Antidepressant-like activity and modulation of brain monoaminergic transmission by blockade of anandamide hydrolysis

Abstract

Although anecdotal reports suggest that cannabis may be used to alleviate symptoms of depression, the psychotropic effects and abuse liability of this drug prevent its therapeutic application. The active constituent of cannabis, delta9-tetrahydrocannabinol, acts by binding to brain CB1 cannabinoid receptors, but an alternative approach might be to develop agents that amplify the actions of endogenous cannabinoids by blocking their deactivation. Here, we show that URB597, a selective inhibitor of the enzyme fatty-acid amide hydrolase, which catalyzes the intracellular hydrolysis of the endocannabinoid anandamide, exerts potent antidepressant-like effects in the mouse tail-suspension test and the rat forced-swim test. Moreover, URB597 increases firing activity of serotonergic neurons in the dorsal raphe nucleus and noradrenergic neurons in the nucleus locus ceruleus. These actions are prevented by the CB1 antagonist rimonabant, are accompanied by increased brain anandamide levels, and are maintained upon repeated URB597 administration. Unlike direct CB1 agonists, URB597 does not exert rewarding effects in the conditioned place preference test or produce generalization to the discriminative effects of delta9-tetrahydrocannabinol in rats. The findings support a role for anandamide in mood regulation and point to fatty-acid amide hydrolase as a previously uncharacterized target for antidepressant drugs.

Fig. 1.

Time-dependent effects of URB597 on endocannabinoid levels in rat brain. (a and b) Effects of URB597 on anandamide (AEA) (a) and 2-AG (b) in hippocampus after single (0.1 mg·kg^-1, i.p.) or repeated injections (0.1 mg·kg^-1, i.p., once daily for 4 days). (c and d) Effects of a single URB597 administration on anandamide and 2-AG in cortex (c) and midbrain (d). Vehicle, open bars; URB597, filled bars. *, P < 0.05 vs. vehicle; **, P < 0.01 vs. vehicle.

Fig. 2.

Antidepressant-like effects of URB597 in (a-d) mouse TST and (e-h) rat FST. (a and c) Effects of URB597 (mg·kg^-1, i.p.) and desipramine (DES, 20 mg·kg^-1, i.p.) in the TST after single (a) or repeated (c) administration (once daily for 4 days). (b and d) Single injection of rimonabant (RIM) (1 mg·kg^-1, i.p., 30 min before URB597, 0.1 mg·kg^-1) prevents the effects of single (b) or repeated (d) URB597 administration. (e-g) Effects of URB597 in the FST: (e) effects of single URB597 injection; (f) single injection of rimonabant prevents the effects of URB597; (g) effects of multiple URB597 injections. (h) Effects of repeated desipramine injections (15 mg·kg^-1, i.p., once daily for 4 days). *, P < 0.05 vs. vehicle; **, P < 0.01 vs. vehicle; ***, P < 0.001 vs. vehicle.

Fig. 3.

Motivational profile of URB597. (a) Effects of URB597 (0.03-0.3 mg·kg^-1, i.p., 2-h treatment) in the rat conditioned place preference. Δ Time, difference in time spent in the nonpreferred compartment between post- and preconditioning sessions. (b and c) Effects of WIN 55,212-2 (▪), Δ⁹-THC (•), URB597 2 h before test session (▾), and URB597 40 min before test session (▴) in rats trained to discriminate 3 mg·kg^-1 Δ⁹-THC from vehicle. Shown are the percent of responses on the Δ⁹-THC associated lever (b) and rate of lever pressing over the entire 30-min session (c). Open symbols represent respective vehicles. **, P < 0.01 vs. vehicle.

Fig. 4.

Effects of URB597 on 5-HT neuron firing in the rat DRN. (a) Integrated firing rate histogram of DRN neurons, illustrating the time-dependent effects of URB597; arrow indicates time of URB597 injection (0.1 mg·kg^-1, i.v.; calibration bar: 1 min). (b) Dose-dependent effects of URB597 on spontaneous firing rate. (c and d) Single administration of rimonabant (RIM) (1 mg·kg^-1, i.v.) prevents the effects of single (0.1 mg·kg^-1) (c) and repeated (d) URB597 injections (0.1 mg·kg^-1, i.p., once daily for 4 days) on 5-HT neuron firing. (e) Repeated URB597 administration does not affect the response of 5-HT neurons to 8-hydroxy-2-(di-n-propylamino)tetralin, expressed as percent inhibition of 5-HT-neuron firing rate. Open symbols represent vehicle. (f and g) Effects of single or repeated URB597 injections on 5-HT outflow over 3 h in hippocampus (f) and prefrontal cortex (g) of awake rats. *, P < 0.05 vs. vehicle; **, P < 0.01, vs. vehicle.

Fig. 5.

Effects of URB597 on NE neuron firing in the rat locus ceruleus. (a) Integrated firing rate histogram of locus ceruleus neurons, illustrating the time-dependent effects of URB597 (0.1 mg·kg^-1, i.v.). Arrow, time of URB597 injection; calibration bar, 1 min. (b) Effects of single injection (0.1 mg·kg^-1, i.v.) (Left) or repeated injections (0.1 mg·kg^-1, i.p., once daily for 4 days) (Right) of URB597 on NE firing activity, and blockade of these effects by single injection of rimonabant (RIM) (1 mg·kg^-1, i.v.). Bars represent mean ± SEM firing activity (Hz) of neurons recorded 20-120 s after injection. **, P < 0.01 vs. vehicle.