The ␥-aminobutyric acid, type B (GABA B ) receptor is well recognized as being composed of two subunits, GABA B1 and GABA B2 . Both subunits share structural homology with other class-III G-protein-coupled receptors. They are composed of two main domains: a heptahelical domain (HD) typical of all G-protein-coupled receptors and a large extracellular domain (ECD). Although GABA B1 binds GABA, GABA B2 is required for GABA B1 to reach the cell surface. However, it is still not demonstrated whether the association of these two subunits is always required for function in the brain. Indeed, GABA B2 plays a major role in the coupling of the heteromer to G-proteins, such that it is possible that GABA B2 can transmit a signal in the absence of GABA B1 . Today only ligands interacting with GABA B1 ECD have been identified. Thus, the compounds acting exclusively on the GABA B2 subunit will be helpful in analyzing the specific role of this subunit in the brain. Here, we explored the mechanism of action of CGP7930, a compound described as a positive allosteric regulator of the GABA B receptor. We showed that it activates the wild type GABA B receptor but with a low efficacy. The GABA B2 HD is necessary for this effect, although one cannot exclude that CGP7930 could also bind to GABA B1 . Of interest, CGP7930 could activate GABA B2 expressed alone and is the first described agonist of GABA B2 . Finally, we show that CGP7930 retains its agonist activity on a GABA B2 subunit deleted of its ECD. This demonstrates that the HD of GABA B2 behaves similar to a rhodopsin-like receptor, because it can reach the cell surface alone, can couple to G-protein, and be activated by agonists. These data open new strategies for studying the mechanism of activation of GABA B receptor and examine any possible role of homomeric GABA B2 receptors.

Background: Mechanosensory information gathered by hair cells of the fish lateral-line system is collected by sensory neurons and sent to the ipsilateral hindbrain. The information is then conveyed to other brain structures through a second-order projection. In the adult, part of the second-order projection extends to the contralateral hindbrain, while another part connects to a midbrain structure, the torus semicircularis.

The GABA(B) (gamma-aminobutyric acid-B) receptor is composed of two subunits, GABA(B1) and GABA(B2). Both subunits share structural homology with other class-III G-protein-coupled receptors. They contain two main domains, a heptahelical domain typical of all G-protein-coupled receptors and a large ECD (extracellular domain). It has not been demonstrated whether the association of these two subunits is always required for function. However, GABA(B2) plays a major role in coupling with G-proteins, and GABA(B1) has been shown to bind GABA. To date, only ligands interacting with GABA(B1)-ECD have been identified. In the present study, we explored the mechanism of action of CGP7930, a compound described as a positive allosteric regulator of the GABA(B) receptor. We have shown that it can weakly activate the wild-type GABA(B) receptor, but also the GABA(B2) expressed alone, thus being the first described agonist of GABA(B2). CGP7930 retains its weak agonist activity on a GABA(B2) subunit deleted of its ECD. Thus the heptahelical domain of GABA(B2) behaves similar to a rhodopsin-like receptor. These results open new strategies for studying the mechanism of activation of GABA(B) receptor and examine any possible role of GABA(B2).