Histamine-caused itch induces Fos-like immunoreactivity in dorsal horn neurons: effect of morphine pretreatment.

It is well known that cutaneous pain causes c-fos gene expression in the dorsal horn neurons. The present study examined whether or not an itching sensation had the same effect on these neurons. In order to produce an itching sensation, cotton balls soaked in histamine solutions of different concentrations were applied to the lower limbs of rats and then the treated area was scratched with the tip of a pair of forceps. After 2 h, the number of neurons showing Fos-like immunoreactivity was significantly increased in a dose-dependent manner in the ipsilateral dorsal horn when compared to saline-treated and scratched controls. When scratching with the forceps was omitted, the number of these neurons was reduced to one-third, but was still significantly higher than in controls treated only with saline. Expression of Fos-like immunoreactivity in these animals was markedly reduced by morphine pretreatment (10 mg/kg, i.p.), suggesting that the transmission of both itch and pain is blocked by morphine.