Boutons Containing Vesicular Zinc Define a Subpopulation of Synapses with Low AMPAR Content in Rat Hippocampus

Abstract

Cortical regions of the brain stand out for their high content in synaptic zinc, which may thus be involved in synaptic function. The relative number, chemical nature and transmitter receptor profile of synapses that sequester vesicular zinc are largely unknown. To address this, we combined pre-embedding zinc histochemistry and post-embedding immunogold electron microscopy in rat hippocampus. All giant mossy fibre (MF) terminals in the CA3 region and approximately 45% of boutons making axospinous synapses in stratum radiatum in CA1 contained synaptic vesicles that stained for zinc. Both types of zinc-positive boutons selectively expressed the vesicular zinc transporter ZnT-3. Zinc-positive boutons further immunoreacted to the vesicular glutamate transporter VGLUT-1, but not to the transmitter gamma-aminobutyric acid. Most dendritic spines in CA1 immunoreacted to alpha-amino-3-hydroxy-5-methylisoxazole-4-propionic acid receptor (AMPAR) subunits GluR1-3 (approximately 80%) and to N-methyl-D-aspartate receptor (NMDAR) subunits NR1 + NR2A/B (approximately 90%). Synapses made by zinc-positive boutons contained 40% less AMPAR particles than those made by zinc-negative boutons, whereas NMDAR counts were similar. Further analysis indicated that this was due to the reduced synaptic expression of both GluR1 and GluR2 subunits. Hence, the levels of postsynaptic AMPARs may vary according to the presence of vesicular zinc in excitatory afferents to CA1. Zinc-positive and zinc-negative synapses may represent two glutamatergic subpopulations with distinct synaptic signalling.

Zinc transporter 2 (SLC30A2) can suppress the vesicular zinc defect of adaptor protein 3-depleted fibroblasts by promoting zinc accumulation in lysosomes

“…For instance, ZnT1 is ubiquitously expressed and targeted to the plasma membrane, where it mediates zinc efflux from the cell [17], and its elimination by targeted disruption of the Znt1 gene in mice has been reported to result in embryonic lethality [13]. ZnT3 is expressed mostly in brain, where it localizes to zinc-containing synaptic vesicles within a subset of glutamatergic neurons [18][19][20]. Knockout mice lacking ZnT3 are viable but display a severe depletion of zinc in synaptic vesicles [21].…”

AMPA receptor inhibition by synaptically released zinc

“…Free zinc is stored in glutamatergic vesicles in many excitatory synapses in the cerebral cortex, limbic, and brainstem nuclei (16). In some brain areas, such as in the hippocampus, 50% of boutons synapsing onto CA1 neurons and all mossy fibers synapsing onto CA3 neurons contain synaptic free zinc (17). Whereas earlier studies demonstrated that exogenous zinc inhibits AMPARs (18)(19)(20)(21), more recent work suggests that endogenously released synaptic zinc does not modulate AMPARs in central synapses (14,22).…”

Zinc transporter ZnT-3 regulates presynaptic Erk1/2 signaling and hippocampus-dependent memory

“…Little is known about the role of synaptic zinc in learning and memory. The zinc transporter ZnT3 is localized in clear synaptic vesicles of cortical glutamatergic terminals (27,28). Targeted deletion of ZnT3 prevents synaptic vesicle zinc uptake (29) and ablates the extracellular release of zinc by action potentials (16).…”