Noradrenaline stimulation of (Na+, K+)ATPase in homogenates of the developing rat brain

In vivo evidence for reduced cortical glutamate-glutamine cycling in rats treated with the antidepressant/antipanic drug phenelzine

Converging evidence has indicated that hyperglutamatergic activity and GABAergic dysfunction may play important roles in the neurobiology and treatment of depression and other mood disorders. In this study, in vivo 1H[13C] magnetic resonance spectroscopy was used to quantify the effects of acute phenelzine administration on cortical energetics, glutamate neurotransmission, and GABA synthesis flux. The time-resolved kinetics of cortical [4-13C]glutamate, [4-13C]glutamine, and [2-13C]GABA turnover from i.v.-infused [1,6-13C2]glucose was measured at 11.7 T in alpha-chloralose anesthetized rats four hours after phenelzine treatment (10 mg/kg, i.p.) and in non-treated controls. The rate of the tricarboxylic acid cycle flux was not affected by phenelzine treatment compared with the non-treated group (0.46+/-0.05 vs. 0.50+/-0.05 micromol/g/min, respectively). The rate of the glutamate-glutamine cycling flux between neurons and glia in the phenelzine-treated group was significantly reduced (from 0.16+/-0.04 to 0.10+/-0.03 micromol/g/min), providing in vivo evidence that phenelzine attenuates glutamate neurotransmission. Following phenelzine treatment, the cortical GABA concentration increased significantly (from 1.02+/-0.17 to 2.30+/-0.26 micromol/g), while the GABA synthesis flux was unchanged (from 0.07+/-0.02 to 0.06+/-0.02 micromol/g/min). The possible role of augmented GABAergic function resulting from elevated GABA levels in the observed modulatory effect of phenelzine on the glutamate-glutamine cycling flux was discussed. The reduced glutamate-glutamine cycling flux observed in this study suggests that, in addition to its effects on monoaminergic and GABAergic systems, the attenuation of glutamate neurotransmission resulting from phenelzine administration may also contribute to its efficacy in the treatment of depression. This study is the first demonstration that the glutamate-glutamine cycling flux, which can be measured non-invasively in the human brain in vivo, was altered due to the action of a psychotropic drug.

Differential effects of K(ATP) channel blockers on [(3)H]-noradrenaline overflow after short- and long-term exposure to (+)-oxaprotiline or desipramine

To test whether prolonged uptake blockade can lead to changes in the function of ATP-dependent potassium (K(ATP)) channels we investigated in rat neocortex slices the effects of K(ATP) channel blockers on electrically evoked [(3)H]-noradrenaline ([(3)H]-NA) overflow after short- (45 min) and long-term (210 min) exposure to the NA uptake blockers (+)-oxaprotiline or desipramine (1 microM each). The K(ATP) channel blocker glibenclamide (1 micro M) increased the evoked [(3)H]-NA overflow by 42% after short-term uptake inhibition. This effect was confirmed by tolbutamide and glipizide, two other K(ATP) channel antagonists. The evoked [(3)H]-NA overflow was enhanced by 73% following short-term uptake blockade (15 min) and by 110% following long-term blockade (180 min). After long-term blockade (210 min), however, glibenclamide failed to further enhance the overflow of [(3)H]-NA. The alpha(2)-autoreceptor-mediated feedback control was not involved in the glibenclamide-induced increase in [(3)H]-NA overflow after short-term uptake blockade or in the increase in [(3)H]-NA overflow due to long-term uptake blockade per se. The Na(+)/K(+)-ATPase inhibitor ouabain diminished the glibenclamide-induced enhancement of [(3)H]-NA overflow after short-term uptake blockade, suggesting that an operative Na(+)/K(+)-ATPase is the prerequisite of activation of K(ATP) channels. These results suggest that short-term uptake blockade activates the Na(+)/K(+)-ATPase, thereby reducing intracellular ATP which allows transient opening of K(ATP) channels. Activation of the Na(+)/K(+)-ATPase may increase the Na(+) gradient, probably over the membrane of noradrenergic nerve terminals. The resulting hyperpolarisation leads to inhibition of the evoked overflow which can be reversed, i.e. enhanced, by K(ATP) channel blockers. In contrast, longer lasting uptake blockade seems to reduce the activity of the Na(+)/K(+)-ATPase and hence the consumption of ATP. As a consequence, reduced Na(+) and K(+) gradients may facilitate transmitter release. Closure of K(ATP) channels by accumulating ATP may further promote membrane depolarisation and transmitter release. The unexpected effect of longer exposure to uptake blockers could be somehow related to the clinical time latency of the antidepressant efficacy of monoamine uptake blockers.

Na+/K(+)-ATPase in rat brain and erythrocytes as a possible target and marker, respectively, for neurotoxicity: studies with chlordecone, organotins and mercury compounds

Due to the inaccessibility of human nerve tissue for direct biochemical evaluation, there appears to be a need to identify peripheral markers which will reflect toxicity to the central nervous system by relatively non-invasive means. The aim of this study was to investigate whether the enzyme Na+/K(+)-ATPase in erythrocytes could be used as a marker for effects on the same enzyme in brain tissue. The compounds chosen to test this hypothesis were the pesticide chlordecone, the organotin compounds triethyltin and tributyltin, mercuric chloride and methyl mercury. All compounds were found to inhibit in vitro Na+/K(+)-ATPase activity in rat brain (IC50s = 0.9-56 microM) and in rat erythrocytes (IC50s = 1.2-66 microM) with similar potencies. However, administration of these compounds in vivo at high doses produced no significant inhibition of either brain or erythrocyte Na+/K(+)-ATPase activity, despite observed symptoms of neurotoxicity. Dialysis experiments indicated that dissociation of the compounds by dilution during tissue preparation was not responsible for the lack of detectable in vivo inhibition. Measurements of metal concentrations in brain by atomic absorption spectrometry after in vivo administration of triethyltin, mercuric chloride and methyl mercury indicated that levels of these compounds were too low to inhibit significantly NA+/K(+)-ATPase activity. These results suggest that inhibition of Na+/K(+)-ATPase activity might not represent the mechanism responsible for the neurotoxicity of these compounds, and that erythrocyte Na+/K(+)-ATPase activity is not a useful marker for neurotoxicity following acute exposures.

Na+, K+-ATPase activity in cultured C6 glioma cells

Rat C6 glioma cells were cultured for 4 days in MEM medium supplemented with 10% bovine serum and Na+, K+-ATPase activity was determined in homogenates of harvested cells. Approximately 50% of enzyme activity was attained at 1.5 mM K+ and the maximum (2.76 +/- 0.13 mumol Pi/h/mg protein) at 5 mM K+. The specific activity of Na+, K+-ATPase was not influenced by freezing the homogenates or cell suspensions before the enzyme assay. Ten minutes' exposure of glioma cells to 10(-4) or 10(-5) M noradrenaline (NA) remained without any effect on NA+, K+-ATPase activity. Neither did the presence of NA in the incubation medium, during the enzyme assay, influence the enzyme activity. The nonresponsiveness of Na+, K+-ATPase of C6 glioma cells to NA is consistent with the assumption that alpha (+) form of the enzyme may be preferentially sensitive to noradrenaline. Na+, K+-ATPase was inhibited in a dose-dependent manner by vanadate and 50% inhibition was achieved at 2 x 10(-7) M concentration. In spite of the fact that Na+, K+-ATPase of glioma cells was not responsive to NA, the latter could at least partially reverse vanadate-induced inhibition of the enzyme. Although the present results concern transformed glial cells, they suggest the possibility that inhibition of glial Na+, K+-ATPase may contribute to the previously reported inhibition by vanadate of Na+, K+-ATPase of the whole brain tissue.

Brain Na+,K+-ATPase activity possibly regulated by a specific serotonin receptor

Na+,K+-ATPase activity in 6 regions of adult brain was measured after incubation with varying concentrations of serotonin. A concentration-dependent increase in enzyme activity was observed in 4 regions, with cerebral cortex and cerebellum showing the largest response. These results together with previous ones suggest that serotonin modulates brain Na+,K+-ATPase activity through a specific receptor located in target neurons or glial cells.

Effect of lead on Na+,K+ATPase activity in the developing brain of intra-uterine growth-retarded rats

Lead (Pb) intoxication in developing mammals, including humans, produces serious brain damage. In addition, it is known that nutritional status influences the susceptibility to Pb toxicity. We developed an in utero undernutrition model based on restriction of blood supply to fetuses on d 17 of pregnancy (IUGR rats). The aim of this study was to investigate in vitro the possible effect of Pb on Na+, K+ATPase activity in the brain of developing IUGR and control rats from 6 to 60 d after birth. In addition, we measured the stimulation of Na+, K+ATPase by the monoamines noradrenaline and serotonin. Our results show that: The neurotoxic effect of Pb is an age-related phenomenon. Both IUGR and control rats were more sensitive to Pb in the first week of life. In adults, Pb had a weak inhibitory potency; the delayed matured brain in IUGR animals seemed less sensitive to Pb when compared to age-paired control rats; in the IUGR group, at 15 and 22 d, low doses of Pb had a stimulatory effect on Na+, K+ATPase instead of an inhibitory effect; noradrenaline and serotonin stimulated Na+, K+ATPase activity to an equivalent extent, but this was greater in IUGR than control rats; and at low Pb concentrations, the studied monoamines reversed Pb-induced inhibition.