Incorporation of RNA-precursors into neuronal and glial cells of rat brain during a learning experiment

Purines and Pyrimidines: Metabolism, Function and Potential as Therapeutic Options in Neurodegenerative Diseases

Various neurodegenerative disorders have various molecular origins but some common molecular mechanisms. In the current scenario, there are very few treatment regimens present for advanced neurodegenerative diseases. In this context, there is an urgent need for alternate options in the form of natural compounds with an ameliorating effect on patients. There have been individual scattered experiments trying to identify potential values of various intracellular metabolites. Purines and Pyrimidines, which are vital molecules governing various aspects of cellular biochemical reactions, have been long sought as crucial candidates for the same, but there are still many questions that go unanswered. Some critical functions of these molecules associated with neuromodulation activities have been identified. They are also known to play a role in foetal neurodevelopment, but there is a lacuna in understanding their mechanisms. In this review, we have tried to assemble and identify the importance of purines and pyrimidines, connecting them with the prevalence of neurodegenerative diseases. The leading cause of this class of diseases is protein misfolding and the formation of amyloids. A direct correlation between loss of balance in cellular homeostasis and amyloidosis is yet an unexplored area. This review aims at bringing the current literature available under one umbrella serving as a foundation for further extensive research in this field of drug development in neurodegenerative diseases.

New perspectives on the roles of pyrimidines in the central nervous system

Since 1956, when exogenous uridine and cytidine were found to be necessary for the maintenance of perfused rat brain function, the co-existence of de novo synthesis, salvage pathways and removal of pyrimidine bases in the CNS has been a controversial subject. Here, we review studies on metabolites and enzymes of pyrimidine metabolism through more than 60 years. In view of known and newly-described inherited pyrimidine and purine disorders - some with complex clinical profiles of neurological impairments - we underline the necessity to investigate how the different pathways work together in the developing brain and then sustain plasticity, regeneration and neuro-transmission in the adult CNS. Experimentally, early incorporation studies in animal brain slices and homogenates with radio-labelled nucleosides or precursors demonstrated salvage activity or de novo synthesis. Later, the nucleoside transporters and organic anionic transporters underlying uptake of metabolites and anti-pyrimidine drugs in the CNS were identified. Recently, the expression of de novo enzymes in glial cells and neurons was verified using (immuno) histochemical and in-situ-hybridization techniques. Adult brain was shown to take up or produce all pyrimidine (deoxy) ribonucleosides or, after uptake and phosphorolysis of nucleosides, to make use of ribose for different purposes, including energy. More recently, non-canonical pyrimidine bases (5mC, 5hmC) have been found most notably in brain, pointing to considerable postreplicative DNA metabolism, with the need for pyrimidine-specific enzymes. Even more perspectives are emerging, with advances in genome analysis and in the manipulation of expression from the gene.

Quantitative EEG and neurochemical aspects of memory and learning

Different, established or putative animal models of learning were studied by electrophysiological (EEG) and neurochemical methodologies. The training of rats to new behavioral patterns proved to result in the stimulation of total RNA synthesis rate in specific brain structures, as well as in modifications of the EEG organization of hippocampus. This two-fold approach to the assessment of modifications in learning-involved brain structures was extended to the study of mirror focus and kindling, to verify the suitability of these phenomena as experimental models of learning. Reduced (3H) uridine incorporation and proportion of poly (A)-associated RNA were found in mirror focus in comparison with control regions. These variations seem related to brain damage rather than be congruent with learning-related modifications. Further studies are necessary to verify whether the kindling phenomena are to be equated to learning models.

Behavioral effects of arginine in male rats

The effects of aminoacid arginine on conditioned and unconditioned behavior were studied in male rats. Arginine was administered orally at different dose levels. Both acute and subchronic (7 days) treatment schedule was performed. Exploratory behavior of the rats was studied in an open field. Acquisition of active avoidance behavior was studied in the shuttle-box test situation, and retention of passive avoidance reaction was studied in a step-through type of passive avoidance behavior. Acute administration of arginine failed to affect the acquisition and the retention of avoidance responses, and exploratory behavior of the rats. A 7-day treatment with the aminoacid caused an increase in ambulation of rats of Wistar strain, and a facilitation of acquisition and retention of avoidance responses in rats of CDR strain with poor learning capacity. It is possible that behavioral effects or arginine depend on its involvement in nucleic acid synthesis.

Allometric nucleo-cytoplasmic volune relations and rotation ellipsoid neuronal bodies of the cat nodosal and superior sympathetic ganglia

Using a Henning and Elias' plot, evidence was obtained for the rotation ellipsoid nature of the three-dimensioinal shape of neuronal bodies of the nodosal and cervical sympathetic ganglia of the cat. Nucleo-cytoplasmic volume relationships for both types of ganglia neurons obeyed the allometry formula. The allometry coefficient (ratio of logarithms of cytoplasmic and nuclear volumes) was found to be similar in the neurons of both ganglia, its value corresponding to 1.38--1.43.

Dopamine and macromolecule synthesis in rat hippocampus

In hippocampus slices both dopamine and apomorphine lead to an increased incorporation of (3H)-fucose into total proteins, whereas the incorporation of (14C)-leucine was unchanged or decreased, respectively. Noradrenaline did not alter the incorporation of both precursors, whereas haloperidol partially reduced the dopamine induced increase in incorporation of fucose. Thus, an induction process of the observed macromolecular changes involving dopaminoceptive structures of hippocampus can be assumed.

Hippocampal activation and incorporation of macromolecule precursors

The effect of rhythmic slow wave activity (theta rhythm) on the incorporation of 3H-leucine and 3H-fucose into the total proteins of different hippocampus areas was studied. The theta rhythm was elicited by electrical stimulation of medial septum nuclei. An increase in 3H-leucine incorporation into the total proteins of CA 3 and CA 1 sectors of the hippocampus was observed, whereas the stimulation had no influence on precursor incorporation into the complex CA 4/area dentata. In contrast to these findings 3H-fucose incorporation into hippocampal proteins was not influenced by electrical stimulation of the medial septum. These findings are discussed in comparison to the results obtained in a learning experiment, which revealed an increased incorporation of both leucine and focuse into hippocampal proteins.

Topographical differences of RNA labelling in rat brain after intraventricular administration of labelled RNA precursors

14C-uridine or 14C-orotic acid was injected into the third or fourth brain ventricle of adult rats. The rate of incorporation of these precursors into the RNA of various brain regions was studied by autoradiography. 0.5 hour, 1 hour, 2 hours and 4 hours after the injection of labelled uridine and/or orotic acid into the third ventricle, a very uneven labelling of different brain regions was observed. The highest grain density was found over the ventricular walls and in the closely adjacent brain tissue; the intensity of labelling decreased sharply with distance from the ventricular lumen. 24 hours after intraventricular injection, a medio-lateral gradient of grain density was no longer observed. An intense labelling of leptomeninx (especially at the base of the brain) and of ependymal cells was observed at all time intervals investigated. At time intervals 0.5-2 hours the grain density of these structures surpassed by a considerable amount the grain density over neurona, glial cells or neuropile. Two hours after the injection of 14C-orotic acid into the fourth ventricle, the grains were mainly localised over leptomeningeal cells and vessels at the base of the brain and in the adjacent narrow strip of brain tissue. The rest of the brain was only very faintly labelled.

Increased fucose incorporation into rat hippocampus during learning. A biochemical and microautoradiographic study

The incorporation of intraventricularly infected L-[1-3H]fucose into proteins of the hippocampus and visual cortex was studied during the acquisition of a shock-motivated brightness discrimination in rats. The labeling of Tris-soluble proteins from both regions was not significantly changed during learning, whereas solubilized insoluble proteins obtained from the hippocampus revealed an increased fucose incorporation in learned animals. A significant enhanced incorporation into some distinct, slow-moving, carbohydrate-rich protein bands, separated by polyacrylamide gel electrophoresis, was observed in material from CA1 and CA3 sectors as well as from area dentata of the hippocampus formation during acquisition. The corresponding gel bands from the visual cortex exhibited no differences between trained animals and controls. Jointly performed microautoradiography showed an increased fucose incorporation into most areas of the hippocampus in learned animals compared with active and passive controls. The most significant differences were found to occur mainly in substructures consisting of densely packed neuronal cells.

The effect of convulsions induced by flurothyl on ribonucleic acid synthesis in rat cerebral cortex during the recovery phase

The effect of convulsions, induced by flurothyl, on RNA synthesis in purified unfractionated nuclei and the cytoplasm of rat cerebral cortex was studied by using a double-label technique involving injection of [3H]- and [14C]-orotate intracisternally. 2. Intact RNA was extracted in 80% yield by an enzymic method by using a proteinase in the presence of sodium dodecyl sulphate followed by deoxyribonuclease. Electrophoresis on 1.5% polyacrylamide-0.5% agarose gels revealed the presence of giant nuclear RNA of size up to approx. 300X 10(6) daltons and mRNA of maximal mol.wt. 9 X 10(6)-16 X 10(6). 3. Nuclear RNA synthesis was decreased to 27% in the first 15 min after convulsions but rapidly increased, so that at 1 1/2 h it was 124% of the control, and at 6 h 147%. 4. Labelling of cytoplasmic RNA was decreased to 15% at 15 min after convulsions but had not recovered to control values by 6 h. 5. Analysis of radioactive gel patterns and the 3H/14C ratio at six time-points (15 min-6h) showed that the major effect was inhibition of the processing of heterogeneous nuclear RNA resulting in a sharp decline in the export of newly synthesized RNA from the nucleus. 6. Cytoplasmic RNA patterns indicated that specific messengers were synthesized at different times during the recovery of the cell after convulsions.