Altered acquisition and extinction of amphetamine-paired context conditioning in genetic mouse models of altered NMDA receptor function.

Repeated intermittent exposure to amphetamine (AMPH) results in the development of persistent behavioral and neurological changes. When drug exposure is paired with a specific environment, contextual cues can control conditioned responses, context-specific sensitization, and alterations in dendritic morphology in the nucleus accumbens (NAc). Intact N-methyl-D-aspartate (NMDA) glutamate receptor signaling is thought to be required for associative learning. The acquisition of context-specific behavioral sensitization to AMPH and extinction of conditioned hyperactivity have been investigated in two genetically modified mouse strains: the serine racemase homozygous knockout (SR-/-) and glycine transporter 1 heterozygous mutant (GlyT1-/+). These strains have reciprocally altered NMDA receptor co-agonists, D-serine and glycine, levels that result in decreased (SR-/-) or increased (GlyT1-/+) NMDA receptor signaling. AMPH-induced changes in dendritic morphology in the NAc were also examined. SR-/- mice showed reduced expression of context-specific sensitization and conditioned hyperactivity. However, the conditioned hyperactivity in these mice is completely resistant to extinction. Extinction reversed AMPH-induced increased in NAc spine density in wild-type but not SR-/- mice. GlyT1 -/+ mice showed a more rapid acquisition of sensitization, but no alteration in the extinction of conditioned hyperactivity. The SR-/- data demonstrate that a genetic model of NMDA receptor hypofunction displays a reduced ability to extinguish conditioned responses to drug-associated stimuli. Findings also demonstrate that the morphological changes in the NAc encode conditioned responses that are sensitive to extinction and reduced NMDA receptor activity. NMDA receptor hypofunction may contribute to the comorbidity of substance abuse in schizophrenia.

Deficiency in L-serine deaminase interferes with one-carbon metabolism and cell wall synthesis in Escherichia coli K-12.

Escherichia coli K-12 provided with glucose and a mixture of amino acids depletes L-serine more quickly than any other amino acid even in the presence of ammonium sulfate. A mutant without three 4Fe4S L-serine deaminases (SdaA, SdaB, and TdcG) of E. coli K-12 is unable to do this. The high level of L-serine that accumulates when such a mutant is exposed to amino acid mixtures starves the cells for C(1) units and interferes with cell wall synthesis. We suggest that at high concentrations, L-serine decreases synthesis of UDP-N-acetylmuramate-L-alanine by the murC-encoded ligase, weakening the cell wall and producing misshapen cells and lysis. The inhibition by high L-serine is overcome in several ways: by a large concentration of L-alanine, by overproducing MurC together with a low concentration of L-alanine, and by overproducing FtsW, thus promoting septal assembly and also by overexpression of the glycine cleavage operon. S-Adenosylmethionine reduces lysis and allows an extensive increase in biomass without improving cell division. This suggests that E. coli has a metabolic trigger for cell division. Without that reaction, if no other inhibition occurs, other metabolic functions can continue and cells can elongate and replicate their DNA, reaching at least 180 times their usual length, but cannot divide.

Cobalamin deficiency results in severe metabolic disorder of serine and threonine in rats.

Dietary cobalamin (vitamin B12; Cbl) deficiency caused significant increases in plasma serine, threonine, glycine, alanine, tyrosine, lysine and histidine levels in rats. In particular, the serine and threonine levels were over five and eight times, respectively, higher in the Cbl-deficient rats than those in the sufficient controls. In addition, some amino acids, including serine and threonine, were excreted into urine at significantly higher levels in the deficient rats. When Cbl was supplemented into the deficient rats for 2 weeks, in coincidence with the disappearance of the urinary excretion of methylmalonic acid (an index of Cbl deficiency), the plasma serine and threonine levels were normalized. These results indicate that Cbl deficiency results in metabolic disorder of certain amino acids, including serine and threonine. The expression level of hepatic serine dehydratase (SDH), which catalyzes the conversion of serine and threonine to pyruvate and 2-oxobutyrate, respectively, was significantly lowered by Cbl deficiency, even though Cbl does not participate directly in the enzyme reaction. The SDH activity in the deficient rats was less than 20% of that in the sufficient controls, and was normalized 2 weeks after the Cbl supplementation. It is thus suggested that the decrease of the SDH expression relates closely with the abnormalities in the plasma and urinary levels of serine and threonine in the Cbl-deficient rats.

Sequential phenotypic changes in hyperplastic areas during hepatocarcinogenesis in the rat.

Sequential phenotypic changes in hyperplastic areas of rat liver during N-2-fluorenylacetamide feeding were studied by enzyme and immunohistochemical methods combined with radioautography. Hyperplastic area showed a marked deficiency of beta-glucuronidase and serine dehydratase during their developing phase, the 6th through the 9th experimental weeks, and were fairly specifically labeled by injections of tritiated thymidine after partial hepatectomy performed at the 9th week. A sequential observation on these labeled hyperplastic areas revealed a considerable elevation of the levels of these marker enzymes in the majority of the labeled areas in 3 to 18 weeks after labeling. On the other hand, there was a small group of hyperplastic areas in which the enzyme deficiency persisted during the observation period. This type of lesion was generally larger than those showing enzymic maturation. Labeled cells were not detectable either in distinct hyperplastic nodules at late phase or in carcinomas. The metabolic regulation in the cells comprising hyperplastic areas was studied by checking the induction and repression of serine dehydratase after dietary stimuli. Serine dehydratase was not inducible in hyperplastic areas during the developing phase or in areas with persistent enzyme deficiency, but it was clearly induced and repressed in areas where there was an elevation of the endogenous enzyme level. The areas of hyperplasia with persistent enzyme deficiency and growth appeared to be more important than the ones of phenotypic maturation in relation to the later development of carcinoma. The phenotypic maturation in hyperplastic areas might represent reversion of altered cells towards normalcy from the condition related with neoplastic transformation.