Responses of superficial dorsal horn neurons to intradermal serotonin and other irritants: comparison with scratching behavior.

Scratching behavior is used to assess itch sensation in animals, but few studies have addressed the relative scratch-inducing capacity of different algesic and pruritic chemicals. Furthermore, central neural mechanisms underlying itch are not well understood. We used electrophysiological and behavioral methods to investigate the ability of several irritant chemicals to excite neurons in the superficial dorsal horn, as well as to elicit scratching, in rats. In anesthetized rats, single neurons in the superficial lumbar dorsal horn, identified by their responsiveness to intracutaneous (ic) histamine, were classified as wide dynamic range (WDR) or nociceptive-specific (NS). Serotonin (5-HT) given ic to the paw excited most (88%) WDR and NS neurons over a prolonged time course (often up to 40 min). 5-HT-evoked responses exhibited significant tachyphylaxis. Most neurons also gave shorter-duration responses to ic capsaicin (92%) and mustard oil (71%). In separate behavioral experiments, significant dose-related hind limb scratching directed at the ic injection site in the back of the neck was elicited by 5-HT over a time course similar to that of evoked neuronal firing. A second 5-HT injection made 40 min later at the same site elicited significantly less scratching. Formalin also elicited scratching that was not dose-related and less than that evoked by 5-HT. 5-HT and Formalin also evoked head or whole-body shakes that were significantly correlated with scratching. Neither histamine, capsaicin, nor vehicle controls elicited significant scratching or shaking. In rats, 5-HT appears to be more pruritogenic than histamine as assessed by scratching and shaking behavior, and excites superficial dorsal horn neurons over a behaviorally relevant time course. However, because most neurons additionally responded to pain-producing stimuli, they are not itch-specific. They might nonetheless contribute to neural pathways that distinguish between pain and itch based on some neural mechanism such as frequency coding.