Differential modulation of anxiety by acute and chronic blockade of the CB1 cannabinoid receptor in mice together with biogenic amine changes in the forebrain

Gunnar Thiemann1, Catherine Ledent2, Areles Molleman1, Ruediger Hasenoehrl1. 1University of Hertfordshire, Hatfield, Herts., United Kingdom, 2Universite Libre de Bruxelles, Brussels, Belgium.

The CB1 cannabinoid receptor has been implicated in the control of fear and anxiety (Pacher et al., 2006). Here, we investigated the effects of a genetic versus pharmacological blockade of the CB1 cannabinoid receptor on the emotional reactivity of mice (3-month-old male and female CB1 receptor-knockout (CB1R-/-) mice and male wild-type (CB1R+/+) controls all weighing 25-30g). Three different ethological models of anxiety were used: the elevated T-maze, the elevated plus-maze, and the open field-test of emotionality. Enhanced anxiety was defined by a decrease in time spent on the open arm of the elevated T- and plus maze, and by decreased sojourn times in the centre of the open field test, respectively. Furthermore, we measured tissue levels of noradrenaline (NA), dopamine (DA), serotonin (5-HT) and their metabolites in anxiety-relevant forebrain regions, i.e., prefrontal cortex, hippocampus, septum, dorsal and ventral striatum using HPLC-ED. Data were analyzed with ANOVA followed by Tukey test or pair-wise with the Mann-Whitney U-test, where applicable. The experiments were approved by the UK Home Office.

The major findings can be summarized as follows: The CB1 receptor antagonist SR141617A (SR) modulated anxiety in a dose-dependent manner. At a dosage of 3mg/kg IP, the compound consistently increased anxiety in all of the three different anxiety tests applied (T-maze; %open arm time: controls, 30.74±4.51, 3mg/kg SR, 15.91±3.90; p<0.05. Plus-maze; time on open arm: controls, 121.30±27.92s, 3mg/kg SR, 49.88±27.24s; p=0.028. Open field; centre sojourn time: controls, 74.13±6.39s, 3mg/kg SR, 30.38±7.77s; p<0.05), while the lower dosage of 1mg/kg SR had no such effects (corresponding p-values>0.05). Independent of dosage, SR caused increased biogenic amine concentrations in most forebrain areas examined. However, the anxiogenic effects of SR appeared to be specifically related to a reduced 5-HT concentration in septum (controls, 2.19±0.39ng/mg, 3mg/kg SR, 1.22±0.25ng/mg; p=0.05), which was paralleled by increased 5-HT turnover rates (5-HIAA/5-HT: controls, 2.87±0.17, 3mg/kg SR, 4.91±0.41; p=0.002). Furthermore, CB1R-/- mice and animals treated with the CB1 receptor antagonist AM251 (3mg/kg IP) did not display any significant alterations in anxiety-related behaviour as measured with the elevated-T and plus-maze as well as the open field test of emotionality, respectively.

Our findings support the general idea of a SR141617A (rimonabant)-sensitive cannabinoid receptive site that is different from the ‘classical’ CB1 receptor and that has a pivotal role in the regulation of different psychological functions. However, with regard to its functional significance in terms of anxiety our findings suggest that under physiological conditions this receptive site is involved in the control of anxiolysis rather than anxiogenesis as suggested previously (Rodgers et al., 2005)

Pacher P. et al. (2006) Pharmacol Rev 58:389-462

Rodgers R.J. et al. (2005) Behav Pharmacol 16:405-413