WHY DO AXOTOMISED C-FIBER NEURONS/NOCICEPTORS NOT FIRE SPONTANEOUSLY DESPITE THEIR LOSS OF MEMBRANE-ASSOCIATED TREK2 AND DEPOLARISED EMS?

ACOSTA, CG.; L. DJOUHRI; SALLY LAWSON

Buenos Aires

Congreso; 15th World Congress on Pain; 2014

IASP

Aim of Investigation: To investigate why axotomised C-fiber nociceptors in a partial nerve injury model of chronic pathological pain do not fire spontaneously despite being depolarised. Background: In partial nerve injury models of chronic pain in which re-innervation of the tissues is prevented, the damaged/axotomised C-fibers do not fire spontaneously (Campbell et al 2006, Djouhri et al 2006, Djouhri et al, 2012, Liu et al 2000, Wu et al 2001) and thus presumably do not contribute to spontaneous pain. We found that axotomised C-neurons after L5 spinal nerve axotomy are depolarised by about 10mV due, at least in part, to loss of membrane-associated TREK2 in the small IB4+ve C-neurons that normally express, and are hyperpolarised by, this channel (Acosta et al., 2014). It is therefore surprising that these neurons do not fire spontaneously, especially given that in uninjured neurons in this model, the amount of SF is greater in neurons with more depolarised Ems (Djouhri et al., 2012). In contrast, C-fiber neurons that remain uninjured after partial nerve injury, with fibers that comingle with degenerating axotomised fibers, do show SF. One explanation is that the SF is generated in the peripheral part of the fiber. However, there is short term (a few days) SF seen in axotomised A-fiber neurons in similar models which have also lost their peripheral fibers. A possible explanation for lack of SF in axotomised C-fibers is that Nav1.7 mRNA and Nav1.8 mRNA/protein (two Nav channels that form a major part of their excitatory machinery) are down-regulated after axotomy. However, this has not been investigated in C-fiber neurons that express TREK2. Methods: In female Wistar rats under anaesthesia, models of moderate neuropathic (NP) pain were generated. This involved L5 spinal nerve axotomy. 7 days later intracellular recordings were made in the ipsilateral L5 sensory dorsal root ganglion (DRG) C-fiber neurons in vivo under deep anaesthesia. The resting membrane potentials (Ems) and presence/rate of spontaneous firing (SF) were examined. We therefore carried out fluorescence immunocytochemistry of ipsilateral L5 DRGs removed under deep anaesthesia, to examine the expression of Nav1.7 and Nav1.8 in IB4+ve small neurons. Results: The membrane potential (Em) of C-fiber neurons was depolarised by ~10mV but no SF was observed (Djourhi et al 2006). Decreased TREK2 expression at the periphery of the soma membrane in these axotomised L5 neurons was observed, with TREK2 being limited to the more perinuclear regions of the soma. There was a very great reduction of both Nav1.7 and Nav1.8 in both, TREK2+ve and TREK2-ve neurons. Conclusions: The lack of SF in axotomised ipsilateral L5 C-fiber neurons, despite them having depolarised Ems, is likely to result from the great reduction in expression of Nav1.7 and Nav1.8, known to be essential to the excitability of C-fiber neurons. Their loss in both TREK2+ve and TREK2-ve small DRG neurons may well make these neurons incapable of firing. The apparent loss of membrane TREK2 may result in energy conservation, by decreasing K+ leakage across the membrane, without risk of increasing/causing action ectopic potential generation. The loss of target-derived trophic factors, such as NGF, may explain these alterations in ion channel expression and distribution. The SF seen in some models where nerve regeneration possible, may result from target-derived trophic factors beginning again to reach the soma, although this remains to be confirmed. References Acosta C. et al. (2014) J Neurosci 34: 1494-1509. Campbell JN, Meyer RA. Neuron 2006;52:77-92. Djouhri L. et al. (2012) Pain 153: 1824-1836. Djouhri L. et al. (2006). J Neurosci 26: 1281-1292. Liu X. et al. (2000) Pain 84:309-318. Liu CN. et al. (2000) Pain 85:503-521. Wu G. et al. (2001) J Neurosci 21:RC140.