Experience-dependent alteration of synaptic zinc in rat somatosensory barrel cortex

Chelatable zinc is co-localized with glutamate in the synaptic vesicles of a distinct population of telencephalic neurons. The present study used a histochemical technique to localize zinc-containing terminals within the somatosensory barrel cortex (S1) of normal adult rats and rats that had been subjected to 4-6 weeks of tactile deprivation produced by simple whisker trimming beginning either at birth or during adulthood. In normal adult rats intense staining for synaptic zinc was observed in laminae I, II/III and V. Lamina VI was moderately stained, particularly within sublamina VIa where thalamic axons terminate. Thalamorecipient barrel centers in lamina IV were distinguished by the relative absence of synaptic zinc and were bounded by darkly stained inter-barrel septa. This pattern was unchanged in rats whose whiskers were trimmed only during adulthood, suggesting that synaptic zinc constitutes a relatively stable pool that is resistant to subtle changes in sensory input. By contrast, neonatal whisker trimming resulted in a marked increase in density of synaptic zinc specifically within the centers of deprived barrels in lamina IV. This redistribution of synaptic zinc appears to be permanent since altered staining of deprived barrels persists after extended periods of tactile experience with regrown whiskers. The results in normal rats indicate that zinc-containing circuits are distributed heterogeneously within S1 where they most likely subserve intracortical vs thalamocortical processing. The altered distribution of zinc-ergic circuits following neonatal whisker trimming suggests that zinc-sequestering neurons in developing S1 are particularly sensitive to early tactile experience.