Trigeminal transition zone/rostral ventromedial medulla connections and facilitation of orofacial hyperalgesia after masseter inflammation in rats.

Abstract

Recent studies have implicated a role for the trigeminal interpolaris/caudalis (Vi/Vc) transition zone in response to orofacial injury. Using combined neuronal tracing and Fos protein immunocytochemistry, we investigated functional connections between the Vi/Vc transition zone and rostral ventromedial medulla (RVM), a key structure in descending pain modulation. Rats were injected with a retrograde tracer, FluoroGold, into the RVM 7 days before injection of an inflammatory agent, complete Freund's adjuvant, into the masseter muscle and perfused at 2 hours postinflammation. A population of neurons in the ventral Vi/Vc overlapping with caudal ventrolateral medulla, and lamina V of the trigeminal subnucleus caudalis (Vc), exhibited FluoroGold/Fos double staining, suggesting the activation of the trigeminal-RVM pathway after inflammation. No double-labeled neurons were found in the dorsal Vi/Vc and laminae I-IV of Vc. Injection of an anterograde tracer, Phaseolus vulgaris leucoagglutinin, into the RVM resulted in labeling profiles overlapped with the region that showed FluoroGold/Fos double labeling, suggesting reciprocal connections between RVM and Vi/Vc. Lesions of Vc with a soma-selective neurotoxin, ibotenic acid, significantly reduced inflammation-induced Fos expression as well as the number of FluoroGold/Fos double-labeled neurons in the ventral Vi/Vc (P<0.05). Compared with control rats, lesions of the RVM (n=6) or Vi/Vc (n=6) with ibotenic acid led to the elimination or attenuation of masseter hyperalgesia/allodynia developed after masseter inflammation (P<0.05-0.01). The present study demonstrates reciprocal connections between the ventral Vi/Vc transition zone and RVM. The Vi/Vc-RVM pathway is activated after orofacial deep tissue injury and plays a critical role in facilitating orofacial hyperalgesia.

• Authors: Shinichi Sugiyo, Motohide Takemura, Ronald Dubner, Ke Ren
• Journal: The Journal of comparative neurology (J Comp Neurol) Vol. 493 Issue 4 Pg. 510-23 (Dec 26 2005) ISSN: 0021-9967 [Print] United States
• PMID: 16304628 (Publication Type: Comparative Study, Journal Article, Research Support, N.I.H., Extramural)
• Copyright: Copyright (c) 2005 Wiley-Liss, Inc.

Chemical References

• Proto-Oncogene Proteins c-fos

Topics

• Animals
• Facial Pain (physiopathology)
• Hyperalgesia (metabolism, physiopathology)
• Inflammation (physiopathology)
• Male
• Masseter Muscle (innervation, physiopathology)
• Neurons (metabolism, physiology)
• Proto-Oncogene Proteins c-fos (metabolism)
• Rats
• Rats, Wistar
• Trigeminal Nerve (cytology, physiopathology)
• Trigeminal Nucleus, Spinal (cytology, metabolism, physiopathology)