In this study, we investigated functional connectivity between ventrolateral PAG and other brain regions during resting state in 100 healthy normal subjects. Results showed that PAG activity is significantly correlated with surrounding brain regions such as midbrain tegmentum, substantia nigra, raphe nucleus, thalamus, striatum, pallidum, hypothalamus, hippocampus, and cerebellum, as well as distant brain regions including bilateral rACC, pACC, RVM, and right anterior insula.
As hypothesized, we found a significant intrinsic functional coherence between PAG and ACC (rACC and pACC) during resting state which provides direct evidence that PAG and ACC form a core intrinsic functional system. We believe elucidation of this ACC-PAG network will further facilitate our understanding of the pathology of chronic pain. For instance, in a recent study, Jensen and colleagues [26
] found that compared with healthy control subjects, fibromyalgia syndrome patients (FMS) failed to respond to pain provocation in rACC. We speculate this decreased activity in rACC of FMS patients may further trigger dysfunction (low activity of PAG) of the pain descending inhibition system. Consistent with results from previous studies [19
], we also found significant coherence between the PAG and RVM. It is known that PAG’s modulation function depends on its descending projection to RVM, which is involved in both the inhibiting and facilitating of pain perception. As such, we believe that dysfunction of the intrinsic functional connectivity of ACC-PAG-RVM may be part of the pathological basis of chronic pain.
Elucidating the intrinsic functional connectivity between ACC and PAG may also deepen our understanding of mood disorders. Prior studies have associated the rACC with lateral and accessory basal nuclei of the amygdala and implicated a role for the rACC during emotional / motivational processing [13
] and positively valenced events such as happiness [57
]. Further studies report the region’s involvement in mood disorders including depression [14
] and post-traumatic stress disorder (PTSD) [49
]. In addition, PAG is also significantly connected with hypothalamus, a key region of Hypothalamic–pituitary–adrenal (HPA) axis. It is known that the HPA axis is also involved in the mood disorder such as stress and anxiety, major depression and sleep disorders [1
]; thus, PAG demonstrates another pathway for involvement in mood and emotional regulation. Considering the last two findings together, we speculate that PAG may be involved in the mood / emotion regulation through two pathways: the rACC, amygdala and prefrontal cortex and the HPA-axis. Further study on these two pathways could considerably advance our understanding of mental disorders. Also, this overlap between chronic pain physiology and the brain regions associated with mood disorders may explain the high degree of co-morbidity between chronic pain and mental disorders [59
Our data showed a strong functional coherence between PAG and brain regions immediately surrounding it. This pattern is very similar to prior observations [37
] reporting that as electric shock grew closer, brain responses shifted from rACC to PAG and surrounding areas including thalamus, striatum, pallidum, hypothalamus, and cerebellum (see original in [37
] for details). In addition, Price [44
] suggested that neuroanatomical systems including hypothalamus, thalamus , ventral pallidum and anterior insula may be involved in the related functions of fear and discernment of the consequences of one's actions, as both are related to a system for control and modulation of visceral functions stretching from the spinal cord. This system is important for the control of emotions and plays a central role in the ability to discern the consequences of one's actions and make appropriate behavioral choices.
In this study, we also found significant functional connectivity between the PAG and anterior insula, a key region in pain process. This result is consistent with a recent publication in which the authors found that the functional connectivity between the anterior insula and PAG before a sensory event reflects the susceptibility to a subsequent noxious stimulus being perceived as painful or non-painful [42
]. In addition, we believe the finding [42
] also demonstrates the physiological significance of the functional connectivity among brain pain matrix.
Previous studies suggest that morphine produces a significantly greater degree of analgesia in males in comparison to females; some investigators believe that the sexually dimorphic function of the PAG-RVM circuit is involved in this process [32
]. Unfortunately, we did not find significant differences in the PAG-RVM functional connectivity between males and females. We speculate that the reason lies in the fact that the sexual-dimorphism of PAG-RVM function exists at the molecular level, thus this difference can not be detected by MRI technique. Nevertheless, we found that females showed more connectivity with mid-cingulate cortex, a region believed to be involved in the affective aspect of pain [46
]; males showed more connectivity with left medial orbital prefrontal cortex, uncus, right insula /operculum and prefrontal cortex. We believe these findings will shed some new light on our understanding of the gender difference in pain perception and modulation.
In addition to positive functional connectivity, we also found that PAG is negatively associated with brain regions including bilateral orbital prefrontal cortex, medial frontal cortex, postcentral gyrus, middle temporal gyrus, middle occipital gyrus, left lateral prefrontal cortex, paracentral lobule superior temporal gyrus, right inferior parietal lobule, and posterior insula. However, as interpretation of negative functional connectivity findings are still widely debated in the literature [38
], a further study clarifying the meaning of these result is needed.
In a recent report, Mobbs and colleagues [36
] investigated the neural organization of two adaptive responses to potential and imminent danger, postencounter and circa-strike defensive, using PPI analysis. They found that during the circa-strike condition, subjects showed increased coupling between the midbrain and mid-dorsal ACC and decreased coupling with the sgACC, amygdala, and hippocampus. Greater activity was observed in the right pregenual ACC for high compared with low probability of capture during circa-strike threat. This region showed decreased coupling with the amygdala, insula, and ventromedial prefrontal cortex. Although their study used different methods and studied a different condition, we believe that their results corroborate our findings, particularly our discovery of the functional connection between the PAG and ACC.
In this study, we did not find significant functional connectivity between the amygdala and PAG at the threshold we set. However, when the threshold was lowered to voxelwise p < 0.001 uncorrected, significant functional connectivity between bilateral amygdala and PAG was observed. It is known that amygdala is an important region in the pain descending inhibition system and plays a role in emotional processing and regulation. We speculate that this relative weak linkage between the PAG and amygdala may due to the fact that the subjects were neither in pain nor perform any emotional processing during resting state, thus the intrinsic coherence between the two region is not as important as ACC. Further study is required at this point.
Although it is well accepted in the field to use the coordinates from a previous fMRI study to locate the seed [23
], one potential concern is that the results from our seed region may not be representative of the connectivity at other parts of ventrolateral PAG. To further address this concern, we chose another coordinate (2 −28 −8) of right ventrolateral PAG, based on the NMI template, and performed an identical functional analysis using this seed as an independent coordinate to validate our findings. Results showed that PAG activity is significantly correlated with surrounding brain regions (including tegmentum, substantia nigra, raphe nucleus, thalamus, striatum, pallidum, hypothalamus, and cerebellum) and distant brain regions (including bilateral rACC and RVM) at the threshold we set. When we lower the threshold to voxelwise p < 0.001 uncorrected, the result is very similar to the results we obtained using our original seed, which provides further support for our results.
One potential limitaiton for this study is that we didn’t use a cardiac gated MRI acquisition, which reduces the effect of cardic pulsation on the variance of voxels around the brain stem. However, our global signal regression during the preprocessing stage is proven to reduce the correlation due to physiological origin [54
]. Thus, we do not believe physiological noise significantly influenced our results.
In summary, we found that PAG is intrinsicly connected with brain regions such as rACC, pACC and RVM. Eluciation of this ACC-PAG-RVM network has the potential to enhance our understanding of the physiological and pathological development and maintenance of chronic pain states and mood disorders. In addition, PAG is also connected to several important regions for visceral control and may play a central role in the ability to discern the consequences of one's actions and make appropriate behavioral choices.