fashion (6, 7). Arc was unique because in contrast to the mRNAs of other IEGs, Arc mRNA rapidly migrates throughout the dendritic arbor of the neuron in which it is induced. This was discovered through in situ hybridization analyses of the distribution of Arc mRNA in the dentate gyrus after a single electroconvulsive shock (ECS). For example, by 2 h after a single ECS, newly synthesized Arc mRNA is distributed throughout the molecular layer of the dentate gyrus, which contains the dendrites of dentate granule cells (Fig. 1) whereas the mRNAs for other IEGs remain tightly localized to the region of the cell body.
Because Arc is expressed as an IEG, the synthesis, intracellular trafficking, localization, and life history of Arc mRNA can be studied in a way that is not possible with mRNAs that are expressed constitutively. Evaluations of Arc mRNA distribution at different times after an ECS indicate that the mRNA reaches the most distal tips of the granule cell dendrites within 1 h after the inducing stimulus. The distance from the granule cell body layer to the distal tips of the dendrites is about 300 µm. Thus, Arc mRNA moves into dendrites at a rate of at least 300 µm per h (8).
The evaluation of Arc expression at various times after ECS also revealed that Arc mRNA was present in dendrites only transiently. Peak levels of Arc mRNA were seen 1–2 h after a single ECS; thereafter, the levels of Arc mRNA declined, returning to near control levels after about 6 h (8). Interestingly, this is approximately the same time interval during which synaptic modifications are sensitive to inhibition of protein synthesis.
Subsequent studies of Arc revealed another remarkable feature—that newly synthesized Arc mRNA is selectively targeted to synapses that have been strongly activated (9). This was discovered initially in studies of Arc mRNA distribution after high-frequency stimulation of the entorhinal cortical projections to the dentate gyrus using a paradigm typically used to induce LTP.
The projection from the entorhinal cortex to the dentate gyrus (the perforant path) terminates in a topographically organized fashion along the dendrites of dentate granule cells. Projections from the medial entorhinal cortex terminate selectively in the middle molecular layer of the dentate gyrus, whereas projections from the lateral entorhinal cortex terminate in the outer molecular layer. By positioning a stimulating electrode in different parts of the entorhinal cortex, it is possible to selectively activate a band of synapses that terminate on particular proximo-distal segments. High-frequency activation of the projections to middle dendritic domains (400-hz trains, eight pulses per train, delivered at a rate of 1/10 sec) strongly induces Arc expression. If high-frequency stimulation is continued as the newly synthesized mRNA migrates into dendrites, the mRNA localizes selectively in the middle molecular layer in exactly the location of the band of synapses that had been activated. This selective localization is evidenced by a prominent band of labeling for Arc mRNA in the middle molecular layer of the dentate gyrus (Fig. 1).
When the medial perforant path is activated, the levels of labeling remain quite low in the outer molecular layer, indicating that newly synthesized Arc mRNA never migrates into the distal dendrites. This finding is in contrast to the situation after an ECS, where there are high levels of labeling through-out the molecular layer (compare Fig. 1 B and C), which suggests that as the mRNA enters the dendrites, it is somehow captured in the activated dendritic segments. An analysis of the distribution of Arc mRNA after various periods of stimulation (Fig. 2) further supports this idea. After 30 min of stimulation, Arc mRNA is still confined to the cell body layer. With continued stimulation, levels of labeling in the activated dendritic lamina increase progressively, whereas there is minimal, if any, increase in labeling in the nonactivated distal dendritic segments (the outer molecular layer). Thus, newly synthesized Arc mRNA appears to be captured by active synapses, preventing the further migration of the mRNA into more distal segments.
The intradendritic distribution of newly synthesized Arc mRNA is determined by the populations of synapses that are activated. For example, high-frequency stimulation of the lateral entorhinal cortex, which innervates distal dendritic segments, produces a band of labeling for Arc mRNA in the outer molecular layer. When the projections to proximal dendritic laminae are strongly activated, newly synthesized Arc mRNA localizes precisely in a band corresponding to the zone of activation. The synapses that terminate in this proximal dendritic lamina originate from large neurons in the hilus of the dentate gyrus. These project bilat-