The incorporation of radioactive amino acids into brain subcellular proteins during training

Posttranscriptional Gene Regulation of the GABA Receptor to Control Neuronal Inhibition

Behavior and higher cognition rely on the transfer of information between neurons through specialized contact sites termed synapses. Plasticity of neuronal circuits, a prerequisite to respond to environmental changes, is intrinsically coupled with the nerve cell’s ability to form, structurally modulate or remove synapses. Consequently, the synaptic proteome undergoes dynamic alteration on demand in a spatiotemporally restricted manner. Therefore, proper protein localization at synapses is essential for synaptic function. This process is regulated by: (i) protein transport and recruitment; (ii) local protein synthesis; and (iii) synaptic protein degradation. These processes shape the transmission efficiency of excitatory synapses. Whether and how these processes influence synaptic inhibition is, however, widely unknown. Here, we summarize findings on fundamental regulatory processes that can be extrapolated to inhibitory synapses. In particular, we focus on known aspects of posttranscriptional regulation and protein dynamics of the GABA receptor (GABAR). Finally, we propose that local (co)-translational control mechanism might control transmission of inhibitory synapses.

Protein homeostasis and synaptic plasticity

It is clear that de novo protein synthesis has an important function in synaptic transmission and plasticity. A substantial amount of work has shown that mRNA translation in the hippocampus is spatially controlled and that dendritic protein synthesis is required for different forms of long-term synaptic plasticity. More recently, several studies have highlighted a function for protein degradation by the ubiquitin proteasome system in synaptic plasticity. These observations suggest that changes in synaptic transmission involve extensive regulation of the synaptic proteome. Here, we review experimental data supporting the idea that protein homeostasis is a regulatory motif for synaptic plasticity.

Sexual differentiation of the zebra finch song system: potential roles for sex chromosome genes

BACKGROUND

Recent evidence suggests that some sex differences in brain and behavior might result from direct genetic effects, and not solely the result of the organizational effects of steroid hormones. The present study examined the potential role for sex-biased gene expression during development of sexually dimorphic singing behavior and associated song nuclei in juvenile zebra finches.

RESULTS

A microarray screen revealed more than 2400 putative genes (with a false discovery rate less than 0.05) exhibiting sex differences in the telencephalon of developing zebra finches. Increased expression in males was confirmed in 12 of 20 by qPCR using cDNA from the whole telencephalon; all of these appeared to be located on the Z sex chromosome. Six of the genes also showed increased expression in one or more of the song control nuclei of males at post-hatching day 25. Although the function of half of the genes is presently unknown, we have identified three as: 17-beta-hydroxysteroid dehydrogenase type IV, methylcrotonyl-CoA carboxylase, and sorting nexin 2.

CONCLUSION

The data suggest potential influences of these genes in song learning and/or masculinization of song system morphology, both of which are occurring at this developmental stage.

In vivo rates of protein synthesis in brain, muscle, and liver of five vertebrate species

To compare cerebral protein metabolism rates in vivo, protein synthesis rates of three organs of five vertebrate species were measured after a single i.p. injection of a flooding dose of [1-14C]valine. In muscle, brain, and liver, the respective average protein synthesis rates, expressed as percent of total protein-bound valine replaced per hour, that is, percent synthesis per hour, in goldfish at 22 degrees C body temperature, were 0.07, 0.23, and 0.57%; in the bullfrog at 20 degrees C, 0.06, 0.18, and 0.55%; in the white Leghorn chicken at 39 degrees C, 0.24, 0.70, and 2.17%; and in the mouse at 38 degrees C, 0.22, 0.65, and 2.0%. In the Tokay lizard at different body temperatures, the synthesis rates were 0.04, 0.13, and 0.43% at 26 degrees C; 0.05, 0.20, and 0.63% at 32 degrees C; and 0.07, 0.27, and 0.81% at 38 degrees C. The results demonstrate differences in protein synthesis rates in organs of the various species examined. The differences among the species seem to be due, to a major extent, to differences in body temperature; rates in lizard are below those in other species at temperatures tried. Protein synthesis rates in brain in all species are almost three times lower than those in liver and almost three times higher than those in muscle.

Recovery of monkey brain after prolonged ischemia. II. Protein synthesis and morphological alterations

Recovery of protein synthesis following 1 h of complete ischemia of the monkey brain was assessed by 3H-labeled amino acid incorporation in vivo at various postischemic periods between 1.5 and 24 h. The regional autoradiographic patterns obtained were compared on the basis of precursor-product relationships determined biochemically at the end of the tracer incorporation studies. Shortly after ischemia, protein synthesis was severely inhibited, but it gradually recovered with increasing recirculation times. In the cerebellum it returned to almost normal levels within 3 h and in the cortex within 24 h. Hippocampal and thalamic regions, however, did not recover control levels of protein synthesis at 24 h. Histoautoradiographic evaluation of amino acid incorporation in individual neurons revealed recovery of pyramidal neurons in the CA1 and CA3 sectors of the hippocampus within 6 h of recirculation, which, however, was followed by secondary inhibition after longer recirculation. Neurons in cortical layer 5 steadily recovered to near control within 24 h, with the exception of those located in arterial border zones, which returned to only 50% of control at 24 h. Incomplete recovery was also observed in thalamic neurons and Purkinje cells. The regional and histoautoradiographic pattern of protein synthesis correlated with the morphological appearance of cells. Ischemic cell changes (mainly of the dark type with microvacuolization and perineuronal glial swelling) were marked after short recirculation times but gradually disappeared in parallel with the return of protein synthesis in most regions of the brain. Only in pyramidal cells of the hippocampus, thalamic neurons, and Purkinje cells were changes not reversed during the observation period. The results obtained corroborate the electrophysiological observations reported in the first part of this investigation and support the notion that the majority of the neurons of monkey brain survive complete cerebrocirculatory arrest of 1 h for at least 1 day.

Brain membrane disordering by administration of a single ethanol dose

There is a bulk of evidence that ethanol exerts an important direct effect on biological membranes, especially in the central nervous system, during chronic administration. Whether membranes are affected after an acute and subacute ethanol administration remains to be demonstrated. Crude synaptic membrane fluidity (checked by fluorescence polarization) together with (Na+ +K+)ATPase activity were therefore examined 18 hours after a single oral ethanol administration (5 g/kg bwt.) to naive rats or to rats previously intubated with ethanol repeatedly during 4 days (increasing the daily dose from 7 to 10 g/kg). The sensitivity of both parameters to different concentrations of ethanol added in vitro (0.175 M-1.400 M) was also determined. Although no changes in the basal intrinsic fluidity were found, (Na+ +K+)ATPase activity increased slightly after administration of ethanol to naive as well as to short-term ethanol intoxicated rats. The fluidizing as well as the ATPase inhibiting effects following the addition of ethanol in vitro were markedly increased 18 hours after ethanol administration to naive rats. Such an hypersensitization seems not to be related to an unspecific stress or to changes in body temperature and was no longer apparent in short-term ethanol intoxicated rats. Disappearance of the acute ethanol induced hypersensitization with further ethanol administration may represent the first stage of tolerance acquisition.

Brain membrane disordering related to acute ethanol administration in naive and short-term ethanol-intoxicated rats

Crude synaptic membrane fluidity (checked by fluorescence polarization) together with (Na+ + K+) ATPase activity were examined 18 hours after a single oral ethanol administration (5 g/kg bwt.) to naive rats and to rats previously intubated with ethanol repeatedly during 4 days. The sensibility of both parameters to different concentrations of ethanol added in vitro (0.175 M-1.400 M) was also determined. Although no changes in the basal intrinsic fluidity were found, (Na+ + K+)ATPase activity increased slightly in both conditions. The fluidizing as well as the ATPase inhibiting effects following the addition of ethanol in vitro were markedly increased 18 hours after ethanol administration to naive rats. This hypersensitization was no longer apparent in rats pretreated with ethanol during 4 days. The acute ethanol-induced hypersensitization found in naive rats appears not to be related to an unspecific stress or to changes in body temperature. The disappearance of this hypersensitization in short-term alcohol-intoxicated animals may represent the first stage of tolerance acquisition.

The characterization and phosphorylation of an actin-like protein in synaptosomal membranes

A protein of 43,000 daltons, named protein 'C', is a component of synaptosomal plasma membranes, vesicular and microsomal membranes, as well as synaptosomal and cellular cytoplasm. Protein 'C' undergoes endogeneous phosphorylation in synaptosomal plasma membranes but not in other subcellular fractions. This phosphorylation is stimulated by papaverine and calcium, inhibited by magnesium and not affected by cyclic nucleotides. Protein 'C' and muscle actin were shown to be very similar by isoelectric focusing, two dimensional gel electrophoresis, and by peptide mapping. This suggests that protein 'C' is an actin-like protein which undergoes endogenous phosphorylation specifically in synaptosomal plasma membranes. Phosphorylation of protein 'C' may be involved in neurotransmitter release.

Role of RNA and protein synthesis in memory formation

A brief review is given of experiments which are concerned with the hypothesis that brain RNA and protein synthesis are directly involved in the establishment of long-term memory. It is concluded that these experiments neither support or refute this hypothesis. A convincing demonstration is lacking of interanimal memory transfer by injection of macromolecular extracts. The majority of experiments which attempt to correlate increased macromolecular synthesis with learning use radioactive precursor methods and these studies do not exclude possible changes in precursor specific activity as the cause of the increased labeling. Although some studies find directly observable changes in brain macromolecules in response to training, their relationship to memory formation is unclear. It is possible that these changes represent only an enhanced production of constitutive macromolecules in response to an increase in cerebral metabolism during training, rather than molecular changes that are directly involved with modifying synaptic connectivity. Inhibitors of cerebral protein synthesis block memory formation, but these drugs are not pharmacologically specific and this complicates the interpretation of these studies.

Relative synthesis of myelin in different brain regions of postnatally undernourished rats

We used a double isotope procedure and starved and normal littermate rats to compare relative protein synthesis in the cerebellar nuclear, myelin, synaptosomal, mitochondrial, and microsomal subfractions of postnatally starved animals. The remaining brain tissue was dissected into 6 additional regions (cerebral cortex, medulla oblongata, midbrain, hippocampus, striatum, and hypothalamus) and these were frozen for similar subcellular fractionation and analysis at a later date. The microsomal fraction derived from frozen tissues was discarded. The results show that early postnatal starvation specifically depresses myelin synthesis to about the same extent in all major brain regions at 18 and 21 days of age.

ACTH and the stress-induced changes of lysine incorporation into brain and liver proteins

When mice were subjected to footshock treatment and subsequently injected with [3H] lysine, the cerebral uptake of [3H] lysine, its incorporation into brain protein and the relative radioactivity (RR = protein radioactivity divided by amino acid radioactivity) were all increased. In the liver, footshocked mice showed decreased free lysine radioactivity, and increased protein radioactivity and relative radioactivity compared to quiet mice. The possibility that ACTH mediated these effects was investigated. The injection of saline had no effect in the brain but partially mimicked the footshock responses in the liver. Injections of ACTH 1--24 mimicked the effects of footshock in the brain, and further augmented the saline-induced effect on the RR in the liver. ACTH 4--10 increased the RR of brain protein, but produced no significant change in brain free lysine radioactivity or in any measure in the liver. Pretreatment of mice with the synthetic glucocorticoid, dexamethasone, did not enhance these effects and diminished the effect of ACTH 4--10 in the brain. ACTH treatment did not alter the profiles of brain polyribosomes. Lysine vasopressin, which is also released during stress, did not alter the incorporation of [3H] lysine into brain or liver protein, except at high doses when it decreased plasma radioactivity. These results suggest that secretion of ACTH at least partially mediates the stress-induced changes of [3H] lysine incorporation into brain and liver proteins, but that it is probably not the only factor involved.

Effects of stressful procedures as ether anesthesia and intracranial injections on amino acid incorporation into brain protein

Ether anesthesia elevates plasma corticosterone levels considerably and interferes severely with the incorporation of centrally applied [3 H] leucine into brain protein. Only minor changes in leucine incorporation are observed in conscious rats with an implanted cannula in the third brain ventricle as compared to noncannulated controls. Injection through this cannula can be regarded as a minor stressful procedure comparable to subcutaneous injection both causing moderate elevations in plasma corticosterone levels. Injection through the cannula per se did not affect the leucine incorporation. In studies which require local application of the radioactive precursor, it is therefore recommended to avoid ether anesthesia and to use a preimplanted cannula in the brain ventricular system.

Effects of imprinting on lysine uptake and incorporation into protein in chick brain

One-day old chicks were exposed for either 30, 60, or 120 min to an imprinting stimulus or kept in darkness in similar conditions. At the end of this time they were injected peripherally with 14C-lysine and killed 20 min later. The radioactivity of free lysine and that incorporated into protein was measured; incorporation was found to differ between exposed and dark birds only in the anterior part of the forebrain roof after 60-min treatment (E/D = 1.25). However, more free radioactive lysine was found in all brain regions of exposed birds at this time. When the specific radioactivity of the free lysine (dpm/nmol lysine) was measured there were no differences between the two types of birds, indicating that the incorporation difference was not due to a change in precursor radioactivity. The use of 14C-2-aminoisobutyrate confirmed that even with a nonincorporated amino acid pool size changes still occurred. The greater lysine incorporation in anterior forebrain roof was largely restricted to the cytoplasmic soluble fraction.

Brain protein metabolism and the acquisition of new behaviors. II. Immunological studies of the alpha, beta and gamma proteins of goldfish brain

In a previous study, the labeling pattern of three proteins (alpha, beta and gamma) in goldfish brain was found to change after the animals successfully acquired a new pattern of behavior. In the present study, these proteins were isolated from the brain cytoplasmic fraction, purified by successive gel electrophoresis and used as antigent to immunize rabbits. Antisera containing antibodies to two of the proteins (beta and gamma) were obtained. These gave single precipitin bands when plated against the antigens and a mixture of the total cytoplasmic proteins. The distribution of beta and gamma in brain subcellular fractions and in a variety of goldfish tissues was determined by immunodiffusion methods. gamma was specific to brain. The beta protein cross-reacted but was not identical to a widely distributed substance in plasma, liver and kidney. Both beta and gamma appear to be species specific in that no cross-reactivity was obtained with mouse, chick or rat brain proteins. Immunological methods, in combination with double labeling experiments were used to establish that the beta and gamma antigens were proteins which were normally present in goldfish brain. Both the beta and gamma antisera were equally capable of specifically precipitating the proteins which were differentially labeled after training as well as purified proteins of the same molecular weight present in the brains of control animals. These results suggest that the acquisition of a new pattern of behavior can increase the demand for the synthesis of specific proteins (beta and gamma) normally present in goldfish brain.

Identification of specific changes in the pattern of brain protein synthesis after training

Double labeling studies with [3H]valine and [14C]valine were used to investigate the pattern of protein synthesis in the brains of goldfish. The protein fractions in three bands (alpha, beta, and gamma) on sodium dodecyl sulfate-polyacrylamide gels indicate that more valine was incorporated in the brains of goldfish that had been trained in a vestibular conditioning task than in the brains of untrained fish or fish trained in a variety of control behavioral situation. Changes in the pattern of labeling were localized in the cytoplasmic fraction of the brain; no increases in labeling occurred in either the nuclear or synaptosomal components. The results suggest that a specific change occurs in the pattern of protein synthesis in the brain after the acquistion of a new behavior.