LTP- and LTD-inducing stimulations cause opposite changes in arc/arg3.1 mRNA level in hippocampal area CA1 in vivo

Immediate early genes (IEGs) typically are the first genetic responders to a variety of cellular activations. The IEG that encodes activity-regulated cytoskeleton-associated protein (arc/arg3.1) has attracted much interest because its mRNA is transported to and translated near activated synapses. Moreover, arc has been implicated in both long-term potentiation (LTP) and long-term depression (LTD). However, little is known about the time course of altered arc expression during LTP and LTD. Here we characterized arc mRNA levels in area CA1 of the adult rat hippocampus in vivo after LTP- and LTD-inducing stimulations that were identical, except for the temporal patterning of the stimulation pulses. We observed a persistent increase in arc mRNA level during LTP. In contrast, during LTD, arc mRNA level first was decreased and then transiently increased relative to control level. These findings demonstrate that arc mRNA is regulated differently during LTP and LTD, and they provide evidence for stimulation-induced downregulation of mRNA availability during LTD. Findings of abbreviated LTD when transcription was inhibited indicate that the prolonged maintenance of the type of N-methyl-D-aspartate receptor-dependent LTD studied here requires de novo transcription. Furthermore, lack of evidence for a LTD-associated change in the mRNA level of the IEG zif268 demonstrates that the decrease in arc mRNA during LTD is not a general genetic response. Thus, the regulation of arc expression not only differs between LTP and LTD, but also diverges from that of other IEGs implicated in activity-dependent synaptic plasticity.