Dynamic activation of K(ATP) channels in rhythmically active neurons

Ionic mechanisms involved in arginine vasopressin-mediated excitation of auditory cortical and thalamic neurons

The axons containing arginine vasopressin (AVP) from the hypothalamus innervate a variety of structures including the cerebral cortex, thalamus, hippocampus and amygdala. A plethora amount of evidence indicates that activation of the V1a subtype of the vasopressin receptors facilitates anxiety-like and fear responses. As an essential structure involved in fear and anxiety responses, the amygdala, especially the lateral nucleus of amygdala (LA), receives glutamatergic innervations from the auditory cortex and auditory thalamus where high density of V1a receptors have been detected. However, the roles and mechanisms of AVP in these two important areas have not been determined, which prevents the understanding of the mechanisms whereby V1a activation augments anxiety and fear responses. Here, we used coronal brain slices and studied the effects of AVP on neuronal activities of the auditory cortical and thalamic neurons. Our results indicate that activation of V1a receptors excited both auditory cortical and thalamic neurons. In the auditory cortical neurons, AVP increased neuronal excitability by depressing multiple subtypes of inwardly rectifying K+ (Kir) channels including the Kir2 subfamily, the ATP-sensitive K+ channels and the G protein-gated inwardly rectifying K+ (GIRK) channels, whereas activation of V1a receptors excited the auditory thalamic neurons by depressing the Kir2 subfamily of the Kir channels as well as activating the hyperpolarization-activated cyclic nucleotide-gated (HCN) channels and a persistent Na+ channel. Our results may help explain the roles of V1a receptors in facilitating fear and anxiety responses. Categories: Cell Physiology.

Glutamate-Evoked Ca2+ Responses in the Rat Suprachiasmatic Nucleus: Involvement of Na+/K+-ATPase and Na+/Ca2+-Exchanger

Glutamate mediates photic entrainment of the central clock in the suprachiasmatic nucleus (SCN) by evoking intracellular Ca2+ signaling mechanisms. However, the detailed mechanisms of glutamate-evoked Ca2+ signals are not entirely clear. Here, we used a ratiometric Ca2+ and Na+ imaging technique to investigate glutamate-evoked Ca2+ responses. The comparison of Ca2+ responses to glutamate (100 μM) and high (20 mM) K+ solution indicated slower Ca2+ clearance, along with rebound Ca2+ suppression for glutamate-evoked Ca2+ transients. Increasing the length of exposure time in glutamate, but not in 20 mM K+, slowed Ca2+ clearance and increased rebound Ca2+ suppression, a result correlated with glutamate-induced Na+ loads. The rebound Ca2+ suppression was abolished by ouabain, monensin, Na+-free solution, or nimodipine, suggesting an origin of activated Na+/K+-ATPase (NKA) by glutamate-induced Na+ loads. Ouabain or Na+-free solution also slowed Ca2+ clearance, apparently by retarding Na+/Ca2+-exchanger (NCX)-mediated Ca2+ extrusion. Together, our results indicated the involvement of glutamate-induced Na+ loads, NKA, and NCX in shaping the Ca2+ response to glutamate. Nevertheless, in the absence of external Na+ (NMDG substituted), Ca2+ clearance was still slower for the Ca2+ response to glutamate than for 20 mM K+, suggesting participation of additional Ca2+ handlers to the slower Ca2+ clearance under this condition.

Gasotransmitter modulation of hypoglossal motoneuron activity

Obstructive sleep apnea (OSA) is characterized by sporadic collapse of the upper airway leading to periodic disruptions in breathing. Upper airway patency is governed by genioglossal nerve activity that originates from the hypoglossal motor nucleus. Mice with targeted deletion of the gene Hmox2, encoding the carbon monoxide (CO) producing enzyme, heme oxygenase-2 (HO-2), exhibit OSA, yet the contribution of central HO-2 dysregulation to the phenomenon is unknown. Using the rhythmic brainstem slice preparation that contains the preBötzinger complex (preBötC) and the hypoglossal nucleus, we tested the hypothesis that central HO-2 dysregulation weakens hypoglossal motoneuron output. Disrupting HO-2 activity increased the occurrence of subnetwork activity from the preBötC, which was associated with an increased irregularity of rhythmogenesis. These phenomena were also associated with the intermittent inability of the preBötC rhythm to drive output from the hypoglossal nucleus (i.e. transmission failures), and a reduction in the input-output relationship between the preBötC and the motor nucleus. HO-2 dysregulation reduced excitatory synaptic currents and intrinsic excitability in inspiratory hypoglossal neurons. Inhibiting activity of the CO-regulated H2S producing enzyme, cystathionine-γ-lyase (CSE), reduced transmission failures in HO-2 null brainstem slices, which also normalized excitatory synaptic currents and intrinsic excitability of hypoglossal motoneurons. These findings demonstrate a hitherto uncharacterized modulation of hypoglossal activity through mutual interaction of HO-2/CO and CSE/H2S, and support the potential importance of centrally derived gasotransmitter activity in regulating upper airway control.

Brain capillary pericytes are metabolic sentinels that control blood flow through a KATP channel-dependent energy switch

Despite the abundance of capillary thin-strand pericytes and their proximity to neurons and glia, little is known of the contributions of these cells to the control of brain hemodynamics. We demonstrate that the pharmacological activation of thin-strand pericyte KATP channels profoundly hyperpolarizes these cells, dilates upstream penetrating arterioles and arteriole-proximate capillaries, and increases capillary blood flow. Focal stimulation of pericytes with a KATP channel agonist is sufficient to evoke this response, mediated via KIR2.1 channel-dependent retrograde propagation of hyperpolarizing signals, whereas genetic inactivation of pericyte KATP channels eliminates these effects. Critically, we show that decreasing extracellular glucose to less than 1 mM or inhibiting glucose uptake by blocking GLUT1 transporters in vivo flips a mechanistic energy switch driving rapid KATP-mediated pericyte hyperpolarization to increase local blood flow. Together, our findings recast capillary pericytes as metabolic sentinels that respond to local energy deficits by increasing blood flow to neurons to prevent energetic shortfalls.

Immunoreactivity of Muscarinic Acetylcholine M2 and Serotonin 5-HT2B Receptors, Norepinephrine Transporter and Kir Channels in a Model of Epilepsy

Epilepsy is characterized by an imbalance in neurotransmitter activity; an increased excitatory to an inhibitory activity. Acetylcholine (ACh), serotonin, and norepinephrine (NE) may modulate neural activity via several mechanisms, mainly through its receptors/transporter activity and alterations in the extracellular potassium (K⁺) concentration via K⁺ ion channels. Seizures may disrupt the regulation of inwardly rectifying K⁺ (Kir) channels and alter the receptor/transporter activity. However, there are limited data present on the immunoreactivity pattern of these neurotransmitter receptors/transporters and K⁺ channels in chronic models of epilepsy, which therefore was the aim of this study. Changes in the immunoreactivity of epileptogenesis-related neurotransmitter receptors/transporters (M2, 5-HT2B, and NE transporter) as well as Kir channels (Kir3.1 and Kir6.2) were determined in the cortex, hippocampus and medulla of adult Wistar rats by utilizing a Pentylenetetrazol (PTZ)-kindling chronic epilepsy model. Increased immunoreactivity of the NE transporter, M2, and 5-HT2B receptors was witnessed in the cortex and medulla. While the immunoreactivity of the 5-HT2B receptor was found increased in the cortex and medulla, it was decreased in the hippocampus, with no changes observed in the M2 receptor in this region. Kir3.1 and Kir6.2 staining showed increase immunoreactivity in the cerebral cortex, but channel contrasting findings in the hippocampus and medulla. Our results suggest that seizure kindling may result in significant changes in the neurotransmitter system which may contribute or propagate to future epileptogenesis, brain damage and potentially towards sudden unexpected death in epilepsy (SUDEP). Further studies on the pathogenic role of these changes in neurotransmitter receptors/transporters and K⁺ channel immunoreactivity may identify newer possible targets to treat seizures or prevent epilepsy-related comorbidities.

α-Synuclein Aggregates Increase the Conductance of Substantia Nigra Dopamine Neurons, an Effect Partly Reversed by the KATP Channel Inhibitor Glibenclamide

Dopaminergic neurons (DNs) in the substantia nigra pars compacta (SNpc) form an important part of the basal ganglia circuitry, playing key roles in movement initiation and coordination. A hallmark of Parkinson’s disease (PD) is the degeneration of these SNpc DNs leading to akinesia, bradykinesia and tremor. There is gathering evidence that oligomeric α-synuclein (α-syn) is one of the major pathologic species in PD, with its deposition in Lewy bodies (LBs) closely correlated with disease progression. However, the precise mechanisms underlying the effects of oligomeric α-syn on DN function have yet to be fully defined. Here, we have combined electrophysiological recording and detailed analysis to characterize the time-dependent effects of α-syn aggregates (consisting of oligomers and possibly small fibrils) on the properties of SNpc DNs. The introduction of α-syn aggregates into single DNs via the patch electrode significantly reduced both the input resistance and the firing rate without changing the membrane potential. These effects occurred after 8–16 min of dialysis but did not occur with the monomeric form of α-syn. The effects of α-syn aggregates could be significantly reduced by preincubation with the ATP-sensitive K⁺ channel (KATP) inhibitor glibenclamide. These data suggest that accumulation of α-syn aggregates in DNs may chronically activate KATP channels leading to a significant loss of excitability and dopamine release.

Energetic substrate availability regulates synchronous activity in an excitatory neural network

Neural networks are required to meet significant metabolic demands associated with performing sophisticated computational tasks in the brain. The necessity for efficient transmission of information imposes stringent constraints on the metabolic pathways that can be used for energy generation at the synapse, and thus low availability of energetic substrates can reduce the efficacy of synaptic function. Here we study the effects of energetic substrate availability on global neural network behavior and find that glucose alone can sustain excitatory neurotransmission required to generate high-frequency synchronous bursting that emerges in culture. In contrast, obligatory oxidative energetic substrates such as lactate and pyruvate are unable to substitute for glucose, indicating that processes involving glucose metabolism form the primary energy-generating pathways supporting coordinated network activity. Our experimental results are discussed in the context of the role that metabolism plays in supporting the performance of individual synapses, including the relative contributions from postsynaptic responses, astrocytes, and presynaptic vesicle cycling. We propose a simple computational model for our excitatory cultures that accurately captures the inability of metabolically compromised synapses to sustain synchronous bursting when extracellular glucose is depleted.

Intense physical exercise potentiates glucose inhibitory effect over food intake of male Wistar rats

NEW FINDINGS

What is the central question of this study? How does an acute session of exercise affect food intake of male Wistar rats? What is the main finding and its importance? Food intake in male Wistar rats is decreased in the first hour after physical exercise independent of the intensity. Moreover, high-intensity exercise potentiates the anorexic effect of peripheral glucose administration. This work raises new feeding-related targets that would explain how exercise drives body weight loss.

ABSTRACT

Obesity has emerged as a critical metabolic disorder in modern society. An adequate lifestyle with a well-oriented programme of diet and physical exercise (PE) can prevent or potentially even cure obesity. Additionally, PE might lead to weight loss by increasing energy expenditure and decreasing hunger perception. In this article, we hypothesize that an acute exercise session would potentiate the glucose inhibitory effects on food intake in male Wistar rats. Our data show that moderate- or high-intensity PE significantly decreased food intake, although no changes in the expression of feeding-related neuropeptide in the arcuate nucleus of the hypothalamus were found. Exercised animals demonstrated a reduced glucose tolerance and increased blood insulin concentration. Intraperitoneal administration of glucose decreased food intake in control animals. In the animals submitted to moderate-intensity PE, the decrease in food intake promoted by glucose was similar to controls; however, an interaction was observed when glucose was injected in the high-intensity PE group, in which food intake was significantly lower than the effect produced by glucose alone. A different pattern of expression was observed for the monocarboxylate transporter isoforms (MCT1, 2 and 4) and 6-phosphofructo-2-kinase/fructose-2,6-biphosphatase 3 (PFKFBP3) in the hypothalamus, which was dependent on the exercise intensity. In conclusion, PE decreases food intake independently of the intensity. However, an interaction between PE and the anorexic effect of glucose is only observed when a high-intensity exercise is performed. These data show an essential role of exercise intensity in the modulation of the glucose inhibitory effect on food intake.

BAD and KATP channels regulate neuron excitability and epileptiform activity

Brain metabolism can profoundly influence neuronal excitability. Mice with genetic deletion or alteration of Bad (BCL-2 agonist of cell death) exhibit altered brain-cell fuel metabolism, accompanied by resistance to acutely induced epileptic seizures; this seizure protection is mediated by ATP-sensitive potassium (K_ATP) channels. Here we investigated the effect of BAD manipulation on K_ATP channel activity and excitability in acute brain slices. We found that BAD’s influence on neuronal K_ATP channels was cell-autonomous and directly affected dentate granule neuron (DGN) excitability. To investigate the role of neuronal K_ATP channels in the anticonvulsant effects of BAD, we imaged calcium during picrotoxin-induced epileptiform activity in entorhinal-hippocampal slices. BAD knockout reduced epileptiform activity, and this effect was lost upon knockout or pharmacological inhibition of K_ATP channels. Targeted BAD knockout in DGNs alone was sufficient for the antiseizure effect in slices, consistent with a ‘dentate gate’ function that is reinforced by increased K_ATP channel activity.

Mitochondria-Bound Hexokinase (mt-HK) Activity Differ in Cortical and Hypothalamic Synaptosomes: Differential Role of mt-HK in H2O2 Depuration

Glucose and oxygen are vital for the brain, as these molecules provide energy and metabolic intermediates that are necessary for cell function. The glycolysis pathway and mitochondria play a pivotal role in cell energy metabolism, which is closely related to reactive oxygen species (ROS) production. Hexokinase (HK) is a key enzyme involved in glucose metabolism that modulates the level of brain mitochondrial ROS by recycling ADP for oxidative phosphorylation (OxPhos). Here, we hypothesize that the control of mitochondrial metabolism by hexokinase differs in distinct areas of the brain, such as the cortex and hypothalamus, in which ROS might function as signaling molecules. Thus, we investigated mitochondrial metabolism of synaptosomes derived from both brain regions. Cortical synaptosomes (CSy) show a predominance of glutamatergic synapses, while in the hypothalamic synaptosomes (HSy), the GABAergic synapses predominate. Significant differences of oxygen consumption and ROS production were related to higher mitochondrial complex II activity (succinate dehydrogenase-SDH) in CSy rather than to mitochondrial number. Mitochondrial HK (mt-HK) activity was higher in CSy than in HSy regardless the substrate added. Mitochondrial O₂ consumption related to mt-HK activation by 2-deoxyglucose was also higher in CSy. In the presence of substrate for complex II, the activation of synaptosomal mt-HK promoted depuration of ROS in both HSy and CSy, while ROS depuration did not occur in HSy when substrate for complex I was used. The impact of the mt-HK inhibition by glucose-6-phosphate (G6P) was the same in synaptosomes from both areas. Together, the differences found between CSy and HSy indicate specific roles of mt-HK and SDH on the metabolism of each brain region, what probably depends on the main metabolic route that is used by the neurons.

Calcium dynamics control K-ATP channel-mediated bursting in substantia nigra dopamine neurons: a combined experimental and modeling study

Burst firing in medial substantia nigra (mSN) dopamine (DA) neurons has been selectively linked to novelty-induced exploration behavior in mice. Burst firing in mSN DA neurons, in contrast to lateral SN DA neurons, requires functional ATP-sensitive potassium (K-ATP) channels both in vitro and in vivo. However, the precise role of K-ATP channels in promoting burst firing is unknown. We show experimentally that L-type calcium channel activity in mSN DA neurons enhances open probability of K-ATP channels. We then generate a mathematical model to study the role of Ca²⁺ dynamics driving K-ATP channel function in mSN DA neurons during bursting. In our model, Ca²⁺ influx leads to local accumulation of ADP due to Ca-ATPase activity, which in turn activates K-ATP channels. If K-ATP channel activation reaches levels sufficient to terminate spiking, rhythmic bursting occurs. The model explains the experimental observation that, in vitro, coapplication of NMDA and a selective K-ATP channel opener, NN414, is required to elicit bursting as follows. Simulated NMDA receptor activation increases the firing rate and the rate of Ca²⁺ influx, which increases the activation of K-ATP. The model suggests that additional sources of hyperpolarization, such as GABAergic synaptic input, are recruited in vivo for burst termination or rebound burst discharge. The model predicts that NN414 increases the sensitivity of the K-ATP channel to ADP, promoting burst firing in vitro, and that that high levels of Ca²⁺ buffering, as might be expected in the calbindin-positive SN DA neuron subpopulation, promote rhythmic bursting pattern, consistent with experimental observations in vivo. NEW & NOTEWORTHY Recently identified distinct subpopulations of midbrain dopamine neurons exhibit differences in their two primary activity patterns in vivo: tonic (single spike) firing and phasic bursting. This study elucidates the biophysical basis of bursts specific to dopamine neurons in the medial substantia nigra, enabled by ATP-sensitive K⁺ channels and necessary for novelty-induced exploration. A better understanding of how dopaminergic signaling differs between subpopulations may lead to therapeutic strategies selectively targeted to specific subpopulations.

Hippocampal Proteome of Rats Subjected to the Li-Pilocarpine Epilepsy Model and the Effect of Carisbamate Treatment

In adult rats, the administration of lithium–pilocarpine (LiPilo) reproduces most clinical and neuropathological features of human temporal lobe epilepsy (TLE). Carisbamate (CRS) possesses the property of modifying epileptogenesis in this model. Indeed, about 50% of rats subjected to LiPilo status epilepticus (SE) develop non-convulsive seizures (NCS) instead of motor seizures when treated with CRS. However, the mechanisms underlying these effects remain unknown. The aim of this study was to perform a proteomic analysis in the hippocampus of rats receiving LiPilo and developing motor seizures or NCS following CRS treatment. Fifteen adult male Sprague–Dawley rats were used. SE was induced by LiPilo injection. CRS treatment was initiated at 1 h and 9 h after SE onset and maintained for 7 days, twice daily. Four groups were studied after video-EEG control of the occurrence of motor seizures: a control group receiving saline (CT n = 3) and three groups that underwent SE: rats treated with diazepam (DZP n = 4), rats treated with CRS displaying NCS (CRS-NCS n = 4) or motor seizures (CRS-TLE n = 4). Proteomic analysis was conducted by 2D-SDS-PAGE. Twenty-four proteins were found altered. In the CRS-NCS group, proteins related to glycolysis and ATP synthesis were down-regulated while proteins associated with pyruvate catabolism were up-regulated. Moreover, among the other proteins differentially expressed, we found proteins related to inflammatory processes, protein folding, tissue regeneration, response to oxidative stress, gene expression, biogenesis of synaptic vesicles, signal transduction, axonal transport, microtubule formation, cell survival, and neuronal plasticity. Our results suggest a global reduction of glycolysis and cellular energy production that might affect brain excitability. In addition, CRS seems to modulate proteins related to many other pathways that could significantly participate in the epileptogenesis-modifying effect observed.

Monitoring ATP dynamics in electrically active white matter tracts

In several neurodegenerative diseases and myelin disorders, the degeneration profiles of myelinated axons are compatible with underlying energy deficits. However, it is presently impossible to measure selectively axonal ATP levels in the electrically active nervous system. We combined transgenic expression of an ATP-sensor in neurons of mice with confocal FRET imaging and electrophysiological recordings of acutely isolated optic nerves. This allowed us to monitor dynamic changes and activity-dependent axonal ATP homeostasis at the cellular level and in real time. We find that changes in ATP levels correlate well with compound action potentials. However, this correlation is disrupted when metabolism of lactate is inhibited, suggesting that axonal glycolysis products are not sufficient to maintain mitochondrial energy metabolism of electrically active axons. The combined monitoring of cellular ATP and electrical activity is a novel tool to study neuronal and glial energy metabolism in normal physiology and in models of neurodegenerative disorders.

DOI:http://dx.doi.org/10.7554/eLife.24241.001

The brain contains an intricate network of nerve cells that receive, process, send and store information. This information travels as electrical impulses along a long, thin part of each nerve cell known as the nerve fiber or axon. The act of sending these electrical signals requires a lot of energy, and energy in cells is most often stored within molecules of adenosine triphosphate (called ATP for short).

Importantly, a better understanding of how the production and consumption of ATP in nerve cells relates to electrical activity would help scientists to better understand how a shortage of energy in the brain contributes to diseases like multiple sclerosis. However, to date, it has been challenging to study the dynamics of ATP in nerve cells that are active.

Now, Trevisiol et al. describe a new system that allows changes in ATP levels to be seen within active nerve cells. First, mice were genetically engineered to produce a molecule that works like an ATP sensor only in their nerve cells. This made it possible to visualize the amount of ATP inside the axons in real-time using a microscope. Measuring ATP levels and recording the electrical signals moving along an axon at the same time allowed Trevisiol et al. to see how ATP content and electrical activity correlate and regulate each other.

The experiments reveal that strong electrical activity reduces the ATP content of the axon. Trevisiol et al. also discovered that nerve cells are unable to generate enough energy on their own to sustain their electrical activity. These results provide evidence that other cells in the brain – most likely non-nerve cells called oligodendrocytes – play an active role in delivering energy-rich substances to the axons of nerve cells.

In the future, the same tools and approaches could be used to monitor ATP levels and electrical activity in mice that model neurological disorders. Such experiments could tell scientists more about how disturbing energy production in nerve cells affects these diseases.

DOI:http://dx.doi.org/10.7554/eLife.24241.002

Low-Dimensional Models of "Neuro-Glio-Vascular Unit" for Describing Neural Dynamics under Normal and Energy-Starved Conditions

The motivation of developing simple minimal models for neuro-glio-vascular (NGV) system arises from a recent modeling study elucidating the bidirectional information flow within the NGV system having 89 dynamic equations (1). While this was one of the first attempts at formulating a comprehensive model for neuro-glio-vascular system, it poses severe restrictions in scaling up to network levels. On the contrary, low-­dimensional models are convenient devices in simulating large networks that also provide an intuitive understanding of the complex interactions occurring within the NGV system. The key idea underlying the proposed models is to describe the glio-vascular system as a lumped system, which takes neural firing rate as input and returns an “energy” variable (analogous to ATP) as output. To this end, we present two models: biophysical neuro-energy (Model 1 with five variables), comprising K_ATP channel activity governed by neuronal ATP dynamics, and the dynamic threshold (Model 2 with three variables), depicting the dependence of neural firing threshold on the ATP dynamics. Both the models show different firing regimes, such as continuous spiking, phasic, and tonic bursting depending on the ATP production coefficient, ɛ_p, and external current. We then demonstrate that in a network comprising such energy-dependent neuron units, ɛ_p could modulate the local field potential (LFP) frequency and amplitude. Interestingly, low-frequency LFP dominates under low ɛ_p conditions, which is thought to be reminiscent of seizure-like activity observed in epilepsy. The proposed “neuron-energy” unit may be implemented in building models of NGV networks to simulate data obtained from multimodal neuroimaging systems, such as functional near infrared spectroscopy coupled to electroencephalogram and functional magnetic resonance imaging coupled to electroencephalogram. Such models could also provide a theoretical basis for devising optimal neurorehabilitation strategies, such as non-invasive brain stimulation for stroke patients.

KATP channels modulate intrinsic firing activity of immature entorhinal cortex layer III neurons

Medial temporal lobe structures are essential for memory formation which is associated with coherent network oscillations. During ontogenesis, these highly organized patterns develop from distinct, less synchronized forms of network activity. This maturation process goes along with marked changes in intrinsic firing patterns of individual neurons. One critical factor determining neuronal excitability is activity of ATP-sensitive K⁺ channels (K_ATP channels) which coupled electrical activity to metabolic state. Here, we examined the role of K_ATP channels for intrinsic firing patterns and emerging network activity in the immature medial entorhinal cortex (mEC) of rats. Western blot analysis of Kir6.2 (a subunit of the K_ATP channel) confirmed expression of this protein in the immature entorhinal cortex. Neuronal activity was monitored by field potential (fp) and whole-cell recordings from layer III (LIII) of the mEC in horizontal brain slices obtained at postnatal day (P) 6–13. Spontaneous fp-bursts were suppressed by the K_ATP channel opener diazoxide and prolonged after blockade of K_ATP channels by glibenclamide. Immature mEC LIII principal neurons displayed two dominant intrinsic firing patterns, prolonged bursts or regular firing activity, respectively. Burst discharges were suppressed by the K_ATP channel openers diazoxide and NN414, and enhanced by the K_ATP channel blockers tolbutamide and glibenclamide. Activity of regularly firing neurons was modulated in a frequency-dependent manner: the diazoxide-mediated reduction of firing correlated negatively with basal frequency, while the tolbutamide-mediated increase of firing showed a positive correlation. These data are in line with an activity-dependent regulation of K_ATP channel activity. Together, K_ATP channels exert powerful modulation of intrinsic firing patterns and network activity in the immature mEC.

Metabolic differences in hippocampal 'Rett' neurons revealed by ATP imaging

Understanding metabolic control of neuronal function requires detailed knowledge of ATP handling in living neurons. We imaged ATP in organotypic hippocampal slices using genetically encoded sensor Ateam 1.03 modified to selectively transduce neurons in the tissue. ATP imaging indicated distinct differences in ATP production and consumption in dentate gyrus and cornu ammonis (CA) areas. Removal of extracellular Mg(2+) from the bath evoked epileptiform-like activity that was accompanied by ATP decline from 2-3 to 1-2mM. The slices fully recovered from treatment and showed persistent spontaneous activity. Neuronal discharges were followed by transient ATP changes and periodic activation of ATP-sensitive K(+) (K-ATP) channels. The biggest ATP decreases during epileptiform-like episodes of activity were observed in CA1 and CA3 neurons. Examination of neurons from the Rett model mice MeCP2(-/y) showed that seizure-like activity had earlier onset and subsequent spontaneous activity demonstrated more frequent discharges. Hippocampal MeCP2(-/y) neurons had higher resting ATP levels and showed bigger ATP decreases during epileptiform-like activity. More intense ATP turnover in MeCP2(-/y) neurons may result from necessity to maintain hippocampal function in Rett syndrome. Elevated ATP may make, in turn, Rett hippocampus more prone to epilepsy due to inadequate activity of K-ATP channels.

The cellular building blocks of breathing

Respiratory brainstem neurons fulfill critical roles in controlling breathing: they generate the activity patterns for breathing and contribute to various sensory responses including changes in O2 and CO2. These complex sensorimotor tasks depend on the dynamic interplay between numerous cellular building blocks that consist of voltage-, calcium-, and ATP-dependent ionic conductances, various ionotropic and metabotropic synaptic mechanisms, as well as neuromodulators acting on G-protein coupled receptors and second messenger systems. As described in this review, the sensorimotor responses of the respiratory network emerge through the state-dependent integration of all these building blocks. There is no known respiratory function that involves only a small number of intrinsic, synaptic, or modulatory properties. Because of the complex integration of numerous intrinsic, synaptic, and modulatory mechanisms, the respiratory network is capable of continuously adapting to changes in the external and internal environment, which makes breathing one of the most integrated behaviors. Not surprisingly, inspiration is critical not only in the control of ventilation, but also in the context of "inspiring behaviors" such as arousal of the mind and even creativity. Far-reaching implications apply also to the underlying network mechanisms, as lessons learned from the respiratory network apply to network functions in general.

K(ATP) channels of parafacial respiratory group (pFRG) neurons are involved in H2S-mediated central inhibition of respiratory rhythm in medullary slices of neonatal rats

We recently found that hydrogen sulfide (H2S) participates in inhibitory regulation of rhythmic respiration by acting on the parafacial respiratory group (pFRG) in medullary slices of neonatal rats. The present study investigated whether ATP-sensitive potassium (KATP) channels are expressed in neurons of the pFRG, and, if so, whether they play a role in central regulation of respiratory activity, in particular the H2S-mediated central inhibition of respiratory rhythm in medullary slices of neonatal rats. Immunohistochemical techniques revealed that KATP channels are expressed in neurons of the pFRG region. Micro-injection of the KATP channel activator, pinacidil, into the pFRG region inhibited the discharge rhythm of hypoglossal rootlets, whereas injection of the KATP channel blocker, glibenclamide (Gl), had no effect. Micro-injection of the H2S donor sodium hydrosulfide (NaHS) into the pFRG region produced identical inhibitory responses to those induced by pinacidil. However, combined micro-injection of Gl and NaHS eliminated inhibitory effects of NaHS and converted to minor excitatory effects on the respiratory rhythm. It can be concluded that KATP channels of pFRG neurons are involved in the central regulation of respiratory rhythm and H2S-mediated inhibitory actions on respiratory rhythm in medullary slices of neonatal rats.

Post-hypoxic recovery of respiratory rhythm generation is gender dependent

The preBötzinger complex (preBötC) is a critical neuronal network for the generation of breathing. Lesioning the preBötC abolishes respiration, while when isolated in vitro, the preBötC continues to generate respiratory rhythmic activity. Although several factors influence rhythmogenesis from this network, little is known about how gender may affect preBötC function. This study examines the influence of gender on respiratory activity and in vitro rhythmogenesis from the preBötC. Recordings of respiratory activity from neonatal mice (P10-13) show that sustained post-hypoxic depression occurs with greater frequency in males compared to females. Moreover, extracellular population recordings from the preBötC in neonatal brainstem slices (P10-13) reveal that the time to the first inspiratory burst following reoxygenation (TTFB) is significantly delayed in male rhythmogenesis when compared to the female rhythms. Altering activity of ATP sensitive potassium channels (KATP) with either the agonist, diazoxide, or the antagonist, tolbutamide, eliminates differences in TTFB. By contrast, glucose supplementation improves post-hypoxic recovery of female but not male rhythmogenesis. We conclude that post-hypoxic recovery of respiration is gender dependent, which is, in part, centrally manifested at the level of the preBötC. Moreover, these findings provide potential insight into the basis of increased male vulnerability in a variety of conditions such as Sudden Infant Death Syndrome (SIDS).

Single KATP channel opening in response to stimulation of AMPA/kainate receptors is mediated by Na+ accumulation and submembrane ATP and ADP changes

Excessive stimulation of glutamatergic receptors (GluRs) can overexcite neurons. This can be dampened by KATP channels linking metabolic and neuronal activities, but the cross-talk has not yet been examined on the single channel level. In the brainstem and hippocampal neurons, GluR agonists augmented the open state probability (Popen) of KATP channels with relative efficacy: kainate AMPA > NMDA > t-ACPD. Inhibition of calcium influx and chelation of intracellular calcium did not modify the effects. Kainate did not augment production of reactive oxygen species measured with roGFP1. H2O2 slightly increased Popen, but GluR effects were not modified. GluR actions were abolished in Na(+)-free solutions and after blockade of Na(+)-K(+)-ATPase. KATP channels in open-cell patch-clamp measurements were inhibited by ATP, stimulated by ADP, and kainate was effective only in the presence of ATP. GluR stimulation enhanced ATP consumption that decreased submembrane ATP levels, whereas metabolic poisoning diminished bulk ATP. Modelling showed strong ATP depletion and ADP accumulation near the membrane, and both effects contributed to Popen increases after GluR stimulation. Kainate and hypoxia activated KATP channels in the functional brainstem slices. Inhibition of aerobic ATP production and GluR stimulation were about equally effective in KATP channel opening during hypoxia. Induction of seizure-like activity in hippocampal slices with Mg(2+)-free solutions was accompanied by ATP decrease and KATP channel opening. We propose that KATP channels and GluRs are functionally coupled that can regulate long-lasting changes of neuronal activity in the CNS neurons.

Neuroprotective role of ATP-sensitive potassium channels in cerebral ischemia

ATP-sensitive potassium (K(ATP)) channels are weak, inward rectifiers that couple metabolic status to cell membrane electrical activity, thus modulating many cellular functions. An increase in the ADP/ATP ratio opens K(ATP) channels, leading to membrane hyperpolarization. K(ATP) channels are ubiquitously expressed in neurons located in different regions of the brain, including the hippocampus and cortex. Brief hypoxia triggers membrane hyperpolarization in these central neurons. In vivo animal studies confirmed that knocking out the Kir6.2 subunit of the K(ATP) channels increases ischemic infarction, and overexpression of the Kir6.2 subunit reduces neuronal injury from ischemic insults. These findings provide the basis for a practical strategy whereby activation of endogenous K(ATP) channels reduces cellular damage resulting from cerebral ischemic stroke. K(ATP) channel modulators may prove to be clinically useful as part of a combination therapy for stroke management in the future.

The ketogenic diet: metabolic influences on brain excitability and epilepsy

A dietary therapy for pediatric epilepsy known as the ketogenic diet has seen a revival in its clinical use during the past decade. Although the underlying mechanism of the diet remains unknown, modern scientific approaches, such as the genetic disruption of glucose metabolism, are allowing for more detailed questions to be addressed. Recent work indicates that several mechanisms may exist for the ketogenic diet, including disruption of glutamatergic synaptic transmission, inhibition of glycolysis, and activation of ATP-sensitive potassium channels. Here, we describe on-going work in these areas that is providing a better understanding of metabolic influences on brain excitability and epilepsy.

Calmodulin and calmodulin kinase II mediate emergent bursting activity in the brainstem respiratory network (preBötzinger complex)

Emergence of persistent activity in networks can be controlled by intracellular signalling pathways but the mechanisms involved and their role are not yet fully explored. Using calcium imaging and patch-clamp we examined the rhythmic activity in the preBötzinger complex (preBötC) in the lower brainstem that generates the respiratory motor output. In functionally intact acute slices brief hypoxia, electrical stimulation and activation of AMPA receptors transiently depressed bursting activity which then recovered with augmentation. The effects were abrogated after chelation of intracellular calcium, blockade of L-type calcium channels and inhibition of calmodulin (CaM) and CaM kinase (CaMKII). Rhythmic calcium transients and synaptic drive currents in preBötC neurons in the organotypic slices showed similar CaM- and CaMKII-dependent responses. The stimuli increased the amplitude of spontaneous and miniature excitatory synaptic currents indicating postsynaptic changes at glutamatergic synapses. In the acute and organotypic slices, CaM stimulated and ADP inhibited calcium-dependent TRPM4 channels and CaMKII augmented synaptic drive currents. Experimental data and simulations show the role of ADP and CaMKII in the control of bursting activity and its relation to intracellular signalling. I propose that CaMKII-mediated facilitation of glutamatergic transmission strengthens emergent synchronous activity within preBötC that is then maintained by periodic surges of calcium during the bursts. This may find implications in restoration and consolidation of autonomous activity in the respiratory disorders.

Understanding the rhythm of breathing: so near, yet so far

Breathing is an essential behavior that presents a unique opportunity to understand how the nervous system functions normally, how it balances inherent robustness with a highly regulated lability, how it adapts to both rapidly and slowly changing conditions, and how particular dysfunctions result in disease. We focus on recent advancements related to two essential sites for respiratory rhythmogenesis: (a) the preBötzinger Complex (preBötC) as the site for the generation of inspiratory rhythm and (b) the retrotrapezoid nucleus/parafacial respiratory group (RTN/pFRG) as the site for the generation of active expiration.

Cytosolic calcium coordinates mitochondrial energy metabolism with presynaptic activity

Most neurons fire in bursts, imposing episodic energy demands, but how these demands are coordinated with oxidative phosphorylation is still unknown. Here, using fluorescence imaging techniques on presynaptic termini of Drosophila motor neurons (MNs), we show that mitochondrial matrix pH (pHm), inner membrane potential (Δψm), and NAD(P)H levels ([NAD(P)H]m) increase within seconds of nerve stimulation. The elevations of pHm, Δψm, and [NAD(P)H]m indicate an increased capacity for ATP production. Elevations in pHm were blocked by manipulations that blocked mitochondrial Ca2+ uptake, including replacement of extracellular Ca2+ with Sr2+ and application of either tetraphenylphosphonium chloride or KB-R7943, indicating that it is Ca2+ that stimulates presynaptic mitochondrial energy metabolism. To place this phenomenon within the context of endogenous neuronal activity, the firing rates of a number of individually identified MNs were determined during fictive locomotion. Surprisingly, although endogenous firing rates are significantly different, there was little difference in presynaptic cytosolic Ca2+ levels ([Ca2+]c) between MNs when each fires at its endogenous rate. The average [Ca2+]c level (329±11 nM) was slightly above the average Ca2+ affinity of the mitochondria (281±13 nM). In summary, we show that when MNs fire at endogenous rates, [Ca2+]c is driven into a range where mitochondria rapidly acquire Ca2+. As we also show that Ca2+ stimulates presynaptic mitochondrial energy metabolism, we conclude that [Ca2+]c levels play an integral role in coordinating mitochondrial energy metabolism with presynaptic activity in Drosophila MNs.

Single K ATP channel opening in response to action potential firing in mouse dentate granule neurons

ATP-sensitive potassium channels (K(ATP) channels) are important sensors of cellular metabolic state that link metabolism and excitability in neuroendocrine cells, but their role in nonglucosensing central neurons is less well understood. To examine a possible role for K(ATP) channels in modulating excitability in hippocampal circuits, we recorded the activity of single K(ATP) channels in cell-attached patches of granule cells in the mouse dentate gyrus during bursts of action potentials generated by antidromic stimulation of the mossy fibers. Ensemble averages of the open probability (p(open)) of single K(ATP) channels over repeated trials of stimulated spike activity showed a transient increase in p(open) in response to action potential firing. Channel currents were identified as K(ATP) channels through blockade with glibenclamide and by comparison with recordings from Kir6.2 knock-out mice. The transient elevation in K(ATP) p(open) may arise from submembrane ATP depletion by the Na(+)-K(+) ATPase, as the pump blocker strophanthidin reduced the magnitude of the elevation. Both the steady-state and stimulus-elevated p(open) of the recorded channels were higher in the presence of the ketone body R-β-hydroxybutyrate, consistent with earlier findings that ketone bodies can affect K(ATP) activity. Using perforated-patch recording, we also found that K(ATP) channels contribute to the slow afterhyperpolarization following an evoked burst of action potentials. We propose that activity-dependent opening of K(ATP) channels may help granule cells act as a seizure gate in the hippocampus and that ketone-body-mediated augmentation of the activity-dependent opening could in part explain the effect of the ketogenic diet in reducing epileptic seizures.

Stimulation of bursting in pre-Bötzinger neurons by Epac through calcium release and modulation of TRPM4 and K-ATP channels

The exchange factor directly activated by cAMP (Epac) can couple cAMP production to the activation of particular membrane and cytoplasmic targets. Using patch-clamp recordings and calcium imaging in organotypic brainstem slices, we examined the role of Epac in pre-Bötzinger complex, an essential part of the respiratory network. The selective agonist 8-(4-chlorophenylthio)-2'-O-methyl-cAMP (8-pCPT) sensitized calcium mobilisation from inositol-1,4,5-trisphosphate-sensitive internal stores that stimulated TRPM4 (transient receptor potential cation channel, subfamily M, Melastatin) channels and potentiated the bursts of action potentials. 8-pCPT actions were abolished after inhibition of phospholipase C with U73122 and depletion of calcium stores with thapsigargin. Caffeine-sensitive release channels were not modulated by 8-pCPT. Epac inhibited ATP-sensitive K(+) channels that also led to the enhancement of bursting by 8-pCPT. Bursting activity, spontaneous calcium transients and activity of TRPM4 and ATP-sensitive K(+) channels were potentiated after brief exposures to bradykinin and incubation with wortmannin produced opposite effects that can be explained by changes in phosphatidylinositol 4,5-bisphosphate levels. 8-pCPT stimulated the respiratory motor output in functionally intact preparations and the effects of bradykinin and wortmannin were identical to those observed in organotypic slices. The data thus indicate a novel pathway of controlling bursting activity in pre-Bötzinger complex neurons through Epac that can involved in reinforcement of the respiratory activity by cAMP.

Outward Currents Contributing to Inspiratory Burst Termination in preBötzinger Complex Neurons of Neonatal Mice Studied in Vitro

We studied preBötzinger Complex (preBötC) inspiratory interneurons to determine the cellular mechanisms that influence burst termination in a mammalian central pattern generator. Neonatal mouse slice preparations that retain preBötC neurons generate respiratory motor rhythms in vitro. Inspiratory-related bursts rely on inward currents that flux Na⁺, thus outward currents coupled to Na⁺ accumulation are logical candidates for assisting in, or causing, burst termination. We examined Na⁺/K⁺ ATPase electrogenic pump current (I_pump), Na⁺-dependent K⁺ current (I_K–Na), and ATP-dependent K⁺ current (I_K–ATP). The pharmacological blockade of I_pump, I_K–Na, or I_K–ATP caused pathological depolarization akin to a burst that cannot terminate, which impeded respiratory rhythm generation and reversibly stopped motor output. By simulating inspiratory bursts with current-step commands in synaptically isolated preBötC neurons, we determined that each current generates approximately 3–8 mV of transient post-burst hyperpolarization that decays in 50–1600 ms. I_pump, I_K–Na, and – to a lesser extent – I_K–ATP contribute to terminating inspiratory bursts in the context of respiratory rhythm generation by responding to activity dependent cues such as Na⁺ accumulation.

Respiratory circuits: function, mechanisms, topology, and pathology

Neuroscientists have long sought to understand how circuits in the nervous system are organized to generate the precise neural outputs that underlie particular behaviors. Recent studies deepened our understanding of the mechanisms responsible for the generation of the rhythmic output for breathing. Here, the author focuses on issues that are pertinent for the respiratory network and considers its organization and how it derives the functional output. The author discusses pacemaker and network mechanisms of rhythm generation, which are now combined into a novel concept of emergent network activity due to coherent excitation of pacemaker groups. He discusses subcellular basis of this hypothesis and possible mechanisms of synchronization within respiratory network. These new findings in respiratory neuroscience are further applied to explain modifications in breathing during hypoxia and possible origins of respiratory disorders that may be acquired during neural development and aging.

Structure-function analysis of rhythmogenic inspiratory pre-Bötzinger complex networks in "calibrated" newborn rat brainstem slices

Inspiratory pre-Bötzinger complex (preBötC) networks remain active in perinatal rodent brainstem slices. Our analysis of (crescendo-like) inspiratory-related population and cellular bursting in novel histologically identified transversal preBötC slices in physiological (3 mM) superfusate [K(+)] revealed: (i) the preBötC extent sufficient for rhythm in thin slices is at most 175 microm. (ii) In 700 microm thick slices with unilaterally exposed preBötC, a <100 microm kernel generates a eupnea-like inspiratory pattern under predominant influence of caudally adjacent structures or thyrotropin-releasing hormone-like transmitters, but a mixed eupnea-sigh-like pattern when influence of rostral structures or substance-P-like transmitters dominates. (iii) Primarily presynaptic processes may underlie inhibition of rhythm by opioids or raising superfusate [Ca(2+)] from lower to upper physiological limits (1-1.5 mM). (iv) High K(+) reverses depression of rhythm by raised Ca(2+), opioids and anoxia. In summary, distinct activity patterns of spatiochemically organized isolated inspiratory networks depend on both an extracellular "Ca(2+)-K(+) antagonism" and slice dimensions. This explains some discrepant findings between studies and suggests use of "calibrated" slices and more uniform experimental conditions.

Asymmetric control of inspiratory and expiratory phases by excitability in the respiratory network of neonatal mice in vitro

Rhythmic motor behaviours consist of alternating movements, e.g. swing-stance in stepping, jaw opening and closing during chewing, and inspiration-expiration in breathing, which must be labile in frequency, and in some cases, in the duration of individual phases, to adjust to physiological demands. These movements are the expression of underlying neural circuits whose organization governs the properties of the motor behaviour. To determine if the ability to operate over a broad range of frequencies in respiration is expressed in the rhythm generator, we isolated the kernel of essential respiratory circuits using rhythmically active in vitro slices from neonatal mice. We show respiratory motor output in these slices at very low frequencies (0.008 Hz), well below the typical frequency in vitro (approximately 0.2 Hz) and in most intact normothermic mammals. Across this broad range of frequencies, inspiratory motor output bursts remained remarkably constant in pattern, i.e. duration, peak amplitude and area. The change in frequency was instead attributable to increased interburst interval, and was largely unaffected by removal of fast inhibitory transmission. Modulation of the frequency was primarily achieved by manipulating extracellular potassium, which significantly affects neuronal excitability. When excitability was lowered to slow down, or in some cases stop, spontaneous rhythm, brief stimulation of the respiratory network with a glutamatergic agonist could evoke (rhythmic) motor output. In slices with slow (<0.02 Hz) spontaneous rhythms, evoked motor output could follow a spontaneous burst at short (<or=1 s) or long (approximately 60 s) intervals. The intensity or timing of stimulation determined the latency to the first evoked burst, with no evidence for a refractory period greater than approximately 1 s, even with interburst intervals >60 s. We observed during inspiration a large magnitude (approximately 0.6 nA) outward current generated by Na(+)/K(+) ATPase that deactivated in 25-100 ms and thus could contribute to burst termination and the latency of evoked bursts but is unlikely to control the interburst interval. We propose that the respiratory network functions over a broad range of frequencies by engaging distinct mechanisms from those controlling inspiratory duration and pattern that specifically govern the interburst interval.

Ketone bodies, glycolysis, and KATP channels in the mechanism of the ketogenic diet

The ketogenic diet (KD) has shown remarkable efficacy in the treatment of drug-resistant childhood epilepsy. Our understanding of how the KD produces its anticonvulsant and antiepileptogenic effects remains incomplete, which is perhaps not surprising for a biological manipulation as sweeping as dietary change. Several hypotheses focus on ketone bodies, fuel molecules that circulate at millimolar concentrations in the blood of patients on a KD, as causative agents. Here I consider some recent evidence for one such hypothesis, involving a possible role for altered glycolysis and consequent activation of a class of potassium channels called K(ATP)channels.

Metabotropic glutamate receptors activate dendritic calcium waves and TRPM channels which drive rhythmic respiratory patterns in mice

Respiration in vertebrates is generated by a compact network which is located in the lower brainstem but cellular mechanisms which underlie persistent oscillatory activity of the respiratory network are yet unknown. Using two-photon imaging and patch-clamp recordings in functional brainstem preparations of mice containing pre-Bötzinger complex (preBötC), we examined the actions of metabotropic glutamate receptors (mGluR1/5) on the respiratory patterns. The agonist DHPG potentiated and antagonist LY367385 depressed respiration-related activities. In the inspiratory neurons, we observed rhythmic activation of non-selective channels which had a conductance of 24 pS. Their activity was enhanced with membrane depolarization and after elevation of calcium from the cytoplasmic side of the membrane. They were activated by a non-hydrolysable PIP(2) analogue and blocked by flufenamate, ATP4- and Gd3+. All these properties correspond well to those of TRPM4 channels. Calcium imaging of functional slices revealed rhythmic transients in small clusters of neurons present in a network. Calcium transients in the soma were preceded by the waves in dendrites which were dependent on mGluR activation. Initiation and propagation of waves required calcium influx and calcium release from internal stores. Calcium waves activated TPRM4-like channels in the soma and promoted generation of inspiratory bursts. Simulations of activity of neurons communicated via dendritic calcium waves showed emerging activity within neuronal clusters and its synchronization between the clusters. The experimental and theoretical data provide a subcellular basis for a recently proposed group-pacemaker hypothesis and describe a novel mechanism of rhythm generation in neuronal networks.

4-Aminopyridine-sensitive outward currents in preBötzinger complex neurons influence respiratory rhythm generation in neonatal mice

We measured a low-threshold, inactivating K+ current, i.e. A-current (I(A)), in respiratory neurons of the preBötzinger complex (preBötC) in rhythmically active slice preparations from neonatal C57BL/6 mice. The majority of inspiratory neurons (21/34 = 61.8%), but not expiratory neurons (1/8 = 12.5%), expressed I(A). In whole-cell and somatic outside-out patches I(A) activated at -60 mV (half-activation voltage measured -16.3 mV) and only fully inactivated above -40 mV (half-inactivation voltage measured -85.6 mV), indicating that I(A) can influence membrane trajectory at baseline voltages during respiratory rhythm generation in vitro. 4-Aminopyridine (4-AP, 2 mm) attenuated I(A) in both whole-cell and somatic outside-out patches. In the context of rhythmic network activity, 4-AP caused irregular respiratory-related motor output on XII nerves and disrupted rhythmogenesis as detected with whole-cell and field recordings in the preBötC. Whole-cell current-clamp recordings showed that 4-AP changed the envelope of depolarization underlying inspiratory bursts (i.e. inspiratory drive potentials) from an incrementing pattern to a decrementing pattern during rhythm generation and abolished current pulse-induced delayed excitation. These data suggest that I(A) opposes excitatory synaptic depolarizations at baseline voltages of approximately -60 mV and influences the inspiratory burst pattern. We propose that I(A) promotes orderly recruitment of constituent rhythmogenic neurons by minimizing the activity of these neurons until they receive massive coincident synaptic input, which reduces the periodic fluctuations of inspiratory activity.

Parameter estimation for bursting neural models

This paper presents work on parameter estimation methods for bursting neural models. In our approach we use both geometrical features specific to bursting, as well as general features such as periodic orbits and their bifurcations. We use the geometry underlying bursting to introduce defining equations for burst initiation and termination, and restrict the estimation algorithms to the space of bursting periodic orbits when trying to fit periodic burst data. These geometrical ideas are combined with automatic differentiation to accurately compute parameter sensitivities for the burst timing and period. In addition to being of inherent interest, these sensitivities are used in standard gradient-based optimization algorithms to fit model burst duration and period to data. As an application, we fit Butera et al.'s (Journal of Neurophysiology 81, 382-397, 1999) model of preBötzinger complex neurons to empirical data both in control conditions and when the neuromodulator norepinephrine is added (Viemari and Ramirez, Journal of Neurophysiology 95, 2070-2082, 2006). The results suggest possible modulatory mechanisms in the preBötzinger complex, including modulation of the persistent sodium current.

Imaging of respiratory network topology in living brainstem slices

Topology of neuronal networks contributes to their functioning but the structure-function relationships are not yet understood. In order to reveal the spatial organisation of the respiratory network, we expressed enhanced green fluorescent proteins in neurons in brainstem slices containing the respiratory kernel (pre-Bötzinger complex). The expression was neuron specific due to use of adeno-associated viral vector driving transgene expression from synapsin 1 promoter. Both neuronal cell bodies and their dendrites were labelled with high efficacy. This labelling allowed for enhanced spatial resolution as compared to conventional calcium-sensitive dyes. Neurons occupied about 10% of tissue volume and formed an interconnected network. Using custom-developed software, we quantified the network structure that had a modular structure consisting of clusters having transverse (dorso-ventral) orientation. They contained in average seven neurons and connections between the cells in different clusters were less frequent. This novel in situ imaging technique is promising to gain new knowledge about the fine structure and function of neuronal networks in living slice preparations.

Disruption of Kir6.2-containing ATP-sensitive potassium channels impairs maintenance of hypoxic gasping in mice

Hypoxic gasping emerges under severe hypoxia/ischemia in various species, exerting a life-protective role by assuring minimum ventilation even in loss of consciousness. However, the molecular basis of its generation and maintenance is not well understood. Here we found that mice lacking Kir6.2- but not Kir6.1-containing ATP-sensitive potassium (K(ATP)) channels [knockout (KO) mice] exhibited few gaSPS when subjected to abrupt ischemia by decapitation, whereas wild-type mice all exhibited more than 10 gaSPS. Under anesthesia, wild-type mice initially responded to severe hypoxic insult with augmented breathing (tachypnea) accompanied by sighs and subsequent depression of respiratory frequency. Gasping then emerged and persisted stably (persistent gasping); if the hypoxia continued, several gaSPS with distinct patterns appeared (terminal gasping) before cessation of breathing. KO mice showed similar hypoxic responses but both depression and the two types of gasping were of much shorter duration than in wild-type mice. Moreover, in the unanesthetized condition, the onset of terminal gasping in KO mice, which was always earlier than in wild-type mice, was unaltered by decreasing O(2) concentrations within the severe range (4.5-7.0%), whereas onset in wild-type mice became earlier in response to lowered O(2) concentrations. Thus, the mechanism responsible for regulating the hypoxic response in accordance with the severity of the hypoxia was dysfunctional in these KO mice, suggesting that Kir6.2-containing K(ATP) channels are critically involved in the maintenance rather than the generation of hypoxic gasping and depression of respiratory frequency.

Ketogenic diet metabolites reduce firing in central neurons by opening K(ATP) channels

A low-carbohydrate ketogenic diet remains one of the most effective (but mysterious) treatments for severe pharmacoresistant epilepsy. We have tested for an acute effect of physiological ketone bodies on neuronal firing rates and excitability, to discover possible therapeutic mechanisms of the ketogenic diet. Physiological concentrations of ketone bodies (beta-hydroxybutyrate or acetoacetate) reduced the spontaneous firing rate of neurons in slices from rat or mouse substantia nigra pars reticulata. This region is thought to act as a "seizure gate," controlling seizure generalization. Consistent with an anticonvulsant role, the ketone body effect is larger for cells that fire more rapidly. The effect of ketone bodies was abolished by eliminating the metabolically sensitive K(ATP) channels pharmacologically or by gene knock-out. We propose that ketone bodies or glycolytic restriction treat epilepsy by augmenting a natural activity-limiting function served by K(ATP) channels in neurons.

ADP regulates movements of mitochondria in neurons

Mitochondria often reside in subcellular regions with high metabolic demands. We examined the mechanisms that can govern the relocation of mitochondria to these sites in respiratory neurons. Mitochondria were visualized using tetramethylrhodamineethylester, and their movements were analyzed by applying single-particle tracking. Intracellular ATP ([ATP](i)) was assessed by imaging the luminescence of luciferase, the fluorescence of the ATP analog TNP-ATP, and by monitoring the activity of K(ATP) channels. Directed movements of mitochondria were accompanied by transient increases in TNP-ATP fluorescence. Application of glutamate and hypoxia reversibly decreased [ATP](i) levels and inhibited the directed transport. Injections of ATP did not rescue the motility of mitochondria after its inhibition by hypoxia. Introduction of ADP suppressed mitochondrial movements and occluded the effects of subsequent hypoxia. Mitochondria decreased their velocity in the proximity of synapses that correlated with local [ATP](i) depletions. Using a model of motor-assisted transport and Monte Carlo simulations, we showed that mitochondrial traffic is more sensitive to increases in [ADP](i) than to [ATP](i) depletions. We propose that consumption of synaptic ATP can produce local increases in [ADP](i) and facilitate the targeting of mitochondria to synapses.

Spontaneous and evoked neuronal activities regulate movements of single neuronal mitochondria

Mitochondria produce ATP and act as internal Ca2+ storage sites in neurons. Their localization at active synapses can be beneficial both for the maintenance of normal neuronal activity and for preventing neurodegeneration. Mitochondrial distribution in neurons is a dynamic process that can, in turn, be determined by their activity. To examine these relationships, we used respiratory neurons that possess persistent rhythmic activity, to which mitochondria substantially contributed. Mitochondria were visualized using potentiometric dyes and two-photon microscopy. The trajectories of mitochondrial movements were obtained by single particle tracking. Spontaneous and evoked synaptic activity and intracellular Ca2+ were measured by using FM 1-43 and fura-2, respectively. Inhibition of synaptic activity with N-type Ca2+ and Na+ channel blockers, omega-conotoxin GVIA, and tetrodotoxin, increased the run-lengths of the directed transport. After brief periods of spontaneous synaptic activity and after membrane depolarization, mitochondrial movements were inhibited in correlation with the duration of intracellular [Ca2+] elevations. Movements of mitochondria were also suppressed after membrane depolarization in Ca2+-free solutions, indicating that the effects of Ca2+ are indirect and other factors, e.g., ATP depletion, may be involved. Through the use of experimentally determined parameters of mitochondrial motions, we modeled the behavior of mitochondrial ensembles and showed a tendency of mitochondria to produce linear aggregates whose formation is enhanced by irregularities of mitochondrial movements. We propose that accumulation and clustering of mitochondria in neurons are caused by interruptions in the directed transport of mitochondria, leading to the inhibition of their movements at the active synapses.

Protein kinase G-induced activation of K(ATP) channels reduces contractility of human prostate tissue

BACKGROUND

Human cultured prostatic stromal cells respond to protein kinase G (PKG) activators and the nitric oxide donor, sodium nitroprusside (SNP) by opening ATP-sensitive potassium channels (K(ATP) channels) to reduce nifedipine-sensitive phorbol ester-induced contractility.

METHODS

PKG activators, SNP, diazoxide, nifedipine, isoprenaline, forskolin, and Sp-8-Br-cAMP were used to inhibit alpha(1)-adrenoceptor-induced contractions in tissue from transurethral resections of the prostate (TURP). The selective K(ATP) and large conductance Ca(2+) activated K(+) (BK(Ca)) channel inhibitors, glibenclamide and charybdotoxin, respectively were used to inhibit responses to PKG activators. RT-PCR identified the K(ATP) channel subunits present in TURP tissue and cultured cells.

RESULTS

The PKG activators, APT-cGMP (1 nM-100 microM) and PET-cGMP (1 nM-100 microM), and also SNP (1 nM-100 microM), forskolin (10 microM), diazoxide (100 microM) and nifedipine (3 microM) inhibited phenylephrine (20 microM)-induced contractions. The effect of APT-cGMP (1 nM-100 microM) could be reversed by glibenclamide, but not by charybdotoxin. TURP tissue contained mRNA for PKG Ialpha, Ibeta, and II and the K(ATP) channel subunits Kir6.1, Kir6.2, SUR2B, and SUR1. Cultured stromal cells contained only Kir6.1 and SUR2B subunit mRNA. SUR1 mRNA was detected in one of five cultured epithelial cell lines.

CONCLUSIONS

PKG activators reduce alpha(1)-adrenoceptor-induced contractility in TURP tissue via the activation of K(ATP) channels. (c) 2005 Wiley-Liss, Inc.

Tuning and playing a motor rhythm: how metabotropic glutamate receptors orchestrate generation of motor patterns in the mammalian central nervous system

Repeated motor activities like locomotion, mastication and respiration need rhythmic discharges of functionally connected neurons termed central pattern generators (CPGs) that cyclically activate motoneurons even in the absence of descending commands from higher centres. For motor pattern generation, CPGs require integration of multiple processes including activation of ion channels and transmitter receptors at strategic locations within motor networks. One emerging mechanism is activation of glutamate metabotropic receptors (mGluRs) belonging to group I, while group II and III mGluRs appear to play an inhibitory function on sensory inputs. Group I mGluRs generate neuronal membrane depolarization with input resistance increase and rapid fluctuations in intracellular Ca(2+), leading to enhanced excitability and rhythmicity. While synchronicity is probably due to modulation of inhibitory synaptic transmission, these oscillations occurring in coincidence with strong afferent stimuli or application of excitatory agents can trigger locomotor-like patterns. Hence, mGluR-sensitive spinal oscillators play a role in accessory networks for locomotor CPG activation. In brainstem networks supplying tongue muscle motoneurons, group I receptors facilitate excitatory synaptic inputs and evoke synchronous oscillations which stabilize motoneuron firing at regular, low frequency necessary for rhythmic tongue contractions. In this case, synchronicity depends on the strong electrical coupling amongst motoneurons rather than inhibitory transmission, while cyclic activation of K(ATP) conductances sets its periodicity. Activation of mGluRs is therefore a powerful strategy to trigger and recruit patterned discharges of motoneurons.

Mitochondrial K(ATP) channels in respiratory neurons and their role in the hypoxic facilitation of rhythmic activity

Hypoxia is damaging in neurons, but it can also produce beneficial effects by consolidating the activity of neural networks such as facilitation of respiratory activity [T.L. Baker-Herman, D.D. Fuller, R.W. Bavis, A.G. Zabka, F.J. Golder, N.J. Doperalski, R.A. Johnson, J.J. Watters, G.S. Mitchell, Nature Neuroscience 7 (2004) 48-55; J.L. Feldman, G.S. Mitchell, E.E. Nattie, Ann. Rev. Neurosci. 26 (2003) 239-266; D.M. Blitz, J.M. Ramirez, J. Neurophysiol. 87 (2002) 2964-2971]. The underlying mechanisms are unknown, and we hypothesized they may be similar to ischemic preconditioning in the heart, involving mitochondrial K(ATP) (mK(ATP)) channels. By measuring the mitochondrial potential (Psi(m)) and Ca2+ ([Ca2+]m) in neurons of pre-Botzinger complex (pBC), we examined the functional expression of mK(ATP) channels in the respiratory network. The opener of mK(ATP) channels diazoxide decreased Psi(m) and [Ca2+]m both in pBC neurons and in isolated immobilized mitochondria. 5-Hydroxydecanoate (5-HD), the blocker of mK(ATP) channels, increased both Psi(m) and [Ca2+]m. Phorbol 12-myristate-13-acetate (PMA) mimicked the effects of diazoxide. Protein kinase C (PKC) was stimulated during hypoxia that occurred mostly at the mitochondria. Brief hypoxia induced facilitation of the respiratory activity, which was prevented after blockade of mK(ATP) channels with 5-HD and PKC with staurosporine. Diazoxide potentiated the motor output and subsequent application of hypoxia was ineffective. We propose that a PKC-induced stimulation of K(ATP) channels in the mitochondria of respiratory neurons is responsible for the hypoxic facilitation of rhythmic activity.

Protective role of neuronal KATP channels in brain hypoxia

During severe arterial hypoxia leading to brain anoxia, most mammalian neurons undergo a massive depolarisation terminating in cell death. However, some neurons of the adult brain and most immature nervous structures tolerate extended periods of hypoxia-anoxia. An understanding of the mechanisms underlying this tolerance to oxygen depletion is pivotal for developing strategies to protect the brain from consequences of hypoxic-ischemic insults. ATP-sensitive K(+) (K(ATP)) channels are good subjects for this study as they are activated by processes associated with energy deprivation and can counteract the terminal anoxic-ischemic neuronal depolarisation. This review summarises in vitro analyses on the role of K(ATP) channels in hypoxia-anoxia in three distinct neuronal systems of rodents. In dorsal vagal neurons, blockade of K(ATP) channels with sulfonylureas abolishes the hypoxic-anoxic hyperpolarisation. However, this does not affect the extreme tolerance of these neurons to oxygen depletion as evidenced by a moderate and sustained increase of intracellular Ca(2+) (Ca(i)). By contrast, a sulfonylurea-induced block of K(ATP) channels shortens the delay of occurrence of a major Ca(i) rise in cerebellar Purkinje neurons. In neurons of the neonatal medullary respiratory network, K(ATP) channel blockers reverse the anoxic hyperpolarisation associated with slowing of respiratory frequency. This may constitute an adaptive mechanism for energy preservation. These studies demonstrate that K(ATP) channels are an ubiquituous feature of mammalian neurons and may, indeed, play a protective role in brain hypoxia.

Metabotropic glutamate receptor activity induces a novel oscillatory pattern in neonatal rat hypoglossal motoneurones

Tongue muscles innervated by the hypoglossal nerves play a crucial role to ensure airway patency and milk suckling in the neonate. Using a slice preparation of the neonatal rat brain, we investigated the electrophysiological characteristics of hypoglossal motoneurones in the attempt to identify certain properties potentially capable of synchronizing motor commands to the tongue. Bath-applied DHPG, a selective agonist of group I metabotropic glutamate receptors (mGluRs), generated persistent, regular electrical oscillations (4-8 Hz) recorded from patch-clamped motoneurones. Under voltage clamp, oscillations were biphasic events, comprising large outward slow currents alternated with fast, repeated inward currents. Electrical oscillations had amplitude and period insensitive to cell membrane potential, and required intact glutamatergic transmission via AMPA receptors. Oscillations were mediated by subtype 1 receptors of group I mGluRs (mGluR1s), and were routinely observed during pharmacological block of glycinergic and GABAergic inhibition, although they could also be recorded in standard saline. Simultaneous recordings from pairs of motoneurones within the same hypoglossal nucleus demonstrated that oscillations were due to their strong electrical coupling and were blocked by the gap junction blocker carbenoxolone. Pacing of slow oscillations apparently depended on the operation of K(ATP) channels in view of the block by tolbutamide or glibenclamide. Under current clamp, oscillations generated more regular spike firing of motoneurones and facilitated glutamatergic excitatory inputs. These data suggest that neonatal motoneurones of the nucleus hypoglossus possess a formerly undisclosed ability to express synchronous electrical oscillations, unveiled by activation of mGluR1s.

Background sodium current stabilizes bursting in respiratory pacemaker neurons

Endogenous pacemaker properties have been proposed to generate rhythmic activity underlying many behaviors including respiration. For pacemakers to generate regenerative bursting, background currents maintain their membrane potential (Vm) within a range where bi-stable properties are expressed, thereby stabilizing rhythmogenesis. We previously found that the baseline Vm of respiratory pacemakers is stabilized against hyperpolarizing shifts in their Vm. In response to prolonged hyperpolarizing current injection synaptically isolated respiratory pacemakers steadily depolarize and resume bursting, suggesting a stabilizing background current is involved. What is the ionic basis of this background current in respiratory pacemakers? Here we demonstrate that in low-[Na(+)](o) ACSF, synaptically isolated respiratory pacemakers hyperpolarized and remained outside the bursting window, but could burst upon depolarizing current injection. These data suggest that pacemakers possess a background sodium current that is necessary to bring their Vm into a bursting range. Low-[Na(+)](o) ACSF also abolished the depolarizing shift evoked during prolonged hyperpolarizing current injection, and bursting did not resume. This depolarizing shift persisted in the presence of I(h)-current blockers, but was abolished in tetrodotoxin. Although, under control conditions, the Vm of synaptically isolated respiratory pacemaker neurons was not significantly affected when [K(+)](o) was changed from 3 to 8 mM, the Vm is altered when [K(+)](o) was raised in low-[Na(+)](o) ACSF. Thus, current-clamp studies suggest that respiratory pacemaker neurons possess a background sodium current that maintains their membrane potential within a range where they express bursting, thereby stabilizing rhythmogenesis.