Synapsin-GFP AAV (Serotype 5)

Craniofacial muscle pain is highly common in temporomandibular joint disorders but is difficult to treat. A deeper understanding of the neurobiology that is unique to craniofacial muscle pain should facilitate the development of novel mechanism-based therapeutic approaches. Nociceptive afferents in craniofacial muscles are primarily peptidergic afferents that are enriched in TRPV1. Signals from peripheral glutamate receptors converge on TRPV1, resulting in mechanical hyperalgesia. Recent developments in neural circuit manipulation methods allow for the silencing of specific overactive neural circuits. Chemical genetic silencing of TRPV1-expressing afferents or neurons in the anteroventral medulla attenuated hyperalgesia during masseter muscle inflammation. Therefore, further delineation of the neural circuits that mediate craniofacial muscle hyperalgesia may enhance treatments for chronic muscle pain conditions.

Chemical genetic approaches may also be used to manipulate central circuits. For example, chemogenetic silencing of RVM neurons attenuated spontaneous and biteevoked pain from masseter inflammation (Figure 1). Adeno-associated virus 5 encoding green fluorescent protein (GFP) or hM4Di fused to mCherry under the neuron-specific promoter synapsin (AAV5-Syn-GFP or AAV5-syn-hM4Di-mCherry) was stereotaxically injected into the rostral ventromedial medulla (RVM) (Figure 1A). GFP+ or mCherry+ axons are densely projected to the dorsal horn of the spinal cord, which is close to the TRPV1+ central terminals of the primary afferents. Four weeks after injection of AAV5-syn-GFP or AAV5-syn-hM4Di, mice expressing hM4Di or GFP showed comparable baseline MGS and bite force (Figure 1B and C). After 1 day of complete Freund's adjuvant (CFA) injection in the masseter muscles bilaterally, both groups showed similar increases in MGS and decreases in bite force. Systemic injection of the DREADD agonist compound 21 reduced MGS and increased bite force in the hM4Di group, but not in the GFP group. These results suggest that functional inhibition of descending projections from the RVM to the Vc following masseter muscle inflammation reduces the net facilitation of spontaneous and bite-evoked pain and mechanical hyperalgesia. This approach could be used to inhibit other excitatory circuits involved in masseter muscle pain.

Figure 1. Chemogenetic inhibition of rostral ventromedial medulla (RVM) neurons attenuates spontaneous and bite-evoked pain during masseter inflammation in mice. (Chung M K, et al., 2020)