UC San Diego

The fundamental question that motivates this thesis is the elusive physical basis of memory. Despite substantial research advances over the last decades, the cellular and molecular mechanisms that allow the brain to create, store and recall memories remain largely unknown, mainly due to the lack of techniques to test the causal role of learning-induced modifications in vivo. We propose an approach to identify an engram - or memory trace - for contextual fear conditioning, a paradigm that involves associating a neutral context (conditioned stimulus, CS) with mild foot shocks (unconditioned stimulus, US), producing an aversive memory to the context. Our approach for identifying an engram is based on three criteria: (1) an observable learning-induced change that (2) once disrupted impairs memory; and (3) if artificially induced can produce a memory independent of associative training. Our hypothesis is that synaptic potentiation in neurons of the basolateral amygdala, a known fear center that responds to both CS and US stimuli, represents an engram for context conditioning. We found that conditioning produced potentiation of excitatory synapses that lasted for at least 7 days specifically onto basolateral amygdala neurons activated during learning (1). We developed a method to disrupt memories by reversing synaptic potentiation specifically in learning-activated neurons using a mutated form of CaMKIIα that induces synaptic depression, demonstrating a causal role between synaptic plasticity and memory recall (2). Lastly, we generated an artificial fear association, without associative Pavlovian conditioning, by high-frequency optogenetic stimulation of the CS and US-activated ensembles or biochemical induction of synaptic potentiation in US-responsive neurons, demonstrating the sufficiency of synaptic potentiation of amygdala neurons in producing context fear memories (3). These results suggest that a necessary and sufficient step in forming context fear associations is plasticity of CS inputs onto US-responsive basolateral amygdala neurons and that context discrimination is determined by the CS-specific amygdala inputs activated during retrieval. Synaptic plasticity found elsewhere in the hippocampus and other cortical regions during context conditioning might be more involved in context encoding, modulating the response to the aversive memory stored in the basolateral amygdala.