Ammonia added in vitro, but not moderate hyperammonemia in vivo, stimulates glutamate uptake and H(+)-ATPase activity in synaptic vesicles of the rat brain

Ammonia as a Potential Neurotoxic Factor in Alzheimer's Disease

Ammonia is known to be a potent neurotoxin that causes severe negative effects on the central nervous system. Excessive ammonia levels have been detected in the brain of patients with neurological disorders such as Alzheimer disease (AD). Therefore, ammonia could be a factor contributing to the progression of AD. In this review, we provide an introduction to the toxicity of ammonia and putative ammonia transport proteins. We also hypothesize how ammonia may be linked to AD. Additionally, we discuss the evidence that support the hypothesis that ammonia is a key factor contributing to AD progression. Lastly, we summarize the old and new experimental evidence that focuses on energy metabolism, mitochondrial function, inflammatory responses, excitatory glutamatergic, and GABAergic neurotransmission, and memory in support of our ammonia-related hypotheses of AD.

Effects of hyperammonemia and liver failure on glutamatergic neurotransmission

Glutamate is the main excitatory neurotransmitter in mammals. Glutamatergic neurotransmission involves several steps, beginning with release of glutamate from the presynaptic neuron. Glutamate in the extracellular space activates glutamate receptors present in the synaptic membranes, leading to activation of signal transduction pathways associated with these receptors. To avoid continuous activation of glutamate receptors, glutamate is removed from the synaptic cleft by specific glutamate transporters located mainly on astrocytes. All these steps are tightly modulated under physiological conditions, and alterations of any of the above steps may result in impairment of glutamatergic neurotransmission, leading to neurological alterations. There are studies in the literature reporting alterations in all these steps in hyperammonemia and/or hepatic failure. Glutamatergic neurotransmission modulates important cerebral processes. Some of these processes are altered in patients with liver disease and hepatic encephalopathy, who show altered sleep-wake patterns, neuromuscular coordination, and decreased intellectual capacity. The alterations in glutamatergic neurotransmission may be responsible for some of these neurological alterations found in hepatic encephalopathy. The effects of hyperammonemia and liver failure on different steps of glutamatergic neurotransmission including alterations of glutamate concentration in the extracellular fluid in brain, transport and transporters of glutamate, the content and function of different types of glutamate receptors and signal transduction pathways. Alterations induced by hyperammonemia and liver failure on the glutamate-nitric oxide-cGMP pathway in brain may result in changes in long-term potetiation and learning ability.

Identification of differential gene expression profiles in rat cortical cells exposed to the neuroactive agents trimethylolpropane phosphate and bicuculline

Recent technological advancements in microfabrication combined with the rapid acquisition of full genome sequence data have led to the development of DNA arrays that have the capacity to monitor the expression levels of thousands of genes simultaneously. The development of this technology enables the use of functional genomics approaches to identify molecular markers associated with cellular responsiveness to cytotoxic exposures. Databases containing unique cell-response profiles associated with specific toxicants or classes of toxicants can then be used in conjunction with cell-based biosensor platforms for environmental surveillance and toxicological assessment. An important issue that must be addressed, however, is whether DNA arrays can be used to identify transient gene modulation events in a reproducible manner. To address this issue, we utilized a primary embryonic rat (day 18) cortical cell model system and examined the RNA of both chemically treated and untreated cells using radioisotope-labeled cDNA probes and commercially available nylon membrane arrays. Using this approach, we examined experimental variability, basal gene expression variability, the occurrence of false positives, and the reproducibility of gene expression profiles obtained after chemical exposure. Minimal differences in gene modulation were observed between RNA samples from independently cultured cortical cells when array experiments were conducted in parallel (Pearson correlation coefficient for gene intensities =0.98). In contrast, significant differences in gene expression were observed between array experiments conducted at different times with an identical RNA source (Pearson correlation coefficient for gene intensities=0.91). Our results suggest the effect of basal gene activity differences in independently isolated cell cultures is negligible and that experimental variability possibly associated with the handling of RNA samples, differences in reverse transcription efficiency, hybridization, and/or signal acquisition are the primary contributors to variability in measurements. Using cDNA array analysis of unexposed cells from three independent cell culture preparations, we calculated false positive gene modulation events as a function of the threshold absolute value of log(2) >1.0. The number of false positives using this criteria was 1-10 gene/ESTs/5109 actively transcribed gene/ESTs represented on the array. Using three independent replicate experiments of untreated cortical cell cultures, we determined that a threshold criterion of absolute value of log(2) >0.63 for triplicate experiments would reduce the expected number of false positives in our experiments to less than one. Using this criterion, reproducible gene expression profiles were identified in cortical cells exposed to the neuroactive agents trimethylolpropane phosphate and bicuculline.

Release of [3H]dopamine from striatal and cerebral cortical slices from rats with thioacetamide-induced hepatic encephalopathy: different responses to stimulation by potassium ions and agonists of ionotropic glutamate receptors

The effects of depolarizing stimuli; high (50 mM) potassium ions and the glutamate receptor agonists N-methyl-D-aspartate, kainate and 2-amino-3-hydroxy-5-methyl-4-isoxazolepropionate (AMPA) on the release of newly-loaded [3H]dopamine were studied in frontal cortical and striatal slices from control rats and from rats with acute hepatic encephalopathy induced with a hepatotoxin, thioacetamide. Hepatic encephalopathy enhanced the stimulatory effect of potassium ions by 20% in striatal slices and by 34% in frontal cortical slices. In striatal slices the stimulatory effects of N-methyl-D-aspartate and kainate were depressed in hepatic encephalopathy by 46% and 21%, respectively, which may be taken to reflect impaired modulation of striatal dopamine release by glutamate acting at N-methyl-D-aspartate or kainate receptors. In frontal cortical slices, the stimulatory effect of kainate was enhanced by 35% in hepatic encephalopathy but N-methyl-D-aspartate-stimulated release was not affected. The release evoked by 2-amino-3-hydroxy-5-methyl-4-isoxazolepropionate was not affected in hepatic encephalopathy in either brain region. Stimulation of dopamine release in the frontal cortex by depolarization or glutamate acting at kainate receptors could inhibit the activity of descending corticostriatal glutamatergic pathways, further impairing regulation of dopamine release by glutamate in the striatum.