Escalation and reinstatement of fentanyl self-administration in male and female rats

Abstract

Rationale: Escalation of drug intake and craving are two DSM-5 hallmark symptoms of opioid use disorder (OUD).

Objectives: This study determined if escalation of intake as modeled by long access (LgA) self-administration (SA) and craving measured by reinstatement are related.

Methods: Adult male and female Sprague-Dawley rats were trained to self-administer fentanyl across 7 daily 1-h short access (ShA) sessions, followed by 21 SA sessions of either 1- or 6-h duration (ShA or LgA). Following 14 1-h extinction sessions, Experiment 1 assessed reinstatement induced by either fentanyl (10 or 30 µg/kg) or yohimbine (1 or 2 mg/kg), and Experiment 2 assessed reinstatement induced by a drug-associated cue light.

Results: Females acquired fentanyl SA faster than males. When shifted to LgA sessions, LgA rats escalated fentanyl intake, but ShA rats did not; no reliable sex difference in the rate of escalation was observed. In extinction, compared to ShA rats, LgA rats initially responded less and showed less decay of responding across sessions. A priming injection of fentanyl induced reinstatement, with LgA rats reinstating more than ShA rats at the 30 µg/kg dose. Yohimbine (1 mg/kg) also induced reinstatement, but there was no effect of access group or sex. With cue-induced reinstatement, LgA females reinstated less than LgA males and ShA females.

Conclusion: Among the different reinstatement tests assessed, escalation of fentanyl SA increased only drug-primed reinstatement, suggesting a limited relationship between escalation of drug intake and craving (reinstatement) for OUD.

Keywords: Cue; Escalation; Female; Fentanyl; Male; Reinstatement; Sex; Yohimbine.

Figure 1:. Acquisition, Escalation, 1^st h Escalation, and Extinction in^{Experiment 1}.

(A) Mean (±SEM) number of active and inactive lever presses for females and males across the 1-h sessions during the Experiment 1 acquisition. The difference between active and inactive lever pressing change was significant for both males and females *(both p < .0001). The difference between the active and inactive lever pressing change across sessions was greater in females than males ^(p < .001). There was a significant difference between active and inactive lever pressing on session 1 for males only +(p < .05). (B) Mean (±SEM) number of active lever presses for ShA and LgA females and males across sessions during the Experiment 1 escalation. LgA rats pressed the active lever more on session 1 than ShA rats #(p < .0001). LgA rats escalated intake across sessions *(p < .0001), but ShA rats did not (p > .05). (C) Mean (±SEM) number of active lever presses for ShA and LgA females and males across sessions during the 1^st h of Experiment 1 escalation. LgA rats escalated intake across sessions *(p < .001), but ShA rats did not (p > .05). (D) Mean (±SEM) number of active lever presses for ShA and LgA females and males across sessions during the Experiment 1 extinction. LgA rats pressed the active lever less than ShA rats on session 1 ^(p ≤ .05). LgA rats showed less decay of active lever pressing than ShA rats *(p < .001). Note that in all figure panels, presses on the active lever during the time-out period were not included.

Figure 2:. Fentanyl- and Yohimbine-induced reinstatement in^{Experiment 1}.

(A) Mean (±SEM) number of active lever presses for ShA and LgA females and males following vehicle, 10 μg/kg fentanyl, and 30 μg/kg fentanyl. All rats reinstated to 10 μg/kg and 30 μg/kg fentanyl in comparison to vehicle *(p < .05). The difference between vehicle and 30 ug/kg fentanyl responding was greater for the LgA group than the ShA group ^(p < .05); note the extreme outlier ShA female following 30 μg/kg fentanyl. Females had higher active lever pressing than males overall +(p < .01). (B) Mean (±SEM) number of active lever presses for ShA and LgA females and males following vehicle, 1 mg/kg yohimbine, and 2 mg/kg yohimbine. The 1 mg/kg yohimbine dose produced reinstatement in comparison to vehicle, collapsed across access group and sex *(p < .001). Note that in both figure panels, presses on the active lever during the time-out period were not included.

Figure 3:. Acquisition, Escalation, 1^st h Escalation, and Extinction in^{Experiment 2}.

(A) Mean (±SEM) number of active and inactive lever presses for females and males across the 1-h sessions during Experiment 2 acquisition. The difference between active and inactive lever pressing change was significant for both males and females *(both p < .0001). The difference between the active and inactive lever pressing change across sessions was greater in females than males ^(p < 0.05). (B) Mean (±SEM) number of active lever presses for ShA and LgA females and males across sessions during Experiment 2 escalation. The LgA group pressed the active lever more on session 1 than the ShA group #(p < .0001). Females had more active lever presses than males overall ^(p < .05). LgA rats escalated intake across sessions *(p < .01), but ShA rats did not (p > .05). (C) Mean (±SEM) number of active lever presses for ShA and LgA females and males across sessions during the 1^st h of Experiment 2 escalation. Females had more active lever presses than males overall ^(p < .05). LgA rats escalated intake across sessions *(p < .001), but ShA rats did not (p > .05). (D) Mean (±SEM) number of active lever presses for ShA and LgA females and males across sessions during Experiment 2 extinction. LgA rats pressed the active lever less than ShA rats on session 1 ^(p < .01). LgA rats showed less decay of active lever pressing than the ShA group *(p < .0001). Note that in all figure panels, presses on the active lever during the time-out period were not included.

Figure 4:. Cue-induced reinstatement in^{Experiment 2}.

Mean (±SEM) number of active lever presses for ShA and LgA females and males following the final extinction session (Ext) or cue. All rats reinstated to the cue in comparison to their final extinction session *(p < .0001). There was also a significant cue × access group × sex interaction, indicating the in reinstatement between access groups was moderated by sex, with LgA females showing less reinstatement than LgA males and ShA females #(p < .01). Note that presses on the active lever during the time-out period were not included.