Biomedical subjects

M K Ticku

Publications and source records attributed to M K Ticku.

At least 109 records · Page 6Linked to original sources

PubMed1985

Comparison of [3H]baclofen binding to GABAB receptors in spontaneously hypertensive and normotensive rats.

Baclofen-sensitive GABAB receptor binding to various brain regions was compared in age-matched spontaneously hypertensive (SHR) and normotensive Wistar-Kyoto (WKY) rat. The specific [3H]baclofen binding was significantly higher in SHR cerebellum, hippocampus and nucleus tractus solitarii (NTS), but lower in medulla, when compared with WKY. Scatchard analysis of the binding isotherms indicated a higher Bmax of the high affinity sites in SHR cerebellum and hippocampus and a lower Bmax of the same sites in SHR medulla. Guanyl nucleotide (GTP) decreased [3H]baclofen binding in brain regions of SHR and WKY, although the percentage inhibition of binding did not differ between SHR and WKY. The basal adenylate cyclase activity in SHR cerebellum was lower, in contrast to higher [3H]baclofen binding, whereas lower baclofen binding in SHR medulla was accompanied with higher adenylate cyclase activity. (+/-)Baclofen inhibited adenylate cyclase activity in both SHR and WKY rat brain. Among various regions, the percentage inhibition of cyclase activity in SHR cerebellum was significant lower, as compared to WKY cerebellum. These results suggest that activity of GABABergic mechanisms may be different in SHR and WKY rat brain.

Adenylyl Cyclases↗

PubMed1985

GABA, depressants and chloride ions affect the rate of dissociation of 35S-t-butylbicyclophosphorothionate binding.

The dissociation of 35S-TBPS was studied from binding sites of rat cerebral cortex. Monophasic dissociation plots became polyphasic and accelerated in the presence of micromolar concentrations of GABA suggesting the involvement of low (or super-low) affinity GABA receptors. The presence of the depressants etazolate, R(-)MPPB and ethanol resulted in similarly accelerated dissociation patterns. In contrast, the convulsants S(+)MPPB and pentamethylenetetrazol did not significantly affect the dissociation of TBPS. Dissociation initiated by dilution was not affected either by an excess of picrotoxin or by varying the equilibrium occupancy of the TBPS sites. These findings rule out the possibility of a kinetic cooperativity for the binding of convulsants. The removal of chloride ions also enhanced the rate of TBPS dissociation. Kinetic heterogeneity of the TBPS binding sites can be interpreted with allosteric interactions mediated by various sites at the GABA receptor complex coupled to different states of the chloride ionophore.

Animals↗

PubMed1985

Separate site(s) of action of optical isomers of 1-methyl-5-phenyl-5-propylbarbituric acid with opposite pharmacological activities at the GABA receptor complex.

The behavioral profile of the optical isomers of 1-methyl-5-phenyl-5-propylbarbituric acid (MPPB) and their interaction with the convulsant binding site at the GABA receptor complex were investigated. R(-)-MPPB produced dose-related loss of righting reflex, whereas S(+)MPPB produced convulsions in a dose-dependent manner. Subconvulsive doses of S(+)MPPB were proconvulsant with a subeffective dose of picrotoxin. S(+)MPPB-induced seizures were blocked by R(-)MPPB and pentobarbital. In contrast, S(+)MPPB did not block the loss of righting reflex produced by R(-)MPPB or pentobarbital. S(+)MPPB inhibited the binding of [35S]t-butylbicyclophosphorothionate (TBPS), a ligand that binds to the picrotoxin site on the oligomeric GABA receptor complex, to rat brain membranes competitively, whereas R(-)MPPB inhibited it noncompetitively. Thus, the optical isomer of MPPB, which have opposite pharmacological effects, interact differently with the convulsant (TBPS) site at the GABA receptor complex. These results suggest that the convulsant S(+)MPPB and the depressant R(-)MPPB may produce their behavioral effects by acting via convulsant and anticonvulsant sites, respectively, at the GABA receptor complex.

Animals↗

PubMed1985

Involvement of a GABAergic mechanism in the anticonvulsant effect of pentobarbital against maximal electroshock-induced seizures in rats.

The interaction between pentobarbital and other modulators of GABAergic transmission (diazepam, ethanol and progabide) was investigated on maximal electroshock seizures and on the loss of righting reflexes in rats. Pentobarbital, diazepam and ethanol produced a dose-dependent protection against electroshock seizures, with pentobarbital being more potent (3- and 50-times) than diazepam and ethanol. Progabide neither provided protection nor caused loss of righting reflex. Subprotective doses of pentobarbital and diazepam, together or when combined with a single ineffective dose of ethanol or progabide, caused protection against seizures and loss of righting reflex for variable durations, while ethanol and progabide combination did not provide protection. The protective effect of diazepam was antagonized by RO15-1788, picrotoxin and bicuculline pretreatments. The antagonism of pentobarbital protection by a specific GABA receptor antagonist, bicuculline suggests involvement of the GABAergic system in the anticonvulsant effect of pentobarbital. These results indicate that, like diazepam, the anticonvulsant effect of pentobarbital appears to be mediated through a GABAergic mechanism.

Animals↗

PubMed1985

A possible role of a GABAergic mechanism in the convulsant action of RO5-4864.

The purpose of the present investigation was to determine the possible role of GABAergic mechanism in the convulsant action of RO5-4864. Benzodiazepines (BZ) and other agents which facilitate central GABAergic transmission delayed the onset of facial and forelimb clonus, whereas tonic hind limb extension was blocked in a dose-dependent manner. RO5-4864-induced convulsions were blocked by diazepam, clonazepam, pentobarbital, ethanol and amino-oxyacetic acid (AOAA). RO5-4864-induced convulsions were not blocked by the BZ antagonist RO15-1788. Specifically, RO15-1788 caused a decrease in the onset of severity component of tonic seizures, which tended to become generalized and precipitated in a tonic extension of the hindlimbs. Further, subconvulsive doses of a direct GABA receptor antagonist, bicuculline, enhanced the proconvulsant action of RO5-4864, indicating thereby a potential antagonism of the central GABAergic transmission. These observations strongly suggest that RO5-4864 probably elicits convulsions by selective impairment of the GABAergic transmission.

Animals↗

PubMed1985

The effect of in vitro and in vivo ethanol administration on [35S]t-butylbicyclophosphorothionate binding in C57 mice.

Previous studies have shown that ethanol may produce some of its effects by facilitation of GABAergic transmission. One of the potential sites of drug action at the GABA receptor complex is the picrotoxin site, which can be studied with [35S]t-butylbicyclophosphorothionate (TBPS). Ethanol inhibited the binding of [35S]TBPS to C57 mice brain regions in vitro. This inhibition appears to be noncompetitive since ethanol decreased the Bmax and not the KD value of [35S]TBPS. C57 mice were chronically treated with ethanol in liquid diet to determine if the sensitivity of TBPS binding is altered following chronic treatment or during withdrawal. Chronic treatment with ethanol and during withdrawal did not alter the KD or Bmax values of [35S]TBPS binding in C57 mice brain regions. It is suggested that the sensitivity of picrotoxin site on the oligomeric GABA receptor complex is not altered during ethanol tolerance or withdrawal. The effects of ethanol on GABA system may be mediated by its interaction with the coupling mechanism(s) or a direct effect on the chloride channels.

Acoustic Stimulation↗

PubMed1985

Dissociation of [35S]t-butylbicyclophosphorothionate binding differentiates convulsant and depressant drugs that modulate GABAergic transmission.

The dissociation of [35S]t-butylbicyclophosphorothionate ([35S]TBPT) from binding sites on membranes from rat cerebral cortex, after addition of saturating concentrations of convulsant and depressant drugs, was studied. The addition of unlabeled TBPT, picrotoxinin, or pentamethylenetetrazol resulted in dissociation patterns that were monophasic and not distinguishable, suggesting that these convulsants bind competitively to the same (convulsant) sites. In contrast, gamma-aminobutyric acid (GABA) greatly facilitated [35S]TBPT dissociation by binding allosterically to the GABA recognition site of the receptor-ionophore complex. TBPT dissociation was similarly accelerated by the depressants etazolate, (+)-etomidate, and barbiturates. The convulsant and depressant S(+) and R(-) stereoisomers of N-methyl-5-phenyl-5-propyl-barbituric acid displayed large stereoselectivity in the acceleration of TBPT dissociation. These results suggest that depressants bind to sites different from the convulsant sites of the allosteric GABA receptor complex, or the binding of depressants to the same population of sites elicits negative cooperativity and dissociates the convulsants.

Animals↗

PubMed1984

Kinetic differences in GABA receptor binding following GABA and bicuculline methiodide pretreatment.

The kinetics of [3H]GABA binding were studied in cortical membranes following GABA and bicuculline methiodide (BCM) pretreatment at 37 degrees C. The slower parts of the biphasic ON and OFF rates can be attributed to the high affinity sites. BCM pretreatment decreased the ON and OFF rates of [3H]GABA binding, including the rapid initial ON- and OFF-phases. Decreased association rates of GABA were independent of GABA and concentration and suggested a rate determining dissociation of bound BCM (t1/2 = 35 min).

Animals↗

PubMed1984

Pretreatment with GABA and modulatory ligands enhances GABA receptor binding.

GABA pretreatment at 37 degrees C but not at 4 degrees C enhanced subsequent [3H]GABA binding in a concentration-dependent manner (EC50 = 440 nM) by increasing the number of high and low affinity sites. The effect mediated by GABAA receptors was attenuated by bicuculline methiodide and picrotoxinin and potentiated by pentobarbital, flurazepam and Triton X-100. These results can be interpreted by the facilitated removal of endogenous substances and/or by a temperature-restricted conversion of the GABA receptor populations.

Animals↗

PubMed1984

Interactions of pentamethylenetetrazole and tetrazole analogues with the picrotoxinin site of the benzodiazepine-GABA receptor-ionophore complex.

The interactions of pentamethylenetetrazole and ten tetrazole analogues with the picrotoxinin site of the benzodiazepine-GABA receptor-ionophore complex was investigated. All the active tetrazole analogues potently inhibited the binding of [35S]t-butyl- bicyclophosphorothionate ( TBPT ), a ligand which binds to the picrotoxinin site. Tetrazole analogues appear to interact with the picrotoxinin site competitively. All the tetrazole analogues tested were more potent in inhibiting TBPT binding than diazepam binding. There is a reasonably good correlation between the tetrazole analogues to inhibit TBPT binding and the doses at which they produce convulsions. Pentamethylenetetrazole inhibited TBPT binding at concentrations which are similar to the in vivo concentrations present during convulsions produced by this drug. These results suggest that tetrazoles may produce convulsions by acting at the picrotoxin-sensitive site of the benzodiazepine-GABA receptor-ionophore complex.

Animals↗

PubMed1984

RO5-4864 inhibits the binding of [35S]t-butylbicyclophosphorothionate to rat brain membranes.

RO5-4864, a 1,4-benzodiazepine, has recently been shown to possess anticonvulsant, convulsant and anxiogenic properties and to inhibit Ca++-calmodulin-stimulated membrane phosphorylation. RO5-4864 inhibited the binding of [35S]t-butylbicyclophosphorothionate (TBPT) to cerebral cortex, cerebellar and hippocampus membranes, with an IC50 value of approximately 20 microM. TBPT binds apparently to the picrotoxinin site of the benzodiazepine-GABA receptor-ionophore complex and appears to be a site of action for several classes of convulsant, depressant and anxiolytic drugs that modulate GABAergic transmission. RO5-4864 inhibited [35S] TBPT binding in cerebral cortex, apparently competitively. Antagonists of GABA and central benzodiazepine sites did not interfere with the ability of RO5-4864 to inhibit [35S] TBPT binding. The properties of RO5-4864 to inhibit TBPT binding are similar to other convulsants and GABA antagonists (except bicuculline) which inhibit TBPT binding. These results suggest that RO5-4864 interacts with the TBPT binding sites of the oligomeric GABA receptor complex.

Androstanes↗

PubMed1984

Differential interactions of GABA agonists, depressant and convulsant drugs with [35S]-t-butylbicyclophosphorothionate binding sites in cortex and cerebellum.

Effects of three GABA agonists, four GABA antagonists and convulsants (picrotoxinin, alpha-dihydropicrotoxinin [DHP], pentamethylenetetrazole [PTZ] and isopropylbicyclophosphate ester) and three depressant drugs (pentobarbital, (+)etomidate and etazolate) were investigated on [35S]t-butylbicyclophosphorothionate (TBPT) in cortex and cerebellum. All the convulsants tested were equipotent in inhibiting [35S]TBPT binding in cortex and cerebellum. Convulsants like picrotoxinin inhibited [35S]-TBPT binding competitively in both cortex and cerebellum. In contrast, gamma-aminobutyric acid (GABA) agonists (muscimol, GABA and 4,5,6,7-tetrahydroisoxazol[5,4-C]pyridine-3-ol [THIP]), and depressants like etazolate, (+)etomidate and pentobarbital were more potent inhibitors of [35S]TBPT binding in cerebellum than in cortex. GABA inhibition of [35S]TBPT binding appears to be mediated through a low-affinity site. GABA and pentobarbital inhibited [35S]TBPT binding in cortex and cerebellum noncompetitively. Depressants like pentobarbital appear to interact with the TBPT sites allosterically. These results suggest that depressant and convulsant drugs that modulate GABAergic transmission interact differently with the TBPT binding sites in cortex and cerebellum.

Animals↗

PubMed1984

Diazotization and thiocyanate differentiate agonists from antagonists for the high- and low-affinity receptors of gamma-aminobutyric acid.

The differentiation of high- and low-affinity postsynaptic gamma-aminobutyric acid (GABA) receptors was examined in a washed cortical membrane preparation of the rat. The selective elimination of the high- and low-affinity GABA sites by the chaotropic anion thiocyanate and diazotization by p-diazobenzenesulfonic acid (DSA), respectively, offered two model systems for the separate sites. The [3H]GABA displacing potencies of some GABA agonists [GABA, 4,5,6,7-tetrahydro- isoxazole [4,5c]pyridine-3-ol (THIP), and muscimol] and antagonists [bicuculline methiodide (BCM), 3-alpha-hydroxy-16-imino-5 beta-17-aza-androstan-11-one (R-5135), and d-tubocurarine] and their slope factors were examined in these model systems and in control membranes. The displacing potency of the agonists was increased in the DSA-pretreated membranes and decreased in the presence of thiocyanate. The displacing potency of the antagonists was shifted in an opposite manner. The chaotropic effect of thiocyanate was reversible and not additive with the inhibitory effect of diazotization on the specific binding of GABA. Inhibition of specific GABA binding by pyridoxal-5-phosphate (PLP) could not be protected by GABA antagonists (BCM and R-5135) but only by agonists. The results can be interpreted in the framework of a dual (agonist-antagonist) receptor model, postulating a hydrophobic accessory site at the low-affinity GABA receptor. The effect of thiocyanate on the GABA receptor may result in the exposure of the hydrophobic accessory sites.

Androstanes↗

PubMed1984

Binding characteristics and interactions of depressant drugs with [35S]t-butylbicyclophosphorothionate, a ligand that binds to the picrotoxinin site.

[35S]t-Butylbicyclophosphorothionate (TBPT), a cage convulsant with picrotoxinin-like activity, binds to rat brain membranes to a single site with an apparent KD of 25.1 +/- 5.6 nM and a Bmax of 1.40 +/- 0.22 pmol/mg protein. TBPT binding to rat brain membranes was inhibited by a variety of convulsant, depressant, anxiolytic, and anticonvulsant drugs that had previously been shown to inhibit [3H]alpha-dihydropicrotoxinin binding. Depressant drugs such as pentobarbital and the nonbarbiturate (+)etomidate inhibited TBPT binding in an uncompetitive manner. Thus, pentobarbital and (+)etomidate decreased both the affinity and the number of binding sites of TBPT to whole brain membranes. The IC50 values of (+)etomidate (9 microM) and pentobarbital (90 microM) are similar to the EC50 values at which they enhance both [3H]gamma-aminobutyric acid and [3H]diazepam binding in cerebral cortex membranes. RO5-4864, which has recently been shown to be a convulsant, also inhibited TBPT binding (IC50 = 10 microM). These results suggest that TBPT binds to the picrotoxinin site and further supports the notion that the picrotoxinin site is an important modulatory site at the benzodiazepine-GABA receptor-ionophore complex.

Animals↗

PubMed1984

Characterization of gamma-aminobutyric acid-benzodiazepine receptor complexes by protection against inactivation by group-specific reagents.

The chemical topography of the gamma-aminobutyric acid (GABA) and benzodiazepine (BZ) receptors was investigated in a thoroughly washed cortical membrane preparation of the rat. Chemical modification by several amino- and tyrosyl-selective reagents and the protection from it by direct and allosteric ligands of the GABA-BZ receptor complex were used to identify the residues at the binding sites. Inhibition of specific GABA binding by p-diazobenzenesulfonic acid (DSA), tetranitromethane (TNM), and N-acetylimidazole and the selective and complete protection from it by GABA and muscimol suggest the presence of a tyrosine residue at the GABAA site. TNM, like DSA, selectively decreased the number of the low-affinity GABA receptors, and this could be completely protected only by GABA concentrations that can saturate the low-affinity sites. TNM pretreatment also abolished the muscimol enhancement of [3H]diazepam binding, which suggests that the low-affinity GABA receptor sites are responsible for this enhancement. Inhibition of GABA binding by pyridoxal-5-phosphate (PLP) and the selective protection by GABA and muscimol support the presence of a lysine residue at the GABAA receptor site. Complete and selective protection from diethylpyrocarbonate (DEP) inhibition of [3H]diazepam binding by flurazepam suggests the presence of a histidine residue at the BZ site. Flurazepam selectively protected from inhibition of [3H]diazepam binding by N-bromosuccinimide and N-acetylimidazole, but not that by DSA and TNM, which does not allow a unanimous conclusion regarding the presence of tyrosine or tryptophan residues at the BZ site.

Animals↗

PubMed1983

Convulsant/depressant site of action at the allosteric benzodiazepine-GABA receptor-ionophore complex.

Several classes of centrally acting convulsant, depressant, anticonvulsant and anxiolytic drugs modulate GABAergic transmission. The postsynaptic receptor with which these drugs interact is an allosteric complex with distinct binding sites for GABA, benzodiazepines, picrotoxinin and related compounds. Convulsants which inhibit GABA transmission (except bicuculline) inhibit competitively the binding of dihydropicrotoxinin (DHP) or t-butylbicyclophosphorothionate (TBPT) to the picrotoxinin site and prevent the allosteric enhancing effect of depressant drugs on GABA and benzodiazepine binding. Depressant drugs give a mixed inhibition of TBPT binding. The possible topography of the picrotoxinin site and its relationship to convulsant/depressant drug action at the benzodiazepine-GABA receptor-ionophore complex is discussed.

Allosteric Regulation↗

PubMed1983

The perspective of GABA replenishment therapy in the epilepsies: a critical evaluation of hopes and concerns.

Impaired GABA-mediated inhibition is probably one of the cellular abnormalities leading to Focal Epilepsy. The role of GABA in generalized seizures, particularly of Petit Mal type, is unknown. Various approaches are available to potentiate GABA function. Merits and flaws of each one of them are critically evaluated. In some forms of epilepsy, GABA agonists may replenish depleted pools, and in some others may nonspecifically raise the general excitability threshold of the brain, yet in other forms they may exert a glutamate/aspartate antagonistic effect. The available experimental evidence suggests that in bilaterally synchronous spike and wave epilepsies, GABA agonists are either ineffective or pejorative.

Epilepsy↗

PubMed1983

GABA and benzodiazepine binding sites in spontaneously hypertensive rat.

The binding of GABA to its central receptor sites and of benzodiazepines to central and peripheral types of binding sites was compared in age-matched spontaneously hypertensive rat (SHR) and normotensive (WKY) rat. The KD and Bmax values of GABA agonist binding were not significantly different between SHR and WKY rat in the forebrain, cerebellum or pons-medulla. The binding of [3H]flunitrazepam to central types of binding sites was also not different in 12 week-old SHR and WKY rat. However, the binding of [3H]flunitrazepam and [3H]RO5-4864 was significantly lower in the SHR kidney, as compared to WKY rat kidney. This decrease was due to a lower Bmax in SHR kidney. In contrast, the binding of these radioligands in the hearts of the two strains was not different. These results indicate that the SHR has lower peripheral benzodiazepine binding sites in the kidney as compared to age-matched normotensive WKY rat kidney.

Animals↗