Compulsive-Like Sufentanil Vapor Self-Administration in Rats

Abstract

Opioid misuse is at historically high levels in the United States, with inhalation (ie, smoking and vaping) being one of the most common routes of consumption. We developed and validated a novel preclinical model of opioid self-administration by inhalation that does not require surgery and reliably produces somatic and motivational signs of dependence. Rats were trained to perform an operant response (nosepoke) to receive 10 s of vaporized sufentanil, a potent opioid, in 2 h daily sessions. Rats readily and concentration-dependently self-administered vaporized sufentanil. Rats exhibited a significant increase in responding for sufentanil when given the preferential μ-opioid receptor inverse agonist naloxone, suggesting the participation of μ-opioid receptors in the reinforcing properties of sufentanil vapor. Serum sufentanil concentrations significantly correlated with the number of sufentanil vapor deliveries. Rats that were given long access (LgA; 12 h/day) but not short access (ShA; 1 h/day) to vaporized sufentanil escalated their drug intake over time and exhibited both naloxone-precipitated somatic signs of opioid withdrawal and spontaneous withdrawal-induced mechanical hypersensitivity. After 6 months of forced drug abstinence, LgA rats returned to pre-escalation baseline levels of responding for sufentanil and mechanical sensitivity. Upon subsequent re-escalation (ie, after the return to extended access to sufentanil vapor), LgA rats again developed naloxone-precipitated somatic signs of withdrawal and spontaneous withdrawal-induced mechanical hypersensitivity. These findings demonstrate that the operant sufentanil vapor self-administration model has both face and construct validity and therefore will be useful for investigating the neurobiological basis of opioid addiction.

Figure 1

Operant sufentanil vapor self-administration. (a) Concentration–response function for sufentanil vapor self-administration. Rats (n=8) self-administered sufentanil vapor (10 s delivery) in 2 h sessions according to the following concentration schedule (in order of presentation): 3.33 mg/ml for six consecutive sessions, 1.65 mg/ml for eight consecutive sessions, and 10 mg/ml for four consecutive sessions. Bars represent the mean number of reinforcers (+SEM) earned across sessions for each of the three different sufentanil concentrations. **p<0.01, different from 3.33 mg/ml. (b) Effect of the opioid receptor inverse agonist naloxone on sufentanil vapor self-administration. The same rats (n=8) that were tested in panel (a) self-administered sufentanil vapor (10 mg/ml) in 2 h sessions. They were subcutaneously injected with 0.12, 0.25, 0.5, and 1 mg/kg naloxone immediately prior to sufentanil self-administration (one self-administration session per concentration). The baseline data are expressed as the average of five baseline sessions that were conducted intercalated with the naloxone tests. *p<0.05, **p<0.01, ***p<0.0001, different from baseline at the same time of the session.

Figure 2

Serum sufentanil concentrations. (a) Timeline for non-contingent delivery of sufentanil vapor. A drug-naive cohort of rats received 2, 5, or 10 non-contingent sufentanil vapor (3 mg/ml) deliveries (n=3 rats per condition) in a 2 h period. The onset of vapor deliveries across the 2 h period was evenly spaced and timed so that the final vapor delivery occurred at minute 118 of the session, regardless of the number of deliveries. (b) Symbols represent mean serum sufentanil concentrations (ng/ml; +SEM) following 2, 5, and 10 sufentanil vapor deliveries. There was a significant correlation between serum sufentanil concentration and the number of sufentanil vapor deliveries (p<0.05). The data are expressed as ng/ml of serum.

Figure 3

Escalation and re-escalation (6 months later) of sufentanil vapor self-administration. (a) Escalation of sufentanil vapor self-administration; symbols represent the mean number of sufentanil vapor deliveries (±SEM) over 13 sessions. Rats that were allowed long access (LgA; 12 h sessions) to sufentanil vapor (3 mg/ml) exhibited dramatic escalation of drug intake over time. Rats that were allowed short access (ShA; 1 h sessions) to sufentanil vapor exhibited a modest decrease, then a modest increase in drug intake over time. n=8 per group. (b) Re-escalation of sufentanil vapor self-administration. The same ShA and LgA rats that were used in the escalation experiment (panel [a]) were used for the re-escalation experiment 6 months later. LgA rats exhibited significant re-escalation of drug deliveries over 13 sessions. ShA rats exhibited more stable drug intake over time. ShA rats, n=7; LgA rats, n=8. *p<0.05, **p<0.01, ***p<0.001, compared with session 1. (c, d) Raster plots that illustrate the number of active nosepokes in one representative ShA rat and one representative LgA rat during sessions 1 and 13 for the escalation (c) and re-escalation (d) experiments. In LgA rats, the increase in sufentanil intake was reflected by a decrease in the inter-infusion interval over the entire 12 h session.

Figure 4

Mechanical sensitivity (a, b) and somatic signs of opioid withdrawal (c, d) after escalation and re-escalation of sufentanil vapor self-administration. (a) Symbols represent mechanical sensitivity measured by paw withdrawal thresholds (g; +SEM) using von Frey filaments. LgA and ShA rats (see Figure 3 for self-administration data) exhibited similar paw withdrawal thresholds at baseline (BL; conducted before any exposure to sufentanil) and test 1 (T1; conducted before the second self-administration session). LgA but not ShA rats exhibited lower paw withdrawal thresholds at T2 (conducted before the tenth self-administration session). (b) The same ShA and LgA rats were tested again for mechanical sensitivity 6 months later. Results that were identical to escalation were found (ie, no group differences for BL and T1 but a significant decrease in mechanical thresholds in LgA rats at T2). *p<0.05, **p<0.01, different from ShA rats; ⁺⁺p<0.01, different from BL. (c, d) Bars represent somatic withdrawal scores (+SEM) following naloxone-precipitated opioid withdrawal after escalation (c) and re-escalation (d). Rats were given a subcutaneous injection of naloxone (1 mg/kg) after the last self-administration session (ie, session 13) of the escalation and re-escalation experiments. LgA rats (n=8) after both the escalation and the re-escalation experiments exhibited significantly higher scores of naloxone-precipitated withdrawal compared with ShA rats (n=7). *p<0.05, **p<0.01, different from ShA rats.