Neuronal damage in the rat hippocampus induced by in vivo hypoxia

Histological and cytochemical analysis of the brain under conditions of hypobaric hypoxia-induced oxygen deficiency in albino rats

High altitude sickness is a life-threatening disease that occurs among acclimatized individuals working or living at a high altitude accompanied by hypobaric hypoxia exposure. The prolonged influence of hypobaric hypoxia on the brain may trigger neuronal damage and cell death due to an oxygen deficiency. The purpose of the current study was to investigate the histomorphological changes in the hippocampus, cerebral cortex, cerebellar cortex, and striatum of the rat's brain following chronic hypobaric hypoxia. Fourteen albino rats were used for this investigation. The animals were exposed to chronic hypobaric hypoxia in the special decompression chamber at an altitude of 7000 m for 7 days. The histological analysis was conducted via toluidine staining and silver impregnation. DNA damage and cell apoptosis were assessed via Feulgen staining. The histochemical assessment revealed increased dark neurons in the hippocampus with cell swelling. Silver impregnation showed increased argyrophilic neurons in the cerebellar cortex, striatum, CA1 subfield of the hippocampus, and cerebral cortex. The cytochemical analysis determined the increased apoptotic cells with hyperchromatic condensation and pyknosis in the hippocampus subfields and cerebral cortex. In addition, it has been observed that hypoxia has resulted in small hemorrhages and perivascular edema within the cerebellar and cerebral cortex. The results indicate brain injury observed in the various parts of the brain towards hypobaric hypoxia, however, the hippocampus showed greater vulnerability against hypoxic exposure in comparison to the striatum, cerebellum, and cerebral cortex. These changes support our insights regarding brain intolerance under conditions of hypoxia-induced oxygen deficiency and its histomorphological manifestations.

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.

Early Transient Mild Hypothermia Attenuates Neurologic Deficits and Brain Damage After Experimental Subarachnoid Hemorrhage in Rats

OBJECTIVE

Metabolic exhaustion in ischemic tissue is the basis for a detrimental cascade of cell damage. In the acute stage of subarachnoid hemorrhage (SAH), a sequence of global and focal ischemia occurs, threatening brain tissue to undergo ischemic damage. This study was conducted to investigate whether early therapy with moderate hypothermia can offer neuroprotection after experimental SAH.

METHODS

Twenty male Sprague-Dawley rats were subjected to SAH and treated by active cooling (34°C) or served as controls by continuous maintenance of normothermia (37.0°C). Mean arterial blood pressure, intracranial pressure, and local cerebral blood flow over both hemispheres were continuously measured. Neurologic assessment was performed 24 hours later. Hippocampal damage was assessed by hematoxylin-eosin and caspase-3 staining.

RESULTS

By a slight increase of mean arterial blood pressure in the cooling phase and a significant reduction of intracranial pressure, hypothermia improved cerebral perfusion pressure in the first 60 minutes after SAH. Accordingly, a trend to increased cerebral blood flow was observed during this period. The rate of injured neurons was significantly reduced in hypothermia-treated animals compared with normothermic controls.

CONCLUSIONS

The results of this series cannot finally answer whether this form of treatment permanently attenuates or only delays ischemic damage. In the latter case, slowing down metabolic exhaustion by hypothermia may still be a valuable treatment during this state of ischemic brain damage and prolong the therapeutic window for possible causal treatments of the acute perfusion deficit. Therefore, it may be useful as a first-tier therapy in suspected SAH.

Identification of brain areas sensitive to the toxic effects of sparteine

Sparteine is one of the most toxic quinolizidine alkaloids found in leguminous plants. Several studies have demonstrated that sparteine affects the nervous system, blocking the nervous ganglion, producing antimuscarinic effects, depressing the central nervous system and causing neuronal necrosis. However, there are no reports identifying the areas of the brain that are sensitive to the toxic effects of this alkaloid. 32 adult Wistar rats were on study, sixteen were implanted with an intracerebral stainless steel cannula and randomly assigned to a control or experimental group (n=8). Animals, control and experimental, received daily intraventricular (ICV) injections of a sparteine or a sterile water solution for five consecutive days. Additionally, two groups of animals (8 rats each) received daily intraperotineal injections (IP) of a sparteine or sterile water solution for five consecutive days. 72h after the last dose, the animals were sacrificed, their brains removed, fixed and embedded in paraffin to obtain 10μm tissue slices. Brain slices were stained with H&E and evaluated under a light microscope. The main brain structures sensitive to sparteine were the cerebral cortex (frontal, fronto-parietal and striate) olfactory and amygdaloid areas, the ventromedial hypothalamic nucleus, the Purkinje cells in the cerebellum, and the CA1, CA3 and dentate gyrus regions of the hippocampus. Administration of sparteine, via ICV or IP, caused neuronal necrosis in brain structures, mainly related with cholinergic pathways.

Karwinskia humboldtiana (buckthorn) fruit causes central nervous system damage during chronic intoxication in the rat

Karwinskia humboldtiana fruit (Kh) causes a neurological disorder 3-4 weeks after ingestion, characterized by flaccid, symmetrical, ascending paralysis, similar to the Guillain-Barre syndrome. In this polyneuropathy the lesion (demyelization) in peripheral nerves has been described in several animal species, both in acute and in chronic intoxication. However, no reports exist about the presence of lesions in the Central Nervous System (CNS), in chronic intoxication. We considered it important to evaluate, with histological techniques, the possible presence of lesions in the brain, by using a model of chronic intoxication that reproduces the same stages present in the human intoxication, to better understanding of this pathological process. In our present work we fed the ground Kh fruit to Wistar rats and samples of brain, cerebellum, and pons were embedded in paraffin. Sections were stained with Hematoxylin & Eosin (HE) and special stains for nerve tissue. Histopathological changes were evaluated in the CNS through the different stages of the polyneuropathy and comparison to a control group. With this methodology, we found lesions in the motor pathway. This is the first report about the presence of neuronal damage caused by Kh in the Central Nervous System in chronic intoxication.

Hypoxic-ischemic changes in SIDS brains as demonstrated by a reduction in MAP2-reactive neurons

Sudden infant death syndrome (SIDS) is characterized by a lack of any known morphological or functional organ changes that could explain the lethal process. In the present study we investigated the hypothesis of an association between hypoxic/ischemic injury and SIDS deaths. In a previous study, we could demonstrate by quantitative immunohistochemistry a distinct drop in microtubule-associated protein (MAP2) reactivity in neurons of adult, human brains secondary to acute hypoxic-ischemic injuries. Here we applied the same method on sections of the frontal cortex and hippocampus of 41 brains of infants younger than 1 year of age. For each brain area 100 selected neurons were evaluated for their MAP2 reactivity in the different layers of the frontal cortex and in the different segments of the hippocampus. Three groups were compared: (1) SIDS victims (n = 17), (2) infants with hypoxia/ischemia (control group one; n = 14), (3) infants without hypoxic/ischemic injury (control group two; n = 10). The SIDS group and hypoxic/ischemic group exhibited a general reduction in the number of MAP2 reactive neurons in comparison with the non-hypoxic/ischemic injury group. The SIDS group also had a significantly lower (P < 0.05) number of reactive neurons in the CA2 and CA3 areas of the hippocampus than did control group two. No difference was detected between the SIDS group and control group one. The SIDS brains were thus found to display hypoxic/ischemic features without however providing evidence as to the cause of the oxygen reduction.

Layer specific changes of astroglial gap junctions in the rat cerebellar cortex by persistent Borna Disease Virus infection

Neonatal Borna Disease Virus (BDV) infection of the Lewis rat brain, leads to Purkinje cell degeneration, in association with astroglial activation. Since astroglial gap junctions (GJ) are known to influence neuronal degeneration, we investigated BDV dependent changes in astroglial GJ connexins (Cx) Cx43, and Cx30 in the Lewis rat cerebellum, 4, and 8 weeks after neonatal infection. On the mRNA level, RT-PCR demonstrated a BDV dependent increase in cerebellar Cx43, and a decrease in Cx30, 8, but not 4 weeks p.i. On the protein level, Western blot analysis revealed no overall upregulation of Cx43, but an increase of its phosphorylated forms, 8 weeks p.i. Cx30 protein was downregulated. Immunohistochemistry revealed a BDV dependent reduction of Cx43 in the granular layer (GL), 4 weeks p.i. 8 weeks p.i., Cx43 immunoreactivity recovered in the GL, and was induced in the molecular layer (ML). Cx30 revealed a BDV dependent decrease in the GL, both 4, and 8 weeks p.i. Changes in astroglial Cxs correlated not with expression of the astrogliotic marker GFAP, which was upregulated in radial glia. With regard to functional coupling, primary cerebellar astroglial cultures, revealed a BDV dependent increase of Cx43, and Cx30 immunoreactivity and in spreading of the GJ permeant dye Lucifer Yellow. These results demonstrate a massive, BDV dependent reorganization of astroglial Cx expression, and of functional GJ coupling in the cerebellar cortex, which might be of importance for the BDV dependent neurodegeneration in this brain region.

Hypoxia modulates cholinergic but not opioid activation of G proteins in rat hippocampus

Intermittent hypoxia, such as that associated with obstructive sleep apnea, can cause neuronal death and neurobehavioral dysfunction. The cellular and molecular mechanisms through which hypoxia alter hippocampal function are incompletely understood. This study used in vitro [(35)S]guanylyl-5'-O-(gamma-thio)-triphosphate ([(35)S]GTP gamma S) autoradiography to test the hypothesis that carbachol and DAMGO activate hippocampal G proteins. In addition, this study tested the hypothesis that in vivo exposure to different oxygen (O(2)) concentrations causes a differential activation of G proteins in the CA1, CA3, and dentate gyrus (DG) regions of the hippocampus. G protein activation was quantified as nCi/g tissue in CA1, CA3, and DG from rats housed for 14 days under one of three different oxygen conditions: normoxic (21% O(2)) room air, or hypoxia (10% O(2)) that was intermittent or sustained. Across all regions of the hippocampus, activation of G proteins by the cholinergic agonist carbachol and the mu opioid agonist [D-Ala(2), N-Met-Phe(4), Gly(5)] enkephalin (DAMGO) was ordered by the degree of hypoxia such that sustained hypoxia > intermittent hypoxia > room air. Carbachol increased G protein activation during sustained hypoxia (38%), intermittent hypoxia (29%), and room air (27%). DAMGO also activated G proteins during sustained hypoxia (52%), intermittent hypoxia (48%), and room air (43%). Region-specific comparisons of G protein activation revealed that the DG showed significantly less activation by carbachol following intermittent hypoxia and sustained hypoxia than the CA1. Considered together, the results suggest the potential for hypoxia to alter hippocampal function by blunting the cholinergic activation of G proteins within the DG.

Underlying mechanism of hypoxic preconditioning decreasing apoptosis induced by anoxia in cultured hippocampal neurons

It is known that hypoxic preconditioning (HP, a brief period of sublethal hypoxia) provides neuroprotection against subsequent severe anoxia, but the mechanisms of this increased tolerance have not been fully elucidated. A hypoxic preconditioning model was established by exposing a 4-day hippocampal culture to 1% O(2) for 20 min/day for 8 days. The preconditioning significantly decreased the number of apoptotic neurons at reoxygenation 24 h after 4 h of severe anoxia (0% O(2)). Further study demonstrated that the degradation of mitochondrial membrane potential (MMP) was greatly inhibited and the expression of B-cell lymphoma protein-2 (Bcl-2) was increased considerably after severe anoxia in the HP groups. These results indicate that the increased anoxic tolerance, which is induced by HP in cultured hippocampal cells, may be correlated with Bcl-2 overexpression and enhanced stability of MMP, which ultimately reduces apoptosis 24 h after reoxygenation.

Microtubule-associated protein 2 (MAP2)--a promising approach to diagnosis of forensic types of hypoxia-ischemia

The loss of neuronal immunoreactivity of the cytoskeletal microtubule-associated protein 2 (MAP2) is known to be a marker of--at least--transient functional failure of neurons following ischemia. Because there are no specific neuropathological findings in forensic types of acute hypoxia-ischemia, detection of this relevant cause of death is often complicated and a reliable ischemic biomarker would be of great importance. We therefore investigated the neuronal immunoreactivity of MAP2 in human cases of forensic significance. A control group (n=27) was compared to a group of cases of hypoxia-ischemia (n=45), comprising death due to hanging (n=19), drowning (n=14) and carbon monoxide (CO) poisoning (n=12). Using immunohistochemical staining, the percentage of MAP2-positive neurons in the hippocampus (areas CA1-CA4) and frontal cortex (layers II-VI) was evaluated and compared. The hypoxia-ischemia group showed decreased MAP2 immunostaining in the hippocampal areas CA2-CA4 (P<0.05) and in cortical layers II-VI (P<0.001) compared to controls. Most vulnerable regions seem to be the hippocampal CA4 area and cortical layers III-V. Within the hypoxia-ischemia group, death due to CO poisoning was characterized by the lowest MAP2 immunoreactivity. The hypoxic-ischemic groups differ from controls by a distinct decrease of MAP2 immunostaining. Thus, the loss of MAP2 immunoreactivity may support the diagnosis of neuronal injury in forensic types of hypoxia-ischemia, although investigations on postmortem tissue must be interpreted cautiously.

In vivo hypoxia-induced neuronal damage in dentate gyrus of rat hippocampus: changes in NMDA receptors and the effect of MK-801

Hypoxia is a major cause of ischaemia-induced neuronal damage. In the present study, we examined the effects of in vivo hypoxia on N-methyl-D-aspartate receptors (NMDAR) in the rat hippocampus. This model of in vivo hypoxia involved placing rats in a hypoxic chamber containing 5% O2 and 95% N2 for 30 min. In the hippocampus, neuronal cells in the CA3, the hilus of the dentate gyrus and the dentate gyrus (DG) were damaged. In the CA1, which is known to be vulnerable to ischaemic damage, neuronal cells did not show hypoxia-induced damage. In vivo hypoxia-induced damage caused morphological changes in neuronal cells, such as shrunken, spindle or triangular shapes accompanied by pyknotic nuclei, but did not induce the loss of neuronal cells. On the other hand, the number of binding sites for [3H]-1-[1-(2-thienyl)cyclohexyl]-3,4-piperidine hydrochloride (TCP) gradually decreased on and after 7 days, and then maximally decreased by 25% at 21 days after hypoxia. The number of NMDAR1-immunopositive cells was decreased by 22% in the DG, but was unchanged in the CA3. Furthermore, we examined the effect of a non-competitive NMDA antagonist, (+)-5-methyl-10, 11-dihydro-5H-dibenzo[a,b] cyclohepten-5,10-imine hydrogen maleate (MK-801), on against in vivo hypoxia. The administration of MK-801 (3 mg/kg, i.p.), 30 min before hypoxia treatment, partly protected against neuronal damage in the DG, but not in the CA3. These results suggest that hypoxia-induced neuronal damage in the DG involves, in part, the activation of NMDAR.

Delayed expression of c-fos protein in rat hippocampus and cerebral cortex following transient in vivo exposure to hypoxia

The time course of c-fos protein expression after hypoxia was examined in rat hippocampus and cerebral cortex using an immunohistochemical method. The rats were exposed to in vivo hypoxia for 30 min in a chamber containing 5% O2 and 95% N2. Immediately after the treatment, c-fos protein-like immunoreactivity was observed in the granule cell layer of the dentate gyrus. The change was transient, and the density of immunoreactive cells returned quickly to a control level 3 h after the exposure. However, the density of positive cells was again increased 1 day after hypoxia and reached the maximum 7 days after. In the cerebral cortex, on the other hand, no change was detected in the pattern of staining at any time, with an exception on 21 days after hypoxia. At this period, positively stained neurons were significantly increased in both density and intensity throughout the entire extent of the cerebral cortex including the cingulate gyrus. These results clearly indicate that hypoxia induces different patterns of c-fos protein expression among various regions of the brain. The biphasic pattern seen in the dentate gyrus as well as the delayed expression in the cerebral cortex may be related to delayed neuronal damages induced by hypoxia.

Changes in protein kinase C isozymes in the rat hippocampus following transient hypoxia

The effects of hypoxia on protein kinase C (PKC) isozymes (alpha, beta I, beta II, and gamma) were examined in the hippocampus from rats subjected to hypoxic conditions (5% O2 in 95% N2) for 30 min in a chamber. Western blot analysis revealed that the total amounts of PKC-alpha (-26.0% of control) and -gamma (-32.7% of control) were decreased significantly at the end of hypoxia, which was followed by the reduction of that of PKC-beta II (-23.7% of control at 7 days after hypoxia). Whereas, the PKC activities, which were measured by the incorporation of [gamma-32P] into a specific PKC substrate peptide, in both the cytosolic and the particulate fractions did not change. The reductions of PKC-gamma and -alpha at the end of hypoxia may be related to the following neuronal degeneration.