Novel cues reinstate cocaine-seeking behavior and induce Fos protein expression as effectively as conditioned cues

Abstract

Cue reinstatement of extinguished cocaine-seeking behavior is a widely used model of cue-elicited craving in abstinent human addicts. This study examined Fos protein expression in response to cocaine cues or to novel cues as a control for activation produced by test novelty. Rats were trained to self-administer cocaine paired with either a light or a tone cue, or received yoked saline and cue presentations, and then underwent daily extinction training. They were then tested for reinstatement of extinguished cocaine-seeking behavior elicited by response-contingent presentations of either the cocaine-paired cue or a novel cue (that is, tone for those trained with a light or vice versa). Surprisingly, conditioned and novel cues both reinstated responding and increased Fos similarly in most brain regions. Exceptions included the anterior cingulate, which was sensitive to test cue modality in saline controls and the dorsomedial caudate-putamen, where Fos was correlated with responding in the novel, but not conditioned, cue groups. In subsequent experiments, we observed a similar pattern of reinstatement in rats trained and tested for sucrose-seeking behavior, whereas rats trained and tested with the cues only reinstated to a novel, and not a familiar, light or tone. The results suggest that novel cues reinstate responding to a similar extent as conditioned cues regardless of whether animals have a reinforcement history with cocaine or sucrose, and that both types of cues activate similar brain circuits. Several explanations as to why converging processes may drive drug and novel cue reinforcement and seeking behavior are discussed.

Figure 1

Cocaine reinforcement and seeking behavior from Experiment 1. (a) Daily cocaine reinforcement rates (infusions±SEM) across sessions where infusions were paired with either a light (circles; n=16) or tone (squares; n=18). Inset graph shows the total number of cocaine infusions (±SEM) earned during self-administration. (b) Responses on the active lever (±SEM) across extinction sessions in animals that had previously received either a light or tone cue paired with cocaine reinforcement during training; responses on the active lever produced no scheduled consequences during extinction. (c) Estimated marginal means for responses on the active lever/90 min (±SEM), with the change in inactive lever presses from baseline to test day as the covariate, for groups tested with novel (open symbols, dashed lines) or conditioned (closed symbols, solid lines) light (circles) or tone (squares) cues (n=7–9/group). ^ Represents a significant difference from tone-trained group (tests of simple effects, p<0.05). ^*Represents a difference from baseline responding (ANOVA, p<0.05). ⁺Represents a difference between test cue modalities (light vs tone; ANOVA, p<0.05).

Figure 2

Fos protein expression data from Experiment 1. (a) Schematic representations illustrating the regions analyzed in brain sections taken at +3.2, +1.6, −2.56, and −5.8 mm from bregma (Paxinos and Watson, 1998). The numbered regions are as follows: (1) Cg1 region of the anterior cingulate cortex (Cg1); (2) Cg2 region of the anterior cingulate cortex (Cg2); (3) dorsomedial caudate/putamen (dmCPu); (4) dorsolateral caudate/putamen (dlCPu); (5) nucleus accumbens core (NAcC); (6) nucleus accumbens shell (NAcS); (7) basolateral amygdala (BlA); and (8) ventral tegmental area (VTA). (b) Representative photomicrographs at × 10 magnification of Fos protein expression in the NAcC with a dashed rectangle indicating the sample region. (c) NAcC samples taken at × 20 magnification from a representative subject/group with arrows indicating Fos-positive nuclei. All sample areas were 0.26 mm² and all photomicrographs were taken at × 20 magnification with the scale bar equal to 100 μm. (d) Number of Fos-positive nuclei/mm² (±SEM) in brain regions that exhibited enhanced Fos expression in rats with a history of cocaine self-administration (n=32) compared with saline controls (n=16) in Experiment 1. Data are grouped by drug history. (e) Fos expression (±SEM) in the Cg1 grouped by drug history and test cue modality (n=7–18/group). Scatter plot of dmCPu for the cocaine (f) novel (n=15) and (g) the CS (n=16) test cue groups with Fos expression vs test session cocaine-seeking behavior. Fos-positive nuclei/mm² and test session responding on the active lever were centered to where ‘0' represents the mean of each test cue modality in order to remove group differences. The line represents a best-fit linear relationship between the two variables. ^*Represents a difference from saline-yoked groups (p<0.05). ^†Represents a difference from the saline tone-tested group (ANOVA, p<0.05). ⁺⁺Represents a positive linear correlation among responding on the active lever and Fos-labeled cells (p<0.01). ac, anterior commissure.

Figure 3

Sucrose reinforcement and seeking behavior from Experiment 2. (a) Daily reinforcement rates (±SEM) for rats trained to lever press for sucrose pellets that were paired with a light (circles) or a tone (squares). Inset graph shows the total number of sucrose pellets (±SEM) attained across all training sessions. Dashed vertical lines represent the transition period from food restriction (∼16–22 g chow/day) to food ab libitium. (b) Responses on the active lever (±SEM) across extinction sessions in animals that had previously received either a light or a tone cue paired with sucrose reinforcement during training; responses on the active lever produced no scheduled consequences during extinction. (c) Estimated marginal means for responses on the active lever/30 min (±SEM), with the change in inactive lever presses from baseline to test day as the covariate, for groups tested with novel (open symbols, dashed lines) or conditioned (closed symbols, solid lines) light (circles) or tone (squares) cues (n=7–9/group). ^Represents a difference from the light-trained group (tests of simple effects, p<0.05). ^*Represents a difference from baseline responding (p<0.05).

Figure 4

Cue reinforcement and seeking behavior from Experiment 3. (a) Daily reinforcement rates (±SEM) for rats trained to lever press for either a light (circles) or a tone (squares) cue presentation. Inset graph shows the total number of cue presentations (±SEM) earned across all training sessions. (b) Responses on the active lever (±SEM) across extinction sessions in animals that had previously received either a light or a tone cue without reinforcement during training; responses on the active lever produced no scheduled consequences during extinction. (c) Estimated marginal means for responses on the active lever/30 min (±SEM), with the change in inactive lever presses from baseline to test day as the covariate, for groups tested with novel (open symbols, dashed lines) or conditioned (closed symbols, solid lines) light (circles) or tone (squares) cues (n=8–9/group) ^#Represents a difference from the tone-trained group in total number of cue presentations (p<0.05). ^Represents a difference from the tone-trained group (tests for simple effects, p<0.05). ^*Represents a difference from baseline responding (tests of simple effects, p<0.05).