GAD67-positive puncta: Contributors to learning-dependent plasticity in the barrel cortex of adult mice

We have previously shown that a classical aversive conditioning paradigm involving stimulation of a row of facial vibrissae (whiskers) in the mouse produced expansion of the cortical representation of the activated vibrissae (“trained row”). This was demonstrated by labeling with 2-deoxyglucose (2DG) in layer IV of the barrel cortex. We have also shown that functional reorganization of the S1 cortex is accompanied by increases in the density of small GABAergic cells, and in GAD67 mRNA in the hollows of barrels representing the “trained row”. The aim of this study was to determine whether GAD67-positive puncta (boutons) are affected by learning. Unbiased optical disector counting was applied to sections from the mouse barrel cortex that had been immunostained using a polyclonal antibody against GAD67. Quantification of the numerical density of GAD67-positive boutons was performed for four groups of mice: those that had been given aversive conditioning, pseudoconditioned mice with random application of the unconditioned stimulus, mice that had received only whisker stimulation, and naive animals. This study is the first to demonstrate that learning-dependent modification of mature somatosensory cortex is associated with a 50% increase in GAD67-positive boutons in the hollows of “trained” barrels compared with those of control barrels. Sensory learning seems to mobilize the activity of the inhibitory transmission system in the cortical region where plastic changes were previously detected by 2DG labeling.