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Biomedical subjects

R W Olsen

Publications and source records attributed to R W Olsen.

At least 127 records · Page 7Linked to original sources
PubMed1986

Benzodiazepine/barbiturate/GABA receptor-chloride ionophore complex in a genetic model for generalized epilepsy.

The inhibitory neurotransmitter gamma-aminobutyric acid (GABA) acts through postsynaptic receptor sites which regulate membrane chloride ion channels. The GABA receptor-ionophore complex also contains modulatory receptor sites for two classes of centrally acting drugs, one for the benzodiazepines, and a second for both barbiturates and related depressants and for picrotoxin and related convulsants. The presence of these drug modulatory sites, directly on the GABA receptor protein, is consistent with other experimental observations; blocking GABA function can cause seizures, and augmenting GABA function can afford protection against seizures. This, and other circumstantial evidence, has suggested the possibility that a functional GABA deficit may be involved in some kinds of human epilepsy. Some neurochemical markers for GABA synapses have been reported to be altered in certain animal models as well as in human temporal lobe epilepsy. We have examined the postsynaptic GABA receptor complex using receptor binding assays for GABA, benzodiazepine (BZ), and barbiturate receptor sites in the seizure-susceptible gerbil, a genetic model of generalized epilepsy. A 30% deficit in BZ receptor binding was observed in the midbrain of seizure-sensitive animals relative to normal controls. This was shown by quantitative brain-slice binding autoradiography to involve a decrease in the number of binding sites in the substantia nigra (SN) and periaqueductal gray regions. A deficit in membrane receptors for BZs (which are linked to a subtype of postsynaptic GABA receptors) in a crucial region of brain might therefore contribute to seizure susceptibility in some kinds of epilepsy.

Animals↗
PubMed1986

Facilitation of recurrent inhibition in rat hippocampus by barbiturate and related nonbarbiturate depressant drugs.

The effects of anticonvulsant, anesthetic and convulsant barbiturates and of related depressant drugs were characterized on excitatory and inhibitory synaptic transmission in slices of rat hippocampus. The duration of recurrent GABAergic inhibition was increased by all of the drugs tested, including the convulsant barbiturate 5-ethyl-5-[1,3-dimethylbutyl]barbituric acid, anesthetic barbiturates such as pentobarbital and nonbarbiturate anesthetics such as (+)-etomidate. Several barbiturates, including phenobarbital and (+)-mephobarbital facilitated inhibition, but the maximal responses to these agents were significantly less than with pentobarbital. In general, there was a good correspondence between the potencies of these drugs in facilitating inhibition and their previously reported abilities to regulate binding at the gamma-aminobutyric acid/benzodiazepine/barbiturate receptor complex. In addition to facilitating recurrent GABAergic inhibition, at successively higher doses most of these drugs induced direct depression of the population spike response, field excitatory postsynaptic potential and presynaptic fiber spike. 5-Ethyl-5-[1,3-dimethylbutyl]barbituric acid, (+)-mephobarbital and pentobarbital facilitated excitatory synaptic transmission at the Schaffer collateral/commissural synapses on the CA1 pyramidal neurons at low doses, but caused depression at higher doses. The net effects observed with each drug tested (facilitation/depression of excitatory transmission, enhancement of GABAergic inhibition) correlated well with the behavioral effects of these agents in vivo.

Animals↗
PubMed1985

Benzodiazepine/gamma-aminobutyric acid receptor deficit in the midbrain of the seizure-susceptible gerbil.

The density of benzodiazepine/gamma-aminobutyric acid receptor binding sites was lower in the midbrain of seizure-susceptible gerbils compared to control seizure-resistant gerbils. Binding of [3H]diazepam to high-affinity brain-specific sites in membrane homogenates of gerbil brain showed a 20-30% lower binding in midbrain (but not other regions) in adult seizure-susceptible gerbils than in controls. This binding deficit was localized by tissue slice autoradiography with [3H]flunitrazepam to the substantia nigra and mesencephalic periaqueductal gray regions, while higher binding was observed in the interpeduncular nucleus. These differences were also seen in animals sacrificed immediately after a seizure. A parallel deficit of [3H]bicuculline methochloride binding to low-affinity gamma-aminobutyric acid receptors also was seen in the same midbrain regions. Scatchard plot analysis showed that the benzodiazepine binding deficit in the nigra was due to a lower number of binding sites with not significant difference in affinity. Lower [3H]flunitrazepam binding was likewise seen in younger animals (29% lower at 30 days of age, 38% at 60 days, and 21% at 90 days), indicating that the midbrain receptor deficit is present in the seizure-susceptible gerbil prior to the age of onset of seizures at 50-100 days. Therefore, these changes are not likely to result from seizures but reflect genetically determined biochemical differences that could play a role in the expression of seizure susceptibility. The deficit in midbrain benzodiazepine/gamma-aminobutyric acid receptors in the seizure-susceptible gerbil would be consistent with the hypothesis that a deficit of gamma-aminobutyric acid-mediated inhibition might contribute to some kinds of epilepsy.

Animals↗
PubMed1985

Avermectin B1a modulation of gamma-aminobutyric acid/benzodiazepine receptor binding in mammalian brain.

The anthelminthic natural product avermectin B1a (AVM) modulates the binding of gamma-aminobutyric acid (GABA) and benzodiazepine (BZ) receptor ligands to membrane homogenates of mammalian brain. The potent (EC50 = 40 nM) enhancement by AVM of [3H]diazepam binding to rat or bovine brain membranes resembled that of barbiturates and pyrazolopyridines in being inhibited (partially) by the convulsants picrotoxin, bicuculline, and strychnine, and by the anticonvulsants phenobarbital and chlormethiazole. The maximal effect of AVM was not increased by pentobarbital or etazolate. However, AVM affected BZ receptor subpopulations or conformational states in a manner different from pentobarbital. Further, unlike pentobarbital and etazolate, AVM did not inhibit allosterically the binding of the BZ receptor inverse agonist [3H]beta-carboline-3-carboxylate methyl ester, nor did it inhibit, but rather enhanced, the binding of the cage convulsant [35S]t-butyl bicyclophosphorothionate to picrotoxin receptor sites. AVM at submicromolar concentrations had the opposite effect of pentobarbital and etazolate on GABA receptor binding, decreasing by half the high-affinity binding of [3H]GABA and related agonist ligands, and increasing by over twofold the binding of the antagonist [3H]bicuculline methochloride, an effect that was potentiated by picrotoxin. AVM also reversed the enhancement of GABA agonists and inhibition of GABA antagonist binding by barbiturates and pyrazolopyridines. These overall effects of AVM are unique and require the presence of another separate drug receptor site on the GABA/BZ receptor complex.

Animals↗
PubMed1985

Influence of hypothermia and circulatory arrest on cerebral temperature distributions.

A finite element model of the bioheat transfer equation has been developed to simulate the temperature distribution in the head of a subhuman primate. Simulations were made of the induction of deep hypothermia and of subsequent hypothermic circulatory arrest (HCA). Simulations of the circulatory arrest phase were performed with different values of surface heat transfer coefficient and tissue metabolic heat generation. Numerical results were compared with experimental data for the same procedure. The simulations indicate the brain cools rapidly to a near isothermal condition in response to an infusion of cold arterial blood. However, extracerebral structures cool much more slowly. The bulk of heat gain by the brain during HCA is due to heat transfer from these warmer extra-cerebral tissues. These results suggest extended cooling by cardiopulmonary bypass (CPB) combined with surface cooling pads should reduce or even prevent the rise of brain temperatures during HCA.

Animals↗
PubMed1984

gamma-Aminobutyric acid receptor binding antagonism by the amidine steroid RU5135.

The novel convulsant amidine steroid RU5135 inhibited gamma-aminobutyric acid receptor binding in membranes from seven regions of rat brain (IC50 = 11 +/- 2 nM against [3H]muscimol and 0.8 +/- 0.2 nM against [3H]bicuculline methochloride), apparently lowering the number of binding sites labeled with gamma-aminobutyric acid receptor agonists or antagonists. The steroid reversed the enhancement of benzodiazepine receptor binding by gamma-aminobutyric acid, pentobarbital, and etazolate, but did not inhibit the binding of the convulsant [35S]t-butyl bicylophosphorothionate. Thus, RU5135 shows very potent in vitro actions more resembling those of the gamma-aminobutyric acid site antagonist, bicuculline, than the chloride channel antagonist, picrotoxin.

Androstanes↗
PubMed1984

Barbiturates allosterically inhibit GABA antagonist and benzodiazepine inverse agonist binding.

Barbiturates and the related depressant drugs, etazolate and etomidate, inhibited both the binding of [3H]bicuculline methochloride (BMC) to gamma-aminobutyric acid (GABA) receptor sites and the binding of [3H] beta-carboline-3-carboxylic acid methyl ester (beta CCM) to benzodiazepine receptor sites in mammalian brain. These concentration-dependent effects were chemically specific and stereospecific in a manner correlating with the activity of barbiturates to enhance GABA responses in neurons and to enhance GABA and benzodiazepine receptor agonist binding in vitro. The barbiturate inhibition of [3H]BMC binding involved a decrease in affinity which at high concentrations of barbiturates results in an effective complete loss of detectable binding. The maximal inhibition of [3H] beta CCM binding involved a more modest decrease in affinity (increase in KD from 1.35 to 1.85 nM). The barbiturate inhibitions of both ligands could be reversed by picrotoxin, suggesting an indirect action at previously defined picrotoxin/barbiturate modulatory sites on the GABA-benzodiazepine receptor/chloride ion channel complex.

Allosteric Regulation↗
PubMed1984

gamma-Aminobutyric acid activation of 36Cl- flux in rat hippocampal slices and its potentiation by barbiturates.

gamma-Aminobutyric acid (GABA) increases the rate of 36Cl- efflux from preloaded rat hippocampal slices in a dose-dependent manner (EC50: 400 microM). This action has the pharmacological specificity expected of activation of GABA receptors in that it is mimicked by the agonists muscimol and 3-aminopropanesulfonic acid, and blocked by the antagonists bicuculline and picrotoxinin. GABA uptake inhibitors, nipecotic acid and 2,4-diaminobutyric acid, fail to increase 36Cl- flux. Pentobarbital produces a dose-dependent activation (EC50 = 1.5 mM) of 36Cl- efflux with maximal response greater than that of GABA. The effect of pentobarbital can be mimicked by 1,3-dimethylbutylbarbiturate, secobarbital, (+)hexobarbital but not (-)hexobarbital, and is blocked by bicuculline and picrotoxinin. Pentobarbital and the other active barbiturates also potentiate the action of GABA. Phenobarbital does not have any effect independently or in combination with GABA. It is suggested that GABA increases 36Cl- permeability by activation of a postsynaptic receptor which is in turn functionally coupled to a barbiturate receptor.

Animals↗
PubMed1984

Solubilization of convulsant/barbiturate binding activity on the gamma-aminobutyric acid/benzodiazepine receptor complex.

Binding activity of the radioactive cage convulsant [35S]t-butylbicyclophosphorothionate was solubilized from rat brain membranes using the zwitterionic detergent 3-[(3-cholamidopropyl)-dimethylammonio] propanesulfonate. Binding (KD = 26 nM, Bmax = 0.4 pmol/mg protein) was inhibited by picrotoxin and related convulsants and by barbiturates and related depressants that interact with gamma-aminobutyric acid and benzodiazepine receptors via the picrotoxinin binding site. The convulsant/barbiturate binding activity chromatographed on gel filtration as a single peak coinciding with the benzodiazepine/gamma-aminobutyric acid receptor protein complex.

Animals↗
PubMed1984

Role of the gamma-aminobutyric acid receptor-ionophore complex in seizure disorders.

The possibility of a role for the inhibitory neurotransmitter gamma-aminobutyric acid (GABA) in seizure disorders has been strengthened by biochemical studies showing that various nervous system depressant drugs can modulate GABA receptor binding in vitro. In particular, two classes of anticonvulsant agents, the benzodiazepines and the barbiturates, have modulatory receptor sites on the GABA receptor-ionophore protein complex of the postsynaptic membrane. Furthermore, it is well established that direct block of GABA function causes seizures and that augmentation of GABA function can protect against seizure activity. Direct evidence for altered GABA synaptic markers has been obtained in some animal models of epilepsy, as well as in human focal epilepsy. We present preliminary evidence for a deficit in benzodiazepine receptor binding in the midbrain of seizure-susceptible Mongolian gerbils. These data would be consistent with an impairment of GABA-mediated inhibitory synaptic transmission that contributes to susceptibility to the genesis or spread of seizures in some kinds of epilepsy.

Animals↗
PubMed1984

Opiate receptor binding in the brain of the seizure sensitive Mongolian gerbil (Meriones unguiculatus).

Opiate receptor binding was studied in seizure sensitive (SS) and seizure resistant (SR) strains of the Mongolian gerbil. Cryostat sections of the brain were labeled with [3H]-dihydromorphine, subjected to autoradiography and analysed by microdensitometry. SS gerbils, prior to seizure induction, demonstrated overall greater brain opiate binding when compared to SR animals. Immediately following a seizure, binding in the interpeduncular nucleus fell to levels found in SR animals. The increased opiate binding in the SS (pre-seizure) compared to SR gerbils could reflect a deficit of endogenous ligand which could underlie the seizure diathesis in the gerbil.

Animals↗
PubMed1984

Biochemical pharmacology of the gamma-aminobutyric acid receptor/ionophore protein.

The synaptic receptor sites for the neurotransmitter gamma-aminobutyric acid (GABA) can be assayed in vitro with several radiolabeled agonists and one antagonist. Numerous criteria of specificity have been met for these binding sites. All of the ligands show heterogeneity in binding affinities. The subpopulations thus defined have a remarkably similar specificity for GABA analogs, which suggests an intimate relationship and possible interconvertibility. Modulation of GABA receptor binding by barbiturates, anions, and other membrane treatments that affect agonists and antagonists in an opposite manner suggests a three-state model of interconvertible affinities. The complex of GABA receptor and chloride ion channel contains modulatory sites for barbiturates and benzodiazepines, drugs that enhance GABA responses in neurons. The receptor complex can be solubilized in detergent with the three mutually interacting receptor activities intact. The complex has an apparent molecular weight of 355,000 and has been partially purified. GABA agonist function has been assayed at the biochemical level by measuring the activation of 36Cl- efflux from preloaded hippocampal slices by GABA, muscimol, and barbiturates. This response is blocked by the antagonists of the GABA site (bicuculline) and the barbiturate site (picrotoxin). Comparison of binding and function on the same tissue should be useful in analyzing the mechanism of action of GABA.

Allosteric Regulation↗
PubMed1983

[3H]bicuculline methochloride binding to low-affinity gamma-aminobutyric acid receptor sites.

The binding of [3H]bicuculline methochloride (BMC) to mammalian brain membranes was characterized and compared with that of [3H] gamma-aminobutyric acid ([3H]GABA). The radiolabeled GABA receptor antagonist showed significant displaceable binding in Tris-citrate buffer that was improved by high concentrations of chloride, iodide, or thiocyanate, reaching greater than 50% displacement in the presence of 0.1 M SCN-. An apparent single class of binding sites for [3H]BMC (KD = 30 nM) was observed in 0.1 M SCN- for fresh or frozen rat cortex or several regions of frozen and thawed bovine brain. The Bmax was about 2 pmol bound/mg of crude mitochondrial plus microsomal membranes from unfrozen washed and osmotically shocked rat cortex, similar to that for [3H]GABA. Frozen membranes, however, showed decreased levels of [3H]BMC binding with no decrease or an actual increase in [3H]GABA binding sites. [3H]BMC binding was inhibited by GABA receptor specific ligands, but showed a higher affinity for antagonists and lower affinity for agonists than did [3H]GABA binding. Kinetics experiments with [3H]GABA binding revealed that low- and high-affinity sites showed a similar pharmacological specificity for a series of GABA receptor ligands, but that whereas all agonists had a higher affinity for slowly dissociating high-affinity [3H]GABA sites, bicuculline had a higher affinity for rapidly dissociating low-affinity [3H]GABA sites. This reverse potency between agonists and antagonists during assay of radioactive antagonists or agonists supports the existence of agonist- and antagonist-preferring conformational states or subpopulations of GABA receptors. The differential affinities, as well as opposite effects on agonist and antagonist binding by anions, membrane freezing, and other treatments, suggest that [3H]BMC may relatively selectively label low-affinity GABA receptor agonist sites. This study, using a new commercially available preparation of [3H]bicuculline methochloride, confirms the report of bicuculline methiodide binding by Möhler and Okada (1978), and suggests that this radioactive GABA antagonist will be a valuable probe in analyzing various aspects of GABA receptors.

Animals↗