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

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.