strychnine and pitrazepin

The natural templates (NT) approach, which is a superimposition-based protocol that has been successfully employed in several studies, is here applied to ligands of the glycine ligand-gated ion channel receptor. Bioactive conformations for glycine and its analogs were obtained using strychnine (a natural and specific competitive antagonist) as template. Experimental evidence was used to guide the superimposition protocol. Three essential regions have been defined in strychnine's structure that serve as a pharmacophore for agonist and antagonist activities. Reasonable alignments of known ligands were found in the majority of the cases. Molecular mechanics (i.e., conformational searches for the relatively flexible ligands) and molecular dynamics (for relatively rigid ligands such as strychnine and 5,6,7,8-tetrahydro-4H-isoxazolo[3,4-d]azepin-3-ol) were used to assess the energetic accessibility of the proposed bioactive conformations.

Topics: Amino Acids; Bicuculline; Binding, Competitive; Computer Simulation; Dibenzazepines; Glycine; Ligands; Models, Molecular; Molecular Conformation; Receptors, Glycine; Strychnine; Thermodynamics

We used molecular modeling techniques to examine six reported antagonists of glycine with varying Ki values against strychnine. We found the data suggest two groups operating with different mechanisms. In group 1 (strychnine, brucine, Pitrazepin, and bicuculline methobromide) the antagonist contains two or three sites that can electrostatically bind to the three comparable groups of opposite charge in the recognition site where the natural neurotransmitter binds, thus opening the chloride channel. In addition, when in this position, the antagonist is able to also block the now opened chloride channel with a different portion of its structure. In many cases, this involves an interaction between a carbonyl group on the antagonist and the guanidinium group of arginine which is part of the polypeptide segment of the outer mouth of the chloride channel (Grenningloh et al., Nature 330:25-26, 1987). In group 2 (R5135 and 1,5-diphenyl-3,7-diazaadamantan-9-ol) the antagonist contains charged sites but when one of these molecules attaches to the recognition site, the chloride channel is not opened. In addition, R5135 contains a carbonyl group which attaches to arginine as pointed out in the text, whereas 1,5-diphenyl-3,7-diazaadamantan-9-ol contains a phenyl group that can block the channel.

Topics: Animals; Binding, Competitive; Dibenzazepines; GABA Antagonists; Glycine; Models, Molecular; Molecular Structure; Receptors, Glycine; Strychnine

Pitrazepin (3-(piperazinyl-1)-9H-dibenz(c,f)triazolo(4,5-a)azepin) is a new GABAA receptor antagonist reported to antagonize electrophysiological effects of GABA. We have investigated in some detail the interaction of pitrazepin with the GABA/benzodiazepine receptor chloride channel complex. Pitrazepin was found to be a competitive inhibitor of the GABAA receptor which is coupled to [3H]diazepam and [35S]TBPS binding sites; the KI value obtained by Schild analyses was 80 nM. Although pitrazepin interacted weakly with BZ receptors the compound did not affect the chloride gating mechanism (labelled with [35S]TBPS or [3H]avermectin B1a). Further, pitrazepin was a non-selective GABA antagonist since glycine receptors, labelled with [3H]strychnine, were affected at low concentrations (the KI values in rat brain-stem were 71-110 nM).

Topics: Animals; Bridged Bicyclo Compounds; Bridged Bicyclo Compounds, Heterocyclic; Cerebral Cortex; Dibenzazepines; In Vitro Techniques; Kinetics; Male; Membranes; Muscimol; Rats; Rats, Inbred Strains; Receptors, GABA-A; Receptors, Glycine; Receptors, Neurotransmitter; Strychnine