Depressive behavior and alterations in receptors for dopamine and 5-hydroxytryptamine in the brain of the senescence accelerated mouse (SAM)-P10

Improvement of Learning and Memory in Senescence-Accelerated Mice by S-Allylcysteine in Mature Garlic Extract

S-allylcysteine (SAC), a major thioallyl compound contained in mature garlic extract (MGE), is known to be a neuroactive compound. This study was designed to investigate the effects of SAC on primary cultured hippocampal neurons and cognitively impaired senescence-accelerated mice prone 10 (SAMP10). Treatment of these neurons with MGE or SAC significantly increased the total neurite length and number of dendrites. SAMP10 mice fed MGE or SAC showed a significant improvement in memory dysfunction in pharmacological behavioral analyses. The decrease of α-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid (AMPA) receptor, N-methyl-d-aspartate (NMDA) receptor, and phosphorylated α-calcium/calmodulin-dependent protein kinase II (CaMKII) in the hippocampal tissue of SAMP10 mice fed MGE or SAC was significantly suppressed, especially in the MGE-fed group. These findings suggest that SAC positively contributes to learning and memory formation, having a beneficial effect on brain function. In addition, multiple components (aside from SAC) contained in MGE could be useful for improving cognitive function by acting as neurotrophic factors.

Depression-like behavior is dependent on age in male SAMP8 mice

Aging is associated with an increased risk of depression in humans. To elucidate the underlying mechanisms of depression and its dependence on aging, here we study signs of depression in male SAMP8 mice. For this purpose, we used the forced swimming test (FST). The total floating time in the FST was greater in SAMP8 than in SAMR1 mice at 9 months of age; however, this difference was not observed in 12-month-old mice, when both strains are considered elderly. Of the two strains, only the SAMP8 animals responded to imipramine treatment. We also applied the dexamethasone suppression test (DST) and studied changes in the dopamine and serotonin (5-HT) uptake systems, the 5-HT2a/2c receptor density in the cortex, and levels of TPH2. The DST showed a significant difference between SAMR1 and SAMP8 mice at old age. SAMP8 exhibits an increase in 5-HT transporter density, with slight changes in 5-HT2a/2c receptor density. In conclusion, SAMP8 mice presented depression-like behavior that is dependent on senescence process, because it differs from SAMR1, senescence resistant strain.

Towards translational rodent models of depression

Rodent models of depression have been developed in an effort to identify novel antidepressant compounds and to further our understanding of the pathophysiology of depression. Various rodent models of depression and antidepressant-like behaviour are currently used but, clearly, none of these current models fully recapitulate all features of depression. Moreover, these models have not resulted in the development of novel non-monoaminergic-based antidepressants with clinical efficacy. Thus, a refinement of the current models of depression is required. The present review outlines the most commonly used models of depression and antidepressant drug-like activity and suggests several factors that should be considered when refining these models.

Senescence-accelerated mouse (SAM) with special references to neurodegeneration models, SAMP8 and SAMP10 mice

The SAM strains, a group of related inbred strains consisting of senescence-prone inbred strains (SAMP) and senescence-resistant inbred strains (SAMR), have been successfully developed by selective inbreeding of the AKR/J strain of mice donated by the Jackson laboratory in 1968. The characteristic feature of aging common to the SAMP and SAMR is accelerated senescence and normal aging, respectively. Furthermore, SAMP and SAMR strains of mice manifest various pathobiological phenotypes spontaneously. Among SAMP strains, SAMP8 and SAMP10 mice show age-related behavioral deterioration such as deficits in learning and memory, emotional disorders (reduced anxiety-like behavior and depressive behavior) and altered circadian rhythm associated with certain pathological, biochemical and pharmacological changes. Here, the previous and recent literature on SAM mice are reviewed with an emphasis on SAMP8 and SAMP10 mice. A spontaneous model like SAM with distinct advantages over the gene-modified model is hoped by investigators to be used more widely as a biogerontological resource to explore the etiopathogenesis of accelerated senescence and neurodegenerative disorders.

Apical vulnerability to dendritic retraction in prefrontal neurones of ageing SAMP10 mouse: a model of cerebral degeneration

The SAMP10 mouse is a model of accelerated ageing in which senescence is characterized by age-related atrophy of the cerebral cortex and limbic structures, poor learning and memory task performance with depressive behaviour and cholinergic and dopaminergic alterations. Here we studied age-related changes in the dendritic arbors and spine density of pyramidal cells in the medial prefrontal cortex of SAMP10 mice using a quantitative Golgi method. Dendrites of prefrontal neurones gradually retracted with ageing towards the soma with the relative preservation of overall complexity. Apical dendrites were much more severely affected than basal dendrites. The combined length of the apical dendrites and spine density were decreased by 45% and 55%, respectively, in mice at 12 months, compared with mice at 3 months of age. Immunohistochemical and immunoblot analyses indicated that expression of microtubule-associated protein (MAP) 2, a marker of dendrites, decreased in an age-related manner not only in the anterior cortex but also in the posterior cortex and olfactory structures in SAMP10 mice. Decreased expression of MAP2 mRNA caused the decrease in MAP2 protein expression. These results suggest that retraction of apical, but not of basal dendrites, with a loss of spines in prefrontal neurones, appears to be responsible for poor learning and memory performance in aged SAMP10 mice. It is also suggested that age-related dendritic retraction occurs in a wide area including the entire cerebral cortex and olfactory structures.

Antidepressant-like effects of selegiline in the forced swim test

Although selegiline, a monoamine oxidase (MAO)-B inhibitor, is reported to exert antidepressant effects in depressant patients, evidence in rodents for effects of selegiline is quite limited. The purpose of the present study was to assess effects of selegiline in the forced swim test (FST) and on locomotor activity, and to investigate whether MAO inhibition or stimulation of receptors contributes to antidepressant-like effects of selegiline. Drugs were subcutaneously administrated. The single administration of reference drug nortriptyline at 5 mg/kg reduced locomotor activity without effects in FST and brain MAO activities. But nortriptyline repeatedly given 24, 5 and 1 h before behavioral tests significantly decreased an immobility time in FST without effects in motor activities, and showed weak brain MAO-B inhibition. Single and following repeated (24, 5 and 1 h before behavioral tests) administrations of selegiline at 10 mg/kg significantly decreased the immobility time in FST, with little motor stimulant effect. In contrast, (+)-methamphetamine caused a marked decrease in immobility time and an increase in locomotor activity. Selegiline at 1 and 3 mg/kg, which failed to decrease immobility time, markedly inhibited brain total-MAO and MAO-B activities. A dopamine D1 receptor antagonist SCH 23390 completely blocked antidepressant-like effects of selegiline, but not dopamine D2, serotonergic or noradrenergic receptor antagonists. These results suggest that selegiline exerts the antidepressant-like effects by prolonging escape-directed behavior rather than by a motor stimulant effect and D1 receptor activation contributes to its effect.

Age-related alterations in the expression of glial cell line-derived neurotrophic factor in the senescence-accelerated mouse brain

Senescence-accelerated mouse prone 8 (SAMP8) and prone 10 (SAMP10) are useful murine model of accelerated aging. SAMP8 shows marked impairment of learning and memory, whereas SAMP10 shows brain atrophy and aging-associated depressive behavior. This study examined the expression of glial cell line-derived neurotrophic factor (GDNF) in SAMP8 and SAMP10 brains, relative to that in SAM resistant 1 (SAMR1) controls, which age normally. Hippocampal GDNF mRNA expression decreased in an age-dependent manner (10- vs 2-month-old animals) in the SAMR1, but not in the SAMP8 or SAMP10 strains. Furthermore, GDNF mRNA expression in 2-month-old SAMP8 and SAMP10 strains was less than in SAMR1 specimens of the same age. The number of surviving neurons in the CA1 region decreased with age in SAMP8 and SAMP10, and also decreased relative to the number of neurons in 10-month-old SAMR1 controls. Immunohistochemistry revealed that cells that were positive for GDNF-like activity in 10-month-old SAMP8 and SAMP10 were diffusely distributed, in part, around the pyramidal cell layer in the hippocampus. These findings suggest that low GDNF expression in young SAMP8 and SAMP10 may be involved in hippocampal dysfunctions, such as age-related learning impairment and neuronal death.