Robust escalation of nicotine intake with extended access to nicotine self-administration and intermittent periods of abstinence

Abstract

Although established smokers have a very regular pattern of smoking behavior, converging lines of evidence suggest that the escalation of smoking behavior is a critical factor in the development of dependence. However, the neurobiological mechanisms that underlie the escalation of smoking are unknown, because there is no animal model of the escalation of nicotine intake. On the basis of the pattern of smoking behavior in humans and presence of monoamine oxidase inhibitors in tobacco smoke, we hypothesized that the escalation of nicotine intake may only occur when animals are given extended-access (21 h per day) self-administration sessions after repeated periods of abstinence (24-48 h), and after chronic inhibition of monoamine oxidase using phenelzine sulfate. Intermittent access (every 24-48 h) to extended nicotine self-administration produced a robust escalation of nicotine intake, associated with increased responding under fixed- and progressive-ratio schedules of reinforcement, and increased somatic signs of withdrawal. The escalation of nicotine intake was not observed in rats with intermittent access to limited (1 h per day) nicotine self-administration or daily access to extended (21 h per day) nicotine self-administration. Moreover, inhibition of monoamine oxidase with daily administration of phenelzine increased nicotine intake by ≈ 50%. These results demonstrate that the escalation of nicotine intake only occurs in animals given intermittent periods of abstinence with extended access to nicotine, and that inhibition of monoamine oxidase may contribute to the escalation of smoking, thus validating both an animal model of the escalation of smoking behavior and the contribution of monoamine oxidase inhibition to compulsive nicotine-seeking.

Figure 1

Before 1 h of each self-administration (SA) session, the rats received either phenelzine (2 mg/kg, intraperitoneally (i.p.)) or vehicle. The rats were first given access to nicotine for 1 h per day (acquisition) and then separated into two groups given either short access (ShA, 1 h per day) or long access (LgA, 21 h per day) to nicotine for 12 consecutive days. The ShA and LgA rats then self-administered nicotine for an additional 18 days either daily or with a 24-h abstinence period between sessions (ie, intermittent schedule). Subsequently, vehicle-treated rats only were allowed an additional 18 days of nicotine self-administration, with the intermittent groups given a 48-h abstinence period between sessions instead of 24 h. After 48 h of the 18th day, the rats self-administered nicotine on a progressive-ratio (PR) schedule. LgA rats then continued to self-administer nicotine on an fixed-ratio (FR)1 schedule for 21 h. Finally, 48 h later, somatic signs of both spontaneous and mecamylamine-precipitated withdrawal were measured in all of the rats.

Figure 2

Nicotine intake (mean±SEM) in rats that self-administered nicotine under a fixed-ratio (FR)1 schedule in either 21 h (long access (LgA)) or 1 h (short access (ShA)) sessions. LgA rats increased their nicotine intake on an intermittent schedule with 24–48 h breaks between sessions, whereas LgA rats on a daily schedule did not. (a) Total number of nicotine infusions per session when the intermittent schedule included 24 h breaks between sessions. (b) Total number of inactive operant responses per session when the intermittent schedule included 24 h breaks between sessions. (c) Total number of nicotine infusions per session when the intermittent schedule included 48 h breaks between sessions. (d) Total number of inactive operant responses per session when the intermittent schedule included 48 h breaks between sessions. (e) Total number of nicotine infusions during baseline (ie, last 3 days of self-administration before separating the rats into daily and intermittent conditions; see Figure 1) vs the last 3 days of daily/intermittent nicotine self-administration. ^#p<0.05, compared with baseline; ^*p<0.05, compared with daily self-administration group. n=8–10 per group.

Figure 3

Withdrawal signs and breakpoints on a progressive-ratio (PR) schedule in long access (LgA) rats that self-administered nicotine either daily or with 48 h abstinence between sessions. (a) Withdrawal signs (mean±SEM) were measured 1 h before nicotine self-administration (ie, following 2 or 47 h of abstinence from nicotine) in rats that received a saline injection (ie, spontaneous withdrawal) or mecamylamine (1.5 mg/kg, subcutaneously (s.c.); ie, precipitated withdrawal). ^*p<0.05, main effect of intermittent schedule; ^#p<0.05, main effect of mecamylamine. (b) LgA rats on an intermittent schedule reached significantly higher breakpoints than LgA rats that self-administered nicotine daily. The data are expressed as mean±SEM. ^*p<0.05. n=7–9 rats per group.

Figure 4

(a) Phenelzine-induced enhancement of nicotine intake in rats with extended access but not limited access to nicotine. The enhancement of nicotine intake in long-access (LgA) rats was observed during the daily nicotine self-administration stage (ie, baseline) and following the transition to intermittent access. (b) Inactive lever presses. ^#p<0.05, compared with baseline; ^*p<0.05, compared with rats that self-administered nicotine daily (analysis of variance (ANOVA) followed by Fisher's protected least significant difference (PLSD) post-hoc test). n=7–10 rats per group. The data are expressed as mean±SEM.