Parallel, solution phase synthesis of dihydropyridine miticides via a versatile multicomponent reaction.

A novel class of highly active dihydropyridine miticides was prepared using a multicomponent reaction process. The initial lead was rapidly optimized using solution phase parallel synthesis techniques and a positional scanning approach. Detailed structure-activity relationships were developed for the amino and carbonyl components of the molecule and used to select the best candidates for broad field testing. The chemistry, biology and toxicology of these compounds will be presented along with numerous structural variants of the reaction products.

Cross-resistance with dieldrin of a novel tricyclic dinitrile GABA receptor antagonist.

A novel tricyclic dinitrile, KN244, blocked the wild-type (dieldrin-sensitive) homo-oligomeric gamma-aminobutyric acid (GABA)-gated chloride channel of Drosophila melanogaster expressed in Xenopus oocytes. Sensitivity to the block by KN244 of the response to 30 microM GABA (IC50=41.6 nM, wild-type RDLac) was reduced abut 100 fold (IC50=4.5 microM) in the dieldrin-resistant (RDLacA302S) form of RDL.

Effects of [3H]-BIDN, a novel bicyclic dinitrile radioligand for GABA-gated chloride channels of insects and vertebrates.

1. The radiolabelled bicyclic dinitrile, [3H]-3,3-bis-trifluoromethyl-bicyclo[2.2.1]heptane-2,2-dicarbonitrile ([3H]-BIDN), exhibited, specific binding of high affinity to membranes of the southern corn rootworm (Diabrotica undecimpunctata howardi) and other insects. A variety of gamma-aminobutyric acid (GABA) receptor convulsants, including the insecticides heptachlor (IC50, 35 +/- 3 nM) and dieldrin (IC50, 93 +/- 7 nM), displaced [3H]-BIDN from rootworm membranes. When tested at 100 microM, 1-(4-ethynylphenyl)-4-n-propyl-2,6,7-trioxabicyclo[2.2.2]oct ane(EBOB), 4-t-butyl-2,6,7-trioxa-1-phosphabicy-clo[2.2.2]octane-1-thio ne (TBPS), 1-phenyl-4-t-butyl-2,6,7-trioxabicyclo[2.2.2]octane (TBOB) and picrotoxin failed to displace 50% of [3H]-BIDN binding to rootworm membranes indicating that the bicyclic dinitrile radioligand probes a site distinct from those identified by other convulsant radioligands. 2. Dissociation studies showed that dieldrin, ketoendrin, toxaphene, heptachlor epoxide and alpha and beta endosulphan displace bound [3H]-BIDN from rootworm membranes by a competitive mechanism. 3. Rat brain membranes were also shown to possess a population of saturable, specific [3H]-BIDN binding sites, though of lower affinity than in rootworm and with a different pharmacological profile. Of the insecticidal GABAergic convulsants that displaced [3H]-BIDN from rootworm, cockroach (Periplaneta americana) and rat brain membranes, many were more effective in rootworm. 4. Functional GABA-gated chloride channels of rootworm nervous system and of cockroach nerve and muscle were blocked by BIDN, whereas cockroach neuronal GABA(B) receptors were unaffected. 5. Expression in Xenopus oocytes of either rat brain mRNA, or cDNA-derived RNA encoding a GABA receptor subunit (Rdl) that is expressed widely in the nervous system of Drosophila melanogaster resulted in functional, homo-oligomeric GABA receptors that were blocked by BIDN. Thus, BIDN probes a novel site on GABA-gated Cl- channels to which a number of insecticidally-active molecules bind.

Polycyclic dinitriles: a novel class of potent GABAergic insecticides provides a new radioligand, [3H]BIDN.

The polycyclic dinitriles are a potent class of insecticides which are non-competitive GABA (gamma-aminobutyric acid) antagonists acting at the convulsant site. Comparison with other classes of GABA convulsant site ligands using molecular modelling has shown significant structural similarities. We have developed a pharmacophore model which unifies this class and some previous classes of GABA convulsants. Key pharmacophore elements are a polarizable functionality separated by a fixed distance from two H-bond accepting elements. This model is based on information from X-ray crystal structures and Sybyl using the Tripos force field. Using this pharmacophore model, numerous structural modifications were explored to enhance understanding of structure-activity relationships at the GABA receptor convulsant site of insects and mammals. A radiolabelled bicyclic dinitrile, [3H]BIDN [3H]3,3-bis-trifluoromethyl-bicyclo[2,2,1]heptane-2,2-dicarbonitrile+ ++), was prepared from this area of chemistry and was used as a probe for the interaction of polycyclic dinitriles at the target site.

Actions of picrotoxinin analogues on an expressed, homo-oligomeric GABA receptor of Drosophila melanogaster.

The actions of picrotoxinin and four of its analogues were tested on a Drosophila melanogaster homo-oligomeric GABA (gamma-aminobutyric acid) receptor formed when RDL (resistance to dieldrin) subunits were expressed in Xenopus oocytes. In agreement with previously reported studies on native insect GABA receptors and native expressed vertebrate GABA receptors, acetylation of the bridgehead hydroxyl group (picrotoxinin acetate) greatly reduced the activity of the molecule, but surprisingly, substitution with flourine at the same position also reduced the activity. Conversion of the terminal isopropenyl group to an acetyl (alpha-picrotoxinone) or hydration of the double bond (picrotin) also reduced activity, in agreement with findings for native insect and mammalian receptors. The present results suggest that interactions of convulsants with homo-oligomeric and multimeric GABA receptors are qualitatively similar. Thus, the RDL homo-oligomer exhibits a pharmacological profile for picrotoxinin analogues resembling that of native GABA receptors.

Blocking actions of picrotoxinin analogues on insect (Periplaneta americana) GABA receptors.

Five picrotoxinin analogues were examined on GABA-gated chloride channels of an identifiable cockroach (Periplaneta americana) motor neurone (Df). Substitution of the bridgehead hydroxyl at the C-6 position of the picrotoxinin molecule by a fluorine atom (fluoropicrotoxinin) had little effect, whereas acetylation of the same functional group (picrotoxinin acetate) substantially reduced the effectiveness of the parent compound. Conversion of the terminal isopropenyl group to an acetyl (alpha-picrotoxinone) or hydration of the double bond (picrotin) also reduced activity. Dendrobine, a naturally-occurring picrotoxinin-like compound had very little effect on GABA-induced responses at concentrations up to 1.0 x 10(-5) M. The present results suggest that the size and the ability of the bridgehead hydroxyl to undergo hydrogen bond formation and the lipophilic nature of the terminal isopropenyl group profoundly affect the inhibitory actions of the picrotoxinin molecule on insect neuronal GABA-gated chloride channels.

Actions of a series of bisquaternary compounds on nicotinic acetylcholine receptors in insects: ligand binding and electrophysiological studies.

A series of bisquaternary ammoniums, with chain lengths of between 4-12 carbon atoms (C4-C12), have been tested for their ability to block acetylcholine-induced responses in the fast coxal depressor motor neurone (Df) of the cockroach (Periplaneta americana) and to displace [125I]alpha-bungarotoxin from membrane preparations of the CNS of the cockroach. The physiological studies showed that tetramethonium was inactive, whereas hexa-, octa- and dodecamethonium showed an enhanced ability to block acetylcholine-induced responses as the chain length increased. Decamethonium resulted in a slight increase in acetylcholine-induced depolarizations. Ligand binding studies showed that the ability of the compounds to inhibit the specific binding of [125I]alpha-bungarotoxin increased with size from C4-C12. The results show that neuronal nicotinic receptors in insects differ in aspects of their pharmacology from both the major subclasses of nicotinic receptors of vertebrates.

Insecticide resistance: challenge to pest management and basic research.

The agricultural use of synthetic insecticides usually protects crops but imposes strong selection pressures that can result in the development of resistance. The most important resistance mechanisms are enhancement of the capacity to metabolically detoxify insecticides and alterations in target sites that prevent insecticides from binding to them. Insect control methods must incorporate strategies to minimize resistance development and preserve the utility of the insecticides. The most promising approach, integrated pest management, includes the use of chemical insecticides in combination with improved cultural and biologically based techniques.

A critical examination of the dual system theory in Ostrinia nubilalis.

Beck's dual system theory (DST) is examined theoretically and experimentally by investigating the oviposition rhythm of Ostrinia nubilalis and its entrainment by light cycles. Several well-known circadian phenomena are not accounted for by the DST. 1) It does not generate transient cycles when light pulses fall during the advance portion of the circadian cycle. This is also reflected in DST-predicted phase-response curves (PRC's) for both Drosophila pseudoobscura and O. nubilalis. Steady-state phase advances are predicted to occur on day 1 after the light pulses by the DST, not several cycles later as has been observed in many cases. 2) It does not account for the observation that the magnitude of a phase shift (delta phi) is often a function of pulse duration of both delays and advances. The DST predicts the same + delta phi, for example, for a 0.5-h and a 6.0-h light pulse beginning 5.0 h after dusk. 3) The DST does not accurately predict steady-state phase relationships between the light cycle and the gating oscillation (P-system) in non-24-h light cycles. 4) The driver (S-system) is given the property of being temperature sensitive whereas the driven rhythm (P-system) is temperature compensated. This is contrary to accumulated data suggesting that the circadian pacemaker is temperature compensated.