Biomedical subjects

D I Kerr

Publications and source records attributed to D I Kerr.

At least 19 recordsLinked to original sources

PubMed2001

Comparative activities of the enantiomeric GABA(B) receptor agonists CGP 44532 and 44533 in central and peripheral tissues.

In neocortical slices maintained in Mg(2+)-free Krebs medium, the gamma-aminobutyric acid (GABA(B)) receptor agonists baclofen, (3-amino-2(S)-hydroxypropyl)methylphosphinic acid (CGP 44532), and its (R)-enantiomer CGP 44533 depressed the frequency of spontaneous discharges in a concentration-dependent manner (EC(50)=10, 6.5, and 50 microM, respectively). These effects were reversibly antagonised by the GABA(B) receptor antagonist (+)-(S)-5,5 dimethylmorpholinyl-2-acetic acid (Sch 50911) (3, 10, and 30 microM) (average pA(2) value=6.0+/-0.2). In neocortical wedges, baclofen, CGP 44532 and CGP 44533 elicited concentration-dependent hyperpolarisations (the EC(50)s were 14, 7.5 and 16 microM, respectively) sensitive to Sch 50911 (1, 5, 10 microM) (average pA(2) value=6.0+/-0.1), whilst they also depressed ileal electrically elicited cholinergic twitch contractions (EC(50)=11, 7, and 50 microM) that were antagonised by Sch 50911 (average pA(2) value=6.0+/-0.1). In electrically stimulated brain slices preloaded with [3H]GABA, baclofen, CGP 44532 and CGP 44533 decreased [3H]GABA release (IC(50)=5, 0.45, and 10 microM); this effect was reversed by Sch 50911 (50 microM). It is concluded that CGP 44532 is a far more potent agonist at GABA(B) autoreceptors than at central or peripheral heteroreceptors.

Animals↗

PubMed2001

CGP 36216 is a selective antagonist at GABA(B) presynaptic receptors in rat brain.

In rat neocortical preparations maintained in Mg(2+)-free Krebs medium, baclofen depressed the frequency of spontaneous discharges in a concentration-dependent manner (EC(50) = 6 microM), sensitive to (3-aminopropyl)ethylphosphinic acid (CGP 36216) (100, 300 and 500 microM) (pA(2) = 3.9 +/- 0.1). By contrast, CGP 36216, up to 1 mM, was ineffective in antagonising baclofen-induced hyperpolarisations, mediated through gamma-aminobutyric acid(B) (GABA(B)) postsynaptic receptors. In electrically stimulated brain slices preloaded with [3H]GABA, CGP 36216 increased [3H]GABA release (IC(50) = 43 microM), which was reversed by baclofen (20 microM). While CGP 36216 is ineffective at GABA(B) postsynaptic receptors, it is appreciably more active at presynaptic receptors.

Animals↗

PubMed2000

Effects of Ca2+ concentration on GABA(B) receptor function in rat neocortical slices.

In rat neocortical slices maintained in Mg2+-free Kreb's medium, the effects of Ca2+ concentration on repetitive spontaneous discharges and on GABA(B) receptor-mediated responses were investigated. Over a concentration range of 0.3-2.4 mM Ca2+, there was a reduction of discharge amplitude, with a 50 +/- 6.5% reduction in 0.3 mM Ca2+, whilst the burst frequency remained unaffected. Baclofen, the GABA(B) receptor agonist, produced a concentration-dependent depression of discharge frequency, reversibly antagonised by the antagonist (+)-(S)-5,5-dimethylmorpholinyl-2-acetic acid (Sch 50911). The EC50 value for baclofen in 2.4 mM Ca2+ was 11 microM, while the EC50 values in 0.3, 0.6, 1.2, and 1.8 mM Ca2+ were 1.3, 2.5, 3.6, and 10 microM, respectively, resulting in 8.5, 4.4, 3.1, and 1.1-fold leftward shifts. This enhanced action of baclofen in low extracellular Ca2+ concentrations in the neocortex may be the result of a lower concentration gradient which reinforces the action of baclofen.

Animals↗

PubMed2000

Recent advances in GABAB receptors: from pharmacology to molecular biology.

Bicuculline-insensitive receptors for the inhibitory neurotransmitter gamma-aminobutyric acid (GABA), GABAB receptors, are a distinct subclass of receptors that mediate depression of synaptic transmission and contribute to neuronal inhibition. When activated, these receptors reduce transmission at excitatory and inhibitory synapses, as a result of an increase in K+ conductance, or a decrease in voltage-dependent Ca2+ currents. They are also linked to G-proteins, or intracellular effector systems in a very complex manner. The recent development of highly specific and potent agonists and antagonists for these receptors has led to a much better understanding of their physiology and pharmacology, including their heterogeneity, as well as their molecular biology. Over the past year, expression and cloning studies have contributed to major advances in characterizing GABAB receptor structure, with the discovery of the amino acid sequences of GABABR1a/R1b splice variants and GABABR2 receptors. These isoforms are widely distributed throughout the nervous system, and can be functionally expressed. Importantly, GABABR2 receptors can form a heteromeric assembly with GABABR1 proteins to operate as a heterodimer that displays robust coupling to inward-rectifying K+ channels, as well as inhibition of forskolin-stimulated adenylate cyclase activity. Further insights underlying the mechanisms of GABAB receptor functions can now be gained, leading ultimately to the therapeutic potential of drugs acting at these sites. It is increasingly clear that new information on GABAB receptor molecular structure will provide a plethora of targets for pharmaceutical intervention in areas such as drug addiction, nociception and absence seizures. This review summarizes the renewed efforts, and highlights the recent advances emerging in this field.

Animals↗

PubMed1999

Pharmacological re-evaluation of a GABA(B) receptor antagonist CGP 47332A in rat brain.

In rat neocortical slices maintained in Mg(2+)-free Krebs medium, the gamma-aminobutyric acid (GABA(B)) receptor agonist baclofen concentration-dependently depressed the frequency of spontaneous discharges (EC(50)=12 microM). This was reversibly antagonised by (R, S)-3-amino-2-hydroxy-propyl-P-n-butyl-phosphinic acid (CGP 47332A) (25, 100, 300 microM) which produced rightwards shifts of the baclofen concentration-response curves (pA(2) value=4.8+/-0.1). In electrically stimulated slices preloaded with [3H]GABA, CGP 47332A increased its release (EC(150)=100 microM) through antagonism of GABA(B) autoreceptors. Although CGP 47332A was some six times weaker on GABA(B) auto- than on heteroreceptors, yet its congener lacking the beta-hydroxy substituent displays equal potency in both binding (IC(50)=38 microM) and GABA(B) autoreceptor functional studies (EC(150)=38 microM) as previously reported [Froestl, W., Mickel, S.J. , Von Sprecher, G., Diel, P.J., Hall, R.G., Maier, L., Strub, D., Melillo, V., Baumann, P.A., Bernasconi, R., Gentsch, C., Hauser, K., Jaekel, J., Karlsson, G., Klebs, K., Maitre, L., Marescaux, C., Pozza, M.F., Schmutz, M., Steinmann, M.W., Van Riezen, H., Vassout, A., Mondadori, C., Olpe, H.R., Waldmeier, P.C., Bittiger, H., Phosphinic acid analogues of GABA: 2. Selective, orally active GABA(B) antagonists. J. Med. Chem. 38 (1995) 3313-3331.].

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PubMed1999

Comparative potencies of CGP 47654A and CGP 46165A as GABA(B) receptor antagonists in rat brain.

In rat neocortical slices maintained in Mg2+-free Krebs medium, the gamma-aminobutyric acid (GABAB) receptor agonist baclofen concentration-dependently depressed the frequency of spontaneous discharges (EC50 = 6.1 microM). This was reversibly antagonised by 3-aminopropyl-(1,1-difluoro-n-butyl)-phosphinic acid (25, 100, 500 microM) (CGP 47654A) and 3-aminopropyl-P-(alpha-hydroxybenzyl)-phosphinic acid (CGP 46165A) (50, 100, 400 microM) which produced rightwards shifts of the baclofen concentration-response curves, with respective pA2 values of 4.9+/-0.2 and 4.6+/-0.15. Although relatively potent on GABAB heteroreceptors studied here, CGP 47654A and CGP 46165A were 5 and 50 times weaker, respectively, as GABAB autoreceptor antagonists [Froestl, W., Mickel, S.J., Von Sprecher, G., Diel, P.J., Hall, R.G., Maier, L., Strub, D., Melillo, V., Baumann, P.A., Bernasconi, R., Gentsch, C., Hauser, K., Jaekel, J., Karlsson, G., Klebs, K., Maitre, L., Marescaux, C., Pozza, M.F., Schmutz, M., Steinmann, M.W., Van Riezen, H., Vassout, A., Mondadori, C., Olpe, H.R., Waldmeier, P.C., Bittiger, H., 1995. Phosphinic acid analogues of GABA. 2. Selective, orally active GABAB antagonists. J. Med. Chem. 38: 3313-3331.], representing potentially useful ligands for differentiating GABA hetero- and autoreceptors.

Animals↗

PubMed1999

Antagonism of GABA(B) receptors by morpholino-2-acetic acid derivatives Sch 54679 and Sch 51324 in rat brain.

In rat neocortical slices maintained in Mg2+-free Krebs medium, baclofen depressed the rate of spontaneous discharges in a concentration-dependent manner (EC50 = 4.5 microM). This depression was reversibly antagonised by 5-(S,R)-hydroxymethyl-5-methylmorpholinyl-2-(R,S)-acetic acid (Sch 54679) and 2-(R,S)-5-[spirocyclopentyl]-morpholinyl-acetic acid (Sch 51324) (respective pA2 values of 5.8+/-0.15 and 5.4+/-0.2). In electrically-stimulated slices preloaded with [3H]gamma-aminobutyric acid (GABA), Sch 54679 (EC50 = 3 microM) was 2.3 times more potent than Sch 51324 (EC50 = 7 microM) in increasing [3H]GABA release through antagonism of GABA(B) autoreceptors. These structurally novel analogues may be pharmacologically useful for elucidating GABA(B) receptor functions.

Animals↗

PubMed1999

Evaluation of 3-aminopropanesulphonamide analogues of GABA as antagonists at GABA(B) receptors in peripheral and central preparations.

Cholinergic twitch responses in the guinea-pig isolated ileum, and spontaneous discharges in rat neocortical slices, were depressed by the GABA(B) receptor agonist baclofen. These actions were reversibly antagonised by the sulphonamide derivatives (R,S)-2-hydroxy-3-phthalimidopropanesulphonamide (HPIPS), 3-amino-N-benzoylpropanesulphonamide (ABPS) and 3-phthalimidopropanesulphonamide (PIPS) which produced rightwards shifts of the baclofen concentration-response curves, with pA2 values ranging between 4.1 and 4.3 in both preparations. From these results, HPIPS, ABPS and PIPS constitute a novel class of antagonists at GABA(B) hetero-receptors.

Animals↗

PubMed1999

Tonic activation of presynaptic GABAB receptors in the opener neuromuscular junction of crayfish.

Release of excitatory transmitter from boutons on crayfish nerve terminals was inhibited by (R,S)-baclofen, an agonist at GABAB receptors. Baclofen had no postsynaptic actions as it reduced quantal content without affecting quantal amplitude. The effect of baclofen increased with concentration producing 18% inhibition at 10 microM; EC50, 50% inhibition at 30 microM; maximal inhibition, 85% at 100 microM and higher. There was no desensitization, even with 200 or 320 microM baclofen. Phaclofen, an antagonist at GABAB receptors, competitively antagonized the inhibitory action of baclofen (KD = 50 microM, equivalent to a pA2 = 4.3 +/- 0.1). Phaclofen on its own at concentrations below 200 microM had no effect on release, whereas at 200 microM phaclofen itself increased the control level of release by 60%, as did 2-hydroxy-saclofen (200 microM), another antagonist at GABAB receptors. This increase was evidently due to antagonism of a persistent level of GABA in the synaptic cleft, since the effect was abolished by destruction of the presynaptic inhibitory fiber, using intra-axonal pronase. We conclude that presynaptic GABAB receptors, with a pharmacological profile similar to that of mammalian GABAB receptors, are involved in the control of transmitter release at the crayfish neuromuscular junction.

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PubMed1998

The morpholino-acetic acid analogue Sch 50911 is a selective GABA(B) receptor antagonist in rat neocortical slices.

The pharmacological properties of (+)-(S)-5,5-dimethylmorpholinyl-2-acetic acid (Sch 50911) were evaluated on GABA(B) receptors in rat neocortical slices. The GABA(B) receptor agonist, baclofen, produced a concentration-dependent depression of the frequency of spontaneous discharges in slices maintained in Mg2+-free Krebs medium with an EC50 of 6 microM, reversibly antagonised by Sch 50911 (5, 10 and 25 microM) with an apparent pA2 of 6.0 +/- 0.1. The (-) enantiomer Sch 50910 (500 microM) and the racemic des-methyl analogue Sch 48588 (500 microM) were inactive. In slices preloaded with [3H]GABA, Sch 50911 antagonised GABA(B) autoreceptors, increasing the electrically-stimulated 3H overflow in a concentration-dependent manner, with an IC50 of 3 microM. The maximal effect (148 +/- 10.5%) was found at 10 microM, but at 50 microM the response was reduced to 67 +/- 19%. In contrast, evoked release was unaffected by Sch 50910 (100 microM) whilst Sch 48588 at 100 microM increased the overflow by 51.3 +/- 11.6%. In summary, Sch 50911 is a relatively potent antagonist of considerable potential in studies of GABA(B) receptor function.

Animals↗

PubMed1998

Morpholin-2-yl-phosphinic acids are potent GABA(B) receptor antagonists in rat brain.

The pharmacological properties of morpholin-2-yl-phosphinic acids were evaluated on GABA(B) receptors. In rat neocortical slices maintained in Mg2+-free Krebs medium, baclofen, a GABA(B) receptor agonist, produced a concentration-dependent depression of the frequency of spontaneous discharges with an EC50 of 14 +/- 5.5 microM, which was antagonised reversibly by the morpholin-2-yl-phosphinic derivatives. The order of potency was 3-[(3S,6R)-6-[(cyclohexylmethyl)hydroxyphosphinoylmethyl- morpholin-3-yl]benzoic acid (CGP 76290A) (pA2 = 7.1 +/- 0.05) > its enantiomer 3-[(3R,6S)-6-[(cyclohexylmethyl)hydroxyphosphinoylmethyl]-++ +morpholin-3-yl]benzoic acid (CGP 76291A) (pA2 = 6.8 +/- 0.1) > cyclohexylmethyl-[(2R',5S')-5-(3-nitrophenyl)-morpholin-2-++ +ylmethyl]phosphinic acid (CGP 71978) (pA2 = 6.5 +/- 0.05) > cyclohexylmethyl-[(2R,5S)-5-phenyl-morpholin-2-ylmethyl++ +]phosphinic acid (CGP 71980) (pA2 = 6.3 +/- 0.15) > its enantiomer cyclohexylmethyl-[(2S,5R)-5-phenyl-morpholin-2-ylmethyl++ +]phosphinic acid (CGP 71979) (pA2 = 5.8 +/- 0.1). An open chain analogue of CGP 76290A, CGP 56999A (3-[1(R)-[(3-cyclohexylmethyl-hydroxyphosphinoyl)-2(S)-hydro xypropyl-amino]-ethyl]benzoic acid lithium salt) gave a pA2 of 6.6 +/- 0.2. In GABA(B) receptor binding assays, CGP 71982 (the racemic mixture of CGP 76290A and CGP 76291A), CGP 76290A, CGP 76291A, CGP 71978, CGP 71980 and CGP 71979 had IC50 values against [3H]CGP 27492 binding of 8, 1.85, 69, 124, 326 and 1460 nM, respectively. In electrically-evoked [3H]GABA release from rat cortical slices, CGP 71982, CGP 71978, CGP 71980 and its enantiomer CGP 71979, antagonised GABA(B) autoreceptors with EC150 values of 2.5, 33, 181 and 474 nM, respectively. These compounds form a novel class of potent GABA(B) receptor antagonists.

Animals↗

PubMed1998

The gamma-aminobutyric acid uptake inhibitor NO-711 potentiates 3-aminopropylphosphinic acid-induced actions in rat neocortical slices.

In rat neocortical slices maintained in Mg2+-free Krebs medium, the GABAB receptor agonists baclofen and 3-aminopropylphosphinic acid dose-dependently reduced the frequency of spontaneous discharges, 3-aminopropylphosphinic acid being 10 times less potent than baclofen. These were sensitive to the antagonist CGP 52432 (3-[[3,4-dichloro-phenyl)methyl]-amino]propyl](-P-diethoxymethyl)- phosphinic acid) (1, 5 and 10 microM). The GABA uptake inhibitor NO-711 (1-(2-(((diphenylmethylene)amino)oxy)ethyl)-1,2,5,6-tetrahydro-3-+ ++pyridinecarboxylic acid) (5 and 10 microM) produced 2.9 and 9 fold increases in the potency of 3-aminopropylphosphinic acid without affecting baclofen-induced responses. In this study, the low potency of 3-aminopropylphosphinic acid when compared to baclofen, may be attributed to its uptake by NO-711-sensitive GABA transporters.

Animals↗

PubMed1998

Differential effects of phosphonic analogues of GABA on GABA(B) autoreceptors in rat neocortical slices.

The effects of five phosphonic derivatives of GABA on the release of [3H]-GABA from rat neocortical slices, preloaded with [3H]-GABA, were investigated. Phaclofen and 4-aminobutylphosphonic acid (4-ABPA) increased the overflow of [3H] evoked by electrical stimulation (2 Hz) in a concentration-dependent manner, with similar potencies (phaclofen EC50=0.3 mmol/l, 4-ABPA EC50=0.4 mmol/l). At 3 mmol/l, phaclofen increased the release of [3H]-GABA by 82.6+/-8.6%, and 4-ABPA increased the release by 81.3+/-9.0%. 2-Amino-ethylphosphonic acid (2-AEPA) increased the overflow of [3H] by 46.8+/-10.9% at the highest concentration tested (3 mmol/l). In contrast, the lower phosphonic homologue 3-aminopropylphosphonic acid (3-APPA), and 2-amino-2-(p-chlorophenyl)-ethylphosphonic acid (2-CPEPA), a baclofen analogue, did not modify the stimulated overflow. These results suggest that phaclofen, 4-ABPA and 2-AEPA are antagonists at GABA(B) autoreceptors, the latter being the weakest antagonist, whilst neither 3-APPA nor 2-CPEPA are active at these receptors. Since phaclofen, 4-ABPA and 2-CPEPA are antagonists and 3-APPA a partial agonist/antagonist on GABA(B) heteroreceptors, the lack of effect of 3-APPA and 2-CPEPA on [3H]-GABA release in this study suggests that GABA(B) autoreceptors may be pharmacologically distinct from the heteroreceptors.

Animals↗

PubMed1997

Actions of thienyl analogs of baclofen at GABA(B) receptors in rat neocortical slices.

In rat neocortical slices maintained in Mg2+-free Krebs medium, baclofen and its thienyl analogs, 4-amino-3-(5-chlorothien-2-yl)-butanoic acid (5h), 4-amino-3-(5-methylthien-2-yl)-butanoic acid (5d), 4-amino-3-(5-bromothien-2-yl)-butanoic acid (5f) and 4-amino-3-(thien-3-yl)-butanoic acid (5j) dose-dependently suppressed the spontaneous discharges, antagonised by the GABA(B) receptor antagonist 2-hydroxysaclofen (200 microM). Their relative potencies were baclofen > 5h > 5d > 5f > 5j. These heterocyclic analogs may prove useful as GABA(B) receptor agonists in functional studies.

Animals↗

PubMed1996

GABAB receptor antagonism by resolved (R)-saclofen in the guinea-pig ileum.

The GABAB receptor antagonist saclofen (3-amino-2-(4-chlorophenyl)propylsulphonic acid) has been resolved by chiral high-performance liquid chromatography. The enantiomer (R)-saclofen, but not (S)-saclofen, reversibly antagonised the (R,S)-baclofen-induced depression of cholinergic twitch contractions in the guinea-pig ileum with an apparent pA2 of 5.3. Also, 2-hydroxy-saclofen was resolved by the same method, its (S)-enantiomer yielding an apparent pA2 of 5.0. This method provides a convenient resolution of these antagonists.

Animals↗

PubMed1995

3-amino-2-hydroxy-N-(4-nitrophenyl)-propanesulphonamide, a new class of GABAB receptor antagonist in central and peripheral preparations.

Racemic 3-amino-2-hydroxy-N-(4-nitrophenyl)-propanesulphonamide (AHPNS), a sulphonamide analog of GABA, reversibly and competitively antagonised the concentration-dependent depression of cholinergic twitch contractions by baclofen, in the electrically stimulated guinea-pig isolated ileum, with a pA2 of 4.0 +/- 0.2. In the rat neocortex, maintained in Mg(2+)-free medium, AHPNS (100-500 microM) also reversibly antagonised the baclofen (10 microM)-induced suppression of spontaneous discharges. AHPNS is a new class of GABAB receptor antagonist that has central and peripheral blocking actions.

Animals↗

PubMed1995

The (S)-enantiomer of 2-hydroxysaclofen is the active GABAB receptor antagonist in central and peripheral preparations.

In the guinea-pig isolated ileum, (RS)-(+/-)-baclofen induced a depression of cholinergic twitch contractions, reversibly and competitively antagonised by (S)-2-hydroxysaclofen (pA2 = 5.2 +/- 0.2), but not by (R)-2-hyroxysaclofen. The depression of excitatory field potentials by baclofen ( 5 mu M) in rat CA1 hippocampal slices was antagonised by (S)-2-hydroxysaclofen (100 mu m) (pA2 = 4.3), whilst in rat neocortex, (S)-2-hyroxysaclofen (50-500 mu M) antagonised the baclofen (10 mu M)-induced suppression of spontaneous discharges, the (R)-enantiomer being inactive. These results show that (S)-2-hydroxysaclofen is the active antagonist at central and peripheral GABAB receptors.

Animals↗

PubMed1995

Interactions of N-ethylmaleimide and aluminium fluoride with GABAB receptor function in rat neocortical slices.

Interactions of N-ethylmaleimide and aluminium fluoride (AlF - 4) with GABAB receptors have been examined using spontaneously discharging rat neocortical slices. The suppression of discharges by the GABAB receptor agonist baclofen (5-10 mu M) was irreversibly prevented by N-ethylmaleimide (10-50 mu M) and its analog N-phenylmaleimide (10-50 mu M), whilst superfusion of slices with NaF (10 mM) and AlCl3 (100 mu M) to form a fluoroaluminate (AlF - 4) complex markedly potentiated the action of baclofen. The lipoxygenase inhibitors, nordihydroguaiaretic acid (10-50 mu M) and eicosatetraynoic acid (10-50 mu M) or the phospholipase A2 inhibitor bromophenacylbromide (50-100 mu M) did not affect the response to baclofen. The depressant action of baclofen is evidently mediated through G-proteins, but is not dependent on arachidonic acid metabolites.

Aluminum Compounds↗