Plasma tryptophan and kynurenine in females with temporomandibular disorders and fibromyalgia-An exploratory pilot study

Background Both temporomandibular disorders myalgia (TMDM) and fibromyalgia (FM) have been linked to central and peripheral changes in serotonin availability. The precursor of serotonin, tryptophan (TRP), is mainly catabolised via another pathway to produce kynurenine (KYN), but whether changes of this pathway are present in TMDM and FM are still unclear. Objective The aim was to explore blood plasma concentrations of TRP and KYN in TMDM and FM in an attempt to identify novel associations for future research. Methods Plasma of 113 female participants (17 TMDM, 40 FM and 56 healthy pain-free controls) were analysed for TRP and KYN concentrations. The degradation of TRP via the KYN pathway was indicated by the KYN to TRP ratio (KYN/TRP). Pain intensities were assessed with the Graded Chronic Pain Scale (GCPS) and Visual Analogue Scale (VAS). Psychological symptoms were evaluated using the Hospital Anxiety and Depression Scale (HADS), Patient Health Questionnaire (PHQ-9) and General Anxiety Disorder scale (GAD-7). Results In TMDM there was a negative correlation between TRP and pain intensity (r(s) = -0.55 P = .023) and positive correlations between KYN/TRP and pain intensity (r(s) = 0.59 P = .013). In FM, KYN/TRP was negatively correlated with anxiety symptoms (r(s) = -0.36 P = .022) and a trend towards significantly lower TRP levels was found compared to controls (P = .05). Conclusion The association between KYN/TRP and pain intensity as well as anxiety ratings in this small exploratory study may indicate that KYN/TRP could be a relevant indicator for symptom severity in TMDM and FM. Further investigations of the KYN pathway in chronic myalgia are warranted.