Dynorphin and κ-Opioid Receptor Dysregulation in the Dopaminergic Reward System of Human Alcoholics

Abstract

Molecular changes induced by excessive alcohol consumption may underlie formation of dysphoric state during acute and protracted alcohol withdrawal which leads to craving and relapse. A main molecular addiction hypothesis is that the upregulation of the dynorphin (DYN)/κ-opioid receptor (KOR) system in the nucleus accumbens (NAc) of alcohol-dependent individuals causes the imbalance in activity of D1- and D2 dopamine receptor (DR) expressing neural circuits that results in dysphoria. We here analyzed post-mortem NAc samples of human alcoholics to assess changes in prodynorphin (PDYN) and KOR (OPRK1) gene expression and co-expression (transcriptionally coordinated) patterns. To address alterations in D1- and D2-receptor circuits, we studied the regulatory interactions between these pathways and the DYN/KOR system. No significant differences in PDYN and OPRK1 gene expression levels between alcoholics and controls were evident. However, PDYN and OPRK1 showed transcriptionally coordinated pattern that was significantly different between alcoholics and controls. A downregulation of DRD1 but not DRD2 expression was seen in alcoholics. Expression of DRD1 and DRD2 strongly correlated with that of PDYN and OPRK1 suggesting high levels of transcriptional coordination between these gene clusters. The differences in expression and co-expression patterns were not due to the decline in neuronal proportion in alcoholic brain and thereby represent transcriptional phenomena. Dysregulation of DYN/KOR system and dopamine signaling through both alterations in co-expression patterns of opioid genes and decreased DRD1 gene expression may contribute to imbalance in the activity of D1- and D2-containing pathways which may lead to the negative affective state in human alcoholics.

Keywords: Alcohol addiction; Co-expression of gene clusters; D1-pathway; D2-pathway; Dynorphin; Dysphoria; Nucleus accumbens; Post-mortem human brain tissue; Reward system; κ-opioid receptor.

Fig. 1

Effects of alcoholism on PDYN and OPRK1 expression levels in NAc. a, d Presented are data on whole tissue PDYN and OPRK1 mRNA levels for 50 controls and 42 alcoholics. b, e Presented are data on whole tissue PDYN and OPRK1 mRNA levels adjusted for changes in neuronal proportion for 32 controls and 24 alcoholics. c, f Correlations between neuronal proportion and PDYN (c) or OPRK1 (f) mRNAs. Statistical analysis was performed by one-way ANCOVA; P values were calculated by ordinary bootstrap with 5 × 10⁵ non-parametric resampling of cases. mRNA levels are shown in arbitrary units. In boxplots, the middle line is the median, box spans the interquartile range (IQR), and whiskers extend 1.5 × IQR from box limits. Lines and shading represent the estimated slopes and 95% confidence intervals (CIs), respectively

Fig. 2

Relationship between PDYN and OPRK1 in alcoholics (N = 42) and controls (N = 50). The slope of the regression line was steeper in alcoholics than in controls (two-way ANCOVA; P values were calculated by ordinary bootstrap with 5 × 10⁵ non-parametric resampling of cases; P = 0.011). mRNA levels are shown in arbitrary units. Lines and shading denote alcoholism slope estimates and 95% CI, respectively

Fig. 3

Effects of alcoholism on DRD1 and DRD2 expression levels in NAc. a, d Presented are data on whole-tissue DRD1 and DRD2 mRNA levels for 50 controls and 42 alcoholics. b, e Presented are data on whole-tissue DRD1 and DRD2 mRNA levels adjusted for changes in neuronal proportion for 32 controls and 24 alcoholics. c, f Correlation between neuronal proportion and DRD1 mRNA (c) or DRD2 mRNA (f). Statistical analysis was performed by one-way ANCOVA; P values were calculated by ordinary bootstrap with 5 × 10⁵ non-parametric resampling of cases. mRNA levels are shown in arbitrary units. In boxplots, middle line is the median, box spans the interquartile range (IQR), and whiskers extend 1.5 × IQR from box limits. Lines and shading represent the estimated slopes and 95% CI, respectively

Fig. 4

Correlation between expression of the opioid PDYN and OPRK1 genes and the dopamine receptor DRD1 and DRD2 genes in NAc. a–d Relationship between PDYN and DRD1 (a), PDYN and DRD2 (b), OPRK1 and DRD1 (c), and OPRK1 and DRD2 (d) in combined sample of controls and alcoholics (N = 92 subjects). Statistical analysis was performed by one-way ANCOVA; P values were calculated by ordinary bootstrap with 5 × 10⁵ non-parametric resampling of cases. mRNA levels are shown in arbitrary units. Lines and shading represent alcoholism slope estimates and 95% CI, respectively

Fig. 5

Functional implications of transcriptional changes in expression and co-expression patterns of the opioid PDYN and OPRK1 genes and the dopamine receptor DRD1 and DRD2 genes, for the shift to aversive and dysphoric state in alcoholics. Downregulation of DRD1 expression and binding potential (present study and [15]) in alcoholics may reflect the decline in DRD1-mediated synaptic activity or the number of D1-MSNs, and that in turn may lead to disinhibition of D2-MSNs, in which activation results in dysphoria and aversive behavior. Dynorphins regulate information processing in the MSN local microcircuitry by inhibiting GABA release from D1- and D2-MSN collaterals [23]. Changes in the PDYN/OPRK1 co-expression pattern suggest that in subgroup of alcoholics with high DYN and KOR expression, the D2-MSN output may be preferentially controlled by dynorphins through inhibition of GABA release from D1-MSN collaterals (a, c). The DYN-induced inhibition at this site would lead to disinhibition of D2-neurons. In contrast, in alcoholics with low DYN/KOR activity, the disinhibition of GABA release from D2-presynaptic terminals may lead to inhibition of D1-MSNs and, consequently, disinhibition of D2-MSNs (b, d)