Abstract:
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.
