Figure 3. Perisomatic boutons are larger and contain proportionally more CB1 receptors compared to dendritic boutons.
(a,b) Perisomatic interneuron terminals are larger than boutons of dendritic interneurons (n = 6 cells/cell type, Mann-Whitney U test, p = 0.004, U = 1 for area, p = 0.002, U = 0 for perimeter). (c) Perisomatic axon terminals also carry more presynaptic CB1 receptors (p = 0.015, U = 3). (d) CB1 NLP shows correlation with bouton size in both interneuron types (n = 279 and 334 boutons, Spearman’s correlation). (e,f) In contrast, the density of CB1 is similar between the two cell types (Mann-Whitney U test, p = 0.7 and 0.3, U = 15 and 11 for NLP over area and perimeter, respectively). (g,j) Biocytin-filled axon terminals of CB1-positive perisomatic (in g) and dendritic (in j) interneurons labeled by a mixture of Alexa488- and Alexa647-conjugated streptavidin were imaged in confocal (g,j) and STORM (h,k) modes. (i,l) A 3D-convex hull was fitted on the localization points to visualize the surface of axon terminals. (m) In agreement with the size difference of CB1 2D-convex hulls (b), the perimeter of biocytin 2D-convex hulls of perisomatic boutons is significantly larger (n = 5 cells/cell type, Mann-Whitney U test, p = 0.012, U = 0). (n) Dual-channel directSTORM imaging of an interneuron bouton labeled by Alexa488-conjugated streptavidin, and immunostained against CB1 using Alexa647. (o) Fitting 3D-convex hulls on the biocytin or on the CB1 localization points results in strikingly similar contours of the structure. Representative images are shown from experiments repeated successfully on two biocytin-filled perisomatic interneurons. Graphs show raw data and median±IQR.