The (S)-enantiomer of 2-hydroxysaclofen is the active GABAB receptor antagonist in central and peripheral preparations

Molecular mechanisms of metabotropic GABAB receptor function

Metabotropic γ-aminobutyric acid G protein-coupled receptors (GABA_B) represent one of the two main types of inhibitory neurotransmitter receptors in the brain. These receptors act both pre- and postsynaptically by modulating the transmission of neuronal signals and are involved in a range of neurological diseases, from alcohol addiction to epilepsy. A series of recent cryo-EM studies revealed critical details of the activation mechanism of GABA_B Structures are now available for the receptor bound to ligands with different modes of action, including antagonists, agonists, and positive allosteric modulators, and captured in different conformational states from the inactive apo to the fully active state bound to a G protein. These discoveries provide comprehensive insights into the activation of the GABA_B receptor, which not only broaden our understanding of its structure, pharmacology, and physiological effects but also will ultimately facilitate the discovery of new therapeutic drugs and neuromodulators.

Structural mechanism of ligand activation in human GABA(B) receptor

Human GABA(B) (γ-aminobutyric acid class B) receptor is a G-protein-coupled receptor central to inhibitory neurotransmission in the brain. It functions as an obligatory heterodimer of the subunits GBR1 and GBR2. Here we present the crystal structures of a heterodimeric complex between the extracellular domains of GBR1 and GBR2 in the apo, agonist-bound and antagonist-bound forms. The apo and antagonist-bound structures represent the resting state of the receptor; the agonist-bound complex corresponds to the active state. Both subunits adopt an open conformation at rest, and only GBR1 closes on agonist-induced receptor activation. The agonists and antagonists are anchored in the interdomain crevice of GBR1 by an overlapping set of residues. An antagonist confines GBR1 to the open conformation of the inactive state, whereas an agonist induces its domain closure for activation. Our data reveal a unique activation mechanism for GABA(B) receptor that involves the formation of a novel heterodimer interface between subunits.

Distribution of GABA(B) receptor mRNAs in the rat brain and peripheral organs

GABA, the predominant inhibitory neurotransmitter present in the mammalian CNS, is also found in the periphery. GABA actions are mediated by the ionotropic GABA(A)/GABA(C) receptors, as well as the metabotropic GABA(B) receptor. The rat GABA(B) receptor has recently been cloned and two cDNA clones have been isolated encoding two isoforms of the receptor, GABA(B)R1a and R1b. Northern blot analysis revealed the presence of both transcripts in the rat brain using specific cDNA probes for GABA(B)R1a and R1b, respectively. However, Northern blot analysis, hybridized with a probe containing a sequence common to both isoforms, revealed specific RNAs in the rat brain and in testis, but not in other peripheral tissues. In the present study, by using the more sensitive reverse transcriptase-polymerase chain reaction with a specific set of primers for each isoform and Southern blot analysis, we found that both isoforms of the GABA(B) receptor are expressed not only throughout the brain but also in all peripheral organs examined, including heart, spleen, lung, liver, small intestine, large intestine, kidney, stomach, adrenal, testis, ovary and urinary bladder. The peripheral distribution of GABA(B)R1 mRNAs supports the notion of a physiological role for GABA in the control of a wide range of peripheral organs.

GABAB receptors in the dorsomedial hypothalamus and heart rate in anesthetized rats

Previous studies have shown that: (1) activation of neurons in the dorsomedial hypothalamus (DMH) of the rat by blockade of local GABAA receptors with bicuculline methiodide (BMI) elicits cardiovascular changes resembling those seen in experimental stress, including marked sympathetically-mediated tachycardia, and (2) inhibition of neurons in the same region by local microinjection of the GABAA receptor agonist muscimol can virtually abolish stress-induced tachycardia. This study examined the possibility that GABAB receptors exist in the neural circuitry of the DMH, and that stimulation of these receptors might suppress the cardiovascular response to local disinhibition with BMI. Microinjection of BMI 10 pmol into the DMH in urethane-anesthetized rats resulted in marked tachycardia with little or no effect on arterial pressure. Simultaneous injection of the GABAB receptor agonist baclofen at doses of 2.5, 5.0 and 10 pmol produced dose-related suppression of BMI induced tachycardia. Coinjection of the GABAB receptor antagonist 2-hydroxysaclofen 100 or 200 pmol had no significant effect on the heart rate response to BMI, but reversed the suppression elicited in the presence of baclofen. These findings indicate that (1) functional GABAB receptors exist in the DMH, and (2) stimulation of these receptors inhibits the tachycardia resulting from blockade of local GABAA receptors.

GABAB receptor antagonism by resolved (R)-saclofen in the guinea-pig ileum

The GABAB receptor antagonist saclofen (3-amino-2-(4-chlorophenyl)propylsulphonic acid) has been resolved by chiral high-performance liquid chromatography. The enantiomer (R)-saclofen, but not (S)-saclofen, reversibly antagonised the (R,S)-baclofen-induced depression of cholinergic twitch contractions in the guinea-pig ileum with an apparent pA2 of 5.3. Also, 2-hydroxy-saclofen was resolved by the same method, its (S)-enantiomer yielding an apparent pA2 of 5.0. This method provides a convenient resolution of these antagonists.