Water maze learning and hippocampal synaptic plasticity in streptozotocin-diabetic rats: effects of insulin treatment

Glucose transporter expression in the central nervous system: relationship to synaptic function

The family of facilitative glucose transporter (GLUT) proteins is responsible for the entry of glucose into cells throughout the periphery and the brain. The expression, regulation and activity of GLUTs play an essential role in neuronal homeostasis, since glucose represents the primary energy source for the brain. Brain GLUTs exhibit both cell type and region specific localizations suggesting that the transport of glucose across the blood-brain barrier is tightly regulated and compartmentalized. As seen in the periphery, insulin-sensitive GLUTs are expressed in the brain and therefore may participate in the central actions of insulin. The aim of this review will be to discuss the localization of GLUTs expressed in the central nervous system (CNS), with a special emphasis upon the recently identified GLUT isoforms. In addition, we will discuss the regulation, activity and insulin-stimulated trafficking of GLUTs in the CNS, especially in relation to the centrally mediated actions of insulin and glucose.

Insulin and the insulin receptor in experimental models of learning and memory

Insulin is best known for its action on peripheral insulin target tissues such as the adipocyte, muscle and liver to regulate glucose homeostasis. In the central nervous system (CNS), insulin and the insulin receptor are found in specific brain regions where they show evidence of participation in a variety of region-specific functions through mechanisms that are different from its direct glucose regulation in the periphery. While the insulin/insulin receptor associated with the hypothalamus plays important roles in regulation of the body energy homeostasis, the hippocampus- and cerebral cortex-distributed insulin/insulin receptor has been shown to be involved in brain cognitive functions. Emerging evidence has suggested that insulin signaling plays a role in synaptic plasticity by modulating activities of excitatory and inhibitory receptors such as glutamate and GABA receptors, and by triggering signal transduction cascades leading to alteration of gene expression that is required for long-term memory consolidation. Furthermore, deterioration of insulin receptor signaling appears to be associated with aging-related brain degeneration such as the Alzheimer's dementia and cognitive impairment in aged subjects suffering type 2 diabetes mellitus.

Increased levels of insulin and insulin-like growth factor-1 hybrid receptors and decreased glycosylation of the insulin receptor alpha- and beta-subunits in scrapie-infected neuroblastoma N2a cells

We have previously shown that ScN2a cells (scrapie-infected neuroblastoma N2a cells) express 2-fold- and 4-fold-increased levels of IR (insulin receptor) and IGF-1R (insulin-like growth factor-1 receptor) respectively. In addition, the IR alpha- and beta-subunits are aberrantly processed, with apparent molecular masses of 128 and 85 kDa respectively, as compared with 136 and 95 kDa in uninfected N2a cells. Despite the 2-fold increase in IR protein, the number of (125)I-insulin-binding sites was slightly decreased in ScN2a cells [Ostlund, Lindegren, Pettersson and Bedecs (2001) Brain Res. 97, 161-170]. In order to determine the cellular localization of IR in ScN2a cells, surface biotinylation was performed, showing a correct IR trafficking and localization to the cell surface. The present study shows for the first time that neuroblastoma N2a cells express significant levels of IR-IGF-1R hybrid receptors, and in ScN2a cells the number of hybrid receptors was 2-fold higher than that found in N2a cells, potentially explaining the apparent loss of insulin-binding sites due to a lower affinity for insulin compared with the homotypic IR. Furthermore, the decreased molecular mass of IR subunits in ScN2a cells is not caused by altered phosphorylation or proteolytic processing, but rather by altered glycosylation. Enzymic deglycosylation of immunoprecipitated IR from N2a and ScN2a cells with endoglycosidase H, peptide N-glycosidase F and neuraminidase all resulted in subunits with increased electrophoretic mobility; however, the 8-10 kDa shift remained. Combined enzymic or chemical deglycosylation using anhydrous trifluoromethane sulphonic acid treatment ultimately showed that the IR alpha- and beta-subunits from ScN2a cells are aberrantly glycosylated. The increased formation of IR-IGF-1R hybrids in ScN2a cells may be part of a neuroprotective response to prion infection. The degree and functional significance of aberrantly glycosylated proteins in ScN2a cells remain to be determined.

Upregulated expression of neuronal nitric oxide synthase by insulin in both neurons and astrocytes

Both insulin and nitric oxide (NO) play important roles in the brain. However, there are no unequivocal evidences pointing to a direct effect of insulin on nitric oxide pathway in the brain. In the present study, the effects of insulin on the expression and activity of neuronal nitric oxide synthase (nNOS) were investigated in the cultured cerebellum cell line R2, cerebral cortical astrocytes, and neurons of rats by using flow cytometry, in situ hybridization, RT-PCR, and electron spin resonance (ESR) techniques. In astrocytes, the expression of nNOS was significantly stimulated by insulin in a concentration-dependent manner, with a maximal increase of about 47.6% compared with the control values (p<0.05, t test, n=5). Furthermore, in situ hybridization analysis showed that the expression of nNOS was also significantly increased by insulin (0.64 ng/ml, 6 h), reaching 134.2+/-9.6% of the control values (p<0.05, t test, n=3). In addition, by using nNOS specific primers, RT-PCR analysis also demonstrated the same effect of insulin (0.64 ng/ml, 6 h) on nNOS mRNA expression. Similarly, significant increase of the expression of nNOS protein and mRNA were also observed in both R2 cells and neurons of rats after incubation with insulin. In addition, significant increase of the activity of nNOS in R2 cells and astrocytes were also detected after incubation with insulin (0.64 ng/ml, 9 h) by using ESR technique. Overall, our results suggested that exogenous insulin could upregulate the expression and activity of nNOS in R2 cells, cerebral cortical astrocytes, and neurons of rats. The phenomena opened new insights for further investigation of the physical and pathological significances of insulin in the brain.

Insulin, C-peptide, hyperglycemia, and central nervous system complications in diabetes

Diabetes is an increasingly common disorder which causes and contributes to a variety of central nervous system (CNS) complications which are often associated with cognitive deficits. There appear to be two types of diabetic encephalopathy. Primary diabetic encephalopathy is caused by hyperglycemia and impaired insulin action, which evolves in a diabetes duration-related fashion and is associated with apoptotic neuronal loss and cognitive decline. This appears to be particularly associated with insulin-deficient diabetes. Secondary diabetic encephalopathy appears to arise from hypoxic-ischemic insults due to underlying microvascular disease or as a consequence of hypoglycemia. This type of cerebral diabetic complication is more common in the type 2 diabetic population. Here, we will review the clinical and experimental data supporting this conceptual division of diabetic CNS complications and discuss the underlying metabolic, molecular, and functional aberrations.

NMDA receptor subunits 2A and 2B decrease and lipid peroxidation increase in the hippocampus of streptozotocin-diabetic rats: effects of insulin and gliclazide treatments

Recent studies indicate that diabetes mellitus changes N-methyl-D-aspartate (NMDA) receptor subunit composition and impairs cognitive functions. It also has been known that diabetes mellitus causes lipid peroxidation. This study examined the effects of streptozotocin-diabetes and insulin or gliclazide treatment on the hippocampal NMDA receptor subunit 2A and 2B (NR2A and NR2B) concentrations. In addition, malondial dehyde (MDA) levels were measured as a marker for lipid peroxidation. Eight weeks after the induction of diabetes MDA, levels were increased, and NR2A and NR2B concentrations were reduced. Insulin and gliclazide treatment partially prevented the reduction of NR2A and NR2B expression and prevented the elevation of MDA levels. There was no significant difference between the effects of insulin and gliclazide. The results suggest that the elevation of lipid peroxidation can be the primary biochemical disturbances in diabetes progression, and that changes in NMDA receptor subunit compositions can be involved in cognitive decline in diabetes.

Repetitive hypoglycemia in young rats impairs hippocampal long-term potentiation

Mechanisms underlying cognitive dysfunction in young diabetic children are poorly understood, and may include synaptic dysfunction from insulin-induced hypoglycemia. We developed a model of repetitive insulin-induced hypoglycemia in young rats and examined hippocampal long-term potentiation, an electrophysiologic assay of synaptic plasticity, 3-5 d after the last hypoglycemic event. Three hypoglycemic events between postnatal d 21-25 produced modest cortical (17 +/- 2.9 dead neurons per section in parasagittal cortex), but not hippocampal, neuron death quantified by Fluoro-Jade B staining. There was no change in neurogenesis in the hippocampal dentate granule cell region by quantification of bromodeoxyuridine incorporation. Although normal baseline hippocampal synaptic responses were elicited from hippocampal slices from hypoglycemic animals, long-term synaptic potentiation could not be induced in hippocampal slices from rats subjected to hypoglycemia. These results suggest that repetitive hypoglycemia in the developing brain can cause selective impairment of synaptic plasticity in the absence of cell death, and without complete disruption of basal synaptic transmission. We speculate that impaired synaptic plasticity in the hippocampus caused by repetitive hypoglycemia could underlie memory and cognitive deficits observed in young diabetic children, and that cortical neuron death caused by repetitive hypoglycemia in the developing brain may contribute to other neurologic, cognitive, and psychological problems sometimes encountered in diabetic children.

Acupuncture increases nitric oxide synthase expression in hippocampus of streptozotocin-induced diabetic rats

In the present study, the effect of acupuncture at Zusanli acupoint on nitric oxide synthase (NOS) expression in the hippocampus of streptozotocin (STZ)-induced diabetic rats was investigated via nicotinamide adenine dinucleotide phosphate-diaphorase (NADPH-d) histochemistry. Animals were divided into four groups: the control group, the nondiabetic and acupunctured group, the STZ-induced diabetes group, and the STZ-induced diabetes and acupunctured group. From the results, NADPH-d-positive neurons in the hippocampus were decreased in STZ-induced diabetic rats, while acupuncture increased NOS expression significantly under diabetic conditions. In the present study, it can be suggested that acupuncture treatment may modulate NOS activity in the hippocampus under diabetic conditions.

Two phases of nitrergic neuropathy in streptozotocin-induced diabetic rats

The distinction between metabolic and structural changes occurring in autonomic neurons during diabetes has not been fully clarified. Here we demonstrate that nitric oxide synthase-containing (nitrergic) neurons innervating the penis and gastric pylorus of streptozotocin-induced diabetic rats undergo a selective degenerative process in two phases. In the first phase, nitrergic nerve fibers lose some of their neuronal nitric oxide synthase content and function. In the second phase, nitrergic degeneration takes place in the cell bodies in the ganglia, leading to complete loss of nitrergic function. The changes in the first phase are reversible with insulin replacement; however, the neurodegeneration in the second phase is irreversible. Neurodegeneration is due to apoptotic cell death in the ganglia, which is selective for the nitrergic neurones.

Altered expression of NCAM in hippocampus and cortex may underlie memory and learning deficits in rats with streptozotocin-induced diabetes mellitus

Neurological and structural changes are paralleled by cognitive deficits in diabetes mellitus. The present study was designed to evaluate the expression of neural cell adhesion molecules (NCAM) in the hippocampus, cortex and cerebellum and to examine cognitive functions in diabetic rats. Diabetes was induced in male albino rats via intraperitoneal streptozotocin injection. Learning and memory behaviors were investigated using a passive avoidance test and a spatial version of the Morris water maze test. NCAM expression was detected in the hippocampus, cortex and cerebellum by an immunoblotting method. The diabetic rats developed significant impairment in learning and memory behaviours as indicated by deficits in passive avoidance and water maze tests as compared to control rats. Expression of NCAM 180 and 120 kDa were found to be higher in hippocampus and cortex of diabetic rat brains compared to those of control, whereas expression of NCAM 140 kDa decreased in these brain regions. Our findings suggest that streptozotocin-induced diabetes impairs cognitive functions and causes an imbalance in expression of NCAM in those brain regions involved in learning and memory. Altered expression of NCAM in hippocampus may be an important cause of learning and memory deficits that occur in diabetes mellitus.

Effects of insulin on long-term potentiation in hippocampal slices from diabetic rats

AIMS/HYPOTHESIS

Cognitive deficits occur commonly in diabetic patients. It is unclear whether these impairments result from hypoglycaemia during intensive insulin therapy, or from the diabetes itself. The aim of this study was to examine if impaired energy utilization resulting from insulin deficiency contributes to impaired long-term potentiation (reflecting impaired synaptic plasticity). As long-term potentiation is considered a candidate cellular mechanism underlying learning and memory, understanding how diabetes alters long-term potentiation may provide insight into mechanisms producing cognitive deficits in diabetes.

METHODS

Electrophysiologic recordings were used to study long-term potentiation in the CA1 region of hippocampal slices from healthy rats and rats with streptozotocin-induced diabetes.

RESULTS

Long-term potentiation was difficult to induce in slices from diabetic rats in standard recording buffer (contains 10 mmol/l glucose). In slices from diabetic rats, increasing extracellular glucose failed to recover long-term potentiation induction, but 10 mmol/l pyruvate added to standard buffer enabled long-term potentiation induction. Moreover, incubation of slices from diabetic rats with insulin enabled long-term potentiation induction in standard buffer. Acute administration of streptozotocin alone did not impair long-term potentiation in slices from healthy animals, and changing extracellular glucose concentrations over the range of 5 mmol/l to 30 mmol/l did not alter long-term potentiation in slices from control rats.

CONCLUSIONS/INTERPRETATION

These observations suggest that impaired energy utilization from insulin deficiency, rather than the accompanying hyperglycaemia, impair long-term potentiation in diabetes. Impaired hippocampal synaptic plasticity could contribute to learning and cognitive impairment in diabetic patients.

Folium mori increases cell proliferation and neuropeptide Y expression in dentate gyrus of streptozotocin-induced diabetic rats

The possibility has been raised that Folium mori is clinically effective for the treatment and prevention of diabetes. In the present study, the effects of Folium mori on cell proliferation and expression of neuropeptide Y (NPY) in the dentate gyrus of rats with streptozotocin (STZ)-induced diabetes were investigated by 5-bromo-2'-deoxyuridine (BrdU) immunohistochemistry and NPY immunohistochemistry. In rats with STZ-induced diabetes, cell proliferation and NPY expression in the dentate gyrus were suppressed, and treatment with Folium mori was shown to increase new cell formation and NPY expression in the dentate gyrus in both normal rats and those with STZ-induced diabetes. In light of previous studies, this result appears to indicate that increased expression of NPY in the dentate gyrus induced by treatment with Folium mori is associated with the observed effect of Folium mori extract on cell proliferation. Based on the present results, it is suggested that Folium mori treatment may aid in the recovery from the central nervous system complications of diabetes mellitus.

Angiotensin converting enzyme inhibition partially prevents deficits in water maze performance, hippocampal synaptic plasticity and cerebral blood flow in streptozotocin-diabetic rats

Vascular dysfunction is important in the pathogenesis of peripheral complications of diabetes. However, the effects of diabetes on cerebral blood flow and the role of vascular deficits in the pathogenesis of diabetic encephalopathy are still unknown. The present study examined whether experimental diabetes is associated with reduced cerebral blood flow and whether treatment with enalapril can improve cerebral perfusion and function (blood flow and functional cerebral deficits). Streptozotocin-diabetic rats were treated with the ACE inhibitor enalapril (24 mg/kg) from onset of diabetes. After 14 weeks of diabetes, 12 enalapril treated and 12 untreated diabetic rats, and 12 nondiabetic age-matched control rats were tested in a spatial version of the Morris water maze. After 16 weeks of diabetes, in the same groups, blood flow in the hippocampus and thalamus was measured by hydrogen clearance microelectrode polarography. In a separate study, hippocampal long-term potentiation was measured after 26 weeks of diabetes. Water maze performance and hippocampal long-term potentiation were impaired in diabetic rats. Furthermore, blood flow in diabetic rats was reduced by 30% (P<0.001) in the hippocampus and by 37% (P<0.005) in the thalamus compared to nondiabetic controls. Enalapril treatment significantly improved water maze performance (P<0.05), hippocampal long term potentiation (P<0.05) and hippocampal blood flow (P<0.05). Cerebral perfusion is reduced in diabetic rats compared to controls. Treatment aimed at the vasculature can improve cerebral blood flow, deficits in Morris maze performance and long term potentiation. These findings suggest that vasculopathy plays a role in the development of cerebral dysfunction in diabetic rats.

Moderate hypoglycemia aggravates effects of hypoxia in hippocampal slices from diabetic rats

We recorded the effects of hypoxia combined with relative hypoglycemia on pre- and post-synaptic potentials in the CA1 area of slices from 4-month-old control and diabetic (streptozotocin-treated) Wistar rats. In experiments on slices kept in 10 or 4 mM glucose (at 33 degrees C), hypoxia was applied until the pre-synaptic afferent volley disappeared--after 12-13 min in most slices, but much earlier (5+/-0.8 min) in diabetic slices kept in 4 mM glucose. When oxygenation was resumed, the afferent volley returned in all slices, for an overall mean recovery of 86.5% (+/-8.8%). Field post-synaptic potentials were fully blocked within 2-3 min of the onset of hypoxia. After the end of hypoxia, they failed to reappear in some slices: overall, their recovery varied between 62 and 68% in control slices, as well as in diabetic slices kept in 10 mM glucose; but recovery was very poor in diabetic slices kept in 4 mM glucose (only 15+/-0.94%). In the latter, hypoxic injury discharges occurred earlier (4.2+/-0.68 min vs. 6.5-8 min for other groups). We conclude that diabetes appears to make hippocampal slices more prone to irreversible loss of synaptic function and early block of axonal conduction when temporary hypoxia is combined with moderate hypoglycemia.

Hippocampal neuronal apoptosis in type 1 diabetes

Duration-related cognitive impairment is an increasingly recognized complication of type 1 diabetes. To explore potential underlying mechanisms, we examined hippocampal abnormalities in the spontaneously type 1 diabetic BB/W rat. As a functional assay of cognition, the Morris water maze test showed significantly prolonged latencies in 8-month diabetic rats not present at 2 months of diabetes. These abnormalities were associated with DNA fragmentation, positive TUNEL staining, elevated Bax/Bcl-x(L) ratio, increased caspase 3 activities and decreased neuronal densities in diabetic hippocampi. These changes were not caused by hypoglycemic episodes or reduced weight in diabetic animals. To explore potential mechanisms responsible for the apoptosis, we examined the expression of the IGF system. Western blotting and in situ hybridization revealed significant reductions in the expression of IGF-I, IGF-II, IGF-IR and IR preceding (2 months) and accompanying (8 months) the functional cognitive impairments and the apoptotic neuronal loss in hippocampus. These data suggest that a duration-related apoptosis-induced neuronal loss occurs in type 1 diabetes associated with cognitive impairment. The data also suggest that this is at least in part related to impaired insulin and/or IGF activities.

Differential learning strategies in spatial and nonspatial versions of the Morris water maze in the C57BL/6J inbred mouse strain

We recently developed a new nonspatial version of the Morris water maze that requires the use of four visually distinct intra-maze patterns to efficiently locate a hidden platform. The nonspatial version was designed to match the spatial version on complexity of cue usage, and differs only on spatiality of cues, thereby allowing more meaningful comparisons between the two versions. Following a previous experiment that demonstrated nonspatial learning with the BXSB inbred mouse strain, C57 inbred mice were tested in this study. They received spatial and nonspatial training in a counter-balanced order so that Test Order and information transfer could be assessed. Subjects that received spatial training first had superior performance in both the spatial and the nonspatial tasks when compared to mice that received nonspatial training first. The mice that received spatial training first used extra-maze cues as a spatial strategy. However, during nonspatial testing they did not use the intra-maze cues to locate the platform; instead, the mice used an egocentric strategy of circling through the platform annulus. Subjects that received spatial testing first were superior on the nonspatial task to those subjects that received nonspatial training first. Moreover, subjects that received nonspatial testing first were unable to learn the spatial version. Overall, C57 mice can learn both the spatial and nonspatial versions of the Morris maze presented here; however, the nonspatial version is more difficult and is solved using an egocentric strategy.

Neuronal Ca2+ disregulation in diabetes mellitus

The Ca(2+) hypothesis of brain ageing and dementia may account for part of the available data on the pathogenesis of dementia and certain neurodegenerative disorders. The hypothesis proposes that disturbances in the homeostasis of neuronal cytosolic free Ca(2+) are part of a final common pathway, ultimately leading to neuronal dysfunction and cell death. The hypothesis also proposes that a small change in cytosolic free Ca(2+) sustained over a long period of time will result in similar damage as a large change over a short period. Diabetes mellitus is associated with neurological complications in the peripheral and central nervous system, as reflected in peripheral neuropathy, modest cognitive impairments and an increased risk of dementia. In animal models of diabetes, learning impairments are associated with alterations in Ca(2+) -dependent forms of hippocampal synaptic plasticity. Disturbances in the homeostasis of cytosolic free Ca(2+) may present a final common pathway in the multifactorial pathogenesis of neurological complications of diabetes, which involves vascular changes, oxidative stress, and non-enzymatic protein glycation. In line with the Ca(2+) hypothesis of neurodegenerative disorders, a prolonged, small increase in basal cytosolic Ca(2+) levels indeed exists in sensory neurones of diabetic animals. In addition, Ca(2+) dynamics are affected. Ca(2+) channel blockers, such as nimodipine, have been shown to improve experimental peripheral neuropathy, through a vascular mechanism, possibly in combination with direct neuronal effects. Preliminary studies indicate that nimodipine may also improve Ca(2+)-dependent forms of synaptic plasticity in the hippocampus of diabetic rats.

Ageing and diabetes: implications for brain function

Diabetes mellitus is associated with moderate cognitive deficits and neurophysiological and structural changes in the brain, a condition that may be referred to as diabetic encephalopathy. Diabetes increases the risk of dementia, particularly in the elderly. The emerging view is that the diabetic brain features many symptoms that are best described as "accelerated brain ageing." The clinical characteristics of diabetic encephalopathy are discussed, as well as behavioural (e.g. spatial learning) and neurophysiological (e.g. hippocampal synaptic plasticity) findings in animal models. Animal models can make a substantial contribution to our understanding of the pathogenesis, which shares many features with the mechanisms underlying brain ageing. By unravelling the pathogenesis, targets for pharmacotherapy can be identified. This may allow treatment or prevention of this diabetic complication in the future. We discuss changes in glutamate receptor subtypes, in second-messenger systems and in protein kinases that may account for the alterations in synaptic plasticity. In addition, the possible role of cerebrovascular changes, oxidative stress, nonenzymatic protein glycation, insulin and alterations in neuronal calcium homeostasis are addressed.

Lipoic acid confers protection against oxidative injury in non-neuronal and neuronal tissue

In the past decade or so, a convincing link between oxidative stress and degenerative conditions has been made and with the knowledge that oxidatiye changes may actually trigger deterioration in cell function, a great deal of energy has focussed on identifying agents which may have possible therapeutic value in combating oxidative changes. One agent which has received attention, because of its powerful antioxidative effects, particularly in neuronal tissue, is lipoic acid.

Learning and memory in streptozotocin-induced diabetic rats in a novel spatial/object discrimination task

Diabetes mellitus is associated with disturbances of cognitive functioning. The aim of this study was to examine cognitive functioning in diabetic rats using the 'Can test', a novel spatial/object learning and memory task, without the use of aversive stimuli. Rats were trained to select a single rewarded can from seven cans. Mild water deprivation provided the motivation to obtain the reward (0.3 ml of water). After 5 days of baseline training, in which the rewarded can was marked by its surface and position in an open field, the animals were divided into two groups. Diabetes was induced in one group, by an intravenous injection of streptozotocin. Retention of baseline training was tested at 2-weekly intervals for 10 weeks. Next, two adapted versions of the task were used, with 4 days of training in each version. The rewarded can was a soft-drink can with coloured print. In a 'simple visual task' the soft-drink can was placed among six white cans, whereas in a 'complex visual task' it was placed among six soft-drink cans from different brands with distinct prints. In diabetic rats the number of correct responses was lower and number of reference and working memory errors higher than in controls in the various versions of the test. Switches between tasks and increases in task complexity accentuated the performance deficits, which may reflect an inability of diabetic rats to adapt behavioural strategies to the demands of the tasks.

Cognitive effects of insulin in the central nervous system

Evidence has been accumulating recently that the hormone insulin may modulate cognitive activity by acting in the central nervous system. Initially derived from the observation that insulin and insulin receptors are found in specific brain areas, this evidence also includes cognitive assessments of humans in insulin-deficient and insulin-resistant disease states and experimental manipulation of rodent models. Additional support is derived from in vivo and in vitro systems that are used to investigate the neurophysiological basis of learning and memory. This article is a brief review of the literature that suggests a connection between insulin and memory and draws together some of the findings relevant to possible physiological mechanisms for this cognitive effect.

Role of insulin and insulin receptor in learning and memory

As one of the most extensively studied protein hormones, insulin and its receptor have been known to play key roles in a variety of important biological functions. Until recent years, the functions of insulin and insulin receptor (IR) in the central nervous system (CNS) have largely remained unclear. IR is abundantly expressed in several specific brain regions that govern fundamental behaviors such as food intake, reproduction and high cognition. The IR from the periphery and CNS exhibit differences in both structure and function. In addition to that from the peripheral system, locally synthesized insulin in the brain has also been identified. Accumulated evidence has demonstrated that insulin/IR plays important roles in associative learning, as suggested by results from both interventive and correlative studies. Interruption of insulin production and IR activity causes deficits in learning and memory formation. Abnormal insulin/IR levels and activities are seen in Alzheimer's dementia, whereas administration of insulin significantly improves the cognitive performance of these patients. The synaptic bases for the action of insulin/IR include modifying neurotransmitter release processes at various types of presynaptic terminals and modulating the activities of both excitatory and inhibitory postsynaptic receptors such as NMDA and GABA receptors, respectively. At the molecular level, insulin/IR participates in regulation of learning and memory via activation of specific signaling pathways, one of which is shown to be associated with the formation of long-term memory and is composed of intracellular molecules including the shc, Grb-r/SOS, Ras/Raf, and MEK/MAP kinases. Cross-talk with another IR pathway involving IRS1, PI3 kinase, and protein kinase C, as well as with the non-receptor tyrosine kinase pp60c-src, may also be associated with memory processing.

Insulin-like peptides are not involved in maturation or functional recovery of neural circuits in the locust flight system

We sought to manipulate maturation and functional recovery of locust flight circuitry by treating locusts with pharmacological doses of bovine anti-insulin and insulin. Anti-insulin treatment of maturing locusts caused reduced growth of the thoracic nervous system, lower body weight, and softer cuticles compared with control locusts. We were unable to block either maturation or recovery of flight circuitry with anti-insulin. We propose that insulin-related peptides are involved in growth and cuticular changes during adult maturation, but have no role in promoting neuronal sprouting during this period or as a result of injury.Key words: insulin, maturation, functional recovery, proprioceptors, flight.

The effect of gamma-linolenic acid-alpha-lipoic acid on functional deficits in the peripheral and central nervous system of streptozotocin-diabetic rats

Diabetes mellitus can lead to functional and structural deficits in both the peripheral and central nervous system. The pathogenesis of these deficits is multifactorial, probably involving, among others, microvascular dysfunction and oxidative stress. The present study examined the effects of 12 weeks of treatment with a conjugate of the essential fatty acid gamma-linolenic acid and the anti-oxidant alpha-lipoic acid (GLA-LA) on functional deficits in the peripheral and central nervous system in streptozotocin-diabetic rats. Treatment was initiated 16 weeks after diabetes induction. Sciatic nerve motor and sensory conduction velocity, brainstem auditory evoked potentials and visual evoked potentials were measured in control, untreated and GLA-LA treated diabetic rats. Also, long-term potentiation, a form of synaptic plasticity used as a model for learning and memory at the cellular level, was examined in hippocampal slices. GLA-LA treatment (50 mg/kg/day) did not reverse established deficits in nerve conduction velocity or in evoked potential latencies in diabetic rats. However, GLA-LA treatment did improve long-term potentiation in the hippocampus. It is concluded that GLA-LA, which is known to improve early deficits in peripheral nerve conduction in diabetic rats, is unable to reverse late deficits. However, the compound does reverse established deficits in long-term potentiation, suggesting that at least part of its activity is specifically directed at synaptic plasticity.

Cognition and synaptic plasticity in diabetes mellitus

Diabetes mellitus is associated with cognitive deficits and an increased risk of dementia, particularly in the elderly. These deficits are paralleled by neurophysiological and structural changes in the brain. In animal models of diabetes, impairments of spatial learning occur in association with distinct changes in hippocampal synaptic plasticity. At the molecular level these impairments might involve changes in glutamate-receptor subtypes, in second-messenger systems and in protein kinases. The multifactorial pathogenesis of diabetic encephalopathy is not yet completely understood, but clearly shares features with brain ageing and the pathogenesis of diabetic neuropathy. It involves both metabolic and vascular changes, related to chronic hyperglycaemia, but probably also defects in insulin action in the brain. Treatment with insulin might therefore not only correct hyperglycaemia, but could also directly affect the brain.

Experimental diabetes in rats causes hippocampal dendritic and synaptic reorganization and increased glucocorticoid reactivity to stress

We report that 9 d of uncontrolled experimental diabetes induced by streptozotocin (STZ) in rats is an endogenous chronic stressor that produces retraction and simplification of apical dendrites of hippocampal CA3 pyramidal neurons, an effect also observed in nondiabetic rats after 21 d of repeated restraint stress or chronic corticosterone (Cort) treatment. Diabetes also induces morphological changes in the presynaptic mossy fiber terminals (MFT) that form excitatory synaptic contacts with the proximal CA3 apical dendrites. One effect, synaptic vesicle depletion, occurs in diabetes as well as after repeated stress and Cort treatment. However, diabetes produced other MFT structural changes that differ qualitatively and quantitatively from other treatments. Furthermore, whereas 7 d of repeated stress was insufficient to produce dendritic or synaptic remodeling in nondiabetic rats, it potentiated both dendritic atrophy and MFT synaptic vesicle depletion in STZ rats. These changes occurred in concert with adrenal hypertrophy and elevated basal Cort release as well as hypersensitivity and defective shutoff of Cort secretion after stress. Thus, as an endogenous stressor, STZ diabetes not only accelerates the effects of exogenous stress to alter hippocampal morphology; it also produces structural changes that overlap only partially with those produced by stress and Cort in the nondiabetic state.

Abnormal synaptic plasticity and impaired spatial cognition in mice transgenic for exon 1 of the human Huntington's disease mutation

Huntington's disease (HD) is an autosomal dominant progressive and fatal neurodegenerative brain disorder caused by an expanded CAG/polyglutamine repeat in the coding region of the gene. Presymptomatic Huntington's disease patients often exhibit cognitive deficits before the onset of classical symptoms. To investigate the possibility that changes in synaptic plasticity might underlie cognitive impairment in HD, we examined hippocampal synaptic plasticity and spatial cognition in a transgenic mouse (R6/2 line) expressing exon 1 of the human Huntington's disease gene containing an expanded CAG repeat. This mouse exhibits a progressive and fatal neurological phenotype that resembles Huntington's disease. We report that R6/2 mice show marked alterations in synaptic plasticity at both CA1 and dentate granule cell synapses, and impaired spatial cognitive performance in the Morris water maze. The changes in hippocampal plasticity were age dependent, appearing at CA1 synapses several weeks before they were observed in the dentate gyrus. Deficits in synaptic plasticity at CA1 synapses occurred before an overt phenotype. This suggests that altered synaptic plasticity contributes to the pre-symptomatic changes in cognition reported in human carriers of the Huntington' disease gene. The temporal and regional changes in synaptic plasticity within the hippocampus mirror the appearance of neuronal intranuclear inclusions, suggesting a relationship between polyglutamine aggregation and dysfunction.

Classically conditioned responses following repeated insulin and glucose administration in humans

This paper describes the neural basis and the role of Pavlovian conditioning in the modification of blood glucose and related endocrine parameters after repeated insulin and glucose administration. Pavlovian conditioning requires that conditioned stimulus (CS) and unconditioned stimulus (US) are both detected in the central nervous system (CNS), where the CS-US association takes place. We will therefore elucidate the detectability of insulin and glucose in the CNS. Since current data focus almost exclusively on animals, we conducted a placebo-controlled insulin conditioning experiment in humans (Experiment 1). Compared with the control group with CS-placebo pairings throughout, the experimental group with previous CS-insulin pairings in the acquisition phase showed a conditioned decrease in blood glucose and a trend for a conditioned baseline insulin increase, and an increase in cortisol levels relative to baseline and cumulative number of neuroglycopenic symptoms in the CS-placebo test session. The conditionability of glucose administration also had to be examined; experiments using an arbitrary CS and glucose are extremely rare, even in animals. Glucose is the natural stimulus for endogenous insulin secretion, so studies on cephalic-phase insulin release (CPIR) will be reviewed in this paper. We implemented a placebo-controlled three-group design (Experiment 2): Subjects received either CS-insulin, CS-glucose or CS-placebo pairings during the acquisition. Together, our results demonstrate the conditionability mainly of insulin, but also of glucose effects in healthy humans. The clinical relevance and future research perspectives are outlined with an emphasis on insulin in the brain and its role in learning and memory.

Pathophysiological implications of the structural organization of the excitatory synapse

The glutamatergic synapse is the key structure in the development of activity-dependent synaptic plasticity in the central nervous system. The analysis of the complex biochemical mechanisms at the basis of the long-term changes in synaptic efficacy have received a tremendous impulse by the observation that the post-synaptic constituents of the synapse can be separated and purified through a simple procedure involving detergent treatment of synaptosomes and differential centrifugation. In this fraction, called post-synaptic density (PSD), the functional interactions of its constituents are preserved. The various subunits of ionotropic glutamate receptors are held in register with the presynaptic active zone through their interaction with linker proteins. N-methyl-D-aspartate (NMDA) subunits NR2A and NR2B, bind to the PSD protein called PSD-95, which in turn binds neuroligins, providing a handle for interacting with neurexin, located in the plasma membrane at the presynaptic active zone. Additional clustering of NMDA receptors is provided through the binding of NRI subunits to the cytoskeletal protein alpha-actinin-2. AMPA (alpha-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid) and kainate receptors are other important constituents of PSDs and bind to different anchoring proteins. Phosphorylation processes have long been known to modulate NMDA receptor functional activity: the finding that several protein kinases, particularly Ca2+/Calmodulin-dependent protein kinase II and protein tyrosine kinases of the src family, are major constituents of PSDs has allowed to demonstrate that these enzymes are localized in a strategic position of the glutamatergic synapse, so that their activation provides a means for NMDA receptor function regulation upon its activation. The relevance of these mechanisms has been demonstrated in experimental models of pathologies involving deficits in synaptic plasticity, such as in streptozotocin-induced diabetes and in an animal model of prenatal induced ablation of hippocampal neurons. Both animal models display disturbances in long-term potentiation and cognitive deficits, thus providing in vivo models to study pathology related changes in both the structure and the function of the excitatory synapse.