Pharmacological effects of the dansylated neuropeptide FF analogues on body temperature and morphine analgesia

In our previous work, the two putative agonists (dansyl-GSRFamide and dansyl-PQRFamide) and the two putative antagonists (dansyl-GSRamide and dansyl-PQRamide) on neuropeptide FF (NPFF) receptors were synthesized to evaluate the importance of Phe8 of NPFF. In the present study, these putative NPFF agonists/antagonists containing different N-terminal sequences were further examined for their pharmacological profiles in thermoregulatory and nociceptive tests. The results indicated that the two dansylated agonists potently possessed similar thermoregulation (rank order of potencies: dansyl-GSRFamide ≫ NPFF > dansyl-PQRFamide) and different modulation of opioid-induced analgesia; in contrast, both of the two putative antagonists exhibited marked hypothermia (rank order of potencies: dansyl-PQRamide > dansyl-GSRamide) and facilitation of morphine analgesia (rank order of potencies: dansyl-PQRamide > dansyl-GSRamide). These data reveal that the difference of the N-terminal residues of the two putative agonists causes their dissociation of pharmacological pro- and anti-opioid effects. In addition, their N-terminal part is important to determine the potency of the dansylated agonists/antagonists. Our work might be helpful to develop a highly potent and fluorescent NPFF ligand.