A chronic-constriction injury of the sciatic nerve reduces bilaterally the responsiveness to formalin in rats: a behavioral and hormonal evaluation.
Application of four loose ligatures to the sciatic nerve of a rat (chronic constriction injury [CCI]) induces clear hypersensitivity to non-noxious stimulation and chemical irritants. However, in this study, an injection of formalin in the hind paw of a rat with CCI-induced mononeuropathy resulted in an ipsilateral decreased flinching and licking or biting behavior in both phases of the formalin testing. The effect was independent of the formalin concentration used. This altered behavior was accompanied with smaller plasma levels of adrenocorticotrope hormone and corticosterone compared with sham and non-operated animals. Formalin injection in the contralateral nonligated hind paw of CCI rats also reduced the licking or biting behavior as compared with sham-operated and non-operated control animals only in the second phase of the formalin test. Thus, CCI reduces the pain reactivity and hypothalamic-pituitary-adrenal-axis activation to ipsilateral and contralateral formalin injection. Further research should investigate whether the decreased pain reactivity by CCI is situated at the peripheral, spinal, or supraspinal level or is result of changes in the stress reactivity and coping strategies.


CONCLUSIONS
We evaluated the changes in the behavioral reactions and the hormonal effects of a noxious chemical stimulus, i.e., formalin injection in animals with previously induced chronic constriction injury to the sciatic nerve. The effect in animals injected at the ipsilateral and contralateral site, sham-operated and controls, were compared.
Vissers
Kris
K
*Multidisciplinary Pain Unit, Ziekenhuis Oost-Limburg, Genk; †R&D, PRD Johnson & Johnson, Beerse; and ‡Department of Anesthesiology, University Hospital Antwerp, Edegem, Belgium.
Adriaensen
Hugo
H
De Coster
Roland
R
De Deyne
Cathy
C
Meert
Theo F
TF
D000324:Adrenocorticotropic Hormone / Q000097:blood; D000818:Animals; D001522:Behavior, Animal; D003080:Cold Temperature; D003345:Corticosterone / Q000097:blood; D005557:Formaldehyde / Q000008:administration & dosage*; D006614:Hindlimb; D006728:Hormones / Q000097:blood*; D007030:Hypothalamo-Hypophyseal System; D008026:Ligation; D008297:Male; D009408:Nerve Compression Syndromes / Q000503:physiopathology; D010146:Pain / Q000139:chemically induced*; D017288:Pain Threshold; D010913:Pituitary-Adrenal System; D011388:Prolactin / Q000097:blood; D051381:Rats; D017207:Rats, Sprague-Dawley; D012584:Sciatic Nerve; D020426:Sciatic Neuropathy / Q000097:blood / Q000503:physiopathology*; D013972:Thyrotropin / Q000097:blood
D006728:Hormones; D005557:Formaldehyde; D000324:Adrenocorticotropic Hormone; D011388:Prolactin; D013972:Thyrotropin; D003345:Corticosterone