Effect of cocaine and sucrose withdrawal period on extinction behavior, cue-induced reinstatement, and protein levels of the dopamine transporter and tyrosine hydroxylase in limbic and cortical areas in rats

Abstract

Lever pressing during tests for resistance to extinction and cue-induced reinstatement of cocaine seeking in rats progressively increases over the first 2 months of withdrawal. In the present report, we investigated the generality of these findings in rats trained to self-administer sucrose, a non-drug reinforcer. We also examined whether the time-dependent changes in cocaine seeking correlate with the levels of the dopamine transporter (DAT) and tyrosine hydroxylase (TH) proteins in the amygdala, nucleus accumbens, prefrontal cortex and orbitofrontal cortex. Rats were trained to self-administer cocaine (0.5 mg/kg/i.v. infusion) or 10% sucrose (0.2 ml/infusion into a liquid drop receptacle) for 10 days (6 h/day); each reward delivery was paired with a tone+light cue. Tests for cocaine seeking were conducted following 1 or 15 reward-free days. On the test day, rats were initially tested for resistance to extinction during 6-7 60-min extinction sessions in the absence of the tone-light cue, until they reached the extinction criterion of less than 15 responses/60 min. Subsequently, rats were tested for cue-induced reinstatement during a 60-min session in which each lever press led to a contingent presentation of the tone-light cue. Lever pressing during the tests for reward seeking was significantly greater on day 15 than on day 1 following withdrawal from both cocaine and sucrose self-administration training. The levels of DAT, but not TH, were greater in the prefrontal cortex of cocaine-trained rats than in sucrose-trained rats on both days 1 and 15 of withdrawal. The levels of DAT and TH in other brain areas were not altered following withdrawal from cocaine or sucrose self-administration. These data suggest that the withdrawal can modulate reward seeking of both drug and non-drug reinforcers, and that alterations in DAT and TH levels in the brain regions examined do not mediate this effect.

FIGURE 1

Cocaine and sucrose self-administration behavior during the training phase. Mean (± SEM) daily number of earned cocaine (0.5 mg/kg/infusion) and 10% sucrose (0.2 ml) rewards, during the 10 days of self-administration training. Rats were trained for two 3-h sessions/day, separated by 3 h (n=36 and 30 for cocaine and sucrose, respectively).

FIGURE 2

Time-dependent changes in resistance to extinction behavior and cue-induced reinstatement following withdrawal from cocaine (A) or sucrose (B). Mean (± SEM) number of responses on the previously active lever during the first six 60-min sessions of extinction [conducted in the absence of the discrete tone–light cue previously paired with cocaine infusions (A) or sucrose presentations (B)] and during the test for cue-induced reinstatement, which followed the extinction sessions. During the test for cue-induced reinstatement, lever presses led to presentations of the tone–light cue. Baseline: The last 60-min extinction session on which the rats reached the extinction criterion prior to the test for cue-induced reinstatement. *Significantly different from day1 withdrawal (Fisher PLSD test, P<0.05), n=9–11 per group. Data only include rats tested for both extinction responding and cue-induced reinstatement.