Blockade of nitric oxide signalling promotes resilience to the effects of social defeat stress on the conditioned rewarding properties of MDMA in mice.

MDMA abuse continues being a serious problem in our society. Environmental factors, such as stress, increase the vulnerability of individuals to develop drug abuse and we have observed that exposure to social defeat (SD) stress alters the sensitivity of mice to the rewarding effects of MDMA in the conditioned place preference (CPP) paradigm. In the present study, we evaluated the role of the nitric oxide (NO) pathway in the effects of SD on the rewarding properties of MDMA. Three groups of mice were treated with an inhibitor of NO synthesis, 7-nitroindazole (0, 7.25 and 12.5 mg/kg), before each exposure to SD and place conditioning with MDMA (1.25 mg/kg) on PND 54, 56, 58, and 60. One control group was not exposed to SD before place conditioning. In addition, we studied the effects of SD on the levels of nitrites in the striatum, hippocampus and frontal cortex. Our results showed that the low dose of 7-nitroindazole blocked the effects of SD on the rewarding properties of MDMA. Moreover, SD exposure increased the nitrites in the prefrontal cortex and hippocampus. These results demonstrated the role of NO signalling in the effects of SD stress in mice and suggested that the inhibition of NO synthesis may confer resilience to the effects of social stress on the rewarding properties of MDMA. The manipulation of the NO signalling pathway could be a useful target for the treatment of MDMA-dependent subjects who experienced high levels of stress.

Role of NMDA and AMPA glutamatergic receptors in the effects of social defeat on the rewarding properties of MDMA in mice.

Exposure to social stress alters the response to drugs of abuse of experimental animals. Changes in the glutamatergic system seem to play a role in the effects of social defeat stress on the rewarding properties of cocaine and amphetamine. The aim of the present study was to evaluate the involvement of N-methyl-D-aspartate (NMDA) and α-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid (AMPA) glutamate receptors in the effects of social defeat on the conditioned place preference induced by 3,4-methylenedioxymethamphetamine (MDMA). Our hypothesis was that changes in these receptors could mediate the effects of social defeat on MDMA reward. Young adult male mice were exposed to an episode of social defeat with an aggressive conspecific immediately before each conditioning session with MDMA (1.25 mg/kg, four sessions on alternating days). According to the treatment received before defeats, six groups were used: saline, 5 or 10 mg/kg of memantine (NMDA antagonist) and 0.25, 1 or 5 mg/kg of 6-cyano-7-nitroquinoxaline-2,3-dione (CNQX) (AMPA antagonist). One control group was exposed to exploration before place conditioning. In two additional defeated and control groups, the membrane expression of NMDA and AMPA receptors was determined in the striatum and the hippocampus. Control and memantine-treated groups developed place preference, but not defeated mice treated with saline or CNQX, suggesting that the blockade of NMDA receptors reversed the effects of social defeat. Social defeat decreased the expression of several subunits of NMDA and AMPA receptors, mainly GluN1 and GluA1. These results demonstrated that glutamatergic plasticity is involved in the effects of social defeat stress on MDMA reward.

Sildenafil citrate improves perinatal outcome in fetuses from pre-eclamptic rats.

OBJECTIVE: To evaluate perinatal outcome after sildenafil citrate (SC) administration at the onset of pregnancy in a rat pre-eclampsia model. DESIGN: In vivo animal experimental study. SETTING: Fundación IVI-Instituto Universitario IVI, Valencia, Spain. SAMPLE: Control and pre-eclampsia-induced pregnant Wistar rats exposed to chronic SC administration. METHODS: We evaluated the use of SC, which was tested as a potential therapeutic tool to maintain vasodilatation in complicated pregnancies. We have demonstrated previously that SC shows a hypotensive selective effect in normal rat pregnancies when compared with nonpregnant animals. MAIN OUTCOME MEASURES: Maternal blood pressure, weight and mortality during pre- and postnatal development, maternal blood cellularity and haemodynamic changes with maternal and fetal Doppler quantitative indices. RESULTS: SC restores normal values of blood pressure, cell count and proteinuria for maternal syndrome. In offspring, SC improves weight gain and increases survival rates without fetotoxic effects. According to the haemodynamic results, SC has a significant effect on the resistance index in the uterine artery in pre-eclamptic animals, as it restores normal values to correlate with an increase in fetal perfusion through the ductus venosus. CONCLUSIONS: These results suggest that SC administration during pregnancy may have a potential benefit for the treatment of hypertension during pregnancy by reversing the maternal effects of pre-eclampsia and by improving uteroplacental and fetal perfusion.

Haemodynamic effects of long-term administration of sildenafil in normotensive pregnant and non-pregnant rats.

OBJECTIVE: To determine the effects of chronic administration of sildenafil citrate on healthy pregnant rats. DESIGN: In vivo animal experimental study. SETTING: Fundación IVI-Instituto Universitario IVI, Valencia, Spain. SAMPLE: Pregnant and non-pregnant Wistar rats exposed to chronic administration of sildenafil. METHODS: Placental cross-barrier and feto-maternal relationship levels, maternal blood pressure, and haemodymamic effects on uterine arteries were evaluated. The effect of growth on weight and fetal tissues, and on perinatal outcome, was investigated. MAIN OUTCOME MEASURES: Maternal blood pressure, blood viscosity, vascular indices of uterine arteries and fetal ductus venosus, plasmatic levels of sildenafil, embryo/fetal and litter weights, perinatal/postnatal survival rates. RESULTS: Sildenafil citrate crossed the placenta. The maternal and fetal levels of sildenafil, and its metabolite desmethyl-sildenafil, demonstrated a positive linear correlation in treated pregnant animals versus controls; a selective maternal hypotensive effect without changes in uterine vascular resistance was noted on days E8 and E11 (embryonic day). The lower pulsatility index of the ductus venosus on day E18 suggests fetal overflow at the end of the pregnancy. Effects on offspring were placental and liver enlargement, and increased fetal weight gain in the second half of pregnancy (irrespective of liver enlargement) and at birth. Perinatal and postnatal survival rates in the sildenafil group remained unaltered. No haemodynamic effects were evident in non-pregnant animals. CONCLUSIONS: In normotensive rats, sildenafil appears to have a selective effect at the onset of pregnancy, implying increased fetal blood supply, and increased fetal weight, and placental and liver enlargement, but no increased perinatal mortality.

Ultrasound bioeffects in rats: quantification of cellular damage in the fetal liver after pulsed Doppler imaging.

OBJECTIVE: To determine whether pulsed Doppler examination of the ductus venosus in rat fetuses could damage exposed tissue. METHODS: On gestational day 18, the livers of a mean of approximately five fetuses per mother (n = 5.14, SD = 1.6), in a cohort of 35 pregnant female rats, were exposed individually to pulsed Doppler and these were considered the 'exposed group'. The remaining fetuses in each pregnant rat (n = 5.16, SD = 2.1) formed the 'control group'. We tested for 600, 300, 60, 20, 15, 10 and 3 s of exposure of the fetal ductus venosus and the damage was evaluated measuring a cell death index of apoptotic activity at 7 h post-exposure (n = 16). In addition, subgroups of mothers were sacrificed at 2, 4, 5, 7, 12 and 24 h post-exposure to determine when the damage appeared and disappeared and whether this depended on the exposure time. RESULTS: After exposure of 20 s or more, we observed significant damage, as assessed by caspase 3 activity (a marker of apoptotic activity related to tissue damage), in all cases; after 15 s of exposure, some samples presented damage (P = 0.4); there was no damage after 10 s or 3 s of exposure (P = 0.87 and P = 0.3, respectively). There was a positive linear correlation between apoptotic index and pulsed Doppler exposure time, (Pearson's coefficient = 0.324, P < 0.01). No liver still showed significant damage at 12 or 24 h post-exposure (P > 0.05 and P > 0.4). CONCLUSIONS: We observed reversible damage after pulsed Doppler imaging in an in-vivo fetal liver tissue rat model and found that longer exposure times produced more tissue damage. We established that 10 s was the maximum exposure time to ensure absence of damage to tissue in this model. It would appear sensible to recommend expert supervision of pulsed Doppler imaging and to have intervals between subsequent examinations.

Expression and traffic of cellular prolyl oligopeptidase are regulated during cerebellar granule cell differentiation, maturation, and aging.

Prolyl oligopeptidase (POP) is an endopeptidase which cleaves short proline-containing neuropeptides, and it is involved in memory and learning. POP also has an intercellular function mediated through the inositol pathway, and has been involved in cell death. POP has been early considered as a housekeeping enzyme, but the recent research indicates that POP expression is regulated across tissues and intracellularly. In the brain, POP is exclusively expressed in neurons and most abundantly in pyramidal neurons of cerebral cortex, in the CA1 field neurons of hippocampus and in cerebellar Purkinje's cells. Intracellularly, POP is mainly present in the cytoplasm and some in intracellular membranes, like rough endoplasmic reticulum and Golgi apparatus. In this paper, we systematically studied the levels of expression of POP along the life of cerebellar granule cells (CGC) in culture and the distribution of POP within different intracellular compartments. We used the tight-binding inhibitor JTP-4819 covalently coupled with fluorescein (FJTP) as a tool to study the changes on expression and localization of POP protein. Our results indicate that POP activity levels are regulated during the life of the neurons. POP was found mainly in cytoplasm and neuronal projections, but at an early developmental phase significant amounts were found also in nuclei. Along the life of the neurons, POP activity fluctuated in 7-day cycles. In young neurons, the cytosolic POP activity was low but increased by maturation so that the activity peak coincided with full differentiation. Over aging, cytoplasmic POP was concentrated around nucleus, but the activity decreased with time. POP was also present in vesicles across the neuron. No major changes were seen in the nuclear or membrane bound POP over aging until activity disappeared upon neuronal death. This is the first time when POP was found in the nuclei of human neuronal cells.

Developmental Exposure to Pesticides Alters Motor Activity and Coordination in Rats: Sex Differences and Underlying Mechanisms.

It has been proposed that developmental exposure to pesticides contributes to increasing prevalence of neurodevelopmental disorders in children, such as attention deficit with hyperactivity (ADHD) and to alterations in coordination skills. However, the mechanisms involved in these alterations remain unclear. We analyzed the effects on spontaneous motor activity and motor coordination of developmental exposure to a representative pesticide of each one of the four main chemical families: organophosphates (chlorpyrifos), carbamates (carbaryl), organochlorines (endosulfan), and pyrethroids (cypermethrin). Pesticides were administered once a day orally, in a sweet jelly, from gestational day 7 to post natal day 21. Spontaneous motor activity was assessed by an actimeter and motor coordination using the rotarod, when rats were adults. The effects were analyzed separately in males and females. Extracellular GABA in cerebellum and NMDA receptor subunits in hippocampus were assessed as possible underlying mechanisms of motor alterations. Motor coordination was impaired by developmental exposure to endosulfan, cypermethrin, and chlorpyrifos in females but not in males. The effect of endosulfan and cypermethrin would be due to increased extracellular GABA in cerebellum, which remains unaltered in male rats. Chlorpyrifos increased motor activity in males and females. Cypermethrin decreased motor activity mainly in males. In male rats, but not in females, expression of the NR2B subunit of NMDA receptor in hippocampus correlated with motor activity. These results show sex-specific effects of different pesticides on motor activity and coordination, associated with neurotransmission alterations. These data contribute to better understand the relationship between developmental exposure to the main pesticide families and motor disorders in children.

Hippocampal long-term potentiation is reduced in mature compared to young male rats but not in female rats.

Aging is associated with a decline in cognitive function which could be due to reduced synaptic plasticity. Hippocampal long-term potentiation (LTP) is an activity-dependent form of increased transmission efficacy at synapses that is considered the basis for some forms of learning and memory. We studied the N-methyl-d-aspartic acid (NMDA) receptor-dependent LTP in the CA1 region of hippocampus in young (2 months) and mature (8 months) male and female rats. We have found that in young male rats the tetanus increased the magnitude of excitatory post-synaptic potentials to 204+/-10% of basal while in mature male rats the magnitude of the LTP was significantly lower reaching only 153+/-11% of basal. This decrease did not occur in female rats. Similar changes occurred in the content of the NMDA receptor subunits NR1 and NR2A in hippocampus. The amount of both subunits was reduced significantly (15-16%) in hippocampus of 8-month-old compared with 2-month-old male rats. This decrease was not observed in female rats. Moreover, there is a significant correlation between the content of NR1 subunit and the magnitude of the potentiation. These data suggest that some of the neurobiological changes induced in hippocampus by aging are different in males and females.

Activation of soluble guanylate cyclase by nitric oxide is increased in lymphocytes from both rats chronically exposed to 2,5-hexanedione and workers chronically exposed to n-hexane.

Although occupational exposure to n-hexane induces neurotoxic effects in the central and peripheral nervous systems, the mechanisms of its neurotoxicity remain unclear. n-Hexane is metabolized to 2,5-hexanedione (2,5-HD), which is the neurotoxic agent and the indicator chosen for the biological monitoring of exposed workers. It has been previously reported that chronic exposure to 2,5-HD impairs the glutamate-nitric oxide-cyclic GMP pathway at the level of activation of soluble guanylate cyclase (sGC) enzyme by nitric oxide (NO), both in cultured neurons and in the cerebellum of rats in vivo. The aim of this study was to assess whether the activation of sGC by NO is also altered in lymphocytes from rats treated with 2,5-HD and/or workers chronically exposed to n-hexane. Lymphocytes were isolated from male Wistar rats treated with 2,5-HD in drinking water, and from blood samples from shoe-factory workers environmentally and chronically exposed to n-hexane. Urine samples were also collected from workers at the end of the shift in order to measure the urinary levels of 2,5-HD. Activation of sGC by NO was significantly higher (p<0.05) in lymphocytes from rats treated with 2,5-HD than in control rats. In isolated lymphocytes from exposed workers the activation of sGC by NO also increases (p<0.05) in contrast to the controls. The results presented here indicate that the activation of lymphocytes could be an indicator of the toxicity produced by being exposed to n-hexane, since the effects observed in workers chronically exposed to n-hexane are similar to those found in rats chronically treated with 2,5-HD in drinking water.

Chronic exposure to ammonia alters basal and NMDA-induced phosphorylation of NMDA receptor-subunit NR1.

Hyperammonemia is responsible for many of the neurological alterations in patients with hepatic encephalopathy by mechanisms that remain unclear. Hyperammonemia alters phosphorylation of brain protein kinase C substrates and impairs N-methyl-d-aspartate (NMDA) receptor-associated signal transduction. The aim of this work was to analyze, in rat cerebellar neurons in culture, the effects of ammonia exposure on NMDA receptor phosphorylation, MK801 binding and surface expression. Ammonia reduces MK801 binding to NMDA receptors and the surface expression of the NR1 and NR2A subunits. As phosphorylation of serines in the NR1 C1 cassette has been implied in receptor trafficking, we assessed whether hyperammonemia alters phosphorylation of these serines. Basal phosphorylation of serines 890, 896 and 897 was increased in neurons exposed to ammonia, while NMDA-induced phosphorylation of S896 and S897 was reduced. Exposure to ammonia also increased basal phosphorylation of Akt but reduced NMDA and BDNF stimulation of Akt phosphorylation. These results suggest that alterations in receptor surface expression and possibly the phosphorylation state of the NR1 subunit of NMDA receptors may contribute to the impairment by ammonia of signal transduction pathways modulated by NMDA receptors.

Chronic hyperammonemia prevents changes in brain energy and ammonia metabolites induced by acute ammonium intoxication.

Acute ammonia toxicity has been attributed to the depletion of energy metabolite intermediates. Ingestion of an ammonium containing diet produces hyperammonemia and protects rats against acute ammonium intoxication. We have tested the effect of chronic hyperammonemia on the brain contents of energy and ammonia metabolite intermediates and on the effect on these contents of acute ammonia intoxication (i.p. injection of 7 mmol/kg of ammonium acetate). Chronic hyperammonemia was induced in rats by feeding them a diet containing 20% ammonium acetate. Control rat were fed the same diet without addition of ammonium acetate. It is shown that chronic hyperammonemia did not affect the content of most metabolites, the only remarkable changes are the increases of the contents of ammonia (46%), glutamine (81%), acetoacetate (31%) and of the mitochondrial NAD+/NADH ratio (32%) as well as the marked decrease of beta-hydroxybutyrate (by 86%). Chronic hyperammonemia prevents most changes in metabolites induced by acute ammonium intoxication (i.p. injection of 7 mmol/kg of ammonium acetate). In control rats it was a marked breakdown of glycogen and increased contents of glucose, lactate and pyruvate, with decreased cytosolic NAD+/NADH ratio and beta-hydroxybutyrate and ATP contents. These changes were nearly completely prevented in hyperammonemic rats. In controls, ammonia increased 12.8-fold while glutamate and aspartate decreased by approximately 40% and glutamine and alanine raised by 37% and 93%, respectively; in hyperammonemic rats ammonia increased 6.9-fold while glutamate, glutamine and alanine were not significantly affected. Also the mitochondrial NAD+/NADH ratio raised by 18-fold in controls and by 6-fold in hyperammonemic rats. These results indicate that chronic hyperammonemia markedly prevents the alterations of the contents of energy and ammonia metabolites induced by acute ammonium intoxication.

Sustained recovery of Na(+)-K(+)-ATPase activity in sciatic nerve of diabetic mice by administration of H7 or calphostin C, inhibitors of PKC.

We have previously shown that intraperitoneal injection of H-7, an inhibitor of PKC, restores completely the activity of Na(+)-K(+)-ATPase in sciatic nerve of diabetic mice; however, the effect was transient, with a half-life of approximately 1 h under the conditions used. This work assessed whether calphostin C, a new more potent and specific inhibitor of PKC, is also able to restore the activity of Na(+)-K(+)-ATPase in sciatic nerve of ALX-induced diabetic mice and also assessed if continuous administration of H-7 or calphostin C can afford sustained recovery of the ATPase. Small amounts of calphostin C (i.e., 2 micrograms/kg) restore entirely the activity of the enzyme. Larger doses (e.g., 30 micrograms/kg) can be administered with equal results. The ED50 was approximately 0.5 micrograms/kg. This indicates that calphostin C is approximately 20,000 times more potent than H-7 in restoring the ATPase activity in diabetic mice. A single intraperitoneal injection of 1 or 10 micrograms/kg of calphostin C maintains the enzyme for 4 and 8 h, respectively. Administration of H-7 by continuous delivery from micro-osmotic pumps implanted in the back of the mice maintains the Na(+)-K(+)-ATPase for 24 h, although the activity decreases thereafter. This is the result of instability of H-7 in solution. Continous administration of calphostin C maintains the activity of the ATPase at nearly normal values for at least 2 wk. The results support the hypothesis that, in sciatic nerve tissue of diabetic animals, the activity of PKC is increased, leading to higher phosphorylation of Na(+)-K(+)-ATPase, which results in the decreased activity observed.(ABSTRACT TRUNCATED AT 250 WORDS)

Chronic exposure to ammonia alters the modulation of phosphorylation of microtubule-associated protein 2 by metabotropic glutamate receptors 1 and 5 in cerebellar neurons in culture.

Hyperammonemia impairs signal transduction associated to glutamate receptors and phosphorylation of some neuronal proteins including microtubule-associated protein 2 (MAP-2). The aim of this work was to analyze the effects of hyperammonemia on modulation of MAP-2 phosphorylation by metabotropic glutamate receptors (mGluRs) in rat cerebellar neurons in culture. Hyperammonemia increased basal phosphorylation of MAP-2 (180%). Activation of mGluRs 1 and 5 with (S)-3,5-dihydroxyphenylglycine (DHPG) increased MAP-2 phosphorylation (170%) in control neurons but not in neurons exposed to ammonia. Activation of mGluRs 2 and 3 with (2S,3S,4S)-CCG/(2S, 1'S,2'S)-2-(carboxycyclopropyl)glycine increased slightly (25%) MAP-2 phosphorylation in neurons exposed to ammonia or not. Activation of mGluR5 with (+/-)-trans-azetidine-2,4-dicarboxylic acid increased MAP-2 phosphorylation (24%) in control neurons but decreased it by 56% in neurons exposed to ammonia. Activation of mGluR1 using 2-methyl-6-(phenylethynyl)pyridine and DHPG increased MAP-2 phosphorylation 183% in control neurons but only 89% in neurons exposed to ammonia. In control neurons mGluR1 activation greatly increases phosphorylation of MAP-2, while activation of mGluRs 5, 2 or 3 increased it slightly. Taken together, hyperammonemia reduces the increase in MAP-2 phosphorylation induced by mGluR1activation. Moreover, in neurons exposed to ammonia activation of mGluR5 reduces MAP-2 phosphorylation. These effects reflect significant alterations in signal transduction associated to mGluR1 and mGluR5 in hyperammonemia that may contribute to altered glutamatergic neurotransmission and to the neurological alterations in hyperammonemia and hepatic encephalopathy.

Bile duct ligation plus hyperammonemia in rats reproduces the alterations in the modulation of soluble guanylate cyclase by nitric oxide in brain of cirrhotic patients.

Modulation of soluble guanylate cyclase (sGC) by nitric oxide (NO) is altered in brain from cirrhotic patients. The aim of this work was to assess whether an animal model of cirrhosis, bile duct ligation, alone or combined with diet-induced hyperammonemia for 7-10 days reproduces the alterations in NO modulation of sGC found in brains from cirrhotic patients. sGC activity was measured under basal conditions and in the presence of NO in cerebellum and cerebral cortex of the following groups of rats: controls, bile duct ligation without or with hyperammonemia and hyperammonemia without bile duct ligation. In cerebellum activation of sGC by NO was significantly lower in bile duct ligated rats with (12 +/- five-fold) or without (14 +/- six-fold) hyperammonemia than in control rats (23 +/- seven-fold). In cerebral cortex activation of sGC by NO was higher in rats with bile duct ligation with hyperammonemia (124 +/- 30-fold) but not without hyperammonemia (59 +/- 15-fold) than in control rats (66 +/- 11-fold). The combination of bile duct ligation and hyperammonemia reproduces the alterations in the modulation of soluble guanylate cyclase by NO found in cerebral cortex and cerebellum of cirrhotic patients while bile duct ligation or hyperammonemia alone reproduces the effects in cerebellum but not in cerebral cortex.

Neuronal and inducible nitric oxide synthase expression in the rat cerebellum following portacaval anastomosis.

In order to determine the role of neuronal nitric oxide synthase (nNOS) and inducible nitric oxide synthase (iNOS) in the pathogenesis of experimental hepatic encephalopathy (HE), the expression of both was analyzed in the cerebellum of rats 1 month and 6 months after performing portacaval anastomosis (PCA). In control cerebella, nNOS immunoreactivity was mainly observed in the molecular layer (ML), whereas the Purkinje cells did not express nNOS. However, nNOS expression was detected in the Purkinje cells at 1 month after PCA, correlating with a decrease in nNOS expression in the ML--part of an overall reduction in cerebellar nNOS concentrations (as determined by Western blotting). At 6 months post-PCA, a significant increase in nNOS expression was observed in the ML, as well as increased nNOS immunoreactivity in the Purkinje cells. nNOS immunoreactivity was also observed in the Bergmann glial cells of PCA-treated rats. While no immunoreactivity for iNOS was seen in the cerebella of control rats, iNOS immunoreactivity was significantly induced in the cerebellum 1 month after PCA. In addition, the expression of iNOS was greater at 6 months than at 1 month post-PCA. Immunohistochemical analysis revealed this iNOS to be localized in the Purkinje cells and Bergmann glial cells. The induction of iNOS in astroglial cells has been associated with pathological conditions. Therefore, the iNOS expression observed in the Bergmann glial cells might play a role in the pathogenesis of HE, the harmful effects of PCA being caused by them via the production of excess nitric oxide. These results show that nNOS and iNOS are produced in the Purkinje cells and Bergmann glial cells following PCA, implicating nitric oxide in the pathology of HE.

Prenatal exposure of rats to ammonia impairs NMDA receptor function and affords delayed protection against ammonia toxicity and glutamate neurotoxicity.

The aim of this work was to assess whether perinatal hyperammonemia impairs the function of NMDA receptors and whether this impairment affords protection against acute ammonia toxicity and glutamate and NMDA neurotoxicity. Rats were exposed to ammonia during the prenatal and lactation periods by feeding the female rats an ammonium-containing diet since day 1 of pregnancy. After weaning (at postnatal day 21), the pups were fed a normal diet with no ammonia added. This treatment resulted in a marked decrease of the growth rate of the animals, which was maintained even 1 month after normalization of ammonia levels. Rats exposed to ammonia were more resistant than controls to acute ammonia toxicity 13 days after feeding a normal diet but not at 3 months. Primary cultures of cerebellar neurons from hyperammonemic rats showed decreased binding of [3H]MK-801 and were remarkably more resistant than controls to glutamate and NMDA toxicities. Also, the increase in aspartate aminotransferase activity induced by low concentrations of NMDA was not produced in such cultures. These results indicate that exposure to ammonia during the prenatal and lactation periods results in long-lasting impairment of NMDA receptor function. This would be the reason for the delayed protection afforded by exposure to low ammonia levels against acute ammonia toxicity in animals and against glutamate and NMDA toxicity in neuronal cultures.

Precursors of mitochondrial proteins are degraded in the cytosol at different rates.

The stability of rat liver mitochondrial protein precursors in the cytosol was investigated. The precursors were synthesized in a post-mitochondrial supernatant from rat liver, and mitochondria were then added at different times. The amount and pattern of proteins incorporated were determined. The precursors have different rates of transport into mitochondria. Some precursors are stable in the cytosol while most are degraded rapidly, with a half-life of about 30 min at 30 degrees C.

Retention of actin synthesis in liver under conditions that inhibit synthesis of almost all other proteins.

As briefly reported [(1986) Fed. Proc. 45, 1771, Abstr. 1690], rats fed a protein-free diet for a few days often show a marked inhibition of protein synthesis in liver cytosol. However the synthesis of a protein of molecular mass approximately 42 kDa is fully retained. We show here on the basis of its molecular mass, number of bands on isoelectric focusing, isoelectric point and immunological reactivity that this protein is actin and also that actin mRNA is not degraded by micrococcal nuclease under conditions which degrade the bulk of other mRNAs.

Adenosine 5'-triphosphate stimulates the release of polypeptides from mitochondria.

There was release of polypeptides to the medium when mitochondria containing labeled proteins were incubated with a rat liver post-mitochondrial supernatant. The release of polypeptides increased with the amount of rat liver extract added. Addition of cycloheximide did not inhibit the effect. Heating the post-mitochondrial supernatant did not inhibit the release of mitochondrial proteins, indicating that it was due to a heat-stable factor. The factor responsible has been isolated and identified as ATP. The presence of EDTA inhibits the release of polypeptides caused by ATP and Mg2+ stimulates it. The possible role of ATP in the turnover of mitochondrial proteins is briefly discussed.

Actinomycin D decreases protein kinase C content and induces neuritogenesis in neuroblastoma cells.

We have previously reported that inhibition of protein kinase C induces differentiation of neuroblastoma cells in culture. It is shown now that actinomycin D, a well known inhibitor of DNA synthesis, reduces selectively the content of protein kinase C and induces neuritogenesis in Neuro 2a cells in culture.