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lthough more than 60 papers in A the past five years have con- 'Cartellian' competition at the neuromuscularjundion tained the words 'synaptic competition' in their title or abstract, there is - as far as we know - no generally accepted definition of this phrase. In Webster's Ninth New Collegiate Dictionary z there are two relevant definitions of competition: (1) 'The effort of two or more parties acting independently to secure the business of a third party by offering the most favorable terms', and (2) 'Active demand by two or more organisms or kinds of organisms for some environmental resource in short supply'. In the first definition, the actions of the parties are independent, meaning that one party does not influence the performance of the other parties. For example, in a diving competition each diver independently attempts to make the most favorable impression on the judges. Ultimately only one diver can win the gold medal, so the actions of one competitor do eventually affect the others, but this only happens at the outcome not during the process of competition. The second definition implies an interdependent competitive mechanism because in this case the actions of one party may affect the performance of the other parties during the process. Thus, in a diving competition of this type, the participants might dive simultaneously for a single gold medal lying on the bottom of the pool. In this case, rather than only affecting the competitors at the outcome, the actions of one diver may affect the other divers' ability to succeed during the process. The two dictionary definitions of competition may also apply to the mechanisms underlying synaptic rearrangements. In neurobiology, one of the most accessible cases in which synapses are rearranged occurs during a process known as synapse elimination at developing neuromuscular junctions. For example, in virtually all mammalian muscles, two or more axons innervate each muscle fiber at birth, but during early postnatal life muscle fibers become innervated by only one motor axon 2'3. Although motor axons may have an inherent tendency to withdraw some of their synaptic terminals 3'4, the consistency with TINS, Vol. 15, No. 6, 1992
which muscle fibers end up singly innervated has led workers in this field to believe strongly that competition must occur between motoneurons that are striving to be the sole source of innervation to each fiber3'5. However, it is less clear what this competition entails. Synapse elimination is often described as an example of 'axonal' or 'synaptic' competition, but neither term describes the competitors adequately. Each axon innervates and competes on many muscle fibers simultaneously. Because the opponents and competitive outcome for each of its branches are different and largely independent, competition at the neuromuscular junction is better described as pitting axonal branches (rather than axons) against each other. Moreover, the contact between a motor axon and a muscle fiber actually comprises a number of synaptic sites 6. Because the synapse elimination process causes all the synapses on the muscle fiber from one axon to be eliminated while the synapses of another axon are maintained, the fates of the set of synapses derived from each axon are linked. Thus, synapse elimination is actually competition between sets of synapses rather than synapses per se. Each set of synapses behaves essentially as a group of likeminded individuals in a business cartel that aims to eliminate its competitors and establish a monopoly. Thus, at the neuromuscular junction synaptic cartels are groups of synapses acting together with the goal of monopolizing the muscle fiber. The members of each cartel are related in two ways. (1) They originate from the same motoneuron (and so are synchronously active), and (2) they innervate the same postsynaptic target. Competition at the neuromuscular junction could be called 'carteUian' to convey the idea that competition is occurring between groups of synapses, each group having a common purpose. The mechanism underlying competitive synapse elimination could either permit one cartel only to be actively maintained or, alternatively, cause all but one cartel to be
actively eliminated. The principal difference here lies in the default state of the synapses. If synapses are prone to be lost unless actively maintained, then competition may select some terminals to be maintained. By contrast, if synapses are prone to remain in place once they are established, then competition must actively eliminate synapses. While either maintenance of one or elimination of all but one synaptic cartel is the ultimate aim of competition, the competitive process may have an intermediate goal of handicapping or aiding the competitors. Feedback may cause a cartel to become stronger or weaker over time as a result of its actions or the actions of its competitors. Thus, understanding the mechanism requires determining whether competing inputs are independent or interdependent, whether synapses are actively maintained or eliminated, whether feedback is present, and whether there is strengthening, weakening, or both. Using an in vitro preparation in which embryonic Xen@us myocytes are innervated by two individual spinal neurons, Lo and Poo 7 recently demonstrated what appears to be a very accessible example of synaptic competition at the neuromuscular junction. They found that after one of the neurons innervating a myocyte is stimulated, the synaptic strength of the other neuron becomes reduced. This effect, which they call 'heterosynaptic suppression', was generally not observed when both inputs were stimulated synchronously. The fact that synchronous activity mitigated competitive interactions suggests that the members of competing synaptic cartels are identified by their activity patterns rather than their cytoplasmic continuity. Within 24 h after culturing muscle cells and spinal neurons from Xen@us embryos 1 day old, functional synaptic contacts were established between the neurons and mononucleated spherical myocytes. Using whole-cell voltageclamp recordings from myocytes innervated by two nearby spinal neurons, the authors were able to
© 1992, ElsevierScience PublishersLtd, (UK) 0166- 2236/92/$05.00
Howard Colman and Jeff W. Lichtman
Deptof Anatomyand Neurobio/ogy, WashinEton UniversityEchoo/of Medicine, PO Box 8108, 660South EudidAve, 5t Louis, MO 63110, USA.
1 97
record the synaptic currents in the myocyte elicited in response to stimulation of each neuron by extracellular microelectrodes. The major finding of the paper is that when one input is activated with a pulse stimulus (50-100 pulses at a frequency of 2-5 Hz), the synaptic efficacy of the unstimulated input is weakened within 10 re_in. This decrease in synaptic strength was often maintained for the duration of the experiments (up to 60 min). Significant heterosynaptic suppression was seen in eight of eight cases in which the stronger (larger initial evoked synaptic current) was stimulated, and in three out of five cases in which the weaker input was stimulated. When both inputs to a myocyte were stimulated concurrently, only three out of 14 synapses showed significant reduction in synaptic strength, suggesting that asynchronous stimulation of the inputs is necessary for reliable heterosynaptic suppression. In other experiments, simultaneous and asynchronous stimulation were presented at different times to the same preparation. Synchronous stimulation generally had little or no effect on the synaptic currents from either axon, while asynchronous stimulation usually resulted in the suppression of one or both inputs. If the basic component processes of competition proposed earlier are considered in relation to the results of Lo and Poo, it is clear that the competition is interdependent, because the actions of one input affect the other, and that feedback is present and leads to the progressive weakening of the less active input. However, in at least a few instances there was also some evidence that strengthening of a stimulated input could occur, especially when it was the weaker input to a cell that was stimulated. Because these experiments did not determine whether there were any structural correlates to heterosynaptic suppression, it is unclear whether the suppression ultimately gives rise to synapse elimination. One similarity between this work and synapse elimination in vivo is that the heterosynaptic suppression observed by Lo and Poo only acted over a limited distance. When the two inputs were spatially separated (by 50198
75 ~m) on spindle-shaped myocytes, stimulation of one input did not result in suppression of the other input. A similar conclusion has been suggested in studies examining multiple innervation in vivo. Several different types of experiments have demonstrated that multiple innervation can sometimes be maintained in adult muscle fibers, but only when the synapses are sufficiently distant from one another a'8-u. Another similarity between the situations in vitro and in vivo is that neuronal activity seems to play an important role in both cases. The results of Lo and Poo demonstrate that in cultured, multiply innervated Xenopus myocytes, the activity pattern of one input can have a rapid and dramatic effect on the synaptic efficacy of the other input. Similarly, short-term heterosynaptic interactions between nerve terminals on multiply innervated rat and Xenopus muscle fibers have been reported 12'13. The importance of neural activity in synaptic plasticity has been demonstrated in several different in vitro preparations 14-16 and in a variety of in vivo systems, including attempts to determine the role of activity in synapse elimination at the neuromuscular junction 17-29. Examinations of the effects of activity on the outcome of synapse elimination have not reached a consensus, with some studies supporting the view that the more active inputs are preferentially maintained26'27, while others have suggested that less active inputs are at an advantage 28'29. If the heterosynaptic suppression observed in vitro is in fact related to synapse elimination, then the results of Lo and Poo are consistent with the notion that more highly active inputs are at an advantage on a multiply innervated muscle fiber. On the other hand, it remains to be seen whether heterosynaptic suppression is part of synapse elimination in vivo because the relationship between synaptic strength and synapse elimination is not yet known. As already mentioned, it is also unclear whether changes in synaptic strength in vitro would eventually lead to structural withdrawal of one axon, as occurs during synapse elimination in vivo. Once all the component processes of competition are identified
in a given system, the next level of analysis presumably would be to understand the cellular and mob ecular phenomena that underlie the various components. Several different mechanisms have been proposed to explain synapse elimination in vivo. One possibility is that there is a direct interaction between the two presynaptic axons, which does not depend on the postsynaptic cell. Thus, it is possible that axon terminals in close proximity to each other act directly to affect each other's synaptic effÉcacy (and ultimate survival) via a presynaptic signal that is regulated in an activity-dependent manner. In contrast to a direct interaction between the presynaptic terminals, the activity of one synaptic set could be translated via a process involving the postsynaptic cell into a message that affects the other input. Various suggestions for such a mechanism of synapse elimination in vivo have been made, such as competition between the axon terminals for an activity-regulated trophic factor released by the postsynaptic cell3°-32, or competition between axon terminals based on postsynaptic or presynaptic proteases that are influenced by postsynaptic activity and presynaptic volume 1s,19,33
news
lthough more than 60 papers in A the past five years have con- 'Cartellian' competition at the neuromuscularjundion tained the words 'synaptic competition' in their title or abstract, there is - as far as we know - no generally accepted definition of this phrase. In Webster's Ninth New Collegiate Dictionary z there are two relevant definitions of competition: (1) 'The effort of two or more parties acting independently to secure the business of a third party by offering the most favorable terms', and (2) 'Active demand by two or more organisms or kinds of organisms for some environmental resource in short supply'. In the first definition, the actions of the parties are independent, meaning that one party does not influence the performance of the other parties. For example, in a diving competition each diver independently attempts to make the most favorable impression on the judges. Ultimately only one diver can win the gold medal, so the actions of one competitor do eventually affect the others, but this only happens at the outcome not during the process of competition. The second definition implies an interdependent competitive mechanism because in this case the actions of one party may affect the performance of the other parties during the process. Thus, in a diving competition of this type, the participants might dive simultaneously for a single gold medal lying on the bottom of the pool. In this case, rather than only affecting the competitors at the outcome, the actions of one diver may affect the other divers' ability to succeed during the process. The two dictionary definitions of competition may also apply to the mechanisms underlying synaptic rearrangements. In neurobiology, one of the most accessible cases in which synapses are rearranged occurs during a process known as synapse elimination at developing neuromuscular junctions. For example, in virtually all mammalian muscles, two or more axons innervate each muscle fiber at birth, but during early postnatal life muscle fibers become innervated by only one motor axon 2'3. Although motor axons may have an inherent tendency to withdraw some of their synaptic terminals 3'4, the consistency with TINS, Vol. 15, No. 6, 1992
which muscle fibers end up singly innervated has led workers in this field to believe strongly that competition must occur between motoneurons that are striving to be the sole source of innervation to each fiber3'5. However, it is less clear what this competition entails. Synapse elimination is often described as an example of 'axonal' or 'synaptic' competition, but neither term describes the competitors adequately. Each axon innervates and competes on many muscle fibers simultaneously. Because the opponents and competitive outcome for each of its branches are different and largely independent, competition at the neuromuscular junction is better described as pitting axonal branches (rather than axons) against each other. Moreover, the contact between a motor axon and a muscle fiber actually comprises a number of synaptic sites 6. Because the synapse elimination process causes all the synapses on the muscle fiber from one axon to be eliminated while the synapses of another axon are maintained, the fates of the set of synapses derived from each axon are linked. Thus, synapse elimination is actually competition between sets of synapses rather than synapses per se. Each set of synapses behaves essentially as a group of likeminded individuals in a business cartel that aims to eliminate its competitors and establish a monopoly. Thus, at the neuromuscular junction synaptic cartels are groups of synapses acting together with the goal of monopolizing the muscle fiber. The members of each cartel are related in two ways. (1) They originate from the same motoneuron (and so are synchronously active), and (2) they innervate the same postsynaptic target. Competition at the neuromuscular junction could be called 'carteUian' to convey the idea that competition is occurring between groups of synapses, each group having a common purpose. The mechanism underlying competitive synapse elimination could either permit one cartel only to be actively maintained or, alternatively, cause all but one cartel to be
actively eliminated. The principal difference here lies in the default state of the synapses. If synapses are prone to be lost unless actively maintained, then competition may select some terminals to be maintained. By contrast, if synapses are prone to remain in place once they are established, then competition must actively eliminate synapses. While either maintenance of one or elimination of all but one synaptic cartel is the ultimate aim of competition, the competitive process may have an intermediate goal of handicapping or aiding the competitors. Feedback may cause a cartel to become stronger or weaker over time as a result of its actions or the actions of its competitors. Thus, understanding the mechanism requires determining whether competing inputs are independent or interdependent, whether synapses are actively maintained or eliminated, whether feedback is present, and whether there is strengthening, weakening, or both. Using an in vitro preparation in which embryonic Xen@us myocytes are innervated by two individual spinal neurons, Lo and Poo 7 recently demonstrated what appears to be a very accessible example of synaptic competition at the neuromuscular junction. They found that after one of the neurons innervating a myocyte is stimulated, the synaptic strength of the other neuron becomes reduced. This effect, which they call 'heterosynaptic suppression', was generally not observed when both inputs were stimulated synchronously. The fact that synchronous activity mitigated competitive interactions suggests that the members of competing synaptic cartels are identified by their activity patterns rather than their cytoplasmic continuity. Within 24 h after culturing muscle cells and spinal neurons from Xen@us embryos 1 day old, functional synaptic contacts were established between the neurons and mononucleated spherical myocytes. Using whole-cell voltageclamp recordings from myocytes innervated by two nearby spinal neurons, the authors were able to
© 1992, ElsevierScience PublishersLtd, (UK) 0166- 2236/92/$05.00
Howard Colman and Jeff W. Lichtman
Deptof Anatomyand Neurobio/ogy, WashinEton UniversityEchoo/of Medicine, PO Box 8108, 660South EudidAve, 5t Louis, MO 63110, USA.
1 97
record the synaptic currents in the myocyte elicited in response to stimulation of each neuron by extracellular microelectrodes. The major finding of the paper is that when one input is activated with a pulse stimulus (50-100 pulses at a frequency of 2-5 Hz), the synaptic efficacy of the unstimulated input is weakened within 10 re_in. This decrease in synaptic strength was often maintained for the duration of the experiments (up to 60 min). Significant heterosynaptic suppression was seen in eight of eight cases in which the stronger (larger initial evoked synaptic current) was stimulated, and in three out of five cases in which the weaker input was stimulated. When both inputs to a myocyte were stimulated concurrently, only three out of 14 synapses showed significant reduction in synaptic strength, suggesting that asynchronous stimulation of the inputs is necessary for reliable heterosynaptic suppression. In other experiments, simultaneous and asynchronous stimulation were presented at different times to the same preparation. Synchronous stimulation generally had little or no effect on the synaptic currents from either axon, while asynchronous stimulation usually resulted in the suppression of one or both inputs. If the basic component processes of competition proposed earlier are considered in relation to the results of Lo and Poo, it is clear that the competition is interdependent, because the actions of one input affect the other, and that feedback is present and leads to the progressive weakening of the less active input. However, in at least a few instances there was also some evidence that strengthening of a stimulated input could occur, especially when it was the weaker input to a cell that was stimulated. Because these experiments did not determine whether there were any structural correlates to heterosynaptic suppression, it is unclear whether the suppression ultimately gives rise to synapse elimination. One similarity between this work and synapse elimination in vivo is that the heterosynaptic suppression observed by Lo and Poo only acted over a limited distance. When the two inputs were spatially separated (by 50198
75 ~m) on spindle-shaped myocytes, stimulation of one input did not result in suppression of the other input. A similar conclusion has been suggested in studies examining multiple innervation in vivo. Several different types of experiments have demonstrated that multiple innervation can sometimes be maintained in adult muscle fibers, but only when the synapses are sufficiently distant from one another a'8-u. Another similarity between the situations in vitro and in vivo is that neuronal activity seems to play an important role in both cases. The results of Lo and Poo demonstrate that in cultured, multiply innervated Xenopus myocytes, the activity pattern of one input can have a rapid and dramatic effect on the synaptic efficacy of the other input. Similarly, short-term heterosynaptic interactions between nerve terminals on multiply innervated rat and Xenopus muscle fibers have been reported 12'13. The importance of neural activity in synaptic plasticity has been demonstrated in several different in vitro preparations 14-16 and in a variety of in vivo systems, including attempts to determine the role of activity in synapse elimination at the neuromuscular junction 17-29. Examinations of the effects of activity on the outcome of synapse elimination have not reached a consensus, with some studies supporting the view that the more active inputs are preferentially maintained26'27, while others have suggested that less active inputs are at an advantage 28'29. If the heterosynaptic suppression observed in vitro is in fact related to synapse elimination, then the results of Lo and Poo are consistent with the notion that more highly active inputs are at an advantage on a multiply innervated muscle fiber. On the other hand, it remains to be seen whether heterosynaptic suppression is part of synapse elimination in vivo because the relationship between synaptic strength and synapse elimination is not yet known. As already mentioned, it is also unclear whether changes in synaptic strength in vitro would eventually lead to structural withdrawal of one axon, as occurs during synapse elimination in vivo. Once all the component processes of competition are identified
in a given system, the next level of analysis presumably would be to understand the cellular and mob ecular phenomena that underlie the various components. Several different mechanisms have been proposed to explain synapse elimination in vivo. One possibility is that there is a direct interaction between the two presynaptic axons, which does not depend on the postsynaptic cell. Thus, it is possible that axon terminals in close proximity to each other act directly to affect each other's synaptic effÉcacy (and ultimate survival) via a presynaptic signal that is regulated in an activity-dependent manner. In contrast to a direct interaction between the presynaptic terminals, the activity of one synaptic set could be translated via a process involving the postsynaptic cell into a message that affects the other input. Various suggestions for such a mechanism of synapse elimination in vivo have been made, such as competition between the axon terminals for an activity-regulated trophic factor released by the postsynaptic cell3°-32, or competition between axon terminals based on postsynaptic or presynaptic proteases that are influenced by postsynaptic activity and presynaptic volume 1s,19,33
