The L-type calcium channel blocker, isradipine, attenuates cue-induced cocaine-seeking by enhancing dopaminergic activity in the ventral tegmental area to nucleus accumbens pathway

Abstract

Previous preclinical and clinical investigations have focused on the L-type calcium channel (LTCC) as a potential therapeutic target for substance abuse. While some clinical studies have examined the ability of LTCC blockers to alter cocaine's subjective effects, very few LTCC studies have examined cocaine relapse. Here, we examined whether ventral tegmental area (VTA)-specific or systemic administration of the LTCC inhibitor, isradipine, altered cocaine-seeking behavior in a rat model. Male Sprague-Dawley rats first received 10 days of cocaine self-administration training (2 h sessions), where active lever depression resulted in delivery of a ∼0.5 mg/kg cocaine infusion paired with a tone + light cue. Rats then underwent 10 days of forced abstinence, without access to cocaine or cocaine cues. Rats were then returned to the opertant chamber for the cue-induced cocaine-seeking test, where active lever depression in the original training context resulted in tone + light cue presentation. We found VTA specific or systemic isradipine administration robustly attenuated cocaine-seeking, without altering cocaine-taking nor natural reward seeking. Dopamine (DA) signaling in the nucleus accumbens (NAc) core is necessary and sufficient for cue-induced drug-seeking. Surprisingly in our study, isradipine enhanced tonic and phasic DA signaling in cocaine abstinent rats, with no change in sucrose abstinent nor naïve rats. Strikingly, isradipine's behavioral effects were dependent upon NAc core DA receptor activation. Together, our findings reveal a novel mechanism by which the FDA-approved drug, isradipine, could act to decrease cocaine relapse.

Fig. 1

Intra-VTA isradipine attenuates cue-induced cocaine-seeking, but not cocaine-taking. a Experimental timeline for cue-induced cocaine-seeking and cocaine taking experiments. b Day 1 to 10 cocaine self-administration training, where rats did not show a-priori differences in active or inactive lever presses based on their experimental (“to be”) assignment for the cocaine-seeking test. c Intra-VTA isradipine administration, on abstinence day 10, dose-dependently decreased cue-induced cocaine-seeking behavior. d Day 1 to 9 cocaine self-administration training, where rats showed no a-priori differences in active or inactive lever presses, based on day 10 (“to-be”) experimental assignment. e Intra-VTA isradipine administration on cocaine-taking day 10 did not alter the number of cocaine infusions and did not alter the number of lever presses. f Histological verification of VTA cannula placements for cocaine experiments. Data are presented as ± the standard error of the mean (SEM). ****p < 0.0001 (active lever presses for 0 vs. 223 pg/side isradipine, post-hoc analysis with a Bonferroni correction)

Fig. 2

Intra-VTA isradipine does not alter cue-induced sucrose-seeking behavior. a Experimental timeline for cue-induced sucrose-seeking experiments. b Day 1 to 10 sucrose self-administration training, represented based on subjects (“to be”) experimental assignment for the subsequent sucrose-seeking test. Active levers were continuously available in the operant chamber throughout the training sessions. c VTA infusion of isradipine on abstinence day 10 did not alter cue-induced sucrose-seeking behavior. d Day 1 to 10 sucrose self-administration training based on subjects subsequent abstinence day 10 (“to-be”) experimental assignment. During self-administration training, each sucrose pellet delivery was followed by active lever retraction for a 6 s timeout period. e VTA isradipine infusion on abstinence day 10 again did not alter cue-induced sucrose-seeking behavior. f Histological verification of VTA cannula placements for sucrose experiments. Data are presented as ± the SEM

Fig. 3

Intra-VTA isradipine enhances phasic dopamine (DA) signaling in the NAc of cocaine abstinent, but not sucrose abstinent, rats. a Experimental timeline for cocaine or sucrose self-administration experiments and subsequent DA analysis with FSCV. b Representative current vs. time color plots (left) and DA concentration vs. time traces (right) in cocaine abstinent rats. c Representative current vs. time color plots (left) and DA concentration vs. time traces (right) in sucrose abstinent rats. d Peak evoked phasic DA release in cocaine abstinent and sucrose abstinent rats. Data are represented as a percentage, compared to the average peak evoked DA concentrations in 6 pre-infusion samples. Intra-VTA isradipine led to increased peak phasic DA release in cocaine-abstinent rats, compared to other groups. e Peak phasic evoked DA release in the NAc of naïve, sucrose abstinent and cocaine abstinent rats prior to VTA infusion. Analysis revealed no differences in phasic DA release. Data are presented as ± the SEM. *p < 0.05 (significant difference between VTA vehicle and VTA isradipine infusion in cocaine abstinent rats, Bonferroni corrected comparisons)

Fig. 4

Intra-VTA isradipine increases tonic DA signaling in the NAc. a Representative current vs. time color plots (left) and DA concentration vs. time traces (right) in cocaine abstinent rats. b Peak evoked tonic DA concentrations in cocaine abstinent rats. Data are represented as a percentage, compared to the average peak evoked DA in 6 pre-infusion samples. During cocaine abstinence, VTA isradipine-infused rats showed higher peak evoked tonic DA release compared to VTA vehicle infused rats. c Within-subject comparisons of 6 pre-VTA infusion tonic DA release to 6 post-VTA infusion tonic DA release. During cocaine abstinence, VTA vehicle infusion led to significant decrease in evoked tonic DA, while VTA isradipine infusion led to a significant increase in evoked tonic DA. Data are presented as ± the SEM. *p < 0.05 (pre vs. post vehicle infusion or pre vs. post-isradipine infusion, paired samples t-test)

Fig. 5

Systemic isradipine administration attenuates cue-induced cocaine-seeking and enhances NAc DA signaling. a Timeline of systemic isradipine experiments for cue-induced cocaine-seeking and voltammetric analysis of DA signaling in the NAc. b Day 1 to 10 cocaine self-administration training, based on “to be” experimental assignments for the cue-induced cocaine-seeking test. c Systemic administration of isradipine, on abstinence day 10, significantly attenuated cue-induced cocaine-seeking. d Average evoked phasic DA release in the NAc of cocaine abstinent rats. Systemic isradipine administration led to significantly greater phasic DA release, compared to systemic vehicle administration. e Averaged evoked tonic DA release in the NAc of cocaine abstinent rats. Systemic isradipine also led to significantly greater tonic DA release, compared to systemic vehicle. Data are presented as ± the SEM. *p < 0.05 (panel c: post-hoc significant decrease in active lever presses for vehicle vs. isradipine rats, Bonferroni corrected comparisons, panels d and e: post-hoc significant DA differences in vehicle vs. isradipine rats, Bonferroni corrected comparisons)

Fig. 6

DA receptor antagonism in the NAc blocks the behavioral effects of systemic isradipine. a Timeline for intra-NAc flupenthixol experiments with cue-induced cocaine-seeking. b Day 1 to 10 cocaine self-administration training based on “to be” experimental assignment for the cue-induced cocaine-seeking test. c Intra-NAc infusion of the DA receptor antagonist, flupenthixol, dose-dependently decreased lever press activity during the cue-induced cocaine-seeking test. d Experimental timeline for combined systemic isradipine and NAc flupenthixol administration prior to cue-induced cocaine-seeking test. e Day 1 to 10 cocaine self-administration training based on abstinence day 10, “to be” experimental assignment. f Systemic administration of isradipine on abstinence day 10 significantly attenuated cue-induced cocaine-seeking. Systemic isradipine differentially influenced cocaine-seeking behavior in the presence versus absence of NAc flupenthixol. In presence of NAc flupenthixol, systemic isradipine did not alter cocaine-seeking behavior. In contrast, rats receiving systemic isradipine alone showed decreased cocaine-seeking compared to rats with systemic isradipine plus NAc flupenthixol. Data are presented as ± the SEM. **p < 0.01, ****p < 0.001 (panel c, Bonferroni corrected post-hoc comparisons of active lever presses for 20 μg/side flupenthixol vs. 0 or 7 μg/side. Bonferroni corrected post-hoc comparison of inactive lever presses for 20 μg/side flupenthixol vs. 0 or 7 μg/side. g Histological verification of NAc cannula placements