Neuronal cell death in Huntington's disease: a potential role for dopamine.

Huntington's disease is an inherited neurodegenerative disorder, the cause of which is unknown. Excitotoxicity, mitochondrial dysfunction and oxidative stress are all likely to contribute to the striatal cell death that occurs in this disorder. There are accumulating data indicating that under specific circumstances, dopamine, which occurs in high concentrations in the basal ganglia, might be neurotoxic. In this article, the current models used to study Huntington's disease are reviewed and the recent findings that implicate dopamine in the pathophysiology of this progressive disorder are discussed. Although many questions remain unanswered, the dopaminergic system could contribute to striatal vulnerability in Huntington's disease and provide a novel avenue for the development of new therapies.

Methamphetamine and human immunodeficiency virus protein Tat synergize to destroy dopaminergic terminals in the rat striatum.

Dysfunction of the dopaminergic system accompanied by loss of dopamine in the striatum is a major feature of human immunodeficiency virus-1-associated dementia. Previous studies have shown that human immunodeficiency virus-1-associated dementia patients with a history of drug abuse have rapid neurological progression, prominent psychomotor slowing, more severe encephalitis and more severe dendritic and neuronal damage in the frontal cortex compared with human immunodeficiency virus-1-associated dementia patients without a history of drug abuse. In a previous study, we showed that methamphetamine and human immunodeficiency virus-1 protein Tat interact to produce a synergistic decline in dopamine levels in the rat striatum. The present study was carried out to understand the underlying cause for the loss of dopamine. Male Sprague-Dawley rats were administered saline, methamphetamine, Tat or Tat followed by methamphetamine 24 h later. Two and seven days later the animals were killed and tissue sections from striatum were processed for silver staining to examine terminal degeneration while sections from striatum and substantia nigra were processed for tyrosine hydroxylase immunoreactivity. Striatal tissue was also analyzed by Western blotting for tyrosine hydroxylase protein levels. Compared with controls, methamphetamine+Tat-treated animals showed extensive silver staining and loss of tyrosine hydroxylase immunoreactivity and protein levels in the ipsilateral striatum. There was no apparent loss of tyrosine hydroxylase in the substantia nigra. Markers for oxidative stress were significantly increased in striatal synaptosomes from Tat+methamphetamine group compared with controls. The results indicate that methamphetamine and Tat interact to produce an enhanced injury to dopaminergic nerve terminals in the striatum with sparing of the substantia nigra by a mechanism involving oxidative stress. These findings suggest a possible mode of interaction between methamphetamine and human immunodeficiency virus-1 infection to produce enhanced dopaminergic neurotoxicity in human immunodeficiency virus-1 infected/methamphetamine-abusing patients.

Dopamine induces proteasome inhibition in neural PC12 cell line.

The autoxidation and enzymatic catabolism of dopamine results in the generation of reactive oxygen species (ROS), which may possibly contribute to oxidative stress in multiple neurodegenerative disorders. Recent studies indicate that proteasome inhibition occurs in numerous neurodegenerative conditions, possibly as the result of oxidative stress, although the effects of dopamine on proteasome activity have not been determined. In the present study we examined the effects of dopamine on proteasome activity in the neural PC12 cell line. Application of dopamine induced a dose- and time-dependent decrease in proteasome activity, which occurred prior to cell death. Application of an antioxidant (gluthathione monoethyl ester), monoamine oxidase inhibitors (deprenyl, clogyline, paragyline), or an inhibitor of dopamine uptake (nomifensine) attenuated dopamine toxicity and dopamine-induced proteasome impairment. Application of the proteasome inhibitor lactacystin increased the toxicity of dopamine and the levels of protein oxidation following administration of dopamine. Together, these data indicate that dopamine induces proteasome inhibition that is dependent, in part, on ROS and dopamine uptake, and suggest a possible role for proteasome inhibition in dopamine toxicity.

Neurons of origin and fiber trajectory of amygdalofugal projections to the medial preoptic area in Syrian hamsters.

The amygdaloid neurons of origin and the trajectory of amygdaloid fibers to the medial preoptic area of the adult male Syrian hamster were identified by using horseradish peroxidase (HRP) histochemistry. After iontophoresis of HRP into the medial preoptic area, retrogradely labeled amygdaloid neurons were located in the dorsal and caudal parts of the medial amygdaloid nucleus and throughout the amygdalohippocampal area. No amygdaloid neurons were labeled after HRP applications confined to the most rostral portion of the medial preoptic area (anterior to the body of the anterior commissure). Following more caudal medial preoptic area injections (body of the anterior commissure to the suprachiasmatic nucleus) the distribution of retrogradely labeled cells in the medial amygdaloid nucleus and the amygdalohippocampal area revealed no topographic organization of the amygdalopreoptic connections. When amygdaloid neurons were labeled, the amygdalohippocampal area contained two to five times as many HRP-filled cells as the medial amygdaloid nucleus. Retrogradely transported HRP could be followed from the medial preoptic area to the amygdala through fibers in the dorsomedial quadrant of the stria terminalis. In addition, electrolytic lesions of the stria terminalis prior to iontophoresis of HRP into the medial preoptic area prevented retrograde transport to neurons in both the dorsocaudal medial amygdaloid nucleus and the amygdalohippocampal area. These results confirm earlier observations describing the location of autoradiographically labeled efferents from the medial amygdaloid nucleus to the medial preoptic area and provide new information about the restricted region within the medial amygdaloid nucleus from which these projections arise. They also suggest that, unlike the projections from the medial amygdaloid nucleus to the bed nucleus of the stria terminalis, the efferents to the medial preoptic area travel entirely in the stria terminalis.

Neuronal injury in hippocampus with human immunodeficiency virus transactivating protein, Tat.

Patients with human immunodeficiency virus infection may develop a dementing illness. Using both in vitro and in vivo models, we investigated the susceptibility of the hippocampal formation to the Tat protein of human immunodeficiency virus. We also determined the pattern of hippocampal injury in patients with human immunodeficiency virus encephalitis. Following exposure of hippocampal slices to Tat, marked susceptibility of CA3 region with relative insensitivity of the CA1/2 region was observed. Injection of Tat into different regions of the rat hippocampus produced similar neuronal loss in both CA3 region and the dentate gyrus. In animals administered Tat, lesions were dose-dependent and immunohistochemical staining showed marked gliosis and loss of microtubule associated protein-2 in the affected areas at 3 days post-injection. Interestingly, synaptophysin staining was relatively preserved. In hippocampal tissue from patients with human immunodeficiency virus encephalitis, loss of microtubule-associated protein-2 staining was reduced in the molecular layer of the dentate gyrus. The results of our experiments demonstrate a unique pattern of hippocampal injury in organotypic culture and rats exposed to Tat. Our observations that patients with human immunodeficiency virus reveal a similar pattern of damage suggests that Tat protein may be pathophysiological relevant in human immunodeficiency virus encephalitis.

Estrogen protects against the synergistic toxicity by HIV proteins, methamphetamine and cocaine.

BACKGROUND: Human immunodeficiency virus (HIV) infection continues to increase at alarming rates in drug abusers, especially in women. Drugs of abuse can cause long-lasting damage to the brain and HIV infection frequently leads to a dementing illness. To determine how these drugs interact with HIV to cause CNS damage, we used an in vitro human neuronal culture characterized for the presence of dopaminergic receptors, transporters and estrogen receptors. We determined the combined effects of dopaminergic drugs, methamphetamine, or cocaine with neurotoxic HIV proteins, gp120 and Tat. RESULTS: Acute exposure to these substances resulted in synergistic neurotoxic responses as measured by changes in mitochondrial membrane potential and neuronal cell death. Neurotoxicity occurred in a sub-population of neurons. Importantly, the presence of 17beta-estradiol prevented these synergistic neurotoxicities and the neuroprotective effects were partly mediated by estrogen receptors. CONCLUSION: Our observations suggest that methamphetamine and cocaine may affect the course of HIV dementia, and additionally suggest that estrogens modify the HIV-drug interactions.

Neurotoxicity of HIV-1 proteins gp120 and Tat in the rat striatum.

HIV-associated dementia complex is a serious disabling disease characterized by cognitive, behavioral and motor dysfunction. Basal ganglia involvement in HIV-1 infection may be responsible for some of the psychomotor symptoms associated with HIV dementia. The objectives of the present study were to determine: (1) whether gp120 and Tat produce striatal toxicity, and (2) whether gp120 and Tat show synergistic toxicity in the striatum. In these studies, the recombinant proteins gp120, Tat, or saline (0.9%) were stereotaxically injected in the striatum of adult male rats. The striatal sections were evaluated for area of tissue loss (Cresyl-violet stained sections) and the number of GFAP immunoreactive cells 7 days after the injections. Doses of gp120 250 ng/microl or higher and Tat 5 microg/microl or higher produced a significant area of tissue loss and significantly increased the number of GFAP reactive cells. We found no toxicity in animals treated with immunoabsorbed gp120 or Tat. Combined gp120 (100 ng/microl)+Tat (1 microg/microl) injections into the rat striatum significantly increased the area of tissue loss and altered morphology and increased number of GFAP reactive cells, as compared to controls. Thus, the present results suggest the involvement of gp120 and Tat in striatal toxicity and provide a model for further studies to fully characterize their role in HIV-1 toxicity and to develop therapeutic strategies for HIV-1 associated dementia complex.

A study of cortical and hippocampal NMDA and PCP receptors following selective cortical and subcortical lesions.

The neuronal localization of glutamate and phencyclidine (PCP) receptors was evaluated in the cerebral cortex and hippocampal formation of rat CNS using quantitative autoradiography. Scatchard analysis of [3H]glutamate binding in the cortex (layers I and II and V and VI) showed no difference in the total number of binding sites (Bmax) or apparent affinity (Kd) 1 week, 1 month and 2 months following unilateral ibotenate lesions to nucleus basalis of Meynert (nbM) compared to the non-lesioned side. Quisqualic acid displacement of [3H]glutamate in layers I and II, 1 week following nbM destruction, revealed both high- and low-affinity binding sites (representing the quisqualate (QA) and N-methyl-D-aspartate (NMDA) sites, respectively). Compared to the control side, there was no difference in binding parameters for either of the receptor sites. In similarly lesioned animals, the NMDA receptor was specifically labelled with [3H]glutamate and the associated PCP receptor labelled with [3H]N-(1-[2-thienyl]cyclohexyl)3,4-piperidine ([3H]TCP) in adjacent brain sections. For both receptors, there was no change in the total number of binding sites in the cortex following destruction of nbM. On the other hand, virtually all binding to NMDA and PCP receptors was eliminated following chemical destruction of intrinsic cortical neurons. These results suggest that the NMDA/PCP receptor complex does not exist on the terminals of cortical cholinergic afferents. One week after knife cuts of the glutamatergic entorhinal pathway to the hippocampal formation only an approximate 10% reduction of NMDA and PCP receptors was seen in the dentate gyrus. Conversely, selective destruction of the dentate granule cells using colchicine caused a near identical loss of NMDA and PCP receptors (84% vs 92% respectively). It is concluded from these experiments that glutamate and PCP receptors exist almost exclusively on neurons intrinsic to the hippocampal formation and that no more than 10% of NMDA and PCP receptors exist as autoreceptors on glutamatergic terminals.

Clorgyline and deprenyl attenuate striatal malonate and 3-nitropropionic acid lesions.

We have previously shown that dopamine depletion reduces striatal damage elicited by the mitochondrial neurotoxins malonate and 3-nitropropionic acid (3NP). Metabolism of dopamine by monoamine oxidase results in the formation of hydrogen peroxide, which may mediate dopamine toxicity. In this study, administration of the monoamine oxidase inhibitors clorgyline and deprenyl resulted in a 42% and 75% reduction in lesion volumes in malonate- and 3NP-treated animals, respectively, compared to controls.

Quantitative autoradiographic localization of NMDA, quisqualate and PCP receptors in the frog tectum.

An organizing role for the N-methyl-D-aspartate (NMDA) receptor/channel has been suggested in the development of the retinotectal projection in Rana pipiens. The regional distributions of NMDA, phencyclidine (PCP) and quisqualic acid (QA) receptors were quantified using in vitro autoradiography in the tectum of normal and surgically produced 3-eyed juvenile frogs. NMDA and QA receptor binding was highest in the pretectum. Of the tectal layers, the superficial retinotectal synaptic zone, layer 9, had the highest amount of NMDA and QA receptor binding. Moderate binding was observed in layer 5, with little binding in the cellular layer 6. No specific [3H]N-(1-[2-thienyl]cyclohexyl) piperidine ([3H]TCP) binding was observed in any of the tectal regions.

Methamphetamine toxicity is attenuated in mice that overexpress human manganese superoxide dismutase.

We have investigated methamphetamine (MA) toxicity in transgenic mice that overexpress the human form of mitochondrial manganese superoxide dismutase (MnSOD). Our results reveal a significant reduction in the long-term depletion of striatal dopamine and protein oxidation following repeated administration of MA in transgenic vs. non-transgenic littermates. These findings support the notion that ROS contribute to MA-induced brain damage and suggest that mitochondria may play an important role in this form of neurodegeneration.

The mitochondrial inhibitor malonate enhances NMDA toxicity in the neonatal rat striatum.

Intra-striatal injections of the mitochondrial inhibitor malonate elicit age-dependent neuronal damage in rat brain; injury is more extensive in older animals than in young adults. We investigated the neurotoxic potential of malonate in the immature rat brain. We found that 7-day-old (P7) rats were highly resistant to malonate neurotoxicity. Yet, although intra-striatal injections of 1 mumol malonate did not elicit overt tissue injury in P7 rats, co-administration of this dose of malonate with a dose of NMDA close to its toxicity threshold (2.5 nmol) doubled the severity of resulting excitotoxic injury.

Resistance to nitroprusside neurotoxicity in perinatal rat brain.

Nitric oxide (NO) may mediate some of the toxic effects of the excitatory amino acid (EAA) glutamate when there is overactivation of the N-methyl-D-aspartate (NMDA) receptor. In the developing rodent nervous system, NMDA neurotoxicity peaks at postnatal day 7. To assess whether NO toxicity exhibits a similar developmental profile, we injected the NO generator sodium nitroprusside into the immature and adult rodent hippocampal formation and striatum, using a dose known to damage the adult nervous system. Contrary to our expectations, we found the immature brain highly resistant to the toxic effects of sodium nitroprusside.

Loss of hippocampal [3H]TCP binding in Alzheimer's disease.

We have previously demonstrated a marked loss in N-methyl-D-aspartate (NMDA) receptors in the hippocampus and cerebral cortex of patients dying with dementia of the Alzheimer type (DAT). In addition, we have found that the dissociative anesthetic N-(1-[2-thienyl]cyclohexyl)3,4-piperidine ([3H]TCP) binds to a site whose regional distribution is highly correlated with that of NMDA receptor sites. We studied the binding of [3H]TCP to sections of hippocampi from 8 controls, 12 patients with DAT and 7 patients with other dementias. [3H]TCP binding was significantly reduced in strata pyramidalia of CA1/CA2, CA3 and subiculum of DAT hippocampal formation compared to that of control. Labelled dissociative anesthetics could potentially be used with positron emission tomography in the diagnosis of DAT.

Glutamate transmission and toxicity in Alzheimer's disease.

1. Despite intensive research, the cause of Alzheimer's disease is unknown. 2. Glutamate is the major excitatory transmitter of the cerebral cortex and hippocampus and it appears to have an important role in learning and memory. In addition to its transmitter function, glutamate is a neurotoxin which has been implicated in the pathogenesis of a variety of neurodegenerative disorders. 3. Glutamate toxicity may play a role in the pathogenesis of Alzheimer's disease. 4. Disruption of glutamatergic neurotransmission may account, in part, for the learning and memory deficits of Alzheimer's disease. 5. Labeling of the glutamate receptor complex may allow in vivo diagnosis by positron emission tomography. 6. Glutamate receptor ligands may provide a means of therapeutic intervention in Alzheimer's disease.

Neurotoxicity and dysfunction of dopaminergic systems associated with AIDS dementia.

Infection with the human immunodeficiency virus (HIV) selectively targets the basal ganglia resulting in loss of dopaminergic neurons. Although frequently asymptomatic, some patients may develop signs of dopamine deficiency de novo. Accordingly, they are highly susceptible to drugs that act on dopaminergic systems. Both neuroleptics and psychostimulants may exacerbate these symptoms. Experimental evidence suggests that viral proteins such as gp120 and Tat can cause toxicity to dopaminergic neurons, and this toxicity is synergistic with compounds such as methamphetamine and cocaine that also act on the dopaminergic system. In addition, other neurotransmitters that modulate dopaminergic function, such as glutamate and opioids, may also modify the susceptibility of the dopamine system to HIV. Therefore, a thorough understanding of the mechanisms that lead to this selective neurotoxicity of dopaminergic neurons would also likely lead to the development of therapeutic modalities for patients with HIV dementia.

Neurotransmitter receptors in Alzheimer disease.

Alzheimer disease (AD) is an exceedingly complex disorder in which numerous populations of neurons and neurotransmitter systems are damaged or destroyed. Effective treatment of the cognitive symptoms of AD does not exist, and new targets for therapeutic intervention are needed desperately. Traditionally, the neurotransmitter receptors have been the focus of new neuropsychopharmacological agents, so it seems reasonable to assess the status of these receptors in the AD brain. In this article, we review the quarter century of receptor research in AD. The limitations of receptor studies, in general, and the particular limitations of studying AD tissue are discussed. The cholinergic and glutamatergic systems have been implicated most directly in normal cognitive function, so the receptors for these neurotransmitters are emphasized in this review. We have attempted to point out the possible neurobiological roles and potential clinical significance of the various receptors in AD. Investigation of neurotransmitter receptors in AD provides a rational approach to the development of therapeutic and diagnostic strategies, and, at a minimum, should lead to a better understanding of the neurobiology of AD.