Dynorphin-(1-13) is a potent in vivo suppressor of vasopressin levels in the rat.

: Intracerebroventricular administration of dynorphin-(1-13) inhibits dose-dependently plasma vasopressin level in normal as well as in water-deprived rats, whereas systemic (subcutaneous) administration of this opioid peptide is ineffective in this respect. Simultaneous subcutaneous, but not intracerebroventricular, administration of naloxone prevents the suppressive effect of dynorphin-(1-13) on plasma vasopressin levels.