Lesioning the fimbria-fornix impairs basolateral amygdala induced reinforcement of LTP in the dentate gyrus

We have recently shown that early-long-term potentiation (LTP) in the denta te gyrus can be reinforced into late LTP by stimulation of the basolateral nucleus of the amygdala [Frey et al., submitted for publication]. The pathw ays and mechanisms for such interactions are unclear, considering that no d irect projection from the amygdala to the dentate gyrus is known. To ascert ain the possible mediation of the septo-hippocampal projection we have tran sected the fimbria-fornix (FF) fiber system in young adult (2 months) male rats. The electrophysiological evaluation a week later showed that the lesi on does not modify the effects of pre-stimulation of the basolateral amygda la (BLA) on the induction of LTP at the perforant pathway (PP)-granule cell s synapses, but impairs its maintenance I h later. This suggests that two d ifferent pathways might mediate different aspects of the amygdala-hippocamp al interactions. One seemed to be anatomically independent from the FF and might influence LTP induction; while the second, probably through the septo -hippocampal fornical projection appeared important for LTP maintenance. (C ) 2000 Elsevier Science B.V. All rights reserved.