181

Brain Research, 527 (1990) 181-191 Elsevier BRES 15837

Research

Reports

actions of dihydropyridines on synaptic Depolarization-dependent transmission in the in vitro rat hippocampus Michael H. O’Regan1p3Jeffery D. Kocsis’Y2and Stephen G. Waxrnanf,3 Departments of lNeurology, ‘Neuroanatomy, and ‘Pharmacology, Yale UniversitySchool of Medicine, New Haven, CT and PVAIEPVA Neuroscience Research Center, VeteransAdministrationMedical Center, WestHaven, CT (U.S.A.) (Accepted 13 March 1990) Key words: Dihydropyridine;

Hippocampus;

Calcium channel; Glutamate; Adenosine

Field potential and intracellular recordings were obtained in the in vitro hippocampal slice to study the effects on synaptic transmission of dihydropyridine (DHP) derivatives. Nimodipine or nifedipine by itself had little effect upon the postsynaptic response as determined by field potentia1 analysis. However, facilitation became evident when DHP application was coupled with manipulations which induced a moderate degree of membrane depolarization. In accordance with the hydrophobic nature of these compounds, extensive washing in normal Krebs’ solution failed to reverse the facilitation indicating that the DHP effects outlasted the induced depolarization. Nifedipine is photolabile and its actions were reversed when intense light was applied to the slice. Application of the DHP Bay K 8644, resulted in a similar depolarization-dependent increase in neuronal excitabiiity which, upon washout and exposure to light, was at first attenuated and then reversed, resulting in a long-lasting depression of the EPSP that was sensitive to caffeine. This depressant action of Bay K 8644 appeared to be mediated at a site presynaptic to the pyramidal cell because the postsynaptic component of the field potential response to pulsed applications of glutamate was not altered. Intracellular recording from CA1 neurons supports a presynaptic locus for the depressant actions of Bay K 8644; spike threshold for synaptically evoked responses was greatly increased whiIe spike threshold to direct depolarization of the soma was unchanged. These results indicate that DHPs can exert effects on synaptic transmission in hippocampal brain slice under conditions of moderate membrane depolarization. INTRODUCTION

There is considerable evidence for the existence of several distinct classes of voltage-sensitive calcium channels in a variety of cell types. The ‘L-type’ channel requires a larger depolarization for activation and shows little inactivation, while ‘T-type’ channels have a lower threshold for activation and do not show sustained activation5r’3. A third type of channel, ‘N-type’, shows properties of both type#O, although its characteristics have not been completely delineated”. It has been demonstrated that dihydropyridines such as nimodipine, nifedipine, and Bay K 8644 have selective actions on the L-channel. The L-channel is associated with high affinity (Kd = lo-” M) DHP binding sites which are abundant in the mammalian brain4*8yM.There is also a lower affinity DHP binding site (I&, = 10F8 M) which appears to be linked to the nucleoside transporter”. In spite of the abundance of high affinity binding sites in brain, directly measured actions of DHPs on neuronal calcium channels are subtle and often require micromolar concentrations of ligand 24,36.In the in vitro hippocampal slice preparation, nifedipine (5-10 yM) has been reported to reduce calcium conductance in CA1 pyramidal neurons, while having no effect on synaptic transmisCorrespondence: J.D. Kocsis, Neuroscience OOO6-8993/901$03.500

sionzs. K+-stimulated calcium uptake by synaptosomes and neurotransmitter release are generally insensitive to DHPs~~ (but also see47*54), results which imply that DHP-resistant calcium channel types (T or N) may regulate neurotransmitter release. In the hippocampus, synaptic transmission is reportedly blocked by omegaconotoxin, an inhibitor of N-type (and possibly L) calcium channels, but is unaffected by 10 PM nifedipinez6. Indeed, the apparent low sensitivity of hippocampal neurons to DHPs has been cited as a reason for their lack of central side-effects at therapeutic dosages when used for treatment of ischemic heart diseaseg. The discrepancy between the observation of high affinity DHP binding sites in the central nervous system and an apparent paucity of physiological effects of DHPs may be due to the ‘use dependency’ of DHP block of calcium channels as in cardiac muscle3. Thus, the inhibitory action of nifedipine (0.1-5 PM) on the L-type calcium current was greatly enhanced when dorsal root ganglion (DRG) neurons were held at depolarized membrane potentials4’. The present experiments were carried out to examine the possibility that membrane depolarization would similarly expose DHP effects on synaptic transmission in the rat hippocampus.

Research Laboratory (127A), VA Medical Center, West Haven, CT 06518, U.S.A.

1990 Elsevier Science Publishers B.V. (Biomedical Division)

182 MATERIALS AND METHODS Hippocampal slices were prepared and maintained in vitro using standard procedures. Female adult Wistar rats (n = 121) were deeply anesthetized with sodium pentobarbital (60 mg/kg, i.p.), decapitated and a block of tissue from one hemisphere was removed. Tissue slices (400 ~m) were cut from the block and incubated in modified Kreb's solution containing (in raM): NaC1, 124; KCI, 3.0; MgCI2, 2.0; CaCI2, 2.0; NaH2PO4, 1.3; NaHCO 3, 26; dextrose, 10; pH 7.4; saturated with 95% 0 2 and 5% CO 2 for at least 60 min. Recording took place in a submersion-type chamber maintained at 34 °C, through which flowed the above solution (NS) or solution with dissolved drug at a flow rate of 5 ml/min. Concentrated solutions of adenosine, caffeine and kynurenate (Sigma) were prepared in distilled water and diluted in NS. Nifedipine (Sigma), nimodipine and Bay K 8644 (provided by Dr. A. Scriabine, Miles Laboratories) were dissolved in dimethylsulfoxide (DMSO) and diluted in NS. Similar final concentrations of DMSO (-60 mV (RP = -70.1 +__1.8 mV; input resistance (RI) = 47 + 3 MQ; mean _+ S.E.M., n = 21) and spike height of >t60 mV were studied via intracellular recording.

RESULTS

D H P effects on synaptic transmission Three dihydropyridines, nifedipine, n i m o d i p i n e and Bay K 8644, were tested for effects mission in the hippocampal slice. T h e 'antagonist' D H P s because in some block L-type Ca 2+ channels. Bay K

o n synaptic transfirst two are called cell systems they 8644 on the other

hand is referred to as an agonist D H P because it promotes Ca 2+ current through these channels 5. The results for nifedipine are presented in Table I; field potential records, both prior to and following drug application, from a representative slice are reproduced in Fig. 1. As is apparent in Fig. 1B, n o significant changes in the field potential were evident in the presence of drug alone ( N I F E D ; 20 #M). However, if the concentration of

TABLE I

The effects of dihydropyridines on population EPSP slope measured as percent of control (X. + S. E. M.) Nifedipine (201~M) A.

n=14 Drug alone 105.1 + 4.0 Drug + 5 mM K+ 116.7+3.8"* Wash 111.9_+3.0 Wash + light 95.3+_5.0 5 mM K + alone 106.1-+3.8

B. Drug alone Drug, post-stim. Wash Wash + light Wash + light, post-stim.

n=15 110.5+3.9

Nimodipine (40~M)

Bay K8644 (O.I ~M)

n = 10 121.7-+11.2

n=7 107.4-+4.3

137.0+13.3' 125.3+7.9 116.6+7.5 109.4+6.6

124.0+6.3" 110.6+2.5 57.4+15.8"** 110.6+11.0

n=5 103.0+5.1

n=7 116.1+5.4

134.1+5.5 ***0 137.0+5.9 ***0*0 143.0+19.5" 125.0+7.2 **°°° 125.6+11.2 133.0+4.5"* 131.7+8.4 **00 127.0+8.2 112.6+9.8 -

65.6+15.8"

Significantly differs from control: *P < 0.05; **P < 0.01; ***P < 0.001; significantly differs from drug alone: °P < 0.05; oop < 0.01; ooop < 0.001, determined by Student-Neuman-Keuls Multiple Range Test.

183 K + in the perfusing solution was elevated from 3.0 to 5.0

mM (a concentration which did not affect long-term excitability by itself; Fig. 1A) the population spike greatly increased in magnitude (Fig. 1C). This effect was not reversed by washing in NS demonstrating that depolarization due to elevated K ÷ was not responsible for the change in synaptic efficacy (Fig. 1D). We took advantage of the photolability48 of nifedipine to demonstrate that the enhanced population spike was due to a specific action of this DHP; thus within 2 min of exposure to light the population spike magnitude returned to control levels (Fig. 1E). A similar pattern of change in synaptic efficacy is evident in the calculated population EPSP slopes in Table I-A (5 mM K ÷) and I-B (20 I-Iz/0.5 s pulse train). In another set of experiments, the Schaffer collaterals were stimulated by a 20 Hz/0.5 s pulse train (given once only, 5 rain into the DHP application) in order to induce membrane depolarization. Following the stimulus train,

5 mM K

A

D

--

the excitant effect of nifedipine was clearly demonstrated by the appearance of a second population spike in response to a single stimulus (Fig. 2C). Upon washing in NS the second spike persisted (Fig. 2D). However, the second spike was abolished with continued wash in the presence of light (Fig. 2E) as expected from the photolability of nifedipine. The slope of the population EPSP was increased significantly in the post-pulse train condition (post-stim, Table I-B) compared to both the control and the drug alone conditions. Recovery was only evident when NS wash was combined with light exposure. Stimulation at 20 Hz/0.5 s, in the absence of drug, had no lasting effect on the postsynaptic response to single stimuli (Fig. 2A). A third manipulation which elicited the excitant effects of nifedipine was a brief increase in the stimulus intensity to 30-40 V, which could lead to massive activation of the pyramidal neurons and a large increase in [K+]o. Brief stimulation at 30-40 V resulted in multiple population spikes being elicited in response to single stimuli at the control stimulus intensity (Fig. 2F) that was reversed by NS wash in the presence of light. Nifedipine was tested

A

--

20 Hz/0.5SEC

~'~

0.5

B

__

NIFED

WASH

D

mV

E NIFED

C

--

.

WASH + L I G H ~ , ~

NIFED+5mMK

S TNIFEDI(40VM 0.5

mV

4 msec

C

--

F

"~

~

o 5

my

msec

Fig. 1. Excitatory effects of nifedipine (20 ~M) on synaptic transmission in stratum radiatum of rat hippocampus are enhanced during perfusion in 5 m M K + Krcbs' (A-E). Extraccllular field recording of the synaptic response to single stimulation (N V) of the Schaffcr collaterals. Baseline potential is given by the short horizontal line. A control response is superimposed upon responses obtained during a sequential series of drug and INS wash perfusates (arrows), I0 min in duration. Neither 5 m M K + (A) nor nifcdipine (NIFED, B) alone significantly altered the extraccllular field response. However, pcrfusion of the slicewith 5 m M K +, following ulfedipine exposure (C), results in an enhancement of the population spike. This effect persisted during washout in NS (D), but was eliminated upon exposure to high intensity light (E).

Fig. 2. The depolarization-dependenteffects of nifedipine (20/~M) following a 20 Hz/0.5 s pulse train (A-E). Nifedipine-evoked increases in synaptic excitability,indicated by the appearance of a second population spike, became evident followinga single, brief, low frequencypulse train (POST-STIM, C). Neither the pulse train delivered in the absence of nifedipine (A) nor nifedipine in the absence of a pulse train (B) affected the synaptic response to single stimuli of the afferent fibers. Prolonged (60 re.in) wash in NS had little effect (D), but the postsynapticresponse recovered to control levels upon exposure to light (E). Alternatively, a high intensity stimulus of the Schaffer collaterals could satisfy the depolarization requirement, with a resultant increase in synaptic excitabilityin the presence of 20/~M nifedipine (40 V STIM, F).

184 NIMOD

WASH ./

,¢ t

NIMOD (POST-STIM)

I/

_,.1/"

WASH + LIGHT

,.~/

,j~,/~S!v,,,..,,~' • e~?'/l.O

mV ~

4 msec Fig. 3. A 20 Hz/0.5 s pulse train reveals the excitant effect of nimodipine (40 ~M) on synaptic transwJssion. During the perfusion of the hippocampal slice with nimodipine, a short pulse train induces an increase in the magnitude of the population EPSP and the appearance of multiple population spikes (POST-STIM, B). The excitant effect of nimodipine was not reversed by either extensive wash in NS (C) or exposure to high intensity light (D). at concentrations ranging from 0.1 to 200 gM. No consistent, significant actions were observed with concentrations below 10 g M , while those above 40 /xM reduced the time to onset of excitant effects (this was not examined in detail). Combined results from the nifedi-

pine experiments indicate that nifedipine-evoked increases in population E P S P magnitude achieved significance on 26/50 slices tested. Increases in population spike amplitude and the occurrence of multiple population spikes following nifedipine application when coupled

C

//,j//.j/~,~'~"/'~'~~"'~. 1 ,~J'~-;~"~'~;

B A Y K 8644

A

WASH

.P' F i .....

B

I

BAYK-~. 5mMK .

D

':~

WASH + LIGHT

///"

/,~j/- ~ / J

;..~//

--J

I.0I mV msec

Fig. 4. K + enhances the excitant action of Bay K 8644 (0.1 #M) on synaptic transmission. By itself, Bay K 8644 slightly increases the magnitude of the population EPSP (A), an effect that is more evident during the concomitant perfusion with 5.0 mM K + (B). Extensive washing in NS largely reduces the Bay K 8644-evoked increase in synaptic efficacy (C), while exposure to light results in a reversal of Bay K 8644 action and the prolonged depression in synaptic activity (D).

185 BAY K 8644

WASH + LIGHT

D

WASH + LIGHT (POST-STIM)

E

5_

7J -

\.d

RECOVERY

WASH

C-

F~

ID.$e c

Fig. 5. A 20 Hz/0.5 s pulse train potentiates both the excitant and depressant effects of Bay K 8644 (0.1/~M) on synaptic transmission (A-F). Perfusion of the slice preparation with Bay K 8644 (in the dark) had little effect (A), but, following the pulse train, there was an increase in the magnitude of both the population EPSP and spike, together with the appearance of a second population spike (POST-STIM, B). Note also the obvious increase in the slope of the population EPSP. This excitant action was largely insensitive to washout (C), but returned to approximately control dimensions following exposure to light (D). Application of a second pulse at this time resulted in a reversal of the effects of Bay K 8644 and a depression of postsynaptic activity (E). Partial recovery was evident 60 min later (F).

with depolarization were observed more frequently (45/ 50 and 37/50 slices, respectively). Similar effects on synaptic transmission were seen in experiments using nimodipine, although consistent effects were not observed at concentrations less than 30 /zM. Nimodipine (40 gM) applied by itself failed to significantly increase population EPSP slope (Table I-A and B), but, following either manipulation designed to cause membrane depolarization, resulted in increased synaptic efficacy. Multiple population spikes in response to a single stimulus (Fig. 3B) appeared following a 20 Hz/0.5 s pulse train, an effect not induced by nimodipine in the absence of the pulse train (Fig. 3A). The nimodipine-evoked increase in synaptic efficacy was not easily reversed by wash in the presence of light (Fig. 3C-D), in keeping with the reduced photosensitivity of this DHP 45. Racemic mixtures of Bay K 8644, which contain optimal isomers that have opposite effects on calcium channels 14, had a biphasic action on synaptic efficacy. When bath applied in the absence of light, Bay K 8644

(0.05-1.0/zM) had excitant effects which, in common with the antagonist DHPs, required either exposure to 5.0 mM K +, stimulation by a 20 Hz/0.5 s pulse train, or a brief high intensity (30-40 V) stimulus of the afferent fibers. Upon wash in NS and exposure to high intensity light the EPSP transiently returned to control levels but was then significantly reduced in amplitude. On occasion, the EPSP was eliminated, although the presynaptic fiber response remained, indicating that all excitability was not lost. As illustrated in Fig. 4, the subsequent perfusion of a slice with 5.0 mM K + enhanced the effect of Bay K 8644 on the population EPSP magnitude (Fig. 4B, and slope, Table I-A). Under conditions of wash and exposure to light (Fig. 4D), the population EPSP magnitude was greatly reduced. This Bay K 8644-evoked depression of synaptic transmission was apparent in all slices tested. A similar sequence of events followed when an 20 Hz/0.5 s pulse train was utilized to induce depolarization instead of elevated [K+]o. Bay K 8644 had little effect by itself (Fig. 5A). In the post-stimulus train interval, the magnitude of both the population EPSP and the popu-

186 lation spike in response to a single stimulus was greatly increased by exposure to Bay K 8644 and a second population spike was evident (Fig. 5B). Interestingly, neither NS wash (Fig. 5C) nor wash with light (Fig. 5D) completely abolished the excitatory action of Bay K 8644 on this cell. A second pulse train, applied at this time, had the effect of reversing the action of Bay K 8644 so that the postsynaptic response was depressed relative to control levels (Fig. 5E). This depressant action of Bay K 8644 was eventually reversible (Fig. 5F) with continued wash in the presence of light (30-90 min), indicating that the synaptic depression was not due to cellular damage. Comparable results were obtained when the calculated population EPSP slope was used to measure synaptic efficacy (Table l-B). Presynaptic versus postsynaptic locus of action of DHPs Alterations in the field potential response to a stimulation of afferent fibers could result from drug actions at either (or both) pre- or postsynaptic sites. To distinguish between pre- and postsynaptic effects, the field response

TABLE II Comparison of the effects of DHPs, kynurenate and adenosine on synaptic vs glutamate-evokedresponses

Values are percent of control (.~ + S.E.M.). Population EPSP Glutamateresponse magnitude magnitude A.

Kynurenate (2mM;n = 11) Recovery

3.8+2.7*** 74.6 _+11.2

Adenosine (0.5 raM; n = 7) Recovery

0.0"** 85.7 + 6.5

56.0+7.3*** 118.6 + 8.8

n.

96.1 + 2.5 97.9 + 2.7

C.

Bay K 8644 (1/~M;n = 7) wash + light recovery kynurenate (same slice)

110.6 + 5.6 36.5 + 14.8"* 24.0 + 15.6"* 0.0"**

100.9 + 3.4 105.5 + 6.9 105.0 + 15.0 48.5 + 18.5"

Population EPSP Glutamateresponse slope magnitude D.

Nifedipine (20#M; n = 8) Post-stimulus Wash Wash+ light

108.5 + 5.1 138.3 + 6.40~ 132.4 + 6.5~ 113.1 + 5.9

111.3 + 14.0 86.5 + 16.9 90.9 + 19.7 108.3 + 28.2

Significantlydiffersfrom control magnitude; *P < 0.05; **P < 0.01; ***P < 0,001 (Student's t-test); significantlydiffers from control slope: op < 0.05; ~P < 0.01; ~'P < 0.001 (SNKtest).

to pulsed application of glutamate and the synapticallyevoked field potential in stratum radiatum were recorded sequentially. An agent acting postsynaptically would be expected to affect the field potentials elicited by both stimulation and glutamate application; one acting presynaptically should depress the synapticaUy-evoked potential but not the response to applied glutamate 33. Adenosine and kynurenate both depress synaptically evoked potentials. Adenosine presumably inhibits glutamate release presynaptically11, while kynurenate is a non-specific glutamate receptor antagonist and therefore acts postsynapticallyTM. We used application of adenosine and kynurenate, together with pulsed local application of the excitatory neurotransmitter glutamate, as a probe of whether DHPs alter synaptic efficacy by a pre- or postsynaptic mechanism. In Fig. 6A (and Table II), kynurenate significantly reduced both the synaptically and glutamate-evoked field responses. Adenosine completely blocked the synaptically evoked potential without altering the glutamate response (Fig. 6B), an action mimicked by Bay K 8644 (during light exposure in NS wash, Fig. 6D). Neither nifedipine nor Bay K 8644 (in the dark) had substantial effects on the glutamate-evoked response (Fig. 6C,E). These results support a presynaptic action in the depressant effect of Bay K 8644 on synaptic transmission. The similarity between the effects of adenosine and Bay K 8644, i.e., selective depression of the synaptically evoked response without reduction of postsynaptic glutamate sensitivity, together with the reported high affinity binding of DHPs to the nucleoside transporter ~7, suggests that Bay K 8644 can depress synaptic activity by elevating extracellular levels of endogenous adenosine. If the action of Bay K 8644 is in part mediated by an elevation of adenosine levels, it would be predicted that the actions of Bay K 8644 should be attenuated by adenosine antagonists. Following Bay K 8644 administration and subsequent exposure to light during NS wash, caffeine (at 200/~M, specifically a competitive adenosine receptor antagonist in this preparation) significantly and reversibly antagonized the depressant effect of Bay K 8644 on the population EPSP slope (Bay K 8644:27.8 + 13.1; Bay K 8644 plus caffeine: 70.0 + 8.7; wash: 32.2 + 6.4, % of control, P < 0.001, n = 4). Intracellular recording Intracellular recordings from CA1 pyramidal cell soma (n = 21) revealed a significant increase in the stimulus intensity required to elicit a synaptically-evoked spike (from 36.3 + 4.8 to 71.5 __. 3.2 V, P < 0.001), associated with the depressant action of Bay K 8644 on postsynaptic field potentials. During the Bay K 8644-evoked decrease in synaptic activation no significant changes were noted

187

KYN

BAY K

S

(LIOHT)

$]

D

A 0.3

mV

50 msec

ADO

$

NIFED

B

E 0.3 I mV 0.5 mV i00

100 msec

msec BAY K

(DARK)

/%

C

Fig. 6. Lack of DHP effect on a glutamate-evoked post-synaptic response. The effects of nifedipine and Bay K 8644 on the synaptically-evoked response to stimulation of the Schaffer afferents (10-20 V, S) was compared to the field response to pressure-applied glutamate (100/aM, 3-5 ms pulse at 10-20 psi, G). For comparison, the differential effects of kynurenate (KYN, 2 mM), a glutamate receptor antagonist, and adenosine (ADO, 0.5 mM), which blocks the release of glutamate, were examined because of their primarily post- and presynaptic mechanisms of action, respectively. Kynurenate (arrows) reduced both the glutamate-evoked and the synaptic response (A), while adenosine selectively reduced the synaptic response. The effect of Bay K 8644 (in the light, D) was similar to that of adenosine, and nifedipine (E) also affected the synaptic response specifically. A in either resting m e m b r a n e potential (-70.1 + 1.8 vs - 6 9 . 8 + 2.3 mV), input resistance (47 + 3 vs 44 + 4 M~2), or spike threshold to direct depolarization of the postsynaptic m e m b r a n e (0.9 _+ 0.14 vs 0.88 + 0.17 nA), although spike height tended to decrease (from 68.1 + 2.1 to 59.5 + 3.0 mV), associated with a decrease in spike latency. Five cells exhibited bursting and spontaneous spiking during Bay K 8644 perfusion, which was reversed upon NS wash and exposure to light. In Fig. 7, a 75-mV spike is elicited via a 1.0 n A depolarizing current both in NS (A1) and during B a y K 8644 washout in the presence of light (A2), while the concurrent spike, elicited by a 35-V stimulus of the Schaffer collaterals fails following

B

_/f\ 101mV 4 msec

Fig. 7. Intracellular recordings from a CA1 neuron; responses to a 10 ms, 1.0 nA depolarizing current delivered via a bridge circuit through the recording microelectrode (1), and to a 35-V stimulus of the Schaffer collaterals (2). In NS, both stimuli elicit a 75-mV spike (A). However, following perfusion with Bay K 8644 (1/aM), and washout in the presence of light (B) the response to stimulation of the afferent fibers fails while the intracelhilar current pulse still elicits an action potential, indicative of a presynaptic locus for the depressant actions of Bay K 8644.

2

2

188 perfusion with Bay K 8644, wash and light exposure (B2). Analysis of the synaptically-evoked EPSP (n = 9) demonstrated the depressant action of Bay K 8644 (in light) with significant decreases in EPSP area (from 53.6 + 9.8 to 13.3 + 0.3 mV.ms, P < 0.01), maximum EPSP amplitude (from 5.6 + 1.1 to 2.3 _+ 0.3 mV, P < 0.05), and EPSP slope (from 2.34 + 0.48 to 0.89 + 0.19 mV/ms, P < 0.05). However, even when the EPSP was depressed, high intensity stimulation of the afferent fibers was capable of eliciting an IPSP. DISCUSSION

Facilitation of synaptic activity The basic result of this study is that micromolar concentrations of DHPs can affect synaptic transmission in the hippocampal slice if moderate levels of depolarization are achieved during drug application. The finding that nifedipine and nimodipine enhanced synaptic transmission in mammalian hippocampus is novel and may relate to the reported nootropic actions of these compounds, e.g., nimodipine's ability to facilitate learning acquisition in aged rabbits 1°. The three DHPs examined, nifedipine, nimodipine and Bay K 8644, facilitated synaptic transmission at the Schaffer collateral-CA1 glutamatergic synapse, as manifested by an increase in the magnitude (or slope) of the population EPSP and population spike, or by the appearance of multiple population spikes. The excitant effects of these DHPs were dependent upon a depolarization elicited by moderate elevation of [K÷]o, a short stimulus pulse train, or brief high intensity stimulation of the Schaffer collaterals. None of the above manipulations alone was capable of significantly modifying synaptic efficacy in the absence of dihydropyridine application. It is important to note that the effects of DHPs on synaptic efficacy were present even after return from 5.0 mM to 3.0 mM K ÷, indicating that the change in synaptic efficacy is not due to depolarization. The failure of nifedipine and nimodipine to significantly affect synaptic efficacy at lower concentrations (