Zinc-mediated attenuation of hippocampal mossy fiber long-term potentiation induced by forskolin

Abstract

The rise in presynaptic calcium induced by high-frequency stimulation activates the calcium-calmodulin-sensitive adenylyl cyclase (AC) 1 followed by the induction of long-term potentiation (LTP) at the hippocampal mossy fiber-CA3 synapse. Zinc is released with glutamate from mossy fiber terminals. However, the role of the zinc in mossy fiber LTP is controversial. In the present study, the mechanism of zinc-mediated attenuation of mossy fiber LTP was examined in that induced by forskolin, an AC activator. Mossy fiber LTP induced by tetanic stimulation (100Hz for 1s) was attenuated in the presence of 5μM ZnCl2, whereas that induced by forskolin under test stimulation (0.1Hz) was not attenuated. Forskolin-induced mossy fiber LTP was attenuated by perfusion with 100μM ZnCl2 prior to the induction. However, the zinc (100μM) pre-perfusion did not attenuate mossy fiber LTP induced by Sp-cAMPS, an activator of protein kinase A, under test stimulation. Zinc is necessary to be taken up into mossy fiber boutons for effectively inhibiting AC activity. In hippocampal slices labeled with ZnAF-2 DA, a membrane-permeable zinc indicator, intracellular ZnAF-2 signal was increased during tetanic stimulation in the presence of 5μM ZnCl2, but not under test stimulation. Intracellular ZnAF-2 signal was increased under test stimulation in the presence of 100μM ZnCl2. These results suggest that zinc taken up into mossy fibers attenuates forskolin-induced mossy fiber LTP via inhibition of AC activity. The significance of endogenous zinc uptake by mossy fibers is discussed focused on tetanus-induced mossy fiber LTP.