Observation of scratching behavior

Scratching behavior was quantified as described previously²². In brief, on the testing day, the mice were individually placed in the observation cage (12×9×14 cm) to permit acclimation for 30 min. Scratching behavior was videotaped for 30 min after administration of chloroquine or for 60 minutes after the administration of delta antagonists. The total numbers of scratch bouts by the hind paws at various body sites during this period were counted by a person who was blinded to treatment.