Olivier J. Manzoni

Endogenous cannabinoids mediate long-term synaptic depression in the nucleus accumbens

Do endocannabinoids (eCBs) participate in long-term synaptic plasticity in the brain? Using pharmacological approaches and genetically altered mice, we show that stimulation of prelimbic cortex afferents at naturally occurring frequencies causes a long-term depression of nucleus accumbens glutamatergic synapses mediated by eCB release and presynaptic CB1 receptors. Translation of glutamate synaptic transmission into eCB retrograde signaling involved metabotropic glutamate receptors and postsynaptic intracellular Ca2+ stores. These findings unveil the role of the eCB system in activity-dependent long-term synaptic plasticity and identify a mechanism by which marijuana can alter synaptic functions in the endogenous brain reward system.

Nitric oxide-induced blockade of NMDA receptors

Abstract We studied the effects of nitric oxide (NO)-producing agents on N-methyl-d-aspartate (NMDA) receptor activation in cultured neurons. 3-Morpholinosydnonimine (SIN-1) blocked both NMDA-induced currents and the associated increase in intracellular Ca 2+ . The actions of SIN-1 were reversible and suppressed by hemoglobin. A degraded SIN-1 solution that did not release NO was unable to block NMDA receptors. This showed that the SIN-1 effects were due to NO and not to another breakdown product. Similar results were obtained with 1-nitrosopyrrolidine(an NO-containing drug) and with NO released from NaNO 2 . Pretreatment with hemoglobil potentiated NMDA-induced effects, demonstrating that endogenous NO modulates NMDA receptors. Since NMDA receptor activation induces NO synthesis, these results suggest a feedback inhibition of NMDA receptors by NO under physiological condition.

The serine hydrolase ABHD6 controls the accumulation and efficacy of 2-AG at cannabinoid receptors

The endocannabinoid 2-arachidonoylglycerol (2-AG) regulates neurotransmission and neuroinflammation by activating CB1 cannabinoid receptors on neurons and CB2 cannabinoid receptors on microglia. Enzymes that hydrolyze 2-AG, such as monoacylglycerol lipase, regulate the accumulation and efficacy of 2-AG at cannabinoid receptors. We found that the recently described serine hydrolase α-β-hydrolase domain 6 (ABHD6) also controls the accumulation and efficacy of 2-AG at cannabinoid receptors. In cells from the BV-2 microglia cell line, ABHD6 knockdown reduced hydrolysis of 2-AG and increased the efficacy with which 2-AG can stimulate CB2-mediated cell migration. ABHD6 was expressed by neurons in primary culture and its inhibition led to activity-dependent accumulation of 2-AG. In adult mouse cortex, ABHD6 was located postsynaptically and its selective inhibition allowed the induction of CB1-dependent long-term depression by otherwise subthreshold stimulation. Our results indicate that ABHD6 is a rate-limiting step of 2-AG signaling and is therefore a bona fide member of the endocannabinoid signaling system.

Transition to Addiction Is Associated with a Persistent Impairment in Synaptic Plasticity

Addicts Lose Plasticity What are the biological mechanisms associated with the transition from occasional drug use to addiction? In rats, like in humans, even after a prolonged period of drug intake, only a limited number of animals develop addiction-like behavior despite the amount of drug taken by all subjects being the same. Kasanetz et al. (p. 1709) compared the expression of N-methyl-d-aspartate (NMDA)–dependent long-term depression (NMDA-LTD) in the nucleus accumbens of addicted and nonaddicted rats. Initially, once drug self-administration had been learned and consolidated, but before the appearance of addiction-like behavior, LTD was suppressed in all animals independently of their vulnerability to addiction at a later stage. However, after 2 months, when addiction-like behavior appears, LTD was persistently lost in the addicted animals. In contrast, normal NMDA-LTD reappeared in animals that maintained a controlled drug intake without becoming addicted. Loss of glutamate receptor–dependent synaptic plasticity in the brain is associated with the transition to cocaine addiction. Chronic exposure to drugs of abuse induces countless modifications in brain physiology. However, the neurobiological adaptations specifically associated with the transition to addiction are unknown. Cocaine self-administration rapidly suppresses long-term depression (LTD), an important form of synaptic plasticity in the nucleus accumbens. Using a rat model of addiction, we found that animals that progressively develop the behavioral hallmarks of addiction have permanently impaired LTD, whereas LTD is progressively recovered in nonaddicted rats maintaining a controlled drug intake. By making drug seeking consistently resistant to modulation by environmental contingencies and consequently more and more inflexible, a persistently impaired LTD could mediate the transition to addiction.

Molecular components and functions of the endocannabinoid system in mouse prefrontal cortex.

Background Cannabinoids have deleterious effects on prefrontal cortex (PFC)-mediated functions and multiple evidences link the endogenous cannabinoid (endocannabinoid) system, cannabis use and schizophrenia, a disease in which PFC functions are altered. Nonetheless, the molecular composition and the physiological functions of the endocannabinoid system in the PFC are unknown. Methodology/Principal Findings Here, using electron microscopy we found that key proteins involved in endocannabinoid signaling are expressed in layers V/VI of the mouse prelimbic area of the PFC: presynaptic cannabinoid CB1 receptors (CB1R) faced postsynaptic mGluR5 while diacylglycerol lipase α (DGL-α), the enzyme generating the endocannabinoid 2-arachidonoyl-glycerol (2-AG) was expressed in the same dendritic processes as mGluR5. Activation of presynaptic CB1R strongly inhibited evoked excitatory post-synaptic currents. Prolonged synaptic stimulation at 10Hz induced a profound long-term depression (LTD) of layers V/VI excitatory inputs. The endocannabinoid -LTD was presynaptically expressed and depended on the activation of postsynaptic mGluR5, phospholipase C and a rise in postsynaptic Ca2+ as predicted from the localization of the different components of the endocannabinoid system. Blocking the degradation of 2-AG (with URB 602) but not of anandamide (with URB 597) converted subthreshold tetanus to LTD-inducing ones. Moreover, inhibiting the synthesis of 2-AG with Tetrahydrolipstatin, blocked endocannabinoid-mediated LTD. All together, our data show that 2-AG mediates LTD at these synapses. Conclusions/Significance Our data show that the endocannabinoid -retrograde signaling plays a prominent role in long-term synaptic plasticity at the excitatory synapses of the PFC. Alterations of endocannabinoid -mediated synaptic plasticity may participate to the etiology of PFC-related pathologies.

MCPG Antagonizes Metabotropic Glutamate Receptors but not Long‐term Potentiation in the Hippocampus

In the CA1 and CA3 regions of the guinea pig hippocampus, we have tested the ability of the new antagonist (RS)‐α‐methyl‐4‐carboxyphenylglycine (MCPG) to inhibit the well‐known effects of (trans)‐1‐amino‐cyclopentyl‐1,3‐dicarboxylate (ACPD), a specific agonist of glutamate metabotropic receptors. Whole‐cell recordings showed that MCPG was able to antagonize the blocking action of ACPD on IAHP in the CA1 region. In addition, we report here that MCPG also antagonized the presynaptic inhibitory actions of ACPD on field excitatory postsynaptic potentials in both areas CA1 and CA3. Thus, MCPG proved to be an effective tool for determining physiological roles of the glutamate metabotropic receptors in synaptic transmission in the hippocampus. We next tested the possible effects of this antagonist on long‐term potentiation (LTP). In completely blind experiments MCPG was without effect on LTP in both areas CA1 and CA3. In conclusion, our results suggest that, although MCPG is a valuable antagonist of the ACPD‐sensitive receptors, it has no inhibitory effect on LTP.

Nutritional omega-3 deficiency abolishes endocannabinoid-mediated neuronal functions

The corollaries of the obesity epidemic that plagues developed societies are malnutrition and resulting biochemical imbalances. Low levels of essential n-3 polyunsaturated fatty acids (n-3 PUFAs) have been linked to neuropsychiatric diseases, but the underlying synaptic alterations are mostly unknown. We found that lifelong n-3 PUFAs dietary insufficiency specifically ablates long-term synaptic depression mediated by endocannabinoids in the prelimbic prefrontal cortex and accumbens. In n-3–deficient mice, presynaptic cannabinoid CB1 receptors (CB1Rs) normally responding to endocannabinoids were uncoupled from their effector Gi/o proteins. Finally, the dietary-induced reduction of CB1R functions in mood-controlling structures was associated with impaired emotional behavior. These findings identify a plausible synaptic substrate for the behavioral alterations caused by the n-3 PUFAs deficiency that is often observed in western diets.

A single in-vivo exposure to delta 9THC blocks endocannabinoid-mediated synaptic plasticity.

Endogenous cannabinoids (eCB) mediate synaptic plasticity in brain regions involved in learning and reward. Here we show that in mice, a single in-vivo exposure to Δ9-tetrahydrocannabinol (THC) abolishes the retrograde signaling that underlies eCB-mediated synaptic plasticity in both nucleus accumbens (NAc) and hippocampus in vitro. This effect is reversible within 3 days and is associated with a transient modification in the functional properties of cannabinoid receptors.

A Single In Vivo Exposure to Cocaine Abolishes Endocannabinoid-Mediated Long-Term Depression in the Nucleus Accumbens

In the nucleus accumbens (NAc), a key structure to the effects of all addictive drugs, presynaptic cannabinoid CB1 receptors (CB1Rs) and postsynaptic metabotropic glutamate 5 receptors (mGluR5s) are the principal effectors of endocannabinoid (eCB)-mediated retrograde long-term depression (LTD) (eCB-LTD) at the prefrontal cortex-NAc synapses. Both CB1R and mGluR5 are involved in cocaine-related behaviors; however, the impact of in vivo cocaine exposure on eCB-mediated retrograde synaptic plasticity remains unknown. Electrophysiological and biochemical approaches were used, and we report that a single in vivo cocaine administration abolishes eCB-LTD. This effect of cocaine was not present in D1 dopamine receptor (D1R) -/- mice and was prevented when cocaine was coadministered with the selective D1R antagonist 8-chloro-2,3,4,5-tetrahydro-3-5-1h-3-benzazepin-7-ol (0.5 mg/kg) or with the NMDA receptor (NMDAR) blocker (+)-5-methyl-10,11-dihydro-5H-dibenzo [a,d] cyclohepten-5,10-imine maleate (1 mg/kg), suggesting the involvement of D1R and NMDAR. We found that the cocaine-induced blockade of retrograde signaling was correlated with enhanced expression levels of Homer scaffolding proteins containing the coiled-coil domain and accompanied by a strong reduction of mGluR5 surface expression. The results suggest that cocaine-induced loss of eCB retrograde signaling is caused by a reduction in the ability of mGluR5 to translate anterograde glutamate transmission into retrograde eCB signaling.

Reelin, Very-Low-Density Lipoprotein Receptor, and Apolipoprotein E Receptor 2 Control Somatic NMDA Receptor Composition during Hippocampal Maturation In Vitro

Reelin is a secreted protein that regulates brain layer formation during embryonic development. Reelin binds several receptors, including two members of the low-density lipoprotein (LDL) receptor family, the apolipoprotein E receptor 2 (ApoER2) and the very-low-density lipoprotein receptor (VLDLR). Despite the high level of expression of Reelin and ApoER2 in the postnatal brain, their functions in the adult CNS remain elusive. Here, using electrophysiological, immunocytochemical, and biochemical approaches in cultured postnatal hippocampal neurons, we show that Reelin controls the change in subunit composition of somatic NMDA glutamate receptors (NMDARs) during maturation. We found that maturation is characterized by the gradual decrease of the participation of NR1/2B receptors to whole-cell NMDAR-mediated currents. This maturational change was mirrored by a timely correlated increase of both Reelin immunoreactivity in neuronal somata and the amount of secreted Reelin. Chronic blockade of the function of Reelin with antisense oligonucleotides or the function-blocking antibody CR-50 prevented the decrease of NR1/2B-mediated whole-cell currents. Conversely, exogenously added recombinant Reelin accelerated the maturational changes in NMDA-evoked currents. The maturation-induced change in NMDAR subunits also was blocked by chronic treatment with an inhibitor of the Src kinase signaling pathway or an antagonist of the LDL receptors, but not by inhibitors of another class of Reelin receptor belonging to the integrin family. Consistent with these results, immunocytochemistry revealed that NR1-expressing neurons also expressed ApoER2 and VLDLR. These data reveal a new role for Reelin and LDL receptors and reinforce the idea of a prominent role of extracellular matrix proteins in postnatal maturation.