Short-term and long-term changes in the postischemic hippocampus

Mild hypoxia-induced structural and functional changes of the hippocampal network

Hypoxia causes structural and functional changes in several brain regions, including the oxygen-concentration-sensitive hippocampus. We investigated the consequences of mild short-term hypoxia on rat hippocampus in vivo. The hypoxic group was treated with 16% O2 for 1 h, and the control group with 21% O2. Using a combination of Gallyas silver impregnation histochemistry revealing damaged neurons and interneuron-specific immunohistochemistry, we found that somatostatin-expressing inhibitory neurons in the hilus were injured. We used 32-channel silicon probe arrays to record network oscillations and unit activity from the hippocampal layers under anaesthesia. There were no changes in the frequency power of slow, theta, beta, or gamma bands, but we found a significant increase in the frequency of slow oscillation (2.1-2.2 Hz) at 16% O2 compared to 21% O2. In the hilus region, the firing frequency of unidentified interneurons decreased. In the CA3 region, the firing frequency of some unidentified interneurons decreased while the activity of other interneurons increased. The activity of pyramidal cells increased both in the CA1 and CA3 regions. In addition, the regularity of CA1, CA3 pyramidal cells' and CA3 type II and hilar interneuron activity has significantly changed in hypoxic conditions. In summary, a low O2 environment caused profound changes in the state of hippocampal excitatory and inhibitory neurons and network activity, indicating potential changes in information processing caused by mild short-term hypoxia.

Neuropeptides in the developing human hippocampus under hypoxic-ischemic conditions

The perinatal period, sensitive for newborn survival, is also one of the most critical moments in human brain development. Perinatal hypoxia due to reduced blood supply to the brain (ischemia) is one of the main causes of neonatal mortality. Brain damage caused by perinatal hypoxia–ischemia (HI) can lead to neuro‐ and psychological disorders. However, its impact seems to be region‐dependent, with the hippocampus being one of the most affected areas. Among the neuronal populations of the hippocampus, some interneuron groups – such as somatostatin‐ or neuropeptide Y‐expressing neurons – seem to be particularly vulnerable. The limited information available about the effects of HI in the hippocampus comes mainly from animal models and adult human studies. This article presents an immunohistochemical analysis of somatostatin (SOM) and neuropeptide Y (NPY) expression in the developing human hippocampus after perinatal HI. Two rostrocaudal sections of the body of the hippocampus were analysed, and the number of immunostained cells in the polymorphic layer of the dentate gyrus (DG) and the pyramidal cell layer and stratumoriens of the CA3, CA2 and CA1 fields of the hippocampus proper were quantified. The results showed a lower density of both neuropeptides in hypoxic compared to control cases. In the HI group, the number of SOM‐immunoreactive cell bodies was statistically significantly lower in the pyramidal cell layer and stratumoriens of CA1, while the number of NPY‐expressing neurons was statistically lower in the pyramidal cell layer of CA2. Besides, the number of SOM‐expressing neurons was significantly higher in the stratumoriens of CA1 compared to that in CA2. In sum, we observed a different vulnerability of SOM‐ and NPY‐containing neurons in the developing human hippocampus following perinatal HI damage. Our results could contribute to a better understanding of the behaviour of these neuronal populations under stressful conditions during the perinatal period.

Neurogenesis and Gliogenesis in the Postischemic Brain

Stem cells are found throughout the adult mammalian brain, including the subventricular zone (SVZ) adjacent to the lateral ventricles, and in the hippocampal dentate subgranular zone (SGZ). Cells born in the SVZ migrate to the olfactory bulb. Those born in the SGZ migrate into the granule cell layer. Following 5 or 10 min of global ischemia in the adult gerbil, there is a tenfold increase in the birth of new cells in the SGZ as assessed using bromo-deoxy-uridine incorporation. This begins at 7 days, peaks at 11 days, and decreases thereafter. Over the ensuing month, approximately one-fourth of the newborn cells disappear. Of the remaining cells, 60% migrate into the granule cell layer where two-thirds of these become NeuN, calbindin, and MAP-2 immunostained neurons. The remaining 40% of the cells migrate into the dentate hilus where one-fourth of these become glial fibrillary acidic protein-labeled astrocytes. Death of CA1 pyramidal neurons does not stimulate neurogenesis because ischemia-induced tolerance—which does not produce CA1 injury—also stimulated cell proliferation. It is proposed that ischemia-induced neurogenesis contributes to the recovery of function, specifically of anterograde and retrograde recent memory function that is lost following global ischemia in man.

Lifelong protection from global cerebral ischemia and reperfusion in long-lived Mclk1(+/)(-) mutants

To achieve a long life span, animals must be resistant to various injuries as well as avoid or delay lethality from age-dependent diseases. Reduced expression of the mitochondrial enzyme CLK-1/MCLK1 (a.k.a. Coq7), a mitochondrial hydroxylase that is necessary for the biosynthesis of ubiquinone (UQ), extends lifespan in Caenorhabditiselegans and in mice. Here, we show that long-lived Mclk1(+/)(-) mutants have enhanced resistance to neurological damage following global cerebral ischemia-reperfusion (I/R) injury induced by transient bilateral common carotid artery occlusion (BCCAO). Both young ( approximately 100days old) and relatively aged ( approximately 450days old) mutants display increased resistance as indicated by a significant decrease in the amount of degenerating cells observed in forebrain cortex and in hippocampal areas after ischemia and reperfusion. Furthermore, less oxidative damage resulting from the procedure was measured in the brain of young Mclk1(+/)(-) animals. The finding that both young and old mutants are protected indicates that this is a basic phenotype of these mutants and not a secondary consequence of their slow rate of aging. Thus, the partial resistance to I/R injury suggests that Mclk1(+/)(-) mutants have an enhanced recovery potential following age-dependant vascular accidents, which correlates well with their longer survival. By relating this neuroprotective effect to previously reported characteristics of the Mclk1(+/)(-) phenotype, including altered mitochondrial metabolism and increased HIF-1alpha expression, this study establishes these mutants as useful models to analyze the mechanisms underlying tolerance to ischemia, particularly those associated with ischemic preconditioning, as well as to clarify the relation between aging and age-dependent diseases.

Subclasses of oligodendrocytes populate the mouse hippocampus

Oligodendrocytes are the myelin-forming cells of the central nervous system that facilitate transmission of axonal electrical impulses. Using transgenic mice expressing 2',3' cyclic nucleotide 3' phosphodiesterase (CNPase)-enhanced green fluorescent protein, a three-dimensional reconstruction tool and analysis, we illustrate that three morphologically different oligodendrocyte types exist in the hippocampus. Those of the ramified type have the most numerous processes, the largest cell body, occupy the largest area and form beaded-like structures, due to mitochondria aggregates, along the processes. Stellar-shaped oligodendrocytes have smaller cell bodies and their processes cover a significantly smaller area. Those of the smooth subtype have a small cell body with at most two processes. In addition to these types, a large number of oligodendrocytes were found that faintly express CNPase-enhanced green fluorescent protein. More than 50% of the faint type colocalized with NG2 and 91% with oligodendrocyte transcription factor-2, whereas 94% of NG2-immunoreactive and 45% of oligodendrocyte transcription factor-2-immunoreactive cells were faintly CNPase-enhanced green fluorescent protein positive. Based on the complexity of the overall structure, the three types probably represent stages of a maturation process such that one subtype can morph into another. Thus, the least complex 'smooth' cell would represent the youngest oligodendrocyte that matures into the stellar type and eventually progresses to become the most complex ramified oligodendrocyte. Investigation of the distribution pattern revealed that the highest density of oligodendrocytes was found in the stratum lacunosum-moleculare and the hilar region. The distribution analysis of oligodendrocyte subclasses revealed a tendency for different cell types to segregate in large non-overlapping areas. This observation suggests that morphologically, and possible functionally, different oligodendrocytes are topographically segregated.

IGF-1 derived small neuropeptides and analogues: a novel strategy for the development of pharmaceuticals for neurological conditions

Insulin-like growth factor-1 (IGF-1) is neuroprotective and improves long-term function after brain injury. However, its clinical application to neurological disorders is limited by its large molecular size, poor central uptake and mitogenic potential. Glycine-proline-glutamate (GPE) is naturally cleaved from the IGF-1 N-terminal and it is also neuroprotective after ischemic injury, which provided a novel strategy of drug discovery for neurological disorders. GPE is not enzymatically stable, thus intravenous infusion of GPE becomes necessary for stable and potent neuroprotection. The broad effective dose range and treatment window of 3-7 h after the lesion suggest its potential for treating acute brain injuries. G-2meth-PE, a GPE analogue designed to be more enzymatic resistant, has a prolonged plasma half-life and is more potent in neuroprotection. Neuroprotection by GPE and its analogue may involve modulation of inflammation, promotion of astrocytosis, inhibition of apoptosis and vascular remodelling. Acute administration of GPE also prevents 6-OHDA-induced nigrostrial dopamine depletion. Delayed treatment with GPE does not prevent dopamine loss, but improves long-term function. Cyclo-glycyl-proline (cyclic Gly-Pro) is an endogenous DKP that may be derived from GPE. Cyclic Gly-Pro and its analogue cyclo-L-glycyl-L-2-allylproline (NNZ 2591) are both neuroprotective after ischaemic injury. NNZ2591 is highly enzymatic resistant and centrally accessible. Its peripheral administration improves somatosensory-motor function and long-term histological outcome after brain injury. Our research suggests that small neuropeptides have advantages over growth factors in the treatment of brain injury, and that modified neuropeptides designed to overcome the limitations of their endogenous counterparts represent a novel strategy of pharmaceutical discovery for neurological disorders.

Late-onset epileptogenesis and seizure genesis: lessons from models of cerebral ischemia

Patients surviving ischemic stroke often express delayed epileptic syndromes. Late poststroke seizures occur after a latency period lasting from several months to years after the insult. These seizures might result from ischemia-induced neuronal death and associated morphological and physiological changes that are only partly elucidated. This review summarizes the long-term morphofunctional alterations observed in animal models of both focal and global ischemia that could explain late-onset seizures and epileptogenesis. In particular, this review emphasizes the change in GABAergic and glutamatergic signaling leading to hyperexcitability and seizure genesis.

Expression of glutamic acid decarboxylase and identification of GABAergic cells in the ischemic rat dentate gyrus

We have investigated the glutamic acid dexcarboxylase (GAD) mRNA and protein isoforms as markers for ischemic loss of GABAergic neurons in the dentate hilus. Stereological counts of these neurons were performed in rats surviving 8 days after 10 min of transient forebrain ischemia, and in control rats (sham-operated and naïve). GAD65 and GAD67 were detected by both in situ hybridization and immunocytochemistry. No differences (three-way ANOVA, P > 0.05) were found between treatments (ischemia, sham-operated or naïve) when cell counts of identical GAD isoforms were compared at the same level (mRNA or protein). However, irrespective of treatments, the number of neurons expressing GAD65 mRNA was significantly higher than the number of neurons expressing GAD65 protein, and the number of neurons expressing GAD67 mRNA was significantly lower than the number of neurons expressing GAD67 protein. In parallel, we investigated the colocalization of the cell death marker Fluorojade B (FJB) with somatostatin- or GAD67-immunoreactivity in hilus of control and ischemic rats. Although the majority of FJB positive cells also contained somatostatin, a small number of GAD67 immunoreactive neurons contained FJB, suggesting that a small number of GABAergic neurons die after ischemia. In conclusion, this study provides direct evidence that a small proportion of GABAergic hilar neurons succumbs to ischemia. Stereological counts of neurons identified from their expression of either GAD isoform of mRNA or protein revealed a high inter-animal variation at any detection level in both naïve, sham-operated and ischemic rats. Therefore, counts of GABAergic neurons should be carefully interpreted in accordance with the marker used for identification.

Ca2+-Permeable Acid-sensing Ion Channels and Ischemic Brain Injury

Acidosis is a common feature of brain in acute neurological injury, particularly in ischemia where low pH has been assumed to play an important role in the pathological process. However, the cellular and molecular mechanisms underlying acidosis-induced injury remain unclear. Recent studies have demonstrated that activation of Ca(2+)-permeable acid-sensing ion channels (ASIC1a) is largely responsible for acidosis-mediated, glutamate receptor-independent, neuronal injury. In cultured mouse cortical neurons, lowering extracellular pH to the level commonly seen in ischemic brain activates amiloride-sensitive ASIC currents. In the majority of these neurons, ASICs are permeable to Ca(2+), and an activation of these channels induces increases in the concentration of intracellular Ca(2+) ([Ca(2+)](i)). Activation of ASICs with resultant [Ca(2+)](i) loading induces time-dependent neuronal injury occurring in the presence of the blockers for voltage-gated Ca(2+) channels and the glutamate receptors. This acid-induced injury is, however, inhibited by the blockers of ASICs, and by reducing [Ca(2+)](o). In focal ischemia, intracerebroventricular administration of ASIC1a blockers, or knockout of the ASIC1a gene protects brain from injury and does so more potently than glutamate antagonism. Furthermore, pharmacological blockade of ASICs has up to a 5 h therapeutic time window, far beyond that of glutamate antagonists. Thus, targeting the Ca(2+)-permeable acid-sensing ion channels may prove to be a novel neuroprotective strategy for stroke patients.

Characterization of calretinin I-II as an EF-hand, Ca2+, H+-sensing domain

Calretinin, a neuronal protein with well-defined calcium-binding properties, has a poorly defined function. The pH dependent properties of calretinin (CR), the N-terminal (CR I-II), and C-terminal (CR III-VI) domains were investigated. A drop in pH within the intracellular range (from pH 7.5 to pH 6.5) leads to an increased hydrophobicity of calcium-bound CR and its domains as reported by fluorescence spectroscopy with the hydrophobic probe 2-(p-toluidino)-6-naphthalenesulfonic acid (TNS). The TNS data for the N- and C-terminal domains of CR are additive, providing further support for their independence within the full-length protein. Our work concentrated on CR I-II, which was found to have hydrophobic properties similar to calmodulin at lower pH. The elution of CR I-II from a phenyl-Sepharose column was consistent with the TNS data. The pH-dependent structural changes were further localized to residues 13-28 and 44-51 using nuclear magnetic resonance spectroscopy chemical shift analysis, and there appear to be no large changes in secondary structure. Protonation of His 12 and/or His 27 side chains, coupled with calcium chelation, appears to lead to the organization of a hydrophobic pocket in the N-terminal domain. CR may sense and respond to calcium, proton, and other signals, contributing to conflicting data on the proteins role as a calcium sensor or calcium buffer.

Neuroprotective effects of the N-terminal tripeptide of insulin-like growth factor-1, glycine-proline-glutamate (GPE) following intravenous infusion in hypoxic-ischemic adult rats

The N-terminal tripeptide of insulin-like growth factor-1, GPE is neuroprotective when given intracerebroventricularly 2 h after hypoxic-ischemic (HI) brain injury in rats. We have now examined whether GPE can cross the blood-brain barrier and exert neuroprotective actions following intravenous administration. Following a single bolus intravenous injection, GPE was rapidly metabolized and cleared from the circulation. The short half-life (<2 min) in blood was subsequently associated with modest and inconsistent neuroprotection. In contrast, potent neuroprotection of GPE was consistently observed in all brain regions examined following 4 h intravenous infusion (12 mg/kg). The neuroprotective effects of GPE after infusion showed a broad effective dose range (1.2-120 mg/kg) and an extended window of treatment to 7-11 h after injury. The central penetration of GPE after intravenous infusion was injury-dependent. GPE also improved long-term somatofunction with a comparable neuronal outcome. GPE reduced both caspase-3-dependent and -independent apoptosis in the hippocampus. Treatment with GPE also inhibited microglial proliferation and prevented the injury-induced loss of astrocytes. In conclusion, the neuroprotective actions of GPE infusion were global, robust and displayed a broad effective dose range and treatment window. GPE's activity included the prevention of neuronal apoptosis, promotion of astrocyte survival and inhibition of microglial proliferation. With injury specific central penetration, GPE has considerable promise as a systemic neuroprotective treatment after acute encephalopathies.

Argyrophilic dark neurons represent various states of neuronal damage in brain insults: some come to die and others survive

Argyrophilic dark neurons (DNs) reflect the early histopathological state of neurons following various brain insults. We examined the fate of DNs, about to either die or recover, following two types (heavy and light damage) of brain insult. Wistar rats were injected ibotenic acid unilaterally into the hippocampal CA1 region (ibotenic acid [IA] injection) or were forced to swim (SWIM). Argyrophil III (DNs)-, activated caspase-3 immuno-, TUNEL- and hematoxylin-eosin (H-E)-staining and ultrastructural examinations were then performed. One to three hours after IA injection, typical DNs (argyrophilic both in somata and dendrites) with corkscrew-like dendrites were densely packed in the pyramidal cell layer of hippocampal CA1 around the injection site. After 12-24 h, DNs were argyrophilic only in the somata and proximal dendrites but absent in distal dendrites in the CA1 region. However, at this time typical DNs were found in remote areas. At 3 h, caspase-3 activation was detected at the injection site, which increased to a peak level after 12 h. Three to 7 days after injection, TUNEL positive cells were detected in the CA1 pyramidal cell layer. Immediately following SWIM, "brown" rather than "dark" neurons were detected in the various areas and most frequently in the CA1 pyramidal cell layer. No typical DNs were detected over the first 3 days. Some activation of caspase-3 was detected in a few CA3 pyramidal cells but no TUNEL-positive cells were detected. Ultrastructural examination revealed a diffuse distribution of aggregated silver particles in the dendrites and cytoplasm of pyramidal cells at the sites of IA injection. After SWIM, silver particles were detected mainly on mitochondria of affected cells. These data suggest that DNs provide a measure of neuronal damage: typically dark neurons with broad damage to the cytoskeleton of dendrites would die, while non-typical brown neurons, that may have a disturbance in mitochondria, predominantly survive.

Interneuron diversity series: containing the detonation--feedforward inhibition in the CA3 hippocampus

Feedforward inhibitory circuits are involved both in the suppression of excitability and timing of action potential generation in principal cells. In the CA3 hippocampus, a single mossy fiber from a dentate gyrus granule cell forms giant boutons with multiple release sites, which are capable of detonating CA3 principal cells. By contrast, mossy fiber terminals form a larger number of Lilliputian-sized synapses with few release sites onto local circuit interneurons, with distinct presynaptic and postsynaptic properties. This dichotomy between the two synapse types endows the circuit with exquisite control over pyramidal cell discharge. Under pathological conditions where feedforward inhibition is compromised, focal excitation is no longer contained, rendering the circuit susceptible to hyperexcitability.

Pharmacokinetics of glycine–proline–glutamate, the N-terminal tripeptide of insulin-like growth factor-1, in rats

Glycine-proline-glutamate (GPE) is the N-terminal tripeptide of insulin-like growth factor-1 and has been shown to be neuroprotective following ischemia-induced brain injury. The pharmacokinetics of GPE were studied in adult rats since GPE is a candidate for use in neuroprotection therapies. To measure plasma concentrations of GPE a novel radioimmunoassay was developed whereby GPE was initially derivatized with Bolton and Hunter reagent before use in a standard homologous assay against the Bolton and Hunter iodinated form. The derivatized GPE radioimmunoassay showed a 83% recovery of unlabeled GPE and complete parallel displacement with rat plasma. The simplicity and speed of the assay described here indicate an exciting new use for a previously described technology. The pharmacokinetic studies were conducted in adult rats using a single bolus intravenous injection of GPE at 30 or 100 mg/kg and showed that GPE was rapidly cleared from the circulation. In addition, evidence of the route of the metabolic degradation of GPE is presented. The findings presented here are the first description of the pharmacokinetics of GPE and suggest that, because of its very short half-life in plasma, continuous intravenous infusion of GPE may be the preferred route of administration for use in future neuroprotection therapies.

Radiosurgery of the Rat Hippocampus: Magnetic Resonance Imaging, Neurophysiological, Histological, and Behavioral Studies

OBJECTIVE

To explore the histological, electrophysiological, radiological, and behavioral effects of radiosurgery using a new model of proton beam radiosurgery (PBR) of the rodent hippocampus.

METHODS

Forty-one rats underwent PBR of the right hippocampus with nominal doses of 5 to 130 cobalt Gray equivalents (CGE). Three control animals were untreated. Three months after PBR, 41 animals were evaluated with the Morris water maze, 23 with T2-weighted magnetic resonance imaging, and 22 with intrahippocampal microelectrode recordings. Animals that were studied physiologically were killed, and their brains were examined with Nissl staining and immunocytochemical staining for glutamic acid decarboxylase, heat shock protein 72 (HSP-72), parvalbumin, calmodulin, calretinin, calbindin, and somatostatin.

RESULTS

Ninety and 130 CGE resulted in decreased performance in the Morris water maze, increased signal on T2-weighted magnetic resonance imaging, diminished granule cell field potentials, and tissue necrosis, which was restricted to the irradiated side. These doses also resulted in ipsilateral up-regulation of calbindin and HSP-72. Parvalbumin was down-regulated at 130 CGE. The 30 and 60 CGE animals displayed a marked increase in HSP-72 staining on the irradiated side but no demonstrable cell loss. No asymmetries were noted in somatostatin, calretinin, and glutamic acid decarboxylase staining. Normal physiology was found in rats receiving up to 60 CGE.

CONCLUSION

This study expands our understanding of the effects of radiosurgery on the mammalian brain. Three months after PBR, the irradiated rat hippocampus demonstrates necrosis at 90 CGE, but not at 60 CGE, with associated abnormalities in magnetic resonance imaging, physiology, and memory testing. HSP-72 was up-regulated at nonnecrotic doses.

Insulin-like growth factor-1 and post-ischemic brain injury

Insulin-like growth factor-1 (IGF-1) is a naturally occurring neurotrophic factor that plays an important role in promoting cell proliferation and differentiation during normal brain development and maturation. The present review examines recent evidence that endogenous IGF-1 also plays a significant role in recovery from insults such as hypoxia-ischemia and that giving additional exogenous IGF-1 can actively ameliorate damage. It is now well established that neurons and other cell types die many hours or even days after initial injury due to activation of programmed cell death pathways. IGF-1 and its binding proteins and receptors are intensely induced within damaged brain regions following brain injury, suggesting a possible a role for IGF-1 in brain recovery. Exogenous administration of IGF-1 within a few hours after brain injury is now known to be protective in both gray and white matter and leads to improved somatic function. In contrast, pre-treatment is ineffective, likely reflecting limited intracerebral penetration of IGF-1 into the uninjured brain. The neuroprotective effects of IGF-1 are mediated by IGF-1 receptors and its binding proteins and are specific to particular cellular phenotypes and brain regions. The window of opportunity for treatment with IGF-1 is limited to a few hours after normothermic brain injury, reflecting its specific actions on early, intracellular events in the apoptotic cascade. However, injury-associated mild post-hypoxic hypothermia, which delays the development of cell death, can shift and dramatically extend the window of opportunity for delayed treatment with IGF-1. Such a combined approach is likely to be essential for any clinical treatment.

Environmental conditions unexpectedly affect the long-term extent of cell death following an hypoxic episode

Previously we reported delayed cell death, defined by clear-cut cell loss 60 days after a nitrite-induced hypoxic episode. The loss of cells was not apparent two weeks after the treatment, although some changes in cellular appearance were observed at that time. A similar delayed loss of neurons in the hippocampus after hypoxia induced by blood vessel occlusion has also been found. In addition, we reported that the amount of methemoglobinemia induced by the sodium nitrite can be reduced by the stress produced by handling and the injection of saline 2 or 24 h before the nitrite administration. The degree of methemoglobin formed is directly related to cell death in certain areas of the brain, including regions within the hippocampus. Considering the many effects that can be produced by chronic and acute stress of several kinds and the length of time during which these effects manifest themselves, we undertook to determine the histologic effects of the stresses of transport on the neuroanatomic effects of sodium nitrite administration 60 days post administration. Comparisons were made of the effects of two methods of transport from the laboratory in which the animals (male CD-1 mice) were injected with the sodium nitrite or saline (Tufts Medical School) to the laboratory in which the histologic evaluations were made (Binghamton University). The animals began their travel several hours after the injections. One transport method was by commuter airline and the other was by automobile. All animals had the same transport from the supplier to the Boston location (truck). Thus, the stress of experimental interest occurred after the nitrite administration. Upon arrival at Binghamton University, the animals were housed at the University in their own colony room for 60 days before sacrifice. After sacrifice, sections from their brains were subjected to a number of histologic staining procedures, including PTAH, the Bielschowsky silver method, GFAP, and the standard Nissl procedure. Although special attention was paid to hippocampal areas, changes in cells in the habenulae and the linings of ventricular areas were also prominent. Surprisingly, the nitrite treatment before transport to Binghamton offered partial protection against the very substantial and lasting effects of the injections, transport, and handling found in the control animals. Differential effects caused by the two methods of transport were also noted.

Neurogenesis following brain ischemia

Following 5 or 10 min of global ischemia in the adult gerbil there is a tenfold increase in the birth of new cells in the subgranular zone of dentate gyrus of the hippocampus as assessed using BrdU incorporation. This begins at 7 days, peaks at 11 days, and decreases thereafter. Over the next month approximately 25% of the newborn cells disappear. Of the remaining cells, 60% migrate into the granule cell layer where two-thirds become NeuN, calbindin and MAP-2 immunostained neurons. The remaining 40% of the cells migrate into the dentate hilus where 25% of these become GFAP labeled astrocytes. It is proposed that ischemia-induced neurogenesis contributes to the recovery of function, and specifically may serve to improve anterograde and retrograde recent memory function that is lost following global ischemia in animals and man.

Effects of global cerebral ischemia and preconditioning on heat shock protein 27 immunocontent and phosphorylation in rat hippocampus

Global cerebral ischemia, with or without preconditioning, leads to an increase in heat shock protein 27 (HSP27) immunocontent and alterations in HSP27 phosphorylation in CA1 and dentate gyrus areas of the hippocampus. We studied different times of reperfusion (1, 4, 7, 14, 21 and 30 days) using 2 min, 10 min or 2+10 min of ischemia. The results showed an increase in HSP27 immunocontent of about 300% after 10 min of ischemia in CA1 and dentate gyrus. CA1, a hippocampal vulnerable area, showed an increase in HSP27 phosphorylation, parallel with immunocontent. In dentate gyrus, a resistant area, the increase in HSP phosphorylation was lower than immunocontent. After preconditioned ischemia (2+10 min), when CA1 neurons are protected to a lethal, 10 min insult, we observed an increase in HSP immunocontent and a decrease in phosphorylation in both regions of the hippocampus, suggesting that, when there is no neuronal death, HSP27 in a vulnerable area responds similarly to the resistant area.When dephosphorylated, HSP27 acts as a chaperone, protecting other proteins from denaturation. As it is markedly expressed in astrocytes, we suggest that HSP27 could be protecting hippocampal astrocytes, which could then be helping neurons to resist to the insult, maintaining tissue normal homeostasis.

Insulin-like growth factor-1 improves somatosensory function and reduces the extent of cortical infarction and ongoing neuronal loss after hypoxia-ischemia in rats

Treatment with insulin-like growth factor-1 has been demonstrated to reduce the extent of cortical infarction 5 days after hypoxic-ischemic brain injury. As neuronal death can be progressive and long lasting after initial injury, the present study examined the long-term effects of insulin-like growth factor-1 on late neuronal loss 20 days after hypoxic-ischemic injury, together with evaluating neurobehavioral outcome as assumed by somatosensory function. Unilateral brain injury was induced in adult rats by carotid artery ligation followed by 10 min of hypoxia (6% O2). A single dose of insulin-like growth factor-1 (50 microg) was administered intracerebroventricularly via a stereotaxically pre-fixed cannula 2 h after injury. A bilateral tactile stimulation test was used to examine the degree of somatosensory function at 3, 5, 10 and 20 days after the hypoxia in both insulin-like growth factor-1- (n=12) and its vehicle- (n=12) treated rats, along with sham-operated rats (n=9). Cortical infarction and percentage of selective neuronal loss in the cerebral cortex were examined 20 days after the hypoxic-ischemic injury in both treatment groups. Hypoxic-ischemic injury resulted in a significant delay in the time taken to contact the patch over the period examined (left/right ratio 5.1+/-0.79), particularly at 3 days (7.0+/-2.8) after the hypoxia, compared to sham-operated rats (1.1+/-0.9, P<0.05). The overall effect of insulin-like growth factor-1 in reducing the time taken to contact the patch was significant (P=0.03, 2.6+/-0.79) compared to the vehicle group. There was a trend towards a reduction of cortical infarction after insulin-like growth factor-1 treatment (P=0.058), however insulin-like growth factor-1 significantly reduced the percentage of selective neuronal loss (P=0.027) 20 days following the hypoxia. From these data we suggest that insulin-like growth factor-1 improves somatosensory function by reducing both the extent of cortical infarction and ongoing progressive neuronal death during brain recovery from hypoxic-ischemic injury.

Autoradiographic localization of N-type VGCCs in gerbil hippocampus and failure of omega-conotoxin MVIIA to attenuate neuronal injury after transient cerebral ischemia

In the mammalian central nervous system, transient global ischemia of specific duration causes selective degeneration of CA1 pyramidal neurons in hippocampus. Many of the ischemia-induced pathophysiologic cascades that destroy the neurons are triggered by pre- and postsynaptic calcium entry. Consistent with this, many calcium channel blockers have been shown to be neuroprotective in global models of ischemia. omega-Conotoxin MVIIA, a selective N-type VGCC blocker isolated from the venom of Conus magus, protects CA1 neurons in the rat model of global ischemia, albeit transiently. The mechanism by which this peptide renders neuroprotection is unknown. We performed high-resolution receptor autoradiography with the radiolabeled peptide and observed highest binding in stratum lucidum of CA3 subfield, known to contain inhibitory neurons potentially important in the pathogenesis of delayed neuronal death. This finding suggested that the survival of stratum lucidum inhibitory neurons might be the primary event, leading to CA1 neuroprotection after ischemia. Testing of this hypothesis required the reproduction of its neuroprotective effects in the gerbil model of global ischemia. Surprisingly, we found that omega-MVIIA did not attenuate CA1 hippocampal injury after 5 min of cerebral ischemia in gerbil. Possible reasons are discussed. Lastly, we show that the peptide can be used as a synaptic marker in assessing short and long-term changes that occur in hippocampus after ischemic injury.

A unifying hypothesis of Alzheimer's disease. III. Risk factors

Normal ageing and Alzheimer's disease (AD) have many features in common and, in many respects, both conditions only differ by quantitative criteria. A variety of genetic, medical and environmental factors modulate the ageing-related processes leading the brain into the devastation of AD. In accordance with the concept that AD is a metabolic disease, these risk factors deteriorate the homeostasis of the Ca(2+)-energy-redox triangle and disrupt the cerebral reserve capacity under metabolic stress. The major genetic risk factors (APP and presenilin mutations, Down's syndrome, apolipoprotein E4) are associated with a compromise of the homeostatic triangle. The pathophysiological processes leading to this vulnerability remain elusive at present, while mitochondrial mutations can be plausibly integrated into the metabolic scenario. The metabolic leitmotif is particularly evident with medical risk factors which are associated with an impaired cerebral perfusion, such as cerebrovascular diseases including stroke, cardiovascular diseases, hypo- and hypertension. Traumatic brain injury represents another example due to the persistent metabolic stress following the acute event. Thyroid diseases have detrimental sequela for cerebral metabolism as well. Furthermore, major depression and presumably chronic stress endanger susceptible brain areas mediated by a host of hormonal imbalances, particularly the HPA-axis dysregulation. Sociocultural and lifestyle factors like education, physical activity, diet and smoking may also modulate the individual risk affecting both reserve capacity and vulnerability. The pathophysiological relevance of trace metals, including aluminum and iron, is highly controversial; at any rate, they may adversely affect cellular defences, antioxidant competence in particular. The relative contribution of these factors, however, is as individual as the pattern of the factors. In familial AD, the genetic factors clearly drive the sequence of events. A strong interaction of fat metabolism and apoE polymorphism is suggested by intercultural epidemiological findings. In cultures, less plagued by the 'blessings' of the 'cafeteria diet-sedentary' Western lifestyle, apoE4 appears to be not a risk factor for AD. This intriguing evidence suggests that, analogous to cardiovascular diseases, apoE4 requires a hyperlipidaemic lifestyle to manifest as AD risk factor. Overall, the etiology of AD is a key paradigm for a gene-environment interaction. Copyright 2000 John Wiley & Sons, Ltd.

Changes in excitatory and inhibitory circuits of the rat hippocampus 12-14 months after complete forebrain ischemia

Changes in interneuron distribution and excitatory connectivity have been investigated in animals which had survived 12-14 months after complete forebrain ischemia, induced by four-vessel occlusion. Anterograde tracing with Phaseolus vulgaris leucoagglutinin revealed massive Schaffer collateral input even to those regions of the CA1 subfield where hardly any surviving pyramidal cells were found. Boutons of these Schaffer collaterals formed conventional synaptic contacts on dendritic spines and shafts, many of which likely belong to interneurons. Mossy fibres survived the ischemic challenge, however, large mossy terminals showed altered morphology, namely, the number of filopodiae on these terminals decreased significantly. The entorhinal input to the hippocampus did not show any morphological alterations. The distribution of interneurons was investigated by neurochemical markers known to label functionally distinct GABAergic cell populations. In the hilus, spiny interneurons showed a profound decrease in number. This phenomenon was not as obvious in CA3, but the spiny metabotropic glutamate receptor 1alpha-positive non-pyramidal cells, some of which contain calretinin or substance P receptor, disappeared from stratum lucidum of this area. In the CA1 region, somatostatin immunoreactivity disappeared from stratum oriens/lacunosum-moleculare-associated cells, while in metabotropic glutamate receptor 1alpha-stained sections these cells seemed unaffected in number. Other interneurons did not show an obvious decrease in number. In stratum radiatum of the CA1 subfield, some interneuron types had altered morphology: the substance P receptor-positive dendrites lost their characteristic radial orientation, and the metabotropic glutamate receptor 1alpha-expressing cells became extremely spiny. The loss of inhibitory interneurons at the first two stages of the trisynaptic loop coupled with a well-preserved excitatory connectivity among the subfields suggests that hyperexcitability in the surviving dentate gyrus and CA3 may persist even a year after the ischemic impact. The dorsal CA1 region is lost; nevertheless hyperactivity, if it occurs, may have a route to leave the hippocampus via the longitudinally extensive axon collaterals of CA3 pyramidal cells, which may activate the subiculum and entorhinal cortex with a relay in the surviving ventral hippocampal CA1 region.

Effects of transient cerebral ischemia on glial fibrillary acidic protein phosphorylation and immunocontent in rat hippocampus

Transient global cerebral ischemia induced in rats by four-vessel occlusion for 20 min produced an increase in the immunocontent of glial fibrillary acidic protein and a protein phosphorylation response that was different in the CA1 and dentate gyrus areas of the hippocampus. We studied different times of reperfusion (one, four, seven, 14 and 30 days) and observed that the immunocontent and in vitro rate of phosphorylation of glial fibrillary acidic protein in the CA1 region was significantly increased at all intervals after the ischemic insult, indicating that the astrocytic response was maintained for at least 30 days. After reperfusion for 14 days a significant increase in the ratio "in vitro phosphorylation rate/immunocontent" in the CA1 region was observed when compared to control values, to other intervals and to the dentate gyrus, suggesting a hyperphosphorylation of this intermediate filament protein at this interval. In the dentate gyrus, an area less vulnerable to the insult, labelling and immunocontent of glial fibrillary acidic protein were equally increased from four days of reperfusion and the increase remained significant until 30 days, confirming that neuronal death is not the only determining factor for gliosis to occur. In control sham-operated animals, neither the CA1 region nor the dentate gyrus showed significant increases in labelling or immunocontent. Changes in the phosphorylation of glial fibrillary acidic protein may be essential for the plastic response of astrocytes to neuronal damage, as neurons and astrocytes can act as functional units involved in homeostasis, plasticity and neurotransmission.

Aurintricarboxylic acid prevents GLUR2 mRNA down-regulation and delayed neurodegeneration in hippocampal CA1 neurons of gerbil after global ischemia

Aurintricarboxylic acid (ATA), an inhibitor of endonuclease activity and other protein-nucleic acid interactions, blocks apoptosis in several cell types and prevents delayed death of hippocampal pyramidal CA1 neurons induced by transient global ischemia. Global ischemia in rats and gerbils induces down-regulation of GluR2 mRNA and increased alpha-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid (AMPA)-induced Ca2+ influx in CA1 before neurodegeneration. This result and neuroprotection by antagonists of AMPA receptors suggests that formation of AMPA receptors lacking GluR2, and therefore Ca2+ permeable, leads to excessive Ca2+ influx in response to endogenous glutamate; the resulting delayed neuronal death in CA1 exhibits many characteristics of apoptosis. In this study, we examined the effects of ATA on expression of mRNAs encoding glutamate receptor subunits in gerbil hippocampus after global ischemia. Administration of ATA by injection into the right cerebral ventricle 1 h before (but not 6 h after) bilateral carotid occlusion prevented the ischemia-induced decrease in GluR2 mRNA expression and the delayed neurodegeneration. These findings suggest that ATA is neuroprotective in ischemia by blocking the transcriptional changes leading to down-regulation of GluR2, rather than by simply blocking endonucleases, which presumably act later after Ca2+ influx initiates apoptosis. Maintaining formation of Ca2+ impermeable, GluR2 containing AMPA receptors could prevent delayed death of CA1 neurons after transient global ischemia, and block of GluR2 down-regulation may provide a further strategy for neuroprotection.

Comparison of the regional expression of nicotinic acetylcholine receptor alpha7 mRNA and [125I]-alpha-bungarotoxin binding in human postmortem brain

Neuronal nicotinic acetylcholine receptors are expressed in the human central nervous system. A specific subtype of this receptor family, the alpha7 nicotinic acetylcholine receptor, is thought to be the principal alpha-bungarotoxin (alphaBTX)-binding protein in mammalian brain. Although the expression of this receptor subtype has been characterized in rat, no study has specifically compared the expression of both the alpha7 gene and the localization of BTX binding sites in human brain. Expression of alpha7 mRNA and receptor protein in human postmortem brain tissue was examined by in situ hybridization and [125I]-alpha-bungarotoxin autoradiography, respectively, with particular emphasis on regions associated with sensory processing. Regions with high levels of both alpha7 gene expression and [125I]-alphaBTX binding include the nucleus reticularis of the thalamus, the lateral and medial geniculate bodies, the basilar pontine nucleus, the horizontal limb of the diagonal band of Broca, the nucleus basalis of Meynert, and the inferior olivary nucleus. High-to-moderate levels of alpha7 probe hybridization were also seen in the hippocampus and the cerebral cortex; however, there was a reduced or variable degree of [125I]-alphaBTX binding in these regions compared with the level of probe hybridization. In most brain regions, [125I]-alphaBTX binding was localized to neuronal cell bodies similar in morphology to those that exhibited alpha7 hybridization, suggesting that the high-affinity [125I]-alphaBTX binding sites in the human brain are likely to be principally composed of alpha7 receptor subtypes.