Figure 5.

Late Arc synthesis is required to stabilize F-actin during LTP. Arc AS- or SC-ODN was locally infused 2 h after HFS, and the brain was fixed by transcardial perfusion 1 h later. Coronal sections were stained with phalloidin–FITC. A, Top left, A band of phalloidin staining specific to the termination zone of the medial perforant path was observed in the dentate gyrus of SC-treated rats. Top right, Contralateral, unstimulated dentate gyrus (CON). Bottom left, Arc AS treatment abolished the band of enhanced phalloidin staining. Bottom right, Comparison of phalloidin staining in SC- and AS-treated rats. OML, Outer molecular layer; MML, middle molecular layer; IML, inner molecular layer; GCL, granule cell layer. Arrowheads mark phalloidin staining in the middle molecular layer. The white bar marks the hippocampal fissure. Representative images based on four AS-treated and five SC-treated rats. The time course of phalloidin labeling is shown in supplemental Figure 1 (available at www.jneurosci.org as supplemental material). B, Profile of phalloidin–FITC fluorescence intensity ratio (ipsilateral/contralateral) in rats receiving SC (gray) or AS (black) infusion. Fluorescence intensity of the phalloidin–FITC signal was measured in 15 μm steps along the shortest line (rectangle in A) extending from 20 μm above the granule cell layer border to the hippocampal fissure. Optical density values from 10 neighboring pixels were averaged. Florescence values at each pixel were normalized relative to a reference area in CA1 (small box in A). Values obtained at three sites along the dentate gyrus inner blade were averaged, and group means were obtained. All sections were from within 300 μm of the recording site. C, Phalloidin staining after 2 h of 400 Hz, eight-pulse bursts applied at 10 s intervals. A bright, sharply demarcated band appears in the middle molecular layer of the dentate gyrus. D, Infusion of the F-actin stabilizing agent jasplakinolide blocks the inhibitory effect of Arc AS on LTP maintenance. Jasplakinolide (Jasp; 1 μm) and Arc AS were infused at the times indicated after HFS. Pretreatment with jasplakinolide did not affect ongoing LTP and abolished the effect of Arc AS on LTP maintenance. fEPSP slope measurements of six consecutive responses obtained immediately before and 10 min after jasplakinolide or Arc infusion were not significantly different (p < 0.05).