Differential modulation of alpha 2-adrenergic and mu-opioid spinal antinociception by neuropeptide FF.

Neuropeptide FF (NPFF) has been found to act as an antiopioid peptide. However, IT NPFF has recently been shown to potentiate the antinociceptive effects of IT morphine and to produce antinociception on its own. The aim of this study was to find out whether pretreatment with NPFF causes a comparable potentiation of dexmedetomidine-induced antinociception. NPFF (0.05-10.0 nmol) produced no antinociceptive effects in the rat tail flick test. NPFF potentiated the antinociceptive effect of IT morphine (7.8 nmol). This potentiation was prevented by IT naltrindole (28 nmol), which did not attenuate the antinociceptive effect of morphine. Dexmedetomidine (1.6-6.4 nmol) produced a dose-dependent antinociceptive effect, which was not potentiated by NPFF. Activation of the endogenous delta-opioid system due to the antiopioid effect of IT NPFF is proposed as an explanation to the reported differential action of NPFF on the mu-opioid and the alpha 2-adrenergic systems.