Concentrations of extracellular free zinc (pZn)_e in the central nervous system during simple anesthetization, ischemia and reperfusion

"Free Zn²⁺" (rapidly exchangeable Zn²⁺) is stored along with glutamate in the presynaptic terminals of specific specialized (gluzinergic) cerebrocortical neurons. This synaptically releasable Zn²⁺ has been recognized as a potent modulator of glutamatergic transmission and as a key toxin in excitotoxic neuronal injury. Surprisingly (despite abundant work on bound zinc), neither the baseline concentration of free Zn²⁺ in the brain nor the presumed co-release of free Zn²⁺ and glutamate has ever been directly observed in the intact brain in vivo. Here, we show for the first time in dialysates of rat and rabbit brain and human CSF samples from lumbar punctures that: (i) the resting or "tonic" level of free Zn²⁺ signal in the extracellular fluid of the rat, rabbit and human being is approximately 19 nM (95% range: 5-25 nM). This concentration is 15,000-fold lower than the "300 μM" concentration which is often used as the "physiological" concentration of free zinc for stimulating neural tissue. (ii) During ischemia and reperfusion in the rabbit, free zinc and glutamate are (as has often been presumed) released together into the extracellular fluid. (iii) Unexpectedly, Zn²⁺ is also released alone (without glutamate) at a variable concentration for several hours during the reperfusion aftermath following ischemia. The source(s) of this latter prolonged release of Zn²⁺ is/are presumed to be non-synaptic and is/are now under investigation. We conclude that both Zn²⁺ and glutamate signaling occur in excitotoxicity, perhaps by two (or more) different release mechanisms.