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 ␣ 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.

In primary sensory cortices, neuronal circuits change throughout life as a function of learning. During associative learning a neutral sensory stimulus acquires the emotional valence of an aversive event or a reward after repetitive contingent pairing. One important consequence is the enlargement of the representational area of the conditioned stimulus in the cortical map of its sensory modality. The details of this phenomenon at the circuit level are still largely unknown. Here, mice were trained in a differential conditioning paradigm where the deflections of one whisker row were paired with tail shocks and the deflections of two others were not. Changes occurring in excitatory circuits of barrel cortex were then examined in brain slices with laser scanning photostimulation mapping. We found that learning affected the projections targeting the supragranular layers in the columns of unpaired whiskers: Pyramidal cells located in layer (L) 3 received enhanced inputs from L5A cells located in their home column and new inputs from L2/3 and L4 cells located in the neighboring column of the paired whisker. In contrast, the excitatory projections impinging onto L2/3 cells in the column of the paired whisker were not altered. Together, these data reveal that associative learning alters the canonical columnar organization of functional ascending L4 projections and strengthens transcolumnar excitatory projections in barrel cortex. These phenomena could participate to the transformation of the whisker somatotopic map induced by associative learning.

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Contiguous skin surfaces that tend to be synchronously stimulated are represented in neighbouring sectors of primary somatosensory maps. Moreover, neuronal receptive fields (RFs) are reshaped through ongoing competitive/cooperative interactions that segregate/desegregate inputs converging onto cortical neuronal targets. The present study was designed to evaluate the influence of spatio-temporal constraints on somatotopic map organization. A vascularized and innervated pedicle flap of the ventrum skin bearing nipples was rotated by 180 degrees . Electrophysiological maps of ventrum skin were elaborated in the same rats at 24 h after surgery and 2 weeks after parturition. Neurones with split RFs resulting from the surgical separation of formerly adjoining skin surfaces were more numerous in non-nursing than nursing rats. RFs that included newly adjacent skin surfaces on both sides of the scar line emerged in nursing rats, suggesting that the spatial contiguity of formerly separated skin surfaces induced a fusion of their cortical representations through nursing-induced stimulation. In addition, nursing-dependent inputs were found to reincorporate the rotated skin flap representation in an updated topographical organization of the cortical map. A skin territory including recipient and translocated skin areas was costimulated for 7 h, using a brushing device. Neural responses evoked by a piezoelectric-induced skin indentation before and after skin brushing confirmed the emergence of RFs crossing the scar line and contraction of non-brushed components of split RFs. Our findings provide further evidence that the spatiotemporal structure of sensory inputs changing rapidly or evolving in a natural context is critical for experience-dependent reorganization of cortical map topography.