- Email: [email protected]
Soft-diet feeding decreases dopamine release and impairs aversion learning in Alzheimer model rats
Neuroscience Letters 439 (2008) 208–211
Contents lists available at ScienceDirect
Neuroscience Letters journal homepage: www.elsevier.com/locate/neulet
Soft-diet feeding decreases dopamine release and impairs aversion learning in Alzheimer model rats S. Kushida a,b , K. Kimoto b,c,∗ , N. Hori a,b , M. Toyoda a , N. Karasawa d , T. Yamamoto b,e , A. Kojo f , M. Onozuka b,f a
Division of Prosthetics, Department of Oral & Maxillofacial Rehabilitation, Kanagawa Dental College, Yokosuka 238-8580, Japan Research Center of Brain & Oral Science, Kanagawa Dental College, Yokosuka 238-8580, Japan Division of Fixed Prosthodontics, Department of Oral & Maxillofacial Rehabilitation, Kanagawa Dental College, Yokosuka 238-8580, Japan d Faculty of Care and Rehabilitations, Seijoh University, Tokai 476-8588, Japan e Department of Human Biology, Kanagawa Dental College, Yokosuka 238-8580, Japan f Department of Physiology and Neuroscience, Kanagawa Dental College, Yokosuka 238-8580, Japan b c
a r t i c l e
i n f o
Article history: Received 25 March 2008 Received in revised form 5 May 2008 Accepted 7 May 2008 Keywords: Dopaminergic system Learning and memory Soft-diet feeding -Amyloid Alzheimer
a b s t r a c t To examine the effects of soft-diet feeding on the dopaminergic system in a model rat for Alzheimer’s disease (AD), we measured dopamine release in the hippocampus using a microdialysis approach and assessed learning ability and memory using step-through passive avoidance tests. Furthermore, we immunohistochemically examined the ventral tegmental area (VTA), which is the origin of hippocampal dopaminergic fibers using tyrosine hydroxylase (TH), a marker enzyme for the dopaminergic nervous system. Feeding a soft diet decreased dopamine release in the hippocampus and impaired learning ability and memory in AD model rats in comparison with rats fed a hard diet; however, TH-immunopositive profiles in the VTA seemed not to be notably different between rats fed a soft diet and those fed a hard diet. These observations suggest that soft-diet feeding enhances the impairment of learning ability and memory through the decline of dopamine release in the hippocampus in AD rats. © 2008 Elsevier Ireland Ltd. All rights reserved.
Elderly people lose many teeth as they age. The loss of teeth not only forces them to eat soft foods [6,22], but also correlates to the development of senile dementia and Alzheimer’s disease (AD) [14,19,29,34]. AD is the main cause of progressive cognitive impairment in elderly people [11,23]. Neuropathological characteristics of AD are the presence of many senile plaques and neurofibrillary tangles [10,20]. Senile plaques are primarily composed of amyloid peptide (A), a 1–40 amino acid peptide fragment of amyloid protein [1]. In these brains, the pathological progression of AD also damages the ascending neurotransmitter systems [3], leading to cholinergic and/or dopaminergic deficiency, both of which are related to defects of memory and cognition in mild AD [7]. Cholinergic agents such as cholinesterase inhibitors and cholinergic agonists are widely applied for the treatment of AD patients [40]. Recent studies showed that dysfunctional mastication produced by cutting off or extracting teeth impaired the cholinergic neural system in the hippocampus and parietal cortex, caused
∗ Corresponding author at: Division of Fixed Prosthodontics, Department of Oral & Maxillofacial Rehabilitation, Kanagawa Dental College, 82 Inaoka-cho, Yokosuka 238-8580, Japan. Tel.: +81 468 22 9532; fax: +81 468 22 9532. E-mail address: [email protected] (K. Kimoto). 0304-3940/$ – see front matter © 2008 Elsevier Ireland Ltd. All rights reserved. doi:10.1016/j.neulet.2008.05.017
deterioration of spatial memory, and reduced the density of neuronal somata in the hippocampal CA1 sector in normal rats and a model for human senile dementia, aged SAMP8 mice [15,30]. Dopaminergic agents, such as a monoamine oxidase inhibitor, are also efficacious in treating AD patients of moderate severity [33]. A recent study using positron-emission tomography (PET) suggested that the availability of hippocampal dopamine D2 receptor correlates with memory function in AD patients [17]. Despite these research efforts, questions remain concerning the relationship between the dopaminergic system and AD. Using A-infused rats as AD model rats, we examined the effects of soft-diet feeding on dopamine release in the hippocampus using a microdialysis approach. Additionally, we assessed tyrosine hydroxylase (TH) immunoreactive profiles in the ventral tegmental area (VTA), which is the origin of hippocampal dopaminergic fibers. Finally, we assessed learning ability and memory by behavioral tests. Animal treatments were performed with permission from the Ethics Committee of Kanagawa Dental College, employing guidelines established by the committee. We kept 38 male Wistar rats in a room with controlled temperature (22 ± 3 ◦ C) and lighting (12 h light/dark cycle). Rats had free access to food and water. They were 3 weeks old at the beginning of the experiment. We divided the 38
S. Kushida et al. / Neuroscience Letters 439 (2008) 208–211
rats into two experimental groups, namely AD rats fed with hard food (hard food; MF solid, Oriental Yeast Co., Ltd., Osaka, Japan) and AD rats fed with soft food (soft food; MF powder: Oriental Yeast Co., Ltd.). After the rats had been fed each food for 7 weeks (the time span was chosen according to previous paper [2]), AD rats were produced by continuous infusion of A peptide (1–40) (300 pmol/day) dissolved in a solution containing 30% acetonitrile and 0.1% trifluoroacetic acid for 14 days according to the method of Nitta et al. [28]. An Alzet mini-osmotic pump, model 2002 (Durect Co., Cupertino, CA) attached to a cannula enabled continuous application of A peptide to rats. The cannula was implanted into the right lateral ventricle at the co-ordinates antero-posterior (AP), −0.8; mediolateral (ML), +1.5; dorso-ventral (DV), +3.5 mm according to the stereotaxic atlas of Paxinos and Watson [31] and the mini-osmotic pump was embedded under the dorsal skin of neck. Synthetic human A peptide (1–40) was purchased from Bachem AG (Bubendorf, Switzerland). After infusion of A peptide for 14 days, we removed the implanted cannula, anesthetized the animals (n = 5 for each group) with thiamylal sodium, and stereotaxically implanted a microdialysis probe into the left hippocampus at the co-ordinates AP, −3.14; ML, +2.0; DV, +3.0 mm according to the atlas of Paxinos and Watson [31]. After 1 day, we obtained data from freely moving rats. Analytical procedures for dopamine by HPLC have been previously described [12]. Rats were directly connected to the HPLC equipment with a dialysis tube for on-line analysis of dopamine (Eicom Co., Kyoto, Japan). A microperfusion pump perfused the hippocampus with normal Ringer’s solution (147 mM NaCl, 4 mM KCl, 2.3 mM CaCl2 ) through the dialysis tube at a flow rate of 1 ml/min. The dialysis sample was injected every 6 min via an autoinjector, EAS-20 (Eicom Co.). The mobile phase consisted of 99% 0.1 M sodium phosphate-buffered solution, 1% methanol, 500 mg/L sodium 1-decanesulfonate, and 50 mg/L EDTA-2Na. An Eicompack PP-ODS column (30 cm in length and 4.6 mm in diameter; Eicom Co.) separated dopamine at 25 ◦ C. The working electrode (HTLC500; Eicom Co.) was graphite and the flow rate was 500 ml/min. After the dopamine level in the dialysate became stable, we stimulated dopamine release by perfusion of high-K-containing Ringer’s solution (51 mM NaCl, 100 mM KCl, 1.25 mM CaCl2 ) in place of normal Ringer’s solution for 30 min, and then returned to normal Ringer’s solution [13]. After experiments, we confirmed the location of the microdialysis tip by light microscopy (Fig. 1). In the TH immunohistochemical study, we deeply anesthetized the rats (n = 3 for each group) with thiamylal sodium, perfused them with cold (4 ◦ C) saline followed by 4% formaldehyde (FA) in 0.1 M sodium phosphate buffer (PB, pH 7.4), dissected out the brain, and post-fixed it for 24 h in 2% FA. The brain was immersed in PB containing 0.9% NaCl (PBS) and graded concentrations (10, 20, and 30%) of sucrose for 24 h each at 4 ◦ C. The part of the frozen brain that contained the VTA was cut into 40-m-thick transverse sections with a cryostat. Primary antiserum was raised in a rabbit against highly purified rat adrenal TH and its specificity was previously published elsewhere [26]. Sections were incubated in the primary antiserum diluted 1:5000 for 48 h at 4 ◦ C. Immunolabeled sections were further processed using a Vectastain kit (Vector Laboratories, Burlingame, CA) and developed with diaminobenzidine tetrahydrochloride. TH immunoreactive structures in the VTA of each group were observed using a Power BX 41 microscope (Olympus, Tokyo, Japan). The number of immunoreactive somata in the VTA was counted using a field (515 mm × 692 mm) printed in a rectangle sized 6.7 cm × 9 cm for each animal. A step-through passive avoidance task was used to evaluate learning ability and memory [27]. The apparatus consisted of a light compartment (10 cm × 245 cm × 240 cm) connected by a guillotine door (10 cm × 10 cm) to a dark compartment (310 cm × 300 cm × 310 cm) with a steel-rod grid floor (36 paral-
209
Fig. 1. A representative photograph showing the placement of the microdialysis probe (arrow). Abbreviations: CPu, caudate putamen; Hip, hippocampus; Hypo, hypothalamus; Thal, thalamus.
lel steel rods, 0.3 cm diameter, set 1.5 cm apart). A lamp (20 W) was centrally positioned 30 cm above the floor of the light compartment. The room was kept dark during the experimental sessions, which were conducted between 09:00 and 17:00. During the acquisition trial, the guillotine door connecting the light and dark compartments was kept closed. When a rat (n = 11 for each group) was placed in the light compartment, the guillotine door was opened. We measured the time until the rat put all four feet on the grid of the dark compartment with a stopwatch. After the rat entered the dark compartment, we closed the guillotine door and applied an inescapable scrambled foot shock (0.5 mA for 2 s) through the grid floor by a shock generator (Neuroscience Inc., Tokyo, Japan). More than 5 s after the electric shock, we transferred the rat from the dark compartment to the home cage until the retention trial, which was carried out 24 h later. We placed the rat once again in the light compartment, opened the guillotine door, and measured the time until the rat entered the dark compartment through the guillotine door as the step-through latency (STL) time. We set 600 s as the upper cut-off time.
Fig. 2. Extracellular levels of dopamine and their changes induced by high-K Ringer’s solution (exposure time is indicated by a double-arrowed bar) in the AD rat hippocampus. The X-axis indicates time and the Y-axis indicates the percentage of basal release. The values are the mean ± SED. *P < 0.05 in comparison with the soft-diet-fed group.
210
S. Kushida et al. / Neuroscience Letters 439 (2008) 208–211
Fig. 3. TH immunoreactive neurons in the right VTA. (A) Hard-diet-fed AD rats. (B) Soft-diet-fed AD rats. mp, mammillary peduncle. Scale bars: 200 mm.
All numerical values were expressed as the mean ± standard error deviation (SED). Differences between groups were statistically evaluated using one-way analysis of variance (ANOVA) and ´ test. Probabilities of <5% post hoc Fisher’s PLSD test or Scheffe’s (P < 0.05) were considered significant. We found no significant differences in the basal level of dopamine release in the hippocampus between soft- and harddiet-fed groups (Fig. 2); however, the dopamine release evoked by high-K Ringer’s solution was significantly different between the soft- and hard-diet-fed groups (Fig. 2). The increase of dopamine release in the hard-diet-fed group reached 420%; however, that in the soft-diet-fed group reached only 260% (Fig. 2). In AD model rats, dopamine release in the soft-diet-fed group was significantly less than that of the hard-diet-fed group (P < 0.05). By immunohistochemistry, we detected TH immunoreactive structures in the VTA of both groups (Fig. 3A and B); however, there were no remarkable differences in TH immunoreactive profiles of the VTA between softdiet- and hard-diet-fed groups at the light microscopic level (Fig. 3A and B). The density of immunoreactive somata in the VTA of softdiet-fed rats (354.2 ± 12.3 somata/1 mm2 ) was similar to that of hard-diet-fed rats (349.5 ± 13.7 somata/1 mm2 ). In the acquisition trial, the STL time was not statistically different between harddiet- and soft-diet-fed groups (Fig. 4A); however, the STL time of the soft-diet-fed group was significantly shorter than that of the hard-diet-fed group (Fig. 4B). The statistical probability was under 0.05. Because some neurotransmitter systems are impaired in the AD brain [18,32], we focused on the possible influence of soft-diet feeding on the hippocampal dopaminergic system. In the present study,
we showed that soft-diet feeding suppressed high-K-stimulated dopamine release in the hippocampus of AD model rats. Dopamine in the hippocampus is supplied by dopaminergic fibers arising from the VTA, where dopaminergic somata are densely distributed [37]; however, a notable difference between the VTA was not observed in these two groups. These results suggest that the ability to synthesize dopamine is not affected by soft-diet feeding and that soft-diet feeding affects only dopamine release from dopaminergic terminals in the hippocampus. Alternatively, the pathological abnormalities in the AD brain started in the hippocampus and parahippocampal gyrus prior to other regions [5]. The present experimental time may be equivalent to this early stage of AD. Our results also suggest that soft-diet feeding impairs learning ability and memory in AD rats; however, the underlying mechanism is unknown. One possible explanation is that the present results may be caused by the changes of activity in sensori-motor pathways of soft-diet-fed rats. Mastication is regulated by the central nervous system. For involuntary mastication, sensory afferents from mechanoreceptors in the jaw muscle and periodontal ligament affect masticatory motoneurons through the mesencephalic trigeminal nucleus and supratrigeminal nucleus [16,24]. For voluntary mastication, sensory afferents from the orofacial areas also affect masticatory motoneurons through the principal and spinal trigeminal sensory nuclei, ventral posterior thalamic nucleus and sensory and motor cortices [16]. These sensory afferent impulses are important during growth and development. Soft-diet feeding reduced the volume of the masticatory muscle and prevented craniofacial and occlusal development [4,38]. It also impaired learning ability and memory, and reduced synaptogenesis in the cerebral
Fig. 4. Acquisition trial time (A) and step-through latency time (B). The X-axis indicates the time in s. The values indicate the mean ± SED. *P < 0.001 in comparison with the hard-diet-fed group.
S. Kushida et al. / Neuroscience Letters 439 (2008) 208–211
cortex [8,41]. Our previous studies have demonstrated that adultborn neurons in the hippocampus are suppressed by soft-diet feeding [2], and are functionally integrated into the appropriate neuronal circuits [9,36]. These results suggest that the amount of sensory afferent impulses relates not only to the development maintenance of oral and maxillofacial units [4,39], but also to the acquisition of learning ability and memory. Afferent sensory impulses from the masticatory organ might affect the dopaminergic system. Alternatively, chronic stress may cause these phenomena. Dysfunctional mastication caused by the bite-raised condition increased plasma corticosterone levels, possibly through the hypothalamus–pituitary–adrenal axis. Furthermore, this dysfunctional mastication impaired spatial memory and decreased the number of neuronal somata in the hippocampal CA3 sector [21]. Administration of a glucocorticoid synthesis inhibitor suppressed these changes [25,30]; therefore, similar to dysfunctional mastication, soft-diet feeding may produce chronic stress and affect the hypothalamus–pituitary–adrenal axis. The intimate association of dopamine with stress has been reported [35]. The implication of the present results is relevant to treatment because the texture of food act on dopaminergic brain function in the AD brain. Further study is required to determine how mastication relates to the dopaminergic system and how it relates to learning and memory. Acknowledgements We express our thanks to Dr. Toshitaka Nabeshima (Department of Chemical Pharmacology, Graduate School of Pharmaceutical Science, Meijo University, Nagoya, Japan) and Dr Atsumi Nitta (Department of Neuropsychopharmacology and Hospital Pharmacy, School of Medicine, Nagoya University, Nagoya, Japan) for instruction on how to infuse rats with amyloid- peptide. This work was supported in part by a Grant-in-Aid for Scientific Research from the Ministry of Education, Science and Culture of Japan (18592149). References [1] C.R. Abraham, D.J. Selkoe, H. Potter, Immunochemical identification of the serine protease inhibitor alpha 1-antichymotrypsin in the brain amyloid deposits of Alzheimer’s disease, Cell 52 (1988) 487–501. [2] H. Aoki, K. Kimoto, N. Hori, M. Toyoda, Cell proliferation in the dentate gyrus of rat hippocampus is inhibited by soft diet feeding, Gerontology 51 (2005) 369–374. [3] H. Arai, K. Kosaka, R. Iizuka, Changes of biogenic amines and their metabolites in postmortem brains from patients with Alzheimer-type dementia, J. Neurochem. 43 (1984) 388–393. [4] R.M. Beecher, R.S. Corruccini, Effects of dietary consistency on craniofacial and occlusal development in the rat, Angle Orthod. 51 (1981) 61–69. [5] H. Braak, E. Braak, Frequency of stages of Alzheimer-related lesions in different age categories, Neurobiol. Aging 18 (1997) 351–357. [6] H.H. Chauncey, M.E. Muench, K.K. Kapur, A.H. Wayler, The effect of the loss of teeth on diet and nutrition, Int. Dent. J. 34 (1984) 98–104. [7] K.L. Davis, R.C. Mohs, D. Marin, D.P. Purohit, D.P. Perl, M. Lantz, G. Austin, V. Haroutunian, Cholinergic markers in elderly patients with early signs of Alzheimer disease, JAMA 281 (1999) 1401–1406. [8] Y. Endo, T. Mizuno, K. Fujita, T. Funabashi, F. Kimura, Soft-diet feeding during development enhances later abilities in female rats, Physiol. Behav. 56 (1994) 629–633. [9] P.S. Eriksson, E. Perfilieva, T. Bjork-Eriksson, A.M. Alborn, C. Nordborg, D.A. Peterson, F.H. Gage, Neurogenesis in the adult human hippocampus, Nat. Med. 4 (1998) 1313–1317. [10] J. Hardy, D. Allsop, Amyloid deposition as the central event in the aetiology of Alzheimer’s disease, Trends Pharmacol. Sci. 12 (1991) 383–388. [11] H.C. Hendrie, Epidemiology of Alzheimer’s disease, Geriatrics 52 (1997) 4–8. [12] N. Hori, K. Kimoto, H. Aoki, S. Kushida, M. Toyoda, Release of dopamine and serotonin levels during the restraint stress change by biting, Bull. Kanagawa Dent. Col. 33 (2005) 130–132. [13] A. Itoh, A. Nitta, M. Nadai, K. Nishimura, M. Hirose, T. Hasegawa, T. Nabeshima, Dysfunctional of cholinergic and dopaminergic neuronal system in -amyloid protein-infused rats, J. Neurochem. 66 (1996) 1113–1117.
211
[14] J.A. Jones, J.N. Lavalle, J. Alman, Caries incidence in patients with dementia, Gerodontology 10 (1993) 76–82. [15] T. Kato, T. Usami, Y. Noda, M. Hasegawa, M. Ueda, T. Nabeshima, The effect of the loss of molar teeth on spatial memory and acetylcholine release from the parietal cortex in aged rats, Behav. Brain Res. 83 (1997) 239–242. [16] Y. Kawamura, Neurogenesis of mastication, in: Y. Kawamura (Ed.), Frontiers of Oral Physiology, vol. 1, Physiology of Mastication, S. Karger AG, Basel, 1974, pp. 77–120. [17] N. Kemppainen, M. Laine, M.P. Laalso, K. Kaasinen, T. Vahlberg, T. Kurki, J.O. Rinne, Hippocampal dopamine D2 receptors correlate with memory functions in Alzheimer’s disease, Eur. J. Neurosci. 18 (2003) 149–154. [18] N. Kemppainen, H. Ruottinen, K. Nagren, J.O. Rinne, PET shows that striatal dopamine D1 and D2 receptors are differentially affected in AD, Neurology 55 (2000) 205–209. [19] K. Kondo, M. Nino, K. Shido, A case–control study of Alzheimer’s disease in Japan-significance of life-style, Dementia 5 (1994) 314–326. [20] K.S. Kosic, Alzheimer plaques and tangles: advances on both fronts, Trends Neurosci. 14 (1991) 218–219. [21] K. Kubo, Y. Yamada, M. Iinuma, F. Iwaku, Y. Tamura, K. Watanabe, H. Nakamura, M. Onozuka, Occlusal disharmony induces spatial memory impairment and hippocampal neuron degeneration via stress in SAMP8 mice, Neurosci. Lett. 414 (2007) 188–191. [22] J.L. Leake, An index of chewing ability, J. Public Health Dent. 50 (1990) 262–267. [23] A. Lobo, L.J. Launer, L. Fratiglioni, K. Andersen, A. Di Carlo, M.M. Breteler, J.R. Copeland, F.J. Dartigues, C. Japper, J. Martinez-Lage, H. Soininen, A. Hofman, Prevalence of dementia and major subtypes in Europe: a collaborative study of population-based cohorts. Neurologic Diseases in the Elderly Research Group, Neurology 54 (Suppl. 5) (2000) S4–S9. [24] J.P. Lund, Mastication and its control by the brain stem, Crit. Rev. Oral Biol. Med. 2 (1991) 33–64. [25] B.S. McEwen, Corticosteroids and hippocampal plasticity, Ann. NY Acad. Sci. 746 (1994) 134–142. [26] I. Nagatsu, Y. Kondo, S. Inagaki, N. Karasawa, T. Kato, T. Nagatsu, Immunofluorescent studies on tyrosine hydroxylase: application for its axoplasmic transport, Acta Histochem. Cytochem. 10 (1977) 494–499. [27] A. Nitta, K. Hayashi, T. Hasegawa, T. Nabeshima, Development of plasticity of brain function with repeated trainings and passage of time after basal forebrain lesions in rats, J. Neural Transm. Gen. Sect. 93 (1993) 37–46. [28] A. Nitta, A. Itoh, T. Hasegawa, T. Nabeshima, -Amyloid protein-induced Alzheimer’s disease animal model, Neurosci. Lett. 170 (1994) 63–66. [29] K. Okimoto, K. Ieiri, K. Matsuo, Y. Terada, The relationship between oral status and the progress of dementia at senile hospital (in Japanese with English summary), J. Jpn. Prosthodont. Soc. 35 (1991) 931–934. [30] M. Onozuka, K. Watanabe, M. Fujita, M. Tomida, S. Ozono, Changes in the septohippocampal cholinergic system following removal of molar teeth in the aged SAMP8 mouse, Behav. Brain Res. 133 (2002) 197–204. [31] G. Paxinos, C. Watson, The Rat Brain in Stereotaxic Coordinates, 6th ed., Academic Press, New York, 2007, 455 pp. [32] J.O. Rinne, N. Sahlberg, H. Ruottinen, K. Nagren, P. Lehikoinen, Striatal uptake of the dopamine reuptake ligand [11C] beta-CFT is reduced in Alzheimer’s disease assessed by positron emission tomography, Neurology 50 (1998) 152– 156. [33] M. Sano, C. Ernesto, R.G. Thomas, M.R. Klauber, K. Schafer, M. Grundman, P. Woodbury, J. Growdon, C.W. Cotman, E. Pfeiffer, L.S. Schneider, L.J. Thal, A controlled trial of selegiline, alpha-tocopherol, or both as treatment for Alzheimer’s disease. The Alzheimer’s disease cooperative Study, N. Engl. J. Med. 336 (1997) 1216–1222. [34] T. Shigetomi, T. Asano, T. Katou, T. Usami, M. Ueda, K. Kawano, A study on oral function and aging. An epidemiological risk factor for dementia (in Japanese with English summary), J. Jpn. Stomatol. Soc. 47 (1998) 403–407. [35] G.D. Stanwood, Dopamine, central, in: G. Fink (Ed.), Encyclopedia of Stress, vol. 1 A-E, Academic Press, New York, 2007, pp. 852–859. [36] H. van Praag, A.F. Schinder, B.R. Christie, N. Toni, T.D. Palmer, F.H. Gage, Functional neurogenesis in the adult hippocampus, Nature 415 (2002) 1030–1034. [37] C. Verney, M. Baulac, B. Berger, C. Alvarez, A. Vigny, K.B. Helle, Morphological evidence for a dopaminergic terminal field in the hippocampal formation of young and adult rat, Neuroscience 14 (1985) 1039–1052. [38] Y. Watanabe, M. Tonogi, G. Yamane, I. Watanabe, H. Hirano, N. Ishiyama, T. Tachikawa, A study of the effect of a semi-fluid diet on the masticatory muscles of aged rats (in Japanese with English summary), Jpn. J. Gerodont. 13 (1998) 29–38. [39] D.G. Watt, H.M. Williams, The effects of the development of the mandible and maxilla of the rat, Am. J. Orthodont. 73 (1951) 895–928. [40] B. Winblad, K. Engedal, H. Soininen, F. Verhaey, G. Waldemar, A. Wimo, A.L. Wetterholm, R. Zhang, A. Haglund, P. Subbiah, A 1-year, randomized, placebocontrolled study of donepezil in patients with mild to moderate AD, Neurology 57 (2001) 489–495. [41] T. Yamamoto, A. Hirayama, Effect of soft-diet feeding on synaptic density in hippocampus and parietal cortex of senescence-accelerated mice, Brain Res. 902 (2001) 255–263.
Contents lists available at ScienceDirect
Neuroscience Letters journal homepage: www.elsevier.com/locate/neulet
Soft-diet feeding decreases dopamine release and impairs aversion learning in Alzheimer model rats S. Kushida a,b , K. Kimoto b,c,∗ , N. Hori a,b , M. Toyoda a , N. Karasawa d , T. Yamamoto b,e , A. Kojo f , M. Onozuka b,f a
Division of Prosthetics, Department of Oral & Maxillofacial Rehabilitation, Kanagawa Dental College, Yokosuka 238-8580, Japan Research Center of Brain & Oral Science, Kanagawa Dental College, Yokosuka 238-8580, Japan Division of Fixed Prosthodontics, Department of Oral & Maxillofacial Rehabilitation, Kanagawa Dental College, Yokosuka 238-8580, Japan d Faculty of Care and Rehabilitations, Seijoh University, Tokai 476-8588, Japan e Department of Human Biology, Kanagawa Dental College, Yokosuka 238-8580, Japan f Department of Physiology and Neuroscience, Kanagawa Dental College, Yokosuka 238-8580, Japan b c
a r t i c l e
i n f o
Article history: Received 25 March 2008 Received in revised form 5 May 2008 Accepted 7 May 2008 Keywords: Dopaminergic system Learning and memory Soft-diet feeding -Amyloid Alzheimer
a b s t r a c t To examine the effects of soft-diet feeding on the dopaminergic system in a model rat for Alzheimer’s disease (AD), we measured dopamine release in the hippocampus using a microdialysis approach and assessed learning ability and memory using step-through passive avoidance tests. Furthermore, we immunohistochemically examined the ventral tegmental area (VTA), which is the origin of hippocampal dopaminergic fibers using tyrosine hydroxylase (TH), a marker enzyme for the dopaminergic nervous system. Feeding a soft diet decreased dopamine release in the hippocampus and impaired learning ability and memory in AD model rats in comparison with rats fed a hard diet; however, TH-immunopositive profiles in the VTA seemed not to be notably different between rats fed a soft diet and those fed a hard diet. These observations suggest that soft-diet feeding enhances the impairment of learning ability and memory through the decline of dopamine release in the hippocampus in AD rats. © 2008 Elsevier Ireland Ltd. All rights reserved.
Elderly people lose many teeth as they age. The loss of teeth not only forces them to eat soft foods [6,22], but also correlates to the development of senile dementia and Alzheimer’s disease (AD) [14,19,29,34]. AD is the main cause of progressive cognitive impairment in elderly people [11,23]. Neuropathological characteristics of AD are the presence of many senile plaques and neurofibrillary tangles [10,20]. Senile plaques are primarily composed of amyloid peptide (A), a 1–40 amino acid peptide fragment of amyloid protein [1]. In these brains, the pathological progression of AD also damages the ascending neurotransmitter systems [3], leading to cholinergic and/or dopaminergic deficiency, both of which are related to defects of memory and cognition in mild AD [7]. Cholinergic agents such as cholinesterase inhibitors and cholinergic agonists are widely applied for the treatment of AD patients [40]. Recent studies showed that dysfunctional mastication produced by cutting off or extracting teeth impaired the cholinergic neural system in the hippocampus and parietal cortex, caused
∗ Corresponding author at: Division of Fixed Prosthodontics, Department of Oral & Maxillofacial Rehabilitation, Kanagawa Dental College, 82 Inaoka-cho, Yokosuka 238-8580, Japan. Tel.: +81 468 22 9532; fax: +81 468 22 9532. E-mail address: [email protected] (K. Kimoto). 0304-3940/$ – see front matter © 2008 Elsevier Ireland Ltd. All rights reserved. doi:10.1016/j.neulet.2008.05.017
deterioration of spatial memory, and reduced the density of neuronal somata in the hippocampal CA1 sector in normal rats and a model for human senile dementia, aged SAMP8 mice [15,30]. Dopaminergic agents, such as a monoamine oxidase inhibitor, are also efficacious in treating AD patients of moderate severity [33]. A recent study using positron-emission tomography (PET) suggested that the availability of hippocampal dopamine D2 receptor correlates with memory function in AD patients [17]. Despite these research efforts, questions remain concerning the relationship between the dopaminergic system and AD. Using A-infused rats as AD model rats, we examined the effects of soft-diet feeding on dopamine release in the hippocampus using a microdialysis approach. Additionally, we assessed tyrosine hydroxylase (TH) immunoreactive profiles in the ventral tegmental area (VTA), which is the origin of hippocampal dopaminergic fibers. Finally, we assessed learning ability and memory by behavioral tests. Animal treatments were performed with permission from the Ethics Committee of Kanagawa Dental College, employing guidelines established by the committee. We kept 38 male Wistar rats in a room with controlled temperature (22 ± 3 ◦ C) and lighting (12 h light/dark cycle). Rats had free access to food and water. They were 3 weeks old at the beginning of the experiment. We divided the 38
S. Kushida et al. / Neuroscience Letters 439 (2008) 208–211
rats into two experimental groups, namely AD rats fed with hard food (hard food; MF solid, Oriental Yeast Co., Ltd., Osaka, Japan) and AD rats fed with soft food (soft food; MF powder: Oriental Yeast Co., Ltd.). After the rats had been fed each food for 7 weeks (the time span was chosen according to previous paper [2]), AD rats were produced by continuous infusion of A peptide (1–40) (300 pmol/day) dissolved in a solution containing 30% acetonitrile and 0.1% trifluoroacetic acid for 14 days according to the method of Nitta et al. [28]. An Alzet mini-osmotic pump, model 2002 (Durect Co., Cupertino, CA) attached to a cannula enabled continuous application of A peptide to rats. The cannula was implanted into the right lateral ventricle at the co-ordinates antero-posterior (AP), −0.8; mediolateral (ML), +1.5; dorso-ventral (DV), +3.5 mm according to the stereotaxic atlas of Paxinos and Watson [31] and the mini-osmotic pump was embedded under the dorsal skin of neck. Synthetic human A peptide (1–40) was purchased from Bachem AG (Bubendorf, Switzerland). After infusion of A peptide for 14 days, we removed the implanted cannula, anesthetized the animals (n = 5 for each group) with thiamylal sodium, and stereotaxically implanted a microdialysis probe into the left hippocampus at the co-ordinates AP, −3.14; ML, +2.0; DV, +3.0 mm according to the atlas of Paxinos and Watson [31]. After 1 day, we obtained data from freely moving rats. Analytical procedures for dopamine by HPLC have been previously described [12]. Rats were directly connected to the HPLC equipment with a dialysis tube for on-line analysis of dopamine (Eicom Co., Kyoto, Japan). A microperfusion pump perfused the hippocampus with normal Ringer’s solution (147 mM NaCl, 4 mM KCl, 2.3 mM CaCl2 ) through the dialysis tube at a flow rate of 1 ml/min. The dialysis sample was injected every 6 min via an autoinjector, EAS-20 (Eicom Co.). The mobile phase consisted of 99% 0.1 M sodium phosphate-buffered solution, 1% methanol, 500 mg/L sodium 1-decanesulfonate, and 50 mg/L EDTA-2Na. An Eicompack PP-ODS column (30 cm in length and 4.6 mm in diameter; Eicom Co.) separated dopamine at 25 ◦ C. The working electrode (HTLC500; Eicom Co.) was graphite and the flow rate was 500 ml/min. After the dopamine level in the dialysate became stable, we stimulated dopamine release by perfusion of high-K-containing Ringer’s solution (51 mM NaCl, 100 mM KCl, 1.25 mM CaCl2 ) in place of normal Ringer’s solution for 30 min, and then returned to normal Ringer’s solution [13]. After experiments, we confirmed the location of the microdialysis tip by light microscopy (Fig. 1). In the TH immunohistochemical study, we deeply anesthetized the rats (n = 3 for each group) with thiamylal sodium, perfused them with cold (4 ◦ C) saline followed by 4% formaldehyde (FA) in 0.1 M sodium phosphate buffer (PB, pH 7.4), dissected out the brain, and post-fixed it for 24 h in 2% FA. The brain was immersed in PB containing 0.9% NaCl (PBS) and graded concentrations (10, 20, and 30%) of sucrose for 24 h each at 4 ◦ C. The part of the frozen brain that contained the VTA was cut into 40-m-thick transverse sections with a cryostat. Primary antiserum was raised in a rabbit against highly purified rat adrenal TH and its specificity was previously published elsewhere [26]. Sections were incubated in the primary antiserum diluted 1:5000 for 48 h at 4 ◦ C. Immunolabeled sections were further processed using a Vectastain kit (Vector Laboratories, Burlingame, CA) and developed with diaminobenzidine tetrahydrochloride. TH immunoreactive structures in the VTA of each group were observed using a Power BX 41 microscope (Olympus, Tokyo, Japan). The number of immunoreactive somata in the VTA was counted using a field (515 mm × 692 mm) printed in a rectangle sized 6.7 cm × 9 cm for each animal. A step-through passive avoidance task was used to evaluate learning ability and memory [27]. The apparatus consisted of a light compartment (10 cm × 245 cm × 240 cm) connected by a guillotine door (10 cm × 10 cm) to a dark compartment (310 cm × 300 cm × 310 cm) with a steel-rod grid floor (36 paral-
209
Fig. 1. A representative photograph showing the placement of the microdialysis probe (arrow). Abbreviations: CPu, caudate putamen; Hip, hippocampus; Hypo, hypothalamus; Thal, thalamus.
lel steel rods, 0.3 cm diameter, set 1.5 cm apart). A lamp (20 W) was centrally positioned 30 cm above the floor of the light compartment. The room was kept dark during the experimental sessions, which were conducted between 09:00 and 17:00. During the acquisition trial, the guillotine door connecting the light and dark compartments was kept closed. When a rat (n = 11 for each group) was placed in the light compartment, the guillotine door was opened. We measured the time until the rat put all four feet on the grid of the dark compartment with a stopwatch. After the rat entered the dark compartment, we closed the guillotine door and applied an inescapable scrambled foot shock (0.5 mA for 2 s) through the grid floor by a shock generator (Neuroscience Inc., Tokyo, Japan). More than 5 s after the electric shock, we transferred the rat from the dark compartment to the home cage until the retention trial, which was carried out 24 h later. We placed the rat once again in the light compartment, opened the guillotine door, and measured the time until the rat entered the dark compartment through the guillotine door as the step-through latency (STL) time. We set 600 s as the upper cut-off time.
Fig. 2. Extracellular levels of dopamine and their changes induced by high-K Ringer’s solution (exposure time is indicated by a double-arrowed bar) in the AD rat hippocampus. The X-axis indicates time and the Y-axis indicates the percentage of basal release. The values are the mean ± SED. *P < 0.05 in comparison with the soft-diet-fed group.
210
S. Kushida et al. / Neuroscience Letters 439 (2008) 208–211
Fig. 3. TH immunoreactive neurons in the right VTA. (A) Hard-diet-fed AD rats. (B) Soft-diet-fed AD rats. mp, mammillary peduncle. Scale bars: 200 mm.
All numerical values were expressed as the mean ± standard error deviation (SED). Differences between groups were statistically evaluated using one-way analysis of variance (ANOVA) and ´ test. Probabilities of <5% post hoc Fisher’s PLSD test or Scheffe’s (P < 0.05) were considered significant. We found no significant differences in the basal level of dopamine release in the hippocampus between soft- and harddiet-fed groups (Fig. 2); however, the dopamine release evoked by high-K Ringer’s solution was significantly different between the soft- and hard-diet-fed groups (Fig. 2). The increase of dopamine release in the hard-diet-fed group reached 420%; however, that in the soft-diet-fed group reached only 260% (Fig. 2). In AD model rats, dopamine release in the soft-diet-fed group was significantly less than that of the hard-diet-fed group (P < 0.05). By immunohistochemistry, we detected TH immunoreactive structures in the VTA of both groups (Fig. 3A and B); however, there were no remarkable differences in TH immunoreactive profiles of the VTA between softdiet- and hard-diet-fed groups at the light microscopic level (Fig. 3A and B). The density of immunoreactive somata in the VTA of softdiet-fed rats (354.2 ± 12.3 somata/1 mm2 ) was similar to that of hard-diet-fed rats (349.5 ± 13.7 somata/1 mm2 ). In the acquisition trial, the STL time was not statistically different between harddiet- and soft-diet-fed groups (Fig. 4A); however, the STL time of the soft-diet-fed group was significantly shorter than that of the hard-diet-fed group (Fig. 4B). The statistical probability was under 0.05. Because some neurotransmitter systems are impaired in the AD brain [18,32], we focused on the possible influence of soft-diet feeding on the hippocampal dopaminergic system. In the present study,
we showed that soft-diet feeding suppressed high-K-stimulated dopamine release in the hippocampus of AD model rats. Dopamine in the hippocampus is supplied by dopaminergic fibers arising from the VTA, where dopaminergic somata are densely distributed [37]; however, a notable difference between the VTA was not observed in these two groups. These results suggest that the ability to synthesize dopamine is not affected by soft-diet feeding and that soft-diet feeding affects only dopamine release from dopaminergic terminals in the hippocampus. Alternatively, the pathological abnormalities in the AD brain started in the hippocampus and parahippocampal gyrus prior to other regions [5]. The present experimental time may be equivalent to this early stage of AD. Our results also suggest that soft-diet feeding impairs learning ability and memory in AD rats; however, the underlying mechanism is unknown. One possible explanation is that the present results may be caused by the changes of activity in sensori-motor pathways of soft-diet-fed rats. Mastication is regulated by the central nervous system. For involuntary mastication, sensory afferents from mechanoreceptors in the jaw muscle and periodontal ligament affect masticatory motoneurons through the mesencephalic trigeminal nucleus and supratrigeminal nucleus [16,24]. For voluntary mastication, sensory afferents from the orofacial areas also affect masticatory motoneurons through the principal and spinal trigeminal sensory nuclei, ventral posterior thalamic nucleus and sensory and motor cortices [16]. These sensory afferent impulses are important during growth and development. Soft-diet feeding reduced the volume of the masticatory muscle and prevented craniofacial and occlusal development [4,38]. It also impaired learning ability and memory, and reduced synaptogenesis in the cerebral
Fig. 4. Acquisition trial time (A) and step-through latency time (B). The X-axis indicates the time in s. The values indicate the mean ± SED. *P < 0.001 in comparison with the hard-diet-fed group.
S. Kushida et al. / Neuroscience Letters 439 (2008) 208–211
cortex [8,41]. Our previous studies have demonstrated that adultborn neurons in the hippocampus are suppressed by soft-diet feeding [2], and are functionally integrated into the appropriate neuronal circuits [9,36]. These results suggest that the amount of sensory afferent impulses relates not only to the development maintenance of oral and maxillofacial units [4,39], but also to the acquisition of learning ability and memory. Afferent sensory impulses from the masticatory organ might affect the dopaminergic system. Alternatively, chronic stress may cause these phenomena. Dysfunctional mastication caused by the bite-raised condition increased plasma corticosterone levels, possibly through the hypothalamus–pituitary–adrenal axis. Furthermore, this dysfunctional mastication impaired spatial memory and decreased the number of neuronal somata in the hippocampal CA3 sector [21]. Administration of a glucocorticoid synthesis inhibitor suppressed these changes [25,30]; therefore, similar to dysfunctional mastication, soft-diet feeding may produce chronic stress and affect the hypothalamus–pituitary–adrenal axis. The intimate association of dopamine with stress has been reported [35]. The implication of the present results is relevant to treatment because the texture of food act on dopaminergic brain function in the AD brain. Further study is required to determine how mastication relates to the dopaminergic system and how it relates to learning and memory. Acknowledgements We express our thanks to Dr. Toshitaka Nabeshima (Department of Chemical Pharmacology, Graduate School of Pharmaceutical Science, Meijo University, Nagoya, Japan) and Dr Atsumi Nitta (Department of Neuropsychopharmacology and Hospital Pharmacy, School of Medicine, Nagoya University, Nagoya, Japan) for instruction on how to infuse rats with amyloid- peptide. This work was supported in part by a Grant-in-Aid for Scientific Research from the Ministry of Education, Science and Culture of Japan (18592149). References [1] C.R. Abraham, D.J. Selkoe, H. Potter, Immunochemical identification of the serine protease inhibitor alpha 1-antichymotrypsin in the brain amyloid deposits of Alzheimer’s disease, Cell 52 (1988) 487–501. [2] H. Aoki, K. Kimoto, N. Hori, M. Toyoda, Cell proliferation in the dentate gyrus of rat hippocampus is inhibited by soft diet feeding, Gerontology 51 (2005) 369–374. [3] H. Arai, K. Kosaka, R. Iizuka, Changes of biogenic amines and their metabolites in postmortem brains from patients with Alzheimer-type dementia, J. Neurochem. 43 (1984) 388–393. [4] R.M. Beecher, R.S. Corruccini, Effects of dietary consistency on craniofacial and occlusal development in the rat, Angle Orthod. 51 (1981) 61–69. [5] H. Braak, E. Braak, Frequency of stages of Alzheimer-related lesions in different age categories, Neurobiol. Aging 18 (1997) 351–357. [6] H.H. Chauncey, M.E. Muench, K.K. Kapur, A.H. Wayler, The effect of the loss of teeth on diet and nutrition, Int. Dent. J. 34 (1984) 98–104. [7] K.L. Davis, R.C. Mohs, D. Marin, D.P. Purohit, D.P. Perl, M. Lantz, G. Austin, V. Haroutunian, Cholinergic markers in elderly patients with early signs of Alzheimer disease, JAMA 281 (1999) 1401–1406. [8] Y. Endo, T. Mizuno, K. Fujita, T. Funabashi, F. Kimura, Soft-diet feeding during development enhances later abilities in female rats, Physiol. Behav. 56 (1994) 629–633. [9] P.S. Eriksson, E. Perfilieva, T. Bjork-Eriksson, A.M. Alborn, C. Nordborg, D.A. Peterson, F.H. Gage, Neurogenesis in the adult human hippocampus, Nat. Med. 4 (1998) 1313–1317. [10] J. Hardy, D. Allsop, Amyloid deposition as the central event in the aetiology of Alzheimer’s disease, Trends Pharmacol. Sci. 12 (1991) 383–388. [11] H.C. Hendrie, Epidemiology of Alzheimer’s disease, Geriatrics 52 (1997) 4–8. [12] N. Hori, K. Kimoto, H. Aoki, S. Kushida, M. Toyoda, Release of dopamine and serotonin levels during the restraint stress change by biting, Bull. Kanagawa Dent. Col. 33 (2005) 130–132. [13] A. Itoh, A. Nitta, M. Nadai, K. Nishimura, M. Hirose, T. Hasegawa, T. Nabeshima, Dysfunctional of cholinergic and dopaminergic neuronal system in -amyloid protein-infused rats, J. Neurochem. 66 (1996) 1113–1117.
211
[14] J.A. Jones, J.N. Lavalle, J. Alman, Caries incidence in patients with dementia, Gerodontology 10 (1993) 76–82. [15] T. Kato, T. Usami, Y. Noda, M. Hasegawa, M. Ueda, T. Nabeshima, The effect of the loss of molar teeth on spatial memory and acetylcholine release from the parietal cortex in aged rats, Behav. Brain Res. 83 (1997) 239–242. [16] Y. Kawamura, Neurogenesis of mastication, in: Y. Kawamura (Ed.), Frontiers of Oral Physiology, vol. 1, Physiology of Mastication, S. Karger AG, Basel, 1974, pp. 77–120. [17] N. Kemppainen, M. Laine, M.P. Laalso, K. Kaasinen, T. Vahlberg, T. Kurki, J.O. Rinne, Hippocampal dopamine D2 receptors correlate with memory functions in Alzheimer’s disease, Eur. J. Neurosci. 18 (2003) 149–154. [18] N. Kemppainen, H. Ruottinen, K. Nagren, J.O. Rinne, PET shows that striatal dopamine D1 and D2 receptors are differentially affected in AD, Neurology 55 (2000) 205–209. [19] K. Kondo, M. Nino, K. Shido, A case–control study of Alzheimer’s disease in Japan-significance of life-style, Dementia 5 (1994) 314–326. [20] K.S. Kosic, Alzheimer plaques and tangles: advances on both fronts, Trends Neurosci. 14 (1991) 218–219. [21] K. Kubo, Y. Yamada, M. Iinuma, F. Iwaku, Y. Tamura, K. Watanabe, H. Nakamura, M. Onozuka, Occlusal disharmony induces spatial memory impairment and hippocampal neuron degeneration via stress in SAMP8 mice, Neurosci. Lett. 414 (2007) 188–191. [22] J.L. Leake, An index of chewing ability, J. Public Health Dent. 50 (1990) 262–267. [23] A. Lobo, L.J. Launer, L. Fratiglioni, K. Andersen, A. Di Carlo, M.M. Breteler, J.R. Copeland, F.J. Dartigues, C. Japper, J. Martinez-Lage, H. Soininen, A. Hofman, Prevalence of dementia and major subtypes in Europe: a collaborative study of population-based cohorts. Neurologic Diseases in the Elderly Research Group, Neurology 54 (Suppl. 5) (2000) S4–S9. [24] J.P. Lund, Mastication and its control by the brain stem, Crit. Rev. Oral Biol. Med. 2 (1991) 33–64. [25] B.S. McEwen, Corticosteroids and hippocampal plasticity, Ann. NY Acad. Sci. 746 (1994) 134–142. [26] I. Nagatsu, Y. Kondo, S. Inagaki, N. Karasawa, T. Kato, T. Nagatsu, Immunofluorescent studies on tyrosine hydroxylase: application for its axoplasmic transport, Acta Histochem. Cytochem. 10 (1977) 494–499. [27] A. Nitta, K. Hayashi, T. Hasegawa, T. Nabeshima, Development of plasticity of brain function with repeated trainings and passage of time after basal forebrain lesions in rats, J. Neural Transm. Gen. Sect. 93 (1993) 37–46. [28] A. Nitta, A. Itoh, T. Hasegawa, T. Nabeshima, -Amyloid protein-induced Alzheimer’s disease animal model, Neurosci. Lett. 170 (1994) 63–66. [29] K. Okimoto, K. Ieiri, K. Matsuo, Y. Terada, The relationship between oral status and the progress of dementia at senile hospital (in Japanese with English summary), J. Jpn. Prosthodont. Soc. 35 (1991) 931–934. [30] M. Onozuka, K. Watanabe, M. Fujita, M. Tomida, S. Ozono, Changes in the septohippocampal cholinergic system following removal of molar teeth in the aged SAMP8 mouse, Behav. Brain Res. 133 (2002) 197–204. [31] G. Paxinos, C. Watson, The Rat Brain in Stereotaxic Coordinates, 6th ed., Academic Press, New York, 2007, 455 pp. [32] J.O. Rinne, N. Sahlberg, H. Ruottinen, K. Nagren, P. Lehikoinen, Striatal uptake of the dopamine reuptake ligand [11C] beta-CFT is reduced in Alzheimer’s disease assessed by positron emission tomography, Neurology 50 (1998) 152– 156. [33] M. Sano, C. Ernesto, R.G. Thomas, M.R. Klauber, K. Schafer, M. Grundman, P. Woodbury, J. Growdon, C.W. Cotman, E. Pfeiffer, L.S. Schneider, L.J. Thal, A controlled trial of selegiline, alpha-tocopherol, or both as treatment for Alzheimer’s disease. The Alzheimer’s disease cooperative Study, N. Engl. J. Med. 336 (1997) 1216–1222. [34] T. Shigetomi, T. Asano, T. Katou, T. Usami, M. Ueda, K. Kawano, A study on oral function and aging. An epidemiological risk factor for dementia (in Japanese with English summary), J. Jpn. Stomatol. Soc. 47 (1998) 403–407. [35] G.D. Stanwood, Dopamine, central, in: G. Fink (Ed.), Encyclopedia of Stress, vol. 1 A-E, Academic Press, New York, 2007, pp. 852–859. [36] H. van Praag, A.F. Schinder, B.R. Christie, N. Toni, T.D. Palmer, F.H. Gage, Functional neurogenesis in the adult hippocampus, Nature 415 (2002) 1030–1034. [37] C. Verney, M. Baulac, B. Berger, C. Alvarez, A. Vigny, K.B. Helle, Morphological evidence for a dopaminergic terminal field in the hippocampal formation of young and adult rat, Neuroscience 14 (1985) 1039–1052. [38] Y. Watanabe, M. Tonogi, G. Yamane, I. Watanabe, H. Hirano, N. Ishiyama, T. Tachikawa, A study of the effect of a semi-fluid diet on the masticatory muscles of aged rats (in Japanese with English summary), Jpn. J. Gerodont. 13 (1998) 29–38. [39] D.G. Watt, H.M. Williams, The effects of the development of the mandible and maxilla of the rat, Am. J. Orthodont. 73 (1951) 895–928. [40] B. Winblad, K. Engedal, H. Soininen, F. Verhaey, G. Waldemar, A. Wimo, A.L. Wetterholm, R. Zhang, A. Haglund, P. Subbiah, A 1-year, randomized, placebocontrolled study of donepezil in patients with mild to moderate AD, Neurology 57 (2001) 489–495. [41] T. Yamamoto, A. Hirayama, Effect of soft-diet feeding on synaptic density in hippocampus and parietal cortex of senescence-accelerated mice, Brain Res. 902 (2001) 255–263.