Effects of Levamisole on Cocaine Self-Administration by Rats

Effects of fentanyl and the adulterant levamisole on the rewarding and locomotor effects of methamphetamine in rats

BACKGROUND

People who use psychostimulant substances can be exposed to unknown adulterants, such as the synthetic opioid fentanyl (FEN) and the anthelmintic cholinergic agent levamisole (LEV). This work explores the rewarding and locomotor effects of methamphetamine (METH) in combination with FEN or LEV.

METHODS

We used adult male Wistar rats in the conditioned-place preference (CPP) paradigm (conditioning, extinction, and reinstatement phases) and in the open field test to study effective doses of METH, FEN, or LEV, or ineffective doses of METH+FEN or METH+LEV in combination.

RESULTS

METH and LEV, at 1mg/kg METH each, and 30µg/kg FEN produced CPP. Extinction to METH- or LEV-induced CPP occurred after eight saline injections, but it took 8-26 sessions to extinguish FEN-induced CPP. A challenge dose of 0.5mg/kg METH reinstated CPP. The same occurred with 15µg/kg FEN but not with 0.5 or 1mg/kg LEV. Training animals with ineffective doses of METH (0.01mg/kg) combined with either FEN (0.3µg/kg) or LEV (0.01mg/kg) produced CPP. Sub-effective doses of METH or FEN alone did not induce reinstatement after extinction. However, animals challenged with LEV, METH+FEN, or METH+LEV mixtures did it. Combining FEN (3µg/kg) with 0.1mg/kg METH increased locomotor activity.

CONCLUSION

Ineffective FEN and LEV doses mixed with METH produce effects larger than would be expected based on the effects of either drug alone. This outcome suggests a supra-additive interaction, which could increase the risk of developing a METH use disorder.

Study design criteria for regulatory-based drug control action: Drug discrimination

This "methods paper" focusses on one specific and limited aspect of drug safety evaluations required for all new drug entities that affect the central nervous system - the drug discrimination (DD) assay. We focus on three critical factors involved in experimental design and protocol development for the conduct of DD studies for abuse liability risk assessment that comply with the Good Laboratory Practice Guidelines (GLPs). The selection of 1) the reference drug(s) choice, 2) training dose selection, and 3) the selected route-of-administration will determine the applicability of the data to meet the regulatory expectations of the 8-factors determinative of schedule control recommendations. The study conduct and resulting data submission to the FDA are intended for drug scheduling review by the Controlled Substances Staff in the Center for Drug Evaluation and Research (CDER) at the US Food & Drug Administration (FDA). These animal studies are required to meet the statutory requirements of the Controlled Substances Act of 1970. The abuse liability study is conducted during Phase II and III of human clinical trials. Procedural or method-based errors this late in drug development can result in a significant economic and business threat to the program.

Discriminative stimulus effects of cocaine-levamisole combinations in Sprague-Dawley rats

BACKGROUND

Levamisole (LVM) is a common adulterant in clandestinely manufactured cocaine (COC), with a range of well-documented deleterious health effects. Although the prevalence of LVM in COC has been widely noted, the subjective effects related to concomitant COC-LVM administration are poorly understood.

AIMS

The present study sought to compare the subjective effects of LVM alone and in combination with COC in male and female Sprague-Dawley rats trained to discriminate COC from vehicle injections.

METHODS

Male and female Sprague-Dawley rats were trained to discriminate COC from vehicle injections using a two-lever, food-reinforced drug discrimination procedure. Subsequent dose-effect curves were generated for COC, LVM, and a variety of COC-LVM combinations.

RESULTS

No significant difference in males and females was observed on any measure of responding. LVM alone dose-dependently decreased response rates and failed to produce substitution for COC. When LVM was administered with COC, previously ineffective doses of COC engendered COC-appropriate responding.

CONCLUSIONS

LVM potentiates the subjective effects of COC when administered concomitantly. These findings are consistent with the popular notion that LVM is added as an adulterant to COC to amplify the subjective effects of COC administration.