Nicotine dependence may result even from passive smoking, a new study on rats has shown. The exposure to tobacco smoke induces brain changes indicative of nicotine dependence.
Their findings of how rats' brains respond to exposure to tobacco smoke have implications for the study of the effects of tobacco smoke on the human brain even from passive exposure to other smokers, and for future studies testing new treatments for tobacco addiction.
Nicotine as well as many other compounds in tobacco smoke act together on the brain reward system and are addicting in smokers, but the effects of passive exposure have not been studied.
In a set of four experiments on male Wistar rats, Dr. Adrie Bruijnzeel and colleagues from the University of Florida in the US investigated whether rats exposed passively to tobacco smoke would become dependent on nicotine.
They specifically looked at how the rats' brains responded to being exposed to tobacco smoke and whether the rats displayed withdrawal symptoms.
For all the experiments, freely moving rats were chronically exposed to tobacco smoke for a few hours per day.
In the first experiment, the rats were fitted with an intracranial probe to measure the emotional aspects of tobacco withdrawal.
The second experiment looked at whether being exposed to tobacco smoke decreased the rats' self-administration of nicotine.
The third experiment investigated whether rats exposed to tobacco smoke were less motivated to eat.
Finally, in the fourth experiment, the researchers looked specifically at the effects of tobacco smoke exposure on the brain's hippocampus, or grey matter - the area of the brain most sensitive to smoke and nicotine-induced changes.
The rats exposed to tobacco smoke showed both affective and physical withdrawal signs, as well as nicotine-induced changes in the hippocampus, which demonstrates that passive exposure to tobacco smoke exposure leads to nicotine dependence.
"These studies suggest that the rat tobacco smoke exposure model can be used to investigate the effects of tobacco smoke on the human brain and to evaluate the efficacy of novel treatments for tobacco addiction," the author said.
The study has been published online in Springer's journal Psychopharmacology.