Evidence for a role of high Km aldehyde reductase in the degradation of endogenous gamma-hydroxybutyrate from rat brain

Mechanisms for the Specific Properties of γ-Hydroxybutyrate in Brain

γ-Hydroxybutyrate (GHB) is both a natural brain compound with neuromodulatory properties at central GABAergic synapses (micromolar concentration range) and also a drug (Xyrem(R) ) clinically used for the treatment of various neurological symptoms (millimolar dose range). However, this drug has abuse potential and can be addictive for some patients. Here, we review the basic mechanistic role of endogenous GHB in brain as well as the properties and mechanisms of action for therapeutic clinical doses of exogenous GHB. Several hypotheses are discussed with a preference for a molecular mechanism that conciliates most of the findings available. This conciliatory model may help for the design of GHB-like drugs active at lower doses and devoid of major side effects.

Metabolism of gamma hydroxybutyrate in human hepatoma HepG2 cells by the aldo-keto reductase AKR1A1

Gamma hydroxybutyrate (GHB) is a recreational and date-rape drug, for which the detection following ingestion is hampered by rapid metabolism and its endogenous presence. GHB catabolism occurs mainly by its oxidation to succinic semialdehyde (SSA), which converts to succinate and enters the tricarboxylic acid cycle. A high Km aldehyde reductase has previously been reported to catalyse the NADP-dependent oxidation of GHB at high concentrations. It is assumed that this enzyme is identical to the aldo-keto reductase AKR1A1, but its role in GHB oxidation has not been fully evaluated. In this study, the extent of AKR1A1 in GHB metabolism has been determined in HepG2 cells using RNA-interference technology. The gene encoding AKR1A1 was targeted by siRNA. Results demonstrate a successful knock-down of the AKR1A1 gene with 92% reduction in total mRNA and 93% reduction in protein expression. Demolishing AKR1A1 expression in HepG2 cells leads to significant 82% decrease in NADP-dependent GHB-dehydrogenase activity at high concentration (10mM) of GHB. Moreover, when exposing the cells to 50 μM of GHB for 24h, and measuring intracellular and extracellular GHB levels by GC/MS, a significant two-fold increase was observed on GHB intracellular level in silenced cells. In contrast, measuring SSA-reductase activity in silenced cells indicated that AKR1A1 is not involved in endogenous GHB production. These findings describe a pathway for GHB metabolism in the liver which should be useful in GHB exposure cases, and will enable a better understanding of the enzymes participating in its metabolism at natural and overexposed levels.

Absorption, distribution, metabolism and excretion pharmacogenomics of drugs of abuse

Pharmacologic and toxic effects of xenobiotics, such as drugs of abuse, depend on the genotype and phenotype of an individual, and conversely on the isoenzymes involved in their metabolism and transport. The current knowledge of such isoenzymes of frequently abused therapeutics such as opioids (oxycodone, hydrocodone, methadone, fentanyl, buprenorphine, tramadol, heroin, morphine and codeine), anesthetics (γ-hydroxybutyric acid, propofol, ketamine and phencyclidine) and cognitive enhancers (methylphenidate and modafinil), and some important plant-derived hallucinogens (lysergide, salvinorin A, psilocybin and psilocin), as well as of nicotine in humans are summarized in this article. The isoenzymes (e.g., cytochrome P450, glucuronyltransferases, esterases and reductases) involved in the metabolism of drugs and some pharmacokinetic data are discussed. The relevance of such data is discussed for predicting possible interactions with other xenobiotics, understanding pharmacokinetic behavior and pharmacogenomic variations, assessing toxic risks, developing suitable toxicological analysis procedures, and finally for interpretating drug testing results.

Enzymes involved in the metabolism of gamma-hydroxybutyrate in SH-SY5Y cells: identification of an iron-dependent alcohol dehydrogenase ADHFe1

The metabolism of the endogenous metabolite gamma-hydroxybutyrate (GHB) has been studied in a human neuroblastoma cell line SH-SY5Y as a model for examining neuronal metabolism. We show that GHB can be synthesized and released from these cells, indicating that pathways for GHB synthesis and secretion are present. Activities for the major enzymes that are involved in GHB metabolism are reported, and transcripts for AKR1A1, AKR7A2, ALDH5A1 and GABA-T can be detected by RT-PCR. We also demonstrate the presence of the ADHFe1 transcript, a gene that has been reported to encode a hydroxyacid-oxoacid transhydrogenase (HOT). We show that the ADHFe1 gene is related to bacterial GHB dehydrogenases and has a conserved NAD-binding site. The potential for using the SH-SY5Y cell line for investigating GHB catabolism is discussed.

Synthesis and catabolism of gamma-hydroxybutyrate in SH-SY5Y human neuroblastoma cells: role of the aldo-keto reductase AKR7A2

gamma-Hydroxybutyrate (GHB) is an endogenous metabolite synthesized in the brain. There is strong evidence to suggest that GHB has an important role as a neurotransmitter or neuromodulator. The human aldo-keto reductase AKR7A2 has been proposed previously to catalyze the NADPH-dependent reduction of succinic semialdehyde (SSA) to GHB in human brain. In this study we have used RNA interference to evaluate the role of AKR7A2 in GHB biosynthesis in human neuroblastoma SH-SY5Y cells. Quantitative reverse transcription-PCR analysis and immunoblotting revealed that short interfering RNA molecules directed against AKR7A2 led to a significant reduction in both AKR7A2 transcript and protein levels 72 h post-transfection. We have shown that reduced expression of AKR7A2 results in a 90% decrease in SSA reductase activity of cell extracts. Furthermore, we have shown using gas chromatography-mass spectrometry that a decrease in the level of AKR7A2 was paralleled with a significant reduction in intracellular GHB concentration. This provides conclusive evidence that AKR7A2 is the major SSA reductase in these cells. In contrast, short interfering RNA-dependent reduction in AKR7A2 levels had no effect on the GHB dehydrogenase activity of the extracts, and inhibitor studies suggest that another enzyme characteristic of an NAD-dependent alcohol dehydrogenase may be responsible for catalyzing this reverse reaction. Together these findings delineate pathways for GHB metabolism in the brain and will enable a better understanding of the relationship between GHB biosynthesis and catabolism in disease states and in drug overdose.

Neurochemical and electrophysiological evidence for the existence of a functional gamma-hydroxybutyrate system in NCB-20 neurons

Clonal neurohybridoma NCB-20 cells express a valproate-insensitive succinic semialdehyde reductase activity that transforms succinic semialdehyde into gamma-hydroxybutyrate. This activity (1.14+/-0.16 nmol/min/mg protein) was similar to the lowest activity existing in adult rat brain. [3H]gamma-Hydroxybutyrate labels a homogeneous population of sites on NCB-20 cell membranes (Kd=250+/-44.4nM, Bmax=180+/-16.2fmol/mg protein) that apparently represents specific gamma-hydroxybutyrate binding sites characterized previously on brain cell membranes. Finally, an Na+-dependent uptake of [3H]gamma-hydroxybutyrate was expressed in NCB-20 cells with a Km of 35+21.1 microM and a Vmax of 80+/-14.2 pmol/min/mg protein. A three-day treatment with 1 mM dibutyryl-cyclic-AMP induced a three-fold increase in the cellular succinic semialdehyde reductase activity. In parallel, a K+-evoked release of [3H]gamma-hydroxybutyrate occurred. This release was Ca2+ dependent and was not present in undifferentiated cells. Cyclic-AMP treatment induced a decrease of [3H]gamma-hydroxybutyrate binding sites, which could be due to spontaneous gamma-hydroxybutyrate release. Patch-clamp experiments carried out on differentiated NCB-20 cells revealed the presence of Ca2+ conductances which were partially inhibited by 50 microM gamma-hydroxybutyrate. This gamma-hydroxybutyrate-induced effect was blocked by the gamma-hydroxybutyrate receptor antagonist NCS-382, but not by the GABA(B) antagonist CGP-55845. These results demonstrate the presence of an active gamma-hydroxybutyratergic system in NCB-20 cells which possesses the ability to release gamma-hydroxybutyrate. These cells express specific gamma-hydroxybutyrate receptors which modulate Ca2+ currents independently of GABA(B) receptors.

Binding characteristics of gamma-hydroxybutyric acid as a weak but selective GABAB receptor agonist

The aim of this study was to reexamine the concept that gamma-hydroxybutyric acid (GHB) is a weak but selective agonist at gamma-aminobutyric acidB (GABAB) receptors, using binding experiments with several radioligands. Ki values of GHB were similar (approximately equal to 100 microM) in three agonist radioligand assays for GABAB receptors, [3H]baclofen (beta-para-chlorophenyl-gamma-aminobutyric acid), [3H]CGP 27492 (3-aminopropyl-phosphinic acid) and [3H]GABA, in the presence of the GABAA receptor agonist isoguvacine with rat cortical, cerebellar and hippocampal membranes. In competition experiments between GHB and the GABAB receptor antagonist, [3H]CGP 54626 (3-N [1-{(S)-3,4-dichlorophenyl}-ethylamino]-2-(S)-hydroxypropyl cyclo-hexylmethyl phosphinic acid), the IC50 values were significantly increased with 300 microM of 5'-guanyl-imidodiphosphate (Gpp(NH)p), which suggested that guanine nucleotide binding proteins (G-proteins) modulate GHB binding on GABAB receptors. The inhibition by GHB of [3H]CGP 27492 binding in cortical membranes was not altered in the presence of 0.3 or 3 mM of the two GHB dehydrogenase inhibitors, valproate and ethosuximide. Thus, GHB is not reconverted into GABA by GHB dehydrogenase. Taken together, the results of this study demonstrated that GHB is an endogenous weak but selective agonist at GABAB receptors.

The gamma-hydroxybutyrate signalling system in brain: organization and functional implications

gamma-Hydroxybutyrate is a metabolite of GABA which is synthesized and accumulated by neurons in brain. This substance is present in micromolar quantities in all brain regions investigated as well as in several peripheral organs. Neuronal depolarization releases gamma-hydroxybutyrate into the extracellular space in a Ca(2+)-dependent manner. Gamma-hydroxybutyrate high-affinity receptors are present only in neurons, with a restricted specific distribution in the hippocampus, cortex and dopaminergic structures of rat brain (the striatum in general, olfactory bulbs and tubercles, frontal cortex, dopaminergic nuclei A9, A10 and A12). Stimulation of these receptors with low amounts of gamma-hydroxybutyrate induces in general hyperpolarizations in dopaminergic structures with a reduction of dopamine release. However, in the hippocampus and the frontal cortex, it seems that gamma-hydroxybutyrate induces depolarization with an accumulation of cGMP and an increase in inositol phosphate turnover. Some of the electrophysiological effects of GHB are blocked by NCS-382, a gamma-hydroxybutyrate receptor antagonist while some others are strongly attenuated by GABAB receptors antagonists. Gamma-hydroxybutyrate penetrates freely into the brain when administered intravenously or intraperitoneally. This is a unique situation for a molecule with signalling properties in the brain. Thus, the gamma-hydroxybutyrate concentration in brain easily can be increased more than 100 times. Under these conditions, gamma-hydroxybutyrate receptors are saturated and probably desensitized and down-regulated. It is unlikely that GABAB receptors could be stimulated directly by GHB. Most probably, GABA is released in part under the control of GHB receptors in specific pathways expressing GABAB receptors. Alternatively, GABAB receptors might be specifically stimulated by the GABA formed via the metabolism of gamma-hydroxybutyrate in brain. In animals and man, these GHBergic and GABAergic potentiations induce dopaminergic hyperactivity (which follows the first phase of dopaminergic terminal hyperpolarization), a strong sedation with anaesthesia and some EEG changes with epileptic spikes. It is presumed that, under pathological conditions (hepatic failure, alcoholic intoxication, succinic semialdehyde dehydrogenase defects), the rate of GHB synthesis or degradation in the peripheral organ is modified and induces increased GHB levels which could interfere with the normal brain mechanisms. This pathological status could benefit from treatments with gamma-hydroxybutyric and/or GABAB receptors antagonists. Nevertheless, the regulating properties of the endogenous gamma-hydroxybutyrate system on the dopaminergic pathways are a cause for the recent interest in synthetic ligands acting specifically at gamma-hydroxybutyrate receptors and devoid of any role as metabolic precursor of GABA in brain.

Gamma-hydroxybutyrate: an overview of the pros and cons for it being a neurotransmitter and/or a useful therapeutic agent

Gamma-hydroxybutyrate (GHB) is a catabolite in brain of gamma-aminobutyrate (GABA) and is also found in nonneuronal tissues. It is present in the brain at about one thousandth of the concentration of its parent compound. High affinity and specific uptake, and energy dependent transport systems for GHB have been described in brain in addition to a class of high affinity binding sites, functional at a rather unphysiologically low pH. Administration of large doses of GHB to animals and man leads to sedation, and at the highest doses, anaesthesia. These effects are prominent when GHB brain levels are over one hundred-fold the endogenous levels. In some animals, GHB administration also induces an electroencephalographic and behavioural changes resembling that of human petit mal epilepsy. GHB has been used in man as an anaesthetic adjuvant. GHB lowers cerebral energy requirements and may play a neuroprotective role. Administered GHB profoundly effects the cerebral dopaminergic system by a mechanism which remains to be unravelled. GHB has been tested with success on alcoholic patients where it attenuates the withdrawal syndrome. It is indicated here that in this situation, it may owe its effect by acting as a pro-drug of the neurotransmitter GABA into which it can be transformed. As administration of GHB, a GABAB receptor agonist and a natural opioid peptide all elicit similar abnormal EEG phenomena, it may be suggested that they are acting via a common pathway. The petit mal epileptic effects of GHB might be ascribed to its direct, or indirect agonist properties after transformation to a pool of GABA at the GABAB receptor or via interactions at its own binding sites linked to a similar series of biochemical events. Some anticonvulsant drugs, the opiate antagonist naloxone and a synthetic structural GHB analogue antagonise certain behavioural effects of GHB administration. It is postulated that GHB exerts some of its effects via transformation to GABA pools, and that substances which inhibit this process antagonise its effects by blocking GABA formation. GHB has been proposed as a neurotransmitter, although straightforward evidence for this role is lacking. Evidence for and against GHB, as a neurotransmitter, is reviewed here together with a discussion of its potential as a therapeutically useful drug.

Significance of gamma-hydroxybutyric acid in the brain

1. Administration of the endogenous compound gamma-hydroxybutyric acid (GHB) can induce a sleep-like state in experimental animals and, indeed, it has been used as a general anaesthetic in clinical medicine. 2. Although GHB appears to be a CNS depressant, there is evidence it possesses epileptiform activity resembling petit mal epilepsy. In the brain GHB is evidently derived from GABA, the final step being catalyzed by succinic semialdehyde reductase, a cytosolic NADP(+)-dependent enzyme. 3. Two different oxidoreductases, GHB dehydrogenase and hydroxyacid-ketoacid dehydrogenase, acting independently, are responsible for the reverse reaction when GHB is being metabolically inactivated. 4. Brain contains a Na(+)-dependent GHB uptake system which exhibits two components, one with a Km of 46 microM and the other with a Km of 325 microM. GHB also binds to receptor sites in brain homogenates and exhibits two distinct affinities. One binding site displays a Kd of 95 nM whereas the second site has a Kd of 16 microM. Binding to both sites is inhibited in the presence of NCS-382, a GHB receptor antagonist. 5. GHB might play a role as a neurotransmitter, particularly being involved in influencing dopamine release in the substantia nigra.

An overview of gamma-hydroxybutyrate catabolism: the role of the cytosolic NADP(+)-dependent oxidoreductase EC 1.1.1.19 and of a mitochondrial hydroxyacid-oxoacid transhydrogenase in the initial, rate-limiting step in this pathway

Two enzymes have been found which catalyze the initial step in the catabolism of GHB. The oxidation of GHB to SSA, catalyzed by both of these enzymes, is coupled to the reduction of an oxoacid. In the case of the mitochondrial transhydrogenase, the coupling is obligatory. Although coupling is not obligatory for the GHB dehydrogenase, the stimulation provided by the coupled reaction, and the nature of the kinetics of the uncoupled reaction, may not only allow the reaction to proceed, but may provide a means of regulating the rate of the reaction under in vivo conditions. Since the oxidation of GHB to SSA is the rate limiting step in the overall catabolic pathway (the rate of conversion of GHB to SSA proceeds at approximately one one thousandth of the rate at which SSA is oxidized to succinate by SSA dehydrogenase (30)), factors which regulate the rate of either or both of these enzymes will, in turn, influence tissue levels of endogenous GHB as well as the duration and magnitude of the physiological effect of a dose of GHB.

In vivo conversion of gamma-aminobutyric acid and 1,4-butanediol to gamma-hydroxybutyric acid in rat brain. Studies using stable isotopes

The formation of 4-[1,4-13C]hydroxybutyric acid ([13C]gamma-hydroxybutyric acid; [13C]GHB) in rat brain was studied following intracerebroventricular (i.c.v.) administration of either 4-[1,4-13C]aminobutyric acid ([13C]GABA or 1,4-[1,4-13C]butanediol ([13C]1,4-BD) to awake, freely moving animals. GHB and [13C]GHB were measured with a gas chromatographic mass spectrometric (GC/MS) technique designed to detect the lactone derivative of GHB with the acid or lactone being determined by conditions of tissue extraction. [13C]GHB was detected following i.c.v. administration of [13C]GABA with a turnover rate of 2.04 nmol/g tissue/hr and [13C]1,4-BD with a turnover rate of 1.4 nmol/g/hr. The formation of [13C]GHB from [13C]GABA was blocked by an inhibitor of GABA-transaminase, but this drug had no effect on the formation of [13C]GHB from [13C]1,4-BD. The latter pathway was also unaffected by alcohol dehydrogenase inhibitors, compounds which block this pathway in the periphery. Further, in the course of these experiments, naturally occurring endogenous gamma-butyrolactone (GBL) was detected in rat brain in a concentration of 200 pmol/g tissue weight, but lactonization in vivo of [13C]GHB formed from either labeled GABA or 1,4-BD was not demonstrated. These data confirm two separate pathways of synthesis for GHB in brain, demonstrate the presence of GBL in brain, and illustrate the utility of a new GC/MS technique for analysis of GHB and for GBL which does not involve extensive derivatization.

Gammahydroxybutyrate: an endogenous regulator of energy metabolism

Gammahydroxybutyrate is a naturally occurring metabolite of many mammalian tissues. Although its administration produces a wide range of pharmacological effects, its normal function has never been clearly defined. GHB can induce NREM and REM sleep, anaesthesia, hypothermia, and a trance-like state which has been considered a model for petit mal epilepsy. It markedly increases brain dopamine levels. It has been touted as a central neurotransmitter or neuromodulator, and high affinity brain receptors, as well as central mechanisms for its synthesis, uptake and release have been demonstrated in support of this. But GHB is also found in many peripheral tissues and in some of these in higher concentrations than in the brain. No explanation has been offered for its presence in these tissues. A number of studies indicate that GHB can reduce energy substrate consumption in both brain and peripheral tissues, and that it can protect these tissues from the damaging effects of anoxia or excessive metabolic demand. Indeed there is some evidence to suggest that endogenous GHB levels rise under these circumstances. GHB appears to act through the endogenous opioid system, since in the brain, at least, GHB raises dynorphin levels and its metabolic and pharmacological effects can be blocked by naloxone. These, and other observations detailed in this review, suggest that GHB may function naturally in the induction and maintenance of physiological states, like sleep and hibernation, in which energy utilization is depressed. GHB may also function naturally as an endogenous protective agent when tissue energy supplies are limited.

3'-5' cyclic-guanosine monophosphate increase in rat brain hippocampus after gamma-hydroxybutyrate administration. Prevention by valproate and naloxone

An increase (123%) of cyclic GMP (cGMP) was observed in the hippocampus of the rat killed by microwave irradiation 45 min after administration of 500 mg/kg gamma-hydroxybutyrate (GHB) IP. This increase is time and dose dependent. No modification in cyclic nucleotide content was observed in striatum and in cerebellum. As the role of GHB has been implicated in neurotransmission, the fact that this compound increases cyclic GMP accumulation in hippocampus in vivo may represent a mechanism by which the actions of GHB are mediated at the cellular level. Valproate (400 mg/kg) or naloxone (10 mg/kg) pretreatment completely abolish the cGMP increase due to GHB. A GABAergic and/or opiate phenomenon may be involved in the mechanism of GHB induced increase of cGMP.