Pertussis toxin in the analysis of receptor mechanisms

OZITX, a pertussis toxin-like protein for occluding inhibitory G protein signalling including Gαz

Heterotrimeric G proteins are the main signalling effectors for G protein-coupled receptors. Understanding the distinct functions of different G proteins is key to understanding how their signalling modulates physiological responses. Pertussis toxin, a bacterial AB₅ toxin, inhibits Gα_i/o G proteins and has proven useful for interrogating inhibitory G protein signalling. Pertussis toxin, however, does not inhibit one member of the inhibitory G protein family, Gα_z. The role of Gα_z signalling has been neglected largely due to a lack of inhibitors. Recently, the identification of another Pertussis-like AB₅ toxin was described. Here we show that this toxin, that we call OZITX, specifically inhibits Gα_i/o and Gα_z G proteins and that expression of the catalytic S1 subunit is sufficient for this inhibition. We identify mutations that render Gα subunits insensitive to the toxin that, in combination with the toxin, can be used to interrogate the signalling of each inhibitory Gα G protein.

A recently identified pertussis toxin-like AB5 toxin, OZITX, is found to inhibit Gαi/o and Gαz G proteins. In combination with directed mutations, it is a useful tool for interrogating Gαi/o/z G protein subunits individually.

Identification of a G-Protein-Independent Activator of GIRK Channels

G-protein-gated inwardly rectifying K⁺ (GIRK) channels are essential effectors of inhibitory neurotransmission in the brain. GIRK channels have been implicated in diseases with abnormal neuronal excitability, including epilepsy and addiction. GIRK channels are tetramers composed of either the same subunit (e.g., homotetramers) or different subunits (e.g., heterotetramers). Compounds that specifically target subsets of GIRK channels in vivo are lacking. Previous studies have shown that alcohol directly activates GIRK channels through a hydrophobic pocket located in the cytoplasmic domain of the channel. Here, we report the identification and functional characterization of a GIRK1-selective activator, termed GiGA1, that targets the alcohol pocket. GiGA1 activates GIRK1/GIRK2 both in vitro and in vivo and, in turn, mitigates the effects of a convulsant in an acute epilepsy mouse model. These results shed light on the structure-based development of subunit-specific GIRK modulators that could provide potential treatments for brain disorders.

The Molecular Basis of Toxins' Interactions with Intracellular Signaling via Discrete Portals

An understanding of the molecular mechanisms by which microbial, plant or animal-secreted toxins exert their action provides the most important element for assessment of human health risks and opens new insights into therapies addressing a plethora of pathologies, ranging from neurological disorders to cancer, using toxinomimetic agents. Recently, molecular and cellular biology dissecting tools have provided a wealth of information on the action of these diverse toxins, yet, an integrated framework to explain their selective toxicity is still lacking. In this review, specific examples of different toxins are emphasized to illustrate the fundamental mechanisms of toxicity at different biochemical, molecular and cellular- levels with particular consideration for the nervous system. The target of primary action has been highlighted and operationally classified into 13 sub-categories. Selected examples of toxins were assigned to each target category, denominated as portal, and the modulation of the different portal’s signaling was featured. The first portal encompasses the plasma membrane lipid domains, which give rise to pores when challenged for example with pardaxin, a fish toxin, or is subject to degradation when enzymes of lipid metabolism such as phospholipases A₂ (PLA₂) or phospholipase C (PLC) act upon it. Several major portals consist of ion channels, pumps, transporters and ligand gated ionotropic receptors which many toxins act on, disturbing the intracellular ion homeostasis. Another group of portals consists of G-protein-coupled and tyrosine kinase receptors that, upon interaction with discrete toxins, alter second messengers towards pathological levels. Lastly, subcellular organelles such as mitochondria, nucleus, protein- and RNA-synthesis machineries, cytoskeletal networks and exocytic vesicles are also portals targeted and deregulated by other diverse group of toxins. A fundamental concept can be drawn from these seemingly different toxins with respect to the site of action and the secondary messengers and signaling cascades they trigger in the host. While the interaction with the initial portal is largely determined by the chemical nature of the toxin, once inside the cell, several ubiquitous second messengers and protein kinases/ phosphatases pathways are impaired, to attain toxicity. Therefore, toxins represent one of the most promising natural molecules for developing novel therapeutics that selectively target the major cellular portals involved in human physiology and diseases.

Pertussis leukocytosis: mechanisms, clinical relevance and treatment

The significant and sometimes dramatic rise in the number of circulating white blood cells (leukocytosis) in infants suffering from pertussis (whooping cough) has been recognized for over a century. Although pertussis is a disease that afflicts people of all ages, it can be particularly severe in young infants, and these are the individuals in whom leukocytosis is most pronounced. Very high levels of leukocytosis are associated with poor outcome in infants hospitalized with pertussis and modern treatments are often aimed at reducing the number of leukocytes. Pertussis leukocytosis is caused by pertussis toxin, a soluble protein toxin released by Bordetella pertussis during infection, but the exact mechanisms by which this occurs are still unclear. In this minireview, I discuss the history of clinical and experimental findings on pertussis leukocytosis, possible contributing mechanisms causing this condition and treatments aimed at reducing leukocytosis in hospitalized infants. Since recent studies have detailed significant associations between specific levels of pertussis leukocytosis and fatal outcome, this is a timely review that may stimulate new thinking on how to understand and combat this problem.

Pertussis toxin targets airway macrophages to promote Bordetella pertussis infection of the respiratory tract

Pertussis toxin (PT), a secreted virulence factor of Bordetella pertussis, ADP ribosylates mammalian G(i) proteins and plays an important early role in respiratory tract infection by this pathogen in a mouse intranasal infection model. To test the hypothesis that PT targets resident airway macrophages (AM) to promote this infection, we depleted AM by intranasal administration of liposome-encapsulated clodronate prior to bacterial inoculation. This treatment enhanced respiratory tract infection by B. pertussis, even though it also induced a rapid influx of neutrophils to the airways. Strikingly, AM depletion also enhanced infection by mutant strains deficient in PT production or activity to the same level as the wild-type infection, indicating that AM may be the primary target cells for PT in promoting infection. The enhancing effect of clodronate-liposome treatment on infection (i) was shown to be due to macrophage depletion rather than neutrophil influx; (ii) was observed for both tracheal infection and lung infection; (iii) was observed during the early and peak phases of the infection but was lost by day 14 postinoculation, during clearance of the infection; (iv) persisted for at least 1 week (prior to bacterial inoculation); and (v) was equivalent in magnitude to the effect of PT pretreatment and the effects were not additive, consistent with the idea that PT targets AM. We found that PT efficiently ADP ribosylated AM G proteins both in vitro and after intranasal administration of PT in mice and that the duration of G protein modification in vivo was equivalent to the duration of the enhancing effect of PT treatment on the bacterial infection. Collectively, these observations indicate that PT targets AM to promote early infection of the respiratory tract by B. pertussis.

Molecular mechanisms for the effect of mastoparan on G proteins in tissues of vertebrates and invertebrates

The peptide toxin mastoparan increased GTP-binding activity of heterotrimeric G proteins in tissues of vertebrate and invertebrate animals, the effect of mastoparan in mussel tissues being less pronounced. The stimulatory effect of mastoparan on GTP binding was not observed after treatment of membranes with pertussis toxin that selectively modulates function of Gi proteins. Activity of mastoparan decreased in the presence of C-terminal peptide 346-355 from the Gi protein alphai2-subunit. Mastoparan dose-dependently decreased the stimulatory effect of hormones on GTP binding in tissues of rats and mussels. The influence of these hormones on the cell is realized via Gi proteins. However, mastoparan did not modulate the effect of Gs protein-activating hormones.

Third Prize:Prostaglandin E2-3 Receptor Is Involved in Ureteral Contractility in Obstruction

BACKGROUND AND PURPOSE

We previously found that prostaglandin (PG) E2 contracts acutely obstructed ureters while relaxing normal ureters. This study investigated the procontractile effects of the PG EP3 receptor in PGE(2)-mediated contractility in obstructed and normal porcine ureters.

MATERIALS AND METHODS

We created unilateral ureteral obstruction laparoscopically using titanium clips in farm pigs; the contralateral ureters were dissected as sham controls. Ureters were harvested 48 hours post-obstruction, cut into 5-mm segments, and suspended in water-jacketed tissue baths in Krebs buffer. Tissues were equilibrated for 1 hour, and spontaneous contractile rates were recorded. After 2 hours of incubation in Krebs (controls) or pertussis toxin (G(alpha)i signaling-protein inhibitor [EP-3 blockade]) 500 ng/mL, a concentration- response curve (10(-9) M-10(-5) M) to PGE(2), PGF(2), sulprostone (EP 3 agonist), or 0.01% ethanol (vehicle) was created (N = 4).

RESULTS

In the normal ureters, PGE(2) relaxed both pertussis toxin-treated and control tissues. In obstructed segments, PGE(2) increased contractions by 60%; this was reversed by pertussis toxin to a 67% reduction in contractile rate. In both obstructed and contralateral segments, sulprostone induced contractility in the controls; this was attenuated by pertussis toxin. The PGF(2) produced a contractile effect in both the controls and the pertussis toxin-treated segments, demonstrating the selectivity of pertussis toxin for EP3 receptors.

CONCLUSION

Our data indicate that the EP3 receptor is involved in hypercontractility during ureteral obstruction. However, it may not be the sole factor behind the condition-dependent effect of PGE(2).

Stromal cell-derived factor-1alpha directly modulates voltage-dependent currents of the action potential in mammalian neuronal cells

Stromal cell-derived factor-1alpha (SDF-1alpha) is a chemokine whose receptor, CXCR4, is distributed in specific brain areas including hypothalamus. SDF-1alpha has recently been found to play important roles in neurons, although direct modulation of voltage-gated ionic channels has never been shown. In order to clarify this issue, we performed patch-clamp experiments in fetal mouse hypothalamic neurons in culture. SDF-1alpha (10 nm) decreased the peak and rising slope of the action potentials and spike discharge frequency in 22% of hypothalamic neurons tested. This effect was blocked by the CXCR4 antagonist AMD 3100 (1 microm) but not by the metabotropic glutamate receptor antagonist MCPG (500 microm), indicating a direct action of SDF-1alpha on its cognate receptor. This effect involved a depression of both inward and outward voltage-dependent currents of the action potential. We confirmed these effects in the human neuroblastoma cell line SH-SY5Y, which endogenously expresses CXCR4. Voltage-clamp experiments revealed that SDF-1alpha induced a 20% decrease in the peak of the tetrodotoxin-sensitive sodium current and tetraethylammonium-sensitive delayed rectifier potassium current, respectively. Both effects were concentration dependent, and blocked by AMD 3100 (200 nm). This dual effect was reduced or blocked by 0.4 mm GTPgammaS G-protein pre-activation or by pre-treatment with the G-protein inhibitor pertussis toxin (200 ng/mL), suggesting that it is mediated via activation of a G(i/o) protein. This study extends the functions of SDF-1alpha to a direct modulation of voltage-dependent membrane currents of neuronal cells.

Pertussis toxin and adenylate cyclase toxin provide a one-two punch for establishment of Bordetella pertussis infection of the respiratory tract

Previously we found that pertussis toxin (PT), an exotoxin virulence factor produced by Bordetella pertussis, plays an important early role in colonization of the respiratory tract by this pathogen, using a mouse intranasal infection model. In this study, we examined the early role played by another exotoxin produced by this pathogen, adenylate cyclase toxin (ACT). By comparing a wild-type strain to a mutant strain (DeltaCYA) with an in-frame deletion of the cyaA gene encoding ACT, we found that the lack of ACT confers a significant peak (day 7) colonization defect (1 to 2 log(10)). In mixed-infection experiments, the DeltaCYA strain was significantly outcompeted by the wild-type strain, and intranasal administration of purified ACT did not increase colonization by DeltaCYA. These data suggest that ACT benefits the bacterial cells that produce it and, unlike PT, does not act as a soluble factor benefiting the entire infecting bacterial population. Comparison of lower respiratory tract infections over the first 4 days after inoculation revealed that the colonization defect of the PT deletion strain was apparent earlier than that of DeltaCYA, suggesting that PT plays an earlier role than ACT in the establishment of B. pertussis infection. Examination of cells in the bronchoalveolar lavage fluid of infected mice revealed that, unlike PT, ACT does not appear to inhibit neutrophil influx to the respiratory tract early after infection but may combat neutrophil activity once influx has occurred.

Suppression of serum antibody responses by pertussis toxin after respiratory tract colonization by Bordetella pertussis and identification of an immunodominant lipoprotein

Pertussis toxin (PT), a virulence factor secreted by Bordetella pertussis, contributes to respiratory tract infection and disease caused by this pathogen. By comparing a wild-type (WT) B. pertussis strain to a mutant strain with an in-frame deletion of the ptx genes encoding PT (DeltaPT), we recently found that the lack of PT confers a significant defect in respiratory tract colonization in mice after intranasal inoculation. In this study, we analyzed serum antibody responses in mice infected with the WT or DeltaPT strain and found that infection with the DeltaPT strain elicited greater responses to several B. pertussis antigens than did infection with the WT, despite the lower colonization level achieved by the DeltaPT strain. The same enhanced antibody response was observed after infection with a strain expressing an enzymatically inactive PT; but this response was not observed after infection with B. pertussis mutant strains lacking filamentous hemagglutinin or adenylate cyclase toxin, nor when purified PT was administered with the DeltaPT inoculum, indicating a specific role for PT activity in this immunosuppressive effect. In particular, there were consistent strong serum antibody responses to one or more low-molecular-weight antigens after infection with the DeltaPT strain. These antigens were Bvg independent, membrane localized, and also expressed by the closely related pathogens Bordetella parapertussis and Bordetella bronchiseptica. Two-dimensional gel electrophoresis and mass spectrometry were used to identify one of the immunodominant low-molecular-weight antigens as a protein with significant sequence homology to peptidoglycan-associated lipoprotein in several other gram-negative bacterial species. However, a serum antibody response to this protein alone did not protect mice against respiratory tract infection by B. pertussis.

Involvement of signal transducing GTP-binding proteins in renal artery alpha 1-adrenoceptor mediated smooth muscle contraction

OBJECTIVE

To determine the participation of GTP-binding proteins (G-proteins) in the cellular mechanism of the phenylephrine-induced renal artery vasospasm by using swine renal artery smooth muscle rings in a standard organ baths, as increased noradrenaline release from perivasal and intramural sympathetic nerve endings during renal ischaemia results in increased vascular smooth muscle tone that is important in the loss of kidney function during renal transplantation and nephron-sparing surgery.

MATERIALS AND METHODS

Fresh swine kidneys were transported in cold calcium-free Tyrode solution to the laboratory. Adipose tissue around the arteries was removed, the organ de-capsulated and interlobar arteries dissected. The contractile properties of renal artery smooth muscle rings were assessed in a standard organ bath, the rings pre-tensioned at 2 g. Contractions were evoked by applying the alpha 1-adrenoceptor selective agonist phenylephrine (1 nmol/L to 0.3 mmol/L). Isometric contractions of the tissue were registered and stored digitally. Dose-response curves were obtained sequentially with a wash-out of 20 min between each concentration; the maximum contractility of an individual muscle ring was set at 100%. Dose-response curves of inhibitory agents (e.g. WB4101, cholera and pertussis toxins) were determined by comparing the remaining contractility after incubating with the respective drug with a control contraction that was evoked three times (10 mumol/L phenylephrine) and the mean set at 100%.

RESULTS

Phenylephrine induced dose-dependent and fully reversible isometric contractions with a threshold concentration of 100 nmol/L and an EC50 of 0.8 mumol/L. The receptor was identified as the alpha 1A-subtype by the selective antagonist WB4101. Pre-treatment of tissue rings with 5 micrograms/mL pertussis toxin (120 min, 37 degrees C) inhibited the control contraction by a mean (SEM) of 52.0 (4.6)%, whereas pre-treatment with 1 microgram/mL cholera toxin (60 min, 37 degrees C), leading to a permanent activation of the Gs-protein via blockade of the GTPase activity, decreased the response by 39.0 (8.2%).

CONCLUSION

These results suggest a coupling of alpha 1A-adrenoceptors in renal vascular tissue to the heterotrimeric Gs-protein and to heterotrimeric G-proteins of the G1- and/or G0-family in the phenylephrine-induced contraction.

Pertussis toxin plays an early role in respiratory tract colonization by Bordetella pertussis

In this study, we sought to determine whether pertussis toxin (PT), an exotoxin virulence factor produced exclusively by Bordetella pertussis, is important for colonization of the respiratory tract by this pathogen by using a mouse intranasal infection model. By comparing a wild-type Tohama I strain to a mutant strain with an in-frame deletion of the ptx genes encoding PT (deltaPT), we found that the lack of PT confers a significant peak (day 7) colonization defect (1 to 2 log(10) units) over a range of bacterial inoculum doses and that this defect was apparent within 1 to 2 days postinoculation. In mixed-strain infection experiments, the deltaPT strain showed no competitive disadvantage versus the wild-type strain and colonized at higher levels than in the single-strain infection experiments. To test the hypothesis that soluble PT produced by the wild-type strain in mixed infections enhanced respiratory tract colonization by deltaPT, we coadministered purified PT with the deltaPT inoculum and found that colonization was increased to wild-type levels. This effect was not observed when PT was coadministered via a systemic route. Intranasal administration of purified PT up to 14 days prior to inoculation with deltaPT significantly increased bacterial colonization, but PT administration 1 day after bacterial inoculation did not enhance colonization versus a phosphate-buffered saline control. Analysis of bronchoalveolar lavage fluid samples from mice infected with either wild-type or deltaPT strains at early times after infection revealed that neutrophil influx to the lungs 48 h postinfection was significantly greater in response to deltaPT infection, implicating neutrophil chemotaxis as a possible target of PT activity promoting B. pertussis colonization of the respiratory tract.

Role of L-type Ca(2+) channels in pertussis toxin induced antagonism of U50,488H analgesia and hypothermia

Previous studies have shown that the kappa-opioid effects are sensitive to pertussis toxin (PTX) and affected by Ca(2+) fluxes. However, the possible involvement of Ca(2+) channels in PTX-induced inhibition of kappa-opioid effects has not been reported. The effect of intracerebroventricular (i.c.v.) treatment of pertussis toxin (1 microg/rat, PTX) or saline on the kappa-opioid agonist, U-50,488H (U5H) induced tail-flick analgesia and hypothermia in rats was determined. The effect of nimodipine (NIM), a dihydropyridine (DHP)-sensitive Ca(2+) channel blocker (CCB), on PTX-induced modulation of U5H effects was examined. The DHP ligand, [3H]PN200-110 binding was also determined in both PTX and saline treated rats to study the possible involvement of L-type Ca(2+) channels in PTX modulation of kappa-opioid agonist effects. The analgesia and change in colonic temperature were determined using tail-flick analgesiometer and telethermometer, respectively. U5H (40 mg/kg, i.p.) produced significant analgesic and hypothermic responses. PTX treatment significantly (P<0.01) antagonized the analgesic and hypothermic effects of U5H. Acute pretreatment of NIM (1 mg/kg, i.p.) 15 min prior significantly (P<0.01) reversed the PTX-induced antagonism of U5H effects. In the binding study, PTX treatment (72 h before) resulted in a significant (P<0.005) upregulation (+45% vs. saline control) of DHP binding (B(max)) with no change in affinity (K(d)). The results showed significant upregulation of DHP binding in accordance with PTX-induced antagonism of U5H effects and this blockade was reversed by NIM. Thus, present results suggest that U5H-induced analgesia and hypothermia may be mediated through PTX-sensitive transducer G-proteins (G(i/o)) coupled to L-type Ca(2+) channels.

Mu-opioid receptor down-regulation and tolerance are not equally dependent upon G-protein signaling

In the present study, the contribution of pertussis toxin (PTX)-sensitive G(i/o)-proteins to opioid tolerance and mu-opioid receptor down-regulation in the mouse were examined. Mice were injected once intracerebroventricularly and intrathecally with PTX (0.1 microg/site). Controls were treated with saline. On the 10th day following PTX treatment, continuous subcutaneous infusion of etorphine (150 or 200 microg/kg/day) or morphine (40 mg/kg/day+25 mg slow-release pellet) was begun. Control mice were implanted with inert placebo pellets. Pumps and pellets were removed 3 days later, and mice were tested for morphine analgesia or mu-opioid receptor density was determined in the whole brain, spinal cord, and midbrain. Both infusion doses of etorphine produced significant tolerance (ED50 shift=approximately 4-6-fold) and down-regulation of mu-opioid receptors (approximately 20-35%). Morphine treatment also produced significant tolerance (ED50 shift= approximately 5-8-fold), but no mu-opioid receptor down-regulation. PTX dramatically reduced the acute potency of morphine and blocked the further development of tolerance by both etorphine and morphine treatments. However, PTX had no effect on etorphine-induced mu-opioid receptor down-regulation in brain, cord, or midbrain. These results suggest that PTX-sensitive G-proteins have a minimal role in agonist-induced mu-opioid receptor density regulation in vivo, but are critical in mediating acute and chronic functional effects of opioids such as analgesia and tolerance.

Pharmacological characterisation of the goldfish somatostatin sst5 receptor

Somatostatin (somatotropin release inhibiting factor, SRIF), exerts its effects via specific G protein coupled receptors of which five subtypes have been cloned (sst1-5). Recently, SRIF receptors have also been cloned from fish tissues. In this study, goldfish sst5 receptors (gfsst5) were expressed and characterised in the Chinese hamster lung fibroblast cell line, that harbours the luciferase reporter gene driven by the serum responsive element (CCL39-SRE-Luci). The agonist radioligands [125I]-LTT-SRIF-28 ([Leu8, DTrp22, 125I-Tyr25]SRIF-28) and [125I][Tyr10]cortistatin-14 labelled similar receptor densities with high affinity and in a saturable manner (pKd: 9.99-9.71; Bmax: 300-350 fmol mg-1). 5'-Guanylyl-imidodiphosphate inhibited radioligand binding to some degree (38.5-57.9%). In competition binding studies, the pharmacological profile of SRIF binding sites defined with [125I]LTT-SRIF-28 and [125I][Tyr10]cortistatin-14 correlated significantly (r2=0.97, n=20). Pharmacological profiles of human and mouse sst5 receptors expressed in CCL39 cells correlated markedly less with those of the gfsst5 profile (r2=0.52-0.78, n > or = b16). Functional expression of the gfsst5 receptor was examined by measurement of agonist-induced luciferase expression and stimulation of [35S]GTPgammaS ([35S]guanosine 5'-O-(3-thiotriphosphate) binding. Profiles were similar to those achieved in radioligand binding studies (r2=0.81-0.93, n=20), although relative potency (pEC50) was reduced compared to pKd values. Relative efficacy profiles of luciferase expression and [35S]GTPgammaS binding, were rather divergent (r2=0.48, n=20) with peptides showing full agonism at one pathway and absence of agonism at the other. BIM 23056 (D-Phe-Phe-Tyr-D-Trp-Lys-Val-Phe-D-Nal-NH2) acted as an antagonist on the effects of SRIF-14 (pKB=6.74 +/- 0.23) on stimulation of [35S]GTPgammaS binding. Pertussis toxin abolished the effect of SRIF-14 on luciferase expression and [35S]GTPgammaS binding suggesting coupling of the receptor to G(i)/G(o) proteins. In summary, the present studies demonstrate that the gfsst5 receptor has a similar pharmacological profile and transductional properties to mammalian sst5 receptors. The difference in efficacy profiles defined using different functional assays suggests numerous, agonist specific, conformational receptor states, and/or ligand-dependent receptor trafficking.

A dual role of protein kinase C in insulin signal transduction via adenylyl cyclase signaling system in muscle tissues of vertebrates and invertebrates

Further decoding of a novel adenylyl cyclase signaling mechanism (ACSM) of the action of insulin and related peptides detected earlier (Pertseva et al. Comp Biochem Physiol B Biochem Mol Biol 1995;112:689-95 and Pertseva et al. Biochem Pharmacol 1996;52:1867-74) was carried out with special attention given to the role of protein kinase C (PKC) in the ACSM. It was shown for the first time that transduction of the insulin signal via the ACSM followed by adenylyl cyclase (AC, EC 4.6.1.1) activation was blocked in the muscle tissues of rat and mollusc Anodonta cygnea in the presence of pertussis toxin, inducing the impairment of G(i)-protein function, wortmannin, an inhibitor of phosphatidylinositol 3-kinase (PI3-K), and calphostin C, a blocker of PKC. The cholera toxin treatment of muscle membranes led to an increase in basal AC activity and a decrease in enzyme insulin reactivity. Phorbol ester and diacylglycerol activation of PKC (acute treatment) induced the inhibition of the insulin AC activating effect. This negative influence was also observed in the case of the AC system activated by biogenic amines. It was first concluded that the ACSM of insulin action involves the following signaling chain: receptor tyrosine kinase => G(i) (betagamma) => PI3-K => PKCzeta (?) => G(s) => AC => adenosine 3',5'-cyclic monophosphate. It was also concluded that the PKC system has a dual role in the ACSM: (1) a regulatory role (PKC sensitive to phorbol esters) that is manifested as a negative feedback modulation of insulin signal transduction via the ACSM; (2) a transductory role, which consists in direct participation of atypical PKC (PKCzeta) in the process of insulin signal transduction via the ACSM.

Modulation of 5-HT(1A) receptor activation by its interaction with wild-type and mutant g(alphai3) proteins

Constitutive and agonist-dependent activation of the recombinant human 5-HT(1A) receptor (RC: 2.1.5HT.01A) was investigated by co-expression with a rat G(alphai3) protein in Cos-7 cells. The interaction between the 5-HT(1A) receptor and rat G(alphai3) protein was modulated by substitution of the G(alphai3) protein site for pertussis toxin-catalysed ADP-ribosylation (cysteine(351)) by each of the natural amino acids. Enhanced basal [(35)S]GTPgammaS binding responses (+24 to +189%) were observed with the mutant G(alphai3) proteins containing at position 351 either a histidine, glutamine, serine, tyrosine or a nonpolar amino acid with the exception of a proline. With each of these mutant G(alphai3) proteins, spiperone (10 microM), but not WAY 100635 (10 microM), reduced (-22 to -60%, p<0.05) the enhanced basal [(35)S]GTPgammaS binding response. 5-HT (10 microM)-mediated [(35)S]GTPgammaS binding responses attained for some of the mutant G(alphai3)Cys(351) proteins (Phe, Met, Val and Ala) more than 300% of that obtained with the wt G(alphai3) protein. Similar results were also obtained with the prototypical 5-HT(1A) agonist 8-OH-DPAT and the partial agonist (-)-pindolol. Fusion proteins assembled from the 5-HT(1A) receptor and either the wt G(alphai3)Cys(351), mutant G(alphai3)Cys(351)Gly or G(alphai3)Cys(351)Ile protein displayed similar observations for these ligands as obtained by co-expression of the 5-HT(1A) receptor with each of these G(alphai3) proteins. Both the degree of 5-HT(1A) receptor activation by 8-OH-DPAT and (-)-pindolol, and its inhibition by spiperone, strongly correlate (r(2): 0.78-0.81) with the octanol/water partition coefficients of the mutated amino acid at position 351 of the G(alphai3) protein. The present data also suggest the wt G(alphai3) protein does not result in maximal activation of the 5-HT(1A) receptor by the agonists being investigated.

Analytical pharmacology of G protein-coupled receptors by stoichiometric expression of the receptor and G(alpha) protein subunits

The description of a new family of recombinant proteins, which are constructed by the covalent fusion of the cDNA encoding a G protein-coupled receptor with that of a G(alpha) protein subunit, has recently been introduced as an original strategy to explore receptor pharmacology under defined experimental conditions. As such, a controlled 1:1 stoichiometry of receptor and G(alpha) protein expression can be achieved, as well as a forced spatial proximity to each other. Fusion proteins have been revealed as active at the receptor ligand binding level and functional at the G(alpha) protein and effector level. Insights on analytical pharmacological data are discussed for wild-type and mutant receptors interacting with a given G(alpha) protein subunit and different subtypes of either wild-type or mutant G(alpha) proteins activated by a single receptor subtype. A possible alteration of the receptor:G(alpha) protein selectivity may occur due either to the spatial proximity of both protein partners or to a constraint receptor state unable to accommodate to different G(alpha) protein states. Coactivation of endogenous G(alpha) proteins in host cells expressing a fusion protein has also been observed, but depends mainly on the coupling efficiency of the receptor and G(alpha) protein engaged in the fusion process. The ligand's apparent intrinsic activity has been shown to be either enhanced, attenuated, or unmodified when the functional responses of a fusion protein are compared to the coexpression of both fusion protein partners.

Dopamine enhances somatostatin receptor-mediated inhibition of adenylate cyclase in rat striatum and hippocampus

Although there is evidence that suggests that dopamine (DA) has stimulatory effects on somatostatinergic transmission, it is unknown to date if DA increases the activity of the somatostatin (SS) receptor-effector system in the rat brain. In this study, we evaluated the effects of the administration of DA and the DA D1-like (D1, D5) receptor antagonist SCH 23390 and the D2-like (D2, D3, D4) receptor antagonist spiperone on the SS receptor-adenylate cyclase (AC) system in the Sprague-Dawley rat striatum and hippocampus. An intracerebroventricular injection of DA (0.5 microgram/rat) increased the number of SS receptors and decreased their apparent affinity in the striatum and hippocampus 15 hr after its administration. The simultaneous administration of the DA receptor antagonists SCH 23390 (0.25 mg/kg, ip) and spiperone (0.1 mg/kg, ip) before DA injection partially prevented the DA-induced increase in SS binding. The administration of SCH 23390 plus spiperone alone produced a significant decrease in the number of SS receptors in both brain areas studied at 15 hr after injection, an effect that disappeared at 24 hr. The increased number of SS receptors in the DA-treated rats was associated with an increased capacity of SS to inhibit basal and forskolin (FK)-stimulated (AC) activity in the striatum and hippocampus at 15 hr after injection. This effect had disappeared at 24 hr. By contrast, basal and FK-stimulated enzyme activities were unaltered after DA injection. No significant changes in the levels of the alpha i (alpha i1 + alpha i2) subunits were found in DA-treated rats as compared with control rats. In addition, the immunodetection of the alpha i1 or alpha i2 subunits showed no significant changes in their levels in DA-treated rats when compared with controls. DA injection also induced an increase in SS-like immunoreactive content in the rat striatum but not hippocampus at 15 hr after administration and returned to control values at 24 hr. These results provide direct evidence of a functional linkage between the dopaminergic and somatostatinergic systems at the molecular level.

Time-dependent effects of in vivo pertussis toxin on morphine analgesia and G-proteins in mice

Previous studies have indicated a long-duration of effect of in vivo pertussis toxin (PTX) on morphine analgesia in the mouse. However, the time-course of potency changes in morphine analgesia as determined in dose-response studies and biochemical correlates of PTX treatment have not been reported to date. Therefore, in the present studies the effects of in vivo PTX on morphine analgesia ED50 and PTX-catalyzed incorporation of [32P]-ADP-ribose and synapsin content in mouse spinal cord were examined. Mice were injected IT & ICV with saline or PTX (total dose = 0.2 microg) and tested for systemic morphine analgesia (tail-flick) 1, 10, 16 & 40 days later. There was no significant decrease in morphine potency 1 day following PTX treatment, whereas PTX produced a significant decrease in morphine potency at 10, 16 & 40 days. Concurrent decreases in the incorporation of [32P]-ADP-ribose in spinal cord by PTX were observed on days 10, 16 & 40. No changes were observed in synapsin content which suggests that the effect was not nonspecific. This study indicates that in vivo PTX produces co-ordinate long-lasting effects in both functional (analgesia) and biochemical (Gi/o-proteins) assays.

Relation of the [3H] gamma-hydroxybutyric acid (GHB) binding site to the gamma-aminobutyric acidB (GABAB) receptor in rat brain

gamma-Hydroxybutyric acid (GHB) is a naturally occurring compound that has the ability to induce generalized absence seizures when given to animals. GHB has been hypothesized to induce this effect via the postsynaptic gamma-aminobutyric acidB (GABAB) receptor. We sought to test this hypothesis by examining the affinity of GABAB agonists and antagonists for the [3H]GHB binding site, the affinity of GHB and a GHB antagonist for the [3H]GABAB binding site, and the effect of guanine nucleotides and pertussis toxin on both, using autoradiographic binding assays. GHB and its antagonist, NCS 382, did not compete for [3H]GABAB binding, nor did (-)-baclofen or the [3H]GABAB antagonists, CGP 35348 or SCH 50911, compete for [3H]GHB binding; however, the GABAB agonist 3-amino-propylphosphinic acid (3-APPA), and the GABAB antagonists phaclofen and 2-hydroxysaclofen (2-OH saclofen) did show a weak affinity for [3H]GHB binding in frontal cortex. GTP and the nonhydrolyzable GTP analogues, GTP gamma S and Gpp(NH)p, depressed [3H]GABAB binding throughout the brain, but increased [3H]GHB binding in frontal cortex and thalamus, those regions involved in GHB-induced absence seizures. Pertussis toxin significantly depressed [3H]GABAB binding throughout the brain, but attenuated [3H]GHB binding only in frontal cortex, and to a lesser degree than [3H]GABAB binding. The guanine nucleotide-induced changes in [3H]GHB and [3H]GABAB binding were due to a change in KD for both. Moreover, GTP gamma S reversed the ability of 3-APPA, phaclofen, and 2-OH saclofen to compete for [3H]GHB binding. These data do not support the hypothesis that GHB acts through the postsynaptic GABAB receptor to produce absence seizures. Rather, they raise the possibility either that the [3H]GHB binding site may be an isoform of the presynaptic GABAB receptor or that an independent GHB site is operative in the GHB model of absence seizures.

gamma-Hydroxybutyrate receptor binding in rat brain is inhibited by guanyl nucleotides and pertussis toxin

gamma-Hydroxybutyrate is an endogenous substance of mammalian brain which is primarily derived from GABA. This compound exhibits neuromodulatory influences on dopamine and serotonin synthesis and release in rat brain. These effects are mediated by specific brain receptors which are mainly distributed in the hippocampus, cortex and striatum. In order to characterize this type of receptor, we have studied the possibility that guanosine triphosphate (GTP) and/or pertussis toxin mediated modification of the affinity for gamma-hydroxybutyrate binding to the receptor. Results presented in this paper favor the presence of guanine nucleotide binding proteins (G0 or Gi family), which are coupled to the gamma-hydroxybutyrate receptor, modifying the high-affinity gamma-hydroxybutyrate binding. We conclude that the gamma-hydroxybutyrate receptor in brain belongs to the G-protein family of receptors.

Regulation of the adenylyl cyclase signaling system in various types of cultured endothelial cells

We studied the effects of modulators of the adenylyl cyclase pathway on the accumulation of cAMP in endothelial cells isolated from bovine aortas, pig pulmonary arteries, human umbilical veins, and human subcutaneous adipose microvessels. In addition to quantitative differences in the basal levels, cAMP stimulation in different endothelial cell types varied in sensitivity and magnitude in response to both the direct adenylyl cyclase activator forskolin and the beta-adrenergic receptor agonist isoproterenol. Furthermore, the ubiquitous phosphodiesterase inhibitor IBMX differentially enhanced both the basal and the stimulated cAMP levels in the various cell types. Histamine caused an elevation of cAMP only in bovine aortic endothelial cells and in human umbilical vein endothelial cells. Treatment of the cells with cholera and pertussis toxins, which uniquely affect G-protein subunits, resulted in divergent elevation of cAMP in the various cells. Thus, in each cell type, a distinct profile of regulation of the cAMP levels was found. Our results suggest that the adenylyl cyclase signaling system in various types of endothelial cells can be differentially regulated at the levels of receptors, G-proteins, adenylyl cyclase, and phosphodiesterase.

Pertussis and cholera toxins modulate kappa-opioid receptor agonists-induced hypothermia and gut inhibition

In mice pretreated intracerebroventricularly (i.c.v.) with either saline (10 microliters/mouse), pertussis (1 microgram/mouse) or cholera (2.5 micrograms/mouse) toxins, effect of kappa-opioid receptor agonists on the colonic temperature and charcoal meal transit time were assessed. The kappa-opioid receptor agonist, trans-(+)-3,4-dichloro-N-methyl-[2-(1- pyrrolidinyl)cyclohexyl]-benzeneacetamide methane sulfonate hydrate (U-50488H, 50, 100 and 200 micrograms/mouse, i.c.v.) produced dose dependent hypothermia. Pertussis toxin pretreatment (72 and/or 144 h before) antagonized (P < 0.05) the hypothermic effect of U-50488H (100 micrograms/mouse) and (+)-trans-N-methyl-N-[2-(1- pyrrolidinyl)cyclohexyl[benz[b]-thio-phene-4-acetamide (PD 117302, 30 micrograms/mouse). In contrast, cholera toxin pretreatment (48 and/or 96 h before) did not antagonize the hypothermic effect of the kappa-opioid receptor agonists. Moreover, both i.c.v. and intrathecal (i.t.) administration of kappa-opioid receptor agonists, U-50488H, }[5R-(5 alpha,7 alpha,8 beta)]-(+/-)-N-methyl-N-[7-(1- pyrrolidinyl)-1-oxaspiro[4,5]dec-8-yl]-benzeneacetamide¿ (U-69593) and PD 117302, produced dose dependent inhibition of the charcoal meal transit. Cholera toxin pretreatment (48 and 96 h before) augmented (P < 0.05) the antitransit effect of i.c.v. administered U-50488H (100 micrograms/mouse), U-69593 (100 micrograms/mouse) and PD 117302 (50 micrograms/mouse). However, pertussis toxin pretreatment did not affect the gastrointestinal inhibitory effect of the kappa-opioid receptor agonists. The present results extend our previous results on the effect of kappa-selective agonists on gastrointestinal motility and indicate, like the prototype opiate agonist morphine, kappa-opioid receptor agonists are effective in inhibiting the gastrointestinal motility when administered either by intrathecal or intracerebroventricular routes.(ABSTRACT TRUNCATED AT 250 WORDS)

GABAB receptors

GABAB receptors are a distinct subclass of receptors for the major inhibitory transmitter 4-aminobutanoic acid (GABA) that mediate depression of synaptic transmission and contribute to the inhibition controlling neuronal excitability. The development of specific agonists and antagonists for these receptors has led to a better understanding of their physiology and pharmacology, highlighting their diverse coupling to different intracellular effectors through Gi/G(o) proteins. This review emphasises our current knowledge of the neurophysiology and neurochemistry of GABAB receptors, including their heterogeneity, as well as the therapeutic potential of drugs acting at these sites.

Spinal and supraspinal effects of pertussis toxin on opioid analgesia

The effects of in vivo pertussis toxin (PTX) treatment on the functional effects of opioid agonists were examined in the mouse. Mice were injected intracerebroventricularly (ICV), or intrathecally (IT), or IT and ICV with PTX, and dose-response studies of the antinociceptive action of systemic (SC) morphine, fentanyl, and etorphine were conducted 10 days later. IT PTX decreased the potency (approximately 4.5-fold) of morphine more than ICV administration (approximately 1.5-fold), whereas the combination of IT and ICV administration produced an additive effect. When PTX was administered spinally and supraspinally, the potency of morphine, fentanyl, and etorphine was reduced similarly (approximately 5-7-fold), indicating that the effect of PTX does does not vary considerably among agonists of different intrinsic efficacies. These studies indicate that in vivo PTX can reduce the potency of opioid agonists with different intrinsic efficacies, and that spinal mechanisms appear to be more sensitive to PTX treatment.

Mechanisms of action of nitrates

Glyceryl trinitrate, isosorbide dinitrate, and isosorbide-5-mononitrate are organic nitrate esters commonly used in the treatment of angina pectoris, myocardial infarction, and congestive heart failure. Organic nitrate esters have a direct relaxant effect on vascular smooth muscles, and the dilation of coronary vessels improves oxygen supply to the myocardium. The dilation of peripheral veins, and in higher doses peripheral arteries, reduces preload and afterload, and thereby lowers myocardial oxygen consumption. Inhibition of platelet aggregation is another effect that is probably of therapeutic value. Effects on the central nervous system and the myocardium have been shown but not scrutinized for therapeutic importance. Both the relaxing effect on vascular smooth muscle and the effect on platelets are considered to be due to a stimulation of soluble guanylate cyclase by nitric oxide derived from the organic nitrate ester molecule through metabolization catalyzed by enzymes such as glutathione S-transferase, cytochrome P-450, and possibly esterases. The cyclic GMP produced by the guanylate cyclase acts via cGMP-dependent protein kinase. Ultimately, through various processes, the protein kinase lowers intracellular calcium; an increased uptake to and a decreased release from intracellular stores seem to be particularly important.

Somatostatin enhances nerve growth factor-induced neurite outgrowth in PC12 cells

The neuropeptide somatostatin has been found to be abundant in numerous developing regions within the central nervous system. In order to understand the role of somatostatin in development, effects of exposure to the neuropeptide were studied in PC12 cells, a well characterized model of neuronal differentiation. Somatostatin increased neurite outgrowth after 2 days in culture and enhanced neurite outgrowth after nerve growth factor (NGF) exposure. This effect was inhibited by somatostatin antibody and pertussis toxin. Somatostatin had no effect on NGF binding or internalization but did cause a decrease in cAMP levels during the time of maximal stimulation of neurite outgrowth. In a protein kinase A-deficient cell line (A126-1B2), somatostatin had no effect on neurite outgrowth. These results indicate that somatostatin may function as a differentiation factor in developing systems through inhibition of cAMP synthesis.

A comparative study of the role of adenylate cyclase in the release of adrenocorticotropin from the ovine and rat anterior pituitary

The interaction between corticotropin-releasing factor (CRF) and arginine vasopressin (AVP) is important in the regulation of adrenocorticotropin (ACTH) release from the anterior pituitary (AP). CRF exerts its effect on the AP by activating the adenylate cyclase (AC) complex whereas AVP increases the turnover of phosphatidylinositol. In the rat and in man, CRF is the most potent ACTH secretagogue whereas AVP alone is only a weak agonist. Since recent studies in the sheep indicate a reversal of this order of potency, these studies were undertaken to test the hypothesis that a functional alteration of the AC in the ovine corticotrope might limit the ability of CRF to release ACTH from these cells. When rat AP cells were incubated with CRF, a dose-dependent increase in AC activity was observed. This effect was potentiated either by AVP or PMA, although neither agent alone altered AC activity. In contrast, CRF alone, or in combination with AVP or PMA, did not increase AC activity in ovine AP cells. Both cholera toxin (CT) and pertussis toxin (PT) caused a dose-dependent release of ACTH from rat and ovine AP cells, but the amount of ACTH released from the ovine AP cells by both agents was relatively reduced. In the ovine cells, however, AVP acted synergistically with CT or PT to markedly increase the release of ACTH to levels which approached those obtained when the rat AP cells were exposed to CT or PT alone. Forskolin increased AC activity in AP cells of both species, but to a much lower extent in ovine cells than in the rat cells. However, when the ovine cells were exposed to AVP, the AC response to forskolin became similar to the response observed in the rat cells when incubated with forskolin alone. Forskolin also released significantly less ACTH from the ovine AP cells, but AVP also acted synergistically with forskolin to greatly enhance the amount of ACTH released from these cells. Finally, 8-bromo-cyclic AMP produced a similar release of ACTH from both ovine and rat AP cells. We conclude that: (1) the decreased ability of CRF to increase ACTH release from the ovine AP reflects a net decrease in AC activity and cannot be ascribed to an ovine corticotropic resistance to cAMP; (2) the decreased activity of the ovine corticotropic AC complex may in turn reflect functional alterations at the level of both the G proteins and the catalytic subunit; (3) since AVP causes protein kinase C substrate phosphorylation in the ovine AP, AVP may increase AC activity in this tissue by phosphorylating the G proteins and/or the catalytic subunit.

C-terminal modifications of pertussis toxin-sensitive G-protein alpha-subunits differentially affect immunoreactivity. Evidence against endogenous ADP-ribosylation in human heart, lung, thrombocytes and adipose tissue

Immunochemical detection of pertussis toxin-sensitive guanine-nucleotide binding proteins has been suggested to represent the most direct approach to quantitate the protein than pertussis toxin-catalysed [32P]ADP-ribosylation. The latter technique is potentially hampered by pre-existing covalent modification of the C-terminus. However, limited data exist as to whether and in what way modifications of the C-terminus affect immunoreactivity of Gi alpha (alpha-subunit of the inhibitory G-protein of adenylyl cyclase). Membranes from human myocardium, thrombocytes, adipose tissue and lung were treated with pertussis toxin or N-ethylmaleimide. Both, conditions prevented high affinity agonist binding to m-cholinoceptors and inhibited [32P]ADP-ribosylation by pertussis toxin consistent with the notion that the modifications took place at the C-terminus. Pertussis toxin treatment increased immunoreactivity to different antisera raised against the C-terminal decapeptide of transducin alpha (KENLKDCGLF, DS 1-4, AS). N-Ethylmaleimide reduced immunoreactivity towards all antisera studied. Pertussis toxin reduced the mobility of Gi alpha on sodium dodecyl sulfate-polyacrylamide gel electrophoresis (SDS-PAGE) depending on the presence of the toxin and sensitivity to inhibition of ADP-ribosylation by nicotinamide. In native membranes from none of the tissues studied, immunoreactive material comigrating with pertussis toxin-modified form of Gi alpha was detected. It is concluded that modification of the C-terminus by pertussis toxin or N-ethylmaleimide resulting in the same functional consequence, i.e. prevention of high affinity agonist receptor binding, is capable of producing opposite changes of immunoreactivity. Pertussis toxin treatment reduces the electrophoretic mobility on SDS-PAGE. Separation of the native and pertussis toxin-modified form of Gi alpha on SDS-PAGE demonstrates that endogenously ADP-ribosylated Gi alpha is lacking in membranes from human myocardium, thrombocytes, lung and adipose tissue.

Age-dependent changes in transmembrane signalling: identification of G proteins in human lymphocytes and polymorphonuclear leukocytes

In human neutrophils (PMNLs) we found that in the elderly IP3 formation was significantly decreased compared to that of young subjects. For FMLP receptor binding affinity and number no measurable differences occurred upon ageing, studying both the low or the high affinity receptors. The amount of ADP-ribosylated G proteins, catalysed by pertussis toxin (PT) or cholera toxin (CT), was significantly increased in PMNLs of the elderly. In lymphocytes, the PT-catalysed ADP ribosylation of G proteins was also increased with ageing, while the CT-catalysed ribosylation was decreased. The autoradiogram of [32P]ADP-ribosylated proteins by CT in lymphocytes of young individuals showed a major polypeptide of 40,000 M(r). In contrast, in lymphocytes of the elderly, the major polypeptide was 45,000 M(r). In PMNLs, CT labelled quite strongly the 45,000 M(r) band, mainly in the elderly. When PT was used, no age-related pattern changes could be demonstrated, while differences could be observed between the two types of cells. The use of antiserum P680 (G alpha common) showed no age-related pattern changes, while the intensity of the labelled proteins varies with age and cell type. The antiserum U46 (Go alpha) could identify in lymphocytes of young subjects two polypeptides 68,000 and 41,000 M(r). The prominent polypeptide in lymphocytes of the elderly was the 70,000 M(r) and no other polypeptides could be recognized. In PMNLs of young subjects the U46 and serum identified a range of species. In PMNLs of the elderly all these bands were weakly labelled. The present data indicate changes in the pattern and the quantity of G proteins in lymphocytes and PMNLs of elderly subjects.

Involvement of nitric oxide and peptides in the inhibitory non-adrenergic, non-cholinergic (NANC) response in bovine mesenteric artery

The cyclic GMP mediated non-adrenergic non-cholinergic (NANC) relaxation in field stimulated bovine mesenteric artery and its modulation by various factors was studied. Electrical field stimulation of precontracted (Phe 2.5 microM or histamine 5 microM) bovine mesenteric arteries resulted in relaxations varying between 10-70% in different preparations. Tetrodotoxin (3 microM) completely blocked the inhibitory NANC response. Preincubation with high concentrations (100 microM-1 mM) of NG-nitro-L-arginine for 15 min. significantly reduced the relaxation induced by electrical field stimulation. Blockade of cyclooxygenases and prostaglandin synthesis by indomethacin had no effect on the relaxatory response to electrical field stimulation. Neither the alpha 2-adrenoceptor selective antagonist yohimbine (1 microM) nor the alpha 2-adrenoceptor selective agonist UK 14,304 (1 microM) had any significant effect on the electrical field stimulation-induced relaxation. Pertussis toxin (100 ng/ml) was without effect on relaxations elicited by electrical field stimulation. GTP in the concentration range 10 microM-1 mM slightly potentiated the relaxant response. N-carboxymethyl-Phe-Leu (an inhibitor of enkephalinase) or aprotinin (an inhibitor of several proteases) had no significant effect on the electrical field stimulation response. Addition of trypsin (100 U/ml) in combination with chymotrypsin (20 U/ml) significantly reduced the electrical field stimulation-induced relaxation. In the present study we have found indications for the involvement of nitric oxide and possibly also peptides in mediating the inhibitory NANC response (relaxation) in bovine mesenteric arteries.

Intramembrane receptor-receptor interactions: integration of signal transduction pathways in the nervous system

During recent years a large number of observations have been made indicating that neuropeptides and other transmitters in various brain areas can regulate the affinity of monoamine receptors via the activation of their own receptors. These "receptor--receptor interactions" can either take place at the plasma membrane level or use intracytoplasmatic loops. This review is mainly focused on the evidence for hetero-regulation of dopamine (DA) D2 receptors in the basal ganglia. The existence of such receptor--receptor interactions increases the plasticity of transmission and opens up the possibility of developing new drugs which indirectly modulate receptor recognition and decoding processes. This would avoid the use of direct receptor agonists or antagonists which induce major side effects such as tolerance and abstinence. Disturbances in the receptor--receptor interactions, including DA D2 receptors, may be involved in the development of neurological and mental diseases such as schizophrenia.

Effects of pertussis toxin on behavioural responses of guinea-pigs to centrally administered substance P, quinpirole, carbachol, U-50,488H, morphine and morphine withdrawal

The effects of pretreatment with pertussis toxin (PTX) on the sedative effect of morphine administered i.c.v. (200 nmol), and on the locomotor and behavioural activation precipitated by naloxone (15 mg/kg s.c.) following treatment with a single dose of morphine (i.c.v., 200 nmol), were investigated in guinea-pigs. Responses to i.c.v. administration of substance P (50 nmol), quinpirole (200 nmol), U50,488H (100 nmol) and carbachol (2 nmol) following PTX pretreatment were also investigated. Following PTX pretreatment, morphine induced mild agitation and the onset of sedation was delayed. Pretreatment with PTX also attenuated the locomotor and some components of behavioural activation induced by substance P, U50,488H, quinpirole and naloxone-precipitated morphine withdrawal, but failed to attenuate the effects induced by carbachol. These results suggest the involvement of PTX-sensitive G-protein-mediated mechanisms in the sedative effect of morphine in guinea-pigs and in the central stimulating actions of acute morphine withdrawal, U50,488H, substance P, and quinpirole.

Evidence for G protein modulation of experimental-generalized absence seizures in rat

Pertussis toxin is known to inhibit G proteins via ADP-ribosylation, and a response to pertussis toxin is presumptive evidence of G protein modulation of the activity being studied. Therefore, in order to test the hypothesis that G protein-mediated mechanisms are involved in the pathogenesis of generalized absence seizures, the effect of pertussis toxin in two pharmacological models of generalized absence seizures in rat was investigated. The experimental absence seizure models used were the gamma-hydroxybutyrate (GHB) model and low dose pentylenetetrazole (PTZ) model in rat. Pretreatment with pertussis toxin administered intracerebroventricularly (i.c.v.) resulted in a significant decrease in duration of seizure in both models. These data suggest that G-protein-mediated mechanisms may be involved in the pathogenesis of the bilaterally synchronous spike wave discharges (SWD) that characterize experimental absence seizures.

Escherichia coli hemolysin is a potent inductor of phosphoinositide hydrolysis and related metabolic responses in human neutrophils

Escherichia coli hemolysin (Hly) is a proteinaceous pore-forming exotoxin that probably represents a significant virulence factor in E. coli infections. We investigated its influence on human polymorphonuclear neutrophils (PMN), previously identified as highly susceptible targets. Hly provoked rapid secretion of elastase and myeloperoxidase, generation of superoxide, and synthesis of platelet-activating factor (PAF) and lyso-PAF. Concomitantly, marked phosphatidylinositol (PtdIns) hydrolysis with sequential appearance of the inositol-phosphates, inositol-phosphates, inositol triphosphate, diphosphate, and monophosphate, respectively, and formation of diacylglycerol, occurred. The metabolic responses displayed distinct bell-shaped dose dependencies, with maximum events noted at low toxin concentrations of 0.1-0.5 hemolytic units per milliliter. PtdIns hydrolysis and metabolic responses elicited by Hly exceeded those evoked by optimal concentrations of formylmethionyl-leucyl phenylalanine, PAF, leukotriene B4, A23187, or staphylococcal alpha-toxin. The toxin-induced effects were sensitive toward modulators of PMN stimulus transmission pathways (pertussis toxin, the kinase C inhibitor H7, and phorbol myristate acetate "priming"). We conclude that the marked capacity of low doses of Hly to elicit degranulation, respiratory burst, and lipid mediator generation in human PMN probably envolves signal transduction via PtdIns hydrolysis.

Synthesis and characterization of a Pertussis toxin-biotin conjugate

We prepared a Pertussis toxin-biotin conjugate and found its biological properties to be similar to those of native Pertussis toxin with respect to the hemagglutination, Chinese hamster ovary cell, and lymphocyte proliferation assays. Direct binding to Chinese hamster ovary and Jurkat cells was observed using fluorescence microscopy. Pertussis toxin-biotin was also found to possess similar glycoconjugate binding specificities as those of 125I-labeled Pertussis toxin.

Failure of [32P]ADP-ribosylation by pertussis toxin to determine Gi alpha content in membranes from various human tissues. Improved radioimmunological quantification using the 125I-labelled C-terminal decapeptide of retinal transducin

The quantitative determination of pertussis-toxin-sensitive guanine-nucleotide-binding proteins (G-proteins) in cell membranes is still a problem. Pertussis-toxin-catalysed [32P]ADP-ribosylation strongly relies on the substrate quality of the alpha-subunits and is influenced by the concentration of nucleotides, beta gamma-subunits, the physicochemical properties of the membranes influencing the availability of Gi alpha for pertussis toxin, and covalent modification of Gi alpha. Quantification of immunoreactive material on Western blots can be only imprecisely performed by two-dimensional densitometry. In order to generate a method for quantification of pertussis-toxin-sensitive G-proteins in membranes we have developed a fast and sensitive radioimmunoassay. The C-terminal decapeptide of retinal transducin alpha (KENLKDCGLF) was 125I-labelled and used as tracer. Polyclonal antiserum (DS 4) was raised against this peptide. Gi alpha proteins were determined by competition of solubilized membranes for 125I-KENLKDCGLF binding to DS 4 using dilutions of retinal transducin alpha as standard. The interassay variation was less than 10%, with a sensitivity of 2.5 micrograms/ml. The density of Gi alpha was highest in human adipose tissue, followed by HL60 cells, lung, mononuclear leucocytes, thrombocytes and left ventricular myocardium. A striking difference was observed between the density of Gi alpha and the amount of incorporation of [32P]ADP-ribose into the 40 kDa membrane proteins by pertussis toxin in the same samples. This is also demonstrated by comparison of the weak [32P]ATP-ribosylation of pertussis toxin substrates with the density of immunoreactive Gi alpha on Western blots in tissues such as lung. This study shows that the Gi alpha content can be exactly determined by a sensitive and fast radioimmunoassay using iodinated synthetic peptide homologues of Gi alpha proteins. Radioimmunological quantification of Gi alpha might be able to detect the 'true' Gi alpha content of membranes without being hampered by influences on the [32P]ADP-ribosylation reaction. It is concluded that this newly developed method may become an important tool for studying expression of Gi alpha proteins in a variety of tissues or cell types, and for precisely quantifying the changes caused by pathological conditions.

Pertussis toxin reduces the day-night rhythm of nociception and mu and kappa opioid peptide-mediated antinociception in the snail, Cepaea nemoralis

There is accumulating evidence that pertussis toxin-sensitive G proteins are associated with the transduction of opioid-mediated antinociception in mammals. The present study examined the effects of hemocel injections of pertussis toxin (0.10 microgram) on the day-night rhythm of nociception and mu and kappa opioid-mediated antinociception in a mollusc, the land snail, Cepaea nemoralis. Five days after treatment, pertussis toxin significantly reduced the naloxone-sensitive, opioid-mediated nocturnal peak in the day-night rhythm of nociception [as measured by the latency of response to a thermal (40 degrees C) stimulus] in Cepaea, without affecting the daytime response latency. Pertussis toxin also significantly decreased the antinociceptive effects of the mu agonist, DAMGO, and blocked those of the kappa opioid agonist, U-69,593. These results suggest that G protein substrates of pertussis toxin are associated with the transduction of opioid-mediated nociception and antinociception in the snail, Cepaea.