Molecular, histological and behavioral evidences for neuroprotective effects of minocycline against nicotine-induced neurodegeneration and cognition impairment: Possible role of CREB-BDNF signaling pathway

Abstract

Aim

Neurodegeneration is one of the serious adverse effects of stimulant agents such as nicotine. Minocycline possess established neuroprotective properties. The role of CREB-BDNF signaling pathway in mediating the neuroprotective effects of minocycline against nicotine-induced neurodegeneration in rats was evaluated in current study.

Methods

Seventy adult male rats were divided randomly into seven groups. Group 1 and 2, received 0.7 ml/rat of normal saline (i.p) and nicotine (10 mg/kg, s.c) respectively. Groups 3, 4, 5 and 6, treated concurrently with nicotine (10 mg/kg) and minocycline (10, 20, 30 and 40 mg/kg, i.p, respectively) for 21 days. Group 7 received minocycline alone (40 mg/kg, i.p) for 21 days. From 17th to 21 st days of experiment, Morris water maze (MWM) was used to evaluate learning and spatial memory in rats treated in different groups. According to the critical role of hippocampus in cognitive behavior, hippocampal neurodegenerative parameters (oxidative stress and inflammatory biomarkers) and also cyclic AMP response element binding protein (CREB) and brain-derived neurotrophic factor (BDNF) levels were evaluated in isolated hippocampus in day 22 of experiment and after drug treatment. Also hippocampal cell density and tissue changes were evaluated by hematoxylin and eosin staining.

Result

Nicotine administration impaired the learning and spatial memory in rats and simultaneous treatment with various doses of minocycline attenuated the nicotine-induced cognition disturbances. In addition, nicotine treatment increased lipid peroxidation and the levels of oxidized form of glutathione (GSSG), interleukin 1 beta (IL-1β), tumor necrosis factor alpha (TNF-α), and Bax protein, while decreasing reduced form of glutathione (GSH), Bcl-2 protein, P-CREB and BDNF levels in the hippocampus of experimental animals. Nicotine also reduced the activity of superoxide dismutase (SOD), glutathione peroxidase (GPx) and glutathione reductase (GR) in the hippocampus. Minocycline attenuated nicotine-induced neurodegeneration and elevating CREB (both forms) and BDNF levels. Also minocycline treatment alone increases the cognitive activity and increased CREB (both forms) and BDNF levels and decreased oxidative stress, inflammation and apoptotic biomarkers. Minocycline at high doses cause inhibition of nicotine induced cell density and changes in both area of dentate gyrus (DG) and CA1 in hippocampus.

Conclusion

It can be concluded that minocycline, probably through activation of P-CREB/BDNF signaling pathway, confers neuroprotection against nicotine-induced neurodegeneration in rat hippocampus.

Introduction

Nicotine is a stimulant mediator with neurodegenerative properties [1,2]. Due to its pharmacological properties, its uses have been increased during recent years [[3], [4], [5]]. The biochemical and behavioral effects of chronic administration of nicotine remain unclear [6,7]. Animal studies have shown that nicotine in high doses induce anxiety, depression and cognitive impairment in animal models [4,[8], [9], [10]]. This may be through nicotine induced neurodegeneration in some area of the brain such as the amygdala and hippocampus which is confirmed by experimental studies [11,12]. According to molecular studies, nicotine administration can lead to DNA fragmentation and production of apoptotic proteins such as Bax and caspase family in brain areas [[13], [14], [15]]. Also nicotine administration can cause inflammation, oxidative stress and mitochondrial dysfunction in brain areas [14,16]. Detailed mechanisms and involved signaling pathways in nicotine induced neurodegeneration remains unclear [15,17]. Many previous studies demonstrated that cigarette smoking and nicotine abuse cause some inflammatory and oxidative stress events in human brain and cause neurodegenerative conditions [[18], [19], [20], [21], [22]]. Delightfully, the uses of new neuroprotective compounds have increased during recent years [23,24]. Minocycline, a broad-spectrum and long-acting antibiotic with anti-inflammatory and antioxidant effects [23,24], is extensively considered as therapeutic agent against neurodegenerative events [25,26]. The antioxidant, anti-inflammatory and anti-apoptotic effects of minocycline were approved in former studies [27,28]. It is also proved to have neural protective effects [27,29]. Minocycline has possible antidepressant and cognitive enhancer properties in animal and human [[30], [31], [32]]. All these properties may contribute to therapeutic potential of minocycline in neurodegenerative disorders in drug abusers [25]. On the other hand, cyclic AMP response element binding protein (CREB) by regulation of some gene expression, such as brain-derived neurotrophic factor (BDNF) plays a critical role in synaptic and survival of neurons [[33], [34], [35]]. CREB/BDNF signaling pathway supports the growth and survival of neurons. This pathway regulates neural growth, cognition, emotions, and reward activities [[36], [37], [38]]. Because of importance of P-CREB/BDNF pathway in conferring neuroprotection and cognitive functions, this study was designed to assess the role of this pathway in mediating minocycline-induced neuroprotection against nicotine induced oxidative stress, inflammation, apoptosis and cognitive disturbances. This study is of particular importance as it provides insight into the disturbances associated with nicotine administration and neuroprotective action of minocycline against these toxicities.