Using a translational approach, we have shown that impaired PDYN mRNA expression in the human PAC is a shared characteristic of the diagnostically distinct but symptomatically related disorders of opiate addiction and MDD. The consistent disturbance of PAC-PDYN expression is intriguing but raises questions as to the potential functional relevance of this impairment, since the PAC, unlike other amygdala nuclei, has not been extensively studied. As such, we expanded insights about the PAC-Pdyn neurons by examining their in vivo functional connectivity. Our use of the molecular in vivo imaging strategy DREAMM in rats highlighted the selective recruitment of the ExA in relation to impaired PAC-Pdyn neuronal activity, therefore linking the Pdyn in the PAC to a functional neural network that regulates negative affect responses relevant to opiate addiction vulnerability.
Our finding of an association between the Pdyn
system and negative affect is in line with a large body of evidence. While much of the published data have been obtained from experimental animal studies (15
), recent clinical investigations have also demonstrated that variants of the PDYN
gene relate to transcriptional alterations of PDYN
mRNA expression and are associated with depressive traits and negative craving in alcoholics (36
). The current study adds to the field by directly examining the human brain to provide neuropathophysiological insights that have not been previously reported. The shared reduction of PDYN
mRNA expression in the amygdala PAC of human heroin and MDD subjects supports the hypothesis that common disturbances in brain areas linked to emotional regulation underlie the high comorbidity rates of drug addiction with MDD. Although human postmortem studies have many challenges, the PDYN
mRNA impairments observed were replicated in multiple cohorts of opiate abusers and disturbances detected in the MDD subjects also matched those previously reported (38
). In addition, unlike the PAC, where PDYN expression was similarly reduced in addiction and MDD subjects, PDYN
mRNA is differentially affected in the striatum of presumably depressed suicide subjects (39
) and heroin abusers (40
The prevailing hypothesis in the field posits that increased dynorphin mediates negative affect. While the reduction of PDYN
mRNA expression observed in the current study might seem contrary, there may be compensatory upregulation of peptide levels. Indeed, it has been demonstrated in the striatum that reduced PDYN
mRNA expression can occur concomitant to elevated dynorphin levels in some heroin abusers (40
). Future insights about the release of PDYN
-derived peptides from PAC-PDYN
neurons will be very informative. It is also important to emphasize that the relationship between PDYN and different aspects of negative affect is complex and still requires further investigation. For example, although stress-induced aversion is established to be mediated by elevated dynorphin (41
), a number of investigations have documented that animals lacking the Pdyn
gene have enhanced anxiety behavior (43
). Moreover, our study underscores the heterogeneity of PDYN expression even within discrete brain regions, suggesting different PDYN circuits could contribute to distinct behavioral components relevant to negative affect.
A notable finding in the present study was that MDD subjects, in addition to the PAC, also had reduced PDYN mRNA expression in the AB and AHA nuclei. These disturbances were not, however, evident in the lateral nucleus (Supplemental Figure 3), which also expresses PDYN, suggesting that the alterations in MDD individuals are not global but instead discretely localized. Further studies are needed to delineate the specific contribution of PDYN AB and AHA neurons to distinct components of MDD neuropathophysiology.
Direct molecular studies of the human brain are of marked importance, but some limitations of human postmortem investigations are the lack of complete knowledge about the subjects’ drug abuse and other confounding factors, including nicotine consumption, medical history, and time of last drug use prior to death, which are relevant for interpretation of the data. However, interestingly, a reduction of PAC-Pdyn
mRNA expression was also found in rats that self-administered heroin at a time point associated with acute withdrawal related to hyperactivation of brain stress systems (upregulation of Crf
expression in the CeA). Indeed, extracellular and mRNA Crf
changes in the CeA are consistently associated with acute withdrawal from opiates (25
). Our human data also underscore the importance of the PAC-PDYN
alterations related to chronic neuroadaptations, given that acute toxicology of heroin metabolites did not correlate with amygdala PDYN
expression. Altogether, these observations are in line with PAC-PDYN
impairments underlying negative affect that may contribute to the motivational drive for drug-seeking and relapse behavior and that have been associated with a decrease in the function of neuronal reward circuits that contributes to addiction, which Koob et al. have characterized as the “dark side” of addiction (46
Our use of the innovative molecular and in vivo imaging strategy of DREAMM in rats also clearly highlighted the striking selective association of PAC-Pdyn
neurons with the ExA that serves as an interface between drug abuse and negative affect (13
). Moreover, the fact that impaired firing of Pdyn
neurons directly enhanced corticosterone levels and also induced behavioral anhedonia and despair provides strong evidence for a direct causal relationship between PAC-Pdyn
neuronal function and negative affect. Recent imaging studies in humans and primates have expanded the concept of the ExA in higher order species and support a role for its dysfunction in responses to antidepressants (48
). As such, the activation of the ExA due to inhibition of Pdyn
neurons in the PAC of behaving animals suggests that chronic PAC-Pdyn
dysregulation may be relevant to relapse that is driven by enhanced negative affect/stress/anxiety that maintains the insidious cycle of drug abuse. Further studies are needed to determine whether PAC-Pdyn
neurons are a feasible target for addiction-related behaviors, particularly during the withdrawal phase.
It is important to note that species differences exist in the expression pattern of Pdyn
in the amygdala of humans and rodents. Whereas humans have pronounced Pdyn
expression in cortical amygdala nuclei such as the PAC and in the AB (22
), rodents have the strongest expression in the CeA, basolateral, and medial nuclei (50
). Despite the species difference, the PAC circuitry variations examined in several primate and nonprimate models have been shown to be complementary (51
). As such, the similar changes in PAC-Pdyn
observed in the rodent heroin self-administration model and in human heroin abusers would be predicted to affect similar types of neural circuitry.
Overall, the common disturbance of PDYN in the PAC in opiate addiction and MDD offers new pathophysiological insights into the amygdala relevant to stress and negative affect endophenotypes common in opiate addiction. Moreover, the in vivo imaging technique of DREAMM to manipulate neurochemically distinct cells in discrete brain areas with high molecular specificity in awake freely moving animals will be a valuable neurobiological tool in helping to enhance knowledge about discrete neuronal circuits relevant to neuropsychiatric disorders.