Elevating Endogenous Cannabinoids Reduces Opioid Withdrawal in Mice

• Main Author: Ramesh, Divya
• Format: Others
• Published: VCU Scholars Compass 2012
• Subjects: endocannabinoid; MAGL; FAAH; Medical Pharmacology; Medical Sciences; Medicine and Health Sciences
• Online Access: http://scholarscompass.vcu.edu/etd/2661; http://scholarscompass.vcu.edu/cgi/viewcontent.cgi?article=3660&context=etd

Delta9-tetrahydrocannbinol (THC), the primary active constituent of Cannabis sativa, has long been known to reduce opioid withdrawal symptoms. Although THC produces most of its pharmacological actions through the activation of CB1 and CB2 cannabinoid receptors, the role these receptors play in reducing opioid withdrawal symptoms remains unknown. The endogenous cannabinoids, N-arachidonoylethanolamine (anandamide; AEA) and 2-arachidonylglycerol (2-AG), activate both cannabinoid receptors, but are rapidly metabolized by fatty acid amide hydrolase (FAAH) and monoacylglycerol lipase (MAGL), respectively. The objective of this dissertation was to test whether increasing AEA or 2-AG, via inhibition of their respective hydrolytic enzymes, reduces morphine withdrawal symptoms in in vivo and in vitro models of opiate dependence. Morphine-dependent ICR mice subjected to acute naloxone challenge or abrupt withdrawal (via pellet removal) reliably displayed a profound withdrawal syndrome, consisting of jumping, paw tremors, head shakes, diarrhea, and weight loss. THC and the MAGL inhibitor, JZL184 dose-dependently reduced the intensity of precipitated withdrawal measures through the activation of CB1 receptors. The FAAH inhibitor, PF-3845, reduced the intensity of a subset of precipitated signs through the activation of CB1 receptors, but did not ameliorate the incidence of diarrhea or weight loss. In the next set of experiments, MAGL inhibition dose-dependently reduced the intensity of all spontaneous withdrawal signs (i.e jumps, paw flutters, head shakes, weight loss and diarrhea) in a CB1 receptor dependent manner. However, FAAH inhibition reduced the intensity of head shakes and paw flutters, but did not affect other signs. Strikingly, a combination of low-dose JZL184 and high-dose PF-3845 reduced abrupt withdrawal signs in a manner similar to complete MAGL inhibition, which suggests potential therapeutic advantages of dual enzyme inhibition. This combination elevated appropriate eCB levels and caused moderate CB1 receptor desensitization, but did not affect receptor number in whole brain. Since MAGL, but not FAAH inhibition, blocked diarrhea during opioid withdrawal in vivo, we investigated whether inhibitors of each enzyme would differentially attenuate naloxone-precipitated contractions and secretion in morphine-dependent ilea in vitro. Both enzyme inhibitors attenuated the intensity of naloxone-induced contractions, and blocked naloxone-precipitated hypersecretion. Thus, these models offer useful tools for investigating in vitro end-ponts of withdrawal, but not for elucidating anti-diarrheal mechanism of these inhibitors.If targeting endocannabinoid catabolic enzymes is indeed a viable approach to treat other abuse disorders, it is important to know whether these inhibitors would themselves have abuse or dependence liability. In the final series of experiments we tested whether prolonged elevation of endocannabinoid leads to the development of cannabinoid dependence, based on the occurrence of somatic withdrawal signs upon challenge with rimonabant, a CB1 receptor antagonist. Repeated treatment with high doses, but not low doses, of JZL184 led to cannabinoid dependnece. These results indicate that the strategy of increasing endogenous cannabinoids through the inhibition of their catabolic enzymes represents a promising approach to ameliorate opioid withdrawal symptoms.