Autoradiographic localization of benzodiazepine receptors in immunocytochemically identified gamma-aminobutyrergic synapses

The legacy of the benzodiazepine receptor: from flumazenil to enhancing cognition in Down syndrome and social interaction in autism

The study of the psychopharmacology of benzodiazepines continues to provide new insights into diverse brain functions related to vigilance, anxiety, mood, epileptiform activity, schizophrenia, cognitive performance, and autism-related social behavior. In this endeavor, the discovery of the benzodiazepine receptor was a key event, as it supplied the primary benzodiazepine drug-target site, provided the molecular link to the allosteric modulation of GABAA receptors and, following the recognition of GABAA receptor subtypes, furnished the platform for future, more selective drug actions. This review has two parts. In a retrospective first part, it acknowledges the contributions to the field made by my collaborators over the years, initially at Hoffmann-La Roche in Basle and later, in academia, at the University and the ETH of Zurich. In the second part, the new frontier of GABA pharmacology, targeting GABAA receptor subtypes, is reviewed with special focus on nonsedative anxiolytics, antidepressants, analgesics, as well as enhancers of cognition in Down syndrome and attenuators of symptoms of autism spectrum disorders. It is encouraging that a clinical trial has been initiated with a partial inverse agonist acting on α5 GABAA receptors in an attempt to alleviate the cognitive deficits in Down syndrome.

Gamma-aminobutyric acidA and benzodiazepine receptor alterations in the rat brain after unilateral 6-hydroxydopamine lesions of the medial forebrain bundle

Gamma-aminobutyric acidA (GABA(A)) and benzodiazepine (BZ) receptors and dopamine uptake sites in 6-hydroxydopamine-treated rat brains were studied by receptor autoradiography using [3H]muscimol, [3H]flunitrazepam and [3H]mazindol binding, respectively. The rats were unilaterally lesioned in the medial forebrain bundle and the brains were analyzed at 1, 2, 4 and 8 weeks post-lesion. Degeneration of the nigrostriatal pathway after 6-hydroxydopamine treatment caused a significant loss of dopamine uptake sites in the ipsilateral striatum and substantia nigra (SN) in the lesioned animals. In the contralateral side, however, dopamine uptake sites showed no significant changes in the brain throughout the experiments. On the other hand, no significant changes in GABA(A) receptors were observed in the brain of both the ipsilateral and contralateral sides during post-lesion. In contrast, BZ receptors were observed significantly increased in the ventromedial part of striatum of the ipsilateral side from 2 to 4 weeks post-lesion. Furthermore, a transient increase in BZ receptors was found in the ipsilateral SN only at 2 weeks post-lesion. In contralateral side, most regions examined showed no significant changes in BZ receptors throughout the experiments except for a transient increase in the SN at 1 week post-lesion. These results demonstrate that 6-hydroxydopamine can cause severe functional damage in dopamine uptake sites in the nigrostriatal pathway. Our results also suggest that the change in BZ receptors is more pronounced than that in GABA(A) receptors in the brain after 6-hydroxydopamine treatment. Furthermore, our findings suggest that the increase in BZ receptors in the brain of 6-hydroxydopamine-treated model may be due to the additional disruption of the nigrostriatal dopamine system. Thus, investigations into possible changes in neurotransmitter receptors other than dopaminergic receptors appear to be important for the elucidation of pathogenesis of Parkinsons disease.

GABA(A) receptor activation triggers a Cl- conductance increase and a K+ channel blockade in cerebellar granule cells

GABA(A) receptor activation in cerebellar granule cells induced a complex physiological response, namely the activation of a Cl- conductance in concert with a blockade of the resting K+ outward conductance (by 71% as compared to controls). Both responses were mediated by the activation of GABA(A) receptors, since they were both mimicked by the GABA(A) receptor agonist muscimol and antagonized by picrotoxin and bicuculline. A substantial decrease of the mean open time of single, outwardly rectifying K+ channels was triggered by GABA as revealed from cell-attached recordings; this finding implies that an intracellular pathway links GABA(A) receptors and K+ channels. Furthermore, this action of GABA is mediated through the cytoplasm, as experiments with the cell-attached patch-clamp technique show. GABA induced a prominent membrane depolarization ranging from 10 to 25 mV as revealed by current-clamp recordings of gramicidin (or nystatin) permeabilized patches, thus selecting conditions not to perturb the physiological Cl- gradient across the cell. Our findings imply that the GABA-activated Cl- current depolarized the membrane as described for immature neurons. The blockade of the resting K+ channel conductance acts in concert and both mechanisms lead to this substantial depolarizing event.

Topics in clinical pharmacology: flumazenil, a benzodiazepine antagonist

Flumazenil is a central antagonist of the sedative effects of benzodiazepines. It has been used to reverse benzodiazepine effects in conscious sedation, general anesthesia, and overdose with restoration of alertness and psychomotor function within minutes of administration. Seizures have followed the use of flumazenil. Overdose patients who have co-ingested cyclic antidepressants are especially at risk for this complication. Flumazenil is administered intravenously in small, incremental doses.

Benzodiazepine receptor sites in the chick optic lobe: development and pharmacological characterization

To investigate the interaction between gamma-aminobutyric acid (GABA) and benzodiazepine (BZD) receptor sites during development, the time-course of appearance of flunitrazepam (FNZ) binding sites and their pharmacological characterization were studied in developing chick optic lobe. At the earliest stage examined, embryonic day (Ed) 12, the receptor density was 30.9% (0.05 +/- 0.01 pmol/mg protein) of that found in the chick optic lobes of adult chicks. The adult value was achieved on Ed 16 (0.16 +/- 0.01 pmol/mg protein). After this stage there was a sharp and transient increase in specific [3H]FNZ binding of about two-fold reaching a maximal value between hatching and the postnatal day (pnd) 2 (0.33 +/- 0.01 pmol/mg protein). Scatchard analysis at different stages of development revealed the presence of a single population of specific FNZ binding sites. The increase in [3H]FNZ binding during development was due to a large number of binding sites while their affinity remained unchanged. Competition experiments in the chick optic lobe revealed that the order of potency for displacement of specific [3H]FNZ binding paralleled the pharmacological potency of the BZDs tested. The IC50s for clonazepam, flunitrazepam, Ro 15-1788 and chlordiazepoxide were 3.02, 4.30, 0.32, and 4778.64 nM respectively. Ro 5-4864, a potent inhibitor of BZD binding to peripheral tissues, had no effect on specific [3H]FNZ binding indicating that only central BZD binding sites are present in the chick optic lobe. The peak of maximal expression of BZD receptor sites precedes in 5-6 days the peak of GABA receptor sites indicating a precocious development of BZD receptor sites.(ABSTRACT TRUNCATED AT 250 WORDS)

Flumazenil: a benzodiazepine antagonist

Although benzodiazepines have been proven safe and effective for the induction and maintenance of sedation, some instances require the reversal of these events prior to the natural process of metabolism and elimination. Flumazenil, a 1,4-imidazobenzodiazepine, is an antagonist that can reduce or terminate benzodiazepine effects in a dose-dependent manner. The antagonist acts by the competitive inhibition of benzodiazepines at their central nervous system receptor sites. When administered intravenously in incremental doses, flumazenil allows for optimal patient response on an individual basis. Despite its short elimination half-life, small doses of flumazenil are usually effective in producing benzodiazepine reversal. Flumazenil's short duration of activity is due to its rapid hepatic metabolism and elimination. Intravenous antagonist doses of 0.2 mg followed by 0.1 mg/min to a total dose of 1 mg have produced significant results in reversing benzodiazepine sedation. As much as 5 mg of flumazenil have been necessary when treating benzodiazepine or mixed-agent intoxications. In such situations, response rarely exceeds a duration of one hour. If resedation occurs, additional doses or an infusion of the antagonist may provide the desired response. Flumazenil is well tolerated locally as well as systemically. Nausea and vomiting occurring after anesthesia is the most documented adverse effect in both placebo and treatment populations. However, there has been no significant difference in the occurrence of vomiting in placebo compared with flumazenil-treated subjects. Careful observation and slow reversal of central nervous system depression is crucial in the avoidance of benzodiazepine withdrawal in those patients dependent upon these agents. Flumazenil appears to provide a mechanism for the safe and effective reversal of benzodiazepine-induced sedation.(ABSTRACT TRUNCATED AT 250 WORDS)

GABAA-receptors: structural requirements and sites of gene expression in mammalian brain

GABAA-receptors, the major synaptic targets for the neurotransmitter GABA, are gated chloride channels. By their allosteric drug-induced modulation they serve as molecular control elements through which the levels of anxiety, vigilance, muscle tension and epileptiform activity can be regulated. Despite their functional prominence, the structural requirements of fully functional GABAA-receptors are still elusive. Expression of cDNAs coding for the alpha 1- beta 1-subunits of rat brain yielded GABA-gated chloride channels which were modulated by barbiturates but displayed only agonistic responses to ligands of the benzodiazepine receptor. GABAA-receptors with fully functional benzodiazepine receptor sites were formed when the alpha 1- and beta 1-subunits were co-expressed with the gamma 2-subunit of rat brain. These receptors, however, failed to show cooperativity of GABA in gating the channel. In order to determine the subunit repertoire available for receptor assembly in different neuronal populations in vivo, the sites of subunit gene expression were (alpha 1, alpha 2, alpha 3, alpha 5, alpha 6, beta 1, beta 2, beta 3, gamma 2) mapped by in situ hybridization histochemistry in brain sections. The mRNAs of the alpha 1-, beta 1- and gamma 2-subunits were co-localized e.g. in mitral cells of olfactory bulb, pyramidal cells of hippocampus as well as granule cells of dentate gyrus and cerebellum. The lack of colocalization in various other brain areas points to an extensive receptor heterogeneity. The presence of multiple GABAA-receptors in brain may contribute to synaptic plasticity, differential responsiveness of neurons to GABA and to variations in drug profiles.

Purification and characterization of a benzodiazepine-like substance from mammalian brain

An endogenous brain ligand which competes with [3H]-flunitrazepam for the binding to benzodiazepine receptor has been isolated and purified to homogeneity. The purification procedures involve the extraction of the endogenous ligand by homogenizing the brain tissue in water containing various protease inhibitors followed by filtration through a PM 10 membrane (exclusion limit: 10,000-dalton), column chromatographies on Sephadex G-50, Bio-Rad P2 and a series of C18 reverse phase HPLC columns. The purified endogenous ligand was eluted as a single and symmetrical peak monitored at either 220 or 280 nm. Furthermore, the ligand activity coincided with the absorption peak. The purified endogenous ligand is thermostable, insensitive to various peptidases and proteolytic enzymes, resistant to DNAse, RNAse, and carbohydrate enzyme e.g. neuraminidase (EC 3.2.1.18) and acid treatment. It has a major absorption peak at 220 nm and a minor one at 313 nm. The endogenous ligand appears to be quite specific since it only inhibits the binding of ligand to the central type benzodiazepine receptor but not to other receptors, e.g. peripheral type benzodiazepine receptor, alpha 1-adrenoceptor, alpha 2-adrenoceptor, beta-adrenoceptor and muscarinic cholinergic receptor. Furthermore, the inhibition of the receptor binding by the endogenous ligand is enhanced by GABA suggesting that the endogenous ligand is a benzodiazepine receptor agonist. The structure of the endogenous ligand is unknown.

Benzodiazepine receptors resolved

To date, attempts to map the distribution and density of benzodiazepine receptors in the CNS have been dominated by radiohistochemical techniques with conventional receptor binding. Their limited resolution, however, prompted us to try an immunohistochemical approach. Purified GABA/benzodiazepine receptors, prepared from bovine cerebral cortex, have been used to raise monoclonal antibodies for this purpose. Immunoreactive sites in rat brain, spinal cord and retina as well as in bovine and post-mortem human brain were found to be concentrated on neuronal cell bodies and processes in those regions known to be innervated by GABAergic neurons. Electron microscopic analysis revealed a selective staining of axosomatic and axodendritic pre- and postsynaptic contacts.

Study of an octadecaneuropeptide derived from diazepam binding inhibitor (DBI): biological activity and presence in rat brain

An endogenous brain neuropeptide with 104 amino acid residues that modulates gamma-aminobutyric acid receptor function was termed DBI because it displaces diazepam from its specific brain binding sites. Tryptic digestion of DBI generates an octadecaneuropeptide (ODN) that is more potent than the parent compound in the displacement of specifically bound beta-[3H]carboline-3-carboxylate methyl ester [( 3H]BCCM) and in proconflict action (Vogel test in thirsty rats). The proconflict action of ODN is antagonized by the imidobenzodiazepinone Ro 15-1788, which is a specific antagonist of beta-carboline and benzodiazepine recognition sites. The ODN amino acid sequence is Gln-Ala-Thr-Val-Gly-Asp-Val-Asn-Thr-Asp-Arg-Pro-Gly-Leu-Leu-Asp-Leu-Lys. The pharmacological properties associated with this sequence were confirmed by comparing the activity of ODN generated from tryptic digestion of DBI with that of ODN obtained by synthesis. Amidation of the terminal lysine of ODN produces a peptide (ODN-NH2) devoid of pharmacological activity. Three peptides containing the COOH-terminal segment of ODN were synthesized. All these peptides [Arg-Pro-Gly-Leu-Leu-Asp-Leu-Lys (octapeptide), Pro-Gly-Leu-Leu-Asp-Leu-Lys (heptapeptide), and Gly-Leu-Leu-Asp-Leu-Lys (hexapeptide)] express the displacing and proconflict actions of ODN. In primary cultures of cerebellar granule cells of rat, DBI, ODN, octapeptide, heptapeptide, and hexapeptide preferentially displace [3H]BCCM over [3H]flunitrazepam; moreover, they displace bound [3H]BCCM completely but [3H]flunitrazepam only by 50%. These data suggest that ODN includes a specific ligand for the gamma-aminobutyric acid receptor regulatory site occupied by beta-carbolines. Using rabbit antibodies directed against the NH2-terminal portion of ODN, we detected ODN-like material in rat brain homogenates. However, whether this material is identical to the ODN generated by tryptic digestion of DBI remains to be established.

The benzodiazepine receptor

The benzodiazepines are among the most widely used drugs in the world. When first introduced, little was known about their mechanism of action. However, in the last 20 years, our understanding of the chemistry and function of the central nervous system (CNS) has increased substantially. This knowledge has shed some light on the mechanism of action of the benzodiazepines and other centrally acting drugs. It is well established that the benzodiazepines act by combining with specific receptors in the central nervous system. These receptors are anatomically in close association with gamma amino butyric acid (GABA) receptors and appear to reside on the neuronal membrane in the same supramolecular protein complex. GABA is the major inhibitory neurotransmitter of the CNS. The benzodiazepines act by increasing the affinity of the GABA receptor for its ligand, thereby augmenting the inhibitory effect of a given concentration of GABA. Two hypotheses of benzodiazepine ligand-receptor interactions in this supramolecular protein complex have been proposed: (1) multiple receptor subtypes analogous to the opioid receptors; (2) single receptor with multiple conformations. The multiple receptor hypothesis suggests that each pharmacologic effect of the benzodiazepines (i.e., anxiolysis) is mediated by interaction with a specific receptor subtype. On the other hand, the alternative hypothesis suggests that only one receptor exists which has a dynamic conformation. Experimental evidence in support of each hypothesis is presented and critically evaluated.

The visualization of neuronal benzodiazepine receptors in the brain by autoradiography and immunohistochemistry

Recent methodological improvements in receptor autoradiography have enabled the in vitro and in vivo binding of the benzodiazepines in the brain to be visualized and pharmacologically characterized with an anatomical resolution unattainable by biochemical radioligand binding assays. This approach, combined with computerized microdensitometry, can be used not only to map the distribution of benzodiazepine receptors in the brain but also to quantify their regional densities. Furthermore, immunohistochemical studies, using monoclonal antibodies directed against the solubilized and purified GABA/benzodiazepine receptor-ionophore complex, have revealed the distribution of antigenic sites on brain neurons and their processes. The brain regions of intense immunoreactivity are known to contain a high density of GABA-ergic efferents and neuronal-type benzodiazepine receptors. Current trends and prospects in this area of receptor research are briefly reviewed.

[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.

Absence of central effects in man of the benzodiazepine antagonist Ro 15-1788

The benzodiazepines are typified by a profile of side effects which includes drowsiness, ataxia and incoordination. Ro 15-1788, an imidazodiazepine derivative, exhibits marked antagonism of the behavioural and biochemical effects of the benzodiazepines in animals and man. It is devoid of any behavioural activity in animals, except at very high doses. In the present study the effects of single rising oral doses of Ro 15-1788 on cognitive, psychomotor and subjective function in man have been assessed using a battery of psychometric tests designed to identify the sedative action of the benzodiazepines. At all doses up to 600 mg, Ro 15-1788 demonstrated none of the classical behavioural effects of the benzodiazepines.

Physicochemical characterization of detergent-solubilized gamma-aminobutyric acid and benzodiazepine receptor proteins from bovine brain

[3H]Muscimol and [3H]flunitrazepam binding activities have been solubilized from bovine cortex using the ionic detergent sodium deoxycholate. The soluble receptor proteins were shown to bind [3H]muscimol with a dissociation constant, Kd, of 12 nM and a binding capacity (Bmax value) of 1.56 pmol/mg protein; gamma-amino[3H]-butyric acid with a Kd of 50 nM and Bmax of 1.55 pmol/mg protein; and [3H]flunitrazepam with a Kd of 8 nM and a Bmax of 0.8 pmol/mg protein. Gel filtration of the soluble receptor proteins showed that the gamma-amino[3H]butyric acid and [3H]flunitrazepam binding activities comigrated with a Stokes radius of 6.8 nm. The two binding activities were also found to comigrate after sedimentation in a sucrose density gradient. The hydrodynamic properties of the assumed protein-detergent complexes were determined by gel filtration and sedimentation through gradients of sucrose in H2O or 2H2O. Under the conditions employed, the parameters for both the putative gamma-aminobutyric acid and benzodiazepine receptors were: partial specific volume, 0.73 ml g-1; sedimentation coefficient, 12.5 S; molecular weight, 355000; and frictional ratio 1.46. These observations are consistent with the conclusion that the majority of both binding activities solubilized in deoxycholate reside in a single macromolecular complex. However, Triton X-100 selectively solubilized the benzodiazepine binding activity. This suggests that the two binding activities can be at least partially separated.

Benzodiazepine antagonist Ro 15-1788: binding characteristics and interaction with drug-induced changes in dopamine turnover and cerebellar cGMP levels

The recently discovered benzodiazepine antagonist Ro 15-1788 was characterized in binding studies, and its potency and selectivity were determined in vivo by interaction with drug-induced changes in dopamine turnover and cerebellar cGMP level. Ro 15-1788 reduced [3H]flunitrazepam binding in the brain in vivo with a potency similar to that of diazepam and effectively inhibited [3H]diazepam binding in vitro (IC50 = 2.3 +/- 0.6 nmol/liter). [3H]Ro 15-1788 bound to tissue fractions of rat cerebral cortex with an apparent dissociation (KD) of 1.0 +/- 0.1 nmol/liter. The in vitro potency of various benzodiazepines in displacing [3H]Ro 15-1788 from its binding site was of the same rank order as found previously in [3H]diazepam binding. Autoradiograms of [3H]Ro 15-1788 binding in sections of rat cerebellum showed the same distribution of radioactivity as with [3H]flunitrazepam. The attenuating effect of diazepam on the chlorpromazine- or stress-induced elevation of homovanillic acid in rat brain was antagonized by Ro 15-1788. Among a series of compounds which either decreased or increased the rat cerebellar cGMP level, only the effect of benzodiazepine receptor ligands (diazepam, zopiclone, CL 218 872) was antagonized by Ro 15-1788. Thus, Ro 15-1788 is a selective benzodiazepine antagonist acting at the level of the benzodiazepine receptor in the central nervous system. Peripheral benzodiazepine binding sites in kidney and schistosomes were not affected by Ro 15-1788.