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Link to original content: https://pubmed.ncbi.nlm.nih.gov/24102293
Extended access of cocaine self-administration results in tolerance to the dopamine-elevating and locomotor-stimulating effects of cocaine - PubMed Skip to main page content
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. 2014 Jan;128(2):224-32.
doi: 10.1111/jnc.12452. Epub 2013 Oct 21.

Extended access of cocaine self-administration results in tolerance to the dopamine-elevating and locomotor-stimulating effects of cocaine

Affiliations

Extended access of cocaine self-administration results in tolerance to the dopamine-elevating and locomotor-stimulating effects of cocaine

Erin S Calipari et al. J Neurochem. 2014 Jan.

Abstract

Tolerance to the neurochemical and psychoactive effects of cocaine after repeated use is a hallmark of cocaine addiction in humans. However, comprehensive studies on tolerance to the behavioral, psychoactive, and neurochemical effects of cocaine following contingent administration in rodents are lacking. We outlined the consequences of extended access cocaine self-administration as it related to tolerance to the psychomotor activating, dopamine (DA) elevating, and DA transporter (DAT) inhibiting effects of cocaine. Cocaine self-administration (1.5 mg/kg/inj; 40 inj; 5 days), which resulted in escalation of first hour intake, caused reductions in evoked DA release and reduced maximal rates of uptake through the DAT as measured by slice voltammetry in the nucleus accumbens core. Furthermore, we report reductions in cocaine-induced uptake inhibition and a corresponding increase in the dose of cocaine required for 50% inhibition of DA uptake (Ki ) at the DAT. Cocaine tolerance at the DAT translated to reductions in cocaine-induced DA overflow as measured by microdialysis. In addition, cocaine-induced elevations in locomotor activity and stereotypy were reduced, while rearing behavior was enhanced in animals with a history of cocaine self-administration. Here, we demonstrate both neurochemical and behavioral cocaine tolerance in an extended-access rodent model of cocaine abuse, which allows for a better understanding of the neurochemical and psychomotor tolerance that develops to cocaine in human addicts. We demonstrate tolerance to the neurochemical and behavioral effects of cocaine following extended-access cocaine self-administration. With respect to neurochemistry, we show reduced cocaine-induced dopamine uptake inhibition, an increased dose of cocaine required for 50% inhibition of the dopamine transporter, and reduced cocaine-induced dopamine overflow. In addition, we show escalation of cocaine intake and reduced cocaine-induced locomotor activity following cocaine self-administration.

Keywords: cocaine; dopamine; rat; self-administration; striatum; tolerance.

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Conflict of interest statement

Conflict of interest: The authors have no conflicts to report.

Figures

Figure 1
Figure 1. Cocaine self-administration results in escalation of first-hour cocaine intake
Group data presented as cocaine injections within the first hour of the self-administration session over five consecutive sessions (n = 16). The graph demonstrates that first-hour cocaine intake increases over session, indicating escalation in the amount of cocaine consumption over days. **, p < 0.01; ***, p < 0.001
Figure 2
Figure 2. Reduced dopamine (DA) release and uptake following cocaine self-administration
(A) Representative DA traces indicating that stimulated DA release is reduced following cocaine self-administration in the nucleus accumbens core. Plots are represented as DA concentration over time. (B) Grouped data showing changes in µM DA release evoked by electrical stimulation of the terminal (n = 9 per group). Cocaine self-administration results in a decrease in stimulated DA release relative to control animals. (C) Representative DA traces shifted to match for peak DA concentration, highlight differences in maximal rate of DA uptake in the nucleus accumbens core. (D) Group data (n = 9 per group) indicate that cocaine self-administration results in reduced maximal rate of uptake as compared to control animals. **, p < 0.01; SA, self-administration; Vmax, maximal rate of dopamine uptake
Figure 3
Figure 3. Cocaine self-administration results in tolerance to the dopamine transporter (DAT)-inhibiting and dopamine (DA)-elevating effects of cocaine
(A) Representative DA traces (normalized peak height) plotted over time highlight reduced ability of cocaine to inhibit DA uptake in the nucleus accumbens core of animals with a history of cocaine self-administration. (B) Group data showing the potency of cocaine in naïve and cocaine self-administering animals, plotted as apparent Km over cocaine concentration (n = 9 controls, 7 self-administration animals). Cocaine potency is reduced over a dose-response (0.3–30 µM) following cocaine self-administration. (C) The Ki for cocaine was significantly increased in animals with a history of cocaine self-administration (n = 9 controls, 7 self-administration animals). Increased Ki values indicate that higher concentrations of drug are needed in cocaine self-administering animals, to achieve the same degree of DA uptake inhibition in naïve animals. (D) DA overflow in the NAc was measured by microdialysis in freely-moving animals (n = 8 controls, 6 self-administration animals). Pre-drug baselines were determined, animals were injected with 15 mg/kg cocaine i.p., and cocaine-induced elevations in DA overflow were monitored. *, p < 0.05; **, p < 0.01; ***, p < 0.001; SA, self-administration; Sec, seconds; Min, minutes
Figure 4
Figure 4. Cocaine self-administration results in tolerance to the locomotor activating effects of acute cocaine administration
(A) Group data of locomotor activity as measured in cm over time plotted in five-minute bins (n = 7 per group). Data indicate that a history of cocaine self-administration results in reduced cocaine (15mg/kg; i.p.)-induced locomotor activity. (A, inset) Saline-induced locomotion was not different between controls and animals with a history of cocaine self-administration. (B) Cocaine-induced locomotor activity represented as percent of saline injection (n = 7 per group). (C) Cocaine-induced stereotypy represented as average stereotypy counts per five-minute bin over the 90-minute session (n = 7 per group). (D) Cocaine-induced vertical activity represented as average vertical counts per five-minute bin over the 90-minute session (n = 7 per group). *, p < 0.05; **, p < 0.01; ***, p < 0.001; SA, self-administration

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