GABAB Receptors
The GABAB receptor was first recognized over 20 years ago, although the selective agonist for the receptor, p-β-chloro-phenyl-GABA (baclofen), had already been marketed as an antispastic agent years earlier with little knowledge about its site of action. The receptor on which baclofen acts is coupled via Gi/Go proteins to calcium and potassium channels as well as adenylyl cyclase in neurons and hence is classified as a metabotropic receptor. Synaptic activation of the receptor in many brain regions produces a slow inhibitory post-synaptic potential (ipsp) contrasting with the fast ipsp produced by GABAA receptor activation. The GABAB receptor is not only located post-synaptically, but is also present on pre-synaptic terminals where its activation modulates the release of neurotransmitters. This is clearly evident in spinal cord where activation of the receptor on primary afferent terminals appears to be important in the modulation of nociceptive inputs, and on terminals of monosynaptic inputs to motoneurons in the production of muscle relaxation.
The first indication of the structure of the GABAB receptor emerged in 1997 when Bettler and colleagues identified a large molecular weight (130 kDa), seven transmembrane spanning receptor protein, GABAB1. This was obtained using an expression cloning technique which was dependent on the development of the high affinity radiolabelled iodinated receptor ligand [125I]-CGP64213. No sequence homology with other seven transmembrane spanning receptors was observed, although 20% similarity to metabotropic glutamate receptors was noted. A year after this initial discovery, it was realized that GABAB1 is not expressed on the surface of cells without the support of a second receptor protein, referred to as GABAB2, which appears to couple to GABAB1 at the level of the endoplasmic reticulum in order to facilitate surface expression. GABAB2 also has a seven transmembrane spanning motif and links to GABAB1 at their intracellular C-terminals. The combination of these two proteins forms a heterodimer that is crucial for full receptor function. However, no GABA binding has been associated with GABAB2, although it appears that this protein may be more than just a 'trafficker' for GABAB1. Numerous isoforms of GABAB1 have been described with at least four forms of human GABAB1 protein. However, whether different combinations of these isoforms produce different pharmacological characteristics is not known. Even definitive evidence for the existence of subtypes of native GABAB receptors has yet to be shown, although there are data which support a separation based on neuropharmacological and neurochemical analysis. A variety of proteins that are unrelated to GABAB receptors, e.g. CREB2, have been shown to independently associate with high affinity to GABAB1 and GABAB2 proteins, although they fail to produce any receptor functionality.
A variety of agonists and antagonists for the GABAB receptor have been developed since the selective action of the archetypal agonist, baclofen, was first described. Notably, high as well as low affinity antagonists (nM - µM affinity), which penetrate the blood brain barrier, have been produced by Froestl and colleagues. However, the potential of any of these compounds as therapeutic agents is still to be fully realized, although basic research studies would suggest that the antagonists may suppress absence epilepsy seizures, improve cognitive impairment and even act as neuroprotective agents. Clinical trials with SGS742 (previously known as CGP36742) are currently in progress for mild cognitive impairment. The agonist, baclofen has already been shown to possess clinical efficacy as an anti-spasticity agent and may have analgesic properties in certain types of pain such as trigeminal neuralgia.
Studies by Urwyler et al. have demonstrated that the GABAB receptor can be positively modulated in an allosteric manner by CGP7930 and by GS39783. Neither compounds are receptor agonists but they appear to act on the heptahelical region of GABAB2 to enhance the action of the receptor agonists, GABA and baclofen.
The Table below contains accepted modulators and additional information. For a list of additional products, see the Materials section below.
Abbreviations
CGP7930: 2,6-Di-tert-butyl-4-(3-hydroxy-2,2-dimethyl-propyl)-phenol
CGP13501: 3-(3',5'-Di-tert-butyl-4'-hydroxy)phenyl-2,2-dimethylpropanal
CGP35348: 3-Aminopropyl-diethoxymethyl-phosphinic acid
CGP36742: 3-Aminopropyl-n-butyl-phosphinic acid
CGP52432: [3-[[(3,4-Dichlorophenyl)methyl]amino]propyl](diethoxy-methyl)phosphinic acid
CGP54626: (3-N[[1-(S)-(3,4-Dichlorophenyl)ethyl]amino-2-(S)-hydroxypropyl)-cyclohexylmethylphosphinic acid
CGP55845: (3-N[[1-(S)-(3,4-Dichlorophenyl)ethyl]amino-2-(S)-hydroxypropyl)-benzyl-phosphinic acid
CGP62349: 3-{1-(R)-[2-(S)-Hydroxy-3-[hydroxy-(4-methoxy-benzyl)-phosphinoyl]-propylamino]-ethyl}-benzoic acid
CGP64213: 3-{1-(R)-[2-(S)-Hydroxy-3-[hydroxy-(5-[3-(4-hydroxy-3-iodo-phenyl)-propionylamino]-pentyl-phosphinoyl)-propylamino]-ethyl}-benzoic acid
CGP71872: 3-{[1-(R)-(3-[[5-(4-Azido-2-hydroxy-5-iodo-benzoylamino)-pentyl]-hydroxy-phosphinoyl]-2-(S)-hydroxy-propylamino)-ethyl}-benzoic acid
SCH-50911: (+)-(S)-5,5-Dimethylmorpholinyl-2-acetic acid
References
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