With its high rates of morbidity, depression is a debilitating mental disorder that poses a serious threat to human health as well as serious social and economic issues [1], [2]. Despite significant research efforts, the exact diagnosis and treatment of depression remain difficult since the mechanism behind depression disease is still unknown due to its complexity [3]. The current depression diagnosis is mostly based on subjective symptoms that might be mistaken for a variety of other psychological issues [4]. Increasing evidence indicated neurotransmitters [5], [6] and excessive reactive oxygen species (ROS) accumulation [7], [8], [9] are closely related to depression disorder. Nitric oxide (NO) as an important gasotransmitter is produced from l-arginine under the catalysis of NO synthase (NOS enzyme), which is involved in the pathophysiology of many neurological diseases [10], [11]. The primary source of NO in the central nervous system is mostly located in the hypothalamus, prefrontal lobe, and locus coeruleus, which are strongly linked to the control of stress responses and the onset of depression [12]. And NO affects the main neurotransmitters implicated in the neurobiology of depression, including glutamate, serotonin, dopamine, and norepinephrine. Studies have found patients with depression generate higher amounts of NO, indicating that nitric oxide levels are different in depressed individuals [13]. Reducing the level of nitric oxide in the brain or blocking its synthesis (blocking NOS) induces antidepressant-like effects [14]. It is commonly acknowledged that NO plays a crucial role as a mediator in the pathophysiology of depression. However, due to the lack of an effective tool, the exact relationship between NO and depression is unclear [15]. Therefore, it is crucial to provide precise and sensitive techniques for measuring and tracking NO levels in depression models.

Compared with other NO detection techniques such as electrochemical and electron paramagnetic resonance spectroscopy [16], [17], [18], fluorescence imaging has many advantages in terms of sensitivity, selectivity, spatiotemporal resolution and experimental feasibility [19], [20], [21]. In recent years, various types of fluorescent probes for NO detection have been created based on different sensing mechanisms such as transition metal complex [22], o-phenylenediamine [23], [24], dihydropyridine derivatives [25], [26] and monoamine [27]. Besides, Zhang et al. developed a coumarin derivative incorporating thiosemicarbazide moiety, which exhibits high specificity in detecting NO [28]. Song et al. devised a DCM-based fluorophore, incorporating 4-(4-nitrophenyl)-thiosemicarbazide for the recognition of NO in idiopathic pulmonary fibrosis [29]. However, their efficiency in the quickly variable NO response and the interference of background autofluorescence in vivo is limited by their slow response time, short excitation and emission wavelength, and small Stokes shift. In addition, due to the low concentration and rapid diffusion of NO, the development of organelle-targeting probes is conducive to the accurate detection of NO in real-time. Particular NO probes that target certain organelles including lysosome- and mitochondria [30], [31] have been developed. There are few reports of Golgi-targetable fluorescent probes for NO detection [32], despite the Golgi apparatus containing an amount of NOS engaged in NO formation. Therefore, it is imperative to develop a NIR fluorescence probe with a large Stokes shift to achieve a NO-specific imaging strategy in Golgi to elucidate the mechanism of nitric oxide in depression.

Herein, we design a NIR fluorescent probe TJ730-Golgi-NO, which consists of 4-(4-nitrophenyl)-thiosemicarbazide and a rhodamine fluorophore TJ730 [33]. The benzenesulfonamide group acts as a Golgi-targeting group [34]. Probe TJ730-Golgi-NO is essentially nonfluorescent due to its spirolactam structure. After reacting with NO, the fluorescence of the probe is turned on, which is attributed that the response group could be changed to an oxadiazole structure and lead to the ring opening of the spirolactam. Probe TJ730-Golgi-NO displays a fast response time (<1 min) and could monitor the NO concentration changes in vitro. The probe exhibits a large Stokes shift (158 nm) and achieves almost complete separation of its excitation and emission peaks. Moreover, TJ730-Golgi-NO is applied to detect the fluctuation of NO levels in PC12, Raw 264.7 cells, and depression-like mice models. These studies suggest that TJ730-Golgi-NO could be a powerful tool for detecting NO in Golgi and depression diagnosis.