Characterization of benzodiazepine receptors in primary cultures of fetal mouse brain and spinal cord neurons

High doses of penicillin decreases [3H]flunitrazepam binding sites in rat neuron primary culture

Penicillin (PC) neurotoxicity (convulsions and encephalopathy) is considered to be due to GABAergic inhibition. The effects of penicillin G(PCG) on [3H]flunitrazepam (FNZ) binding in rat neuron-enriched primary cultures was examined to assess the role of the benzodiazepine (BDZ) receptor in the neurotoxicity. PCG application for 24 h induced a significant decrease in [3H]FNZ binding activity at 10(-3) M, and a decrease in available receptor number (Bmax) at 10(-2) M, without obvious cell damage. Pre-application of the BDZ receptor antagonist, Ro-15-1788, prevented the PC-induced decrease in [3H]FNZ binding activity. Therefore, PC seems to reduce the number of BDZ receptors through a direct effect on this receptor, which is a part of the major inhibitory system in mammalian brain; the GABAergic macromolecular receptor complex. This decrease in BDZ receptors may play a role in PC-induced neurotoxicity, especially encephalopathy.

Chronic GABA exposure down-regulates GABA-benzodiazepine receptor-ionophore complex in cultured cerebral cortical neurons

Cerebral cortical cultured neurons were characterized for GABA-benzodiazepine (BZ) receptor complex, and the effect of chronic exposure of cortical neurons to GABA on GABA-BZ receptor system was investigated. In the intact cells, the [3H]flunitrazepam binding was rapid and saturable, with an apparent Kd of 4.2 +/- 1.5 nM and Bmax of 776 +/- 54 fmol/mg protein. Specifically bound [3H]flunitrazepam was displaced in a concentration-dependent manner by various BZ receptor ligands such as Ro15-1788, DMCM, Ro15-4513, clonazepam, alprazolam, diazepam and zolpidem, and enhanced by GABA, muscimol and pentobarbital. GABA induced enhancement of 36Cl-influx in a concentration-dependent manner (EC50 = 9 +/- 2 microM). Chronic exposure of the cultured neurons to GABA resulted in a reduced [3H]flunitrazepam, [3H]GABA, [3H]Ro15-1788, [3H]Ro15-4513 and [35S]TBPS binding, a reduced enhancement of [3H]flunitrazepam binding by GABA, and a reduced GABA-induced 36Cl-influx susceptible to reversal by concomitant exposure of the cultures to R 5135, a GABAA-receptor antagonist. These findings indicate that cerebral cortical cultured neurons provide an ideal model to study GABA-BZ receptor complex using binding and 36Cl-influx assays, and chronic exposure of cortical cultures to GABA leads to a down-regulation of GABA-BZ receptor system. It is a GABAA receptor-mediated slow process.

The GABAA/benzodiazepine receptor complex in rat brain neuronal cultures. Characterization by immunoprecipitation

The expression of the GABAA/benzodiazepine receptor (GABAR/BZDR) complex in primary neuronal cultures from rat brain embryos has been investigated. The GABAR/BZDR complex was photoaffinity labeled with [3H]flunitrazepam [3H]FNZ and immunoprecipitated with subunit specific antibodies. These were the mAb 62-3G1 which is specific for the 57-kDa GABA binding subunit, and the rabbit antiserum A which recognizes the 51-kDa [3H]FNZ binding subunit. The results indicate that the cultured neurons express 5 different peptides of 51, 53, 54, 57 and 59 kDa that can be photoaffinity labeled with [3H]FNZ and that all of them are physically coupled to the GABAA receptor. Most of the [3H]FNZ photolabeled peptides have similar mobilities to those found in the brain of the newborn rat. Nevertheless, some of the quantitative changes in the photolabeled peptides observed during the normal development of the rat brain were not observed or occurred at much slower pace in the cultured neurons.

Biomechanical basis of diazepam-induced neural tube defects in early chick embryos: a morphometric study

The biomechanical basis of diazepam (Valium/Roche)-induced neural tube defects in the chick was investigated using a combination of electron microscopy and morphometry. Embryos at stage 8 (four-somite stage) of development were explanted and grown for 6 hr in nutrient medium containing 400 micrograms/ml diazepam. Nearly 80% of these embryos exhibited neural tube defects that were most pronounced in the forming midbrain region and typified by a "relaxation" or "collapse" of neural folds. The hindbrain and spinal cord regions were less affected. Electron microscopy revealed that neuroepithelial cells in diazepam-treated embryos had smoother apical surfaces and broader apical widths than did controls. Morphometric measurements supported this observation and further showed that these effects were focused at sites within the wall of the forming neural tube that typically exhibit the greatest degree of bending and apical constriction (i.e., the floor and midlateral walls). Overall results indicate that neural tube defects associated with exposure to diazepam are due largely to a general inhibition of the contractile activity of apical microfilament bundles in neuroepithelial cells. These findings 1) emphasize the important contribution of microfilament-mediated apical constriction of neuroepithelial cells in providing the driving forces for bending of the neuroepithelium during neural tube formation and 2) suggest that agents or conditions that impair their contractile activity could play a role in the pathogenesis of certain types of neural tube defects.

Effect of thyroid hormones on benzodiazepine receptors in neuron-enriched primary cultures

The effect of administration of thyroid hormones on central benzodiazepine receptors was investigated using neuron-enriched primary cultures obtained from the neopallium of 16-day-old embryonic rats. Addition of L-triiodothyronine for 3 days decreased the maximal number of benzodiazepine receptor binding sites without any change in affinity at 10(-5) and 10(-6) M. L-Thyroxine administered for 3 days had the same effect at 10(-5) M. No significant change was observed over periods of less than 3 days, a finding indicating that this inhibition was not a direct in vitro effect. This down-regulation seems to be a direct modulatory effect of thyroid hormones on cerebral cortical neurons. Addition for 3 days of D-thyroxine and D-triiodothyronine, which are physiologically inactive isomers of the thyroid hormones, did not induce any significant alterations in benzodiazepine receptors. The decrease in number of cerebral cortical neuronal benzodiazepine receptors due to L-isomers of thyroid hormones may be related to the convulsions and anxiety observed in thyrotoxicosis in humans.

'GABA shift' in vivo: enhancement of benzodiazepine binding in vivo by modulation of endogenous GABA

The enhancement of benzodiazepine binding by gamma-aminobutyric acid (GABA) and its analogues has been described in detail in brain membrane preparations, but results in in vivo preparations such as tissue slices or animals treated with GABA modulators are conflicting. This 'GABA shift' in vitro has been reported for compounds with agonist effects at the benzodiazepine receptor but not for antagonists. We examined the effects of modulators of endogenous GABA on benzodiazepine receptor binding in vivo as determined by specific uptake of the benzodiazepine antagonist [3H]Ro 15-1788. Enhancement of radioligand uptake was observed in cortex, hypothalamus, hippocampus and pons-medulla 4 h after treatment with aminooxyacetic acid (AOAA), in cortex, cerebellum, hypothalamus, hippocampus and pons-medulla 0.5 h after treatment with valproic acid, and in cortex, cerebellum, hypothalamus and hippocampus 6 h after treatment with gamma-vinyl-GABA. GABA concentrations were increased at each of these points, as were synaptosomal GABA concentrations in prior studies. In contrast, no changes in radioligand uptake or GABA concentrations were observed 12 and 24 h after gamma-vinyl-GABA treatment. Increases in binding appeared to be due to increased apparent affinity at the receptor rather than a change in receptor number. These data indicate that binding of a benzodiazepine antagonist undergoes a GABA shift in vivo analogous to that observed with agonists in vitro.

GABA induces down-regulation of the benzodiazepine-GABA receptor complex in the rat cultured neurons

Cultured neurons from embryonic rat brain display central type benzodiazepine receptors characterized by high-affinity binding of [3H]flunitrazepam which is allosterically enhanced in the presence of gamma-aminobutyric acid (GABA). A 48 h treatment of the cultured neurons with 1 microM diazepam, 0.1 microM clonazepam or 0.1 microM beta-carboline ester derivatives did not change either Bmax or KD values of the [3H]flunitrazepam specific binding. A 48 h incubation in the presence of GABA (1 mM) or muscimol (0.1 mM) induced a 30% decrease of the Bmax value of [3H]flunitrazepam specific binding without change of the KD value. The down-regulation was dependent on GABA concentrations and temperature, and was partially inhibited by bicuculline but not by the benzodiazepine antagonist Ro 15-1788. The other subunits of the benzodiazepine-GABA-chloride channel receptor complex also seemed to be down-regulated by GABA since there was a decrease of the specific binding of [3H]muscimol and [35S]t-butylbicyclophosphorothionate (TBPS) to the GABAA and chloride channel sites respectively. The GABA-induced down-regulation of the GABA-benzodiazepine receptor seems to be selective since the specific binding of ligands to other receptors was not affected. Our results suggests that activation of the low-affinity GABA subunit which is involved in cellular electrophysiological responses, induced the receptor down-regulation.

Characteristics of flunitrazepam binding to intact primary cultured spinal cord neurons and its modulation by GABAergic drugs

The interaction of [3H]flunitrazepam and its modulation by various drugs was studied in intact primary cultured spinal cord neurons. In the intact cells, the [3H]-flunitrazepam binding was rapid and saturable. The benzodiazepine binding sites exhibited high affinity and saturability, with an apparent KD of 6.1 +/- 1.6 nM and Bmax of 822 +/- 194 fmol/mg protein. The association and dissociation of [3H]flunitrazepam binding exhibited monoexponential kinetics. Specifically bound [3H]flunitrazepam was displaced in a concentration-dependent manner by benzodiazepines like flunitrazepam, clonazepam, diazepam, Ro 15-1788, and beta-carbolines like methyl-6,7-dimethoxy-4-ethyl-beta-carboline-3'-carboxylate. Specific [3H]flunitrazepam binding to intact cells was enhanced in a concentration-dependent manner by gamma-aminobutyric acid (GABA) agonists and drugs which facilitate GABAergic transmission like etazolate, (+)-etomidate, and pentobarbital. The enhancing effect of GABA agonists was antagonized by bicuculline and picrotoxinin. These results suggest that the intact cultured spinal cord neurons exhibit the properties of benzodiazepine GABA receptor-ionophore complex. Since these cells can also be studied in parallel for characterizing GABA-induced 36Cl-influx, they provide an ideal in vitro assay preparation to study GABA synaptic pharmacology.

Electrophysiological studies in cultured mouse CNS neurones of the actions of an agonist and an inverse agonist at the benzodiazepine receptor

The action of agents which bind with the benzodiazepine (BZ) receptor has been investigated by use of intracellular recordings from dissociated mouse neurones grown in tissue culture. The agents tested were midazolam (an agonist at the BZ receptor) and methyl 6,7-dimethoxy-4-ethyl-beta-carboline-3-carboxylate (DMCM-an inverse agonist at the BZ receptor). These were applied to the neurone under study by one of the following methods: iontophoresis; pressure application of known concentrations from blunt pipettes; directly in the perfusing medium. On only very few occasions did midazolam or DMCM have a direct effect on the membrane potential (EM) or conductance (GM) of the impaled neurone. For the neurones where direct effects were present, there was no consistent pattern of response. Neither substance affected the threshold for action potential generation. The effect of midazolam and DMCM on responses evoked by iontophoretic application of gamma-aminobutyric acid (GABA) was also investigated. Three parameters were used to quantify GABA responses: the depolarization (VGABA); the increase in GM (gGABA) measured with constant current pulses; using voltage clamp, the GABA current (IGABA). The GABA response should be quantified by a parameter which is linearly related to the number of GABA-operated channels which are conducting at any instant. VGABA does not fulfil this criterion. gGABA is an appropriate parameter, but is difficult to determine for large responses where the membrane is nearly short circuited. IGABA measured during voltage clamp fulfils this criterion. Midazolam (greater than 10(-6) M) reliably potentiated GABA responses with a parallel shift to the left of the dose-response curve. This is in agreement with biochemical studies where BZs increase the affinity of the GABA receptor for its ligand. The effect of DMCM on GABA responses was more variable. In the majority of cases GABA responses were reduced by DMCM. The threshold dose for this depression was usually around 10(-6) M, but was sometimes as low as 10(-8) M. Dose-response curves of IGABA or gGABA showed the inhibition to be of a non-competitive nature. The maximum inhibition achieved was around 70%. For a given neurone, and at doses which did not necessarily cause a reduction of the response to GABA, DMCM could antagonize the potentiating action of midazolam on GABA responses. A possible interpretation is that more than one BZ site per receptor complex must be occupied by a BZ agonist (or inverse agonist) before the functional changes for the complex as a whole can occur. Desensitization to GABA was increased by midazolam.

Benzodiazepine binding characteristics of embryonic rat brain neurons grown in culture

The binding of [3H]diazepam to cell homogenates of embryonic rat brain neurons grown in culture was examined. Under the conditions used to prepare and maintain these neurons, only a single, saturable, high-affinity binding site was observed. The binding of [3H]diazepam was potently inhibited by the CNS-specific benzodiazepine clonazepam (Ki = 0.56 +/- 0.08 nM) but was not affected by the peripheral-type receptor ligand Ro5-4864. The KD for [3H]diazepam bound specifically to cell homogenates was 2.64 +/- 0.24 nM, and the Bmax was 952 +/- 43 fmol/mg of protein. [3H]Diazepam binding to cell membranes washed three times was stimulated dose-dependently by gamma-aminobutyric acid (GABA), reaching 112 +/- 7.5% above control values at 10(-4) M. The rank order for potency of drug binding to the benzodiazepine receptor site in cultured neurons was clonazepam greater than diazepam greater than beta-carboline-3-carboxylate ethyl ester greater than Ro15-1788 greater than CL218,872 much greater than Ro5-4864. The binding characteristics of this site are very similar to those of the Type II benzodiazepine receptors present in rat brain. These data demonstrate that part, if not all, of the benzodiazepine-GABA-chloride ionophore receptor complex is being expressed by cultured embryonic rat brain neurons in the absence of accompanying glial cells and suggest that these cultures may serve as a model system for the study of Type II benzodiazepine receptor function.

CL 218,872 binding to benzodiazepine receptors in rat spinal cord: modulation by gamma-aminobutyric acid and evidence for receptor heterogeneity

The binding of the triazolopyridazine CL 218,872 to central benzodiazepine receptors identified with [3H]Ro 15-1788 was studied in extensively washed homogenates of rat spinal cord and cerebral cortex. CL 218,872 displacement curves were shallow in both spinal cord (nH = 0.67) and cortex (nH = 0.54), suggesting the presence of type 1 and type 2 benzodiazepine receptors in both tissues. CL 218,872 had lower affinity in spinal cord (IC50 = 825 nM) than cortex (IC50 = 152 nM), possibly reflecting the presence of fewer type 1 sites in the cord. Activating gamma-aminobutyric acid (GABA) receptors with 10 microM muscimol resulted in a two- to threefold increase in CL 218,872 affinity in both tissues without changes in the displacement curve slope. This indicates that GABA enhances CL 218,872 affinity for both type 1 and type 2 sites in both spinal cord and cerebral cortex.

Benzodiazepine binding to cultured human pituitary cells

Benzodiazepine receptors were investigated in a cell line of human pituitary cells (18-54,SF) grown in serum-free medium. Preparations of 18-54,SF whole cells and cell membranes were shown to possess saturable [3H]diazepam binding sites. Membrane sites were found to have a KD of 20 nM for diazepam while whole cells possessed a twofold higher value. The KD values determined from Rosenthal, Hill, and kinetic analyses were consistent for each preparation. Whole-cell binding of [3H]diazepam was observed to be more stable than binding to membranes at higher temperatures (37 degrees C) and when longer incubation times (60 min) were employed at 4 degrees C. The rank order potency of various benzodiazepines to inhibit [3H]diazepam binding to whole cells and membranes was Ro 5-4864, flunitrazepam, diazepam, and clonazepam. Representatives of other drug classes did not inhibit this benzodiazepine binding. When 18-54,SF cells were grown for 24 h with 100 nM diazepam and then extensively washed membranes prepared, the KD for diazepam increased to 38 nM whereas the Bmax was unchanged when compared with untreated controls. Overall, these findings indicate that pituitary cells possess a peripheral-type benzodiazepine receptor and that the whole cell receptor differs quantitatively when compared with the membrane receptor.

Benzodiazepine receptor ligand actions on GABA responses. Beta-carbolines, purines

The effects of several beta-carboline and purine ligands for benzodiazepine receptors were studied upon GABA (4-aminobutyric acid) responses and upon diazepam enhancement of GABA responses, using mouse spinal cord neurons in dissociated cell culture. While the potent convulsant beta-carboline DMCM (methyl-6,7-dimethyoxy-4-ethyl-carboline-3-carboxylate), reduced GABA responses, methyl-carboline-3-carboxylate (beta CCMe) and the corresponding ethyl ester (beta CCEt) did not alter GABA responses. The propyl ester (beta CCPr) enhanced GABA responses in a concentration-dependent fashion, while both beta CCMe and beta CCPr blocked diazepam enhancement of GABA responses. beta CCPr may thus have partial agonist activity. Two purines with moderate benzodiazepine receptor affinity, 1-methylisoguanosine (MeIG) and 6-dimethylaminopurine (DMAP), weakly enhanced GABA responses. MeIG also significantly antagonized diazepam enhancement of GABA responses. Inosine and hypoxanthine had no apparent actions upon GABA responses or upon diazepam enhancement of such responses. The results with beta-carbolines are consistent with their behavioural profile in vivo and with neurochemical studies of their effects upon GABA-benzodiazepine receptor complexes. Furthermore, certain purines are also able to interact with these complexes.

Properties of [3H]diazepam binding sites on cultured murine glia and neurons

[3H]Diazepam binding was assayed in situ on living cultures of fetal mouse cerebral cortex or glia in an attempt to further characterize the high and low affinity binding sites. Mixed neuronal-glial cultures were found to have a high (Kd approximately equal to 10 nM) as well as a low (Kd approximately equal to 240 nM) affinity binding site. Glial cultures also had a similarly high affinity site (Kd 13 nM). In both types of cultures, the high affinity site was Ro 5-4864 sensitive and clonazepam resistant. Since Ro 5-4864 has particular affinity for non-neuronal elements and clonazepam for neuronal elements, the data suggest that the high affinity binding site may be localized to glial elements and the low affinity site primarily neuronal.

Binding of [3H]flunitrazepam to the LM cell, a transformed murine fibroblast

LM cells have a saturable, high affinity binding site for [3H]flunitrazepam with a KD of 13 nM and a Bmax of 19 pmoles/mg protein. The IC50 values for Ro 5-4864, flunitrazepam and clonazepam against [3H]flunitrazepam were 6, 23 and 2800 nM, respectively, indicating that this receptor is of the peripheral type. A decrease of 37, 26 and 26% in Bmax was associated with substituting dimethylethanolamine, monomethylethanolamine or ethanolamine, respectively, for choline in the cell culture medium. These treatments did not change either the KD of [3H]flunitrazepam binding or the IC50 values of the different benzodiazepine drugs. Metastatic cell lines of the LM cell obtained from either athymic or C3H/Hef mice exhibited alterations in the binding parameters of [3H]flunitrazepam. There was a reduction in the Bmax values of the athymic (34%) and the C3H/Hef (44%) cell lines compared to the LM cell. In both groups there was a 90% increase in the KD. In the C6 astrocytoma, the peripheral type receptor appears to regulate plasma membrane mediated synthesis of phosphatidylcholine from phosphatidylethanolamine. However, this was not observed in the LM cell. Nor did it modulate cyclic AMP metabolism as assessed by measurement of cyclic AMP levels in whole cells after drug treatment.

Development and differentiation of the benzodiazepine receptor in cultures of fetal mouse spinal cord

Cultures of fetal mouse spinal cord were assayed for the development of the benzodiazepine (BDZ) receptor on intact cells in situ. At 2 and 3 weeks in culture, levels of binding were low and Scatchard analysis was linear revealing a single binding site with Kds of 115 and 71 nM, respectively. After 4 weeks in culture, coincident with large increases in neuronal (clonazepam displaceable) and nonneuronal (Ro5-4864 displaceable) binding. Scatchard plots became non-linear revealing 2 distinct binding sites with Kds of 11 and 175 nM.

Studies on the mechanisms of neurulation in the chick: possible involvement of myosin in elevation of neural folds

The possible involvement of myosin in elevation of neural folds in the chick was studied. Indirect immunofluorescence revealed the presence of myosin in the neuroepithelium as early as the neural-plate stage and was concentrated in the apical regions of neuroepithelial cells where microfilaments are known to be organized into discrete bundles. This fluorescent pattern persisted until closure of the neural tube. Actin-specific fluorescence followed a similar distribution pattern as myosin. Diazepam (Valium/Roche), at 400 micrograms/ml, was found to preferentially inhibit elevation of neural folds in explanted stage 8 embryos within 6 hr of incubation. Affected neuroepithelial cells were often less elongated, contained thinner and less conspicuous microfilament bundles, and had apical surfaces which were smoother and broader than the controls. These morphological changes were accompanied by a considerable reduction in the intensity of myosin-specific fluorescence, particularly in the cell apices. Results suggest that (1) diazepam inhibits elevation of neural folds through its disruptive effects on the organization and contractility of apical microfilament bundles in developing neuroepithelial cells and (2) myosin may be directly involved in elevation of neural folds.

Benzodiazepines and beta-adrenergic binding to primary cultures of astrocytes and neurons

Characteristics of primary cultures of neurons or of astrocytes are discussed which suggest that such cultures are appropriate models for their in vivo counterparts. Advantages or disadvantages of these cultures for studies of receptor binding are: Neuronal and astrocytic binding can be studied separately. Binding can be studied to intact cells or after homogenization. Apparent binding to intact cells may include unspecific retention. This seems to be a problem for beta-adrenergic ligands but not for benzodiazepines.

Multiple actions of picomolar concentrations of flurazepam on the excitability of cultured mouse spinal neurons

Intracellular recordings from mouse spinal neurons grown dissociated in tissue culture were used to study the effects of the water soluble benzodiazepine, flurazepam, upon neuronal excitability. Low concentrations of this drug (1 pM to 10 nM) depressed excitability in three distinctly different ways: (1) by directly increasing Cl- conductance, (2) by potentiating responses to GABA, and (3) by elevating spike threshold and/or depressing repetitive spike firing. Bathing neurons with picrotoxin induced 'convulsive-like' activity which was attenuated by flurazepam. The direct effects of flurazepam on the passive and active properties of membrane excitability were insensitive to picrotoxin. However, when the dose of flurazepam was increased to 10 nM or greater this drug lost its effectiveness. These results show that flurazepam is a potent drug with multiple sites of action all of which are likely to contribute to its pharmacological actions in vivo.

Diazepam binding of dissociated hippocampal cultures from fetal mice

Primary cultures of dissociated hippocampi from fetal mice examined for the presence of binding sites for [3H]diazepam. The binding assays were done with living cells still attached to the culture dish. The cells contain high affinity binding sites for [3H]diazepam, Kd = 5 nM, which are completely inhibited with 20 nM R05-4864 but only 26% with 20 nM lorazepam. The binding was inhibited by purinergic compounds and by quinidine. The living cell did not exhibit increased binding of [3H]diazepam in the presence of GABA and in fact a slight decrease in binding was found. This was also found when live, intact C6 glial cells were investigated. These observations suggest that the use of living cells to study the benzodiazepine receptor is valuable and maybe necessary to fully characterize this receptor.

Benzodiazepine binding and interactions with the GABA receptor complex in living cultures of rat cerebral cortex

Benzodiazepines bind to living cultures of dissociated rat cerebral cortex. This binding is saturable, and kinetic analyses indicate that the binding is to a single class on sites with kinetic constants very close to those obtained using neuronal membrane preparations. The efficacy of a number of benzodiazepines, xanthine derivatives and other drugs in competition experiments is similar to that seen in neuronal membrane preparations, and suggests that the benzodiazepine binding site studied in these investigations is the same as that found in neuronal membrane preparations and believed to be the pharmacologically active benzodiazepine binding site. GABA agonists increase the binding of benzodiazepines, and this increase has the same order of efficacy as their ability to hyperpolarize the neurons when applied at known concentrations with muscimol greater than GABA greater than THIP. At high concentrations THIP potentials benzodiazepine binding to the same level as GABA. Diazepam increases the ability of both GABA and THIP to hyperpolarize the neurons as well as the amplitude of spontaneous IPSPS which, in this system, are GABA-mediated. The competitive GABA antagonist bicuculline methiodide slightly decreased benzodiazepine binding and also antagonized the increase due to GABA. The non-competitive GABA antagonist picrotoxinin had no effect on benzodiazepine binding but did antagonize the GABA-induced increase in benzodiazepine binding. Replacement of Cl- in the incubation medium by acetate, which does not permeate the GABA-mediated Cl-- ionophore, increases benzodiazepine binding, and GABA no longer increases the binding. Picrotoxinin decreases the increase in benzodiazepine binding is Cl--free media, and this decrease is blocked by GABA. These results are discussed in terms of interactions at the GABA receptor complex consisting of a GABA recognition site, a benzodiazepine recognition site, a picrotoxinin recognition site, and a Cl- ionophore.

Diazepam-induced neural tube closure defects in explanted early chick embryos

The effects of diazepam on the development of explanted stage 4 chick embryos were investigated. Diazepam, at 10-120 micrograms/ml, preferentially inhibited closure of the neural tube. This effect was reversible. Concentrations of 150-200 micrograms/ml inhibited not only neural tube closure but also blastodermal expansion, somite formation, and heart development in 52% of the embryos. Concentrations above 200 micrograms/ml were highly embryotoxic. Electron microscopy of affected neuroepithelial cells revealed that 1) apical surfaces were much smoother than usual and 2) apical filament bundles, which are generally thought to provide motive forces for uplifting of neural folds, were not well organized and often lacked alternating dark and light areas along their length. These findings and the fact that changes in cell surface topography reflect the contractile activities of underlying filament bundles suggest that the observed "smoothing" effect on apical cell surfaces and neural tube closure defects are due, at least in part, to the impaired ability of these filament bundles to contract.

Benzodiazepines have high-affinity binding sites and induce melanogenesis in B16/C3 melanoma cells

We found that two markers of differentiation, tyrosinase (monophenol, dihydroxyphenylalanine:oxygen oxidoreductase, EC 1.14.18.1) activity and melanin synthesis, are induced by diazepam in B16/C3 mouse melanoma cells. We also demonstrated high-affinity binding sites for [3H]diazepam in these cells by radioreceptor assay, and we visualized binding to the cell surface by fluorescence microscopy with a benzodiazepine analog conjugated to a fluorescein-labeled protein. Our studies also showed that there are differences between the binding characteristics in intact cells and in membrane fractions prepared from these cells. Scatchard analysis of the binding data from membrane fractions gave a linear plot (Kd = 9.1 X 10(-8) M). With intact cells, a curvilinear Scatchard plot was obtained. This was resolved into two components defining binding sites with affinity constants of 1.7 X 10(-9) M and 4.6 X 10(-7) M. Thus, it appears that [3H]diazepam binding in intact cells is more complex than in isolated membranes. Several related benzodiazepines, including flunitrazepam, Ro-5-4864, nitrazepam, oxazepam, lorazepam, Ro-5-3072, chlordiazepoxide, and clonazepam also induced melanogenesis. When these compounds were tested for their ability to inhibit [3H]diazepam binding, flunitrazepam, diazepam, and Ro-5-4864 were found to be the most effective inhibitors. These three compounds were also the most potent in inducing melanogenesis. Our results suggest that the benzodiazepines modulate cell differentiation. The presence of high-affinity binding sites in this homogeneous, easily grown cell line may provide a useful model for studies on the mechanism of action of these compounds.