University Laboratory of Physiology, Parks Road, Oxford OX1 3PT, UK and , 1 Institute of Neuroinformatics, University/ETH Zurich, Winterthurerstrasse 190, 8057 Zurich, Switzerland
Address correspondence to K.A.C. Martin, Institute of Neuroinformatics, University/ETH Zurich, Winterthurerstrasse 190, 8057 Zurich, Switzerland. Email : kevan{at}ini.phys.ethz.ch.
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Abstract |
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Introduction |
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There is now substantial anatomical and physiological evidence for recurrent excitatory circuits in cat visual cortex (Martin, 1988; Douglas et al., 1989
, 1995
; Douglas and Martin, 1991
; Ahmed et al., 1994
; Anderson et al., 1994
). Theoretical studies have indicated that recurrent microcircuits may mediate a number of the emergent' properties of visual cortical neurons in the cat, such as contrast gain control, orientation and direction selectivity, and receptive field structure (Douglas and Martin, 1991
; Ben-Yishai et al., 1995
; Somers et al., 1995
; Ahmed et al., 1997
; Chance et al., 1998
).
Both experimental and theoretical studies have largely ignored the dynamics and efficacy of the different types of synapses that contribute to the layer 4 circuits. One reason is that it is only recently that any experimental evidence has been obtained concerning identified neurons in the cat (Stratford et al., 1996; Buhl et al., 1997
; Tarczy-Hornoch et al., 1998
). In the rat somatomotor cortex far more is known. Synaptic depression is a prominent feature of connections between pyramidal cells (Thomson and West, 1993
; Thomson et al., 1993a
; Markram and Tsodyks, 1996
; Tsodyks and Markram, 1997
; Varela et al., 1997
, 1999
). Depression is also seen at synapses from layer 4 spiny neurons to smooth cells in the rat somatosensory cortex (Reyes et al., 1998
) and in the cat visual cortex (Tarczy-Hornoch et al., 1998
), and between spiny neurons in layer 4 of cat visual cortex (Stratford et al., 1996
). Facilitating synapses are found between pyramidal cells and smooth cells in the rat (Thomson et al., 1993b
), and between one class of pyramidal cells and spiny stellate cells in the cat (Ferster and Lindström, 1985a
,b
; Stratford et al., 1996
). It has been suggested that temporal variations in synaptic function have significant implications for the way in which information is coded (Tsodyks and Markram, 1997
; Tsodyks et al., 1998
), and recently the rat data have been used in models of cortical gain control (Abbott et al., 1997
), and of the temporal response characteristics of neurons in primary visual cortex (Chance et al., 1998
).
The experiments reported here consider further aspects of the heterogenous group of excitatory synapses found in layer 4 of the primary visual cortex in the cat. We investigated excitatory synaptic connections between spiny cells in layer 4, and between layer 6 excitatory neurons and their spiny neuron targets in layer 4. We analysed the release probabilities and the dynamic behaviour of these synaptic connections. A subset of these data has previously been summarized briefly (Stratford et al., 1996).
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Materials and Methods |
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Recordings were made in slices of visual cortex taken from cats aged 1216 weeks (1.01.6 kg). Anaesthesia was induced with pentobarbitone (Sagatal, Sigma, 60 mg/kg, i.p. or i.m.) and maintained with Saffan (Sigma, i.v., as needed). The visual cortex was accessed by craniotomy, and a block of cortex excised after removal of the dura. The cat was subsequently killed by an overdose of anaesthetic. The slice preparation and maintenance techniques were similar to those reported previously (Mason et al., 1991). Slices 400 or 500 µm thick were cut on a vibrating microtome (Vibroslice, Campden Instruments) and maintained at 3436°C in an interface-type recording chamber supplied with ACSF and warmed, humidified carbogen (95% O2/5% CO2). Slices remained in the recording chamber for at least 2 h before intracellular impalements were attempted. In some cases, slices were held in an interface holding' chamber, and transferred to the recording chamber during the experiment. The composition of the ACSF used for slice preparation and recovery was (in mM) NaCl, 124; KCl, 2.3; MgSO4, 1.0; KH2PO4, 1.3; CaCl2, 2.5; NaHCO3, 26; glucose, 10 (pH 7.4). For recording, 50 µM DL-2-amino-5-phosphopentanoic acid (AP-5, Sigma) was added to the ACSF to block N-methyl-D-aspartate (NMDA)-mediated currents.
Intracellular Recording
Recording electrodes were filled with 0.5 M potassium methyl sulphate with 5 mM potassium chloride and 2% biocytin (2 mg/100 µl) (Sigma). The first electrode, positioned in the upper portion of layer 4 parallel to the cortical surface, was advanced obliquely using a microdrive (SCAT-01, Digitimer, Welwyn Garden City, UK) until we obtained a stable impalement. To study connections within layer 4, a second electrode approaching from the opposite direction was then placed on the slice surface parallel to the first, but slightly further from the layer 3/4 border. This strategy was adopted in light of the known axonal arborization patterns of spiny stellate neurons in layer 4, which project mainly to superficial layers. The pipette entry points were 200 µm apart laterally on average. The second electrode was advanced until a second neuron was recorded. We tested whether this second neuron was synaptically connected to the first. If no connection was found in either direction, we abandoned the second neuron after a total duration of only a few minutes (i.e. usually insufficient for biocytin to fill the cell). The process was repeated until a connection was found or the first neuron was lost, after which we moved to a new area of the slice, to avoid possible ambiguity in matching physiology to labelled neurons. Positions of the pipette penetrations in the slice were plotted precisely with the aid of an eyepiece micrometer, and the angles of penetration noted, before moving to a new site. In studying connections between layer 6 and layer 4, we varied the search strategy, beginning sometimes with a layer 6 impalement. Within layer 4, the ratio of connections found (all types inhibitory or excitatory) to formally tested pairs ranged from 3% to 28% in different experiments. Layer 6 to layer 4 connections were much harder to find than connections within layer 4.
All excitatory postsynaptic potentials (EPSPs) were recorded at postsynaptic membrane potentials between 65 and 75 mV. Presynaptic activity was evoked by current injections of varying amplitude, adjusted to trigger single or multiple action potentials. We did not usually use separate current steps to control spike timing, so the intervals used to study paired-pulse behaviour were variable. Where multiple intervals were studied, we report the interval nearest to 50 ms.
Histology
Following recording from cells, slices were immediately fixed in 4% paraformaldehyde/0.5% glutaraldehyde/0.2% picric acid in 0.1 M phosphate buffer, pH 7.4 (PB), and left overnight. Slices were then washed several times in PB and then transferred through a graded series of sucrose solutions (1030%) before being freeze-thawed in liquid nitrogen. Each slice was then resectioned to a thickness of 60 or 80 µm, preceding a standard biocytin reaction technique. The sections were incubated in a solution of avidinbiotin complex (ABC; Vector Labs Ltd, Peterborough, UK), 2% in PB, overnight at 4°C. After several washes in PB the sections were transferred to a 0.16% solution of HankerYates reagent (Sigma) in PB for 15 min. A solution of hydrogen peroxide was then added to give a final concentration of 0.004% and the reaction monitored under a dissecting microscope. When the cell had become sufficiently dark, the reaction was terminated by rinsing several times with PB. The sections were then osmicated in a 1% solution of OsO4 (in PB), dehydrated through a graded series of ethanol and then mounted on microscope slides in epoxy resin (Araldite or Durcupan; Fluka) with coverslips. In later experiments diaminobenzidine rather than HankerYates was used, and the ABC reaction was preceded by a 10 min preincubation in 3% hydrogen peroxide to decrease background staining. Cells were drawn and reconstructed using the TRAKA reconstruction system (Anderson et al., 1994).
Most neurons impaled in layer 4 were spiny stellate cells and star pyramidal cells, though ~10% of recovered neurons impaled in layer 4 were pyramidal cells of lower layer 3 or upper layer 5; these have been excluded. Presumed layer 4 cells that were not identified anatomically have been included.
Layer 4 neurons were provisionally identified at the time of recording as spiny (i.e. excitatory) or smooth (i.e. inhibitory) on the basis of their firing patterns. In the cat confirmed smooth or inhibitory neurons responded to increments of suprathreshold current injection with increases in firing rate three to four times greater than those seen in confirmed spiny or excitatory neurons (K. Tarczy-Hornoch, unpublished Ph.D. thesis). Since the classification on the basis of firing pattern was invariably confirmed when further information concerning synaptic influence or morphology became available, it has been used to identify postsynaptic targets as spiny' neurons when no morphological data were obtained.
Electrophysiological Data Analysis
All synaptic responses were filtered at 2 kHz and recorded with 5 kHz digitization using a CED 1401 interface (Cambridge Electronic Design) and SIGAVG software (Cambridge Electronic Design). In-house software was used to derive input resistances from the voltage responses to small 0.5 ms current pulses.
For analysis of synaptic responses, we used in-house software to extract the amplitudes of individual synaptic events from raw data traces. These were measured as differences between averages taken over short windows at baseline and at signal peak, whose positions were defined relative to presynaptic action potential peak. In pairs or trains of EPSPs with short interpulse intervals, the measurement windows for later events fall on the decay phase of previous EPSPs; individual trial measurements were corrected for this decaying baseline on the basis of a template average waveform derived from the responses to repeated single action potentials. For every trace, we also measured the amplitude difference, during the baseline period, between windows of the same size and separation as used for measuring the response, to determine the variance of the background noise. To improve accuracy, four separate determinations of noise variance were made using different regions of the baseline, and the average of these variances was used. Noise-corrected coefficients of variation (CV) of EPSPs were calculated as
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The number of trials used to calculate CV is indicated in the tables; this was a subset of consecutive trials at 1 Hz, usually drawn from a larger set of recorded trials exploring paired pulses, trains and different repeat rates (total trials recorded ranged from 100 to >1500 except for EPSPs 8, 10, 27, 35). In deriving CV of pulse 2 for analysis of paired-pulse changes in CV, we used between 100 and 400 trials for pulse 2 CV (except for EPSP 37, using 50 trials for pulse 2). Means and CVs reported are for periods of data in which the mean was stable over time.
Analysis of Statistics of Neurotransmitter Release
In the theory of quantal analysis of synaptic transmission in the central nervous system (Jack et al., 1990, 1994
; Redman, 1990
), the binomial parameter n is thought to be the number of synapses, or release sites, mediating the synaptic connection, and on this assumption is an integer (Korn and Faber, 1987
). The likelihood that neurotransmitter release (i.e. release of at least one vesicle) will take place at a given release site is given by the probability p. In this paper we make the simple assumption that the release probability at each release site is the same and that the quantal size, q, recorded from the soma, is similar for all release sites of a given connection. Our observations, in rat hippocampus and rat visual cortex, are in support of that assumption (Larkman et al., 1997
) (N.J. Bannister, N. Hardingham and A.U. Larkman, unpublished observations). On this basis, we can use the measures of the mean and standard deviation (SD) of our samples as an estimate of the above parameters in the following way :
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![]() | (2) |
![]() | (3) |
![]() | (4) |
For each n, a range of p and q values will define a limited area on the plot of CV against mean (see Figs 3A and 5C). Our approach has been to find the minimum ranges for both p and q that embrace all our data points.
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For Figure 4F, release probabilites are derived from two pairs of values for mean and CV that might reasonably be assumed to differ only in release probability, since neither n nor q would be expected to change at repeat rates <1 Hz [AMPA recovery from desensitization occurs over tens of milliseconds (Trussell and Fischbach, 1989
; Colquhoun et al., 1992
)]. On this assumption, the release probabilities are derived as follows : using equations (1) and (3) above it can be shown, by simple algebraic manipulation, that
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Results |
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Of the postsynaptic targets of excitatory connections between layer 4 spiny neurons, 14 were identified morphologically as spiny stellate cells, and 9 as pyramidal cells of layer 4. Of the presynaptic sources, 12 were spiny stellate cells and 6 were layer 4 pyramidal cells. The remaining cells were not identified morphologically, but their electrophysiological properties were characteristic (see Materials and Methods) of excitatory neurons, and presumably were other examples of either the spiny stellate or pyramidal cells we had identified morphologically.
Excitatory Synaptic Connections Between Layer 4 Spiny Neurons
The properties of the 14 layer 4 excitatory synaptic connections onto spiny stellate cells are summarized in Table 1. In seven pairs, the presynaptic neurons were also identified : five were spiny stellate cells, and two were pyramidal cells. A reconstruction of the morphology of a connected pair of spiny stellate neurons is shown in Figure 1
, along with the EPSP recorded. Comparison of EPSP variation to baseline noise suggests multiquantal transmitter release with trial-to-trial fluctuations in quantal content (Fig. 1B
). The absence of low-amplitude events indicates a high probability of neurotransmitter release. The average EPSP amplitude recorded in spiny stellate cells was 1018 µV and the average CV was 0.25. Amplitudes tended to be larger and CVs lower for the subset of EPSPs originating from identified spiny stellate neurons : these EPSPs averaged 1739 ± 790 µV in amplitude, with an average CV of 0.19 ± 0.16.
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Figure 3B shows the relationship between EPSP amplitude and the net lateral distance (i.e. excluding separation along the white matterpial axis) separating synaptically connected cells in layer 4. Large amplitude connections (>1 mV) were rarely seen at intersomatic distances >100 µm. Few connections at all were detected beyond 150 µm, but the sample is biased since cell pairs were deliberately sought close together to minimize subsequent ambiguity in cell identification. Most presynaptic cells were nearer to the white matter than their postsynaptic targets (not shown). While we deliberately sought presynaptic cells further from the layer 3/4 border after first establishing an impalement in a cell at the top of layer 4, we were able to test for connections in both directions in more than two-thirds of cases. Within this subset, more than three times as many connections from ascending projections were found as from descending ones.
Dynamic Properties of Excitatory Synaptic Connections Between Layer 4 Spiny Neurons
The synapses between layer 4 spiny cells showed either depression or no change on the second EPSP when pairs of action potentials were evoked in the presynaptic cell (n = 24; see Tables 13). Nine of the 15 connections that showed depression were analysed for paired-pulse changes in CV. In seven of these, the ratio of 1/CV2 (pulse 1/pulse 2) was greater than the ratio of mean amplitudes (Fig. 3C
), which suggests an important presynaptic contribution to paired-pulse depression (see Materials and Methods).
In eight connections we also analysed responses to third and subsequent spikes in trains of action potentials. EPSPs were heterogeneous with respect to the magnitude of depression seen with repetitive stimulation. Some, such as EPSP 5 (Fig. 4A,B), were robust during repetitive firing, showing little second pulse depression except at very short interpulse intervals (Fig. 4B
, compare second pulse in trains of six versus three spikes), and little overall depression during trains. Other synaptic responses, such as EPSP 20 (Fig. 4C,F
), depressed more markedly under similar conditions (compare Fig. 4D
, filled circles, and 4B
). We consistently observed that average amplitude decreased cumulatively with successive pulses in a train, but tended to converge to a stable fraction of the first pulse amplitude (e.g. Fig. 4B,D
). This was accompanied by an opposite trend in CV (Fig. 4E
). However, EPSP amplitude depended also on the trial repeat rate, increasing with slower repeat rates in six of the seven cases examined. This is shown for EPSP 20, where the first pulse amplitude was larger and CV smaller, if presynaptic trains were evoked at 0.2 Hz rather than 1 Hz (Fig. 4D,E
).
We can calculate the release probabilities for the first pulse for the 1 and 0.2 Hz repeat rate conditions if we assume that the difference in first pulse response amplitude between the two conditions is due to a change in release probability (see Materials and Methods). We can then derive n from the CV, and thereby calculate release probabilities for successive pulses within trains. For EPSP 20, depression within the train is accounted for by a fall in release probability without change in quantal size, with p falling ~70% by the end of the train from starting values of 0.45 and 0.94 for trains at 1 and 0.2 Hz respectively. Figure 4F illustrates the trajectory of 1/CV2 against mean amplitude over the course of a train of pulses for each trial repeat rate, closely matching in each case the predicted hyperbolic trajectory for pure changes in release probability.
Reciprocal Synaptic Connections Between Excitatory Neurons of Layer 4
Of the 20 pairs of layer 4 excitatory cells tested for reciprocity, four were reciprocally connected. One of these was the pair of pyramidal cells from which we recorded EPSPs 19 and 20 (Table 2). Another pair, consisting of a spiny stellate and an unidentified neuron, produced EPSPs 12 and 26 (Tables 1 and 3
). Although reciprocity was established for the remaining two pairs, adequate data were only collected for one direction in each case before one cell was lost. For these two pairs the reciprocal EPSPs do not appear in the tabulated data, but the reciprocal of EPSP 14 (Table 1
), from a spiny stellate to an unidentified neuron, was ~1 mV in amplitude and the reciprocal of EPSP 27 (Table 3
), from an unidentified neuron to a pyramidal cell, was ~200 µV.
Excitatory Synaptic Connections Between Layer 6 and Layer 4 Spiny Cells
Seven connections from layer 6 to layer 4 were recorded, as well as one connection from layer 4 to layer 6. The properties of the EPSPs recorded from the ascending connections are summarized in Table 4. The postsynaptic cells were recovered in three cases : two spiny stellates and one pyramidal cell. No connections were obtained in which both the presynaptic and the postsynaptic cell were recovered for morphological reconstruction. The presynaptic layer 6 cells, when recovered, had the dendritic and axonal morphology typical of the lateral geniculate nucleus-projecting' layer 6 pyramidal cells described by Katz (Katz, 1987
), which form the majority of layer 6 neurons.
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Dynamic Properties of Layer 6 to Layer 4 Excitatory Connections
The EPSPs recorded from the seven postsynaptic cells in layer 4 all showed facilitation (Table 4). This facilitation persisted throughout a train of multiple presynaptic spikes (Fig. 6A
). Figure 6B
shows the shift in amplitude distribution occurring between pulse 1 and pulse 2 for EPSP 34. Figure 6C
shows the rise in mean amplitude and fall in CV occurring over trains of three spikes. Figure 6D
plots 1/CV2 against mean amplitude for these trains. The solid line would be the predicted trajectory for a pure change in p without an increase in q if we were to assume a q of 200 µV, based on the regular interpeak spacing of the histograms in Figure 6B
(Larkman et al., 1991
). A plot of the trajectories of 1/CV2 against the mean during paired-pulse facilitation is shown for five different layer 6 to layer 4 connections in Figure 6E
.
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One EPSP was recorded in a layer 6 pyramidal cell during a presynaptic impalement in layer 4. The layer 4 cell was not recovered, but the layer 6 cell had the morphology of the layer 6 pyramidal cells described by Katz (Katz, 1987) that project to the claustrum. These pyramidal cells have a single unbranched apical dendrite extending through layer 2/3, one very thick dominant basal dendrite giving the soma a fusiform appearance, an asymmetric basal dendritic tree with branches extending horizontally, and basal dendrites descending into the white matter. The local projections of the axons of the claustrally projecting cells are largely confined to layers 5 and 6. Consistent with this general pattern, the layer 6 cell was not reciprocally connected to the layer 4 cell.
This EPSP is shown in Figure 7. The mean amplitude, with single action potentials evoked at 1 Hz, was 497 µV, with a CV of 0.48. Unlike the layer 6 pyramidal cells that project to layer 4, it showed 15% depression at an interpulse interval of 46 ms, without a change in CV. Pulse 1 amplitude increased to 851 µV at a lower trial repeat rate, with a CV of only 0.23, and under these circumstances pulse 2 depression was greater and associated with an increase in CV. If the difference in pulse 1 amplitudes at different repeat rates were due to release probability alone, then pulse 1 release probability at 1 Hz would be 0.36.
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Discussion |
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Our findings in vitro regarding the different dynamics of the excitatory synapses in layer 4 are consistent with the findings of Ferster and Lindström who examined the responses of simple cells in layer 4 in vivo (Ferster and Lindström, 1985a,b
). After destroying the relay cells of the lateral geniculate nucleus, they antidromically activated the recurrent collaterals of populations of layer 6 geniculocortical cells and observed augmentation', or facilitation, of the excitatory responses recorded in layer 4 neurons. The layer 6 pyramidal synapses with layer 4 neurons, therefore, are the likely origin of the augmenting response seen in electrically evoked cortical potentials in vivo (Dempsey and Morison, 1943
; Bishop et al., 1961
; Spencer and Brookhart, 1961
; Grossman et al., 1967
; Sasaki et al., 1970
; Morin and Steriade, 1981
). Ferster and Lindström also noted the slow rising phase of the responses, which they thought reflected latency scatter due to the slow conduction velocity of the corticogeniculate pyramidal cells (Ferster and Lindström, 1985b
). They supposed that the lack of discrete inflections on the depolarization was due to the convergence of many inputs from the layer 6 pyramidal cells, each giving rise to a very small unitary EPSP'. Consistent with this interpretation, we have shown that the each layer 6 pyramid evokes a small, facilitating EPSP in layer 4 postsynaptic cells. Ferster and Lindström also observed depression in both monosynaptic and the disynaptic EPSPs evoked by electrically stimulating relay neurons in the intact lateral geniculate nucleus (Ferster and Lindström, 1985b
). The disynaptic EPSP was probably generated by the excitatory connections between layer 4 cells that we also found to depress. The monosynaptic EPSP was due to the geniculocortical synapse, which we have previously shown to depress in vitro (Stratford et al., 1996
). Thus, the basic characteristics of the different excitatory synapses seen in layer 4 in vivo seem to be preserved, despite the insults involved in preparing and maintaining slices in vitro.
Morphology and Local Connections of Layer 4 Excitatory Neurons
Our frequent finding of pyramidal cells in layer 4 of cat visual cortex is at odds with the common assertion that layer 4 largely contains spiny stellate cells' (Peters and Payne, 1993), but is consistent with the findings of Martin and Whitteridge, who filled 17 spiny stellates and 14 star pyramidal cells in layer 4 during blind' in vivo recording (Martin and Whitteridge, 1984
). However, the spiny stellate neurons do have much richer projections within layer 4 than the star pyramids (Martin and Whitteridge, 1984
), so the spiny stellates are the major local source of excitation within layer 4 (Anderson et al., 1994
). Within the layer 4 connections, our data do not suggest marked differences between the synapses involving pyramidal versus spiny stellate neurons; there was, however, a tendency for the synaptic connections between spiny stellate cells to produce among the largest and most reliable EPSPs.
It has been estimated that at least 60% of the layer 6 pyramidal neurons in the cat have their major collateral axonal projection in layer 4 and form asymmetric synapses with the dendritic shafts of spiny and smooth neurons (McGuire et al., 1984; Katz, 1987
; Ahmed et al., 1994
). Seven synaptic connections we studied were of this circuit. We also recorded one connection from layer 4 to layer 6. The existence of a functional descending pathway from layer 4 to layer 6 has not been described, but the axons of layer 4 spiny neurons do send branches into the deep layers (Lund et al., 1979
; Martin and Whitteridge, 1984
). Spiny stellate cells also form synapses with the ascending dendrites of layer 6 pyramidal cells (J.C. Anderson, K.A.C. Martin and J.C. Nelson, unpublished observations) although the waveform of the EPSP we recorded did not suggest passive cable filtering of a distal input.
There is now anatomical (Ahmed et al., 1994) and physiological evidence (Ferster and Lindström, 1985a
,b
; Stratford et al., 1996
) that at least three distinct groups of presynaptic excitatory neurons form synapses with layer 4 spiny stellate cells in cat. The physiological properties of two of these types of excitatory synapses described here appear to depend on the identity of the presynaptic neuron. Studies in rat cortex have shown that synapses originating from a given class of cells either facilitate or depress according to the nature of the postsynaptic target (Thomson, 1997
; Markram et al., 1998
, Reyes et al., 1998
). Galarreta and Hestrin found that prolonged activation produced stronger depression at excitatory than at inhibitory cortical synapses in rat layer 5 cells (Galarreta and Hestrin, 1998
). Clearly, therefore, neither the presynaptic nor the postsynaptic neuron alone determines synaptic behaviour.
Our study focused on local connectivity, in contrast to the long-range horizontal connections known to exist in cat visual cortex. Over the range studied, we noted a decrease in EPSP amplitude with increasing physical separation of the neurons in a connected pair. This qualitatively supports a model of connectivity in which synapses between the cloud of axonal boutons of one neuron and of the dendritic arbour of another occur by chance (Braitenberg and Schuz, 1991). For spiny stellate cells the densities of synaptic boutons and their dendritic target sites approximate three-dimensional Gaussian distributions, so assuming random connectivity, the average number of release sites per synaptic connection decreases as a function of distance (Douglas et al., 1995
). The patterns of axonal arborization, meanwhile, impose broad constraints on connectivity, as seen in the predominance of ascending over descending connections within layer 4.
Estimates of Release Probabilities
For each population of synaptic connections, we have examined the relationship of mean amplitude to coefficient of variation within the population, in order to estimate the range of release probabilities at the synapses in question. Our estimates are robust with respect to our assumptions regarding the range of quantal size, such that our conclusions would vary little if we were to assume a much larger range of q. The release probabilities seen at the synapses mediating recurrent excitation in layer 4 are striking, and seem to pertain even when only small numbers of release sites mediate the connections. The actual release probabilities are probably even higher than estimated, because it has been assumed that the CV is due entirely to variation in quantal content. In reality, however, the stochastic variation in channel opening (Sigworth, 1980) may also contribute to the variance of the postsynaptic response, because AMPA channels can have opening probabilities not of 1.0 but nearer 0.7 (Hestrin, 1992
; Silver et al., 1996
) [see also Jack et al. (Jack et al., 1994
)]. This contribution to overall variance will be all the greater at high release probabilities, when quantal content varies relatively little. The synapses from layer 6 to layer 4 excitatory neurons, under the same stimulation conditions, operate at lower release probabilities, with no overlap between the two populations.
Smetters and Zador (1996) concluded in their review of available literature that, In the cortex, individual synapses seem to be extremely unreliable : the probability of transmitter release in response to a single action potential can be as low as 0.1 or lower' (Smetters and Zador, 1996). Similarly, in hippocampal pyramidal cells probabilities in the range 0.190.37 are considered to be high (Hessler et al., 1993
; Huang and Stevens, 1997
). Our quantitative estimates indicate that the excitatory synapses in layer 4 of cat visual cortex have release probabilities that are out of this range and we have had to scale our qualitative descriptions accordingly. Thus, the release probability for layer 6 pyramidal cell synapses in layer 4 is moderately high (0.370.56) and is very high (0.690.98) for layer 4 cell synapses. The thalamic synapses also fall in the very high range (Stratford et al., 1996
). It is clear that the synaptic transmission involved in the first stage of processing in the cat is much more secure than that reported in the rat.
Short-term Synaptic Plasticity
It has been suggested that the release probability determines the degree of short-term synaptic plasticity that may be effected. The reasoning is that a low initial release probability results in little depletion of putative synaptic resources, and thus less depression with repeated stimulation (Tsodyks and Markram, 1997) [see also Korn and Faber (Korn and Faber, 1987
), Thomson and West (Thomson and West, 1993
) and Thomson et al. (Thomson et al., 1993a
)], perhaps even unmasking facilitation. For a given synaptic connection this principle may operate, since we consistently observed that the factors that seemingly influenced the first pulse release probability (e.g. trial repeat rate), also changed the relative magnitude of depression on the second pulse. It is also true that the population of synapses showing facilitation was that with lower initial release probabilities. However, synaptic connections between layer 4 cells do not, on average, show marked depression, despite very high starting release probabilities. Furthermore, there is diversity even within this population, with some connections showing a greater tendency to depress than do others. This indicates that factors other than resource depletion must vary between synapses and contribute to net paired-pulse behaviour.
In investigating short-term synaptic plasticity in our two populations of synaptic connections, we studied paired-pulse changes in CV and mean. In normalized plots of 1/CV2 against mean, trajectories above the diagonal do not exclude changes in quantal size, e.g. from postsynaptic AMPA receptor desensitization (Trussell and Fischbach, 1989; Colquhoun et al., 1992
; Hestrin, 1992
). However, in the absence of knowledge of the precise hyperbolic trajectory for pure probability change, only trajectories with slope less than the diagonal necessarily involve changes in quantal size. For both depression at synapses between layer 4 cells and facilitation at layer 6 to layer 4 synapses, it is mainly modifications of presynaptic release probability that make the largest contribution to the dynamic behaviour of the synapses. This mechanism allows rapid modulation of synaptic efficacy for a given synaptic connection.
Relevance of Synaptic Dynamics to In Vivo Circuits
Benevento et al. wrote : it should be realized that excitatory interaction between cortical neurons may play a role in cortical organization.... This excitatory intracortical interaction could be viewed as modulating the intracortical inhibition which shapes the suprathreshold trigger feature properties of cortical neurons' (Benevento et al., 1972). We now understand that there are many factors that govern this interaction between inhibition and excitation. The characteristic strength and reliability of different synaptic connections, as well as their dynamic properties, must be included in consideration of how a given cortical cell is influenced by the stimulus-evoked patterns of discharge observed in vivo in the different neuronal populations constituting the cortical network. Dynamic synapses transmit different aspects of presynaptic activity depending on the pattern of activity and synaptic parameters (Markram et al., 1998
; Tsodyks et al., 1998
). Dynamic synaptic behaviour also implies that the balance of weights' of converging inputs shifts with time. The temporal properties of the connections discussed here, along with those of interactions involving inhibitory neurons in layer 4 (Tarczy-Hornoch et al., 1998
) and LGN inputs (Stratford et al., 1996
), are represented schematically in Figure 8
.
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Acknowledgments |
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References |
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