gamma-Butyrolactone-sensitive and -insensitive dopamine release, and their relationship to dopamine metabolism in three rat brain regions

Anti-aggressive effects of GHB in OF.1 strain mice: involvement of dopamine D2 receptors

Numerous studies indicate that gamma-hydroxybutyric acid (GHB) influences the endogenous dopamine system. Both GHB and most dopaminergic D(2) receptor antagonists are effective anti-aggressive agents in animal models. The present study aimed to investigate the effects of GHB on agonistic behaviour and to implicate D(2) dopamine receptor on these behaviours. For this purpose, the effects of GHB (80, 120 and 160 mg/kg, IP) and tiapride (60 mg/kg) administered alone or in combination were examined on agonistic behaviour elicited by 'isolation' in male mice. Individually housed mice were exposed to anosmic "standard opponents" 30 min after drug administration, and the encounters were videotaped and evaluated using an ethologically based analysis. The administration of 80 and 120 mg/kg of GHB reduced threat without impairing motor activity, but the administration of 160 mg/kg of GHB or the co-administration of GHB+tiapride (a selective D(2) receptor antagonist) significantly reduced threat and attack but concomitantly increased immobility. The co-administration of GHB+tiapride had different effects to those observed by the administration of these drugs separately. It is concluded that the anti-aggressive effect of GHB appears to be mediated, at least in part, by D(2) dopamine receptors. This anti-dopaminergic activity is an indirect effect, probably induced by the activation of GHB receptors of low affinity, and in this way, this compound would reduce levels of dopamine without blockading of D(2) postsynaptic dopamine receptors.

Sites of action of gamma-hydroxybutyrate (GHB)--a neuroactive drug with abuse potential

OBJECTIVE

This review highlights the biochemistry, pharmacology, and toxicology of the naturally-occurring fatty acid derivative, gamma-hydroxybutyrate (GHB). GHB is derived from gamma-aminobutyric acid (GABA) and is proposed to function as an inhibitory chemical transmitter in the central nervous system.

CONTENT

When administered in pharmacological doses, its powerful central nervous system depressant effects are readily observed. Although some of the neurophysiological actions of GHB could involve alterations in dopaminergic transmission in the basal ganglia, both its physiological and pharmacological actions are probably mediated through specific brain receptors for GHB. In addition, GHB might mediate some of its effects through interaction with the GABA(B) receptor. Experimentally, GHB has been used as a model for petit mal epilepsy; clinically, it has been used as a general anesthetic and as a drug to treat certain sleep disorders and related conditions. Owing to the purported ability of GHB to induce a state of euphoria, recreational use of this substance is popular. Although no deaths or long-term problems have been associated with GHB abuse, symptoms of GHB intoxication can be severe. The continued potential for GHB abuse makes it imperative for clinical toxicologists to be aware of the effects of this agent. Future research on the mechanism of action of GHB is needed to elucidate both its central nervous system depressant properties and its ability to effect a state of well-being.

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.

Effects of chronic neuroleptic treatment on dopamine release: insights from studies using 3-methoxytyramine

Antipsychotic medications appear to exert their therapeutic effects by blocking D2 receptors. While D2 blockade occurs rapidly, reduction in psychotic symptoms is often delayed. This time discrepancy has been attributed to the relatively slow development of depolarization inactivation (DI) of dopaminergic neurons. The reduced firing rates associated with DI has been hypothesized to reduce dopamine release and thus psychotic symptoms. Studies assessing changes in dopamine release during chronic neuroleptic treatment, using microdialysis and voltammetry, have been inconsistent. This may be due to methodological differences between studies, the invasive nature of these procedures, or other confounds. To investigate the effects of DI on dopamine release, 3-MT accumulation, an index of dopamine release that does not involve disruption of brain tissue, was measured during acute and chronic neuroleptic treatment. These results are compared with those using other techniques. 3-MT levels remained elevated after chronic treatment, suggesting that DI does not markedly reduce release. Regulation of dopamine release during DI was examined using two techniques known to block dopamine neuronal impulse flow. 3-MT levels were markedly reduced by both, implying that DI does not alter the portion of dopamine release mediated by neuronal impulse flow. Overall, studies to date suggest that the delayed therapeutic effects of neuroleptics are not due to reductions in impulse dependent dopamine release. Recent studies using a neurodevelopmental animal model of schizophrenia have pointed to altered pre- and post-synaptic indices of dopamine neurotransmission. The results suggest that neuroleptics may exert their therapeutic effects, in part, by limiting the fluctuations in dopamine release, and raise new issues for future research.

Effect of gamma-hydroxybutyrate and its antagonist NCS-382 on spontaneous cell firing in the prefrontal cortex of the rat

Gamma-hydroxybutyrate (GHB) at low doses (5-10 mg/kg i.p.) increased and at high doses (160-320 mg/kg i.p.) decreased the spontaneous firing rate of prefrontal cortex (PFC) neurons recorded in urethane-anesthetized rats. Only excitations were blocked by NCS-382, a specific GHB receptor antagonist; this suggests that the excitatory effect of low doses of GHB is mediated by a GHB receptor whereas the inhibitory effect of high doses of GHB involves a more complex mechanism.

A study of the mechanism of MDMA ('ecstasy')-induced neurotoxicity of 5-HT neurones using chlormethiazole, dizocilpine and other protective compounds

1. An investigation has been made in rats into the neurotoxic effect of the relatively selective 5-hydroxytryptamine (5-HT) neurotoxin, 3,4-methylenedioxymethamphetamine (MDMA or 'Ecstasy') using chlormethiazole and dizocilpine, both known neuroprotective compounds and also gamma-butyrolactone, ondansetron and pentobarbitone. 2. Administration of MDMA (20 mg kg-1, i.p.) resulted in a 50% loss of cortical and hippocampal 5-HT and 5-hydroxyindole acetic acid (5-HIAA) 4 days later. This reflects the long term neurotoxic loss of 5-HT that occurs. Injection of gamma-butyrolactone (GBL; 400 mg kg-1, i.p.) 5 min before and 55 min after the MDMA provided substantial protection. Pentobarbitone (25 mg kg-1, i.p.) using the same dose regime was also protective, but ondansetron (0.5 mg kg-1 or 0.1 mg kg-1, i.p.) was without effect. 3. MDMA (20 mg kg-1) had no significant effect on striatal dopamine concentration 4 days later but did produce a small decrease in 3,4-dihydroxyphenylacetic acid (DOPAC) content. There were few significant changes in rats given MDMA plus GBL, ondansetron or pentobarbitone. 4. A single injection of MDMA (20 mg kg-1, i.p.) resulted in a greater than 80% depletion of 5-HT in hippocampus and cortex 4 h later, reflecting the initial rapid release that had occurred. None of the neuroprotective compounds (chlormethiazole, 50 mg kg-1; dizocilpine, 1 mg kg-1; GBL, 400 mg kg-1; pentobarbitone, 25 mg kg-1) given 5 min before and 55 min after the MDMA injection, altered the degree of 5-HT loss. 5. Acute MDMA injection increased striatal dopamine content (28%) and decreased the DOPAC content. In general, administration of the drugs under investigation did not significantly alter these MDMA-induced changes. Both chlormethiazole and GBL produced a greater increase in dopamine than MDMA alone, but this was apparently an additive effect to the action of either drug alone. 6. The 5-HT loss 4 h following administration of the neurotoxin p-chloroamphetamine (2.5 mg kg-1,i.p.) was not affected by chlormethiazole or dizocilpine. p-Chloroamphetamine did not appear to alter striatal dopamine metabolism.7. None of the protective drugs inhibited the initial 5-HT loss following MDMA, rendering unlikely any proposal that they are protective because they inhibit 5-HT release and the subsequent formation ofa toxic indole derivative. All the protective compounds (unlike ondansetron) probably inhibit dopamine release in the striatum. Since the neurotoxic action of some substituted amphetamines is dependent on the integrity of nigro-striatal neurones, this fact may go some way to explain the protective action of this diverse group of compounds.

Interacting presynaptic kappa-opioid and GABAA receptors modulate dopamine release from rat striatal synaptosomes

The presynaptic regulation of stimulated dopamine release from superfused rat striatal synaptosomes by opioids and gamma-aminobutyric acid (GABA) was studied. It was found that in addition to dopamine D2 autoreceptors, calcium-dependent K(+)-stimulated [3H]dopamine release was inhibited through activation of a homogeneous population of kappa-opioid receptors in view of the potent inhibitory effect of the kappa-selective agonist U69,593 (EC50 0.2 nM) and its antagonism by norbinaltorphimine. Neither mu- nor delta-selective receptor agonists affected release of [3H]-dopamine. In addition, GABA potently inhibited the evoked [3H]dopamine release (EC50 0.4 nM) through activation of GABAA receptors in view of the GABA-mimicking effect of muscimol, the sensitivity of its inhibitory effect to picrotoxin and bicuculline, and the absence of an effect of the GABAB receptor agonist baclofen. In the presence of a maximally effective concentration of GABA, U69,593 did not induce an additional release-inhibitory effect, indicating that these receptors and the presynaptic D2 receptor are colocalized on the striatal dopaminergic nerve terminals. The excitatory amino acid agonists N-methyl-D-aspartate and kainate, as well as the cholinergic agonist carbachol, stimulated [3H]dopamine release, which was subject to kappa-opioid receptor-mediated inhibition. In conclusion, striatal dopamine release is under regulatory control of multiple excitatory and inhibitory neurotransmitter by activation of colocalized presynaptic receptors for excitatory amino acids, acetylcholine, dopamine, dynorphins, and GABA within the dopaminergic nerve terminals. Together, these receptors locally control ongoing dopamine neurotransmission.

Haloperidol and clozapine increase intraneuronal dopamine metabolism, but not gamma-butyrolactone-resistant dopamine release

In a previous study, two coexisting mechanisms of dopamine release were identified in dopamine neuron terminals. One can be blocked with gamma-butyrolactone, suggesting it is impulse flow-dependent, while the other one cannot and is apparently impulse flow-independent. The goal of this study was to further characterize the gamma-butyrolactone-resistant mechanism and its relation to dopamine metabolism. Following acute and chronic haloperidol or clozapine treatment, gamma-butyrolactone was given to block dopamine neuronal impulse flow. In all groups, 3-methoxytyramine levels after monoamine oxidase inhibition with pargyline (an index of dopamine release) were measured in the frontal cortex, nucleus accumbens and striatum. Regional steady-state levels of dopamine, 3,4-dihydroxyphenylacetic acid (DOPAC) and homovanillic acid (HVA) were also measured in the rats treated acutely with neuroleptics. In all three regions, gamma-butyrolactone reduced the 3-methoxytyramine levels by over 50% after chronic neuroleptic treatment. This indicates that dopamine release from the terminals is primarily impulse flow-dependent during chronic neuroleptic treatment, both in the dopamine neurons which do undergo depolarization block, and in those that do not. No neuroleptic effect on the gamma-butyrolactone-resistant dopamine release was observed, while DOPAC and HVA were markedly elevated by the acute treatment, suggesting a predominant production of these metabolites from unreleased dopamine.