Functional and structural changes in mammalian sympathetic neurones following colchicine application to post-ganglionic nerves

Synaptic Plasticity on Motoneurons After Axotomy: A Necessary Change in Paradigm

Motoneurons axotomized by peripheral nerve injuries experience profound changes in their synaptic inputs that are associated with a neuroinflammatory response that includes local microglia and astrocytes. This reaction is conserved across different types of motoneurons, injuries, and species, but also displays many unique features in each particular case. These reactions have been amply studied, but there is still a lack of knowledge on their functional significance and mechanisms. In this review article, we compiled data from many different fields to generate a comprehensive conceptual framework to best interpret past data and spawn new hypotheses and research. We propose that synaptic plasticity around axotomized motoneurons should be divided into two distinct processes. First, a rapid cell-autonomous, microglia-independent shedding of synapses from motoneuron cell bodies and proximal dendrites that is reversible after muscle reinnervation. Second, a slower mechanism that is microglia-dependent and permanently alters spinal cord circuitry by fully eliminating from the ventral horn the axon collaterals of peripherally injured and regenerating sensory Ia afferent proprioceptors. This removes this input from cell bodies and throughout the dendritic tree of axotomized motoneurons as well as from many other spinal neurons, thus reconfiguring ventral horn motor circuitries to function after regeneration without direct sensory feedback from muscle. This process is modulated by injury severity, suggesting a correlation with poor regeneration specificity due to sensory and motor axons targeting errors in the periphery that likely render Ia afferent connectivity in the ventral horn nonadaptive. In contrast, reversible synaptic changes on the cell bodies occur only while motoneurons are regenerating. This cell-autonomous process displays unique features according to motoneuron type and modulation by local microglia and astrocytes and generally results in a transient reduction of fast synaptic activity that is probably replaced by embryonic-like slow GABA depolarizations, proposed to relate to regenerative mechanisms.

Regulation of axonal regeneration following spinal cord injury in the lamprey

Following rostral spinal cord injury (SCI) in larval lampreys, injured descending brain neurons, particularly reticulospinal (RS) neurons, regenerate their axons, and locomotor behavior recovers in a few weeks. However, axonal regeneration of descending brain neurons is mostly limited to relatively short distances, but the mechanisms for incomplete axonal regeneration are unclear. First, lampreys with rostral SCI exhibited greater axonal regeneration of descending brain neurons, including RS neurons, as well as more rapid recovery of locomotor muscle activity right below the lesion site, compared with animals with caudal SCI. In addition, following rostral SCI, most injured RS neurons displayed the "injury phenotype," whereas following caudal SCI, most injured neurons displayed normal electrical properties. Second, following rostral SCI, at cold temperatures (~4-5°C), axonal transport was suppressed, axonal regeneration and behavioral recovery were blocked, and injured RS neurons displayed normal electrical properties. Cold temperatures appear to prevent injured RS neurons from detecting and/or responding to SCI. It is hypothesized that following rostral SCI, injured descending brain neurons are strongly stimulated to regenerate their axons, presumably because of elimination of spinal synapses and reduced neurotrophic support. However, when these neurons regenerate their axons and make synapses right below the lesion site, restoration of neurotrophic support very likely suppress further axonal regeneration. In contrast, caudal SCI is a weak stimulus for axonal regeneration, presumably because of spared synapses above the lesion site. These results may have implications for mammalian SCI, which can spare synapses above the lesion site for supraspinal descending neurons and propriospinal neurons.NEW & NOTEWORTHY Lampreys with rostral spinal cord injury (SCI) exhibited greater axonal regeneration of descending brain neurons and more rapid recovery of locomotor muscle activity below the lesion site compared with animals with caudal SCI. In addition, following rostral SCI, most injured reticulospinal (RS) neurons displayed the "injury phenotype," whereas following caudal SCI, most injured neurons had normal electrical properties. We hypothesize that following caudal SCI, the spared synapses of injured RS neurons might limit axonal regeneration and behavioral recovery.

Peripheral Nerve Nanoimaging: Monitoring Treatment and Regeneration

Accidental and iatrogenic trauma are major causes of peripheral nerve injury. Healing after nerve injury is complex and often incomplete, which can lead to acute or chronic pain and functional impairment. Current assessment methods for nerve regeneration lack sensitivity and objectivity. There is a need for reliable and reproducible, noninvasive strategies with adequate spatial and temporal resolution for longitudinal evaluation of degeneration or regeneration after injury/treatment. Methods for noninvasive monitoring of the efficacy and effectiveness of neurotherapeutics in nerve regeneration or of neuropathic pain are needed to ensure adequacy and responsiveness to management, especially given the large variability in the patient populations, etiologies, and complexity of nerve injuries. Surrogate biomarkers are needed with positive predictive correlation for the dynamics and kinetics of neuroregeneration. They can provide direct real-time insight into the efficacy and mechanisms of individualized therapeutic intervention. Here, we review the state-of-the-art tools, technologies, and therapies in peripheral nerve injury and regeneration as well as provide perspectives for the future. We present compelling evidence that advancements in nanomedicine and innovation in nanotechnology such as nanotheranostics hold groundbreaking potential as paradigm shifts in noninvasive peripheral nerve imaging and drug delivery. Nanotechnology, which revolutionized molecular imaging in cancer and inflammatory disease, can be used to delineate dynamic molecular imaging signatures of neuroinflammation and neuroregeneration while simultaneously monitoring cellular or tissue response to drug therapy. We believe that current clinical successes of nanotechnology can and should be adopted and adapted to the science of peripheral nerve injury and regeneration.

The Roles of Microtubule-Based Transport at Presynaptic Nerve Terminals

Targeted intracellular movement of presynaptic proteins plays important roles during synapse formation and, later, in the homeostatic maintenance of mature synapses. Movement of these proteins, often as vesicular packages, is mediated by motor complexes travelling along intracellular cytoskeletal networks. Presynaptic protein transport by kinesin motors in particular plays important roles during synaptogenesis to bring newly synthesized proteins to establish nascent synaptic sites. Conversely, movement of proteins away from presynaptic sites by Dynein motors enables synapse-nuclear signaling and allows for synaptic renewal through degradation of unwanted or damaged proteins. Remarkably, recent data has indicated that synaptic and protein trafficking machineries can modulate each other's functions. Here, we survey the mechanisms involved in moving presynaptic components to and away from synapses and how this process supports presynaptic function.

Reduction in nerve growth factor availability leads to a conditioning lesion-like effect in sympathetic neurons

Axotomized peripheral neurons are capable of regeneration, and the rate of regeneration can be enhanced by a conditioning lesion (i.e., a lesion prior to the lesion after which neurite outgrowth is measured). A possible signal that could trigger the conditioning lesion effect is the reduction in availability of a target-derived factor resulting from the disconnection of a neuron from its target tissue. We tested this hypothesis with respect to nerve growth factor (NGF) and sympathetic neurons by administering an antiserum to NGF to adult mice for 7 days prior to explantation or dissociation of the superior cervical ganglion (SCG) and subsequently measuring neurite outgrowth. The antiserum treatment dramatically lowered the concentration of NGF in the SCG and increased the rate of neurite outgrowth in both explants and cell cultures. The increase in neurite outgrowth was similar in magnitude to that seen after a conditioning lesion. To determine if exogenous NGF could block the effect of a conditioning lesion, mice were injected with NGF or cytochrome C immediately prior to unilateral axotomy of the SCG, and for 7 days thereafter. A conditioning lesion effect of similar magnitude was seen in NGF-treated and control animals. While NGF treatment increased NGF levels in the contralateral control ganglion, it did not significantly elevate levels in the axotomized ganglion. The results suggest that the decreased availability of NGF after axotomy is a sufficient stimulus to induce the conditioning lesion effect in sympathetic neurons. While NGF administration did not prevent the conditioning lesion effect, this may be due to the markedly decreased ability of sympathetic neurons to accumulate the growth factor after axotomy.

Functions and mechanisms of retrograde neurotrophin signalling

Neuronal connections are established and refined through a series of developmental programs that involve axon and dendrite specification, process growth, target innervation, cell death and synaptogenesis. Many of these developmental events are regulated by target-derived neurotrophins and their receptors, which signal retrogradely over long distances from distal-most axons to neuronal cell bodies. Recent work has established many of the cellular and molecular events that underlie retrograde signalling and the importance of these events for both development and maintenance of proper neural connectivity.

Cerebellar grafting in the oculomotor system as a model to study target influence on adult neurons

In the last decades, there have been many efforts directed to gain a better understanding on adult neuron-target cell relationships. Embryonic grafts have been used for the study of neural circuit rewiring. Thus, using several donor neuronal tissues, such as cerebellum or striatum, developing grafted cells have been shown to have the capability of substituting neural cell populations and establishing reciprocal connections with the host. In addition, different lesion paradigms have also led to a better understanding of target dependence in neuronal cells. Thus, for example, axotomy induces profound morphofunctional changes in adult neurons, including the loss of synaptic inputs and discharge alterations. These alterations are probably due to trophic factor loss in response to target disconnection. In this review, we summarize the different strategies performed to disconnect neurons from their targets, and the effects of target substitution, performed by tissue grafting, upon neural properties. Using the oculomotor system-and more precisely the abducens internuclear neurons-as a model, we describe herein the effects of disconnecting a population of central neurons from its natural target (i.e., the medial rectus motoneurons at the mesencephalic oculomotor nucleus). We also analyze target-derived influences in the structure and physiology of these neurons by using cerebellar embryonic grafts as a new target for the axotomized abducens internuclear neurons.

Grafting of a new target prevents synapse loss in abducens internuclear neurons induced by axotomy

The loss of afferent synaptic boutons is a prominent alteration induced by axotomy on adult central neurons. In this work we attempted to prove whether synapse loss could be reverted by reconnection with a new target. We severed the medial longitudinal fascicle of adult cats and then transplanted embryonic cerebellar primordia at the lesion site immediately after lesion. As previously shown, the transected axons from abducens internuclear neurons penetrate and reinnervate the graft [J Comp Neurol 444 (2002) 324]. By immunocytochemistry and electron microscopy we studied the synaptology of abducens internuclear neurons under three conditions: control, axotomy and transplant (2 months of survival time). Semithin sections of the abducens nucleus were immunostained against calretinin, to identify abducens internuclear neurons, and either synaptophysin (SF), to label synaptic terminals, or glial fibrillary acidic protein (GFAP) to detect the astrocytic reaction. Optical and linear density of SF and GFAP immunostaining were measured. Data revealed a significant decrease in the density of SF-labeled terminals with a parallel increase in GFAP-immunoreactive elements after axotomy. On the contrary, in the transplant group, the density of SF-labeled terminals was found similar to control, and the astrocytic reaction induced by lesion was significantly reduced. At the ultrastructural level, synaptic coverage and linear density of boutons were measured around the somata of abducens internuclear neurons. Whereas a significant reduction in both parameters was found after axotomy, cells of the transplant group received a normal density of synaptic endings. The ratio between F- and S-type boutons was found similar in the three groups. Therefore, these findings indicate that the grafting of a new target can prevent the loss of afferent synaptic boutons produced by the axotomy.

Functional alterations of cat abducens neurons after peripheral tetanus neurotoxin injection

Tetanus neurotoxin (TeNT) cleaves synaptobrevin, a protein involved in synaptic vesicle docking and fusion, thereby preventing neurotransmitter release and causing a functional deafferentation. We injected TeNT into the lateral rectus muscle of adult cats at 0.5 or 5 ng/kg (low and high dose, respectively). In the periphery, TeNT slightly slowed motor axon conduction velocity, and at high doses, partially blocked neuromuscular transmission. TeNT peripheral actions displayed time courses different to the more profound and longer-lasting central actions. Central effects were first observed 2 days postinjection and reversed after 1 mo. The low dose induce depression of inhibitory inputs, whereas the high dose produce depression of both inhibitory and excitatory inputs. Simultaneous recordings of eye movement and neuronal firing revealed that low-dose injections specifically reduced inhibition of firing during off-directed saccadic movements, while high-dose injections of TeNT affected both inhibitory and excitatory driven firing patterns. Motoneurons and abducens interneurons were both affected in a similar way. These alterations resulted in modifications in all discharge characteristic analyzed such as background firing, threshold for recruitment, and firing sensitivities to both eye position and velocity during spontaneous movements or vestibulo-ocular reflexes. Removal of inhibition after low-dose injections also altered firing patterns, and although firing activity increased, it did not result in muscle tetanic contractions. Removal of inhibition and excitation by high-dose injections resulted in a decrease in firing modulation with eye movements. Our findings suggest that the distinct behavior of oculomotor and spinal motor output following TeNT intoxication could be explained by their different interneuronal and proprioceptive control.

Firing properties of axotomized central nervous system neurons recover after graft reinnervation

Axotomy produces changes in the electrical properties of neurons and in their synaptic inputs, leading to alterations in firing pattern. We have considered the possibility that these changes occur as a result of the target deprivation induced by the lesion. Thus, we have provided a novel target to axotomized central neurons by grafting embryonic tissue at the lesion site to study the target dependence of discharge characteristics. The extracellular single-unit electrical activity of abducens internuclear neurons was recorded in the alert behaving cat in control, after axotomy, and after axotomy plus the implantation of cerebellar primordium. As recently characterized (de la Cruz et al. [2000] J. Comp. Neurol. 427:391-404), firing alterations induced by axotomy included an overall decrease in firing rate and a loss of eye-related signals, i.e., eye position and velocity neuronal sensitivities, that do not resume to normality with time. The grafting of a novel target to the injured abducens internuclear neurons restored the normal firing and sensitivities as recorded in the majority of units. To study the reinnervation of the implant, we performed anterograde labeling with biocytin combined with electron microscopy visualization. Axons of abducens internuclear neurons grew into the transplant sprouting into granule cell and molecular layers, as characterized by the immunostaining for gamma-aminobutyric acid and calbindin D-28k. Ultrastructural examination of labeled axons and boutons revealed the establishment of synaptic contacts, mainly axodendritic, with different cell types of the grafted cerebellar cortex. Therefore, these data indicate that axotomized central neurons resume to normal firing after the reinnervation of a novel target.

Retrograde signaling at central synapses

Transcellular retrograde signaling from the postsynaptic target cell to the presynaptic neuron plays critical roles in the formation, maturation, and plasticity of synaptic connections. We here review recent progress in our understanding of the retrograde signaling at developing central synapses. Three forms of potential retrograde signals-membrane-permeant factors, membrane-bound factors, and secreted factors-have been implicated at both developing and mature synapses. Although many of these signals may be active constitutively, retrograde factors produced in association with activity-dependent synaptic plasticity, e.g., long-term potentiation and long-term depression, are of particular interest, because they may induce modification of neuronal excitability and synaptic transmission, functions directly related to the processing and storage of information in the nervous system.

Regulation of Synaptic Function by Neurotrophic Factors in Vertebrates and Invertebrates: Implications for Development and Learning

Recent studies have demonstrated that neurotrophic factors contribute to the molecular events involved in synaptic plasticity, both during vertebrate development and in the mature nervous system. Although it is well established that many of the cellular and molecular mechanisms underlying synaptic plasticity are conserved between invertebrates and vertebrates, there are, as yet, very few neurotrophic factors identified in invertebrate species. Nonetheless, vertebrate neurotrophins can influence invertebrate neuronal growth and plasticity. In addition, homologs of neurotrophic factor receptors have been identified in several invertebrate species. These studies may indicate that the roles of neurotrophins in both developmental and adult plasticity are highly conserved across diverse phyla.

Retrograde signaling in the development and modification of synapses

Retrograde signaling from the postsynaptic cell to the presynaptic neuron is essential for the development, maintenance, and activity-dependent modification of synaptic connections. This review covers various forms of retrograde interactions at developing and mature synapses. First, we discuss evidence for early retrograde inductive events during synaptogenesis and how maturation of presynaptic structure and function is affected by signals from the postsynaptic cell. Second, we review the evidence that retrograde interactions are involved in activity-dependent synapse competition and elimination in developing nervous systems and in long-term potentiation and depression at mature synapses. Third, we review evidence for various forms of retrograde signaling via membrane-permeant factors, secreted factors, and membrane-bound factors. Finally, we discuss the evidence and physiological implications of the long-range propagation of retrograde signals to the cell body and other parts of the presynaptic neuron.

Changes in the receptive fields of cat retinal ganglion cells affected by pressure on the optic nerve

In the cat optic nerve a lesion was induced by brief application of pressure. It selectively blocked impulse conduction in large diameter fibres of the retinal ganglion cells. Electrophysiological examination of single optic axons several weeks later demonstrated a gross alteration of the visual properties of the affected BT/Y ganglion cells. It is suggested that the alteration of receptive field properties may reflect the cellular and dendritic response to distant focal injury of the axon.

NT-3 increases amplitude of EPSPs produced by axotomized group Ia afferents

We tested the hypothesis that neurotrophin-3 (NT-3) in adult cats can rescue the central synapses made by muscle afferents from the effects of peripheral axotomy. The medial gastrocnemius (MG) muscle nerve in cats was axotomized and capped or axotomized and the distal end provided with either saline or NT-3 by mini-osmotic pump. Four to five weeks later monosynaptic excitatory postsynaptic potentials (EPSPs) elicited by electrical stimulation of the axotomized MG nerve were recorded in intact lateral gastrocnemius/soleus (LGS) motoneurons. The axotomized MG afferents without NT-3 treatment generated EPSPs averaging one-half of the amplitude of those generated by normal intact MG afferents. Axotomized MG afferents treated with NT-3 elicited EPSPs averaging 2.5 times normal amplitude and 5 times the amplitude of those from afferents axotomized but not treated. The very large EPSPs generated by NT-3-treated afferents remained as susceptible to depression during high-frequency stimulation (32 shocks at 167 Hz) as those elicited by untreated axotomized afferents. The arrival of the afferent volley of the cord dorsum potential and the onset of EPSPs were both delayed by axotomy of the group Ia afferents and were both restored by exposure to NT-3. This result suggests that the conduction velocity and thus the caliber of group Ia afferents are also controlled by NT-3. We conclude that the neurotrophin NT-3 has a continuing role in the maintenance of physiological function of muscle afferents in adult mammals.

Postnatal addition of satellite cells to parasympathetic neurons

We have examined the postnatal development of satellite cells associated with parasympathetic neurons of mouse salivary duct ganglia. The number of satellite cells associated with each neuron was found to increase during the first 8 weeks after birth but remained constant thereafter. This corresponds to the period of maximal growth of the salivary gland that serves as the target organ innervated by these neurons. At all ages examined, the number of satellite cells associated with each neuron was found to be highly correlated with neuronal volume. The development of satellite cells associated with individual identified neurons was followed directly by in vivo video microscopy over several months, and the number of satellite cell nuclei was found to increase in regions of the neuronal surface with increasing numbers of synaptic boutons. These results indicate that the postnatal addition of satellite cells to parasympathetic neurons is linked to neuronal enlargement and that synaptic remodeling occurs in concert with satellite cell development.

Priming events and retrograde injury signals. A new perspective on the cellular and molecular biology of nerve regeneration

Successful axon regeneration requires that signals from the site of injury reach the nucleus to elicit changes in transcription. In spite of their obvious importance, relatively few of these signals have been identified. Recent work on regeneration in the marine mollusk Aplysia californica has provided several insights into the molecular events that occur in neurons after axon injury. Based on these findings, we propose a model in which axon regeneration is viewed as the culmination of a series of temporally distinct but overlapping phases. Within each phase, specific signals enter the nucleus to prime the cell for the arrival of subsequent signals. The first phase begins with the arrival of injury-induced action potentials, which act via calcium and cAMP to turn on genes used in the early stages of repair. In the next phase, MAP-kinases and other intrinsic constituents activated at the injury site are retrogradely transported through the axon to the nucleus, informing the nucleus of the severity of the axonal injury, reinforcing the earlier events, and triggering additional changes. The third phase is characterized by the arrival of signals that originate from extrinsic growth factors and cytokines released by cells at the site of injury. In the last phase, signals from target-derived growth factors arrive in the cell soma to stop growth. Because many of these events appear to be universal, this framework may be useful in studies of nerve repair in both invertebrates and vertebrates.

Influence of the postsynaptic target on the functional properties of neurons in the adult mammalian central nervous system

In this review we have attempted to summarize present knowledge concerning the regulatory role of target cells on the expression and maintenance of the neuronal phenotype during adulthood. It is well known that in early developmental stages the survival of neurons is maintained by specific neurotrophic factors derived from their target tissues. Neuronal survival is not the only phenotype that is regulated by target-derived neurotrophic factors since the expression of electrophysiological and cytochemical properties of neurons is also affected. However, a good deal of evidence indicates that the survival of neurons becomes less dependent on their targets in the adult stage. The question is to what extent are target cells still required for the maintenance of the pre-existing or programmed state of the neuron; i.e., what is the functional significance of target-derived factors during maturity? Studies addressing this question comprise a variety of neuronal systems and technical approaches and they indicate that trophic interactions, although less apparent, persist in maturity and are most easily revealed by experimental manipulation. In this respect, research has been directed to analyzing the consequences of disconnecting a group of neurons from their target-by either axotomy or selective target removal using different neurotoxins-and followed (or not) by the implant of a novel target, usually a piece of embryonic tissue. Numerous alterations have been described as taking place in neurons following axotomy, affecting their morphology, physiology and metabolism. All these neuronal properties return to normal values when regeneration is successful and reinnervation of the target is achieved. Nevertheless, most of the changes persist if reinnervation is prevented by any procedure. Although axotomy may represent, besides target disconnection, a cellular lesion, alternative approaches (e.g., blockade of either the axoplasmic transport or the conduction of action potentials) have been used yielding similar results. Moreover, in the adult mammalian central nervous system, neurotoxins have been used to eliminate a particular target selectively and to study the consequences on the intact but target-deprived presynaptic neurons. Target depletion performed by excitotoxic lesions is not followed by retrograde cell death, but targetless neurons exhibit several modifications such as reduction in soma size and in the staining intensity for neurotransmitter-synthesizing enzymes. Recently, the oculomotor system has been used as an experimental model for evaluating the functional effects of target removal on the premotor abducens internuclear neurons whose motoneuronal target is destroyed following the injection of toxic ricin into the extraocular medial rectus muscle. The functional characteristics of these abducens neurons recorded under alert conditions simultaneously with eye movements show noticeable changes after target loss, such as a general reduction in firing frequency and a loss of the discharge signals related to eye position and velocity. Nevertheless, the firing pattern of these targetless abducens internuclear neurons recovers in parallel with the establishment of synaptic contacts on a presumptive new target: the small oculomotor internuclear neurons located in proximity to the disappeared target motoneurons. The possibility that a new target may restore neuronal properties towards a normal state has been observed in other systems after axotomy and is also evident from experiments of transplantation of immature neurons into the lesioned central nervous system of adult mammals. It can be concluded that although target-derived factors may not control neuronal survival in the adult nervous system, they are required for the maintenance of the functional state of neurons, regulating numerous aspects of neuronal structure, chemistry and electro-physiology.(ABSTRUCT TRUNCATED)

Effects of axotomy, deafferentation, and reinnervation on sympathetic ganglionic synapses: a comparative study

The main physiological and morphological features of the synapses in the superior cervical ganglia of mammals and the last two abdominal ganglia of the frog sympathetic chain are summarized. The effects of axotomy on structure and function of ganglionic synapses are then reviewed, as well as various changes in neuronal metabolism in mammals and in the frog, in which the parallel between electrophysiological and morphological data leads to the conclusion that a certain amount of synaptic transmission occurs at "simple contacts." The effects of deafferentation on synaptic transmission and ultrastructure in the mammalian ganglia are reviewed: most synapses disappear, but a number of postsynaptic thickenings remain unchanged. Moreover, intrinsic synapses persist after total deafferentation and their number is strongly increased if axotomy is added to deafferentation. In the frog ganglia, the physiological and morphological evolution of synaptic areas is comparable to that of mammals, but no intrinsic synapses are observed. The reinnervation of deafferented sympathetic ganglia by foreign nerves, motor or sensory, is reported in mammals, with different degrees of efficiency. In the frog, the reinnervation of sympathetic ganglia with somatic motor nerve fibers is obtained in only 20% of the operated animals. The possible reasons for the high specificity of ganglionic connections in the frog are discussed.

Multiple signals underlie the axotomy-induced up-regulation of c-JUN in adult sensory neurons

We examined the axotomy-induced expression of the immediate-early gene (proto-oncogene) c-jun in the Ola mouse mutant (which exhibits a dramatic delay in Wallerian degeneration) using immunocytochemistry to c-JUN (the protein product of the protooncogene c-jun). c-JUN-like protein immunoreactivity was present in a similar proportion (ca. 60%) of L4 dorsal root ganglion (DRG) neuronal cell bodies from normal (C57/6J/BL) and Ola mice at 1 week following a sciatic nerve crush (axotomy). In normal mice, the intensity and extent of staining declined at 3 weeks, correlating with regeneration. In contrast, Ola mice exhibited a marked reduction (by 77%) in the extent of staining at 2 weeks. At 3 weeks (coinciding to the onset of extensive axonal degeneration in this mutant), staining levels were increased to 1 week levels. Taken together, these findings suggest that multiple signals (both independent and dependent on axonal degeneration) regulate c-jun expression in DRG neuronal cell bodies.

Galanin expression increases in adult rat sympathetic neurons after axotomy

Changes in neuropeptide expression occur in sensory, motor, and sympathetic neurons following axotomy. The particular pattern of peptide changes that occurs varies among the three cell types. We have studied the regulation in the rat superior cervical ganglion of the expression of galanin, a peptide previously shown to increase in axotomized sensory and motor neurons. While normally only an occasional neuron exhibiting galanin-like immunoreactivity is found in this ganglion, at two days after transection of the postganglionic internal and external carotid nerves, immunostaining can be observed in many neurons throughout the ganglion. Similar changes are found when ganglia are placed in organ culture for two days. The distribution of immunostained neurons after section of only one of the postganglionic trunks suggests that changes in galanin-like immunoreactivity occur only within neurons whose axons are transected. None the less, even when both nerve trunks are transected, only about half of the neurons in the ganglion exhibit galanin-like immunoreactivity, indicating that only a proportion of the axotomized neurons exhibit a detectable response. The few immunostained neurons seen after section of the cervical sympathetic trunk may also represent axotomized neurons. Galanin-like immunoreactivity extracted from the ganglion co-chromatographs with authentic galanin, and the level of this immunoreactivity increases dramatically after axotomy and explantation, and modestly after decentralization. These same manipulations produce parallel increases in the level of galanin messenger RNA. Together, the findings indicate that the expression of galanin increases in sympathetic neurons after axotomy. Galanin is thus the first neuropeptide whose expression has been shown to increase after transection of all three types of peripheral axons that have been studied.

In vitro modulation of somatic glycine-like immunoreactivity in presumed glycinergic neurons

Previous studies indicate that tuberculoventral and cartwheel cells in the dorsal cochlear nucleus as well as a group of stellate cells in the ventral cochlear nucleus are likely to be glycinergic. To test whether these neurons contain higher levels of free glycine than cells that are probably not glycinergic, immunocytochemical studies with antibodies against glycine conjugates were undertaken on slices of the murine cochlear nuclear complex. Present results show that the cell bodies of all three groups of neurons are immunolabeled. However, the somatic labeling of the tuberculoventral and cartwheel cells can be modulated by experimental conditions. In slices fixed immediately after cutting, many cell bodies in the deep layer of the dorsal cochlear nucleus (DCN), presumably tuberculoventral neurons, are labeled. As a slice is incubated in vitro, cell bodies in the deep layer of the DCN lose their glycine-like immunoreactivity. After 7 hours in vitro, labeled cells are absent in the deep DCN, but the immunoreactivity can be regained by electrically stimulating the auditory nerve for 20 minutes. The loss of immunoreactivity is prevented by electrical stimulation, by axotomy, and by inclusion of 0.8 microM tetrodotoxin, or 1 microM strychnine, or 50 microM colchicine or 50 microM beta-lumicolchicine in the bathing saline. Cartwheel cells retain their immunoreactivity during incubation in vitro without electrical stimulation, but lose it under two conditions. One is following a cut across the ventral cochlear nucleus (VCN) that severs most of their granule cell input, and the other is the inclusion of tetrodotoxin in the bathing saline. The labeling of cell bodies in the ventral cochlear nucleus and of puncta and processes is not changed by any of these experimental manipulations.

A descriptive and quantitative morphometric study of long-term mouse adrenal medulla grafts implanted into the putamen: effect of nerve growth factor injected at grafting

Mouse adrenal medulla grafts were evaluated morphologically and quantitatively after implantation into the mouse putamen, either alone or with nerve growth factor (NGF) injected at grafting. Specific antibodies were used to determine the expression of neurofilaments, dopamine (DA) and phenylethanolamine-N-methyl transferase (PNMT). Three months after grafting, the survival rate and size volume of chromaffin cells were significantly greater in the grafts containing NGF, and increasing numbers of intermediate cell types (e.g. chromaffin cells transforming into neurons), and of neuron-like cells seemed to have formed. Chromaffin cells stained positively for DA and PNMT, but only a few chromaffin-like processes stained for neurofilaments. A neuronal network of adrenal medulla grafts was observed, consisting of non-myelinated nerve fibers, nerve terminals and chromaffin-like processes. In all grafts the synapses on chromaffin cells were mainly small, symmetrical or asymmetrical (about 1-2 microns in diameter) with round, small clear synaptic vesicles. Nerve terminals were not immunoreactive to dopamine or PNMT. These results show that a single injection of NGF at grafting influences the survival and differentiation of chromaffin cells. This study suggests that adrenal medulla grafts may integrate into the putamen.

PC12 cell neuronal differentiation is associated with prolonged p21ras activity and consequent prolonged ERK activity

Expression of oncogenic ras in PC12 cells causes neuronal differentiation and sustained protein tyrosine phosphorylation and activity of extracellular signal-regulated kinases (ERKs), p42erk2 and p44erk1. Oncogenic N-ras-induced neuronal differentiation is inhibited by compounds that block ERK protein tyrosine phosphorylation or ERK activity, indicating that ERKs are not only activated by p21ras but serve as the primary downstream effectors of p21ras. Treatment of PC12 cells with nerve growth factor or fibroblast growth factor results in neuronal differentiation and in a sustained elevation of p21ras activity, of ERK activity, and of ERK tyrosine phosphorylation. Epidermal growth factor, which does not cause neuronal differentiation, stimulates only transient (< 1 hr) activation of p21ras and ERKs. These data indicate that transient activation of p21ras and, consequently, ERKs is not sufficient for induction of neuronal differentiation. Prolonged ERK activity is required: a consequence of sustained activation of p21ras by the growth factor receptor protein tyrosine kinase.

Target dependence of motoneuronal survival: the current status

The concept that target-derived molecules are essential for the maintenance of motoneuronal survival has now received considerable support from different sources. One source of information arises from the target-related motoneuron death that occurs naturally at early developmental stages. Another source of information arises from axotomy-induced motoneuron death in adulthood. During development, the survival of motoneurons is initially target-independent. Its target dependence is expressed at a certain embryonic stage and, subsequently, motoneuronal survival becomes less dependent upon the target. It is not known how the state-switch of motoneurons is induced during development. Also, it is not certain whether naturally occurring motoneuron death during development and axotomy-induced motoneuron death in adulthood are based on the same mechanisms. Axotomy induces injury-associated disturbance in the motoneurons, in addition to elimination of the target-derived trophic supply. At present, there is no direct evidence that axotomy-induced motoneuron death in adulthood results solely from the deprivation of trophic factors from the target. The survival of motoneurons in adulthood appears to be maintained by multiple mechanisms. Some of the tropic factors that are involved in the maintenance of neuronal phenotypic expression are distinct from those involved in the maintenance of neuronal survival. There are multiple target-derived trophic factors for a given neuron.

The ontogeny of specific retrograde transport of nerve growth factor by motoneurons of the brainstem and spinal cord

Radiolabeled Nerve Growth Factor (NGF) was injected into either the mandibular process of the first visceral arch or the limb bud of chick embryos at Days 3.5-14 or Days 4-13 of incubation, respectively. Control embryos received injections of labeled cytochrome-C or labeled NGF plus an excess of unlabeled NGF. The tissues were then processed for autoradiography. The 125I-NGF was retrogradely transported by motoneurons of the trigeminal (V) motor nucleus on Days 3.5-8 of incubation, but not at later stages. Similar transport was seen in motoneurons of the spinal cord lateral motor column from Days 4-10 of incubation, but not at later stages. Sensory neurons of the V ganglion and of the dorsal root ganglia transported NGF at all injection ages. In no instance was the 125I-cytochrome-C transported by sensory or motor neurons. The injection of an excess of cold NGF along with labeled NGF resulted in no evidence of retrograde transport of the labeled NGF indicating that the transport was saturable. The time of transport by these brainstem and spinal cord motoneurons corresponds closely to the points during development at which they have been found to exhibit specific NGF binding. The present results, then, provide further evidence for a possible biological role for NGF during early developmental stages of these motoneuron populations.

The response of cultured trigeminal and spinal cord motoneurons to nerve growth factor

Dissociated neurons from the trigeminal (V) region of the metencephalic basal plate or the ventral spinal cord from chick embryos of Day 4 (V basal plate) or Day 5 (spinal cord) were cultured on a laminin substratum either in the presence of nerve growth factor (NGF) or in control medium. Assessment was made of neuronal survival, the amount of neurite elaborated, and the percentage of neurons initiating neurites. The presence of motoneurons was verified by retrograde labeling with the fluorescent dye diI. NGF was found to significantly increase the quantity of neuritic processes produced by the spinal cord dissociates at both 24 and 48 hr in vitro. The percentage of neurons initiating neuritic processes was significantly increased by NGF in the trigeminal population at 48 hr in vitro. Neuronal survival was not enhanced by NGF in either group. Both trigeminal and spinal cord neurons were also found to specifically bind 125I-NGF in culture. These results provide direct evidence for an influence of NGF on process formation of early embryonic motoneurons in culture.

Synaptic efficacy of inhibitory synapses and repetitive firing in the reinnervating trigeminal and hypoglossal motoneurons

The synaptic efficacy and repetitive firing in masseteric motoneurons after the self-union operation and in tongue protruder motoneurons after their cut axons were reunited to tongue retractor muscles, the styloglossus muscle, were studied in cats. To ensure the correct identification of reinnervating motoneurons, the muscle response produce by an induced spike in a motoneuron by intracellularly injected depolarizing current was recorded. In both masseteric and tongue protruder motoneurons there were no differences on the patterns of postsynaptic potentials produced in reinnervating and non-reinnervating motoneurons by peripheral nerve stimulation, suggesting that the recovery of the synaptic efficacy of inhibitory synapses is time-dependent rather than muscle reinnervation. However, the present study demonstrated that the recovery of processes that control rhythmical firing of motoneurons is probably dependent on muscle reinnervation.

Neurotrophic molecules

Neurotrophic molecules have a profound influence on developmental events such as naturally occurring cell death, differentiation, and process outgrowth. Despite their striking effects on developing neurons, a role for these molecules in the pathogenesis or therapy of neurological disease has not yet been defined. However, a variety of recent advances promise to provide the techniques necessary to assess the potential relevance of neurotrophic molecules to clinical neurology. In this article we review recent investigations into the biological effects, regulation of production, and mechanisms of action of the best characterized trophic molecule, nerve growth factor. In addition we review studies characterizing brain-derived neurotrophic factor and other putative neurotrophic molecules. Finally, we discuss how pharmacological effects of these molecules may be relevant to the therapy of disease states as well as neural regeneration.

Comparison of the neurotoxic effects of colchicine, the vinca alkaloids, and other microtubule poisons

Previous studies have revealed that colchicine is selectively toxic to certain neuronal populations in the CNS, particularly granule cells of the dentate gyrus. The present study evaluates whether other microtubule poisons exhibit similar neurotoxic effects. Equimolar solutions of colchicine, colcemid, podophyllotoxin, vinblastine, vincristine and lumicolchine, the non-binding analog of colchicine, were injected into the dentate gyrus. Neurotoxicity was evaluated histologically. As previously reported, colchicine selectively destroyed dentate granule cells with minimal damage to other neurons including hippocampal pyramidal cells. Vincristine was very toxic and was not selective for granule cells. Vinblastine was relatively selective in destroying granule cells, but was not as potent as colchine. Colcemid and podophyllotoxin had minimal toxic effects. Lumicolchine injections caused no more damage than injections of vehicle. This ordering appears to correlate with the reversibility of binding tubulin.

Effects of intraneural injection of taxol on retrograde axonal transport and morphology of corresponding nerve cell bodies

Taxol exerts a potent effect on the assembly and stability of cellular microtubules. In the present study this drug was injected into the facial nerve of mice, and its influence on retrograde axonal transport and on morphology of the facial nerve cell bodies was monitored. A reduction in the amount of retrogradely transported fluorescein isothiocyanate-conjugated wheat germ agglutinin from the peripheral field of innervation to neuronal perikarya was demonstrated by cytofluorometry. Transport was not completely blocked, since some degree of tracer accumulation was found in most neurons. Morphometric analysis was employed to determine the volume fraction of cells and cell nuclei as well as nucleolar size on micrographs of the facial nucleus. After facial nerve transection the reaction in nerve cell bodies was similar in taxol-injected animals and in animals not exposed to this substance. Furthermore, intraneural injection of taxol without prior nerve section resulted in nucleolar enlargement. The present data show that taxol-induced disturbances in microtubule organisation interferes with the retrograde axonal transport and suggest that changes associated with the retrograde nerve cell reaction may develop when the transfer of material from the peripheral field of innervation is disturbed.

Neuron/glia relationships observed over intervals of several months in living mice

Identified neurons and glial cells in a parasympathetic ganglion were observed in situ with video-enhanced microscopy at intervals of up to 130 d in adult mice. Whereas the number and position of glial cells associated with particular neurons did not change over several hours, progressive differences were evident over intervals of weeks to months. These changes involved differences in the location of glial nuclei on the neuronal surface, differences in the apparent number of glial nuclei associated with each neuron, and often both. When we examined the arrangement of neurons and glial cells in the electron microscope, we also found that presynaptic nerve terminals are more prevalent in the vicinity of glial nuclei than elsewhere on the neuronal surface. The fact that glial nuclei are associated with preganglionic endings, together with the finding that the position and number of glial nuclei associated with identified neurons gradually changes, is in accord with the recent observation that synapses on these neurons are normally subject to ongoing rearrangement (Purves, D., J. T. Voyvodic, L. Magrassi, and H. Yawo. 1987. Science (Wash. DC). 238:1122-1126). By the same token, the present results suggest that glial cells are involved in synaptic remodeling.

Effect on the rat hypoglossal nucleus of vinblastine and colchicine applied to the intact or transected hypoglossal nerve

The possibility that interruption of axonal transport in otherwise intact axons induces retrograde neuronal and nonneuronal reactions was examined. In addition, the proposal that blockade of axonal transport proximal to nerve injury might inhibit or delay the axon reaction was examined. Cuffs containing various doses of vinblastine were applied to the intact hypoglossal nerve. Colchicine was applied in a similar way to the intact hypoglossal nerve, injected directly into the intact nerve, or administered proximal to the site of hypoglossal nerve transection. The effect on retrograde axonal transport in the nerve was evaluated in the vinblastine experiments by the retrograde horseradish peroxidase (HRP) technique following injection of HRP or wheat germ agglutinin-conjugated HRP into the tongue. A dose of 0.01% caused an almost complete, but transient, blockade of the retrograde transport of the tracer, and induced a clearcut chromatolytic reaction in hypoglossal neurons. The chromatolytic changes were accompanied by a significant increase in the number of glial cells, many of which were identified as microglia. Similar results were obtained with colchicine alone or in combination with nerve transection. Signs of Wallerian degeneration after vinblastine treatment (0.01%) were observed only in a small number of myelinated fibers. The findings are compatible with the view that depletion of retrogradely transported factors from the peripheral innervation territory (including the distal nerve stump) to the perikaryon and/or a premature return of anterogradely transported substances at the site of drug exposure are factors inducing retrograde neuronal and nonneuronal changes.

Chronic block of the cervical trunk increases synaptic efficacy in the superior and stellate ganglia of the guinea-pig

1. The effects of chronic conduction block with tetrodotoxin (TTX) of the cervical sympathetic trunk on synapses in the superior and stellate ganglia were examined in vitro with intracellular recording techniques. 2. The mean maximum amplitude of excitatory post-synaptic potentials (e.p.s.p.s) evoked in superior cervical ganglion neurones by stimulation of the cervical sympathetic trunk was increased significantly after 2 or 4 days of block. The electrical properties of the ganglion cells were not appreciably changed by the period of inactivity. 3. Chronic conduction block for 4 days also resulted in a significant increase in the amplitude of the e.p.s.p.s. evoked in stellate ganglion cells by active collaterals of the blocked fibres. 4. The number of steps in the synaptic response elicited in individual stellate ganglion neurones by graded stimulation of the ansa subclavia and the subclavian trunk (which join to form the cervical sympathetic trunk) was increased after 4 days of conduction block. 5. These results show that elimination of impulse activity in the cervical sympathetic trunk increased synaptic efficacy at the disused synapses in the superior cervical ganglion and at the active synapses in the stellate ganglion. In the latter case, collaterals of the blocked fibres apparently formed new synaptic connexions on stellate ganglion cells.

Outgoing synapses of small granule-containing cells in the rat superior cervical ganglion after post-ganglionic axotomy

Small granule-containing cells are intrinsic and interneurone-like in the rat superior cervical ganglion, being innervated by preganglionic axons and giving outgoing synapses of asymmetrical type to the principal neurones. A quantitative ultrastructural investigation has been made of the effect on these outgoing synapses of axotomy of the major post-ganglionic nerve trunks 18.5 h-390 days previously. Cutting, or cutting and ligating, the internal and external carotid nerves 2-3 mm from the ganglion in rats aged 1.5-5.5 months resulted in a statistically significant mean loss of up to 85% of the asymmetrical synapses given by small granule-containing cells in the injured ganglion. The reduction of synapses was maximal 5-9 days post-operatively, and thereafter the incidence of synapses showed significant signs of progressive recovery. The time course and magnitude of the change in incidence of these synapses resembled those found earlier (Matthews & Nelson, 1975) for the loss of preganglionic synapses to principal neurones in the same ganglia, and after an identical post-ganglionic lesion. Control experiments showed that there was no loss of outgoing synapses from the small granule-containing cells as a result of surgical stress or of simple ageing. Older rats (5.5 and 13 months) showed a small but significant increase in the incidence of these synapses. Unilateral post-ganglionic axotomy produced the same reaction in the injured ganglia as did bilateral lesions. Uninjured ganglia contralateral to unilateral axotomies, however, also showed some deficit of outgoing synapses from small granule-containing cells, but this was slight, amounting to 9.9% over-all in comparison to normal values in young rats, and this difference did not reach statistical significance. Cutting the cervical sympathetic trunk to produce preganglionic denervation 2 days before surgical removal of ganglia for analysis did not alter the incidence of outgoing synapses of the small granule-containing cells, either in ganglia post-ganglionically axotomized 5-128 days earlier or in contralateral ganglia, indicating that at no stage was any significant proportion of these synapses given to preganglionic axons. These findings suggest that most of the outgoing synapses from the intra-ganglionic small granule-containing cells are directed to principal neurones whose axons leave with the injured branches, the internal and external carotid nerves.(ABSTRACT TRUNCATED AT 400 WORDS)

Distribution of subgroups of noradrenaline neurons in the coeliac ganglion of the guinea-pig

The distributions within the coeliac ganglion of different chemically coded subgroups of noradrenaline neurons, and the relationships between these neurons and nerve fibres projecting to the ganglion from the intestine, have been assessed quantitatively by use of an immunohistochemical double-staining method. Noradrenaline (NA) neurons made up 99% of all cell bodies. Of these, 21% were also reactive for somatostatin (NA/SOM neurons), 53% were also reactive for NPY (NA/NPY neurons), and 26% were not reactive for either peptide. NA neurons without reactivity for any of the peptides whose localization was tested have been designated NA/-. A small percentage, about 1%, of neurons were reactive for both NPY and SOM. The three major types of NA neurons were arranged in clumps or ribbons throughout the ganglia, with a tendency for NA/SOM neurons to be medial and NA/NPY neurons to be lateral in the ganglia. A small group of neurons (less than 1%) encoded with dynorphin, NPY and vasoactive intestinal peptide (VIP) was encountered. VIP-immunoreactive nerve terminals, projecting to the ganglion from cell bodies in the intestine, ended around NA/SOM and NA/- neurons but not around NA/NPY neurons. Thus, the VIP axons from the intestine end selectively around neurons that modify intestinal function (NA/SOM and NA/- neurons) but not around neurons, the terminals of which supply blood vessels (NA/NPY neurons).

The density of reinnervation of adult rat superior cervical sympathetic ganglionic neurons is limited by the number of available postsynaptic sites

The adult rat superior cervical ganglion has about 27,000 neurons and is innervated by about 9000 preganglionic axons which make a total of nearly 11 million synapses. Surgical removal of the upper part of the ganglion, reducing the number of neurons to about 20%, causes an overall reduction of the number of synapses to about 30%, but has no effect on the numbers of preganglionic axons. Thus, a 5-fold increase in the axon/neuron ratio causes an increase of only about 50% in the number of synapses per cell. Axotomy followed by regeneration of the preganglionic axons causes no further increase in the number of synapses per cell, even though the average number of synapses per axon is reduced to about one-quarter of the normal. This suggests that the ganglionic neurons can only accept a limited number of synapses, and that in the normal situation there is only possibility for a relatively minor increase before this limit is reached. This study is complementary to a previous one in which the numbers of preganglionic axons were surgically reduced and it was found that, when allowed to regenerate into an entire denervated ganglion, the remaining axons could not increase their numbers of synapses. Thus, in the normal rat superior cervical sympathetic ganglion the total number of synapses is such that while the preganglionic axons are probably expressing close to their full synaptogenic potential, the ganglionic neurons express only about two-thirds of their ability to receive synapses.

Denervation-induced formation of adrenergic synapses in the superior cervical sympathetic ganglion of the rat and the enhancement of this effect by postganglionic axotomy

A study has been made at the ultrastructural level of the effects of denervation and axotomy on the synapse population of the rat superior cervical ganglion. Superior cervical ganglia were subjected unilaterally to acute (survival, 48 h) or chronic preganglionic denervation (survival, 41-189 days) by cutting the cervical sympathetic trunk; in chronic denervation experiments regeneration of preganglionic nerve fibres into the ganglion was prevented by suturing the proximal (caudal) stump of the trunk into the sternomastoid muscle. In some chronic experiments the preganglionic denervation was combined with simultaneous crush axotomy of the major postganglionic branches of the ganglion, the internal and external carotid nerves (axotomized-denervated ganglia). Control observations were made in contralateral ganglia and in ganglia from normal rats. After excision and before fixation, ganglia were incubated briefly in the presence of 5-hydroxydopamine to label adrenergic vesicles. Chronic denervation caused a statistically significant 12% decrease from control values in the cytoplasmic minor axes of the principal ganglionic neurones; axotomy combined with chronic denervation led to a 6% increase in this dimension, which was not statistically significant. The minor axes of the neuronal nuclei did not differ significantly from control values in either type of experiment. Axotomy combined with denervation led however to a 36% decrease in the incidence of nucleated neuronal profiles per unit area of ganglion. Counts of synapses were made in the various classes of ganglia and their incidence was expressed per nucleated neuronal profile, to permit comparison within and between experiments. Normal and control ganglia showed a high incidence of synapses of preganglionic cholinergic type. Nerve terminal profiles and synapses containing small dense-cored vesicles, as distinct from the efferent synapses of small granule-containing cells, were not found to be present on the principal neurones or their dendrites in these ganglia, despite strong 5-hydroxydopamine labelling of small dense-cored vesicles within cell bodies and dendrites. After acute denervation extremely few residual synapses were found in the ganglion, in areas remote from small granule-containing cells, and these residual synapses were of the cholinergic type. Acute denervation led to the appearance of vacated or isolated postsynaptic densities; such densities were also found, but were fewer in number, in chronically denervated and axotomized-denervated ganglia.(ABSTRACT TRUNCATED AT 400 WORDS)

Peripheral nerve grafts to the frog optic tectum: a morphological study of the axon reaction in trigeminal motor and sensory neurons

The mandibular branch of the trigeminal nerve was severed and the proximal stump was grafted onto the optic tectum in adult Rana pipiens. The resultant changes occurring in the cell bodies of origin in the ipsilateral trigeminal motor and mesencephalic nuclei were studied qualitatively and quantitatively. Nucleolar, nuclear, and somal cross-sectional areas increased in size significantly approximately 3 days after surgery and peaked at 6 weeks postsurgery. This swelling, in which the nucleolus was most severely affected, gradually reversed itself and disappeared by 24 weeks after surgery. Despite the cell enlargement, cytoplasmic basophilia was maintained or even slightly increased. These morphologic changes suggest a strong anabolic reaction. Two differences were found between the motoneurons and the sensory neurons. First, the morphometric cell changes occurred at a faster rate in neurons of the trigeminal motor nucleus than in those of the mesencephalic nucleus. The time course of the motoneuron response correlated well with that of axonal regeneration from the nerve graft. Second, there was a delayed loss of mesencephalic nucleus cells between 12 and 24 weeks after surgery, whereas cells of the trigeminal motor nucleus were maintained at all survival times studied. Taken together with sensory cell loss in the trigeminal ganglion, this suggests a greater viability of regenerating motoneurons.

Effect of colchicine and vinblastine on identified leech neurons

An identified neuron of the leech central nervous system is affected by the application of colchicine or vinblastine to its axon. It develops characteristic changes of membrane electrical properties, which are similar to those observed after surgical axotomy. The ionic mechanisms associated with the impulses induced by axotomy and colchicine treatment are not equivalent.

A theory on the lability and stability of spinal motoneuron soma size and induction of synaptogenesis in the adult spinal cord

There exists a dynamic relationship between the soma size of a motoneuron and its motor unit size. Adult motoneuron soma size can be experimentally increased if the neuron is allowed to innervate more muscle fibers than it normally innervates. In postnatal mammals a transition from polyneuronal to mononeuronal innervation of limb muscle fibers occurs which is temporally related to a plastic change in the perikaryal size. This lability of postnatal motoneuron size is temporally related to growth of synaptic connections on the motoneuron. In adult mammal, regenerating motor axons polyneuronally innervate the muscle fibers for a transient period. This hypothesis proposes that a plastic change in soma size occurs in these adult motoneurons. This short-lived labile state may revive the embryonic properties and evoke growth of synaptic boutons. Experimentally induced labile state in motoneuron pools and spinal ganglion neurons in the adult mammal should offer a basis for the study of mechanisms of synaptogenesis in the spinal cord.

The effects of microtubule dissociating agents on the physiology and cytology of the sensory neuron in the femoral tactile spine of the cockroach, periplaneta americana L

Microtubules are prominent cellular components of the mechanosensory and chemosensory sensilla associated with the insect cuticle, and a range of hypotheses have been proposed to account for their role in sensory transduction. Chemical agents such as colchicine and vinblastine, which dissociate microtubules, also interfere with transduction in these sensilla, and this has been attributed to their anti-microtubule activity. We have now examined the dynamic properties of sensory transduction in the mechanosensitive neuron of the cockroach femoral tactile spine, after the application of colchicine, vinblastine and lumicolchicine. Concurrently we have examined the ultrastructure of the same sensory ending by transmission electron microscopy. All of the drugs reduced the mechanical sensitivity o the receptor. Colchicine and vinblastine achieved this reduction without altering the dynamic properties of the receptor but lumicolchicine changed the dynamic response, and increased the relative sensitivity to rapid movements. Conduction velocity, another measure of neuronal function, which relies upon ionic currents flowing through the membrane, was reduced by all three drugs. The effects of the drugs upon the ultrastructure of the sensory ending were also disparate. In the case of colchicine there was complete dissociation of microtubules in the tubular body and distal dendrite before a total loss of mechanical sensitivity. Vinblastine was less effective in dissociating microtubules, although more effective in the reduction of mechanical sensitivity. With lumicolchicine the dominant morphological effect was a severe disruption of the dendritic membrane. We conclude from these experiments that microtubules are not essential in the transduction of mechanical stimuli by cuticular receptors and that the effects of these drugs upon mechanosensitivity are not directly related to their dissociation of the microtubules in the tubular body, but are more likely to arise from actions upon the cell membrane. These actions could include effects upon tubulin in the membrane or upon other membrane components.

Evidence for a motor nerve growth factor

We review the evidence that a motor nerve growth factor released from muscle has wide ranging effects on the development and maintenance of muscle innervation. The actions of this putative factor on motor neurons are analogous to the actions of the well known nerve growth factor (NGF) on sympathetic and sensory neurons.

Changes in unmyelinated fibers including sympathetic postganglionic fibers of a skin nerve after peripheral neuroma formation

Conduction velocities of unmyelinated fibers and proportions of postganglionic fibers which can be activated from the lumbar sympathetic trunk were determined for the superficial peroneal nerve of the cat's hindlimb. The nerve was either left intact (4 control experiments) or cut and ligated peripherally 6-245 days before the experiments so that neuromata developed (15 experiments). Additionally, 3 control experiments were performed on the sural nerve. Our findings are that about 20-30% of all unmyelinated fibers in intact skin nerves are postganglionic sympathetic fibers. This percentage of postganglionic axons, which can be activated from the preganglionic side, decreases to 0-7% 100 days or more after the nerve section. In intact skin nerves the conduction velocity of the afferent unmyelinated fibers is greater (mean +/- 1 S.D.: 0.88 +/- 0.17 m/s) than that of postganglionic axons (0.69 +/- 0.14 m/s). The conduction velocities in both types of axons decrease by 25-30% in nerves with peripheral neuroma. This occurs largely during the first 15 days following the nerve section described.

The effects of electrical stimulation on sprouting after partial denervation of guinea-pig sympathetic ganglion cells

1. The effects of electrical stimulation of presynaptic and post-synaptic cells on sprouting after partial denervation were examined in the guinea-pig superior cervical ganglion with intracellular recording. The partial denervation reduced the mean number of preganglionic axons innervating each ganglion cell from about eleven to about two (Maehlen & Njå, 1981). 2. One week after partial denervation alone, the number of residual preganglionic axons innervating each superior cervical ganglion cell was increased by about 30%. This means that the number of ganglion cells contacted by each residual preganglionic axon was increased by the same amount, which represents an early stage of sprouting. 3. Preganglionic stimulation for 1 hr immediately after the partial denervation (with 100 pulses at 20 Hz every 25 sec) increased the rate of sprouting. Thus 1 week after partial denervation and preganglionic stimulation the number of residual preganglionic axons innervating each ganglion cell was increased by about 70%. Preganglionic stimulation had no similar effect on the innervation of normal ganglia. 4. The acceleration of sprouting caused by preganglionic stimulation was abolished by blocking ganglionic transmission with hexamethonium (30--60 mg kg-1 hr-1 I.V.) during the stimulation. 5. Furthermore, the rate of sprouting of residual preganglionic axons was increased by electrical stimulation of the ganglion cells alone. 6. These results show that after partial denervation of the superior cervical ganglion, a period of impulse activity in ganglion cells enhances their subsequent ability to receive innervation from sprouts arising from residual preganglionic axons.

Intraocular colchicine selectively destroys immature ganglion cells in chicken retina

Low doses of colchicine (greater than 0.5 microgram) injected into the eyes of young chickens irreversibly inhibit axonal transport of material labelled with [3H]fucose. This is due to almost total destruction of the retinal ganglion cells, while other retinal cell types, including the displaced amacrine cells found in the same retinal layer, survive the treatment. The neurotoxic effect of colchicine on chick retinal ganglion cells decreases after hatching, and it is suggested that the decrease in sensitivity may be related to myelination of the retinal ganglion cell axons.

Selective synapse formation during sprouting after partial denervation of the guinea-pig superior cervical ganglion

1. The synaptic connexions established by sprouting of intact preganglionic sympathetic axons were examined by intracellular recording in vitro and by observing the sympathetic end organ responses to ventral root stimulation in vivo. 2. The superior cervical ganglion of the guinea-pig was partially denervated (70-85%) by crushing the cervical sympathetic trunk at the level of the subclavian artery, leaving the ansa subclavia intact. The intact nerve carried some preganglionic axons arising from each of the eight spinal cord segments (C8-T7) contributing innervation to the ganglion. 3. During the first 4 weeks after the operation, there was a two-to threefold increase in the number of steps in the synaptic response elicited in individual ganglion cells by graded stimulation of the ansa subclavia. There was also an increase in the amplitude of the synaptic potential elicited by each preganglionic axon. 4. This increase in the synaptic contribution of the intact nerve to neurones in the superior cervical ganglion after partial denervation was attributed to sprouting of residual preganglionic axons. A major contribution from collateral connexions between ganglion cells was ruled out by intracellular recording form neurones during antidromic stimulation of their axons in the inferior post-ganglionic nerve. 5. After sprouting, the specificity of the sympathetic end organ responses elicited by stimulation of the ventral roots of spinal segments T1 and T4 in vivo was indistinguishable from normal, although the strength of these responses increased from just perceptible acutely after partial denervation to near normal 3-6 weeks after the operation, when sprouting was largely complete. 6. These results show that intact preganglionic axons arising from different spinal levels established selective connexions with different classes of ganglion cells during sprouting.