Studies on AMP deaminase and 5'-nucleotidase in rat brain under different experimental conditions

Nucleoside triphosphate diphosphohydrolases role in the pathophysiology of cognitive impairment induced by seizure in early age

Studies have shown that seizures in young animals lead to later cognitive deficits. There is evidence that long-term potentiation (LTP) and long-term depression (LTD) might contribute to the neural basis for learning and memory mechanism and might be modulated by ATP and/or its dephosphorylated product adenosine produced by a cascade of cell-surface transmembrane enzymes, such as E-NTPDases (ecto-nucleoside triphosphate diphosphohydrolases) and ecto-5'-nucleotidase. Thus, we have investigated if hippocampal ecto-nucleotidase activities are altered at different time periods after one episode of seizure induced by kainic acid (KA) in 7 days old rats. We also have evaluated if 90 day-old rats previously submitted to seizure induced by KA at 7 days of age presented cognitive impairment in Y-maze behavior task. Our results have shown memory impairment of adult rats (Postnatal day 90) previously submitted to one single seizure episode in neonatal period (Postnatal day 7), which is accompanied by an increased ATP hydrolysis in hippocampal synaptosomes. The metabolism of ATP evaluated by HPLC confirmed that ATP hydrolysis was faster in adult rats treated with KA in neonatal period than in controls. Surprisingly, the mRNA and protein levels as seen by PCR and Western blot, respectively, were not altered by the KA administration in early age. Since we have found an augmented hydrolysis of ATP and this nucleotide seems to be important to LTP induction, we could assume that impairment of memory and learning observed in adult rats which have experienced a convulsive episode in postnatal period may be a consequence of the increased ATP hydrolysis. These findings correlate the purinergic signaling to the cognitive deficits induced by neonatal seizures and contribute to a better understanding about the mechanisms of seizure-induced memory dysfunction.

Pentylenetetrazol kindling alters adenine and guanine nucleotide catabolism in rat hippocampal slices and cerebrospinal fluid

Pentylenetetrazol (PTZ) is commonly used as a convulsant drug. The enhanced seizure susceptibility induced by kindling is probably attributable to plastic changes in the synaptic efficacy. Adenosine and guanosine act both as important neuromodulators and neuroprotectors with mostly inhibitory effects on neuronal activity. Adenosine and guanosine can be released per se or generated from released nucleotides (ATP, ADP, AMP, GTP, GDP, and GMP) that are metabolized and rapidly converted to adenosine and guanosine. The aim of this study was to evaluate nucleotide hydrolysis by ecto- and soluble nucleotidases (hippocampal slices and CSF, respectively) after PTZ-kindling (stages 3, 4, or 5 seizures) or saline treatment in rats. Additionally, the levels of purines in rat cerebrospinal fluid (CSF), as well as ecto-NTPDases (1, 2, 3, 5, 6 and 8) and ecto- 5'-nucleotidase expression were determined. Ecto-enzyme assays demonstrated that ATP, AMP, GDP, and GMP hydrolysis enhanced when compared with controls. In addition, there was an increase of ADP, GDP, and GMP hydrolysis by soluble nucleotidases in PTZ-kindling rats compared to control group. The HPLC analysis showed a marked increase in PTZ-kindled CSF concentrations of GTP, ADP, and uric acid, but GDP, AMP, and hypoxanthine concentrations were decreased. Such alterations indicate that the modulatory role of purines in CNS could be affected by PTZ-kindling. However, the physiological significance of these findings remains to be elucidated.

Effects of deoxycoformycin on adenosine, inosine, hypoxanthine, xanthine, and uric acid release from the hypoxemic rat cerebral cortex

The effects of the adenosine deaminase inhibitor, deoxycoformycin, on purine release from the rat cerebral cortex were studied with the cortical cup technique. Deoxycoformycin (5 and 500 micrograms/kg i.v.) enhanced the hypoxia/ischemia-evoked release of adenosine from the cerebral cortex, indicating a marked rise in the adenosine content of interstitial fluid in the cerebral cortex. Inosine and hypoxanthine release were attenuated at the higher dose of deoxycoformycin. Uric acid release into the cortical perfusates was enhanced at the higher dose level. These results demonstrate that low doses of deoxycoformycin can be used to elevate interstitial levels of adenosine in the brain during hypoxia, and to depress the formation of some of its metabolites. The elevation of hypoxia/ischemia-evoked adenosine levels can account for the previously reported potentiation of hypoxia-evoked increases in rat cerebral blood flow after deoxycoformycin administration. The potential therapeutic utility of these findings is discussed.

Acute and short term effects of ethanol on membrane enzymes in rat brain

Changes in the biophysical and biochemical character of membranes brought about by ethanol have been emphasized in the underlying mechanism of alcohol toxicity. Membrane enzymes such as Na+, K+ activated ATPase, 5'-nucleotidase, and gamma-glutamyl transpeptidase were studied in cerebral cortex, cerebellum, and brain stem of rats subjected to acute and short term ethanol toxicity. Acute ethanol toxicity was induced by intraperitoneal injection of 1 ml of 7M ethanol per 100 g body weight of rat and the animals were sacrificed half an hour after the administration. Short term ethanol toxicity was induced by intraperitoneal injections of 0.5 ml (7 M ethanol) per 100 g weight of the rat for 7 days and the animals were sacrificed half an hour after the last injection. In acute ethanol toxicity the activity of Na+, K+-activated ATPase was found to decrease significantly in cerebral cortex and brain stem, while in short term alcohol toxicity, the activity was found to increase in cerebral cortex and cerebellum. The activity of gamma-glutamyl transpeptidase was found to increase in all the three regions in acute and short term ethanol toxicity. No change in the activity of 5'-nucleotidase was observed in any of the regions either in acute or in chronic ethanol toxicity. While a significant increase in the activity of adenosine deaminase was found in cerebral cortex, cerebellum, and brain stem in acute ethanol toxicity, the same was found to decrease significantly in cerebral cortex and a persistent increase in brain stem in short term ethanol toxicity. The above changes in the activities of the enzyme were discussed with reference to the well known changes in the membrane structure and consequent alteration in brain function.

Possible occurrence of ornithine-omega-aminotransferase in GABAergic neurons

The specific precursors for neurotransmitter pools of glutamate giving rise to GABA in GABAergic neurons and nerve endings have not been clearly established. Glutamate is the immediate precursor for the production of GABA and it is suggested that ornithine (from arginine) might be serving as one of the precursors of glutamate for the formation of neurotransmitter pool of GABA. Damage to GABAergic neurons in different regions of the brain in anoxia is well known. If arginine and ornithine act as precursors for GABA in GABAergic neurons, a decrease in the activities of arginase and ornithine-omega-transferase (Orn-T) is possible in areas having the lesions involving the GABAergic neurons due to anoxia. Estimation of Orn-T and arginase in different regions of the brain of rats exposed to anoxia revealed such a possibility.

Adenosine deaminase in convulsions along with its regional, cellular and synaptosomal distribution in rat brain

Adenosine deaminase activity was determined in enriched neuronal, glial and synaptosomal fractions in three regions of rat brain. It was found to be equally distributed in cortical neurons and synaptosomes. Appreciable activity was observed in glial cells. A significant increase in the activity of this enzyme was observed in three brain regions in rats subjected to leptazol convulsions. No significant change, however, was found in the pre- or post-convulsive periods. These results are discussed in relation to the possible degradation of adenosine, a neuroinhibitory substance and to the possible production of inosine, an initiator of seizure activity in low amounts and depressor of the same in high amounts.

Intracellular content of thiol compounds, thiobarbituric acid reactive substances and gamma-glutamyl transpeptidase in rat brain during anoxia

Intracellular thiol compounds like glutathione play an important role in scavenging free radicals. Free radical reactions occur in the respiratory chain, and free radicals are known to accumulate in hypoxia and ischaemia. The content of thiobarbituric acid (TBA) reactive substances, thiol and non-protein thiol (reduced glutathione) concentrations along with the activity of gamma-glutamyl transpeptidase (GGTP), a membrane enzyme, were estimated in three regions of the brain in rats exposed to anoxia. It was observed that no change occurred in the content of TBA-reactive substances in any one of the three brain regions, although a significant increase in the non-protein thiol concentration occurred uniformly during anoxia. A significant increase in GGTP occurred in all the three brain regions during anoxia. The increase in the activity of GGTP is suggestive of an alteration in the function of blood-brain barrier during anoxia. These results are suggestive of an absence of lipid peroxidative damage during anoxia.