GABA(B) receptors function as a heteromeric assembly of the subunits GABA(B)R1 and GABA(B)R2

Spatially restricted expression of candidate taste receptors in the Drosophila gustatory system

BACKGROUND

Taste is an important sensory modality in most animals. In Drosophila, taste is perceived by gustatory neurons located in sensilla distributed on several different appendages throughout the body of the animal. Here we show that the gustatory receptors are encoded by a family of at least 54 genes (Gr genes), most of which are expressed exclusively in a small subset of taste sensilla located in narrowly defined regions of the fly's body.

RESULTS

BLAST searches with the predicted amino acid sequences of 6 7-transmembrane-receptor genes of unknown function and 20 previously identified, putative gustatory receptor genes led to the identification of a large gene family comprising at least 54 genes. We investigated the expression of eight genes by using a Gal4 reporter gene assay and found that five of them were expressed in the gustatory system of the fly. Four genes were expressed in 1%-4% of taste sensilla, located in well-defined regions of the proboscis, the legs, or both. The fifth gene was expressed in about 20% of taste sensilla in all major gustatory organs, including the taste bristles on the anterior wing margin. Axon-tracing experiments demonstrated that neurons expressing a given Gr gene project their axons to a spatially restricted domain of the subesophageal ganglion in the fly brain.

CONCLUSIONS

Our findings suggest that each taste sensillum represents a discrete, functional unit expressing at least one Gr receptor and that most Gr genes are expressed in spatially restricted domains of the gustatory system. These observations imply the potential for high taste discrimination of the Drosophila brain.

Constitutive expression of functional GABA(B) receptors in mIL-tsA58 cells requires both GABA(B(1)) and GABA(B(2)) genes

Studies of gamma-aminobutyric acid (GABA)(B) receptor function in heterologous cell systems have suggested that expression of two distinct seven transmembrane G-protein coupled receptor subunits is necessary for receptor activation and signal transduction. Some results suggest that both receptor proteins must be inserted into the plasma membrane to create heterodimers; however, it is possible that subunit monomers or homodimers are functional in cells which constitutively express GABA(B) receptors. A new pituitary intermediate lobe melanotrope cell clone (mIL tsA58) has been isolated which constitutively expresses GABA(B), D(2) and corticotrophin releasing factor receptors. Here, we report on characterization of the GABA(B) receptors. Solution hybridization-nuclease protection assays reveal the presence of GABA(B(1)) and GABA(B(2)) transcripts. Western blots show GABA(B(1a)) and one of two GABA(B(2)) proteins. Addition of the GABA(B) agonist baclofen to cultured mIL-tsA58 (mIL) cells inhibits high voltage activated Ca(2+) channels, as measured by agonist-induced inhibition of the K(+)-depolarization-stimulated increase in Ca(2+) influx. CGP55845, a GABA(B) antagonist, blocks the response to baclofen. Knockdown of either GABA(B(1)) or GABA(B(2)) subunits with selective antisense oligodeoxynucleotides reduced GABA(B) protein levels and completely abolished the GABA(B) receptor response in the mIL cells. Taken together, these results indicate that functionally active GABA(B) receptors in mIL cells require the constitutive expression of both GABA(B) genes. This is a physiologic validation of results from recombinant overexpression in naive cells and shows that the mIL cell line is a useful model for studying GABA(B) receptor expression, regulation and function.

Effect of MPTP-induced denervation on basal ganglia GABA(B) receptors: correlation with dopamine concentrations and dopamine transporter

We investigated the effect of MPTP-induced lesion of the substantia nigra pars compacta (SNpc) dopaminergic neurons on GABA(B) receptors in the basal ganglia of mice and monkeys using receptor autoradiography and in situ hybridization. The extent of the lesion was measured with striatal catecholamine content, striatal binding of (125)I-RTI-121 to dopamine transporter (DAT), and DAT expression in the SNpc. GABA(B) receptors in mice brain were evaluated using (3)H-CGP54626 and its expression was measured with oligonucleotides probes targeting the mRNAs of GABA(B(1a+b)), GABA(B(1a)), GABA(B(1b)), GABA(B(2)) subunits. In monkeys, (125)I-CGP64213 and selective probes for GABA(B(1a+b)) and GABA(B(2)) mRNAs were used. In mice, dopamine content, (125)I-RTI-121 binding, and DAT expression were reduced by 44%, 40%, and 39% after a dose of 40 mg/kg of MPTP and 74%, 70%, and 34% after 120 mg/kg of MPTP, respectively. In monkeys, dopamine content and DAT expression were decreased by more than 90% and 80%, respectively. In the striatum and the subthalamic nucleus, GABA(B) receptors were unchanged following MPTP in both species. In the SNpc of mice, MPTP (120 mg/kg) induced a significant decrease of (3)H-CGP54626 binding (-10%) and of the expression of GABA(B(1a+b)) mRNA (-13%). The decrease of the expression of GABA(B(1a+b)) mRNA was correlated with dopamine content, (125)I-RTI-121 binding and DAT expression. In MPTP-treated monkeys, (125)I-CGP64213 binding (-40%), GABA(B(1a+b)) mRNA (-69%) and GABA(B(2)) mRNA (-66%) were also significantly decreased in the SNpc. Our results suggest that MPTP-induced denervation is associated with a decrease of GABA(B) receptors restricted to the SNpc. These observations may be relevant to the pathophysiology of motor disorders involving dysfunction of the basal ganglia such as Parkinson disease.

Site of action of GABA(B) receptor for vagal motor control of the lower esophageal sphincter in ferrets and rats

BACKGROUND & AIMS

Stimulation of gamma-aminobutyric acid B metabotropic receptors (GBRs) by baclofen reduces the incidence of transient lower esophageal sphincter (LES) relaxations. The GBR effect may be a result of a central site of action in the dorsal vagal complex, where upper gastrointestinal vagal reflexes are integrated. Therefore, we first localized GBR immunostaining in the dorsal vagal complex. Next, we tested the hypothesis that baclofen modulates LES motor tone via GBR expressed by vagal efferent neurons.

METHODS

An antibody against the human GBR1b isoform was characterized and used for immunocytochemistry in rats and ferrets. Functional studies involved microinjection of L-glutamate into the caudal dorsal motor nucleus of the vagus to evoke an LES relaxation in decerebrate unanesthetized ferrets.

RESULTS

In both species, GBR1b was expressed in preganglionic motor neurons and, in ferrets, the receptor was highly expressed in identified LES-projecting preganglionic neurons. GBR1b immunostaining was also pronounced in the subnucleus centralis of the nucleus tractus solitarius. This distribution implicates GBR in control of the esophageal phase of swallowing at the level of the central program generator. In functional studies, centrally evoked LES relaxation (-73% +/- 8% mm Hg) was significantly attenuated after 7 micromol/kg intravenous baclofen (-37% +/- 10%; N = 5).

CONCLUSIONS

These data all suggest that GBR agonists inhibit LES relaxation via a site of action associated with vagal motor outflow to the LES.

Epileptogenesis and enhanced prepulse inhibition in GABA(B1)-deficient mice

The recent cloning of two GABA(B) receptor subunits, GABA(B1) and GABA(B2), has raised the possibility that differences in GABA(B) receptor subunit composition may give rise to pharmacologically or functionally distinct receptors. If present, such molecular diversity could permit the selective targeting of GABA(B) receptor subtypes specifically involved in pathologies such as drug addiction, spasticity, pain, and epilepsy. To address these issues we have developed a GABA(B1) subunit knockout mouse using gene targeting techniques. In the brains of GABA(B1) null mice, all pre- and postsynaptic GABA(B) receptor function was absent demonstrating that the GABA(B1) subunit is essential for all GABA(B) receptor-mediated mechanisms. Despite this, GABA(B1) null mice appeared normal at birth, although by postnatal week four their growth was retarded and they developed a generalized epilepsy that resulted in premature death. In addition, GABA(B1) heterozygote animals showed enhanced prepulse inhibition responses compared to littermate controls, suggesting that GABA(B1) deficient mice exhibit increased sensorimotor gating mechanisms. These data suggest that GABA(B) receptor antagonists may be of benefit in the treatment of psychiatric and neurological disorders in which attentional processing is impaired.

Metabotropic glutamate 1alpha and adenosine A1 receptors assemble into functionally interacting complexes

Recently, evidence has emerged that seven transmembrane G protein-coupled receptors may be present as homo- and heteromers in the plasma membrane. Here we describe a new molecular and functional interaction between two functionally unrelated types of G protein-coupled receptors, namely the metabotropic glutamate type 1alpha (mGlu(1alpha) receptor) and the adenosine A1 receptors in cerebellum, primary cortical neurons, and heterologous transfected cells. Co-immunoprecipitation experiments showed a close and subtype-specific interaction between mGlu(1alpha) and A1 receptors in both rat cerebellar synaptosomes and co-transfected HEK-293 cells. By using transiently transfected HEK-293 cells a synergy between mGlu(1alpha) and A1 receptors in receptor-evoked [Ca(2+)](i) signaling has been shown. In primary cultures of cortical neurons we observed a high degree of co-localization of the two receptors, and excitotoxicity experiments in these cultures also indicate that mGlu(1alpha) and A1 receptors are functionally related. Our results provide a molecular basis for adenosine/glutamate receptors cross-talk and open new perspectives for the development of novel agents to treat neuropsychiatric disorders in which abnormal glutamatergic neurotransmission is involved.

Mutant G protein alpha subunit activated by Gbeta gamma: a model for receptor activation?

How receptors catalyze exchange of GTP for GDP bound to the Galpha subunit of trimeric G proteins is not known. One proposal is that the receptor uses the G protein's betagamma heterodimer as a lever, tilting it to pull open the guanine nucleotide binding pocket of Galpha. To test this possibility, we designed a mutant Galpha that would bind to betagamma in the tilted conformation. To do so, we excised a helical turn (four residues) from the N-terminal region of alpha(s), the alpha subunit of G(S), the stimulatory regulator of adenylyl cyclase. In the presence, but not in the absence, of transiently expressed beta(1) and gamma(2), this mutant (alpha(s)Delta), markedly stimulated cAMP accumulation. This effect depended on the ability of the coexpressed beta protein to interact normally with the lip of the nucleotide binding pocket of alpha(s)Delta. We substituted alanine for an aspartate in beta(1) that binds to a lysine (K206) in the lip of the alpha subunit's nucleotide binding pocket. Coexpressed with alpha(s)Delta and gamma(2), this mutant, beta(1)-D228A, elevated cAMP much less than did beta(1)-wild type; it did bind to alpha(s)Delta normally, however, as indicated by its unimpaired ability to target alpha(s)Delta to the plasma membrane. We conclude that betagamma can activate alpha(s) and that this effect probably involves both a tilt of betagamma relative to alpha(s) and interaction of beta with the lip of the nucleotide binding pocket. We speculate that receptors use a similar mechanism to activate trimeric G proteins.

Differential expression of GABA(B)R1 and GABA(B)R2 receptor immunoreactivity in neurochemically identified neurons of the rat neostriatum

Gamma-aminobutyric acid (GABA) is the major inhibitory neurotransmitter in the neostriatum. Functions of GABA are known to mediate GABA(A) and GABA(B) receptors. A functional GABA(B) receptor is known to compose of heteromeric subunits, namely the GABA(B)R1 and GABA(B)R2 subunits. Our previous report (Yung et al. [1999] Brain Res. 830:345-352) has demonstrated that all major subpopulations of striatal neurons express GABA(B)R1 immunoreactivity. The cellular localization of the second subunit of GABA(B) receptor protein, i.e., GABA(B)R2 immunoreactivity, in the rat neostriatum is not yet known. By using a new commercially available specific antibody against GABA(B)R2, immunofluorescence was performed to investigate the cellular expression of GABA(B)R2 in neurochemically identified subpopulations of neurons in the rat neostriatum. Immunoreactivity for GABA(B)R2 was primarily found in the neuropil of the rat neostriatum. Double labeling revealed that those perikarya that expressed immunoreactivity for parvalbumin, choline acetyltransferase, nitric oxide synthase, glutamate receptor two, N-methyl-D-aspartate receptor one, or GABA(A)alpha1 receptor, respectively, did not express GABA(B)R2 immunoreactivity. In addition, perikarya and most of the neuropilar elements in the neostriatum that expressed glutamic acid decarboxylase 67 immunoreactivity were found to be GABA(B)R2-negative. In contrast, immunoreactivity for GABA(B)R1 was found to be expressed by all of the above neuronal subpopulations. Moreover, a vast number of SV2-immunoreactive profiles and a number of tyrosine hydroxylase-immunoreactive profiles in the neuropil of the neostriatum were found to display GABA(B)R2 immunoreactivity. The present results indicate that there is a differential expression of GABA(B)R2 and GABA(B)R1 immunoreactivity in different subpopulations of striatal neurons that are identified by their specific neurochemical markers. Immunoreactivity for GABA(B)R2 is likely to localize in neuropilar elements of the neostriatum that may belong to non-GABAergic elements. These findings provide anatomical evidence of GABA(B)R2 receptor localization in the neostriatum that may have an important functional implication of the GABA(B)-mediated functions in neurons of the neostriatum.

Regulation of transport of the dopamine D1 receptor by a new membrane-associated ER protein

Many structural determinants for G protein-coupled receptor (GPCR) functions have been defined, but little is known concerning the regulation of their transport from the endoplasmic reticulum (ER) to the cell surface. Here we show that a carboxy-terminal hydrophobic motif, FxxxFxxxF, which is highly conserved among GPCRs, functions independently as an ER-export signal for the dopamine D1 receptor. A newly identified ER-membrane-associated protein, DRiP78, binds to this motif. Overexpression or sequestration of DRiP78 leads to retention of D1 receptors in the ER, reduced ligand binding, and a slowdown in the kinetics of receptor glycosylation. Our results indicate that DRiP78 may regulate the transport of a GPCR by binding to a specific ER-export signal.

Advances in the molecular understanding of GABA(B) receptors

The molecular nature of the metabotropic GABA(B) receptor was for some time a mystery, however it was recently discovered that two related G-protein-coupled receptors have to heterodimerize to form the functional GABA(B) receptor at the cell surface. This review discusses the most recent findings in the rapidly expanding field of GABA(B) receptor research, and includes a summary of all splice variants of both receptor subunits identified to date. It also evaluates emerging evidence that certain splice variants might play a role in determining pharmacologically distinguishable receptors, and reviews receptor localization at the sub-cellular level and involvement in neuronal development.

Allosteric interactions between GB1 and GB2 subunits are required for optimal GABA(B) receptor function

Recent studies on G-protein-coupled receptors revealed that they can dimerize. However, the role of each subunit in the activation process remains unclear. The gamma-amino-n-butyric acid type B (GABA(B)) receptor is comprised of two subunits: GB1 and GB2. Both consist of an extracellular domain (ECD) and a heptahelical domain composed of seven transmembrane alpha-helices, loops and the C-terminus (HD). Whereas GB1 ECD plays a critical role in ligand binding, GB2 is required not only to target GB1 subunit to the cell surface but also for receptor activation. Here, by analysing chimeric GB subunits, we show that only GB2 HD contains the determinants required for G-protein signalling. However, the HD of GB1 improves coupling efficacy. Conversely, although GB1 ECD is sufficient to bind GABA(B) ligands, the ECD of GB2 increases the agonist affinity on GB1, and is necessary for agonist activation of the receptor. These data indicate that multiple allosteric interactions between the two subunits are required for wild-type functioning of the GABA(B) receptor and highlight further the importance of the dimerization process in GPCR activation.

Identification of a novel member of the T1R family of putative taste receptors

In the gustatory system, the recognition of sugars, amino acids and bitter-tasting compounds is the function of specialized G protein-coupled receptors. Recently, two members of novel subfamily of G protein-coupled receptors were proposed to function as taste receptors based on their specific expression in taste receptor cells. Here, we report the identification of a third member, T1R3, of this family of receptors. T1R3 maps near the telomere of mouse chromosome 4 rendering it a candidate for the Sac locus, a primary determinant of sweet preference in mice. Consistent with its candidacy for the Sac locus, T1R3 displays taste receptor cell-specific expression. In addition, taster and non-taster strains of mouse harbor different alleles of T1R3.

An RGD sequence in the P2Y(2) receptor interacts with alpha(V)beta(3) integrins and is required for G(o)-mediated signal transduction

The P2Y(2) nucleotide receptor (P2Y(2)R) contains the integrin-binding domain arginine-glycine-aspartic acid (RGD) in its first extracellular loop, raising the possibility that this G protein-coupled receptor interacts directly with an integrin. Binding of a peptide corresponding to the first extracellular loop of the P2Y(2)R to K562 erythroleukemia cells was inhibited by antibodies against alpha(V)beta(3)/beta(5) integrins and the integrin-associated thrombospondin receptor, CD47. Immunofluorescence of cells transfected with epitope-tagged P2Y(2)Rs indicated that alpha(V) integrins colocalized 10-fold better with the wild-type P2Y(2)R than with a mutant P2Y(2)R in which the RGD sequence was replaced with RGE. Compared with the wild-type P2Y(2)R, the RGE mutant required 1,000-fold higher agonist concentrations to phosphorylate focal adhesion kinase, activate extracellular signal-regulated kinases, and initiate the PLC-dependent mobilization of intracellular Ca(2+). Furthermore, an anti-alpha(V) integrin antibody partially inhibited these signaling events mediated by the wild-type P2Y(2)R. Pertussis toxin, an inhibitor of G(i/o) proteins, partially inhibited Ca(2+) mobilization mediated by the wild-type P2Y(2)R, but not by the RGE mutant, suggesting that the RGD sequence is required for P2Y(2)R-mediated activation of G(o), but not G(q). Since CD47 has been shown to associate directly with G(i/o) family proteins, these results suggest that interactions between P2Y(2)Rs, integrins, and CD47 may be important for coupling the P2Y(2)R to G(o).

Identification of GABA(B) receptor in rat testis and sperm

gamma-Aminobutyric acid (GABA) can mimic and potentiate the action of progesterone in initiating the acrosome reaction (AR) of mammalian sperm, indicating that sperm contain receptors for GABA. This contention was validated by identifying the receptor (R) subtype, GABA(A)R, in mammalian sperm. In the present study a second subtype, GABA(B)R, was identified in rat testis and sperm. Total RNAs of rat testis and sperm were prepared and used as template to synthesize the respective cDNAs by the RT-PCR method. Two splice variants of the cDNA coding GABA(B)R1 (GABA(B)R1a and GABA(B)R1c) and GABA(B)R2 were identified. Extracts of rat testis, spermatogenic cells and sperm contained two proteins with estimated molecular sizes of 130 and 100 kDa, corresponding to GABA(B)R1a and GABA(B)R1c/lb, respectively, determined by Western blot using polyclonal anti-GABA(B)R1 antibody. By an indirect immunofluorescence technique, GABA(B)R1 was located on the head of rat sperm. The present finding is the first direct demonstration that mammalian sperm contain GABA(B)R.

GABA(B) receptors couple directly to the transcription factor ATF4

The inhibitory neurotransmitter gamma-aminobutyric acid (GABA), acts at ionotropic (GABA(A) and GABA(C)) and metabotropic (GABA(B)) receptors. Functional GABA(B) receptors are heterodimers of GABA(B(1)) and GABA(B(2)) subunits. Here we show a robust, direct, and specific interaction between the coiled-coil domain present in the C-terminus of the GABA(B(1)) subunit and the transcription factor ATF4 (also known as CREB2). ATF4 and GABA(B(2)) binding to the GABA(B(1)) subunit were mutually exclusive. In rat hippocampal neurons native GABA(B(1)) showed surprisingly little similarity to GABA(B(2)) in its subcellular distribution. GABA(B(1)) and ATF4, however, were highly colocalized throughout the cell and displayed a punctate distribution within the dendrites. Activation of GABA(B) receptors in hippocampal neurons caused a dramatic translocation of ATF4 out of the nucleus into the cytoplasm. These data suggest a novel neuronal signaling pathway that could regulate the functional expression of GABA(B) receptors and/or modulate gene transcription.

Both GABA(B) receptor agonist and antagonists decreased brain stimulation reward in the rat

The present experiments were designed to determine the role of GABA(B) receptor function on brain stimulation reward. Using a discrete-trial current-intensity threshold procedure, dose-effect functions were generated for the GABA(B) receptor agonist CGP 44532 (0-1.0 mg/kg, s.c.) and the GABA(B) receptor antagonists CGP 56433A (0-10.0 mg/kg, s.c.) and CGP 51176 (0-300.0 mg/kg, s.c.) on brain reward thresholds in rats. The GABA(B) receptor antagonists CGP 56433A and CGP 51176 were used also to examine interaction effects with the GABA(B) receptor agonist CGP 44532 on reward thresholds. Administration of the highest doses of both the GABA(B) receptor agonist and antagonists elevated reward thresholds. Thus, both the agonist and antagonists used induced a reward decrement when administered separately. In addition, the co-administration of either of the two receptor antagonists with the agonist induced an additive effect on thresholds, rather than blocking the agonist-induced threshold elevations. These results suggest that activation of GABA(B) receptors modulates intracranial self-stimulation behavior in a complex fashion, possibly through differential effects of GABA(B) agonists and antagonists on pre- and post-synaptic GABA(B) receptors.

Oligomerization of G-protein-coupled transmitter receptors

Examples of G-protein-coupled receptors that can be biochemically detected in homo- or heteromeric complexes are emerging at an accelerated rate. Biophysical approaches have confirmed the existence of several such complexes in living cells and there is strong evidence to support the idea that dimerization is important in different aspects of receptor biogenesis and function. While the existence of G-protein-coupled-receptor homodimers raises fundamental questions about the molecular mechanisms involved in transmitter recognition and signal transduction, the formation of heterodimers raises fascinating combinatorial possibilities that could underlie an unexpected level of pharmacological diversity, and contribute to cross-talk regulation between transmission systems. Because G-protein-coupled receptors are major pharmacological targets, the existence of dimers could have important implications for the development and screening of new drugs. Here, we review the evidence supporting the existence of G-protein-coupled-receptor dimerization and discuss its functional importance.

Oligomerisation of G-protein-coupled receptors

A range of approaches have recently provided evidence that G-protein-coupled receptors can exist as oligomeric complexes. Both homo-oligomers, comprising multiple copies of the same gene product, and hetero-oligomers containing more than one receptor have been detected. In several, but not all, examples, the extent of oligomerisation is regulated by the presence of agonist ligands, and emerging evidence indicates that receptor hetero-oligomers can display distinct pharmacological characteristics. A chaperonin-like role for receptor oligomerisation in effective delivery of newly synthesised receptors to the cell surface is a developing concept, and recent studies have employed a series of energy-transfer techniques to explore the presence and regulation of receptor oligomerisation in living cells. However, the majority of studies have relied largely on co-immunoprecipitation techniques, and there is still little direct information on the fraction of receptors existing as oligomers in intact cells.

Cloning and tissue distribution of a novel isoform of the rat GABA(B)R1 receptor subunit

We have identified a novel splice variant of the metabotropic GABA(B) receptor (R) subunit I, designated GABA(B)R1f, from a rat hippocampus cDNA library screen. GABA(B)R1f shares sequence homology with rat GABA(B)R1a, with the exception of an in-frame deletion of exon 4, resulting in a 21 bp deletion in the coding sequence of the N-terminal extracellular domain. In addition, GABA(B)R1f also contains a 93 bp in-frame insertion in a region of the sequence corresponding to the second extracellular loop and the fifth transmembrane domain, similar to that found in rat GABA(B)R1c. While being ubiquitously (but variably) expressed, reverse-transcription polymerase chain reaction analysis revealed the GABA(B)R1f isoform to be most prevalent in peripheral vs central tissues, suggesting a potential role for this novel isoform in either the mediation of inhibitory transmission in these various tissues, or in as yet defined actions unrelated to central synaptic regulatory mechanisms attributable to GABA(B)R.

Retinylidene proteins: structures and functions from archaea to humans

Retinylidene proteins, containing seven membrane-embedded alpha-helices that form an internal pocket in which the chromophore retinal is bound, are ubiquitous in photoreceptor cells in eyes throughout the animal kingdom. They are also present in a diverse range of other organisms and locations, such as archaeal prokaryotes, unicellular eukaryotic microbes, the dermal tissue of frogs, the pineal glands of lizards and birds, the hypothalamus of toads, and the human brain. Their functions include light-driven ion transport and phototaxis signaling in microorganisms, and retinal isomerization and various types of photosignal transduction in higher animals. The aims of this review are to examine this group of photoactive proteins as a whole, to summarize our current understanding of structure/function relationships in the best-studied examples, and to report recent new developments.

Subcellular localization of GABA(B) receptor subunits in rat visual cortex

Although studies in the visual cortex have found gamma-aminobutyric acid B (GABA(B)) receptor-mediated pre- and postsynaptic inhibitory effects on neurons, the subcellular localization of GABA(B) receptors in different types of cortical neurons and synapses has not been shown directly. To provide this information, we have used antibodies against the GABA(B) receptor (R)1a/b and GABA(B)R2 subunits and have studied the localization of immunoreactivities in rat visual cortex. Light microscopic analyses have shown that both subunits are expressed in cell bodies and dendrites of 65-92% of corticocortically projecting pyramidal neurons and in 92-100% of parvalbumin (PV)-, calretinin (CR)-, and somatostatin (SOM)-containing GABAergic neurons. Electron microscopic analyses of immunoperoxidase- and immunogold-labeled tissue revealed staining in the nucleus, cytoplasm and cell surface membranes with both antibodies. Colocalization of both subunits was observed in all of these structures. GABA(B)R1a/b and GABA(B)R2 were concentrated in excitatory and inhibitory synapses and in extrasynaptic membranes. In GABAergic synapses, GABA(B)R1a/b and GABA(B)R2 were more strongly expressed postsynaptically on pyramidal and nonpyramidal cells than presynaptically. In type 1 synapses GABA(B)R1a/b and GABA(B)R2 was found in pre- and postsynaptic membranes. The nuclear localization of GABA(B)R1 and GABA(B)R2 subunits suggests a novel role for neurotransmitter receptors in controlling gene expression. The synaptic colocalization of GABA(B)R1 and GABA(B)R2 indicates that subunits form heteromeric assemblies of the functional receptor in inhibitory and excitatory synapses. Subunit coexpression in GABAergic synapses that include PV-containing and PV-deficient terminals suggests that pre- and postsynaptic GABA(B) receptor activation is provided by several different types of interneurons. The coexpression of both subunits in excitatory synapses suggests a role for GABA(B) receptors in the regulation of glutamate release and raises the question how these receptors are activated in the absence of pre-or postsynaptic GABAergic synaptic inputs to excitatory synapses.

Distribution of GABA(B(1a)), GABA(B(1b)) and GABA(B2) receptor protein in cerebral cortex and thalamus of adult rats

The distribution of GABA(B) receptor subunits GABA(B(1a)), GABA(B(1b)) and GABA(B2), has been examined in the cerebral cortex and thalamus of adult rats using an immunocytochemical technique. GABA(B(1a)) and GABA(B(1b)) subunits co-localized with GABA(B2) in the cortex, where afferent thalamic GABAergic axons project to pyramidal neurones. The expression patterns of GABA(B(1a)), GABA(B(1b)) and GABA(B2) were similar throughout the thalamus. The data suggest that the GABA(B(1b)) subunit might be the presynaptic isoform in the thalamo-cortical pathway with the GABA(B(1a)) subunit possibly present at postsynaptic sites on cell bodies. This contrasts with our previous data, obtained in cerebellum and spinal cord which indicate opposite locations. Thus, it seems unlikely that functional role along with cellular location can be assigned in a general manner to specific GABA(B) receptor subunit splice variants.

GABA(B) receptors: altered coupling to G-proteins in rats sensitized to amphetamine

Modified dopamine and glutamate neurotransmission in discrete brain regions is implicated in stimulant-induced behavioral sensitization. Release of both neurotransmitters is influenced by GABA(B) metabotropic receptors for the principal inhibitory neurotransmitter GABA. Accordingly, GABA(B) receptors were examined in rats sensitized to amphetamine by measuring receptor density and coupling to G-proteins indicated as [(3)H]baclofen binding and baclofen-mediated [(35)S]GTP gamma S binding. Repeated treatment with (+)-amphetamine (5mg/kg per day, i.p., for five days) sensitized the rats to amphetamine challenge (1mg/kg) at 14 days, but not one day, later. GABA(B) receptor density was not altered at either time. Baclofen-mediated [(35)S]GTP gamma S binding, however, was selectively augmented in the prefrontal cortex and attenuated in the nucleus accumbens at 14 days, but not one day, after amphetamine treatment. Changes in GABA(B) receptor coupling to G-proteins in rats sensitized to amphetamine, but not in similarly treated but unsensitized rats, lead us to suggest that altered GABA(B) receptor functioning may contribute to the expression of amphetamine-induced behavioral sensitization.

Ontogenic expression of anterior pituitary GABA(B) receptor subunits

gamma-Aminobutyric acid (GABA) is involved in the neuroendocrine control of hypophyseal secretion, acting both in the central nervous system and directly at the pituitary. We have characterized the properties of anterior pituitary GABA(B) receptors. In this work the ontogeny of rat anterior pituitary GABA(B) receptors and the pattern of subunit expression in rats of both sexes were determined. Western blot analysis showed a temporal decrease in GABA(B) subunits GABA(B(1a)) and GABA(B(1b)) expression in female anterior pituitary membranes from day 4 to adulthood, with GABA(B(1a)) being significantly more abundant than GABA(B(1b)) at early stages of development; the GABA(B(2)) subunit was barely detectable. In the male, GABA(B(1a)) followed a similar pattern and appeared to be significantly less abundant than in 4- and 12-day-old females; GABA(B(1b)) and GABA(B(2)) expression in the male was barely detectable. Scatchard plot analysis showed a temporal decrease in binding sites in female anterior pituitary membranes, in agreement with the western blot results. The number of binding sites was significantly higher in female than in male 4-day-old membranes. Dissociation constant values were similar for both sexes at all ages studied. This study reports for the first time the ontogeny of anterior pituitary GABA(B) receptors, showing a particular developmental pattern of subunit expression and a clear sexual dimorphism.

Inverse, protean, and ligand-selective agonism: matters of receptor conformation

Concepts regarding the mechanisms by which drugs activate receptors to produce physiological response have progressed beyond considering the receptor as a simple on-off switch. Current evidence suggests that the idea that agonists produce only varying degrees of receptor activation is obsolete and must be reconciled with data to show that agonist efficacy has texture as well as magnitude. Thus, agonists can block system constitutive response (inverse agonists), behave as positive and inverse agonists on the same receptor (protean agonists), and differ in the stimulus pattern they produce in physiological systems (ligand-selective agonists). The molecular mechanism for this seemingly diverse array of activities is the same, namely, the selective microaffinity of ligands for different conformational states of the receptor. This paper reviews evidence for the existence of the various types of agonism and the potential therapeutic utility of different agonist types.-Kenakin, T. Inverse, protean, and ligand-selective agonism: matters of receptor conformation.

The expression of GABA(B1) and GABA(B2) receptor subunits in the cNS differs from that in peripheral tissues

GABA(B) receptors are G-protein-coupled receptors that mediate the slow and prolonged synaptic actions of GABA in the CNS via the modulation of ion channels. Unusually, GABA(B) receptors form functional heterodimers composed of GABA(B1) and GABA(B2) subunits. The GABA(B1) subunit is essential for ligand binding, whereas the GABA(B2) subunit is essential for functional expression of the receptor dimer at the cell surface. We have used real-time reverse transcriptase-polymerase chain reaction to analyse expression levels of these subunits, and their associated splice variants, in the CNS and peripheral tissues of human and rat. GABA(B1) subunit splice variants were expressed throughout the CNS and peripheral tissues, whereas surprisingly GABA(B2) subunit splice variants were neural specific. Using novel antisera specific to individual GABA(B) receptor subunits, we have confirmed these findings at the protein level. Analysis by immunoblotting demonstrated the presence of the GABA(B1) subunit, but not the GABA(B2) subunit, in uterus and spleen. Furthermore, we have shown the first immunocytochemical analysis of the GABA(B2) subunit in the brain and spinal cord using a GABA(B2)-specific antibody. We have, therefore, identified areas of non-overlap between GABA(B1) and GABA(B2) subunit expression in tissues known to contain functional GABA(B) receptors. Such areas are of interest as they may well contain novel GABA(B) receptor subunit isoforms, expression of which would enable the GABA(B1) subunit to reach the cell surface and form functional GABA(B) receptors.

Coexistence of kappa- and delta-opioid receptors in rat spinal cord axons

It has been found that heterodimers of kappa- and delta-opioid receptors can occur in vitro, but it has been unclear whether they also occur in intact animals. In the present study we examined whether kappa-delta heterodimers might occur in vivo by staining for these receptors with two-color fluorescence immunocytochemistry. Sections of rat spinal cord were double-stained using rabbit anti-kappa opioid receptor combined with rat anti-delta-opioid receptor. It was found that axons in the superficial dorsal horn of the spinal cord were double-labeled. In addition, structures within axonal varicosities were sometimes double-labeled. We conclude that single axons, and single structures within axons, express both kappa- and delta-opioid receptors. These observations are consistent with heterodimers of these receptors existing in vivo.

Heterodimerization of G-protein-coupled receptors in the CNS

Over the last year the combinations of G-protein-coupled receptors that are known to form heterodimeric complexes has rapidly increased. For example, dopamine receptors can dimerize with both somatostatin and adenosine receptors. These studies have been aided by improved technologies to monitor protein/protein interactions in living cells. Crosstalk at the level of the receptors might explain some of the known physiological interactions of these neurotransmitter systems and also provide new approaches for therapeutic intervention.

Cloning and functional expression of GABA(B) receptors from Drosophila

The neurotransmitter GABA (gamma-aminobutyric acid) functions as the major inhibitory neurotransmitter in the central nervous system of vertebrates and invertebrates. In vertebrates GABA signals both through ionotropic receptors (GABA(A), GABA(C)), which induce fast synaptic inhibitory responses, and through metabotropic receptors (GABA(B)), which play a fundamental role in the reduction of presynaptic transmitter release and postsynaptic inhibitory potentials. Whilst GABA(A) and GABA(C) receptors have been cloned from vertebrates as well as invertebrates, GABA(B) receptors have only been identified in vertebrate species to date, although indirect evidence suggests their existence in arthropods, too. Here we report the cloning of three putative invertebrate GABA(B) receptor subtypes (D-GABA(B)R1, R2 and R3) isolated from Drosophila melanogaster. Whilst D-GABA(B)R1 and R2 show high sequence identity to mammalian GABA(B)R1 and R2, respectively, the receptor D-GABA(B)R3 seems to be an insect-specific subtype with no known mammalian counterpart so far. All three D-GABA(B)R subtypes are expressed in the embryonic central nervous system. In situ hybridization of Drosophila melanogaster embryos shows that two of the D-GABA(B)Rs (D-GABA(B)R1 and R2) are expressed in similar regions, suggesting a coexpression of the two receptors, whilst the third D-GABA(B)R (D-GABA(B)R3) displays a unique expression pattern. In agreement with these results we have only been able to functionally characterize D-GABA(B)R1 and R2 when the two subtypes are coexpressed either in Xenopus laevis oocytes or mammalian cell lines, whilst D-GABA(B)R3 was inactive in any combination. The pharmacology of the coexpressed D-GABA(B)R1/2 receptor was different from the mammalian GABA(B)Rs: e.g. baclofen, an agonist of mammalian GABA(B)Rs, showed no effect.

Association of GABA(B) receptors and members of the 14-3-3 family of signaling proteins

Two GABA(B) receptors, GABA(B)R1 and GABA(B)R2, have been cloned recently. Unlike other G protein-coupled receptors, the formation of a heterodimer between GABA(B)R1 and GABA(B)R2 is required for functional expression. We have used the yeast two hybrid system to identify proteins that interact with the C-terminus of GABA(B)R1. We report a direct association between GABA(B) receptors and two members of the 14-3-3 protein family, 14-3-3eta and 14-3-3zeta. We demonstrate that the C-terminus of GABA(B)R1 associates with 14-3-3zeta in rat brain preparations and tissue cultured cells, that they codistribute after rat brain fractionation, colocalize in neurons, and that the binding site overlaps partially with the coiled-coil domain of GABA(B)R1. Furthermore we show a reduced interaction between the C-terminal domains of GABA(B)R1 and GABA(B)R2 in the presence of 14-3-3. The results strongly suggest that GABA(B)R1 and 14-3-3 associate in the nervous system and begin to reveal the signaling complexities of the GABA(B)R1/GABA(B)R2 receptor heterodimer.

GABA(B(1)) mRNA expression in hippocampal sclerosis associated with human temporal lobe epilepsy

GABA(B) receptors act to inhibit neurotransmitter release from presynaptic terminals, and mediate the late inhibitory postsynaptic potential. Studies of GABA(B) receptor function in rodent models of temporal lobe epilepsy (TLE) suggest that GABA(B) receptor expression and/or function may be perturbed. GABA(B(1)) mRNA levels were investigated in 10 hippocampal resection samples obtained at surgery from intractable hippocampal sclerosis (HS) associated TLE patients and five neurologically normal post-mortem (PM) control samples. In situ hybridisation with a 35S-dATP-labelled oligonucleotide was carried out to measure mRNA levels, along with three-dimensional cell counting, for assessment of neuronal density in hippocampal subregions. GABA(B(1)) mRNA was significantly up-regulated in the subiculum of HS samples as compared with PM controls. When adjusted for the characteristic neuronal density changes observed in HS, GABA(B(1)) mRNA was significantly up-regulated in CA1, hilus and dentate gyrus granule cell layer of HS samples as compared with PM controls. The possibility of increased GABA(B(1)) expression suggests that changes in GABA(B) receptor mechanisms may be involved in the pathogenesis of human HS-associated TLE.

Oligomerization of opioid receptors with beta 2-adrenergic receptors: a role in trafficking and mitogen-activated protein kinase activation

G-protein-coupled receptors (GPCRs) have recently joined the list of cell surface receptors that dimerize. Dimerization has been shown to alter the ligand-binding, signaling, and trafficking properties of these receptors. Recent studies have shown that GPCRs heterodimerize with closely related members, resulting in the modulation of their function. In this study, we have attempted to determine whether members of GPCR superfamilies that couple to different families of G-proteins can associate and form oligomers. We chose the beta2 adrenergic receptor that couples to stimulatory G-proteins and delta & kappa opioid receptors that couple to inhibitory G-proteins. beta2 and delta receptors undergo robust agonist-mediated endocytosis, whereas kappa receptors do not. We find that when coexpressed, beta2 receptors can form heteromeric complexes with both delta and kappa receptors. This heterooligomerization does not significantly alter the ligand binding or coupling properties of the receptors. However, it affects the trafficking properties of the receptors. For example, we find that delta receptors, when coexpressed with beta2 receptors, undergo isoproterenol-mediated endocytosis. Conversely, beta2 receptors in these cells undergo etorphine-mediated endocytosis. However, beta2 receptors, when coexpressed with kappa receptors, undergo neither opioid- nor isoproterenol-mediated endocytosis. Moreover, these cells exhibit a substantial decrease in the isoproterenol-induced phosphorylation of mitogen-activated protein kinases. Taken together, these results provide direct evidence of heteromerization of GPCRs that couple to different types of G-proteins, which results in the modulation of receptor trafficking and signal transduction.

Oligomerization of opioid receptors with beta 2-adrenergic receptors: a role in trafficking and mitogen-activated protein kinase activation

G-protein-coupled receptors (GPCRs) have recently joined the list of cell surface receptors that dimerize. Dimerization has been shown to alter the ligand-binding, signaling, and trafficking properties of these receptors. Recent studies have shown that GPCRs heterodimerize with closely related members, resulting in the modulation of their function. In this study, we have attempted to determine whether members of GPCR superfamilies that couple to different families of G-proteins can associate and form oligomers. We chose the beta2 adrenergic receptor that couples to stimulatory G-proteins and delta & kappa opioid receptors that couple to inhibitory G-proteins. beta2 and delta receptors undergo robust agonist-mediated endocytosis, whereas kappa receptors do not. We find that when coexpressed, beta2 receptors can form heteromeric complexes with both delta and kappa receptors. This heterooligomerization does not significantly alter the ligand binding or coupling properties of the receptors. However, it affects the trafficking properties of the receptors. For example, we find that delta receptors, when coexpressed with beta2 receptors, undergo isoproterenol-mediated endocytosis. Conversely, beta2 receptors in these cells undergo etorphine-mediated endocytosis. However, beta2 receptors, when coexpressed with kappa receptors, undergo neither opioid- nor isoproterenol-mediated endocytosis. Moreover, these cells exhibit a substantial decrease in the isoproterenol-induced phosphorylation of mitogen-activated protein kinases. Taken together, these results provide direct evidence of heteromerization of GPCRs that couple to different types of G-proteins, which results in the modulation of receptor trafficking and signal transduction.

Pharmacology of [3H]R(+)-7-OH-DPAT binding in the rat caudate-putamen

Dopamine D3 receptors may be involved in drug addiction and in disorders such as schizophrenia and Parkinson's disease. To determine the pharmacological properties of dopamine D3 receptors in the rat caudate-putamen, we have investigated R(+)-[3H]7-hydroxy-N,N-di-n-propyl-2-aminotetralin ([3H]R(+)-7-OH-DPAT) binding to membrane preparations from the rat caudate-putamen. Kinetic analyses showed that [3H]R(+)-7-OH-DPAT binding reached equilibrium in approximately 1 h and that both association and dissociation curves were composed of at least two components. Likewise, saturation curves showed at least two binding components with a combined Bmax value of about 600 fmol/mg protein, which is three times higher than what is present in the subcortical limbic area. Competition curves were performed with agonists such as R(-)-propylnorapomorphine, dopamine, PD 128907, quinpirole, and bromocriptine, and antagonists such as haloperidol, raclopride, clozapine, GR 218231x, remoxipride, and U99194A. These experiments revealed that [3H]R(+)-7-OH-DPAT binding could be resolved into three specific binding sites (R1-R3) and one nonspecific binding site, with R1-R2 probably representing D3 receptor binding and the minor R3 representing D2 receptor binding. The low affinities of (+/-)-8-OH-DPAT and 1,3-di(2-tolyl)guanidine to inhibit [3H]R(+)-7-OH-DPAT binding indicate negligible involvement of 5-HT1A or sigma binding sites, respectively. The pharmacological profile of [3H]R(+)-7-OH-DPAT (2 nM) binding in the caudate-putamen was similar to that of dopamine on [125I]iodosulpride binding in the cerebellar lobule X, which contain D3 but not D2 receptors. Mg2+ increased and GTP and Na+ decreased the binding of [3H]R(+)-7-OH-DPAT, suggesting a coupling of endogenous D3 receptors to G proteins. Taken together, these results suggest that dopamine D3 receptors display multiple agonist binding states, and that D3 receptors are present in high concentrations in the rat caudate-putamen. These results may have implications for the physiological and pathological roles of dopamine D3 receptors in the brain.

Comparative immunohistochemical localisation of GABA(B1a), GABA(B1b) and GABA(B2) subunits in rat brain, spinal cord and dorsal root ganglion

GABA(B) receptors are G-protein-coupled receptors mediating the slow onset and prolonged synaptic actions of GABA in the CNS. The recent cloning of two genes, GABA(B1) and GABA(B2), has revealed a novel requirement for GABA(B) receptor signalling. Studies have demonstrated that the two receptor subunits associate as a GABA(B1)/GABA(B2) heterodimer to form a functional GABA(B) receptor. In this study we have developed polyclonal antisera specific to two splice variants of the GABA(B1) subunit, GABA(B1a) and GABA(B1b), as well as an antiserum to the GABA(B2) subunit. Using affinity-purified antibodies derived from these antisera we have mapped out the distribution profile of each subunit in rat brain, spinal cord and dorsal root ganglion. In brain the highest areas of GABA(B1a), GABA(B1b) and GABA(B2) subunit expression were found in neocortex, hippocampus, thalamus, cerebellum and habenula. In spinal cord, GABA(B1) and GABA(B2) subunits were expressed in the superficial layers of the dorsal horn, as well as in motor neurones in the deeper layers of the ventral horn. GABA(B) receptor subunit immunoreactivity in dorsal root ganglion suggested that expression of GABA(B1b) was restricted to the large diameter neurones, in contrast to GABA(B1a) and GABA(B2) subunits which were expressed in both large and small diameter neurones. Although expression levels of GABA(B1) and GABA(B2) subunits varied we found no areas in which GABA(B1) was expressed in the absence of GABA(B2). This suggests that most, if not all, GABA(B1) immunoreactivity may represent functional GABA(B) receptors. Although our data are in general agreement with functional studies, some discrepancies in GABA(B1) subunit expression occurred with respect to other immunohistochemical studies. Overall our data suggest that GABA(B) receptors are widely expressed throughout the brain and spinal cord, and that GABA(B1a) and GABA(B1b) subunits can associate with GABA(B2) to form both pre- and post-synaptic receptors.

Postnatal alterations of GABA receptor profiles in the rat superior colliculus

Midbrain sections taken from Sprague-Dawley rats of varying ages within the first four postnatal weeks were used to determine, immunocytochemically, putative changes of GABA(A) receptor beta2/3 subunits, GABA(B) receptor (R1a and R1b splice variants), and GABA(C) receptor rho1 subunit expression and distribution in the superficial, visual layers of the superior colliculus. Immunoreactivity for the GABA(A) receptor beta2/3 subunits was found in the superficial grey layer from birth. The labelling changed with age, with an overall continuous reduction in the number of cells labelled and a significant increase in the labelling intensity distribution (neuropil vs soma). Further analysis revealed an initial increase in the labelling intensity between postnatal days 0 and 7 in parallel with an overall reduction of labelled neurones. This was followed by a significant decrease in labelling intensity distribution between postnatal days 7 and 16, and a subsequent increase in intensity between postnatal days 16 and 28. The labelling profiles for GABA(B) receptors (R1a and R1b splice variants) and GABA(C) receptors (rho1 subunit) showed similar patterns. Both receptors could be found in the superficial layers of the superior colliculus from birth, and the intensity and distribution of labelling remained constant during the first postnatal month. However, the cell body count showed a significant decrease between postnatal days 7 and 16. These changes may be related to the time-point of eye opening, which occurred approximately two weeks after birth. For all three receptor types, the cell body count remained constant after postnatal day 16. By four weeks of age, there was no significant difference between the cell numbers obtained for the different receptors. Both GABA itself and neurofilament labelling were also obtained in the superficial superior colliculus at birth. Neurofilament, although found at birth, showed very little ordered arrangement until 16days after birth. When slices were double labelled for GABA(C) receptors and neurofilament, some overlap was observed. Double labelling for the presynaptic protein synaptophysin and GABA(C) receptors showed proximity in some places, indicative of a partly synaptic location of GABA(C) receptors. When GABA(C) and GABA(A) receptors were labelled simultaneously, some but not all neurones showed immunoreactivity for both receptor types. In conclusion, all three GABA receptor types were found to be present in the superior colliculus from birth, and all show some form of postnatal modification, with GABA(A) receptors demonstrating the most dramatic changes. However, GABA(B) and GABA(C) receptors are modified significantly around the onset of input-specific activity. Together, this points towards a contribution of the GABAergic system to processes of postnatal maturation in the superficial superior colliculus.

The murine GABA(B) receptor 1: cDNA cloning, tissue distribution, structure of the Gabbr1 gene, and mapping to chromosome 17

GABA (gamma-aminobutyric acid) is a major inhibitory neurotransmitter in the central nervous system (CNS) which activates both ionotropic (GABA(A)/GABA(C)) and metabotropic (GABA(B)) receptor systems. We identified two alternatively spliced cDNA variants of the murine GABA(B) receptor 1 that are predominantly expressed in the CNS. Deduced protein structures are highly homologous to the previously characterized rat and human receptors. Comparison of the genomic structures of mouse and human revealed that alternative splicing occurred at the same position, whereas the mouse gene has an additional 5' exon. Radiation hybrid mapping, combined with database searches, indicated that the GABA(B) receptor gene (Gabbr1) is located on mouse chromosome 17, adjacent to the marker D17Mit24 in a region homologous to human chromosome 6p21.3.

GABA(B) receptor autoradiography in hippocampal sclerosis associated with human temporal lobe epilepsy

1. Metabotropic gamma-aminobutyric acid receptors (GABA(B)) exist both pre- and postsynaptically throughout the brain, mediating the suppression of neurotransmitter release and late inhibitory postsynaptic potentials. Investigation of GABA(B) receptors in rodent models of temporal lobe epilepsy (TLE) suggests that expression or function of these receptors may be altered in the disorder. 2. The aim of the present study was to investigate the expression of GABA(B) receptors in samples of hippocampus surgically resected from patients with hippocampal sclerosis (HS) related intractable TLE, and compare this expression with samples of neurologically normal post-mortem (PM) control hippocampal tissue. Appropriate measures of neuronal loss associated with HS were investigated for comparison with receptor binding data. 3. Receptor autoradiography with [(3)H]-GABA in the presence of isoguvacine, and quantitative densitometric analysis were used to investigate GABA(B) receptor expression (B(max)) and affinity (K(D)) in 11 HS samples and eight controls. A three-dimensional cell counting technique was used to assess neuronal density in both groups. 4. GABA(B) receptor density was significantly reduced in CA1, CA2, CA3, hilus and dentate gyrus, and increased in the subiculum, of HS cases as compared with PM controls. Neuronal loss was significant in all regions measured. When adjusted for neuronal loss, CA1 GABA(B) receptor expression appeared significantly upregulated (P:<0.05). 5. In HS/TLE, GABA(B) receptor expression per remaining neurone appears increased in CA1. This finding, and increased [(3)H]-GABA affinity at CA3 and hilar GABA(B) receptors, suggests altered GABA(B) receptor function may occur in human HS/TLE, possibly as a result of synaptic reorganization.

Extracellular calcium sensing and extracellular calcium signaling

The cloning of a G protein-coupled extracellular Ca(2+) (Ca(o)(2+))-sensing receptor (CaR) has elucidated the molecular basis for many of the previously recognized effects of Ca(o)(2+) on tissues that maintain systemic Ca(o)(2+) homeostasis, especially parathyroid chief cells and several cells in the kidney. The availability of the cloned CaR enabled the development of DNA and antibody probes for identifying the CaR's mRNA and protein, respectively, within these and other tissues. It also permitted the identification of human diseases resulting from inactivating or activating mutations of the CaR gene and the subsequent generation of mice with targeted disruption of the CaR gene. The characteristic alterations in parathyroid and renal function in these patients and in the mice with "knockout" of the CaR gene have provided valuable information on the CaR's physiological roles in these tissues participating in mineral ion homeostasis. Nevertheless, relatively little is known about how the CaR regulates other tissues involved in systemic Ca(o)(2+) homeostasis, particularly bone and intestine. Moreover, there is evidence that additional Ca(o)(2+) sensors may exist in bone cells that mediate some or even all of the known effects of Ca(o)(2+) on these cells. Even more remains to be learned about the CaR's function in the rapidly growing list of cells that express it but are uninvolved in systemic Ca(o)(2+) metabolism. Available data suggest that the receptor serves numerous roles outside of systemic mineral ion homeostasis, ranging from the regulation of hormonal secretion and the activities of various ion channels to the longer term control of gene expression, programmed cell death (apoptosis), and cellular proliferation. In some cases, the CaR on these "nonhomeostatic" cells responds to local changes in Ca(o)(2+) taking place within compartments of the extracellular fluid (ECF) that communicate with the outside environment (e.g., the gastrointestinal tract). In others, localized changes in Ca(o)(2+) within the ECF can originate from several mechanisms, including fluxes of calcium ions into or out of cellular or extracellular stores or across epithelium that absorb or secrete Ca(2+). In any event, the CaR and other receptors/sensors for Ca(o)(2+) and probably for other extracellular ions represent versatile regulators of numerous cellular functions and may serve as important therapeutic targets.

Cellular and synaptic adaptations mediating opioid dependence

Although opioids are highly effective for the treatment of pain, they are also known to be intensely addictive. There has been a massive research investment in the development of opioid analgesics, resulting in a plethora of compounds with varying affinity and efficacy at all the known opioid receptor subtypes. Although compounds of extremely high potency have been produced, the problem of tolerance to and dependence on these agonists persists. This review centers on the adaptive changes in cellular and synaptic function induced by chronic morphine treatment. The initial steps of opioid action are mediated through the activation of G protein-linked receptors. As is true for all G protein-linked receptors, opioid receptors activate and regulate multiple second messenger pathways associated with effector coupling, receptor trafficking, and nuclear signaling. These events are critical for understanding the early events leading to nonassociative tolerance and dependence. Equally important are associative and network changes that affect neurons that do not have opioid receptors but that are indirectly altered by opioid-sensitive cells. Finally, opioids and other drugs of abuse have some common cellular and anatomical pathways. The characterization of common pathways affected by different drugs, particularly after repeated treatment, is important in the understanding of drug abuse.

Mapping the agonist-binding site of GABAB type 1 subunit sheds light on the activation process of GABAB receptors

The gamma-amino-n-butyric acid type B (GABA(B)) receptor is composed of two subunits, GABA(B)1 and GABA(B)2, belonging to the family 3 heptahelix receptors. These proteins possess two domains, a seven transmembrane core and an extracellular domain containing the agonist binding site. This binding domain is likely to fold like bacterial periplasmic binding proteins that are constituted of two lobes that close upon ligand binding. Here, using molecular modeling and site-directed mutagenesis, we have identified residues in the GABA(B)1 subunit that are critical for agonist binding and activation of the heteromeric receptor. Our data suggest that two residues (Ser(246) and Asp(471)) located within lobe I form H bonds and a salt bridge with carboxylic and amino groups of GABA, respectively, demonstrating the pivotal role of lobe I in agonist binding. Interestingly, our data also suggest that a residue within lobe II (Tyr(366)) interacts with the agonists in a closed form model of the binding domain, and its mutation into Ala converts the agonist baclofen into an antagonist. These data demonstrate the pivotal role played by the GABA(B)1 subunit in the activation of the heteromeric GABA(B) receptor and are consistent with the idea that a closed state of the binding domain of family 3 receptors is required for their activation.

Evidence for an accessory protein function for Toll-like receptor 1 in anti-bacterial responses

Members of the Toll-like receptor (TLR) family are components of the mammalian anti-microbial response, signaling with a domain closely related to that of IL-1 receptors. In this report the expression and function of TLR1, a TLR of unknown function, are examined. TLR1 is expressed by monocytes, as demonstrated using a novel mAb. Monocytes also express TLR2. TLR1 transfection of HeLa cells, which express neither TLR1 nor TLR2, was not sufficient to confer responsiveness to several microbial extracts. However, cotransfection of TLR1 and TLR2 resulted in enhanced signaling by HeLa cells to soluble factors released from Neisseria meningitidis relative to the response with either TLR alone. This phenomenon was also seen with high concentrations of some preparations of LPS. The N. meningitidis factors recognized by TLR1/TLR2 were not released by N. meningitidis mutant in the LpxA gene. Although LpxA is required for LPS biosynthesis, because cooperation between TLR1 and TLR2 was not seen with all LPS preparations, the microbial component(s) TLR1/2 recognizes is likely to be a complex of LPS and other molecules or a compound metabolically and chemically related to LPS. The functional IL-1R consists of a heterodimer; this report suggests a similar mechanism for TLR1 and TLR2, for certain agonists. These data further suggest that mammalian responsiveness to some bacterial products may be mediated by combinations of TLRs, suggesting a mechanism for diversifying the repertoire of Toll-mediated responses.

The GABAB receptor interacts directly with the related transcription factors CREB2 and ATFx

gamma-Aminobutyric acid type B (GABA(B)) receptors mediate the metabotropic actions of the inhibitory neurotransmitter GABA. These seven-transmembrane receptors are known to signal primarily through activation of G proteins to modulate the action of ion channels or second messengers. The functional GABA(B) receptor is made up of a heterodimer consisting of two subunits, GABA(B)-R1 and GABA(B)-R2, which interact via coiled-coil domains in their C-terminal tails. By using a yeast two-hybrid approach, we have identified direct interactions between the C-terminal tails of GABA(B)-R1 and GABA(B)-R2 with two related transcription factors, CREB2 (ATF4) and ATFx. In primary neuronal cultures as well in recombinant Chinese hamster ovary cells expressing GABA(B) receptors, CREB2 is localized within the cytoplasm as well as the nucleus. Activation of the GABA(B) receptor by the specific agonist baclofen leads to a marked translocation and accumulation of CREB2 from the cytoplasm into the nucleus. We demonstrate that receptor stimulation results in activation of transcription from a CREB2 responsive reporter gene. Such a signaling mechanism is unique among Family C G protein-coupled receptors and, in the case of the GABA(B) receptor and CREB2, may play a role in long-term changes in the nervous system.

A novel family of candidate pheromone receptors in mammals

Pheromonal activation of the vomeronasal organ (VNO) elicits genetically preprogrammed behaviors and physiological changes in mammals. We have identified a novel gene family encoding over one hundred VNO specific receptors, the V3Rs. V3R sequences are highly similar to each other and appear distantly related to the putative pheromone receptors, V1Rs, and the taste receptors, T2Rs. Within the VNO, V3R-positive neurons are distinct from neurons expressing the pheromone receptor families V1R and V2R. The V3Rs are likely to represent a new large family of pheromone receptors in mammals. Multiple V3R-related human sequences have been identified, including one clone retaining the capacity to create a complete and functional transcript. Our data uncover a striking complexity in the molecular and cellular organization of the VNO and provide an essential framework for the study of pheromone signaling in mammals.

Characterization of [(3)H]-CGP54626A binding to heterodimeric GABA(B) receptors stably expressed in mammalian cells

Functional human GABA(B(1a,2)) and GABA(B(1b,2)) receptors have been stably expressed in mammalian CHO K1 cells. Detailed characterization of GABA(B) ligand binding at each of the receptors has been compared using [(3)H]-CGP54626A. In cell membranes fractions, [(3)H]-CGP54626A bound to a single site with a K(D) of 1. 51+/-1.12 nM, B(max) of 2.02+/-0.17 pmoles mg protein(-1) and 0. 86+/-0.20 nM, B(max) of 5.19+/-0.57 pmoles mg protein(-1) for GABA(B(1a,2)) and GABA(B(1b,2)) respectively. In competition binding assays the rank order was identical for both GABA(B) receptors. For known GABA(B) agonists the rank order was CGP27492>SKF97541=CGP46381>GABA>Baclofen and for GABA(B) antagonists the rank order was CGP54262A>CGP55845>CGP52432>SCH 50911>CGP51176>CGP36742=CGP35348 > or =2-OH Saclofen > or =ABPA. The allosteric effect of calcium cations was also investigated. The effect of removal of CaCl(2) from the binding assay conditions was ligand dependent to either cause a decrease in ligand affinity or to have no significant effect. However, these effects were similar for both GABA(B) receptors. A whole cell, scintillation proximity binding assay was used to determine agonist affinity at exclusively heterodimeric GABA(B) receptors. In competition assays, the rank order was the same for both GABA(B(1a,2)) and GABA(B(1b,2)) and consistent with that seen with cell membrane fractions. These data suggest that, in terms of ligand binding, the currently identified isoforms of the GABA(B) receptor are pharmacologically indistinguishable.

Antagonistic oxytocin/alpha2-adrenoreceptor interactions in the nucleus tractus solitarii: relevance for central cardiovascular control

The modulation of the central cardiovascular effects of alpha2-adrenoceptor activation by oxytocin in the nucleus tractus solitarii has been evaluated by cardiovascular analysis and by quantitative receptor autoradiography. Microinjections in the nucleus tractus solitarii of a threshold dose of oxytocin effectively and significantly counteracted the vasodepressor and bradycardic actions of an ED50 dose of the alpha2-adrenoceptor agonist clonidine. The coinjection of a threshold dose of oxytocin with a threshold dose of clonidine did not produce any changes in the mean arterial pressure but a tachycardic response was observed. Receptor autoradiographical experiments showed that oxytocin (3 nM) significantly increased the Kd and Bmax values of [3H]p-aminoclonidine binding sites in the nucleus tractus solitarii compatible with a possible antagonistic interaction with the alpha2-adrenoceptors, and this effect was blocked by the presence of the specific oxytocin receptor antagonist 1-deamino-2-D-Tyr-(OEt)-4-Thr-8-Orn-oxytocin. These findings suggest the existence of an antagonistic oxytocin/alpha2-adrenoceptor interaction in nucleus tractus solitarii that may be of relevance for the demonstrated modulation of alpha2-adrenoceptor induced cardiovascular responses by oxytocin.