Ethanol pre-exposure does not increase delay discounting in P rats, but does impair the ability to dynamically adapt behavioral allocation to changing reinforcer contingencies

Abstract

Increased subjective discounting of delayed rewards is associated with substance abuse, and individuals tend to discount their drug of choice at a greater rate compared to monetary rewards. While there is evidence indicating that increased delay discounting (DD) is a risk factor for substance abuse, some results suggest that exposure to drugs of abuse also increases DD, but effects are mixed. The current study examined whether ethanol pre-exposure increases DD and if an ethanol reinforcer would be discounted at a greater rate than sucrose. Alcohol preferring (P) rats were pre-exposed to either ethanol or sucrose using an intermittent access protocol (IAP) for 8 weeks. Then animals completed an operant fixed choice procedure where each pre-exposure group was split into either an ethanol or sucrose reinforcer group. Afterwards, animals completed an adjusting delay DD task using the same groups as the fixed choice task. Animals that received access to ethanol in the IAP showed increased delayed reward preference in a delay and session dependent manner. Specifically, ethanol pre-exposed animals took more sessions to decrease their preference for the delayed reward at longer delays. In the adjusting delay task, no differences in mean adjusting delays were seen, but ethanol pre-exposure impaired animals' ability to reach stability criteria. The observed results are not consistent with ethanol pre-exposure causing a change in DD. Rather they indicate ethanol pre-exposure impaired animals' ability to reallocate their behavior in response to a change in reinforcer contingencies. The current findings extend prior results showing alcohol naïve P rats exhibit both increased DD and decreased response inhibition (Beckwith and Czachowski 2014, 2016) by demonstrating that after alcohol exposure they exhibit a form of behavioral inflexibility. Hence, a "two-hit" genetic vulnerability/environmental acceleration of addictive behavior is supported.

Keywords: Behavioral flexibility; Ethanol exposure; Impulsivity; Intermittent access protocol; Rat; Selected line.

Figure 1:

Ethanol and sucrose intake during the 8-week IAP. Both 24hr (top panels) and 1hr (bottom panels) showed no differences as a function of future DD reinforcer and were collapsed accordingly. However, significant effects of access period were seen for all time periods and solutions. A) Mean (±SEM) ethanol intake (g/kg) plotted as a function of access period along with the best fitting polynomial functions. B) Mean (±SEM) sucrose solution intake graphed by access period with best fitting polynomial functions.

Figure 2:

Arcsine (ASIN) transformed delayed lever preference (DLP) from the fixed choice delay discounting task. Data are collapsed across both DDR and IAP as no significant interactions of these two factors were seen at any individual delay. A) Mean (±SEM) ASIN DLP plotted as a function of session by delay by IAP. *(grey) trend for 10E-IAP vs. 1S-IAP; *p<.05 10E-IAP vs. 1S-IAP; #p<.05 for IAP by session interaction. B) Mean (±SEM) ASIN DLP plotted as a function of session, delay, and DDR. C) Mean (±SEM) plotted as a function of session inside the 8sD along with the best fitting polynomial. D) Mean (±SEM) plotted as a function of session inside the 16sD along with the best fitting polynomial. The inset shows the same data with a different Y axis to aid in visualization. E) Mean (±SEM) ASIN DLP in the first and last 10 trials within 10E-IAP animals as a function of session. *p<.05 for first 10 trials vs. last 10 trials; **p<.01 for first 10 trials vs. last 10 trials. F) Mean (±SEM) ASIN DLP in the first and last 10 trials within 1S-IAP animals as a function of session. ***p<.001 for first 10 trials vs. last 10 trials.

Figure 3:

Free choice trial completion and trial initiation latencies from the fixed choice DD task plotted as a function of session and delay. A) Mean (±SEM) free choice trials plotted as a function of IAP. B) Mean (±SEM) free choice trials plotted as a function of DDR. *(grey) trend for DDR; *p<.05 for 10E-DD versus 1S-DD; **p<.01 for 10E-DD versus 1S-DD; ‡ p_scheffe<.05 for 16sD versus all other delays inside 1S-DD. C) Mean (±SEM) initiation latencies plotted as a function IAP. D) Mean (±SEM) initiation latencies plotted as a function of DDR. *(grey) trend for DDR. #p<.05 session by DDR; #(grey) trend for session by DDR.

Figure 4:

Log 10 transformed choice latencies from the fixed choice delay discounting task. Data are collapsed across either IAP or DDR due to a lack of an interaction between these factors. A) Mean (±SEM) Log 10 choice latencies plotted as a function of session, delay, and IAP. *(grey) trend for 10E-IAP vs. 1S-IAP; # IAP by session interaction p<.05. B) Bar graph of mean (±SEM) log 10 choice latencies as a function of delay and IAP. *(grey) trend; *p<.05; **p<.01; ***p<.001. C) Log 10 mean (±SEM) choice latencies plotted as function of session, delay, and DDR. *(grey) trend for 10E-DD vs. 1S-DD; # session by DDR p<.05.

Figure 5:

Sucrose and ethanol intake during the fixed choice delay discounting task and BEC determination. A) Mean (±SEM) sucrose intake in g/kg plotted as a function of session, delay, and IAP. B) Mean (±SEM) ethanol intake in g/kg graphed by session, delay, and IAP. C) Scatter plot of blood ethanol concentration (BEC) plotted as a function of ethanol intake (g/kg) pending whether animals were allowed to complete the whole session or only the first 10 trials. Dotted lines indicate the 95% confidence band. D) Mean (±SEM) BEC graphed by when blood was sampled.

Figure 6:

Mean adjusting delay (MAD) scores from the adjusting delay DD task. Mean (±SEM) scores graphed by group, delay to the immediate reward, and by IAP (inset). ***p<.001 0 vs. 4 second delay to the immediate reward.

Figure 7:

Analysis of animals’ ability to meet stability criteria. A) Percentage of animals reaching stability criteria (survival) graphed by delay and IAP. *p<.05 for IAP. B) Survival percentage for meeting stability criteria based on DDR and delay to the immediate reward. C) Mean (±SEM) number of sessions to reach criteria graphed based on delay to immediate reward and IAP. **p<.01 for main effect of IAP; ## p<.01 for 1S-IAP vs. 10E-IAP at the 4 second delay. D) Total subjects that failed to reach stability criteria graphed based on IAP and cause of instability. **p<.01 for MAD score variability vs. trial completion.