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Electrophysiological characterization of potent agonists and antagonists at pre‐ and postsynaptic GABA B receptors on neurones in rat brain slices
Author(s) -
Seabrook G.R.,
Howson W.,
Lacey M.G.
Publication year - 1990
Publication title -
british journal of pharmacology
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 2.432
H-Index - 211
eISSN - 1476-5381
pISSN - 0007-1188
DOI - 10.1111/j.1476-5381.1990.tb14186.x
Subject(s) - chemistry , bicuculline , substantia nigra , agonist , gabab receptor , postsynaptic potential , neuroscience , glutamate receptor , pharmacology , medicine , endocrinology , gabaa receptor , biology , receptor , dopamine , biochemistry , dopaminergic
1 Intracellular recordings were made from neurones in striatum (caudate‐putamen) and substantia nigra pars compacta in rat brain slices. Three GABA B agonists, baclofen, 3‐aminopropylphosphinic acid (3‐APPA) and 3‐aminopropyl(methyl)phosphinic acid (SK&F 97541), depressed excitatory postsynaptic potentials (e.p.s.ps) mediated by glutamate in the striatum, and hyperpolarized neurones in the substantia nigra. The ability of 3‐aminopropyl(diethoxymethyl)phosphinic acid (CGP 35348), 3‐aminopropyl (hexyl)phosphinic acid (3‐APHPA) and phaclofen to antagonize these responses was assessed. 2 Striatal e.p.s.ps, studied in the presence of bicuculline (30 μ m ), were reduced in amplitude by 92% with 6,7‐dinitroquinoxaline‐2,3‐dione (DNQX; 30 μ m ). These e.p.s.ps were depressed by up to 95% by SK&F 97541 and baclofen with EC 50 s of 0.092 μ m and 1.25 μ m respectively. The maximal effect of 3‐APPA was 67% with an EC 50 of 0.83 μ m . Agonist concentration‐effect data fitted a single‐site logistic model. GABA B agonists were without effect on striatal neurone membrane potential, input resistance or depolarizations induced by applied glutamate. 3 The depression of striatal e.p.s.ps by SK&F 97541 was reversibly antagonized by CGP 35348, 3‐APHPA and phaclofen with estimated equilibrium dissociation constants ( K B ) of 11.2 ± 1.7 μ m ( n = 4), 13.3 ± 0.4 μ m ( n = 3), and 405 ± 43 μ m ( n = 3) respectively. CGP 35348 and 3‐APHPA appeared to act competitively (Schild plot slopes of 0.99 and 1.01 respectively). 4 Nigral neurones were hyperpolarized by up to 25 mV by SK&F 97541 and baclofen with EC 50 s of 0.15 μ m and 3.6 μ m respectively. The maximum hyperpolarization by 3‐APPA was only 84% that of the other agonists, with an EC 50 of 9.0 μ m . Agonist concentration‐effect data fitted a single‐site logistic model. 5 The SK&F 97541‐induced hyperpolarization was reversibly antagonized by CGP 35348, 3‐APHPA and phaclofen with estimated K B s of 17.6 ± 4.4 ( n = 3), 14.0 ± 1.5 ( n = 4), and >400 μ m ( n = 1) respectively. CGP 35348 appeared competitive (Schild plot slope of 0.99). Antagonists were also tested with baclofen as agonist, yielding similar K B estimates as for SK&F 97541. 6 It is concluded that at both the presynaptic and postsynaptic sites examined, SK&F 97541 was about 10 fold more potent than baclofen or 3‐APPA. The antagonists CGP 35348 and 3‐APHPA ( K B 10–20 μ m ) were about 20 fold more potent than phaclofen. The similarities in relative agonist potency and estimated antagonist affinity between these two functionally distinct GABA B receptors renders them pharmacologically indistinguishable at present.