Brain, Vol. 125, No. 4, 844-860,
April 2002
© 2002 Guarantors of Brain
Post-ischaemic long-term synaptic potentiation in the striatum: a putative mechanism for cell type-specific vulnerability
1 Clinica Neurologica, Dipartimento di Neuroscienze, Università di Tor Vergata and IRCCS Fondazione Santa Lucia, Rome, Italy
Correspondence to: Paolo Calabresi, Clinica Neurologica, Dipartimento di Neuroscienze, Università di Tor Vergata, Via di Tor Vergata 135, 00133 Rome, Italy E-mail: calabre{at}uniroma2.it
Received September 17, 2001. Revised October 14, 2001. Accepted October 22, 2001.
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In the present in vitro study of rat brain, we report that transient oxygen and glucose deprivation (in vitro ischaemia) induced a post-ischaemic long-term synaptic potentiation (i-LTP) at corticostriatal synapses. We compared the physiological and pharmacological characteristics of this pathological form of synaptic plasticity with those of LTP induced by tetanic stimulation of corticostriatal fibres (t-LTP), which is thought to represent a cellular substrate of learning and memory. Activation of N-methyl-D-aspartate (NMDA) receptors was required for the induction of both forms of synaptic plasticity. The intraneuronal injection of the calcium chelator BAPTA [bis(2-aminophenoxy)ethane-N,N,N',N'-tetraacetate] and inhibitors of the mitogen-activated protein kinase pathway blocked both forms of synaptic plasticity. However, while t-LTP showed input specificity, i-LTP occurred also at synaptic pathways inactive during the ischaemic period. In addition, scopolamine, a muscarinic receptor antagonist, prevented the induction of t-LTP but not of i-LTP, indicating that endogenous acetylcholine is required for physiological but not for pathological synaptic potentiation. Finally, we found that striatal cholinergic interneurones, which are resistant to in vivo ischaemia, do not express i-LTP while they express t-LTP. We suggest that i-LTP represents a pathological form of synaptic plasticity that may account for the cell type-specific vulnerability observed in striatal spiny neurones following ischaemia and energy deprivation.
Keywords: excitotoxicity; NMDA receptors; oxygen/glucose deprivation; synaptic plasticity
Abbreviations: AMPA = 
-amino-3-hydroxy-5-methyl-isoxazole-4-propionate; APV = D-2-amino-5-phosphonovalerate; BAPTA = bis(2-aminophenoxy)ethane-N,N,N',N'-tetraacetate; CNQX = 6-cyano-7-nitroquinoxaline-2,3-dione; EPSC = excitatory post-synaptic current; EPSP = excitatory post-synaptic potential; LTP = long-term potentiation; MAP = mitogen-activated protein; NMDA = N-methyl-D-aspartate
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Introduction |
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Post-tetanic long-term potentiation (t-LTP) has been considered to be a physiological form of synaptic plasticity, and its occurrence either in cortical or in subcortical areas has been regarded as a cellular substrate for memory and learning (Bliss and Collingridge, 1993
; Lovinger, 1993; Nicoll and Malenka, 1995; Calabresi et al., 1992, 1996). More recently, it has been reported that pathological events such as ischaemia and energy deprivation may also induce long-term changes in excitatory synaptic transmission in hippocampal CA1 pyramidal neurones (Crépel et al., 1993; Hammond et al., 1994; Tekkok and Krnjevic, 1995; Hsu and Huang, 1997). Long-term pathological changes of synaptic transmission induced by ischaemia and energy deprivation may also underlie the differential neuronal vulnerability expressed in different brain areas or even in different neuronal subtypes in the same structure. However, the differential expression of pathological forms of synaptic plasticity in relation to the vulnerability of specific neuronal subtypes has never been addressed. Interestingly, these pathological forms of enduring synaptic changes may share some common mechanisms with the plasticity induced by high frequency stimulation. Among these mechanisms, the rise in intracellular calcium levels during the induction phase is a critical requirement for both forms of synaptic plasticity (Lynch et al., 1983; Bliss and Collingridge, 1993; Crépel and Ben-Ari, 1996). Accordingly, intracellular levels of calcium are a critical determinant of excitotoxicity (Reynolds, 1998) and ischaemia-induced neuronal death (Lee et al., 1999). Virtually all the studies dealing with the induction of LTP following energy deprivation have been performed at the hippocampal Schaffer collateral/commissural–CA1 cell synapse. Nevertheless, these studies have generated a significant amount of discordant data. A major controversy concerns the expression of ischaemic LTP (i-LTP), i.e. is i-LTP expressed on both -amino-3-hydroxy-5-methyl-isoxazole-4-propionate (AMPA)- and N-methyl-D-aspartate (NMDA)-mediated components of excitatory transmission (Hsu and Huang, 1997), or is it selectively expressed on the NMDA-mediated component (Hammond et al., 1994)? Since, in most of the studies, i-LTP and t-LTP are mutually occlusive, one can argue that these forms of synaptic plasticity must share some common mechanisms. At this point, a second critical question can be raised: is there any pharmacological approach whereby one form of synaptic plasticity but not the other is selectively affected? A third issue concerns the possibility of utilizing the knowledge obtained by these in vitro experiments performed in the hippocampal region to understand the pathophysiology of in vivo brain ischaemia (Rothman and Olney, 1986; Lee et al., 1999). This latter point requires the description and characterization of post-ischaemic forms of synaptic plasticity in brain areas other than the hippocampal CA1 region. In addition, the hypothesis that i-LTP represents a synaptic mechanism underlying the ischaemia-induced cell type-specific neuronal vulnerability requires the demonstration that this form of pathological synaptic plasticity is selectively expressed in vulnerable neurones but not in cells resistant to the ischaemic insult. To address all these issues, we have performed electrophysiological recordings from striatal spiny neurones, which share with CA1 hippocampal pyramidal cells a high vulnerability towards ischaemic insults (Pulsinelli, 1985), and we have induced in this neuronal subtype a characteristic form of i-LTP. Moreover, we have compared some of the features of this form of synaptic plasticity with those shown by striatal t-LTP. Finally, we have provided evidence that striatal cholinergic interneurones, which are resistant to in vivo ischaemia (Chesselet et al., 1990), do not express this form of pathological potentiation while they express t-LTP. |
Material and methods |
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Preparation and maintenance of rat brain slices
Male Wistar rats (150–250 g) were used. The preparation and maintenance of coronal slices have been described previously (Calabresi et al., 1995
, 1997a, b, 1998). Briefly, corticostriatal coronal slices (200–300 µm thick) were prepared from tissue blocks of the brain with the use of a vibratome. A single slice was transferred to a recording chamber and submerged in a continuously flowing Krebs’ solution (35°C, 2–3 ml/min) gassed with 95% O2/5% CO2. To study ischaemia in striatal neurones, slices were deprived of glucose by totally removing glucose from the perfusate and by adding sucrose to balance the osmolarity. This solution was gassed with 95% N2/5% CO2 instead of the usual gas mixture. In some experiments the osmolarity was balanced by increasing the concentration of NaCl (Jiang and Haddad, 1992). This procedure did not affect the resting membrane potential or the input resistance of the recorded neurones. Since experiments performed using these different procedures to replace glucose gave similar results, all data were pooled. Ischaemic solutions entered the recording chamber no later than 20 s after turning a three-way tap. Complete replacement of the medium in the chamber took 
90 s as determined by the speed of diffusion of a coloured solution. The composition of the control solution was (in mM): 126 NaCl, 2.5 KCl, 1.2 NaH2PO4, 2.4 CaCl2, 11 glucose, 1.2 MgCl2 and 25 NaHCO3. Magnesium ions were omitted from the bathing solution in the experiments performed with intracellular electrodes to better disclose the NMDA-mediated component of the corticostriatal glutamatergic transmission at the resting membrane potential. In contrast, all the whole-cell patch–clamp experiments were performed in a medium containing 1.2 mM MgCl2.
Intracellular recordings
Intracellular recording electrodes were filled with 2 M KCl (30–60 M
). Signals were recorded with the use of an Axoclamp 2A amplifier (Axon Instruments, Foster City, CA, USA), displayed on a separate oscilloscope and stored on a digital system. The input resistance of the recorded neurones was monitored by delivering hyperpolarizing current pulses (0.3–1 s duration, 100–300 pA intensity). Bipolar electrodes were used for synaptic stimulation. These stimulating electrodes were located either in the cortical areas close to the recording electrode or in the white matter between the cortex and the striatum, in order to activate corticostriatal fibres. As the conditioning tetanus, we used three trains (3 s duration, 100 Hz frequency, at 20 s intervals). This protocol induced a stable LTP (Calabresi et al., 1992, 1996) that occluded further potentiation when a second induction protocol was applied at the same synaptic pathway. The duration of each individual pulse was 0.01–0.3 ms and the intensity was 3–10 V. Under the control condition, the frequency of stimulation was 0.1–0.05 Hz. During tetanic stimulation, the intensity was increased to generate a single action potential during the excitatory post-synaptic potential (EPSP). In a subset of experiments, stimulus injections were accomplished using a two-pathway design. Two orthodromic stimuli were delivered in an alternative manner. The intensity of the baseline current for each pathway was fixed at 50% of the maximum amplitude obtained for each EPSP response. All experimental protocols were administered to both pathways equally to eliminate any experimental bias due to electrode placement.
Whole-cell patch–clamp recordings
Whole-cell patch–clamp recordings were made with borosilicate glass pipettes (1.8 mm o.d.; 3–5 M) containing (in mM): 125 K+-gluconate, 10 NaCl, 1.0 CaCl2, 2.0 MgCl2, 0.5 1,2-bis(2-aminophenoxy)ethane-N,N,N,N-tetraacetic acid (BAPTA), 19 HEPES [N-(2-hydroxyethyl)-piperazine-N-s-ethanesulphonic acid], 0.3 GTP and 1.0 Mg-ATP, adjusted to pH 7.3 with KOH. The striatum could be readily identified under low-power magnification, whereas individual neurones were visualized in situ using a differential interference contrast (Nomarski) optical system. This employed an Olympus BX50WI (Olympus Optical Co. Ltd, Japan) non-inverted microscope with x40 water immersion objective combined with an infra-red filter, a monochrome CCD camera (COHU 4912, COHU Inc., USA) and a PC compatible system for analysis of images and contrast enhancement (WinVision 2000, Delta Sistemi, Rome, Italy). Recording pipettes were advanced towards individual cells in the slice under positive pressure and, on contact, tight G seals were made by applying negative pressure. The membrane patch was then ruptured by suction and membrane current and potential monitored using an Axopatch 1D patch–clamp amplifier (Axon Instruments, Foster City, Calif., USA). Whole-cell access resistances measured in voltage clamp were in the range of 5–30 M prior to electronic compensation (60–80% was routinely used). Glutamate-mediated excitatory post-synaptic inward currents (EPSCs) were evoked at a frequency of 0.1 Hz and at the holding potential of –80 or –50 mV when recorded from striatal spiny neurones, and of –50 mV when recorded from large aspiny cholinergic interneurones.
Data analysis and drug applications
Quantitative data on post-tetanic or post-ischaemic modifications were expressed as a percentage of the controls, the latter representing the mean of responses recorded during a stable period (15–30 min) before the ischaemic episode or the tetanic stimulation. Values given in the text and in the figures are mean ± SEM (standard error of the mean) of changes in the respective cell populations. Student’s t-test (for paired and unpaired observations) was used to compare the means. The characteristics of action potentials and of current–voltage curves in different experimental conditions were studied by using a fast chart recorder and a digital system (Nicolet System 400, Instrument Corporation, Madison, WI, USA, Benchtop Waveform Acquisition System, Sekowie Co. Ltd, Japan). Drugs were applied by dissolving them to the desired final concentration in saline and by switching the perfusion from control saline to drug-containing saline. D-2-amino-5-phosphonovalerate (APV) was from Sigma, Milan, Italy, while 6-cyano-7-nitroquinoxaline-2,3-dione (CNQX), scopolamine, BAPTA and PD98059 were from RBI, Milan, Italy. Picrotoxin was from Tocris, Bristol, UK.
About 50% of the recordings were obtained in the presence of 50 µM picrotoxin in order to rule out a possible contamination of the EPSPs by depolarizing potentials mediated by GABAA receptors. Since these experiments gave similar results to those obtained in the absence of this drug, all the data were pooled.
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Electrophysiological properties of striatal spiny neurones and large aspiny cholinergic interneurones
Intracellular recordings were performed from striatal spiny neurones and large aspiny cholinergic interneurones in corticostriatal slices. Striatal neurones were recognized by means of an electrophysiological characterization of the intrinsic membrane properties and firing properties of the two neuronal subtypes. The large majority (82) of the recorded cells were spiny neurones. They had high resting membrane potential (–85 ± 5 mV), relatively low apparent input resistance (36 ± 10 M
) when measured at rest from the amplitude of small (<10 mV) hyperpolarizing electrotonic potentials, action potentials of short duration (1.1 ± 0.3 ms) and high amplitude (102 ± 4 mV). These cells were silent at rest and showed membrane rectification and tonic firing activity during depolarizing current pulses (Fig. 1). Some of these properties have been described previously, both in vivo and in vitro (Kita et al., 1984; Jiang and North, 1991; Cepeda et al., 1994; Calabresi et al., 1995, 1997a, b, 1998) and resemble those reported for intracellularly stained striatal spiny neurones.
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Twelve cells had different electrophysiological characteristics and showed properties that have previously been attributed to large aspiny interneurones (Wilson et al., 1990
; Jiang and North, 1991; Kawaguchi, 1992, 1993; Kawaguchi et al., 1995; Calabresi et al., 1997a, 1998; Pisani et al., 1999). These cells had low resting membrane potential (–58 ± 5 mV) and high input resistance (152 ± 45 M). Spontaneous firing occurred in three of the 12 cells. In these neurones, depolarizing current pulses of small amplitude (200–400 pA) and short duration (5–30 ms) elicited a few action potentials followed by a long-lasting after-hyperpolarization (amplitude 8.9 ± 9 mV, duration 350 ± 130 ms). The amplitude of the action potential was 70.5 ± 3 mV and the duration of spike at half-amplitude was 0.70 ± 0.05 ms. The hyperpolarizing electrotonic potential declined after about the first 100 ms (Fig. 1). For this reason, the input resistance values were calculated either from the peak amplitude of the electrotonic potential evoked by a 300–400 pA current pulse or at the steady state. The decline in the electrotonic hyperpolarizing potential was blocked by 2 mM caesium in the external medium. This finding suggests that it might be attributed to the activation of an Ih current (Jiang and North, 1991).
Short-term effects of in vitro ischaemia on striatal spiny neurones
A short period of oxygen and glucose deprivation induces a membrane depolarization/inward current associated with a decrease of input resistance in striatal spiny neurones (Calabresi et al., 1999a, b). In this study, we applied 3 min of in vitro ischaemia to the recorded cells to test the effect of energy deprivation on glutamatergic corticostriatal transmission. In the absence of external Mg2+, spiny neurones respond to single cortical stimulation by producing an EPSP mediated by the activation of both AMPA and NMDA glutamate receptors (Calabresi et al., 1996, 1998). As already shown for hypoxia (Calabresi et al., 1995) and aglycaemia (Calabresi et al., 1997a) alone, in vitro ischaemia caused a reversible and complete blockade of synaptic transmission within 2 min of the onset of the ischaemic episode, coupled with significant changes in resting membrane potential and apparent input membrane resistance. In all neurones used for data analysis, resting membrane potential, apparent input membrane resistance and EPSP amplitude recovered to the control values within 3–6 min of wash (Fig. 2).
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Long-term effects of in vitro ischaemia on striatal spiny neurones
When Mg2+ was omitted from the external medium, on return to control solution, the EPSP amplitude recorded from spiny neurones recovered and was subsequently and progressively potentiated to reach a plateau after 15–20 min of wash (n = 45, P < 0.001) (Fig. 2A and B). This i-LTP lasted throughout the period of observation (usually >30 min), whereas pre-incubation of the slices with the NMDA receptor antagonist APV (50 µM) fully prevented its induction (n = 6, P > 0.05) (Fig. 3). Accordingly, this phenomenon was absent in the presence of 1.2 mM external Mg2+ (n = 11; data not shown).
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In order to test whether i-LTP was selectively expressed on the NMDA receptor-mediated component of the corticostriatal transmission or could also be affected by the AMPA receptor-mediated component, we performed two series of experiments. In the first set of experiments we incubated the slices in the presence of 10 µM CNQX, an AMPA receptor antagonist. This experimental condition did not affect the characteristics of the ischaemic depolarization (n = 7) and did not prevent the formation of i-LTP (n = 7, P < 0.001) (Fig. 4A). These experiments show that the activation of AMPA receptors is not required for the induction of i-LTP and that the pharmacologically isolated NMDA receptor-mediated component of the corticostriatal EPSP can undergo post-ischaemic long-lasting synaptic changes. In the second set of experiments we investigated whether the AMPA receptor-mediated component of the EPSP could also express this form of synaptic plasticity. We first evaluated the amplitude of the AMPA component in pharmacological isolation (in 50 µM APV) before the ischaemic period (Fig. 4B, a). Then, we washed out the APV for at least 20 min (Fig. 4B, b). After this period, we applied the ischaemic solution to induce i-LTP. When i-LTP reached a steady level (Fig. 4B, c), we returned to the solution containing APV in order to measure the amplitude of the AMPA-mediated component of the EPSP after the induction of i-LTP (Fig. 4B, d). In all the cells tested (n = 6), this component was significantly enhanced by the ischaemic period (compare trace a and d and see the histogram in the right panel of Fig. 4B; P < 0.001). To confirm that the enhanced component of this EPSP was fully mediated by AMPA receptors, we added 10 µM CNQX to the solution containing APV. As shown in Fig. 4B (e), under this condition the synaptic potential was completely blocked.
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Whole-cell analysis of i-LTP in 1.2 mM magnesium
In the presence of Mg2+ ions, the blockade of NMDA receptors is usually relieved by membrane depolarization. Using the whole-cell patch–clamp technique, even in the presence of physiological concentrations of Mg2+ ions (1.2 mM), an NMDA-mediated component of corticostriatal EPSCs could be unmasked by clamping membrane potential of the recorded striatal cells at membrane values significantly positive to their resting membrane potential. Accordingly, when recorded at holding potentials of –50 mV, but not at –80 mV, striatal spiny neurones responded to single stimulation of corticostriatal fibres by producing an EPSC partially blocked by 50 M APV (19 ± 4%, n = 6, P < 0.01).
Combined oxygen and glucose deprivation (4 min) invariably produced inward currents in striatal neurones clamped at –80 mV (–180 ± 38 pA, n = 6), while it caused inward currents (–40 ± 10 pA, n = 5), outward currents (+20 ± 13 pA, n = 3) or no effect (n = 2) in striatal neurones clamped at –50 mV. These data are consistent with previous reports in which the reversal potential of the ischaemic current was estimated at about –40 mV (Calabresi et al., 1999a).
When spiny neurones were held at –50 mV during the application of the ischaemic insult, we detected an i-LTP of similar time course and amplitude to that observed in Mg2+-free solution (see above). Interestingly, this experimental condition led to the enhancement not only of EPSCs recorded at –50 mV, but also those recorded at –80 mV (n = 10, P < 0.01 for both holding potentials) (Fig. 5A). EPSCs evoked at –80 mV remained insensitive to APV, even after their potentiation (n = 4, P > 0.05) (not shown). Conversely, either EPSCs recorded at –50 mV or EPSCs recorded at –80 mV were unchanged following the application of the ischaemic challenge at holding potential of –80 mV (n = 6) (Fig. 5B). Taken together, these data indicate that in the presence of physiological concentrations of Mg2+, reactivation of NMDA receptors by membrane depolarization is a crucial requirement to induce i-LTP and that, after its induction, this form of synaptic plasticity is dependent, at least in part, on the potentiation of AMPA currents.
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Occlusion experiments between t-LTP and i-LTP in striatal spiny neurones
Tetanic stimulation of glutamatergic corticostriatal fibres can induce a LTP of the amplitude of the EPSP recorded from striatal spiny neurones (Fig. 6A). This form of t-LTP is blocked by antagonists of NMDA receptors (Calabresi et al., 1996
). Occlusion experiments were performed in order to examine whether striatal t-LTP and i-LTP might share some cellular mechanisms. As shown in Fig. 6A, induction of t-LTP prevented the subsequent formation of i-LTP (n = 6, P > 0.05). Figure 6B shows the converse situtation (n = 6, P > 0.05).
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Intracellular BAPTA in t-LTP and i-LTP
In order to test the dependence of both t-LTP and i-LTP on a rise in postsynaptic intracellular Ca2+ concentration, we investigated the effects of the intraneuronal injection of the Ca2+ chelator BAPTA (100 mM). Under this condition, neither the high frequency tetanic stimulation of the corticostriatal fibres nor the brief ischaemic episode were able to induce a LTP in the recorded neurone (n = 5 and P > 0.05 for each experimental condition) (Fig. 7A and B). It is important to note that in BAPTA-loaded cells, the membrane depolarization induced by tetanic stimulation as well as the ischaemia-induced membrane depolarization did not significantly differ from those recorded with 2M KCl electrodes (data not shown).
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Inhibition of the MAP kinase pathway on i-LTP
One of the protein kinase families that has been implicated in the expression of long-term changes of excitatory transmission (English and Sweatt, 1997
; Coogan et al., 1999) as well as in the NMDA-mediated neurotoxicity (Ghosh and Greenberg, 1995) is the mitogen-activated protein (MAP) kinases. They can be activated in response to increases in intracellular calcium levels or stimulation of glutamate receptors (Bading and Greenberg, 1991). For this reason, we tested the effect of PD98059, a specific inhibitor of MAP kinase kinase (MEK) and p42/44 MAP kinase activation (Alessi et al., 1995; Pang et al., 1995), on the induction and expression of i-LTP. As shown in Fig. 7A and B, long-term incubation (2 h) of the slices in 10 µM PD98059 prevented the induction of both striatal i-LTP and t-LTP (n = 5 and P > 0.05 for each experimental condition). This drug did not affect any of the measured intrinsic membrane properties of the striatal spiny neurones per se. Moreover, the physiological and pharmacological characteristics of the corticostriatal EPSPs evoked by a single stimulation were not altered by this inhibitor.
Effects of muscarinic receptor blockade
Acetylcholine, via muscarinic receptors, plays a major role in memory formation (Whishaw, 1989; Graybiel, 1995) and physiological synaptic plasticity (Auerbach and Segal, 1996; Calabresi et al., 2000). It has been shown that cholinergic interneurones, with widely branching axonal terminals, provide levels of endogenous acetylcholine that are among the highest in any brain region (Graybiel, 1990). Thus, we have investigated whether endogenous acetylcholine might influence striatal synaptic plasticity induced either by physiological events or by a pathological condition induced by energy deprivation. As shown in Fig. 7C and D, the muscarinic receptor antagonist scopolamine (0.1–10 µM) exerted a dose-dependent inhibition of t-LTP (n = 6 for each concentration tested; P > 0.05 at 1, 3 and 10 µM), while it did not affect i-LTP (n = 6 and P < 0.001 for each concentration tested). These doses of scopolamine by themselves did not influence resting membrane potential (n = 32), input resistance (n = 32) and ischaemia-induced membrane depolarization (n = 18) of striatal spiny neurones (not shown).
Input specificity of t-LTP and i-LTP
The interaction between t-LTP and i-LTP was further investigated by using a two-pathway design of stimulation. As shown in Fig. 8A, two different stimulating electrodes were positioned in the white matter between the cortex and the striatum at opposite sides of the recording electrode. The different (test and conditioning) stimuli were delivered in an alternative manner and the intensity of the baseline current for each pathway was fixed at 50% of the maximum amplitude obtained for each EPSP response. t-LTP was input-specific, i.e. it occurred only at the synapse that underwent the conditioning high-frequency electrical stimulation (n = 5, P < 0.001). The amplitude of the EPSP elicited by the test stimulating electrode to which tetanic stimulation was not delivered did not show any significant long-term changes compared with the control (P > 0.05) (Fig. 8B and D). Conversely, i-LTP occurred not only at the stimulated synapse, but also at the test synaptic pathway, which was not active during the application of the ischaemic medium (n = 5 and P < 0.001 for both contitioning and test EPSPs) (Fig. 8C and D). These results imply that striatal i-LTP does not depend on stimulation and is not pathway specific.
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Striatal large aspiny cholinergic interneurones express t-LTP but not i-LTP
As already reported by our group (Calabresi et al., 1997
a; Pisani et al., 1999), a short episode of energy deprivation causes a membrane hyperpolarization/outward current associated with a decrease of input resistance in striatal large aspiny cholinergic interneurones. Interestingly, this ischaemia-induced electrophysiological event is associated with a significant increase in intracellular calcium levels (Pisani et al., 1999). In this striatal cell type, the activation of corticostriatal fibres evoked an EPSP mediated by both AMPA and NMDA receptors in the absence of external Mg2+ (Calabresi et al., 1998). We tested the effects of 3 min of in vitro ischaemia on the synaptic transmission of these cholinergic interneurones. The EPSP was reversibly abolished within 2 min of the onset of the ischaemic episode in all cells tested (n = 12) and, on return to the oxygenated solution, recovered to the control value without showing any significant long-term modifications (n = 7, P > 0.05) (Fig. 9A and B), suggesting that i-LTP is selectively expressed in striatal spiny neurones, but not in cholinergic interneurones. In four out of four interneurones the manual clamp of the membrane to the resting membrane potential or even to more depolarized levels (+20 mV from the rest) during the ischaemic period failed to reveal i-LTP (data not shown). Conversely, as recently reported (Suzuki et al., 2001), these interneurones were able to generate t-LTP following a repetitive synaptic stimulation (three trains: 3 s duration, 100 Hz frequency, 20 s intervals) similar to the one utilized to induce LTP in spiny neurones (n = 7; P < 0.01) (Fig. 9B, right graph).
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Post-i-LTP is expressed in striatal spiny neurones but not in cholinergic aspiny interneurones: a possible mechanism for differential neuronal vulnerability
In the present study we have found that i-LTP is selectively expressed in striatal spiny neurones, but not in cholinergic interneurones. The expression of this pathological form of synaptic plasticity in a specific neuronal type may represent a novel mechanism underlying cell-type-specific vulnerability to ischaemia (Pulsinelli et al., 1985
; Chesselet et al., 1990). Interestingly, this differential neuronal vulnerability between spiny neurones and cholinergic interneurones has also been observed in Huntington’s disease, a pathological condition in which energy metabolism is impaired (Ferrante et al., 1985, 1987). We are aware that several other factors might account for the differential vulnerability of these two neuronal subtypes: the distribution of ionotropic glutamate receptor subunits (Calabresi et al., 1998), the response to metabotropic glutamate receptors (Calabresi et al., 1999c), the presence of intracellular Ca2+-binding proteins (Sloviter, 1989) and the expression of superoxide dismutase (Inagaki et al., 1991), an enzyme involved in the degradation of free radicals. We believe, however, that all these factors may converge to enable the expression of a long-term enhancement of excitatory transmission in one neuronal subtype but not the other. Although striatal spiny neurones and cholinergic interneurones express opposite short-term membrane responses to hypoxia (Calabresi et al., 1995), aglycaemia (Calabresi et al., 1997a) and ischaemia (Calabresi et al., 1999a; Pisani et al., 1999), the absence of i-LTP in cholinergic interneurones cannot simply depend on these differential short-term ionic mechanisms. In fact, a dramatic intracellular calcium elevation, which represents the main trigger for the induction of i-LTP, has been detected by microfluorometric measurements in both these neuronal subtypes following the ischaemic period (Calabresi et al., 1999a; Pisani et al., 1999). To confirm this thesis, we found that manual clamp of the membrane of cholinergic interneurones at depolarized membrane potentials during ischaemia failed to reveal i-LTP in this neuronal subtype. Interestingly, we also found that, as reported recently (Suzuki et al., 2001), cholinergic interneurones express t-LTP. This observation seems to indicate that although cholinergic cells possess the intracellular machinery necessary for synaptic plasticity, energy deprivation is unable to engage it.
Activation of NMDA receptors is required for the induction of i-LTP
While striatal spiny neurones are highly polarized in the in vitro brain-slice preparation, they show a characteristic membrane oscillatory behaviour between a depolarized ‘up state’ and a hyperpolarized ‘down state’ when recorded intracellularly in vivo (Calabresi et al., 1996; Wilson and Kawaguchi, 1996). During this depolarized up state, the voltage-dependent blockade of the NMDA receptor–channel complex exerted by Mg2+ can be removed enabling the induction of i-LTP during an ischaemic episode. Accordingly, we found that i-LTP can be induced either by omitting Mg2+ ions from the extracellular medium (experiments with sharp microelectrodes) or by holding the cells at depolarized membrane potentials (–50 mV) by utilizing whole-cell patch–clamp recordings. Conversely, ischaemia was unable to induce LTP when the cell was held at negative potentials (–80 mV).
The critical pharmacological demonstration that the induction of i-LTP depends on activation of NMDA receptors is the observation that this form of pathological synaptic plasticity is blocked by APV, a NMDA receptor antagonist. Accordingly, the induction of anoxic LTP described in CA1 hippocampal neurones also requires the activation of NMDA receptors (Crépel and Ben-Ari, 1996; Hsu and Huang, 1997). Interestingly, the concept that NMDA receptors are crucial in the excitotoxic ischaemia-induced brain damage is convergent with our findings and pioneering observations indicating that NMDA receptor antagonists reduce neuronal death in animal models of ischaemic or hypoglycaemic brain injury (Simon et al., 1984; Wieloch, 1985).
Post-i-LTP is expressed both on NMDA- and AMPA-mediated components of corticostriatal EPSP
Pharmacological manipulation of ionotropic glutamate receptors supports the idea that both NMDA- and AMPA-mediated components of corticostriatal EPSPs are enhanced by ischaemia. Similar conclusions can be drawn on the basis of the whole-cell patch–clamp recordings showing that i-LTP, once induced by concomitant membrane depolarization and energy deprivation, can be detected even at hyperpolarized potentials. This latter evidence also indicates that the AMPA receptor-mediated component of corticostriatal EPSCs undergoes this form of synaptic plasticity.
The issue concerning the glutamate receptor subtype involved in i-LTP has been a matter of controversy in previous studies dealing with anoxic LTP in the CA1 hippocampal area. In fact, Ben-Ari’s group found that ischaemia selectively enhanced NMDA-mediated transmission (Crépel et al., 1993). The anoxic LTP induced in the studies of Hsu and Huang (1997) did not require NMDA receptors, since anoxic LTP of the pharmacologically isolated AMPA EPSP could be induced in the presence of APV. The experimental condition utilized in our study strongly resembles the protocol utilized by Ben-Ari’s group. Thus, we have to assume that the different ischaemia-induced long-term synaptic changes observed in CA1 pyramidal cells and in striatal spiny neurones are not merely dependent on the severity (ischaemia versus anoxia) or on the duration of the insults, but rather are due to different expression mechanisms.
The expression of i-LTP on both components of corticostriatal synaptic transmission leads to two considerations. First, since the magnitude of the ischaemia-induced potentiation is similar when the two components of the EPSP are recorded in pharmacological isolation, we can argue that a presynaptic mechanism is mainly involved in the expression of striatal i-LTP. Alternatively, we have to assume that the ischaemic episode triggers a cascade of postsynaptic events leading to a similar enhancement of both NMDA and AMPA postsynaptic currents. Secondly, it is likely that a long-term enhancement of both components of the EPSP has more profound consequences on neuronal survival. In fact, while the expression of i-LTP on the NMDA component would simply affect corticostriatal transmission during the up state, the expression of i-LTP on the AMPA component alters this excitatory transmission even during the down state (Wilson and Kawaguchi, 1996). AMPA receptors are highly permeable to Ca2+ ions after the loss of the glutamate receptor 2 subunit following ischaemia (Pellegrini-Giampietro et al., 1997). Thus, the ischaemic enhancement of AMPA transmission may dramatically amplify the excitatory signal leading to ischaemic neuronal death.
Post-i-LTP and -t-LTP might share some post-receptor mechanisms
Occlusion experiments have shown that striatal i-LTP and t-LTP are mutually occlusive, suggesting that these two synaptic events might share some cellular mechanisms. However, we cannot exclude the possibility that the lack of new synaptic changes in this particular protocol of occlusion could result from a saturation of the synaptic strength induced by the first conditioning stimulus. We observed that the buffering of intracellular Ca2+ by BAPTA prevents the induction of both forms of striatal synaptic plasticity, indicating that an elevation in intracellular Ca2+ is the first critical step for the induction of i-LTP and t-LTP. This observation also indicates that the induction of both forms of synaptic plasticity requires a critical postsynaptic mechanism. The buffering of intracellular calcium by BAPTA altered neither the amplitude and the time course of the ischaemia-induced membrane changes nor the characteristics of the tetanus-induced depolarizations. This finding suggests that the BAPTA-induced blockade of both i-LTP and t-LTP is not simply due to different membrane changes achieved during the induction phase. It is worth noting that hippocampal anoxic LTP also occluded post-t-LTP and was prevented by Ca2+ chelating agents (Crépel and Ben-Ari, 1996; Hsu and Huang, 1997), suggesting that these features of pathological forms of synaptic transmission may be common in various neuronal subtypes.
In the present study, we also found that PD98059, a specific inhibitor of MAP kinase activity (Alessi et al., 1995; Pang et al., 1995) blocks both i-LTP and t-LTP. Accordingly, this inhibitor has been shown to alter hippocampal LTP (English and Sweatt, 1997) and to block long-term facilitation in the mollusk Aplysia (Martin et al., 1997). In addition, we have recently reported that the same concentration of PD98059 employed in this study (10 µM) fully inhibited MAP kinase activity and synaptic potentiation observed in striatal neurones following the application of mitochondrial toxins (Calabresi et al., 2001). At present, however, very little is known about the role of the MAP kinase pathway on the synaptic processes following ischaemia. It has been hypothesized that cerebral ischaemia leads to the activation of this signal transduction cascade via glutamate release and activation of NMDA receptors, which, in turn, causes calcium entry (Wieloch et al., 1996; Xia et al., 1996). However, studies of the role of MAP kinase activity in ischaemia have generated discordant opinions. Sustained MAP kinase activation following ischaemia was suggested to mediate selective resistance to ischaemia in CA3 pyramidal neurones (Hu and Wieloch, 1994). Conversely, it has been proposed that MAP kinase activity might cause events favouring neuronal death, such as inappropriate protein phosphorylation in CA3 pyramidal cells and disruption of the cytoskeleton in the CA1 neurones (Runden et al., 1998). We have provided the first evidence that activation of the MAP kinase pathway is required for the generation of the ischaemia-induced long-term enhancement of excitatory transmission in the brain.
Post-i-LTP lacks input specificity
A major characteristic of physiological forms of synaptic plasticity such as hippocampal LTP (Malenka, 1994), cerebellar long-term depression (LTD) (Linden, 1994; Daniel et al., 1998) and striatal LTD (Calabresi et al., 1992) is the input specificity; only the synaptic pathway activated during the conditioning event leads to long-term changes of synaptic efficacy. In the present study, we were able to demonstrate that striatal t-LTP also shows a clear input specificity. Conversely, we observed that striatal i-LTP lacks this phenomenon. Thus, a brief ischaemic episode may enhance corticostriatal excitatory transmission not only at the specific activated pathway, but also at synapses that were not active during the induction phase. This difference between t-LTP and i-LTP may have dramatic consequences in the synaptic control of striatal function. In fact, the striatal spiny neurone is the main integrating element of the striatum (Graybiel, 1990; Gerfen, 1992). Compared with the aspiny interneurone, which receives only a sparse innervation, the spiny neurone is the major target of both local and extrinsic afferents. Moreover, this neuronal subtype integrates information from a wide variety of afferents and conveys the results of this complex neuronal computation outside the striatum. The integrative role of the spiny neurone implies a precise hierarchical distribution of different chemospecific afferents that interact in a highly ordered manner. We can assume that after the induction of i-LTP this ordered distribution is completely lost and the filtering activity of striatal neurones in the basal ganglia is impaired.
Endogenous acetylcholine is required for t-LTP but not for i-LTP
Clinical and experimental evidence suggest a major role of acetylcholine in learning and memory. Cognitive deficits occurring within certain senile dementias and Alzheimer’s disease are in fact associated with a reduction in cholinergic activity. In animal models, lesion of the brain cholinergic systems results in learning deficits that can be mimicked by cholinergic antagonists (Whishaw, 1989). Since long-term changes in the efficacy of synaptic transmission have been identified in many brain regions and are considered as possible cellular substrates underlying learning and memory, a modulatory action of acetylcholine in these forms of synaptic plasticity can be postulated (Auerbach and Segal, 1996; Calabresi et al., 2000). In accordance with this idea, we found that scopolamine fully prevented physiological LTP induced by repetitive corticostriatal stimulation, while it did not affect synaptic potentiation induced by ischaemia, suggesting that this pathological event is refractory to the modulation of specific neurotransmitter systems.
Post-i-LTP may trigger apoptotic neuronal death
Excitotoxicity plays an important role in the neuronal necrosis induced by ischaemia (Greene and Greenamyre, 1996). Moreover, growing evidence now indicates that several neuronal subtypes undergo apoptosis after ischaemic attacks (Lee et al., 1999). We can assume that glutamate-induced neuronal depolarizations cause necrotic death in cells located in the ischaemic core, while sub-lethal glutamate concentrations initiate apoptotic processes in neurones located in the ischaemic penumbra. Thus, the induction of i-LTP in striatal spiny neurones following a brief episode of ischaemia may represent a synaptic mechanism leading to apoptosis via a long-term but controlled increase of intracellular calcium. The pharmacological modulation of the MAP kinase pathway by altering i-LTP may provide an alternative way of rescuing neurones in the ischaemic penumbra.
Future studies will be necessary to address at the cellular level the short- and long-term effects of energy deprivation on the other subclasses of striatal interneurones, namely parvalbumin- and nitric oxide synthase-positive interneurones. These cells also show differential sensitivity to ischaemic insults (Pulsinelli, 1985). Thus, the analysis of their electrophysiological behaviour following energy deprivation may further validate the link between long-term changes in excitatory inputs and neuronal death.
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Acknowledgements |
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We wish to thank Mr Massimo Tolu for excellent technical assistance. This study was supported by Telethon (E. 729) and CNR (Invecchiamento) grants to P.C., and by MURST/CNR (legge 95/95) and MURST/COFIN (1998) grants to G.B.
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