Consolidation of passive avoidance learning is associated with transient increases of polysialylated neurons in layer II of the rat medial temporal cortex

Within the rat medial temporal lobe, transient modulations of neural cell a dhesion molecule (NCAM) polysialylation have been observed to follow spatia l learning. These have been attributed to neuroplastic events associated wi th the processing of information destined for long term memory consolidatio n. To determine if similar events are associated with avoidance learning, w e investigated change in polysialylated cell number in the entorhinal, peri rhinal, and piriform cortex, following acquisition of a passive avoidance t ask in the rat. Direct quantification of polysialylated neurons in layer IZ of these cortical regions revealed a significant increase in polysialylate d cell frequency at 12 h following passive avoidance training. Unlike spati al learning, the increased expression of polysialylated neurons persisted f or up to 24-48 h following training. In the more dorsal aspect of the perir hinal/entorhinal cortex, this increase was found to be specific to learning , as it was not observed in animals rendered amnesic with scopolamine, By c ontrast, change in polysialylated cell frequency in the ventral aspect of t he medial temporal lobe was only partially reduced by amnesic doses of scop olamine, The persisting activation of NCAM polysialylation in the more dors al aspects of the perirhinal and entorhinal cortex is suggested to reflect the need for more extensive synaptic alterations, as compared to those requ ired for the consolidation of spatial learning. Moreover, the neuroplastic modulations observed in the more ventral regions of the entorhinal and peri rhinal cortex appear to be a unique aspect of avoidance conditioning that r eflects the activation of alternative learning strategies associated with m otivational and/or contextual parameters of the task. (C) 2000 John Wiley & Sons. Inc.