Lack of β2-AR Increases Anxiety-Like Behaviors and Rewarding Properties of Cocaine

Abstract

It is well known that β-adrenoceptors (β-ARs) play a critical role in emotional arousal and stressful events, but the specific contributions of the β2-AR subtype to the psychological disorders are largely unknown. To investigate whether β2-AR are involved in anxiety-like behavior and reward to addictive drugs, we conducted a series of behavioral tests on β2-AR knock-out (KO) mice. β2-AR KO mice exhibited increased preference for the dark compartment and closed arm in tests of Light/Dark box and elevated plus maze, indicating that β2-AR deletion elevates level of anxiety or innate fear. β2-AR KO mice also showed decreased immobility in tail suspension test (TST), suggesting that β2-AR deletion inhibits depression-like behavior. Interestingly, β2-AR ablation did not change basal locomotion but significantly increased locomotor activity induced by acute cocaine administration. β2-AR KO mice showed enhanced place preference for cocaine, which could be attenuated by β1-selective AR antagonist betaxolol. Consistently, β2-AR agonist suppressed cocaine-conditioned place preference (CPP). These data indicate that β2-AR deletion enhances acute response and reward to cocaine. Our results suggest that β2-AR regulates anxiety level, depression-like behavior and hedonic properties of cocaine, implicating that β2-AR are the potential targets for the treatment of emotional disorders and cocaine addiction.

Keywords: anxiety; cocaine; depression; reward; β2-AR.

Figure 1

Sequence of behavioral tests. The anxiety level and depression level tests were carried out following the first and second cohorts, respectively. Acute cocaine response and cocaine conditioned place preference (CPP) were tested following the third and fourth cohorts, respectively (the first cohort: wild type (WT) n = 26–27, heterozygous (HET) n = 17, knock-out (KO) n = 27; the second cohort: WT n = 12, HET n = 18, KO n = 16; the third cohort: WT n = 9, HET n = 6, KO n = 9; the forth cohort: WT n = 18, HET n = 8, KO n = 20).

Figure 2

Anxiety level was elevated in β2-adrenoceptors (β2-AR) KO mice. (A) Representative images of β2-AR and β1-AR expression in the hippocampus of the WT and β2-AR KO mice by WB analysis and the quantification of β1-AR and β2-AR expression in hippocampus membrane fractions. (B) Representative images of β2-AR and β1-AR expression in the medial prefrontal cortex (mPFC) of the WT and β2-AR KO and the quantification of β1-AR and β2-AR expression in mPFC membrane fractions. (C,D) The anxiety level of WT, HET and β2-AR KO mice. (C) Light/Dark box test WT: n = 27, HET: n = 17, β2-AR KO: n = 27. (D) Elevated plus maze test. WT: n = 26, HET: n = 17, β2-AR KO: n = 27. Data are presented as mean ± SEM *p < 0.05 vs. WT, **p < 0.01 vs. WT, ***p < 0.001 vs. WT.

Figure 3

Depression-like behavior was suppressed in β2-AR KO mice. The performance of WT, HET and β2-AR KO mice in Tail-suspension (A,B) and Forced swimming (C,D) tests. Total immobility time during 5-min period (A,C) in each 1-min block (B,D) are plotted. For tail-suspension test (TST): WT: n = 12; HET: n = 18; KO: n = 16. For Forced swimming test (FST): WT: n = 12; HET: n = 18; KO: n = 16). Data are presented as mean ± SEM *p < 0.05 vs. WT, **p < 0.01 vs. HET.

Figure 4

Locomotor response to acute cocaine administration increased in β2-AR KO mice. The performance of WT, HET and β2-AR KO mice in Open field (OF) (A–E). (A) Total distance traveled in the whole arena for 30 min. (B) Distance traveled during each 5-min block in the whole arena. (C) Time spent and (D) distance traveled in the center arena (one-quarter of the total area around the center of the arena). (E) Number of entrances into center arena (WT: n = 27, HET: n = 17, KO: n = 27). Data are presented as the mean ± S.E.M. The locomotor activity induced by an injection of cocaine (20 mg/kg, i. p.) was tested after 30-min adaptation in OF in WT, HET and β2-AR KO mice (F,G). (F) Total distance traveled in the whole arena for a 60-min duration after cocaine injection. (G) Distance traveled during each 5-min block in the whole arena (WT: n = 9, HET: n = 6, KO: n = 9). Data are presented as the mean ± SEM. *p < 0.05 vs. WT and HET, **p < 0.01 vs. WT and HET, ***p < 0.001 vs. WT and HET.

Figure 5

Preference for cocaine paired side was enhanced in β2-AR KO mice. The cocaine-CPP was conducted in WT, HET and β2-AR KO mice. After three daily conditioning, mice were tested for the preference for cocaine paired side. (A,B) Cocaine-CPP procedure (A) and the memory retention test (B). WT: n = 18, HET: n = 8, KO: n = 20. *p < 0.05 vs. WT. (C,D) Cocaine-CPP procedure (C) and the memory retention test (D) for C57 mice. Clenbuterol (Clen, 5 mg/kg, i.p.) or saline was administrated 30 min before cocaine conditioning. n = 14–20 per group. *p < 0.05 vs. saline group. (E,F) Cocaine-CPP procedure (E) and the memory retention test (F) for WT (left) and β2-AR KO (right) mice. Betaxolol (Bet, 10 mg/kg, i.p.) or saline was injected 30 min before cocaine conditioning. WT/Sal: n = 24, WT/Bet: n = 18, KO/Sal: n = 17, KO/Bet: n = 14). Data are presented as the mean ± SEM. **p < 0.01 and ***p < 0.001 vs. saline group.