Insular excitatory-inhibitory balance and amplification of functional connectivity during sustained pressure pain is associated with hyperalgesia and temporal summation in chronic pain

Chronic pain is characterized by hyperalgesia and heightened temporal summation of pain, however, the brain mechanisms underlying these phenomena remain understudied. We sought to understand the relationship of hyperalgesia and temporal summation with the balance between basal excitatory and inhibitory neurotransmission (E-I balance) and brain functional connectivity during sustained pressure pain. Sixty-four females with Fibromyalgia (FM, age=43±11, M±SD) and 19 controls (age=42±16) were recruited for the study. Brain imaging included proton magnetic resonance spectroscopy (1H-MRS) to measure Glutamate+Glutamine (Glx) and gamma-Aminobutyric acid (GABA) in the right anterior insula (aINS), followed by individually calibrated 6-minute tonic pressure pain (PAIN) over the left leg (40/100 pain). Pain ratings were obtained for initial, middle, and final 2-minutes of PAIN. Temporal Summation Index (TSI) was calculated as ((Final/Initial Rating)/mmHg) x 100. aINS E-I balance was calculated as Glx/GABA ratio. Functional connectivity amplification of the primary somatosensory cortical representation of the leg (S1leg) was calculated as the contrast between the final 2 minutes of PAIN versus the initial 2 minutes of PAIN (final-initial). FM patients required lower pressure (p=0.0002) but reported higher TSI (p=0.0031) relative to controls. In FM, TSI was associated with clinical pain (r=0.47). During PAIN, FM showed amplified S1leg-aINS connectivity, and this amplification was associated with clinical pain (r=0.44), TSI (r=0.34), and cuff pressure (r=-0.37). aINS E-I balance was also associated with cuff pressure (r=-0.34) and S1leg-aINS connectivity (r=0.29) in FM. Mediation analysis showed that the effect of aINS E-I balance on clinical pain was serially mediated by cuff pressure and amplification of S1leg-aINS connectivity (indirect effect, β=0.053, BootSE=0.027, BootCI=[0.005, 0.114]). Mechanistically, in chronic pain, greater basal aINS excitability (E-I balance) necessitates lower pressure to achieve target experimental pain, resulting in greater amplification of S1leg and aINS communication during tonic experimental pain. This project was supported by R01AT007550 (NIH-NCCIH) awarded to R. E. Harris and V. Napadow and F99DK126121 (NIH-NIDDK) training grant awarded to I. Mawla.