Impairment of acquisition of i.v. cocaine self-administration by RNAinterference of dopamine D1 receptors in the nucleus accumbens shell

A large body of evidence indicates that dopamine (DA) is the main neurochemical substrate of reinforcing actions and abuse liability of cocaine. Discrete lesions of DA terminals and local infusion of DA receptor antagonists have indicated the nucleus accumbens (NAc) as the DA projecting area responsible for cocaine reinforcement. Anatomical studies have shown that in the NAc citoarchitecture, connections and distribution of neurotransmitters, as well as their enzymes and receptors, outline heterogeneous subdivisions. Thus, a medio-ventral part, the shell, and a dorso-lateral part, the core, have been distinguished and have been assigned different functions in behavior motivated by food and by drugs of abuse. Nonetheless compelling evidence for a differential role of NAc shell and core DA in cocaine reinforcement has not yet been given and would require studies on the effect of local impairment of DA transmission on i.v. cocaine self-administration. Different studies directly addressed this issue, but not neat differences1,2 or sometimes opposite conclusions were drawn3,4. Such discrepancies and inconsistencies might result from the use of local infusion of DA antagonists as a tool to manipulate DA transmission in the NAc shell and core. These highly lipofilic compounds rapidly spread out across target boundaries, particularly after repeated administration. These difficulties call for the use of other means to impair DA transmission. RNA interference (RNAi), an evolutionary conserved mechanism of genome protection against viral infections and mutation, has been used to silence protein expression. For in vivo application, viral mediated expression of specifically designed siRNAs triggers stable long term mediated RNAi against targeted mRNA, thus silencing relative genes and offering higher temporal and spatial control of manipulations compared to conventional knockout or pharmacological strategies. In the present study DA D1 receptor (D1R) silencing by lentiviral vectors expressing specific siRNAs (Lv-siRNAs) in discrete regions of NAc shell or core was used to investigate the role of D1R in cocaine reinforcement, in an intravenous cocaine self-administration paradigm in rats. Lv-siRNAs or control GFP-expressing vectors (Lv-GFP) were stereotaxically injected in the medial NAc shell or the dorsal NAc core of rats. After two weeks of recovery, rats were trained to acquire cocaine SA, at the dose of 0.25 mg/kg, under FR-1 schedule of operant responding, then increased to FR-5 protocol during maintenance phase, in 1-h daily sessions. Injection of Lv-siRNAs in the medial shell consistently impaired acquisition and maintenance of i.v cocaine SA, while infusion of Lv-GFP in the same area did not affect acquisition nor maintenance. In contrast, injection of Lv-siRNAs in the NAc core did not affect acquisition or maintenance of operant responding behaviour and drug intake, during i.v. cocaine SA, if compared with Lv-GFP control group. In order to verify extent and localization of Drd1a silencing, D1R expression and functional activity was analyzed by immunohistochemical detection of D1R protein and amphetamine-induced cFos expression. Both markers were reduced in the shell or in the core areas transfected by Lv-siRNAs, but only Lv-siRNAs shell group showed a delayed and reduced pattern of operant behavior and cocaine intake. These results strongly suggest that D1R in the NAc shell, but not in the core, are crucial for acquisition and maintenance of response-contingent i.v. cocaine SA, since silencing of Drd1a mRNA in the NAc core did not modify cocaine reward and/or instrumental learning in this self-administration paradigm. 1Bari & Pierce, 2005. Neuroscience, 135(3):959-68. 2Bachtell et al., 2005. Psychopharmacology (Berl), 183(1):41-53. 3Di Ciano, 2008. Eur. Neuropsychopharmacol., 18(12):888-96. 4Veenemanet al., 2012. Neuropsychopharmacology, 37(2):487-98.