Perinatal changes of transport systems for amino acids in slices of mouse brain

As Verified with the Aid of Biotinylated Spermine, the Brain Cannot Take up Polyamines from the Bloodstream Leaving It Solely Dependent on Local Biosynthesis

The importance of polyamines (PAs) for the central nervous system (CNS) is well known. Less clear, however, is where PAs in the brain are derived from. Principally, there are three possibilities: (i) intake by nutrition, release into the bloodstream, and subsequent uptake from CNS capillaries, (ii) production by parenchymatous organs, such as the liver, and again uptake from CNS capillaries, and (iii) uptake of precursors, such as arginine, from the blood and subsequent local biosynthesis of PAs within the CNS. The present investigation aimed to unequivocally answer the question of whether PAs, especially the higher ones like spermidine (SPD) and spermine (SPM), can or cannot be taken up into the brain from the bloodstream. For this purpose, a biotin-labelled analogue of spermine (B-X-SPM) was synthesized, characterized, and used to visualize its uptake into brain cells following application to acute brain slices, to the intraventricular space, or to the bloodstream. In acute brain slices there is strong uptake of B-X-SPM into protoplasmic and none in fibrous-type astrocytes. It is also taken up by neurons but to a lesser degree. Under in vivo conditions, astrocyte uptake of B-X-SPM from the brain interstitial fluid is also intense after intraventricular application. In contrast, following intracardial injection, there is no uptake from the bloodstream, indicating that the brain is completely dependent on the local synthesis of polyamines.

Effect of gamma-glutamyl transpeptidase inhibitors on the transport of glutamate into neuronal and glial primary cultures

Inhibitors of gamma-glutamyl transpeptidase (mixture of serine and borate--13 mM, kainic acid--5 mM and 6-diazo-5-oxo-L-norleucine--2 mM) significantly suppressed glutamate uptake into cultured neurones and glial cells. The simultaneous application of any of these inhibitors with ouabain resulted in a further decline in glutamate uptake. It can be speculated that gamma-glutamyl transpeptidase significantly contributes to glutamate transport into nerve cells in the early period of brain development until the Na(+)-K(+)-gradient is fully constituted.

Differential taurine uptake in central and peripheral regions of goldfish retina

The transport system of taurine was investigated in fragments of goldfish retina prepared from the total tissue and from concentric regions: center and periphery. A high-affinity, saturable, sodium-dependent system was demonstrated in the three types of fragments. The Km for one-site analysis was similar in the two regions and the total retina. The analysis for two sites revealed a significant higher Km for the high-affinity site in fragments from the central region. The maximal uptake rate was higher in the central zone than in the total retina or the periphery. The Hill slopes obtained from saturation experiments of fragments of total retina, center, and periphery were similar to one other and near to 1. The slope of the time course uptake was intermediate for total retina and higher in the center than in the periphery. Hypotaurine and beta-alanine were found to inhibit taurine uptake, but GABA was a weak inhibitor. The values of Ki for hypotaurine by one- and two-site analysis were lower in the central region. The disruption of photoreceptors by shaking did not modify significantly the uptake of the amino acid. Remotion of endogenous taurine by dialysis of central and peripheral fragments increased the uptake in the periphery, but not in the center. The differences observed among the three samples revealed less affinity and high capacity for taurine uptake in the center, plus a higher sensitivity of inhibition. In addition, the peripheral zone had a greater affinity for taurine, and the maximal velocity of the entrance seems to be inhibited by the higher concentration of the amino acid in this zone. These observations may reflect differences between proliferating and non-proliferating regions of the retina (i.e., periphery and center).

Changes with aging in the levels of amino acids in rat CNS structural elements: III. Large neutral amino acids

This paper reports the concentrations of valine, leucine, isoleucine, phenylalanine, and tyrosine as determined in 53 discrete brain areas of 3- and 29-month-old male Fischer 344 rats microdissected by the punch technique. The levels of other amino acids in the same areas are reported in other papers of this series. The essential amino acids reported in this paper showed great regional heterogeneity, with concentrations in areas of the highest content being 9 to 12 times higher than levels in areas of the lowest content. The relative distributions of these amino acids were fairly similar in that in a number of areas the amino acids were all at very high levels and in some other areas they were all at low levels, with a few exceptions. With aging, levels of some amino acids increased and levels of some others did not change, but the predominant change was a significant decrease in levels with age. The results indicate high regional heterogeneity in amino acid levels and in the changes of these levels with age.

Changes with aging in the levels of amino acids in rat CNS structural elements. I. Glutamate and related amino acids

Glutamate and related amino acids were determined in 53 discrete brain areas of 3- and 29-month-old male Fischer 344 rats microdissected with the punch technique. The levels of amino acids showed high regional variation - the ratio of the highest to lowest level was 9 for aspartate, 5 for glutamate, 6 for glutamine, and 21 for GABA. Several areas were found to have all four amino acids at very high or at very low level, but also some areas had some amino acids at high, others at low level. With age, in more than half of the areas, significant changes could be observed; decrease occurred 5 times more frequently than increase. Changes occurred more often in levels of aspartate and GABA than in those of glutamate or glutamine. The regional levels of glutamate and its related amino acids show severalfold variations, with the levels tending to decrease in the aged brain.

Free amino acids of rat astrocytes in primary culture: changes during cell maturation

The concentrations of free amino acids were analysed in cultured primary astrocytes during cell maturation and in the starting material, i.e. the cerebral hemispheres of newborn rats. Taurine was the most abundant amino acid in all samples, the content of glutamine being comparable only in immature astrocytes (7 days in culture). The intracellular levels of most amino acids significantly decreased during the first 2 weeks in culture, remaining fairly stable during the third week.

Accumulation and metabolism of pipecolic acid in the developing brain of the mouse

In newborn mice, following i.p. injections of D,L-[3H]PA (pipecolic acid, 28 micrograms/kg), accumulation of radioactivity continues to increase up to 24 h. In adults, radioactivity peaks at 5 min and remains approximately constant up to 5 h, and then declines slowly to 24 h. Fifteen-day-old mice follow the newborn pattern, while 30-day-old mice show the same trend as the adult. Radioactivity in plasma shows essentially the same pattern of accumulation in adult and newborn animals with some quantitative differences. Secretion of radioactivity in the urine is significantly higher in the adult than in the newborn during the interval between 10 min and 5 h. Accumulation of radioactivity at 24 h in the newborn brain shows a preferential localization to the olfactory bulb, the anterior telencephalon and the diencephalon. Two hours after the i.p. injection, approximately 70% of the radioactivity recovered in brain is due to PA. This percentage increases to 75% and 87% at 5 and 24 h respectively. Alpha-aminoadipic acid (alpha-Aaa) a major metabolite of PA was identified in brain extracts at 5 h. The maximal formation of alpha-Aaa in relation to PA occurs approximately at 5 h. No other brain metabolites of PA could be identified with this chromatographic system. The present results show that access of PA to the brain is easier in the newborn mouse than in the adult. In addition, our results demonstrate, for the first time, the presence of PA metabolism in the newborn mouse.

Tritiated leucine incorporation in the developing hamster facial nucleus with injury: a liquid scintillation study

Tritiated leucine incorporation was examined after either crush or axotomy of the hamster facial nerve at specific stages in the maturation of the neuronal nucleolus. Changes in the neuronal metabolic response to injury in development were demonstrated with liquid scintillation examination of tritiated leucine incorporation into the trichloroacetic acid (TCA)-insoluble and TCA-soluble fractions derived from whole reactive and normal facial nuclear groups. Changes in incorporation seen in the developmental sequence were attributed to actual changes in neuronal protein metabolism, and not to changes in the amino acid pool, glial changes or hyperemic capillary changes. The ability to increase leucine incorporation over the normal as a result of injury in development coincided with the time of final nucleolar maturation in the facial motor neurons, beginning at approximately 20 days postnatal age. Thus, there is a correlation between a specific morphological event, the attainment of the mature nucleolar configuration, and the acquisition of the mature synthetic capacity as indicated by the ability to respond to injury in the mature manner.

Brain slice protein degradation and development

Protein degradation rates were measured in brain slices prepared from rats of various ages. This was done by adding the protein synthesis rate, determined by incorporation of a labeled precursor, and the net protein degradation rate, determined by measuring the changes with time of total free amino acids. These rates are about 30% higher than those previously calculated from data on protein synthesis rates and protein accumulation rates in vivo. The protein degradation rates in brain slices diminish with age; i.e., 2-day cerebellum greater than 2-day cerebral hemisphere greater than 12-day cerebral hemisphere greater than young adult cerebral hemisphere. Protein degradation rates in slices from young brain are initially slightly higher than protein synthesis rates, resulting in a small net degradation with time. Unlike slices from adult brain, the protein degradation rates in slices from young brain decline only modestly with time for as much as 100 min of incubation. The characteristics of protein degradation in brain slices from young animals are roughly similar to some of the data calculated for protein degradation in vivo, suggesting that this system may prove useful for studying factors which control or affect brain protein degradation.

Tryptophan and phenylalanine transport in rat cerebral cortex slices as influenced by sodium ions

Tryptophan and phenylalanine transport in rat cerebral cortex slices was studied in sodium-free media and during influx and efflux of sodium ions. Choline as a substitute for sodium in incubation media increased efflux and decreased influx of tryptophan and phenylalanine. Exchange of intracellular [3H]tryptophan and [3H]phenylalanine with extracellular unlabeled histidine, phenylalanine, and tryptophan was sodium-independent. Efflux of sodium ions from the slices had no immediate effects on phenylalanine and tryptophan efflux, but influx decreased. Influx of sodium into the sodium-depleted slices provoked a transient increase in tryptophan and phenylalanine efflux and also enhanced influx. The results are interpreted to indicate that sodium ions may possibly affect the function of the primary transport sites for aromatic amino acids at cerebral membranes by controlling the orientation of their reactive sites towards the intracellular and extracellular sides, rather than by being directly involved in the binding of amino acids to the carriers.

Regional distribution of membrane-bound gamma-glutamyl transpeptidase activity in mouse brain. Comparison with rabbit brain

Activity of membrane-bound gamma-glutamyl transpeptidase (gamma-GTP) was examined in various regions of mouse brain, in capillaries of the cerebral cortex and in telencephalic choroid plexuses. The level of activity in the capillaries was double and that of the choroid plexus nine times that of the gamma-GTP activity found in the brain, septum, hippocampus, hypothalamus, thalamus, cerebellum, frontal cortex, pons, medulla oblongata, and amygdala. Histochemically the gamma-GTP activity was demonstrated in the surface membranes of choroidal cells and in the endothelium of small capillaries. The activities of gamma-GTP of cerebral cortex, choroid plexus, and capillaries from rabbit were 5--17 times greater than those from corresponding areas of mouse brain. While 30 mM methionine stimulated (in vitro) the enzyme from mouse brain, no such effect was observed with the enzyme activity from rabbit brain. The gamma-GTP activity from the capillaries of cerebral cortex of both mouse and rabbit was not affected by the presence of methionine. These findings suggest existence of differences in the specificity of gamma-GTP activity in these two species.

Cerebral uptake of nicotine and of amino acids

Nicotine is among the compounds that enter the brain very rapidly (blood-flow-limited). It also leaves the brain rapidly; by five minutes, 90% exits, an exit somewhat slower than that of water. In spite of rapid exit, brain levels remain higher than levels in blood over a wide range of blood concentrations. Nicotine enters the fetal circulation from the maternal blood; it enters fetal brain but to a smaller extent than adult brain. Nicotine entry is different from that of amino acid: No interaction of amino acid transport and nicotine could be detected. Most close analogs have no effect on nicotine uptake, but at higher concentrations nicotine uptake is saturable. Nicotine and morphine mutually inhibit each other's uptake. The results suggest an uptake compartment (lipid space) for nicotine shared by morphine.

The effect of age upon the entry of some amino acids into the brain, and their incorporation into cerebral protein

The way in which the influx of amino acids into the brain changes as the age of rats increases from one to 25 weeks is reported. Most of the amino acids have a very high rate of influx during the first weeks of life, which falls progressively until an adult is reached by about 8 to 10 weeks of age. The high influx in early life is due to carrier-mediated transport systems which act more rapidly in the young than in the adult. These systems can be saturated by raised levels of amino acid in the blood, both in young and adult animals. The high influx in early life is necessary to provide abundant supplies of amino acids for cerebralprotein formation during the period of rapid brain growth. In adult life, influx declines to a level that is adequate to support the synthesis of cerebral protein, which is still being continuously broken down and renewed at a remarkably high rate.