Loss of inhibitory tone on spinal cord dorsal horn spontaneously and nonspontaneously active neurons in a mouse model of neuropathic pain

Abstract

Plasticity of inhibitory transmission in the spinal dorsal horn (SDH) is believed to be a key mechanism responsible for pain hypersensitivity in neuropathic pain syndromes. We evaluated this plasticity by recording responses to mechanical stimuli in silent neurons (nonspontaneously active [NSA]) and neurons showing ongoing activity (spontaneously active [SA]) in the SDH of control and nerve-injured mice (cuff model). The SA and NSA neurons represented 59% and 41% of recorded neurons, respectively, and were predominantly wide dynamic range (WDR) in naive mice. Nerve-injured mice displayed a marked decrease in the mechanical threshold of the injured paw. After nerve injury, the proportion of SA neurons was increased to 78%, which suggests that some NSA neurons became SA. In addition, the response to touch (but not pinch) was dramatically increased in SA neurons, and high-threshold (nociceptive specific) neurons were no longer observed. Pharmacological blockade of spinal inhibition with a mixture of GABAA and glycine receptor antagonists significantly increased responses to innocuous mechanical stimuli in SA and NSA neurons from sham animals, but had no effect in sciatic nerve-injured animals, revealing a dramatic loss of spinal inhibitory tone in this situation. Moreover, in nerve-injured mice, local spinal administration of acetazolamide, a carbonic anhydrase inhibitor, restored responses to touch similar to those observed in naive or sham mice. These results suggest that a shift in the reversal potential for anions is an important component of the abnormal mechanical responses and of the loss of inhibitory tone recorded in a model of nerve injury-induced neuropathic pain.

Enhanced analgesic cholinergic tone after neuropathy

“…However, we first performed a set of recordings of lamina II neurons in transverse slices, in order to put our results in context with the literature, mostly focusing on this layer. Indeed, while the cuff model of mouse peripheral neuropathy has been well described in behavioral terms (Benbouzid, et al, 2008; Yalcin, Bohren, et al, 2011), we were the first to analyze the plasticity occurring in the spinal cord with electrophysiology in vivo (Medrano, Dhanasobhon, Yalcin, Schlichter, & Cordero-Erausquin, 2016) and now in vitro . Recordings were performed in the ipsilateral (to cuff insertion) DH in the presence of 0.5 μM TTX (Fig.…”

“…Neurons were selected for their localization in the DH (Fig. 7A) and their response to mechanical stimulation of the hindpaw, either non-nociceptive (touch) and/or nociceptive (pinch) (see methods and (Medrano, et al, 2016)). Local application of 100 μM mecamylamine (on the spinal cord) induced an increased response to mechanical stimulation in neurons recorded from both sham and cuff mice (Fig.…”

“…The cuff, a neuropathic pain model chosen for this study, was originally developed for rats (Mosconi & Kruger, 1996) and then adapted for mice (Benbouzid, et al, 2008). In a previous study, we characterized by in vivo recordings the loss of inhibitory tone in the spinal DH in this model (Medrano, et al, 2016). We provide here for the first time in vitro recordings in the spinal cord of these mice and show that overall, the changes observed in synaptic inputs of DH lamina II neurons in other models are similarly observed in the cuff model.…”

“…Single-unit extracellular recordings from spinal dorsal horn neurons were performed as previously described (Medrano, et al, 2016). Only neurons responding to mechanical stimulation of the ipsilateral hind paw were included in this study.…”

“…The cutaneous receptive field of the recorded neuron was identified by touching the ipsilateral hind paw. The response to a non-nociceptive mechanical stimulus, touch, and to a nociceptive one, pinch, was determined as described before (Medrano, et al, 2016)). The touch stimulus was applied by brushing the skin with a camel’s hair brush for 10 seconds (8-10 times).…”