Impact of spontaneous synaptic activity on the resting properties of cat neocortical pyramidal neurons In vivo

The frequency of spontaneous synaptic events in vitro is probably lower than in vivo because of the reduced synaptic connectivity present in cortical slices and the lower temperature used during in vitro experiments. Because this reduction in background synaptic activity could modify the integrative properties of cortical neurons, we compared the impact of spontaneous synaptic events on the resting properties of intracellularly recorded pyramidal neurons in vivo and in vitro by blocking synaptic transmission with tetrodotoxin (TTX). The amount of synaptic activity was much lower in brain slices (at 34 degrees C), as the standard deviation of the intracellular signal was 10-17 times lower in vitro than in vivo. Input resistances (Rins) measured in vivo during relatively quiescent epochs ("control Rins") could be reduced by up to 70% during periods of intense spontaneous activity. Further, the control Rins were increased by approximately 30-70% after TTX application in vivo, approaching in vitro values. In contrast, TTX produced negligible Rin changes in vitro (approximately 4%). These results indicate that, compared with the in vitro situation, the background synaptic activity present in intact networks dramatically reduces the electrical compactness of cortical neurons and modifies their integrative properties. The impact of the spontaneous synaptic bombardment should be taken into account when extrapolating in vitro findings to the intact brain.     

Determinants of voltage attenuation in neocortical pyramidal neuron dendrites

How effectively synaptic and regenerative potentials propagate within neurons depends critically on the membrane properties and intracellular resistivity of the dendritic tree. These properties therefore are important determinants of neuronal function. Here we use simultaneous whole-cell patch-pipette recordings from the soma and apical dendrite of neocortical layer 5 pyramidal neurons to directly measure voltage attenuation in cortical neurons. When combined with morphologically realistic compartmental models of the same cells, the data suggest that the intracellular resistivity of neocortical pyramidal neurons is relatively low ( approximately 70 to 100 Omegacm), but that voltage attenuation is substantial because of nonuniformly distributed resting conductances present at a higher density in the distal apical dendrites. These conductances, which were largely blocked by bath application of CsCl (5 mM), significantly increased steady-state voltage attenuation and decreased EPSP integral and peak in a manner that depended on the location of the synapse. Together these findings suggest that nonuniformly distributed Cs-sensitive and -insensitive resting conductances generate a "leaky" apical dendrite, which differentially influences the integration of spatially segregated synaptic inputs.     

Acute alcohol blocks neurosteroid modulation of synaptic transmission and long-term potentiation in the rat hippocampal slice

Effects of ethanol (22 mM) on the modulation of synaptic transmission and long-term potentiation (LTP) by the neurosteroid dehydroepiandrosterone sulfate (DHEAS; 10 microM) was examined in the in vitro rat hippocampal slice preparation. The synaptic responses were elicited by Schaffer collateral stimulation and recorded extracellularly in the somatic and dendritic regions of CA1 pyramidal neurons. LTP induction produced an increase (approximately 55% to 75%) in the amplitude of synaptic responses in ethanol and ethanol plus DHEAS (ethanol/DHEAS) treated slices. These increases were significantly smaller than the approximately 130% increase observed previously in slices treated with DHEAS, but were not significantly different from the approximately 82% increase observed in control slices. These results indicate that an ethanol/DHEAS interaction prevents the enhancement of LTP normally observed with DHEAS treatment of hippocampal slices. An ethanol/DHEAS interaction also altered DHEAS's effects on individual synaptic components of the synaptic response to Schaffer collateral stimulation. Ethanol applied before but not after DHEAS prevented DHEAS's enhancement of the NMDA receptor-mediated synaptic component. DHEAS's depression of the GABAA receptor-mediated synaptic component was also blocked by ethanol. Ethanol or DHEAS individually had no effect on the AMPA receptor-mediated synaptic component, but application of ethanol after DHEAS resulted in a small enhancement of this synaptic component, an effect that was not observed if ethanol was applied before DHEAS. These results show that ethanol and DHEAS interact, altering DHEAS's effects on synaptic transmission and LTP in the hippocampus. Such an interaction may be involved in ethanol's actions on the CNS and raises the possibility that ethanol and DHEAS may act via a common site or pathway.     

Dendritic properties of hippocampal CA1 pyramidal neurons in the rat: intracellular staining in vivo and in vitro

Dendritic morphology and passive cable properties determine many aspects of synaptic integration in complex neurons, together with voltage-dependent membrane conductances. We investigated dendritic properties of CA1 pyramidal neurons intracellularly labeled during in vivo and in vitro physiologic recordings, by using similar intracellular staining and three-dimensional reconstruction techniques. Total dendritic length of the in vivo neurons was similar to that of the in vitro cells. After correction for shrinkage, cell extent in three-dimensional representation was not different between the two groups. Both in vivo and in vitro neurons demonstrated a variable degree of symmetry, with some neurons showing more cylindrical symmetry around the main apical axis, whereas other neurons were more elliptical, with the variation likely due to preparation and preservation conditions. Branch order analysis revealed no difference in the number of branch orders or dendritic complexity. Passive conduction of dendritic signals to the soma in these neurons shows considerable attenuation, particularly with higher frequency signals (such as synaptic potentials compared with steady-state signals), despite a relatively short electrotonic length. Essential aspects of morphometric appearance and complex dendritic integration critical to CA1 pyramidal cell functioning are preserved across neurons defined from the two different hippocampal preparations used in this study.     

Synaptic effects of identified interneurons innervating both interneurons and pyramidal cells in the rat hippocampus

GABAergic interneurons sculpt the activity of principal cells and are themselves governed by GABAergic inputs. To determine directly some of the sources and mechanisms of this GABAergic innervation, we have used dual intracellular recordings with biocytin-filled microelectrodes and investigated synaptic interactions between pairs of interneurons in area CA1 of the adult rat hippocampus. Of four synaptically-coupled interneuron-to-interneuron cell pairs, three presynaptic cells were identified as basket cells, preferentially innervating somata and proximal dendrites of pyramidal cells, but one differing from the other two in the laminar distribution of its dendritic and axonal fields. The fourth presynaptic interneuron was located at the border between strata lacunosum moleculare and radiatum, with axon ramifying within stratum radiatum. Action potentials evoked in all four presynaptic interneurons were found to elicit fast hyperpolarizing inhibitory postsynaptic potentials (mean amplitude 0.35 +/- 0.10 mV at a membrane potential of -59 +/- 2.8 mV) in other simultaneously recorded interneurons (n=4). In addition, three of the presynaptic interneurons were also shown to produce similar postsynaptic responses in subsequently recorded pyramidal cells (n=4). Electron microscopic evaluation revealed one of the presynaptic basket cells to form 12 synaptic junctions with the perisomatic domain (seven somatic synapses and five synapses onto proximal dendritic shafts) of the postsynaptic interneuron in addition to innervating the same compartments of randomly-selected local pyramidal cells (50% somatic and 50% proximal dendritic synapses, n=12). In addition, light microscopic analysis also indicated autaptic self-innervation in basket (12 of 12) and bistratified cells (six of six). Electron microscopic investigation of one basket cell confirmed six autaptic junctions made by five of its boutons. Together, these data demonstrate that several distinct types of interneuron have divergent output to both principal cells and local interneurons of the same (basket cells) or different type. The fast synaptic effects, probably mediated by GABA in both postsynaptic interneurons and principal cells are similar. These additional sources of GABA identified here in the input to GABAergic cells could contribute to the differential temporal patterning of distinct GABAergic synaptic networks.     

Estimating the time course of the excitatory synaptic conductance in neocortical pyramidal cells using a novel voltage jump method

We introduce a method that permits faithful extraction of the decay time course of the synaptic conductance independent of dendritic geometry and the electrotonic location of the synapse. The method is based on the experimental procedure of Pearce (1993), consisting of a series of identical somatic voltage jumps repeated at various times relative to the onset of the synaptic conductance. The progression of synaptic charge recovered by successive jumps has a characteristic shape, which can be described by an analytical function consisting of sums of exponentials. The voltage jump method was tested with simulations using simple equivalent cylinder cable models as well as detailed compartmental models of pyramidal cells. The decay time course of the synaptic conductance could be estimated with high accuracy, even with high series resistances, low membrane resistances, and electrotonically remote, distributed synapses. The method also provides the time course of the voltage change at the synapse in response to a somatic voltage-clamp step and thus may be useful for constraining compartmental models and estimating the relative electrotonic distance of synapses. In conjunction with an estimate of the attenuation of synaptic charge, the method also permits recovery of the amplitude of the synaptic conductance. We use the method experimentally to determine the decay time course of excitatory synaptic conductances in neocortical pyramidal cells. The relatively rapid decay time constant we have estimated (tau approximately 1.7 msec at 35 degrees C) has important consequences for dendritic integration of synaptic input by these neurons.     

Inhibitory control of somatodendritic interactions underlying action potentials in neocortical pyramidal neurons in vivo: an intracellular and computational study

The effect of synaptic inputs on somatodendritic interactions during action potentials was investigated, in the cat, using in vivo intracellular recording and computational models of neocortical pyramidal cells. An array of 10 microelectrodes, each ending at a different cortical depth, was used to preferentially evoke synaptic inputs to different somatodendritic regions. Relative to action potentials evoked by current injection, spikes elicited by cortical microstimuli were reduced in amplitude and duration, with stimuli delivered at proximal (somatic) and distal (dendritic) levels evoking the largest and smallest decrements, respectively. When the inhibitory postsynaptic potential reversal was shifted to around -50 mV by recording with KCl pipettes, synaptically-evoked spikes were significantly less reduced than with potassium acetate or cesium acetate pipettes, suggesting that spike decrements are not only due to a shunt, but also to voltage-dependent effects. Computational models of neocortical pyramidal cells were built based on available data on the distribution of active currents and synaptic inputs in the soma and dendrites. The distribution of synapses activated by extracellular stimulation was estimated by matching the model to experimental recordings of postsynaptic potentials evoked at different depths. The model successfully reproduced the progressive spike amplitude reduction as a function of stimulation depth, as well as the effects of chloride and cesium. The model revealed that somatic spikes contain an important contribution from proximal dendritic sodium currents up to approximately 100 microm and approximately 300 microm from the soma under control and cesium conditions, respectively. Proximal inhibitory postsynaptic potentials can present this dendritic participation thus reducing the spike amplitude at the soma. The model suggests that the somatic spike amplitude and shape can be used as a "window" to infer the electrical participation of proximal dendrites. Thus, our results suggest that inhibitory postsynaptic potentials can control the participation of proximal dendrites in somatic sodium spikes.     

Synaptic target selectivity and input of GABAergic basket and bistratified interneurons in the CA1 area of the rat hippocampus

To assess the position of interneurons in the hippocampal network, fast spiking cells were recorded intracellularly in vitro and filled with biocytin. Sixteen non-principal cells were selected on the basis of 1) cell bodies located in the pyramidal layer and in the middle of the slice, 2) extensive labeling of their axons, and 3) a branching pattern of the axon indicating that they were not axo-axonic cells. Examination of their efferent synapses (n = 400) demonstrated that the cells made synapses on cell bodies, dendritic shafts, spines, and axon initial segments (AIS). Statistical analysis of the distribution of different postsynaptic elements, together with published data (n = 288) for 12 similar cells, showed that the interneurons were heterogeneous with regard to the frequency of synapses given to different parts of pyramidal cells. When the cells were grouped according to whether they had less or more than 40% somatic synaptic targets, each population appeared homogeneous. The population (n = 19) innervating a high proportion of somata (53 +/- 10%, SD) corresponds to basket cells. They also form synapses with proximal dendrites (44 +/- 12%) and rarely with AISs and spines. One well-filled basket cell had 8,859 boutons within the slice, covering an area of 0.331 mm2 of pyramidal layer tangentially and containing 7,150 pyramidal cells, 933 (13%) of which were calculated to be innervated, assuming that each pyramidal cell received nine to ten synapses. It was extrapolated that the intact axon probably had about 10,800 boutons innervating 1,140 pyramids. The proportion of innervated pyramidal cells decreased from 28% in the middle to 4% at the edge of the axonal field. The other group of neurons, the bistratified cells (n = 9), showed a preference for dendritic shafts (79 +/- 8%) and spines (17 +/- 8%) as synaptic targets, rarely terminating on somata (4 +/- 8%). Their axonal field was significantly larger (1,250 +/- 180 microns) in the medio-lateral direction than that of basket cells (760 +/- 130 microns). The axon terminals of bistratified cells were smaller than those of basket cells. Furthermore, in constrast to bistratified cells, basket cells had a significant proportion of dendrites in stratum lacunosum-moleculare suggesting a direct entorhinal input. The results define two distinct types of GABAergic neuron innervating pyramidal cells in a spatially segregated manner and predict different functions for the two inputs. The perisomatic termination of basket cells is suited for the synchronization of a subset of pyramidal cells that they select from the population within their axonal field, whereas the termination of bistratified cells in conjunction with Schaffer collateral/commissural terminals may govern the timing of CA3 input and/or voltage-dependent conductances in the dendrites.     

Characterization of trichomonad species and strains by PCR fingerprinting

Changes in the number of spines on apical dendrites of pyramidal neurons in layers III and V of the motor area of the cerebral cortex were examined in the young adult mice (30-60 days old) by a modified Golgi-Cox method and laser scanning microscopy on the 1st, 6th, 10th and 12th days after callosotomy. The anterior part of the callosal body, including rostrum, genu and truncus corporis callosi, was sectioned with a razor blade. The numbers of spines on apical dendrites and their oblique branches of pyramidal neurons were counted at the level of layer II for layer III pyramidal neurons and at the level of layer III for layer V pyramidal neurons. The density of spines was increased on all these dendritic parts during 10 days after callosotomy; the diameters of spine stems and dendrites were also increased. These changes, however, appeared to be transient as they were decreased almost to normal level on the 12th day after callosotomy.     

Axonal sprouting in layer V pyramidal neurons of chronically injured cerebral cortex

We performed experiments to determine whether axonal sprouting occurs in neurons of chronic neocortical epileptogenic lesions. Partially isolated somatosensory cortical islands with intact pial blood supply were prepared in mature rats. Neocortical slices from these lesions, studied 6-39 d later, generated spontaneous and/or evoked epileptiform field potentials (Prince and Tseng, 1993) during which neurons displayed prolonged polyphasic excitatory and inhibitory synaptic potentials/currents. Single electrophysiologically characterized layer V pyramidal neurons in control and epileptogenic slices were filled with biocytin using sharp and patch-electrode techniques, their axonal arbors reconstructed and compared quantitatively. Neurons in injured cortex had a 56% increase in total axonal length, a 64% increase in the number of axonal collaterals and more than a doubling (115% increase) of the number of axonal swellings. The presumed boutons were smaller and more closely spaced than those of control cells. In some neurons the main descending axon had hypertrophic segments from which branches arose. These highly significant changes were most marked in the perisomatic region of layer V. The axonal sprouting was associated with a decrease in somatic area but no significant change in dendritic arbors. Results suggest that a significant degree of axonal reorganization takes place in the chronically injured cortex where it might be an adaptive mechanism for recovery of function after injury, or might be maladaptive and play an important role in the generation of epileptiform events by increasing the numbers and density of synaptic contacts between neurons.     

A quantitative analysis of the dendritic organization of pyramidal cells in the rat hippocampus

The three dimensional organization of the dendritic trees of pyramidal cells in the rat hippocampus was investigated using intracellular injection of horseradish peroxidase in the in vitro hippocampal slice preparation and computer-aided reconstruction. The total dendritic length, dendritic length in each of the hippocampal laminae, and the number of dendritic branches were measured in 20 CA1 pyramidal cells, 7 neurons in CA2 and 20 CA3 pyramidal cells. The total dendritic length of CA3 pyramidal cells varied in a consistent fashion depending on their position within the field. Cells located close to the dentate gyrus had the smallest dendritic trees which averaged 9,300 microns in total length. Cells in the distal part of CA3 (near CA2) had the largest dendritic trees, averaging 15,800 microns. The CA2 field contained cells which resembled CA3 pyramidal cells in most respects except for the absence of thorny excrescences on their proximal dendrites. There were also smaller pyramidal cells that resembled CA1 neurons. CA1 pyramidal cells tended to be more homogeneous. Pyramidal neurons throughout the transverse extent of CA1 had a total dendritic length on the order of 13,500 microns. The quantitative analysis of the laminar distribution of dendrites demonstrated that the stratum oriens and stratum radiatum contained significant portions of the pyramidal cell dendritic trees. In Ca3, for example, 42-51% of the total dendritic length was located in stratum oriens; about 34% of the dendritic tree was located in stratum radiatium. The amount of dendritic length in stratum lacunosum-moleculare of CA3 varied depending on the location of the cell. Many CA3 cells located within the limbs of the dentate gyrus, for example, had no dendrites extending into stratum lacunosum-moleculare whereas those located distally in CA3 had about the same percentage of their dendritic tree in stratum lacunosum-moleculare as in stratum radiatum. In CA1, nearly half of the dendritic length was located in stratum radiatum, 34% was in stratum oriens and 18% was in stratum lacunosum-moleculare. These studies identified distinctive dendritic branching patterns, in the stratum radiatum and stratum lacunosum-moleculare, which clearly distinguished CA3 from CA1 neurons.     

Active summation of excitatory postsynaptic potentials in hippocampal CA3 pyramidal neurons

The manner in which the thousands of synaptic inputs received by a pyramidal neuron are summed is critical both to our understanding of the computations that may be performed by single neurons and of the codes used by neurons to transmit information. Recent work on pyramidal cell dendrites has shown that subthreshold synaptic inputs are modulated by voltage-dependent channels, raising the possibility that summation of synaptic responses is influenced by the active properties of dendrites. Here, we use somatic and dendritic whole-cell recordings to show that pyramidal cells in hippocampal area CA3 sum distal and proximal excitatory postsynaptic potentials sublinearly and actively, that the degree of nonlinearity depends on the magnitude and timing of the excitatory postsynaptic potentials, and that blockade of transient potassium channels linearizes summation. Nonlinear summation of synaptic inputs could have important implications for the computations performed by single neurons and also for the role of the mossy fiber and perforant path inputs to hippocampal area CA3.     

GABAergic cells are the major postsynaptic targets of mossy fibers in the rat hippocampus

Dentate granule cells communicate with their postsynaptic targets by three distinct terminal types. These include the large mossy terminals, filopodial extensions of the mossy terminals, and smaller en passant synaptic varicosities. We examined the postsynaptic targets of mossy fibers by combining in vivo intracellular labeling of granule cells, immunocytochemistry, and electron microscopy. Single granule cells formed large, complex "mossy" synapses on 11-15 CA3 pyramidal cells and 7-12 hilar mossy cells. In contrast, GABAergic interneurons, identified with immunostaining for substance P-receptor, parvalbumin, and mGluR1a-receptor, were selectively innervated by very thin (filopodial) extensions of the mossy terminals and by small en passant boutons in both the hilar and CA3 regions. These terminals formed single, often perforated, asymmetric synapses on the cell bodies, dendrites, and spines of GABAergic interneurons. The number of filopodial extensions and small terminals was 10 times larger than the number of mossy terminals. These findings show that in contrast to cortical pyramidal neurons, (1) granule cells developed distinct types of terminals to affect interneurons and pyramidal cells and (2) they innervated more inhibitory than excitatory cells. These findings may explain the physiological observations that increased activity of granule cells suppresses the overall excitability of the CA3 recurrent system and may form the structural basis of the target-dependent regulation of glutamate release in the mossy fiber system.     

Dendritic spines as basic functional units of neuronal integration

Most excitatory synaptic connections occur on dendritic spines. Calcium imaging experiments have suggested that spines constitute individual calcium compartments, but recent results have challenged this idea. Using two-photon microscopy to image fluorescence with high resolution in strongly scattering tissue, we measured calcium dynamics in spines from CA1 pyramidal neurons in slices of rat hippocampus. Subthreshold synaptic stimulation and spontaneous synaptic events produced calcium accumulations that were localized to isolated spines, showed stochastic failure, and were abolished by postsynaptic blockers. Single somatic spikes induced fast-peaking calcium accumulation in spines throughout the cell. Pairing of spikes with synaptic stimulation was frequently cooperative, that is, it resulted in supralinear calcium accumulations. We conclude: (1) calcium channels exist in spine heads; (2) action potentials invade the spines; (3) spines are individual calcium compartments; and (4) spines can individually detect the temporal coincidence of pre- and postsynaptic activity, and thus serve as basic functional units of neuronal integration.     

Oscillatory coupling of hippocampal pyramidal cells and interneurons in the behaving Rat

We examined whether excitation and inhibition are balanced in hippocampal cortical networks. Extracellular field and single-unit activity were recorded by multiple tetrodes and multisite silicon probes to reveal the timing of the activity of hippocampal CA1 pyramidal cells and classes of interneurons during theta waves and sharp wave burst (SPW)-associated field ripples. The somatic and dendritic inhibition of pyramidal cells was deduced from the activity of interneurons in the pyramidal layer [int(p)] and in the alveus and st. oriens [int(a/o)], respectively. Int(p) and int(a/o) discharged an average of 60 and 20 degrees before the population discharge of pyramidal cells during the theta cycle, respectively. SPW ripples were associated with a 2.5-fold net increase of excitation. The discharge frequency of int(a/o) increased, decreased ("anti-SPW" cells), or did not change ("SPW-independent" cells) during SPW, suggesting that not all interneurons are innervated by pyramidal cells. Int(p) either fired together with (unimodal cells) or both before and after (bimodal cells) the pyramidal cell burst. During fast-ripple oscillation, the activity of interneurons in both the int(p) and int(a/o) groups lagged the maximum discharge probability of pyramidal neurons by 1-2 msec. Network state changes, as reflected by field activity, covaried with changes in the spike train dynamics of single cells and their interactions. Summed activity of parallel-recorded interneurons, but not of pyramidal cells, reliably predicted theta cycles, whereas the reverse was true for the ripple cycles of SPWs. We suggest that network-driven excitability changes provide temporal windows of opportunity for single pyramidal cells to suppress, enable, or facilitate selective synaptic inputs.     

Connectivity and convergence of single corticostriatal axons

The distribution of synapses formed by corticostriatal neurons was measured to determine the average connectivity and degree of convergence of these neurons and to search for spatial inhomogeneities. Two kinds of axonal fields, focal and extended, and two striatal tissue compartments, the patch (striosome) and matrix, were analyzed separately. Electron microscopic examination revealed that both kinds of corticostriatal axons made synapses at varicosities that could be identified in the light microscope, and each varicosity made a single synapse. Thus, the distribution of varicosities was a good estimate of the spatial distribution of synapses. The distance between axonal varicosities was measured to determine the density of synaptic connections formed by one axon within the volume occupied by a striatal neuron. Intersynaptic distances were distributed exponentially, except that synapses were rarely located <4 microm apart. The mean distance between synapses was approximately 10 microm, so axons made a maximum of 40 synapses within the dendritic volume of a spiny neuron. There are approximately 2840 spiny neurons located within the volume of the dendrites of one spiny cell (Oorschot, 1996), so each axon must contact 相似文献   

The highly irregular firing of cortical cells is inconsistent with temporal integration of random EPSPs

How random is the discharge pattern of cortical neurons? We examined recordings from primary visual cortex (V1; Knierim and Van Essen, 1992) and extrastriate cortex (MT; Newsome et al., 1989a) of awake, behaving macaque monkey and compared them to analytical predictions. For nonbursting cells firing at sustained rates up to 300 Hz, we evaluated two indices of firing variability: the ratio of the variance to the mean for the number of action potentials evoked by a constant stimulus, and the rate-normalized coefficient of variation (Cv) of the interspike interval distribution. Firing in virtually all V1 and MT neurons was nearly consistent with a completely random process (e.g., Cv approximately 1). We tried to model this high variability by small, independent, and random EPSPs converging onto a leaky integrate-and-fire neuron (Knight, 1972). Both this and related models predicted very low firing variability (Cv < 1) for realistic EPSP depolarizations and membrane time constants. We also simulated a biophysically very detailed compartmental model of an anatomically reconstructed and physiologically characterized layer V cat pyramidal cell (Douglas et al., 1991) with passive dendrites and active soma. If independent, excitatory synaptic input fired the model cell at the high rates observed in monkey, the Cv and the variability in the number of spikes were both very low, in agreement with the integrate-and-fire models but in strong disagreement with the majority of our monkey data. The simulated cell only produced highly variable firing when Hodgkin-Huxley-like currents (INa and very strong IDR) were placed on distal dendrites. Now the simulated neuron acted more as a millisecond-resolution detector of dendritic spike coincidences than as a temporal integrator. We argue that neurons that act as temporal integrators over many synaptic inputs must fire very regularly. Only in the presence of either fast and strong dendritic nonlinearities or strong synchronization among individual synaptic events will the degree of predicted variability approach that of real cortical neurons.     

Cholinergic excitation of dendrites in neocortical neurons

Discharge patterns were studied in response to iontophoretic application of acetylcholine to the soma and dendrites of 128 neocortical pyramidal neurons of layer V. Extracellular recordings were obtained from slices of the guinea-pig parietal cortex. All responses found were excitatory and were better expressed in spontaneously firing cells than in silent ones. Sensitivity to acetylcholine was approximately the same at somatic and dendritic sites in all the cells. Activation of muscarinic receptors gave rise to firing patterns with equal latencies and intensities when applied to both soma and dendrites. The latter suggests that membrane excitation elicited in dendrites by binding of acetylcholine to muscarinic cholinoreceptors is likely to propagate towards the soma through intracellular biochemical processes. Modulating effect of acetylcholine on output firing patterns, elicited by dendritic application of excitatory amino acids, included shortening of the somatic response latency and increase of response intensity and duration. We propose that, in contrast to glutamatergic excitation, the spread of cholinergic excitation along dendrites involves intra-cellular chemical signalling and results in changing the electrical properties of dendrites all over their length.     

Dendritic voltage and calcium-gated channels amplify the variability of postsynaptic responses in a Purkinje cell model

The dendrites of most neurons express several types of voltage and Ca2+-gated channels. These ionic channels can be activated by subthreshold synaptic input, but the functional role of such activations in vivo is unclear. The interaction between dendritic channels and synaptic background input as it occurs in vivo was studied in a realistic computer model of a cerebellar Purkinje cell. It previously was shown using this model that dendritic Ca2+ channels amplify the somatic response to synchronous excitatory inputs. In this study, it is shown that dendritic ion channels also increased the somatic membrane potential fluctuations generated by the background input. This amplification caused a highly variable somatic excitatory postsynaptic potential (EPSP) in response to a synchronous excitatory input. The variability scaled with the size of the response in the model with excitable dendrite, resulting in an almost constant coefficient of variation, whereas in a passive model the membrane potential fluctuations simply added onto the EPSP. Although the EPSP amplitude in the active dendrite model was quite variable for different patterns of background input, it was insensitive to changes in the timing of the synchronous input by a few milliseconds. This effect was explained by slow changes in dendritic excitability. This excitability was determined by how the background input affected the dendritic membrane potentials in the preceding 10-20 ms, causing changes in activation of voltage and Ca2+-gated channels. The most important model variables determining the excitability at the time of a synchronous input were the Ca2+-activation of K+ channels and the inhibitory synaptic conductance, although many other model variables could be influential for particular background patterns. Experimental evidence for the amplification of postsynaptic variability by active dendrites is discussed. The amplification of the variability of EPSPs has important functional consequences in general and for cerebellar Purkinje cells specifically. Subthreshold, background input has a much larger effect on the responses to coherent input of neurons with active dendrites compared with passive dendrites because it can change the effective threshold for firing. This gives neurons with dendritic calcium channels an increased information processing capacity and provides the Purkinje cell with a gating function.     

The patterns and synaptic properties of horizontal intracortical connections in the rat motor cortex

1. The laminar distribution of synaptic activity in the primary motor cortex, elicited by stimulation of intracortical, horizontal afferents, was studied in young (12-17 days old) and adult rats using the in vitro brain slice preparation. Connectivity patterns were deduced from current-source density (CSD) analyses of field potential depth profiles and were confirmed by anatomic data of retrograde cell labeling after focal injections of a fluorescent tracer. 2. According to the CSD distributions, horizontal axons in layer II/III provide strong monosynaptic input to dendrites of layer II and III pyramidal cells in a distant column, and weaker monosynaptic input to layer V and VI cells by synapsing on dendritic fields at the border of layer III and V and in deep layer V. When these pathways are activated, layer II/III cells may relay excitatory activity to upper and deep layer V, as well as to other cells in layer II/III of the same column. Axons arising from layer V provide monosynaptic input to pyramidal cells in all layers of neighboring columns, by synapsing in two dendritic fields: one in the superficial layers and the other in middle layer V. Activation of these pathways may generate a disynaptic intracolumnar input from layer II/III cells to middle layer V, as well as to other cells in layer II/III. Similar patterns of synaptic activity were elicited by stimulation from 0.45 to 2 mm distal to the recorded column. There were no apparent differences between young and adult rats in the connectivity patterns revealed by the CSD analyses. 3. Tracer injections in layer III resulted in retrograde labeling of cells in layers II/III and V, at distances > 2 mm from the injection site, whereas injections in layer V resulted in retrograde labeling of cells at long distances in layer V and to a lesser extent in layer II/III. These findings indicate that neurons in layer V project, via horizontal axon collaterals, for long distances within layers III and V, whereas the horizontal axon collaterals of layer III cells are restricted, for the most part, to the superficial layers. 4. Suppression of inhibitory activity by bath application of the gamma-aminobutyric acid-A (GABAA) receptor antagonist bicuculline methiodide (BMI) did not alter the pattern of the CSD distributions. All synaptic currents present in the control medium were enhanced by application of BMI, although the effect was more pronounced on the polysynaptic components.(ABSTRACT TRUNCATED AT 400 WORDS)