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Sustained microglial immunoreactivity in the caudal spinal trigeminal nucleus after formalin injection
1Department of Oral & Maxillofacial Surgery, National University of Singapore, Singapore. omsyeojf@nus.edu.sg
Abstract:
Recent studies indicate that glia may be involved in altered nociceptive processing after a peripheral inflammatory lesion produced by injection of inflammatory reagents such as formalin and zymosan. Most of these studies, however, confined their observations to a period shortly after the injections. This study investigated the immunohistochemical responses of microglia in the caudal part of the spinal trigeminal nucleus for up to 60 days after subcutaneous injection of formalin into the lateral faces of Wistar rats. The results showed obvious up-regulation of microglial markers such as OX-18, OX-42 and OX-6 up to 21 days after formalin injection. These were somewhat reduced at 30 days after injection. Electron microscope investigation revealed no evidence of significant phagocytosis of degenerative neuronal elements by microglia in the nucleus at the time--that is, 7 days after formalin injection, when microglial activation was at its peak. Significantly, however, the period of microglial activation corresponded closely to that showing enhanced nociceptive behavior after perioral formalin injection (Cadet et al., 1995). This study indicates a microglial role in the genesis of enhanced nociceptive behavior.
Insights
Microglia activation in the spinal trigeminal nucleus persists for weeks after formalin injection, correlating with enhanced pain sensitivity. This suggests a role for microglia in chronic pain development.
Area of Science:
- Neuroscience
- Immunology
- Pain Research
Background:
- Glia, particularly microglia, are implicated in processing pain signals following peripheral inflammation.
- Previous research often focused on short-term inflammatory responses, limiting understanding of long-term glial involvement.
Purpose of the Study:
- To investigate the long-term immunohistochemical changes in microglia within the spinal trigeminal nucleus.
- To correlate microglial activation with nociceptive behavior for up to 60 days post-formalin injection.
Main Methods:
- Subcutaneous formalin injections into the lateral faces of Wistar rats.
- Immunohistochemical analysis of microglial markers (OX-18, OX-42, OX-6) at various time points up to 60 days.
- Electron microscopy to assess microglial phagocytosis.
- Behavioral assessment of nociceptive responses.
Main Results:
- Significant upregulation of microglial markers observed up to 21 days post-formalin, with a reduction by day 30.
- Peak microglial activation at 7 days showed no significant phagocytosis of neuronal debris.
- The duration of microglial activation closely matched the period of enhanced nociceptive behavior.
Conclusions:
- Microglial activation in the spinal trigeminal nucleus is a prolonged response to peripheral inflammation.
- This sustained microglial activation is closely linked to the development of enhanced nociceptive behavior.
- Findings suggest a critical role for microglia in the genesis and maintenance of inflammatory pain.
