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.