Target cell-specific modulation of neuronal activity by astrocytes

Interaction between astrocytes and neurons enriches the behavior of brain circuits. By releasing glutamate and ATP, astrocytes can directly excite neurons and modulate synaptic transmission. In the rat olfactory bulb, we demonstrate that the release of GABA by astrocytes causes long-lasting and synchronous inhibition of mitral and granule cells. In addition, astrocytes release glutamate, leading to a selective activation of granule-cell NMDA receptors. Thus, by releasing excitatory and inhibitory neurotransmitters, astrocytes exert a complex modulatory control on the olfactory network.

Er81 is expressed in a subpopulation of layer 5 neurons in rodent and primate neocortices

Laminar organization is a fundamental cytoarchitecture in mammalian CNS and a striking feature of the neocortex. ER81, a transcription factor, has recently been utilized as a marker of cells in the layer 5 of the neocortex. We further pursued the distribution of ER81 to investigate the identity of the ER81-expressing cells in the brain. Er81 transcript was expressed in a subset of pyramidal cells that were scattered throughout the entire width of layer 5. In the rat cortex, Er81 transcripts were first detected in the ventricular zone at E15, remained expressed in putative prospective layer 5 neurons during infant and juvenile stages. The ER81-expressing subpopulation in adult layer 5 neurons did not segregate with the phenotypes of the projection targets. By retrograde labeling combined with immunohistochemistry or reverse transcription-polymerase chain reaction analysis, we found ER81 expression in nearly all of the layer 5 neurons projecting to the spinal cord or to the superior colliculus, while in only one-third of the layer 5 neurons projecting to the contralateral cortex. Er81 was also detected in layer 5 neurons in a P2 Japanese macaque monkey but not in adult monkey cortices. These findings suggest that a neuron class defined by a molecular criterion does not necessarily segregate with that defined by an anatomical criterion, that ER81 is involved in cell differentiation of a subset of layer 5 projection neurons and that this mechanism is conserved among rodents and primates.

Action potential propagation in dendrites of rat mitral cells in vivo

Odors evoke beta-gamma frequency field potential oscillations in the olfactory systems of awake and anesthetized vertebrates. In the rat olfactory bulb, these oscillations reflect the synchronous discharges of mitral cells that result from both their intrinsic membrane properties and their dendrodendritic interactions with local inhibitory interneurons. Activation of dendrodendritic synapses is purportedly involved in odor memory and odor contrast enhancement. Here we investigate in vivo to what extent action potentials propagate to remote dendrodendritic sites in the entire dendritic tree and if this propagation is changed during discharges at 40 Hz. By combining intracellular recording and two-photon microscopy imaging of intracellular calcium ([Ca2+]i), we show that in remote branches of the apical tuft and basal dendrites, transient Ca2+ changes are triggered by single sodium action potentials. Neither the amplitude of these Ca2+ transients nor that of action potentials obtained from intradendritic recordings showed a significant attenuation as a function of the distance from the soma. Calcium channel density seemed homogeneous; however, propagating action potentials occasionally failed to trigger a Ca2+ transient at a site closer to the soma whereas it did farther. This suggests that measurements of calcium transients underestimate the occurrence of sodium action potentials. During 40 Hz bursts of action potentials, [Ca2+]i increases with the number of action potentials in all dendritic compartments. These results suggest that the presence of release sites in dendrites is accompanied by an "axonal-like behavior" of the entire dendritic tree of mitral cells, including their most distal dendritic branches.

Climbing Fibre Responses in Olivo-cerebellar Slice Cultures. I. Microelectrode Recordings from Purkinje Cells

Cerebellar slices prepared from newborn rats were co-cultured with slices derived from the inferior olive of 4-day-old rats. After several weeks in vitro olivary fibres projecting into the cerebellar tissue could be assessed by anterograde labelling with the fluorescent dye 1,1-dioctadecyl-3,3,3,3'-tetramethylindocarbocyanine perchlorate (Dil). Following electrical field stimulation of the olivary tissue, all-or-nothing complex spikes were generated in Purkinje cells, which closely resembled climbing fibre responses as seen in situ. These responses were completely and reversibly abolished by 6-cyano-7-nitroquinoxaline-2-3-dione (CNQX, 5 microM), an antagonist of non-N-methyl-d-aspartate excitatory amino acid receptors. Wash in of smaller concentrations of CNQX (0.5 - 2 microM) resulted in a graded dose-dependent depression of the climbing fibre-induced postsynaptic potentials and in a consecutive failure of distinct active components of the complex spikes. With climbing fibre synaptic transmission blocked by CNQX, complex spike-like potentials could, however, still be evoked by intrasomatic injection of depolarizing current pulses. Increasing the concentration of Mg2+ in the bathing solution from 0.5 to up to 8 mM depressed regenerative complex-spike components. Olivary stimulation elicited only monophasic postsynaptic potentials in Purkinje cells under these conditions. These observations indicate that voltage-gated conductances which are substantially involved in the generation of the complex spike, are gated by the climbing fibre synaptic depolarization rather than directly by the climbing fibre transmitter.

Kainate receptors regulate unitary IPSCs elicited in pyramidal cells by fast-spiking interneurons in the neocortex

Unitary IPSCs elicited by fast-spiking (FS) interneurons in layer V pyramidal cells of the neocortex were studied by means of dual whole-cell recordings in acute slices. FS to pyramidal cell unitary IPSCs were depressed by (RS)-S-amino-3-(3-hydroxy-5-tert-butylisoxazol-4-yl) (ATPA), a kainate (KA) receptor agonist, and by the endogenous agonist l-glutamate in the presence of AMPA, NMDA, mGluR, and GABA(B) receptor antagonists. This effect was accompanied by an increase in failure rate of synaptic transmission, in the coefficient of variation, and in the paired pulse ratio, indicating a presynaptic origin of the IPSC depression. Pairing the activation of the presynaptic neuron with a depolarization of the postsynaptic cell mimicked the decrease of unitary IPSCs, and this effect persisted when postsynaptic sodium action potentials were blocked with the local anesthetic QX314. The effects of ATPA, glutamate, and of the pairing protocol were almost totally blocked by CNQX. These data suggest that KA receptors located on presynaptic FS cell terminals decrease the release of GABA and can be activated by glutamate released from the somatodendritic compartment of the postsynaptic pyramidal cells.

Classification of fusiform neocortical interneurons based on unsupervised clustering

A classification of fusiform neocortical interneurons (n = 60) was performed with an unsupervised cluster analysis based on the comparison of multiple electrophysiological and molecular parameters studied by patch-clamp and single-cell multiplex reverse transcription-PCR in rat neocortical acute slices. The multiplex reverse transcription-PCR protocol was designed to detect simultaneously the expression of GAD65, GAD67, calbindin, parvalbumin, calretinin, neuropeptide Y, vasoactive intestinal peptide (VIP), somatostatin (SS), cholecystokinin, alpha-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid, kainate, N-methyl-d-aspartate, and metabotropic glutamate receptor subtypes. Three groups of fusiform interneurons with distinctive features were disclosed by the cluster analysis. The first type of fusiform neuron (n = 12), termed regular spiking nonpyramidal (RSNP)-SS cluster, was characterized by a firing pattern of RSNP cells and by a high occurrence of SS. The second type of fusiform neuron (n = 32), termed RSNP-VIP cluster, predominantly expressed VIP and also showed firing properties of RSNP neurons with accommodation profiles different from those of RSNP-SS cells. Finally, the last type of fusiform neuron (n = 16) contained a majority of irregular spiking-VIPergic neurons. In addition, the analysis of glutamate receptors revealed cell-type-specific expression profiles. This study shows that combinations of multiple independent criteria define distinct neocortical populations of interneurons potentially involved in specific functions.

Identification of sleep-promoting neurons in vitro

The neurons responsible for the onset of sleep are thought to be located in the preoptic area and more specifically, in the ventrolateral preoptic nucleus (VLPO). Here we identify sleep-promoting neurons in vitro and show that they represent an homogeneous population of cells that must be inhibited by systems of arousal during the waking state. We find that two-thirds of the VLPO neurons are multipolar triangular cells that show a low-threshold spike. This proportion matches that of cells active during sleep in the same region. We then show, using single-cell reverse transcriptase followed by polymerase chain reaction, that these neurons probably contain gamma-aminobutyric acid (GABA). We also show that these neurons are inhibited by noradrenaline and acetylcholine, both of which are transmitters of wakefulness. As most of these cells are also inhibited by serotonin but unaffected by histamine, their overall inhibition by transmitters of wakefulness is in agreement with their relative inactivity during waking with respect to sleep. We propose that the reciprocal inhibitory interaction of such VLPO neurons with the noradrenergic, serotoninergic and cholinergic waking systems to which they project is a key factor for promoting sleep.

Postsynaptic glutamate receptors and integrative properties of fast-spiking interneurons in the rat neocortex

The glutamate-mediated synaptic responses of neocortical pyramidal cell to fast-spiking interneuron (pyramidal-FS) connections were studied by performing paired recordings at 30-33 degrees C in acute slices of 14- to 35-day-old rats (n = 39). Postsynaptic fast-spiking (FS) cells were recorded in whole cell configuration with a patch pipette, and presynaptic pyramidal cells were impaled with sharp intracellular electrodes. At a holding potential of -72 mV (near the resting membrane potential), unitary excitatory postsynaptic potentials (EPSPs) had a mean amplitude of 2.1 +/- 1.3 mV and a mean width at half-amplitude of 10.5 +/- 3.7 ms (n = 18). Bath application of the N-methyl-D-aspartate (NMDA) receptor antagonist D(-)2-amino-5-phosphonovaleric acid (D-AP5) had minor effects on both the amplitude and the duration of unitary EPSPs, whereas the alpha-amino-3-hydroxy-5-methyl-4-isoxazole-propionate (AMPA)/kainate receptor antagonist 6-cyano-7-nitroquinoxaline-2,3-dione (CNQX) almost completely blocked the synaptic responses. In voltage-clamp mode, the selective antagonist of AMPA receptors 1-(4-aminophenyl)-3-methylcarbamyl-4-methyl-7,8-methylenedioxy-3, 4-dihydro-5H-2,3-benzodiazepine (GYKI 53655; 40-66 microM) blocked 96 +/- 1.9% of D-AP5-insensitive unitary excitatory postsynaptic currents (EPSCs), confirming the predominance of AMPA receptors, as opposed to kainate receptors, at pyramidal-FS connections (n = 3). Unitary EPSCs mediated by AMPA receptors had fast rise times (0.29 +/- 0.04 ms) and amplitude-weighted decay time constants (2 +/- 0.8 ms; n = 16). In the presence of intracellular spermine, these currents showed the characteristic rectifying current-voltage (I-V) curve of calcium-permeable AMPA receptors. A slower component mediated by NMDA receptors was observed when unitary synaptic currents were recorded at a membrane potential more positive than -50 mV. In response to short trains of moderately high-frequency (67 Hz) presynaptic action potentials, we observed only a limited temporal summation of unitary EPSPs, probably because of the rapid kinetics of AMPA receptors and the absence of NMDA component in these subthreshold synaptic responses. By combining paired recordings with extracellular stimulations (n = 11), we demonstrated that EPSPs elicited by two different inputs were summed linearly by FS interneurons at membrane potentials below the action potential threshold. We estimated that, in our in vitro recording conditions, 8 +/- 5 pyramidal cells (n = 18) should be activated simultaneously to make FS interneurons fire an action potential from -72 mV. The low level of temporal summation and the linear summation of excitatory inputs in FS cells favor the role of coincidence detectors of these interneurons in neocortical circuits.

Selective excitation of subtypes of neocortical interneurons by nicotinic receptors

The cellular mechanisms by which neuronal nicotinic cholinergic receptors influence many aspects of physiology and pathology in the neocortex remain primarily unknown. Whole-cell recordings and single-cell reverse transcription (RT)-PCR were combined to analyze the effect of nicotinic receptor agonists on different types of neurons in acute slices of rat neocortex. Nicotinic receptor agonists had no effect on pyramidal neurons and on most types of interneurons, including parvalbumin-expressing fast spiking interneurons and somatostatin-expressing interneurons, but selectively excited a subpopulation of interneurons coexpressing the neuropeptides vasoactive intestinal peptide (VIP) and cholecystokinin. This excitation persisted in the presence of glutamate, GABA, and muscarinic receptor antagonists and in the presence of tetrodotoxin and low extracellular calcium, suggesting that the depolarization was mediated through the direct activation of postsynaptic nicotinic receptors. The responses were blocked by the nicotinic receptor antagonists dihydro-beta-erythroidine and mecamylamine and persisted in the presence of the alpha7 selective nicotinic receptor antagonist methyllycaconitine, suggesting that the involved nicotinic receptors lacked the alpha7 subunit. Single-cell RT-PCR analysis indicated that the majority of the interneurons that responded to nicotinic stimulation coexpressed the alpha4, alpha5, and beta2 nicotinic receptor subunits. Therefore, these results provide a role for non-alpha7 nicotinic receptors in the selective excitation of a subpopulation of neocortical interneurons. Because the neocortical interneurons expressing VIP have been proposed previously to regulate regional cortical blood flow and metabolism, these results also provide a cellular basis for the neuronal regulation of cortical blood flow mediated by acetylcholine.

Developmental synaptic changes increase the range of integrative capabilities of an identified excitatory neocortical connection

Excitatory synaptic transmission between pyramidal cells and fast-spiking (FS) interneurons of layer V of the motor cortex was investigated in acute slices by using paired recordings at 30 degrees C combined with morphological analysis. The presynaptic and postsynaptic properties at these identified central synapses were compared between 3- and 5-week-old rats. At these two postnatal developmental stages, unitary EPSCs were mediated by the activation of AMPA receptors with fast kinetics at a holding potential of -72 mV. The amplitude distribution analysis of the EPSCs indicates that, at both stages, pyramidal-FS connections consisted of multiple functional release sites. The apparent quantal size obtained by decreasing the external calcium ([Ca2+]e) varied from 11 to 29 pA near resting membrane potential. In young rats, pairs of presynaptic action potentials elicited unitary synaptic responses that displayed paired-pulse depression at all tested frequencies. In older animals, inputs from different pyramidal cells onto the same FS interneuron had different paired-pulse response characteristics and, at most of these connections, a switch from depression to facilitation occurred when decreasing the rate of presynaptic stimulation. The balance between facilitation and depression endows pyramidal-FS connections from 5-week-old animals with wide integrative capabilities and confers unique functional properties to each synapse.

Properties of bipolar VIPergic interneurons and their excitation by pyramidal neurons in the rat neocortex

In the rat neocortex, a subset of GABAergic interneurons express the neuropeptide vasoactive intestinal peptide (VIP). Previously, we demonstrated that a population of VIPergic interneurons could be accurately identified by their irregular spiking (IS) pattern and their bipolar morphology. IS interneurons were studied in neocortical slices from 16-22-day-old rats using whole-cell recordings, intracellular labelling and single-cell RT-PCR. In response to a depolarizing pulse, IS interneurons typically discharged a burst of action potentials followed by spikes emitted at an irregular frequency. Several seconds of depolarization, micromolar concentrations of 4-aminopyridine, and nanomolar concentrations of either dendrotoxin I or K converted this irregular pattern to a sustained discharge, suggesting the involvement of an ID-like K+ current. The main glutamate receptor subunits detected in IS cells were GluR1 flop and GluR2 flop, GluR5 and GluR6, and NR2B and NR2D for the alpha-amino-3-hydroxyl-5-methyl-4-isoxazolepropionic acid (AMPA), kainate and N-methyl-D-aspartic acid (NMDA) subtypes, respectively. Paired whole-cell patch-clamp recordings indicated that pyramidal neurons provide intracortical glutamatergic inputs onto IS interneurons. Most connections had high probabilities of response and exhibited frequency-dependent paired pulse depression. Comparison of the amplitude distribution of paired responses suggested that most of these connections consisted of multiple functional release sites. Finally, two discrete subpopulations of IS cells could be identified based on the duration of the initial burst of action potentials and the differential expression of calretinin and choline acetyltransferase.

Cardiac arrest in rodents: maximal duration compatible with a recovery of neuronal activity

We report here that during a permanent cardiac arrest, rodent brain tissue is "physiologically" preserved in situ in a particular quiescent state. This state is characterized by the absence of electrical activity and by a critical period of 5-6 hr during which brain tissue can be reactivated upon restoration of a simple energy (glucose/oxygen) supply. In rat brain slices prepared 1-6 hr after cardiac arrest and maintained in vitro for several hours, cells with normal morphological features, intrinsic membrane properties, and spontaneous synaptic activity were recorded from various brain regions. In addition to functional membrane channels, these neurons expressed mRNA, as revealed by single-cell reverse transcription-PCR, and could synthesize proteins de novo. Slices prepared after longer delays did not recover. In a guinea pig isolated whole-brain preparation that was cannulated and perfused with oxygenated saline 1-2 hr after cardiac arrest, cell activity and functional long-range synaptic connections could be restored although the electroencephalogram remained isoelectric. Perfusion of the isolated brain with the gamma-aminobutyric acid A receptor antagonist picrotoxin, however, could induce self-sustained temporal lobe epilepsy. Thus, in rodents, the duration of cardiac arrest compatible with a short-term recovery of neuronal activity is much longer than previously expected. The analysis of the parameters that regulate this duration may bring new insights into the prevention of postischemic damages.

Subunit composition, kinetic, and permeation properties of AMPA receptors in single neocortical nonpyramidal cells

Native AMPA receptors (AMPARs) were investigated in neocortical fast-spiking (FS) and regular-spiking nonpyramidal (RSNP) cells. The onset of and recovery from desensitization as well as current rectification and single-channel conductance were studied by using fast glutamate application to outside-out patches. The GluR1-4 subunit, flip/flop splicing, and R/G editing expression patterns of functionally characterized cells were determined by single-cell reverse transcription-PCR to correlate the subunit composition of native AMPARs with their functional properties. Our sample, mostly constituted by RSNP neurons, predominantly expressed GluR3 flip and GluR2 flop. In individual cells, flip/flop splicing of each subunit appeared to be regulated independently, whereas for R/G editing all subunits were either almost fully edited or unedited. We confirmed that the relative GluR2 expression controls the permeation properties of native AMPARs, whereas none of the single molecular parameters considered appeared to be a key determinant of the kinetics. FS neurons displayed AMPARs with relatively homogeneous functional properties characterized by fast desensitization, slow recovery from desensitization, marked inward rectification, and large single-channel conductance. In contrast, these parameters varied over a wide range in RSNP neurons, and their combination resulted in various AMPAR functional patterns. Indeed, in different cells, fast or slow desensitization was found to be associated with either slow or fast recovery from desensitization. Similarly, fast or slow kinetics was associated with either strong or weak rectification. Our results suggest that kinetic and permeation properties of native AMPARs can be regulated independently in cortical neurons and probably do not have the same molecular determinants.

Molecular and physiological diversity of cortical nonpyramidal cells

The physiological and molecular features of nonpyramidal cells were investigated in acute slices of sensory-motor cortex using whole-cell recordings combined with single-cell RT-PCR to detect simultaneously the mRNAs of three calcium binding proteins (calbindin D28k, parvalbumin, and calretinin) and four neuropeptides (neuropeptide Y, vasoactive intestinal polypeptide, somatostatin, and cholecystokinin). In the 97 neurons analyzed, all expressed mRNAs of at least one calcium binding protein, and the majority (n = 73) contained mRNAs of at least one neuropeptide. Three groups of nonpyramidal cells were defined according to their firing pattern. (1) Fast spiking cells (n = 34) displayed tonic discharges of fast action potentials with no accommodation. They expressed parvalbumin (n = 30) and/or calbindin (n = 19) mRNAs, and half of them also contained transcripts of at least one of the four neuropeptides. (2) Regular spiking nonpyramidal cells (n = 48) displayed a firing behavior characterized by a marked accommodation and presented a large diversity of expression patterns of the seven biochemical markers. (3) Finally, a small population of vertically oriented bipolar cells, termed irregular spiking cells (n = 15), fired bursts of action potentials at an irregular frequency. They consistently co-expressed calretinin and vasoactive intestinal polypeptide. Additional investigations of these cells showed that they also co-expressed glutamic acid decarboxylase and choline acetyl transferase. Our results indicate that neocortical nonpyramidal neurons display a large diversity in their firing properties and biochemical patterns of co-expression and that both characteristics could be correlated to define discrete subpopulations.

Functional and molecular analysis of glutamate-gated channels by patch-clamp and RT-PCR at the single cell level

In the central nervous system (CNS) rapid excitatory neurotransmission is mainly mediated by ligand gated, cationic channels activated by glutamate. Three main subtypes of glutamate-gated channels have been characterized by pharmacological studies. They have been named according to their preferred agonist, N-methyl-D-aspartate (NMDA), high affinity kainate and alpha-amino-3-hydroxy-5-methyl-4-isoxazolepropionate (AMPA). Furthermore, a large diversity within each class of glutamate-gated channels has been revealed by the molecular cloning of multiple subunits and their spliced and edited variants (for review see Wisden and Seeburg, 1993). These subunits can potentially form different oligomeric complexes with diverging properties. A crucial question is therefore to determine the actual subunit composition of naturally occurring glutamate receptors. We have combined patch-clamp recording, reverse transcription (RT) and PCR to correlate, at the single cell level, the pattern of subunits expression with the functional properties of native glutamate receptors. We describe here results obtained on the AMPA receptors of hippocampal neurones and on the NMDA receptors of cerebellar granule cells which show that the subunit composition of these two types of receptors explains some of their functional properties. Furthermore, our data also indicate that the expression of NMDA receptor subunits during the postnatal development of cerebellar granule cells is regulated by an activity-dependent mechanism.

Neuronal activity differentially regulates NMDA receptor subunit expression in cerebellar granule cells

Reverse-transcription PCR assays were used to measure levels of NMDA receptor (NR) subunit mRNAs encoding splice variants of NR1 (NR1a, -exon 5; NR1b, +exon 5) and the major NR2 subunits (NR2A, NR2B, and NR2C) in dissociated cerebellar granule cell cultures. Cultures chronically exposed to 25 mM KCl or 100 microM NMDA/15 mM KCl, which promote survival by stimulating Ca2+ influx through voltage-sensitive Ca2+ channels or NRs, were compared with 5 mM KCl culture conditions, which results in limited cell survival attributable to a lower level of NR stimulation by ambient glutamate. In situ granule-cell maturation is associated with downregulation of NR2B and increases both of NR2A and NR2C and in the ratio of NR1b/NR1a mRNAs. In culture, 25 mM KCl or NMDA rapidly induced NR2A and downregulated NR2B, followed by gradual induction of NR2C. In 5 mM KCl, a similar, rapid increase in NR2A was observed, but disappearance of NR2B occurred over a longer time course. By 9-12 d in vitro in 5 mM KCl, the relative proportions of all three NR2 mRNAs in surviving cells were not significantly different from cells cultured in 25 mM KCl. NR1a mRNA predominated at every stage of culture in 25 mM KCl or NMDA, however, whereas gradual induction of the mature-form NR1b was observed in 5 mM KCl. Although using high potassium- or NMDA-containing media enhanced granule cell survival, it did not reproduce the pattern of expression of NR mRNAs observed in situ, whereas this pattern was observed in granule cells surviving in 5 mM KCl.

Diversity of glutamate receptors in neocortical neurons: implications for synaptic plasticity

The biochemical and functional characteristics of the AMPA subtype of the glutamate receptors expressed by pyramidal and non-pyramidal neurons of the neocortex have been studied in acute slices by means of single-cell RT-PCR and fast applications of glutamate on outside-out patches. Our results suggest that the predominant expression of the flop splice variants of the GluR1-4 AMPA subunits contributes to the faster desensitization of these receptors in non-pyramidal neurons compared to pyramidal cells where flip variants of GluR1-4 are dominant. Alternative splicing of AMPA receptors may therefore play an important role in regulating synaptic function in a cell-type specific manner.

Calcium-dependent, slowly inactivating potassium currents in cultured neurons of rat neocortex

Slowly inactivating outward currents were examined in neurons from rat anterior cortex dissociated at postnatal day 1 and recorded after 7-48 days in vitro by the use of whole-cell patch-clamp technique, in the presence of 0.5-0.8 microM tetrodotoxin (TTX). 50 microM carbachol and 1-5 mM CsCl2. Experiments were often carried out in the additional presence of 1-5 mM CsCl2, which blocks the anomalous, inwardly rectifying IQ, the fast Ca(2+)-dependent K+ current (IC), and 50 microM carbachol, which depresses the IM current. These currents were evoked by depolarizing steps to -40 +/- 5 mV from a conditioning hyperpolarization to -110 +/- 10 mV. Their sensitivity to elevation from 2.5 to 12.5 mM in extracellular K+ concentration, together with their sensitivity to 5-15 mM tetraethylammonium, suggests that they are mainly carried by K+ ions. Their activation and inactivation curves show that the threshold for activation is -65 mV, that their inactivation is achieved at -75 mV and that potentials more negative than -120 mV are needed to abolish it. The time-dependence of de-inactivation gives a maximal current amplitude for conditioning hyperpolarizations of 2 s and is best described by a monoexponential function with a time constant of 0.7 s. Slow transient K+ currents were depressed by low doses of 4-aminopyridine (30-100 microM), which indicates the occurrence of an ID-type component in the recorded K+ currents. No slowly declining K+ current was expressed when a recording solution containing 10 mM 1,2-bis (2-aminophenoxy)ethane-N,N,N'-N'-tetraacetic acid (BAPTA), instead of 1-5 mM BAPTA, was used. When recorded without Ca2+ chelator in the pipette, slowly declining K+ currents were blocked by bath-applied 40-50 microM BAPTA-aminoethoxy, revealing a large-amplitude, rapidly inactivating outward current. This residual component is insensitive to 50 microM 4-aminopyridine and may include a current more related to the IA-type. Our data provide evidence that, in cultured cortical neurons from rat, the expression of an ID-like K+ current is highly dependent on internal Ca2+ concentration.

Single cell RT-PCR proceeds without the risk of genomic DNA amplification

We have previously described a method for detection of mRNAs expressed in single cells after patch-clamp recordings. The method, termed single cell RT-PCR, involves aspiration of the cell content, a reverse transcription (RT) step, and a polymerase chain reaction (PCR) using specific primers. Since the nucleus is frequently harvested together with the cytosol, genomic DNA may generate false positive results. Thus, we demonstrated that dilutions containing a few copies of plasmid could be detected by PCR in a range which, according to the Poisson law, suggests that the PCR method can amplify from the two genomic alleles. We performed single cell RT-PCR of intronless GluR2 or GluR5 fragments by comparing cerebellar cell types where these mRNAs are known to be present or absent. For each cell the nucleus was harvested together with the cytosol. Following RT-PCR with GluR5 primers, all Purkinje cells (n = 6) yielded the expected PCR product, whereas it was not generated from any of the granule cells (n = 5). In corresponding experiments with GluR2 primers, we obtained the GluR2 product from all Purkinje cells (n = 5), but not from any of the glial cells (n = 5). These results are in agreement with the known cellular expression of GluR2 and GluR5 mRNAs. We conclude that the single cell RT-PCR method does not amplify the genomic DNA when the nucleus is aspirated together with the cytosol. We suggest that genomic DNA amplification is avoided, because the genomic alleles are not exposed during the procedure.

Evidence for two types of non-NMDA receptors in rat cerebellar Purkinje cells maintained in slice cultures

Pharmacological properties of non-NMDA receptors were investigated in Purkinje cells grown in rat cerebellar slice cultures and recorded in the whole-cell configuration of the patch-clamp technique. Dose-response curves for AMPA and domoate suggest that AMPA, in the concentration range tested, activated only AMPA receptors whereas, domoate activated two types of receptors, probably AMPA and kainate receptors, with EC50 values of 8 and 0.5 microM, respectively. The Scatchard analysis of the dose-response relationship for domoate also suggest that both kainate and AMPA receptors were activated by domoate with approximate affinities of 5 and 0.07 microM-1, respectively. The non-competitive non-NMDA receptors antagonist, GYKI 52466, reduced the amplitude of both AMPA- and domoate-activated currents, with a greater potency in reducing currents evoked by AMPA (IC50 = 10 microM) than those induced by domoate (IC50 = 105 microM). These results suggest that, in addition to AMPA receptors, Purkinje cells express kainate receptors and that these two types of non-NMDA receptors can be distinguished from each other on the basis of several pharmacological properties, including affinity for AMPA, domoate and GYKI 52466.

Afferent connections of the medial frontal cortex of the rat. II. Cortical and subcortical afferents

In order to compare the frontal cortex of rat and macaque monkey, cortical and subcortical afferents to subdivisions of the medial frontal cortex (MFC) in the rat were analyzed with fluorescent retrograde tracers. In addition to afferent inputs common to the whole MFC, each subdivision of the MFC has a specific pattern of afferent connections. The dorsally situated precentral medial area (PrCm) was the only area to receive inputs from the somatosensory cortex. The specific pattern of afferents common to the ventrally situated prelimbic (PL) and infralimbic (IL) areas included projections from the agranular insular cortex, the entorhinal and piriform cortices, the CA1-CA2 fields of the hippocampus, the subiculum, the endopiriform nucleus, the amygdalopiriform transition, the amygdalohippocampal area, the lateral tegmentum, and the parabrachial nucleus. In all these structures, the number of retrogradely labeled cells was larger when the injection site was located in area IL. The dorsal part of the anterior cingulate area (ACd) seemed to be connectionally intermediate between the adjacent areas PrCm and PL; it receives neither the somatosensory inputs characteristic of area PrCm nor the afferents characteristic of areas PL and IL, with the exception of the afferents from the caudal part of the retrosplenial cortex. A comparison of the pattern of afferent and efferent connections of the rat MFC with the pattern of macaque prefrontal cortex suggests that PrCm and ACd areas share some properties with the macaque premotor cortex, whereas PL and IL areas may have characteristics in common with the cingulate or with medial areas 24, 25, and 32 and with orbital areas 12, 13, and 14 of macaques.

Activity-dependent regulation of N-methyl-D-aspartate receptor subunit expression in rat cerebellar granule cells

The glutamate receptor channels of the N-methyl-D-aspartate (NMDA) subtype are composed of different subunits named NR1 and NR2A-D. These subunits can combine in different oligomers with diverging properties and their expression is developmentally regulated. We have used rat cerebellar slice cultures to test the involvement of bioelectrical activity and synaptic transmission in the changes in NR2A-C expression observed in developing granule cells. A correlation between the functional properties of the NMDA receptors and expression of the NR2A-C mRNAs was obtained in single granule cells by coupling patch-clamp recording and reverse transcription followed by polymerase chain reaction. Granule cells grown under standard culture conditions expressed mainly NR2A mRNA when examined after 15-40 days in vitro. Consistent with this observation, their responses to NMDA were only weakly reduced by 3 microM ifenprodil, a non-competitive antagonist which discriminates between NR2A and NR2B subunits in expression systems. In cerebellar cultures chronically exposed to tetrodotoxin to eliminate spontaneous electrical activity, granule cells maintained a predominant expression of NR2B subunits and their responses to NMDA were largely inhibited by 3 microM ifenprodil. These results provide evidence that the expression of the NR2A and B subunits is regulated through an activity-dependent mechanism leading to the formation of NMDA receptors with different pharmacological properties. Finally, the NR2C subunit, abundantly expressed in vivo by adult granule cells, was only rarely detected in slice cultures, even when excitatory synapses were formed between granule cells and fibres originating from co-cultured brainstem explants. These data suggest that the induction of NR2C expression observed in vivo requires an additional factor(s) that remains to be identified.

Cellular locus of the nitric oxide-synthase involved in cerebellar long-term depression induced by high external potassium concentration

The cellular location of the NO-synthase involved in long-term depression (LTD) of parallel fiber (PF)-mediated EPSCs induced by raising the external potassium (K) concentration has been investigated by using both whole-cell patch-clamp recordings (WCR) of Purkinje cells (PCs) in thin slices in vitro, and reverse transcription followed by polymerase chain reaction (PCR) applied to mRNAs harvested from these single PCs during WCR. In all tested cells in the control group, a large LTD of PF-mediated EPSCs was induced by perfusing the slices for 3 min with a high (30 mM) K perfusing medium. In a second group of cells for which the protein kinase C (PKC) inhibitor peptide 19-36 was added to the intrapipette solution at a concentration of 10 microM, the LTD following complete wash out of the high K solution was significantly less prominent than in the control group. Very similar results were also obtained when 30 microM NG-methyl-L-arginine (L-NMMA) was added to the perfusing medium. In contrast, when both the PKC inhibitor peptide 19-36 and L-NMMA were added to the intrapipette solution at a concentration of 10 and 30 microM respectively, no LTD was revealed following wash out of the high K solution. Finally, the PCR amplification of mRNAs harvested from these single PCs during WCR, as well as from granule cells from the same slices, confirms that mRNAs encoding the NO-synthase are expressed by granule cells, whereas they are not detected in PCs.

Subunit composition at the single-cell level explains functional properties of a glutamate-gated channel

The diversity of known glutamate-gated channels has been markedly increased by the discovery of multiple subunits and their spliced and edited variants. These subunits can potentially form different oligomeric complexes with diverging properties. A crucial question is therefore to determine the actual subunit composition of naturally occurring glutamate receptors. We have coupled patch-clamp recordings and reverse transcription followed by PCR amplification to correlate the presence of mRNAs for each subunit and the functional properties of native glutamate receptors at the single-cell level. In a homogeneous population of functionally identified hippocampal neurons (type II) in culture bearing a glutamate receptor of the AMPA subtype with a high calcium permeability, we found that, among the multiple subunits, only two, the flop forms of GluR1 and GluR4, were expressed. In particular, GluR2 was never detected. This composition explains the uncommon properties of AMPA receptors in type II neurons.

AMPA receptor subunits expressed by single Purkinje cells

Several subunits of the glutamate receptor of the AMPA subtype have been cloned recently. These subunits, named GluR1, GluR2, GluR3, and GluR4, exist as two splicing variants (flip and flop). We have determined the subset of AMPA receptor subunits expressed by single cerebellar Purkinje cells in culture. This was achieved by combining whole-cell patch-clamp recordings and a molecular analysis, based on the polymerase chain reaction, of the messenger RNAs harvested into the patch pipette at the end of each recording. We found that each single cell expresses the messenger RNAs encoding the following five subunits: the flip and flop versions of GluR1 and GluR2 as well as GluR3flip, GluR2 being the most abundant. In addition, GluR3flop and GluR4flip were scarcely expressed in half of these neurons, and GluR4flop was never detected.

Excitatory synaptic potentials in neurons of the deep nuclei in olivo-cerebellar slice cultures

Excitatory postsynaptic potentials evoked in neurons of the deep cerebellar nuclei, either by electrical stimulation within the nuclei in cerebellar slice cultures or by electrical stimulation of olivary explants in olivo-cerebellar co-cultures, were investigated in the rat by means of intracellular recordings. In neurons of the deep cerebellar nuclei, stimulation of the nuclear tissue, as well as stimulation of the olivary tissue, induced a fast rising excitatory postsynaptic potential, followed by an inhibitory postsynaptic potential and a long-lasting excitation. The fast rising excitatory postsynaptic potential and the following inhibitory postsynaptic potential were blocked by 6-cyano-7-nitroquinoxaline-2,3-dione. The remaining depolarization was abolished by D-(-)-2-amino-5-phosphonovalerate, suggesting that this potential was mediated by N-methyl-D-aspartate receptors. With only D-(-)-2-amino-5-phosphonovalerate added to the bath, the slow excitation was depressed, whereas the fast excitatory and inhibitory postsynaptic potentials were not affected. In the presence of bicuculline, the 6-cyano-7-nitroquinoxaline-2,3-dione- and the D-(-)-2-amino-5-phosphonovalerate-sensitive excitatory postsynaptic potentials elicited by stimulation of the olivary tissue had the same latency, and were both graded with stimulation strength. The time-to-peak and the duration of the D-(-)-2-amino-5-phosphonovalerate-sensitive excitatory postsynaptic potentials were considerably longer than those of the 6-cyano-7-nitroquinoxaline-2,3-dione-sensitive excitatory postsynaptic potentials.(ABSTRACT TRUNCATED AT 250 WORDS)

Reduction of desensitization of a glutamate ionotropic receptor by antagonists

The glutamate receptor channel subtype that responds to both quisqualate (QA) and alpha-amino-3-hydroxy-5-methyl-isoxazole-4-propionate (AMPA) was expressed in Xenopus oocytes injected with rat cerebral cortex mRNA. Voltage-clamp current responses to QA, AMPA, and glutamate (GLU) exhibited a rapid increase followed by a decrease to a desensitized steady state (DS). Perfusion with high agonist concentrations produced smaller DS responses than perfusion with low concentrations. During the DS, the current was increased by lowering of the concentration of agonist or by application of low concentrations of a competitive antagonist, 6,7-dinitroquinoxaline-2,3-dione (DNQX). This paradoxical increase of the agonist-induced currents during the DS was also observed in cultured Purkinje cells with another competitive antagonist, 6-cyano-7-nitro-quinoxaline-2,3-dione (CNQX). Dose-response curves obtained in oocytes were bell shaped, with a negative slope for high concentrations of QA. DNQX shifted these bell-shaped curves to the right. Together, these results indicate that the agonists are able to reversibly inhibit the AMPA receptor. The classical desensitization model of Katz and Thesleff [J. Physiol. (Lond.) 138:63-80 (1957)] cannot account for our observations.

Responses to excitatory amino acids of Purkinje cells' and neurones of the deep nuclei in cerebellar slice cultures

1. The actions of the endogenous excitatory amino acids (EAAS) glutamate (Glu), aspartate (Asp) and homocysteate (HCA) on Purkinje cells and neurones of the deep nuclei in cerebellar slice cultures were investigated using intracellular recordings in the single-electrode voltage-clamp mode and the whole-cell configuration of the patch-clamp technique. 2. Purkinje cells and neurones of deep cerebellar nuclei were identified according to their localization in the living cultures, their morphology as revealed by intracellular injections of Lucifer Yellow and their immunoreactivity to antibodies to the 28 kDa Ca2(+)-binding protein. 3. When Purkinje cells were voltage-clamped near their resting membrane potential in a TTX-containing salt solution, Glu, Asp and HCA induced inward currents which were abolished by 6-cyano-7-nitroxaline-2,3-dione (CNQX), a selective antagonist of the non-N-methyl-D-aspartate (NMDA) subtype of EAA receptors. The selective antagonist of NMDA receptors, D-(-)-2-amino-5-phosphonovaleric acid (D-APV), was ineffective in blocking the responses induced by these three amino acids. NMDA, even at high concentrations and in magnesium-free bathing solution, had no detectable effect on membrane properties of Purkinje cells grown in culture during 11-34 days. 4. In magnesium-containing saline, the amplitude of the responses induced by Glu, Asp and HCA was a linear function of the membrane potential. 5. In contrast, neurones of the deep cerebellar nuclei were responsive to NMDA and the inward currents induced by Glu, Asp and HCA were partially blocked both by CNQX and by D-APV. 6. In magnesium-containing saline, the amplitude of the currents induced by NMDA as well as by the three endogenous EAAs decreased at hyperpolarizing holding potentials whereas the current-voltage relation of the responses induced by quisqualate (QA) was strictly linear. 7. It is concluded that Purkinje cells in cerebellar slice cultures do not express NMDA receptors and that excitation of these neurones by the endogenous amino acids Glu, Asp and HCA is mediated exclusively through the activation of non-NMDA receptors. In the same preparation, neurones of the deep cerebellar nuclei possess NMDA and non-NMDA receptors which can be both activated by the three endogenous excitatory amino acids.

Afferent connections of the medial frontal cortex of the rat. A study using retrograde transport of fluorescent dyes. I. Thalamic afferents

The present study of the medial frontal cortex of the rat was undertaken with two objectives. First, to compare the pattern of afferent thalamic neurons for each of the three subdivisions of the medial frontal cortex: the medial precentral (PrCm), dorsal anterior cingulate (ACd) and prelimbic (PL) areas. Second, to provide a firmer basis for anatomical comparisons of cortical regions between rat and monkey. Focal injections of retrogradely transported fluorescent tracers, true blue and diamidino yellow, were placed in different regions of the medial frontal cortex, to reveal the organization of afferent thalamic neurons. The PL area can be readily distinguished from PrCm and ACd areas because it receives afferents from a large number of neurons from both the medial and the lateral parts of the mediodorsal nucleus (MD) whereas only a few neurons, from the lateral MD exclusively, project to PrCm and ACd areas. Moreover, the paratenial and the paraventricular thalamic nuclei project only to the PL area, and the central medial nucleus projects mostly to the PL area. The ventrolateral nucleus projects only to the dorsal part of the medial frontal cortex. The rhomboid, reuniens, ventromedial, intralaminar, posterior and laterodorsal nuclei project to the whole medial frontal cortex. On the basis of these findings, the pattern of thalamic afferents to the PL area was compared to the pattern of thalamic afferents to cingulate and retrosplenial cortices in rat. The conclusion is that the PL area has a pattern of thalamic afferents which is different not only from those of PrCm and ACd areas but also from those of cingulate and retrosplenial cortices. On the basis of its rich innervation from the mediodorsal nucleus, the prelimbic area could very likely be a part of the prefrontal cortex of rat.

Neurotensin-induced excitation of neurons of the rat's frontal cortex studied intracellularly in vitro

The actions of neurotensin (NT) on frontal pyramidal neurons were studied in vitro in slices of rat cerebral cortex using current clamp and single electrode voltage clamp (SEVC) techniques. Bath application of NT (0.1 microM-10 microM) induced a depolarization (2-13 mV) in 88% of the pyramidal cells, this effect was associated with a decrease in input conductance of 5-35% and its reversal potential was estimated at -88 +/ -9.7 mV. Typically, this depolarizing effect of NT was transient, since no cell responded to a second application of the peptide within 20 min after the first one. NT also induced an increase in the rate of firing of pyramidal cells evoked by direct stimulation, even when an hyperpolarizing current was applied to prevent the depolarization induced by NT. This effect could neither be explained by a decrease of the post-spike after-hyperpolarization, nor by an increase of the persistent sodium current which sustains the spiking of pyramidal cells, since the former was not affected consistently by NT and the later was insensitive to the peptide. This excitation of pyramidal neurons by NT persisted after blockade of synaptic transmission. On the other hand, NT also enhanced the synaptic noise recorded in pyramidal cells in standard perfusing medium. Furthermore, dopaminergic antagonists and noradrenergic antagonists failed to block these effects of NT. Finally, the inactive fragment of the peptide, NT(1-8), did not affect membrane properties of pyramidal cells. All together, these results suggest that NT excites frontal cortical neurons through the activation of specific NT receptors.

Cortico-cortical connections of the limbic cortex of the rat

By means of the retrograde transport of fluorescent tracers (Fast Blue, True Blue, Fluorogold and Diamidino Yellow), the cortico-cortical connections of prelimbic, insular, anterior and posterior cingulate (retrosplenial) areas have been studied. Our results demonstrate that there are, in the cortex of the adult rat, a few cells which have branched axons with connections in the ipsilateral hemisphere (associational neurons) or in the contralateral hemisphere (callosal neurons). The callosal neurons could be separated into two categories: "double callosal" neurons which project both axon collaterals to two cortical areas of the contralateral hemisphere, and "associational-callosal" neurons which send axon collaterals to both hemispheres.

Excitation of rat prefrontal cortical neurons by dopamine: an in vitro electrophysiological study

The effects of dopamine (DA) on prefrontal pyramidal neurons were studied in vitro on rat cerebral cortex slices using intracellular recordings. Pyramidal neurons were first identified by Lucifer yellow and some of their basic bioelectrical properties were analysed. At resting potential, white matter stimulation mainly evoked depolarizing inhibitory postsynaptic potentials (IPSPs) which reversed between -60 and -50 mV and were almost totally abolished by bicuculline. Furthermore, pyramidal cells often exhibited spontaneous depolarizing IPSPs abolished by bicuculline. Under tetrodotoxin (TTX) this synaptic noise was partly blocked suggesting that it was due both to the spontaneous firing of presynaptic gamma-aminobutyric acid (GABA)ergic neurons and to a spontaneous quantal release from these afferent fibers. In pyramidal cells, DA enhanced the number of spikes evoked by depolarizing current pulses, and the input resistance was increased by 10-20%. DA also clearly increased the inhibitory synaptic noise. This effect was blocked by fluphenazine. In contrast, evoked IPSPs were not consistently affected by DA. Taken altogether, these results suggest, that in the prefrontal cortex, dopamine has a mild excitatory effect on both pyramidal cells and GABAergic interneurons impinging on them.

Electrophysiological properties of neurons recorded intracellularly in slices of the pigeon optic tectum

The electrical properties of pigeon's optic tectum neurons located in the non-retinorecipient region of layer II have been studied in vitro slice preparations by using intracellular recordings. As judged from the somatodendritic characteristics of cells intracellularly labeled with horseradish peroxidase recordings were obtained from pyramidal neurons, the main morphological type, as well as from ganglion cells. When stimulated with depolarizing current pulses of 300-500 ms duration, three distinct modes of firing were observed. Most neurons (Type I) responded with a continuous firing of fast action potentials whose frequency rate increased regularly when current strength was raised. Another group of cells (Type II) also exhibited sustained firing. However, in Type II cells, grouped discharges formed by 2-6 fast action potentials per group fired in rapid succession were elicited within a certain range of current intensity. Finally, another group of cells (Type III) responded at all intensities tested by a short train of fast action potentials only at the onset of the current step. At current strength close to threshold the spike undershoot of type I neurons was followed by a slow hyperpolarizing afterpotential while the spike undershoot of Type II cells was followed by a hump-like depolarization and a slow hyperpolarizing afterpotential. In Type II cells, we have also observed a pronounced increase of the hyperpolarizing afterpotential after a grouped discharge. Type III cells were characterized by a small amplitude and short duration hyperpolarizing afterpotential, barely visible in most of them. In Type I and II cells the slow hyperpolarizing afterpotential was blocked by replacing Ca2+ with Mg2+ or Cd2+ in the saline. These results support the idea that in these two types of neurons the slow hyperpolarizing afterpotential is primarily caused by a Ca2+-dependent K+ conductance. Furthermore, blocking the slow hyperpolarizing afterpotential provoked a pronounced increase of the firing frequency of Type I cells. In Type II cells blockade of the slow hyperpolarizing afterpotential had a greater effect on firing behavior: i.e. when Ca2+ was replaced with Mg2+ or Cd2+, Type II neurons exhibited repetitively fired action potentials at high frequency but were incapable of discharging repetitive grouped discharges. These observations indicate that the Ca2+-dependent K+ conductance involved in the generation of the slow hyperpolarizing afterpotential is the main modulator of the firing behavior of both types of cells.(ABSTRACT TRUNCATED AT 400 WORDS)

Synaptic organization of inhibitory circuits in the pigeon's optic tectum

The synaptic organization of inhibitory systems in the pigeon's optic tectum was studied with intracellular recording techniques. An extrapolation procedure based on response latency was used to determine the synaptic delay of the postsynaptic potentials (PSPs) and the velocity of conduction of the associated retinal axons. Tectal cells receive mostly disynaptic, trisynaptic or polysynaptic inhibition from retinal ganglion cells. However, evidence was found which together with previous studies raised the possibility of the existence of a direct inhibitory retino-tectal path. Our present results also suggest that inhibition is transmitted from the retina to the tectal cells by way of both, feedforward and feedback pathways.

Synaptic transmission of excitation from the retina to cells in the pigeon's optic tectum

Intracellular recordings were used to study the synaptic excitation of optic tectum neurons in the pigeon. Electrical stimulation of both contralateral optic nerve and ipsilateral optic tract evoked in the tectal neurons EPSPs which in most cases were followed by an IPSP. An extrapolation procedure based on response latency was used to reveal that the EPSPs were mediated by way of mono-, di- and polysynaptic connections with the retinal endings. The laminar location of the recorded cells was estimated according to the field potential and the recording depth with the exception of one cell which was intracellularly stained with HRP. Monosynaptic EPSPs were recorded from cells in the retinorecipient region (sublayers IIa-f) as well as in the non-retinorecipient region (sublayers IIg-j and layer III) of the tectum, while di- and polysynaptic EPSPs were never recorded from the input layers. Tectofugal projections arise largely from layer III neurons. Thus, these results indicate that retinal excitation is transmitted to the output tectal cells by way of mono-, di- or polysynaptic pathways. The conduction velocities of most retinal fibers mediating the EPSP ranged from 4 to 22 m/s (average 12 m/s). However, in a number of retinal fibers the conduction velocities were in a faster range, up to 36 m/s.