Methods to Study the Behavioral Effects and Expression of CB2 Cannabinoid Receptors and Its Gene Transcripts in Chronic Mild Stress Model of Depressio

Behavioral and molecular methods were used to study and determine whether there is a link between depression that may be a factor in drug/alcohol addiction, and the endocannabinoid hypothesis of substance abuse. Depression is a lack of interest in the pleasurable things of life (termed anhedonia) and depressed mood. It is unknown whether CB₂ cannabinoid receptors are expressed in the brain and whether they are involved in depression and substance abuse. Therefore, mice were subjected daily for 4 wk to chronic mild stress (CMS), and anhedonia was measured by the consumption of 2% sucrose solution. Behavioral and rewarding effects of abused substances were determined in the CMS and control animals. The expression of CB₂ receptors and their gene transcripts was compared in the brains of CMS and control animals by Western blotting using CB₂ receptor antibody and reverse transcriptase-polymerase chain reaction (RT-PCR). Furthermore, the expression and immunocytochemical identification of CB₂ cannabinoid receptor in the rat brain were determined. CMS induced gender-specific aversions, which were blocked by WIN55,212-2, a nonspecific CB₁ and CB₂ cannabinoid receptor agonist. Direct CB₂ antisense oligonucleotide microinjection into the mouse brain induced anxiolysis, indicating that CB₂ or CB₂ like receptors are present in the brain and may influence behavior. The major finding from these studies was the expression of CB₂ receptor and its gene transcript in the mouse brain, which was enhanced by CMS. These preliminary results, if confirmed, suggest that the CB₂ receptors are expressed in the mammalian brain and may be nvolved in depression and substance abuse.