In the current study cocaine did not
In the current study, cocaine did not significantly increase negative subjective effects and nepicastat did not increase these responses. This outcome is somewhat surprising given that the most commonly reported effect to date has been to increase the aversive properties of cocaine such as anxiety, paranoia, and psychosis in humans or in rodent models (Gaval-Cruz et al., 2012, Mutschler et al., 2009, Schank et al., 2006). Furthermore, genetic thyroid hormone receptor of DBH enhances cocaine-induced paranoia in humans (Cubells et al., 2000, Kalayasiri et al., 2007) and transforms a cocaine conditioned place preference into a place aversion in mice (Schank et al., 2006). This is important since a preferential increase in the aversive effects of cocaine could explain why a medication that enhances cocaine responses (e.g., Haile et al., 2003, Hameedi et al., 1995, Manvich et al., 2013, McCance-Katz et al., 1998a, McCance-Katz et al., 1998b) reduces cocaine use. The reason for a lack of effect nepicastat on cocaine-induced negative subjective effects in the current study may be a function of the small sample size, but also the relatively low doses of cocaine evaluated.
The primary limitation of the present study was a small sample size, although the final N is typical of many inpatient studies. The sample size contributes to the possibility that the study was underpowered to detect an effect of nepicastat on some outcome measures. A second limitation was that efficacy data were based solely on participants randomized to nepicastat using a within-subjects study design for statistical analyses. While a more favorable comparison would have been with a group of participants who received placebo for the same study duration (13days), the within subject comparison was prospectively planned, and placebo was included in the design to maintain the double-blind. A final limitation was that subjects received multiple randomized doses of cocaine at 1-hour intervals within the same session, thus the report for each individual dose of cocaine may have been influenced by previous infusions.
Dopamine β-hydroxylase (DBH) is an enzyme that catalyzes the conversion of dopamine into norepinephrine. An imbalance of these neurotransmitters can lead to many neurological and psychiatric disorders. A key step in the mechanism involves an electron transfer between two copper centers. Based on the structure of other hydroxylases, mechanisms invoking organized water networks have been proposed, where the electron is transferred over a distance greater than 10 Å. X-ray crystallography has recently shown that DBH is a homodimer in which the distance between the two copper centers can span both short (5 Å) and long (15 Å) distances. Our hypothesis is that the protein can switch between open and closed states at a rate that is compatible with catalysis. In the closed state, the short-distance between the two-copper center is exploited to transfer the electron, whereas the open state facilitates substrate binding and release. To test this hypothesis, we used molecular dynamic simulations. We have developed the parameters necessary to describe the copper centers with the CHARMM molecular mechanics force field, reconstructed regions missing in the structure, and refined the structure to better account for disulfide bond formation and hydrogen bond networks. Finally, we used free energy perturbations of Na, K, Mg, and Ca to determine the most stable ion configuration in the putative ion binding site in the DOMON domain. The results from our initial computer simulations are compatible with the proposed hypothesis. If our hypothesis stands further testing, it would provide an alternative mechanism that can explain the electron transfer between the copper centers without the intervention of organized water networks.
Cocaine dependence is common, with over 1.5 million actively cocaine-dependent people in 2011 who have substantial social and economic morbidity from it, but it has no US Food and Drug Administration-approved pharmacotherapy (, ). In methadone maintenance programs rates of cocaine use range from 30% to 50% and lead to poorer outcomes and higher incidence of human immunodeficiency virus risk behaviors (, , , , , , , , , , ). Although a number of innovative pharmacological approaches have had limited success in reducing cocaine use (, , ), disulfiram has shown some initial promise in treating cocaine dependence in both nonopioid-dependent (, , ) and opioid-dependent cocaine abusers (, ).