Release properties of isolated neuromuscular boutons of the garter snake

1. Motor nerve terminals innervating fibres in the transversus abdominis muscle of the garter snake comprise discrete boutons. Using a combination of enzymatic digestion and mechanical manipulation, individual boutons were removed from living terminals for study in isolation. 2. Boutons freed from terminals were usually allowed to remain in their original location on the endplate ('attached' one-bouton synapse). Alternatively, they were removed from the endplate, and then placed on the same or another vacant endplate site to form a 'reconstructed' one-bouton synapse. When removed from the endplate, boutons were 2-4 microns in diameter and nearly spherical in shape, in contrast to the variety of complex shapes seen among boutons still in contact with muscle fibre endplates. 3. Transmitter release was assessed by intracellular recording from the postsynaptic fibre. Boutons produced spontaneous miniature endplate potentials (MEPPs) of nearly normal amplitude; extracellular stimulation elicited endplate potentials (EPPs) which resembled MEPPs. Typical EPP amplitudes fluctuated between zero and five quanta per stimulus. For low-frequency stimulation under normal physiological conditions, mean quantal content, m, averaged 1.4; the binomial number of release sites, n, averaged 2.4; and the binomial probability of release, p, averaged 0.57. Statistics of the quantal fluctuations recorded from single boutons agreed only approximately with predictions of simple binomial theory, the discrepancy being that the apparent number of quanta released exceeded n in 5% of the events. 4. In separate experiments, activity-dependent probes were used to locate rare naturally occurring nerve terminals comprising a single bouton. Activation of these small synapses evoked quantal responses similar to those of attached and reconstructed one-bouton synapses described above.

Schwann cells induce and guide sprouting and reinnervation of neuromuscular junctions

The "terminal' Schwann cells that sit atop the neuromuscular junction sense neuromuscular transmission and respond to perturbations of this transmission by extending long processes. These processes have the ability to induce nerve growth and serve as substrates to guide this growth. These processes thus play major roles in muscle reinnervation and in sprouting. An absence of nerve sprouting is correlated with the apoptotic death of terminal Schwann cells at denervated endplates in neonatal muscles. Thus, Schwann cells appear to participate actively in the maintenance and repair of neuromuscular synapses.

Schwann cell processes guide regeneration of peripheral axons

Terminal Schwann cells overlying the neuromuscular junction sprout elaborate processes upon muscle denervation. We show here that motor axons use these processes as guides/substrates during regeneration; in so doing, they escape the confines of endplates and grow between endplates to generate polyneuronal innervation. We also show that Schwann cells in the nerve provide similar guidance. Axons extend from the cut end of a nerve in association with Schwann cell processes and appear to navigate along them. The processes extend from axotomized nerves at the same rate and in the same manner as they do from axon-containing nerves. The rate of process extension limits the rate at which axons regenerate. Thus, Schwann cell processes lead and guide peripheral regeneration.

Nerve sprouting in muscle is induced and guided by processes extended by Schwann cells

Partial denervation or paralysis with botulinum toxin, manipulations that induce sprouting of nerve terminals in muscle, also induced terminal Schwann cells to extend processes. These processes were associated with every nerve sprout and in some cases were longer than the sprouts that appeared to be growing along them. Following partial denervation, more than 70% of the nerve sprouts that grew to innervate nearby denervated endplates were associated with Schwann cell processes that had extended from the denervated endplates, i.e., in the direction opposite to nerve growth. Implantation of Schwann cells into an innervated muscle induced sprouting upon contact of an axon or nerve terminal by Schwann cell processes. These observations show that Schwann cells induce and guide axonal sprouting in muscle.

Differential neural regulation of a neuromuscular junction-associated antigen in muscle fibers and Schwann cells

Monoclonal antibodies 3G2 and 4E2 recognize a postsynaptic component of rat neuromuscular junctions. In contrast to many other postsynaptic junctional antigens, expression of this antigen is nerve-dependent: immunoreactivity disappears from junctions following denervation and returns upon reinnervation (Astrow et al., 1992 J. Neurosci. 12:1602-1615). Here we show that the epitope is also expressed by Schwann cells and that this expression is also neurally regulated. Weak mAb 3G2/4E2 immunoreactivity was found in myelinating Schwann cells but was not detected in either nonmyelinating Schwann cells or in terminal Schwann cells at the neuromuscular junction. Following axotomy, immunoreactivity increased in myelinating Schwann cells, and nonmyelinating and terminal Schwann cells became immunopositive. Moreover, the immunoreactivity in terminal Schwann cells revealed their extensive sprouting in response to denervation (Reynolds and Woolf, 1992, J. Neurocytol. 21: 50-66). After nerve regeneration, mAb 3G2/4E2 immunoreactivity in all Schwann cells returned towards normal: it disappeared from terminal Schwann cells, returned to low levels in myelinating Schwann cells, and decreased in nonmyelinating Schwann cells. Immunoblots of axotomized nerve and cultured muscle fibers revealed the same set of immunoreactive bands. Therefore, Schwann cells and muscle fibers share the expression of an epitope that is under neural control, but is regulated differently at each site. In Schwann cells, the presence of the nerve suppresses expression of the epitope, whereas in muscle fibers, the nerve terminal promotes this expression. The differential regulation of mAb 3G2/4E2 immunoreactivity in terminal Schwann cells and muscle fibers suggests that the epitope may be involved in interactions between nerve terminals and these cells.