Protein Metabolism of The Nervous System

Temporal phases of long-term potentiation (LTP): myth or fact?

Long-term potentiation (LTP) remains the most widely accepted model for learning and memory. In accordance with this belief, the temporal differentiation of LTP into early and late phases is accepted as reflecting the differentiation of short-term and long-term memory. Moreover, during the past 30 years, protein synthesis inhibitors have been used to separate the early, protein synthesis-independent (E-LTP) phase and the late, protein synthesis-dependent (L-LTP) phase. However, the role of these proteins has not been formally identified. Additionally, several reports failed to show an effect of protein synthesis inhibitors on LTP. In this review, a detailed analysis of extensive behavioral and electrophysiological data reveals that the presumed correspondence of LTP temporal phases to memory phases is neither experimentally nor theoretically consistent. Moreover, an overview of the time courses of E-LTP in hippocampal slices reveals a wide variability ranging from <1 h to more than 5 h. The existence of all these conflictual findings should lead to a new vision of LTP. We believe that the E-LTP vs. L-LTP distinction, established with protein synthesis inhibitor studies, reflects a false dichotomy. We suggest that the duration of LTP and its dependency on protein synthesis are related to the availability of a set of proteins at synapses and not to the de novo synthesis of plasticity-related proteins. This availability is determined by protein turnover kinetics, which is regulated by previous and ongoing electrical activities and by energy store availability.

The entry of acidic amino acids into brain and CSF during development, using in situ perfusion in the rat

Previous studies using the rapid single pass blood to tissue uptake of substances by the capillaries of the blood-brain barrier, have failed to show significant uptake of acidic amino acids. However, by the use of a bilateral in situ brain perfusion in neonatal and adult rats, extending the perfusion time to 30 min, the carrier-mediated uptake of aspartate and glutamate into brain and CSF has been demonstrated. The ratios of 14C-acidic amino acids in the brain and CSF to that in perfusate were measured and represented as Rbrain and RCSF respectively, after 30 min, neonatal (1-week-old) Rbrain values for both amino acids were approximately twice that of adults, while neonatal RCSF for aspartate and glutamate were 3 to 5 times that of the adult. In contrast, there was no significant entry of NMDA into either compartment for both adults and neonates. The transfer coefficient, Kin into brain and CSF was also measured in relation to stages of development. In general the Kin values for brain and CSF for aspartate and glutamate were higher in the younger age groups than the adult group (1 week > 2 week > 3 week > or = adult). In 1- and 2-week-old rats entry into CSF appears to be higher than that of brain, whereas for adults entry into the brain tissue was dominant.

Relative postmortem stability of spinal motoneuronal proteins detectable by two-dimensional electrophoresis

The suitability of using spinal tissue several hours after death for analysis by high resolution two-dimensional electrophoresis has been examined. It was found that many of the proteins of bovine spinal motoneurons detectable on two-dimensional polyacrylamide gels appear to be relatively stable in situ at room temperature during the first postmortem day. When extracts of total proteins from ventral roots and motoneuronal cell bodies isolated from 1-d-old tissue were examined, all spots could be matched to control gels. Upon visual inspection of the gels, postmortem changes in the amount of stain associated with a spot were obvious in three of 364 proteins from isolated motoneuronal cell bodies and none of 237 proteins from ventral roots. Other proteins underwent quantitative changes that were detected only after computer-assisted densitometry on the gels, whereas some did not appear to change at all. In the neuropil surrounding the motoneuron cell bodies, more pronounced changes in protein patterns occurred during the postmortem period. We conclude that properly controlled two-dimensional electrophoretic analyses of postmortem spinal tissue can provide reliable qualitative and quantitative information about the antemortem protein composition of spinal motoneurons.

The blood-retinal barriers

The Blood-Retinal Barrier (BRB) is a situation of restricted permeability which is present between the blood and the retina. This barrier has a well defined anatomic substrate, particular permeability characteristics and appears to play a role of major importance in the pathophysiology and therapeutics of retinal disease. The BRB phenomenon operates fundamentally at two levels, retinal vessels and chorioepithelial interface, forming which may be better called an inner BRB and an outer BRB. The main structures involved are, for the inner BRB, the endothelial membrane of the retinal vessels, and for the outer BRB, the retinal pigment epithelium. 'Zonulae occludentes' are present in these membranes forming complete belts around the cells, sealing off the spaces between them. Other structures appear to play an accessory role. Both barriers show an apparent predominance of processes of active transport over mechanisms of passive transfer, these being extremely restricted. Much information on the pathophysiology of the BRB mechanism has been obtained from studies of its experimental breakdown. In this way, a breakdown of the inner BRB may be induced by acute distension of the vessel walls, ischaemia, chemical influences, defects in the endothelial cells and failure of the active transport system, whereas experimental ischaemia, mechanical distension of the pigment epithelial membrane, defects in the pigment epithelium and failure of the active transport systems can cause a breakdown of the outer BRB. The increased permeability of the inner BRB, and of the outer BRB, appears to be related to changes in the vascular endothelial membrane and retinal pigment epithelium, respectively. In clinical ophthalmology there are two methods for the diagnosis of breakdown of the BRB, fundus fluorescein angiography and vitreous fluorophotometry. Vitreous fluorophotometry being capable of detecting functional alterations of the barrier before any pathological changes are apparent. There is evidence of an intimate relationship between breakdown of the BRB and almost every retinal disease, particularly the vascular retinopathies and the pigment epitheliopathies. Diabetic retinopathy, hypertensive retinopathy, retinal vein obstruction, blood diseases, trauma or surgery to the eye, temporary arterial obstruction, perivasculitis, Behçet's and Coats' diseases, retinoblastoma, hemangioblastoma and retinal neovascularization are examples of situations where a breakdown of the inner BRB has been demonstrated. On the other hand, examples of breakdown of the outer BRB include situations of choroidal ischaemia, detachment of the pigment epithelium, choroidal neovascularization, photocoagulation, retinal detachment, Koyanagi's disease, central serous choroidopathy, multifocal inner choroiditis and acute placoid pigment epitheliopathy.

Influence of gonadal hormones on the developing amphibian brain: changes in ribonucleic acid, protein and activity levels of acetylcholinesterase on in vivo administration of progesterone

The levels of RNA and protein and activity levels of acetylcholinesterase decreased in the brain of developing tadpoles of 13-15-day-old Bufo melanostictus on in vivo administration of progesterone (200 mug/0.1 ml refined peanut oil). These changes suggest decelaration in the activity of the protein-synthetic machinery in progesterone administered animals.

Rapid eye movement (rem) sleep deprivation: effect on acid mucopolysaccharides in rat brain

The effect of rapid eye movement (REM) sleep deprivation on the total content and proportion of different mucopolysaccharides (AMPS) containing uronic acid in rat brain was studied. REM sleep deprivation was induced by the water tank methods. Five experimental groups of animals were used: control, stressed, REM sleep deprived, post-stress sleeping and post-deprivation sleeping rats. No changes of AMPS were observed in any of the experimental groups when the whole brain was analysed. A significant increase of AMPS was found in the cerebral hemispheres of stressed and REM deprived rats. A significant decrease of AMPS was observed in the cerebellum and brain stem. A further increase of AMPS was found in the cerebral hemispheres after the rebound of REM sleep following its deprivation, and after the recovery sleep following the stress. A significant increase of AMPS was found in the brain stem of rats allowed to recuperate after REM deprivation or stress as compared with the stressed and REM deprived animals. Recovery sleep induced a significant increase of AMPS in the cerebellum in previously stressed rats, while previously REM deprived rats exhibited a further decrease of AMPS from control values. The possible functional meaning of these results is discussed in relation to the role of REM sleep in protein synthesis and learning and memory processes. Intriguing, well-controlled positive findings and the fact that no experimental design is known where stress is minimal while REM deprivation is 100 per cent, justify and encourage continued efforts in studying the biochemical state of the brain during sleep and/or its alterations.

Inhibition of entry of some amino acids into the brain, with observations on mental retardation in the aminoacidurias

Abnormally high levels of various amino acids were maintained in the bloodstream of rats, causing saturation of amino acid transport into the brain and partial exclusion from the brain of other amino acids which are necessary for protein synthesis. Excluded amino acids could be made to enter the brain by raising their concentration in the bloodstream. The possible relevance of these findings to improvements in the dietary treatment of some inborn errors of metabolism is discussed.