Behavior of knock-in mice with a cocaine-insensitive dopamine transporter after virogenetic restoration of cocaine sensitivity in the striatum

Highlights

• Mutant DAT knock-in mice do not display cocaine-induced reward or hyperlocomotion.

• Expressing wild-type DAT in the striatum of these mice restores cocaine stimulation.

• Expressing wild-type DAT in striatal subregions does not restore cocaine reward.

Abstract

Cocaine's main pharmacological actions are the inhibition of the dopamine, serotonin, and norepinephrine transporters. Its main behavioral effects are reward and locomotor stimulation, potentially leading to addiction. Using knock-in mice with a cocaine-insensitive dopamine transporter (DAT-CI mice) we have shown previously that inhibition of the dopamine transporter (DAT) is necessary for both of these behaviors. In this study, we sought to determine brain regions in which DAT inhibition by cocaine stimulates locomotor activity and/or produces reward. We used adeno-associated viral vectors to re-introduce the cocaine-sensitive wild-type DAT in specific brain regions of DAT-CI mice, which otherwise only express a cocaine-insensitive DAT globally.

Viral-mediated expression of wild-type DAT in the rostrolateral striatum restored cocaine-induced locomotor stimulation and sensitization in DAT-CI mice. In contrast, the expression of wild-type DAT in the dorsal striatum, or in the medial nucleus accumbens, did not restore cocaine-induced locomotor stimulation. These data help to determine cocaine's molecular actions and anatomical loci that cause hyperlocomotion. Interestingly, cocaine did not produce significant reward – as measured by conditioned place-preference – in any of the three cohorts of DAT-CI mice with the virus injections. Therefore, the locus or loci underlying cocaine-induced reward remain underdetermined. It is possible that multiple dopamine-related brain regions are involved in producing the robust rewarding effect of cocaine.

Introduction

Cocaine is an inhibitor of the dopamine (DA), norepinephrine (NE), and serotonin (5-HT) transporters (Han and Gu, 2006, Ritz et al., 1987). It is simultaneously an addictive drug with euphorigenic effects, and adverse cardiovascular and psychiatric effects (Rotheram-Fuller et al., 2007). While inhibition of each of the monoamine transporters is likely to contribute to each of cocaine's effects in some way, there has been much effort to determine the specific role of each target in producing a behavioral effect. Knock-out mice with each of the monoamine transporters deleted still self-administer cocaine, indicating that none of these targets are individually required for its rewarding effect (Hall et al., 2002). Double knock-out mice lacking the dopamine transporter (DAT) and the serotonin transporter (SERT) do not show cocaine reward, suggesting that the monoamines are mutually or redundantly involved in producing cocaine reward (Uhl et al., 2002). However, the knock-out mice may have substantial adaptive changes. In contrast, using knock-in mice with a functional yet cocaine-insensitive dopamine transporter (DAT-CI mice), we determined that DAT inhibition is necessary for cocaine's rewarding and hyperlocomotive effects (Chen et al., 2006).

It is well established that the mesolimbic dopamine system, containing dopaminergic projections from ventral tegmental area (VTA) to the nucleus accumbens (NAc) and other forebrain structures, plays a critical role in reward/reinforcement and that most addictive drugs elevate extracellular dopamine in the NAc (Carboni et al., 1989, Cass et al., 1992, Di Chiara, 1995, Koob, 1998). It was shown that rats self-administer amphetamine directly into the NAc (Hoebel et al., 1983), however, cocaine infusion to the NAc did not produce place-conditioning (Hemby et al., 1992). Additionally, cocaine was found to be readily self-administered into the prefrontal cortex (PFC) (Goeders and Smith, 1983). Interestingly, cocaine self-administration to the dorsal, and also to the ventral striatum (nucleus accumbens), is sufficient to induce hyperlocomotion (Delfs et al., 1990, Mao and Wang, 2000). It is therefore important to directly test how cocaine inhibition of DAT in various brain regions contributes to different behavioral responses induced by cocaine.

Here, we set out to determine specifically where DAT inhibition in the brain is involved in producing cocaine reward and hyperlocomotion. We tested the function of DAT inhibition in a specific region by restoring expression of the wild-type dopamine transporter (DAT_wt) in DAT-CI mice using adeno-associated viruses (AAV). We injected DAT-CI mice with AAV-DAT_wt in the dorsal striatum (dCPu), medial nucleus accumbens (mNAc), and in both the dorsal and ventral portions of the lateral striatum (lCPu). After expression of DAT_wt in these regions, we exposed the DAT-CI mice to cocaine, and tested for restoration of the reward and locomotor behaviors.