The peach potato aphid, Myzus persicae is a globally distributed crop pest with a host range of over 400 species including many economically important crop plants. The intensive use of insecticides to control this species over many years has led to populations that are now resistant to several classes of insecticide. Work spanning over 40 years has shown that M. persicae has a remarkable ability to evolve mechanisms that avoid or overcome the toxic effect of insecticides with at least seven independent mechanisms of resistance described in this species to date. The array of novel resistance mechanisms, including several 'first examples', that have evolved in this species represents an important case study for the evolution of insecticide resistance and also rapid adaptive change in insects more generally. In this review we summarise the biochemical and molecular mechanisms underlying resistance in M. persicae and the insights study of this topic has provided on how resistance evolves, the selectivity of insecticides, and the link between resistance and host plant adaptation.
Title: Pharmacology of the nicotinic acetylcholine receptor from fetal rat muscle expressed in Xenopus oocytes Cooper JC, Gutbrod O, Witzemann V, Methfessel C Ref: European Journal of Pharmacology, 309:287, 1996 : PubMed
The fetal rat muscle nicotinic acetylcholine receptor was expressed in Xenopus oocytes. Using the voltage-clamp technique, the response to a range of agonists was measured, listed in order of (decreasing) activity efficacy: anatoxin > or = epibatidine > acetylcholine > DMPP (1,1-dimethyl-4-phenylpiperazinium) > > cytisine > pyrantel > nicotine > coniine > tubocurare > lobeline. The agonist responses were compared with the steric and electrostatic properties of the molecules, using molecular modelling. Single-channel current were measured in outside-out patches for acetylcholine, nicotine, cytisine, anatoxin and epibatidine. The conductance of the single channels was independent of the type of agonist. The mean open times were characteristic of the agonist applied. Tubocurare, better known for its antagonist properties, was also a partial agonist. Single-channel currents were also observed for tubocurare, and for methyllycaconitine in patches with a very high density of the muscle nicotinic acetylcholine receptor, and these were blocked by alpha-bungarotoxin. The agonist properties of physostigmine, galanthamine and their methyl derivatives were also investigated. The conductance of the channels observed in outside-out patches was similar to that obtained for the classical agonists. The single-channel currents observed for physostigmine, galanthamine and their methyl derivatives were blocked by alpha-bungarotoxin, methyllycaconitine and mecamylamine, in contrast to previously reported studies on neuronal and adult muscle nicotinic acetylcholine receptors.
The alkaloids (-)physostigmine (Phy), galanthamine (Gal) and codeine (Cod), and several derivatives and homologous compounds, can act as noncompetitive agonists (NCA) of nicotinic acetylcholine receptors (nAChR) from Torpedo electrocytes, frog and mammalian muscle cells, clonal rat pheochromocytoma cells, cultured hippocampal neurons and several ectopic expression systems, by interacting with a binding site on the alpha-subunits of these nAChRs that is insensitive to the natural transmitter, acetylcholine (ACh), and ACh-competitive agonists and antagonists. Several endogenous ligands, including opioid-type compounds, can also act via this site, albeit at higher concentrations than is typical for the interaction with their cognate receptors. The NCA-evoked responses can be observed at the single-channel level but they do not summate to significant macroscopic currents, suggesting that the major role of NCAs is to act as "co-agonists", thereby potentiating nAChR channel activation by the natural transmitter. In more general terms, noncompetitive agonists may constitute part of a "chemical network", by which intercellular messengers, in addition to serving their cognate receptors, could modulate the sensitivity of other neuroreceptors to their archetypic ligands. Such a mode of action would make centrally acting NCAs interesting candidate drugs in the treatment of neuro-degenerative diseases.
The acetylcholine esterase inhibitor (-)-physostigmine has been shown to act as agonist on nicotinic acetylcholine receptors from muscle and brain, by binding to sites on the alpha-polypeptide that are distinct from those for the natural transmitter acetylcholine (Schrder et al., 1994). In the present report we show that (-)-physostigmine, galanthamine, and the morphine derivative codeine activate single-channel currents in outside-out patches excised from clonal rat pheochromocytoma (PC12) cells. Although several lines of evidence demonstrate that the three alkaloids act on the same channels as acetylcholine, the competitive nicotinic antagonist methyllycaconitine only inhibited channel activation by acetylcholine but not by (-)-physostigmine, galanthamine or codeine. In contrast, the monoclonal antibody FK1, which competitively inhibits (-)-physostigmine binding to nicotinic acetylcholine receptors, did not affect channel activation by acetylcholine but inhibited activation by (-)-physostigmine, galanthamine and codeine. The three alkaloids therefore act via binding sites distinct from those for acetylcholine, in a 'noncompetitive' fashion. The potency of (-)-physostigmine and related compounds to act as a noncompetitive agonist is unrelated to the level of acetylcholine esterase inhibition induced by these drugs. (-)-Physostigmine, galanthamine and codeine do not evoke sizable whole-cell currents, which is due to the combined effects of low open-channel probability, slow onset and slow inactivation of response. In contrast, they sensitize PC12 cell nicotinic receptors in their submaximal response to acetylcholine. While the abundance of nicotinic acetylcholine receptor isoforms expressed in PC12 cells excludes identification of specific nicotinic acetylcholine receptor subtypes that interact with noncompetitive agonists, the identical patterns of single-channel current amplitudes observed with acetylcholine and with noncompetitive agonists suggested that all PC12 cell nicotinic acetylcholine receptor subtypes that respond to acetylcholine also respond to noncompetitive agonist. The action of noncompetitive agonists therefore seems to be highly conserved between nicotinic acetylcholine receptor subtypes, in agreement with the high level of structural conservation in the sequence region harboring major elements of this site.
