Increased number and size of dendritic spines in ipsilateral barrel field cortex following unilateral whisker trimming in postnatal rat.

The barrel field area of the primary somatosensory cortex of rodents is a fertile ground for investigating experience-dependent plasticity and its mechanisms, because the neurons in its layer IV are distributed in groups (barrels) which correspond somatotopically to the vibrissae of the contralateral facial pad. After removal of three rows of whiskers from the right facial pad of young rats during the first two postnatal months, we looked for eventual changes in dendritic spine number and morphology in the corresponding barrels ipsi- and contralateral to the deprivation. Intact littermate controls were also examined. Spine number was determined by means of the unbiased disector method in electron micrographs from serial thin sections processed for post-embedding gamma-aminobutyric acid (GABA) immunocytochemistry. The volume and surface area of spine head, surface area of postsynaptic density and length of spine neck were measured from computerized three-dimensional reconstructions. Even though there was no significant side-to-side difference in the numerical density of dendritic spines in the experimental animals, the total number of spines in the ipsilateral barrels had increased by 67%, in view of the greater thickness of layer IV on this side. Moreover, spine head volume and surface area of postsynaptic densities were increased, and the length of spine neck was reduced in the ipsilateral compared to the contralateral cortex, and similar differences were noticeable between ipsilateral and control cortex. These changes apparently involved not only the predominant population of relatively small, dendritic spines innervated by asymmetrical synaptic terminals, but also the relatively small contingent of larger spines receiving symmetrical synapses formed by GABA terminals. The most likely explanation for such ipsilateral changes was an increased use of the intact (contralateral) facial pad during postnatal life, in keeping with the notion that activation of a peripheral sensory apparatus during the early postnatal period may have profound effects on the neuronal morphology and structural design of the primary somatosensory cortex. A possible mechanism in this case might be the excessive early activation of thalamic afferents, resulting in increased production of trophic factors, such as brain-derived nerve growth factor.