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'Mirror pain' in the formalin test: behavioral and 2-deoxyglucose studies
Anna Maria Aloisi1, Carlo Adolfo Porro, Milena Cavazzuti
1Istituto di Fisiologia Umana, Universitá di Siena, I-53100 SienaItaly Dipartimento di Scienze e Tecnologie Biomediche, Università di Udine, I-33100 UdineItaly Clinica Neurologica, Università di Modena, 1-41100 ModenaItaly Istituto di Fisiologia Umana, Università di Modena, 1-41100 ModenaItaly.
Pain
|November 1, 1993
Summary
Formalin injections in rats caused "mirror pain," observed as licking the opposite paw. This pain response correlated with increased functional activity in the lumbar spinal cord, suggesting referred pain mechanisms.
Area of Science:
- Neuroscience
- Pain Research
- Animal Models
Background:
- Formalin-induced pain is a common model for studying nociception.
- Referred pain, pain felt at a location other than the site of the painful stimulus, is a complex phenomenon.
- Understanding the neural mechanisms underlying referred pain is crucial for effective pain management.
Purpose of the Study:
- To investigate the phenomenon of 'mirror pain' following formalin injection in rats.
- To examine the functional activity changes in the lumbar spinal cord associated with this contralateral pain response.
- To explore the potential neural pathways involved in referred pain.
Main Methods:
- Subcutaneous injection of dilute formaldehyde solution (5% or 10%) into the hind paw of rats.
- Observation and quantification of licking behavior on both ipsilateral and contralateral hind paws.
- Assessment of functional activity levels in the lumbar spinal cord using the 2-deoxyglucose (2-DG) technique in unanesthetized, freely moving rats.
Main Results:
- Formalin injection induced 'mirror pain,' characterized by licking of the contralateral hind paw in a majority of rats.
- Contralateral licking was less frequent but positively correlated in intensity with ipsilateral licking.
- Increased bilateral functional activity in the lumbar spinal cord (dorsal/ventral horns, central gray, funiculi) was observed within the first hour post-injection.
Conclusions:
- Formalin-induced 'mirror pain' in rats suggests a referred pain phenomenon.
- Bilateral spinal cord metabolic changes indicate widespread neural activation, potentially involving interneurons and supraspinal pathways.
- These findings provide insights into the neurobiological basis of referred pain and spinal cord processing of nociceptive information.