Employing a novel, rapid and sensitive method for evaluation of phospholipase C (PLC) activity, the present study characterized the actions of diverse agonists and antagonists at human (h)5-HT2C receptors expressed in Chinese Hamster Ovary (CHO) cells. In addition, affinities and efficacies at these sites were compared with those obtained at h5-HT2B receptors.5-HT elicited a robust and rapid reduction in levels of the pre-labelled, membrane-bound substrate of PLC, [3H]phosphatidylinositols ([3H]PI). The time-course of [3H]PI depletion paralleled that of [3H]inositol phosphate ([3H]IP) accumulation, as determined by conventional anion exchange chromatography. Inactivation of h5-HT2C receptors with the alkylating agent, N-ethoxycarbonyl-2-ethoxy-1,2-dihydroquinoline (EEDQ), revealed a large receptor reserve, with half-maximal PLC activation induced by a concentration of 5-HT occupying only 5% of sites. In analogy to 5-HT ( Emax=100%), DOI, MK212 and mCPP, as well as the novel ligands, Ro600332, Ro600175 and BW723C86, showed "full" efficacy at h5-HT2C sites. Their efficacies were similar at h5-HT2B sites, with the exception of mCPP and MK212, which acted as partial agonists. Further, lisuride and Ro600869 behaved as partial agonists and antagonists at h5-HT2C and h5-HT2B receptors, respectively. As concerns functional selectivity (potency for induction of [3H]PI depletion), only Ro600175 preferentially activated h5-HT2B sites. In contrast, Ro600332 preferentially activated h5-HT2C receptors. Amongst antagonists, RS102221 and SB242084 displayed a marked preference for h5-HT2C sites, whereas LY266097, S33526 and SB204741 behaved as selective antagonists at h5-HT2B receptors. At both h5-HT2C and h5-HT2B receptors, antagonist potency (p Kb) and binding affinity (p Ki) were highly correlated. In conclusion, this rapid and innovative method for determination of PLC activity permitted characterization of an extensive range of novel ligands at h5-HT2C receptors. Although several antagonists clearly differentiated h5-HT2C from h5-HT2B receptors under these conditions, highly selective agonists remain to be identified.

Acid-catalyzed cyclizations under thermodynamically and kinetically controlled conditions of the hydroxyenol ethers 18-21 are reported. Thermodynamically controllcd cyclizations of 18, 19, and 21 produced only the more stable corresponding spiroacetals 22 and 27. Thermodynamically controlled cyclization of compound 20 produced a 1:l mixture of non-epimerisable spiroacetals 24 and 26. On the other hand, kinetically controlled cyclizations of the same four hydroxyenol ethers produced, along with the more stable spiroacetals mentioned above, the less stable spiroacetals 23,25, and 28. These results show that the kinetically controlled cyclizalion takes place via an early transition state which produces a mixture of the less stable and the more stable isomers. These results are explained by an early transition state taking into account the principle of stereoelectronic control while following the antiperiplanar lone-pair hypothesis (Burgi-Dunirz angle of attack of a nucleophile on a n systcm).Introduction. -We previously reported [ 11 a study which revealed that the unsubstituted 1,7-dioxaspiro[5.5]undecane exists exclusively in conformation A, (Scheme I ), even at room temperature. This experimental observation was explained by the fact that conformation A, is stereoelectronically and sterically more stable than conformations A, and A, which were estimated to be less stable by a value of 2.4 and 4.8 kcal/mol, respectively'). This result was further confirmed experimentally by comparing the behavior of l-oxaspiro[5.5]undecane which was shown to exist as an equilibrium mixture of two conformers [2].

No abstract

A new series of imidazolyl nitrones spin traps has been synthesized and evaluated pharmacologically. The salient structural feature of these molecules is the presence of an imidazole moiety substituted by aromatic or heteroaromatic cycles. This connectivity imparts to the nitrone superior neuroprotective properties in vivo and in parallel reduced side effects and toxicity. Thus compound 6a (a 2-phenylimidazolyl nitrone) administered intraperitoneally protects (80%) mice from lethality induced by an intracerebroventricular administration of tert-butyl hydroperoxide (t-BHP) an oxidant capable of inducing neurodegenerative processes. Administration of the archetypal nitrone phenyl-tert-butyl nitrone (PBN) at an equimolar dose also affords some protection (60%) in this test. However, this activity is accompanied by hypothermia, whereas no such effect is apparent for 6a. Moreover, previously prepared nonsubstituted or alkyl-substituted imidazolyl nitrones were shown to be extremely toxic to rats in contrast to the compounds prepared in this study. The observed activities in vivo correlate well with the calculated partition coefficients (ClogP) and HOMO energy level.