Cocaine sensitization in male rats requires activation of estrogen receptors

Abstract

Gonadal steroids play a modulatory role in cocaine use disorders, and are responsible for many sex differences observed in the behavioral response to cocaine. In females, it is well established that estradiol enhances the behavioral response to cocaine. In males, we have recently shown that testosterone enhances sensitization to cocaine but its mechanism of action remains to be elucidated. The current study investigated the contribution of DHT, a non-aromatizable androgen, and of estradiol, in regulating cocaine-induced sensitization in male rats. Gonadectomized (GDX) male rats treated with estradiol sensitized to repeated cocaine administration, while GDX rats treated with DHT did not, implicating estradiol in cocaine sensitization. Furthermore, intact male rats treated with the antiestrogen ICI 182,780 did not show sensitization to repeated cocaine. This study demonstrates the pivotal role of estradiol in cocaine-induced neuroplasticity and neuroadaptations in the rodent brain.

Keywords: ICI 182 780; Sensitization; androgens; antiestrogen; cocaine; dihydrotestosterone; estradiol; faslodex; males; testosterone.

Figure 1.. Behavioral sensitization protocol

Five days after arrival to the Animal Facilities, rats were gonadectomized (or sham gonadectomized) and received a Silastic implant (Day −7). The INT-Veh and INT-ICI groups were also subjected to stereotaxic surgery and received an Alzet^™ osmotic mini-pump. All animals were allowed a 7 day recovery period (Day −7 to Day 0). On Day 0, rats were habituated to the locomotor activity chamber for 1 hour. From Days 1 through 5 and on Day 13, rats were injected with saline or with cocaine (15 mg/kg). On days 1, 5 and 13 rats were injected in the locomotor activity chamber, on days 2, 3 and 4 animals were injected in their home cages (see context of injection). From days 6 through 12, rats remained undisturbed in their home cages.

Figure 2.. Gonadectomy increases the acute locomotor response to cocaine, and impairs sensitization in male rats.

Data are represented by the average and the error bars represent the S.E.M. * p<0.05; ** p<0.01; *** p<0.001; **** P<0.0001. (See Tables 4 and 5 in the Appendix for complete statistical results). A and B: Time course of the locomotor activity of Intact-Sham or GDX rats injected daily with saline or with cocaine (15 mg/kg). An increase in cocaine-induced locomotor activity with time (days) was observed in intact-sham rats, and to a lesser extent, in gonadectomized (GDX), rats. The asterisks (*) represents time points in which both Day 5 and Day 13 were significantly different than Day 1 (p<0.05). The plus signs (+) represent time points in which only one of the days, Day 5 or Day 13, were significantly different than Day 1 (P<0.05). C: LMA represented as area under the curve (AUC) during the first 20 minutes after the first cocaine injection in intact-sham (INT-Sham) and gonadectomized (GDX) males. LMA of GDX males is significantly higher than that of INT-Sham rats D: Percent change of LMA of Days 5 and 13 to Day 1 of INT-Sham and GDX rats. Data represents AUC during the first 20 min after cocaine injection. INT-Sham rats showed a significant increase in cocaine-induced LMA on Days 5 and 13 compared to that displayed on Day 1, whereas GDX show an increase on Day 5, but not on Day 13.

Figure 3.. Dihydrotestosterone (DHT) has little or no effect on the acute locomotor response to cocaine and on sensitization.

Data are represented by the average and the error bars represent the S.E.M. * p<0.05; ** p<0.01; *** p<0.001; **** P<0.0001. (See Tables 4 and 5 in the Appendix for complete statistical results). A: Time course of the locomotor activity of GDX-DHT males injected daily with saline or with cocaine (15 mg/kg). An increase in cocaine-induced locomotor activity in a single time point (5 min after cocaine administration) was observed on Days 5 and 13 compared to Day 1. The asterisks (*) represents time points in which both Day 5 and Day 13 were significantly different than day 1 (p<0.05). B: LMA represented as area under the curve (AUC) during the first 20 minutes after the first cocaine injection in GDX, GDX-DHT and intact-sham (INT-Sham) males. LMA of GDX and GDX-DHT males is significantly higher than that of INT-Sham rats. C: Percent change in LMA represented as AUC during the first 20 minutes after cocaine injection in GDX and GDX-DHT males. A RM ANOVA reveals no differences across time between GDX and GDX-DHT rats. However post hoc analysis indicate that GDX males show sensitization whereas GDX-DHT males do not (See Table 4 for complete statistical results). This issue is further analyzed in the discussion section. D: Percent change in LMA represented as AUC during the first 20 minutes after cocaine injection in INT-Sham and GDX-DHT males. A RM ANOVA reveals a significant difference across time between INT-Sham and GDX-DHT rats, INT-Sham rats display sensitization whereas GDX-DHT do not.

Figure 4.. Locomotor response to cocaine of gonadectomized male rats treated with estradiol.

Data are represented by the average and the error bars represent the S.E.M. * p<0.05; ** p<0.01; *** p<0.001; **** P<0.0001. (See Tables 4 and 5 in the Appendix for complete statistical results). A: Time course of the locomotor activity of GDX estradiol-treated (EB) males injected daily with saline or with cocaine (15 mg/kg). An increase in cocaine-induced locomotor activity at all time points was observed on days 5 and 13 (except min 90) compared to Day 1. The asterisks (*) represents time points in which both Day 5 and Day 13 were significantly different than Day 1 (p<0.05). The plus sign (+) represents time points in which only one of the days, Day 5 or Day 13, was significantly different than Day 1 (P<0.05). B: LMA represented as area under the curve (AUC) during the first 20 minutes after the first cocaine injection in GDX, GDX-EB and intact-sham (INT-Sham) males. LMA of GDX and GDX-EB males is higher than that of INT-Sham rats, but this difference reached significance only in GDX males. C: Percent change in LMA represented as AUC during the first 20 minutes after cocaine injection in GDX and GDX-EB males. A RM ANOVA reveals dramatic differences across time between GDX and GDX-EB rats, indicating a more robust sensitization in GDX-EB rats. D: Percent change in LMA represented as AUC during the first 20 minutes after cocaine injection in INT-Sham and GDX-EB males. A RM ANOVA reveals no differences across time between INT-Sham and GDX-EB rats, indicating that estrogen restores sensitization to that of INT-Sham animals.

Figure 5.. The antiestrogen ICI-182,780 blocks sensitization to cocaine in intact male rats.

Data are represented by the average and the error bars represent the S.E.M. * p<0.05; ** p<0.01; *** p<0.001; **** P<0.0001. (See Tables 4 and 5 in the Appendix for complete statistical results). A and B: Time course of the locomotor activity of intact-vehicle (INT-Veh) or INT-ICI rats injected daily with saline or with cocaine (15 mg/kg). An increase in cocaine-induced locomotor activity with time (days) was observed in INT-Veh, and not in rats treated with ICI 182,780. The asterisks (*) represents time points in which both Day 5 and Day 13 were significantly different than Day 1 (p<0.05). The plus signs (+) represent time points in which only one of the days, Day 5 or Day 13, were significantly different than Day 1 (P<0.05). C: LMA represented as area under the curve (AUC) during the first 20 minutes after the first cocaine injection in INT-Veh and INT-ICI males. No difference was observed in the acute locomotor response to cocaine between these groups. D: Percent change in LMA represented as AUC during the first 20 minutes after cocaine injection in INT-Veh and INT-ICI males. A RM ANOVA reveals no differences across time between INT-Veh and INT-ICI rats. However post hoc analysis indicate that INT-Veh show sensitization that reached significance by day 13 whereas INT-ICI males do not sensitize to cocaine. E: LMA represented as area under the curve (AUC) during the first 20 minutes after the first cocaine injection in INT-Veh and INT-Sham males. INT-Veh males show a higher locomotor response to cocaine on Day 1 than INT-Sham rats. F: Percent change in LMA represented as AUC during the first 20 minutes after cocaine injection in INT-Veh and INT-Sham males. A RM ANOVA reveals no differences across time between INT-Veh and INT-ICI rats. However post hoc analysis indicate that INT-Veh show sensitization that reached significance by Day 13 whereas sensitization in INT-Sham males is evident on Days 5 and 13.