Rat NPFF₁ receptor-mediated signaling: functional comparison of neuropeptide FF (NPFF), FMRFamide and PFR(Tic)amide

Neuropeptide FF (NPFF) participates in many physiological functions associated with opioids in the mammalian CNS. We established a pheochromocytoma PC-12 cell line clone stably expressing rat NPFF₁ receptors. Both NPFF and FMRFamide activated NPFF₁ receptors to couple with Gi/o protein and inhibited adenylyl cyclase activity in PC-12/rNPFF₁ cells, but there were no effects on MAPKs (ERK1/2 and p38 MAPK) or PI3K/Akt pathway. FMRFamide also inhibited DARPP-32/Thr34 phosphorylation in the presence of forskolin. Similarly, PFR(Tic)amide, a 'super-agonist' of NPFF receptors, inhibited the production of cAMP and slightly decreased DARPP-32/Thr34 phosphorylation in PC-12/rNPFF₁ cells. Intracerebroventricular injections of PFR(Tic)amide blocked behavioral sensitization of locomotor activity to amphetamine, and intrathecal injection of PFR(Tic)amide caused a dose-dependent antinociception in vivo in rats. Thus, "over-activation" of NPFF receptors by PFR(Tic)amide induced different bio-effects from those induced by NPFF in vivo.