Characterization of a functional role of the neurokinin-3 receptor in behavioral effects of cocaine

Abstract

The tachykinin NK-3 receptor is a G-protein coupled receptor activated by mammalian tachykinin neuropeptides, which can modulate dopaminergic neurotransmission, and alter dopamine-mediated behaviors. The NK-3 receptor is currently under investigation as a novel therapeutic target for cocaine addiction. Our studies, as outlined in this dissertation, sought to determine if NK-3 receptors have a functional role in the acute as well as long-term behavioral effects of cocaine. Administration of NK-3 receptor agonists or antagonists potentiates or attenuates dopamine-mediated behaviors, respectively. Based on these findings, we hypothesized that blockade of neurokinin-3 receptors would alter acute and long-term behavioral responses to cocaine. We investigated whether acute and repeated administration of the NK-3 receptor antagonist SB 222200 altered hyperactivity induced by cocaine, and determined a possible mechanism involving dopamine D1 receptors in the striatum. We also determined whether NK-3 receptor blockade altered the development and expression of behavioral sensitization after repeated cocaine administration. Lastly, we investigated whether modulation of behavioral effects of acute and repeated cocaine by NK-3 receptors involved GSK3 phosphorylation in the nucleus accumbens. As described in this dissertation, we show that acute administration of the NK-3 receptor antagonist SB 222200 before a cocaine injection attenuated stereotypic responses produced by cocaine. Repeated administration of SB 222200 enhanced stereotypic activity produced by either cocaine or a low dose of SKF 82958 (0.125 mg/kg, i.p.) when administered seven days later. Dopamine receptor binding studies were performed to determine the mechanism of enhanced stereotypic responses. Binding studies showed a 19.7% increase in dopamine D1 receptor density in the striatum seven days later after repeated SB 222200 administration. These findings demonstrate that acute blockade of NK-3 receptors attenuated cocaine-induced behaviors in agreement with previous studies. Furthermore, these studies also show novel effects of repeated blockade of NK-3 receptors, which causes subsequent enhancement of cocaine and dopamine D1 receptor-mediated behaviors, possibly resulting from dopamine D1 receptor up-regulation in the striatum. In order to determine a role of NK-3 receptors in the development of cocaine-induced behavioral sensitization, the NK-3 receptor antagonist SB 222200 (2.5 or 5 mg/kg, s.c.) was administered prior to daily cocaine injections for 5 days. After a 7-day drug-free period, behavioral responses to a cocaine challenge were measured. Repeated administration of cocaine for 5 days induced a sensitized response upon a cocaine challenge 7 days later. Administration of SB 222200 prior to daily cocaine attenuated the development of behavioral sensitization. Moreover, administration of SB 222200 prior to the cocaine challenge blocked the expression of behavioral sensitization. These findings demonstrate that NK-3 receptor activity is involved in the development and expression of behavioral sensitization to cocaine. Lastly, we examined GSK3 phosphorylation in the nucleus accumbens induced by acute and repeated cocaine administration and determined if phosphorylation was altered by NK-3 receptor blockade. Similar to the drug administration regimens used in the behavioral studies, the NK-3 receptor antagonist SB 222200 was administered 30 mins prior to an acute cocaine injection. The nucleus accumbens was examined for changes in GSK3 phosphorylation by Western blot analysis. Increases in phosphorylation of the isoforms, GSK3α and GSK3β in the nucleus accumbens were detected 20 mins after an acute injection of cocaine. NK-3 receptor blockade prior to cocaine administration did not alter the cocaine-induced increase in GSK3 phosphorylation. Similar to the behavioral sensitization studies, SB 222200 was administered prior to repeated cocaine for 5 days, and 7 days later GSK3 phosphorylation was measured after a subsequent cocaine challenge. In contrast to the increases in GSK3α and GSK3β in the nucleus accumbens after an acute cocaine injection, no regulation of GSK3 phosphorylation was found after prior repeated cocaine administration and cocaine challenge. Administration of SB 222200 prior to repeated cocaine produced an increase in GSK3α and GSK3β phosphorylation after a cocaine challenge. Collectively, these data point to involvement of NK-3 receptor activity in changes in the phosphorylation of GSK3 in the nucleus accumbens produced by cocaine. In summary, functional involvement of NK-3 receptors in acute and long-term behavioral effects of cocaine was investigated. In agreement with previous findings, studies in this dissertation demonstrate that acute blockade of NK-3 receptors attenuates cocaine-induced behaviors. In addition, we found novel effects of repeated blockade of NK-3 receptors on cocaine-induced hyperactivity. There is enhancement of subsequent cocaine and dopamine D1 receptor-mediated behaviors possibly due to dopamine D1 receptor up-regulation in the striatum. NK-3 receptor activity was shown to be involved in long-term behavioral effects of cocaine and molecular changes in GSK3 phosphorylation in the nucleus accumbens. Blockade of NK-3 receptors prevented the development and expression of behavioral sensitization to cocaine and also blocked the changes in the phosphorylation of GSK3 in the nucleus accumbens. This dissertation has demonstrated a role of NK-3 receptors in modulating acute as well long-term cocaine-induced behavioral hyperactivity. Therefore, there is potential clinical relevance of NK-3 receptors in cocaine abuse and dependence as a therapeutic target for treatment, which warrants further characterization in future preclinical and clinical investigations.