Impaired periamygdaloid-cortex prodynorphin is characteristic of opiate addiction and depression

Abstract

Negative affect is critical for conferring vulnerability to opiate addiction as reflected by the high comorbidity of opiate abuse with major depressive disorder (MDD). Rodent models implicate amygdala prodynorphin (Pdyn) as a mediator of negative affect; however, evidence of PDYN involvement in human negative affect is limited. Here, we found reduced PDYN mRNA expression in the postmortem human amygdala nucleus of the periamygdaloid cortex (PAC) in both heroin abusers and MDD subjects. Similar to humans, rats that chronically self-administered heroin had reduced Pdyn mRNA expression in the PAC at a time point associated with a negative affective state. Using the in vivo functional imaging technology DREAMM (DREADD-assisted metabolic mapping, where DREADD indicates designer receptors exclusively activated by designer drugs), we found that selective inhibition of Pdyn-expressing neurons in the rat PAC increased metabolic activity in the extended amygdala, which is a key substrate of the extrahypothalamic brain stress system. In parallel, PAC-specific Pdyn inhibition provoked negative affect-related physiological and behavioral changes. Altogether, our translational study supports a functional role for impaired Pdyn in the PAC in opiate abuse through activation of the stress and negative affect neurocircuitry implicated in addiction vulnerability.

Figure 1. PDYN is reduced in the PAC of postmortem chronic heroin subjects and MDD subjects.

(A–C) Representative film autoradiograms of PDYN mRNA expression on coronal cryosections of the amygdala and corresponding bar graphs of PDYN mRNA expression levels in the PAC. Values are expressed in DPM/mg (mean ± SEM) and the PAC is encircled in red. Scale bar: 5 mm (A) Cohort I of heroin abusers and matched controls. (B) Cohort II of heroin abusers and matched controls. (C) MDD subjects and matched controls. *P < 0.05.

Figure 2. Reduced Pdyn mRNA in the PAC and concomitant sensitization of a neural stress marker induced in rats with chronic heroin self-administration.

(A) Rodent PAC schematic and in situ hybridization image of Pdyn in the PAC. Scale bar: 2 mm. (B) Lever-pressing behavior (active versus inactive lever presses) of rats receiving heroin or saline euthanized 24 hours after last self-administration session. Data shown as mean ± SEM. *P < 0.05 between active lever (AL) and inactive lever (IL) presses of animals receiving heroin. ^#P < 0.05 in active lever presses between heroin and saline animals. (C) Reduction of PAC-Pdyn mRNA expression in rats 24 hours after final heroin self-administration session versus saline animals. Data represent mean ± SEM, fold change difference. *P < 0.05. (D) Increased Crf mRNA expression in the CeA 24 hours following heroin self-administration. Values are expressed in DPM/mg (mean ± SEM). *P < 0.05.

Figure 3. DREAMM imaging reveals that neuronal inhibition of Pdyn neurons in the PAC increases metabolic activity in the ExA.

(A) Axial and right sagittal views showing that activation of G_i-mediated signaling in Pdyn neurons of the PAC with CNO leads to a profound induction of neuronal activity primarily in the right ExA as compared with vehicle administration in the same animal (red, relative increase; blue, relative decrease). LH, lateral hypothalamus; NAcSh, NAc shell; SI, substantia inominata; VMH, ventral medial hypothalamus; IPAC, interstitial nucleus of posterior limb of anterior commissure; SC/MC, sensory cortex/motor cortex; ON, olfactory nuclei; CeA, central amygdala; MVePC,: medival vestibular nucleus, parvicellular part; VP, ventral pallidum; LEnt, lateral entorhinal cortex; BLA, basolateral amygdala; MPA, medial preoptic area. (B) Schematic diagram of the ExA circuit and its substructures.

Figure 4. Voxel analysis of ExA substructures confirms increase of metabolic activity following PAC-Pdyn inhibition.

(A) Representative coronal images of ExA substructures analyzed for voxel analysis (B) Quantitative bar graphs of 3D voxel intensities in the right ExA substructures. Evaluation of each substructure demonstrated that CNO administration enhanced voxel intensity selectively in the right substructures as compared with vehicle administration. BNST, P = 0.002; NAc shell, P = 0.031; MeA, P = 0.008; CeA, P = 0.1, paired t test.

Figure 5. Inhibitory modulation of PAC-Pdyn neurons induces depression-related behavioral and physiological phenotypes.

(A) There is a significant increase in corticosterone levels in PAC-Pdyn-hM4Di rats administered CNO as compared with vehicle administration in the same animal. Values represent ng/ml, *P < 0.05. (B) PAC-Pdyn-hM4Di animals consumed significantly less sucrose than PAC-Pdyn-GFP animals (P = 0.032, ANOVA) although they did not differ in water consumption. Data are shown in ml, *P < 0.05 (C) PAC-Pdyn-hM4Di rats show increased immobility compared with Pdyn-GFP animals in the forced swim test. Data represent cumulative time in seconds, **P < 0.01.