Figure 6: Ruboxistaurin inhibits dopamine overflow elicited by amphetamine (AMPH) and cocaine by two different mechanisms. a. (Cocaine, left side).

Cocaine increases extracellular dopamine by blocking the dopamine transporter⁶⁴. Because cocaine blocks the reuptake of dopamine at the dopamine transporter that has been released by exocytosis, its effect will be regulated by D2-like autoreceptor activity. PKC activation normally inhibits dopamine autoreceptor activity by reducing surface expression of D2 receptors which will contribute to the concentration of extracellular dopamine^{24–25, 65}. a. (Cocaine + rubox, right side) Ruboxistaurin inhibits PKCβ, increasing the surface expression of D2 autoreceptors by blocking their internalization thus enhancing their effectiveness in inhibiting exocytosis of dopamine. This leads to a reduction in extracellular dopamine (dopamine overflow). b. (AMPH, left side) The mechanism of amphetamine action is dependent on the dopamine transporter. Amphetamine is a substrate for the dopamine transporter and will elicit dopamine efflux through reversal of the transporter⁶⁶. Amphetamine activity does not depend on dopamine exocytosis and is not significantly regulated by dopamine autoreceptors^41–42. Amphetamine that is taken up through the transporter activates PKCβ^6–7 which potentiates reversal of the transporter and dopamine efflux^{5, 9}. b. (AMPH + Rubox, right side) Blockade of PKCβ reduces the outward transport of dopamine but does not affect inward transport^{2–4, 10–11, 54–55}. The end result of ruboxistaurin action on activation by either cocaine or amphetamine is a reduction in extracellular dopamine (or norepinephrine) and a reduction in stimulant-induced locomotor activity.