Heterogeneous chloride homeostasis and GABA responses in the median preoptic nucleus of the rat

Physiological roles of chloride ions in bodily and cellular functions

Physiological roles of Cl-, a major anion in the body, are not well known compared with those of cations. This review article introduces: (1) roles of Cl- in bodily and cellular functions; (2) the range of cytosolic Cl- concentration ([Cl-]c); (3) whether [Cl-]c could change with cell volume change under an isosmotic condition; (4) whether [Cl-]c could change under conditions where multiple Cl- transporters and channels contribute to Cl- influx and efflux in an isosmotic state; (5) whether the change in [Cl-]c could be large enough to act as signals; (6) effects of Cl- on cytoskeletal tubulin polymerization through inhibition of GTPase activity and tubulin polymerization-dependent biological activity; (7) roles of cytosolic Cl- in cell proliferation; (8) Cl--regulatory mechanisms of ciliary motility; (9) roles of Cl- in sweet/umami taste receptors; (10) Cl--regulatory mechanisms of with-no-lysine kinase (WNK); (11) roles of Cl- in regulation of epithelial Na+ transport; (12) relationship between roles of Cl- and H+ in body functions.

Mild KCC2 Hypofunction Causes Inconspicuous Chloride Dysregulation that Degrades Neural Coding

Disinhibition caused by Cl⁻ dysregulation is implicated in several neurological disorders. This form of disinhibition, which stems primarily from impaired Cl⁻ extrusion through the co-transporter KCC2, is typically identified by a depolarizing shift in GABA reversal potential (E_GABA). Here we show, using computer simulations, that intracellular [Cl⁻] exhibits exaggerated fluctuations during transient Cl⁻ loads and recovers more slowly to baseline when KCC2 level is even modestly reduced. Using information theory and signal detection theory, we show that increased Cl⁻ lability and settling time degrade neural coding. Importantly, these deleterious effects manifest after less KCC2 reduction than needed to produce the gross changes in E_GABA required for detection by most experiments, which assess KCC2 function under weak Cl⁻ load conditions. By demonstrating the existence and functional consequences of “occult” Cl⁻ dysregulation, these results suggest that modest KCC2 hypofunction plays a greater role in neurological disorders than previously believed.

Efficacy of synaptic inhibition depends on multiple, dynamically interacting mechanisms implicated in chloride homeostasis

Chloride homeostasis is a critical determinant of the strength and robustness of inhibition mediated by GABA(A) receptors (GABA(A)Rs). The impact of changes in steady state Cl(-) gradient is relatively straightforward to understand, but how dynamic interplay between Cl(-) influx, diffusion, extrusion and interaction with other ion species affects synaptic signaling remains uncertain. Here we used electrodiffusion modeling to investigate the nonlinear interactions between these processes. Results demonstrate that diffusion is crucial for redistributing intracellular Cl(-) load on a fast time scale, whereas Cl(-)extrusion controls steady state levels. Interaction between diffusion and extrusion can result in a somato-dendritic Cl(-) gradient even when KCC2 is distributed uniformly across the cell. Reducing KCC2 activity led to decreased efficacy of GABA(A)R-mediated inhibition, but increasing GABA(A)R input failed to fully compensate for this form of disinhibition because of activity-dependent accumulation of Cl(-). Furthermore, if spiking persisted despite the presence of GABA(A)R input, Cl(-) accumulation became accelerated because of the large Cl(-) driving force that occurs during spikes. The resulting positive feedback loop caused catastrophic failure of inhibition. Simulations also revealed other feedback loops, such as competition between Cl(-) and pH regulation. Several model predictions were tested and confirmed by [Cl(-)](i) imaging experiments. Our study has thus uncovered how Cl(-) regulation depends on a multiplicity of dynamically interacting mechanisms. Furthermore, the model revealed that enhancing KCC2 activity beyond normal levels did not negatively impact firing frequency or cause overt extracellular K(-) accumulation, demonstrating that enhancing KCC2 activity is a valid strategy for therapeutic intervention.

Perirhinal cortex hyperexcitability in pilocarpine-treated epileptic rats

The perirhinal cortex (PC), which is heavily connected with several epileptogenic regions of the limbic system such as the entorhinal cortex and amygdala, is involved in the generation and spread of seizures. However, the functional alterations occurring within an epileptic PC network are unknown. Here, we analyzed this issue by using in vitro electrophysiology and immunohistochemistry in brain tissue obtained from pilocarpine-treated epileptic rats and age-matched, nonepileptic controls (NECs). Neurons recorded intracellularly from the PC deep layers in the two experimental groups had similar intrinsic and firing properties and generated spontaneous depolarizing and hyperpolarizing postsynaptic potentials with comparable duration and amplitude. However, spontaneous and stimulus-induced epileptiform discharges were seen with field potential recordings in over one-fifth of pilocarpine-treated slices but never in NEC tissue. These network events were reduced in duration by antagonizing NMDA receptors and abolished by NMDA + non-NMDA glutamatergic receptor antagonists. Pharmacologically isolated isolated inhibitory postsynaptic potentials had reversal potentials for the early GABA(A) receptor-mediated component that were significantly more depolarized in pilocarpine-treated cells. Experiments with a potassium-chloride cotransporter 2 antibody identified, in pilocarpine-treated PC, a significant immunostaining decrease that could not be explained by neuronal loss. However, interneurons expressing parvalbumin and neuropeptide Y were found to be decreased throughout the PC, whereas cholecystokinin-positive cells were diminished in superficial layers. These findings demonstrate synaptic hyperexcitability that is contributed by attenuated inhibition in the PC of pilocarpine-treated epileptic rats and underscore the role of PC networks in temporal lobe epilepsy.

Endogenous angiotensin II facilitates GABAergic neurotransmission afferent to the Na+-responsive neurons of the rat median preoptic nucleus

The median preoptic nucleus (MnPO) is densely innervated by efferent projections from the subfornical organ (SFO) and, therefore, is an important relay for the peripheral chemosensory and humoral information (osmolality and serum levels ANG II). In this context, controlling the excitability of MnPO neuronal populations is a major determinant of body fluid homeostasis and cardiovascular regulation. Using a brain slice preparation and patch-clamp recordings, our study sought to determine whether endogenous ANG II modulates the strength of the SFO-derived GABAergic inputs to the MnPO. Our results showed that the amplitude of the inhibitory postsynaptic currents (IPSCs) were progressively reduced by 44 ± 2.3% by (Sar1, Ile8)-ANG II, a competitive ANG type 1 receptor (AT1R) antagonist. Similarly, losartan, a nonpeptidergic AT1R antagonist decreased the IPSC amplitude by 40.4 ± 5.6%. The facilitating effect of endogenous ANG II on the GABAergic input to the MnPO was not attributed to a change in GABA release probability and was mimicked by exogenous ANG II, which potentiated the amplitude of the muscimol-activated GABAA/Cl− current by 53.1 ± 11.4%. These results demonstrate a postsynaptic locus of action of ANG II. Further analysis reveals that ANG II did not affect the reversal potential of the synaptic inhibitory response, thus privileging a cross talk between postsynaptic AT1 and GABAA receptors. Interestingly, facilitation of GABAergic neurotransmission by endogenous ANG II was specific to neurons responding to changes in the ambient Na+ level. This finding, combined with the ANG II-mediated depolarization of non-Na+-responsive neurons reveals the dual actions of ANG II to modulate the excitability of MnPO neurons.

Application of muscle biopotential measurement for sustained, noninvasive blood glucose survey

Biopotential, the electric potential generated by living tissues, is affected by changes in extracellular electrolyte and glucose concentrations. We aimed to apply correlation between blood glucose concentrations (BGC) and biopotential of peripheral muscles for noninvasive blood glucose measurement. The study included 58 Wistar rats. In part of them, diabetes was induced by streptozotocin injection. Group 1, comprising 19 normal and 5 diabetic rats, received glucose-challenging protocol (intraperitoneal injection of 1 g/ml glucose). Group 2, 24 normal and 6 diabetic rats, received insulin-challenging protocol (three 30 IU insulin injections with 15-min intervals). Four control rats, group 3, were injected with 2-ml saline. BGC were measured by a standard ACCU-CHEK-Sensor Meter and compared with those estimated by biopotential sensor, further designated as GlucoSat, placed around proximal parts of the tails of the anaesthetized animals. GlucoSat results were calculated using the following biopotential equation: BGC(t) = k1 F1(t) + k2 F2(t) k3 F3(t) + k4, based on an experimental model involving estimation of pH, muscle metabolism, and tissue conductance, where t is time, k1-k4 are coefficients, and F1-F4 are functions. Mean biopotential system measured BGC was 181.7 +/- 4.3 mg/dl, not differing statistically from 187.9 +/- 4.3 mg/dl estimated by ACCU-CHEK. Pearson's correlation coefficient (r(2)) was 0.961 (P < 0.00001), indicating strong, direct correlation between the results. Within the nondiabetic group, r(2) was 0.944 (P < 0.00001), while, within the diabetic group, r(2) was 0.974 (P < 0.00001). No significant, adverse skin reactions were concomitantly observed in any experimental group. Biopotential measurements may be used for continuous, noninvasive estimation of changes in BGC. Further studies are needed to evaluate the applicability of this method to humans.

Altered chloride homeostasis removes synaptic inhibitory constraint of the stress axis

In mammals, stress elicits a stereotyped endocrine response that requires an increase in the activity of hypothalamic parvocellular neuroendocrine neurons. The output of these cells is normally constrained by powerful GABA-mediated synaptic inhibition. We found that acute restraint stress in rats released the system from inhibitory synaptic drive in vivo by down-regulating the transmembrane anion transporter KCC2. This manifested as a depolarizing shift in the reversal potential of GABA(A)-mediated synaptic currents that rendered GABA inputs largely ineffective. Notably, repetitive activation of GABA synapses after stress resulted in a more rapid collapse of the anion gradient and was sufficient to increase the activity of neuroendocrine cells. Our data indicate that hypothalamic neurons integrate psychological cues to mount the endocrine response to stress by regulating anion gradients.

Spinal cord injury-induced attenuation of GABAergic inhibition in spinal dorsal horn circuits is associated with down-regulation of the chloride transporter KCC2 in rat

Most spinal cord injury (SCI) patients suffer from chronic pain. Effective therapy for this pain is lacking, and the underlying mechanisms are poorly understood. The spinal superficial dorsal horn (SDH) contains neuronal circuits capable of modulating primary afferent information involved in pain processing. KCC2 is an isoform of the K(+)-Cl(-) cotransporter that contributes to the regulation of transmembrane anion gradient which plays a key role in shaping GABA(A) receptor-mediated signalling in the CNS. We tested the hypothesis that SCI causes down-regulation of KCC2 distal to the injury and contributes to the neuronal hyperresponsiveness and pain-related behaviours. SCI was a hemisection at T(13) level of adult Sprague-Dawley rats. Spinal sagittal slices with attached dorsal roots (DR) were prepared from L(4) to L(6) level. The reversal potentials of GABA responses (E(GABA)) and DR-evoked IPSPs and EPSPs of L(4-6) SDH neurones in sham-operated and SCI rats were compared using gramicidin-perforated patch-clamp recordings. Here we report that thoracic SCI-induced down-regulation of KCC2 in the lumbar SDH parallels the development of allodynia. The subsequent changes of E(GABA) in SDH neurones attenuate the GABA(A) receptor-mediated inhibitory synaptic transmission. These changes cause certain normally subthreshold primary A and C fibre inputs to evoke action potential output in SDH neurones. We conclude that SCI induces KCC2 down-regulation and subsequent changes of E(GABA) in the SDH below the injury site. The resulting disinhibition unmasks normally ineffective SDH neuronal circuits and may contribute to the below-level central pain-related behaviours after incomplete SCI.

Postsynaptic mu-opioid receptor response in the median preoptic nucleus is altered by a systemic sodium challenge in rats

The median preoptic nucleus (MnPO) is an integrator site for the chemosensory and neural signals induced by a perturbation in the hydromineral balance, and it is highly involved in controlling fluid and electrolyte ingestion. Here, we hypothesize that opioid peptides, previously recognized to control ingestive behaviors, may regulate the excitability of MnPO neurons and that this regulatory action may depend on the natriuric (Na(+)) status of body fluid compartments. Our results show that activation of mu-, but not delta-, opioid receptors (OR) triggered a membrane hyperpolarization by recruiting a G-protein-regulated inward-rectifier K(+) (GIRK) conductance in 41% of the neurons tested. Interestingly, 24 h Na(+) depletion strengthened this opioid-mediated control of neuronal excitability. In Na(+)-depleted animals, the neuronal population displaying the mu-OR-induced hyperpolarization expanded to 60% (Z-test, P = 0.012), whereas Na(+) repletion restored this population to the control level (39%; Z-test, P = 0.037). Among the neurons displaying mu-OR-induced hyperpolarization, Na(+) depletion specifically increased the neuronal population responsive to variation in ambient Na(+) (from 27% to 43%; Z-test, P = 0.029). In contrast, Na(+) repletion dramatically reduced the population that was unresponsive to Na(+) (from 17% to 3%; Z-test, P = 0.031). Neither the basic properties of the neurons nor the characteristics of the mu-OR-induced response were altered by the body Na(+) challenge. Our results indicate that an episode of Na(+) depletion/Na(+) repletion modifies the organization of the opioid-sensitive network of the MnPO. Such network plasticity might be related to the avid salt ingestion triggered by repeated Na(+) depletion.

Inhibitory postsynaptic potentials in lumbar motoneurons remain depolarizing after neonatal spinal cord transection in the rat

GABA and glycine are excitatory in the immature spinal cord and become inhibitory during development. The shift from depolarizing to hyperpolarizing inhibitory postsynaptic potentials (IPSPs) occurs during the perinatal period in the rat, a time window during which the projections from the brain stem reach the lumbar enlargement. In this study, we investigated the effects of suppressing influences of the brain on lumbar motoneurons during this critical period for the negative shift of the reversal potential of IPSPs (E(IPSP)). The spinal cord was transected at the thoracic level on the day of birth [postnatal day 0 (P0)]. E(IPSP), at P4-P7, was significantly more depolarized in cord-transected than in cord-intact animals (E(IPSP) above and below resting potential, respectively). E(IPSP) at P4-P7 in cord-transected animals was close to E(IPSP) at P0-P2. K-Cl cotransporter KCC2 immunohistochemistry revealed a developmental increase of staining in the area of lumbar motoneurons between P0 and P7 in cord-intact animals; this increase was not observed after spinal cord transection. The motoneurons recorded from cord-transected animals were less sensitive to the experimental manipulations aimed at testing the functionality of the KCC2 system, which is sensitive to [K(+)](o) and blocked by bumetanide. Although bumetanide significantly depolarized E(IPSP), the shift was less pronounced than in cord-intact animals. In addition, a reduction of [K(+)](o) affected E(IPSP) significantly only in cord-intact animals. Therefore influences from the brain stem may play an essential role in the maturation of inhibitory synaptic transmission, possibly by upregulating KCC2 and its functionality.

Subiculum network excitability is increased in a rodent model of temporal lobe epilepsy

In this study, we used in vitro electrophysiology along with immunohistochemistry and molecular techniques to study the subiculum--a limbic structure that gates the information flow from and to the hippocampus--in pilocarpine-treated epileptic rats. Comparative data were obtained from age-matched nonepileptic controls (NEC). Subicular neurons in hippocampal-entorhinal cortex (EC) slices of epileptic rats were: (i) hyperexcitable when activated by CA1 or EC inputs; and (ii) generated spontaneous postsynaptic potentials at higher frequencies than NEC cells. Analysis of pharmacologically isolated, GABA(A) receptor-mediated inhibitory postsynaptic potentials revealed more positive reversal potentials in epileptic tissue (-67.8 +/- 6.3 mV, n = 16 vs. -74.8 +/- 3.6 mV in NEC, n = 13; P < 0.001) combined with a reduction in peak conductance (17.6 +/- 11.3 nS vs. 41.1 +/- 26.7 nS in NEC; P < 0.003). These electrophysiological data correlated in the epileptic subiculum with (i) reduced levels of mRNA expression and immunoreactivity of the neuron-specific potassium-chloride cotransporter 2; (ii) decreased number of parvalbumin-positive cells; and (iii) increased synaptophysin (a putative marker of sprouting) immunoreactivity. These findings identify an increase in network excitability within the subiculum of pilocarpine-treated, epileptic rats and point at a reduction in inhibition as an underlying mechanism.