Methamphetamine addiction: involvement of CREB and neuroinflammatory signaling pathways

Abstract

Rationale and objectives: Addiction to psychostimulant methamphetamine (METH) remains a major public health problem in the world. Animal models that use METH self-administration incorporate many features of human drug-taking behavior and are very helpful in elucidating mechanisms underlying METH addiction. These models are also helping to decipher the neurobiological substrates of associated neuropsychiatric complications. This review summarizes our work on the influence of METH self-administration on dopamine systems, transcription and immune responses in the brain.

Methods: We used the rat model of METH self-administration with extended access (15 h/day for eight consecutive days) to investigate the effects of voluntary METH intake on the markers of dopamine system integrity and changes in gene expression observed in the brain at 2 h-1 month after cessation of drug exposure.

Results: Extended access to METH self-administration caused changes in the rat brain that are consistent with clinical findings reported in neuroimaging and postmortem studies of human METH addicts. In addition, gene expression studies using striatal tissues from METH self-administering rats revealed increased expression of genes involved in cAMP response element binding protein (CREB) signaling pathway and in the activation of neuroinflammatory response in the brain.

Conclusion: These data show an association of METH exposure with activation of neuroplastic and neuroinflammatory cascades in the brain. The neuroplastic changes may be involved in promoting METH addiction. Neuroinflammatory processes in the striatum may underlie cognitive deficits, depression, and parkinsonism reported in METH addicts. Therapeutic approaches that include suppression of neuroinflammation may be beneficial to addicted patients.

Keywords: Cortex; Dopamine; Gene expression; Neuroinflammation; Neurotoxicity; Self-administration; Striatum.

Fig. 1

Changes in METH intake and body weight over the course of METH self-administration in rats. (A) Increase of METH intake in rats. Symbols show average METH intake during 8 consecutive self-administration sessions (means ± SEM; n = 11). Rats increased METH intake on sessions 3–8 compared with sessions 1 and 2. Animals further increased their drug intake on session 6 in comparison to session 3 and stayed at that level until cessation of METH self-administration. Data were analyzed by one-way ANOVA for repeated measures, followed by Tukey’s multiple comparison tests: * p<0.05 in comparison with sessions 1 and 2, # p<0.05 in comparison with session 3. (B) Decreases in body weight in animals that self-administered METH (means ± SEM; n = 6–9). Data analyzed by two-way ANOVA for repeated measures, followed by Tukey’s multiple comparison test: * p<0.05 in comparison to saline group. Figure adapted from Krasnova et al. 2010.

Fig. 2

Changes in the markers of DA system integrity and GFAP in the dorsal striatum following METH self-administration. Rats self-administered METH for eight 15-h sessions and were euthanized at 2 h, 24 h, 7 days, 14 days or 1 month withdrawal. (A) METH caused significant decreases in striatal DA levels up to 1 month post-drug. DOPAC concentrations were decreased at 2 h, 24 h, and 7 days, but returned to control levels after 14 days of withdrawal. HVA levels were transiently decreased at 7 days after cessation of METH self-administration, but normalized at 14 days post-drug. (B) Representative immunoblots showing TH and DAT protein levels in the striatum at 14 days after METH withdrawal. (C) Quantitative analyses demonstrated reductions in DAT and TH protein levels in the striatum of METH-treated rats. (D) Representative immunoblots of D1 and D2 DA receptor protein levels. (E) METH self-administration caused increases in D1 DA receptor protein levels at 2 hours post-drug; D2 DA receptor protein levels were decreased after 1 month withdrawal. (F) A representative immunoblot demonstrating GFAP levels in rat striatum 7 days following cessation of METH self-administration. (G) Quantitative analyses of the Western blots show increases in GFAP levels in the METH-treated rats. N = 7–11 per group. Data are expressed as percent (protein optical density, monoamine levels) of mean ± SEM values of saline group. Data were analyzed by ANOVA followed by PLSD. * p < 0.05, significantly different from saline group. Figure adapted from Krasnova et al. 2010, .

Fig. 3

Effects of METH self-administration on the markers of dopamine system integrity and GFAP in the cortex. (A) METH self-administration resulted in long-term reductions in DA concentrations in the cortex. (B) Representative immunoblots of TH and DAT protein expression in the cortex 14 days after cessation of METH exposure. (C) Quantification of METH effects showed decreases in TH and DAT protein levels in the cortex. (D) A representative immunoblot showing GFAP expression in the cortex 7 days post-drug. (E) Quantitative analyses showed increases in GFAP levels in the METH self-administering rats. Data are shown as percent (protein optical density, monoamine levels) of mean ± SEM values of saline group. Data were analyzed by ANOVA followed by PLSD. * p < 0.05, significantly different from saline group. Figure adapted from Krasnova et al. 2010.

Fig. 4

Transcriptional and epigenetic mechanisms are involved in METH addiction and toxicity. Biochemical and behavioral effects of METH include activation of dopaminergic and glutamatergic pathways together with other neurotransmitter systems that might participate in the development of addiction and toxicity. Activation of these neurotransmitter systems is followed by activation or inhibition of transcriptional and epigenetic events that underlie compulsive abuse of the drug. These behaviors might be secondary to subcortical hyperconnection syndrome caused by cortical disinhibition and specific cognitive changes in human METH addicts.

Fig. 5

Validation of METH-induced expression of genes identified by microarray experiments using RT-PCR. Values represent means ± SEM in comparison to saline group. Consistent with microarray data, METH self-administration induced increases in the expression of C-jun and Crem transcription factors, Fst, Nts and Pdyn neuropeptides and Stx1A involved in the synaptic functions at 2 h, which were normalized at 24 h after cessation of drug exposure. N = 6 per group. * p < 0.05 in comparison to saline group (Student’s t-test). Figure adapted from Krasnova et al. 2013.

Fig. 6

METH self-administration causes differential expression of genes involved in cell-to-cell signaling and interaction at 2 hours after cessation of drug exposure. This network includes genes involved in G-protein coupled receptor signaling, dopamine-DARPP32 signaling, axonal guidance signaling, synaptic long-term depression and CREB signaling. Figure adapted from Cadet et al. 2015; Krasnova et al. 2013.

Fig. 7

24 hour withdrawal from METH self-administration causes upregulated expression of genes involved in inflammatory response, inflammatory disease and neurological disease. This network includes genes involved in interleukin-6 signaling, interleukin-10 signaling and NFAT-regulated immune response. Figure adapted from Cadet et al. 2015; Krasnova et al. 2013.