Atsuyoshi Shimada

Age-related deterioration in conditional avoidance task in the SAM-P/10 mouse, an animal model of spontaneous brain atrophy.

A novel inbred strain of mouse SAM-P/10 (Senescence Accelerated Mouse) is a model of age-related brain atrophy characterized by age-related loss and shrinkage of neurons in the cerebral neocortex. Age-related changes in learning and memory skills of SAM-P/10 mice were investigated using a newly developed conditional avoidance task in a T-maze. Comparisons were made with findings in the SAM-R/1 strain which shows a little loss and no shrinkage of neocortical neurons. Four-month-old SAM-R/1 and SAM-P/10 performed well during a 10-day training schedule of the conditional avoidance task. SAM-R/1 mice over 17 months of age were slower learners than younger SAM-R/1 mice but reached nearly the same high percentage avoidance as seen in the 4-month-old mice during the last 4 days of the schedule. Performance of the SAM-P/10 mice gradually worsened with aging and 10- to 12-month-old SAM-P/10 mice could not reach the percentage avoidance seen with the 4-month-old mice, even after the 10-day training. When the mean percentage of successful avoidance or escape behavior on every training day was plotted, the curves were much the same for both SAM-R/1 and SAM-P/10 mice, of any age. These results show that aged SAM-P/10 mice retained the left-right turning discrimination in the T-maze and lost the ability to predict the forthcoming aversive shock by associating conditioned stimulus and unconditioned stimulus.

Localization of atrophy-prone areas in the aging mouse brain: Comparison between the brain atrophy model SAM-P/10 and the normal control SAM-R/1

Mouse inbred strain SAM-P/10 (Senescence Accelerated Mouse) is a model of age-related brain atrophy. In this strain there is an earlier and more severe age-related deterioration in the conditional avoidance learning than the normal control inbred SAM-R/1 strain. The present study analysed age-related changes in brain area size using a computerized morphometric method. The region most vulnerable to age-related atrophy in SAM-P/10 was the frontal region of the cerebral cortex, including the prefrontal cortex. Other neocortical regions underwent diffuse atrophy. Posterior piriform cortex, entorhinal cortex, anterior olfactory nucleus, amygdala, caudate-putamen, nucleus accumbens and cerebellar cortex were atrophy-prone regions. The septum also underwent atrophy but other basal forebrain structures were intact. The hippocampus, diencephalon and brainstem structures showed no atrophic change. White matter structures did not change in size with aging except for the forceps minor of the corpus callosum, which showed age-related atrophy. On the contrary, SAM-R/1 showed a significant age-related atrophy only in a restricted part of the cerebral cortex, mainly in the parietal region. Other cortical regions, subcortical structures, diencephalon, brainstem structures, cerebellum and white matter were atrophy-resistant in SAM-R/1. The prefrontal cortex, entorhinal cortex, piriform cortex and striatum are closely interconnected and also connect with the amygdala which plays a key role in conditioning in the rodent. Age-related atrophy in all these structures in SAM-P/10 presumably accounts for the age-related deficits in conditional avoidance learning in this strain of mouse. Comparison between SAM-P/10 and SAM-R/1 or other well-known rodents indicates that SAM-P/10 is a unique rodent that spontaneously and rapidly develops progressive generalized cerebral atrophy, which is considered to be a pathological process rather than an accelerated aging process.

Age-related hearing impairment in senescence-accelerated mouse (SAM)

The auditory brainstem response and histopathology of the cochlea were investigated in an accelerated senescence-prone strain, SAM-P/1 mice and a senescence-resistant strain, SAM-R/1 mice. Each strain displayed an age-related auditory loss expressed as elevated thresholds similar to human hearing loss in that high-frequency losses occurred earlier than middle- or low-frequency losses. SAM-P/1 showed a more rapid decline of hearing with age than did SAM-R/1. Interpeak intervals I-III and I-IV were prolonged with age in both strains, especially at high frequency. The prolongation was more marked in SAM-P/1 than in SAM-R/1. The decrease in amplitude of wave I observed in both strains was greater in SAM-P/1 than in SAM-R/1. The auditory function assessed by thresholds, interpeak intervals and amplitudes of wave I in SAM-P/1 at 12 months of age corresponded roughly to that in SAM-R/1 at 20 months of age. In morphological studies, there was an age-related decrease in the cell density as well as in the size of spiral ganglion neurons in both strains, but these changes were more pronounced in SAM-P/1 than in SAM-R/1. These results reveal that age-related hearing impairment associated with morphological changes in the cochlea is manifested earlier and progresses more rapidly in SAM-P/1 than in SAM-R/1. Thus, the SAM-P/1 strain should prove useful as a model of presbycusis.

Age-related cochlear degeneration in senescence-accelerated mouse

Age-related hair cell loss and strial atrophy were investigated in an accelerated senescence-prone strain, SAMP1 mice, and an accelerated senescence-resistant strain, SAMR1 mice. The loss of inner and outer hair cells in SAMP1 progressed more rapidly than that in SAMR1 with age. In both strains, areas of the loss of inner and outer hair cells were located mainly in the apex and base. Atrophy of the stria vascularis was observed in both strains, but in SAMP1 it appeared to increase earlier than in SAMR1. These results reveal that age-related hair cell loss and atrophy of the stria vascularis comparable to that in the human cochlea occur earlier and progress more rapidly in SAMP1 than in SAMR1. Hearing impairment in SAM may be due to a combination of sensory and strial presbycusis as well as to neural presbycusis, as reported previously. The morphological changes in the cochlea observed in SAMP1 and SAMR1 make these strains suitable for the study of the mechanisms of presbycusis.

Age-related deterioration of long-term potentiation in the CA3 and CA1 regions of hippocampal slices from the senescence-accelerated mouse

Long-term potentiation (LTP) is one candidate for the mechanism underlying memory storage. In the present study, we carried out electrophysiological studies on hippocampal slices prepared from the senescence-accelerated mouse (SAM-P/8), a strain which shows accelerated senescence and failure of certain types of learning in behavioral tests. The findings were compared with those noted in the SAM-R/1 substrain without severe symptoms of senescence. No significant differences were found between SAM-R/1 and SAM-P/8 of the same ages in responses in the absence of tetanic stimulation, and in LTP after tetanic stimulation. However, there were marked decreases in the degree of potentiation with aging in both strains.

Highly selective localization of leukotriene C4 synthase in hypothalamic and extrahypothalamic vasopressin systems of mouse brain

While leukotriene C4 (LTC4) was originally identified as a potent bronchoconstrictor, the compound has versatile biological activities besides inflammatory reactions. Although the high content of LTC4 has been reported in the hypothalamus and median eminence, the precise localization of the compound remained obscure. To elucidate its possible functions in the neuroendocrine systems, we determined immunohistochemical localization of LTC4 synthase, a key enzyme to produce LTC4 using mouse brains. Light microscopy and confocal laser scanning microscopy showed that LTC4 synthase was selectively localized in the vasopressinergic magnocellular neurons of the hypothalamic paraventricular, supraoptic and suprachiasmatic nuclei and in the retrochiasmatic area, as well as in axons that emanated from these neurons to the pars nervosa of the pituitary gland. At subcellular level, however, LTC4 synthase and arginine vasopressin appeared to localize differently within individual neurons. LTC4 synthase immunoreactivity was also observed in the axons of the extrahypothalamic system including the bed nucleus of the stria terminalis, lateral habenular nucleus, midbrain central gray, medial amygdaloid nucleus and ventral septal area, although this immunoreactivity was relatively minor. The other brain regions did not contain LTC4 synthase immunoreactivity. The distribution of LTC4 synthase did not overlap with that of either oxytocin or luteinizing hormone releasing hormone. Therefore, LTC4 is considered to be involved in neural functions of the brain magnocellular vasopressinergic system such as water retention. LTC4 may also be involved in extrahypothalamic vasopressinergic neural functions including the regulation of learning and memory, social recognition memory, sexual and aggressive behavior, etc.

Sensory system-predominant distribution of leukotriene A4 hydrolase and its colocalization with calretinin in the mouse nervous system

Leukotriene B4 is a potent lipid mediator, which has been identified as a potent proinflammatory and immunomodulatory compound. Although there has been robust evidence indicating that leukotriene B4 is synthesized in the normal brain, detailed distribution and its functions in the nervous system have been unclear. To obtain insight into the possible neural function of leukotriene B4, we examined the immunohistochemical distribution of leukotriene A4 hydrolase, an enzyme catalyzing the final and committed step in leukotriene B4 biosynthesis, in the mouse nervous system. Immunoreactivity for leukotriene A4 hydrolase showed widespread distribution with preference to the sensory-associated structures; i.e. neurons in the olfactory epithelium and vomeronasal organ, olfactory glomeruli, possibly amacrine cells, neurons in the ganglion cell layer and three bands in the inner plexiform layer of the retina, axons in the optic nerve and tract up to the superior colliculus, inner and outer hair cells and the spiral ganglion cells in the cochlea, vestibulocochlear nerve bundle, spinal trigeminal tract, and lamina II of the spinal cord. Double immunofluorescence staining demonstrated that most of the leukotriene A4-hydrolase-immunopositive neurons coexpressed calretinin, a calcium-binding protein in neurons. The ubiquitous distribution of leukotriene A4 hydrolase was in sharp contrast with the distribution of leukotriene C4 synthase [Shimada A, Satoh M, Chiba Y, Saitoh Y, Kawamura N, Keino H, Hosokawa M, Shimizu T (2005) Highly selective localization of leukotriene C4 synthase in hypothalamic and extrahypothalamic vasopressin systems of mouse brain. Neuroscience 131:683-689] which was confined to the hypothalamic and extrahypothalamic vasopressinergic neurons. These results suggest that leukotriene B4 may exert some neuromodulatory function mainly in the sensory nervous system, in concert with calretinin.

The SAM Strain of Mice, a Higher Oxidative Stress, Age-Dependent Degenerative Disease, and Senescence Acceleration Model

The Senescence-Accelerated Mouse (SAM) strain of mice is a group of related inbred strains consisting of a series of SAMP (accelerated senescence-prone) and SAMR (accelerated senescence-resistant) strains. Compared with SAMR strains, SAMP strains show accelerated senescence processes, shorter life spans, and earlier onset and more rapid progression of age-associated pathologic phenotypes similar to human geriatric disorders. Based on these observations, the SAM strain was developed as a model for senescence acceleration and age-associated disorders. Numerous studies using this strain of mice have been conducted for more than 30 years to clarify mechanisms of senescence acceleration and pathogenesis of age-associated disorders. Many of the aforementioned mechanisms highlight (a) the oxidative stress status that results from (b) mitochondrial dysfunction as critical factors in possible mechanisms that accelerate the senescence process and cause and/or aggravate age-dependent degeneration of various body tissues. Because the SAM strain was developed as an animal model of geriatric disorders, many experiments to intervene with senescent phenotypes were done from the very beginning. Recently, many substrates with protective effects on mitochondria were tested to determine their effectiveness for treating senescent phenotypes and age-associated disorders seen in SAMP mice. The aforementioned SAM strains can serve as useful tools to understand cellular mechanisms of age-dependent degeneration and to develop clinical interventions.

Effects of rearing conditions on aging characteristics and pathobiological phenotypes in SAMP10 mice

Abstract The purpose of this study was to obtain background data regarding aging characteristics and pathobiological phenotypes in SAMP10/Ta (P10) and SAMR1TA (R1) mice reared under specific pathogen-free (SPF) conditions and compare the data with those of P10 and R1 mice reared under conventional conditions at Kyoto University. Senescence scores were suppressed, and life span was prolonged in both strains under SPF conditions. However, there was still a significant difference between P10 and R1 in both life span and senescence scores under SPF conditions, suggesting a well preserved “accelerated senescence” in P10 mice even under SPF conditions. In P10 mice that died naturally, the incidence of amyloidosis, abscess and contracted kidney was reduced significantly under SPF conditions. However, the incidence of lymphoid cell neoplasms and pulmonary congestion and edema was increased significantly under SPF conditions. In P10 mice, loss of brain weight manifested somewhat later under SPF conditions. Morphometrically, age-related involutional changes were evident mainly in the frontal portion of the cerebrum. The tail suspension test revealed an age-related increase in immobility times. Thus, P10-specific pathologies, brain atrophy and emotional disorder were also preserved even under SPF conditions. In R1 mice, these phenotypes were not observed under either set of conditions.

Centripetal retraction of dendrites with apical vulnerability in the prefrontal neurons of aged SAMP10 mice

Abstract SAMP10 strain is characterized by age-related brain atrophy and by poor learning and memory performance. Here we studied age-related changes in the dendritic arbors and spine density in layer II pyramidal cells of the prefrontal cortex in SAMP10 mice using a quantitative Golgi method, as well as changes in the protein and mRNA expression of MAP2, a microtubule-associated protein. The dendrites of the prefrontal neurons retracted toward the somata with aging, with the apical dendrites being more affected. Spine density of the apical dendrites decreased with aging as well. Immunohistochemical staining indicated a widespread decrease in MAP2 immunoreactivity in the prefrontal cortex of aged SAMP10 mice. Immunoblot analysis indicated a decrease in MAP2 protein expression not only in the anterior but also posterior cortex in aged SAMP10 mice. Reverse transcriptase-polymerase chain reaction (RT-PCR) analysis indicated a decreased MAP2 mRNA expression in the anterior and posterior cortex of aged SAMP10 mice. We concluded that (1) retracting change of the apical dendrites with loss of spines was a manifestation of age-related degeneration of the prefrontal neurons in SAMP10 mice, (2) the dendritic degeneration occurred in a widespread manner affecting the entire cerebral cortex, and was related to a decrease in MAP2 expression of the neurons.