Mice and rats differ with respect to activity-dependent slowing of conduction velocity in the saphenous peripheral nerve.

We assess in mice, the electrophysiological criteria developed in humans and rats in vivo for unmyelinated (C) fibre differentiation into sub-classes, derived from the activity-induced latency increase ("slowing") in response to electrical stimulation during 6 min at 0.25 Hz followed by 3 min at 2 Hz. Fibres are considered nociceptors if they show more than 10% slowing at 2 Hz; nociceptors are further divided into mechanosensitive ("polymodal") and mechanoinsensitive ("silent") ones according to a latency shift of less and more than 1% during the first minute at 0.25 Hz, respectively. Sympathetic postganglionics are recognised by 2-10% slowing at 2 Hz; units slowing less than 2% at 2 Hz remain uncategorised. For assessment of these criteria, we also developed a novel in vivo technique for recording of peripheral single-fibres in the mouse. We compared the theoretical slowing-rate discriminator criteria with experimental data obtained from mice in vivo/in vitro and rats in vitro. Out of 69 cutaneous mouse C-fibres in vitro and 19 in vivo, only 38 (67%) and 9 (47%) met the above 1% criterion, respectively; sympathetics were not identified. In contrast, out of 20 rats nerve fibres in vitro, 19 (95%) met this criterion. We conclude that (A) our novel electrophysiological technique is a practical method for examining mouse cutaneous single-fibres in vivo and (B) the published criterion for identifying silent nociceptors in rats and humans is not applicable in mice.