John W. Olney

Brain lesions, obesity, and other disturbances in mice treated with monosodium glutamate.

In newborn mice subcutaneous injectionis of monosodium glutamate induced acute neuronal necrosis in several regions of developing brain including the hypothanamus. As adults, treated animals showed stunted skeletal development, marked obesity, and female sterility. Pathological changes were also found in several organs associated with endocrine function. Studies of food consumption failed to demonstrate hyperphagia to explain the obesity. It is postulated that the aduls syndrome represents a multifacted nueroendocrine disturbance arising from the disruption of developing nueral centers concered in the mediation of endocrine function.

Excitotoxity and the NMDA receptor

Abstract The same receptors for excitatory amino acids (EAA) that mediate direct neuronal depolarization can also be responsible for neuronal injury. Prolonged stimulation of EAA receptors of either the N-methyl-d-aspartate (NMDA) or non-NMDA types eventually results in the death of most central neurons. The exact mechanism(s) of cell injury is complicated, since depolarization and neuronal swelling, calcium influx, and possibly second messengers all contribute. Evidence is accumulating that the brain damage associated with anoxia, stroke, hypoglycemia, epilepsy, and perhaps neurodegenerative illnesses such as Huntingtons disease may be at least partially produced by excessive activation of NMDA receptors. To the extent that the pathophysiology can be explained by this mechanism, it may be amenable to rational therapies now under development.

NMDA receptor hypofunction model of schizophrenia

Several decades of research attempting to explain schizophrenia in terms of the dopamine hyperactivity hypothesis have produced disappointing results. A new hypothesis focusing on hypofunction of the NMDA glutamate transmitter system is emerging as a potentially more promising concept. In this article, we present a version of the NMDA receptor hypofunction hypothesis that has evolved from our recent studies pertaining to the neurotoxic and psychotomimetic effects of PCP and related NMDA antagonist drugs. In this article, we examine this hypothesis in terms of its strengths and weaknesses, its therapeutic implications and ways in which it can be further tested.

Cytotoxic effects of acidic and sulphur containing amino acids on the infant mouse central nervous system

SummaryThe brains and retinas of infant mice were examined following subcutaneous administration of monosodium glutamate (MSG) and structurally related compounds in an attempt to clarify the molecular specificity of MSG-induced neuropathology. Based on the effects on the infant retina and hypothalamus all compounds could be placed into one of four groups: 1. Those equipotent with L-MSG in necrosing neurons. 2. Those substantially more potent than L-MSG in necrosing neurons. 3. Those which affect non-neuronal components (glial, ependymal, Muller cells) without appreciable effects on neurons. 4. Those with no cytotoxic effects. Except for L-cysteine, all neurotoxic compounds were acidic amino acids known to excite neurones, the most potent neurotoxic compounds being those which are powerful neuroexcitants (N-methyl DL-aspartic and DL-homocysteic acids). The exception posed by L-cysteine may be more apparent than real in that the in vivo conversion of the SH terminal to a more acidic group (SO2H or SO3H) could account for its neurotoxicity. The close correspondence in molecular specificities associated with neurotoxic and neuroexcitatory properties of simple amino acids suggests the two phenomena may be governed by similar mechanisms of action.

Widespread patterns of neuronal damage following systemic or intracerebral injections of kainic acid: A histological study

The effect of systemic and intracerebral injections of kainic acid, a potent neuroexcitatory and neurotoxic analogue of glutamate, has been studied in the rat using a variety of light- and electron-microscopic techniques. Systemic injections of 12 mg/kg produce a high incidence of ‘wet dog shakes’ and convulsions; seizure activity is consistently associated with neuronal damage. The initial neuropathological reactions include dendritic and glial dilations in discrete areas of the neuropil; at this time and subsequently, affected neuronal somata either appear swollen and pale, or are shrunken with dark cytoplasm. In the most severely affected areas, the lesion progresses to severe disruption of the neuropil. A vvidespread pattern of damage is seen throughout the forebrain, which is relatively consistent from brain to brain and is characterized by the parallel degeneration of pairs or groups of structures with extensive axonal interconnections. The commonly affected areas include the olfactory cortex, amygdaloid complex, hippocampus, and related parts of the thalamus and neocortex. Intracerebral injections of 2–6 nmol produce extensive neuronal damage in distant structures, as well as at the injection site. The pattern of distant damage varies with the site of the injection and appears to reflect axonal connections between the affected areas near the injection and the distant areas of damage. For example, injections into the olfactory cortex tend to produce damage in the amygdala, the mediodorsal thalamic nucleus, and related areas of the frontal cortex, as well as in the olfactory cortex itself. Injections into the posterior part of the olfactory cortex which involve the entorhinal cortex tend to produce severe degeneration in field CA1 of the hippocampus, although field CA3 is more severely damaged following intraventricular, intrahippocampal or intrastriatal injections. These observations suggest that kainic acid may exert at least part of its toxic effects via axonal connections. Furthermore, the widespread damage induced by intracerebral injections indicates that caution must be exercised in the use of kainic acid to produce circumscribed lesions, and that careful histological evaluation of such lesions is required.

Antiepileptic drugs and apoptotic neurodegeneration in the developing brain.

Epilepsy is the most common neurological disorder of young humans. Each year 150,000 children in the United States experience their first seizure. Antiepileptic drugs (AEDs), used to treat seizures in children, infants, and pregnant women, cause cognitive impairment, microcephaly, and birth defects. The cause of unwanted effects of therapy with AEDs is unknown. Here we reveal that phenytoin, phenobarbital, diazepam, clonazepam, vigabatrin, and valproate cause apoptotic neurodegeneration in the developing rat brain at plasma concentrations relevant for seizure control in humans. Neuronal death is associated with reduced expression of neurotrophins and decreased concentrations of survival-promoting proteins in the brain. β-Estradiol, which stimulates pathways that are activated by neurotrophins, ameliorates AED-induced apoptotic neurodegeneration. Our findings present one possible mechanism to explain cognitive impairment and reduced brain mass associated with prenatal or postnatal exposure of humans to antiepileptic therapy.

Ketamine-Induced NMDA Receptor Hypofunction as a Model of Memory Impairment and Psychosis

N-methyl-D-aspartate (NMDA) glutamate receptor antagonists are reported to induce schizophrenia-like symptoms in humans, including cognitive impairments. Shortcomings of most previous investigations include failure to maintain steady-state infusion conditions, test multiple doses and/or measure antagonist plasma concentrations. This double-blind, placebo-controlled, randomized, within-subjects comparison of three fixed subanesthetic, steady-state doses of intravenous ketamine in healthy males (n = 15) demonstrated dose-dependent increases in Brief Psychiatric Rating Scale positive (F[3,42] = 21.84; p < 0.0001) and negative symptoms (F[3,42] = 2.89; p = 0.047), and Scale for the Assessment of Negative Symptoms (SANS) total scores (F[3,42] = 10.55; p < 0.0001). Ketamine also produced a robust dose-dependent decrease in verbal declarative memory performance (F[3,41] = 5.11; p = 0.004), and preliminary evidence for a similar dose-dependent decrease in nonverbal declarative memory, occurring at or below plasma concentrations producing other symptoms. Increasing NMDA receptor hypofunction is associated with early occurring memory impairments followed by other schizophrenia-like symptoms.

Brain Lesions in an Infant Rhesus Monkey Treated with Monosodium Glutamate

In an infant rhesus monkey brain damage resulted from subcutaneously administered monosodium glutamate. Although a relatively high dose of monosodium glutamate was used, the infant was asymptomatic for a 3-hour observation period during which time hypothalamic neurons were undergoing a process of acute cell death. With the electron microscope it was observed that dendrites and cell bodies of neurons are the tissue components primarily affected in brain damage induced by monosodium glutamate.

Isoflurane-induced Neuroapoptosis in the Neonatal Rhesus Macaque Brain

Background:Brief isoflurane anesthesia induces neuroapoptosis in the developing rodent brain, but susceptibility of non-human primates to the apoptogenic action of isoflurane has not been studied. Therefore, we exposed postnatal day 6 (P6) rhesus macaques to a surgical plane of isoflurane anesthesia for 5 h, and studied the brains 3 h later for histopathologic changes. Method:With the same intensity of physiologic monitoring typical for human neonatal anesthesia, five P6 rhesus macaques were exposed for 5 h to isoflurane maintained between 0.7 and 1.5 end-tidal Vol% (endotracheally intubated and mechanically ventilated) and five controls were exposed for 5 h to room air without further intervention. Three hours later, the brains were harvested and serially sectioned across the entire forebrain and midbrain, and stained immunohistochemically with antibodies to activated caspase-3 for detection and quantification of apoptotic neurons. Results:Quantitative evaluation of brain sections revealed a median of 32.5 (range, 18.0-48.2) apoptotic cells/mm3 of brain tissue in the isoflurane group and only 2.5 (range, 1.1-5.2) in the control group (difference significant at P = 0.008). Apoptotic neuronal profiles were largely confined to the cerebral cortex. In the control brains, they were sparse and randomly distributed, whereas in the isoflurane brains they were abundant and preferentially concentrated in specific cortical layers and regions. Conclusion:The developing non-human primate brain is sensitive to the apoptogenic action of isoflurane and displays a 13-fold increase in neuroapoptosis after 5 h exposure to a surgical plane of isoflurane anesthesia.

Potential of ketamine and midazolam, individually or in combination, to induce apoptotic neurodegeneration in the infant mouse brain

Recently, it was reported that anesthetizing infant rats for 6 h with a combination of anesthetic drugs (midazolam, nitrous oxide, isoflurane) caused widespread apoptotic neurodegeneration in the developing brain, followed by lifelong cognitive deficits. It has also been reported that ketamine triggers neuroapoptosis in the infant rat brain if administered repeatedly over a period of 9 h. The question arises whether less extreme exposure to anesthetic drugs can also trigger neuroapoptosis in the developing brain. To address this question we administered ketamine, midazolam or ketamine plus midazolam subcutaneously at various doses to infant mice and evaluated the rate of neuroapoptosis in various brain regions following either saline or these various drug treatments. Each drug was administered as a single one‐time injection in a dose range that would be considered subanesthetic, and the brains were evaluated by unbiased stereology methods 5 h following drug treatment. Neuroapoptosis was detected by immunohistochemical staining for activated caspase‐3. It was found that either ketamine or midazolam caused a dose‐dependent, statistically significant increase in the rate of neuroapoptosis, and the two drugs combined caused a greater increase than either drug alone. The apoptotic nature of the neurodegenerative reaction was confirmed by electron microscopy. We conclude that relatively mild exposure to ketamine, midazolam or a combination of these drugs can trigger apoptotic neurodegeneration in the developing mouse brain.