How the brain transitions into a seizure is poorly understood. Recurrent seizure-like events (SLEs) in low-Mg 2ϩ /high-K ϩ perfusate were measured in the CA3 region of the intact mouse hippocampus. The SLE was divided into a "preictal phase," which abruptly turns into a higher frequency "ictal" phase. Blockade of GABA A receptors shortened the preictal phase, abolished interictal bursts, and attenuated the slow preictal depolarization, with no effect on the ictal duration, whereas SLEs were blocked by glutamate receptor blockade. In CA3 pyramidal cells and stratum oriens non-fast-spiking and fast-spiking interneurons, recurrent GABAergic IPSCs predominated interictally and during the early preictal phase, synchronous with extracellularly measured recurrent field potentials (FPs). These IPSCs then decreased to zero or reversed polarity by the onset of the higher-frequency ictus. However, postsynaptic muscimol-evoked GABA A responses remained intact. Simultaneously, EPSCs synchronous with the FPs markedly increased to a maximum at the ictal onset. The reversal potential of the compound postsynaptic currents (combined simultaneous EPSCs and IPSCs) became markedly depolarized during the preictal phase, whereas the muscimol-evoked GABA A reversal potential remained unchanged. During the late preictal phase, interneuronal excitability was high, but IPSCs, evoked by local stimulation, or osmotically by hypertonic sucrose application, were diminished, disappearing at the ictal onset. We conclude that the interictal and early preictal states are dominated by GABAergic activity, with the onset of the ictus heralded by exhaustion of presynaptic release of GABA, and unopposed increased glutamatergic responses.

] c 8.9 ± 0.7% and 3.8 ± 0.3%, while decreasing normoxic whole-cell NMDAR currents by 41.1 ± 6.7% and 55.4 ± 10.2%, respectively. These changes were also blocked by 5HD or glibenclamide, BAPTA, or spermine. Blockade of mitochondrial Ca 2+ -uptake decreased normoxic NMDAR currents 47.0 ± 3.1% and this change was blocked by BAPTA but not by 5HD. Taken together, these data suggest mK ATP channel activation in the anoxic turtle cortex uncouples mitochondria and reduces mitochondrial Ca 2+ uptake via the uniporter, subsequently increasing [Ca 2+ ] c and decreasing NMDAR activity.

The mammalian brain undergoes rapid cell death during anoxia that is characterized by uncontrolled Ca(2+) entry via N-methyl-D-aspartate receptors (NMDARs). In contrast, the western painted turtle is extremely anoxia tolerant and maintains close-to-normal [Ca(2+)](i) during periods of anoxia lasting from days to months. A plausible mechanism of anoxic survival in turtle neurons is the regulation of NMDARs to prevent excitotoxic Ca(2+) injury. However, studies using metabolic inhibitors such as cyanide (NaCN) as a convenient method to induce anoxia may not represent a true anoxic stress. This study was undertaken to determine whether turtle cortical neuron whole-cell NMDAR currents respond similarly to true anoxia with N(2) and to NaCN-induced anoxia. Whole-cell NMDAR currents were measured during a control N(2)-induced anoxic transition and a control NaCN-induced transition. During anoxia with N(2) normalized, NMDAR currents decreased to 35.3%+/-10.8% of control values. Two different NMDAR current responses were observed during NaCN-induced anoxia: one resulted in a 172%+/-51% increase in NMDAR currents, and the other was a decrease to 48%+/-14% of control. When responses were correlated to the two major neuronal subtypes under study, we found that stellate neurons responded to NaCN treatment with a decrease in NMDAR current, while pyramidal neurons exhibited both increases and decreases. Our results show that whole-cell NMDAR currents respond differently to NaCN-induced anoxia than to the more physiologically relevant anoxia with N(2).

Without oxygen, all mammals suffer neuronal injury and excitotoxic cell death mediated by overactivation of the glutamatergic N-methyl-D-aspartate receptor (NMDAR). The western painted turtle can survive anoxia for months, and downregulation of NMDAR activity is thought to be neuroprotective during anoxia. NMDAR activity is related to the activity of another glutamate receptor, the ␣-amino-3-hydroxy-5-methylisoxazole-4-propionic acid receptor (AMPAR). AMPAR blockade is neuroprotective against anoxic insult in mammals, but the role of AMPARs in the turtle's anoxia tolerance has not been investigated. To determine whether AMPAR activity changes during hypoxia or anoxia in the turtle cortex, whole cell AMPAR currents, AMPAR-mediated excitatory postsynaptic potentials (EPSPs), and excitatory postsynaptic currents (EPSCs) were measured. The effect of AMPAR blockade on normoxic and anoxic NMDAR currents was also examined. During 60 min of normoxia, evoked peak AMPAR currents and the frequencies and amplitudes of EPSPs and EPSCs did not change. During anoxic perfusion, evoked AMPAR peak currents decreased 59.2 Ϯ 5.5 and 60.2 Ϯ 3.5% at 20 and 40 min, respectively. EPSP frequency (EPSP ƒ) and amplitude decreased 28.7 Ϯ 6.4% and 13.2 Ϯ 1.7%, respectively, and EPSCƒ and amplitude decreased 50.7 Ϯ 5.1% and 51.3 Ϯ 4.7%, respectively. In contrast, hypoxic (PO2 ϭ 5%) AMPAR peak currents were potentiated 56.6 Ϯ 20.5 and 54.6 Ϯ 15.8% at 20 and 40 min, respectively. All changes were reversed by reoxygenation. AMPAR currents and EPSPs were abolished by 6-cyano-7-nitroquinoxaline-2,3-dione (CNQX). In neurons pretreated with CNQX, anoxic NMDAR currents were reversibly depressed by 49.8 Ϯ 7.9%. These data suggest that AMPARs may undergo channel arrest in the anoxic turtle cortex.N-methyl-D-aspartate receptor; excitotoxic cell death; spike arrest THE WESTERN PAINTED TURTLE (Chrysemys picta belli) can survive without oxygen for days to months (43). This tolerance is predicated on the turtle's ability to maintain neuronal function by using anaerobically produced ATP. In contrast, mammals cannot survive on anaerobic metabolism alone. Oxygen deprivation induces elevations in excitatory amino acids, which leads to overexcitation of glutamate receptors, ATP loss, anoxic depolarization, excessive Ca 2ϩ influx, and rapid excitotoxic cell death (ECD) (6,11,24,36,41). Two glutamate receptors, the N-methyl-D-aspartate receptor (NMDAR) and the ␣-amino-3-hydroxy-5-methyl-4-isoxazole propionic acid receptor (AMPAR), are the principle mediators of ECD. In mammals, the activity of both receptors is increased during hypoxia (14, 29), and blockade of either receptor is neuroprotective against global or focal ischemia (15,42).A proposed mechanism used by facultative anaerobes to prolong anoxia tolerance and prevent ECD is a decrease in membrane permeability and downregulation of ion channel or receptor activity termed "channel arrest" (17). Indeed, in contrast to the commonly observed anoxic potentiation of mammalian NMDAR currents, turtle NMDARs underg...

No abstract

No abstract

Shin DS, Carlen PL. Enhanced I h depresses rat entopeduncular nucleus neuronal activity from high-frequency stimulation or raised K e ϩ . J Neurophysiol 99: 2203-2219, 2008. First published February 27, 2008 doi:10.1152/jn.01065.2007. High-frequency stimulation (HFS) is used to treat a variety of neurological diseases, yet its underlying therapeutic action is not fully elucidated. Previously, we reported that HFS-induced elevation in [K ϩ ] e or bath perfusion of raised K e ϩ depressed rat entopeduncular nucleus (EP) neuronal activity via an enhancement of an ionic conductance leading to marked depolarization. Herein, we show that the hyperpolarization-activated (I h ) channel mediates the HFS-or K ϩ -induced depression of EP neuronal activity. The perfusion of an I h channel inhibitor, 50 M ZD7288 or 2 mM CsCl, increased input resistance by 23.5 Ϯ 7% (ZD7288) or 35 Ϯ 10% (CsCl), hyperpolarized cells by 3.4 Ϯ 1.7 mV (ZD7288) or 2.3 Ϯ 0.9 mV (CsCl), and decreased spontaneous action potential (AP) frequency by 51.5 Ϯ 12.5% (ZD7288) or 80 Ϯ 13.5% (CsCl). The I h sag was absent with either treatment, suggesting a block of I h channel activity. Inhibition of the I h channel prior to HFS or 6 mM K ϩ perfusion not only prevented the previously observed decrease in AP frequency, but increased neuronal activity. Under voltage-clamp conditions, I h currents were enhanced in the presence of 6 mM K ϩ . Calcium is also involved in the depression of EP neuronal activity, since its removal during raised K e ϩ application prevented this attenuation and blocked the I h sag. We conclude that the enhancement of I h channel activity initiates the HFS-and K ϩ -induced depression of EP neuronal activity. This mechanism could underlie the inhibitory effects of HFS used in deep brain stimulation in output basal ganglia nuclei.

No abstract