Cannabinoid-derived drugs are promising agents for the development of novel neuroprotective strategies. Activation of neuronal CB₁ cannabinoid receptors attenuates excitotoxic glutamatergic neurotransmission, triggers prosurvival signalling pathways and palliates motor symptoms in animal models of neurodegenerative disorders. However, in Huntington's disease there is a very early downregulation of CB₁ receptors in striatal neurons that, together with the undesirable psychoactive effects triggered by CB₁ receptor activation, foster the search for alternative pharmacological treatments. Here, we show that CB₂ cannabinoid receptor expression increases in striatal microglia of Huntington's disease transgenic mouse models and patients. Genetic ablation of CB₂ receptors in R6/2 mice, that express human mutant huntingtin exon 1, enhanced microglial activation, aggravated disease symptomatology and reduced mice lifespan. Likewise, induction of striatal excitotoxicity in CB₂ receptor-deficient mice by quinolinic acid administration exacerbated brain oedema, microglial activation, proinflammatory-mediator state and medium-sized spiny neuron degeneration. Moreover, administration of CB₂ receptor-selective agonists to wild-type mice subjected to excitotoxicity reduced neuroinflammation, brain oedema, striatal neuronal loss and motor symptoms. Studies on ganciclovir-induced depletion of astroglial proliferation in transgenic mice expressing thymidine kinase under the control of the glial fibrillary acidic protein promoter excluded the participation of proliferating astroglia in CB₂ receptor-mediated actions. These findings support a pivotal role for CB₂ receptors in attenuating microglial activation and preventing neurodegeneration that may pave the way to new therapeutic strategies for neuroprotection in Huntington's disease as well as in other neurodegenerative disorders with a significant excitotoxic component.

Endocannabinoids (eCBs) have recently been identified as axon guidance cues shaping the connectivity of local GABAergic interneurons in the developing cerebrum. However, eCB functions during pyramidal cell specification and establishment of long-range axonal connections are unknown. Here, we show that eCB signaling is operational in subcortical proliferative zones from embryonic day 12 in the mouse telencephalon and controls the proliferation of pyramidal cell progenitors and radial migration of immature pyramidal cells. When layer patterning is accomplished, developing pyramidal cells rely on eCB signaling to initiate the elongation and fasciculation of their long-range axons. Accordingly, CB ₁ cannabinoid receptor (CB ₁ R) null and pyramidal cell-specific conditional mutant (CB ₁ R ^f/f,NEX-Cre ) mice develop deficits in neuronal progenitor proliferation and axon fasciculation. Likewise, axonal pathfinding becomes impaired after in utero pharmacological blockade of CB ₁ Rs. Overall, eCBs are fundamental developmental cues controlling pyramidal cell development during corticogenesis.

Endocannabinoids exert an important neuromodulatory role via presynaptic cannabinoid CB₁receptors and may also participate in the control of neural cell death and survival. The function of the endocannabinoid system has been extensively studied in differentiated neurons, but its potential role in neural progenitor cells remains to be elucidated. Here we show that the CB₁receptor and the endocannabinoid-inactivating enzyme fatty acid amide hydrolase are expressed, bothin vitroandin vivo, in postnatal radial glia (RC2⁺cells) and in adult nestin type I (nestin⁺GFAP⁺) neural progenitor cells. Cell culture experiments show that CB₁receptor activation increases progenitor proliferation and differentiation into astroglial cellsin vitro.In vivoanalysis evidences that, in postnatalCB₁^−/−mouse brain, progenitor proliferation and astrogliogenesis are impaired. Likewise, in adult CB₁-deficient mice, neural progenitor proliferation is decreased but is increased in fatty acid amide hydrolase-deficient mice. In addition, endocannabinoid signaling controls neural progenitor differentiation in the adult brain by promoting astroglial differentiation of newly born cells. These results show a novel physiological role of endocannabinoids, which constitute a new family of signaling cues involved in the regulation of neural progenitor cell function.

Cannabinoids, the active components of marijuana and their endogenous counterparts, act on the brain and many other organs through the widely expressed CB ₁ cannabinoid receptor. In contrast, the CB ₂ cannabinoid receptor is abundant in the immune system and shows a restricted expression pattern in brain cells. CB ₂ ‐selective agonists are, therefore, very attractive therapeutic agents as they do not cause CB ₁ ‐mediated psychoactive effects. CB ₂ receptor expression in brain has been partially examined in differentiated cells, while its presence and function in neural progenitor cells remain unknown. Here we show that the CB ₂ receptor is expressed, both in vitro and in vivo , in neural progenitors from late embryonic stages to adult brain. Selective pharmacological activation of the CB ₂ receptor in vitro promotes neural progenitor cell proliferation and neurosphere generation, an action that is impaired in CB ₂ ‐deficient cells. Accordingly, in vivo experiments evidence that hippocampal progenitor proliferation is increased by administration of the CB ₂ ‐selective agonist HU‐308. Moreover, impaired progenitor proliferation was observed in CB ₂ ‐deficient mice both in normal conditions and on kainate‐induced excitotoxicity. These findings provide a novel physiological role for the CB ₂ cannabinoid receptor and open a novel therapeutic avenue for manipulating neural progenitor cell fate.—Palazuelos, J., Aguado, T., Egia, A., Mechoulam, R., Guzmán, M., Galve‐Roperh, I. Non‐psychoactive CB2 cannabinoid agonists stimulate neural progenitor proliferation. FASEB J. 20, E1773–E1779 (2006)

Endocannabinoids act as neuromodulatory and neuroprotective cues by engaging type 1 cannabinoid receptors. These receptors are highly abundant in the basal ganglia and play a pivotal role in the control of motor behaviour. An early downregulation of type 1 cannabinoid receptors has been documented in the basal ganglia of patients with Huntington’s disease and animal models. However, the pathophysiological impact of this loss of receptors in Huntington’s disease is as yet unknown. Here, we generated a double-mutant mouse model that expresses human mutant huntingtin exon 1 in a type 1 cannabinoid receptor-null background, and found that receptor deletion aggravates the symptoms, neuropathology and molecular pathology of the disease. Moreover, pharmacological administration of the cannabinoid Δ⁹-tetrahydrocannabinol to mice expressing human mutant huntingtin exon 1 exerted a therapeutic effect and ameliorated those parameters. Experiments conducted in striatal cells show that the mutant huntingtin-dependent downregulation of the receptors involves the control of the type 1 cannabinoid receptor gene promoter by repressor element 1 silencing transcription factor and sensitizes cells to excitotoxic damage. We also provide in vitro and in vivo evidence that supports type 1 cannabinoid receptor control of striatal brain-derived neurotrophic factor expression and the decrease in brain-derived neurotrophic factor levels concomitant with type 1 cannabinoid receptor loss, which may contribute significantly to striatal damage in Huntington’s disease. Altogether, these results support the notion that downregulation of type 1 cannabinoid receptors is a key pathogenic event in Huntington’s disease, and suggest that activation of these receptors in patients with Huntington’s disease may attenuate disease progression.