Increase In Neuronal Firing Research Articles

Detecting memory content in firing rate signals using a machine learning approach: A fractal analysis

While top-down modulation can be observed by increases in neuronal firing in some brain areas, modulation in other areas demonstrates complex nonlinear dynamics. When these signal modifications cannot be identified by classical methods, fractal analysis can be used to better describe the resulting changes. It was previously shown that the Higuchi dimension could detect such modification. However, these changes likely extend to multiple fractal dimensions. In this paper, machine learning algorithms were employed to extract the fractal features of neuronal firing and detect the occurrence of top-down modulation. We assessed the performance of classifiers trained on different fractal features as well as the Fourier transform and wavelet transform-based features obtained from the firing rate signals during a spatial memory task to measure the ability of individual neurons in the middle temporal cortex to hold memory information. By comparing the results of the different classifiers, Higuchi fractal dimension was confirmed as the strongest indicator of working memory content in cortical firing patterns compared to other fractal features. We demonstrated the utility of this approach and observed that the middle temporal cortex likely encodes spatial working memory based on linear and nonlinear temporal features rather than simply increasing the average firing rate.

Nitric oxide signalling underlies salicylate-induced increases in neuronal firing in the inferior colliculus: A central mechanism of tinnitus?

The anti-inflammatory drug salicylate induces tinnitus in animals and man. Salicylate reduces cochlear output but causes hyperactivity in higher auditory centres, including the inferior colliculus (the auditory midbrain). Using multi-electrode recording in anaesthetised guinea pigs (Cavia porcellus), we addressed the hypothesis that salicylate-induced hyperactivity in the inferior colliculus involves nitric oxide signalling secondary to increased ascending excitatory input.Systemic salicylate (200 mg/kg i.p., 0 h) markedly increased spontaneous and sound-driven neuronal firing in the inferior colliculus (3-6 h post drug), with both onset and sustained responses to pure tones being massively increased. Reverse microdialysis of increasing concentrations of salicylate directly into the inferior colliculus (100 µM-10 mM, from 0 h) failed to mimic systemic salicylate. In contrast, it caused a small, transient, increase in sound-driven firing (1 h), followed by a larger sustained decrease in both spontaneous and sound-driven firing (2-5 h). When salicylate was given systemically, reverse microdialysis of the neuronal nitric oxide synthase inhibitor L-methyl arginine into the inferior colliculus (500 mM, 2-6 h) completely blocked the salicylate-induced increase in spontaneous and sound-driven neuronal firing.Our data indicate that systemic salicylate induces neuronal hyperactivity in the auditory midbrain via a mechanism outside the inferior colliculus, presumably upstream in the auditory pathway; and that the mechanism is ultimately dependent on nitric oxide signalling within the inferior colliculus.Given that nitric oxide is known to mediate NMDA receptor signalling in the inferior colliculus, we propose that salicylate activates an ascending glutamatergic input to the inferior colliculus and that this is an important mechanism underlying salicylate-induced tinnitus.

Redox-sensitive calcium/calmodulin-dependent protein kinase IIα in angiotensin II intra-neuronal signaling and hypertension

Dysregulation of brain angiotensin II (AngII) signaling results in modulation of neuronal ion channel activity, an increase in neuronal firing, enhanced sympathoexcitation, and subsequently elevated blood pressure. Studies over the past two decades have shown that these AngII responses are mediated, in part, by reactive oxygen species (ROS). However, the redox-sensitive target(s) that are directly acted upon by these ROS to execute the AngII pathophysiological responses in neurons remain unclear. Calcium/calmodulin-dependent protein kinase II (CaMKII) is an AngII-activated intra-neuronal signaling protein, which has been suggested to be redox sensitive as overexpressing the antioxidant enzyme superoxide dismutase attenuates AngII-induced activation of CaMKII. Herein, we hypothesized that the neuronal isoform of CaMKII, CaMKII-alpha (CaMKIIα), is a redox-sensitive target of AngII, and that mutation of potentially redox-sensitive amino acids in CaMKIIα influences AngII-mediated intra-neuronal signaling and hypertension. Adenoviral vectors expressing wild-type mouse CaMKIIα (Ad.wtCaMKIIα) or mutant CaMKIIα (Ad.mutCaMKIIα) with C280A and M281V mutations were generated to overexpress either CaMKIIα isoform in mouse catecholaminergic cultured neurons (CATH.a) or in the brain subfornical organ (SFO) of hypertensive mice. Overexpressing wtCaMKIIα exacerbated AngII pathophysiological responses as observed by a potentiation of AngII-induced inhibition of voltage-gated K+ current, enhanced in vivo pressor response following intracerebroventricular injection of AngII, and sensitization to chronic peripheral infusion of AngII resulting in a more rapid increase in blood pressure. In contrast, expressing the mutant CaMKIIα in CATH.a neurons or the SFO failed to intensify these AngII responses. Taken together, these data identify neuronal CaMKIIα as a redox-sensitive signaling protein that contributes to AngII-induced neuronal activation and hypertension.

Regulating nociceptive transmission by VGluT2‐expressing spinal dorsal horn neurons

Vesicular glutamate transporter-2 (VGluT2) mediates the uptake of glutamate into synaptic vesicles in neurons. Spinal cord dorsal horn interneurons are highly heterogeneous and molecularly diverse. The functional significance of VGluT2-expressing dorsal horn neurons in physiological and pathological pain conditions has not been explicitly demonstrated. Designer receptors exclusively activated by designer drugs (DREADDs) are a powerful chemogenetic tool to reversibly control neuronal excitability and behavior. Here, we used transgenic mice with Cre recombinase expression driven by the VGluT2 promoter, combined with the chemogenetic approach, to determine the contribution of VGluT2-expressing dorsal horn neurons to nociceptive regulation. Adeno-associated viral vectors expressing double-floxed Cre-dependent Gαq-coupled human M3 muscarinic receptor DREADD (hM3D)-mCherry or Gαi-coupled κ-opioid receptor DREADD (KORD)-IRES-mCitrine were microinjected into the superficial spinal dorsal horn of VGluT2-Cre mice. Immunofluorescence labeling showed that VGluT2 was predominantly expressed in lamina II excitatory interneurons. Activation of excitatory hM3D in VGluT2-expressing neurons with clozapine N-oxide caused a profound increase in neuronal firing and synaptic glutamate release. Conversely, activation of inhibitory KORD in VGluT2-expressing neurons with salvinorin B markedly inhibited neuronal activity and synaptic glutamate release. In addition, chemogenetic stimulation of VGluT2-expressing neurons increased mechanical and thermal sensitivities in naive mice, whereas chemogenetic silencing of VGluT2-expressing neurons reversed pain hypersensitivity induced by tissue inflammation and peripheral nerve injury. These findings indicate that VGluT2-expressing excitatory neurons play a crucial role in mediating nociceptive transmission in the spinal dorsal horn. Targeting glutamatergic dorsal horn neurons with inhibitory DREADDs may be a new strategy for treating inflammatory pain and neuropathic pain.

CACHD1 is an α2δ-Like Protein That Modulates CaV3 Voltage-Gated Calcium Channel Activity.

The putative cache (Ca2+ channel and chemotaxis receptor) domain containing 1 (CACHD1) protein has predicted structural similarities to members of the α2δ voltage-gated Ca2+ channel auxiliary subunit family. CACHD1 mRNA and protein were highly expressed in the male mammalian CNS, in particular in the thalamus, hippocampus, and cerebellum, with a broadly similar tissue distribution to CaV3 subunits, in particular CaV3.1. In expression studies, CACHD1 increased cell-surface localization of CaV3.1, and these proteins were in close proximity at the cell surface, consistent with the formation of CACHD1-CaV3.1 complexes. In functional electrophysiological studies, coexpression of human CACHD1 with CaV3.1, CaV3.2, and CaV3.3 caused a significant increase in peak current density and corresponding increases in maximal conductance. By contrast, α2δ-1 had no effect on peak current density or maximal conductance in CaV3.1, CaV3.2, or CaV3.3. A comparison of CACHD1-mediated increases in CaV3.1 current density and gating currents revealed an increase in channel open probability. In hippocampal neurons from male and female embryonic day 19 rats, CACHD1 overexpression increased CaV3-mediated action potential firing frequency and neuronal excitability. These data suggest that CACHD1 is structurally an α2δ-like protein that functionally modulates CaV3 voltage-gated calcium channel activity.SIGNIFICANCE STATEMENT This is the first study to characterize the Ca2+ channel and chemotaxis receptor domain containing 1 (CACHD1) protein. CACHD1 is widely expressed in the CNS, in particular in the thalamus, hippocampus, and cerebellum. CACHD1 distribution is similar to that of low voltage-activated (CaV3, T-type) calcium channels, in particular to CaV3.1, a protein that regulates neuronal excitability and is a potential therapeutic target in conditions such as epilepsy and pain. CACHD1 is structurally an α2δ-like protein that functionally increases CaV3 calcium current. CACHD1 increases the presence of CaV3.1 at the cell surface, forms complexes with CaV3.1 at the cell surface, and causes an increase in channel open probability. In hippocampal neurons, CACHD1 causes increases in neuronal firing. Thus, CACHD1 represents a novel protein that modulates CaV3 activity.

S200 Paroxyzmal depolarization shift – Is it an interictal phenomenon is humans?

Paroxysmal depolarization shift (PDS) was extensively studied in animal models of acute and subacut epilepsy. Based on the observation that PDS occurred between behavioral seizures it had been complemented by interictal discharges, that can be recorded in humans in the seizure onset zone (SOZ) and around. PDS is characterized by excessive discharge of large number of neurons within a certain area of the cortex. Studying human SOZ with microelectrodes revealed that selective or no neuronal firing increase happen during IIDs. In this lecture we demonstrate interictal and ictal neuronal discharge patterns in humans and compare to PDS recordings from animals. Based on similarities and discrepancies we try to conclude how neuronal hyperexcitability can be interpreted in human chronic epilepsy compared to our understanding from animal models.

Activation of β-noradrenergic receptors enhances rhythmic bursting in mouse olfactory bulb external tufted cells.

The main olfactory bulb (MOB) receives a rich noradrenergic innervation from the nucleus locus coeruleus. Despite the well-documented role of norepinephrine and β-adrenergic receptors in neonatal odor preference learning, identified cellular physiological actions of β-receptors in the MOB have remained elusive. β-Receptors are expressed at relatively high levels in the MOB glomeruli, the location of external tufted (ET) cells that exert an excitatory drive on mitral and other cell types. The present study investigated the effects of β-receptor activation on the excitability of ET cells with patch-clamp electrophysiology in mature mouse MOB slices. Isoproterenol and selective β2-, but not β1-, receptor agonists were found to enhance two key intrinsic currents involved in ET burst initiation: persistent sodium (INaP) and hyperpolarization-activated inward (Ih) currents. Together, the positive modulation of these currents increased the frequency and strength of ET cell rhythmic bursting. Rodent sniff frequency and locus coeruleus neuronal firing increase in response to novel stimuli or environments. The increase in ET excitability by β-receptor activation may better enable ET cell rhythmic bursting, and hence glomerular network activity, to pace faster sniff rates during heightened norepinephrine release associated with arousal.

Ketogenic diet prevents neuronal firing increase within the substantia nigra during pentylenetetrazole-induced seizure in rats.

The mechanism responsible for the anti-seizure effect of ketogenic diets is poorly understood. Because the substantia nigra pars reticulata (SNr) is a "gate" center for seizures, the aim of the present experiment was to evaluate if a ketogenic diet modifies the neuronal response of this nucleus when a seizure-inducing drug is administered in rats. Two groups of rats were given a standard diet (group 1) or a ketogenic diet (group 2) for four weeks, then the threshold for seizure induction and the firing rate of putative GABAergic neurons within the SNr were evaluated with progressive infusion of pentylenetetrazole under general anesthesia. The results demonstrated that the ketogenic diet abolished the correlation between the firing rate response of SNr-neurons and the seizure-threshold. This result suggests that the anti-seizure effect of ketogenic diets can be due to a decrease in reactivity of GABAergic SNr-neurons.

Effect of Intramuscular Protons, Lactate, and ATP on Muscle Hyperalgesia in Rats.

Chronic muscle pain is a significant health problem leading to disability[1]. Muscle fatigue can exacerbate muscle pain. Metabolites, including ATP, lactate, and protons, are released during fatiguing exercise and produce pain in humans. These substances directly activate purinergic (P2X) and acid sensing ion channels (ASICs) on muscle nociceptors, and when combined, produce a greater increase in neuron firing than when given alone. Whether the enhanced effect of combining protons, lactate, and ATP is the sum of individual effects (additive) or more than the sum of individual effects (synergistic) is unknown. Using a rat model of muscle nociceptive behavior, we tested each of these compounds individually over a range of physiologic and supra-physiologic concentrations. Further, we combined all three compounds in a series of dilutions and tested their effect on muscle nociceptive behavior. We also tested a non-hydrolyzable form of ATP (α,β-meATP) alone and in combination with lactate and acidic pH. Surprisingly, we found no dose-dependent effect on muscle nociceptive behavior for protons, lactate, or ATP when given alone. We similarly found no effect after application of each two-metabolite combination. Only pH 4 saline and α,β-meATP produced hyperalgesia when given alone. When all 3 substances were combined, however, ATP (2.4μm), lactate (10mM), and acidic pH (pH 6.0) produced an enhanced effect greater than the sum of the effects of the individual components, i.e. synergism. α,β me ATP (3nmol), on the other hand, showed no enhanced effects when combined with lactate (10mM) or acidic pH (pH 6.0), i.e. additive. These data suggest that combining fatigue metabolites in muscle produces a synergistic effect on muscle nociception.

Estimation of the synaptic input firing rates and characterization of the stimulation effects in an auditory neuron.

To understand information processing in neuronal circuits, it is important to infer how a sensory stimulus impacts on the synaptic input to a neuron. An increase in neuronal firing during the stimulation results from pure excitation or from a combination of excitation and inhibition. Here, we develop a method for estimating the rates of the excitatory and inhibitory synaptic inputs from a membrane voltage trace of a neuron. The method is based on a modified Ornstein-Uhlenbeck neuronal model, which aims to describe the stimulation effects on the synaptic input. The method is tested using a single-compartment neuron model with a realistic description of synaptic inputs, and it is applied to an intracellular voltage trace recorded from an auditory neuron in vivo. We find that the excitatory and inhibitory inputs increase during stimulation, suggesting that the acoustic stimuli are encoded by a combination of excitation and inhibition.

The lidocaine metabolite N-ethylglycine has antinociceptive effects in experimental inflammatory and neuropathic pain.

Glycine transporter 1 (GlyT1) plays a crucial role in regulating extracellular glycine concentrations and might thereby constitute a new drug target for the modulation of glycinergic inhibition in pain signaling. Consistent with this view, inhibition of GlyT1 has been found to induce antinociceptive effects in various animal pain models. We have shown previously that the lidocaine metabolite N-ethylglycine (EG) reduces GlyT1-dependent glycine uptake by functioning as an artificial substrate for this transporter. Here, we show that EG is specific for GlyT1 and that in rodent models of inflammatory and neuropathic pain, systemic treatment with EG results in an efficient amelioration of hyperalgesia and allodynia without affecting acute pain. There was no effect on motor coordination or the development of inflammatory edema. No adverse neurological effects were observed after repeated high-dose application of EG. EG concentrations both in blood and spinal fluid correlated with an increase of glycine concentration in spinal fluid. The time courses of the EG and glycine concentrations corresponded well with the antinociceptive effect. Additionally, we found that EG reduced the increase in neuronal firing of wide-dynamic-range neurons caused by inflammatory pain induction. These findings suggest that systemically applied lidocaine exerts antihyperalgesic effects through its metabolite EG in vivo, by enhancing spinal inhibition of pain processing through GlyT1 modulation and subsequent increase of glycine concentrations at glycinergic inhibitory synapses. EG and other substrates of GlyT1, therefore, may be a useful therapeutic agent in chronic pain states involving spinal disinhibition.

What is a seizure network? Long-range network consequences of focal seizures.

What defines the spatial and temporal boundaries of seizure activity in brain networks? To fully answer this question a precise and quantitative definition of seizures is needed, which unfortunately remains elusive. Nevertheless, it is possible to ask under conditions where clearly divergent patterns of activity occur in large-scale brain networks whether certain activity patterns are part of the seizure while others are not. Here we examine brain network activity during focal limbic seizures, including diverse regions such as the hippocampus, subcortical arousal systems and fronto-parietal association cortex. Based on work from patients and from animal models we describe a characteristic pattern of intense increases in neuronal firing, cerebral blood flow, cerebral blood volume, blood oxygen level dependent functional magnetic resonance imaging (BOLD fMRI) signals and cerebral metabolic rate of oxygen consumption in the hippocampus during focal limbic seizures. Similar increases are seen in certain closely linked subcortical structures such as the lateral septal nuclei and anterior hypothalamus, which contain inhibitory neurons. In marked contrast, decreases in all of these parameters are seen in the subcortical arousal systems of the upper brainstem and intralaminar thalamus, as well as in the fronto-parietal association cortex. We propose that the seizure proper can be defined as regions showing intense increases, while those areas showing opposite changes are inhibited by the seizure network and constitute long-range network consequences beyond the seizure itself. Importantly, the fronto-parietal cortex shows sleep-like slow wave activity and depressed metabolism under these conditions, associated with impaired consciousness. Understanding which brain networks are directly involved in seizures versus which sustain secondary consequences can provide new insights into the mechanisms of brain dysfunction in epilepsy, hopefully leading to innovative treatment approaches.

Distinct muscarinic acetylcholine receptor subtypes mediate pre- and postsynaptic effects in rat neocortex

BackgroundCholinergic transmission has been implicated in learning, memory and cognition. However, the cellular effects induced by muscarinic acetylcholine receptors (mAChRs) activation are poorly understood in the neocortex. We investigated the effects of the cholinergic agonist carbachol (CCh) and various agonists and antagonists on neuronal activity in rat neocortical slices using intracellular (sharp microelectrode) and field potential recordings.ResultsCCh increased neuronal firing but reduced synaptic transmission. The increase of neuronal firing was antagonized by pirenzepine (M1/M4 mAChRs antagonist) but not by AF-DX 116 (M2/M4 mAChRs antagonist). Pirenzepine reversed the depressant effect of CCh on excitatory postsynaptic potential (EPSP) but had marginal effects when applied before CCh. AF-DX 116 antagonized the depression of EPSP when applied before or during CCh. CCh also decreased the paired-pulse inhibition of field potentials and the inhibitory conductances mediated by GABAA and GABAB receptors. The depression of paired-pulse inhibition was antagonized or prevented by AF-DX 116 or atropine but only marginally by pirenzepine. The inhibitory conductances were unaltered by xanomeline (M1/M4 mAChRs agonist), yet the CCh-induced depression was antagonized by AF-DX 116. Linopirdine, a selective M-current blocker, mimicked the effect of CCh on neuronal firing. However, linopirdine had no effect on the amplitude of EPSP or on the paired-pulse inhibition, indicating that M-current is involved in the increase of neuronal excitability but neither in the depression of EPSP nor paired-pulse inhibition.ConclusionsThese data indicate that the three effects are mediated by different mAChRs, the increase in firing being mediated by M1 mAChR, decrease of inhibition by M2 mAChR and depression of excitatory transmission by M4 mAChR. The depression of EPSP and increase of neuronal firing might enhance the signal-to-noise ratio, whereas the concomitant depression of inhibition would facilitate long-term potentiation. Thus, this triade of effects may represent a “neuronal correlate” of attention and learning.

Angiotensin (1–7) Antagonizes Angiotensin II Action in SHR Hypothalamic Neurons via Stimulation of PTEN Expression

It was demonstrated previously that Ang (1–7) counter‐regulates Ang II action in neuronal activity of SHR neurons. However, the underlying molecular mechanisms are still not fully known. Our objective in the present study was to characterize the signal transduction mechanism of phosphatidylinositol 3‐kinase (PI3K) and phosphatase and tensin homologue deleted on chromosome ten (PTEN) involvement in the counter‐regulatory effect of Ang (1–7) on Ang II action in neurons cultured from hypothalamus of WKY vs. SH rats using current clamp recording.Application of Ang II produced a 44% greater increase in neuronal firing in SHR vs. WKY neurons. Chronic treatment with Ang (1–7) (100nM, 24 hrs) and inhibition of PI3K using lentiviral vector (LV‐DNp85α) significantly abolished Ang II mediated increase in neuronal firing rate in SHR vs. WKY neurons. Pretreatment of Ang (1–7) attenuated Ang II‐induced increase in the phosphorylation of Akt by 83% in SHR vs WKY neurons. Treatment with Ang (1–7) (100nM, 24 hrs) significantly enhanced PTEN expression in both WKY and SHR neuronal cultures using western blot and real‐time PCR. These effects of Ang (1–7) were blocked by A‐779 (10 μM), a Mas‐R antagonist. The data demonstrate that Ang (1–7) induces PTEN expression via Mas‐R, and depresses PI3K signaling via PKB/Akt pathway, which leads to a counter‐regulatory effect of Ang (1–7) on Ang II action in neuronal activity of SHR neurons.

Interictal high frequency oscillations in an animal model of infantile spasms

While infantile spasms is the most common catastrophic epilepsy of infancy and early-childhood, very little is known about the basic mechanisms responsible for this devastating disorder. In experiments reported here, spasms were induced in rats by the chronic infusion of TTX into the neocortex beginning on postnatal days 10–12. Studies of focal epilepsy suggest that high frequency EEG oscillations (HFOs) occur interictally at sites that are most likely responsible for seizure generation. Thus, our goal was to determine if HFOs occurred and where they occurred in cortex in the TTX model. We also undertook multiunit recordings to begin to analyze the basic mechanisms responsible for HFOs. Our results show that HFOs occur most frequently during hypsarrhythmia and NREM sleep and are most prominent contralateral to the TTX infusion site in the homotopic cortex and anterior to this region in frontal cortex. While HFOs were largest and most frequent in these contralateral regions, they were also commonly recorded synchronously across multiple and widely-spaced recordings sites. The amplitude and spatial distribution of interictal HFOs were found to be very similar to the high frequency bursts seen at seizure onset. However, the latter differed from the interictal events in that the high frequency activity was more intense at seizure onset. Microwire recordings showed that neuronal unit firing increased abruptly with the generation of HFOs. A similar increase in neuronal firing occurred at the onset of the ictal events. Taken together, results suggest that neocortical networks are abnormally excitable, particularly contralateral to TTX infusion, and that these abnormalities are not restricted to small areas of cortex. Multiunit firing coincident with HFOs is fully consistent with a neocortical hyperexcitability hypothesis particularly since they both occur at seizure onset.

Dual Mechanism of Neuronal Ensemble Inhibition in Primary Auditory Cortex

Inhibition plays an essential role in shaping and refining the brain's representation of sensory stimulus attributes. In primary auditory cortex (A1), so-called "sideband" inhibition helps to sharpen the tuning of local neuronal responses. Several distinct types of anatomical circuitry could underlie sideband inhibition, including direct thalamocortical (TC) afferents, as well as indirect intracortical mechanisms. The goal of the present study was to characterize sideband inhibition in A1 and to determine its mechanism by analyzing laminar profiles of neuronal ensemble activity. Our results indicate that both lemniscal and nonlemniscal TC afferents play a role in inhibitory responses via feedforward inhibition and oscillatory phase reset, respectively. We propose that the dynamic modulation of excitability in A1 due to the phase reset of ongoing oscillations may alter the tuning of local neuronal ensembles and can be regarded as a flexible overlay on the more obligatory system of lemniscal feedforward type responses.

Postsynaptic Spiking Homeostatically Induces Cell-Autonomous Regulation of Inhibitory Inputs via Retrograde Signaling

Developing neural circuits face the dual challenge of growing in an activity-induced fashion and maintaining stability through homeostatic mechanisms. Compared to our understanding of homeostatic regulation of excitatory synapses, relatively little is known about the mechanism mediating homeostatic plasticity of inhibitory synapses, especially that following activity elevation. Here, we found that elevating neuronal activity in cultured hippocampal neurons for 4 h significantly increased the frequency and amplitude of mIPSCs, before detectable change at excitatory synapses. Consistently, we observed increases in presynaptic and postsynaptic proteins of GABAergic synapses, including GAD65, vGAT, and GABA(A)Rα1. By suppressing activity-induced increase of neuronal firing with expression of the inward rectifier potassium channel Kir2.1 in individual neurons, we showed that elevation in postsynaptic spiking activity is required for activity-dependent increase in the frequency and amplitude of mIPSCs. Importantly, directly elevating spiking in individual postsynaptic neurons, by capsaicin activation of overexpressed TRPV1 channels, was sufficient to induce increased mIPSC amplitude and frequency, mimicking the effect of elevated neuronal activity. Downregulating BDNF expression in the postsynaptic neuron or its extracellular scavenging prevented activity-induced increase in mIPSC frequency, consistent with a role of BDNF-dependent retrograde signaling in this process. Finally, elevating activity in vivo by kainate injection increased both mIPSC amplitude and frequency in CA1 pyramidal neurons. Thus, spiking-induced, cell-autonomous upregulation of GABAergic synaptic inputs, through retrograde BDNF signaling, represents an early adaptive response of neural circuits to elevated network activity.

Neuronal hyperexcitability in the dorsal horn after painful facet joint injury

Excessive cervical facet capsular ligament stretch has been implicated as a cause of whiplash-associated disorders following rear-end impacts, but the pathophysiological mechanisms that produce chronic pain in these cases remain unclear. Using a rat model of C6–C7 cervical facet joint capsule stretch that produces sustained mechanical hyperalgesia, the presence of neuronal hyperexcitability was characterized 7days after joint loading. Extracellular recordings of spinal dorsal horn neuronal activity between C6 and C8 (117 neurons) were obtained from anesthetized rats, with both painful and non-painful behavioral outcomes established by the magnitude of capsule stretch. The frequency of neuronal firing during noxious pinch (p<0.0182) and von Frey filaments applications (4–26g) to the forepaw was increased (p<0.0156) in the painful group compared to the non-painful and sham groups. In addition, the incidence and frequency of spontaneous and after discharge firing were greater in the painful group (p<0.0307) relative to sham. The proportion of cells in the deep laminae that responded as wide dynamic range neurons also was increased in the painful group relative to non-painful or sham groups (p<0.0348). These findings suggest that excessive facet capsule stretch, while not producing visible tearing, can produce functional plasticity of dorsal horn neuronal activity. The increase in neuronal firing across a range of stimulus magnitudes observed at day 7 post-injury provides the first direct evidence of neuronal modulation in the spinal cord following facet joint loading, and suggests that facet-mediated chronic pain following whiplash injury is driven, at least in part, by central sensitization.

N-desmethylclozapine (NDMC) is an antagonist at the human native muscarinic M1 receptor

N-desmethylclozapine (NDMC) has been reported to display partial agonism at the human recombinant and rat native M(1) mAChR, a property suggested to contribute to the clinical efficacy of clozapine. However, the profile of action of NDMC at the human native M(1) mAChR has not been reported. The effect of NDMC on M(1) mAChR function was investigated in human native tissues by assessing its effect on (1) M(1) mAChR-mediated stimulation of [(35)S]-GTPgammaS-G(q/11)alpha binding to human post mortem cortical membranes and (2) the M(1) mAChR-mediated increase in neuronal firing in human neocortical slices. NDMC displayed intrinsic activities of 46+/-9%, compared to oxo-M, at the human recombinant M(1) receptor, in FLIPR studies and 35+/-4% at rat native M(1) receptors in [(35)S]-GTPgammaS-G(q/11)alpha binding studies. In [(35)S]-GTPgammaS-G(q/11)alpha binding studies in human cortex, oxo-M stimulated binding by 240+/-26% above basal with a pEC(50) of 6.56+/-0.05. In contrast, NDMC did not stimulate [(35)S]-GTPgammaS-G(q/11)alpha binding to human cortical membranes but antagonised the response to oxo-M (2microM) showing a pK(B) of 6.8, comparable to its human recombinant M(1) mAChR affinity (pK(i)=6.9) derived from [(3)H]-NMS binding studies. In human, contrary to the rat neocortical slices, NDMC did not elicit a significant increase in M(1) mAChR-mediated neuronal firing, and attenuated a carbachol-induced increase in neuronal firing when pre-applied. These data indicate that, whereas NDMC displays moderate to low levels of partial agonism at the human recombinant and rat native M(1) mAChR, respectively, it acts as an antagonist at the M(1) mAChR in human cortex.

Activation of presynaptic α7 nicotinic receptors evokes an excitatory response in hippocampal CA3 neurones in anaesthetized rats: an in vivo iontophoretic study

alpha7 Nicotinic receptors have been suggested to play an important role in hippocampal learning and memory. However, the direct action of this receptor subtype on hippocampal pyramidal neurones in vivo has not yet been fully investigated. The availability of selective agonists for alpha7 receptors [AR-R17779 and (R)-(-)-5'-phenylspiro[1-azabicyclo[2.2.2] octane-3,2'-(3'H)furo[2,3-b]pyridine (PSAB-OFP)] has now allowed this role to be investigated. Single-cell extracellular recordings were made from hippocampal CA3 pyramidal neurones in anaesthetized rats. The effects of nicotine, AR-R17779 and PSAB-OFP, applied either systemically or iontophoretically, were studied on the activity of these neurones. Intravenous injection of cumulative doses of nicotine and PSAB-OFP induced dose-related, significant increases in neuronal firing in the majority of neurones tested. This excitation could be inhibited by intravenous administration of methyllycaconitine (MLA), a selective alpha7 nicotinic receptor antagonist. Furthermore, iontophoretic application of nicotine, AR-R17779 and PSAB-OFP each evoked current-dependent excitation of most CA3 pyramidal neurones studied, and this excitation was antagonized by co-iontophoretic application of MLA. In addition, the excitation induced by iontophoretic application of nicotine, AR-R17779 or PSAB-OFP was also blocked by co-iontophoretic application of either 6,7-dinitroquinoxaline-2,3-dione (DNQX) or D(2)-2-amino-5-phosphonopentanoate (D-AP5), selective N-methyl-D-aspartic acid (NMDA) and non-NMDA receptor antagonists respectively. CA3 pyramidal neurones are modulated by activation of presynaptic alpha7 nicotinic receptors, which, at least in part, enhances glutamate release onto post-synaptic (RS)-alpha-amino-3-hydroxy-5-methyl-4-isoxazole proprionic acid and NMDA receptors on these CA3 neurones. This mechanism probably contributes to the effects of nicotine on hippocampal learning and memory.