Presynaptic evidence for zinc release at the mossy fiber synapse of rat hippocampus

✍ Scribed by Joshua K. Ketterman; Yang V. Li

• Publisher: John Wiley and Sons
• Year: 2008
• Tongue: English
• Weight: 508 KB
• Volume: 86
• Category: Article
• ISSN: 0360-4012
• DOI: 10.1002/jnr.21488

✦ Synopsis

Abstract

Vesicular zinc (Zn^2+^) is found in a subset of glutamatergic nerve terminals throughout the mammalian forebrain and is colocalized with glutamate. Despite well‐documented neuromodulatory roles, exocytosis of endogenous Zn^2+^ from presynaptic terminals has never been directly demonstrated, because existing studies have measured elevated Zn^2+^ concentrations by examining the perfusate. Thus, the specific origin of synaptic Zn^2+^ remains a controversial subject. Here, we describe synaptic Zn^2+^ trafficking between cellular compartments at hippocampal mossy fiber synapses by using the fluorescent indicator Zinpyr‐1 to label the hippocampal mossy fiber boutons. We determined endogenous Zn^2+^ exocytosis by direct observation of vesicular Zn^2+^ as decreasing fluorescence intensity from presynaptic axonal boutons in the stratum lucidum of CA3 during neural activities induced by the stimulation of membrane depolarization. This presynaptic fluorescence gradually returned to a level near baseline after the withdrawal of moderate stimulation, indicating an endogenous mechanism to replenish vesicular Zn^2+^. The exocytosis of the synaptic Zn^2+^ was also dependent on extracellular Ca^2+^ and was sensitive to Zn^2+^‐specific chelators. Vesicular Zn^2+^ loading was sensitive to the vacuolar‐type H^+^‐ATPase inhibitor concanamycin A, and our experiments indicated that blockade of vesicular reloading with concanamycin A led to a depletion of that synaptic Zn^2+^. Furthermore, synaptic Zn^2+^ translocated to the postsynaptic cell body upon release to produce increases in the concentration of weakly bound Zn^2+^ within the postsynaptic cytosol, demonstrating a feature unique to ionic substances released during neurotransmission. Our data provide important evidence for Zn^2+^ as a substance that undergoes release in a manner similar to common neurotransmitters. © 2007 Wiley‐Liss, Inc.