Behavioral and neural substrates of habit formation in rats intravenously self-administering nicotine

Abstract

Tobacco addiction involves a transition from occasional, voluntary smoking towards habitual behavior that becomes increasingly resistant to quitting. The development of smoking habits may reflect a loss of behavioral control that can be modeled in rats. This study investigated the behavioral and neural substrates of habit formation in rats exposed to either brief (10 days) or extended (47 days) intravenous (IV) nicotine self-administration training. Following training, the first cohort of rats were exposed to a nicotine devaluation treatment, which involved repeated pairings of IV nicotine with lithium injection. They were then tested for sensitivity of responding to nicotine devaluation under extinction and reinstatement conditions. The second cohort of rats received equivalent self-administration training followed by processing of brain tissue for c-Fos immunohistochemistry. After brief training, devaluation suppressed nicotine-seeking during tests of extinction and reinstatement, confirming that responding is initially sensitive to current nicotine value, and therefore, goal directed. In contrast, after extended training, devaluation had no effect on extinction or reinstatement of responding, indicating that responding had become habitual. Complementary neuroanalysis revealed that extended nicotine self-administration was associated with increased c-Fos expression in brain regions implicated in habitual control of reward seeking, including activation of the dorsolateral striatum and substantia nigra pars compacta. These findings provide evidence of direct devaluation of an IV drug reward, that nicotine self-administration is initially goal-directed but becomes habitual with extended training, and that this behavioral transition involves activation of brain areas associated with the nigrostriatal system.

Figure 1

Schematic representations of coronal sections illustrating the brain regions analyzed for c-Fos protein expression in Experiment 2. Counts included sections beginning at +3.72, +1.68, −2.04, and −4.68 mm from the bregma (Paxinos and Watson, 1998). The numbered regions and the size of the area counted are as follows: (1) Cg1 region of the anterior cingulate cortex (Cg1, 0.5 mm²); (2) prelimbic cortex (PrL, 1.0 mm²); (3) infralimbic cortex (IL, 1.0 × 0.6 mm); (4) orbitofrontal cortex (OFC, 1.0 mm²); (5) dorsomedial striatum (DMS, 1.0 mm²); (6) dorsolateral striatum (DLS, 1.0 mm²); (7, 8) nucleus accumbens core (NAcC, 2 × 0.4 mm²); (9, 10) nucleus accumbens shell (NAcSh, 2 × 0.4 mm²); (11) central amygdala (CEA, 0.5 mm²); (12) basolateral amygdala (BLA, 0.5 mm²); (13) ventral tegmental area (VTA, 1.0 mm²); (14) substantia nigra pars compacta (SNPC, whole structure; (15) substantia nigra pars reticulata (SNPR, whole structure).

Figure 2

Latency to the first response (seconds) at the beginning of the session and conditioned responses (active minus inactive) as a proportion of baseline during the extinction (a and b), reinstatement (c and d) and reacquisition (e and f) tests. Rats had received nicotine paired with lithium (Paired: P) or with saline (non-paired: NP) after brief or extended self-administration training. Data represents group means±SEM. Asterisks indicate significant difference between Paired and Non-Paired when p<0.05.

Figure 3

Mean number of Fos-positive nuclei in the (a) nucleus accumbens core, (b) nucleus accumbens shell, (c) dorsomedial striatum, (d) dorsolateral striatum, (e) ventral tegmental area, and (f) substantia nigra pars compacta for rats self-administering saline (SAL) or nicotine (NIC) after either brief (12 days) or extended (42 days) training. Bars indicate group mean±SEM. Asterisks indicate significant main effect of drug treatment (saline vs nicotine) on figures (a, b, and e), main effect of training on figure (d) or drug × treatment interaction on figure (c and f) when p<0.05.

Figure 4

Photomicrographs of Fos-positive nuclei in sections from the (a) nucleus accumbens core, (b) nucleus accumbens shell, (c) dorsomedial striatum, (d) dorsolateral striatum, (e) ventral tegmental area, and (f) substantia nigra pars compacta. Rats intravenously self-administered either saline or nicotine for either brief (12 days) or extended (42 days) training periods. Images were captured at × 20 magnification using an Olympus BX53 Microscope and DP72 Digital Camera. Images were captured using Image-Pro Insight Software (Media Cybernetics, MD, USA) and processed for brightness and contrast using Adobe Photoshop CS6 (Adobe Systems Incorporated, CA, USA). Scale bar represents 500 μm.