Experience-dependent plasticity of zinc-containing cortical circuits during a critical period of postnatal development.

Distinctive subsets of glutamatergic neurons in cerebral cortex sequester the transition metal zinc within the synaptic vesicles of their axon terminals. In the present study we used histochemical localization of synaptic zinc to investigate normal postnatal development and experience-dependent plasticity of zinc-containing circuits in somatosensory barrel cortex of rats. First, we found that zinc-containing cortical circuits are dynamically reorganized between postnatal day (P) 0 and P28. Whereas most cortical laminae exhibited idiosyncratic increases in zinc histochemical staining with advancing age, lamina IV barrels were darkly reactive early in life and then lost much of their complement of synaptic zinc during postnatal weeks 2-4. Second, we established that sensory experience plays a major role in sculpting the zinc-containing innervation of cortical barrels. Trimming a particular facial whisker arrested the normal postnatal decline in synaptic zinc in the corresponding, deprived barrel. This resulted in more intense zinc staining in deprived barrels compared with adjacent, nondeprived barrels. Notably, the influence of experience on development of zinc circuits was most robust during a critical period extending from about P14, when an effect of whisker trimming first could be observed, through P28, after which time chronic deprivation no longer resulted in heightened levels of synaptic zinc in lamina IV. These findings indicate that sensory input can have a marked influence on development of cortical circuits, including those within lamina IV, throughout the first postnatal month.