In this experiment, by detecting the changes in behavioral biology and targeted metabolomics in the brain reward circuits, we demonstrated the functional activation of the brain reward circuits via a classical cocaine-induced self-administration (SA) model for freely moving rats. In our experiment, UHPLC-MS/MS targeted metabolomics was used to determine 23 neurotransmitters metabolites. The measurement results showed that there were many changes for many neurotransmitters metabolites in the key nuclei of brain reward circuits (the striatum, the NAc, the hippocampus, the PFC, etc.), but there was no change in the cerebellum during the reconsolidation of drug addiction memory. These findings strongly indicated that many neurotransmitter metabolites in the key nuclei of brain reward circuits (the striatum, the NAc, the hippocampus, the PFC) participated in the process of cocaine-induced behavioral sensitization for self-administering rats.
Addiction can be considered a metabolic disease because it is triggered by the destruction of metabolism and causes persistent neurochemical disorders, which leads to addiction [3]. Another previous study showed that after intraperitoneal injection of cocaine, serotonin, norepinephrine, glucose, dopamine, 3, 4-dihydroxy Phenylacetic acid (DOPAC) and 5-hydroxyindoleacetic acid in several brain regions such as thalamus, striatum and frontal cortex, have undergone significant changes [25]. In addition, the study also showed that cocaine has an effect on thalamic glycolysis [25].
The classic mechanism of cocaine addiction is increased dopaminergic transmission and altered glutamate transmission in the striatum [5, 6]. Previous drug abuse studies using metabolomics methods have demonstrated the potential of metabolomics and the concept of a multi-event mechanism for cocaine addiction. Patkar et al. used a liquid chromatography electrochemical array platform to perform metabolomics analysis on cocaine-dependent plasma samples and drug-free plasma samples, and found that N-methyl serotonin and guanine were significantly up-regulated, while hypoxanthine and Xanthine is down-regulated [26, 27]. Li et al. used 1 H-NMR to study metabolomics in cocaine rat brain tissue [27, 28], and the result showed that changes in tissue concentrations of creatine, taurine, and N-acetylaspartate were observed after cocaine treatment, meanwhile providing metabolic changes related to neurotransmitters, oxidative stress, and mitochondrial disorders.
Exploring changes in neurotransmitters in the key nuclei of brain reward circuits in the process of cocaine addiction by targeted metabolomics, our experimental results were as follows:
1) Striatum: L-Histidine increases, while 5-Hydroxyindoleacetic acid, L-Kynurenine, Homovanillic acid, L-Dopa and 3,4-Dihydroxyphenylacetic Acid all decreased.
2) Nucleus accumbens: Acetylcholine, L-Histidine, L-Arginine, Histamine, L-Glutamine increased significantly, while Homovanillic acid decreased.
3) Prefrontal cortex (PFC): L-Histidine and L-Glutamic acid increased significantly.
4) Hippocampus: L-Arginine and L-Glutamine increased significantly.
5) Cerebellum: there was no obvious changes.
The results showed that there were many changes for many neurotransmitters metabolites in the key nuclei of brain reward circuits (the striatum, the NAc, the hippocampus, the PFC, etc.), but there was no change in cerebellum. From the results of neurotransmitter targeted metabolomics, many neurotransmitter metabolites in the key nuclei of brain reward circuits (the striatum, the NAc, the hippocampus, the PFC) participated in the process of cocaine addiction.
Although the role of the neurotransmitters system in cocaine addiction has been recognized [21–23], the mechanism of toxicity and addiction has not been fully evaluated [24, 26]. Similar studies have shown that repeated administration of MAP can cause obvious neurotransmitters disturbances, oxidative stress, and membrane destruction in the hippocampus, NAc, and PFC. These metabolites can be used as biological indicators of the MAP-induced behavior sensitization mechanism. Environmental factors could affect specific proteins and metabolites, and these chemicals could become potential biomarkers, help diagnose addiction, and could even be used as treatment targets.
Although a large number of proteins and metabolites changes related to drug abuse have been detected, so far, it has been difficult to identify a certain molecule as a potential biomarker. There was increasing evidence that these metabolites were inherently involved in multiple metabolic pathways in our body, and the relative quantification of metabolites in biological fluids could provide the organism's overall metabolic status and provide new insights into the underlying mechanisms of disease. Using metabolomics to identify disturbances in biochemical processes may provide a deeper understanding of the biological and molecular pathways of exposure, addiction, and withdrawal, meanwhile helping to reveal potential biomarkers for diagnosis or monitoring treatment.
According to the neurotransmitters metabolomics results, many neurotransmitter metabolites in the key nuclei of brain reward circuits (the striatum, the NAc, the hippocampus, the PFC, etc.) participated in the process of cocaine addiction. Our experiment only showed the phenomenon of neurotransmitters metabolites changes, and no further and deeper research has been done on the neurotransmitter metabolic pathways, nor do we know how to explain the mechanism in depth, but it aimed to provide a new perspective for the study of drug addiction. Above findings may contribute to a better understanding of metabolic changes in the process of cocaine-induced behavioral sensitization and provided a summary of cocaine-induced metabolites, which may be helpful to provide a more integrated view of the molecular underpinnings and to ultimately find biomarkers to assist clinical diagnosis and treatment.