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4-Aminopyridine-mediated increase in long-term potentiation in CA1 of the rat hippocampus
Neuroscwnce Letters, 70 (1986) 106 il)~
106
lilse~ icl
NSL 04143
4-Aminopyridine-mediated increase in long-term potentiation in CA1 of the rat hippocampus W a i - L i n g Lee l, R o g e r A n w y l I a n d Mich'.el R o w a n 2 JDepartment of Physiology, and 2Department of Pharmacology and Therapeutics, Trinity College, Dubtin 2 (Ireland) (Received 24 April 1986; Revised version received 4 June 1986; Accepted 6 June 1986)
Key words." Hippocampal slice
Long-term potentiation - Rat
DL-2-Amino-5-phosphonovalerate 4-
Aminopyridine The effect of 4-aminopyridine (4-AP) on long-term potentiation (LTP) was studied in the hippocampal slice preparation of the rat. Field excitatory postsynaptic potentials (EPSPs) were recorded and evoked in the stratum radiatum of the CA~. Both the low frequency EPSP and LTP of the EPSP were significantly increased by treatment with 4-AP. These effects were inhibited by increasing the magnesium concentration from 1 to 4 mM. Pretreatment with 20 a M OL-2-amino-5-phosphonovalerate antagonized only the increase in LTP produced by 4-AP. It is suggested that 4-AP enhances Ca influx either pre- or postsynaptically and thereby increases LTP.
Tetanic stimulation of different pathways in the hippocampus produces a longterm potentiation (LTP) of synaptic transmission [2, 15]. This LTP has been proposed to act as a substrate for memory [13] and also for kindling-produced epileptogenesis [16]. In the present studies we have investigated the action of 4-aminopyridine (4-AP) on LTP. Through its well-known block of K conductance [5, 17], 4-AP increases Ca influx into neurons [17]. Ca has a very important role in LTP, with a Ca influx above that necessary for synaptic transmission being required for LTP [6]. Moreover, intracellular injection of a Ca chelator into pyramidal cells in CAj blocks LTP [12], and simply raising extracellular Ca for a few minutes can induce LTP [18]. Hippocampal slices were prepared from adult Wistar rats as previously described [I] with the slices superfused at 33"C with a medium containing (raM): NaCI 120, KCI 2.5, CaC12 2.0, MgSO4 1.0, NaHCO3 26, NaH2PO4 1.3, glucose 10. Extracellular recordings were obtained from the stratum radiatium of CAl. Stimulating and recording electrodes were glass microelectrodes filled with 200 mM NaCI and a resistance of l Mff2. In control media, stimulation of the Schaffer collateral-commissural pathway evoked constant amplitude field excitatory postsynaptic potentials (EPSPs) in the radiatum layer. Brief high frequency stimulation caused LTP (observed as a stable Correspondence." R. Anwyl, Department of Physiology, Trinity College, Dublin 2, Ireland.
107
"~
200 4-AP
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CONTROL TETANUS
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TETANUS IN 4-AP " ~
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0 -10
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10 TIME(min)
20
30
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,
post
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=! , " (~)
40
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~0.2mV 5msec
Fig. 1. Induction of LTP by high frequency stimulation. A: EPSP amplitude plotted as a function of time, before and after tetanic stimulation (arrow), in control media, in 10 #m 4-AP, and in 10 #m 4-AP plus 20 #M APV. B: individual EPSPs (i) before and (ii) 20 min after the tetanus in control media, and (iii) before, and (iv) after the tetanus in 10 #M 4-AP.
increase in the EPSP lasting for over I h) providing the EPSP amplitude was greater than 800 900 #V (this was also the threshold for spike generation). LTP was studied by giving test pulses which evoked 300 #V amplitude EPSPs at 0.02 Hz, and then giving a tetanus (20 trains of 8 stimuli at 200 Hz, 2 s interval between each train) w i t h a n E P S P a m p l i t u d e o f 900 ,uV. T h i s s t i m u l a t i o n e v o k e d m a x i m u m
LTP. LTP
was measured 20 rain after the tetanus, and was expressed as the percentage increase in amplitude of the EPSP evoked by the test pulse. The EPSP amplitude, rather than slope, was measured, since the amplitude was linearly related to slope up to the spiking threshold of 900 #V. In control media, LTP was 42 + 7%, n = 11, mean + S.E.M. (Fig. 1). The input-output curve was shifted to the left by the tetanus (Fig. 2). Perfusion of 4-AP caused a sustained increase in the amplitude of the control low frequency EPSP. The increase commenced within 2 5 min, and continued until a maximum was reached after 30 40 rain. 10,uM 4-AP caused an increase of 158 + 14%, n = 9, of the 300-#V EPSP, These observations support those previously made [3]. This action o r 4 - A P could be reversed by 45 min of washout in control media, which is probably the time taken to remove 4-AP from its intracellular site of action [17]. ] ) l . - 2 - a m i n o - 5 - p h o s p l l o n o v a l e r a t e ( A P V ) a t 20 # M h a d n o effect o n t h e a m p l i t u d e 1.0
LTP
IN 4 - A P
CONTROL Epsp
(mY)
0.6
O'4
0"2
i
i
i
t
J
0
20
40
60
80
100
PRE-SYNAPTIC
VOLLEY(%)
Fig. 2. Input output curve of the EPSP amplitude plotted as a function of the presynaptic nerve volley in control media and after LTP in the control media, and also in 10 #M 4-AP after reducing stimulation current, and after LTP in 10 #m 4-AP.
108 of the low frequency control EPSP in control solution. Moreover, it did not inhibit the increase in the amplitude of the low lYequency EPSP evoked by 10 pM 4-AP. this increase being 152+ 6%, n = 4. This effect of 4-AP on the low tYequency EPSP is probably similar to that occurring at peripheral synapses, at which 4-AP enhances Ca influx presynaptically [ll, 14, 17]. In the presence of 4-AP, LTP was investigated under conditions in which the amplitude of the low frequency EPSP was reduced to the control level by lowering the stimulation current (Fig. 1). In 10/~M 4-AP, LTP was significantly increased by 156+ 17%, n = 6 (P<0.001) (Figs. 1 and 2). LTP was also increased in two slices in 10 ~M 4-AP (mean increase, 110%). 4-AP did not affect the threshold amplitude at which LTP was evoked. LTP in 4-AP could be antagonized by high magnesium (Mg) and APV. Thus in 10 ~M 4-AP, increasing Mg from 1 to 4 mM reduced the low frequency EPSP to an amplitude close to control over the whole range of the input-output curve, probably by blocking transmitter release. In 10 #M 4-AP and 4 mM Mg, LTP was reduced significantly to 26±4%, n = 4 (P<0.001). In 20/tM APV, the LTP evoked in 10/~M 4-AP was nearly abolished, measuring only 8 _ I%, n = 5 (Fig. 1). The LTP increase by 4-AP is probably mediated by 4-AP causing a K channel block which results in a prolonged action potential and thereby an enhanced Ca entry. This could be presynaptic, and enhancing transmitter release [17] or postsynaptic, leading to cellular alterations [14]. The LTP produced in 4-AP appears to be mediated by similar if not identical mechanisms to the LTP in control media. Thus the threshold amplitude of the EPSP for LTP is the same in 4-AP as in control media, and high Mg and APV block LTP in 4-AP and in control media [4, 9]. Mg is a well-known blocker of Ca channels, and the antagonism of the LTP by Mg in 4-AP is probably due to the reduction in Ca influx. Dendritic Ca spikes are known to be present in hippocampal pyramidal cells [21] and dendritic Ca spikes in thalamic neurons are enhanced by 4-AP [10]. The reduction of LTP in 4-AP by APV is evidence for the involvement of N-methyl-Daspartate (NMDA) receptors in the LTP. In normal media, it has been suggested that during low frequency transmission, the transmitter binds to the N M D A receptors, but a voltage-dependent block of the receptor channel by Mg occurs. Tetanic stimulation relieves the Mg block and full expression of the N M D A receptor activation occurs [9]. An identical mechanism could be operating during the increased LTP in 4-AP. The present results clearly demonstrate that 4-AP caused a large increase in maximum LTP, i.e. 4-AP enhanced the capacity for LTP. Thus there is a very large reserve capacity for LTP which is not recruited in control media by a single tetanus. However, during kindling stimulation patterns, greatly increased LTP, at least of the population spike, does occur [16]. The only other agents reported to increase hippocampal LTP are picrotoxin and bicuculline [20]. However, these agents only facilitated the induction of LTP, and did not enhance maximal LTP [20]. A previous study on the action of 4-AP failed to find a significant increase of LTP caused by 4-AP. This result may have been due to the higher concentrations of 4-AP or to much larger EPSP being used in that study [8].
109
Diaminopyridines are known to improve a spatial working memory in aged rats [7] and to have a beneficial action on memory in patients with Alzheimer's disease [19]. An increase in LTP mediated by the 4-AP could be responsible for this improvement in memory, since the hippocampus and LTP are known to play a very important role in working memory [13]. Supported by MRC Ireland. 1 Anwyl, R. and Rowan, M.J., Frequency dependent block of field potentials in the rat hippocampal slice caused by tricylic antidepressants, Br. J. Pharmacol., 86 (1985) 201 208. 2 Bliss. T.V.P. and Lomo, T., Long-lasting potentiation of synaptic transmission of the dentate area of the anaesthetised rabbit following stimulation of the perforant pathway, J. Physiol. (London), 232 (1973) 331 356. 3 Buckle, P.J. and Haas, H.L., Enhancement of synaptic transmission by 4-aminopyridine in hippocampal slices of the rat, J. Physiol. (London), 326 (1981) 109 122. 4 Collingridge, G.L., Kehl, S.J. and McLennan, It., Excitatory amino acids in synaptic transmission in the Schaffer collateral-commissural pathway of the rat hippocampus, J. Physiol. (London), 334 (1983) 33 46. 5 Dubois, J.M., Potassium currents in the frog node of Ranvier, Prog. Biophys. Mol. Biol., 42 (19831 I 20. 6 Dunwiddie, T.V. and Lynch, G., The relationship between extracellular calcium concentration and the induction ofhippocampal long-term potentiation, Brain Res., 169 (1979) 103 110. 7 Eppich, C., Barnes, C.A. and Baldwin, J., 3,4-Diaminopyridine improves spatial working memory but not spatial reference memory in aged rats, Soc. Neurosci. Abstr., 11 (1985) 724. 8 Haas, H.L. and Greene, R.W., Long-term potentiation and 4-aminopyridine, Cell Mol. Neurobiol., 5([985)297 301. 9 tterron. C.E., Lester, R.A.J., Coan, E.J. and Collingridge, G.L., Intracellular demonstration of an Nmethyl-I)-aspartate receptor-mediated component of synaptic transmission in the rat hippocampus, Neurosci. Letl., 60 (1985) 19 23. 10 Jahnsen, t|. and Llinas, R., Electrophysiological properties of guinea-pig thalamus neurones: an in vitro study, J. Physiol. (London) 349 (1984) 205 226. 11 Lundh, H., Effects of 4-aminopyridine on statistical parameters of transmitter release at the neuromuscular junction, Acta Pharmacol. Toxicol., 44 (1979) 343 346. 12 Lynch, G., Larson, J., Kelso, S., Barrionuevo, E. and Schottler, F., lntracellular injection of EGTA blocks induction of hippocampal long-term potentiation, Nature (London), 305 (19831 719 721. 13 Morris, R.G.M., Anderson, E., Lynch, G.S. and Baudry, M., Selective impairment of learning and blockade of long-term potentiation by an N-methyl-D-aspartate receptor antagonist, AP-5, Nature (London), 319 (1986) 774 776. 14 Rogawski, M.A. and Barker, J.L., Effects of 4-aminopyridine on calcium action potentials and calcium current under voltage clamp in spinal neurons, Brain Res., 280 (1983) 180 185. 15 Schwartzkroin, P.A. and Wester, K., Long-lasting facilitation of a synaptic potential following tetanization in the "in vitro" hippocampal slice, Brain Res., 89 (1975) 107 119. 16 Slater, N.T., Stelzer, A. and Galvan, M., Kindling-like stimulus patterns induce epileptiform discharges in the guinea pig in vitro hippocampus, Neurosci. Lett., 6(1 (1985) 25 31. 17 Thesleff, S., Aminopyridines and synaptic transmission, Neuroscience, 5 (1980) 1413 1419. 18 Turner. R.W., Bainbridge, K.G. and Miller, J.J., Calcium induced long-term potentiation in the hippocampus, Neuroscience, 7 (1982) 1411 1416. 19 Wesseling, H., Agoston, S., Van Dam, G.B.P., Pasma, J., De Wit, J.D. and Havinga, H., Effects of 4-aminopyridine in elderly patients with Alzheimer's disease, N. Engl. J. Med., 310 (19851 297 3(14. 20 Wigstrom, H. and Gustafsson, B., Facilitation of hippocampal long-lasting potentiation by GABA antagonists, Acta Physiol. Scand., 125 (19851 154-172. 21 Wong, R.K.S., Prince, D.A. and Basbaum, A.I., Intradendritic recordings from hippocampal neurons, Proc. Natl. Acad. Set. USA, 76 (19791 986 990.
106
lilse~ icl
NSL 04143
4-Aminopyridine-mediated increase in long-term potentiation in CA1 of the rat hippocampus W a i - L i n g Lee l, R o g e r A n w y l I a n d Mich'.el R o w a n 2 JDepartment of Physiology, and 2Department of Pharmacology and Therapeutics, Trinity College, Dubtin 2 (Ireland) (Received 24 April 1986; Revised version received 4 June 1986; Accepted 6 June 1986)
Key words." Hippocampal slice
Long-term potentiation - Rat
DL-2-Amino-5-phosphonovalerate 4-
Aminopyridine The effect of 4-aminopyridine (4-AP) on long-term potentiation (LTP) was studied in the hippocampal slice preparation of the rat. Field excitatory postsynaptic potentials (EPSPs) were recorded and evoked in the stratum radiatum of the CA~. Both the low frequency EPSP and LTP of the EPSP were significantly increased by treatment with 4-AP. These effects were inhibited by increasing the magnesium concentration from 1 to 4 mM. Pretreatment with 20 a M OL-2-amino-5-phosphonovalerate antagonized only the increase in LTP produced by 4-AP. It is suggested that 4-AP enhances Ca influx either pre- or postsynaptically and thereby increases LTP.
Tetanic stimulation of different pathways in the hippocampus produces a longterm potentiation (LTP) of synaptic transmission [2, 15]. This LTP has been proposed to act as a substrate for memory [13] and also for kindling-produced epileptogenesis [16]. In the present studies we have investigated the action of 4-aminopyridine (4-AP) on LTP. Through its well-known block of K conductance [5, 17], 4-AP increases Ca influx into neurons [17]. Ca has a very important role in LTP, with a Ca influx above that necessary for synaptic transmission being required for LTP [6]. Moreover, intracellular injection of a Ca chelator into pyramidal cells in CAj blocks LTP [12], and simply raising extracellular Ca for a few minutes can induce LTP [18]. Hippocampal slices were prepared from adult Wistar rats as previously described [I] with the slices superfused at 33"C with a medium containing (raM): NaCI 120, KCI 2.5, CaC12 2.0, MgSO4 1.0, NaHCO3 26, NaH2PO4 1.3, glucose 10. Extracellular recordings were obtained from the stratum radiatium of CAl. Stimulating and recording electrodes were glass microelectrodes filled with 200 mM NaCI and a resistance of l Mff2. In control media, stimulation of the Schaffer collateral-commissural pathway evoked constant amplitude field excitatory postsynaptic potentials (EPSPs) in the radiatum layer. Brief high frequency stimulation caused LTP (observed as a stable Correspondence." R. Anwyl, Department of Physiology, Trinity College, Dublin 2, Ireland.
107
"~
200 4-AP
u ~'e , .
lOC
. .s""
CONTROL TETANUS
.e"
o. E <
k
=
TETANUS IN 4-AP " ~
pre
pre
*
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FETANUS
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~
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0 -10
0
10 TIME(min)
20
30
!
,
post
i~
," f
=! , " (~)
40
~
~0.2mV 5msec
Fig. 1. Induction of LTP by high frequency stimulation. A: EPSP amplitude plotted as a function of time, before and after tetanic stimulation (arrow), in control media, in 10 #m 4-AP, and in 10 #m 4-AP plus 20 #M APV. B: individual EPSPs (i) before and (ii) 20 min after the tetanus in control media, and (iii) before, and (iv) after the tetanus in 10 #M 4-AP.
increase in the EPSP lasting for over I h) providing the EPSP amplitude was greater than 800 900 #V (this was also the threshold for spike generation). LTP was studied by giving test pulses which evoked 300 #V amplitude EPSPs at 0.02 Hz, and then giving a tetanus (20 trains of 8 stimuli at 200 Hz, 2 s interval between each train) w i t h a n E P S P a m p l i t u d e o f 900 ,uV. T h i s s t i m u l a t i o n e v o k e d m a x i m u m
LTP. LTP
was measured 20 rain after the tetanus, and was expressed as the percentage increase in amplitude of the EPSP evoked by the test pulse. The EPSP amplitude, rather than slope, was measured, since the amplitude was linearly related to slope up to the spiking threshold of 900 #V. In control media, LTP was 42 + 7%, n = 11, mean + S.E.M. (Fig. 1). The input-output curve was shifted to the left by the tetanus (Fig. 2). Perfusion of 4-AP caused a sustained increase in the amplitude of the control low frequency EPSP. The increase commenced within 2 5 min, and continued until a maximum was reached after 30 40 rain. 10,uM 4-AP caused an increase of 158 + 14%, n = 9, of the 300-#V EPSP, These observations support those previously made [3]. This action o r 4 - A P could be reversed by 45 min of washout in control media, which is probably the time taken to remove 4-AP from its intracellular site of action [17]. ] ) l . - 2 - a m i n o - 5 - p h o s p l l o n o v a l e r a t e ( A P V ) a t 20 # M h a d n o effect o n t h e a m p l i t u d e 1.0
LTP
IN 4 - A P
CONTROL Epsp
(mY)
0.6
O'4
0"2
i
i
i
t
J
0
20
40
60
80
100
PRE-SYNAPTIC
VOLLEY(%)
Fig. 2. Input output curve of the EPSP amplitude plotted as a function of the presynaptic nerve volley in control media and after LTP in the control media, and also in 10 #M 4-AP after reducing stimulation current, and after LTP in 10 #m 4-AP.
108 of the low frequency control EPSP in control solution. Moreover, it did not inhibit the increase in the amplitude of the low lYequency EPSP evoked by 10 pM 4-AP. this increase being 152+ 6%, n = 4. This effect of 4-AP on the low tYequency EPSP is probably similar to that occurring at peripheral synapses, at which 4-AP enhances Ca influx presynaptically [ll, 14, 17]. In the presence of 4-AP, LTP was investigated under conditions in which the amplitude of the low frequency EPSP was reduced to the control level by lowering the stimulation current (Fig. 1). In 10/~M 4-AP, LTP was significantly increased by 156+ 17%, n = 6 (P<0.001) (Figs. 1 and 2). LTP was also increased in two slices in 10 ~M 4-AP (mean increase, 110%). 4-AP did not affect the threshold amplitude at which LTP was evoked. LTP in 4-AP could be antagonized by high magnesium (Mg) and APV. Thus in 10 ~M 4-AP, increasing Mg from 1 to 4 mM reduced the low frequency EPSP to an amplitude close to control over the whole range of the input-output curve, probably by blocking transmitter release. In 10 #M 4-AP and 4 mM Mg, LTP was reduced significantly to 26±4%, n = 4 (P<0.001). In 20/tM APV, the LTP evoked in 10/~M 4-AP was nearly abolished, measuring only 8 _ I%, n = 5 (Fig. 1). The LTP increase by 4-AP is probably mediated by 4-AP causing a K channel block which results in a prolonged action potential and thereby an enhanced Ca entry. This could be presynaptic, and enhancing transmitter release [17] or postsynaptic, leading to cellular alterations [14]. The LTP produced in 4-AP appears to be mediated by similar if not identical mechanisms to the LTP in control media. Thus the threshold amplitude of the EPSP for LTP is the same in 4-AP as in control media, and high Mg and APV block LTP in 4-AP and in control media [4, 9]. Mg is a well-known blocker of Ca channels, and the antagonism of the LTP by Mg in 4-AP is probably due to the reduction in Ca influx. Dendritic Ca spikes are known to be present in hippocampal pyramidal cells [21] and dendritic Ca spikes in thalamic neurons are enhanced by 4-AP [10]. The reduction of LTP in 4-AP by APV is evidence for the involvement of N-methyl-Daspartate (NMDA) receptors in the LTP. In normal media, it has been suggested that during low frequency transmission, the transmitter binds to the N M D A receptors, but a voltage-dependent block of the receptor channel by Mg occurs. Tetanic stimulation relieves the Mg block and full expression of the N M D A receptor activation occurs [9]. An identical mechanism could be operating during the increased LTP in 4-AP. The present results clearly demonstrate that 4-AP caused a large increase in maximum LTP, i.e. 4-AP enhanced the capacity for LTP. Thus there is a very large reserve capacity for LTP which is not recruited in control media by a single tetanus. However, during kindling stimulation patterns, greatly increased LTP, at least of the population spike, does occur [16]. The only other agents reported to increase hippocampal LTP are picrotoxin and bicuculline [20]. However, these agents only facilitated the induction of LTP, and did not enhance maximal LTP [20]. A previous study on the action of 4-AP failed to find a significant increase of LTP caused by 4-AP. This result may have been due to the higher concentrations of 4-AP or to much larger EPSP being used in that study [8].
109
Diaminopyridines are known to improve a spatial working memory in aged rats [7] and to have a beneficial action on memory in patients with Alzheimer's disease [19]. An increase in LTP mediated by the 4-AP could be responsible for this improvement in memory, since the hippocampus and LTP are known to play a very important role in working memory [13]. Supported by MRC Ireland. 1 Anwyl, R. and Rowan, M.J., Frequency dependent block of field potentials in the rat hippocampal slice caused by tricylic antidepressants, Br. J. Pharmacol., 86 (1985) 201 208. 2 Bliss. T.V.P. and Lomo, T., Long-lasting potentiation of synaptic transmission of the dentate area of the anaesthetised rabbit following stimulation of the perforant pathway, J. Physiol. (London), 232 (1973) 331 356. 3 Buckle, P.J. and Haas, H.L., Enhancement of synaptic transmission by 4-aminopyridine in hippocampal slices of the rat, J. Physiol. (London), 326 (1981) 109 122. 4 Collingridge, G.L., Kehl, S.J. and McLennan, It., Excitatory amino acids in synaptic transmission in the Schaffer collateral-commissural pathway of the rat hippocampus, J. Physiol. (London), 334 (1983) 33 46. 5 Dubois, J.M., Potassium currents in the frog node of Ranvier, Prog. Biophys. Mol. Biol., 42 (19831 I 20. 6 Dunwiddie, T.V. and Lynch, G., The relationship between extracellular calcium concentration and the induction ofhippocampal long-term potentiation, Brain Res., 169 (1979) 103 110. 7 Eppich, C., Barnes, C.A. and Baldwin, J., 3,4-Diaminopyridine improves spatial working memory but not spatial reference memory in aged rats, Soc. Neurosci. Abstr., 11 (1985) 724. 8 Haas, H.L. and Greene, R.W., Long-term potentiation and 4-aminopyridine, Cell Mol. Neurobiol., 5([985)297 301. 9 tterron. C.E., Lester, R.A.J., Coan, E.J. and Collingridge, G.L., Intracellular demonstration of an Nmethyl-I)-aspartate receptor-mediated component of synaptic transmission in the rat hippocampus, Neurosci. Letl., 60 (1985) 19 23. 10 Jahnsen, t|. and Llinas, R., Electrophysiological properties of guinea-pig thalamus neurones: an in vitro study, J. Physiol. (London) 349 (1984) 205 226. 11 Lundh, H., Effects of 4-aminopyridine on statistical parameters of transmitter release at the neuromuscular junction, Acta Pharmacol. Toxicol., 44 (1979) 343 346. 12 Lynch, G., Larson, J., Kelso, S., Barrionuevo, E. and Schottler, F., lntracellular injection of EGTA blocks induction of hippocampal long-term potentiation, Nature (London), 305 (19831 719 721. 13 Morris, R.G.M., Anderson, E., Lynch, G.S. and Baudry, M., Selective impairment of learning and blockade of long-term potentiation by an N-methyl-D-aspartate receptor antagonist, AP-5, Nature (London), 319 (1986) 774 776. 14 Rogawski, M.A. and Barker, J.L., Effects of 4-aminopyridine on calcium action potentials and calcium current under voltage clamp in spinal neurons, Brain Res., 280 (1983) 180 185. 15 Schwartzkroin, P.A. and Wester, K., Long-lasting facilitation of a synaptic potential following tetanization in the "in vitro" hippocampal slice, Brain Res., 89 (1975) 107 119. 16 Slater, N.T., Stelzer, A. and Galvan, M., Kindling-like stimulus patterns induce epileptiform discharges in the guinea pig in vitro hippocampus, Neurosci. Lett., 6(1 (1985) 25 31. 17 Thesleff, S., Aminopyridines and synaptic transmission, Neuroscience, 5 (1980) 1413 1419. 18 Turner. R.W., Bainbridge, K.G. and Miller, J.J., Calcium induced long-term potentiation in the hippocampus, Neuroscience, 7 (1982) 1411 1416. 19 Wesseling, H., Agoston, S., Van Dam, G.B.P., Pasma, J., De Wit, J.D. and Havinga, H., Effects of 4-aminopyridine in elderly patients with Alzheimer's disease, N. Engl. J. Med., 310 (19851 297 3(14. 20 Wigstrom, H. and Gustafsson, B., Facilitation of hippocampal long-lasting potentiation by GABA antagonists, Acta Physiol. Scand., 125 (19851 154-172. 21 Wong, R.K.S., Prince, D.A. and Basbaum, A.I., Intradendritic recordings from hippocampal neurons, Proc. Natl. Acad. Set. USA, 76 (19791 986 990.