Investigations Of The Mechanism And Modulation Of Gaba(A) Receptors And Their Potentiation By Dihydropyrimidinones
Gamma-aminobutyric acid type-A (GABAA) receptors are ligandgated chloride channels essential for regulating signal transmission within the mammalian central nervous system (CNS). The chloride currents due to these pentameric receptors can be positively modulated (potentiated) by a variety of molecules including barbiturates, diazepams, and endogenous neurosteroids, some of which are used as therapeutics. Molecules that selectively modulate GABAA receptors of specific subunit compositions are scarce but would be valuable tools for elucidating the function of GABAA receptor subtypes within the CNS and may aid in developing therapeutics with greater target specificity. Described here is the discovery of several dihydropyrimidinone (DHPM) small molecules that preferentially potentiate GABA induced whole-cell currents from [delta] subunit-containing GABAA receptors. Some of the DHPMs, such as JM-II-43A, had potentiation efficacies comparable to those of non-subtype selective modulators phenobarbital and tracazolate, but the DHPMs did not directly induce receptor currents. JM-II-43A was further tested with [alpha]1[beta]2[delta] GABAA receptors using flash-photolysis transient kinetic techniques as well as single-channel and multi-channel current recording patch-clamp techniques to determine how DHPMs modulate these receptors. The results lead to the proposal of a novel mechanism of receptor function involving an equilibrium of GABAA receptors between an active and inactive state before the presence of agonist. This equilibrium is shifted by potentiating compounds, such as DHPMs, towards the active state to increase the total number of receptors that reach an open-channel (ion conducting) state. This hypothesis also explains the characteristic lack of desensitization and low peak current amplitudes associated with [delta] subunit-containing receptors. Also discussed is the purification and characterization of several photo labile GABA derivatives containing a coumarin-based caging group for their quantum yield, rate of photolysis, solubility, thermal stability and sensitivity to visible light in the 350- to 450-nm region. These compounds were intended for transient kinetic investigations of the modulation of GABAA receptors by DHPMs discussed above. One of these compounds was determined to have a quantum yield of 0.1, however the rate of photolysis was not suitable for transient kinetic measurements in the microto milli-second time region.
GABA(A) Receptors; ligand-gated ion channel; dihydropyrimidinone
Hess, George P
Wilson, David B; Harris-Warrick, Ronald Morgan
Ph.D. of Biochemistry
Doctor of Philosophy
dissertation or thesis