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Priming stimulation of group II metabotropic glutamate receptors inhibits the subsequent induction of rat hippocampal long-term depression in vitro
Neuroscience Letters 307 (2001) 13±16
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Priming stimulation of group II metabotropic glutamate receptors inhibits the subsequent induction of rat hippocampal long-term depression in vitro Christian Mellentin, Wickliffe C. Abraham* Department of Psychology and the Neuroscience Research Centre, University of Otago, Box 56, Dunedin, New Zealand Received 19 February 2001; received in revised form 19 April 2001; accepted 7 May 2001
Abstract The ability of priming activation of metabotropic glutamate receptors (mGluRs) to regulate long-term depression (LTD) was studied in area CA1 of hippocampal slices taken from young adult male rats. Pharmacological activation of Group I mGluRs 30±40 min prior to low-frequency stimulation at 3 Hz failed to affect LTD. Activation of Group II mGluRs, however, signi®cantly inhibited the LTD by . 50%, while activation of Group III mGluRs had no statistically signi®cant effect on LTD. The inhibition of LTD by activation of Group II mGluRs was even stronger when the Group II agonist was applied during the low-frequency stimulation. Because activation of Group II mGluRs is also known to inhibit LTP, the net effect of such stimulation is the induction of a metaplasticity that greatly restricts the effective range of stimuli that can evoke synaptic plasticity in the hippocampus. q 2001 Elsevier Science Ireland Ltd. All rights reserved. Keywords: Hippocampus; Long-term depression; Metabotropic glutamate receptor; Metaplasticity; Low-frequency stimulation; Priming
Metabotropic glutamate receptors (mGluRs) play an important modulatory role in the induction of long-term potentiation (LTP) and long-term depression (LTD), but the precise roles played by these receptors have remained controversial. This may be due in part to the diversity of mGluR subtypes, and the associated wide range of signalling cascades that they regulate. In the hippocampus, activation of Group I mGluRs, which activate phospholipase C, generally facilitates LTP induction and persistence in area CA1 of the hippocampus [2,17]. However, activation of these receptors may not be obligatory for certain forms of LTP [6,20]. Group I mGluR agonists can also directly elicit LTD [18], but the necessity for these receptors to be active during low-frequency stimulation (LFS) to induce LTD varies with the experimental conditions [16,20]. In contrast, agonists of Group II and Group III mGluRs, which inhibit adenylate cyclase, impair LTP [10,15]. Where it has been studied in vivo, Group II mGluR activation appears to promote LTD, since antagonists block its induction [12], but Group III mGluR activation inhibits LTD [13]. Interestingly, Group I agonists can be applied well before * Corresponding author. Tel.: 164-3-479-7648; fax: 1 64-3-4798335. E-mail address: [email protected] (W.C. Abraham).
tetanic stimulation, and yet still promote, i.e. prime, LTP [5,8]. Thus, at least this subtype of mGluR can trigger longlasting changes in synaptic function that impinge on LTP mechanisms; such changes have been termed collectively metaplasticity [1]. Here, we address whether priming stimulation of Group I mGluRs affects LTD in a complementary fashion. Our ®ndings indicate that Group I receptor activation in fact has no effect on subsequent LTD, whereas activation of Group II mGluRs does produce a metaplastic change that effectively inhibits the induction of LTD. Hippocampal slices (400 mm) were prepared from young adult (8±12 week) male Sprague±Dawley rats, as described previously [8]. Slices were submerged in a brain slice chamber and pre-incubated for at least 2 h in a continuous ¯ow (2±3 ml/min) of arti®cial cerebrospinal ¯uid (ACSF, containing in mM: 124 NaCl, 3.2 KCl, 1.25 NaH2PO4, 26 NaHCO3, 2.5 CaCl2, 1.3 MgCl2, and 10 d-glucose, equilibrated with 95% O2/5% CO2) at 32.58C. Field excitatory synaptic potentials (fEPSPs) were recorded from stratum radiatum in area CA1 using glass microelectrodes (1±3 MV) ®lled with 2 M NaCl. Baseline synaptic responses were evoked by stimulation of the Schaffer collateral/ commissural pathway at 0.033 Hz (diphasic pulses, 0.1 ms half-wave duration) with a 50 mm tungsten monopolar electrode. The stimulation intensity was adjusted to elicit
0304-3940/01/$ - see front matter q 2001 Elsevier Science Ireland Ltd. All rights reserved. PII: S03 04 - 394 0( 0 1) 01 91 5- 2
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C. Mellentin, W.C. Abraham / Neuroscience Letters 307 (2001) 13±16
fEPSPs with an amplitude of 1 mV. LFS for inducing LTD consisted of 600 or 1200 pulses delivered at 3 Hz and with a stimulus intensity at population spike threshold. Initial slopes of the fEPSPs were measured off-line and expressed as percentage change from baseline, calculated as the average of the last 15 min of baseline recordings. Percent LTD was calculated by averaging the 10 responses recorded 55± 60 min post-LFS. Slices which had baseline drift of more than 7% over the 20 min baseline period, as determined by linear regression, were discarded from the analysis. Twotailed unpaired Student's t-tests were performed to determine group differences in LTD at the 95% con®dence level. Data are presented as group means ^ SEM. Reagents were obtained from the following vendors: all salts from BDH Laboratory supplies; (R,S)-3,5-dihydroxyphenylglycine (DHPG), (2S,2 0 R,3 0 R)-2-(2 0 ,3 0 -dicarboxycyclopropyl)glycine (DCG-IV) and L(1)-2-amino-4phosphonobutyric acid (L-AP4) from Tocris (Bristol). Drugs were dissolved in either 100 mM NaOH (DCG-IV, L-AP4) or dH2O (DHPG), and diluted 100±1000-fold to their ®nal concentration in ASCF. Because DHPG facilitates subsequent LTP [8,19], we ®rst tested whether the same dose regimen of this Group I agonist would cause a complementary inhibition of LTD. The control level of LTD, elicited by 1200 pulses at 3 Hz, was 230 ^ 4% (n 7; Fig. 1A). Administration of 20 mM DHPG for 10 min caused a marked response depression (43 ^ 7%) and a smaller residual LTD after 15 min of wash-out, which can persist for at least 90 min after drug treatment in control slices ([8,18], S. Webb and W.C. Abraham, unpublished observations). At this point the stimulus strength was increased to return the fEPSP to its original 1 mV amplitude. After a further 15 min recording period, LFS was delivered, generating an LTD that was not signi®cantly different from controls (231 ^ 5%, n 8, P . 0:5). These data indicate that Group I mGluR activation does not inhibit subsequent LTD. We then considered the alternative possibility that Group I mGluR activation may in fact facilitate LTD induction, but that this could not be observed in experiments where strong LTD was already induced by the control LFS. To test this new hypothesis, a mild LFS (600 pulses, 3 Hz) was employed that produced only a small LTD in control slices (27 ^ 4%, n 6; Fig. 1B). DHPG treatment again produced a marked initial response depression (243 ^ 4%) that resolved as a moderate level of depression following wash-out. After readjustment of the stimulus strength to regain the baseline response amplitude, the mild LFS failed to produce any greater LTD than observed in control slices (24 ^ 5%, n 6, P . 0:5; Fig. 1B). Thus, although Group I receptor activation by DHPG can induce LTD on its own, it does not affect subsequent LFS-induced LTD, in contrast to its robust ability to prime LTP. Results from in vivo studies indicate that Group II and Group III mGluR activation can modify LTD induction and depotentiation in CA1, although the direction of change has been controversial [10,12]. In the present study, we tested
whether prior administration of the Group II agonist DCGIV (1 mM) or the Group III agonist L-AP4 (500 mM) would modify LTD induction by the strong LFS protocol (1200 pulses). Administration of DCG-IV for 10 min caused a transient response depression (218 ^ 4%, n 6) that fully recovered to baseline in four of the slices. The stimulus current was raised in the remaining two slices to return the response amplitude to 1 mV. Subsequent LFS generated only 212 ^ 6% LTD, signi®cantly less than that observed in control slices (P , 0:05; Fig. 2A). Like DCG-IV, L-AP4 induced an initial depression of 221 ^ 3% (n 6), but this effect was transient and only small stimulus current adjustments were required to renormalize the baseline response (Fig. 2B). However, the LTD induced by subsequent LFS
Fig. 1. Lack of effect by DHPG on LTD. (A) DHPG (20 mm, bar) was bath-applied for 10 min, and washed out for 30 min prior to LFS (3 Hz, 1200 pulses). DHPG caused a strong initial response depression that did not completely recover during drug washout. The stimulus amplitude was increased (arrow) to adjust the response back to the baseline level of 1 mV. DHPG had no effect on the level of LFS compared to non-drug-treated controls. Inset waveforms are averages of 10 responses recorded just prior to LFS (larger response), and 55±60 min post-LFS, for sample control and DHPG-treated slices. Scale bars: 0.5 mV, 5 ms. (B) DHPG, applied as described in (A) above, failed to facilitate LTD induction by a moderate LFS consisting of 600 pulses, which produced a weak LTD in control slices.
C. Mellentin, W.C. Abraham / Neuroscience Letters 307 (2001) 13±16
Fig. 2. (A) Bath administration of the Group II agonist DCG-IV (1 mM) for 10 min inhibited the induction of LTD when applied 40 min prior to LFS. DCG-IV caused a transient mild depression of the fEPSP, which largely washed out quickly upon return to the control solution. In some slices, a small adjustment of the stimulus strength was required to bring the response back to baseline levels (arrow). Inset waveforms are averages of 10 responses recorded before and after LFS, at times as described in (A). Scale bars: 0.5 mV, 5 ms. (B) Administration of the Group III agonist L-AP4 (500 mM) did not signi®cantly affect LTD, although it produced a small, transient response depression equivalent to that observed following DCG-IV, shown in (A).
was not statistically different from that in control slices (218 ^ 8%; P , 0:02). Since priming activation of the Group II receptors only partially inhibited LTD, we tested whether their direct activation during LFS would give a more complete block. Indeed, when DCG-IV was applied for 10 min before and during the LFS, no signi®cant LTD was induced (26 ^ 4%, n 5; data not shown). Taken together, these data indicate that Group II mGluRs can exert a potent inhibitory in¯uence on LTD induction through mechanisms that decay very slowly over time after drug wash-out. Metaplasticity refers to activity-dependent modi®cations in the ability to produce synaptic plasticity [1]. Previously, it has been shown that prior activation of Group I mGluRs can
15
facilitate the induction of LTP [8] and set a molecular switch that renders LTP insensitive to the mGluR antagonist a-methyl-4-carboxyphenylglycine (MCPG) [5]. These effects are in addition to any LTD induced directly by Group I mGluR activation, and appear to be due to activation of several signalling cascades that ultimately increase cellular excitability and increase local protein synthesis [7,19]. We addressed here whether the effects of Group I receptor activation would in¯uence LTD in a way that was complementary to the facilitation of LTP, for example, by an inhibition of LTD. However, we were unable to detect either a facilitation or an inhibition of LTD by prior administration of DHPG. This failure to in¯uence LTD is in accord with a recent study showing no effect of the broad spectrum mGluR antagonist LY341495 on LTD in vitro [9], but con¯icts with a ®nding in vivo that Group I antagonists impair the maintenance of LTD [12]. This latter result may indicate that LFS-induced LTD in vivo at least partly re¯ects an mGluR-mediated LTD, similar to that which is observed in vitro following DHPG application alone, but which is different from the N-methyl-d-aspartate receptordependent LTD that is induced by LFS in the present studies [11,16]. In contrast to the lack of effects by DHPG, the Group II agonist DCG-IV potently inhibited LTD. An inhibition of LTD by a Group II agonist has not previously been reported to our knowledge, but these ®ndings are in general concordance with earlier in vivo studies showing that a Group II agonist can inhibit depotentiation [10], a process related to LTD. Taking these ®ndings together with the fact that Group II agonists inhibit LTP [4,10], it appears that Group II mGluR activation greatly reduces the range of stimuli that elicit synaptic plasticity. This may relate to the generally inhibitory effects of Group II mGluRs on the function of voltage-dependent calcium channels, which have been linked to the induction of both LTP and LTD [3]. However, it should be noted that Group II antagonists have also been shown to inhibit LTD in vivo [12]. Furthermore, L-AP4 blocks LTD in vivo [13], but it had no signi®cant effect in the present in vitro experiments. The reasons for these discrepancies remain unclear, but they may relate to different drug concentrations, a strain difference in LTD mechanisms [14], or to a different kind of LTD being generated between in vivo and in vitro preparations. While a near complete inhibition of LTD was detected when DCG-IV was given concurrently with the LFS, LTD was still reduced by greater than 50% even when the DCGIV was given 30 min prior to the LFS. The duration of this effect indicates that the drug induced a persistent metaplastic change in the state of the synapses being activated. The persistence of such changes allows for an integration of synaptic events that occur widely in time, and may be important for normal learning and memory functions. However, it remains a challenge to predict the direction and degree of metaplasticity that would arise from synaptically released glutamate, since it is now clear that LTP and
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C. Mellentin, W.C. Abraham / Neuroscience Letters 307 (2001) 13±16
LTD can be both up- and down-regulated by prior activation of different glutamatergic receptor subtypes ([1], present results). Resolution of this issue will require further understanding of the factors regulating glutamate diffusion and access to its various receptors under different activation conditions, the in¯uence of co-active neuromodulatory transmitters, and whether speci®c plasticity-inducing protocols are selectively sensitive to the biochemical state changes wrought by activation of particular glutamate receptor subtypes. [1] Abraham, W.C. and Bear, M.F., Metaplasticity: The plasticity of synaptic plasticity, Trends Neurosci., 19 (1996) 126± 130. [2] Bashir, Z.I., Bortolotto, Z.A., Davies, C.H., Berretta, N., Irving, A.J., Seal, A.J., Henley, J.M., Jane, D.E., Watkins, J.C. and Collingridge, G.L., Induction of LTP in the hippocampus needs synaptic activation of glutamate metabotropic receptors, Nature, 363 (1993) 347±350. [3] Bear, M.F. and Abraham, W.C., Long-term depression in the hippocampus, Ann. Rev. Neurosci., 19 (1996) 437±462. [4] Behnisch, T., Wilsch, V.W., Balschun, D. and Reymann, K.G., The role of group II metabotropic glutamate receptors in hippocampal CA1 long-term potentiation in vitro, Eur. J. Pharmacol., 356 (1998) 159±165. [5] Bortolotto, Z.A., Bashir, Z.I., Davies, C.H. and Collingridge, G.L., A molecular switch activated by metabotropic glutamate receptors regulates induction of long-term potentiation, Nature, 368 (1994) 740±743. [6] Chinestra, P., Aniksztejn, L., Diabira, D. and Ben-Ari, Y., (RS)-a-methyl-4-carboxyphenylglycine neither prevents induction of LTP nor antagonizes metabotropic glutamate receptors in CA1 hippocampal neurons, J. Neurophysiol., 70 (1993) 2684±2689. [7] Cohen, A.S., Coussens, C.M., Raymond, C.R. and Abraham, W.C., Long-lasting increase in cellular excitability associated with the priming of LTP induction in rat hippocampus, J. Neurophysiol., 82 (1999) 3139±3148. [8] Cohen, A.S., Raymond, C.R. and Abraham, W.C., Priming of long-term potentiation induced by activation of metabotropic glutamate receptors coupled to phospholipase C, Hippocampus, 8 (1998) 160±170. [9] Fitzjohn, S.M., Bortolotto, Z.A., Palmer, M.J., Doherty, A.J., Ornstein, P.L., Schoepp, D.D., Kingston, A.E., Lodge, D. and
[10]
[11]
[12]
[13]
[14]
[15]
[16] [17] [18]
[19]
[20]
Collingridge, G.L., The potent mGlu receptor antagonist LY341495 identi®es roles for both cloned and novel mGlu receptors in hippocampal synaptic plasticity, Neuropharmacology, 37 (1998) 1445±1458. HoÈlscher, C., Anwyl, R. and Rowan, M.J., Activation of group-II metabotropic receptors blocks induction of longterm potentiation and depotentiation in area CA1 of the rat in vivo, Eur. J. Pharmacol., 322 (1997) 155±163. Kerr, D.S. and Abraham, W.C., Cooperative interactions among afferents govern the induction of homosynaptic LTD in the hippocampus, Proc. Natl. Acad. Sci., USA, 92 (1995) 11637±11641. Manahan-Vaughan, D., Group 1 and 2 metabotropic glutamate receptors play differential roles in hippocampal longterm depression and long-term potentiation in freely moving rats, J. Neurosci., 17 (1997) 3303±3311. Manahan-Vaughan, D., Group III metabotropic glutamate receptors modulate long-term depression in the hippocampal CA1 region of two rat strains in vivo, Neuropharmacology, 39 (2000) 1952±1958. Manahan-Vaughan, D., Long-term depression in freely moving rats is dependent upon strain variation, induction protocol and behavioral state, Cereb. Cortex, 10 (2000) 482± 487. Manahan-Vaughan, D. and Reymann, K.G., Regional and developmental pro®le of modulation of hippocampal synaptic transmission and LTP by AP4-sensitive mGluRs in vivo, Neuropharmacology, 34 (1995) 991±1001. Oliet, S.H.R., Malenka, R.C. and Nicoll, R.A., Two distinct forms of long-term depression coexist in CA1 hippocampal pyramidal cells, Neuron, 18 (1997) 969±982. Otani, S. and Ben-Ari, Y., Metabotropic receptor-mediated long-term potentiation in rat hippocampal slices, Eur. J. Pharmacol., 205 (1991) 325±326. Palmer, M.J., Irving, A.J., Seabrook, G.R., Jane, D.E. and Collingridge, G.L., The group I mGlu receptor agonist DHPG induces a novel form of LTD in the CA1 region of the hippocampus, Neuropharmacology, 36 (1997) 1517± 1532. Raymond, C.R., Thompson, V., Tate, W.P. and Abraham, W.C., Metabotropic glutamate receptors trigger homosynaptic protein synthesis to prolong LTP, J. Neurosci., 20 (2000) 969±976. Selig, D.K., Lee, H.-K., Bear, M.F. and Malenka, R.C., Reexamination of the effects of MCPG on hippocampal LTP, LTD, and depotentiation, J. Neurophysiol., 74 (1995) 1075±1082.
www.elsevier.com/locate/neulet
Priming stimulation of group II metabotropic glutamate receptors inhibits the subsequent induction of rat hippocampal long-term depression in vitro Christian Mellentin, Wickliffe C. Abraham* Department of Psychology and the Neuroscience Research Centre, University of Otago, Box 56, Dunedin, New Zealand Received 19 February 2001; received in revised form 19 April 2001; accepted 7 May 2001
Abstract The ability of priming activation of metabotropic glutamate receptors (mGluRs) to regulate long-term depression (LTD) was studied in area CA1 of hippocampal slices taken from young adult male rats. Pharmacological activation of Group I mGluRs 30±40 min prior to low-frequency stimulation at 3 Hz failed to affect LTD. Activation of Group II mGluRs, however, signi®cantly inhibited the LTD by . 50%, while activation of Group III mGluRs had no statistically signi®cant effect on LTD. The inhibition of LTD by activation of Group II mGluRs was even stronger when the Group II agonist was applied during the low-frequency stimulation. Because activation of Group II mGluRs is also known to inhibit LTP, the net effect of such stimulation is the induction of a metaplasticity that greatly restricts the effective range of stimuli that can evoke synaptic plasticity in the hippocampus. q 2001 Elsevier Science Ireland Ltd. All rights reserved. Keywords: Hippocampus; Long-term depression; Metabotropic glutamate receptor; Metaplasticity; Low-frequency stimulation; Priming
Metabotropic glutamate receptors (mGluRs) play an important modulatory role in the induction of long-term potentiation (LTP) and long-term depression (LTD), but the precise roles played by these receptors have remained controversial. This may be due in part to the diversity of mGluR subtypes, and the associated wide range of signalling cascades that they regulate. In the hippocampus, activation of Group I mGluRs, which activate phospholipase C, generally facilitates LTP induction and persistence in area CA1 of the hippocampus [2,17]. However, activation of these receptors may not be obligatory for certain forms of LTP [6,20]. Group I mGluR agonists can also directly elicit LTD [18], but the necessity for these receptors to be active during low-frequency stimulation (LFS) to induce LTD varies with the experimental conditions [16,20]. In contrast, agonists of Group II and Group III mGluRs, which inhibit adenylate cyclase, impair LTP [10,15]. Where it has been studied in vivo, Group II mGluR activation appears to promote LTD, since antagonists block its induction [12], but Group III mGluR activation inhibits LTD [13]. Interestingly, Group I agonists can be applied well before * Corresponding author. Tel.: 164-3-479-7648; fax: 1 64-3-4798335. E-mail address: [email protected] (W.C. Abraham).
tetanic stimulation, and yet still promote, i.e. prime, LTP [5,8]. Thus, at least this subtype of mGluR can trigger longlasting changes in synaptic function that impinge on LTP mechanisms; such changes have been termed collectively metaplasticity [1]. Here, we address whether priming stimulation of Group I mGluRs affects LTD in a complementary fashion. Our ®ndings indicate that Group I receptor activation in fact has no effect on subsequent LTD, whereas activation of Group II mGluRs does produce a metaplastic change that effectively inhibits the induction of LTD. Hippocampal slices (400 mm) were prepared from young adult (8±12 week) male Sprague±Dawley rats, as described previously [8]. Slices were submerged in a brain slice chamber and pre-incubated for at least 2 h in a continuous ¯ow (2±3 ml/min) of arti®cial cerebrospinal ¯uid (ACSF, containing in mM: 124 NaCl, 3.2 KCl, 1.25 NaH2PO4, 26 NaHCO3, 2.5 CaCl2, 1.3 MgCl2, and 10 d-glucose, equilibrated with 95% O2/5% CO2) at 32.58C. Field excitatory synaptic potentials (fEPSPs) were recorded from stratum radiatum in area CA1 using glass microelectrodes (1±3 MV) ®lled with 2 M NaCl. Baseline synaptic responses were evoked by stimulation of the Schaffer collateral/ commissural pathway at 0.033 Hz (diphasic pulses, 0.1 ms half-wave duration) with a 50 mm tungsten monopolar electrode. The stimulation intensity was adjusted to elicit
0304-3940/01/$ - see front matter q 2001 Elsevier Science Ireland Ltd. All rights reserved. PII: S03 04 - 394 0( 0 1) 01 91 5- 2
14
C. Mellentin, W.C. Abraham / Neuroscience Letters 307 (2001) 13±16
fEPSPs with an amplitude of 1 mV. LFS for inducing LTD consisted of 600 or 1200 pulses delivered at 3 Hz and with a stimulus intensity at population spike threshold. Initial slopes of the fEPSPs were measured off-line and expressed as percentage change from baseline, calculated as the average of the last 15 min of baseline recordings. Percent LTD was calculated by averaging the 10 responses recorded 55± 60 min post-LFS. Slices which had baseline drift of more than 7% over the 20 min baseline period, as determined by linear regression, were discarded from the analysis. Twotailed unpaired Student's t-tests were performed to determine group differences in LTD at the 95% con®dence level. Data are presented as group means ^ SEM. Reagents were obtained from the following vendors: all salts from BDH Laboratory supplies; (R,S)-3,5-dihydroxyphenylglycine (DHPG), (2S,2 0 R,3 0 R)-2-(2 0 ,3 0 -dicarboxycyclopropyl)glycine (DCG-IV) and L(1)-2-amino-4phosphonobutyric acid (L-AP4) from Tocris (Bristol). Drugs were dissolved in either 100 mM NaOH (DCG-IV, L-AP4) or dH2O (DHPG), and diluted 100±1000-fold to their ®nal concentration in ASCF. Because DHPG facilitates subsequent LTP [8,19], we ®rst tested whether the same dose regimen of this Group I agonist would cause a complementary inhibition of LTD. The control level of LTD, elicited by 1200 pulses at 3 Hz, was 230 ^ 4% (n 7; Fig. 1A). Administration of 20 mM DHPG for 10 min caused a marked response depression (43 ^ 7%) and a smaller residual LTD after 15 min of wash-out, which can persist for at least 90 min after drug treatment in control slices ([8,18], S. Webb and W.C. Abraham, unpublished observations). At this point the stimulus strength was increased to return the fEPSP to its original 1 mV amplitude. After a further 15 min recording period, LFS was delivered, generating an LTD that was not signi®cantly different from controls (231 ^ 5%, n 8, P . 0:5). These data indicate that Group I mGluR activation does not inhibit subsequent LTD. We then considered the alternative possibility that Group I mGluR activation may in fact facilitate LTD induction, but that this could not be observed in experiments where strong LTD was already induced by the control LFS. To test this new hypothesis, a mild LFS (600 pulses, 3 Hz) was employed that produced only a small LTD in control slices (27 ^ 4%, n 6; Fig. 1B). DHPG treatment again produced a marked initial response depression (243 ^ 4%) that resolved as a moderate level of depression following wash-out. After readjustment of the stimulus strength to regain the baseline response amplitude, the mild LFS failed to produce any greater LTD than observed in control slices (24 ^ 5%, n 6, P . 0:5; Fig. 1B). Thus, although Group I receptor activation by DHPG can induce LTD on its own, it does not affect subsequent LFS-induced LTD, in contrast to its robust ability to prime LTP. Results from in vivo studies indicate that Group II and Group III mGluR activation can modify LTD induction and depotentiation in CA1, although the direction of change has been controversial [10,12]. In the present study, we tested
whether prior administration of the Group II agonist DCGIV (1 mM) or the Group III agonist L-AP4 (500 mM) would modify LTD induction by the strong LFS protocol (1200 pulses). Administration of DCG-IV for 10 min caused a transient response depression (218 ^ 4%, n 6) that fully recovered to baseline in four of the slices. The stimulus current was raised in the remaining two slices to return the response amplitude to 1 mV. Subsequent LFS generated only 212 ^ 6% LTD, signi®cantly less than that observed in control slices (P , 0:05; Fig. 2A). Like DCG-IV, L-AP4 induced an initial depression of 221 ^ 3% (n 6), but this effect was transient and only small stimulus current adjustments were required to renormalize the baseline response (Fig. 2B). However, the LTD induced by subsequent LFS
Fig. 1. Lack of effect by DHPG on LTD. (A) DHPG (20 mm, bar) was bath-applied for 10 min, and washed out for 30 min prior to LFS (3 Hz, 1200 pulses). DHPG caused a strong initial response depression that did not completely recover during drug washout. The stimulus amplitude was increased (arrow) to adjust the response back to the baseline level of 1 mV. DHPG had no effect on the level of LFS compared to non-drug-treated controls. Inset waveforms are averages of 10 responses recorded just prior to LFS (larger response), and 55±60 min post-LFS, for sample control and DHPG-treated slices. Scale bars: 0.5 mV, 5 ms. (B) DHPG, applied as described in (A) above, failed to facilitate LTD induction by a moderate LFS consisting of 600 pulses, which produced a weak LTD in control slices.
C. Mellentin, W.C. Abraham / Neuroscience Letters 307 (2001) 13±16
Fig. 2. (A) Bath administration of the Group II agonist DCG-IV (1 mM) for 10 min inhibited the induction of LTD when applied 40 min prior to LFS. DCG-IV caused a transient mild depression of the fEPSP, which largely washed out quickly upon return to the control solution. In some slices, a small adjustment of the stimulus strength was required to bring the response back to baseline levels (arrow). Inset waveforms are averages of 10 responses recorded before and after LFS, at times as described in (A). Scale bars: 0.5 mV, 5 ms. (B) Administration of the Group III agonist L-AP4 (500 mM) did not signi®cantly affect LTD, although it produced a small, transient response depression equivalent to that observed following DCG-IV, shown in (A).
was not statistically different from that in control slices (218 ^ 8%; P , 0:02). Since priming activation of the Group II receptors only partially inhibited LTD, we tested whether their direct activation during LFS would give a more complete block. Indeed, when DCG-IV was applied for 10 min before and during the LFS, no signi®cant LTD was induced (26 ^ 4%, n 5; data not shown). Taken together, these data indicate that Group II mGluRs can exert a potent inhibitory in¯uence on LTD induction through mechanisms that decay very slowly over time after drug wash-out. Metaplasticity refers to activity-dependent modi®cations in the ability to produce synaptic plasticity [1]. Previously, it has been shown that prior activation of Group I mGluRs can
15
facilitate the induction of LTP [8] and set a molecular switch that renders LTP insensitive to the mGluR antagonist a-methyl-4-carboxyphenylglycine (MCPG) [5]. These effects are in addition to any LTD induced directly by Group I mGluR activation, and appear to be due to activation of several signalling cascades that ultimately increase cellular excitability and increase local protein synthesis [7,19]. We addressed here whether the effects of Group I receptor activation would in¯uence LTD in a way that was complementary to the facilitation of LTP, for example, by an inhibition of LTD. However, we were unable to detect either a facilitation or an inhibition of LTD by prior administration of DHPG. This failure to in¯uence LTD is in accord with a recent study showing no effect of the broad spectrum mGluR antagonist LY341495 on LTD in vitro [9], but con¯icts with a ®nding in vivo that Group I antagonists impair the maintenance of LTD [12]. This latter result may indicate that LFS-induced LTD in vivo at least partly re¯ects an mGluR-mediated LTD, similar to that which is observed in vitro following DHPG application alone, but which is different from the N-methyl-d-aspartate receptordependent LTD that is induced by LFS in the present studies [11,16]. In contrast to the lack of effects by DHPG, the Group II agonist DCG-IV potently inhibited LTD. An inhibition of LTD by a Group II agonist has not previously been reported to our knowledge, but these ®ndings are in general concordance with earlier in vivo studies showing that a Group II agonist can inhibit depotentiation [10], a process related to LTD. Taking these ®ndings together with the fact that Group II agonists inhibit LTP [4,10], it appears that Group II mGluR activation greatly reduces the range of stimuli that elicit synaptic plasticity. This may relate to the generally inhibitory effects of Group II mGluRs on the function of voltage-dependent calcium channels, which have been linked to the induction of both LTP and LTD [3]. However, it should be noted that Group II antagonists have also been shown to inhibit LTD in vivo [12]. Furthermore, L-AP4 blocks LTD in vivo [13], but it had no signi®cant effect in the present in vitro experiments. The reasons for these discrepancies remain unclear, but they may relate to different drug concentrations, a strain difference in LTD mechanisms [14], or to a different kind of LTD being generated between in vivo and in vitro preparations. While a near complete inhibition of LTD was detected when DCG-IV was given concurrently with the LFS, LTD was still reduced by greater than 50% even when the DCGIV was given 30 min prior to the LFS. The duration of this effect indicates that the drug induced a persistent metaplastic change in the state of the synapses being activated. The persistence of such changes allows for an integration of synaptic events that occur widely in time, and may be important for normal learning and memory functions. However, it remains a challenge to predict the direction and degree of metaplasticity that would arise from synaptically released glutamate, since it is now clear that LTP and
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C. Mellentin, W.C. Abraham / Neuroscience Letters 307 (2001) 13±16
LTD can be both up- and down-regulated by prior activation of different glutamatergic receptor subtypes ([1], present results). Resolution of this issue will require further understanding of the factors regulating glutamate diffusion and access to its various receptors under different activation conditions, the in¯uence of co-active neuromodulatory transmitters, and whether speci®c plasticity-inducing protocols are selectively sensitive to the biochemical state changes wrought by activation of particular glutamate receptor subtypes. [1] Abraham, W.C. and Bear, M.F., Metaplasticity: The plasticity of synaptic plasticity, Trends Neurosci., 19 (1996) 126± 130. [2] Bashir, Z.I., Bortolotto, Z.A., Davies, C.H., Berretta, N., Irving, A.J., Seal, A.J., Henley, J.M., Jane, D.E., Watkins, J.C. and Collingridge, G.L., Induction of LTP in the hippocampus needs synaptic activation of glutamate metabotropic receptors, Nature, 363 (1993) 347±350. [3] Bear, M.F. and Abraham, W.C., Long-term depression in the hippocampus, Ann. Rev. Neurosci., 19 (1996) 437±462. [4] Behnisch, T., Wilsch, V.W., Balschun, D. and Reymann, K.G., The role of group II metabotropic glutamate receptors in hippocampal CA1 long-term potentiation in vitro, Eur. J. Pharmacol., 356 (1998) 159±165. [5] Bortolotto, Z.A., Bashir, Z.I., Davies, C.H. and Collingridge, G.L., A molecular switch activated by metabotropic glutamate receptors regulates induction of long-term potentiation, Nature, 368 (1994) 740±743. [6] Chinestra, P., Aniksztejn, L., Diabira, D. and Ben-Ari, Y., (RS)-a-methyl-4-carboxyphenylglycine neither prevents induction of LTP nor antagonizes metabotropic glutamate receptors in CA1 hippocampal neurons, J. Neurophysiol., 70 (1993) 2684±2689. [7] Cohen, A.S., Coussens, C.M., Raymond, C.R. and Abraham, W.C., Long-lasting increase in cellular excitability associated with the priming of LTP induction in rat hippocampus, J. Neurophysiol., 82 (1999) 3139±3148. [8] Cohen, A.S., Raymond, C.R. and Abraham, W.C., Priming of long-term potentiation induced by activation of metabotropic glutamate receptors coupled to phospholipase C, Hippocampus, 8 (1998) 160±170. [9] Fitzjohn, S.M., Bortolotto, Z.A., Palmer, M.J., Doherty, A.J., Ornstein, P.L., Schoepp, D.D., Kingston, A.E., Lodge, D. and
[10]
[11]
[12]
[13]
[14]
[15]
[16] [17] [18]
[19]
[20]
Collingridge, G.L., The potent mGlu receptor antagonist LY341495 identi®es roles for both cloned and novel mGlu receptors in hippocampal synaptic plasticity, Neuropharmacology, 37 (1998) 1445±1458. HoÈlscher, C., Anwyl, R. and Rowan, M.J., Activation of group-II metabotropic receptors blocks induction of longterm potentiation and depotentiation in area CA1 of the rat in vivo, Eur. J. Pharmacol., 322 (1997) 155±163. Kerr, D.S. and Abraham, W.C., Cooperative interactions among afferents govern the induction of homosynaptic LTD in the hippocampus, Proc. Natl. Acad. Sci., USA, 92 (1995) 11637±11641. Manahan-Vaughan, D., Group 1 and 2 metabotropic glutamate receptors play differential roles in hippocampal longterm depression and long-term potentiation in freely moving rats, J. Neurosci., 17 (1997) 3303±3311. Manahan-Vaughan, D., Group III metabotropic glutamate receptors modulate long-term depression in the hippocampal CA1 region of two rat strains in vivo, Neuropharmacology, 39 (2000) 1952±1958. Manahan-Vaughan, D., Long-term depression in freely moving rats is dependent upon strain variation, induction protocol and behavioral state, Cereb. Cortex, 10 (2000) 482± 487. Manahan-Vaughan, D. and Reymann, K.G., Regional and developmental pro®le of modulation of hippocampal synaptic transmission and LTP by AP4-sensitive mGluRs in vivo, Neuropharmacology, 34 (1995) 991±1001. Oliet, S.H.R., Malenka, R.C. and Nicoll, R.A., Two distinct forms of long-term depression coexist in CA1 hippocampal pyramidal cells, Neuron, 18 (1997) 969±982. Otani, S. and Ben-Ari, Y., Metabotropic receptor-mediated long-term potentiation in rat hippocampal slices, Eur. J. Pharmacol., 205 (1991) 325±326. Palmer, M.J., Irving, A.J., Seabrook, G.R., Jane, D.E. and Collingridge, G.L., The group I mGlu receptor agonist DHPG induces a novel form of LTD in the CA1 region of the hippocampus, Neuropharmacology, 36 (1997) 1517± 1532. Raymond, C.R., Thompson, V., Tate, W.P. and Abraham, W.C., Metabotropic glutamate receptors trigger homosynaptic protein synthesis to prolong LTP, J. Neurosci., 20 (2000) 969±976. Selig, D.K., Lee, H.-K., Bear, M.F. and Malenka, R.C., Reexamination of the effects of MCPG on hippocampal LTP, LTD, and depotentiation, J. Neurophysiol., 74 (1995) 1075±1082.