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Open Access Highly Accessed Research article

Loss of molars early in life develops behavioral lateralization and impairs hippocampus-dependent recognition memory

Masatsuna Kawahata1, Yumie Ono12*, Akinori Ohno1, Shoichi Kawamoto1, Katsuhiko Kimoto1 and Minoru Onozuka3

Author Affiliations

1 Department of Prosthodontics & Oral Rehabilitation, Graduate School of Dentistry, Kanagawa Dental University, Yokosuka, Japan

2 Department of Electronics and Bioinformatics, School of Science and Technology, Meiji University, Kawasaki, Japan

3 Nittai Jyusei Medical College for Judo Therapeutics, Tokyo, Japan

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BMC Neuroscience 2014, 15:4  doi:10.1186/1471-2202-15-4

Published: 4 January 2014

Abstract

Background

Using senescence-accelerated mouse prone 8 (SAMP8), we examined whether reduced mastication from a young age affects hippocampal-dependent cognitive function. We anesthetized male SAMP8 mice at 8 weeks of age and extracted all maxillary molar teeth of half the animals. The other animals were treated similarly, except that molar teeth were not extracted. At 12 and 24 weeks of age, their general behavior and their ability to recognize novel objects were tested using the open-field test (OFT) and the object-recognition test (ORT), respectively.

Results

The body weight of molarless mice was reduced significantly compared to that of molar-intact mice after the extraction and did not recover to the weight of age-matched molar-intact mice throughout the experimental period. At 12 weeks of age, molarless mice showed significantly greater locomotor activity in the OFT than molar-intact mice. However, the ability of molarless mice to discriminate a novel object in the ORT was impaired compared to that of molar-intact mice. The ability of both molarless and molar-intact SAMP8 mice to recognize objects was impaired at 24 weeks of age. These results suggest that molarless SAMP8 mice develop a deficit of cognitive function earlier than molar-intact SAMP8 mice. Interestingly, both at 12 and 24 weeks of age, molarless mice showed a lateralized preference of object location in the encoding session of the ORT, in which two identical objects were presented. Their lateralized preference of object location was positively correlated with the rightward turning-direction preference, which reached statistical significance at 24 weeks of age.

Conclusions

Loss of masticatory function in early life causes malnutrition and chronic stress and impairs the ability to recognize novel objects. Hyperactivation and lateralized rotational behavior are commonly observed with dysfunction of the dopaminergic system, therefore, reduced masticatory function may deplete the mesolimbic and mesocorticolimbic dopaminergic systems to impair the cognitive functions of selective attention and recognition memory in the prefrontal cortex and the hippocampus.

Keywords:
Hippocampus; Senescence-accelerated mouse; Cognitive function; Open-field test; Object-recognition test; Behavioral laterality; Dopamine; Chronic stress