Infant rats can acquire, but not retain contextual associations in object-in-context and contextual fear conditioning paradigms.

Abstract

Context learning in postnatal day (PD) 16-18 rats has been taken by Revillo, Cotella, Paglini, and Arias (2015, Physiology & Behavior, 148, 6-21) to challenge the view that the ontogeny of contextual learning is related to the development of the hippocampal system (Rudy, 1993, Behavioral Neuroscience, 107(5), 887-891; Schiffino, Murawski, Rosen, & Stanton, 2011 Neurobiology of Learning and Memory, 95(2), 190-198). Whether context learning is "incidental" or "reinforcement-driven" may determine the ontogeny and neural systems involved (Rudy, 2009, Learning & Memory (Cold Spring Harbor, N.Y.), 16, 573-585). However, we have shown differential ontogeny of two different forms of incidental context learning, the context pre-exposure facilitation effect (CPFE; Jablonski, Schiffino, & Stanton,2012, Developmental Psychobiology, 54(7), 714-722), which emerges between PD 17 and 21; and object-in-context recognition (OiC, Ramsaran, Westbrook, & Stanton, 2016, Developmental Psychobiology, 58(7), 883-895; Ramsaran, Sanders, & Stanton, 2016, Behavioural Brain Research, 298, 37-47), which is present on PD17. We investigated whether this task-dissociation reflects an encoding or a retention deficit, by varying the sample-to-testing intervals for both tasks. Experiment 1A found that PD17 rats were able to perform the OiC task after short (5min) but not long (24hr) sample-to-test intervals. Experiments 1B and 1C found that PD17 rats trained on the CPFE are able to acquire and express context-shock associations after short but not long retention intervals. These findings suggest that pre-weanling rats encode contexts but show poor consolidation or retrieval after longer retention intervals.