Our results show that in control conditions less than 25% of mechanosensitive lamina I projection neurons produce action potentials in response to low-threshold tactile stimuli, consistent with previous studies in rats  and cats [14, 24]. In contrast to the situation in naïve animals we show here for the first time that peripheral nerve injury causes a functional switch in the sensory specificity of this subpopulation of spinal output neurons whereby the majority of these neurons respond to low-threshold tactile stimuli. Finally, the same qualitative functional switch is triggered in naïve animals by acute local, spinal, application of ATP-stimulated microglia or disruption of chloride homeostasis.
Hyperalgesia can be explained by a quantitative change in response properties of nociceptive relay neurons whereby the same nociceptive input generates a greater action potential output. However, given that the majority of lamina I neurons respond to noxious input only, a quantitative change in nociceptive responsiveness appears insufficient to explain allodynia. Rather, allodynia implies a qualitative change, a miscoding of information such that innocuous inputs are converted into a nociceptive message. The switch in modality specificity we observed in lamina I output neurons is such a qualitative change in response properties and thus may be sufficient to explain allodynia. After peripheral nerve injury, administering ATP-stimulated microglia or disrupting chloride transport, innocuous inputs are transformed in the dorsal horn and become encoded by lamina I projection neurons. Consequently, action potential discharge is now generated by these neurons and sent to higher brain structures through output neurons that were previously nociceptive specific. It is thus logical to infer that such signals will be interpreted as noxious at the supraspinal level, providing a substrate to explain tactile allodynia. Similar logic can be applied to the finding of the appearance of spontaneous bursts of spikes in lamina I output neurons after nerve injury, treatment with ATP-stimulated microglia, or disruption of chloride homeostasis, providing a substrate of spontaneous pain that occur in neuropathic conditions.
A comparable loss of selectivity for noxious stimuli has been observed on unidentified nociceptive specific neurons in the superficial dorsal horn following application of mustard oil or capsaicin on the receptive field of the cells in the periphery [25, 26]. Similarly, brushing of the skin after sciatic nerve crush leads to c-fos expression in the superficial dorsal horn . Under those conditions, however, as with nerve injury, the possibility remains that the central response to low-threshold stimuli is due to a peripheral change in selectivity (e.g. peripheral nociceptors responding to low-threshold mechanical stimuli). In contrast, the evidence presented here, using local spinal administration of agents, shows conclusively that responses to low threshold inputs by lamina I neurons can result from purely central mechanisms.
Based on previous studies, the proposed central mechanism that was affected in the current experimental conditions is a disruption of anion homeostasis [7, 8], effectively weakening inhibition . The site at which the disinhibition occurs appears to be within the circuitry intrinsic to the dorsal horn, not within the network of afferent or descending terminals entering the dorsal horn. A loss of KCC2, which normally extrudes Cl- from the cells, appears to be the underlying mechanism [7, 8, 28] and KCC2 is not present in synaptic terminals nor on primary afferents . The other principal regulator of Cl- in the dorsal horn is NKCC1, which normally causes Cl- accumulation into cells. NKCC1 is very weakly expressed in adult dorsal horn neurons [29, 30] but is the dominant cation-chloride co-transporter in primary afferents , and thus NKCC1 leads to GABA exerting a depolarizing, albeit inhibitory, action on sensory terminals. Abnormal presynaptic excitation of sensory terminals has been proposed to occur via upregulation of NKCC1 in small diameter afferents after an inflammatory peripheral insult . This is thought to produce suprathreshold GABAergic depolarization in sensory terminals yielding cross excitation between low and high threshold afferents. The latter mechanism is unlikely to contribute to the effects observed in the present study for the following reasons. First, nerve injury is associated with a loss of KCC2 expression  as mentioned above. Second, ATP-stimulated microglia has been shown to cause tactile allodynia via the release of BDNF , and BDNF-trkB signalling is linked to downregulation of KCC2 [28, 32]. Third, the blocker of cation-chloride co-transport used in the present study, DIOA, preferentially inhibits KCC2 and not NKCC1 [33, 34]. Even if one doubts the specificity of DIOA and assumes that it also antagonized NKCC1 , this site of action could not account for the effect of DIOA we observed, unmasking low-threshold input to lamina I neurons, because blocking NKCC1 would work against exaggerated depolarization in primary afferents and thus prevent rather than produce cross talk between them as proposed for inflammatory insult . In summary, our findings indicate that selective impairment of postsynaptic Cl- homeostasis in the spinal dorsal horn is sufficient to unmask the relay of innocuous input through normally nociceptive specific pathways.
This aberrant relay of innocuous input may occur via unmasking polysynaptic connections in the superficial dorsal horn [37–39] functionally linking low threshold afferents and nociceptive lamina I projection neurons . This can be mediated either via disinhibition of feedforward excitatory interneurons such that they can convey this input onto normally nociceptive lamina I neurons or by lowering the threshold of nociceptive lamina I neurons to normally subthreshold polysynaptic input . In addition, unmasking of low-threshold input to lamina I output neurons may occur via inversion of normally inhibitory post-synaptic events from inhibitory interneurons into excitatory ones in a subset of cells .
The finding that altered chloride homeostasis compromises inhibitory control in dorsal horn neurons raises the question of the therapeutic avenues to compensate for this form of disinhibition. Indeed, a weakening of the hyperpolarizing action of GABAA/glycine receptor activation suggests that drugs aimed at enhancing GABAA/glycine receptor-mediated inhibition may be ineffective in reversing nerve injury-induced allodynia. However, three elements must be considered before making such inference. First, while raising intracellular [Cl-] suppresses the component of inhibition caused by hyperpolarizing the neuron, it only minimally affects the component of inhibition caused by shunting the membrane (as discussed in detail in ). Second, it is particularly important to note that a small depolarizing shift in reversal potential for GABAA currents will cause a loss of inhibition without necessarily causing GABAA-mediated net excitation. Thus, when intracellular Cl- homeostasis is altered, activation of GABAA and glycine receptors may continue to be inhibitory albeit less inhibitory [9, 10], allowing for unmasking of latent excitatory inputs. Finally, it must be kept in mind that because presynaptic GABAA receptor-mediated inhibition remains intact (see above), drugs activating or enhancing the function of GABAA receptors may remain analgesic by inhibiting afferent input at its entry point into the spinal cord. This is consistent with reports of antiallodynic effect of intrathecally-applied GABAA receptor agonists [41, 42]. Thus, at least in some cases, sufficient residual inhibition may remain in neuropathic pain conditions permitting GABAergic drugs to be analgesic. Results from modelling studies show that while small reductions in anion gradient may be effectively compensated for by potentiating GABAA/glycine receptor-mediated input, this can occur at the expense of stability of the system, and compensation will fail as the reduction of anion gradient exceeds a critical value . Depending on the extent of the pathology, compensation by enhancing GABAergic transmission may therefore be effective or not. Thus, measuring intracellular [Cl-] may be important to guide treatments based on GABA-modulating agents, and restoring normal anion homeostasis or targeting excitatory transmission may represent more effective therapeutic strategies .