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 dentate gyrus can be reinforced into late LTP by stimulation of the basolateral nucleus of the amygdala [Frey et al., submitted for publication]. The pathways and mechanisms for such interactions are unclear, considering that no direct projection from the amygdala to the dentate gyrus is known. To ascertain the possible mediation of the septo-hippocampal projection we have transected the fimbria-fornix (FF) fiber system in young adult (2 months) male rats. The electrophysiological evaluation a week later showed that the lesion does not modify the effects of pre-stimulation of the basolateral amygdala (BLA) on the induction of LTP at the perforant pathway (PP)-granule cells synapses, but impairs its maintenance 1 h later. This suggests that two different pathways might mediate different aspects of the amygdala-hippocampal 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.