Inositol phosphate formation and chloride current responses induced by acetylcholine and serotonin through GTP-binding proteins in Xenopus oocyte after injection of rat brain messenger RNA

Use of an electrophysiological technique for stepwise detection of trace agonist constituents of Hochuekkito in Xenopus oocytes injected with serotonin 2C receptor mRNA

An electrophysiological bioassay was used to isolate the active compound from Hochuekkito (HET), which the current authors previously described as having potent agonist action against serotonin 2C receptors (5-HT2CR). Synthetic 5-HT2CR mRNA was injected into Xenopus oocytes to specifically express these receptors. Crude extracts and purified products were subjected to an electrophysiological bioassay using the voltage clamp method. HET stimulated a 5-HT2CR-induced current response, whereas Juzentaohoto (JTT), which has anti-depressive action similar to that of HET, did not. Current responses were not observed with an extract mixed with five types of herbal medicines common to HET and JTT but were detected with an extract with the five types of herbal medicines found in HET alone (Hoc5). When the responses to each of the five types of Hoc5 were examined, current responses were noted with Cimicifugae rhizoma (CR) and Citrus unshiu Markovich extracts. Since efficacy and the EC₅₀ value were higher for CR, its constituents were separated using three-dimensional high-performance liquid chromatography and the current response at each of the isolated peaks was examined. One constituent displayed a strong response and was identified as a single substance with a molecular weight of 283.1393 based on liquid chromatography/mass spectrometry. These results will contribute to the isolation of 5-HT2CR-stimulating constituents in HET and the identification of trace constituents with agonist action.

Differential role of STIM1 and STIM2 during transient inward (T in) current generation and the maturation process in the Xenopus oocyte

BACKGROUND

The Xenopus oocyte is a useful cell model to study Ca2+ homeostasis and cell cycle regulation, two highly interrelated processes. Here, we used antisense oligonucleotides to investigate the role in the oocyte of stromal interaction molecule (STIM) proteins that are fundamental elements of the store-operated calcium-entry (SOCE) phenomenon, as they are both sensors for Ca2+ concentration in the intracellular reservoirs as well as activators of the membrane channels that allow Ca2+ influx.

RESULTS

Endogenous STIM1 and STIM2 expression was demonstrated, and their synthesis was knocked down 48-72 h after injecting oocytes with specific antisense sequences. Selective elimination of their mRNA and protein expression was confirmed by PCR and Western blot analysis, and we then evaluated the effect of their absence on two endogenous responses: the opening of SOC channels elicited by G protein-coupled receptor (GPCR)-activated Ca2+ release, and the process of maturation stimulated by progesterone. Activation of SOC channels was monitored electrically by measuring the T in response, a Ca2+-influx-dependent Cl- current, while maturation was assessed by germinal vesicle breakdown (GVBD) scoring and electrophysiology.

CONCLUSIONS

It was found that STIM2, but not STIM1, was essential in both responses, and T in currents and GVBD were strongly reduced or eliminated in cells devoid of STIM2; STIM1 knockdown had no effect on the maturation process, but it reduced the T in response by 15 to 70%. Thus, the endogenous SOCE response in Xenopus oocytes depended mainly on STIM2, and its expression was necessary for entry into meiosis induced by progesterone.

[Cellular and molecular pharmacological studies on membrane receptor-signaling and stress-responses in the brain]

Studies on the cellular and molecular mechanism of neurotransmitter receptor-signaling and of neuronal and glial cell responses to stresses seem to be important to elucidate the action mechanism of centrally-acting drugs and to develop novel therapeutics against several diseases in the brain. The present review shows our findings with regard to the membrane receptor-signaling mechanism including serotonin, noradrenaline, glutamate receptors, ion channels, G-proteins, protein kinases and drug actions in Xenopus oocytes injected with rat brain mRNA, NG108-15 cells and brain membranes. Regarding the results of studies on the inter- and intra-cellular mechanism of neurons and glial cells against cerebral ischemia/hypoxia, we review the involvement of a transcription factor NF-kappa B in LPS-elicited inducible NO synthase (iNOS) expression in rat astroglial cells. Then we describe possible involvement of: 1) ADP-ribosylation/nitrosylation of glyceraldehyde-3-phosphate dehydrogenase (GAPDH) and 2) decrease in mitochondrial membrane potential, release of caspase-3 from mitochondria and degradation of the inhibitor of caspase-activated DNase by activated caspase in NO-induced neuronal apoptosis. We observed that hypoxia results in expression of a molecular chaperon such as protein disulfide isomerase (PDI) and HSP70 in astroglial cells. Our recent findings indicate that overexpression of PDI in the rat hippocampus (in vivo) and in neuroblastoma SK-N-MC cells (in vitro) significantly suppress the hypoxia-induced neuronal death. From physiological/pathophysiological and pharmacological aspects, we review the importance of studies on the cellular and molecular mechanism of membrane receptor-signaling and of stress-responses in the brain to identify functional roles of neuro-glial- as well as neuro-neuronal interaction in the brain.

Activation of inositol 1,4,5-trisphosphate receptor is essential for the opening of mouse TRP5 channels

We studied the opening mechanism of Ca(2+)-permeable channels formed with mouse transient receptor potential type 5 (mTRP5) using Xenopus oocytes. After stimulation of coexpressed muscarinic M(1) receptors with acetylcholine (ACh) in a Ca(2+)-free solution, switching to 2 mM Ca(2+)-containing solution evoked a large Cl(-) current, which reflects the opening of endogenous Ca(2+)-dependent Cl(-) channels following Ca(2+) entry through the expressed channels. The ACh-evoked response was not affected by a depletion of Ca(2+) store with thapsigargin but was inhibited by preinjection of antisense oligodeoxynucleotides (ODNs) to G(q), G(11), or both. The mTRP5 channel response was also induced by a direct activation of G proteins with injection of guanosine 5'-3-O-(thio)triphosphate (GTP gamma S). The ACh- and GTP gamma S-evoked responses were inhibited by either pretreatment with a phospholipase C inhibitor, U73122, or an inositol-1,4,5-trisphosphate (IP(3)) receptor inhibitor, xestospongin C (XeC). An activation of IP(3) receptors with injection of adenophostin A (AdA) evoked the mTRP5 channel response in a dose-dependent manner. The AdA-evoked response was not suppressed by preinjection of antisense ODNs to G(q/11) or U73122 but was suppressed by either preinjection of XeC or a peptide mimicking the IP(3) binding domain of Xenopus IP(3) receptor. These findings suggest that the activation of IP(3) receptor is essential for the opening of mTRP5 channels, and that neither G proteins, phosphoinositide metabolism, nor depletion of the Ca(2+) store directly modifies the IP(3) receptor-linked opening of mTRP5 channels.

Serotonergic effects of dotarizine in coronary artery and in oocytes expressing 5-HT2 receptors

In strips of pig coronary arteries incubated in oxygenated Krebs-bicarbonate solution at 37 degrees C, dotarizine blocked the phasic contractions evoked by 5-HT (0.5 microM) or K+ depolarization (35 mM K+) with an IC50 of 0.22 and 3.7 microM, respectively. Flunarizine inhibited both types of contractions with IC50 values of 1.7 microM for 5-HT and 2.4 microM for K+ responses. In Xenopus oocytes injected with in vitro transcribed RNA encoding for 5-HT2A or 5-HT2C receptors, 5-HT (100 nM for 20 s) applied every 10 min caused, in both cases, a reproducible inward current through Ca2(+)-activated Cl- channels (ICl). Dotarizine inhibited the 5-HT2A response in a concentration-dependent manner, with an IC50 of 2.2 nM. In contrast, the 5-HT2C response was unaffected by 1 microM dotarizine and blocked around 62% by 10 microM of this drug. The ICl activated either by intracellular injection of inositol 1,4,5-trisphosphate (IP3) in oocytes or by direct photorelease of Ca2+ in DM-nitrophen-injected oocytes was unaffected by 10 microM dotarizine. It is concluded that dotarizine blocks 5-HT2A receptors with a high affinity; the compound is devoid of intracellular effects on any further steps of the transduction pathway (i.e., IP3 receptor). Contrary to flunarizine that blocks equally well the serotonergic and the K+ vascular responses, dotarizine exhibits 17-fold higher affinity for vascular 5-HT receptors. These findings might be relevant to an understanding of the mechanism involved in the use of dotarizine and flunarizine as prophylactic agents in migraine.

Interaction between capacitative Ca2+ influx and Ca2+-dependent Cl- currents in Xenopus oocytes

The relationship between capacitative Ca2+ influx and activation of Ca2+-dependent Cl- channels was monitored in intact Xenopus oocytes following stimulation of 5-hydroxytryptamine (5-HT) receptors, through the activity of Ca2+-dependent Cl- channels using the double-electrode voltage-clamp technique. Under voltage-clamp conditions, 5-HT evoked a rapid transient inward current followed by a slowly developing secondary inward current. The secondary current reflected depletion-activated Ca2+ entry. Hyperpolarising pulses evoked sustained Ca2+-dependent Cl- currents when applied during the transient inward current, but evoked hump-like currents which inactivated rapidly when applied during the secondary inward current. Hump currents arose from Ca2+ entering through the depletion-activated pathway. The hump currents inactivated with hyperpolarising pulses at < 5-s intervals, and recovered monoexponentially with a time constant of around 8 s. Currents in response to hyperpolarising pulses during the transient current did not inactivate, suggesting that inactivation was associated with Ca2+ entry. When ca2+ release evoked by inositol 1,4,5-triphosphate [ins(1,4,5)p3] was prevented by heparin injection, hyperpolarising pulses during ca2+ ionophore application also generated hump currents that were dependent on external ca2+, inactivated and recovered from inactivation with a similar time course as the humps following 5-ht treatment. Pretreatment with the Ca2+ adenosine 5'-triphosphatase (Ca2+ATPase) inhibitor thapsigargin reduced the rate of rise of the hump current, increased the time-to-peak of the current and slowed the rate of decay. Pharmacological interventions to disrupt the cytoskeleton reduced the amplitude of the hump current. It is suggested that, following hyperpolarisation in the presence of Ca2+ entry, the ensuing Ca2+ influx interacts with Cl- channels in a way that might reflect both Ca2+ inhibition of Ca2+ entry and clustering of Cl- channels in the plasma membrane.

Molecular biology of the opioid receptors: structures, functions and distributions

Opiates like morphine and endogenous opioid peptides exert their pharmacological and physiological effects through binding to their endogenous receptors, opioid receptors. The opioid receptors are classified into at least three types, mu-, delta- and kappa-types. Recently, cDNAs of the opioid receptors have been cloned and have greatly advanced our understanding of their structure, function and expression. This review focuses on the recent advances in the studies on opioid receptors using the cloned cDNAs. We describe the molecular cloning of the opioid receptor gene family and studies of the structure-function relationships, modes of coupling to second messenger systems, pharmacological effects of antisense oligonucleotide and anatomical distributions of opioid receptors.

Protein kinase C modulates exogenous acetylcholine current in Xenopus oocytes

The modulation of acetylcholine-activated current (IACh) by protein kinase C (PKC) was studied in Xenopus laevis oocytes microinjected with either mRNA extracted from C2C12 myotubes (C2C12 mRNA) or RNAs encoding murine alpha beta gamma delta subunits of the nicotinic ACh receptor (nAChR). Voltage-clamped oocytes were treated for 90 sec with 12-O-tetradecanoylphorbol-13-acetate (TPA, 300 nM), a potent PKC activator. Transient increase in the amplitude and acceleration in the decay of IACh were invariably observed within minutes of TPA application, and were independent of extracellular Ca2+ concentration. Both parameters recovered to control within 20-30 min; then a slight depression of IACh developed. By this time, an initial PKC down regulation was observed. At the peak of TPA-induced potentiation, dose-response relations suggested an increased binding affinity of nAChR for the neurotransmitter. 4 alpha-phorbol 12,13-didecanoate (300 nM), a biologically inactive analogue of TPA, did not affect IACh, while staurosporine (5-10 microM), a potent inhibitor of PKC activity, suppressed the action of TPA on IACh. In oocytes co-injected with C2C12 mRNA and with rat brain mRNA, IACh was potentiated by 5-hydroxy-tryptamine (10 microM), whose receptors are coupled to phosphoinositide hydrolysis. The nAChR-channel activity in cell-attached patches increased when TPA was applied to the oocytes. In 50% of the oocytes examined, a sustained depression of the single channel activity followed. We conclude that in Xenopus oocytes an endogenous PKC system regulates the function of embryonic-type muscle nAChRs.

Aspects of calcium-activated chloride currents: a neuronal perspective

Ca(2+)-activated Cl- channels are expressed in a variety of cell types, including central and peripheral neurones. These channels are activated by a rise in intracellular Ca2+ close to the cell membrane. This can be evoked by cellular events such as Ca2+ entry through voltage- and ligandgated channels or release of Ca2+ from intracellular stores. Additionally, these Ca(2+)-activated Cl currents (ICl(Ca)) can be activated by raising intracellular Ca2+ through artificial experimental procedures such as intracellular photorelease of Ca2+ from "caged" photolabile compounds (e.g. DM-nitrophen) or by treating cells with Ca2+ ionophores. The potential changes that result from activation of Ca(2+)-activated Cl- channels are dependent on resting membrane potential and the equilibrium potential for Cl-. Ca2+ entry during a single action potential is sufficient to produce substantial after potentials, suggesting that the activity of these Cl- channels can have profound effects on cell excitability. The whole cell ICl(Ca) can be identified by sensitivity to increased Ca2+ buffering capacity of the cell, anion substitution studies and reversal potential measurements, as well as by the actions of Cl- channel blockers. In cultured sensory neurones, there is evidence that the ICl(Ca) deactivates as Ca2+ is buffered or removed from the intracellular environment. To date, there is no evidence in mammalian neurones to suggest these Ca(2+)-sensitive Cl- channels undergo a process of inactivation. Therefore, ICl(Ca) can be used as a physiological index of intracellular Ca2+ close to the cell membrane. The ICl(Ca) has been shown to be activated or prolonged as a result of metabolic stress, as well as by drugs that disturb intracellular Ca2+ homeostatic mechanisms or release Ca2+ from intracellular stores. In addition to sensitivity to classic Cl- channel blockers such as niflumic acid, derivatives of stilbene (4,4'diisothiocyanostilbene-2,2'-disulphonic acid, 4-acetamido-4'-isothiocyanostilbene-2,2'-disulphonic acid) and benzoic acid (5-nitro 2-(3-phenylpropylamino) benzoic acid), ICl(Ca) are also sensitive to polyamine spider toxins and some of their analogues, particularly those containing the amino acid residue arginine. The physiological role of Ca(2+)-activated Cl- channels in neurones remains to be fully determined. The wide distribution of these channels in the nervous system, and their capacity to underlie a variety of events such as sustained or transient depolarization or hyperpolarizations in response to changes in intracellular Ca2+ and variations in intracellular Cl- concentration, suggest the roles may be subtle, but important.

Molecular pharmacology of the opioid receptors

Opioid receptors are the primary sites of actions of opiates and endogenous opioid peptides, which have a wide variety of pharmacological and physiological effects. The opioid receptors are classified into at least three subtypes, mu, delta, and kappa, and their cDNAs have been cloned. In this review, we describe the molecular cloning of opioid receptor gene family and studies of the structure-function relationships, modes of coupling to second messenger systems, pharmacological effects of antisense oligonucleotides, and anatomical distribution of opioid receptor mRNAs.

Mobilization of intracellular Ca2+ and stimulation of cyclic AMP production by kappa opioid receptors expressed in Xenopus oocytes

The intracellular metabotropic pathway, following kappa opioid receptor activation, was investigated in the Xenopus oocyte translation system. When oocytes were injected with cRNA for kappa opioid receptor cDNA, U50488H rarely evoked phospholipase C-mediated, oscillatory Cl- current responses. However, after the oocytes were incubated with staurosporine, both the occurrence and the amplitude of U50488H-evoked responses were increased. The U50488H-evoked response was antagonized by naloxone and inhibited by pretreatment of the oocytes with pertussis toxin. When oocytes were coinjected with RNAs encoding kappa opioid receptor and cystic fibrosis transmembrane conductance regulator (CFTR), treatment of the oocytes with forskolin and 3-isobutyl-1-methylxanthine (IBMX) evoked a smooth-shaped Cl- current flowing through the CFTR channels. The forskolin/IBMX-evoked response was never inhibited but was greatly potentiated in the presence of U50488H, indicating stimulation of adenylyl cyclase by U50488H. This U50488H-induced potentiation of CFTR channel opening was antagonized by naloxone and inhibited by pretreatment with pertussis toxin. These results suggest that kappa opioid receptors mobilize intracellular Ca2+ and stimulate cyclic AMP production by coupling positively to both phospholipase C and adenylyl cyclase via pertussis toxin-sensitive GTP-binding proteins in the oocytes.

Functional expression of Ca(2+)-mobilizing opioid receptors in Xenopus oocytes injected with rat brain mRNA

Functional expression of opioid receptors was detected in the Xenopus oocyte translation system by a voltage-clamp recording. After injection of poly(A)+ RNA isolated from 3-week-old rat striatum or whole brain, the oocytes often demonstrated intracellular Ca(2+)-mediated oscillatory responsiveness to [D-Ala2, N-methyl-Phe4, Gly5-ol]enkephalin (DAMGO), [D-Pen2, D-Pen5]enkephalin (DPDPE) and U50488H at a concentration of 1 microM. These responses were very transiently expressed after injection of the mRNA, however, water-injected oocytes never responded to any of these opioid agonists. After fractionation by a sucrose-density gradient, an RNA size of about 3-4 kb encoded these opioid receptors. In the oocytes injected with size-selected striatal mRNA, DPDPE evoked the fluctuating current with higher probability and larger amplitude than other agonists, whereas oocytes injected with size-selected whole brain mRNA produced DAMGO and U50488H responses predominantly. The DPDPE response of striatal mRNA-injected oocytes was antagonized by naloxone as well as the delta-specific antagonist ICI 174864. The DAMGO and U50488H responses have not been characterized yet because of a strong desensitizing property making repeated recordings impossible. These observations suggest that putative mu, delta and kappa subtypes of opioid receptors mobilizing intracellular Ca2+ are expressed in Xenopus oocytes by rat brain mRNA.

Effects of an activator and an inhibitor of protein kinase C on serotonin receptors induced by rat brain mRNA in Xenopus oocytes

The effects of bath-applied phorbol esters, protein kinase C (PKC) activators, and sphingosine, a PKC inhibitor, on membrane currents evoked by serotonin were studied in Xenopus oocytes injected with rat brain mRNA. The oscillatory Cl- current in response to bath-applied 50 nM serotonin was significantly depressed by 200 nM phorbol dibutyrate and remarkably augmented by 100 microM sphingosine. Both drugs have little effect either on the nonspecific Cl- current response induced by aluminum fluoride or on the cationic current response induced by 1 mM kainate.

Cloning and expression of a cDNA for the rat kappa-opioid receptor

We cloned a cDNA for the rat kappa-opioid receptor from a rat thalamus cDNA library. The deduced amino acid sequence consists of 380 residues with features shared by members of the G protein-coupled receptor family. The specific binding of [3H]bremazocine to the membrane of COS-7 cells transfected with the cDNA was displaced by kappa-specific opioid ligands, but not by mu- and delta-specific ligands. Xenopus oocytes injected with the in vitro transcribed mRNA responded to opioid ligands with the same subtype specificity. Northern blot analysis demonstrated that kappa-opioid receptor mRNA is expressed in a regionally specific manner in rat brain.

Depletion of InsP3 stores activates a Ca2+ and K+ current by means of a phosphatase and a diffusible messenger

In non-excitable cells, release of Ca2+ from the inositol 1,4,5-trisphosphate (InsP3)-sensitive store can activate Ca2+ entry. Very little is known about the signal mechanism relating store emptying to plasma membrane Ca2+ influx. It has been suggested that the signal may be either a diffusible messenger like an inositol phosphate, or the InsP3 receptor itself, which, by physically coupling to some component of Ca2+ entry in the plasma membrane, may link store release to Ca2+ entry. The nature of the Ca2+ entry pathway is also unclear. Only in mast cells has a very selective Ca2+ current been observed after store emptying. Activation of exogenous 5-hydroxytryptamine (5-HT) receptors expressed in Xenopus oocytes or direct injection of InsP3 evokes Ca2+ entry activated by InsP3 pool depletion. Here we investigate the nature of this influx pathway and find a current activated by pool depletion. This has an unusual selectivity in that it is more permeable to Ca2+ ions than to other divalent cations (Ba2+, Sr2+ or Mn2+). Moreover, a K+ permeability is also stimulated after pool depletion. The activation of this store depletion current involves both a phosphatase and an unidentified diffusible messenger. Both the Ca2+ entry pathway and the activating factors found here may be relevant to pool-depleted Ca2+ entry in a variety of non-excitable cells.

Calcium-activated currents in cultured neurones from rat dorsal root ganglia

1. Voltage-activated Ca2+ currents and caffeine (1 to 10 mM) were used to increase intracellular Ca2+ in rat cultured dorsal root ganglia (DRG) neurones. Elevation of intracellular Ca2+ resulted in activation of inward currents which were attenuated by increasing the Ca2+ buffering capacity of cells by raising the concentration of EGTA in the patch solution to 10 mM. Low and high voltage-activated Ca2+ currents gave rise to Cl- tail currents in cells loaded with CsCl patch solution. Outward Ca2+ channel currents activated at very depolarized potentials (Vc + 60 mV to + 100 mV) also activated Cl- tail currents, which were enhanced when extracellular Ca2+ was elevated from 2 mM to 4 mM. 2. The Ca(2+)-activated Cl- tail currents were identified by estimation of tail current reversal potential by use of a double pulse protocol and by sensitivity to the Cl- channel blocker 5-nitro 2-(3-phenyl-propylamino) benzoic acid (NPPB) applied at a concentration of 10 microM. 3. Cells loaded with Cs acetate patch solution and bathed in medium containing 4 mM Ca2+ also had prolonged Ca(2+)-dependent tail currents, however these smaller tail currents were insensitive to NPPB. Release of Ca2+ from intracellular stores by caffeine gave rise to sustained inward currents in cells loaded with Cs acetate. Both Ca(2+)-activated tail currents and caffeine-induced inward currents recorded from cells loaded with Cs acetate were attenuated by Tris based recording media, and had reversal potentials positive to 0 mV suggesting activity of Ca(2+)-activated cation channels.4. Our data may reflect (a) different degrees of association between Ca2+-activated channels with voltage-gated Ca2+ channels, (b) distinct relationships between channels and intracellular Ca2" stores and Ca2+ homeostatic mechanisms, (c) regulation of Ca2+-activated channels by second messengers, and (d) varying channel sensitivity to Ca2 , in the cell body of DRG neurones.

Metabotropic responses to acetylcholine and serotonin of Xenopus oocytes injected with rat brain mRNA are transduced by different G-protein subtypes

To assign the GTP-binding protein (G-protein) subtype involved in the signal transduction from exogenous receptors to phospholipase C in the Xenopus oocyte translation system, antisense DNA complementary to rat G-protein alpha-subunit mRNA was designed and injected together with rat brain poly(A)+ RNA. Current response of mRNA-injected oocytes to acetylcholine (ACh) was suppressed dose-dependently by a co-injection of Gil alpha-antisense DNA, but response of the same oocytes to serotonin (5-HT) was not inhibited. In the oocytes co-injected with Go alpha-antisense DNA, the 5-HT response was more effectively suppressed than the ACh response. These results suggest that Go alpha but not Gil alpha intermediates brain 5-HT1C receptor function, and in contrast, muscarinic receptors derived from rat brain utilize Gil alpha rather than Go alpha to activate phospholipase C.

Angiotensin II and acetylcholine differentially activate mobilization of inositol phosphates in Xenopus laevis ovarian follicles

Angiotensin II (AII) evokes a Ca(2+)-dependent Cl- current in Xenopus laevis ovarian follicles that appears to involve a pertussis-toxin-sensitive G protein mediating phosphoinositide hydrolysis and Ca2+ mobilization from intracellular stores. Follicle responses to AII closely resemble the two-component response stimulated by acetylcholine (ACh) in this tissue. Intraoocyte injections of phytic acid, heparin, and inositol 1,4,5-trisphosphate [Ins(1,4,5)P3], acting as inhibitors of Ins(1,4,5)P3-induced Ca(2+)-release, resulted in loss of responsiveness to AII and ACh. As previously reported for ACh [Moriarty et al. (1988) Proc Natl Acad Sci USA 85: 8865-8869], pertussis toxin and microinjected GTP[gammaS] were found to inhibit follicle responses to AII, implying the involvement of a G protein. However, ACh and AII responses differ strikingly in the way they mobilize inositol phosphates and in densitization characteristics. We have previously been unable to find significant increases in inositol phosphates after 60 min stimulation (with Li+) by AII, although ACh potently activated increases in these [McIntosh and McIntosh (1990) Arch Biochem Biophys 283: 135-140]. In the present paper, AII was found to activate rapid increases in inositol bis- and trisphosphates after 1 min stimulation without Li+. ACh and AII also exerted different actions on follicle adenylate-cyclase-dependent responses. We conclude that at least two separate inositol-phosphate-linked receptor mechanisms may exist in ovarian follicles, resulting from involvement of one or more pertussis-toxin-sensitive G protein(s).

Inhibitory effects of KN-62, a specific inhibitor of Ca/calmodulin-dependent protein kinase II, on serotonin-evoked C1-current and 36-C1-efflux in Xenopus oocytes

To clarify the details of the involvement of Ca(2+)-calmodulin in gating of C1-channels, effects of a novel calmodulin-dependent protein kinase II (CaMK II) inhibitor, KN-62, on C1-current and C1-efflux induced by serotonin (5-HT) were investigated in Xenopus oocyte injected with rat brain mRNA. 5-HT evoked inward current on voltage clamp condition at -60 mV in a concentration-dependent manner. The 5-HT (1 microM)-evoked current was blocked by preperfusion with 1 microM KN-62 also inhibited acetylcholine- and inositol 1,4,5-trisphosphate-evoked current. Furthermore, 5-HT enhanced Cl- efflux about 2.5-fold from the oocyte preinjected with 36Cl-, and the effects were inhibited by KN-62 as well. These results suggest that CaMK II is activated by Ca(2+)-calmodulin and opens Cl- channels to induce CL- efflux in Xenopus oocytes.

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.

Light-induced, GTP-binding protein mediated membrane currents of Xenopus oocytes injected with rhodopsin of cephalopods

Xenopus oocytes that were injected with rhabdomeric membranes of squid and octopus photoreceptors acquired light sensitivity. The injected oocytes showed a light-induced current having characteristics similar to other G-protein-mediated Cl- currents induced by the activation of other membrane receptors. Pretreatment of the oocytes with pertussis toxin before the injection suppressed the generation of the light-induced current, indicating an ability of cephalopod rhodopsin to cross-react with an endogenous G-protein of Xenopus oocytes.

Ins(1,4,5)P3 formation and fluctuating chloride current response induced by external ATP in Xenopus oocytes injected with embryonic guinea pig brain mRNA

In voltage-clamped Xenopus oocytes injected with embryonic guinea pig mRNA, effective concentrations of extracellular ATP elicited an inward fluctuating current. This current, carried by Cl-ions, was mainly dependent upon liberation of Ca2+ ions from stores as demonstrated by experiments using intracellular EGTA loading and TMB-8 superfusion. Neomycin inhibited these fluctuating currents indicating that the transplanted purinoceptor is linked to phospholipase C activity and triggers Ins(1,4,5)P3 formation. Ins(1,4,5)P3 production evoked by external ATP was clearly demonstrated by directly measuring the water-soluble Ins(1,4,5)P3 level in injected oocytes. Finally, it is suggested that the ATP effect was mediated by a Ca2+ release from Ins(1,4,5)P3 sensitive pools since heparin blocked the ATP responsiveness. The acquired purinoceptor may be made apparent to a P2 subtype since ATP and ADP were equipotent in eliciting Cl- current while AMP and Adenosine were ineffective in injected oocytes.

Two types of intrinsic muscarinic responses in Xenopus oocytes. I. Differences in latencies and 45Ca efflux kinetics

Oocytes of 40% of Xenopus laevis frogs respond to acetylcholine (ACh). Oocytes of the majority of responders exhibit the common two-component depolarizing muscarinic response (mean amplitude of the rapid component, 54 nA). Oocytes of approximately 10% of the responders ("variant" donors) exhibit a muscarinic response characterized by a very large transient, rapid current (mean amplitude 1242 nA, reversal potential -33 mV). Responses in oocytes of variant donors exhibit further qualitative differences: pronounced desensitization (absent in oocytes of common donors), characteristic prolonged latency (5.4 vs 0.9 s in oocytes of common donors) and marked inhibition of the response by activators of protein kinase C. Rapid responses in oocytes of variant donors are usually increased by treatment with collagenase, which, in common oocytes, often results in a complete loss of the response that correlates with the loss of muscarinic ligand binding. The number of muscarinic receptors was similar in oocytes of both types of donors (2.2 vs 3.0 fmol/oocyte). Also, the responses of oocytes of variant donors to microinjections of CaCl2 or inositol 1,4,5-trisphosphate were similar to those found in cells of common donors. These findings imply that altered receptor number, calcium stores and/or chloride channel density are not responsible for the variant responses. However, ACh caused an sixteen-fold greater efflux of 45Ca in oocytes of variant donors (35 vs 2.2% of total label in oocytes of common donors). Hence, the characteristics of the variant response may be related to a more efficient coupling between receptor stimulation and the mobilization of cellular calcium.

Diversity in responses from endogenous and expressed mammalian receptors which cause chloride ion efflux from ovarian follicles of Xenopus laevis

Inositol phosphates are produced in ovarian follicles of Xenopus laevis on activation of endogenous acetylcholine receptors, which also stimulates Ca2+ release and efflux of Cl- ions detected electrophysiologically. Inositol phosphates were not detectable on activation of endogenous angiotensin II receptors which did, however, stimulate both a dose-dependent Ca2+ efflux and a depolarizing current very similar in maximum size and other characteristics to those caused by acetylcholine action. In contrast, activation of exogenous receptors for angiotensin II expressed by microinjected mRNA extracted from bovine adrenal did form measurable inositol phosphates. Also, the endogenous electrophysiological responses to angiotensin II and acetylcholine desensitize homologously but fail to cross-desensitize (Lacy, McIntosh, and McIntosh, 1989, Biochem. Biophys. Res. Commun, 159, 658-663). It appears that endogenous ovarian angiotensin II receptors in Xenopus activate a different transduction mechanism from endogenous acetylcholine receptors and expressed mammalian adrenal angiotensin II receptors and/or may be sited in the electrically connected follicular cells rather than in the oocyte itself.

GABAB receptors expressed in Xenopus oocytes by guinea pig cerebral mRNA are functionally coupled with Ca2(+)-dependent Cl- channels and with K+ channels, through GTP-binding proteins

Transmembrane currents induced by (-)-baclofen (BAC), a specific agonist of the gamma-aminobutyric acid-B (GABAB) receptor, in Xenopus oocytes injected with guinea pig cerebral mRNA were electrophysiologically and pharmacologically characterized under a voltage-clamp condition. The oocytes injected with mRNA acquired responsiveness to BAC and showed two types of currents at a holding potential of -50 mV. One was the slow and smooth inward current which had a short latency and associated with a decrease in membrane conductance, and its amplitude was decreased by hyperpolarization and increased by depolarization. The other was the large fast oscillatory inward current with a long-latency, which was decreased in amplitude by depolarization and reversed at -26 mV. Both currents were not blocked by bicuculline but were depressed by 2-hydroxysaclofen (2-OH-SAC), though the smooth current was less sensitive to 2-OH-SAC; about 40% blockade at the 2-OH-SAC concentration capable of abolishing the oscillatory current. The smooth current was depressed by Ba2+. The oscillatory current was time-dependently attenuated and almost abolished by intracellularly injected pertussis toxin (PTX), while the smooth current was not depressed by this toxin even when the oscillatory current was nearly abolished. The intracellular injection of GTP-gamma-S into oocytes attenuated both oscillatory and smooth currents. These results suggest the possibility that GABAB receptors expressed in Xenopus oocytes by cerebral mRNA are functionally coupled with two signal transduction systems, one is the opening of Ca2(+)-dependent Cl- channels mediated by PTX-sensitive GTP-binding protein(s) and the other is the closure of K+ channels through PTX-insensitive GTP-binding protein(s).

Expression of Histamine HReceptors in Xenopus Oocytes Injected with Messenger Ribonucleic Acid from Bovine Adrenal Medulla: Pertussis Toxin Insensitive Activation of Membrane Chloride Currents

Abstract Histamine H(2)-receptors have been identified in Xenopus oocytes previously microinjected with poly(A) + ribonucleic acid from bovine adrenal glands. Bath application of histamine to ribonucleic acid-primed oocytes evoked concentration-dependent, oscillating membrane currents under voltage-clamp conditions. H(1)-receptor specific antagonists clemastine, doxepin, pyrilamine, promethacine, diphenylhydramine, dephenylpyraline and chlorpheniramine, but not H(2)-receptor antagonists, cimetidine and ranitidine, inhibited histamine-induced responses. Membrane currents evoked by bath-applied histamine were insensitive to pertussis toxin, carried by chloride ions and dependent on intracellular but not extracellular calcium.

Interaction between injected Ca2+ and intracellular Ca2+ stores in Xenopus oocytes

Upon two repetitive deep injections of Ca2+ into Xenopus oocyte (200-300 microns under the membrane), the amplitude of the transient Cl- current induced by the second injection is several-fold higher than that of the first one. This 'potentiation' persists even at 60-90 min intervals between injections. However, in oocytes permeabilized to Ca2+ by the ionophore A23187 in a Ca2(+)-free solution, the potentiation completely disappears after 30 min. It is proposed that the injected Ca2+ is largely taken up by the stores, whereas following the second injection, a higher proportion of Ca2+ reaches the membrane, since the stores are already loaded. In ionophore-treated oocytes, the stores lose the accumulated Ca2+ over several minutes and are then ready to take up Ca2+ again, hindering its arrival at the membrane.

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.

Functional expression of Ca2(+)-mobilizing alpha-thrombin receptors in mRNA-injected Xenopus oocytes

alpha-Thrombin (TH) initiates a program of intracellular events that lead to DNA replication in quiescent CCL39 Chinese hamster lung fibroblasts via membrane receptors that have yet to be characterized at a molecular level. Functional TH receptors were expressed in Xenopus laevis oocytes following injection of poly(A)+ RNA from TH-responsive CCL39 cells; their presence was demonstrated by TH-stimulated 45Ca2+ efflux or Ca2(+)-dependent Cl- channel activation. In voltage clamp experiments on microinjected oocytes a Ca2(+)-activated Cl- current was detected in response to TH (0.2-10 U/ml). The TH response was blocked by a specific TH inhibitor, and potentiated by addition of FGF or intracellular injection of GTP-gamma-S.

Serotonin receptors expressed in Xenopus oocytes by mRNA from brain mediate a closing of K+ membrane channels

Membrane currents evoked by serotonin (5-HT) were studied in Xenopus oocytes injected with rat brain mRNA. Intracellular EGTA was used to abolish the Ca2(+)-dependent oscillatory Cl- current to 5-HT, revealing an underlying smooth inward current. This was associated with a decreased membrane conductance, was antagonized by Ba2+ and Zn2+ (but not TEA), and probably arises through a closing of K+ channels. Half-maximal responses were obtained with 30 nM 5-HT, while 8-hydroxy-2-(di-n-propylamino)-tetralin (8-OH-DPAT) was ineffective. Furthermore, methysergide, mianserin and lysergic acid antagonized the K(+)-closing response to 5-HT, consistent with it being mediated through 5-HT1C receptors. The largest K(+)-closing responses were induced by a size fraction of mRNA which also induced a large K+ conductance, suggesting that the response requires expression of both receptors and K+ channels. The K(+)-closing response induced in the oocyte resembles the M- and S-type currents described in, respectively, mammalian and invertebrate neurons.

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.

Regional distribution of metabotropic glutamate response in the rat brain using Xenopus oocytes

The regional distribution of metabotropic L-glutamate responses was investigated in Xenopus oocytes injected with poly(A)+-RNA from a rat brain which was separated into 3 parts: cerebrum, cerebellum and brainstem. Under voltage-clamp, oscillatory current responses were induced more in cerebellum or brainstem poly (A)+-RNA-injected oocytes, and less in cerebrum poly(A)+-RNA-injected oocytes. These results suggest that the metabotropic glutamate receptor is distributed mostly in cerebellum and brainstem.

Modulation of the voltage-dependent sodium channel by agents affecting G-proteins: a study in Xenopus oocytes injected with brain RNA

The effects of agents known to affect G-proteins on voltage-dependent, tetrodotoxin-sensitive Na+ channels were studied in Xenopus oocytes injected with rat brain RNA, using two-electrode voltage-clamp technique. The non-hydrolysable analogue of GTP, GTP-gamma-S, known to activate G-proteins, inhibited the Na+ current (INa). The decrease in the amplitude of INa was not accompanied by changes in activation or inactivation characteristics of the channel. The non-hydrolysable analogue of GDP, GDP-beta-S, had no effect on INa. The responses to gamma-aminobutyric acid and kainate in the same oocytes were also attenuated by GTP-gamma-S. Pertussis toxin, which inactivates some G-proteins by catalyzing their ADP-ribosylation, enhanced INa, but did not prevent the inhibition of INa by GTP-gamma-S. We conclude that the Na+ channel, and possibly the GABA and kainate receptors and/or channels, are coupled to a G-protein. The activation of the G-protein modulates the channels either directly, or via activation of biochemical cascade possibly involving production of second messengers and channel phosphorylation.

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.

A specific quisqualate agonist inhibits kainate responses induced in Xenopus oocytes injected with rat brain RNA

Electrophysiological recording was used to study non-N-methyl-D-aspartate (NMDA) excitatory amino acid (EAA) receptors after injection of rat brain ribonucleic acid (RNA) in Xenopus laevis oocytes. Quisqualate (QA) induced two types of responses, a smooth one and an oscillatory one. These responses are probably mediated by the ionotropic (QAi, a cationic channel) and the metabotropic (QAp, a newly discovered receptor coupled to phospholipase C) QA receptors respectively. alpha-Amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid (AMPA) only induced a smooth inward current suggesting that it acts only on QAi. Kainate (KA) also induced a smooth inward current, the maximal KA response being 10-fold higher than the maximal AMPA. AMPA inhibited the KA response in a dose-dependent and competitive manner. Amongst various complex hypotheses the simplest to explain these results would be that KA and AMPA both activate the same receptor-channel complex, AMPA inducing a smaller response than KA.

Angiotensin II stimulates an endogenous response in Xenopus laevis ovarian follicles

While responses to angiotensin II have previously been induced in Xenopus laevis oocytes after injection of messenger RNA extracted from mammalian tissue, no endogenous responses of ovarian tissue to this hormone have been reported. Here we describe such an endogenous dose-dependent response to angiotensin II, detected by conventional electrophysiological techniques, in follicular oocytes. The ED50 of the response was estimated to be 0.15 +/- 0.07 microM (S.E.M.). Maximal depolarization, obtained at 1 microM angiotensin II, was 18.3 +/- 1.4 mV (n = 18, three experiments using oocytes from two toads, mean resting membrane potential = -42 +/- 2 mV). The response was absent from collagenase-treated oocytes or follicular oocytes treated with octanol, suggesting that the receptors are predominantly in the follicular layer surrounding the oocytes.

Molecular cloning and sequence determination of a cDNA coding for the alpha-subunit of a Go-type protein of Xenopus laevis oocytes

Xenopus laevis oocytes are cells ideally suited to the study of signal transduction and of the G-proteins that are involved in this process. A X. laevis cDNA library in lambda gt10 has been screened with a mixture of three oligonucleotide probes designed to detect sequences found in various mammalian alpha-subunits of G-proteins. One of these clones has been purified through tertiary screening and the DNA insert has been sequenced. This clone was found to include the total sequence coding for a 354 amino acid protein that is 89% identical to the sequence of alpha-subunit of rat Go. The differences with the mammalian protein were clustered in amino acids 290-315, which have been postulated to define the region interacting with the receptor and effector molecule. The homology with the alpha-subunits of other mammalian G-proteins is lower (65-70% to Gi and 42% to Gs). On this basis, this clone can be classified as Go-like.

G-proteins and egg activation

G-proteins are present in eggs, and experiments in which GTP-gamma-S, GDP-beta-S, cholera toxin and pertussis toxin have been injected into eggs have indicated the involvement of G-proteins in egg activation at fertilization and in oocyte maturation. Eggs into which serotonin or muscarinic acetylcholine receptors have been introduced by mRNA injection produce fertilization-like responses when exposed to serotonin or acetylcholine; since these neurotransmitter receptors act by way of G-proteins, this observation further supports the conclusion that a G-protein is involved in the fertilization process.