Department of Physiology and Clinical Neurological Sciences, University of Western Ontario, London, Ontario, Canada N6A 5A5
Evoked field potentials and current-source-density analysis
were used to study the olfactory, entorhinal, and perirhinal
projections to the hippocampus. In urethane-anesthetized rats,
various structures were electrically stimulated, and evoked
potentials were mapped using glass micropipettes or multichannel
silicon probes. Stimulation of the olfactory bulb, lateral olfactory
tract, piriform cortex, amygdala-entorhinal transition, lateral
entorhinal cortex, or lateral perforant path (LPP) evoked an
outer molecular layer sink (inferred distal dendritic excitation)
in the dentate gyrus, with progressively decreasing onset latency.
Medial perforant path (MPP) stimulation evoked a middle molecular
layer sink (mid-dendritic excitation) in the dentate gyrus.
LPP and MPP were also inferred to monosynaptically excite the
distal dendrites of CA3, often resulting in a population spike
in CA3. CA3 spiking, in turn, was often followed by excitation
at the inner molecular layer of the dentate gyrus. LPP and MPP
evoked distal dendritic sinks but no population spikes in CA1.
Stimulation of the perirhinal cortex activated a sink in the
subiculum/CA1 border without activating the dentate gyrus. In
addition, reverberatory activity through a hippocampal-entorhinal-hippocampal
pathway may be activated by MPP or CA3 stimulation. It is suggested
that the parallel projections of the entorhinal and perirhinal
inputs to the distal dendrites of hippocampal principal neurons
enhance local and distributed processing as characterized by
CA3 to dentate gyrus feedback, and hippocampal-entorhinal reverberation.
