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Title: Neurophysical effects of essential oil constituents in insects and molluscs : do they target octopamine receptors?
Author: Price, David Neil
Awarding Body: University of Wales, Swansea
Current Institution: Swansea University
Date of Award: 2006
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Essential oils and their constituents are potential pesticides whose mechanism of action is unknown. Some, such as eugenol, are claimed to work at micromolar doses in insects by activation of neuronal octopamine receptors, though there is no physiological evidence. The actions of eugenol, citral and geraniol on the neurophysiology of cockroaches and snails were examined, focusing on comparisons with octopamine. Cockroaches were killed by topical application or intra-abdominal injection of the oils. Minimum lethal injection doses indicated haemolymph concentrations exceeding 5mM. Eugenol depressed impulse activity recorded extracellularly in the abdominal nerve cord whereas citral and geraniol produced biphasic effects (low-dose excitation). Similar effects on spiking occurred in dorsal unpaired median (DUM) neurons, recorded intracellularly in the isolated terminal abdominal ganglion. The oils reduced spike undershoot and decreased excitability of depolarized DUM neurons, and eugenol induced plateau potentials. Octopamine did not reduce spike undershoot or produce plateau potentials; it had opposing effects to eugenol on DUM neurons and foregut activity, and its effects on DUM neurons were not blocked by eugenol. Thus eugenol did not activate or block octopamine receptors. Snails were killed by the oils dissolved in the aquarium water (ca. 5 x Krtvl). Citral and geraniol mimicked octopamine in activating rhythmic movements of the buccal mass and initiating burst activity in intracellularly recorded neurons in isolated buccal ganglia (fictive feeding). Eugenol generally reduced spike activity, acting like a local anaesthetic. Octopamine, applied by perfusion or iontophoresis, hyperpolarized an identified giant neuron, whereas the oils excited it. Metoclopramide blocked excitatory responses to octopamine but not to the oils. Thus the oils did not appear to activate either depolarizing or hyperpolarizing octopamine receptors. Below 10'3M the oils did not block octopamine (or dopamine) receptors, or affect electrotonic coupling, though acetylcholine receptors were blocked at 5 x lO^M. The results indicate no specific targeting by essential oils of octopamine receptors in insects or snails, and the high doses required to produce effects suggest that a range of non-specific factors could contribute to their pesticidal actions.
Supervisor: Not available Sponsor: Not available
Qualification Name: Thesis (Ph.D.) Qualification Level: Doctoral
EThOS ID:  DOI: Not available