Extended access to nicotine leads to a CRF1 receptor dependent increase in anxiety-like behavior and hyperalgesia in rats

Abstract

Tobacco dependence is associated with the emergence of negative emotional states during withdrawal, including anxiety and nociceptive hypersensitivity. However, the current animal models of nicotine dependence have focused on the mechanisms that mediate the acute reinforcing effects of nicotine and failed to link increased anxiety and pain during abstinence with excessive nicotine self-administration. Here, we tested the hypothesis that the activation of corticotropin-releasing factor-1 (CRF1 ) receptors and emergence of the affective and motivational effects of nicotine abstinence only occur in rats with long access (>21 hours/day, LgA) and not short (1 hour/day, ShA) access to nicotine self-administration. ShA and LgA rats were tested for anxiety-like behavior, nociceptive thresholds, somatic signs of withdrawal and nicotine intake after 3 days of abstinence. The role of CRF1 receptors during abstinence was tested using systemic or intracerebral infusion of MPZP (N,N-bis(2-methoxyethyl)-3-(4-methoxy-2-methylphenyl)-2,5-dimethyl-pyrazolo(1,5α)pyrimidin-7-amine), a CRF1 receptor antagonist, in the central nucleus of the amygdala (CeA). LgA but not ShA rats exhibited abstinence-induced increases in anxiety-like behavior and nociceptive hypersensitivity, which both predicted subsequent excessive nicotine intake and were prevented by systemic administration of MPZP. Intra-CeA MPZP infusion prevented abstinence-induced increases in nicotine intake and nociceptive hypersensitivity. These findings demonstrate that the model of short access to nicotine self-administration has limited validity for tobacco dependence, highlight the translational relevance of the model of extended-intermittent access to nicotine self-administration for tobacco dependence and demonstrate that activation of CRF1 receptors is required for the emergence of abstinence-induced anxiety-like behavior, hyperalgesia and excessive nicotine intake.

Keywords: addiction; emotion; stress; tobacco; withdrawal.

Figure 1

Experimental design. In all of the experiments, the rats were first given access to nicotine for 1 h per day (acquisition) until nicotine self-administration was stabilized and then separated into two groups given either short access (ShA, 1 h/day) or long access (LgA, 21–23 h/day) to nicotine. In Experiments 2–3 (B, C), LgA rats self-administered nicotine for 21 h instead of 23 h to allow time to run the ShA rats in the same boxes. Each week, in all of the experiments, LgA and ShA rats self-administered nicotine for 4 days/sessions (Monday 10:00 AM to Friday 10:00 AM), followed by 3 days of abstinence from nicotine (Friday-Monday). Behavioral testing was performed before and/or following 72 h of abstinence at 10:00 AM. SA, self-administration; LgA, long access; ShA, short access; PR, progressive-ratio; FR, fixed-ratio.

Figure 2

Following 72 h of abstinence, LgA rats (n = 8) increased their nicotine intake and exhibited higher motivation for nicotine than ShA rats (n = 14). However, both groups had similar levels of mecamylamine-induced somatic signs of withdrawal. Panels A-C describe the patterns of nicotine self-administration during the pre-abstinence daily intake sessions (i.e., Tuesday-Friday) vs. the post-abstinence sessions (i.e., Monday), with the data referring to the average performance during the first 2 weeks of the study. (A) Total nicotine intake. (B) Cumulative nicotine intake during the 23 h session in LgA rats. (C) Hour-by-hour nicotine infusions in LgA rats. (D) Breakpoints on a progressive-ratio (PR) schedule of reinforcement. (E) Mecamylamine-induced somatic signs of withdrawal after 72 h of abstinence. *p < 0.05, **p < 0.01.

Figure 3

Anxiety-like behavior in the elevated plus maze is increased following 72 h of abstinence from nicotine self-administration and is CRF₁ receptor-dependent. (A) Percentage of time spent on the open arms of the elevated plus maze (normalized with a log transformation; Y = log[x+1]) after 72 h of abstinence in rats pretreated with vehicle (LgA, n = 14; ShA, n = 8) or MPZP (LgA, n = 9; ShA, n = 7) and naive rats (n = 7). *p < 0.05, compared with LgA-Vehicle. For raw values see table 3. (B) Correlation between the percentage of time spent on the open arms (normalized) and nicotine self-administration after 72 h of abstinence.

Figure 4

Mechanical hypersensitivity is increased following 72 h of abstinence from nicotine self-administration and is CRF₁ receptor-dependent. (A) Paw withdrawal thresholds (grams) tested after 72 h of abstinence in LgA rats (n = 9) and ShA rats (n = 8) following administration of either MPZP or vehicle and naive rats (n = 8). *p < 0.05, compared with ShA rats. (B) Correlation between paw withdrawal thresholds and nicotine self-administration (fixed-ratio 1) in LgA rats after 72 h of abstinence. *p < 0.05. The data are expressed as mean ± SEM.

Figure 5

(A) MPZP microinfusions into the CeA attenuated the abstinence-induced increase in nicotine intake and mechanical hypersensitivity. (B) Nicotine self-administration (n = 8) and (C) paw withdrawal thresholds (n = 6) in LgA rats after 72 h of abstinence from nicotine self-administration. The data are expressed as mean ± SEM. *p < 0.02.