Targeted disruption of serine racemase affects glutamatergic neurotransmission and behavior.

A subset of glutamate receptors that are specifically sensitive to the glutamate analog N-methyl-D-aspartate (NMDA) are molecular coincidence detectors, necessary for activity-dependent processes of neurodevelopment and in sensory and cognitive functions. The activity of these receptors is modulated by the endogenous amino acid D-serine, but the extent to which D-serine is necessary for the normal development and function of the mammalian nervous system was previously unknown. Decreased signaling at NMDA receptors has been implicated in the pathophysiology of schizophrenia based on pharmacological evidence, and several human genes related to D-serine metabolism and glutamatergic neurotransmission have been implicated in the etiology of schizophrenia. Here we show that genetically modified mice lacking the ability to produce D-serine endogenously have profoundly altered glutamatergic neurotransmission, and relatively subtle but significant behavioral abnormalities that reflect hyperactivity and impaired spatial memory, and that are consistent with elevated anxiety.

Oxidative stress, glutamate, and neurodegenerative disorders.

There is an increasing amount of experimental evidence that oxidative stress is a causal, or at least an ancillary, factor in the neuropathology of several adult neurodegenerative disorders, as well as in stroke, trauma, and seizures. At the same time, excessive or persistent activation of glutamate-gated ion channels may cause neuronal degeneration in these same conditions. Glutamate and related acidic amino acids are thought to be the major excitatory neurotransmitters in brain and may be utilized by 40 percent of the synapses. Thus, two broad mechanisms--oxidative stress and excessive activation of glutamate receptors--are converging and represent sequential as well as interacting processes that provide a final common pathway for cell vulnerability in the brain. The broad distribution in brain of the processes regulating oxidative stress and mediating glutamatergic neurotransmission may explain the wide range of disorders in which both have been implicated. Yet differential expression of components of the processes in particular neuronal systems may account for selective neurodegeneration in certain disorders.

Methylazoxymethanol treatment of fetal rats results in abnormally dense noradrenergic innervation of neocortex.

A single injection of methylazoxymethanol in pregnant rats at 15 days of gestation results in severe cortical atrophy in the offspring. In the adult offspring, the neurochemical markers for the cortical gamma-aminobutyric acid-containing neurons are severely reduced, whereas the noradrenergic markers are minimally altered. Immunohistofluorescence microscopy demonstrates a marked increase in the density of noradrenergic axons which have an abnormal pattern of distribution in the atrophic cortex. The results suggest that the central noradrenergic neurons determine the number of axons to be formed early in brain development, but local factors in the terminal field regulate the ultimate distribution of the noradrenergic axons.

Major innervation of newborn rat cortex by monoaminergic neurons.

A major monoaminergic innervation in infant rat neocortex, predominantly in layer IV, has been demonstrated by ultrastructural and biochemical studies after the administration of exogenous catecholamine precursors and congeners. One-third of all cortical synapses have an uptake-storage mechanism for catecholamines. In newborn cortex, the storage capacity for catecholamines is tenfold greater than the endogenous levels, and the uptake-storage mechanism matures earlier than the ability to synthesize neurotransmitter.

Antiparkinsonian drugs: inhibition of dopamine uptake in the corpus striatum as a possible mechanism of action.

A variety of antiparkinsonian drugs are potent, noncompetitive inhibitors of dopamine uptake into synaptosomes in homogenates of rat corpus striatum. Inhibition of dopamine uptake may potentiate the synaptic actions of dopamine in the striatum and could explain the antiparkinsonian effects of these drugs. This hypothesis accounts for several clinical features of Parkinson's disease and predicts compounds which may be new therapeutic agents.

Selective destruction of neurons by a transmitter agonist.

Microinjection of nanomole amounts of kainic acid, a heterocyclic analog of glutamate, into the cerebellums of adult hamsters and rats causes rapid degeneration of Purkinje, basket, stellate, and Golgi II cells, neurons that receive synaptic input from granule cells, whereas the granule cells themselves are spared. This selectivity is consistent with the evidence that glutamate is the granule cell transmitter and supports the hypothesis that kainic acid exerts its neurotoxic effects through glutamate receptors.

Alzheimer's disease: a disorder of cortical cholinergic innervation.

Great emphasis is being placed on identification of neurotransmitter systems involved in the symptomatic manifestations of neurological and psychiatric disorders. In the case of Alzheimer's disease, which now seems to be one of the most common causes of mental deterioration in the elderly, compelling evidence has been developed that acetylcholine-releasing neurons, whose cell bodies lie in the basal forebrain, selectively degenerate. These cholinergic neurons provide widespread innervation of the cerebral cortex and related structures and appear to play an important role in cognitive functions, especially memory. These advances reflect a close interaction between experimental and clinical neuroscientists in which information derived from basic neurobiology is rapidly utilized to analyze disorders of the human brain.

Alzheimer's disease and senile dementia: loss of neurons in the basal forebrain.

Recent evidence indicates that the nucleus basalis of Meynert, a distinct population of basal forebrain neurons, is a major source of cholinergic innervation of the cerebral cortex. Postmortem studies have previously demonstrated profound reduction in the presynaptic markers for cholinergic neurons in the cortex of patients with Alzheimer's disease and senile dementia of the Alzheimer's type. The results of this study show that neurons of the nucleus basalis of Meynert undergo a profound (greater than 75 percent) and selective degeneration in these patients and provide a pathological substrate of the cholinergic deficiency in their brains. Demonstration of selective degeneration of such neurons represents the first documentation of a loss of a transmitter-specific neuronal population in a major disorder of higher cortical function and, as such, points to a critical subcortical lesion in Alzheimer's patients.

Unintended changes in cognition, mood, and behavior arising from cell-based interventions for neurological conditions: ethical challenges.

The prospect of using cell-based interventions (CBIs) to treat neurological conditions raises several important ethical and policy questions. In this target article, we focus on issues related to the unique constellation of traits that characterize CBIs targeted at the central nervous system. In particular, there is at least a theoretical prospect that these cells will alter the recipients' cognition, mood, and behavior-brain functions that are central to our concept of the self. The potential for such changes, although perhaps remote, is cause for concern and careful ethical analysis. Both to enable better informed consent in the future and as an end in itself, we argue that early human trials of CBIs for neurological conditions must monitor subjects for changes in cognition, mood, and behavior; further, we recommend concrete steps for that monitoring. Such steps will help better characterize the potential risks and benefits of CBIs as they are tested and potentially used for treatment.

Glutamate toxicity in a neuronal cell line involves inhibition of cystine transport leading to oxidative stress.

Glutamate binds to both excitatory neurotransmitter binding sites and a Cl(-)-dependent, quisqualate- and cystine-inhibited transport site on brain neurons. The neuroblastoma-primary retina hybrid cells (N18-RE-105) are susceptible to glutamate-induced cytotoxicity. The Cl(-)-dependent transport site to which glutamate and quisqualate (but not kainate or NMDA) bind has a higher affinity for cystine than for glutamate. Lowering cystine concentrations in the cell culture medium results in cytotoxicity similar to that induced by glutamate addition in its morphology, kinetics, and Ca2+ dependence. Glutamate-induced cytotoxicity is directly proportional to its ability to inhibit cystine uptake. Exposure to glutamate (or lowered cystine) causes a decrease in glutathione levels and an accumulation of intracellular peroxides. Like N18-RE-105 cells, primary rat hippocampal neurons (but not glia) in culture degenerate in medium with lowered cystine concentration. Thus, glutamate-induced cytotoxicity in N18-RE-105 cells is due to inhibition of cystine uptake, resulting in lowered glutathione levels leading to oxidative stress and cell death.

N-acetylaspartate in neuropsychiatric disorders.

N-Acetyl aspartate (NAA) is the second most abundant amino acid in the human brain. NAA is synthesized by L-aspartate N-acetyl transferase or by cleavage from N-acetyl aspartyl glutamate by N-acylated alpha-linked L-amino dipeptidase (NAALADase); and it is catabolized to acetate and aspartate by N-acetyl aspartate amino hydrolase (amino acylase II). NAA is localized primarily to neurons, where it is concentrated in the cytosol. Although NAA is devoid of neurophysiological effects, it serves as an acetyl donor, an initiator of protein synthesis or a carbon transfer source across the mitochondrial membrane. The concentration of NAA in human brain increases 3-fold between midgestation and adulthood. In Canavan's Disease, an autosomal recessive disorder due to a null mutation in amino acylase II, NAA levels in brain are markedly increased and disrupt myelination. NAA levels have been found to be reduced in neurodegenerative disorders, including Alzheimer's Disease and Huntington's Disease. Since endogenous NAA can be readily detected in human brain by magnetic resonance spectroscopy, it is increasingly being exploited as a marker for functional and structural integrity of neurons in an expanding number of disorders.

My Life in Clinical Neuroscience: The Beginning.

This chapter recounts the author's life from childhood until he opened his research laboratory as an Assistant Professor in the Department of Pharmacology and Experimental Therapeutics at Johns Hopkins School of Medicine in 1976. It emphasizes the importance of chance opportunities and generous mentoring in the initiation of his career in neuroscience and psychiatric research.

Intracellular modulation of NMDA receptor function by antipsychotic drugs.

The present study deals with the functional interaction of antipsychotic drugs and NMDA receptors. We show that both the conventional antipsychotic drug haloperidol and the atypical antipsychotic drug clozapine mediate gene expression via intracellular regulation of NMDA receptors, albeit to different extents. Data obtained in primary striatal culture demonstrate that the intraneuronal signal transduction pathway activated by haloperidol, the cAMP pathway, leads to phosphorylation of the NR1 subtype of the NMDA receptor at (897)Ser. Haloperidol treatment is likewise shown to increase (897)Ser-NR1 phosphorylation in rats in vivo. Mutation of (896)Ser and (897)Ser to alanine, which prevents phosphorylation at both sites, inhibits cAMP-mediated gene expression. We conclude that antipsychotic drugs have the ability to modulate NMDA receptor function by an intraneuronal signal transduction mechanism. This facilitation of NMDA activity is necessary for antipsychotic drug-mediated gene expression and may contribute to the therapeutic benefits as well as side effects of antipsychotic drug treatment.

Serum levels of anticholinergic drugs in treatment of acute extrapyramidal side effects.

A simple, sensitive, and specific radioreceptor assay has been developed for the measurement of anticholinergic drugs in human serum. The assay is based on the competitive inhibition by free anticholinergic drugs in a 0.2-mL sample of serum with the specific binding of the potent muscarinic antagonists, tritiated quinuclidinyl benzilate, to solubilized brain muscarinic receptors. Anticholinergic activity could be detected regardless of drug structure and was quantified against atropine standards. Although the serum levels of anticholinergic drugs varied considerably in 35 patients receiving both neuroleptic and anticholinergic drugs, there was a highly significant inverse correlation between the presence of acute extrapyramidal side effects due to neuroleptics and the serum levels of anticholinergics.

A placebo-controlled trial of D-cycloserine added to conventional neuroleptics in patients with schizophrenia.

BACKGROUND: In a preliminary dose-finding study, D-cycloserine, a partial agonist at the glycine modulatory site of the glutamatergic N-methyl-D-aspartate (NMDA) receptor, improved negative symptoms and cognitive function when added to conventional neuroleptics at a dose of 50 mg/d. METHODS: Forty-seven patients with schizophrenia meeting criteria for deficit syndrome were randomized to D-cycloserine, 50 mg/d (n=23) or placebo (n=24) added to their conventional neuroleptic for an 8-week, double-blind trial. Clinical assessments were performed at baseline and at weeks 1, 2, 4, 6, and 8. Serum concentrations of D-cycloserine, relevant amino acids, and homovanillic acid were assayed at baseline and at weeks 4 and 8. A cognitive battery was performed at baseline and at week 8. RESULTS: Thirty-nine patients completed the 8-week trial. Seven dropouts occurred in the D-cycloserine group and 1 in the placebo group. The mean reduction in negative symptoms with D-cycloserine (23%) was significantly greater than with placebo (7%) as calculated by slopes representing Scale for the Assessment of Negative Symptoms (SANS) total scores. Improvement of negative symptoms was predicted by low neuroleptic dose and low baseline SANS total score. No differences were found in performance on any cognitive test between groups or in changes in any other clinical measure. Clinical response did not correlate significantly with serum amino acid concentrations at baseline or with concentrations of D-cycloserine at weeks 4 and 8. CONCLUSION: These results support the hypothesis that agents acting at the glycine modulatory site of the NMDA receptor improve primary negative symptoms.

Abnormal excitatory neurotransmitter metabolism in schizophrenic brains.

BACKGROUND: Schizophrenia has been hypothesized to be caused by a hypofunction of glutamatergic neurons. Findings of reduced concentrations of glutamate in the cerebrospinal fluid of patients with schizophrenia and the ability of glutamate-receptor antagonists to cause psychotic symptoms lend support to this hypothesis. N-acetylaspartylglutamate (NAAG), a neuropeptide that is highly concentrated in glutamatergic neurons, antagonizes the effects of glutamate at N-methyl-D-aspartate receptors. Moreover, NAAG is cleaved to glutamate and N-acetylaspartate by a specific peptidase, N-acetyl-alpha-linked acidic dipeptidase (NAALADase). To test the glutamatergic hypothesis of schizophrenia, we studied the NAAG-related glutamatergic variables in postmortem brains from patients with schizophrenia, neuroleptic-treated controls, and normal individuals, with particular emphasis on the prefrontal cortex and hippocampus. METHOD: Different regions of frozen brain tissue from three different groups (patients with schizophrenia, neuroleptic-treated controls, and normal controls) were assayed to determine levels of NAAG, N-acetylaspartate, NAALADase, and several amino acids, including aspartate and glutamate. RESULTS: Our study demonstrates alterations in brain levels of aspartate, glutamate, and NAAG and in NAALADase activity. Levels of NAAG were increased and NAALADase activity and glutamate levels were decreased in the schizophrenic brains. Notably, the changes in NAAG level and NAALADase activity in schizophrenic brains were more selective than those for aspartate and glutamate. In neuroleptic-treated control brains, levels of aspartate, glutamate, and glycine were found to be increased. CONCLUSIONS: The changes in levels of aspartate, glutamate, NAAG, and NAALADase are prominent in the prefrontal and hippocampal regions, where previous neuropathological studies of schizophrenic brains demonstrate consistent changes. These findings support the hypothesis that schizophrenia results from a hypofunction of certain glutamatergic neuronal systems. They also suggest that the therapeutic efficacy of neuroleptics may be related to increased glutamatergic activity.

Local and distant neuronal degeneration following intrastriatal injection of kainic acid.

After intrastriatal injection, the neurotoxin, kainic acid, was cleared from the rat forebrain in a biphasic manner with 70% eliminated within 2 hours; by 24 hours after infusion, less than 1% of the kainic acid remained in the forebrain. The kainic acid diffused into adjacent brain structures, achieving mu molar concentrations in several regions ipsilateral to the injected striatum. At various times after intrastriatal injection of 9.3 nmoles of kainic acid, the brain was serially sectioned; the sections were stained for Nissl substance with cresyl violet or for degenerating neurons with the ammoniacal silver method. Neuronal degeneration spread unevenly into contiguous structures from the central sphere in the injected striatum and affected the ipsilateral pyriform cortex and amygdala, the deep layers of the overlying cerebral cortex, and the medial aspects of the bed nucleus of the stria terminalis and of the nucleus accumbens. In half of the rats, the pyriform cortex contralateral to the side of injection also underwent degeneration. A subpopulation of pyramidal cells in layer IV of the lateral neocortex and the CA3-CA4 pyramidal cells in the ipsilateral hippocampus were selectively affected, whereas adjacent neurons remained intact. The distribution of agyrophilic fibers and terminals in subcortical structures was consistent with the degeneration of neurons of origin in the affected striatal and extrastriatal regions. Brain sections stained by the gold sublimate technique from rats perfused 20 days after injection revealed an intense astrocytic response in all areas affected by acute neuronal degeneration. Extrastriatal damage could be markedly reduced by injection of lower doses of kainic acid (2.3 nmoles) with brief anesthesia; under these conditions, however, the subpopulation of large striatal neurons were relatively resistant, as compared to the Golgi II neurons. These studies demonstrate significant and variable neuronal degeneration beyond the primary site of the lesion after intracerebral injection of kainic acid; several factors affect the pattern of degeneration, including the amount of kainic acid injected, its biological activity, its diffusion, duration of anesthesia, and variable sensitivity of neurons. Consequently, care must be exercised in the use of this neurotoxin to determine the extent and selectivity of neuronal damage, particularly with reference to neuronal vulnerability beyond the central sphere of intrinsic neuronal degeneration.

Cortical degeneration with swollen chromatolytic neurons: its relationship to Pick's disease.

We report two cases of dementia in which cortical degeneration with widespread swollen chromatolytic neurons (SCN) was the dominant pathologic feature. Each patient had received the diagnosis of Alzheimer's disease on the basis of clinical findings. There was no deficit of cortical choline acetyltransferase activity, assayed in one case, or lesions of the nucleus basalis of Meynert. The brains had moderate to marked frontal atrophy. Comparison of SCN with several other cerebral degenerative disorders indicates a similarity with certain features of the transmissible spongiform encephalopathies and with corticodentatonigral degeneration. The pathologic features of our cases are those of a number of other cases reported as "Pick's disease," and may represent an earlier stage in the pathogenetic process than the severe, sharply circumscribed atrophy with "nonspecific" cell loss and gliosis as the only microscopic residuals. Our findings re-emphasize the need to search for pathogenetically distinct subgroups which have been wholly or partially subsumed into the concept of Pick's disease.