Abstract
Hind-paw scratching elicited by an intradermal injection of serotonin (5-HT) into the nape of the neck is used increasingly as a rodent itch model. In the present study, we examined if the itch model was useful tool for studying itch sensation in humans, especially for its negative affective aspects. Using adult male Sprague-Dawley rats, we first confirmed that intradermal 5-HT injection evokes hind-paw scratching as a measure of sensory aspects of itch sensation. Second, we evaluated negative affective aspects by combining 5-HT treatment with the conditioned place aversion paradigm. In both experiments, the rats were injected with either 5-HT or saline (vehicle) into the nape of the neck, and also another set of rats were injected with 5-HT into an area of the back where the rats cannot reach with the hind-paws to remove the itch by scratching. 5-HT injection into either the nape of the neck or the back significantly increased scratching compared to vehicle injection although scratching elicited by 5-HT into the back was significantly less when compared to that into the nape of the neck. In the place-conditioning, 5-HT injections, irrespective of the injection site, produced a significant place aversion compared to vehicle injection, suggesting that itch produces an aversive sensation whether or not scratching temporarily relieves it. These data provide supporting evidence that the rodent itch model is a reliable method to study itch sensation in humans.
