aversive stimulus (Lammel and Malenka, 2010). The exclusively DAergic nature of VTA projection neurons has also been questioned. Using optogenetic tools unambiguous functional evidence for co-release of DA and glutamate from VTA projection neurons has been provided (Tecuapetla et al., 2010; Stuber et al., 2010). In addition, pure glutamatergic and GABAergic projection neurons also exist in the VTA (Yamaguchi et al., 2007; Margolis et al., 2008). Whether the activity of the latter is affected by addictive drugs and how co-release of glutamate affects drug action is largely unknown.
