Peptides. Vol. 10, pp. 735-739. © Pergamon Pressplc, 1989.Printed in the U.S.A. 0196-9781/89$3.00 + .00 Peripheral Injection of DNS-RFa, A FMRFa Agonist, Suppresses Morphine-Induced Analgesia in Rats ARJEN B. BRUSSAARD, KAREL S. KITS, ANDRIES TER MAAT, ARIE H. MULDER'I" AND ANTON N. M. SCHOFFELMEER'I" Neurophysiology, Department of Biology, Free University P.O. Box 7161, 1007 MC Amsterdam, The Netherlands and :-Department of Pharmacology, Medical Faculty, Free University Van der Boechorstraat 7, 1081 BT Amsterdam, The Netherlands Received 23 January 1989 BRUSSAARD, A. B., K. S. KITS, A. TER MAAT. A. H. MULDER AND A. N. M. SCHOFFELMEER. Peripheralinjection of DNS-RFa, a FMRFaagonist, suppresses morphine-induced analgesia in rats. PEPTIDES 10(4) 735-739, 1989.--The present results demonstrate an antagonistic effect of DNS-RFaon morphine-induced analgesiain rats. This confirmspreviousevidencepresented by others on the effects of FMRFa-related peptides when applied centrally. Unlike these peptides, however, it is shown here that DNS-RFa is effective upon peripheral injection. The effects of DNS-RFa on morphine-induced analgesia were dose-dependent (ED~o=0.5 mg/kg). DNS-RFa alone (5 mg/kg) did not affect the control level of nociception. Peripheral injection of FMRFa (5 mg/kg) did not affect morphine-induced analgesia. DNS-RFadefinesthe minimalconfiguration to activate neuronalFMRFareceptors in the pond snail (4). The present report suggests also that in vertebrates the Arg-Phe-NH 2 sequence is essentialand that DNS-RFa readily penetrates the blood-brainbarrier. Analgesia DNS-RFa FMRFa Neuropeptide Opioid antagonism Pharmacology FMRFa-like immunoreactivity has a broad phylogenetic distribu- tion (2). Extensive FMRFa-like immunoreactivity has been re- ported in the central nervous system of rats, with highest concentra- tions present in the spinal cord and hypothalamus (5,15). Three mammalian FMRFa-like peptides have been chemically charac- terized: Leu-Pro-Leu-Arg-Phe-NH 2 from chicken brain (6) and Phe-Leu-Phe-Gln-Pro-Gln-Arg-Phe-NH 2 (F-8-F-NH2) and Ala- Gly-Glu-Gly-Leu-Ser-Ser-Pro-Phe-Trp-Ser-Leu-Ala- Ala-Pro- Gln-Arg-Phe-NH2 (A-18-F-NH2) in dorsal spinal cord, periaque- ductal grey and medulla pons (14,27). Although structurally more complex than FMRFa [(18), Fig. 1], the above mentioned peptides share the characteristic C-terminal dipeptide amide (RFa) and they display biological activity similar to that of the original tetrapep- tide (23,27). Specific FMRF receptors have been demonstrated in various invertebrate species, especially in molluscs (4, 10, 16, 17, 19, 20). Recently we have reported on the structure-activity relationship of the FMRFa receptor type on the neurosecretory caudo dorsal cells (CDCs) of the pond snail Lymnaea stagnalis (4). FMRFa is one of the native peptides in Lymnaea (7) and FMRFa-like immunoreactivity is closely associated with the CDCs (3). FMRFa inhibits the CDCs via a dual membrane action (3), mediated by a single receptor type (4). Our analysis included a new synthetic FMRFa analog: Dansyl-protected Arg-Phe-NH 2 (DNS-RFa) [(4), Fig. 1]. This substance, having a large hydro- phobic domain (naphthalene-group), mimicked all FMRFa effects in CDCs and showed a clear cross desensitization with the native peptide. It was concluded that DNS-RFa defines the minimal configuration required for activation of the neuronal FMRFa receptor. This parallels the results of a recent binding study of FMRFa receptors in the brain of Helix aspersa (17). Evidence has been presented that FMRFa or a family of immunoreactive FMRFa-like peptides may function as endoge- nous peptides with antiopiate effects (physiological antagonists of the opioid peptides). Firstly, intracerebroventricular injections of FMRFa blocked morphine-induced analgesia in mice (12) while intrathecal injections of antibodies directed against mammalian FMRFa-like peptides produced long lasting analgesia in rats, which was reversible by naloxone (23,25). Secondly intracere- broventricular injection of FMRFa reduced immobilization-in- duced analgesia, as well as the opioid-mediated analgesic and ingestive responses arising from intraspecific aggressive interac- tions and defeat in mice (11,12). However, it is unlikely that the antagonistic effects of FMRFa and related peptides are due to a direct blockade of opioid receptors. Cross-desensitization between FMRFa and naloxone is not likely [cf. (9)]. Firstly, binding studies have revealed that FMRFa has very low affinity for opioid receptors (28) and secondly, Kavaliers and co-workers (13) recently reported that calcium channel antagonists significantly reduced the inhibitory effects of FMRFa, but had no effects on naloxone-mediated inhibition of either morphine- or immobiliza- tion-induced analgesia. These findings suggested that. in verte- brates too, the effects of FMRFa-related peptides are mediated 735