Chronic psychoemotional stress impairs cannabinoid-receptor-mediated control of GABA transmission in the striatum

Exposure to stressful events has a myriad of consequences in animals and in humans, and triggers synaptic adaptations in many brain areas. Stress might also alter cannabinoid-receptor-mediated transmission in the brain, but no physiological study has addressed this issue so far. In the present study, we found that social defeat stress, induced in mice by exposure to aggression, altered cannabinoid CB1-receptor-mediated control of synaptic transmission in the striatum. In fact, the presynaptic inhibition of GABAergic IPSCs induced by the cannabinoid CB1 receptor agonist HU210 [(6aR)-trans-3-(1,1-dimethylheptyl)-6a, 7,10,10a-tetrahydro-1-hydroxy-6,6-dimethyl-6H- dibenzo[b, d] pyran-9-methanol] was reduced after a single stressful episode and fully abolished after 3 and 7 d of stress exposure. Repeated psychoemotional stress also impaired the sensitivity of GABA synapses to endocannabinoids mobilized by group I metabotropic glutamate receptor stimulation, whereas the cannabinoid CB1-mediated control of glutamate transmission was unaffected by repeated exposure to an aggressor. Corticosteroids released in response to the activation of the hypothalamic-pituitary-adrenal axis played a major role in the synaptic defects observed in stressed animals, because these alterations were fully prevented by pharmacological blockade of glucocorticoid receptors and were mimicked by corticosterone injections. The recovery of stress-induced synaptic defects was favored when stressed mice were given access to a running wheel or to sucrose consumption, which function as potent natural rewards. A similar rescuing effect was obtained by a single injection of cocaine, a psychostimulant with strong rewarding properties. Targeting cannabinoid CB1 receptors or endocannabinoid metabolism might be a valuable option to treat stress-associated neuropsychiatric conditions.