Long lasting functional alterations in the rat dentate gyrus following entorhinal cortex lesion: a current source density analysis.

The functional consequences of lesions of the entorhinal cortex of rats were studied by analysing laminar distributions of stimulus induced field potentials in the dentate gyrus with a subsequent current source density analysis. Stimulation of the inner molecular layer elicits large excitatory postsynaptic potentials with small if any population spikes in the stratum granulare both in normal and lesioned animals. In lesioned animals middle molecular layer stimulation causes large excitatory sinks in the stratum moleculare without generation of population spikes in stratum granulare, while the same stimulation in slices from normal animals readily induces population spikes. The current source density analysis revealed a shift of current sinks induced by stimulation of either the inner or the middle molecular layer to common site. The N-methyl-D-aspartate receptor contribution to the current sink and source was found to be more prominent after middle molecular layer stimulation in comparison to inner molecular layers stimulation in the control group, while such a distinction could not be made in the lesioned group. Activation of mossy fibers did not reveal any significant differences between normal and lesioned animals. Following entorhinal cortex lesion sprouting of remaining afferents (e.g. commissural fibers) into the termination zones of the degenerated perforant path has been reported suggesting a compensatory replacement of excitatory synaptic input. However, persistent transneuronal dendritic alterations of neurons in the dentate gyrus have been observed which might result in altered dentate gyrus function. Our findings suggest that the reorganization process after entorhinal cortex lesion does not lead to full functional compensation of the lost perforant path input, resulting in an altered balance between excitation and inhibition.