UBC Theses and Dissertations
Actions of isovaline and endogenous amino acids on inhibitory receptors in ventrobasal thalamus Cooke, James Edward
This thesis consists of three manuscripts that examine the effects of amino acids and inhibitory neurotransmission in ventrobasal thalamus, a region of the brain responsible for processing nociceptive information. In the first manuscript we examined the possibility that a proposed antagonist of receptors for endogenous amino acids was selective for β-amino acids. In the second manuscript, we determined the ionic mechanism of action of isovaline, a non-biogenic amino acid with chemical similarity to glycine and GABA. In the third manuscript, we determined that the inhibitory action of isovaline is mediated by metabotropic receptors, likely GABAB. In the first manuscript we used whole-cell patch clamp electrophysiology and immunohistochemistry to examine the differential antagonism of GABAAergic IPSCs by a proposed β-amino acid antagonist, TAG. In IPSCs that were attributable to both GABAergic and glycinergic stimulation, TAG significantly reduced both components. TAG had no effect in purely GABAAergic IPSCs. Our data supports the hypothesis that a specific GABAA subunit, α4, is sensitive to the β-amino acid antagonist, TAG. The second manuscript examines the ionic mechanism of action of isovaline, demonstrated to have analgesic properties in animal models. Isovaline inhibited action potential firing of thalamocortical neurons by activating a long-lasting potassium conductance that was insensitive to the glycine antagonist, strychnine. The sensitivity of isovaline currents to K⁺ channel blockers, their reversal near EK and Nernstian behavior on changing the extracellular [K⁺] confirmed K⁺ current involvement in isovaline inhibition. Since glycine receptors were not apparently involved in isovaline action, we proceeded to determine whether the actions of isovaline were mediated by a metabotropic receptor. In the third manuscript we showed that the long-lasting inhibition of isovaline was eliminated by preventing activation of G-proteins and by antagonism of GABAB receptors. Alteration of GABAB receptor function by CGP7930, an allosteric GABAB modulator, resulted in a potentiation of isovaline’s actions, and the current-voltage relationship of isovaline was similar to that of baclofen, a GABAB agonist. However isovaline also had actions that were different from baclofen. For example, isovaline increased a transient ‘A-type’ potassium current and did not activate some neurons that responded to baclofen.
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