Insulin and insulin-like growth factors in the CNS

Location of phosphotyrosine-containing proteins by immunocytochemistry in the rat forebrain corresponds to the distribution of the insulin receptor

Cellular regulation by certain growth factor receptors and protooncogene products involves tyrosine kinase activity with the resultant tyrosine phosphorylation of protein substrates. In the present report we describe the distribution of phosphotyrosine-containing material detected by immunocytochemistry (ICC) in the rat forebrain. Specificity of the affinity-purified antibody against phosphotyrosine used in the ICC technique was demonstrated by the ability of phosphotyrosine and p-nitrophenyl phosphate but not phosphoserine, phosphothreonine, or L-tyrosine to inhibit the immunostaining reaction. With ICC, relatively high amounts of phosphotyrosine-positive material were observed in neurons in specific structures that included the supraoptic, paraventricular, and arcuate nuclei; the median eminence; medial habenula; subfornical organ; and piriform cortex. Moderate to high amounts were present in the cerebral cortical layers II-IV and in the pyramidal cell layer of the hippocampus. Small to moderate amounts were detected in a few other locations. Glial elements showed minimal staining. Other areas of the rat forebrain failed to react with this antibody. Importantly, the distribution of the areas positive for phosphotyrosine agreed to a remarkable extent with the distribution of the brain insulin receptor, which itself has tyrosine kinase activity. These findings suggest a relationship between the insulin receptor and the increased phosphotyrosine content of these neurons and support the concept that the brain insulin receptor is active in vivo.

Release of immunoreactive insulin from rat brain synaptosomes under depolarizing conditions

Synaptosome preparations were utilized to characterize the release and compartmentalization of immunoreactive insulin (IRI) in the adult rat brain. Depolarization of synaptosomes by elevation of the external potassium ion concentration elicited release of IRI from the synaptosomes into the incubation medium. This release was reduced or eliminated under three conditions known to prevent depolarization-induced Ca2+ flux: elevating the external MgCl2, adding CoCl2, and eliminating external Ca2+ with EGTA. Depolarization of synaptosomes by veratridine also elicited release of synaptosomal IRI. This release was inhibited by tetrodotoxin. The amount of IRI released under depolarizing conditions represented 3-7% of that contained in the synaptosomes. High levels of IRI release also were observed upon removal of external Na+ to allow depolarization-independent influx of external Ca2+ into the synaptosomal compartment. The Ca2+ dependency of synaptosomal IRI release suggests IRI is stored in the adult rat brain in synaptic vesicles within nerve endings from which it can be mobilized by exocytosis in association with neural activity.

Trophic effects of basic fibroblast growth factor on fetal rat hypothalamic cells: interactions with insulin-like growth factor I

The existence of different growth factors within a single brain region suggests that developing brain cells are exposed to a variety of trophic factors throughout neurogenesis. Cooperative interactions between growth factors are known to orchestrate growth and differentiation of various cell types. We explored the possibility that two growth factors may interact in promoting in vitro growth in fetal hypothalamic cells. We found that basic fibroblast growth factor (b-FGF) exerts trophic effects on primary mixed hypothalamic cell cultures, on enriched hypothalamic neuronal cultures, and on hypothalamic glial cultures. In addition, b-FGF increased the growth rate of two virally transformed hypothalamic cell lines. Since insulin-like growth factor I (IGF-I) also promotes growth of rat hypothalamic cells in vitro, we examined the combined effects of b-FGF and IGF-I on hypothalamic cells. Significantly higher numbers of neurite-bearing cells were present in primary mixed hypothalamic cultures when b-FGF and IGF-I were added together than were added separately. The effect was additive. These results establish b-FGF as a putative hypothalamic neurotrophic factor and demonstrate potential coordinate interactions between IGF-I and b-FGF in stimulating the growth or survival of developing hypothalamic cells.

Increased incidence of neuroleptic-induced perioral movements in the rat by hyperglycemia

It has been suggested that Tardive dyskinesia (TD) is associated with abnormal glucose metabolism. To investigate further the issue the effects of alloxan-induced hyperglycemia on the incidence and severity of haloperidol-induced perioral movements were studied in the rat. Hyperglycemic rats showed significantly higher incidence and severity of rating of abnormal perioral movements than did control rats. Severity ratings of perioral movements were significantly correlated with blood glucose levels in the hyperglycemic rats. These findings suggest that hyperglycemia may increase the severity of neuroleptic-induced perioral movements, and support the possibility that glucose intolerance may increase the risk of TD.

Specific binding of insulin to membranes from dendrodendritic synaptosomes of rat olfactory bulb

The external plexiform layer of the olfactory bulb is among the brain regions where insulin receptors are most abundant. In vitro binding of porcine 125I-insulin to membranes of dendrodendritic synaptosomes isolated from adult rat olfactory bulbs was studied to test the hypothesis that dendrodendritic synapses are major insulin-receptive sites in the external plexiform layer of olfactory bulbs. Of the specific insulin binding sites present in a total particulate fraction from the olfactory bulbs, approximately half were recovered in the dendrodendritic synaptosome fraction. The only other subcellular fraction to which substantial insulin binding was observed was the conventional (axodendritic/axosomatic) synaptosome fraction. Analysis of equilibrium binding of insulin to dendrodendritic synaptosomal membranes, at total insulin concentrations of 0.5-1,000 nM, revealed binding site heterogeneity consistent with a two-site model for insulin binding to a high-affinity (KD = 6 nM), low-capacity (Bmax = 110 fmol/mg of protein) site and a low-affinity (KD = 190 nM), high-capacity (Bmax = 570 fmol/mg of protein) site. The results indicate that the intense labeling of the external plexiform layer of the olfactory bulb in autoradiographic studies of insulin binding can be attributed to insulin receptors on dendrodendritic synaptic membranes in this region.

Trophic effects of insulin-like growth factor-I on fetal rat hypothalamic cells in culture

The hypothesis that insulin-like growth factor-I is a trophic factor for primary fetal rat hypothalamic cells was tested, since we previously reported a potent mitogenic effect of this peptide on virally-transformed hypothalamic cells. It was found that insulin-like growth factor-I produced significant and dose-dependent increases in the survival of fetal hypothalamic neurons in primary mixed glial/neuronal cultures. By 48 h in vitro, cultures treated with insulin-like growth factor-I (6 nM) had twice as many neurite-bearing cells as controls, while by day 15 a five-fold difference was present. The peptide was similarly active in promoting neuronal survival in neuron-enriched (98% neurons) hypothalamic cultures. Mixed hypothalamic cultures had specific binding sites for insulin-like growth factor-I. In addition, the neurons grown in the presence of insulin-like growth factor-I had a more differentiated morphology and had significantly higher levels of protein kinase C, an enzyme that increases during neurite formation and synaptogenesis. Finally, glial-enriched cultures (greater than 99% glial cells) obtained from the fetal hypothalamus showed increased [3H]thymidine incorporation in response to insulin-like growth factor-I. These results further support the contention that insulin-like growth factor-I is a neurotrophic factor and suggest that it may participate in the normal development of the hypothalamus by increasing neuronal survival/differentiation and stimulating glial growth.

Regulation of cerebroside sulfotransferase activity in cultured oligodendrocytes: effect of growth factors and insulin

Cerebroside sulfotransferase (EC 2.8.2.11, CST) specific activity has been determined in oligodendrocyte (OL)-enriched glial cell cultures from newborn rat brain grown in serum supplemented medium. This activity is detectable at 5 days in vitro (DIV) and reaches its maximum value at 12 DIV. This period corresponds to that of oligodendrocyte precursor proliferation in these cultures. The activity decreases thereafter and remains nearly constant after 24 DIV. The developmental curve of CST activity is parallel in pure oligodendrocyte subcultures but twice higher than in primary cultures. These data confirm that CST is highly enriched in OL. Basic fibroblast growth factor (bFGF) (15 ng/ml) and platelet derived growth factor (PDGF) (0.75 U/ml) both enhance CST activity by 90% and 72%, respectively. This increase is in the same range than that of DNA content in treated cultures, whereas protein increase is smaller (50% and 22%, respectively). In contrast, transforming growth factor beta 1 (TGF beta 1, 0.5 and 5 ng/ml) does not significantly enhance CST activity nor DNA content of OL cultures. Insulin at high concentrations (5 micrograms/ml) also enhances CST activity but has no effect at physiological concentrations (20 ng/ml). These results show that CST activity can be controlled by growth factors. They suggest that CST activity is more closely related to OL and OL precursor proliferation than to myelination itself since its maximal activity preceeds myelination in vitro.

The olfactory system and Alzheimer's disease

Alzheimer's disease (AD) is considered to be the number one health problem and seems to be reaching epidemic proportion in the USA. The cause of AD is not known, a reliable animal model of the disease has not been found and appropriate treatment of this dementia is wanting. The present review focuses on the possibility that a virus or exogenous toxic materials may gain access to the CNS using the olfactory mucosa as a portal of entry. Anterograde and retrograde transport of the virus/zeolites to olfactory forebrain regions, which receive primary and secondary projections from the main olfactory bulb (MOB) and which, in turn, project centrifugal axons to the MOB, may initiate cell degeneration at such loci. Pathological changes may, thus, be initially confined to projecting and intrinsic neurons localized in cortical and subcortical olfactory structures; arguments are advanced which favor the view that excitotoxic phenomena could be mainly responsible for the overall degenerative picture. Neurotoxic activity may follow infection by the virus itself, be facilitated by loss of GABAergic terminals in olfactory cortex, develop following repeated episodes of physiological long term potentiation (which unmasks NMDA receptors) or be due to excessive release, faculty re-uptake or altered glutamate receptor sensitivity. Furthermore, a reduction in central inhibitory inputs to the MOB might then result in disinhibition of mitral/tufted neurons and enhance the excitotoxic phenomena in the MOB projecting field. Within this context, and in line with recent studies, it is believed that pathology begins at cortical (mainly olfactory) regions, basal forebrain neurons being secondarily affected due to retrograde degeneration. In addition, failure to produce a critical level of neurotrophic factors by a damaged MOB and olfactory cortex, could adversely affect survival of basal cholinergic neurons which innervate both regions. Support for these hypothesis is provided, first, by recent reports on pathological findings in AD brains which seem to involve preferentially the olfactory and entorhinal cortices, the olfactory amygdala and the hippocampus, all of which receive primary or secondary projections from the MOB; secondly, by the presence of severe olfactory deficits in the early stages of the disease, mainly of a discriminatory nature, which points to a malfunction of central olfactory structures.

Insulin binding in the hypothalamus of lean and genetically obese Zucker rats

Recent reports have suggested that the obesity and hyperphagia of the genetically obese Zucker rat may be related to defective insulin action or binding in the hypothalamus. We used quantitative autoradiography to determine if insulin binding is altered in specific hypothalamic nuclei associated with food intake. Insulin binding was measured in the arcuate (ARC), dorsomedial (DMN), and ventromedial (VMN) hypothalamic nuclei of 3-4-month-old lean (Fa/Fa) and genetically obese (fa/fa) Zucker rats. A consistently reproducible 15% increase in the total specific binding of 0.1 nM [125I]-insulin was found in the ARC of the obese genotype. A slight increase in insulin binding in the DMN was also found. No difference in specific insulin binding was found between genotypes in the VMN. Nonlinear least squares analysis of competitive binding studies showed that the Kd of the ARC insulin binding site was 33% higher in the lean rats than in the obese rats, indicating an increased affinity for insulin. No difference in site number (Bmax) was found in the ARC, DMN or VMN, and no evidence was found for reduced insulin binding in the hypothalamus of the obese (fa/fa) genotype. The results suggest that hyperphagia and obesity of the obese (fa/fa) Zucker rat genotype may be associated with increased insulin binding in the arcuate nucleus.

Reduction of insulin binding in the arcuate nucleus of the rat hypothalamus after 6-hydroxydopamine treatment

Insulin receptors are present in the hypothalamus, but the cell types bearing them are unknown. In order to test the hypothesis that some insulin receptors in the hypothalamus are associated with catecholamine terminals, rats were injected with 50 micrograms or 75 micrograms doses (intracerebroventricular) of 6-hydroxydopamine (6-OHDA). Control rats received vehicle only. The animals were sacrificed 7 days after injection, and catecholamine and indolamine levels in the hypothalamus were measured by high performance liquid chromatography with electrochemical detection. Localization of specific binding sites for [125I]-insulin in the arcuate (ARC), dorsomedial (DMN) and ventromedial (VMN) nuclei were determined by quantitative film autoradiography. Treatment with 6-OHDA resulted in a 70% reduction in hypothalamic norepinephrine content as compared to vehicle-treated controls (P less than 0.01). A slight depletion of epinephrine, dopamine and indolamines was also detected. Computerized image analysis of the autoradiograms was used to determine radioactivity bound (DPM/mm2) in each nucleus. Highest binding was in the ARC and DMN, with much lower binding in the VMN. Insulin binding in the ARC of the 6-OHDA-treated group was decreased by 25% compared to controls (P less than 0.01). No significant change in insulin binding was observed in the DMN or VMN. The 6-OHDA treatment had no significant effect on weight gain or on plasma insulin levels. The reduction of insulin binding in the ARC after 6-OHDA treatment supports the hypothesis that some insulin binding sites are located on catecholamine terminals in the arcuate nucleus.

Neurotrophic molecules

Neurotrophic molecules have a profound influence on developmental events such as naturally occurring cell death, differentiation, and process outgrowth. Despite their striking effects on developing neurons, a role for these molecules in the pathogenesis or therapy of neurological disease has not yet been defined. However, a variety of recent advances promise to provide the techniques necessary to assess the potential relevance of neurotrophic molecules to clinical neurology. In this article we review recent investigations into the biological effects, regulation of production, and mechanisms of action of the best characterized trophic molecule, nerve growth factor. In addition we review studies characterizing brain-derived neurotrophic factor and other putative neurotrophic molecules. Finally, we discuss how pharmacological effects of these molecules may be relevant to the therapy of disease states as well as neural regeneration.

Function of neurotrophic factors in the adult and aging brain and their possible use in the treatment of neurodegenerative diseases

This review summarizes the current knowledge of characterized neurotrophic factors, including nerve growth factor (NGF) which serves as paradigmatic example when studying novel molecules. Special consideration is given to the function of neurotrophic factors in the adult and aging brain. Strategies are discussed for the eventual development of pharmacological applications of these molecules in the treatment of neurodegenerative diseases.

Tyrosine phosphorylated proteins accumulate in junctional regions of the developing chick neural retina

Antibodies specific for protein phosphotyrosyl residues were used to localize sites of action of tyrosine-specific protein kinases in developing chick neural retina by immunoperoxidase staining. Phosphotyrosine-modified proteins became prominent in growth cone- and process-rich regions of embryonic retina during neuronal differentiation. Maximal levels accumulated in the synaptic layers and limiting membranes of the adult retina, where numerous junctional complexes reside. Two major phosphotyrosine-modified proteins in adult retina (80, 42 kDal) increased markedly during maturation. In contrast, the synaptic layers of optic tectum and other brain regions exhibited low protein phosphotyrosine levels. These results suggest a specific role for protein tyrosine phosphorylation in the retina at sites of synapses and other intercellular junctions.

Insulin-like growth factor-1 (somatomedin-C) receptors in the rat brain: distribution and interaction with the hippocampal cholinergic system

The present work characterizes the autoradiographic distribution of insulin-like growth factor-1 (IGF-1)/somatomedin-C binding sites in neonatal and adult rat brain, and attempts to correlate the distribution of IGF-1 sites, in certain regions of the rat brain, with functional IGF-1 receptors. In neonatal brain, [125I]IGF-1 binding sites are especially concentrated in superficial cortical layers, nucleus accumbens and hippocampus. In the adult rat brain, the distribution of IGF-1 sites is broader, with a high density of sites observed in superficial and deep cortical layers, olfactory bulb, endopiriform nucleus, basomedial nucleus of the amygdala, thalamic nuclei and hippocampus. Specific binding of [125I]IGF-1 to its sites in these brain regions was almost completely inhibited by 100 nM nonradioactive IGF-1. In contrast, similar concentrations of either IGF-2 or insulin did not significantly alter [125I]IGF-1 binding to its sites. Therefore, under our incubation conditions, [125I]IGF-1 appears to label specifically the type-I IGF receptor. In the hippocampus, which is highly enriched with specific [125I]IGF-1 binding sites in both neonatal and adult rat brain, IGF-1 significantly altered the potassium-evoked (25 mM) release of acetylcholine (ACh) from slices of adult, but not immature (6- and 18-day-old), rat brain. This IGF-1-induced decrease in ACh release from adult rat brain slices was concentration-dependent and appeared to be specific to hippocampus; ACh release from frontal cortical slices was not affected by this GF. The spontaneous release of ACh in the presence of IGF-1 in either tissue was not significantly different from control.(ABSTRACT TRUNCATED AT 250 WORDS)

A substance resembling somatomedin C in the American cockroach

Material antigenically resembling somatomedin C (type I insulin-like growth factor, IGF-I) is demonstrated in the American cockroach Periplaneta americana by means of a monoclonal antibody immunoperoxidase technique. It was localized histochemically in neuronal cell somata and axonal fibers (probably interneurons) of the central nervous/neuroendocrine system and in 'endocrine-type' cells lining the midgut epithelium. The IGF-I-like substance is different from vertebrate insulin and also distinct from materials immunostained by different insulin antibodies in the brain and neurohaemal complex of this insect species. These findings are viewed in the light of recent reports on the presence and action of insulin-like chemicals in insects, and with respect to the existence of an insect brain-midgut system similar to the mammalian brain-gastroenteropancreatic system.

Insulin and insulin-like growth factor receptors in the nervous system

Insulin and the insulin-like growth factors (I and II) are homologous peptides essential to normal metabolism as well as growth. These peptide hormones are present in the brain, and, based on biosynthetic labeling studies as well as evidence for local gene expression, they are synthesized by nervous tissue as well as being taken up by the brain from the peripheral circulation. Furthermore, the presence of insulin and IGF receptors in the brain, on both neuronal and glial cells, also suggests a role for these peptides in the nervous system. Thus, these ligands affect brain electrical activity, either as neurotransmitters or as neuromodulators, altering the release and re-uptake of other neurotransmitters. The insulin and IGF-I and -II receptors found in the brain exhibit a lower molecular weight than corresponding receptors on peripheral tissues, primarily caused by alterations in glycosylation. Despite these alterations, both brain insulin and IGF-I receptors exhibit tyrosine kinase activity in cell-free systems, as do their peripheral counterparts. Brain insulin and IGF-I receptors are developmentally regulated, with the highest levels appearing in fetal or perinatal life. However, the altered glycosylation of brain receptors does not appear until late in fetal development. The receptors are widely distributed in the brain, but especially enriched in the circumventricular organs, choroid plexus, hypothalamus, cerebellum, and olfactory bulb. These studies on the insulin and IGF receptor in brain, add strong support to the suggestion that insulin and IGFs are important neuroactive substances, regulating growth, development, and metabolism in the brain.

Growth promoting effects of IGF-I on fetal hypothalamic cell lines under serum-free culture conditions

Recent evidence indicates that the insulin-like family of peptides may act as endogenous trophic factors in the central nervous system. To further examine this possibility we have investigated the effects of three insulin-like peptides on the in vitro growth of fetal hypothalamic cell lines. Two virally transformed rat hypothalamic cell lines which have been developed in our laboratory (A-6 and F-12) were used. Cells were plated at varying densities and cultured in the presence or absence of either insulin-like growth factor I (IGF-I), insulin, or multiplication stimulating activity (MSA or IGF-II), in serum-free medium for 1 wk. Cell growth was assessed by counting or by measuring cellular incorporation of 3H-thymidine. Of the three peptides tested IGF-I was the most potent in eliciting cell growth. Insulin also stimulated growth of both cell lines, but was 100 times less potent for A-6 cells while it was equipotent with IGF-I in F-12 cells. MSA had no effect on either cell line. Both IGF-I and insulin showed dose-response effects in increasing cell growth. We also found that the two cell lines had the greatest response to IGF-I at low cell densities. Finally, time-course experiments suggested that a continued presence of the peptide is essential for the growth-promoting effects. We conclude that IGF-I is a potent growth factor for virally transformed cell lines derived from the rat fetal hypothalamus. Since both IGF-I immunoreactivity and IGF-I receptors have been located in this diencephalic area these results suggest that IGF-I may constitute a mitogenic signal for hypothalamic cells during neurogenesis.

Brain neuropeptides: actions on central cardiovascular control mechanisms

The many peptides we have not considered (e.g. gastrin, motilin, FMRFamide, carnosine, litorin, dermorphin, casomorphin, eledoisin, prolactin, growth hormone, neuromedin U, proctolin, etc.) were omitted due to lack of information as far as any putative central cardiovascular effects are concerned. However, even for some of these peptide pariahs intriguing snippets of information are available now (e.g. ref. 85), although as we write, the list of possible candidates for investigation grows longer. On an optimistic note, it is becoming clear that many brain neuropeptides may have important effects on cardiovascular regulation. It seems feasible that 'chemically coded' pathways in the brain might be the neuroanatomical correlate of a 'viscerotopic' organization of cardiovascular control mechanisms, whereby the activity of the heart and flows through vascular beds are individually controlled, but in an integrated fashion, utilizing particular combinations of neurotransmitters and neuropeptides within the brain. Such possibilities can only be investigated, properly, by measurement of changes in cardiac output and regional haemodynamics in response to appropriate interventions, in conscious, unrestrained animals.

Developmental regulation of protein tyrosine phosphorylation in rat brain

Proteins phosphorylated at tyrosine residues in the developing rat brain have been identified with a focus on the nerve growth cone and synaptic terminal. Endogenous protein phosphorylation in membranes from a subcellular growth cone fraction of fetal rat brain revealed prominent 55-60 kD phosphotyrosine-containing proteins. Proteins of similar size were recognized by phosphotyrosine antibodies in isolated growth cone membranes, indicating that they contained phosphotyrosine in vivo. Proteins of 55-60 kD were not highly phosphorylated in synaptosomes from adult brain, suggesting a growth cone-specific function. Generally, tyrosine phosphorylation was much lower in adult brain than in fetal brain fractions. Although some synaptosomal membrane proteins that contained phosphotyrosine corresponded in size with those in growth cone membranes (92 kD, 41 kD), others were unique to synaptosomal membranes (38 kD and 30 kD). Immunoperoxidase staining of fetal rat neocortex with phosphotyrosine antibodies at embryonic day 19 revealed immunoreactivity in presumptive migratory neuroblasts in the intermediate zone and in processes of the molecular layer. Proliferating neuroepithelial cells of the ventricular zone showed little immunoreactivity. Lower levels of phosphotyrosine immunoreactivity were seen until postnatal day 10, correlating with the period of maximal process outgrowth. These results indicate that protein tyrosine phosphorylation in the developing nervous system may be functionally significant in an aspect of neuronal differentiation such as growth cone-mediated process extension and cell migration. An analogous role in the mature brain may be related to synaptic plasticity or function.

Characterization of specific insulin binding sites on chromaffin cells from bovine adrenal medulla

1. Insulin receptors were investigated in isolated chromaffin cells from bovine adrenal medulla. 2. The cells were incubated with [125I]insulin in HEPES buffer, pH 7.8 at 15 degrees C for 180 min to obtain steady state binding. Specific binding was linearly related to the number of cells in the range 0.5-10 x 10(6) cells/ml. Insulin and proinsulin caused half maximal displacement of specifically bound tracer in concentrations of 0.18 and 2.46 nM, respectively. 3. Computer analysis of the binding data gave a linear Scatchard plot, consistent with a single class of non-interacting receptors with an affinity constant of 5.6 nM-1, the total number of receptors per cell being 1700. 4. The apparent MW of the insulin binding subunit of the receptor was 135,000, determined by affinity crosslinking and SDS gel electrophoresis under reducing conditions.