Intracisternal, But Not Intrathecal, Injection of Naloxone Inhibits Cutaneous Itch-Related Response in Mice

The present study was conducted to determine whether cutaneous itch involves mu-opioid receptors in either of the spinal cord or lower brainstem or in both regions in mice. An intraplantar injection of serotonin hydrochloride (100 nmol/site) induced biting, an itch-related behavior. The behavior was inhibited by subcutaneous (0.3-1 mg/kg) and intracisternal (1-10 nmol/site), but not intrathecal (1-10 nmol/site), injections of naloxone hydrochloride. An intradermal injection of serotonin (100 nmol/site) to the rostral back induced scratching, an itch-related behavior, which was inhibited by subcutaneous (1 mg/kg) and intracisternal (10 nmol/site) injections of naloxone. These results suggest that mu-opioid receptor in the lower brainstem, but not spinal cord, is a site of central pruritogenic action of opioids and is involved in the facilitatory regulation of itch signaling.