Investigating the Role of Protein Kinase C Inhibitors in Amphetamine-Mediated Behaviors

Abstract

Amphetamines (AMPH) are among the most widely misused drugs in the United States and worldwide. Currently, there are no pharmacological interventions for amphetamine-use disorder. A close examination of mechanism of action of AMPH identified protein kinase C (PKCβ) as a potential target for a new therapeutic intervention. The reinforcing effects of AMPH are elicited through an increase in extracellular dopamine levels in the brain. PKCβ inhibition attenuates AMPH-stimulated increases in dopamine levels in vivo and ex vivo, leading to the possibility that PKCβ inhibition decreases the reinforcing effects of AMPH. These findings highlight the potential for developing PKCβ inhibitors as a pharmacological intervention for AMPH-use disorder. To assess the feasibility of utilizing PKCβ inhibitors as a treatment for AMPH-use disorders, I examined whether pharmacological inhibition of PKCβ attenuated AMPH-stimulated behaviors. I used the PKCβ inhibitors in proof-of-concept studies demonstrating that PKCβ inhibitors decrease AMPH-stimulated locomotor activity and AMPH self-administration. Using a self-administration model to investigate the reinforcing properties of AMPH, I showed that an intracerebroventricular injection of a PKCβ inhibitor 18 hr prior to a self-administration session decreased responding for AMPH under a fixed-ratio 5 schedule of reinforcement and shifted the ascending limb of the AMPH dose-effect curve for self-administration to the right. Furthermore, PKCβ inhibitors decreased responding for AMPH under a progressive-ratio schedule of reinforcement, demonstrating that PKCβ inhibition decreased the reinforcing strength of AMPH and the motivation to work for AMPH. PKCβ inhibitors did not decrease responding for sucrose under a fixed-ratio or progressive ratio schedule, indicating that the doses tested did not inhibit general behavior. These properties are desirable for a pharmacological intervention for substance-use disorders and support the development of PKCβ inhibitors as new therapeutics for the treatment of AMPH-use disorder. Interestingly, my findings suggested that PKCβ inhibitors acted acutely to decrease AMPH-stimulated behaviors but also decreased AMPH-stimulated behaviors through a secondary mechanism. To probe these mechanisms, I compared locomotor activity and protein expression at different time points following administration of a PKCβ inhibitor into the nucleus accumbens (NAc) or ventral tegmental area (VTA). I found that PKCβ inhibition in the NAc acutely decreased AMPH-stimulated locomotor activity whereas PKCβ administration into the VTA required an extended period of time before attenuating locomotor activity. Furthermore, PKCβ levels were decreased in the VTA 18 hr following administration of a PKCβ inhibitor. These findings support the hypothesis that PKCβ inhibitors act through a secondary mechanism, possibly the downregulation of PKCβ, to decrease AMPH-mediated behaviors. Altogether, these findings support the use of PKCβ inhibitors as a new therapeutic for AMPH-use disorder and further our understanding of the mechanism by which the inhibitors act to decrease AMPH-stimulated behaviors.