Acute tolerance to the excitatory effect of enkephalin microinjections into hippocampus.

Microinjections of 1–20 μg of leu-enkephalin were done in the dorsal hippocampus of cats and rats in acute experiments. Long lasting epileptiform changes in the hippocampal EEG were produced. These changes were prevented by previous microinjections of naloxone at the same site. When repeated microinjections of the same dose of enkephalin which initially was epileptogenic were done a progressive loss of effectiveness of the drug was observed. Also, progressively higher doses of enkephalin were necessary to produce effects comparable to the effect of the initial injection. These results suggest that acute tolerance may develop to the epileptogenic effect of leu-enkephalin in hippocampus.     

The Entamoeba histolytica Dnmt2 Homolog (Ehmeth) Confers Resistance to Nitrosative Stress

Nitric oxide (NO) has antimicrobial properties against many pathogens due to its reactivity as an S-nitrosylating agent. It inhibits many of the key enzymes that are involved in the metabolism and virulence of the parasite Entamoeba histolytica through S-nitrosylation of essential cysteine residues. Very little information is available on the mechanism of resistance to NO by pathogens in general and by this parasite in particular. Here, we report that exposure of the parasites to S-nitrosoglutathione (GSNO), an NO donor molecule, strongly reduces their viability and protein synthesis. However, the deleterious effects of NO were significantly reduced in trophozoites overexpressing Ehmeth, the cytosine-5 methyltransferase of the Dnmt2 family. Since these trophozoites also exhibited high levels of tRNA^Asp methylation, the high levels suggested that Ehmeth-mediated tRNA^Asp methylation is part of the resistance mechanism to NO. We previously reported that enolase, another glycolytic enzyme, binds to Ehmeth and inhibits its activity. We observed that the amount of Ehmeth-enolase complex was significantly reduced in GSNO-treated E. histolytica, which explains the aforementioned increase of tRNA methylation. Specifically, we demonstrated via site-directed mutagenesis that cysteine residues 228 and 229 of Ehmeth are susceptible to S-nitrosylation and are crucial for Ehmeth binding to enolase and for Ehmeth-mediated resistance to NO. These results indicate that Ehmeth has a central role in the response of the parasite to NO, and they contribute to the growing evidence that NO is a regulator of epigenetic mechanisms.     

Similarities between the akinesia induced by carbachol microinjections into the pontine reticular formation and neuroleptic catalepsy.

We reported previously that microinjections of carbachol directly into the pontine reticular formation of rats induced intense akinesia. In the present article we report results of tests for rigidity, righting, bracing and clinging which were conducted with the purpose to characterize behaviorally this type of akinesia. After injections of 5-15 micrograms/0.5 microliter of carbachol into the pontine reticular formation the rats were cataleptic, were not rigid when equilibrium was not challenged, had strong righting reflexes and strong bracing and clinging responses. This type of akinesia is different from the catatonia induced by systemic morphine (20 mg/kg IP), but similar to the catalepsy induced by systemic injections of haloperidol (5 mg/kg IP). It is thus suggested that the cataleptic state produced by topical carbachol in the pons is related to the dopaminergic mechanisms important for the cataleptic effect of the neuroleptic drugs.     

Tumor Reliance on Cytosolic versus Mitochondrial One-Carbon Flux Depends on Folate Availability

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