1. ¹DBSF, Pharmacology Section and Center of Neuroscience, University of Insubria, Busto Arsizio, Varese, Italy
2. ²Department of Pharmacology, Chemotherapy and Medical Toxicology, Faculty of Sciences, University of Milan, Milan, Italy

Correspondence: Dr T Rubino, DBSF, Pharmacology Section and Center of Neuroscience, University of Insubria, via A. da Giussano 10, 21052 Busto Arsizio, Varese, Italy. Tel: +39 0331 339416; Fax: +39 0331 339459; E-mail: tiziana.rubino@uninsubria.it

³These two authors contributed equally to this work

Received 10 April 2006; Revised 21 November 2006; Accepted 11 December 2006; Published online 7 February 2007.

Abstract

We investigated the effect of low doses of intraperitoneal ⁹-tetrahydrocannabinol (THC) on anxiety behavior in rats using the elevated plus maze (EPM). An anxiolytic effect was obtained in a range of doses between 0.075 and 1.5 mg/kg, the 0.75 dose being the most effective. Pretreatment with the CB1 receptor antagonist AM251 fully reversed THC's effect, suggesting CB1 receptors were involved. In order to elucidate the neuroanatomical substrates underlying the effect of the maximal effective dose of THC, we investigated cFos expression in anxiety-related brain regions (prefrontal cortex, nucleus accumbens, amygdala, and hippocampus) of rats exposed to the EPM. THC significantly lowered the amount of cFos in prefrontal cortex and amygdala without affecting the other cerebral areas. As there is increasing evidence that CREB function regulates anxiety-like behavior in rats, the second biochemical parameter we measured was phosphorylated CREB in the same brain areas. Rats treated with THC showed a significant increase in CREB activation in the prefrontal cortex and hippocampus. In the prefrontal cortex this increased activation was linked to an increase in ERK activation, whereas in the hippocampus there was a drop in the activity of CAMKII, a kinase with inhibitory effect on CREB activation. All these effects were reversed by AM251 pretreatment, suggesting that stimulation of CB1 receptors is fundamental for triggering the biochemical events. Our results suggest that the stimulation of these receptors in the prefrontal cortex, amygdala, and hippocampus with the subsequent activation of different signaling pathways is the first event underlying the effects of cannabinoids on anxious states.