Cannabinoids Promote Embryonic and Adult Hippocampus Neurogenesis and Produce Anxiolytic and Antidepressant Like Effects

Tuesday, February 1, 2011

Cannabis (marijuana, hashish, or cannabinoids) has been used for medical and recreational purposes for many centuries and is likely the only medicine or illicit drug that has constantly evoked tremendous interest or controversy within both the public domain and medical research. Cannabinoids appear to be able to modulate pain, nausea, vomiting, epilepsy, ischemic stroke, cerebral trauma, multiple sclerosis, tumors, and other disorders in humans and/or animals (17). However, marijuana has been the most commonly used illicit drug in developed countries, producing acute memory impairment and dependence/withdrawal symptoms in chronic users and animal models (6, 810). Cannabis acts on 2 types of cannabinoid receptors, the CB1 and CB2 receptors, which are distributed mainly in the brain and immune system, respectively. In the brain, CB1 receptors are also targeted by endogenous cannabinoids (i.e., endocannabinoids) such as anandamide (AEA), 2-arachidonylglycerol, and arachidonylethanolamide (1, 6, 10, 11).

The recent discovery that the hippocampus is able to generate new neurons (i.e., neurogenesis) throughout the lifespan of mammals, including humans, has changed the way we think about the mechanisms of psychiatric disorders (12) and drug addiction (13). The subgranular zone of the dentate gyrus (SGZ) in the adult brain contains neural stem/progenitor cells (NS/PCs) capable of producing thousands of new granule cells per day (14). We, and others, have shown that these newborn hippocampal neurons are functionally integrated into the existing neuroanatomical circuitry (15, 16) and are positively correlated with hippocampus-dependent learning and memory processes (17) and the developmental mechanisms of stress and mood disorders (12). Recent studies have further shown that chronic fluoxetine treatment produced antidepressant and anxiolytic effects (18, 19) and the anxiolytic effects are likely achieved by promoting hippocampal neurogenesis (18).

Chronic administration of the major drugs of abuse including opiates, alcohol, nicotine, and cocaine has been reported to suppress hippocampal neurogenesis in adult rats (2023), suggesting a potential role of hippocampal neurogenesis in the initiation, maintenance, and treatment of drug addiction (13). The recent finding of prominently decreased hippocampal neurogenesis in CB1-knockout mice (24) suggests that CB1 receptor activation by endogenous, plant-derived, or synthetic cannabinoids may promote hippocampal neurogenesis. However, endogenous cannabinoids have been reported to inhibit adult hippocampal neurogenesis (25). Nevertheless, it is possible that exo- and endocannabinoids could differentially regulate hippocampal neurogenesis, as exo- and endocannabinoids act as full or partial agonists on CB1 receptors, respectively (11).

The goal of the present study was to test the hypothesis that the potent synthetic cannabinoid HU210 is able to promote hippocampal neurogenesis, leading to the anxiolytic and antidepressant effects of cannabinoids. We demonstrate here that both HU210 and the endocannabinoid AEA promote proliferation of embryonic hippocampal NS/PCs without significant effects on their differentiation, resulting in more newborn neurons. The effects of HU210 on adult hippocampal neurogenesis were quantified in freely moving rats and were correlated with behavioral testing. We show that 1 month after chronic HU210 treatment, rats display increased newborn neurons in the hippocampal dentate gyrus and significantly reduced measures of anxiety- and depression-like behavior. Thus, cannabinoids appear to be the only illicit drug whose capacity to produce increased hippocampal newborn neurons is positively correlated with its anxiolytic- and antidepressant-like effects.


Expression of CB1 receptors in embryonic and adult hippocampal NS/PCs. In the mammalian brain, the CB1 receptor is one of the most abundant G protein–coupled receptors, accounting for most, if not all, of the centrally mediated effects of cannabinoids (5). We reasoned that if cannabinoids were able to regulate neurogenesis, the NS/PCs capable of producing new neural cells would contain CB1 receptors. We therefore employed CB1 antibody immunocytochemistry, Western blotting, and PCR to examine CB1 protein and gene expression in cultured NS/PCs isolated from the hippocampus of E17 rat embryos. About 95% of the total neurosphere cells labeled with Hoeschst stain were also labeled with both CB1 and nestin (a marker for NS/PCs) antibodies (Figure 1A). Some Hoechst-labeled cells in the neurospheres exhibited the shape of glial cells, with small round nuclei, and were CB1 immunoreactive but without nestin staining (Figure 1A). The staining of CB1 antibody appears specific for 2 reasons. First, Western blots with the same antibody and cultured NS/PC revealed a strong protein band with the molecular weight of 60 kDa (Figure 1B), which corresponds to the CB1 receptor (26). Second, we could not detect the positive immunostaining or 60-kDa protein band using the CB1 antibody preabsorbed with the antigen. Using PCR, we further identified a band of the predicted size (1,440 bp) corresponding to the full encoding region of CB1 (Figure 1C), suggesting the presence of CB1 transcripts in NS/PCs. Similar results, i.e., CB1 protein and gene expression, were seen in both second and sixth passages of NS/PCs. We then examined adult naive rats sacrificed 2 hours after receiving a single dose of BrdU to label dividing cells. We found that about 90% of BrdU-stained cells in the SGZ were also doubly labeled with CB1 (Figure 1D; n = 3). These results suggest that both embryonic and adult hippocampal NS/PCs express CB1 receptors.

Wen Jiang1,2, Yun Zhang1, Lan Xiao1, Jamie Van Cleemput1, Shao-Ping Ji1, Guang Bai3 and Xia Zhang1

1Neuropsychiatry Research Unit, Department of Psychiatry, University of Saskatchewan, Saskatoon, Saskatchewan, Canada.
2Department of Neurology, Xijing Hospital, Fourth Military Medical University, Xi’an, People’s Republic of China.
3Department of Biomedical Sciences, Dental School, Program in Neuroscience, University of Maryland, Baltimore, Maryland, USA.

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