Proceedings of The Physiological Society
University of Oxford (2011) Proc Physiol Soc 23, PC302
CCL2 excites a subgroup of inflamed C-fibre axons in the rat
N. Richards1, A. Dilley1
1. Clinical and Laboratory Investigation, Brighton and Sussex Medical School, Falmer, Brighton, United Kingdom.
There is evidence that chemokines may play an important role in inflammatory pain pathways (Oh et al., 2001). In particular, the chemokine CCL2 and its cognate receptor CCR2 have been implicated in the excitation of injured peripheral neurons (White et al. 2005). The aim of this in vivo study was to examine the electrophysiological effects of CCL2 on both untreated and inflamed C-fibre neurons and to determine the neuronal expression of CCR2 following inflammation. The sciatic nerve was locally inflamed (neuritis) in anaesthetised adult male Sprague Dawley rats (n=31; isoflurane, 1.75% in O2) as previously described (Dilley & Bove, 2008). Three to six days following neuritis (n=26), and in untreated animals (n=32), dorsal root single unit recordings were carried out under terminal anaesthesia (1.5g/kg urethane i.p.). CCL2 (in 0.1% bovine serum albumin/saline; BSA saline) or BSA saline were suspended for 15 minutes around the sciatic nerve at the neuritis or equivalent site in untreated animals. Axons were assessed for ongoing activity (OA) as a measure of excitability. CCR2 immunofluorescence was carried out on ipsilateral L5 dorsal root ganglia (DRG). OA developed in 38% of neurons following neuritis (median rate=16.8 spikes/min) compared to 7% in the untreated group (p<0.05). There was also a significant slowing of conduction velocity (median=0.71 m/sec (IQR 0.33)) compared to the untreated group (0.89 m/sec (IQR 0.28); p<0.05). The test agents had negligible effects on untreated neurons (1/10 developed OA post-CCL2, 0/12 post-BSA saline), whereas CCL2 caused 27% (6/22) of neurons in the neuritis group to develop or increase their rate of OA compared to none (0/21) for BSA saline (p<0.05). The ongoing responders had significantly slower baseline firing rates (1.7 (IQR 2.7) spikes/min) compared to non-responders (16.3 (IQR 36.0) spikes/min; p<0.05) as well as normal conduction velocities (0.94 (IQR 0.49 m/sec)). In a subgroup of untreated animals (n=26), the periphery underwent repeated noxious mechanical stimulation during receptive field searches. Receptive field testing took place during in vivo electrophysiological procedures, under terminal anaesthesia. In this group, neurons developed slow OA over time (median rate=2.8 spikes/min). Twenty seven percent (6/22) of these neurons responded to CCL2 treatment compared to 0/19 for BSA saline (p<0.05). The majority of responders (5/6) were recorded late in the experiments (>151 min from setup) and were probably sensitised. CCR2 was expressed at extremely low levels within L5 DRG cell bodies in both the untreated (2.1%) and neuritis groups (0.6%). In summary, CCL2 excites a subgroup of subtly inflamed C-fibre neurons that are either silent or slowly ongoing and have unaltered conduction velocities. The lack of CCR2 expression on inflamed axons suggests that this receptor is not involved in a direct axonal mechanism in the neuritis model. Since CCL2 effects were rapid, other axonal CCL2 receptors may be involved.
Where applicable, experiments conform with Society ethical requirements