Clinical neuroscience (New York, N.Y.)最新文献

Recently, it has been demonstrated that unstable trinucleotide repeats are the etiologic factor in myotonic dystrophy, fragile-X syndrome, Kennedy's disease, Huntington's disease, spinocerebellar ataxia type 1, and dentatorubral-pallidoluysian atrophy. All available evidence suggests that these expanded trinucleotide repeats, or unstable DNA, are the biological basis of the clinical phenomenon of genetic anticipation. Two components of anticipation, increased severity and earlier age of onset in subsequent generations, have been widely observed in schizophrenia. We review the evidence for and against genetic anticipation in schizophrenia. Although the major criticisms of the anticipation hypothesis can be questioned, so can the evidence in favor of it. We conclude that molecular genetic approaches might be the most useful means of resolving ambiguity in clinical arguments about the origin of the anticipation-like phenomenon in schizophrenia.

One difficulty in reproducing the core neurobiological features of schizophrenia in experimental animals is that most neurobiological data about the illness are inclusive: neither the inducing conditions nor the neurobiological mechanisms have been made clear. We review the advantages and limitations of animal models of schizophrenia based on neurodevelopmental hypotheses that implicate early, probably prenatal age, as the time at which the fundamental disease process occurs. These models, although principally founded on circumstantial clinical evidence of early developmental neuropathology, seem to reproduce a surprisingly broad spectrum of prominent neurobiological aspects of the disorder, and may help explain mechanisms that underlie the pathophysiology of this illness. In particular, the model based on neonatal excitotoxic hippocampal damage has provided data indicating the neurobiological plausibility of the notion that a developmental cortical defect has a delayed effect on cortical function and dopamine regulation (i.e., the neurodevelopmental hypothesis).

This article is introductory to the special articles devoted to various aspects of myoclonus that appear in this issue and that cover both clinical and basic aspects. The definition and the classification of myoclonus are provided. Myoclonus can be classified in various ways depending on which aspect is focused upon, but classification based on the pathophysiological mechanisms proposed in this article might be of practical use in-terms of both diagnosis and treatment. The physiological mechanisms underlying generation of spontaneous cortical or cortical reflex myoclonus have been partially elucidated through various electrophysiological techniques, although the mechanism of pathologically exaggerated excitability of the sensorimotor cortex remains to be clarified. The mechanisms of subcortical myoclonus of various kinds are still unknown. Elucidation of the pathophysiology of each kind of myoclonus will lead to its most appropriate treatment.

The integration of gene transfer techniques with neuroscience provides new opportunities to study the cellular and molecular aspects of neuron survival, dysfunction, and degeneration. A variety of viral vectors have been used to probe the effects of new gene expression in neurons, glial cells, and progenitor cells. These vectors provide an avenue to deliver genes with potential therapeutic value as shown by neuroprotective effects in animal models subsequent to the transduction of selected cell types. In addition, basic research in this field provides insight into the signalling mechanisms that dictate cell survival and programmed cell death and helps shape future strategies for gene therapy in the central nervous system. New generations of viral vectors will offer improved gene expression and move the field of gene delivery/therapy closer to clinical reality for the prevalent neurodegenerative conditions that include Alzheimer's and Parkinson's.

The pathogenesis of sporadic amyotrophic lateral sclerosis (ALS) is unknown, but several observations suggest that glutamate could participate in selective motor neuron degeneration. Extracellular levels of glutamate are elevated in ALS. Synaptic concentrations of glutamate are regulated by high-affinity glutamate transport, and defects in glutamate transport have also been observed in ALS tissue. Three sodium-dependent glutamate transporters have now been identified: a neuronal transporter EAAC1, and two astroglial transporters GLT-1 and GLAST. The defect in glutamate transport in ALS appears to be relatively specific for the GLT-1 subtype. The role of chronic excess glutamate and glutamate transporter loss has been investigated in experimental paradigms, where it was found that excitotoxicity could account for selective motor neuron degeneration. These culture paradigms have demonstrated that motor neurons are sensitive to glutamate toxicity via non-NMDA receptors and that various agents (e.g., antioxidants, glutamate release inhibitors, non-NMDA receptor antagonists) can be neuroprotective. These experimental studies will provide a basis for understanding the primary and secondary role of glutamate in motor neuron death and will provide important insight into possible therapeutic interventions.

The autosomal dominant cerebellar ataxias are a clinically and genetically heterogeneous group of disorders. In one unique form, early loss of color discrimination with macular degeneration is followed by gradual progression of cerebellar dysfunction and development of pyramidal signs. Pathology shows degeneration of cerebellum, basis pontis, inferior olive, and retinal ganglion cells. This disorder is genetically distinct from the other autosomal dominant cerebellar ataxias, consistent with the unique clinicopathologic features of this form of ADCA. Profound anticipation is noted in families with this phenotype and suggests that a trinucleotide repeat expansion may be the cause of this disease. Genetic characterization of this unique disorder may allow better understanding of the pathophysiology seen in these patients and provide insight into the nature of this and other neurodegenerative disorders.

Oxidative phosphorylation (OXPHOS) diseases can be caused by mutations in nuclear genes or mitochondrial DNA (mtDNA) genes. mtDNA mutations include complex mtDNA rearrangements in which large segments of mtDNA are duplicated or deleted and point mutations in which single nucleotide substitutions occur within transfer RNA (tRNA) genes, ribosomal RNA (rRNA) genes, or mitochondrial genes encoding OXPHOS polypeptides. Although over 30 pathogenic mtDNA point mutations and over 60 different types of mtDNA deletions are known (Shoffner and Wallace, 1995; Wallace et al., 1994), only a subset of these mutations are associated with cerebellar ataxia. This review focuses on the clinical, biochemical, and genetic features of OXPHOS diseases caused by mtDNA mutations in which ataxia is a common manifestation.

Spinocerebellar ataxia type 1 (SCA1) is an autosomal dominant progressive neurodegenerative disorder characterized by ataxia, dysarthria, ophthalmoparesis, and variable degrees of amyotrophy and neuropathy. Symptoms usually develop in the third or fourth decade but anticipation has been noted in juvenile onset cases. Neuropathologic findings include severe neuronal loss in the cerebellum and brainstem as well as degeneration of spinocerebellar tracts. The SCA1 gene which maps to the short arm of human chromosome 6 was identified using a positional cloning approach. The disease causing mutation is an expansion of a CAG trinucleotide repeat which lies within the coding region of a novel protein, ataxin-1, and encodes a polyglutamine tract. The number of CAG repeats varies from 6-39 repeats on normal alleles and 40-81 repeats on SCA1 alleles. The repeat has a perfect CAG configuration on expanded alleles whereas it is interrupted by 1-3 CAT units on normal alleles. Both wild type and expanded alleles are transcribed, ruling out impairment of transcriptional efficiency in SCA1. A pathogenetic model is proposed based on the findings in SCA1 and other neurodegenerative diseases caused by expansion of polyglutamine tracts. The expanded polyglutamine tract in ataxin-1 may lead to neurodegeneration through a gain of function mechanism involving aberrant interactions with other molecules in the involved neurons.

The molecular mechanisms of myoclonus are unknown. Drugs used in the symptomatic treatment of myoclonus were developed for other indications, such as epilepsy. Antimyoclonic drugs are not a single family of compounds but rather constitute a heterogeneous group of agents that act at various sites along the metabolic pathway of neurotransmitters or as receptor agonists or antagonists. For some drugs, the mechanism of antimyoclonic action is obscure despite many known actions. Myoclonus is affected by manipulation of more than one neurotransmitter system, and the neurotransmitters most linked to myoclonus are gamma-aminobutyric acid (GABA), glutamate, glycine, and serotonin. This is a review of the pharmacology of drugs acting on those neurotransmitters that are known or potential antimyoclonic drugs. A time of continuing advances in molecular biology and drug development is propitious for the pharmacotherapy of disorders that historically have been so refractory to conventional drug treatment.

The concept of propriospinal myoclonus is briefly reviewed in this report. Myoclonus can be classified according to the site of the generator into cortical, subcortical but supraspinal, and spinal myoclonus. Propriospinal myoclonus is a special type of spinal myoclonus characterized by rhythmic or arrhythmic, spontaneous or sometimes stimulus-sensitive flexion or extension movements of the axial muscles of the body with or without spread to the limbs excluding the cranially innervated muscles. Neurophysiologic study shows a characteristic pattern of order of recruitment of electromyographic bursts which are initially noted in the midthoracic segments (myoclonic generator) followed by propagation up and down the spinal cord via slowly conducting (3-11 M/s) pathways, such as the propriospinal systems. The role of spinal cord in functioning as a stepping generator and in locomotion has been strengthened by this new concept of propriospinal myoclonus.