Characterization of methionine-enkephalin release in the rat striatum by in vivo dialysis: effects of gamma-hydroxybutyrate on cellular and extracellular methionine-enkephalin levels

Comparison of N-methyl-2-pyrrolidone (NMP) and the "date rape" drug GHB: behavioral toxicology in the mouse model

N-methyl-2-pyrrolidone (NMP) and γ-hydroxybutyrate acid (GHB) are synthetic solvents detected in the recreational drug market. GHB has sedative/hypnotic properties and is used for criminal purposes to compromise reaction ability and commit drug-facilitated sexual assaults and other crimes. NMP is a strong solubilizing solvent that has been used alone or mixed with GHB in case of abuse and robberies. The aim of this experimental study is to compare the acute pharmaco-toxicological effects of NMP and GHB on neurological signs (myoclonia, convulsions), sensorimotor (visual, acoustic, and overall tactile) responses, righting reflex, thermoregulation, and motor activity (bar, drag, and accelerod test) in CD-1 male mice. Moreover, since cardiorespiratory depression is one of the main adverse effects related to GHB intake, we investigated the effect of NMP and GHB on cardiorespiratory changes (heart rate, breath rate, oxygen saturation, and pulse distension) in mice. The present study demonstrates that NMP inhibited sensorimotor and motor responses and induced cardiorespiratory depression, with a lower potency and efficacy compared to GHB. These results suggest that NMP can hardly be used alone as a substance to perpetrate sexual assault or robberies.

Dynorphin and its role in alcohol use disorder

The dynorphin / kappa opioid receptor (KOR) system has been implicated in many aspects that influence neuropsychiatric disorders. Namely, this system modulates neural circuits that primarily regulate reward seeking, motivation processing, stress responsivity, and pain sensitivity, thus affecting the development of substance and alcohol use disorder (AUD). The effects of this system are often bidirectional and depend on projection targets. To date, a majority of the studies focusing on this system have examined the KOR function using agonists and antagonists. Indeed, there are studies that have examined prodynorphin and dynorphin levels by measuring mRNA and tissue content levels; however, static levels of the neuropeptide and its precursor do not explain complete and online function of the peptide as would be explained by measuring dynorphin transmission in real time. New and exciting methods using optogenetics, chemogenetics, genetic sensors, fast scan cyclic voltammetry are now being developed to detect various neuropeptides with a focus on opioid peptides, including dynorphin. In this review we discuss studies that examine dynorphin projections in areas involved in AUD, its functional involvement in AUD and vulnerability to develop AUD at various ages. Moreover, we discuss dynorphin's role in promoting AUD by dysregulation motivation circuits and how advancements in opioid peptide detection will further our understanding.

Effects of acute ethanol administration on methionine-enkephalin expression and release in regions of the rat brain

The dopaminergic mesolimbic system plays a key role in mediating the reinforcing properties of ethanol and other drugs of abuse. Ethanol reinforcement and high alcohol drinking behaviour have been suggested to involve the ethanol-induced activation of endogenous opioid systems. Ethanol may alter opioidergic transmission at different levels, including opioid peptide biosynthesis and release, as well as binding to opioid receptors. The aim of this work was to investigate the effects of different ethanol doses on methionine-enkephalin (Met-enk) release from the rat nucleus accumbens (NAcc). Ethanol effects were also studied on Met-enk content in the NAcc, prefrontal cortex (PFC) and caudate-putamen (CP). Met-enk release was studied by microdialysis in Wistar anesthetized rats and peptide concentrations were quantitated by radioimmunoassay. Ethanol was administered by intraperitoneal injection after a 2-h basal release period. Ethanol doses of 0.5, 1 and 2.5 g/kg induced a 2.7-, 4.9- and 3.4-fold increase in Met-enk release from the NAcc. However, ethanol responses followed different kinetics, with earliest effects observed with the highest ethanol dose. In comparison, a 2.5-fold increase in peptide release was produced by 100 mM KCl. Ethanol, at a dose of 2.5 g/kg, induced a significant 66.7% decrease in Met-enk content in the NAcc, as well as a 76.4% reduction in peptide levels in the CP. Lower ethanol doses did not alter Met-enk content in these regions. On the other hand, an ethanol dose of 0.5 g/kg produced a non-significant decrease in Met-enk levels in the PFC. Our results suggest that ethanol-induced changes in enkephalin expression and release in regions of the mesocorticolimbic and nigrostriatal pathways could be involved in ethanol central effects. Released enkephalins by ethanol may modulate the dopaminergic activity of mesolimbic neurons and play a critical role in ethanol reinforcement mechanisms.

A single acute pharmacological dose of γ-hydroxybutyrate modifies multiple gene expression patterns in rat hippocampus and frontal cortex

γ-Hydroxybutyrate (GHB) is a natural brain neuromodulator that has its own enzymatic machinery for synthesis and degradation, release, and transport systems and several receptors that belong to the G protein-coupled receptor (GPCR) family. Targeting of this system with exogenous GHB is used in therapy to induce sleep and anesthesia and to reduce alcohol withdrawal syndrome. GHB is also popular as a recreational drug for its anxiolytic and mild euphoric effects. However, in both cases, GHB must be administered at high doses in order to maintain GHB concentrations in brain of ∼800-1,000 μM. These high concentrations are thought to be necessary for interactions with low-affinity sites on GABA(B) receptor, but the molecular targets and cellular mechanisms modulated by GHB remain poorly characterized. Therefore, to provide new insights into the elucidation of GHB mechanisms of action and open new tracks for future investigations, we explored changes of GHB-induced transcriptomes in rat hippocampus and prefrontal cortex by using DNA microarray studies. We demonstrate that a single acute anesthetic dose of 1 g/kg GHB alters a large number of genes, 121 in hippocampus and 53 in prefrontal cortex; 16 genes were modified simultaneously in both brain regions. In terms of molecular functions, the majority of modified genes coded for proteins or nucleotide binding sites. In terms of Gene Ontology (GO) functional categories, the largest groups were involved in metabolic processing for hippocampal genes and in biological regulation for prefrontal cortex genes. The majority of genes modified in both structures were implicated in cell communication processes. Western blot and immunohistochemical studies carried out on eight selected proteins confirmed the microarray findings.

Modulation of MDMA-induced behavioral and transcriptional effects by the delta opioid antagonist naltrindole in mice

The delta opioid system is involved in the behavioral effects of various drugs of abuse. However, only a few studies have focused on the possible interactions between the opioid system and the effects of 3,4-methylenedioxymethamphetamine (MDMA). In order to examine the possible role of the delta opioid system in MDMA-induced behaviors in mice, locomotor activity and conditioned place preference (CPP) were investigated in the presence of naltrindole (NTI), a selective delta opioid antagonist. Moreover, the consequences of acute and chronic MDMA administration on pro-enkephalin (Penk) and pro-opiomelanocortin (Pomc) gene expression were assessed by real-time quantitative polymerase chain reaction (QPCR). The results showed that, after acute MDMA administration (9 mg/kg; i.p.), NTI (5 mg/kg, s.c.) was able to totally block MDMA-induced hyperlocomotion. Penk gene expression was not modulated by acute MDMA, but a decrease of Pomc gene expression was observed, which was not antagonized by NTI. Administration of the antagonist prevented the acquisition of MDMA-induced CPP, suggesting an implication of the delta opioid receptors in this behavior. Following chronic MDMA treatment, only the level of Pomc was modulated. The observed increase was totally blocked by NTI pre-treatment. All these results confirm the interactions between the delta opioid system (receptors and peptides) and the effects of MDMA.

GHB ameliorates naloxone-induced conditioned place aversion and physical aspects of morphine withdrawal in mice

RATIONALE

Gamma-hydroxybutyric acid (GHB) is a naturally occurring substance in the brain, the administration of which has proved useful in the treatment of the opiate withdrawal symptoms in humans.

OBJECTIVES

The aim of the present work was to validate this beneficial effect on the physical and motivational aspects of morphine withdrawal in mice.

METHODS

In a first experiment, animals rendered morphine-dependent were conditioned to develop a place aversion (CPA) to the compartment paired with naloxone administration in a two-chamber apparatus. The conditioning phase consisted of three pairings of either naloxone (0.250 mg/kg) or vehicle in one compartment, both with similar time allotments during the preconditioning test. During the testing phase, mice were again allowed to explore the entire apparatus. GHB (6, 12.5, 25, and 50 mg/kg) was administered during either the acquisition or expression phase of this conditioning. In a second experiment, the capacity of GHB to ameliorate the intensity of physical signs of morphine withdrawal was evaluated.

RESULTS

GHB blocked CPA in both phases: administered during acquisition (from 12.5 mg/kg and higher) as well as in the expression phase (from 6 mg/kg, except for 25 mg/kg). It also decreased the intensity of physical signs of morphine withdrawal to near control levels measured by the modified Gellert-Holtzman scale (25 mg/kg and higher). Decreases in jumping, body shakes, and paw tremor were also observed.

CONCLUSIONS

Our results support the idea that GHB ameliorates both aspects of morphine withdrawal, physical as well as motivational signs.

GHB: a new and novel drug of abuse

There has been increasing attention in the United States to problems of abuse of gamma-hydroxybutyrate (GHB), with some evidence for problems in other parts of the world as well. In vitro and animal research show that, while GHB shares some properties with abused central nervous system depressant drugs, it has unique aspects of its pharmacology as well, including actions at a specific neural receptor which probably mediates many of its effects. Abuse potential assessment of GHB using standard animal models has not yielded a picture of a highly abusable substance, but little human testing has yet been done. Very little systematic data exist on tolerance and dependence with GHB, but both have been seen in human users. Quantitative data on the prevalence of GHB abuse is incomplete, but various qualitative measures indicate that a mini-epidemic of abuse began in the late 1980s and continues to the present. GHB is often included with the group of 'club drugs', and can be used as an intoxicant. It also has been used as a growth promoter and sleep aid and has been implicated in cases of 'date rape', usually in combination with alcohol. Undoubtedly the easy availability of GHB and some of its precursors has contributed to its popularity. Recent changes in the control status of GHB in the US may reduce its availability with as yet unknown consequences for the scope of the public health problem. Drug abuse experts need to familiarize themselves with GHB as possibly representing a new type of drug abuse problem with some unique properties.

Gamma-hydroxybutyric acid: an evaluation of its rewarding properties in rats and mice

Gamma-hydroxybutyric acid, an endogenous compound present in mammalian brain and supposed to be a neurotransmitter or neuromodulator, has been shown to affect several aspects of dependence from some drugs of abuse. It has been successfully used in clinical practice to alleviate both alcohol and opiate withdrawal symptoms. The aim of this study was to investigate whether gamma-hydroxybutyric acid possesses rewarding properties by means of conditioned place preference and intravenous self-administration paradigms. In the present study, gamma-hydroxybutyric acid induced conditioned place preference in rats, was intravenously self-administered by drug-naive mice, and altered cocaine intravenous self-administration in rats. Although to date the physiological role of this compound still remains unclear, there is no doubt that gamma-hydroxybutyric acid, in addition to its proved effect on alcohol and opiate dependence, possesses reinforcing properties of its own and may interfere with the neurochemical events in the rewarding effects produced by psychostimulant drugs. Our investigation points out the abuse liability of this drug, suggesting the use of particular precaution in handling gamma-hydroxybutyric acid as a clinically useful drug.

Prodynorphin and proenkephalin mRNAs are increased in rat brain after acute and chronic administration of gamma-hydroxybutyrate

The effects of gamma-hydroxybutyrate (GHB) on prodynorphin (PD) and proenkephalin (PE) mRNA expression were examined using in situ hybridization histochemistry in discrete rat brain structures rich in GHB receptors. A single dose of GHB (500 mg/kg i.p.) increased striatal PE mRNA levels (+60%) between 15 and 90 min after injection. An increase in PD mRNA expression was observed in the frontal cortex (+90%) 6 h after GHB administration. Chronic exposure to GHB (500 mg/kg i.p. twice a day) for 10 days induced significant increases in both PE and PD mRNA levels in different brain regions examined, suggesting that PD and PE mRNA expressions are modulated by the endogenous GHBergic system.

Gamma-hydroxybutyric acid decreases intravenous cocaine self-administration in rats

Gamma-hydroxybutyric acid (GHB) is an endogenous compound present in mammalian brain suggested as a putative neurotransmitter, which has been shown to affect several aspects of dependence from various classes of drugs of abuse. In the present study, two sets of experiments were performed to investigate the effects of acute pretreatment with GHB on intravenous cocaine self-administration in rats. In the first experiment GHB was administered intragastrically at the doses of 175, 350, and 700 mg/kg to Long-Evans rats trained to self-administer cocaine using nose-poke as operandum. In the second experiment, GHB was administered intraperitoneally at the doses of 100, 200, and 400 mg/kg to Wistar rats trained to self-administer cocaine intravenously using lever-pressing as operandum. In both experiments acute pretreatment with GHB significantly and dose dependently reduced cocaine self-administration. The effectiveness of GHB was similar in both experiments, indicating that the effect of GHB on cocaine self-administration is independent of animal strain. route of administration, and type of operant response required. These results indicate that GHB reduces cocaine-seeking behavior in rats, modulating the acute reinforcing effect of cocaine. The clinical effectiveness of GHB in dependence from various classes of abused drugs warrants further studies to evaluate the possibility that GHB might represent a useful therapeutic agent for cocaine addiction in humans.

The anxiolytic effect of gamma-hydroxybutyrate in the elevated plus maze is reversed by the benzodiazepine receptor antagonist, flumazenil

The effects of gamma-hydroxybutyrate (GHB), a product of gamma-aminobutyric acid (GABA) metabolism which possesses neuromodulatory properties in brain, were investigated in the elevated plus maze in rats. The number of entries and the time spent in the open arms of the maze were increased by GHB (50, 150, 250 mg/kg i.p.). This is classically considered as indicative of an anxiolytic effect of the drug. There was no sedative effect at these doses as measured by the spontaneous locomotor activity in the actimeter or the total number of arm entries. The anxiolytic properties of GHB were reversed by neither the GHB receptor antagonist, NCS-382 (6,7,8,9-tetrahydro-5(H)-5-olylidene acetic acid) (300 mg/kg i.p.), nor the opioid receptor antagonist, naloxone (10 mg/kg i.p.). However the anti-anxiety effect of GHB was antagonized by the benzodiazepine receptor antagonist, flumazenil (10 mg/kg i.p.), suggesting an interaction of GHB with the GABA(A) receptor complex which mediates the anti-anxiety effect of benzodiazepines.

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.

Effect of gamma-hydroxybutyrate on central dopamine release in vivo. A microdialysis study in awake and anesthetized animals

gamma-Hydroxybutyrate (GHB) is generally considered to be an inhibitor of striatal dopamine (DA) release. However, a number of recent reports and at least one major review suggest that GHB enhances rather than inhibits striatal DA release. To examine this discrepancy, the effect of GHB on striatal DA release was monitored for 2 hr by microdialysis in awake and urethane-anesthetized rats. GHB (500 mg/kg, i.p.) significantly inhibited striatal DA release in conscious animals. However, anesthetic pretreatment completely abolished the inhibitory effect of GHB on DA release. In urethane-anesthetized animals, intraperitoneal injections of GHB resulted in a dialysis DA output that was the same as basal and saline control levels for all but the last three intervals where DA release was elevated slightly. In contrast to the intraperitoneal route, subcutaneous injections of GHB in anesthetized animals produced significant elevations of DA release above baseline levels. The increases ranged from 125 to 133% of basal levels. These results indicate that while GHB enhances striatal DA release in anesthetized animals, it inhibits rather than enhances this release in awake animals. This would explain why GHB induces an inhibition of DA-release-dependent behaviors rather than an enhancement. The results also indicate that the route of GHB administration influences its effects on striatal DA release, at least in anesthetized animals.

Does gamma-hydroxybutyrate inhibit or stimulate central DA release?

Gamma-hydroxybutyrate (GHB), a four-carbon fatty acid and anaesthetic, is widely considered to be a relatively specific inhibitor of central dopamine (DA) release. The inhibitory effect of GHB on the latter is thought to occur as a consequence of its diminution of impulse flow in central dopaminergic neurons. However, a number of studies have recently reported that GHB primarily stimulates rather than inhibits central DA release, with any inhibitory effect produced of a modest and transitory nature. GHB has been and continues to be widely used as an important research tool largely because it is one of only a few drugs available that acts primarily on DA release. Consequently, it is important to determine whether GHB inhibits DA release as previously thought, or stimulates DA release, as more recently suggested. Following a critical review of the literature, the present report suggests that GHB does inhibit rather than stimulate presynaptic DA release in consonance with its behavioral and pharmacological activity. Recent in vivo studies indicating that GHB stimulates DA release were done under anaesthesia or in the presence of a high concentration of calcium. Both conditions have been found to spuriously enhance striatal DA release in vivo, which may account for the failure of some studies to observe an inhibitory effect of GHB on DA release in vivo.

Naloxone reverses the inhibitory effect of gamma-hydroxybutyrate on central DA release in vivo in awake animals: a microdialysis study

gamma-Hydroxybutyrate (GHB) is a 4-carbon anesthetic that acts primarily by inhibiting presynaptic dopamine (DA) release in vivo. A number of studies have reported a reversal of many of the central effects of GHB by the allegedly pure opiate antagonist naloxone (NX) but its mechanism of action is unclear. In vivo microdialysis performed in the present preliminary study disclosed a significant inhibitory effect of GHB (500 mg/kg) on striatal DA release which was completely reversed by a low dose of NX (0.8 mg/kg). The results indicate that NX likely inhibits many of the central effects produced by GHB primarily through its reversal of the GHB induced inhibition of central DA release.

Lack of effect of gamma-hydroxybutyrate on mu, delta and kappa opioid receptor binding

gamma-Hydroxybutyrate (GHB) and morphine induce a number of similar effects. Moreover, the effects they elicit can be reversed by the opiate antagonist naloxone (NX), suggesting that GHB may produce at least some of its central effects by acting as an opiate agonist. The present study considered this possibility by examining the effect of GHB on mu, delta, and kappa-opioid receptor binding in concentrations of 1 nM-0.1 mM. GHB was inactive in each instance, at every dose examined. GHB is consequently not a direct opiate receptor agonist. It is also unlikely to be an indirect (enkephalin or dynorphin release-stimulating) agonist. The mechanism of action involved whereby NX can reverse the effects of GHB must therefore not involve opioid mechanisms; at least not directly.

Unilateral 6-hydroxydopamine lesions of meso-striatal dopamine neurons and their physiological sequelae

One of the primary approaches in experimental brain research is to investigate the effects of specific destruction of its parts. Here, several neurotoxins are available which can be used to eliminate neurons of a certain neurochemical type or family. With respect to the study of dopamine neurons in the brain, especially within the basal ganglia, the neurotoxin 6-hydroxydopamine (6-OHDA) provides an important tool. The most common version of lesion induced with this toxin is the unilateral lesion placed in the area of mesencephalic dopamine somata or their ascending fibers, which leads to a lateralized loss of striatal dopamine. This approach has contributed to neuroscientific knowledge at the basic and clinical levels, since it has been used to clarify the neuroanatomy, neurochemistry, and electrophysiology of mesencephalic dopamine neurons and their relationships with the basal ganglia. Furthermore, unilateral 6-OHDA lesions have been used to investigate the role of these dopamine neurons with respect to behavior, and to examine the brain's capacity to recover from or compensate for specific neurochemical depletions. Finally, in clinically-oriented research, the lesion has been used to model aspects of Parkinson's disease, a human neurodegenerative disease which is neuronally characterized by a severe loss of the meso-striatal dopamine neurons. In the present review, which is the first of two, the lesion's effects on physiological parameters are being dealt with, including histological manifestations, effects on dopaminergic measures, other neurotransmitters (e.g. GABA, acetylcholine, glutamate), neuromodulators (e.g. neuropeptides, neurotrophins), electrophysiological activity, and measures of energy consumption. The findings are being discussed especially in relation to time after lesion and in relation to lesion severeness, that is, the differential role of total versus partial depletions of dopamine and the possible mechanisms of compensation. Finally, the advantages and possible drawbacks of such a lateralized lesion model are discussed.

Displacement of [3H] gamma-hydroxybutyrate binding by benzamide neuroleptics and prochlorperazine but not by other antipsychotics

Since gamma-hydroxybutyrate receptor agonists exhibit dopaminergic regulatory properties and neuroleptic-like effects in neuropharmacological tests, the common neuroleptics were tested for [3H] gamma-hydroxybutyrate binding activity on rat brain membranes. (-)-Sulpiride, sultopride, amisulpride and prochlorperazine possess affinity for the gamma-hydroxybutyrate site(s), consistent with their therapeutic dosage. This study has revealed that gamma-hydroxybutyrate receptors represent an additional target for antipsychotics.