Knee joint inflammation attenuates spinal FOS expression after unilateral paw formalin injection in rat

Carrageenan-induced knee joint inflammation evoked a transient spinal FOS protein expression in neurons localized in the apical region of laminae I-III with peak activity observed 2 h after inflammation. Consistent with previously published observations, paw formalin injection evoked a distinct pattern of FOS protein expression in L3-L5 spinal segments. The majority of FOS-positive neurons were localized in the superficial dorsal horn (laminae I-II). Laminae V and VI contained moderate numbers of labeled neurons and only a few labeled nuclei were visible in laminae VII-X. In contrast, ipsilateral paw formalin injection, if administered 4 h after carrageenan-induced knee inflammation, evoked significantly fewer FOS positive neurons in all laminar and segmental levels analyzed as compared with formalin injected animals but without previous knee joint inflammation. These data indicate that primary acute or subacute nociceptive input may evoke central processes that are characterized by an inducible form of central inhibition which then may serve to modulate the subsequent spinal effect of superimposed nociceptive peripheral stimulation.