Orthodromic activation of hippocampal CA1 region of the rat.

(1) The posterior alveus (PA), the anterior alveus (AA) and the Schaffer collaterals (SCH) evoked field potential components which were organized as parasagittal strips of various widths. Spatially continuous and interactive lamellae are suggested. (2) By correlation with unit activities, the early postsynaptic components evoked by PA, AA and SCH were inferred to be extracellular excitatory postsynaptic potentials (EPSPs) and the late, long-duration components, the inhibitory postsynaptic potentials (IPSPs). The hypothesis that interneurons as well as pyramidal cells generate the field is proposed and discussed. (3) One- and two-dimensional profiles of deep evoked potentials and current source-sink analysis revealed excitatory synapses in stratum oriens for the PA and AA inputs and in stratum radiatum for the SCH input. The late dipole field evoked by PA and AA possessed current sources in strata radiatum and pyramidale, the sites of the inhibitory synapses. The late dipole field evoked by SCH had another component possibly generated by recurrent activity, afterpotentials or relayed activity through CA3.