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 CB 1 receptor (CB 1 R) antagonism, several in vitro functional studies recently have suggested non-CB 1 R 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 CB 1 R antagonists, different methods of in vivo microdialysis, CB 1 R knockout (KO) mice, tissue measurements of ACh, and immunochemistry. First, we showed that systemic injections of the CB 1 R antagonists N-(pi-dose-dependently increased hippocampal ACh efflux. Likewise, local hippocampal, but not septal, infusions of SR141716A or AM251 increased hippocampal ACh release. It is noteworthy that the stimulatory effects of systemically administered CB 1 R antagonists on hippocampal ACh release were completely abolished in CB 1 R KO mice. CB 1 R KO mice had similar basal but higher stress-enhanced hippocampal ACh levels compared with wild-type controls. It is interesting that dopamine D 1 receptor antagonism counteracted the stimulatory effect of CB 1 R blockade on hippocampal ACh levels. Finally, immunohistochemical methods revealed that a high proportion of CB 1 R-positive nerve terminals were found in hippocampus and confirmed the colocalization of CB 1 receptors with cholinergic and dopaminergic nerve terminals. In conclusion, hippocampal ACh release may specifically be controlled through CB 1 Rs located on both cholinergic and dopaminergic neuronal projections, and CB 1 R antagonism increases hippocampal ACh release, probably through both a direct disinhibition of ACh release and an indirect increase in dopaminergic neurotransmission at the D 1 receptors.Animal studies have shown that cannabinoid receptor activation and blockade impair and enhance cognitive performance, respectively, under certain experimental conditions (Chaperon and Thiebot, 1999). These effects on cognition have been correlated with fluctuating extracellular acetylcholine (ACh) levels in the hippocampus, in which an abundance of cannabinoid receptors and cholinergic nerve terminals reside (Tzavara et al., 2003b;Inui et al., 2004). The effect of cannabinoid receptor stimulation on hippocampal ACh efflux can vary depending on dosage and site of administration. In general, high doses of cannabinoid agonists decrease whereas lower doses increase hippocampal ACh release, and these effects are mediated through the hippocampus and septum, respectively (Tzavara et al., 2003b). Two types of cannabinoid receptors have been identified: the cannabinoid receptor type 1 (CB 1 R) and type 2 (CB 2 R) (Devane et al., 1988). The modulation of hippocampal ACh levels induced by cannabinoids is probably mediated through CB 1 R given their distribution in the septohippocampal pathway (Howlett et al., 2004).The effect of cannabinoid receptor sti...
