Long-term cilostazol administration ameliorates memory decline in senescence-accelerated mouse prone 8 (SAMP8) through a dual effect on cAMP and blood-brain barrier
Introduction
Senescence-accelerated mice (SAM) are lines of mice originally developed from AKR/J mice having a common genetic background (Takeda et al., 1981). Currently, there are seven substrains of SAM: six senescence-accelerated mice prone (SAMP) and one senescence-accelerated mice resistant (SAMR) substrains. SAMPs are characterized by early onset of age-related phenotypic alternations, such as decreased activity, hair loss, dull hair, increased lordokyphosis, and shortened life span compared to control SAMR (Takeda et al., 1981, Takeda et al., 2013a). None of these phenotypic alterations has been successfully reversed yet (Takeda et al., 1981, Takeda et al., 2013a). Among the six substrains of SAMP, SAMP8 possess a distinct feature typical of early-onset memory impairment in several established behavioral tasks (Cheng et al., 2014, Flood and Morley, 1998, Miyamoto et al., 1986, Takeda et al., 1991, Takeda et al., 2013a). In addition, SAMP8 exhibit changes in some biological markers of aging, such as increased oxidative stress (Baltanás et al., 2013, Smith et al., 2013), impaired or diminished mitochondrial function (Eckert et al., 2013, Shi et al., 2010), and increased permeability and/or disruption of the blood-brain barrier (BBB; Hosokawa and Ueno, 1999, Pelegrí et al., 2007, Ueno et al., 1996, Ueno et al., 2001, Vorbrodt et al., 1995). As these changes are common phenomena observed in aged animals (Albers and Beal, 2000, Elahy et al., 2015, Paradies et al., 2011), SAMP8 has been used not only as a mouse model of cognitive aging but also as a model of aging in general.
Recently, there has been a growing interest in therapeutic interventions aimed at overcoming or delaying the onset of memory decline in the aging population (Cacabelos and Torrellas, 2014, Stella et al., 2015, Yanai and Endo, 2015). One of these interventions focuses on the cellular cascade triggered by activation of 3′, 5’-cyclic adenosine monophosphate (cAMP). The cAMP-protein kinase A (PKA)-cAMP response element-binding protein (CREB) signaling pathway mediates long-term neuronal plasticity that underlies learning and memory (Borlikova and Endo, 2009, Florian et al., 2006, Kandel, 2012, Ota et al., 2008). This is the case in invertebrates and mammals, including Aplysia (Bartsch et al., 1995, Goelet et al., 1986), Drosophila (Davis, 1996, Yin et al., 1995), and rodents (Kida, 2012, Sano et al., 2014). Phosphodiesterases (PDEs) are enzymes that hydrolyze cyclic nucleotides including cAMP and/or 3′, 5’-cyclic guanosine monophosphate (cGMP). PDE inhibitors enhance synaptic plasticity by elevating the concentration of intracellular cAMP (for recent review, Sanderson and Sher, 2013). Because of these facts, PDE inhibitors have attracted much attention for use as possible therapeutic treatments for cognitive disorders (Blokland et al., 2006, Reneerkens et al., 2009, Rodefer et al., 2012, Yanai et al., 2014, Yanai and Endo, 2015). Recent studies demonstrate that several inhibitors specific for certain PDEs improve or enhance memory and cognitive functions in rodent models. Examples include inhibitors of PDE2 (bay 60–7550; Bollen et al., 2015, Rodefer et al., 2012), PDE4 (rolipram; Monti et al., 2006, Rodefer et al., 2012, Rutten et al., 2009, Zhang and O'Donnell, 2000), PDE5 (sildenafil; Boccia et al., 2011, Cuadrado-Tejedor et al., 2011, Puzzo et al., 2009, Rodefer et al., 2012), PDE7 (S14; Perez-Gonzalez et al., 2013), PDE9 (bay 73–6691; van der Staay et al., 2008), and PDE10 (papaverine; Rodefer et al., 2012).
One of these PDE inhibitors, cilostazol [6-[-4-(1-cyclohexyl-1H-tetrazol-5-yl)butoxy]-3,4-dihydro-2-(1H)-quinolinone], a selective PDE3 inhibitor, was originally prescribed as an antiplatelet agent to treat ischemic stroke and intermittent claudication (O'Donnell et al., 2009). Recent studies show that cilostazol also significantly ameliorates cognitive impairment induced by intracerebral infusion of the amyloid peptide fragment Aβ25-35 (Hiramatsu et al., 2010, Park et al., 2011), in addition to enhancing cognitive performance of a hippocampus-dependent memory task in young mice (Yanai et al., 2014). Furthermore, recent studies showed the beneficial roles of cilostazol to improve memory impairment caused by hypoperfusion in rat (Godinho et al., 2015, Lee et al., 2007, Watanabe et al., 2006). Given that PDE3 is distributed in the central nervous system (CNS) (Xu et al., 2011) and that the retrospective study of mild dementia patients taking cilostazol slowed cognitive decline (Taguchi et al., 2013), cilostazol is potentially a good candidate drug for therapeutic improving cognitive impairment (Arai and Takahashi, 2009, Hiramatsu et al., 2010, Ihara et al., 2014, Maki et al., 2014, Park et al., 2011, Saito and Ihara, 2014, Taguchi et al., 2013, Yanai and Endo, 2015).
In the present study, we used SAMP8 as a mouse model of cognitive aging to determine how the selective PDE3 inhibitor cilostazol affects learning ability. For comprehensive understanding of the effect of cilostazol in SAMP8 in the CNS, we also evaluated the cilostazol-induced cerebral glucose metabolism using positron emission tomography (PET) with 2-deoxy-2-18F-fluoro-d-glucose (18F-FDG) (Heiss et al., 1992, Hoyer, 2003). Further, molecular mechanisms are elucidated for the functions of cilostazol on the blood brain barrier. Our preliminary study revealed that acute administration of cilostazol had no significant effects on performing a Pavlovian fear conditioning task (see Supplementary Materials). Therefore, in the present study, we adopted a long-term strategy, one that administers cilostazol chronically for 3 months.
Section snippets
Subjects
Experimentally naive male SAMR1/TaSlc (SAMR1) and SAMP8/TaSlc (SAMP8) (Japan SLC, Hamamatsu, Japan) were used. Mice were housed in groups of five per cage (GM500; Tecniplast, Buguggiate (VA), Italy) in a vivarium maintained at 22 ± 1 °C and 55 ± 5% humidity under a 12-hr light-dark cycle (light on at 7:00 a.m.). The number of mice used in this study and their assignment to the different experimental conditions are presented in Table 1. The experiments were carried out in a blind manner so that
Cilostazol concentration in serum
To examine the pharmacokinetics of cilostazol throughout the diurnal period when the experiments were conducted, we investigated serum cilostazol concentrations in SAMP8 and SAMR1 after long-term cilostazol administration. With the 0.3% cilostazol dose, serum cilostazol concentration in both strains was relatively stable, being below or around the clinically effective threshold (i.e., 0.76 μg/ml) during the diurnal period (Fig. 1A), threshold that reduce platelet coagulation in human (Takase
Discussion
In the present study, we demonstrated for the first time that long-term administration of cilostazol restores impaired fear memory in SAMP8 (Fig. 2), a mouse model of cognitive aging. This memory improvement was accompanied by an increased number of pCREB-positive cells in the DG of the hippocampal formation (Fig. 3). Furthermore, we discovered that long-term cilostazol administration also restores BBB integrity, which is accompanied by an increased amount of tight junction proteins, and in
Acknowledgements
The authors acknowledge the significant and essential contribution of Mr. Kai Kojima for carrying out the research. The authors also thank Ms. Masako Suzuki, Tomoko Arasaki, and Kazuko Nakanishi for their excellent technical help. This work is supported in part by JSPS KAKENHI (24730642, 25293331, 25560382, 26115532, 15H03103); the Naito Foundation; and Japan Foundation for Aging.
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