The processes underpinning post-developmental neurogenesis in the mammalian brain continue to be defined. Such processes involve the proliferation of neural stem cells and neural progenitor cells (NPCs), neuronal migration, differentiation and integration into a network of functional synapses within the brain. Both intrinsic (cell signalling cascades) and extrinsic (neurotrophins, neurotransmitters, cytokines, hormones) signalling molecules are intimately associated with adult neurogenesis and largely dictate the proliferative activity and differentiation capacity of neural cells. Cannabinoids are a unique class of chemical compounds incorporating plant-derived cannabinoids (the active components of Cannabis sativa), the endogenous cannabinoids and synthetic cannabinoid ligands, and these compounds are becoming increasingly recognized for their roles in neural developmental processes. Indeed, cannabinoids have clear modulatory roles in adult neurogenesis, probably through activation of both CB1 and CB2 receptors. In recent years, a large body of literature has deciphered the signalling networks involved in cannabinoid-mediated regulation of neurogenesis. This timely review summarizes the evidence that the cannabinoid system is intricately associated with neuronal differentiation and maturation of NPCs and highlights intrinsic/extrinsic signalling mechanisms that are cannabinoid targets. Overall, these findings identify the central role of the cannabinoid system in adult neurogenesis in the hippocampus and the lateral ventricles and hence provide insight into the processes underlying post-developmental neurogenesis in the mammalian brain.
Abbreviations2-AG, 2-arachidonoylglycerol; AA, arachidonic acid; ACEA, arachidonyl-2′-chloroethylamide; AEA, anandamide; BDNF, brain-derived neurotrophic factor; BMP, bone morphogenetic protein; BrdU, 5-bromo-2'deoxyuridine; CB, cannabinoid receptor; CBC, cannabichromene; CBD, cannabidiol; CREB, cAMP response element-binding protein; DAGL, DAG lipase; DCX, double cortin; FAAH, fatty acid amide hydrolase; GFAP, glial fibrillary acidic protein; IGF-1, insulin-like growth factor-1; mTORC1, mammalian target of rapamycin complex 1; NGF, nerve-growth factor; NPC, neural progenitor cell; NSC, neural stem cell; Ptc1, patched 1; RMS, rostral migratory stream; SGZ, subgranular zone; Shh, Sonic Hedgehog; Smo, smoothened; SVZ, subventricular zone; THC, Δ
IntroductionFor decades, the true plasticity of the mammalian CNS was underestimated and the adult brain was long considered to be a post-mitotic organ incapable of self-regeneration. However, pioneering work in the 1960s by Joseph Altman and colleagues challenged this long-standing dogma (Altman and Das, 1965). In this groundbreaking publication, Altman provided the first evidence that new neurons were generated in the adult rat hippocampus. Subsequent experiments demonstrated that adult neurogenesis was not specific to the hippocampus, with the adult olfactory bulb identified as another brain region where new neurons are added to existin...