beta-funaltrexamine and normorphine

The antagonist beta-funaltrexamine (beta-FNA), known to bind covalently to mu-opioid receptors by a two-step, doubly discriminating sequence, was used as a sensitive gauge to compare wildtype to mutant H297Q mu-opioid receptors. We addressed whether this mutation, which enhances the intrinsic activities of alkaloid mu-receptor agents, affects both the reversible and covalent phases of beta-FNA binding. Such altered binding serves as a reporter for the dimensions and topography of the receptor's recognition site. Using the voltage-clamped Xenopus oocyte expression system with coexpressed GIRK potassium channels, we found that beta-FNA blocked the wildtype and mutant H297Q receptors both reversibly and irreversibly, indicating overall conserved tertiary structure in the mutant. The mutant H297Q receptor, however, was more resistant to both phases of blockade, indicating some disturbance of the mutant H297Q receptor recognition site. beta-FNA acted as a partial agonist at the mutant H297Q receptor expressed in both oocytes, as measured by the activation of GIRK channels, and in COS-7 cells assayed by GTPgamma(35)S binding. beta-FNA showed no activity at the wildtype receptor expressed in oocytes, but surprisingly induced binding of GTPgamma(35)S in transfected COS-7 cells. Thus, the topography of the mutant H297Q receptor recognition site is sufficiently conserved to allow the selective binding of beta-FNA, but the decrease in binding affinity and increase in efficacy in oocytes demonstrates clear differences from the wildtype receptor.

Topics: Animals; COS Cells; Dose-Response Relationship, Drug; G Protein-Coupled Inwardly-Rectifying Potassium Channels; Guanosine 5'-O-(3-Thiotriphosphate); Membrane Potentials; Morphine Derivatives; Mutation; Naltrexone; Narcotic Antagonists; Oocytes; Patch-Clamp Techniques; Potassium Channels; Potassium Channels, Inwardly Rectifying; Receptors, Opioid, mu; Transfection; Xenopus

Intracellular recordings were made from neurones of the rat locus coeruleus (LC) which were located in a slice of pons superfused in vitro. Opioid agonists and antagonists were applied by adding them to the superfusing solution; normorphine and enkephalin analogues were also applied by ejecting a few nanoliters of a solution which contained the drugs from a pipette situated above the tissue slice. Opioid agonists hyperpolarized LC neurones. This has been shown previously to result from an increase in the membrane potassium conductance. The lowest concentration of normorphine which was effective was 30 nM, the EC50 was 1 microM, and the maximum effect was observed with 30 microM. The irreversible antagonist beta-funaltrexamine (beta-FNA) was used to estimate the dissociation equilibrium constants; these ranged from 9-16 microM for normorphine and [Met5]enkephalin and was about 2 microM for [D-Ala2,D-Leu5]enkephalin. beta-FNA also blocked the hyperpolarization caused by [D-Ala2,D-Leu5]enkephalin, ethylketacyclazocine, and [D-Ser2,D-Leu5] enkephalin-Thr. Naloxone reversibly antagonized the hyperpolarizations caused by normorphine and [D-Ala2,D-Leu5]enkephalin, with a dissociation equilibrium constant of 2 nM. It is suggested that the opioid hyperpolarization of LC neurones is mediated by a receptor having a high affinity for naloxone, previously termed a mu-receptor. The affinity of this receptor for normorphine appears to be 3 to 4 orders or magnitude lower than its affinity for naloxone.

Topics: Action Potentials; Animals; Enkephalin, Methionine; In Vitro Techniques; Locus Coeruleus; Morphine Derivatives; Naloxone; Naltrexone; Narcotic Antagonists; Narcotics; Neurons; Rats; Receptors, Opioid

Application of 100 nM beta-FNA for 60 minutes to isolated longitudinal muscles-myenteric plexus preparations from the guinea pig ileum caused a marked antagonism of the inhibitory action of normorphine and leucine enkephalin without greatly affecting the inhibitory potency of dynorphin or ethylketocyclazocine. The interaction of beta-FNA with the normorphine (mu-opiate receptors) appears to be non-equilibrium. Pretreatment with beta-FNA caused a significant increase in the apparent naloxone dissociation constant for normorphine and leucine enkephalin but not for dynorphin or ethylketocyclazocine. The results lend further support to the hypothesis that normorphine and the enkephalins activate preferentially mu-opiate receptors on the ileum, whereas dynorphin interacts predominantly at k-opiate sites.

Topics: Animals; Binding, Competitive; Dose-Response Relationship, Drug; Dynorphins; Electric Stimulation; Endorphins; Guinea Pigs; Morphine Derivatives; Muscle, Smooth; Naloxone; Naltrexone; Receptors, Opioid; Time Factors