Perception of Thermal Pain and the Thermal Grill Illusion Is Associated with Polymorphisms in the Serotonin Transporter Gene

AIM: The main aim of this study was to assess if the perception of thermal pain thresholds is associated with genetically inferred levels of expression of the 5-HT transporter (5-HTT). Additionally, the perception of the so-called thermal grill illusion (TGI) was assessed. Forty-four healthy individuals (27 females, 17 males) were selected a-priori based on their 5-HTTLPR/rs25531 ('tri-allelic 5-HTTLPR') genotype, with inferred high or low 5-HTT expression. Thresholds for heat- and cold-pain were determined along with the sensory and affective dimensions of the TGI. RESULTS: Thresholds to heat- and cold-pain correlated strongly (rho  = -0.58, p<0.001). Individuals in the low 5-HTT-expressing group were significantly less sensitive to heat-pain (p = 0.02) and cold-pain (p = 0.03), compared to the high-expressing group. A significant gender-by-genotype interaction also emerged for cold-pain perception (p = 0.02); low 5-HTT-expressing females were less sensitive. The TGI was rated as significantly more unpleasant (affective-motivational dimension) than painful (sensory-discriminatory dimension), (p<0.001). Females in the low 5-HTT expressing group rated the TGI as significantly less unpleasant than high 5-HTT expressing females (p<0.05), with no such differences among men. CONCLUSION/SIGNIFICANCE: We demonstrate an association between inferred low 5-HTT expression and elevated thresholds to thermal pain in healthy non-depressed individuals. Despite the fact that reduced 5-HTT expression is a risk factor for chronic pain we found it to be related to hypoalgesia for threshold thermal pain. Low 5-HTT expression is, however, also a risk factor for depression where thermal insensitivity is often seen. Our results may thus contribute to a better understanding of the molecular underpinnings of such paradoxical hypoalgesia. The results point to a differential regulation of thermoafferent-information along the neuraxis on the basis of 5-HTT expression and gender. The TGI, suggested to rely on the central integration of thermoafferent-information, may prove a valuable tool in probing the affective-motivational dimension of these putative mechanisms

Etude des modifications temporelles des otoémissions acoustiques provoquées lors d'une stimulation acoustique controlatérale, chez l'humain normo-entendant

In 32 subjects, the medial olivo-cochlear efferent system was activated by a contralateral acoustic stimulation (CAS) which is able to mimic the inhibitory effects of electrical stimulation of the crossed olivo-cochlear bundle. A white noise CAS of 35dB SL induced temporal alterations in otoacoustic emissions evoked by clicks of 63dB SPL, called phase shift effect (PSE). The PSE appeared as an advance of signals in 80% of cases and occurred after a latency of about 8ms. Two different signal processing methods (in time and frequency domains), showed that the phase shift was mainly located on lower frequencies with a maximum around 1.5kHz and a lack around 4kHz. When contralateral stimulation intensity was varying from 25 to 45 dB SPL, the PSE, increased linearly ; the shape of the curve allowed to rule out a middle ear reflex. On the contrary, when ipsilateral stimulation intensity was varying from 57 to 69 dB SPL, the PSE decreased linearly, indicating that PSE originated in inner ear mechanisms. Lastly, a significant correlation had been evidenced between the amplitude of the suppressive (inhibitory) effect of CAS, and the amplitude of the phase shift effect, allowing one to suggest that both effects may have a common physiological origin