Novel cyclic gamma-hydroxybutyrate (GHB) analogs with high affinity and stereoselectivity of binding to GHB sites in rat brain

CaMKIIα as a Promising Drug Target for Ischemic Grey Matter

Ca2+/calmodulin-dependent protein kinase II (CaMKII) is a major mediator of Ca2+-dependent signaling pathways in various cell types throughout the body. Its neuronal isoform CaMKIIα (alpha) centrally integrates physiological but also pathological glutamate signals directly downstream of glutamate receptors and has thus emerged as a target for ischemic stroke. Previous studies provided evidence for the involvement of CaMKII activity in ischemic cell death by showing that CaMKII inhibition affords substantial neuroprotection. However, broad inhibition of this central kinase is challenging because various essential physiological processes like synaptic plasticity rely on intact CaMKII regulation. Thus, specific strategies for targeting CaMKII after ischemia are warranted which would ideally only interfere with pathological activity of CaMKII. This review highlights recent advances in the understanding of how ischemia affects CaMKII and how pathospecific pharmacological targeting of CaMKII signaling could be achieved. Specifically, we discuss direct targeting of CaMKII kinase activity with peptide inhibitors versus indirect targeting of the association (hub) domain of CaMKIIα with analogues of γ-hydroxybutyrate (GHB) as a potential way to achieve more specific pharmacological modulation of CaMKII activity after ischemia.

Discovery and Optimization of 5-Hydroxy-Diclofenac toward a New Class of Ligands with Nanomolar Affinity for the CaMKIIα Hub Domain

The Ca²⁺/calmodulin-dependent protein kinase II α (CaMKIIα) is a brain-relevant kinase involved in long-term potentiation and synaptic plasticity. We have recently pinpointed the CaMKIIα hub domain as the long-sought-after high-affinity target of γ-hydroxybutyrate ligands substantiated with a high-resolution cocrystal of 5-hydroxydiclofenac (3). Herein, we employed in silico approaches to rationalize and guide the synthesis and pharmacological characterization of a new series of analogues circumventing chemical stability problems associated with 3. The oxygen-bridged analogue 4d showed mid-nanomolar affinity and notable ligand-induced stabilization effects toward the CaMKIIα hub oligomer. Importantly, 4d displayed superior chemical and metabolic stability over 3 by showing excellent chemical stability in phosphate-buffered saline and high resistance to form reactive intermediates and subsequent sulfur conjugates. Altogether, our study highlights 4d as a new CaMKIIα hub high-affinity ligand with enhanced pharmacokinetic properties, representing a powerful tool compound for allosteric regulation of kinase activity with subtype specificity.

GHB analogs confer neuroprotection through specific interaction with the CaMKIIα hub domain

GHB is a natural brain metabolite of GABA, previously reported to be neuroprotective. However, the high-affinity binding site for GHB has remained elusive for almost 40 y. We here unveil CaMKIIα, a highly important neuronal kinase, as the long-sought-after GHB high-affinity target. Via a specific interaction within the central hub domain of CaMKIIα, GHB analogs act to stabilize the hub oligomer complex. This interaction potentially explains pronounced neuroprotective effects of GHB analogs in cultured neurons exposed to a chemical insult and in mice exposed to ischemia. The postischemic treatment effects of GHB analogs underline these compounds as selective and high-affinity potential drug candidates and CaMKIIα as a relevant pharmacological target for stroke therapy.

Ca²⁺/calmodulin-dependent protein kinase II alpha subunit (CaMKIIα) is a key neuronal signaling protein and an emerging drug target. The central hub domain regulates the activity of CaMKIIα by organizing the holoenzyme complex into functional oligomers, yet pharmacological modulation of the hub domain has never been demonstrated. Here, using a combination of photoaffinity labeling and chemical proteomics, we show that compounds related to the natural substance γ-hydroxybutyrate (GHB) bind selectively to CaMKIIα. By means of a 2.2-Å x-ray crystal structure of ligand-bound CaMKIIα hub, we reveal the molecular details of the binding site deep within the hub. Furthermore, we show that binding of GHB and related analogs to this site promotes concentration-dependent increases in hub thermal stability believed to alter holoenzyme functionality. Selectively under states of pathological CaMKIIα activation, hub ligands provide a significant and sustained neuroprotection, which is both time and dose dependent. This is demonstrated in neurons exposed to excitotoxicity and in a mouse model of cerebral ischemia with the selective GHB analog, HOCPCA (3-hydroxycyclopent-1-enecarboxylic acid). Together, our results indicate a hitherto unknown mechanism for neuroprotection by a highly specific and unforeseen interaction between the CaMKIIα hub domain and small molecule brain-penetrant GHB analogs. This establishes GHB analogs as powerful tools for investigating CaMKII neuropharmacology in general and as potential therapeutic compounds for cerebral ischemia in particular.

GABAB Receptor Chemistry and Pharmacology: Agonists, Antagonists, and Allosteric Modulators

The GABA_B receptors are metabotropic G protein-coupled receptors (GPCRs) that mediate the actions of the primary inhibitory neurotransmitter, γ-aminobutyric acid (GABA). In the CNS, GABA plays an important role in behavior, learning and memory, cognition, and stress. GABA is also located throughout the gastrointestinal (GI) tract and is involved in the autonomic control of the intestine and esophageal reflex. Consequently, dysregulated GABA_B receptor signaling is associated with neurological, mental health, and gastrointestinal disorders; hence, these receptors have been identified as key therapeutic targets and are the focus of multiple drug discovery efforts for indications such as muscle spasticity disorders, schizophrenia, pain, addiction, and gastroesophageal reflex disease (GERD). Numerous agonists, antagonists, and allosteric modulators of the GABA_B receptor have been described; however, Lioresal^® (Baclofen; β-(4-chlorophenyl)-γ-aminobutyric acid) is the only FDA-approved drug that selectively targets GABA_B receptors in clinical use; undesirable side effects, such as sedation, muscle weakness, fatigue, cognitive deficits, seizures, tolerance and potential for abuse, limit their therapeutic use. Here, we review GABA_B receptor chemistry and pharmacology, presenting orthosteric agonists, antagonists, and positive and negative allosteric modulators, and highlight the therapeutic potential of targeting GABA_B receptor modulation for the treatment of various CNS and peripheral disorders.

DARK Classics in Chemical Neuroscience: Gamma-Hydroxybutyrate (GHB)

Gamma-hydroxybutyrate (GHB) is a naturally occurring short-chain fatty acid that rose to prominence as a popular club drug in the 1990s. Originally developed as an anesthetic in the early 1960s, it was later sold as an over-the-counter dietary supplement before becoming a rising substance of abuse in the following decades as one of the "date rape" drugs. Despite its abuse potential, there has been a recent surge in therapeutic interest in the drug due to its clinical viability in the treatment of narcolepsy and alcohol abuse/withdrawal. Its interactions with the GABAergic framework of higher mammals has made it the prototypical example for the study of the chief inhibitory mechanism in the human central nervous system. Though relatively obscure in terms of popular culture, it has a storied history with widespread usage in therapeutic, recreational ("Chemsex"), and some disturbingly nefarious contexts. This Review aims to capture its legacy through review of the history, synthesis, pharmacology, drug metabolism, and societal impact of this DARK classic in chemical neuroscience.

GAT3 selective substrate l-isoserine upregulates GAT3 expression and increases functional recovery after a focal ischemic stroke in mice

Ischemic stroke triggers an elevation in tonic GABA inhibition that impairs the ability of the brain to form new structural and functional cortical circuits required for recovery. This stroke-induced increase in tonic inhibition is caused by impaired GABA uptake via the glial GABA transporter GAT3, highlighting GAT3 as a novel target in stroke recovery. Using a photothrombotic stroke mouse model, we show that GAT3 protein levels are decreased in peri-infarct tissue from 6 h to 42 days post-stroke. Prior studies have shown that GAT substrates can increase GAT surface expression. Therefore, we aimed to assess whether the GAT3 substrate, L-isoserine, could increase post-stroke functional recovery. L-Isoserine (38 µM or 380 µM) administered directly into the infarct from day 5 to 32 post-stroke, significantly increased motor performance in the grid-walking and cylinder tasks in a concentration-dependent manner, without affecting infarct volumes. Additionally, L-isoserine induced a lasting increase in GAT3 expression in peri-infarct regions accompanied by a small decrease in GFAP expression. This study is the first to show that a GAT3 substrate can increase GAT3 expression and functional recovery after focal ischemic stroke following a delayed long-term treatment. We propose that enhancing GAT3-mediated uptake dampens tonic inhibition and promotes functional recovery after stroke.

Pharmacological Characterization of [3H]ATPCA as a Substrate for Studying the Functional Role of the Betaine/GABA Transporter 1 and the Creatine Transporter

The betaine/γ-aminobutyric acid (GABA) transporter 1 (BGT1) is one of the four GABA transporters (GATs) involved in the termination of GABAergic neurotransmission. Although suggested to be implicated in seizure management, the exact functional importance of BGT1 in the brain is still elusive. This is partly owing to the lack of potent and selective pharmacological tool compounds that can be used to probe its function. We previously reported the identification of 2-amino-1,4,5,6-tetrahydropyrimidine-5-carboxylic acid (ATPCA), a selective substrate for BGT1 over GAT1/GAT3, but also an agonist for GABA_A receptors. With the aim of providing new functional insight into BGT1, we here present the synthesis and pharmacological characterization of the tritiated analogue, [³H]ATPCA. Using traditional uptake assays at recombinant transporters expressed in cell lines, [³H]ATPCA displayed a striking selectivity for BGT1 among the four GATs ( K_m and V_max values of 21 μM and 3.6 nmol ATPCA/(min × mg protein), respectively), but was also found to be a substrate for the creatine transporter (CreaT). In experiments with mouse cortical cell cultures, we observed a Na⁺-dependent [³H]ATPCA uptake in neurons, but not in astrocytes. The neuronal uptake could be inhibited by GABA, ATPCA, and a noncompetitive BGT1-selective inhibitor, indicating functional BGT1 in neurons. In conclusion, we report [³H]ATPCA as a novel radioactive substrate for both BGT1 and CreaT. The dual activity of the radioligand makes it most suitable for use in recombinant studies.

Radiosynthesis and Evaluation of [11C]3-Hydroxycyclopent-1-enecarboxylic Acid as Potential PET Ligand for the High-Affinity γ-Hydroxybutyric Acid Binding Sites

γ-Hydroxybutyric acid (GHB) is an endogenous neuroactive substance and proposed neurotransmitter with affinity for both low- and high-affinity binding sites. A radioligand with high and specific affinity toward the high-affinity GHB binding site would be a unique tool toward a more complete understanding of this population of binding sites. With its high specific affinity and monocarboxylate transporter (MCT1) mediated transport across the blood-brain barrier in pharmacological doses, 3-hydroxycyclopent-1-enecarboxylic acid (HOCPCA) seems like a suitable PET radiotracer candidate. Here, we report the ¹¹C-labeling and subsequent evaluation of [¹¹C]HOCPCA in a domestic pig, as a PET-radioligand for visualization of the high-affinity GHB binding sites in the live pig brain. To investigate the regional binding of HOCPCA in pig brain prior to in vivo PET studies, in vitro quantitative autoradiography on sections of pig brain was performed using [³H]HOCPCA. In vivo evaluation of [¹¹C]HOCPCA showed no brain uptake, possibly due to a limited uptake of HOCPCA by the MCT1 transporter at tracer doses of [¹¹C]HOCPCA.

Autoradiographic imaging and quantification of the high-affinity GHB binding sites in rodent brain using 3H-HOCPCA

GHB (γ-hydroxybutyric acid) is a compound endogenous to mammalian brain with high structural resemblance to GABA. GHB possesses nanomolar-micromolar affinity for a unique population of binding sites, but the exact nature of these remains elusive. In this study we utilized the highly selective GHB analogue, 3-hydroxycyclopent-1-enecarboxylic acid (HOCPCA) as a tritiated version (³H-HOCPCA) to radioactively label the specific GHB high-affinity binding site and gain further insight into the density, distribution and developmental profile of this protein. We show that, in low nanomolar concentrations, ³H-HOCPCA displays excellent signal-to-noise ratios using rodent brain autoradiography, which makes it a valuable ligand for anatomical quantification of native GHB binding site levels. Our data confirmed that ³H-HOCPCA labels only the high-affinity specific GHB binding site, found in high density in cortical and hippocampal regions. The experiments revealed markedly stronger binding at pH 6.0 (K_d 73.8 nM) compared to pH 7.4 (K_d 2312 nM), as previously reported for other GHB radioligands but similar B_max values. Using ³H-HOCPCA we analyzed the GHB binding protein profile during mouse brain development. Due to the high sensitivity of this radioligand, we were able to detect low levels of specific binding already at E15 in mouse brain, which increased progressively until adulthood. Collectively, we show that ³H-HOCPCA is a highly sensitive radioligand, offering advantages over the commonly used radioligand ³H-NCS-382, and thus a very suitable in vitro tool for qualitative and quantitative autoradiography of the GHB high-affinity site.

In Vitro and In Vivo Evidence for Active Brain Uptake of the GHB Analog HOCPCA by the Monocarboxylate Transporter Subtype 1

γ-Hydroxybutyric acid (GHB) is a recreational drug, a clinically prescribed drug in narcolepsy and alcohol dependence, and an endogenous substance that binds to both high- and low-affinity sites in the brain. For studying the molecular mechanisms and the biologic role of the GHB high-affinity binding sites, ligands with high and specific affinity are essential. The conformationally restricted GHB analog HOCPCA (3-hydroxycyclopent-1-enecarboxylic acid) is one such compound. The objective of this study was to investigate the transport of HOCPCA across the blood-brain barrier in vitro and in vivo and to investigate the hypothesis that HOCPCA, like GHB, is a substrate for the monocarboxylate transporters (MCTs). For in vitro uptake studies, MCT1, -2, and -4 were recombinantly expressed in Xenopus laevis oocytes, and the previously reported radioligand [(3)H]HOCPCA was used as substrate. HOCPCA inhibited the uptake of the endogenous MCT substrate l-[(14)C]lactate, and [(3)H]HOCPCA was shown to act as substrate for MCT1 and 2 (Km values in the low- to mid-millimolar range). Introducing single-point amino acid mutations into positions essential for MCT function supported that HOCPCA binds to the endogenous substrate pocket of MCTs. MCT1-mediated brain entry of HOCPCA (10 mg/kg s.c.) was further confirmed in vivo in mice by coadministration of increasing doses of the MCT inhibitor AR-C141990 [(R)-5-(3-hydroxypyrrolidine-1-carbonyl)-1-isobutyl-3-methyl-6-(quinolin-4-ylmethyl)thieno[2,3-d]pyrimidine-2,4(1H,3H)-dione], which inhibited brain penetration of HOCPCA in a dose-dependent manner (ID50 = 4.6 mg/kg). Overall, our study provides evidence that MCT1 is an important brain entry site for HOCPCA and qualifies for future in vivo studies with HOCPCA.

γ-Hydroxybutyric acid (GHB) is not an agonist of extrasynaptic GABAA receptors

γ-Hydroxybutyric acid (GHB) is an endogenous compound and a drug used clinically to treat the symptoms of narcolepsy. GHB is known to be an agonist of GABAB receptors with millimolar affinity, but also binds with much higher affinity to another site, known as the GHB receptor. While a body of evidence has shown that GHB does not bind to GABAA receptors widely, recent evidence has suggested that the GHB receptor is in fact on extrasynaptic α4β1δ GABAA receptors, where GHB acts as an agonist with an EC50 of 140 nM. We investigated three neuronal cell types that express a tonic GABAA receptor current mediated by extrasynaptic receptors: ventrobasal (VB) thalamic neurons, dentate gyrus granule cells and striatal medium spiny neurons. Using whole-cell voltage clamp in brain slices, we found no evidence that GHB (10 µM) induced any GABAA receptor mediated current in these cell types, nor that it modulated inhibitory synaptic currents. Furthermore, a high concentration of GHB (3 mM) was able to produce a GABAB receptor mediated current, but did not induce any other currents. These results suggest either that GHB is not a high affinity agonist at native α4β1δ receptors, or that these receptors do not exist in classical areas associated with extrasynaptic currents.

New synthesis and tritium labeling of a selective ligand for studying high-affinity γ-hydroxybutyrate (GHB) binding sites

3-Hydroxycyclopent-1-enecarboxylic acid (HOCPCA, 1) is a potent ligand for the high-affinity GHB binding sites in the CNS. An improved synthesis of 1 together with a very efficient synthesis of [(3)H]-1 is described. The radiosynthesis employs in situ generated lithium trimethoxyborotritide. Screening of 1 against different CNS targets establishes a high selectivity, and we demonstrate in vivo brain penetration. In vitro characterization of [(3)H]-1 binding shows high specificity to the high-affinity GHB binding sites.

γ-Hydroxybutyric acid induces actions via the GABAB receptor in arousal and motor control-related nuclei: implications for therapeutic actions in behavioral state disorders

γ-Hydroxybutyric acid (GHB) is used as an effective therapeutic for reducing the hypersomnolence and cataplexy (loss of motor control) of the sleeping disorder, narcolepsy, with an immediate pharmacologic behavioral action of inducing a natural sleep-like state. Despite its clinical use, few studies have examined the cellular actions of this drug on behavioral state-related neurons. Therefore, we monitored GHB-induced responses using calcium imaging within the laterodorsal tegmentum (LDT) and the dorsal raphe (DR), two pontine nuclei important in state and motor control. In addition, we recorded GHB-induced membrane responses using whole cell, patch clamp electrophysiology of immunohistochemically-identified principal neurons within these nuclei. GHB induced GABAB receptor-mediated rises in calcium in neurons of the LDT and the DR. However, the pattern and amplitude of calcium rises differed greatly between these two nuclei. GHB induced GABAB receptor antagonist-sensitive outward currents/hyperpolarizations in immunohistochemically-identified cholinergic LDT and serotonergic DR neurons. However, GHB had this action in a greater proportion of DR cells than LDT neurons. Further, larger inhibitory currents were induced in DR cells when compared to the amplitude of GHB-induced current in LDT-responding cells. Finally, NCS-382 and HOCPCA, a reported antagonist and agonist specific to activity at the putative GHB receptor, respectively, with no demonstrated binding at the GABAB receptor, failed to block GHB-induced effects or elicit any discernible electrophysiological action when applied alone, indicating a lack of involvement of a GHB receptor in mediating GHB actions. Taken together, our data support the conclusion that GHB may be exerting its actions on state and motor control, in part, via an acutely mediated strong inhibition of serotonergic DR neurons and a more modest inhibitory action on a smaller proportion of LDT cholinergic neurons. Given the roles played by these nuclei, these actions are consistent with acute pharmacologic effects of GHB: hypotonia and promotion of sleep, including presence of REM, a sub-state of sleep. Differences in GHB-mediated calcium suggest differential regulation of calcium-dependent processes, which may also contribute to functioning of the LDT and DR in state and motor control and the therapeutic pharmacologic actions of GHB, which develop following chronic administration. These findings add to knowledge of cellular actions of GHB and it is hoped that, combined with findings from other studies examining GHB neurotransmission, these data can contribute to development of highly targeted therapeutics at the GABAB receptor for management of human disorders presenting with alterations in motor and arousal control.

α4βδ GABA(A) receptors are high-affinity targets for γ-hydroxybutyric acid (GHB)

γ-Hydroxybutyric acid (GHB) binding to brain-specific high-affinity sites is well-established and proposed to explain both physiological and pharmacological actions. However, the mechanistic links between these lines of data are unknown. To identify molecular targets for specific GHB high-affinity binding, we undertook photolinking studies combined with proteomic analyses and identified several GABA(A) receptor subunits as possible candidates. A subsequent functional screening of various recombinant GABA(A) receptors in Xenopus laevis oocytes using the two-electrode voltage clamp technique showed GHB to be a partial agonist at αβδ- but not αβγ-receptors, proving that the δ-subunit is essential for potency and efficacy. GHB showed preference for α4 over α(1,2,6)-subunits and preferably activated α4β1δ (EC(50) = 140 nM) over α4β(2/3)δ (EC(50) = 8.41/1.03 mM). Introduction of a mutation, α4F71L, in α4β1(δ)-receptors completely abolished GHB but not GABA function, indicating nonidentical binding sites. Radioligand binding studies using the specific GHB radioligand [(3)H](E,RS)-(6,7,8,9-tetrahydro-5-hydroxy-5H-benzocyclohept-6-ylidene)acetic acid showed a 39% reduction (P = 0.0056) in the number of binding sites in α4 KO brain tissue compared with WT controls, corroborating the direct involvement of the α4-subunit in high-affinity GHB binding. Our data link specific GHB forebrain binding sites with α4-containing GABA(A) receptors and postulate a role for extrasynaptic α4δ-containing GABA(A) receptors in GHB pharmacology and physiology. This finding will aid in elucidating the molecular mechanisms behind the proposed function of GHB as a neurotransmitter and its unique therapeutic effects in narcolepsy and alcoholism.

3-Hydroxy-2'-methoxy-6-methylflavone: a potent anxiolytic with a unique selectivity profile at GABA(A) receptor subtypes

Genetic and pharmacological studies have demonstrated that α2- and α4-containing GABA(A) receptors mediate the anxiolytic effects of a number of agents. Flavonoids are a class of ligands that act at GABA(A) receptors and possess anxiolytic effects in vivo. Here we demonstrate that the synthetic flavonoid, 3-hydroxy-2'-methoxy-6-methylflavone (3-OH-2'MeO6MF) potentiates GABA-induced currents at recombinant α1/2β2, α1/2/4/6β1-3γ2L but not α3/5β1-3γ2L receptors expressed in Xenopus oocytes. The enhancement was evident at micromolar concentrations (EC(50) values between 38 and 106 μM) and occurred in a flumazenil-insensitive manner. 3-OH-2'MeO6MF displayed preference for β2/3- over β1-containing receptors with the highest efficacy observed at α2β2/3γ2L, displaying a 4-11-fold increase in efficacy over α2β1γ2L and α1/4/6-containing subtypes. In contrast, 3-OH-2'MeO6MF acted as a potent bicuculline-sensitive activator, devoid of potentiation effects at extrasynaptic α4β2/3δ receptors expressed in oocytes. The affinity of 3-OH-2'MeO6MF for α4β2/3δ receptors (EC(50) values between 1.4 and 2.5 μM) was 10-fold higher than at α4β1δ GABA(A) receptors. 3-OH-2'MeO6MF acted as a full agonist at α4β2/3δ (105% of the maximal GABA response) but as a partial agonist at α4β1δ (61% of the maximum GABA response) receptors. In mice, 3-OH-2'MeO6MF (1-100 mg/kg i.p.) induced anxiolytic-like effects in two unconditioned models of anxiety: the elevated plus maze and light/dark paradigms. No sedative or myorelaxant effects were detected using holeboard, actimeter and horizontal wire tests and only weak barbiturate potentiating effects on the loss of righting reflex test. Taken together, these data suggest that 3-OH-2'MeO6MF is an anxiolytic without sedative and myorelaxant effects acting through positive allosteric modulation of the α2β2/3γ2L and direct activation of α4β2/3δ GABA(A) receptor subtypes.

Design, synthesis, and in vitro pharmacology of new radiolabeled gamma-hydroxybutyric acid analogues including photolabile analogues with irreversible binding to the high-affinity gamma-hydroxybutyric acid binding sites

Gamma-hydroxybutyric acid (GHB) is a psychotropic compound endogenous to the brain. Despite its potential physiological significance, the complete molecular mechanisms of action remain unexplained. To facilitate the isolation and identification of the high-affinity GHB binding site, we herein report the design and synthesis of the first (125)I-labeled radioligands in the field, one of which contains a photoaffinity label which enables it to bind irreversibly to the high-affinity GHB binding sites.

Novel radioiodinated {gamma}-hydroxybutyric acid analogues for radiolabeling and Photolinking of high-affinity {gamma}-hydroxybutyric acid binding sites

γ-Hydroxybutyric acid (GHB) is a therapeutic drug, a drug of abuse, and an endogenous substance that binds to low- and high-affinity sites in the mammalian brain. To target the specific GHB binding sites, we have developed a (125)I-labeled GHB analog and characterized its binding in rat brain homogenate and slices. Our data show that [(125)I]4-hydroxy-4-[4-(2-iodobenzyloxy)phenyl]butanoate ([(125)I]BnOPh-GHB) binds to one site in rat brain cortical membranes with low nanomolar affinity (K(d), 7 nM; B(max), 61 pmol/mg protein). The binding is inhibited by GHB and selected analogs, but not by γ-aminobutyric acid. Autoradiography using horizontal slices from rat brain demonstrates the highest density of binding in hippocampus and cortical regions and the lowest density in the cerebellum. Altogether, the findings correlate with the labeling and brain regional distribution of high-affinity GHB sites or [(3)H](E,RS)-(6,7,8,9-tetrahydro-5-hydroxy-5H-benzocyclohept-6-ylidene)acetic acid ([(3)H]NCS-382) binding sites. Using a (125)I-labeled photoaffinity derivative of the new GHB ligand, we have performed denaturing protein electrophoresis and detected one major protein band with an apparent mass of 50 kDa from cortical and hippocampal membranes. [(125)I]BnOPh-GHB is the first reported (125)I-labeled GHB radioligand and is a useful tool for in vitro studies of the specific high-affinity GHB binding sites. The related photoaffinity linker [(125)I]4-hydroxy-4-[4-(2-azido-5-iodobenzyloxy)phenyl]butanoate can be used as a probe for isolation of the elusive GHB binding protein.

Phenylacetic acids and the structurally related non-steroidal anti-inflammatory drug diclofenac bind to specific gamma-hydroxybutyric acid sites in rat brain

Gamma-Hydroxybutyric acid (GHB) is a proposed neurotransmitter or neuromodulator with a yet unresolved mechanism of action. GHB binds to both specific high-affinity GHB binding sites and to gamma-aminobutyric acid subtype B (GABA(B)) receptors in the brain. To separate specific GHB effects from GABA(B) receptor effects, it is imperative to develop GHB selective and potent compounds. We generated the compound, 4-(biphen-4-yl)-4-hydroxybutyric acid, which is the 4-hydroxyl analogue of the non-steroidal anti-inflammatory drug (NSAID) fenbufen (referred to as gamma-hydroxyfenbufen). When measured in a rat brain homogenate [(3)H]NCS-382 binding assay, gamma-hydroxyfenbufen inhibited [(3)H]NCS-382 binding with a 10-fold higher affinity than GHB (K(i) 0.44 microM), thus establishing it as a novel lead structure. The active metabolite of fenbufen, 4-biphenylacetic acid inhibited [(3)H]NCS-382 binding with a twofold higher affinity than GHB. Measuring the affinities of structurally related NSAIDs for the [(3)H]NCS-382 site identified diclofenac, a clinically relevant NSAID (Voltaren, Diclon) of the phenylacetic acid (PAA) type, as a GHB ligand (K(i) value of 5.1 microM). Other non-NSAID PAAs also exhibited affinities similar to GHB. Our data raise the interesting possibility that the widely used over-the-counter drug compound, diclofenac, might affect GHB binding at relevant clinical dosages. Furthermore, the identification of PAAs as GHB ligands supplies new information about the structural preferences of the GHB ligand-binding site.

Narcolepsy and depression and the neurobiology of gammahydroxybutyrate

A voluminous literature describes the relationship between disturbed sleep and depression. The breakdown of sleep is one of the cardinal features of depression and often also heralds its onset. Frequent arousals, periods of wakefulness and a short sleep onset REM latency are typical polysomnographic features of depression. The short latency to REM sleep has been attributed to the combination of a monoaminergic deficiency and cholinergic supersensitivity and these irregularities have been proposed to form the biological basis of the disorder. A similar imbalance between monoaminergic and cholinergic neurotransmission has been found in narcolepsy, a condition in which frequent awakenings, periods of wakefulness and short sleep onset REM latencies are also characteristic findings during sleep. In many cases of narcolepsy, this imbalance appears to result from a deficiency of hypocretin but once established, whether in depression or narcolepsy, this disequilibrium sets the stage for the dissociation or premature appearance of REM sleep and for the dissociation of the motor inhibitory component of REM sleep or cataplexy. In the presence of this monoaminergic/cholinergic imbalance, gammahydroxybutyrate (GHB) may acutely further reduce the latency of REM sleep and induce cataplexy, in both patients with narcolepsy or depression. On the other hand, the repeated nocturnal application of GHB in patients with narcolepsy improves the continuity of sleep, prolongs the latency to REM sleep and prevents cataplexy. Evidence to date suggests that GHB may restore the normal balance between monoaminergic and cholinergic neurotransmission. As such, the repeated use of GHB at night and the stabilization of sleep over time makes GHB an effective treatment for narcolepsy and a potentially effective treatment for depression.

gamma-Hydroxybutyrate (GHB) induces GABA(B) receptor independent intracellular Ca2+ transients in astrocytes, but has no effect on GHB or GABA(B) receptors of medium spiny neurons in the nucleus accumbens

We report on cellular actions of the illicit recreational drug gamma-hydroxybutyrate (GHB) in the brain reward area nucleus accumbens. First, we compared the effects of GHB and the GABA(B) receptor agonist baclofen. Neither of them affected the membrane currents of medium spiny neurons in rat nucleus accumbens slices. GABAergic and glutamatergic synaptic potentials of medium spiny neurons, however, were reduced by baclofen but not GHB. These results indicate the lack of GHB as well as postsynaptic GABA(B) receptors, and the presence of GHB insensitive presynaptic GABA(B) receptors in medium spiny neurons. In astrocytes GHB induced intracellular Ca(2+) transients, preserved in slices from GABA(B) receptor type 1 subunit knockout mice. The effects of tetrodotoxin, zero added Ca(2+) with/without intracellular Ca(2+) store depletor cyclopiazonic acid or vacuolar H-ATPase inhibitor bafilomycin A1 indicate that GHB-evoked Ca(2+) transients depend on external Ca(2+) and intracellular Ca(2+) stores, but not on vesicular transmitter release. GHB-induced astrocytic Ca(2+) transients were not affected by the GHB receptor-specific antagonist NCS-382, suggesting the presence of a novel NCS-382-insensitive target for GHB in astrocytes. The activation of astrocytes by GHB implies their involvement in physiological actions of GHB. Our findings disclose a novel profile of GHB action in the nucleus accumbens. Here, unlike in other brain areas, GHB does not act on GABA(B) receptors, but activates an NCS-382 insensitive GHB-specific target in a subpopulation of astrocytes. The lack of either post- or presynaptic effects on medium spiny neurons in the nucleus accumbens distinguishes GHB from many drugs and natural rewards with addictive properties and might explain why GHB has only a weak reinforcing capacity.

Novel high-affinity and selective biaromatic 4-substituted gamma-hydroxybutyric acid (GHB) analogues as GHB ligands: design, synthesis, and binding studies

Gamma-hydroxybutyrate (GHB) is a metabolite of gamma-aminobutyric acid (GABA) and has been proposed to function as a neurotransmitter or neuromodulator. GHB is used in the treatment of narcolepsy and is a drug of abuse. GHB binds to both GABA(B) receptors and specific high-affinity GHB sites in brain, of which the latter have not been linked unequivocally to function, but are speculated to be GHB receptors. In this study, a series of biaromatic 4-substituted GHB analogues, including 4'-phenethylphenyl, 4'-styrylphenyl, and 4'-benzyloxyphenyl GHB analogues, were synthesized and characterized pharmacologically in a [3H](E,RS)-(6,7,8,9-tetrahydro-5-hydroxy-5H-benzocyclohept-6-ylidene)acetic acid ([3H]NCS-382) binding assay and in GABA(A) and GABA(B) receptor binding assays. The compounds were selective for the high-affinity GHB binding sites and several displayed Ki values below 100 nM. The affinity of the 4-[4'-(2-iodobenzyloxy)phenyl] GHB analogue 17b was shown to reside predominantly with the R-enantiomer (Ki = 22 nM), which has higher affinity than previously reported GHB ligands.

Behavioral analyses of GHB: receptor mechanisms

GHB is used therapeutically and recreationally, although the precise mechanism of action responsible for its different behavioral effects is not entirely clear. The purpose of this review is to summarize how behavioral procedures, especially drug discrimination procedures, have been used to study the mechanism of action of GHB. More specifically, we will review several different drug discrimination procedures and discuss how they have been used to qualitatively and quantitatively study different components of the complex mechanism of action of GHB. A growing number of studies have provided evidence that the behavioral effects of GHB are mediated predominantly by GABAB receptors. However, there is also evidence that the mechanisms mediating the effects of GHB and the prototypical GABAB receptor agonist baclofen are not identical, and that other mechanisms such as GHB receptors and subtypes of GABAA and GABAB receptors might contribute to the effects of GHB. These findings are consistent with the different behavioral profile, abuse liability, and therapeutic indications of GHB and baclofen. A better understanding of the similarities and differences between GHB and baclofen, as well as the pharmacological mechanisms of action underlying the recreational and therapeutic effects of GHB, could lead to more effective medications with fewer adverse effects.

Validation of high-affinity binding sites for succinic acid through distinguishable binding of gamma-hydroxybutyric acid receptor-specific NCS 382 antipodes

Gamma-hydroxybutyric acid (GHB) binding to multiple sites for the tricarboxylic acid cycle intermediate succinic acid (SUC) has been disclosed recently. In order to better characterize these targets, distinguishable binding of GHB receptor-specific NCS 382 antipodes to [(3)H]-SUC or [(3)H]-GHB labelled sites in rat brain synaptic membranes was explored. Eutomer binding parameters suggest identity of the high-affinity target for SUC with a synaptic GHB receptor subtype.

Metabolic GHB precursor succinate binds to gamma-hydroxybutyrate receptors: characterization of human basal ganglia areas nucleus accumbens and globus pallidus

Binding of the metabolic gamma-hydroxybutyrate (GHB) precursor succinate to NCS-382-sensitive [3H]GHB-labeled sites in crude synaptosomal or purified synaptic membrane fractions prepared from the human nucleus accumbens (NA), globus pallidus (GP) and rat forebrain has been shown. This site can be characterized by binding of ethyl hemisuccinate and gap-junction blockers, including carbenoxolone hemisuccinate and beta-GRA. There was no significant binding interaction between GABAB receptor ligands (CGP 55845, (R)-baclofen) and these [3H]GHB-labeled sites. GHB, NCS-382 and succinate binding profile of [3H]GHB-labeled sites in rat forebrain, human NA or GP synaptic membranes were similar. The synaptic fraction isolated from the rat forebrain was characterized by GHB binding inhibition constants: Ki,NCS-382 = 1.2 +/- 0.2 microM, Ki,GHB = 1.6 +/- 0.3 microM and Ki,SUCCINATE = 212 +/- 66 microM. In crude membranes containing mainly extrasynaptic membranes, distinct GHB and GABAB receptor sites were found in the NA. By contrast, extrasynaptic GABAB receptor sites of rat forebrain and GP were GHB- and succinate-sensitive, respectively. The heterogeneity of GABAB sites found in native membranes indicates GABAB receptor-dependent differences in GHB action. Based on these findings, we suggest that succinate (and possibly drugs available as succinate salt derivatives) can mimic some of the actions of GHB.

Gamma-hydroxybutyrate--a drug of abuse

Gamma-hydroxybutyrate (GHB) is a drug of abuse that causes euphoria, anxiolysis, and hypnosis. The recent rise in the recreational intake of GHB, as well as its association with 'drug rape', has turned the attention to GHB in acute hospital settings. Acutely admitted GHB intoxicated patients may display various levels of sedation or coma, but may also show paradoxical agitation, combativeness, or self-injurious behaviors. The symptoms can be nonspecific and the definite diagnosis therefore normally relies on the detection of GHB in blood or body fluids, which is an analysis that may not be promptly available. As a basis for understanding the clinical features of GHB intoxication and abuse, we here review the pharmacological and neurophysiological knowledge about GHB, which stems from decades of clinical and basic GHB research. In addition, we discuss the latest discoveries in the quest for distinct GHB receptors in the brain, and their possible implications for future therapies of GHB abuse.

Unravelling the brain targets of gamma-hydroxybutyric acid

Gamma-hydroxybutyric acid (GHB) is a naturally occurring gamma-aminobutyric acid (GABA) metabolite that has been proposed as a neurotransmitter/neuromodulator that acts via its own receptor (GHBR). Its exogenous administration, however, elicits central nervous system-dependent effects (e.g. memory impairment, increase in sleep stages 3 and 4, dependence, seizures and coma) that are mostly mediated by GABAB receptors. The past few years have seen important developments in our understanding of GHB neurobiology: a putative GHBR has been cloned; a transgenic model of GHB aciduria has been developed; GABAB receptor knockout mice and novel GHB analogs have helped to characterize the vast majority of exogenous GHB actions mediated by GABAB receptors; and some of the cellular mechanisms underlying the dependence/abuse properties of GHB, and its ability to elicit absence seizures and an increase in sleep stages 3 and 4, have been clarified. Nevertheless, the physiological significance of a brain GHB signaling pathway is still unknown, and there is an urgent need for a well-validated functional assay for GHBRs. Moreover, as GHB can also be metabolized to GABA, it remains to be seen whether the many GABAB receptor-mediated actions of GHB are caused by GHB itself acting directly on GABAB receptors or by a GHB-derived GABA pool (or both).