Endogenous kappa-opioid receptor systems regulate mesoaccumbal dopamine dynamics and vulnerability to cocaine - PubMed (original) (raw)

Comparative Study

Endogenous kappa-opioid receptor systems regulate mesoaccumbal dopamine dynamics and vulnerability to cocaine

Vladimir I Chefer et al. J Neurosci. 2005.

Abstract

Genetic and pharmacological approaches were used to examine kappa-opioid receptor (KOR-1) regulation of dopamine (DA) dynamics in the nucleus accumbens and vulnerability to cocaine. Microdialysis revealed that basal DA release and DA extraction fraction (Ed), an indirect measure of DA uptake, are enhanced in KOR-1 knock-out mice. Analysis of DA uptake revealed a decreased Km but unchanged Vmax in knock-outs. Knock-out mice exhibited an augmented locomotor response to cocaine, which did not differ from that of wild-types administered a behavioral sensitizing cocaine treatment. The ability of cocaine to increase DA was enhanced in knock-outs, whereas c-fos induction was decreased. Although repeated cocaine administration to wild types produced behavioral sensitization, knock-outs exhibited no additional enhancement of behavior. Administration of the long-acting KOR antagonist nor-binaltorphimine to wild-type mice increased DA dynamics. However, the effects varied with the duration of KOR-1 blockade. Basal DA release was increased whereas Ed was unaltered after 1 h blockade. After 24 h, release and Ed were increased. The behavioral and neurochemical effects of cocaine were enhanced at both time points. These data demonstrate the existence of an endogenous KOR-1 system that tonically inhibits mesoaccumbal DA neurotransmission. Its loss induces neuroadaptations characteristic of "cocaine-sensitized" animals, indicating a critical role of KOR-1 in attenuating responsiveness to cocaine. The increased DA uptake after pharmacological inactivation or gene deletion highlights the plasticity of mesoaccumbal DA neurons and suggests that loss of KOR-1 and the resultant disinhibition of DA neurons trigger short- and long-term DA transporter adaptations that maintain normal DA levels, despite enhanced release.

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Figures

Figure 1.

Basal DA dynamics in the NAc of WT, HET, and KOR-1 KO mice. A, Plot of the average gain or loss of DA (_C_in - _C_out) to or from perfusate and the average linear regression fit of the data from no-net-flux microdialysis for each experimental group. The slope of the regression line represents the _E_d. The point when no DA is gained or lost from the perfusate (_C_in - _C_out = 0) represents an unbiased estimate of DAext concentration. The dotted line represents the average regression line for KO mice and has a significantly greater slope relative to WT controls (solid line). B, Bar graph of basal dialysate levels (nanomolar) expressed as means ± SEM; n indicates number of animals per experimental group. * indicates significant difference in dialysate DA levels between WT and KO animals (Newman-Keuls multiple comparison test). C, Bar graph of basal DAext (nanomolar) expressed as means ± SEM. ANOVA revealed no significant difference in this parameter between genotypes (F(2,47) = 0.83; p = 0.44). D, Bar graph of _E_d expressed as the mean ± SEM. * denotes significant difference in _E_d (Newman-Keuls multiple comparison test).

Figure 2.

Unaltered levels of DAT in KOR-1 KO mice of mixed (C57BL/6J × 129S6) and congenic (C57BL/6J) backgrounds are shown by Western blot analysis. For quantification, the intensity of each DAT band was normalized to the β-actin band for each lane. The mean relative density and SEM were calculated for each pair. Comparisons between genotypes were made using an unpaired t test with Welch's correction. For mixed background, the mean relative density for WT is 0.73 ± 0.21 versus a mean of 0.76 ± 0.07 for KO (p = 0.914). For the congenic background, WT has a mean of 0.65 ± 0.02 versus KO with a mean of 0.79 ± 0.13 (p = 0.468). No statistically significant differences were seen. The immunoblots are representative comparisons.

Figure 3.

Basal and cocaine-induced locomotor activity and DA levels in the NAc in WT, HET, and KOR-1 KO mice. A, Time course of dialysate DA levels in the NAc in WT (filled squares), HET (filled triangles), and KOR-1 KO (open circles) mice. Each point represents the mean ± SEM; n indicates the number of animals per experimental group. Abscissa, Microdialysis fractions (15 min) before and after cocaine injections. Vertical dotted lines correspond to the time of saline (sal) and cocaine (coc) (5, 10, and 15 mg/kg) injections. B, AUC values for cocaine-evoked DA levels expressed as means ± SEM. * denotes significant difference between WT and KO mice. C, The time course of locomotor activity before and after cocaine challenge in WT (filled squares), HET (filled triangles), and KOR-1 KO (open circles) mice. Each point represents the mean ± SEM; n indicates the number of animals per experimental group. Abscissa, Time (minutes). Vertical dotted lines represent the time of saline and cocaine (5, 10, and 15 mg/kg) injections. D, AUC values for cocaine-evoked locomotor expressed as means ± SEM. * denotes significant difference between WT and KO mice. ** denotes significant difference between HET and KO mice.

Figure 4.

Induction of IEGs after acute cocaine challenge in WT and KOR-1 KO mice. A, A representative RPA gel shows protected fragments from WT and KOR-1 KO mice after acute injection of saline or cocaine (20 mg/kg). The housekeeping genes L32 and GAPDH were used to quantitate levels of RNA per lane. Quantitative decreases in induction of c-fos were observed in KOR-1 KO animals after this dose of cocaine. B, c-fos is significantly increased in both WT and KOR-1 KO mice after cocaine injection, but the induction is significantly less robust in KOR-1 KO mice. * denotes significant difference between saline- and cocaine-treated animals; # denotes significant difference between WT (filled bars) and KO (open bars) mice.

Figure 5.

Development of cocaine-induced behavioral sensitization in WT but not KOR-1 KO mice. A, C, Time course of locomotor activity in WT (A) and KO (C) animals (C57BL/6J background) before and after intraperitoneal injections of saline (0.1 ml/10 g) and cocaine (15 mg/kg) on different treatment days. Each point represents the mean ± SEM. Ordinate, Ambulatory distance. Abscissa, Time (minutes). Vertical dotted lines represent the time of saline and cocaine injections. B, D, AUC values for cocaine-evoked locomotor activity response expressed as means ± SEM. * denotes significant difference between WT and KO mice. ** denotes significant difference between the first and the last day of treatment in WT mice.

Figure 6.

Influence of acute or prolonged KOR-1 blockade on basal NAc DA dynamics. DA dynamics were quantified 1 h (A) and 24 h (B) after pretreatment with nor-BNI. A, B, Plots of the average gain or loss of DA (_C_in - _C_out) to or from perfusate and the average linear regression fit of the data from no-net-flux microdialysis for each experimental group. The slope of the regression line represents the _E_d. The point when no DA is gained or lost from the perfusate (_C_in - _C_out = 0) represents an unbiased estimate of DAext concentration.

Figure 7.

DA levels in the NAc and locomotor activity response to an acute cocaine challenge. A, Time course of dialysate DA levels in the NAc in control (open squares) and nor-BNI-treated (filled squares) animals. Each point represents the mean ± SEM; n indicates the number of animals per experimental group. Abscissa, Microdialysis fractions (10 min) before and after (0.1 ml/10 g, i.p.) saline and cocaine (15 mg/kg, i.p.) injections. Vertical dotted lines represent the time of saline and cocaine injections. B, AUC values for cocaine-evoked DA dialysate levels expressed as means ± SEM. * denotes significant difference between control and nor-BNI-treated animals. C, The time course of locomotor activity before and after cocaine challenge in control (open circles) and nor-BNI-treated (filled circles) animals. Each point represents the mean ± SEM; n indicates the number of animals per experimental group. Ordinate, Ambulatory distance. Abscissa, Time (minutes). Vertical dotted lines represent the time of saline and cocaine injections. D, AUC values for cocaine-evoked locomotor activity expressed as means ± SEM. * denotes significant difference between control and nor-BNI-treated animals.

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Endogenous kappa-opioid receptor systems regulate mesoaccumbal dopamine dynamics and vulnerability to cocaine - PubMed (original) (raw)