GABA receptor
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GABA receptors are proteins utilized for the primary inhibitory neurotransmitter in vertebrate central nervous systems, gamma-aminobutyric acid or GABA (Kerr, 1995). Upon binding with GABA, GABAB receptors utilize a second messenger amplification pathway that ultimately results in an inhibitory signal for neuronal transmission. This pathway for signal transmission differs from GABAA receptors, which are considered ligand-gated ion channels as the binding of GABA results in the opening of ion channels leading to the inhibition of a neuronal signal.
StructureStructure
There are two major classes of GABA receptors abundant throughout neuronal cell types, ionotropic and metabotropic (Cryan, 2005). Metabotropic GABAB receptors are a specific division of the GABA receptor that induce a change in membrane potential through the action of a second messenger pathway (Kerr, 1995). The GABAB receptor functions as a heterodimer of two subunits, GABAB1 and GABAB2 (Figure 1: GABAB1 - gray; GABAB2 - green). Heterodimerization is accomplished using interactions of the coiled-coil motifs within the C-termini and interactions between the transmembrane and extracellular domains (Cryan, 2005). Additionally, there are two GABAB1 isoforms that differ at the N-termini where there are two sushi domains (Cryan, 2005). These sushi domains on the GABAB1 subunit are key to the receptor's interaction with other proteins as well as axonal signaling (Cryan, 2005). The two GABAB subunits link together as GABAB2 connects to GABAB1 at the endoplasmic reticulum via their intracellular C-terminus to form the heterodimer GABAB receptor (GABA B Receptors, 2015).
The GABAB receptor exists in two different forms - in the resting state (Figure 1) and the active state (Figure 2)(Geng, 2013). Geng et. al. has found, using the GABAB crystal structures, that both subunits exist in open conformations while at rest. Upon binding with the agonist, the GABAB1 subunit closes(Geng, 2013) (see below). Additionally, it was found that the agonist is bound in the interdomain crevice of the GABAB1 subunit due to an overlap of amino acid residues (Geng, 2013). This conformation change is highlighted in Figures 1 and 2 as one may notice the reduction in the space between GABAB subunits upon binding with GABA.
FunctionFunction
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It has been found that GABAB receptors provide an inhibitory function through the coupling to G-proteins and the recruitment of second messengers (Bettler, 2004). Presynaptic GABAB receptors effectively repress the influx of calcium ions (Ca2+) via the inhibition of voltage gated Ca2+ channels through the activation of the Gβγ subunits (Bettler, 2004). Postsynaptic GABAB receptors then activate the opening of potassium ion (K+) channels, again through the activation of Gβγ subunits (Bettler, 2004). The efflux of potassium ions results in the hyperpolarization of the neuronal membrane due to the greatly negative Nernst value of potassium common to cerebrospinal fluid. This hyperpolarization of the neuronal membrane causes the neuron’s membrane potential to move away from threshold, thus inhibiting the GABA neuronal function (Bettler, 2004). This functions in opposition of the GABAA receptor in order to control and slow the inhibitory postsynaptic potentials as the GABAA receptor activation causes a more transient inhibitory signal (Cryan, 2005). Aside from the interaction with ion channels, GABAB receptors also inhibit adenylyl cyclase through the Giα/Goα subunits and activate adenylyl cyclase through Gβγ subunits (Bettler, 2004). This control of adenylyl cyclase is expected to control neuronal function for a longer period of time compared to the control via ion channels (Geng, 2013).
DiseaseDisease
GABAB receptors are targeted for a number of treatments in the clinical setting of neurodegenerative and pathophysiolocial disorders including epilepsy, spasticity, chronic pain, anxiety, depression, schizophrenia, cognitive function, gastro-esophageal reflux and drug dependence and addiction (Kerr 2005).
There is increasing evidence that links schizophrenia directly to GABAB receptor deficits. GABBR1, the gene associated with the expression of the GABAB1 receptor was found to have a high amount of methylation in receptors tested for patients with schizophrenia (Citrine et al. 2009). The receptor 1 gene is on chromosome 6 where the locus is susceptible for disorders such as multiple sclerosis, epilepsy, and schizophrenia (GABA B Receptors, 2015). Presynaptic dopaminergic terminals (i.e. neuronal terminals that secrete dopamine) have GABAB receptors that are involved in the release of dopamine along with modulation of glutaminergic regulation of dopamine (Citrome et al. 2009).
A possible therapeutic approach utilizing GABAB receptors would be for the treatment of substance use disorder (i.e. drug addition). Since the GABAB receptor plays a crucial role in mediating behavioral and molecular effects of drug abuse, the GABAB receptor can be utilized as a potential anti-addictive therapeutic strategy (Flip et. al, 2015). Agonists at GABAB receptors can promote abstinence or decrease and control the reinforcing effects of drugs on the mind (Kerr 2005).
ReferencesReferences
Bettler, B., Kaupmann, K., Mosbacher, J., & Gassmann, M. (2004). Molecular structure and physiological functions of GABAB receptors. Physiological reviews, 84(3), 835-867.