An analysis of ethanol-induced behavioural plasticity in Caenorhabditis elegans

Ethanol is one of the most widely used and socially acceptable drugs in the world.<br/>However its chronic use can lead to serious problems including the development of<br/>dependence. Alcohol dependence is a chronic, relapsing disorder characterised by<br/>tolerance, withdrawal, preoccupation with obtaining alcohol, loss of control over its<br/>consumption and impairment in social and occupational functioning. In humans this<br/>develops over years, primarily driven by adaptations in many distinct signalling<br/>pathways and neural circuits as a result of continued heavy drinking. Whilst alcohol<br/>dependence has been extensively studied our understanding of how its distinct targets<br/>integrate to produce various behavioural responses remains far from clear.<br/><br/>The nematode worm Caenorhabditis elegans is a model genetic organism with a<br/>simple nervous system and well-defined behaviour. These nematodes can display<br/>plasticity in the form of tolerance to, and withdrawal from, 5-HT or nicotine. They are<br/>thus a genetically tractable system in which to investigate the neural substrates of<br/>adaptive responses to ethanol. In this simple system the impact of changes at the<br/>molecular level on signalling in defined neural circuits and the resultant animal<br/>behaviour can be investigated. The aims of this thesis were to establish a C. elegans<br/>paradigm for alcohol dependence and to use this to define the genetic basis of the<br/>ethanol-dependent behaviours of intoxication, tolerance and withdrawal.<br/><br/>Evidence was provided that ethanol equilibrates rapidly across the worm cuticle<br/>indicating that the internal concentration closely approximates to the external<br/>concentration in which the animal is placed. Ethanol-dependent behaviours were<br/>carefully characterised using a variety of behavioural assays. C. elegans exhibit<br/>distinct behavioural states, corresponding to intoxication and withdrawal, which<br/>impair the ability to navigate towards food. Visual and automated analysis defined a<br/>sub-behaviour, an increased tendency to form spontaneous deep body bends, which<br/>was specifically associated with withdrawal. This was ameliorated by a low dose of<br/>alcohol supporting the contention that it arises from ethanol-induced neuroadaptation.<br/><br/>A series of loss of function mutants, were analysed for alterations in ethanoldependent<br/>behaviour. The absence of withdrawal in a strain of worms depleted in<br/>neuropeptides (egl-3) demonstrated that peptidergic signalling is key to the chronic<br/>adaption to, but not to the acute effects of, ethanol. However the neuropeptide<br/>receptor NPR-1, previously shown to impact on ethanol responses in C. elegans, had<br/>no effect on withdrawal behaviour in these assays. Alterations in intoxication and<br/>withdrawal behaviour in strains of worms depleted in 5-HT (tph-1) and dopamine<br/>(cat-2) indicated that serotonergic and dopaminergic signalling may also be involved<br/>in the ethanol response in C. elegans. This study has therefore provided a quantitative<br/>analysis of distinct ethanol-induced behavioural states and highlighted a role for<br/>neuropeptides and major classes of neuromodulatory transmitters. In particular this<br/>data is consistent with the emerging role of neuropeptides in ethanol withdrawal.<br/>

Name dc:type.qualificationname

Ph.D.

Level dc:type.qualificationlevel

doctoral

Grantor dc:publisher.institution

University of Southampton

Year dc:date.issued

2009

Author dc:creator

• Mitchell, Philippa Helen

Advisors dc:contributor.advisor

• Holden Dye, L.M.
• O'Connor, V.