Is conditioned pain modulation (CPM) affected by negative emotional state?

BACKGROUND: Conditioned pain modulation (CPM) is an experimental paradigm, which describes the inhibition of responses to a noxious or strong-innocuous stimulus, the test stimulus (TS), by the additional application of a second noxious or strong-innocuous stimulus, the conditioning stimulus (CS). As inadequate CPM efficiency has been assumed to be predisposing for clinical pain, the search for moderating factors explaining inter-individual variations in CPM is ongoing. Psychological factors have received credits in this context. However, research concerning associations between CPM and trait factors relating to negative emotions has yielded disappointing results. Yet, the influence of anxious or fearful states on CPM has not attracted much interest despite ample evidence that negative affective states enhance pain. Our study aimed at investigating the effect of fear induction by symbolic threat on CPM. METHODS: Thirty-seven healthy participants completed two experimental blocks: one presenting aversive pictures showing burn wounds (high-threat block) and one presenting neutral pictures (low-threat block). Both blocks contained a CPM paradigm with contact heat as TS and hot water as CS; subjective numerical ratings as well as contact-heat evoked potentials (CHEPs) were assessed. RESULTS: We detected an overall inhibitory CPM effect for CHEPs amplitudes but not for pain ratings. However, we found no evidence for a modulation of CPM by threat despite threat ratings indicating that our manipulation was successful. DISCUSSION: These results suggest that heat/thermal CPM is resistant to this specific type of symbolic threat induction and further research is necessary to examine whether it is resistant to fearful states in general. SIGNIFICANCE: The attempt of modulating heat conditioned pain modulation (CPM) by emotional threat (fear/anxiety state) failed. Thus, heat CPM inhibition again appeared resistant to emotional influences. Pain-related brain potentials proved to be more sensitive for CPM effects than subjective ratings.

Interactive effects of conditioned pain modulation and temporal summation of pain-the role of stimulus modality.

Conditioned pain modulation (CPM) and temporal summation of pain (TSP) are 2 experimental paradigms capturing endogenous pain modulation, which have repeatedly demonstrated clinical relevance. Conditioned pain modulation describes the inhibition of the pain response to a test stimulus (Ts) when a second noxious stimulus, the conditioning stimulus (CS), is concurrently applied. Temporal summation of pain describes the enhanced pain response to a series of stimuli compared with single stimuli. Temporal summation of pain-limiting effects of CPM are likely but may depend on the stimulus modality of the Ts. This study aimed at investigating these differential effects of stimulus modality. Thirty-five healthy volunteers completed 2 experimental blocks (Ts modality: pressure vs heat) in balanced order. Both blocks consisted of 3 conditions: baseline (no CS), CPM1 (nonpainful CS: 42°C water bath), and CPM2 (painful CS: 46°C water bath). Single stimuli and series of stimuli were alternatingly applied to assess TSP by means of a Numerical Rating Scale. Both TSP and CPM were successfully induced with no difference between the 2 Ts modalities. We also detected a significant interaction between TSP and CPM, with higher pain reduction for a series of Ts compared with single Ts during the painful CS. Interestingly, this interaction was modality-dependent: TSP for heat Ts was completely abolished by CPM, whereas this was not the case for pressure Ts. Our findings suggest different forms of central sensitization induced by TSP using either heat or pressure stimuli, which differ in their susceptibility to CPM. Clinical implications and directions for future research are discussed.