JOURNALOF Vol. 67, No.

NEUROPHYSIOLOGY 2, February 1992.

Printed

in U.S.A.

Low-Threshold Ca2+ Channels Mediate Induction of Long-Term Potentiation in Kitten Visual Cortex YUKIO KOMATSU AND MICHIYO IWAKIRI Department of Physiology, Kyoto Prefectural University of Medicine, Kawaramachi-Hirokoji, Kamigyoku, Kyoto 602, Japan SUMMARY

AND

CONCLUSIONS

I. The induction mechanism of long-term potentiation (LTP) in developing visual cortex was studied by recording intracellular responses from layer III-IV cells in slice preparations of kitten visual cortex at 30-40 days after birth. 2. Strong stimulation of white matter produced a late depolarizing response after an orthodromic action potential. This depolarizing response was abolished by membrane depolarization or hyperpolarization caused by current injection through the recording electrode. In addition, this response was reduced by bath application of a low concentration ( 100 PM) of Ni2+ without any changes in the rising slope of the excitatory postsynaptic potential (EPSP) or orthodromic action potential. This suggests that this response is mediated by low-threshold Ca2+ channels ( LTCs). 3. The involvement of LTCs in the induction of LTP was tested. White matter was stimulated at 2 Hz for 15 min as a conditioning stimulus to induce LTP, and the resultant changes in EPSPs were tested by low-frequency (0.1 Hz) stimulation of white matter. Conditioning stimulation produced a large N-methyl-Daspartate (NMDA) receptor-mediated depolarizing response in these cells, which obscured the presence of the late depolarization. Therefore the test was conducted in a solution containing an NMDA antagonist 2-amino-5-phosphonovalerate ( APV) . 4. Weak conditioning stimulation, which evoked no LTC responses, never induced LTP; whereas strong conditioning stimulation, which evoked LTC responses, always induced LTP. Strong conditioning stimulation failed to induce LTP when LTC responses were prevented either by membrane depolarization or hyperpolarization or by a bath application of 100 PM Ni2+ 5. In a solution without APV, the application of Ni2+ also prevented the induction of LTP. 6. When cells were impaled by an electrode containing a Ca2+ chelator 1,2-bis- ( o-aminophenoxy )ethane - N, N, N’, IV’-tetraacetic acid (BAPTA), LTP was never induced, even though LTC responses were evoked by conditioning stimulation. These results indicate that Ca2+ influx into postsynaptic cells through LTCs induces the LTP. 7. The responses mediated by LTCs, which were evoked by the injection of current pulses into the cells, were maximum at the critical period of visual cortical plasticity, suggesting that LTCs in postsynaptic cells regulate the plastic changes in developing visual cortex. INTRODUCTION

Neuronal responses in visual cortex are modified by visual experience during the early postnatal period (Blakemore and Cooper 1970; Hirsch and Spinelli 1970; Wiesel and Hubel 1963). Long-term potentiation (LTP) may be one of the synaptic mechanisms underlying this experiencedependent modification, because LTP in cat visual cortex is 0022-3077/92

$2.00 Copyright

most easily induced during the critical period when visual responses are modifiable (Komatsu et al. 198 1, 1988). It has been supposed that activation of the N-methyl+aspartate (NMDA) receptor is essential to the induction of LTP in developing visual cortex, as in hippocampus (Collingridge et al. 1983 ) , because the injection of an NMDA antagonist into visual cortex prevents the effect of monocular deprivation on ocular-dominance preference of visual cortical cells (Kleinschmidt et al. 1987) and because the visual responses are reduced by the NMDA antagonist more in young cats than in mature cats (Fox et al. 1989; Tsumoto et al. 1987). This supposition has been supported by tests in rat visual cortex ( Artola and Singer 1987, 1990; Kimura et al. 1989). However, our previous study using slice preparations of kitten visual cortex indicated that the activation of NMDA receptors was not required for LTP induction (Komatsu et al. 199 1). In this study, we examined the involvement of voltage-gated Ca2+ channels in LTP induction because the intracellular Ca2+ increase often triggers plastic changes of synaptic transmission (Lynch et al. 1983; Malenka et al. 1988; Sakurai 1990) and because Ca2’ channels are known to be rich in kitten visual cortical cells (Bode-Greuel and Singer 1988). The results demonstrate that LTP induction is mediated by low-threshold Ca2+ channels (LTCs) in postsynaptic cells and that the responses mediated by LTCs are maximum in the critical period. METHODS

The slice preparations and the experimental arrangement for stimulation and recording are similar to those previously described (Komatsu et al. 199 1). Cats (~1 = 3, 5-7 days; n = 44, 30-40 days; n = 2,65-70 days; n = 2, 126-l 58 days; and n = 3, adult) were deeply anesthetized by intraperitoneal injection of a mixture of urethan ( 1 g/kg) and cu-chloralose ( 100 mg/kg), and coronal slices (thickness, 500 pm) were dissected from visual cortex (area 17). Slices were perfused with oxygenated (95% O,-5% C02) Krebs-Ringer solution [containing (in mM) 124 NaCl, 5 KCl, 1.3 MgSO,, 2.4 CaCl,, 1.24 KH2P04, 26 NaHCO,, and 10 glucose] at 33°C and CaCl, was replaced with MgCl, in a Ca2+free solution. A pair of bipolar stimulating electrodes was placed in white matter near the border between layer VI and white matter to investigate postsynaptic potentials. The stimulating electrode delivered constant-current pulses with an intensity of 0.0 l-2 mA at a low frequency (0.1 Hz) when postsynaptic potentials were analyzed. A higher-frequency (2 Hz) stimulation was applied for 15 min to induce LTP. To determine whether the excitatory postsynaptic potentials (EPSPs) are monosynaptic or polysynaptic, we

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placed another pair of bipolar stimulating electrodes in white matter, separated ~0.6 mm from the former electrode. Glass microelectrodes filled with 2 M K-methylsulfate (electrical resistance 1OO- 160 Mfi) were used for the intracellular recording. Cells with resting membrane potentials deeper than -55 mV were selected for the analysis. The identification of monosynaptic EPSPs was based on the central delay, which is the time spent in synaptic transmission after afferent impulses arrived at axon terminals. The conduction time of afferent impulses was evaluated from the conduction velocity of afferent impulses, which was obtained by dividing the distance between the two stimulating electrodes by the difference in latencies of EPSPs evoked by the two electrodes. The central delay was determined by subtracting the conduction time of afferent impulses from the EPSP latency. EPSPs with a central delay < 1 ms were considered to be monosynaptic (Komatsu et al. 199 1). The drugs used were bicuculline methiodide (Pierce), 2amin&-phosphonovalerate (APV; Sigma), tetrodotoxin (TTX; Sigma), nifedipine ( Sigma), tetraethylammonium (TEA; Sigma), 4-aminopyridine (Sigma), 6,7dinitroquinoxahne-2,3dione (DNQX; Tocris Neuramin) , and 1,2-bis- ( o-aminophenoxy)ethaneIVJVJV’,N’-tetraacetic acid (BAPTA; Dojin). The data were presented as means 2 SD and tested by Student’s t test or paired t test. RESULTS

Intracellular analysis was conducted in visual cortical cells sampled from kittens at 30-40 days after birth, when LTP could be most easily induced (Komatsu et al. 1988). Cells were sampled from layer IV and the lower part of layer

A 0.09

M. IWAKIRI

III, where it is kn own that most cells receive monosy naptic excitatory inputs from cells in the lateral geniculate nucleus (Toyama et al. 1974). Cells activated monosynaptically by white-matter stimulation were selected for the analysis according to the criteria described previously (Komatsu et al. 199 1). Furthermore, considering the possibility that the synaptic potentials contained some polysynaptic components, especially in their later part, we used the slope rather than the amplitude of the EPSP as an index of synaptic strength to minimize the contribution of any polysynaptic components.

Late depolarizing responses Figure 1A illustrates the typical postsynaptic potentials evoked by stimulation of white matter in a cell perfused with a solution containing a y-aminobutyric acid, (GABA,) antagonist, bicuculline methiodide ( 1 PM). In this solution, LTP of EPSPs was frequently induced (Komatsu et al. 199 1) . Weak stimulation produced an EPSP. Stimulation with an intermediate intensity produced a larger EPSP followed by an inhibitory postsynaptic potential and an orthodromic action potential on the rising phase of the EPSP. Strong stimulation added a late depolarizing potential as a hump (arrow in Fig. 1A) after the action potential. The late depolarization, which was commonly observed in the cells sampled at 30-40 days after birth, seems to be generated by LTCs, as described below.

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20 ms FIG. 1. Postsynapticresponses mediated by LTCs. A : superimposed traces (~1 = 2) of intracellular responses evoked in a cortical cell by white-matter stimulation with intensities of 0.09,O. 14,0.3, and 0.5 mA. Arrow shows the late depolarizing potential. B: voltage dependence of the late depolarization. Membrane potential was changed by current injection of +OS (top), 0 (middle), and -2 nA (bottom). Dotted line in the top trace represents the resting membrane potential ( -63 mV). C: responses evoked by white-matter stimulation before (top) and 15 min after a bath application of 100 PM Ni2+ (middle); bottom, superimposed traces. In A-C, the perfusate contained 1 PM bicuculline methiodide. In B and C, 100 PM APV were added to the perfusate to avoid contribution of NMDA receptors to the voltage dependence and the effect of Ni2+.

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The late depolarization showed a strong dependence on the membrane potential. The EPSPs of kitten visual cortical cells contain an NMDA receptor-mediated component (Komatsu et al. 199 1) that is known to be dependent on membrane potential (Ault et al. 1980; Mayer et al. 1984; Nowak et al. 1984). To avoid the possible contribution of NMDA receptors to the voltage dependence, we added an NMDA antagonist, APV ( 100 PM), to the perfusate. When membrane potential was depolarized to the range between -40 and -50 mV or hyperpolarized to the range between -90 and - 125 mV by current injection through the recording electrode, the late depolarization disappeared in all of the 10 tested cells (Fig. 1B). This suggests that the late depolarization is generated by the activation of LTCs, because it has been demonstrated in various kinds of neurons that LTCs are activated above the hyperpolarized range and are almost completely inactivated in the depolarized voltage range (Carbone and Lux 1984; Fedulova et al. 1985; Llinas and Yaron 1981; Nowycky et al. 1985). To test the above possibility, we examined the effect of a low concentration ( 100 PM) of Ni2+, which is known to substantially block LTCs but only slightly block highthreshold Ca2+ channels (HTCs) (Fox et al. 1987; Narahashi et al. 1987). The late depolarization was significantly reduced by Ni2+ (7 t 2 mV, mean t SD; n = 5; P < 0.0 1) without any changes in the rising slope of EPSP ( 100 t 7% of control, n = 5; P > 0.8 ) and orthodromic action potentials (Fig. 1C) . In contrast, the late depolarization was not affected by a dihydropyridine-sensitive Ca2+ channel antagonist, nifedipine ( 10 PM), which blocks sustained-type HTCs (Fox et al. 1987 ) . Therefore the late depolarization is likely to be mediated by LTCs.

Ca2+ channel-mediated responsesevoked by current pulses To verify that LTCs are present in these cells, we examined responses evoked by depolarizing current pulses in the presence of a Na+-channel blocker, TTX ( 1 PM), and a K+-channel blocker, TEA ( 10 mM), in the perfusate. Depolarizing pulses produced small (+ in Fig. 2A ) and large depolarizing responses (+) with low and high thresholds, respectively. These responses are thought to be mediated by Ca2+ channels because they disappeared in a Ca2+-free solution, as shown in Fig. 2 B ( n = 3). Ni2+ ( 100 PM) reduced the amplitude of the low-threshold component to one-third of the control (35 t 9%, n = 6; P < 0.0 1), but it only very slightly reduced maximum rate of rise in the high-threshold component (94 t 3%, n = 6; P < 0.05; Fig. 2, D-F). In contrast, 10 PM nifedipine did not affect the lowthreshold component ( 10 1 t 6%, n = 4; P > 0.4), but it did reduce the high-threshold component ( 82 t lo%, n = 4; P < 0.05 ). Further decrease in the high-threshold component was not observed even with the application of 100 PM nifedipine (84 t 6%, n = 3; P < 0.05)) suggesting that the remaining high-threshold component is mediated by transient-type HTCs (Fox et al. 1987). These observations indicate that both LTCs and HTCs are present in these cells and that 100 PM Ni 2+selectively blocks the responses mediated by LTCs in these cells. Therefore we conclude that the late depolarization is mediated by LTCs.

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Ca free cc FIG. 2. Responses evoked by depolarizing current pulses. A : responses evoked in a cell by depolarizing current pulses in the control solution. Top and bottom traces represent voltage responses and injection currents, respectively. B: as in A, but 30 min after perfusate was changed to a Ca*+free solution. C: superimposed average (n = 4) responses to 0.6-nA pulses in control and Ca*+ -free solutions. Time and voltage calibrations are common to A-C and current calibration is common to A and B. D-F: as in A-C, but the effect of Ni*+ on the responses mediated by Ca*+ channels. Amplitude of injected current in F was 0.6 nA. Arrow in F indicates the amplitude of the LTC response. Time and voltage calibrations are common to D-F and current calibration is common to D and E. Solutions contained 1 PM TTX and 10 mM TEA in A-F.

Involvement of LTCs in LTP induction The involvement of the LTC in the induction of LTP was tested. The perfusate contained 1 PM bicuculline methiodide, which was used to facilitate the induction of LTP (Komatsu et al. 199 1). The intensity of test stimulation (0.1 Hz) was adjusted to evoke only EPSPs, and conditioning stimulation (2 Hz) strong enough to evoke orthodromic action potentials was applied to white matter for 15 min. The conditioning stimulation evoked large NMDA receptor-mediated responses after orthodromic action potentials (Komatsu et al. 199 1) . Because these large responses obscured the presence of LTC responses (cf. Fig. 4C), 100 PM APV was added to the perfusate, which also excluded the possible contribution of the NMDA receptors to the LTP. LTP was induced in all cells by the strong conditioning stimulation, which evoked LTC responses, but not by weak conditioning stimulation, which did not evoke any LTC responses, as shown in Fig. 3, B and A, respectively. The

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FIG. 3. Dependence of LTP induction on intensity of stimulation and membrane potential. A: effect of weak conditioning stimulation. Traces show superimposed average EPSPs (n = 5 ) evoked by test stimulation before and 30 min after conditioning stimulation (left) and responses to conditioning stimulation ( right). Graph plots the rising slope of the EPSP evoked by test stimulations against time after the initiation of conditioning stimulation. Each dot represents the average value of 5 consecutive responses. Filled bar indicates the period of conditioning stimulation. B: effect of strong conditioning stimulation. C: effect of depolarizing current injection (0.3 nA) into the cell during strong conditioning stimulation. Intensity of the conditioning stimulation was adjusted to elicit the LTC responses at the resting membrane potential by briefly applying 2 Hz stimulation. D: as in C, but for the effect of hyperpolarization (-2 nA). The perfusate contained 1 PM bicuculline methiodide and 100 PM APV in A-D. Time and voltage calibrations are common to A-D, except for the voltage calibration of 40 mV for the response to conditioning stimulation in D.

l l l

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strong conditioning stimulation failed to induce LTP if LTC responses were prevented by membrane depolarization (Fig. 3C) or hyperpolarization (Fig. 3 D). In addition, LTP induction was prevented by 100 PM Ni2+ in conditions in which the intensity of the conditioning stimulation was adjusted to produce EPSPs with a rising slope ( 8.5 t 2.7 V/s, n = 6) comparable with those for strong conditioning stimulation without Ni2+ (8.3 t 2.0 V/s, n = 8; Fig. 4A). LTP induction was not affected by 10 PM nifedipine, which had no effect on the LTC responses (Fig. 4 B). These results, summarized in Table 1, indicate that the activation of LTCs is necessary for LTP induction in conditions in which NMDA receptors are blocked by APV. There is a possibility that Ca2+ influx through either NMDA receptor channels or LTCs is able to induce LTP. To test this, we examined the effect of Ni2+ on LTP induction in a solution without APV. However, it is known that Ni2+ as well as Mg2+ blocks NMDA receptor channels in a voltage-dependent manner (Ault et al. 1980; Mayer and Westbrook 1985 ) . Therefore we first examined the effect of Ni 2+on NMDA receptor-mediated EPSPs. Ni 2+( 100 PM) did not affect the amplitude of EPSPs (98 t 17% of control, n = 5; P > 0.6) recorded from cells perfused with a solution containing a non-NMDA antagonist, DNQX (40 PM).

Under this condition EPSPs have been demonstrated to be mediated by NMDA receptors (Komatsu et al. 199 1). This suggests that 100 PM Ni2+ provides little additional block of the NMDA receptor channels in the presence of 1.3 mM m 2+. Thus it seemspossible to perform a test, in a solution containing 100 PM Ni2+, to determine whether LTP can be 1. Involvement of LTCs in induction of LTP in solutions containing APV

TABLE

Magnitude Weak Strong Depolarization H yperpolarization Ni2+ Nifedipine BAPTA

95 167 99 108 96 167 98

of LTP, %

z!I 10 (8) + 30 (8)* f. 5 (5) + 10 (6) + 6 (6) + 8 (4)* k 7 (5)

Values are means t SD of percentage of the excitatory postsynaptic potential slope measured at 30 min after conditioning stimulation; numbers in parentheses are numbers of cells tested. LTC, low-threshold Ca2+ channels; LTP, long-term potentiation; APV, 2-amino-5-phosphonovalerate; BAPTA, 1,2-bis-(o-aminophenoxy)ethane-N,N,N’,N’-tetraacetic acid. *Statistically significant (P < 0.01) changes.

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TABLE

Control Ni2+ Conventions

are the same as for Table 1.

I 50

(min)

induced by the activation of NMDA receptors without the activation of LTCs. The LTP was prevented by the application of 100 PM Ni2+ (Fig. 4, C and D; Table 2)) although the amplitude of slow depolarization evoked by conditioning stimulation (+ in Fig. 4, C and D) was as large in the solution containing Ni2+ (32 t 10 mV, n = 6) as it was in the solution without Ni2+ (21 t 4 mV, II = 8). The slow depolarization is thought to be mediated by NMDA receptors because it was blocked by APV (Komatsu et al. 199 1). These results support the idea that the activation of NMDA receptors will not induce LTP under conditioning stimulation as employed in this experiment. However, we cannot completely rule out the possibility that Ni2+ indirectly reduced NMDA

Magnitude

I 30

of LTP, %

151 + 24 (8)* 103 t 14 (6)

receptor-mediated components during conditioning stimulation by a blockade of LTC responses, which may have reduced polysynaptic transmissions.

Efect of a Ca2+ chelator on LTP To confirm that the induction of LTP requires Ca2+ influx into postsynaptic cells, we examined LTP in conditions in which a Ca2+ chelator, BAPTA (Tsien 1980)) was injected into the recorded cell. After penetration of the cells, the falling phase of the action potentials was slowed, as was also described in CA1 pyramidal cells (Lancaster and Nicolll987 ) . The change was ascribed to the reduction of a fast Ca2+-activated-K+ current (Lancaster and Nicoll 1987). The half-width of spikes increased from 0.8 t 0.1 to 1.1 to.1 ms(n = 4; P < 0.05) and reached almost a steady value within 30 min after the penetration, indicating that BAPTA was substantially diffused into the cell. Therefore the test was started 30 min after the penetration of the cell. LTP was not induced in any of the tested cells, even though large LTC responses were evoked by conditioning stimulation (Fig. 5; Table 1). Therefore we conclude that Ca2+ influx through the LTCs is responsible for the LTP induction.

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BAPTA 13 mV 5%

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FIG. 5. Effect of a Ca2+ chelator on LTP induction. Similar to Fig. 3A, but responses were recorded by an electrode containing 200 mM BAPTA.

Development of responsesmediated by LTCs The LTP in cat visual cortex occurs mostly around 1 mo after birth (Komatsu et al. 1988). This age dependence might be due to postnatal change in LTCs, because the activation of LTCs is required to induce LTP as described above. To test this possibility, we measured the maximal amplitude of LTC responses evoked by current pulses. The examination was made in the presence of 1 PM TTX, in which conditions LTC responses were evoked by current pulses as in the solution containing TTX and TEA, but HTC responses were rarely evoked. Maximal LTC responses were usually evoked as an anodal break when 0.61.6 nA hyperpolarizing current pulses (duration 300 ms) were injected into the cell at the membrane potential between -40 and -50 mV. Typical examples are shown in Fig. 6A. The maximal amplitude changed considerably with age and peaked at 5 wk after birth (Fig. 6 B), when LTP is known to be most easily induced. In the adults, the addition of the K+-channel blockers, TEA ( 10 mM) and 4-aminopyridine ( 1 mM) , did not significantly (P > 0.2) increase the amplitude of LTC responses, indicating that the postnatal change occurs in the LTC itself rather than in the K+ channels. DISCUSSION

Responsesmediated by LTCs It has been reported that LTCs are present in various cells in the vertebrate nervous system (Carbone and Lux 1984; Fedulova et al. 1985; Fox et al. 1987; Jahnsen and Llinas 1984; Llinas and Yaron 1981; Murase and Randic 1983; Nowycky et al. 1985; Yaari et al. 1987). They have a common voltage range for activation and inactivation: both are between about -90 and -50 mV. In addition, a low con-

AND

M. IWAIURI

centration of Ni2+ substantially blocks these channels but only slightly blocks HTCs in all of the neurons examined so far (Blaxter et al. 1989; Crunelli et al. 1989; Fox et al. 1987; Narahashi et al. 1987; Ozawa et al. 1989). The late depolarizing potential evoked by white-matter stimulation is thought to be mediated by the LTCs because it demonstrated these properties characteristic to the responses mediated by LTCs. The late depolarizing potential seems to be elicited by EPSPs rather than by orthodromic sodium spikes because the spikes could be evoked without accompanving the late depolarization when weak stimulation was applied or when strong stimulation was applied during membrane hyperpolarization. If the LTCs are located in dendrites, the sodium spike may possibly fail to evoke the late depolarization. The sodium spike, which is generally thought to be generated in soma, may be greatly attenuated in dendrites, as demonstrated in Purkinje cells (Llinas and Sugimori 1980). The LTCs play an important role in generating rhythmic firing of cells in various brain areas (Greene et al. 1986; Jahnsen and Llinas 1984; Llinas and Yaron 198 1; Wilcox et al. 1988). The present study demonstrated another functional role of LTCs. The induction of LTP was shown to be dependent on postsynaptic membrane potential, and it was also shown to be blocked by intracellular injection of Ca2+ chelator into postsynaptic cells. This indicates that an increase in intracellular Ca2+ concentration, which is due to Ca 2+influx through voltage-sensitive channels, is responsible for the LTP induction. Because NMDA receptor channels are unlikely to contribute here (Komatsu et al. 199 1), voltage-gated Ca2+ channels are expected to be involved. The fact that the induction of LTP was prevented by membrane depolarization over -50 mV and also prevented by a bath application of Ni2+ indicates that the LTCs are responsible for the LTP induction. Because nifedipine did not affect the induction of LTP, it is likely that sustainedtype HTCs are not required for LTP induction. The highthreshold component of Ca spikes evoked by depolarizing current pulses was only partly reduced by nifedipine. The remaining response may be generated by transient-type HTCs. At present we cannot test the involvement of the transient-type HTCs in LTP induction because synaptic transmission may be blocked by transient-type HTC antagonists (Miller 1987).

Comparison of LTPs with d@erent induction mechanisms NMDA receptors are involved in the induction of LTP in Schaffer collateral-commissural fiber pathway to CA 1 pyramidal cells, association-commissural fiber pathway to CA3 pyramidal cells, perforant path to dentate granule cells, and some pathways in neocortex (Artola and Singer 1987; Collingridge et al. 1983; Harris and Cotman 1986; Kimura et al. 1989; Morris et al. 1986; Sah and Nicoll 199 1; Sutor and Hablitz 1989; Zalutsky and Nicoll 1990). In these synapses EPSPs comprise non-NMDA receptor-mediated early and NMDA receptor-mediated late components, and LTP induction is prevented by the application of APV. The synaptic transmission from mossy fiber to CA3 pyramidal cells contains the non-NMDA receptor-mediated component but not the NMDA receptor-mediated component (Mon-

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LOW-THRESHOLD

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adult ----------------

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FIG. 6. Development of responses mediated by LTCs. A: responses evoked by hyperpolarizing current pulses in cells sampled from kittens at 1 (top) and 5 wk after birth (middle) and an adult cat (bottom). Circle in the middle trace indicates the amplitude of LTC responses. Bar indicates the period (300 ms) of hyperpolarizing current pulses. Amplitude of current pulses was 0.6 nA for the top trace and 0.8 nA for the middIe and bottom traces. Solution contained 1 PM TTX. B: maximal amplitude of LTC responses plotted against the age of the cats. Hollow and solid circles represent values obtained in the presence of 1 PM TTX and in the presence of 1 PM TTX, 10 mM TEA and 1 mM 4-aminopyridine, respectively. Points represent the mean f: SD. Number of cells for age group: 10 for 1 wk, 10 for 5 wk, 8 for 10 wk, 14 for 20 wk, and 15 (7 with and 8 without K+ channel blockers) for adults.

aghan and Cotman 1985). NMDA receptors are not involved in the LTP induction in this synapse (Harris and Cotman 1986; Zalutsky and Nicoll 1990). Therefore it is generally thought that LTP induction requires activation of the NMDA receptors in the synaptic transmission with the NMDA component. The LTP in kitten visual cortex seems to be exceptional. Although the EPSPs have both nonNMDA and NMDA components and the NMDA component is relatively large, APV does not affect the induction of LTP (Komatsu et al. 199 1). The two induction mechanisms, dependent on NMDA receptors and LTCs, have some common characteristics. They are both present in postsynaptic cells, and the Ca2’ influx through NMDA receptor channels or LTCs associated with postsynaptic cellular activities is required to trigger the LTP. It is well documented that the NMDA receptor-dependent LTP has cooperativity and associativity: there is a threshold for the intensity of conditioning stimulation necessary to induce LTP (Levy and Steward 1979; McNaughton et al. 1978 ) . This property is explained by the voltage dependence of the NMDA receptor itself (Collingridge and Bliss 1987). The LTP demonstrated by the present study also showed a clear threshold, which was the threshold for the activation of LTCs. Both types of LTP induction depended on the membrane

potential of postsynaptic cells. When membrane potential was hyperpolarized far from resting membrane potential, LTP could not be induced in kitten visual cortex or in hippocampal CA 1 (Malinow and Miller 1986). As for the LTP in CA1 pyramidal cells, depolarization strongly enhances LTP induction, and low-frequency stimulation paired with the depolarization is sufficient to induce LTP (Gustafsson et al. 1987). In contrast, LTP was never induced in kitten visual cortical cells when the membrane potential was only slightly depolarized. The difference in conditioning stimulation parameters may reflect the difference in the induction mechanism. NMDA receptors are relieved from Mg2+ blockade by the temporal summation of non-NMDA receptor-mediated EPSPs (Ault et al. 1980; Mayer et al. 1984; Nowak et al. 1984). Therefore high-frequency ( 50- 100 Hz) stimulation is favorble for NMDA receptor-dependent LTP. In contrast, such high-frequency stimulation may be ineffective for the induction of LTP in kitten visual cortical cells because LTCs are thought to be inactivated soon after depolarization produced by the first stimulus. Because the duration of NMDA receptor-mediated EPSPs evoked by 2 Hz stimulation is a few hundred milliseconds in kitten visual cortical cells (Komatsu et al. 199 1) , LTCs may be deinactivated by 500 ms after stimulation. This may be one reason that a

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AND

relatively low-frequency (2 Hz) stimulation is effective for the induction of LTP. The LTP in CA 1 pyramidal cells is confined to the conditioned pathway (Andersen et al. 1977; Lynch et al. 1977). In our previous study in which conditioning stimulation was applied to one side of white matter separated at its center, LTP of field potentials was induced in the conditioned pathway but not in the unconditioned pathway (Komatsu et al. 199 1). This was also observed in LTP of EPSPs recorded intracellularly (unpublished observation). Therefore it is likely that LTP in kitten visual cortex has a pathway specificity similar to that of LTP in CAl. If the LTCs are located in dendrites, as suggested above, then input specificity for the spatially separated inputs to dendrites could be explained by the localized increase of Ca2+ concentration in dendrites receiving inputs from the conditioned pathway. It has not yet been tested whether the input specificity holds to the spatially overlapped inputs. If this is the case, there is a possibility that some additional mechanisms are required to discriminate synapses that are either activated or not activated by conditioning stimulation. This issue presented here seems to require more knowledge of the distribution of LTCs in postsynaptic cells and the spatial relation between excitatory synapses and LTCs. Relevance of LTP to visual cortical plasticity Our studies have demonstrated that the LTP in cat visual cortex is most easily induced during the critical period when visual responses are modified by visual experience (Komatsu et al. 198 1, 1988). This suggests that the LTP is one of the cellular bases of the plastic changes in visual responsiveness. This inference is supported by the present finding that the mechanism for LTP induction resides in postsynaptic cells, because the activity-dependent modification of photic responses in visual cortex is also gated by the activities in postsynaptic cells (Fregnac et al. 1988; Rauschecker and Singer 198 1; Reiter and Stryker 1988 ) . In addition, the development of responses mediated by LTCs is paralleled in time with that of the susceptibility of the visual responses of cortical cells to the visual experience (Blakemore and Van Sluyters 1974; Hubel and Wiesel 1970; Olson and Freeman 1980). The effect of monocular deprivation on the ocular-dominance preference is prevented by the application of APV in kitten visual cortex (Kleinschmidt et al. 1987 ) . In addition, LTP in rat visual cortex is prevented by APV (Artola and Singer 1987, 1990; Kimura et al. 1989). These observations have strongly suggested that NMDA receptors are generally involved in LTP induction in developing visual cortex. However, our previous and present experiments have demonstrated that the activation of NMDA receptors is not required for LTP induction in kitten visual cortex. At present, we do not have a clear explanation for the disagreement of induction mechanisms between cat and rat LTPs. However, the difference in conditioning stimulation parameters might be one reason. In rat visual cortex, LTP could be induced by intermediate-frequency ( -2 Hz) and high-frequency ( - 50 Hz) stimulation (Berry et al. 1989). The effect of APV was examined in LTP induced by high-

M. IWAIURI

frequency stimulation applied for a short period (Artola and Singer 1990; Kimura et al. 1989). The LTP induced by intermediate-frequency stimulation applied for a long period might be insensitive to APV. If this is the case, Ca2+ influx through either NMDA receptor channels or voltagegated Ca 2+channels can initiate the LTP, as demonstrated in LTP of CA1 pyramidal cells (Aniksztejn and Ben-Ari 199 1; Grover and Teyler 1990). However, in kitten visual cortex, Ca2+ influx through NMDA receptor channels is unlikely to induce LTP, because the application of Ni2+ prevented LTP induction in the solution without APV, even though the conditioning stimulation produced large depolarizing responses mediated by NMDA receptors. Furthermore, high-frequency stimulation rarely induced LTP in kitten visual cortex (unpublished observation). In addition, we failed to induce long-term depression of EPSP by high-frequency stimulation in the presence of APV (unpublished observation), which has been reported in rat visual cortex (Artola et al. 1990). Therefore it is likely that the activity-dependent modification of excitatory synaptic transmission in kitten visual cortex is induced by intermediate-frequency stimulation rather than high-frequency stimulation and is controlled by LTCs rather than NMDA receptors. Another possible explanation for the discrepancy is the existence of species differences in LTP. It is known that visual cortical cells are less sensitive to the manipulation of visual experience during the postnatal early period in rodent and rabbit than in cat and monkey (Drager 1978; Van Sluyters and Stewart 1974). The LTCs might be specifically expressed in cells of the latter species, which have well-developed binocular vision in comparison with the former species, during the critical period. Although NMDA receptors are not involved in LTP of kitten visual cortical cells, visual responses are more sensitive to the NMDA antagonist in young cats than in adult cats (Fox et al. 1989; Tsumoto et al. 1987)) and application of the NMDA antagonist prevents plastic changes of visual responses in kitten visual cortex (Kleinschmidt et al. 1987). This suggests that NMDA receptors play an important role in plasticity in developing visual cortex but through processes other than LTP. Because the voltage range of inactivation of LTCs is near the resting membrane potential, slight changes of membrane potential may strongly influence the modifiability of synaptic transmission. It has been proposed that noradrenergic and cholinergic fibers control the plasticity in kitten visual cortex (Bear and Singer 1986; Pettigrew and Kasamatsu 1978 ) . There is a possibility that these monoaminergic systems modulate the modifiability of visual responsiveness by changing the membrane potentials of cortical cells. In addition, monoamines might directly modify the LTCs, as proposed in Ca2+ channels of CA3 pyramidal cells (Fisher and Johnston 1990). This work was supported by Grants-in-Aid for Scientific Research Project 02679956 from the Japanese Ministry of Education, Science, and Culture. Address reprint requests to Y. Komatsu. Received 15 July 199 1; accepted in final form 20 September 199 1.

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LOW-THRESHOLD

cA2+

CHANNELS

REFERENCES ANDERSEN, P., SUNDBERG, S. H., SVEEN, O., AND WIGSTR~M,

H. Specific long-lasting potentiation of synaptic transmission in hippocampal slices. Nature Lond. 266: 736-737, 1977. ANIKSZTEJN, L. AND BEN-ARI, Y. Novel form of long-term potentiation produced by a K+ channel blocker in the hippocampus. Nature Lond. 349:67-69,1991. ARTOLA, A., BR&HER,

S., AND SINGER, W. Different voltage-dependent thresholds for inducing long-term depression and long-term potentiation in slices of rat visual cortex. Nature Land. 347: 69-72, 1990. ARTOLA, A. AND SINGER, W. Long-term potentiation and NMDA receptors in rat visual cortex. Nature Lond. 330: 649-652, 1987. ARTOLA, A. AND SINGER, W. The involvement of N-methyl-D-aspartate receptors in induction and maintenance of long-term potentiation in rat visual cortex. Eur. J. Neurosci. 2: 254-269, 1990. AULT, B., EVANS, R. H., FRANCIS, A. A., OAKES, D. J., AND WATKINS, J. C. Selective depression of excitatory amino acid induced depolarizations by magnesium ions in isolated spinal cord preparations. J. Physiol. Land 307:4 13-428,198O. BEAR, M. F. AND SINGER, W. Modulation of visual cortical plasticity by acetylcholine and noradrenaline. Nature Land. 320: 172- 175, 1986. BERRY, R. L., TEYLER, T. J., AND TAIZHEN, H. Induction of LTP in rat primary visual cortex: tetanus parameters. Brain Res. 48 1: 22 l-227, 1989. BLAKEMORE, C. AND COOPER, G. F. Development of the brain depends on the visual environment. Nature Land. 228: 477-478, 1970. BLAKEMORE, C. AND VAN SLUYTERS, R. C. Reversal of the physiological effects of monocular deprivation in kittens: further evidence for a sensitive period. J. Physiol. Lond. 237: 195-2 16, 1974. BLAXTER, T. J., CARLEN, P. L., AND NIESEN, C. Pharmacological and anatomical separation of calcium currents in rat dentate granule neurones in vitro. J. Physiol. Land. 4 12: 93-l 12, 1989. BODE-GREUEL, K. M. AND SINGER, W. Developmental changes of the distribution of binding sites for organic Ca2+-channel blockers in cat visual cortex. Exp. Brain Res. 70: 266-275, 1988. CARBONE, E. AND Lux, H. D. A low voltage-activated, fully inactivating Ca channel in vertebrate sensory neurones. Nature Land. 3 10: 50 l-502, 1984. COLLINGRIDGE, G. L. AND BLISS, T. V. B. NMDA receptors-their role in long-term potentiation. Trends Neurosci. 10: 288-293, 1987. COLLINGRIDGE, G. L., KEHL, S. J., AND MCLENNAN, H. Excitatory amino acids in synaptic transmission in the Schaffer collateral-commissural pathway of the rat hippocampus. J. Physiol. Lond. 334: 33-46, 1983. CRIJNELLI, V., LIGHTOWLER, S., AND POLLARD, C. E. A T-type Ca2+ current underlies low-threshold Ca2+ potentials in cells of the cat and rat lateral geniculate nucleus. J. Physiol. Lond. 4 13: 543-56 1, 1989. DRXGER, U. C. Observations on monocular deprivation in mice. J. Neurophysiol. 4 1: 28-42, 1978. FEDULOVA, S. A., KOSTYUIC, P. G., AND VESELOVSKY, N. S. Two types of calcium channels in the somatic membrane of newborn rat dorsal root ganglion neurones. J. Physiol. Land. 359: 431-446, 1985. FISHER, R. AND JOHNSTON, D. Differential modulation of single voltagegated calcium channels by choline& and adrenergic agonists in adult hippocampal neurons. J. Neurophysiol. 64: 129 1- 1302, 1990. Fox, A. P., NOWCKY, M. C., AND TSIEN, R. W. Kinetic and pharmacological properties distinguishing three types of calcium currents in chick sensory neurones. J. Physiol. Land. 394: 149- 172, 1987. Fox, K., SATO, H., AND DAW, N. The location and function of NMDA receptors in cat and kitten visual cortex. J. Neurosci. 9: 2443-2454, 1989. FR~GNAC, Y., SHULZ, D., THORPE, S., AND BIENENSTOCK, E. A cellular

analogue of visual cortical plasticity. Nature Lond. 333: 367-370, 1988. R. W. A low threshold calcium spike mediates firing pattern alterations in pontine reticular neurons. Science Wash. DC 234: 738-740, 1986. GROVER, L. M. AND TEYLER, T. J. Two components of long-term potentiation induced by different patterns of afferent activation. Nature Land. 347: 477-479, 1990. GUSTAFSSON, B., WIGSTR~M, H., ABRAHAM, W. C., AND HUNG, Y.-Y. Long-term potentiation in the hippocampus using depolarizing current pulses as the conditioning stimulus to single volley synaptic potentials. J. Neurosci. 7: 774-780, 1987. GREENE, R, W., HAAS, H. L., AND MCCARLEY,

AND

LTP IN KITTEN

VISUAL

CORTEX

409

HARRIS, E. W. AND COTMAN, C. W. Long-term

potentiation of guinea pig mossy fiber responses is not blocked by N-methyl-D-aspartate antagonists. Neurosci. Lett. 70: 132-l 37, 1986. HIRSCH, H. V. B. AND SPINELLI, D. N. Visual experience modifies distribution of horizontally and vertically oriented receptive fields in cats. Science Wash. DC 168: 869-87 I, 1970. HUBEL, D. H. AND WIESEL, T. N. The period of susceptibility to the physiological effects of unilateral eye closure in kittens. J. Physiol. Land. 206: 419-436,197O. JAHNSEN, H. AND LLIN~S, R. Ionic basis for the electroresponsiveness

and oscillatory properties of guinea-pig thalamic neurones in vitro. J. Physiol. Lond. 349: 227-247, 1984. KIMURA, F., NISHIGORI, A., SHIROKAWA, T., AND TSUMOTO, T. Longterm potentiation and N-methyl-D-aspartate receptors in the visual cortex of young rats. J. Physiol. Lond. 4 14: 125-144, 1989. KLEINSCHMIDT, A., BEAR, M. F., AND SINGER, W. Blockade of ‘NMDA’ receptors disrupts experience-dependent plasticity of kitten striate cortex. Science Wash. DC238: 355-358, 1987. KOMATSU, Y., FUJII, K., MAEDA, J., SAKAGUCHI, H., AND TOYAMA, K. Long-term potentiation of synaptic transmission in kitten visual cortex. J. Neurophysiol. 59: 124-141, 1988. KOMATSU, Y., NAKAJIMA, S., AND TOYAMA, K. Induction of long-term potentiation without participation of N-methyl-D-aspartate receptors in kitten visual cortex. J. Neurophysiol. 65: 20-32, 199 1. KOMATSU, Y., TOYAMA, K., MAEDA, J., AND SAKAGUCHI, H. Long-term potentiation investigated in a slice preparation of striate cortex of young kittens. Neurosci. Lett. 26: 269-274, 198 1. LANCASTER, B. AND NICOLL, R. A. Properties of two calcium-activated hyperpolarizations in rat hippocampal neurons. J. Physiol. Lond. 389: 187-203, 1987. LEVY, W. B. AND STEWARD, 0. Synapses as associative memory elements in the hippocampal formation. Brain Res. 175: 233-245, 1979. LLIN&, R. AND SUGIMORI, M. Electrophysiological properties of in vitro

Purkinje cell dendrites in mammalian cerebellar slices. J. Physiol. Lond. 305: 197-2 13, 1980. LLIN~, R. AND YARON, Y. Properties and distribution of ionic conductances generating electroresponsiveness of mammalian inferior olivary neurones in vitro. J. Physiol. Lond. 3 15: 569-584, 198 1. LYNCH, G., DUNWIDDIE, T., AND GRIBKOFF, V. Heterosynaptic depression: a postsynaptic correlate of long-term potentiation. Nature Land. 266: 737-739, 1977. LYNCH, G., LARSON, J., KELSO, S., BARRIONUEVO,

G., AND SCHOTLER, F. Intracellular injections of EGTA block induction of hippocampal longterm potentiation. Nature Land. 305: 7 19-720, 1983. MALENKA, R. C., KAUER, J. A., ZUCKER, R. S., AND NICOLL, R. A. Postsynaptic calcium is sufficient for potentiation of hippocampal synaptic transmission. Science Wash. DC 242: 8 l-84, 1988. MALINOW, R. AND MILLER, J. P. Postsynaptic hyperpolarization during conditioning reversibly blocks induction of long-term potentiation. Nature Land. 320: 529-530, 1986. MAYER, M. L. AND WESTBROOK, G. L. The action of N-methyl+aspartic acid on mouse spinal neurones in culture. J. Physiol. Lond. 36 1: 65-90, 1985. MAYER, M. L., WESTBROOK, G. L., AND GUTHRIE, P. B. Voltagedependent block by Mg2+ of NMDA responses in spinal cord neurones. NatureLond. 309: 261-263,1984. MCNAUGHTON, B. L., DOUGLAS, R. M., AND GODDARD, G. V. Synaptic enhancement in fascia dent&a: cooperativity among coactive afferents. Brain Res. 157: 277-293, 1978. MILLER, R. J. Multiple calcium channels and neuronal function. Science Wash. DC2351 46-52, 1987. MONAGHAN, D. T. AND COTMAN, C. W. Distribution of N-methyl-Daspartate-sensitive, L- [ 3H] -glutamate-binding sites in rat brain as determined by quantitative autoradiography. J. Neurosci. 5: 2909-29 19, 1985. MORRIS, R. G. M., ANDERSON, E., LYNCH, G. S., AND BAUDRY, M. Selec-

tive impairment of learning and blockade of long-term potentiation by an N-methyl+aspartate receptor antagonist, AP5. Nature Lond. 3 19: 774-776, 1986. MURASE, K. AND RANDI~, M. Electrophysiological properties of rat spinal dorsal horn neurones in vitro: calcium-dependent action potentials. J. Physiol. Lond. 334: 141-153, 1983. NARAHASHI, T., Tsmoo, A., AND YOSHII, M. Characterization of two

Downloaded from www.physiology.org/journal/jn by ${individualUser.givenNames} ${individualUser.surname} (129.081.226.078) on January 15, 2019.

410

Y. KOMATSU

types of calcium channels in mouse neuroblastoma cells. J. Physiol. Land. 383: 231-249, 1987. NOWAK, L., BREGESTOVSKI, P., ASCHER, P., HERBET, A., AND PROCHIANTZ, A. Magnesium gates glutamate-activated channels in mouse central neurones. Nature Lond. 307: 462-465, 1984. NOWYCKY, M. C., Fox, A. P., AND TSIEN, R. W. Three types of neuronal calcium channel with different calcium agonist sensitivity. Nature Land. 316: 440-443, 1985. OLSON, C. R. AND FREEMAN, R. D. Profile of the sensitive period for monocular deprivation in kittens. Exp. Brain Res. 39: 17-2 1, 1980. OZAWA, S., TSUZUKI, K., 11~0, M., OGURA, A., AND KUDO, Y. Three types of voltage-dependent calcium current in cultured rat hippocampal neurons. Brain Res. 495: 329-336, 1989. PETTIGREW, J. D. AND KASAMATSU, T. Local perfusion of noradrenaline maintains visual cortical plasticity. Nature Lond. 27 1: 76 l-763, 1978. RAUSCHECKER, J. P. AND SINGER, W. The effects of early visual experience of the cat’s visual cortex and their possible explanation by Hebb synapses. J. Physiol. Lond. 3 10: 2 15-239, 198 1. REITER, H. 0. AND STRYKER, M. P. Neural plasticity without postsynaptic action potentials: less-active inputs become dominant when kitten visual cortical cells are pharmacologically inhibited. Proc. Natl. Acad. Sci. USA 85: 3623-3627, 1988. SAH, P. AND NICOLL, R. A. Mechanism underlying potentiation of synaptic transmission in rat anterior cingulate cortex in vitro. J. Physiol. Lond. 433: 6 15-630, 199 1. SAKURAI, M. Calcium is an intracellular mediator of the climbing fiber in induction of cerebellar long-term depression. Proc. Nat/. Acad. Sci. USA 87: 3383-3385, 1990.

AND

M. IWAKIRI

SUTOR, B. AND HABLITZ, J. J. EPSPs in rat neocortical neurons in vitro. II.

Involvement of N-methyl+aspartate receptors in the generation of EPSPs. J. Neurophysiol. 6 1: 62 l-634, 1989. TOYAMA, K., MATSUNAMI, K., OHNO, T., AND TOKASHIKI, S. An intracellular study of neuronal organization in the visual cortex. Exp. Brain Res. 21: 45-66, 1974. TSIEN, R. Y. New calcium indicators and buffers with high selectivity against magnesium and protons: design, synthesis, and properties of prototype structures. Biochemistry 19: 2396-2404, 1980. TSUMOTO, T., HAGIHARA, K., SATO, H., AND HATA, Y. NMDA receptors in the visual cortex of young kittens are more effective than those of adult cat. Nature Lond. 327: 5 13-5 14, 1987. VAN SLUYTERS, R. C. AND STEWART, D. L. Binocular neurons of the rab bit’s visual cortex: effects of monocular sensory deprivation. Exp. Brain Res. 19: 196-204, 1974. WIESEL, T. N. AND HUBEL, D. H. Single-cell responses in striate cortex of kittens deprived of vision in one eye. J. Neurophysiol. 26: 1003- 10 17, 1963. WILCOX, K. S., GUTNICK, M. J., AND CHRISTOPH, G. R. Electrophysiological properties of neurons in the lateral habenula nucleus: an in vitro study. J. Neurophysiol. 59: 2 12-225, 1988. YAARI, Y., HAMON, B., AND Lux, H. D. Development of two types of calcium channels in cultured mammalian hippocampal neurons. Science Wash. DC235: 680-682, 1987. ZALUTSKY, R. A. AND NICOLL, R. A. Comparison of two forms of longterm potentiation in single hippocampal neurons. Science Wash. DC 248: 1619-1624, 1990.

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