Institute of Physiology at the Charité, Department of Neurophysiology, Humboldt University, 10117 Berlin, Germany
There are multiple connections from the entorhinal cortex (EC)
to the hippocampus that carry the information from the EC to
the hippocampus. Layer II cells of the medial EC innervating
the dentate gyrus (DG)-molecular layer possess K
+-outward currents
and inward rectifier currents that are potentially modulated
by changes in intracellular second messengers. Layer II cells
responded to synaptic stimulation with a rather flat input-output
curve, and much stronger stimuli are required to generate action
potentials in these neurons than in EC layer III cells. During
repetitive stimulation at frequencies of 10 Hz and more, EC
layer II cells respond with increased likelihood to generate
action potentials. Two different NMDA conductances can be demonstrated
in these neurons. A slow, less Mg, less voltage-dependent component
is responsible for the transient depolarization between the
fast and slow IPSP. A second group of neurons also projects
to the DG. These are either pyramidal or nonpyramidal cells
in the deep layers of the EC. At least part of these neurons
also possess rhythmogenic properties. In contrast to layer II
cells, layer III neurons have a steep input-output curve and
show during repetitive synaptic activation a tendency to repolarize
and to display long-lasting inhibitions dependent on GABA
B-,
atropine-, and naloxone-sensitive components. As a consequence,
they are readily activated during low frequency stimulation,
but project only a few action potentials to area CA1 initially
during higher (more than 10 Hz) frequency synaptic stimulation.
