Dopamine Modulation of Avoidance Behavior in Caenorhabditis elegans Requires the NMDA Receptor NMR-1

The nematode C. elegans utilizes a relatively simple neural circuit to mediate avoidance responses to noxious stimuli such as the volatile odorant octanol. This avoidance behavior is modulated by dopamine. cat-2 mutant animals that are deficient in dopamine biosynthesis have an increased response latency to octanol compared to wild type animals, and this defect can be fully restored with the application of exogenous dopamine. Because this avoidance behavior is mediated by glutamatergic signaling between sensory neurons and premotor interneurons, we investigated the genetic interactions between dopaminergic signaling and ionotropic glutamate receptors. cat-2 mutant animals lacking either the GLR-1 or GLR-2 AMPA/kainate receptors displayed an increased response latency to octanol, which could be restored via exogenous dopamine. However, whereas cat-2 mutant animals lacking the NMR-1 NMDA receptor had increased response latency to octanol they were insensitive to exogenous dopamine. Mutants that lacked both AMPA/kainate and NMDA receptors were also insensitive to exogenous dopamine. Our results indicate that dopamine modulation of octanol avoidance requires NMR-1, consistent with NMR-1 as a potential downstream signaling target for dopamine.     

Role of local nonspiking interneurons in the generation of rhythmic motor activity in the stick insect

Local nonspiking interneurons in the thoracic ganglia of insects are important premotor elements in posture control and locomotion. It was investigated whether these interneurons are involved in the central neuronal circuits generating the oscillatory motor output of the leg muscle system during rhythmic motor activity. Intracellular recordings from premotor nonspiking interneurons were made in the isolated and completely deafferented mesothoracic ganglion of the stick insect in preparations exhibiting rhythmic motor activity induced by the muscarinic agonist pilocarpine. All interneurons investigated provided synaptic drive to one or more motoneuron pools supplying the three proximal leg joints, that is, the thoraco-coxal joint, the coxa-trochanteral joint and the femur-tibia joint. During rhythmicity in 83% (n=67) of the recorded interneurons, three different kinds of synaptic oscillations in membrane potential were observed: (1) Oscillations were closely correlated with the activity of motoneuron pools affected; (2) membrane potential oscillations reflected only certain aspects of motoneuronal rhythmicity; and (3) membrane potential oscillations were correlated mainly with the occurrence of spontaneous recurrent patterns (SRP) of activity in the motoneuron pools. In individual interneurons membrane potential oscillations were associated with phase-dependent changes in the neuron's membrane conductance. Artificial changes in the interneurons' membrane potential strongly influenced motor activity. Injecting current pulses into individual interneurons caused a reset of rhythmicity in motoneurons. Furthermore, current injection into interneurons influenced shape and probability of occurrence for SRPs. Among others, identified nonspiking interneurons that are involved in posture control of leg joints were found to exhibit the above properties. From these results, the following conclusions on the role of nonspiking interneurons in the generation of rhythmic motor activity, and thus potentially also during locomotion, emerge: (1) During rhythmic motor activity most nonspiking interneurons receive strong synaptic drive from central rhythm-generating networks; and (2) individual nonspiking interneurons some of which underlie sensory-motor pathways in posture control, are elements of central neuronal networks that generate alternating activity in antagonistic leg motoneuron pools. © 1995 John Wiley & Sons, Inc.     

Simulation of Gamma Rhythms in Networks of Interneurons and Pyramidal Cells

Networks of hippocampal interneurons, with pyramidal neuronspharmacologically disconnected, can generate gamma-frequency(20 Hz and above) oscillations. Experiments and models have shownhow the network frequency depends on excitation of the interneurons,and on the parameters of GABA_{{\rm A}}-mediated IPSCs betweenthe interneurons (conductance and time course). Herewe use network simulations to investigate how pyramidal cells, connected tothe interneurons and to each other throughAMPA-type and/or NMDA-type glutamatereceptors, might modify the interneuron network oscillation. With orwithout AMPA-receptor mediated excitation of the interneurons, the pyramidal cells and interneurons fired in phaseduring the gamma oscillation. Synaptic excitation of the interneuronsby pyramidal cellscaused them to fire spike doublets or short bursts at gammafrequencies, thereby slowing the population rhythm.Rhythmic synchronized IPSPs allowed the pyramidal cells toencode their mean excitation by their phase of firing relativeto the population waves.Recurrent excitation between the pyramidal cells couldmodify the phase of firing relative to the population waves.Our model suggests that pools of synaptically interconnectedinhibitory cells are sufficient to produce gamma frequency rhythms,but the network behavior can be modified by participation ofpyramidal cells.     

Alleviation by GABA<Subscript>B</Subscript> Receptors of Neurotoxicity Mediated by Mitochondrial Permeability Transition Pore in Cultured Murine Cortical Neurons Exposed to <Emphasis Type="Italic">N</Emphasis>-Methyl-<Emphasis Type="SmallCaps">d</Emphasis>-aspartate

Mitochondrial permeability transition pore (PTP) is supposed to at least in part participate in molecular mechanisms underlying the neurotoxicity seen after overactivation of N-methyl-d-aspartate (NMDA) receptor (NMDAR) in neurons. In this study, we have evaluated whether activation of GABA_B receptor (GABA_BR), which is linked to membrane G protein-coupled inwardly-rectifying K⁺ ion channels (GIRKs), leads to protection of the NMDA-induced neurotoxicity in a manner relevant to mitochondrial membrane depolarization in cultured embryonic mouse cortical neurons. The cationic fluorescent dye 3,3′-dipropylthiacarbocyanine was used for determination of mitochondrial membrane potential. The PTP opener salicylic acid induced a fluorescence increase with a vitality decrease in a manner sensitive to the PTP inhibitor ciclosporin, while ciclosporin alone was effective in significantly preventing both fluorescence increase and viability decrease by NMDA as seen with an NMDAR antagonist. The NMDA-induced fluorescence increase and viability decrease were similarly prevented by pretreatment with the GABA_BR agonist baclofen, but not by the GABA_AR agonist muscimol, in a fashion sensitive to a GABA_BR antagonist. Moreover, the GIRK inhibitor tertiapin canceled the inhibition by baclofen of the NMDA-induced fluorescence increase. These results suggest that GABA_BR rather than GABA_AR is protective against the NMDA-induced neurotoxicity mediated by mitochondrial PTP through a mechanism relevant to opening of membrane GIRKs in neurons.     

Synaptic NMDA Receptor-Dependent Ca2+ Entry Drives Membrane Potential and Ca2+ Oscillations in Spinal Ventral Horn Neurons

During vertebrate locomotion, spinal neurons act as oscillators when initiated by glutamate release from descending systems. Activation of NMDA receptors initiates Ca²⁺-mediated intrinsic membrane potential oscillations in central pattern generator (CPG) neurons. NMDA receptor-dependent intrinsic oscillations require Ca²⁺-dependent K⁺ (K_Ca2) channels for burst termination. However, the location of Ca²⁺ entry mediating K_Ca2 channel activation, and type of Ca²⁺ channel – which includes NMDA receptors and voltage-gated Ca²⁺ channels (VGCCs) – remains elusive. NMDA receptor-dependent Ca²⁺ entry necessitates presynaptic release of glutamate, implying a location at active synapses within dendrites, whereas VGCC-dependent Ca²⁺ entry is not similarly constrained. Where Ca²⁺ enters relative to K_Ca2 channels is crucial to information processing of synaptic inputs necessary to coordinate locomotion. We demonstrate that Ca²⁺ permeating NMDA receptors is the dominant source of Ca²⁺ during NMDA-dependent oscillations in lamprey spinal neurons. This Ca²⁺ entry is synaptically located, NMDA receptor-dependent, and sufficient to activate K_Ca2 channels at excitatory interneuron synapses onto other CPG neurons. Selective blockade of VGCCs reduces whole-cell Ca²⁺ entry but leaves membrane potential and Ca²⁺ oscillations unaffected. Furthermore, repetitive oscillations are prevented by fast, but not slow, Ca²⁺ chelation. Taken together, these results demonstrate that K_Ca2 channels are closely located to NMDA receptor-dependent Ca²⁺ entry. The close spatial relationship between NMDA receptors and K_Ca2 channels provides an intrinsic mechanism whereby synaptic excitation both excites and subsequently inhibits ventral horn neurons of the spinal motor system. This places the components necessary for oscillation generation, and hence locomotion, at glutamatergic synapses.     

NMDA modulation of dopamine dynamics is diminished in the aged striatum: an in vivo voltametric study

The technique of in vivo voltametry and a paired recording paradigm were employed to study the age-related changes in N-methyl-d-aspartate (NMDA) function in regulating the striatal dopaminergic transmission in male Sprague-Dawley rats. Microinjection of NMDA (100pmol) consistently elicited larger striatal dopamine (DA) overflows from young rats (3-4 months old) than from aged rats (27-28 months old). Furthermore, the rate of clearance (T(c)) of the NMDA-evoked dopamine release was lower in the aged rats. Local application of dopamine evoked reversible electrochemical signals with similar amplitudes in both young and aged rats. However, T(c) was reduced and time course parameters were prolonged in the aged rats. While microejection of NMDA (1pmol) did not induce any dopamine overflow, simultaneous administration of NMDA and K(+) evoked larger dopamine releases than K(+) alone in the young striatum. Concomitant application of NMDA did not potentiate the K(+)-evoked dopamine release in the aged striatum. Taken together, with the reduced dopamine release in response to depolarizing stimuli, our in vivo electrochemical data suggest that age-related changes in NMDA function contribute to the impaired dopaminergic dynamics, including an attenuation of NMDA-evoked dopamine release and a diminished augmentation by K(+) of NMDA-induced dopamine release during the normal aging process.     

Requirement for the homeobox gene Hb9 in the consolidation of motor neuron identity.

The homeobox gene Hb9, like its close relative MNR2, is expressed selectively by motor neurons (MNs) in the developing vertebrate CNS. In embryonic chick spinal cord, the ectopic expression of MNR2 or Hb9 is sufficient to trigger MN differentiation and to repress the differentiation of an adjacent population of V2 interneurons. Here, we provide genetic evidence that Hb9 has an essential role in MN differentiation. In mice lacking Hb9 function, MNs are generated on schedule and in normal numbers but transiently acquire molecular features of V2 interneurons. The aberrant specification of MN identity is associated with defects in the migration of MNs, the emergence of the subtype identities of MNs, and the projection of motor axons. These findings show that HB9 has an essential function in consolidating the identity of postmitotic MNs.     

Biogenic amines modulate synaptic transmission between identified giant interneurons and thoracic interneurons in the escape system of the cockroach

In the escape system of the cockroach, Periplaneta americana, a population of uniquely identifiable throacic interneurons (type A or TI_As) receive information about wind via chemical synapses from a population of ventral giant interneurons (vGIs). The TI_As are involved in the integration of sensory information necessary for orienting the animal during escape. It is likely that there are times in an animal's life when it is advantageous to modify the effectiveness of synaptic transmission between the vGIs and the TI_As. Given the central position of the TI_As inthe escape system, this would greatly alter associated motor outputs. We tested the ability of octopamine, serotonin, and dopamine to modulate synaptic transmission between vGIs and TI_As. Both octopamine and dopamine significantly increased the amplitude of vGI-evoked excitatory postsynaptic potentials (EPSPs) in TI_As at 10^?4?10^?2 M, and 10^?3 M, respectively. On the other hand, serotonin significantly decreased the vGI-evoked EPSPs in TI_As at 10^?4?10^?3 M. These results indicate that octopamine, serotonin, and dopamine are capable of modulating the efficacy of transmission of important neural connections within this circuit. © 1992 John Wiley & Sons, Inc.     

Differential regulation of the excitability of prefrontal cortical fast-spiking interneurons and pyramidal neurons by serotonin and fluoxetine

Serotonin exerts a powerful influence on neuronal excitability. In this study, we investigated the effects of serotonin on different neuronal populations in prefrontal cortex (PFC), a major area controlling emotion and cognition. Using whole-cell recordings in PFC slices, we found that bath application of 5-HT dose-dependently increased the firing of FS (fast spiking) interneurons, and decreased the firing of pyramidal neurons. The enhancing effect of 5-HT in FS interneurons was mediated by 5-HT₂ receptors, while the reducing effect of 5-HT in pyramidal neurons was mediated by 5-HT₁ receptors. Fluoxetine, the selective serotonin reuptake inhibitor, also induced a concentration-dependent increase in the excitability of FS interneurons, but had little effect on pyramidal neurons. In rats with chronic fluoxetine treatment, the excitability of FS interneurons was significantly increased, while pyramidal neurons remained unchanged. Fluoxetine injection largely occluded the enhancing effect of 5-HT in FS interneurons, but did not alter the reducing effect of 5-HT in pyramidal neurons. These data suggest that the excitability of PFC interneurons and pyramidal neurons is regulated by exogenous 5-HT in an opposing manner, and FS interneurons are the major target of Fluoxetine. It provides a framework for understanding the action of 5-HT and antidepressants in altering PFC network activity.     

Gap-junction blocker carbenoxolone differentially enhances NMDA-induced cell death in hippocampal neurons and astrocytes in co-culture

The beneficial or detrimental role of gap junction communication in the pathophysiology of brain injury is still controversial. We used co-cultures of hippocampal astrocytes and neurons, where we identified homocellular astrocyte-astrocyte and heterocellular astrocyte-neuron coupling by fluorescence recovery after photobleaching, which was decreased by the gap junction blocker carbenoxolone (CBX). In these cultures, we determined the cell type-specific effects of CBX on the excitotoxic damage caused by N-methyl-D-aspartate (NMDA). We determined in both astrocytes and neurons the influence of CBX, alone or together with NMDA challenge, on cytotoxicity using propidium iodide labeling. CBX alone was not cytotoxic, but CBX treatment differentially accelerated the NMDA-induced cell death in both astrocytes and neurons. In addition, we measured mitochondrial potential using rhodamine 123, membrane potential using the oxonol dye bis(1,3-diethylthiobarbituric acid)trimethine oxonol, cytosolic Ca(2+) level using fura-2, and formation of reactive oxygen species (ROS) using dihydroethidium. CBX alone induced neither an intracellular Ca(2+) rise nor a membrane depolarization. However, CBX elicited a mitochondrial depolarization in both astrocytes and neurons and increased the ROS formation in neurons. In contrast, NMDA caused a membrane depolarization in neurons, coinciding with intracellular Ca(2+) rise, but neither mitochondrial depolarization nor ROS production seem to be involved in NMDA-mediated cytotoxicity. Pre-treatment with CBX accelerated the NMDA-induced membrane depolarization and prevented the repolarization of neurons after the NMDA challenge. We hypothesize that these effects are possibly mediated via blockage of gap junctions, and might be involved in the mechanism of CBX-induced acceleration of excitotoxic cell death, whereas the CBX-induced mitochondrial depolarization and ROS formation are not responsible for the increase in cytotoxicity. We conclude that both in astrocytes and neurons gap junctions provide protection against NMDA-induced cytotoxicity.     

Role of Caspases in N-Methyl-d-Aspartate-Induced Apoptosis in Cerebrocortical Neurons

Abstract: Overactivation of glutamate receptors mediates neuronal death in several acute and chronic neurodegenerative diseases. The intracellular processes underlying this form of death, however, remain poorly understood. Depending on the severity of insult, N-methyl-d -aspartate (NMDA) receptor activation induces either apoptosis or necrosis. Cysteine proteases related to interleukin-1β-converting enzyme (ICE), recently termed caspases, appear necessary for neuronal apoptosis in vivo and in vitro. To determine whether caspases play a role in NMDA-induced apoptosis, we used two functionally distinct approaches to decrease substrate cleavage by caspases. One is a novel peptide (V-ICE_inh) that contains the caspase catalytic site and acts as a pseudoenzyme that binds caspase substrates and prevents their cleavage. The other is a pseudosubstrate peptide (Z-VAD·fmk) that inhibits caspase activity. Pretreatment with either V-ICE_inh or Z-VAD·fmk protects cerebrocortical neurons from NMDA-induced apoptosis, suggesting a role for caspases in NMDA-induced apoptosis. To explore the signaling pathways involved, we looked at the effects of NMDA receptor activation on Ca²⁺ influx, production of reactive oxygen species (ROS), mitochondrial membrane potential, and lipid peroxidation. Neither NMDA-induced Ca²⁺ influx nor the initial collapse of mitochondrial membrane potential could be prevented by pretreatment with V-ICE_inh or Z-VAD·fmk. In contrast, ROS formation and lipid peroxidation were completely blocked by both V-ICE_inh and Z-VAD·fmk. Taken together, our results suggest that Ca²⁺ influx and mitochondrial depolarization occur upstream from caspase activation, whereas ROS formation and lipid peroxidation may be downstream events in the cascade leading to cortical neuronal apoptosis.     

Dopamine D4 receptor activation increases hippocampal gamma oscillations by enhancing synchronization of fast-spiking interneurons

Background

Gamma oscillations are electric activity patterns of the mammalian brain hypothesized to serve attention, sensory perception, working memory and memory encoding. They are disrupted or altered in schizophrenic patients with associated cognitive deficits, which persist in spite of treatment with antipsychotics. Because cognitive symptoms are a core feature of schizophrenia it is relevant to explore signaling pathways that potentially regulate gamma oscillations. Dopamine has been reported to decrease gamma oscillation power via D1-like receptors. Based on the expression pattern of D4 receptors (D4R) in hippocampus, and pharmacological effects of D4R ligands in animals, we hypothesize that they are in a position to regulate gamma oscillations as well.

Methodology/Principal Findings

To address this hypothesis we use rat hippocampal slices and kainate-induced gamma oscillations. Local field potential recordings as well as intracellular recordings of pyramidal cells, fast-spiking and non-fast-spiking interneurons were carried out. We show that D4R activation with the selective ligand PD168077 increases gamma oscillation power, which can be blocked by the D4R-specific antagonist L745,870 as well as by the antipsychotic drug Clozapine. Pyramidal cells did not exhibit changes in excitatory or inhibitory synaptic current amplitudes, but inhibitory currents became more coherent with the oscillations after application of PD168077. Fast-spiking, but not non-fast spiking, interneurons, increase their action potential phase-coupling and coherence with regard to ongoing gamma oscillations in response to D4R activation. Among several possible mechanisms we found that the NMDA receptor antagonist AP5 also blocks the D4R mediated increase in gamma oscillation power.

Conclusions/Significance

We conclude that D4R activation affects fast-spiking interneuron synchronization and thereby increases gamma power by an NMDA receptor-dependent mechanism. This suggests that converging deficits on fast-spiking interneurons may lead to decreased network function and thus aberrant gamma oscillations and cognitive decline in schizophrenia.     

NR2A-containing NMDA receptors depress glutamatergic synaptic transmission and evoked-dopamine release in the mouse striatum

NMDA receptors play essential roles in the physiology and pathophysiology of the striatum, a brain nucleus involved in motor control and reward-motivated behaviors. NMDA receptors are composed of NR1 and NR2A–D subunits. Functional properties of NMDA receptors are determined by the type of NR2 subunit they contain. In this study, we have examined the involvement of NR2B and NR2A in the modulatory effect of NMDA on glutamatergic and dopaminergic synaptic transmission in the striatum. We found that bath application of NMDA decreased the amplitude of the field excitatory post-synaptic potential/population spike (fEPSP/PS) measured in corticostriatal mouse brain slices. This depression was not affected by the NR2B-selective antagonists Ifenprodil and Ro 25-6981, but was abolished by the NR2A antagonist NVP-AAM077. Activation of corticostriatal neurons by NMDA did not contribute to synaptic depression because similar results were obtained in decorticated striatal slices. Synaptic depression was not dependent on GABA release because the GABA_A receptor antagonist bicuculline did not affect NMDA-induced decrease of the fEPSP/PS. NMDA also depressed evoked-dopamine release through NR2A- but not NR2B-containing NMDA receptors. Our results identify an important role for NR2A-containing NMDA receptors intrinsic to the striatum in regulating glutamatergic synaptic transmission and evoked-dopamine release.     

The iron component of sodium nitroprusside blocks NMDA-induced glutamate accumulation and intracellular Ca2+ elevation

These studies were designed to compare the effects of nitric oxide (NO) generating compounds with those of several iron containing, compounds which do not generate NO on glutamate receptor function. Stimulation of primary cultures of cerebellar granule cells with N-methyl-D-aspartate (NMDA) or kainate results in the elevation of intracellular calcium ([Ca²⁺]_i) and cGMP and the release of glutamate. The iron containing compounds, sodium nitroprusside (SNP), potassium ferrocyanide (K₄Fe(CN)₆) and potassium ferricyanide (K₃Fe(CN)₆) decrease the NMDA-induced release of glutamate. SNP is the only compound of the above 3 agents which generates NO. A non-iron, NO generating compound, S-nitroso-N-acetylpenicillamin (SNAP), has no effect on the NMDA-induced glutamate release. Potassium ferrocyanide (Fe II), but not potassium ferricyanide (Fe III), blocks NMDA-induced cGMP elevations after 3 min exposure times. This contrasts with the NO generating compounds (both SNP and SNAP) which elevate cGMP levels. Furthermore, both potassium ferrocyanide (Fe II) and SNP (Fe II) suppress the elevation of [Ca²⁺]_i induced by NMDA but neither potassium ferricyanide (Fe III) nor SNAP are effective in this regard. These effects are also independent of cyanide as another Fe II compound, ferrous sulfate (FeSO₄) is also able to suppress NMDA-induced elevations of [Ca²⁺]_i SNP was unable to suppress kainate receptor functions. Collectively, these results indicate that Fe II, independently of NO, has effects on NMDA receptor function.     

Activation of NMDA receptors leads to phosphorylation of TRPV1 S800 by protein kinase C and A-Kinase anchoring protein 150 in rat trigeminal ganglia

A-Kinase anchoring protein 150 (AKAP150) is required for the phosphorylation of transient receptor potential cation channel subfamily V member 1 (TRPV1) by PKA or PKC in sensory neurons and, hence, affects TRPV1-dependent hyperalgesia under pathological conditions. Recently, we showed that the activation of N-methyl-d-aspartate (NMDA) receptors sensitizes TRPV1 by enhancing serine phosphorylation through PKC in trigeminal nociceptors. In this study, we extended this observation by investigating whether AKAP150 mediates NMDA-induced phosphorylation of TRPV1 via PKC in native sensory neurons in the rat. By adopting a phospho-specific antibody combined with a surface biotinylation assay, we first assessed NMDA-induced changes in the phosphorylation level of serine 800 residues (S800) in TRPV1 delimited to cell surface membrane in cultured trigeminal ganglia (TG). The biotinylation assay yielded that the application of NMDA significantly increased the phosphorylation of S800 (p-S800) of TRPV1 at time points correlating with the development of NMDA-induced mechanical hyperalgesia [10]. We then obtained a siRNA sequence against AKAP150 that dose-dependently down-regulated the AKAP150 protein. Pretreatment of TG culture with the siRNA, but not mismatch sequences, prevented the NMDA-induced phosphorylation of serine residues of total TRPV1 as well as S800 of membrane bound TRPV1. We confirmed that AKAP150 co-immunoprecipitated with TRPV1 and demonstrated that it also co-immunoprecipitated with NMDA receptor subunits (NR1 and NR2B) in TG. These data offer novel information that the activation of NMDA-induced TRPV1 sensitization involves p-S800 of TRPV1 in cell surface membrane in native sensory neurons and that AKAP150 is required for NMDA-and PKC-mediated phosphorylation of TRPV1 S800. Therefore, we propose that the NMDA receptor, AKAP150, and TRPV1 forms a signaling complex that underlies the sensitization of trigeminal nociceptors by modulating phosphorylation of specific TRPV1 residues.     

A possible pivotal role of mitochondrial free calcium in neurotoxicity mediated by N-methyl-d-aspartate receptors in cultured rat hippocampal neurons

We have previously shown that mitochondrial membrane potential disruption is involved in mechanisms underlying differential vulnerabilities to the excitotoxicity mediated by N-methyl-d-aspartate (NMDA) receptors between primary cultured neurons prepared from rat cortex and hippocampus. To further elucidate the role of mitochondria in the excitotoxicity after activation of NMDA receptors, neurons were loaded with the fluorescent dye calcein diffusible in the cytoplasm and organelles for determination of the activity of mitochondrial permeability transition pore (mPTP) responsible for the leakage of different mitochondrial molecules. The addition of CoCl₂ similarly quenched the intracellular fluorescence except mitochondria in both cultured neurons, while further addition of NMDA led to a leakage of the dye into the cytoplasm in hippocampal neurons only. An mPTP inhibitor prevented the NMDA-induced loss of viability in hippocampal neurons, while an activator of mPTP induced a similarly potent loss of viability in cortical and hippocampal neurons. Although NMDA was more effective in increasing rhodamine-2 fluorescence as a mitochondrial calcium indicator in hippocampal than cortical neurons, a mitochondrial calcium uniporter inhibitor significantly prevented the NMDA-induced loss of viability in hippocampal neurons. Expression of mRNA was significantly higher for the putative uniporter uncoupling protein-2 in hippocampal than cortical neurons. These results suggest that mitochondrial calcium uniporter would be at least in part responsible for the NMDA neurotoxicity through a mechanism relevant to promotion of mPTP orchestration in hippocampal neurons.     

Halothane Anesthesia Affects NMDA-Stimulated Cholinergic and GABAergic Modulation of Striatal Dopamine Efflux and Metabolism in the Rat In Vivo

Microdialysis of the striatum of halothane-anesthetized rats was used to study the participation of local cholinergic and GABAergic neurotransmission in NMDA receptor-modulated striatal dopamine release and metabolism. Reverse dialysis.of NMDA (1 mM) evoked a 10-fold increase in dopamine efflux and reduced DOPAC and HVA to > 20% of basal values. The effect of NMDA on dopamine efflux was abolished by atropine (10 microM) but unaffected by (+)-bicuculline (50 microM). NMDA-induced decrease in DOPAC (but not HVA) efflux was potentiated by atropine, whereas (+)-bicuculline attenuated the decrease in DOPAC and HVA. Compared to our previous studies in unanesthetised rats, our data suggest that halothane anesthesia alters the balance between NMDA-stimulated cholinergic and GABAergic influences on striatal dopamine release and metabolism. Differential sensitivity to halothane of NMDA receptors expressed by the neurones mediating these modulatory influences, or loss of specific NMDA receptor populations through voltage-dependent Mg2+ block under anesthesia, could underlie these observations.     

Computer simulation of the segmental neural network generating locomotion in lamprey by using populations of network interneurons

Realistic computer simulations of the experimentally established local spinal cord neural network generating swimming in the lamprey have been performed. Populations of network interneurons were used in which cellular properties, like cell size and membrane conductance including voltage dependent ion channels were randomly distributed around experimentally obtained mean values, as were synaptic conductances (kainate/AMPA, NMDA, glycine) and delays. This population model displayed more robust burst activity over a wider frequency range than the more simple subsample model used previously, and the pattern of interneuronal activity was appropriate. The strength of the reciprocal inhibition played a very important role in the regulation of burst frequency, and just by changing the inhibitory bias the entire physiological range could be covered. At the lower frequency range of bursting the segmental excitatory interneurons provide stability as does the activation of voltage dependent NMDA receptors. Spike frequency adaptation by means of summation of afterhyperpolarization (AHP) serves as a major burst terminating factor, and at lower rates the membrane properties conferred by the NMDA receptor activation. The lateral interneurons were not of critical importance for the burst termination. They may, however, be of particular importance for inducing a rapid burst termination during for instance steering and righting reactions. Several cellular factors combine to provide a secure and stable motor pattern in the entire frequency range.     

Involvement of Nitric Oxide in Dopaminergic Transmission in Rat Striatum: An In Vivo Electrochemical Study

Abstract: In vivo electrochemical detection with a Nafion-coated carbon fiber working electrode, which provides information on the spatial and temporal dynamics of dopamine overflow, was used to investigate the involvement of nitric oxide (NO) in the dopaminergic transmission in the striatum of urethane-anesthetized Sprague-Dawley rats. A mixture of N -methyl- d -aspartate (NMDA) and nomifensine, a dopamine uptake blocker, was locally pressure-ejected to elicit a transient dopamine overflow from the dopamine-containing nerve terminals in the striatum. Local application of N ^ω-nitro- l -arginine methyl ester ( l -NAME), which blocks endogenous NO formation, increased the magnitude of dopamine release evoked by a subsequent NMDA and nomifensine application but resulted in no significant alteration in the time course. Furthermore, microejection of l -arginine, an NO precursor, or sodium nitroprusside (SNP), an NO generator, did not cause detectable changes in dopamine level in the striatal extracellular space. However, NMDA-induced dopamine release was profoundly inhibited with l -arginine or SNP pretreatment. In addition, NO affects dopamine uptake in rat striatum. Exogenous dopamine applied through a micropipette, reversibly and reproducibly, elicited an electrochemical signal. The time course of these signals was significantly prolonged by l -NAME treatment. These data suggest that NO is diversely involved in regulating dopaminergic transmission in rat striatum.     

Identified nonspiking interneurons in leg reflexes and during walking in the stick insect

In the stick insect Carausius morosus identified nonspiking interneurons (type E4) were investigated in the mesothoracic ganglion during intraand intersegmental reflexes and during searching and walking.In the standing and in the actively moving animal interneurons of type E4 drive the excitatory extensor tibiae motoneurons, up to four excitatory protractor coxae motoneurons, and the common inhibitor 1 motoneuron (Figs. 1–4).In the standing animal a depolarization of this type of interneuron is induced by tactile stimuli to the tarsi of the ipsilateral front, middle and hind legs (Fig. 5). This response precedes and accompanies the observed activation of the affected middle leg motoneurons. The same is true when compensatory leg placement reflexes are elicited by tactile stimuli given to the tarsi of the legs (Fig. 6).During forward walking the membrane potential of interneurons of type E4 is strongly modulated in the step-cycle (Figs.8–10). The peak depolarization occurs at the transition from stance to swing. The oscillations in membrane potential are correlated with the activity profile of the extensor motoneurons and the common inhibitor 1 (Fig. 9).The described properties of interneuron type E4 in the actively behaving animal show that these interneurons are involved in the organization and coordination of the motor output of the proximal leg joints during reflex movements and during walking.Abbreviations CLP reflex, compensatory leg placement reflex - CI1 common inhibitor I motoneuron - fCO femoral chordotonal organ - FETi fast extensor tibiae motoneuron - FT femur-tibia - SETi slow extensor tibiae motoneuron