Electron microscopy of severed motor fibers in the crayfish

Survival of functional synapses on crustacean neurons lacking cell bodies

Previous workers have demonstrated that some crustacean neurons remain capable of spike propagation and transmitter release and replenishment for months after removal of their perikarya. Here, it is shown that postsynaptic reactions to chemical synaptic input can also persist for months after removal of the soma of the postsynaptic neuron. Interneuron A of the crayfish abdominal cord receives chemically transmitting terminals of ipsilateral tactile afferents of the tail fan. The neuron's soma lies contralateral to its axon and dendrites at the caudal margin of the last abdominal ganglion. The region containing the soma was removed. Interneuron A unambiguously identified by receptive field, location, and size, survived and continued to respond sensitively to tactile input in better than 50% of the cases examined for more than 8 weeks. Cobalt filling of the active fiber in several 8-week-old preparations ruled out the possibility that the severed neurite had reconnected with a foreign soma.

Structural and functional changes in an identified cricket neuron after separation from the soma. I. Structural changes

The morphological effects of separation from the soma were examined in isolated arborization and isolated axon segments of an identified motor neuron in the Polynesian field cricket, Teleogryllus oceanicus. The identified neuron, the contralateral dorsal longitudinal motor neuron of the metathoracic ganglion (CDLM), possesses an arborization most of which lies contralateral to its soma within the metathoracic ganglion. Midline surgical lesions in the metathoracic ganglion separated CDLM into a distal segment composed of the axon and most of the arborization, and a proximal segment comprised of the remaining arborization, neuritie, and soma. Isolated axonal segments were produced by cutting the nerve root containing the axon of CDLM close to the ganglion. The normal anatomy of CDLM was determined by axonal dye-fills using cobaltous chloride. Morphological changes in the isolated arborization of CDLM were examined by axonal dye-fills at successive time intervals. Changes in the isolated CDLM axon were examined via dissection and histological cross-sections of the distal nerve at graded time intervals. In one example, a remnant of the isolated CDLM arborization survived to 168 days postoperative, a time comparable to the longest previously-reported physiological and morphological survival times of distal axonal segments of invertebrates. In general the isolated arborization does not survive this long. Normally-occurring branches of the arborization can be preserved about 0 to 50 days. After this period branches of the arborization seem to be lost in progressive fashion from smaller to larger, leading to complete loss of the arborization and axon in most cases at 100 or more postoperative days. There is evidence for the presence of supernumerary fibers in the isoalted CDLM arborization between 0 to 63 days postoperative. Such supernumerary fibers indicate an independent capacity for outgrowth of the isolated arborization without connection to the nucleus. The distal axonal segment of CDLM degenerates physiologically and morphologically within 4 to 15 days after peripheral nerve section. This time course is close to that of Wallerian degeneration of vertebrate peripheral nerve axons.

Protein metabolism in transected peripheral nerves of the crayfish

The significance of the protein metabolism in crayfish peripheral nerve was studied in relation the ability of crayfish motor axons to survive for over 200 days following axotomy. In contrast to frog peripheral nerves, the crayfish nerves appear to more closely resemble ganglia in their profiles of synthesis expressed on sodium dodecyl sulfate (SDS) gels, and have higher incorporation rates of [3H]leucine into protein than ganglia. Since anisomycin inhibits over 95% of protein synthesis in crayfish peripheral nerve, it was concluded that this local protein synthesis was dependent upon a eukaryotic ribosomal mechanism. Radioautography of isolated nerves reveals newly synthesized proteins in glial sheaths, and also within the axoplasm of large motor fibers. Based upon the data available at present, a hypothesis that the glia surrounding the axons are responsible for the local protein synthesis, and that some of these newly synthesized proteins are transported into the axon, is presented. Transection of crayfish peripheral nerves proximal to the neuron cell bodies produced a more than two-fold increase in [3H]leucine incorporation, but no significant changes in labeling profiles of the proteins on SDS gels. The data suggest that while an active local protein synthesis may be necessary for the maintenance of several crayfish motor axons, it is not a sufficient condition.

Evidence for the local synthesis of a transmitter enzyme (glutamic acid decarboxylase) in crayfish peripheral nerve

The activities of three enzymes of neurotransmitter metabolism (choline acetyl-transferase, CAT; acetylcholinesterase, AChE; and glutamic acid decarboxylase, GAD) were studied in normal, transected, and organ cultured crayfish nerves. CAT (to a lesses extent AChE) was dramatically decreased in activity when the nerve was cut proximal to the nerve cell bodies. GAD activity was unaffected by such procedures. In organ cultured nerve, where both motor and sensory axons degenerated, the CAT and AChE activities were virtually absent, whereas GAD activity remained close to normal levels. Inhibition of protein synthesis in cultured nerve caused the GAD activity to decrease rapidly. In view of these data, and the well documented fact that motor axons survive axotomy whereas sensory axons do not, a hypothesis that GAD is synthesized in the peripheral nerve is presented.

Trans-glial channels in ventral nerve roots of crayfish

The sheath around the roots of the sixth abdominal ganglion in the ventral nerve cord of the crayfish consists of concentric layers of thin glial processes alternating with wide clefts containing filamentous connective tissue. Regions of each glial lamella are perforated by single, short, tubular channels: the trans-glial channels. In thin plastic sections examined in the electron microscope, the channels appear as slits that are 240 A wide and 450-550 A long which traverse glial lamellae less than 1,500 A thick. Branched tubular channels cross glial sheets that are thicker than 1,500 A. The thickest glial wrap is adaxonal; it closely encapsulates individual axons and its cell membrane is separated from the axolemma by a collagen-free space of only 150 A. The adaxonal glial cytoplasm contains unique, three-dimensional networks of interconnected tubules. Separate tubular lattices occur along these thicker processes. In replicas of freeze-fractured sheaths, the outer half of the plasma membrane belonging to the thin glial sheets exhibits many volcano-like protrusions which represent cross fractures through the necks of trans-glial channels. Corresponding depressions on the inner half of these membranes are sites where the plasma membrane invaginates to form the channels. Although some channels are randomly dispersed, others are lineraly positioned in restricted areas across successive glial layers. The number of channels is far more readily appreciated in replicas than in thin sections. The average frequency of channels is 16 per mu2 (range 8 to 33) in normal roots and does not differ significantly from the average of 13 per mu2 in proximal stumps of roots fixed three to four weeks after the roots were cut. The channels are not precisely aligned from one glial layer to the next but do appear to coincide approximately with the adaxonal tubular lattice. The combination of trans-glial channels and adaxonal tubular lattices may provide a complex conduit that could facilitate a rapid, passive flow of electrolytes and nutrients across the nerve sheath to the axonal surface. Horseradish peroxidase solutions bathing the ventral roots enter the trans-glial channels, extracellular clefts and finally the tubular lattices. This distribution supports the proposed role of the channels in a rapid extracellular passage of solutes. The channel profiles have a range of forms consistent with the supposition that they are not static but continually reforming. There are indications that, proximal to the cut, the areas of glial plasma membrane with channel profiles contain more junctional complexes between regenerating cells than between glial cells of normal sheaths. The channel profiles and aggregates of particles belonging to junctions are closely associated when they occupy the same region of the membrane.

Neurotrophic activity of brain extracts in forelimb regeneration of the urodele, Triturus

The loss in protein synthesis which the regenerating forelimb of the newt suffers after denervation can be recovered by infusing into it an extract of newt soluble brain protein. Moreover, the synthesis of basic protein shows a greater response to the active brain principle than does that of acidic protein. The active agent of the nervous tissue is destroyed by heat and trypsin digestion. Extracts of liver and spleen, similarly prepared, do not evoke recovery of lost protein synthesis. Synaptosomal extracts of the frog brain also cause recovery of protein synthesis in the denervated regenerate, demonstrating the likelihood that the active agent is not species-specific within these amphibians, that it is a constituent of the neuronal fraction of nervous tissue, and that it is present in axonal terminals. Additional experiments showed that the nervous agent is likely a basic protein, and that the amount of protein infused is of the order of only 1.0% of the total regenerate protein. The significance of the findings is discussed in relation to the nature of the effect on protein synthesis and the nature of the active principle.

Distribution of ultrastructural tracers in crustacean axons

Ruthenium red and horseradish peroxidase were used to compare the uptake of exogenous molecules into crayfish motor axons and their sheaths in severed and intact peripheral nerves. Both tracers penetrated the axonal sheath and were subsequently seen lining small vesicles and tubules in the axoplasm. Tracer appeared to enter the axon via pinocytotic vesicles. There were no perceptible quantitative or qualitative differences in ruthenium red uptake between intact and severed axons. However, counts of tracer-filled vesicles in axons exposed to peroxidase showed that at least three times as much tracer penetrated the severed as opposed to the intact axons.

Degeneration of sensory and motor axons in transplanted segments of a crustacean peripheral nerve

Segments of sensory and motor axons 0.3-0.5 mm in length were taken from crayfish peripheral limb nerves and transplanted into the abdominal cavity of the same animal. Transplanted sensory axons showed relatively few ultra-structural changes after one week, many had undergone complete lysis within two weeks, and almost all degenerated within three weeks. Transplanted motor axons appeared normal after one week, except for some hypertrophy of their surrounding glial sheaths. After two weeks, glial sheaths were grossly hypertrophied around motor axons; axonal mitochondria had increased in number and many had migrated from the periphery to the centre of the axon. The axonal membranes of all motor axons were still intact after three weeks, although most were no longer continuous after four weeks. By five weeks, all axonal material had completely disintegrated. These data suggest that axonal synthetic processes in crayfish sensory (and presumably motor) axons can maintain the axons relatively intact for 7-14 days and that transfer of substances form hypertrophied glial cells to motor axons may account for the longer survival times of transplanted motor axons.

A multisomatic axon in the central nervous system of the leech

There is one particularly large axon in the medial bundle of the nerve cord of the leech. It extends along the entire length of the cord and is connected to a single cell body in each ganglion. The cell bodies in adjacent ganglia are tightly electrically coupled, and dye injected into one cell body can diffuse along the axon and into the cell body of the next ganglion. If the nerve cord is cut between two ganglia, neither end of the axon degenerates. Repair of the axon appears to occur by end-to-end fusion.

The sensitivity to glutamate of denervated muscles of the crayfish

The effect of denervation on the sensitivity of muscle fibres to glutamate was studied in leg muscles of crayfish.1. When the motor nerve was cut close to the proximal accessory flexor muscle the distal end of the nerve degenerated. Neuromuscular transmission failed and spontaneous miniature potentials disappeared after two months. Several stages of nerve terminal degeneration were seen in muscles denervated between 1 and 3 months, and after 4 months no remains of synapses could be found.2. Following denervation for periods up to 8 months there was no significant change in sensitivity to glutamate, a substance that mimics the action of the neural transmitter. Depolarizations produced by various concentrations of glutamate in the bathing solution were the same in denervated and control muscle fibres. Moreover, the sensitivity to iontophoretically applied glutamate was localized to discrete patches as in innervated muscles.3. Supersensitivity of muscles apparently does not occur after denervation in the crayfish.

Transfer of newly synthesized proteins from Schwann cells to the squid giant axon

The squid giant axon is presented as a model for the study of macromolecular interaction between cells in the nervous system. When the isolated giant axon was incubated in sea water containing [(3)H]leucine for 0.5-5 hr, newly synthesized proteins appeared in the sheath and axoplasm as demonstrated by: (i) radioautography, (ii) separation of the sheath and axoplasm by extrusion, and (iii) perfusion of electrically excitable axons. The absence of ribosomal RNA in the axoplasm [Lasek, R. J. et al. (1973) Nature 244, 162-165] coupled with other evidence indicates that the labeled proteins that are found in the axoplasm originate in the Schwann cells surrounding the axon. Approximately 50% of the newly synthesized Schwann cell proteins are transferred to the giant axon. These transferred proteins are soluble for the most part and range in molecular size from 12,000 to greater than 200,000 daltons. It is suggested that proteins transferred from the Schwann cell to the axon have a regulatory role in neuronal function.