Rapid, experience-dependent changes in levels of synaptic zinc in primary somatosensory cortex of the adult mouse

Abstract

Electrophysiological studies have established that the adult cerebral cortex undergoes immediate functional reorganizations after perturbations of the sensory periphery. These activity-dependent modifications are thought to be mediated via the rapid regulation of the synaptic strength of existing connections. Recent studies have implicated synaptic zinc as contributing to activity-dependent mechanisms of cortical plasticity, such as long-term potentiation and long-term depression, by virtue of its potent ability to modulate glutamatergic neurotransmission. To investigate the role of synaptic zinc in cortical plasticity, we examined changes in the barrel-specific distribution of zinc in axon terminals innervating the primary somatosensory cortex of adult mice at different time points after whisker plucking. In layer IV of normal adult mice, zinc staining in the barrel field was characterized by intense staining in inter-barrel septae and low levels of staining in barrel hollows. Within 3 hr, and up to 1 week after the removal of a row of whiskers, zinc staining increased significantly in barrel hollows corresponding to the plucked whiskers. With longer survival times, levels of zinc staining gradually declined in deprived barrel hollows, returning to normal levels by 2-3 weeks after whisker removal. Increased levels of zinc staining in deprived barrel hollows were highly, negatively correlated with the length of whiskers as they regrew. These results indicate that levels of synaptic zinc in the neocortex are rapidly regulated by changes in sensory experience and suggest that zinc may participate in the plastic changes that normally occur in the cortex on a moment-to-moment basis.

Fig. 1.

Distribution of histochemically reactive zinc in a section through layer IV of the somatosensory cortex in control mice (A). There are five rows (A–E) of whisker-related cortical barrels that delineate the location of the posteriomedial barrel subfield in the somatosensory cortex. Barrel compartments are characterized by high levels of zinc staining in inter-barrel septae and low levels of staining in barrel hollows. A, Anterior;P, posterior; L, lateral;M, medial. Scale bar, 500 μm. B, Mean (±SEM) zinc-staining intensity for each row relative to all other rows. Note that barrel rows A, B, andE have positive percentage difference values indicating that, on average, the staining intensity of these rows was greater than the mean staining intensity of the other rows. By contrast, negative percentage difference scores corresponding to rows C andD indicate that the level of zinc staining in these rows was lower than the other rows.

Fig. 2.

Changes in zinc staining in row C at survival times ranging from 3 hr to 2 weeks after the removal of whiskers from row C of the contralateral face. The position of row C is indicated by the white arrow in each panel. Subtle increases in zinc staining were apparent in row C within 3 hr of whisker removal (A). At 6 (B), 12 (C), and 24 hr (D), and 1 week (E) after whisker removal, levels of zinc staining in the deprived row C were robustly increased relative to adjacent nondeprived barrel rows. Two weeks after whiskers were removed (F), levels of zinc staining in row C were normal. Scale bar, 500 μm.

Fig. 3.

Quantitative analysis of zinc staining in barrel row C at different survival times after removal of row C whiskers. Relative to control mice, levels of zinc staining increased significantly at 3, 6, 12, and 24 hr, and 1 week after whisker plucking. However, with longer survival times (2 and 3 weeks), the level of zinc staining in row C was not significantly different from baseline levels. *p < 0.05; **p < 0.001.

Fig. 4.

Alterations to zinc staining levels in somatosensory cortex 24 hr after other patterns of whisker removal. Animals either had rows A, B, and C removed (A) or had all but the C2 whisker removed (B). Zinc staining in cortical barrels associated with plucked whiskers (white arrows) appeared much darker than that observed for nondeprived barrels (black arrows). At higher magnification, zinc staining appeared punctate in both nondeprived (C) and deprived (D) barrel hollows. In deprived barrel hollows (D), zinc-stained punctae appeared much more numerous and more densely clustered than that observed in the nondeprived barrel hollow (C). Scale bar (shown in B):A, B, 500 μm; (shown inD): C, D, 20 μm.

Fig. 5.

Scatterplot showing the inverse relationship between levels of zinc staining in the deprived barrel hollow and the length of the regrowing whisker. This significant correlation (r² = 0.66;p < 0.001) demonstrates that higher levels of zinc staining in deprived barrel hollows are associated with shorter whiskers, whereas fully regrown whiskers are associated with a return of zinc staining to normal levels.