Mechanical, thermal and formalin-induced nociception is differentially altered in 5-HT1A-/-, 5-HT1B-/-, 5-HT2A-/-, 5-HT3A-/- and 5-HTT-/- knock-out male mice

Extensive studies in rodents suggest that serotonin (5-HT) modulates nociceptive responses through the stimulation of several receptor types. However, it remains to demonstrate that these receptors participate in the control of nociception under physiological conditions. Pain behaviors of mutants which do not express 5-HT_1A, 5-HT_1B, 5-HT_2A or 5-HT_3A receptors, or lacking the 5-HT transporter, compared to paired wild-type mice of the same genetic background, were examined using validated tests based on different sensory modalities. Mechanical (von Frey filaments, tail pressure, tail clip tests), thermal (radiant heat, 46 °C water bath, hot-plate test) and formalin-induced nociception were determined in 2- to 3-month-old males. 5-HT_1A knock-out mice differed from wild-types by higher thermal sensitivity (hot-plate test only), and 5-HT_1B knock-out mice by higher thermal and formalin sensitivity. Both 5-HT_2A and 5-HT_3A knock-out mice differed from wild-types by a dramatic decrease in the formalin-induced nociceptive responses for phase II (16–45 min after injection/inflammatory phase). In contrast, neither mechanical, thermal nor formalin-induced nociception differed between mutants lacking the 5-HT transporter and paired wild-type mice. Although differences in spontaneous locomotor activity in 5-HT_1B?/? (increase) and 5-HT_3A?/? (decrease) knock-out mice versus paired wild-types might have confounded differences in nociception, acute 5-HT receptor blockade by selective antagonists was found to replicate in wild-type mice the effects on pain behavior, but not on locomotor activity, of the respective gene knock-out in mutants. These results support the conclusion that the complex control of pain mechanisms by 5-HT, acting at multiple receptors, is physiologically relevant in mice.