Recruitment of functional GABA(A) receptors to postsynaptic domains by insulin

GABA Receptors on Orexin and Melanin-Concentrating Hormone Neurons Are Differentially Homeostatically Regulated Following Sleep Deprivation

Though overlapping in distribution through the hypothalamus, orexin (Orx) and melanin-concentrating hormone (MCH) neurons play opposite roles in the regulation of sleep-wake states. Orx neurons discharge during waking, whereas MCH neurons discharge during sleep. In the present study, we examined in mice whether GABAA and GABAB receptors (Rs) are present on Orx and MCH neurons and might undergo differential changes as a function of their different activities following sleep deprivation (SD) and sleep recovery (SR). Applying quantitative stereological image analysis to dual-immunofluorescent stained sections, we determined that the proportion of Orx neurons positively immunostained for GABAARs was significantly higher following SD (∼48%) compared with sleep control (SC; ∼24%) and SR (∼27%), and that the luminance of the GABAARs was significantly greater. In contrast, the average proportion of the MCH neurons immunostained for GABAARs was insignificantly lower following SD (∼43%) compared with SC (∼54%) and SR (56%), and the luminance of the GABAARs was significantly less. Although, GABABRs were observed in all Orx and MCH neurons (100%), the luminance of these receptors was differentially altered following SD. The intensity of GABABRs in the Orx neurons was significantly greater after SD than after SC and SR, whereas that in the MCH neurons was significantly less. The present results indicate that GABA receptors undergo dynamic and differential changes in the wake-active Orx neurons and the sleep-active MCH neurons as a function of and homeostatic adjustment to their preceding activity and sleep-wake state.

The Rationale for Insulin Therapy in Alzheimer's Disease

Alzheimer's disease (AD) is the most common form of dementia, with a prevalence that increases with age. By 2050, the worldwide number of patients with AD is projected to reach more than 140 million. The prominent signs of AD are progressive memory loss, accompanied by a gradual decline in cognitive function and premature death. AD is the clinical manifestation of altered proteostasis. The initiating step of altered proteostasis in most AD patients is not known. The progression of AD is accelerated by several chronic disorders, among which the contribution of diabetes to AD is well understood at the cell biology level. The pathological mechanisms of AD and diabetes interact and tend to reinforce each other, thus accelerating cognitive impairment. At present, only symptomatic interventions are available for treating AD. To optimise symptomatic treatment, a personalised therapy approach has been suggested. Intranasal insulin administration seems to open the possibility for a safe, and at least in the short term, effective symptomatic intervention that delays loss of cognition in AD patients. This review summarizes the interactions of AD and diabetes from the cell biology to the patient level and the clinical results of intranasal insulin treatment of cognitive decline in AD.

Central Nervous Insulin Signaling in Sleep-Associated Memory Formation and Neuroendocrine Regulation

The neurochemical underpinnings of sleep's contribution to the establishment and maintenance of memory traces are largely unexplored. Considering that intranasal insulin administration to the CNS improves memory functions in healthy and memory-impaired humans, we tested whether brain insulin signaling and sleep interact to enhance memory consolidation in healthy participants. We investigated the effect of intranasal insulin on sleep-associated neurophysiological and neuroendocrine parameters and memory consolidation in 16 men and 16 women (aged 18-30 years), who learned a declarative word-pair task and a procedural finger sequence tapping task in the evening before intranasal insulin (160 IU) or placebo administration and 8 h of nocturnal sleep. On the subsequent evening, they learned interfering word-pairs and a new finger sequence before retrieving the original memories. Insulin increased growth hormone concentrations in the first night-half and EEG delta power during the second 90 min of non-rapid-eye-movement sleep. Insulin treatment impaired the acquisition of new contents in both the declarative and procedural memory systems on the next day, whereas retrieval of original memories was unchanged. Results indicate that sleep-associated memory consolidation is not a primary mediator of insulin's acute memory-improving effect, but that the peptide acts on mechanisms that diminish the subsequent encoding of novel information. Thus, by inhibiting processes of active forgetting during sleep, central nervous insulin might reduce the interfering influence of encoding new information.

Naringin Improves Neuronal Insulin Signaling, Brain Mitochondrial Function, and Cognitive Function in High-Fat Diet-Induced Obese Mice

The epidemic and experimental studies have confirmed that the obesity induced by high-fat diet not only caused neuronal insulin resistance, but also induced brain mitochondrial dysfunction as well as learning impairment in mice. Naringin has been reported to posses biological functions which are beneficial to human cognitions, but its protective effects on HFD-induced cognitive deficits and underlying mechanisms have not been well characterized. In the present study Male C57BL/6 J mice were fed either a control or high-fat diet for 20 weeks and then randomized into four groups treated with their respective diets including control diet, control diet + naringin, high-fat diet (HFD), and high-fat diet + naringin (HFDN). The behavioral performance was assessed by using novel object recognition test and Morris water maze test. Hippocampal mitochondrial parameters were analyzed. Then the protein levels of insulin signaling pathway and the AMP-activated protein kinase (AMPK) in the hippocampus were detected by Western blot method. Our results showed that oral administration of naringin significantly improved the learning and memory abilities as evidenced by increasing recognition index by 52.5% in the novel object recognition test and inducing a 1.05-fold increase in the crossing-target number in the probe test, and ameliorated mitochondrial dysfunction in mice caused by HFD consumption. Moreover, naringin significantly enhanced insulin signaling pathway as indicated by a 34.5% increase in the expression levels of IRS-1, a 47.8% decrease in the p-IRS-1, a 1.43-fold increase in the p-Akt, and a 1.89-fold increase in the p-GSK-3β in the hippocampus of the HFDN mice versus HFD mice. Furthermore, the AMPK activity significantly increased in the naringin-treated (100 mg kg(-1) d(-1)) group. These findings suggest that an enhancement in insulin signaling and a decrease in mitochondrial dysfunction through the activation of AMPK may be one of the mechanisms that naringin improves cognitive functions in HFD-induced obese mice.

α6-Containing GABAA Receptors Are the Principal Mediators of Inhibitory Synapse Strengthening by Insulin in Cerebellar Granule Cells

Activity-dependent strengthening of central synapses is a key factor driving neuronal circuit behavior in the vertebrate CNS. At fast inhibitory synapses, strengthening is thought to occur by increasing the number of GABAA receptors (GABARs) of the same subunit composition to preexisting synapses. Here, we show that strengthening of mouse cerebellar granule cell GABAergic synapses occurs by a different mechanism. Specifically, we show that the neuropeptide hormone, insulin, strengthens inhibitory synapses by recruiting α6-containing GABARs rather than accumulating more α1-containing receptors that are resident to the synapse. Because α6-receptors are targeted to functionally distinct postsynaptic sites from α1-receptors, we conclude that only a subset of all inhibitory synapses are strengthened. Together with our recent findings on stellate cells, we propose a general mechanism by which mature inhibitory synapses are strengthened. In this scenario, α1-GABARs resident to inhibitory synapses form the hardwiring of neuronal circuits with receptors of a different composition fulfilling a fundamental, but unappreciated, role in synapse strengthening.

Interneuron- and GABA(A) receptor-specific inhibitory synaptic plasticity in cerebellar Purkinje cells

Inhibitory synaptic plasticity is important for shaping both neuronal excitability and network activity. Here we investigate the input and GABA(A) receptor subunit specificity of inhibitory synaptic plasticity by studying cerebellar interneuron-Purkinje cell (PC) synapses. Depolarizing PCs initiated a long-lasting increase in GABA-mediated synaptic currents. By stimulating individual interneurons, this plasticity was observed at somatodendritic basket cell synapses, but not at distal dendritic stellate cell synapses. Basket cell synapses predominantly express β2-subunit-containing GABA(A) receptors; deletion of the β2-subunit ablates this plasticity, demonstrating its reliance on GABA(A) receptor subunit composition. The increase in synaptic currents is dependent upon an increase in newly synthesized cell surface synaptic GABA(A) receptors and is abolished by preventing CaMKII phosphorylation of GABA(A) receptors. Our results reveal a novel GABA(A) receptor subunit- and input-specific form of inhibitory synaptic plasticity that regulates the temporal firing pattern of the principal output cells of the cerebellum.

Activation of the insulin-signaling pathway in sciatic nerve and hippocampus of type 1 diabetic rats

Peripheral neuropathy is a major complication associated with diabetes and central neuropathy characterized by Alzheimer's disease-like features in the brain is associated with increased dementia risk for patients with diabetes. Although glucose uptake into the cells of the nervous system is insulin-independent, contribution of impaired insulin support is clearly recognized to play a role, however not yet fully understood, in the development of neuropathy. In this study, we assessed the direct role of insulin on the peripheral nervous system (PNS) and central nervous system (CNS) of insulin-dependent type 1 diabetic rats. Fresh sciatic nerve and hippocampus from control and diabetic rats were incubated with varied ex vivo concentrations of insulin and phosphorylation levels of insulin receptor and glycogen synthase kinase-3 (GSK3β) were assessed by Western blot analysis. Both the sciatic nerve and hippocampus from type 1 diabetic rats were highly responsive to exogenous insulin with a significantly increased phosphorylation of insulin receptor and GSK3 compared to tissues from control rats. Further, sustained in vivo insulin delivery, not sufficient to restore normal blood glucose, normalized the activation of both insulin receptor and GSK3 in both PNS and CNS tissues. These results suggest that the insulin-signaling pathway is responsive to exogenous insulin in the nervous system of insulin-deficient type 1 diabetic rats and that constant insulin delivery restore normal nerve function and may protect PNS and CNS from damage.

Intracranial pancreatic islet transplantation increases islet hormone expression in the rat brain and attenuates behavioral dysfunctions induced by MK-801 (dizocilpine)

The treatment of rodents with non-competitive antagonist of the N-Methyl-D-aspartate (NMDA) receptor, MK-801 (dizocilpine), induces symptoms of psychosis, deficits in spatial memory and impairment of synaptic plasticity. Recent studies have suggested that insulin administration might attenuate the cognitive dysfunctions through the modulatory effect on the expression of NMDA receptors and on the brain insulin signaling. Intrahepatic pancreatic islet transplantation is known as an efficient tool for correcting impaired insulin signaling. We examined the capacity of syngeneic islets grafted into the cranial subarachnoid cavity to attenuate behavioral dysfunctions in rats exposed to MK-801. Animals were examined in the open field (OF) and the Morris Water Maze (MWM) tests following acute or subchronic administration of MK-801. We found well-vascularized grafted islets expressing insulin, glucagon and somatostatin onto the olfactory bulb and prefrontal cortex. Significantly higher levels of insulin were detected in the hippocampus and prefrontal cortex of transplanted animals compared to the non-transplanted rats. All animals expressed normal peripheral glucose homeostasis for two months after transplantation. OF tests revealed that rats exposed to MK-801 treatment, showed hyper-responsiveness in motility parameters and augmented center field exploration compared to intact controls and these effects were attenuated by the grafted islets. Moreover, in the MWM, the rats treated with MK-801 showed impairment of spatial memory that were partially corrected by the grafted islets. In conclusion, intracranial islet transplantation leads to the expression of islet hormones in the brain and attenuates behavioral and cognitive dysfunctions in rats exposed to MK-801 administration without altering the peripheral glucose homeostasis.

High-fat diet induces hepatic insulin resistance and impairment of synaptic plasticity

High-fat diet (HFD)-induced obesity is associated with insulin resistance, which may affect brain synaptic plasticity through impairment of insulin-sensitive processes underlying neuronal survival, learning, and memory. The experimental model consisted of 3 month-old C57BL/6J mice fed either a normal chow diet (control group) or a HFD (60% of calorie from fat; HFD group) for 12 weeks. This model was characterized as a function of time in terms of body weight, fasting blood glucose and insulin levels, HOMA-IR values, and plasma triglycerides. IRS-1/Akt pathway was assessed in primary hepatocytes and brain homogenates. The effect of HFD in brain was assessed by electrophysiology, input/output responses and long-term potentiation. HFD-fed mice exhibited a significant increase in body weight, higher fasting glucose- and insulin levels in plasma, lower glucose tolerance, and higher HOMA-IR values. In liver, HFD elicited (a) a significant decrease of insulin receptor substrate (IRS-1) phosphorylation on Tyr608 and increase of Ser307 phosphorylation, indicative of IRS-1 inactivation; (b) these changes were accompanied by inflammatory responses in terms of increases in the expression of NFκB and iNOS and activation of the MAP kinases p38 and JNK; (c) primary hepatocytes from mice fed a HFD showed decreased cellular oxygen consumption rates (indicative of mitochondrial functional impairment); this can be ascribed partly to a decreased expression of PGC1α and mitochondrial biogenesis. In brain, HFD feeding elicited (a) an inactivation of the IRS-1 and, consequentially, (b) a decreased expression and plasma membrane localization of the insulin-sensitive neuronal glucose transporters GLUT3/GLUT4; (c) a suppression of the ERK/CREB pathway, and (d) a substantial decrease in long-term potentiation in the CA1 region of hippocampus (indicative of impaired synaptic plasticity). It may be surmised that 12 weeks fed with HFD induce a systemic insulin resistance that impacts profoundly on brain activity, i.e., synaptic plasticity.

Neuronal gamma-aminobutyric acid (GABA) type A receptors undergo cognate ligand chaperoning in the endoplasmic reticulum by endogenous GABA

GABA_A receptors mediate fast inhibitory neurotransmission in the brain. Dysfunction of these receptors is associated with various psychiatric/neurological disorders and drugs targeting this receptor are widely used therapeutic agents. Both the efficacy and plasticity of GABA_A receptor-mediated neurotransmission depends on the number of surface GABA_A receptors. An understudied aspect of receptor cell surface expression is the post-translational regulation of receptor biogenesis within the endoplasmic reticulum (ER). We have previously shown that exogenous GABA can act as a ligand chaperone of recombinant GABA_A receptors in the early secretory pathway leading us to now investigate whether endogenous GABA facilitates the biogenesis of GABA_A receptors in primary cerebral cortical cultures. In immunofluorescence labeling experiments, we have determined that neurons expressing surface GABA_A receptors contain both GABA and its degradative enzyme GABA transaminase (GABA-T). Treatment of neurons with GABA-T inhibitors, a treatment known to increase intracellular GABA levels, decreases the interaction of the receptor with the ER quality control protein calnexin, concomittantly increasing receptor forward-trafficking and plasma membrane insertion. The effect of GABA-T inhibition on the receptor/calnexin interaction is not due to the activation of surface GABA_A or GABA_B receptors. Consistent with our hypothesis that GABA acts as a cognate ligand chaperone in the ER, immunogold-labeling of rodent brain slices reveals the presence of GABA within the rough ER. The density of this labeling is similar to that present in mitochondria, the organelle in which GABA is degraded. Lastly, the effect of GABA-T inhibition on the receptor/calnexin interaction was prevented by pretreatment with a GABA transporter inhibitor. Together, these data indicate that endogenous GABA acts in the rough ER as a cognate ligand chaperone to facilitate the biogenesis of neuronal GABA_A receptors.

Role of IGF-1 in cortical plasticity and functional deficit induced by sensorimotor restriction

In the adult rat, sensorimotor restriction by hindlimb unloading (HU) is known to induce impairments in motor behavior as well as a disorganization of somatosensory cortex (shrinkage of the cortical representation of the hindpaw, enlargement of the cutaneous receptive fields, decreased cutaneous sensibility threshold). Recently, our team has demonstrated that IGF-1 level was decreased in the somatosensory cortex of rats submitted to a 14-day period of HU. To determine whether IGF-1 is involved in these plastic mechanisms, a chronic cortical infusion of this substance was performed by means of osmotic minipump. When administered in control rats, IGF-1 affects the size of receptive fields and the cutaneous threshold, but has no effect on the somatotopic map. In addition, when injected during the whole HU period, IGF-1 is interestingly implied in cortical changes due to hypoactivity: the shrinkage of somatotopic representation of hindlimb is prevented, whereas the enlargement of receptive fields is reduced. IGF-1 has no effect on the increase in neuronal response to peripheral stimulation. We also explored the functional consequences of IGF-1 level restoration on tactile sensory discrimination. In HU rats, the percentage of paw withdrawal after a light tactile stimulation was decreased, whereas it was similar to control level in HU-IGF-1 rats. Taken together, the data clearly indicate that IGF-1 plays a key-role in cortical plastic mechanisms and in behavioral alterations induced by a decrease in sensorimotor activity.

The GLP-1 Receptor Agonist Exendin-4 and Diazepam Differentially Regulate GABAA Receptor-Mediated Tonic Currents in Rat Hippocampal CA3 Pyramidal Neurons

Glucagon-like peptide-1 (GLP-1) is a metabolic hormone that is secreted in a glucose-dependent manner and enhances insulin secretion. GLP-1 receptors are also found in the brain where their signalling affects neuronal activity. We have previously shown that the GLP-1 receptor agonists, GLP-1 and exendin-4 enhanced GABA-activated synaptic and tonic currents in rat hippocampal CA3 pyramidal neurons. The hippocampus is the centre for memory and learning and is important for cognition. Here we examined if exendin-4 similarly enhanced the GABA-activated currents in the presence of the benzodiazepine diazepam. In whole-cell recordings in rat brain slices, diazepam (1 μM), an allosteric positive modulator of GABA_A receptors, alone enhanced the spontaneous inhibitory postsynaptic current (sIPSC) amplitude and frequency by a factor of 1.3 and 1.6, respectively, and doubled the tonic GABA_A current normally recorded in the CA3 pyramidal cells. Importantly, in the presence of exendin-4 (10 nM) plus diazepam (1 μM), only the tonic but not the sIPSC currents transiently increased as compared to currents recorded in the presence of diazepam alone. The results suggest that exendin-4 potentiates a subpopulation of extrasynaptic GABA_A receptors in the CA3 pyramidal neurons.

Postsynaptic distribution of IRSp53 in spiny excitatory and inhibitory neurons

The 53 kDa insulin receptor substrate protein (IRSp53) is highly enriched in the brain. Despite evidence that links mutations of IRSp53 with autism and other neuropsychiatric problems, the functional significance of this protein remains unclear. We used light and electron microscopic immunohistochemistry to demonstrate that IRSp53 is expressed throughout the adult rat brain. Labeling concentrated selectively in dendritic spines, where it was associated with the postsynaptic density (PSD). Surprisingly, its organization within the PSD of spiny excitatory neurons of neocortex and hippocampus differed from that within spiny inhibitory neurons of neostriatum and cerebellar cortex. The present data support previous suggestions that IRSp53 is involved in postsynaptic signaling, while hinting that its signaling role may differ in different types of neurons.

Cellular and metabolic alterations in the hippocampus caused by insulin signalling dysfunction and its association with cognitive impairment during aging and Alzheimer's disease: studies in animal models

A growing body of animal and epidemiological studies suggest that metabolic diseases such as obesity, insulin resistance, metabolic syndrome and type 2 diabetes mellitus are associated with the development of cognitive impairment, dementia and Alzheimer's disease, particularly in aging. Several lines of evidence suggest that insulin signalling dysfunction produces these metabolic alterations and underlie the development of these neurodegenerative diseases. In this article, we address normal insulin function in the synapse; we review and discuss the physiopathological hallmarks of synaptic insulin signalling dysfunction associated with metabolic alterations. Additionally, we describe and review the major animal models of obesity, insulin resistance, metabolic syndrome and type 2 diabetes mellitus. The comprehensive knowledge of the molecular mechanisms behind the association of metabolic alterations and cognitive impairment could facilitate the early detection of neurodegenerative diseases in patients with metabolic alterations, with treatment that focus on neuroprotection. It could also help in the development of metabolic-based therapies and drugs for using in dementia and Alzheimer's disease patients to alleviate their symptoms in a more efficient and comprehensive way.

GLP-1 and exendin-4 transiently enhance GABAA receptor-mediated synaptic and tonic currents in rat hippocampal CA3 pyramidal neurons

Glucagon-like peptide-1 (GLP-1) is a hormone that stimulates insulin secretion. Receptors for GLP-1 are also found in the brain, including the hippocampus, the center for memory and learning. Diabetes is a risk factor for decreased memory functions. We studied effects of GLP-1 and exendin-4, a GLP-1 receptor agonist, on γ-aminobutyric acid (GABA) signaling in hippocampal CA3 pyramidal neurons. GABA is the main inhibitory neurotransmitter and decreases neuronal excitability. GLP-1 (0.01-1 nmol/L) transiently enhanced synaptic and tonic currents, and the effects were blocked by exendin (9-39). Ten pmol/L GLP-1 increased both the spontaneous inhibitory postsynaptic current (sIPSC) amplitudes and frequency by a factor of  1.8. In 0.1, 1 nmol/L GLP-1 or 10, 50, or 100 nmol/L exendin-4, only the sIPSC frequency increased. The tonic current was enhanced by 0.01-1 nmol/L GLP-1 and by 0.5-100 nmol/L exendin-4. When action potentials were inhibited by tetrodotoxin (TTX), inhibitory postsynaptic currents decreased and currents were no longer potentiated by GLP-1 or exendin-4. In contrast, although the tonic current decreased in TTX, it was still enhanced by GLP-1 or exendin-4. The results demonstrate GLP-1 receptor regulation of hippocampal function and are consistent with GLP-1 receptor agonists enhancing GABAA signaling by pre- and postsynaptic mechanisms.

Regulation of GABAARs by phosphorylation

γ-Aminobutyric acid type A receptors (GABAARs) are the principal mediators of fast synaptic inhibition in the brain as well as the low persistent extrasynaptic inhibition, both of which are fundamental to proper brain function. Thus unsurprisingly, deficits in GABAARs are implicated in a number of neurological disorders and diseases. The complexity of GABAAR regulation is determined not only by the heterogeneity of these receptors but also by its posttranslational modifications, the foremost, and best characterized of which is phosphorylation. This review will explore the details of this dynamic process, our understanding of which has barely scratched the surface. GABAARs are regulated by a number of kinases and phosphatases, and its phosphorylation plays an important role in governing its trafficking, expression, and interaction partners. Here, we summarize the progress in understanding the role phosphorylation plays in the regulation of GABAARs. This includes how phosphorylation can affect the allosteric modulation of GABAARs, as well as signaling pathways that affect GABAAR phosphorylation. Finally, we discuss the dysregulation of GABAAR phosphorylation and its implication in disease processes.

MHC class I limits hippocampal synapse density by inhibiting neuronal insulin receptor signaling

Proteins of the major histocompatibility complex class I (MHCI) negatively regulate synapse density in the developing vertebrate brain (Glynn et al., 2011; Elmer et al., 2013; Lee et al., 2014), but the underlying mechanisms remain largely unknown. Here we identify a novel MHCI signaling pathway that involves the inhibition of a known synapse-promoting factor, the insulin receptor. Dominant-negative insulin receptor constructs decrease synapse density in the developing Xenopus visual system (Chiu et al., 2008), and insulin receptor activation increases dendritic spine density in mouse hippocampal neurons in vitro (Lee et al., 2011). We find that genetically reducing cell surface MHCI levels increases synapse density selectively in regions of the hippocampus where insulin receptors are expressed, and occludes the neuronal insulin response by de-repressing insulin receptor signaling. Pharmacologically inhibiting insulin receptor signaling in MHCI-deficient animals rescues synapse density, identifying insulin receptor signaling as a critical mediator of the tonic inhibitory effects of endogenous MHCI on synapse number. Insulin receptors co-immunoprecipitate MHCI from hippocampal lysates, and MHCI unmasks a cytoplasmic epitope of the insulin receptor that mediates downstream signaling. These results identify an important role for an MHCI-insulin receptor signaling pathway in circuit patterning in the developing brain, and suggest that changes in MHCI expression could unexpectedly regulate neuronal insulin sensitivity in the aging and diseased brain.

Insulin induces phosphorylation of the AMPA receptor subunit GluR1, reversed by ZIP, and over-expression of Protein Kinase M zeta, reversed by amyloid beta

Insulin receptor (IR) in the brain plays a role in synaptic plasticity and cognitive functions. Phosphorylation of α-amino-3-hydroxy-5-methylisoxazole-4-propionic acid (AMPA) receptors GluR1 subunit at Serine 831 is regulated by calcium-calmodulin-dependent protein kinase II and protein kinase C that underlie long-term potentiation and learning/memory. Recent studies have shown that the novel Protein Kinase M zeta (PKMζ) underlies synaptic plasticity and may regulate AMPAr. In this study, we show that insulin induces phosphorylation of Serine 831 GluR1 subunit of AMPAr and induces over-expression of PKMζ; pre-treatment with either the IR inhibitor 3-Bromo-5-t-butyl-4-hydroxy-benzylidenemalonitrile (AG1024) or PKMζ inhibitor protein kinase C zeta pseudo-substrate inhibitor returned the phosphorylation value of GluR1 to control level. Amyloid beta (Aβ) peptide in the form of oligomers interferes with IR signaling. Pre-treating neuronal cultures with Aβ following incubation with insulin, we found a reduction of insulin-dependent PKMζ over-expression and MAPK/Erk (1/2) phosphorylation, i.e., signaling pathways involved in synaptic plasticity and learning/memory. These results indicate a new intracellular insulin signaling pathway, and, additionally, that insulin resistance in Alzheimer's disease is a response to the production and accumulation of Aβ.

Association of insulin receptor H1085H C>T, insulin receptor substrate 1 G972R and insulin receptor substrate 2 1057G/A polymorphisms with refractory temporal lobe epilepsy in Han Chinese

PURPOSE

Insulin/insulin receptor (INSR) signaling plays diverse roles in the central nervous system, including regulation of blood glucose, synaptic plasticity, dendritic growth, modulation of electrophysiological activity, proliferation of astrocytes and neuronal apoptosis. Interestingly, many of these and/or related processes represent biological mechanisms associated with temporal lobe epilepsy (TLE). Thus, insulin signaling may play a role in the development of TLE and its therapeutic responses. We hypothesized that functional polymorphisms in the insulin pathway genes INSR, insulin receptor substrate 1 (IRS1), and IRS2 may be associated with the therapeutic responses of TLE. Therefore, in this study we analyzed the association of three single nucleotide polymorphisms (SNPs) showing a risk for TLE drug resistance using a hospital-based case-control design.

METHOD

Two hundred and one patients with refractory TLE and one hundred and seventy-five drug-responsive TLE patients were recruited for the study. Polymerase chain reaction-restriction fragment length polymorphism was used to detect the genotypes of INSR His1085His, IRS1 G972R and IRS2 1057G/A.

RESULTS

No significant differences between refractory and drug-responsive TLE patients were observed for the IRS1 G972R and IRS2 1057G/A polymorphisms (P>0.05), but a significant association was found for the INSR His1085His polymorphism for both genotypes (P=0.035) and alleles (P=0.011). IRS2 1057G/A combined with the INSR His 1085 His polymorphism increased the odds ratio of drug resistance in TLE (P=0.011, OR=2.263, 95% CI: 1.208-4.239).

CONCLUSION

These results suggest that a genetic variation in the insulin signaling pathway genes may affect the therapeutic response of TLE.

The mother as hunter: significant reduction in foraging costs through enhancements of predation in maternal rats

In previous laboratory investigations, we have identified enhanced cognition and reduced stress in parous rats, which are likely adaptations in mothers needing to efficiently exploit resources to maintain, protect and provision their immature offspring. Here, in a series of seven behavioral tests on rats, we examined a natural interface between cognition and resource gathering: predation. Experiment 1 compared predatory behavior (toward crickets) in age-matched nulliparous mothers (NULLs) and postpartum lactating mothers (LACTs), revealing a highly significant enhancement of predation in LACT females (mean = -65s in LACTs, vs. -270s in NULLs). Experiment 2 examined the possibility that LACTs, given their increased metabolic rate, were hungrier, and thus more motivated to hunt; doubling the length of time of food deprivation in NULLs did not decrease their predatory latencies. Experiments 3-5, which examined sensory regulation of the effect, indicated that olfaction (anosmia), audition (blockade with white noise), and somatosensation (trimming the vibrissae) appear to play little role in the behavioral enhancement observed in the LACTs; Experiment 6 examined the possibility that visual augmentations may facilitate the improvements in predation; testing LACTs in a 0-lux environment eliminated the behavioral advantage (increasing their latencies from -65s to -212s), which suggests that temporary augmentation to the visual system may be important, and with hormone-neural alterations therein a likely candidate for further study. In contrast, testing NULLS in the 0-lux environment had the opposite effect, reducing their latency to catch the cricket (from -270s to -200s). Finally, Experiment 7 examined the development of predatory behavior in Early-pregnant (PREG), Mid-PREG, and Late-PREG females. Here, we observed a significant enhancement of predation in Mid-PREG and Late-PREG females--at a time when maternity-associated bodily changes would be expected to diminish predation ability--relative to NULLs. Therefore, as with the increasing reports of enhancements to the maternal brain, it is apparent that meaningful behavioral adaptations occur that likewise promote the survival of the mother and her infants at a crucial stage of their lives.

Inflammation and insulin/IGF-1 resistance as the possible link between obesity and neurodegeneration

Obesity is a growing epidemic that contributes to several brain disorders including Alzheimer's, Parkinson's, and Huntington's diseases. Obesity could promote these diseases through several different mechanisms. Here we review evidence supporting the involvement of two recently recognized factors linking obesity with neurodegeneration: the induction of pro-inflammatory cytokines and onset of insulin and insulin-like growth factor 1 (IGF-1) resistance. Excess peripheral pro-inflammatory mediators, some of which can cross the blood brain barrier, may trigger neuroinflammation, which subsequently exacerbates neurodegeneration. Insulin and IGF-1 resistance leads to weakening of neuroprotective signaling by these molecules and can contribute to onset of neurodegenerative diseases.

Leptin potentiates GABAergic synaptic transmission in the developing rodent hippocampus

It is becoming increasingly clear that leptin is not only a hormone regulating energy homeostasis but also a neurotrophic factor impacting a number of brain regions, including the hippocampus. Although leptin promotes the development of GABAergic transmission in the hypothalamus, little is known about its action on the GABAergic system in the hippocampus. Here we show that leptin modulates GABAergic transmission onto developing CA3 pyramidal cells of newborn rats. Specifically, leptin induces a long-lasting potentiation (LLP-GABAA) of miniature GABAA receptor-mediated postsynaptic current (GABAA-PSC) frequency. Leptin also increases the amplitude of evoked GABAA-PSCs in a subset of neurons along with a decrease in the coefficient of variation and no change in the paired-pulse ratio, pointing to an increased recruitment of functional synapses. Adding pharmacological blockers to the recording pipette showed that the leptin-induced LLP-GABAA requires postsynaptic calcium released from internal stores, as well as postsynaptic MAPK/ERK kinases 1 and/or 2 (MEK1/2), phosphoinositide 3 kinase (PI3K) and calcium-calmodulin kinase kinase (CaMKK). Finally, study of CA3 pyramidal cells in leptin-deficient ob/ob mice revealed a reduction in the basal frequency of miniature GABAA-PSCs compared to wild type littermates. In addition, presynaptic GAD65 immunostaining was reduced in the CA3 stratum pyramidale of mutant animals, both results converging to suggest a decreased number of functional GABAergic synapses in ob/ob mice. Overall, these results show that leptin potentiates and promotes the development of GABAergic synaptic transmission in the developing hippocampus likely via an increase in the number of functional synapses, and provide insights into the intracellular pathways mediating this effect. This study further extends the scope of leptin's neurotrophic action to a key regulator of hippocampal development and function, namely GABAergic transmission.

CX3CL1 is up-regulated in the rat hippocampus during memory-associated synaptic plasticity

Several cytokines and chemokines are now known to play normal physiological roles in the brain where they act as key regulators of communication between neurons, glia, and microglia. In particular, cytokines and chemokines can affect cardinal cellular and molecular processes of hippocampal-dependent long-term memory consolidation including synaptic plasticity, synaptic scaling and neurogenesis. The chemokine, CX₃CL1 (fractalkine), has been shown to modulate synaptic transmission and long-term potentiation (LTP) in the CA1 pyramidal cell layer of the hippocampus. Here, we confirm widespread expression of CX₃CL1 on mature neurons in the adult rat hippocampus. We report an up-regulation in CX₃CL1 protein expression in the CA1, CA3 and dentate gyrus (DG) of the rat hippocampus 2 h after spatial learning in the water maze task. Moreover, the same temporal increase in CX₃CL1 was evident following LTP-inducing theta-burst stimulation in the DG. At physiologically relevant concentrations, CX₃CL1 inhibited LTP maintenance in the DG. This attenuation in dentate LTP was lost in the presence of GABA_A receptor/chloride channel antagonism. CX₃CL1 also had opposing actions on glutamate-mediated rise in intracellular calcium in hippocampal organotypic slice cultures in the presence and absence of GABA_A receptor/chloride channel blockade. Using primary dissociated hippocampal cultures, we established that CX₃CL1 reduces glutamate-mediated intracellular calcium rises in both neurons and glia in a dose dependent manner. In conclusion, CX₃CL1 is up-regulated in the hippocampus during a brief temporal window following spatial learning the purpose of which may be to regulate glutamate-mediated neurotransmission tone. Our data supports a possible role for this chemokine in the protective plasticity process of synaptic scaling.

Effects of estradiol and genistein on the insulin signaling pathway in the cerebral cortex of aged female rats

Menopause leads to a decrease in estrogen production that increases central insulin resistance, contributing to the development of neurodegenerative diseases. We have evaluated the influence of aging and estradiol or genistein treatments on some key stages of the insulin signaling pathway in the cerebral cortex. Young and aged female Wistar rats were ovariectomized and treated acutely with 17β-estradiol (1.4μg/kg body weight), two doses of genistein (10 or 40mg/kg body weight), or vehicle. The cortical expression of several key insulin signaling pathway components was analyzed by western blotting. Our results showed an age-related deterioration in the interactions between the regulatory subunit of phosphatidylinositol 3-kinase (p85α) and the activated form of insulin receptor substrate 1 (p-IRS1tyr612), as well as between p85α and the 46kDa isoform of the estrogen receptor α (ERα46). Moreover, aging also decreased the translocation of glucose transporter-4 (GLUT4) to the plasma membrane. 17β-Estradiol but not genistein reduced the negative impact of aging on central insulin sensitivity by favoring this GLUT4 translocation, and therefore could be neuroprotective against the associated neurodegenerative diseases. However, protein kinase B (Akt) activation by genistein suggests that other possible mechanisms are involved in the neuroprotective effects of this phytoestrogen during the aging process.

Inflammation, defective insulin signaling, and mitochondrial dysfunction as common molecular denominators connecting type 2 diabetes to Alzheimer disease

A growing body of evidence supports an intriguing clinical/epidemiological connection between Alzheimer disease (AD) and type 2 diabetes (T2D). T2D patients have significantly increased risk of developing AD and vice versa. Recent studies have begun to reveal common pathogenic mechanisms shared by AD and metabolic disorders, notably obesity and T2D. In T2D and obesity, low-grade chronic inflammation is a key mechanism leading to peripheral insulin resistance, which progressively causes tissue deterioration and overall health decline. In the brain, proinflammatory signaling was recently found to mediate impaired neuronal insulin signaling, synapse deterioration, and memory loss. Here, we review evidence indicating that inflammation, insulin resistance, and mitochondrial dysfunction are common features in AD and T2D. We further propose the hypothesis that dementia and its underlying neuronal dysfunction are exacerbated or driven by peripheral inflammation. Identification of central and peripheral inflammation as potential mediators of brain dysfunction in AD may lead to the development of effective treatments for this devastating disease.

Insulin action in brain regulates systemic metabolism and brain function

Insulin receptors, as well as IGF-1 receptors and their postreceptor signaling partners, are distributed throughout the brain. Insulin acts on these receptors to modulate peripheral metabolism, including regulation of appetite, reproductive function, body temperature, white fat mass, hepatic glucose output, and response to hypoglycemia. Insulin signaling also modulates neurotransmitter channel activity, brain cholesterol synthesis, and mitochondrial function. Disruption of insulin action in the brain leads to impairment of neuronal function and synaptogenesis. In addition, insulin signaling modulates phosphorylation of tau protein, an early component in the development of Alzheimer disease. Thus, alterations in insulin action in the brain can contribute to metabolic syndrome, and the development of mood disorders and neurodegenerative diseases.

Benzodiazepine ligands rapidly influence GABAA receptor diffusion and clustering at hippocampal inhibitory synapses

Benzodiazepines (BZDs) are widely used in the treatment of a variety of neurological and psychiatric conditions including anxiety, insomnia and epilepsy. BZDs are thought to act predominantly by affecting the gating of GABAA receptor channels, resulting in enhanced GABA-mediated currents in neurons. However, mutations mimicking the effect of BZDs on GABAAR channel gating have been shown to also impact the membrane dynamics and synaptic anchoring of the receptors. Here, using single molecule tracking combined with electrophysiological recordings, we show that BZD ligands rapidly influence the dynamic behavior of GABAARs in hippocampal neurons. Application of the inverse BZD agonist DMCM rapidly increased the diffusion and reduced the clustering of GABAARs at synapses, resulting in reduced postsynaptic currents. Conversely, the BZD full agonist diazepam had little effect at rest but reduced lateral diffusion and increased synaptic stabilization and clustering of GABAARs upon sustained neuronal activity, resulting in enhanced potency of inhibitory synapses. These effects occurred in the absence of detectable changes in gephyrin clusters, suggesting they did not reflect a rapid dispersion of the synaptic scaffold. Thus, alterations of the diffusion and synaptic anchoring of GABAARs represent a novel, unsuspected mechanism through which BZDs rapidly modulate GABA signaling in central neurons.

Short-term high-fat-and-fructose feeding produces insulin signaling alterations accompanied by neurite and synaptic reduction and astroglial activation in the rat hippocampus

Chronic consumption of high-fat-and-fructose diets (HFFD) is associated with the development of insulin resistance (InsRes) and obesity. Systemic insulin resistance resulting from long-term HFFD feeding has detrimental consequences on cognitive performance, neurogenesis, and long-term potentiation establishment, accompanied by neuronal alterations in the hippocampus. However, diet-induced hippocampal InsRes has not been reported. Therefore, we investigated whether short-term HFFD feeding produced hippocampal insulin signaling alterations associated with neuronal changes in the hippocampus. Rats were fed with a control diet or an HFFD consisting of 10% lard supplemented chow and 20% high-fructose syrup in the drinking water. Our results show that 7 days of HFFD feeding induce obesity and InsRes, associated with the following alterations in the hippocampus: (1) a decreased insulin signaling; (2) a decreased hippocampal weight; (3) a reduction in dendritic arborization in CA1 and microtubule-associated protein 2 (MAP-2) levels; (4) a decreased dendritic spine number in CA1 and synaptophysin content, along with an increase in tau phosphorylation; and finally, (5) an increase in reactive astrocyte associated with microglial changes. To our knowledge, this is the first report addressing hippocampal insulin signaling, as well as morphologic, structural, and functional modifications due to short-term HFFD feeding in the rat.

The Potential Role of Insulin on the Shank-Postsynaptic Platform in Neurodegenerative Diseases Involving Cognition

Loss of synaptic function is critical in the pathogenesis of Alzheimer's disease (AD) and other central nervous system (CNS) degenerations. A promising candidate in the regulation of synaptic function is Shank, a protein that serves as a scaffold for excitatory synaptic receptors and proteins. Loss of Shank alters structure and function of the postsynaptic density (PSD). Shank proteins are associated with N-methyl-d-aspartate and α-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid receptor loss at the PSD in AD; mutations in Shank also lead to autism spectrum disorders (ASDs) and schizophrenia, both of which affect cognition, suggesting that Shank may play a common pathologic role in AD, ASD, and schizophrenia. Shank protein directly associates with insulin receptor substrate protein p53 in PSD. Insulin and insulin sensitizers have been used in clinical trials for these diseases; this suggests that insulin signals may alter protein homeostasis at the shank-postsynaptic platform in PSDs; insulin could improve the function of synapses in these diseases.

Is Alzheimer's disease a systemic disease?

Although Alzheimer's disease (AD) is the most common neurodegenerative disease, the etiology of AD is not well understood. In some cases, genetic factors explain AD risk, but a high percentage of late-onset AD is unexplained. The fact that AD is associated with a number of physical and systemic manifestations suggests that AD is a multifactorial disease that affects both the CNS and periphery. Interestingly, a common feature of many systemic processes linked to AD is involvement in energy metabolism. The goals of this review are to 1) explore the evidence that peripheral processes contribute to AD risk, 2) explore ways that AD modulates whole-body changes, and 3) discuss the role of genetics, mitochondria, and vascular mechanisms as underlying factors that could mediate both central and peripheral manifestations of AD. Despite efforts to strictly define AD as a homogeneous CNS disease, there may be no single etiologic pathway leading to the syndrome of AD dementia. Rather, the neurodegenerative process may involve some degree of baseline genetic risk that is modified by external risk factors. Continued research into the diverse but related processes linked to AD risk is necessary for successful development of disease-modifying therapies.

Insulin resistance and gray matter volume in neurodegenerative disease

The goal of this study was to compare insulin resistance in aging and aging-related neurodegenerative diseases, and to determine the relationship between insulin resistance and gray matter volume (GMV) in each cohort using an unbiased, voxel-based approach. Insulin resistance was estimated in apparently healthy elderly control (HC, n=21) and neurodegenerative disease (Alzheimer's disease (AD), n=20; Parkinson's disease (PD), n=22) groups using Homeostasis Model Assessment of Insulin Resistance 2 (HOMA2) and intravenous glucose tolerance test (IVGTT). HOMA2 and GMV were assessed within groups through General Linear Model multiple regression. We found that HOMA2 was increased in both AD and PD compared to the HC group (HC vs. AD, p=0.002, HC vs. PD, p=0.003), although only AD subjects exhibited increased fasting glucose (p=0.005). Furthermore, our voxel-based morphometry analysis revealed that HOMA2 was related to GMV in all cohorts in a region-specific manner (p<0.001, uncorrected). Significant relationships were observed in the medial prefrontal cortex (HC), medial temporal regions (AD), and parietal regions (PD). Finally, the directionality of the relationship between HOMA2 and GMV was disease-specific. Both HC and AD subjects exhibited negative relationships between HOMA2 and brain volume (increased HOMA2 associated with decreased brain volume), while a positive relationship was observed in PD. This cross-sectional study suggests that insulin resistance is increased in neurodegenerative disease, and that individuals with AD appear to have more severe metabolic dysfunction than individuals with PD or PD dementia.

Deregulation of brain insulin signaling in Alzheimer's disease

Contrary to the previous belief that insulin does not act in the brain, studies in the last three decades have demonstrated important roles of insulin and insulin signal transduction in various functions of the central nervous system. Deregulated brain insulin signaling and its role in molecular pathogenesis have recently been reported in Alzheimer's disease (AD). In this article, we review the roles of brain insulin signaling in memory and cognition, the metabolism of amyloid β precursor protein, and tau phosphorylation. We further discuss deficiencies of brain insulin signaling and glucose metabolism, their roles in the development of AD, and recent studies that target the brain insulin signaling pathway for the treatment of AD. It is clear now that deregulation of brain insulin signaling plays an important role in the development of sporadic AD. The brain insulin signaling pathway also offers a promising therapeutic target for treating AD and probably other neurodegenerative disorders.

GABAergic neurogliaform cells represent local sources of insulin in the cerebral cortex

Concentrations of insulin in the brain are severalfold higher than blood plasma levels. Insulin in the brain regulates the metabolism, molecular composition, and cognitive performance of microcircuits and reduces food intake; cerebral insulin levels are altered in diabetes, aging, obesity, and Alzheimer's disease. Released by pancreatic β cells, insulin passes the blood-brain barrier, but sources of locally released insulin still remain unclear. We find that insulin is strongly expressed in GABAergic neurogliaform cells in the cerebral cortex of the rat detected by single-cell digital PCR. Focal application of glucose or glibenclamide to neurogliaform cells mimics the excitation suppressing effect of external insulin on local microcircuits via insulin receptors. Thus, neurogliaform cells might link GABAergic and insulinergic action in cortical microcircuits.

Apoptosis signal regulating kinase 1 (ASK1): potential as a therapeutic target for Alzheimer's disease

Alzheimer's disease (AD) is the most common form of dementia, characterized by a decline in memory and cognitive function. Clinical manifestations of AD are closely associated with the formation of senile plaques and neurofibrillary tangles, neuronal loss and cognitive decline. Apoptosis signal regulating kinase 1 (ASK1) is a mediator of the MAPK pathway, which regulates various cellular responses such as apoptosis, cell survival, and differentiation. Accumulating evidence indicates that ASK1 plays a key role in the pathogenesis of neurodegenerative disorders such as Huntington's disease and AD. Of particular interest, ASK1 is associated with many signaling pathways, which include endoplasmic reticulum (ER) stress-mediated apoptosis, Aβ-induced neurotoxicity, tau protein phosphorylation, and insulin signal transduction. Here, we review experimental evidence that links ASK1 signaling and AD pathogenesis and propose that ASK1 might be a new point of therapeutic intervention to prevent or treat AD.

Brain effect of insulin and clonazepam in diabetic rats under depressive-like behavior

Diabetes mellitus is characterized by hyperglycemia resulting from defects on insulin secretion, insulin action, or both. It has recently become clear that the central nervous system is not spared from the deleterious effects of diabetes, since diabetic encephalopathy was recognized as a complication of this heterogeneous metabolic disorder. There is a well recognized association between depression and diabetes, once prevalence of depression in diabetic patients is higher than in general population, and clonazepam is being used to treat this complication. Oxidative stress is widely accepted as playing a key mediatory role in the development and progression of diabetes and its complications. In this work we analyzed DNA damage by comet assay and lipid damage in prefrontal cortex, hippocampus and striatum of streptozotocin-induced diabetic rats submitted to the forced swimming test. It was verified that the diabetic group presented DNA and lipid damage in the brain areas evaluated, when compared to the control groups. Additionally, a significant reduction of the DNA and lipid damage in animals treated with insulin and/or clonazepam was observed. These data suggest that the association of these two drugs could protect against DNA and lipid damage in diabetic rats submitted to the forced swimming test, an animal model of depression.

ErbB4 reduces synaptic GABAA currents independent of its receptor tyrosine kinase activity

ErbB4 signaling in the central nervous system is implicated in neuropsychiatric disorders and epilepsy. In cortical tissue, ErbB4 associates with excitatory synapses located on inhibitory interneurons. However, biochemical and histological data described herein demonstrate that the vast majority of ErbB4 is extrasynaptic and detergent-soluble. To explore the function of this receptor population, we used unbiased proteomics, in combination with electrophysiological, biochemical, and cell biological techniques, to identify a clinically relevant ErbB4-interacting protein, the GABAA receptor α1 subunit (GABAR α1). We show that ErbB4 and GABAR α1 are robustly coexpressed in hippocampal interneurons, and that ErbB4-null mice have diminished cortical GABAR α1 expression. Moreover, we characterize a Neuregulin-mediated ErbB4 signaling modality, independent of receptor tyrosine kinase activity, that couples ErbB4 to decreased postsynaptic GABAR currents on inhibitory interneurons. Consistent with an evolving understanding of GABAR trafficking, this pathway requires both clathrin-mediated endocytosis and protein kinase C to reduce GABAR inhibitory currents, surface GABAR α1 expression, and colocalization with the inhibitory postsynaptic protein gephyrin. Our results reveal a function of ErbB4, independent of its tyrosine kinase activity, that modulates postsynaptic inhibitory control of hippocampal interneurons and may provide a novel pharmacological target in the treatment of neuropsychiatric disorders and epilepsy.

GABAA receptor plasticity in Jurkat T cells

GABAA receptors (GABAAR) mediate inhibitory neurotransmission in the human brain. Neurons modify subunit expression, cellular distribution and function of GABAAR in response to different stimuli, a process named plasticity. Human lymphocytes have a functional neuronal-like GABAergic system with GABAAR acting as inhibitors of proliferation. We here explore if receptor plasticity occurs in lymphocytes. To this end, we analyzed human T lymphocyte Jurkat cells exposed to different physiological stimuli shown to mediate plasticity in neurons: GABA, progesterone and insulin. The exposure to 100 μM GABA differently affected the expression of GABAAR subunits measured at both the mRNA and protein level, showing an increase of α1, β3, and γ2 subunits but no changes in δ subunit. Exposure of Jurkat cells to different stimuli produced different changes in subunit expression: 0.1 μM progesterone decreased δ and 0.5 μM insulin increased β3 subunits. To identify the mechanisms underlying plasticity, we evaluated the Akt pathway, which is involved in the phosphorylation of β subunits and receptor translocation to the membrane. A significant increase of phosphorylated Akt and on the expression of β3 subunit in membrane occurred in cells exposed 15 h to GABA. To determine if plastic changes are translated into functional changes, we performed whole cell recordings. After 15 h GABA-exposure, a significantly higher percentage of cells responded to GABA application when compared to 0 and 40 h exposure, thus indicating that the detected plastic changes may have a role in GABA-modulated lymphocyte function. Our results reveal that lymphocyte GABAAR are modified by different stimuli similarly and by similar mechanisms to those in neurons. This property is of significance for the development of future therapies involving pharmacological modulation of the immune response.

Thyroid hormone signaling and homeostasis during aging

Studies in humans and in animal models show negative correlations between thyroid hormone (TH) levels and longevity. TH signaling is implicated in maintaining and integrating metabolic homeostasis at multiple levels, notably centrally in the hypothalamus but also in peripheral tissues. The question is thus raised of how TH signaling is modulated during aging in different tissues. Classically, TH actions on mitochondria and heat production are obvious candidates to link negative effects of TH to aging. Mitochondrial effects of excess TH include reactive oxygen species and DNA damage, 2 factors often considered as aging accelerators. Inversely, caloric restriction, which can retard aging from nematodes to primates, causes a rapid reduction of circulating TH, reducing metabolism in birds and mammals. However, many other factors could link TH to aging, and it is these potentially subtler and less explored areas that are highlighted here. For example, effects of TH on membrane composition, inflammatory responses, stem cell renewal and synchronization of physiological responses to light could each contribute to TH regulation of maintenance of homeostasis during aging. We propose the hypothesis that constraints on TH signaling at certain life stages, notably during maturity, are advantageous for optimal aging.

Molecular links between Alzheimer's disease and diabetes mellitus

Substantial epidemiological evidence shows an increased risk for developing Alzheimer's disease (AD) in people with diabetes. Yet the underlying molecular mechanisms still remain to be elucidated. This article reviews the current studies on common pathological processes of Alzheimer's disease and diabetes with particular focus on potential mechanisms through which diabetes affects the initiation and progression of Alzheimer's disease. Impairment of insulin signaling, inflammation, oxidative stress, mitochondrial dysfunction, advanced glycation end products, APOEε4 and cholesterol appear to be important mediators and are likely to act synergistically in promoting AD pathology.

Food for thought: the role of appetitive peptides in age-related cognitive decline

Through their well described actions in the hypothalamus, appetitive peptides such as insulin, orexin and leptin are recognized as important regulators of food intake, body weight and body composition. Beyond these metabolic activities, these peptides also are critically involved in a wide variety of activities ranging from modulation of immune and neuroendocrine function to addictive behaviors and reproduction. The neurological activities of insulin, orexin and leptin also include facilitation of hippocampal synaptic plasticity and enhancement of cognitive performance. While patients with metabolic disorders such as obesity and diabetes have greater risk of developing cognitive deficits, dementia and Alzheimer's disease (AD), the underlying mechanisms that are responsible for, or contribute to, age-related cognitive decline are poorly understood. In view of the importance of these peptides in metabolic disorders, it is not surprising that there is a greater focus on their potential role in cognitive deficits associated with aging. The goal of this review is to describe the evidence from clinical and pre-clinical studies implicating insulin, orexin and leptin in the etiology and progression of age-related cognitive decline. Collectively, these studies support the hypothesis that leptin and insulin resistance, concepts normally associated with the hypothalamus, are also applicable to the hippocampus.

Regulation of synaptic functions in central nervous system by endocrine hormones and the maintenance of energy homoeostasis

Energy homoeostasis, a co-ordinated balance of food intake and energy expenditure, is regulated by the CNS (central nervous system). The past decade has witnessed significant advances in our understanding of metabolic processes and brain circuitry which responds to a broad range of neural, nutrient and hormonal signals. Accumulating evidence demonstrates altered synaptic plasticity in the CNS in response to hormone signals. Moreover, emerging observations suggest that synaptic plasticity underlies all brain functions, including the physiological regulation of energy homoeostasis, and that impaired synaptic constellation and plasticity may lead to pathological development and conditions. Here, we summarize the current knowledge on the regulation of postsynaptic receptors such as AMPA (α-amino-3-hydroxy-5-methylisoxazole-4-propionic acid), NMDA (N-methyl-D-aspartate) and GABA (γ-aminobutyric acid) receptors, and the presynaptic components by hormone signals. A detailed understanding of the neurobiological mechanisms by which hormones regulate energy homoeostasis may lead to novel strategies in treating metabolic disorders.

Insulin: an emerging treatment for Alzheimer's disease dementia?

Accumulating evidence indicates a role for metabolic dysfunction in the pathogenesis of Alzheimer's disease (AD). It is widely reported that Type 2 diabetes (T2D) increases the risk of developing AD, and several postmortem analyses have found evidence of insulin resistance in the AD brain. Thus, insulin-based therapies have emerged as potential strategies to slow cognitive decline in AD. The main methods for targeting insulin to date have been intravenous insulin infusion, intranasal insulin administration, and use of insulin sensitizers. These methods have elicited variable results regarding improvement in cognitive function. This review will discuss the rationale for targeting insulin signaling to improve cognitive function in AD, the results of clinical studies that have targeted insulin signaling, and what these results mean for future studies of the role of insulin-based therapies for AD.

No effect of adjunctive, repeated-dose intranasal insulin treatment on psychopathology and cognition in patients with schizophrenia

OBJECTIVE

This study examined the effect of adjunctive intranasal insulin therapy on psychopathology and cognition in patients with schizophrenia.

METHODS

Each subject had a Diagnostic and Statistical Manual of Mental Disorders, Fourth Edition, diagnosis of schizophrenia or schizoaffective disorder and been on stable antipsychotics for at least 1 month. In an 8-week randomized, double-blind, placebo-controlled study, subjects received either intranasal insulin (40 IU 4 times per day) or placebo. Psychopathology was assessed using the Positive and Negative Syndrome Scale and the Scale for Assessment of Negative Symptoms. A neuropsychological battery was used to assess cognitive performance. The assessment for psychopathology and cognition was conducted at baseline, week 4, and week 8.

RESULTS

A total of 45 subjects were enrolled in the study (21 in the insulin group and 24 in the placebo group). The mixed model analysis showed that there were no significant differences between the 2 groups at week 8 on various psychopathology and cognitive measures (P > 0.1).

CONCLUSIONS

Adjunctive therapy with intranasal insulin did not seem to be beneficial in improving schizophrenia symptoms or cognition in the present study. The implications for future studies were discussed.

Involvement of insulin-like peptide in long-term synaptic plasticity and long-term memory of the pond snail Lymnaea stagnalis

The pond snail Lymnaea stagnalis is capable of learning taste aversion and consolidating this learning into long-term memory (LTM) that is called conditioned taste aversion (CTA). Previous studies showed that some molluscan insulin-related peptides (MIPs) were upregulated in snails exhibiting CTA. We thus hypothesized that MIPs play an important role in neurons underlying the CTA-LTM consolidation process. To examine this hypothesis, we first observed the distribution of MIP II, a major peptide of MIPs, and MIP receptor and determined the amounts of their mRNAs in the CNS. MIP II was only observed in the light green cells in the cerebral ganglia, but the MIP receptor was distributed throughout the entire CNS, including the buccal ganglia. Next, when we applied exogenous mammalian insulin, secretions from MIP-containing cells or partially purified MIPs, to the isolated CNS, we observed a long-term change in synaptic efficacy (i.e., enhancement) of the synaptic connection between the cerebral giant cell (a key interneuron for CTA) and the B1 motor neuron (a buccal motor neuron). This synaptic enhancement was blocked by application of an insulin receptor antibody to the isolated CNS. Finally, injection of the insulin receptor antibody into the snail before CTA training, while not blocking the acquisition of taste aversion learning, blocked the memory consolidation process; thus, LTM was not observed. These data suggest that MIPs trigger changes in synaptic connectivity that may be correlated with the consolidation of taste aversion learning into CTA-LTM in the Lymnaea CNS.

Regulation of nuclear TDP-43 by NR2A-containing NMDA receptors and PTEN

The dysfunction of TAR DNA-binding protein-43 (TDP-43) is implicated in neurodegenerative diseases. However, the function of TDP-43 is not fully elucidated. Here we show that the protein level of endogenous TDP-43 in the nucleus is increased in mouse cortical neurons in the early stages, but return to basal level in the later stages after glutamate accumulation-induced injury. The elevation of TDP-43 results from a downregulation of phosphatase and tensin homolog (PTEN). We further demonstrate that activation of NR2A-containing NMDA receptors (NR2ARs) leads to PTEN downregulation and subsequent reduction of PTEN import from the cytoplasm to the nucleus after glutamate accumulation. The decrease of PTEN in the nucleus contributes to its reduced association with TDP-43, and thereby mediates the elevation of nuclear TDP-43. We provide evidence that the elevation of nuclear TDP-43, mediated by NR2AR activation and PTEN downregulation, confers protection against cortical neuronal death in the late stages after glutamate accumulation. Thus, this study reveals a NR2AR-PTEN-TDP-43 signaling pathway by which nuclear TDP-43 promotes neuronal survival. These results suggest that upregulation of nuclear TDP-43 represents a self-protection mechanism to delay neurodegeneration in the early stages after glutamate accumulation and that prolonging the upregulation process of nuclear TDP-43 might have therapeutic significance.