The formation of synapses in amphibian striated muscle during development

Seasonal factors influence quantal transmitter release and calcium dependence at amphibian neuromuscular junctions

Amphibian neuromuscular junctions (NMJs) are composed of hundreds of neurotransmitter release sites that exhibit nonuniform transmitter release probabilities and demonstrated seasonal modulation. We examined whether recruitment of release sites is variable when the extracellular calcium concentration ([Ca²⁺]_o) is increased in the wet and dry seasons. The amount of transmitter released from the entire nerve terminal increases by approximately the fourth power as [Ca²⁺]_o is increased. Toad (Bufo marinus) NMJs were visualized using 3,3'-diethyloxardicarbocyanine iodide [DiOC₂(5)] fluorescence, and focal loose patch extracellular recordings were used to record the end-plate currents (EPCs) from small groups of release sites. Quantal content (m̄_e ), average probability of quantal release (p_e ), and the number of active release sites (n_e ) were determined for different [Ca²⁺]_o Our results indicated that the recruitment of quantal release sites with increasing [Ca²⁺]_o differs spatially (between different groups of release sites) and also temporally (in different seasons). These differences were reflected by the nonuniform alterations in p_e and n_e Most release site groups demonstrated an increase in both p_e and n_e when [Ca²⁺]_o increased. In ~30% of release site groups examined, p_e decreased while n_e increased only during the active period (wet season). Although the dry season induced parallel right shift in the quantal release versus extracellular calcium concentration when compared with the wet season, the dependence of quantal content on [Ca²⁺]_o was not changed. These results demonstrate the flexibility, reserve, and adaptive capacity of neuromuscular junctions in maintaining appropriate levels of neurotransmission.

Physiology of afferent neurons in larval zebrafish provides a functional framework for lateral line somatotopy

Fishes rely on the neuromasts of their lateral line system to detect water flow during behaviors such as predator avoidance and prey localization. Although the pattern of neuromast development has been a topic of detailed research, we still do not understand the functional consequences of its organization. Previous work has demonstrated somatotopy in the posterior lateral line, whereby afferent neurons that contact more caudal neuromasts project more dorsally in the hindbrain than those that contact more rostral neuromasts (Gompel N, Dambly-Chaudiere C, Ghysen A. Development 128: 387-393, 2001). We performed patch-clamp recordings of afferent neurons that contact neuromasts in the posterior lateral line of anesthetized, transgenic larval zebrafish (Danio rerio) to show that larger cells are born earlier, have a lower input resistance, a lower spontaneous firing rate, and tend to contact multiple neuromasts located closer to the tail than smaller neurons, which are born later, have a higher input resistance, a higher spontaneous firing rate, and tend to contact single neuromasts. We suggest that early-born neurons are poised to detect large stimuli during the initial stages of development. Later-born neurons are more easily driven to fire and thus likely to be more sensitive to local, weaker flows. Afferent projections onto identified glutamatergic regions in the hindbrain lead us to hypothesize a novel mechanism for lateral line somatotopy. We show that afferent fibers associated with tail neuromasts respond to stronger stimuli and are wired to dorsal hindbrain regions associated with Mauthner-mediated escape responses and fast, avoidance swimming. The ability to process flow stimuli by circumventing higher-order brain centers would ease the task of processing where speed is of critical importance. Our work lays the groundwork to understand how the lateral line translates flow stimuli into appropriate behaviors at the single cell level.

The stimulus–response curve and motor unit variability in normal subjects and subjects with amyotrophic lateral sclerosis

The behavior and stability of motor units (MUs) in response to electrical stimulation of different intensities can be assessed with the stimulus-response curve, which is a graphical representation of the size of the compound muscle action potential (CMAP) in relation to stimulus intensity. To examine MU characteristics across the whole stimulus range, the variability of CMAP responses to electrical stimulation, and the differences that occur between normal and disease states, the curve was studied in 11 normal subjects and 16 subjects with amyotrophic lateral sclerosis (ALS). In normal subjects, the curve showed a gradual increase in CMAP size with increasing stimulus intensity, although one or two discrete steps were sometimes observed in the upper half of the curve, indicating the activation of large MUs at higher intensities. In ALS subjects, large discrete steps, due to loss of MUs and collateral sprouting, were frequently present. Variability of the CMAP responses was greater than baseline variability, indicating variability of MU responses, and at certain levels this variability was up to 100 microVms. The stimulus-response curve shows differences between normal and ALS subjects and provides information on MU activation and variability throughout the curve.

Developmental consequences of neuromuscular junctions with reduced presynaptic calcium channel function

Evoked neurotransmitter release at the Drosophila neuromuscular junction (NMJ) is regulated by the amount of calcium influx at the presynaptic nerve terminal, as for most chemical synapses. Calcium entry occurs via voltage-gated calcium channels. The temperature-sensitive Drosophila mutant, cac(TS2), has a reduced amount of calcium entry during evoked stimulation. We have used this mutation to examine homeostatic regulatory mechanisms during development of the NMJ on muscle 6 within the developing larva. The amplitude of the excitatory postsynaptic potentials are reduced for both the Ib and Is motor neurons in 3rd instar larvae which have been raised at 33 degrees C from the 1st instar stage. Larvae raised at 25 degrees C and larvae pulsed at 33 degrees C from the late 2nd instar for various lengths of time show a reduced synaptic efficacy as a 3rd instar. The results indicate that the nerve terminal cannot fully compensate physiologically in the regulation of synaptic transmission during larval life for a reduced amount of evoked calcium entry. Morphological comparisons of Ib and Is terminals in relation to length and numbers of varicosities are significantly reduced in cac(TS2), which also suggests a lack in homeostatic ability. These findings are relevant since many deficits in synaptic transmission in various systems are compensated for either physiologically or structural over development, but not in this case for reduced calcium entry during evoked transmission.

Formation and function of synapses with respect to Schwann cells at the end of motor nerve terminal branches on mature amphibian (Bufo marinus) muscle

A study has been made of the formation and regression of synapses with respect to Schwann cells at the ends of motor nerve terminal branches in mature toad (Bufo marinus) muscle. Synapse formation and regression, as inferred from the appearance and loss of N-(3-triethylammoniumpropyl)-4-(4-(dibutylamino)styryl) pyridinium dibromide (FM1-43)-stained vesicle clusters, occurred at the ends of terminal branches over a 16 hr period. Multiple microelectrodes placed in an array about FM1-43 blobs at the ends of terminal branches detected the electrical signs of neurotransmitter being released onto receptors. Injection of a calcium indicator (Oregon Green 488 BAPTA-1) into the motor nerve with subsequent imaging of the calcium transients, in response to stimulation, often showed a reduced calcium influx in the ends of terminal branches. Injection of a fluorescent dye into motor nerves revealed the full extent of their terminal branches and growing processes. Injection of the terminal Schwann cells (TSCs) often revealed pseudopodial TSC processes up to 10-microm-long. Imaging of these TSC processes over minutes or hours showed that they were highly labile and capable of extending several micrometers in a few minutes. Injection of motor nerve terminals with a different dye to that injected into their TSCs revealed that terminal processes sometimes followed the TSC processes over a few hours. It is suggested that the ends of motor nerve terminals in vivo are in a constant state of remodeling through the formation and regression of processes, that TSC processes guide the remodeling, and that it can occur over a relatively short period of time.

Formation and function of synapses with respect to Schwann cells at the end of motor nerve terminal branches on mature amphibian (Bufo marinus) muscle

A study has been made of the formation and regression of synapses with respect to Schwann cells at the ends of motor nerve terminal branches in mature toad (Bufo marinus) muscle. Synapse formation and regression, as inferred from the appearance and loss of N-(3-triethylammoniumpropyl)-4-(4-(dibutylamino)styryl) pyridinium dibromide (FM1-43)-stained vesicle clusters, occurred at the ends of terminal branches over a 16 hr period. Multiple microelectrodes placed in an array about FM1-43 blobs at the ends of terminal branches detected the electrical signs of neurotransmitter being released onto receptors. Injection of a calcium indicator (Oregon Green 488 BAPTA-1) into the motor nerve with subsequent imaging of the calcium transients, in response to stimulation, often showed a reduced calcium influx in the ends of terminal branches. Injection of a fluorescent dye into motor nerves revealed the full extent of their terminal branches and growing processes. Injection of the terminal Schwann cells (TSCs) often revealed pseudopodial TSC processes up to 10-microm-long. Imaging of these TSC processes over minutes or hours showed that they were highly labile and capable of extending several micrometers in a few minutes. Injection of motor nerve terminals with a different dye to that injected into their TSCs revealed that terminal processes sometimes followed the TSC processes over a few hours. It is suggested that the ends of motor nerve terminals in vivo are in a constant state of remodeling through the formation and regression of processes, that TSC processes guide the remodeling, and that it can occur over a relatively short period of time.

Synapse formation molecules in muscle and autonomic ganglia: the dual constraint hypothesis

In 1970 it was thought that if the motor-nerve supply to a muscle was interrupted and then allowed to regenerate into the muscle, motor-synaptic terminals most often formed presynaptic specializations at random positions over the surface of the constituent muscle fibres, so that the original spatial pattern of synapses was not restored. However, in the early 1970s a systematic series of experiments were carried out showing that if injury to muscles was avoided then either reinnervation or cross-reinnervation reconstituted the pattern of synapses on the muscle fibres according to an analysis using the combined techniques of electrophysiology, electronmicroscopy and histology on the muscles. It was thus shown that motor-synaptic terminals are uniquely restored to their original synaptic positions. This led to the concept of the synaptic site, defined as that region on a muscle fibre that contains molecules for triggering synaptic terminal formation. However, nerves in developing muscles were found to form connections at random positions on the surface of the very short muscle cells, indicating that these molecules are not generated by the muscle but imprinted by the nerves themselves; growth in length of the cells on either side of the imprint creates the mature synaptic site in the approximate middle of the muscle fibres. This process is accompanied at first by the differentiation of an excess number of terminals at the synaptic site, and then the elimination of all but one of the terminals. In the succeeding 25 years, identification of the synaptic site molecules has been a major task of molecular neurobiology. This review presents an historical account of the developments this century of the idea that synaptic-site formation molecules exist in muscle. The properties that these molecules must possess if they are to guide the differentiation and elimination of synaptic terminals is considered in the context of a quantitative model of this process termed the dual-constraint hypothesis. It is suggested that the molecules agrin, ARIA, MuSK and S-laminin have suitable properties according to the dual-constraint hypothesis to subserve this purpose. The extent to which there is evidence for similar molecules at neuronal synapses such as those in autonomic ganglia is also considered.

Cholinergic stimulation increases thrombin activity and gene expression in cultured mouse muscle

Activity-dependent synapse reduction is a major determinant of neuromuscular innervation. Previous research has shown that nanomolar concentrations of hirudin, a specific thrombin antagonist, significantly attenuates this reduction, and protease nexin 1 (PN1), an endogenous thrombin inhibitor closely localized to the neuromuscular synapse, can inhibit synapse reduction at similar concentrations. Protease inhibitors which do not inhibit thrombin, including cystatin and aprotinin, had no effect on synapse reduction. We present a series of experiments examining whether prothrombin and/or PN1 gene expression, as well as thrombin activity, are regulated in muscle cultures by acetylcholine (ACh) receptor activation. We also studied the effect of exogenous thrombin on synapse elimination in co-cultures of muscle and cholinergic neurons. Cultured muscle cells were electrically blocked with tetrodotoxin (TTX), or co-treated with ACh in order to isolate ACh receptor activation. Electrical blockade resulted in a decrease in thrombin release to about two-thirds of control values. The application of ACh to electrically blocked muscle cultures resulted in a 2.5-fold increase in thrombin activity released into the medium and a 2-fold increase in prothrombin gene expression. In contrast, ACh treatment in the presence of TTX had no effect on PN1 gene expression compared to treatment with TTX alone. In addition, exogenous thrombin significantly increased synapse elimination in unstimulated muscle/cholinergic neuron co-cultures. These results suggest that thrombin or a thrombin-like molecule released from muscle is required for activity-dependent synapse elimination and is regulated by neuromuscular activity.

Autonomic neuromuscular transmission at a varicosity

This review attempts to clarify the definition of what constitutes an autonomic neuromuscular function formed by a varicosity. Ultrastructural studies of serial sections through varicosities, partly or wholly bare of Schwann cell covering, show that areas of close apposition occur between varicosities and muscle cell membrane that vary between 20 and 150 nm, depending on the muscle considered. Consideration of the diffusion of purine transmitters and their receptor kinetics after secretion in a packet show that the number of purinergic receptor channels opened at a site of 150 nm apposition by a varicosity is about 15% of that at a site of 50 nm apposition. These results, together with the analysis of the stochastic fast component and the deterministic slow components of the rising phase of the EJP suggest that the stochastic fast component is due to varicosities that form especially close appositions (20-50 nm), whereas the deterministic slow component is due to the large number of varicosities at distances up to about 150 nm. Varicosities forming appositions of 20-150 nm with muscle cells several hundred micrometers long possess junctional receptor types distinct from extrajunctional receptors. According to this argument, then, there are two different classes of varicosities: one that gives rise to a relatively large junctional current and another that is responsible for a very small junctional current. Present evidence suggests that two subclasses of varicosities can be discerned amongst the varicosities that generate large junctional currents. One of these subclasses of varicosity possesses relatively few post-junctional receptors compared with the amount of transmitter reaching the receptors from the varicosity, so that the junctional current generated is determined by the size of the receptor population; in this case, the size of the transmitter packages released from these varicosities is unknown and the size of the junctional current is relatively constant. The other subclass of varicosity possesses large receptor patches, sufficient to accommodate the largest amounts of transmitter released from the varicosities: in this case, the size of the transmitter packages is shown to be highly non-uniform. These speculations await confirmation by direct labelling of the receptor patches beneath varicosities, a possibility that is likely to be realized in the near future.

Quantal transmitter release at somatic motor-nerve terminals: stochastic analysis of the subunit hypothesis

Here we analyze the problem of determining whether experimentally measured spontaneous miniature end-plate currents (MEPCs) indicate that quanta are composed of subunits. The properties of MEPCs at end plates with or without secondary clefts at the neuromuscular junction are investigated, using both stochastic and deterministic models of the action of a quantum of transmitter. It is shown that as the amount of transmitter in a quantum is increased above about 4000 acetylcholine (ACh) molecules there is a linear increase in the size of the MEPC. It is possible to then use amplitude-frequency histograms of such MEPCs to detect a subunit structure, as there is little potentiation effect above 4000 ACh molecules. Autocorrelation and power spectral analyses of such histograms establish that their subunit structure can be detected if the coefficient of variation of the subunit size is less than about 0.12 or, if electrical noise is added, about 0.1. Positive gradients relate the rise time and half-decay times of MEPCs to their amplitude, even in the absence of potentiating effects; these gradients are shallower at motor nerve terminals that possess secondary clefts. The effect of asynchronous release of subunits is also investigated. The criteria determined by this analysis for identifying a subunit composition in the quantum are applied to an amplitude-frequency histogram of MEPCs recorded from a small group of active zones at a visualized amphibian motor-nerve terminal. This did not provide evidence for a subunit structure.

Probabilistic secretion of quanta: spontaneous release at active zones of varicosities, boutons, and endplates

The amplitude-frequency histogram of spontaneous miniature endplate potentials follows a Gaussian distribution at mature endplates. This distribution gives the mean and variance of the quantum of transmitter. According to the vesicle hypothesis, this quantum is due to exocytosis of the contents of a single synaptic vesicle. Multimodal amplitude-frequency histograms are observed in varying degrees at developing endplates and at peripheral and central synapses, each of which has a specific active zone structure. These multimodal histograms may be due to the near synchronous exocytosis of more than one vesicle. In the present work, a theoretical treatment is given of the rise of intraterminal calcium after the stochastic opening of a calcium channel within a particular active zone geometry. The stochastic interaction of this calcium with the vesicle-associated proteins involved in exocytosis is then used to calculate the probability of quantal secretions from one or several vesicles at each active zone type. It is shown that this procedure can account for multiquantal spontaneous release that may occur at varicosities and boutons, compared with that at the active zones of motor nerve terminals.

The origin of Gaussian distributions of synaptic potentials

Spontaneous synaptic potentials were identified at the motor endplate 40 years ago. These were shown to possess amplitudes that could be described by a Gaussian distribution as could the amplitudes of evoked synaptic potentials under conditions of very low probability for secretion. As these Gaussians were identical, the idea of a unit or quantum of transmission was conceived. The failure to obtain similar Gaussian distributions for both spontaneous and low-probability evoked potentials during development of endplates indicated that a unit of transmission was not operating. However both the spontaneous and very low-probability evoked potentials could each be described by mixtures of Gaussians indicating a subunit of transmission might be operative. There are no ganglionic or central synapses at which comparisons have been made between spontaneous and low-probability evoked potentials that show each can be described by a Gaussian distribution, let alone that these are the same indicating a unit of transmission as originally conceived. There is some evidence that mixtures of Gaussians can be used to describe both spontaneous and very low-probability evoked synaptic potential amplitudes, opening up the possibility for a subunit of transmission at these synapses. The vesicle hypothesis, that the quantum of transmission at the endplate is due to the exocytosis of the contents of a synaptic vesicle, was also enunciated nearly 40 years ago. The existence of subunits of transmission has required reconsideration of this hypothesis. Three alternatives are considered: in one, the calcium-transient hypothesis, the subunit of secretion is due to the release of calcium from one of several calcium stores in the nerve terminal, so that several subunits are released when a number of these calcium stores are engaged in a regenerative response to the terminal action potential; a second alternative, the mediatophore hypothesis, is that a subunit of secretion occurs when a single transmitter transport protein channels transmitter across the terminal membrane, several such mediatophore proteins acting in concert then give multiple subunit release; finally, there is the vesicle fusion-pore hypothesis, in which individual transient openings of a fusion-pore channel joining a synaptic vesicle to the terminal membrane are responsible for secretion of a transmitter subunit, with multiple transients giving several subunits. Perhaps we will have distinguished between these possibilities before the quantal hypothesis is 50 years old.

Sexual differentiation in the CNS of the moth, Manduca sexta. II. Target dependence for the survival of the imaginal midline neurons

While the majority of neurons in the adult nervous system of the moth Manduca sexta are produced postembryonically, little is known about how these cells interact with their targets during development. Few of these cells are motor neurons; most of Manduca's adult motor neurons are respecified larval motor neurons that developed embryonically. A few motor neurons do develop postembryonically, including a large class of mixed neurosecretory and motor neurons called the imaginal midline neurons (IMNs). A subset of these cells show an unusual pattern of sex-specific development and survival (Thorn and Truman, 1994, J. Neurobiol. in press), which led us to suspect that factors extrinsic to the cells were controlling their fates. We analyzed one such potential factor by altering the contacts between a subset of these developing IMNs and their adult-specific target, the male sperm duct. When we transected the nerve that innervated the sperm duct in the pupa, we observed a loss of many sperm duct IMNs. In contrast, a transection of the same nerve in larvae showed no neuron loss. Immunocytochemistry showed that the pupal nerve transections were accompanied by a loss of axon endings on the sperm duct, while the larval nerve transections showed no such loss. Using local hormone application to slow the development of the sperm duct while leaving the nerve intact still resulted in a loss of IMNs. These results suggest that these IMNs need contact with a robust developing target in the pupa to survive metamorphosis.

Synapse elimination from the mouse neuromuscular junction in vitro: a non-Hebbian activity-dependent process

The effect of action potentials on elimination of mouse neuromuscular junctions (NMJ) was studied in a three-compartment cell culture preparation. Axons from superior cervical ganglion or ventral spinal cord neurons in two lateral compartments formed multiple neuromuscular junctions with muscle cells in a central compartment. The loss of synapses over a 2-7-day period was determined by serial electrophysiological recording and a functional assay. Electrical stimulation of axons from one side compartment during this period, using 30-Hz bursts of 2-s duration, repeated at 10-s intervals, caused a significant increase in synapse elimination compared to unstimulated cultures (p < 0.001). The extent of homosynaptic and heterosynaptic elimination was comparable, i.e., of the 226 functional synapses of each type studied, 111 (49%) of the synapses that had been stimulated were eliminated, and 87 (39%) of unstimulated synapses on the same muscle cells were eliminated. Also, simultaneous bilateral stimulation caused significantly greater elimination of synapses than unilateral stimulation (p < 0.005). These observations are contrary to the Hebbian hypothesis of synaptic plasticity. A spatial effect of stimulus-induced synapse elimination was also evident following simultaneous bilateral stimulation. Prior to stimulation, most muscle cells were innervated by axons from both side compartments, but after bilateral stimulation, muscle cells were predominantly unilaterally innervated by axons from the closer compartment. These experiments suggest that synapse elimination at the NMJ is an activity-dependent process, but it does not follow Hebbian or anti-Hebbian rules of synaptic plasticity. Rather, elimination is a consequence of postsynaptic activation and a function of location of the muscle cell relative to the neuron. An interaction between spatial and activity-dependent effects on synapse elimination could help produce optimal refinement of synaptic connections during postnatal development.

Synapse formation and elimination during growth of the pectoral muscle in Xenopus laevis

1. Synapse formation and synapse elimination were studied in the pectoral muscle of Xenopus laevis. 2. Histology showed that fibres were not added during postmetamorphic growth. Most fibres were innervated at two widely separated junctions and this number did not change as frogs grew. 3. Intracellular recording revealed that fibres with two junctions could be mononeuronally innervated, or innervated in one of three different polyneuronal patterns. A growth-related shift in innervation pattern was observed, with the polyneuronal patterns replaced by mononeuronal innervation. 4. Endplate potentials (EPPs) evoked by low-frequency nerve stimulation were simultaneously measured at both junctions on individual fibres. For each fibre, the ratio of EPP amplitudes (smaller/larger) was calculated. When the two junctions were innervated by different motoneurones (A-B), the median EPP ratio was smaller than when the two junctions were innervated by the same motoneurone (A-A), although the difference was not significant. 5. The difference in the ratio of EPP amplitudes became significant, however, if junctions were conditioned by a train of fifty stimuli at 10 Hz. Immediately after such a train, EPP ratios for A-B fibres were significantly smaller than ratios for A-A fibres. This difference was due to greater synaptic depression at one of the junctions on A-B fibres. 6. We concluded that enhanced depression of the EPP upon repetitive stimulation is a physiological correlate of the competition that underlies synapse elimination.

Redeployment of trigeminal motor axons during metamorphosis

As a consequence of the degeneration and replacement of the jaw muscle fibers in the leopard frog, Rana pipiens, trigeminal motoneurons innervate different targets before and after metamorphosis. This investigation examined the morphological correlates of the reassignment of trigeminal motoneurons during the initial phases of myofiber turnover. Specifically, silver-cholinesterase histochemistry and electron microscopy were used to 1) identify the fate of motor axons within the neuromuscular junctions (NMJs) applied to degenerating larval myofibers and 2) to determine the origin(s) of the motor axons that innervate the postmetamorphic muscle fibers of the jaw. The results demonstrate that the NMJs are retained on larval myofibers throughout their degeneration and are readily identifiable on the residual larval basal laminae that remain after involution of the sarcoplasm. Light and electron microscopic observations provide evidence that both pre- and post-synaptic elements are present on the degenerating fibers. Furthermore, morphometric analyses indicate that the preponderance (86%) of motor axons supplying adult muscle fibers originates from the larval NMJs. This condition suggests that metamorphic redeployment of trigeminal motoneurons occurs through the resumption of growth at the axon terminal supplying larval muscle rather than through the proximal collateralization of these axons and resorption of larval terminals.

Repeated, in vivo observation of frog neuromuscular junctions: remodelling involves concurrent growth and retraction

The fluorescent dye 4-(4-diethylaminostyryl)-N-methylpyridinium iodide was used as a vital stain to study remodelling of motor nerve terminals in sartorius muscles of living frogs (Rana pipiens). Identified terminals were observed twice in vivo at intervals of 87-192 days. After the second observation, muscles were fixed and stained with the nitroblue tetrazolium method for nerve terminals and with cholinesterase stain. Observations were made of 243 junctions in 26 frogs. Most nerve terminals grew during the observation interval, with an average increase in total terminal length of 29%. This growth involved substantial remodelling. Within single junctions, the change in size was the net result of differing degrees of growth or shrinkage in individual nerve terminal branches. At least one new terminal branch appeared in 25% of the junctions. Terminal retraction was also common, with branch shortening seen in 60% of junctions and the complete disappearance of a branch in 12%. In one case the original axonal input retracted completely and the junction was partially reinnervated by a terminal sprout from a junction on an adjacent fibre. Some discrepancies between histological and in vivo observations of remodelling were noted. These observations confirm that frog neuromuscular junctions are highly dynamic synapses, subject to profound structural remodelling throughout adult life.

Mechanisms of elimination, remodeling, and competition at frog neuromuscular junctions

Mechanisms governing synapse elimination, synaptic remodeling, and polyneuronal innervation were examined in anatomical and electrophysiological studies of frog neuromuscular junctions. There was a substantial level of polyneuronal innervation in adult junctions and this varied seasonally. Nerve terminal retraction and synapse elimination occurred during normal growth and following reinnervation. Synapse elimination was not inevitable, however. Repeated in vivo observations of some identified junctions showed that polyneuronal innervation could persist for over a year, while at other junctions it arose de novo by terminal sprouting. We concluded that polyneuronal innervation in adult muscles was governed by an equilibrium between processes of retraction and elimination on one hand, and sprouting and synaptogenesis on the other. Other observations revealed that structural remodeling was a common feature of adult junctions. Most often, remodeling involved the simultaneous growth and retraction of different parts of the same junction. The net result was usually junctional growth that, in small frogs, appeared to provide a good match between synaptic size and the electrical demands of transmission. In larger animals, pre- and postsynaptic sizes were not as well matched, providing morphological evidence for a growth-associated decline in synaptic efficacy. Finally, electrophysiology was used to describe some of the functional correlates and consequences of competitive interactions between the terminals of different axons. These results are explained by a hypothetical mechanism that involves trophic support provided by the muscle to the motoneuron, the overall level of nerve-muscle activity, and the synchrony of pre- and postsynaptic activity.

Expression of the growth cone specific epitope CDA 1 and the synaptic vesicle protein SVP38 in the developing mammalian cerebral cortex

CDA 1 is a novel antigen that within the brain is present specifically in neuronal growth cones. Electron microscope immunohistochemistry and subcellular fractionation showed the CDA 1 epitope to be on a cytosolic molecule. In cultured neurons, it is abundant in growth cones and not detectable in neurites or cell bodies. The development of the rat cerebral cortex was investigated by using the monoclonal antibody to CDA 1 and an antibody to SVP38, the synaptic vesicle glycoprotein. CDA 1 immunoreactivity in the rat cerebral cortex peaks just before birth and disappears by postnatal day 12, a few days before the major increase in the number of mature synapses. In contrast, SVP38 is expressed in parallel with the appearance of mature synapses. CDA 1 and SVP38 thus are markers of growth cones and synapses, respectively. Their expression during development reflects some of the structural and functional changes that occur during synapse formation.

Electrophysiology and dye-coupling are sexually dimorphic characteristics of individual laryngeal muscle fibers in Xenopus laevis

Sex differences at the laryngeal neuromuscular junction of Xenopus laevis were examined by recording intracellularly from muscle fibers in response to nerve stimulation. Male laryngeal muscle contains 2 physiologically distinct fiber types. Type I fibers generate postsynaptic potentials in response to low-magnitude stimulus pulses and action potentials in response to higher-magnitude stimulus pulses. Type II muscle fibers require repetitive stimulation for action potential production, probably because of facilitation. Subthreshold events in type I and II fibers suggest that these neuromuscular synapses have low safety factor junctions. Female laryngeal muscle contains one fiber type (III), which is physiologically distinct from those found in the male. Type III fibers produce an action potential in response to a single-stimulus pulse of suprathreshold voltage delivered to the laryngeal nerve; subthreshold events were not observed. Iontophoretic injection of Lucifer yellow into a single female muscle fiber resulted in as many as 43 labeled fibers. In males, only one fiber was labeled. Dye-coupling was not observed in adult females treated with the androgenic steroid hormone, testosterone. We have previously reported that laryngeal muscle fibers are recruited throughout a stimulus train presented to the laryngeal nerve in males, but are not recruited in females (Tobias and Kelly, 1987). Sex differences in the frequency of electrophysiological fiber types described here may account for sex differences in fiber recruitment. Synchronous activity of dye-coupled fibers may increase the effectiveness of muscle contraction in females.

The development of topographical maps and fibre types in toad (Bufo marinus) glutaeus muscle during synapse elimination

1. The toad glutaeus muscle consists of two muscle compartments. A study has been made of the topographical distribution of motor units in these compartments, in relation to the fibre types which arise during different stages of development. 2. Monoclonal antibodies to myosin allowed the distribution of fibre types to be determined. In mature muscles (from toads of greater than 30 g body weight) clusters of type 5 (tonic) fibres were found exclusively at the dorsal surface of the muscle, surrounded by a layer of type 3 (slow-twitch) fibres. A homogeneous layer of type 2 (fast-twitch red) fibres was found beneath this dorsal rind of slow and tonic fibres. The rest of the muscle, including the ventral surface, consisted of a mosaic of type 1 (fast-twitch white) and type 2 fibres. 3. Glycogen-depletion methods, together with the myosin antibodies, allowed the distribution of single motor units and their fibre types to be determined. In mature muscles, axons originating from rostral spinal cord possessed muscle units located in a band extending from the ventral surface to beyond the middle of the muscle; these units consisted of 78% type 1 and 22% type 2 fibres found amongst the mosaic of type 1 and type 2 fibres. Intermediate axons possessed muscle units located primarily in the middle and dorsal half of the muscle. These units consisted mostly of type 2 fibres (29% type 1, 71% type 2) also found amongst the mosaic of type 1 and type 2 fibres. Thus rostral and intermediate units were of mixed fibre type, with type 1 fibres predominating in the former units and type 2 in the latter. Caudal axons possessed muscle units located mostly in the homogeneous layer of type 2 fibres, beneath the dorsal rind of tonic fibres; these units were almost always composed entirely of type 2 fibres. 4. The distribution of single motor units and their fibre types were determined for the caudal axons during development. In juvenile animals (toads of about 10 g body weight) the dorsal rind of tonic and slow fibres, together with the underlying homogeneous layer of type 2 fibres, were still present, but the rest of the muscle to the ventral surface consisted almost entirely of type 1 fibres. Caudal axons innervated the type 2 fibre layer at the dorsal surface as they do in mature animals. 5. The glutaeus in post-metamorphic toads (0.15 g body weight) had only a small number of tonic and slow-twitch fibres in the very dorsal layer of cells; the muscle was largely type 1.(ABSTRACT TRUNCATED AT 400 WORDS)

Spontaneous release of multiquantal miniature excitatory junction potentials induced by a Drosophila mutant

1. Intracellular recordings were made from muscle fibre No. 6 of the dorsal longitudinal flight muscle (DLM) of Drosophila melanogaster in both wild-type flies and the temperature-sensitive paralytic mutant, shibirets-1 (shi). 2. Continuous recordings of the miniature excitatory junction potentials (MEJPs) in this fibre were made as the temperature was changed from 19 to 29 degrees C, and back to 19 degrees C. In shi flies, synapses become depleted of vesicles at 29 degrees C due to a temperature-dependent blockage in the recycling process, while transmitter release proceeds normally. When the temperature is lowered to 19 degrees C, recycling is allowed to proceed and recovery of the full complement of synaptic vesicles gradually occurs in about 20 min. 3. It was observed that the MEJP amplitude distribution in shi flies was unimodal at 19 degrees C prior to heating (as was wild-type), but during recovery from 8 min exposure to 29 degrees C became multimodal, with peaks at roughly integral multiples of the original peak prior to heating. This effect was never seen in wild-type flies. 4. Also, during recovery, the MEJP did not occur randomly, but rather occurred in a clustered fashion. 5. It is concluded that during recovery from depletion in shi neuromuscular junctions, a condition exists which causes the synchronization of spontaneous release, causing multiquantal MEJPs or clustering of MEJPs, depending on the degree of synchronization. 6. The possible role of Ca2+ in this phenomenon is discussed.

Quantal secretion at release sites of nerve terminals in toad (Bufo marinus) muscle during formation of topographical maps

1. The number of quanta secreted from selected sites along terminal branches at suppressed synapses in the developing toad (Bufo marinus) gluteus muscle has been determined. The topographical projection from segmental nerves 8 and 9 to the ventral surface of this muscle matures slowly as toads develop in size from 12 to 40 g. Terminal branches of nerves 8 and 9 were visualized by prior staining with the fluorescent dye, 3-3-diethyloxardicarbocyanine iodide (DiOC2(5]. 2. The evoked quantal release recorded with an extracellular electrode (m(e) at different positions along the length of terminal branches at synaptic sites innervated either by nerve 8 (me,8) or nerve 9 (me,9) was determined in an external Ca2+ concentration, [Ca2+]o, of 0.35-0.45 mM. For over 90% of branches longer than 80 microns, me declined along exponential curves from a relatively large value at the proximal end of branches for both nerve 8 and nerve 9 terminals; the exponent for these exponential curves gave quantal length constants that varied from 26 to 80 microns (48 +/- 4 microns, mean +/- S.E.M.) depending on the length of the branch. 3. The evoked quantal release recorded with an intracellular electrode (m) at synaptic sites dually innervated by nerve 8 and nerve 9 was nearly always (greater than 90%) greater for nerve 8 terminals than for nerve 9 terminals. At singly innervated sites the value of m per 100 microns length of terminal declined approximately exponentially with an increase in total terminal length (length constant 400 microns). However, at dually innervated sites the value of m per 100 microns length of nerve 9 terminal was very low at all total terminal lengths compared with singly innervated sites; this indicates that nerve 9 terminals were suppressed at dually innervated sites. 4. At five dually innervated sites, seven out of nine terminal branches of nerve 8 showed an exponential decline in me,8 along their length, from a relatively large value near the proximal end of the branches (length constant 35 +/- 3 microns, mean +/- S.E.M.). In contrast, all the terminal branches of nerve 9 greater than 80 microns showed a uniformly low value of me,9 along their length. 5. It is suggested that the suppression of nerve 9 terminals at dually innervated sites is primarily due to a decrease in the probability of secretion of normally highly secreting release sites at the proximal end of terminal branches.

Electrophysiology and dye-coupling are sexually dimorphic characteristics of individual laryngeal muscle fibers in Xenopus laevis

Sex differences at the laryngeal neuromuscular junction of Xenopus laevis were examined by recording intracellularly from muscle fibers in response to nerve stimulation. Male laryngeal muscle contains 2 physiologically distinct fiber types. Type I fibers generate postsynaptic potentials in response to low-magnitude stimulus pulses and action potentials in response to higher-magnitude stimulus pulses. Type II muscle fibers require repetitive stimulation for action potential production, probably because of facilitation. Subthreshold events in type I and II fibers suggest that these neuromuscular synapses have low safety factor junctions. Female laryngeal muscle contains one fiber type (III), which is physiologically distinct from those found in the male. Type III fibers produce an action potential in response to a single-stimulus pulse of suprathreshold voltage delivered to the laryngeal nerve; subthreshold events were not observed. Iontophoretic injection of Lucifer yellow into a single female muscle fiber resulted in as many as 43 labeled fibers. In males, only one fiber was labeled. Dye-coupling was not observed in adult females treated with the androgenic steroid hormone, testosterone. We have previously reported that laryngeal muscle fibers are recruited throughout a stimulus train presented to the laryngeal nerve in males, but are not recruited in females (Tobias and Kelly, 1987). Sex differences in the frequency of electrophysiological fiber types described here may account for sex differences in fiber recruitment. Synchronous activity of dye-coupled fibers may increase the effectiveness of muscle contraction in females.

Electrophysiological and morphological study of polyneuronal innervation in the cutaneous pectoris muscle of adult frog (Rana esculenta)

The incidence of polyneuronal innervation in the cutaneous pectoris muscle of the adult frog, Rana esculenta, was determined quantitatively using electrophysiological and morphological techniques. The mean percentages of multiple innervated endplates obtained with both techniques from a series of 19 muscles examined at all times of the year were in very good correlation: 30.7% (196/639 endplates) by electrophysiology and 30.5% (478/1569 endplates) by morphology. In nine muscles examined during the period from December to March the mean percentages, 36.8% (110/299) by electrophysiology and 38.6% (281/727) by morphology, were significantly higher than those obtained for 10 muscles investigated during the period from May to November, 25.3% (86/340) and 23.4% (197/842) with both techniques respectively. The higher incidence of collateral sprouted branches detected at polyinnervated endplate sites in muscles of winter frogs might be related to these seasonal variations. Most of the 1688 fibres from 26 muscles examined throughout the year exhibited one centrally located endplate. However, around 11% of them were found to be innervated at two separate endplate sites. Muscle fibres exhibiting this type of innervation were invariably the largest fibres in each muscle tested, having an apparent diameter greater than 48 micron. The distance between the endplates of these fibres represented between 10 and 30% of their total length. No significant seasonal variations were observed in the incidence of these dually innervated fibres. In conclusion, both electrophysiological and morphological results show that the normal incidence of polyneuronal innervation in the cutaneous pectoris muscle of adult Rana esculenta is affected by seasonal related factors which influence the nodal sprouting activity. Moreover, they show that a dual pattern of innervation is a common feature in large fibres of this muscle.

Synaptic rearrangement during postembryonic development in the cricket

Synaptic rearrangement during development is a characteristic of the vertebrate nervous system and was thought to distinguish vertebrates from the invertebrates. However, examination of the wind-sensitive cercal sensory system of the cricket demonstrates that some identified synaptic connections systematically decrease in strength as an animal matures, while others increase in strength over the same period. Moreover, a single sensory neuron could increase the strength of its synaptic connection with one interneuron while decreasing the strength of its connection with another interneuron. Thus, rather than being a hallmark of the vertebrate nervous system, synaptic rearrangement is probably characteristic of the development of many if not all nervous systems.

The formation of topographical maps in developing rat gastrocnemius muscle during synapse elimination

1. The rat lateral gastrocnemius muscle (LG) is a complex of four muscle compartments, each defined in terms of its unique innervation by a single primary nerve branch of the muscle nerve. A study has been made of the topographical distribution of motor units in the medial compartment of the LG (LGM) both before and after the loss of polyneuronal innervation that accompanies development. 2. Glycogen depletion methods showed that the distribution of single motor units depended on the rostro-caudal origins of their axons in the spinal cord: rostral axons possessed motor units almost exclusively confined to the medial half of the LGM; intermediate axons possessed motor units primarily in the intermediate and lateral part of the LGM; caudal axons possessed motor units that were not restricted to any particular part of the LGM. 3. Myosin ATPase staining showed that about 80% of the LGM consists of type II A fibres, whilst the remainder are type II B. Physiological determination of the contractile properties of motor units indicated two classes of units: those that were relatively fatigue resistant and did not show a sag property (like fast-twitch, fatigue-resistant fibres or FR) and those that were relatively fatigable and did show a sag property (like fast-twitch, fatigable fibres or FF). 4. Glycogen depletion was also used to determine the distribution of motor units in the LGM at 7 days post-natal, when most fibres still receive a polyneuronal innervation. The LGM primary nerve branch innervated a confined sub-volume of muscle fibres which is similar to the mature pattern. However, rostral axons possessed motor units that extended into the lateral half of the LGM, a position from which they are excluded in the adult. 5. These observations suggest that the axons of rostral and intermediate units form a topographical map within adult FR motor units (type II A fibres) in the LGM. The results suggest that competition between axon terminals for synaptic sites plays a role in the elimination of inappropriately positioned terminals and subsequent emergence of the topographical map.

Loss of polyneuronal innervation and establishment of a topographical map in the glutaeus muscle of Bufo marinus during generation of secondary muscle cells

The development of synaptic connections to the toad (Bufo marinus) glutaeus magnus from segmental nerves 8 (N8) and 9 (N9) was determined in the postmetamorphic period. Three different-size toads were studied: small (0.3-2.0 g), medium-size (5-15 g) and large (greater than 20 g). The number of cells in the glutaeus increased about 9-fold during development; this involved the appearance and subsequent maturation of secondary fibres throughout the muscle. The glutaeus in small toads, which consisted almost entirely of primary fibres, was innervated to a similar extent by N8 and N9 as assessed by tetanic contraction measurements. During late development there was a progressive increase in the percentage of the muscle innervated by N9 and a decrease in the percentage innervated by N8. This change in the segmental innervation was accompanied by changes in the innervation of the ventral glutaeus as assessed by intracellular recording. In small toads this surface of the muscle was innervated predominantly by N8, with N9 frequently appearing as a low-efficacy terminal on dually innervated fibres. With further development there was a progressive reduction in the percentage of dually innervated fibres and a concomitant decrease in the percentage innervation of the entire ventral glutaeus by N8. These results suggest that the topographical projection is established by the initial distribution of N9 terminals on the primary fibres of the muscle. The multiple innervation of newly generated fibres and the on-going process of terminal elimination results in N9 terminals, many of which were initially weak, preserving their position in the muscle. This occurs at the expense of N8 terminals, whose relative incidence declines during development. The competitive advantage of N9 motoneurones may be due to their greater capacity to lay down axon collaterals and preferentially innervate newly generated fibres; alternatively N9 terminals may displace N8 terminals, which were initially more efficacious, from dually innervated fibres. Secondary muscle fibres generated throughout the muscle are thus incorporated into an increasingly precise topographical map.

Non-uniform release at the frog neuromuscular junction: evidence of morphological and physiological plasticity

The frog neuromuscular junction (NMJ) is a fusiform structure parallel to the muscle fiber with a few secondary and tertiary branches. Both sprouting and regression can occur on the same nerve terminal, suggesting a continuous on-going remodelling of the mature neuromuscular junction. Thus, the frog NMJ is a dynamic structure. Ultrastructural observations of the nerve terminal suggest that the active zones are distributed equally along the mature nerve terminal. Disorganized active zones have however been observed in distal regions. The density of synaptic vesicles is also uniform throughout the whole structure. However, mitochondria appear to be more abundant in the very distal regions of the nerve terminal. The postjunctional folds and the cholinergic receptors are also uniformly distributed along the NMJ. However, during remodelling periods, the distributions of postjunctional folds and of cholinergic receptors are not uniform in the degenerating and regenerating regions. Fig. 1 summarizes these morphological data. The frequency of spontaneous release (MEPPs) at the NMJ is higher in the proximal region than in the distal regions and recent evidence suggests that the mean MEPP amplitude is higher in the proximal than in the distal portions. Evoked transmitter release is also non-uniform along the frog NMJ. As for spontaneous release, it is higher in the proximal regions than in the distal regions. Failures of the active propagation of the PNAP at low safety points, such as the end of the myelinated axon and the branching points, may be one of the mechanisms responsible for unequal evoked release. It is also possible that the PNAP does not actively invade the whole extend of the nerve terminal since Na+ channels are absent from the distal regions. Fig. 2 summarizes these physiological data.

Competition between segmental nerves at end-plates in rat gastrocnemius muscle during loss of polyneuronal innervation

1. The segmental innervation of the rat lateral gastrocnemius (l.g.) muscle is from lumbar nerves L4 and L5. A study has been made of changes in the innervation of the l.g. muscle by nerves L4 and L5 before and after removal of L4 during the loss of polyneuronal innervation which occurs over the first 2 weeks postnatal. 2. The rat l.g. muscle is a complex of three unipennate muscles; the lateral, intermediate and medial heads. Each of these has a characteristic distribution of type I and type II muscle fibres of different diameter by 12 days postnatal. Following the removal of nerve L4 at birth there is no change in the distribution of fibre types in the different heads of the l.g. muscle at 12 days postnatal. 3. The number of fibres in the l.g. muscle doubled between birth and 12 days postnatal and the maximum tetanic force generated by the muscle increased by fourfold. Removal of nerve L4 at birth did not alter the increase in fibre number and there was no difference between the tension generated by the l.g. muscle in the ipsilateral and contralateral limbs of operated animals. 4. The number of detectable motor units in a extracellular calcium concentration [( Ca]o) of 2 mM remained constant in L4 at about fifteen between birth and 12 days postnatal; the number of detectable motor units in L5 declined over this period from about fifteen to six in a [Ca]o of 2 mM. Following the removal of L4 at birth, the number of motor units in L5 increased to twenty by 3 days postnatal and remained there for at least 12 days in a [Ca]o of 2 mM. 5. The mean size of motor units and the range of motor unit sizes declined for both L4 and L5 during the postnatal period. Following the removal of L4, the mean size of motor units and the range of motor unit sizes in L5 failed to decline. 6. The number of motor units detected at birth in a raised [Ca]o of 3-5 mM increased in L4 and L5 to about twenty; this estimate in high [Ca]o remained constant during the postnatal period. Estimates of the size of the large, low threshold motor units declined whether these were made in low or high [Ca]o.(ABSTRACT TRUNCATED AT 400 WORDS)

An ultrastructural comparison of neuromuscular junctions in normal and developmentally arrested Rana pipiens larvae: limited maturation in the absence of metamorphosis

Neuromuscular junctions in the rectus abdominis muscles of normal and developmentally arrested Rana pipiens larvae were studied with freeze fracture and conventional electron microscopy to determine whether structural aspects of junctional maturation depend on metamorphosis. Comparison was made between junctions in premetamorphic larvae 1-3 months old and junctions in larvae that had remained in premetamorphosis for more than a year (more than four times as long as normal). In most respects, junctions from the two groups of larvae were similar. Unlike adult junctions, nerve-muscle contacts in both larval groups were pleomorphic and often involved more than one neuronal process; Schwann cell processes very rarely extended between nerve and muscle. Active zone structure ranged from total disorganization to an adult pattern of highly ordered double rows of particles aligned over junctional folds. Only quantitative analysis revealed differences between junctions in old and young larvae. The older larvae had fewer nerve-muscle contact sites involving multiple neuronal elements and a higher ratio of active zone length to presynaptic membrane area, although the mean active zone length was the same in the two groups. The results indicate that the maturation of junctional shape, the branching pattern of the axons, and the relationship of presynaptic axons to Schwann cells must be directly or indirectly dependent on the hormonal or behavioral changes associated with metamorphosis.

Development of the mature distribution of synapses on fibres in the frog sartorius muscle

Most of the fibres in mature frog sartorius muscle possess two or more synapses separated by up to one-third the length of the muscle. The aim of the present work was to determine how the relative distances between these synapses changes during development in the frog (Limnodynastes tasmaniansis), as the fibres increase in length from 2 mm (stage 56) to 20 mm (1 year postmetamorphosis). At the earliest stage investigated (fibres 2.0-4.0 mm in length; stages 56-57) about 80% of the fibres were innervated at two endplates. The percentage of fibres with two endplates then remained approximately constant with further development. The polyneuronal innervation of endplates was almost eliminated by stage 57. Muscle fibres with two endplates had each situated on average about one-third the length of the fibre from a tendinous insertion; these relative positions did not change throughout development. Thus the distance between endplates increased linearly with an increase in fibre length. The size of terminals and the complexity of their branching also increased continually throughout development, independently of the location of the terminals on the fibres. The observations suggest that the distance between terminals increases during development because of the intercalation of new plasma membrane and basal lamina associated with the increase in length and diameter of fibres.

Formation of the active zone at developing neuromuscular junctions in larval and adult bullfrogs

Development of the presynaptic active zone was studied at neuromuscular junctions with freeze-fracture electron microscopy in larval and adult bullfrogs. In rudimentary larval neuromuscular junctions, clusters of active zone particles were scattered over the P-face of the presynaptic membrane. Vesicle openings were observed at these terminals even though active zone particles lacked the mature pattern of two double rows. Gradually, active zone particles became organized into rows, but they were still randomly located and oriented. Once junctional folds were observed in replicas, developing active zones were located opposite to the folds, as in mature terminals. Multiple terminals occupying the same junctional gutters were also observed. At the end of metamorphosis, most active zones were still immature in appearance and had only grown to one third of their mature length. After metamorphosis, the number of active zone segments aligned at the same junctional fold increased. These discontinuous short active zones then elongated, joined together, and finally formed the mature active zones. Signs suggesting synapse elimination such as disorganization of active zones, absence of intramembrane particles in varicosities, and exposed muscle membranes with patches of acetylcholine receptor aggregates were observed. In some multiply innervated junctions, one terminal had mature active zones with vesicle openings, the other in the same gutter displayed disorganized active zones without vesicle openings, although both terminals showed similar sizes and distributions of background particles. This study suggests that developing active zones, as is the case for regenerating active zones in the adult, are functional before the mature organization is formed. The sequence of development of active zones is also similar to that of regeneration except for the random location and orientation of early active zones in tadpoles. The comparison between regeneration and development further indicates that the process of active zone formation is related to junctional folds and/or associated structures. It is also suggested that synapse elimination may involve degenerative changes in presynaptic membranes, although direct evidence remains to be provided.

Octopamine enhances neuromuscular transmission in developing and adult moths, Manduca sexta

The effect of octopamine on neuromuscular transmission was examined in developing and adult Manduca sexta. Intracellular recordings were made from the dorsal longitudinal muscle (DLM), superfused with solutions containing DL-octopamine or other amines. In untreated adult moths and pharate adults nearly ready to enclose (stage Day 19), stimulation of the motor nerve evokes a large excitatory junction potential (EJP), an active membrane response, and a twitch. In adults and Day 19 animals DL-octopamine (10(-7) to 10(-4)M) has no effect on the amplitude and rise-time of the electrical response in normal saline, but 10(-6) to 10(-4) M DL-octopamine increases the amplitude of the excitatory junction potential recorded in saline containing one-third the normal calcium concentration. Immature (Day 16) muscle, which normally produces only small EJPs following stimulation of its motor nerve, responds to 10(-6) to 10(-4) M DL-octopamine by an increase in the EJP above threshold for an active membrane response and a contraction. When the muscle has developed sufficiently to spike and contract in response to nerve stimulation in the absence of exogenous octopamine (Days 17 and 18), application of DL-octopamine increases the maximum rate at which the muscle contracts in response to each stimulus in a train (designated the maximum following frequency, MFF). The threshold dose for an effect on the MFF of Day 18 immature moths is less than 10(-10) M. At this stage 10(-8) M DL-octopamine increases the MFF four-fold. The effect on the MFF is dose-dependent over the range 10(-10) M to 10(-6) M. The biogenic amines DL-epinephrine, DL-norepinephrine, tyramine, DL-phenylethanolamine, 2-phenylethylamine, and dopamine, applied at concentrations of 10(-8) or 10(-4) M, do not change the MFF. Both DL-synephrine (10(-8) M) and serotonin (10(-7) M) mimic the action of 10(-10) M DL-octopamine on the MFF. The action of DL-octopamine (10(-7) M) is blocked by phentolamine (10(-4)M) but not by propranolol (10(-4)M). The octopamine content of hemolymph was determined with a radioenzymtic assay. The concentration of octopamine in the hemolymph increases 3.6-fold, from 5 X 10(-8) M on Day 18 (duration of adult development is 19 days) to 1.85 X 10(-7) M one day following eclosion.(ABSTRACT TRUNCATED AT 400 WORDS)

Specificity of innervation among Xenopus twitch muscle fibers

More than 50% of the Xenopus pectoralis twitch muscle fibers with two distant endplates are innervated at both sites by the same neuron. This study indicates that there are three separable twitch motor unit types in this muscle which show very little overlap in innervation. This study also shows that each motor unit is topographically localized and that similar type units are shifted relative to one another. It is concluded that these two factors may contribute importantly to the observed high incidence of mononeuronal innervation.

Profiles of evoked release along the length of frog motor nerve terminals

In order to determine the relative probability of evoked transmitter release from different parts of frog motor nerve terminals, a technique has been developed in which single quantum end-plate potentials (e.p.p.s) are recorded by two intracellular electrodes, located at opposite ends of identified junctions. The log of the ratio of the amplitudes recorded simultaneously at the two electrodes is a linear function of the distance of the site of origin of the event from each of the two electrodes. Using online computer data acquisition and analysis, and current pulses at known locations for spatial calibration, it is possible to localize the site of single quantum e.p.p.s to within +/- 10-20 micron. Using the frog cutaneous pectoris neuromuscular preparation and a low calcium, high magnesium Ringer solution to ensure mostly single quantum events and failures, several thousand responses were recorded from each junction, allowing construction of a profile of the numbers of single quantum events arising from each portion of the junction. By comparison of junctional morphology and release profiles, it is possible to construct a probability of release per unit length profile for the entire junction. This technique has several advantages over localization of release events by measurements of extracellular synaptic currents. It was found that, for most junctions, the central 60-90% of the terminal exhibited relatively uniform probability of release, with highest levels typically near the point where the axon first contacted the muscle fibre, or in regions with many short terminal branches. However, no instances have been found in which a small region of terminal (10% or less) showed extraordinarily high release levels (30-50% of the total release from the junction). Characteristically, but not invariably, there is reduced release near the ends of terminal branches, especially the longer branches, where release per unit length could be as little as 5-10% of that in proximal portions. Some junctions had large regions of terminal that released very little transmitter. These also showed multiple myelineated axonal inputs, and may have been polyneuronally innervated junctions in which one of the inputs was much weaker than the other.

Motor axon sprouting in frog sartorius muscles is not altered by contralateral axotomy

Sartorius muscles of the frog Rana pipiens were used to study the incidence of motor nerve sprouting in normal unoperated muscles, in experimental muscles contralateral to axotomy of the sartorius nerve, and in sham-operated control muscles. Muscles were stained with either a combination of nitroblue tetrazolium nerve terminal stain and cholinesterase stain or with a combination of silver nerve terminal stain and cholinesterase stain. Each endplate that could be clearly seen was classified into one or more of the following categories: normal endplates without sprouts, three types of terminal sprouts, preterminal sprouts, nodal sprouts, sprouts of unknown origin and destination, and doubly innervated gutters. A quantitative study of 318 endplates from nine unoperated muscles, 779 endplates from 45 experimental muscles, and 694 endplates from 41 control muscles showed that all muscles had a high incidence of motor nerve sprouting and other forms of remodelling (20-28% of all endplates). There were, however, no significant differences between experimental, control, and unoperated muscles when results obtained with the same stains were compared. Results obtained with the two different stains were only slightly different. We conclude that sprouting is a very common but highly variable feature of normal frog neuromuscular junctions, and in the sartorius, contralateral axotomy does not alter this ongoing remodelling.

Innervation pattern of muscles of one-legged Xenopus laevis supplied by motoneurons from both sides of the spinal cord

In Xenopus tadpoles one limb bud was removed before innervation and motoneurons from both sides of the spinal cord were induced to innervate the remaining limb. When examined after metamorphosis the motor innervation of the limb had the following characteristics. In agreement with previous findings a large proportion of contralateral motoneurons survived (51-82% of the ipsilateral numbers) and sent axons to the limb. By acetylcholinesterase staining and intracellular recording from muscle fibers of the response to electrical stimulation of the two limb innervations, the neuromuscular junctions from contralateral motoneurons were indistinguishable from those from the ipsilateral side in their morphology, spacing along the fiber, and physiological properties. Many single muscle fibers shared innervation from both sides of the cord by symmetrically placed spinal nerves. By the same techniques junctions in one-legged frogs were morphologically indistinguishable from those in normal frogs, but the quantal content of transmitter release was increased by up to 63%. Recording twitch and tetanic tensions from individual motor units from the gastrocnemius muscle showed that the one-legged animals had many more and smaller motor units than do normal frogs. We confirm that the hind-limb musculature has the ability, normally unexpressed, to sustain, through the period of normal developmental cell death, up to twice the usual number of motoneurons. In maturity, motoneurons accommodate themselves to the limb muscles by making fewer than the normal number of synapses. The above suggests that developmental motoneuron death is not primarily a mechanism for adjusting the number of motoneurons to the size of the peripheral musculature and is likely to be related to mechanisms for securing specific neuromuscular connections.

Acetylcholine-sensitivity and local regenerative activity in denervated frog slow muscle fibres

The distribution of acetylcholine (ACh)-sensitive membrane areas was determined in 11-75 days denervated slow muscle fibres of Rana temporaria by iontophoretic application of acetylcholine. The fibres were also stimulated directly, and their electrical activity was recorded with an intra- and an extracellular electrode. During the first two weeks following denervation the size of the ACh-sensitive fibre surface was similar to that of normal slow fibres, but a spreading out of ACh-sensitivity occurred between the 13th and 20th day. The slow fibre membrane did not become homogeneously ACh-sensitive; even after long periods of denervation large local sensitivity gradients could be observed. Throughout the denervation period maximum values of ACh-sensitivity were in the same range as in normal slow fibres. Action potentials were fully developed when ACh-sensitivity started to spread out. Extracellularly recorded inward currents varied in amplitude along the fibre surface, and either one or two peaks were observed in individual fibres. The spatial relationship between inward current peaks and peaks of ACh-sensitivity was investigated in 12 fibres. Fifteen inward current peaks were located at distances of 30-640 microns from points of maximum ACh-sensitivity; only once did the centers of ACh-sensitivity and excitability coincide. It is concluded that Na channels are incorporated into nonjunctional membrane areas of denervated slow fibres; this process precedes the incorporation of ACh receptors by approximately one week.

Segmental motor projections to rat muscles during the loss of polyneuronal innervation

The pattern of innervation of three muscles from rat (lateral gastrocnemius, biceps brachii, pectoralis minor) has been ascertained in the neonatal period (birth to 4 days) and from two to three weeks postnatal. Contraction, electrophysiological and histological methods were used to determine the pattern of innervation. There is a change in the segmental innervation during development which establishes the topographical projection of different segmental nerves found in mature muscles. Development of such topographical projections by different segmental nerves onto the muscles studied emerges as a consequence of elimination of terminals arising from the inappropriate segmental nerve.

Bimodal miniature and evoked end-plate potentials in adult mouse neuromuscular junctions

Intracellular recordings of spontaneous miniature end-plate potentials (m.e.p.p.s) in muscles from adult CBF-1 mice revealed a population of muscle fibres in which the amplitude distribution of m.e.p.p.s was bimodal. The large mode m.e.p.p.s were similar to those from fibres having unimodal amplitude distributions and the small mode m.e.p.p.s were about one-half to one-quarter the amplitude of the large mode. In five diverse muscle groups (extensor digitorum communis, gluteus maximus, diaphragm, extensor digitorum longus, and soleus) from mice 10-12 or 31 months of age, bimodal m.e.p.p. amplitude distributions were present in about 20% of fibres sampled. In the common bimodal distribution (type 1), the rise times of small mode m.e.p.p.s were similar to those of large mode m.e.p.p.s. A rare class of small mode m.e.p.p.s (type 2) having long rise times was also observed. Amplitudes and half-decay times of type 1 small mode m.e.p.p.s increased in the presence of an anticholinesterase (edrophonium). Increasing extracellular potassium concentration led to an increase in large mode m.e.p.p. frequency but had more variable effects on small mode frequency. In the few cases available for study, type 2 small mode m.e.p.p.s disappeared after addition of edrophonium or increased potassium. When the extracellular calcium/magnesium ratio was reduced, large mode but not small mode m.e.p.p. frequency decreased. In almost all muscle fibres in which end-plate potentials (e.p.p.s) were evoked by nerve stimulation at 20 Hz in low calcium/high magnesium solution, small mode e.p.p.s similar to small mode m.e.p.p.s appeared during 'failures' of large mode m.e.p.p.s. Also, in twelve out of fifteen fibres which had unimodal m.e.p.p. amplitude distributions, small mode e.p.p.s appeared which were similar in amplitude to small mode m.e.p.p.s in fibres with type 1 bimodal m.e.p.p.s. Thus, if both spontaneous and evoked potentials are included, small mode m.e.p.p.s are present at most CBF-1 mouse adult neuromuscular junctions independent of muscle type or animal age. Small and large mode m.e.p.p.s differ in certain responses but both are evoked by nerve stimulation at physiological frequencies and therefore participate in normal neuromuscular synaptic activity. The possible origin of small mode m.e.p.p.s is discussed.

An anatomical and electrophysiological study of synapse elimination at the developing frog neuromuscular junction

Synapse elimination was examined in the developing frog cutaneous pectoris muscle using histological and electrophysiological techniques. Morphological synapse elimination occurred in two phases. The first phase, which began at the time of metamorphosis and continued until the second to third postmetamorphic week, was characterized by a rapid decline in the number of endplates receiving greater than or equal to 3 synaptic inputs. However, 50% of the muscle fibers still remained dually innervated. This dual innervation decreased with a much slower time course; approximately 20% of the muscle fibers were dually innervated in 1- to 2-year-old frogs. During the first phase of synapse elimination no difference was noted between the distribution of acetylcholine receptors or acetylcholinesterase activity associated with the terminal arborizations formed by separate axons at one synaptic site. However, terminal arborizations formed by small diameter axons and consisting of varicosities separated by thin interconnectives became apparent during this period. Such varicose arborizations responded to nerve stimulation and released acetylcholine in proportion to their terminal length as did the nonvaricose arborizations. In addition, the number of morphological and physiological inputs at one endplate site was well correlated throughout the first phase of synapse elimination.

Presynaptic active zones at neuromuscular junctions of larval frogs

Freeze-fracturing presynaptic membranes at tadpole neuromuscular junctions display small clusters of large P-face particles, including short double linear arrays. Short pairs of double particle rows are randomly oriented at some junctions. At others, presynaptic membranes are crossed at regular intervals by long pairs of double rows indistinguishable from those characterizing the active zones of adult amphibian neuromuscular junctions. Formation of double particles rows, pairing of the double rows, and transverse alignments of the pairs are shown to be independent processes.