GABAB Receptor-Mediated Inhibition of Histamine H1-Receptor-Induced Inositol Phosphate Formation in Slices of Rat Cerebral Cortex

Histamine-stimulated accumulation of [3H]inositol monophosphate ([3H]IP1) in lithium-treated slices of rat cerebral cortex was inhibited by gamma-aminobutyric acid (GABA) (IC50 0.30 +/- 0.03 mM). The maximum level of inhibition was 69 +/- 2%. GABA alone caused a small stimulation of basal accumulation of [3H]IP1. The inhibitory action of GABA on the response to histamine was mimicked by the GABAB agonist (-)-baclofen, IC50 0.69 +/- 0.04 microM, which was 430-fold more potent as an inhibitor than the (+)-isomer. (-)-Baclofen also inhibited histamine-induced formation of [3H]inositol bisphosphate ([3H]IP2) and [3H] inositol trisphosphate ([3H]IP3). Inhibition curves for GABA and for (-)-and and (+)-baclofen had Hill coefficients greater than unity. (-)-Baclofen, at concentrations that caused inhibition of histamine-induced [3H]IP1 accumulation, did not alter the basal level of [3H]IP1 or the incorporation of [3H]inositol into total inositol phospholipids. Isoguvacine, a GABAA agonist, had no effect on either the histamine-stimulated or basal accumulation of [3H]IP1. GABA had no effect on carbachol-stimulated [3H]IP1 formation.     

Electroconvulsive Shock Increases α1b-but Not α1a-Adrenoceptor Binding Sites in Rat Cerebral Cortex

Repeated administration of electroconvulsive shock (ECS) increases [3H]prazosin binding to alpha 1-adrenoceptors in rat cerebral cortex. In contrast, [3H]WB4101 binding in cortex has been reported to be unchanged after ECS. [3H]Prazosin labels two alpha 1-adrenoceptor subtypes, termed alpha 1a and alpha 1b, whereas [3H]WB4101 labels the alpha 1a subtype preferentially. The purpose of this study was to determine whether ECS increases one or both alpha 1-adrenoceptor subtypes in rat cerebral cortex. We found that treatment of rats with ECS once daily for 10-12 days increased [3H]prazosin binding in cortex by about 25% but did not significantly alter [3H]WB4101 binding to alpha 1-adrenoceptors. Measurement of alpha 1a and alpha 1b receptors by competition analysis of the selective alpha 1a antagonist 5-methylurapidil against [3H]prazosin and measurement of [3H]prazosin binding in homogenates preincubated with chlorethylclonidine, which alkylates alpha 1b binding sites, also indicated that the ECS-induced increase in alpha 1-adrenoceptors is confined to the alpha 1b subtype. In contrast to its effect on [3H]prazosin binding, ECS did not increase phosphoinositide hydrolysis as measured by [3H]inositol 1-phosphate accumulation in slices of rat cerebral cortex stimulated by either norepinephrine or phenylephrine. The failure of ECS to increase [3H]inositol 1-phosphate accumulation stimulated by phenylephrine, which is a partial agonist for this response, suggests that spare receptors do not account for the apparent absence of effect of ECS on alpha 1-adrenoceptor-mediated phosphoinositide hydrolysis.     

Muscarinic Agonists Evoke Neurotransmitter Release: Possible Roles for Phosphatidyl Inositol Bisphosphate Breakdown Products in Neuromodulation

Carbachol (CCh), a muscarinic agonist that elicits the formation of inositol trisphosphate (IP3) and diacylglycerol (DG), induces a calcium-dependent [3H]norepinephrine ([3H]NE) release [IC50 = (2.7 +/- 0.5) X 10(-4) M] in rat brain slices. Similarly, other muscarinic agonists evoke [3H]NE release which is specifically inhibited by muscarinic antagonists such as 3-quinuclidinyl benzilate, atropine, and N-methyl-4-piperidyl benzilate. The atropine-sensitive evoked release is effectively inhibited by neomycin (IC50 = 50 microM), a phospholipase C inhibitor that interferes with IP3-dependent cellular processes. In addition, polymyxin B, a rather selective inhibitor of protein kinase C (PK-C), abolishes the agonist-mediated release with a half-maximal effective concentration of 0.53 microM (750 ng/ml). These results have a significant implication for the mechanism by which agonists generating IP3 and DG act as inducers of neurotransmitter release in the CNS. However, since both neomycin and polymyxin B act also as N-calcium-channel blockers, other possible mechanisms are discussed. The CCh-induced release suggests that in the CNS an agonist-receptor interaction leads to a calcium-dependent neurotransmitter release, most likely via promoting the IP3/DG as second messengers followed by activation of PK-C.     

Characterization of the subtype of muscarinic receptor coupled to the stimulation of phosphoinositide hydrolysis in 132-1N1 human astrocytoma cells.

Stimulation of muscarinic receptors increases phosphoinositide (PI) hydrolysis in 132-1N1 human astrocytoma cells. To evaluate the subtype of receptors which mediate PI hydrolysis in 132-1N1 cells, the effects of: a) the nonselective M1 agonist, carbachol; b) the selective M1 agonist, 4-hydroxy-2-butynyl-trimethylammonium chloride-m-chlorocarbinilate (McN-343); c) the nonselective antagonists, atropine and scopolamine; d) the relatively selective M1 antagonist, pirenzepine; e) the relatively selective M2 antagonists, AF-DX 116 (11-2-diethylaminomethyl-1-piperidinylacetyl-5, 11-dihydro-6H-pyrido-2,3-b-1,4-benzodiazepine-6-one) and methoctramine and f) the relatively selective M3 antagonist, hexahydrosila-difenidol (HHSiD) on PI hydrolysis in 132-1N1 cells were studied. The cell pools of inositol-phospholipids were prelabelled by incubating 132-1N1 cells in a low inositol containing medium (CMRL-1066) supplemented with [3H]inositol (2 microCi/ml) for 20-24 hours at 37 degrees C. The cells were washed and resuspended in a physiological salt solution, and PI hydrolysis was measured by accumulation of [3H]inositol-1-phosphate (IP) in the presence of 10 mM LiCl. Carbachol produced time and concentration dependent PI hydrolysis (EC50, 37 microM). McN-A343 did not cause significant hydrolysis of PI in 132-1N1 cells indicating that the receptor was not of M1 type. All the above muscarinic antagonists caused a concentration dependent decrease in the level of IP in response to carbachol (100 microM). The rank order of their affinities (pA2 values) was: atropine (8.8) > HHSiD (7.6) > pirenzepine (6.8) > methoctramine (6.0) > AF-DX 116 (5.8). This rank order supports the concept that M3 (other names, M2 beta, glandular M2) receptors are linked to PI hydrolysis in 132-1N1 cells. HHSiD, which is selective for M3 receptors of the smooth muscle has higher affinity for muscarinic receptors in 132-1N1 cells than AF-DX 116 which is selective for M2 receptors in cardiac tissue. If the receptor in 132-1N1 cells had been M2, part of the rank order for affinities would have been methoctramine > AF-DX 116 > HHSiD > pirenzepine. From all of these observations, the muscarinic receptor for PI hydrolysis in 132-1N1 cells is tentatively characterized as of M3 type.     

Potentiation of sympathetic neurotransmission in bovine isolated irides by isoprostanes.

Isoprostanes (IsoP) are formed by free radical catalyzed peroxidation of arachidonic acid independent of the cyclooxygenase enzyme. In the present study, we examined the effect of IsoP on norepinephrine (NE) release from the bovine isolated iris. Furthermore, we studied the role of IsoP's in hydrogen peroxide (H2O2)-induced enhancement of NE release from this tissue. Isolated bovine irides were prepared for studies of [3H]NE release using the superfusion method. Release of [3H]NE was induced via electrical field stimulation. Both 8-iso-prostaglandin E2 (E2-IsoP) and 8-iso-prostaglandin F2 alpha (F2-IsoP) produced a concentration-related enhancement of field-stimulated [3H]NE release from isolated bovine irides, an effect that was mimicked by the thromboxane (Tx) receptor agonist, U46619 and by H2O2. The Tx-receptor antagonist, SQ 29548 inhibited responses to E2-IsoP (10 microM) with an IC50 of 370 +/- 50 nM. SQ 29548 (10 microM) also blocked the enhancement of electrically-evoked [3H]NE release induced by U46619 (10 microM) but not that caused by H2O2 (300 microM). The Tx synthetase inhibitor, carboxyheptylimidazole (10 microM) prevented the stimulatory effect of E2-IsoP on evoked [3H]NE release without affecting responses induced by H2O2. We conclude that IsoP's can enhance sympathetic neurotransmission in the bovine isolated iris, an effect that can be blocked by a Tx-receptor antagonist. Furthermore, endogenously produced Tx's mediate the stimulatory effect of IsoP's on NE release. However, endogenously generated IsoP's or Tx's are not involved in H2O2-induced potentiation of sympathetic neurotransmission.     

Uncoupling of Rat Cerebral Cortical α2-Adrenoceptors from GTP-Binding Proteins by N-Ethylmaleimide

Pretreatment of membranes from rat cerebral cortex with N-ethylmaleimide (NEM) decreased [3H]-clonidine binding in a concentration-dependent manner. The Bmax values of high-affinity sites for [3H]clonidine were reduced by 50 microM NEM treatment. Treatment with 500 microM NEM diminished the sum of Bmax of both high- and low-affinity components. GTP, Na+, and Mn2+ exerted little effect on [3H]clonidine binding in NEM-treated membranes. The addition of purified GTP-binding proteins caused an increase in the binding to the membranes pretreated with 50 microM NEM, but did not increase [3H]-clonidine binding in membranes treated with 500 microM NEM. In contrast, NEM pretreatment inhibited islet activating protein (IAP)-catalyzed ADP ribosylation of membrane-bound (41,000-dalton) and purified (39,000/41,000-dalton) GTP-binding proteins. From these results, it is suggested that two or three categories of essential sulfhydryl groups are involved in the coupling between agonist, alpha 2-adrenoceptor, and GTP-binding protein. One is a highly sensitive site to NEM (a concentration range of 1-50 microM), which is probably a cysteine residue, IAP-catalyzed ADP-ribosylating site on the alpha-subunit of GTP-binding protein. Other sites have low sensitivity to NEM (a concentration range of 0.1-1 mM), and are the binding domain of agonist and/or the coupling domain of GTP-binding protein on the alpha 2-adrenoceptor. In addition, Ki-ras p21 protein may lack the capacity to couple with the alpha 2-adrenoceptor.     

Pharmacologic characterization of cirazoline-activated inositol phospholipid hydrolysis in rat brain cortical slices

Interaction of cirazoline, an imidazoline derivative, with alpha 1-adrenoceptor coupled inositol phospholipid hydrolysis was characterized in rat brain cortical slices. Norepinephrine, a full alpha 1-agonist, and phenylephrine, a partial alpha 1-agonist, on inositol phospholipid hydrolysis were included for comparison. Norepinephrine produced a fourfold stimulation of inositol phospholipid hydrolysis, whereas cirazoline and phenylephrine caused only submaximal responses (40-60%) when compared with norepinephrine. The stimulation of inositol phospholipid hydrolysis by cirazoline was completely blocked by the alpha 1-adrenoceptor antagonist prazosin, but not by selective alpha 2- or beta-adrenoceptor antagonists. Furthermore, the norepinephrine dose-response curve was shifted to the right in the presence of cirazoline, without affecting the maximal response. These results suggest that cirazoline behaves as a partial agonist at brain alpha 1-adrenoceptors linked to inositol phospholipid hydrolysis.     

Antagonism by 8-hydroxy-2(di-n-propylamino)tetraline and other serotonin agonists of muscarinic M1-type receptors coupled to inositol phospholipid breakdown in human IMR-32 and SK-N-MC neuroblastoma cells.

IMR-32 and SK-N-MC cells were found to contain [3H]quinuclidinyl benzilate specific binding sites inhibited by pirenzepine in a manner suggesting the presence of both M1-type and M2-type muscarinic receptor recognition sites. Neither cell had detectable [3H]8-OH-DPAT binding sites. Carbachol stimulated the rate of inositol phospholipid breakdown in IMR-32 and SK-N-MC human neuroblastoma cells with an EC50 value of about 50 microM in both cases. Pirenzepine inhibited the carbachol (100 microM)-stimulated inositol phospholipid breakdown in both cells with Hill slopes of unity and IC50 values of 15 nM (IMR-32) and 12 nM (SK-N-MC). The 5-HT1A receptor agonist 8-OH-DPAT competitively inhibited carbachol-stimulated inositol phospholipid breakdown with pA2 values of 5.78 (IMR-32) and 5.61 (SK-N-MC). These values are consistent with the inhibitory potency of 8-OH-DPAT towards [3H]quinuclidinyl benzilate binding in these cells. The 5-HT agonists 5-MeODMT and buspirone at micromolar concentrations inhibited carbachol-stimulated breakdown in IMR-32 cells. The inhibition by 8-OH-DPAT and 5-MeODMT was not affected by preincubation with (-)alprenolol. 5-HT (10-100 microM) was without effect on either basal or carbachol-stimulated breakdown. It is concluded that IMR-32 and SK-N-MC neuroblastoma cells express muscarinic M1-type but not serotoninergic receptors coupled to phosphoinositide-specific phospholipase C. 8-OH-DPAT acts as a weak antagonist at these muscarinic receptors.     

3H]Benzophenone photolabeling identifies state-dependent changes in nicotinic acetylcholine receptor structure

Interactions of benzophenone (BP) with the Torpedo nicotinic acetylcholine receptor (nAChR) were characterized by electrophysiological analyses, radioligand binding assays, and photolabeling of nAChR-rich membranes with [3H]BP to identify the amino acids contributing to its binding sites. BP acted as a low potency noncompetitive antagonist, reversibly inhibiting the ACh responses of nAChRs expressed in Xenopus oocytes (IC50 = 600 microM) and the binding of the noncompetitive antagonist [3H]tetracaine to nAChR-rich membranes (IC50 = 150 microM). UV irradiation at 365 nm resulted in covalent incorporation of [3H]BP into the nAChR subunits (delta > alpha approximately beta > gamma), with photoincorporation limited to the nAChR transmembrane domain. Comparison of nAChR photolabeling in the closed state (absence of agonist) and desensitized state (equilibrated with agonist) revealed selective desensitized state labeling in the delta subunit of deltaPhe-232 in deltaM1 and deltaPro-286/deltaIle-288 near the beginning of deltaM3 that are within a pocket at the interface between the transmembrane and extracellular domains. There was labeling in the closed state within the ion channel at position M2-13 (alphaVal-255, betaVal-261, and deltaVal-269) that was reduced by 90% upon desensitization and labeling in the transmembrane M3 helices of the beta and gamma subunits (betaMet-285, betaMet-288, and gammaMet-291) that was reduced by 50-80% in the desensitized state. Labeling at the lipid interface (alphaMet-415 in alphaM4) was unaffected by agonist. These results provide a further definition of the regions in the nAChR transmembrane domain that differ in structure between the closed and desensitized states.     

Glycerol-3-phospho-D-myo-inositol 4-phosphate (Gro-PIP) is an inhibitor of phosphoinositide-specific phospholipase C

The deacylated forms of the phosphoinositides were used to determine whether the guinea pig uterus phosphoinositide-specific phospholipase C (PI-PLC I, Mr 60,000) required fatty acids at the sn-1 and sn-2 positions for the hydrolysis of the sn-3 phosphodiester bond. L-alpha-Glycerophospho-D-myo-inositol 4-phosphate (Gro-PIP), but not glycerol 3-phosphate (Gro-3-P), L-alpha-glycerophospho-D-myo-inositol (Gro-PI), or L-alpha-glycerophospho-D-myo-inositol 4,5-bisphosphate (Gro-PIP2), inhibited PI-PLC I in a concentration-dependent manner. Assays performed with 10 microM [3H]phosphatidylinositol ([3H]PI), 10 microM [3H]phosphatidylinositol 4-phosphate ([3H]PIP) or 10 microM [3H]phosphatidylinositol 4,5-bisphosphate ([3H]PIP2) as substrates, with increasing [Gro-PIP] revealed an IC50 = 380 microM. Kinetic studies with increasing [3H]PI substrate concentrations in the presence of 100 microM and 300 microM Gro-PIP demonstrated that Gro-PIP exhibited competitive inhibition; Kis = 40 microM. Ca2+ concentrations over the range 1.1 microM to 1 mM did not effect inhibition, suggesting that Gro-PIP inhibition of [3H]PI hydrolysis was calcium-independent. To determine whether Gro-PIP was a substrate, 20 microM and 500 microM [3H]Gro-PIP were incubated with PI-PLC I. Anion-exchange HPLC analysis revealed no [3H]IP2 product formation, indicating that [3H]Gro-PIP was not hydrolyzed. Assays performed with [3H]PI and [3H]PIP substrates in the presence of 500 microM [3H]Gro-PIP revealed approx. 75% less [3H]inositol 1-phosphate ([3H]IP1) and [3H]inositol 1,4-bisphosphate ([3H]IP2) product formation, respectively, indicating that [3H]Gro-PIP inhibited the hydrolysis of the substrates by PI-PLC I. These data suggest that Gro-PIP does not serve as a substrate, and that it inhibits PI-PLC I by competitive inhibition in a Ca2(+)-independent fashion.     

Phorbol ester inhibits phosphoinositide hydrolysis and calcium mobilization in cultured astrocytoma cells

In cultured human 1321N1 astrocytoma cells, muscarinic receptor stimulation leads to phosphoinositide hydrolysis, formation of inositol phosphates, and mobilization of intracellular Ca2+. Treatment of these cells with 1 microM 4 beta-phorbol 12 beta-myristate 13 alpha-acetate (PMA) completely blocks the carbachol-stimulated formation of [3H]inositol mono-, bis-, and trisphosphate ( [3H]InsP, [3H]InsP2, and [3H]InsP3). The concentrations of PMA that give half-maximal and 100% inhibition of carbachol-induced [3H]InsP formation are 3 nM and 0.5 microM, respectively. Inactive phorbol esters (4 alpha-phorbol 12,13-didecanoate and 4 beta-phorbol), at 1 microM, do not inhibit carbachol-stimulated [3H]InsP formation. The KD of the muscarinic receptor for [3H]N-methyl scopolamine is unchanged by PMA treatment, while the IC50 for carbachol is modestly increased. PMA treatment also abolishes carbachol-induced 45Ca2+ efflux from 1321N1 cells. The concomitant loss of InsP3 formation and Ca2+ mobilization is strong evidence in support of a causal relationship between these two responses. In addition, our finding that PMA blocks hormone-stimulated phosphoinositide turnover suggests that there may be feedback regulation of phosphoinositide metabolism through the Ca2+- and phospholipid-dependent protein kinase.     

Mechanism of vascular relaxation by thaligrisine: functional and binding assays

In the present study we examine the mechanism by which thaligrisine, a bisbenzyltetrahydroisoquinoline alkaloid, inhibits the contractile response of vascular smooth muscle. The work includes functional studies on rat isolated aorta and tail artery precontracted with noradrenaline or KCl. In other experiments rat aorta was precontracted by caffeine in the presence or absence of extracellular Ca2+. In order to assess whether thaligrisine interacts directly with calcium channel binding sites or with alpha-adrenoceptors we examined the effect of the alkaloid on [3H]-(+)-cis diltiazem, [3H]-nitrendipine and [3H]-prazosin binding to cerebral cortical membranes. The functional studies showed that the alkaloid inhibited in a concentration-dependent manner the contractile response induced by depolarization in rat aorta (IC50 = 8.9+/-2.9 microM, n=5) and in tail artery (IC50 = 3.04+/-0.3 microM, n=6) or noradrenaline induced contraction in rat aorta (IC50 = 23.0+/-0.39 microM, n=9) and in tail artery (IC50 = 3.8+/-0.9 microM, n=7). In rat aorta, thaligrisine concentration-dependently inhibited noradrenaline-induced contraction in Ca2+-free solution (IC50 = 13.3 microM, n=18). The alkaloid also relaxed the spontaneous contractile response elicited by extracellular calcium after depletion of noradrenaline-sensitive intracellular stores (IC50 = 7.7 microM, n=4). The radioligand receptor-binding study showed that thaligrisine has higher affinity for [3H]-prazosin than for [3H]-(+)-cis-diltiazem binding sites, with Ki values of 0.048+/-0.007 microM and 1.5+/-1.1 microM respectively. [3H]-nitrendipine binding was not affected by thaligrisine. The present work provides evidence that thaligrisine shows higher affinity for [3H]-prazosin binding site than [3H]-(+)-cis-diltiazem binding sites, in contrast with tetrandrine and isotetrandrine that present similar affinity for both receptors. In functional studies thaligrisine, acted as an alpha1-adrenoceptor antagonist and as a Ca2+ channel blocker, relaxing noradrenaline or KCl-induced contractions in vascular smooth muscle. This compound specifically inhibits the refilling of internal Ca2+-stores sensitive to noradrenaline, by blocking Ca2+-entry through voltage-dependent Ca2+-channels.     

Enhanced arterial contractility to noradrenaline in diabetic rats is associated with increased phosphoinositide metabolism.

The purpose of this study was to investigate whether the increased contractile responsiveness of aortae from male rats with 12-14 week streptozotocin-induced diabetes to noradrenaline is associated with alterations in phosphoinositide metabolism. The contractile response to noradrenaline (10 microM) in both the presence and absence of extracellular calcium was significantly enhanced in aortae from diabetic rats. No significant differences were found between control and diabetic arteries in the basal incorporation of 32P and [3H]myo-inositol into phosphoinositides, or in the basal accumulation of [32P]phosphatidic acid and [3H]inositol phosphates. However, noradrenaline (10 microM) caused significantly greater breakdown of [32P]phosphatidylinositol 4,5-bisphosphate and formation of [32P]phosphatidic acid and [3H]inositol phosphates in diabetic aortae than in control preparations. The production of [3H]inositol phosphates induced by noradrenaline was selectively reduced by the alpha 1-adrenoceptor antagonist, prazosin, in both control and diabetic tissues. These results indicate that phosphoinositide metabolism in response to noradrenaline via stimulation of alpha 1-adrenoceptors is enhanced in aortae from chronic streptozotocin-diabetic rats. The increase in inositol 1,4,5-trisphosphate and 1,2-diacylglycerol production that presumably results could be responsible, at least in part, for the enhanced contractile response of aortae from diabetic rats to noradrenaline.     

Characteristics of Phorbol Ester- and Agonist-Induced Down-Regulation of Astrocyte Receptors Coupled to Inositol Phospholipid Metabolism

We have examined some of the characteristics of phorbol ester- and agonist-induced down-regulation of astrocyte receptors coupled to phosphoinositide metabolism. Our results show that preincubation of [3H]inositol-labelled astrocyte cultures with phorbol 12-myristate 13-acetate (PMA) resulted in a time- (t 1/2, 1-2 min) and concentration-dependent (IC50, 1 nM) decrease in the accumulation of [3H]inositol phosphates (IP) evoked by muscarinic receptor stimulation. Much longer (30-40 min) preincubation periods with higher concentrations (IC50, 600 microM) were required to elicit the same effect with the receptor agonist carbachol. Following preincubation, agonist-stimulated [3H]IP accumulation recovered with time; in both cases pretreatment levels of inositol lipid metabolism were attained within 2 days. Both phorbol ester and agonist pretreatments were also effective in reversing the carbachol-evoked mobilisation of 45Ca2+ in these cells. However, their effects on phosphoinositide metabolism were found not to be additive. Although neither pretreatment affected the incorporation of [3H]inositol into phosphoinositides, both resulted in a loss of membrane muscarinic receptors as assessed by [3H]N-methylscopolamine binding. In washed membranes prepared from [3H]inositol-labelled cultures, the guanine nucleotide analogue, guanosine 5'-O-thiotriphosphate (GTP-gamma-S), caused a dose-dependent increase in [3H]IP formation. This response was enhanced when carbachol was also included in the incubation medium, although the agonist alone was without effect. Pretreatment with either PMA or carbachol had no effect on GTP-gamma-S-stimulated [3H]IP accumulation but did reduce the ability of carbachol to augment this response. Similar findings were obtained when membranes were exposed directly to PMA.(ABSTRACT TRUNCATED AT 250 WORDS)     

Dopamine Stimulates [3H]Phorbol 12,13-Dibutyrate Binding in Cultured Striatal Cells

The effect of dopamine (DA) on the binding of [3H]phorbol 12,13-dibutyrate ([3H]PdBu) in cultured rat striatal cells was examined. DA maximally increased specific [3H]PdBu binding by 70 +/- 10%, an increase comparable to that observed with norepinephrine (NE). This finding suggests that DA activates protein kinase C in cultured striatal cells, because increases in [3H]PdBu binding reflect translocation of protein kinase C. Half-maximal stimulation was observed with 10(-6) M DA. The peak response was observed at 2-3 min after addition of 10(-4) M DA, but [3H]PdBu binding was still increased above basal at 30 min. DA was not acting via an adrenergic receptor. Prazosin (10(-6) M) blocked the response to NE, suggesting mediation by an alpha 1-adrenergic receptor, but had little effect on the response to DA. Conversely, the D1 receptor antagonist SCH-23390 (10(-6) M) blocked the response to DA, but only partially inhibited the response to NE. Morphine (10(-6) M) inhibited the response to DA by 46 +/- 14%, but did not affect significantly the response to NE. The DA effect on [3H]PdBu binding is apparently independent of the increase in cyclic AMP seen on D1 receptor activation. Forskolin, apomorphine, and the D1 agonist SKF-38393 all increased cyclic AMP in striatal cells, but were less effective than DA in stimulating [3H]PdBu binding. The D2 agonist quinpirole was ineffective in stimulating either cyclic AMP or [3H]PdBu binding.     

Purification of a histamine H3 receptor negatively coupled to phosphoinositide turnover in the human gastric cell line HGT1.

The histamine H3 receptor agonist (R)alpha-methylhistamine (MeHA) inhibited, in a nanomolar range, basal and carbachol-stimulated inositol phosphate formation in the human gastric tumoral cell line HGT1-clone 6. The inhibition was reversed by micromolar concentrations of the histamine H3 receptor antagonist thioperamide and was sensitive to cholera or pertussis toxin treatment. Using [3H]N alpha-MeHA as specific tracer, high affinity binding sites were demonstrated with a Bmax of 54 +/- 3 fmol/mg of protein and a KD of either 0.61 +/- 0.04 or 2.2 +/- 0.4 nM, in the absence or presence of 50 microM GTP[gamma]S, respectively. The binding sites were solubilized by Triton X-100 and prepurified by gel chromatography. They were separated from the histamine H2 receptor sites by filtration through Sepharose-famotidine and finally retained on Sepharose-thioperamide. The purified sites concentrated in one single silver-stained protein band of 70 kDa in SDS-polyacrylamide gel electrophoresis. They specifically bound [3H]N alpha-MeHA with a KD of 1.6 +/- 0.1 nM and a Bmax of 12,000 +/- 750 pmol/mg of protein. This corresponds to a 90,225-fold purification over cell lysate and a purity degree of 84%. Binding was competitively displaced by N alpha-MeHA (IC50 = 5.8 +/- 0.7 nM), (R) alpha-MeHA (IC50 = 9 +/- 1 nM), and thioperamide (IC50 = 85 +/- 10 nM), but not by famotidine (H2 antagonist) or by mepyramine (H1 antagonist). These findings provide the first evidence for solubilization, purification, and molecular mass characterization of the histamine H3 receptor protein and for the negative coupling of this receptor phosphatidylinositol turnover through a so far unidentified G protein.     

Effects of neuropeptide Y on alpha 1-and beta-adrenoceptor-stimulated second messenger systems in rat frontal cortex

Neuropeptide Y (NPY) (1 microM) significantly reduced the basal cAMP concentration in slices of rat frontal cortex. However, NPY (10(-9)-10(-6)M) did not alter the isoproterenol-stimulated (10(-9)-10(-5) M) accumulation of cAMP in the frontal cortical slices, showing that Y2 NPY receptors do not modulate the beta-adrenoceptor-stimulated adenylase cyclase activity. NPY (10(-8)-2.5 x 10(-5) M) was also demonstrated to stimulate inositol phosphate accumulation in rat frontal cortex slices in a dose-dependent manner. However, NPY (1 microM) did not potentiate the ability of phenylephrine (5 X 10(-8)-10(-4) M), an alpha 1-adrenoceptor agonist, to stimulate inositol phosphate hydrolysis. The combined effects of phenylephrine and NPY (1 microM) on inositol phosphate hydrolysis were additive, suggesting that the alpha 1-adrenoceptor and NPY Y1 receptor sites are located on different postsynaptic sites in rat frontal cortex. This study demonstrates the existence of both Y2 and Y1 NPY receptors in the rat frontal cortex based on second messenger systems, but there does not appear to be an interaction of NPY with either alpha 1- or beta-adrenoceptors.     

Characterization of alpha-adrenoceptor subtypes by [3H]prazosin and [3H]rauwolscine binding to canine venous smooth muscle membranes

Postsynaptic alpha-adrenoceptor subtypes were studied using [3H]prazosin and [3H]rauwolscine binding to plasmalemma-enriched microsomal fractions isolated from dog saphenous veins and mesenteric veins. Both radioligands showed saturable binding consistent with the presence of a single homogeneous binding site in each case, based on Scatchard analysis. The Kd values of [3H]prazosin and [3H]rauwolscine, calculated from kinetic studies were similar to those from equilibrium binding data in both venous muscle membranes. The microsomal membranes of dog saphenous vein and mesenteric vein contained about a fourfold higher density of the high affinity [3H]rauwolscine binding sites than those for [3H]prazosin binding. In competition studies, IC50 values for displacement of rauwolscine or prazosin suggested that the sites of interaction for the antagonists prazosin and rauwolscine were independent. Phenylephrine, a functionally selective alpha-adrenoceptor agonist, competed with a similar IC50 value for the specific binding sites of [3H]prazosin and [3H]rauwolscine; but B-HT 920, a functionally selective alpha 2-adrenoceptor agonist, competed for [3H]rauwolscine and [3H]prazosin binding with distinctly different IC50 values. Our data show the existence of two populations of alpha-adrenoceptor antagonist binding sites in the plasma membranes of dog saphenous vein and mesenteric vein, and raise the question whether agonist selectively depends on different affinities or on differential efficacies at one or two sites.     

Dopamine Receptor Stimulation Does Not Affect Phosphoinositide Hydrolysis in Slices of Rat Striatum

The effect of dopamine receptor stimulation on the accumulation of labelled inositol phosphates in rat striatal slices under basal and stimulated conditions was examined following preincubation with [3H]inositol. Incubation of striatal slices with the selective D-1 agonist SKF 38393 or the selective D-2 agonist LY 171555 for 5 or 30 min did not affect the basal accumulation of labelled inositol mono-, bis-, tris-, and tetrakisphosphate. Resolution by HPLC of inositol trisphosphate into inositol-1,3,4-tris-phosphate and inositol-1,4,5-trisphosphate isomers revealed that under basal conditions dopamine did not influence the accumulation of inositol-1,4,5-trisphosphate. Depolarisation evoked by KCl, or addition of the muscarinic receptor agonist carbachol, produced a marked increase in the accumulation of labelled inositol phosphates in both the presence and absence of lithium. Addition of dopamine did not reduce the ability of KCl or carbachol to increase inositol phospholipid hydrolysis. In the presence of lithium, dopamine (100 microM) enhanced KCl-stimulated inositol phospholipid hydrolysis, but this effect appears to be mediated by alpha 1 adrenoceptors because it was blocked by prazosin. SKF 38393 (10 microM) or LY 171555 (10 microM) also did not affect carbachol-stimulated inositol phospholipid hydrolysis. These data, in contrast to recent reports, suggest that striatal dopamine receptors do not appear to be linked to inositol phospholipid hydrolysis.     

Guanosine 5''-O-Thiotriphosphate and Sodium Fluoride Activate Polyphosphoinositide Hydrolysis in Rat Cortical Membranes by Distinct Mechanisms

NaF and guanosine 5'-O-thiotriphosphate [GTP(S)] stimulated the accumulation of [3H]inositol monophosphate ([3H]InsP) in rat brain cortical membranes, with half-maximal stimulation at 2 mM and 1 microM, respectively. Calcium also increased basal [3H]InsP formation over a range of concentrations from 10(-7) to 10(-4) M. The stimulatory effect of GTP(S) (30 microM) on [3H]InsP production was insensitive to Ca2+, whereas NaF-evoked [3H]InsP formation was dependent on Ca2+ concentrations. Guanosine 5'-O-thiodiphosphate significantly attenuated GTP(S)- but not NaF-stimulated [3H]InsP production. Coincubation of GTP(S) (30 microM) and submaximal concentrations of NaF (1 or 3 mM) stimulated [3H]InsP formation to a degree that was nearly additive with that produced by either drug alone. However, the resultant accumulation of [3H]InsP in the presence of maximally effective concentrations of GTP(S) and NaF was not different from that produced by NaF alone. Incubation of cortical membranes with GTP(S) and NaF for 1 min stimulated the accumulation of [3H]inositol bisphosphate (InsP2) but not [3H]InsP. [3H]InsP2 production elicited by GTP(S) was markedly enhanced by the muscarinic cholinergic agonist carbachol. In contrast, NaF-stimulated [3H]InsP2 formation was not potentiated by carbachol. Our findings of different characteristics of GTP(S) and fluoride activation of polyphosphoinositide (PPI) hydrolysis suggest that separate regulatory mechanisms are involved in these two modes of stimulation in brain membranes. Activation of PPI hydrolysis by fluoride may be mediated by a direct stimulation of PPI phosphodiesterase or by activating a putative guanine nucleotide regulatory protein at a location distinct from the GTP-binding site.