Investigating a pharmacological agent against tobacco smoke addiction using the rat as animal model / Linda Nel
It is widely accepted that the majority of people who smoke tobacco do so to experience the psychopharmacological effects of the nicotine present in the smoke. Drugs of abuse activate the brain area called the nucleus accumbens (Nacc), which is putatively the brain's "reward centre" or "pleasure centre" (Melichar et al., 2001 and Balfour & Fagerstrom, 1996). A shared feature of drugs of abuse is their ability to increase dopamine neurotransmission in the brain. Because the underlying mechanism of many addictive drugs is thought to be similar, we hypothesized that nicotinamide adenine dinucleotide (NAD), which is currently being used in the treatment of alcoholism, may also be effective to terminate the craving for tobacco. The purpose of this study was to find appropriate methods to determine withdrawal symptoms in tobacco smoke addiction in a rat model and to determine if NAD would terminate the craving for tobacco. Two methods were used in this study to determine withdrawal symptoms: locomotor activity and acoustic startle response (ASR). Locomotor activity is widely used to study nicotine's behavioural actions in rodents and the dopamine activation produced by nicotine is associated with elevated locomotor activity in rats. The acoustic startle response (ASR) is a reflex response and consists of a reflex contraction of the skeletal musculature in response to an intense, abrupt stimulus. A special device was designed to smoke cigarettes and to trap compounds that are usually inhaled by smokers. Rats were exposed to the smoke extract via subcutaneously implanted Alzet osmotic minipumps for 28 days to accomplish addiction. On day 28 the minipumps were removed and the experimental group was injected with NAD (the control rats were injected with saline) for four days during which time the ASR of the rats was measured. The locomotor activity of the rats was monitored on specific days throughout the experiment with a Digiscan Animal Activity Monitor. All the rats were sacrificed on day 42 and their brains removed. The concentrations of dopamine, serotonin and their metabolites were determined in the nucleus accumbens by HPLC-analysis using an electrochemical detector. To determine DNA damage, the Single cell gel electrophoresis (Comet) assay was performed on the striata of all the rats. The rats that received tobacco smoke extract and injected with NAD displayed an increase in locomotor activity after the osmotic minipumps were removed when compared to the control group (received tobacco smoke extract) injected with saline after removal of the minipumps, indicating that the control group was experiencing withdrawal symptoms. The results of the ASR experiments also showed that the NAD treated group experienced higher startle levels than the saline treated group, indicating that the treated group experienced less craving than the control group. According to the comet assay, the treated rats had more DNA damage than the control rats. This might he the result of the higher levels of NAD that activates the electron transport chain, causing a release of electrons (hydroxy-radicals) which may cause more damage to the brain cells. The locomotor activity and acoustic startle response recorded during smoking, withdrawal and treatment can be used as parameters for addiction to tobacco smoke and to test novel drugs for their potential to cure addiction. The above results indicate that NAD shows definite potential for treating tobacco smoke addiction.
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