Opioid modulation of scratching and spinal c-fos expression evoked by intradermal serotonin.
We investigated a spinal site for opioid modulation of itch-related scratching behavior in rats. Intradermal 5-HT (2%, 10 microl) elicited intermittent bouts of hindlimb scratching directed toward the injection site (nape of neck) beginning within minutes and lasting >1 hr. 5-HT-evoked scratching was significantly reduced by systemic administration of the opiate antagonist naltrexone but was not affected by systemic morphine at a dosage (3 mg/kg) that induces analgesia. Intradermal 5-HT elicited a significant increase in c-fos-like immunoreactivity (FLI) in superficial laminas I-III at the lateral aspect of the cervical C3-C6 dorsal horn compared with controls receiving intradermal saline. Neither systemic morphine nor naltrexone significantly affected counts of 5-HT-evoked FLI. The lack of effect of morphine suggests that intradermal 5-HT activates dorsal horn neurons, signaling itch but not pain. Attenuation of 5-HT-evoked scratching but not spinal FLI by naltrexone suggests a supraspinal site for its antipruritic action. In contrast, morphine significantly attenuated FLI elicited by intradermal capsaicin, a chemical that induces pain but not scratching.
Nojima
Hiroshi
H
Department of Applied Pharmacology, Faculty of Pharmaceutical Sciences, Toyama Medical and Pharmaceutical University, Toyama 930-0194, Japan.
Simons
Christopher T
CT
Cuellar
Jason M
JM
Carstens
Mirela Iodi
MI
Moore
Justin A
JA
Carstens
Earl
E
D000701:Analgesics, Opioid / Q000494:pharmacology*; D000818:Animals; D001522:Behavior, Animal / Q000187:drug effects / Q000502:physiology*; D006614:Hindlimb / Q000502:physiology; D007271:Injections, Intradermal; D008297:Male; D008845:Microinjections; D009020:Morphine / Q000494:pharmacology; D009271:Naltrexone / Q000494:pharmacology; D009292:Narcotic Antagonists / Q000494:pharmacology; D009333:Neck; D020671:Posterior Horn Cells / Q000187:drug effects / Q000378:metabolism; D016760:Proto-Oncogene Proteins c-fos / Q000378:metabolism*; D011537:Pruritus / Q000139:chemically induced / Q000503:physiopathology; D051381:Rats; D017207:Rats, Sprague-Dawley; D012701:Serotonin / Q000008:administration & dosage*; D013116:Spinal Cord / Q000187:drug effects / Q000378:metabolism*
D000701:Analgesics, Opioid; D009292:Narcotic Antagonists; D016760:Proto-Oncogene Proteins c-fos; D012701:Serotonin; D009271:Naltrexone; D009020:Morphine