Nicotine (NC) is the substance which sustains the addictive use of tobacco, and tobacco results in numerous harmful health effects and continues to be the leading cause of preventable death [1, 2]. It has been reported that addicted tobacco users suffer from NC-induced cognitive impairments in some conditions of smoking, as well as modulated moods such as anxiety- and depression-related symptoms [3–5]. Cognitive impairments including deficits in working memory, a process for maintaining temporary active information , have been regarded as being among the representative symptoms of NC withdrawal observed in NC-dependent human and rodent models [3, 7, 8]. Furthermore, the direct neurotoxic effects of NC have also been reported depending on the treatment conditions such as dose, period and paradigm, and this neurotoxicity has been suggested to induce memory impairments, particularly at earlier periods in development [9–12]. However, in some clinical and experimental animal studies, cognitive improvements or absence of any effects have been demonstrated [13–17]. Negative, positive or no effects of NC have also been reported against the anxiety-related behaviors [18–20].
Working memory impairments have been reported for various stressors such as restraint stress (immobilization stress) in both humans and rodent models [21–23]. Like certain NC treatments, such stressors also induce and exacerbate the anxiety-like behavioral responses in rodent models [24, 25]. Furthermore, it has been suggested that brain regions such as the medial prefrontal cortex, for which NC-induced modulations have been demonstrated [26, 27], are concurrently involved in the development of stress-induced working memory impairments and anxiety [28–31]. However, there are only a few studies investigating the characteristic effects of NC as a stressor, particularly those on cognitive function [32, 33].
A considerable number of studies have implicated the relationship between NC and stress. For example, in some rodent models, repeated or acute stress has been shown to aggravate the behavioral and neuronal effects of NC [34–36]. Recent human studies have shown some directly-exacerbated mood symptoms induced by stress in smokers [37, 38]. However, against the behavioral and neuronal impairments caused by stress, antagonistic effects of subsequently administered NC have been shown in some rodent models [39–41]. With respect to cognitive function, NC has also been reported to block stress-induced impairments in several experimental conditions in rodents [41, 42]. Nevertheless, the above-mentioned neurotoxic effects of NC which could lead to cognitive dysfunction [10–12] may be correlated with the possibility that NC and stress augment each other’s unfavorable effects on cognitive function.
In previous studies, a strong involvement of brain cannabinoid (CB) receptors, typically CB type 1 (CB1) receptors, was reported in the representative emotion-related behaviors (anxiety- and depression-like behaviors) induced by NC [18, 43, 44] and stress  in rodents. This is consistent with the prominent behavioral alterations induced by NC in CB1 knockout mice , and the overlapping distribution of CB1 receptors and nicotinic acetylcholine receptors (nAChRs) in some brain regions which supports functional interactions between these receptors [47, 48]. Furthermore, recent reviews suggest that CB1 receptors contribute to deficits in memory including working memory by demonstrating that CB1 agonists impair memory formation and CB1 antagonists reverse these impairments [49, 50]. However, there have been a limited number of studies on the direct contribution of brain CB receptors to the memory-related effects of NC [51, 52]. The participation of CB1 receptors has also been reported in anxiety processes, but the roles of CB1 agonist are contradictory in that both anxiolytic-like and anxiogenic-like effects have been induced depending on the treatment conditions [53, 54]. Against the NC-induced anxiety-related behaviors, inconsistent and contradictory effects of CB1 agonists and other CB ligands have also been demonstrated [18, 43].
In the present study, using behavioral tests in mice (Y-maze and elevated plus-maze (EPM) test), the working memory- and anxiety-related behavioral alterations caused by NC were assessed and compared with those caused by immobilization stress (IM), a typical stressor. The interactions between the NC- and IM-induced behavioral effects were also examined. Furthermore, considering the possible involvement of brain CB receptors in these NC- and/or IM-induced memory- and anxiety-related behavioral alterations, the effects of selected CB ligands (the CB1 antagonist AM 251, the non-selective CB agonist CP 55,940, and the CB1 partial agonist/antagonist virodhamine) were evaluated against these behavioral alterations, as described in previous studies [43, 51, 52].