Organization of pERK-immunoreactive cells in trigeminal spinal nucleus caudalis, upper cervical cord, NTS and Pa5 following capsaicin injection into masticatory and swallowing-related muscles in rats

Many phosphorylated extracellular signal-regulated kinase (pERK)-immunoreactive (IR) cells are expressed in the trigeminal spinal subnucleus caudalis (Vc), upper cervical spinal cord (C1-C2), nucleus tractus solitarii (NTS) and paratrigeminal nucleus (Pa5) after capsaicin injection into the whisker pad (WP), masseter muscle (MM), digastric muscle (DM) or sternohyoideus muscle (SM). The pERK-IR cells also showed NeuN immunoreactivity, indicating that ERK phosphorylation occurs in neurons. The pERK-IR cells were significantly reduced after intrathecal injection of MEK 1/2 inhibitor PD98059. The pERK-IR cells expressed bilaterally in the Vc and C1-C2 after capsaicin injection into the unilateral DM or SM, whereas unilaterally in the Vc and C1-C2 after unilateral WP or MM injection. After capsaicin injection into the WP or MM, the pERK-IR cell expression in the Vc was restricted rostrocaudally within a narrow area. However, the distribution of pERK-IR cells was more wide spread without a clear peak in the Vc and C1-C2 after capsaicin injection into the DM or SM. In the NTS, the unimodal pERK-IR cell expression peaked at 0-720 μm rostral from the obex following capsaicin injection into WP, MM, DM or SM. In the ipsilateral Pa5, many pERK-IR cells were observed following capsaicin injection into the SM. The number of swallows elicited by distilled water administration was significantly smaller after capsaicin injection into the WP, MM or DM but not SM compared to that of vehicle-injected rats. Various noxious inputs due to the masticatory or swallowing-related muscle inflammation may be differentially involved in muscle pain and swallowing reflex activity.