Inhibition of Neuronal Nitric Oxide Synthase and Soluble Guanylate Cyclase Prevents Depression-Like Behaviour in Rats Exposed to Chronic Unpredictable Mild Stress Yusufhan Yazir 1 , Tijen Utkan 2 and Feyza Aricioglu 3 1 Department of Histology and Embryology, Faculty of Medicine, Kocaeli University, Kocaeli, Turkey, 2 Department of Pharmacology and Experimental Medical Research and Application Unit, Faculty of Medicine, Kocaeli University, Kocaeli, Turkey and 3 Department of Pharmacology and Psychopharmacology Research Unit, Faculty of Pharmacy, Marmara University, Istanbul, Turkey (Received 17 October 2011; Accepted 27 February 2012) Abstract: Depression is the most common psychiatric disorder. It is well established that endogenous nitric oxide (NO) contrib- utes to chronic unpredictable mild stress (CUMS)-induced depression. The aim of this study was to investigate brain-derived neurotropic factor (BDNF) expression in CUMS-induced depression-like behaviour in rats. Rats were exposed to CUMS for 5 weeks. A specific and selective nNOS inhibitor, 3-bromo-7-nitroindazole (3-Br-7-NI; 20 mg/kg/day, i.p.), and a specific soluble guanylate cyclase (sGC) inhibitor, 1H-(1,2,4)oxadiazolo(4,3-a)quinoxalin-1-one (ODQ; 10 mg/kg/day, i.p.), were administered during CUMS. The forced swimming test (FST) was used to assess despair and sucrose consumption, and sucrose preference test was used to assess anhedonia that are the main symptoms of the depression. We show that both 3-Br-7-NI and ODQ administra- tion during CUMS suppressed CUMS-induced, depression-like behavioural changes, including reduced sucrose preference, body-weight and locomotor activity as well as increased immobility time in the FST. CUMS also significantly decreased BDNF protein levels in the CA1 and CA3 regions of the hippocampus, which was reversed by 3-Br-7-NI and ODQ administration. Our findings suggest a novel role for nNOS and sGC-cGMP in the development of the CUMS model of depression. Introduction Major depressive disorder is a common and chronic mental illness. It is important to understand the neurobiology of this illness to provide better therapeutic strategies [1]. Preclinical studies have shown that depending on the species and the stressors, acute and chronic stress produces structural and functional changes in the brain [2,3]. The chronic unpredict- able mild stress (CUMS) model of depression was developed in an attempt to mimic some of the environmental factors con- tributing to the induction of depressive disorders in human beings [4–6]. Furthermore, Willner et al. [7] described an experimental model in which rats were exposed for several weeks to a variety of mild and unpredictable stressors. The model was initially designed to mimic anhedonia, a core symptom of clinical depression. Brain-derived neurotrophic factor (BDNF) is involved in synaptic plasticity, neuronal circuit formation and neuronal sur- vival [8]. Knockdown of BDNF expression in the dentate gyrus (DG) induced depression-like behaviours in animals [9,10], and selective loss of BDNF in the DG attenuated the efficacy of antidepressants in the forced swimming test (FST) [11]. Also, the CUMS procedure significantly decreased BDNF levels in the DG, CA1 and CA3 regions of the hippocampus in rats [12]. At least three distinct isoforms of nitric oxide synthase (NOS) have been cloned and located: endothelial NOS (eNOS), neuronal NOS (nNOS) and inducible NOS (iNOS) [13]. Nitric oxide (NO) has been suggested to have multiple targets, among which soluble guanylate cyclase (sGC) is the most extensively characterized. sGC converts guanosine tri- phosphate (GTP) to the important intracellular messenger cyc- lic guanosine monophosphate (cGMP) [14]. Moreover, NO has been implicated in the regulation of various behavioural, cog- nitive and emotional processes, for example, learning [15], locomotion [16], anxiety [17] and depression [18,19]. Previous findings have shown antidepressant-like effects of chemically distinct NOS and GC inhibitors under physiological conditions in the FST in animals [20–22]. Recent studies provide further data to suggest the involvement of NO in chronic stress- induced, depression-like behaviour in the FST. Although the inhibition of eNOS [23], nNOS [24] and iNOS [25] prevented the development of depression-like behaviour induced by chronic stress in rodents, it has not been clearly shown whether the effect of NO in the FST is mediated through changes in cGMP. This study was designed to investigate whether the effect of NO inhibition in CUMS-induced depression could be mediated through a subsequent decrease in cGMP by a direct inhibition of the NO-sGC-cGMP pathway using the NO-sensitive inhibitor of sGC, 1H-(1,2,4)oxadiazolo(4,3-a)quinoxalin-1-one (ODQ), and a direct and highly selective inhibition of nNOS using 3-bromo-7-nitroindazole (3-Br-7-NI). Materials and Methods Animals. Adult male Wistar rats (Kocaeli University, Experimental Medical Research and Application Center, Turkey) weighing 250–300 g Author for correspondence: Yusufhan Yazir, Department of Histology and Emryology, Medical School, Kocaeli University, 41380 Kocaeli, Turkey (fax + 90 262 3037003, e-mail yusufhanyazir@yahoo.com). © 2012 The Authors Basic & Clinical Pharmacology & Toxicology © 2012 Nordic Pharmacological Society Basic & Clinical Pharmacology & Toxicology, 2012, 111, 154–160 Doi: 10.1111/j.1742-7843.2012.00877.x