Excitatory Currents Research Articles

Optically Selective Two-Photon Uncaging of Glutamate at 900 nm

We have synthesized a 7-diethylaminocoumarin (DEAC) derivative that allows wavelength-selective two-photon uncaging at 900 nm versus 720 nm. This new caging chromophore, called DEAC450, has an extended π-electron moiety at the 3-position that shifts the absorption spectrum maximum of DEAC from 375 to 450 nm. Two-photon excitation at 900 nm was more than 60-fold greater than at 720 nm. Two-photon uncaging of DEAC450-Glu at 900 nm at spine heads on pyramidal neurons in acutely isolated brain slices generated postsynaptic responses that were similar to spontaneous postsynaptic excitatory miniature currents, whereas significantly higher energies at 720 nm evoked no currents. Since many nitroaromatic caged compounds are two-photon active at 720 nm, optically selective uncaging of DEAC450-caged biomolecules at 900 nm may allow facile two-color optical interrogation of bimodal signaling pathways in living tissue with high resolution for the first time.

M‐type K+ Channel Openers: In vivo Neuroprotective Role during Cerebrovascular Stroke

Stroke is the third leading cause of human death and many models have been used to study the mechanisms of tissue damage and neuronal protection. Investigations of neuroprotection during ischemic stroke through “M‐channels” could underlie potential treatments, since K+ channels stabilize resting potentials by counterbalancing the depolarizing effects of excitatory cation currents. These voltage‐gated channels, with distinct electrophysiological and pharmacological properties, play critical roles in control of neuronal excitability and action potential firing. A known M‐channel activator, retigabine (RTG) and other novel openers, up‐regulate M‐channels by stabilizing their open states through distinct molecular mechanisms. We hypothesize that M current‐mediated neuronal silencing has a neuroprotective role by increasing opening of KCNQ channels, thus decreasing neuronal activity. We used two in vivo mouse models; a cerebral infarct produced by laser‐controlled photothrombosis and the second the middle cerebral artery occlusion (MCAo). M‐current activity was pharmacologically altered in both models and application of M‐channel activators significantly reduced lesion area. Furthermore, retigabine enhanced protection from cerebral inflammatory responses. Thus, this study uses powerful models which may provide novel therapeutics for commonly‐occurring ischemic attacks.

Short‐term sustained hypoxia affects synaptic transmission in the nucleus tractus solitarius neurons of juvenile rats

In the present study we evaluated the effects of a short‐term sustained hypoxia [(SH) 24 hours ‐ 10% O2] on intrinsic properties and synaptic transmission of nucleus tractus solitary (NTS) neurons of juvenile Wistar rats (P21). After 24 hours inside hypoxic or normoxic chambers rats were anesthetized, the brains removed and brainstems sliced. Intrinsic properties and excitatory currents of NTS neurons were recorded using whole‐cell patch clamp. SH did not change the resting membrane potential (control: −78.7 ± 2.4 mV n=15; SH: −72.3 ± 3.7 mV n=15), input resistance (control: 1.4 ± 0.4 G Ohm n=8; SH: 1.4 ± 0.4 G Ohm n=9) or capacitance of neurons (control: 14.2 ± 0.9 pF n=17; SH: 12.4 ± 0.8 pF n=15), indicating that SH does not change the intrinsic properties of NTS neurons. However, it was observed an increase in frequency of spontaneous currents (control: 3.6 ± 0.4 Hz n=18; SH: 7.1 ± 1.5 Hz n=15) and in peak amplitude of evoked currents (control: −289 ± 51 pA n=15; SH: −472 ± 64 pA n=15). Moreover, after a train of 5 stimuli, NTS neurons from SH group presented higher depression in glutamatergic events (relative amplitude of evoked currents; control: 100, 44, 34, 32 and 28%; SH: 100, 40, 28, 22 and 17%, *p<0.001, n=15), suggesting that SH affects the vesicular release probability. The data show that short‐term SH increases the spontaneous and evoked neurotransmitter release in the NTS neurons, which may explain, at least in part, the autonomic and respiratory changes observed in this experimental model.Financial support: FAPESP and CNPq.

Tonic inhibition sets the state of excitability in olfactory bulb granule cells

GABAergic granule cells (GCs) regulate, via mitral cells, the final output from the olfactory bulb to piriform cortex and are central for the speed and accuracy of odour discrimination. However, little is known about the local circuits in which GCs are embedded and how GCs respond during functional network activity. We recorded inhibitory and excitatory currents evoked during a single sniff-like odour presentation in GCs in vivo. We found that synaptic excitation was extensively activated across cells, whereas phasic inhibition was rare. Furthermore, our analysis indicates that GCs are innervated by a persistent firing of deep short axon cells that mediated the inhibitory evoked responses. Blockade of GABAergic synaptic input onto GCs revealed a tonic inhibitory current mediated by furosemide-sensitive GABA(A) receptors. The average current associated with this tonic GABAergic conductance was 3-fold larger than that of phasic inhibitory postsynaptic currents. We show that the pharmacological blockage of tonic inhibition markedly increased the occurrence of supra-threshold responses during an odour-stimulated sniff. Our findings suggest that GCs mediate recurrent or lateral inhibition, depending on the ambient level of extracellular GABA.

Sphingosine-1-Phosphate-Induced Nociceptor Excitation and Ongoing Pain Behavior in Mice and Humans Is Largely Mediated by S1P3 Receptor

The biolipid sphingosine-1-phosphate (S1P) is an essential modulator of innate immunity, cell migration, and wound healing. It is released locally upon acute tissue injury from endothelial cells and activated thrombocytes and, therefore, may give rise to acute post-traumatic pain sensation via a yet elusive molecular mechanism. We have used an interdisciplinary approach to address this question, and we find that intradermal injection of S1P induced significant licking and flinching behavior in wild-type mice and a dose-dependent flare reaction in human skin as a sign of acute activation of nociceptive nerve terminals. Notably, S1P evoked a small excitatory ionic current that resulted in nociceptor depolarization and action potential firing. This ionic current was preserved in "cation-free" solution and blocked by the nonspecific Cl(-) channel inhibitor niflumic acid and by preincubation with the G-protein inhibitor GDP-β-S. Notably, S1P(3) receptor was detected in virtually all neurons in human and mouse DRG. In line with this finding, S1P-induced neuronal responses and spontaneous pain behavior in vivo were substantially reduced in S1P(3)(-/-) mice, whereas in control S1P(1) floxed (S1P(1)(fl/fl)) mice and mice with a nociceptor-specific deletion of S1P(1)(-/-) receptor (SNS-S1P(1)(-/-)), neither the S1P-induced responses in vitro nor the S1P-evoked pain-like behavior was altered. Therefore, these findings indicate that S1P evokes significant nociception via G-protein-dependent activation of an excitatory Cl(-) conductance that is largely mediated by S1P(3) receptors present in nociceptors, and point to these receptors as valuable therapeutic targets for post-traumatic pain.

Levetiracetam Has Opposite Effects on Alcohol- and Cocaine-Related Behaviors in C57BL/6J Mice

The antiepileptic drug levetiracetam (LEV) is a potential treatment for alcohol use disorders, yet few preclinical studies exist on its effects in animal models relevant to drug or alcohol abuse. We investigated the effects of LEV on locomotor stimulation following acute and repeated administration of alcohol or cocaine and on alcohol- and cocaine-mediated changes in responding for brain stimulation reward (BSR) in C57BL/6J mice. LEV alone (10.0-100.0 mg/kg intraperitoneally) had no significant effect on locomotor activity or intracranial self-stimulation. Pretreatment with LEV reduced acute locomotor stimulation by 2.0 g/kg alcohol, attenuated the development of locomotor sensitization to alcohol with repeated exposure, and produced a shift in the dose-response curve for alcohol on BSR threshold without affecting maximum operant response rate (MAX). Conversely, LEV pretreatment enhanced both acute locomotor stimulation by 15 mg/kg cocaine and development of locomotor sensitization following repeated exposure and produced a leftward shift in the dose-response curve for cocaine on BSR threshold without affecting MAX. Electrophysiological recordings in vitro showed that LEV reduced excitatory currents in both ventral tegmental area (VTA) dopamine neurons and nucleus accumbens (NAc) medium spiny neurons, consistent with a presynaptic effect. The opposite effects of LEV pretreatment on alcohol- and cocaine-related behaviors may predict its clinical utility in the treatment of patients with alcohol, but not psychostimulant abuse disorders.

An optimized fluorescent probe for visualizing glutamate neurotransmission

We describe an intensity-based glutamate-sensing fluorescent reporter (“iGluSnFR”) with signal-to-noise ratio and kinetics appropriate for in vivo imaging. We engineered iGluSnFR in vitro to maximize its fluorescence change, and validated its utility for visualizing glutamate release by neurons and astrocytes in increasingly intact neurological systems. In hippocampal culture, iGluSnFR detected single field stimulus-evoked glutamate release events. In pyramidal neurons in acute brain slices, glutamate uncaging at single spines showed that iGluSnFR responds robustly and specifically to glutamate in situ, and responses correlate with voltage changes. In mouse retina, iGluSnFR-expressing neurons showed intact light-evoked excitatory currents, and the sensor revealed tonic glutamate signaling in response to light stimuli. In worms, glutamate signals preceded and predicted post-synaptic calcium transients. In zebrafish, iGluSnFR revealed spatial organization of direction-selective synaptic activity in the optic tectum. Finally, in mouse forelimb motor cortex, iGluSnFR expression in layer V pyramidal neurons revealed task-dependent single-spine activity during running.

Rapid surface accumulation of NMDA receptors increases glutamatergic excitation during status epilepticus

After 1h of lithium-pilocarpine status epilepticus (SE), immunocytochemical labeling of NMDA receptor NR1 subunits reveals relocation of subunits from the interior to the cell surface of dentate gyrus granule cells and CA3 pyramidal cells. Simultaneously, an increase in NMDA-miniature excitatory postsynaptic currents (mEPSC) as well as an increase in NMDA receptor-mediated tonic currents is observed in hippocampal slices after SE. Mean-variance analysis of NMDA-mEPSCs estimates that the number of functional postsynaptic NMDA receptors per synapse increases 38% during SE, and antagonism by ifenprodil suggests that an increase in the surface representation of NR2B-containing NMDA receptors is responsible for the augmentation of both the phasic and tonic excitatory currents with SE. These results provide a potential mechanism for an enhancement of glutamatergic excitation that maintains SE and may contribute to excitotoxic injury during SE. Therapies that directly antagonize NMDA receptors may be a useful therapeutic strategy during refractory SE.

Stable learning of functional maps in self-organizing spiking neural networks with continuous synaptic plasticity

This study describes a spiking model that self-organizes for stable formation and maintenance of orientation and ocular dominance maps in the visual cortex (V1). This self-organization process simulates three development phases: an early experience-independent phase, a late experience-independent phase and a subsequent refinement phase during which experience acts to shape the map properties. The ocular dominance maps that emerge accommodate the two sets of monocular inputs that arise from the lateral geniculate nucleus (LGN) to layer 4 of V1. The orientation selectivity maps that emerge feature well-developed iso-orientation domains and fractures. During the last two phases of development the orientation preferences at some locations appear to rotate continuously through ±180° along circular paths and referred to as pinwheel-like patterns but without any corresponding point discontinuities in the orientation gradient maps. The formation of these functional maps is driven by balanced excitatory and inhibitory currents that are established via synaptic plasticity based on spike timing for both excitatory and inhibitory synapses. The stability and maintenance of the formed maps with continuous synaptic plasticity is enabled by homeostasis caused by inhibitory plasticity. However, a prolonged exposure to repeated stimuli does alter the formed maps over time due to plasticity. The results from this study suggest that continuous synaptic plasticity in both excitatory neurons and interneurons could play a critical role in the formation, stability, and maintenance of functional maps in the cortex.

Muscarinic acetylcholine receptor activation prevents disinhibition-mediated LTP in the hippocampus

Disinhibition-mediated long-term potentiation (LTP) in the CA1 region of the hippocampus involves GABAergic synaptic plasticity at feedforward inhibitory inputs, resulting in the reduced shunting of glutamatergic excitatory currents. The GABAergic plasticity which underlies disinhibition-mediated LTP results from a Ca2+-dependent decrease in the activity of the K+–Cl− cotransporter (KCC2), depolarizing the reversal potential for GABAA receptor-mediated currents (EGABA), thereby attenuating inhibition. Muscarinic acetylcholine receptor (mAChR) activation has previously been shown to regulate classic glutamatergic LTP, modulate intracellular [Ca2+] and signaling, and facilitate the excitability of GABAergic interneurons in the CA1. Based on these effects, and the ability of mAChR activation to regulate CA1 pyramidal neuron KCC2 expression, we proposed that mAChR activation would modulate disinhibition-mediated LTP. To test this prediction, we made whole cell recordings from CA1 pyramidal neurons in hippocampal slices. Disinhibition-mediated LTP was induced using a spike timing-dependent plasticity (STDP) protocol, which involved coincident pre-synaptic stimulation and post-synaptic current injection (at 5 Hz for 60 s). We found that mAChR activation via carbachol (CCh) prevented the induction of disinhibition-mediated LTP. Moreover, in the presence of CCh, EGABA failed to depolarize following plasticity induction. Lastly, we recorded the paired-pulse ratio (PPR) during the induction of disinhibition-mediated LTP and found that in the presence of CCh, plasticity induction induced a significant paired-pulse depression. This suggests that pre-synaptic mAChR activation may prevent the post-synaptic expression of disinhibition-mediated LTP.

Chronic Intermittent Hypoxia Depresses Afferent Neurotransmission in NTS Neurons by a Reduction in the Number of Active Synapses

Long-term synaptic plasticity has been recently described in brainstem areas associated to visceral afferent sensory integration. Chronic intermittent hypoxia (CIH), an animal model for studying obstructive sleep apnea in humans, depresses the afferent neurotransmission in nucleus tractus solitarii (NTS) neurons, which affect respiratory and autonomic regulation. Here we identified the synaptic mechanisms of CIH-induced depression of the afferent neurotransmission in NTS neurons in juvenile rats. We verified that CIH reduced the amplitude of both NMDA and non-NMDA glutamatergic excitatory currents (eEPSCs) evoked by tractus solitarii stimulation (TS-eEPSC) of second-order neurons in the NTS. No changes were observed in release probability, evidenced by absence of any CIH-elicited effects on short-term depression and failures in EPSCs evoked in low calcium. CIH also produced no changes in TS-eEPSC quantal size, since the amplitudes of both low calcium-evoked EPSCs and asynchronous TS-eEPSCs (evoked in the presence of Sr(2+)) were unchanged. Using single TS afferent fiber stimulation in slices from control and CIH rats we clearly show that CIH reduced the quantal content of the TS-eEPSCs without affecting the quantal size or release probability, suggesting a reduction in the number of active synapses as the mechanism of CIH induced TS-eEPSC depression. In accordance with this concept, the input-output relationship of stimulus intensity and TS-eEPSC amplitude shows an early saturation in CIH animals. These findings open new perspectives for a better understanding of the mechanisms underlying the synaptic plasticity in the brainstem sensory neurons under challenges such as those produced by CIH in experimental and pathological conditions.

Differential modulation of retinal ganglion cell light responses by orthosteric and allosteric metabotropic glutamate receptor 8 compounds

To investigate the role of mGluR8 in modulating the synaptic responses of retinal ganglion cells, we used a recently identified positive allosteric modulator of mGluR8, AZ12216052 (AZ) and the mGluR8-specific orthosteric agonist (S)-3,4-dicarboxyphenylglycine (DCPG). These agents were applied to whole-cell voltage-clamped ganglion cells from an isolated, superfused mouse retina preparation. DCPG reduced OFF-ganglion cell excitatory currents, whereas AZ enhanced the peak excitatory currents in ON-, OFF-, and ON–OFF-ganglion cells. The effects on ganglion cell inhibitory currents were more varied. The effects of the allosteric modulator were stronger for bright stimuli than for dim stimuli, consistent with receptor stimulation by endogenous glutamate being stronger during bright light stimulation and with mGluR8 receptors mainly being localized away from glutamate release sites, immuno-labeled with VGLUT1. The differential sensitivity of ganglion cell light responses to DCPG and AZ supports multiple sites where mGluR8 modulates the light responses of ganglion cells.

Visual Stimulation Reverses the Directional Preference of Direction-Selective Retinal Ganglion Cells

Direction selectivity in the retina is mediated by direction-selective ganglion cells. These cells are part of a circuit in which they are asymmetrically wired to inhibitory neurons. Thus, they respond strongly to an image moving in the preferred direction and weakly to an image moving in the opposite (null) direction. Here, we demonstrate that adaptation with short visual stimulation of a direction-selective ganglion cell using drifting gratings can reverse this cell's directional preference by 180°. This reversal is robust, long lasting, and independent of the animal's age. Our findings indicate that, even within circuits that are hardwired, the computation of direction can be altered by dynamic circuit mechanisms that are guided by visual stimulation.

Abstract 412: Electrophysiological Properties of Kidney-related Neurons in the Rostral Ventrolateral Medulla

Presympathetic neurons in the rostral ventrolateral medulla (RVLM) play an important integrative role in the neuronal network mediating cardiovascular regulation. In this study, we employed a retrograde, transsynaptic pseudorabies viral label (PRV-152), a PRV construct which expresses enhanced green fluorescent protein (EGFP), to identify kidney-related neurons in the RVLM. Whole-cell, patch-clamp recordings were made from 86 kidney-related neurons in the RVLM in transverse brainstem slices from 4-7-week-old rats. Under control conditions, we identified two populations of PRV-labeled RVLM neurons based on their electrophysiological properties. The resting membrane potential of large kidney-related neurons was -48 ± 2 mV and the input resistance was 205 ± 28 MΩ. The resting membrane potential of small kidney-related neurons was -51 ± 2 mV and the input resistance was 430 ± 36 MΩ. Spontaneous firing was observed in 2 out of 15 large cells with frequency of firing 0.008 and 0.05 Hz and 7 out of 16 small cells with average of firing 0.4 ± 0.2 Hz. Both spontaneous and miniature inhibitory and excitatory postsynaptic currents (i.e., sEPSCs, sIPSCs and mEPSCs, mIPSCs) were observed in small and large neurons. Tonic inhibitory and excitatory currents were identified in large kidney-related neurons of the RVLM. Application of the type A γ-aminobutyric acid (GABA A ) receptor-linked Cl - channel blocker, bicuculline methiodide (30 μM), blocked sIPSCs and revealed a robust tonic inhibitory current with an average amplitude of 145.3 ± 30 pA. The cocktail of potent N-methyl-D-aspartate (NMDA) and α-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid receptor (AMPA)/kainate ionotropic glutamate receptor antagonist, AP-5 (50 μM) and CNQX (10 μM), blocked sEPSCs and revealed a two-component tonic excitatory current mediated by NMDA and AMPA/kainate ionotropic glutamate receptors with overall amplitude of 65.2 ± 11.3 pA. These data demonstrate the synaptic complexity involved in the regulation of kidney-related RVLM neurons.

Cross inhibition from ON to OFF pathway improves the efficiency of contrast encoding in the mammalian retina

The retina is divided into parallel and mostly independent ON and OFF pathways, but the ON pathway "cross" inhibits the OFF pathway. Cross inhibition was thought to improve signal processing by the OFF pathway, but its effect on contrast encoding had not been tested experimentally. To quantify the effect of cross inhibition on the encoding of contrast, we presented a dark flash to an in vitro preparation of the mammalian retina. We then recorded excitatory currents, inhibitory currents, membrane voltages, and spikes from OFF α-ganglion cells. The recordings were subjected to an ideal observer analysis that used Bayesian methods to determine how accurately the recordings detected the dark flash. We found that cross inhibition increases the detection accuracy of currents and membrane voltages. Yet these improvements in encoding do not fully reach the spike train, because cross inhibition also hyperpolarizes the OFF α-cell below spike threshold, preventing small signals in the membrane voltages at low contrast from reaching the spike train. The ultimate effect of cross inhibition is to increase the accuracy with which the spike train detects moderate contrast, but reduce the accuracy with which it detects low contrast. In apparent compensation for the loss of accuracy at low contrast, cross inhibition, by hyperpolarizing the OFF α-cell, reduces the number of spikes required to detect the dark flash and thereby increases encoding efficiency.

Drosophila neuroligin 1 regulates synaptic growth and function in response to activity and phosphoinositide-3-kinase

Neuroligins are postsynaptic neural cell adhesion molecules that mediate synaptic maturation and function in vertebrates and invertebrates, but their mechanisms of action and regulation are not well understood. At the Drosophila larval neuromuscular junction (NMJ), previous analysis demonstrated a requirement for Drosophila neuroligin 1 (dnlg1) in synaptic growth and maturation. The goal of the present study was to better understand the effects and mechanisms of loss-of-function and overexpression of dnlg1 on synapse size and function, and to identify signaling pathways that control dnlg1 expression. Consistent with reduced synapse size, evoked excitatory junctional currents (EJCs) were diminished in dnlg1 mutants but displayed normal Ca(2+) sensitivity and short-term plasticity. However, postsynaptic function was also perturbed, in that glutamate receptor staining and the distribution of amplitudes of miniature excitatory junctional currents (mEJCs) were abnormal in mutants. All the above phenotypes were rescued by a genomic transgene. Overexpression of dnlg1 in muscle resulted in synaptic overgrowth, but reduced the amplitudes of EJCs and mEJCs. Overgrowth and reduced EJC amplitude required Drosophila neurexin 1 (dnrx1) function, suggesting that increased DNlg1/DNrx1 signaling attenuates synaptic transmission and regulates growth through a retrograde mechanism. In contrast, reduced mEJC amplitude was independent of dnrx1. Synaptic overgrowth, triggered by neuronal hyperactivity, absence of the E3 ubiquitin ligase highwire, and increased phosphoinositide-3-kinase (PI3K) signaling in motor neurons reduced synaptic DNlg1 levels. Likewise, postsynaptic attenuation of PI3K, which increases synaptic strength, was associated with reduced DNlg1 levels. These observations suggest that activity and PI3K signaling pathways modulate growth and synaptic transmission through dnlg1-dependent mechanisms.

Inositol hexakisphosphate suppresses excitatory neurotransmission via synaptotagmin-1 C2B domain in the hippocampal neuron

Inositol hexakisphosphate (InsP(6)) levels rise and fall with neuronal excitation and silence, respectively, in the hippocampus, suggesting potential signaling functions of this inositol polyphosphate in hippocampal neurons. We now demonstrate that intracellular application of InsP(6) caused a concentration-dependent inhibition of autaptic excitatory postsynaptic currents (EPSCs) in cultured hippocampal neurons. The treatment did not alter the size and replenishment rate of the readily releasable pool in autaptic neurons. Intracellular exposure to InsP(6) did not affect spontaneous EPSCs or excitatory amino acid-activated currents in neurons lacking autapses. The InsP(6)-induced inhibition of autaptic EPSCs was effectively abolished by coapplication of an antibody to synaptotagmin-1 C2B domain. Importantly, preabsorption of the antibody with a GST-WT synaptotagmin-1 C2B domain fragment but not with a GST-mutant synaptotagmin-1 C2B domain fragment that poorly reacted with the antibody impaired the activity of the antibody on the InsP(6)-induced inhibition of autaptic EPSCs. Furthermore, K(+) depolarization significantly elevated endogenous levels of InsP(6) and occluded the inhibition of autaptic EPSCs by exogenous InsP(6). These data reveal that InsP(6) suppresses excitatory neurotransmission via inhibition of the presynaptic synaptotagmin-1 C2B domain-mediated fusion via an interaction with the synaptotagmin Ca(2+)-binding sites rather than via interference with presynaptic Ca(2+) levels, synaptic vesicle trafficking, or inactivation of postsynaptic ionotropic glutamate receptors. Therefore, elevated InsP(6) in activated neurons serves as a unique negative feedback signal to control hippocampal excitatory neurotransmission.

Postsynaptic mGluR mediated excitation of neurons in midbrain periaqueductal grey

Metabotropic glutamate (mGlu) receptors modulate pain from within the midbrain periaqueductal grey (PAG). In the present study, the postsynaptic mGlu receptor mediated effects on rat PAG neurons were examined using whole-cell patch-clamp recordings in brain slices. The selective group I agonist DHPG (10 μM) produced an inward current in all PAG neurons tested which was associated with a near parallel shift in the current–voltage relationship. By contrast, the group II and III mGlu receptor agonists DCG-IV (1 μM) and l-AP4 (3 μM) produced an outward current in only 10–20% of PAG neurons tested. The DHPG induced current was concentration dependent (EC50 = 1.4 μM), was reduced by the mGlu1 antagonist CPCCOEt (100 μM), and was further reduced by CPCCOEt in combination with the mGlu5 antagonist MPEP (10 μM). The glutamate transport blocker TBOA (30 μM) also produced an inward current, however, this was largely abolished by CNQX (10 μM) plus AP5 (25 μM). Slow EPSCs were evoked following train, but not single shock stimulation, which were enhanced by TBOA (30 μM). The TBOA enhancement of slow EPSCs was abolished by MPEP plus CPCCOEt. These findings indicate that endogenously released glutamate, under conditions in which neurotransmitter spill-over is enhanced, activates group I mGlu receptors to produce excitatory currents within PAG. Thus, postsynaptic group I mGlu receptors have the potential to directly modulate the analgesic, behavioural and autonomic functions of the PAG.This article is part of a Special Issue entitled ‘Metabotropic Glutamate Receptors’.

Features of chaotic activity in a balanced network of Type II neuronal oscillators

The taxonomy of collective states in models of neuronal networks must grow in tandem with relevant measures of their function to progress our understanding. One network state has emerged as an explanation for why many real neuronal networks display irregular, asynchronous network activity, despite being composed of reliable units that each process many inputs. This robust ‘balanced state’ attractor arises when the sum of the mean excitatory and mean inhibitory currents to each neuron is on average balanced and subthreshold, leaving fluctuations to drive spiking[1]. Chaos in a network’s dynamics, or lack thereof, is a useful functional measure because it can strongly constrain how the network will process its inputs[2]. Rates of information loss due to chaotic dynamics are connected to the stability metric known as the Lyapunov spectrum. Methods for its calculation have only recently appeared[3], where the application furthered our understanding of chaos in balanced networks. This and previous work have focused on cortical network models of neurons with integrator (Type I) excitability. Here, we extend this research to neurons with resonator (Type II) excitability. These include the mitral cells of the olfactory bulb, which exhibit intrinsic subthreshold membrane potential oscillations whose frequency matches that of spiking[4]. The mitral cell network displays irregular, yet precisely-timed activity. It also receives excitatory sensory activity from the olfactory mucosa and potentially balancing inhibitory recurrent input. We have performed the first Lyapunov analyses of a balanced resonator network, whose stability properties we seek to understand, and which we believe will help explain the role of olfactory bulb collective oscillations in behavioural discriminations[5]. We study a generic 2-dimensional linear threshold neuron model, which produces leaky integrator or resonator [6] dynamics, depending on the parameters. The terms: a leakage term, a scalable coupling term, an input term from recurrent input, X , distributed across V and W according to δ, γ∈ [0,1], and finally external driving term, which determines to where the state is attracted. V is a somatic voltage variable whose membrane time constant sets the units of time. W is an auxiliary current variable that takes on different biologically meaningful roles depending on the parameter values. We consider δ-pulse coupling with coupling strength matrix, Jij. For evolution between spikes, we have obtained the analytic solution to this model and implemented an efficient and precise root finding algorithm to obtain the next threshold-crossing time. Using this, we can perform numerically exact, event-based network simulations, iterating from one spike in the network to the next. We also analytically calculated the single spike Jacobian of this iterative map that describes how perturbations evolve between spikes and which we use to compute the full Lyapunov spectrum. We first have reproduced results from a separate implementation of the correlated leaky integrate-and-fire neuron. Then, by progressively increasing the frequency of subthreshold oscillations from 0, we will present our results on how the Lyapunov spectrum of the network changes across the transition from Type I to Type II neuron dynamics.

Motifs in health and disease: the promise of circuit interrogation by optogenetics

Identifying the dominant dynamical motifs in cortical circuits and determining their functional relevance is of the utmost importance to understand the underlying mechanisms of psychiatric diseases and to develop effective therapies. Optogenetics can be used to interrogate cortical circuits to determine the dominant motif and thereby identify the relevant biophysical time scales that set the oscillation frequency. We review how computational models of cortical networks can help guide optogenetics experiments. We focus our attention on the pyramidal interneuron gamma motif, which is comprised of reciprocally connected excitatory and inhibitory neurons, and determine how the different biophysical time scales of the circuit components are reflected in the resonance of the power in the local field potential at the frequency of stimulation as a function of that frequency. Cardin et al. [J.A. Cardin et al. (2009)Nature, 459, 663-667] find that periodic stimulation of inhibitory cells leads to a resonance at gamma frequencies (30-80 Hz), but that stimulation of excitatory cells does not lead to a resonance. We can account for these results when the pyramidal cells are endowed with an intrinsic frequency preference due to a slow hyperpolarizing current. Furthermore, when fast α-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid (AMPA)-mediated excitatory currents are replaced by slow N-methyl-d-aspartate (NMDA)-mediated ones in inhibitory cells, the gamma frequency resonance is reduced; however, when the same replacement is made in excitatory cells, gamma oscillations are enhanced. The results are relevant to schizophrenia, because there is evidence that NMDA receptors on parvalbumin-positive cells are primarily affected and that the regulation of gamma oscillations is impaired.