A Differential Role for Neuropeptides in Acute and Chronic Adaptive Responses to Alcohol: Behavioural and Genetic Analysis in Caenorhabditis elegans

Abstract

Prolonged alcohol consumption in humans followed by abstinence precipitates a withdrawal syndrome consisting of anxiety, agitation and in severe cases, seizures. Withdrawal is relieved by a low dose of alcohol, a negative reinforcement that contributes to alcohol dependency. This phenomenon of ‘withdrawal relief’ provides evidence of an ethanol-induced adaptation which resets the balance of signalling in neural circuits. We have used this as a criterion to distinguish between direct and indirect ethanol-induced adaptive behavioural responses in C. elegans with the goal of investigating the genetic basis of ethanol-induced neural plasticity. The paradigm employs a ‘food race assay’ which tests sensorimotor performance of animals acutely and chronically treated with ethanol. We describe a multifaceted C. elegans ‘withdrawal syndrome’. One feature, decrease reversal frequency is not relieved by a low dose of ethanol and most likely results from an indirect adaptation to ethanol caused by inhibition of feeding and a food-deprived behavioural state. However another aspect, an aberrant behaviour consisting of spontaneous deep body bends, did show withdrawal relief and therefore we suggest this is the expression of ethanol-induced plasticity. The potassium channel, slo-1, which is a candidate ethanol effector in C. elegans, is not required for the responses described here. However a mutant deficient in neuropeptides, egl-3, is resistant to withdrawal (although it still exhibits acute responses to ethanol). This dependence on neuropeptides does not involve the NPY-like receptor npr-1, previously implicated in C. elegans ethanol withdrawal. Therefore other neuropeptide pathways mediate this effect. These data resonate with mammalian studies which report involvement of a number of neuropeptides in chronic responses to alcohol including corticotrophin-releasing-factor (CRF), opioids, tachykinins as well as NPY. This suggests an evolutionarily conserved role for neuropeptides in ethanol-induced plasticity and opens the way for a genetic analysis of the effects of alcohol on a simple model system.

The Genetics of Alcohol Responses of Invertebrate Model Systems

“…Animals in withdrawal also displayed a unique body posture with exaggerated bends that is not seen in naïve or ethanol-exposed animals. Both phenotypes could be rescued with a low dose of ethanol (Mitchell et al, 2010).…”

“…Mutants in this gene have normal acute ethanol responses but reduced withdrawal responses (Mitchell et al, 2010).…”

Meeting report: 2010 Caenorhabditis elegans Neurobiology Meeting, University of Wisconsin, USA

“…EGL-3 is a pre-peptide processing enzyme. Egl-3 mutants failed to show ethanol withdrawal behaviour (Mitchell et al 2010). Further work will focus on defining which parts of the neural network are modulated by the neuroadaptive, peptide-dependent responsedefining chronic ethanol behaviour.…”

A Novel Cholinergic Action of Alcohol and the Development of Tolerance to That Effect inCaenorhabditis elegans

“…Acute functional tolerance, which is unrelated to metabolism of the drug, develops rapidly to the effects of ethanol on the speed of locomotion (Davies et al 2004;Bettinger et al 2012;Raabe et al 2014). Longer ethanol exposures followed by removal from the drug can lead to withdrawal-related phenotypes (Davies et al 2004;Mitchell et al 2010). Therefore, these drug-induced behavioral effects are indicative of ethanol having activating and depressing actions in C. elegans as it does in mammals and demonstrate that tolerance can develop to acute and long-term ethanol exposures.…”