Inflammatory Pain Reduces C Fiber Activity-Dependent Slowing in a Sex-Dependent Manner, Amplifying Nociceptive Input to the Spinal Cord

Abstract

C fibers display activity-dependent slowing (ADS), whereby repetitive stimulation (≥1 Hz) results in a progressive slowing of action potential conduction velocity, which manifests as a progressive increase in response latency. However, the impact of ADS on spinal pain processing has not been explored, nor whether ADS is altered in inflammatory pain conditions. To investigate, compound action potentials were made, from dorsal roots isolated from rats with or without complete Freund's adjuvant (CFA) hindpaw inflammation, in response to electrical stimulus trains. CFA inflammation significantly reduced C fiber ADS at 1 and 2 Hz stimulation rates. Whole-cell patch-clamp recordings in the spinal cord slice preparation with attached dorsal roots also demonstrated that CFA inflammation reduced ADS in the monosynaptic C fiber input to lamina I neurokinin 1 receptor-expressing neurons (1–10 Hz stimulus trains) without altering the incidence of synaptic response failures. When analyzed by sex, it was revealed that females display a more pronounced ADS that is reduced by CFA inflammation to a level comparable with males. Cumulative ventral root potentials evoked by long and short dorsal root stimulation lengths, to maximize and minimize the impact of ADS, respectively, demonstrated that reducing ADS facilitates spinal summation, and this was also sex dependent. This finding correlated with the behavioral observation of increased noxious thermal thresholds and enhanced inflammatory thermal hypersensitivity in females. We propose that sex/inflammation-dependent regulation of C fiber ADS can, by controlling the temporal relay of nociceptive inputs, influence the spinal summation of nociceptive signals contributing to sex/inflammation-dependent differences in pain sensitivity.SIGNIFICANCE STATEMENT The intensity of a noxious stimulus is encoded by the frequency of action potentials relayed by nociceptive C fibers to the spinal cord. C fibers conduct successive action potentials at progressively slower speeds, but the impact of this activity-dependent slowing (ADS) is unknown. Here we demonstrate that ADS is more prevalent in females than males and is reduced in an inflammatory pain model in females only. We also demonstrate a progressive delay of C fiber monosynaptic transmission to the spinal cord that is similarly sex and inflammation dependent. Experimentally manipulating ADS strongly influences spinal summation consistent with sex differences in behavioral pain thresholds. This suggests that ADS provides a peripheral mechanism that can regulate spinal nociceptive processing and pain sensation.

Melatonin limits paclitaxel‐induced mitochondrial dysfunction in vitro and protects against paclitaxel‐induced neuropathic pain in the rat

“…Here, we demonstrate that in peripheral nerve tissue from paclitaxel‐treated rats, C‐fibres had significantly lower ADS, an effect that was prevented in those rats co‐treated with melatonin. We have very recently reported that C‐fibre ADS alters the temporal relay of pain input to the spinal cord and that a reduction in ADS, as we have observed in the paclitaxel model, facilitates central pain processing, and likely contributes to pain hypersensitivity …”

Morphological and functional properties distinguish the substance P and gastrin-releasing peptide subsets of excitatory interneuron in the spinal cord dorsal horn

“…For those involving SP neurons, we used Tac1 Cre mice that had received intraspinal injection of either AAV.flex.eGFP or AAV.flex.tdTom between 1 and 3 weeks previously. Spinal cord slices were obtained from 40 injected Tac1 Cre mice and from 105 GRP::eGFP mice aged 4 to 10 weeks old, as described previously [14,34]. The spinal cord was removed either following laminectomy performed under isoflurane anaesthesia, or in ice-cold dissection solution after decapitation of the mice under brief isoflurane or tribromoethanol anaesthesia.…”

“…Mice from which the cord was removed under anaesthesia were decapitated immediately afterwards. Parasagittal (300 – 500 μm), transverse (400 – 600 μm) or horizontal (400 μm) slices from lumbar spinal cord were cut with a vibrating blade microtome and allowed to recover in recording or modified dissection solution for at least 30 minutes at room temperature [14,34]. In some cases the slices were placed in a NaCl-based recovery solution or an N -methyl-D-glucamine (NMDG) -based recovery solution [64] at 32°C for 15 minutes before being placed in recording solution at room temperature.…”

Etiology and Pharmacology of Neuropathic Pain

“…This is compounded by epigenetic modifications in humans and by the limited ability of rodent models to predict clinical efficacy (Mogil, 2017;Patel et al, 2017;Sexton et al, 2017;Yekkirala et al, 2017). Despite the realization that pain processing in males differs from that in females (Mogil, 2012b;Mifflin and Kerr, 2013;Sorge et al, 2015;Dodds et al, 2016;Melchior et al, 2016;Dickie et al, 2017;Sorge and Totsch, 2017), many preclinical studies have been done on male rodents to avoid possible complications imposed by the estrous cycle. It is also established that neonatal injury can affect manifestation of pain in adults (Beggs et al, 2012a).…”