Docking studies suggest ligand-specific delta-opioid receptor conformations

Interaction and regulatory functions of μ- and δ-opioid receptors in nociceptive afferent neurons

μ-opioid receptor (MOR) agonists such as morphine are powerful analgesics used for pain therapy. However, the use of these drugs is limited by their side-effects, which include antinociceptive tolerance and dependence. Earlier studies reported that MOR analgesic tolerance is reduced by blockade of δ-opioid receptors (DORs) that interact with MORs. Recent studies show that the MOR/DOR interaction in nociceptive afferent neurons in the dorsal root ganglion may contribute to morphine analgesic tolerance. Further analysis of the mechanisms for regulating the trafficking of receptors, ion channels and signaling molecules in nociceptive afferent neurons would help to understand the nociceptive mechanisms and improve pain therapy.

Coexpression of delta- and mu-opioid receptors in nociceptive sensory neurons

Morphine-induced analgesia and antinociceptive tolerance are known to be modulated by interaction between delta-opioid receptors (DORs) and mu-opioid receptors (MORs) in the pain pathway. However, evidence for expression of DORs in nociceptive small-diameter neurons in dorsal root ganglia (DRG) and for coexistence of DORs with MORs and neuropeptides has recently been challenged. We now report, using in situ hybridization, single-cell PCR, and immunostaining, that DORs are widely expressed not only in large DRG neurons but in small ones and coexist with MORs in peptidergic small DRG neurons, with protachykinin-dependent localization in large dense-core vesicles. Importantly, both DOR and MOR agonists reduce depolarization-induced Ca(2+) currents in single small DRG neurons and inhibit afferent C-fiber synaptic transmission in the dorsal spinal cord. Thus, coexistence of DORs and MORs in small DRG neurons is a basis for direct interaction of opioid receptors in modulation of nociceptive afferent transmission and opioid analgesia.

INTERACTION OF BRIDGED PIPERAZINE DERIVATIVES WITH THE μ-OPIOID RECEPTOR — A THEORETICAL MODEL

The flexible molecular docking was used to study interactions between a series of 3,6-diazabicyclo[3.1.1]heptanes, 9,10-diazatricyclo[4.2.1.1]decanes, and 2,7-diazatricyclo [4.4.0.0]decanes N-substituted by propanoyl and by arylalkenyl groups, and a model of the μ-opioid receptor. It has been found that the optimal position and orientation of the compounds in the ligand–receptor complex resemble that of fentanyl analogs described earlier.1 This model explains stereochemical effects on binding of the two series of 3,6-diazabicyclo[3.1.1]heptanes, suggesting that the steric interaction of the bridge methylenic group plays the major role in modulating μ-receptor affinity of those molecules. Ab initio B3LYP method was used to determine electrostatic potentials of different bridged piperazine derivatives, and to estimate electrostatic contribution to the ligand–receptor complex stability.

Structural requirements for ligands of the δ-opioid receptor

The ?-opioid receptor is sensitive to ligand geometry. In order to assist the synthesis of new ?-selective opioid ligands, the structure elements of ?-selective opioid ligands necessary for their effective binding were investigated. The automated docking procedure with a flexible ligand was used to simulate the binding of 17 ?-selective ligands to the ?-receptor. It was found that voluminous N-alkyl groups reduce the binding potency of naltrindole derivatives by preventing the ligands from adopting the preferred conformation in the receptor. This was confirmed by enantiospecific binding of chiral compounds where only one enantiomer adopts the naltrindole-like preferred conformation in the binding pocket. Voluminous groups replacing the hydroxyl group in the 3-hydroxybenzyl fragment of naltrindole analogs reduce the binding potency due to unfavorable steric interactions with the receptor. The two diastereoisomers of the potent ?-opioid ligand SNC80 confirmed the preferred binding conformation and the major receptor-ligand interactions.