Increased binding of [3H]muscimol and [3H]flunitrazepam in the rat brain under hypoxia

Acute hypoxia differentially affects the γ-aminobutyric acid type A receptor α1, α2, β2, and γ2 subunit mRNA levels in the developing chick optic tectum: Stage-dependent sensitivity

This investigation analyzes the effect of an acute hypoxic treatment on the level of four (alpha(1), alpha(2), beta(2), and gamma(2)) subunit mRNAs of the GABA(A) receptor in layer "i" of the developing chick optic tectum. Our results show that 1 hr of normobaric acute hypoxia significantly changes the subunit mRNA levels. Different subunit mRNAs display different sensitivity to hypoxia: alpha(1), beta(2), and gamma(2) mRNAs are highly sensitive, whereas alpha(2) mRNA is almost not affected. The sensitivity of the mRNA levels to hypoxia is stage dependent. The mean percentages of variation produced by the hypoxia in the level of expression of the four subunits were 20% at ED12, 5% at ED16, and only 2% at ED18. These changes in the mean percentages of expression modify the probability of coexpression. In the case of double mRNA combinations, the hypoxia produced a mean variation in the probability of coexpression of 37% at ED12, 8% at ED16, and only 4% at ED18. With regard to the triple subunit mRNAs combinations, the variations were 206% at ED12, 11% at ED16, and only 7% at ED18. The quadruple combination values were 1,500% at ED12, 21% at ED16, and only 11% at ED18. This study demonstrates that the subunit mRNA levels are highly sensitive during the early stages, suggesting that GABA(A) receptor composition might undergo environment-dependent plastic changes providing a high degree of plasticity to the GABA neurotransmitter system development.

The effects of short-term hypoxia on motor cortex excitability and neuromuscular activation

The effects of acute hypoxia on motor cortex excitability, force production, and voluntary activation were studied using single- and double-pulse transcranial magnetic stimulation techniques in 14 healthy male subjects. Electrical supramaximal stimulations of the right ulnar nerve were performed, and transcranial magnetic stimulations were delivered to the first dorsal interosseus motor cortex area during short-term hypoxic (HX) and normoxic (NX) condition. M waves, voluntary activation, F waves, resting motor threshold (rMT), recruitment curves (100-140% of rMT), and short-interval intracortical inhibition and intracortical facilitation were measured. Moreover, motor-evoked potentials (MEPs) and cortical silent periods were determined during brief isometric maximum right index finger abductions. Hypoxia was induced by breathing a fraction of inspired oxygen of 12% via a face mask. M waves, voluntary activation, and F waves did not differ between NX and HX. The rMT was significantly lower in HX (55.79 +/- 9.40%) than in NX (57.50 +/- 10.48%) (P < 0.01), whereas MEP recruitment curve, short-interval intracortical inhibition, intracortical facilitation, maximum right index finger abduction, and MEPs were unaffected by HX. In contrast, the cortical silent periods in HX (158.21 +/- 33.96 ms) was significantly shortened compared with NX (169.42 +/- 39.69 ms) (P < 0.05). These data demonstrate that acute hypoxia results in increased cortical excitability and suggest that acute hypoxia alters motor cortical ion-channel function and GABAergic transmission.

Hypoxia alters GABAA receptor function and subunit expression in NT2-N neurons

Hypoxia causes dysfunction of excitatory and inhibitory neurotransmission, often resulting in encephalopathy, seizures or myoclonus. We evaluated the effects of hypoxia on GABAA receptor (GABAAR) function and expression in an in vitro model of neuronal hypoxia. NT2-N cells, derived from the human NT2 teratocarcinoma cell line, were exposed to < or =1% O2 for 8 h and then used immediately for experiments or allowed to recover under normoxic conditions (95% air/5% CO2) for 24, 48 or 96 h. Hypoxic treatment did not cause obvious morphological changes or cell death. In whole-cell patch-clamp recordings, the GABA current EC50 was unchanged, however, maximal GABA-evoked currents changed in a biphasic manner. Maximal GABA currents were significantly increased immediately after hypoxia, but were significantly reduced after 48 h normoxic recovery, and then returned to baseline after 96 h recovery. Maximal potentiation of 10 microM GABA currents by diazepam was increased 48 h after hypoxia, but potentiation by zolpidem was decreased. Barbiturate enhancement and zinc inhibition of GABA currents were unchanged. Semiquantitative reverse transcriptase (RT)-PCR showed decreased alpha1, alpha5, beta2 and gamma2 subunit mRNA after hypoxia. Hypoxic exposure altered GABAAR physiology and subunit mRNA expression, which may correlate with symptoms observed after hypoxia in vivo.

Prolonged exposure to hypobaric hypoxia transiently reduces GABA(A) receptor number in mice cerebral cortex

The central nervous system is severely affected by hypoxic conditions, which produce alterations in neural cytoarchitecture and neurotransmission, resulting in a variety of neuropathological conditions such as convulsive states, neurobehavioral impairment and motor CNS alterations. Some of the neuropathologies observed in hypobaric hypoxia, corresponding to high altitude conditions, have been correlated with a loss of balance between excitatory and inhibitory neurotransmission, produced by alterations in glutamatergic and GABAergic receptors. In the present work, we have studied the effect of chronic hypobaric hypoxia (506 hPa, 18 h/day x 21 days) applied to adult male mice on GABA(A) receptors from cerebral cortex, to determine whether hypoxic exposure may irreversibly affect central inhibitory neurotransmission. Saturation curves for [3H]GABA specifically bound to GABA(A) receptors in isolated synaptic membranes showed a 30% decrease in maximal binding capacity after hypoxic exposure (Bmax control, 4.70+/-0.19, hypoxic, 3.33+/-0.10 pmol/mg protein), with no effect on GABA binding sites affinity (Kd control: 159.3+/-13.3 nM, hypoxic: 164.2+/-15.1 nM). Decreased B(max) values were observed up to the 10th post-hypoxic day, returning to control values by the 15th post-hypoxic day. Pharmacological properties of GABA(A) receptor were also affected by hypoxic exposure, with a 45 to 51% increase in the maximal effect by positive allosteric modulators (pentobarbital and 5alpha-pregnan-3alpha-ol-20-one). We conclude that long-term hypoxia produces a significant but reversible reduction on GABA binding to GABA(A) receptor sites in cerebral cortex, which may reflect an adaptive response to this sustained pathophysiological state.

Acute hypoxic hypoxia transiently reduces GABA(A) binding site number in developing chick optic lobe

The Central Nervous System is known to be critically affected in the prenatal-perinatal period by hypoxic-ischemic insults, which produce several disorders such as loss of neural projections, increased susceptibility to seizures, apoptosis and an imbalance in normal activity of glutamatergic and GABAergic neurones, resulting in acute cell excitotoxicity. The aim of the present work was to establish a chick embryo model of normobaric acute hypoxic hypoxia as well as to evaluate modifications in GABA(A) receptor complex from chick optic lobe, that may result from this injury. Fertile chicken (Gallus gallus domesticus) eggs from White Leghorn were incubated and at embryonic days (ED) 12 to 18, subjected to a stream of 8%O(2)/92%N(2) during1 h, and then were either returned to their shelves in the incubator for recovery, or immediately processed for biochemical studies. Hypoxic treatment produced a significant age dependent reduction in GABA binding sites showing the greatest decrease at the earliest stages studied (ED12-ED16). Saturation curves of GABA binding performed at ED12 showed a decrease in B(max), (control, 5.48+/-0.20, hypoxic, 3.90+/-0.39 pmol/mg protein), but no significant change in K(d). Following 48 h in normoxic atmosphere post-hypoxia reduction in [3H]GABA binding was reversed. Pharmacological properties of GABA(A) receptor at ED12 showed that positive allosteric modulation effects of the steroid 3alpha-hydroxy-5alpha-pregnan-20-one and the barbiturate pentobarbital sodium were enhanced by the treatment. This model of acute prenatal hypoxic hypoxia produced marked alterations in inhibitory CNS neurotransmission that proved reversible and age dependent.

Effects of GABA receptor blockade on the ventilatory response to hypoxia in hypothermic newborn piglets

Hypothermic newborn piglets have a depressed ventilatory response to hypoxia, and this may be due to an increase in CNS gamma-aminobutyric acid (GABA) levels. To evaluate the effects of GABA(A) receptor blockade on the ventilatory response to hypoxia in hypothermic piglets, 31 anesthetized paralyzed mechanically ventilated newborn piglets (2-7 d) were studied at a brain temperature of 38.5 +/- 0.5 degrees C [normothermia (NT), n = 15] or 34 +/- 0.5 degrees C [hypothermia (HT), n = 16]. The central respiratory output was evaluated by measuring burst frequency and moving time average area of phrenic nerve activity. Measurements of minute phrenic output (MPO), arterial blood pressure, heart rate, oxygen consumption, and arterial blood gases were obtained at room air and during 20 min of isocapnic hypoxia [fraction of expired oxygen (FiO2) = 0.10]. After 10 min of hypoxia, a bolus injection of 20 microL of bicuculline methiodide (BM; 10 microg) or Ringer's solution was administered into the cisterna magna over a 1-min period, and the piglets remained in hypoxia for an additional 10 min. There was an initial increase of 50 +/- 6% in MPO during the first minute of hypoxia followed by a decrease to values 24 +/- 8% above baseline at 10 min in the NT group. In contrast, in the HT group, the initial increase in MPO with hypoxia was eliminated, and, at 10 min, there was a decrease to a mean value 35 +/- 4% below baseline level (NT versus HT, p < 0.03). After administration of BM, a significant increase in MPO with hypoxia was observed in both groups compared with their placebo groups (p < 0.002 in NT-BM group, p < 0.0001 in HT-BM group). However, the magnitude of the increase in MPO during hypoxia was significantly greater in the HT group after administration of BM (NT versus HT, p < 0.0001). Changes in oxygen consumption, arterial blood pressure, heart rate, pH, partial pressure of oxygen (PaO2), and base excess with hypoxia were not different between NT and HT groups before and after the administration of BM. The cardiorespiratory response to hypoxia was not modified after administration of Ringer's solution to NT and HT placebo groups. These data suggest that the depression in hypoxic ventilatory response produced by HT is in part modulated by an increased CNS GABA concentration.

Cabergoline, a long-acting dopamine D2-like receptor agonist, produces a sustained antiparkinsonian effect with transient dyskinesias in parkinsonian drug-naive primates

Continuous dopaminergic receptor stimulation is now considered as an interesting approach for the control of motor complications often seen in parkinsonian patients treated chronically with levodopa. Cabergoline, which is a long-acting dopamine D2-like receptor agonist, has been tried recently with good results as an adjunct in patients already on levodopa-therapy. Thus, the present study was designed to test the effects of repeated s.c. administration of cabergoline as sole therapeutic agent during a month in 3 drug-naive MPTP parkinsonian monkeys to see whether or not cabergoline, given every other day at 0.25 mg/kg, would have a sustained antiparkinsonian effect and would induce dyskinesias. The animals were rated to quantify the antiparkinsonian as well as the dyskinetic response and gross locomotor activity was monitored by photocells. The averaged locomotor response, initially greatly increased (approximately 9 times higher than after saline treatment in the same animals), decreased by approximately 50% after 2 weeks but was thereafter maintained at this level until the end of the study. The parkinsonian features were improved in a sustained manner in all monkeys and transient dyskinesias (week 1 and 2) were present in 2 of 3 monkeys. After sacrifice receptor binding assays were performed on striatal and pallidal tissues homogenates with tritiated selective ligands and compared with those of 3 normal and 3 MPTP-exposed monkeys otherwise untreated. A significant decrease in dopamine D2-like receptor density in the putamen (-36% on average vs. untreated MPTP-exposed monkeys) may be involved in the behavioral partial tolerance to antiparkinsonian effect of cabergoline and the disappearance of dyskinesias. A reversal of the supersensitivity of GABAA receptor in the internal segment of the globus pallidus (-15% on average vs. untreated MPTP-exposed monkeys) may also be implicated in this latter behavioral effect.

Anoxia tolerant animals from a neurobiological perspective

This paper discusses the mechanisms for brain anoxia survival seen in crucian carp (Carassius carassius) and a few species of freshwater turtle (Chrysemys and Trachemys species). Comparisons are made with the hypoxic tolerant mammalian neonate brain. In the anoxic tolerant species the basic strategy for anoxia survival appears to be the maintenance of ion gradients, and thereby the avoidance of anoxic depolarization. Important facilitating factors involve having huge glycogen stores, increased blood supply to the brain, the suppression of electrical activity, increased release of inhibitory neuromodulators and neurotransmitters, upregulation of inhibitory neuroreceptors, the down-regulation of excitatory ion conductance and the down-regulation of Ca2+ channels. By contrast, for the mammalian neonate the most important causes of its increased hypoxia tolerance may be just simple consequences of the comparatively undifferentiated state of the brain of the newborn, with its lower energy requirements, slower decline in ATP and lower excitability levels acting to delay depolarization.

Unchanged [3H]MK-801 binding and increased [3H]flunitrazepam binding in turtle forebrain during anoxia

In order to determine if functional changes in N-methyl-D-aspartate receptors and GABAA receptors play a role in the remarkable anoxia tolerance of freshwater turtle brain, we used autoradiographic techniques to assay [3H]MK-801 and [3H]flunitrazepam binding in turtle forebrain after turtles had been subjected to anoxia for 2 or 6 h. The effects of glutamate, glycine, competitive N-methyl-D-aspartate antagonists, glycine antagonists, polyamines, magnesium, and zinc on [3H]MK-801 binding were the same in anoxic and control turtle forebrains. These results indicate that NMDA receptor regulation plays no role in the adaptive responses to anoxia in turtle brain. In contrast, [3H]flunitrazepam binding was significantly increased in the anoxic dorsal cortex and striatum. The most parsimonious explanation for elevated benzodiazepine receptor binding is that the rise in extracellular GABA levels known to accompany anoxia enhances benzodiazepine receptor affinity. It is possible, however, that GABAA receptor upregulation during anoxia increases the effectiveness of the inhibitory action of released GABA and contributes to the anoxia tolerance of turtles.

Amino acid levels and gamma-aminobutyric acidA receptors in rat neostriatum, cortex, and thalamus after neonatal 6-hydroxydopamine lesion

The amino acid gamma-aminobutyric acid (GABA) is the main inhibitory neurotransmitter in brain, and GABAergic neurons have been proposed to play a major role in basal ganglia physiology. In the neostriatum (caudate putamen), medium-sized aspiny interneurons, as well as neostriatal output neurons that project to several brain regions, use GABA as their neurotransmitter. Dopamine fibers arising from the substantia nigra represent a major input to the neostriatum where, besides their classic neurotransmitter role, they are seemingly involved in the regulation of amino acid neurotransmitter release. To further characterize the nature of some of the amino acid/dopamine interactions, selective dopaminergic deafferentations were produced in neonatal rats (3 days postnatal) by intraventricular administration of the neurotoxin 6-hydroxydopamine (6-OHDA); the noradrenergic neurons were protected by prior administration of desmethylimipramine. After a 3-month survival, levels of catecholamines, indoleamines, and amino acids were determined in cingulate cortex, thalamus, and neostriatum. In addition, GABAA receptors were measured in membrane preparations from these three regions, using the specific agonist [3H]muscimol. In the 6-hydroxydopamine-lesioned rats, levels of dopamine and its metabolites homovanillic acid, 3,4-dihydroxyphenylacetic acid, and 3-methoxytyramine were decreased, as expected, in cortex and neostriatum, but remained unmodified in thalamus. In all three regions, serotonin content was increased; its metabolite, 5-hydroxyindole-3-acetic acid, was also elevated, but only in cortex and neostriatum. The levels of GABA were increased in neostriatum and thalamus, but remained unmodified in cortex. Glycine was increased in all three regions examined. There were also increases of phosphatidylethanolamine and serine in thalamus, and of aspartic acid and alanine in neostriatum. The density of GABAA binding sites was increased in neostriatum, but remained unchanged in cortex and thalamus. The changes in amino acid levels and [3H]muscimol binding sites induced by a neonatal 6-hydroxydopamine treatment differ from those found after similar lesions in adult animals, possibly because of the plastic and synaptic rearrangements that can still occur during early postnatal development. The present results also demonstrate that adaptations occur in response to a dopaminergic deafferentation at an early age and that these exhibit a regional specificity.

Protective effect of synaptic inhibition during cerebral ischemia in rats and rabbits

BACKGROUND

Excitatory neurotransmitters appear to cause cell death during ischemia by inducing depolarization, influx of ions, and metabolic failure in the postsynaptic neuron. If this hypothesis is correct, then postsynaptic membrane hyperpolarization and inhibition of metabolism may be protective. Antagonists of the excitotoxic amino acid glutamate protect neurons in culture and in animal models of stroke but appear to cause unacceptable side effects in humans. We propose an alternative strategy of protection using agonists of the inhibitory neurotransmitter gamma-aminobutyric acid.

METHODS

We caused multifocal cerebral ischemia in rats and rabbits by injecting microspheres into the carotid circulation. We administered saline, muscimol, or MK-801 within 5 minutes of stroke onset. We used a bioassay to measure outcome. In rats, we also used learning to assess cortical function, and we performed detailed quantitative brain morphometry 3 months after infarction.

RESULTS

Using the bioassay, we found that muscimol exerted a protective effect in rats (p less than 0.01). There was a dose-response effect seen in muscimol-treated rabbits. Rats treated with muscimol or MK-801 exhibited significantly better visual-spatial learning compared with saline-treated subjects (p less than 0.001). Hemisphere volume after ischemia was comparable in all groups.

CONCLUSIONS

Agonists of gamma-aminobutyric acid and antagonists of glutamate appear to protect brain during ischemia. Since agonists of gamma-aminobutyric acid are known to have fewer side effects in humans, they may prove more useful in the clinical setting as neuroprotective agents.

A method to maintain normal respiratory and metabolic state in artificially respired rats

Analysis of arterial blood gases (ABG) in awake, paralyzed, locally anesthetized, and artificially respired rats revealed the development with time of severe hypoxemia associated with metabolic acidosis despite adequate ventilation as assessed by normal PaCO2. These respiratory and metabolic disturbances may underlie the progressive deterioration experienced with this preparation frequently used in neuropharmacological experiments. We report here that the intravascular infusion of bicarbonated artificial plasma, associated with continuous positive pressure ventilation, prevents the deterioration of the respiratory and metabolic state in this preparation, which can be maintained within the range of that of the freely moving animal. This stabilized preparation may thus be highly suitable for neuropharmacological experiments extending for several hours.

Oxygen-induced modifications of benzodiazepine receptors and D2 dopamine receptors in the rat under hyperoxia

Peripheral-type benzodiazepine Receptors (PBR) in the kidney and Central-type Benzodiazepine Receptors (CBR) in the cerebral cortex were not affected in rats exposed to chronic hyperoxia (85% O2, ATA, 6 days). Nevertheless, cortical CBR showed a significant decrease (29%) after hyperbaric hyperoxia (100% O2, 3.5 ATA, 2 h) in rats at a preconvulsive stage, with no concomitant alteration of kidney PBR. A similar down-regulation of striatal D2 dopamine receptors was noticed (27%)--after hyperbaric hyperoxia--without any modification of cortical PBR. On the contrary, an up regulation of liver PBR was obtained in the same conditions (20%). It is likely that receptors implicated in neurotransmission are particularly down regulated or altered under hyperbaric hyperoxia.

Hypoxia-induced neurotransmitter deficits in neonatal rats are partially corrected by exogenous GM1 ganglioside

Exposure of 7-day-old rats to 7% oxygen/balance nitrogen for 2 h results in selective changes of cholinergic, serotonergic, and dopaminergic neuronal markers in the frontal cortex, hippocampus, and striatum when evaluated 3 weeks after the insult. There is also about a 15% deficiency in brain weight. Treatment with GM1 ganglioside, 50 mg/kg i.p., for 2 days before and for 3 weeks after the hypoxic insult partially corrects the neurodevelopmental abnormalities including the deficiency in brain weight. We conclude that GM1 ganglioside might have therapeutic potential for treating suspected neonatal hypoxia.

Effect of Plasmodium yoelii infection on GABA metabolism of mouse brain

Plasmodium yoelii infection in albino mice decreased the activity of brain glutamic acid decarboxylase (GAD) by about 30 and 48% in crude homogenate and its synaptosomal fraction, respectively. The decrease was evident from 20% parasitemia and remained more or less constant up to 80% parasitemia. The Km values of GAD in normal and infected animals were 1.2 x 10(-2) and 3.3 x 10(-2) mM, respectively, indicating a decrease in enzyme substrate affinity due to infection. The lowered GAD activity rose to slightly above normal by treatment of infected animals with chloroquine. Decrease in GAD activity reflected lower gamma-aminobutyric acid (GABA) levels in the infected brain; however, GABA-transaminase activity was not significantly influenced by infection. It has been proposed that impaired GABA synthesis may be due to hypoxia induced by malarial infection.

Effects of oxygen depletion on norepinephrine- and carbachol-stimulated phosphoinositide turnover in rat brain slices

We examined the effects of in vitro anoxia and in vivo hypoxia (8% O2/92% N2) on norepinephrine (NE)- and carbachol-stimulated phosphoinositide (PI) turnover in rat brain slices. The formation of 3H-labeled polyPI in cortical slices was impaired by in vitro anoxia and fully restored by reoxygenation. Accumulation of 3H-labeled myo-inositol phosphates (3H-IPs) stimulated by 10(-5) M NE was significantly reduced by anoxia (control at 60 min, 1,217 +/- 86 cpm/mg of protein; anoxia for 60 min, 651 +/- 82 cpm/mg; mean +/- SEM; n = 5; p less than 0.01), and reoxygenation following anoxia resulted in overshooting of the accumulation (control at 120 min, 1,302 +/- 70 cpm/mg; anoxia for 50 min plus oxygenation for 70 min, 1,790 +/- 126 cpm/mg; n = 5; p less than 0.01). The underlying mechanisms for the two phenomena--the decrease caused by anoxia and the overshooting caused by reoxygenation following anoxia--seemed to be completely different because of the following observations. (a) Although the suppression of NE-stimulated accumulation at low O2 tensions was also observed in Ca2+-free medium, the overshooting in response to reoxygenation was not. (b) Carbachol-stimulated accumulation was significantly reduced by anoxia and was restored by reoxygenation only to control levels. Thus, the postanoxic overshooting in accumulation of 3H-IPs seems to be a specific response to NE. (c) The decrease observed at low O2 tensions was due to a decrease in Emax value, whereas the postanoxic overshooting was due to a decrease in EC50 value.(ABSTRACT TRUNCATED AT 250 WORDS)