Sensory system development influences the ontogeny of hippocampal associative coding and trace eyeblink conditioning

- Prevalence of trial event-responsive neurons in CA1 increases with age.
- Magnitude of CA1 trial event-related activity is greatest during learning.
- Magnitude of CA1 trial event-related activity increases with age.
- Proportion of event-responsive neurons decreases across training days.
- Early-developing CS modalities facilitate learning and hippocampal responding.

Until recently, it was believed that hippocampal development was the primary rate-limiting factor in the developmental emergence of hippocampal forms of learning, such as trace eyeblink conditioning (EBC). Indeed, hippocampal neuronal activity shows an age-related increase in both complexity and task responsiveness during trace EBC. However, recent work from our laboratory suggests that sensory system development may also play a role. Training with the earlier-developing somatosensory system results in an earlier emergence of trace EBC in rats, suggesting that the development of sensory input to the hippocampus may influence the development of trace EBC. The goal of the current study was to examine the activity of hippocampal CA1 pyramidal cells during acquisition of trace EBC with an early-developing somatosensory CS. Rat pups were trained with a vibration CS on postnatal days (P) 17-19, P21-23, and P24-26 while CA1 pyramidal cell activity was recorded. Results indicated that CA1 neurons show an age-related increase in responsiveness to trial events. Although the magnitude of neuronal responding showed age-related increases in activity, all three age groups demonstrated learning-related increases in firing rate magnitude and peaks in firing rate were evident both at CS onset and offset. These findings suggest that the ontogeny of trace eyeblink conditioning is related to both hippocampal and sensory system development.