Fig. 6. Ultrastructural evidence for spine motility in synapses that have experienced intense synaptic activation. Illustrated are synapses in the middle molecular layer of the dentate gyrus on the control nonstimulated side (A) and after 2 h of high-frequency stimulation of the medial perforant path (B). Note that on the stimulated side, spines exhibit a chalise-like form that is remarkably similar to the form of highly motile spines. Animals received medial perforant path stimulation as described for 2 h and then were perfused with 2% paraformaldehyde/2% glutaraldehyde and prepared for electron microscopy. Photomicrographs then were taken in the middle molecular layer on the stimulated and control nonstimulated sides, den, dendrite; s, spine; t, terminal.

activation used herein to induce Arc expression and targeting. Electron micrographs of synapses in the middle molecular layer of the dentate gyrus after 2 h of medial perforant path stimulation reveal striking modifications of spine shape (Fig. 6 compare A, control side, with B, stimulated). In particular, the synapses in the activated zone undergo a dramatic shape change and assume a chalice-like configuration that is highly reminiscent of the shapes exhibited by highly motile spines (20, 21). Similar shape changes have been described after brief periods of stimulation in a standard LTP paradigm (22). These shapes invite the speculation that high-frequency stimulation induces a period of intense spine motility. It is noteworthy, however, that one does not find obvious examples of polyribosomes near or embedded within the psd. Indeed, polyribosomes are difficult to find in the spines that exhibit the dramatic shape changes.

These observations recall an earlier quantitative evaluation of synapse morphology after the induction of LTP that was, until now, rather curious—that fewer polyribosomes are detectable in and around synapses after inducing LTP (23). One interpretation of these observations is that strong synaptic activation triggers a translocation of ribosomes from the spine base or head to the psd, and that ribosomes that embedded in the electrondense psd become undetectable by conventional electron microscopy.

It is important to note that our hypothesis of local synthesis of psd proteins revisits an hypothesis proposed in 1981 (24). As part of a study of synaptogenesis in the cerebellar cortex, Palacios-Pru et al. (24) provided electron microscopic images of what appeared to be ribosomes in close association with immature psds on developing spines of Purkinje cells in the cerebellum. Based on these images, it was suggested that during early development, the psd was synthesized by ribosomes that were actually in immediate contact with it (see also ref. 25). If it turns out that ribosomes are embedded within mature psds and mediate cotranslational assembly of components of the NRC, what was then a controversial hypothesis will have been vindicated. Clearly it is now important to explore this issue with modern immunocyto-chemical or other techniques.

Studies of Arc reveal elements of a mechanism that is well-suited to mediate the sorts of molecular changes in synapses that are believed to underlie long-term synaptic plasticity.

1. The expression of the Arc gene is triggered by the patterns of synaptic activity that lead to enduring synaptic modification (LTP).

2. Arc mRNA encodes a protein that is targeted to synapses, and also to the nucleus.

3. The activity-dependent induction of Arc protein occurs during a time window that extends for a few hours after the inducing stimulus.