Yohimbine impairs extinction of cocaine-conditioned place preference in an alpha2-adrenergic receptor independent process

Abstract

Extinction, a form of learning that has the ability to reshape learned behavior based on new experiences, has been heavily studied utilizing fear learning paradigms. Mechanisms underlying extinction of positive-valence associations, such as drug self-administration and place preference, are poorly understood yet may have important relevance to addiction treatment. Data suggest a major role for the noradrenergic system in extinction of fear-based learning. Employing both pharmacological and genetic approaches, we investigated the role of the alpha(2)-adrenergic receptor (alpha(2)-AR) in extinction of cocaine-conditioned place preference (CPP) and glutamatergic transmission in the bed nucleus of the stria terminalis (BNST). We found that pre-extinction systemic treatment with the alpha(2)-AR antagonist yohimbine impaired cocaine CPP extinction in C57BL/6J mice, an effect that was not mimicked by the more selective alpha(2)-AR antagonist, atipamezole. Moreover, alpha(2A)-AR knockout mice exhibited similar cocaine CPP extinction and exacerbated extinction impairing effects of yohimbine. Using acute brain slices and electrophysiological approaches, we found that yohimbine produces a slowly evolving depression of glutamatergic transmission in the BNST that was not mimicked by atipamezole. Further, this action was extant in slices from alpha(2A)-AR knockout mice. Our data strongly suggest that extinction-modifying effects of yohimbine are unlikely to be due to actions at alpha(2A)-ARs.

Figure 1.

C57BL/6J mice administered yohimbine 5 mg/kg (i.p.) during extinction sessions display impaired extinction. (A) Schematic of cocaine CPP extinction. Mice are trained with 20 mg/kg cocaine during CPP training. After mice show a preference for the cocaine-paired side (CS+), they are subjected to extinction training. (B,C) C57BL/6J mice acquire place preference for the cocaine-paired side (n = 30, saline group; n = 18, yohimbine group). (D,E) Mice were extinguished over 6 d. Shown is time spent on the cocaine-paired side and saline-paired side during post-test and extinction. Mice were given an injection of saline or yohimbine and placed in home cage; 30–35 min later, mice were given an injection of saline and placed in chambers. Error bars, ±SEM. **P < 0.01 comparison of time spent on white side of chamber in comparison to the CS+ post-test value (solid black line). Dotted line in panels B and C indicates half of test period (450 sec).

Figure 2.

Effects of yohimbine on extinction of fear in C57BL/6J mice. (A) Schematic of fear extinction time line. (B) Yohimbine was administered to mice 20 min prior to extinction session. Shown is the percentage of time freezing during the test during conditioned stimulus exposures (five exposures per bin). ψ is P < 0.01 saline vs. yoh (2.5 mg/kg). Φ is P < 0.01 saline vs. yoh (5.0 mg/kg). Ω is P < 0.05 saline vs. yoh 2.5 mg/kg. (C) Shown is the percentage of time freezing on day 3 of extinction. (n = 8–9 per group). Error bars, ±SEM.

Figure 3.

Yohimbine (5 mg/kg) administered during extinction sessions impairs extinction of cocaine CPP in α_2A-AR KO and WT littermates. Mice were administered either saline or yohimbine and placed in the home cage; 30–35 min later, mice were given an injection of saline and placed in chambers. (A) WT littermates obtain place preference. (n = 16). (B) Shown is time spent on each side of the chamber of WT cocaine trained and saline extinguished. (n = 8). (C) Shown is time spent on each side of chamber of WT cocaine trained and yohimbine extinguished. (n = 8). (D) KO mice obtain place preference. (n = 22). (E) Shown is time spent on each side of the chamber of KO mice cocaine trained and saline extinguished. (n = 12). (F) Shown is time spent on each side of chamber of KO mice cocaine trained and yohimbine extinguished. (n = 10). Error bars, ±SEM. *P < 0.05, **P < 0.01. Dotted line (panels A,D) indicates half of test period (450 sec).

Figure 4.

KO mice extinguished with yohimbine exhibit impaired extinction compared to saline extinguished KO mice. Percentage difference from preconditioning and extinction days for each genotype on 2, 4, and 6 for cocaine trained mice. Shown is the percentage of time on CS+ side during preconditioning minus percentage of time on CS+ during extinction. Bars, ±SEM. WT (n = 8), KO (n = 10–12).

Figure 5.

Effect of atipamezole (s.c.) on fear extinction in C57BL/6J mice. (A) Atipamezole (ati) was administered to mice 20 min prior to extinction session. Shown is the percentage of time freezing during the test during conditioned stimulus exposures (five exposures per bin). (B) Shown is the percentage of time freezing on the third test day of extinction. Error bars, ±SEM. (n = 13–14 per group).

Figure 6.

C57BL/6J mice administered atipamezole extinguish place preference. (A) Mice acquire place preference for the cocaine-paired side. (B) Mice were given atipamezole (n = 12) and placed in home cage; 30–35 min later, mice were given an injection of saline and placed in chambers. Error bars, ±SEM. **P < 0.01 comparison of time spent on white side of chamber in comparison to the CS+ post-test value (solid black line).

Figure 7.

EPSCs in the dlBNST: (A) 1 μM UK14,304 (n = 1); (B) 10 μM yohimbine (n = 10); (C) 10 μM yohimbine in α_2A-AR KO mouse (n = 6); (D) 1 μM atipamezole (n = 10); (E) 1 μM UK14,304 in the presence of 1 μM atipamezole (n = 3). Error bars, ±SEM.