of the area under the AP (AP area) and log₁₀ AHP duration in Aδ neurones. All SP- and most CGRP-containing Aδ neurones had AP shapes similar to those previously described for nociceptive neurones. However, a few without peptide also showed such properties, raising the possibility that some nociceptive neurones did not express these peptides.