Introduction
Neurogenesis is a process within neuronal plasticity, the capacity of the brain to reorganize its structure, function, and connections in response to extrinsic or intrinsic stimuli (Baptista & Andrade, 2018). In this regard, new neurons are produced in the brain via a manner known as adult hippocampal neurogenesis (AHN). Among the neurogenic areas studied in the human brain, hippocampal formation emerges as crucial, exerting a noteworthy role in brain function (Spalding et al., 2013; Toda, Parylak, Linker, & Gage, 2019). In the dentate gyrus (DG) of the hippocampus is located the neurogenic niche, aimed to maintain AHN and implicated in many processes under normal physiological conditions during adulthood. In fact, AHN is demonstrated to be preserved throughout human life, even until the ninth decade of life, being essential to keep related physiological functions (Boldrini et al., 2018; Moreno-Jimenez et al., 2019; Terreros-Roncal et al., 2022). Furthermore, a functional separation has been demonstrated in the hippocampus, where stress and modulation of emotional behavior is processed by the anterior portion in humans (ventral, in rodents) while cognitive functions and memory is related to the posterior part (dorsal, in rodents) (Fanselow & Dong, 2010; Kheirbek et al., 2013; Tanti & Belzung, 2013).
Intriguingly, dysregulation of AHN is related to several brain disorders, such as major depressive disorder (MDD), age-dependent cognitive decline, Alzheimer’s disease (AD), amyotrophic lateral sclerosis, Huntington’s disease, Parkinson’s disease, dementia with Lewy bodies, and frontotemporal dementia (I. B. Kim & Park, 2021; Martos, Tuka, Tanaka, Vecsei, & Telegdy, 2022; Moreno-Jimenez et al., 2019; Terreros-Roncal et al., 2022). MDD is one of the prominent mental health conditions in the world with more than 300 million people diagnosed. MDD is defined by a collection of behavioral, emotional and cognitive symptoms, and confer a challenge for the medical community by increasing the risk of suicidality and death. Suicide is considered the worst outcome or consequence of MDD, over 700,000 human lives lost every year (Elias, Zhang, & Manners, 2022). Moreover, during the COVID-19 pandemic, multiple challenges have arisen, such as loneliness or financial hardship, producing about 34% prevalence of depression in general population, with 5–15% suicidal ideation in that period (Giner, Vera-Varela, de la Vega, Zelada, & Guija, 2022).
The majority of current antidepressants target monoamines, with remarkable shortcomings such as adverse events and delayed onset of efficacy (Harmer, Duman, & Cowen, 2017). Notably, studies determine that approximately 50% of such patients fail to respond and about 65% of them fail to achieve remission, which classifies these patients with treatment-resistant depression (TRD) (Chen, 2019). Recently, the discovery of ketamine as an effective antidepressant led to the authorization of a nasal spray form ofesketamine(Hashimoto, 2020). Esketamine provides fast-acting symptomatic relief in TRD patients, but the significant risks associated with esketamine limit its use for a broad patient population. Overall, nowadays MDD patients have no adequate treatment options, implicating that additional underlying mechanisms need to be considered in order to improve the efficacy of treatments. In recent decades, progress in the MDD field has been made possible in part through the use of rodent models (Planchez, Surget, & Belzung, 2019). Accordingly, boosting hippocampal neurogenesis in these patients emerges as a potential therapeutic approach (Colucci-D’Amato, Speranza, & Volpicelli, 2020; Miller & Hen, 2015). Critical components of AHN are cell proliferation, neuronal differentiation, and survival, strictly controlled by multiple intrinsic or extrinsic epigenetic factors that could promote or suppress neurogenesis (Toda et al., 2019). These factors confer a pivotal role in understanding the importance of adult neurogenesis on physiological and pathological conditions (Kempermann et al., 2018). In this way, the neurogenic-promoting effects of neurotransmitters/neuropeptides and neurotrophic factors are crucial regulators of neurogenic niche activities in health and disease (Kuhn, 2015).
Among them,neuropeptide Y (NPY) is one of the most abundant neuropeptides in the nervous system. NPY is a 36 amino acid polypeptide neurotransmitter highly conserved in mammals and involved in basic biological and pathophysiological functions, such as neuroendocrine secretions, mood regulation, feeding behavior, circadian rhythms, neuronal excitability, neuroplasticity and memory (Kormos & Gaszner, 2013; Zaben & Gray, 2013). Regarding hippocampal neurogenesis, a pro-neurogenic role of NPY on hippocampal stem cells has been evidenced both in vitro (Howell et al., 2003; Howell et al., 2007) and in vivo (Decressac et al., 2011; Geloso, Corvino, Di Maria, Marchese, & Michetti, 2015). In preclinical models, it was found reduced brain NPY in genetic and environmental models of MDD (Cohen, Vainer, Zeev, Zohar, & Mathe, 2018; Jimenez Vasquez, Salmi, Ahlenius, & Mathe, 2000; Jimenez-Vasquez, Overstreet, & Mathe, 2000). Reliable with the animal data, brain NPY is reduced in postmortem brains from MDD patients who committed suicide (Kautz, Charney, & Murrough, 2017; Sah & Geracioti, 2013). With regard to treatment effects, all antidepressant procedures tested preclinically to date increase brain NPY (Bjornebekk, Mathe, & Brene, 2006; Husum, Mikkelsen, Hogg, Mathe, & Mork, 2000). In line with these findings, transgenic rats overexpressing hippocampal NPY show decreased depression-like behaviors (Thorsell et al., 2000). Furthermore, recently was demonstrated that intranasal NPY and the NPYY1 receptor (Y1R) agonist administration had antidepressant effects in rodents (Nahvi et al., 2021; Serova, Mulhall, & Sabban, 2017) and in MDD patients (Mathe, Michaneck, Berg, Charney, & Murrough, 2020). In this respect, the Y1R have been proposed as a critical target by mediating dentate neurogenesis and antidepressant effects (Rana et al., 2022).
Brain-derived neurotrophic factor (BDNF) is a pivotal molecule involved in the neuroprotective effects of the antidepressants by regulating different neurogenic processes in the hippocampus (Castren & Kojima, 2017; Colucci-D’Amato et al., 2020; Miranda, Morici, Zanoni, & Bekinschtein, 2019). Moreover, a reduction in BDNF was reported in the hippocampus of post-mortem brain tissues of MDD and suicide victims (Dwivedi, 2012; Pandey et al., 2008) while injection of BDNF in the hippocampus reduces depression-like behavior in rodents (Hoshaw, Malberg, & Lucki, 2005). Besides, BDNF was significantly increased 24 h following treatment with NPY in the trimethyltin (TMT)-induced model of hippocampal neurodegeneration (Corvino et al., 2014).
We have demonstrated NPY andgalanin(GAL) interactions in different limbic system regions at molecular-, cellular-, and behavioral-specific levels (Mirchandani-Duque et al., 2022; Narvaez et al., 2016; Narvaez et al., 2018; Narvaez et al., 2015). GAL is broadly distributed in the central nervous system contributing to numerous physiological effects (Katsetos et al., 2001). Regarding hippocampal neurogenesis, theGAL2 receptor agonist,GAL 2–11 was involved in proliferative and trophic actions in vitro (Abbosh, Lawkowski, Zaben, & Gray, 2011). Concerning depression, GALR2 activation induced antidepressant effects in rodents and the increased GALR2 expression in the ventral hippocampus was related to antidepressant effects (Kuteeva et al., 2008; Luo et al., 2019). Moreover, GalR2-knockout mice displayed depression-like behaviors (Lu, Ross, Sanchez-Alavez, Zorrilla, & Bartfai, 2008). Recently, we described a facilitatory interaction between NPY and GAL through the Y1R-GALR2 heteroreceptor complexes formation. Intranasally combined GALR2 and Y1R agonists improved spatial memory performance related to increased cell proliferation in the dentate gyrus of the dorsal hippocampus (Borroto-Escuela et al., 2022).
The purpose of the current work was to assess the role of the NPY and GAL interaction in the neurogenic actions on the ventral hippocampus. Following GALR2 and Y1R agonists intranasal administration, we analyzed the ventral hippocampal activation and their proliferating actions through c-Fos expression and proliferating cell nuclear antigen (PCNA). To examine the associated cellular mechanism we assessed the expression of the brain-derived neurothrophic factor (BDNF) on the ventral hippocampal dentate gyrus (DG). Moreover, we studied the formation of Y1R-GALR2 heteroreceptor complexes with in situ proximity ligation assay (PLA) and analyzed morphological changes on hippocampal neuronal cells. Finally, the functional outcome of the NPY and GAL interaction on the ventral hippocampus was evaluated in the forced swimming test, employing the intranasal procedure that takes advantage of a direct nose-to-brain transport of therapeutics.