Metabolite activity in the anterior cingulate cortex during a painful stimulus using functional MRS

Abstract

To understand neurochemical brain responses to pain, proton magnetic resonance spectroscopy (¹H-MRS) is used in humans in vivo to examine various metabolites. Recent MRS investigations have adopted a functional approach, where acquisitions of MRS are performed over time to track task-related changes. Previous studies suggest glutamate is of primary interest, as it may play a role during cortical processing of noxious stimuli. The objective of this study was to examine the metabolic effect (i.e., glutamate) in the anterior cingulate cortex during noxious stimulation using fMRS. The analysis addressed changes in glutamate and glutamate + glutamine (Glx) associated with the onset of pain, and the degree by which fluctuations in metabolites corresponded with continuous pain outcomes. Results suggest healthy participants undergoing tonic noxious stimulation demonstrated increased concentrations of glutamate and Glx at the onset of pain. Subsequent reports of pain were not accompanied by corresponding changes in glutamate of Glx concentrations. An exploratory analysis on sex revealed large effect size changes in glutamate at pain onset in female participants, compared with medium-sized effects in male participants. We propose a role for glutamate in the ACC related to the detection of a noxious stimulus.

Figure 1

Glutamate levels in the anterior cingulate cortex (ACC). (A) Location of MRS voxel in the ACC. (B) Example spectrum with raw data (black), LCModel fit (blue), and the contribution of glutamate determined by LCModel (shown in red).

Figure 2

MR data acquisition and pain intervention. The functional scan was uninterrupted for 22.4 min. Participants provided pain ratings via an MRI compatible clicker every 2 min. T₂-weighted images were acquired before and after the functional MRS scan series to confirm stability of voxel placement by absence of subject motion.

Figure 3

In-scanner pain model. (1) Capsaicin (0.075) was applied on the skin. (2) An inactive thermo-pad was placed on the skin covering the area where capsaicin was applied. 3) The thermo-pad was water-activated (~ 41 °C).

Figure 4

Overview of individual (coloured lines) and mean (black line) baseline-subtracted spectra of included data (n = 15). The functional MRS scan series had 3 different condition (8-min pre-heat, 4-min heat, 8-min post-heat). A consistent high spectral quality was evidenced by high signal to noise ratio (SNR), and narrow line width (LW).

Figure 5

Pain intensity ratings (NRS) and glutamate and Glx concentrations. Values during the functional scan (n = 15).

Figure 6

NAA, tCr, Glx and Glu ACC concentrations during rest (no pain perception) and pain onset for each volunteer (n = 15). Exploratory sex-based analysis showed a large effect for change in glutamate at pain onset in females (Cohen’s d = 0.88, CI − 0.43 to 2.21, n = 7), compared with a medium sized effect for males (Cohen’s d = 0.60 CI − 0.57 to 1.73, n = 8). Changes in Glx at pain onset were medium sized for both men (Cohen’s d = 0.65 CI − 0.53 to 1.83) and women (Cohen’s d = 0.58 CI − 0.69 to 1.87). There was no difference in pain rating at onset between men and women (t = − 0.24, p-value = 0.80 CI − 1.2 to 0.9).

Figure 7

Linear mixed effects model during the 20 min fMRS scan. There was no relationship within subjects over the entire duration of data acquisition between pain rating and glutamate or Glx.