Increase of delayed rectifier potassium currents in large aspiny neurons in the neostriatum following transient forebrain ischemia

Kir6.2-deficient mice develop somatosensory dysfunction and axonal loss in the peripheral nerves

Glucose-responsive ATP-sensitive potassium channels (K_ATP) are expressed in a variety of tissues including nervous systems. The depolarization of the membrane potential induced by glucose may lead to hyperexcitability of neurons and induce excitotoxicity. However, the roles of K_ATP in the peripheral nervous system (PNS) are poorly understood. Here, we determine the roles of K_ATP in the PNS using K_ATP-deficient (Kir6.2-deficient) mice. We demonstrate that neurite outgrowth of dorsal root ganglion (DRG) neurons was reduced by channel closers sulfonylureas. However, a channel opener diazoxide elongated the neurite. K_ATP subunits were expressed in mouse DRG, and expression of certain subunits including Kir6.2 was increased in diabetic mice. In Kir6.2-deficient mice, the current perception threshold, thermal perception threshold, and sensory nerve conduction velocity were impaired. Electron microscopy revealed a reduction of unmyelinated and small myelinated fibers in the sural nerves. In conclusion, K_ATP may contribute to the development of peripheral neuropathy.

Hypoxia with inflammation and reperfusion alters membrane resistance by dynamically regulating voltage-gated potassium channels in hippocampal CA1 neurons

Hypoxia typically accompanies acute inflammatory responses in patients and animal models. However, a limited number of studies have examined the effect of hypoxia in combination with inflammation (Hypo-Inf) on neural function. We previously reported that neuronal excitability in hippocampal CA1 neurons decreased during hypoxia and greatly rebounded upon reoxygenation. We attributed this altered excitability mainly to the dynamic regulation of hyperpolarization-activated cyclic nucleotide-gated cation (HCN) channels and input resistance. However, the molecular mechanisms underlying input resistance changes by Hypo-Inf and reperfusion remained unclear. In the present study, we found that a change in the density of the delayed rectifier potassium current (IDR) can explain the input resistance variability. Furthermore, voltage-dependent inactivation of A-type potassium (IA) channels shifted in the depolarizing direction during Hypo-Inf and reverted to normal upon reperfusion without a significant alteration in the maximum current density. Our results indicate that changes in the input resistance, and consequently excitability, caused by Hypo-Inf and reperfusion are at least partially regulated by the availability and voltage dependence of KV channels. Moreover, these results suggest that selective KV channel modulators can be used as potential neuroprotective drugs to minimize hypoxia- and reperfusion-induced neuronal damage.

A- and D-type potassium currents regulate axonal action potential repolarization in midbrain dopamine neurons

Midbrain dopamine neurons (DANs) regulate various brain functions such as motor control and motivation. Alteration of spiking activities of these neurons could contribute to severe brain disorders including Parkinson's disease and depression. Previous studies showed important roles of somatodendritic voltage-gated K⁺ channels (Kv) of DANs in governing neuronal excitability and dopamine release. However, it remains largely unclear about the biophysical properties and the function of Kv channels distributed at DAN axons. We performed whole-cell recordings from the axons of DANs in acute mouse midbrain and striatal slices. We detected both rapidly activating/inactivating Kv current (i.e. A-current) and rapidly activating but slowly inactivating current (i.e. D-current) in DAN axons. Pharmacological experiments with channel blockers revealed that these currents are predominantly mediated by Kv1.4 and Kv1.2 subunits, respectively. Blocking these currents could substantially prolong axonal action potentials (APs) via a reduction of their repolarization slope. Together, our results show that Kv channels mediating A- and D-currents shape AP waveforms in midbrain DAN axons, through this regulation they may control dopamine release at the axonal terminals. Therefore, these axonal Kv channels could be drug targets for brain disorders with abnormal dopamine release.

In vivo Two-Photon Imaging Reveals Acute Cerebral Vascular Spasm and Microthrombosis After Mild Traumatic Brain Injury in Mice

Mild traumatic brain injury (mTBI), or concussion, is reported to interfere with cerebral blood flow and microcirculation in patients, but our current understanding is quite limited and the results are often controversial. Here we used longitudinal in vivo two-photon imaging to investigate dynamic changes in cerebral vessels and velocities of red blood cells (RBC) following mTBI. Closed-head mTBI induced using a controlled cortical impact device resulted in a significant reduction of dwell time in a Rotarod test but no significant change in water maze test. Cerebral blood vessels were repeatedly imaged through a thinned skull window at baseline, 0.5, 1, 6 h, and 1 day following mTBI. In both arterioles and capillaries, their diameters and RBC velocities were significantly decreased at 0.5, 1, and 6 h after injury, and recovered in 1 day post-mTBI. In contrast, decreases in the diameter and RBC velocity of venules occurred only in 0.5–1 h after mTBI. We also observed formation and clearance of transient microthrombi in capillaries within 1 h post-mTBI. We concluded that in vivo two-photon imaging is useful for studying earlier alteration of vascular dynamics after mTBI and that mTBI induced reduction of cerebral blood flow, vasospasm, and formation of microthrombi in the acute stage following injury. These changes may contribute to early brain functional deficits of mTBI.

Involvement of HCN Channel in Muscarinic Inhibitory Action on Tonic Firing of Dorsolateral Striatal Cholinergic Interneurons

The striatum is the most prominent nucleus in the basal ganglia and plays an important role in motor movement regulation. The cholinergic interneurons (ChIs) in striatum are involved in the motion regulation by releasing acetylcholine (ACh) and modulating the output of striatal projection neurons. Here, we report that muscarinic ACh receptor (M receptor) agonists, ACh and Oxotremorine (OXO-M), decreased the firing frequency of ChIs by blocking the hyperpolarization-activated cyclic nucleotide-gated (HCN) channels. Scopolamine (SCO), a nonselective antagonist of M receptors, abolished the inhibition. OXO-M exerted its function by activating the Gi/o cAMP signaling cascade. The single-cell reverse transcription polymerase chain reaction (scRT-PCR) revealed that all the five subtypes of M receptors and four subtypes of HCN channels were expressed on ChIs. Among them, M2 receptors and HCN2 channels were the most dominant ones and expressed in every single studied cholinergic interneuron (ChI).Our results suggest that ACh regulates not only the output of striatal projection neurons, but also the firing activity of ChIs themselves by activating presynaptic M receptors in the dorsal striatum. The activation of M2 receptors and blockage of HCN2 channels may play an important role in ACh inhibition on the excitability of ChIs. This finding adds a new G-protein coupled receptor mediated regulation on ChIs and provides a cellular mechanism for control of cholinergic activity and ACh release in the dorsal striatum.

Simultaneous recording of brain extracellular glucose, spike and local field potential in real time using an implantable microelectrode array with nano-materials

Glucose is the main substrate for neurons in the central nervous system. In order to efficiently characterize the brain glucose mechanism, it is desirable to determine the extracellular glucose dynamics as well as the corresponding neuroelectrical activity in vivo. In the present study, we fabricated an implantable microelectrode array (MEA) probe composed of platinum electrochemical and electrophysiology microelectrodes by standard micro electromechanical system (MEMS) processes. The MEA probe was modified with nano-materials and implanted in a urethane-anesthetized rat for simultaneous recording of striatal extracellular glucose, local field potential (LFP) and spike on the same spatiotemporal scale when the rat was in normoglycemia, hypoglycemia and hyperglycemia. During these dual-mode recordings, we observed that increase of extracellular glucose enhanced the LFP power and spike firing rate, while decrease of glucose had an opposite effect. This dual mode MEA probe is capable of examining specific spatiotemporal relationships between electrical and chemical signaling in the brain, which will contribute significantly to improve our understanding of the neuron physiology.

Mechanisms of gain control by voltage-gated channels in intrinsically-firing neurons

Gain modulation is a key feature of neural information processing, but underlying mechanisms remain unclear. In single neurons, gain can be measured as the slope of the current-frequency (input-output) relationship over any given range of inputs. While much work has focused on the control of basal firing rates and spike rate adaptation, gain control has been relatively unstudied. Of the limited studies on gain control, some have examined the roles of synaptic noise and passive somatic currents, but the roles of voltage-gated channels present ubiquitously in neurons have been less explored. Here, we systematically examined the relationship between gain and voltage-gated ion channels in a conductance-based, tonically-active, model neuron. Changes in expression (conductance density) of voltage-gated channels increased (Ca²⁺ channel), reduced (K⁺ channels), or produced little effect (h-type channel) on gain. We found that the gain-controlling ability of channels increased exponentially with the steepness of their activation within the dynamic voltage window (voltage range associated with firing). For depolarization-activated channels, this produced a greater channel current per action potential at higher firing rates. This allowed these channels to modulate gain by contributing to firing preferentially at states of higher excitation. A finer analysis of the current-voltage relationship during tonic firing identified narrow voltage windows at which the gain-modulating channels exerted their effects. As a proof of concept, we show that h-type channels can be tuned to modulate gain by changing the steepness of their activation within the dynamic voltage window. These results show how the impact of an ion channel on gain can be predicted from the relationship between channel kinetics and the membrane potential during firing. This is potentially relevant to understanding input-output scaling in a wide class of neurons found throughout the brain and other nervous systems.

Voltage-gated potassium channels at the crossroads of neuronal function, ischemic tolerance, and neurodegeneration

Voltage-gated potassium (Kv) channels are widely expressed in the central and peripheral nervous system and are crucial mediators of neuronal excitability. Importantly, these channels also actively participate in cellular and molecular signaling pathways that regulate the life and death of neurons. Injury-mediated increased K(+) efflux through Kv2.1 channels promotes neuronal apoptosis, contributing to widespread neuronal loss in neurodegenerative disorders such as Alzheimer's disease and stroke. In contrast, some forms of neuronal activity can dramatically alter Kv2.1 channel phosphorylation levels and influence their localization. These changes are normally accompanied by modifications in channel voltage dependence, which may be neuroprotective within the context of ischemic injury. Kv1 and Kv7 channel dysfunction leads to neuronal hyperexcitability that critically contributes to the pathophysiology of human clinical disorders such as episodic ataxia and epilepsy. This review summarizes the neurotoxic, neuroprotective, and neuroregulatory roles of Kv channels and highlights the consequences of Kv channel dysfunction on neuronal physiology. The studies described in this review thus underscore the importance of normal Kv channel function in neurons and emphasize the therapeutic potential of targeting Kv channels in the treatment of a wide range of neurological diseases.

Mature and precursor brain-derived neurotrophic factor have individual roles in the mouse olfactory bulb

Background

Sensory deprivation induces dramatic morphological and neurochemical changes in the olfactory bulb (OB) that are largely restricted to glomerular and granule layer interneurons. Mitral cells, pyramidal-like neurons, are resistant to sensory-deprivation-induced changes and are associated with the precursor to brain-derived neurotrophic factor (proBDNF); here, we investigate its unknown function in the adult mouse OB.

Principal Findings

As determined using brain-slice electrophysiology in a whole-cell configuration, brain-derived neurotrophic factor (BDNF), but not proBDNF, increased mitral cell excitability. BDNF increased mitral cell action potential firing frequency and decreased interspike interval in response to current injection. In a separate set of experiments, intranasal delivery of neurotrophic factors to awake, adult mice was performed to induce sustained interneuron neurochemical changes. ProBDNF, but not BDNF, increased activated-caspase 3 and reduced tyrosine hydroxylase immunoreactivity in OB glomerular interneurons. In a parallel set of experiments, short-term sensory deprivation produced by unilateral naris occlusion generated an identical phenotype.

Conclusions

Our results indicate that only mature BDNF increases mitral cell excitability whereas proBDNF remains ineffective. Our demonstration that proBDNF activates an apoptotic marker in vivo is the first for any proneurotrophin and establishes a role for proBDNF in a model of neuronal plasticity.

Up-regulation of A-type potassium currents protects neurons against cerebral ischemia

Excitotoxicity is the major cause of many neurologic disorders including stroke. Potassium currents modulate neuronal excitability and therefore influence the pathological process. A-type potassium current (I(A)) is one of the major voltage-dependent potassium currents, yet its roles in excitotoxic cell death are not well understood. We report that, following ischemic insults, the I(A) increases significantly in large aspiny (LA) neurons but not medium spiny (MS) neurons in the striatum, which correlates with the higher resistance of LA neurons to ischemia. Activation of protein kinase Cα increases I(A) in LA neurons after ischemia. Cultured neurons from transgenic mice lacking both Kv1.4 and Kv4.2 subunits exhibit an increased vulnerability to ischemic insults. Increase of I(A) by recombinant expression of Kv1.4 or Kv4.2 is sufficient in improving the survival of MS neurons against ischemic insults both in vitro and in vivo. These results, taken together, provide compelling evidence for a protective role of I(A) against ischemia.

Excitatory roles of protein kinase C in striatal cholinergic interneurons

Protein kinase C (PKC) plays critical roles in neuronal activity and is widely expressed in striatal neurons. However, it is not clear how PKC activation regulates the excitability of striatal cholinergic interneurons. In the present study, we found that PKC activation significantly inhibited A-type potassium current (I(A)), but had no effect on delayed rectifier potassium currents. Consistently, application of PKC activator caused an increase of firing in response to depolarizing currents in cholinergic interneurons, which was persistent in the presence of both excitatory and inhibitory neurotransmission blockers. These excitatory effects of PKC could be partially mimicked and occluded by blockade of I(A) with potassium channel blocker 4-aminopyridine. In addition, immunostaining demonstrated that PKCalpha, but not PKCgamma and PKCepsilon, was expressed in cholinergic interneurons. Furthermore, activation of group I metabotropic glutamate receptors (mGluRs) led to an inhibition of I(A) through a PKC-dependent pathway. These data indicate that activation of PKC, most likely PKCalpha, increases the neuronal excitability of striatal cholinergic interneurons by down-regulating I(A). Group I mGluR-mediated I(A) inhibition might be important for the glutamatergic regulation of cholinergic tone in the neostriatum.

Dopamine D1-Like Receptors Depress Excitatory Synaptic Transmissions in Striatal Neurons After Transient Forebrain Ischemia

Background and Purpose— Spiny neurons in the neostriatum are highly vulnerable to ischemia. Despite an enormous body of research suggesting that dopamine is involved in ischemia-induced neuronal loss in the striatum, it remains unclear how dopamine interacts with the glutamatergic excitotoxicity that is widely accepted as a major cause of ischemic cell death. Our study was designed to investigate the effects of dopamine D1 receptor (D1R) activation on excitatory neurotransmission in postischemic striatal neurons.

Methods— We used the 4-vessel occlusion ischemia model and brain slice preparations. Whole-cell voltage-clamp recording was performed on striatal neurons to measure excitatory postsynaptic currents (EPSCs). Systemic administration of a D1R agonist after ischemia and hematoxylin/eosin staining were performed to evaluate the effects of D1R activation on ischemia-induced neuronal degeneration in the striatum.

Results— D1R activation depressed EPSCs in postischemic striatal neurons. The depression was attributable to inhibition of presynaptic release. An activator of cAMP-dependent protein kinase A (PKA) mimicked the depressive effects of D1R activation. Bath application of a PKA inhibitor blocked the depression of EPSCs, whereas intracellular postsynaptic application of the PKA inhibitor had no effect. The D1R agonist failed to reduce EPSC amplitude in the presence of an adenosine A1 receptor antagonist. Systemic administration of a D1R agonist after ischemia significantly attenuated ischemia-induced cell death in the striatum.

Conclusions— These results indicate that D1R activation presynaptically depresses excitatory synaptic transmission in striatal neurons after ischemia through activation of PKA and adenosine A1 receptors and thus demonstrate a novel mechanism of D1R-mediated protection against ischemia.

Inhibition of Ih in striatal cholinergic interneurons early after transient forebrain ischemia

Striatal cholinergic interneurons are relatively resistant to ischemic insults. These neurons express hyperpolarization-activated cation current (I(h)) that profoundly regulates neuronal excitability. Changes in neuronal excitability early after ischemia may be crucial for determining neuronal injury. Here we report that I(h) in cholinergic interneurons was decreased 3 h after transient forebrain ischemia, which was accompanied by a negative shift of the voltage dependence of activation. The inhibition of I(h) might be due to the tonic activation of adenosine A1 receptors, as blockade of A1 receptors significantly increased I(h) in postischemic neurons, but had no effect on control neurons. Consistent with the inhibition of I(h), postischemic neurons showed a reduction in both spontaneous firing and hyperpolarization-induced rebound depolarization. These findings indicate that I(h) may play excitatory roles in striatal cholinergic interneurons. Postischemic inhibition of I(h) might be a novel mechanism by which adenosine confers neuronal resistance to cerebral ischemia.

Involvement of I(h) in dopamine modulation of tonic firing in striatal cholinergic interneurons

Striatal cholinergic interneurons are tonically active neurons and respond to sensory stimuli by transiently suppressing firing that is associated with sensorimotor learning. The pause in tonic firing is dependent on dopaminergic activity; however, its cellular mechanisms remain unclear. Here, we report evidence that dopaminergic inhibition of hyperpolarization-activated cation current (I(h)) is involved in this process. In neurons exhibiting regular firing in vitro, exogenous application of dopamine caused a prolongation of the depolarization-induced pause and an increase in the duration of slow afterhyperpolarization (sAHP) after depolarization. Partially blocking I(h) with specific blocker ZD7288 (4-ethylphenylamino-1,2-dimethyl-6-methylaminopyrimidinium chloride) reduced firing and mimicked the effects of dopamine on sAHP. The I(h), being active at membrane potentials negative than -50 mV, was inhibited by dopamine via activation of the D2-like receptor, but not D1-like receptor. The inhibitory effects of the D2 receptor activation on I(h) were mediated through a protein kinase A-independent cyclic AMP pathway. Consistently, D2-like receptor agonist quinpirole showed comparable effects on sAHP and firing rate as those induced by I(h) channel blocker. Moreover, dopamine was unable to further affect the sAHP duration in neurons when I(h) was blocked. These findings indicate that D2 receptor-dependent inhibition of I(h) may be a novel mechanism for modulating the pause response in tonic firing in cholinergic interneurons.

Enhancement of Excitatory Synaptic Transmission in Spiny Neurons After Transient Forebrain Ischemia

Spiny neurons in the neostriatum are highly vulnerable to ischemia. Enhancement of excitatory synaptic transmissions has been implicated in ischemia-induced excitotoxic neuronal death. Here we report that evoked excitatory postsynaptic currents in spiny neurons were potentiated after transient forebrain ischemia. The ischemia-induced potentiation in synaptic efficacy was associated with an enhancement of presynaptic release as demonstrated by an increase in the frequency of miniature excitatory postsynaptic currents (mEPSCs) and a decrease in the paired-pulse ratio. The amplitude of inward currents evoked by exogenous application of glutamate did not show significant changes after ischemia, suggesting that postsynaptic mechanism is not involved. The ischemia-induced increase in mEPSCs frequency was not affected by blockade of voltage-gated calcium channels, but it was eliminated in the absence of extracellular calcium. Bath application of ATP P2X receptor antagonist pyridoxal-phosphate-6-azophenyl-2′,4′-disulfonic acid (PPADS) significantly reduced mEPSC frequency in ischemic neurons but had no effects on the control ones. Furthermore, the inhibitory effect of PPADS on ischemic neurons was abolished in Ca2+-free external solution. These results indicate that excitatory synaptic transmissions in spiny neurons are potentiated after ischemia via presynaptic mechanisms. Activation of P2X receptors and the consequent Ca2+ influx might contribute to the ischemia-induced facilitation of glutamate release.

Calcium- and metabolic state-dependent modulation of the voltage-dependent Kv2.1 channel regulates neuronal excitability in response to ischemia

Ischemic stroke is often accompanied by neuronal hyperexcitability (i.e., seizures), which aggravates brain damage. Therefore, suppressing stroke-induced hyperexcitability and associated excitoxicity is a major focus of treatment for ischemic insults. Both ATP-dependent and Ca2+-activated K+ channels have been implicated in protective mechanisms to suppress ischemia-induced hyperexcitability. Here we provide evidence that the localization and function of Kv2.1, the major somatodendritic delayed rectifier voltage-dependent K+ channel in central neurons, is regulated by hypoxia/ischemia-induced changes in metabolic state and intracellular Ca2+ levels. Hypoxia/ischemia in rat brain induced a dramatic dephosphorylation of Kv2.1 and the translocation of surface Kv2.1 from clusters to a uniform localization. In cultured rat hippocampal neurons, chemical ischemia (CI) elicited a similar dephosphorylation and translocation of Kv2.1. These events were reversible and were mediated by Ca2+ release from intracellular stores and calcineurin-mediated Kv2.1 dephosphorylation. CI also induced a hyperpolarizing shift in the voltage-dependent activation of neuronal delayed rectifier currents (IK), leading to enhanced IK and suppressed neuronal excitability. The IK blocker tetraethylammonium reversed the ischemia-induced suppression of excitability and aggravated ischemic neuronal damage. Our results show that Kv2.1 can act as a novel Ca2+- and metabolic state-sensitive K+ channel and suggest that dynamic modulation of IK/Kv2.1 in response to hypoxia/ischemia suppresses neuronal excitability and could confer neuroprotection in response to brief ischemic insults.