Nonquantum release of acetylcholine in frog neuromuscular junction after blocking of axoplasmic transport with colchicine

Standard microelectrode techniques were used to evaluate the effect of d-tubocurarine chloride on membrane potential of junctional and extrajunctional areas of muscle fibers in a potassium-free Ringer solution. The experiments were made on frogs after inactivation of acetylcholinesterase. d-Tubocurarine chloride hyperpolarized the membrane of muscle fibers only in the junctional area. Blockade of axoplasmic transport with colchicine did not affect the magnitude of the hyperpolarization response of the membrane end plate to the presence of d-tubocurarine chloride, but at the same time it significantly reduced the membrane rest potential of muscle fibers, and gave rise to the appearance of extrajunctional sensitivity to acetylcholine. It is concluded that the blockade of axoplasmic transport does not affect the pattern of non-quantum acetylcholine release from nerve terminals. Therefore, this is unlikely to cause denervation-like changes in the muscle under the conditions described.     

Effect of postsynaptic blockade of neuromuscualar transmission on properties of the frog fast muscle fiber membrane

Sensitivity of the postsynpatic membrane to acetylcholine, the resting membrane potential, input resistance, and membrane time constant of fast muscle fibers were measured in experiments on frogs. Complete immobilization of the animals with D-tubocurarine or local immobilization of a muscle with α-bungarotoxin was found not to affect these parameters of the muscle membrane, whereas denervation of the muscle widens the zone of postsynaptic sensitivity to acetylcholine, lowers the resting membrane potential, and increases the input resistance and time constant of the muscle membrane. These results are evidence that neurotrophic control of the frog fast muscle fiber membrane is achieved mainly by substances reaching the muscle via axoplasmic transport and not by the character of the neuronal discharge and motor activity or by synaptic acetylcholine.     

Depletion of substance P and somatostatin in the upper dorsal horn after blockade of axoplasmic transport

Summary In the upper dorsal horn of the rat lumbosacral spinal cord, substance P and somatostatin are present in two distinct and different populations of primary central afferent terminals. Substance-P-positive terminals are mainly concentrated in lamina I, while somatostatin-positive terminals are confined to lamina II. Although these two populations of primary afferent terminals differ at light- and electron-microscopic level, they are equally affected by transganglionic degenerative atrophy (TDA) which is induced by the blockade of axoplasmic transport in the segmentally related, ipsilateral sensory nerve by the local application of Vinblastin, a microtubule inhibitor. In consequence, substance P and somatostatin are depleted in the medial and intermediate portions of the upper dorsal horn, while the lateralmost area, which represents the postaxial portion of the dermatome, remains virtually intact. Substance P and somatostatin in propriospinal elements and the axonal meshwork within the dorsolateral funicle are not affected by TDA. Neurotensine, a propriospinal neuropeptide, does not show any alterations in the affected spinal segments.This work was supported by research grant no. 06/4-01/449 from the Hungarian Ministry of Health and no. 375/82/3.2 from the Hungarian Academy of Sciences     

Mediator liberation at the myoneural junctions of the frog sartorius muscle when axoplasmic transport has been disrupted with colchicine]

Colchicine application to the nerve innervating the frog sartorius muscle leads to reduction of the differences in functional condition of the myoneural synapses of this muscle by the signs of quantum content of the end plate potential (EPP) and by the changes of the EPP amplitudes in high frequency stimulation. Along with this there occurs a rarefaction of the frequency of miniature potential of the end plate, and reduction of its amplitude. Apparently, the latter is caused by reduction of effective resistance of the postsynaptic membrane. Changes of the induced and spontaneous secretion of the mediator are connected with disturbances of the axoplasmic transport just in the somatic nerve fibers.     

Effect of fast axoplasmic transport on activity of chloride transfer in membrane of rat skeletal muscle fibers

Experiments on innervated fibers of the diaphragmatic muscle of rat led to the conclusion that the furosemide-sensitive chloride transport is inactive under normal conditions due to the depressing effect of the substances transferred to the muscle by fast axoplasmic transport. The neurotrophic control disturbance arising when the axonic flow is blocked by colchicine led to intensification of active chloride transport, increase in the intracellular Cl^– concentration, and decrease in the equilibrium chloride potential, which resulted in a denervation-like depolarization of the muscle membrane.Neirofiziologiya/Neurophysiology, Vol. 25, No. 4, pp. 278–280, July–August, 1993.     

Nonquantal acetylcholine release from motor nerve endings and denervation changes in rat muscle fiber membranes after axonal transport blockade

Microelectrode experiments on the rat diaphragm showed that application of colchicine, which disturbs axonal transport, to the motor nerve leads after 5 days to a decrease in resting potential and an increase in input resistance of the electrogenic membrane, disappearance of differences of input resistance between the postsynaptic and extrasynaptic membranes, the appearance of extrasynaptic sensitivity to acetylcholine, and the appearance of anode-break action potentials resistant to tetrodotoxin. Similar changes develop in the muscle membrane after division of the motor nerve. Application of colchicine to the nerve, unlike its division, does not cause cessation of contractile activity of the muscle or disturbance of quantal and reduction of nonquantal acetylcholine secretion in motor nerve endings, as reflected in the degree of hyperpolarization of the postsynaptic membrane (H effect) in response to the action of D-tubocurarine chloride on the muscle after inhibition of acetylcholinesterase. The results confirmed the view that neurotrophic control of the mammalian muscle fiber membrane is effected mainly by means of substances carried to the muscle by axonal transport. Synaptic acetylcholine, secreted from nerve endings in nonquantal form, does not play a leading role in neurotrophic control of the muscle membrane.S. V. Kurashov Medical Institute, Ministry of Health of the RSFSR, Kazan'. Translated from Neirofiziologiya, Vol. 16, No. 2, pp. 231–238, March–April, 1984.     

Electrophysiological study of mediator secretion in the frog neuromuscular synapse under a colchicine block of axoplasmic transport

Two weeks after colchicine nerve treatment the evoked transmitter release was blocked in part of the frog sartorius synapses, with spontaneous activity being absent from some of them. In the synapses with evoked and spontaneous transmitter release preserved within this period of time, the magnitudes of the absolute refractory phase of nerve terminals were significantly higher than the control ones, while in part of synapses, the frequency of miniature end plate potentials (MEPP) was considerably increased. Nerve stimulation (5 imp.s-1) led to a rise of the amplitude of evoked potentials and of MEPP frequency followed by irreversible blockade of synaptic activity. It is concluded that substances transported by rapid axonal flow control the level of membrane potential of nerve terminals and are fairly important for presynaptic membrane integrity.     

Effect of motor function disorder on the properties of the muscle fiber membrane

Experiments on frogs with the use of the microelectrode techniques were made to study the effect of tenotomy and immobilization of a limb with a metal cast in the extension position on the properties of the membrane of muscle fibers. Two weeks after tenotomy there were no changes in the magnitude of the membrane rest potential, input resistance and time constant of the membrane of muscle fibers or in the pattern of its sensitivity to acetylcholine. Two and three weeks after the limb immobilization no changes in the membrane rest potential and passive electrical properties of the muscle membrane were recorded either. However, if the time elapsed after immobilization was 2 and 3 weeks, the zone of the sensitivity of muscle fibers to acetylcholine was slightly greater than in the control. It is suggested that the motor activity in the frog per se is not the determinant of the muscle fiber differentiation preset by the nervous system.     

Calcium and the mechanism of axoplasmic transport

Using desheathed cat peroneal nerves in in vitro studies, Ca2+ was recently shown to be required to maintain axoplasmic transport. Calmodulin was also shown to be present in nerve and to participate in transport. These findings open up new possibilities for a better understanding of the underlying mechanism of transport. In the transport filament model, the materials transported are bound to a common carrier, the transport filaments, which are moved along the microtubules by means of an interaction with the side arms of the microtubules. This is an energy-requiring process that depends on a supply of ATP, which is utilized by the Ca2+,Mg2+-ATPase associated with the side arms of the microtubules. The Ca2+,Mg2+-ATPase is activated by calmodulin at the low micromolar levels of free Ca2+ present in the axon. The level is kept low by calcium-regulatory mechanisms that include mitochondria, endoplasmic reticulum, and calcium-binding proteins. Nerves exposed to higher-than-normal concentrations of Ca2+ in the medium show an increased number of particles in these organelles as expected of their Ca2+-regulatory role. The nature of the calmodulin-Ca,Mg-ATPase complex associated with the side arms is discussed on the basis of the transport model. Also discussed is slow transport, which is explained on the basis of the model as a differential binding affinity to the transport filaments.     

Block of axoplasmic transport in vitro by vinca alkaloids

The three potent antimitotic vinca alkaloids: vincristine (VCR), vinblastine (VLB), and vindesine (VDS) were compared for their effect in blocking axoplasmic transport in vitro using a desheathed preparation of the peroneal branch of cat sciatic nerve. A range of vinca alkaloid concentrations from 1–100μM was examined. The relative order of potency in blocking axoplasmic transport was VCR > VLB > VDS at a concentration of 25μM. At the higher concentrations block occurred so rapidly that a statistically significant difference between these agents could not be obtained. The relation of vinca block ot the transport mechanism is discussed.     

Electrical properties and structure of the frog arachnoid membrane

We have used an in vitro preparation of the frog arachnoid membrane to study the role of this membrane in the maintenance of the “blood-cerebrospinal fluid (csf) barrier.” Electron microscopy showed that the membrane was made up of 10–15 layers of flat epithelial cells joined together by numerous cell junctions. The electrical resistance of the preparation was about 2000 ohms cm². The steady-state transmural potential difference (pd) ranged up to 45 mV, csf positive, and this was eliminated by either the addition of ouabain to the csf, or by replacing the NaCl with TEA Cl. The pd across the membrane increased when bicarbonate was added to the external bathing solutions. We conclude that this pd is due to the active transport of sodium from the subural fluid to the csf. In some preparations the transmural pd was reversed, i.e., csf negative, and this was also abolished by the addition of ouabain to the csf, or by replacing chloride with isethionate. We conclude that this pd is related to active chloride transport. These, and other experiments, lead us to the conclusion that the arachnoid membrane is involved in the production of the cerebrospinal fluid and the maintenance of the blood-cerebrospinal fluid barrier.     

B型酪氨酸激酶受体的轴浆转运

脑源性神经营养因子(brain-derived neurotrophic factor,BDNF)在发育及成熟的中枢神经系统(central nervoussystem,CNS)中起到举足轻重的调节作用,而其中绝大部分作用由其B型酪氨酸激酶受体(tyrosine kinase receptortype B,TrkB)介导,因此TrkB在神经元中的轴浆转运过程显得尤为重要。本文从动力蛋白、潜在调节分子、细胞骨架蛋白等方面对TrkB轴浆转运分子机制的研究进展进行综述,并就其进一步研究提出一系列的问题与展望。     

Origin of axoplasmic RNA in the squid giant fiber

The origin of axoplasmic RNA in the squid giant fiber was investigated after exposure of the giant axon or of the giant fiber lobe to [3H]uridine. The occurrence of a local process of synthesis was indicated by the accumulation of labeled axoplasmic RNA in isolated axons incubated with the radioactive precursor. Similar results were obtained in vivo after injection of [3H]uridine near the stellate nerve at a sizable distance from the ganglion. Exposure of the giant fiber lobe to [3H]uridine under in vivo and in vitro conditions was followed by the appearance of labeled RNA in the axoplasm and in the axonal sheath. While the latter process is attributed to incorporation of precursor by sheath cells, a sizable fraction of the radioactive RNA accumulating in the axoplasmic is likely to originate from neuronal perikarya by a process of axonal transport.