Structural factors governing agonist and antagonist activity in the GABAA system

Activation of single heteromeric GABA(A) receptor ion channels by full and partial agonists

The linkage between agonist binding and the activation of a GABA(A) receptor ion channel is yet to be resolved. This aspect was examined on human recombinant alpha1beta2gamma2S GABA(A) receptors expressed in human embryonic kidney cells using the following series of receptor agonists: GABA, isoguvacine, 4,5,6,7-tetrahydroisoxazolo[5,4-c]pyridin-3-ol (THIP), isonipecotic acid, piperidine-4-sulphonic acid (P4S), imidazole-4-acetic acid (IAA), 5-(4-piperidyl)-3-isothiazolol (thio-4-PIOL) and 5-(4-piperidyl)-3-isoxazolol (4-PIOL). Whole-cell concentration-response curves enabled the agonists to be categorized into four classes based upon their maximum responses. Single channel analyses revealed that the channel conductance of 25-27 pS was unaffected by the agonists. However, two open states were resolved from the open period distributions with mean open times reduced 5-fold by the weakest partial agonists. Using saturating agonist concentrations, estimates of the channel shutting rate, alpha, ranged from 200 to 600 s(-1). The shut period distributions were described by three or four components and for the weakest partial agonists, the interburst shut periods increased whilst the mean burst durations and longest burst lengths were reduced relative to the full agonists. From the burst analyses, the opening rates for channel activation, beta, and the total dissociation rates, k(-1), for the agonists leaving the receptor were estimated. The agonist efficacies were larger for the full agonists (E approximately 7-9) compared to the weak partial agonists ( approximately 0.4-0.6). Overall, changes in agonist efficacy largely determined the different agonist profiles with contributions from the agonist affinities and the degree of receptor desensitization. From this we conclude that GABA(A) receptor activation does not occur in a switch-like manner since the agonist recognition sites are flexible, accommodating diverse agonist structures which differentially influence the opening and shutting rates of the ion channel.

Superimposition-based protocol as a tool for determining bioactive conformations. II. Application to the GABA(A) receptor

The natural templates (NT) superimposition method is used to determine the pharmacophoric requirements of the A subtype of the gamma-aminobutyric acid (GABA) receptor. Bioactive conformations for antagonists and agonists are found by superimposing them on a relatively rigid alkaloid bicuculline, which itself is a competitive antagonist at this ligand-gated ion channel receptor. As has been usual in the application of this modeling method, consideration of available experimental data is the cornerstone for obtaining realistic models. The identification of two substructural fragments of bicuculline permitted classification of the ligands. Analysis of the antagonists and agonists with respect to the two substructural fragments revealed two bioactive conformations of the highly flexible GABA molecule, one of which is extended with the nonhydrogenic atoms roughly coplanar torsional angles of -37 and -179 degrees at N-C-C-C and C-C-C-C (carboxyl), respectively. The second bioactive compound is clearly non planar (torsional angles of -81 and -109 degrees at N-C-C-C and C-C-C-C (carboxyl), respectively).

GABAA receptor pharmacology

gamma-Aminobutyric acid (GABA)A receptors for the inhibitory neurotransmitter GABA are likely to be found on most, if not all, neurons in the brain and spinal cord. They appear to be the most complicated of the superfamily of ligand-gated ion channels in terms of the large number of receptor subtypes and also the variety of ligands that interact with specific sites on the receptors. There appear to be at least 11 distinct sites on GABAA receptors for these ligands.