The key finding of the present study is that sustained loss of masticatory stimulation since an early age not only accelerated the aging process of hippocampal-dependent cognitive function, but also developed abnormal behavior of locomotor hyperactivation and lateralized behavior at their advanced age. Previous studies using the same strain of mice revealed that 7 to 10 days of reduced mastication, caused either by extraction or reduction of molar teeth, impairs spatial learning ability in aged subjects (more than 36 weeks old) with morphological and functional deterioration of hippocampal neurons [14, 15, 17, 18], although the same duration of reduced mastication in young subjects (20–24 weeks at the extraction) did not affect their cognitive function [14, 15]. Our results further demonstrated that, even in younger subjects, the sustained reduction of masticatory stimulation for more than two weeks impairs hippocampal-dependent cognitive function in this strain.
Permanent loss of teeth causes loss of somatosensory stimuli from the oral cavity, as well as malnutrition, both of which induce sustained increase of circulating corticosterone concentration [18, 19]. Our result in the ORT demonstrated that loss of molar teeth for a long period of time may be a chronic stress that accelerates age-related cognitive impairment. The sustained hyperactivation of the hippocampal neurons via activation of stress-induced glucocorticoid receptors (GRs) causes the functional and morphological decline of these neurons . Attenuated neuronal responsiveness to new stimuli in the hippocampus impairs the spatial-learning ability, possibly interfering with the induction of LTP in hippocampal neurons . Tsutsui et al.  reported that the sustained reduction of masticatory stimulation induced loss of pyramidal cells in the hippocampal CA1 and CA3 regions. As CA1 and CA3 regions are critical to recognizing familiar item or familiar spatial information  and to detecting a novel visual object in a spatial-temporal context , respectively, permanent loss of teeth may impair hippocampal cognitive functions of both spatial and episodic memory.
Attenuated hippocampal function may further cause over-secretion of corticosteroids as the hippocampus is a target brain region of corticosterone regulating its negative-feedback system . Nyakas et al.  reported that activation of GR is essential for the expression of exploratory activity in a novel environment, and that animals with partial lesions in their hippocampi engage in enhanced exploratory activity with increased uptake of glucocorticoids in the remaining neurons in the hippocampus. The locomotor hyperactivation observed in molarless mice in the OFT suggests that chronic stress impairs the function of hippocampal neurons, resulting in hyperactivation of GRs.
Chronic stress derived from reduced mastication has also been reported to enhance oxidative stress  and reduce the response of the dopamine (DA) neurons in the hippocampus, impairing the learning ability in the step-through passive avoidance test . The DA projection from the midbrain dopamine cells of the ventral tegmental area (VTA) to the hippocampal neurons mainly regulates the late phase of LTP, which is essential for the settlement of long-term memory . Although we did not measure the hippocampal LTP in this study, accumulating results suggest that the chronic stress caused by reduced mastication has at least two possible neuronal pathways to impair the hippocampal-dependent learning ability: (1) increased corticosterone directly interferes with the induction of hippocampal LTP, and (2) reduced DA responsiveness in the hippocampus prevents the settlement of the LTP.
Lateralized behavior of molarless mice observed in the ORT also suggests the possible involvement of an attenuated DA system in the hippocampus. The prefrontal cortex (PFC) is another projection site of DA neurons from the VTA, and chronic stress causes a hypodopaminergic state in the PFC , as well as in the hippocampus  with an impaired spatial working memory . Interestingly, exposure to an uncontrollable stressor causes a lateralized alteration of DA consumption in the PFC , which closely relates to lateralized rotational preferences . The age-dependent development of the lateralized preference of rotational behavior in molarless mice may indicate the selective and progressive reduction of neuronal transmission of the DA system.
Another possible hypothesis supporting the attenuated DA system in the molarless mice is iron deprivation caused by extraction-induced reduction of gastric acid secretion . The iron is a key element in DA synthesis, and DA deficit in the brain is generally recognized as a pivotal for hyperactivity and pathologic lateralization of cognitive functioning that are commonly observed in attention-deficit/hyperactivity disorder (ADHD) . In fact, an excessive spontaneous motor activity and a deficit in selective attention are key signatures of the rodent model of ADHD , which corresponds to the behavior of molarless mice at 24 weeks of age.
A limitation of this study is that it is difficult to separate the influence of malnutrition from psychological stress on the development of cognitive and behavioral deficit in the molarless mice. Considering that either food deprivation with intact masticatory function or molar extraction with powder-diet feeding causes impairment of the hippocampus-dependent memory function, as well as attenuation of the DA system [36, 37], these two factors might have an additive effect. Further examination of the serum ferritin levels or administration of DA agonist/antagonist in the current rodent model is needed to elucidate the effect of masticatory function on the systemic development in detail. Uneven dimensions of subdivisions defined in the OFT might dismiss the preference for peripheral and/or intermediate areas, although it would not affect the interpretation of the result of comparable number of entrances to the subdivisions between molarless and control groups.