Reduced contents of CB1 receptors and G protein-coupled receptor kinases in the prefrontal cortex of human cocaine addicts

María Álvaro-Bartolomé, Jesús A. García-Sevilla. University of the Balearic Islands, Lab Neuropharmacology, IUNICS, 07122, Spain

Recent evidence indicates that endocannabinoid system is implicated in the effects of drugs of abuse including those of the psychostimulant cocaine. Chronic exposure to cocaine has been shown to decrease the expression of CB1 receptor mRNA without altering the number of agonist (3H-CP-55,940) binding sites in rat cerebral cortex. Chronic cocaine, through stimulation of dopamine D2-like receptors, was also reported to increase the content of striatal anandamide (AEA) which may involve stimulation of AEA synthesis and/or inhibition of its degradation. On the other hand, the homologous regulation of neurotransmitter receptors is induced by a family of G protein-coupled receptor kinases (GRKs) that phosphorylate the agonist (endogenous ligand)-activated receptor which finally leads to receptor desensitization and downregulation. In this context, little is known on the status of CB1 receptors and GRKs in brains of human cocaine addicts. Therefore, this postmortem study assessed the contents of CB1 receptors, GRK2 (the main GRK in brain), GRK3 (a close GRK2 homolog) and GRK5 (a GRK with different structural and regulatory properties) in the prefrontal cortex (PFC) of a well-defined cohort of cocaine addicts. Specimens of PFC (Brodmann area 9) were collected from 10 cocaine addicts (6M/4F; age: 37±4 years; postmortem interval: 27±7 hours) and 10 healthy matched-controls (6M/4F; age: 40±4 years; postmortem interval:22±5 hours). Blood samples of cocaine abusers revealed high concentrations of cocaine (0.02-14 μg/ml) and its active metabolite benzoylecgonine (0.2-11 μg/ml). Cocaine and benzoylecgonine were also detected in hair samples (1.5-310 ng/mg), which indicated a chronic exposure to cocaine over the last 6-12 months. Cocaine abusers had no recent history of opiate abuse as revealed by absence of opiate drugs in blood and hair samples. Target proteins were quantified in PFC/BA9 by Western blot analysis with specific and validated antibodies. The immunodensity of CB1 receptor protein was markedly reduced in the PFC/BA9 of cocaine addicts (55%, p<0.01) when compared with that in age-, sex-, and postmortem interval-matched controls. Similarly, the contents of GRK2 (32%, P<0.05), GRK3 (13%, P>0.05) and GRK5 (24%, P<0.05) were decreased, although to a lesser extent, in the same brain samples of cocaine addicts. These results indicate that chronic exposure to cocaine is associated with reduced CB1 receptor protein in human brain, which suggests the participation of the endocannabinoid system (and a role of the endogenous agonist AEA regulating the receptor) in cocaine addiction. The parallel downregulation of GRK2 (translocation to membrane induced by agonist) and GRK5 (also localized in the cell nucleus) in brains of cocaine addicts also suggests that these kinases are involved in the homologous regulation of CB1 and/or dopamine receptors (long-term adaptations of receptor desensitization/downregulation and regulatory GRKs).

Supported by SAF2011-299181 and RETICS RD06/0001/0003 (MINECO-FEDER, Spain).