Fig 1.
CONSORT flow-algorithm for human subjects.
Four subjects were allocated to a feasibility study. One subject was excluded from this part due to an unsuspected finding of an abnormal ultrasound cardiography. Fifteen subjects were included and randomized. Three subjects were excluded for reasons not related to the study. Twelve subjects completed the study and data were examined in an interim analysis by an independent statistician.
Table 1.
Inclusion and exclusion criteria.
Fig 2.
Study timeline in the human MHI-model.
Day 1/Day 3 include induction of mild heat injury (MHI) with baseline assessments (green rectangle), heat injury (red) and post-injury assessments (blue). Day 2/Day 4 include a pre-drug assessment (blue; post-injury 165 hrs), drug-infusions (naloxone or placebo; grey) and post-infusion assessments (magenta). Stars indicate assessments of mechanical pain thresholds (red star), thermal thresholds (yellow) and secondary hyperalgesia areas (green).
Table 2.
BMI = body mass index.
Fig 3.
Hyperalgesia was assessed by a decrease in paw withdrawal threshold (PWT50%) in a model of mild heat injury (MHI).
Line graphs illustrate response to von Frey (vF) stimulation at baseline (BL) and at 1, 2, 3, 72, and 168 hrs after MHI. Naloxone reinstated primary hyperalgesia (A-B) at a dose of 0.3 mg/kg (60 min, P = 0.0015), 3 mg/kg (30 min, P = 0.0011; 60 min, P = 0.0002), and 10 mg/kg (30 min, P < 0.0001; 60 min, P < 0.0001; 120 min, P < 0.0001) and secondary hyperalgesia (C-D) at a dose of 0.3 mg/kg (60 min, P = 0.0148), 3 mg/kg (30 min, P = 0.0004; 60 min, P = 0.0002), and 10 mg/kg (30 min, P < 0.0001; 60 min, P < 0.0001; 120 min, P = 0.0013). A,C: Line drawings. B,D: Histograms of the post-injection time points (plotted as the mean of the 30–120 min time points). Values represent mean ± 95% CI. * P < 0.05.
Fig 4.
Heat pain thresholds and secondary hyperalgesia areas before and after a mild heat injury.
Heat pain thresholds (white circles; left y-axis) and secondary hyperalgesia areas (red circles; right y-axis) before and, 60, 120 and 180 min, after a mild heat injury (MHI). Values represent mean of Day 1 and Day 3 values (mean ± 95% CI). * P < 0.01; ** P < 0.005; **** P < 0.0005.
Fig 5.
Individual trajectories of secondary hyperalgesia areas during the early and late phase after a mild heat injury.
Individual trajectories of secondary hyperalgesia areas (n = 12) during the early phase (0–3 hrs) and the late phase (165–169 hrs) after induction of the mild heat injury (MHI). Data are from naloxone (A) and placebo (B) sessions. The trajectories of the four naloxone responders, indicating presence of endogenous opioid masked sensitization, are delineated by red circles and the trajectories from the eight non-responders by blue filled circles. MHI and drug administration are indicated by vertical arrows. One (#11) of the responders demonstrated residual SHA at 165 hrs after the induction of MHI (indicated by oblique arrow; B), but not during placebo. Secondary hyperalgesia areas developed in 4/12 subjects (#2,6,7,11) after naloxone (A) and in 0/12 after placebo (B).