Role of metabotropic glutamate receptor 5 in the procholinergic effects of neuropsychotherapeutic compounds

Endocannabinoid Actions on Cortical Terminals Orchestrate Local Modulation of Dopamine Release in the Nucleus Accumbens

Dopamine (DA) transmission mediates numerous aspects of behavior. Although DA release is strongly linked to firing of DA neurons, recent developments indicate the importance of presynaptic modulation at striatal dopaminergic terminals. The endocannabinoid (eCB) system regulates DA release and is a canonical gatekeeper of goal-directed behavior. Here we report that extracellular DA increases induced by selective optogenetic activation of cholinergic neurons in the nucleus accumbens (NAc) are inhibited by CB1 agonists and eCBs. This modulation requires CB1 receptors on cortical glutamatergic afferents. Dopamine increases driven by optogenetic activation of prefrontal cortex (PFC) terminals in the NAc are similarly modulated by activation of these CB1 receptors. We further demonstrate that this same population of CB1 receptors modulates optical self-stimulation sustained by activation of PFC afferents in the NAc. These results establish local eCB actions on PFC terminals within the NAc that inhibit mesolimbic DA release and constrain reward-driven behavior.

Cued and contextual fear conditioning in BTBR mice is improved with training or atomoxetine

The BTBR T+tf/J (BTBR) strain of mice is a model for autism spectrum disorders (ASDs). These mice display reduced social behavior, altered communication, and high levels of repetitive behavior. BTBR mice have shown a deficit in learning cued and contextual fear conditioning. In this study, experiments were conducted to determine if either changes in training or drug administration would improve learning in BTBR mice when compared to C57BL/6 (B6) mice in contextual and cued fear conditioning. The first experiment examined the effects of three conditioned stimulus-unconditioned stimulus (CS-US) training paradigms; a 1P (1 CS-US pairing), 4P (4 CS-US pairings), and 10P (10 CS-US pairings). Increasing the number of CS-US pairings to 10 caused an increase in freezing behavior by the BTBR mice in contextual and cued conditioning indicating that more training facilitated BTBR learning. B6 mice had a more complex reaction to the increased training; the mice increased freezing behavior in the cued fear conditioning but not contextual fear conditioning. The second experiment determined whether atomoxetine, a noradrenergic reuptake inhibitor that has been shown to improve attention and decrease hyperactivity, impulsivity, and social withdrawal, would enhance learning. There was a significant increase in freezing behavior in contextual fear conditioning following atomoxetine administration in BTBR mice but not in B6 mice. Our data demonstrates that contextual and cued learning in BTBR mice is facilitated by increased training. Furthermore, contextual learning is improved in BTBR mice with use of atomoxetine, which helps to improve attention. Both increased training and pharmacological intervention improved learning in the BTBR mice suggesting a role for the combination of the two.

Interactions between the cannabinoid and dopaminergic systems: evidence from animal studies

There is a prominent role of the cannabinoid system to control basal ganglia function, in respect to reward, psychomotor function and motor control. Cannabinoid dysregulations might have a pathogenetic role in dopamine- and basal ganglia related neuropsychiatric disorders, such as drug addiction, psychosis, Parkinson's disease and Huntington's disease. This review highlights interactions between cannabinoids, and dopamine, to modulate neurotransmitter release and synaptic plasticity in the context of drug addiction, psychosis and cognition. Modulating endocannabinoid function, as a plasticity based therapeutic strategy, in the above pathologies with particular focus on cannabinoid receptor type 1 (CB1 receptor) antagonists/inverse agonists, is discussed. On the basis of the existing literature and of new experimental evidence presented here, CB1 receptor antagonists might be beneficial in disease states associated with hedonic dysregulation, and with cognitive dysfunction in particular in the context of psychosis. It is suggested that this effects might be mediated via a hyperglutamatergic state through metabotropic glutamate activation. Indications for endocannabinoid catabolism inhibitors in psychiatric disorders, that might be CB1 receptor independent and might involve TRPV1 receptors, are also discussed.

CB1 receptor knockout mice are hyporesponsive to the behavior-stimulating actions of d-amphetamine: role of mGlu5 receptors

Blockade of the cannabinoid CB1 receptors (CB1R) has been shown to reduce psychostimulant-induced hyperactivity, an effect that we sought to further characterize here. The CB1R antagonist SR141716A dose-dependently decreased d-amphetamine-induced hyperactivity.Also, d-amphetamine-induced hyperlocomotion was reduced in CB1R knockout (KO) mice. However, CB1R KO and wild-type mice showed a similar d-amphetamine-induced increase in nucleus accumbens DA release. Hence, we investigated whether CB1R antagonism/invalidation reduces d-amphetamine-induced hyperlocomotion through a mechanism involving changes in glutamatergic neurotransmission. Blockade of metabotropic-glutamate-receptors-5 (mGluR5)with MPEP, but not blockade of N-methyl-D-aspartate-receptors (NMDA) with MK-801,restored to a great extent the blunted d-amphetamine-induced hyperlocomotion seen after CB1R antagonism/invalidation. Thus, hyporesponsiveness to the psychostimulant effects of d-amphetamine as a result of CB1R antagonism/invalidation is not due to an ensuing decrease in d-amphetamine-induced DA release in the nucleus accumbens, but rather due to a hyperglutamatergic state and facilitation of glutamatergic neurotransmission at the mGlu5, but not NMDA, receptors.