Cerebral Cortex Advance Access originally published online on May 25, 2005
Cerebral Cortex 2006 16(3):386-393; doi:10.1093/cercor/bhi117
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Distinct Ca2+ Channels Mediate Transmitter Release at Excitatory Synapses Displaying Different Dynamic Properties in Rat Neocortex
University Laboratory of Physiology, University of Oxford, Parks Road, Oxford OX1 3PT, UK
Address correspondence to A.B. Ali, Department of Pharmacology, School of Pharmacy, 29/39 Brunswick Square, London WC1N 1AX, UK. Email: afia.ali{at}ulsop.ac.uk.
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Abstract |
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To study the type of presynaptic calcium channels controlling transmitter release at synaptic connections displaying depression or facilitation, dual whole cell recordings combined with biocytin labelling were performed in acute slices from motor cortex of 17- to 22-day-old rats. Layer V postsynaptic interneurons displayed either fast spiking (FS) (n = 12) or burst firing (BF) (n = 12) behaviour. The axons of FS cells ramified preferentially around pyramidal cell somata, while BF cell axons ramified predominately around pyramidal cell dendrites. Synapses between pyramidal cells and FS cells displayed brief train depression (n = 12). Bath application of
-Agatoxin IVA (0.5 µM), blocking P/Q-type calcium channels, decreased mean peak amplitudes of the EPSPs to 40% of control EPSPs (n = 8). Failure rate of the EPSPs after the first presynaptic action potential increased from 9 ± 11 to 28 ± 15%. This was associated with an increase in paired pulse ratio of 152 ± 44%. -Conotoxin GVIA (1–10 µM), selectively blocking N-type calcium channels, had no effect on peak amplitudes or frequency dependent properties of these connections (n = 5). Synapses from pyramidal cells to BF cells displayed brief train facilitation (n = 8). Application of -Conotoxin in these connections decreased peak amplitudes of the EPSPs to 15% of control EPSPs (n = 6) and decreased the paired pulse ratio by 41 ± 30%. -Agatoxin did not have any significant effect on the EPSPs elicited in BF cells. This study indicates that P/Q-type calcium channels are associated with transmitter release at connections displaying synaptic depression, whereas N-type channels are predominantly associated with connections displaying facilitation.
Key Words: Ca2+ channels • excitatory synapses • rat neocortex • depression • facilitation
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Introduction |
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Short-term synaptic depression acts as a low-pass filter and synapses displaying short-term depression can transfer information most efficiently at low frequencies of presynaptic activation. In contrast, synaptic facilitation acts as high-pass filter and synapses displaying facilitation require a burst of presynaptic activation to maximize transfer of information. In the rat neocortex and the CA1 region of hippocampus synaptic depression is a prominent feature of excitatory connections between pyramidal cells (Thomson et al., 1993a
; Markram and Tsodyks, 1996; Deuchars and Thomson, 1996). Depression is also observed in excitatory connections between layer 4 spiny stellate cells in the cat visual cortex (Stratford et al., 1996; Tarczy-Hornoch et al., 1999). In contrast, pyramidal cell axons targeting inhibitory interneurons can show paired-pulse and brief-train facilitation or synaptic depression dependent on the class of postsynaptic target neuron (Thomson et al., 1993b, 1995; Ali and Thomson, 1998; Ali et al., 1998; Reyes et al., 1998). Depressing inputs are usually onto fast spiking (FS) and regular spiking interneurons, whose axons are proximally targeting (Gil et al., 1999; Ali et al., 2001; Beierlein et al., 2003), including parvalbumin positive interneurons (Reyes et al., 1998; Thomson and West, 2003). However, facilitating inputs are found on somatostatin-containing cells (Reyes et al., 1998) with ascending axonal arbors, which prefer to target dendritic regions of other cells (Thomson et al., 1995). Thus, the postsynaptic target neuron plays an important role in determining the type of temporal pattern of neurotransmitter release it receives.
Different synaptic dynamics may be associated with multiple cellular mechanisms, perhaps acting simultaneously. These mechanisms might include: specific vesicular proteins acting as Ca2+ sensors at release sites, several types of presynaptic Ca2+ channels and G-protein modulation of Ca2+ channel activity and autoreceptor activation (for reviews, see Geppert and Sudhof, 1998; Thomson, 2000). Calcium channels mediating release of neurotransmitter at central synapses include N-, P/Q- and R-type high-voltage activated channels. There is usually more than one type at a given synapse; for example, transmission at CA3 to CA1 synapses involves N-, P/Q- and R-type channels (Wu and Saggau, 1995) in autapses from hippocampal cultures, N- and P/Q-type channels contributed equally (Reid et al., 1998). Others have shown that the release of GABA involves N-type Ca2+ channels at one type of synapse and P-type at another (Poncer et al., 1997). Human N-type channels expressed in cell lines have been shown to inactivate upon repetitive stimulation by voltage steps mimicking APs (McNaughton et al., 1998). In contrast to N-type Ca2+ channels, currents mediated by P-type Ca2+ channels (often indistinguishable from Q type, therefore referred to as P/Q-type) facilitate in a Ca2+- dependent manner on repetitive stimulation with AP-like depolarizing voltage steps at the Calyx of Held (Cuttle et al., 1998). A simplistic prediction therefore would be that release sites employing N-type channels would exhibit depression, while those employing P/Q type channels would exhibit facilitation. To test this prediction, paired recordings were conducted between identified pyramidal cells and two classes of interneurons to determine whether presynaptic Ca2+ channels are associated with the different types of release patterns.
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Materials and Methods |
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Slice Preparation
Sprague–Dawley rats (17–22 postnatal days) were killed by cervical dislocation. After decapitation the brain was rapidly removed under ice-cold conditions and 300-µm-thick coronal sections of cerebral cortex were obtained. The slices were incubated for 1 h in a physiological extracellular saline solution containing (in mM): 121 NaCl, 2.5 KCl, 1.25 NaH2PO4, 2 CaCl2, 1 MgCl2, 26 NaHCO3, 20 glucose and 5 pyruvate, and with a mixture of 95% O2 and 5% CO2. For recordings, they were transferred to a submerged-style chamber and perfused at 1–2 ml/min with the same physiological extracellular solution at room temperature (20–24°C).
Paired Recordings
Simultaneous somatic dual whole cell recordings were made in current clamp in layer V of rat motor cortex between electrophysiologically identified pyramidal cells and interneurons. The cell types were selected using videomicroscopy under near-infrared differential interference contrast (DIC) illumination. Interneurons were selected with round or oval somata and further characterized from their firing properties recorded in current clamp. Experiments were conducted at room temperature with patch pipettes (resistance 8–10 M
) pulled from borosilicate glass tubing and filled with an internal solution containing (in mM): 144 K-gluconate, 3 MgCl2, 0.2 EGTA, 10 HEPES, 2 Na2-ATP, 0.2 Na2-GTP and 0.02% w/v of biocytin (pH 7.2–7.4, 300 mOsm).
Pairs of presynaptic action potentials (APs) with an interspike interval of 15 ms and/or 50ms or a train of APs were elicited by injecting short (5–10 ms) pulses of depolarizing current repeated at 0.2 Hz (SEC 05L/H, npi electronics, Germany). Postsynaptic responses were amplified, low-pass filtered at 2 kHz, digitized at 5KHz using CED 1401 interface and recorded on computer for of-line analysis. -Conotoxin (Sigma, UK) and -Agatoxin (Tocris, UK) were administered by bath application.
Data Analysis
Data were acquired and analysed using Signal software (Cambridge, UK). Individual sweeps were observed and either accepted, edited or rejected according to the trigger points that would trigger measurements and averaging of the EPSPs during subsequent data analysis. Averaging of EPSPs was triggered by the rising phase of the first presynaptic spike for the first EPSP, the rising phase of the second presynaptic spike for the second EPSP and so on. The paired pulse ratios were obtained from second EPSPs elicited at an interspike interval of 50ms for both types of connections studied. For some of the averages illustrated, individual records in which second and third spikes fell within a set time window (e.g. 15–20 or 35–40 ms) following the first spike were selected into data subsets and averaged. Average EPSP amplitudes were measured between the baseline and the peak of the EPSP. At short interspike intervals, average EPSP amplitude was measured from the decay phase of the preceding EPSP and the peak of the EPSP. Averages are given as mean ± SD from 50–350 sweeps. The electrophysiological characteristics of the recorded cells were measured from their voltage response to 500 ms current pulses between –2 and +1 nA in amplitude. The EPSP rise times (10–90%) were measured from averaged EPSPs elicited by a single action potential (AP). Apparent failures of synaptic transmission were counted manually by eye and EPSP amplitude in the range of the synaptic noise were taken as failures. Statistical significance was analysed by Student's paired t-test.
Morphology
Slices containing biocytin-filled cells were fixed overnight in 4% paraformaldehyde in phosphate buffer (PB, 0.1 M). Extensive rinses were carried out between each step using phosphate buffered saline (PBS, 0.01 M). The sections were freeze-thawed over liquid nitrogen after cryoprotecting in 10, 20 and 30% sucrose in distilled water (the sections sink when infiltrated). The endogenous peroxidase activity was then blocked using 10% methanol/90% PBS/1% H2O2. The sections were incubated overnight in Vector ABC-peroxidase (1:200) at 4°C. The peroxidase group was revealed using 3'3 diaminobenzidine as the chromogen (Vector DAB kit). The visualized cells were intensified with 0.1% osmium tetroxide and the sections were cleared by dehydration in an ascending series of alcohols to 100%, then embedded in araldite resin (Agar Scientific). The cells were reconstructed using a Zeiss Axioskop with attached camera lucida.
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Results |
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Dual whole cell recordings were performed between pyramidal cells and two subclasses of electrophysiologically identified interneurons (fast spiking and burst firing cell), which formed local circuits in layer V of the motor cortex.
Electrophysiological and Morphological Characteristics of Postsynaptic Target Interneurons
The postsynaptic target neurons were selected according to the shape and size of their somata, which were usually round or oval and distinct from pyramidal cells observed under DIC. Two populations of interneurons were distinguished electrophysiologically, fast spiking (FS, n = 12) and burst firing (BF, n = 12).
FS cells displayed very fast APs that were terminated with deep, fast spike after-hyperpolarization (AHP) and trains of spikes showed little accommodation or adaptation. From a holding potential of –70 mV the mean peak action potential amplitude was 72 ± 10 mV (from threshold to peak), with a mean width at half amplitude of 0.25 ± 0.2 ms. The mean AHP from spike threshold to peak was –15 ± 7 mV, with a width at half amplitude of 6 ± 2 ms. The mean input resistance of FS cells was 226 ± 29 M, with a time constant of 12.2 ± 5.2 ms (see also Connors and Gutnick, 1990; Angulo et al., 1999; Ali et al., 2001). An example of a typical FS cell studied is illustrated in Figure 1B.
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BF cells responded to depolarizing current steps with a burst of action potentials followed by tonic firing (n = 12). The resting membrane potentials of these cells were between –67 and – 80 mV. Their action potential durations were significantly slower than those of FS cells and were terminated with shallow, more complex AHPs, as shown in Figure 1E. From a membrane potential of –70 mV the mean peak AP amplitude was of 69 ± 15 mV (from threshold to peak), with a mean width at half amplitude of 0.45 ± 0.14 ms. The mean AHP amplitude was –5.2 ± 3.5 mV from spike threshold to peak with the width at half amplitude of 9.2 ± 6.5 ms (See Ali et al., 1998
). The mean input resistance of BF cells was 270 ± 65 M, with a time constant of 20 ± 3.7 ms. These values were significantly different from FS cell parameters (P < 0.05, n = 12). Morphologically the somata of FS cells were round or oval with smooth and beaded dendrites. The axons of these cells ramified with a preferential horizontal elongation in layer V. Postsynaptic pyramidal cell somata were always located in the core terminal field of the presynaptic interneuron axon (n = 12). The probability of finding that the interneuron was postsynaptically connected with a neighbouring pyramidal cell was high. On average only two pyramidal cells were tested with each FS cell before a connection was found.
The somata of BF cells were usually located deep in layer V. An example is shown in Figure 1F. These cells were bipolar in appearance and their dendrites were smooth to sparsely spiny (n = 10). The axons of these cells originated from the soma, ascended to layer IV and were finer in appearance compared with those of FS cell axons (n = 3). On average five pyramidal cells were tested with each BF cell before a connected pair in which the pyramidal cell was presynaptic to the interneuron was found.
Synaptic Dynamics of Layer V Pyramidal Inputs to Interneurons
Unitary EPSPs were elicited by pyramidal cells in FS cells (n = 12) or BF cells (n = 8) by single or double spikes, or trains of presynaptic pyramidal cell action potentials repeated every 5 s. Excitatory connections from pyramidal cells to FS cells consistently displayed paired-pulse and brief-train depression (for properties of depression, see also Ali et al., 1998). First EPSPs at a membrane potential of –60 mV were 538 ± 329 µV in average amplitude (range, 100 to 1200 µV). Second EPSPs elicited at an interspike interval of 50 ms, were smaller, 258 ± 169 µV in average amplitude (range, 115–300 µV) (n = 12). The average 10–90% rise time for the first EPSP was 1.2 ± 0.8 ms, with an average width at half amplitude of 18 ± 3.5 ms. Figures 1A and 2 show examples of the EPSPs elicited by pyramidal cells in FS cells. Figure 2C illustrates the fluctuation in amplitude of the first (EPSP1) and second EPSPs (EPSP2) elicited at an interspike interval of 50 ms. The paired pulse ratio (PPR; EPSP2 amplitude/mean EPSP1 amplitude) was 39 ± 8.4% (n = 12). Apparent failures of transmission were more commonly observed following the second presynaptic AP (31.70 ± 12.38%, n = 12) than following the first EPSPs (4.2 ± 3.6%, n = 12)). The coefficients of variation (CVs) were typically smaller for the first EPSPs (CV range, 0.2–0.9) than for the second EPSPs (CV range, 0.4–2).
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Connections from pyramidal cells to BF interneurons consistently displayed paired-pulse and brief-train facilitation (n = 8, see Fig. 1D; for time dependent properties of facilitation, see Ali and Thomson, 1998
). The proportions of apparent failures of transmission after the first presynaptic action potential at these connections were very high, on average 35 ± 17% for the first and 10 ± 7% for the second EPSPs. Peak amplitudes for the first and second EPSPs at –60 mV were 147 ± 152 (range, 40–460 µV) and 340 ± 217 µV (obtained at an interspike interval of 50 ms; range, 150–760 µV), respectively. The 10–90% rise times and width at half amplitudes of these EPSPs were much slower than the EPSPs elicited in FS cells, at 4 ± 2.3 and 29 ± 12 ms respectively (P < 0.05, n = 8). The average PPR was 388 ± 80% (significantly different from FS cell PPR at P < 0.01, n = 8), with a large CV after the first presynaptic AP (range, 0.5–1.5) and commensurate with their larger amplitudes the CVs for second EPSPs were smaller than the first (range, 0.4–0.8). Pyramid to FS Cell Transmission Displaying Synaptic Depression Is Predominantly Mediated by P/Q-type Ca2+ Channels
Figure 2 illustrates an example of EPSPs elicited in a FS cell in control conditions, after the application of Conotoxin, then the addition of Agatoxin and finally following the addition of NiCl. Following bath application of -Conotoxin (1 µM after 
25 min), a selective N-type Ca2+ blocker, there was no significant effect on the EPSPs elicited in FS cells. The average peak amplitude of the first EPSP was 93 ± 17% of the control EPSP (P = 0.95, n = 4).
However, with the bath application of a P/Q type Ca2+ channel blocker, -Agatoxin (0.5 µM), the peak amplitude decreased to 42 ± 24% of the control first EPSPs and second EPSPs were reduced to 60 ± 29% of control second EPSP amplitudes (paired t-test, P < 0.05, n = 8 for the first EPSPs, in which Agatoxin was the first blocker applied). This effect was seen 8–10 min after toxin application and did not reverse during 45 min of washout. Subsequent addition of NiCl (100 µM) almost totally abolished the N- and P/Q-type calcium channel blocker-resistant EPSPs (decreased to 2% of control first EPSPs, P = 0.01, n = 5), suggesting an involvement of R-type calcium channels (see Fig. 2). To demonstrate that Conotoxin was effective and could penetrate the tissue to reach the synapses, reciprocal connections were studied as an internal control. Reciprocal connections between fast spiking and pyramidal cells are shown in Figure 3. Unitary IPSPs were totally blocked by 1 µM Conotoxin (Fig. 3A) and continued to be ineffective at the reciprocal EPSPs elicited in the FS cell (Fig. 3B) (n = 3). In contrast, Agatoxin affected the EPSPs but had no effect on the IPSP amplitudes (shown in Fig. 3C,D; P = 0.05, n = 3).
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Pyramid to BF Cell Transmission Displaying Synaptic Facilitation Is Mediated by N-type Ca2+ Channels
Synapses from pyramidal cells to BF interneurons, which consistently displayed synaptic facilitation were not significantly affected by bath application of the P/Q-type Ca2+ channel blocker, -Agatoxin (0.5 µM, after 40 min of bath application; see Fig. 4). The average peak amplitudes for the first and second EPSPs were 98.6 ± 4.7 (P = 0.98, n = 3) and 90 ± 14.3% (P = 0.35, n = 3) of the control EPSPs, respectively. However, with the selective N-type Ca2+ blocker -Conotoxin (1 µM), the average peak amplitudes decreased to 15 ± 16% (P = 0.05, n = 6) and 36 ± 17.4% (P = 0.02, n = 6) of the control first and second EPSPs, respectively. Subsequent addition of NiCl (100 µM; Fig. 4A) almost completely abolished the EPSPs (decreased to 3% of control first EPSPs, P = 0.01, n = 4).
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The EPSP amplitude distributions before and during Conotoxin are illustrated in Figure 4C. Since only a relatively small number of events (50–300) for the first and second EPSPs are included in the plot in Figures 2C and 4C, data are binned coarsely and no attempt was made here to distinguish peaks. The amplitude distributions were relatively evenly distributed about the mean in each condition. There was a clear shift of the amplitude distribution towards smaller values after toxin application in the examples studied here.
Changes in EPSPs Elicited in FS or BF Cells with Calcium Channel Blockers Are Presynaptic in Origin
Figure 5 summarizes the data obtained with bath application of P/Q- and N-type calcium channel blockers. Agatoxin had a significant effect on the EPSPs elicited in FS cells, while EPSPs elicited in BF cells were significantly affected by Conotoxin (see Fig. 5A).
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To determine whether the toxins had in fact exerted their effects presynaptically, two tests were performed. First, the proportions of apparent failures of transmission were calculated for control and toxin-blocked EPSPS. In all cases a reduction in average first EPSP amplitude was accompanied by an increase in EPSP failure rate (see Fig. 5), suggesting a presynaptic site for blockade for both toxins. In postsynaptic FS cells, Agatoxin increased first EPSP failure rate by 654.3 ± 500% (P < 0.05, n = 6). The proportional increase in second EPSP failure rate was less (94 ± 35%; P = 0.1, n = 6) than for the first EPSPs and second EPSP amplitudes were reduced less than first EPSP amplitudes. Paired pulsed ratios were therefore increased by Agatoxin (from 39 ± 8.4 to 78 ± 16.0%; P < 0.05, n = 6), suggesting that in FS cells paired pulse facilitation can, to an extent, overcome the effects of P/Q channel blockade. In contrast, in BF cells, Conotoxin had a greater effect upon the second than upon first EPSPs. It produced a larger increase in second than in first EPSP failure rates and actually reduced PPRs from 338 ± 132 to 149 ± 51% (P < 0.05, n = 6), shown in Figure 5C. Conotoxin-blocked terminals thus appear less able to support facilitation, despite the reduction in release probability this toxin is expected to produce.
In the second test, the proportional change in the inverse square of the coefficient of variation (CV–2) was compared with the proportional change in the mean EPSP amplitude (M) to determine whether the quantal amplitude (q), the release probability (p) or the number of release sites (n) had changed. In a binomial distribution, CV–2 = [np/(1 – p)], and is therefore independent of q, while M = npq. When mean EPSP amplitude changes, therefore, no change in CV–2 indicates that only q has changed, a larger proportional change in CV–2 than in M indicates that p has changed, while a similar proportional change in the two parameters indicates that n has changed (Clements, 1990; Faber and Korn, 1991; Larkman et al., 1991). In the majority of the pairs tested, application of either of the toxins resulted in a greater proportional decrease in CV–2 than in M (Fig. 5E; each point represents a pair), indicating that, as expected, Agatoxin and Conotoxin had reduced release probability.
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Discussion |
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Local circuit excitatory connections from pyramidal cells to inhibitory interneurons were studied to investigate the types of presynaptic calcium channels associated with different synaptic release patterns. Interneurons with specific electrophysiological and morphological characteristics probably play very different roles in the local circuitry, and the patterns of transmitter release by which they are activated are very different. This study provides evidence that distinct presynaptic Ca2+ channels are associated with specific neurotransmitter release patterns.
The two classes of interneurons studied were distinct electrophysiologically and morphologically and were similar to members of two broad classes of interneurons reported previously (Kawaguchi and Kubota, 1997; Xiang et al., 2002; Bacci et al., 2003). The FS cell axons in the present study resembled basket cells axons reported previously (Reyes et al., 1998; Gil et al., 1999; Ali et al., 2001; Beierlein et al., 2003; Thomson and West, 2003). The morphology of BF cells reported here are those of dendrite-targeting cells, reported previously (Deuchars and Thomson, 1995; Reyes et al., 1998).
Postsynaptic FS cells received EPSPs that exhibited synaptic depression (see also Angulo et al., 1999; Ali et al., 2001). This is probably mediated presynaptically, since the apparent transmission failure rates were larger after second and third than after first presynaptic action potentials. In marked contrast, postsynaptic BF cells received EPSPs that exhibited facilitation which is also at least in part of presynaptic origin, since failure rates were much higher after the first than subsequent presynaptic action potentials in trains (see also Deuchars and Thomson, 1995). These data support previous observations of target-cell-specific synaptic dynamics (Ali et al., 1998; Ali and Thomson, 1998; Reyes et al., 1998, Rozov et al., 2001; Thomson et al., 2002).
A previous study looked at the actions of calcium channels blockers on pyramidal inputs onto two different types of layer II/III interneurons; multipolar neurons that received depressing inputs and bitufted interneurons that received facilitating inputs (Rozov et al., 2001). Agatoxin and Conotoxin each produced similar degrees of blockade of EPSPs in the two types of connections; hence the authors concluded that N-, P/Q- type channels mediated release from both types of terminals. Following studies of the effects of presynaptic calcium chelators, the difference in the dynamic properties of the two types of synapses was ascribed to differences in the diffusional distance from points of calcium entry to the calcium binding sites on the release machinery.
In the present study, a significant difference was found in the sensitivities of two classes of synapse to these calcium channel blockers. Facilitating pyramidal inputs onto burst firing, bipolar cells were blocked by Conotoxin, but relatively unaffected by Agatoxin. In contrast, depressing inputs onto FS multipolar neurons were blocked by Agatoxin but little affected by Conotoxin, although in reciprocal connections the inhibitory outputs of FS cells onto pyramidal cells was totally blocked by Conotoxin. Whether the different findings in the two studies reflect the different neocortical layers studied, the two different age groups studied or differences in the class of postsynaptic interneurons investigated is unclear. The FS spiking, multipolar neurons that are shown in both the present and the Rozov et al. studies, albeit in different layers, probably belong to the same broad class (although the morphology of these interneurons is not described in the layer II/III study). However, the bitufted layer II/III cells previously reported to display an accommodating firing patterns (Reyes et al., 1998) and the burst firing bipolar layer V interneurons clearly represent members of different interneuronal classes, as are the presynaptic pyramidal cells, which are certainly different in these two studies.
The results of the present study are surprising in another way. As discussed in the introduction the properties of recombinant N- and P-type channels (i.e. kinetic properties of the channels and/or diffusion distance between N- and P-type channels and Ca2+ sensors that trigger release) would predict that synapses dominated by N-type channels would exhibit depression, while those dominated by P-type channels would exhibit facilitation (Umemiya and Berger, 1995; Forsythe et al., 1998; McNaughton et al., 1998). This is the obverse of what was observed. Facilitation was dramatically reduced by N-type channel blockade (despite a decrease in release probability), while P-type channel blockade allowed functional expression of facilitation in previously depressing connections.
Therefore, taking the results of the present and previous studies together, whether a given connection exhibits a low release probability at low frequencies and facilitation at high frequencies or a high release probability at low frequencies and depression depends on factors other than the type(s) of calcium channel that initiate transmitter release. If the distance between the channels and the release machinery is the deciding factor (Rozov et al., 2001), the present study would suggest that at facilitating inputs onto burst firing cells, N-type channels are distant from the release site, whereas at depressing inputs onto layer V fast spiking cells, P-type channels are close to the machinery.
This study demonstrates that P/Q- and possibly R-type calcium channels are involved in transmitter release at connections displaying short term synaptic depression with the involvement of N-type calcium channels in synaptic connections displaying facilitation in layer V of neocortex. Therefore, specific presynaptic calcium channels seem to be selectively associated with specific types of synaptic connections.
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Acknowledgments |
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This work was funded by the Wellcome Trust (London) and Novartis Pharma (Basel).
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