**1. Introduction**

3,3- -Diindolylmethane (DIM) is a metabolite of indole-3-carbinol, which is present in Brassicaceae vegetables containing glucosinolates (206–3895 mg/kg) [1]. Hydrolysis products of glucosinolates, such as isothiocyanates and indol-3-carbinol, are well known for exerting health-promoting effects, including for neurodegenerative diseases [2]. DIM, a dimer of indole-3-carbinol that is converted in

the acidic conditions of stomach after intake [3], is known to possess beneficial effects especially in antioxidant [4], anti-cancer [5], and hepatoprotective actions [6]. Recently, it was reported that DIM could protect neuronal cells against inflammation [7,8] and ischemia [7,8] in brain tissue. Moreover, DIM at the doses used did not show any serious side effects in healthy volunteers [9]. Nonetheless, the effect of DIM on neurodegenerative diseases has not been elucidated clearly.

Neurodegenerative diseases, which are increasing along with the extension of life spans and the occurrence of metabolic diseases, are known to be associated with the generation of reactive oxygen species (ROS) [10], which are responsible for the apoptosis of neuronal cells [11,12]. Therefore, the regulation of ROS generation is suggested as a key target for the treatment or prevention of neurodegenerative diseases. Brain-derived neurotrophic factor (BDNF), which is mainly produced from the central nervous system [13–15] and is concerned with the preservation of neuronal cells in in vivo systems, is known to play a crucial role [16]; it participates in the proliferation and differentiation of neuronal cells through the activation of tropomyosin-related kinase receptor B (TrkB) [17]. In addition, BDNF protects neuronal cells against oxidative stress [16,18]. Recently, it was observed that some specific antioxidant compounds ameliorate scopolamine-induced memory impairment by promoting BDNF production and suppressing oxidative damage in scopolamine-exposed mice [11,12,19]. Therefore, the regulation of BDNF formation may be a key point for the treatment or prevention of neurodegenerative diseases such as Alzheimer's disease.

Currently, inhibitors of acetylcholinesterase (AChE) have been used for treatment of Alzheimer's disease [20]. However, these inhibitors only delay the progression of Alzheimer's disease and also produce side effects [20]. In this respect, phytochemicals derived from vegetables may be preferable alternatives for the prevention or treatment of neurodegenerative diseases such as Alzheimer's disease.

In this study, we investigated whether DIM was able to protect hippocampal neuronal cells against oxidative stress-induced apoptosis, and then we examined how DIM protects neuronal cells against oxidative stress-induced apoptosis in both in vitro and in vivo models. Herein, we report that the neuroprotective and antioxidant properties of DIM are closely associated with the production of BDNF and antioxidant enzymes by activation of the pathways involving TrkB/protein kinase B (Akt) in hippocampal neuronal cells. Such an effect of DIM may help to address the neuroprotective action of indole-3-carbinol and some vegetables of the brassicaceae family and may provide information for the application of DIM as a therapeutic or preventive supplement for neurodegenerative diseases.
