Tachykinin systems have been highly conserved throughout evolution, and SP is found in the basal ganglia of all vertebrates (Holmgren and Jensen, 2001; Medina and Reiner, 1995; Smeets et al., 2000). The activity of SP in these regions suggests that it contributes to the execution of motivated behaviors. SP and its NK1R are therefore positioned at the intersection of appetitive and aversive behaviors, and provide a substrate by which these behaviors can interact. In considering specific effects of manipulating this system on drug seeking and taking, there is therefore a need to carefully consider whether effects are produced through actions that impact reward- or stress-related circuitry, or both.
