Chronic escalating cocaine exposure, abstinence/withdrawal, and chronic re-exposure: effects on striatal dopamine and opioid systems in C57BL/6J mice

Abstract

Cocaine addiction is a chronic relapsing disease with periods of chronic escalating self-exposure, separated by periods of abstinence/withdrawal of varying duration. Few studies compare such cycles in preclinical models. This study models an "addiction-like cycle" in mice to determine neurochemical/molecular alterations that underlie the chronic, relapsing nature of this disease. Groups of male C57BL/6J mice received acute cocaine exposure (14-day saline/14-day withdrawal/13-day saline + 1-day cocaine), chronic cocaine exposure (14 day cocaine) or chronic re-exposure (14-day cocaine/14-day withdrawal/14-day cocaine). Escalating-dose binge cocaine (15-30 mg/kg/injection × 3/day, i.p. at hourly intervals) or saline (14-day saline) was administered, modeling initial exposure. In "re-exposure" groups, after a 14-day injection-free period (modeling abstinence/withdrawal), mice that had received cocaine were re-injected with 14-day escalating-dose binge cocaine, whereas controls received saline. Microdialysis was conducted on the 14th day of exposure or re-exposure to determine striatal dopamine content. Messenger RNA levels of preprodynorphin (Pdyn), dopamine D1 (Drd1) and D2 (Drd2) in the caudate putamen were determined by real-time PCR. Basal striatal dopamine levels were lower in mice after 14-day escalating exposure or re-exposure than in those in the acute cocaine group and controls. Pdyn mRNA levels were higher in the cocaine groups than in controls. Long-term adaptation was observed across the stages of this addiction-like cycle, in that the effects of cocaine on dopamine levels were increased after re-exposure compared to exposure. Changes in striatal dopaminergic responses across chronic escalating cocaine exposure and re-exposure are a central feature of the neurobiology of relapsing addictive states.

Figure 1

The mean (±SEM) locomotor activity of mice that received escalating-dose binge cocaine or saline administration in each period (exposure, withdrawal and re-exposure) is shown in Figure 1. Cocaine resulted in significantly increased horizontal locomotor activity on all days of injection compared to saline controls, p < 0.000001.

Figure 2

In 2A, a photomicrograph of a tissue section from a mouse used in this study shows the location of the microdialysis probe in the brain. The mean (±SEM) levels of dopamine in the dialysate from the striatum during the baseline, in the hour after each injection of mice which received either binge cocaine or saline injections, are shown in Figure 2B. The arrows indicate cocaine or saline injections. Basal (pre-injection) dopamine levels in both the chronic cocaine and chronic cocaine re-exposure groups were significantly lower than in the saline/saline re-exposure controls. Levels of dopamine expressed as percent increases over baselines in the 3 hours after cocaine or saline injections are shown in Figure 2C.

Figure 3

The mean (±SEM) percent increases in dopamine over the 3 hours after cocaine or saline injections are shown in Figure 3. The increase in striatal dopamine levels over basal levels was significantly higher in the mice from the chronic cocaine group and chronic cocaine re-exposure group than in saline/saline re-exposure controls. Mice in the chronic cocaine re-exposure group showed a greater percent increase in dopamine compared to that observed in the chronic cocaine group.

Figure 4

The increase of dopamine in response to each cocaine or saline injection across the binge is shown in Figure 4. 4A shows absolute dopamine increases whereas 4B shows percent increases over pre-injection basal levels. Both the chronic cocaine and chronic cocaine re-exposure groups showed increases in dopamine over basal levels after the first injection. The chronic cocaine group showed no further elevation after the second and third injections. The chronic cocaine re-exposure group showed an increasing response over the course of the binge. The dopamine levels in the chronic cocaine re-exposure group were significantly higher than those of the chronic cocaine group.

Figure 5

The mean (+SEM) levels of Pdyn (A), DAT (B), Drd1 (C) and Drd2 (D) receptor mRNA levels in the caudate putamen in response to saline, chronic cocaine exposure and re-exposure are shown in Figure 5. Mice that had acute cocaine exposure, chronic cocaine exposure or chronic cocaine re-exposure had significantly higher levels of Pdyn mRNA compared to the saline/saline re-exposure control mice with no significant difference in Pdyn mRNA between chronic cocaine and chronic cocaine re-exposure groups (Figure 5A). Although one-way ANOVA showed no significant main effect of cocaine on DAT mRNA, a planned comparison found that dopamine transporter in the caudate putamen was significantly lower in the chronic cocaine re-exposure group than in the chronic cocaine group (Figure 5B).