SUMMARY
Marijuana has been used for thousands of years as a treatment for medical conditions. However, untoward side effects limit its medical value. Here we show that synaptic and cognitive impairments following repeated exposure to Δ9-tetrahydrocannabinol (Δ9-THC) are associated with the induction of cyclooxygenase-2 (COX-2), an inducible enzyme that converts arachidonic acid to prostanoids, in the brain. COX-2 induction by Δ9-THC is mediated via CB1 receptor-coupled G-protein βγ subunits. Pharmacological or genetic inhibition of COX-2 blocks down-regulation and internalization of glutamate receptor subunits and alterations of the dendritic spine density of hippocampal neurons induced by repeated Δ9-THC exposures. Ablation of COX-2 also eliminates Δ9-THC-impaired hippocampal long-term synaptic plasticity, spatial, and fear memories. Importantly, the beneficial effects of decreasing β-amyloid plaques and neurodegeneration by Δ9-THC in Alzheimer’s disease animals are retained in the presence of COX-2 inhibition. These results suggest that the applicability of medical marijuana would be broadened by concurrent inhibition of COX-2.
Cyclooxygenase-2 (COX-2), a rate-limiting enzyme converting arachidonic acid to prostaglandins and a key player in neuroinflammation, has been implicated in the pathogenesis of neurodegenerative diseases such as multiple sclerosis, Parkinson's and Alzheimer's diseases, and in traumatic brain injury-and ischemia-induced neuronal damage, and epileptogenesis. Accumulated information suggests that the contribution of COX-2 to neuropathology is associated with its involvement in synaptic modification. Inhibition or elevation of COX-2 has been shown to suppress or enhance excitatory glutamatergic neurotransmission and long-term potentiation (LTP). These events are mainly mediated via PGE 2 , the predominant reaction product of COX-2, and the PGE 2 subtype 2 receptor (EP 2 )-protein kinase A pathway. Recent evidence shows that endogenous cannabinoids are substrates for COX-2 and can be oxygenated by COX-2 to form new classes of prostaglandins (prostaglandin glycerol esters and prostaglandin ethanolamides). These COX-2 oxidative metabolites of endocannabinoids, as novel signaling mediators, modulate synaptic transmission and plasticity and cause neurodegeneration. The actions of these COX-2 metabolites are likely mediated by mitogen-activated protein kinase (MAPK) and inositol 1,4,5-trisphosphate (IP 3 ) signal transduction pathways. These discoveries suggest that the contributions of COX-2 to neurotransmission and brain malfunction result not only from its conversion of arachidonic acid to classic prostaglandins but also from its oxidative metabolism of endocannabinoids to novel prostaglandins. Thus, elucidation of COX-2 in synaptic signaling may provide a mechanistic basis for designing new drugs aimed at preventing, treating or alleviating neuroinflammation-associated neurological disorders.
J. Neurochem. (2010) 112, 691–702.
Abstract
Chronic use of marijuana impairs synaptic plasticity and cognitive function. However, the molecular mechanisms by which marijuana alters long‐term synaptic plasticity are largely unknown. Here, we show that repeated in vivo exposures to Δ9‐THC for 7 consecutive days significantly impaired hippocampal long‐term potentiation (LTP) of excitatory glutamatergic synaptic transmission. The Δ9‐THC exposure‐induced decrease in LTP was prevented by pharmacological inhibition or deletion of the cannabinoid 1 receptor (CB1R). To determine the molecular mechanisms underlying Δ9‐THC‐altered LTP, we targeted expression and function of the glutamate receptors (GluR) and phosphorylation status of cAMP‐response element‐binding protein (CREB). Chronic in vivo exposure to Δ9‐THC produced CB1R‐dependent decreases in expression of hippocampal GluR1, NR2A, and NR2B, the ratio of α‐amino‐3‐hydroxy‐5‐methylisoxazole‐4‐propionate (AMPA)/NMDA receptor‐gated currents, and phosphorylation of CREB. Our results suggest that reduced expression and function of the GluR subunits and phosphorylation of CREB may underlie the impaired long‐term synaptic plasticity induced by repeated in vivo exposure to Δ9‐THC.
Acupuncture is one of the most promising modalities in complimentary medicine. However, the underlying mechanisms are not well understood yet. We found that in TRPV2 knockout male mice, acupuncture-induced analgesia was suppressed with a decreased activation of mast cells in the acupoints stimulated. The mast cell stabilizer sodium cromolyn could suppress the release of adenosine in the acupoints on male rats. A direct injection of adenosine A1 receptor agonist or histamine H1 receptor agonist increased β-endorphin in the cerebral-spinal fluid in the acute adjuvant arthritis male rats and thus replicated the analgesic effect of acupuncture. These observations suggest that the mast cell is the central structure of acupoints and is activated by acupuncture through TRPV2 channels. The mast cell transduces the mechanical stimuli to acupuncture signal by activating either H1 or A1 receptors, therefore triggering the acupuncture effect in the subject. These findings might open new frontiers for acupuncture research.
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