Colocalization of CB1 receptors with L1 and GAP-43 in forebrain white matter regions during fetal rat brain development: Evidence for a role of these receptors in axonal growth and guidance

• Published in: Neuroscience 2008-05, Vol.153 (3), p.687-699
• Main Authors: Gómez, M, Hernández, M.L, Pazos, M.R, Tolón, R.M, Romero, J, Fernández-Ruiz, J
• Format: Article
• Language: English
• Subjects: Animals; Biological and medical sciences; Blotting, Western; Female; Fetus; Fluorescent Antibody Technique; Fundamental and applied biological sciences. Psychology; GAP-43 Protein - metabolism; Growth Cones - metabolism; Immunohistochemistry; Neural Cell Adhesion Molecule L1 - metabolism; Neurology; Prosencephalon - embryology; Prosencephalon - metabolism; Rats; Rats, Wistar; Receptor, Cannabinoid, CB1 - metabolism; Synaptophysin - metabolism; Vertebrates: nervous system and sense organs
• ISSN: 0306-4522; 1873-7544
• DOI: 10.1016/j.neuroscience.2008.02.038

Abstract There is recent evidence supporting the notion that the cannabinoid signaling system plays a modulatory role in the regulation of cell proliferation and migration, survival of neural progenitors, neuritic elongation and guidance, and synaptogenesis. This assumption is based on the fact that cannabinoid 1-type receptors (CB1 receptors) and their ligands emerge early in brain development and are abundantly expressed in certain brain regions that play key roles in these processes. We have recently presented in vivo evidence showing that this modulatory action might be exerted through regulating the synthesis of the cell adhesion molecule L1 that is also a key element for those processes. To further explore this issue, we conducted here immunohistochemical studies aimed at determining the cellular substrates of CB1 receptor–L1 interactions in the rat brain during late fetal development. In this period, we previously found that the activation of CB1 receptors increased L1 synthesis in several forebrain white matter regions but not in gray matter areas. Using double labeling studies, we observed here colocalization of both proteins in fiber tracts including the corpus callosum, the adjacent subcortical white matter, the internal capsule and the anterior commissure. Experiments conducted with cultures of fetal rat cortical nerve cells revealed that L1 is present mainly in neurons but not in glial cells. This fact, together with the results obtained in the double labeling studies, would indicate that L1 and CB1 receptors should possibly be present in axons elongating through these white matter tracts, or, alternatively, in migrating neurons. Further experiments confirmed the presence of CB1 receptors in elongating axons, since these receptors colocalized with growth-associated protein 43 (GAP-43), a marker of growth cones, but not with synaptophysin, a marker of active synaptic terminals, in the same forebrain white matter regions. Lastly, using cultured fetal rat cortical neurons, we also observed that the activation of cannabinoid receptors increased the levels of the full-length L1 and altered those of some active proteolytic fragments of this protein whose generation has been associated with specific steps in the process of neuritic elongation in cultured neurons. In summary, we have demonstrated that the effects caused by cannabinoid agonists on L1 are facilitated by the colocalization of this cell adhesion molecule with CB1 receptors in several fore