Gamma-aminobutyric acidA receptor function is desensitised in rat cultured cerebellar granule cells following chronic flunitrazepam treatment

Cannabigerolic acid, a major biosynthetic precursor molecule in cannabis, exhibits divergent effects on seizures in mouse models of epilepsy

Background and Purpose

Cannabis has been used to treat epilepsy for millennia, with such use validated by regulatory approval of cannabidiol (CBD) for Dravet syndrome. Unregulated artisanal cannabis‐based products used to treat children with intractable epilepsies often contain relatively low doses of CBD but are enriched in other phytocannabinoids. This raises the possibility that other cannabis constituents might have anticonvulsant properties.

Experimental Approach

We used the Scn1a^+/− mouse model of Dravet syndrome to investigate the cannabis plant for phytocannabinoids with anticonvulsant effects against hyperthermia‐induced seizures. The most promising, cannabigerolic acid (CBGA), was further examined against spontaneous seizures and survival in Scn1a^+/− mice and in electroshock seizure models. Pharmacological effects of CBGA were surveyed across multiple drug targets.

Key Results

The initial screen identified three phytocannabinoids with novel anticonvulsant properties: CBGA, cannabidivarinic acid (CBDVA) and cannabigerovarinic acid (CBGVA). CBGA was most potent and potentiated the anticonvulsant effects of clobazam against hyperthermia‐induced and spontaneous seizures, and was anticonvulsant in the MES threshold test. However, CBGA was proconvulsant in the 6‐Hz threshold test and a high dose increased spontaneous seizure frequency in Scn1a^+/− mice. CBGA was found to interact with numerous epilepsy‐relevant targets including GPR55, TRPV1 channels and GABA_A receptors.

Conclusion and Implications

These results suggest that CBGA, CBDVA and CBGVA may contribute to the effects of cannabis‐based products in childhood epilepsy. Although these phytocannabinoids have anticonvulsant potential and could be lead compounds for drug development programmes, several liabilities would need to be overcome before CBD is superseded by another in this class.

The effects of zolpidem treatment on GABA(A) receptors in cultured cerebellar granule cells: changes in functional coupling

AIMS

Hypnotic zolpidem is a positive allosteric modulator of γ-aminobutyric acid (GABA) action, with preferential although not exclusive binding for α1 subunit-containing GABA(A) receptors. The pharmacological profile of this drug is different from that of classical benzodiazepines, although it acts through benzodiazepine binding sites at GABA(A) receptors. The aim of this study was to further explore the molecular mechanisms of GABA(A) receptor induction by zolpidem.

MAIN METHODS

In the present study, we explored the effects of two-day zolpidem (10 μM) treatment on GABA(A) receptors on the membranes of rat cerebellar granule cells (CGCs) using [(3)H]flunitrazepam binding and semi-quantitative PCR analysis.

KEY FINDINGS

Two-day zolpidem treatment of CGCs did not significantly affect the maximum number (B(max)) of [(3)H]flunitrazepam binding sites or the expression of α1 subunit mRNA. However, as shown by decreased GABA [(3)H]flunitrazepam binding, two-day exposure of CGCs to zolpidem caused functional uncoupling of GABA and benzodiazepine binding sites at GABA(A) receptor complexes.

SIGNIFICANCE

If functional uncoupling of GABA and benzodiazepine binding sites at GABA(A) receptors is the mechanism responsible for the development of tolerance following long-term administration of classical benzodiazepines, chronic zolpidem treatment may induce tolerance.

The effects of zolpidem treatment and withdrawal on the in vitro expression of recombinant alpha1beta2gamma2s GABA(A) receptors expressed in HEK 293 cells

Zolpidem, a widely used hypnotic drug which acts through benzodiazepine binding sites, is a positive allosteric modulator of gamma-aminobutyric acid (GABA) action with preferential affinity for GABA(A) receptors containing alpha1 subunit. The pharmacological profile of zolpidem is different from that of classical benzodiazepines. The aim of this study was to find out whether zolpidem treatment triggers adaptive changes in the recombinant alpha1 subunit-containing GABA(A) receptors other than those observed following treatment with classical benzodiazepine-diazepam. Radioligand binding studies showed that 2-day exposure of human embryonic kidney (HEK) 293 cells stably expressing recombinant alpha1beta2gamma2s GABA(A) receptors to zolpidem (10 muM) up-regulated the maximum number (B (max)) of [(3)H]flunitrazepam, [(3)H]muscimol, and [(3)H]t-butylbicycloorthobenzoate ([(3)H]TBOB) binding sites without changing their affinity (K (d)), suggesting an increase in total GABA(A) receptor number. Semi-quantitative RT-PCR analysis demonstrated increased levels of alpha1 subunit mRNA, while Western blot demonstrated up-regulated gamma2 subunit proteins, suggesting that zolpidem induced de novo synthesis of receptors proteins, at both the transcriptional and translational levels. GABA-induced potentiation of [(3)H]flunitrazepam binding to membranes obtained from zolpidem-treated cells was markedly reduced, indicating allosteric uncoupling between GABA and benzodiazepine binding sites. The number of benzodiazepine and convulsant binding sites as well as the functional coupling between GABA and benzodiazepine binding sites normalized in 24 h following discontinuation of zolpidem treatment. The results of our in vitro studies suggest that a 2-day exposure of recombinant alpha1 subunit-containing GABA(A) receptors stably transfected in HEK 293 cells to zolpidem induces adaptive changes in this selective GABA(A) receptor subtype, which are not substantially different from those obtained after prolonged exposure of cells to high concentrations of diazepam.

Regulation of GABA(A) receptor subunit expression by pharmacological agents

The gamma-aminobutyric acid (GABA) type A receptor system, the main fast-acting inhibitory neurotransmitter system in the brain, is the pharmacological target for many drugs used clinically to treat, for example, anxiety disorders and epilepsy, and to induce and maintain sedation, sleep, and anesthesia. These drugs facilitate the function of pentameric GABA(A) receptors that exhibit widespread expression in all brain regions and large structural and pharmacological heterogeneity as a result of composition from a repertoire of 19 subunit variants. One of the main problems in clinical use of GABA(A) receptor agonists is the development of tolerance. Most drugs, in long-term use and during withdrawal, have been associated with important modulations of the receptor subunit expression in brain-region-specific manner, participating in the mechanisms of tolerance and dependence. In most cases, the molecular mechanisms of regulation of subunit expression are poorly known, partly as a result of neurobiological adaptation to altered neuronal function. More knowledge has been obtained on the mechanisms of GABA(A) receptor trafficking and cell surface expression and the processes that may contribute to tolerance, although their possible pharmacological regulation is not known. Drug development for neuropsychiatric disorders, including epilepsy, alcoholism, schizophrenia, and anxiety, has been ongoing for several years. One key step to extend drug development related to GABA(A) receptors is likely to require deeper understanding of the adaptational mechanisms of neurons, receptors themselves with interacting proteins, and finally receptor subunits during drug action and in neuropsychiatric disease processes.

Treatment with haloperidol and diazepam alters GABA(A) receptor density in the rat brain

A significant body of data suggests that GABA(A) receptors are altered in the CNS of subjects with schizophrenia. However, subjects with schizophrenia are treated with antipsychotic drugs and, in some cases, antipsychotic drugs and benzodiazepines. It has therefore been suggested that the changes in GABA(A) receptors in the CNS of subjects with schizophrenia are due to such drug treatments. Surprisingly, there appear to be no studies to determine the effect of a combined antipsychotic-benzodiazepine treatment on GABA(A) receptors. We therefore measured both the GABA binding site ([3H]muscimol) and the benzodiazepine binding site ([3H]flumazenil) in the CNS of rats treated with either haloperidol, diazepam or a combination of the two drugs. The main findings of our study are that treatment with diazepam or the combination of diazepam and haloperidol results in regionally selective increases GABA binding sites but treatment with haloperidol alone decreases the GABA binding site in the thalamus but increases these sites in the hypothalamus. By contrast, treatment with diazepam, haloperidol and a combination of the two drugs resulted in widespread decreases in the number of benzodiazepine binding sites in the rat CNS. The notable exception to this outcome was increased numbers of benzodiazepine binding sites in the frontal cortex of rats that had received diazepam. Our data suggests that there are complex changes in the GABA(A) receptor following treatment with haloperidol, diazepam or a combination of these drugs. This outcome may be relevant to the therapeutic benefits of using both drugs in conjunction early in the treatment of a psychotic episode.

Allosteric uncoupling and up-regulation of benzodiazepine and GABA recognition sites following chronic diazepam treatment of HEK 293 cells stably transfected with alpha1beta2gamma2S subunits of GABA (A) receptors

Benzodiazepines are drugs known to produce tolerance and dependence and also to be abused and co-abused. The aim of this study was to further explore the mechanisms that underlie adaptive changes in GABA(A) receptors following prolonged exposure to these drugs. Human embryonic kidney (HEK 293) cells stably expressing recombinant alpha1beta2gamma2s GABA(A) receptors were exposed for 72 h to a high concentration of diazepam (50 microM) in the absence or presence of other drugs. Radioligand binding studies were used to determine the parameters of [(3)H]flunitrazepam and [(3)H]muscimol binding sites and allosteric interactions between these sites. Prolonged treatment with diazepam increased the maximum number (B (max)) of [(3)H]flunitrazepam and [(3)H]muscimol binding sites in the membranes, and of [(3)H]muscimol binding sites on the surface of HEK 293 cells. There was no change in the affinity (K (d)) of binding sites. The diazepam-induced increase in the B (max) value of [(3)H]flunitrazepam binding sites was reduced by two GABA(A) receptor antagonists, gabazine (1 and 10 microM) and picrotoxin (100 microM). In addition, it was reduced by cycloheximide (5 microg/ml), a protein synthesis inhibitor, and actinomycin D (7.5 microg/ml), an RNA synthesis inhibitor. Flumazenil (5 microM), the antagonist of benzodiazepine binding sites, also up-regulated [(3)H]flunitrazepam recognition sites. Simultaneous treatment with diazepam and flumazenil failed to produce an additive up-regulation. GABA (1 nM - 1 mM)-induced potentiation of [(3)H]flunitrazepam binding to membranes obtained from diazepam (50 microM)-pretreated cells was markedly reduced, suggesting functional uncoupling between GABA and benzodiazepine binding sites. The results suggest that diazepam up-regulated benzodiazepine binding sites on stably expressed GABA(A) receptors by stimulating their synthesis at both the transcriptional and translational levels. A comparable increase of [(3)H]muscimol binding sites expressed on the surface of intact HEK 293 cells suggests that internalisation of surface receptors presumably can not explain the uncoupling.

Changes of brain neuropeptide Y and its receptors in rats with flurazepam tolerance and dependence

AIM

Anticonvulsant tolerance and dependence are two obstacles that restrict the clinical use of benzodiazepines (BDZ). In order to explore the mechanism of these two adverse reactions, changes of neuropeptide Y (NPY) and its receptors in the hippocampus of rat models, in relation to flurazepam (FZP, a member of BDZ) tolerance and dependence, were investigated.

METHODS

The mRNA of preproNPY and its receptors (Y1, Y2, and Y5) in the hippocampus were determined by competitive RT-PCR, and the distribution of NPY in the hippocampus was examined by immunohistochemistry.

RESULTS

A decrease of preproNPY mRNA in the hippocampus was found in tolerant and dependent rats. The level of preproNPY mRNA in the hippocampus was reversely correlated with the degree of tolerance and dependence, measured by the threshold of pentylenetetrazol-induced seizures. Immunohistochemistry indicated a decrease of NPY-immunoreactive material in neurons of the CA1, CA3, and dentate gyrus regions of both tolerant and dependent rats. The mRNA of NPY receptors Y1 and Y5 decreased in tolerant rats but did not change in dependent rats. The mRNA of NPY receptor Y2 increased in tolerant rats but decreased in dependent rats.

CONCLUSION

A decrease of NPY in the hippocampus might be involved in anticonvulsant tolerance and dependence following long-term treatment with FZP. Y1, Y2, and Y5 mRNA were also altered in FZP tolerance and dependence.