IPSILATERAL ASSOCIATIONAL PATHWAY IN THE DENTATE GYRUS - AN EXCITATORY FEEDBACK-SYSTEM THAT SUPPORTS N-METHYL-D-ASPARTATE-DEPENDENT LONG-TERM POTENTIATION

Axons from granule cells in the dentate gyrus of the rat hippocampus p roject to cells in the hilar region, including messy cells, which proj ect along the longitudinal axis of the hippocampus and synapse in the inner (proximal) one-third of the molecular layer of the dentate gyrus . To study this feedback system, multiple recording electrodes were lo cated along the longitudinal (septo-temporal) axis in the dorsal leaf of the dentate gyrus in urethane-anesthetized rats. Single pulse elect rical stimuli delivered to the hilar region evoked negative-going, mon osynaptic field potentials that were largest in the inner one-third of the molecular layer (commissural zone). These evoked field potentials (EFPs) were recorded simultaneously at three to five locations. The l atency to onset and peak amplitude of the EFP varied linearly with dis tance from point of stimulation, and EFPs were elicited in both direct ions along the longitudinal axis. The transmission speed was estimated to be 1.4 m/s. Tetanic stimulation of the hilar region potentiated th e EFP slopes (mean = 26%). Potentiation lasted at least 2 hours and wa s specific to responses from the tetani:red stimulating electrode; the responses to other stimulating electrodes in the hilus and the angula r bundle of the perforant path changed less than 4%. Combined stimulat ion of the hilus and the medial perforant path increased the magnitude of recorded field potentials and population spikes, demonstrating tha t both pathways are excitatory. NMDA antagonist NPC-17742 blocked pote ntiation of EFP slopes in both the medial perforant path and hilus pat hways. The results suggest that tile ipsilateral associational system of the dentate gyrus is excitatory and capable of supporting long-last ing, NMDA-dependent, synapse-specific plasticity. (c) 1994 Wiley-Liss, Inc.