Sustained glutamate receptor activation down-regulates GABAB receptors by shifting the balance from recycling to lysosomal degradation

Somatostatin and the pathophysiology of Alzheimer's disease

Among the central features of Alzheimer's disease (AD) progression are altered levels of the neuropeptide somatostatin (SST), and the colocalisation of SST-positive interneurons (SST-INs) with amyloid-β plaques, leading to cell death. In this theoretical review, I propose a molecular model for the pathogenesis of AD based on SST-IN hypofunction and hyperactivity. Namely, hypofunctional and hyperactive SST-INs struggle to control hyperactivity in medial regions in early stages, leading to axonal Aβ production through excessive presynaptic GABAB inhibition, GABAB1a/APP complex downregulation and internalisation. Concomitantly, excessive SST-14 release accumulates near SST-INs in the form of amyloids, which bind to Aβ to form toxic mixed oligomers. This leads to differential SST-IN death through excitotoxicity, further disinhibition, SST deficits, and increased Aβ release, fibrillation and plaque formation. Aβ plaques, hyperactive networks and SST-IN distributions thereby tightly overlap in the brain. Conversely, chronic stimulation of postsynaptic SST2/4 on gulutamatergic neurons by hyperactive SST-INs promotes intense Mitogen-Activated Protein Kinase (MAPK) p38 activity, leading to somatodendritic p-tau staining and apoptosis/neurodegeneration - in agreement with a near complete overlap between p38 and neurofibrillary tangles. This model is suitable to explain some of the principal risk factors and markers of AD progression, including mitochondrial dysfunction, APOE4 genotype, sex-dependent vulnerability, overactive glial cells, dystrophic neurites, synaptic/spine losses, inter alia. Finally, the model can also shed light on qualitative aspects of AD neuropsychology, especially within the domains of spatial and declarative (episodic, semantic) memory, under an overlying pattern of contextual indiscrimination, ensemble instability, interference and generalisation.

Restoring GABAB receptor expression in the ventral tegmental area of methamphetamine addicted mice inhibits locomotor sensitization and drug seeking behavior

Repeated exposure to psychostimulants such as methamphetamine (METH) induces neuronal adaptations in the mesocorticolimbic dopamine system, including the ventral tegmental area (VTA). These changes lead to persistently enhanced neuronal activity causing increased dopamine release and addictive phenotypes. A factor contributing to increased dopaminergic activity in this system appears to be reduced GABAB receptor-mediated neuronal inhibition in the VTA. Dephosphorylation of serine 783 (Ser783) of the GABAB2 subunit by protein phosphatase 2A (PP2A) appears to trigger the downregulation GABAB receptors in psychostimulant-addicted rodents. Therefore, preventing the interaction of GABAB receptors with PP2A using an interfering peptide is a promising strategy to restore GABAB receptor-mediated neuronal inhibition. We have previously developed an interfering peptide (PP2A-Pep) that inhibits the GABAB receptors/PP2A interaction and thereby restores receptor expression under pathological conditions. Here, we tested the hypothesis that restoration of GABAB receptor expression in the VTA of METH addicted mice reduce addictive phenotypes. We found that the expression of GABAB receptors was significantly reduced in the VTA and nucleus accumbens but not in the hippocampus and somatosensory cortex of METH-addicted mice. Infusion of PP2A-Pep into the VTA of METH-addicted mice restored GABAB receptor expression in the VTA and inhibited METH-induced locomotor sensitization as assessed in the open field test. Moreover, administration of PP2A-Pep into the VTA also reduced drug seeking behavior in the conditioned place preference test. These observations underscore the importance of VTA GABAB receptors in controlling addictive phenotypes. Furthermore, this study illustrates the value of interfering peptides targeting diseases-related protein-protein interactions as an alternative approach for a potential development of selective therapeutic interventions.

ERK1/2-Dependent Phosphorylation of GABAB1(S867/T872), Controlled by CaMKIIβ, Is Required for GABAB Receptor Degradation under Physiological and Pathological Conditions

GABAB receptor-mediated inhibition is indispensable for maintaining a healthy neuronal excitation/inhibition balance. Many neurological diseases are associated with a disturbed excitation/inhibition balance and downregulation of GABAB receptors due to enhanced sorting of the receptors to lysosomal degradation. A key event triggering the downregulation of the receptors is the phosphorylation of S867 in the GABAB1 subunit mediated by CaMKIIβ. Interestingly, close to S867 in GABAB1 exists another phosphorylation site, T872. Therefore, the question arose as to whether phosphorylation of T872 is involved in downregulating the receptors and whether phosphorylation of this site is also mediated by CaMKIIβ or by another protein kinase. Here, we show that mutational inactivation of T872 in GABAB1 prevented the degradation of the receptors in cultured neurons. We found that, in addition to CaMKIIβ, also ERK1/2 is involved in the degradation pathway of GABAB receptors under physiological and ischemic conditions. In contrast to our previous view, CaMKIIβ does not appear to directly phosphorylate S867. Instead, the data support a mechanism in which CaMKIIβ activates ERK1/2, which then phosphorylates S867 and T872 in GABAB1. Blocking ERK activity after subjecting neurons to ischemic stress completely restored downregulated GABAB receptor expression to normal levels. Thus, preventing ERK1/2-mediated phosphorylation of S867/T872 in GABAB1 is an opportunity to inhibit the pathological downregulation of the receptors after ischemic stress and is expected to restore a healthy neuronal excitation/inhibition balance.

Ethanol blocks a novel form of iLTD, but not iLTP of inhibitory inputs to VTA GABA neurons

The ventral tegmental area (VTA) is an essential component of the mesocorticolimbic dopamine (DA) circuit that processes reward and motivated behaviors. The VTA contains DA neurons essential in this process, as well as GABAergic inhibitory cells that regulate DA cell activity. In response to drug exposure, synaptic connections of the VTA circuit can be rewired via synaptic plasticity-a phenomenon thought to be responsible for the pathology of drug dependence. While synaptic plasticity to VTA DA neurons as well as prefrontal cortex to nucleus accumbens GABA neurons are well studied, VTA GABA cell plasticity, specifically inhibitory inputs to VTA GABA neurons, is less understood. Therefore, we investigated the plasticity of these inhibitory inputs. Using whole cell electrophysiology in GAD67-GFP mice to identify GABA cells, we observed that these VTA GABA cells experience either inhibitory GABAergic long-term potentiation (iLTP) or inhibitory long-term depression (iLTD) in response to a 5 Hz stimulus. Paired pulse ratios, coefficient of variance, and failure rates suggest a presynaptic mechanism for both plasticity types, where iLTP is NMDA receptor-dependent and iLTD is GABAB receptor-dependent-this being the first report of iLTD onto VTA GABA cells. As illicit drug exposure can alter VTA plasticity, we employed chronic intermittent exposure (CIE) to ethanol (EtOH) vapor in male and female mice to examine its potential impact on VTA GABA input plasticity. Chronic EtOH vapor exposure produced measurable behavioral changes illustrating dependence and concomitantly prevented previously observed iLTD, which continued in air-exposed controls, illustrating the impact of EtOH on VTA neurocircuitry and suggesting physiologic mechanisms at play in alcohol use disorder and withdrawal states. Taken together, these novel findings of unique GABAergic synapses exhibiting either iLTP or iLTD within the mesolimbic circuit, and EtOH blockade specifically of iLTD, characterize inhibitory VTA plasticity as a malleable, experience-dependent system modified by EtOH.

Protein phosphatase 2A regulation of GABAB receptors normalizes ischemia-induced aberrant receptor trafficking and provides neuroprotection

One major factor regulating the strength of GABA_B receptor signaling and thereby neuronal excitability is the dynamic control of their cell surface expression. GABA_B receptors are constitutively internalized and recycled back to the plasma membrane to maintain a stable number of receptors at cell surface for appropriate signaling. Protein phosphatase 2A (PP2A) dependent dephosphorylation of serine 783 (S783) in the GABA_B2 subunit is a key event for downregulating GABA_B receptor cell surface expression particularly under conditions associated with excitotoxicity. Here, we investigated the role of PP2A in regulating GABA_B receptor cell surface expression under physiological and excitotoxic conditions. For this purpose, we developed an interfering peptide (PP2A-Pep) that inhibits the interaction of GABA_B receptors with PP2A. Using cultured cortical neurons, we found that PP2A downregulates GABA_B receptor cell surface expression by inhibiting recycling of the receptors and thereby promoting degradation of the receptors. Inhibition of the GABA_B receptor/PP2A interaction by PP2A-Pep in cultured cortical neurons restored GABA_B receptor cell surface expression after excitotoxic stress and inhibited progressing neuronal death even when added 48 h after the insult. To explore the therapeutic potential of PP2A-Pep, we further analyzed effect of PP2A-Pep in the middle cerebral artery occlusion (MCAO) mouse model of cerebral ischemia. Incubation of brain slices prepared from MCAO-treated mice with PP2A-Pep restored normal GABA_B receptor expression and GABA_B receptor-mediated inhibition, reduced ischemic-induced overexcitability of neurons, and prevented neuronal death in the ischemic penumbra. This data illustrates the crucial role of regulating GABA_B receptor phosphorylation by PP2A for controlling neuronal inhibition and excitability. The results further suggest that interfering with the GABA_B receptor/PP2A interaction is a promising strategy for the development of specific therapeutic interventions to treat neurological diseases associated with a disturbed excitation/inhibition balance and downregulation of GABA_B receptors.

Homotopic contralesional excitation suppresses spontaneous circuit repair and global network reconnections following ischemic stroke

Understanding circuit-level manipulations that affect the brain's capacity for plasticity will inform the design of targeted interventions that enhance recovery after stroke. Following stroke, increased contralesional activity (e.g. use of the unaffected limb) can negatively influence recovery, but it is unknown which specific neural connections exert this influence, and to what extent increased contralesional activity affects systems- and molecular-level biomarkers of recovery. Here, we combine optogenetic photostimulation with optical intrinsic signal imaging to examine how contralesional excitatory activity affects cortical remodeling after stroke in mice. Following photothrombosis of left primary somatosensory forepaw (S1FP) cortex, mice either recovered spontaneously or received chronic optogenetic excitation of right S1FP over the course of 4 weeks. Contralesional excitation suppressed perilesional S1FP remapping and was associated with abnormal patterns of stimulus-evoked activity in the unaffected limb. This maneuver also prevented the restoration of resting-state functional connectivity (RSFC) within the S1FP network, RSFC in several networks functionally distinct from somatomotor regions, and resulted in persistent limb-use asymmetry. In stimulated mice, perilesional tissue exhibited transcriptional changes in several genes relevant for recovery. Our results suggest that contralesional excitation impedes local and global circuit reconnection through suppression of cortical activity and several neuroplasticity-related genes after stroke, and highlight the importance of site selection for targeted therapeutic interventions after focal ischemia.

Targeting the interaction of GABAB receptors with CaMKII with an interfering peptide restores receptor expression after cerebral ischemia and inhibits progressive neuronal death in mouse brain cells and slices

Cerebral ischemia is the leading cause for long-term disability and mortality in adults due to massive neuronal death. Currently, there is no pharmacological treatment available to limit progressive neuronal death after stroke. A major mechanism causing ischemia-induced neuronal death is the excessive release of glutamate and the associated overexcitation of neurons (excitotoxicity). Normally, GABA_B receptors control neuronal excitability in the brain via prolonged inhibition. However, excitotoxic conditions rapidly downregulate GABA_B receptors via a CaMKII-mediated mechanism and thereby diminish adequate inhibition that could counteract neuronal overexcitation and neuronal death. To prevent the deleterious downregulation of GABA_B receptors, we developed a cell-penetrating synthetic peptide (R1-Pep) that inhibits the interaction of GABA_B receptors with CaMKII. Administration of this peptide to cultured cortical neurons exposed to excitotoxic conditions restored cell surface expression and function of GABA_B receptors. R1-Pep did not affect CaMKII expression or activity but prevented its T286 autophosphorylation that renders it autonomously and persistently active. Moreover, R1-Pep counteracted the aberrant downregulation of G protein-coupled inwardly rectifying K⁺ channels and the upregulation of N-type voltage-gated Ca²⁺ channels, the main effectors of GABA_B receptors. The restoration of GABA_B receptors activated the Akt survival pathway and inhibited excitotoxic neuronal death with a wide time window in cultured neurons. Restoration of GABA_B receptors and neuroprotective activity of R1-Pep was verified by using brain slices prepared from mice after middle cerebral artery occlusion (MCAO). Treatment with R1-Pep restored normal GABA_B receptor expression and GABA receptor-mediated K⁺ channel currents. This reduced MCAO-induced neuronal excitability and inhibited neuronal death. These results support the hypothesis that restoration of GABA_B receptor expression under excitatory conditions provides neuroprotection and might be the basis for the development of a selective intervention to inhibit progressive neuronal death after ischemic stroke.

Targeting the Interaction of GABAB Receptors With CHOP After an Ischemic Insult Restores Receptor Expression and Inhibits Progressive Neuronal Death

GABA_B receptors control neuronal excitability via slow and prolonged inhibition in the central nervous system. One important function of GABA_B receptors under physiological condition is to prevent neurons from shifting into an overexcitation state which can lead to excitotoxic death. However, under ischemic conditions, GABA_B receptors are downregulated, fostering over-excitation and excitotoxicity. One mechanism downregulating GABA_B receptors is mediated via the interaction with the endoplasmic reticulum (ER) stress-induced transcription factor CHOP. In this study, we investigated the hypothesis that preventing the interaction of CHOP with GABA_B receptors after an ischemic insult restores normal expression of GABA_B receptors and reduces neuronal death. For this, we designed an interfering peptide (R2-Pep) that restored the CHOP-induced downregulation of cell surface GABA_B receptors in cultured cortical neurons subjected to oxygen and glucose deprivation (OGD). Administration of R2-Pep after OGD restored normal cell surface expression of GABA_B receptors as well as GABA_B receptor-mediated inhibition. As a result, R2-Pep reduced enhanced neuronal activity and inhibited progressive neuronal death in OGD stressed cultures. Thus, targeting diseases relevant protein-protein interactions might be a promising strategy for developing highly specific novel therapeutics.

Medial prefrontal cortex Notch1 signalling mediates methamphetamine-induced psychosis via Hes1-dependent suppression of GABAB1 receptor expression

Methamphetamine (METH), a widely abused stimulant drug, induces psychosis in approximately half of abusers; this effect is becoming a major concern for society. Although the Notch1 signalling pathway has been shown to play a part in the pathogenesis of some psychiatric disorders, its role in METH-induced psychosis (MIP) is still unknown. Here, the METH-induced locomotor sensitization model in rodents is considered to represent the underlying neurochemical changes driving psychoses. We found that the Notch1 signalling was downregulated in the medial prefrontal cortex (mPFC) in sensitized mice. Direct genetic and pharmacological manipulations of Notch1 signalling bidirectionally altered METH-induced locomotor sensitization and other MIP-related behaviours through governing neuronal activity in the mPFC. Moreover, Notch1 signalling negatively regulated GABA_B1 receptor expression in the mPFC of METH-sensitized mice through Hes1, a transcriptional repressor in Notch1 signalling. Further, we show that Hes1 can directly bind to the GABA_B1 receptor promoter. Notably, pharmacological regulation of the GABA_B receptor in the mPFC reversed the changes in METH-induced locomotor sensitization caused by the dysfunction of Notch1 signalling. Together, our findings uncover a previously unrecognised Notch1-Hes1-GABA_B1 receptor-dependent mechanism involved in regulating mPFC neuronal activity and behavioural phenotypes in MIP. Our work provides mechanistic insight into the aetiology and pathophysiology of MIP.

GABAB Receptors and Pain

A substantial fraction of the human population suffers from chronic pain states, which often cannot be sufficiently treated with existing drugs. This calls for alternative targets and strategies for the development of novel analgesics. There is substantial evidence that the G protein-coupled GABA_B receptor is involved in the processing of pain signals and thus has long been considered a valuable target for the generation of analgesics to treat chronic pain. In this review, the contribution of GABA_B receptors to the generation and modulation of pain signals, their involvement in chronic pain states as well as their target suitability for the development of novel analgesics is discussed.

Sustained Baclofen-Induced Activation of GABA B Receptors After Cerebral Ischemia Restores Receptor Expression and Function and Limits Progressing Loss of Neurons

One important function of GABA_B receptors is the control of neuronal activity to prevent overexcitation and thereby excitotoxic death, which is a hallmark of cerebral ischemia. Consequently, sustained activation of GABA_B receptors with the selective agonist baclofen provides neuroprotection in in vitro and in vivo models of cerebral ischemia. However, excitotoxic conditions severely downregulate the receptors, which would compromise the neuroprotective effectiveness of baclofen. On the other hand, recent work suggests that sustained activation of GABA_B receptors stabilizes receptor expression. Therefore, we addressed the question whether sustained activation of GABA_B receptors reduces downregulation of the receptor under excitotoxic conditions and thereby preserves GABA_B receptor-mediated inhibition. In cultured neurons subjected to oxygen and glucose deprivation (OGD), to mimic cerebral ischemia, GABA_B receptors were severely downregulated. Treatment of the cultures with baclofen after OGD restored GABA_B receptor expression and reduced loss of neurons. Restoration of GABA_B receptors was due to enhanced fast recycling of the receptors, which reduced OGD-induced sorting of the receptors to lysosomal degradation. Utilizing the middle cerebral artery occlusion (MCAO) mouse model of cerebral ischemia, we verified the severe downregulation of GABA_B receptors in the affected cortex and a partial restoration of the receptors after systemic injection of baclofen. Restored receptor expression recovered GABA_B receptor-mediated currents, normalized the enhanced neuronal excitability observed after MCAO and limited progressive loss of neurons. These results suggest that baclofen-induced restoration of GABA_B receptors provides the basis for the neuroprotective activity of baclofen after an ischemic insult. Since GABA_B receptors regulate multiple beneficial pathways, they are promising targets for a neuroprotective strategy in acute cerebral ischemia.

Reelin signaling modulates GABAB receptor function in the neocortex

Reelin is a protein that is best known for its role in controlling neuronal layer formation in the developing cortex. Here, we studied its role for post-natal cortical network function, which is poorly explored. To preclude early cortical migration defects caused by Reelin deficiency, we used a conditional Reelin knock-out (RelncKO ) mouse, and induced Reelin deficiency post-natally. Induced Reelin deficiency caused hyperexcitability of the neocortical network in vitro and ex vivo. Blocking Reelin binding to its receptors ApoER2 and VLDLR resulted in a similar effect. Hyperexcitability in RelncKO organotypic slice cultures could be rescued by co-culture with wild-type organotypic slice cultures. Moreover, the GABAB receptor (GABAB R) agonist baclofen failed to activate and the antagonist CGP35348 failed to block GABAB Rs in RelncKO mice. Immunolabeling of RelncKO cortical slices revealed a reduction in GABAB R1 and GABAB R2 surface expression at the plasma membrane and western blot of RelncKO cortical tissue revealed decreased phosphorylation of the GABAB R2 subunit at serine 892 and increased phosphorylation at serine 783, reflecting receptor deactivation and proteolysis. These data show a role of Reelin in controlling early network activity, by modulating GABAB R function. Cover Image for this issue: https://doi.org/10.1111/jnc.15054.

Mechanisms and Regulation of Neuronal GABAB Receptor-Dependent Signaling

γ-Aminobutyric acid B receptors (GABA_BRs) are broadly expressed throughout the central nervous system where they play an important role in regulating neuronal excitability and synaptic transmission. GABA_BRs are G protein-coupled receptors that mediate slow and sustained inhibitory actions via modulation of several downstream effector enzymes and ion channels. GABA_BRs are obligate heterodimers that associate with diverse arrays of proteins to form modular complexes that carry out distinct physiological functions. GABA_BR-dependent signaling is fine-tuned and regulated through a multitude of mechanisms that are relevant to physiological and pathophysiological states. This review summarizes the current knowledge on GABA_BR signal transduction and discusses key factors that influence the strength and sensitivity of GABA_BR-dependent signaling in neurons.

Haploinsufficiency of Tsc2 Leads to Hyperexcitability of Medial Prefrontal Cortex via Weakening of Tonic GABAB Receptor-mediated Inhibition

Loss-of-function mutation in one of the tumor suppressor genes TSC1 or TSC2 is associated with several neurological and psychiatric diseases, including autism spectrum disorders (ASDs). As an imbalance between excitatory and inhibitory neurotransmission, E/I ratio is believed to contribute to the development of these disorders, we investigated synaptic transmission during the first postnatal month using the Tsc2+/- mouse model. Electrophysiological recordings were performed in acute brain slices of medial prefrontal cortex. E/I ratio at postnatal day (P) 15-19 is increased in Tsc2+/- mice as compared with wildtype (WT). At P25-30, facilitated GABAergic transmission reduces E/I ratio to the WT level, but weakening of tonic GABAB receptor (GABABR)-mediated inhibition in Tsc2+/- mice leads to hyperexcitability both at single cell and neuronal network level. Short (1 h) preincubation of P25-30 Tsc2+/- slices with baclofen restores the GABABR-mediated inhibition and reduces network excitability. Interestingly, the same treatment at P15-19 leads to weakening of GABABR-mediated inhibition. We hypothesize that a dysfunction of tonic GABABR-mediated inhibition might contribute to the development of ASD symptoms and suggest that GABABR activation within an appropriate time window may be considered as a therapeutic target in ASD.

Cross-modal reaction of auditory and visual cortices after long-term bilateral hearing deprivation in the rat

Visual cortex (VC) over-activation analysed by evoked responses has been demonstrated in congenital deafness and after long-term acquired hearing loss in humans. However, permanent hearing deprivation has not yet been explored in animal models. Thus, the present study aimed to examine functional and molecular changes underlying the visual and auditory cross-modal reaction. For such purpose, we analysed cortical visual evoked potentials (VEPs) and the gene expression (RT-qPCR) of a set of markers for neuronal activation (c-Fos) and activity-dependent homeostatic compensation (Arc/Arg3.1). To determine the state of excitation and inhibition, we performed RT-qPCR and quantitative immunocytochemistry for excitatory (receptor subunits GluA2/3) and inhibitory (GABAA-α1, GABAB-R2, GAD65/67 and parvalbumin-PV) markers. VC over-activation was demonstrated by a significant increase in VEPs wave N1 and by up-regulation of the activity-dependent early genes c-Fos and Arc/Arg3.1 (thus confirming, by RT-qPCR, our previously published immunocytochemical results). GluA2 gene and protein expression were significantly increased in the auditory cortex (AC), particularly in layers 2/3 pyramidal neurons, but inhibitory markers (GAD65/67 and PV-GABA interneurons) were also significantly upregulated in the AC, indicating a concurrent increase in inhibition. Therefore, after permanent hearing loss in the rat, the VC is not only over-activated but also potentially balanced by homeostatic regulation, while excitatory and inhibitory markers remain imbalanced in the AC, most likely resulting from changes in horizontal intermodal regulation.

GABAB receptor activation attenuates inflammatory orofacial pain by modulating interleukin-1β in satellite glial cells: Role of NF-κB and MAPK signaling pathways

Orofacial inflammation could activate satellite glial cells (SGCs) in the trigeminal ganglion (TG) to produce interleukin 1β (IL-1β) which plays crucial roles in the development of inflammatory pain. Recent studies have shown that gamma-amino butyric acid-B (GABA_B) receptor could modulate the expression of inflammatory cytokines in microglia and astrocytes in the spinal cord. The objective of this study was to investigate whether GABA_B receptors in TG SGCs attenuate inflammatory facial pain via mediating IL-1β following inflammation and its mechanisms. Complete Freund's adjuvant (CFA) was injected into the whisker pad of rats to induce inflammation in vivo. Lipopolysaccharide (LPS) was added to culture medium to activate SGCs in vitro. Behavioral measures showed that microinjection of baclofen (a selective GABA_B receptor agonist) into the TG ameliorated the mechanical allodynia of CFA-treated rats. Interestingly, baclofen pretreatment inhibited SGC activation and IL-1β production, however, preserved the decreased expression of GABA_B receptors in SGCs activated by CFA in vivo and LPS in vitro. In addition, baclofen suppressed the increased expression of p-NF- κ B p65, p-I κ Bα, and p-p38 MAPK, while reversed the decreased production of I κ Bα, and further enhanced the increased expression of p-ERK(1/2) in LPS-treated SGCs in vitro. Finally, those effects of baclofen were abolished by saclofen (a specific GABA_B receptor antagonist) co-administration. Altogether, these results demonstrated for the first time that activation of GABA_B receptor might inhibit IL-1β production by suppressing NF- κ B and p38 MAPK signaling pathway activation and restore GABA_B receptor expression in SGCs to attenuate inflammatory facial pain.

Heterosynaptic modulation in the paraventricular nucleus of the hypothalamus

The stress response-originally described by Hans Selye as "the nonspecific response of the body to any demand made upon it"-is chiefly mediated by the hypothalamic-pituitary-adrenal (HPA) axis and is activated by diverse sensory stimuli that inform threats to homeostasis. The diversity of signals regulating the HPA axis is partly achieved by the complexity of afferent inputs that converge at the apex of the HPA axis: this apex is formed by a group of neurosecretory neurons that synthesize corticotropin-releasing hormone (CRH) in the paraventricular nucleus of the hypothalamus (PVN). The afferent synaptic inputs onto these PVN-CRH neurons originate from a number of brain areas, and PVN-CRH neurons respond to a long list of neurotransmitters/neuropeptides. Considering this complexity, an important question is how these diverse afferent signals independently and/or in concert influence the excitability of PVN-CRH neurons. While many of these inputs directly act on the postsynaptic PVN-CRH neurons for the summation of signals, accumulating data indicates that they also modulate each other's transmission in the PVN. This mode of transmission, termed heterosynaptic modulation, points to mechanisms through which the activity of a specific modulatory input (conveying a specific sensory signal) can up- or down-regulate the efficacy of other afferent synapses (mediating other stress modalities) depending on receptor expression for and spatial proximity to the heterosynaptic signals. Here, we review examples of heterosynaptic modulation in the PVN and discuss its potential role in the regulation of PVN-CRH neurons' excitability and resulting HPA axis activity. This article is part of the Special Issue entitled 'Hypothalamic Control of Homeostasis'.

Long-Term Stress Disrupts the Structural and Functional Integrity of GABAergic Neuronal Networks in the Medial Prefrontal Cortex of Rats

Clinical and experimental data suggest that fronto-cortical GABAergic deficits contribute to the pathophysiology of major depressive disorder (MDD). To further test this hypothesis, we used a well characterized rat model for depression and examined the effect of stress on GABAergic neuron numbers and GABA-mediated synaptic transmission in the medial prefrontal cortex (mPFC) of rats. Adult male Wistar rats were subjected to 9-weeks of chronic mild stress (CMS) and based on their hedonic-anhedonic behavior they were behaviorally phenotyped as being stress-susceptible (anhedonic) or stress-resilient. Post mortem quantitative histopathology was used to examine the effect of stress on parvalbumin (PV)-, calretinin- (CR), calbindin- (CB), cholecystokinin- (CCK), somatostatin-(SST) and neuropeptide Y-positive (NPY+) GABAergic neuron numbers in all cortical subareas of the mPFC (anterior cingulate (Cg1), prelimbic (PrL) and infralimbic (IL) cortexes). In vitro, whole-cell patch-clamp recordings from layer II–III pyramidal neurons of the ventral mPFC was used to examine GABAergic neurotransmission. The cognitive performance of the animals was assessed in a hippocampal-prefrontal-cortical circuit dependent learning task. Stress exposure reduced the number of CCK-, CR- and PV-positive GABAergic neurons in the mPFC, most prominently in the IL cortex. Interestingly, in the stress-resilient animals, we found higher number of neuropeptide Y-positive neurons in the entire mPFC. The electrophysiological analysis revealed reduced frequencies of spontaneous and miniature IPSCs in the anhedonic rats and decreased release probability of perisomatic-targeting GABAergic synapses and alterations in GABA_B receptor mediated signaling. In turn, pyramidal neurons showed higher excitability. Anhedonic rats were also significantly impaired in the object-place paired-associate learning task. These data demonstrate that long-term stress results in functional and structural deficits of prefrontal GABAergic networks. Our findings support the concept that fronto-limbic GABAergic dysfunctions may contribute to emotional and cognitive symptoms of MDD.

Ca2+/Calmodulin-Dependent Protein Kinase II (CaMKII) β-Dependent Phosphorylation of GABAB1 Triggers Lysosomal Degradation of GABAB Receptors via Mind Bomb-2 (MIB2)-Mediated Lys-63-Linked Ubiquitination

The G protein-coupled GABA_B receptors, constituted from GABA_B1 and GABA_B2 subunits, are important regulators of neuronal excitability by mediating long-lasting inhibition. One factor that determines receptor availability and thereby the strength of inhibition is regulated protein degradation. GABA_B receptors are constitutively internalized from the plasma membrane and are either recycled to the cell surface or degraded in lysosomes. Lys-63-linked ubiquitination mediated by the E3 ligase Mind bomb-2 (MIB2) is the signal that sorts GABA_B receptors to lysosomes. However, it is unknown how Lys-63-linked ubiquitination and thereby lysosomal degradation of the receptors is regulated. Here, we show that Ca²⁺/calmodulin-dependent protein kinase II (CaMKII) promotes MIB2-mediated Lys-63-linked ubiquitination of GABA_B receptors. We found that inhibition of CaMKII in cultured rat cortical neurons increased cell surface GABA_B receptors, whereas overexpression of CaMKIIβ, but not CaMKIIα, decreased receptor levels. This effect was conveyed by Lys-63-linked ubiquitination of GABA_B1 at multiple sites mediated by the E3 ligase MIB2. Inactivation of the CaMKII phosphorylation site on GABA_B1(Ser-867) strongly reduced Lys-63-linked ubiquitination of GABA_B receptors and increased their cell surface expression, whereas the phosphomimetic mutant GABA_B1(S867D) exhibited strongly increased Lys-63-linked ubiquitination and reduced cell surface expression. Finally, triggering lysosomal degradation of GABA_B receptors by sustained activation of glutamate receptors, a condition occurring in brain ischemia, was accompanied with a massive increase of GABA_B1(Ser-867) phosphorylation-dependent Lys-63-linked ubiquitination of GABA_B receptors. These findings indicate that CaMKIIβ-dependent Lys-63-linked ubiquitination of GABA_B1 at multiple sites controls sorting of GABA_B receptors to lysosomes for degradation under physiological and pathological condition.

Neuroadaptations of presynaptic and postsynaptic GABAB receptor function in the paraventricular nucleus in response to chronic unpredictable stress

BACKGROUND AND PURPOSE

Chronic stress impairs GABA_A (GABA type A) receptor-mediated inhibition in the hypothalamic paraventricular nucleus (PVN). It is not clear whether GABA_B receptor function is also altered. We hypothesize that chronic stress alters GABA_B receptor function in PVN corticotrophin-releasing hormone (CRH) neurons to control hypothalamus-pituitary-adrenal axis activity.

EXPERIMENTAL APPROACH

Whole-cell patch clamp recordings were made of PVN-CRH neurons expressing eGFP driven by CRH promoter in brain slices from unstressed rats and rats exposed to chronic unpredictable mild stress (CUMS).

KEY RESULTS

CUMS elevated the basal circulating corticosterone levels and increased the basal firing activity of PVN-CRH neurons. Microinjection of GABA_B receptor agonist baclofen into the PVN suppressed the increased corticosterone levels in CUMS rats compared with unstressed rats. CUMS blunted the baclofen-induced inhibition on PVN-CRH neurons and outward currents in these neurons. Furthermore, CUMS reduced expression of GABA_B1 (GABA_B R1) protein in the PVN. Blocking NMDA receptors with AP5 restored the reduced baclofen-induced currents in CUMS rats but had no effect on GABA_B1 expression. Furthermore, CUMS treatment augmented the baclofen-induced decrease in the frequency of glutamatergic excitatory postsynaptic currents (EPSCs) and GABAergic inhibitor postsynaptic currents in PVN-CRH neurons. The GABA_B receptor antagonist CGP55845 increased the firing activity of PVN-CRH neurons only in CUMS-treated rats and not in unstressed rats.

CONCLUSIONS AND IMPLICATIONS

These findings suggest that chronic stress impairs postsynaptic GABA_B receptor function but augments presynaptic GABA_B receptor function in controlling glutamatergic and GABAergic synaptic inputs in PVN-CRH neurons.

GABAb Receptor Mediates Opposing Adaptations of GABA Release From Two Types of Prefrontal Interneurons After Observational Fear

The observational fear (OF) paradigm in rodents, in which the subject is exposed to a distressed conspecific, elicits contextual fear learning and enhances future passive avoidance learning, which may model certain behavioral traits resulting from traumatic experiences in humans. As these behaviors affected by the OF require dorso-medial prefrontal cortex (dmPFC), we searched for synaptic adaptations in dmPFC resulting from OF in mice by recording synaptic responses in dmPFC layer V pyramidal neurons elicited by repeated 5 Hz electrical stimulation of dmPFC layer I or by optogenetic stimulation of specific interneurons ex vivo 1 day after OF. OF increased depression of inhibitory postsynaptic currents (IPSCs) along IPSC trains evoked by the 5 Hz electrical stimulation, but, surprisingly, decreased depression of dendritic IPSCs isolated after blocking GABAa receptor on the soma. Subsequent optogenetic analyses revealed increased depression of IPSCs originating from perisomatically projecting parvalbumin interneurons (PV-IPSCs), but decreased depression of IPSCs from dendritically projecting somatostatin cells (SOM-IPSCs). These changes were no longer detectable in the presence of a GABAb receptor antagonist CGP52432. Meanwhile, OF decreased the sensitivity of SOM-IPSCs, but not PV-IPSCs to a GABAb receptor agonist baclofen. Thus, OF causes opposing changes in GABAb receptor mediated suppression of GABA release from PV-positive and SOM-positive interneurons. Such adaptations may alter dmPFC connectivity with brain areas that target its deep vs superficial layers and thereby contribute to the behavioral consequences of the aversive experiences.

Multimodal Imaging of Neurometabolic Pathology due to Traumatic Brain Injury

The impact of traumatic brain injury (TBI) involves a combination of complex biochemical processes beginning with the initial insult and lasting for days, months and even years post-trauma. These changes range from neuronal integrity losses to neurotransmitter imbalance and metabolite dysregulation, leading to the release of pro- or anti-apoptotic factors which mediate cell survival or death. Such dynamic processes affecting the brain's neurochemistry can be monitored using a variety of neuroimaging techniques, whose combined use can be particularly useful for understanding patient-specific clinical trajectories. Here, we describe how TBI changes the metabolism of essential neurochemical compounds, summarize how neuroimaging approaches facilitate the study of such alterations, and highlight promising ways in which neuroimaging can be used to investigate post-TBI changes in neurometabolism.

Lys-63-linked Ubiquitination of γ-Aminobutyric Acid (GABA), Type B1, at Multiple Sites by the E3 Ligase Mind Bomb-2 Targets GABAB Receptors to Lysosomal Degradation

GABAB receptors are heterodimeric G protein-coupled receptors, which control neuronal excitability by mediating prolonged inhibition. The magnitude of GABAB receptor-mediated inhibition essentially depends on the amount of receptors in the plasma membrane. However, the factors regulating cell surface expression of GABAB receptors are poorly characterized. Cell surface GABAB receptors are constitutively internalized and either recycled to the plasma membrane or degraded in lysosomes. The signal that sorts GABAB receptors to lysosomes is currently unknown. Here we show that Mind bomb-2 (MIB2)-mediated Lys-63-linked ubiquitination of the GABAB1 subunit at multiple sites is the lysosomal sorting signal for GABAB receptors. We found that inhibition of lysosomal activity in cultured rat cortical neurons increased the fraction of Lys-63-linked ubiquitinated GABAB receptors and enhanced the expression of total as well as cell surface GABAB receptors. Mutational inactivation of four putative ubiquitination sites in the GABAB1 subunit significantly diminished Lys-63-linked ubiquitination of GABAB receptors and prevented their lysosomal degradation. We identified MIB2 as the E3 ligase triggering Lys-63-linked ubiquitination and lysosomal degradation of GABAB receptors. Finally, we show that sustained activation of glutamate receptors, a condition occurring in brain ischemia that down-regulates GABAB receptors, considerably increased the expression of MIB2 and Lys-63-linked ubiquitination of GABAB receptors. Interfering with Lys-63-linked ubiquitination by overexpressing ubiquitin mutants or GABAB1 mutants deficient in Lys-63-linked ubiquitination prevented glutamate-induced down-regulation of the receptors. These findings indicate that Lys-63-linked ubiquitination of GABAB1 at multiple sites by MIB2 controls sorting of GABAB receptors to lysosomes for degradation under physiological and pathological conditions.

Phospho-dependent Accumulation of GABABRs at Presynaptic Terminals after NMDAR Activation

Here, we uncover a mechanism for regulating the number of active presynaptic GABAB receptors (GABABRs) at nerve terminals, an important determinant of neurotransmitter release. We find that GABABRs gain access to axon terminals by lateral diffusion in the membrane. Their relative accumulation is dependent upon agonist activation and the presence of the two distinct sushi domains that are found only in alternatively spliced GABABR1a subunits. Following brief activation of NMDA receptors (NMDARs) using glutamate, GABABR diffusion is reduced, causing accumulation at presynaptic terminals in a Ca(2+)-dependent manner that involves phosphorylation of GABABR2 subunits at Ser783. This signaling cascade indicates how synaptically released glutamate can initiate, via a feedback mechanism, increased levels of presynaptic GABABRs that limit further glutamate release and excitotoxicity.

Post-endocytotic Deubiquitination and Degradation of the Metabotropic γ-Aminobutyric Acid Receptor by the Ubiquitin-specific Protease 14

Mechanisms controlling the metabotropic γ-aminobutyric acid receptor (GABAB) cell surface stability are still poorly understood. In contrast with many other G protein-coupled receptors (GPCR), it is not subject to agonist-promoted internalization, but is constitutively internalized and rapidly down-regulated. In search of novel interacting proteins regulating receptor fate, we report that the ubiquitin-specific protease 14 (USP14) interacts with the GABAB(1b)subunit's second intracellular loop. Probing the receptor for ubiquitination using bioluminescence resonance energy transfer (BRET), we detected a constitutive and phorbol 12-myristate 13-acetate (PMA)-induced ubiquitination of the receptor at the cell surface. PMA also increased internalization and accelerated receptor degradation. Overexpression of USP14 decreased ubiquitination while treatment with a small molecule inhibitor of the deubiquitinase (IU1) increased receptor ubiquitination. Treatment with the internalization inhibitor Dynasore blunted both USP14 and IU1 effects on the receptor ubiquitination state, suggesting a post-endocytic site of action. Overexpression of USP14 also led to an accelerated degradation of GABABin a catalytically independent fashion. We thus propose a model whereby cell surface ubiquitination precedes endocytosis, after which USP14 acts as an ubiquitin-binding protein that targets the ubiquitinated receptor to lysosomal degradation and promotes its deubiquitination.

Sodium salicylate reduces the level of GABAB receptors in the rat's inferior colliculus

Previous studies have indicated that sodium salicylate (SS) can cause hearing abnormalities through affecting the central auditory system. In order to understand central effects of the drug, we examined how a single intraperitoneal injection of the drug changed the level of subunits of the type-B γ-aminobutyric acid receptor (GABAB receptor) in the rat's inferior colliculus (IC). Immunohistochemical and western blotting experiments were conducted three hours following a drug injection, as previous studies indicated that a tinnitus-like behavior could be reliably induced in rats within this time period. Results revealed that both subunits of the receptor, GABABR1 and GABABR2, reduced their level over the entire area of the IC. Such a reduction was observed in both cell body and neuropil regions. In contrast, no changes were observed in other brain structures such as the cerebellum. Thus, a coincidence existed between a structure-specific reduction in the level of GABAB receptor subunits in the IC and the presence of a tinnitus-like behavior. This coincidence likely suggests that a reduction in the level of GABAB receptor subunits was involved in the generation of a tinnitus-like behavior and/or used by the nervous system to restore normal hearing following application of SS.

Cross-talk and regulation between glutamate and GABAB receptors

Brain function depends on co-ordinated transmission of signals from both excitatory and inhibitory neurotransmitters acting upon target neurons. NMDA, AMPA and mGluR receptors are the major subclasses of glutamate receptors that are involved in excitatory transmission at synapses, mechanisms of activity dependent synaptic plasticity, brain development and many neurological diseases. In addition to canonical role of regulating presynaptic release and activating postsynaptic potassium channels, GABA_B receptors also regulate glutamate receptors. There is increasing evidence that metabotropic GABA_B receptors are now known to play an important role in modulating the excitability of circuits throughout the brain by directly influencing different types of postsynaptic glutamate receptors. Specifically, GABA_B receptors affect the expression, activity and signaling of glutamate receptors under physiological and pathological conditions. Conversely, NMDA receptor activity differentially regulates GABA_B receptor subunit expression, signaling and function. In this review I will describe how GABA_B receptor activity influence glutamate receptor function and vice versa. Such a modulation has widespread implications for the control of neurotransmission, calcium-dependent neuronal function, pain pathways and in various psychiatric and neurodegenerative diseases.

Influence of JuA in evoking communication changes between the small intestines and brain tissues of rats and the GABAA and GABAB receptor transcription levels of hippocampal neurons

ETHNOPHARMACOLOGICAL RELEVANCE

Jujuboside A (JuA) is a main active ingredient of semen ziziphi spinosae, which can significantly reduce spontaneous activity in mammals, increase the speed of falling asleep, prolong the sleeping time as well as improve the sleeping efficiency. In this study, the mechanism and the pathway of the sedative and hypnotic effect of JuA were investigated.

MATERIALS AND METHODS

After being treated with JuA (in vitro), the rat׳s small intestine tissues cultures were used to stimulate the brain tissues. Then 27 cytokine levels were detected in the two kinds of tissue culture via liquid protein chip technology; In addition, the cultured hippocampal neurons of rat were treated with JuA, and γ-aminobutyric acid (GABA) receptor subunits (GABAAα1, GABAAα5, GABAAβ1 and GABABR1) mRNAs were evaluated by Real-time PCR.

RESULTS

The levels of IL-1α, MIP-1α, IL-1β and IL-2 were reduced significantly after 3h of treating the small intestine tissues with JuA (200µl/ml), and the concentration change rates, in order, were -59.3%, -3.59%, -50.1% and -49.4%; these cytokines were transmitted to brain tissues 2h later, which could lead to significant levels of reduction of IL-1α, IFN-γ, IP-10 and TNF-α; the concentration change rates were -62.4%, -25.7%, -55.2% and -38.5%, respectively. Further, the intercellular communication network diagram was mapped out, which could suggest the mechanism and the pathway of the sedative and hypnotic effect of JuA. The results also indicated that JuA (50µl/ml) increased significantly GABAAα1 receptor mRNAs and reduced GABABR1, mRNAs in hippocampal neurons after 24h of stimulation; however, all the mRNA transcription levels of GABAAα1,GABAAα5, GABAAβ1 and GABABR1 receptors increased significantly after 48h.

CONCLUSION

JuA performed its specific sedative and hypnotic effect through not only adjusting GABA receptors subunit mRNAs expression, but also down-regulating the secretion of relevant inflammation cytokines on the intestinal mucosal system to affect the intercellular cytokine network between nerve cells in the brain. This mechanism is similar to that of melatonin.

Expression of glutamatergic genes in healthy humans across 16 brain regions; altered expression in the hippocampus after chronic exposure to alcohol or cocaine

We analyzed global patterns of expression in genes related to glutamatergic neurotransmission (glutamatergic genes) in healthy human adult brain before determining the effects of chronic alcohol and cocaine exposure on gene expression in the hippocampus. RNA-Seq data from 'BrainSpan' was obtained across 16 brain regions from nine control adults. We also generated RNA-Seq data from postmortem hippocampus from eight alcoholics, eight cocaine addicts and eight controls. Expression analyses were undertaken of 28 genes encoding glutamate ionotropic (AMPA, kainate, NMDA) and metabotropic receptor subunits, together with glutamate transporters. The expression of each gene was fairly consistent across the brain with the exception of the cerebellum, the thalamic mediodorsal nucleus and the striatum. GRIN1, encoding the essential NMDA subunit, had the highest expression across all brain regions. Six factors accounted for 84% of the variance in global gene expression. GRIN2B (encoding GluN2B), was up-regulated in both alcoholics and cocaine addicts (FDR corrected P = 0.008). Alcoholics showed up-regulation of three genes relative to controls and cocaine addicts: GRIA4 (encoding GluA4), GRIK3 (GluR7) and GRM4 (mGluR4). Expression of both GRM3 (mGluR3) and GRIN2D (GluN2D) was up-regulated in alcoholics and down-regulated in cocaine addicts relative to controls. Glutamatergic genes are moderately to highly expressed throughout the brain. Six factors explain nearly all the variance in global gene expression. At least in the hippocampus, chronic alcohol use largely up-regulates glutamatergic genes. The NMDA GluN2B receptor subunit might be implicated in a common pathway to addiction, possibly in conjunction with the GABAB1 receptor subunit.

Differential regulation of GABAB receptor trafficking by different modes of N-methyl-D-aspartate (NMDA) receptor signaling

Inhibitory GABAB receptors (GABABRs) can down-regulate most excitatory synapses in the CNS by reducing postsynaptic excitability. Functional GABABRs are heterodimers of GABAB1 and GABAB2 subunits and here we show that the trafficking and surface expression of GABABRs is differentially regulated by synaptic or pathophysiological activation of NMDA receptors (NMDARs). Activation of synaptic NMDARs using a chemLTP protocol increases GABABR recycling and surface expression. In contrast, excitotoxic global activation of synaptic and extrasynaptic NMDARs by bath application of NMDA causes the loss of surface GABABRs. Intriguingly, exposing neurons to extreme metabolic stress using oxygen/glucose deprivation (OGD) increases GABAB1 but decreases GABAB2 surface expression. The increase in surface GABAB1 involves enhanced recycling and is blocked by the NMDAR antagonist AP5. The decrease in surface GABAB2 is also blocked by AP5 and by inhibiting degradation pathways. These results indicate that NMDAR activity is critical in GABABR trafficking and function and that the individual subunits can be separately controlled to regulate neuronal responsiveness and survival.

Complex GABAB receptor complexes: how to generate multiple functionally distinct units from a single receptor

The main inhibitory neurotransmitter, GABA, acts on both ligand-gated and G protein-coupled receptors, the GABA_A/C and GABA_B receptors, respectively. The later play important roles in modulating many synapses, both at the pre- and post-synaptic levels, and are then still considered as interesting targets to treat a number of brain diseases, including addiction. For many years, several subtypes of GABA_B receptors were expected, but cloning revealed only two genes that work in concert to generate a single type of GABA_B receptor composed of two subunits. Here we will show that the signaling complexity of this unit receptor type can be largely increased through various ways, including receptor stoichiometry, subunit isoforms, cell-surface expression and localization, crosstalk with other receptors, or interacting proteins. These recent data revealed how complexity of a receptor unit can be increased, observation that certainly are not unique to the GABA_B receptor.

Proteasomal degradation of γ-aminobutyric acidB receptors is mediated by the interaction of the GABAB2 C terminus with the proteasomal ATPase Rtp6 and regulated by neuronal activity

Regulation of cell surface expression of neurotransmitter receptors is crucial for determining synaptic strength and plasticity, but the underlying mechanisms are not well understood. We previously showed that proteasomal degradation of GABAB receptors via the endoplasmic reticulum (ER)-associated protein degradation (ERAD) machinery determines the number of cell surface GABAB receptors and thereby GABAB receptor-mediated neuronal inhibition. Here, we show that proteasomal degradation of GABAB receptors requires the interaction of the GABAB2 C terminus with the proteasomal AAA-ATPase Rpt6. A mutant of Rpt6 lacking ATPase activity prevented degradation of GABAB receptors but not the removal of Lys(48)-linked ubiquitin from GABAB2. Blocking ERAD activity diminished the interaction of Rtp6 with GABAB receptors resulting in increased total as well as cell surface expression of GABAB receptors. Modulating neuronal activity affected proteasomal activity and correspondingly the interaction level of Rpt6 with GABAB2. This resulted in altered cell surface expression of the receptors. Thus, neuronal activity-dependent proteasomal degradation of GABAB receptors by the ERAD machinery is a potent mechanism regulating the number of GABAB receptors available for signaling and is expected to contribute to homeostatic neuronal plasticity.

Pore positioning: current concepts in Pannexin channel trafficking

Pannexins (Panxs) are a multifaceted family of ion and metabolite channels that play key roles in a number of physiological and pathophysiological settings. These single membrane large-pore channels exhibit a variety of tissue, cell type, and subcellular distributions. The lifecycles of Panxs are complex, yet must be understood to accurately target these proteins for future therapeutic use. Here we review the basics of Panx function and localization, and then analyze the recent advances in knowledge regarding Panx trafficking. We examine several intrinsic features of Panxs including specific post-translational modifications, the divergent C-termini, and oligomerization, all of which contribute to Panx anterograde transport pathways. Further, we examine the potential influence of extrinsic factors, such as protein-protein interactions, on Panx trafficking. Finally, we highlight what is currently known with respect to Panx internalization and retrograde transport, and present new data illustrating Panx1 internalization following an activating stimulus.

Endoplasmic reticulum-associated degradation controls cell surface expression of γ-aminobutyric acid, type B receptors

Metabotropic GABAB receptors are crucial for controlling the excitability of neurons by mediating slow inhibition in the CNS. The strength of receptor signaling depends on the number of cell surface receptors, which is thought to be regulated by trafficking and degradation mechanisms. Although the mechanisms of GABAB receptor trafficking are studied to some extent, it is currently unclear whether receptor degradation actively controls the number of GABAB receptors available for signaling. Here we tested the hypothesis that proteasomal degradation contributes to the regulation of GABAB receptor expression levels. Blocking proteasomal activity in cultured cortical neurons considerably enhanced total and cell surface expression of GABAB receptors, indicating the constitutive degradation of the receptors by proteasomes. Proteasomal degradation required Lys(48)-linked polyubiquitination of lysines 767/771 in the C-terminal domain of the GABAB2 subunit. Inactivation of these ubiquitination sites increased receptor levels and GABAB receptor signaling in neurons. Proteasomal degradation was mediated by endoplasmic reticulum-associated degradation (ERAD) as shown by the accumulation of receptors in the endoplasmic reticulum upon inhibition of proteasomes, by the increase of receptor levels, as well as receptor signaling upon blocking ERAD function, and by the interaction of GABAB receptors with the essential ERAD components Hrd1 and p97. In conclusion, the data support a model in which the fraction of GABAB receptors available for plasma membrane trafficking is regulated by degradation via the ERAD machinery. Thus, modulation of ERAD activity by changes in physiological conditions may represent a mechanism to adjust receptor numbers and thereby signaling strength.

Sushi domains confer distinct trafficking profiles on GABAB receptors

GABA(B) receptors mediate slow inhibitory neurotransmission in the brain and feature during excitatory synaptic plasticity, as well as various neurological conditions. These receptors are obligate heterodimers composed of GABA(B)R1 and R2 subunits. The two predominant R1 isoforms differ by the presence of two complement control protein modules or Sushi domains (SDs) in the N terminus of R1a. By using live imaging, with an α-bungarotoxin-binding site (BBS) and fluorophore-linked bungarotoxin, we studied how R2 stabilizes R1b subunits at the cell surface. Heterodimerization with R2 reduced the rate of internalization of R1b, compared with R1b homomers. However, R1aR2 heteromers exhibited increased cell surface stability compared with R1bR2 receptors in hippocampal neurons, suggesting that for receptors containing the R1a subunit, the SDs play an additional role in the surface stability of GABA(B) receptors. Both SDs were necessary to increase the stability of R1aR2 because single deletions caused the receptors to be internalized at the same rate and extent as R1bR2 receptors. Consistent with these findings, a chimera formed from the metabotropic glutamate receptor (mGluR)2 and the SDs from R1a increased the surface stability of mGluR2. These results suggest a role for SDs in stabilizing cell surface receptors that could impart different pre- and postsynaptic trafficking itineraries on GABA(B) receptors, thereby contributing to their physiological and pathological roles.

Regulation of neuronal GABA(B) receptor functions by subunit composition

GABA(B) receptors (GABA(B)Rs) are G protein-coupled receptors for GABA, the main inhibitory neurotransmitter in the CNS. In the past 5 years, notable advances have been made in our understanding of the molecular composition of these receptors. GABA(B)Rs are now known to comprise principal and auxiliary subunits that influence receptor properties in distinct ways. The principal subunits regulate the surface expression and the axonal versus dendritic distribution of these receptors, whereas the auxiliary subunits determine agonist potency and the kinetics of the receptor response. This Review summarizes current knowledge on how the subunit composition of GABA(B)Rs affects the distribution of these receptors, neuronal processes and higher brain functions.

Chronic pain states: pharmacological strategies to restore diminished inhibitory spinal pain control

Potentially noxious stimuli are sensed by specialized nerve cells named nociceptors, which convey nociceptive signals from peripheral tissues to the central nervous system. The spinal dorsal horn and the trigeminal nucleus serve as first relay stations for incoming nociceptive signals. At these sites, nociceptor terminals contact a local neuronal network consisting of excitatory and inhibitory interneurons as well as of projection neurons. Blockade of neuronal inhibition in this network causes an increased sensitivity to noxious stimuli (hyperalgesia), painful sensations occurring after activation of non-nociceptive fibers (allodynia), and spontaneous pain felt in the absence of any sensory stimulation. It thus mimics the major characteristics of chronic pain states. Diminished inhibitory pain control in the spinal dorsal horn occurs naturally, e.g., through changes in the function of inhibitory neurotransmitter receptors or through altered chloride homeo-stasis in the course of inflammation or nerve damage. This review summarizes our current knowledge about endogenous mechanisms leading to diminished spinal pain control and discusses possible ways that could restore proper inhibition through facilitation of fast inhibitory neurotransmission.

Methamphetamine-evoked depression of GABA(B) receptor signaling in GABA neurons of the VTA

Psychostimulants induce neuroadaptations in excitatory and fast inhibitory transmission in the ventral tegmental area (VTA). Mechanisms underlying drug-evoked synaptic plasticity of slow inhibitory transmission mediated by GABA(B) receptors and G protein-gated inwardly rectifying potassium (GIRK/Kir(3)) channels, however, are poorly understood. Here, we show that 1 day after methamphetamine (METH) or cocaine exposure both synaptically evoked and baclofen-activated GABA(B)R-GIRK currents were significantly depressed in VTA GABA neurons and remained depressed for 7 days. Presynaptic inhibition mediated by GABA(B)Rs on GABA terminals was also weakened. Quantitative immunoelectron microscopy revealed internalization of GABA(B1) and GIRK2, which occurred coincident with dephosphorylation of serine 783 (S783) in GABA(B2), a site implicated in regulating GABA(B)R surface expression. Inhibition of protein phosphatases recovered GABA(B)R-GIRK currents in VTA GABA neurons of METH-injected mice. This psychostimulant-evoked impairment in GABA(B)R signaling removes an intrinsic brake on GABA neuron spiking, which may augment GABA transmission in the mesocorticolimbic system.

Modulation of cell surface GABA(B) receptors by desensitization, trafficking and regulated degradation

Inhibitory neurotransmission ensures normal brain function by counteracting and integrating excitatory activity. γ-Aminobutyric acid (GABA) is the main inhibitory neurotransmitter in the mammalian central nervous system, and mediates its effects via two classes of receptors: the GABA(A) and GABA(B) receptors. GABA(A) receptors are heteropentameric GABA-gated chloride channels and responsible for fast inhibitory neurotransmission. GABA(B) receptors are heterodimeric G protein coupled receptors (GPCR) that mediate slow and prolonged inhibitory transmission. The extent of inhibitory neurotransmission is determined by a variety of factors, such as the degree of transmitter release and changes in receptor activity by posttranslational modifications (e.g., phosphorylation), as well as by the number of receptors present in the plasma membrane available for signal transduction. The level of GABA(B) receptors at the cell surface critically depends on the residence time at the cell surface and finally the rates of endocytosis and degradation. In this review we focus primarily on recent advances in the understanding of trafficking mechanisms that determine the expression level of GABA(B) receptors in the plasma membrane, and thereby signaling strength.

Downregulation of GABAB receptors in the spinal cord dorsal horn in diabetic neuropathy

Diabetic neuropathic pain is a common clinical problem and remains difficult to treat with classic analgesics. Spinal dorsal horn neurons are important in mediating nociceptive signaling, and the hyperactivity of these neurons is critical in diabetic neuropathy. In this study, we determined the GABA(B) receptor expression level in dorsal horn neurons in streptozotocin (STZ)-induced diabetes in rats by using reverse-transcription polymerase chain reaction (RT-PCR) and western blot analyses. Mean blood glucose concentrations were significantly higher and the paw withdrawal threshold was significantly lower in STZ-treated rats than in saline-treated rats. Immunohistochemical staining showed that the GABA(B) receptor was extensively expressed in the spinal dorsal horn neurons. The GABA(B1) mRNA level decreased in a time-dependent manner in STZ-treated rats compared with saline-treated controls. Furthermore, the protein expression level revealed by western blot analysis was lower in STZ-treated rats than in saline-treated rats. These data suggest that GABA(B) receptors are downregulated in the spinal dorsal horn in this model of STZ-induced diabetic neuropathic pain. The reduction of GABA(B) expression may contribute to the hyperactivity of spinal dorsal horn neurons and diabetic neuropathic pain.