LOW-AFFINITY OF FMRFAMIDE AND 4 FARPS (FMRFAMIDE-RELATED PEPTIDES), INCLUDING THE MAMMALIAN-DERIVED FARPS F-8-FAMIDE (NPFF) AND A-18-FAMIDE, FOR OPIOID-MU, OPIOID-DELTA, OPIOID-KAPPA(1), OPIOID-KAPPA(2A), OR OPIOID-KAPPA(2B) RECEPTORS

The binding affinities at opioid receptor subtypes in rat or guinea pig brain membranes were determined for the neuropeptide FMRFamide (Phe-Met-Arg-Phe-NH2), the two mammalian-derived FMRFamide-related peptides F-8-Famide (NPFF; Phe-Leu-Phe-Gln-Pro-Gln-Arg-Phe-NH2) and A-18-Famide (Ala-Gly-Glu-Gly-Leu-Ser-Ser-Pro-Phe-Trp-Ser-Leu-Ala-Ala-Pro-Gln-Arg-Phe-NH2), and the two other FMRFamide-related peptides Tyr-Phe-Met-Arg-Phe-NH2 (Tyr-FMRFamide) and Pro-Gln-Arg-Phe-NH2, (Pro-Gln-RFamide). The mu and delta sites were labeled in rat brain membranes using tritiated [D-Ala(2),N-MePhe(4),Gly-ol(5)]enkephalin ([H-3]DAMGO) and [D-Ala(2),D-Leu(5)]enkephalin ([H-3]DADLE), respectively. The kappa sites were labeled in guinea pig brain using [H-3]U-69,593 after treatment with BIT and FIT for kappa(1) and [H-3]bremazocine after pretreatment with BIT and FIT for kappa(2). The kappa(2a) binding sites were assayed using [Leu(5)]enkephalin to block kappa(2b) sites and the kappa(2b) sites were assayed using (-)-(1S,2S)-U50,488 to block kappa(2a) sites. Neither FMRFamide nor any of the FMRFamide-related peptides (up to 61.0 mu M) displayed significant affinity at any of the subtypes of opioid receptor. Hence, the known ability of FMRFamide and FaRPs to interact with the opioid system does not appear to be related to direct binding to these opioid receptors.