The morphological and physiological properties of a regenerating synapse in the C.N.S. of the leech

Ultrastructure of an identified molluscan neuron in organ culture and cell culture following axotomy

We examined the ultrastructure of neuron 5 from the buccal ganglion of the mollusc Helisoma trivolvis after axotomy and organ culture, and after isolation of the same neuron in culture. Buccal ganglia containing axotomized neurons 5 were cultured either in host snails or in Leibovitz medium conditioned with ganglia. In addition, some neurons 5 were isolated from buccal ganglia by micro-dissection and plated into culture. Neuron 5 and its processes were identified in both whole mounts and plastic sections of buccal ganglia after intracellular injection with Lucifer Yellow or horseradish peroxidase. Five days after axotomy of neuron 5, thick sections of buccal ganglia stained with toluidine blue revealed that densely staining basophilic bodies (Nissl bodies) within the cytoplasm had dispersed, i.e., they had undergone chromatolysis. Coincident with chromatolysis was an overall increase in diffuse basophilic staining within the cytoplasm of neuron 5 when maintained in organ culture. The dispersion of Nissl bodies viewed by light microscopy correlated with a more freely arranged rough endoplasmic reticulum and associated polysomes within neuron 5 as seen by electron microscopy. Isolated neurons 5 did not possess densely staining Nissl bodies when examined after 2 days in vitro, thus indicating that chromatolysis occurred earlier in isolated neurons. Furthermore, no increase in diffuse cytoplasmic basophilia was observed within isolated neurons 5 cultured in vitro. However, isolated neurons 5 exhibited a marked increase in the number of lipid-like bodies (0.5-1.5 micron in diameter) that were particularly evident in scanning electron micrographs. Scanning and transmission electron micrographs revealed that the isolated neurons were free of associated glia, but non-neuronal cells (hemocytes) would attach themselves to the somata and neurites. Glia surrounding neuron 5 within buccal ganglia exhibited a marked hypertrophy following axotomy and organ culture. Hypertrophy of glia was absent, however, if ganglia were axotomized and left within the animal or axotomized ganglia were implanted into host animals and examined 5 days later by electron microscopy. These observations indicate that, following axotomy, a molluscan neuron may exhibit different morphological features depending on its microenvironment. In addition, the hypertrophy of glia surrounding neurons in Helisoma was not associated with axotomy per se, but with organ culture.

Individual microglia move rapidly and directly to nerve lesions in the leech central nervous system

Small cells called microglia, which collect at nerve lesions, were tracked as they moved within the leech nerve cord to crushes made minutes or hours before. The aim of this study was to determine whether microglia respond as a group and move en masse or instead move individually, at different rates, and whether they move along axons directly to the lesion or take another route, such as along the edges of the nerve cord. Cell nuclei in living nerve cords were stained with Hoechst 33258 dye and observed under dim ultraviolet illumination using fluorescence optics, a low-light video camera, and computer-assisted signal enhancement. Muscular movements of the cord were selectively reduced by bathing in 23 mM MgCl2. Regions of nerve cord within 300 microns of the crush were observed for 2-6 hr. Only a fraction of microglia, typically less than 50%, moved at any time, traveling toward the lesion at speeds up to 7 microns/min. Cells were moving as soon as observation began, within 15 min of crushing, and traveled directly toward the lesion along axons or axon tracts. Movements and roles of leech microglia are compared with their vertebrate counterparts, which are also active and respond to nerve injury.

Developing axons continue to grow at their tip after synapsing with their appropriate target

The contacts a growing neuron's axon makes with its synaptic targets are believed to inhibit further growth at the axon tip. Inhibition of axon growth has been difficult to examine in vivo, where studies have focused on populations of neurons with multiple targets, making the influence of a single target difficult to determine. Results of a direct test of the influence of synapse formation on axon growth are presented for the axon of the S interneuron in the leech, which has a single synaptic target that can decidedly inhibit growth at the axon's tip during regeneration in adults. Surprisingly, in embryos, after synapsing with its target, each S cell axon grew for several days, including growth at its tip, nearly doubling its length. Therefore, synaptic contact with the target does not stop further growth at the axon's tip.

Recovery of escape locomotion following a CNS lesion in Aplysia

The recovery of escape locomotion in Aplysia following a CNS lesion was investigated. The connectives between the cerebral and pleural ganglia were crushed in anesthetized animals, producing a specific behavioral deficit. Animals with lesions failed to initiate escape locomotion in response to tail shock. Tail withdrawal and inking which were also evoked by tail shock were still present. Other behaviors such as normal locomotion and feeding were not impaired. There was gradual recovery from the effects of the lesion. Animals with lesions began to respond to tail shock with weak pedal waves at long latencies after 7-13 days. The responses grew more vigorous and the latencies decreased over subsequent days. Full escape locomotor responses were observed as early as 15 days postlesion. By Postlesion Day 27, all of the animals had completely recovered and gave full escape responses. The mean latency of the escape locomotor response in recovered animals was not significantly different from prelesion control values. In behaviorally recovered animals, retrograde tracing from a point distal to the lesion site stained neurons in the cerebral ganglion. Intracellular dye injections of individual neurons revealed sprouting of new processes. Stimulation of the tail nerve and individual neurons demonstrated synaptic connections between cerebral and pleural ganglia neurons. These results suggest that the observed behavioral recovery was due to pleural ganglia neurons regenerating and forming appropriate synaptic connections in the cerebral ganglion.

Regeneration studies on a crayfish neuromuscular system. I. Connectivity changes after intersegmental nerve transplants

The superficial flexor muscles of the crayfish are a neuromuscular system of a few muscle cells innervated by six neurons in a precise position-dependent pattern. The neurons are capable of regenerating their normal connectivity patterns within a short span of time when conditions are favorable. The superficial flexor muscles of the second and third segments, despite their similarities in neuronal and muscle cell size and number, have distinctive connectivity patterns; some homologous neurons form similar patterns but other homologous neurons form patterns that are reversed between segments. We transplanted each segment's nerve into each other's muscle in order to observe regeneration of the nerves into a target area that differed in connectivity patterns from their original muscle. During the first weeks of regeneration all neurons formed a connectivity pattern with more connections medially and declining connections laterally, a pattern determined by the medial location of the nerve transplant. This pattern is maintained for most of the neurons, but for some there is an eventual reduction in medial connections as maximum synapse formation shifts to the lateral muscle fibers. Three of the eight neurons studied were able to regenerate connectivity patterns that corresponded to their segment of origin and not to their host muscle. This suggests that intersegmental muscle differences are not influencing the formation of these connectivity patterns, so the neurons will follow their inherent synaptogenesis program.

Serotonin storage and uptake by identified neurons in the leech Haementeria ghilianii

Characterization of serotonin-containing neurons in the glossiphoniid leech Haementeria ghilianii was undertaken to provide a reference for developmental studies of their differentiation and for comparative studies of their distribution and function. Five types of serotonin-containing neurons were identified with an antiserum against serotonin and by radioenzymatic assay of individual isolated somata. They contain high concentrations of serotonin (in some cases exceeding saturation in aqueous solution) and their serotonin content increases with growth of the animal. Each type is capable of taking serotonin up from the extracellular fluid, as demonstrated autoradiographically. They exhibit segment-specific, and on comparison with hirudinid leeches, species-specific, differences in distribution, morphology, and the expression of serotonin metabolism.

Morphological and physiological survival of goldfish Mauthner axons isolated from their somata by spinal cord crush

Axon segments isolated from their somata degenerate within days or months depending on species and neuronal type. To better understand the time course of morphological and physiological changes associated with degeneration of axon segments of vertebrate central neurons, we have studied the goldfish Mauthner axon (M-axon) when it has been separated from its soma by spinal cord crush. M-axon segments survive morphologically for at least 77 days at 14 degrees C. Cross-sectional areas of isolated M-axon segments (measured 25-30 mm caudal to the wound site at postoperative days 64 and 77) were greater than those of control axons at the same level. Sheath areas did not change. Electron microscopic observations at the same spinal cord location indicated no clear changes in the configuration or number of neurofilaments or any other organelle. M-axon segments studied morphologically after 87 postoperative days had all degenerated. Mauthner axon segments were capable of conducting action potentials and eliciting ipsilateral EMG responses. Repetitive firing of the M-axon segments elicited EMG responses that fatigued more easily and remained fatigued over a longer interval than did those of control axons. The long duration of M-axon segment survival is unusual in a vertebrate and may be due to the low temperature at which the experiments were conducted (14 degrees C) and/or temperature-independent factors. The increased susceptibility to synaptic depression, which has not reported previously, may represent an early sign of the degenerative process.

Chemical synapses, particle arrays, pseudo-gap junctions and gap junctions of neurons and glia in the buccal ganglion of Helisoma

The nervous system of the snail, Helisoma trivolvis, has been utilized for a wide range of studies of neuronal plasticity; however, the ultrastructural features of this tissue were previously unknown. The present study examined the nature of synaptic interactions of neurons and glia and considered several plasma membrane specializations of these cells. The symmetrical pair of buccal ganglia consisted of a ring of unipolar neurons surrounding a central neuropil. The neurons were separated by two morphologically distinct types of glia: type I were most numerous and possessed an electron-dense homogeneous cytoplasm, whereas type II glia were of lower electron density, possessed a heterogeneous cytoplasm, and appeared to be phagocytic. Gap junctions were abundant between glia and were occasionally found between neuronal processes, including those of neurons 19 injected with horseradish peroxidase (HRP). Comparison of neuron and glial gap junction widths (16.4 and 17.6 nm, respectively) in thin sections and their intramembrane particle diameters (13.1 and 13.7 nm, respectively) by freeze fracture, did not elucidate significant differences. A heterogeneous population of putative chemical synapses, similar to those reported in other molluscs, was also observed between axonal collaterals in the neuropil. Additionally, examination of freeze-fractured neuropil revealed rhombic arrays of particles localized on neuronal membranes; these arrays do not appear to form intercellular junctions but may represent postsynaptic receptor sites. Freeze fracture also revealed small, square arrays consisting of 7-9 nm diameter particles on glial membranes which may correspond to pentalaminar membrane contacts (pseudo-gap junctions) seen in thin sections between glia situated around dilated extracellular spaces (lacunae).

Differential time course of the response to axotomy induced by cut or crush in the leech AP cell

The time course of the reaction to axotomy in the leech AP cell was determined by measuring the duration of the spontaneous spikes at different times after the operation. The axotomy performed by section of the segmental roots containing the AP axon induced an increase of the spike duration, which persisted over 30 days. A different time course was found when the axotomy was performed by nerve crush: the changes in duration of the spontaneous spikes, which occurred during the early 2 weeks, were significantly reduced afterwards. Dye staining of some cells axotomized by crushing revealed that the reversion of the changes, which had been set up by axotomy, was in some cases concomitant with the reconnection between proximal and distal axon stumps. The section of a single axonal branch was never sufficient to affect the membrane properties of the AP cells. It is concluded that the changes observed in axotomized AP cells are not produced by simple axonal injury and that the maintainance of normal properties in the somatic membrane requires the presence of at least part of the distal axon arborization.

Morphogenesis of an identified leech neuron: segmental specification of axonal outgrowth

We have investigated the development of segmental diversity in an identified leech neuron, the Retzius cell. Retzius cells in the genital segments differ from those in other segments in lacking central axons and contacting different peripheral targets: the genitalia. These differences are not apparent during initial axon outgrowth, when all Retzius cells follow the same morphogenetic pattern. Rather, they first appear about the time the peripheral axons of the genital segment Retzius cells contact the genital primordia. This suggests that the pattern of central and peripheral axonal outgrowth may be modified by an interaction with peripheral targets.

Reconnection of severed nerve axons with polyethylene glycol

Severed medial giant axons in crayfish can be rejoined in vitro with polyethylene glycol (PEG) to produce axoplasmic continuity and through transmission of action potentials. Severed axon-like processes of a mammalian neuroblastoma/glioma cell line seem to be rejoined to the cell body using PEG in tissue culture. Our data suggest that PEG might be used to rejoin severed axons in vivo in various organisms.

Age-dependent occurrence of an ascending axon on the omega neuron of the cricket, Teleogryllus oceanicus

The omega neurons (ON1s) are a mirror-symmetrical pair of identified prothoracic auditory interneurons of crickets which have been previously described as intraganglionic. Using intracellular techniques we stained ON1s of female Teleogryllus oceanicus and found that many ON1s have axons which project anteriorly out of the prothoracic ganglion. The ascending axon arises contralateral to the soma at the most anteriolateral bend of the bow-shaped process of an otherwise "archetypical" ON1 and travels up the neck connective in a ventral position just inside the connective tissue sheath. The occurrence of the ascending axon is age-dependent. Seventy-five percent of ON1s stained in late nymphal stages and in young adults had an ascending axon while only 30% of ON1s in older adults had an ascending axon. Evidence is presented to show that ON1s having ascending axons are developmental variants of the "archetypical" ON1 and do not represent a separate neuron type. The two morphological types of ON1s are not distinguishable on the basis of their responses to sound stimuli having carrier frequencies of 3.5-60 kHz. Although we know that the ascending axon conducts action potentials, its target and terminal morphology are not yet known.

Differential ability of two adult molluscan neurons to regenerate electrical synapses

The ability of two electrical synapses (neuron L4-R4 and neuron L19-R19) to regenerate in the adult Helisoma nervous system was examined. The L4-R4 electrical connection exhibited rapid restoration to 50% of normal strength, whereas L19-R19 reconnection was weak or absent. This disparity is attributable to the inability of neuron 19 to sprout effectively across a crush site in the buccal commissure, although peripheral sprouting was pronounced. The factors which underlie the inability of neuron 19 (but not neuron 4) to sprout into a central environment are unknown, but dictate differential synapse restoration in this adult nervous system.

Regeneration of functional synapses between individual recognizable neurons in the lamprey spinal cord

In 4- to 5-year-old sea lamprey larvae that had recovered from complete transection of the spinal cord, pairs of giant interneurons on opposite sides of the scar were impaled with microelectrodes. In 4 of 30 pairs, stimulation of the caudal cell elicited a monosynaptic electrochemical excitatory postsynaptic potential in the rostral cell. Fifty percent of such pairs were synaptically linked in control lampreys without transections. These results show regeneration of functional synaptic connections between individual neurons in a vertebrate central nervous system.

Sprouting and functional regeneration of an identified serotonergic neuron following axotomy

An identified serotonergic neuron (C1) in the cerebral ganglion of Helisoma trivolvis sprouts following axotomy and rapidly (seven to eight days) regenerates to recover its regulation of feeding motor output from neurons of the buccal ganglia. The morphologies of normal and regenerated neurons C1 were compared. Intracellular injection of the fluorescent dye, Lucifer Yellow, into neuron C1 was compared with serotonin immunofluorescent staining of the cerebral and buccal ganglia. The two techniques revealed different and complimentary representations of the morphology of neuron C1. Lucifer Yellow provided optimal staining of the soma, major axon branches, and dendritic arborization. Immunocytochemical staining revealed terminal axon branches on distant targets and showed an extensive plexus of fine fibers in the sheaths of ganglia and nerve trunks. In addition to C1, serotonin-like immunoreactivity was localized in approximately 30 other neurons in each of the paired cerebral ganglia. Only cerebral neurons C1 had axons projecting to the buccal ganglia. No neuronal somata in the buccal ganglia displayed serotonin-like immunoreactivity. Observations of regenerating neurons C1 demonstrated: Actively growing neurites, both in situ and in cell culture, displayed serotonin-like immunoreactivity; severed distal axons of C1 retained serotonin-like immunoreactivity for up to 28 days; axotomized neurons C1 regenerated to restore functional control over the feeding motor program.

Axon growth from limb motorneurons in the locust embryo: the effect of target limb removal on the pattern of axon branching in the periphery

Metathoracic limb buds have been unilaterally ablated from locust embryos at 25 to 30% of embryonic development and the effect of this operation on the axon morphology of the motorneuron fast extensor tibiae (FETi) observed at later embryonic stages. In control embryos this neuron sends a single axon out the main leg nerve, nerve 5, to the extensor tibiae muscle in the femur. In limb ablated embryos the axon of FETi is found in a wide variety of aberrant peripheral nerve pathways and projects to a wide range of foreign muscles. There is a degree of apparent selectivity, but no rigid hierarchy, in the choice of pathway and muscle made by FETi. A high degree of variability is found between one embryo and another in the extent and pattern of axon branching. The axon of FETi is generally found in pathways that correspond to nerves in control embryos but on occasion grows along novel routes. An anteriorly directed dendritic branch, seldom seen in control FETi neurons, is frequently seen in experimental FETis. These findings are discussed in terms of the rules for specific axon growth in normal development.

Selective loss of neurites during differentiation of cells in the leech central nervous system

The arborizations of annulus erector (AE) motoneurons in the central nervous system of the leech, Hirudo medicinalis, have been examined during embryogenesis to determine how segmental differences in their branching patterns arise. Early in development AE motoneurons all along the ganglionic chain had a similar central arborization, with major branches extending both rostrally and caudally along the connectives that link adjacent ganglia. As the embryo grew, the processes in the connectives elongated but failed to increase significantly in caliber and eventually atrophied and were lost. This sequence of events did not occur uniformly along the cord, however. AE motoneurons in midbody ganglia lost both anterior and posterior branches, cells near the head lost only their posterior branch, while cells near the tail lost only their anterior branch. In this way, the selective atrophy of neurites during development produced a systematic segmental difference in the morphology of homologous cells.

Segmental variation in the arborization of identified neurons in the leech central nervous system

Mechanosensory and motor neurons in the central nervous system of the leech have been examined by intracellular injection of horseradish peroxidase and electrophysiological mapping of their peripheral fields to determine how the arborizations of homologous cells are influenced by their segmental position. The branching patterns of annulus erector (AE) motoneurons in ganglia near the head and tail were found to be more extensive than those of cells in midbody ganglia. As in midbody ganglia, the peripheral fields of AE motoneurons in adjacent ganglia near the head and tail overlapped extensively, but the subfields innervated by individual branches of a single AE motoneuron showed little or no overlap. No AE motoneurons were found in the head ganglion or in the 20th and 21st free segmental ganglia. The branching pattern of touch-sensitive mechanosensory cells showed a similar segmental variation; touch cells in ganglia near the head and tail had more extensive arborizations than those in midbody ganglia. The rostrocaudal position along the cord at which the branching pattern changed from that characteristic of midbody ganglia to one with a more extensive arborization differed for different types of neurons. These findings demonstrate that a cell's pattern of arborization is not determined by a simple segmental difference between ganglia and suggest that during development neurons respond individually to cues that vary along the length of the cord.

Response to axotomy of an identified leech neuron, in vivo and in culture

Membrane properties of identified leech neurons with non-spiking somata were studied after axotomy. Two distinct procedures were used: the section of ganglionic roots in vivo and the isolation of cell somata in culture. The results indicated that axotomized neurons progressively developed the excitability of somatic membrane, both in vivo and in culture.

Accurate regeneration of an electrical synapse between two leech neurones after destruction of the ensheathing glial cell

An interneurone, the S cell in the central nervous system of the leech, regenerates its severed axon and forms an electrical synapse with its target, another S cell, entirely within the ensheathment of two glial cells. After the two glial cells were killed selectively by intracellular injection of protease, axonal regeneration and synapse formation occurred in a normal fashion during the month following nerve injury. Soon after reconnexion of S cells, the conduction of impulses across the non-rectifying electrical junction between the cells was more reliable from the target than into it from the thinner regenerating axon. The distal segments of severed S-cell axons survived for weeks or months after destruction of their glial cells, indicating that the ensheathing glia is not required for long-term survival of axon segments. The distal axon segment of the S cell remained connected to the target axon at the normal region of synapse midway between ganglia within the nerve cord. In about half the cases in which reconnexion between injured S cell and target S cell occurred between 10 and 25 days following nerve crush, the regenerating neurone had formed an electrical synapse with its severed distal axon and had thereby become reconnected, indirectly, with its target. In the other cases, reconnexion was by direct contact. By 4 weeks, the proportion of injured S cells that were coupled and making direct contact with their targets rose to more than 80% of the total population, indicating that regeneration continued until the two S cells contacted one another directly. This is similar to the course of S-cell regeneration in the presence of the ensheathing glia. Microscopy of the regenerating neurone and both its distal axon segment and its target showed that the site of synapse formation in the absence of the usual glial sheath was normal. Fluorescence microscopy following intracellular injection of Lucifer Yellow dye, which crosses between S cells at the electrical synapse, showed that the regenerated synapse formed specifically between S cells. Moreover, the target did not form alternative synapses when regeneration failed.

Distinctions between gap junctions and sites of intermediate filament attachment in the leech C.N.S

Freeze-fracture studies on the nerve cord of the leech Hirudo medicinalis reveal that the plasma membranes of various cells, including glial and muscle cells, contain at least two distinct types of aggregated intramembrane particles, identified as hemidesmosomes and gap junctions. Hemidesmosomes consist of angular particles irregularly arranged in circular or elongate patches in external leaflets (E-faces), and are associated with a bundle of intermediate filaments extending into the cytoplasm. Hemidesmosomes of specific axons abut on extracellular space at openings in the surrounding glial sheath. Gap junctions are patches of rounder particles in cytoplasmic leaflets (P-faces) and are more uniformly spaced; they have a corresponding array of pits in the complementary E-face. Gap junctions connect processes of adjacent smooth muscle cells, and apparently interconnect glial processes. Thus, different types of cells in the leech C.N.S. have similar intramembrane specializations. Moreover, the hemidesmosomes and gap junctions might, on superficial examination, be confused.

Gap junctions and septate-like junctions between neurons of the opisthobranch mollusc Navanax inermis

The buccal ganglia of Navanax inermis were studied by thin section, lanthanum infiltration and freeze fracture. Freeze fracture clearly demonstrates small gap junctions between neuronal processes in the neuropil, many of which are known to be electrotonically coupled. Junctional particles cleave with the P-face. In thin section, gap junctions appeared as small blurred contacts, presumably because of the small size of the junctions. Lanthanum infiltration was poor and failed to aid in identifying gap junctions. However, it did reveal septate-like junctions whose septa were not osmiophilic. Corresponding E-face grooves and ridges were seen in freeze fracture, sometimes adjacent to gap junctions. The septate-like junctions have parallel membranes and may have been mistaken for gap junctions in several other thin section studies of invertebrate neurons.

Morphological evidence that regenerating axons can fuse with severed axon segments

Regenerating axons of sensory neurons in the leech nerve cord usually reconnect with their normal targets by growing the entire distance from the site of lesion to the target. However, in less than 1% to nearly 10% of cases a rapid restoration of the normal arborization occurs when the regenerating axon connects with the severed distal segment of the same cell or another cell of the same modality. The passage of horseradish peroxidase (mol. wt approximately 40,000 daltons) from the regenerating axon selectively into the axon or cell with which it has connected indicates that the two have joined or fused, rather than become linked by an electrical synapse, as sometimes occurs for other neurons in the leech. These results support the conclusions, based largely on physiological data from regenerating motor axons in crayfish, that unusually rapid and complete regeneration can occur when a growing axon fuses with its severed distal segment.

Frequency-dependent coupling between rhythmically active neurons in the leech

The heart excitor (HE) cells, a set of rhythmically active motor neurons, drive the heartbeat of the medicinal leech. Their activity is gated by inhibitory input from a network of interneurons, but that influence may be modified locally by electrotonic coupling between the HE cells. In this paper I analyze that electrotonic coupling by applying direct current and alternating current signals, and compare the results with predictions based on linear cable theory. The electrotonic junction itself appears to be conventional, but because of the membrane properties of the HE cells, the coupling strength depends upon both the frequency and polarity of the signal and the phase of heartbeat cycle when the signal is applied.

Neuronal organization in fly optic lobes altered by laser ablations early in development or by mutations of the eye

The role of afferent and efferent connections in the differentiation of optic lobe interneurons was investigated by using laser ablations of neuronal precursors in the brain of Musca domestica and analysis of two eye mutants of the same species. The first mutant, split eye, had no connections between the retina and the optic lobes. In this case the optic lobes were drastically reduced in volume and the neural organization within the neuropil regions was altered. The other mutant, spindle, had reduced retinae that innervated reduced optic lobes with a normal-appearing orderly arrangement of neurons. In addition disordered neuropil, composed of identified visual interneurons, was found that had no afferent innervation. Three main types of alterations resulting from laser ablations were analyzed. These ablations removed entire neuropil regions or parts of these: (1) removal of the first optic neuropil region (the lamina) resulting in receptor axons projecting directly to the second neuropil (the medulla) and sprouting of medulla neurons toward the receptor layer; (2) removal of one part of the third optic neuropil (the lobula plate) and severe alteration of the other part (the lobula) resulting in sprouting of lobula neurons into the medulla neuropil; and (3) removal of the entire optic lobe resulting in reduction of the volume of the lateral midbrain and photoreceptor axons forming a tangle beneath the retina. Our findings confirm that afferent retinal input is essential for normal differentiation and maintenance of many optic lobe interneurons. Furthermore, it was seen that a normal columnar organization of the neuropils and the dendritic patterns of visual interneurons are dependent on afferent inputs. A common response to removal of inputs was a reorganization of axonal and dendritic projections.

Synapses between neurons regenerate accurately after destruction of ensheathing glial cells in the leech

Individual glial cells that ensheathe axons in the central nervous system of the leech were destroyed by intracellular injection of protease. The axons were then severed, and regeneration by particular neurons was studied physiologically and morphologically. Although certain axons sprouted more in the absence of the glial cell, functional synapses were accurately regenerated with normal frequency.

Long term survival of enucleated segments of glial cytoplasm in the leech Macrobdella decora

Enucleated cytoplasmic segments of the giant connective glial cell (GCGC) survive morphologically intact for at least 10 weeks in the leech Macrobdella decora. Enucleated GCGC segments isolated from regenerating nerve axons show some degenerative changes after 4 weeks compared to GCGC segments which surround intact or regenerating nerve axons. Survival of GCGC cytoplasm is associated with an increase in the number of microglia. Relatively few (10-30%) nerve axons degenerate after severance from their cell body.

Long-term survival of glial segments during nerve regeneration in the leech

Nerve injury that severs axons also disrupts ensheathing glial cells. Specifically, crushing or cutting the leech nerve cord separates the glial cell's nucleated portion from an anucleate recording, by intracellular injection of Lucifer Yellow dye and horseradish peroxidase (HRP) as tracers, and by electron microscopy. The nucleated portion of the glial cell did not divide, degenerate, or grow appreciably. The severed glial stump remained isolated from the nucleated portion but maintained its resting potential and normal morphology for months. Stumps typically began to deteriorate after 3 months. Small macrophage-like cells, or 'microglia' increased in number after injury and ensheathed axons, thus partially replacing the atrophying glial stump. Some axons in the nerve cord degenerated; the remainder appeared morphologically and physiologically normal. Thus, both nucleated and anucleate glial segments persisted throughout the one to two months required for axons to regenerate functional connections. Glial cells in the leech are therefore available to guide physically the growing axons or to contribute in other ways to nerve regeneration.

Enhanced spinal cord regeneration in lamprey by applied electric fields

After a weak, steady electric current of approximately 10 microamperes was imposed across the completely severed spinal cord of the larval lamprey Petromyzon marinus, enhanced regeneration was observed in the severed giant reticulospinal neurons. The current was applied with implanted wick electrodes for 5 to 6 days after transection (cathode distal to lesion). The spinal cords were examined 44 to 63 days after the operation by means of intracellular fluorescent dye injections and electrophysiology. Extracellular stimulation of whole cords showed that action potentials in most of the electrically treated preparations were conducted in both directions across the lesion, but they were not conducted in either direction in most of the sham-treated controls. In most of the electrically treated animals, processes from giant axons with swollen irregular tips, indicating active growth, were seen in or across the lesion. Only a few of the sham-treated controls showed these features. It is possible that these facilitated regenerative responses were mediated by the effects of the artificially applied electric fields on the natural steady current of injury entering the spinal lesion.

Mapping of neuronal contacts with intracellular injection of horseradish peroxidase and Lucifer yellow in combination

A technique for the simultaneous visualization in the light microscope of processes of neurons filled with horseradish peroxidase and Lucifer yellow in combination has been developed. The technique is applied to determine the location, number and distribution of presumptive synaptic sites between neurons in the leech central nervous system.

Lucifer dyes--highly fluorescent dyes for biological tracing

Lucifer dyes are intensity fluorescent 4-aminonaphthalimides which are readily visible in living cells at concentrations and levels of illumination at which they are nontoxic. Because of their low molecular weight they frequently pass from one cell to another; this widespread phenomenon, termed dye-coupling, is thought to reveal functional relationships between cells. Lucifer dyes can also be used for ultrastructural tracing by comparison of electron micrographs with light micrographs of the same thin section. In addition, they show promise for backfilling neurones through cut nerves, for visualizing the results of retrograde axonal transport and for the covalent labeling of macromolecules.

Membrane properties and selective connexions of identified leech neurones in culture

1. Individual, identified neurones, dissected from the central nervous system of the leech and maintained in culture for several weeks, sprouted processes and formed synaptic connexions.2. The action potentials of isolated touch (T), pressure (P), nociceptive (N) cells and Retzius cells resembled those of their counterparts in situ, enabling them to be recognized unambiguously. Their input resistances were approximately 4 times greater than those of corresponding cells within the animal. In T, P and N cells trains of impulses were followed by a pronounced after-hyperpolarization, as in the animal.3. In certain cells, notably the L motoneurones, membrane properties became altered in culture. The current-voltage relation showed novel rectification and action potentials became much larger.4. Numerous neurites often extended for hundreds of micrometres from isolated neurones and ended in typical growth cones. Electron micrographs revealed that many fine axons were braided together to form thicker fascicles. Frequently, the processes were orientated between two neighbouring cells rather than at random. The fine structure of the cytoplasm, nucleus and organelles in cultured cells resembled those of their counterparts in situ. The glial cell that normally surrounds the neurones was, however, absent.5. Pairs of Retzius cells in culture usually became coupled electrically after about 6 days. Similarly L motoneurones became coupled in vitro. These junctions allowed current to pass in both directions and resembled those seen in the animal.6. Selective connexions were made by certain types of cells. Thus, P sensory neurones did not become coupled with Retzius cells but did develop electrical connexions with L motoneurones, as in the animal.7. Novel synaptic interactions not obvious in the animal could appear in culture. Retzius and L cells became electrically coupled and, in some instances where electrical coupling between Retzius cells failed to develop, chemically mediated inhibitory potentials became apparent.8. Isolated, identified leech neurones not only survive but regenerate processes and are capable of forming selective connexions in culture. The ability to define interactions between isolated pairs of cells offers the opportunity to explore in detail problems relating to synapse formation and cell-cell recognition.

Synaptic regeneration and glial reactions in the transected spinal cord of the lamprey

We have examined axonal growth and synaptic regeneration in identified giant neurons of the transected lamprey spinal cord using intracellular injection of horseradish peroxidase. Wholemounts together with serial section light and electron microscopy, show that axons from identified Müller and Mauthner reticulospinal neurons grow across the lesion and regenerate new synaptic contacts. Relatively normal swimming returns in these animals by 3-4 weeks after spinal transection. This occurs despite the formation of regenerated synapses in regions of the cord that are not usually occupied by these neurons. The regenerating axons branch profusely in contrast to their unbranched state in the normal animal. In addition to showing the two synaptic configuration found normally, synapses may be formed by slender sprouts from the growing giant axon. These 'sprout' type synaptic contacts appear unique to the regenerating neuron. Only regenerated chemical synapses were seen; the morphologically mixed chemical and electrical (gap junction) synaptic complex common in the normal animal was not observed at regenerated synapses. The site of spinal transection in the functionally recovered animal shows an increase in the number of ependymal and glial cells. Ependymal-like cells appear in regions away from the central canal. The expanded ependymal and glial processes covering the peripheral surface of the injured cord become convoluted, in contrast to their normal smooth configuration. There is no collagen within the cord at the site of transection but a considerable deposition is seen external to the cord surface. Axonal growth across a spinal lesion and subsequent synaptic regeneration can be examined in single identifiable giant interneurons in the spinal cord of the larval lamprey. This preparation may be used as an assay to investigate factors that could contribute to functional recovery following central nervous system injury in the higher vertebrates.

Transmission at a 'direct' electrical connexion mediated by an interneurone in the leech

1. Touch sensory neurones in the leech excite a rapidly conducting interneurone called the S-cell. Although the electrical synaptic connexion between the two cells is monosynaptic by physiological criteria, intracellular staining reveals that the touch cells and the S-cell do not make contact, but instead are linked by a pair of small interneurones. 2. The electrical coupling between touch cells and S-cells rectifies, in that depolarizing current but not hyperpolarizing current passes from the touch cell into the S-cell. The rectifying junction is between the touch cells and coupling interneurones, while the connexion between coupling interneurones and the S-cell passes current in both directions. 3. Selective destruction of the coupling interneurones by intracellular injection of a protease interrupts the disynaptic electrical connexion between touch and S-cells. 4. The touch cell's geometry and membrane properties account for the failure of impulses that are generated in certain portions of the receptive field in the skin to propagate beyond the first branch-points of the touch cell axon within the ganglion. Conduction block at branch-points is used to examine physiologically the spatial distribution of contacts between the touch cell and the coupling interneurones. In addition, it is shown that under natural conditions branch-point failure presynaptically reduces the effectiveness of the electrical synaptic connexions.

Ultrastructural studies of severed medial giant and other CNS axons in crayfish

The distal stumps of severed medial giant axons (MGAs) and of nongiant axons (NGAs) in the CNS of the crayfish Procambarus clarkii show long-term (5--9 months) survival associated with disorientation of mitochondria and thickening of the glial sheath. However, the morphological responses of the two axonal types differ in that neither the proximal nor the distal stump of severed MGAs ever fills with mitochondria as is observed in some severed NGAs. Furthermore, the adaxonal glial layer never completely encircles portions of MGA axoplasm as occurs in many severed NGAs; in fact, ultrastructural changes in the adaxonal layer around severed MGAs are often difficult to detect. No multiple axonal profiles are ever seen within the glial sheath of the proximal or distal stumps of severed MGAs whereas these structures are easily located within severed NGAs.

Differential discrimination of appropriate pathways by regenerating identified neurons in Helisoma

Pathway selection by two different identified neurons (neuron 4 and neuron 5) in the pond snail Helisoma was studied by intracellular injection of the fluorescent dye, Lucifer Yellow CH. The axonal projections of these neurons in normal animals are remarkably constant. Axons of neuron 4 and 5 share a common nerve, the esophageal trunk (ET), which bifurcates to form the gastric nerve (GN) and the salivary nerve (SN). Neuron 5's axon traverses the GN and "avoids" when neuron 4 enters the GN it may make right angle turns, leave the gastric nerve cylinder, and extend directly to its target, the salivary glands. Anomalously located neuron 4 somata also may extend axons to their correct targets via unusual routes. Following ET crush both neurons 4 and 5 regenerate. Sprouts from neuron 5 encountering the bifurcation of the ET basically recapitulate the normal neuron 5 axonal projection. In contrast, neuron 4 sprouts indiscriminately into "correct" and "incorrect" branches of the ET. These experiments indicate that pathway selection by growing neurites can be differentially and specifically regulated for different neurons of the same nerve trunk. The fidelity of different regenerating neurons for selecting nerves which normally contain their axons differs. Though the mechanism of pathway selection cannot yet be identified, the regenerate axonal projections cannot be accounted for by direct guidance of sprouts by the surviving distal axon segments, as has been reported for some identified neurons in the leech.

Removal of the synaptic target permits terminal sprouting of a mature intact axon

When the central nervous system (CNS) develops, neurones send out axons to make contact with appropriate synaptic target cells and then stop growing. If its usual target is missing, an axon may continue to grow until it synapses with a suitable but inappropriate target. This suggests that contact with a synaptic target is important in stopping axonal growth during development. Many classes of neurone in the adult CNS retain a capacity to grow towards denervated targets, but it is not known whether the synaptic contacts established during development continue to regulate the growth of individual mature, intact axons. This has been a difficult problem to investigate; in vertebrates most studies necessarily involve large populations of neurones, and the most direct approach, removal of a synaptic target, usually damages many neurones, including the axons that are to be studied. We report here a demonstration of target cell influences on the growth of a single mature, intact axon in the CNS of the leech by selectively destroying the axon's synaptic target without injuring the axon itself. Target removal, which itself does not trigger sprouting of intact axons, permits the intact axon to grow at its tip in response to injury of other axonal branches of the same cell.

Correct axonal regeneration after target cell removal in the central nervous system of the leech

The unique target neuron of a severed axon in the leech was selectively eliminated by intracellular injection of protease. In the absence of the target, the severed axon regenerated normally along its original pathway to the usual site of synapse, where it stopped growing without forming alternative connections.