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Differentiation is a complex process by which a terminal cell phenotype is determined. During neural development, in vivo cells of the nervous system reach this terminal phenotype through both preprogrammed genetic signaling and epigenetic signaling. This genetic program can set up initial organizational planes and an initial temporal sequence of events, but epigenetic signals drive much of the later gene expression and subsequent protein expression that determines different phases of differentiation. This epigenetic signaling can stimulate pluripotent cells to become more restricted in their fate, usually leading to multipotent cells and eventually to a final terminal phenotype. Epigenetic signals include a number of morphogenic and neurotrophic molecules that determine phenotype based on (1) the level of exposure to these endogenous substances, (2) the order of exposure, and (3) the mixture of exposure to these different epigenetic signaling molecules. These complex signaling events are being elucidated with advances in stem cell research in which the signals that stimulate multipotent cells to become neurons, glia, muscle, or bone are starting to be revealed (1). Because of this complexity, it is difficult to tease these different signaling events apart in many in vivo systems and this is why a reductionist approach with in vitro systems is often favored. [ABSTRACT FROM AUTHOR]

Advances in molecular genetics over the last decade have resulted in the identification of genetic mutations responsible for several inherited neurological diseases. Not only has cloning of these genes led to methods for diagnosis of patients and identification of carriers but also to the establishment of animal and cell culture models to study mechanisms by which mutant proteins induce toxicity in vulnerable cell types. Early neuropathological studies of autopsy tissue from patients with degenerative neurological diseases commonly revealed the presence of inclusion bodies in affected neuronal populations (see Table 1; 1-44). These include tangles and plaques in Alzheimer's disease, Lewy bodies in Parkinson's disease, and nuclear or cytoplasmic aggregates in the trinucleotide repeat diseases (spinal bulbar muscular atrophy, Huntington's disease, spinocerebellar ataxia 1 and 3, dentato-pallidoluysian atrophy) and cytoplasmic inclusions in familial and sporadic motor neuron diseases (45,46). That similar inclusions are observed in both sporadic and hereditary forms of neurological diseases suggested that similar pathways might be involved in pathogenesis whether protein abnormalities result from inherited sequence differences, DNA damage, or posttranslational modifications. The presence in inclusions of ubiquitin, a stress protein required for targeting abnormal proteins for degradation, suggested failure of proteolytic processing to rid cells of aberrant proteins. However, the primary or secondary role of these inclusions in the pathogenesis of disease could not be surmised from studies of postmortem tissue at end-stage disease. [ABSTRACT FROM AUTHOR]

It is often necessary to use a variety of techniques to more completely assess a specific area of study. In recent years, this has been increasingly true for the field of neuroendocrinology. Anatomical and molecular methodologies are often used in conjunction with in vitro and in vivo physiological approaches to gain meaningful information regarding factors controlling or altering neuroendocrine functions. When used together in a given study, in vitro and in vivo methods can be complimentary to one another and, thus, provide vital information. The focus of this chapter will be to demonstrate how both in vitro and in vivo techniques can be used in this complimentary fashion to provide new and important insights into basic neuroendocrine events and their mechanisms of action. Furthermore, we will demonstrate how these techniques can be used to help better understand the sites of action and the effects of specific toxic substances that alter neuroendocrine function. [ABSTRACT FROM AUTHOR]

Because of the limited accessibility of the brain for experimentation, but also for ethical and economical reasons, there is considerable interest in culture models suitable for neurotoxicological research. Although it is generally accepted that in vitro models cannot cover the entire spectrum of brain functions, they have proven to be indispensable for investigations in the life sciences since the early work of Harrison (1). To date, many in vitro models of various complexity are available, ranging from monolayer cultures of immortalized cell lines to organotypic cultures. Each of these culture systems has its particularities, therefore, it is of great importance to select the model that is most appropriate for the question to be solved. [ABSTRACT FROM AUTHOR]

Of all known neurotoxicants, lead (Pb) has received by far the most research attention. Because of increasing awareness of its untoward effects, investigation of the metal's central nervous system (CNS) actions has extended over several decades and across multiple experimental species, methods, and approaches. As a result, numerous actions of lead on the brain have been uncovered at the cellular and systems levels (e.g., ref. 1). Nonetheless, progress toward defining the specific bases of the neurotoxicity observed in exposed young children has been slow and inefficient and has not been commensurate with the magnitude of effort invested. Multiple factors have limited the development of this new information, but one of the most prominent has been the difficulty in linking findings obtained with in vitro approaches to neurotoxicity present in vivo. [ABSTRACT FROM AUTHOR]

Cells that make up the nervous system interact in complex, dynamic structural and biochemical contexts to generate organ function. A neurotoxicant that alters the activities of a particular cell type also induces secondary changes in the interactions between this cell and other cells. All types of cell in the nervous system are potential primary or secondary targets for damage by neurotoxic substances. The purpose of this chapter is to examine the reported cell-specific effects of an archetypal environmental neurotoxicant, inorganic lead (Pb), on neurons and neuroglia. Pb is an archetype in the broad sense that, like several environmental neurotoxicants, it affects multiple cell types, employs multiple mechanisms of toxic action, produces sublethal functional impairment to cells at low doses, is widespread in the environment, and is metabolically nonessential. Pb was perhaps the earliest environmental contaminant to be recognized as a neurotoxicant and is the most thoroughly studied to date in vitro. Pb neurotoxicologists have charted their own courses, often guided by progress in neuroscience and cell biology and sometimes pointing out new directions for neurobiology. The approaches that Pb neurotoxicologists have taken or not taken, the roads, paths, and blind alleys, will be discussed in this chapter, in the hope that telling the story will facilitate in vitro studies with other neurotoxicants. This work will be limited to effects of Pb on neurons, astroglia, and myelinating glia (oligodendroglia and Schwann cells), as the effects of Pb on microglia are virtually unstudied. [ABSTRACT FROM AUTHOR]

In order to perform neurotoxicological studies, model systems have to be established and techniques developed to analyze relevant parameters. The present chapter describes the in vitro effects of the neurotoxicants aminooxyacetic acid (AOAA), 3-nitropropionic acid (3-NPA), and methylmercury (MeHg) on glial cell neurotransmitters, and for 3-NPA, we also describe the effects on cultured neurons. Major emphasis is directed at the effects of these compounds on glutamate metabolism. [ABSTRACT FROM AUTHOR]

The role of apoptosis in development and neurodegeneration has become increasingly apparent in the past 10 yr. Normal apoptosis occurs in the central nervous system (CNS) from the embryonic stage through senescence, with different cells in each region of the nervous system having characteristic temporal patterns of programmed cell death. Several different stimuli trigger the apoptotic cascade, initiating diverse intracellular signaling pathways. These include mitochondrial calcium overload, generation of reactive oxygen species, and alterations in neurotrophic factor signaling. Neurotrophic factor-mediated signaling is achieved though the interaction of p75NTR and Trk receptors to modulate apoptotic cell death in developing and adult neural tissues. Both in vivo and in vitro experiments have suggested that exposure to a number of neurotoxicants results in apoptosis. For example, exposure of organogenesis-stage mouse embryos to a wide variety of xenobiotics, including ethanol, arsenic, hydroxyurea, chemotheraputic drugs, or haloacetic acids, results in increased apoptosis during neurulation. Ethanol, a widely used neurotoxicant, has been shown to affect apoptosis, as well as proliferation and migration, in developing animals. Exposure to heavy metal contaminants has been implicated in apoptotic cell death. This has been demonstrated with methylmercury with in vivo exposure producing apoptosis in the cerebellum and in vitro studies in PC12 cells implicating a neurotrophic factor dependent mechanism for this effect. [ABSTRACT FROM AUTHOR]

Over the past decades, the risk assessment of chemicals has frequently been considered a pseudoscientific process, primarily determined by public policy that masquerades as science, rather than representing a process well grounded in firm biological principles. Many toxicologists are uncomfortable when their work is linked to risk assessment or even when asked how their work will influence the risk assessment process. This chapter discusses the integration of toxicological data in risk assessment, emphasizing the applications of molecular toxicology (toxicogenomics) for improving the risk assessments of all chemical especially neurotoxicants. The chapter discusses the integration of toxicology information via an exposure-dose paradigm, provides a brief synopsis of the emerging consensus to increase the scientific basis of "neuroactive" chemical risk assessment, and notes the areas where molecular toxicology will likely contribute to refinements of the current risk assessment process. [ABSTRACT FROM AUTHOR]

Homeostasis in neurons is regulated by interactions among many signaling pathways. We will loosely define the term "signaling pathways" to include any molecular mechanisms that transduce external environmental stimuli (e.g., neurotransmitters, hormones, or contact with other cells) and/or intracellular metabolic conditions (e.g., intracellular free Ca2+ ion concentrations, redox status, or ATP demand) into cellular responses such as process growth, synthesis of neurotransmitters and/or their receptors, or changes in cellular respiration. This definition includes the interlinked pathways that lead to alterations in protein kinase or phosphatase activity and activation or repression of gene transcription and, perhaps less familiar mechanisms such as the stimulation of mitochondrial matrix enzymes by elevations in intramitochondrial Ca2+. [ABSTRACT FROM AUTHOR]

Risk assessment has been broadly defined as the characterization of the adverse health effects of human exposures to environmental hazards and can be divided into four major steps: hazard identification, dose-response assessment, exposure assessment, and risk characterization (1). Hazard identification is defined as determining whether human exposure to an agent can cause an increased incidence of an adverse health effect (e.g., neurotoxicity). Dose-response assessment is the process of characterizing the relationship between the administered or effective dose of an agent and the incidence of an adverse health effect in exposed populations, estimating the incidence of the effect as a function of human exposure to the agent. A dose-response assessment should account for exposure intensity and duration, developmental age, and other factors that may modify the response (e.g., gender, diet). Exposure assessment is the process of measuring or estimating the intensity, frequency, and duration of human exposure to an agent found in the environment or an agent that may be released into the environment. Risk characterization integrates these preceding steps by estimating the incidence of a health effect under various conditions of human exposure. [ABSTRACT FROM AUTHOR]

The history of neuroscience is punctuated by oracular disclosures from in vitro systems. In 1907, a pivotal tissue culture study by Harrison proved that Ramón y Cajal's theory on the developmental origin of nerve fibers was correct. Cajal had proposed in 1890, based on microscopic analysis of static histologic tissue sections, that the immature neuronal cell body sends out an axon that elongates freely, bearing a motile growth cone at its tip. Competing theories held that free growth of neurites did not occur, but that the neurites formed from the fusion of elements produced by other cells or from the stretching of a protoplasmic bridge between central and peripheral cell bodies of a multinucleated cell (1). These theories could not be tested by the histologic methods of the time, because axonal growth by a living neuron could not be directly observed. Harrison (2) pioneered a culture system for long-term microscopic observation of neuronal differentiation in living tadpole neural tube tissue. His observation that neurites grow out from cell bodies has been hailed as "one of the most revolutionary results in experimental biology" (3). [ABSTRACT FROM AUTHOR]