Potassium Channels In Hippocampal Neurons Research Articles

α-Synuclein forms non-selective cation channels and stimulates ATP-sensitive potassium channels in hippocampal neurons.

In Parkinson's disease, the protein α-synuclein (αS) is produced within neurons and also appears in the extracellular fluid. In this study in hippocampal neurons, αS formed non-selective cation channels with multiple levels of conductance and rectification depending on their insertion site. αS channels induced local spontaneous increases in intracellular Na(+) and Ca(2+), depolarized neurons, augmented bursting activity and stimulated the opening of ATP-sensitive K(+) channels. Non-selective channels were also observed in neurons transfected with either wild-type or mutant A53T αS, and after extracellular application of these proteins. The properties of αS channels in neuronal membranes suggest that extracellular αS is more toxic than αS produced within neurons. In Parkinson's disease and several other neurodegenerative diseases, the protein α-synuclein (αS) is produced within neurons and accumulates in the extracellular fluid. Several mechanisms of αS action are proposed, one of which is the formation of cation-permeable pores that may mediate toxicity. αS induces non-selective cation channels in lipid bilayers, but whether this occurs in living neurons and which properties the channels possess have not yet been examined. In this study the properties of αS channels in dissociated hippocampal neurons are documented. In cell-attached recordings the incorporation of αS into membranes was driven by applied negative potentials. These channels exhibited multiple levels of conductance (30, 70 and 120 pS at -100 mV) and inward rectification. The persistent activity of αS channels induced local changes in intracellular Na(+) and Ca(2+), depolarized neurons and augmented bursting activity. αS channels formed by adding αS to the intracellular membrane in inside-out patches exhibited outward rectification. αS channels were equally permeable to Na(+), K(+) and Ca(2+). These channels were also observed in neurons transfected with wild-type or mutant A53T αS, and after extracellular application of wild-type or mutant A53T αS proteins. Opening of αS channels stimulated opening of ATP-sensitive K(+) (KATP ) channels and did not interfere with the activity of delayed rectifier K(+) channels. The properties of αS channels in neuronal membranes suggest stronger toxicity of extracellularly applied αS than intracellular αS. Enhancement of neuronal excitability and distortions in ion homeostasis may underlie the toxic effects of αS that can be dampened by KATP channels.

Alterations of A-Type Potassium Channels in Hippocampal Neurons after Traumatic Brain Injury

Traumatic brain injury (TBI) is associated with cognitive deficits, memory impairment, and epilepsy. Previous studies have reported neuronal loss and neuronal hyperexcitability in the post-traumatic hippocampus. A-type K+ currents (I(A)) play a critical role in modulating the intrinsic membrane excitability of hippocampal neurons. The disruption of I(A) is reportedly linked to hippocampal dysfunction. The present study investigates the changes of I(A) in the hippocampus after TBI. TBI in rats was induced by controlled cortical impact. The impact induced a reproducible lesion in the cortex and an obvious neuronal death in the ipsilateral hippocampus CA3 region. At one week after TBI, immunohistochemical staining and Western blotting showed that the expression of I(A) channel subunit Kv4.2 was markedly decreased in the ipsilateral hippocampus, but remained unchanged in the contralateral hippocampus. Meanwhile, electrophysiological recording showed that I(A) currents in ipsilateral CA1 pyramidal neurons were significantly reduced, which was associated with an increased neuronal excitability. Furthermore, there was an increased sensitivity to bicuculline-induced seizures in TBI rats. At 8 weeks after TBI, immunohistochemical staining and electrophysiological recording indicated that I(A) returned to control levels. These findings suggest that TBI causes a transient downregulation of I(A) in hippocampal CA1 neurons, which might be associated with the hyperexcitability in the post-traumatic hippocampus, and in turn leads to seizures and epilepsy.

In vitro and in vivo characterization of the novel GABA B receptor positive allosteric modulator, 2-{1-[2-(4-chlorophenyl)-5-methylpyrazolo[1,5-a]pyrimidin-7-yl]-2-piperidinyl}ethanol (CMPPE)

There is preclinical evidence supporting the finding that the GABA B receptor orthosteric agonist, baclofen, has significant effects on eating behavior suggesting the potential therapeutic application of this compound for the treatment of eating related disorders. However, the wide clinical use of baclofen might be limited by the appearance of sedative and motor impairment effects. The identification of positive allosteric modulators (PAMs) of GABA B receptors represents a novel therapeutic approach to reduce the centrally-mediated adverse effects typical of the GABA B receptor orthosteric agonist. In the present work, we report the in vitro profile of a novel chemical structure, 2-{1-[2-(4-chlorophenyl)-5-methylpyrazolo[1,5-a]pyrimidin-7-yl]-2-piperidinyl}ethanol (CMPPE) identified by screening the GSK compound collection. CMPPE potentiates GABA-stimulated [ 35S]GTPγS binding to membranes of human recombinant cell line and of rat brain cortex. GABA concentration-response curves (CRC) in the presence of fixed concentrations of CMPPE, in rat native tissue, revealed an increase of both the potency and maximal efficacy of GABA. A similar modulatory effect was observed in GABA B receptor-mediated activation of inwardly rectifying potassium channels in hippocampal neurons. CMPPE (30–100 mg/kg) and GS39783 (100 mg/kg) significantly decreased food consumption in rat without impairment on the animal locomotor activity. On the contrary, baclofen (2.5 mg/kg) decreased both food intake and motor performance. All together these findings confirm the role of GABA B system in controlling animal food intake and for the first time demonstrate that GABA B receptor PAMs may represent a novel pharmacological approach to treat eating disorders without unwanted sedative effects.

Glucose Deprivation Activates Diversity of Potassium Channels in Cultured Rat Hippocampal Neurons

1. Glucose is one of the most important substrates for generating metabolic energy required for the maintenance of cellular functions. Glucose-mediated changes in neuronal firing pattern have been observed in the central nervous system of mammals. K(+) channels directly regulated by intracellular ATP have been postulated as a linkage between cellular energetic metabolism and excitability; the functional roles ascribed to these channels include glucose-sensing to regulate energy homeostasis and neuroprotection under energy depletion conditions. The hippocampus is highly sensitive to metabolic insults and is the brain region most sensitive to ischemic damage. Because the identity of metabolically regulated potassium channels present in hippocampal neurons is obscure, we decided to study the biophysical properties of glucose-sensitive potassium channels in hippocampal neurons. 2. The dependence of membrane potential and the sensitivity of potassium channels to glucose and ATP in rat hippocampal neurons were studied in cell-attached and excised inside-out membrane patches. 3. We found that under hypoglycemic conditions, at least three types of potassium channels were activated; their unitary conductance values were 37, 147, and 241 pS in symmetrical K(+), and they were sensitive to ATP. For K(+) channels with unitary conductance of 37 and 241, when the membrane potential was depolarized the longer closed time constant diminished and this produced an increase in the open-state probability; nevertheless, the 147-pS channels were not voltage-dependent. 4. We propose that neuronal glucose-sensitive K(+) channels in rat hippocampus include subtypes of ATP-sensitive channels with a potential role in neuroprotection during short-term or prolonged metabolic stress.

Osmolarity modulates K+ channel function on rat hippocampal interneurons but not CA1 pyramidal neurons.

1. Whole-cell and single-channel recording methods were used in conjunction with infrared video microscopy techniques to examine the properties of voltage-activated potassium channels in hippocampal neurons during the application of hyposmolar solutions to hippocampal slices from rats. 2. Hyposmolar external solutions (osmolarity reduced by 10% to 267 mosmol l-1) produced a significant potentiation of voltage-activated K+ current on lacunosum/moleculare (L/M) hippocampal interneurons, but not on CA1 and subiculum pyramidal neurons. Hyperpolarization-activated (IH) and leak currents were not altered during the application of hyposmolar solutions in all cell types. 3. Mean channel open time and the probability of channel opening were dramatically increased under hyposmolar recording conditions for outside-out patches from L/M interneurons; no changes were observed for patches from CA1 pyramidal neurons. Mean current amplitude and the threshold for channel activation were not affected by hyposmotic challenge. 4. Hyposmolar external solutions produced a significant reduction in the firing frequency of L/M interneurons recorded in current-clamp mode. Hyposmolar solutions had no effect on resting membrane potential, action potential amplitude or duration, and spike after-hyperpolarization amplitude. 5. These results indicate that selective modulation of interneuron ion channel activity may be a critical mechanism by which osmolarity can regulate excitability in the central nervous system.

Effects of divalent cations on the activation of a calcium-dependent potassium channel in hippocampal neurons.

The activation of a calcium-dependent K+ channel K(Ca) in cultured hippocampal neurons has been studied after the addition, to the internal solution, of the divalent cations magnesium (Mg2+), strontium (Sr2+) or barium (Ba2+). With physiological K+ across inside-out patches and Ca2+ present at 0.2 mM in the bath solution, a 90-pS channel was activated with an open probability in excess of 75%. When the internal Ca2+ was reduced to levels near 5 microM, the channel-open probability was significantly diminished. However, if 0.2 mM concentrations of either Mg2+ or Sr2+ were added to the internal solution, the open probability was increased to a value close to original level. In the presence of internal Ca2+ at 0.1 microM, the K(Ca) channel was not active and was not activated with the addition of 0.2 mM Mg2+ or Sr2+ to the internal solution. Thus, Mg2+ or Sr2+ were not able to active K(Ca) in the absence of Ca2+; however, both of these divalent cations could potentiate the Ca(2+)-induced activation of K(Ca) if internal Ca2+ was near 5 microM. The results indicate that Mg2+ could have a role as an internal modulator of K(Ca) in the Ca(2+)-dependent regulation of excitability in nerve membrane. Replacement of Ca2+ with Ba2+, or addition of Ba2+ to Ca-containing solutions, caused significant decreases in the channel-open probability for K(Ca). The action of Ba2+ was primarily mediated by a decrease in the frequency of channel opening. At a concentration of 5 microM, Ba2+ diminished the channel-open probability by one-half.