Switch time-point for rapid experience-dependent changes in zinc-containing circuits in the mouse barrel cortex

Zinc histochemistry reveals circuit refinement and distinguishes visual areas in the developing ferret cerebral cortex

A critical question in brain development is whether different brain circuits mature concurrently or with different timescales. To characterize the anatomical and functional development of different visual cortical areas, one must be able to distinguish these areas. Here, we show that zinc histochemistry, which reveals a subset of glutamatergic processes, can be used to reliably distinguish visual areas in juvenile and adult ferret cerebral cortex, and that the postnatal decline in levels of synaptic zinc follows a broadly similar developmental trajectory in multiple areas of ferret visual cortex. Zinc staining in all areas examined (17, 18, 19, 21, and Suprasylvian) is greater in the 5-week-old than in the adult. Furthermore, there is less laminar variation in zinc staining in the 5-week-old visual cortex than in the adult. Despite differences in staining intensity, areal boundaries can be discerned in the juvenile as in the adult. By 6 weeks of age, we observe a significant decline in visual cortical synaptic zinc; this decline was most pronounced in layer IV of areas 17 and 18, with much less change in higher-order extrastriate areas during the important period in visual cortical development following eye opening. By 10 weeks of age, the laminar pattern of zinc staining in all visual areas is essentially adultlike. The decline in synaptic zinc in the supra- and infragranular layers in all areas proceeds at the same rate, though the decline in layer IV does not. These results suggest that the timecourse of synaptic zinc decline is lamina specific, and further confirm and extend the notion that at least some aspects of cortical maturation follow a similar developmental timecourse in multiple areas. The postnatal decline in synaptic zinc we observe during the second postnatal month begins after eye opening, consistent with evidence that synaptic zinc is regulated by sensory experience.

Chronic treatment with zinc and antidepressants induces enhancement of presynaptic/extracellular zinc concentration in the rat prefrontal cortex

Zinc exhibits antidepressant-like activity in preclinical tests/models. Moreover, zinc homeostasis is implicated in the pathophysiology of affective disorders. The aim of the present study was to examine the effect of chronic zinc, citalopram and imipramine intraperitoneal administration on the presynaptic and extracellular zinc concentration in the rat prefrontal cortex and hippocampus. We used two methods: zinc-selenium histochemistry (which images the pool of presynaptic-vesicle zinc) and anodic stripping voltammetry (ASV) for zinc determination in microdialysate (which assays the extracellular zinc concentration). We report that chronic (14 ×) zinc (hydroaspartate, 10 and 65 mg/kg) and citalopram (20 mg/kg) administration increased the pool of presynaptic zinc (by 34, 50 and 37%, respectively) in the rat prefrontal cortex. The 21% increase induced by imipramine (20 mg/kg) was marginally significant. Likewise, zinc (hydroaspartate, 65 mg/kg), citalopram and imipramine increased the extracellular zinc (although with a different pattern: time point, area under the curve and/or basal level) in this brain region. Furthermore, zinc induced an increase in presynaptic (by 65%) and extracellular zinc (by 90%) in the hippocampus, while both citalopram and imipramine did not. These results indicate that all of the treatments increase presynaptic/extracellular zinc concentrations in the rat prefrontal cortex, which may then contribute to their antidepressant mechanisms. Alterations induced by zinc (but not antidepressants) administration in the hippocampus may be related to specific zinc mechanisms. All the data (previous and present) on the effect of antidepressant treatments on the presynaptic/extracellular zinc concentrations suggest the involvement of this biometal presynaptic/synaptic homeostasis in the antidepressant mechanism(s).

Transient synaptic zinc-positive thalamocortical terminals in the developing barrel cortex

In rat barrel cortex, layer 4 has a transiently high density of zinc-positive terminations from postnatal day (P)9 to P12 [P.W. Land & L. Shamalla-Hannah (2002)J. Comp. Neurol., 447, 43-56]. These terminations have been proposed to originate from cortico-cortical connections, but their exact origin is unknown. To determine their sources, we injected sodium selenite into the barrel cortex of two adult rats and 32 pups, from P5 to P28. As predicted, abundant zinc-positive cortically projecting neurons were visible around the injection sites and in distant cortical areas. From P9 to P13, however, neurons retrogradely labeled by zinc selenite occurred in the thalamus, in topographically appropriate regions of the ventroposterior medial (VPM) and posterior nuclei (Po). Because there are no previous reports of zinc-positive sensory thalamocortical connections, we sought corroboration of this unexpected finding by electron microscopy. This revealed a subset of boutons in layers 4 and 1, positive for both zinc and vesicular glutamate transporter 2, a protein used by thalamocortical terminations. Finally, in an additional nine rats, we carried out in situ hybridization for zinc transporter 3 mRNA. Moderate signal was detected in VPM and Po at P10, but this disappeared by P28. In contrast, a strong signal was apparent in the anterodorsal nucleus, which projects to limbic areas, and this persisted at P28. The timing of the transient zinc-positive terminations in the sensory thalamus roughly coincides with the onset of exploratory and whisking behavior in the middle of the second postnatal week; and this suggests zinc is important for activity-related refinement of circuitry.

Increase in synaptic hippocampal zinc concentration following chronic but not acute zinc treatment in rats

Electroconvulsive seizures (ECS), one of the most effective treatments of depression, induce mossy fiber sprouting (when assayed by means of synaptic zinc method), and this indicates an increase in the synaptic zinc level in the hippocampus following such therapy. The aim of the present study was to investigate the influence of acute and chronic zinc hydroaspartate administration on the synaptic and total zinc level in the rat hippocampus. We used two methods of zinc determination: (1) zinc-selenium method, which images the pool of synaptic zinc, and (2) flame atomic absorption spectrometry, which assays the total concentration of zinc. Our results indicate that chronic (14 x 65 mg/kg), but not acute, zinc hydroaspartate administration intraperitoneally (i.p.) increases the pool of synaptic zinc in the majority of rat hippocampal layers (by 72-190%), except for the stratum moleculare and stratum radiatum CA, and perforant path DG. On the other hand, no changes were found in total hippocampal zinc level, measured by flame atomic absorption spectrometry. These data suggest that chronic zinc treatment increases the pool of synaptic zinc in the hippocampus, and this effect is similar to that observed following chronic ECS treatment. The measurement of zinc concentration in the whole hippocampus by the flame atomic absorption spectrometry method is not sensitive enough to detect such subtle alteration.

Zinc-rich transient vertical modules in the rat retrosplenial cortex during postnatal development

The rat retrosplenial cortex is part of a heavily interconnected limbic circuit, considered to have an important role in spatial memory. Interestingly, the granular retrosplenial cortex has an exceptionally distinct system of dendritic bundles, originating from callosally projecting pyramidal neurons in layer II. These can be detected as early as postnatal day 5; and, although their functional significance remains to be elucidated, the existence of these bundles makes the granular retrosplenial cortex an attractive model system for a wide range of development and functional investigations. Here, we report four results concerning the development of modularity in the granular retrosplenial cortex in rats as investigated by neurochemical markers associated to cortico-cortical and thalamo-cortical connections. Emphasis is placed on zinc, an activity-related substance associated with glutamatergic, non-thalamic terminations. 1) Zinc shows a transient strong expression during early postnatal development, but later than the appearance of the upper layer bundles (at postnatal day 5). By postnatal day 11 to postnatal day 15 staining for zinc achieved its most complex pattern; such that layer I had an elaborate organization both in the tangential and radial dimensions. Three sublaminae were distinguished (layers Ia-c): a superficial, thin tier (Ia) with patchy, moderate staining which periodically intruded into the underlying layer Ib ("funnel" modules), a middle band of variable width and light staining (Ib), and a deep, thin band with heavy and patchy staining (Ic) which, at rostral levels, spread upward into layer Ib (as "dome-like" modules). 2) At postnatal day 15, immunohistochemical methods showed that layers Ia, b zinc-funnels were co-localized with glutamate receptor subunits 2/3, GABA receptor type A alpha1 subunit and the thalamo-cortical marker, vesicular glutamate transporter 2. Layer Ic and the zinc dome-like modules were co-labeled for the cortico-cortical marker, vesicular glutamate transporter 1 and calretinin. 3) The spatial coincidence between zinc funnels in layers Ia, b and vesicular glutamate transporter 2 was further investigated by electron microscopy, which demonstrated co-localization of zinc and vesicular glutamate transporter 2 in synaptic boutons. The unusual co-localization of zinc and thalamo-cortical terminations was confirmed by retrograde transport of zinc to neurones in the anterodorsal thalamic nucleus at postnatal day 9 and postnatal day 13, and can thus be considered a transient zinc expression in thalamo-cortical boutons. This was not observed at postnatal day 28 or later. 4) After postnatal day 18, zinc staining started to fade in all layers. Before postnatal day 21, the heavy staining for zinc in the domes had completely disappeared. Zinc staining in layer Ia and the funnels virtually disappeared after postnatal day 28. A transient expression of zinc is reported in at least one other cortical area (layer IV of barrel cortex from postnatal day 5 to postnatal day 14, maximal at postnatal days 9-11). We conclude that the transient expression of zinc can occur in both limbic and sensory areas, and that down-regulation of zinc in cortical modules might be related to synaptic plasticity and remodeling during development.

Dissociation of synaptic zinc level and zinc transporter 3 expression during postnatal development and after sensory deprivation in the barrel cortex of mice

In the neocortex, synaptic zinc level is regulated by sensory experience. Previously, we found that trimming of mystacial vibrissae resulted in an increase of synaptic zinc level in corresponding deprived barrels in the cortex of mice. The present study focused on the relationship between synaptic zinc and zinc transporter 3 (ZnT3) protein expression in the barrel cortex of mice during postnatal development and after sensory deprivation of selected vibrissae. Using immunocytochemistry and western blot analysis, we found that ZnT3 expression is delayed as compared with the onset of synaptic zinc and presynaptic markers, such as synapsin I and synaptophysin. Further, neither long-term deprivation in young mice nor short deprivation in adult mice, that resulted in an increase of synaptic zinc level, produced alterations in ZnT3, synapsin I or synaptophysin expression in deprived barrels. These results suggest that in the barrel cortex ZnT3, synapsin I or synaptophysin are not determinant for the activity-dependent regulation of the synaptic zinc level.