Regeneration of neuromuscular connections in crayfish allotransplanted neurons

Depressing effects of caffeine at crayfish neuromuscular synapses I. Dosage response and Ca++ gradient effects

The response of crayfish synaptic terminals to drugs began to be studied to characterize the terminal's physiological characteristics. Caffeine, the first drug to be studied, was selected to enhance synaptic transmission because of its ability to increase calcium release from internal stores.1. The largest excitor neuron to the superficial flexor muscle system of Procambarus clarkii was stimulated at 10 Hz while recording junction potentials from several lateral muscle fibers.2. Caffeine unexpectedly decreased synaptic transmission in this system in a dosage-dependent manner. The depressing effect of caffeine was observed at 5 mM caffeine and junction potentials disappeared completely at 50 mM. Washing the preparation in fresh control Ringers did not restore the amplitudes of the junction potentials.3. Changes in extracellular calcium concentrations delayed or depressed the caffeine effect depending on the calcium gradient across the membrane or the caffeine dosage. The data suggest that calcium is involved in caffeine's response in this system in a way yet to be determined.

Allotransplanted nerves regenerate inhibitory synapses on a crayfish muscle: Possible postsynaptic specification

Donor nerves of different origins, when transplanted onto a previously denervated adult crayfish abdominal superficial flexor muscle (SFM), regenerate excitatory synaptic connections. Here we report that an inhibitory axon in these nerves also regenerates synaptic connections based on observation of nerve terminals with irregular to elliptically shaped synaptic vesicles characteristic of the inhibitory axon in aldehyde fixed tissue. Inhibitory terminals were found at reinnervated sites in all 12 allotransplanted-SFMs, underscoring the fact that the inhibitory axon regenerates just as reliably as the excitatory axons. At sites with degenerating nerve terminals and at sparsely reinnervated sites, we observe densely stained membranes, reminiscent of postsynaptic membranes, but occurring as paired, opposing membranes, extending between extracellular channels of the subsynaptic reticulum. These structures are not found at richly innervated sites in allotransplanted SFMs, in control SFMs, or at several other crustacean muscles. Although their identity is unknown, they are likely to be remnant postsynaptic membranes that become paired with collapse of degenerated nerve terminals of excitatory and inhibitory axons. Because these two axons have uniquely different receptor channels and intramembrane structure, their remnant postsynaptic membranes may therefore attract regenerating nerve terminals to form synaptic contacts selectively by excitatory or inhibitory axons, resulting in postsynaptic specification.

Regenerated synaptic terminals on a crayfish slow muscle identify with transplanted phasic or tonic axons

Phasic or tonic nerves transplanted onto a denervated slow superficial flexor muscle in adult crayfish regenerated synaptic connections that displayed large or small excitatory postsynaptic potentials (EPSPs), respectively, suggesting that the neuron specifies the type of synapse that forms (Krause et al., J Neurophysiol 80:994-997, 1998). To test the hypothesis that such neuronal specification would extend to the synaptic structure as well, we examined the regenerated synaptic terminals with thin serial section electron microscopy. There are distinct differences in structure between regenerated phasic and tonic innervation. The phasic nerve provides more profuse innervation because innervation sites occurred more frequently and contained larger numbers of synaptic terminals than the tonic nerve. Preterminal axons of the phasic nerve also had many more sprouts than those of the tonic nerve. Phasic terminals were thinner and had a lower mitochondrial volume than their tonic counterparts. Phasic synapses were half the size of tonic ones, although their active zone-dense bars were similar in length. The density of active zones was higher in the phasic compared with the tonic innervation, based on estimates of the number of dense bars per synapse, per synaptic area, and per nerve terminal volume. Because these differences mirror those seen between phasic and tonic axons in crayfish muscle in situ, we conclude that the structure of the regenerated synaptic terminals identify with their transplanted axons rather than with their target muscle. Therefore, during neuromuscular regeneration in adult crayfish, the motoneuron appears to specify the identity of synaptic connections.

Regeneration of phasic motor axons on a crayfish tonic muscle: neuron specifies synapses

Motor neurons are matched to their target muscles, often forming separate phasic and tonic systems as in the abdomen of crayfish where they are used for rapid escape and slow postural movements, respectively. To assess the role of motor neuron and muscle fiber in forming synapses we attempted a mismatch experiment by allotransplanting a phasic nerve attached to its ganglion to a denervated tonic muscle. Regenerating motor axons sprouted 10-30 branches (typical of phasic motor neurons, as tonic ones sprout far fewer branches) to reinnervate muscle fibers and form synapses that produced large excitatory postsynaptic potentials (typical of phasic motor neurons, as tonic synapses give small potentials). Therefore motor neurons, not muscle fibers, appear to specify one of the major properties of regenerating neuromuscular synapses.

Muscle fibers in regenerating crayfish motor nerves

Single discrete muscle fibers were found in regenerating motor nerves in adult crayfish. The regenerating nerves were from native or transplanted ganglia in the third abdominal segments and consisted of several motor axons. The proximal end of these motor axons showed numerous sprouts. Muscle fibers in these regenerating nerves appeared newly developed and were innervated by excitatory nerve terminals. A likely source of these novel muscle fibers may be blood cells in the nerve or satellite cells from neighboring muscle. Contacts made by axon sprouts with other axon sprouts, glia, and muscle fiber, in the form of a dense bar with clustered clear vesicles, characterized the regenerating nerve. These contacts may provide a possible signaling pathway for axon regeneration and myogenesis.

Structure of allotransplanted ganglia and regenerated neuromuscular connections in crayfish

In adult crayfish, Procambarus clarkii, motoneurons to a denervated abdominal superficial flexor muscle regenerate long-lasting and highly specific synaptic connections as seen from recordings of excitatory postsynaptic potentials, even when they arise from the ganglion of another crayfish. To confirm the morphological origins of these physiological connections we examined the fine structure of the allotransplanted tissue that consisted of the third abdominal ganglion and the nerve to the superficial flexor muscle (the fourth ganglion and the connecting ventral nerve cord were also included). Although there is considerable degeneration, the allotransplanted ganglia display intact areas of axon tracts, neuropil, and somata. Thus in both short (6-8 weeks) and long (24-30 weeks) term transplants approximately 20 healthy somata are present and this is more than the five axons regenerated to the host muscle. The principal neurite and dendrites of these somata receive both excitatory and inhibitory synaptic inputs, and these types of synaptic contacts also occur among the dendritic profiles of the neuropil. Axon tracts in the allotransplanted ganglia and ventral nerve cord consist largely of small diameter axons; most of the large axons including the medial and lateral giant axons are lost. The transplanted ganglia have many blood vessels and blood lacunae ensuring long-term survival. The transplanted superficial flexor nerve regenerates from the ventral to the dorsal surface of the muscle where it has five axons, each consisting of many profiles rather than a single profile. This indicates sprouting of the individual axons and accounts for the enlarged size of the regenerated nerve. The regenerated axons give rise to normal-looking synaptic terminals with well-defined synaptic contacts and presynaptic dense bars or active zones. Some of these synaptic terminals lie in close proximity to degenerating terminals, suggesting that they may inhabit old sites and in this way ensure target specificity. The presence of intact somata, neuropil, and axon tracts are factors that would contribute to the spontaneous firing of the transplanted motoneurons.