cDNA of eight nuclear encoded subunits of NADH:ubiquinone oxidoreductase: human complex I cDNA characterization completed

Brief elevation in postsynaptic calcium in hippocampal CA1 neurons leads to prolonged changes in synaptic strength. The calcium may enter the postsynaptic neuron via different routes, such as voltage-gated calcium channels or glutamate receptor channels of N-methyl-D-aspartate type, and/or be released from intracellular stores. The manner in which the synapse is altered, leading to the expression of an enhanced/depressed synaptic strength, is still unclear. The present study, performed using whole-cell recording from CA1 pyramidal cells of three- to five-week-old guinea-pigs, shows that postsynaptic depolarization alone, allowing for calcium influx through voltage-gated calcium channels, leads to a synaptic potentiation characterized by an altered time-course of the evoked excitatory synaptic response, an unaltered coefficient of variation of that response and a decreased paired-pulse facilitation likely related to a postsynaptic mechanism. These characteristics contrasted with those of long-term potentiation induced via activation of N-methyl-D-aspartate receptor channels, where the time-course was unaltered, the coefficient of variation was decreased and no change in paired-pulse facilitation was observed. Synapses can thus have mechanistically separate, but co-existent, potentiations of synaptic transmission initiated from separate sources for postsynaptic calcium.     

Associative synaptic plasticity in hippocampus and visual cortex: cellular mechanisms and functional implications

Synchronous pre- and postsynaptic neuronal activity results in long-term potentiation (LTP) of excitatory synaptic transmission in the hippocampus and the neocortex. Induction of this form of potentiation requires calcium influx mediated by NMDA receptors. Experimental evidence is reviewed for induction of long-term depression (LTD) of synaptic transmission in the hippocampus in vitro and neocortical neurons in vivo, when the discharge of the postsynaptic neuron is temporally decorrelated from the presynaptic stimulation. Homosynaptic LTD induced by low frequency tetani in the hippocampus in vitro requires NMDA receptor activation and a moderate calcium influx. The role of postsynaptic calcium as a key parameter in the encoding of temporal contiguity of neural activity and its possible implications in the formation of engrams during specific learning tasks are discussed.     

Glutamate mediates an inhibitory postsynaptic potential in dopamine neurons

Rapid information transfer within the brain depends on chemical signalling between neurons that is mediated primarily by glutamate and GABA (gamma-aminobutyric acid), acting at ionotropic receptors to cause excitatory or inhibitory postsynaptic potentials (EPSPs or IPSPs), respectively. In addition, synaptically released glutamate acts on metabotropic receptors to excite neurons on a slower timescale through second-messenger cascades, including phosphoinositide hydrolysisl. We now report a unique IPSP mediated by the activation of metabotropic glutamate receptors. In ventral midbrain dopamine neurons, activation of metabotropic glutamate receptors (mGluR1) mobilized calcium from caffeine/ryanodine-sensitive stores and increased an apamin-sensitive potassium conductance. The underlying potassium conductance and dependence on calcium stores set this IPSP apart from the slow IPSPs described so far. The mGluR-induced hyperpolarization was dependent on brief exposure to agonist, because prolonged application of exogenous agonist desensitized the hyperpolarization and caused the more commonly reported depolarization. The rapid rise and brief duration of synaptically released glutamate in the extracellular space can therefore mediate a rapid excitation through activation of ionotropic receptors, followed by inhibition through the mGluR1 receptor. Thus the idea that glutamate is solely an excitatory neurotransmitter must be replaced with a more complex view of its dual function in synaptic transmission.     

Calcium-permeable AMPA receptors mediate long-term potentiation in interneurons in the amygdala

Fear conditioning is a paradigm that has been used as a model for emotional learning in animals. The cellular correlate of fear conditioning is thought to be associative N-methyl-D-aspartate (NMDA) receptor-dependent synaptic plasticity within the amygdala. Here we show that glutamatergic synaptic transmission to inhibitory interneurons in the basolateral amygdala is mediated solely by alpha-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid (AMPA) receptors. In contrast to AMPA receptors at inputs to pyramidal neurons, these receptors have an inwardly rectifying current-voltage relationship, indicative of a high permeability to calcium. Tetanic stimulation of inputs to interneurons caused an immediate and sustained increase in the efficacy of these synapses. This potentiation required a rise in postsynaptic calcium, but was independent of NMDA receptor activation. The potentiation of excitatory inputs to interneurons was reflected as an increase in the amplitude of the GABA(A)-mediated inhibitory synaptic current in pyramidal neurons. These results demonstrate that excitatory synapses onto interneurons within a fear conditioning circuit show NMDA-receptor independent long-term potentiation. This plasticity might underlie the increased synchronization of activity between neurons in the basolateral amygdala after fear conditioning.     

Glutamate inhibits GABA excitatory activity in developing neurons

In contrast to the mature brain, in which GABA is the major inhibitory neurotransmitter, in the developing brain GABA can be excitatory, leading to depolarization, increased cytoplasmic calcium, and action potentials. We find in developing hypothalamic neurons that glutamate can inhibit the excitatory actions of GABA, as revealed with fura-2 digital imaging and whole-cell recording in cultures and brain slices. Several mechanisms for the inhibitory role of glutamate were identified. Glutamate reduced the amplitude of the cytoplasmic calcium rise evoked by GABA, in part by activation of group II metabotropic glutamate receptors (mGluRs). Presynaptically, activation of the group III mGluRs caused a striking inhibition of GABA release in early stages of synapse formation. Similar inhibitory actions of the group III mGluR agonist L-AP4 on depolarizing GABA activity were found in developing hypothalamic, cortical, and spinal cord neurons in vitro, suggesting this may be a widespread mechanism of inhibition in neurons throughout the developing brain. Antagonists of group III mGluRs increased GABA activity, suggesting an ongoing spontaneous glutamate-mediated inhibition of excitatory GABA actions in developing neurons. Northern blots revealed that many mGluRs were expressed early in brain development, including times of synaptogenesis. Together these data suggest that in developing neurons glutamate can inhibit the excitatory actions of GABA at both presynaptic and postsynaptic sites, and this may be one set of mechanisms whereby the actions of two excitatory transmitters, GABA and glutamate, do not lead to runaway excitation in the developing brain. In addition to its independent excitatory role that has been the subject of much attention, our data suggest that glutamate may also play an inhibitory role in modulating the calcium-elevating actions of GABA that may affect neuronal migration, synapse formation, neurite outgrowth, and growth cone guidance during early brain development.     

GABAergic input to cholinergic nucleus basalis neurons

The potential influence of GABAergic input to cholinergic basalis neurons was studied in guinea-pig basal forebrain slices. GABA and its agonists were applied to electrophysiologically-identified cholinergic neurons, of which some were labelled with biocytin and confirmed to be choline acetyltransferase-immunoreactive. Immunohistochemistry for glutamate decarboxylase was also performed in some slices and revealed GABAergic varicosities in the vicinity of the biocytin-filled soma and dendrites of electrophysiologically-identified cholinergic cells. From rest (average - 63 mV), the cholinergic cells were depolarized by GABA. The depolarization was associated with a decrease in membrane resistance and diminution in firing. The effect was mimicked by muscimol, the specific agonist for GABA(A) receptors, and not by baclofen, the specific agonist for GABA(B) receptors, which had no discernible effect. The GABA- and muscimol-evoked depolarization and decrease in resistance were found to be postsynaptic since they persisted in the presence of solutions containing either high Mg2+/low Ca2+ or tetrodotoxin. They were confirmed as being mediated by a GABA(A) receptor, since they were antagonized by bicuculline. The reversal potential for the muscimol effect was estimated to be approximately -45 mV, which was -15 mV above the resting membrane potential. Finally, in some cholinergic cells, spontaneous subthreshold depolarizing synaptic potentials (average 5 mV in amplitude), which were rarely associated with action potentials, were recorded and found to persist in the presence of glutamate receptor antagonists but to be eliminated by bicuculline. These results suggest that GABAergic input may be depolarizing, yet predominantly inhibitory to cholinergic basalis neurons.     

Developmental influence of glycinergic transmission: regulation of NMDA receptor-mediated EPSPs

The influence of excitatory transmission on postsynaptic structure is well established in developing animals, but little is known about the role of synaptic inhibition. We addressed this issue in developing gerbils with two manipulations designed to decrease glycinergic transmission in an auditory nucleus, the lateral superior olive (LSO), before the onset of sound-evoked activity. First, contralateral cochlear ablation functionally denervated the glycinergic pathway from the medial nucleus of the trapezoid body (MNTB) to the LSO, while leaving the excitatory pathway intact. Second, continuous release of a glycine receptor antagonist, strychnine (SN), was used to decrease transmission. The strength of excitatory and inhibitory synapses was examined with whole-cell recordings from LSO neurons in a brain-slice preparation. The percentage of LSO neurons exhibiting MNTB-evoked IPSPs was reduced in both ablated and SN-treated animals. In those neurons displaying IPSPs, the amplitude was significantly reduced. This decrease was accompanied by an 8 mV depolarization in the IPSP equilibrium potential. In contrast, the ipsilaterally evoked EPSPs were of unusually long duration in experimental animals. These long-duration EPSPs were significantly shortened by hyperpolarizing the neuron to -90 mV or exposing them to aminophosphonopentanoic acid (AP-5), an NMDA receptor antagonist. Membrane hyperpolarization and AP-5 had little effect in control neurons. In addition, LSO neurons from ablated or SN-treated animals displayed broad rebound depolarizations after membrane hyperpolarization, and these were abolished in the presence of Ni2+. Because both cochlear ablation and SN-rearing were initiated before the onset of sound-evoked activity, the results suggest that spontaneous glycinergic transmission influences the development of postsynaptic properties, including the IPSP reversal potential, NMDA receptor function, and a Ca2+ conductance.     

Mechanism and kinetics of heterosynaptic depression at a cerebellar synapse

High levels of activity at a synapse can lead to spillover of neurotransmitter from the synaptic cleft. This extrasynaptic neurotransmitter can diffuse to neighboring synapses and modulate transmission via presynaptic receptors. We studied such modulation at the synapse between granule cells and Purkinje cells in rat cerebellar slices. Brief tetanic stimulation of granule cell parallel fibers activated inhibitory neurons, leading to a transient elevation of extracellular GABA, which in turn caused a short-lived heterosynaptic depression of the parallel fiber to Purkinje cell EPSC. Fluorometric calcium measurements revealed that this synaptic inhibition was associated with a decrease in presynaptic calcium influx. Heterosynaptic inhibition of synaptic currents and calcium influx was eliminated by antagonists of the GABAB receptor. The magnitude and time course of the depression of calcium influx were mimicked by the rapid release of an estimated 10 microM GABA using the technique of flash photolysis. We found that inhibition of presynaptic calcium influx peaked within 300 msec and decayed in <3 sec at 32 degrees C. These results indicate that presynaptic GABAB receptors can sense extrasynaptic GABA increases of several micromolar and that they rapidly regulate the release of neurotransmitter primarily by modulating voltage-gated calcium channels.     

GABAB receptor-mediated inhibition of GABAA receptor calcium elevations in developing hypothalamic neurons

In the CNS, gamma-aminobutyric acid (GABA) affects neuronal activity through both the ligand-gated GABAA receptor channel and the G protein-coupled GABAB receptor. In the mature nervous system, both receptor subtypes decrease neural excitability, whereas in most neurons during development, the GABAA receptor increases neural excitability and raises cytosolic Ca2+ levels. We used Ca2+ digital imaging to test the hypothesis that GABAA receptor-mediated Ca2+ rises were regulated by GABAB receptor activation. In young, embryonic day 18, hypothalamic neurons cultured for 5 +/- 2 days in vitro, we found that cytosolic Ca2+ rises triggered by synaptically activated GABAA receptors were dramatically depressed (>80%) in a dose-dependent manner by application of the GABAB receptor agonist baclofen (100 nM-100 microM). Coadministration of the GABAB receptor antagonist 2-hydroxy-saclofen or CGP 35348 reduced the inhibitory action of baclofen. Administration of the GABAB antagonist alone elicited a reproducible Ca2+ rise in >25% of all synaptically active neurons, suggesting that synaptic GABA release exerts a tonic inhibitory tone on GABAA receptor-mediated Ca2+ rises via GABAB receptor activation. In the presence of tetrodotoxin the GABAA receptor agonist muscimol elicited robust postsynaptic Ca2+ rises that were depressed by baclofen coadministration. Baclofen-mediated depression of muscimol-evoked Ca2+ rises were observed in both the cell bodies and neurites of hypothalamic neurons taken at embryonic day 15 and cultured for three days, suggesting that GABAB receptors are functionally active at an early stage of neuronal development. Ca2+ rises elicited by electrically induced synaptic release of GABA were largely inhibited (>86%) by baclofen. These results indicate that GABAB receptor activation depresses GABAA receptor-mediated Ca2+ rises by both reducing the synaptic release of GABA and decreasing the postsynaptic Ca2+ responsiveness. Collectively, these data suggest that GABAB receptors play an important inhibitory role regulating Ca2+ rises elicited by GABAA receptor activation. Changes in cytosolic Ca2+ during early neural development would, in turn, profoundly affect a wide array of physiological processes, such as gene expression, neurite outgrowth, transmitter release, and synaptogenesis.     

Ibogaine and a total alkaloidal extract of Voacanga africana modulate neuronal excitability and synaptic transmission in the rat parabrachial nucleus in vitro

Ibogaine is a natural alkaloid of Voacanga africana that is effective in the treatment of withdrawal symptoms and craving in drug addicts. As the synaptic and cellular basis of ibogaine's actions are not well understood, this study tested the hypothesis that ibogaine and Voacanga africana extract modulate neuronal excitability and synaptic transmission in the parabrachial nucleus using the nystatin perforated patch-recording technique. Ibogaine and Voacanga africana extract dose dependently, reversibly, and consistently attenuate evoked excitatory synaptic currents recorded in parabrachial neurons. The ED50 of ibogaine's effect is 5 microM, while that of Voacanga africana extract is 170 micrograms/ml. At higher concentrations, ibogaine and Voacanga africana extract induce inward currents or depolarization that are accompanied by increases in evoked and spontaneous firing rate. The depolarization or inward current is also accompanied by an increase in input resistance and reverses polarity around 0 mV. The depolarization and synaptic depression were blocked by the dopamine receptor antagonist haloperidol. These results indicate that ibogaine and Voacanga africana extract 1) depolarize parabrachial neurons with increased excitability and firing rate; 2) depress non-NMDA receptor-mediated fast synaptic transmission; 3) involve dopamine receptor activation in their actions. These results further reveal that the Voacanga africana extract has one-hundredth the activity of ibogaine in depressing synaptic responses. Thus, ibogaine and Voacanga africana extract may produce their central effects by altering dopaminergic and glutamatergic processes.     

Modulation by substance P of synaptic transmission in the mouse hippocampal slice

The modulatory action of substance P on synaptic transmission of CA1 neurons was studied using intra- or extracellular recording from the mouse hippocampal slice preparation. Bath-applied substance P (2-4 microM) or the selective NK1 receptor agonist substance P methylester (SPME, 10 nM-5 microM) depressed field potentials (recorded from stratum pyramidale) evoked by focal stimulation of Schaffer collaterals. This effect was apparently mediated via NK1 receptors since it was completely blocked by the selective NK1 antagonist SR 140333. The field potential depression by SPME was significantly reduced in the presence of bicuculline. Intracellular recording from CA1 pyramidal neurons showed that evoked excitatory postsynaptic potentials (EPSPs) and evoked inhibitory postsynaptic potentials (IPSPs) were similarly depressed by SPME, which at the same time increased the frequency of spontaneous GABAergic events and reduced that of spontaneous glutamatergic events. The effects of SPME on spontaneous and evoked IPSPs were prevented by the ionotropic glutamate receptor blocker kynurenic acid. In tetrodotoxin (TTX) solution, no change in either the frequency of spontaneous GABAergic and glutamatergic events or in the amplitude of responses of pyramidal neurons to 4 microM alpha-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid (AMPA) or 10 microM N-methyl-D-aspartate (NMDA) was observed. On the same cells, SPME produced minimal changes in passive membrane properties unable to account for the main effects on synaptic transmission. The present data indicate that SPME exerted its action on CA1 pyramidal neurons via a complex network mechanism, which is hypothesized to involve facilitation of a subset of GABAergic neurons with widely distributed connections to excitatory and inhibitory cells in the CA1 area.     

L-arginine potentiates GABA-mediated synaptic transmission by a nitric oxide-independent mechanism in rat dopamine neurons

Effects of L-arginine in the nervous system are often attributed to nitric oxide. Using whole-cell patch pipettes to record membrane currents in voltage-clamp from dopamine neurons in the rat midbrain slice, the present studies found that L-arginine potentiates GABA-dependent membrane currents via a nitric oxide-independent mechanism. L-Arginine (0.3-10 mM) increased the peak amplitude, half-width duration and time constant of decay of GABA(B) receptor-mediated inhibitory postsynaptic currents in a concentration-dependent manner. In the presence of CGP 35348 (300 microM), a GABA(B) receptor antagonist, L-arginine also prolonged the duration of inhibitory postsynaptic currents mediated by GABA(A) receptors, but their amplitudes were reduced. L-Arginine (10 mM) also evoked 17+/-3 pA of outward current (at -60 mV) which was significantly increased in the presence of exogenous GABA (100 microM). Pressure-ejection of GABA from micropipettes produced outward currents mediated by GABA(B) receptors (recorded in bicuculline) or GABA(A) receptors (recorded in CGP 35348); both types of receptor-mediated currents were increased by L-arginine (10 mM). In contrast, outward currents evoked by baclofen, a GABA(B) receptor agonist, were not potentiated by L-arginine. The GABA transport inhibitors NO 711 (1 microM) and nipecotic acid (1 mM) significantly increased the half-width duration and time-constant of decay of GABA(B)-mediated inhibitory postsynaptic currents, thus mimicking effects of L-arginine. However, nitric oxide donors failed to mimic effects of L-arginine on GABA(B) inhibitory postsynaptic currents, and inhibitors of nitric oxide synthesis failed to selectively block the action of L-arginine. These findings suggest that L-arginine potentiates GABA synaptic transmission by a nitric oxide-independent mechanism. Similarities between effects of L-arginine, NO 711 and nipecotic acid suggest that L-arginine inhibits a GABA transporter.     

Synaptic strengthening through activation of Ca2+-permeable AMPA receptors

Postsynaptic Ca2+ elevation during synaptic transmission is an important trigger for short- and long-term changes in synaptic strength in the vertebrate central nervous system. The AMPA (alpha-amino-3-hydroxy-5-methyl-4-isoxazoleproprionate) receptors, a subfamily of glutamate receptors, mediate much of the excitatory synaptic transmission in the brain and spinal cord. It has been shown that a subtype of the AMPA receptor is Ca2+-permeable and is present in the subpopulations of neurons. When synaptically localized, these receptors should mediate postsynaptic Ca2+ influx, providing a trigger for changes in synaptic strength. Here we show that Ca2+-permeable AMPA receptors are synaptically localized on a subpopulation of dorsal horn neurons, and that they provide a synaptically gated route of Ca2+ entry, and that activation of these receptors strengthens synaptic transmission mediated by AMPA receptors. This pathway for postsynaptic Ca2+ influx may provide a new form of activity-dependent modulation of synaptic strength.     

Differential effects of a benzodiazepine on synaptic transmissions in rat hippocampal neurons in vitro

The effects of midazolam, one of the most popular benzodiazepines, on synaptic transmissions were compared with intracellular recordings between CA1 pyramidal cells (CA1-PCs) and dentate gyrus granule cells (DG-GCs) in rat hippocampal slices. First, we studied the effects of midazolam on orthodromically evoked spikes, membrane properties and synaptic potentials. Secondly, the effects of a GABA(A) receptor agonist, muscimol, were examined on membrane properties to determine whether or not the densities of GABA(A) receptors are different between CA1-PCs and DG-GCs. Midazolam (75 microM) markedly depressed orthodromically evoked spikes in CA1-PCs, compared with those in DG-GCs. A GABA(A) receptor antagonist, bicuculline (10 microM), almost completely antagonized the depressant effects of midazolam on spike generation in CA1-PCs, whereas it had little effect on midazolam in dentate gyrus granule cells. Midazolam produced either depolarizing or hyperpolarizing effects on resting membrane potentials (Vm) with an input resistance decrease in CA1-PCs, whereas it produced depolarized Vm in DG-GCs. Midazolam significantly increased the amplitude of monosynaptic inhibitory postsynaptic potentials in CA1-PCs, whereas midazolam slightly decreased these in DG-GCs. Midazolam significantly decreased the amplitude of excitatory postsynaptic potentials both in CA1-PCs and DG-GCs. Muscimol (100 microM) produced either depolarizing or hyperpolarizing effects on Vm with an input resistance decrease in CA1-PCs, and it depolarized Vm with an input resistance decrease in DG-GCs. These results demonstrate that midazolam has differential effects on excitatory and inhibitory synaptic transmissions in hippocampal neurons. The mechanism of this difference could be partly due to the different types of GABA(A) receptors between CA1-PCs and DG-GCs.     

Calcium channel types contributing to excitatory and inhibitory synaptic transmission between individual hypothalamic neurons

The contribution of L-, N-, P- and Q-type Ca2+ channels to excitatory and inhibitory synaptic transmission and to whole-cell Ba2+ currents through Ca2+ channels (Ba2+ currents) was investigated in rat hypothalamic neurons grown in dissociated cell culture. Excitatory and inhibitory postsynaptic currents (EPSCs and IPSCs) were evoked by stimulating individual neurons under whole-cell patch-clamp conditions. The different types of high-voltage-activated (HVA) Ca2+ channels were identified using nifedipine, omega-Conus geographus toxin VIA (omega-CTx GVIA), omega-Agelenopsis aperta toxin IVA (omega-Aga IVA), and omega-Conus magus toxin VIIC (omega-CTx MVIIC). N-, but not P- or Q-type Ca2+ channels contributed to excitatory as well as inhibitory synaptic transmission together with Ca2+ channels resistant to the aforementioned Ca2+ channel blockers (resistant Ca2+ channels). Reduction of postsynaptic current (PSC) amplitudes by N-type Ca2+ channel blockers was significantly stronger for IPSCs than for EPSCs. In most neurons whole-cell Ba2+ currents were carried by L-type Ca2+ channels and by at least two other Ca2+ channel types, one of which is probably of the Q-type and the others are resistant Ca2+ channels. These results indicate a different contribution of the various Ca2+ channel types to excitatory and inhibitory synaptic transmission and to whole-cell currents in these neurons and suggest different functional roles for the distinct Ca2+ channel types.     

In vivo activity-dependent plasticity at cortico-striatal connections: evidence for physiological long-term potentiation

The purpose of the present study was to investigate in vivo the activity-dependent plasticity of glutamatergic cortico-striatal synapses. Electrical stimuli were applied in the facial motor cortex and intracellular recordings were performed in the ipsilateral striatal projection field of this cortical area. Recorded cells exhibited the typical intrinsic membrane properties of striatal output neurons and were identified morphologically as medium spiny type I neurons. Subthreshold cortical tetanization produced either short-term posttetanic potentiation or short-term depression of cortically-evoked excitatory postsynaptic potentials. When coupled with a postsynaptic depolarization leading the membrane potential to a suprathreshold level, the tetanus induced long-term potentiation (LTP) of cortico-striatal synaptic transmission. Induction of striatal LTP was prevented by intracellular injection of a calcium chelator suggesting that this synaptic plasticity involves an increase of postsynaptic free calcium concentration. Contrasting with previous in vitro studies our findings demonstrate that LTP constitutes the normal form of use-dependent plasticity at cortico-striatal synapses. Since excitation of striatal neurons produces a disinhibition of premotor networks, LTP at excitatory striatal inputs should favor the initiation of movements and therefore could be critical for the functions of basal ganglia in motor learning.     

Alpha-subunit of calcium/calmodulin-dependent protein kinase II enhances gamma-aminobutyric acid and inhibitory synaptic responses of rat neurons in vitro

1. Here we report that in acutely isolated rat spinal dorsal horn neurons, the gamma-aminobutyric acid-A (GABAA) receptor can be regulated by calcium/calmodulin-dependent protein kinase II (CaM-KII). Intracellularly applied, the alpha-subunit of CaM-KII enhanced GABAA-receptor-activated current recorded with the use of the whole cell patch-clamp technique. This effect was associated with reduced desensitization of GABA responses. 2. GABA-induced currents are also potentiated by calyculin A, an inhibitor of protein phosphatases 1 and 2A. 3. Conventional intracellular recordings were made from hippocampal CA1 neurons in slices to determine the effect of intracellular application of CaM-KII on inhibitory synaptic potentials evoked by electrical stimulation of the stratum oriens/alveus. The inhibitory synaptic potential was enhanced by CaM-KII; this mechanism may contribute to long-term enhancement of inhibitory synaptic transmission and may also play a role in other forms of plasticity in the mammalian brain.     

Glutamate currents in morphologically identified human dentate granule cells in temporal lobe epilepsy

Glutamate-receptor-mediated synaptic transmission was studied in morphologically identified hippocampal dentate granule cells (DGCs; n = 31) with the use of whole cell patch-clamp recording and intracellular injection of biocytin or Lucifer yellow in slices prepared from surgically removed medial temporal lobe specimens of epileptic patients (14 specimens from 14 patients). In the current-clamp recording, low-frequency stimulation of the perforant path generated depolarizing postsynaptic potentials that consisted of excitatory postsynaptic potentials and phase-inverted inhibitory postsynaptic potentials mediated by the gamma-aminobutyric acid-A (GABA(A)) receptor at a resting membrane potential of -62.7 +/- 2.0 (SE) mV. In the voltage-clamp recording, two glutamate conductances, a fast alpha-amino-3-hydroxy-5-methylisoxazole-4-propionic acid (AMPA)-receptor-mediated excitatory postsynaptic current (EPSC; AMPA EPSC) and a slowly developing N-methyl-D-aspartate (NMDA)-receptor-mediated EPSC (NMDA EPSC), were isolated in the presence of a GABA(A) receptor antagonist. NMDA EPSCs showed a voltage-dependent increase in conductance with depolarization by exhibiting an N-shaped current-voltage relationship. The slope conductance of the NMDA EPSC ranged from 1.1 to 9.4 nS in 31 DGCs, reaching up to twice the size of the AMPA conductance. This widely varying size of the NMDA conductance resulted in the generation of double-peaked EPSCs and a nonlinear increase of the slope conductance of up to 37.5 nS with positive membrane potentials, which resembled "paroxysmal currents," in a subpopulation of the neurons. In contrast, AMPA EPSCs, which were isolated in the presence of an NMDA receptor antagonist (2-amino-5-phosphonovaleric acid), showed voltage-independent linear changes in the current-voltage relationship and were blocked by 6-cyano-7-nitroquinoxaline-2,3-dione. The AMPA conductance showed little variance, regardless of the size of the NMDA conductance of a given neuron. The average AMPA slope conductance was 5.28 +/- 0.65 (SE) nS in 31 human DGCs. This value was similar to AMPA EPSC conductances in normal rat DGCs (5.35 +/- 0.52 nS, mean +/- SE; n = 55). Dendritic morphology and spine density were quantified in the individual DGCs to assess epileptic pathology. Dendritic spine density showed an inverse correlation (r2 = 0.705) with a slower rise time and a longer half-width of the excitatory postsynaptic potentials mediated by the NMDA receptor. It is concluded that both AMPA and NMDA EPSCs contribute to human DGC synaptic transmission in epileptic hippocampus. However, a wide range of changes in the slope conductance of the NMDA EPSCs suggests that the NMDA-receptor-mediated conductance could be altered in human epileptic DGCs. These changes may influence the generation of chronic subthreshold epileptogenic synaptic activity and give rise to pathological excitation leading to epileptic seizures and dendritic pathology.     

Corticofugal amplification of subcortical responses to single tone stimuli in the mustached bat

Since 1962, physiological data of corticofugal effects on subcortical auditory neurons have been controversial: inhibitory, excitatory, or both. An inhibitory effect has been much more frequently observed than an excitatory effect. Recent studies performed with an improved experimental design indicate that corticofugal system mediates a highly focused positive feedback to physiologically "matched" subcortical neurons, and widespread lateral inhibition to "unmatched" subcortical neurons, in order to adjust and improve information processing. These results lead to a question: what happens to subcortical auditory responses when the corticofugal system, including matched and unmatched cortical neurons, is functionally eliminated? We temporarily inactivated both matched and unmatched neurons in the primary auditory cortex of the mustached bat with muscimol (an agonist of inhibitory synaptic transmitter) and measured the effect of cortical inactivation on subcortical auditory responses. Cortical inactivation reduced auditory responses in the medial geniculate body and the inferior colliculus. This reduction was larger (60 vs. 34%) and faster (11 vs. 31 min) for thalamic neurons than for collicular neurons. Our data indicate that the corticofugal system amplifies collicular auditory responses by 1.5 times and thalamic responses by 2.5 times on average. The data are consistant with a scheme in which positive feedback from the auditory cortex is modulated by inhibition that may mostly take place in the cortex.     

Physiological functions of GABA-induced depolarizations in the developing rat spinal cord

Gamma-aminobutyric acid (GABA) is one of the principle inhibitory neurotransmitters in the mature spinal cord. It effectively suppresses synaptic transmission by mechanisms of postsynaptic and presynaptic inhibition. The function of GABA is less well understood early in spinal cord development, when the amino acid is transiently expressed in most neurons, and it depolarizes instead of hyperpolarizes neurons. This article reviews the possible physiological roles of GABA in modulating synaptic transmission, promoting neuronal development, and regulating neuronal pH during early stages of spinal cord differentiation. It is proposed that despite its depolarizing action, GABA acts as an inhibitory neurotransmitter that may also function as a neurotrophic agent.