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Compound trans-4-(p-bromophenyl)-4-(dimethylamino)-1-(2-thiophen-2-yl-ethyl)-cyclohexanol (C8813), structurally unrelated to morphine, is a novel analgesic. The present study examined the antinociception, opioid receptor selectivity and in vitro activity of C8813. The antinociceptive activity was evaluated using mouse hot plate and acetic acid writhing tests. In mouse hot plate test, the antinociceptive ED(50) of C8813 was 11.5 microg/kg, being 591 times and 3.4 times more potent than morphine and fentanyl respectively. In mouse writhing test, the antinociceptive ED(50) of C8813 was 16.9 microg/kg, being 55 times and 2.3 times more active than morphine and fentanyl respectively. In the opioid receptor binding assay, C8813 showed high affinity for mu-opioid receptor (K(i) = 1.37 nM) and delta-opioid receptor (K(i) = 3.24 nM) but almost no affinity for kappa-opioid receptor (at 1 microM). In the bioassay, the inhibitory effect of C8813 in the guinea-pig ileum (GPI) was 16.5 times more potent than in the mouse vas deferens (MVD). The inhibitory effects of C8813 in the GPI and MVD could be antagonized by mu-opioid receptor antagonist naloxone and delta-opioid receptor antagonist ICI174,864 respectively. However, the inhibitory effect of C8813 in the rabbit vas deferens was very weak. These results indicated that C8813 was a potent analgesic and a high affinity agonist for the mu- and delta-opioid receptors.

The effect of l-12-chloroscoulerine (l-CSL), a novel ligand with dual dopamine D1 receptor agonistic and D2 receptor antagonistic actions, on the development of morphine-induced conditioned place preference (CPP) was investigated in mice. Morphine (10 mg/kg)-induced place preference was dose dependently suppressed by coadministration of l-CSL (5, 10 and 20 mg/kg), which induced neither place preference nor place aversion when administered alone at a dose of 20 mg/kg. The D1 receptor antagonist SCH23390 (0.1 mg/kg) suppressed, whereas the D2 receptor agonist (+/-)-2-(N-phenylethyl-N-propyl)-amino-5-hydroxytetralin (PPHT) (0.5 mg/kg) had no influence on the development of morphine-induced place preference. However, SCH23390 (0.1 mg/kg) did not affect, whereas PPHT (0.5 mg/kg) reversed the suppressive effect of l-CSL on the development of morphine-induced place preference. These results indicate that l-CSL suppresses the development of place preference of morphine by blocking D2 receptors.

Ohmefentanyl is a very potent and highly selective agonist for mu-opioid receptors. We now study analgesia, in vitro activity and opioid receptor affinity of the stereoisomers of ohmefentanyl isothiocyanate. We found that some isomers of ohmefentanyl isothiocyanate had a potent analgesic effect and that all isomers except (3R,4S,2'S)-ohmefentanyl isothiocyanate had a more potent inhibitory action on the electrically evoked contractions of mouse vas deferens than of guinea pig ileum. The inhibitory actions could be antagonized by naloxone. However, compared with the activity of the corresponding stereoisomers of ohmefentanyl, these ohmefentanyl isothiocyanates had significantly reduced analgesia and in vitro activity. They also inhibited the binding of [3H]DPDPE ([D-Pen(2),D-Pen(5)]enkephalin) and [3H]DAGO ([D-Ala(2),Mephe(4),Gly-ol(5)]enkephalin) to opioid receptors in mouse brain membranes. The inhibitory effect of stereoisomers of ohmefentanyl isothiocyanate at mu-opioid receptors was markedly lower than that of their parent compounds. The affinity of stereoisomers of ohmefentanyl isothiocyanate for delta-opioid receptors was, however, greater than or equal to that of their corresponding stereoisomers of ohmefentanyl. The results showed that the introduction of an isothiocyanato group into the phenyl ring in position-1 of ohmefentanyl reduced bioactivity and affinity to mu-opioid receptors but that the selectivity of these compounds for delta-opioid receptors was enhanced. Isomer (3R,4S,2'R)-ohmefentanyl isothiocyanate showed highest selectivity for delta-opioid receptors (K(i)(mu)/K(i)(delta)=13.6) and potent analgesic activity (ED(50)=0.25 mg/kg).

Differences of analgesia and withdrawal response among ohmefentanyl stereoisomers have been studied. In the present study, Quantitative comparison of reinforcing effects of ohmefentanyl stereoisomers and morphine was performed by using a conditioned place preference design in mice. Results showed that morphine and ohmefentanyl stereoisomers were able to increase significantly the time spent in the drug-paired side with respect to vehicle treated animals. A good linear correlation between doses of drugs and number of mice with place preference was found within a given dose range. On the basis of the dose-response curve analysis, ohmefentanyl stereoisomers displayed a significant difference in place preference ED50. The addictive index (analgesic ED50/place preference ED50) was used to assess the addictive potential of drugs. It was demonstrated that the addictive potential of ohmefentanyl stereoisomers did not exhibit a large difference as addictive index. Among these stereoisomers, the addictive potential of compound F9208 was markedly lower than that of morphine.

Based on our previous result of the three-dimensional model of the μ-opioid receptor, binding conformations of 13 fentanyl analogs and three-dimensional structures for the complexs of these analogs with μ-opioid receptor were constructed employing the molecular modeling method and our binding conformation search program for ligands (BCSPL). Energetic calculation and quantitative structure–activity relationship (QSAR) analysis indicated a good correlation between the calculated binding energies of fentanyl analogs and their binding affinities, pKi's and pK's, and analgesic activities, − log ED50's. Based on the three-dimensional models, the possible interaction mechanism of fentanyl analogs with μ-opioid receptor can be illustrated and the available structure–activity relationship of these analgesic agents can be explained reasonably. © 2000 John Wiley & Sons, Inc. Int J Quant Chem 78: 285–293, 2000

The present study was designed to examine the ventral tegmental area (VTA) dopamine neuronal activity in nicotine withdrawal rats by means of in vivo single-unit extracellular recordings. Animals were treated with nicotine base (6 mg/kg/day, s.c.) four times daily for 12 days. One day after the last nicotine administration, the firing rates of the VTA dopamine neurons were found to be significantly decreased. Following 2, 3, 5 and 10 days of nicotine withdrawal, however, the firing rates returned to the control levels. The number of spontaneously active dopamine cells was not altered at any time points. The results indicate that the VTA dopamine neuronal activity is reduced during the first day of nicotine withdrawal.

N-[1-(2-Hydroxy-2-phenylethyl)-3-methyl-4-piperidyl]-N-phenylpropanamide (ohmefentanyl, 1) is an extremely potent analgesic agent with high affinity and selectivity for opioid μ receptors. There are three chiral carbons in 1, so eight optically active isomers are possible. Respective reaction of optically active 3-methyl-N-phenyl-4-piperidinamines (5a-d) with (R)- or (S)-styrene oxide produced eight optically active intermediates which were subsequently converted to eight optically active isomers of 1 (1a-h). The absolute configurations of 1a-h were determined by X-ray analysis of (3R,4S,2'R)-(-)-cis-1a and (3R,4R,2'S)-(-)-trans-1g. The analgesic activity (mice, ip, hot plate) revealed their extreme stereodifferences ; the ED 50 values of (3R,4S,2'R)-(-)-cis-1a and (3R,4S,2'S)-(+)-cis-1b, which are the most potent isomers among eight isomers, were 0.004 65 (2990 times that of morphine) and 0.001 06 mg/kg (13 100 times that of morphine), respectively, while the corresponding antipodes 1d,c were the least potent compounds among the eight isomers. In agreement with pharmacological results, both 1a,b also had the highest receptor affinity and selectivity for the opioid μ receptor. The ratio of K i (DPDPE)/K i (DAMGO) was 22 800 for 1a and 22 500 for 1b. All isomers except 1c,d strongly inhibited the electrically evoked smooth muscle contraction of GPI and MVD but not that of RVD, and the inhibitory effects could be reversed by naloxone, which indicated that these compounds were potent μ agonists in GPI and MVD. There was a good linear correlation between the analgesic potencies (ED 50 ) and the receptor affinities (K i (DAMGO)) or inhibitory effects (IC 50 ) to GPI and MVD. These results suggested that the analgesic effects of ohmefentanyl are mediated by interaction between the agents and opioid μ receptors in the central nervous system and that the 3R,4S configuration at the piperidine 3- and 4-carbon atoms and the S configuration at the phenylethyl 2-carbon atom are beneficial for analgesic potency and inhibitory effects in GPI and MVD and the same for an S or R configuration at the phenylethyl 2-carbon atom besides the 3R,4S configuration for receptor μ affinity and selectivity.

The binding affinity of ohmefentanyl stereoisomers for μ-opioid receptors and the effect of chronic ohmefentanyl stereoisomers pretreatments on intracellular cAMP formation were investigated in Sf9 insect cells expressing human μ-opioid receptors (Sf9-μ cells). Competitive assay of [ 3 H]ohmefentanyl binding revealed that these isomers had high affinity for μ-opioid receptors in Sf9-μ cells. Isomer F9204 had the highest affinity for μ-opioid receptors with the K i value of 1.66 ± 0.28 nM. After pretreated Sf9-μ cells with increasing concentrations of these isomers for 6 h, addition of naloxone (1 μM) precipitated an overshoot of foskolin-stimulated cAMP accumulation. The ability of these isomers to induce cAMP overshoot differed greatly with the order of F9202 > F9205 > F9208 > F9206 > F9204 > F9207. Of these isomers, F9202 was 2.7-fold less potent than F9204 in receptor binding affinity, but 71.5-fold more potent in ability to induce cAMP overshoot. These results suggested that there was a significant stereo-structural difference among ohmefentanyl stereoisomers in ability to induce naloxone-precipitated cAMP overshoot in Sf9-μ cells.

To investigate the receptor binding affinity and naloxone-precipitated cAMP overshoot of dihydroetorphine, fentanyl, heroin, and pethidine in Sf9 insect cells expressing human mu-opioid receptor (Sf9-mu cells).Competitive binding assay of [3H]ohmefentanyl was used to reveal the affinity for mu-opioid receptor in Sf9-mu cells. [3H]cAMP RIA was used to determine cAMP level. Antinociceptive activity was evaluated using degree 55 mouse hot plate test. Naloxone-precipitated withdrawal jumping was used to reflect physical dependence in mice.All drugs displayed antinociceptive activity and produced physical dependence in mice. The K(i) values of dihydroetorphine, fentanyl, heroin, and pethidine in competitive binding assay were (0.85+/-0.20) nmol, (59.1+/-11.7) nmol, (0.36+/-0.13) micromol, and (12.2+/-3.8) micromol respectively. The binding affinities of these drugs for mu-opioid receptor in Sf9-mu cells were paralleled to their antinociceptive activities in mice. After chronic pretreatment with these drugs, naloxone induced cAMP withdrawal overshoot in Sf9-mu cells. The dependence index in Sf9-mu cells was calculated as K(i) value in competitive binding assay over EC(50) value in naloxone-precipitated cAMP assay. The physical dependence index in mice was calculated as antinociceptive ED(50)/withdrawal jumping cumulative ED(50). There was a good linear correlation between dependence index in Sf9-mu cells and physical dependence index in mice.The Sf9-mu cells could be used as a cell model to evaluate the receptor binding affinity and physical dependent liability of analgesic agents.

To express dopamine D1 receptor in baculovirus-Sf9 cell system, and to investigate the effects of l-12-chloroscoulerine (l-CSL) on the recombinant D1 receptor (D1R).The recombinant baculovirus, Autographa californica nuclear polyhedrosis virus bearing D1R (AcNPV- D1R) was generated, and then was used to produce recombinant D1R in Sf9 insect cells. Expression of D1R in Sf9 cells was monitored by [3H]SCH23390 binding assay. The effects of l-CSL on recombinant D1R were investigated by [3H]SCH23390 binding assay and cAMP assay.The recombinant baculovirus AcNPV bearing D1R cDNA was generated, and was successfully expressed in Sf9 insect cells. The expression level of (Bmax) was (0.94+/-0.06) nmol/g protein. The Kd value of [3H]SCH23390 was (1.9+/-0.3) nmol/L, which was consistent with the previous results from calf striatum tissues. l-CSL had a high affinity to recombinant D1R with Ki value of (6.3+/-1.4) nmol/L, and increased the intracellular cAMP level in a concentration-dependent manner with EC50 value of 0.72 micromol/L and 95 % confidence limit was 0.67-0.77 micromol/L. Thus l-CSL has the D1 receptor agonism.An efficient baculovirus-Sf9 insect cell system for dopamine D1 receptor was constructed and l-CSL presented the D1 receptor agonism on cellular-molecular level directly.

Stereo-structural difference of ohmefentanyl stereoisomers on analgesic action and receptor affinity has been studied. To assess the difference of ohmefentanyl stereoisomers in physical dependence, the potency of physical dependence was quantified by estimating the ED50 value of ohmefentanyl stereoisomers in the naloxone-precipitated jumping test in mice. Morphine was used to assess the method and as a drug of comparison. The results indicate that the degree of physical dependence of morphine can been quantified by estimating the ED50 value of morphine withdrawal jumping induced by naloxone. A significant difference was observed in withdrawal jumping ED50 values among ohmefentanyl stereoisomers. Of these isomers, F9202 and F9204 had similarly potent analgesic action, but very significant difference in naloxone precipitated withdrawal response. Dependent potency index of F9204 was 618-fold weaker than that of F9202. It is concluded that a stereo-structural difference in physical dependence is found to exist among ohmefentanyl stereoisomers. Compound F9204 displayed a strong analgesic action and weak physical dependent potency.