Distribution of GABA receptors in the rat cerebellum

The distribution of GABA receptors in the cerebellum is not homogeneous. In comparison with detergent-treated membranes from the whole tissue the number of [3H]muscimol binding sites per mg protein (Bmax) is about doubled in preparations enriched in large fragments of the cerebellar glomeruli, and it is about one-third in the dissected deep nuclei. On the other hand, the apparent affinity (Kd) is similar in the different preparations. Comparison of the results with earlier studies suggests a heterogeneity in cerebellar GABA receptors and/or their control.

Thyroid deficiency and cell death in the rat cerebellum during development

The [3H]-DNA content of the cerebellum remained constant during a period from 4 h to 8--9 days after the administration of [3H]-thymidine both to normal rats at 6 and 12 days of age and to thyroid deficient rats at 6 days. On the other hand, when [3H]-thymidine was given to 12-day-old thyroid deficient rats, a progressive decrease in the cerebellar content of [3H]-DNA was observed during the first 4 days after the injection. These findings, together with previous results showing a markedly elevated pyknotic index in the internal granular layer of the 12-day-old hypothyroid cerebellum, are consistent with there being an increased death of newly formed and differentiating granule cells in thyroid deficiency towards the end of the second week of postnatal life.

Effects of albumin, amino acids, and clofibrate on the uptake of tryptophan by the rat brain

The influences of total tryptophan concentration, albumin binding, and amino acid competition on the rate of tryptophan influx into rat brain were compared using a single-pass injection technique with tritiated water as a freely diffusible reference. Omission of 3% bovine albumin froma bolus containing tryptophan in Krebs-Ringer bicarbonate buffer injected into the carotid artery increased non-albumin bound (free) tryptophan concentration threefold but tryptophan uptake by only 35% and 30% into forebrain and hypothalamus, respectively. However, tryptophan uptake from injected rat plasma was more markedly elevated when free tryptophan concentration was raised. Thus, when free tryptophan was doubled, but total tryptophan unchanged, by in vitro addition of clofibrate to a plasma bolus, uptake was increased by 53% and 28% into forebrain and hypothalamus respectively. When clofibrate was injected in vivo so that plasma total tryptophan concentration was decreased by 45% but neither free tryptophan nor competing amino acid concentrations were altered, then uptake from a bolus of the rat's own plasma was unchanged. Addition of competing amino acids at physiological concentrations to tryptophan in Krebs-Ringer buffer significantly reduced tryptophan influx into both brain regions, but did not increase the effect of albumin binding. The results indicate that tryptophan uptake into rat forebrain is substantially influenced by albumin binding and competition from other amino acids, but that hypothalamic uptake is less influenced by these factors.

Effects of thyroxine on postnatal cell acquisition in the rat brain

The effects of treatment with L-thyroxine (3 micrograms by subcutaneous injection daily from birth) on cell acquisition in the rat brain were studied during the first 3 postnatal weeks. In the forebrain, thyroxine has no effect on cell proliferation in the first 6 days, but it causes decreased cell acquisition from 12 to 21 days so that cell number becomes significantly reduced. Estimates of cell proliferation kinetics and of cell death in the lateral ventricular subependymal layer show no apparent abnormality. In the cerebellum, treatment from birth leads to increased cell proliferation during the first week: in comparison with controls, the rate of [3H]thymidine incorporation into DNA, thymidine kinase activity, and the number of cells both in the major germinal site (external granular layer: EGL) and in the whole cerebellum are elevated. This initial effect of thyroxine appears by day 3 and is short-lived, being no longer evident after day 6. The build-up of cell numbers in the EGL at day 6 seems to be related to a preceding, transient retardation of cell migration from this layer rather than to an acceleration of cell replication, since cell cycle parameters are normal. From day 12 the rate of [3H]thymidine incorporation into DNA is severely reduced in treated rats. Advancement of cellular differentiation rather than increased cell death in the EGL appears to be involved in this phenomenon.

Effect of thyroid deficiency on the growth of the hippocampus in the rat. A combined biochemical and morphological study

The growth of the hippocampus was studied in normal and hypothyroid rats using both biochemical and morphological techniques, and the results were compared with observations on the whole forebrain or on the cerebral cortex. The longitudinal growth, area and the volume of the hippocampus was severely reduced in thyroid deficiency. In the cerebral cortex the longitudinal growth and some of the parameters of transverse development were significantly decreased at the rostral but not at the caudal level. On rehabilitation from day 35 to 160 the longitudinal growth remained decreased in both the brain parts while the transverse growth was restored to normal in the cerebral cortex but not in the hippocampus. In the normal hippocampus about 60% of the cells were formed during the first 3 postnatal weeks. This developmental increase was significantly depressed in hypothyroid rats; the final deficit in cell number was about 13%. The rate of cell acquisition was calculated from the slopes of the logistic curves fitted to the data of DNA content. At the age of maximal cell acquisition (at day 2-3) the daily deposition was 0.44x106 cells in controls and 0.34x106 cells in the hypothyroid rats. In controls the concentration of DNA decreased in the hippocampus during maturation. Thyroid deficiency did not influence this developmental trend. In contrast, a severe reduction was observed in the cellular composition of RNA and protein.

Effect of undernutrition on the regional development of transmitter enzymes: glutamate decarboxylase and choline acetyltransferase

The effect of undernutrition on the activity of glutamate decarboxylase (GAD) and choline acetyltransferase (ChAc) (markers for the GABA-ergic and the cholinergic transmitter system, respectively) was studied in various parts of the rat brain at the age of 10, 15 and 21 days, and at day 54 following 33 days of rehabilitation. The brain regions investigated were the olfactory bulbs, cerebellum, pons-medulla, hypothalamus, colliculi, cerebral cortex hippocampus and the residual brain. Undernutrition resulted in a marked retardation of the developmental rise of the activities of both enzymes, expressed in terms of either total brain part or unit weight or protein. The effect diminished with age even during the period of nutritional deprivation. In most brain regions the enzyme activities were restored to normal after rehabilitation. In the cerebral cortex the total activity of both enzymes was persistently reduced, although the concentration of GAD exceeded the control levels. A negative correlation was manifested between the activities of GAD and ChAc in the different brain parts (except the cerebellum) during development. The correlation became significant by day 21 in the controls, but only after postweaning rehabilitation of the undernourished rats. The results showed therefore that undernutrition caused a reversible retardation in the development of these two transmitter enzymes, and they suggested that even the balance of the GABA-ergic and cholinergic systems throughout the brain can be restored to normal by rehabilitation.

Effect of undernutrition on cell generation in the adult rat brain

Cell proliferation and cell death in the lateral ventricular subependymal layer were studied in acutely food-deprived and chronically undernourished 6-week-old rats. The length of the cell cycle was increased by 40-75%, while the DNA synthesis phase of the cycle was prolonged by 70-85%. The rate of cell acquisition was reduced to approximately half the control level, and the proportion of degenerating cells was significantly increased. The response to undernutrition of proliferating cells in the subependymal layer of adult rats differs from that in animals in the first two weeks of life, and resembles that seen in other tissues.

Effect of reserpine on cell proliferation in the developing rat bran: a biochemical study

Reserpine, a well-known CNS depressant which depletes central monoamine stores, was found to produce in the brains of 11-day-old rats a severe depression in cell proliferation in terms of the rate of [3H]thymidine incorporation into DNA. The effect was studied in detail 12 h after ther administration of the drug (2.5 mg/kg, s.c.) when the rate of in vivo DNA synthesis in the forebrain was about one-third of control: the decrease was less marked in the cerebellum (rate about two-thirds of control). It was possible to exclude side effects of the drug, such as restricted food intake, hypothermia and an elevation of the level of blood corticosteroids being responsible for the reduction of [3H]thymidine incorporation into DNA. Kinetic studies showed that reserpine had no marked effect on the entry of [3H]thymidine from blood to brain, but it caused some retardation in the rate of [3H]thymidine conversion into [3H]thymidine nucleotides. Nevertheless, the severe depression of DNA labelling was evident even after correcting the values on the basis of [3H]thymidine nucleotide concentrations. In contrast to these effects, thymidine kinase activity was normal in the brain of reserpine-treated animals.

Effect of reserpine on cell proliferation in the developing rat brain: a quantivative histological study

Rats aged 11 days were injected with reserpine (2.5 mg/kg body wt.) and the rate of [3H]thymidine incorporation into brain DNA was followed over a period of 36h. In the forebrain this was significantly depressed by 2h, and it reached a nadir of about 30% of the control level at 4h, at which it remained for another 26h. A partial recovery occurred by 36h. The effect was less pronounced in the cerebellum. On the basis of this information brains of rats were examined histologically and by autoradiography between 7 and 36 h after reserpine to obtain estimates of cell cycle parameters and of rates of cell proliferation and cell loss. In the forebrain lateral ventricular subependymal layer the labelling index was markedly reduced in comparison withe controls. Cell cycle time was prolonged by 50% and turnover time increased by 60%. In the cerebellar external granular layer, the mitotic index was reduced and increased numbers of degenerate postmitotic nuclei were found, notably in the latter part of the experimental period. These effects are potentially of functional and clinical significance.

Do drugs acting on the nervous system affect cell proliferation in the developing brain

Drugs which alter the balance of neurotransmitter activity may, while failing to cause gross structural malformations of the brain, produce long-lasting functional disturbances if given when the brain is developing. Subtle anatomical changes may underlie such disturbances; reserpine has been shown to interfere with cell proliferation in the brain of suckling rats, and long-term alterations in behaviour reported after treatment with reserpine may be related to this effect of the drug. Neurotransmitters, apart from their conventional role, may also function as neurohumours and be involved in the regulation of cell proliferation in the nervous system. Thus drugs influencing central neurotransmitter activity, such as phenothiazines and adrenergic agonists and antagonists, which in clinical practice are often given to pregnant mothers, may affect the developing brain through mechanisms similar to those reported for reserpine. More experimental information is needed about the influence of such drugs on cell proliferation in the brain, and about their "behavioural teratogenicity".

Effect of thyroid deficiency on postnatal cell formation in the rat brain: a biochemical investigation

In thyroid deficiency, interference with postnatal cell formation seems to be confined to those regions of the brain where postnatal neurogenesis is significant. In comparison with controls the increase in cell number in the cerebellum is retarded in the second week of life, but a normal number is reached by 35 days. In contrast the DNA content of the olfactory bulbs is apparently irreversibly depressed. Mitotic activity, in terms of incorporation of [2-14C]thymidine into DNA, is mainly affected in the cerebellum: in thyroid deficient rats, it is depressed below control levels at day 12, but it is about 4 times higher than in controls at day 21 when, under normal conditions, cell proliferation has virtually ceased. The time course (15-240 min) of [14C]thymidine metabolism at day 14 shows regional differences in control rats. The rate of conversion of [14C]thymidine to [14C]thymidine nucleotides, and of these in turn to [14C]DNA is slower in the forebrain - where cell proliferation occurs on a smaller scale - than in the cerebellum. Consequently, in the forebrain nearly linear DNA synthesis rate is maintained for a longer time than in the cerebellum (1 h vs. 0.5 h), and since less 14C is conserved in DNA a significant efflux of unconverted [14C]thymidine is evident during the experimental period. The effect of thyroid deficiency on [14C]thymidine metabolism in the brain is only slight, and is due to an abnormally large supply of [14C]thymidine consequent to depressed systemic utilization of this precursor.

Effect of thyroid deficiency on cell acquistion in the postnatal rat brain: a quantitative histological study

The mechanisms underlying transient reduction of cell number in the developing cerebellum of thyroid-deficient rats have been studied by quantitative histological methods. Thyroid deficiency has no significant effect on the generation cycle of dividing cells in either the subependymal layer of the lateral ventricular walls or the external granulay layer of the cerebellum: the length of the cell cycle and the duration of the different phases of the cycle, including the DNA synthesis time appears to be normal. However, the external granular layer of the cerebellum contains fewer cells than in control at 12 days. Pyknotic nuclei are prominent in the granule cell layer of the hypothyroid cerebellum at this age. These amount to an estimated loss of about 1% of the total cerebellar cell population in 24 h. It is suggested that death of granule cells is for the most part a consequence of reduced Purkinje cell dendritic arborization, with failure of parallel fibre/Purkinje cell synaptogenesis. In the second postnatal week, granule cell death and reduced numbers of cells in the germinal zone can account to a great extent for the observed shortfall in cerebellar DNA content. The eventual attainment of normal cell numbers in the cerebellum of hypothyroid rats is related to a persistent external granular layer in the forth and fifth postnatal weeks.

Effect of undernutrition on metabolic compartmentation of glutamate and on the incorporation of [14C] leucine into protein in the developing rat brain

The effect of undernutrition on the rate of protein synthesis and the development of metabolic compartmentation of glutamate in the brain was investigated by using [U -14 C] leucine as precursor. In the brain of normal rats the incorporation rate of [14C] leucine into protein was at a maximum during the 3rd week after birth, but in the undernourished animal this rate was markedly lower. The biochemical maturation of the brain, followed in terms of the age-dependent increase in the glutamine/glutamate specific radioactivity ratio, was severly retarded in the undernourished animals, mainly as a result of a marked depression in the conversion of leucine carbon into glutamine. However these biochemical effects of undernutrition were reversible: on rehabilitation from Day 21-35 the rate of conversion of leucine carbon, both into proteins and glutamate and glutamine, was restored to normal.

The effect of undernutrition in early life on cell generation in the rat brain

In undernourished rats aged up to 21 days, the DNA synthesis period in dividing cells of the subependymal and external granular layers is consistently and markedly prolonged, while rates of cell production from these layers are only slightly altered. Cell cycle times are unchanged up to the end of the first week of life and prolonged from day 12. The G1 phase is markedly shortened at 1, 6 and 12 days of age. It would appear that, in comparison with controls, disappearance of the external granular layer is delayed, and cell numbers in both germinal layers may be reduced.