nuevo. Amplification of perforant-path EPSPs in CA3 pyramidal Transverse hippocampal slices (400-500 mm) were prepared cells by LVA calcium and sodium channels. J. Neurophysiol. 80: from 3-to 6-wk-old male Sprague-Dawley rats (Aghajanian and 1558(Aghajanian and -1561 (Aghajanian and , 1998 . The perforant path forms a monosynaptic con-Rasmussen 1989; Urban and Barrionuevo 1996) . While the slices nection between the cells of layer II of the entorhinal cortex and remained in the vibratome

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... , a cut was made through the the pyramidal cells in hippocampal area CA3. Although this projec-CA3b region, from the alveus to the suprapyramidal blade of the tion is prominent anatomically, very little is known about the physidentate gyrus. This cut transected the mossy fiber pathway and ological properties of this input. The distal location of these synprevented disynaptic activation of CA3 after perforant-path stimuapses suggests that somatically recorded perforant-path excitatory lation (data not shown, n ú 25). postsynaptic potentials (EPSPs) may be influenced by the activa-During recordings, slices were submerged and perfused by nortion of voltage-dependent channels in CA3 cells. We observed mal artificial cerebrospinal fluid (ACSF) at 32-34ЊC. The rethat perforant-path EPSPs are reduced (by Ç25% ) by blockade of cording solution contained (in mM) 125 NaCl, 3 KCl, 10 dextrose, postsynaptic low-voltage-activated calcium and sodium channels, 26 NaHCO 3 , 3 MgCl 2 , 3 CaCl 2 , 0.025 D-2-amino-5-phosphonovalindicating that perforant-path EPSPs are amplified by the activation eric acid (D-APV; Tocris), 0.01 bicuculline (Sigma), and 0.5 of of these channels. These data suggest that the perforant path may the g-aminobutyric acid (GABA B ) antagonist CGP35348 (gift of represent an important and highly modifiable direct connection Ciba-Geigy). 2-Amino-5-phosphonovaleric acid (APV) was inbetween the entorhinal cortex and area CA3. cluded in all recordings to prevent the confounding of the experiments by the blockade of N-methyl-D-aspartate receptors by nickel (J. G. Dilmore and J. W. Johnson, personal communication). Care I N T R O D U C T I O N was taken to avoid exposing stock solutions or ACSF containing The axons of cells in layer II of the entorhinal cortex, light-sensitive compounds (nifedipine) to the light. Nifedipine which form the perforant path, make excitatory synapses on stock solution (10 mM) was made in 100% ethanol. the distal apical dendrites of hippocampal CA3 pyramidal Whole cell electrodes (3-7 MV) were filled with a solution containing (in mM) 130 potassium gluconate, 20 KCl, 10 N-2-neurons (Berzhanskaya et al. 1998; Steward 1976). Because hydroxyethylpiperazine-N-2-ethanesulfonic acid, 1.0 ethylene of the location of these synapses, cable theory predicts that glycol-bis(b-aimonethyl ether)-N,N,N,N,-tetraacetic acid, 4.0 perforant-path excitatory postsynaptic current (EPSC) peak Mg adenosinetriphosphate, 0.3 guanosine 5-triphosphate, and 10 amplitude will be attenuated Ç90% en route to the soma sodium phosphocreatine. Data were collected with an Axopatch (Henze et al. 1996). The attenuation of excitatory postsyn-1C amplifier (Axon Instruments) and custom software. Series reaptic potentials (EPSPs) is even greater, often reaching 99% sistance (voltage clamp) and input resistance (current clamp) were (unpublished observations). Thus the observation that in monitored throughout the experiments. vivo the perforant-path input to CA3 can elicit short latency Stimulation electrodes were placed in the stratum lacunosum population spikes (Yeckel and Berger 1990) is surprising moleculare of the CA1 region. In slices in which mossy fibers were and suggests that the passive model of CA3 pyramidal cells cut, stimulation from this position results in selective activation of the perforant-path synapses in CA3 (Berzhanskaya et al. 1998). may be inadequate. In one set of experiments, somatic depolarization was elicited by Recent work reviewed by Yuste and Tank (1996) showed direct (10-20 ms) depolarization of distal dendrites in the presence that dendritic voltage-dependent channels can be activated of glutamate and GABA-receptor antagonists (see RESULTS ). All by subthreshold synaptic stimulation (Huguenard et al. 1989; values are reported as mean { SE. Magee and Johnston 1995b). These channels were shown to amplify the propagation EPSPs (Gillessen and Alzheimer R E S U L T S 1997; Lipowsky et al. 1996) to the soma. Activation of these channels partially counteracts the passive attenuation of dis-Blockade of voltage-dependent calcium channels tal inputs (Andreasen and Lambert 1998). Here we test the hypothesis that the amplification of perforant-path EPSPs To determine whether voltage-dependent calcium chanby voltage-dependent conductances may in part explain the nels amplify perforant-path EPSPs, we bath applied calcium strength of the perforant-path input to CA3 pyramidal cells. channel antagonists while recording perforant-path a-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid (AMPA) re-The costs of publication of this article were defrayed in part by the ceptor-mediated (see METHODS ) EPSPs. EPSPs of different payment of page charges. The article must therefore be hereby marked amplitudes will activate different types of voltage-dependent "advertisement" in accordance with 18 U.S.C. Section 1734 solely to indicate this fact. channels. Thus for each antagonist applied, we examined its 1558 0022-3077/98 $5.00