Differential and irreversible CNS ontogenic reduction in maximal MK-801 binding site number in the NMDA receptor after acute hypoxic hypoxia.

CNS exposure to hypoxia impairs excitatory and inhibitory neurotransmission. Our aim was to determine variations induced by normobaric acute hypoxic hypoxia (8% O(2) for 60 min) on the NMDA receptor complex, as well as their potential reversibility after normoxic recovery. To this end, [3H]MK-801 binding assays to a synaptic membrane fraction isolated from chick optic lobes were performed. Previous studies throughout development had disclosed a characteristic age-dependent pattern. Results at embryonic day (ED) 12 and 18 indicated two distinct MK-801 binding sites. Hypoxic treatment failed to alter either the high affinity site dissociation constant (K(d)) or its maximal binding capacity (B(max)), whereas the low affinity site B(max) was significantly decreased (50% and 30% at ED12 and 18, respectively), without alteration in its K(d) values. Hypoxic embryos restored for 48 h at ED12 to normoxic conditions displayed unchanged MK-801 binding reduction, unlike those treated likewise at ED18 whose values fully recovered control levels. To conclude, hypoxic treatment reduces low affinity MK-801 B(max) in the NMDA receptor which proves irreversible up to ED12. Such early neuronal vulnerability may be due to post-transcriptional changes, to endocytosis followed by receptor degradation, or alternatively to cell death.

Acute hypoxic hypoxia alters GABA(A) receptor modulation by allopregnanolone and pentobarbital in embryonic chick optic lobe.

Using a previously developed model of acute normobaric hypoxic hypoxia on chick embryos, here we studied at embryonic day 12 the in vitro effect of two positive allosteric modulators of GABA binding, the barbiturate sodium pentobarbital and the neurosteroid allopregnanolone. In both cases an increase in E(max) values in membranes obtained from hypoxic embryos was observed. Studies of GABA-gated chloride influx showed that there were no differences in maximal chloride uptake between hypoxic and control membranes. We have already demonstrated that maximal density of GABA binding sites was decreased after hypoxia, suggesting that each of the remaining GABA(A) receptors display a greater chloride flux than controls. To further characterize GABA(A) receptor alterations, GABA-gated chloride influx modulated by the above barbiturate and neurosteroid was determined, finding that E(max) values were increased 60% and 42%, respectively. The increase in Cl(-) influx per receptor subsequent to hypoxic trauma, and the enhancement in the modulatory properties studied, may mediate neuronal damage by potential changes in subunit interaction at the GABA(A) receptor level.

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.

Synaptic membrane freezing affects modulatory sites in avian central nervous system GABA(A) receptor.

Studies were carried out to determine whether barbiturates and neurosteroids share common recognition sites at the GABA(A) receptor complex in avian CNS. To achieve this, differentially prepared fresh and frozen synaptic membranes were used. Both the barbiturate, pentobarbital, and the neurosteroid, 3alpha-hydroxy-5alpha-pregnan-20-one, were able to stimulate GABA binding in both types of membranes. Stimulation differed markedly when both drugs were added jointly to different treated tissue. In frozen membranes drugs acted synergistically and were differentially displaced by picrotoxinin, while in fresh ones, where both compounds were inhibited by the convulsant, this additivity was absent. Post-freezing wash supernatants were collected and used as a source of putative endogenous factors involved in the above mentioned membrane differences. Addition of a high molecular weight fraction from supernatants to frozen synaptic membranes led to an inhibition of barbiturate and neurosteroid potentiation, as well as a loss of their additive effect. Our results indicate that GABA(A) receptor modulation by barbiturates and neurosteroids is affected by synaptic membrane treatment, with a common modulatory site in fresh membranes and separate recognition sites after a freeze-thawing procedure. There may also be endogenous factors involved in overlapping of modulatory sites, which would thus regulate GABA(A) receptor functionality by direct interaction with the complex.

Pentobarbital modulatory effect on GABA binding sites in developing chick optic lobe.

Barbiturates are allosteric modulators of the CNS GABAA receptor, increasing [3H]-GABA binding to its receptor sites. In the present work we have studied the modulatory effect of the barbiturate pentobarbital on low-affinity GABA binding sites during ontogenetic development of the chick optic lobe. Our results indicate that [3H]-GABA binding enhancement by pentobarbital shows a differential profile during development, following a two-component enhancement model at early stages of development and a single-component enhancement model in the adult stage. Kinetic analysis performed at different stages of development showed that barbiturate enhancement was invariably due to an increase in [3H]-GABA binding affinity, while maximal binding capacity remained unchanged. Using GABA antagonists, picrotoxinin and bicuculline, convulsant sensitivity of high-affinity barbiturate modulatory sites was found at early stages. These data suggest that barbiturate action displays receptor heterogeneity during development, with high- and low-affinity modulatory sites only at early stages, while the high-affinity sites disappear between hatching and adulthood. Kinetic data indicate that both barbiturate modulatory sites are coupled to the GABAA receptor at early stages. The presence of high-affinity modulatory sites at early stages and at hatching suggests a major role during visual pathway maturation.

Methods for removing endogenous factors from CNS membrane preparations: differences in [3H]GABA binding parameters and developmental-related effects.

The present report describes a systematic study comparing and combining methods currently used for the removal of endogenous factors known to affect the interaction of GABA with its receptor. The effects of these methods were analyzed by performing [3H]GABA binding studies, and by measuring the amount of residual GABA left in the different membrane preparations. The effectiveness of these methods were also applied to different developmental stages. The results show that: 1) an exhaustive buffer washing procedure is necessary to accurately measure the maximal binding capacity (Bmax) of the low-affinity GABA binding site, and 2) the use of more drastic methods, including freeze-thawing and Triton treatment allows a clear demonstration of receptor heterogeneity and a precise measurement of the Bmax of the high-affinity GABA binding site as well as increases the affinity of the low-affinity site. The analysis of the Bmax values obtained with these different procedures in relation to the values of GABA removal, strongly indicates that the exhaustive washing procedure removes some unknown endogenous substances required for Triton treatment to exhibit its maximal effectiveness. Finally, a detailed analysis of Kd and Bmax values obtained with these three methods in the developing nervous tissue shows the existence of significant differences with regard to their effectiveness in removing endogenous substances when applied in different developmental stages.

The effect of acute ethanol administration on GABA receptor binding in cerebellum and hypothalamus.

The effect of the intraperitoneal administration of ethanol on [3H]GABA binding and glutamic acid decarboxylase (GAD) in cerebellum and hypothalamus was investigated. Acute ethanol administration produced an increase in the binding capacity of the high affinity GABA binding sites and a decrease in the binding capacity of low affinity sites. A decrease in the binding capacity of the high affinity GABA binding sites and an increase in the binding capacity of the low affinity sites were observed in the hypothalamus. No apparent changes were detected in the binding affinities for the two types of GABA receptor sites in both brain areas following ethanol treatment. Ethanol enhanced GAD activity in the cerebellum and reduced GAD activity in the hypothalamus. Changes in GABA binding may be involved in some of the neuropharmacological effects of ethanol.

gamma-Aminobutyric acid receptors in anterior pituitary and brain areas after median eminence lesions.

Previous results have indicated that the median eminence (ME) plays a key role in the regulation of estrogen and dopamine receptors in the anterior pituitary (AP). Since 3H-labeled gamma-aminobutyric acid ([3H]GABA) receptors have been described in the pituitary as well as in the brain, the aim of the present work was to investigate whether those receptors are also under hypothalamic control. Ovariectomized adults rats were divided into two groups. In the first one, the ME was lesioned by an anodic current (MEL); the second group consisted of sham-operated controls. Animals were used 7-14 days thereafter to study [3H]GABA binding in a crude mitochondrial membrane fraction prepared from the anterior pituitary, the hypothalamic-preoptic suprachiasmatic area, or the frontoparietal cortex. Binding of [3H]GABA was a saturable process, with high affinity in all studied structures. In AP Scatchard analysis of the saturation curves indicated similar Kd values in sham and MEL rats; the maximal number of binding sites increased from 0.65 +/- 0.005 pmol/mg protein in controls to 1.05 +/- 0.006 pmol/mg protein in MEL rats (P less than 0.001). In the hypothalamus, Kd values were similar in both groups, but the maximum number of binding sites (Bmax) increased about 50% in MEL animals. In the frontoparietal cortex, Kd values were similar in control and lesioned animals; but the Bmax decreased by 40% after the lesion. Animals with lesions showed hyperprolactinemia and a reduction in serum levels of LH, FHS, and TSH. Thus, Kd values were of similar magnitude in all studied regions and were not altered by ME destruction. Bmax values in controls were higher in the cerebral cortex than in the hypothalamus and AP. After ME lesions, there was an increment of [3H]GABA binding in AP and hypothalamus and a reduction in cerebral cortex. The data indicate that the ME participates in the regulation of [3H]GABA binding in AP and also suggest that it plays a role in regulating GABA receptors in various regions of the brain.