Molecular pattern of a decrease in the rewarding effect of cocaine after an escalating-dose drug regimen

Abstract

Background: Long-term cocaine exposure leads to dysregulation of the reward system and initiates processes that ultimately weaken its rewarding effects. Here, we studied the influence of an escalating-dose cocaine regimen on drug-associated appetitive behavior after a withdrawal period, along with corresponding molecular changes in plasma and the prefrontal cortex (PFC).

Methods: We applied a 5 day escalating-dose cocaine regimen in rats. We assessed anxiety-like behavior at the beginning of the withdrawal period in the elevated plus maze (EPM) test. The reinforcement properties of cocaine were evaluated in the Conditioned Place Preference (CPP) test along with ultrasonic vocalization (USV) in the appetitive range in a drug-associated context. We assessed corticosterone, proopiomelanocortin (POMC), β-endorphin, CART 55-102 levels in plasma (by ELISA), along with mRNA levels for D2 dopaminergic receptor (D2R), κ-receptor (KOR), orexin 1 receptor (OX1R), CART 55-102, and potential markers of cocaine abuse: miRNA-124 and miRNA-137 levels in the PFC (by PCR).

Results: Rats subjected to the escalating-dose cocaine binge regimen spent less time in the cocaine-paired compartment, and presented a lower number of appetitive USV episodes. These changes were accompanied by a decrease in corticosterone and CART levels, an increase in POMC and β-endorphin levels in plasma, and an increase in the mRNA for D2R and miRNA-124 levels, but a decrease in the mRNA levels for KOR, OX1R, and CART 55-102 in the PFC.

Conclusions: The presented data reflect a part of a bigger picture of a multilevel interplay between neurotransmitter systems and neuromodulators underlying processes associated with cocaine abuse.

Keywords: Cocaine binge; HPA axis; KOR; Neuromodulators; OX1R; microRNA.

Fig. 1

The experimental scheme. PP natural place preference assessment, EPM elevated plus maze test, CPP condition place preference, USV ultrasonic vocalization

Fig. 2

A Time spent in open arms in the EPM test after cocaine ‘binge’; B Shift of time spent in the cocaine-associated compartment during the CPP test; C Locomotor activity in the CPP test; D 50-kHz USVs in the cocaine-associated context during the CPP test. CPP conditioned place preference test, EPM elevated plus maze test. The time spent in the open arms in the EPM test and the total distance in the CPP test were analyzed with the Student t test. The shift of time spent in cocaine-associated compartment during the CPP test and the number of 50-kHz USVs in the cocaine-associated context during the CPP test were analyzed by the Mann–Whitney U test. Control-group receiving saline (n = 7–9), cocaine ‘binge’–cocaine ‘binge’ group (n = 9). The data are shown as the means + SEM

Fig. 3

A Corticosterone; B CART 55–102; C β-endorphin; D POMC levels in the plasma. CART—cocaine amphetamine regulated transcript 55–102; POMC—proopiomelanocortin peptide. The β-endorphin levels were analyzed with the Student t test. The corticosterone, CART 55–102, and POMC levels were analyzed by the Mann–Whitney U test. Control-group receiving saline (n = 8–9), cocaine ‘binge’–cocaine ‘binge’ group (n = 8–9). The data are shown as the means + SEM

Fig. 4

mRNA levels in the prefrontal cortex (PFC) for A Dopamine type 2 receptor (D2R); B Kappa opioid receptor (KOR); C Cocaine and amphetamine-regulated transcript 55–102 (CART 55–102); D Orexin type 1 receptor (OX1R). The presented data were analyzed by the Mann–Whitney U test. Control-group receiving saline (n = 8–9), cocaine ‘binge’–cocaine ‘binge’ group (n = 8–9). The data are shown as the means + SEM

Fig. 5

A miRNA-124 and B miRNA-137 levels in the prefrontal cortex (PFC). The presented data were analyzed with Student t-test. Control-group receiving saline (n = 9), cocaine ‘binge’–cocaine ‘binge’ group (n = 9). The data are shown as the means + SEM