| Summary: | In central neurons, the endogenous amino acid γ-aminobutyric acid (GABA) exerts synaptic inhibition mediated through ionotropic GABAA-, or metabotropic GABAB-receptors. These receptors exist on both pre- and postsynaptic membranes. The synthetic structural analogues of GABA, 6-aminomethyl-3-methyl-4H-1,2,4-benzothiadiazine-1,1-dioxide (AMBD) and R-isovaline have received little study on synaptic inhibition in the mammalian thalamus. AMBD was originally proposed as a taurine antagonist whereas R-isovaline is a non-biogenic amino acid that increases postsynaptic K⁺ conductance of thalamocortical neurons. The aim of this work was to assess the prediction that AMBD and R-isovaline would affect presynaptic release of GABA onto neurons of ventrobasal nuclei.
AMBD and R-isovaline were applied by perfusion of thalamic slices obtained from juvenile Sprague-Dawley rats (P10 -13). During whole-cell patch clamp recording from thalamocortical neurons, we voltage-clamped neurons at a holding potential of -70 mV. Miniature inhibitory postsynaptic currents (mIPSCs) were recorded in the presence of tetrodotoxin (TTX). Kynurenic acid and internal Cs⁺ were used to block postsynaptic glutamate receptors and K⁺ conductances. We used the GABAA antagonist bicuculline to identify GABAergic mIPSCs, without affecting a possible presynaptic GABAB-component.
Applied alone at 250 μM, AMBD had no effect on the passive and active membrane properties of neurons. In the range of 10 µM to 1 mM, AMBD had no effect on amplitude or decay time constant of GABAergic mIPSCs. Acting with an IC₅₀ of 232 μM, AMBD reversibly reduced the frequency of GABAergic mIPSCs. The above observations implied that AMBD reduced presynaptic release of GABA. In a range of 25 to 200 µM, R-isovaline had no effect on the holding current or frequency, amplitude and decay time constant of GABAergic mIPSCs. Hence, R-isovaline did not affect release of GABA and did not affect receptors on nerve terminals.
In summary, AMBD reversibly decreased the presynaptic release of GABA, likely by an action on nerve terminals while having no effects on postsynaptic membrane properties that could account for the reduced frequency of GABAergic mIPSCs. The exact mechanism whereby AMBD decreased GABA release remains unclear. R-isovaline had no effect on GABA release in ventrobasal nuclei.
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