DAMGO

The mu-opioid receptor (MOR) couples to multiple G proteins, of which coupling to Gs has long been debated. As expected, in opioid naive Chinese hamster ovary cells expressing recombinant MOR, the predominant action of [D-Ala2,N-Me-Phe4,Gly5-ol]-enkephalin (DAMGO) is inhibitory. However, inactivation of Gi/Go proteins via pertussis toxin (PTX) unmasks its ability to facilitate forskolin activation of adenylyl cyclase (AC) activity. Tolerance develops to this effect of DAMGO, which can also be attenuated by cholera toxin (CTX). The latter suggests G mediation. D-Phe-Cys-Tyr-D-Trp-Arg-Thr-Pen-Thr-NH2 (CTAP), previously considered to be a neutral MOR antagonist, also produces a facilitation of forskolin (FSK) activation of AC that is augmented by chronic morphine. Facilitative effects of CTAP in naive as well as its augmentation in tolerant membranes are both substantially reduced by CTX. This suggests that not only Gs mediation but also G(salpha)-linked signaling is critical to the chronic morphine-induced enhanced facilitative action of CTAP. Interestingly, the (augmented) CTAP facilitation of FSK-stimulated AC activity that is observed in opioid tolerant (but not in naive) membranes is also sensitive to PTX.

The mu and delta opioid receptors (MOR and DOR) are highly homologous members of the opioid family of GPCRs. There is evidence that MOR and DOR interact, however the extent to which these interactions occur in vivo and affect synaptic function is unknown. There are two stable DOR subtypes: DPDPE sensitive (DOR1) and deltorphin II sensitive (DOR2); both agonists are blocked by DOR selective antagonists. Robust motivational effects are produced by local actions of both MOR and DOR ligands in the ventral tegmental area (VTA). Here we demonstrate that a majority of both dopaminergic and non-dopaminergic VTA neurons express combinations of functional DOR1, DOR2, and/or MOR, and that within a single VTA neuron, DOR1, DOR2, and MOR agonists can differentially couple to downstream signaling pathways. As reported for the MOR agonist DAMGO, DPDPE and deltorphin II produced either a predominant K;+; dependent hyperpolarization or a Ca;v;2.1 mediated depolarization in different neurons. In some neurons DPDPE and deltorphin II produced opposite responses. Excitation, inhibition, or no effect by DAMGO did not predict the response to DPDPE or deltorphin II, arguing against a MOR-DOR interaction generating DOR subtypes.

Biphalin, a synthetic opioid peptide with a broad affinity for all opioid receptors (δ, μ, and κ) and high antinociceptive activity, has been under extensive study as a potential analgesic drug. This study presents the synthesis and biological properties of four new analogues of biphalin containing amphiphilic α-alkylserines in position 2 and 2'. The incorporation of bulky α,α-disubstituted amino acids in the peptide chain using standard peptide chemistry is often unsuccessful. We synthesized depsipeptides, and then, the desired peptides were obtained by internal O,N-migration of the acyl residue from the hydroxyl to the amino group under mild basic conditions. The potency and selectivity of the new analogues were evaluated by a competitive receptor-binding assay in the rat brain using [(3)H]DAMGO (a μ ligand) and [(3)H]DELT (a δ ligand). Their binding affinity is strongly dependent on the chirality of α-alkylserine, as analogues containing (R)-α-alkylserines displayed higher μ receptor affinity and selectivity than those incorporating the (S)-isomers.