Arousal from hibernation alters contextual learning and memory

Hibernation is a unique and highly regulated physiological state characterized by profound, albeit periodically reversible, depression in body temperature, metabolism, and consciousness. Hippocampal synapses undergo pronounced remodeling in concert with torpor and arousal. During hibernation, the number of postsynaptic densities, apical dendritic branches, and spine densities decreases substantially in the hippocampus. Upon arousal these parameters increase beyond pre-hibernation levels and peak within 2-3 h. By 24 h after arousal, dendritic parameters remain elevated but have started to subside, consistent with pruning and differentiation. The present study examined the functional consequences of these natural changes in synaptic structure. Wild-caught Arctic ground squirrels (AGS) were trained in a hippocampal-dependent contextual fear conditioning task at 3 h, 24 h, or 4 weeks after arousal (warm-adapted euthermic control group). All groups acquired the fear conditioned response similarly on the training day. During a subsequent retention test session, AGS in the 24 h group exhibited enhanced expression of contextual fear compared to the other two groups. These data suggest that the morphological and biochemical changes occurring at 24 h after arousal from hibernation affect hippocampal-dependent learning and memory. The natural change in synaptic structure during hibernation may provide a unique opportunity to assess the neural substrates underlying cognitive enhancement. (c) 2005 Elsevier B.V. All rights reserved.