Intracisternal Injection of Opioids Induces Itch-Associated Response through μ-Opioid Receptors in Mice

Abstract

We examined whether opioids, especially morphine, would centrally elicit scratching in mice and determined some characteristics of the scratch-inducing action of opioids. When intracisternally (i.e.) injected, morphine (0.1–3 nmol/mouse) produced a dose-dependent increase in scratching of the face, but not of the ears, head and body trunk. When injected intradermally into the rostral part of the back, morphine (at most potent i.e. dose of 3 nmol/mouse or higher) did not increase the scratching of the injected site. Facial scratching of the mouse induced by i.e. injection of morphine (0.3 nmol/mouse) was almost abolished by distraction and by naloxone (1 mg/kg, s.c). [D-Ala2, 7V-Me-Phe4, Gly5-ol]Enkephalin (DAMGO) (0.03-2 nmol), but not [D-Pen2,5]enkephalin (DPDPE) and U-50,488, dose-dependently elicited facial scratching by i.e. injection. These results suggest that morphine and DAMGO increased facial scratching, probably mediated by central opioid μ-receptors in mice, and such scratching was due to a sensation, probably itching. The present animal model may be useful for analyzing opioid-mediated central itching.