Developmental changes of agonist affinity at GABABR1 receptor variants in rat brain

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.

GABAB receptor-dependent bidirectional regulation of critical period ocular dominance plasticity in cats

Gama amino butyric acid (GABA) inhibition plays an important role in the onset and offset of the critical period for ocular dominance (OD) plasticity in the primary visual cortex. Previous studies have focused on the involvement of GABA_A receptors, while the potential contribution of GABA_B receptors to OD plasticity has been neglected. In this study, the GABA_B receptor antagonist SCH50911 or agonist baclofen was infused into the primary visual cortex of cats concurrently with a period of monocular deprivation (MD). Using single-unit recordings we found that the OD shift induced by four days of MD during the critical period was impaired by infusion of the antagonist SCH50911, but enhanced by infusion of the agonist baclofen. In contrast, seven days of MD in adult cats did not induce any significant OD shift, even when combined with the infusion of SCH50911 or baclofen. Together, these findings indicate that an endogenous GABA_B receptor-mediated inhibition contributes to juvenile, but not adult, OD plasticity.

Modular composition and dynamics of native GABAB receptors identified by high-resolution proteomics

GABAB receptors, the most abundant inhibitory G protein-coupled receptors in the mammalian brain, display pronounced diversity in functional properties, cellular signaling and subcellular distribution. We used high-resolution functional proteomics to identify the building blocks of these receptors in the rodent brain. Our analyses revealed that native GABAB receptors are macromolecular complexes with defined architecture, but marked diversity in subunit composition: the receptor core is assembled from GABAB1a/b, GABAB2, four KCTD proteins and a distinct set of G-protein subunits, whereas the receptor's periphery is mostly formed by transmembrane proteins of different classes. In particular, the periphery-forming constituents include signaling effectors, such as Cav2 and HCN channels, and the proteins AJAP1 and amyloid-β A4, both of which tightly associate with the sushi domains of GABAB1a. Our results unravel the molecular diversity of GABAB receptors and their postnatal assembly dynamics and provide a roadmap for studying the cellular signaling of this inhibitory neurotransmitter receptor.

Identifying the role of pre-and postsynaptic GABA(B) receptors in behavior

Although many reviews exist characterizing the molecular differences of GABAB receptor isoforms, there is no current review of the in vivo effects of these isoforms. The current review focuses on whether the GABAB1a and GABAB1b isoforms contribute differentially to behaviors in isoform knockout mice. The roles of these receptors have primarily been characterized in cognitive, anxiety, and depressive phenotypes. Currently, the field supports a role of GABAB1a in memory maintenance and protection against an anhedonic phenotype, whereas GABAB1b appears to be involved in memory formation and a susceptibility to developing an anhedonic phenotype. Although GABAB receptors have been strongly implicated in drug abuse phenotypes, no isoform-specific work has been done in this field. Future directions include developing site-specific isoform knockdown to identify the role of different brain regions in behavior, as well as identifying how these isoforms are involved in development of behavioral phenotypes.

GABA transporters control GABAergic neurotransmission in the mouse subplate

The subplate is a transient layer between the cortical plate and intermediate zone in the developing cortex. Thalamo-cortical axons form temporary synapses on subplate neurons (SPns) before invading the cortical plate. Neuronal activity within the subplate is of critical importance for the development of neocortical circuits and architecture. Although both glutamatergic and GABAergic inputs on SPns were reported, short-term plasticity of GABAergic transmission has not been investigated yet. GABAergic postsynaptic currents (GPSCs) were recorded from SPns in coronal neocortical slices prepared from postnatal day 3-4 mice using whole-cell patch-clamp technique. Evoked GPSCs (eGPSCs) elicited by electrical paired-pulse stimulation demonstrated paired-pulse depression at all interstimulus intervals tested. Baclofen, a specific GABAB receptor (GABABR) agonist, reduced eGPSC amplitudes and increased paired-pulse ratio (PPR), suggesting presynaptic location of functional GABABRs. Baclofen-induced effects were alleviated by (2S)-3-[[(1S)-1-(3,4-dichlorophenyl)ethyl]amino-2-hydroxypropyl](phenylmethyl)phosphinic acid (CGP55845), a selective GABABR blocker. Moreover, CGP55845 increased eGPSC amplitudes and decreased PPR even under control conditions, indicating that GABABRs are tonically activated by ambient GABA. Because extracellular GABA concentration is mainly regulated by GABA transporters (GATs), we asked whether GATs release GABA. 1,2,5,6-tetrahydro-1-[2-[[(diphenylmethylene)amino]oxy]ethyl]-3-pyridinecarboxylic acid (NNC-711) (10μM), a selective GAT-1 blocker, increased eGPSC decay time, decreased eGPSC amplitudes and PPR. The two last effects but not the first one were blocked by CGP55845, indicating that GAT-1 blockade causes an elevation of extracellular GABA concentration and in turn activation of extrasynaptic GABAARs and presynaptic GABABRs. 1-[2-[tris(4-methoxyphenyl)methoxy]ethyl]-(S)-3-piperidinecarboxylic acid (SNAP-5114), a specific GAT-2/3 blocker, failed to affect eGPSC kinetics. However, in contrast to NNC-711 SNAP-5114 increased eGPSC amplitudes and decreased PPR. In the presence of SNAP-5114 CGP55845 did not influence GABAergic transmission, indicating that GABABRs are not activated any longer. We conclude that in the subplate GAT-2/3 operates in reverse mode. GABA released via GAT-2/3 activates presynaptic GABABRs on GABAergic synapses and tonically inhibits GABAergic inputs on SPns.

GABA suppresses neurogenesis in the adult hippocampus through GABAB receptors

Adult neurogenesis is tightly regulated through the interaction of neural stem/progenitor cells (NSCs) with their niche. Neurotransmitters, including GABA activation of GABAA receptor ion channels, are important niche signals. We show that adult mouse hippocampal NSCs and their progeny express metabotropic GABAB receptors. Pharmacological inhibition of GABAB receptors stimulated NSC proliferation and genetic deletion of GABAB1 receptor subunits increased NSC proliferation and differentiation of neuroblasts in vivo. Cell-specific conditional deletion of GABAB receptors supports a cell-autonomous role in newly generated cells. Our data indicate that signaling through GABAB receptors is an inhibitor of adult neurogenesis.

Activation of presynaptic GABA(B(1a,2)) receptors inhibits synaptic transmission at mammalian inhibitory cholinergic olivocochlear-hair cell synapses

The synapse between olivocochlear (OC) neurons and cochlear mechanosensory hair cells is cholinergic, fast, and inhibitory. The inhibitory sign of this cholinergic synapse is accounted for by the activation of Ca(2+)-permeable postsynaptic α9α10 nicotinic receptors coupled to the opening of hyperpolarizing Ca(2+)-activated small-conductance type 2 (SK2)K(+) channels. Acetylcholine (ACh) release at this synapse is supported by both P/Q- and N-type voltage-gated calcium channels (VGCCs). Although the OC synapse is cholinergic, an abundant OC GABA innervation is present along the mammalian cochlea. The role of this neurotransmitter at the OC efferent innervation, however, is for the most part unknown. We show that GABA fails to evoke fast postsynaptic inhibitory currents in apical developing inner and outer hair cells. However, electrical stimulation of OC efferent fibers activates presynaptic GABA(B(1a,2)) receptors [GABA(B(1a,2))Rs] that downregulate the amount of ACh released at the OC-hair cell synapse, by inhibiting P/Q-type VGCCs. We confirmed the expression of GABA(B)Rs at OC terminals contacting the hair cells by coimmunostaining for GFP and synaptophysin in transgenic mice expressing GABA(B1)-GFP fusion proteins. Moreover, coimmunostaining with antibodies against the GABA synthetic enzyme glutamic acid decarboxylase and synaptophysin support the idea that GABA is directly synthesized at OC terminals contacting the hair cells during development. Thus, we demonstrate for the first time a physiological role for GABA in cochlear synaptic function. In addition, our data suggest that the GABA(B1a) isoform selectively inhibits release at efferent cholinergic synapses.

Plasma membrane density of GABA(B)-R1a, GABA(B)-R1b, GABA-R2 and trimeric G-proteins in the course of postnatal development of rat brain cortex

With the aim to understand the onset of expression and developmental profile of plasma membrane (PM) content /density of crucial components of GABA(B)-R signaling cascade, GABA(B)-R1a, GABA(B)-R1b, GABA(B)-R2, G(i)1/G(i)2alpha, G(i)3alpha, G(o)alpha, G(z)alpha and Gbeta subunit proteins were determined by quantitative immunoblotting and compared in PM isolated from brain cortex of rats of different ages: between postnatal-day-1 (PD1) and 90 (PD90). PM density of GABA(B)-R1a, GABA(B)-R2, G(i)1/G(i)2alpha, G(i)3alpha, G(o)alpha, G(z)alpha and Gbeta was high already at birth and further development was reflected in parallel decrease of both GABA(B)-R1a and GABA(B)-R2 subunits. The major decrease of GABA(B)-R1a and GABA(B)-R2 occurred between the birth and PD15: to 55 % (R1a, **) and 51 % (R2, **), respectively. Contrarily, PM level of the cognate G-proteins G(i)1/G(i)2alpha, G(i)3alpha, G(o)alpha, G(z)alpha and Gbeta was unchanged in the course of the whole postnatal period of brain cortex development. Maturation of GABA(B)-R cascade was substantially different from ontogenetic profile of prototypical plasma membrane marker, Na, K-ATPase, which was low at birth and further development was reflected in continuous increase of PM density of this enzyme. Major change occurred between the birth and PD25. In adult rats, membrane content of Na, K-ATPase was 3-times higher than around the birth.

Modulation of synaptic depression of the calyx of Held synapse by GABA(B) receptors and spontaneous activity

The calyx of Held synapse of the medial nucleus of the trapezoid body is a giant axosomatic synapse in the auditory brainstem, which acts as a relay synapse showing little dependence of its synaptic strength on firing frequency. The main mechanism that is responsible for its resistance to synaptic depression is its large number of release sites with low release probability. Here, we investigated the contribution of presynaptic GABA(B) receptors and spontaneous activity to release probability both in vivo and in vitro in young-adult mice. Maximal activation of presynaptic GABA(B) receptors by baclofen reduced synaptic output by about 45% in whole-cell voltage clamp slice recordings, which was accompanied by a reduction in short-term depression. A similar reduction in transmission was observed when baclofen was applied in vivo by microiontophoresis during juxtacellular recordings using piggyback electrodes. No significant change in synaptic transmission was observed during application of the GABA(B) receptor antagonist CGP54626 both during in vivo and slice recordings, suggesting a low ambient GABA concentration. Interestingly, we observed that synapses with a high spontaneous frequency showed almost no synaptic depression during auditory stimulation, whereas synapses with a low spontaneous frequency did depress during noise bursts. Our data thus suggest that spontaneous firing can tonically reduce release probability in vivo. In addition, our data show that the ambient GABA concentration in the auditory brainstem is too low to activate the GABA(B) receptor at the calyx of Held significantly, but that activation of GABA(B) receptors can reduce sound-evoked synaptic depression.

Anticonvulsant and behavioral effects of GABA(B) receptor positive modulator CGP7930 in immature rats

Possible anticonvulsant action of GABAB receptor positive allosteric modulator CGP7930 was studied in cortical epileptic afterdischarges (ADs) in rat pups 12, 18, and 25 days old. Afterdischarges were induced by six series of stimulation of sensorimotor cortex, and CGP7930 (20 or 40 mg/kgi.p.) was administered after the first AD. In addition, the effects of CGP7930 on sensorimotor performance and behavior in open field and elevated plus maze were assessed. CGP7930 decreased duration of ADs in 12-day-old but not in older rats. Motor phenomena (movements accompanying stimulation and clonic seizures) were not changed. CGP7930 only moderately affected sensorimotor performance, altered slightly spontaneous behavior in the open field, and did not influence behavior in the elevated plus maze in terms of an adaptive form of learning or anxiety-like behavior. Marked anticonvulsant action with subtle deficits in sensorimotor performance in 12-day-old rats suggests a possible use of CGP7930 as an age-specific anticonvulsant.

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.

Cortical inhibition in attention deficit hyperactivity disorder: new insights from the electroencephalographic response to transcranial magnetic stimulation

Attention deficit hyperactivity disorder is one of the most frequent neuropsychiatric disorders in childhood. Transcranial magnetic stimulation studies based on muscle responses (motor-evoked potentials) suggested that reduced motor inhibition contributes to hyperactivity, a core symptom of the disease. Here we employed the N100 component of the electroencephalographic response to transcranial magnetic stimulation as a novel marker for a direct assessment of cortical inhibitory processes, which has not been examined in attention deficit hyperactivity disorder so far. We further investigated to what extent affected children were able to regulate motor cortical inhibition, and whether effects of age on the electroencephalographic response to transcranial magnetic stimulation were compatible with either a delay in brain maturation or a qualitatively different development. N100 amplitude evoked by transcranial magnetic stimulation and its age-dependent development were assessed in 20 children with attention deficit hyperactivity disorder and 19 healthy control children (8-14 years) by 64-channel electroencephalography. Amplitude and latency of the N100 component were compared at rest, during response preparation in a forewarned motor reaction time task and during movement execution. The amplitude of the N100 component at rest was significantly lower and its latency tended to be shorter in children with attention deficit hyperactivity disorder. Only in controls, N100 amplitude to transcranial magnetic stimulation was reduced by response preparation. During movement execution, N100 amplitude decreased while motor evoked potential amplitudes showed facilitation, indicating that the electroencephalographic response to transcranial magnetic stimulation provides further information on cortical excitability independent of motor evoked potential amplitudes and spinal influences. Children with attention deficit hyperactivity disorder showed a smaller N100 amplitude reduction during movement execution compared with control children. The N100 amplitude evoked by transcranial magnetic stimulation decreased with increasing age in both groups. The N100 reduction in children with attention deficit hyperactivity disorder at all ages suggests a qualitative difference rather than delayed development of cortical inhibition in this disease. Findings further suggest that top-down control of motor cortical inhibition is reduced in children with attention deficit hyperactivity disorder. We conclude that evoked potentials in response to transcranial magnetic stimulation are a promising new marker of cortical inhibition in attention deficit hyperactivity disorder during childhood.

Basal GABA regulates GABA(B)R conformation and release probability at single hippocampal synapses

Presynaptic GABA(B) receptor (GABA(B)R) heterodimers are composed of GB(1a)/GB(2) subunits and critically influence synaptic and cognitive functions. Here, we explored local GABA(B)R activation by integrating optical tools for monitoring receptor conformation and synaptic vesicle release at individual presynaptic boutons of hippocampal neurons. Utilizing fluorescence resonance energy transfer (FRET) spectroscopy, we detected a wide range of FRET values for CFP/YFP-tagged GB(1a)/GB(2) receptors that negatively correlated with release probabilities at single synapses. High FRET of GABA(B)Rs associated with low release probability. Notably, pharmacological manipulations that either reduced or increased basal receptor activation decreased intersynapse variability of GB(1a)/GB(2) receptor conformation. Despite variability along axons, presynaptic GABA(B)R tone was dendrite specific, having a greater impact on synapses at highly innervated proximal branches. Prolonged neuronal inactivity reduced basal receptor activation, leading to homeostatic augmentation of release probability. Our findings suggest that local variations in basal GABA concentration are a major determinant of GB(1a)/GB(2) conformational variability, which contributes to heterogeneity of neurotransmitter release at hippocampal synapses.

A mouse model for visualization of GABA(B) receptors

GABA(B) receptors are the G-protein-coupled receptors for the neurotransmitter gamma-aminobutyric acid (GABA). Receptor subtypes are based on the subunit isoforms GABA(B1a) and GABA(B1b), which combine with GABA(B2) subunits to form heteromeric receptors. Here, we used a modified bacterial artificial chromosome (BAC) containing the GABA(B1) gene to generate transgenic mice expressing GABA(B1a) and GABA(B1b) subunits fused to the enhanced green fluorescence protein (eGFP). We demonstrate that the GABA(B1)-eGFP fusion proteins reproduce the cellular expression patterns of endogenous GABA(B1) proteins in the brain and in peripheral tissue. Crossing the GABA(B1)-eGFP BAC transgene into the GABA(B1) (-/-) background restores pre and postsynaptic GABA(B) functions, showing that the GABA(B1)-eGFP fusion proteins substitute for the lack of endogenous GABA(B1) proteins. Finally, we demonstrate that the GABA(B1)-eGFP fusion proteins replicate the temporal expression patterns of native GABA(B) receptors in cultured neurons. These transgenic mice therefore provide a validated tool for direct visualization of native GABA(B) receptors.

Molecular basis for amino acid sensing by family C G-protein-coupled receptors

Family C of human G-protein-coupled receptors (GPCRs) is constituted by eight metabotropic glutamate receptors, two gamma-aminobutyric acid type B (GABA(B1-2)) subunits forming the heterodimeric GABA(B) receptor, the calcium-sensing receptor, three taste1 receptors (T1R1-3), a promiscuous L-alpha;-amino acid receptor G-protein-coupled receptor family C, group 6, subtype A (GPRC6A) and seven orphan receptors. Aside from the orphan receptors, the family C GPCRs are dimeric receptors characterized by a large extracellular Venus flytrap domain which bind the endogenous agonists. Except from the GABA(B1-2) and T1R2-3 receptor, all receptors are either activated or positively modulated by amino acids. In this review, we outline mutational, biophysical and structural studies which have elucidated the interaction of the amino acids with the Venus flytrap domains, molecular mechanisms of receptor selectivity and the initial steps in receptor activation.

The GABAB1a isoform mediates heterosynaptic depression at hippocampal mossy fiber synapses

GABA(B) receptor subtypes are based on the subunit isoforms GABA(B1a) and GABA(B1b), which associate with GABA(B2) subunits to form pharmacologically indistinguishable GABA(B(1a,2)) and GABA(B(1b,2)) receptors. Studies with mice selectively expressing GABA(B1a) or GABA(B1b) subunits revealed that GABA(B(1a,2)) receptors are more abundant than GABA(B(1b,2)) receptors at glutamatergic terminals. Accordingly, it was found that GABA(B(1a,2)) receptors are more efficient than GABA(B(1b,2)) receptors in inhibiting glutamate release when maximally activated by exogenous application of the agonist baclofen. Here, we used a combination of genetic, ultrastructural and electrophysiological approaches to analyze to what extent GABA(B(1a,2)) and GABA(B(1b,2)) receptors inhibit glutamate release in response to physiological activation. We first show that at hippocampal mossy fiber (MF)-CA3 pyramidal neuron synapses more GABA(B1a) than GABA(B1b) protein is present at presynaptic sites, consistent with the findings at other glutamatergic synapses. In the presence of baclofen at concentrations >or=1 microm, both GABA(B(1a,2)) and GABA(B(1b,2)) receptors contribute to presynaptic inhibition of glutamate release. However, at lower concentrations of baclofen, selectively GABA(B(1a,2)) receptors contribute to presynaptic inhibition. Remarkably, exclusively GABA(B(1a,2)) receptors inhibit glutamate release in response to synaptically released GABA. Specifically, we demonstrate that selectively GABA(B(1a,2)) receptors mediate heterosynaptic depression of MF transmission, a physiological phenomenon involving transsynaptic inhibition of glutamate release via presynaptic GABA(B) receptors. Our data demonstrate that the difference in GABA(B1a) and GABA(B1b) protein levels at MF terminals is sufficient to produce a strictly GABA(B1a)-specific effect under physiological conditions. This consolidates that the differential subcellular localization of the GABA(B1a) and GABA(B1b) proteins is of regulatory relevance.

Roles of GABAB receptor subtypes in presynaptic auto- and heteroreceptor function regulating GABA and glutamate release

Gamma-Aminobutyric acid B (GABA B) receptors are heterodimers composed of two subunits GABA B(1) and GABA B(2), the former existing in two isoforms GABA B(1a) and GABA B(1b). The contributions of individual receptor subunits and isoforms to GABA B auto- and heteroreceptor functions were investigated, using release experiments in cortical slice preparations from corresponding knockout mice. Presynaptic GABA B autoreceptors are located on GABAergic terminals and inhibit GABA release, whereas presynaptic GABA B heteroreceptors control the release of other neurotransmitters (e.g. glutamate). Neither baclofen nor the selective antagonist CGP55845 at maximally active concentrations affected [3H]GABA release in slices from GABA B(1)-/- mice. The amount of [3H]GABA released per pulse was unaffected by the stimulation frequency in slices from GABA B(1)-/- and GABA B(2)-/- demonstrating a loss of GABA B autoreceptor function in these knockout animals. The GABA B receptor agonist baclofen was ineffective in modulating glutamate release in cortical slices from GABA B(2)-/- mice, showing that heteroreceptor function was abolished as well. Next we investigated knockout mice for the two predominant GABA B(1) isoforms expressed in brain, GABA B(1a) and GABA B(1b). In cortical, hippocampal and striatal slices from both GABA B(1a)-/- and GABA B(1b)-/- mice, the frequency dependence of [3H]GABA released per pulse was maintained, suggesting that both isoforms participate or can substitute for each other in GABA B autoreceptor function. By contrast, the efficacy of baclofen to inhibit glutamate release was substantially reduced in GABA B(1a)-/-, but essentially unaltered in GABA B(1b)-/- mice. Our data suggest that functional GABA B heteroreceptors regulating glutamate release are predominantly, but not exclusively composed of GABA B(1a) and GABA B(2) subunits.

Discriminative stimulus effects of tiagabine and related GABAergic drugs in rats

RATIONALE

Tiagabine is an anticonvulsant drug which may also have sleep-enhancing properties. It acts by inhibiting reuptake at the gamma-aminobutyric acid (GABA) transporter (GAT-1).

OBJECTIVES

The aim of the study was to determine whether tiagabine acted as a discriminative stimulus and, if so, whether other GABAergic compounds would generalise to it.

MATERIALS AND METHODS

Rats were trained to discriminate tiagabine (30 mg/kg p.o.) from vehicle, and generalisation to drugs that modulate GABA was assessed.

RESULTS

Gaboxadol (5-20 mg/kg p.o.), a selective extrasynaptic GABA A agonist, generalised to tiagabine, although the extent of the generalisation was inconclusive. Indiplon (1 mg/kg p.o.), a benzodiazepine-like hypnotic, also partially generalised to tiagabine, although zolpidem and S-zopiclone did not. Baclofen, a GABA B receptor agonist, and gabapentin, which increases synaptic GABA, did not generalise to tiagabine. (+)-Bicuculline (3 mg/kg i.p.), a GABA A receptor antagonist, blocked the tiagabine cue, but the less brain-penetrant salt form, bicuculline methochloride, had no effect.

CONCLUSIONS

These data suggest that tiagabine generates a discriminative stimulus in rats, and provides a central GABA-mediated cue, but is distinct from the other GABAergic compounds tested.

Subcellular regulation of metabotropic GABA receptors in the developing cerebellum

Our understanding of GABAergic and glutamatergic neurotransmission in the CNS has been greatly influenced with the discovery and subsequent investigations of the metabotropic gamma-aminobutyric acid (B) (GABA(B)) receptors. These G-protein coupled receptors mediate slow inhibitory neurotransmission and are widely expressed and distributed in the cerebellum, where they play critical roles in neuronal excitability and modulation of synaptic neurotransmission. Their function is modulated by interaction with effector ion channels, notably inwardly rectifying K(+) channels and voltage-gated Ca(2+) channels. The receptors are encoded by two distinct subunits, GABA(B1) and GABA(B2), both of which are required in order to function normally in vivo, as shown in recombinant expression systems and in GABA(B1) -/- mice. The GABA(B1) and GABA(B2) subunits exhibit overlapping distributions in the cerebellar cortex, both at pre- and postsynaptic sites, during development and adulthood. They are in particular abundant in Purkinje cells prior to synaptogenesis and throughout postnatal development. Using high-resolution immunohistochemical techniques at the electron microscopic level in combination with quantitative analysis and three-dimensional reconstructions, it has recently been demonstrated that GABA(B) receptors undergo changes in localization on the surface of Purkinje cell dendrites and spines during postnatal development in association with the establishment and maturation of excitatory synapses. Due to this dynamic regulation, the highest densities of GABA(B1) and GABA(B2) subunits occur around the glutamatergic synapses between Purkinje cell spines and parallel fibre varicosities. This review highlights recent studies that have shed further light on the subcellular localization during postnatal development and the cell surface dynamics of GABA(B) receptors.

Gamma-aminobutyric acid type B receptors are constitutively internalized via the clathrin-dependent pathway and targeted to lysosomes for degradation

Receptor internalization is recognized as an important mechanism for rapidly regulating cell surface numbers of receptors. However, there are conflicting results on the existence of rapid endocytosis of gamma-aminobutyric acid, type B (GABAB) receptors. Therefore, we analyzed internalization of GABAB receptors expressed in HEK 293 cells qualitatively and quantitatively using immunocytochemical, cell surface enzyme-linked immunosorbent assay, and biotinylation methods. The data indicate the existence of rapid constitutive receptor internalization, with the first endocytosed receptors being observed in proximity of the plasma membrane after 10 min. After 120 min, a loss of about 40-50% of cell surface receptors was detected. Stimulation of GABAB receptors with GABA or baclofen did not enhance endocytosis of receptors, indicating the lack of agonist-induced internalization. The data suggest that GABAB receptors were endocytosed via the classical dynamin- and clathrin-dependent pathway and accumulated in an endosomal sorting compartment before being targeted to lysosomes for degradation. No evidence for recycling of receptors back to the cell surface was found. In conclusion, the results indicate the presence of constitutive internalization of GABAB receptors via clathrin-coated pits, which resulted in lysosomal degradation of the receptors.

Postnatal development of GABAB-receptor-mediated modulation of potassium currents in brainstem respiratory network of mouse

The GABA(B)-receptor is known to activate a potassium conductance that is inwardly-rectifying, Ba(2+)-sensitive and mediated by G-protein-coupled mechanism. The network that generates respiratory rhythm is located in the brainstem and is modulated by GABA(B)-receptors. The present study investigated the mechanisms by which GABA(B)-receptor activation modulates respiratory rhythm and how these effects change during the first 2 weeks of postnatal development (P0-P15). Whole-cell patch clamp recordings were obtained from inspiratory neurons in the ventral respiratory column of acute brain stem slice of mouse. In presence of TTX and cadmium, application of baclofen, a GABA(B)-receptor agonist, activated an inwardly-rectifying potassium current. The reversal potential of the current was around -78 mV, which was close to the calculated equilibrium potential of potassium. The action of baclofen was dose-dependent and could be partially blocked (>85%) by a selective GABA(B)-receptor antagonist CGP 55845A. The current density of the baclofen-activated potassium currents increased over the first 2 postnatal weeks. At the cellular level, baclofen-activated potassium currents hyperpolarized inspiratory neurons in a concentration- and age-dependent manner. At the network level, the frequency of the respiratory rhythm decreased or was abolished depending on the concentration of baclofen applied. Our results indicate that the endogenous modulation of respiratory rhythm by GABA(B)-receptors that we have demonstrated previously is mediated at least in part through activation of an inwardly rectifying K(+) conductance and that this effect increases postnatally.

Opposite effects of a GABAB antagonist in two models of epileptic seizures in developing rats

The action of a GABA(B) antagonist CGP 35348 and a GABA(B) agonist baclofen was studied in two models of epileptic seizures characterized by EEG spike-and-wave rhythm in freely moving immature rats. Rhythmic metrazol activity (RMA, model of human absences) was induced by low systemic dose of pentylenetetrazol (PTZ) in 18- and 25-day-old rats, epileptic after discharges (ADs, model of human myoclonic seizures) were elicited by electrical stimulation of sensorimotor cortex in rat pups 12, 18, and 25 days old. CGP 35348 (50, 100 and 200 mg/kg i.p.) suppressed RMA in both age groups in a dose-dependent manner. Simultaneously it increased the incidence of clonic seizures, potentiating thus an effect of PTZ. Baclofen (1, 3 and 6 mg/kg i.p.) augmented markedly RMA in 25-day-old rats. On the contrary, baclofen suppressed RMA in a part of 18-day-old animals. Incidence of seizures was not changed by baclofen in either age group. As ADs are concerned CGP 35348 (100 and 200 mg/kg i.p.) exhibited a proconvulsant action, baclofen (3, 6 or 12 mg/kg i.p.) was anticonvulsant, but again an irregularity of action was found in 18-day-old rats. The role of GABA(B)-mediated inhibition in epileptogenesis depends on the type of seizures and also on the stage of maturation.

GABAB1 receptor subunit isoforms exert a differential influence on baseline but not GABAB receptor agonist-induced changes in mice

GABA(B) receptor agonists produce hypothermia and motor incoordination. Two GABA(B(1)) receptor subunit isoforms exist, but because of lack of specific molecular or pharmacological tools, the relevance of these isoforms in controlling basal body temperature, locomotor activity, or in vivo responses to GABA(B) receptor agonists has been unknown. Here, we used mice deficient in the GABA(B(1a)) and GABA(B(1b)) subunit isoforms to examine the influence of these isoforms on both baseline motor behavior and body temperature and on the motor-incoordinating and hypothermic responses to the GABA(B) receptor agonists l-baclofen and gamma-hydroxybutyrate (GHB). GABA(B(1b))(-/-) mice were hyperactive in a novel environment and showed slower habituation than either GABA(B(1a))(-/-) or wild-type mice. GABA(B(1b))(-/-) mice were hyperactive throughout the circadian dark phase. Hypothermia in response to l-baclofen (6 and 12 mg/kg) or GHB (1 g/kg), baclofen-induced ataxia as determined on the fixed-speed Rotarod, and GHB-induced hypolocomotion were significantly, but for the most part similarly, attenuated in both GABA(B(1a))(-/-) and GABA(B(1b))(-/-) mice. We conclude that l-baclofen and GHB are nonselective for either GABA(B(1)) receptor isoform in terms of in vivo responses. However, GABA(B(1)) receptor isoforms have distinct and different roles in mediating locomotor behavioral responses to a novel environment. Therefore, GABA(B(1a)) and GABA(B(1b)) isoforms are functionally relevant molecular variants of the GABA(B(1)) receptor subunit, which are differentially involved in specific neurophysiological processes and behaviors.

Differential compartmentalization and distinct functions of GABAB receptor variants

GABAB receptors are the G protein-coupled receptors for the main inhibitory neurotransmitter in the brain, gamma-aminobutyric acid (GABA). Molecular diversity in the GABAB system arises from the GABAB1a and GABAB1b subunit isoforms that solely differ in their ectodomains by a pair of sushi repeats that is unique to GABAB1a. Using a combined genetic, physiological, and morphological approach, we now demonstrate that GABAB1 isoforms localize to distinct synaptic sites and convey separate functions in vivo. At hippocampal CA3-to-CA1 synapses, GABAB1a assembles heteroreceptors inhibiting glutamate release, while predominantly GABAB1b mediates postsynaptic inhibition. Electron microscopy reveals a synaptic distribution of GABAB1 isoforms that agrees with the observed functional differences. Transfected CA3 neurons selectively express GABAB1a in distal axons, suggesting that the sushi repeats, a conserved protein interaction motif, specify heteroreceptor localization. The constitutive absence of GABAB1a but not GABAB1b results in impaired synaptic plasticity and hippocampus-dependent memory, emphasizing molecular differences in synaptic GABAB functions.

Differential compartmentalization and distinct functions of GABAB receptor variants

GABAB receptors are the G protein-coupled receptors for the main inhibitory neurotransmitter in the brain, gamma-aminobutyric acid (GABA). Molecular diversity in the GABAB system arises from the GABAB1a and GABAB1b subunit isoforms that solely differ in their ectodomains by a pair of sushi repeats that is unique to GABAB1a. Using a combined genetic, physiological, and morphological approach, we now demonstrate that GABAB1 isoforms localize to distinct synaptic sites and convey separate functions in vivo. At hippocampal CA3-to-CA1 synapses, GABAB1a assembles heteroreceptors inhibiting glutamate release, while predominantly GABAB1b mediates postsynaptic inhibition. Electron microscopy reveals a synaptic distribution of GABAB1 isoforms that agrees with the observed functional differences. Transfected CA3 neurons selectively express GABAB1a in distal axons, suggesting that the sushi repeats, a conserved protein interaction motif, specify heteroreceptor localization. The constitutive absence of GABAB1a but not GABAB1b results in impaired synaptic plasticity and hippocampus-dependent memory, emphasizing molecular differences in synaptic GABAB functions.

Adenohypophyseal and hypothalamic GABA B receptor subunits are downregulated by estradiol in adult female rats

Gamma-aminobutyric acid (GABA) participates in neuroendocrine regulation. Since steroid hormones have been shown to modulate the GABAergic system, here we evaluated the effect of chronic in vivo estradiol administration on GABA B receptor (GABA(B)R) expression. GABA(B1) and GABA(B2) subunits were analyzed by Western Blot and RT-PCR, in hypothalami and anterior pituitaries of adult female rats: a) treated for 1 week with estradiol-valerate (a single dose of 100 mug /kg: E1), b) implanted with a 10 mg pellet of estradiol-benzoate for 5 weeks (E5) or c) on proestrous (P), d) ovariectomized (OVX). Pituitary GABA(B)R levels were correlated to a biological effect: baclofen, a GABA(B)R agonist, action on intracellular calcium titers ([Ca(2+)](i)) in pituitary cells. E5 pituitaries showed a significant decrease in the expression of GABA(B1) and GABA(B2) mRNAs compared to P. The GABA(B1a) splice variant of GABA(B1) was always more abundant than GABA(B1b) in this tissue. Similar to the pituitary, hypothalamic GABA(B1) and GABA(B2) mRNAs decreased in E5; this was confirmed at the protein level. In the hypothalamus GABA(B1b) was the main variant expressed in P rats, and was the one significantly sensitive to estradiol-induced decrease, as determined by Western Blots. Castration did not modify GABA(B)R expression with regards to P in either tissue. In P pituitary cells baclofen induced a decrease in [Ca(2+)](i), in contrast this effect was lost in E5 cells. We conclude that chronic estradiol treatment negatively regulates the expression of the GABA(B)R subunits in the pituitary and the hypothalamus. This effect is coupled to a loss of baclofen action on intracellular calcium in pituitary cells.

Molecular and cellular diversity of neuronal G-protein-gated potassium channels

Neuronal G-protein-gated potassium (GIRK) channels mediate the inhibitory effects of many neurotransmitters. Although the overlapping distribution of GIRK subunits suggests that channel composition varies in the CNS, little direct evidence supports the existence of structural or functional diversity in the neuronal GIRK channel repertoire. Here we show that the GIRK channels linked to GABAB receptors differed in two neuron populations. In the substantia nigra, GIRK2 was the principal subunit, and it was found primarily in dendrites of neurons in the substantia nigra pars compacta (SNc). Baclofen evoked prominent barium-sensitive outward current in dopamine neurons of the SNc from wild-type mice, but this current was completely absent in neurons from GIRK2 knock-out mice. In the hippocampus, all three neuronal GIRK subunits were detected. The loss of GIRK1 or GIRK2 was correlated with equivalent, dramatic reductions in baclofen-evoked current in CA1 neurons. Virtually all of the barium-sensitive component of the baclofen-evoked current was eliminated with the ablation of both GIRK2 and GIRK3, indicating that channels containing GIRK3 contribute to the postsynaptic inhibitory effect of GABAB receptor activation. The impact of GIRK subunit ablation on baclofen-evoked current was consistent with observations that GIRK1, GIRK2, and GABAB receptors were enriched in lipid rafts isolated from mouse brain, whereas GIRK3 was found primarily in higher-density membrane fractions. Altogether, our data show that different GIRK channel subtypes can couple to GABAB receptors in vivo. Furthermore, subunit composition appears to specify interactions between GIRK channels and organizational elements involved in channel distribution and efficient receptor coupling.

Age-related changes of the GABA-B receptor in the lumbar spinal cord of male rats and penile erection

Dorsal horn neurons of lumbosacral spinal cord innervate penile vasculature and regulate penile erection. GABAergic system is involved in the regulation of male sexual behavior. Because aging is frequently accompanied by a progressive decline in erectile function, the aim of this work was to examine age-related changes of the GABA-B receptor in the lumbar spinal cord. Sprague-Dawley rats of 10 and 21 days old, 3, 9 and 20 months old were used. GABA-B receptors were evaluated by quantitative autoradiography using [3H]-Baclofen as ligand with or without GABA (10 microM) to determine the non-specific binding. Ten days after birth a homogeneous neuroanatomical distribution pattern was found in the gray matter, however at 20-day-old adult distribution emerged becoming heterogeneous with the highest binding values at layers II-III and X. In dorsal layers a significant decrease was observed in 9-month-old rats while layer X showed an earlier decrease (21-day-old). GABA-B receptor affinity showed significant age-dependent and regional increase. The GABA-B receptor decrease in aged rats seems not to be related to this receptor inhibitory function in penile erection. Moreover the changes found in GABA-B receptor binding anatomical distribution may indicate its role in the morphological development of the lumbar spinal cord rather than in the decline of the erectile function.

Expression of gamma-aminobutyric acid B receptor subunits in hypothalamus of male and female developing rats

GABA and its receptors show particular ontogenic distributions in different rat brain areas. Recently, GABAB receptors (GBR) have been described to assemble as heterodimers formed by a GBR1a/b and a GBR2 subunit. Here, the ontogeny of rat GBRs and the pattern of subunit expression in both sexes were determined in the hypothalamus, a critical area for homeostatic regulation. Male and female rats were sacrificed at 1, 4, 12, 20, 28, 38 days of life and at adulthood and hypothalami were removed and frozen. Western blots analysis for GBR1 and GBR2 subunits showed that both were expressed in male and female hypothalamic membranes from day 1 to adulthood. In females, both GBR1a and GBR1b were maximally expressed in newborns and decreased towards adulthood. At birth, expression of GBR1a was significantly higher than GBR1b, while at 38 days, GBR1b was more abundant. In males, GBR1a and GBR1b expression was higher in young animals and decreased gradually showing adult levels between the second and third weeks of age without differences between isoforms. Comparing GBR1 variants levels in hypothalamus between sexes, GBR1a was significantly more abundant in females at birth while at 38 days its expression was higher in males; GBR1b showed no sex differences along development. GBR2 was detected in hypothalami of females and males at all ages; maximum levels were observed at 12 days and adult levels were attained at 38 days, without sex differences. This is the first report on the ontogeny of hypothalamic GABAB receptors in male and female rats, with a particular developmental pattern of subunit and isoform expression and presenting some sex differences.

GABAB-receptor-mediated suppression of sympathetic outflow from the spinal cord of neonatal rats

Using a splanchnic nerve-spinal cord preparation in vitro that could spontaneously generate sympathetic nerve discharge (SND), we investigated the roles of intraspinal GABAB receptors in the regulation of SND. Despite an age-dependent difference in sensitivity, bath applications of baclofen (Bac; GABAB-receptor agonist) consistently reduced SND in a concentration-dependent manner. The drug specificity of Bac in activation of GABAB receptors was verified by application of its antagonist saclofen (Sac) or CGP-46381 (CGP). Sac or CGP alone did not change SND. However, in the presence of Sac or CGP, the effects of Bac on SND inhibition were reversibly attenuated. The splanchnic sympathetic preganglionic neuron (SPN) was recorded by blind whole cell, patch-clamp techniques. We examined Bac effects on electrical membrane properties of SPNs. Applications of Bac reduced excitatory synaptic events, induced membrane hyperpolarizations, and inhibited SPN firing. In the presence of 12 mM Mg2+ or 0.5 μM TTX to block Ca2+- or action potential-dependent synaptic transmissions, applications of Bac induced an outward baseline current that reversed at −29 ± 6 mV. Because the K+ equilibrium potential in our experimental conditions was −100 mV, the Bac-induced currents could not simply be attributed to an alteration of K+ conductance. On the other hand, applications of Bac to Cs+-loaded SPNs reduced Cd2+-sensitive and high-voltage-activated inward currents, indicating an inhibition of voltage-gated Ca2+ currents. Our results suggest that the activation of intraspinal GABAB receptors suppresses SND via a mixture of ion events that may link to a change in Ca2+ conductance.

Determination of the minimal functional ligand-binding domain of the GABAB1b receptor

In the mammalian central nervous system, slow inhibitory neurotransmission is largely mediated by metabotropic GABA(B) receptors (where GABA stands for gamma-aminobutyric acid), which belong to the G-protein-coupled receptor gene family. Functional GABA(B) receptors are assembled from two subunits GABA(B1) (GABA(B) receptor subtype 1) and GABA(B2). For the GABA(B1) subunit, which binds the neurotransmitter GABA, two variants GABA(B1a) (GABA(B) receptor subtype 1 variant a) and GABA(B1b) have been identified. They differ at the very N-terminus of their large glycosylated ECD (extracellular domain). To simplify the structural characterization, we designed truncated GABA(B1) receptors to identify the minimal functional domain which still binds a competitive radioligand and leads to a functional, GABA-responding receptor when co-expressed with GABA(B2). We show that it is necessary to include all the portion of the ECD encoded by exon 6 to exon 14. Furthermore, we studied mutant GABA(B1b) receptors, in which single or all potential N-glycosylation sites are removed. The absence of oligosaccharides does not impair receptor function, suggesting that the unglycosylated ECD of GABA(B1) can be used for further functional or structural investigations.

GABAB receptor subunit mRNAs are differentially regulated in pituitary melanotropes during development and detection of functioning receptors coincides with completion of innervation

This study examines the developmental expression of GABAB receptor subunits (GABAB(1a), GABAB(1b), GABAB(2)) in the pituitary intermediate lobe using in situ hybridization, reverse transcriptase-polymerase chain reaction, immunohistochemistry, and Western blots. Receptor functionality was studied by baclofen-stimulated GTPgammaS binding. In the adult rat pituitary all three transcripts were detected in melanotropes, but not in glia, of the intermediate lobe. No transcripts of any subunit were detected in the neural lobe. Transcripts of GABAB(1a) and GABAB(1b), but not of GABAB(2), were detected in specific subpopulations of cells in the anterior lobe. All three transcripts were detected in melanotropes on gestational day 18 using in situ hybridization. Reverse transcriptase-polymerase chain reactions comparing postnatal day 2 and adult transcript levels in the neurointermediate lobe support in situ hybridization data that GABAB(1a) mRNA levels do not change, GABAB(1b) levels increase, and GABAB(2) levels decrease as the rat matures. Thus, GABAB receptor subunit transcripts are differentially regulated in melanotropes during development. In the adult rat both GABAB(1) and GABAB(2) proteins were detected in the neurointermediate lobe using Western blotting and in melanotropes by immunohistochemistry. Developmentally, GABAB(1) protein was not detected until postnatal day 7, but was clearly expressed by postnatal day 15 while GABAB(2) protein could not be detected until postnatal day 15. Functional receptors were found in the intermediate lobe at postnatal day 15 and in the adult. The demonstration of transcripts for GABAB(1a), GABAB(1b) and GABAB(2) subunits at gestational day 18 contrasted with the failure to detect any protein before postnatal day 7, suggesting that the regulation of GABAB subunit isoforms occurs differentially at both the transcriptional and translational level as development progresses. The disparity in the regulation of the receptor subunits may suggest that GABAB(1) could have other functions besides being part of the GABAB receptor heterodimer.

The RXR-type endoplasmic reticulum-retention/retrieval signal of GABAB1 requires distant spacing from the membrane to function

Functional gamma-aminobutyric acid type B (GABA(B)) receptors are normally only observed upon coexpression of GABA(B1) with GABA(B2) subunits. A C-terminal arginine-based endoplasmic reticulum (ER) retention/retrieval signal, RSRR, prevents escape of unassembled GABA(B1) subunits from the ER and restricts surface expression to correctly assembled heteromeric receptors. The RSRR signal in GABA(B1) is proposed to be shielded by C-terminal coiled-coil interaction of the GABA(B1) with the GABA(B2) subunit. Here, we investigated whether the RSRR motif in GABA(B1) remains functional when grafted to ectopic sites. We found that the RSRR signal in GABA(B1) is inactive in any of the three intracellular loops but remains functional when moved within the distal zone of the C-terminal tail. C-terminal deletions that position the RSRR signal closer to the plasma membrane drastically reduce its effectiveness, supporting that proximity to the membrane restricts access to the RSRR motif. Functional ectopic RSRR signals in GABA(B1) are efficiently inactivated by the GABA(B2) subunit in the absence of coiled-coil dimerization, supporting that coiled-coil interaction is not critical for release of the receptor complex from the ER. The data are consistent with a model in which removal of RSRR from its active zone rather than its direct shielding by coiled-coil dimerization triggers forward trafficking. Because arginine-based intracellular retention signals of the type RXR, where X represents any amino acid, are used to regulate assembly and surface transport of several multimeric complexes, such a mechanism may apply to other proteins as well.

cAMP response element-binding protein, activating transcription factor-4, and upstream stimulatory factor differentially control hippocampal GABABR1a and GABABR1b subunit gene expression through alternative promoters

Expression of metabotropic GABA(B) receptors is essential for slow inhibitory synaptic transmission in the CNS, and disruption of GABA(B) receptor-mediated responses has been associated with several disorders, including neuropathic pain and epilepsy. The location of GABA(B) receptors in neurons determines their specific role in synaptic transmission, and it is believed that sorting of subunit isoforms, GABA(B)R1a and GABA(B)R1b, to presynaptic or postsynaptic membranes helps to determine this role. GABA(B)R1a and GABA(B)R1b are thought to arise by alternative splicing of heteronuclear RNA. We now demonstrate that alternative promoters, rather than alternative splicing, produce GABA(B)R1a and GABA(B)R1b isoforms. Our data further show that subunit gene expression in hippocampal neurons is mediated by the cAMP response element-binding protein (CREB) by binding to unique cAMP response elements in the alternative promoter regions. Double-stranded oligonucleotide decoys selectively alter levels of endogenous GABA(B)R1a and GABA(B)R1b in primary hippocampal neurons, and CREB knock-out mice show changes in levels of GABA(B)R1a and GABA(B)R1b transcripts, consistent with decoy competition experiments. These results demonstrate a critical role of CREB in transcriptional mechanisms that control GABA(B)R1 subunit levels in vivo. In addition, the CREB-related factor activating transcription factor-4 (ATF4) has been shown to interact directly with GABA(B)R1 in neurons, and we show that ATF4 differentially regulates GABA(B)R1a and GABA(B)R1b promoter activity. These results, together with our finding that the depolarization-sensitive upstream stimulatory factor (USF) binds to a composite CREB/ATF4/USF regulatory element only in the absence of CREB binding, indicate that selective control of alternative GABA(B)R1 promoters by CREB, ATF4, and USF may dynamically regulate expression of their gene products in the nervous system.

Redistribution of GABAB(1) protein and atypical GABAB responses in GABAB(2)-deficient mice

GABAB receptors mediate slow synaptic inhibition in the nervous system. In transfected cells, functional GABAB receptors are usually only observed after coexpression of GABAB(1) and GABAB(2) subunits, which established the concept of heteromerization for G-protein-coupled receptors. In the heteromeric receptor, GABAB(1) is responsible for binding of GABA, whereas GABAB(2) is necessary for surface trafficking and G-protein coupling. Consistent with these in vitro observations, the GABAB(1) subunit is also essential for all GABAB signaling in vivo. Mice lacking the GABAB(1) subunit do not exhibit detectable electrophysiological, biochemical, or behavioral responses to GABAB agonists. However, GABAB(1) exhibits a broader cellular expression pattern than GABAB(2), suggesting that GABAB(1) could be functional in the absence of GABAB(2). We now generated GABAB(2)-deficient mice to analyze whether GABAB(1) has the potential to signal without GABAB(2) in neurons. We show that GABAB(2)-/- mice suffer from spontaneous seizures, hyperalgesia, hyperlocomotor activity, and severe memory impairment, analogous to GABAB(1)-/- mice. This clearly demonstrates that the lack of heteromeric GABAB(1,2) receptors underlies these phenotypes. To our surprise and in contrast to GABAB(1)-/- mice, we still detect atypical electrophysiological GABAB responses in hippocampal slices of GABAB(2)-/- mice. Furthermore, in the absence of GABAB(2), the GABAB(1) protein relocates from distal neuronal sites to the soma and proximal dendrites. Our data suggest that association of GABAB(2) with GABAB(1) is essential for receptor localization in distal processes but is not absolutely necessary for signaling. It is therefore possible that functional GABAB receptors exist in neurons that naturally lack GABAB(2) subunits.

Effect of androgens on sexual differentiation of pituitary gamma-aminobutyric acid receptor subunit GABA(B) expression

Previous work demonstrated a sexually dimorphic ontogenic expression of gamma-aminobutyric acid receptors (GABA(B)R) in rat pituitary. As sex steroids determine sex-specific expression patterns, we now studied the effect of sex hormones on pituitary GABA(B)R expression. GABA(B)R subunits, measured by Western blot and by semi-quantitative RT-PCR and luteinizing hormone (LH), follicle-stimulating hormone (FSH) and testosterone measured by RIA were determined in two experimental designs: First experimental design: 8- and 15-day-old females (8F, 15F); 8F and 15F treated with 100 mug testosterone propionate (TP) on day 1 of life (8F100TP, 15F100TP), 8- and 15-day-old males (8M, 15M) and 8M and 15M castrated on day 1 (8MC, 15MC). Second experimental design: 8-day-old female and male animals: 8F, 8F100TP, 8F treated with 1 mug/day TP on days 1-4 (8F1TP), 8F treated with the androgen antagonist Flutamide (Flut: 2.5 mg/100 g BW of pregnant mother on days E17-E23) (8F-Flut), 8M, 8MC, 8M treated with Flut as above (8M-Flut) and 8MC-Flut. In these animals, in addition, GABA, glutamate, aspartate and taurine were measured by HPLC in hypothalami and cortex. In the first set of experiments, GABA(B1)R mRNA/protein expression was higher in 8F than in 15F, 8M or 15M. In 8F100TP, GABA(B1)R mRNA/protein decreased to male levels. TP treatment did not alter GABA(B1)R expression in 15F. There was no difference in GABA(B1)R expression between 8M and 15M and neonatal castration did not modify its expression. In the second set of experiments, TP (1 mug) or Flut did not modify GABA(B1)R in 8F, while 100 microg TP continued to decrease GABA(B1)R expression. In 8M, Flut, alone or with castration, increased GABA(B1)R mRNA/protein expression to 8F. Hypothalamic GABA content followed the same pattern as pituitary GABA(B)R expression in 8-day-old animals, suggesting a cross-regulation. With regard to hormonal levels, 100 microg, but not 1 microg TP altered gonadotropins at 8 days, although both treatments effectively androgenized females as evidenced by lack of cycling. We conclude that androgens, acting pre- and postnatally, decrease pituitary GABA(B)R subunit expression.

GABA(B) modulation of GABA(A) and glycine receptor-mediated synaptic currents in hypoglossal motoneurons

We studied the effects of GABA(B) receptor activation on either glycine or GABA(A) receptor-mediated synaptic transmission to hypoglossal motoneurons (HMs, P8-13) using a rat brainstem slice preparation. Activation of GABA(B) receptors with baclofen, a GABA(B) receptor agonist, inhibited the amplitude of evoked glycine and GABA(A) receptor-mediated inhibitory postsynaptic currents. Additionally, with blockade of postsynaptic GABA(B) receptors baclofen decreased the frequency of both glycine and GABA(A) receptor-mediated spontaneous miniature inhibitory postsynaptic currents (mIPSCs), indicating a presynaptic site of action. Conversely, the GABA(B) receptor antagonist CGP 35348 increased the frequency of glycine receptor-mediated mIPSCs. Application of the GABA transport blocker SKF 89976A decreased the frequency of glycinergic mIPSCs. Lastly, we compared the effects of baclofen on the frequency of glycine and GABA(A) receptor-mediated mIPSC during HM development. At increased postnatal ages (P8-13 versus P1-3) mIPSC frequency was more strongly reduced by baclofen. These results show that presynaptic GABA(B) receptors inhibits glycinergic and GABAergic synaptic transmission to HMs, and the presynaptic sensitivity to baclofen is increased in P8-13 versus P1-3 HMs. Further, endogenous GABA is capable of modulating inhibitory synaptic transmission to HMs.

Molecular structure and physiological functions of GABA(B) receptors

GABA(B) receptors are broadly expressed in the nervous system and have been implicated in a wide variety of neurological and psychiatric disorders. The cloning of the first GABA(B) receptor cDNAs in 1997 revived interest in these receptors and their potential as therapeutic targets. With the availability of molecular tools, rapid progress was made in our understanding of the GABA(B) system. This led to the surprising discovery that GABA(B) receptors need to assemble from distinct subunits to function and provided exciting new insights into the structure of G protein-coupled receptors (GPCRs) in general. As a consequence of this discovery, it is now widely accepted that GPCRs can exist as heterodimers. The cloning of GABA(B) receptors allowed some important questions in the field to be answered. It is now clear that molecular studies do not support the existence of pharmacologically distinct GABA(B) receptors, as predicted by work on native receptors. Advances were also made in clarifying the relationship between GABA(B) receptors and the receptors for gamma-hydroxybutyrate, an emerging drug of abuse. There are now the first indications linking GABA(B) receptor polymorphisms to epilepsy. Significantly, the cloning of GABA(B) receptors enabled identification of the first allosteric GABA(B) receptor compounds, which is expected to broaden the spectrum of therapeutic applications. Here we review current concepts on the molecular composition and function of GABA(B) receptors and discuss ongoing drug-discovery efforts.

Increased baclofen-stimulated G protein coupling and deactivation in rat brain cortex during development

The number and affinity of GABA(B) receptors (assayed by the specific antagonist [(3)H]CGP54626A) was unchanged when compared in carefully washed cerebrocortical membranes from young (12-day-old) and adult (90-day-old) rats. In contrast, high-affinity GTPase activity, both basal and baclofen-stimulated was significantly higher (by 45% and 56%, respectively) in adult than in young rats. Similar results were obtained by concomitant determination of agonist (baclofen)-stimulated GTP gamma S binding. Under standard conditions, baclofen-stimulated GTPase activity was further considerably enhanced by exogenously added regulator of G protein function, RGS1, but not by RGS16. RGS16 was able to affect agonist-stimulated GTPase activity only in the presence of markedly increase substrate (GTP) concentrations. RGS1 alone slightly increased GTPase activity in adult rats, but neither RGS1 nor RGS16 influenced GTPase activity in membrane preparations isolated from young animals. These findings indicate increasing functional activity of trimeric G protein(s) involved in GABAergic transmission in the developing rat brain cortex and suggest a high potential of RGS1 in regulation of high-affinity GTPase activity.

Ontogeny of GABA(B) receptor subunit expression and function in the rat spinal cord

Little is known about the chronology of expression, cellular localization and function of GABA(B) subunits in the developing rat spinal cord. In the present study, in situ hybridization, immunohistochemistry and quantitative RT-PCR analysis were used to examine this issue. At embryonic day 18, in situ hybridization reveals that all three transcripts, GABA(B(1a)), GABA(B(1b)), and GABA(B(2)), are present throughout the gray matter. At postnatal day (PN) 2, while overall expression appears to decrease, it becomes more highly concentrated in motoneurons of the ventral horn. By PN 7, distinct subpopulations of cells expressing the transcripts become heavily expressed in motoneurons. Immunohistochemical analysis revealed that, unlike mRNA, GABA(B(1)) protein is more highly concentrated in the dorsal horn as compared to the motoneurons. Analysis using RT-PCR demonstrates that in spinal cord GABA(B(1a)) mRNA expression remains constant throughout development, GABA(B(1b)) increases from PN 2 to adult, and GABA(B(2)) decreases from PN 2 to adult. The distribution of functional GABA(B) receptors, as measured by baclofen-stimulated [35S]GTPgammaS binding, in the spinal cord during development generally follows the distribution of subunit expression, being widely distributed throughout the gray matter in embryonic spinal cord slices and becoming more concentrated in the dorsal horn during postnatal development, similar to the distribution of subunit proteins as measured by immunohistochemistry. These findings suggest that spinal cord GABA(B(1a)), GABA(B(1b)), and GABA(B(2)) transcripts are differentially regulated during development with the chronology of this expression suggesting that GABA(B) receptor subunits, in addition to forming functional GABA(B) receptors, may have a trophic function or participate in synaptogenesis.

Altered expression of GABAB receptors in the hippocampus after kainic-acid-induced seizures in rats

Epilepsy is closely related to an altered transmission of GABA, the major inhibitory transmitter in the brain. GABA acts through two classes of receptors, ionotropic GABA(A) receptors and metabotropic GABA(B) receptors. Using in situ hybridization, receptor autoradiography and immunocytochemistry, we now investigated temporal changes in the expression the GABA(B)-1 and GABA(B)-2 subunits (GABA(B)-1R and GABA(B)-1R, respectively) in the hippocampus following kainic-acid-induced seizures. Significant decreases (by about 40%) in mRNA levels of both splice variants (a and b) of GABA(B)-1R and of GABA(B)-2R were observed in the principal cell layer of the hippocampus 6-12 h after kainic acid injection in the rat. Whereas mRNA levels in the granule cell layer returned to basal after 24 h, the decreases persisted in sectors CA1 and CA3, presumably due to progressing neurodegeneration. In the sector CA3, GABA(B)-R mRNA levels and GABA(B)-R1 immunoreactivity partially recovered 30 days after the initial kainic acid seizures indicating receptor upregulation in surviving neurons.

Modulation of adenylyl cyclase activity by baclofen in the developing rat brain: difference between cortex, thalamus and hippocampus

Ontogenetic changes in the levels of GABA(B) receptors and their ability to modulate adenylyl cyclase (AC) activity were analyzed in rat cortex, thalamus and hippocampus. The relative numbers of GABA(B) receptors (measured as saturable, high-affinity [(3)H](-)baclofen binding sites) in cortex and thalamus were high already at postnatal day 1 (PD 1) and they reached a maximum at PD 25 and PD 12, respectively. There were no detectable high-affinity [(3)H](-)baclofen binding sites in hippocampus between birth and PD 12 and low-affinity [(3)H](-)baclofen binding attained at PD 12 did not change in adulthood (PD 90). Whereas GTP-stimulated AC activity in cortex and thalamus was depressed by baclofen, it was enhanced in hippocampus. These data indicate that the inhibitory effect of baclofen on AC in cortex and thalamus is primarily mediated through the alpha subunits of G(i)/G(o) proteins. The stimulatory effect of baclofen in hippocampus may be explained by engagement of Gbetagamma subunits.

Differential ontogeny of GABA(B)-receptor-mediated pre- and postsynaptic modulation of GABA and glycine transmission in respiratory rhythm-generating network in mouse

Rhythm generation in mature respiratory networks is influenced strongly by synaptic inhibition. In early neonates, GABA(A)-receptor- and glycine-receptor-mediated inhibition is not present, thus the question arises as to whether GABA(B)-receptor-mediated inhibition plays an important role. Using brainstem slices of neonatal mice (postnatal day, P0-P15), we analysed the role of GABA(B)-mediated modulation of GABA and glycine synaptic transmission in the respiratory network. Blockade of GABA uptake by nipecotic acid (0.25-2 mM) reduced the respiratory frequency. This reduction was prevented by the selective GABA(B) receptor antagonist CGP55845A (CGP) alone at P0-P3, but by bicuculline as well as CGP at P7-P15. Blockade of GABA(B) receptors by CGP increased the respiratory frequency at P0-P3, whereas it caused a reduction of frequency in older animals. The effect of CGP on respiratory frequency was diminished in the presence of bicuculline and strychnine in older but not in younger animals. The relative contribution of GABA(B)-receptor-mediated pre- and postsynaptic modulation was examined by analysing the effect of GABA(B) receptors on spontaneous and miniature IPSCs. In younger animals (P0-P3), the GABA(B) receptor agonist baclofen had no detectable effect on IPSC frequency, but caused a significant decrease in the amplitude. In older animals (P7-P15), baclofen decreased both the frequency and amplitude of spontaneous and miniature IPSCs. These results demonstrate that GABA(B)-receptor-mediated postsynaptic modulation plays an important role in the respiratory network from P0 on. GABA(B)-receptor-mediated presynaptic modulation develops with a longer postnatal latency, and becomes predominant within the first postnatal week.

A chronic model of atypical absence seizures: studies of developmental and gender sensitivity

Treatment of Long Evans hooded rats during post-natal brain development with the cholesterol synthesis inhibitor, AY-9944 (AY) results in the occurrence of atypical absence seizures, which are frequent, recurrent, and life-long. AY induced slow spike-and-wave discharges (SSWD) are significantly more frequent and prolonged in female Long Evans rats than males. Three groups of experiments were performed in order to characterize further the AY model of atypical absence seizures, (1) a developmental study was performed to ascertain whether AY-induced seizures appear before or after the onset of puberty; (2) male/female differences in severity of response to AY was determined in order to answer the question whether the gender specificity was a pre- or postpubertal phenomenon; (3) a time course study was done to determine the minimum number of postnatal AY doses needed to induce the life-long atypical absence seizure state. The data indicate that AY-induced atypical absence seizures emerge before the onset of puberty. Further, we show that the gender difference in severity of AY-induced seizures also is a pre-pubertal phenomenon. Finally, a single dose of AY (7.5 mg/kg) administered on post-natal day (P) 5 was sufficient to induce SSWD on the electrocorticogram (ECoG). Our results suggest that sex hormones are important in the AY model, although the exact role of cholesterol derived steroid hormones in the regulation and maintenance of AY induced atypical absence seizures remains to be determined.