Aim: To investigate whether blood serotonin (5-hydroxytryptamine) (5-HT) modulates musculoskeletal pain differently in seropositive and seronegative rheumatoid arthritis (RA).Methods: Patients with temporomandibular joint (TMJ) involvement of seropositive RA (33 patients) or seronegative RA (28 patients) and 26 healthy individuals were included. TMJ pain, general musculoskeletal pain, plasma and serum 5-HT, acute phase reactants and thrombocyte count were investigated.Results: The patients with seropositive RA had higher serum (median = 1130 nmol/l) and plasma (55 nmol/l) levels of 5-HT than the healthy individuals (704 nmol/l, p=0.044 and 23 nmol/l, p<0.001, respectively), and higher plasma levels of 5-HT than the seronegative patients (14 nmol/l, p<0.001). There was no significant correlation between serum and plasma levels of 5-HT in any group.In the seropositive RA patients, positive correlations were found between serum levels of 5-HT and the number of painful mandibular movements
(rs=0.36, n=33, p=0.042), as well as pain on maximum mouth opening (rs=0.41, n=24, p=0.047) and tenderness to digital palpation (rs=0.49, n=33, p = 0.003).In the healthy individuals, there was a negative correlation between plasma level of 5-HT and the TMJ pressure pain threshold
(rs=−0.47
, n=20
, p=0.037).Conclusion: Peripheral serotonergic pain mechanisms seem to be activated by blood 5-HT in patients with seropositive RA, in contrast to seronegative patients.
