TREK2, AN ENDOGENOUS PAIN CONTROL MECHANISM, HYPERPOLARISES C-NOCICEPTORS, DECREASING THEIR SPONTANEOUS FIRING, AND LIMITS SPONTANEOUS PAIN

ACOSTA, CG.; L. DJOUHRI; ROGER WATKINS; C. BERRY; SALLY LAWSON

Bueno Aires

Congreso; 15th World Congress on Pain; 2014

IASP

Aim of Investigation: Background: In rat dorsal root ganglia, TREK2, a two pore domain K+ (K2P) channel, is selectively expressed in IB4-binding C-fiber nociceptors. K2Ps make major contributions to Em (Acosta et al 2014). We previously showed that siRNA knockdown of TREK2 caused depolarisation in small (C-fiber) DRG neurons in culture (Acosta et al., 2014). The extent of spontaneous pain is related to the rate of pathological spontaneous firing (SF) in uninjured C-fiber nociceptors (Djouhri et al 2006, 2012). Aim of investigation: To investigate the role of TREK2 in controlling C-fiber nociceptor membrane potentials (Ems), C-fiber spontaneous firing and pathological spontaneous pain behaviour. Methods: Moderate pain models for neuropathic (NP) or inflammatory pain (intradermal CFA) were generated under anaesthesia in female Wistar rats. The NP pain model involved L5 spinal nerve axotomy plus L4 spinal nerve loose ligation with inflammation-inducing chronic gut. After 1 (CFA) or 7 days (NP) we examined spontaneous foot lifting (SFL) (as an indication of spontaneous pain); in some rats under deep anaesthesia made intracellular recordings in identified nociceptors in DRGs in vivo, and studied the immunoreactivity of these neurons for TREK2. Relationship (and correlation analysis) of membrane potential, SF and SFL with TREK2 immunoreactivity were determined. We tested the effects of reducing TREK2 expression by in vivo intradermal injection of TREK2 siRNA. This caused decreased TREK2 expression (as shown by lower immunoreactivity in the local nerve fibers and in the skin). Finally, we examined in vivo if i.p. administration of a TREK2 opener (Baicalein) had an effect on SFL 1 day after CFA. Results: The Ems of C-nociceptors was related to the intensity of TREK2 immunostaining at the perimeter of the soma (probably membrane-related TREK2). The amount of SF in conducting L4 DRG C-nociceptors in the NP model was correlated with their Ems, with the more depolarised neurons showed greater SF rates. In vivo siRNA knockdown of TREK2 significantly increased the SFL observed after intradermal CFA supporting a protective effect of TREK2 against ongoing/spontaneous pain. In agreement with this, intraperitoneal administration of a low dose of the TREK2 opener (activator) Baicalein significantly reduced SFL. Conclusions: TREK2, which is expressed in IB4+ C-nociceptors, keeps their Ems hyperpolarised; hyperpolarised Ems limit SF rate. Since spontaneous pain behaviour (SFL) is related to the rate of SF in uninjured C-nociceptors in chronic pain models (Djouhri et al., 2006;Djouhri et al., 2012), limiting SF firing rate in these models should protect against spontaneous pain. Our data show that knocking down TREK2 locally in vivo increased CFA-induced spontaneous pain behaviour. This shows that TREK2 provides an endogenous protection against pathological pain. This view is supported by the finding that Baicalein, which opens TREK2 channels, reduced this behaviour providing further circumstantial support for a protective role of TREK2 against pathological pain. While Baicalein is not selective for TREK2, there are as yet no selective TREK2 channels openers. This highlights the importance of developing such tools to help explore the potential therapeutic benefits for chronic pain sufferers of an increased TREK2 activity. Reference List Acosta C, Djouhri L, Watkins R, Berry C, Bromage K, Lawson SN (2014) J Neurosci 34: 1494-1509. Djouhri L, Fang X, Koutsikou S, Lawson SN (2012) Pain 153: 1824-1836. Djouhri L, Koutsikou S, Fang X, McMullan S, Lawson SN (2006). J Neurosci 26: 1281-1292.