Identification of inosine and hypoxanthine as endogenous inhibitors of [3H] diazepam binding in the central nervous system

Benzodiazepines modulate the A2 adenosine binding sites on 108CC15 neuroblastoma X glioma hybrid cells

We have demonstrated high affinity diazepam binding sites of the Ro5-4864 benzodiazepine receptor subtype on 108CC15 neuroblastoma X glioma hybrid cells. These cells were previously shown to have purinoceptors of the A2 adenosine subtype and we have now found that [3H]-adenosine can be displaced from this binding site by the benzodiazepines and related compounds that can also bind to the Ro5-4864 site. Diazepam was found to have no intrinsic activity at the A2-receptor as measured by the stimulation of adenosine 3':5'-cyclic monophosphate (cyclic AMP) production in this cell line. At concentrations sufficient to compete for the A2-receptor, diazepam was shown to facilitate, by approximately 2 fold, the stimulation of cyclic AMP by adenosine. These effects are not due to inhibition of adenosine uptake or phosphodiesterase activity, but are probably a consequence of modulation of the coupling of the A2-receptor to cyclic AMP production in this hybrid cell line.

Adenosine deaminase in convulsions along with its regional, cellular and synaptosomal distribution in rat brain

Adenosine deaminase activity was determined in enriched neuronal, glial and synaptosomal fractions in three regions of rat brain. It was found to be equally distributed in cortical neurons and synaptosomes. Appreciable activity was observed in glial cells. A significant increase in the activity of this enzyme was observed in three brain regions in rats subjected to leptazol convulsions. No significant change, however, was found in the pre- or post-convulsive periods. These results are discussed in relation to the possible degradation of adenosine, a neuroinhibitory substance and to the possible production of inosine, an initiator of seizure activity in low amounts and depressor of the same in high amounts.

Food intake: opioid/purine interactions

The exogenous opioids butorphanol tartrate (BT) and ethylketocyclazocine (EKC) have been reported to stimulate feeding in rats. In this study we evaluated the effects of purines (known to suppress feeding) and the adenosine antagonist, caffeine, on opioid induced feeding. Adenosine and inosine significantly suppressed BT and EKC induced feeding at various doses and time points. Caffeine enhanced food consumption was suppressed by various doses of naloxone, but was not suppressed by adenosine or inosine. Although caffeine itself induced further feeding, it did not enhance BT induced food consumption. Adenosine and inosine failed to suppress BT induced feeding when 12.5 mg/kg of caffeine was administered to the rats suggesting blockade of the adenosine receptor by caffeine. In contrast to 12.5 mg/kg caffeine, high dose caffeine (50 mg/kg) suppressed BT induced feeding over a 4 hour time period. Adenosine (50 mg/kg) and inosine (50 mg/kg) injected one hour after injection of BT and caffeine (50 mg/kg) reversed the suppressive effect of high dose caffeine in BT induced feeding. These studies indicate that opioid induced feeding can be suppressed by adenosine and inosine. Also, caffeine can reverse the suppressive effect of adenosine and inosine on feeding and vice versa. Naloxone's suppression of caffeine enhanced food consumption indicate that at least part of caffeine's effect on food intake may be mediated through an opioid mechanism.

Effects of various 2-amino-6-alkyldithiopurines on brain specific [3H]diazepam binding

Various derivatives of 2-amino-6- methylthiopurine with substituents at the 6-position of purine were tested for their abilities to displace [3H]diazepam binding to rat brain membranes. The potency was dependent on the carbon chain-length in the 6-position of purine. Among the derivatives tested, 2-amino-6-n- pentyldithiopurine had the highest potency, with a Ki value of 0.92 microM.

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.

Mechanism of potentiation by diazepam of adenosine response

Diazepam (3 microM) potentiates the adenosine-induced relaxation of caecum. Other benzodiazepine receptor ligands, such as inosine and hypoxanthine, failed to modify adenosine responses. Diazepam failed to further modify the dose-response curve to adenosine obtained in the presence of dipyridamole, and uptake inhibitor. Diazepam and dipyridamole did not affect the responses to 2-chloroadenosine. The relaxant effect of adenosine was not blocked by Ro 15-1788, a benzodiazepine receptor antagonist. These observations indicated that diazepam potentiated adenosine response by inhibiting the uptake of purine nucleotide and, that the benzodiazepine receptor is not purinergic in nature in the rat caecum.

Molecular aspects of the action of benzodiazepine and non-benzodiazepine anxiolytics: a hypothetical allosteric model of the benzodiazepine receptor complex

The availability of radiolabeled benzodiazepines has resulted in the identification of high affinity receptors in the central nervous system for this class of psychotherapeutic agent which are linked to recognition sites for the inhibitory neurotransmitter, GABA. Evaluation of new, synthetic compounds in the benzodiazepine radioligand binding assay has resulted in the identification of nine classes of non-benzodiazepine putative anxiolytic agents, some of which may be more anxioselective than the benzodiazepines. At least three and possibly five subclasses of benzodiazepine receptor have been identified in mammalian tissues using radioligand binding assays. The possibility exists that one of these receptor subclasses may mediate the anxiolytic effects of the benzodiazepines while the remainder may be involved in the mediation of the sedative, ataxic and anticonvulsant properties associated with benzodiazepine-like agents. Several endogenous ligands for the benzodiazepine receptor(s) have been postulated. These include various proteins and peptides, purines and the beta-carbolines. This latter group, which competitively antagonizes the pharmacological and biochemical effects of the benzodiazepines, has the highest affinity for the benzodiazepine receptor of all compounds thus far examined; however, none of these compounds has been conclusively identified as the endogenous ligand akin to the enkephalins and endorphins at the opiate receptor. The majority of available evidence would indicate that the endogenous ligand for the benzodiazepine receptor(s) is an antagonist of the benzodiazepines and other putative anxiolytic agents.

In vitro uptake of benzodiazepines by rat pineal gland

As a part of a study aiming to characterize the physiological and pharmacological significance of the high affinity pineal benzodiazepine (BZP) binding sites reported previously, we examined the uptake of the BZP derivative 3H-flunitrazepam (FNZP) by rat pineal glands in vitro. At 37 degrees C, 3H-radioactivity was taken up by tissue up to a pineal/medium concentration of about 12, while at 0 degrees C the uptake amounted to only one-third that at 37 degrees C. Reciprocal of uptake analyzed by Lineweaver-Burk plots indicated apparent Km's of 1.74 and 1.45 microM, and Vmax's of 1.32 and 1.04 pmol per min per mg tissue, for control and superior cervical ganglionectomized rats, respectively, suggesting that the neural compartment does not participate significantly in 3H-FNZP uptake. Cerebral cortex explants of similar size and weight as the pineal ones took up 3H-FNZP to a maximum tissue/medium concentration of about 2. Neither pineal nor cerebral cortex 3H-radioactivity uptake exhibited significant changes as a function of time of day. A number of agents, including several BZP analogues, cocaine, desipramine, melatonin, fluoxetine, nomifensine, and dipiridamol, as well as changes in the ionic environment or metabolic inhibitors, did not affect 3H-FNZP uptake significantly. Other tissues, such as liver, muscle, kidney, adrenal gland, or anterior pituitary, took up 3H-radioactivity to tissue concentrations slightly lower than those of the cerebral cortex, suggesting that drug liposolubility accounted only to a limited extent for the high in vitro uptake detected in incubated pineals.

On a brain polypeptide functioning as a putative effector for the recognition sites of benzodiazepine and beta-carboline derivatives

Stimulation of benzodiazepine recognition sites by various ligands can elicit opposite types of responses such as proconvulsant and anticonvulsant or proconflict and anticonflict actions. The study of the pharmacological profile of various ligands makes it possible to distinguish three classes of compounds: (1) those that elicit anticonflict responses in the Vogel test, inhibit the convulsions due to an impairment of GABAergic transmission and increase the Bmax of the high affinity recognition site for GABA; (2) those that displace benzodiazepines from specific binding sites, facilitate convulsions due to impairment of GABAergic mechanisms, elicit proconflict responses in Vogel's test and inhibit the facilitation by benzodiazepines of GABA binding and (3) those that displace benzodiazepines and beta-carboline-derivatives from specific binding sites, antagonize the anticonflict, the proconflict, the anticonvulsant and the proconvulsant actions of the two preceding groups of substances and in very large doses elicit a small proconvulsant action. Examples of the latter are an imidazobenzodiazepine (RO 15-1788) and a pyrazolquinolinone derivative (CGS 8216). The nomenclature for these three classes of drugs should be kept flexible until the action of the endogenous ligand that functions as the physiological effector of the benzodiazepine/beta-carboline recognition site is known. A putative ligand for this site (DBI = diazepam binding inhibitor) has been isolated and purified to homogeneity. It includes 104 amino acid residues, the sequence of the last 45 amino acids has been determined. This compound elicits a proconflict action, displaces beta-carboline derivatives more than anxiolytic benzodiazepines, but its high molecular weight and relative low affinity for the binding sites in brain might suggest that it is a precursor, rather than the putative effector for benzodiazepine and/or beta-carboline recognition sites.

The neuropharmacology of various diazepam antagonists

Recently, compounds which bind avidly to benzodiazepine binding sites have been shown to possess diazepam antagonist properties. For example, the benzodiazepine RO 15-1788 and the pyrazoloquinoline CGS 8216 can antagonize the anxiolytic, sedative, muscle relaxant and anticonvulsant properties of diazepam. The beta-carbolines have also been shown to antagonize several actions of diazepam. Other compounds including physostigmine, naloxone, bicuculline, picrotoxin, caffeine and theophylline, lack appreciable affinity for benzodiazepine binding sites but do antagonize at least some of the behavioral actions of diazepam. Their antagonist properties are probably the result of opposing pharmacological actions rather than direct receptor antagonism. Clinically, a potent safe diazepam antagonist could be used to reverse effects of diazepam overdose and to speed recovery of diazepam-treated patients after various out-patient procedures.

Inhibitors of peripheral-type benzodiazepine receptors present in human urine and plasma ultrafiltrates

Several endogenous substances that inhibit central-type benzodiazepine (BZD) receptor binding have recently been identified. We have found that ultrafiltrates of human uremic plasma, normal plasma, and urine contain competitive inhibitors of peripheral-type benzodiazepine receptors. Using urine as source, we have partially purified a peripheral-type BZD receptor inhibitor(s) by adsorption to and selective elution from small octadecyl-silane (Sep-pak) columns and thin layer chromatography. The inhibitor has a 125-fold greater affinity for peripheral-type than central-type BZD receptors and has been purified 8000-fold from urine.

Prenatal diazepam: distribution and metabolism in perinatal rats

Previous investigations have revealed that prenatal exposure to diazepam (DZ) alters brain development and behavior in the offspring of rats and mice. In order to understand how DZ may affect the developing nervous system it is necessary to examine its metabolic fate in the neonate. It is therefore the aim of this study to investigate the disposition, metabolism, and persistence of DZ in the neonate. Dams were injected s.c. with 2.5 mg/kg of 14C DZ (10 muCi/day) on days 13-20 of gestation and their litters were fostered at birth. Dams killed within 24 hours postpartum and neonates killed at postnatal days 0, 10, and 20 were analyzed for 14C activity. Brain levels (pmoles DZ and metabolites/100 mg tissue SE) were 3.4 +/- 0.3 in the dam and in the neonates were 3.2 +/- 0.3 (day 0), 3.4 +/- 0.3 (day 10), and undetectable at day 20. Neonatal peripheral tissue 14C activity was undetectable by day 10. Brain regional analysis indicates 14C is highest in the colliculi at day 0, but not at day 10. Brain levels of DZ, oxazepam (OXA), N-desmethyldiazepam (NDZ), and the glucuronide (GLU) determined by high-performance liquid chromatography (HPLC), were GLU (49%), DZ (28%), and NDZ (24%) in the dam; GLU (52%), DZ (24%), and NDZ (25%) in the day 0 neonate; and GLU (32%), DZ (12%), NDZ (39%), and OXA (19%) at day 10. The distribution and metabolism of 14C DZ that persists in the neonate following prenatal exposure differs from that which occurs in the dam.(ABSTRACT TRUNCATED AT 250 WORDS)

Actions of 6-aminonicotinamide on benzodiazepine receptors in rat CNS

Since nicotinamide, 6-aminonicotinamide and harmaline can modify the ability of 3-acetylpyridine to cause CNS lesions, and nicotinamide and harmane alkaloids have been proposed as ligands at the benzodiazepine receptor, we have examined the effects of these agents individually or in combinations on [3H]diazepam binding to rat cortex membranes. No interaction was observed between any of the compounds, but 6-aminonicotinamide is over 100-fold more potent than nicotinamide in displacing diazepam.

Isolation, characterization, and purification to homogeneity of an endogenous polypeptide with agonistic action on benzodiazepine receptors

A brain polypeptide termed diazepam-binding inhibitor (DBI) and thought to be chemically and functionally related to the endogenous effector of the benzodiazepine recognition site was purified to homogeneity. This peptide gives a single band of protein on NaDodSO4 and acidic urea gel electrophoresis. A single UV-absorbing peak was obtained by HPLC using three different columns and solvent systems. DBI has a molecular mass of approximately equal to 11,000 daltons. Carboxyl-terminus analysis shows that tyrosine is the only residue while the amino-terminus was blocked. Cyanogen bromide treatment of DBI yields three polypeptide fragments, and the sequences of two of them have been determined for a total of 45 amino acids. DBI is a competitive inhibitor for the binding of [3H]diazepam, [3H]flunitrazepam, beta-[3H]carboline propyl esters, and 3H-labeled Ro 15-1788. The Ki for [3H]-diazepam and beta-[3H]carboline binding were 4 and 1 microM, respectively. Doses of DBI that inhibited [3H]diazepam binding by greater than 50% fail to change [3H]etorphine, gamma-amino[3H]butyric acid, [3H]-quinuclidinyl benzilate, [3H]dihydroalprenolol, [3H]adenosine, and [3H]imipramine binding tested at their respective Kd values. DBI injected intraventricularly at doses of 5-10 nmol completely reversed the anticonflict action of diazepam on unpunished drinking and, similar to the anxiety-inducing beta-carboline derivative FG 7142 (beta-carboline-3-carboxylic acid methyl ester), facilitated the shock-induced suppression of drinking by lowering the threshold for this response.

Benzodiazepine inhibition of nucleoside transport in human erythrocytes

The interaction of several benzodiazepines (BDZs) with the nucleoside transport system of fresh erythrocytes from humans was investigated. The affinities of BDZs for the nucleoside transport system were estimated by measuring BDZ inhibition of (a) the site-specific binding of nitrobenzylthioinosine, a potent and specific inhibitor of nucleoside transport, and (b) the uridine transport processes, zero-trans influx, zero-trans efflux, and equilibrium exchange influx. The BDZs inhibited both the inward and outward transport processes, and, for individual agents, inhibition constants (Ki) were similar for the inhibition of each transport process and for the inhibition of the site-specific binding of nitrobenzylthioinosine. The order of potencies of the BDZs in their interactions with the nucleoside transport mechanism (Ro 5-4864 greater than diazepam greater than clonazepam greater than lorazepam greater than flurazepam) is distinct from the potencies of these compounds at BDZ recognition sites. The affinities of the BDZs for the nucleoside transport system, which are about 1000-fold lower than for BDZ recognition sites, suggest that significant inhibition is unlikely to occur with the plasma concentrations (less than 1 microM) that result from usual anxiolytic doses of these agents.

Electroshock raises pentylenetetrazol threshold: possible role of inosine

A series of 15 single electroshocks administered through electrodes applied to the scalp of mice raised the threshold to pentylenetetrazol-induced seizures as determined by a tail vein infusion method. The same stimulus increased brain inosine and hypoxanthine. Phenytoin, which blocks the increase in inosine produced by this stimulus, reversed the elevation in threshold may be electroshock. The effect of electroshock on threshold may be mediated by inosine.

Theophylline antagonizes diazepam-induced psychomotor impairment

Eight healthy men received an oral dose of 0.25 mg/kg diazepam followed 40 min later by an intravenous infusion of 100 ml physiological sodium chloride solution, with or without 4.4 mg/kg theophylline. Psychomotor function was assessed after each blood sampling up to 5 h post-infusion. Thirty min after diazepam psychomotor performance measured by Card Sorting test and Digit Symbol Substitution test was impaired and subjects felt sleepy and could think less clearly (two factors of the Clyde Mood Scale). Theophylline antagonized the diazepam-induced impairment statistically significantly for up to 5 h and subjects felt less tense and less apprehensive (State Anxiety Inventory). Since pharmacokinetic parameters of diazepam seemed not to be different after theophylline, interaction at receptor level can be assumed.

Purines interact with 'central' but not 'peripheral' benzodiazepine binding sites

The effects of several purines and the purine uptake inhibitor, dipyridamole, on the binding, to rat brain membranes, of 4 benzodiazepines with different pharmacological specificities were studied. While all purines tested displaced the binding of [3H](+)-3-methyl-clonazepam and [3H]Ro15-1788, selective agonist and antagonist ligands respectively for 'central' benzodiazepine receptors, purines had little or no affinity for [3H]Ro5-4864 'peripheral'-type binding sites in brain, heart or kidney. These results suggest that purines interact with a pharmacologically relevant class of central benzodiazepine 'receptors', and not with central and peripheral 'acceptor' sites labelled by the benzodiazepine Ro5-4864.

Contrasting regulation by GABA of the displacement of benzodiazepine antagonist binding by benzodiazepine agonists and purines

Gaba increases the potency of the benzodiazepines chlordiazepoxide, clonazepam, diazepam, nitrazepam and oxazepam, and the triazolopyridazine CL 218,872 in displacing specific binding of the benzodiazepine antagonist [3H]Ro 15-1788. In contrast, the potencies of the purines 1-methyl- and 1-ethylisoguanosine for benzodiazepine antagonist binding sites were decreased by GABA, while the potencies of inosine, hypoxanthine, 6-dimethylaminopurine, and the non-benzodiazepine anxiolytic, zopiclone, were unaltered by GABA. The results suggest that the purines and 'classical' benzodiazepine agonists may bind to different conformations or populations of receptors.

Binding of beta-carbolines and caffeine on benzodiazepine receptors: correlations to convulsions and tremor

Compounds from both the beta-carboline (BC) and xanthine groups have been suggested to be the natural ligands for benzodiazepine (BZ) receptors. In this study we examined the effects of several BC's and caffeine, 1,3,7-trimethylxanthine, on the binding of 3H-flunitrazepam (3H-FZ) and beta-3H-carboline-3-carboxylic acid ethyl ester (3H-BCCE) to the BZ receptors of rat and mouse brain. In mice, convulsion-producing doses of caffeine (120 mg/kg intravenously) and harmane (30 mg/kg intravenously) lowered the specific binding of 3H-FZ in vivo by 12-31%. A tremorogenic dose of harmaline (30 mg/kg intravenously) increased binding by 31%. Caffeine and harmane also slightly decreased the in vivo binding of 3H-BCCE, a compound that binds preferentially to the cerebellar type of BZ receptors. Harmaline stimulated the binding of 3H-BCCE only in the forebrain. Both harmaline and harmane increased by 41-111% the amount of 3H-BCCE that was distributed to the brain. In vitro BC's and caffeine displaced 3H-FZ from receptors in the rat brain with various Ki values (4.7 to 206.9 microM). The antagonism for BZ binding was competitive and in Scatchard analysis produced linear plots. Exceptions were harmaline and caffeine in the forebrain: both exhibited curvilinear plots for 3H-FZ binding. Harmaline increased the binding, and caffeine decreased it by altering the affinity of a subgroup of BZ receptors. In the hindbrain both harmaline and caffeine inhibited binding and produced linear plots. BC-induced tremor and convulsions unveil a large number of spare receptors in the brain, and these seem to be of the cerebellar type of BZ receptors. In addition, our results show that tremorogenic and convulsive BC's act differently on BZ receptors: during harmaline-induced tremor the affinity of some BZ receptors is increased, while harmane-induced convulsions are connected to direct occupation of BZ receptors.

Effect of synthetic purines and purine nucleosides on [3H]diazepam binding in brain

We have compared fifteen synthetic purines and purine nucleosides on their ability to displace [3H]diazepam binding to rat brain membranes. Among these analogs, 6-methylthioguanine was found to be most potent, inhibiting competitively the specific binding of [3H]diazepam with a Ki value of 16 micro M. At a concentration of 50 micro M, 6-methyl-thioguanine increased tha apparent Kd of specific diazepam binding from 4.3 nM to 13.3 nM without affecting the Bmax, nor had it any effect on the non-specific binding. Binding with membrane preparations from developing rat brain was slightly less sensitive to 6-methylthioguanine inhibition than that with membranes prepared from mature brain.

Purinergic regulation of food intake

Inosine peripherally administered to rats markedly suppressed spontaneous food intake and food intake induced by diazepam, muscimol, insulin, and food deprivation. The purines 2-deoxyguanosine and 2-deoxyinosine also suppressed food deprivation-induced feeding, whereas 7-methylinosine, which does not bind to the benzodiazepine binding site in vitro, had no effect on food intake when compared with controls. These results suggest that purines may represent endogenous substances that regulate food intake through interactions with the benzodiazepine receptor.

Baseline exploratory activity predicts anxiolytic responsiveness to diazepam in five mouse strains

Inbred mouse strains showing variability in spontaneous exploratory behaviors displayed differential responsiveness to diazepam in an anxiety-related exploration model. C57B1/6J, BALB/cJ, Swiss Webster/NIH, Swiss Webster/Harlan Sprague-Dawley, and CF-1 mice demonstrated a significant correlation between baseline exploratory activity and maximal percentage increase in exploratory behavior induced by diazepam. No correlation was seen between those behavioral responses and the characteristics of brain benzodiazepine binding sites in the different strains. Anti-anxiety responsiveness appears to be a function of some genetically-determined substrate for spontaneous exploratory behaviors which may have multiple neurochemical substrates.

Pharmacologic control of pemoline induced self-injurious behavior in rats

Administration of oral Pemoline produces long lasting amphetamine-type stereotyped behavior and persistent self-biting behavior in rats. The effects of haloperidol, pimozide, diazepam, and serotonin depletion by pretreatment with p-chlorophenylalanine (PCPA) or chronic pretreatment with p-chloroamphetamine (PCA) on abnormal behavior produced by pemoline were studied. Diazepam consistently increased the duration of stereotyped behavior. It also reduced licking/biting and self-biting but the latter effects were not consistent. Pretreatment with PCA had negligible effects on stereotyped behavior. Pretreatment with PCPA dramatically increased locomotion and rearing without affecting the other components of stereotypy--stereotyped head movements, licking/biting, and self-biting. Haloperidol (0.2 and 0.3 mg/kg) produced a dose related normalization of pemoline induced behaviors, including elimination of self-biting. Pimozide (0.5, 0.8 and 1.3 mg/kg) had little or no effect on behaviors such as locomotions, rears, licking/biting, or stereotyped head movements but eliminated self-biting at 1.3 mg/kg. These data suggest that pemoline, like amphetamine, produces stereotyped behavior through central dopaminergic mechanisms. Dopaminergic mechanisms also appear to be involved in pemoline induced self-biting. pemoline is apparently pharmacologically and behaviorally very similar to amphetamine. Pemoline may provide a useful animal model for syndromes characterized by self-injurious behavior and other repetitive behaviors.

Prostaglandins A as possible endogenous ligands of benzodiazepine receptor

We carried out biochemical studies on several prostaglandins to determine if whey would displace [3H]diazepam binding to the membranes of bovine cerebral cortex. Prostaglandins A1 and A2 were the most potent inhibitors of this binding and all the others tested were either less potent or not effective. As prostaglandins A1 and A2 competitively inhibited [3H]diazepam binding with Ki values of 7.1 +/- 0.1 and 15 +/- 1 microM, respectively, their possible function as endogenous ligands of benzodiazepine receptors warrants further attention.

Stimulatory and protective effects of benzodiazepines on GABA receptors labeled with [3H]muscimol

We investigated the effects of benzodiazepines on [3H]muscimol binding to rat brain membranes and on heat inactivation of GABA receptors. Scatchard analysis of [3H]muscimol binding to frozen and 0.05% Triton X-100 treated membranes revealed two components; a higher affinity (Kd = 2.2 nM, Bmax = 1.2 pmol/mg protein) and a lower affinity component (Kd = 15.9 nM, Bmax = 4.4 pmol/mg protein). Diazepam and flurazepam (3 microM) increases significantly the specific binding of 40 nM but not of 2 nM [3H]muscimol. This stimulation was attributed to an increase in the affinity of the lower affinity component for GABA receptors. The time course of heat inactivation of GABA receptors revealed rapidly and then slowly denaturating Phases. These observations would suggest that there are multiple GABA receptors with different sensitivities to the heat treatment. Diazepam depressed remarkably the slowly denaturating phase(s). After heat treatment for 50 min, the single component of GABA receptors with Kd of 14.3 nM and Bmax of 0.6 pmol/mg protein survived, whereas in the membranes preincubated with 3 microM diazepam, the Kd and Bmax of the still viable GABA receptors were 14.8 nM and 1.14 pmol/mg protein, respectively. In light of these findings, the stimulation of the lower affinity component of GABA receptors may be related to the protective effects of these drugs against heat inactivation.

Chronic clonazepam administration induces benzodiazepine receptor subsensitivity

Clonazepam and chlordiazepoxide were administered chronically in increasing doses for three weeks in two different strains of mice. Forebrain [3H]diazepam binding was assayed in groups of mice sacrificed at 2, 26, 50 hr and 10 days following the last dose. Scatchard and single point analyses revealed a significant decrease in the number of [3H]diazepam binding sites [Bmax] which persisted for at least two days following chronic clonazepam treatment. The Bmax changes observed following chlordiazepoxide treatment were less pronounced than those elicited by clonazepam. No significant changes in receptor binding affinity (Kd) were detected with either drug. In the clonazepam-treated animals, Bmax values returned to normal by day 10 after drug treatment. Chronic benzodiazepine administration therefore induced a decrease in the apparent number of benzodiazepine binding sites in the mouse forebrain. The magnitude and duration of the observed subsensitivity appears to depend on the potency of the administered benzodiazepine.

Inhibition of [3H]adenosine binding by stereoisomers of oxazepam hemisuccinate in guinea-pig brain synaptosomes

1. The stereoisomers of oxazepam sodium hemisuccinate have been tested for their ability to inhibit the [3H]adenosine binding in guinea-pig brain synaptosomes. 2. All three stereoisomers were able to inhibit the [3H]adenosine binding in a dose-dependent fashion. 3. The IC20 values of the D-, DL- and L-stereoisomers were approximately 5 X 10(-5) M, 7.5 X 10(-5) M and 10(-4) M respectively. 4. The results are consistent with hypothesis that adenosine may be the endogenous substrata mediating some actions of the benzodiazepines.

[3H]Diazepam displacing activity in human cerebrospinal fluid

Pooled human cerebrospinal fluid was separated by Sephadex G-50 chromatography. The presence of three peaks, A, B and C, was demonstrated by monitoring absorbance at 254 and 280 nm. All peaks showed [3H]diazepam displacing activity in the membrane receptor test. Peak B was further separated on Bio-Gel P-4. At least two major fractions free of salt and GABA in the molecular weight range of approximately 700--3600 were shown to displace [3H]diazepam in the receptor test. This activity was enhanced by a factor of 3 in the presence of 10 microM-GABA.

Electroshock seizures in mice: effect on brain adenosine and its metabolites

Adenosine and its immediate metabolites, inosine and hypoxanthine, were measured in mouse brain following the induction of electroshock seizures and after a subconvulsive series of electric shocks. Electroshock seizures resulted in a marked and prolonged rise in inosine, with maximal values at 5 min. Hypoxanthine increased more slowly but reached high levels by 10 min. Adenosine was unchanged. Phenytoin and to a lesser extent phenobarbital reduced these effects. Following the subconvulsive stimulus, 15 single shocks over an interval of 5 sec, inosine increased rapidly, adenosine rose slightly, and hypoxanthine did not change. Both phenytoin and phenobarbital blocked these increases in adenosine and inosine. Early elevations in inosine may play some role in seizure generation and propagation. The high levels of inosine and hypoxanthine found after recovery may be involved in the termination of epileptic activity, possibly by interacting with the benzodiazepine receptor for which they are ligands.

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.

An endogenous ligand to the benzodiazepine receptor: preliminary evaluation of its bioactivity

Chromatographic separation of aqueous brain extracts yields a peptide containing fraction which competitively inhibits 3H-diazepam binding to its receptor. An intracerebral-ventricular injection of this isolated fraction results in altered responses in pharmacological and behavioral tests which are similar to those observed when diazepam is administered in the same fashion. The most pronounced effect was obtained in the conflict test. Changes observed in other tests, such as blocking pentylenetetrazole convulsions, altering motility or reducing hyperthermia, were also consistent with the actions of diazepam. At the dose used, neither diazepam nor the brain extract altered muscular co-ordination in two ataxia evaluations. Thus, the animals' performance in the other paradigms would not be adversely influenced by immobilization side-effects. The results reported here support the notion that an endogenous factor does exist in brain which can act like the benzodiazepine drugs when tested for bioactivity in animal studies.