Received for publication April 17, 2006.
Revised July 19, 2006.
Accepted for publication July 19, 2006.

CB₁ Receptor Antagonism Increases Hippocampal Acetylcholine Release: Site and Mechanism of Action

Abstract

Evidence indicates that blockade of cannabinoid receptors increases acetylcholine (ACh) release in brain cortical regions. Although it is assumed that this type of effect is mediated through CB₁ receptor (CB₁R) antagonism, recently several in vitro functional studies have suggested non-CB₁R involvement. In addition, neither the precise neuroanatomical site nor the exact mechanisms underlying this effect are known. We thoroughly examined these issues using a combination of systemic and local administration of CB₁R antagonists, different methods of in vivo microdialysis, CB₁R knockout (KO) mice, tissue measurements of ACh, and immunochemistry. First, we showed that systemic injections of the CB₁R antagonists SR141716A and AM251 dose-dependently increased hippocampal ACh efflux. Similarly, local hippocampal, but not septal, infusions of SR141716A or AM251 increased hippocampal ACh release. Importantly, the stimulatory effects of systemically administered CB₁R antagonists on hippocampal ACh release were completely abolished in CB₁R KO mice. CB₁R KO mice had similar basal, but higher stress-enhanced, hippocampal ACh levels compared to wild-type controls. Interestingly, dopamine D₁ receptor antagonism counteracted the stimulatory effect of CB₁R blockade on hippocampal ACh levels. Finally, immunohistochemical methods revealed that a high proportion of CB₁R positive nerve terminals were found in hippocampus and confirmed the co-localization of CB₁ receptors with cholinergic and dopaminergic nerve terminals. In conclusion, hippocampal ACh release may specifically be controlled through CB₁Rs located on both cholinergic and dopaminergic neuronal projections, and CB₁R antagonism increases hippocampal ACh release probably through both a direct disinhibition of ACh release and an indirect increase in dopaminergic neurotransmission at the D₁ receptors.

Key words: Cannabinoid, Immunocytochemistry, Knockout