Molecular marker of memory formation reveals complex mechanisms of developmental learning.

The intersection of age- and experience-dependent processes influence learning throughout development, and developmental learning can have long-term effects on behavior. Phosphorylation of ribosomal protein S6 phosphorylation (pS6) is required in active ribosomes, and new protein synthesis is a conserved mechanism that supports long term memory formation. As such, pS6 fluctuations in brain regions processing experience can provide insight into shifts in the ability for learning and memory. Juvenile male and female zebra finch songbirds (Taeniopygia guttata) perform sensory song learning in ways that affect their adult behaviors. As adults, both sexes perform song recognition learning. Both juvenile and adult types of sensory learning invoke the auditory forebrain. Prior reports established a pS6 song response in the auditory forebrain in Posthatch day 30 juvenile males but not females, and not in younger birds. This was intriguing because the experience-dependent pS6 increase tracked with the onset of the critical period for juvenile sensory song learning in males, and behavioral data indicated that females also effectively learn at P30, though they may not have a critical period. Further, by adulthood (> Posthatch day 90), both male and female auditory forebrains showed an equivalent increase in pS6 after hearing song in patterns consistent with effective recognition learning. Here, to further test the relationships between a crucial molecular marker of active learning processes and the developmental trajectory of juvenile sensory song learning and the emergence of adult-like song recognition learning, we assessed the effect of age and sex, as well as the absence of tutor experience, a manipulation that extends the critical period for developmental learning in males, on the phosphorylation of S6 within the auditory forebrain. Outcomes highlight the complexity of molecular mechanisms across developmental learning and reveal questions to be addressed by further inquiry.