Cocaine-Induced Behavioral Sensitization Is Associated With Changes in the Expression of Endocannabinoid and Glutamatergic Signaling Systems in the Mouse Prefrontal Cortex

Abstract

Background:Endocannabinoids modulate the glutamatergic excitatory transmission by acting as retrograde messengers. A growing body of studies has reported that both signaling systems in the mesocorticolimbic neural circuitry are involved in the neurobiological mechanisms underlying drug addiction.Methods:We investigated whether the expression of both endocannabinoid and glutamatergic systems in the prefrontal cortex (PFC) were altered by an acute and/or repeated cocaine administration schedule that resulted in behavioral sensitization. We measured the protein and mRNA expression of the main endocannabinoid metabolic enzymes and the cannabinoid receptor type 1 (CB1). We also analyzed the mRNA expression of relevant components of the glutamate-signaling system, including glutamate-synthesizing enzymes, metabotropic receptors, and ionotropic receptors.Results:Although acute cocaine (10mg/kg) produced no significant changes in the endocannabinoid-related proteins, repeated cocaine administration (20mg/kg daily) induced a pronounced increase in the CB1 receptor expression. In addition, acute cocaine administration (10mg/kg) in cocaine-sensitized mice (referred to as cocaine priming) induced a selective increase in the endocannabinoid-degrading enzymes fatty acid amide hydrolase (FAAH) and monoacylglycerol lipase (MAGL). These protein changes were accompanied by an overall decrease in the ratios of endocannabinoid synthesis/degradation, especially the N-acyl phosphatidylethanolamine phospholipase D/FAAH and diacylglycerol lipase alpha/MAGL ratios.Regarding mRNA expression, while acute cocaine administration produced a decrease in CB1 receptors and N-acyl phosphatidylethanolamine phospholipase D, repeated cocaine treatment enhanced CB1 receptor expression. Cocaine-sensitized mice that were administered priming injections of cocaine mainly displayed an increased FAAH expression.These endocannabinoid changes were associated with modifications in glutamatergic transmission-related genes. An overall decrease was observed in the mRNA expression of the glutamate-synthesizing gene kidney-type glutaminase (KGA), the metabotropic glutamate receptors (mGluR3 and GluR), and subunits of NMDA ionotropic receptors (NR1, NR2A, NR2B and NR2C) after acute cocaine administration, while mice repeatedly exposed to cocaine only displayed an increase in NR2C. However, in cocaine-sensitized mice primed with cocaine, this inhibition was reversed and a strong increase was detected in the mGluR5, NR2 subunits, and both GluR1 and GluR3.Conclusions:These findings indicate that cocaine sensitization is associated with an endocannabinoid downregulation and a hyperglutamatergic state in the PFC that, overall, contribute to an enhanced glutamatergic input into PFC-projecting areas.

Presynaptic G Protein-Coupled Receptors: Gatekeepers of Addiction?

“…Reduced CB1 receptor expression has been observed in postmortem brain samples from human cocaine addicts (Álvaro-Bartolomé and García-Sevilla, 2013), and this was recapitulated in mice exposed to cocaine. Some studies report increased CB1 receptor expression levels despite reduced function, and this is likely due to the decreased eCB tone producing a compensatory upregulation of receptor levels (Blanco et al, 2014). A single in vivo exposure to the opiate drug oxycodone eliminates eCB-LTD in the dorsal striatum (Atwood et al, 2014a).…”

The acute effect of cannabis on plasma, liver and brain ammonia dynamics, a translational study

“…Samples were solubilized with TX-100 at a final concentration of 1% (v/v) and then spun at 100,000g for 30 min at 4°C (Blanco et al 2015). Protein concentration was measured by the Pierce™ BCA Protein Assay Kit.…”

“…Protein concentration was measured by the Pierce™ BCA Protein Assay Kit. The enzyme activity was assayed as previously described by Heini et al and Blanco et al (Blanco et al 2015; Heini et al 1987) Briefly, 25 μL supernatant was incubated for 1 h at 37°C with 35 μL reagent one containing 100 mM potassium phosphate, 171 mM L-glutamine and 1.5 mM NH 4 Cl, pH 8.0. The reaction was terminated by adding 10 μL 10% tricarboxylic acid (TCA).…”

“…After incubation (45 min) at room temperature in the dark, the absorbance of the ammonia produced was measured at 405 nm and compared to the absorbance generated by a standard curve of NH 4 CL treated with reagent two. GA activity is expressed as mmol ammonia/min/mg of proteins (Blanco et al 2015). …”

Effects of Cocaine Sensitization on Drug Self-Administration, Mesocorticolimbic SAPAP Levels, and Prefrontal Ionotropic Glutamate Receptors

“…Although these effects are thought to stem from changes in prefrontal glutamate, the mechanisms of how the populations of AMPA/NMDA ionotropic glutamate receptors (iGluRs) are affected in this region is currently a mystery. Despite this, it is known that the end result of cocaine-induced changes in the mPFC creates a region hyper-excitable to glutamate, indicating that there may be an increase in the contribution of excitatory AMPA receptors [171]. This enhanced response to glutamate in not solely restricted to the mPFC, and thus the study of other regions of this mesocorticolimbic system may reveal how glutamate receptors are altered in this region.…”