Neurotensin and acetylcholine evoke common responses in frog oocytes injected with rat brain messenger ribonucleic acid

Signaling of lysophosphatidic acid-evoked chloride current: calcium release from inositol trisphosphate-sensitive store

In Xenopus oocytes, lysophosphatidic acid (LPA) evoked inward currents at the holding potential of -60 mV, which were quickly desensitized upon repeated challenges of the compound at 10 nM or 1 microM. This desensitization was prevented by pretreatment with protein kinase A inhibitor or recovered by its post-treatment, but not by the pretreatment with an inhibitor of protein kinase C or calmodulin kinase II. From pharmacological studies, the LPA-evoked currents were found to be mediated by phospholipase C, calcium-mobilization from thapsigargin-sensitive Ca2+ stores, araguspongine E-sensitive inositol trisphosphate receptor, and calcium-dependent chloride channels.

Mechanisms of regulation of neurotensin receptors

Since its discovery in 1973, the neuropeptide neurotensin has been demonstrated to be involved in the control of a broad variety of physiological activities in both the central nervous system and in the periphery. Pharmacological studies have shown that the biological effects elicited by neurotensin result from its specific binding to cell membrane neurotensin receptors that have been characterized in various tissue and in cell preparations. In addition, it is now well documented that most of these responses are subject to rapid desensitization. Such desensitization results in transient responses to sustained peptide applications, or to tachyphylaxis during successive stimulations in the same conditions. More recently, desensitization of neurotensin signalling was investigated at the cellular and molecular levels. In cultured cells, regulation at the second messenger level, receptor internalization, and receptor down-regulation processes have been reported. These are proposed to play a critical role in the control of cell responsiveness to neurotensin. This review aims to compile recent data on the different biochemical processes involved in the regulation of the neurotensin receptor and to discuss the physiological consequences of this regulation in vivo.

The thrombin receptor second cytoplasmic loop confers coupling to Gq-like G proteins in chimeric receptors. Additional evidence for a common transmembrane signaling and G protein coupling mechanism in G protein-coupled receptors

Thrombin activates human platelets and other cells in part by cleaving an unusual G protein-coupled receptor. Thrombin cleavage of this receptor's amino-terminal exodomain unmasks a new amino terminus. This then binds intramolecularly to the body of the receptor to trigger transmembrane signaling and activation of Gi- and Gq-like G proteins. Toward identifying the domains responsible for thrombin receptor-G protein interactions, we examined the signaling properties of chimeric receptors in which thrombin receptor cytoplasmic sequences replaced the cognate sequences in the Gs-coupled beta2-adrenergic receptor (beta2AR) or the Gi-coupled dopamine D2 receptor (D2R). In Xenopus oocytes, a chimeric beta2AR bearing the thrombin receptor second cytoplasmic (C2) loop gained the ability to trigger intracellular Ca2+ release in response to adrenergic agonist, whereas a beta2AR bearing the cognate C2 loop from the D2R did not. Similarly, in COS-7 cells, a chimeric D2R bearing the thrombin receptor C2 loop gained the ability to trigger phosphoinositide hydrolysis in response to dopaminergic agonist, apparently by coupling to a Gq-like G protein. No detectable Gs coupling was seen. Thus, the thrombin receptor C2 loop was able to confer Gq-like coupling in several different receptor contexts. These observations suggest that the thrombin receptor C2 loop specifies Gq coupling by directly contacting Gq or by contributing to a structure required for Gq coupling. The ability of the thrombin receptor C2 loop to function in the context of the D2R and beta2AR strongly suggests that the transmembrane switching and G protein activation strategies used by the thrombin receptor must be very similar to those used by the D2R and beta2AR despite the thrombin receptor's strikingly different liganding mechanism.

Cardiovascular effects of neurotensin microinjections into the nucleus of the solitary tract

Neurotensin (NT) immunoreactivity and binding sites have been demonstrated to be extensively distributed throughout the caudal nucleus of the solitary tract (NTS). In this study, the cardiovascular effects of microinjecting the tridecapeptide neurotensin (NT) or its analogues NT 1-8 and [D-Trp11]NT into NTS were investigated in the chloralose-anesthetized, paralyzed and artificially ventilated rat. Microinjection of NT (10 pmol) elicited decreases in arterial pressure (AP) (-34 +/- 3 mm Hg) and heart rate (HR) (-28 +/- 2 beats/min), whereas microinjection of equimolar amounts of the NT fragment NT 1-8 elicited a significantly smaller depressor response (-14 +/- 3 mm Hg), but the bradycardic (-22 +/- 4 beats/min) response was similar in magnitude to that elicited by NT. On the other hand, microinjection of [D-Trp11]NT did not elicit cardiovascular responses from sites in NTS. In addition, the prior injection of [D-Trp11]NT into cardiovascular responsive sites in the NTS did not significantly reduce the AP or HR response to NT. The depressor response elicited by NT was not affected by bilateral vagotomy but was abolished by either C1-C2 spinal cord transection or the i.v. administration of the nicotinic receptor blocker hexamethonium bromide. The cardiac slowing was partially attenuated by either bilateral vagotomy (-19 +/- 2 beats/min), i.v. administration of atropine methyl bromide (-17 +/- 4 beats/min), i.v. administration of hexamethonium bromide (-11 +/- 4 beats/min) or by spinal cord transection (-12 +/- 3 beats/min), and completely abolished after total autonomic blockade or by combined bilateral vagotomy and spinal cord transection. These data have demonstrated that within a restricted region of the caudal NTS NT activates neurons that contribute to vasodepressor responses as a result of sympatho-inhibition and to bradycardia responses as a result of vagal excitation and sympatho-inhibition. Furthermore, these data suggest that NT may act as a neurotransmitter or modulator in central cardiovascular reflex pathways.

Desensitization of neurotensin-induced phosphoinositide hydrolysis in transfected CHO cells

The regulation of neurotensin-induced phosphoinositide turnover was studied in transfected CHO cells expressing the rat neurotensin receptor. Stimulation of these cells with neurotensin resulted in an important, but transient, increase in inositol phosphate cell content. Preincubation of the cells with neurotensin dramatically decreased their response to further stimulation. This diminution, which was time-dependent and not related to the availability of phospholipase C substrate, is though to reflect a progressive homologous desensitization of the recombinant neurotensin receptor.

The second intracellular loop of metabotropic glutamate receptor 1 cooperates with the other intracellular domains to control coupling to G-proteins

Metabotropic glutamate receptors (mGluR) share no sequence homology with any other G-protein-coupled receptors (GPCRs). The characterization of their G-protein coupling domains will therefore help define the general rules for receptor-G-protein interaction. To this end, the intracellular domains of mGluR3 and mGluR1, receptors coupled negatively to adenylyl cyclase and positively to phospholipase C, respectively, were systematically exchanged. The ability of these chimeric receptors to induce Ca2+ signals were examined in Xenopus oocytes and HER 293 cells. The chimeric receptors that still possessed the second intracellular loop (i2) of these proteins were targeted correctly to the plasma membrane. Consistent Ca2+ signals could be recorded only with chimeric mGluR3 receptors that contains i2 and at least one other intracellular domains of mGluR3 have to be replaced by their mGluR1 equivalent to produce optimal coupling to G protein. These observations indicate that i2 of mGluR1 is a critical element in determining the transduction mechanism of this receptor. These results suggest that i2 of mGluRs may play a role similar to i3 of most other GPCRs in the specificity of coupling to the G-proteins. Moreover, as in many other GPCRs, our data revealed cooperation between the different mGluR intracellular domains to control efficient coupling to G-proteins.

Light-induced currents in Xenopus oocytes expressing bovine rhodopsin

1. We have investigated the functioning of bovine rod opsin, which is efficiently synthesized from RNA made by in vitro transcription, following injection into Xenopus oocytes. We found that oocytes expressing the gene for opsin exhibit light-dependent ionic currents only after pigment generation by incubation with 11-cis-retinal. These currents are similar to the endogenous muscarinic acetylcholine (ACh) response of oocytes, but their amplitude is substantially smaller. 2. In order to optimize the conditions for obtaining light-induced currents in RNA-injected oocytes, the native ACh response was examined under several conditions. It was found that elevated external calcium markedly enhances the muscarinic response and that these currents have a non-linear dependence on membrane voltage, increasing substantially with depolarization. 3. Using the optimal conditions for evoking the largest ACh responses, (28 mM [Ca2+]o, 0 mV, omission of serum and Hepes from the media), the light-evoked currents obtained in RNA-injected oocytes were remarkably enhanced, and responses to multiple light stimuli could be obtained. 4. The light response appeared to desensitize, even after long periods of recovery and pigment regeneration. By contrast, the ACh responses continued to appear normal. These results suggest that desensitization of photoresponses expressed in Xenopus oocytes involve changes at early stages of the pathway, resulting in a reduced ability of rhodopsin to couple to the endogenous signalling system.

Alternative splicing generates metabotropic glutamate receptors inducing different patterns of calcium release in Xenopus oocytes

A splice variant of the metabotropic glutamate receptor (mGluR) 1a, named mGluR1c, was isolated. Compared to mGluR1a, the predicted mGluR1c protein is 302 amino acids shorter at its C-terminal end. Despite this difference, mGluR1c activates phospholipase C in Xenopus oocytes with a pharmacological profile identical to that of mGluR1a. However, in contrast to the large fast transient responses induced by mGluR1a, mGluR1c receptors elicit a small more slowly generated long-lasting oscillatory current, suggesting that these two receptors do not generate the same pattern of Ca2+ release in Xenopus oocytes. In situ hybridization data show that mGluR1c mRNA is expressed at a lower level than the other splice variants of mGluR1. Some differences in the regional distribution of these transcripts were observed in the cerebellum, the olfactory bulb, and the striatum.

Actions of neurotensin: a review of the electrophysiological studies

Three effects of NT were observed on midbrain DA cells. The modulatory effect of NT, that is, the attenuation of DA-induced inhibition, has been most extensively examined. Studies indicate that this effect of NT was not simply due to a nonspecific excitation. NT selectively attenuated DA-induced inhibition without affecting either GABA-induced inhibition or glutamate-induced excitation of the same cells, and the attenuation of DA-induced inhibition could be observed at the doses at which the basal activity of DA cells was not changed by NT. The attenuation of DA-induced inhibition by NT is also unlikely to result from the formation of a DA-NT complex, since neuromedin N, which competes with NT for the same receptor but does not bind to DA, mimicked the effects, and neurotensin(1-11), which forms a complex with DA but is inactive in competing for NT receptors, did not. The similarities between the effects of NT and those of 8-bromo-cAMP and forskolin suggest that intracellular cAMP and protein kinase A may be involved. This suggestion was supported by the findings that IBMX (an inhibitor of phosphodiesterases) potentiated the effect of NT; and SQ22536 (an inhibitor of adenylate cyclase) and H8 (an inhibitor of protein kinase A) antagonized it. Phorbal-12,13-dibutyrate (an activator of protein kinase C) did not mimic the effect of neurotensin, and H7 (an inhibitor of protein kinase C) did not reduce the effect, suggesting that protein kinase C is unlikely to be involved in the modulatory effect of neurotensin. Experiments in vitro indicated that the excitatory effect of NT on DA cells occurred at higher concentrations (> 10 nM) than those needed for producing the modulatory effect. Its persistence during DA receptor blockade by sulpiride suggests that this effect was not entirely mediated by an attenuation of the inhibition induced by endogenously released DA. At even higher concentrations (> 100 nM), a sudden cessation of cell activity preceded by an increase in firing rate was observed. Whether this effect of NT was due to depolarization inactivation or a toxic effect of the peptide at high concentrations remains to be determined. In most other areas studied, the excitatory effect of NT was most commonly observed. In many areas, this excitatory effect was apparently a direct postsynaptic effect of NT. However, different mechanisms may be involved (see Table 1). For example, in some areas NT acted through a decrease in membrane conductance, while in others no change or an increase in the membrane conductance was observed.(ABSTRACT TRUNCATED AT 400 WORDS)

Estimation by an electrophysiological method of the expression of oxytocin receptor mRNA in human myometrium during pregnancy

In order to evaluate the changes in uterine oxytocin receptor-specific mRNA during pregnancy, receptor expression in Xenopus oocytes are examined electrophysiologically following microinjection of mRNA from human uterus. In voltage-clamped oocytes injected with term myometrial mRNA, oxytocin elicited an inward current response. The amplitude of the oxytocin-induced current increased with increasing dose of oxytocin, but no current was elicited following stimulation with vasopressin. The oxytocin-induced current was completely eliminated as a result of pretreatment with a specific oxytocin antagonist. 21 of 27 oocytes injected with term myometrial mRNA showed a large amplitude (77.0 +/- 16.1 nA) reaction to oxytocin. In comparison, only 3 of 13 oocytes injected with early gestational myometrial mRNA exhibited a small amplitude (4.6 +/- 1.4 nA) reaction to oxytocin. No oxytocin response was observed in oocytes injected with non-pregnant myometrial mRNA. These results indicate that the striking increment in oxytocin sensitivity in term uterus depends on the increase in mRNA encoding oxytocin receptors.

Structure and expression of a human oxytocin receptor

Just before the onset of labour, uterine myometrium becomes extremely sensitive to oxytocin, for which it is a primary target tissue, because of a dramatic increase in the number of oxytocin receptors. We report here the structure and expression of the human oxytocin receptor complementary DNA isolated by expression cloning. The encoded receptor is a 388-amino-acid polypeptide with 7 transmembrane domains typical of G protein-coupled receptors. The oxytocin receptor, expressed in Xenopus oocytes, specifically responds to oxytocin and induces an inward membrane current. Messenger RNAs for the receptor are of two sizes, 3.6 kilobases in breast, and 4.4 kilobases in ovary, uterine endometrium and myometrium. The mRNA level in the myometrium is very high at term. We conclude that the increase in receptor number in the myometrium at labour is, at least in part, due to the increase in mRNA.

Reducing inositol lipid hydrolysis, Ins(1,4,5)P3 receptor availability, or Ca2+ gradients lengthens the duration of the cell cycle in Xenopus laevis blastomeres

We have microinjected a mAb specifically directed to phosphatidylinositol 4,5-bisphosphate (PIP2) into one blastomere of two-cell stage Xenopus laevis embryos. This antibody binds to endogenous PIP2 and reduces its rate of hydrolysis by phospholipase C. Antibody-injected blastomeres undergo partial or complete arrest of the cell cycle whereas the uninjected sister blastomeres divided normally. Since PIP2 hydrolysis normally produces diacylglycerol (DG) and inositol 1,4,5-triphosphate (Ins[1,4,5]P3), we attempted to measure changes in the levels of DG following stimulation of PIP2 hydrolysis in antibody-injected oocytes. The total amount of DG in antibody-injected oocytes was significantly reduced compared to that of water-injected ones following stimulation by either acetylcholine or progesterone indicating that the antibody does indeed suppress PIP2 hydrolysis. We also found that the PIP2 antibodies greatly reduced the amount of intracellular Ca2+ released in the egg cortex during egg activation. As an indirect test for Ins(1,4,5)P3 involvement in the cell cycle we injected heparin which competes with Ins(1,4,5)P3 for binding to its receptor, and thus inhibits Ins(1,4,5)P3-induced Ca2+ release. Microinjection of heparin into one blastomere of the two-cell stage embryo caused partial or complete arrest of the cell cycle depending upon the concentration of heparin injected. We further investigated the effect of reducing any [Ca2+]i gradients by microinjecting dibromo-BAPTA into the blastomere. Dibromo-BAPTA injection completely blocked mitotic cell division when a final concentration of 1.5 mM was used. These results suggest that PIP2 turnover as well as second messenger activity influence cell cycle duration during embryonic cell division in frogs.

Functional properties of strychnine-sensitive glycine receptors expressed in Xenopus oocytes injected with a single mRNA

Mature rat spinal cord cDNA libraries constructed in lambda gt10 and lambda ZAPII were screened with an oligonucleotide probe (39 mer), and 4 clones that possess DNA-inserts encoding a glycine receptor subunit were obtained. The cloned cDNAs were used to reconstruct the nucleotide sequence of the full-length open reading frame consisting of 1350 base pairs (bp) as well as the 5'-(184 bp) and 3'-(591 bp) non-coding regions. Synthetic RNA transcribed in vitro from the glycine receptor cDNA induced Xenopus oocytes to synthesize functional glycine receptor that generated large Cl- currents. The electrophysiological properties of the wild-type receptor and some mutant receptors produced by site-directed mutagenesis were analyzed.

The four dimensions of calcium signalling in Xenopus oocytes

This review focuses on the inositol phosphate/Ca2+ signalling pathway in Xenopus oocytes. The known characteristics of the individual elements of this cascade--from the membrane receptors to the intracellular Ca2+ stores--will be covered. Based on this knowledge, a simple model will then try to account for the behaviour of the newly recognized oscillations of free intracellular Ca2+ and propagated Ca2+ waves. Finally, some of the potential physiological functions of the inositol phosphate pathway will be summarized. Although there is no systematic attempt to contrast the findings in the oocyte to those in other cells, the readers of this journal will not fail to notice a high degree of similarity. Although this may seem unexciting at first, it suggests that the inositol phosphate signalling pathway may be strikingly conserved across species.

Possible heterogeneity of metabotropic glutamate receptors induced in Xenopus oocytes by rat brain mRNA

Pharmacological properties of metabotropic glutamate receptors were studied in Xenopus oocytes injected with rat brain mRNA. trans-1-Amino-cyclopentyl-1,3-dicarboxylic acid (t-ACPD), a conformationally restricted analog of glutamate, induced oscillatory inward currents in mRNA-injected oocytes. These t-ACPD responses showed several characteristics identical to those of the other metabotropic responses including the metabotropic glutamate responses stimulated by quisqualate. D,L-2-Amino-3-phophonopropionate (D,L-AP3) effectively suppressed the t-ACPD and ibotenate responses. However, quisqualate responses were not affected substantially by D,L-AP3. These findings suggest that the metabotropic glutamate receptors in the oocytes may be classified into at least two subtypes according to their pharmacological properties: one preferentially activated by quisqualate and insensitive to AP3, and the other activated by t-ACPD and ibotenate and antagonized by AP3.

Neurotensin and neuromedin N elevate the cytosolic calcium concentration via transiently appearing neurotensin binding sites in cultured rat cortex cells

Through assessment of the changes in the intracellular free-calcium concentration ([Ca2+]i), which was measured using the calcium sensitive dye, fura-2, the character of the neurotensin (NT) binding sites which appeared transiently during the early ontogenetic stage in the rat cerebral cortex was analyzed in primary cultures of cerebral cortex cells from neonatal rats. NT (1-1000 nM) elevated [Ca2+]i of the cells even when extracellular calcium was chelated with 1 mM ethylene glycol-bis(beta-aminoethylether)-N,N,N',N'-tetraacetic acid (EGTA). These findings suggest that the transiently appearing NT-binding sites in the cortex are receptors for NT and that some of the NT-induced increase in [Ca2+]i is due to mobilization from the intracellular calcium store. Further application of NT after 10 min washing caused an increase in [Ca2+]i again. This is in contrast to the findings for cortical slices from adult rats and mRNA-injected oocytes; desensitization due to NT was of long duration and further application of NT failed to activate the neurons which had responded the first time to NT. These facts suggest that the character of the NT-binding sites in the cerebral cortex differs between neonatal and adult rats. In addition, we showed that neuromedin N had a similar property to NT as to mobilization of [Ca2+]i and acted only on NT-responsive cells, suggesting the interaction between NT and neuromedin N at the postsynaptic level via the same receptor.

Structure and functional expression of the cloned rat neurotensin receptor

A functional cDNA clone for the rat neurotensin receptor was isolated by combining molecular cloning in an RNA expression vector with an electrophysiological assay in Xenopus oocytes. The neurotensin receptor consists of 424 amino acids with seven putative transmembrane domains and belongs to the family of G protein-coupled receptors. The cloned receptor expressed in mammalian cells or in Xenopus oocytes shows a selective and high-affinity binding to neurotensin peptides and undergoes potent desensitization by repeated application of neurotensin. The neurotensin receptor mRNA is expressed in both the brain and the peripheral tissues at different levels. This investigation discloses the molecular nature of the neurotensin receptor, which mediates the diverse neuronal and peripheral actions of neurotensin by effecting the G protein-associated second messenger system.

Characterization of a Na(+)-K(+)-2Cl- cotransport system in oocytes from Xenopus laevis

In order to characterize the transport systems mediating K+ uptake into oocytes, flux studies employing 86Rb were performed on Xenopus oocytes stripped of follicular cells by pretreatment with Ca2(+)-Mg2(+)-free Barth's medium. Total Rb+ uptake consisted of an ouabain-sensitive and an ouabain-insensitive flux. In the presence of 100 mmol/l NaCl and 0.1 mmol/l ouabain the ouabain-insensitive flux amounted to 754.7 +/- 59.9 pmol/oocyte per h (n = 30 cells, i.e., 10 cells each from three different animals). In the absence of Na+ (Na+ substituted by N-methylglucamine) or when Cl- was replaced by NO3- the ouabain-insensitive flux was reduced to 84.4 +/- 42.9 and 79.2 +/- 12.1 pmol/oocyte per h, respectively (n = 50 cells). Furthermore, this Na(+)- and Cl(-)-dependent flux was completely inhibited by 10(-4) mol/l bumetanide, a specific inhibitor of the Na(+)-K(+)-2Cl- cotransport system. These results suggest that K+ uptake via a bumetanide-sensitive Na(+)-K(+)-2Cl- cotransport system represents a major K+ pathway in oocytes.

Short- and long-term desensitization of serotonergic response in Xenopus oocytes injected with brain RNA: roles for inositol 1,4,5-trisphosphate and protein kinase C

In Xenopus oocytes injected with rat brain RNA, serotonin (5HT) and acetylcholine (ACh) evoke membrane responses through a common biochemical cascade that includes activation of phospholipase C, production of inositol 1,4,5-trisphosphate (Ins1,4,5-P3), release of Ca2+ from intracellular stores, and opening of Ca-dependent Cl- channels. The response is a Cl- current composed of a transient component (5HT1 or ACh1) and a slow, long-lasting component (5HT2 or ACh2). Here we show that only the fast, but not the slow, component of the response is subject to desensitization that follows a previous application of the transmitter. The recovery of 5HT1 from desensitization is biphasic, suggesting the existence of two types of desensitization: short-term desensitization (STD), which lasts for less than 0.5 h; and long-term desensitization (LTD) lasting for up to 4 h. The desensitization between 5HT and ACh is heterologous and long-lasting. We searched for (a) the molecular target and (b) the cause of desensitization. (a) Pre-exposure to 5HT does not reduce the response evoked by intracellular injection of Ca2+ and by Ca2+ influx. Cl- current evoked by intracellular injection of Ins1,4,5-P3 was reduced shortly after application of 5HT, but fully recovered 30 min later. Thus, the Cl- channel is not a target for desensitization. Neither Ins1,4,5-P3 receptor nor the Ca2+ store is a target of LTD but they may be the targets of STD. (b) Ca2+ injection did not inhibit the 5HT response, suggesting that Ca2+ is not a sole cause of STD or LTD.(ABSTRACT TRUNCATED AT 250 WORDS)

A long-lasting potentiation of calmodulin-mediated chloride channel activity without a mediation of protein kinase C in Xenopus oocytes injected with rat brain mRNA

When Xenopus oocytes injected with rat brain poly(A)+RNA were voltage-clamped in a recording solution containing Ca2+, a depolarization pulse induced a transient current, ICl(Ca), which reflects calmodulin-mediated opening of endogenous Cl- channels in response to a Ca2+ influx through Ca2+ channels of brain origin. ICl(Ca) could be repetitively observed with a steady amplitude over 1 h, whereas the response was greatly potentiated for more than 30 min after a brief stimulation of muscarinic or other Ca2(+)-mobilizing receptors. The enhancement of ICl(Ca) was mimicked by an injection of inositol-1,4,5-trisphosphate or by a treatment with A23187, but not affected by treatments that stimulate or inhibit protein kinase C activity. Isolated Ba2+ current flowing through voltage-sensitive Ca2+ channels was not augmented during the facilitation of ICl(Ca). These observations indicate that the endogenous calmodulin/Cl- channel system may memorize an over-threshold increase in the intracellular Ca2+ concentration and potentiate the Ca2(+)-sensitiveness of the Cl- channel. A long-lasting autoregulation of Ca2(+)-dependent ion channel activity is suggested.

Calcium-dependent chloride currents elicited by injection of ethanol into Xenopus oocytes

Electrophysiological techniques were used to study the response of native Xenopus laevis oocytes to intracellular injection and bath application of ethanol. Injection of ethanol produced dose-dependent transient inward currents accompanied by large current fluctuations, with estimated intracellular concentrations ranging from 10 to 300 mM. The response duration varied between 2 and 15 min, with an onset delay of 2-15 s. The inward current sometimes consisted of a fast and a slow component. Bath application of equivalent concentrations elicited similar but considerably smaller responses. The current showed a reversal potential of -20 +/- 10 mV, corresponding to an increase in chloride permeability. The response was eliminated in the presence of low chloride saline and was blocked by the chloride channel inhibitors SITS and DIDS. Ethanol responses were inhibited by the intracellular injection of the calcium chelator EGTA and were unaffected when the extracellular calcium was lowered. It is concluded that ethanol injection into Xenopus oocytes elicits a release of calcium from intracellular stores, which then activates an increased membrane permeability to chloride.

Effects of antidepressants on serotonin-evoked current in Xenopus oocytes injected with rat brain mRNA

Serotonin (5-HT, 1 microM) induced an inward current in Xenopus oocytes injected with rat brain mRNA under steady voltage clamp conditions of -60 mV. The 5-HT response was blocked by 0.1 microM mianserin, but neither buspirone, 8-hydroxy-dipropylaminotetralin (8-OH-DPAT), trifluoromethylphenyl piperazine (TFMPP), pindolol, propranolol, spiperone, ketanserin nor ICS 205-930 exerted effects, suggesting that the 5-HTIC subtype of 5-HT receptors is involved in the current. The 5-HT-induced current was inhibited by imipramine or desipramine with IC50 values of 60 nM or 20 microM, respectively. The response was also inhibited by setiptilline, maprotiline or amoxapine at a dose of 10 microM. Imipramine at 10 microM had no effect on the acetylcholine (ACh, 1 mM)-induced current response. It has been reported that inositol triphosphate (IP3) formation and intracellular Ca2+ are involved in the 5-HT- as well as ACh-induced current and that intracellular injection of either 50 pmol IP3 or 50 pmol Ca2+ mimics the 5-HT-induced current. The response induced by intracellular injection of either 50 pmol IP3 or 50 pmol Ca2+ was not affected by 10 microM imipramine. It is suggested that imipramine, and perhaps other antidepressant drugs tested, blocks 5-HTIC receptors, subsequently inhibiting the 5-HT-evoked current.

Glutamate receptor subtypes may be classified into two major categories: a study on Xenopus oocytes injected with rat brain mRNA

Three major subtypes of glutamate receptors that are coupled to cation channels are known. Recently an additional subtype that is coupled to G proteins and stimulates inositol phospholipid metabolism (the metabotropic glutamate receptor) has been proposed. The pharmacological characteristics of this receptor have now been examined. Although it shares some agonists with N-methyl-D-aspartate- and quisqualate-subtype receptors, it shares virtually no antagonists with any of the three cation channel-coupled receptor subtypes. Thus the metabotropic glutamate receptor belongs to a receptor category that is completely different from that of the other three receptor subtypes, not only functionally, but also pharmacologically.

Excitatory amino acids: the involvement of second messengers in the signal transduction process

1. Excitatory amino acids (EAA) can activate second messenger systems in addition to a direct gating of ion channels. A discrete coupling between novel EAA receptor subtypes and second messenger systems has been previously proposed. 2. EAAs have been suggested to activate both adenylate and guanylate cyclases and also to induce phosphoinositide (PI) turnover. The increased PI turnover was observed in both central neurons and glia, and a "quisqualate-type" receptor has been most frequently involved, which may differ from the quisqualate receptor previously defined by electrophysiological studies. 3. The roles of EAA-induced calcium influx into neurons and raised intracellular calcium levels are discussed regarding the activation of phosphoinositide turnover. 4. This review examines the data supporting a link between EAA receptors and second messengers and considers whether there is any need for adopting new EAA receptor subtypes. Also, the use of the Xenopus laevis oocyte for expressing EAA receptors and studying any putative links to second messenger systems is discussed.

Protein kinase C modulates neurotransmitter responses in Xenopus oocytes injected with rat brain RNA

Oocytes of the frog Xenopus laevis express various exogenous neurotransmitter receptors and ion channels when injected with RNA from excitable tissues. The oocytes serve as a convenient model system in which modulation of neurotransmitter responses can be studied. We examined the effects of activators and an inhibitor of protein kinase C (PKC) on responses to serotonin (5-HT), acetylcholine (ACh), kainate, and gamma-aminobutyric acid (GABA) in oocytes injected with RNA from rat brain. The PKC activators beta-phorbol esters 4 beta-phorbol-12-myristate-13-acetate (PMA) and 4 beta-phorbol-12,13-dibutyrate (PDBu), as well as the synthetic diacylglycerol, 1-oleyl-2-acetylglycerol (OAG), significantly inhibited the responses to 5-HT and ACh (both known to be mediated by mobilization of intracellular Ca2+); the first (transient) phase of these responses was affected stronger than the second, slow phase. PKC activators also reduced the response to GABA. The effect of PDBu on the response to kainate was dual; either inhibition or potentiation were observed at different concentrations of PDBu. The inactive analogue of PMA, the alpha-PMA, was without effect on the responses to 5-HT and GABA. The PKC inhibitor 1,5-isoquinolinesulfonyl-2-methylpiperazine (H7) suppressed the inhibitory effect of PDBu on 5-HT response. Amiloride, a blocker of the Na+/H+ exchange (which is known to be activated by PKC in some tissues), did not suppress the effects of PDBu. We concluded that activation of PKC down-regulates the responses to 5-HT, ACh and GABA, and has a dual effect on response to kainate. Possible mechanisms of these effects are discussed.

Two calcium-activated chloride conductances in Xenopus laevis oocytes permeabilized with the ionophore A23187

1. Currents evoked by elevated intracellular free Ca2+ in Xenopus laevis oocytes were studied using the two-electrode voltage clamp technique. The elevation in Ca2+ concentration was achieved in three ways: by the use of the divalent cation ionophore A23187; by application of Ca2+-mobilizing neurotransmitters serotonin and acetylcholine (ACh); by the entry of Ca2+ through voltage-dependent channels. 2. In most experiments, the membrane was permeabilized to Ca2+ by a 15 min pretreatment with A23187 in a Ca2+-free solution. Exposure of the ionophore-treated oocytes to external Ca2+ elicited an inward current (at holding potentials of -40 to -60 mV). At external Ca2+ concentrations ([Ca2+]) between 0.1 and 1 mM, the current had a time-to-peak of at least 10 s, and slowly decayed over tens of seconds. At [Ca2+] greater than 2 mM, the inward current had two distinct kinetic components, a fast and transient one (Ifast) and a slow one (Islow). 3. The main carrier of the Ca2+-evoked inward current was Cl-. Several data indicate the existence of a tetraethylammonium (TEA)-sensitive K+ conductance. No evidence for a Na+ current was found. 4. The two components of the Ca2+-evoked inward current in ionophore-permeabilized oocytes, and the two components of the current evoked by ACh and serotonin (the latter in oocytes injected with rat brain RNA but untreated with A23187), were blocked by intracellular injection of the Ca2+ chelator, ethyleneglycolbis-(beta-aminoethyl ether)-N,N,N'N'-tetraacetic acid (EGTA). The two components of these currents displayed different sensitivity to Ca2+ buffering; higher doses of EGTA were necessary to inhibit the slow component than the fast one. 5. One to two minutes of treatment with 2 mM-9-anthracene carboxylic acid (9-AC) fully blocked Ca2+-dependent Cl- current evoked by Ca2+ influx through voltage- dependent Ca2+ channels in intact (untreated with A23187) oocytes. In ionophore-treated oocytes, block of Ifast was observed at holding potentials at which the current was outward (i.e. due to Cl- influx); Islow was inhibited only partially. The block of Ca2+-evoked Cl- efflux by 9-AC developed much more slowly and was less potent. to explain these results, the existence of two sites of 9-AC action is proposed. 6. Exposure of the ionophore-permeabilized oocytes to 0.1-0.2 mM [Ca2+] strongly reduced the response to higher concentrations of Ca2+. Ifast displayed stronger Ca2+-dependent inactivation than Islow.(ABSTRACT TRUNCATED AT 400 WORDS)

Characterization of glutamate receptors induced in Xenopus oocytes after injection of rat brain mRNA

Xenopus oocytes in which poly(A)+ mRNA isolated from rat brains were previously injected, exhibited at least 3 categories of current responses to excitatory amino acids. They were oscillatory responses to glutamate (Glu) or quisqualate (QA), smooth large responses to kainate (KA), and smooth small responses to Glu and QA. Oscillatory responses were mediated by a metabotropic type of Glu receptor which is coupled to a G-protein but not directly to an ionic channel. Amplitudes of smooth Glu responses and smooth QA responses were similar in size, and were not additive to each other, suggesting a common receptor mediating both responses. L-Glutamylglycine inhibited KA responses in a competitive manner without affecting smooth Glu/QA responses, indicating that KA and smooth Glu/QA responses were mediated by separate receptors. From these results, it was concluded that the injection of rat brain mRNA induced at least 3 different glutamatergic receptors: receptors mediating (a) KA responses and (b) smooth Glu/QA responses, and (c) the metabotropic Glu receptor. The former two may most likely correspond to Glu receptor subtypes preferring KA and QA, respectively, as seen in the brain.

Inositol trisphosphate-induced membrane potential oscillations in Xenopus oocytes

1. The role of inositol 1,4,5-trisphosphate (Ins1,4,5P3) in controlling the membrane potential oscillations induced by acetylcholine in Xenopus oocytes was investigated by studying the effect of injecting Ins1,4,5P3. 2. Perfusing Xenopus oocytes with low concentrations of acetylcholine (less than or equal to 1 x 10(-7) M) induced regular oscillations in membrane potential. The frequency of these oscillations accelerated as the concentration of acetylcholine was increased. 3. Ionophoretic application of low doses of Ins1,4,5P3 stimulated membrane depolarization in the form of an initial brief spike which was followed by a burst of oscillations when the amount of Ins1,4,5P3 injected was increased. 4. When low doses of Ins1,4,5P3 were injected at 30 s intervals, there was rapid desensitization of the early response which recovered if the interval between injections was extended to 2 min or longer. 5. In comparison to the vegetal pole, the animal pole was much more sensitive to Ins1,4,5P3. This localization of Ins1,4,5P3 sensitivity in the animal pole may contribute to the electrical field which surrounds Xenopus oocytes. 6. A model is presented to explain these oscillations based on the phenomenon of calcium-induced calcium release. It is proposed that Ins1,4,5P3 releases calcium from an Ins1,4,5P3-sensitive pool which is then periodically taken up and released by an Ins1,4,5P3-insensitive pool. It is the overloading of this Ins1,4,5P3-insensitive pool which may provide the trigger to spontaneously release calcium back into the cytoplasm.

Expression of amino acid transport systems in Xenopus oocytes injected with mRNA of rat small intestine and kidney

Xenopus and Cynops oocytes were injected with exogenous mRNA prepared from rat small intestine and kidney and their electrical responses to amino acids were measured by both the current clamped and the voltage clamped methods. Oocytes injected with mRNA of rat small intestine showed a depolarization response to several neutral and basic amino acids, and almost no response to acidic amino acids. The responses to amino acids increased with incubation time after injection of mRNA, and followed Michaelis-Menten type kinetics. The responses were dependent on both Na+ concentration and membrane potential, and were inactivated by a sulfhydryl reagent, 5,5-dithiobis(2-nitrobenzoate). These results are interpreted as due to the expression of Na+/amino acid cotransporter(s) in oocytes injected with rat small intestine mRNA. On the other hand, the oocyte injected with rat kidney mRNA showed a hyperpolarization response to neutral amino acids, a depolarization response to basic ones, and almost no response to acidic ones in frog Ringer solution. These responses were independent of Na+ concentration and followed Michaelis-Menten type kinetics. These amino acid response characteristics in oocytes injected with rat kidney mRNA are interpreted as due to the expression of facilitated diffusion carrier protein(s) (uniporter) of amino acids in the oocyte.

Functional expression of brain cholecystokinin and bombesin receptors in Xenopus oocytes

Total RNA was extracted from 15-day-old whole rat brains. Microinjection of the RNA into Xenopus laevis oocytes induced electrophysiological responsiveness to cholecystokinin-8 (CCK) and bombesin (BBS) but not to corticotropin-releasing factor (CRF) or somatostatin. The responses to CCK and BBS were similar in shape, time course, and reversal potential to that induced by receptor mediated phospholipid breakdown and that which is induced by intracellular injection of IP3. These responses were not blocked by atropine or by mianserin, did not require extracellular Ca2+ and were completely suppressed by intracellular injection of EGTA.

Activation of protein kinase C differentially modulates neuronal Na+, Ca2+, and gamma-aminobutyrate type A channels

Xenopus oocytes were used to study the interaction of neuronal quisqualate receptors with neuronal ion channels. Total mRNA was isolated from chick forebrain and injected into Xenopus oocytes. This technique led to the expression of functional voltage-gated Na+ and Ca2+ channels, of ligand-gated gamma-aminobutyrate and kainate receptor channels, and of quisqualate receptors that could activate endogenous chloride channels by means of inositol trisphosphate-mediated Ca2+ release. Exposure of the oocytes to quisqualate decreased the amplitude of the Na+ current and of the gamma-aminobutyrate type A-gated current and increased the amplitude of the Ba2+ current through Ca2+ channels. This modulation of neuronal ion channels by quisqualate could be mimicked by the protein kinase C activator phorbol 12-myristate 13-acetate and the diacylglycerol analogue 1,2-oleoylacetylglycerol. The kainate-gated channel was not affected by these agents. Phorbol esters that do not activate protein kinase C, alpha-phorbol 12-myristate 13-acetate and alpha-phorbol, were without effect. The inhibitor of protein kinase C, tamoxifen, prevented the modulatory effects of phorbol 12-myristate 13-acetate. The present evidence suggests that the activity of the neuronal Na+ and Ca2+ channels and the ligand-gated gamma-aminobutyrate type A receptor channel are under the control of protein kinase C and that neurotransmitters that activate protein kinase C could profoundly affect neuronal signaling.

Phorbol ester inhibition of current responses and simultaneous protein phosphorylation in Xenopus oocyte injected with brain mRNA

The role of protein kinase C activation in a coupling of Ca2+-mobilizing receptors/GTP-binding protein/phospholipase C was examined using Xenopus oocytes before and after microinjection of mRNA purified from rat brains. Under voltage-clamp conditions, although the phorbol ester TPA per se never elicited any changes in ionic conductance, chloride current responses of mRNA-injected cells to 5-hydroxytryptamine and acetylcholine (ACh) were suppressed by an 8-min pretreatment of 12-O-tetradecanoyl-4 beta-phorbol-13-acetate (TPA), at nanomolar concentrations. Native ACh response in intact follicular oocytes was also inhibited by the TPA treatment. However, similar current responses triggered by the direct activation of their intracellular signalling pathway with guanosine-5'-O-(3-thio)triphosphate or Ca2+ were not affected by TPA. Biochemical analyses indicated that phosphorylation of 33,000- and 45,000-dalton proteins was markedly enhanced by TPA in vivo, and that stimulation of receptors with agonists as well as TPA treatment increased phosphoproteins in the membrane fraction of mRNA-injected oocytes. These observations suggest that protein kinase C may switch off the signal transduction from receptors to GTP-binding proteins and may participate in the negative feedback modulation of receptor-operated ion channel responses.

Roles of protein kinases in neurotransmitter responses in Xenopus oocytes injected with rat brain mRNA

Microinjection of rat brain mRNA in Xenopus oocytes induced acetylcholine, neurotensin, serotonin, and glutamate receptors in the cells. These receptors stimulate an intracellular reaction pathway, including G-protein activation, inositol trisphosphate (IP3) formation, and Ca2+-dependent Cl- channels. In the present study, we examined the roles of several protein kinases in these responses by means of inhibitors and activators of these kinases. Isoquinolinesulfonamides, inhibitors of protein kinases, caused no current responses and affected no receptor-mediated responses when injected into the oocytes at low doses (30-50 pmol), which inhibit cyclic nucleotide-dependent kinases or kinase C specifically, but abolished the receptor-mediated responses at a higher dose (300 pmol), which inhibit most protein kinases nonspecifically. Calmodulin inhibitors blocked the receptor-mediated responses strongly. Activation of cyclic nucleotide-dependent kinases or kinase C by injection of cAMP (or cGMP) or perfusion with phorbol esters caused no direct current responses but suppressed receptor-mediated responses. Current responses triggered by IP3 injection were not suppressed by these treatments. These results suggest that cAMP- (or cGMP-)dependent kinases or kinase C may not be involved in the pathway directly but may modulate it by inhibiting the initial part of the pathway (receptors, G-proteins, and/or phospholipase C), and they suggest that calmodulin may most likely be involved in the activation of Ca2+-dependent Cl- channels.

Vasoactive intestinal peptide (VIP) induces a K+ current in the Xenopus oocyte membrane

Evidence, based on voltage-clamp and current-clamp recording, is presented for endogenous receptors for vasoactive intestinal polypeptide (VIP) on the cell membrane of the Xenopus oocyte. At normal resting potential a hyperpolarisation is produced by VIP, associated with an increase in K+ conductance. The response is dose-dependent, with a threshold near 10(-9)M VIP. The effect is potentiated by forskolin. Of several other types of neuropeptide tested, only avian pancreatic polypeptide gives a response: this is similar to that of VIP, but this peptide can also potentiate the response to VIP.

GTP-binding proteins Gi and Go transplanted onto Xenopus oocyte by rat brain messenger RNA

After injection with messenger RNA (mRNA) isolated from rat brain, Xenopus laevis oocytes acquired electrophysiological responsiveness to externally perfused acetylcholine (ACh) or serotonin (5-HT), and elevated responsiveness to internally applied guanosine 5'-(3-O-thio)triphosphate (GTP gamma S). Compared with the membranes of native oocytes, those of mRNA-injected oocytes contained increased amounts of 39 and 41 kDa proteins, which could be [32P]ADP-ribosylated by pertussis toxin (PTX). The amplitude of the GTP gamma S response and the amounts of the 39 and 41 kDa proteins increased in a parallel manner for at least 3 days following mRNA injection. Current responses to internally applied GTP gamma S showed properties common to those of responses to ACh or 5-HT perfusion: both responses had reversal potentials close to the Cl- potential, were mimicked by intracellular injection of IP3, desensitized by a large dose of IP3, and inhibited by a simultaneous injection of neomycin or EGTA. Incubation of mRNA-injected cells with PTX inhibited both the 5-HT response and the [32P]ADP-ribosylation of the 39 and 41 kDa proteins in a parallel, dose-dependent manner. After pretreatment of oocytes with PTX followed by mRNA injection, the levels of the 39 and kDa proteins and the 5-HT response appeared to be similar to those of non-treated cells injected with mRNA, whereas no detectable amounts of these proteins were induced when PTX-pretreated cells were analyzed under the same conditions without mRNA injection.(ABSTRACT TRUNCATED AT 250 WORDS)

A new type of glutamate receptor linked to inositol phospholipid metabolism

Receptors for excitatory amino acids in the mammalian central nervous system are classified into three major subtypes, ones which prefer N-methyl-D-aspartate (NMDA), quisqualate (QA), or kainate (KA) as type agonists respectively. These receptors are considered to mediate fast postsynaptic potentials by activating ion channels directly (ionotropic type). Recently it was reported that exposure of mammalian brain cells to glutamate (Glu) or its analogues causes enhanced hydrolysis of inositol phospholipids, but it is not clear whether the enhanced hydrolysis is the cause or effect of physiological responses. Membrane depolarization or Ca2+ influx, which can result from Glu receptor activation, can induce enhanced hydrolysis of inositol phospholipids. We have characterized the functional properties of two types of excitatory amino-acid responses, those activated by QA (or Glu) and those activated by KA, induced in Xenopus oocytes injected with rat-brain messenger RNA. We report evidence for a new type of Glu receptor, which prefers QA as agonist, and which directly activates inositol phospholipid metabolism through interaction with GTP-binding regulatory proteins (Gi or Go), leading to the formation of inositol 1,4,5-trisphosphate (InsP3) and mobilization of intracellular Ca2+. This QA/Glu reaction is inhibited by islet-activating protein (IAP, pertussis toxin), but was not blocked by Joro spider toxin (JSTX), a specific blocker of traditional ionotropic QA/Glu receptors.