- The human nucleus accumbens is highly susceptible to G protein down-regulation by methamphetamine and heroin. McLeman, E.R., Warsh, J.J., Ang, L., Li, P.P., Kalasinsky, K.S., Ross, B.M., Tong, J., Schmunk, G., Adams, V., Kish, S.J. J. Neurochem. (2000)