Abstract: Objective To investigate the effect of morphine on scratching response evoked by intracutaneous injection of serotonin (5-HT) and formalin in rats,in order to establish an animal model for peripherally elicited pruritus.Methods Forty male Sprague-Dawley rats were randomly divided into 5 equal groups,injected intracutaneously in the nape of the neck with 10μL of 0.9% sterile saline,0.5% 5-HT,1% 5-HT,2% 5-HT,and 5% formalin,respectively.Then,the rats were observed for 1 h using a video camera.A bout of scratching was defined as one time of the hind paws' scratching around the injection site.Five days later,the experiment in all groups was repeated with morphine pretreatment.Results The total numbers of scratching bouts per hour in 1%5-HT and 2%5-HT groups were significantly higher than that in saline group (both P<0.01).Although no significant difference was found in scratching behavior between the formalin group and the saline group (P>0.05),the scratching bouts were significantly less in formalin group than in the 2% 5-HT group (P<0.05).The morphine pretreatment significantly decreased the scratching numbers in formalin groups but had no significant effect on the 5-HT groups (P<0.01,P>0.05,respectively).Conclusions The scratching behavior induced by 5-HT and formalin in rats is associated with itch and pain,respectively.A rat model for pruritus can be established by intradermal injection with 10μL of 2% 5-HT at the nape of the neck.

Key words: Rats, Serotonin, Formaldehyde, Pruritus, Models, animal