Smell and taste dissociations in the modulation of tonic pain perception induced by a capsaicin cream application

Abstract

Background: Pain is a subjective experience characterized by sensory (intensity) and emotional (unpleasantness) aspects. Although literature reports behavioural effects on pain due to smell and taste influence, to our knowledge the relationship between tonic pain induced by a capsaicin cream and these chemosensory systems has never been explored before. The aim of this study was to investigate the modulation of olfactory and gustatory substances having different valence on tonic pain perception mediated by a capsaicin cream application. Methods: Sixty healthy volunteers were included in two separated experiments (N = 30 smell; N = 30 taste) and underwent different valence smell and taste stimulations, while receiving painful stimuli. Perception of pain intensity (the sensory component) and unpleasantness (the affective component) was measured with a numerical rating scale, both during the two aforementioned experiments. Results: Pain unpleasantness rating showed differences only in the smell experiment between the two odourous conditions. In particular, pleasant odour induced lower ratings of pain unpleasantness, while no significant results were found for intensity. Regarding taste, we could not observe significant effects nor for pain unpleasantness or intensity. Conclusions: These findings highlight the potential role of pleasant odours in influencing the affective aspects of pain perception induced by this kind of tonic pain. Such evidence might provide new insight for using chemosensory substances as analgesics for modulating the cognitive aspects of neuropathic pain. Significance: This work shows the effect of smell on the emotional component of tonic pain, experimentally induced by capsaicin cream application. Previous literature investigated tonic pain in interaction with smell and/or taste stimuli, but mainly with physical methods such as temperature. Our findings add new information in this field, contributing to a deeper insight on the role of olfaction on this particular kind of tonic pain perception. This approach could open to new investigations aimed to consider odours for pain management.

Pain, Smell, and Taste in Adults: A Narrative Review of Multisensory Perception and Interaction

“…There is an ample body of literature available on the administration of smell and taste substances (e.g., functional magnetic resonance imaging [fMRI], olfactory/gustatory event-related potentials). One of the most commonly used devices to deliver odors is the olfactometer: subjects inhale odors through a nasal canula inserted in the nose [33,37,39,41,42,46,48]. When substances are delivered via a diffuser or humidifier in the testing room [34,36,40], participants are not always aware of the stimulation.…”

“…Finally, a pen-like odor-dispensing device can be used, which works with a tampon imbibed with a liquid odorant or an odorant substance dissolved in a solution [47]. In gustatory stimulation, subjects are asked to ingest or to retain either something solid [49,55,58,61] or a liquid substance in their mouth [50-54, 56, 59, 60] after drinking from a cup or exposure to a liquid spray [46,48].…”

“…This can be done in a pilot study phase to select a preferred/disliked substance for each participant (for example [32,33]) or to choose the substances or the optimal concentration in experiments later involving a different pool of participants [36,38,40]. Alternatively, evaluation may take place after a substance is administered to determine whether it was perceived as intended [43,46,48].…”

“…Researchers have often been found to fail to check study participants' perceptual status of smell and taste. Few studies have reported that subjects had been screened with either validated tests [43,[46][47][48]57] or an experimental paradigm devised to detect gross dysfunction in their perception of the stimuli [33,39,42].…”

“…Reporting of qualitative pain measures of pain intensity has sometimes been inconsistent. For example, pain intensity was reportedly not modified by odors in studies in which pain was induced by hot or cold thermal stimulation [33,38,39] or with capsaicin cream applied to the back of the right hand at the level of the first dorsal interosseous muscle [48] or with phasic nicotine stimuli to induce burning and stinging pain [41]. One study applied three different modes of stimulation (thermode, pressure algometer, ischemic) and reported reduced pain intensity associated with essential oil of lavender only in men [34].…”