Terminal sprouting in mouse neuromuscular junctions poisoned with botulinum type A toxin: morphological and electrophysiological features

Functional properties of terminal sprouts elicited by an in vivo injection of Clostridium botulinum type A toxin were studied in endplates of the Levator auris longus muscle of the mouse poisoned from a few days to 28 days beforehand. For this purpose, morphological observations of the extent of terminal sprouts and localization of acetylcholine receptors was performed in whole mount preparations. Sprouts appeared as thin unmyelinated filaments that run usually parallel to the longitudinal axis of the muscle fibres; labelling acetylcholine receptors revealed their line-shaped accumulation co-localized with the sprouts. In addition, presynaptic membrane currents elicited by nerve stimulation were recorded by external electrodes applied under visual control onto the membrane of pre-existing motor endings and newly formed sprouts. These recordings showed the presence of widespread triphasic waveforms which indicated active impulse propagation of the action potential over most of the length of the poisoned endings. Ca2+ influx and Ca2(+)-dependent K+ currents in the sprout membrane were found to be similar to those described in unpoisoned endings. The presence of normal Ca2+ influx, upon active depolarization, in the terminal sprout membranes together with the localization of acetylcholine receptors in front of these membranes, indicates that the terminal sprouts may play a role in the recovery of neuromuscular transmission after Clostridium botulinum poisoning.

Advantages of the triangularis sterni muscle of the mouse for investigations of synaptic phenomena

The triangularis sterni muscle (TS) of the mouse is a thin trapezoidal sheet of fibres in which individual neuromuscular junctions are easily observed with Nomarski optics. Thus, microelectrodes are readily positioned to accurately record various synaptic phenomena. For example, miniature end-plate currents were easily recorded with a focally positioned extracellular electrode and the end-plate sensitivity to acetylcholine averaged 2062 mV/nC. In addition, the intercostal nerves segmentally innervate the TS. Electrophysiologic and histologic analysis showed that each nerve innervates a sharply defined territory of the muscle surface. These preliminary observations suggest that the TS may be ideal for studies of synaptic function and the processes underlying synapse stabilization in the mammal.

Nerve terminal sprouting in botulinum type-A treated mouse levator auris longus muscle

The marked outgrowth of the motor nerve terminal arborization triggered by an in vivo local injection of Clostridium botulinum type-A toxin in the mouse levator auris longus muscle was studied with morphological and immunochemical approaches. The increase in total nerve terminal length depended on the time elapsed after toxin administration and was due to both increased number of terminal branches and branch length as revealed by a quantitative morphological analysis of whole mounts using the combined cholinesterase-silver stain. Nerve terminal sprouts increased in number, length and complexity even after the functional recovery of neuromuscular transmission had occurred as revealed by electrophysiological examination. Although we cannot exclude that transmitter release sites from the original nerve terminal arborization may still be functional after botulinum type-A toxin (BoTx-A) treatment, it is likely that newly formed functional release sites on the sprouts play a major role in the functional recovery of neuromuscular transmission. The presence of an immunoreactivity to synaptophysin and synaptotagmin-II, integral proteins of synaptic vesicles, gives support to our previous findings suggesting that nerve terminal sprouts have the molecular machinery for acetylcholine release.

The levator auris longus muscle of the mouse: a convenient preparation for studies of short- and long-term presynaptic effects of drugs or toxins

We describe here the levator auris longus muscle of the mouse as a convenient neuromuscular preparation for the in vitro study of presynaptic effects of drugs and toxins applied in vivo in young or adult mice. The good visibility of its motor axons and terminals using Nomarski optics allows accurate electrophysiological studies of presynaptic signals. In addition, the levator auris longus muscle is sufficiently thin to be stained as a whole mount preparation. Preliminary results indicate that some correlation can be established between changes in time course of the presynaptic signal and the morphology of motor endings after poisoning the levator auris longus muscle with botulinum type A toxin.

The dorsal column system: II. Functional properties and bulbar relay of the postsynaptic fibres of the cat's fasciculus gracilis

Microelectrode recordings were made from dorsal column postsynaptic (DCPS) fibres, in the fasciculus gracilis of the cat, at thoracic level Th 12, and from single cells in the nucleus gracilis. The sensitivity of the fibres (Th 12) and cells (bulbar level) to both gentle and noxious stimuli was studied and a classification of the units was made on the basis of their responses to these natural stimuli. The DCPS fibres have been classified into three groups: i) 16.3% of them were activated only by light mechanical stimuli, ii) a few responded to nothing but noxious mechanical stimuli, iii) 77% were characterized by their sensitivity to both gentle and noxious stimuli and constituted the polymodal group. The study of the nucleus gracilis cells revealed that 31.2% of the total number of cells investigated in the nucleus were also characterized by a high degree of modality convergence (polymodal cells). Besides their convergence, the polymodal DCPS fibres and the polymodal cells of the nucleus gracilis were functionally in contrast to the properties of the specific fibres and cells of the dorsal column system in other respects: i) they responded with a slowly adapting response to constant mechanical stimulation of the skin, ii) they were sensitive to noxious mechanical and thermal stimuli. It is argued that the polymodal cells could be the bulbar relay of the impulses conveyed through the DCPS fibres of the fasciculus gracilis. The possible role of the dorsal column system in nociception is discussed in the light of the results.

Changes of quantal transmitter release caused by gadolinium ions at the frog neuromuscular junction

1. The actions of the trivalent cation, gadolinium (Gd3+), were studied on frog isolated neuromuscular preparations by conventional electrophysiological techniques. 2. Gd3+ (450 microM) applied to normal or formamide-treated cutaneous pectoris nerve-muscle preparations induced, after a short delay, a complete block of neuromuscular transmission. The reversibility of the effect was dependent on the time of exposure. 3. Gd3+ (5-450 microM) had no consistent effect on the resting membrane potential of the muscle fibres. 4. Gd3+ (5-40 microM) applied to preparations equilibrated in solutions containing high Mg2+ and low Ca2+ reduced the mean quantal content of endplate potentials (e.p.ps) in a dose-dependent manner. Under those conditions, 3,4-diaminopyridine (10 microM) consistently reversed the depression of evoked quantal release. 5. The calcium current entering motor nerve terminals, revealed after blocking presynaptic potassium currents with tetraethylammonium (10 mM) in the presence of elevated extracellular Ca2+ (8 mM), was markedly reduced by Gd3+ (0.2-0.5 mM). 6. Gd3+ (40-200 microM) increased the frequency of spontaneous miniature endplate potentials (m.e.p.ps) in junctions bathed either in normal Ringer solution or in a nominally Ca(2+)-free medium supplemented with 0.7 microM tetrodotoxin. This effect may be due to Gd3+ entry into the nerve endings since it is not reversed upon removal of extracellular Gd3+ with chelators (1 mM EGTA or EDTA). Gd3+ also enhanced the frequency of me.p.ps appearing after each nerve stimulus in junctions bathed in a medium containing high Mg2+ and low Ca2+. 7. Gd3+, in concentrations higher than 100 microM, decreased reversibly the amplitude of m.e.p.ps suggesting a postsynaptic action. 8. It is concluded that the block of nerve-impulse evoked quantal release caused by Gd3 + is related to its ability to block the calcium current entering the nerve endings, supporting the view that Gd3 + blocks N-type Ca2+ channels; while the enhancement of spontaneous quantal release is probably the result of Gd3 + entry into motor nerve endings. Besides its dual prejunctional effects on quantal release it is suggested that Gd3 + exerts a postsynaptic action on the endplate acetylcholine receptor-channel complex.

Nerve terminal excitability and neuromuscular transmission in T(X;Y)V7 and Shaker mutants of Drosophila melanogaster

We investigated the neuromuscular transmission in relation with genetic neuronal excitability changes in mutants T(X;Y)V7 and ShK,S133 of Drosophila. These mutations affect two different genes belonging to the Shaker gene complex which encode different yet functionally related proteins. Experiments were performed on neuromuscular junctions from Drosophila larvae by recording pre- and postsynaptic membrane currents using external electrodes. It was found that the neuromuscular electrophysiological phenotype of T(X;Y)V7 is caused by presynaptic membrane hyperexcitability probably in relation with a Ca2(+)-dependent down regulation of voltage dependent K channels. By contrast, the ShKS133 phenotype can be explained solely by action potential widening due to the absence of type-A K channels.

The interference of truncated with normal potassium channel subunits leads to abnormal behaviour in transgenic Drosophila melanogaster

The Shaker locus of Drosophila melanogaster encodes a family of A-type potassium channel subunits. Shaker mutants behave as antimorphs in gene dosage tests. This behaviour is due to the production of truncated A-channel subunits. We propose that they interfere with the function of their normal counterpart by forming multimeric A-channel structures. This hypothesis was tested by constructing transgenic flies carrying a heat-inducible gene encoding a truncated A-type potassium channel subunit together with a normal wild type doses of A-type potassium channel subunits. The altered subunit leads at larval, pupal or adult stages to the transformation of wild type into Shaker flies. The transformed flies exhibited a heat-inducible abnormal leg shaking behaviour and a heat-inducible facilitated neurotransmitter release at larval neuromuscular junctions. By the overexpression of an aberrant A-channel subunit the normal behaviour of transgenic D. melanogaster can be altered in a predictable way.

The dorsal column system: I. Existence of long ascending postsynaptic fibres in the cat's fasciculus gracilis

Microelectrode recordings were made from dorsal column fibres at the 12th thoracic segment in cats. Two kinds of activity were elicited by electric shocks applied on the sciatic nerve in the popliteal fossa: primary afferent activity and trans-synaptically evoked activity. The dorsal column postsynaptic (DCPS) fibres represented 9.3% of the total population of fibres studied in the fasciculus gracilis and 14.5% of those with receptors in the skin. They were found to lie between the primary fibres of cutaneous and those of deep origin. The fastest fibres of the alpha range contributed to their activation and it is likely that C fibres contributed as well. 87% of the DCPS fibres studied at Th 12 were antidromically activated from the first cervical segment, and their conduction velocities measured between cervical and thoracic levels ranged from 16 to 71 m/sec.

Enhanced neurotransmitter release is associated with reduction of neuronal branching in a Drosophila mutant overexpressing frequenin

Frequenin is a Drosophila Ca2+ binding protein whose overexpression causes a chronic facilitation of transmitter release at the larval neuromuscular junction and multiple firing of action potentials. These functional abnormalities are similar to those found in other hyperexcitable mutants (Shaker, ether-a-gogo, Hyperkinetic) which, in turn, exhibit increased branching at the motor nerve endings. We report here that mutants which overexpress frequenin have motor nerve terminals with reduced number and length of branches as well as number of synaptic boutons. Similar defects are observed in transgenic flies which have additional copies of the frequenin gene indicating that the phenotype can be adscribed to the overexpression of the protein. The ultrastructure of boutons, however, appears indistinguishable from wild type. In addition, we show here that frequenin overexpression leads also to a down regulation of Shaker proteins expression. The contrast between the observations in frequenin and the other hyperexcitable mutants indicates that nerve terminal morphology and enhanced transmitter release do not have a direct causal relationship.

Dual innervation of end-plate sites and its consequences for neuromuscular transmission in muscles of adult Xenopus laevis

1. Electrophysiological study of dually innervated end-plate sites was carried out in Xenopus laevis pectoral muscle fibres. End-plate potentials (e.p.p.s) and miniature end-plate potentials (m.e.p.p.s) have been recorded in Mg-blocked preparations. 2. The mean quantal content (m) of each e.p.p. at dually innervated end-plates was significantly smaller than the corresponding values obtained at singly innervated ones. At a given doubly innervated end-plate site the values of m tended to be inversely related, so that the compound value of m (obtained by adding them) was in the same range as that found in singly innervated junctions. These findings were taken to suggest the existence of an upper limit in the average amount of transmitter released at a synaptic site. 3. It was found that neither intermittent presynaptic conduction block, nor particular muscle fibre properties could account for the low values of m in dual end plates. The small size of the nerve terminals appears to be the most likely explanation. 4. The sensitivity to acetylcholine and muscle fibre electrical properties were investigated; no differences were found between fibres with sub- or suprathreshold e.p.p.s. 5. The nature of the factors responsible for this presumed small size of the nerve endings (competition between nerve endings for a limited synaptic space on the muscle membrane or reciprocal interaction between closely located terminals) as well as the possible origins of polyinnervation are discussed.

Abnormal reinnervation of skeletal muscle in a tenascin-C-deficient mouse

The possible involvement of tenascin-C in the reinnervation of a skeletal muscle was investigated in the tenascin-C-deficient mouse (T-/-) produced by Saga et al. (1992; Genes Dev 6:1821-1831). The pattern of reinnervation, observed after denervation of the triangularis sterni muscle, differs in T-/- and wild-type muscles in several traits. Axonal growth and stability of terminal arbors are impaired in the T-/- muscle: Some axons in mutant muscles grow beyond their original targets and reinnervate other synaptic sites, which may become dually innervated. In contrast to wild type, polyinnervation increases with time after denervation in T-/- muscles and is still present 7 months after nerve crush. The expression of a tenascin-C mRNA product disappears between 1 and 2 months after nerve crush. Of interest is that this transcriptional regulation in T-/- muscles occurs when major alterations in the morphology of regenerating endings become obvious. These observations strongly implicate tenascin-C in the formation, maturation, and stabilization of the neuromuscular junction.

Electrophysiological and morphological studies of a motor nerve in 'motor endplate disease' of the mouse

Motor nerves of mice affected with hereditary 'motor endplate disease' were examined by means of electrophysiological and morphological techniques. Conduction velocity was slower, the refractory period was prolonged and the temperature sensitivity higher in mutants as compared to controls of the same age. Most of the axons examined in the electron microscope showed signs of paranodal demyelination. The relationship between the morphological and the electrophysiological findings is discussed. We conclude that alterations in the motor axons can account for the failures in neuromuscular transmission described previously in the med mutation.

Fiber types in the mouse levator auris longus muscle: a convenient preparation to study muscle and nerve plasticity

The histochemical composition of the levator auris longus (LAL) muscle has been investigated in adult NMRi mice. Histochemical reaction for myofibrillar adenosine triphosphatase (ATPase) after preincubation in alkaline and acidic media, nicotine amideadenine-dinucleotide dehidrogenase (NADH-dehydrogenase), and alpha-glycerophosphate dehydrogenase were performed on cryosections of LAL muscle. Expression of myosin heavy chain (MyHC) isoforms was detected with the immunoperoxidase method applying monoclonal antibodies against MyHC isoforms -1, -2a, -2x/d, and -2b, as well as by sodium dodecylsulfate (SDS) glycerol gel electrophoresis. The muscle was proven to be a pure fast-twitch muscle. The most numerous fibers in LAL muscles contained MyHC-2b and some MyHC-2a. Histochemically, pure IIA fibers with oxidative metabolism and pure IIB fibers with glycolytic metabolism were detected. In contrast to the majority of mature control muscles, numerous hybrid fibers coexpressing MyHC-2x/d with MyHC-2a or MyHC-2b were present. Both hybrids were oxidative-glycolytic; additionally, some hybrids containing MyHC-2a were oxidative. In one out of six muscles, traces of MyHC-1 were detected both with immunoperoxidase staining and with SDS glycerol gel electrophoresis. Rare fibers that exceptionally expressed small amounts of MyHC-1 always coexpressed MyHC-2a, which is an additional proof that pure type I fibers do not exist in LAL. Due to these histochemical characteristics and to its previously described morphological features, the use of the LAL muscle as a model for various studies, particularly muscle and nerve interactions, is emphasized.

Is the internal calcium regulation altered in type A botulinum toxin-poisoned motor endings?

The hypothesis according to which botulinum A toxin blocks acetylcholine release from motor endings by stimulating intracellular Ca2+ disposal systems was tested by recording presynaptic membrane currents from poisoned muscles. Calcium and calcium-activated potassium currents displayed amplitudes, time courses and stimulation frequency-dependent inactivation similar to those observed in unpoisoned preparations. This indicates that poisoned endings are no more efficient than normal ones in dealing with Ca2+ overloads.

Nitric oxide and peroxynitrite affect differently acetylcholine release, choline acetyltransferase activity, synthesis, and compartmentation of newly formed acetylcholine in Torpedo marmorata synaptosomes

Recent reports proposed that nitric oxide was a modulator of cholinergic transmission. Here, we examined the role of NO on cholinergic metabolism in a model of the peripheral cholinergic nervous synapse: synaptosomes from Torpedo electric organ. The presence of NO synthase was immunodetected in the cell bodies, in the nerve ending area of nerve-electroplate tissue and in the electroplates. Exogenous source of NO was provided from SIN1, a donor of NO and O2-., and an end-derivative peroxynitrite (ONOO-). SIN1 increased calcium-dependent acetylcholine (ACh) release induced by KCl depolarization or a calcium ionophore A23187. The formation of ONOO- was continuously followed by a new chemiluminescent assay. The addition of superoxide dismutase, that decreases the formation of ONOO-, did not impair the stimulation of ACh release, suggesting that NO itself was the main stimulating agent. When the endogenous source of NO was blocked by proadifen, an inhibitor of cytochrome P450 activity of NO synthase, both KCl- and A23187-induced ACh release were abolished; nevertheless, the inhibitor Ng-monomethyl-L-arginine did not modify ACh release when applied in a short time duration of action. Both NO synthase inhibitors reduced the synthesis of ACh from the radioactive precursor acetate and its incorporation into synaptic vesicles as did ONOO- chemically synthesized or formed from SIN1. In addition, choline acetyltransferase activity was strongly inhibited by ONOO- and SIN1 but not by the NO donors SNAP and SNP or, by NO synthase inhibitors. Altogether these results indicate that NO and ONOO modulate presynaptic cholinergic metabolism in the micromolar range, NO (up to 100 microM) being a stimulating agent of ACh release and ONOO- being an inhibitor of ACh synthesis and choline acetyltransferase activity.

Mouse motor nerve terminal immunoreactivity to synaptotagmin II during sustained quantal transmitter release

An antibody directed against the lumenal NH2-terminus of synaptotagmin II was used to examine the distribution of this vesicular protein either after spontaneous acetylcholine release or after sustained release induced by La3+ or alpha-latrotoxin, in conditions that prevent endocytosis. The detection of the epitope was examined in the presence or absence of Triton X-100. We show that, in resting conditions of transmitter release, permeabilization of nerve terminal membranes is required for obvious detection of synaptotagmin Ii immunoreactivity whereas during sustained rates of quantal release, permeabilization is not necessary. These data indicate that, in the latter conditions, synaptotagmin II is incorporated into the terminal axolemma and its intravesicular domain exposed at the extracellular nerve terminal surface.

Reflex modulation of motoneurone activity in the cheliped of the crayfish Astacus leptodactylus

1. The reflex activity elicited by movement of the mero-carpopodite (M-C) joint in the cheliped of the crayfish Astacus leptodactylus is investigated and the role of the different proprioceptors (chordotonal and myochordotonal organs) separately studied. 2. The reflex discharge involves mainly the tonic motoneurones of the extensor (E), the flexor (F) and the accessory flexor (AF) muscles. 3. M-C joint posture is also regulated by the cuticular stress detector (CSD2) afferents: they increase mainly the F discharge and secondarily the AF command. 4. The activity of the motor axons supplying the muscles of the meropodite can be also influenced by a variety of natural stimuli applied to other appendages. The effect usually produced is a general flexion reaction which is characterized by a reciprocity between E and F involving both central and peripheral mechanisms. 5. The AF muscle is innervated by two antagonistic motoneurones, an excitatory neurone functionally linked in its discharge with one of the four excitors supplying F and an inhibitory motoneurone, common with E. The resulting competitive effect between these two neurones has been recorded intracellularly in AF muscle fibres. 6. The role of the myochordotonal organ (MCO) in the crayfish is discussed. In particular the modulation of the AF command in relation to the discharges of the motor nerves to the main muscle E and F is studied.

Electrical activity of mouse motor endings during muscle reinnervation

An in vitro study of electrical activity of regenerating motor endings was performed 11-15 days after the crushing of one motor nerve supplying the triangularis sterni muscle in the adult mouse. For this purpose, presynaptic membrane currents elicited by electrical stimulation of the regenerating nerve were recorded by external electrodes. Ionic channel distribution along the length of the endings was deduced from wave form configuration in normal perfusing fluid together with changes produced by application of specific channel blocking agents. The sharp negative deflection which was shown to correspond to inward Na+ current by its sensitivity to tetrodotoxin application could be recorded along most of the length of the endings indicating a widespread distribution of Na channels. Frequent absence of the late wave form component which signals K+ current was taken to indicate an even K+ current density in the few last nodes, the heminode and the distal part of the endings. Therefore, it appears that regenerating motor endings are characterized by an overlap of Na and K conductances all along their length. In the course of regeneration, the heminode loses the sensitivity to K channel blocking agents and the remainder of the terminal becomes insensitive to tetrodotoxin, the former change occurring usually earlier than the latter.

Plasticity of motor nerve terminals in Drosophila T (X,Y)V7 mutant: effect of deregulation of the novel calcium-binding protein frequenin

The Drosophila T(X,Y)V7 mutant is characterized by abnormally large motor responses that build up upon repetitive stimulation. Genetically it is characterized by a chromosomal breakpoint located at the proximal end of the Shaker gene complex. This mutation affects a gene which encodes a novel calcium-binding protein: the frequenin. Since neuronal activity is known to affect neurite elongation we looked for the geometry of motor terminal arborization in this mutant. Our results show a significant reduction in number and length of motor terminal branches in mutants as compared to wild type. This observation is opposite to the effect of other hyperexcitable mutations such as Shaker or ether-a-gogo or Hyperkinetic. Thus the V7 phenotype cannot be interpreted as a result of changes in motoneuron firing pattern. According to results obtained on transformed larvae in which frequenin cDNA expression was under the control of a heat shock promoter, it appears that the morphological phenotype of V7 may be due to specific effects of deregulation of this calcium-binding protein.

Synaptotagmin II immunoreactivity in normal and botulinum type-A treated mouse motor nerve terminals

The distribution of synaptotagmin II, a synaptic vesicle protein, was examined by immunohistochemistry at normal mouse motor nerve terminals and after botulinum type-A treatment. An immunoreactivity to synaptotagmin II was detected in both control and botulinum type-A treated motor nerve terminals including newly formed sprouts. These data, together with other reports showing the absence of synaptotagmin I at the neuromuscular junction, suggest that synaptotagmin II is the isoform involved in transmitter release at motor nerve terminals.

Membrane currents in lizard motor nerve terminals and nodes of Ranvier

Presynaptic membrane currents were recorded by external electrodes and nodal membrane currents were obtained by the voltage clamp technique in motor nerve endings and nodes of Ranvier of the lizard Anolis carolinensis. Although of compact shape, lizard motor endings display relatively long terminal branches; they exhibit, in agreement with previous findings in mouse and frog motor terminals, Na, Ca and K conductances, the latter consisting of a voltage- and a Ca-dependent type. Lizard nodes of Ranvier, like those of the frog, but unlike those of the mouse, exhibit a K conductance. These observations provide an explanation for the differences and similarities in presynaptic wave form configuration between the lizard and the other two species.

Incorporation of synaptotagmin II to the axolemma of botulinum type-A poisoned mouse motor endings during enhanced quantal acetylcholine release

The involvement of terminal sprouts in neurotransmitter release by in vivo botulinum type-A toxin poisoned motor endings was investigated 15 to 40 days after a single injection of the toxin onto the levator auris longus muscle of the mouse. Enhanced quantal acetylcholine release was induced by alpha-latrotoxin or La3+ in conditions that prevent endocytosis, and an antibody directed against the lumenal domain of synaptotagmin II (Syt II) was used in the presence or absence of Triton X-100. We showed that, under resting conditions, the intravesicular domain of Syt II requires Triton X-100 to be labelled, whereas it becomes exposed to the outside of the axolemma of both the original terminal arborization and the newly formed sprouts during enhanced exocytosis. These data were taken to indicate that, when sprouting is prominent, the whole modified terminal arborization, including the original branches and the sprouts, possesses the machinery for Ca2+-independent exocytosis.

In botulinum type A-poisoned frog motor endings ouabain induces phasic transmitter release through Na+-Ca2+ exchange

Ouabain (100 microM) applied for 60 min to botulinum A (BoTx) poisoned motor junctions increases, in a time-dependent manner, the mean number of acetylcholine quanta released by nerve stimulation and enhances the delayed transmitter release. The drug does not affect spontaneous quantal release. The observed effects on evoked transmitter release cannot be explained by changes in the configuration of presynaptic currents recorded from motor terminals. They suggest that in BoTx-poisoned motor endings the level of intraterminal Ca2+, lower than that required for the activation of quantal transmitter release, can be effectively increased through the reversed operation of an Na+-Ca2+ exchange system that normally uses the Na+ gradient to extrude Ca2+.