Impaired synaptic functions with aging as characterized by decreased calcium influx and acetylcholine release

Biomolecular Markers of Brain Aging

Characterized by the gradual loss of physiological integrity, impaired function, and increased susceptibility to death, aging is considered the primary risk factor for major human diseases, such as cancer, diabetes, cardiovascular disorders, and neurodegenerative diseases. The time-dependent accumulation of cellular damage is widely considered the general cause of aging. While the mechanism of normal aging is still unresolved, researchers have identified different markers of aging, including genomic instability, telomere attrition, epigenetic alterations, loss of proteostasis, deregulated nutrient-sensing, mitochondrial dysfunction, cellular senescence, stem cell exhaustion, and altered intercellular communication. Theories of aging can be divided into two categories: (1) aging is a genetically programmed process, and (2) aging is a random process caused by gradual damage to the organism over time as a result of its vital activities. Aging affects the entire human body, and aging of the brain is undoubtedly different from all other organs, as neurons are highly differentiated postmitotic cells, and the lifespan of most neurons in the postnatal period is equal to the lifespan of the brain. In this chapter, we discuss the conserved mechanisms of aging that may underlie the changes observed in the aging brain, with a focus on mitochondrial function and oxidative stress, autophagy and protein turnover, insulin/IGF signaling, target of rapamycin (TOR) signaling, and sirtuin function.

DNA Methyltransferase 1 (DNMT1) Function Is Implicated in the Age-Related Loss of Cortical Interneurons

Increased life expectancy in modern society comes at the cost of age-associated disabilities and diseases. Aged brains not only show reduced excitability and plasticity, but also a decline in inhibition. Age-associated defects in inhibitory circuits likely contribute to cognitive decline and age-related disorders. Molecular mechanisms that exert epigenetic control of gene expression contribute to age-associated neuronal impairments. Both DNA methylation, mediated by DNA methyltransferases (DNMTs), and histone modifications maintain neuronal function throughout lifespan. Here we provide evidence that DNMT1 function is implicated in the age-related loss of cortical inhibitory interneurons. Dnmt1 deletion in parvalbumin-positive interneurons attenuates their age-related decline in the cerebral cortex. Moreover, conditional Dnmt1-deficient mice show improved somatomotor performance and reduced aging-associated transcriptional changes. A decline in the proteostasis network, responsible for the proper degradation and removal of defective proteins, is implicated in age- and disease-related neurodegeneration. Our data suggest that DNMT1 acts indirectly on interneuron survival in aged mice by modulating the proteostasis network during life-time.

Altered acetylcholine release in the hippocampus of dystrophin-deficient mice

Mild cognitive impairments have been described in one-third of patients with Duchenne muscle dystrophy (DMD). DMD is characterized by progressive and irreversible muscle degeneration caused by mutations in the dystrophin gene and lack of the protein expression. Previously, we have reported altered concentrations of α7- and β2-containing nicotinic acetylcholine receptors (nAChRs) in hippocampal membranes of dystrophic (mdx) mice. This suggests that alterations in the central cholinergic synapses are associated with dystrophin deficiency. In this study, we examined the release of acetylcholine (ACh) and the level of the vesicular ACh transporter (VAChT) using synaptosomes isolated from brain regions that normally have a high density of dystrophin (cortex, hippocampus and cerebellum), in control and mdx mice at 4 and 12months of age. ACh release evoked by nicotinic stimulation or K(+) depolarization was measured as the tritium outflow from superfused synaptosomes preloaded with [(3)H]-choline. The results showed that the evoked tritium release was Ca(2+)-dependent and mostly formed by [(3)H]-ACh. β2-containing nAChRs were involved in agonist-evoked [(3)H]-ACh release in control and mdx preparations. In hippocampal synaptosomes from 12-month-old mdx mice, nAChR-evoked [(3)H]-ACh release increased by 57% compared to age-matched controls. Moreover, there was a 98% increase in [(3)H]-ACh release compared to 4-month-old mdx mice. [(3)H]-ACh release evoked by K(+) depolarization was not altered, while the VAChT protein level was decreased (19%) compared to that of age-matched controls. In cortical and cerebellar preparations, there was no difference in nAChR-evoked [(3)H]-ACh release and VAChT levels between mdx and age-matched control groups. Our previous findings and the presynaptic alterations observed in the hippocampi of 12-month-old mdx mice indicate possible dysfunction of nicotinic cholinergic synapses associated with dystrophin deficiency. These changes may contribute to the cognitive and behavioral abnormalities described in dystrophic mice and patients with DMD.

Decreased response to acetylcholine during aging of aplysia neuron R15

How aging affects the communication between neurons is poorly understood. To address this question, we have studied the electrophysiological properties of identified neuron R15 of the marine mollusk Aplysia californica. R15 is a bursting neuron in the abdominal ganglia of the central nervous system and is implicated in reproduction, water balance, and heart function. Exposure to acetylcholine (ACh) causes an increase in R15 burst firing. Whole-cell recordings of R15 in the intact ganglia dissected from mature and old Aplysia showed specific changes in burst firing and properties of action potentials induced by ACh. We found that while there were no significant changes in resting membrane potential and latency in response to ACh, the burst number and burst duration is altered during aging. The action potential waveform analysis showed that unlike mature neurons, the duration of depolarization and the repolarization amplitude and duration did not change in old neurons in response to ACh. Furthermore, single neuron quantitative analysis of acetylcholine receptors (AChRs) suggested alteration of expression of specific AChRs in R15 neurons during aging. These results suggest a defect in cholinergic transmission during aging of the R15 neuron.

Neuronal dysfunction with aging and its amelioration

The author focused on the functional decline of synapses in the brain with aging to understand the underlying mechanisms and to ameliorate the deficits. The first attempt was to unravel the neuronal functions of gangliosides so that gangliosides could be used for enhancing synaptic activity. The second attempt was to elicit the neuronal plasticity in aged animals through enriched environmental stimulation and nutritional intervention. Environmental stimuli were revealed neurochemically and morphologically to develop synapses leading to enhanced cognitive function. Dietary restriction as a nutritional intervention restored the altered metabolism of neuronal membranes with aging, providing a possible explanation for the longevity effect of dietary restriction. These results obtained with aging and dementia models of animals would benefit aged people.

Preparation of (13)C-labeled ceramide by acetic acid bacteria and its incorporation in mice

We prepared 2-hydroxypalmitoyl-sphinganine (dihydroceramide) labeled with a stable isotope by culturing acetic acid bacteria with (13)C-labeled acetic acid. The GC/MS spectrum of the trimethylsilyl derivative of (13)C-labeled dihydroceramide gave molecular ions with an increased mass of 12-17 Da over that of nonlabeled dihydroceramide. The fragment ions derived from both sphinganine base and 2-hydroxypalmitate were confirmed to be labeled with the stable isotope in the spectrum. Therefore, (13)C-labeled dihydroceramide can be an extremely useful tool for analyzing sphingolipid metabolism. The purified [(13)C]dihydroceramide was administered orally to mice for 12 days, and the total sphingoid base fractions in various tissues were analyzed by GC/MS. The spectrum patterns specific to (13)C-labeled sphingoids were detected in the tissues tested. Sphinganine pools in skin epidermis, liver, skeletal muscle, and synapse membrane in brain were replaced by [(13)C]sphinganine at about 4.5, 4.0, 1.0, and 0.3%, respectively. Moreover, about 1.0% of the sphingosine pool in the liver was replaced by [(13)C]sphingosine, implying that exogenous dihydroceramide can be converted to sphingosine. These results clearly indicate that ingested dihydroceramide can be incorporated into various tissues, including brain, and metabolized to other sphingolipids.

Motor coordination impairment in aged heterozygous rolling Nagoya, Cav2.1 mutant mice

Although rolling Nagoya mice exhibit ataxia and carry a mutation in the alpha1 subunit of the Cav2.1 channel regulating neurotransmitter release, heterozygous mice have not received a great deal of attention. Given the pivotal role of Cav2.1 channels in controlling neurotransmitter release, age-dependent alterations in Cav2.1 channel function may result in aberrant synaptic signaling, leading to motor dysfunction. To examine age-related motor alterations in heterozygous mice, we used a battery of tests (e.g., motor activity, footprint, traction, wire suspension, balance beam, rotating rod, hind-limb extension analysis) in 2- and 22-month-old mice and examined expression patterns of the alpha1 gene in their cerebellum. No significant difference was observed between 2-month-old heterozygous and wild-type mice in the any of the behavioral tests or in the alpha1 expression levels. Although 22-month-old heterozygous and wild-type mice exhibited no significant difference in motor activity, footprint, or traction tests, 22-month-old heterozygous mice showed deficits in the wire hanging, balance beam, and rotating rod tests. Additionally, 22-month-old heterozygous mice displayed clasping behavior in the hind-limb extension test. Expression analysis showed that wild-type Cav2.1alpha(1) mRNA was lower in aged mice than in young mice and that mutant-type Cav2.1alpha(1) mRNA was higher in aged mice than in young mice. These findings suggest that heterozygous mice show age-related motor changes due to mutant-type Cav2.1 and that heterozygous mice may represent a new model for examining motor function.

Aging modifies brain region-specific vulnerability to experimental oxidative stress induced by low dose hydrogen peroxide

Our previous studies demonstrated a significant decline in brain function and behavior in Fischer 344 (F344) rats with age. The present study was designed to test the hypothesis that dysregulation in calcium homeostasis (as assessed through (45)Ca flux) may contribute to the increase in age-related vulnerability to oxidative stress in brain regions, and result in a deficit in behavior-mediated signaling. Crude membrane (P-2) and more purified synaptosomal fractions were isolated from the striatum, hippocampus, and frontal cortex of young (6 months) and old (22 months) F344 rats and were assessed for calcium flux and extracellular-regulated kinase activity 1 (ERK) under control and oxidative stress conditions induced by low dose hydrogen peroxide (final concentration 5 microM). The level of oxidative stress responses was monitored by measuring reactive oxygen species (ROS) and glutathione (GSH). The results showed a significant difference in oxidative stress responses between young and old rats in evaluated brain regions. Old rats showed higher sensitivity to oxidative stress than young rats. The present findings show the differential effects of oxidative stress on calcium flux in brain regions with age that are dependent upon the brain areas examined and the fraction assessed. The accumulation of ROS and the decrease in GSH in the frontal cortex were sufficient to decrease ERK activity in old rats. This is the first study, to our knowledge, that demonstrates age-related differential sensitivity to oxidative stress expressed as a function of behavior-mediated signaling and stress levels among different fractions isolated from brain regions controlling behavior.

Comparison of the prevalence and risk factors for depressive symptoms among elderly nursing home residents in Taiwan and Hong Kong

BACKGROUND

Depression in the elderly has become a serious health care issue worldwide. However, no studies have compared the prevalence and risk factors for depressive symptoms among institutionalized Chinese elders living in different regions.

OBJECTIVES

To explore and compare the prevalence and risk factors for depressive symptoms among elderly residents of nursing homes in Taiwan and Hong Kong.

METHODS

Random sampling was used to recruit participants from eight nursing homes in each region. A total of 150 elders from Taiwan and 214 elders from Hong Kong participated.

RESULTS

The prevalence of depressive symptoms was significantly higher in participants from Hong Kong (65.4%) than in Taiwan (43.3%). Logistic regression analysis indicated that gender, satisfaction with living situation, perceived health condition, and perceived income adequacy significantly predicted depressive symptoms in elderly nursing home residents in Taiwan. Significant predictors of depressive symptoms in the Hong Kong sample were satisfaction with living situation, cognitive status, and functional status.

CONCLUSIONS

It is important to consider risk factors specific to a target population when developing depression intervention programs.

Age-related changes in the levels of voltage-dependent calcium channels and other synaptic proteins in rat brain cortices

Neurotransmitter release from synapses is one of the most important interneuronal signaling in the nervous system. We previously reported that aging decreases depolarization-induced acetylcholine release in rat brain synaptosomes. To investigate the mechanisms underlying the age-related decrements of neurotransmission, we determined the levels of the alpha1 subunit proteins of voltage-dependent calcium channels (VDCCs) and three synaptic proteins that relate to exocytotic processes using synaptosomes prepared from cerebral cortices of young (6-month-old) and aged (27-month-old) rats. Immunoblotting analyses revealed that the protein levels of alpha1A (P/Q-type) and alpha1B (N-type) subunits in aged rats were 38% and 43% lower than the levels of young rats, respectively, but the levels of the alpha1C (L-type) subunit were not different between young and aged. On the contrary, the levels of synaptotagmin-1, synaptophysin and syntaxin were not significantly different between the two age groups in the synaptosomal preparations. These results suggest that synaptic density does not change much in the cerebral cortex in normal aging, and that the reduction of P/Q-type and N-type VDCCs, both of which participate in neurotransmitter release, is one of the causes for the decrease of neurotransmission at aged synapses.

α-Sialylcholesterol enhances the depolarization-induced release of acetylcholine and glutamate in rat hippocampus: in vivo microdialysis study

The effects of alpha-sialylcholesterol (alpha-SC), a synthetic ganglioside analogue, on synaptic neurotransmission were studied using in vivo microdialysis technique. Application of alpha-SC through a microdialysis probe enhanced high potassium-evoked release of acetylcholine and glutamate in the hippocampal CA3 region of Wistar rats. The experiments using synaptosomes and FM1-43, a fluorescent styryl dye used for studies of neurotransmitter release mechanisms, showed that alpha-SC increased depolarization-induced loss of dye but it did not evoke the dye loss at resting condition. These results indicate that alpha-SC promotes a depolarization-induced exocytotic neurotransmitter release in the brain under in vivo conditions. Application of alpha-SC increased the level of glutamate but not that of acetylcholine, suggesting that alpha-SC affects spontaneous glutamate release and/or transport system at the brain region.

3,4-DAP-evoked transmitter release in hippocampal slices of aged rats with impaired memory

Based on a slice superfusion technique, this study investigated the release of acetylcholine, noradrenaline and serotonin in the hippocampus of aged rats (25-27 months) showing no or severe deficits in a spatial reference-memory task (water maze). Young adults (3-5 months) were used as controls. 3,4-diaminopyridine (3,4-DAP), a potassium channel antagonist which increases neuronal excitability, was used to evoke the overflow of the three neurotransmitters. The release of [3H]noradrenaline induced by stimulation of presynaptic nicotinic receptors was also assessed. The experiment compared the accumulation and 3,4-DAP-evoked (or nicotine-evoked) overflow of [3H] in hippocampal slices preincubated with [3H]choline, [3H]noradrenaline, or [3H]serotonin. In aged rats, only the accumulation of [3H]serotonin was reduced significantly (-17%). In percent of tissue-[3H], the 3,4-DAP-evoked overflow of [3H]serotonin was increased (+28%), and that of [3H]acetylcholine was reduced (-23%) in the aged rats. The nicotine-evoked overflow of [3H]noradrenaline was not altered in aged rats. There was a significant correlation of water-maze performance (distance to platform) and evoked overflow of [3H]serotonin. It is concluded that hippocampal cholinergic functions are more altered by aging than noradrenergic or serotonergic ones. Excessive excitability of serotonergic terminals, perhaps in addition to cholinergic dysfunction, might be a crucial factor accounting for age-related cognitive deficits in the present population of rats.

Presynaptic regulation of neurotransmitter release in the cortex of aged rats with differential memory impairments

Cluster analysis of water-maze reference-memory performances of 25-27-month-old (compared to 3-5-month-old) rats distinguished subpopulations of young adult rats (YOUNG), aged rats with no significant impairment (AU), aged rats with moderate impairment (AMI), and aged rats with severe impairment (ASI). In the frontoparietal cortex, we subsequently assessed the electrically evoked release of tritium in slices preloaded with [3H]choline, [3H]noradrenaline (NA), or [3H]serotonin (5-HT) and the effects of an agonist (oxotremorine, UK 14,304, and CP 93,129) of the respective autoreceptors. Cholinergic and monoaminergic markers were measured in homogenates. Overall, aged rats exhibited reduced accumulation of [3H]choline (-25%) and weaker evoked transmitter release (in % of accumulated tritium: -44%, -20%, and -34%, for [3H]acetylcholine, [3H]NA, and [3H]5-HT, respectively). In all rats, the inhibitory effects of the autoreceptor agonists on the evoked release of [3H] were comparable. Acetylcholinesterase (AChE), not choline acetyltransferase (ChAT), activity was reduced. The results suggest age-related modifications in the cholinergic, noradrenergic, and serotonergic innervation of the frontoparietal cortex, alterations of evoked transmitter release, but no interference with presynaptic autoinhibition of the release. Neither of these alterations seemed to account for the cognitive impairment assessed.

Turnover of synaptic membranes: age-related changes and modulation by dietary restriction

We examined age-related changes in the turnover rates of synaptic membrane components that might underlie the decrease in synaptic functions in senescence. Synaptic membrane constituents were labeled in vivo with deuterium and the disappearance of the deuterated molecules from synaptic membranes was measured by mass spectrometry. The turnover rates of phosphatidylcholine, phosphatidylethanolamine, cholesterol, and synaptophysin were all shown to slow down with aging. Dietary restriction, which is known to retard various aging processes, was found to decrease the turnover rates of membrane lipid species. Consequently, the fatty acid composition in phospholipids remained unchanged in the synaptic plasma membranes of food restricted mice. In contrast, the turnover rate of synaptophysin was accelerated under dietary restriction. This may mean that increased turnover enhances the removal of damaged proteins from membranes.

Assessment of choline uptake for the synthesis and release of acetylcholine in brain slices by a dynamic autoradiographic technique using [11C]choline

The uptake of choline for the synthesis and release of acetylcholine was investigated in brain slices by dynamic positron autoradiography using [11C]choline. Brain slices (330 microm) were incubated with [11C]choline in oxygenated Krebs-Ringer medium at 34 degrees C and serial two-dimensional time-resolved images of the uptake and release of radioactivity were recorded on Storage Phosphor screens. [11C]choline uptake increased with the period of incubation and was 1.9 times higher in the striatum than cerebral cortex. The uptake in the striatum was significantly diminished by hemicholinium-3 (HC-3), an inhibitor of high-affinity choline uptake. Pretreatment of brain slices with 50 mM K(+) for 20 min enhanced the uptake in striatum. The uptake of [11C]choline in brain slices was saturable using nonlabeled choline. Two uptake systems, a high-affinity and a low-affinity system, were confirmed to exist by kinetic analysis using Lineweaver-Burk plots. The 11C radioactivity that had accumulated in the striatum disappeared on treatment with veratridine, a depolarization agent, in the presence of HC-3. This pattern of disappearance was consistent with that of the appearance of unlabeled and labeled acetylcholine in the medium. These results indicate that this method is useful for obtaining information regarding the uptake of choline for the synthesis and release of acetylcholine in live brain tissues.

Oral administration of soybean lecithin transphosphatidylated phosphatidylserine improves memory impairment in aged rats

Soybean lecithin transphosphatidylated phosphatidylserine (SB-tPS) was prepared from soybean lecithin and L-serine by a transphosphatidylation reaction, and its effect on age-related memory impairment was evaluated in rats by the Morris water maze test. Continuous oral administration of SB-tPS (60 mg x kg(-1) x d(-1) for 60 d) to male aged rats (24-25 mo) significantly improved performance in the water maze escape test (P < 0.01 vs. control aged rats) similar to bovine brain cortex-derived phosphatidylserine, which restores cognitive function in patients with senile dementia. SB-tPS also increased acetylcholine release and the Na(+), K(+)-ATPase activity of the synaptosomes prepared from these aged rats to the level in young rats. The nootropic actions of SB-tPS in the present study can be partly explained by the changes in these biochemical activities.

Enhancement of learning capacity and cholinergic synaptic function by carnitine in aging rats

The effects of a carnitine derivative, acetyl-L-carnitine (ALCAR), on the cognitive and cholinergic activities of aging rats were examined. Rats were given ALCAR (100 mg/kg) per os for 3 months and were subjected to the Hebb-Williams tasks and a new maze task, AKON-1, to assess their learning capacity. The learning capacity of the ALCAR-treated group was superior to that of the control. Cholinergic activities were determined with synaptosomes isolated from the cortices. The high-affinity choline uptake by synaptosomes, acetylcholine synthesis in synaptosomes, and acetylcholine release from synaptosomes on membrane depolarization were all enhanced in the ALCAR group. This study indicates that chronic administration of ALCAR increases cholinergic synaptic transmission and consequently enhances learning capacity as a cognitive function in aging rats.

Age-related changes in the subtypes of voltage-dependent calcium channels in rat brain cortical synapses

Age-related changes in the relative contribution of voltage-dependent calcium channel (VDCC) subtypes to depolarization-induced Ca(2+) influx and in the density of VDCC subtypes in cortical synapses were investigated using synaptosomes and their membrane preparations from brain cortices of Wistar rats. The relative contribution of VDCC subtypes to Ca(2+) influx was determined by measuring the inhibition of depolarization-induced Ca(2+) influx with four VDCC subtype-specific peptide blockers. In adult rat synaptosomes, L-, N-, P- and Q-type channels accounted for 24, 32, 27 and 12% of the total Ca(2+) influx, respectively. Brain aging significantly reduced the relative contributions of N- and P-type channels and increased the contribution of the channels resistant to the four blockers used. The densities of VDCC subtypes, determined by binding experiments using radiolabeled PN200 -110, omega-conotoxin GVIA and omega-conotoxin MVIIC, were found to be significantly decreased in aged synaptic plasma membranes. On the contrary, the dissociation constants of the blockers were not changed except for PN200-110-sensitive L-type channels. These results suggest that aging alters the relative contributions of each VDCC subtype to depolarization-induced Ca(2+) influx and decreases the number of VDCCs in rat brain cortical synapses. These changes in VDCCs may lead to age-related hypofunction of synaptic neurotransmission in brain cortices.

The effects of soybean transphosphatidylated phosphatidylserine on cholinergic synaptic functions of mice

The effects of soybean transphosphatidylated phosphatidylserine (SB-tPS) on cholinergic synaptic functions were investigated using cerebral cortical synaptosomes from mice. Treatment of the synaptosomes with SB-tPS increased high K+-induced acetylcholine (ACh) release in a bell-shaped, dose-dependent manner without affecting ACh synthesis. SB-tPS (10 and 50microM) also enhanced synaptosomal synthesis of sn-glycero-3-phosphocholine, but did not affect phosphorylcholine synthesis. In contrast, the choline synthesis was significantly reduced as SB-tPS concentration increased. The present result that SB-tPS modified the cholinergic pathway can partly explain its nootropic functions.

Contribution of cholinergic and gabaergic functions to memory processes in BALB/cANnCrlBR mice

Several lines of evidence indicate that glucose influences on memory depend on interactions between glucose, glucoregulation and hippocampal cholinergic function. We previously demonstrated that glucose and scopolamine differentially affected memory consolidation for an operant bar pressing task in two closely-related BALB/c mouse strains. Whereas glucose normally improves memory in several animal strains, memory consolidation was not effected by systemic glucose injections in BALB/cANnCrlBR mice. Moreover, these mice were relatively insensitive to the normally observed amnestic effects of scopolamine. We therefore sought to determine whether cholinergic mechanisms in the dorsal hippocampus were involved in such atypical drug effects on memory processing in that strain of mice. In Experiment 1, we examined whether post-training oxotremorine would also atypically influence memory consolidation for an appetitively reinforced operant bar pressing task following microinjection in the dorsal hippocampus. In Experiment 2, we examined the effects of intrahippocampal GABAA drugs on memory consolidation. The non-selective muscarinic agonist, oxotremorine, dose-dependently impaired memory and the GABAA antagonist, bicuculline, improved retention in BALB/cANnCrlBR mice. It was concluded that GABA-mediated influences on hippocampal pyramidal output in BALB/cANnCrlBR mice and other strains are similar; but the amnestic effects of oxotremorine from the dorsal hippocampus were opposite to facilitating effects normally observed in other animal strains. Results are discussed relative to possible altered septo-hippocampal cholinergic neurotransmission in BALB/cANnCrlBR mice.

Metabotropic glutamate receptor agonist (1S,3R-ACPD) increased frontal cortex dopamine release in aged but not in young rats

The effects of the metabotropic glutamate (mGlu) receptor agonist (1S,3R)-1-Amino cyclopentane-1,3-dicarboxylic acid (1S,3R-ACPD) infusion on frontal cortex dopamine extracellular levels were studied by microdialysis in young (3 months) and aged (24 months) rats. Basal dopamine levels were significantly higher in young than in aged rats. (1S,3R)-ACPD (1 mM) significantly increased dopamine efflux in aged rats, an effect which was antagonized by the mGlu receptor antagonist, (S)-alpha-methyl-4-carboxypheniylglycine (MCPG) (2 mM). On the contrary, (IS,3R)-ACPD up to the concentration of 2 mM failed to influence dopamine extracellular levels in young rats. These results suggest that the agonist of mGlu receptor group I and/or II can improve dopamine release under conditions of deficiency of extracellular dopamine concentration as observed in aging.

Gangliosides and sialylcholesterol as modulators of synaptic functions

Gangliosides were shown to enhance the release of acetylcholine from synaptosomes on stimulation. The influx of calcium ion into synaptosomes on membrane depolarization was increased by gangliosides. This was hypothesized to be an underlying mechanisms for the enhancement of acetylcholine release. Studies using calcium channel blockers revealed that four distinct types of voltage-dependent calcium channels occurred in cerebrocortical synapses, and that the N-type was primarily responsible for the evoked release of acetylcholine. An additional result suggests that gangliosides may act mainly on the N-type calcium channel. Cholinergic-specific gangliosides, Chol-1 alpha, were assumed to participate in the mechanism of high-affinity choline uptake. These two different actions of gangliosides were found to be mimicked by synthetic ganglioside analogs. Calcium influx was increased by alpha-sialylcholesterol, and choline uptake was accelerated by beta-sialylcholesterol. Gangliosides and sialylcholesterol having these apparently beneficial effects were shown to ameliorate decreased functions of synapses from aged brains.

Effect of the long-term feeding of dietary lipids on the learning ability, fatty acid composition of brain stem phospholipids and synaptic membrane fluidity in adult mice: a comparison of sardine oil diet with palm oil diet

The effect of 12 month feeding of 5% palm oil or sardine oil diet on the maze-learning ability, fatty acid composition of brain stem phospholipids and synaptic membrane fluidity in mice was studied. The time required to reach the maze exit and the number of times that a mouse strayed into blind alleys in the maze were measured three times every 4 days. The time and number of mice fed on the sardine oil diet were less than those of animals fed on the palm oil diet in the first and second trials. The results of fatty acid composition analysis of brain stem phosphatidylethanolamine showed that the percentage of docosahexaenoic acid (22:6, n-3; DHA) was higher, but the arachidonic acid (20:4, n-6; AA) and docosatetraenoic acid (22:4, n-6; DTA) were lower in the sardine oil diet fed-mice than in the palm oil diet fed-animals. Moreover, the microviscosity of the synaptic plasma membrane in the sardine oil diet group was lower than that in the palm oil diet group. These results suggest that the adult mice fed on the sardine oil diet for a long period maintain higher levels of docosahe xaenoic acid in brain phospholipids, synaptic membrane fluidity and maze-learning ability than animals fed on the palm oil diet.

Gangliosides enhance KCl-induced Ca2+ influx and acetylcholine release in brain synaptosomes

Effects of gangliosides GM1 and GQ1b on cholinergic synaptic functions were investigated using synaptosomes prepared from mouse brain cortices. Treatment of synaptosomes with GM1 and GQ1b increased high K(+)-evoked acetylcholine (ACh) release in a bell-shaped dose-dependent manner. The peaks of the effects were found to be at 1-5 microM for GM1 and 5-10 microM for GQ1b. ACh synthesis and the levels of ACh in synaptosomes were not affected by the ganglioside treatment. Both gangliosides enhanced depolarization-induced influx of calcium ions into synaptosomes. These results indicate that GM1 and GQ1b gangliosides increase evoked ACh release by modulating voltage-dependent calcium channels in the synaptic plasma membranes. The effect of GM1 on calcium ion influx remained after repetitive washings, but was almost completely abolished when the bound GM1 was removed by trypsin. This indicates that the fraction of GM1 which was tightly bound to, but not incorporated in synaptic plasma membranes, is responsible for activating the calcium channels.

Decrease in exploratory behavior in naturally occurring cholecystokinin (CCK)-A receptor gene knockout rats

We recently found a specific strain of rats (Otsuka Long Evans Tokushima Fatty; OLETF rats) lacking cholecystokinin (CCK)-A receptor because of a genetic abnormality. Since CCK is an abundant neurotransmitter peptide in the brain, we examined whether these animals exhibit any behavioral abnormalities by conducting open-field tests. OLETF rats showed hypolocomotor activity and a decrease in the incidence of rearing in open-field tests. CCK release from the synaptosomes, and levels of CCK and CCK-B receptor mRNAs were the same as those observed in normal rats. It is suggested that the decreased exploration in OLETF rats may be due to the lack of CCK-A receptor.