Weakened Intracellular Zn2+-Buffering in the Aged Dentate Gyrus and Its Involvement in Erasure of Maintained LTP.

Memory is lost by the increased influx of extracellular Zn2+ into neurons. It is possible that intracellular Zn2+ dynamics is modified even at non-zincergic medial perforant pathway-dentate granule cell synapses along with aging and that vulnerability to the modification is linked to age-related cognitive decline. To examine these possibilities, vulnerability of long-term potentiation (LTP) maintenance, which underlies memory retention, to modification of synaptic Zn2+ dynamics was compared between young and aged rats. The influx of extracellular Zn2+ into dentate granule cells was increased in aged rats after injection of high K+ into the dentate gyrus, but not in young rats. This increase impaired maintained LTP in aged rats. However, the impairment was rescued by co-injection of CaEDTA, an extracellular Zn2+ chelator, or CNQX, an AMPA receptor antagonist, which suppressed the Zn2+ influx. Maintained LTP was also impaired in aged rats after injection of ZnAF-2DA into the dentate gyrus that chelates intracellular Zn2+, but not in young rats. Interestingly, the capacity of chelating intracellular Zn2+ with intracellular ZnAF-2 was almost lost in the aged dentate gyrus 2 h after injection of ZnAF-2DA into the dentate gyrus, suggesting that intracellular Zn2+-buffering is weakened in the aged dentate gyrus, compared to the young dentate gyrus. In the dentate gyrus of aged rats, maintained LTP is more vulnerable to modification of intracellular Zn2+ dynamics than in young rats, probably due to weakened intracellular Zn2+-buffering.