The regulation of intracellular chloride, [Cl
-]
i, in DRG neurons has a distinct impact on the detection and transmission of the peripheral nociceptive signals. Measurements with isolated DRG neurons have shown that Cl
- is accumulated into the cytoplasm, supporting a Cl
- equilibrium potential, E
Cl, near -40 mV [
1‐
3]. The opening of Cl
- permeable channels, therefore, induces a depolarizing Cl
- efflux. Presynaptic GABA
A receptors in the dorsal horn conduct Cl
- efflux and cause primary afferent depolarization (PAD) [
4,
5]. PAD mediates presynaptic inhibition of nociceptive afferents, but intense afferent stimulation can cause excitatory Cl
- efflux which contributes to dorsal root reflexes, hyperalgesia, and neurogenic inflammation [
4‐
8]. In addition to these well-characterized spinal processes, excitatory Cl
- effects were also reported for the cutaneous endings of nociceptors [
9,
10]. Together, these observations raise the possibility that depolarizing Cl
- currents contribute to signal generation in the periphery as well as to synaptic transmission. Interestingly, [Cl
-]
i can increase under certain conditions, as was recently reported for nociceptors upon nerve damage and regeneration (
e.g. from 31 to 68 mM [
11]). A rising level of [Cl
-]
i is expected to amplify peripheral and presynaptic Cl
- efflux and its effect on sensory signal generation and transmission. As a consequence of these findings, the regulation of Cl
- transport proteins has become an important topic of nociceptor research [
12]. Indeed, a number of recent studies has documented a link between pain behaviour and Cl
- accumulation in cutaneous and visceral nociceptors. Genetic ablation of the Na
+-K
+-2Cl
- cotransporter NKCC1 was associated with reduced Cl
- accumulation in DRG neurons and caused a reduced thermal nociceptive response [
13] as well as attenuation of Aβ-mediated allodynia [
14]. TRPV1-dependent, referred abdominal allodynia [
15], itch and flare responses [
16], as well as nocifensive behaviour in phase II of the formalin test [
17], were all inhibited upon spinal or peripheral application of the NKCC1-inhibitor bumetanide. Similarly, dorsal root reflexes and neurogenic inflammation were attenuated by intrathecal bumetanide injection [
18].
Cl
- regulation in neurons is mediated by cation-coupled Cl
--cotransporters (CCCs) [
19‐
21]. NKCC1 (SLC12A2) is the main key player of active Cl
- uptake. In contrast, Cl
- extrusion in neurons is mediated by the K
+-Cl
- cotransporter KCC2 (SLC12A5). For DRG neurons, the expression of NKCC1 is well documented [
13,
22], while the expression of KCC2 is controversial [
23,
24]. The intracellular Cl
- concentration in neurons is mainly determined by the functional expression of these CCCs. In addition, regulatory mechanisms that control the activity of Cl
- transporters include phosphorylation [
25‐
27] and dimerization [
28]. Importantly, small shifts in the balance between Cl
- import and Cl
- extrusion can cause changes of E
Cl which substantially alter a neuron's response to the opening of Cl
- channels. A recent approach to model the GABAergic response of a spinal lamina 1 neuron revealed that a shift of E
Cl as small as 10 mV to less negative values is sufficient to turn GABA-induced currents from inhibitory to excitatory [
29]. Interestingly, a recent thermodynamic analysis of Cl
- transport in DRG neurons indicated that the regulation of NKCC1 activity by phosphorylation/dephosphorylation may account for substantial changes in E
Cl [
3]. Thus, Cl
- levels in DRG neurons are probably not constant, but they are subject to regulation. Processes that control the dynamics of Cl
- accumulation, consequently, play an important role in current concepts for the plasticity of the afferent pain pathway [
3,
5,
8,
12,
30‐
35].