biphalin and naltrindole

Biphalin expresses almost equal affinity for μ- and δ-opioid receptors. The aim of this study was to delineate a possible role of δ-opioid receptors in the cardio-respiratory effects of systemic injection of biphalin in anesthetized, spontaneously breathing rats. In control animals, an intravenous bolus of biphalin (0.3 μmol/kg) evoked apnea, followed by a decreased breathing rate and increased tidal volume, hypotension and bradycardia. Blockade of δ-opioid receptors with naltrindole (4.2 μmol/kg) significantly reduced the duration of apnea, slowdown of respiration, immediate post-challenge hypotension and bradycardia induced by biphalin administration. These results indicate that the activation of δ-opioid receptors adds to the depressive response produced by biphalin.

Topics: Analgesics; Animals; Cardiovascular System; Enkephalins; Injections, Intravenous; Male; Naltrexone; Narcotic Antagonists; Rats; Rats, Wistar; Receptors, Opioid, delta; Respiratory System

Mono iodinated analogues of biphalin [(Tyr-D-Ala-Gly-Phe-NH-)2], both nonradioactive [I-Tyr1]biphalin and radioactive [125I-Tyr1]biphalin have been synthesized. The radioligand binding profiles of these compounds for two types of tissues, rat brain membranes, and NG108-15 cell membranes were identical to the parent biphalin. This is additional evidence for the hypothesis that biphalin behaves like a monomeric ligand and that only one intact tyrosine is necessary for high biological activity. The second tyrosine could be used for successful radioiodination which may greatly simplify biochemical and pharmacological studies of biphalin. The results of receptor binding studies show that the binding of both biphalin and [I-Tyr1]biphalin to the delta and mu opioid receptors are not independent. [125I-Tyr1]Biphalin binds to delta receptors as shown in NG108-15 cell membranes. Nevertheless, [125I]biphalin binding to delta receptors in rat brain membranes was hardly evident and mu receptor binding predominated or at least was much more readily detectable in this preparation.

Topics: Animals; Brain; Cell Line; Cell Membrane; Enkephalins; Iodine Radioisotopes; Morphine; Naltrexone; Narcotic Antagonists; Oligopeptides; Radioligand Assay; Rats; Receptors, Opioid

Biphalin [(Tyr-D-Ala-Gly-Phe-NH)2] is a bivalent, opioid peptide containing two pharmacophores linked by a hydrazine bridge. When administered intracerebroventricularly, it has been shown to be more potent than morphine and etorphine at eliciting antinociception. Biphalin has also been shown to cross both the blood-brain and blood-cerebrospinal fluid barriers. To understand the basis of biphalin's potency, regional brain and spinal cord distribution studies with [125I-Tyr1]biphalin were performed 5, 20, and 40 min after intravenous bolus injections. A statistically greater amount of [125I-Tyr1]biphalin was detected in the nucleus accumbens compared with other brain regions (p < 0.05). This correlates with the high density of delta- and mu-opioid receptor mRNA and binding sites shown to be expressed in the nucleus accumbens. Also, a statistically greater amount of [125I-Tyr1] biphalin was detected in two other circumventricular organs, the choroid plexus and pituitary, when compared with other brain regions. These studies provide evidence that biphalin can reach not only brain sites, but also spinal sites to elicit antinociception. The overall CNS distribution of [125I-Tyr1]biphalin was decreased with naloxone, D-Phe-Cys-Tyr-D-Trp-Arg-Thr-Pen-Thr-NH2, or naltrindole pretreatment, showing that biphalin detected in the brain and spinal cord is binding to delta- and mu-opioid receptors. Additional in situ brain perfusion experiments identified a saturable component contributing to CNS entry of [125I-Tyr1]biphalin, which could be described by Michaelis-Menten kinetics with a Km of 2.6 +/- 4.8 microM, Vmax of 14.6 +/- 2.89 pmol(-1) x min(-1) x g(-1), and Kd of 0.568 +/- 0.157 microl x min(-1) x g(-1). Brain entry of [125I-Tyr1]biphalin was sensitive to 2-aminobicyclo[2.2.1]heptane-2-carboxylic acid and L-phenylalanine, suggesting use of the large neutral amino acid carrier. This work provides evidence that biphalin is a promising, potent analgesic that has a unique mechanism for reaching both spinal and supraspinal opioid receptor sites.

Topics: Analgesics; Animals; Binding, Competitive; Brain; Enkephalins; Iodine Radioisotopes; Kinetics; Models, Chemical; Naloxone; Naltrexone; Narcotic Antagonists; Organ Specificity; Peptides; Radioisotope Dilution Technique; Rats; Receptors, Opioid, delta; Receptors, Opioid, mu; Spinal Cord; Tissue Distribution