Characterization of scratching responses in rats following centrally administered morphine or bombesin

The aim of this study was to characterize scratching behavior elicited by central administration of morphine or bombesin in rats, and to determine the role of opioid receptors in scratching induced by both pruritogenic agents. Central administration included intracisternal (i.c.), intrathecal (i.t.), and intracerebroventricular (i.c.v.) routes. Scratching events made with hind paws were counted by observers blinded to treatment conditions. Intracisternal morphine (0.01-0.1 μg) produced dose-dependent increases in scratching; the maximum response to i.c. morphine 0.1 μg was approximately 500 scratches within a 1-hour period. Neither i.t. nor i.c.v. morphine significantly increased scratching. Bombesin (0.01-0.32 μg) elicited robust scratching following i.c. administration. The maximum response to i.c. bombesin 0.32 μg was approximately 4000 scratches within a 1-hour period. Both i.t and i.c.v. bombesin produced profound scratching at similar doses. Antagonist studies confirmed that mu-opioid receptors selectively mediate i.c. morphine-induced scratching. However, selective mu-, kappa-, and delta-opioid antagonists did not attenuate i.c. bombesin-induced scratching. These results demonstrate that morphine and bombesin elicit scratching through different receptor mechanisms, at different central sites, and to different degrees.