1. Introduction
Cigarette smoke contains approximately 8000 toxic chemicals including nicotine, carbon monoxide, heavy metals, hydrogen cyanide, and polycyclic aromatic hydrocarbons [1] and [2]. Smoking is a global health problem. In the US, approximately 42 million adults smoke cigarettes (see http://www.cdc.gov/) and in EU countries 15–55% of the adults smoke cigarettes depending on their gender and country of origin [3]. While self-reports suggest that smoking tobacco can alleviate negative affective states or cognitive impairments [4], others report abnormal hippocampal neurochemistry in smokers [5].
The hippocampal dentate gyrus has a unique regenerative capacity as it is one of the brain areas where a remarkable number of neurons are produced every day. It has been shown that these adult–born neurons of the dentate are essential for normal cognitive functioning and memory formation [6] and [7]. Deficits in the generation and function of adult–born neurons have been suggested to contribute to the pathophysiology of various neuropsychiatric disorders including drug seeking and addiction [8] and [9]. These ideas are based on the observations that different types of drugs of abuse, including nicotine, can influence the generation of neurons in the dentate.
The first reports investigating the effect of nicotine on adult hippocampal neurogenesis used intravenous self-administration in rats and demonstrated an inhibitory effect, but only for high doses [10]. This has been confirmed by others who used different application protocols for nicotine, but again only high doses had inhibitory effect [11]. However, the effects of nicotine injections might be significantly different compared to the situation when animals are exposed to tobacco smoke. In a well-designed study, Bruijnzeel et al. [12] exposed adolescent rats to tobacco smoke for 4 h/day for 14 days and investigated hippocampal cell proliferation as well as the survival rate of the newly formed neurons by 5-bromo-2′-deoxyuridine (BrdU) immunohistochemistry. They found that tobacco smoke exposure decreased both the number of dividing progenitor cells and the number of surviving new cells [12]. They used the whole body smoke exposure model, and they subjected the rats to a rather high dose of tobacco smoke, i.e., after a 3-day initiation period with increasing dosage, from experimental day-4 onwards, they exposed the animals to 15 cigarettes/h for 4 h every day [12]. This means that the animals were exposed to the smoke of 60 cigarettes per day, for 11 days.
Here we also employed the whole body smoke exposure model [13] that is ideal for investigating smoking intensity- and duration-dependent changes simultaneously in different organ systems. In this model, we subjected adult mice to a low intensity smoke exposure, but over long time periods. In this model, we analyzed cell proliferation in the dentate gyrus using the exogenous marker BrdU [14] and quantified the population of immature neurons after labeling them with doublecortin (DCX) for immunohistochemistry [15] and [16].