Mechanisms of metabonomic for a gateway drug: nicotine priming enhances behavioral response to cocaine with modification in energy metabolism and neurotransmitter level

Abstract

Nicotine, one of the most commonly used drugs, has become a major concern because tobacco serves as a gateway drug and is linked to illicit drug abuse, such as cocaine and marijuana. However, previous studies mainly focused on certain genes or neurotransmitters which have already been known to participate in drug addiction, lacking endogenous metabolic profiling in a global view. To further explore the mechanism by which nicotine modifies the response to cocaine, we developed two conditioned place preference (CPP) models in mice. In threshold dose model, mice were pretreated with nicotine, followed by cocaine treatment at the dose of 2 mg/kg, a threshold dose of cocaine to induce CPP in mice. In high-dose model, mice were only treated with 20 mg/kg cocaine, which induced a significant CPP. (1)H nuclear magnetic resonance based on metabonomics was used to investigate metabolic profiles of the nucleus accumbens (NAc) and striatum. We found that nicotine pretreatment dramatically increased CPP induced by 2 mg/kg cocaine, which was similar to 20 mg/kg cocaine-induced CPP. Interestingly, metabolic profiles showed considerable overlap between these two models. These overlapped metabolites mainly included neurotransmitters as well as the molecules participating in energy homeostasis and cellular metabolism. Our results show that the reinforcing effect of nicotine on behavioral response to cocaine may attribute to the modification of some specific metabolites in NAc and striatum, thus creating a favorable metabolic environment for enhancing conditioned rewarding effect of cocaine. Our findings provide an insight into the effect of cigarette smoking on cocaine dependence and the underlying mechanism.

Figure 1. Threshold dose of cocaine-induced CPP is obtained by CPP test and nicotine enhances cocaine-induced CPP (A and B).

(A) To explore threshold dose of cocaine-induced CPP, we designed two doses of cocaine (5 mg/kg and 2 mg/kg) to treat mice once a day for 3 days (n = 12 in each group). (B) To detect effects of nicotine on cocaine, four experimental groups were designed (n = 8 for all groups).

Figure 2. 600 MHz CPMG ¹HNMR spectra of the NAc from mice.

(A) control; (B) 2 mg/kg cocaine; (C) 20 mg/kg cocaine; (D) nicotine; (E) nicotine +2 mg/kg cocaine.

Figure 3. OSC-PLS scores plots of NAc from mice.

(A) control vs. 20 mg/kg cocaine; (B) control vs. nicotine +2 mg/kg cocaine; (C) 2 mg/kg cocaine vs. nicotine +2 mg/kg cocaine; (D) control vs. 2 mg/kg cocaine.

Figure 4. OSC-PLS scores plots of striatum from mice.

(A) control vs. 20 mg/kg cocaine; (B) control vs. nicotine +2 mg/kg cocaine; (C) 2 mg/kg cocaine vs. nicotine +2 mg/kg cocaine; (D) control vs. nicotine; (E) control vs. 2 mg/kg cocaine.

Figure 5. Validation plots of the PLS models after application of OSC in NAc.

(A) control vs. 20 mg/kg cocaine; (B) control vs. nicotine +2 mg/kg cocaine; (C) 2 mg/kg cocaine vs. nicotine +2 mg/kg cocaine; (D) control vs. 2 mg/kg cocaine.

Figure 6. Validation plots of the PLS models after application of OSC in striatum.

(A) control vs. 20 mg/kg cocaine; (B) control vs. nicotine +2 mg/kg cocaine; (C) 2 mg/kg cocaine vs. nicotine +2 mg/kg cocaine; (D) control vs. nicotine; (E) control vs. 2 mg/kg cocaine.

Figure 7. Comparison of NAc from mice by applying PLS loading plots for the region 9.4–5.1 and 4.6-0.2 ppm (440 segments) after application of OSC data filter.

(A) control vs. 20 mg/kg cocaine; (B) control vs. nicotine +2 mg/kg cocaine; (C) 2 mg/kg cocaine vs. nicotine +2 mg/kg cocaine;(D) control vs. 2 mg/kg cocaine.

Figure 8. Comparison of striatum from mice by applying PLS loading plots for the region 9.4–5.1 and 4.6-0.2 ppm (440 segments) after application of OSC data filter.

(A) control vs. 20 mg/kg cocaine; (B) control vs. nicotine +2 mg/kg cocaine; (C) 2 mg/kg cocaine vs. nicotine +2 mg/kg cocaine; (D) control vs. nicotine; (E) control vs. 2 mg/kg cocaine.

Figure 9. Box-and-whisker plots illustrate progressive changes of the metabolites among 5 treatment groups: saline (control), 2 mg/kg cocaine (II), nicotine (III), nicotine +2 mg/kg cocaine (IV) and 20 mg/kg cocaine (V).

Horizontal line in the middle portion of the box, median; bottom and top boundaries of boxes, lower and upper quartile; whiskers, 5th and 95th percentiles; open circles, outliers.

Figure 10. Changed metabolites in brain NAc and altered metabolic pathways for the most relevant distinguishing metabolites with nicotine or cocaine treatment.

(A) In the left, changed metabolites in 20 mg/kg cocaine group exhibit similar changes with combinational drugs group, sharing 10 overlapped metabolites. In the right, a part of modified metabolites showed considerable overlap between 20 mg/kg cocaine group and 2 mg/kg cocaine group or nicotine group. Among them, two shared metabolites between 2 mg/kg cocaine group and nicotine group are Glu and 1-methylhistidine; four shared metabolites between 2 mg/kg cocaine group and 20 mg/kg cocaine group are Gln, L-methionine, α-ketoglutaric acid and phosphocholine; four shared metabolites between nicotine group and 20 mg/kg cocaine group are tryptamine, Lac, Cre and proline. (B) Blue metabolites are upregulated, and red metabolites are downregulated.