Isonipecotic acid
Isonipecotic acid, also known as piperidine-4-carboxylic acid, is a conformationally constrained derivative of γ-aminobutyric acid and a moderately potent GABAA receptor partial agonist. It consists of a piperidine ring with a carboxylic acid moiety in the iso position. The drug showed moderate-efficacy partial agonism of α1, α2, α3, and α5 subunit-containing GABAA receptors, but showed full or near-full agonism of α4 and α6 subunit-containing GABAA receptors. Isonipecotic acid is unable to cross the blood–brain barrier. It was first described in the scientific literature by at least 1944 and was identified as a GABAA receptor agonist by 1978.