Memory for the ‘what’, when’ and ‘where’ of serial events, termed ‘episodic memory’, is a critical element inhuman cognition and is particularly disturbed in conditions of congenital intellectual disability (ID) includingautism spectrum disorders (ASDs). The encoding of episodic-like memory depends upon the entorhinal cortexwith medial (MEC) and lateral (LEC) fields supporting processing of spatial and non-spatial memories,respectively. With the goal of understanding the neurobiological processes contributing to ID in autismdisorders and other forms of congenital cognitive dysfunction, we have evaluated mechanisms of transmissionand enduring synaptic plasticity in LEC projections to hippocampus in the Fmr1 KO mouse model for Fragile-XSyndrome (FXS), the most common inherited form of ID which is also co-morbid for autism. Our results showthat Fmr1 KOs have particularly severe deficits in the expression of Long-Term Potentiation (LTP) in the LEC-hippocampal connection (the lateral perforant path, LPP) and fail to learn episodic memory tasks that, in wildtype (WT) mice, depend upon the LPP. Proposed studies build on these results with goals to identifymechanisms underlying the failure of LTP in Fmr1 KOs and to test manipulations predicted to rescue bothpotentiation and episodic memory. The project takes advantage of our recent discovery that LTP in the LPPinvolves novel substrates: LPP potentiation is induced postsynaptically but expressed presynaptically, viaincreased transmitter release, with the endocannabinoid (eCB) 2-arachidonoylglycerol (2-AG) as the criticalretrograde messenger. The presynaptic adjustments underlying this eCB-dependent LTP (ecLTP) involve CB1–mediated signaling and cytoskeletal reorganization within LPP terminals. Collectively, the results suggest thatencoding episodic memories depends upon an unusual, pathway specific-form of synaptic plasticity. The threespecific aims will test the hypothesis that mechanisms of this ecLTP are severely impaired in Fmr1 KO mice,thus (i) accounting for disturbances in episodic memory and (ii) identifying therapeutic targets to improvelearning in this and potentially other forms of ID associated with autism. Aim 1 will identify postsynapticprocesses required for ecLTP that are defective in Fmr1 KOs: this aim builds upon preliminary resultsindicating that on-demand production of 2-AG is impaired in Fmr1 KO mice. Aim 2 will test if presynapticevents that regulate the expression of ecLTP are impaired in the KOs and, in particular, if there aredisturbances in the regulation of 2-AG breakdown and CB1 signaling to actin. Finally, Aim 3 will test if in Fmr1KOs ecLTP, and episodic memory that depends upon it, are rescued by manipulations that enhance 2-AGlevels. These studies use a new learning paradigm that tests the `what' component of episodic memory forwhich WT learning depends on the LPP and Fmr1 KO encoding is severely impaired. Together, results willprovide unique insights into the bases of disturbances in episodic memory with congenital ID and identify noveltherapeutic targets, and candidate treatments, for enhancement of this specific component of cognition.
Interagency Autism Coordinating Committee (IACC)
Autism Research Database (AFD)
