Insulin and insulin-like growth factors in the CNS

Role of insulin and insulin receptor in learning and memory

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

Roles of growth hormone and insulin-like growth factor 1 in mouse postnatal growth

To examine the relationship between growth hormone (GH) and insulin-like growth factor 1 (IGF1) in controlling postnatal growth, we performed a comparative analysis of dwarfing phenotypes manifested in mouse mutants lacking GH receptor, IGF1, or both. This genetic study has provided conclusive evidence demonstrating that GH and IGF1 promote postnatal growth by both independent and common functions, as the growth retardation of double Ghr/Igf1 nullizygotes is more severe than that observed with either class of single mutant. In fact, the body weight of these double-mutant mice is only approximately 17% of normal and, in absolute magnitude ( approximately 5 g), only twice that of the smallest known mammal. Thus, the growth control pathway in which the components of the GH/IGF1 signaling systems participate constitutes the major determinant of body size. To complement this conclusion mainly based on extensive growth curve analyses, we also present details concerning the involvement of the GH/IGF1 axis in linear growth derived by a developmental study of long bone ossification in the mutants.

Effects of voluntary ethanol drinking on [125I]insulin-like growth factor-I, [125I]insulin-like growth factor-II and [125I]insulin receptor binding in the mouse hippocampus and cerebellum

Chronic exposure to ethanol can induce widespread cell loss in the brain, in some cases even causing dementia. Although the underlying mechanism associated with ethanol toxicity has not yet been established, it is suggested that one of the ways in which ethanol disrupts neuronal functioning/survival is by targeting the actions of mitogenic growth factors. Insulin-like growth factors-I and -II and insulin are structurally related polypeptides with potent mitogenic and metabolic effects on the central and peripheral nervous systems. These growth factors and their respective receptors are widely distributed throughout the brain, including the hippocampus and cerebellum. Evidence indicates that ethanol can decrease plasma levels of insulin-like growth factors and can also inhibit the growth-promoting and cell survival effects of these growth factors under in vitro conditions. The present study was designed to determine if voluntary ethanol consumption over a 21-day period could alter [125I]insulin-like growth factor-I, [125I]insulin-like growth factor-II and [125I]insulin receptor-binding sites in the hippocampus and cerebellum-areas known to be severely affected following chronic exposure to ethanol. C57BL/6 mice were presented with either water only or a choice of water and a 10% v/v ethanol solution. Mice with access to the ethanol solution drank an average of 5.35+/-0.77 g of ethanol/kg body weight per day. [125I]Insulin-like growth factor-I receptor-binding sites were found to be significantly increased in all subfields of the hippocampal formation, but not in the cerebellum, of ethanol-treated mice compared to controls. [125I]Insulin-like growth factor-II and [125I]insulin receptor-binding sites, on the other hand, did not exhibit any alterations either in the hippocampus or cerebellum following chronic exposure to ethanol. These results, in keeping with earlier reports, suggest that hippocampal insulin-like growth factor-I is more sensitive to ethanol treatment than either insulin-like growth factor-II or insulin, and the observed increase in the [125I]insulin-like growth factor-I receptor levels possibly reflects an activity-dependent response to prevent/slow down neuronal degeneration and/or to regulate subtle functional alterations that follow chronic exposure to ethanol.

Biomedicine. Staying slim with insulin in mind

Striking the delicate balance between energy intake in the form of food and energy expenditure in the form of metabolic activity keeps the body extremely busy. As Schwartz explains in his enlightening Perspective, the finding that insulin signals the brain to promote weight loss (Brüning et al.) flies in the face of the notion that insulin is involved solely in glucose storage, its conversion to fat, and weight gain.

Integration of metabolism and intake regulation: a review focusing on periparturient animals

There has been great interest in dry matter intake regulation in lactating dairy cattle to enhance performance and improve animal health and welfare. Predicting voluntary dry matter intake (VDMI) is complex and influenced by numerous factors relating to the diet, management, housing, environment and the animal. The objective of this review is to identify and discuss important metabolic factors involved in the regulation of VDMI and their integration with metabolism. We have described the adaptations of intake and metabolism and discussed mechanisms of intake regulation. Furthermore we have reviewed selected metabolic signals involved in intake regulation. A substantial dip in VDMI is initiated in late pregnancy and continues into early lactation. This dip has traditionally been interpreted as caused by physical constraints, but this role is most likely overemphasized. The dip in intake coincides with changes in reproductive status, fat mass, and metabolic changes in support of lactation, and we have described metabolic signals that may play an equally important role in intake regulation. These signals include nutrients, metabolites, reproductive hormones, stress hormones, leptin, insulin, gut peptides, cytokines, and neuropeptides such as neuropeptide Y, galanin, and corticotrophin-releasing factor. The involvement of these signals in the periparturient dip in intake is discussed, and evidence supporting the integration of the regulation of intake and metabolism is presented. Still, much research is needed to clarify the complex regulation of VDMI in lactating dairy cows, particularly in the periparturient animal.

Differential regulation of leptin receptor expression by insulin and leptin in neuroblastoma cells

Leptin exerts its effects by interacting with specific membrane receptors (Ob-R). We studied the exact localization of long intracellular domain form (Ob-Rb) in human brain. In addition, we analyzed the regulatory features of Ob-Rb expression in two neuroblastoma cell lines. The Ob-Rb mRNAs were abundant in putamen, frontal lobe, medulla, cerebral cortex, cerebellum, thalamus, hippocampus, corpus callosum, caudate nucleus, and amygdala, indicating that Ob-Rb transcripts are expressed differently from that of other Ob-R isoforms. In SK-N-MC cells, the expression of Ob-Rb mRNA was induced by increasing doses of insulin, and the maximum amount of mRNA expression was 9.4-fold higher in the presence of insulin (100 nM for 24 h), compared to the absence of insulin. In IMR32 cells, the transcripts were increased 4.0-fold when cells were incubated with 1 nM of insulin for 48 h. In contrast, Ob-Rb expression in IMR32 cells decreased to 18% of control following a 24-h incubation period with 50 ng/mL of leptin, compared to incubation in the absence of leptin. These results indicate that expression of Ob-Rb is differentially regulated by inhibitory signals of energy balance in neuroblastoma cells. The identification of the novel regulatory mechanisms involving the Ob-Rb isoform by insulin and leptin now makes it possible to elucidate the underlying mechanisms involving increased food intake and uncontrolled energy balance associated with leptin resistance in obese individuals.

Central nervous system control of food intake

New information regarding neuronal circuits that control food intake and their hormonal regulation has extended our understanding of energy homeostasis, the process whereby energy intake is matched to energy expenditure over time. The profound obesity that results in rodents (and in the rare human case as well) from mutation of key signalling molecules involved in this regulatory system highlights its importance to human health. Although each new signalling pathway discovered in the hypothalamus is a potential target for drug development in the treatment of obesity, the growing number of such signalling molecules indicates that food intake is controlled by a highly complex process. To better understand how energy homeostasis can be achieved, we describe a model that delineates the roles of individual hormonal and neuropeptide signalling pathways in the control of food intake and the means by which obesity can arise from inherited or acquired defects in their function.

Choroid plexus recovery after transient forebrain ischemia: role of growth factors and other repair mechanisms

1. Transient forebrain ischemia in adult rats, induced by 10 min of bilateral carotid occlusion and an arterial hypotension of 40 mmHg, caused substantial damage not only to CA-1 neurons in hippocampus but also to epithelial cells in lateral ventricle choroid plexus. 2. When transient forebrain ischemia was followed by reperfusion (recovery) intervals of 0 to 12 hr, there was moderate to severe damage to many frond regions of the choroidal epithelium. In some areas, epithelial debris was sloughed into cerebrospinal fluid (CSF). Although some epithelial cells were disrupted and necrotic, their neighbors exhibited normal morphology. This patchy response to ischemia was probably due to regional differences in reperfusion or cellular metabolism. 3. Between 12 and 24 hr postischemia, there was marked restoration of the Na+, K+, water content, and ultrastructure of the choroid plexus epithelium. Since there was no microscopical evidence for mitosis, we postulate that healthy epithelial cells either were compressed together on the villus or migrated from the choroid plexus stalk to more distal regions, in order to "fill in gaps" along the basal lamina caused by necrotic epithelial cell disintegration. 4. Epithelial cells of mammalian choroid plexus synthesize and secrete many growth factors and other peptides that are of trophic benefit following injury to regions of the cerebroventricular system. For example, several growth factors are upregulated in choroid plexus after ischemic and traumatic insults to the central nervous system. 5. The presence of numerous types of growth factor receptors in choroid plexus allows growth factor mediation of recovery processes by autocrine and paracrine mechanisms. 6. The capability of choroid plexus after acute ischemia to recover its barrier and CSF formation functions is an important factor in stabilizing brain fluid balance. 7. Moreover, growth factors secreted by choroid plexus into CSF are distributed by diffusion and convection into brain tissue near the ventricular system, e.g., hippocampus. By this endocrine-like mechanism, growth factors are conveyed throughout the choroid plexus-CSF-brain nexus and can consequently promote repair of ischemia-damaged tissue in the ventricular wall and underlying brain.

Growth hormone, insulin-like growth factor I and cognitive function in adults

This review focuses on the possible contribution of the growth hormone (GH)-insulin-like growth factor I (IGF-I) axis to cognitive function. Binding sites for GH and IGF-I are found in various areas of the brain. Their distribution suggests that GH and IGF-I contribute to the function of the hippocampus, a brain structure important for the maintenance of cognitive functions such as learning and memory. Evidence for cognitive deficits in GH-deficient individuals has been found in various studies, some of which have shown that these deficits can be reversed by GH substitution therapy. In addition to examining conditions of GH deficiency, this article reviews studies evaluating the correlation between the cognitive deficits associated with ageing and age-related decreases in GH or IGF-I secretion. Based on the available data, one might hypothesize that relative GH or IGF-I deficiency could contribute to the deterioration of cognitive functions observed in the elderly.

The window of opportunity for neuronal rescue with insulin-like growth factor-1 after hypoxia-ischemia in rats is critically modulated by cerebral temperature during recovery

Insulin-like growth factor (IGF-1) is induced in damaged brain tissue after hypoxia-ischemia, and exogenous administration of IGF-1 shortly after injury has been shown to be neuroprotective. However, it is unknown whether treatment with IGF-1 delayed by more than a few hours after injury may be protective. Hypothermia after brain injury has been reported to delay the development of ischemic neuronal death. The authors therefore hypothesize that a reduction in the environmental temperature during recovery from hypoxia-ischemia could prolong the window of opportunity for IGF-1 treatment. Unilateral brain damage was induced in adult rats using a modified Levine model of right carotid artery ligation followed by brief hypoxia (6% O2 for 10 minutes). The rats were maintained in either a warm (31 degrees C) or cool (23 degrees C) environment for the first 2 hours after hypoxia. All rats were subsequently transferred to the 23 degrees C environment until the end of the experiment. A single dose of IGF-1 (50 microg) or its vehicle was given intracerebroventricularly at either 2 or 6 hours after hypoxia. Histologic outcome in the lateral cortex was quantified 5 days after hypoxia. Finally, cortical temperature was recorded from 1 hour before and 2 hours after hypoxia in separate groups of rats exposed to the "warm" and "cool" protocols. In rats exposed to the warm recovery environment, IGF-1 reduced cortical damage (P < 0.05) when given 2 hours but not 6 hours after insult. In contrast, with early recovery in the cool environment, a significant protective effect of IGF-1 in the lateral cortex (P < 0.05) was found with administration 6 hours after insult. In conclusion, a reduction in cerebral temperature during the early recovery phase after severe hypoxia-ischemia did not significantly reduce the severity of injury after 5 days' recovery; however, it markedly shifted and extended the window of opportunity for delayed treatment with IGF-1.

Effect of cerebroventricular infusion of insulin and (or) glucose on hypothalamic expression of leptin receptor and pituitary secretion of LH in diet-restricted ewes

The objective was to determine the effect of central infusion of insulin and (or) glucose on hypothalamic expression of leptin receptor and pituitary secretion of LH in the ewe. Twenty-two ovariectomized ewes (32 wk of age) were fitted with two lateral cerebroventricular (LCV) cannulae and fed 33% of NRC requirements for 8 wk. Ewes (n> or =5/group) were then infused, via LCV cannulae, with artificial cerebrospinal fluid (aCSF) or aCSF containing physiological concentrations of insulin (INS), glucose (GLU), or INS + GLU; the mass of each increasing linearly from Day 0 (mass = 0 units/h) to Day 8 (mass of INS = 80 mIU/hr and GLU = 10 mg/hr). Jugular serum was collected every 12 min for 4 hr on Days 0, 2, and 4. Ewes treated with INS or INS + GLU had greater (P<0.06) mean concentrations of LH than aCSF treated ewes on Day 2 (13.8+/-1.8 and 12.5+/-1.3 > 8.0+/-3.3 ng/ml). Furthermore, on Day 4, concentrations of LH in INS treated ewes exceeded that (P<0.07) of aCSF treated ewes (14.8+/-2.0 > 7.4+/-3.0 ng/ml). Expression of NPY mRNA did not differ between treatments (P = 0.87). Leptin receptor mRNA expression was dramatically reduced (P<0.0002) in INS+GLU versus aCSF treated ewes. These data provide evidence to suggest that insulin may be an important component of hypothalamic mechanisms regulating secretion of LH and expression of leptin receptors in undernourished ruminants.

Neurotrophic factors in neurodegenerative disorders: model of Parkinson's disease

Neurotrophic factors are compounds that enhance neuronal survival and differentiation. Most of these compounds exert their pharmacological actions on selective types of neurons, and therefore, are considered promising new therapeutic agents for the treatment of different neurodegenerative disorders characterized by selective degeneration of certain neuronal groups. Those compounds have been used in humans for several neurological disorders including amyotrophic lateral sclerosis--ciliary derived neurotrophic factor (CNTF) and brain derived neurotrophic factor (BDNF), Alzheimer's disease and peripheral neuropathy--nerve growth factor (NGF) and Parkinson's disease (PD)--glial derived neurotrophic factor (GDNF). In spite of well founded clinical experiments by previous experimental work in animal models some of these trials have been negative. For instance, animal models of PD have shown that several neurotrophic factors, including GDNF and other compounds, reduce apoptosis and increase resistance of dopamine neurons to neurotoxins in vitro. These compounds prevent or recover the damage to dopamine neurons of rodents and primates produced by chemical or mechanical acute lesions including 6-OH-DA, MPTP, methamphetamine and axotomy. The differences between the promising results obtained in experimental models and the lack of clinical results or excessive toxicity found in humans could be attributed to the following reasons: (a) Lack of relevance between the pathogenesis of the experimental lesion and the corresponding neurodegenerative disorder. (b) Poor correlation between results obtained in acute, self-limited, selective deficit produced to experimental animals and those available in more complex, chronic and progressive disorders involving patients. (c) Inadequate delivery of the active product to the target area in the human brain. (d) Poor information from acute experiments in animals which does not predict long-term effects of chronic infusion in humans. Further experimental work, therefore, is needed to transfer these neurotrophic factors to the clinic.

Insulin and leptin: dual adiposity signals to the brain for the regulation of food intake and body weight

Insulin and leptin are hypothesized to be 'adiposity signals' for the long-term regulation of body weight by the brain. Accordingly, a change in the plasma levels of leptin or insulin indicates a state of altered energy homeostasis and adiposity, and the brain responds by adjusting food intake to restore adipose tissue mass to a regulated level. The candidate site for the brain's detection of leptin adiposity signaling is the hypothalamic arcuate nucleus, where leptin inhibits expression neuropeptide Y and increases expression of the pro-opiomelanocortin (POMC) precursor of alphaMSH. Insulin also inhibits arcuate nucleus expression of neuropeptide Y but its effects on other hypothalamic signaling systems are not known. Leptin-responsive neurons in the arcuate nucleus are hypothesized to project to the paraventricular nucleus and lateral hypothalamic area where they are proposed to influence the expression of peptides that regulate food intake. Future development of this model will incorporate brain pathways for integration of leptin and insulin adiposity signaling to the hypothalamus with meal-related signals that act in the caudal brainstem. Recent research showing that leptin and insulin enhance the satiety action of peripheral CCK, thereby causing meals to be terminated earlier and reducing cumulative food intake, suggests that hypothalamic pathways that are sensitive to leptin and insulin adiposity signals have anatomical connections with caudal brainstem neurons that respond to meal-related signals and regulate meal size. The recent findings that insulin alters the expression and function of neural transporters for dopamine and norepinephrine indicate that adiposity signals may influence food intake by acting on non-peptide neurotransmitter systems.

Potential interactions of zinc in the neuroendocrine-endocrine disturbances of diabetes mellitus type 2

An accumulation of evidence implicates leptin, insulin, glucocorticoids, proopiomelanocortin (POMC), and neuropeptide Y (NPY) interactions as being integral to metabolic control associated with neuroendocrine-endocrine functioning. Dysfunction of neuroendocrine-endocrine interactions contributes to the metabolic disturbances of diabetes mellitus type 2 (DM-2). Since Zn has a direct impact on the healthy functioning of hormonal and neuropeptide balance, it is possible that altered Zn status and metabolism in DM-2 are involved in some of the metabolic dysfunctions of DM-2.Key words: zinc, insulin, leptin, neuropeptide Y, glucocorticoids, proopiomelanocortin (POMC), diabetes, obesity.

Tyrosine phosphorylation of tub and its association with Src homology 2 domain-containing proteins implicate tub in intracellular signaling by insulin

A mutation in the tub gene leads to maturity-onset obesity, insulin resistance, and progressive retinal and cochlear degeneration in mice. tub is a member of a growing family of genes that encode proteins of unknown function that are remarkably conserved across species. The absence of obvious transmembrane domain(s) or signal sequence peptide motif(s) suggests that Tub is an intracellular protein. Additional sequence analysis revealed the presence of putative tyrosine phosphorylation motifs and Src homology 2 (SH2)-binding sites. Here we demonstrate that in CHO-IR cells, transfected Tub is phosphorylated on tyrosine in response to insulin and insulin-like growth factor-1 and that in PC12 cells, insulin but not EGF induced tyrosine phosphorylation of endogenous Tub. In vitro, Tub is phosphorylated by purified insulin receptor kinase as well as by Abl and JAK 2 but not by epidermal growth factor receptor and Src kinases. Furthermore, upon tyrosine phosphorylation, Tub associated selectively with the SH2 domains of Abl, Lck, and the C-terminal SH2 domain of phospholipase Cgamma and insulin enhanced the association of Tub with endogenous phospholipase Cgamma in CHO-IR cells. These data suggest that Tub may function as an adaptor protein linking the insulin receptor, and possibly other protein-tyrosine kinases, to SH2-containing proteins.

Possible mechanisms for insulin-induced attenuation of the antinociceptive effect of [D-Ala2, N-MePhe4, Gly-ol5]enkephalin

The effects of pretreatment with protein kinase C and protein kinase A inhibitors on the intraventricular insulin-induced attenuation of the antinociceptive effect of [D-Ala2, N-MePhe4, Gly-ol5]enkephalin (DAMGO) were studied in mice. Intracerebroventricular (i.c.v.) pretreatment with insulin dose- and time-dependently attenuated the antinociceptive effect of i.c.v. DAMGO (5.6 ng) in mice. Intracerebroventricular pretreatment with a highly selective tyrosine kinase inhibitor, herbimycin A, at doses of 200 and 600 ng for 70 min, dose-dependently reversed the attenuation of the antinociceptive effect of DAMGO (5.6 ng, i.c.v.) caused by insulin. Furthermore, i.c.v. pretreatment with serine/threonin kinase inhibitor, 1-(5-isoquinolinesulfonyl)-2-methylpiperazine hydrochloride (H7), at doses of 3-30 nmol for 60 min, dose-dependently reversed the attenuation of the antinociceptive effect of DAMGO (5.6 ng, i.c.v.) caused by insulin. Intracerebroventricular pretreatment with selective protein kinase C inhibitor, calphostin C, at doses of 1 and 3 pmol for 60 min, but not with a highly protein kinase A inhibitor, (8R, 9S, 11S)-(-)-9-hydroxy-9-n-hexyloxy-carbonyl-8-methyl-2, 3, 9, 20-tetrahydro-8, 11-epoxy-1H, 8H, 11H-2, 7b, 11a-triaqzadibenzo[a, g]cycloocta[c, d, e]-trinden-1-one (KT5720), at dose of 10 pmol for 60 min, reversed the attenuation of the antinociceptive effect of DAMGO (5.6 ng, i.c.v.) caused by insulin. These results suggest that the reduction of DAMGO-induced antinociception by insulin in mice may be, in part, due to the activation of protein kinase C followed by the activation of tyrosine kinase.

Model for the regulation of energy balance and adiposity by the central nervous system

In 1995, we described a new model for adiposity regulation. Since then, data regarding the biology of body weight regulation has accumulated at a remarkable rate and has both modified and strengthened our understanding of this homeostatic system. In this review we integrate new information into a revised model for further understanding this important regulatory process. Our model of energy homeostasis proposes that long-term adiposity-related signals such as insulin and leptin influence the neuronal activity of central effector pathways that serve as controllers of energy balance.

Brain insulin response to feeding in the rat is both macronutrient and area specific

Using microdialysis, we showed recently that hypothalamic immuno-reactive insulin (IRI) levels increased after a meal of chow and decreased in response to a fat meal. In the present study, we have compared extracellular hypothalamic and extrahypothalamic basal IRI levels and investigated the effect of meals composed exclusively of either carbohydrates (85% starch, 15% sucrose) or casein on both plasma and medial hypothalamic (PVN-VMH) insulin. The response of IRI to a carbohydrate meal was also investigated in the cerebellum. Basal hypothalamic IRI was twofold higher in the hypothalamus as compared to the cerebellum (33 +/- 4 and 15 +/- 2 pg/mL, respectively). Hypothalamic IRI increased twofold in response to the carbohydrate meal (72 +/- 15 pg/mL) but remained unchanged during the casein meal. No IRI change was found in the cerebellum after a meal of carbohydrates (16 +/- 2 pg/mL). Insulinemia was increased by both the carbohydrate and the casein meal. However, the protein-induced increase was less pronounced (maximum + 359% compared to 1650% for carbohydrates). The present data show a dual specificity of brain insulin response to feeding; in addition to the macronutrient specific variations, a regional specificity was also observed. Taken together with previous observations, the present data are in favor of an involvement of PVN-VMH insulin in the control of feeding and macronutrient-specific appetites.

Stimulation of immunoreactive insulin release by glucose in rat brain synaptosomes

The effect of glucose on the release of immunoreactive insulin (IRI) in synaptosomes isolated from rat brain was studied. In the absence of glucose synaptosomes release about 4% (0.77 microIU/mg protein) of total content. Glucose increases significantly the IRI released by synaptosomes. Addition of the glycolytic inhibitor iodoacetic acid (IAA), decreased the glucose-induced release of IRI by about 50%, suggesting that glucose metabolism is involved. The observation that glucose provides a concentration related signal for IRI release indicates that this synaptosomal preparation may be useful as a model for research on the mechanism of insulin release in brain.

Uptake and degradation of blood-borne insulin by the olfactory bulb

Insulin found within the brain is derived from the blood and can affect various central nervous system (CNS) functions. The olfactory bulb contains one of the highest concentrations of insulin and insulin receptors within the CNS. To determine the mechanism underlying this high concentration of insulin, we used radioactively iodinated insulin to compare the blood to tissue transport rates and tissue degradation rates for the olfactory bulb, whole brain and spinal cord. We found that the olfactory bulb had both the highest transport rate across the blood-brain barrier (BBB) and the highest rate of degradation. Because a higher degradation rate would decrease, not increase, tissue concentrations of insulin, BBB transport may be the primary mechanism by which high concentrations of insulin are maintained within the olfactory bulb. This illustrates an adaptive aspect of the BBB in its regulation of the exchange of information molecules between the blood and the CNS.

Insulin-like growth factor-1 ameliorates age-related behavioral deficits

Insulin-like growth factor-1 has been found to be involved in the regulation of several aspects of brain metabolism, neural transmission, neural growth and differentiation. Because decreased insulin-like growth factor-1 and/or its receptors are likely to contribute to age-related abnormalities in behavior, the strategy of replacing this protein is one potential therapeutic alternative. The present study was designed to assess whether cognitive deficits with ageing may be partially overcome by increasing the availability of insulin-like growth factor-1 in the brain. Fischer-344 x Brown Norway hybrid (F1) male rats of two ages (four-months-old and 32-months-old) were preoperatively trained in behavioral tasks and subsequently implanted with osmotic minipumps to infuse the insulin-like growth factor-1 (23.5 microg/pump) or a vehicle, i.c.v. Animals were retested at two weeks and four weeks after surgery. Insulin-like growth factor-1 improved working memory in the repeated acquisition task and in the object recognition task. An improvement was also observed in the place discrimination task, which assesses reference memory. Insulin-like growth factor-1 had no effect on sensorimotor skills nor exploration, but mildly reversed some age-related deficits in emotionality. These data indicate a potentially important role for insulin-like growth factor-1 in the reversal of age-related behavioral impairments in rodents.

Brain insulin response to feeding in the rat is both macronutrient and area specific

Using microdialysis, we showed recently that hypothalamic immunoreactive insulin (IRI) levels increased after a meal of chow and decreased in response to a fat meal. In the present study, we have compared extracellular hypothalamic and extrahypothalamic basal IRI levels and investigated the effect of meals composed exclusively of either carbohydrates (85% starch, 15% sucrose) or casein on both plasma and medial hypothalamic (PVN-VMH) insulin. The response of IRI to a carbohydrate meal was also investigated in the cerebellum. Basal hypothalamic IRI was twofold higher in the hypothalamus as compared to the cerebellum (33 +/- 4 and 15 +/- 2 pg/mL, respectively). Hypothalamic IRI increased twofold in response to the carbohydrate meal (72 +/- 15 pg/mL) but remained unchanged during the casein meal. No IRI change was found in the cerebellum after a meal of carbohydrates (16 +/- 2 pg/mL). Insulinemia was increased by both the carbohydrate and the casein meal. However, the protein-induced increase was less pronounced (maximum + 359% compared to 1650% for carbohydrates). The present data show a dual specificity of brain insulin response to feeding; in addition to the macronutrient specific variations, a regional specificity was also observed. Taken together with previous observations, the present data are in favor of an involvement of PVN-VMH insulin in the control of feeding and macronutrient-specific appetites.

The role of neuronal growth factors in neurodegenerative disorders of the human brain

Recent evidence suggests that neurotrophic factors that promote the survival or differentiation of developing neurons may also protect mature neurons from neuronal atrophy in the degenerating human brain. Furthermore, it has been proposed that the pathogenesis of human neurodegenerative disorders may be due to an alteration in neurotrophic factor and/or trk receptor levels. The use of neurotrophic factors as therapeutic agents is a novel approach aimed at restoring and maintaining neuronal function in the central nervous system (CNS). Research is currently being undertaken to determine potential mechanisms to deliver neurotrophic factors to selectively vulnerable regions of the CNS. However, while there is widespread interest in the use of neurotrophic factors to prevent and/or reduce the neuronal cell loss and atrophy observed in neurodegenerative disorders, little research has been performed examining the expression and functional role of these factors in the normal and diseased human brain. This review will discuss recent studies and examine the role members of the nerve growth factor family (NGF, BDNF and NT-3) and trk receptors as well as additional growth factors (GDNF, TGF-alpha and IGF-I) may play in neurodegenerative disorders of the human brain.

Signals that regulate food intake and energy homeostasis

Feeding behavior is critical for survival. In addition to providing all of the body's macronutrients (carbohydrates, lipids, and proteins) and most micronutrients (minerals and vitamins), feeding behavior is a fundamental aspect of energy homeostasis, the process by which body fuel stored in the form of adipose tissue is held constant over long intervals. For this process to occur, the amount of energy consumed must match precisely the amount of energy expended. This review focuses on the molecular signals that modulate food intake while integrating the body's immediate and long-term energy needs.

Insulin-like growth factor-I modulation of cerebellar cell populations is developmentally stage-dependent and mediated by specific intracellular pathways

Although development of transgenic animals overexpressing insulin-like growth factor-I has allowed the establishment of a role of this trophic factor in brain growth, detailed knowledge of the action of insulin-like growth factor-I on different brain areas is still lacking. We now provide evidence for a pleiotrophic role of this growth factor on cerebellar development. Insulin-like growth factor-I produced by cerebellar cultures is a survival factor for Purkinje cells and a mitogen/differentiation factor for cerebellar glioblasts. Trophic effects of insulin-like growth factor-I were observed only during specific developmental stages. In addition, insulin-like growth factor-I increased intracellular Ca2+ levels in Purkinje cells and c-Fos in dividing glioblasts. Survival-promoting effects of insulin-like growth factor-I on Purkinje cells required activation of protein kinase C, while glioblast division induced by insulin-like growth factor-I depended on phosphatidylinosytol 3-kinase activation. We conclude that insulin-like growth factor-I is a paracrine/autocrine pleiotrophic factor for both glia and neurons in the cerebellum. Its effects are mediated by distinct intracellular signals and appear to be specific to the developmental stage of the target cell. Since development of the different cell populations that compose a specific brain territory is not synchronized, the pleiotrophic action of growth factors such as insulin-like growth factor-I may be essential to ontogenetic processes underlying normal brain growth.

Insulin-like growth factors-I and -II differentially regulate endogenous acetylcholine release from the rat hippocampal formation

Insulin-like growth factors-I and -II (IGF-I and -II) are structurally related mitogenic polypeptides with potent growth promoting effects. These peptides and their corresponding IGF-I and -II receptors are selectively localized in the brain. To date, most of the effects of IGFs are believed to be mediated by IGF-I receptors whereas the significance of IGF-II receptor in mediating biological responses remains unclear. In the present study, we characterized the distribution of IGF-I and IGF-II receptor sites and investigated the effects of both factors on endogenous acetylcholine (ACh) release in adult rat hippocampus. [125I]IGF-I receptor binding sites are recognized by IGF-I> IGF-II> insulin, whereas [125I]IGF-II binding was competed potently by IGF-II> IGF-I but not by insulin. At the cellular level, IGF-I receptor sites were primarily noted in the molecular layer of the dentate gyrus and the CA2-CA3 subfields of the Ammon's horn whereas IGF-II sites were localized predominantly in the pyramidal cell layer of the CA1-CA3 subfields and in the granular cell layer of the dentate gyrus. IGF-I (10(-14)-10(-8) M) and des(1-3) IGF-I (10(-10)-10(-8) M) were found to inhibit whereas IGF-II (10(-14)-10(-8) M) potentiated K+-evoked ACh release from hippocampal slices. Tetrodotoxin altered the effects of IGF-I but not those of IGF-II suggesting that IGF-I acts indirectly via the release of other modulators whereas IGF-II acts directly on or in close proximity to the cholinergic terminals. The inhibitory effects of IGF-I were also observed in the frontal cortex but not in the striatum. In contrast, the stimulatory effects of IGF-II were evident both in the frontal cortex and striatum. Taken together, these results reveal the differential localization of IGF-I and IGF-II receptor sites in the hippocampal formation and the opposite role for these growth factors in the acute regulation of ACh release likely via two distinct mechanisms. Additionally, these data provide the first evidence for a direct role for IGF-II and its receptors in the regulation of transmitter release in the central nervous system.

Insulin-like growth factors in central nervous system tumors

Insulin-like growth factors (IGFs) appear to play a role in the development of tumors in general and brain tumors in particular. Specific receptors for IGFs have been identified in normal human and rat brain, and evidence suggests that components of the IGF signal transduction system may play a role in the transformation process. Secretion of IGFs by a variety of human brain tumors has been confirmed, and these growth factors appear to have an autocrine stimulatory effect on these tumors. IGFs circulate in the blood stream bound to at least six distinct binding proteins which may modulate the effects of these growth factors on target tissues. Sex steroids may also regulate the behavior of certain brain tumors such as meningiomas at least in part through their effects on the expression of IGFs and their binding proteins. Recently, antisense gene technology against certain IGFs or their receptors have resulted in potent antitumor effects in the case of several gliomas, although the mechanism for this remains unclear.

Effect of experimental diabetes on monoamine oxidase activity from discrete areas of rat brain: relationship with diabetes associated reproductive failure

The effect of alloxan-induced diabetes was studied on the activity of monoamine oxidase (MAO), the oxidative deaminating enzyme of monoamine neurotransmitters. MAO was assayed from discrete brain regions like medial preoptic area and median eminence--arcuate region of hypothalamus, septum, amygdala, thalamus, hippocampus, pons and medulla. In all these areas studied, the induction of diabetes resulted in significant increase in MAO activity at 3, 8, 15 and 28 day intervals, whereas, the treatment of diabetic rats with insulin led to recovery in the enzyme activity. Blood glucose levels increased significantly after induction of diabetes and the recovery was seen after insulin treatment. These data suggest the involvement of MAO in diabetes associated alterations in physiological and endocrinological disorders.

Insulin-like growth factor-I (IGF-I) immunoreactivity in the Alzheimer's disease temporal cortex and hippocampus

IGF-I has been shown to enhance neuronal survival and inhibit apoptosis. IGF-I immunoreactivity was examined in the Alzheimer's disease and normal post-mortem human hippocampus and temporal cortex to determine whether IGF-I protein levels are altered in response to neurodegeneration. IGF-I immunoreactivity was induced in a subpopulation of GFAP-immunopositive astroglia in the Alzheimer's disease temporal cortex. These observations raise the possibility that IGF-I has a neuroprotective role in the Alzheimer's disease brain.

Distribution and levels of [125I]IGF-I, [125I]IGF-II and [125I]insulin receptor binding sites in the hippocampus of aged memory-unimpaired and -impaired rats

The insulin-like growth factors (IGF-I and IGF-II) and insulin are localized within distinct brain regions and their respective functions are mediated by specific membrane receptors. High densities of binding sites for these growth factors are discretely and differentially distributed throughout the brain, with prominent levels localized to the hippocampal formation. IGFs and insulin, in addition to their growth promoting actions, are considered to play important roles in the development and maintenance of normal cell functions throughout life. We compared the anatomical distribution and levels of IGF and insulin receptors in young (five month) and aged (25 month) memory-impaired and memory-unimpaired male Long Evans rats as determined in the Morris water maze task in order to determine if alterations in IGF and insulin activity may be related to the emergence of cognitive deficits in the aged memory-impaired rat. In the hippocampus, [125I]IGF-I receptors are concentrated primarily in the dentate gyrus (DG) and the CA3 sub-field while high amounts of [125I]IGF-II binding sites are localized to the pyramidal cell layer, and the granular cell layer of the DG. [125I]insulin binding sites are mostly found in the molecular layer of the DG and the CA1 sub-field. No significant differences were found in [125I]IGF-I. [125I]IGF-II or [125I]insulin binding levels in any regions or laminae of the hippocampus of young vs aged rats. and deficits in cognitive performance did not relate to altered levels of these receptors in aged memory-impaired vs aged memory-unimpaired rats. Other regions. including various cortical areas, were also examined and failed to reveal any significant differences between the three groups studied. It thus appears that IGF-I, IGF-II and insulin receptor sites are not markedly altered during the normal ageing process in the Long Evans rat, in spite of significant learning deficits in a sub-group (memory-impaired) of aged animals. Hence. recently reported changes in IGF-I receptor messenger RNA levels in aged memory-impaired rats are apparently not reflected at the level of the translated protein.

Systemic administration of kainic acid induces selective time dependent decrease in [125I]insulin-like growth factor I, [125I]insulin-like growth factor II and [125I]insulin receptor binding sites in adult rat hippocampal formation

Administration of kainic acid evokes acute seizure in hippocampal pathways that results in a complex sequence of functional and structural alterations resembling human temporal lobe epilepsy. The structural alterations induced by kainic acid include selective loss of neurones in CA1-CA3 subfields and the hilar region of the dentate gyrus followed by sprouting and permanent reorganization of the synaptic connections of the mossy fibre pathways. Although the neuronal degeneration and process of reactive synaptogenesis have been extensively studied, at present little is known about means to prevent pathological conditions leading to kainate-induced cell death. In the present study, to address the role of insulin-like growth factors I and II, and insulin in neuronal survival as well as synaptic reorganization following kainate-induced seizure, the time course alterations of the corresponding receptors were evaluated. Additionally, using histological preparations, the temporal profile of neuronal degeneration and hypertrophy of resident astroglial cells were also studied. [125I]Insulin-like growth factor I binding was found to be decreased transiently in almost all regions of the hippocampal formation at 12 h following treatment with kainic acid. The dentate hilar region however, exhibited protracted decreases in [125I]insulin-like growth factor I receptor sites throughout (i.e. 30 days) the study. [125I]Insulin-like growth factor II receptor binding sites in the hippocampal formation were found to be differentially altered following systemic administration of kainic acid. A significant decrease in [125I]insulin-like growth factor II receptor sites was observed in CA1 subfield and the pyramidal cell layer of the Ammon's horn at all time points studied whereas the hilar region and the stratum radiatum did not exhibit alteration at any time. A kainate-induced decrease in [125I]insulin receptor binding was noted at all time points in the molecular layer of the dentate gyrus whereas binding in CA1-CA3 subfields and discrete layers of the Ammon's horn was found to be affected only after 12 h of treatment. These results, when analysed with reference to the observed histological changes and established neurotrophic/protective roles of insulin-like growth factors and insulin, suggest possible involvement of these growth factors in the cascade of neurotrophic events that is associated with the reorganization of the hippocampal formation observed following kainate-induced seizures.

Insulin responses to a fat meal in hypothalamic microdialysates and plasma

In a recent microdialysis study in freely-behaving rats, we observed changes in immunoreactive insulin (IRI) in hypothalamic dialysates after a meal of standard laboratory chow. These changes did not always parallel plasma insulin variations, suggesting a partial independence from peripheral insulin. In the present study, we have attempted to assess the profile of medial hypothalamus (VMPH-PVN) extracellular insulin and peripheral insulin before and after a fat meal (lard). In contrast to the increase we previously observed with chow meals, hypothalamic extracellular IRI decreased during the fat meal and fell to 60% 30 min after the meal. Plasma insulin levels did not change. The intake of the lard meal, provided in unlimited amounts, was much larger in calories than the intake of a chow meal under the same conditions. However, when rats were offered a meal of chow after they had eaten a meal averaging 6.7 g of fat (61 calories), they immediately began eating the chow. Thus, the meal of fat produced no general satiation. On the contrary, the rats consumed a second chow meal only after a delay of approximately 40 min after the first one. The present data, in conjunction with our previous observations with chow fed rats, suggest that the level of extracellular hypothalamic IRI may decrease independently of plasma insulin levels and may, at least partially, account for the observed lack of satiation.

Insulin-like growth factor-1 (IGF-1) improves both neurological motor and cognitive outcome following experimental brain injury

We evaluated the efficacy of insulin-like growth factor-1 (IGF-1) in attenuating neurobehavioral deficits following lateral fluid percussion (FP) brain injury. Male Sprague-Dawley rats (345-425 g, n = 88) were anesthetized and subjected to FP brain injury of moderate severity (2.4-2.9 atm). In Study 1, IGF-1 (1.0 mg/kg, n = 9) or vehicle (n = 14) was administered by subcutaneous injection at 15 min postinjury and similarly at 12-h intervals for 14 days. In animals evaluated daily for 14 days, IGF-1 treatment attenuated motor dysfunction over the 2-week period (P < 0.02). In Study 2, IGF-1 (4 mg/kg/day, n = 8 uninjured, n = 13 injured) or vehicle (n = 8 uninjured, n = 13 injured) was administered for 2 weeks via a subcutaneous pump implanted 15 min postinjury. IGF-1 administration was associated with increased body weight and mild, transient hypoglycemia which was more pronounced in brain-injured animals. At 2 weeks postinjury (P < 0.05), but not at 48 h or 1 week, brain-injured animals receiving IGF-1 showed improved neuromotor function compared with those receiving vehicle. IGF-1 administration also enhanced learning ability (P < 0.03) and memory retention (P < 0.01) in brain-injured animals at 2 weeks postinjury. Taken together, these data suggest that chronic, posttraumatic administration of the trophic factor IGF-1 may be efficacious in ameliorating neurobehavioral dysfunction associated with traumatic brain injury.

Central nervous system resuscitation

Traumatic injury to the central nervous system induces delayed neuronal death, which may be mediated by acute and chronic neurochemical changes. Experimental identification of these injury mechanisms and elucidation of the neurochemical cascade following trauma may provide enhanced opportunities for treatment with novel neuroprotective strategies.

Presence and differential internalization of two distinct insulin-like growth factor receptors in rat hippocampal neurons

The pharmacological characteristics, localization and process of internalization of the insulin-like growth factor I and II receptors were studied in rat primary hippocampal cultured neurons grown under serum-free conditions. [125I]insulin-like growth factor-I binding was specific with an apparent affinity (Kd) of 0.1 nM and IC50 values of 0.1, 2.9 and 99.7 nM for insulin-like growth factor-I, insulin-like growth factor-II and insulin, respectively. The competition by insulin suggests the presence of genuine insulin-like growth factor-I receptors and not insulin-like growth factor binding proteins. In contrast, [125I]insulin-like growth factor-II binding showed a Kd of 0.1 nM and IC50 values of 0.2 and 20.5 nM for insulin-like growth factor-II and insulin-like growth factor-I while insulin was inactive, a well established characteristic of the insulin-like growth factor-II receptor. Using emulsion autoradiography, specific binding sites for [125I]insulin-like growth factor-I and -II were over the whole cultured neurons. The use of selective insulin-like growth factor-I and -II receptor antibodies further confirmed the existence of these receptors in rat hippocampal cultured neurons. To investigate the respective internalization profile of [125I]insulin-like growth factor-I and [125I]insulin-like growth factor-II receptor-ligand complexes in neurons, a technique of acid stripping was used. The apparent rate of endocytosis was found to be greater for the insulin-like growth factor-II than for the insulin-like growth factor-I receptor complexes. The internalization of [125I]insulin-like growth factor-I and [125I]insulin-like growth factor-II ligand-receptor complexes was confirmed using phenylarsine oxide which significantly blocked both internalization processes. In order to eliminate possible receptor recycling, monensin was used and shown to have no effect on the internalization of either ligand. Since the insulin-like growth factor-I receptor is coupled to tyrosine kinase activity, tyrphostin 47, a specific tyrosine kinase inhibitor. was used and shown to decrease [125I]insulin-like growth factor-I but not the [125I]insulin-like growth factor-II receptor internalization profile. Accordingly, insulin-like growth factor-I is apparently internalized mostly via the insulin-like growth factor-I tyrosine kinase type receptor, while insulin-like growth factor-II is not. The insulin-like growth factor-II receptor ligand complex is likely internalized via a pathway possibly related to mannose-phosphorylated residues as the insulin-like growth factor-II/mannose-6-phosphate receptor has been implicated in the intracellular targeting of lysosomal proteins containing glycosylated residues. Taken together, our results indicate that primary hippocampal cultured neurons represent a unique model for investigating the differential role and intracellular trafficking of both insulin-like growth factor-I and insulin-like growth factor-II receptor ligand complexes and their relevance to the respective functional role of these two-related trophic factors in the central nervous system.

Insulin-like growth factors and insulin stimulate erythropoietin production in primary cultured astrocytes

Erythropoietin (EPO) is established as a major regulator of erythropoiesis. However, we and others have shown that neurons express erythropoietin receptor (EPO-R), that astrocytes produce EPO and that EPO may act as a neurotrophic factor in the CNS. We also found that EPO production is activated by insulin and insulin-like growth factors (IGFs) in astrocytes in a dose-dependent manner and that IGF-I was the most potent activator. The concentrations required for half-maximal activation were 3 nM IGF-I, 10 nM IGF-II and 100 nM insulin. The oxygen concentration regulates EPO production; hypoxia stimulates EPO production in astrocytes. The stimulatory effect of IGFs and insulin on EPO production in astrocytes was not affected by the oxygen concentration of astrocyte culture. Insulin and IGFs did not increase the total protein synthesis of astrocytes but increased EPO mRNA levels, indicating that EPO production is stimulated at the mRNA level. It appeared that the growth factor-induced accumulation of EPO mRNA in astrocytes was caused by activation of the tyrosine kinase-signal transduction pathway, because tyrosine phosphorylation of receptors for IGF-I and insulin was activated when astrocytes were stimulated by these growth factors.

Effect of diabetes mellitus on the permeability of the blood-brain barrier to insulin

Insulin derived from the peripheral circulation has been shown to exert various effects on the brain due to its ability to cross the blood-brain barrier (BBB). The relation between diabetes mellitus and insulin has been extensively studied for peripheral tissues but not for central nervous system tissues. We examined the effects that streptozotocin- or alloxan-induced diabetes have on the transport of insulin across the murine BBB. We used multiple-time regression analysis to measure the unidirectional influx rate constant (Ki) and vascular association (Vi) of intravenously injected, radioactively labeled human insulin (I-Ins). Treatment with streptozotocin induced an enhancement of both the Ki and Vi of I-Ins that correlated with the onset of diabetes. Brain perfusion showed that the enhanced uptake was not due to altered vascular space or levels of insulin in the serum. Alloxan enhanced Ki and Vi after 5 days but the early phase of diabetes was associated with a decreased Ki. Hyperglycemia induced by the intraperitoneal injection of glucose elevated the Vi but abolished the Ki. Furthermore, altered I-Ins uptake by brain was not associated with changes in brain or body weight. These results show that there is an increased uptake of I-Ins by the brain in the diabetic state that is not due to acute changes in the serum levels of glucose or insulin, altered vascular space, or catabolic events. Chronic changes in levels of glucose, insulin or other hormone or neuroendocrine agents are likely to underlie the altered rate of transport of insulin across the BBB of diabetic mice.

Selective, physiological transport of insulin across the blood-brain barrier: novel demonstration by species-specific radioimmunoassays

Insulin in blood is thought to cross the blood-brain barrier (BBB) to act within the brain to help control appetite. We examined the ability of blood-borne insulin to cross the BBB. Human insulin was infused for 48 h subcutaneously at several doses into mice and the amount of human and murine insulin in serum and brain measured with species-specific radioimmunoassays. For the exogenous human insulin, both brain and blood concentrations increased with increasing doses of infused insulin, whereas for the endogenous murine insulin, brain and blood concentrations decreased. Since the mouse cannot make human insulin, blood was the only source for the human insulin in brain, demonstrating that insulin does indeed cross the BBB. The relationship between the concentrations of human insulin in brain and blood was nonlinear, showing that passage is by a saturable mechanism. Partial saturation of the transporter occurred at euglycemic concentrations of serum insulin. Thus, insulin enters the brain by a saturable transport system that is operational primarily at physiological levels of serum insulin.

Involvement of protein kinase C and nitric oxide in the modulation by insulin-like growth factor-I of glutamate-induced GABA release in the cerebellum

Insulin-like growth factor-I elicits a long-term depression of the glutamate-induced GABA release in the adult rat cerebellum that lasts at least several hours. We studied whether protein kinase C and nitric oxide may be involved in this effect of insulin-like growth factor-I on GABA release since both signalling pathways have been implicated in other forms of neuromodulation in the cerebellum. By using microdialysis in the adult rat cerebellum, we found that either an inhibitor of protein kinase C (staurosporine) or of nitric oxide synthase (Nw-nitro-L-arginine methyl ester) counteracted the long-term, but not the acute effects of insulin-like growth factor-I on glutamate-induced GABA release. On the contrary, when either an activator of protein kinase C (phorbol ester), or an nitric oxide donor (L-arginine), were given with glutamate, they mimicked only the acute effects of insulin-like growth factor-I on glutamate-induced GABA release. Finally, when both protein kinase C and nitric oxide-synthase were simultaneously inhibited by conjoint administration of staurosporine and Nw-nitro-L-arginine methyl ester, a complete blockage of both the short and the long-term effects of insulin-like growth factor-I on GABA release was obtained. These results, indicate that: (i) activation by insulin-like growth factor-I of either the protein kinase C or nitric oxide-signalling pathways is sufficient for the short-term inhibition of glutamate-induced GABA release; and (ii) simultaneous activation of both the protein kinase C and the nitric oxide signalling pathways is necessary for insulin-like growth factor-I to induce a long-term depression of GABA responses to glutamate. Thus, long-term depression of glutamate-induced GABA release by insulin-like growth factor-I in the cerebellum is mediated by simultaneous activation of both protein kinase C and nitric oxide-signalling pathways.

Insulin-like growth factor-1 is a potent neuronal rescue agent after hypoxic-ischemic injury in fetal lambs

This study was designed to determine the potential of IGF-1 as a neuronal rescue agent after cerebral ischemia. Unanesthetized late gestation fetal sheep were subjected to 30-min cerebral ischemia by inflation of carotid artery occluder cuffs. 2 h later either 0.1 microgram rhIGF-1, 1 microgram rhIGF-1, 10 micrograms rhIGF-1, or vehicle was infused into a lateral cerebral ventricle over 1 h. Histologic outcome was assessed 5 d later. Overall neuronal loss was reduced with 0.1 microgram (P < 0.05) and 1 microgram (P < 0.002) rhIGF-1, but treatment with 10 micrograms was not effective. With 1 microgram rhIGF-1 neuronal loss scores were significantly lower in brain regions examined including cortex, hippocampus, and striatum, whereas with 0.1 microgram rhIGF-1 the parietal cortex and thalamus were not improved and the improvement seen in other regions was less than with 1 microgram rhIGF-1. Treatment with 1 microgram rhIGF-1 also delayed the onset of seizures and reduced their incidence. Moreover, the secondary phase of cytotoxic edema was reduced and delayed in onset. We conclude that low dose rhIGF-1 therapy promotes neuronal rescue after cerebral hypoxic-ischemic injury in utero, but the effect is dose dependent. Importantly, rhIGF-1 is effective and nontoxic when administered 2 h after the hypoxic ischemic insult. This distinguishes IGF-1 from most other neuroprotective therapies and suggests clinical application may be possible.

Hypothalamic monoamines and insulin in relation to feeding in the genetically obese Zucker rat as revealed by microdialysis

Dynamic changes in VMH and PVN monoamines and immunoreactive insulin (IRI) were investigated by microdialysis in freely-moving genetically obese Zucker rats in order to relate possible disturbances to the impaired regulation of food intake of this model. Serotonin (5-HT), 5-HIAA and dopamine (DA) increased at the beginning of spontaneous meals while DOPAC decreased. Although similar in normal and obese rats, these changes were much more dramatic in the latter, as if more "signal" for satiety were necessary at the VMH-PVN level. Glucoprivic feeding or satiety are induced in normal rats by intravenous infusions of insulin or insulin+glucose respectively. The Zucker rat is resistant to these treatments. The monoaminergic changes brought about by these infusions were similar in obese and normal rats (decreases in 5-HT and DA and increases in 5-HIAA and DOPAC), but the occurrence of meals, in the obese, showed a superim-position of monoaminergic changes resembling those related to spontaneous feeding. The monoaminergic effects of insulin must therefore be dissociated from its effects on feeding. Hypothalamic insulin itself might be the brain signal. At the beginning of meals presented for the first time, VMH-PVN IRI increased earlier and with a smaller magnitude in the obese. When the rats were accustomed to scheduled meals, a similar anticipatory increase in IRI was found in both obese and lean rats. This suggests that brain insulin is more than a satiety signal. In addition, in response to an i.v. insulin infusion, IRI increased twice as much in obese rats despite lower basal levels. Whatever the origin of hypothalamic insulin, the larger response of the obese Zucker rat, known to be insulin resistant, may reflect the inefficiency of the peptide in reducing feeding and body weight in this pathological model.

Synthesis and secretion of insulin-like growth factor (IGF)-II and IGF binding protein-2 by cultivated brain meningeal cells

The meninges (the pia mater and the arachnoid) covering the surface of brain parenchyma are known to participate in the regulation of blood circulation and the blood-cerebrospinal fluid barrier. In the present study, we isolated and characterized some of the major proteins secreted into the conditioned medium of the meningeal cell cultures prepared from 1-2-day-old rats. Five protein molecules (7 kDa, 16 kDa, 18 kDa, 27 kDa and 32 kDa) were recognized as major proteins in the conditioned medium by SDS-PAGE. The major proteins were isolated and purified to homogeneity, respectively. Their N-terminal sequences, except that of 27 kDa protein, were successfully determined. Homology search has revealed that the N-terminal sequences of the 7 kDa protein and the 32 kDa protein were identical with those of insulin-like growth factor-II (IGF-II) and IGF-binding protein-2 (IGFBP-2), respectively. The N-terminal sequences of the 16 kDa and 18 kDa molecules were identical with those of the corresponding fragments of IGFBP-2. The present study demonstrates that cultured meningeal cells produce and secrete large amounts of IGF-II and IGFBP-2. The meninges may, therefore, be one of the main sources of these proteins in the cerebrospinal fluid and involved in the modulation of neuronal and/or glial cell survival or functioning.

Insulin-like growth factor I is an afferent trophic signal that modulates calbindin-28kD in adult Purkinje cells

Recent evidence suggests that Purkinje cells are specific targets of insulin-like growth factor I (IGF-I) through their entire life span. During development, Purkinje cell numbers and their calbindin-28kD content increase after IGF-I treatment in culture. In the adult, part of the IGF-I present in the cerebellum is transported from the inferior olive, and modulates Purkinje cell function. We investigated whether IGF-I produced by inferior olive neurons and transported to the contralateral cerebellum through climbing fibers may modulate the levels of calbindin-28kD in the cerebellum of adult animals. Twenty-four hr after injection of an antisense oligonucleotide of IGF-I into the inferior olive, both IGF-I and calbindin-28kD levels in the contralateral cerebellar lobe were significantly reduced, while the number of calbindin-positive Purkinje cells was unchanged. The effect of the antisense on IGF-I levels was fully reversed 3 days after its injection into the inferior olive, with a postinhibitory rebound observed at this time, while calbindin-28kD levels slowly returned to control values. A control oligonucleotide did not produce any change in either IGF-I or calbindin-28kD content in the cerebellum. These results indicate that normal levels of IGF-I in the inferior olive are necessary to maintain appropriate levels of IGF-I in the cerebellum and of calbindin-28kD in the Purkinje cell. These results also extend our previous findings on the existence of an olivo-cerebellar IGF-I-containing pathway with trophic influence on the adult Purkinje cell.

Truncation of IGF-I yields two mitogens for retinal Müller glial cells

In the CNS only a truncated form of insulin-like growth factor-I (IGF-I) is detected. Although truncated IGF-I (t-IGF-I) retains mitogenicity, growth promoting activities have not been detected for the tripeptide that is cleaved from IGF-I during truncation. Here, we asked whether the tripeptide is itself a growth factor. Using cultured Müller glial cells from the adult human retina, we found that the cleaved tripeptide, glycine-proline-glutamate, stimulated the proliferation of these cells. Pharmacological experiments indicated that this proliferative effect involves activation of N-methyl-D-aspartate (NMDA) receptors. In addition, t-IGF-I was also mitogenic in our culture system and had an EC50 markedly less than that for IGF-I. Thus, truncation of IGF-I may be a mechanism to augment the mitogenic effect of this gene product by creating a more potent variant and a cleaved tripeptide that is itself a mitogen.

Four-day hyperinsulinemia in euglycemic conditions alters local cerebral glucose utilization in specific brain nuclei of freely moving rats

Although insulin is a well known regulator of peripheral tissue glucose metabolism, there is little agreement over its effects on brain glucose metabolism. Several investigators report that peripheral insulin may enter the brain via several routes. The presence of insulin receptors specific to brain, coupled to diverse reports of the effect of acute insulin administration on brain glucose use, led us to carry out a 4-day hyperinsulinemic euglycemic clamp in freely moving rats with subsequent labelled 2-deoxyglucose metabolic mapping studies. It was found that after 4 days of peripheral insulin infusion, several brain regions (Anterior Hypothalamic area, Suprachiasmatic nucleus, Basolateral Amygdaloid nucleus, Supramammillary bodies, Medial Geniculate nucleus and Locus Coeruleus) had an altered local cerebral glucose utilization. Upon subsequent analysis of their anatomical and functional connections it is proposed that insulin may regulate an integrated circuit of pathways within the central nervous system.

The effects of centrally administered porcine relaxin on drinking behaviour in male and female rats

Of the reproductive hormones it has been suggested that relaxin may play an important role in the increased sodium appetite of pregnancy. ICV injection of porcine relaxin caused water-replete male and female Wistar rats with access to water and 0.9% or 2.7% NaCl to drink on average about 3 to 8 ml of water within 1 h of injection. By 24 h the cumulative intake of water was no different from the control intake. The amounts of water drunk were similar after doses of 50, 100, 250 and 500 ng of relaxin. A dose of 5 ng was ineffective. Male rats generally drank more water than female rats after ICV injection of angiotensin or relaxin. Male SH rats which drink more water than male WKY rats in response to ICV angiotensin also drank more after ICV relaxin. Intakes of 0.9% or 2.7% NaCl were unaffected for up to 24 h after injection of relaxin, whereas angiotensin-injected rats showed a significant increase in 0.9% NaCl 1 h after injection though this difference was no longer evident in the 24 h cumulative intake. Relaxin did not cause any increase in NaCl intake in SH rats. Insulin, which is similar in structure and molecular weight to relaxin, was without effect on drinking when doses comparable to dipsogenically effective doses of relaxin were injected ICV. In male Wistar rats treated with DOCA for 5-15 days, relaxin retained its weak stimulatory action on water intake but did not affect NaCl intake despite the increased baseline NaCl intake during DOCA. These results indicate that relaxin is a dipsogen in the rat but that it seems to have little short-term effect on sodium appetite.

The effect of insulin and insulin-like growth factor-1 on the expression of calretinin and calbindin D-28k in rat embryonic neurons in culture

In a primary culture of rat embryonic neurons, insulin (5 mg/ml) promoted neurite formation between neuron clusters in serum/glial-free simplified medium with a high concentration of transferrin (100 micrograms/ml). Insulin growth factor-1 (IGF-1) exerted a similar effect in the same culture system at a lower dose (100 ng/ml) of insulin. Calretinin has recently been identified as a calcium binding protein expressed predominantly in sensory neurons, which has six calcium binding domains and shows molecular similarity to calbindin D-28k, an intestinal calcium transporter protein also found in the CNS. The effects of insulin and IGF-1 on the expression of calretinin and calbindin D-28k were investigated in rat embryonic neuronal cell culture. When cells were cultured for 2 days, insulin and IGF-1 promoted the expression of both proteins; when cultured for more than 2 days, IGF-1 still exerted a growth factor effect, but insulin decreased the expression of calretinin. Using the present culture system, we demonstrated that the effects of closely related molecules, insulin and IGF-1 which share receptors reciprocally, differed in calretinin and calbindin D-28k expression. Therefore, these two calcium binding proteins may play different physiological roles in the nervous system based either on different molecular mechanisms or switching roles at different stages of CNS development. Amplification based on Western blot employing the streptoavidin-horseradish peroxidase method was applied to detect calretinin and calbindin D-28k in the culture system by an immunoblotting technique.

The developing CNS: a scenario for the action of proinsulin, insulin and insulin-like growth factors

The multifunctional cytokines of the family of insulin and insulin-like growth factors (IGFs) have not yet gained general recognition as essential cell signals for the development of the vertebrate nervous system. This is, in part, a consequence of previous constraints in our thinking, focused for many years on the endocrine roles of these factors in late mammalian development and postnatal stages. The cellular distribution of the components of the insulin and IGFs signalling system in the developing mammalian and avian CNS is remarkably conserved. While receptors are widespread, the much less abundant factors and modulatory proteins are highly regulated in time and space. Progression of neural development through the steps of cell proliferation, differentiation, maturation and survival is stimulated, at least in culture, by proinsulin and insulin and the IGFs. Thus, these factors might be important autocrine and paracrine signals during development of the CNS.