NG-nitro-L-arginine protects against hypoxia/hypoglycemia-induced decrease in CA1 presynaptic spikes in rat hippocampal slices

Contribution of nitric oxide to the depression of neuronal activity induced by temperature increase in the rat hippocampal CA1 area

Using optical recording techniques, we examined whether nitric oxide (NO) is implicated in the impairment of the activity of hippocampal CA1 neurons induced by mild heat stress. A temperature increase from 32 to 38 degrees C reversibly depressed the neuronal activity in hippocampal slices. L-Arginine (1 mM), an NO donor, enhanced the heat-induced depression of the activity of hippocampal CA1 neurons. N(omega)-Nitro-L-arginine methyl ester, an inhibitor of nitric oxide synthase, attenuated the inhibition of the neuronal activity induced by a temperature increase. Methylcobalamin (10 microM), a vitamin B(12) analogue that reduces NO production, reduced the heat-induced depression of the neuronal activity. These results suggest that NO contributes, at least in part, to the heat-induced depression of the neuronal activity in the hippocampal CA1 region.

Nitric oxide signalling is required for the generation of anoxia-induced long-term potentiation in the hippocampus

The involvement of nitric oxide in anoxia-induced long-term potentiation (anoxic LTP) of synaptic transmission was investigated in CA1 neurons of rat hippocampal slices using intracellular recording techniques in vitro. In response to superfusion of an anoxic artificial cerebral spinal fluid saturated with 95% N2-5% CO2, the excitatory postsynaptic potential (EPSP) generated in hippocampal CA1 neurons by stimulation of the Schaffer collateral/commissural afferent pathway was completely abolished within 10 min of anoxia. On return to reoxygenated medium, the EPSP returned to the control value within 10 min and was subsequently and progressively potentiated to reach a plateau 15-20 min after return to oxygen. This anoxia-induced persistent increase in synaptic transmission lasted for more than 1 h. Application of the nitric oxide synthase inhibitors 7-nitroindazole (7-NI) or L-N(G)-nitroarginine (NOARG) produced no effects on the baseline EPSP amplitude, but effectively attenuated the anoxic LTP. The inhibitory effects of both 7-NI and NOARG on the anoxic LTP were blocked by L-arginine, a substrate for nitric oxide synthase. These results suggest that nitric oxide is required for the generation of anoxia-induced LTP of glutamatergic synaptic transmission in the CA1 region of the rat hippocampus.

Nitric oxide participates in the stimulatory and neurotoxic action of endothelin on rat striatal dopaminergic neurons

1. Our method of real-time monitoring of dopamine release from rat striatal slices revealed that endothelin (ET)-3-induced dopamine release was inhibited by NG-methyl-L-arginine (L-NMMA; 1 mM), an inhibitor of nitric oxide (NO) synthase, while NG-methyl-D-arginine (D-NMMA; 1 mM), an inactive isomer of L-NMMA, had no effect. 2. The inhibition of L-NMMA (0.1 mM) became apparent when tissues were pretreated with tetrodotoxin (1 microM) for 30 min and subsequently exposed to ET-3 (4 microM). 3. L-NMMA (0.1 and 1 mM) dose dependently protected against ET-3-triggered hypoxic/hypoglycemic impairment of striatal responses to high K+. 4. Thus, NO may work as a promoter in mediation of the stimulatory and neurotoxic action of ET-3 on the striatal dopaminergic system, presumably by interacting with interneurons in the striatum.

Influence of 7-nitroindazole on the anticonvulsive action of conventional antiepileptic drugs

7-Nitroindazole (a selective neuronal nitric oxide (NO) synthase inhibitor) at 25 and 50 mg/kg, 30 min before the test, did not influence the electroconvulsive threshold. However, at 50 mg/kg, it enhanced the anticonvulsive activity of phenobarbital against maximal electroshock and did not affect that of carbamazepine, diphenylhydantoin and valproate. L-Arginine (500 mg/kg) did not modify the protective activity of phenobarbital alone or the 7-nitroindazole-induced enhancement of its anticonvulsive potency against maximal electroshock. 7-Nitroindazole did not alter the plasma levels of antiepileptic drugs, so a pharmacokinetic interaction, in terms of total and free plasma levels, is not probable. 7-Nitroindazole combined with the antiepileptics resulted in motor disturbances, except for the combination with phenobarbital. On the other hand, the combined treatment of 7-nitroindazole with carbamazepine or phenobarbital produced effects superior to those produced by single drugs, as regards long-term memory. Our results indicate that the protective activity of carbamazepine, diphenylhydantoin, or valproate against maximal electroshock may be not dependent upon the central NO level. The enhancement of the anticonvulsive action of phenobarbital by 7-nitroindazole is probably not related to the decrease of NO in the central nervous system.

Methylcobalamin attenuates the hypoxia/hypoglycemia- or glutamate-induced reduction in hippocampal fiber spikes in vitro

The effects of methylcobalamin, a vitamin B12 analogue, on the hypoxia/hypoglycemia- or glutamate-induced reduction in hippocampal CA1 presynaptic fiber spikes elicited by Schaffer collateral stimulation in rat brain slices were evaluated. Hippocampal slices were exposed to 15 min of hypoxia/hypoglycemia, and then these slices were returned to oxygenated and glucose-containing buffer for 3 h. Hypoxia/hypoglycemia reduced CA1 presynaptic potentials in vitro. Treatment with 10 microM methylcobalamin attenuated the impairment of CA1 presynaptic potentials induced by hypoxia/hypoglycemia or glutamate application (10 mM). Daily injection of methylcobalamin (0.5 mg/kg i.p./day) for 3 days in vivo also attenuated the hypoxia/hypoglycemia- or glutamate-induced reduction in presynaptic potentials in hippocampal slices. Pretreatment with cyanocobalamin at 10 microM failed to attenuate the impairment of CA1 presynaptic potentials. However, daily injection of cyanocobalamin (0.5 mg/kg i.p./day) for 3 days caused a protective action against the hypoxia/hypoglycemia- or glutamate-induced functional deficit. Furthermore, co-treatment of L-arginine (100 microM), a substrate for nitric oxide synthase, with methylcobalamin in vitro reversed the methylcobalamin-induced functional recovery. The present results demonstrate that methylcobalamin application in vivo or in vitro leads to functional recovery from hypoxia/hypoglycemia- or glutamate-induced impairment of CA1 presynaptic potentials. Neuroprotection was obtained by in vivo application of cyanocobalamin, but not by its in vitro application. It is reported that in vivo injected cyanocobalamin converted to methylcobalamin in the hepatic cells. Therefore, the results suggest that a transmethylation reaction in the hippocampal regions may be involved in the methylcobalamin-induced functional recovery from ischemic impairment.