We examined 17 healthy subjects using a novel CPM-protocol. We recorded PREP (including the assessment of the PREP-induced pain) over Cz referred to linked ear lobes, stimulating the dominant hand with the two-fold of the individual pain threshold (2PT) using three custom-built concentric surface electrodes. We than evaluated the CPM-effect in a cross-over design with immersion of the non-dominant hand into either 10 °C or 24 °C (control) cold water as CS. As TS we recorded PREP stimulating with pain intensity of 40–60 on the NRS 0–100 before, and repeatedly during and after CS application. Statistics: Pearson correlation, ANOVA, paired t-test, linear regression.
At 2PT the PREP-induced pain and PREP-amplitude did not correlate. With increasing stimulus intensity from 2.3 ± 0.3 mN to 6.7 ± 1.4 mN, the PREP-induced pain raised from 24.5 ± 3.8 to 52.1 ± 1.1 (p < 0.001), while the PREP-amplitude remained unchained. Only in the 10 °C-condition, both PREP-induced pain intensity (52.6 ± 4.4 vs. 30.3 ± 12.5) and PREP-amplitude (42.1 ± 13.4 μV vs. 28.7 ± 10.5 μV) decreased during and increased after CS termination (all p < 0.001). The changes of PREP-induced pain and PREP-amplitudes correlated only in the 10 °C-condition (r = 0.5; p = 0.042).
The presented novel CPM-protocol with hand immersion in painful cold water as conditioning stimulus can be used to modulate the PREP-induced pain intensity in healthy subjects and, more importantly, the PREP-recordings are able to depict quantitatively the CPM-effect electrophysiologically in an objective way.