δ-Selective compounds 1 and 2 (DS1, compound 22; DS2, compound 16) were introduced as functionally selective modulators of δ-containing GABA type A receptors (GABAR). In our hands, [H]EBOB-binding experiments with recombinant GABAR and compound 22 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 (29, 38, 39, and 41). For all of them, the major factors that distinguished them from compound 22 were variations at the para-positions of their benzamide groups. However, two compounds (29 and 39), when tested in the presence of GABA, revealed effects at several additional GABAR. The newly synthesized compounds will still serve as useful tools to investigate α6β3δ receptors.

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 receptors. With the aim of providing new functional insight into BGT1, we here present the synthesis and pharmacological characterization of the tritiated analogue, [H]ATPCA. Using traditional uptake assays at recombinant transporters expressed in cell lines, [H]ATPCA displayed a striking selectivity for BGT1 among the four GATs ( K and V 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-dependent [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 [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.

Imbalances in GABA-mediated tonic inhibition are involved in several pathophysiological conditions. A classical way of controlling tonic inhibition is through pharmacological intervention with extrasynaptic GABA receptors that sense ambient GABA and mediate a persistent GABAergic conductance. An increase in tonic inhibition may, however, also be obtained indirectly by inhibiting glial GABA transporters (GATs). These are sodium-coupled membrane transport proteins that normally act to terminate GABA neurotransmitter action by taking up GABA into surrounding astrocytes. The aim of the review is to provide an overview of glial GATs in regulating tonic inhibition, especially in epilepsy and stroke. This entails a comprehensive summary of changes known to occur in GAT expression levels and signalling following epileptic and ischemic insults. Further, we discuss the accumulating pharmacological evidence for targeting GATs in these diseases.

γ-Hydroxybutyric acid (GHB) is a neuroactive substance with specific high-affinity binding sites. To facilitate target identification and ligand optimization, we herein report a comprehensive structure-affinity relationship study for novel ligands targeting these binding sites. A molecular hybridization strategy was used based on the conformationally restricted 3-hydroxycyclopent-1-enecarboxylic acid (HOCPCA) and the linear GHB analog trans-4-hydroxycrotonic acid (T-HCA). In general, all structural modifications performed on HOCPCA led to reduced affinity. In contrast, introduction of diaromatic substituents into the 4-position of T-HCA led to high-affinity analogs (medium nanomolar K) for the GHB high-affinity binding sites as the most high-affinity analogs reported to date. The SAR data formed the basis for a three-dimensional pharmacophore model for GHB ligands, which identified molecular features important for high-affinity binding, with high predictive validity. These findings will be valuable in the further processes of both target characterization and ligand identification for the high-affinity GHB binding sites.

γ-Aminobutyric acid (GABA) neurotransmission is terminated by the GABA transporters (GATs) via uptake of GABA into neurons and surrounding glial cells. Four different transporters have been identified: GAT1, GAT2, GAT3, and the betaine/GABA transporter 1 (BGT1). The GAT1 subtype is the most explored transporter due to its high abundance in the brain and the existence of selective and potent GAT1 inhibitors. Consequently, less is known about the role and therapeutic potential of the non-GAT1 subtypes. Emerging pharmacological evidence suggests that some of these transporters pose interesting targets in several brain disorders. Pharmacological non-GAT1-selective tool compounds are important to further investigate the involvement of GATs in different pathological conditions. Extensive medicinal chemistry efforts have been put into the development of subtype-selective inhibitors, but truly selective and potent inhibitors of non-GAT1 subtypes are still limited. This review covers the advances within the medicinal chemistry area and the structural basis for obtaining non-GAT1-selective inhibitors.