Abstract
Rats were chronically implanted with stimulation electrodes in the perforant pathway (pp) bilaterally and a recording electrode in the dentate gyrus (DG) unilaterally. Evoked field potentials (EPs) were recorded upon alternating stimulation of the pp on both sides, and long‐term potentiation (LTP) was induced. Besides the EP after ipsilateral stimulation, an EP with a latency of approximately 5.5–6.5 ms was also seen upon stimulation of the contralateral pp. This potential was reversibly abolished during pentobarbital anesthesia and irreversibly after lesioning of the ipsilateral angular bundle. Paired‐pulse facilitation and paired‐pulse depression, depending on interstimulus interval and intensity, were also observed. Therefore, this long‐latency potential could be characterized as polysynaptic and induced perhaps by transsynaptic activation via the ipsilateral entorhinal cortex. Ipsilateral tetanization induced strong E/S potentiation of both the ipsilaterally and contralaterally evoked EP, but with different time courses. Tetanization of the contralateral pp did not induce LTP of the ipsilaterally induced EP in the first 4 h. But afterwards a late and slowly developing potentiation occurred. The contralaterally induced EP also showed potentiation of the population spike, which was not immediately detectable but developed slowly over time. The results can be interpreted such that, after stimulation of the pp, the DG on the opposite side cannot only be activated via the weak crossed entorhinal projection but also transsynaptically via an entorhino/entorhinal connection. Hippocampus 2001;11:157–167. © 2001 Wiley‐Liss, Inc.