Lower levels of tryptophan also were associated with increased pain intensity in people with muscle pain from temporomandibular disorders affecting the movement of the jaw, the researchers found.
Titled “Plasma tryptophan and kynurenine in females with temporomandibular disorders and fibromyalgia—An exploratory pilot study,” the study was published in the Journal of Oral Rehabilitation.
Chronic pain can manifest itself as a widespread or localized phenomenon. Fibromyalgia (FM) is a chronic condition that’s characterized by widespread pain, fatigue, and disturbed sleep, ultimately leading to poor quality of life.
Despite sharing some of fibromyalgia’s features, temporomandibular disorders myalgia (TMDM) are defined by pain localized to the chewing muscles. Anxiety and depression symptoms are commonly found in both chronic pain diseases.
Evidence indicates TMDM and FM are associated with changes in serotonin metabolism. Tryptophan (TRP) is an essential amino acid and serotonin’s precursor, whose main metabolic route is the kynurenine (KYN) pathway. Some studies have suggested that tryptophan degradation via this pathway is associated with various chronic pain and psychological conditions.
However, scientists have yet to establish if changes in the kynurenine pathway are present, and what their role is, in temporomandibular disorders and fibromyalgia.
Investigators at the Karolinska Institutet and Scandinavian Center for Oralfacial Neurosciences, both in Sweden, now sought to investigate the blood plasma concentrations of tryptophan and kynurenine in TMDM and FM.
The study’s participants were all female: 17 had temporomandibular disorders muscle pain (mean age of 26 years), 40 had fibromyalgia (mean age of 51 years), and 56 were healthy, pain-free, age-matched individuals (controls).
A total of 113 plasma samples were screened for tryptophan and kynurenine concentrations. Scientists also examined the subjects’ pain intensity and psychological symptoms.
In TMDM patients, lower levels of tryptophan were significantly associated with worst pain intensity. Conversely, tryptophan’s degradation was found to be positively correlated with both worst and average pain intensity.
Fibromyalgia patients had normal values in scores of depression and anxiety, but these were still higher compared with controls, along with a greater use of antidepressants and sleep medications. The women with fibromyalgia also showed a trend toward significantly lower tryptophan levels, in comparison with the control sample.
Importantly, and unexpectedly, tryptophan degradation was found be negatively correlated with anxiety symptoms. This suggests that, when tryptophan’s metabolism via the kynurenine pathway is stimulated — meaning, when it is overly active — the symptoms of anxiety are reduced, or vice-versa.
“As some participants used antidepressants, which may influence TRP [tryptophan] concentrations, we tested to exclude these from the analysis, but the results and significant differences or correlations remained similar,” the researchers said.
“The majority of FM patients in this study were also on different medications simultaneously and so, perhaps, the interactions between many different medications in these patients have an effect on TRP, KYN or other mechanisms underlying this correlation. Nevertheless, the reason for our finding is elusive and warrants further investigation,” they added.
Although the findings indicate the kynurenine/tryptophan molecular pathway may play a role in symptom severity in these conditions, further research is necessary to fully understand the pathway’s involvement in chronic muscle pain, the researchers said.