Ca ²⁺ activity at GABA _B receptors constitutively promotes metabotropic glutamate signaling in the absence of GABA

Abstract

Type B ␥-aminobutyric acid receptor (GABABR) is a G proteincoupled receptor that regulates neurotransmitter release and neuronal excitability throughout the brain. In various neurons, GABA BRs are concentrated at excitatory synapses. Although these receptors are assumed to respond to GABA spillover from neighboring inhibitory synapses, their function is not fully understood. Here we show a previously undescribed function of GABA BR exerted independent of GABA. In cerebellar Purkinje cells, interaction of GABA BR with extracellular Ca 2؉ (Ca o 2؉ ) leads to a constitutive increase in the glutamate sensitivity of metabotropic glutamate receptor 1 (mGluR1). mGluR1 sensitization is clearly mediated by GABA BR because it is absent in GABABR1 subunitknockout cells. However, the mGluR1 sensitization does not require G i/o proteins that mediate the GABABR's classical functions. Moreover, coimmunoprecipitation reveals complex formation between GABA BR and mGluR1 in the cerebellum. These findings demonstrate that GABA BR can act as Ca o 2؉ -dependent cofactors to enhance neuronal metabotropic glutamate signaling.T he type B ␥-aminobutyric acid receptor (GABA B R) is a G protein-coupled receptor (GPCR) distributed throughout the brain (1-3). GABA B R regulates neurotransmitter release and neuronal excitability via G i/o proteins (4). In the classic view, GABA B R responds to GABA released from inhibitory presynaptic terminals (4). However, in some central neurons including cerebellar Purkinje cells, postsynaptic GABA B Rs are concentrated perisynaptically at the excitatory synapses and present sparsely at the inhibitory synapses (5-7). Because GABA B Rs are insensitive to the excitatory neurotransmitter glutamate, a physiological role of GABA B R at excitatory synapses was assumed to depend on ␥-aminobutyric acid (GABA) spillover from neighboring inhibitory synapses (8, 9).The extracellular domain of GABA B R has an amino acid sequence homology to that of Ca 2ϩ -sensing receptor (CaR) (10). Some studies in the heterologous expression systems (11,12) revealed that GABA B R indeed interacts with extracellular Ca 2ϩ (Ca o 2ϩ ). Point mutation experiments indicate that the proximity of the GABA-binding site of GABA B R1 subunit (GBR1) is responsible for this interaction (11). Although Ca o 2ϩ -GABA B R interaction does not activate G proteins (12), it causes a remarkable conformational change of GABA B R as Ca o 2ϩ allosterically shifts GABA-GABA B R affinity (11,12 (22-25), and developmental synapse elimination (24, 26). In cerebellar Purkinje cells, mGluR1 colocalizes with GABA B R at the annuli of the dendritic spines innervated by excitatory parallel fibers (7,27). We have previously shown that mGluR1 signaling in Purkinje cells is enhanced as a consequence of interaction between Ca o 2ϩ and an unknown surface molecule(s) (28). For the reasons mentioned above, we considered GABA B R as a likely candidate for such a surface molecule.In Purkinje cells, mGluR1 outnumbers the other mGluR subtypes (16) and operates an inward ca...

Type-1 metabotropic glutamate receptor in cerebellar Purkinje cells: a key molecule responsible for long-term depression, endocannabinoid signalling and synapse elimination

“…GABA B R is localized perisynaptically at excitatory parallel fibre synapses on Purkinje cell dendritic spines where mGluR1 is concentrated (Kulik et al 2002). In addition to the G i/o protein-mediated enhancement, Tabata et al (2004) have shown that GABA B R activation enhances the mGluR1-mediated responses of cultured Purkinje cells through the G i/o protein-independent direct interaction between GABA B R and mGluR1. Surprisingly, this GABA B R-mGluR1 interaction occurs in the absence of GABA but is caused by extracellular Ca 2C (Tabata et al 2004), indicating that GABA B R can act as Ca 2C -dependent cofactor of mGluR1 signalling in the Purkinje cells.…”

“…In addition to the G i/o protein-mediated enhancement, Tabata et al (2004) have shown that GABA B R activation enhances the mGluR1-mediated responses of cultured Purkinje cells through the G i/o protein-independent direct interaction between GABA B R and mGluR1. Surprisingly, this GABA B R-mGluR1 interaction occurs in the absence of GABA but is caused by extracellular Ca 2C (Tabata et al 2004), indicating that GABA B R can act as Ca 2C -dependent cofactor of mGluR1 signalling in the Purkinje cells. These data suggest that LTD induction may be influenced by the GABA B R activity through G i/o protein-dependent and/or -independent pathway.…”

Localization and Expression of Group I Metabotropic Glutamate Receptors in the Mouse Striatum, Globus Pallidus, and Subthalamic Nucleus: Regulatory Effects of MPTP Treatment and Constitutive Homer Deletion

“…For instance, activation of mGluR5 enhances GABA A -mediated currents in a Ca 2ϩ -and PKC-dependent manner in retinal amacrine cells (Hoffpauir and Gleason, 2002). Activation of GABA B receptors can also enhance mGluR1-mediated EPSCs in cerebellar Purkinje neurons (Hirono et al, 2001;Tabata et al, 2004), from which these receptors coimmunoprecipitate (Tabata et al, 2004). Although not demonstrated in the basal ganglia, such functional interactions between group I mGluRs and GABA receptors could potentially influence neuronal activity of GP and STN neurons.…”

Loss of GABAB Receptors in Cochlear Neurons: Threshold Elevation Suggests Modulation of Outer Hair Cell Function by Type II Afferent Fibers

“…3). However, recent work in cerebellar Purkinje cells suggests that the GABA B receptor, whose extracellular domain has sequence homology to that of the Ca 2+ -sensing receptor, can bind Ca 2+ and in the absence of GABA and can modulate the glutamate sensitivity of mGluR1 receptors (Tabata et al 2004). Thus, GABA B1 signaling in type I or type II terminals could directly modulate (glutamatergic) afferent transmission from the hair cells without GABAergic input from efferent terminals.…”