Anandamide administration alone and after inhibition of fatty acid amide hydrolase (FAAH) increases dopamine levels in the nucleus accumbens shell in rats
Marcello Solinas, Zuzana Justinova, Steven R. Goldberg and Gianluigi Tanda
Journal of Neurochemistry, 2006, 98, 408–419.
Although endogenous cannabinoid systems have been implicated in the modulation of the rewarding effects of abused drugs and food, little is known about the direct effects of endogenous ligands for cannabinoid receptors on brain reward processes. Here we show for the first time that the intravenous administration of anandamide, an endogenous ligand for cannabinoid receptors, and its longer-lasting synthetic analog methanandamide, increase the extracellular dopamine levels in the nucleus accumbens shell of awake, freely moving rats, an effect characteristic of most drugs abused by humans. Anandamide produced two distinctly different effects on dopamine levels: (1) a rapid, transient increase that was blocked by the cannabinoid CB1 receptor antagonist rimonabant, but not by the vanilloid VR1 receptor antagonist capsazepine, and was magnified and prolonged by the fatty acid amide hydrolase (FAAH) enzyme inhibitor, URB597; (2) a smaller delayed and long-lasting increase, not sensitive to CB1, VR1 or FAAH blockade. Both effects were blocked by infusing either tetrodotoxin (TTX, 1 lM) or calciumfree
Ringer’s solution through the microdialysis probe, demonstrating that they were dependent on the physiologic activation of dopaminergic neurotransmission. Thus, these results indicate that anandamide, through the activation of the mesolimbic dopaminergic system, participates in the signaling of brain reward processes.
Keywords : anandamide, dopamine release, endogenous cannabinoids, FAAH enzyme, nucleus accumbens shell, reward.
Brain reward processes are believed to be primarily mediated by the dopaminergic mesolimbic system that has its origin in the ventral tegmental area and projects to the nucleus accumbens (Koob 2000; Di Chiara 2002; Wise 2002; Tanda and Goldberg 2003). A common feature of most drugs abused by humans is their ability to activate dopaminergic neurotransmission in this system, resulting in increased extracellular levels of dopamine (DA) in the nucleus accumbens and, in particular, in its ventromedial portion, the shell (Di Chiara and Imperato 1988; Pontieri et al. 1995; Pontieri et al. 1996). D-9-Tetra-hydro-cannabinol (THC), the main psychoactive ingredient in marijuana, and synthetic cannabinoid CB1 receptor agonists increase DA levels in the nucleus accumbens shell, and these effects are often considered central for their rewarding effects (Tanda et al. 1997; Tanda and Goldberg 2003; Gardner 2005; Justinova et al. 2005a). In addition, more natural rewards such as the consumption of palatable food similarly increase the DA levels in the nucleus accumbens shell (Tanda and Di Chiara 1998). Thus, elevations in dopamine levels in the shell of the accumbens may represent a common event in the signaling of rewarding events.
Anandamide is the best-characterized endogenous ligand for cannabinoid CB1 receptors (Piomelli 2003) and, at higher concentrations, it is also an endogenous ligand for vanilloid VR1 receptors (Di Marzo et al. 2001a). The metabolic formation and degradation of anandamide have been described and demonstrated in vivo (Freund et al. 2003; Piomelli 2003; De Petrocellis et al. 2004; Di Marzo et al. 2004). Although anandamide is present in the brain in very low basal concentrations, it is believed to be synthesized on demand in neural membranes and released extracellularly to activate cannabinoid receptors. It is then quickly eliminated by a twostep process consisting of either re-uptake or diffusion into neurons where it undergoes catabolic cleavage, primarily by fatty acid amide hydrolase (FAAH) (Freund et al. 2003; Piomelli 2003; De Petrocellis et al. 2004; Di Marzo et al. 2004). These features make anandamide an important candidate for the modulation of neuronal activity. Indirect in vivo evidence for a role of endogenous cannabinoid systems in the modulation of brain reward processes comes from behavioral studies of the effects of cannabinoid CB1 receptor agonists and antagonists on food consumption, electrical brain stimulation reward or intravenous (i.v.) drug self-administration (Tanda et al. 1997; Tanda and Goldberg 2003; Gardner 2005; Justinova et al. 2005a). More specifically, a role for endogenous cannabinoid systems in mediating the actions of drugs abused by humans on the mesolimbic dopaminergic brain reward system is suggested by studies showing that both pharmacological blockade and genetic deletion of CB1 receptors impairs nicotine-, ethanol- and morphine-induced DA release in the nucleus accumbens shell (Mascia et al. 1999; Cohen et al. 2002). Also, either chronic (in the case of THC, nicotine and ethanol) or acute (in the case of morphine) treatment of rats with drugs of abuse increases the levels of endocannabinoids (either anandamide or 2-arachidonylglycerol, or both) in the limbic forebrain and/or nucleus accumbens (Di Marzo et al. 2000; Gonzalez et al. 2002; Vigano et al. 2004). In addition, the reinforcing effects of non-drug stimuli such as food are blunted by the administration of the cannabinoid antagonist rimonabant (Arnone et al. 1997; Colombo et al. 1998; Solinas and Goldberg 2005). Finally, direct in vivo evidence for a role of endogenous cannabinoids in the modulation of brain reward processes has been suggested by our recent findings that anandamide and its longer lasting, metabolically stable, synthetic analog methanandamide
serve as effective reinforcers of drug-taking behavior when self-administered intravenously by squirrel
monkeys, and that these reinforcing effects are mediated by cannabinoid CB1 receptors (Justinova et al. 2005b). However, there is presently no direct evidence that endogenous ligands for cannabinoid receptors can either activate or facilitate dopaminergic neurotransmission in the mesolimbic
brain reward system. In this study, we demonstrate for the first time that the endogenous cannabinoid anandamide and its metabolically stable synthetic analog methanandamide increase neurotransmission
in the mesolimbic DA reward system, using in vivo microdialysis procedures in rats to measure DA levels in the shell of the nucleus accumbens. We further demonstrate that the effects of anandamide are mediated by cannabinoid CB1 but not by vanilloid VR1 receptor activation. Finally, we demonstrate that the effects of anandamide on dopaminergic neurotransmission can be markedly potentiated by blocking its metabolism by FAAH with the recently developed FAAH enzyme inhibitor URB597 (Kathuria et al. 2003).