Modulation of spontaneous motility by GABAA receptor antagonism in the guinea pig isolated ileum

Cortisone: a potent GABAA antagonist in the guinea-pig isolated ileum

In the guinea-pig isolated ileum, cortisone at 0.001-10 nM induced a non-competitive, dose-dependent antagonism of GABAA-receptor-mediated contractile responses to applied GABA, depressing the maximum contractile response to GABA (100 microM), without affecting contractile responses to acetylcholine or cholinergic twitch contractions. At higher concentrations (greater than 10 nM), cortisone depressed contractile responses to acetylcholine (10-100 nM) and cholinergic twitch responses to transmural stimulation. Cortisone is thus the most potent non-competitive antagonist at GABAA-receptor complexes in the guinea-pig ileum. From molecular modelling, sterically there appeared little difference between cortisone and cortisol, the latter being an enhancer of GABAA-receptor-mediated action in the ileum. However, there were significant differences in electrostatic potentials between the two steroids, due to the different levels of oxidation at C11 which may contribute to such opposing actions.

GABA-receptors in peripheral tissues

Gamma-aminobutyric acid (GABA) and its receptors are found in a wide range of peripheral tissues, including parts of the peripheral nervous system, endocrines, and non-neural tissues such as smooth muscle and the female reproductive system. In all these, both GABAA- and GABAB-receptor types are found, with good evidence for a physiological role in the gut, pancreatic islets and the urinary bladder. In some tissues, the pharmacology of GABA-induced actions is quite atypical and should be further explored with the newer ligands and modulators for GABAA- and GABAB-receptors.