Anxiolytic-like effect of cannabinoids injected into the rat dorsolateral periaqueductal gray Fabrı ´cio Arau ´jo Moreira * ,1 , Daniele Cristina Aguiar 1 , Francisco Silveira Guimar ~ aes Department of Pharmacology, FMRP, Campus USP, 14049-900 Ribeir~ ao Preto, SP, Brazil Received 8 June 2006; received in revised form 11 October 2006; accepted 20 October 2006 Abstract Contradictory results exist concerning the effects of systemic injections of CB 1 cannabinoid receptor agonists on anxiety-related behaviors. Direct drug administration into brain structures related to aversive responses can potentially help to clarify the role of cannabinoids on anxiety. One such structure is the midbrain dorsolateral periaqueductal gray (dlPAG). Therefore, the aim of this study was to test the hypothesis that the activation of the CB 1 receptor in the dlPAG would attenuate anxiety-related behaviors. Male Wistar rats with cannula aimed at the dlPAG received injections of the endogenous cannabinoid anandamide, the anandamide transport inhibitor AM404, the anandamide analogue ACEA or the CB 1 receptor antagonist AM251, and were submitted to the elevated plus maze (EPM), an animal model of anxiety. Anandamide (0.5e50 pmol) and ACEA (0.05e5 pmol) induced anxiolytic-like effects with bell-shaped doseeresponse curves, the higher doses being inef- fective. The anandamide anxiolytic effect was potentiated by AM404 (50 pmol) and prevented by AM251 (100 pmol). Neither AM404 (0.5e 50 pmol) nor AM251 (1e100 pmol) alone modified the animal behavior in the EPM. The present study suggests that the dlPAG is a possible neuroanatomical site for anxiolytic-like effects mediated by CB 1 agonists. Furthermore, this work supports the importance of neuronal uptake as a mechanism that limits the in vivo actions of anandamide. Ó 2006 Elsevier Ltd. All rights reserved. Keywords: Cannabinoids; Endocannabinoids; Anandamide; Cannabinoid receptor; Anxiety; Periaqueductal gray 1. Introduction The herb Cannabis sativa has been used for centuries both in medical and recreational circumstances. This plant may in- duce an emotional state of euphoria, often accompanied by re- laxation and an increase in sociability, although aversive reactions are also likely to occur (Hall and Solowij, 1998; Zuardi et al., 1982). These paradoxal effects are mainly medi- ated by D 9 -tetrahydrocannabinol (D 9 -THC), which activates at least two G-protein coupled receptors, named by their order of discovery as CB 1 and CB 2 cannabinoid receptors (Howlett et al., 2002). Their better-characterized endogenous ligands are the arachidonic acid derivates arachidonoyl-ethanolamide (anandamide) and 2-arachidonoyl-glycerol (2-AG), collec- tively termed as endocannabinoids (Howlett et al., 2002). In the central nervous system (CNS), these substances are synthe- sized on-demand from post-synaptic neurons after an increase in calcium influx and immediately diffuse to the synaptic cleft, activating CB 1 receptors located in pre-synaptic terminals (Wilson and Nicoll, 2002). As for anandamide, its action is limited by re-uptake with subsequent metabolism by the intra- cellular enzyme Fatty Acid Amide Hydrolase (FAAH) located in post-synaptic neurons (Beltramo et al., 1997; Di Marzo et al., 1994; Piomelli et al., 1999). Although the molecular identity of the anandamide transporter (AT) remains to be de- termined, specific pharmacological tools have been developed to inhibit this re-uptake process (Piomelli et al., 1999). The CB 1 receptor as well as the AT and the FAAH are ex- pressed in several regions of the CNS (Egertova et al., 2003; * Corresponding author. Tel.: þ55 16 36023209; fax: þ55 16 36332301. E-mail address: farmoreira@yahoo.com.br (F.A. Moreira). 1 These authors have contributed equally to this work. 0028-3908/$ - see front matter Ó 2006 Elsevier Ltd. All rights reserved. doi:10.1016/j.neuropharm.2006.10.013 Neuropharmacology 52 (2007) 958e965 www.elsevier.com/locate/neuropharm