Molecular Control of Circuit Plasticity and the Permanence of Imprinted Odor Memory

Behavioral imprinting is a distinct form of learning that has a lifelong impact on social interactions and affectional behaviors1-4. Unlike other forms of memory, imprinting does not require conspicuous association of stimuli; exposure per se appears sufficient to induce memories that neither undergo extinction nor are altered by experience later in life. The site of storage of imprinted memory and the mechanisms that control its formation and permanence are unknown. Here we uncover a molecular mechanism that controls olfactory imprinting, which underlies behaviors including kin and nest recognition, maternal attachment, and homing5-10. We show that odor exposure during the perinatal period converts an innately aversive odor into a homing signal. The behavioral change is associated with odor-induced changes in the projection patterns of olfactory sensory neuron (OSN) expressing the cognate receptors for the exposed odor. We show that the Wnt signaling receptor Frizzled1 (Fzd1) acts as a master regulator of the critical period of OSN development and is responsible for closing the critical period to prevent further changes in the neural circuit. In Fzd1 knockout mice axon projection patterns are continually modified by sensory experience. As Fzd1 knockout abolishes the developmental critical period, it also abolishes odor imprinting. Specific knockout of Fzd1 in the OSNs have the same effect. Mechanistically, Fzd1 controls the critical period through an autoregulated shutdown and by controlling an activity-driven regulon in the OSNs. The transient expression and the subsequent downregulation of Fzd1 leads to the irreversible closure of the critical period to lock in circuits established during the critical period. The evidence suggests that imprinted odor memory is stored in the patterns of connectivity at the first synapse in the olfactory bulb. Early odor experience induces changes in the OSN projection to alter connectivity with innate circuits to establish a life-long memory.