Intracellular zinc accumulates in apoptotic neurons after target deprivation in vivo

Excessive accumulation of cytoplasmic zinc is closely associated with a cascade of events leading to neuronal cell death. In many in vivo models of zinc-induced cell death, toxic concentrations of extracellular, synaptically released zinc enters vulnerable neurons via neurotransmitter-or voltage-gated ion channels. In vitro studies, however, clearly demonstrate that zinc can be liberated from intracellular stores following oxidative stress, and contribute to cell death processes, including apoptosis. Here we evaluated the accumulation of intracellular zinc in an in vivo model of cell death induced by target deprivation. We focused on the lateral geniculate nucleus (LGN) for our study because 1) LGN neurons undergo apoptosis upon removal of their target, the primary visual cortex (V1) and 2) unlike cortex and hippocampus, the LGN is mostly devoid of zinc-containing synapses. Postnatal day (P)10 or P60 rats and mice received aspiration lesions of V1. One to 14 days later brains were stained histochemically for zinc or immunochemically to reveal activated caspase-3. In both juvenile and adult subjects, V1 lesions led to accumulation of cytoplasmic zinc in LGN neurons. In juveniles, zinc-stained neurons were evident medially in the LGN by 24 hrs. post-lesion, and were present throughout the nucleus by 48 hours. Caspase-immunoreactive LGN cells occurred in similar spatiotemporal patterns, and correlated with the appearance of reactive brain macrophages within the target-deprived nucleus. A similar sequence was observed in adults, but the onset occurred several days later. Our data indicate that 1) like neurotrauma and excitotoxicity, target deprivation leads to accumulation of zinc in apoptotic neurons, and 2) intracellular zinc accumulation in vivo can occur in the absence of presynaptic zinc release. Together the findings suggest that accumulation of intracellular zinc is a ubiquitous component of the apoptotic cascade in neurons.