Intra CA1 insulin microinjection improves memory consolidation and retrieval

Although the brain was considered as an insulin-insensitive organ, recent studies have shown that insulin receptors exist in the brain and insulin modulates some of the brain tasks. Insulin and its receptor are found in specific areas of CNS with a variety of region-specific functions different from its direct glucose regulation in the periphery. The hippocampus and cerebral cortex distributed insulin/insulin receptor has been shown to be involved in brain cognitive functions. The improving effect of insulin on spatial memory acquisition has been shown. In the present study, the effect of insulin microinjection into the CA1 region of rat hippocampus on spatial memory consolidation and retrieval has been investigated. Insulin in 12 MU (but not in 0.5 and 6 MU) improved both memory retrieval and consolidation.     

Spatial Memory Impairment Is Associated with Hippocampal Insulin Signals in Ovariectomized Rats

Estrogen influences memory formation and insulin sensitivity. Meanwhile, glucose utilization directly affects learning and memory, which are modulated by insulin signals. Therefore, this study investigated whether or not the effect of estrogen on memory is associated with the regulatory effect of this hormone on glucose metabolism. The relative expression of estrogen receptor β (ERβ) and glucose transporter type 4 (GLUT4) in the hippocampus of rats were evaluated by western blot. Insulin level was assessed by ELISA and quantitative RT-PCR, and spatial memory was tested by the Morris water maze. Glucose utilization in the hippocampus was measured by 2-NBDG uptake analysis. Results showed that ovariectomy impaired the spatial memory of rats. These impairments are similar as the female rats treated with the ERβ antagonist tamoxifen (TAM). Estrogen blockade by ovariectomy or TAM treatment obviously decreased glucose utilization. This phenomenon was accompanied by decreased insulin level and GLUT4 expression in the hippocampus. The female rats were neutralized with hippocampal insulin with insulin antibody, which also impaired memory and local glucose consumption. These results indicated that estrogen blockade impaired the spatial memory of the female rats. The mechanisms by which estrogen blockade impaired memory partially contributed to the decline in hippocampal insulin signals, which diminished glucose consumption.     

Permissive role of insulin in the expression of long-term potentiation in the hippocampus of immature rats

Many studies indicate that impairment in insulin signaling leads to learning and memory deficits. However, previous studies failed to establish a clear role of insulin in long-term potentiation (LTP), the best cellular model of memory formation. Here we show that while insulin pretreatment did not affect LTP magnitude in the adult rat hippocampus, it facilitated LTP expression in the immature hippocampus. The tyrosine kinase inhibitor AG-1024 abolished the effect of insulin in young rats, suggesting the involvement of the insulin receptor. On the other hand, increasing extracellular glucose concentration failed to facilitate LTP and application of an insulin-responsive glucose transporter-4 inhibitor did not impair the effect of insulin. These results suggest that the facilitatory action of insulin on LTP is not an indirect effect on glucose homeostasis/utilization. Involvement of the MAPK/ERK pathway, a known downstream pathway of insulin signaling, was revealed by pretreatment with PD98059, which blocked the insulin-mediated LTP facilitation. Consistent with this, high-frequency stimulation induced a significant increase in the level of phosphorylated Erk-2 in insulin-treated hippocampus. Taken together, these results suggest that insulin may be an essential factor in the immature brain, allowing the expression of LTP to facilitate learning and memory.     

Age-related Changes in the Insulin Receptor β in the Gerbil Hippocampus

The insulin receptor has been reported to be associated with memory formation via the hippocampus. In this study, we observed age-related changes in the insulin receptor β immunoreactivity and its protein levels in the hippocampus of gerbils of various ages in order to identify the correlation between the insulin receptor β and aging processes in the hippocampus. Insulin receptor β immunoreactivity was mainly detected in the molecular and polymorphic layers of the dentate gyrus, and in mossy fibers, Schaffer collaterals, alveus and stratum lacunosum-moleculare of the hippocampus proper (CA1-3) of gerbils at postnatal month 1 (PM 1). Insulin receptor β immunoreactivity decreased with age in all of these structures, except for the alveus. Reduction of the insulin receptor β immunoreactivity was prominent in the molecular layer of the dentate gyrus at PM 6 and in the stratum lacunosum-moleculare of the CA1 region at PM 12, while insulin receptor β immunoreactivity was decreased in other regions in the PM 18 groups. In addition, insulin receptor β protein level in the whole hippocampus was slightly increased at PM 3, and it decreased in an age-dependent manner from PM 6 to PM 24. These reductions of the insulin receptor β in the hippocampus may be associated with age-related memory deficits in gerbils.     

Involvement of nitric oxide in insulin induced memory improvement

Although brain was considered as an insulin-insensitive organ, recently it has appeared that insulin has some interesting effects on some brain regions like hippocampus. It has been known that intra-hippocampally administered insulin can improve learning and memory. Knowing that insulin can stimulate nitric oxide (NO) synthesis via eNOS activation and also that NO synthase (NOS) inhibitors can affect learning and memory, the aim of this study was to assess if NO is involved in insulin induced memory improvement. Wistar male rats were intra-CA1 cannulated and the effect of post-training and pre-probe trial intra-hippocampal administration of N-nitro-l-arginine methyl ester (l-NAME) (5, 10, 30 μg), insulin + l-NAME ± l-arginine were assessed in a single-day testing version of Morris water maze (MWM) task. Our results show that, l-NAME can prevent insulin induced memory improvement. This drug had no effect on escape latency of a non-spatial visual discrimination task. Therefore, it seems that endogenous nitric oxide has a role in spatial learning and memory improvement caused by insulin.     

Role of Somatodendritic and Postsynaptic 5-HT1A Receptors on Learning and Memory Functions in Rats

Memory impairment is a major problem afflicting mankind. The association between memory functions and neurotransmitter functions is of great interest for understanding brain function. Serotonergic pathways play an important role in the modulation of memory functions but the importance of its receptor types and subtypes on memory functions is still unclear. Activation and blockade of various serotonin (5-HT) receptors has been reported to alter cognitive processes and 5-HT receptor antagonism could be beneficial in the treatment of cognitive diseases. The role of 5-HT on memory functions is complicated. Among the 5-HT receptors subtypes, 5-HT(1A) receptors are of special interest because these receptors are present in the brain areas involved in learning and memory functions such as hippocampus and cortex. The present study was therefore designed to investigate the effect of activation and blockade of somatodendritic and/or postsynaptic 5-HT(1A) receptor on learning and memory functions in rats using modified version of water maze. In this study, 8-OH-DPAT (8-hydroxy-2-(di-N-propylamino) tetralin) at 0.3?mg/kg significantly enhanced learning acquisition (LA), short-term memory (STM) and long term memory (LTM) of rats pre-injected with saline suggesting that the activation of pre-synaptic 5-HT(1A) receptors by its agonist enhanced the memory functions of rats. Conversely, rats injected with 8-OH-DPAT at 1.0?mg/kg exhibited impaired LA and STM and had no effect on LTM. It was also shown in this study that blockade of 5-HT(1A) receptors by spiperone enhanced LA, had no effect on STM but impaired the LTM, which showed that the blockade of 5-HT(1A) receptors by its antagonist exerts different effect on different types of memory. This study suggests that 5-HT(1A) receptor could be used as a significant pharmacological target for the treatment of CNS diseases. Unraveling the role of serotonin in cognition and memory disorders could provide better therapy and it may lead to new insights in our understandings of learning and memory.     

Effects of estrogen and progesterone treatment on rat hippocampal NMDA receptors: relationship to Morris water maze performance

Estrogen modulates NMDA receptors function in the brain. It increases both dendritic spine density and synapse number in the hippocampus, an effect that can be blocked by NMDA antagonist. In this study, we investigated the effect of 17beta-estradiol and progesterone treatment on NMDA receptors in ovariectomized rats. Two different doses were used for 10 weeks. Receptor autoradiography was done on brain sections using [(3)H] MK-801 as a ligand. Our results showed a significant increase in [(3)H] MK-801 binding in the dentate gyrus, CA3 and CA4 areas of the hippocampus of ovariectomized compared to sham operated rats. In addition, we observed similar changes in CA1. 17beta-estradiol treatment in both doses reduced the binding back to the normal level while progesterone treatment did not show any effect. Spatial reference memory was tested on Morris water maze task. Ovariectomy severely impaired spatial reference memory. Estradiol but not progesterone treatment significantly improved the memory performance of the ovariectomized rats. Low dose treatment showed better learning than high dose estrogen treatment. The decrease in the antagonist sites by estradiol treatment could result in an increase in the sensitivity of the hippocampus to the excitatory stimulation by glutamate system and hence the effect of estradiol on learning and memory. The changes of NMDA receptors in the hippocampus support the concept that estrogen-enhancing effect on spatial reference memory could be through the enhancing of NMDA function.     

Neuroprotective role of Indirubin-3′-monoxime,a GSKβ inhibitor in high fat diet induced cognitive impairment in mice

Recent studies have highlighted that diabetes mellitus (DM) is a strong risk factor for Alzheimer’s disease (AD). Insulin resistance and/or hyperinsulinemia is one of the main characteristics of type 2 DM. Numerous epidemiological studies have demonstrated that insulin resistance contributes to AD pathogenesis. However the molecular mechanisms of association between these still remain elusive. Among the various possible mechanisms, the GSK-3β activity has been reported to be impaired in insulin-resistance, type 2 DM and AD. Thus, the present study was designed to explore the neuroprotective role of GSK3 β inhibitor, Indirubin-3′-monoxime (IMX) in insulin resistance induced cognitive impairment. Further, we have explored the possible molecular mechanism involved in cognitive impairment associated with insulin resistance. The mice subjected to high fat diet exhibited characteristic features of insulin resistance viz. increased serum glucose, triglycerides, cholesterol, insulin levels and impaired spatial learning and memory ability along with reduced brain insulin level, elevated oxidative stress and acetylcholinesterase (AChE) activity. The observed changes occurred concurrently with reduced brain derived neurotrophic factor. In contrast, the mice treated with IMX showed a significant reduction in plasma glucose, triglycerides, cholesterol, insulin levels and improvement in learning and memory performance, attenuated the oxidative stress and AChE activity. Moreover, IMX dose dependently augment the brain insulin and BDNF levels in HFD fed mice. Based upon these findings it could be suggested that GSK3 β inhibition could prove to be beneficial in insulin resistance induced cognitive deficit and this neuroprotection could be the result of enhanced BDNF based synaptic plasticity.     

Brain Insulin Signaling and Alzheimer's Disease: Current Evidence and Future Directions

Insulin receptors in the brain are found in high densities in the hippocampus, a region that is fundamentally involved in the acquisition, consolidation, and recollection of new information. Using the intranasal method, which effectively bypasses the blood-brain barrier to deliver and target insulin directly from the nose to the brain, a series of experiments involving healthy humans has shown that increased central nervous system (CNS) insulin action enhances learning and memory processes associated with the hippocampus. Since Alzheimer's disease (AD) is linked to CNS insulin resistance, decreased expression of insulin and insulin receptor genes and attenuated permeation of blood-borne insulin across the blood-brain barrier, impaired brain insulin signaling could partially account for the cognitive deficits associated with this disease. Considering that insulin mitigates hippocampal synapse vulnerability to amyloid beta and inhibits the phosphorylation of tau, pharmacological strategies bolstering brain insulin signaling, such as intranasal insulin, could have significant therapeutic potential to deter AD pathogenesis.     

Hormonal regulation of hippocampal dendritic morphology and synaptic plasticity

The peripheral functions of hormones such as leptin, insulin and estrogens are well documented. An important and rapidly expanding field is demonstrating that as well as their peripheral actions, these hormones play an important role in modulating synaptic function and structure within the CNS. The hippocampus is a major mediator of spatial learning and memory and is also an area highly susceptible to epileptic seizure. As such, the hippocampus has been extensively studied with particular regard to synaptic plasticity, a process thought to be necessary for learning and memory. Modulators of hippocampal function are therefore of particular interest, not only as potential modulators of learning and memory processes, but also with regard to CNS driven diseases such as epilepsy. Hormones traditionally thought of as only having peripheral roles are now increasingly being shown to have an important role in modulating synaptic plasticity and dendritic morphology. Here we review recent findings demonstrating that a number of hormones are capable of modulating both these phenomena.Key words: synaptic plasticity, leptin, estrogen, insulin, hippocampus, LTD, LTP     

Potential role of insulin on the pathogenesis of depression

The regulation of insulin on depression and depression‐like behaviour has been widely reported. Insulin and activation of its receptor can promote learning and memory, affect the hypothalamic‐pituitary‐adrenal axis (HPA) balance, regulate the secretion of neurotrophic factors and neurotransmitters, interact with gastrointestinal microbiome, exert neuroprotective effects and have an impact on depression. However, the role of insulin on depression remains largely unclear. Therefore, in this review, we summarized the potential role of insulin on depression. It may provide new insight for clarifying role of insulin on the pathogenesis of depression.     

Biological activity of a fragment of insulin

Insulin controls or alters glucose, protein, and fat metabolism as well as other cellular functions. Insulin binds to a specific receptor on the cell membrane initiating a protein phosphorylation cascade that controls glucose uptake and metabolism and long-term effects such as mitogenesis. This process also initiates insulin uptake and ultimate cellular metabolism in all insulin sensitive cells. The effects of insulin on other cellular metabolic properties have not been clearly related to this mechanism. Here we show that intracellular metabolism of insulin may be related to some aspects of insulin actions, specifically control of fat metabolism. A normal intracellular degradation product of insulin has been synthesized and tested for actions on fat turnover in cultured adipocytes. This 7-peptide, B-chain fragment (HLVEALY) inhibits both basal and stimulated lipolysis as measured by glycerol release, but does not inhibit FFA release because of a lack of effect on FFA reesterification in the adipocyte. HLVEALY also enhances insulin's effects on lipogenesis. This study shows that a fragment of insulin produced by the action of the insulin-degrading enzyme has both independent biological effects and interactions with insulin. This supports a biologically important effect of insulin metabolism and insulin degradation products on insulin action on non-glucose pathways.     

Role of 17beta-estradiol administration on insulin sensitivity in the rat: implications for the insulin receptor

The role of 17beta-estradiol in the early steps of insulin action is only partially known, although its effect on glucose homeostasis has been reported. In this paper, we attempt to prove the influence of 17beta-estradiol on the insulin receptor of ovariectomized rats treated with different hormonal doses. Our results show that high doses of estradiol impair insulin sensitivity while low doses improve it. We think that these results are the consequence of changes at a molecular level, because high doses of estradiol produced lower expression of the insulin receptor gene, lower content of this receptor in target tissues, and lower phosphorylation of insulin receptor in these tissues. However, low doses of estradiol seem to produce just the opposite. The possible existence of consensus response elements in the insulin receptor gene promoter to estradiol could be controlling the expression of this gene, this control being dose and timing dependent. Moreover, we cannot discard a possible effect of estradiol on the activity of protein tyrosine phosphatases, and therefore, on the activity of the insulin receptor. These new findings improve knowledge about the possible risk for insulin resistance in women taking oral contraceptives or receiving hormonal replacement therapy around the menopause, but could also open the door towards the possible utilization of 17beta-estradiol in some diabetes cases.     

Brain insulin system dysfunction in streptozotocin intracerebroventricularly treated rats generates hyperphosphorylated tau protein

The intracerebroventricular (icv) application of streptozotocin (STZ) in low dosage was used in 3-month-old rats to explore brain insulin system dysfunction. Three months following STZ icv treatment, the expression of insulin-1 and -2 mRNA was significantly reduced to 11% in hippocampus and to 28% in frontoparietal cerebral cortex, respectively. Insulin receptor (IR) mRNA expression decreased significantly in frontoparietal cerebral cortex and hippocampus (16% and 33% of control). At the protein/activity level, different abnormalities of protein tyrosine kinase activity (increase in hippocampus), total IR beta-subunit (decrease in hypothalamus) and phosphorylated IR tyrosine residues (increase) became apparent. The STZ-induced disturbance in learning and memory capacities was not abolished by icv application of glucose transport inhibitors known to prevent STZ-induced diabetes mellitus. The discrepancy between reduced IR gene expression and increase in both phosphorylated IR tyrosine residues/protein tyrosine kinase activity may indicate imbalance between phosphorylation/dephosphorylation of the IR beta-subunit causing its dysfunction. These abnormalities may point to a complex brain insulin system dysfunction after STZ icv application, which may lead to an increase in hyperphosphorylated tau-protein concentration. Brain insulin system dysfunction is discussed as possible pathological core in the generation of hyperphosphorylated tau protein as a morphological marker of sporadic Alzheimer's disease.     

A role for the angiotensin AT4 receptor subtype in overcoming scopolamine-induced spatial memory deficits.

There is increasing interest in the role of the brain angiotensin AT4 receptor subtype in cognitive processing. This receptor subtype is activated by angiotensin IV (AngIV), is heavily distributed in the mammalian hippocampus, neocortex, and cerebellum, and has been linked with a learning and memory function. The present investigation utilized intracerebroventricular (i.c.v.)-infused scopolamine hydrobromide (scop), a muscarinic receptor antagonist, to disrupt acquisition of the circular water maze task of spatial memory. All animals received 2 days of training trials (five trials/day) using a visible platform in an effort to preclude subsequent confounding by scopolamine-induced sensory and/or motor impairments. In the first experiment, i.c.v.-infused scopolamine (70 nmol) was followed by 0, 10, 100, or 1000 pmol i.c.v. doses of Nle(1)-AngIV in separate groups of rats. Results indicated that each dose of Nle(1)-AngIV improved the poor acquisition of this task induced by scopolamine treatment. However, the 100- and 1000-pmol doses were most effective with respect to latency and distance to find the submerged pedestal. A second experiment demonstrated that treatment with a specific AT4 receptor antagonist, Nle(1), Leual(3)-AngIV (1000 pmol), blocked the ability of Nle(1)-AngIV (100 pmol) to improve the performance of scopolamine-compromised rats. These results support the notion that hippocampal AT4 receptors are involved in spatial memory processing, and that activation of these binding sites can overcome the disruption of spatial memory accompanying treatment with a muscarinic receptor antagonist.     

Effects of testosterone on spatial learning and memory in adult male rats

A male advantage over females for spatial tasks has been well documented in both humans and rodents, but it remains unclear how the activational effects of testosterone influence spatial ability in males. In a series of experiments, we tested how injections of testosterone influenced the spatial working and reference memory of castrated male rats. In the eight-arm radial maze, testosterone injections (0.500 mg/rat) reduced the number of working memory errors during the early blocks of testing but had no effect on the number of reference memory errors relative to the castrated control group. In a reference memory version of the Morris water maze, injections of a wide range of testosterone doses (0.0625-1.000 mg/rat) reduced path lengths to the hidden platform, indicative of improved spatial learning. This improved learning was independent of testosterone dose, with all treatment groups showing better performance than the castrated control males. Furthermore, this effect was only observed when rats were given testosterone injections starting 7 days prior to water maze testing and not when injections were given only on the testing days. We also observed that certain doses of testosterone (0.250 and 1.000 mg/rat) increased perseverative behavior in a reversal-learning task. Finally, testosterone did not have a clear effect on spatial working memory in the Morris water maze, although intermediate doses seemed to optimize performance. Overall, the results indicate that testosterone can have positive activational effects on spatial learning and memory, but the duration of testosterone replacement and the nature of the spatial task modify these effects.     

Hormonal regulation of hippocampal dendritic morphology and synaptic plasticity

The peripheral functions of hormones such as leptin, insulin and estrogens are well documented. An important and rapidly expanding field is demonstrating that as well as their peripheral actions, these hormones play an important role in modulating synaptic function and structure within the CNS. The hippocampus is a major mediator of spatial learning and memory and is also an area highly susceptible to epileptic seizure. As such, the hippocampus has been extensively studied with particular regard to synaptic plasticity, a process thought to be necessary for learning and memory. Modulators of hippocampal function are therefore of particular interest, not only as potential modulators of learning and memory processes, but also with regard to CNS driven diseases such as epilepsy. Hormones traditionally thought of as only having peripheral roles are now increasingly being shown to have an important role in modulating synaptic plasticity and dendritic morphology. Here we review recent findings demonstrating that a number of hormones are capable of modulating both these phenomena.     

Effects of exercise and poor indoor air quality on learning,memory and blood IGF-1 in adolescent mice

It is known that regular aerobic exercise enhances cognitive functions and increases blood insulin-like growth factor 1 (IGF-1) levels. People living in urban areas spend most of their time indoors and indoor air quality can affect health. We investigated the effects of aerobic exercise in poor and good air quality environments on hippocampus and prefrontal cortex (PFC) neurons, anxiety, and spatial learning and memory in adolescent mice. Poor air quality impaired spatial learning and memory; exercise did not affect learning or memory impairment. Exercise in a good air quality environment improved spatial learning and memory. Poor air quality increased apoptosis in the hippocampus and PFC. Both exercised and sedentary groups living in a poor air quality environment had lower serum IGF-1 levels than those living in a good air quality environment. Living in a poor air quality environment has negative effects on the hippocampus, PFC and blood IGF-1 levels in adolescent mice, but exercise did not alter the negative effects of poor air quality.     

Effects of melatonin on oxidative stress and spatial memory impairment induced by acute ethanol treatment in rats

Melatonin has recently been suggested as an antioxidant that may protect neurons from oxidative stress. Acute ethanol administration produces both lipid peroxidation as an indicator of oxidative stress in the brain and impairs water-maze performance in spatial learning and memory tasks. The present study investigated the effect of melatonin against ethanol-induced oxidative stress and spatial memory impairment. The Morris water maze was used to evaluate the cognitive functions of rats. Thiobarbituric acid reactive substances (TBARS), which are the indicators of lipid peroxidation, and the activities of antioxidative enzymes (glutathione peroxidase and superoxide dismutase) were measured in the rat hippocampus and prefrontal cortex which form interconnected neural circuits for spatial memory. Acute administration of ethanol significantly increased TBARS levels in the hippocampus. Combined melatonin-ethanol treatment caused a significant increase in glutathione peroxidase activities and a significant decrease of TBARS in the rat hippocampus. In the prefrontal cortex, there was only a significant decrease of TBARS levels in the combined melatonin-ethanol receiving group as compared to the ethanol-treated group. Melatonin did not affect the impairment of spatial memory due to acute ethanol exposure, but melatonin alone had a positive effect on water maze performances. Our study demonstrated that melatonin decreased ethanol-induced lipid peroxidation and increased glutathione peroxidase activity in the rat hippocampus.     

Evidence for hippocampal role in place avoidance other than merely memory storage

Spatial navigation is used as a popular animal model of higher cognitive functions in people. The data suggest that the hippocampus is important for both storing spatial memories and for performing spatial computations necessary for navigation. Animals use multiple behavioral strategies to solve spatial tasks often using multiple memory systems. We investigated how inactivation of the rat hippocampus affects performance in a place avoidance task to determine if the role of the hippocampus in this task could be attributed to memory storage/retrieval or to the computations needed for navigation. Injecting tetrodotoxin (TTX) into both hippocampi impaired conditioned place avoidance, but after injecting only one hippocampus, the rats learned the place avoidance as well as without any injections. Retention of the place avoidance learned with one hippocampus was not impaired when the injection was switched to the hippocampus that had not been injected during learning. The result suggests that during learning, the hippocampus did not store the place avoidance memory.