2 research outputs found
Pharmacological Characterization of [<sup>3</sup>H]ATPCA as a Substrate for Studying the Functional Role of the Betaine/GABA Transporter 1 and the Creatine Transporter
The
betaine/γ-aminobutyric acid (GABA) transporter 1 (BGT1)
is one of the four GABA transporters (GATs) involved in the termination
of GABAergic neurotransmission. Although suggested to be implicated
in seizure management, the exact functional importance of BGT1 in
the brain is still elusive. This is partly owing to the lack of potent
and selective pharmacological tool compounds that can be used to probe
its function. We previously reported the identification of 2-amino-1,4,5,6-tetrahydropyrimidine-5-carboxylic
acid (ATPCA), a selective substrate for BGT1 over GAT1/GAT3, but also
an agonist for GABA<sub>A</sub> receptors. With the aim of providing
new functional insight into BGT1, we here present the synthesis and
pharmacological characterization of the tritiated analogue, [<sup>3</sup>H]ATPCA. Using traditional uptake assays at recombinant transporters
expressed in cell lines, [<sup>3</sup>H]ATPCA displayed a striking
selectivity for BGT1 among the four GATs (<i>K</i><sub>m</sub> and <i>V</i><sub>max</sub> values of 21 μM and 3.6
nmol ATPCA/(min × mg protein), respectively), but was also found
to be a substrate for the creatine transporter (CreaT). In experiments
with mouse cortical cell cultures, we observed a Na<sup>+</sup>-dependent
[<sup>3</sup>H]ATPCA uptake in neurons, but not in astrocytes. The
neuronal uptake could be inhibited by GABA, ATPCA, and a noncompetitive
BGT1-selective inhibitor, indicating functional BGT1 in neurons. In
conclusion, we report [<sup>3</sup>H]ATPCA as a novel radioactive
substrate for both BGT1 and CreaT. The dual activity of the radioligand
makes it most suitable for use in recombinant studies
Structure–Function Evaluation of Imidazopyridine Derivatives Selective for δ‑Subunit-Containing γ‑Aminobutyric Acid Type A (GABA<sub>A</sub>) Receptors
δ-Selective
compounds 1 and 2 (DS1, compound <b>22</b>; DS2, compound <b>16</b>) were introduced as functionally
selective modulators of δ-containing GABA type A receptors (GABA<sub>A</sub>R). In our hands, [<sup>3</sup>H]EBOB-binding experiments
with recombinant GABA<sub>A</sub>R and compound <b>22</b> showed
no proof of δ-selectivity, although there was a minimally higher
preference for the α4β3δ and α6β2/3δ
receptors with respect to potency. In order to delineate the structural
determinants of δ preferences, we synthesized 25 derivatives
of DS1 and DS2, and investigated their structure–activity relationships
(SAR). Four of our derivatives showed selectivity for α6β3δ
receptors (<b>29</b>, <b>38</b>, <b>39</b>, and <b>41</b>). For all of them, the major factors that distinguished
them from compound <b>22</b> were variations at the <i>para</i>-positions of their benzamide groups. However, two compounds
(<b>29</b> and <b>39</b>), when tested in the presence
of GABA, revealed effects at several additional GABA<sub>A</sub>R.
The newly synthesized compounds will still serve as useful tools to
investigate α6β3δ receptors