Isothiouronium compounds as γ‐aminobutyric acid agonists

147

115

1 The pl protein, which we previously cloned from retina, assembles as a homooligomer that transduces the binding of y-aminobutyric acid (GABA) into robust chloride currents. However, its insensitivity to bicuculline, pentobarbitone and benzodiazepines, all potent agents at typical GABAA receptors, suggested that it may react atypically to other GABA agonists and antagonists.2 cDNAs for the pl and the a5p1 receptors for GABA were expressed as homo-and heterooligomers, respectively, in Xenopus oocytes. The selectivities of the respective receptors for various agonists were investigated using concentration-response experiments in voltage clamped cells. 3 The most potent agonists at the p1 receptor were trans-4-aminocrotonic acid (TACA) > GABA > muscimol; at the a5p, receptor the rank order was muscimol > GABA > 4,5,6,7-tetrahydroisoxazole[4,5-c]pyridine-3-ol (THIP). The most specific agonists were cis-(2-(aminomethyl)-cyclopropylcarboxylic acid (CAMP) and THIP for the pl and the a53, receptors, respectively. 4 Comparing GABA, TACA and cis-aminocrotonic acid (CACA) at pl receptors expressed in COS cells gave results almost indistinguishable from those found at oocytes; the pharmacology of p1 seems independent of the expression system. 5 Agonists THIP, piperidine-4-sulphonic acid (P4S), and isoguvacine, whose C-C-C-N chains are constrained by rings into a folded conformation and were potent at the a501 receptor, were among the weakest at the pl receptor. However CACA and CAMP, which align better with the extended than the folded conformation, were weakest at the a5pI receptor but moderately potent at the pl receptor. These findings suggest that the pl receptor recognizes agonists in the extended conformation, in contrast to GABAA receptors, which are believed to recognize agonists in the partially folded conformation.6 In contrast to the o5p1 receptor, gradations in maximum responses were apparent in the pl receptor, suggesting various degrees of partial agonism. In particular, imidazole-4-acetic acid (I4AA), whose maximum response was only 3% of GABA's maximum, had an apparent Kd for activating the pl receptor of 16f1M; but it had an apparent Kd for competitively blocking the receptor of 0.64 1AM. This difference suggests that steric constraints in the activated (open channel) receptor are tighter than in the resting receptor.7 Hill coefficients approached 2 at the pl receptor, but were closer to unity at the (0I1 receptor. Thus, the pl receptor displayed higher cooperativity. 8 Unlike typical GABAA receptors, the pl receptor was insensitive to the competitive antagonists bicuculline, SR95531, securinine, and (+)-tubocurarine.

Section: Discussionmentioning

confidence: 99%

Pharmacology of GAB A ρ1 and GAB A α/β receptors expressed in Xenopus oocytes and COS cells

Kusama

Spivak

Whiting

et al. 1993

147

115

“…However, the agonist profile of the Drosophila homo-oligomers reflects that the GABAA receptors where the above compounds act as agonists and have a similar order of potency (Barker & Mathers, 1978;Kusama et al, 1993;Woodward et al, 1993). For example, ZAPA is a potent agonist of both RDLac and GABAA receptors (Allan et al, 1986). Similarly, isoguvacine is a more potent agonist of RDLac homo-oligomers and GABAA receptors (Woodward et al, 1993) than is isonipecotic acid, indicating that a decrease in the planarity of the piperidine ring reduces agonist activity on both preparations.…”

Section: Comparison Of the Agonist Pharmacophores Of Drosophila And Vmentioning

confidence: 99%

Agonist pharmacology of two Drosophila GABA receptor splice variants

Hosie

Sattelle

1996

1 The Drosophila melanogaster y-aminobutyric acid (GABA) receptor subunits, RDLaC and DRC 17-1-2, form functional homo-oligomeric receptors when heterologously expressed in Xenopus laevis oocytes. The subunits differ in only 17 amino acids, principally in regions of the N-terminal domain which determine agonist pharmacology in vertebrate ionotropic neurotransmitter receptors. A range of conformationally restricted GABA analogues were tested on the two homo-oligomers and their agonist pharmacology compared with that of insect and vertebrate iontropic GABA receptors. 2 The actions of GABA, isoguvacine and isonipecotic acid on RDLaC and DRC 17-1-2 homo-oligomers were compared, by use of two-electrode voltage-clamp. All three compounds were full agonists of both receptors, but were 4-6 fold less potent agonists of DRC 17-1-2 homo-oligomers than of RDLaC. However, the relative potencies of these agonists on each receptor were very similar.3 A more complete agonist profile was established for RDLac homo-oligomers. The most potent agonists of these receptors were GABA, muscimol and trans-aminocrotonic acid (TACA), which were approximately equipotent. RDLaC homo-oligomers were fully activated by a range of GABA analogues, with the order of potency: GABA> ZAPA ((Z)-3-[(aminoiminomethyl)thio]prop-2-enoic acid)> isoguvacine > imidazole-4-acetic acid > isonipecotic acid > cis-aminocrotonic acid (CACA) > P-alanine. 3-Aminopropane sulphonic acid (3-APS), a partial agonist of RDLLC homo-oligomers, was the weakest agonist tested and 100 fold less potent than GABA. 4 SR95531, an antagonist of vertebrate GABAA receptors, competitively inhibited the GABA responses of RDLaC homo-oligomers, which have previously been found to be insensitive to bicuculline. However, its potency (IC50 500 gM) was much reduced when compared to GABAA receptors.5 The agonist pharmacology of Drosophila RDLac homo-oligomers exhibits aspects of the characteristic pharmacology of certain native insect GABA receptors which distinguish them from vertebrate GABA receptors. The high potency and efficacy of isoguvacine and ZAPA distinguishes RDLIC homo-oligomers from bicuculline-insensitive vertebrate GABAC receptors, while the low potency of SR95531 and 3-APS distinguishes them from GABAA receptors. The differences in the potency of agonists on RDLaC and DRC 17-1-2 homo-oligomers observed in the present study may assist in identification of further molecular determinants of GABA receptor function.

“…The present studies indicate that the substituents on C4' and C3' on the phthalide system in bicuculline and (+)-hydrastine are important to the GABA antagonist actions of these alkaloids. GABA binds to 2 kinetically and pharmacologically distinct classes of bicuculline-sensitive GABAA binding sites on rat brain membranes, with the low affinity binding site being that linked to benzodiazepine receptors (Skerritt et al, 1982b;Johnston, 1986;Allan et al, 1986). The present study indicates that both bicuculline and (+)-hydrastine are more potent antagonists of the low affinity GABAA binding sites than of the high affinity sites.…”

Section: Convulsant Potenciesmentioning

confidence: 58%

(+)‐Hydrastine, a potent competitive antagonist at mammalian GABA_A receptors

Huang

Johnston

1990

Self Cite

(+)‐Hydrastine is a phthalide isoquinoline alkaloid, isolated from Corydalis stricta. It has the same 1S,9R configuration as the competitive GABAA receptor antagonist bicuculline and is the enantiomer of the commercially available (−)‐hydrastine. (+)‐Hydrastine (CD50 0.16 mg kg−1, i.v.) was twice as potent as bicuculline (CD50 0.32 mg kg−1, i.v.) as a convulsant in mice. This action was stereoselective in that (+)‐hydrastine was 180 times as potent as (−)‐hydrastine. (+)‐Hydrastine was a selective antagonist at bicuculline‐sensitive GABAA receptors in the guinea‐pig isolated ileum. It did not influence phaclofen‐sensitive GABAB receptors or acetylcholine receptors in this tissue. (+)‐Hydrastine was a competitive antagonist of GABAA responses (pA2 6.5) more potent than bicuculline (pA2 6.1). When tested against the binding of [3H]‐muscimol to high affinity GABAA binding sites in rat brain membranes, (+)‐hydrastine (IC50 2.37 μm) was 8 times more potent than bicuculline (IC50 19.7 μm). As an antagonist of the activation of low affinity GABAA receptors as measured by the stimulation by GABA of [3H]‐diazepam binding to rat brain membranes, (+)‐hydrastine (IC50 0.4 μm) was more potent than bicuculline (IC50 2.3 μm). (+)‐Hydrastine, 10 nm to 1 mm, did not inhibit the binding of [3H]‐(−)‐baclofen to GABAB binding sites in rat brain membranes.