Methoxsalen inhibits the acquisition of nicotine self-administration: attenuation by cotinine replacement in male rats.

INTRODUCTION: Cigarette smoking remains the leading preventable cause of disease and death. Nicotine is the primary reinforcing ingredient in cigarettes sustaining addiction. Cotinine is the major metabolite of nicotine that produces a myriad of neurobehavioral effects. Previous studies showed that cotinine supported self-administration in rats and rats with a history of cotinine self-administration exhibited relapse-like drug-seeking behavior, suggesting that cotinine may also be reinforcing. To date, whether cotinine may contribute to nicotine reinforcement remains unknown. Nicotine metabolism is mainly catalyzed by hepatic CYP2B1/2 enzymes in rats and methoxsalen is a potent CYP2B1/2 inhibitor.

METHODS: The study examined nicotine metabolism, self-administration, and locomotor activity. The hypothesis is that methoxsalen inhibits nicotine self-administration and cotinine replacement attenuates the inhibitory effects of methoxsalen in male rats.

RESULTS: Methoxsalen decreased plasma cotinine levels following a subcutaneous nicotine injection. Repeated daily methoxsalen treatments reduced the acquisition of nicotine self-administration, leading to fewer nicotine infusions, lower nicotine intake, and lower plasma cotinine levels. However, methoxsalen did not alter the maintenance of nicotine self-administration despite a significant reduction of plasma cotinine levels. Cotinine replacement by mixing cotinine with nicotine for self-administration dose-dependently increased plasma cotinine levels and enhanced the acquisition of self-administration. Neither basal nor nicotine-induced locomotor activity was altered by methoxsalen.

CONCLUSIONS: These results indicate that methoxsalen inhibition of cotinine formation impaired the acquisition of nicotine self-administration, and cotinine replacement attenuated the inhibitory effects of methoxsalen on the acquisition of self-administration, suggesting that cotinine may contribute to the initial development of nicotine reinforcement.

IMPLICATIONS: Smoking cessation medications targeting nicotine's effects are only moderately effective, making it imperative to better understand the mechanisms of nicotine misuse. Methoxsalen inhibited nicotine metabolism to cotinine and impaired the acquisition of nicotine self-administration. Cotinine replacement restored plasma cotinine and attenuated the methoxsalen inhibition of nicotine self-administration in rats. These results suggest that (1) the inhibition of nicotine metabolism may be a viable strategy in reducing the development of nicotine reinforcement, (2) methoxsalen may be translationally valuable, and (3) cotinine may be a potential pharmacological target for therapeutic development given its important role in the initial development of nicotine reinforcement.