Voltage-dependent calcium channels in glial cells

The electrophysiological properties of glial cells were examined in primary culture in the presence of tetraethylammonium and Ba2+, a treatment that reduces K+ permeability of the membrane and enhances currents through voltage-dependent Ca2+ channels. Under these conditions, glial cells showed both spontaneous action potentials and action potentials evoked by the injections of current. These responses appear to represent entry of Ba2+ through Ca2+ channels because they were resistant to tetrodotoxin but were blocked by Mn2+ or Cd2+.     

Crayfish muscle: permeability to sodium induced by calcium depletion

Membrane of crayfish muscle fibers becomes selectively permeable to sodium when the calcium concentration of the bathing medium is reduced. Removal of calcium or its reduction below 1 or 2 millimole per liter causes large transient depolarizations up to 70 millivolts in amplitude. They resemble pro longed action potentials and occur only in the presence of sodium. The responses are abolished when tris(hydroxymethyl)aminomethane or lithium is substituted for sodium, and are blocked by tetrodotoxin even in the presence of sodium.     

Prenatal tetrodotoxin infusion blocks segregation of retinogeniculate afferents

In the adult mammalian visual system, ganglion cell axons from the two eyes are segregated from each other into separate layers within their principal target, the lateral geniculate nucleus. The involvement of spontaneously generated action potential activity in the process of segregation was investigated during the fetal period in which segregation normally occurs in the cat, between embryonic day 45 (E45) and birth (E65). Tetrodotoxin, which blocks the voltage-sensitive sodium channel, was used to prevent action potentials. Fetuses received continuous intracranial infusions of tetrodotoxin from osmotic minipumps implanted in utero on E42. After a 2-week infusion, intraocular injections of anterograde tracers revealed that tetrodotoxin prevented segregation. The contralateral projection filled the lateral geniculate nucleus uniformly, and the ipsilateral projection expanded to occupy most of what would normally be contralaterally innervated layer A. Thus, in the fetus, long before the onset of vision, spontaneous action potential activity is likely to be present in the visual system and to contribute to the segregation of the retinogeniculate pathway.     

Tetraethylammonium and tetrodotoxin: effects on cochlear potentials

Tetraethylammonium chloride, which is believed to decrease potassium conductance, and tetrodotoxin, which apparently decreases sodium conductance in nerve fibers, were introduced iontophoretically into the organ of Corti or the scala media of guinea pig cochlea. The former depressed the direct-current endocochlear potential and also the alternating-current cochlear microphonics (the receptor potential of the ear), but tetrodotoxin was ineffective except on the nerve impulses.     

急性心肌缺血大鼠左心室流出道自发性慢反应电位去极离子流变化分析

目的探讨急性心肌缺血大鼠左室流出道自发性慢反应电位去极离子流的变化。方法常规玻璃微电极细胞内方法观测急性心肌缺血大鼠的离体心脏最大舒张电位(MDP)、0相除极幅度(APA)、0相最大除极速度(Vmax)、4相自动除极速度(VDD)、复极50%(APD50)和90%(APD90)的时间以及自发放电频率(RPF)。结果与给药前相比①1.2mmol.L-1河豚毒(TTX)使APA和Vmax有所减小(P<0.05),VDD和RPF明显减慢(P<0.01);②1.0μmol.L-1维拉帕米(VER)可使该慢电位的APA、Vmax、VDD明显减小,RPF减慢(P<0.01);APD、APD90延长(P<0.05);③2mmol.L-1 4-氨基吡啶(4-AP)使该慢电位的MDP的绝对值、APA、Vmax减小,VDD和RPF加快(P<0.01);④1.5mmol.L-1 CsCl作用6min时,VDD和RPF明显降低(P<0.05),10min时恢复。结论①左心室流出道的自发慢电位0相主要去极离子流除Ca2+内流外,还有少量Na+内流。②4相去极离子流中,除Ca2+、Na+的内流和Ik衰减外,If电流可能也起部分作用。     

Tetrodotoxin derivatives: chemical structure and blockage of nerve membrane conductance

The nerve-impuilse-blocking actions of derivatives of tetrodotoxin have been tested on lobster and squid axons. The block produced by deoxytetrodotoxin was similar to that produced by tetrodotoxin and was probably caused by tetrodotoxin contamination. Tetrodaminotoxin and anhydrotetrodotoxin also produced a similar block but at such high concentrations that tetrodotoxin contamination cannot be ruled out. The hydroxyl group of C(4) and the hemilactal oxygen links play an important role for the nerve-blocking action.     

Tetrodotoxin and manganese ion: effects on action potential of the frog heart

Tetrodotoxin, which effectively suppresses the permeability of various tissues to sodium ions, has no effect on the calcium action potential of the fiber of barnacle muscle, which potential is produced by increase in permeability to calcium ions. Manganese ions, however, suppress the action potential. When applied to fiber of frog cardiac ventricle, tetrodotoxin suppresses the rate of rise of the action potential without affecting the overshoot; the suppressive effect of manganese ion is mainly on the overshoot of the action potential. This suggests that, in the action potential of the car-diac ventricle of the frog, the platealu phase is related primarily to the increase in permeability of the membrane to calcium ions.     

Regulation of dendritic protein synthesis by miniature synaptic events

We examined dendritic protein synthesis after a prolonged blockade of action potentials alone and after a blockade of both action potentials and miniature excitatory synaptic events (minis). Relative to controls, dendrites exposed to a prolonged blockade of action potentials showed diminished protein synthesis. Dendrites in which both action potentials and minis were blocked showed enhanced protein synthesis, suggesting that minis inhibit dendritic translation. When minis were acutely blocked or stimulated, an immediate increase or decrease, respectively, in dendritic translation was observed. Taken together, these results reveal a role for miniature synaptic events in the acute regulation of dendritic protein synthesis in neurons.     

Neurosecretory cell: capable of conducting impulse in rats

Electriic vtinulation of the rat neurohypophlysis in Situ evoked the unit action potentials. in the supraoptic nucleus. Compound and unit action potentials. were recorded from the isolated neurohypophysis stimulation. The applicaition of solutions high in potassium or devoid of sodium made the neurohypophysis inexcitable. Mammalian neurosecretsecretory cells have the neuronal properties generation and conduction of action potentials.     

Aplysia giant cell: soma-axon voltage clamp current differences

Under voltage clamp, local membrane currents have been measured in several regions of the soma. The early inward current appears to pass largely through the membrane of the axon and of the soma near the axon in normal media. After 10(-5) molar tetrodotoxin (TTX) is added to the bathing medium the larger inward current is found in the somatic membrane away from the axon. The late currents are larger at the soma in both normal and TTX media.     

Tetrodotoxin does not block excitation from inside the nerve membrane

Tetrodotoxin does not block the action potential or membrane sodium current when internally perfused through the giant axon of a squid at much higher concentrations than those required for blocking by external application. It is suggested that the gate for the sodium channel is located on the exterior surface of the axon, because tetrodotoxin is not lipid soluble.     

DDT: interaction with nerve membrane conductance changes

The falling phase of action potentials of lobster giant axons is prolonged by DDT; finally a plateau phase is produced like cardiac action potentials. In axons poisoned with DDT, peak transient (sodium) currents associated with step depolarizations are turned off very slowly, and steady-state (potassium) currents are markedly suppressed. These two changes would cause the prolongation of action potentials and are considered the major ionic mechanisms of DDT action.     

DNA synthesis in cartilage cells is stimulated by oscillating electric fields

External oscillating electric fields (1166 volts per centimeter, 5 hertz) enhanced the incorporation of [3H] thymidine into the DNA of chondrocytes isolated from the proliferative layer of embryonic (16 days) chick epiphysis. Verapamil or tetrodotoxin at 10(-6)M concentrations completely blocked the electric field effect. Tetracaine reduced the incorporation of [3H] thymidine in both control and electrically stimulated cells. The findings support the hypothesis that Na and Ca2 fluxes generated by the electrical perturbation trigger DNA synthesis in these cells.     

Specialized inhibitory synaptic actions between nearby neocortical pyramidal neurons

We found that, in the mouse visual cortex, action potentials generated in a single layer-2/3 pyramidal (excitatory) neuron can reliably evoke large, constant-latency inhibitory postsynaptic currents in other nearby pyramidal cells. This effect is mediated by axo-axonic ionotropic glutamate receptor-mediated excitation of the nerve terminals of inhibitory interneurons, which connect to the target pyramidal cells. Therefore, individual cortical excitatory neurons can generate inhibition independently from the somatic firing of inhibitory interneurons.     

Modulation of calcium channels in cardiac and neuronal cells by an endogenous peptide

Calcium channels mediate the generation of action potentials, pacemaking, excitation-contraction coupling, and secretion and signal integration in muscle, secretory, and neuronal cells. The physiological regulation of the L-type calcium channel is thought to be mediated primarily by guanine nucleotide-binding proteins (G proteins). A low molecular weight endogenous peptide has been isolated and purified from rat brain. This peptide regulates up and down the cardiac and neuronal calcium channels, respectively. In cardiac myocytes, the peptide-induced enhancement of the L-type calcium current had a slow onset (half-time approximately 75 seconds), occurred via a G protein-independent mechanism, and could not be inhibited by alpha 1-adrenergic, beta-adrenergic, or angiotensin II blockers. In neuronal cells, on the other hand, the negative effect had a rapid onset (half-time less than 500 milliseconds) and was observed on both T-type and L-type calcium channels.     

Internally perfused axons: effects of two different anions on ionic conductance

Voltage-clamizped giant axons of squid, internally perfused with potassiumt chloride solutions, showed reduced initial transient memnbrane conductance to voltage and increased overall (leakage) conductance. Unclamtped axons showed reduced action and resting potentials. Ionic conductances and memtbrane potentials were maintained or restored by perfusion with potassiuwn fluoride solutions. As much as 90 percent of internal fluoride could be replaced with chloride without alterationt of normal properties of membrane.     

Importance of the axial gradients of electric potentials in the life activity of plants in microgravitation conditions

An installation that changes the unilateral action of the Earth’s gravitational field on a plant has been developed. Microgravitation is provided by simulating the weightlessness condition by rotating the vessel with the growing plants in a vertical plane with a constant slow speed (2 rpm), at which the action of centrifugal forces does not significantly affect the spatial orientation of the plants in the process of their development. Furthermore, the plants are affected by the gravitation vector from different sides, and it does not have time to adapt to the gravitation force. It is shown in terms of the KVS-701 corn hybrid that, in the conditions of microgravitation, the depolarization of the axial bioelectric polarity of the plant and depression of the gradients of the bioelectric potentials (BEP) to the values 0.1–0.2 mV/cm—under which a significant deterioration of the growth and development of the plants is observed—takes place. The restoration of the axial bioelectric polarity of the plants by the external low-energy electric potentials (LEP) to the natural level (approximately 1 mV/cm) leads to an increase of the adaptive capabilities of the grain crop to the microgravitation and restoration of the growth indicators, which is confirmed by the biometric data.     

Transmembrane action potentials from smooth muscle in turtle arteries and veins

Transmembrane potential and active tension were measured in isolated segments of turtle aorta and inferior vena cava. Changes in tension were associated with action potentials, but the potentials had a different pattern in the two tissues. When the frequency of the action potentials increased, the contractions summated, resulting in a steadily maintained contraction.     

Tetrodotoxin: effects on brain metabolism in vitro

A 3-micromolar concentration of tetrodotoxin completely inhibits the stimulation of respiration of rat brain cortex that takes place upon application of electrical impulses. It also inhibits increase in the rate of the respiration that occurs when calcium ions are omitted from the incubation medium. No effect of tetrodotoxin on brain respiration takes place when stimulation is brought about by the addition of 100 millimolar potassium chloride. Tetrodotoxin prevents the fall in the rate of oxidation of cerebral acetate that occurs during electrical stimulation but does not affect the increased rate of the oxidation that occurs in the presence of an increased concentration of potassium chloride. The data indicate that oxidation of cerebral acetate is diminished by influx of sodium ions, which is prevented by tetrodotoxin, and is increased by influx of potassium ions, which is unaffected by tetrodotoxin.     

Neuroscience. Noise makes sense in neuronal computing

The united efforts of assemblies of neurons in the brain's primary visual cortex translate incoming visual signals into action potentials. These action potentials encode, for example, the contrast and orientation of different parts of the image. Some neurons are sensitive to one particular orientation, other are sensitive to other orientations, but all neurons respond equally well to the image contrast. In a Perspective, Volgushev and Eysel explain the finding (Anderson et al.) that neurons are able to maintain this sensitivity to the orientation of a stimulus regardless of the contrast by adding noise to the membrane potential, such that action potentials can also be generated in response to weak signals at low contrast.