On the orientation of foreign neurotransmitter receptors in Xenopus oocytes

Xenopus oocytes can be made to incorporate into their membrane foreign neurotransmitter receptors and voltage-activated sodium channels. In their original location the receptors are normally activated by the extracellular action of transmitter substances. Tests were made to see if some of the newly synthesized foreign receptors were inserted in the oocyte membrane with their active site facing inwards. Since intracellular injections of acetylcholine, gamma-aminobutyric acid, serotonin and kainic acid and tetrodotoxin into the oocyte failed to elicit a response, we conclude that very few, or none, of the receptor molecules expressed in the oocyte by the exogenous mRNA are inserted with the wrong orientation in the membrane.

Post-synaptic calcium influx at the giant synapse of the squid during activation by glutamate

Changes in free calcium were monitored in the post-synaptic axon of the giant synapse of the squid, using the calcium indicators aequorin and Arsenazo III. The peak size of the calcium-dependent optical signals recorded from aequorin and Arsenazo III both showed a linear relation with the amount of calcium injected ionophoretically into the axon, but the Arsenazo signal had a slower time course than the aequorin. Ionophoretic application of glutamate to the post-synaptic axon depolarized the axon and caused a rise in intracellular free calcium. Aequorin signals were detected in natural sea water, and their size increased when the calcium concentration in the sea water was raised. Arsenazo signals could be detected only in high-calcium (55 mM) sea water. Intracellular calcium signals were detected also during bath application of several glutamate analogues, including kainate, ibotenate, and aspartate. The peak amplitude of the intracellular calcium signal, monitored with both indicators, increased with increasing ionophoretic glutamate dose, and varied linearly with the integral of the glutamate-induced membrane depolarization. No calcium signals were detected when depolarizations, similar to those produced by glutamate, were induced by current injection in the absence of glutamate. We conclude that glutamate increases the calcium permeability of the post-synaptic membrane, independently of the glutamate-induced depolarization. The glutamate-induced depolarization and the rise in intracellular free calcium increased roughly linearly as the membrane potential was made more negative. Extrapolation of these data indicated that the glutamate depolarization would reduce to zero at about -30 mV, while the calcium signals would be suppressed at about +50 mV.

Intracellular Ca2+-dependent and Ca2+-independent responses of rat brain serotonin receptors transplanted to Xenopus oocytes

Xenopus oocytes injected with messenger RNA extracted from rat brain are induced to acquire a variety of neurotransmitter receptors and voltage-operated membrane channels. Activation of the receptors to serotonin, acetylcholine (muscarinic) and glutamate elicits oscillatory membrane currents carried by chloride ions. These currents are not abolished by removing external calcium, but are completely abolished after EGTA is injected into the oocytes to chelate intracellular calcium. A smooth current response to serotonin remained in EGTA-loaded oocytes, indicating that this response does not require intracellular calcium. In contrast to the oscillatory chloride currents, the chloride currents activated by GABA or glycine are not abolished by intracellular injection of EGTA. Thus, there appear to be two classes of chloride channels one of which requires intracellular calcium to open.

Messenger RNA from bovine retina induces kainate and glycine receptors in Xenopus oocytes

The retina contains several types of nerve cells that communicate through chemical synapses. The transmitter and receptor molecules that mediate signal transmission across these synapses need further characterization. For this purpose, poly (A)+ mRNA was isolated from bovine retinas and injected into Xenopus laevis oocytes. Translation of the foreign mRNA induced the oocyte membrane to acquire functional receptors to kainate and, to a lesser extent, also receptors to glycine, gamma-aminobutryic acid (GABA), aspartate and glutamate. Thus, the cells in the retina must contain different messengers coding for these neurotransmitter receptors. Activation of the kainate receptors opens membrane channels, generating an ionic current which has an equilibrium potential close to 0 mv. The current is well maintained during prolonged application of kainate, and hence these receptors may be involved in the neurotoxic effects produced by kainate in the retina.

A limited survey of retail apple and grape juices for the mycotoxin patulin

A retail survey (38 samples) of apple, grape and other juice-based products for the mycotoxin patulin has been carried out using HPLC analysis. Positive results were confirmed by GC-MS of the TMS derivative using chemical ionization selected ion monitoring. No grape juice or mixed juice product contained detectable patulin (limit 5 micrograms/l), nor did the majority (58%) of the apple juices. Low levels of patulin (in the range 5-10 micrograms/l) were present in six apple juices and the remaining four positive samples contained 16, 18, 30 and 56 micrograms/l.

A transient inward current elicited by hyperpolarization during serotonin activation in Xenopus oocytes

Activation of serotonin, glutamate or muscarinic receptors, incorporated into the membrane of Xenopus oocytes following injection of messenger RNA from rat brain, caused the development of a transient inward (Tin) current when the membrane was hyperpolarized. A detailed study was made of the Tin current induced during serotonin activation. The current is due principally to efflux of chloride ions, and is presumably activated by an influx of calcium ions, because it was blocked by removal of calcium from the bathing medium, by addition of manganese, cobalt or lanthanum, or by intracellular injection of EGTA. During application of serotonin, the amplitude of the Tin current increased slowly, and after washing it persisted for longer than the direct serotonin-induced current. The amplitude of the Tin current was sensitive to temperature and pH, and was abolished at pH 6.5 or by cooling to 12 degrees C. The Tin current may be of importance in regulating the excitability of neurons in the central nervous system.

Effect of Prazosin on the Efflux of ³H-Norepinephrine and Metabolites from the Intima and Adventitia of the Rabbit Ear Artery

The spontaneous and stimulation-induced (SI) effluxes of ³H-norepinephrine (³H-NE) and its metabolites from the intimal and adventitial surfaces of perfused segments of rabbit ear arteries were determined; vessels were previously incubated with ³H-NE (0.6 µM). The total SI adventitial efflux of ³H was approximately 10-fold greater than the intimal efflux, contained a higher percentage of unchanged ³H-NE (48 vs. 12%) and a lower percentage of O-methylated metabolites (17 vs. 55%); there was little difference between the percentages of deaminated catechols (35 vs. 31 %). Prazosin, at a concentration (0.24 µM) which prevented the arteries constricting during stimulation, had little effect on the composition of the SI effluxes; however, it caused 2- to 3-fold increases in the effluxes of ³H-NE and its metabolites into the lumen during the period of stimulation. This effect is attributed to the failure of the vessel wall to thicken during stimulation, thus facilitating diffusion of ³H-NE and its intra-neuronally formed metabolites across the media. Prazosin decreased the percentage of unchanged ³H-NE and increased that of the deaminated catechols in the spontaneous efflux; these effects are attributed to a direct effect of prazosin on the intra-neuronal metabolism of ³H-NE.

Messenger RNA from rat brain induces noradrenaline and dopamine receptors in Xenopus oocytes

Xenopus oocytes were induced to acquire sensitivity to noradrenaline and dopamine, by injecting them with poly(A)+ mRNA isolated from rat brain. In mRNA-injected oocytes, both neurotransmitters elicited a smooth inward membrane current on which was superimposed an oscillatory inward current, which was carried mainly by chloride ions. This contrasts with the native responses that are sometimes seen in non-injected oocytes, where noradrenaline and dopamine both elicit smooth outward currents that are carried mainly by potassium ions. The serotonin antagonist methysergide blocked the induced responses to both noradrenaline and dopamine, and the noradrenaline response was blocked by propranolol.

Partial purification and functional expression of brain mRNAs coding for neurotransmitter receptors and voltage-operated channels

Poly(A)+ mRNAs extracted from embryonic chicken brain and from adult rat brain were fractionated on sucrose density gradients. The fractions were subsequently injected into Xenopus oocytes where the mRNA was translated. The products were processed and incorporated into the oocyte membrane where they formed functional neurotransmitter receptors and voltage-operated channels. Different mRNA fractions induced the incorporation of different transmitter receptors and voltage-operated channels into the oocyte membrane. These experiments provide a useful step towards the understanding of the structure and function of neurotransmitter receptors and channels.

Chloride current induced by injection of calcium into Xenopus oocytes

Membrane currents of Xenopus oocytes were studied with the membrane under voltage clamp. Intracellular injection of the calcium-chelating agent EGTA reduced, or abolished, the transient outward chloride current normally activated by membrane depolarization. Intracellular injection of calcium ions evoked large membrane currents, which inverted direction close to the chloride equilibrium potential. Injections of strontium, or barium, were less effective than calcium, while magnesium was ineffective. Large chloride currents could be evoked by calcium injections in oocytes which showed only small or no transient outward currents. The current activated by calcium injection increased with increasing depolarization up to high (ca. +60 mV) positive potentials, even though the transient outward current was suppressed by strong depolarization. The results indicate that the transient outward current depends upon entry of calcium through voltage-gated calcium ion channels and show that the oocyte membrane contains numerous chloride channels which are activated by intracellular calcium. Only a few of these chloride channels are activated by depolarization.

Separate fractions of mRNA from Torpedo electric organ induce chloride channels and acetylcholine receptors in Xenopus oocytes

Poly(A)+ mRNA extracted from the electric organ of Torpedo was fractionated by sucrose density gradient centrifugation. After injection into Xenopus oocytes one mRNA fraction induced the appearance of chloride channels in the oocyte membrane. Many of these channels were normally open, and the ensuing chloride current kept the resting potential of injected oocytes close to the chloride equilibrium potential. When the membrane was hyperpolarized, the chloride current was reduced. A separate fraction of mRNA induced the incorporation of acetylcholine receptors into the oocyte membrane. When translated in a cell-free system this fraction directed the synthesis of the alpha, beta, gamma, and delta subunits of the acetylcholine receptor. In contrast, the mRNA fraction that induced the chloride channels caused the synthesis of the delta subunit, a very small amount of alpha, and no detectable beta or gamma subunits. This suggests that the size of the mRNA coding for the chloride channel is similar to the preponderant species of mRNA coding for the delta subunit of the acetylcholine receptor.

Slowly inactivating potassium channels induced in Xenopus oocytes by messenger ribonucleic acid from Torpedo brain

Poly(A+) messenger RNA was extracted from the electric lobe and medulla of Torpedo and injected into oocytes of Xenopus laevis. The synthesis and processing of proteins coded by the injected messenger RNA led to the incorporation of voltage-activated channels in the oocyte membrane. A large, well maintained outward current was recorded from injected oocytes in response to depolarization. Non-injected oocytes did not show this current. The reversal potential of the current varied according to the Nernst equation with external potassium concentration, indicating that it was largely carried by potassium ions. The maintained potassium current was not reduced by manganese (5 mM) or lanthanum ions (0.1 mM). Tetraethylammonium and aminopyridines blocked the potassium current. The block produced by 3,4-diaminopyridine was enhanced by previous activation, but diminished by strong depolarization. The amplitude of the potassium current increased over the approximate voltage range -30 to +50 mV, but reduced at more positive potentials. The decline of the current during maintained depolarization became slower as the membrane potential was made more positive, and the rate of onset of the current became faster. Estimates from noise analysis indicated that the slow potassium current passes through channels with a mean lifetime of about 14 ms and conductance of 14 pS (at -10 mV and room temperature). Injection of the messenger RNA also induced the formation of fast sodium and potassium channels activated by voltage, and channels activated by kainate.

Extracellular ions and excitation-contraction coupling in frog twitch muscle fibres

Intracellular calcium transients were recorded from voltage-clamped frog twitch muscle fibres using Arsenazo III. The possible role of extracellular ions in excitation-contraction (e.-c.) coupling was examined using ion substitutions and blocking drugs in the bathing medium. Parameters measured included the Arsenazo response size to a standard depolarizing pulse (5 ms, 0 mV) and the strength-duration curve for threshold Arsenazo signal. Addition of tetrodotoxin (TTX) decreased the response size to small (-30 mV, 5 ms), but not large (+30 mV, 10 ms) depolarizations, probably because of poor voltage clamp of the tubular membrane in the absence of TTX. Clamping TTX-treated fibres with the wave form of a recorded action potential gave an Arsenazo response similar to that elicited by the normal action potential (at 10 degrees C). Complete substitution of sodium (by choline, lithium or Tris) or chloride (by methyl sulphate or maleate) in the bathing solution gave no appreciable changes in the size of the Arsenazo response. Reduction of extracellular free [Ca2+] to low levels using EGTA caused a slight reduction in the calcium signal elicited by the standard depolarization (to 74% after a few hours, and to 62% after 2 days; temperature 5-10 degrees C). The strength-duration curve was unchanged. Arsenazo responses about 75% of the control size could be elicited in high potassium solution (42 mM-K2SO4) by strong (+80 mV, 20 ms) depolarizations, after re-polarizing the fibres to -90 mV for a few minutes. The voltage dependence of activation was shifted to more positive potentials in this solution. Tetraethylammonium (TEA) bromide at a concentration of 20 mM did not alter the Arsenazo signal, whilst 120 mM-TEA reduced the response by 25%. 3,4-diaminopyridine (DAP) reduced the size of the Arsenazo signal at a concentration of 5 mM, and caused spontaneous release of calcium from the sarcoplasmic reticulum (s.r.) in the absence of membrane potential changes. The Arsenazo signal elicited by an action potential was enhanced by 1 mM-DAP, because of prolongation of the action potential, but was depressed by higher concentrations. We conclude that e.-c. coupling does not involve the influx of any external ions into the muscle fibre. If a current flow between the T-tubules and the s.r. is involved in e.-c. coupling, then this is probably carried by an efflux of potassium ions.

Properties of human brain glycine receptors expressed in Xenopus oocytes

Glycine and gamma-aminobutyric acid (GABA) receptors from the foetal human brain were 'transplanted' into the Xenopus oocyte membrane by injecting the oocytes with poly(A)+-mRNA extracted from the cerebral cortex. Activation of both glycine and GABA receptors induced membrane currents carried largely by chloride ions. However, unlike the GABA-activated current, the glycine current was blocked by strychnine, and was not potentiated by barbiturate. At low doses, the glycine current increased with concentration following a 2.7th power relation, suggesting that binding of three molecules of glycine may be required to open a single membrane channel. The current induced by steady application of glycine decreased with hyperpolarization beyond about -60 mV.

Glutamate and kainate receptors induced by rat brain messenger RNA in Xenopus oocytes

Xenopus laevis oocytes injected with poly(A)+ mRNA extracted from rat brain became sensitive to serotonin, glutamate, kainate, acetylcholine and gamma-aminobutyrate. Application of these substances to mRNA-injected oocytes elicited membrane currents. The glutamate- and acetylcholine-induced currents usually showed oscillations, while the kainate current was smooth. The current oscillations during glutamate application reversed direction at about the chloride equilibrium potential (-24 mV), but the reversal potential for the kainate current was close to 0 mV. The current-voltage relation for the glutamate-induced current oscillations showed strong rectification at hyperpolarized potentials, while that for the kainate current was nearly linear. In some oocytes, glutamate elicited smooth membrane currents, with oscillations either absent, or appearing after a delay. The reversal potential of this component was close to 0 mV, and was clearly different from that of the oscillatory component. The appearance of glutamate and kainate sensitivity in the oocyte membrane is due to the translation of the foreign messenger RNA, and not to activation of the oocytes' own genome, because oocytes still become sensitive when transcription is prevented by enucleation or by treatment with actinomycin D. It appears that mRNA from rat brain contains translationally active messengers which code for various neurotransmitter receptors. When this mRNA is injected into Xenopus oocytes, the messengers are translated and receptors are inserted into the oocyte membrane, where they form functionally active receptor-channel complexes.

Messenger RNA from human brain induces drug- and voltage-operated channels in Xenopus oocytes

Sodium channels and receptors to serotonin and kainate were 'transplanted' from human brain into frog oocytes, by isolating messenger RNA from a fetal brain, and injecting it into Xenopus laevis oocytes. The mRNA was translated by the oocyte and induced the appearance of functional receptors and channels in its membrane. This approach renders drug- and voltage-operated channels of the human brain more amenable to detailed study.

Voltage-operated channels induced by foreign messenger RNA in Xenopus oocytes

Poly(A)+ messenger RNA (mRNA) extracted from rat brains or from cat muscles was injected into Xenopus laevis oocytes. This led to the incorporation of voltage-operated Na+ and K+ channels into the oocyte membrane. These channels are not normally present in the oocyte and presumably result from the synthesis and processing of proteins coded by the injected mRNA. Tetrodotoxin blocked the Na+ channels induced by mRNA derived from either innervated or denervated muscle.

Changes in threshold for calcium transients in frog skeletal muscle fibres owing to calcium depletion in the T-tubules

Strength-duration curves were measured for voltage-clamp depolarizations required to elicit a just detectable rise in intracellular calcium, as monitored using arsenazo III, in frog twitch muscle fibres. In normal Ringer solution, the threshold for a 5 sec duration depolarization was about 5 mV more negative than for a 200 msec duration pulse. The shift in threshold comparing 200 msec and 5 sec pulses was almost abolished in bathing solutions including magnesium or nickel (4 mM), or where the free calcium concentration was buffered. The shift in threshold was little changed by substitution of barium for calcium. These results can be explained by supposing that the 5 sec depolarization activates an inward calcium flux across the T-tubule membrane, which decreases the calcium concentration in the tubules, and hence alters the threshold for activation of excitation-contraction (e.-c.) coupling because of surface charge effects.

Serotonin receptors induced by exogenous messenger RNA in Xenopus oocytes

When poly(A)+-mRNA, extracted from rat brain, was injected into Xenopus laevis oocytes, it induced the appearance of serotonin receptors in the oocyte membrane. Application of serotonin to injected oocytes elicited, after a long delay, oscillations in membrane current. The equilibrium potential of this current corresponded with the chloride equilibrium potential. It appears that rat brain mRNA encodes the translation of serotonin receptors into the oocyte membrane. The combination of serotonin with these receptors leads to the opening of membrane channels.

Recording of single gamma-aminobutyrate- and acetylcholine-activated receptor channels translated by exogenous mRNA in Xenopus oocytes

High resolution ('giga-seal') patch clamp recording in Xenopus oocytes was used to measure single channel currents from ACh- and GABA-activated receptors. The proteins that make up these receptors had been translated from mRNA derived from, respectively, denervated cat muscle and chick optic lobe.

Calcium transients studied under voltage-clamp control in frog twitch muscle fibres

1. Intracellular calcium transients were recorded from frog twitch muscle fibres in response to voltage-clamped depolarizing pulses, using arsenazo III as an intracellular calcium monitor. The object was to investigate the time- and voltage-dependent characteristics of the coupling process between membrane depolarization and calcium release from the sarcoplasmic reticulum (s.r.)2. To examine the extent to which the T-tubule membrane potential was controlled during clamp pulses, the dye NK 2367 was used as an optical probe of tubular potential. This indicated that the tubular time constant is about 0.6 msec.3. Strength-duration curves were obtained for depolarizing pulses required to give both threshold mechanical contraction and calcium signal. Curves measured in these two ways were closely similar.4. Changes in holding potential altered the strength-duration curve for calcium release so that at more positive holding potentials a shorter pulse was needed to obtain a response for any given pulse amplitude.5. A latency of a few milliseconds was observed between the onset of depolarization and the initial rise of the calcium signal. This became shorter with stronger depolarizations, but approached a minimum at potentials above about +25 mV.6. Subthreshold depolarizations applied before a test pulse increased the size and decreased the latency of the calcium signal. Conditioning hyperpolarizations had opposite effects.7. The rate of build-up of potentiation or depression of response size seen with subthreshold de- and hyperpolarizing conditioning pulses was examined using conditioning pulses of different durations. For both pulses this process showed a time constant of about 3 msec (at 10 degrees C).8. The rate of decay of potentiation or depression was similarly measured, using a gap of variable duration between conditioning and test pulses. For both de- and hyperpolarizing pulses this showed a time constant of about 5 msec (10 degrees C).9. The relationship between conditioning pulse potential, and the size of calcium signal elicited by a following test pulse was non-linear.10. Subthreshold pulses immediately following a brief test pulse affected the size of the calcium signal in a similar way to preceding conditioning pulses.11. The relationship between potential and size of the calcium signal was examined using pulses of 3 and 20 msec duration. With the long pulse the relation was roughly sigmoid, but with the short pulse continued to rise even at strongly positive potentials.12. The results are discussed in terms of a model in which the exponential build-up of a hypothetical coupler in the excitation-contraction (e.-c.) coupling process is presumed to lead to calcium release when a threshold level is exceeded.

Calcium transients in frog skeletal muscle fibres following conditioning stimuli

1. Intracellular Ca(2+) transients were recorded from frog twitch muscle fibres, using arsenazo III as a Ca(2+) monitor. When fibres were stimulated by two action potentials, the arsenazo signal to the second stimulus was smaller than the first, for stimulus intervals of up to several seconds.2. The recovery of the amplitude of the second response followed two exponential time courses; a fast one with a time constant of about 70 msec giving recovery to about 90% of the control value, followed by a slow recovery to 100%, with a time constant of about 12 sec (at 10 degrees C).3. The time constant of the fast recovery component was strongly temperature-dependent, with a Q(10) of approximately 2.7, whilst the Q(10) of the slow component was about 1.4.4. Removal of Ca(2+) in the bathing medium lengthened the time constant of the slow recovery component by a factor of three, but had little effect on the fast recovery component. The lengthening of the slow component was not reversed by addition of Mg(2+), but Sr(2+) ions could substitute for Ca(2+).5. The influence of membrane potential on the recovery time-course was investigated after blocking action potentials with tetrodotoxin, using a voltage clamp to control membrane potential. Paired depolarizing stimuli were used, with the potential held to either low (-60 or -80 mV) or high (-110 or -140 mV) potentials between stimuli. No differences were apparent in either the fast or slow recovery components at these holding potentials.6. The arsenazo response elicited by an action potential following a conditioning tetanus was reduced in size even more strongly than following a single action potential. The time course of recovery of the response following a tetanus again comprised two exponential components. After a 20 Hz tetanus for 0.5 sec, the fast component had a time constant of about 400 msec, and gave a recovery to about 60% of the control value. Subsequent recovery to 100% occurred with a time constant of about 12 sec.7. The time constant of the fast recovery component increased markedly with increasing frequency or duration of the conditioning tetanus. The time constant of the slow component was not appreciably altered by conditioning tetani varying between one impulse and sixty impulses. However, the reduction in response size due to the slow component, extrapolated to zero stimulus interval, increased with increasing number of impulses in the tetanus.8. The time constant of the fast recovery component corresponded closely with the decay time constant of the arsenazo response to the conditioning stimulus. This correspondence held over a nearly fifty-fold range of time constants, and for two different conditions which affected the decay time constant (temperature, and frequency of tetanic stimulation).9. The decay time constant of the arsenazo response elicited by an action potential was slowed by a preceding impulse or tetanus. Following a 20 Hz tetanus for 0.5 sec, recovery of the half decay time appeared to follow an exponential time course, with a time constant of about 12 sec.10. These results suggest that the fast recovery component reflects the re-filling of release stores in the sarcoplasmic reticulum by Ca(2+) ions taken up from the cytoplasm. The origin of the slow component is less clear, but it may arise from inactivation of the excitation-contraction (e-c) coupling process between T-tubule depolarization and Ca(2+) release from the sarcoplasmic reticulum.

Synthesis of chick brain GABA receptors by frog oocytes

Poly(A)-mRNA, extracted from the optic lobe of chick embryos, directs the synthesis of gamma-aminobutyric acid (GABA) receptors in Xenopus laevis oocytes. The receptors are inserted into the oocyte membrane, where they form receptor--channel complexes. When activated by GABA, and related agonists, the chick brain receptors open membrane channels that are permeable to chloride ions. Thus, Xenopus oocytes provide a novel and useful approach to the study of brain receptors.

Properties of acetylcholine receptors translated by cat muscle mRNA in Xenopus oocytes

Poly(A)+ mRNA, extracted from denervated skeletal muscles of the cat, directs the synthesis of acetylcholine receptors in Xenopus laevis oocytes. The receptors are inserted in the oocyte membrane where they form acetylcholine receptor-channel complexes which have properties like those of the native receptors in the muscle membrane.