c-Fos expression in the spinal cord and pain-related symptoms induced by chronic arthritis in the rat are prevented by pretreatment with Freund adjuvant.

We previously showed that pathological adjuvant-induced arthritis (AIA), a chronic pain model, is associated with an increase in the number of Fos-like immunoreactivity (Fos-LI) in lumbar spinal cord neurons; maximal Fos-LI corresponds to the peak of pathological and behavioral signs of the disease. On the other hand, AIA is prevented in rats that have previously been injected with diluted complete Freund adjuvant (CFA). Here, the effects of pretreatment with CFA were studied in parallel on both pathological parameters and pain-related tests, and on Fos-LI, 3 weeks after the inoculation of the arthritogenic solution during the acute phase of the disease, that is, when hyperalgesia is maximal. One group of rats (vaccinated group) was pretreated with diluted CFA, 3 weeks and 1 week before inoculation (AIA) (with the concentrated arthritogenic solution), while a second group (arthritic group) was pretreated with the vehicle for the CFA solution and then inoculated with the concentrated arthritogenic solution. Control groups consisted of rats treated with various combinations of either saline or vehicle solutions during the pretreatment periods or at the time of inoculation. The arthritic group developed all pathological symptoms of AIA (i.e., loss of weight, difficulties in walking, increases in paw diameter, and decreases in the vocalization thresholds to mechanical pressure of the hindpaw, and finally an increase in the vocalization response to flexion and extension). In contrast, the vaccinated group did not develop AIA symptoms.(ABSTRACT TRUNCATED AT 250 WORDS)