Time-dependent increases in brain-derived neurotrophic factor protein levels within the mesolimbic dopamine system after withdrawal from cocaine: implications for incubation of cocaine craving

Abstract

Using a rat model of drug craving, we found that the responsiveness to cocaine cues progressively increases or incubates over the first 60 d of cocaine withdrawal. Here we studied whether alterations in brain-derived neurotrophic factor (BDNF) protein levels within the mesolimbic dopamine system are associated with this incubation phenomenon. BDNF is involved in synaptic plasticity and was found to enhance responding for cues associated with natural rewards. Rats were trained to press a lever to receive intravenous cocaine or oral sucrose for 6 hr/d for 10 d; each earned reward was paired with a tone-light cue. Resumption of lever-pressing behavior was then assessed on days 1, 30, or 90 of reward withdrawal. First, resistance to extinction was assessed during 6 hr in which lever presses were not reinforced and the cue was absent. Second, cue-induced reinstatement was assessed after extinction during 1 hr in which responding led to cue presentations. Other rats were killed without testing on days 1, 30, and 90 of reward withdrawal, and BDNF and nerve growth factor (NGF) protein levels were measured in the ventral tegmental area (VTA), accumbens, and amygdala. Lever pressing during extinction and cue-induced reinstatement tests of cocaine craving progressively increased after cocaine withdrawal. Time-dependent changes also were observed during the tests for sucrose craving, with maximal responding on day 30. BDNF, but not NGF, levels in the VTA, accumbens, and amygdala progressively increased after cocaine, but not sucrose, withdrawal. Time-dependent increases in BDNF levels may lead to synaptic modifications that underlie enhanced responsiveness to cocaine cues after prolonged withdrawal periods.

Fig. 1.

Cocaine. A, Extinction, total responses: mean ± SEM responses on the previously active-lever and inactive-lever responses during the six 60 min sessions of extinction (conducted in the absence of the discrete tone–light cue previously paired with cocaine infusions). B, Extinction, responses per hour: mean active-lever responses at each session of extinction. C, Cue-induced reinstatement: mean active-lever responses during the 60 min session that followed the extinction sessions. During the test for cue-induced reinstatement, lever presses led to presentations of the tone–light cue. No cue refers to the last 60 min extinction session on which the rats reached the extinction criterion. *Different from day 1 withdrawal;p < 0.05; Fisher PLSD (n = 11–12 per withdrawal period).

Fig. 2.

Sucrose. A, Extinction, total responses: mean ± SEM responses on the previously active-lever and inactive-lever responses during the six 60 min sessions of extinction (conducted in the absence of the discrete tone–light cue previously paired with sucrose infusions). B, Extinction, responses per hour: mean active-lever responses at each session of extinction. C, Cue-induced reinstatement: mean active-lever responses during the 60 min session that followed the extinction sessions. During the test for cue-induced reinstatement, lever presses led to presentations of the tone–light cue. No cue refers to the last 60 min extinction session on which the rats reached the extinction criterion. *Different from day 1 withdrawal;p < 0.05 (n = 10–14 per withdrawal period).

Fig. 3.

BDNF and NGF protein levels in VTA (A), accumbens (B), and amygdala (C) after reward withdrawal. Data are presented as the percentage of mean values of naive control rats that were not exposed to sucrose or cocaine. *Different from the sucrose-trained groups at each withdrawal day; p < 0.05. ^#Different from day 1 cocaine withdrawal;p < 0.05 (n = 5–7 per group).