Regular and photodamage-enhanced remodelling in vitally stained frog and mouse neuromuscular junctions

Structural dynamics of synapses in living animals

Recently, there has been increasing interest in the use of in vivo imaging approaches in the study of the way that synaptic circuits become established and the degree to which they stabilize in mature brains. We review progress since the first efforts, two decades ago, at in vivo synaptic imaging and highlight the more recent advances in molecular biology, optics and neurobiological imaging that have fueled a mini-renaissance in this line of inquiry. Many of the technical problems that limited early efforts still remain, but the rapid pace of molecular and optical innovation might soon transform this specialized field into one that is more 'mainstream'.

The neuronal naturalist: watching neurons in their native habitat

Dynamic processes of neural development, such as migrations of precursor cells, growth of axons and dendrites, and formation and modification of synapses, can be fully analyzed only with techniques that monitor changes over time. Although there has been long-standing motivation for following cellular and synaptic events in vivo (intravital microscopy), until recently few preparations have been studied, and then often only with great effort. Innovations in low-light and laser-scanning microscopies, coupled with developments of new dyes and of genetically encoded indicators, have increased both the breadth and depth of in situ imaging approaches. Here we present the motivations and challenges for dynamic imaging methods, offer some illustrative examples and point to future opportunities with emerging technologies.

Development of the vertebrate neuromuscular junction

We describe the formation, maturation, elimination, maintenance, and regeneration of vertebrate neuromuscular junctions (NMJs), the best studied of all synapses. The NMJ forms in a series of steps that involve the exchange of signals among its three cellular components--nerve terminal, muscle fiber, and Schwann cell. Although essentially any motor axon can form NMJs with any muscle fiber, an additional set of cues biases synapse formation in favor of appropriate partners. The NMJ is functional at birth but undergoes numerous alterations postnatally. One step in maturation is the elimination of excess inputs, a competitive process in which the muscle is an intermediary. Once elimination is complete, the NMJ is maintained stably in a dynamic equilibrium that can be perturbed to initiate remodeling. NMJs regenerate following damage to nerve or muscle, but this process differs in fundamental ways from embryonic synaptogenesis. Finally, we consider the extent to which the NMJ is a suitable model for development of neuron-neuron synapses.

Stability of frog motor nerve terminals in the absence of target muscle fibers

Using repeated in vivo imaging, we addressed the role of target muscle fibers in the maintenance of frog motor nerve terminals at synaptic sites. Target-deprived nerve terminals were generated by selective and permanent removal of muscle fibers without damage to the innervation. Individual nerve terminals, stained with the dye FM1-43, were imaged before and again during the subsequent 1-9 months of target deprivation and the stability of the nerve terminal arbors over time was determined. Repeated observation of motor nerve terminals showed that nerve terminals were well maintained at synaptic sites during the first 1-2 months after target loss; the original number of nerve terminal segments was retained at 85% of the synaptic sites after muscle damage. After long periods of target deprivation, 6-9 months, loss or retraction of nerve terminal segments resulted in a reduction in the arbor of most but not all nerve terminals. This apparent nerve terminal destabilization was not a result of illumination or irradiation because a similar decrease in the extent of nerve terminal arbors was not observed at control irradiated neuromuscular junctions. The persistence of many complete target-deprived nerve terminal arbors at synaptic sites long after target degeneration suggested that the cues that confer stability to frog motor nerve terminals likely reside external to muscle fibers and may be associated with the synaptic basal lamina or the terminal Schwann cell. Since the arbors of many target-deprived nerve terminals were eventually reduced, the nonmuscle stabilization cues may not persist indefinitely at target-deprived synaptic sites.

In vivo approaches to neuromuscular structure and function

Approaches that permit direct observation and manipulation of skeletal muscle and its innervation in living animals will continue to contribute to our understanding of neural influences on muscle function in developing and mature animals. Understanding how motor neurons interact with each other, with supporting cells such as Schwann cells, and with their target muscle fibers are fundamental issues in neuroscience, as similar mechanisms are likely to underlie the formation and plasticity of synaptic connections in the less easily accessible central nervous system.

Precision of reinnervation and synaptic remodeling observed in neuromuscular junctions of living frogs

Repeated in vivo observations were used to study regenerated nerve terminals in neuromuscular junctions of the adult frog Rana pipiens. Sartorius junctions in living animals were stained with the fluorescent vital dye RH414 and viewed with video fluorescence microscopy. Each junction was observed in the intact muscle and then again 7, 10, and 13 weeks after nerve crush. At 13 weeks, junctions were determined to be mono- or polyneuronally innervated using intracellular recording. Between 7 and 13 weeks, most identified junctions were reinnervated less precisely and completely than described previously. Although some of the original synaptic gutters were reoccupied by regenerated terminal branches, other gutters were only partially occupied, and many appeared abandoned. Junctions showing precise recapitulation of original terminal arborizations comprised a small number of the total examined, as did those where reinnervation was very imprecise. Striking differences in the precision of reinnervation were found within the muscle such that distal terminals regenerated more precisely and completely than did proximal terminals. Terminals in reinnervated muscles were more dynamic than terminals in unoperated muscles over equivalent times. In singly innervated junctions, terminal growth was favored over regression. In doubly innervated junctions, regressive events were more common. Imprecise reinnervation is explained in terms of multisite innervation of muscle fibers and the activity dependence of synaptic stability. We hypothesize that when axons reinnervate the second or third junctions on a fiber, they do so less precisely, because the activity restored by reinnervation of the first junction renders later sites less attractive or less stable.

Synaptic and extrasynaptic distribution of two distinct populations of nicotinic acetylcholine receptor clusters in the frog cardiac ganglion

We examined the distribution of neuronal nicotinic acetylcholine receptor clusters in relation to synaptic sites on autonomic neurons in the frog heart using immunofluorescence techniques and laser scanning confocal microscopy. Acetylcholine receptor clusters were visualized using the rat anti-Electrophorus acetylcholine receptor monoclonal antibody no. 22 and cyanine 3.18-labelled goat anti-rat secondary antibody. Synaptic boutons were labelled with the mouse anti-synaptic vesicle protein SV2, monoclonal antibody no. 10h and cyanine 5.18-labelled goat anti-mouse secondary antibody. Acetylcholine receptor clusters on the neuronal surface exist in two populations that vary in size, staining intensity, and surface distribution. The more prominent population consists of large, brightly stained clusters numbering 30 +/- 15 per cell, while the second class is smaller and less brightly stained and numbers over 100 per cell. The large clusters tend to be organized into groups of 2-6 members. This arrangement results from the fact that 80% of the large clusters colocalize at synaptic boutons and that single boutons can have several associated clusters. The remaining 20% of large/bright acetylcholine receptor clusters are extrasynaptic, but they, too, are clustered and are found in close proximity to synaptic boutons. The small/dim acetylcholine receptor clusters are randomly distributed over the cell surface. The large/bright synaptic acetylcholine receptor clusters presumably underlie fast excitatory synaptic transmission. The small/dim clusters and the large/bright extrasynaptic clusters may represent intermediates in the metabolism of large/bright synaptic clusters.

Long-term synapse loss induced by focal blockade of postsynaptic receptors

Focal application in vivo of alpha-bungarotoxin to block neurotransmission in a small region of a neuromuscular junction causes long-lasting synapse elimination at that site. In contrast, blockade of neurotransmission throughout a junction does not cause synapse elimination. These and related experiments indicate that active synaptic sites can destabilize inactive synapses in their vicinity.

Polyclonal antibodies against NCAM and tenascin delay endplate reinnervation

Experiments were performed to block molecules with antibodies which are upregulated in nerve and muscle following denervation. The delay in endplate reinnervation was taken as a measure for their involvement in regeneration. Gluteus maximus muscles of 86 male CBA/J mice were hemidenervated by freezing the caudal gluteal nerve at a defined position. The degree of reinnervation was evaluated in identified endplates by repeated vital staining of ACh receptors with rhodaminated alpha-bungarotoxin and of axons with 4Di-2ASP. Normally, endplates were completely reinnervated by 13-14 days (108 endplates in seven muscles). After daily application of polyclonal antibodies against NCAM or tenascin, reinnervation was significantly delayed. Preimmune serum, rabbit immunoglobulins or saline did not show this effect. Several monoclonal antibodies against NCAM (H-28) and tenascin (576, 578, 630, 633) showed a tendency but no significant effect. It is concluded that both NCAM and tenascin, upregulated after denervation, are involved in axon guidance and/or endplate reinnervation.

Effects of testosterone on a sexually dimorphic frog muscle: repeated in vivo observations and androgen receptor distribution

In the present study the sexually dimorphic, androgen-sensitive flexor carpi radialis muscle (FCR) in male Xenopus laevis was viewed repeatedly in vivo to assess the influence of testosterone on muscle fiber size over a period of up to 12 weeks. Regions of the muscle innervated by different spinal nerves responded differently to testosterone treatment. Muscle fibers innervated by spinal nerve 2 (SN2) hypertrophied within 7 days in frogs that had been castrated and given testosterone-filled implants. This initial hypertrophy was followed by a return to normal fiber size a week later, after which fiber size slowly increased again. In castrated males with empty implants, muscle fibers innervated by SN2 gradually atrophied. Fibers innervated by spinal nerve 3 (SN3) were not affected by androgen replacement or withdrawal. The sartorius, a control muscle that is neither sexually dimorphic nor particularly androgen sensitive, was also unaffected. The in vivo observations were confirmed by measurements of muscle fiber cross-sectional areas in frozen sections of whole forelimbs. At 8 and 12 weeks after castration, cross-sectional areas of fibers innervated by SN2 were significantly larger in frogs provided with testosterone than in castrates without testosterone. No difference was found in the SN3 region or in the anconeus caput scapulare (triceps), another control muscle. Immunocytochemistry employing an antibody against the androgen receptor (AR) indicated that the receptor is present in myonuclei of all muscles of the forelimb. While no difference in labeling intensity was detected, the number of AR-containing nuclei per muscle fiber cross-section was higher in fibers innervated by SN2 than in those innervated by SN3, and was yet lower in the triceps. This suggests that regulation of androgen sensitivity may occur via muscle fiber ARs, although an influence of the nerve may also contribute.