Electrophysiological and pharmacological characterization of perforant path synapses in CA1: mediation by glutamate receptors.

1. With the use of hippocampal slices from adult rats, we studied monosynaptic potentials in CA1 evoked by stimulating either the perforant pathway or the Schaffer collaterals. Excision of region CA3 and the dentate gyrus prevented polysynaptic excitation of CA1 and facilitated interpretation of the extracellular potentials. 2. Laminar profiles distinguished the population excitatory postsynaptic potentials (pEPSPs) in CA1 evoked by stimulating the Schaffer collaterals and the perforant path. Stimulating the perforant path evoked short-latency negative-going pEPSPs in s. lacunosum-moleculare of CA1 and positive-going pEPSPs in s. radiatum. Stimulating the Schaffer collaterals evoked negative-going pEPSPs in s. radiatum. 3. A pharmacological manipulation also distinguished the two pathways in CA1. The selective GABAB agonist baclofen greatly decreased the slope of pEPSPs evoked by stimulating the Schaffer collaterals but did not decrease the slope of pEPSPs evoked by stimulating the perforant path. 4. Combined bath application of the glutamate receptor antagonists 6,7-dinitroquinoxaline-2,3-dione (DNQX) and 2-amino-5-phosphonopentanoic acid (APV) abolished the negative-going pEPSPs in s. lacunosum-moleculare evoked by stimulating the perforant pathway. This application of DNQX and APV revealed a positive-going field potential that was blocked by bath application of the gamma-aminobutyric acid (GABA) receptor antagonists picrotoxin or bicuculline. 5. Although the glutamate-mediated component of the response evoked by stimulating the perforant path was apparently excitatory, we never observed a population spike in s. pyramidal evoked by stimulating the perforant path.(ABSTRACT TRUNCATED AT 250 WORDS)