Figure 5

Effect of LTP on local polyribosomes, SER, spine density, and total synaptic weight. (A) Synaptic clusters (yellow) surrounded by asynaptic segments (light blue) showing shaft SER extending into SA (green) and polyribosomes (PR) at that spine base and in the dendritic shaft (1 dark blue sphere per ribosome). Double-headed black arrows show oblique segment lengths. (B) Density of PR in dendrites (Den) was significantly lower in LTP vs control (ANOVA, F_(1,16) = 5.4, p = 0.03, η² = 0.25, red asterisk). Asynaptic segments (Asyn) had fewer PR than synaptic clusters (Syn), (ANOVA, F_(1,275) = 18.8, p = 0.00002, η² = 0.064, black asterisks), not differing by condition. Control SA or Tub clusters had more PR than SER− clusters (LSD, p = 0.02, blue asterisk). Control SA clusters had more PR than LTP (LSD, p = 0.04, red asterisk). (Six outliers were excluded with inflated PR densities (>6 PR/µm), due to short segment lengths (<0.4 µm): 1 Asyn, 4 SER− control, 1 Tub LTP). (C) Diagram SER branch points (arrows, #ShSER Br, green numbers) summed across lengths (black numbers). (EM sections, dotted lines). (D) Number of dendritic microtubules (MT) did not differ significantly by condition (ANOVA F_(1,14) = 1.07, p = 0.32, n = 18). (E) Normalized shaft SER volume did not differ significantly between Asyn and Syn (ANOVA, F_(1,275) = 0.13, p = 0.71). For LTP, shaft SER volume was greater in SA than SER− clusters (LSD, p = 0.001). LTP did not differ from Control for Syn (ANOVA F_(1,135) = 2.7, p = 0.07), SER− (LSD, p = 0.11), Tub (LSD, p = 0.09), or SA (LSD, p = 0.26) clusters. (White numbers are total clusters by category for (E,F). (F) Shaft SER branch density was greater overall in control (7.4 ± 0.9/µm) than LTP (3.5 ± 0.4/µm) (ANOVA, F_(1,277) = 21.3, p < 0.0001, η² = 0.07), Asyn (LSD, p < 0.0007), and Syn (LSD, p < 0.004). Syn shaft SER branch density was higher in control vs LTP (ANOVA, F_(1,135) = 5.5, p = 0.02, η² = 0.03), reaching statistical significance only for SER− clusters (LSD, p = 0.010). (G) Spine number correlated with shaft SER branches was greater across Syn in LTP than control (p < 0.0001 for both correlations, ANCOVA, F_(1,128) = 4.44, p = 0.03, η² = 0.03). (H) The summed synaptic area was also correlated with shaft SER branches (p < 0.0001) and greater across Syn clusters in the LTP condition (ANCOVA, F_(1,128) = 4.5, p = 0.03, η² = 0.03). (SA, Tub, and SER- refer to clusters that had spines with SA, Tub or for which all spines lacked SER, respectively).