strychnine and tert-butylbicyclophosphorothionate

Control over stress protects against many of the deleterious effects of stress exposure, but the endogenous mediators responsible for these prophylactic effects have remained elusive. Using behavioral pharmacology, in vitro radioligand binding and neurochemical analyses, we demonstrate that exposure to escapable stress results in brain and behavior changes reminiscent of benzodiazepine administration. The stress control group shows significant protection against picrotoxinin-induced seizures, reductions in [35S]t-butylbicyclophosphorothionate (TBPS) binding and a 3-fold increase of benzodiazepine-like substances in brain in comparison to both yoked-inescapable shock and non-shock controls. These observations suggest that coping behavior leads to the release of endogenous benzodiazepine-like compounds in brain which protect the organism from stress pathology.

Topics: Analysis of Variance; Animals; Brain; Bridged Bicyclo Compounds; Bridged Bicyclo Compounds, Heterocyclic; Cerebral Cortex; Convulsants; Electroshock; Escape Reaction; Flumazenil; GABA-A Receptor Agonists; Hippocampus; Male; Picrotoxin; Radioligand Assay; Rats; Rats, Sprague-Dawley; Receptors, GABA-A; Seizures; Sesterterpenes; Stress, Psychological; Strychnine; Sulfur Radioisotopes; Tritium

The characteristics of [3H]strychnine and t-[35S]-butylbicyclophosphorothionate ([35S]TBPS) binding to sites associated with glycine- and gamma-aminobutyric acid (GABA)-gated chloride channels were compared in the presence of a series of anions with known permeabilities through these channels. Good correlations were found between (a) the potencies (EC50) of these anions to stimulate radioligand binding and their permeabilities relative to chloride; (b) the affinities (KD) of these radioligands in the presence of fixed concentrations of these anions and their relative permeabilities; (c) the potencies (EC50) of these anions to stimulate [35S]TBPS and [3H]strychnine binding; and (d) the affinities (KD) of [3H]strychnine and [35S]TBPS measured at a fixed concentration of these anions. These studies support electrophysiological and biochemical observations demonstrating similarities between glycine- and GABA-gated chloride channels, and suggest that anions enhance [3H]strychnine and [35S]TBPS binding through specific anion binding sites located at the channels.

Topics: Animals; Anions; Bridged Bicyclo Compounds; Bridged Bicyclo Compounds, Heterocyclic; Bridged-Ring Compounds; Cell Membrane; Cell Membrane Permeability; Chlorides; gamma-Aminobutyric Acid; Glycine; Ion Channels; Male; Medulla Oblongata; Rats; Rats, Inbred Strains; Spinal Cord; Strychnine; Sulfur Radioisotopes; Tritium

The inhibitory neurotransmitters glycine and gamma-aminobutyric acid (GABA) both activate transmembrane chloride channels of similar physical characteristics. A common ion channel component has therefore been postulated for both the glycine and GABA receptor proteins. Different convulsant drugs as picrotoxin and t-butyl bicyclophosphorothionate (TBPS) have been reported as channel-blocking ligands of the GABA receptor. Here, we show that the distribution of [35S]TBPS binding sites parallels the binding of the GABA receptor ligand [3H]flunitrazepam, but not that of the glycine receptor antagonist [3H]strychnine. Binding was examined in membrane fractions from different regions of the rat CNS and of the mutant mouse spastic, an animal deficient in glycine receptors. Also, affinity purification of the glycine receptor on aminostrychnine-agarose resulted in almost complete removal of [35S]TPBS binding sites from the receptor preparation. It is concluded that TBPS selectively binds to the GABA, but not glycine, receptor chloride channel complex.

Topics: Animals; Bridged Bicyclo Compounds; Bridged Bicyclo Compounds, Heterocyclic; Bridged-Ring Compounds; Cerebellum; Flunitrazepam; Glycine; Mice; Mice, Neurologic Mutants; Rats; Receptors, GABA-A; Receptors, Glycine; Receptors, Neurotransmitter; Spinal Cord; Strychnine; Synaptic Membranes

Twelve substances which appear to be GABA antagonists, judging by their ability to reverse the inhibitory effect of GABA on 35S-TBPS binding to rat brain membranes, were tested iontophoretically on population spikes in the rat hippocampus. Eight of them, including seven which completely reversed the inhibitory action of GABA on 35S-TBPS binding, caused a marked enhancement of population spikes, with slow onset and long duration and they antagonized the inhibition of population spikes by GABA. These effects were similar to those produced by bicuculline. Electrophysiologically, the most potent of the "complete reversers" were bathophenanthroline disulfonate and brucine. In vitro, amoxapine and brucine most effectively reversed the inhibitory action of GABA on 35S-TBPS binding. Of the five substances which only partly reversed the inhibitory effect of GABA on 35S-TBPS binding, four depressed the population spikes and potentiated the inhibitory action of GABA. The fifth "partial reverser", pipazethate, potently increased the population spikes, like the "complete reversers". Although other interpretations are possible the results are consistent with the existence of several GABA-A receptor types in brain, only some of which are blocked by certain partial reversers.

Topics: Action Potentials; Amoxapine; Animals; Benzothiadiazines; Bridged Bicyclo Compounds; Bridged Bicyclo Compounds, Heterocyclic; GABA Antagonists; gamma-Aminobutyric Acid; Hippocampus; Iontophoresis; Phenanthrolines; Rats; Strychnine; Thiazines

Pitrazepin (3-(piperazinyl-1)-9H-dibenz(c,f)triazolo(4,5-a)azepin) is a new GABAA receptor antagonist reported to antagonize electrophysiological effects of GABA. We have investigated in some detail the interaction of pitrazepin with the GABA/benzodiazepine receptor chloride channel complex. Pitrazepin was found to be a competitive inhibitor of the GABAA receptor which is coupled to [3H]diazepam and [35S]TBPS binding sites; the KI value obtained by Schild analyses was 80 nM. Although pitrazepin interacted weakly with BZ receptors the compound did not affect the chloride gating mechanism (labelled with [35S]TBPS or [3H]avermectin B1a). Further, pitrazepin was a non-selective GABA antagonist since glycine receptors, labelled with [3H]strychnine, were affected at low concentrations (the KI values in rat brain-stem were 71-110 nM).

Topics: Animals; Bridged Bicyclo Compounds; Bridged Bicyclo Compounds, Heterocyclic; Cerebral Cortex; Dibenzazepines; In Vitro Techniques; Kinetics; Male; Membranes; Muscimol; Rats; Rats, Inbred Strains; Receptors, GABA-A; Receptors, Glycine; Receptors, Neurotransmitter; Strychnine