The effect of (D-Ala2)-Met-enkephalin on the contraction of nictitating membrane in cats.

The effects of (D-Ala2)-Met-enkephalin (DAME) on the nictitating membrane contraction induced by either cervical sympathetic nerve stimulation or i.v. administration of norepinephrine were assessed in anesthetized cats. The contraction induced by sympathetic nerve stimulation (0.5--16 Hz) was greatly reduced by pretreatment with DAME (300 micrograms/kg, i.v.). The reduction due to DAME was readily reversed by administration of naloxone (1 mg/kg, i.v.). The contraction, enhanced after administration of cocaine, was also suppressed by DAME. On the other hand, the contraction induced by i.v. application of norepinephrine was unaffected by DAME pretreatment. The results indicate that DAME exhibit a morphine-like effect of the contraction of the nictitating membrane.