Behavioral differentiation between itch and pain in mouse

Abstract

The standard rodent model of itch uses scratching with the hind limb as a behavioral response to pruritic stimuli applied to the nape of the neck. The assumption is that scratching is an indicator of the sensation of itch. But because only one type of site-directed behavior is available, one cannot be certain that scratching is not a response to nociceptive or other qualities of sensations in addition to, or instead of, itch. To extend the model, we administered chemical stimuli to the cheek of the mouse and counted scratching with the hind limb as an indicator of itch and wiping with the forelimb as an indicator of pain. An intradermal injection of histamine and capsaicin, known to evoke predominantly itch and pain, respectively, in humans, each elicited hind limb scratching behavior when injected into the nape of the neck of the mouse. In contrast, when the same chemicals were injected into the cheek of the mouse, there were two site-directed behaviors: histamine again elicited scratching with the hind limb, but capsaicin evoked wiping with the forelimb. We conclude that the "cheek model of itch" in the mouse provides a behavioral differentiation of chemicals that elicit predominantly itch in humans from those that evoke nociceptive sensations. That is, the model provides a behavioral differentiation between itch and pain in the mouse.

Fig. 1

Line drawings of a mouse showing (a) wiping of the cheek with the forelimb, (b) grooming with forelimbs, and (c) scratching of the cheek with the hind limb. Although grooming involved wipes over the cheek, this behavior was not counted as a response to chemical application to the cheek. Cross-hatched area is the area of the cheek that was shaved prior to experiments.

Fig. 2

Scratching directed toward the site of injection of capsaicin and histamine into the nape of the neck. (a) The time course of scratching before and after the injection (at time = 0). The mean number of bouts of scratching/min after an injection of 10 μg of histamine (n = 8, solid circles) or capsaicin (n = 12, open squares). (b) Mean total number of bouts of scratching during the 20 min period after injection of vehicle and different doses of histamine (n = 8 for all groups, mean ± SEM). Each dose of histamine evoked significantly more bouts than the vehicle alone (^*p < 0.05; ^**p < 0.01). There were no significant differences in bouts among doses. (c) Mean total number of bouts of scratching/20 min evoked by vehicle and two doses of capsaicin. The only significant difference in number of bouts was between the largest dose of capsaicin and the vehicle (p < 0.05; n = 12 for all groups, mean ± SEM).

Fig. 3

The time course of scratching and wiping directed toward the site of injection of capsaicin and histamine into the cheek. Wiping and scratching were simultaneously recorded in responses to each stimulus. All injections occurred at time = 0. (a) and (b) The mean number of bouts of scratching/min (a) and mean wipes/min (b) after an injection of 50 μg of histamine (n = 8). (c) and (d) The mean number of bouts of scratching/min (c) and mean wipes/min (d) after an injection of 40 μg of capsaicin (n = 8).

Fig. 4

Cumulative scratching and wiping directed toward the site of injection of capsaicin or histamine into the cheek. Scratching with the hind limb (left column) and wiping with the forelimb (right column) were simultaneously recorded in response to each stimulus. (a) and (b) The mean number of bouts of scratching (a) and mean number of wipes (b) occurring within 20 min after an injection of vehicle (saline) or different doses of histamine. Significant differences between means obtained for each dose and vehicle are indicated with asterisks (^*p < 0.05; ^**p < 0.01). (c) and (d) Mean number of bouts of scratching (c) and wipes (d) occurring during the 20 min after an injection of vehicle of tween-80 saline and different doses of capsaicin in vehicle.

Fig. 5

Scatter gram that summarizes the cumulative scratching and wiping evoked in each mouse by each dose of histamine and capsaicin. These data were used to obtain the means in Fig. 3. For each dose of histamine (solid symbols) and capsaicin (open symbols), the total number of bouts of scratching/20 min evoked in each mouse are plotted against the total number of wipes/20 min. The different shaped symbols represent different doses. Solid circle = 10, solid square = 20, and solid triangle = 50 μg histamine; open circle = 1, open square = 10, and open triangle = 40 μg capsaicin.