Irreversible binding of [3H]flunitrazepam to different proteins in various brain regions

GABAA Receptors of Cerebellar Granule Cells in Culture: Interaction with Benzodiazepines

GABAA receptor mediated inhibition plays an important role in modulating the input/output dynamics of cerebellum. A characteristic of cerebellar GABAA receptors is the presence in cerebellar granule cells of subunits such as α6 and δ which give insensitivity to classical benzodiazepines. In fact, cerebellar GABAA receptors have generally been considered a poor model for testing drugs which potentially are active at the benzodiazepine site. In this overview we show how rat cerebellar granule cells in culture may be a useful model for studying new benzodiazepine site agonists. This is based on the pharmacological separation of diazepam-sensitive α1 β2/3 γ2 receptors from those which are diazepam-insensitive and contain the α6 subunit. This is achieved by utilizing furosemide/Zn2+ which block α6 containing and incomplete receptors.

Covalent modification of GABAA receptor isoforms by a diazepam analogue provides evidence for a novel benzodiazepine binding site that prevents modulation by these drugs

Classical benzodiazepines, for example diazepam, interact with alpha(x)beta(2)gamma(2) GABA(A) receptors, x = 1, 2, 3, 5. Little is known about effects of alpha subunits on the structure of the binding pocket. We studied here the interaction of the covalently reacting diazepam analog 7-Isothiocyanato-5-phenyl-1,3-dihydro-2H-1,4-benzodiazepin-2-one (NCS compound) with alpha(1)H101Cbeta(2)gamma(2) and with receptors containing the homologous mutation, alpha(2)H101Cbeta(2)gamma(2), alpha(3)H126Cbeta(2)gamma(2) and alpha(5)H105Cbeta(2)gamma(2). This comparison was extended to alpha(6)R100Cbeta(2)gamma(2) receptors as this mutation conveys to these receptors high affinity towards classical benzodiazepines. The interaction was studied at the ligand binding level and at the functional level using electrophysiological techniques. Results indicate that the geometry of alpha(6)R100Cbeta(2)gamma(2) enables best interaction with NCS compound, followed by alpha(3)H126Cbeta(2)gamma(2), alpha(1)H101Cbeta(2)gamma(2) and alpha(2)H101Cbeta(2)gamma(2), while alpha(5)H105Cbeta(2)gamma(2) receptors show little interaction. Our results allow conclusions about the relative apposition of alpha(1)H101 and homologous positions in alpha(2), alpha(3), alpha(5) and alpha(6) with the position occupied by -Cl in diazepam. During this study we found evidence for the presence of a novel site for benzodiazepines that prevents modulation of GABA(A) receptors via the classical benzodiazepine site. The novel site potentially contributes to the high degree of safety to some of these drugs. Our results indicate that this site may be located at the alpha/beta subunit interface pseudo-symmetrically to the site for classical benzodiazepines located at the alpha/gamma interface.

Development of subtype selective GABAA modulators

Drugs modulating gamma-aminobutyric acid (GABA) transmission via the benzodiazepine (BZ) site on the gamma-aminobutyric acid type A (GABAA) receptor have been in widespread use for more than 40 years to treat anxiety, epilepsy, and sleep disorders. These drugs have been shown to be safe, well tolerated, and effective although the mechanism by they produce a myriad of pharmacologic effects remains elusive. In recent years it has been discovered that, although the GABAA receptor is widely distributed in the brain, the substructure and composition of the receptor differs from between brain regions. Termed "GABAA receptor subtypes" their discovery leads to speculation that different subtypes may mediate specific effects of BZs such as anxiety or sedation. The phenotypic analysis of transgenic knock-in and knock-out mice in which particular GABAA receptors were rendered insensitive to the effects of BZ while others were unaffected confirmed this speculation. Subsequently, subtype-specific GABAA ligands were developed that, for example, retained the anxiolytic effects of BZs but were devoid of their sedative effects. Therefore, it may be possible to develop effective anxiolytic compounds that have a much reduced side-effect profile compared with existing drugs.

Hippocampal neurons express GABA A receptor insensitive to diazepam in hyperexcitable conditions

PURPOSE

To define the properties of gamma-aminobutyric acid-type A (GABAA) receptors expressed on dentate granule cells in neonatal rats and to define the impact of prolonged seizures on GABAA receptors in 28- to 35-day-old rats.

METHODS

Whole GABAA receptor currents were recorded from acutely isolated dentate granule cells. Cells were isolated from 7- to 14-day-old rats for the first experiment. For the second experiment, cells were isolated from 28- to 35-day-old naive rats and rats that had undergone 45 minutes of status epilepticus. Modulation of GABAA receptor currents by diazepam (DZP), zinc, and zolpidem was studied.

RESULTS

In 7- to 14-day-old rats, dentate granule cells express DZP- and zolpidem-insensitive, zinc-sensitive GABAA receptors. In 28- to 35-day-old rats, dentate granule cells express DZP-sensitive GABAA receptors. At the latter age, prolonged seizures render GABAA receptors DZP-insensitive.

CONCLUSION

Hippocampal dentate granule cells express DZP-insensitive receptors in hyperexcitable states. These receptors are likely to contain alpha4 subunit.

Subtle changes in residue 77 of the gamma subunit of alpha1beta2gamma2 GABAA receptors drastically alter the affinity for ligands of the benzodiazepine binding site

Recombinant alpha1beta2gamma2 gamma-aminobutyric acid type A (GABAA) receptors were functionally expressed in Xenopus oocytes. Upon the mutation F77L, diazepam and Ro 15-1788 retained the ability to interact with the benzodiazepine binding site, but zolpidem lost this ability. To quantify these data, radioligand binding experiments were performed using membrane preparations of transiently transfected human embryonic kidney 293 cells. The amino acid gamma77, phenylalanine, was also mutated to tyrosine, tryptophan, and isoleucine. Although there was little effect on Ro 15-1788 binding upon mutation to tyrosine, the loss in affinity for diazepam was from 12 to 2,720 nM. The change to leucine, in contrast, resulted in little change in the diazepam affinity, whereas there was a strongly reduced affinity for zolpidem from 17 to 4,870 nM and for methyl 6,7-dimethoxy-4-ethyl-beta-carboline-3-carboxylate (DMCM) from 1.9 to 1,780 nM, respectively. The change to tryptophan resulted in two-phasic displacement curves, and only about 50% of the [3H]flunitrazepam binding could be displaced by zolpidem, DMCM, and Ro 15-1788, respectively, whereas midazolam and diazepam still resulted in 100% displacement, indicating the presence of two sites upon expression of this mutant receptor. Functional expression in Xenopus oocytes showed that all mutant channels displayed a comparatively small change (<4.3-fold) in their apparent agonist affinity and that these channels could still be functionally modulated by ligands of the benzodiazepine binding site. We conclude that subtle changes in gammaF77 drastically affect benzodiazepine pharmacology and that this residue probably interacts directly with most ligands of the benzodiazepine binding site and therefore defines part of the benzodiazepine binding pocket.

Brain GABAA receptors studied with subunit-specific antibodies

Brain GABAA/benzodiazepine receptors are highly heterogeneous. This heterogeneity is largely derived from the existence of many pentameric combinations of at least 16 different subunits that are differentially expressed in various brain regions and cell types. This molecular heterogeneity leads to binding differences for various ligands, such as GABA agonists and antagonists, benzodiazepine agonists, antagonists, and inverse agonists, steroids, barbiturates, ethanol, and Cl- channel blockers. Different subunit composition also leads to heterogeneity in the properties of the Cl- channel (such as conductance and open time); the allosteric interactions among subunits; and signal transduction efficacy between ligand binding and Cl- channel opening. The study of recombinant receptors expressed in heterologous systems has been very useful for understanding the functional roles of the different GABAA receptor subunits and the relationships between subunit composition, ligand binding, and Cl- channel properties. Nevertheless, little is known about the complete subunit composition of the native GABAA receptors expressed in various brain regions and cell types. Several laboratories, including ours, are using subunit-specific antibodies for dissecting the heterogeneity and subunit composition of native (no reconstituted) brain GABAA receptors and for revealing the cellular and subcellular distribution of these subunits in the nervous system. These studies are also aimed at understanding the ligand-binding, transduction mechanisms, and channel properties of the various brain GABAA receptors in relation to synaptic mechanisms and brain function. These studies could be relevant for the discovery and design of new drugs that are selective for some GABAA receptors and that have fewer side effects.

Identification of domains in human recombinant GABAA receptors that are photoaffinity labelled by [3H]flunitrazepam and [3H]Ro15-4513

We have used [3H]flunitrazepam and [3H]Ro15-4513 as photoaffinity labelling agents in combination with a chemical cleavage technique to localize the benzodiazepine recognition sites of specific human recombinant alpha 1 beta 1 gamma 2, alpha 1 beta 3 gamma 2 and alpha 6 beta 3 gamma 2 GABAA receptor subtypes. The chemical agent utilized was hydroxylamine, whose substrate is a relatively rare asparagine-glycine amide bond that occurs only in the alpha subunits of the receptors examined in this study. Cleavage products were resolved using sodium dodecyl sulphate-polyacrylamide gel electrophoresis (SDS-PAGE). The results of these experiments show that, in the alpha 1 subunit-containing receptors, incorporation of [3H]flunitrazepam occurs within residues 1-103 of the alpha 1 subunit, while incorporation of [3H]Ro15-4513 occurs within the region of the alpha 1 subunit that lies between residue 104 and the C-terminus. Photolabelling of membranes prepared from the alpha 6 beta 3 gamma 2-expressing cell line with [3H]Ro15-4513 resulted in the incorporation of radiolabel into two major protein species of M(r) 56,000 and M(r) 48,000, indicating incorporation into the alpha 6 subunit and possibly also the gamma 2 subunit. Hydroxylamine cleavage of alpha 6-containing receptors labelled with [3H]Ro15-4513 produced a gel profile consistent with the incorporation of the label occurring between residue 125 and the C-terminal. Thus, we have shown that the recognition sites for the agonist [3H]flunitrazepam and the inverse agonist [3H]Ro15-4513 occur within distinct domains of the human GABAA receptor.

GABAA receptor subtypes: ligand binding heterogeneity demonstrated by photoaffinity labeling and autoradiography

Heterogeneity of binding affinities for a variety of ligands was observed for gamma-aminobutyric acid type A (GABAA) receptors in the rat CNS, at both GABA and benzodiazepine recognition sites. Photoaffinity labeling by [3H]flunitrazepam and [3H]muscimol to affinity column-purified receptor proteins was examined by gel electrophoresis in sodium dodecyl sulfate. Anesthetic barbiturates (pentobarbital) and steroids (alphaxalone) both differentially stimulated the incorporation of [3H]flunitrazepam more so into the 51-kDa alpha 1 subunit than into the 53-kDa alpha 2 polypeptide, and incorporation of [3H]muscimol into the 55-kDa beta 2 subunit more so than the 58-kDa beta 3 polypeptide. Binding to these polypeptides was also affected differentially by other allosteric modulators and competitive inhibitors, including the benzodiazepine "type 1" selective ligand CL218,872. Heterogeneity in affinity of this drug for the single 51-kDa alpha 1 polypeptide strongly suggests that type I receptors, like type II, are heterogeneous. In brain sections, the extent of enhancement of [3H]muscimol binding showed significant regional variation, similar for both steroids and barbiturates, and the GABA analogues THIP and taurine inhibited muscimol binding with regional variations in affinity that were almost opposites of each other. Modulation of [3H]flunitrazepam binding by steroids, barbiturates, and THIP significantly varied with regions. Taken together, ligand binding heterogeneity exhibited by photoaffinity labeling and autoradiography demonstrate the existence of multiple pharmacological-binding subtypes resulting from the combination of multiple polypeptide gene products into several oligomeric isoreceptors. Comparison of the regional distribution of binding subtypes with that of different subunit gene products allows the following conclusions about possible subunit compositions of native pharmacological receptor subtypes present in the brain: Benzodiazepine pharmacology of the oligomeric receptor isoforms is dependent on the nature of alpha and subunits other than alpha, GABA-benzodiazepine coupling is dependent on the nature of the alpha subunits, GABA site pharmacology is dependent on the nature of the beta subunits, and several subunits including alpha and beta contribute to the degree of sensitivity to steroids and barbiturates. Finally, the presence of discrete subunits may be necessary but is not sufficient to postulate a defined pharmacological property.

The GABAA receptor channel mediated chloride ion translocation through the plasma membrane: new insights from 36Cl- ion flux measurements

GABAA receptors in plasma membranes of neurons are integral oligomers which form chloride channels. The binding of GABA molecules at recognition sites for channel opening triggers a transient increase in transmembrane chloride ion flux. The multiplicity and drug specificity of GABAA receptor, kinetics of channel opening, and desensitization of GABAA receptor and its short- and long-term regulation have been investigated by the use of tracer amounts of the radioactive chloride isotope, 36Cl- ion. Results and new insights from 36Cl- ion flux measurements have been reviewed.

Comparative molecular neuroanatomy of cloned GABAA receptor subunits in the rat CNS

gamma-Aminobutyric acidA (GABAA) receptors in the mammalian central nervous system (CNS) are members of a family of ligand-gated ion channels consisting of heterooligomeric glycoprotein complexes in synaptic and extrasynaptic membranes. Although molecular cloning studies have identified 5 subunits (with approximately 40% amino acid homology) and isoforms thereof (approximately 70% homology), namely alpha 1-6, beta 1-4, gamma 1-3, delta, and rho, the subunit composition and stoichiometry of native receptors are not known. The regional distribution and cellular expression of GABAA receptor messenger RNAs (mRNAs) in the rat CNS have now been investigated by in situ hybridization histochemistry with subunit-specific 35S-labelled oligonucleotide probes on adjacent cryostat sections. Whereas alpha 1, beta 2, and gamma 2 transcripts were the most abundant and ubiquitous in the rat brain--correlating with the radioautographic distribution of GABAA receptors revealed by an ionophore ligand--others had a more restricted expression while often being abundant. For example, alpha 2 transcripts were found only in the olfactory bulb, cerebral cortex, caudate putamen, hippocampal formation, and certain lower brain stem nuclei; alpha 3 only in the olfactory bulb and cerebral cortex; alpha 5 in the hippocampal formation; and alpha 6 only in cerebellar granule cells. In addition, beta 1, beta 3, gamma 1, and delta mRNAs were also uniquely expressed in restricted brain regions. Moreover, in the spinal cord, alpha 1-3, beta 2,3, and gamma 2 mRNAs were differently expressed in Rexed layers 2-9, with alpha 2, beta 3, and gamma 2 transcripts most prominent in motoneurons of layer 9. Although differential protein trafficking could lead to the incorporation of some subunits into somatic membranes and others into dendritic membranes, some tentative conclusions as to the probable composition of native proteins in various regions of the CNS may be drawn.(ABSTRACT TRUNCATED AT 400 WORDS)

Separation of alpha 1, alpha 2 and alpha 3 subunits of the GABAA-benzodiazepine receptor complex by immunoaffinity chromatography

Polyclonal antibodies raised to synthetic amino acid sequences of the rat gamma-aminobutyric acid (GABAA) receptor alpha 1-, alpha 2- and alpha 3-subunits selectively recognized single proteins with apparent molecular weight 51 kDa (P51), 53 kDa (P53) and 59 kDa (P59), respectively, in GABAA receptor preparations affinity purified from the brains of 5-10-day-old rats. The antibodies were coupled to Affigel 10, and the resulting immunoaffinity columns were used to isolate these proteins from affinity purified GABAA receptors.

Kindling does not induce persistent changes in fluorographic labeling patterns of benzodiazepine binding proteins in various rat brain regions

The GABAA receptor has been implicated in the mechanisms underlying the phenomenon of kindling. Photoaffinity labeling with 3H-flunitrazepam followed by sodium dodecyl sulfate-polyacrylamide gel electrophoresis allows the fluorographic visualization of GABAA receptor proteins with benzodiazepine binding sites which presumably correspond to different alpha-subtypes. This method offers an opportunity to investigate whether up- or down-regulation of single benzodiazepine binding proteins occurs. In the present study, labeling patterns of benzodiazepine binding proteins were determined in 12 brains regions of amygdala-kindled rats (2 weeks after the last fully kindled seizure) and sham-operated controls. For most brain regions, labeling patterns were separately determined for the ipsi- and contralateral side. A comparison of the labeling patterns thus obtained revealed no persistent changes between kindled animals and controls in any of the brain regions, including amygdala, substantia nigra and hippocampus. Thus, we conclude that kindling does not induce fluorographically detectable changes in the expression patterns of the benzodiazepine binding proteins. The results confirm the existence of regional heterogeneity of benzodiazepine binding proteins and extend the findings to brain regions which had previously not been investigated.

Immunohistochemical localization of the alpha 1, alpha 2 and alpha 3 subunit of the GABAA receptor in the rat brain

The immunohistochemical distribution of the alpha 1, alpha 2 and alpha 3 subunit of the gamma-aminobutyric acid-A (GABAA) receptor was investigated in the rat brain using affinity-purified antibodies against unique parts of the amino acid sequence of the respective subunits. The distribution of the 3 subunits differed markedly from each other indicating heterogeneity of the GABAA-receptor composition in different brain regions and at various receptive compartments (dendrites or somata) of neuronal cells.

Peptide heterogeneity of GABAA/benzodiazepine receptors in bovine cerebral cortex and cerebellum

The GABAA/benzodiazepine receptor complex has been purified from both bovine cerebral cortex and cerebellum by immunoaffinity chromatography on immobilized monoclonal antibody 62-3G1. Sodium dodecyl sulfate-polyacrylamide gel electrophoresis of the purified receptor from either cerebral cortex or cerebellum revealed 3 main bands corresponding to 51,000, 55,000 and 57,000 Mr silver-stained peptides. In addition, a minor band corresponding to a 53,000 Mr peptide was also found. The differences between the two receptor preparations were: (1) that the main silver-stained 55,000 Mr subunit was present in a relative smaller quantity in cerebellum than in cerebral cortex, and (2) when the membrane-bound receptor was photoaffinity-labeled with [3H]flunitrazepam and subsequently immunoaffinity-purified, two photolabeled peptide bands of 51,000 and 57,000 Mr were found in cerebral cortex while only the 51,000 Mr photolabeled peptide was detected in cerebellum following one-dimension sodium dodecyl sulfate-polyacrylamide gel electrophoresis. Peptide maps of the 57,000 Mr [3H]flunitrazepam photoaffinity-labeled peptide indicated that it was composed of two closely migrating photolabeled peptides of 55,000 Mr and 57,000 Mr. Peptide mapping and deglycosylation experiments using the [3H]flunitrazepam photolabeled receptor suggested that the photolabeled peptides commonly present in cerebellum and cerebral cortex are qualitatively similar if not identical. The results suggest that there are subunits of some type(s) of GABAA R/BZDR complex(es) which are more abundant in cerebral cortex than in cerebellum. Photoaffinity labeling with [3H]muscimol showed similar photolabeled peptides in both cerebral cortex and cerebellum: two main peptides of 54,000 and 57,000 Mr were photolabeled with [3H]muscimol to a similar extent in both receptor preparations.(ABSTRACT TRUNCATED AT 250 WORDS)

Purification of the gamma-aminobutyric acidA/benzodiazepine receptor complex by immunoaffinity chromatography

The bovine gamma-aminobutyric acidA/benzodiazepine receptor complex has been purified by a novel immunoaffinity chromatography method on immobilized monoclonal antibody 62-3G1. Immunopurification of the complex was achieved in a single step with an improved yield over affinity chromatography on the benzodiazepine Ro 7-1986/1. High-resolution sodium dodecyl sulfate-polyacrylamide gel electrophoresis (SDS-PAGE) of the immunoaffinity-purified receptor revealed three major peptide bands of 51,000, 55,000, and 57,000 Mr which were also present in the Ro 7-1986/1 affinity-purified receptor. Peptide mapping, immunoblotting with subunit specific antibodies, and photoaffinity labeling with [3H]flunitrazepam and [3H]muscimol have been used for the identification of receptor subunits, including several which comigrated in a single band in SDS-PAGE.

Calpain inhibitor I prevents rapid postmortem degradation of benzodiazepine binding proteins: fluorographic and immunological evidence

Endogenous proteolysis of the major central benzodiazepine (BZ) binding protein of 53K occurs rapidly postmortem and leads to a fragment of 47K. To determine indirectly the protease responsible for this proteolysis, membranes of porcine cortex were prepared from homogenates, which were either frozen immediately or left at room temperature for 12 h in the presence or absence of various representative protease inhibitors. Membranes were subsequently photolabeled with [3H]flunitrazepam, and subjected to sodium dodecyl sulfate-polyacrylamide gel electrophoresis and fluorography or immunoblotted using an alpha-subunit-specific monoclonal antibody bd-24. Both fluorographs and immunoblots revealed that calpain inhibitor I, Ep-459 (E-64 analogue), and EDTA (greater than or equal to 1 mM) prevent endogenous proteolysis. In future studies one of these inhibitors should be added to receptor preparations. The results indicate that calpain is the responsible protease.

Chronic benzodiazepine antagonist treatment and its withdrawal upregulates components of GABA-benzodiazepine receptor ionophore complex in cerebral cortex of rat

Effect of chronic administration of benzodiazepine (BZ) receptor antagonist Ro 15-1788 (flumazenil) (4 mg/kg once daily for 14 days) treatment and its withdrawal on locomotor activity, body temperature, and the binding pattern of receptor ligands that bind to GABA-BZ receptor ionophore complex in different regions of the brain of the rat was studied. Ro 15-1788 (x 14 d) increased the specific binding of [3H]ethyl-8-fluoro-5-6-dihydro-5-methyl-6-oxo-4H- imidazo[1,5 alpha][1,4]benzodiazepine-3-carboxylate [( 3H]Ro 15-1788), [3H]ethyl-8-azido-5-6-dihydro-5-methyl-6-oxo-4H- imidazo[1,5 alpha][1,4]benzodiazepine-3-carboxylate [( 3H]Ro 15-4513), [3H]flunitrazepam, and [35S]t-butylbicyclophosphorothionate [( 35S]TBPS) in cerebral cortex, and this increase in binding remained upregulated during the drug withdrawal at 24 h. The binding of [3H]Ro 15-1788 was also found significantly increased in the hippocampus, but not in cerebellum and striatum. The chronic Ro 15-1788 treatment did not alter the specific binding of [3H]GABA. Rosenthal analysis of the saturation isotherms indicated that the observed upregulation in the binding pattern of [3H]Ro 15-1788 and [3H]Ro 15-4513 in the cerebral cortex was due to an increase in the binding capacity (Bmax). The receptor affinity (Kd) was not changed. The withdrawal of Ro 15-1788 following its chronic administration also enhanced locomotor activity. However, no apparent change in body temperature was observed either due to chronic treatment or withdrawal. These data indicate that chronic Ro 15-1788 treatment and its withdrawal may produce an upregulation of subunits which bind the positive (benzodiazepines), negative (inverse agonist), and neutral (antagonist) ligands of benzodiazepine receptor.(ABSTRACT TRUNCATED AT 250 WORDS)

Diazepam sensitivity of the binding of an imidazobenzodiazepine, [3H]Ro 15-4513, in cerebellar membranes from two rat lines developed for high and low alcohol sensitivity

Ro 15-4513 (ethyl-8-azido-5,6-dihydro-5-methyl-6-oxo-4H-imidazo[1,5-a] [1,4]benzodiazepine-3-carboxylate), a partial inverse agonist of brain benzodiazepine receptors, has been shown to antagonize some actions of ethanol. In addition to conventional benzodiazepine binding sites, Ro 15-4513 binds to a specific cerebellar protein, the binding of which has been shown to be insensitive to diazepam. The binding of [3H]Ro 15-4513 was studied in washed membranes of the cerebellum, hippocampus, and cerebral cortex of two rat lines developed for differences in their sensitivity to ethanol-induced motor impairment. Only minor differences were found in the estimated parameters (KD and Bmax) for the total specific binding between the rat lines. The main difference between the rat lines was, however, observed in the characteristics of the cerebellar binding, all of which was displaced by diazepam in most of the alcohol-sensitive [alcohol-nontolerant (ANT)] rats, in contrast to only approximately 75% displacement in most of the alcohol-insensitive [alcohol-tolerant (AT)] ones. The following cerebellar results were obtained with the major subgroups of both lines, i.e., with the AT rats chosen for the presence of the diazepam-insensitive binding and with the ANT rats chosen for its absence. The KD for the total specific [3H]Ro 15-4513 binding in the ANT animals was about half of that in the AT animals. No line difference was found in the Bmax of the binding in these rats. Photolabeling with [3H]Ro 15-4513 showed that the diazepam-insensitive binding was in a protein with a molecular weight of 55,000.(ABSTRACT TRUNCATED AT 250 WORDS)

Postmortem degradation alters fluorographic labeling patterns and affinities of benzodiazepine binding proteins

To investigate the effect of endogenous proteolysis on the molecular weights of the benzodiazepine binding proteins, brains of trout, chicken, and rat were removed immediately after death and stored at room temperature for various periods of time before they were frozen. Photoaffinity labeling of membranes with [3H]flunitrazepam, followed by sodium dodecyl sulfate-polyacrylamide gel electrophoresis and fluorography, revealed proteolytic fragments of 47K in trout, chicken, and rat. The proteolysis set in rapidly after death. Seemingly in parallel with the degradation observed fluorographically, the affinity for [3H]flunitrazepam increased without systematic changes in receptor density. The degradation pattern was not identical to that of the photolabeled trypsinized benzodiazepine binding proteins. The endogenous proteolytic fragments were deglycosylated in two steps. In conclusion, proteolytic effects must be taken into account when interpreting labeling patterns and binding parameters.

Immunological identification of multiple alpha-like subunits of the gamma-aminobutyric acidA receptor complex purified from neonatal rat cortex

Antibodies were prepared against a synthetic peptide corresponding to amino acid sequences 174-203 of the bovine gamma-aminobutyric acidA (GABAA) receptor alpha 1-subunit. The antibodies recognized this synthetic alpha 1-peptide, but failed to react with the homologous peptide sequence, 170-199, of the bovine beta 1-subunit. On Western blots, anti-alpha 1-subunit antibody recognized a 50-kilodalton (kDa) protein in affinity-purified receptor preparations from adult rat cortex and cerebellum. In receptor purified from neonatal cortex, the anti-alpha 1-antibody reacted with 50-kDa, 53-54-kDa, and 59-kDa proteins. After digestion with endoglycosidase F, these three protein bands retained differing electrophoretic mobilities. The 50-kDa and 59-kDa subunits of affinity-purified neonatal receptor, which were photoaffinity-labeled with [3H]flunitrazepam, were immunoprecipitated to different extents by alpha-subunit antibody. These data suggest the existence in GABAA receptor from neonatal cortex of three proteins (50 kDa, 53 kDa, and 59 kDa) which have immunological homology to alpha 1-subunit of bovine GABAA receptor. The presence of an alpha- and a beta-like subunit with similar mobility on sodium dodecyl sulfate-polyacrylamide gel electrophoresis may account for the relatively high concentration of protein in the 53-54-kDa band which has been observed in receptor purified from neonatal cortex. The presence of multiple alpha-like subunits may be related to the presence of a relatively high concentration of type II GABA receptor in this tissue.

Type I and type II GABAA-benzodiazepine receptors produced in transfected cells

GABAA (gamma-aminobutyric acid A)-benzodiazepine receptors expressed in mammalian cells and assembled from one of three different alpha subunit variants (alpha 1, alpha 2, or alpha 3) in combination with a beta 1 and a gamma 2 subunit display the pharmacological properties of either type I or type II receptor subtypes. These receptors contain high-affinity binding sites for benzodiazepines. However, CL 218 872, 2-oxoquazepam, and methyl beta-carboline-3-carboxylate (beta-CCM) show a temperature-modulated selectivity for alpha 1 subunit-containing receptors. There were no significant differences in the binding of clonazepam, diazepam, Ro 15-1788, or dimethoxy-4-ethyl-beta-carboline-3-carboxylate (DMCM) to all three recombinant receptors. Receptors containing the alpha 3 subunit show greater GABA potentiation of benzodiazepine binding than receptors containing the alpha 1 or alpha 2 subunit, indicating that there are subtypes within the type II class. Thus, diversity in benzodiazepine pharmacology is generated by heterogeneity of the alpha subunit of the GABAA receptor.

Type I and type II gamma-aminobutyric acid/benzodiazepine receptors: purification and analysis of novel receptor complex from neonatal cortex

The gamma-aminobutyric acid (GABA) type A receptor was purified several thousandfold by affinity chromatography from rat cerebellum, adult cortex, and neonatal cortex. Competition for the benzodiazepine binding site by CL 218872 indicated that cerebellar receptors were predominantly type I, adult cortical receptors were a mixture of subtypes, and neonatal cortex was enriched in type II receptor. The receptor purified from neonatal cortex contained predominantly a 54-kilodalton (kDa), beta-subunit-like protein, whereas receptors from cerebellum and adult cortex contained nearly equal amounts of a 50-kDa, alpha-subunit-like protein and a 54-kDa polypeptide. Peptide maps of trypsin-digested 54-kDa subunits from cerebellum, adult cortex, and neonatal cortex exhibited very similar profiles, a result indicating considerable homology between these proteins in the receptor subtypes. A 59-kDa subunit protein was detected in the receptor complex purified from neonatal cortex. Like the 50-kDa, alpha-subunit of the type I receptor, this protein was photolabeled with [3H]flunitrazepam. The photolabeled peptide fragments, produced by trypsin digestion of these alpha 50- and alpha 59-subunits, exhibited the same retention times on reverse-phase HPLC. A less highly purified GABAA receptor preparation from adult rat spinal cord possessed characteristics that were very similar to those of the receptors purified from neonatal cortex.

Identification of the alpha 3-subunit in the GABAA receptor purified from bovine brain

Polyclonal antibodies have been raised to synthetic amino acid sequences of the bovine GABAA receptor alpha 1 and alpha 3 subunits. Anti-alpha 1 subunit antibodies recognise a polypeptide of 53 kDa whereas anti-alpha 3 subunit antibodies recognise a polypeptide of 59-60 kDa, in Western blots of GABAA receptor purified from adult bovine cerebral cortex, cerebellum and 12-day calf cerebral cortex.

Photoaffinity labeling of [3H]flunitrazepam- and [3H]Ro15-4513-bound pellets in rat cerebral cortex and cerebellum

Irreversible incorporation of [3H]flunitrazepam and [3H]Ro15-4513 into GABA/benzodiazepine receptor subunits was studied by UV irradiation using ligand-bound membrane pellets from rat cerebral cortical and cerebellar synaptic membranes. Specific incorporation for [3H]flunitrazepam was greater in the pellet than in the suspension. The incorporation was identical for [3H]Ro15-4513 in both pellet and suspension. With the ligand-bound pellets, 50% of the available binding sites were photolabeled by both ligands in cortex and cerebellum. SDS polyacrylamide gel electrophoresis and fluorography of [3H]flunitrazepam photo-labeled receptor revealed the same number of major sites in both brain regions. In contrast, [3H]Ro15-4513 appears to label fewer sites in cortex and cerebellum. Photoaffinity labeling with [3H]flunitrazepam in ligand-bound membrane pellet provides a more selective and reliable method for studying the subunit structure of GABA/benzodiazepine receptor complex.

Antibodies directed against a nonapeptide sequence of the gamma-aminobutyrate (GABA)/benzodiazepine receptor alpha-subunit. Detection of a distinct alpha-like subunit in pig cerebral cortex but not cerebellum

A synthetic peptide, corresponding to amino acid residues 101-109 of the bovine gamma-aminobutyrate/benzodiazepine receptor alpha-subunit, was used to raise a polyclonal antiserum. The reactivity of this antiserum towards polypeptides of both bovine and pig receptor preparations was established by immunoprecipitation and immunoblotting. Anti-peptide antibodies recognized the alpha-subunit (51 kDa) of receptor prepared from pig cerebellum or cerebral cortex. However, a polypeptide of 57 kDa was additionally recognized in cortical, but not cerebellar, preparations. This alpha-like polypeptide appeared larger than the band of polypeptides labelled irreversibly with [3H]muscimol (beta-subunit, 55-57 kDa) and corresponds to a polypeptide detected only in cortex after silver-staining or irreversible labelling with [3H]flunitrazepam. These results support the idea that the distinct regional patterns of polypeptides labelled irreversibly with [3H]flunitrazepam reflect the existence of heterologous distributions of distinct alpha-like subunits.

Various proteins from rat brain, specifically and irreversibly labeled by [3H]flunitrazepam, are distinct alpha-subunits of the GABA-benzodiazepine receptor complex

gamma-Aminobutyric acid (GABA)-benzodiazepine receptors were purified from the brains of 5 to 10-day-old rats and photolabeled by [3H]flunitrazepam. After sodium dodecyl sulphate-polyacrylamide gel electrophoresis (SDS-PAGE), 3 different proteins with apparent molecular weight 51,000, 53,000 and 59,000 Da specifically and irreversibly labeled by [3H]flunitrazepam were revealed by autoradiography. The same 3 proteins were recognized by the alpha-subunit-specific monoclonal antibody bd-28. In contrast, a different protein with apparent molecular weight 56,000 Da was recognized by the beta-subunit-specific antibody bd-17. These results indicate that the various proteins specifically labeled by [3H]flunitrazepam are different alpha-subunits and distinct and different from the beta-subunit of the GABA-benzodiazepine receptor complex.

Comparison of two different benzodiazepine binding proteins by peptide mapping after limited proteolysis

Proteins P51 and P55 were photolabeled by [3H]flunitrazepam, [3H]clonazepam or 3H-Ro 15-4513 and then compared by peptide mapping after limited digestion with various proteases. Results indicated that [3H]flunitrazepam or [3H]clonazepam irreversibly bind to the same and [3H]Ro 15-4513 to a different part of these proteins. The different radiolabeled peptide patterns obtained from P51 or P55, irrespective of the photolabel used, suggest a difference in the molecular structure of these proteins.

Modification of the apparent molecular weight of different benzodiazepine binding proteins from rat brain membranes by various endoglycosidases

Membranes from rat cerebellum or hippocampus were photolabeled by [3H]flunitrazepam and incubated with various endoglycosidases. The largest reduction in apparent molecular weight of photolabeled proteins P51 (mol. wt. 51,000) and P55 (mol. wt. 55,000) was obtained using endoglycosidase F or glycopeptidase F. However, even after extensive treatment with these enzymes the deglycosylated products of proteins P51 (apparent mol. wt. 44,000) and P55 (two peptides with apparent mol. wt. 48,000 and 51,000) still differed in their apparent molecular weight. This might indicate that structural features other than N-linked carbohydrates are responsible for the difference in apparent molecular weight of these proteins.

Phylogenetic conservation of the benzodiazepine binding sites: pharmacological evidence

Phylogenetic research can help to elucidate the structure of the GABA/benzodiazepine receptor complex. In this study the evolution of the beta-carboline binding site was traced to see whether it paralleled that of the benzodiazepine binding site. The ratio of [3H]ethyl-beta-carboline-3-carboxylate (beta-CCE) to [3H]flunitrazepam (FNZ) binding sites was determined in several nonmammalian species. The results further substantiate the tight link between these two binding sites. Photoaffinity labelling of the benzodiazepine receptor (BZR) has revealed phylogenetic variation of the molecular weight of the benzodiazepine binding proteins. The IC50 values for inhibition of [3H]FNZ by various compounds which are active at the central benzodiazepine receptors were determined in three phylogenetically distant species that each showed distinct subunit patterns. In these species, the respective affinities of the compounds were remarkably similar, suggesting that the binding sites for benzodiazepines are conserved in higher bony fishes and tetrapods. The conserved binding sites, in addition to recent immunological results obtained in other research groups, provide further evidence for the existence of the GABA/BZR as an isoreceptor complex.

Further characterization of the avian benzodiazepine receptor subunits including phylo- and ontogenetic aspects

The two avian benzodiazepine binding proteins offer an opportunity for further studies concerning their regional variation and their phylo- and ontogenetic development. Accordingly, regional variation of the benzodiazepine binding proteins is investigated further in two reptiles and chicken using photoaffinity labeling with [3H]flunitrazepam followed by sodium dodecyl sulfate-polyacrylamide gel electrophoresis and fluorography. Whereas regional heterogeneity is pronounced in chicken, it is not readily apparent in the two reptiles. The ontogeny of the benzodiazepine binding proteins in chicken forebrain and cerebellum is remarkably similar to that previously reported in rodents. The results are discussed in light of the possible existence of the gamma-aminobutyric acid/benzodiazepine receptor as an isoreceptor complex.

Comparison of tryptic peptides of benzodiazepine binding proteins photolabeled with [3H]flunitrazepam or [3H]Ro 15-4513

When rat brain membranes were incubated with the benzodiazepine agonist [3H]flunitrazepam or the partial inverse benzodiazepine agonist [3H]Ro 15-4513 in the presence of ultraviolet light one protein (P51) was specifically and irreversibly labeled in cerebellum and at least two proteins (P51 and P55) were labeled in hippocampus. After digestion of the membranes with trypsin, protein P51 was degraded into several peptides. When P51 was photolabeled with [3H]Ro 15-4513, four peptides with apparent molecular weights of 39,000, 29,000, 21,000, and 17,000 were observed. When P51 was labeled with [3H]flunitrazepam, only two peptides with apparent molecular weights of 39,000 and 25,000 were obtained. Protein P55 was only partially degraded by trypsin, and whether it was labeled with [3H]flunitrazepam or [3H]Ro 15-4513 it yielded the same two proteolytic peptides with apparent molecular weights of 42,000 and 45,000. These results support the existence of at least two different benzodiazepine receptor subtypes associated with proteins P51 and P55. The different receptors seem to be differentially protected against treatment with trypsin. In addition, these results indicate that in the benzodiazepine receptor subtype associated with P51 benzodiazepine agonists and partial inverse benzodiazepine agonists irreversibly bind to different parts of the molecule.

Phylogenetic comparison of the photoaffinity-labeled benzodiazepine receptor subunits

The late evolutionary appearance of the benzodiazepine receptor (BZR) allows an experimental approach for evaluation of the qualitative development of its subunits. Photoaffinity labeling of brain membranes with [3H]flunitrazepam followed by sodium dodecyl sulfate-polyacrylamide gel electrophoresis and fluorography offers a suitable method for tracing the qualitative evolution of the BZR. A systematic comparison of the subunit patterns in fishes, amphibians, reptiles, birds, and mammals revealed that the subunit of 53K is phylogenetically the oldest photoaffinity labeled subunit; whereas it is the only band present in the lungfish and most amphibians, additional bands are apparent in higher tetrapods. In fishes, the evolution of the BZR subunits leads to the loss of the 53K subunit. KD values are discussed in relation to specific subunit patterns. Possible explanations for the observed variation of the subunits are discussed, with special emphasis placed on the possible evolution by gene duplication and subsequent divergence.

Photoaffinity labeling of benzodiazepine receptor proteins with the partial inverse agonist [3H]Ro 15-4513: a biochemical and autoradiographic study

Photolabeling of the benzodiazepine receptor, which to date has been done with benzodiazepine agonists such as flunitrazepam, can also be achieved with Ro 15-4513, a partial inverse agonist of the benzodiazepine receptor. [3H]Ro 15-4513 specifically and irreversibly labeled a protein with an apparent molecular weight of 51,000 (P51) in cerebellum and at least two proteins with apparent molecular weights of 51,000 (P51) and 55,000 (P55) in hippocampus. Photolabeling was inhibited by 10 microM diazepam but not by 10 microM Ro 5-4864. The BZ1 receptor-selective ligands CL 218872 and beta-carboline-3-carboxylate ethyl ester preferentially inhibited irreversible binding of [3H]Ro 15-4513 to protein P51. Not only these biochemical results but also the distribution and density of [3H]Ro 15-4513 binding sites in rat brain sections were similar to the findings with [3H]flunitrazepam. Thus, the binding sites for agonists and inverse agonists appear to be located on the same proteins. In contrast, whereas [3H]flunitrazepam is known to label only 25% of the benzodiazepine binding sites in brain membranes, all binding sites are photolabeled by [3H]Ro 15-4513. Thus, all benzodiazepine receptor sites are associated with photolabeled proteins with apparent molecular weights of 51,000 and/or 55,000. In cerebellum, an additional protein (MW 57,000) unrelated to the benzodiazepine receptor was labeled by [3H]Ro 15-4513 but not by [3H]flunitrazepam. In brain sections, this component contributed to higher labeling by [3H]Ro 15-4513 in the granular than the molecular layer.

Autoradiographic localization of subcomponents of the macromolecular GABA receptor complex

The autoradiographic localization of subcomponents of the gamma-aminobutyric acid (GABA) receptor-chloride ionophore complex has provided insight into the distribution of this macromolecular system. GABA inhibits neurons by preferentially increasing the permeability of the affected membrane to chloride ions. This inhibition can be modified by the presence of other substances which bind to the GABA receptor complex. Autoradiographic localization of specific receptor subtypes associated with this complex has been accomplished in the central nervous system. This type of analysis has been performed on high and low affinity GABAA, benzodiazepine (BZ; both BZ1 and BZ2) and convulsant sites. These receptor sites are situated in distinct brain regions and co-exist in several areas. Other receptor subtypes, which may be influenced by the presence of GABA, can be analyzed for comparison in order to define regions of the brain where GABA may be exerting independent effects (i.e., those not associated with chloride channels). Microscopic localization of receptor sites indicates specific areas to investigate in further studies concerning the characterization of subcomponents of the macromolecular GABA complex associated with chloride ion channels.

Benzodiazepine receptor subunits in avian brain

Using sodium dodecyl sulfate-polyacrylamide gel electrophoresis and fluorography of the photoaffinity-labeled benzodiazepine receptor in duck brain, two equally intensive bands of relative molecular masses of 53,000 and 54,000 were observed, thus being identical to the subunit pattern previously described for chicken brain. An attempt was made to characterize further the avian subunits. Comparison of the fluorographic subunit pattern in cerebellum and telencephalon revealed a pronounced quantitative heterogeneity. Addition of ethyl beta-carboline-3-carboxylate resulted in the selective inhibition of the 53,000 subunit. These results are compared with the mammalian subunit pattern in the corresponding brain regions. The findings are discussed in view of the apparent contradiction between the existence of regional heterogeneity when photoaffinity labeling is applied and its lack when monoclonal antibodies are used.

Comparison of benzodiazepine receptors in cerebellum and inferior colliculus

Reversible and irreversible binding of [3H]flunitrazepam was investigated in membranes from cerebellum and inferior colliculus of young and adult rats. Results indicate that in adult animals predominantly BZ1 receptors are enriched in both brain regions. In the brains of newborn animals, however, additional benzodiazepine receptor subtypes seem to exist in cerebellum as well as in inferior colliculus.

The gamma-aminobutyrate/benzodiazepine receptor from pig brain. Purification and characterization of the receptor complex from cerebral cortex and cerebellum

The gamma-aminobutyrate/benzodiazepine-receptor complex has been purified from a Triton X-100 extract of crude synaptic membranes from pig cerebral cortex and cerebellum by a combination of affinity and ion-exchange chromatography. [3H]Flunitrazepam binding activity was purified 2200-fold from cortex with an overall yield of 2%. The dissociation constants for the binding of [3H]muscimol and [3H]flunitrazepam to the receptor complex were 14 +/- 3 nM and 14 +/- 2 nM respectively. The ratio of [3H]muscimol to [3H]flunitrazepam binding sites was in the range 2.2-2.8. There appeared to be no selective inactivation of either binding site during the purification procedure. Sodium dodecyl sulphate/polyacrylamide-gel electrophoresis revealed two major polypeptides of Mr 49 000 and 55 000 from both cortex and cerebellum. When the receptor from cortex was photoaffinity labelled with [3H]flunitrazepam, radioactivity was incorporated predominantly into the Mr-49 000 polypeptide, although some radioactivity was detectable in the Mr-55 000 band. The cerebellar receptor was photoaffinity labelled on the 49 000-Mr polypeptide but not on the polypeptide of Mr 55 000. In addition, some radioactivity was detected in a minor polypeptide of Mr 43 000. When purified in the presence of 3-[(3-cholamidopropyl)dimethylammonio]propanesulphonate the same major polypeptide components (Mr 49 000 and 55 000) were isolated, but the receptor now retained its ability to be modulated by secobarbital and by the anaesthetic propanidid.

Postnatal development of proteins associated with different benzodiazepine receptors

The postnatal development of several proteins irreversibly labeled by [3H]flunitrazepam in membranes from rat cerebral cortex was investigated. It was demonstrated that in the early postnatal days proteins with apparent molecular weights 55,000 and 59,000 were predominantly labeled whereas irreversible labeling of a protein with apparent molecular weight 51,000 started to predominate only in the second postnatal week. Irreversible labeling of another protein with apparent molecular weight 62,000 was weak throughout development. All these proteins seem to be associated with central benzodiazepine receptors. Irreversible labeling at various time points after birth seems to parallel the postnatal development of these proteins, and the different time course of development and different binding properties of the individual proteins support the hypothesis that these proteins are associated with separate and distinct benzodiazepine receptor subtypes. The pharmacological properties of the individual receptor subtypes seem to be fully developed in the early postnatal days, and therefore newborn animals seem to be a good model system for the investigation of properties and function of these various benzodiazepine receptor subtypes.

Heterogeneity of benzodiazepine binding sites: a review of recent research

This paper reviews selected aspects of benzodiazepine binding site heterogeneity. These include receptor heterogeneity revealed by biochemical determinations of receptor numbers, autoradiographic localization in histological sections of brain, lesion studies, solubilization of receptors, and photoaffinity labelling. The data summarized support the concept of benzodiazepine receptor multiplicity. In addition, we have reviewed recent work on peripheral-type benzodiazepine binding sites and suggest that further study of these sites may increase our understanding of both the central and peripheral actions of benzodiazepines and other ligands.

Benzodiazepine receptors: multiple receptors or multiple conformations?

Several lines of evidence from reversible binding studies seem to indicate there are at least two "central" benzodiazepine receptor subtypes, the BZ1 and BZ2 receptors. Irreversible binding studies, using 3H-flunitrazepam as a photoaffinity label for benzodiazepine receptors, not only are in perfect agreement with the data from reversible binding studies but extend these studies by identifying P51, a protein with apparent molecular weight 51,000, as a protein associated with the BZ1 receptor and by suggesting that the BZ2 receptor might actually consist of several different benzodiazepine receptors associated with different and distinct proteins irreversibly labeled by 3H-flunitrazepam. Other reversible binding studies have accumulated indicating the existence of several different conformations of benzodiazepine receptors. Irreversible binding studies support this conclusion and in addition suggest the existence of four different benzodiazepine binding sites within the GABA-benzodiazepine receptor complex. It is therefore hypothesized that there are several different GABA-benzodiazepine receptor subtypes all of which have four distinct benzodiazepine binding sites which can exist in at least three different but freely interconvertible conformations. This hypothesis can account for all experimental observations obtained so far and might partially explain the distinct clinical effects of structurally similar benzodiazepines.

Comparison of benzodiazepine receptor binding in membranes from human or rat brain

Specific high affinity binding of [3H]flunitrazepam to membranes from human brain was stimulated by gamma-aminobutyric acid (GABA), pentobarbital, 1-ethyl-4-(isopropylidene-hydrazino)-1H-pyrazolo[3,4b]pyridine-5-carboxy lic acid ethyl ester hydrochloride (SQ 20009) and avermectin B1a and was unaffected by 2 microM 4'-chlorodiazepam (Ro 5-4864) indicating that [3H]flunitrazepam in human brain as well as in rat brain predominantly binds to benzodiazepine receptors specific to brain, which was associated with a GABA receptor and several modulatory binding sites for drugs. The potency of several selective and non-selective ligands for benzodiazepine receptors for inhibition of the binding of [3H]flunitrazepam was compared in membranes from human or rat brain cerebellum, hippocampus and cerebral cortex. It was demonstrated that all these compounds, derived from different chemical structures, had a remarkably similar potency for inhibition of the binding of [3H]flunitrazepam in the corresponding regions of the human or rat brain. However, irreversible labelling of benzodiazepine binding sites with [3H]flunitrazepam and subsequent SDS-polyacrylamide gel electrophoresis and fluorography revealed more photolabelled protein bands in human than in rat cerebellum and hippocampus. The results seem to indicate that, although the pharmacological properties of reversible binding of [3H]flunitrazepam are remarkably similar in membranes from rat or human brain, the molecular heterogeneity of benzodiazepine binding sites is even greater in human than in rat brain.

Differential degradation of different benzodiazepine binding proteins by incubation of membranes from cerebellum or hippocampus with trypsin

When rat brain membranes were incubated with [3H]flunitrazepam in the presence of UV light, predominantly one protein (P51) was irreversibly labeled in cerebellum and at least two proteins (P51 and P55) were labeled in hippocampus. On digestion of membranes with increasing concentrations of trypsin up to 40% of radioactivity irreversibly bound to proteins was removed from the membranes. In addition, P51 was nearly completely degraded to a peptide with apparent molecular weight 39,000 and this peptide was further degraded to a peptide with apparent molecular weight 25,000. In contrast, protein P55 was only partially degraded by trypsin and yielded two proteolytic peptides with apparent molecular weights 42,000 and 45,000 which seemed to be rather stable against further attack by trypsin. Membranes treated with trypsin still had the capacity to bind [3H]-flunitrazepam reversibly with an affinity similar to that of membranes not previously treated with trypsin. When these membranes were irradiated with UV light, the same proteolytic peptides were detected as in membranes first photolabeled and then digested with trypsin. These results suggest a close association between reversible and irreversible benzodiazepine binding sites and indicate that membrane-associated proteins P51 and P55 are differentially protected against degradation by trypsin.