In this study, we revealed a relatively comprehensive description of the mechanism of HZ chronification. First, we identified within-BGN and DMN-BGN connectivities by HZ patient outcome, with PHN patients showing increased DMN-BGN connectivity compared with AHZ and RHZ patients, and RHZ patients showing increased within-BGN connectivity compared with AHZ and PHN patients. Moreover, DMN-BGN connectivity was associated with the ID pain score in the AHZ patients and could act as a neuroimaging biomarker to predict HZ patient outcomes.
It is increasingly being recognized that higher-grade brain functions rely on the collaboration of large-scale networks rather than on single regions in isolation(Bassett and Sporns, 2017; Fox and Raichle, 2007; Smith et al., 2013). Researchers have reported the involvement of some RSNs in other multiple chronic pain populations and emotional disorders, such as chronic orofacial neuropathic pain, temporomandibular disorder, spondyloarthritis, and major depressive disorder(Alshelh et al., 2018; Hemington et al., 2018; Kucyi et al., 2014; Yu et al., 2019). Our previous research also showed that PHN patients could be characterized by abnormal DMN-BGN and within-BGN connectivity compared with RHZ patients(Wu et al., 2022), which are also the key points of this present study. So we continue to discover the mechanism of HZ pain chronification from the angle of RSNs connectivity.
The principal anatomical component of BGN is the striatum, which is divided into the caudate, putamen, and nucleus accumbens (NAc). The BGN receives afferent inputs from different cortical regions and projects to the cortex via the thalamus, forming a thalamo-cortico-striatal loop(Borsook et al., 2010; Luscher and Malenka, 2011). This neuronal loop serves as the basis of pain processing, which encompasses sensory-discriminative, emotional/affective, and cognitive dimensions of pain(Borsook et al., 2010; Chudler and Dong, 1995; Cropley et al., 2006). In particular, the pivotal role of the NAc, a central node of the mesocorticolimbic system, in reward-aversion processing makes it a key component in the emotional pain network(Baliki et al., 2013). The striatum of the BGN (especially NAc) is rich in dopaminergic neurons and is considered a hedonic node, involving reward, addiction, and euphoric mood(Serafini et al., 2020). Studies have demonstrated that the prominent connectivity between NAc and dorsal striatum (caudate/putamen) is related to emotional learning(Chang et al., 2014; Voorn et al., 2004). NAc structural and functional abnormalities exist in pain and depression independently and within comorbid states(Pittenger and Duman, 2008; Serafini et al., 2020; Sheng et al., 2017). The strength of the within-BGN connectivity may be one of the central mechanisms involved in the pain-depression comorbidity. Our finding of a higher strength of within-BGN connectivity and positive affective scores in RHZ patients suggested that positive emotion may contribute to the process of pain recovery, which can be partly accounted by the within-BGN dopaminergic neurons. In contrast, the lower-level within-BGN connectivity and higher negative emotional scores in AHZ and PHN patients suggested that the persistent low-level within-BGN connectivity may be involved in the pain-depression vicious circle and may promote pain development and maintenance. But unfortunately, the correlation between within-BGN connectivity and clinical scores did not survive, mainly bacause the pain and emotional scores in same group are over centralized and can’t widen the gap, meanwhile the times of correlation analysis are too many to achieve significant value. But as reported previously(Cao et al., 2020), the similar tendency of within-BGN connectivity and affective score may provide a heuristic cue that within-BGN connectivity potentially accounted for condition of pain alleviating in RHZ group.
Another network emphasized in our results is DMN, which is involved in internally directed processes, including theory of mind and memory, attention, and mind-wandering(Peer et al., 2015; Raichle, 2015; Spreng and Grady, 2010). Previous studies showed that the DMN dynamic was disrupted in several chronic pain diseases(Baliki et al., 2008; Kucyi et al., 2013), such as migraine(Xue et al., 2012), fibromyalgia(Napadow et al., 2010), complex regional pain syndrome(Bolwerk et al., 2013), and chronic back pain(Loggia et al., 2013). The imbalanced DMN pattern of these diseases is probably associated with pain attention, rumination, and pain-related negative emotion(Baliki et al., 2008; Erpelding and Davis, 2013; Kucyi et al., 2013; Kucyi et al., 2014). In addition, abnormal DMN activity has been linked to the difficulty in regulating negative thinking in depressive disorders(Malhi et al., 2018; Malhi et al., 2019; Rogers and Joiner, 2017), indirectly suggesting that the DMN is involved in the integration of pain and depression. In general, these findings indicate that chronic pain does indeed have a widespread effect on the DMN balances and suggested that DMN disruptions may underlie the emotional and cognitive impairments accompanying chronic pain(Baliki et al., 2008). Furthermore, several human and animal studies have also identified the role of DMN-BGN (medial prefrontal cortex-NAc, mPFC-NAc) circuitry in chronic pain persistence and its emotional regulation, which have also been emphasized in our study(Chang et al., 2014; Lee et al., 2015; Ren et al., 2016; Vachon-Presseau et al., 2016; Zhou et al., 2018). Our current results showed higher DMN-BGN connectivity in PHN patients compared with AHZ patients, suggesting that the frequent information exchange between emotion-related BGN and advanced cognition-related DMN during the disease course of PHN play an important role in pain chronification, probably partly by aggravating the pain rumination degree and pain-related negative mood. Our study is consistent with longitudinal study showing a variation in the mPFC-NAc connectivity in a chronic back pain population(Baliki et al., 2012). The authors also elucidated the mechanisms of the reorganizational properties of NAc, demonstrating that local neural networks from the NAc to the dorsal striatum and cortex showed minimal disruption at day 5 in spared nerve injury (SNI) animals and more extensive reorganization at day 28 post-injury(Baliki et al., 2014). This limbic-cortical connectivity in the SNI model was linked to dopamine receptor gene expression and the degree of tactile allodynia, which together mediated the development of chronic pain(Chang et al., 2014). These suggested that the limbic dopaminergic system is part of the rearrangement of cortical circuits associated with the transition to chronic pain.
Further research revealed that the limbic-cortical functional and anatomical connection not only plays a role in the initial development of pain chronification, but also can be a biomarker to predict future disease symptoms in chronic back pain and urologic chronic pelvic pain syndrome(Baliki et al., 2012; Vachon-Presseau et al., 2016). In our study, the DMN-BGN connectivity was positively correlated with ID pain scores and was also able to predict the outcome of AHZ patients, with sensitivity and specificity of 77.8% and 63.2%, respectively. This further confirmed the significance of the role of limbic-cortical connectivity in HZ pain chronification and may provide a neuroimaging basis for the early and active treatment of acute HZ patients and may reduce the incidence of refractory PHN.
Limitations
Although these are the first findings of network-level connectivities in HZ patients of different disease courses, the present study still has some limitations. First, due to a lack of sufficient follow-up data from acute HZ patients and the difficulty of a longitudinal study, we simply compared the network connectivity among different populations of different HZ disease courses and did not follow the alterations in the same AHZ population. Future longitudinal study would emphasise the connectivity imbalance within the same acute population. Second, some other networks (such as the sensorimotor network, ascending and descending pain modulation pathways) and their within-/cross-networks connectivities could also be studied and we are presently exploring these options. In addition, we could re-run the analyses by using the independent component analysis (ICA) method to verify our results. These questions would be addressed with prospective longitudinal imaging studies.