Kai Kaila

The K+/Cl-co-transporter KCC2 renders GABA hyperpolarizing during neuronal maturation

GABA (γ-aminobutyric acid) is the main inhibitory transmitter in the adult brain, and it exerts its fast hyperpolarizing effect through activation of anion (predominantly Cl−)-permeant GABAA receptors. However, during early neuronal development, GABA A-receptor-mediated responses are often depolarizing,, which may be a key factor in the control of several Ca2+ −dependent developmental phenomena, including neuronal proliferation, migration and targeting. To date, however, the molecular mechanism underlying this shift in neuronal electrophysiological phenotype is unknown. Here we show that, in pyramidal neurons of the rat hippocampus, the ontogenetic change in GABAA-mediated responses from depolarizing to hyperpolarizing is coupled to a developmental induction of the expression of the neuronal Cl−-extruding K+/Cl − co-transporter, KCC2 (ref. 7). Antisense oligonucleotide inhibition of KCC2 expression produces a marked positive shift in the reversal potential of GABAA responses in functionally mature hippocampal pyramidal neurons. These data support the conclusion that KCC2 is the main Cl− extruder to promote fast hyperpolarizing postsynaptic inhibition in the brain.

Cation–chloride co-transporters in neuronal communication, development and trauma

Electrical signaling in neurons is based on the operation of plasmalemmal ion pumps and carriers that establish transmembrane ion gradients, and on the operation of ion channels that generate current and voltage responses by dissipating these gradients. Although both voltage- and ligand-gated channels are being extensively studied, the central role of ion pumps and carriers is largely ignored in current neuroscience. Such an information gap is particularly evident with regard to neuronal Cl- regulation, despite its immense importance in the generation of inhibitory synaptic responses by GABA- and glycine-gated anion channels. The cation-chloride co-transporters (CCCs) have been identified as important regulators of neuronal Cl- concentration, and recent work indicates that CCCs play a key role in shaping GABA- and glycine-mediated signaling, influencing not only fast cell-to-cell communication but also various aspects of neuronal development, plasticity and trauma.

Ionic basis of GABAA receptor channel function in the nervous system

Abbreviations

Cation-Chloride Cotransporters and Neuronal Function

Recent years have witnessed a steep increase in studies on the diverse roles of neuronal cation-chloride cotransporters (CCCs). The versatility of CCC gene transcription, posttranslational modification, and trafficking are on par with what is known about ion channels. The cell-specific and subcellular expression patterns of different CCC isoforms have a key role in modifying a neurons electrophysiological phenotype during development, synaptic plasticity, and disease. While having a major role in controlling responses mediated by GABA(A) and glycine receptors, CCCs also show close interactions with glutamatergic signaling. A cross-talk among CCCs and trophic factors is important in short-term and long-term modification of neuronal properties. CCCs appear to be multifunctional proteins that are also involved in shaping neuronal structure at various stages of development, from stem cells to synaptogenesis. The rapidly expanding work on CCCs promotes our understanding of fundamental mechanisms that control brain development and functions under normal and pathophysiological conditions.

Modulation of pH by neuronal activity

Although the requirement for a strict regulation of pH in the brain is frequently emphasized, recent studies indicate that neuronal activity gives rise to significant changes in intracellular and extracellular pH. Given the sensitivity of many ion channels to hydrogen ions, this modulation of local pH might influence brain function, particularly where pH shifts are sufficiently large and rapid. Studies using pH-sensitive microelectrodes have demonstrated marked cellular and regional variability of activity-dependent pH shifts, and have begun to uncover several of their underlying mechanisms. Accumulating evidence suggests that regional and subcellular pH dynamics are governed by the respective localization of glial cells, ligand-gated ion channels, and extracellular and intracellular carbonic anhydrase.

Phase synchrony among neuronal oscillations in the human cortex

Synchronization of neuronal activity, often associated with network oscillations, is thought to provide a means for integrating anatomically distributed processing in the brain. Neuronal processing, however, involves simultaneous oscillations in various frequency bands. The mechanisms involved in the integration of such spectrally distributed processing have remained enigmatic. We demonstrate, using magnetoencephalography, that robust cross-frequency phase synchrony is present in the human cortex among oscillations with frequencies from 3 to 80 Hz. Continuous mental arithmetic tasks demanding the retention and summation of items in the working memory enhanced the cross-frequency phase synchrony among α (∼10 Hz), β (∼20 Hz), and γ (∼30-40 Hz) oscillations. These tasks also enhanced the “classical” within-frequency synchrony in these frequency bands, but the spatial patterns of α, β, and γ synchronies were distinct and, furthermore, separate from the patterns of cross-frequency phase synchrony. Interestingly, an increase in task load resulted in an enhancement of phase synchrony that was most prominent between γ- and α-band oscillations. These data indicate that cross-frequency phase synchrony is a salient characteristic of ongoing activity in the human cortex and that it is modulated by cognitive task demands. The enhancement of cross-frequency phase synchrony among functionally and spatially distinct networks during mental arithmetic tasks posits it as a candidate mechanism for the integration of spectrally distributed processing.

BDNF-induced TrkB activation down-regulates the K+–Cl− cotransporter KCC2 and impairs neuronal Cl− extrusion

Pathophysiological activity and various kinds of traumatic insults are known to have deleterious long-term effects on neuronal Cl− regulation, which can lead to a suppression of fast postsynaptic GABAergic responses. Brain-derived neurotrophic factor (BDNF) increases neuronal excitability through a conjunction of mechanisms that include regulation of the efficacy of GABAergic transmission. Here, we show that exposure of rat hippocampal slice cultures and acute slices to exogenous BDNF or neurotrophin-4 produces a TrkB-mediated fall in the neuron-specific K+–Cl− cotransporter KCC2 mRNA and protein, as well as a consequent impairment in neuronal Cl− extrusion capacity. After kindling-induced seizures in vivo, the expression of KCC2 is down-regulated in the mouse hippocampus with a spatiotemporal profile complementary to the up-regulation of TrkB and BDNF. The present data demonstrate a novel mechanism whereby BDNF/TrkB signaling suppresses chloride-dependent fast GABAergic inhibition, which most likely contributes to the well-known role of TrkB-activated signaling cascades in the induction and establishment of epileptic activity.

Perturbed Chloride Homeostasis and GABAergic Signaling in Human Temporal Lobe Epilepsy

Changes in chloride (Cl−) homeostasis may be involved in the generation of some epileptic activities. In this study, we asked whether Cl− homeostasis, and thus GABAergic signaling, is altered in tissue from patients with mesial temporal lobe epilepsy associated with hippocampal sclerosis. Slices prepared from this human tissue generated a spontaneous interictal-like activity that was initiated in the subiculum. Records from a minority of subicular pyramidal cells revealed depolarizing GABAA receptor-mediated postsynaptic events, indicating a perturbed Cl− homeostasis. We assessed possible contributions of changes in expression of the potassium–chloride cotransporter KCC2. Double in situ hybridization showed that mRNA for KCC2 was absent from ∼30% of CaMKIIα (calcium/calmodulin-dependent protein kinase IIα)-positive subicular pyramidal cells. Combining intracellular recordings with biocytin-filled electrodes and KCC2 immunochemistry, we observed that all cells that were hyperpolarized during interictal events were immunopositive for KCC2, whereas the majority of depolarized cells were immunonegative. Bumetanide, at doses that selectively block the chloride-importing potassium–sodium–chloride cotransporter NKCC1, produced a hyperpolarizing shift in GABAA reversal potentials and suppressed interictal activity. Changes in Cl− transporter expression thus contribute to human epileptiform activity, and molecules acting on these transporters may be useful antiepileptic drugs.

Mechanism of Activity-Dependent Downregulation of the Neuron-Specific K-Cl Cotransporter KCC2

GABA-mediated fast-hyperpolarizing inhibition depends on extrusion of chloride by the neuron-specific K-Cl cotransporter, KCC2. Here we show that sustained interictal-like activity in hippocampal slices downregulates KCC2 mRNA and protein expression in CA1 pyramidal neurons, which leads to a reduced capacity for neuronal Cl- extrusion. This effect is mediated by endogenous BDNF acting on tyrosine receptor kinase B (TrkB), with down-stream cascades involving both Shc/FRS-2 (src homology 2 domain containing transforming protein/FGF receptor substrate 2) and PLCγ (phospholipase Cγ)-cAMP response element-binding protein signaling. The plasmalemmal KCC2 has a very high rate of turnover, with a time frame that suggests a novel role for changes in KCC2 expression in diverse manifestations of neuronal plasticity. A downregulation of KCC2 may be a general early response involved in various kinds of neuronal trauma.

Interstimulus interval dependence of the auditory vertex response and its magnetic counterpart: Implications for their neural generation

Auditory vertex responses elicited by short tone bursts were compared with their magnetic counter parts. Special attention was paid to the behaviour of the N100 deflection of the response. Electrical responses were recorded from scalp locations Fp2, Fz, Cz, Pz, C4 and T4 and the magnetic responses half way between P4 and T6, at a point where the response has one of its amplitude extrema. Different ISIs (from 1 to 16 sec) were applied in order to differentiate specific and nonspecific evoked potential components from each other. The main results were as follows: (1) The scalp distsribution of the electical vertex response depends on the ISI used: with frequent stimulation there are no marked differences in the amplitudes of N100 between frontal and central areas but with long ISIs the amplitude maxima move to the vertex. (2) The magnetic responses also show a clear ISI dependence. The magnetic counterpart of N100 saturates at shorter ISIs than N100 recorded from the vertex. Independent of the ISI the magnetic counterpart of P200 is constantly very small. On the basis of the different sensitivities of the EEG and MEG to current sources of different orientations it is concluded that the auditory vertex response contains both modality specific and non-specific components. Experimental conditions, especially the ISI used, determine the relative contributions of these components to the potential recorded on the scalp.