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

A CRE/DRE dual recombinase transgenic mouse reveals synaptic zinc-mediated thalamocortical neuromodulation

Synaptic zinc is a neuromodulator that shapes synaptic transmission and sensory processing. The maintenance of synaptic zinc is dependent on the vesicular zinc transporter, ZnT3. Hence, the ZnT3 knockout mouse has been a key tool for studying the mechanisms and functions of synaptic zinc. However, the use of this constitutive knockout mouse has notable limitations, including developmental, compensatory, and brain and cell type specificity issues. To overcome these limitations, we developed and characterized a dual recombinase transgenic mouse, which combines the Cre and Dre recombinase systems. This mouse allows for tamoxifen-inducible Cre-dependent expression of exogenous genes or knockout of floxed genes in ZnT3-expressing neurons and DreO-dependent region and cell type-specific conditional ZnT3 knockout in adult mice. Using this system, we reveal a neuromodulatory mechanism whereby zinc release from thalamic neurons modulates N-methyl-d-aspartate receptor activity in layer 5 pyramidal tract neurons, unmasking previously unknown features of cortical neuromodulation.

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.

Pathway-specific utilization of synaptic zinc in the macaque ventral visual cortical areas

Synaptic zinc is an activity-related neuromodulator, enriched in hippocampal mossy fibers and a subset of glutamatergic cortical projections, exclusive of thalamocortical or corticothalamic. Some degree of pathway specificity in the utilization of synaptic zinc has been reported in rodents. Here, we use focal injections of the retrograde tracer sodium selenite to identify zinc-positive (Zn+) projection neurons in the monkey ventral visual pathway. After injections in V1, V4, and TEO areas, neurons were detected preferentially in several feedback pathways but, unusually, were restricted to deeper layers without involvement of layers 2 or 3. Temporal injections resulted in more extensive labeling of both feedback and intratemporal association pathways. The Zn+ neurons had a broader laminar distribution, similar to results from standard retrograde tracers. After anterograde tracer injection in area posterior TE, electron microscopic analysis substantiated that a proportion of feedback synapses was co-labeled with zinc. Nearby injections, Zn+ intrinsic neurons concentrated in layer 2, but in temporal areas were also abundant in layer 6. These results indicate considerable pathway and laminar specificity as to which cortical neurons use synaptic zinc. Given the hypothesized roles of synaptic zinc, this is likely to result in distinct synaptic properties, possibly including differential synaptic plasticity within or across projections.