Dendritic Voltage Recordings Explain Paradoxical Synaptic Plasticity: A Modeling Study.

Experiments have shown that the same stimulation pattern that causes Long-Term Potentiation in proximal synapses, will induce Long-Term Depression in distal ones. In order to understand these, and other, surprising observations we use a phenomenological model of Hebbian plasticity at the location of the synapse. Our model describes the Hebbian condition of joint activity of pre- and postsynaptic neurons in a compact form as the interaction of the glutamate trace left by a presynaptic spike with the time course of the postsynaptic voltage. Instead of simulating the voltage, we test the model using experimentally recorded dendritic voltage traces in hippocampus and neocortex. We find that the time course of the voltage in the neighborhood of a stimulated synapse is a reliable predictor of whether a stimulated synapse undergoes potentiation, depression, or no change. Our computational model can explain the existence of different -at first glance seemingly paradoxical- outcomes of synaptic potentiation and depression experiments depending on the dendritic location of the synapse and the frequency or timing of the stimulation.

Encoding of Odor Fear Memories in the Mouse Olfactory Cortex.

Odor memories are exceptionally robust and essential for animal survival. The olfactory (piriform) cortex has long been hypothesized to encode odor memories, yet the cellular substrates for olfactory learning and memory remain unknown. Here, using intersectional, cFos-based genetic manipulations ("Fos tagging"), we show that olfactory fear conditioning activates sparse and distributed ensembles of neurons in the mouse piriform cortex. We demonstrate that chemogenetic silencing of these Fos-tagged piriform ensembles selectively interferes with odor fear memory retrieval but does not compromise basic odor detection and discrimination. Furthermore, chemogenetic reactivation of piriform neurons that were Fos tagged during olfactory fear conditioning causes a decrease in exploratory behavior, mimicking odor-evoked fear memory recall. Together, our experiments identify specific ensembles of piriform neurons as critical components of an olfactory fear memory trace.