Significance of Low Nanomolar Concentration of Zn2+ in Artificial Cerebrospinal Fluid.

Much less attention has been paid to Zn2+ in artificial cerebrospinal fluid (ACSF), i.e., extracellular medium, used for in vitro slice experiments than divalent cations such as Ca2+. Approximately 2 mM Ca2+ is added to conventional ACSF from essentiality of Ca2+ signaling in neurons and glial cells. However, no Zn2+ is added to it, even though the importance of Zn2+ signaling in them is recognizing. On the other hand, synaptic Zn2+ homeostasis is changed during brain slice preparation. Therefore, it is possible that not only neuronal excitation but also synaptic plasticity such as long-term potentiation is modified in ACSF without Zn2+, in which original physiology might not appear. The basal (static) levels of intracellular (cytosolic) Zn2+ and Ca2+ are not significantly different between brain slices prepared with conventional ACSF without Zn2+ and pretreated with ACSF containing 20 nM ZnCl2 for 1 h. In the case of mossy fiber excitation, however, presynaptic activity assessed with FM 4-64 is significantly suppressed in the stratum lucidum of brain slices pretreated with ACSF containing Zn2+, indicating that hippocampal excitability is enhanced in brain slices prepared with ACSF without Zn2+. The evidence suggests that low nanomolar concentration of Zn2+ is necessary for ACSF. Furthermore, exogenous Zn2+ has opposite effect on LTP induction between in vitro and in vivo experiments. It is required to pay attention to extracellular Zn2+ concentration to understand synaptic function precisely.