Nursing-induced somatosensory cortex plasticity: temporally decoupled changes in neuronal receptive field properties are accompanied by modifications in activity-dependent protein expression.

This study is an attempt to gain insight into the malleability of representational maps in the primary somatosensory cortex in relation to the expression of proteins involved in inhibitory and excitatory neurotransmitter systems that contribute to maintain these maps in a dynamic state. Malleability of somatosensory maps is characterized by changes in the sizes of neuron receptive fields (RFs) affecting the representational grain and in the locations and submodalities of these RFs modifying the map extent. The concomitance of these alterations remains so far hypothetical. We used nursing as an evolving source of ethologically significant cutaneous stimulation. This cyclic behavior is particularly suited to investigating the time course of experience-dependent cortical changes. Electrophysiological maps of the ventrum skin were recorded twice in the same lactating rats between nursing initiation and several weeks after nursing. We found that reduction in RF size occurred earlier than map expansion. As nursing time declined, the map expansion was maintained longer than the RF sharpening. Based on this difference in time course, we compared the expression patterns of several activity-dependent proteins in relation to the RF plasticity. Western blot analysis showed an increase in glutamic acid decarboxylase expression that was concomitant with RF contraction. In contrast, NR2A subunit of NMDA and alpha calcium/calmodulin kinase type II were upregulated at times when map expansion was observed. We propose that inhibitory and excitatory plasticity mechanisms operating with different time courses may contribute to the temporal dissociation of nursing-induced RF reshaping and map expansion.