Differential time course and spatial expression of Fos, Jun, and Krox‐24 proteins in spinal cord of rats undergoing subacute or chronic somatic inflammation

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Abstract
We have used the evoked expression of both immediate early gene (IEG)‐encoded proteins (Krox‐24, c‐Fos, Fos B, Jun D, Jun B, c‐Jun), and dynorphin to monitor sensory processing in the spinal cords of rats undergoing subacute or chronic somatic inflammation (i.e., subcutaneous inflammation of the plantar foot and monoarthritis, respectively). Behavioral and immunocytochemical approaches were conducted in parallel up to 15 weeks postinjection in order to detect possible relationships between clinical evolution and spatiotemporal pattern of IEG‐encoded protein expression. Each disease had specific characteristics both in terms of their clinical evolution and pattern of evoked protein expression. All IEG proteins were expressed in both cases. Most of the staining was observed in both the superficial layers of the dorsal horn and deep dorsal horn (laminae V–VII and X). Monoarthritis was distinguished by a high level of total protein expression. Staining was especially dense in the deep dorsal horn. More labelled cells were observed at 1–2 days and at 2 weeks postinjection, corresponding to the initiation and progressive phases of the disease, respectively. Subcutaneous inflammation was characterized by a moderate level of total IEG expression. More labelled cells were observed in the first day following injection. It is the relative degree of expression of each IEG‐encoded protein with regard to the others that characterized the progression of the diseases. Early stages of the diseases coincided with the expression of all Fos and Jun proteins, while late stages showed an increase in Jun D and Fos B involvement; Krox‐24 was induced mostly during the early phases and/or periods of paroxysm of the diseases. Persistent stimulation was characterized by a predominant expression in deep versus superficial layers of the dorsal horn. Evoked expression of c‐Jun in motoneurons was only observed in monoarthritis. The peak of dynorphin expression was late in regard to both the induction of inflammation and period of maximal IEG‐encoded protein expression. The present work indicates that the neural processing that takes place during progression of these diseases can be monitored well at the spinal cord level by using the expression of an array of IEG‐encoded proteins. Study of long term evolutive diseases and especially those that evolve into chronicity can largely benefit from such an approach.