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J Physiol Volume 556, Number 1, 147-157, April 1, 2004 DOI: 10.1113/jphysiol.2003.053306
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Augmentation of corticogeniculate EPSCs in principal cells of the dorsal lateral geniculate nucleus of the rat investigated in vitro

Björn Granseth and Sivert Lindström

Department of Biomedicine and Surgery, Faculty of Health Sciences, Linköping University, SE-581 85 Linköping, Sweden


   Abstract
Top
 Abstract
Introduction
 Methods
 Results
 Discussion
 References
 
Augmentation is a component of short-term synaptic plasticity with a gradual onset and duration in seconds. To investigate this component at the corticogeniculate synapse, whole cell patch-clamp recordings were obtained from principal cells in a slice preparation of the rat dorsal lateral geniculate nucleus. Trains with 10 stimuli at 25 Hz evoked excitatory postsynaptic currents (EPSCs) that grew in amplitude, primarily from facilitation. Such trains also induced augmentation that decayed exponentially with a time constant {tau}= 4.6 ± 2.6 s (mean ± standard deviation). When the trains were repeated at 1–10 s intervals, augmentation markedly increased the size of the first EPSCs, leaving late EPSCs unaffected. The magnitude of augmentation was dependent on the number of pulses, pulse rate and intervals between trains. Augmented EPSCs changed proportionally to basal EPSC amplitudes following alterations in extracellular calcium ion concentration. The results indicate that augmentation is determined by residual calcium remaining in the presynaptic terminal after repetitive spikes, competing with fast facilitation. We propose that augmentation serves to maintain a high synaptic strength in the corticogeniculate positive feedback system during attentive visual exploration.

(Received 12 August 2003; accepted after revision 14 January 2004; first published online 14 January 2004)
Correspondence address B. Granseth: MRC Laboratory of Molecular Biology, Hills Road, Cambridge CB2 2QH, UK.  Email: bjogr{at}mrc-lmb.cam.ac.uk


   Introduction
Top
 Abstract
 Introduction
Methods
 Results
 Discussion
 References
 
Principal cells in the dorsal lateral geniculate nucleus (dLGN) receive a massive feedback projection from pyramidal cells in layer 6 of the primary visual cortex (Gilbert & Kelly, 1975; Schmielau & Singer, 1977; Ahlsén et al. 1982; Ferster & Lindström, 1982; Murphy & Sillito, 1987; Guillery & Sherman, 2002). The corticogeniculate neurones form glutermatergic synapses that display a pronounced frequency enhancement with repetitive activation (Ahlsén et al. 1985; Lindström & Wróbel, 1990; Turner & Salt, 1998). Two components of facilitation with decay time constants of tens and hundreds of milliseconds have recently been found to underlie this form of short-term synaptic plasticity (Granseth et al. 2002; Granseth, 2004). However, in the accompanying study (Granseth, 2004) a third, much slower component was encountered when using train stimulation at frequencies <5 Hz. This could be augmentation, a type of short-term plasticity that grows and persists during seconds, originally identified at the neuromuscular junction (Magleby & Zengel, 1976; Magleby, 1987; Zucker & Regehr, 2002).

The term augmentation, as used in the present study, should not be confused with the older concept augmenting response (Dempsey & Morison, 1943), which denotes the build-up of excitation in primary cortical areas in response to repetitive stimulation of the corresponding specific thalamic nuclei. For the visual system, it has been shown that the augmenting response is due to antidromic activation of corticogeniculate neurones and their intracortical axon collaterals that terminate on spiny stellate cells in layer 4 (Ferster & Lindström, 1985a,b). Thus, the augmenting response is the result of short-term synaptic enhancement at intracortical terminals of the very same cells that project to the dLGN.

Investigations of augmentation at central synapses have so far been limited to a few studies pertinent to the hippocampus (McNaughton, 1982; Regehr et al. 1994; Stevens & Wesseling, 1999; Rosenmund et al. 2002). It would therefore be of interest to examine this type of short-term synaptic plasticity at yet another synapse, the corticogeniculate, considering its exceptionally large facilitation and resistance to synaptic depression (Granseth, 2004). To that end, whole cell patch-clamp recordings were obtained from dLGN principal cells in acute brain slices from young pigmented rats. Some results have been presented in abstract form (Granseth et al. 2000).


   Methods
Top
 Abstract
 Introduction
 Methods
Results
 Discussion
 References
 
The preparation of dLGN slices and recording procedures are described in detail in the accompanying paper (Granseth, 2004). In brief, dLGN slices (250–300 µm) were prepared from 23- to 37-day-old pigmented DA-HAN rats (BK Universal, Sollentuna, Sweden) of either sex, following anaesthesia with halothane (ISC Chemicals, Avonmouth, UK) and decapitation. Recordings were made in a chamber with slices submerged in Krebs solution containing (mM): NaCl, 124; NaH2PO4, 1.25; NaHCO3, 26; KCl, 3.0; MgCl2 2.0; CaCl2, 2.0; and glucose, 10; at 34°C equilibrated with 95% O2 in 5% CO2. Picrotoxin and DL-APV (Sigma, St Louis, MO, USA) were routinely added to final concentrations of 100 µM. To maintain constant fibre thresholds, magnesium concentration of the Krebs solution was adjusted as described earlier when changes were made in calcium ion concentration (Granseth et al. 2002). The Committee for Ethics in Animal Research of Linköping approved experiments in accordance with Swedish animal-welfare legislation.

EPSCs were recorded by whole cell patch-clamp technique using borosilicate glass microelectrodes (tip resistance 3–6 M{Omega}) filled with electrode buffer containing (mM): caesium gluconate, 100; NaCl, 10; Hepes, 10; TEA-Cl, 20; QX-314, 5.0; EGTA, 0.10; and MgATP 1.0; pH adjusted to 7.3 and osmolality to 300 mmol kg–1. Principal cells in the dLGN were identified by their location, size and activation properties. Most cells provided data also for the accompanying paper (Granseth, 2004). Neurones were voltage clamped at –70 mV, adjusted for a liquid junction potential of 8 mV, access resistance was <25 M{Omega} and not allowed to vary >10%.

Corticogeniculate fibres were stimulated with amplitude graded voltage pulses of 0.2ms duration. The standard stimulation protocol consisted of 10 trains of 10 pulses, each at 25 Hz with 1 s intervals between trains (Fig. 1A). For averaging, 4–10 sets were given with 60 s resting periods between sets. In such stimulation protocols, intervals between trains were varied between 1 and 15 s, train frequencies between 1 and 50 Hz and pulse numbers between 5 and 10. All estimates of augmentation and/or facilitation are relative to the amplitude of first EPSC in the first train (EPSCn: n/EPSC1: 1, subscripts defined in Fig. 1A). Functions were fitted using a least-sum-of-squares method in IgorPro (Wavemetrics, Lake Oswego, OR, USA) or Origin (MicroCal, Northamption, MA, USA). Functions describing facilitation were fitted as described in the accompanying paper (Granseth, 2004). For augmentation, the relative peak amplitudes of the first EPSC in each repeated train were fitted with the functions f(n) =Ass–Kx exp(–n/k) (n= train sequential number) or f(t) =Ass–Kx exp(–t/{tau}) (t= cumulative time in between trains), weighed by the reciprocal of the train sequential number. For the decay of augmentation f(t) = 1 +Ax exp(–t/{tau}) was fitted to the relative amplitude of a test pulse at different times after a train. Data are given as mean ± standard deviation unless otherwise stated. Student t test, paired or unpaired, was used when appropriate to test for significance P < 0.05. Multiple comparisons were evaluated by analysis of variance (ANOVA) with {alpha}= 0.05.



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  		Figure 1.  Augmentation of corticogeniculate EPSCs by repeated trains of stimulation
A, convention used for labelling evoked EPSCs. First subscript refers to position in train while second subscript is train sequential number. B, sample records illustrating increase in first EPSCs in trains with repetitive train stimulation. Records are averages of 10 sets of trials with 10 pulses at 25 Hz per train repeated 10 times at 1 s intervals. C, development of EPSC amplitudes with successive trains as in A. Amplitudes are normalized to EPSC1 of the first train. Mean ±S.E.M. of 12 cells. Note the gradual increase in amplitude of EPSC1 in successive trains paralleled with a reduction in fast facilitation. Lines are double exponential functions (see Methods) with time constants {tau}1= 39ms and {tau}2= 470ms fitted to the data points of each train.

 

   Results
Top
 Abstract
 Introduction
 Methods
 Results
Discussion
 References
 
Different aspects of augmentation of corticogeniculate EPSCs were investigated in 29 dLGN cells from 24 animals. The stimulation protocols (Methods) included 60 s resting periods between sets of trains to ascertain that EPSC amplitudes returned to baseline. Thus, it was not feasible to use unitary EPSCs that are very small with many failures and require large numbers of trials for reliable quantification (Granseth & Lindström, 2003). To reduce variability we have evoked compound EPSCs, using stimuli that recruit many corticogeniculate axons, a procedure justified by the uniform properties of corticogeniculate synapses (Granseth et al. 2002; Granseth, 2004).

Augmentation with repetitive train stimulation

As shown in an accompanying study (Granseth, 2004), the growth in EPSC amplitude during a 25Hz train with 10 impulses could be described by a sum of two exponential functions. Time constants of build-up were {tau}1= 39 ± 19ms and {tau}2= 470 ± 270ms (mean ± standard deviation, 12 cells). This EPSC growth has been assigned to two components of facilitation representing short-term synaptic plasticity at time scales of tens and hundreds of milliseconds, found at several types of synapses (Katz & Miledi, 1968; Magleby, 1987; Zucker & Regehr, 2002). When the same impulse train was repeated at 1 s intervals, the first EPSCs of each train increased markedly in amplitude, while later EPSCs remained largely unchanged (Fig. 1). This growth of EPSC1 cannot be accounted for by the two components of facilitation. The time constants of decay of paired pulse facilitation in a previous study were 12 and 160ms (Granseth et al. 2002). It is possible that these time constants are prolonged when stimulus trains induce massive facilitation, but even if time constants were 3 times larger than the above less than 10% of the slow component would remain after 1.0 s, while the fast component that accounts for most facilitation would have decayed to zero (Granseth, 2004). In contrast, there was an almost fivefold increase in EPSC1 with repeated trains (Fig. 1C). In fact, it seemed as if the fast component of facilitation was virtually obliterated by this progressive increase in EPSC1.

The necessity to infer an additional component of short-term synaptic plasticity with longer duration, i.e. augmentation (Magleby & Zengel, 1976), became even more apparent when the interval between trains was increased. When plotting the growth of EPSC1 in repeated trains with different train intervals, a gradual increase in the ratio EPSC1: n to EPSC1: 1 (n= train sequential number) was still apparent with 10 s between trains (Fig. 2C, see grey symbols). Obviously, there was a component of enhancement lasting >10 s.



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  		Figure 2.  Augmentation of EPSC1 with repetitive trains at different train intervals
A, schematic illustration of stimulation protocol constituting 10 trains with 10 pulses at 25 Hz. Train interval was varied between trials in experiments below. B, sample records from a representative cell of the 1st, 3rd and 6th train with 1 s train intervals. EPSC1 in each train is highlighted in grey. Averages of 5 traces. C, relative increase in EPSC1 compared to that of first train, with different train intervals; 1 s ({blacksquare}); 2 s ({square}); 5s ( ); 10 s ({square}); 15 s ( ). Continuous lines are least-sum-of-squares fits of the exponential function f(n) =Ass–Kx exp(–n/k) with k constrained to the value 1.6 obtained for 1 s interval. Mean ±S.E.M. of 6 cells.

 
Augmentation presumably involves a presynaptic mechanism that increases the transmitter release probability (Magleby & Zengel, 1976). To test if postsynaptic activity was important for the change in EPSC1 with repetitive trains, ionotropic glutamate receptors were partially blocked with 1.0 mM kyunurenic acid (4 cells). Although the evoked EPSCs were only 20 ± 5% of the original amplitude, the growth of EPSC1 with trains at 1 s intervals was unchanged (P= 0.67, two-factor ANOVA with replication). Since corticogeniculate synapses also harbour metabotropic glutamate receptors (McCormick & von Krosigk, 1992; Turner & Salt, 1998) the group I/II mGlu antagonist (S)-{alpha}-methyl-4-carboxyphenylglycine (200 µM; 3 cells) or the group II/III mGlu antatagonist (RS)-{alpha}-cyclopropyl-4-phosponophenylglycine (100 µM) were investigated in separate experiments (4 cells). Neither drug affected the enhancement of EPSC1 in repeated trains (P= 0.20 and 0.60, respectively, two-factor ANOVA with replication). Thus, postsynaptic mechanisms seem not to be involved in the increase in EPSC1 amplitude. Presynaptic feedback mechanisms relying on glutamate signalling can also be excluded (Scanziani et al. 1997).

The decay time course was investigated with a single test pulse at different intervals following a 10-impulse, 25-Hz train (Fig. 3). It could be described by a single exponential function with time constant {tau}= 4.6 ± 2.6 s (9 cells). Only single test pulses followed by 60 s resting periods were used since repetitive stimulation faster than 0.25 Hz seemed to prolong the duration of the effect. The obtained time constant agrees well with the time course of augmentation at other synapses (Magleby & Zengel, 1976; McNaughton, 1982; Magleby, 1987; Stevens & Wesseling, 1999; Rosenmund et al. 2002; Zucker & Regehr, 2002). Thus it is safe to conclude that augmentation is present also at the corticogeniculate synapse.



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  		Figure 3.  Augmentation decay time course after a 10-impulse, 25-Hz train
A, relative increase in amplitude of EPSCs evoked by a test pulse at different intervals after a 25 Hz train of 10pulses. Continuous line is the decay time course described by a single exponential function with time constant {tau}= 4.6 s, corresponding to augmentation. Inset is a schematic illustration of stimulation protocol used. Mean ±S.E.M. of 9 cells. B, sample records from a representative cell with the test pulse at 1, 5 and 10 s interval after train. Averages of 4 traces.

 
The build-up of augmentation for EPSC1 with repeated trains was well described by an exponential function of the type f(n) =Ass–Kx exp(–n/k) (Fig. 2C). Interestingly, the denominator in the exponential, k, was similar at train intervals between 1 and 10 s (1.6 ± 0.2; 5 cells). Transformed into the temporal domain (f(t) =Ass–Kx exp(–t/{tau})) time constants were 1.6–14.4 s, with higher values for longer train intervals. The decay of augmentation at the neuromuscular junction usually deviates slightly from a monoexponential function, with augmentation decaying slower at longer intervals (Magleby & Zengel, 1976). A similar trend was seen at the corticogeniculate synapse (Fig. 3A). The observed increase in time constant of build-up with longer train intervals could be explained by this deviation. A correspondence between build-up and decay time constants would only have been seen if the augmentation induced by single trains had a true monoexponential decay and added linearly with repetitive trains (Helmchen et al. 1996). The steady-state augmentation, defined by Ass in the build-up functions, decreased from 4.0 ± 0.7 at 1 s intervals to 1.5 ± 0.2 with 10 s in between trains (Fig. 2C).

Magnitude of augmentation at different train frequencies

EPSC1 displayed pronounced augmentation for trains at 10–50 Hz with a threshold effect at about 1 Hz (Fig. 4C). The magnitude of augmentation was frequency dependent. Exponential functions with the same k (1.7 ± 0.1; 6 cells) and increasing Ass could describe the build-up in amplitude. In other investigations, augmentation has been reported to increase linearly with stimulation rate (Delaney & Tank, 1994; Stevens & Wesseling, 1999). A similar tendency was evident in the present study although a sigmoidal relationship might seem more fitting (Fig. 4C, inset). In the previous studies, the train duration was kept constant when the frequency was altered. In that case, the effect from a larger number of stimulation pulses further enhanced the augmentation at high frequencies, as described below, and the relationship would become more linear. In contrast to facilitation that is optimal at 25 Hz (Granseth, 2004), the augmentation at the corticogeniculate synapse continued to grow when the train frequency increased from 25 to 50 Hz (Fig. 4C).



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  		Figure 4.  Change in magnitude of augmentation with train pulse frequency
A, schematic illustration of stimulation protocol constituting trains with 10 pulses repeated 10 times with 1 s interval. Train frequency was varied between trials. B, EPSC amplitudes normalized to EPSC1 at first train with 25Hz (•, upper panels) and 10 Hz trains ( , lower panels). Note difference in time scale during trains while train interval is constant (1 s) throughout. Mean ±S.E.M. of 5 cells. C, augmentation of EPSC1 for the same cells as in A; 50 Hz ({square}); 25 Hz ({blacksquare}); 17 Hz ( ); 10 Hz ({square}); 1 Hz ( ). Continuous lines are least-sum-of-squares fits as in Fig. 2C. Inset shows augmentation (Aug) for first EPSC in train 10 (EPSC1: 10/EPSC1: 1; {blacktriangleup}) and lack of augmentation of EPSC10 (EPSC10: 10/EPSC10: 1; {triangleup}) at different train frequencies (Freq.). Continuous lines are linear regressions with slopes 0.1 for EPSC1 and 0.002 for EPSC10. Same cells as in B.

 
One important finding is that late EPSCs in trains were negligibly affected by augmentation. This cannot be explained by EPSC saturation, as the same pattern was visible also at low train frequencies with smaller EPSC amplitudes (Fig. 4B). For instance, the average amplitude of EPSC10 at 10 Hz was only half that at 25 Hz, yet there was virtually no augmentation of the last EPSC in the trains (Fig. 4A). Furthermore, a plot of the ratio EPSC10: 10/EPSC10: 1 against train frequency gave a regression line with a slope close to zero ({Delta}y= 0.002; Fig. 4C, inset). As EPSC1 was substantially increased by augmentation whilst late EPSCs were not, there was an apparent reduction in facilitation with augmentation. Accordingly, the two forms of short-term EPSC enhancement are not independent phenomena at the corticogeniculate synapse.

Effect of pulse number on augmentation

When the number of pulses in the trains was reduced from 10 to 5, augmentation was less prominent (Figs 5B and C; 4 cells). An exponential function fitted to the EPSC1 growth for 10-pulse trains moved towards Ass= 4.0 ± 0.4, with k= 1.4 ± 0.2 (Fig. 5C). Using 5-pulse trains in the same cells, the exponential function approached a lower Ass= 2.9 ± 0.6 more gradually, k= 2.2 ± 0.1. Apparently, the build-up of augmentation with each train depends on the number of impulses in the trains.



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  		Figure 5.  Augmentation depends on the number of impulses
A, schematic illustration of stimulation protocol constituting 25 Hz trains repeated 10 times. Number of pulses in trains and interval between trains were varied in experiments below. B, EPSC amplitudes normalized to EPSC1 at first train for trains with 10 (•, upper panel) and 5 pulses ({circ}, middle panel) at 1 s train intervals and for trains with 10 pulses at 2 s train intervals ( , lower panel). Same time scale in all diagrams but note difference in time of train onset. Mean ±S.E.M. of 4 cells. C, augmentation of EPSC1 for the same cells as in B; 10 pulses at 1 s train intervals ({blacksquare}); 5 pulses at 1 s train intervals ({square}); 10 pulses at 2 s train intervals ({square}). Continuous lines are least-sum-of-squares fits as in Fig. 2C with k= 1.4 and 1.3 for 10 pulse-trains at 1 and 2 s intervals, and k= 2.2 for 5 pulse trains. D, augmentation of EPSC1 as in C for 10 pulse trains at 2 s train intervals ({square}) and 5 pulse trains at 1 s train intervals ({square}), plotted against cumulative time in between trains. Note the similar time constants for the two modes of stimulation, {tau}= 2.2 s and 2.5 s for 5 and 10 pulse trains, respectively.

 
To further test this notion, augmentation with 5-pulse trains at 1 s intervals was compared to that of 10-pulse trains with 2 s separations (Fig. 5D), as the average number of spikes per unit time would be alike. The rate of EPSC growth in time was similar for these two stimulation patterns: {tau}= 2.2 ± 0.1 s for 5-pulse trains and 2.5 ± 0.4 s for 10-pulse trains. The above results imply that the number of spikes per train defines the build-up of augmentation with each train, while the average number of spikes per unit time determines the temporal aspects of augmentation.

Augmentation at different [Ca2+]o

Changes in extracellular calcium ion concentration ([Ca2+]o) have profound effects on basal corticogeniculate EPSC amplitudes and facilitation (Granseth et al. 2002; Granseth, 2004). When [Ca2+]o was lowered to 1.0 mM, the amplitude of EPSC1 of the first train was reduced to 37% of that at 2.0 mM (4 cells; Fig. 6A) while it was increased to 247% at 3.0 mM (3 cells; Fig. 6B). Also the facilitation during the trains was much affected, while the parameters k or {tau} and Ass characterizing the augmentation of EPSC1 did not change significantly (P > 0.2, paired Student's t tests).



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  		Figure 6.  Augmentation at different [Ca2+]o
A, EPSC amplitudes at 2.0 (•, upper panel) and 1.0 mM[Ca2+]o ({circ}, upper panel) normalized to amplitude of EPSC1 for the first train at 2.0 mM. Augmentation of EPSC1 at 2.0 ({blacksquare}, lower panel) and 1.0 mM[Ca2+]o ({square}, lower panel). Trains of 10 pulses at 25 Hz repeated at 1 s intervals, mean ±S.E.M. of 4 cells. B, normalized EPSC amplitudes as in A at 2.0 (•, upper panel) and 3.0 mM[Ca2+]o ({circ}, upper panel) and augmentation ({blacksquare} and {square}, lower panel). Same stimulation pattern as in A, mean ±S.E.M. of 3 cells.

 
The relatively stable augmentation (EPSC1: n/EPSC1: 1) with the different [Ca2+]o investigated is well illustrated by records from a representative cell, normalized with respect to EPSC1 of the first train (Fig. 7). The augmentation values for EPSC1: 5 and EPSC1: 10 were 3.1 and 3.4 at 1.0 mM, 3.3 and 3.2 at 2.0 mM and 3.2 and 3.1 at 3.0 mM. In contrast, the facilitation for EPSC10 in the first train decreased from 8.1 to 2.3 when going from 1.0 to 3.0 mM[Ca2+]o. One consequence of the different [Ca2+]o dependency of augmentation and facilitation was that augmented first EPSCs (EPSC1: 5 and EPSC1: 10) actually became larger than later EPSCs in the trains at 3.0 mM[Ca2+]o (Fig. 7, lower traces). There was no augmentation of EPSC10 in the trains at any [Ca2+]o (Figs 6 and 7).



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  		Figure 7.  Augmentation of EPSC1 is proportional to basal EPSC1 amplitude at different [Ca2+]o
Records are averages of 10 traces of 25 Hz trains at 1 s intervals with 1.0, 2.0 and 3.0 mM[Ca2+]o. First, 5th and 10th trains are shown. Traces are scaled so that EPSC1 traces of train 1 are equal in size at different [Ca2+]o (see scale bars). Note that the augmentation of EPSC1 (shaded area) is similar at different [Ca2+]o while facilitation markedly decreases with increasing [Ca2+]o. All traces are from the same cell with the same stimulation intensity used throughout, shock artefacts are truncated for clarity.

 

   Discussion
Top
 Abstract
 Introduction
 Methods
 Results
 Discussion
References
 
Augmentation was identified as a third component of short-term plasticity at the corticogeniculate synapse, the other two being fast and slow components of facilitation (Granseth et al. 2002; Granseth, 2004). It was manifest as a progressive increase in the first EPSCs in each train with repeated train stimulation, leaving late EPSCs unaffected. The augmentation of EPSC1 could empirically be described by an exponential function with steady-state levels determined by the interval separating the trains, pulse frequency and number of pulses. The decay time constant was 4.6 s after a 10-pulse train at 25 Hz, i.e. about one and two orders of magnitude longer than that of the two components of facilitation. Although changes in [Ca2+]o had profound effects on corticogeniculate EPSC facilitation, augmented EPSCs changed in proportion to the basal EPSC amplitude.

Facilitation and augmentation are not independent at the corticogeniculate synapse

Early studies on short-term synaptic plasticity were performed at the neuromuscular junction under conditions of reduced transmitter release, usually obtained by lowering the extracellular Ca2+: Mg2+ ratio (Katz & Miledi, 1968; Magleby & Zengel, 1976; Magleby, 1987). In this preparation, facilitation and augmentation are independent phenomena, which means the relative enhancement from facilitation is the same whether augmentation has increased the basal synaptic strength or not. This situation contrasts to the corticogeniculate synapse at normal [Ca2+]o, where augmentation enhanced the first EPSCs in trains with a proportional reduction in the apparent facilitation. Primarily the fast component of facilitation, accounting for most of the EPSC growth during trains, was affected. A diminished facilitation with augmentation has also been observed at synapses in the hippocampus (McNaughton, 1982). This interaction has been ascribed to depletion of release-competent synaptic vesicles when augmentation steps up transmitter release. Such a mechanism is attractive for being simple and distinct but cannot explain why augmentation at the corticogeniculate synapse still competes with facilitation with slow train stimulation or decreased [Ca2+]o, when the overall transmitter release is low.

Augmentation can, in principle, step up transmitter release either by increasing the number of release-competent synaptic vesicles or by increasing the efficacy by which presynaptic spikes induce exocytosis of such vesicles. For cultured hippocampal neurones, where the readily releasable pool of vesicles can be probed by hypertonic buffer challenges, augmentation has been ascribed to an increase in efficacy of vesicle release (Stevens & Wesseling, 1999). More recently, a small subpopulation of hippocampal synapses has been identified, where augmentation seems to be associated with an increased number of release-competent vesicles (Rosenmund et al. 2002). These synapses utilize Munc13-2 instead of Munc13-1 for initiating SNARE interactions at the presynaptic terminal (Augustin et al. 1999) and display augmentation at normal [Ca2+]o when Munc13-1 synapses show synaptic depression. The proposed mechanism implies that residual [Ca2+]i activates phopholipase C which leads to direct diacylglycerol-mediated activation of Munc13-2 and more releasable vesicles (Jeong-Seop et al. 2002; Rosenmund et al. 2002). The relevance of this scheme for the corticogeniculate synapse is at present difficult to evaluate. Other presynaptic proteins can be influenced by residual [Ca2+]i, for instance GAP-43 (DeGraan et al. 1994). This molecule is selectively expressed at the corticogeniculate synapse while being absent at the frequency depressing retinogeniculate synapse (Bickford, 1999).

An increase in the number of release-competent vesicles with augmentation is difficult to associate with the fact that the end-point corticogeniculate synaptic strength was unchanged by augmentation. However, the possibility remains that augmentation induces a subpopulation of highly releasable vesicles. Since the basal efficacy of transmitter release is low at the corticogeniculate synapse (psyn < 0.1; Granseth & Lindström, 2003), the addition of only a few highly efficient vesicles would have a marked effect on transmitter release (Hanse & Gustafsson, 2001, 2002). Accordingly, EPSC1 in each train would be much enhanced while the augmentation of subsequent EPSCs would disappear as the induced vesicles are consumed. Late EPSC amplitudes in the trains would then rely solely on facilitation, acting on the regular release-competent vesicles. Such a heterogeneity in vesicle release properties has recently been observed in two giant synapses in the central nervous system where the presynaptic terminals can be patch-clamped (Sakaba & Neher, 2001; Burrone et al. 2002).

If augmentation and facilitation act on separate vesicle populations, they would be expected to uncouple in certain situations. This was not seen; on the contrary the augmentation and the reduction in facilitation were perfectly balanced. Such consistent behaviour suggests that augmentation and the fast component of facilitation interact at a shared mechanism. Fast facilitation at the corticogeniculate synapse seems related to a saturable high-affinity Ca2+ buffer that chelates most of the Ca2+ that enters the presynaptic terminal with the first spike (Neher, 1998; Rozov et al. 2001; Trommershäuser et al. 2003; Granseth, 2004). With buffer saturation, more Ca2+ will be available for transmitter release at subsequent spikes giving facilitation of EPSCs. Augmentation is widely acknowledged to be dependent on residual [Ca2+]i that accumulates and distributes in the presynaptic terminal with repetitive spikes (Magleby & Zengel, 1976; Magleby, 1987; Delaney & Tank, 1994; Kamiya & Zucker, 1994; Zucker & Regehr, 2002). Such a global increase in [Ca2+]i could maintain the high-affinity Ca2+ buffer more or less saturated and reduce its capacity to capture Ca2+ at the release site (Meinrenken et al. 2003; Trommershäuser et al. 2003). As a result, EPSC1 in the trains will be augmented due to increased efficacy of transmitter release with a parallel decrease in facilitation.

Is augmentation of functional importance in vivo?

In his searchlight hypothesis, Crick (1984) suggested that recurrent excitation of dLGN principal cells would provide a neuronal correlate of visual attention. Such feedback excitation could be supplied by corticogeniculate neurones that fire consistently only in awake animals (Livingstone & Hubel, 1981). The pronounced short-term synaptic enhancement of this pathway (Ferster & Lindström, 1985a,b; Lindström & Wróbel, 1990; McCormick & von Krosigk, 1992; Turner & Salt, 1998; Granseth et al. 2002) would allow the corticogeniculate feedback system to function as a neuronal amplifier that potentiates the geniculate relay of visual information (Ahlsén et al. 1985; Granseth, 2004). Fast facilitation, which induces a manyfold increase synaptic strength during the first few hundred milliseconds of cell firing, would primarily determine the gain of the neuronal amplifier (Granseth, 2004). Since facilitation involves a presynaptic mechanism (Granseth & Lindström, 2003) it acts exclusively at those synapses that are activated by a particular visual stimulus, thus making the neuronal amplifier strictly stimulus specific. A low basal synaptic strength ascertains that random single spikes in converging corticogeniculate neurones will be virtually ineffective at the target cell. This property would protect the positive feedback system from self-generated cyclic activity.

Augmentation, also being presynaptic in nature, would have a similar effect to facilitation but over a longer time scale. The physiological role of this component of short-term synaptic enhancement is more uncertain since fixation points are normally changed 3–5 times per second during active vision (Fuller, 1985; Chelazzi et al. 1989; Martinez-Conde et al. 2002). As augmentation enhances the early EPSCs in spike trains, it reduces the time required to reach an effective level of synaptic strength. Thus augmentation may serve to preserve a high gain of the amplifier during attentive visual exploration when the gaze may return repeatedly to the same fixation point (Yarbus, 1967).


   References
Top
 Abstract
 Introduction
 Methods
 Results
 Discussion
 References
 
Ahlsén G, Grant K & Lindström S (1982). Monosynaptic excitation of principal cells in the lateral geniculate nucleus by corticofugal fibers. Brain Res 234, 454–458.[CrossRef][Medline]

Ahlsén G, Lindström S & Lo F-S (1985). Interaction between inhibitory pathways to principal cells in the lateral geniculate nucleus of the cat. Exp Brain Res 58, 134–143.[Medline]

Augustin I, Rosenmund C, Südhof TC & Brose N (1999). Munc13–1 is essential for fusion competence of glutamatergic synaptic vesicles. Nature 400, 457–461.[CrossRef][Medline]

Bickford ME (1999). Growth-associated protein 43 is located in type I corticothalamic terminals in the cat visual thalamus. J Neurosci 19, RC22 (1–5).[Abstract/Free Full Text]

Burrone J, Neves G, Gomis A, Cooke A & Lagnado L (2002). Endogenous calcium buffers regulate fast exocytosis in the synaptic terminal of retinal bipolar cells. Neuron 33, 101–112.[CrossRef][Medline]

Chelazzi L, Rossi F, Tempia F, Ghirardi M & Strata P (1989). Saccadic eye movements and gaze holding in the head-restrained pigmented rat. Eur J Neurosci 1, 639–646.[CrossRef][Medline]

Crick F (1984). Function of the thalamic reticular complex: The searchlight hypothesis. Proc Natl Acad Sci U S A 81, 4586–4590.[Abstract/Free Full Text]

DeGraan PN, Hens JJ & Gispen WH (1994). Presynaptic PKC substrate B-50 (GAP-43) and neurotransmitter release: studies with permeated synaptosomes. Neurotoxicology 15, 41–47.[Medline]

Delaney KR & Tank DW (1994). A quantitative measurement of the dependence of short-term synaptic enhancement on presynaptic residual calcium. J Neurosci 14, 5885–5902.[Abstract]

Dempsey EW & Morison RS (1943). The electrical activity of a thalamocortical relay system. Am J Physiol 138, 283–296.[Free Full Text]

Ferster D & Lindström S (1982). An intracellular analysis of geniculo-cortical connectivity in area 17 of the cat. J Physiol 342, 181–215.

Ferster D & Lindström S (1985a). Augmenting responces evoked in area 17 of the cat by intracortical axon collaterals of cortico-geniculate cells. J Physiol 367, 217–232.[Abstract/Free Full Text]

Ferster D & Lindström S (1985b). Synaptic excitation of neurones in area 17 of the cat by intracortical axon collaterals of cortico-geniculate cells. J Physiol 367, 233–252.[Abstract/Free Full Text]

Fuller JH (1985). Eye and head movements in the pigmented rat. Vision Res 25, 1121–1128.[CrossRef][Medline]

Gilbert CD & Kelly JP (1975). The projection of cells in different layers of the cat's visual cortex. J Comp Neurol 163, 81–106.[CrossRef][Medline]

Granseth B (2004). Dynamic properties of corticogeniculate excitation in the rat dorsal lateral geniculate nucleus in vitro. J Physiol 000, 000–000.

Granseth B, Ahlstrand E & Lindström S (2000). Augmentation of EPSCs at corticogeniculate synapses in the dorsal lateral geniculate nucleus in vitro. Abstract Soc Neurosci 26, 1195.

Granseth B, Ahlstrand E & Lindström S (2002). Paired pulse facilitation of corticogeniculate EPSCs in the dorsal lateral geniculate nucleus of the rat investigated in vitro. J Physiol 544, 477–486.[Abstract/Free Full Text]

Granseth B & Lindström S (2003). Unitary EPSCs of corticogeniculate fibers in the rat dorsal lateral geniculate nucleus in vitro. J Neurophysiol 89, 2952–2960.[Abstract/Free Full Text]

Guillery RW & Sherman SM (2002). Thalamic relay functions and their role in corticocortical communicarion: generalizations from the visual from the visual system. Neuron 33, 163–175.[CrossRef][Medline]

Hanse E & Gustafsson B (2001). Paired-pulse plasticity at the single release site level: an experimental and computational study. J Neurosci 21, 8362–8369.[Abstract/Free Full Text]

Hanse E & Gustafsson B (2002). Release dependence to a paired stimulus at a synaptic release site with a small variable pool of immediately releasable vesicles. J Neurosci 22, 4381–4387.[Abstract/Free Full Text]

Helmchen F, Imoto K & Sakmann B (1996). Ca2+ buffering and action potential-evoked Ca2+ signaling in dendrites of pyramidal neurons. Biophys J 70, 1069–1081.[Abstract/Free Full Text]

Jeong-Seop R, Betz A, Pyott S, Reim K, Varoqueaux F, Augustin I, Hesse D, Südhof TC, Takahashi M, Rosenmund C & Brose N (2002). ß phorbol ester and diacylglycerol-induced augmentation of transmitter release is mediated by Munc13s and not by PKCs. Cell 108, 121–133.[CrossRef][Medline]

Kamiya H & Zucker RS (1994). Residual Ca2+ and short-term synaptic plasticity. Nature 371, 603–606.[CrossRef][Medline]

Katz B & Miledi R (1968). The role of calcium in neuromuscular facilitation. J Physiol 195, 481–492.[Abstract/Free Full Text]

Lindström S & Wróbel A (1990). Frequency dependent corticofugal excitation of principal cells in the cat's dorsal lateral geniculate nucleus. Exp Brain Res 79, 313–318.[CrossRef][Medline]

Livingstone MS & Hubel DH (1981). Effects of sleep and arousal on the processing of visual information in the cat. Nature 291, 554–561.[CrossRef][Medline]

McCormick DA & von Krosigk M (1992). Corticothalamic activation modulates thalamic firing through glutamate ‘metabotropic’ receptors. Proc Natl Acad Sci U S A 89, 2774–2778.[Abstract/Free Full Text]

McNaughton BL (1982). Long-term synaptic enhancement and short-term potentiation in rat fascia dentata act through different mechanisms. J Physiol 324, 249–262.[Abstract/Free Full Text]

Magleby KL (1987). Short-term changes in synaptic efficacy. In Synaptic Function, ed. Edelman GM, Gall WE & Cowan, WM, pp. 21–56. John Wiley & Sons, New York, NY, USA.

Magleby KL & Zengel JE (1976). Augmentation: a process that acts to increase transmitter release at the frog neuromuscular junction. J Physiol 257, 449–470.[Abstract/Free Full Text]

Martinez-Conde S, Macknic SL & Hubel DH (2002). The function of bursts of spikes during visual fixation in the awake primate lateral geniculate nucleus and primary visual cortex. Proc Natl Acad Sci U S A 99, 13920–13925.[Abstract/Free Full Text]

Meinrenken CJ, Borst GG & Sakmann B (2003). Local routes revisited: the space and time dependence of the Ca2+ signal for phasic transmitter release at the rat calyx of Held. J Physiol 547, 665–689.[Abstract/Free Full Text]

Murphy PC & Sillito AM (1987). Corticofugal feedback influences the generation of length tuning in the visual pathway. Nature 329, 727–729.[CrossRef][Medline]

Neher E (1998). Vesicle pools and Ca2+ microdomains: new tools for understanding their roles in neurotransmitter release. Neuron 20, 389–399.[CrossRef][Medline]

Regehr WG, Delaney KR & Tank DW (1994). The role of presynaptic calcium in short-term enhancement at the hippocampal mossy fiber synapse. J Neurosci 14, 523–537.[Abstract]

Rosenmund C, Sigler A, Augustin I, Reim K, Brose N & Jeong-Seop R (2002). Differential control of vesicle priming and short-term plasticity by Munc13 isoforms. Neuron 33, 411–424.[CrossRef][Medline]

Rozov A, Burnashev N, Sakmann B & Neher E (2001). Transmitter release modulation by intracellular Ca2+ buffers in facilitating and depressing nerve terminals of pyramidal cells in layer 2/3 of the rat neocortex indicates a target-cell specific difference in presynaptic calcium dynamics. J Physiol 531, 807–826.[Abstract/Free Full Text]

Sakaba T & Neher E (2001). Quantitative relationship between transmitter release and calcium current at the calyx of Held synapse. J Neurosci 21, 462–476.[Abstract/Free Full Text]

Scanziani M, Salin PA, Vogt KE, Malenka RC & Nicoll RA (1997). Use-dependent increases in glutamate concentration activate presynaptic metabotropic glutamate receptors. Nature 385, 630–634.[CrossRef][Medline]

Schmielau F & Singer W (1977). The role of visual cortex for binocular interactions in the cat lateral geniculate nucleus. Brain Res 120, 354–361.[CrossRef][Medline]

Stevens CF & Wesseling JF (1999). Augmentation is a potentiation of the exocytotic process. Neuron 22, 139–146.[CrossRef][Medline]

Trommershäuser J, Schneggenburger R, Zippelius A & Neher E (2003). Heterogenous presynaptic release probabilities: functional relevance for short-term plasticity. Biophys J 84, 1563–1579.[Abstract/Free Full Text]

Turner JP & Salt TE (1998). Characterization of sensory and corticothalamic excitatory inputs to rat thalamocortical neurones in vitro. J Physiol 510, 829–843.[Abstract/Free Full Text]

Yarbus AL (1967). Eye movements and vision. Trans. Haigh B, ed. Riggs LA. Plenum Press, New York, NY, USA.

Zucker RS & Regehr WG (2002). Short-term synaptic plasticity. Annu Rev Physiol 64, 355–405.[CrossRef][Medline]


   Acknowledgements
 
We thank Dr Erik Ahlstrand for helpful assistance in early experiments. This work was supported by a grant from the Swedish Medical Research Council (Project no. 4767) and the Committee for Medical Research in Östergötland and the Lions Foundation. B.G. was a PhD student at Forum Scientum, supported by the Swedish Foundation for Strategic Research and Linköping University.

Author's present address
B. Granseth: MRC Laboratory of Molecular Biology, Hills Road, Cambridge CB2 2QH, UK.




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