Decreased convulsant potency of picrotoxin and pentetrazol and enhanced [3H]flunitrazepam cortical binding following stressful manipulations in rats

Various stressful manipulations in rats (cold-water swim, electric foot-shock administration, imparied access to food reward) were found to reduce the convulsant potency of drugs which interfere with GABA or benzodiazepine central processes. The convulsant threshold dosages of picrotoxin (0.4 mg/ml) or pentetrazol (10 mg/ml) administered after the stress by infusion (0.2 ml/min) via a vein of the tail were enhanced. The onset of generalized seizures induced by isoniazid (800 mg/kg) or by thiosemicarbazide (64 mg/kg) i.p. was delayed after cold-water swim. However, convulsant threshold dosages of bemegride or strychnine perfused at 2 and 0.2 mg/ml respectively were not changed by stress. Cold-water swim increased the number of cortical (but not cerebellar) [3H]flunitrazepam binding sites (+ 24%) but failed to alter cortical [3H]muscimol binding. This post-stress enhancement of binding sties, although suppressed by bicuculline (10(-4) M) seems not to be dependent on GABAergic mechanisms. Indeed cold-water stress did not reduce the ability of muscimol (10(-6) and 10(-5) M) and GABA (5 x 10(-6) and 5 x 10(-5) M) to increase flunitrazepam binding. Finally, this post-stress enhancement of benzodiazepine binding was not found to be paralleled by changes in the protective effects of diazepam against picrotoxin- or pentetrazol-induced seizures.

Urinary and brain beta-carboline-3-carboxylates as potent inhibitors of brain benzodiazepine receptors

Benzodiazepines probably exert their anxiolytic, hypnotic, and anticonvulsant effects by interacting with brain-specific high-affinity benzodiazepine receptors. In searching for possible endogenous ligands for these receptors we have purified a compound 10(7)-fold from human urine by extractions, treatment with hot ethanol, and column chromatography. The compound was identified as beta-carboline-3-carboxylic acid ethyl ester (IIc) by mass spectrometry, NMR spectrometry, and synthesis; IIc was also isolated from brain tissues (20 ng/g) by similar procedures. Very small concentrations of IIc displaced [3H]diazepam completely from specific cerebral receptors, but not from liver and kidney binding sites; the concentration causing 50% inhibition of specific [3H]diazepam binding (IC50) was 4-7 nM compared to ca. 5 nM for the potent benzodiazepine lorazepam. Specific binding sites for quinuclidinyl benzilate, naloxone, spiroperidol, serotonin, muscimol, and WB 4101 were not affected by IIc. In contrast to benzodiazepines, IIc exhibits "mixed type" competitive inhibition of forebrain benzodiazepine receptors (negative cooperativity). We surmise that an endogenous ligand for benzodiazepine receptors may be a derivative of beta-carboline-3-carboxylic acid.

Interaction of SQ 20009 and GABA-like drugs as modulators of benzodiazepine receptor binding

SQ 20009, a new anxiolytic drug structurally unrelated to GABA or benzodiazepines, has two modulatory effects on the binding of 3H-flunitrazepam to membranes from rat cerebellum. Thus, SQ 20009 has a direct and chloride ion dependent stimulatory effect on benzodiazepine receptor binding which in turn appears to be modulated by the associated GABA receptor. In addition SQ 20009 has indirect effects on benzodiazepine receptor binding since it enhances the potency of GABA and muscimol to stimulate 3H-flunitrazepam binding.

Benzodiazepine receptor protein identified and visualized in brain tissue by a photoaffinity label

Flunitrazepam, a potent benzodiazepine, reversibly binds to the benzodiazepine receptor with high affinity. When irradiated with UV light, flunitrazepam was irreversibly linked to brain tissue. Incorporation of [3H]flunitrazepam was inhibited by other benzodiazepines with a potency corresponding to their affinity for the benzodiazepine receptor. Photolabeling with flunitrazepam reduced the number of benzodiazepine receptors determined by reversible binding of benzodiazepines, whereas the apparent affinity of the remaining receptors was unchanged. Half-maximal incorporation of flunitrazepam occurred at a concentration similar to the apparent dissociation constant of flunitrazepam. Thus, flunitrazepam appears to be a photoaffinity label for the benzodiazepine receptor. The receptor component photolabeled with flunitrazepam was a protein of molecular weight 50,000. Its location in cerebral and cerebellar cortex slices could be visualized by electron microscopic autoradiography. A predominant localization of benzodiazepine receptors in regions of synaptic constants, including those formed by GABAergic neurons (GABA is gamma-aminobutyric acid), was observed.

Radiohistochemical localization of benzodiazepine receptors in rat brain

The benzodiazepine receptor was localized in rat brain with light microscopic autoradiographic methods. The binding of [3H]flunitrazepam to slide-mounted tissue sections had all of the characteristics associated with the benzodiazepine receptor. It was saturable, or a high affinity, affected by chloride ion and gamma-aminobutyric acid and showed an appropriate pharmacology. Autoradiograms were generated by the apposition of emulsion-coated coverslips. Receptor distribution showed striking variation throughout the brain and spinal cord. Some areas showing high densities include the cerebral cortex, the molecular layer of the cerebellum, parts of the limbic system, olfactory bulb and hypothalamus and substantiae gelatinosae of the spinal trigeminal nucleus and spinal cord. White matter areas showed negligible levels of receptor. These localizations show which parts of the brain are affected by benzodiazepine administration and contribute to our understanding of the mechanism of the well-known properties of the drugs such as their anxiolytic and anticonvulsant effects.

Diazepam receptor: specific nuclear binding of [3H]flunitrazepam

Autoradiographic localization of [3H]flunitrazepam in nuclei of the rat cerebral cortex was further confirmed by biochemical analysis of specific nuclear binding. Highly purified rat cerebral cortex nuclei were shown to bind [3H]flunitrazepam specifically. The Kd(app) for nuclear binding was 28 nM for the nuclei compared with a Kd(app) of 1.1 nM for binding of [3H] flunitrazepam to synaptosomal membrane fractions of the same tissue. Inhibition of the nuclear binding with inosine and hypoxanthine was greater than inhibition of the synaptic membrane fractions. These results lead to to conclude that specific binding may occur at both the synaptic membrane and the nuclear levels and that different endogenous ligands may compete at each site for binding. Furthermore, the possibility exists for translocation and alteration of the bound ligand complex from membrane site to nuclear site.

Pharmacokinetic and clinical observations on prolonged administration of flunitrazepam

Eight patients were given flunitrazepam 2 mg orally, once daily for 28 consecutive days. The time-course of the plasma concentration of unchanged flunitrazepam and its principal metabolites were studied in detail after the first and last doses. Additional blood samples were collected immediately before administration of the tablet on days 4, 7, 11, 14, 18, 21 and 25. Clinically there were not changes during the trial period in the onset of sleep, duration of sleep, depth of sleep measured as number of spontaneous awakenings, or in the patients' condition on awakening. The time-course of the plasma concentration of flunitrazepam could be described by a three-compartment model, assuming that the rate constants remained unchanged during treatment. Maximal plasma concentrations of unchanged flunitrazepam, found two hours after intake, reached 10-15 ng/ml after the first and 15-20 ng/ml after the last dose. The beta-half-life was found to be between 20 and 36 h.

Attenuation of the effects of punishment by ethanol: comparisons with chlordiazepoxide

Ethanol (ETOH), like chlordiazepoxide (CDZ), significantly attenuated the suppressive effect of punishment on licking behavior in water-deprived rats and mice. In rats, the greatest effects of ETOH (1.5 g/kg) were observed between 30 and 60 min following IP administration. tert-Butanol also attenuated the effects of punishment, suggesting that acetaldehyde was not contributing to this effect of ETOH. Since a dose of ETOH that increased punished drinking did not increase unpunished drinking, alteration in thirst motivation would not appear to be responsible for its antipunishment action. However, doses of ETOH or CDZ that significantly increased punished responding increased jump thresholds to aversive shock, suggesting that decreased sensitivity to aversive stimulation may contribute to the anti-punishment action of both agents. In addition to these similarities between ethanol and CDZ, several differences were noted in their effects. For example, CDZ decreased serum corticosterone concentration, whereas ETOH did not. Further, ETOH impaired aerial righting reflex and reduced rectal temperature, whereas CDZ had no effect on these parameters at doses that had anti-punishment activity. Finally, specific binding of [3H]flunitrazepam to crude brain cortical membranes was decreased by CDZ, but not ETOH. Although ETOH and CDZ similarly alter punished behavior, results suggest that ETOH does not act through a direct interaction with a benzodiazepine binding site.

In vivo receptor occupation by benzodiazepines and correlation with the pharmacological effect

The existence of specific receptor sites for benzodiazepines has been well documented by in vitro binding studies. In this study, using a highly radiolabelled [3H]-flunitrazepam, we investigated the binding of benzodiazepines to their receptor sites under in vivo conditions. Tracer doses of [3H]flunitrazepam (0.001 mg/kg) were injected i.v. into mice and the concentration of the drug in the brain was monitored. The accumulation of [3H]flunitrazepam 20 min after injection was found to be highest in the hippocampus, cortex, hypothalamus; to be intermediate in the striatum, medulla oblongata/pons and midbrain and to be lowest in the cerebellum. This corresponds well with the different densities of benzodiazepine receptors which we found in in vitro studies, with the exception of medulla oblongata/pons and cerebellum. When increasing doses (0.01--10 mg/kg) of non-labelled benzodiazepine derivatives (flunitrazepam, clonazepam, the 3S and 3R enantiomers of 5-(o-fluorophenyl)-1,3-dihyrdo-1,3-dimethyl-7-nitro-2H-1,4-benzodiazepine-2-one, and chlordiazepoxide) were injected simultaneously with [3H]flunitrazepam, a dose-dependant, saturable and and stereo-specific decrease of [3H]flunitrazepam concentration in the mouse hippocampus was observed. The dose range in which the unlabelled benzodiazepines decreases the levels of [3H]flunitrazepam in the hippocampus corresponds closely to that which inhibited pentylenetetrazol- or picrotoxin-induced seizures, indicating that this in vivo method determines the occupation of pharmacologically relevant receptors.

THIP and isoguvacine are partial agonists of GABA-stimulated benzodiazepine receptor binding

The effects of THIP and isoguvacine on 3H-flunitrazepam binding to washed membranes prepared from the cerebral cortex of adult rats have been examined. THIP, which has only minimal stimulatory effects on benzodiazepine (BZ) receptor binding, has been found to inhibit the stimulation induced by small concentrations (2 microM) of exogenous GABA. While isoguvacine stimulates BZ receptor binding, although to a smaller extent than GABA, it also antagonizes the stimulation of BZ receptor binding induced by GABA. Thus THIP and isoguvacine exhibit the properties of a partial agonist of GABA-stimulated BZ receptor binding.

Practical experiences with flunitrazepam in anesthesia

Flunitrazepam was used in general and local anesthesia. Little effect was noted on the respiratory and cardiovascular systems. In our patients there was an impressive period of amnesia and often we observed a central anticholinergic syndrome, which could be reversed by the use of physostigmine salicylate (0.04/kg body weight).

GABA receptors in clonal cell lines: a model for study of benzodiazepine action at molecular level

A "recptor unit" for gamma-aminobutyric acid (GABA), which includes brainlike receptor binding sites for tritium-labeled GABA and benzodiazepines (diazepam, clonazepam, and flunitrazepam) and a thermostable endogenous protein (GABA modulin) that inhibits both GABA and benzodiazepine binding, has been demonstrated in membranes prepared from NB2a neuroblastoma and C6 glioma clonal cell lines. In these cells, as in brain, diazepam (1 micromolar) prevents the effect of GABA modulin, and in turn GABA (0.oma and, to a lesser extent, the glioma cells represent a suitable model to study the interactions and the sequence of membrane and intracellular events triggered by the stimulation of benzodiazepine and GABA receptors.

Benzodiazepine receptors: effect of tissue preincubation at 37 degrees C

[3H]Flunitrazepam binding to rat brain homogenates was assayed in two ways: by 37-0 degrees C incubations or 0 degrees C only incubations. The affinity of brain benzodiazepine receptors for [3H]flunitrazepam was significantly greater in 37-0 degrees C incubated samples than in 0 degrees C incubated samples, while there was no change in the number of benzodiazepine receptors. The possible mechanisms for this enhanced binding affinity as well as the methodological implications for this observation are discussed.