Modest reduction of benzodiazepine binding in rat brain in vivo induced by antisense oligonucleotide to GABAA receptor gamma 2 subunit subtype

Intrahippocampal infusion of antisense oligodeoxynucleotide to the GABA(A) receptor gamma2 subunit enhances neuropeptide Y gene expression

The effects of hippocampal treatment with a phosphorothioate oligodeoxynucleotide (ODN) antisense to the gamma-aminobutyric acid (GABA)A receptor gamma2 subunit on neuropeptide Y (NPY) were studied. Adult male Wistar rats were treated with unilateral intrahippocampal infusion of gamma2 subunit antisense ODN for 5 days. Rats infused with mismatch ODN and naïve rats served as controls. Brain sections were analysed for levels of NPY mRNA by in situ hybridisation, NPY-immunoreactivity (NPY-ir) by means of immunocytochemistry, and specific NPY binding sites by in vitro receptor autoradiography. Following infusion of antisense ODN, a marked increase in cytoplasmic NPY-ir was observed in hilar neurones of the fascia dentata. Further, intense NPY-ir was visualised in the mossy fibres and in cell bodies of the entorhinal cortex and throughout the neocortex. High levels of NPY mRNA were detected in the same cortical areas of antisense treated rats. A very large increase was observed in the piriform and parietal areas. NPY gene expression also occurred in the granular cell layer, in which no NPY mRNA could be detected in normal animals. The level and distribution of cells displaying high levels of NPY mRNA differed among animals, perhaps as a result of the distinct anatomical location of ODN infusion. Finally, hippocampal levels of NPY specific binding increased, suggesting that NPY neurotransmission is markedly increased. These findings are reminiscent of reported changes in the expression of NPY mRNA and immunoreactivity in conditions of increased neuronal excitation and support the usefulness of the present animal model for the study of epileptic phenomena.

Antidepressant effects on GABA-stimulated 36Cl(-) influx in rat cerebral cortex are altered after treatment with GABA(A) receptor antisense oligodeoxynucleotides

Antidepressants act at the GABA(A) receptor to inhibit GABA-stimulated 36Cl(-) influx and GABA reduction of [35S]TBPS binding. This study examined how selective knock-down (via antisense oligodeoxynucleotides, aODNs) of GABA(A) receptor subunits modified antidepressant activity. The specific aODNs used were for the alpha1, beta1, beta2 or gamma2 subunits of the GABA(A) receptor. The aODN microinjections reduced corresponding GABA(A) receptor subunit mRNA levels by 30-40% as assessed by RT-PCR. The inhibitory effect of the antidepressants amitriptyline and mianserin on GABA-stimulated 36Cl(-) influx was decreased after microinjections of alpha1, beta1, or beta2 subunit aODNs but potentiated after microinjections of gamma2 subunit aODNs. This pattern of aODNs effect on amitriptyline and mianserin modulation of GABA-stimulated 36Cl(-) influx was the same for both antidepressants and similar to GABA but different than that of diazepam and bicuculline. We conclude that multiple subunits of the GABA(A) receptor regulate the effect of amitriptyline and mianserin on the GABA(A) receptor chloride ionophore complex. However, the exact identity of the subunit mediating the direct or allosteric modulation of the antidepressant effect on GABA-stimulated 36Cl(-) influx remains unclear.

GABA-transaminase antisense oligodeoxynucleotide modulates cocaine- and pentylenetetrazol-induced seizures in mice

The mechanism of action of many anticonvulsive agents is to increase the function of the GABAergic system. Inhibition of GABA-Transaminase (GABA-T), the degradative enzyme for GABA, increases GABA levels in the brain. In this study, antisense oligodeoxynucleotides (ASO) targeted at the start codon region of GABA-Transaminase mRNA were used to modify seizure activity. Mice were treated, by intracerebroventricular injection, with antisense oligos or appropriate controls. At various times after treatment, the animals were challenged with cocaine (70 mg/kg, i.p.) and observed for seizure activity. At 15 hours after treatment, 1.152 and 1.44 nmol antisense oligo blocked cocaine-induced seizures. There was no effect of antisense oligo 8 or 36 hours after treatment. In addition, treatment with 7.2 nmol antisense oligo prevented pentylenetetrazol-induced seizures. These data demonstrate the modulation of seizure threshold using antisense oligodeoxynucleotides to GABA-T.

An update on GABAA receptors

Recent advances in molecular biology and complementary information derived from neuropharmacology, biochemistry and behavior have dramatically increased our understanding of various aspects of GABAA receptors. These studies have revealed that the GABAA receptor is derived from various subunits such as alpha1-alpha6, beta1-beta3, gamma1-gamma3, delta, epsilon, pi, and rho1-3. Furthermore, two additional subunits (beta4, gamma4) of GABAA receptors in chick brain, and five isoforms of the rho-subunit in the retina of white perch (Roccus americana) have been identified. Various techniques such as mutation, gene knockout and inhibition of GABAA receptor subunits by antisense oligodeoxynucleotides have been used to establish the physiological/pharmacological significance of the GABAA receptor subunits and their native receptor assemblies in vivo. Radioligand binding to the immunoprecipitated receptors, co-localization studies using immunoaffinity chromatography and immunocytochemistry techniques have been utilized to establish the composition and pharmacology of native GABAA receptor assemblies. Partial agonists of GABAA receptors are being developed as anxiolytics which have fewer and less severe side effects as compared to conventional benzodiazepines because of their lower efficacy and better selectivity for the GABAA receptor subtypes. The subunit requirement of various drugs such as anxiolytics, anticonvulsants, general anesthetics, barbiturates, ethanol and neurosteroids, which are known to elicit at least some of their pharmacological effects via the GABAA receptors, have been investigated during the last few years so as to understand their exact mechanism of action. Furthermore, the molecular determinants of clinically important drug-targets have been investigated. These aspects of GABAA receptors have been discussed in detail in this review article.

Antisense inhibition of striatal GABAA receptor proteins decreases GABA-stimulated chloride uptake and increases cocaine sensitivity in rats

The functional status of striatal GABAA receptors appears to be inversely related to the magnitude of cocaine-induced behaviors. Exposure of striatum to antisense oligodeoxynucleotides (ASODNs) targeted to the mRNAs for the alpha 2 and the beta 3 subunits of the GABAA receptor should decrease expression of receptor proteins and therefore might be expected to increase cocaine sensitivity. ASODNs, scrambled ODNs or saline were injected into right lateral ventricle of rats and behavioral responses to cocaine were tested 18-20 h after treatment. Animals injected separately with alpha 2 or beta 3 ASODNs exhibited increased behavioral sensitivity to cocaine compared to rats injected with saline or scrambled ODNs including performing more 360 degrees turns to the left than to the right. There was significantly less GABA-stimulated Cl uptake in right striatum compared to left striatum of ASODN-treated rats with no significant difference between sides in control animals. Specific binding to benzodiazepine and convulsant sites on the GABAA receptor was not selectively altered by ASODN treatment. Combined alpha 2 beta 3 ASODN treatment did not affect either cocaine sensitivity or GABAA receptor function. There was no difference between the density of Nissl stained cells in the left and right edges of striatum in control or ASODN-treated rats indicating the absence of significant neurotoxic effects of the ASODN treatment. Injection of fluorescein-conjugated ASODNs indicated that ASODN is present in striatum at times during which behavioral and neurochemical indices of GABA receptor function are decreased. Thus, the functional status of GABAA receptors in striatum may be involved in determining cocaine sensitivity.

Differential changes in induced seizures after hippocampal treatment of rats with an antisense oligodeoxynucleotide to the GABA(A) receptor gamma2 subunit

Gamma-aminobutyric acid (GABA) is the principal inhibitory neurotransmitter in the brain. Impairment of GABAergic neurotransmission may be involved in the pathogenesis of epileptic phenomena. We have previously characterized biochemical and histological changes following unilateral intrahippocampal infusion of a phosphorothioate antisense oligodeoxynucleotide to the GABA(A) receptor gamma2 subunit in rats in vivo. The aim of the present study was to investigate the behavioral changes of rats following unilateral hippocampal antisense 'knockdown' of the GABA(A) receptor gamma2 subunit. Antisense, but not mismatch control oligodeoxynucleotide treated rats had a significant weight loss (10%) during 6 d of treatment. Antisense treated rats exhibited no changes in spontaneous behavior, including anxiety-like behavior as measured in the social interaction test, compared to mismatch oligodeoxynucleotide treated rats. However, antisense treated rats developed pronounced changes in induced seizure activity. Seizures induced by subcutaneously injected pentylenetetrazol were markedly accentuated in antisense treated rats compared to treatment naive rats, whereas mismatch treated rats showed a lower seizure score than that of naive rats. Antisense treated rats had a significantly elevated threshold for seizures induced by electrical stimulation in the maximal electroshock seizure threshold test. The results suggest that intrahippocampal infusion of antisense oligodeoxynucleotide to the GABA(A) receptor gamma2 subunit leads to specific alterations in the sensitivity to induced seizures. The results are viewed as consequences of selective down-regulation of GABA(A) receptors and diminished inhibitory neurotransmission in the hippocampus.

The use of in vivo antisense oligonucleotide technology for the investigation of brain GABA receptors

Antisense oligodeoxynucleotides (ODN) can be used as selective inhibitors of in vivo gene expression in the central nervous system (CNS) of experimental animals. The gamma-aminobutyric acid type A (GABAA) receptor is a member of the ligand-gated ion channel superfamily of neurotransmitter receptors. GABAA receptor function is allosterically modulated by several clinically important compounds, e.g. 1,4-benzodiazepines, barbiturates and certain neurosteroids, which recognize binding sites within the receptor complex. GABAA receptor chloride channel complexes are probably pentamers of different polypeptide subunits. The number of known subunit families and isoforms (six alpha s, four beta s, three gamma s, one delta and two rho s) indicates an extensive heterogeneity of GABAA receptors. The gamma 2 subunit is a functionally integral part of the GABAA receptor, necessary for the high affinity binding of benzodiazepines. The infusion of phosphorothioate ODN antisense to the gamma 2 subunit mRNA, but not control sense or mismatch ODN, into the lateral cerebral ventricle or into the hippocampus of rats leads to significant decreases in benzodiazepine receptor radioligand binding. In the hippocampus this is accompanied by a decrease in the number of GABAA receptors and by a loss of neurones, the latter possibly being due to reduced GABAergic inhibitory neurotransmission. Autoradiographic analysis following continuous intrahippocampal infusion of antisense ODN shows the regional extent of the effect on [3H]flunitrazepam binding. The continuous infusion of antisense ODN, but not of mismatch control ODN, into the right lateral cerebral ventricle induced a significant decrease in benzodiazepine binding and [3H]muscimol binding to membranes of the right cortex. Antisense ODN infused into the striatum decreased benzodiazepine binding and binding to the GABA binding site of the GABAA receptor to an extent similar to that found in the hippocampus. It is concluded that the preferred route of administration of antisense ODN for in vivo studies of the GABAA receptor may be by infusion into defined rat brain regions. The reported data support the idea that antisense ODN can be used as a valuable tool for the investigation of the contribution of individual GABAA receptor subunits to the properties of the receptor complex and of mechanisms of receptor subunit assembly.

Diazepam protects against rat hippocampal neuronal cell death induced by antisense oligodeoxynucleotide to GABA(A) receptor gamma2 subunit

Antisense oligodeoxynucleotides (ODNs) are used for the selective inhibition of gene expression. Antisense ODNs are promising tools for the investigation of physiological implications of proteins in the central nervous system of rodents in vivo. We have previously demonstrated that a phosphorothioate antisense ODN to the GABA(A) receptor gamma2 subunit, but not sense or mismatch control ODNs, induces a decrease in ex vivo benzodiazepine receptor radioligand binding in rat hippocampus when infused into the hippocampus in vivo [Karle et al., Neurosci. Lett., 202 (1995) 97-100]. This effect is parallelled by a decrease in the number of GABA(A) receptors and an extensive loss of hippocampal neurones. There is increasing awareness of risks of toxic 'non-antisense' effects induced by ODNs, and in particular phosphorothioate ODNs. The present experiments were designed to investigate the specificity of effects induced by the gamma2 subunit antisense ODN. The temporal development of changes in [3H]flunitrazepam and [3H]quinuclidinyl benzilate binding as well as in tissue protein levels supports the notion that the antisense ODN primarily acts by blocking the expression of the targeted receptor subunit protein. Furthermore, it is shown that a threshold for the elicitation of neurodegenerative changes exists. Finally, it is demonstrated that diazepam treatment of rats protects against the development of neuronal cell death induced by the antisense ODN. Collectively, the results support the hypothesis that the neurodegeneration induced by the antisense ODN is a consequence of diminished GABAergic inhibitory tonus following a selective down-regulation of gamma2 subunit-containing GABA(A) receptor complexes.

Treatment with an antisense oligodeoxynucleotide to the GABAA receptor gamma 2 subunit increases convulsive threshold for beta-CCM, a benzodiazepine "inverse agonist', in rats

The gamma 2 subunit of the gamma-aminobutyric acid type-A (GABAA) receptor is associated with the actions of benzodiazepines and related drugs. A phosphorothioate-modified antisense oligodeoxynucleotide directed against the gamma 2 subunit was given by i.c.v. injection (18 micrograms in 2 microliters saline) to male Sprague-Dawley rats every 12 h for 3 days. Controls received the corresponding sense oligodeoxynucleotide. 4-6 h after the last i.c.v. treatment, rats were given methyl-beta-carboline-3-carboxylate (beta-CCM), a benzodiazepine "inverse agonist', by slow i.v. infusion. Compared to naive rats, the beta-CCM threshold dose was not affected by the sense oligodeoxynucleotide, but was increased 87% in antisense oligodeoxynucleotide-treated rats. The treatment had no effect on the seizure threshold for picrotoxin. Both antisense and sense oligodeoxynucleotide treatments slightly increased the threshold for strychnine seizures. The results suggest that antisense oligodeoxynucleotide treatment altered GABAA receptor composition and interfered with the actions of a benzodiazepine receptor ligand in vivo, and may provide a tool for studying regulation of receptor structure and function.

Antisense oligonucleotide to GABAA receptor gamma 2 subunit induces loss of neurones in rat hippocampus

The binding site for 1,4-benzodiazepines in the brain is part of the hetero-oligomeric gamma-aminobutyric acid (GABA)A receptor complex which regulates a chloride ion channel. The presence of the gamma 2 subunit in the complex is necessary for the binding of benzodiazepines to their binding site. This study demonstrates a reduction of benzodiazepine receptor radioligand binding by 43% compared to control following infusion of phosphorothioate antisense oligodeoxynucleotide to gamma 2 subunit into rat hippocampus. Reduction of benzodiazepine binding sites was paralleled by a decrease in [35S]tert-butyl-bicyclo-phosphorothionate ([35S]TBPS) binding (51%) and [3H]muscimol binding (37%), indicating a reduction in the number of GABAA receptors. Changed macroscopic appearance, reduced protein content and severe loss of neurones in antisense-treated hippocampi suggests that the reduced formation of GABAA receptors leads to neuronal cell death.