Affinity of neuropeptide FF analogs to opioid receptors in the rat spinal cord.

Several high-affinity analogs of neuropeptide FF (Phe-Leu-Phe-Gln-Pro-Gln-Arg-Phe-NH2, NPFF) exhibiting both supraspinal anti-opioid and spinal analgesic activities were studied for their abilities to interact with specific mu, delta, and kappa opioid binding in the rat spinal cord. Measurements by quantitative receptor autoradiography in the superficial layers of the spinal cord revealed that NPFF analogs tested have only a low affinity for opioid receptors since Ki values ranged from 5 to 400 microM. Taking into account the high efficacy of NPFF after intrathecal injection, these results indicate that analgesic effects of NPFF did not result from opioid receptor stimulation.