Potentiation of excitatory synaptic transmission in the normal and in the reinnervated dentate gyrus of the rat.

Following destruction of the ipsilateral temporo-ammonic tract, which originates in the entorhinal cortex, and terminates on the granule cells of the dentate gyrus, fibers from the surviving contralateral entorhinal area proliferate forming extensive new connections with the denervated dentate granule cells. Utlizing extracellular recording techniques, we have compared the characteristics of synaptic transmission in the lesion induced afferents with the characteristics of the normal ipsilateral afferents by analyzing the responses of dentate granule cells to paired pulse activation of temporo-dentate circuitry. In the dentate gyrus of the normal rat, and extracellularly recorded EPSP evoked by stimulation of the ipsilateral entorhinal cortex is enhanced by as much as 100% by a "conditioning" pulse to the same afferent system. This is called paired pulse potentiation. In the reinnervated dentate gyrus, the extracellular EPSP evoked by a test stimulus delivered to the contralateral entorhinal cortex is also potentiated by a conditioning pulse. Thepaired pulse potentiation in the reinnervated dentate gyrus has a time course which is comparable to that of the normal ipsilateral afferent system, but the magnitude of the potentiation is somewhat less, averaging approximately 140% of control...