ABSTRACT
"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 microM" 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.
Subject(s)
Anesthetics/metabolism , Brain Ischemia/metabolism , Central Nervous System/metabolism , Extracellular Space/metabolism , Reperfusion , Zinc/metabolism , Animals , Central Nervous System/cytology , Central Nervous System/drug effects , Chromatography, High Pressure Liquid/methods , Dialysis/methods , Electrochemistry/methods , Extracellular Space/drug effects , Female , Glutamic Acid/metabolism , Humans , Male , Rabbits , Rats , Time FactorsABSTRACT
We used antibodies directed against rat heart connexin43 (Cx43) to perform immunoblot and immunohistochemical (IHC) analyses of the catfish retina. The antibodies recognized a retinal protein which co-migrated with mouse brain Cx43. IHC staining resulted in punctate labeling of the external limiting membrane and the outer nuclear layer. Although infrequent, labeling was also observed in the inner nuclear layer. These results suggest that a Cx43 isoform may be present in Muller glial cells and neurons of the distal catfish retina.
