To investigate whether blood serotonin (5-hydroxytryptamine) (5-HT) modulates musculoskeletal pain differently in seropositive and seronegative rheumatoid arthritis (RA).
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.
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 (r(s) = 0.36, n = 33, p = 0.042), as well as pain on maximum mouth opening (r(s) = 0.41, n = 24, p = 0.047) and tenderness to digital palpation (r(s) = 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 (r(s) = -0.47, n = 20, p = 0.037).
Peripheral serotonergic pain mechanisms seem to be activated by blood 5-HT in patients with seropositive RA, in contrast to seronegative patients.