A magnesium-sensitive post-synaptic potential in rat cerebral cortex resembles neuronal responses to N-methylaspartate

Circuits and Synapses: Hypothesis, Observation, Controversy and Serendipity - An Opinion Piece

More than a century of dedicated research has resulted in what we now know, and what we think we know, about synapses and neural circuits. This piece asks to what extent some of the major advances - both theoretical and practical - have resulted from carefully considered theory, or experimental design: endeavors that aim to address a question, or to refute an existing hypothesis. It also, however, addresses the important part that serendipity and chance have played. There are cases where hypothesis driven research has resulted in important progress. There are also examples where a hypothesis, a model, or even an experimental approach - particularly one that seems to provide welcome simplification - has become so popular that it becomes dogma and stifles advance in other directions. The nervous system rejoices in complexity, which should neither be ignored, nor run from. The emergence of testable "rules" that can simplify our understanding of neuronal circuits has required the collection of large amounts of data that were difficult to obtain. And although those collecting these data have been criticized for not advancing hypotheses while they were "collecting butterflies," the beauty of the butterflies always enticed us toward further exploration.

NMDARs in prefrontal cortex - Regulation of synaptic transmission and plasticity

In this review we consider the various roles played by N-methyl-d-aspartate receptors (NMDARs) located on pyramidal neurones in medial prefrontal cortex (mPFC). We focus on recent data from our lab that has investigated how NMDARs contribute to ongoing synaptic transmission in a frequency dependent manner, the plasticity of NMDARs and how this impacts their contribution to synaptic transmission, and finally consider how NMDARs contribute to plasticity induced by synchronous activation of two separate inputs to mPFC.

Magnesium homeostasis in cattle: absorption and excretion

Magnesium (Mg2+) is an essential mineral without known specific regulatory mechanisms. In ruminants, plasma Mg2+ concentration depends primarily on the balance between Mg2+ absorption and Mg2+ excretion. The primary site of Mg2+ absorption is the rumen, where Mg2+ is apically absorbed by both potential-dependent and potential-independent uptake mechanisms, reflecting involvement of ion channels and electroneutral transporters, respectively. Transport is energised in a secondary active manner by a basolateral Na+/Mg2+ exchanger. Ruminal transport of Mg2+ is significantly influenced by a variety of factors such as high K+ concentration, sudden increases of ammonia, pH, and the concentration of SCFA. Impaired Mg2+ absorption in the rumen is not compensated for by increased transport in the small or large intestine. While renal excretion can be adjusted to compensate precisely for any surplus in Mg2+ uptake, a shortage in dietary Mg2+ cannot be compensated for either via skeletal mobilisation of Mg2+ or via up-regulation of ruminal absorption. In such situations, hypomagnesaemia will lead to decrease of a Mg2+ in the cerebrospinal fluid and clinical manifestations of tetany. Improved knowledge concerning the factors governing Mg2+ homeostasis will allow reliable recommendations for an adequate Mg2+ intake and for the avoidance of possible disturbances. Future research should clarify the molecular identity of the suggested Mg2+ transport proteins and the regulatory mechanisms controlling renal Mg excretion as parameters influencing Mg2+ homeostasis.

Effects of acoustic stimuli on neuronal activity in the auditory cortex of the rat

Spontaneous activity of cortical neurons exhibits alternative fluctuations of membrane potential consisting of phased depolarization called "up-state" and persistent hyperpolarization called "down-state" during slow wave sleep and anesthesia. Here, we examined the effects of sound stimuli (noise bursts) on neuronal activity by intracellular recording in vivo from the rat auditory cortex (AC). Noise bursts increased the average time in the up-state by 0.81+/-0.65 s (range, 0.27-1.74 s) related to a 10 s recording duration. The rise times of the spontaneous up-events averaged 69.41+/-18.04 ms (range, 40.10-119.21 ms), while those of the sound-evoked up-events were significantly shorter (p<0.001) averaging only 22.54+/-8.81 ms (range, 9.31-45.74 ms). Sound stimulation did not influence ongoing spontaneous up-events. Our data suggest that a sound stimulus does not interfere with ongoing spontaneous neuronal activity in auditory cortex but can evoke new depolarizations in addition to the spontaneous ones.

Glutamate receptor subtypes mediating synaptic activation of prefrontal cortex neurons: relevance for schizophrenia

Schizophrenia may involve hypofunction of NMDA receptor (NMDAR)-mediated signaling, and alterations in parvalbumin-positive fast-spiking (FS) GABA neurons that may cause abnormal gamma oscillations. It was recently hypothesized that prefrontal cortex (PFC) FS neuron activity is highly dependent on NMDAR activation and that, consequently, FS neuron dysfunction in schizophrenia is secondary to NMDAR hypofunction. However, NMDARs are abundant in synapses onto PFC pyramidal neurons; thus, a key question is whether FS neuron or pyramidal cell activation is more dependent on NMDARs. We examined the AMPAR and NMDAR contribution to synaptic activation of FS neurons and pyramidal cells in the PFC of adult mice. In FS neurons, EPSCs had fast decay and weak NMDAR contribution, whereas in pyramidal cells, EPSCs were significantly prolonged by NMDAR-mediated currents. Moreover, the AMPAR/NMDAR EPSC ratio was higher in FS cells. NMDAR antagonists decreased EPSPs and EPSP-spike coupling more strongly in pyramidal cells than in FS neurons, showing that FS neuron activation is less NMDAR dependent than pyramidal cell excitation. The precise EPSP-spike coupling produced by fast-decaying EPSCs in FS cells may be important for network mechanisms of gamma oscillations based on feedback inhibition. To test this possibility, we used simulations in a computational network of reciprocally connected FS neurons and pyramidal cells and found that brief AMPAR-mediated FS neuron activation is crucial to synchronize, via feedback inhibition, pyramidal cells in the gamma frequency band. Our results raise interesting questions about the mechanisms that might link NMDAR hypofunction to alterations of FS neurons in schizophrenia.

Concentration of calcium and magnesium and trace elements (zinc, copper, iron and manganese) in cerebrospinal fluid: a try of a pathophysiological classification

The aim of this study is to analyze the variation of the elements (Ca, Mg, Cu, Fe, Zn and Mn) in normal and pathological CSF and develop a classification basing on the increases in cells and proteins and taking into account these variations. A total of 173 cerebrospinal fluids were analyzed. Of these, 37 fulfilled the criteria of normality and, after clinical exploration, were considered to be healthy (control group). The remaining 136 CSFs (pathological group) belonged to people for whom some neurological pathology had been observed in the clinical exploration and whose CSF analysis presented some abnormality. CSF was extracted by puncture in the lumbar cistern. The analysis of metals was performed by atomic absorption spectrophotometry. The statistical values (mean±standard deviation) obtained for each element analyzed in control group were as follows: Ca (mg/dL): 4.95±0.70; Mg (mg/dL): 2.74±0.10; Cu (μg/dL): 15.70±13.50; Fe (μg/dL): 13.10±3.60; Zn (μg/dL): 17.40±9.50 and Mn (μg/dL): 2.50±0.70. In the pathological CSFs, significant increases were found (p<0.050) in relation to the control group for Ca, Cu, Fe, Zn and Mn in groups with an increase of both cells and proteins. A significant decrease of Mg (p<0.050) was found in the groups with cell and protein increases. Given the results obtained in the different subgroups of the proposed classification, we conclude that it is necessary to further categorize the patients' diagnostics in the different subgroups. This would help to validate the classification.

GABAergic and pyramidal neurons of deep cortical layers directly receive and differently integrate callosal input

We studied the involvement of deep cortical layer neurons in processing callosal information in the rat. We observed with electron microscopy that both parvalbumin (PV)-labeled profiles and unlabeled dendritic spines of deep cortical layer neurons receive synapses from the contralateral hemisphere. Stimulation of callosal fibers elicited monosynaptic excitatory postsynaptic currents in both layer VI pyramidal neurons and gamma-aminobutyric acidergic (GABAergic) interneurons immunopositive for the vesicular GABA transporter and PV. Pyramidal cells had intrinsic electrophysiological properties and synaptic responses with slow kinetics and a robust N-metyhl-D-aspartate (NMDA) component. In contrast, GABAergic interneurons had intrinsic membrane properties and synaptic responses with faster kinetics and a less pronounced NMDA component. Consistent with these results, the temporal integration of callosal input was effective over a significantly longer time window in pyramidal neurons compared with GABAergic interneurons. Interestingly, callosal stimulation did not evoke feedforward inhibition in all GABAergic interneurons and in the majority of pyramidal neurons tested. Furthermore, retrogradely labeled layer VI pyramidal neurons of the contralateral cortex responded monosynaptically to callosal stimulation, suggesting interconnectivity between callosally projecting neurons. The data show that pyramidal neurons and GABAergic interneurons of deep cortical layers receive interhemispheric information directly and have properties supporting their distinct roles.

Alterations in the expression of lipid and mechano-gated two-pore domain potassium channel genes in rat brain following chronic cerebral ischemia

To determine the gene expression level of the lipid-sensitive mechano-gated 2P domain K(+) channels in rat cortex and hippocampus after chronic cerebral ischemia, TREK-1, TREK-2 and TRAAK were detected in rats at 3 and 30 days after permanent bilateral carotid artery ligation (BCAL). By using reverse transcription-polymerase chain reaction, we found that TREK-1 and TREK-2 mRNA had no obvious changes in cortex after ischemia, while TRAAK mRNA was significantly increased by 70% after 30 days. In hippocampus, the gene expressions of TREK-1 and TRAAK were increased markedly at 3 days (97% and 87%, respectively) and 30 days (63% and 47%, respectively) after ischemia. However, TREK-2 gene expression level had no change. In-situ hybridization results also showed that TREK-1 gene expressing-positive neurons were increased significantly in cortex and hippocampus after permanent BCAL for 3 and 30 days. These molecular studies provide evidence for an involvement of the lipid-sensitive mechano-gated 2P domain K(+) channels in the BCAL model, which might have neuroprotective effects in cerebral ischemia.

Glutamate transporters regulate excitability in local networks in rat neocortex

Excitatory postsynaptic currents (EPSCs) in the neocortex are principally mediated by glutamate receptors. Termination of excitation requires rapid removal of glutamate from the synaptic cleft following release. Glutamate transporters are involved in EPSC termination but the effect of uptake inhibition on excitatory neurotransmission varies by brain region. Epileptiform activity is largely mediated by a synchronous synaptic activation of cells in local cortical circuits, presumably associated with a large release of glutamate. The role of glutamate transporters in regulating epileptiform activity has not been addressed. Here we examine the effect of glutamate transport inhibition on EPSCs and epileptiform events in layer II/III pyramidal cells in rat neocortex. Inhibiting glutamate transporters with DL-threo-beta-benzyloxyaspartic acid (TBOA; 30 microM) had no effect on the amplitude or decay time of evoked, presumably alpha-amino-3-hydroxyl-5-methyl-isoxazolepropionic acid-mediated, EPSCs. In contrast, the amplitude and duration of epileptiform discharges were significantly enhanced. TBOA resulted also in a decreased threshold for evoking epileptiform activity and an increased probability of occurrence of spontaneous epileptiform discharges. TBOA's effects were not inhibited by the group I and II metabotropic glutamate receptors antagonist (S)-alpha-methyl-4-carboxyphenylglycine or the kainate receptor antagonist [(3S,4aR, 6S, 8aR)-6-((4-carboxyphenyl)methyl-1,2,3,4,4a,5,6,7,8,8a-decahydroisoquinoline-3-carboxylic acid]. D-(-)-2-amino-5-phosphonovaleric acid could both prevent excitability changes by TBOA and block already induced changes. Dihydrokainate (300 microM) had effects similar to TBOA suggesting involvement of the glial transporter GLT-1. Inhibiting glutamate transport increases local network excitability under conditions where there is an enhanced release of glutamate. Our results indicate that uptake inhibition produces an elevation of extracellular glutamate levels and activation of N-methyl-D-aspartate receptors.

The Involvement of N-Methyl-D-Aspartate Receptors in Induction and Maintenance of Long-Term Potentiation in Rat Visual Cortex

Pyramidal neurons from layers II and III of rat visual cortex slices were studied with intracellular recordings. The involvement of N-methyl-D-aspartate (NMDA) receptors was investigated: (1) in the synaptic response to white matter stimulation; (2) in the induction of long-term potentiation (LTP); and (3) in the maintenance of LTP. Bath application of 25 microM of 2-amino-5-phosphonovalerate (APV), an NMDA receptor antagonist, caused a slight (< 10%) reduction of the amplitude of the synaptic response elicited by white matter stimulation. The APV-sensitive excitatory postsynaptic potential (EPSP) had a longer peak latency and duration than the APV-resistant EPSP. Bath application of 10 microM of 6-cyano-7-nitroquinoxaline-2,3-dione (CNQX), a non-NMDA glutamate receptor antagonist, revealed a CNQX-resistant EPSP in response to white matter stimulation which was APV-sensitive. The time course of the CNQX-resistant EPSP was similar to that of the APV-sensitive EPSP and its onset latency was similar to that of the synaptic response in normal medium. Bath application of the GABA-A antagonist bicuculline (0.1 to 0.5 microM) led to a progressive enhancement of the amplitude of the APV-sensitive EPSP. At bicuculline concentrations above 0.3 microM the amplitude of this EPSP increased with membrane depolarization as was the case for the CNQX-resistant EPSP implying that the NMDA receptors were located on the recorded neuron. The susceptibility of the cells to undergo LTP was tested at various concentrations of bicuculline. The effectiveness of bicuculline treatment was quantified by comparing the amplitudes of the synaptic response to just subthreshold stimuli at two post-stimulus delays: (i) at 22 ms, which corresponds to the time to peak of both the initial inhibitory postsynaptic potential and the APV-sensitive EPSP; and (ii) at 8 - 11 ms post-stimulus, which corresponds to the peak of the postsynaptic potential (PSP) in normal medium. Bath application of APV, 20 min after the conditioning tetanus, allowed the authors to measure the amplitude of the APV-sensitive EPSP in the potentiated response. In normal medium, the ratio of the late over the early PSP amplitude was 33.6 +/- 4.1% and tetanic stimulation failed to induce LTP. The conditions remained the same at bicuculline concentrations of 0.1 to 0.2 microM. At higher concentrations of bicuculline the amplitude ratio of late versus early PSP increased and tetanic stimulation induced LTP. In cells, in which bicuculline had caused small ratio increases, only the APV-sensitive EPSP underwent LTP. In cells in which bicuculline had caused large ratio changes, both the APV-resistant and the APV-sensitive EPSP showed LTP. Together with the previous finding that blockade of NMDA receptors prevents LTP (Artola and Singer, 1987) these results suggest that there is a threshold for LTP induction, which is only reached if NMDA receptor-gated channels are sufficiently activated. The data indicate further that the NMDA receptor-mediated EPSP is itself susceptible to LTP whereby its LTP threshold is lower than that of the APV-resistant EPSP. Given the different LTP thresholds of the APV-resistant and APV-sensitive EPSPs, the possibility is raised that their potentiation depends on different mechanisms.

The Effect of Calcium Channel Antagonists on Spontaneous and Evoked Epileptiform Activity in the Rat Neocortex In Vitro

Calcium influx through voltage-activated calcium channels may play a crucial role in the propagation and maintenance of seizure activity. We have examined the contribution of various types of calcium currents to epileptogenesis by studying the effects of various calcium channel blockers on epileptiform activity. N-methyl-d-aspartate receptor-mediated epileptiform activity was induced by removal of magnesium ions superfusing the cortex, or by low-frequency stimulation of the underlying white matter. CoCl2, CdCl2 and omega-conotoxin, acting at the N- and L-type calcium channels, significantly reduced epileptiform activity. L-channel antagonists nifedipine and verapamil, and the agonist BAY K 8644, increased spontaneous bursting in cortical wedges, but had no effect upon evoked activity. The T-channel blocker NiCl2 had variable effects on epileptiform activity, whereas phenytoin consistently reduced such activity. These results suggest that calcium influx underlying epileptiform activity in the rat neocortex may occur at least partially via the activation of the N-type calcium channel. However, contributions from other calcium channel types cannot be excluded.

Excitatory Connections Between CA1 Pyramidal Cells Revealed by Spike Triggered Averaging in Slices of Rat Hippocampus are Partially NMDA Receptor Mediated

Spike triggered averaging was used to record local circuit connections between pairs of CA1 pyramidal neurons in isolated slices of rat hippocampus. Of 795 pairs of neurons tested, six were connected. These epsps were only partially blocked by 2-amino-5-phosphonovalerate (AP-5), which decreased the amplitude and half width of the epsp, but did not affect the early rising phase. In contrast, 6-cyano-7-nitroquinoxaline-2,3-dione (CNQX) blocked all phases of the epsp and combinations of AP-5 and CNQX blocked the epsp almost entirely. These results indicate that these epsps were mediated by both N-methyl-d-aspartate (NMDA) and non-NMDA excitatory amino acid receptors. Moreover, they exhibited a voltage relation typical of neuronal responses to NMDA, increasing in amplitude and duration as the postsynaptic cell was depolarized. These epsps were brief (10 - 90% rise time < 5 ms, width at half amplitude < 20 ms), indicating a proximal location. Increasing presynaptic firing rate (1 - 4 spikes/s) reduced average epsp amplitude by almost 50%. When epsps were evoked by pairs of spikes (interval 3 - 25 ms), a large response to the first spike precluded a large response to the second. No evidence for selective enhancement of the NMDA receptor component by paired spike activation was found. It is concluded that a significant NMDA receptor mediated input to CA1 is provided by local circuit CA1 - CA1 connections and that these synapses can be demonstrated under control conditions.

Interaction between P2Y and NMDA receptors in layer V pyramidal neurons of the rat prefrontal cortex

In the first part of this study, monosynaptic excitatory postsynaptic potentials (EPSPs) in layer V of the rat prefrontal cortex were evoked by electrical stimulation of layer I. Recordings by intracellular sharp microelectrodes showed that EPSPs were concentration-dependently facilitated by the P2 receptor antagonistic ATP analogue 2-methylthio ATP (2-MeSATP), while ATP itself depressed the synaptic potentials. The inhibitory effect of ATP turned into facilitation in the presence of the adenosine A(1) receptor antagonist DPCPX. The 2-MeSATP-induced potentiation of EPSP amplitudes were prevented by the P2 receptor antagonists PPADS and Suramin. The EPSP was almost abolished by coapplication of the NMDA receptor antagonist AP-5 and the AMPA/kainate receptor antagonist CNQX. After blockade of the NMDA receptor-mediated part of the EPSP by AP-5, the stimulatory effect of 2-MeSATP disappeared. When NMDA or AMPA were pressure-applied onto pyramidal cells, only the NMDA-induced depolarization was potentiated by 2-MeSATP. In the second part of the study, NMDA-induced currents were measured by whole-cell patch-clamp pipettes. ATP, 2-MeSATP, UDP and UTP potentiated the response to NMDA, while ADP-beta-S was inactive. PPADS antagonized the effect of ATP. Synaptic isolation of pyramidal neurons by a Ca(2+)-free medium or tetrodotoxin did not alter the effect of ATP which, however, was markedly depressed when GTP in the micropipette was replaced by GDP-beta-S. These observations suggest that in layer V pyramidal neurons of the prefrontal cortex postsynaptically localized P2Y receptors interact with NMDA receptor-channels.

Post-tetanic modification of the efficiency of excitatory transmission in neural networks including interhemispheric connections

This is the first report of modifiable reciprocal transcallosal monosynaptic excitatory connections, detected in in vivo experiments in the rat motor cortex by recording of multineuron activity and cross-correlation analysis. High-frequency microstimulation of a small group of cortical neurons in one hemisphere was shown to alter the efficiency of transcallosal excitatory connections, and also altered the efficiency of ipsilateral connections in both hemispheres. Post-tetanic changes consisted of long-term potentiation and depression. Neurons producing spike trains were found to have better conditions (compared with other neurons) for long-term potentiation of inputs converging on them. Synapses formed by axon collaterals of a given callosal cell on several neurons could simultaneously induce both long-term potentiation and long-term depression, while a given callosal neuron could simultaneously show long-term potentiation in some synapses and long-term depression in others. After microstimulation there were increases in the number of background-active callosal neurons, along with increases in the number and efficiency of transcallosal connections, while the number and efficiency of ipsilateral connections decreased. These data lead to the conclusion that ipsilateral inhibition is more effective than transcallosal inhibition. Microstimulation modified the pattern of initially existing connections between the many elements of ensembles including callosal cells in both hemispheres.

Properties of excitatory synaptic connections mediated by the corpus callosum in the developing rat neocortex

Despite the major role of excitatory cortico-cortical connections in mediating neocortical activities, little is known about these synapses at the cellular level. Here we have characterized the synaptic properties of long-range excitatory-to-excitatory contacts between visually identified layer V pyramidal neurons of agranular frontal cortex in callosally connected neocortical slices from postnatal day 13 to 21 (P13-21) rats. Midline stimulation of the corpus callosum with a minimal stimulation paradigm evoked inward excitatory postsynaptic currents (EPSCs) with an averaged peak amplitude of 56.5 +/- 5 pA under conditions of whole cell voltage clamp at -70 mV. EPSCs had fixed latencies from stimulus onset and could follow stimulus trains (1-20 Hz) without changes in kinetic properties. Bath application of 2,3-dihydro-6-nitro-7-sulfamoyl-benzo(F)quinoxaline (NBQX) abolished these responses completely, indicating that they were mediated by alpha-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid (AMPA) receptors (AMPARs). Evoked responses were isolated in picrotoxin to yield purely excitatory PSCs, and a low concentration of NBQX (0.1 microM) was used to partially block AMPARs and prevent epileptiform activity in the tissue. Depolarization of the recorded pyramidal neurons revealed a late, slowly decaying component that reversed at approximately 0 mV and was blocked by D-2-amino-5-phosphonovaleric acid. Thus AMPA and N-methyl-D-aspartate receptors (NMDARs) coexist at callosal synapses and are likely to be activated monosynaptically. The peak amplitudes and decay time constants for EPSCs evoked using minimal stimulation (+/-40 mV) were similar to spontaneously occurring sEPSCs. Typical conductances associated with AMPA and NMDAR-mediated components, deduced from their respective current-voltage (I-V) relationships, were 525 +/- 168 and 966 +/- 281 pS, respectively. AMPAR-mediated responses showed age-dependent changes in the rectification properties of their I-V relationships. While I-Vs from animals >P15 were linear, those in the younger (<P16) age group were inwardly rectifying. Although Ca2+ permeability in AMPARs can be correlated with inward rectification, outside-out somatic patches from younger animals were characterized by Ca2+-impermeable receptors, suggesting that somatic receptors might be functionally different from those located at synapses. While the biophysical properties of AMPAR components of callosally-evoked EPSCs were similar to those evoked by stimulation of local excitatory connections, the NMDA component displayed input-specific differences. NMDAR-mediated responses for local inputs were activated at more hyperpolarized holding potentials in contrast with those evoked by callosal stimulation. Paired stimuli used to assay presynaptic release properties showed paired-pulse depression (PPD) in animals <P16, which converted to facilitation (PPF) in older animals, suggesting a developmental transition from low probability of transmitter release to high P(r) at these synapses and/or alterations in the properties of the underlying postsynaptic receptors. Physiologic properties of neocortical e-e connections are thus input specific and subject to developmental changes in their postsynaptic receptors.

Evidence for proportional synaptic scaling in neocortex of intact animals

Neocortical neurons in culture can modify their global synaptic strength according to their recent activity level. This process has been termed synaptic scaling. We explored possible effects of synaptic scaling in intact animals by comparing EPSPs of two identified synaptic tracts on single cortical neurons in acute slices. We show that the amplitude and rise-time of unitary thalamocortical and intracortical EPSPs vary among different neurons, but are highly correlated on single cells. Likewise, the AMPA-to-NMDA receptor ratio in EPSPs evoked from the two tracts is correlated in single neurons, although intracortical pathways within layer 5 exhibit a significantly larger NMDA component. These findings demonstrate that, under physiological conditions, cortical neurons actively regulate their own response magnitude, regardless of the input source.

Development of NMDA and non-NMDA receptor-mediated excitatory synaptic transmission in geniculocortical and corticocortical connections in organotypic coculture preparations

Development of N-methyl-D-aspartate (NMDA) and non-NMDA receptor-mediated excitatory synaptic transmission was studied in the visual cortex using organotypic slice cocultures. A slice of visual cortex (VC) dissected from newborn rats was cocultured with either a chunk of embryonic lateral geniculate nucleus (LGN) or another VC. During 7-38 days in vitro (DIV), geniculocortical monosynaptic excitatory postsynaptic potentials (EPSPs) were recorded from layer IV neurons in response to stimulation of the LGN in cocultures of the VC with the LGN. Similarly, corticocortical monosynaptic EPSPs were recorded from layers II/III and V/VI neurons in cocultures of two VCs when stimulating the partner VC. The initial slopes of the non-NMDA and NMDA receptor-mediated components of the EPSPs, which were dissociated pharmacologically, were assessed and compared among three different culture stages, early (7-11 DIV), middle (12-15 DIV) and late (17-38 DIV). With progression of the culture stage, the non-NMDA component tended to increase in both the geniculocortical and corticocortical connections. In contrast, the NMDA component exhibited distinct developmental changes. The NMDA component in layer IV neurons, which receive geniculate inputs, showed a transient increase in the middle stage. In the corticocortical connection, the magnitude of the NMDA component was large in the early stage and maintained through all culture stages in layer V/VI cells, whereas in layer II/III cells it decreased sharply by the late stage. Our results suggest that glutamatergic transmission in the visual cortex develops differently in the geniculocortical and corticocortical connections.

Molecular frequency filters at central synapses

During the 1950s to 70s most of the mechanisms that control transmitter release from presynaptic nerve terminals were described at the neuromuscular junction. It was not, however, until the 1990s that the multiplicity of protein-protein interactions that govern this process began to be identified. The sheer numbers of proteins and the complexity of their interactions at first appears excessive, even redundant. However, studies of identified central synapses indicate that this molecular diversity may underlie a important functional diversity. The task of the neuromuscular junction is to relay faithfully the rate and pattern code generated by the motoneurone. To demonstrate phenomena such as facilitation and augmentation that are apparent only when the probability of release is low, experimental manipulation is required. In the cortex, however, low probability synapses displaying facilitation can be recorded in parallel with high probability synapses displaying depression. The mechanisms are largely the same as those displayed by the neuromuscular junction, but some are differentially expressed and controlled. Central synapses demonstrate exquisitely fine tuned information transfer, each of the many types displaying its own repertoire of pattern- and frequency-dependent properties. These appear tuned to match both the discharge pattern in the presynaptic neurone and the integrative requirements of the postsynaptic cell. The molecular identification of these differentially expressed frequency filters is now just coming into sight. This review attempts to correlate these two aspects of synaptic physiology and to identify the components of the release process that are responsible for the diversity of function.

Confocal imaging of N-methyl-D-aspartate receptors in living cortical neurons

The fluorescence-conjugated N-methyl-D-aspartate receptor-selective antagonist, BODIPY-conantokin-G, was employed to label N-methyl-D-aspartate receptors in living neurons derived from the visual cortex of embryonic rats. The fluorescent labeling was visualized and analysed using confocal microscopy and digital imaging techniques. BODIPY-conantokin-G binding sites were homogeneously distributed across somata four days after neurons (E17-20) were placed in culture. In five-day-old cultures, BODIPY-conantokin-G binding sites became clusters of fluorescently labeled spots which were arranged irregularly on somata and proximal neurites. Distal neurites displayed fluorescent labeling after 10-15 days in culture. Displacement experiments showed that spermine and unlabeled conantokin-G compete with BODIPY-conantokin-G labeling at the N-methyl-D-aspartate receptor-associated polyamine site. The N-methyl-D-aspartate receptor antagonist 2-amino-5-phosphonovaleric acid also depressed the labeling but with a weaker effect, probably due to interactions occurring between the N-methyl-D-aspartate receptor agonist binding site and the polyamine modulatory site. The fluorescent dyes FM 1-43 and FM 4-64 were used in double-labeling studies to compare the distribution of nerve terminals with that of BODIPY-conantokin-G binding sites. BODIPY-conantokin-G binding clusters were associated with presynaptic nerve terminals while isolated BODIPY-conantokin-G binding sites were not always opposed to terminals. The aggregation of receptors to form clusters may lead to the functional formation of excitatory synapses. To investigate whether modulation of membrane potentials affected the formation of N-methyl-D-aspartate receptor clusters, cultured neurons were chronically treated for a week with either tetrodotoxin (to block membrane action potentials) or a high concentration of potassium to depolarize the membrane. While neurons in the tetrodotoxin-treated group showed a similar number of fluorescently labeled clusters compared with the control group, neurons in the high potassium group exhibited a higher number of fluorescently labeled receptor clusters. These results suggest that more active neurons may tend to form more N-methyl-D-aspartate synapses during early development.

Pathophysiology of grass tetany and other hypomagnesemias. Implications for clinical management

Magnesium is an essential mineral with many physiologic and biochemical functions. Surprisingly, Mg homeostasis is not regulated by a hormonal feedback system, but simply depends on inflow (absorption) from the gastrointestinal tract and outflow (endogenous secretion, requirement for milk production, uptake by tissues). Any surplus (inflow greater than outflow) is excreted via urine. Conversely, if the outflow (mainly milk secretion and endogenous loss) exceeds inflow, hypomagnesemia occurs because of the lack of hormonal mechanisms of homeostasis. The major reason for insufficient inflow is a reduced absorption of Mg from the forestomachs. Recent studies from our laboratory and data from the literature permit the proposal of a putative transport model for the secondary active transport of Mg across the rumen epithelium. This model includes two uptake mechanisms across the luminal membrane (PD-dependent and PD-independent) and basolateral extrusion via a Na/Mg exchange. The well-known negative interaction between ruminal K concentration and Mg absorption can be explained on the basis of this model: an increase of ruminal K depolarizes the potential difference of the luminal membrane, PDa, and as the driving force for PD-dependent (or K-sensitive) Mg uptake. Because Na deficiency causes an increase of K concentration in saliva and ruminal fluid, Na deficiency should be considered a potentially important risk factor. The data obtained from in vitro and in vivo studies on the association of Mg transport, changes of ruminal K concentration, and PDa are extensive and confirm the model, if the ruminal Mg concentrations are below 2 to 3 mM. It is further proposed by the model that the PD-independent Mg uptake mechanism is primarily working at high ruminal Mg concentration (above 2 mM). Mg absorption becomes more and more independent of ruminal K with increasing Mg concentration, which can be considered as an explanation for the well-known prophylaxis of hypomagnesemia by increasing oral Mg intake. Fermentation products, NH4+ and SCFA, influence Mg absorption. The possible meaning regarding the pathogenesis of hypomagnesemia is not quite clear. A sudden increase of ruminal NH4+ should be avoided, because high NH4+ concentrations transiently reduce Mg absorption. The most prominent signs of hypomagnesemia are excitations and muscle cramps, which are closely correlated with the Mg concentration in the CSF. It is suggested that the clinical signs are caused by spontaneous activation of neurons in the CNS at low Mg concentrations, which leads to tetany. Prophylactic measures are discussed in context with the known effects on ruminal Mg absorption.

Dorsal hippocampal kindling produces long-lasting changes in the origin of spontaneous discharges in the piriform versus perirhinal cortex in vitro

In an in vitro slice preparation of the amygdala-piriform-perirhinal cortex (A-P area), it was shown previously (McIntyre, D.C., Plant, J. R., 1993. Long-lasting changes in the origin of spontaneous discharges from amygdala-kindled rats: piriform vs. perirhinal cortex in vitro, Brain Res. 624, 268-276) that the infrequent spontaneous field potentials that initially originated in or near the perirhinal (PRh) cortex of slices from control rats began instead in the piriform (Pir) cortex of amygdala-kindled rats. This change in onset was only observed in the A-P area ipsilateral to the kindled amygdala. In the present experiment, we determined whether similar changes in activity were evident following kindling from a different limbic site, the dorsal hippocampus (DH). Kindling of the DH resulted in changes in the origin of the spontaneous discharges in the A-P area similar to amygdala kindling but, importantly, the changes involved both hemispheres. In addition, the origin of spontaneous discharges in slices from partial kindled rats (those that received as many hippocampal afterdischarges as the fully kindled rats but had not developed generalized convulsive responses) initially were similar to control tissue, but, during 0 Mg(2+) perfusion, changed more quickly than control tissue to mimic the profile of generalized kindled rats. The enduring changes in A-P area excitability caused by previous generalized kindling highlights the importance of the A-P area in convulsive generalization of limbic-kindled seizures.

Neuropeptide Y reduces epileptiform discharges and excitatory synaptic transmission in rat frontal cortex in vitro

Neuropeptide Y reduced spontaneous and stimulation-evoked epileptiform discharges in rat frontal cortex slices perfused with a magnesium-free solution and with the GABA(A) receptor antagonist picrotoxin. To investigate the mechanism of that action, effects of neuropeptide Y on intrinsic membrane properties and synaptic responses of layer II/III cortical neurons were studied using intracellular recording. Neuropeptide Y (1 microM) had no detectable effect on the membrane properties of neurons. The evoked synaptic potentials were attenuated by neuropeptide Y. Moreover, the pharmacologically isolated excitatory postsynaptic potentials, mediated by N-methyl-D-aspartate and non-N-methyl-D-aspartate receptors, were reversibly depressed by neuropeptide Y. The most pronounced inhibitory effect of neuropeptide Y was observed on late polysynaptic excitatory postsynaptic potentials. To assess a putative postsynaptic action of neuropeptide Y, N-methyl-D-aspartate was locally applied in the presence of tetrodotoxin. The N-methyl-D-aspartate-evoked depolarizations were unaffected by neuropeptide Y, which suggests that the depression of excitatory postsynaptic potentials was due to an action at sites presynaptic to the recorded neurons. These data show that neuropeptide Y attenuates epileptiform discharges and the glutamate receptor-mediated synaptic transmission in the rat frontal cortex. The above results indicate that neuropeptide Y may regulate neuronal excitability within the cortex, and that neuropeptide Y receptors are potential targets for an anticonvulsant therapy.

Effects of blocking non-N-methyl-D-aspartate receptors on visual responses of neurons in the cat visual cortex

To elucidate the function of non-N-methyl-D-aspartate types of glutamate receptors in the primary visual cortex of the adult cat, we studied the effects of the iontophoretically applied glutamate receptor antagonists 6-cyano-7-nitroquinoxaline-2,3-dione and D-amino-5-phosphonovalerate. Antagonists were applied with ejecting currents that selectively blocked non-N-methyl-D-aspartate receptors. Among 93 cells in which stable recordings were obtained, 6-cyano-7-nitroquinoxaline-2,3-dione reduced the visual response in all cells. The average response magnitude during 6-cyano-7-nitroquinoxaline-2,3-dione administration was reduced to 11.7% of the control (average ejecting current: 41.2 nA). The effect of 6-cyano-7-nitroquinoxaline-2,3-dione was obvious throughout all cortical layers. The effect of D-amino-5-phosphonovalerate on the visual response was tested in 14 cells and it was also effective in blocking the visual response: the average response magnitude during D-amino-5-phosphonovalerate administration was 45.0% of the control (average ejecting current: 41.4 nA). The effect of 6-cyano-7-nitroquinoxaline-2,3-dione on the response was compared in individual cells at both high and low firing rates in order to determine whether a differential effect exists on the level of firing activity of cells due to secondary inactivation of voltage-dependent N-methyl-D-aspartate receptors. However, no indication of response dependency on firing rate was seen with 6-cyano-7-nitroquinoxaline-2,3-dione. We suggest that excitatory transmission at the geniculocortical and corticocortical synapses seems to be strongly dependent on non-N-methyl-D-aspartate receptors throughout the primary visual cortex of the adult cat, and that both non-N-methyl-D-aspartate and N-methyl-D-aspartate type glutamate receptors function additively.

Laminar and cellular distribution of AMPA, kainate, and NMDA receptor subunits in monkey sensory-motor cortex

In situ hybridization histochemistry and immunocytochemistry were used to examine lamina- and cell-specific expression of glutamate receptor (GluR) mRNAs and polypeptide subunits in motor and somatosensory cortex of macaque monkeys. Radioactive complementary RNA (cRNA) probes were prepared from cDNAs specific for alpha-amino-3-hydroxy-5-methylisoxozolepropionate (AMPA)/kainate (GluR1-GluR4), kainate (GluR5-GluR7), and N-methyl-D-aspartate (NMDA; NR1, NR2A-NR2D) receptor subunits. AMPA/kainate and NR1, NR2A, and NR2B receptor transcripts show higher expression than other transcripts. All transcripts show lamina-specific patterns of distribution. GluR2 and GluR4 mRNAs show higher expression than do GluR1 and GluR3 mRNAs. GluR6 transcript expression is higher than that of GluR5 and GluR7. NR1 mRNA expression is much higher than that of NR2 mRNAs. NR2C subunit expression is very low except for a very distinct band of high expression in layer IV of area 3b. Immunocytochemistry, using subunit-specific antisera and double labeling for calbindin, parvalbumin, or alpha type II Ca2+/calmodulin-dependent protein kinase (CaMKII-alpha), allowed identification of cell types expressing different subunit genes. GluR1 and GluR5/6/7 immunoreactivity is found in both pyramidal cells and gamma-amino butyric acid (GABA) cells; GluR2/3 immunoreactivity is preferentially found in pyramidal cells, whereas GluR4 immunoreactivity is largely restricted to GABA cells; NMDA receptor subunit immunoreactivity is far greater in excitatory cells than in GABA cells. The density of expression of AMPA/kainate, kainate, and NMDA receptor subunit mRNAs differed within and across the architectonic fields of sensory-motor cortex. This finding and the lamina- and cell-specific patterns of expression suggest assembly of functional receptors from different arrangements of available subunits in specific neuronal populations.

NMDA and AMPA receptors on neocortical neurons are differentially distributed

The distribution of glutamate receptor subtypes on the surface of neurons is highly relevant for synaptic activation and signal processing in the neocortex. As a novel approach we have used infra-red videomicroscopy in combination with photostimulation or microiontophoresis in brain slices of rat neocortex to map the distribution of N-methyl-D-aspartate (NMDA) and alpha-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid (AMPA) receptors on pyramidal neurons of layer V. Both modes of application revealed a spatially distinct distribution of glutamate receptor subtypes: the soma and the proximal dendrite of neurons are highly sensitive to NMDA, whereas the more distal parts of the dendrite are more sensitive to AMPA. An implication is that NMDA receptors near the soma might regulate the amplification of synaptic signals resulting from AMPA receptor activation on remote dendritic sites.

Spike-wave complexes and fast components of cortically generated seizures. II. Extra- and intracellular patterns

In the previous paper we have demonstrated, by means of field potential and extracellular unit recordings, that bicuculline-induced seizures, which include spike-wave (SW) or polyspike-wave (PSW) complexes, are initiated intracortically and survive ipsilateral thalamectomy. Here, we used multisite field potential and extracellular recordings to validate the patterns of cortical SW/PSW seizures in chronically implanted, behaving cats. To investigate the cellular patterns and excitability during spontaneously occurring and electrically elicited cortical seizures, we used single and dual intracellular recordings from regular-spiking (RS) and fast-rhythmic-bursting (FRB) cortical neurons, in conjunction with field potential recordings from neocortex and related thalamic nuclei, in cats maintained under ketamine-xylazine anesthesia. 1) Invariably, the spontaneous or electrically induced seizures were initiated within the cortex of both behaving and anesthetized animals. Spontaneously occurring, compound seizures consisting of SW/PSW complexes at 2-4 Hz and fast runs at 10-15 Hz, developed without discontinuity from the slow (mainly 0.5-0.9 Hz), sleeplike, cortically generated oscillation. 2) During SW/PSW complexes, RS neurons discharged spike trains during the depth-negative component of the cortical "spike" component of field potentials and were hyperpolarized during the depth-positive field wave. The FRB neurons fired many more action potentials than RS cells during SW/PSW complexes. Averaged activities triggered by the spiky field potentials or by the steepest slope of depolarization in cortical neurons demonstrated similar relations between intracellular activities and field potentials during sleep and seizure epochs, the latter-being an exaggeration of the depolarizing and hyperpolarizing components of the slow sleep oscillation. 3) During the fast runs, RS cells were tonically depolarized and discharged single action potentials or spike doublets (usually with pronounced spike inactivation), whereas FRB cells discharged rhythmic spike bursts, time locked with the depth-negative field potentials. 4) Neuronal excitability, tested by depolarizing current pulses applied throughout the seizures and compared with pre- and postseizure epochs, showed a decreased number of evoked action potentials during both seizure components (SW/PSW complexes and fast runs), eventually leading to null responses during the postictal depression. 5) Data suggest that interconnected FRB neurons may play an important role in the initiation of cortical seizures. We discuss the similarities between the electrographic patterns described in this study and those found in different forms of clinical seizures.

Characterization of neuronal migration disorders in neocortical structures. II. Intracellular in vitro recordings

Neuronal migration disorders (NMD) are involved in a variety of different developmental disturbances and in therapy-resistant epilepsy. The cellular mechanisms underlying the pronounced hyperexcitability in dysplastic cortex are not well understood and demand further clinical and experimental analyses. We used a focal freeze-lesion model in cerebral cortex of newborn rats to study the functional consequences of NMD. Intracellular recordings from supragranular regular spiking cells in cortical slices from adult sham-operated rats revealed normal passive and active intrinsic membrane properties and normal stimulus-evoked excitatory and inhibitory postsynaptic potentials (EPSPs and IPSPs, respectively). Regular spiking neurons recorded in rat dysplastic cortex showed on average a significantly smaller action potential amplitude, a slower spike rise, and a less steep primary frequency-current relationship. Stimulus-elicited EPSPs in NMD-affected cortex consisted of multiphasic burst discharges, which coincided with extracellular field potentials and lasted 150-800 ms. These epileptiform responses could be recorded at membrane potentials between -50 and -110 mV and were blocked by -2-amino-5-phosphonovaleric acid (APV), indicating the involvement of N-methyl--aspartate (NMDA) receptors. Isolated NMDA-mediated and APV-sensitive EPSPs could be recorded at membrane potentials negative to -70 mV, suggesting that NMDA receptors are activated at relatively negative membrane potentials. In comparison with the controls, polysynaptic IPSPs mediated by the gamma-aminobutyric acid (GABA) type A and B receptor were either absent or reduced in peak conductance in microgyric cortex by 27% (P < 0.05) and 17%, respectively. However, monosynaptic IPSPs recorded in the presence of ionotropic glutamate receptor antagonists revealed a similar efficacy in NMD and control cortex, indicating that GABAergic neurons in microgyric cortex get a weaker excitatory input. Our data indicate that the expression of epileptiform activity in NMD-affected cortex rather results from an imbalance between excitatory and inhibitory synaptic transmission than from alterations in the intrinsic membrane properties. This imbalance is caused by an increase in NMDA-receptor-mediated excitation in pyramidal neurons and a concurrent decrease of glutamatergic input onto inhibitory interneurons.

GABA-ergic transmission in deep cerebellar nuclei

gamma-Aminobutyric acid (GABA) is the inhibitory transmitter released at Purkinje cell axon terminals in deep cerebellar nuclei (DCN). Neurons in DCN also receive excitatory glutamatergic inputs from the inferior olive. The output of DCN neurons, which depends on the balance between excitation and inhibition on these cells, is involved in cerebellar control of motor coordination. Plasticity of synaptic transmission observed in other areas of the mammalian central nervous system (CNS) has received wide attention. If GABA-ergic and/or glutamatergic synapses in DCN also undergo plasticity, it would have major implications for cerebellar function. In this review, literature evidence for GABA-ergic synaptic transmission in DCN as well as its plasticity are discussed. Studies indicate that fast inhibitory postsynaptic potentials (IPSPs) and currents (IPSCs) in neurons of DCN are mediated by GABAA receptors. While GABAB receptors are present in DCN, they do not appear to be activated by Purkinje cell axons. The IPSPs undergo paired-pulse, as well as frequency-dependent, depressions. In addition, tetanic stimulation of inputs can induce a long-term depression (LTD) of the IPSPs and IPSCs. Excitatory synapses do not appear to undergo long-term potentiation or LTD. The LTD of the IPSP is not input-specific, as it can be induced heterosynaptically and is associated with a reduced response of DCN neurons to a GABAA receptor agonist. Postsynaptic Ca2+ and protein phosphatases appear to contribute to the LTD. The N-methyl-D-aspartate receptor-gated, as well as the voltage-gated Ca2+ channels are proposed to be sources of the Ca2+. It is suggested that LTD of GABA-ergic transmission, by regulating DCN output, can modulate cerebellar function.

Stimulus intensity, cell excitation and the N-methyl-D-aspartate receptor component of sensory responses in the rat spinal cord in vivo

The importance of receptors for N-methyl-D-aspartate in synaptic plasticity and in triggering long-term pronociceptive changes is explained by their voltage-dependence. This suggests that their contribution to acute nociceptive responses would be determined both by the magnitude of synaptic input and by the level of background excitation. We have now examined the role of N-methyl-D-aspartate receptors in acute nociceptive transmission in the spinal cord. Drugs selectively affecting activity mediated by these receptors were tested on responses of dorsal horn neurons to noxious stimuli of different intensities and at different levels of ongoing spike discharge. The drugs used were the N-methyl-D-aspartate receptor channel blocker ketamine; the competitive antagonists, 3-((R)-2-carboxypiperazin-4-yl)-propyl-1-phosphonic acid (D-CPP) and D-2-amino-5-phosphonopentanoic acid (D-AP5), and the positive modulator thyrotropin-releasing hormone. The activity of dorsal horn wide dynamic range neurons was recorded extracellularly in alpha-chloralose-anaesthetized spinalized rats. Their responses to noxious stimuli (pinch, heat and electrical) were monitored in parallel with responses to iontophoretic N-methyl-D-aspartate and (RS)-alpha-amino-3-hydroxy-5-methyl-4-isoxazole-propionic acid (AMPA). Drugs were given i.v. or (D-AP5) iontophoretically. At doses that selectively inhibited responses to exogenous N-methyl-D-aspartate, ketamine (4 or 8, mean 5 mg/kg i.v.) reduced the nociceptive responses of the majority of the cells in deep dorsal horn. Ketamine also reduced wind-up of the responses to repetitive electrical stimulation. Ketamine (4 or 8 mg/kg). D-CPP (2 mg/kg), D-AP5 (iontophoretically) and thyrotrophin-releasing hormone (1 mg/kg) were tested on different magnitude nociceptive responses evoked by alternating intensities of noxious heat or pinch. In percentage terms, the less vigorous responses were affected by all four drugs as much as or more than the more vigorous responses. When background activity of neurones was enhanced by continuous activation of C-fibres with cutaneous application of mustard oil, ketamine was less effective against superimposed noxious pinch responses. Ongoing background activity was affected in parallel with evoked responses. When background discharge of the cells was maintained at a stable level with continuous ejection of kainate, neither the N-methyl-D-aspartate antagonists nor thyrotrophin-relasing hormone affected the responses to noxious pinch or heat, although responses to exogenous N-methyl-D-aspartate were still blocked. The wind-up of the electrical responses was, however, reduced by ketamine irrespective of the level of background activity. The results indicate that under these conditions in vivo, N-methyl-D-aspartate receptors mediate ongoing low-frequency background activity rather than phasic high-frequency nociceptive responses. The effects of N-methyl-D-aspartate antagonists and positive modulators on nociceptive responses are evidently indirect, being secondary to changes in background synaptic excitation. These results cannot be explained simply in relation to the voltage-dependence of N-methyl-D-aspartate receptor-mediated activity; other factors, such as modulation by neuropeptides, must be involved.

Muscarinic reduction of GABAergic synaptic potentials results in disinhibition of the AMPA/kainate-mediated EPSP in auditory cortex

The present study is concerned with the ability of muscarinic actions of acetylcholine (ACh) to modulate glutamate and gamma-aminobutyric acid (GABA)-mediated synaptic transmission in the in vitro rat auditory cortex. Whole-cell patch clamp recordings were obtained from layer II-III pyramidal neurons, and the fast-EPSP (AMPA/kainate), fast-IPSP (GABA(A)), and slow-IPSP (GABA(B)), were elicited following a stimulus to deep gray/white matter. Acetyl-beta-methylcholine (MCh), a muscarinic receptor agonist, applied by either superfusion or iontophoresis, produced an atropine-sensitive increase or decrease in the amplitude of the fast-EPSP. The effect of MCh could be predicted by the response of the fast-EPSP to paired-pulse stimulation (i.e. a conditioning pulse followed 300 ms later by a test pulse). The fast-EPSP was decreased in amplitude by MCh in cases where the test-EPSP was suppressed in the pre-MCh condition, and increased in amplitude when the test-EPSP was facilitated. The fast- and slow-IPSPs were always reduced by MCh. In several experiments, the strength of synaptic inhibition was systematically modified by varying stimulus intensity. When the fast-EPSP was elicited in the absence of IPSPs, it was decreased in amplitude by MCh. However, when the fast-EPSP was elicited in conjunction with large IPSPs it was increased in amplitude during MCh. Because the magnitude of the fast-EPSP is influenced by the degree of temporal overlap with IPSPs, it was hypothesized that enhancement of the fast-EPSP was the result of disinhibition produced as a consequence of muscarinic reduction of GABAergic IPSPs. This view was supported by the finding that MCh could reduce the amplitude of pharmacologically isolated GABAergic IPSPs (i.e. elicited in the absence of glutamatergic transmission). Our results suggest that ACh at muscarinic receptors can modify fast glutamatergic neurotransmission differently as a function of strength of inhibition, to suppress that produced by 'weak' inputs and enhance that produced by 'strong' inputs.

Physiology and pharmacology of corticothalamic stimulation-evoked responses in rat somatosensory thalamic neurons in vitro

Whole cell current- and voltage-clamp recording techniques were employed in a rat thalamocortical slice preparation to characterize corticothalamic stimulation-evoked responses in thalamic neurons. Three types of corticothalamic stimulation-evoked responses were observed in thalamic neurons. Of thalamic neurons, 57% responded to corticothalamic stimulation with purely excitatory synaptic responses, whereas 27% had inhibitory synaptic responses and 16% had mixed excitatory/inhibitory responses. This suggested corticothalamic activation of multiple distinct synaptic circuits, presumably involving both nucleus reticularis thalami (NRT) and thalamus, because the rat ventrobasal complex is virtually devoid of GABAergic interneurons. Corticothalamic-stimulation-evoked excitatory postsynaptic currents (EPSCs) were predominantly slow rising currents that showed nonlinear voltage dependence, characteristics of an N-methyl-D-aspartate (NMDA)-receptor-mediated synaptic current. These slow rising EPSCs were blocked by the NMDA antagonist 2-amino-5-phosphonovaleric acid (APV). A minority of corticothalamic EPSCs had faster kinetics, and were blocked by 6-cyano-7 nitroquinoxaline-2,3-dione (CNQX). Corticothalamic stimulation of varying frequency optimally activated burst responses in thalamic neurons at low frequencies (3-6 Hz). The optimal 3- to 6-Hz response was reduced by ethosuximide, by APV, and by detaching the neocortex from the thalamocortical slice, suggesting that T current, NMDA receptors, and neocortical properties all contributed to generation of this 3- to 6-Hz frequency preference. In contrast to corticothalamic EPSCs, medial-thalamic-stimulation-evoked responses consisted of fast CNQX-sensitive EPSCs that were predominantly voltage insensitive, with no 3- to 6-Hz frequency preference. In thalamic neurons in which corticothalamic stimulation evoked predominantly inhibitory synaptic responses, this inhibitory postsynaptic potential (IPSP) had early and late phases, often followed by a rebound burst. The early IPSP reversed at -95 mV and was bicuculline sensitive, whereas the late IPSP reversed at -113 mV and was blocked by the gamma-aminobutyric acid-B (GABA(B)) antagonist 3-N[1-(S)-(3,4-dichlorophenyl)ethyl]amino-2-(S)-hydroxypropyl-P-benzy lphoshinic acid (CGP-55845A). In thalamic neurons in which corticothalamic stimulation evoked a mixed excitatory postsynaptic potential (EPSP)/IPSP response, repetitive corticothalamic stimulation rapidly reduced IPSPs and enhanced EPSPs at higher frequencies. This resulted in burst firing being triggered in these mixed response neurons at frequencies >6 Hz. Corticothalamic feedback onto thalamic relay neurons activated diverse responses due to differing relative activation of NRT and "feedforward" inhibitory responses. These multiple in vitro corticothalamic responses differ from responses encountered in other in vitro thalamic preparations lacking a synaptically connected neocortex, but are similar to results evident in thalamic neurons in response to cortical stimulation in vivo. In addition, the thalamocortical 3- to 6-Hz frequency preference was conserved, suggesting that many factors critical for this emergent property of the thalamocortical system are maintained in vitro.

Neuronal and glial localization of NMDA receptors in the cerebral cortex

The crucial role of glutamate receptors of the N-methyl-D-aspartate (NMDA) type in many fundamental cortical functions has been firmly established, as has its involvement in several neuropsychiatric diseases, but until recently, very little was known of the anatomical localization of NMDA receptors in the cerebral cortex of mammals. The recent application of molecular biological techniques to the study of NMDA receptors has allowed the production of specific tools, the use of which has much increased our understanding of the localization of NMDA receptors in the cerebral cortex. In particular, immunocytochemical studies on the distribution of cortical NMDA receptors have: 1. Demonstrated the preferential localization of NMDA receptors in dendritic spines, in line with previous work; 2. Disclosed a thus far unknown fraction of presynaptic NMDA receptors on both excitatory and inhibitory axon terminals: and 3. Shown that cortical astrocytes express NMDA receptors. These studies indicate that the effects of cortical NMDA receptor activation are not caused exclusively by the opening of NMDA channels on neuronal postsynaptic membranes, as previously assumed, and that the activation of presynaptic and glial NMDA receptors can contribute significantly to these effects.

Properties of convergent thalamocortical and intracortical synaptic potentials in single neurons of neocortex

We explored differences in the properties of convergent afferent inputs to single neurons in the barrel area of the neocortex. Thalamocortical slices were prepared from mature mice. Recordings were made from neurons in layer V, and either thalamocortical afferents or horizontal intracortical axons were stimulated. Monosynaptic EPSPs from both sources had latencies shorter than 1.8 msec and low shape variance. Disynaptic thalamocortical IPSPs had latencies longer than 1.8 msec. All neuronal types, as defined by intrinsic firing patterns, received both thalamocortical and intracortical monosynaptic input. The shape parameters (rate of rise and half-width) of monosynaptic EPSPs from the two inputs did not differ significantly. The rate of rise of EPSPs varied considerably across cells, but the rates of rise of thalamocortical and intracortical EPSPs onto single cells were strongly correlated. The relative thresholds for activation of synaptic excitation and inhibition were strikingly different between the two tracts: thalamocortical stimulation induced GABAA-dependent IPSPs at stimulus intensities equal to or less than those required for evoking EPSPs in 35% (24 of 68) of the cells. In contrast, the threshold response to intracortical stimulation was always an EPSP, and only stronger stimuli could generate di- or polysynaptic IPSPs. We suggest that postsynaptic factors may tend to equalize the waveforms of EPSPs from thalamocortical and intracortical synapses onto single neurons. A major difference between the two convergent tracts is that the thalamocortical pathway much more effectively activates feedforward inhibitory circuits than does the horizontal intracortical pathway.

The origins of aperiodicities in sensory neuron entrainment

Aperiodic entrainment to rhythmic sensory input was obtained with either a single neuron or an excitatory network model, without addition of a stochastic or "noisy" element. The entrainment properties of primary sensory neurons were well captured by the dynamics of the Hodgkin-Huxley ordinary differential equations with a quiescent resting state or threshold for spike output. The frequency-amplitude parameter space was compressed and aperiodic regimes were small in comparison to those of periodically activated pacemaker-like neurons. Transitions between phase-locked and aperiodic entrainment patterns were predictable and determined by the equation dynamics, supporting the contention that some aperiodicities observed in situ arise from the inherent membrane properties of neurons. When the rhythmically activated neuron was embedded in an excitatory network of Hodgkin-Huxley neurons with heterogeneous synaptic delays, aperiodic entrainment patterns were more frequently encountered and these were associated with asynchronous output from the network. Embedding the rhythmically activated neuron in a network with synaptic delays greatly reduced the range of entrained spike frequencies and increased the variability in the neuronal firing. The temporal coding of sensory stimuli may be dependent on these findings. Sensory stimuli are signaled in the periphery by a mixture of periodic and irregular interspike intervals. Most models of such temporal codes assume intrinsic rhythmicity arising from the ionic currents, with variations attributed to membrane or synaptic noise. In contrast, we demonstrate irregular neural codes that arise completely in the absence of noise. In the proposed model, the sources of these irregular sensory patterns are the extensive cross-connections and resultant interactions between neurons. The balance between the regular and irregular entrainment of a neuron in situ could uniquely identify a stimulus. Other biological mechanisms of modifying the entrainment properties and promoting aperiodic entrainment are discussed.

Properties of convergent thalamocortical and intracortical synaptic potentials in single neurons of neocortex

We explored differences in the properties of convergent afferent inputs to single neurons in the barrel area of the neocortex. Thalamocortical slices were prepared from mature mice. Recordings were made from neurons in layer V, and either thalamocortical afferents or horizontal intracortical axons were stimulated. Monosynaptic EPSPs from both sources had latencies shorter than 1.8 msec and low shape variance. Disynaptic thalamocortical IPSPs had latencies longer than 1.8 msec. All neuronal types, as defined by intrinsic firing patterns, received both thalamocortical and intracortical monosynaptic input. The shape parameters (rate of rise and half-width) of monosynaptic EPSPs from the two inputs did not differ significantly. The rate of rise of EPSPs varied considerably across cells, but the rates of rise of thalamocortical and intracortical EPSPs onto single cells were strongly correlated. The relative thresholds for activation of synaptic excitation and inhibition were strikingly different between the two tracts: thalamocortical stimulation induced GABAA-dependent IPSPs at stimulus intensities equal to or less than those required for evoking EPSPs in 35% (24 of 68) of the cells. In contrast, the threshold response to intracortical stimulation was always an EPSP, and only stronger stimuli could generate di- or polysynaptic IPSPs. We suggest that postsynaptic factors may tend to equalize the waveforms of EPSPs from thalamocortical and intracortical synapses onto single neurons. A major difference between the two convergent tracts is that the thalamocortical pathway much more effectively activates feedforward inhibitory circuits than does the horizontal intracortical pathway.

Expression of N-methyl-D-aspartate glutamate receptor subunits in the prefrontal cortex of the rat

The laminar distribution and cellular levels of expression of mRNAs encoding N-methyl-D-aspartate receptor subunits (NMDAR1, NMDAR2A-D and the alternatively spliced isoforms of NMDAR1) were examined in prefrontal cortex of rat by in situ hybridization using film and emulsion autoradiography. Film autoradiograms demonstrated a distinctive laminar distribution of hybridization signals for each of the probes recognizing NMDAR1, NMDAR2A, and NMDAR2B messenger RNA; hybridization with probes for NMDAR2C and NMDAR2D resulted in scattered signals without laminar organization. Grain counting disclosed that neurons in layer V displayed the highest and neurons in layer IV the lowest absolute number of grains for all probes examined. Correction for cell size demonstrated statistically significant differences in cellular labelling density of up to 50% across neurons in different cortical layers. The cellular density profiles across cortical laminae differed between probes. Hybridization with a probe recognizing all isoforms of NMDAR1 resulted in significantly lower densities of cellular labelling in neurons of layer IV than of layers II/III, V and VI. Cellular labelling densities following hybridization with probes recognizing alternatively spliced segments of NMDAR1 were examined. Densities were low in neurons of the upper cortical layers II/III and IV using probes for the messenger RNA encoding the amino terminal insert, NMDAR11XX and the second carboxy terminal deletion, NMDAR1XX1; hybridization with a probe for the messenger RNA encoding the first carboxy terminal deletion, NMDAR1X1X, resulted in low cellular signal densities in neurons of layers IV and VIb. NMDAR2A messenger RNA expression was of relatively uniform intensity in neurons of layers II-V but significantly lower in neurons of the inner part of layer VI. NMDAR2B expression was most dense in layer II neurons. These data indicate that neurons in different cortical laminae express distinct N-methyl-D-aspartate receptor subunit messenger RNA phenotypes. In addition, the observed differences in density of N-methyl-D-aspartate receptor subunit messenger RNA expression suggest that cortical laminae differ in the relative contribution of N-methyl-D-aspartate receptors to their excitatory responses.

Stimulation of the prefrontal cortex in the rat induces patterns of activity in midbrain dopaminergic neurons which resemble natural burst events

Evidence suggests that excitatory amino acid-containing afferents from the prefrontal cortex (PFC) play an important role in the induction of burst firing in midbrain dopaminergic (DA) neurons. In the present study, the extracellular activity of individual DA neurons (A10 and A9 cell groups) was recorded during single pulse electrical stimulation (0.25 and 1 mA) of the PFC. The majority of cells were responsive, and two main patterns of activity were elicited: responses characterised by an initial excitation (E responses; 41.8% of responses at 0.25 mA and 26.6% at 1 mA; cell groups combined) and responses characterised by excitation following an initial inhibition (IE responses; 43.3% of responses at 0.25 mA and 56.6% at 1 mA; cell groups combined). Burst analysis performed on the excitatory phase of E and IE responses revealed that the excitation contained events which fulfilled the criteria for natural bursts in DA neurons. A procedure was developed for assessing whether these bursts were time-locked to the stimulus. This showed that 27.9% of E responses and 33.3% of IE responses were accompanied by time-locked bursts (currents and cell groups combined). It is argued that time-locked bursts during IE responses were produced by rebound activation of a low threshold calcium conductance, whereas time-locked bursts during E responses were produced by excitatory afferents. Since natural bursts in DA neurons also seem to involve cortically induced excitation, the hypothesis that the PFC plays a role in the production of natural bursts in DA neurons is strengthened.

Short-term synaptic enhancement and long-term potentiation in neocortex

Repetitive stimuli reliably induce long-term potentiation (LTP) of synapses in the upper layers of the granular somatosensory cortex but not the agranular motor cortex of rats. Herein we examine, in these same cortical areas, short-term changes in synaptic strength that occur during the LTP induction period. theta-Burst stimulation produced a strong short-term enhancement of synapses in the granular area but only weak enhancement in the agranular area. The magnitude of enhancement during stimulation was strongly correlated with the magnitude of LTP subsequently expressed. Short-term enhancement was abolished by an antagonist of N-methyl-D-aspartate (NMDA) receptors but remained in the presence of a non-NMDA receptor antagonist. Inhibitory postsynaptic potentials of the granular and agranular areas displayed similar frequency sensitivity, but the frequency sensitivity of NMDA receptor-dependent excitatory postsynaptic potentials differed significantly between areas. We propose that pathway-specific differences in short-term enhancement are due to variations in the frequency dependence of NMDA currents; different capacities for short-term enhancement may explain why repetitive stimulation more readily induces LTP in the somatosensory cortex than in the motor cortex.

Ischemia and lesion induced imbalances in cortical function

Cortical structures are often critically affected by ischemic and traumatic lesions which may cause transient or permanent functional disturbances. These disorders consist of changes in the membrane properties of single cells and alterations in synaptic network interactions within and between cortical areas including large-scale reorganizations in the representation of the peripheral input. Prominent functional modifications consisting of massive membrane depolarizations, suppression of intracortical inhibitory synaptic mechanisms and enhancement of excitatory synaptic transmission can be observed within a few minutes following the onset of cortical hypoxia or ischemia and probably represent the trigger signals for the induction of neuronal hyperexcitability, irreversible cellular dysfunction and cell death. Pharmacological manipulation of these early events may therefore be the most effective approach to control ischemia and lesion induced disturbances and to attenuate long-term neurological deficits. The complexity of secondary structural and functional alterations in cortical and subcortical structures demands an early and powerful intervention before neuronal damage expands to intact regions. The unsatisfactory clinical experience with calcium and N-methyl-D-aspartate antagonists suggests that this result might be achieved with compounds that show a broad spectrum of actions at different ligand-activated receptors, voltage-dependent channels and that also act at the vascular system. Whether the same therapy strategies developed for the treatment of ischemic injury in the adult brain may be applied for the immature cortex is questionable, since young cortical networks with a high degree of synaptic plasticity reveal a different response pattern to hypoxic and ischemic insults. Age-dependent molecular biological, morphological and physiological parameters contribute to an enhanced susceptibility of the immature brain to these noxae during early ontogenesis and have to be investigated in more detail for the development of adequate clinical therapy.

Glutamatergic and GABAergic postsynaptic responses of striatal spiny neurons to intrastriatal and cortical stimulation recorded in slice preparations

Glutamatergic and GABAergic responses of the neostriatal spiny neurons to intrastriatal and cortical stimulation were characterized by intracellular recording in brain slice preparations. This study also demonstrated the role of each response in the spike activity of the spiny neuron. Single neostriatal stimulation induced postsynaptic potentials consisting of multiple components. The early part of the postsynaptic potential, which was isolated by the GABAA antagonist bicuculline methiodide and the N-methyl-D-aspartate antagonist 3-(2-carboxypiperzin-4-yl)-propyl-1-phosphonic acid (CPP), was mainly an alpha-amino-3-hydroxy-5-methyl-4-isoxazolepropionate (AMPA)/kainate receptor-mediated response. Perfusion of magnesium-free medium containing bicuculline methiodide and the AMPA/kainate antagonist 3-dihydroxy-6-nitro-7-sulfamoyl-benzo[f]quinoxaline (NBQX) disclosed a large, slow N-methyl-D-aspartate receptor-mediated response. The N-methyl-D-aspartate response in magnesium-containing perfusing medium was small in neurons at the resting membrane potential, but became a significant component when the neurons were depolarized to subthreshold membrane potential. The duration of the N-methyl-D-aspartate response was over 300 ms. The nicotinic antagonists dihydro-beta-erythroidine hydrobromide and mecamylamine failed to change responses to single stimulation. Repetitive intrastriatal stimulation induced a large, long-duration depolarization with action potentials in the spiny neurons. This stimulation-induced response resembles that of the depolarization stage observed in anesthetized animals. Bicuculline methiodide increased the response amplitude. In contrast, CPP reduced the amplitude of the response to the below the spike generation threshold. The CPP-sensitive N-methyl-D-aspartate response was large and lasted several hundred milliseconds after the termination of repetitive stimulation. Responses of the neostriatal neurons to cortical stimulation were similar to those induced after intrastriatal stimulation. CPP greatly reduced both the response amplitude and the number of spikes triggered from the response. Bicuculline methiodide, on the other hand, greatly increased the response amplitude and the number of spikes. The AMPA/kainate response alone, which was isolated by application of bicuculline methiodide and CPP, did not induce sustained depolarization in spiny neurons to repetitive cortical stimulation. Application of NBQX diminished GABAA response to cortical stimulation. This observation indicates that, for neostriatal spiny neurons to respond with GABAA response after cortical stimulation, the AMPA/kainate response must be induced in the GABAergic secondary neurons in the neostriatum. This study indicates that the main synaptic driving forces of neostriatal spiny neurons include AMPA/kainate, N-methyl-D-aspartate and GABAA responses. Although AMPA/kainate response is the main synaptic input, the generation of the action potentials in neostriatal neurons is greatly influenced by both GABAA and N-methyl-D-aspartate responses.

Functions of ionotropic and metabotropic glutamate receptors in sensory transmission in the mammalian thalamus

The thalamic relay nuclei play a pivotal role in gating and processing sensory information en route to the cerebral cortex. The major ascending sensory afferents and the descending cortico-fugal afferents to the thalamus almost certainly use the excitatory amino acid L-glutamate as their transmitter. This paper reviews the nature of this transmission in terms of the receptor types which may be used (NMDA, AMPA, kainate and metabotropic glutamate receptors), their electrophysiological and pharmacological properties, and their differential location in the thalamus on neurones, terminals and glial elements. Whilst AMPA receptors, probably of more than one variety, are likely to mediate fast transmission in the thalamus, the contributions of NMDA receptors and metabotropic glutamate receptors to sensory responses under different stimulus conditions may be more varied. This is discussed in the context of the possible functional significance of the interplay of L-glutamate-gated currents with intrinsic membrane currents of thalamic neurones. The interaction of L-glutamate transmission with other modulators (acetylcholine, noradrenaline, serotonin, glycine, D-serine, nitric oxide, arginine, redox agents) is considered.

Intracellular calcium oscillation in cultured rat hippocampal neurons: a model for glutamatergic neurotransmission

Neurons can form a synaptic network in culture and show spontaneous oscillation of intracellular Ca2+ concentration ([Ca2+]i). In the present study, spontaneous oscillation of [Ca2+]i was characterized in cultured hippocampal neurons. The oscillation was blocked completely by tetrodotoxin, 6-cyano-7-nitroquinoxaline-2,3-dione (CNQX) and nicardipine, while DL-2-amino-5-phosphonovaleric acid (APV) showed only a partial depression of the increase in [Ca2+]i. These results suggest that the oscillation in [Ca2+]i is mainly mediated by non-N-methyl-D-aspartate (NMDA) type glutamatergic transmission. The oscillation of [Ca2+]i may be a good model for analyzing glutamatergic transmission and synapse formation.

Synaptic integration fields and associative plasticity of visual cortical cells in vivo

Two major constraints in connectivity decide the spatial extent of visual cortical receptive fields, both during development and adult functioning: 1) feedforward input, extrinsic to visual cortex, is organized in an orderly projection to form a point-to-point mapping of the retina onto the cortical mantle and constitutes the core of the minimal discharge field (MDF) after amplification by local intracortical circuits; and 2) a second type of connectivity consists of long distance horizontal intracortical connections which are thought to favor the binding of local visual operations occurring simultaneously in different parts of the visual field. We review here possible factors, intrinsic to the considered cortical cell, that may control the effectiveness of post-synaptic integration. Their expression during sensory recognition could depend on the spatio-temporal distribution of active inputs onto the target cell dendrite and on their interplay with non-linearities of the membrane properties. On the basis of intracellular recordings in cat area, 17, we have observed that peripheral responses (excitatory and inhibitory) could be boosted by coincident postsynaptic depolarization. This effect is lost if the response and the postsynaptic depolarization are mismatched by 1,000 ms, suggesting that temporal correlation of central and peripheral responses could regulate in a non-linear manner the gain of center-surround interactions. This temporal selectivity is compatible with up-regulation of composite potentials by the transient voltage-dependent activation of slowly inactivating conductances in visual cortical neurons. A direct consequence of these different considerations is that the receptive field (RF) of neurons in visual pathways should not be considered as a static hardwired window probing the outer environment, but as an active filter which may continuously adapt and be updated as a function of global context and past experience.

NMDA and non-NMDA receptors contribute to synaptic transmission between the medial geniculate body and the lateral nucleus of the amygdala

We examined whether the NMDA class of excitatory amino acid receptors contribute to synaptic transmission in the pathway connecting the medial geniculate body (MGB) with the lateral nucleus of the amygdala (LA) using extracellular single unit recordings and microiontophoresis. Cells were identified in LA on the basis of responsivity to electrical stimulation of the MGB. For each cell, a level of current was found for the iontophoretic ejection of the NMDA antagonist AP5 that blocked responses elicited by iontophoresis of NMDA, but had no effect on responses elicited by AMPA. Iontophoresis of AP5 with this level of current blocked the excitatory response elicited by MGB stimulation in most cells tested. Microinfusion of AP5 (25, 50, or 100 microM) also blocked the responses. Additional studies tested individual cells with both AP5 and the AMPA antagonist CNQX and showed that blockade of either NMDA or AMPA receptors interferes with synaptic transmission. Finally, iontophoretic ejection of either AP5 or CNQX blocked short-latency (< 25 ms) responses elicited in LA by peripheral auditory stimulation. Together, these results suggest that the synaptic evocation of action potentials in the thalamo-amygdala pathway depends on both NMDA and non-NMDA receptors. We hypothesize that non-NMDA receptors are most likely required to depolarize the cell sufficiently to remove the blockade of NMDA channels by magnesium and NMDA receptors are required to further depolarize the membrane to the level required for action potential generation.

Human immunodeficiency virus type 1 tat activates non-N-methyl-D-aspartate excitatory amino acid receptors and causes neurotoxicity

The human immunodeficiency virus type 1 (HIV-1) protein Tat is known to be released from HIV-1-infected cells. We show that micromolar concentrations of Tat depolarized young rat and adult human neurons. In addition, Tat, at similar concentrations, was toxic to human fetal neurons in culture. Tat-induced responses were insensitive to the Na+ channel blocker tetrodotoxin, suggesting a direct effect of Tat on neurons. Tat-induced depolarizations and cytotoxicity were blocked by the excitatory amino acid antagonist kynurenate. The N-methyl-D-aspartate receptor antagonist D-2-amino-5-phosphonovalerate had little effect on Tat-induced depolarizations but did provide protection from Tat neurotoxicity. These results suggest that Tat, released from HIV-1-infected cells, may be an important mediator of neurotoxicity observed in HIV-1 encephalopathy.

Spatio-temporal summation of synaptic activity in visual cortical pyramidal cells in vitro

We investigated the spatio-temporal summation of excitatory postsynaptic potentials (EPSPs) in supragranular pyramidal cells of the rat extrastriate visual cortex. EPSPs were evoked orthodromically from different locations within the white matter (WM) via an 8-fold multi-electrode array. Stimuli were applied either sequentially from electrodes 1 to 8 or vice versa at defined interstimulus intervals (ISIs) or separately from each electrode. Maximum EPSP amplitudes were evoked from the WM just below the intracellularly recorded neuron. Even 800 microns lateral to this location, small EPSPs could be elicited. A sequential stimulation resulted in a large compound EPSP. In 79% (n = 34) of the cells tested, the compound response was non-directional and could be predicted from responses evoked by single stimulation electrodes. However, 21% (n = 9) of the neurons showed a non-linear spatial summation and a clear preference for the direction of the stimulation sequence. ISI-tuning curves revealed either a sharply tuned, a bandpass, a highpass or a lowpass characteristic for the non-directional as well as directional cells. This feature, together with the clear directional responses observed in some neurons, may be a correlate of the response preference to moving stimuli of cortical cells found in vivo.