We observed that PC-PLC activity was positively correlated with the number of comorbid conditions in migraineurs but not in controls, supporting our hypothesis that PC-PLC activity is part of a common pathway in the comorbid conditions of migraineurs. This first description of a shared involvement of CSF PC-PLC in these comorbid conditions is significant when it is considered that many helpful medications modulate the biochemical pathways that also share involvement in these conditions, such as serotonin  or noradrenaline [17, 18]. This inference raises the possibility that modulators of PC-PLC may help lessen migraine, as well as other comorbid conditions.
We also observed greater PC-PLC activity in CSF in females in both migraineurs and controls and, among controls only, in younger participants. Given the higher PC-PLC activity in migraineurs compared to controls, the sex difference we observed is consistent with the higher incidence of migraine in the female population.
In considering how PLC activity may relate to migraine and its comorbidities, many of these distinct conditions share a multifaceted pathophysiology, with alterations in molecules as diverse as serotonin, adrenaline, estrogen, cannabinoids, and glutamate, and include the activation of several GPCRs, protein kinase mediated signaling, and early gene activation (c-fos) [36, 37]. PC-PLC plays a role in this process by generating the protein kinase activator, diacylglycerol, and affecting Ca2+ influx into cells [38, 39]. We hypothesize that varying PC-PLC levels, as we observed in our study, reflect fluctuations from their modulation of many G protein coupled receptors, and that any changes of PC-PLC activity in the brain are reflected in CSF. Once altered, PC-PLC can have diverse effects. For example, signaling pathways induced by PC-PLC can form inflammatory or pain molecules; also, PC-PLC can hydrolyze PC, a major component of membrane lipids, altering membrane properties and the functions of ion channels imbedded in the lipid bilayer . Our data suggest that the diverse pain, inflammatory, and behavioral features common to migraine and its comorbidities may arise at least partially from altered PC-PLC activity. Many different CSF PLC isoforms are yet to be defined, but their roles in vascular endothelial dysfunction and inflammation can be explored for therapy . Thus, future studies of isoform-specific PLC modulation may lead to treatments for migraine and its comorbid conditions.
The invasiveness of collecting lumbar CSF in a prospective study led to a relatively small study population (n = 35) and interpreting results from a multivariable analysis can lead to not only bias but to under- or over-estimated variance; this, combined with the clinical differences we observed between controls and migraineurs (such as differences in medication use), necessitates cautious interpretation of our findings. For example, the small numbers for each individual comorbidity (Table 2) means we were unable to analyze the CSF PC-PLC activity levels for any individual comorbidity or to fully explore possible confounders and modifiers of the association between PC-PLC activity and comorbidities. We used physician-diagnosed comorbidities which probably underrepresent some diagnoses; however, this should have occurred randomly in both migraineurs and controls and without regard for PC-PLC activity levels, likely leading to attenuated estimates of the relationships between migraine, comorbidities, and PC-PLC activity. Furthermore, using physician-diagnosed comorbidities minimized the potential bias associated with unblinded participant interviews to ascertain medical histories.