Inhibitory influences of FMRFamide and PLG on stress-induced opioid analgesia and activity

The effects of i.c.v. administrations of the peptide FMRFamide (Phe-Met-Arg-Phe-NH₂), as well as i.p. injections of PLG (Pro-Leu-Gly-NH₂) and the opiate antagonist, naloxone, on immobilization-induced analgesia and locomotor activity were examined in CF-1 and C57BL strains of mice. Both naloxone (1.0 mg/kg) and FMRFamide (0.10–1.0 μg) blocked the experimentally induced analgesia and activity, whereas PLG (0.10–10 mg/kg) suppressed only analgesia. These results indicate that FMRFamide (or FMRFamide-like neuropeptides) and PLG may function as differential antagonists of the behavioral and physiological consequences of endogenous opioid activation.