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Anticonvulsant action of the peripheral-type benzodiazepine antagonist PK 11195 on the El mouse
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EFFECTS OF ENDOGENOUS STEROIDS ON GABA* RECEPTOR RESPONSES RYUZO SHINGAI’,SHAHID H. ZAh4AN2,MARGARET SUTHER~,KATSUNORI HOSHI’ AND ERIC A. BARNARD’, ‘Department of Information Science, Faculty of Engineering Iwate University, “MNU, MRC Center,
Cambridge UK. Steroids 5a-pregnan-3a-ol-20-oone @a-OH-DHP) and pregnenolone sulfate (PS) are endogenous in the brain. 3u-OHDHP potentiates, while PS inhibits GABA activation of GABA* receptors on neurons. Messenger RNAs were made by in vi&o transcription from cloned bovine GABA+, receptor subunit (al or a2 or a3 and fil and y2) cDNAs. Combinations of one of the a subunits with fi (binary) or of one of the as with the p plus y (ternary) were expressed in Xenopus oocytes. The dose-response curve was determined on each combination for 3a-OH-DHP in potentiating the response to a GABA concentration which corresponds to 20% of the maximum GABA response. This potentiation was greater in al-combinations than in a2 or a3-combinations. The presence of the y subunit increased the steroid potency inalp1 and a2B1,butthe combination a3ply2 becameless steroid-sensitive. GABA at a concentration corresponding to40-500/o of the maximum response to GABA, was applied withPSto oocytes. Inall combinations PS showedan inhibitory effect in the concentration rangeabove10"M. The addition of they subunit changedthesensitivity toPS in different directions depending on the subunit combinations. Co-applications of 3a-OH-DHP and PS suggest that the potentiation by 3a-OH-DHP is through a mechanism almost independent of the inhibitory effect of PS.
ANTICONVULSANT ACTION OF THE PERIPHERAL-TYPE BENZODIAZEPINE ANTAGONIST PK 11195ON THE El MOUSE. YVRIE NAKAMOTO AND MITSVNOBV YOSHII, Department of Neurophysiology, Psychiatric Research Institute of Tokyo, 2-l-8 Kamikitazawa, Setagaya-ku, Tokyo 156, Japan. It has been demonstrated that “central-type” knzodiazepines (BZDs) such as clonazepam act as anticonvulsants whereas “peripheral-type” BZDs such as Ro 5-4864 act in the opposite way as convulsants. The anticonvulsant action of the central-type BZD is well known as a consequence of potentiation of
GABAergic transmission. On the other hand, the proconvulsant action of the peripheral-type BZD is less understood. In the present study. we have examined if PK 11195, an antagonist of the peripheral-type BZD receptor, can prevent seizures in El mice as compared with anticonvulsant actions of clonazepam and diazepam. El mouse seizures were induced by natural sensory stimulation (tossing). The seizure “threshold” had been determined for each animal at weekly stimulation. Seizures were prevented by clonazepam at doses of above 0.03 mglKg (i.v.) or by diazepam at doses of above 0.1 mg/Kg (i.v.). PK 11195, on the other hand, did not prevent seizures at a higher dose of 6 mg/Kg (i.v.), but raised the seizure threshold significantly. The results suggest that peripheral-type BZD receptors may play a proconvulsive role in the brain.
CHARACTERIZATION OF ANTAGONISTIC ACTIVITY AND BINDING PROPERTIES OF SR 95531 IN RAT BRAIN YOSHIHISA ITO, TAMAMI KOSHIBA, MASAYO DO1 and HIDEOMI FUKUDA, Department of Pharmacv. Nihon University, 7-7-l Narashinodai, Pharmacoloqy, Collese of Funabashi-shi, Chiba 274, Japan. Experiments were performed to characterize antagonistic activity and binding SR 95531 and properties of SR 95531, a synthetic GABAA antagonist, in rat brain. JGCl- uptake bicuculline methiodide muscimol-stimulated in (BMI) inhibited The inhibitory cortical synaptoneurosomes in a concentration-dependent manner. potency of SR 95531 for the muscimol-stimulated 36Cl- uptake was 15 times higher The IC50 value of SR 95531 for the muscimol-stimulated 36Clthan that of BMI. uptake in cortical synaptosomes was in close agreement with the KD value for lowaffinity binding sites of [3HISR 95531 in the frontal cortex. Pretreatment of the Aa (PLase A,) frontal cortical and cerebellar membranes with phospholipase invariably decreased [3HlSR 95531 binding, although the treatment significantly The products generated by the increased [3HIGABA binding in these regions. catalytic activity of PLase A,, such as arachidonic acid and lysophosphatidylcholine mimicked the effects of PLase A, on the binding of [3HlGABA and [3HlSR 95531. These results suggest that SR 95531 exerts GABA antagonistic action through its low-affinity binding sites. It is also suggested that GABAA agonist and antagonist bind differentially to GABAA receptors and that exogenously added PLase A, causes opposite modulation of antagonist binding to that of agonist.
EFFECTS OF ENDOGENOUS STEROIDS ON GABA* RECEPTOR RESPONSES RYUZO SHINGAI’,SHAHID H. ZAh4AN2,MARGARET SUTHER~,KATSUNORI HOSHI’ AND ERIC A. BARNARD’, ‘Department of Information Science, Faculty of Engineering Iwate University, “MNU, MRC Center,
Cambridge UK. Steroids 5a-pregnan-3a-ol-20-oone @a-OH-DHP) and pregnenolone sulfate (PS) are endogenous in the brain. 3u-OHDHP potentiates, while PS inhibits GABA activation of GABA* receptors on neurons. Messenger RNAs were made by in vi&o transcription from cloned bovine GABA+, receptor subunit (al or a2 or a3 and fil and y2) cDNAs. Combinations of one of the a subunits with fi (binary) or of one of the as with the p plus y (ternary) were expressed in Xenopus oocytes. The dose-response curve was determined on each combination for 3a-OH-DHP in potentiating the response to a GABA concentration which corresponds to 20% of the maximum GABA response. This potentiation was greater in al-combinations than in a2 or a3-combinations. The presence of the y subunit increased the steroid potency inalp1 and a2B1,butthe combination a3ply2 becameless steroid-sensitive. GABA at a concentration corresponding to40-500/o of the maximum response to GABA, was applied withPSto oocytes. Inall combinations PS showedan inhibitory effect in the concentration rangeabove10"M. The addition of they subunit changedthesensitivity toPS in different directions depending on the subunit combinations. Co-applications of 3a-OH-DHP and PS suggest that the potentiation by 3a-OH-DHP is through a mechanism almost independent of the inhibitory effect of PS.
ANTICONVULSANT ACTION OF THE PERIPHERAL-TYPE BENZODIAZEPINE ANTAGONIST PK 11195ON THE El MOUSE. YVRIE NAKAMOTO AND MITSVNOBV YOSHII, Department of Neurophysiology, Psychiatric Research Institute of Tokyo, 2-l-8 Kamikitazawa, Setagaya-ku, Tokyo 156, Japan. It has been demonstrated that “central-type” knzodiazepines (BZDs) such as clonazepam act as anticonvulsants whereas “peripheral-type” BZDs such as Ro 5-4864 act in the opposite way as convulsants. The anticonvulsant action of the central-type BZD is well known as a consequence of potentiation of
GABAergic transmission. On the other hand, the proconvulsant action of the peripheral-type BZD is less understood. In the present study. we have examined if PK 11195, an antagonist of the peripheral-type BZD receptor, can prevent seizures in El mice as compared with anticonvulsant actions of clonazepam and diazepam. El mouse seizures were induced by natural sensory stimulation (tossing). The seizure “threshold” had been determined for each animal at weekly stimulation. Seizures were prevented by clonazepam at doses of above 0.03 mglKg (i.v.) or by diazepam at doses of above 0.1 mg/Kg (i.v.). PK 11195, on the other hand, did not prevent seizures at a higher dose of 6 mg/Kg (i.v.), but raised the seizure threshold significantly. The results suggest that peripheral-type BZD receptors may play a proconvulsive role in the brain.
CHARACTERIZATION OF ANTAGONISTIC ACTIVITY AND BINDING PROPERTIES OF SR 95531 IN RAT BRAIN YOSHIHISA ITO, TAMAMI KOSHIBA, MASAYO DO1 and HIDEOMI FUKUDA, Department of Pharmacv. Nihon University, 7-7-l Narashinodai, Pharmacoloqy, Collese of Funabashi-shi, Chiba 274, Japan. Experiments were performed to characterize antagonistic activity and binding SR 95531 and properties of SR 95531, a synthetic GABAA antagonist, in rat brain. JGCl- uptake bicuculline methiodide muscimol-stimulated in (BMI) inhibited The inhibitory cortical synaptoneurosomes in a concentration-dependent manner. potency of SR 95531 for the muscimol-stimulated 36Cl- uptake was 15 times higher The IC50 value of SR 95531 for the muscimol-stimulated 36Clthan that of BMI. uptake in cortical synaptosomes was in close agreement with the KD value for lowaffinity binding sites of [3HISR 95531 in the frontal cortex. Pretreatment of the Aa (PLase A,) frontal cortical and cerebellar membranes with phospholipase invariably decreased [3HlSR 95531 binding, although the treatment significantly The products generated by the increased [3HIGABA binding in these regions. catalytic activity of PLase A,, such as arachidonic acid and lysophosphatidylcholine mimicked the effects of PLase A, on the binding of [3HlGABA and [3HlSR 95531. These results suggest that SR 95531 exerts GABA antagonistic action through its low-affinity binding sites. It is also suggested that GABAA agonist and antagonist bind differentially to GABAA receptors and that exogenously added PLase A, causes opposite modulation of antagonist binding to that of agonist.