Table 1

Volume-of-interest analysis of endogenous opioid release during sustained masseter muscle pain. Volumes of interest (VOIs) were selected in the thalamus and the amygdala of each volunteer after alignment to the intercommisural line, but before anatomical normalization to stereotactic coordinates. The brain regions selected had been previously implicated in μ-opioid–mediated antinociception in animal models and were readily identifiable in the MR images. The thalamus was divided, on the basis of MRI landmarks, into anterior, posterior, dorsomedial, dorsolateral, ventromedial, and ventrolateral sections. Spheric volumes of identical size, 9 mm in diameter, were centered in the MR images in the anterior and ventrolateral divisions of the thalamus and in the amygdala, bilaterally, and then transferred to the coregistered μ-opioid receptor availability maps. Data are expressed as the means ± SD for each condition. Percent change refers to the percent average change (± SD) between B max/K dvalues obtained in the placebo and pain scans, within subjects.

RegionsPlacebo (Bmax/Kd)Pain (Bmax/Kd)Percent changet valueP value
Ipsilateral
Amygdala1.90 ± 0.431.77 ± 0.34–5.1 ±13.42.050.05*
Thalamus
Anterior2.13 ± 0.392.15 ± 0.371.5 ± 11.40.330.7
Ventrolateral1.00 ± 0.181.01 ± 0.192.2 ± 13.20.570.6
Contralateral
Amygdala1.80 ± 0.331.75 ± 0.37–2.1 ± 15.20.760.4
Thalamus
Anterior2.13 ± 0.472.02 ± 0.39–3.5 ± 12.31.530.1
Ventrolateral1.04 ± 0.270.96 ± 0.25–7.1 ± 10.52.930.009*
  • * Significant changes in μ-opioid receptor availability in vivo from placebo to sustained pain. Paired, two-tailed t-tests, df = 19, P < 0.05.