In this study, we confirmed changes in NM and functional connectivity of the FST circuit and found a significant association between these changes in schizophrenia patients. In the circuit, functional connectivity impairment was primarily in the cortex, striatum, and substantia nigra, among which left SMA-left caudate connectivity was associated with NM. This association indicates that the two pathological changes are interrelated in schizophrenia pathophysiology. Moreover, given that NM-MRI provides a proxy measure for dopamine activity, these results suggest that dopamine underlies the connectivity changes in the FST circuit.
We observed a higher NM-MRI signal in schizophrenia patients than in healthy controls; a higher signal indicates increased NM accumulation in the SN18. This result is consistent with the findings of previous NM-MRI and postmortem studies that reported increased NM levels in schizophrenia patients38–41. In dopamine metabolism, excessive dopamine induces an increase in NM synthesis22. Moreover, previous multimodal studies reported a positive correlation between NM-MRI signals and PET dopamine measures, such as dopamine release and dopamine receptor availability18,24,25. In this context, the higher NM-MRI signal in our study may suggest an increase in dopaminergic function. As mentioned above, increased dopamine is one of the most prominent abnormalities in schizophrenia8.
Our study found a hypoconnected FST circuit subnetwork in schizophrenia patients, which includes the cortex, striatum, and substantia nigra. Consistent with this, previous studies have shown hypoconnectivity in cortico-cortical, striato-cortical, and nigro-striatal connections in schizophrenia patients12,42,43. Preclinical models of schizophrenia pathophysiology explain this hypoconnectivity: spontaneous phasic dopamine release in schizophrenia patients leads to increased noise in dopamine signalling in the striatum, which in turn reduces functional connectivity in the FST circuit8,44. In contrast, some studies have reported increased connectivity, such as in the thalamic subnuclei, in schizophrenia patients 13,14. These connections may not have been found in our study due to network-level statistics or the inclusion of the total thalamus rather than the subnuclei.
The dysconnected FST subnetwork included many striatal connections. The striatum is where dopaminergic projections from the midbrain arrive, and connectivity changes in this region are among the most consistent findings in schizophrenia8. Specifically, striatal connectivity alterations were prominent in the caudate14. In addition, cortical connectivity changes based on the aberrant striatum regions were primarily in the sensorimotor cortex12. These findings are in line with our results showing multiple caudate-sensorimotor cortex dysconnectivity. In our study, we also found connectivity changes in the nigro-striatal pathway. Previous studies suggested that reduced nigro-striatal connectivity may reflect reduced tonic activity and increased phasic activity in the pathway, given that rs-fMRI measures low-frequency fluctuations that are central to tonic activity12.
The FST circuit connectivity significantly predicted NM levels in schizophrenia patients. More specifically, lower left SMA-left caudate connectivity was associated with higher NM levels. Considering that NM levels reflect dopamine function, this result is consistent with previous findings showing an association between dopamine measures and striatal connectivity in schizophrenia patients14–16. In addition, our results of the association primarily identified in the SMA-caudate connection correspond with regional characteristics. According to recent neurochemistry studies, dopaminergic aberrations were greater in the dorsal striatum than in the limbic striatum5,45,46. The dorsal striatum encompasses the caudate region and receives cortical projections mainly from the SMA, motor area, and dorsolateral prefrontal cortex10. Taken together, this study added to other evidence suggesting that midbrain dopamine changes underlie aberrant connectivity of the FST circuit.
This study suggests the possibility of using NM-MRI for various investigations of the dopamine system. For example, treatment response studies reported a linkage between responsiveness and dopamine and noted a need for using molecular imaging for response prediction47–49. PET is a typical molecular imaging method but has limitations in clinical settings due to its invasiveness, cost, etc. NM-MRI is safer in terms of radiation exposure and has better accessibility. In this study, we confirmed that NM-MRI results are consistent with existing molecular imaging results and examined the association between NM-MRI signals and functional connectivity for the first time. Overall, this study has implications for the future use of NM-MRI to examine dopamine function in research and clinical practice.
The main strength of this study is that it examined the association between FST circuit individual connectivity and dopamine system molecules at the network level in schizophrenia patients. Furthermore, this study showed the results of investigating the dopamine system using NM-MRI, a promising molecular imaging method. This study has several limitations. Schizophrenia patients in our study were medicated. Although there was no significant association between medication status and variables, the effect of medication should be considered when interpreting the results. Next, some studies reported different patterns of connectivity among subregions in the FST circuit, particularly in the thalamic subregions14. Future studies using higher resolution are needed to examine the subregions. Finally, our association results do not indicate a causal relationship. Further investigations, for example, using sophisticated models, are required to determine causality within the pathophysiology.
We demonstrated a negative association between NM and FST circuit connectivity in schizophrenia patients using NM-MRI and fMRI. This shows a linkage between abnormal neurochemistry and functional abnormalities in the pathophysiology of schizophrenia. Furthermore, considering that the NM-MRI signal reflects dopamine function, our results suggest that dopamine underlies connectivity changes in the FST circuit. Together, these results expand our understanding of schizophrenia pathophysiology by providing experimental evidence supporting the dopamine hypothesis. In addition, this study has implications for the future use of NM-MRI in studies on the dopamine system.