Age and supraspinal modulation of pain: emotional controls of nociception (ECON)

Are there sex differences in affective modulation of spinal nociception and pain?

Sex differences in the processing and experience of emotion exist. The present study examined whether sex differences in emotion lead to sex differences in affective modulation of pain and spinal nociception (assessed by nociceptive flexion reflex, NFR). Participants were healthy men (n = 47) and women (n = 73). Prior to affective modulation testing, electrocutaneous pain sensitivity was assessed (NFR threshold, pain threshold, pain tolerance). Affective modulation of pain and NFR was then assessed by presenting pictures that vary in emotional valence and arousal (mutilation, attack, death, neutral, families, adventure, erotica) during which suprathreshold electrocutaneous stimulations were delivered. Subjective emotional reactions were assessed after every picture, and nociceptive reactions were assessed after every suprathreshold stimulus. Results indicated women had greater pain sensitivity and also responded more negatively to attack pictures and less positively to erotic pictures. But despite these differences, affective modulation of pain/NFR was not moderated by sex: erotic pictures inhibited pain/NFR and mutilation pictures enhanced pain/NFR. Together, this implies subjective emotional experience does not completely mediate picture-evoked modulation of pain/NFR, a supposition that was further supported by exploratory analyses that demonstrated picture-evoked modulation of pain/NFR was present even after controlling for intra- and inter-individual differences in emotional reactions to pictures. Implications and limitations of these findings are discussed.

PERSPECTIVE

Evidence suggests that women are more sensitive to experimental and clinical pain, but the mechanisms contributing to these sex differences are poorly understood. Affective processes are known to play a role in regulating pain signaling and pain experience; therefore, the present study examined whether sex differences in affective experience contribute to sex differences in pain. Results indicate that in healthy individuals affective processes may not contribute to sex differences in pain.

Habituation, sensitization, and emotional valence modulation of pain responses

The Emotional Controls of Nociception (ECON) paradigm involves the presentation of emotionally-charged pictures during which painful stimuli are delivered. Across several ECON studies, unpleasant pictures enhanced pain and nociception, whereas pleasant pictures inhibited pain and nociception. However, at this time it is unknown whether emotional valence (unpleasant, neutral, pleasant) influences the habituation or sensitization of pain responses that occurs within a testing session. Indeed, ECON assumes that emotional valence modulation of pain is consistent throughout testing; otherwise the interpretation of valence modulation (unpleasant>neutral>pleasant) could be threatened. To address this issue, the present study (N=120) presented 108 pictures that varied in emotional valence. During and in between pictures, 52 suprathreshold electrocutaneous stimuli were delivered to evoke pain, the nociceptive flexion reflex [NFR], and pain-evoked skin conductance response [SCR]. Mixed effects ANOVAs verified that within-subject changes in pain responses were influenced by stimulus repetition (NFR and SCR habituated, pain ratings sensitized) and emotional valence (responses were highest during unpleasant pictures, intermediate during neutral pictures, and lowest during pleasant pictures). However, habituation/sensitization slopes were unaffected by emotional valence, thus indicating emotional valence modulation was consistently observed throughout the testing session. These results provide additional validation for the ECON paradigm and suggest that the circuit responsible for emotional modulation of pain and nociception is less susceptible to habituation or sensitization than the circuits responsible for responses to suprathreshold shocks.