Enhanced Archaerhodopsin Fluorescent Protein Voltage Indicators

A longstanding goal in neuroscience has been to develop techniques for imaging the voltage dynamics of genetically defined subsets of neurons. Optical sensors of transmembrane voltage would enhance studies of neural activity in contexts ranging from individual neurons cultured in vitro to neuronal populations in awake-behaving animals. Recent progress has identified Archaerhodopsin (Arch) based sensors as a promising, genetically encoded class of fluorescent voltage indicators that can report single action potentials. Wild-type Arch exhibits sub-millisecond fluorescence responses to trans-membrane voltage, but its light-activated proton pump also responds to the imaging illumination. An Arch mutant (Arch-D95N) exhibits no photocurrent, but has a slower, ~40 ms response to voltage transients. Here we present Arch-derived voltage sensors with trafficking signals that enhance their localization to the neural membrane. We also describe Arch mutant sensors (Arch-EEN and -EEQ) that exhibit faster kinetics and greater fluorescence dynamic range than Arch-D95N, and no photocurrent at the illumination intensities normally used for imaging. We benchmarked these voltage sensors regarding their spike detection fidelity by using a signal detection theoretic framework that takes into account the experimentally measured photon shot noise and optical waveforms for single action potentials. This analysis revealed that by combining the sequence mutations and enhanced trafficking sequences, the new sensors improved the fidelity of spike detection by nearly three-fold in comparison to Arch-D95N.     

Propagation of action potentials in squid giant axons. Repetitive firing at regions of membrane inhomogeneities

Effects of reduction in potassium conductance on impulse conduction were studied in squid giant axons. Internal perfusion of axons with tetraethylammonium (TEA) ions reduces G K and causes the duration of action potential to be increased up to 300 ms. This prolongation of action potentials does not change their conduction velocity. The shape of these propagating action potentials is similar to membrane action potentials in TEA. Axons with regions of differing membrane potassium conductances are obtained by perfusing the axon trunk and one of its two main branches with TEA after the second branch has been filled with normal perfusing solution. Although the latter is initially free of TEA, this ion diffuses in slowly. Up until a large amount of TEA has diffused into the second branch, action potentials in the two branches have very different durations. During this period, membrane regions with prolonged action potentials are a source of depolarizing current for the other, and repetitive activity may be initiated at transitional regions. After a single stimulus in either axon region, interactions between action potentials of different durations usually led to rebound, or a short burst, of action potentials. Complex interactions between two axon regions whose action potentials have different durations resembles electric activity recorded during some cardiac arrhythmias.     

"Action potentials" in Neurospora crassa, a mycelial fungus.

Occasional spontaneous "action potentials" are found in mature hyphae of the fungus Neurospora crassa. They can arise either from low-level sinusoidal oscillations of the membrane potential or from a linear slow depolarization which accelerates into a rapid upstroke at a voltage 5-20 mV depolarized from the normal resting potential (near-180 mV). The "action potentials" are long-lasting, 1-2 min and at the peak reach a membrane potential near-40 mV. A 2-to 8-fold increase of membrane conductance accompanies the main depolarization, but a slight decrease of membrane conductance occurs during the slow depolarization. Two plausible mechanisms for the phenomenon are (a) periodic increases of membrane permeability to inorganic ions, particularly H+ or Cl- and (b) periodic decreases in activity of the major electrogenic pump (H+) or the Neurospora membrane, coupled with a nonlinear (inverse signoid) current-boltage relationship. Identification of action potential-like disturbances in fungi means that such behavior has now been found in all major biologic taxa which have been probed with suitable electrodes. As yet there is no obvious function for the events in fungi.     

Selective photostimulation of genetically chARGed neurons.

To permit direct functional analyses of neural circuits, we have developed a method for stimulating groups of genetically designated neurons optically. Coexpression of the Drosophila photoreceptor genes encoding arrestin-2, rhodopsin (formed by liganding opsin with retinal), and the alpha subunit of the cognate heterotrimeric G protein--an explosive combination we term "chARGe"--sensitizes generalist vertebrate neurons to light. Illumination of a mixed population of neurons elicits action potentials selectively and cell-autonomously in its genetically chARGed members. In contrast to bath-applied photostimulants or caged neurotransmitters, which act indiscriminately throughout the illuminated volume, chARGe localizes the responsiveness to light. Distributed activity may thus be fed directly into a circumscribed population of neurons in intact tissue, irrespective of the spatial arrangement of its elements.     

Transmitter control of voltage-dependent currents

At one time neurotransmitters were thought of as chemical agents that simply depolarized or hyperpolarized a postsynaptic cell. Now it is known that transmitters can do much more. Biochemical processes, most notably the consequences of activation of adenylate cyclase, are subject to neurotransmitter control. Transmitters can alter a cell's sensitivity to another neurotransmitter; this is exemplified by the action of aspartate in enhancing responses to glutamate. Another action of transmitters is the subject of this review: control of voltage-dependent neuronal currents.Voltage-dependent processes are necessary for the normal function of neuronal systems. Potassium, sodium, and calcium currents that turn on and off with changes in membrane potential are responsible for action potentials and slow-wave (or burst firing) activity. Transmitter control of these ionic currents allows direct synaptic regulation of basic electrophysiological events.Discussion of the voltage-dependent actions of transmitters in neuronal systems will be divided into four areas: (a) broadening and narrowing of action potentials, (b) modulation of burst firing activity, (c) blockade of a voltage-dependent potassium conductance, and (d) induction of a voltage-dependent calcium current. The membrane currents underlying voltage-dependent events will be reviewed only as necessary to understanding transmitter effects. The reader is referred to a recent review for further details on some of these currents (1).     

Functional modulation of brain sodium channels by cAMP-dependent phosphorylation.

Voltage-gated Na+ channels, which are responsible for the generation of action potentials in brain, are phosphorylated by cAMP-dependent protein kinase in vitro and in intact neurons. Phosphorylation by cAMP-dependent protein kinase reduces peak Na+ currents 40%--50% in membrane patches excised from rat brain neurons or from CHO cells expressing type IIA Na+ channels. Inhibition of basal cAMP-dependent protein kinase activity by transfection with a plasmid encoding a dominant negative mutant regulatory subunit increases Na+ channel number and activity, indicating that even the basal level of kinase activity is sufficient to reduce Na+ channel activity significantly. Na+ currents in membrane patches from kinase-deficient cells were reduced up to 80% by phosphorylation by cAMP-dependent protein kinase. These effects could be blocked by a specific peptide inhibitor of cAMP-dependent protein kinase and reversed by phosphoprotein phosphatases. Convergent modulation of brain Na+ channels by neurotransmitters acting through the cAMP and protein kinase C signaling pathways may result in associative regulation of electrical activity by different synaptic inputs.     

Effects of external ions on membrane potentials of a lobster giant axon

The effects of varying external concentrations of normally occurring cations on membrane potentials in the lobster giant axon have been studied and compared with data presently available from the squid giant axon. A decrease in the external concentration of sodium ions causes a reversible reduction in the amplitude of the action potential and its rate of rise. No effect on the resting potential was detected. The changes are of the same order of magnitude, but greater than would be predicted for an ideal sodium electrode. Increase in external potassium causes a decrease in resting potential, and a decrease in potassium causes an increase in potential. The data so obtained are similar to those which have been reported for the squid giant axon, and cannot be exactly fitted to the Goldman constant field equation. Lowering external calcium below 25 mM causes a reduction in resting and action potentials, and the occasional occurrence of repetitive activity. The decrease in action potential is not solely attributable to a decrease in resting potential. Increase of external calcium from 25 to 50 mM causes no change in transmembrane potentials. Variations of external magnesium concentration between zero and 50 mM had no measurable effect on membrane potentials. These studies on membrane potentials do not indicate a clear choice between the use of sea water and Cole's perfusion solution as the better external medium for studies on lobster nerve.     

Excitable membranes--object for evaluating the effect of heavy metal pollution

In acute experiments the interaction of heavy metals (CdCl2 and HgCl2) with neurotransmitters (ACh and 5HT) was studied on the excitable membrane of the identified neurons in the central nervous system of Helix pomatia L. (Gastropoda, Mollusca). It was shown that cadmium and mercury ions exert different influence on both resting and action potentials as well as on the responsiveness of the neural membranes to ACh and 5HT. The selective blocking effect of cadmium and mercury ions can be interpreted on the basis of specificity of postsynaptic receptor structures responsible for the transmitter action and of ion-channels involved in the excitatory processes. The heavy metal effect was not uniform for the different types of neurons, suggesting that pollutants can modify various functions to a different degree. The results show that testing on nerve cell membranes can serve as a useful method and model in investigating the effect of sublethal environmental contamination, as they may cause a profound modulation on the elements of the neural circuitry responsible for the regulation of the animal's behavior.     

Electrophysiology of identified neurosecretory and non-neurosecretory cells in the cockroach pars intercerebralis

Two cell types can be distinguished with intracellular recording from the pars intercerebralis of the American cockroach (Periplaneta americana). The first type, which corresponds morphologically to the medial neurosecretory cell, always had spontaneously occurring, overshooting action potentials. These action potentials are probably endogenously produced. Tetrodotoxin experiments revealed that sodium is the dominant ion of the action potential. The action potentials are followed by a relatively long after-hyperpolarization. The input resistance of these cells ranged from 120 to 390 M omega. A mathematical model, based on cellular morphology and response to current pulses, revealed a membrane time constant of about 100 msec and an axonal:somatic conductance ratio of approximately 13. Area-specific membrane resistance was estimated at 33 k omega cm2. These cells also often had reversible and spontaneous inhibitory postsynaptic potentials. The second cell type, which is non-neurosecretory, never produced spontaneous action potentials and rarely had synaptic potentials. Action potentials could be evoked by current injection into the cell body or by extracellular stimulation of their axons in the posteroventral portion of the the protocerebrum. These action potentials also depend on sodium ions. Their input resistance ranged from 16 to 35 M omega. They had a membrane time constant of approximately 15 msec and an axonal:somatic conductance ratio of about 9. Their area specific membrane resistance was estimated at 14 k omega cm2.     

GABAergic neurotransmission and retinal ganglion cell function

Ganglion cells are the output retinal neurons that convey visual information to the brain. There are ~20 different types of ganglion cells, each encoding a specific aspect of the visual scene as spatial and temporal contrast, orientation, direction of movement, presence of looming stimuli; etc. Ganglion cell functioning depends on the intrinsic properties of ganglion cell’s membrane as well as on the excitatory and inhibitory inputs that these cells receive from other retinal neurons. GABA is one of the most abundant inhibitory neurotransmitters in the retina. How it modulates the activity of different types of ganglion cells and what is its significance in extracting the basic features from visual scene are questions with fundamental importance in visual neuroscience. The present review summarizes current data concerning the types of membrane receptors that mediate GABA action in proximal retina; the effects of GABA and its antagonists on the ganglion cell light-evoked postsynaptic potentials and spike discharges; the action of GABAergic agents on centre-surround organization of the receptive fields and feature related ganglion cell activity. Special emphasis is put on the GABA action regarding the ON–OFF and sustained–transient ganglion cell dichotomy in both nonmammalian and mammalian retina.     

Modulation of electrical activity of neuron RPa2 ofHelix pomatia

Neuron RPa2 ofHelix pomatia can generate rhythmic (beating) or periodic (bursting) activity. A spontaneous switch from beating to bursting activity takes place in the course of tens of minutes. Similar changes in electrical activity can be induced by the addition of the water-soluble fraction obtained from a homogenate of snail ganglia to the experimental chamber. Artificial polarization of the membrane of neuron RPa2 by asteady inward current leads to an increase in the duration of intervals between bursts and to a decrease in the number of action potentials in the burst. With an increase in amplitude of the polarizing current, action potential generation ceases completely, but generation of waves of membrane potential persists. If the voltage on the neuron membrane is clamped, periodic fluctuations of membrane current disappear. It is suggested that action potential generation by neurons RPa2 is determined by the properties of the potential-dependent conductance of its membrane, i.e., that it is endogenous in origin and can be regulated by compounds acting on the membrane. These compounds, secreted by other neurons, resemble neurotransmitters or neurohormones.A. A. Bogomolets Institute of Physiology, Academy of Sciences of the Ukrainian SSR, Kiev. Translated from Neirofiziologiya, Vol. 13, No. 4, pp. 406–412, July–August, 1981.     

Slow potentials in taste cells induced by frog glossopharyngeal nerve stimulation

Intracellular recordings were made from the taste cells of atropinized bullfrogs while the glossopharyngeal (GP) nerve fibres were electrically stimulated. Two types of slow potential, slow hyperpolarizing potentials (HPs) and slow depolarizing potentials (DPs), were induced in the taste cells. The slow HPs appeared when the lingual capillary blood flow was kept above 0.7 mm/s, whereas the slow DPs appeared when the blood flow was slowed down below 0.7 mm/s. The membrane resistance of a taste cell increased during the generation of a slow HP, but decreased during the generation of a slow DP. The reversal potentials for the slow HPs and the slow DPs were recorded at the same membrane potential (-11 to approximately -13 mV). Activation of non-selective cation channels possibly induced the slow DP and inactivation of those channels possibly induced the slow HP in the taste cell membrane. Electrical stimulation of the GP nerve activated a population of C fibres in the nerve and possibly released neurotransmitters from the nerve terminals. Released neurotransmitters might cause modulation of the membrane conductance in taste cells that leads to generation of the slow potentials. The present data suggest that slow HPs and slow DPs evoked in the taste cells of atropinized frogs by GP nerve stimulation are induced by putative neurotransmitters in the taste disc.     

The effects of cations upon the action potentials recorded from neurohaemal tissue of the stick insect

Summary The ionic requirement for the action potentials recorded from the neurohaemal tissue on the lateral branch of the median nerve inCarausius morosus has been studied using extracellular electrodes. Sodium-free, magnesium-free, or calcium-free salines produce irreversible block of the action potentials following prolonged exposure to the nerves. Reducing the sodium concentration to 4 mM has little effect on the amplitude of the action potentials, whilst increasing the sodium concentration to 100 mM reduces the amplitude by 50%. Neither tetrodotoxin nor procaine has any effect on these action potentials.Reducing the magnesium concentration to 1 mM increases the amplitude of the action potentials, whilst increasing the concentration of magnesium reduces the amplitude.The amplitude of the action potentials is linearly related to the log of the external calcium concentration, and the action potentials are blocked by both cobalt ions and lanthanum ions.It is concluded that calcium is the major charge carrier of the inward current in these neurosecretory axons which is the first report of calcium dependent action potentials in a nerve axon. Furthermore, small amounts of sodium and magnesium are necessary to maintain electrical activity. Magnesium is a competitive inhibitor of the calcium currents.We are grateful to the Science Research Council for financial support, and to Mrs. J. Birch for the printing of the electron micrographs.     

Early differentiation of vertebrate spinal neurons in the absence of voltage-dependent Ca2+ and Na+ influx

The development of the action potential and responses to neurotransmitters have been described for a population of embryonic spinal neurons developing in vivo. A comparable pattern is seen for spinal neurons developing in dissociated cell culture. The impulse appears very early in this developmental sequence, and the action potential involves a large inward Ca2+ current. Since Ca2+ is a ubiquitous intracellular regulator, we questioned whether a large influx of Ca2+ is necessary for the subsequent differentiation of membrane properties. Embryonic Xenopus neurons grown in normal culture medium do not make Ca2+- or Na+-dependent action potentials in their cell bodies in a Ca2+-free saline containing tetrodotoxin (TTX). To achieve a chronic blockade of impulse activity, neurons were grown in a medium in which Ca2+ was replaced by Mg2+, and to which 1 mM EGTA was added. In some instances TTX was present. Neurons grown in these experimental culture media extend neurites more rapidly than controls. Action potentials cannot be elicited from neurons when examined in experimental medium. However, examination in saline reveals that the change in the ionic dependence of the impulse is indistinguishable from that observed in neurons grown in normal medium. Furthermore, the time of onset of responses to GABA is unaffected by this experimental treatment. Thus the expression of Ca2+- and Na+-dependent action potentials seems not to play a part in the early differentiation of these membrane properties. However, the later development of GABA sensitivity is reduced.     

Influence of lithium ions on the electrical activity of nerve cells of the leech

The influence of lithium ions on the electrical activity of the nerve cells of the leech,Hirudo medicinalis, was investigated by the method of microelectrode recording. It was discovered that the electrical activity of the giant nerve cells disappears rapidly in a solution with lithium ions. In such a solution, the membrane of these cells is depolarized by 5–7 mV; however, polarization of the cells with direct current does not lead to a restoration of their activity. Possible mechanisms of the inability of lithium ions to replace sodium ions in the generation of the action potentials of leech nerve cells are discussed.I. P. Pavlov Institute of Physiology, Academy of Sciences of the USSR, Leningrad. Translated from Neirofiziologiya, Vol. 2, No. 6, pp. 636–642, November–December, 1970.     

Timing of potential and metabolic brain energy

The temporal relationship between cerebral electro-physiological activities, higher brain functions and brain energy metabolism is reviewed. The duration of action potentials and transmission through glutamate and GABA are most often less than 5 ms. Subjects may perform complex psycho-physiological tasks within 50 to 200 ms, and perception of conscious experience requires 0.5 to 2 s. Activation of cerebral oxygen consumption starts after at least 100 ms and increases of local blood flow become maximal after about 1 s. Current imaging technologies are unable to detect rapid physiological brain functions. We introduce the concepts of potential and metabolic brain energy to distinguish trans-membrane gradients of ions or neurotransmitters and the capacity to generate energy from intra- or extra-cerebral substrates, respectively. Higher brain functions, such as memory retrieval, speaking, consciousness and self-consciousness are so fast that their execution depends primarily on fast neurotransmission (in the millisecond range) and action-potentials. In other words: brain functioning requires primarily maximal potential energy. Metabolic brain energy is necessary to restore and maintain the potential energy.     

The ionic dependence of voltage-activated inward currents in the pharyngeal muscle of Caenorhabditis elegans

The pharynx of Caenorhabditis elegans consists of a syncytium of radially orientated muscle cells that contract synchronously and rhythmically to ingest and crush bacteria and pump them into the intestine of the animal. The action potentials that support this activity are superficially similar to vertebrate cardiac action potentials in appearance with a long, calcium-dependent plateau phase. Although the pharyngeal muscle can generate action potentials in the absence of external calcium ions, action potentials are absent when sodium is removed from the extracellullar solution (Franks et al. 2002). Here we have used whole cell patch clamp recordings from the pharynx and show low voltage-activated inward currents that are present in zero external calcium and reduced in zero external sodium ions. Whilst the lack of effect of zero calcium when sodium ions are present is not surprising in view of the known permeability of voltage-gated calcium channels to sodium ions, the reduction in current in zero sodium when calcium ions are present is harder to explain in terms of a conventional voltage-gated calcium channel. Inward currents were also recorded from egl-19 (n582) which has a loss of function mutation in the pharyngeal L-type calcium channel and these were also markedly reduced in zero external sodium. Despite this apparent dependence on external sodium ions, the current was partially blocked by the divalent cations, cadmium, barium and nickel. Using single-channel recordings we identified a cation channel for which the open-time duration was increased by depolarisation. In inside-out patches, the single-channel conductance was highest in symmetrical sodium solution. Further studies are required to determine the contribution of these channels to the pharyngeal action potential.     

Characteristics of apple snail giant neurons capable of generating action potentials in sodium-free solutions

The ability of apple snail giant neurons to generate action potentials in solutions that lack sodium ions is associated with the input resistance of these neurons in such a way that the higher the input resistance is, the more pronounced is this ability. Neurons in which this ability is well expressed usually exhibit low resting potential values and a slow repolarization phase. When calcium ions are replaced with barium ions, the neurons retain their excitability in a sodium-free medium for a longer period of time. Raising the calcium ion concentration to 30 µmole may exert a restorative effect on neurons that have lost their excitability in a solution that originally lacked sodium ions but contained 10 µmole of calcium ions. Increasing the calcium ion concentration to 60 µmole leads to loss of excitability, which under these conditions can be restored by means of depolarizing the neuron with an outward current. The results are discussed from the point of view of the theory of ionic conductivity of the surface membrane of neurons. It is hypothesized that the ability of the surface membrane of neurons to make use of sodium or calcium ions in generation of action potentials depends upon its permeability to potassium ions.A. A. Bogomolets Institute of Physiology, Academy of Sciences of the Ukrainian SSR, Kiev. Translated from Neirofiziologiya, Vol. 2, No. 1, pp. 100–106, January–February, 1970.     

Permeation of sodium through calcium channels of an insect muscle membrane.

The contribution of Na ions to the electrically excited response was studied in the muscle fibres of mealworm larvae, Tenebrio molitor, using microelectrode techniques. When Ca ions were omitted from the external solution, no action potential could be elicited. However, addition of Na ions to Ca-free medium rendered the fibre excitable again. The amplitude of these action potentials increased with a slope of about 40 mV for a 10-fold elevation of external Na concentrations. Tetrodotoxin had no effect on the initiation of the spike, and Co ions completely suppressed it. Therefore, it seems likely that a Ca-channel, which is utilized by both Na and Ca ions, is the sole factor responsible for the action potential in the mealworm larval muscle fibre membrane.     

Role of the "little brain" in the gut in water and electrolyte homeostasis

The enteric nervous system plays a key role in maintenance of body fluid homeostasis by regulating the transport of ions by the intestinal epithelium. The epithelial cells normally absorb large volumes of fluid and ions daily, but tonically active submucosal neurons continuously suppress ion transport and limit the absorptive capacity of the intestine. Specialized nerve endings detect chemical, osmotic, or thermal alterations of the luminal contents or mechanical activity of the gut wall and encode this information as action potentials that propagate along nerve processes to the ganglia. Information transfer within the ganglia occurs at nicotinic cholinergic or other synapses. Ion transport is altered when neurotransmitters released from motor neurons interact with receptors on epithelial cells to initiate stimulus-response coupling. The signals that transduce changes in epithelial ion transport are largely unknown, except for acetylcholine, but may include vasoactive intestinal peptide or other peptides. These trigger changes in intracellular messengers that influence the state of ionic channels in the epithelial cells and thereby inhibit absorptive processes or stimulate secretory mechanisms. When conservation of salt and water is necessary, command signals from the central nervous system, and perhaps from the myenteric ganglia, will shut down the synaptic circuits in the submucosal ganglia and enhance the absorptive capacity of the bowel.