The reversal potential of Ca(2+)-activated Cl(-) currents indicates that chick sensory neurons accumulate intracellular Cl(-)

Anoctamin 1 mediates thermal pain as a heat sensor

Vertebrates can sense and avoid noxious heat that evokes pain. Many thermoTRP channels are associated with temperature sensation. TRPV1 is a representative ion channel that is activated by noxious heat. Anoctamin 1 (ANO1) is a Cl- channel activated by calcium that is highly expressed in small sensory neurons, colocalized with markers for nociceptors, and most surprisingly, activated by noxious heat over 44oC. Although ANO1 is a Cl- channel, opening of this channel leads to depolarization of sensory neurons, suggesting a role in nociception. Indeed, the functional deletion of ANO1 in sensory neurons triggers the reduction in thermal pain sensation. Thus, it seems clear that ANO1 is a heat sensor in a nociceptive pathway. Since ANO1 modulators are developed for the purpose of treating chronic diseases such as cystic fibrosis, this finding is likely to predict unwanted effects and provide a guide for better developmental strategy

Transgenic mouse lines for non-invasive ratiometric monitoring of intracellular chloride

Chloride is the most abundant physiological anion and participates in a variety of cellular processes including trans-epithelial transport, cell volume regulation, and regulation of electrical excitability. The development of tools to monitor intracellular chloride concentration ([Cli]) is therefore important for the evaluation of cellular function in normal and pathological conditions. Recently, several Cl-sensitive genetically encoded probes have been described which allow for non-invasive monitoring of [Cli]. Here we describe two mouse lines expressing a CFP-YFP-based Cl probe called Cl-Sensor. First, we generated transgenic mice expressing Cl-Sensor under the control of the mouse Thy1 mini promoter. Cl-Sensor exhibited good expression from postnatal day two (P2) in neurons of the hippocampus and cortex, and its level increased strongly during development. Using simultaneous whole-cell monitoring of ionic currents and Cl-dependent fluorescence, we determined that the apparent EC 50 for Cli was 46 mM, indicating that this line is appropriate for measuring neuronal [Cli] in postnatal mice. We also describe a transgenic mouse reporter line for Cre-dependent conditional expression of Cl-Sensor, which was targeted to the Rosa26 locus and by incorporating a strong exogenous promoter induced robust expression upon Cre-mediated recombination. We demonstrate high levels of tissue-specific expression in two different Cre-driver lines targeting cells of the myeloid lineage and peripheral sensory neurons. Using these mice the apparent EC 50 for Cli was estimated to be 61 and 54 mM in macrophages and DRG, respectively. Our data suggest that these mouse lines will be useful models for ratiometric monitoring of Cli in specific cell types in vivo.

Persistent inflammation increases GABA-induced depolarization of rat cutaneous dorsal root ganglion neurons in vitro

Persistent inflammation is associated with a shift in spinal GABA(A) signaling from inhibition to excitation such that GABA(A)-receptor activation contributes to inflammatory hyperalgesia. We tested the hypothesis that the primary afferent is the site of the persistent inflammation-induced shift in GABA(A) signaling which is due to a Na(+)-K(+)-Cl(-)-co-transporter (NKCC1)-dependent depolarization of the GABA(A) current equilibrium potential (E(GABA)). Acutely dissociated retrogradely labeled cutaneous dorsal root ganglion (DRG) neurons from naïve and inflamed (3 days after a subcutaneous injection of complete Freund's adjuvant) adult male rats were studied with Ca(2+) imaging, western blot and gramicidin-perforated patch recording. GABA evoked a Ca(2+) transient in a subpopulation of small- to medium-diameter capsaicin-sensitive cutaneous neurons. Inflammation was associated with a significant increase in the magnitude of GABA-induced depolarization as well as the percentage of neurons in which GABA evoked a Ca(2+) transient. There was no detectable change in NKCC1 protein or phosphoprotein at the whole ganglia level. Furthermore, the increase in excitatory response was comparable in both HEPES- and HCO(3)(-)-buffered solutions, but was only associated with a depolarization of E(GABA) in HCO(3)(-)-based solution. In contrast, under both recording conditions, the excitatory response was associated with an increase in GABA(A) current density, a decrease in low threshold K(+) current density, and resting membrane potential depolarization. Our results suggest that increasing K(+) conductance in afferents innervating a site of persistent inflammation may have greater efficacy in the inhibition of inflammatory hyperalgesia than attempting to drive a hyperpolarizing shift in E(GABA).

Modulation of chloride homeostasis by inflammatory mediators in dorsal root ganglion neurons

Background

Chloride currents in peripheral nociceptive neurons have been implicated in the generation of afferent nociceptive signals, as Cl^- accumulation in sensory endings establishes the driving force for depolarizing, and even excitatory, Cl^- currents. The intracellular Cl^- concentration can, however, vary considerably between individual DRG neurons. This raises the question, whether the contribution of Cl^- currents to signal generation differs between individual afferent neurons, and whether the specific Cl^- levels in these neurons are subject to modulation. Based on the hypothesis that modulation of the peripheral Cl^- homeostasis is involved in the generation of inflammatory hyperalgesia, we examined the effects of inflammatory mediators on intracellular Cl^- concentrations and on the expression levels of Cl^- transporters in rat DRG neurons.

Results

We developed an in vitro assay for testing how inflammatory mediators influence Cl^- concentration and the expression of Cl^- transporters. Intact DRGs were treated with 100 ng/ml NGF, 1.8 μM ATP, 0.9 μM bradykinin, and 1.4 μM PGE₂ for 1–3 hours. Two-photon fluorescence lifetime imaging with the Cl^--sensitive dye MQAE revealed an increase of the intracellular Cl^- concentration within 2 hours of treatment. This effect coincided with enhanced phosphorylation of the Na⁺-K⁺-2Cl^- cotransporter NKCC1, suggesting that an increased activity of that transporter caused the early rise of intracellular Cl^- levels. Immunohistochemistry of NKCC1 and KCC2, the main neuronal Cl^- importer and exporter, respectively, exposed an inverse regulation by the inflammatory mediators. While the NKCC1 immunosignal increased, that of KCC2 declined after 3 hours of treatment. In contrast, the mRNA levels of the two transporters did not change markedly during this time. These data demonstrate a fundamental transition in Cl^- homeostasis toward a state of augmented Cl^- accumulation, which is induced by a 1–3 hour treatment with inflammatory mediators.

Conclusion

Our findings indicate that inflammatory mediators impact on Cl^- homeostasis in DRG neurons. Inflammatory mediators raise intracellular Cl^- levels and, hence, the driving force for depolarizing Cl^- efflux. These findings corroborate current concepts for the role of Cl^- regulation in the generation of inflammatory hyperalgesia and allodynia. As the intracellular Cl^- concentration rises in DRG neurons, afferent signals can be boosted by excitatory Cl^- currents in the presynaptic terminals. Moreover, excitatory Cl^- currents in peripheral sensory endings may also contribute to the generation or modulation of afferent signals, especially in inflamed tissue.

Differential maturation of chloride homeostasis in primary afferent neurons of the somatosensory system

Recent research into the generation of hyperalgesia has revealed that both the excitability of peripheral nociceptors and the transmission of their afferent signals in the spinal cord are subject to modulation by Cl(-) currents. The underlying Cl(-) homeostasis of nociceptive neurons, in particular its postnatal maturation, is, however, poorly understood. Here we measure the intracellular Cl(-) concentration, [Cl(-)]i, of somatosensory neurons in intact dorsal root ganglia of mice. Using two-photon fluorescence-lifetime imaging microscopy, we determined [Cl(-)]i in newborn and adult animals. We found that the somatosensory neurons undergo a transition of Cl(-) homeostasis during the first three postnatal weeks that leads to a decline of [Cl(-)]i in most neurons. Immunohistochemistry showed that a major fraction of neurons in the dorsal root ganglia express the cation-chloride co-transporters NKCC1 and KCC2, indicating that the molecular equipment for Cl(-) accumulation and extrusion is present. RT-PCR analysis showed that the transcription pattern of electroneutral Cl(-) co-transporters does not change during the maturation process. These findings demonstrate that dorsal root ganglion neurons undergo a developmental transition of chloride homeostasis during the first three postnatal weeks. This process parallels the developmental "chloride switch" in the central nervous system. However, while most CNS neurons achieve homogeneously low [Cl(-)]i levels - which is the basis of GABAergic and glycinergic inhibition - somatosensory neurons maintain a heterogeneous pattern of [Cl(-)]i values. This suggests that Cl(-) currents are excitatory in some somatosensory neurons, but inhibitory in others. Our results are consistent with the hypothesis that Cl(-) homeostasis in somatosensory neurons is regulated through posttranslational modification of cation-chloride co-transporters.

GABA-induced calcium signaling in cultured enteric neurons is reinforced by activation of cholinergic pathways

GABA is an important inhibitory transmitter in the CNS. In the enteric nervous system, however, both excitatory and inhibitory actions have been reported. Here, we investigated the effects of GABA on the intracellular Ca2+ concentration of guinea-pig myenteric neurons (at 35 degrees C) using Fura-2-AM. Neurons were identified by 75 mM K+ depolarization (5 s), which evoked a transient intracellular Ca2+ concentration increase. GABA (10 s) induced a dose dependent (5 nM-1 microM) transient intracellular Ca2+ concentration rise in the majority of neurons (500 nM GABA: 251+/-17 nM, n=232/289). Interestingly, the response to 5 microM GABA (n=18) lasted several minutes and did not fully recover. GABA response amplitudes were significantly (P<0.001) reduced by GABAA and GABAB receptor antagonists (10 microM) bicuculline and phaclofen. The GABAA agonist isoguvacine (10 microM) and GABAB agonist baclofen (10 microM) induced similar responses as 50 nM GABA, while the GABAC agonist cis-4-aminocrotonic acid (CACA) (10 microM) only elicited small responses in a minority of neurons. Removal of extracellular Ca2+ abolished all responses while depletion of intracellular Ca2+ stores by thapsigargin (5 microM) did not alter the responses to 500 nM GABA (n=13), but reduction of Ca2+ influx through voltage-dependent Ca2+ channels did. The nicotinic antagonist hexamethonium (100 microM) also reduced GABA responses by almost 70% suggesting that GABA stimulates cholinergic pathways, while the purinergic receptor blocker pyridoxal-phosphate-6-azophenyl-2',4'-disulfonic acid (PPADS) and the 5-HT3 receptor blocker ondansetron only had minor effects.

CONCLUSION

GABA elicits transient intracellular Ca2+ concentration responses in the majority of myenteric neurons through activation of GABAA and GABAB receptors and much of the response can be attributed to facilitation of ACh release. Thus GABA may act mainly as a modulator that sets the state of excitability of the enteric nerve network. A concentration of 5 microM GABA, although frequently used in pharmacological experiments, seems to cause a detrimental response reminiscent of the neurotoxic effects glutamate has in the CNS.

K(+) current regulates calcium-activated chloride current-induced after depolarization in axotomized sensory neurons

One of the major electrophysiological effects of axotomy is a hyperexcitability of injured afferents that is thought to be involved in peripheral neuropathic pain. The molecular determinants of injured sensory neuron excitability are complex and not all have been identified. We have previously shown that sciatic nerve section upregulates the Ca(2+)-activated Cl(-) current in subsets of medium and large sensory neurons. In the peripheral nervous system, the Ca(2+)-activated Cl(-) current can promote after depolarization (ADP) and may therefore be involved in excitability. In this study, we set the conditions for Ca(2+)-activated Cl(-) current activation during the electrical activity of axotomized sensory neurons. We used the whole-cell patch-clamp technique and Ca(2+) fluorescence measurements to record electrical activity or ionic currents associated with intracellular Ca(2+) transients. An analysis of Ca(2+) fluorescence variation under Ca(2+)-activated Cl(-) current activation showed that the Ca(2+) sensitivity of the Ca(2+)-activated Cl(-) current did not allow activation upon one action potential (AP) but instead necessitated intracellular Ca(2+) loading under high-frequency electrical activity or AP lengthening. Nevertheless, ADP was exclusively recorded under AP lengthening following K(+) current inhibition with either extracellular tetraethylammonium or intracellular Cs(+). The measurement of APs and ionic currents associated with the use of niflumic acid to inhibit Cl(-) currents showed that the Ca(2+)-activated Cl(-) current was responsible for the ADP observed during K(+) current inhibition. Thus, the Ca(2+)-activated Cl(-) current-induced ADP in axotomized sensory neurons is regulated by K(+) current density.

Ca2+-activated Cl- current in cultured myenteric neurons from murine proximal colon

Whole cell patch-clamp recordings were made from cultured myenteric neurons taken from murine proximal colon. The micropipette contained Cs(+) to remove K(+) currents. Depolarization elicited a slowly activating time-dependent outward current (I(tdo)), whereas repolarization was followed by a slowly deactivating tail current (I(tail)). I(tdo) and I(tail) were present in approximately 70% of neurons. We identified these currents as Cl(-) currents (I(Cl)), because changing the transmembrane Cl(-) gradient altered the measured reversal potential (E(rev)) of both I(tdo) and I(tail) with that for I(tail) shifted close to the calculated Cl(-) equilibrium potential (E(Cl)). I(Cl) are Ca(2+)-activated Cl(-) current [I(Cl(Ca))] because they were Ca(2+) dependent. E(Cl), which was measured from the E(rev) of I(Cl(Ca)) using a gramicidin perforated patch, was -33 mV. This value is more positive than the resting membrane potential (-56.3 +/- 2.7 mV), suggesting myenteric neurons accumulate intracellular Cl(-). omega-Conotoxin GIVA [0.3 microM; N-type Ca(2+) channel blocker] and niflumic acid [10 microM; known I(Cl(Ca)) blocker], decreased the I(Cl(Ca)). In conclusion, these neurons have I(Cl(Ca)) that are activated by Ca(2+) entry through N-type Ca(2+) channels. These currents likely regulate postspike frequency adaptation.