Our previous studies have shown that CFA-induced masseter inflammation and intra-Vi/Vc microinjection of IL-1β result in contralateral orofacial hyperalgesia
[12, 13]. We have also shown that lesions in the RVM prevented the development of contralateral hyperalgesia
. The present study further shows that the development of contralateral orofacial hyperalgesia involves IL-1R activation in the ipsilateral Vi/Vc that ultimately leads to NK1-R activation in the RVM and 5-HT3 receptor activation in the contralateral Vi/Vc.
The RVM is a critical site in descending modulation of pain
[32, 33]. Much research has focused on the descending inhibitory affects of the opioid system within the RVM
[34, 35]. Our lab has shown that upregulation and phosphorylation of AMPA receptors in the RVM can lead to descending inhibition of inflammatory pain
[36, 37]. However, there has been mounting evidence of RVM-mediated descending facilitation
[38–42]. The RVM has been shown to have reciprocal connections with trigeminal neurons in the Vi/Vc transition zone
. Our previous studies indicated that intra-masseter muscle CFA injection and intra-Vi/Vc IL-1β injection produces bilateral orofacial hyperalgesia. The RVM was previously shown to mediate the IL-1β-induced hyperalgesia on the contralateral side
The contralateral effect was also produced by an IL-1β injection into the Vi/Vc but was not produced after IL-1β injection into the Vc
. Rats with masseter muscle inflammation exhibit increased IL-1β expression in Vi/Vc astroglia
. Furthermore, IL-1 receptor (IL-1R) activation induces a protein kinase-C dependent signaling cascade that leads to N-methyl-D-aspartate receptor phosphorylation and ultimately behavioral hyperalgesia
. In our present study, inhibition of IL-1R activation in the ipsilateral Vi/Vc before CFA treatment prevented the development of contralateral orofacial hyperalgesia. Similarly, inhibition of IL-1R activation in the ipsilateral Vi/Vc after CFA treatment resulted in an attenuation of the contralateral hyperalgesia. These results support the view that activation of IL-1R in the ipsilateral Vi/Vc is involved in the development and maintenance of deep tissue orofacial pain on the contralateral site.
Substance P has been shown to be involved in RVM descending modulation of pain
[22, 25]. NK1-R activation in the RVM enhances excitatory glutamatergic inputs to RVM neurons in rats with persistent pain
. NK1-R antagonism in the RVM attenuated CFA-induced thermal hyperalgesia in the hind paw
. Our previous studies have shown that NK1-R expression is increased in the RVM following hind paw inflammation and direct injection of substance P into the RVM can induce bilateral thermal hyperalgesia in the hind paw
. We also showed that Substance P-induced hyperalgesia can be mediated by 5-HT, NMDA, and GABAA circuits
. We have now shown that NK1-R inhibition in the RVM attenuated inflammatory orofacial hyperalgesia bilaterally. These results suggest that NK1-R activation in the RVM contributes to descending facilitation that mediates inflammatory orofacial hyperalgesia to the ipsilateral and contralateral side.
In the spinal cord, administration of a 5-HT3 receptor antagonist inhibits evoked responses of dorsal horn neurons after noxious stimulation
 and intra-RVM cholecystokinin-induced mechanical and thermal hyperalgesia
. Similarly, intrathecal administration of a 5-HT3 receptor antagonist attenuates formalin-induced hind paw flinching and spinal ERK activation
. We have previously shown that after the depletion of endogenous 5-HT while maintaining the viability and function of serotonergic neurons, descending facilitation is dependent on the activation of serotonergic neurons in the RVM
. The present study shows that descending serotonergic facilitation is necessary for the development and maintenance of contralateral orofacial hyperalgesia after CFA-induced masseter inflammation. Furthermore, activation of Vi/Vc 5-HT3 receptors is necessary for induction of the CFA-induced contralateral orofacial hyperalgesia to develop.
Interestingly, our results show that IL-1R inhibition and 5-HT3 receptor blockade in the ipsilateral Vi/Vc does not affect ipsilateral hyperalgesia. This finding suggests that Vc activation from afferent input is important for the ipsilateral hyperalgesia while the development of contralateral hyperalgesia requires Vi/Vc-RVM circuitry. Supporting this hypothesis, lesions of the Vc inhibited the development of ipsilateral hyperalgesia without affecting contralateral hyperalgesia. It appears that the afferent drive into the Vc is the primary mechanism involved in ipsilateral hyperalgesia consistent with our previous findings
A puzzling observation from this study is that shRNA treatment of Tph-2 in the RVM, which down regulates 5-HT bilaterally, did not affect ipsilateral hyperalgesia. One would expect that hyperalgesia would be attenuated bilaterally. One possible explanation is that ipsilateral hyperalgesia is dominated by hyperexcitability and central sensitization in the Vc driven by peripheral input which masks the loss of descending facilitation. An alternative explanation is that the absence of attenuation of ipsilateral hyperalgesia may be related to the finding of varying degrees of descending inhibitory and facilitatory input into the contralateral and ipsilateral medullary/spinal dorsal horn. It has been shown that descending pain modulation is in dynamic balance between inhibitory and facilitatory signals
. After hind paw inflammation, there is a time-dependent enhancement of descending inhibition on the ipsilateral side
[48, 49]. Descending 5-HT projections from the RVM also produce dual inhibitory and facilitatory effects due to activation of different 5-HT receptor subtypes
. The net effect of descending modulation may be facilitatory on the contralateral side and inhibitory on the ipsilateral side in this masseter inflammation model. Thus, depletion of descending 5-HT on the ipsilateral side would reduce predominant inhibitory signals and the hyperalgesia would be sustained.