In this study, the behavioral, histological and metabolic dynamic changes of toothache were longitudinally evaluated for up to 14 days in a rat DPI model. Interestingly, a perfect correlation among the behavioral, histological examinations and PET imaging was identified during the natural process of toothache, and therefore, our study provided a comprehensive understanding for the neurofunctional patterns underlying toothache. More importantly, the nerve conduction pathways related to toothache, especially the emotional reaction to toothache are identified. The related features provide rich resources to inspect toothache-induced brain changes.
The upload of toothache stimulation signal caused transient and large-scale reactive activation of rat brain nuclei, and there was a relationship between the functional connectivity of each nucleus and the strength of pain signals. According to previous literatures  and the results of the current study (Fig. 2F and G), the ascending pathway of toothache is postulated to be conducted through the trigeminal nerve complex to PAG, cc and VPM, and then to the subcortical area (nucleus accumbens, striatum, hippocampus and amygdala) and cerebral cortex (S1&S2, M1&M2, PFC, Cg, RSD). Among them, pain signals cross cc and PAG, which activate most of the brain regions bilaterally, thus regulating the transmission of pain in the left maxillary teeth. Different from previous studies , the visual signals related to VPM, superior colliculus and LGN, as well as RSD may also be involved in the transmission of toothache. Among them, the superior colliculus and LGN can make a visual reflex response to tactile information , thus reducing perception and emotion caused by pain, suggesting that the signals of toothache might activate the eye branch when ascending through the trigeminal ganglion, which explains why some patients with acute toothache often experience symptoms of eye discomfort and even tears . The RSD is involved in the control of spatial navigation, episodic memory and pain related emotional behaviors . It is speculated that RSD might be related to the cognitive and spatial memory of toothache in rats. Here, PET showed, for the first time, that the degree of activation and inhibition of the 12 brain regions involved in toothache is closely related to the changes of its pain intensity, and may also be related to the cognition and memory of toothache in rats. The changes of brain network caused by toothache are very complex and elusive and need to be further investigated.
Toothache induces anxiety- and depression-related symptoms, thus may cause the patients to miss the best time window to get treatments. Therefore, it is of great significance, to understand how the toothache related emotion was regulated in order to alleviate the negative emotions. ACC, including rostral anterior cingulate cortex (rACC) and cACC, is central to pain processing. rACC participates in the process of somatic pain emotional response through NR2B activation. According to our results, we chose to intervene the cACC at 3rd day after DPI. APV, an antagonist of NMDA receptor, can block the synaptic response of ACC neurons . We found that the glucose metabolism level of bilateral cACC but not rACC increased significantly 3 days after DPI. Interestingly, APV reversed the emotional response related to toothache. These results suggest that cACC but not rACC, may mediate the emotional reaction to toothache by phosphorylating NR2B receptor, which is the difference between emotional regulation of toothache and somatic pain. This might provide a novel insight for relieving the pain affect induced by toothache.