Re-establishment of electrophysiologically functional entorhinal cortical input to the dentate gyrus deafferented by ipsilateral entorhinal lesions: Innervation by the contralaeral entorhinal cortex

Summary We have unilaterally ablated the entorhinal cortex of the developing rat, thereby removing the major synaptic input to the ipsilateral dentate gyrus. We have then examined the efferent projections of the remaining contralateral entorhinal cortex to determine if these might reoccupy the synaptic territory vacated by the ipsilateral entorhinal fibers. By placing lesions in the remaining contralateral entorhinal cortex, and tracing the resulting degeneration products, we show that the contralateral entorhinal projection is reorganized, establishing an anomalous terminal projection to the dentate gyrus deafferented by the initial lesion. The result is a contralateral entorhinal innervation of the dentate gyrus which normally receives only ipsilateral entorhinal afferents.In addition, we investigate the functional capacity of these anomalous contralateral projections. We show that stimulation of the entorhinal cortex in the normal animal never results in short latency activation of the granule cells contralateral to the stimulating electrode, whereas in the lesioned animals, the contralateral entorhinal stimulation results in a short latency, apparently monosynaptic, evoked potential in the dentate gyrus which had been deprived of its ipsilateral entorhinal innervation. Furthermore, this stimulation results in the discharge of cells in the granule cell layer.Therefore, following unilateral entorhinal lesion, the remaining contralateral entorhinal cortex extends its efferent projection to establish electrophysiologically functional synapses with the granule cells deafferented by the initial lesions.The material in this paper was included in a dissertation submitted in partial fulfillment of the requirements for the degree of Doctor of Philosophy at the University of California at Irvine, Irvine Calif.