Endocrine modulation of the neurotoxicity of gp120: implications for AIDS-related dementia complex

2022 ISPNE Bruce McEwen Lifetime Achievement award: Stress, from molecules to societies

The International Society for Psychoneuroendocrinology meeting in Chicago in 2022 was thrilled to recognize Dr. Robert Sapolsky with the Bruce McEwen Lifetime Achievement award. This is the second year for the award to be named to honor Bruce McEwen and it marks the completion of a special issue edited by Blazej Miziak and Robert Paul Juster in the journal Psychoneuroendocrinology dedicated to Bruce's legacy and the unfathomable contribution of Allostatic Load to the stress field. Yet, as our award winner writes, Bruce's legacy is more than scientific as he was well known for mentorship and being an exemplary person, theorist, and scientist. Perhaps understandably for a career favored by humble introverts and shy reclusives, the science shines in the spotlight and personal reflections are cut to accommodate word count limits. For scholars entering the field, stargazing at larger than life luminaries in the field is thrilling yet intimidating as it feels impossible that these experts have the same doubts and distractions as the rest of us primates. Thus, Psychoneuroendocrinology is thrilled to kick off the first perspectives piece in the Cell to Selves series with Dr. Robert Sapolsky sharing that, like his Baboon troops in Kenya, he too sometimes has a bad-hair day. This paper is a written version of a lecture I gave on September 8th, 2022, when receiving the first Bruce McEwen Lifetime Achievement Award from the ISPNE. This was a bittersweet honor; Bruce was my graduate advisor at Rockefeller University and over the next forty years, he was my mentor, teacher and father figure. His death in 2020 left a hole in my life.

Effects of chronic HIV-1 Tat exposure in the CNS: heightened vulnerability of males versus females to changes in cell numbers, synaptic integrity, and behavior

HIV-associated damage to the central nervous system results in cognitive and motor deficits. Anti-retroviral therapies reduce the severity of symptoms, yet the proportion of patients affected has remained the same or increased. Although approximately half of HIV-infected patients worldwide are women, the question of whether biological sex influences outcomes of HIV infection has received little attention. We explored this question for both behavioral and cellular/morphologic endpoints, using a transgenic mouse that inducibly expresses HIV-1 Tat in the brain. After 3 months of HIV-1 Tat exposure, both sexes showed similar reduced open field ambulation. Male Tat⁺ mice also showed reduced forelimb grip strength and enhanced anxiety in a light–dark box assay. Tat⁺ males did not improve over 12 weeks of repeated rotarod testing, indicating a motor memory deficit. Male mice also had more cellular deficits in the striatum. Neither sex showed a change in volume or total neuron numbers. Both had equally reduced oligodendroglial populations and equivalent microglial increases. However, astrogliosis and microglial nitrosative stress were higher in males. Dendrites on medium spiny neurons in male Tat⁺ mice had fewer spines, and levels of excitatory and inhibitory pre- and post-synaptic proteins were disrupted. Our results predict sex as a determinant of HIV effects in brain. Increased behavioral deficits in males correlated with glial activation and synaptic damage, both of which are implicated in cognitive/motor impairments in patients. Tat produced by residually infected cells despite antiretroviral therapy may be an important determinant of the synaptodendritic instability and behavioral deficits accompanying chronic infection.

Human immunodeficiency virus type 1 clade B and C gp120 differentially induce neurotoxin arachidonic acid in human astrocytes: implications for neuroAIDS

HIV-1 clades (subtypes) differentially contribute to the neuropathogenesis of HIV-associated dementia (HAD) in neuroAIDS. HIV-1 envelop protein, gp120, plays a major role in neuronal function. It is not well understood how these HIV-1 clades exert these neuropathogenic differences. The N-methyl-D: -aspartate (NMDA) receptor-reduced glutamine synthesis could lead to secretion of neurotoxins such as arachidonic acid (AA) which plays a significant role in the neuropathogenic mechanisms in neuroAIDS. We hypothesize that clade B and C gp120 proteins exert differential effects on human primary astrocytes by production of the neurotoxin arachidonic acid. Our results indicate that clade B gp120 significantly downregulated NMDA receptor gene and protein expression, and level of glutamine while increasing expression of prostaglandin E2 (PGE(2)) and thromboxane A2 receptor (TBXA(2) R) compared to HIV-1 clade C gp120 protein. Thus, our studies for the first time demonstrate that HIV-1 clade B-gp120 protein appears to induce higher levels of expression of the neuropathogenic molecule cyclooxygenase-2 (COX-2)-mediated arachidonic acid by-products, PGE(2), and TBXA(2) R compared to HIV-1 clade C gp120 protein. These studies suggest that HIV-1 clade B and C gp120 proteins may play a differential role in the neuropathogenesis of HAD in neuroAIDS.

HIV neurotoxicity: potential therapeutic interventions

Individuals suffering from human immunodeficiency virus type 1 (HIV-1) infection suffer from a wide range of neurological deficits. The most pronounced are the motor and cognitive deficits observed in many patients in the latter stages of HIV infection. Gross postmortem inspection shows cortical atrophy and widespread neuronal loss. One of the more debilitating of the HIV-related syndromes is AIDS-related dementia, or HAD. Complete understanding of HIV neurotoxicity has been elusive. Both direct and indirect toxic mechanisms have been implicated in the neurotoxicity of the HIV proteins, Tat and gp120. The glutamatergic system, nitric oxide, calcium, oxidative stress, apoptosis, and microglia have all been implicated in the pathogenesis of HIV-related neuronal degeneration. The aim of this review is to summarize the most recent work and provide an overview to the current theories of HIV-related neurotoxicity and potential avenues of therapeutic interventions to prevent the neuronal loss and motor/cognitive deficits previously described.

Neighbor effects of neurons bearing protective transgenes

Viral vectors bearing protective transgenes can decrease neurotoxicity after varied necrotic insults. A neuron that dies necrotically releases glutamate, calcium and reactive oxygen species, thereby potentially damaging neighboring neurons. This raises the possibility that preventing such neuron death via gene therapy can secondarily protect neighboring neurons that, themselves, do not express a protective transgene. We determined whether such "good neighbor" effects occur, by characterizing neurons that, while uninfected themselves, are in close proximity to a transgene-bearing neuron. We tested two genes whose overexpression protects against excitotoxicity: anti-apoptotic Bcl-2, and a calcium-activated K(+) channel, SK2. Using herpes simplex virus type 2-mediated transgene delivery to hippocampal cultures, we observed "good neighbor" effects on neuronal survival following an excitotoxic insult. However, in the absence of insult, "bad neighbor" effects could also occur (i.e., where being in proximity to a neuron constitutively expressing one of those transgenes is deleterious). We also characterized the necessity for cell-cell contact for these effects. These phenomena may have broad implications for the efficacy of gene overexpression strategies in the CNS.

Evidence for caspase effects on release of cytochrome c and AIF in a model of ischemia in cortical neurons

Neuronal apoptosis following ischemia can be mediated by a caspase-dependent pathway, which involves the mitochondrial release of cytochrome c that initiates a cascade of caspase activation. In addition, there is a caspase-independent pathway, which is mediated by the release of apoptosis-inducing factor (AIF). Using caspase inhibitor gene therapy, we investigated the roles of caspases on the mitochondrial release of cyt c and the release of AIF. Specifically, we used herpes simplex virus-1 amplicon vectors to ectopically express a viral caspase inhibitor (crmA or p35) in mixed cortical cultures exposed to oxygen/glucose deprivation. Overexpression of either crmA or p35 (but not the caspase-3 inhibitor DEVD) inhibited the release of AIF; this suggests that there can be cross-talk between the caspase-dependent and the ostensibly caspase-independent pathway. In addition, both crmA overexpression and DEVD inhibited cyt c release, suggesting a positive feedback loop involving activated caspases stimulating cyt c release.

NAP protects against cytochrome c release: inhibition of the initiation of apoptosis

NAPVSIPQ (NAP), an 8 amino acid peptide derived from activity-dependent neuroprotective protein (ADNP), provides neuroprotection through interaction with microtubules. Previous results have demonstrated NAP protection against oxygen-glucose deprivation in hippocampal cells in culture. Furthermore, in vivo studies have shown that NAP reduces caspase 3 activation in rats subjected to permanent mid-cerebral artery occlusion (a rat model of stroke). Oxygen-glucose deprivation (ischemia) has been associated with microtubule breakdown and cytochrome c release from mitochondria leading to apoptosis. Here, NAP in concentrations ranging from 10(-14)M to 10(-8)M completely blocked cytochrome c release in cortical neurons subjected to oxygen-glucose deprivation. Furthermore, quantitative microscopy coupled to microtubule immunocytochemistry suggested that NAP prevented microtubule degradation under oxidative stress. As cytochrome c release is a known initiator of the apoptotic pathway, it is suggested that NAP inhibits the early events of apoptosis.

The impact of environmental risk factors on HIV-associated cognitive decline in children

Both the human immunodeficiency virus (HIV) and environmental stress have been independently associated with decreased cognitive functioning in children. Given that they are also known to have a strong relationship with each other, the present study sought to test the hypothesis that children in conditions of high environmental risk would be at greater risk for the cognitive complications related to immunosuppression. A retrospective review was conducted to examine the records of 141 children treated at a large pediatric AIDS clinic from 1993 to 2000. CD4+ lymphocyte levels were recorded from laboratory results and IQ scores were recorded from routine psychological evaluations. Key indicators of environmental risk were collected and combined into one measure of overall environmental risk. Pearson product moment correlations were conducted to examine the relationship between environmental risk, age-adjusted CD4 and IQ. Results indicated a significant correlation between CD4 and IQ, with higher levels of immunocompetence predicting higher IQ scores. When subjects were dichotomized based on their environmental risk score, there was no relationship between CD4 count and IQ in the low environmental risk group. In contrast, CD4 was positively associated with IQ in the high environmental risk group. It is proposed that this may be due to gp120 levels in immunocompromised children being particularly toxic to the hippocampus and cortex under conditions of high stress but not so under conditions of low stress.

Myocardial disease in human immunodeficiency virus (HIV) infection: a review

Heart muscle disease is the most important cardiovascular manifestation of HIV infection and is likely to become even more prevalent as HIV infected patients live longer. This may present as myocarditis, dilated cardiomyopathy or isolated left or right ventricular dysfunction. Myocardial involvement in HIV infection is multifactorial and may arise as a result of myocardial invasion with HIV itself, opportunistic infections, viral infections, autoimmune response to viral infection, drug-related cardiac toxicity, nutritional deficiencies, and prolonged immunosuppression. Both adults and children are affected with severity ranging from incidental microscopic inflammatory findings at autopsy to clinically significant cardiac disease with chronic cardiac dysfunction. It is associated with a poor prognosis, and results in symptomatic heart failure in up to 5% of HIV patients. Clinical pathological studies from the pre-HAART era show a 30% prevalence of cardiomyopathy in patients with AIDS. The introduction of highly active antiretroviral therapy (HAART) regimens has substantially modified the course of HIV disease by lengthening survival and improving quality of life of HIV-infected patients. There is also good evidence that HAART significantly reduces the incidence of cardiovascular manifestations of HIV infection. By preventing opportunistic infections and reducing the incidence of myocarditis, HAART regimens have reduced the prevalence of HIV-associated cardiomyopathy by almost 7-fold from the pre-HAART era. HAART is however only available to a minority of HIV infected individuals in most areas of the world and studies from the pre-HAART period still apply. In this review, the aetiopathogenesis and presentation of HIV related myocardial disease were reviewed and measures taken to improve survival discussed.

Anti-apoptotic therapy with a Tat fusion protein protects against excitotoxic insults in vitro and in vivo

A number of gene therapy approaches have been developed for protecting neurons from necrotic neurological insults. Such therapies are limited by the need for transcription and translation of the protective protein, delaying therapeutic impact. As an alternative, we explore the neuroprotective potential of protein therapy, using a fusion protein comprised of the death-suppressing BH4 domain of the Bcl-xL protein and the protein transduction domain of the human immunodeficiency virus Tat protein. This fusion protein decreased neurotoxicity caused by the excitotoxins glutamate and kainic acid in primary hippocampal cultures, and decreased hippocampal damage in vivo in an excitotoxic seizure model.

Oxidative stress and protein oxidation in the brain of water drinking and alcohol drinking rats administered the HIV envelope protein, gp120

Possible roles of oxidative stress and protein oxidation on alcohol-induced augmentation of cerebral neuropathy in gp120 administered alcohol preferring rats drinking either pure water (W rats) or a free-choice ethanol and water (E rats) for 90 days. This study showed that peripherally administered gp120 accumulated into the brain, liver, and RBCs samples from water drinking - gp120 administered rats (Wg rats) and ethanol drinking - gp120 administered rats (Eg rats), although gp120 levels in samples from Eg rats were significantly greater than the levels in samples from Wg rats. The brain samples from ethanol drinking-saline administered (EC) and Wg rats exhibited comparable levels of free radicals that were significantly lower than the levels in Eg rats. Peroxiredoxin-I (PrxI) activity in the brain samples exhibited the following pattern: Wg >> >> WC >> EC > Eg. Total protein-carbonyl and carbonylated hippocampal cholinergic neurostimulating peptide precursor protein levels, but not N-acetylaspartate or N-acetyl aspartylglutamate or total protein-thiol levels, paralleled the free radical levels in the brain of all four groups. This suggests PrxI inhibition may be more sensitive indicator of oxidative stress than measuring free radicals or metabolites. As PrxI oxidation in WC, Wg, and EC rats was reversible, while PrxI oxidation in Eg rats was not, we suggest that alcohol drinking and gp120 together hyperoxidized and inactivated PrxI that suppressed free radical neutralization in the brain of Eg rats. In conclusion, chronic alcohol drinking, by carbonylating and hyperoxidizing free radical neutralization proteins, augmented the gp120-induced oxidative stress that may be associated with an increase in severity of the brain neuropathy.

Mineralocorticoid receptor overexpression differentially modulates specific phases of spatial and nonspatial memory

Glucocorticoids (GCs) and stress modulate specific phases of information processing. The modulatory affects of GCs on hippocampal function are thought to be mediated by the mineralocorticoid receptor (MR) and the glucocorticoid receptor (GR). The GR plays a critical role in mediating the impairing effects of GCs on hippocampal function. Conversely, activation of MR facilitates hippocampal function. The high affinity of MR for GCs suggests that the receptor protein levels play a key role in regulating the beneficial effects of MR-mediated gene transcription. Using herpes simplex vectors, we transiently increased MR levels in dentate gyrus granule cells, which in turn enhanced MR signaling. We then examined its effects on spatial and nonspatial memory consolidation and retrieval using the object placement and object recognition task. Additionally, we assessed whether an increased MR signal could block the impairing effects of high GCs on memory retrieval. Rats overexpressing MR displayed an enhancement in the consolidation of nonspatial memory relative to rats expressing green fluorescent protein and suggest the potential for gene transfer techniques for enhancing cognition during stress. Moreover, rats overexpressing MR were spared from the disruptive effects of high GCs on the retrieval of nonspatial memory. Thus, this study illustrates the critical role of MR in mediating the retrieval and consolidation of nonspatial memory.

Antioxidant enzyme gene delivery to protect from HIV-1 gp120-induced neuronal apoptosis

Human immunodeficiency virus-1 (HIV-1) infection in the central nervous system (CNS) may lead to neuronal loss and progressively deteriorating CNS function: HIV-1 gene products, especially gp120, induce free radical-mediated apoptosis. Reactive oxygen species (ROS), are among the potential mediators of these effects. Neurons readily form ROS after gp120 exposure, and so might be protected from ROS-mediated injury by antioxidant enzymes such as Cu/Zn-superoxide dismutase (SOD1) and/or glutathione peroxidase (GPx1). Both enzymes detoxify oxygen free radicals. As they are highly efficient gene delivery vehicles for neurons, recombinant SV40-derived vectors were used for these studies. Cultured mature neurons derived from NT2 cells and primary fetal neurons were transduced with rSV40 vectors carrying human SOD1 and/or GPx1 cDNAs, then exposed to gp120. Apoptosis was measured by terminal deoxynucleotidyl transferase-mediated nick end labeling (TUNEL) assay. Transduction efficiency of both neuron populations was >95%, as assayed by immunostaining. Transgene expression was also ascertained by Western blotting and direct assays of enzyme activity. Gp120 induced apoptosis in a high percentage of unprotected NT2-N. Transduction with SV(SOD1) and SV(GPx1) before gp120 challenge reduced neuronal apoptosis by >90%. Even greater protection was seen in cells treated with both vectors in sequence. Given singly or in combination, they protect neuronal cells from HIV-1-gp120 induced apoptosis. We tested whether rSV40 s can deliver antioxidant enzymes to the CNS in vivo: intracerebral injection of SV(SOD1) or SV(GPx1) into the caudate putamen of rat brain yielded excellent transgene expression in neurons. In vivo transduction using SV(SOD1) also protected neurons from subsequent gp120-induced apoptosis after injection of both into the caudate putamen of rat brain. Thus, SOD1 and GPx1 can be delivered by SV40 vectors in vitro or in vivo. This approach may merit consideration for therapies in HIV-1-induced encephalopathy.

Gene therapy in the nervous system with superoxide dismutase

Neuronal death following necrotic insults involves the generation of reactive oxygen species (ROS). We investigated the effects of antioxidant gene therapy on ROS accumulation after exposure to either sodium cyanide, kainic acid or oxygen glucose deprivation (OGD). Specifically, we generated herpes simplex virus-1 amplicon vector expressing the gene for the antioxidant enzyme CuZnSOD. Overexpression of this gene in primary hippocampal cultures resulted in increased enzymatic activity of the corresponding protein. CuZnSOD significantly protected hippocampal neurons against sodium cyanide insult and the subsequent lipid peroxidation. However, it did not protect against OGD- or kainic-acid-induced toxicity. Moreover, CuZnSOD significantly worsened the toxicity, hydrogen peroxide accumulation and lipid peroxidation induced by kainic acid. As a possible explanation for this surprising worsening, CuZnSOD overexpression increased glutathione peroxidase activity in the presence of sodium cyanide but had no effect on catalase or glutathione peroxidase activity in the presence of kainic acid. Thus, cells were unlikely to be able to detoxify the excess hydrogen peroxide produced as a result of the CuZnSOD overexpression. These studies can be viewed as a cautionary note concerning gene therapy intervention against necrotic insults.

A 24 h corticosterone exposure exacerbates excitotoxic insult in rat hippocampal slice cultures independently of glucocorticoid receptor activation or protein synthesis

Elevations in circulating concentrations of glucocorticoids (GC) may increase the expression and/or sensitivity of ionotropic transmitter receptors in brain. For example, recent evidence suggests that acute and chronic GC exposure may alter the number and/or function of N-methyl-D-aspartate (NMDA)-type glutamate receptors, effects that may sensitize the brain to excitotoxic insults. The present studies examined the ability of short-term (24 h) corticosterone (CORT) exposure to potentiate NMDA-induced cytotoxicity in rat hippocampal slice cultures. Additional studies evaluated the role of mineralocorticoid (MR) and glucocorticoid receptor (GR) function, as well as de novo protein synthesis, in potentiation of toxicity by corticosterone exposure. Hippocampal slice cultures were exposed to NMDA (20 microM) for 24 h with cytotoxicity assessed by fluorescent detection of propidium iodide uptake. Exposure to NMDA caused significant propidium iodide uptake in each hippocampal region, while 24 h CORT (0.001-1 microM) exposure alone did not significantly increase propidium iodide uptake. Co-exposure of cultures to CORT and NMDA synergistically increased propidium iodide uptake in each hippocampal region, effects that were prevented by co-exposure to a non-toxic concentration of MK-801 (20 microM). In contrast, 24 h exposure with the MR antagonist spironolactone (1-10 microM), the GR antagonist RU-486 (1-10 microM), or the protein synthesis inhibitor cycloheximide (1 microM) failed to reduce the significant increase in propidium iodide uptake. These data suggest that relatively brief elevations in CORT levels may sensitize the hippocampus to injury independently of GC receptor activity and protein synthesis.

17beta-estradiol reduces neuronal apoptosis induced by HIV-1 gp120 in the neocortex of rat

The human immunodeficiency virus type 1 (HIV-1) coat glycoprotein gp120 represents a likely contributor to the development of HIV-1 associated dementia (HAD), a neurological syndrome often observed in AIDS patients and characterised by significant neuronal loss in the neocortex. Since recent studies have highlighted that female sex hormones represent potential neuroprotective agents against damage produced by acute and chronic injuries in the adult brain, we have investigated whether estrogens exert protection in a rat model of gp120 neurotoxicity. Our results demonstrate that systemic administration of 17beta-estradiol (E2, 0.02-0.2 mg/kg) significantly reduces apoptotic cell death observed in the neocortex of rat following subchronic i.c.v. administration of gp120 (100 ng/rat/day). Furthermore, both tamoxifen and ICI182,780, two selective antagonists of estrogen receptors (ER) in the brain, reverted the neuroprotective effect of E2. The molecular mechanism of estrogenic neuroprotection does not appear to involve modulation of the antiapoptotic Bcl-2 or the proapoptotic Bax since we failed to observe changes in the levels of the two proteins in the neocortical tissue after gp120 and/or E2 treatment. However, we detected increased levels of IL-1beta in the neocortex of rats injected with gp120, as early as 6h after drug administration, and this effect was potentiated following pretreatment with E2. Taken together, our results demonstrate that E2 exerts neuroprotection against gp120 neurotoxicity in vivo through a mechanism involving ER activation and, possibly, via modulation of neocortical levels of IL-1beta.

17beta-estradiol protects SH-SY5Y Cells against HIV-1 gp120-induced cell death: evidence for a role of estrogen receptors

Despite the large body of experimental evidence demonstrating the neuroprotective properties of 17beta-estradiol (17beta-E2) both in vitro and in vivo experimental models of neuronal injury, the exact mechanisms implicated in neuroprotection have not been fully delineated. Some experimental evidence highlight a role for the antioxidant properties of 17beta-E2 in mediating protection against oxidative injury. Parallel to these, evidence also exist which point to alternative mechanisms involving estrogen receptors (ER). The HIV-1 coat protein, gp120, has been implicated in the progression of central nervous system damage caused by HIV-1 infection. The neurotoxic effects induced by gp120 are triggered via an excitotoxic mechanism of cell death which implicates alteration of calcium homeostasis, activation of calcium-dependent pathways, mitochondrial uncoupling and membrane lipid peroxidation. In the present study, we demonstrate that 17beta-E2 protects human SH-SY5Y neuroblastoma cells from cell death elicited by gp120. Tamoxifen and ICI 182,780, two ER antagonists, both antagonized 17beta-E2-mediated inhibition of cell death. Exposure of SH-SY5Y cells to gp120 for 30min caused a significant accumulation of intracellular reactive oxygen species (ROS) and this was abrogated by 17beta-E2; however, the ability of 17beta-E2 to counteract ROS generation induced by gp120 does not account for the reported prevention of cell death because ICI 182,780 failed to revert intracellular ROS reduction caused by 17beta-E2 though it was able to revert prevention of cell death. Furthermore, by using 17alpha-E2, the isomer unable to stimulate ER which, however, retains the antioxidant effects, we observed that a pre-treatment with 17alpha-E2 was effective in preventing gp120-induced accumulation of ROS but it failed to affect cell death caused by the viral protein. Collectively, these data demonstrate that neuroprotection afforded by 17beta-E2 is receptor-mediated and ROS scavenging effects may not be implicated.

Marked differences in the efficacy of post-insult gene therapy with catalase versus glutathione peroxidase

It is now recognized that the generation of reactive oxygen species (ROS) following necrotic neurological insults plays a central role in the subsequent neuron death. A key step in ROS detoxification is the conversion of hydrogen peroxide to water and oxygen by either catalase (CAT) or glutathione peroxidase (GPX). We have previously shown that overexpression of CAT or GPX protects cultured neurons against subsequent excitotoxic insults. Because of the unpredictability of most acute neurological insults, gene therapy will most often need to be carried out after rather than in anticipation of an insult. Thus, we have tested whether herpes virus amplicon vectors expressing CAT or GPX still protect cultured hippocampal neurons from oxygen/glucose deprivation if introduced following an insult. CAT-expressing vectors were protective even when introduced 8 h post-insult. In contrast, there was no post-insult time window in which GPX overexpression protected. While CAT requires no cofactor, GPX action requires glutathione as a cofactor. Thus, we speculated that the post-insult decline in glutathione compromises the protective potential of GPX. Supporting this, reversing the post-insult glutathione decline with glutathione supplementation was neuroprotective.

From clinical evidence to molecular mechanisms underlying neuroprotection afforded by estrogens

Recent studies have highlighted that female sex hormones represent potential neuroprotective agents against damage produced by acute and chronic injuries in the adult brain. Clinical reports have documented the effectiveness of estrogens to attenuate symptoms associated with Parkinson's disease, and to reduce the risk of Alzheimer's disease and cerebrovascular stroke. This evidence is corroborated by numerous experimental studies documenting the protective role of female sex hormones both in vitro and in vivo. Accordingly, estrogens have been shown to promote survival and differentiation of several neuronal populations maintained in culture, and to reduce cell death associated with excitotoxicity, oxidative stress, serum deprivation or exposure to beta-amyloid. The neuroprotective effects of estrogens have been widely documented in animal models of neurological disorders, such as Alzheimer's and Parkinson's diseases, as well as cerebral ischemia. Although estrogens are known to exert several direct effects on neurones, the cellular and molecular mechanisms implicated in their protective actions on the brain are not completely understood. Thus, on the basis of clinical and experimental evidence, in this review, we discuss recent findings concerning the neuronal effects of estrogens that may contribute to their neuroprotective actions. Both estrogen receptor-dependent and -independent mechanisms will be described. These include modulation of cell death regulators, such as Bcl-2, Akt and calpain, as well as interaction with growth factors, such as BDNF, NGF, IGF-I and their receptors. The anti-inflammatory effects of estrogens will also be described, namely their ability to reduce brain levels of inflammatory mediators, cytokines and chemokines. Finally, a brief overview about receptor-independent mechanisms of neuroprotection will aim at describing the antioxidant effects of estrogens, as well as their ability to modulate neurotransmission.

Glucocorticoids worsen excitotoxin-induced expression of pro-inflammatory cytokines in hippocampal cultures

Glucocorticoids (GCs), the adrenal steroid hormones released during stress, have well-known anti-inflammatory actions. Despite that, there is increasing evidence that GCs are not uniformly anti-inflammatory in the injured nervous system and, in fact, can be pro-inflammatory. The present report continues this theme. Primary hippocampal cultures were treated with GC concentrations approximating basal, acute (1 h) stress or chronic (24 h) stress conditions and were then exposed to the excitotoxin kainic acid (KA). KA induced expression of the pro-inflammatory cytokines IL-1 beta and TNF-alpha, and chronic high dose GC exposure excacerbated this induction. In a second study, cultures were exposed to the physiological range of GC concentrations for 24 h prior to KA treatment. Low- to mid-range GC concentrations were anti-inflammatory, decreasing expression of IL-1 beta and TNF-alpha, while the highest GC doses either failed to be anti-inflammatory or even potentiated expression further. These findings add to the growing picture of these classically anti-inflammatory hormones potentially having pro-inflammatory effects in the injured CNS.

Estrogenic protection against gp120 neurotoxicity: Role of microglia

HIV infection of the nervous system can cause neurotoxicity, and the glycoprotein gp120 of HIV seems to play a key role in this. gp120 is neurotoxic through a multi-cellular pathway, stimulating microglia to release excitotoxins, cytokines and reactive oxygen species, which then damage neurons. We have previously shown that estrogen decreases the neurotoxicity of gp120 in mixed neuronal/glial cultures. In this study, we determine whether estrogen a) decreases the collective neurotoxicity of the factors released by gp120-treated microglia, and/or b) enhances the ability of neurons to survive such factors. To do so, we established microglial cultures, mixed neuronal/glial hippocampal cultures, and neuron-enriched cultures, independently manipulating gp120 and estrogen exposure in each type of culture, and inducing neurotoxicity in neuron-containing cultures by introducing conditioned media from gp120-treated microglial cultures. We observe that estrogen can exert some small protective effects at the level of bolstering neuronal resistance, but that the bulk of its protective effects arise at the level of decreasing the neurotoxicity of factors released by microglia.

Corticosterone Increases Damage and Cytosolic Calcium Accumulation Associated With Ethanol Withdrawal in Rat Hippocampal Slice Cultures

BACKGROUND

Evidence suggests that stress hormones (i.e., glucocorticoids) may be increased during acute or chronic consumption of ethanol and during withdrawal from ethanol consumption, effects that may contribute to the development of cognitive impairment. The goal of the current studies was to examine the hypothesis that increased glucocorticoid levels in conjunction with ethanol exposure and withdrawal may cause hippocampal damage.

METHODS

Organotypic hippocampal slice cultures were exposed to 50 mM ethanol for 10 days and withdrawn for 1 day. After withdrawal, cytotoxicity and cytosolic Ca2+ accumulation were measured using the nucleic acid stain propidium iodide and Calcium Orange, AM, respectively. Cultures were also treated with nontoxic concentrations of corticosterone (0.001-1 microM) during ethanol exposure and withdrawal or only during withdrawal. Additional cultures were coexposed to corticosterone and RU486 (0.1-10.0 microM), spironolactone (0.1-10.0 microM), or MK-801 (20 microM) during ethanol exposure and/or withdrawal.

RESULTS

Ethanol withdrawal did not increase propidium iodide fluorescence and cytosolic Ca2+ levels. However, significant increases in propidium iodide fluorescence and in cytosolic Ca2+ accumulation were observed in cultures when corticosterone (> or = 100 nM) was exposed during ethanol treatment and/or withdrawal. These effects of corticosterone on ethanol withdrawal were attenuated by RU486 and MK-801 but not by spironolactone coexposure.

CONCLUSIONS

This report demonstrated that corticosterone exposure during ethanol treatment and/or withdrawal resulted in significant hippocampal damage, possibly via activation of glucocorticoid receptors and enhancement of the glutamatergic cascade. The findings from these studies suggest that glucocorticoids contribute to the neuropathological consequences of alcohol dependence in humans.

Dual-gene, dual-cell type therapy against an excitotoxic insult by bolstering neuroenergetics

Increasing evidence suggests that glutamate activates the generation of lactate from glucose in astrocytes; this lactate is shuttled to neurons that use it as a preferential energy source. We explore this multicellular "lactate shuttle" with a novel dual-cell, dual-gene therapy approach and determine the neuroprotective potential of enhancing this shuttle. Viral vector-driven overexpression of a glucose transporter in glia enhanced glucose uptake, lactate efflux, and the glial capacity to protect neurons from excitotoxicity. In parallel, overexpression of a lactate transporter in neurons enhanced lactate uptake and neuronal resistance to excitotoxicity. Finally, overexpression of both transgenes in the respective cell types provided more protection than either therapy alone, demonstrating that a dual-cell, dual-gene therapy approach gives greater neuroprotection than the conventional single-cell, single-gene strategy.

Over-expression of antioxidant enzymes protects cultured hippocampal and cortical neurons from necrotic insults

There is now considerable knowledge concerning neuron death following necrotic insults, and it is believed that the generation of reactive oxygen species (ROS) and oxidative damage play a pivotal role in the neuron death. Prompted by this, we have generated herpes simplex virus-1 amplicon vectors over-expressing the genes for the antioxidant enzymes catalase (CAT) or glutathione peroxidase (GPX), both of which catalyze the degradation of hydrogen peroxide. Over-expression of each of these genes in primary hippocampal or cortical cultures resulted in increased enzymatic activity of the cognate protein. Moreover, each enzyme potently decreased the neurotoxicity induced by kainic acid, glutamate, sodium cyanide and oxygen/glucose deprivation. Finally, these protective effects were accompanied by parallel decreases in hydrogen peroxide accumulation and the extent of lipid peroxidation. These studies not only underline the key role played by ROS in the neurotoxicity of necrotic insults, but also suggest potential gene therapy approaches.

Decreased iNOS synthesis mediates dexamethasone-induced protection of neurons from inflammatory injury in vitro

Brain inflammation is accompanied by transection of axons and death of neurons in the acute lesions of multiple sclerosis. We explored mechanisms of inflammatory damage to neurons in vitro using cocultures of rat embryonal cortical neurons with microglia activated by interferon-gamma (IFNgamma) and lipopolysaccharide (LPS). Previously, we have demonstrated that microglia are highly toxic to neurons and that nitric oxide (NO) derived from inducible nitric oxide synthase (iNOS) is necessary and sufficient to mediate this toxicity. Here, we show that addition of dexamethasone (1 micro M) to activated cocultures provides effective neuroprotection. We demonstrate that dexamethasone down-regulates NO production of primary microglia by approximately 50% and reduces steady-state iNOS protein and mRNA expression by approximately 70%. These changes were reversed by the glucocorticoid receptor blocker RU-486. Furthermore, we analysed the stability of iNOS protein and show that whilst inhibitors of the proteasome blocked iNOS degradation they did not reverse the dexamethasone effect. Our results indicate that the main mechanism of corticosteroid activity on iNOS is reduction in protein synthesis, not destabilization as previously suggested.

(−)-nicotine ameliorates corticosterone's potentiation of N-methyl-d-aspartate receptor-mediated cornu ammonis 1 toxicity

Hypercortisolemia, long-term exposure of the brain to high concentrations of stress hormones (i.e. cortisol), may occur in patients suffering from depression, alcoholism, and other disorders. This has been suggested to produce neuropathological effects, in part, via increased function or sensitivity of N-methyl-d-aspartate (NMDA)-type glutamate receptors. Given that cigarette smoking is highly prevalent in some of these patient groups and nicotine has been shown to reduce toxic consequences of NMDA receptor function, it may be suggested that nicotine intake may attenuate the neurotoxic effects of hypercortisolemia. To investigate this possibility, organotypic hippocampal slice cultures derived from rat were pre-treated with corticosterone (0.001-1 microM) alone or in combination with selective glucocorticoid receptor antagonists for 72-h prior to a brief (1-h) NMDA exposure (5 microM). Pre-treatment with corticosterone (0.001-1 microM) alone did not cause hippocampal damage, while NMDA exposure produced significant cellular damage in the cornu ammonis (CA)1 subregion. No significant damage was observed in the dentate gyrus or CA3 regions following NMDA exposure. Pre-treatment of cultures with corticosterone (0.1-1 microM) markedly exacerbated NMDA-induced CA1 and dentate gyrus region damage. This effect in the CA1 region was prevented by co-administration of the glucocorticoid receptor antagonist RU486 (>or=1 microM), but not spironolactone (1-10 microM), a mineralocorticoid receptor antagonist. In a second series of studies, both acute and pre-exposure of cultures to (-)-nicotine (1-10 microM) significantly reduced NMDA toxicity in the CA1 region. Co-administration of cultures to (-)-nicotine (1-10 microM) with 100 nM corticosterone prevented corticosterone's exacerbation of subsequent CA1 insult. This protective effect of (-)-nicotine was not altered by co-exposure of cultures to 10 microM dihydro-beta-erythroidine but was blocked by co-exposure to 100 nM methyllycaconitine, suggesting the involvement of nicotinic acetylcholine receptors possessing the alpha7* subunit. The present studies suggest a role for hypercortisolemia in sensitizing the hippocampal NMDA receptor system to pathological activation and indicate that prolonged nicotine exposure attenuates this sensitization. Thus, it is possible that one consequence of heavy smoking in those suffering from hypercortisolemia may be a reduction of neuronal injury and sparing of cellular function.

p38 MAP kinase-mediated negative inotropic effect of HIV gp120 on cardiac myocytes

Myocardial dysfunction leading to dilated cardiomyopathy has been documented with surprisingly high frequency in human immunodeficiency virus (HIV)-infected individuals. p38 MAP kinase has been implicated as a mediator of myocardial dysfunction. We previously reported p38 MAP kinase activation by the HIV coat protein gp120 in neonatal rat cardiac myocytes. We now report the direct inotropic effects of HIV gp120 on adult rat ventricular myocytes (ARVM). ARVM were continuously superfused with gp120, and percent fractional shortening (FS) was determined by automated border detection and simultaneous intracellular ionized free Ca2+concentration ([Ca2+]i) measured by fura 2-AM fluorescence: gp120 alone increased FS and increased [Ca2+]iwithin 5 min and then depressed FS without a decrease in [Ca2+]iby 20–60 min, which persisted for at least 2 h. Exposure of ARVM to gp120 also resulted in the phosphorylation of the upstream regulator of p38 MAP kinase MKK3/6, p38 MAP kinase itself, and its downstream effector, ATF-2, over a similar time course. ERK (p44/42) and JNK stress signaling pathways were not similarly activated. The effects of the p38 MAP kinase inhibitor were concentration dependent. SB-203580 (10 μM) blocked both p38 MAP kinase phosphorylation and the delayed negative inotropic effect of gp120. SB-203580 (5 μM) selectively blocked phosphorylation of ATF-2 without blocking the phosphorylation of MKK3/6 or p38 MAP kinase itself. SB-203580 (5 μM) administered before, with, or after gp120 blocked the negative inotropic effect of gp120 in ARVM. p38 MAP kinase activation may be a common stress-response mechanism contributing to myocardial dysfunction in HIV and other nonischemic as well as ischemic cardiomyopathies.

Potassium channel gene therapy can prevent neuron death resulting from necrotic and apoptotic insults

Necrotic insults such as seizure are excitotoxic. Logically, membrane hyperpolarization by increasing outwardly conducting potassium channel currents should attenuate hyperexcitation and enhance neuron survival. Therefore, we overexpressed a small-conductance calcium-activated (SK2) or voltage-gated (Kv1.1) channel via viral vectors in cultured hippocampal neurons. We found that SK2 or Kv1.1 protected not only against kainate or glutamate excitotoxicity but also increased survival after sodium cyanide or staurosporine. In vivo overexpression of either channel in dentate gyrus reduced kainate-induced CA3 lesions. In hippocampal slices, the kainate-induced increase in granule cell excitability was reduced by overexpression of either channel, suggesting that these channels exert their protective effects during hyperexcitation. It is also important to understand any functional disturbances created by transgene overexpression alone. In the absence of insult, overexpression of Kv1.1, but not SK2, reduced baseline excitability in dentate gyrus granule cells. Furthermore, while no behavioral disturbances during spatial acquisition in the Morris water maze were observed with overexpression of either channel, animals overexpressing SK2, but not Kv1.1, exhibited a memory deficit post-training. This difference raises the possibility that the means by which these channel subtypes protect may differ. With further development, potassium channel vectors may be an effective pre-emptive strategy against necrotic insults.

gp120 neurotoxicity fails to induce heat shock defenses, while the over expression of hsp70 protects against gp120

gp120, the coat glycoprotein of HIV, can damage CNS neurons. This appears to mostly involve an indirect pathway in which gp120 infects microglia, triggering the release of cytokines and glutamatergic excitotoxins which then damage neurons. A well-characterized response of cells to insults is to mobilize the heat stress response, a defense that has a number of protective consequences. We tested the capacity of gp120, at a dose well-documented to be neurotoxic, to activate the heat shock response in cultures from cortex and hippocampus, two brain regions sensitive to the neurotoxic effects of gp120. We found that gp120 failed to induce expression of hsp70, hsp25 or hsp90 in cortical or hippocampal cultures, under conditions where induction can be demonstrated in response to other insults. The failure of gp120 to induce a heat shock response is significant because we subsequently demonstrated that such an induction would have been beneficial. Specifically, over expression of hsp70 with a herpes viral amplicon vector protected cultured hippocampal neurons from gp120 neurotoxicity.

Effects of glucocorticoids in the gp120-induced inhibition of glutamate uptake in hippocampal cultures

Studies examining the development of AIDS Related Dementia have concentrated on neurotoxic properties of the HIV viral coat protein, gp120. We have previously shown that this neurotoxicity can be exacerbated by glucocorticoids (GCs), the stress hormones secreted by the adrenal. Moreover, GCs also worsen several of the mechanisms mediating gp120 neurotoxicity, such as increased calcium flux, ROS generation, and energy depletion. Gp120 interferes with the reuptake of glutamate in glia cultures, another possible mechanism by which it can be neurotoxic. This paper examines the role of GCs in exacerbating this phenomenon. It was found that while GCs do not exacerbate the decrease in reuptake of glutamate in glia cultures, they do enhance the decrease in mixed neuronal cultures and this latter effect appears to be energy-dependent.

Protection against gp120-induced neurotoxicity by an array of estrogenic steroids

gp120, the coat protein of HIV, can be neurotoxic and is thought to contribute to AIDS-related dementia complex. Such toxicity involves activation of glutamate receptors, mobilization of free cytosolic calcium, and generation of oxygen radicals. We have previously shown that the estrogen 17beta-estradiol, in concentrations of 100 nM or higher, lessens the neurotoxicity of gp120 in hippocampal and cortical cultures, blunts gp120-induced calcium mobilization, and lessens the oxidative consequences. In this study, we examined the protective potential of other estrogens. We found gp120 neurotoxicity in hippocampal cultures to be significantly lessened by estrone, equilin and estriol, although with an order of magnitude less potent than 17beta-estradiol. We also found all four estrogens to blunt gp120-induced calcium mobilization, with estriol being more efficacious than the other three estrogens. These findings give insight both into the mechanisms of estrogenic protection (e.g. receptor-dependent versus independent actions) as well as into the potential therapeutic use of estrogens against AIDS-related dementia complex.

The anti-inflammatory activity of estrogen in glial cells is regulated by the PKC-anchoring protein RACK-1

It has recently been suggested that estrogen inhibits glial activation and the release of neurotoxic mediators. The mechanisms involved in this anti-inflammatory effect are unclear. We found that an nM concentration of 17-beta estradiol inhibits protein kinaseC-betaII translocation induced by lipopolysaccharide in primary astrocytes. Estradiol treatment did not change the total content of kinaseC-betaII or of lipopolysaccharide receptor, but dose-dependently reduced the levels of receptors for activated C kinases-1 (RACK-1), the anchoring protein involved in protein kinase C (PKC) shuttling. This decrease could thus account for the defective protein kinaseC-betaII activation. Pre-treatment with 1 nmbeta-estradiol, which reduced by approximately 35% the expression of RACK-1, prevented the lipopolysaccharide-induced expression of tumour necrosis factor-alpha mRNA and of the inducible form of nitric oxide (NO) synthase. As a consequence, the production of tumour necrosis factor-alpha and NO were decreased. An antisense oligonucleotide for RACK-1 also reduced tumour necrosis factor-alpha and nitric oxide production on lipopolysaccharide stimulation. These results demonstrate that estrogen reduction of the RACK-1 expression, leading to a defective protein kinase-C activation counteracts the inflammatory response in astrocytes.

Neuroprotective effects of bcl-2 overexpression in hippocampal cultures: interactions with pathways of oxidative damage

Overexpression of bcl-2protects neurons from numerous necrotic insults, both in vitro and in vivo. While the bulk of such protection is thought to arise from Bcl-2 blocking cytochrome c release from mitochondria, thereby blocking apoptosis, the protein can target other steps in apoptosis, and can protect against necrotic cell death as well. There is evidence that these additional actions may be antioxidant in nature, in that Bcl-2 has been reported to protect against generators of reactive oxygen species (ROS), to increase antioxidant defenses and to decrease levels of ROS and of oxidative damage. Despite this, there are also reports arguing against either the occurrence, or the importance of these antioxidant actions. We have examined these issues in neuron-enriched primary hippocampal cultures, with overexpression of bcl-2 driven by a herpes simplex virus amplicon: (i) Bcl-2 protected strongly against glutamate, whose toxicity is at least partially ROS-dependent. Such protection involved reduction in mitochondrially derived superoxide. Despite that, Bcl-2 had no effect on levels of lipid peroxidation, which is thought to be the primary locus of glutamate-induced oxidative damage; (ii) Bcl-2 was also mildly protective against the pro-oxidant adriamycin. However, it did so without reducing levels of superoxide, hydrogen peroxide or lipid peroxidation; (iii) Bcl-2 protected against permanent anoxia, an insult likely to involve little to no ROS generation. These findings suggest that Bcl-2 can have antioxidant actions that may nonetheless not be central to its protective effects, can protect against an ROS generator without targeting steps specific to oxidative biochemistry, and can protect in the absence of ROS generation. Thus, the antioxidant actions of Bcl-2 are neither necessary nor sufficient to explain its protective actions against these insults in hippocampal neurons.

17beta-estradiol enhances cortical cholinergic innervation and preserves synaptic density following excitotoxic lesions to the rat nucleus basalis magnocellularis

Estradiol exerts beneficial effects on neurodegenerative disorders associated with the decline of cognitive performance. The present study was designed to further investigate the effect of 17beta-estradiol on learning and memory, and to evaluate its neuroprotective action on cholinergic cells of the nucleus basalis magnocellularis, a neural substrate of cognitive performance. Female rats were ovariectomized at an age of 6 months. Three weeks later they received injections of either a mid-physiological dose of 17beta-estradiol or vehicle (oil), every other day for 2 weeks. The effect of estradiol on cognitive performance was tested in two associative learning paradigms. In the two-way active shock avoidance task estradiol-replaced animals learned significantly faster, while in the passive shock avoidance test no differences were observed between the experimental groups. Subsequent unilateral infusion of N-methyl-D-aspartate in the nucleus basalis magnocellularis resulted in a significant loss of cholinergic neurons concomitant with the loss of their fibers invading the somatosensory cortex. Estradiol treatment did not affect the total number of choline-acetyltransferase-immunoreactive neurons and their coexpression of the p75 low-affinity neurotrophin receptor either contralateral or ipsilateral to the lesion. In contrast, cholinergic fiber densities in estradiol-treated animals were greater both in the contralateral and ipsilateral somatosensory cortices as was detected by quantitative choline-acetyltransferase and vesicular acetylcholine transporter immunocytochemistry. However, estradiol treatment did not affect the lesion-induced relative percentage loss of cholinergic fibers. A significant decline of synaptophysin immunoreactivity paralleled the cholinergic damage in the somatosensory cortex of oil-treated animals, whereas an almost complete preservation of synaptic density was determined in estradiol-treated rats. Our results indicate that estradiol treatment enhances the cortical cholinergic innervation but has no rescuing effect on cholinergic nerve cells in the basal forebrain against excitotoxic damage. Nevertheless, estradiol may restore or maintain synaptic density in the cerebral cortex following cholinergic fiber loss. This estradiol effect may outweigh the lack of cellular protection on cholinergic cells at the functional level.

Effect of GP120 on glutathione peroxidase activity in cortical cultures and the interaction with steroid hormones

GP120 (the protein component of the HIV viral coat) is neurotoxic and may contribute to the cell loss associated with AIDS-related dementia. Previously, it has been shown in rat cortical mixed cultures that gp120 increased the accumulation of hydrogen peroxide and superoxide, two reactive oxygen species (ROS). We now demonstrate that gp120 increased activity of the key antioxidant glutathione peroxidase (GSPx), presumably as a defensive mechanism against the increased ROS load. Both estrogen and glucocorticoids (GCs), the adrenal steroid released during stress, blunted this gp120 effect on GSPx activity. The similar effects of estrogen and of GCs are superficially surprising, given prior demonstrations that GCs exacerbated and estrogens protected against gp120 neurotoxicity. We find that these similar effects of estrogen and GCs on GSPx regulation arose, in fact, from very different routes, which are commensurate with these prior reports. Specifically, estrogen has demonstrated antioxidant properties that may prevent the ROS increase (therefore acting as a neuroprotective agent) and rendered unnecessary the compensatory GSPx increased activity. To verify this we have added H2O2 to estrogen + gp120-treated cells, and GSPx activity was increased. However, with addition of H2O2 to GCs + gp120-treated cells there was no increase in activity. GCs appeared to decrease enzyme production and or activity and therefore under insult conditions ROS levels rose in the cell resulting in increased neurotoxicity. Overexpression of GSPx enzyme via herpes vector system reversed the GCs-induced loss of enzyme and eliminated the GCs exacerbation of gp120 neurotoxicity.

Cerebral thrombosis and microcirculation of the rat during the oestrous cycle and after ovariectomy

1. The effects of oestrogen on thrombogenesis and the cerebral microcirculation of the female rat were studied during the oestrous cycle and after ovariectomy. 2. Serum levels of oestradiol (E2) and plasma concentrations of nitric oxide (NO) metabolites were significantly greater at pro-oestrus than at dioestrus. Blood vessel diameter, mean red cell velocity, wall shear rate and blood flow at pro-oestrus were significantly higher than at dioestrus. Thrombotic tendency, assessed using a He-Ne laser-induced thrombosis model, was significantly decreased at pro-oestrus compared with dioestrus. 3. The long-term deprivation of oestrogen by ovariectomy significantly depressed serum levels of E2 and plasma concentrations of NO metabolites. Thrombotic tendency was significantly increased 4 weeks after ovariectomy. Vessel diameter, mean red cell velocity, wall shear rate and blood flow in pial arterioles were significantly reduced after ovariectomy. 4. Exogenous administration of oestrogen (17 beta-oestradiol) after surgery reversed the increased thrombotic tendency mediated by ovariectomy. 5. These results strongly indicate that oestrogen mediates beneficial effects on the cerebral microcirculation and moderates cerebral thrombotic mechanisms in the female rat.

Invited review: Estrogens effects on the brain: multiple sites and molecular mechanisms

Besides their well-established actions on reproductive functions, estrogens exert a variety of actions on many regions of the nervous system that influence higher cognitive function, pain mechanisms, fine motor skills, mood, and susceptibility to seizures; they also appear to have neuroprotective actions in relation to stroke damage and Alzheimer's disease. Estrogen actions are now recognized to occur via two different intracellular estrogen receptors, ER-alpha and ER-beta, that reside in the cell nuclei of some nerve cells, as well as by some less well-characterized mechanisms. In the hippocampus, such nerve cells are sparse in number and yet appear to exert a powerful influence on synapse formation by neurons that do not have high levels of nuclear estrogen receptors. However, we also find nonnuclear estrogen receptors outside of the cell nuclei in dendrites, presynaptic terminals, and glial cells, where estrogen receptors may couple to second messenger systems to regulate a variety of cellular events and signal to the nuclear via transcriptional regulators such as CREB. Sex differences exist in many of the actions of estrogens in the brain, and the process of sexual differentiation appears to affect many brain regions outside of the traditional brain areas involved in reproductive functions. Finally, the aging brain is responsive to actions of estrogens, which have neuroprotective effects both in vivo and in vitro. However, in an animal model, the actions of estrogens on the hippocampus appear to be somewhat attenuated with age. In the future, estrogen actions over puberty and in pregnancy and lactation should be further explored and should be studied in both the hypothalamus and the extrahypothalamic regions.

Interactions among ascorbate, dehydroascorbate and glucose transport in cultured hippocampal neurons and glia

There is an increasing recognition of the damaging role played by oxygen radicals in mediating necrotic neuronal injury. As such, it becomes important to understand the transport mechanisms that help maintain appropriate levels of small molecule antioxidants such as ascorbate in the brain. It has long been known that the transport of dehydroascorbate (DHA) into a variety of cell types is accomplished through the Glut-1 glucose transporter. In this paper, we characterize interactions among the transports of ascorbate, DHA and glucose in hippocampal cultures. We find: (a) sodium-dependent transport of ascorbate in mixed neuronal/glial, pure glial, and neuron-enriched hippocampal cultures; in contrast, we observed no such transport of DHA; (b) such ascorbate transport appeared to be independent of the glucose transporter, in that glucose did not compete for such transport, and overexpression of the Glut-1 glucose transporter did not alter ascorbate uptake; (c) in contrast, ascorbate, at concentrations ranging from 1 to 20 mM inhibited 2-dexogyglucose transport in mixed, glial and enriched neuronal hippocampal cultures; (d) potentially, ascorbate, by acting as an electron donor, could impair the function of molecules involve in the transport or metabolism of glucose. We observed mild inhibition of glucose transport by one unrelated electron donor (glutathione). Moreover, transport was also inhibited by an ascorbate analog which is not an electron donor. Thus, we conclude that ascorbate transport in hippocampal neurons and glia occurs independent of the glucose transporter but that, nevertheless, ascorbate, at concentrations generally thought to be supraphysiological, has the potential for disrupting glucose transport.

Neurotrophic and neuroprotective actions of estrogens and their therapeutic implications

Originally known for its regulation of reproductive functions, estradiol, a lipophilic hormone that can easily cross plasma membranes as well as the blood-brain barrier, maintains brain systems subserving arousal, attention, mood, and cognition. In addition, both synthetic and natural estrogens exert neurotrophic and neuroprotective effects. There is increasing evidence that estrogen actions are mediated by nongenomic as well as direct and indirect genomic pathways. Although in vitro models have provided the most extensive evidence for neurotrophic and neuroprotective actions to date, there are also in vivo studies that support these actions.

Interactions among glucose, lactate and adenosine regulate energy substrate utilization in hippocampal cultures

Glucose is the major energy source during normal adult brain activity. However, it appears that glial-derived lactate is preferred as an energy substrate by neurons following hypoxia-ischemia. We examined factors influencing this switch in energetic bias from glucose to lactate in cultured hippocampal neurons, focusing on the effects of the physiological changes in lactate, glucose and adenosine concentrations seen during hypoxia-ischemia. We show that with typical basal concentrations of lactate and glucose, lactate had no effect on glucose uptake. However, at the concentrations of these metabolites found after hypoxia-ischemia, lactate inhibited glucose uptake. Reciprocally, glucose had no effect on lactate utilization regardless of glucose and lactate concentrations. Furthermore, we find that under hypoglycemic conditions adenosine had a small, but significant, inhibitory effect on glucose uptake. Additionally, adenosine increased lactate utilization. Thus, the relative concentrations of glucose, lactate and adenosine, which are indicative of the energy status of the hippocampus, influence which energy substrates are used. These results support the idea that after hypoxia-ischemia, neurons are biased in the direction of lactate rather than glucose utilization and this is accomplished through a number of regulatory steps.

Mechanisms of estrogenic protection against gp120-induced neurotoxicity

gp120, an HIV coat glycoprotein that may play a role in AIDS-related dementia complex (ADC), induces neuronal toxicity characterized by NMDA receptor activation, accumulation of intracellular calcium, and downstream degenerative events including generation of reactive oxygen species and lipid peroxidation. We have previously demonstrated estrogenic protection against gp120 neurotoxicity in primary hippocampal cultures. We here characterize the mechanism of protection by blocking the classical cytosolic estrogen receptors and by measuring oxidative end points including accumulation of extracellular superoxide and lipid peroxidation. Despite blocking ERalpha and ERbeta with 1 microM tamoxifen, we do not see a decrease in the protection afforded by 100 nM 17 beta-estradiol against 200 pM gp120. Additionally, 17alpha-estradiol, which does not activate estrogen receptors, protects to the same extent as 17beta-estradiol. 17beta-Estradiol does, however, decrease gp120-induced lipid peroxidation and accumulation of superoxide. Together the data suggest an antioxidant mechanism of estrogen protection that is independent of receptor binding.

Stimulating effect of HIV-1 coat protein gp120 on corticotropin-releasing hormone and arginine vasopressin in the rat hypothalamus: involvement of nitric oxide

Subjects with human immunodeficiency virus type 1 (HIV-1) infection display increased activity of the hypothalamo-pituitary-adrenal (HPA) axis, which may play a role in both HIV-related neurodegenerative processes and disease progression. It has been speculated that the HIV coat protein gp120 may be responsible for these changes, and previous experimental evidence in both transgenic and nontransgenic mice supports this view. We speculated that one of the effects of gp120 in the CNS is to act within the hypothalamus to affect both corticotropin-releasing hormone (CRH) and arginine vasopressin (AVP), the principal regulators of HPA axis. We therefore administered i.p. gp120 (100 ng/rat) or vehicle to male Wistar rats and then detected Fos protein (an index of neuronal activation), CRH, and AVP immunoreactivity in the cellular compartments of the hypothalamic paraventricular nucleus (PVN). In addition, we tested the direct effect of various concentrations of gp120 on the release of CRH and AVP from rat hypothalamic explants maintained in vitro. Any modulation of gp120 effects by nitric oxide (NO) pathways was also sought by coadministering i.p. to rats or adding to the hypothalamic preparations the NO synthase inhibitor N(G)-methyl-l-arginine (l-NMMA). Gp120 induced the expression of Fos protein in both the parvo- and the magnocellular PVN, which was significantly attenuated by l-NMMA 10(-6) nM/L (P < 0.001 vs gp120 alone). Double immunochemistry showed costaining for Fos protein and CRH or AVP in the PVN following gp120; the number of double-labeled CRH and AVP cells for Fos protein was markedly reduced (P < 0.001) by coadministration of l-NMMA 10(-6) nM/L. In the in vitro studies, addition of gp120 to the hypothalamic explants in the dose range of 10 pM-1 nM resulted in a clear stimulation of both CRH and AVP release (P < 0.05-0.001 compared to control); in the presence of l-NMMA at 10-fold higher concentrations the stimulatory effect of gp120 on the release of both peptides was completely lost. It would therefore appear that gp120 activates CRH and AVP-producing neurons in the hypothalamic PVN and stimulates the release of both peptides in vitro via NO-dependent mechanisms. These findings, in line with previous evidence, further suggest that the increased activity of the HPA axis associated with HIV infection may be of central origin, due to the effects of gp120 on hypothalamic CRH and AVP release.

The effects of steroid hormones in HIV-related neurotoxicity: a mini review

This review examines the interaction of steroid hormones, glucocorticoids and estrogen, and gp120, a possible causal agent of acquired immune deficiency syndrome-related dementia complex. The first part of the review examines the data and mechanisms by which gp120 may cause neurotoxicity and by which these steroid hormones effect cell death in general. The second part of the review summarizes recent experiments that show how these steroid hormones can modulate the toxic effects of gp120 and glucocorticoids exacerbating toxicity, and estrogen decreasing it. We then examine the limited in vivo and clinical data relating acquired immune deficiency syndrome-related dementia complex and steroid hormones and speculate on the possible clinical significance of these findings with respect to acquired immune deficiency syndrome-related dementia complex.

gp120 induces cell death in human neuroblastoma cells through the CXCR4 and CCR5 chemokine receptors

To infect target cells, the human immunodeficiency virus (HIV) type I (HIV-1) must engage not only the well-known CD4 molecule, but it also requires one of several recently described coreceptors. In particular, the CXCR4 (LESTR/fusin) receptor allows fusion and entry of T-tropic strains of HIV, whereas CCR5 is the major coreceptor used by primary HIV-1 strains that infect macrophages and CD4(+) T-helper cells (M-tropic viruses). In addition, the alpha chemokine SDF1alpha and the beta chemokines MIP1alpha, MIP1beta, and RANTES, natural ligands of CXCR4 and CCR5, respectively, are potent soluble inhibitors of HIV infection by blocking the binding between the viral envelope glycoprotein gp120 and the coreceptors. Approximately two-thirds of individuals with acquired immunodeficiency syndrome (AIDS) show neurologic complications, which are referred to a syndrome called AIDS dementia complex or HIV-1-associated cognitive/motor complex. The HIV-1 coat glycoprotein gp120 has been proposed as the major etiologic agent for neuronal damage, mediating both direct and indirect effects on the CNS. Furthermore, recent findings showing the presence of chemokine receptors on the surface of different cell types resident in the CNS raise the possibility that the association of gp120 with these receptors may contribute to the pathogenesis of neurological dysfunction. Here, we address the possible role of alpha and beta chemokines in inhibiting gp120-mediated neurotoxicity using the human neuroblastoma CHP100 cell line as an experimental model. We have previously shown that, in CHP100 cells, picomolar concentrations of gp120 produce a significant increase in cell death, which seems to proceed through a Ca(2+) - and NMDA receptor-dependent cascade. In this study, we gained insight into the mechanism(s) of neurotoxicity elicited by the viral glycoprotein. We found that CHP100 cells constitutively express both CXCR4 and CCR5 receptors and that stimulation with phorbol 12-myristate 13-acetate down-regulates their expression, thus preventing gp120-induced cell death. Furthermore, all the natural ligands of these receptors exerted protective effects against gp120-mediated neuronal damage, although with different efficiencies. These findings, together with our previous reports, suggest that the neuronal injury observed in HIV-1 infection could be due to direct (or indirect) interactions between the viral protein gp120 and chemokine and/or NMDA receptors.

Dexamethasone pre-treatment interferes with apoptotic death in glioma cells

Glucocorticoids are known to influence the ability of cells to undergo apoptosis, directly inducing apoptosis in thymocytes while inhibiting it in hepatoma and carcinoma cells. Dexamethasone, a synthetic glucocorticoid, is reported to induce partial resistance to certain anticancer drugs in glioma cell lines. In the present study, the effect of dexamethasone on apoptosis of glioma and astrocytoma cell lines was investigated. Exposure of D384 human astrocytoma and C6 rat glioma cells to staurosporine induced apoptosis as judged by the formation of condensed nuclei and caspase activation. Pre-treatment of cells with dexamethasone caused a reduction in staurosporine-induced apoptosis. In addition, dexamethasone also conferred protection against the induction of apoptosis by anticancer agents including camptothecin and etoposide. The protective effect of dexamethasone was dose and time dependent, with maximal protection obtained with concentrations equal to or greater than 100 nM and a pre-incubation period of at least 24h. The earliest significant inhibition was seen with a pre-incubation period of 8h. Co-treatment with the glucocorticoid receptor antagonist RU38486 abolished the effect of dexamethasone, indicating that the protection due to dexamethasone is mediated via this receptor. Dexamethasone was found to induce a time-dependent up-regulation of Bcl-x(L) protein expression. However, the ability of cytochrome c/dATP to activate the caspase cascade in cytosolic extracts of D384 cells was unaffected by prior exposure of the cells to dexamethasone (1 microM) for 48 h. In conclusion, dexamethasone inhibits the induction of apoptosis in astrocytoma cells, probably via an up-regulation of Bcl-x(L), which could prevent cytochrome c release from mitochondria and subsequent caspase activation. Since glucocorticoids are often used in the treatment of gliomas to relieve cerebral oedema, the inhibition of apoptosis by these compounds could potentially interfere with the efficacy of chemotherapeutic drugs.

Glucocorticoids exacerbate the deleterious effects of gp120 in hippocampal and cortical explants

Glucocorticoids (GCs), the adrenal steroids secreted during stress, can compromise the ability of hippocampal neurons to survive numerous necrotic insults. We have previously observed that GCs worsen the deleterious effects of gp120, the glycoprotein of the acquired immune deficiency syndrome virus, which can indirectly damage neurons and which is thought to play a role in the neuropathological features of human immunodeficiency virus infection. Specifically, GCs augment gp120-induced calcium mobilization, ATP depletion, decline in mitochondrial potential, and neurotoxicity in fetal monolayer cultures from a number of brain regions. In the present report, we demonstrate a similar gp120/GC synergy in adult hippocampal and cortical explants. We generated explants from rats that were either adrenalectomized, adrenally intact, or intact and treated with corticosterone to produce levels seen in response to major stressors. Metabolic rates in explants were then indirectly assessed with silicon microphysiometry, and cytosolic calcium concentrations were assessed with fura-2 fluorescent microscopy. We observed that basal levels of GCs tonically augment the disruptive effects of gp120 on metabolism in the CA1 cell field of the hippocampus and in the cortex. Moreover, raising GC concentrations into the stress range exacerbated the ability of gp120 to mobilize cytosolic calcium in a number of hippocampal cell fields. Finally, we observed that the synthetic GC prednisone had similarly exacerbating effects on gp120. Thus, GCs can worsen the deleterious effects of gp120 in a system that is more physiologically relevant than the fetal monolayer culture and in a region-specific manner.

A cautionary note: the actions of adenosine agonists and antagonists may be reversed under certain conditions in primary cultures

It is now generally accepted that adenosine has a neuroprotective role in the central nervous system. Agonists of adenosine such as 2-chloroadenosine (2-ClA) have been shown to be neuroprotective, while antagonists such as 8-phenyltheophylline (8-PT) increase neurotoxicity. However, paradoxical results have been reported with adenosine analogues, especially with respect to length of time of administration. We observe similarly contradictory findings with respect to 2-ClA and 8-PT actions in primary hippocampal cultures exposed to glutamate or kainic acid. We found 8-PT and 2-ClA had antagonist and agonist actions, respectively, only with acute (1 h) treatment; with chronic treatment (24 h), 2-ClA had no effects, while 8-PT had significant agonist actions. We also show that with variations in the type of culturing system, concentration, and pH that 8-PT's neurotoxic antagonist actions could be dramatically changed. We, therefore, present this paper as a cautionary note in experimenting with adenosine analogues.

Glucocorticoids, stress, and their adverse neurological effects: relevance to aging

Glucocorticoids, the adrenal steroids secreted during stress, while critical for successful adaptation to acute physical stressors, can have a variety of deleterious effects if secreted in excess. It has come to be recognized that glucocorticoid excess can have adverse effects in the nervous system, particularly the hippocampus. These effects include disruption of synaptic plasticity, atrophy of dendritic processes, compromising the ability of neurons to survive a variety of coincident insults and, at an extreme, overt neuron death. This review considers the current cellular and molecular bases underlying these adverse glucocorticoid actions, and their relevance to brain aging.