Hepatocyte growth factor (HGF/SF) is a muscle-derived survival factor for a subpopulation of embryonic motoneurons

Muscle-derived factors are known to be important for the survival of developing spinal motoneurons, but the molecules involved have not been characterized. Hepatocyte growth factor/scatter factor (HGF/SF) plays an important role in muscle development and motoneuron axon outgrowth. We show that HGF/SF has potent neurotrophic activity (EC50=2 pM) for a subpopulation (40%) of purified embryonic rat motoneurons. Moreover, HGF/SF is an essential component of muscle-derived support for motoneurons, since blocking antibodies to HGF/SF specifically inhibited 65% of the trophic activity of media conditioned by C2/C7 skeletal myotubes, but did not inhibit the trophic activity secreted by Schwann cell lines. High levels of expression of the HGF/SF receptor c-Met in the spinal cord are restricted to subsets of motoneurons, mainly in limb-innervating segments. Consistent with this distribution, cultured motoneurons from limb-innervating brachial and lumbar segments showed a more potent response to HGF/SF than did thoracic motoneurons. By the end of the period of motoneuron cell death, levels of c-Met mRNA in motoneurons were markedly reduced, suggesting that the effects of HGF/SF may be limited to the period of motoneuron cell death. HGF/SF may play an important role during motoneuron development as a muscle-derived survival factor for a subpopulation of limb-innervating motoneurons.     

The functional activity of the portal vein as a reflection of the metabolic changes in experimental chronic kidney failure

Current therapeutic efforts to treat chronic and progressive neurodegenerative disease include, for the first time, attempts to regenerate affected nervous tissue using neurotrophic factors. The rationale for using trophic factors includes the understanding that they support neuronal survival and regrowth processes. The potential benefits of trophic factor therapy will be no more realized in the near future than in the treatment of amyotrophic lateral sclerosis (ALS). ALS is pathologically characterized by the selective degeneration of specific populations of cranial and spinal motoneurons. Evidence for the existence of factors that support motoneurons has come from studies demonstrating that motoneurons receive trophic influences from various tissues, both central and peripheral, within their local environment. Although the identity of these putative tissue-derived factors has remained enigmatic, recent studies have demonstrated that several previously characterized trophic factors exhibit trophic influences on motoneurons. Among these are several members of the neurotrophin family, most notably brain-derived neurotrophic factor. These neurotrophins meet most of the criteria to be considered motoneuron trophic factors: they are locally available to motoneurons in vivo; motoneurons express specific receptors for these factors; and exogenous application of these factors mimicks the effects of the uncharacterized endogenous agents. The clinical use of these factors for the treatment of ALS, therefore, appears to be scientifically justified.     

Effects of chronic spinalization on ankle extensor motoneurons. I. Composite monosynaptic Ia EPSPs in four motoneuron pools

1. We examined the effects of 6-wk chronic spinalization at the L1-L2 level on composite monosynaptic Ia excitatory postsynaptic potentials (EPSPs) recorded in medial gastrocnemius (MG), lateral gastrocnemius (LG), soleus (SOL), and plantaris (PL) motoneurons. Amplitudes, rise times, and half-widths of composite monosynaptic Ia EPSPs evoked by low-strength electrical stimulation of peripheral nerves were measured in barbiturate-anesthetized cats and compared between unlesioned and chronic spinal preparations. 2. The mean amplitude of homonymous composite Ia EPSPs evoked by 1.2 times threshold (1.2T) stimulation and recorded in all four ankle extensor motoneuron pools increased 26% in chronic spinal animals compared with unlesioned controls. There was also an increased incidence of large-amplitude, short-rise time EPSPs. When the same data were separated according to individual motoneuron species, homonymous EPSP amplitudes in MG motoneurons were found to be unchanged. EPSPs recorded in LG motoneurons and evoked by stimulation of the combined LG and SOL nerve were increased by 46%. Mean EPSP amplitudes recorded in both SOL and PL motoneurons were larger after spinalization but statistical significance was only achieved when values from SOL and PL were combined to produce a larger sample size. 3. In LG motoneurons from chronic spinal animals, all EPSPs evoked by 1.2T stimulation of the LGS nerve were > or = 0.5 mV in amplitude. In unlesioned preparations, one fourth of the LG cells had EPSPs that were < or = 0.2 mV. 4. The mean amplitude of heteronymous EPSPs evoked by 2T stimulation of LGS and MG nerves and recorded in MG and LG motoneurons, respectively, doubled in size after chronic spinalization. Because homonymous EPSP amplitudes were unchanged in MG motoneurons, synaptic mechanisms and not passive membrane properties are likely responsible for increased heteronymous EPSP amplitudes in MG. 5. The mean 10-90% rise time of homonymous composite Ia EPSPs in pooled data from all motoneurons decreased 21% in 6-wk chronic spinal animals. Unlike EPSP amplitude, significant rise time decreases were found in all four motoneuron pools. Compared with the other motoneuron species, the mean homonymous rise time recorded in MG motoneurons was shortest and decreased the least in chronic spinal animals. Rise times of heteronymous Ia EPSPs in MG and LG motoneurons also decreased. The maximum rate of rise of homonymous EPSPs increased in all four motoneuron species. 6. The mean half-widths of Ia composite EPSPs decreased in 6-wk spinalized preparations in all motoneuron species.(ABSTRACT TRUNCATED AT 400 WORDS)     

The segmental precision of the motor projection to the intercostal muscles in the developing chicken embryo. A differential labelling study using fluorescent tracers

Each skeletal muscle in the vertebrate is innervated by a group of motoneurons called a motoneuron pool. Retrograde labelling of single motoneuron pools has suggested that the arrangement of motoneuron pools innervating different limb muscles does not change during the embryonic period when more than 50% of the motoneurons die. In this study we retrogradely labelled neighbouring intercostal motoneuron pools differentially with latex microspheres or dextran amines coupled to fluorescent dyes. We then mapped the positions of the differentially labelled motoneurons in whole-mount preparations using a computer-aided drawing system. While the intercostal motoneuron pools are clearly segregated even at early stages, there is some intermingling at the rostral and caudal ends. We used a logistic regression to determine the extent of segmental overlap, and to facilitate a quantitative comparison of the overlap at different stages. Statistical analysis shows that the overlap (expressed as the percentage of the length of the overlapping motoneuron pools) decreases modestly during the period of motoneuron death. Computer simulations suggest that this decrease does not result from random motoneuron death alone; one alternative possibility is selective death of motoneurons in the overlap zone. Occasional "rogue" motoneurons, that is, motoneurons of one pool that scatter into the neighbouring pool, are still present at the end of the period of cell death, representing a potential source of "noise" in the establishment of segmental patterns of connectivity.     

Motoneurons deprived of trophic support in vitro require new gene expression to undergo programmed cell death

During normal development, large numbers of neurons die by programmed cell death. This phenomena has been extensively studied in the lateral motor column of chick embryos, where approximately 50% of the motoneurons that are initially produced, subsequently die due in part to competition for a limited supply of target-derived trophic support. Inhibitors of RNA and protein synthesis block this cell loss in vivo, indicating a requirement for new gene expression (Oppenheim et al., 1990). Prior to their commitment to death, motoneurons can be isolated as a relatively pure population from chick spinal cord for in vitro study. Cells plated with muscle extract, a potent source of target-derived trophic support, survive, and have large, phase-bright cell bodies and extensive neurite outgrowth. In contrast, motoneurons cultured in the absence of muscle extract die within 48 h. This death can be blocked by the RNA synthesis inhibitor actinomycin D, at the time when the cells become committed to die, suggesting that new gene expression is required for cell death. DNA fragmentation and nuclear condensation indicate that some of these cells die by apoptosis. Therefore, it appears that many aspects of motoneuron development observed in vivo can be reconstituted in vitro. These cultures can be used as a model system for studying neuronal death and may contribute to an understanding of the molecular mechanisms that mediate programmed cell death during neuronal development.     

Effects of botulinum neurotoxin type A on abducens motoneurons in the cat: ultrastructural and synaptic alterations

The synaptic alterations induced in abducens motoneurons by the injection of 3 ng/kg of botulinum neurotoxin type A into the lateral rectus muscle were studied using ultrastructural and electrophysiological techniques. Motoneurons identified by the retrograde transport of horseradish peroxidase showed a progressive synaptic stripping already noticeable by four days post-injection which increased over the study period. By 35 days post-injection, the normal coverage of motoneurons by synaptic boutons (66.4 +/- 4.0%) significantly decreased to 27.2 +/- 4.0%. Synaptic boutons detached by a widening of the subsynaptic space but remained apposed by synaptic contacts and desmosomes to the motoneuron. Detachment did not affect equally flat and round vesicle-containing boutons. The control motoneuron had almost equal numbers of both types of boutons, but after 35 days post-injection the ratio of round to flat vesicle-containing boutons was 1.20 +/- 0.01. Synaptic boutons impinging on motoneurons showed signs of alterations in membrane turnover, as indicated by an increase in the number of synaptic vesicles and a decrease in the number of coated vesicles and synaptic vesicles near the active zone. Abducens motoneurons had a transient increase in soma size by 15 days that returned to normal at 35 days, but no signs of chromatolysis or organelle degeneration were seen. Accompanying the swelling of motoneurons, a 15-fold increase in the number of spines, very infrequent in controls, was observed. Spines located in the soma and proximal dendritic trunk received synaptic contacts from both flat and round vesicle-containing boutons that could be either partly detached or completely attached to the motoneuron. An increased turnover of the plasmatic membrane of the motoneuron was observed, as indicated by a four-fold increase in the number of somatic coated vesicles. Animals were implanted with bipolar electrodes in the ampulla of both horizontal semicircular canals for evoking contralateral excitatory and ipsilateral inhibitory postsynaptic potentials. Motoneurons were antidromically identified from the lateral rectus muscle. Synaptic potentials of vestibular origin were recorded in abducens motoneurons. In the period between two and six days post-injection, a complete abolition of inhibitory synaptic potentials was observed. By contrast, excitatory synaptic potentials remained, but were reduced by 82%. The imbalance between excitatory and inhibitory inputs to motoneurons induced a progressive increase of firing frequency within a few stimuli applied to the contralateral canal. Between 7 and 15 days post-injection, both excitatory and inhibitory postsynaptic potentials were virtually abolished and remained so up to the longest time checked (105 days). Some motoneurons recorded beyond 60 days post-injection showed signs of recovery of excitatory postsynaptic potentials. During the whole time-span studied, presynaptic wavelets were present, indicating no affecting of the conduction of afferent volleys to the abducens nucleus. Taken together, these data indicate that botulinum neurotoxin at high doses causes profound synaptic alterations in motoneurons responsible for the effects seen in the behavior of motoneurons recorded in alert animals.     

Brachially innervated ectopic hindlimbs in the chick embryo. I. Limb motility and motor system anatomy during the development of embryonic behavior

The functional status of brachially innervated hindlimbs, produced by transplanting hindlimb buds of chick embryos in place of forelimb buds, was quantified by analyzing the number and temporal distribution of spontaneous limb movements. Brachially innervated hindlimbs exhibited normal motility until E10 but thereafter became significantly less active than normal limbs and the limb movements were more randomly distributed. Contrary to the findings with axolotls and frogs, functional interaction between brachial motoneurons and hindlimb muscles cannot be sustained in the chick embryo. Dysfunction is first detectable at E10 and progresses to near total immobility by E20 and is associated with joint ankylosis and muscular atrophy. Although brachially innervated hindlimbs were virtually immobile by the time of hatching (E21), they produced strong movements in response to electrical stimulation of their spinal nerves, suggesting a central rather than peripheral defect in the motor system. The extent of motoneuron death in the brachial spinal cord was not significantly altered by the substitution of the forelimb bud with the hindlimb bud, but the timing of motoneuron loss was appropriate for the lumbar rather than brachial spinal cord, indicating that the rate of motoneuron death was dictated by the limb. Measurements of nuclear area indicated that motoneuron size was normal during the motoneuron death period (E6-E10) but the nuclei of motoneurons innervating grafted hindlimbs subsequently became significantly larger than those of normal brachial motoneurons. Although the muscle mass of the grafted hindlimb at E18 was significantly less than that of the normal hindlimb (and similar to that of a normal forelimb), electronmicroscopic examination of the grafted hindlimbs and brachial spinal cords of E20 embryos revealed normal myofiber and neuromuscular junction ultrastructure and a small increase in the number of axosomatic synapses on cross-sections of motoneurons innervating grafted hindlimbs compared to motoneurons innervating normal forelimbs. The anatomical data indicate that, rather than being associated with degenerative changes, the motor system of the brachial hindlimb of late-stage embryos is intact, but inactive.     

Decreased axosomatic input to motoneurons and astrogliosis in the spinal cord of aged rats

An increasing body of evidence indicates that aging-related impairments of nervous functions are caused by damage to neuron integrity rather than by loss of neurons. By using electron microscopy, we have examined axosomatic boutons on spinal cord motoneurons derived from aged and young adult Sprague-Dawley rats. The main finding was that about half of the examined motoneuron somata from aged rats had a reduced (50%) bouton coverage, which seemed to be caused by a smaller number of axosomatic bouton profiles. Long stretches of the cell body plasma membrane were apposed by pale processes, and immunolabeling for glial fibrillary acidic protein (GFAP) disclosed that a number of the aged motoneurons appeared embedded in GFAP immunopositive processes. Lumbar motoneurons seemed to be more severely affected than cervical motoneurons. At the ultrastructural level, affected motoneurons disclosed plasma membrane irregularities with appendages/sprout-like extensions that in some cases were sites for axosomatic contacts.     

The role of immune processes in amyotrophic lateral sclerosis pathogenesis

Despite many efforts, the etiopathogenesis of ALS remains unknown. During the last decade evidence for an autoimmune involvement in motoneuron degeneration and death has remarkably increased. Multiple reports have documented significant expression of proteins associated with immune function in affected areas of ALS patients. Two animal models of immune-mediated motoneuron destruction have been developed that closely resemble clinical, electrophysiological and morphological features of human ALS. Inflammatory foci within the spinal cord, and IgG at the neuromuscular junction as well as within upper and lower motoneurons found in the animal models support the role of autoimmune mechanisms of motoneuron destruction in this model. IgG from ALS patients and from the animal models can passively transfer physiological changes at the neuromuscular junction in mice. That ALS IgG interact with calcium channels and induce an alteration of their function is now electrophysiologically and biochemically evident. Furthermore, it has been documented that motoneurons may be selectively vulnerable since they have a deficient calcium buffering capacity. Although further research efforts are necessary to elucidate the interaction of the ALS antibodies with the calcium channel function and how defective calcium handling by the motoneurons is important in their degeneration, the current data strongly suggest the involvement of autoimmune mechanisms in ALS etiopathogenesis.     

Central generation of grooming motor patterns and interlimb coordination in locusts

Coordinated bursts of leg motoneuron activity were evoked in locusts with deefferented legs by tactile stimulation of sites that evoke grooming behavior. This suggests that insect thoracic ganglia contain central pattern generators for directed leg movements. Motoneuron recordings were made from metathoracic and mesothoracic nerves, after eliminating all leg motor innervation, as well as all input from the brain, subesophageal ganglion, and prothoracic ganglion. Strong, brief trochanteral levator motoneuron bursts occurred, together with silence of the slow and fast trochanteral depressor motoneurons and activation of the common inhibitor motoneuron. The metathoracic slow tibial extensor motoneuron was active in a pattern distinct from its activity during walking or during rhythms evoked by the muscarinic agonist pilocarpine. Preparations in which the metathoracic ganglion was isolated from all other ganglia could still produce fictive motor patterns in response to tactile stimulation of metathoracic locations. Bursts of trochanteral levator and depressor motoneurons were clearly coordinated between the left and right metathoracic hemiganglia and also between the mesothoracic and the ipsilateral metathoracic ganglia. These data provide clear evidence for centrally generated interlimb coordination in an insect.     

Role of neurotrophic factors in motoneuron development

More than 10 factors from different gene families are now known to enhance motoneuron survival, and to be expressed in a manner consistent with a role in regulating motoneuron numbers during development. We provide evidence that: a) different factors may act on different sub-populations of motoneurons; b) different factors may act in synergy on a given motoneuron. Thus, the functional diversity of motoneurons, and the cellular complexity of their environment, may be reflected in the mechanisms that have evolved to keep them alive.     

Operant conditioning of H-reflex changes synaptic terminals on primate motoneurons

Operant conditioning of the primate triceps surae H-reflex, the electrical analog of the spinal stretch reflex, creates a memory trace that includes changes in the spinal cord. To define the morphological correlates of this plasticity, we analyzed the synaptic terminal coverage of triceps surae motoneurons from animals in which the triceps surae H-reflex in one leg had been increased (HRup mode) or decreased (HRdown mode) by conditioning and compared them to each other and to motoneurons from unconditioned animals. Motoneurons were labeled by intramuscular injection of cholera toxin-horseradish peroxidase. A total of 5055 terminals on the cell bodies and proximal dendrites of 114 motoneurons from 14 animals were studied by electron microscopy. Significant differences were found between HRup and HRdown animals and between HRup and naive (i.e., unconditioned) animals. F terminals (i.e., putative inhibitory terminals) were smaller and their active zone coverage on the cell body was lower on motoneurons from the conditioned side of HRup animals than on motoneurons from the conditioned side of HRdown animals. C terminals (i.e., terminals associated with postsynaptic cisterns and rough endoplasmic reticulum) were smaller and the number of C terminals in each C complex (i.e., a group of contiguous C terminals) was larger on motoneurons from the conditioned side of HRup animals than on motoneurons either from the conditioned side of HRdown animals or from naive animals. Because the treatment of HRup and HRdown animals differed only in the reward contingency, the results imply that the two contingencies had different effects on motoneuron synaptic terminals. In combination with other recent data, they show that H-reflex conditioning produces a complex pattern of spinal cord plasticity that includes changes in motoneuron physiological properties as well as in synaptic terminals. Further delineation of this pattern should reveal the contribution of the structural changes described here to the learned change in behavior.     

Spatial and temporal features of recurrent facilitation among motoneurons innervating synergistic muscles of the cat

1. The temporal features and strength of recurrent facilitatory potentials were examined in pairs of lumbosacral motoneurons that were separated by a known distance and were identified by antidromic stimulation of muscle nerves. One motoneuron was stimulated by injecting depolarizing current pulses, and responses were recorded in the second motoneuron. The distance between motoneurons in pairs was also measured to assess the spatial distribution in strength of recurrent facilitation in motor pools. All motoneurons in these pairs innervated muscles that act as hip or ankle extensors. 2. Recurrent facilitatory potentials were found frequently among motoneurons innervating the hindlimb extensor muscles examined. Several categories of recurrent facilitatory responses were identified. One category was composed of facilitation responses that followed an inhibition response. A second category was composed of facilitation responses that were not preceded by a significant inhibition and consisted of a monophasic response. There was a considerable range of latencies in this category. 3. Responses in which recurrent facilitatory potentials were preceded by recurrent inhibitory postsynaptic potentials (RIPSPs) among close motoneuron pairs demonstrated an inverse correlation between the durations of the facilitatory and the inhibitory phases. In addition, the duration of inhibition responses without facilitation was longer on average, than the duration of inhibitory responses that were followed by facilitation. It was suggested that recurrent facilitation may restrict the time course of RIPSPs. 4. In contrast to the topographic distribution of RIPSPs described in the previous report, amplitudes of monophasic facilitations were directly correlated with the distance separating motoneurons in pairs, rather than inversely correlated as was the case for RIPSP amplitudes.(ABSTRACT TRUNCATED AT 250 WORDS)     

Improved DNA-repair parameters in PHA-stimulated peripheral blood lymphocytes of human subjects with low body mass index

Effects of spinal cord transection on the synaptology of zebrafish spinal motoneurons were studied. The transection was made at the level of the 14th vertebra and the synaptology of motoneuron somata and dendrites was analysed at the level of the 21st to the 23rd vertebrae at one month and three months after transection. Horseradish peroxidase, applied to the myotomal muscle, was used to label motoneuron somata and dendritic branches in central and in lateral areas of the neuropil (referred to as central and lateral dendritic profiles). Boutons impinging on motoneurons were classified according to the morphology of the vesicles. We discerned R-boutons with spherical vesicles, F-boutons with flat vesicles and DC-boutons with at least one dense core vesicle. The apposition lengths of R-, F- and DC-boutons and the circumference of labelled profiles were determined to assess the proportional covering of boutons on somata and dendrites. Ratio's of covering with R- and F-boutons (R/F ratio) for somata, central and lateral dendritic profiles were 1.1, 2.1, and 2.1 in control fish and 0.5, 0.5 and 0.9 in lesioned fish at one month after transection, respectively. The total covering of motoneurons in lesioned fish was decreased by 20% on somata and by 30% on lateral dendritic profiles, whereas central dendritic profiles did not change significantly. At three months after transection the R/F ratio's for somata, central and lateral dendritic profiles were 0.5, 0.7 and 0.6, respectively. The total covering on somata and central and lateral dendritic profiles was at control levels. The anatomical aspects of the changes in synaptology indicate that in control fish 50 to 60% of the R-boutons on the motoneuron surface originate from descending axons. In contrast, almost all F-boutons seem to be from local origin.     

Behavior and the law reconsidered: psychological syndromes and profiles

Three months after facial nerve transection, total numbers of motoneurons in the facial nucleus of six month (adult) Fischer 344 and Wistar rats were reduced to 83% and 75% of contralateral values, respectively (P < 0.05). This procedure in 22-26 month (ageing) Fischer 344 rats and Wistar rats resulted in a reduction of motoneuron numbers to 77% and 60% of the respective contralateral values (P < 0.05). Compared to adults, contralateral facial nuclei of aging Fischer 344 rats contained 10% fewer motoneurons (non-significant), while ageing Wistar rats had 22% fewer (P < 0.05). No significant changes were found in the proportion of surviving motoneurons expressing calcitonin gene-related peptide, galanin, receptor tyrosine kinase-C or the alpha subunit of the ciliary neurotrophic factor receptor. We conclude that ageing reduces facial motoneuron number and increases their vulnerability to axotomy in Wistar rats, but not in Fischer 344 rats. In neither strain, however, does the proportion of surviving motoneurons expressing the above neuropeptides or neurotrophic factor receptors change. This information may be relevant to those hypotheses of age-related neuronal degenerations which assume that decreased neurotrophic support renders ageing neurons more vulnerable to injury.     

Short-latency synaptic patterns among cat peroneal motoneurons

Motoneurons innervating peroneal muscles in the cat leg (PB, PT and PL, respectively, for peroneus brevis, tertius and longus) were examined for their connections with afferents from these and other leg muscles and with cutaneous afferents. The aim was to investigate (1) whether inputs from nearby muscles and cutaneous areas are likely to assist or oppose the excitation elicited in peroneal motoneurons by PB contractions, and (2) whether reflex connectivity might allow distinction of alpha (i.e. motoneurons innervating skeletal muscle fibres) and beta (i.e. motoneurons innervating both skeletal and intrafusal muscle fibres) subgroups among PB and PT motoneurons. In the three peroneal pools, every motoneuron had excitatory monosynaptic connections with Ia afferents from each of the three peroneal muscles, and nearly every motoneuron received di- or trisynaptic excitation from low-threshold cutaneous afferents in sural or superficial peroneal nerves. Inputs from these sources might facilitate the contraction-induced positive feedback. In contrast, the patterns of short-latency synaptic connections with group I afferents from pretibial flexor and post-tibial extensor muscles were heterogeneous among peroneal motoneurons but did not point to any specific beta pattern.     

Cell death of spinal motoneurons in the chick embryo following deafferentation: rescue effects of tissue extracts, soluble proteins, and neurotrophic agents

In the absence of descending spinal and supraspinal afferent inputs, neurons in the developing lumbar spinal cord of the chick embryo undergo regressive changes including cellular atrophy and degeneration between embryonic days 10 and 16. There are significant decreases in the number of motoneurons, interneurons, and sensory (dorsal root ganglion) neurons. Although there are several possible explanations for how afferents might regulate the maintenance of neuronal viability, we have focused attention on the putative role of neurotrophic agents in these events. Previous studies have shown that specific tissue extracts (e.g., muscle, brain), soluble proteins, growth factors, and trophic agents can promote the in vitro and in vivo survival of avian motoneurons during the period of natural cell death (embryonic days 6-10). Several of these agents were also effective following deafferentation. These included brain extract (BEX), muscle extract (MEX), conditioned medium from astrocyte cultures (ACM), as well as the following neurotrophic agents: nerve growth factor (NGF), brain-derived neurotrophic factor (BDNF), neurotrophin-3 (NT-3), S-100, insulin-like growth factor-I (IGF-I), ciliary neurotrophic factor (CNTF), platelet-derived growth factor (PDGF), basic fibroblast growth factor (bFGF), and leukemia inhibitory factor (CDF/LIF). Both transforming growth factor-beta (TGF-beta) and acidic fibroblast growth factor (aFGF) were ineffective. Although considerable more work is needed to determine which (and how) specific CNS-derived trophic agents regulate motoneuron survival, the present results are consistent with the notion that neurotrophic agents released from or modulated by synaptic inputs to target neurons promote neuronal differentiation and survival in the CNS.     

Inhibitory synaptic drive patterns motoneuronal activity in rhythmic preparations of isolated thoracic ganglia in the stick insect

During active leg movements of an insect leg, the activity of the motoneuron pools of each individual leg joint is generated by the interaction between signals from central rhythm generating sources, peripheral signals as well as coordinating signals from other leg joints and legs. The nature of the synaptic drive from the central rhythm generators onto the motoneuron pools of the individual leg joints during rhythmic motor activity of the stick insect (Carausius morosus) middle leg has been investigated. In the isolated mesothoracic ganglion central rhythm generators were activated pharmacologically by topical application of the muscarinic agonist pilocarpine. Motoneurons supplying the femur-tibia (FT) joint were investigated in detail. Recordings from neuropil processes of these motoneurons revealed that patterning of their rhythmic activity is based on cyclic hyperpolarizing synaptic inputs. These inputs are in clear antiphase for extensor and flexor motoneurons. DCC (discontinuous current clamp) and dSEVC (discontinuous single electrode voltage clamp) recordings showed reversal potentials of the inhibitory inputs between -80 to -85 mV (FETi, N=7; Flex MN, N=3). After intracellular injection of TEA rhythmic inhibition in FETi was decreased by about 84% (N=4). Both findings indicate that the cyclic inhibition is mediated by potassium ions. Thus, it appears that central rhythm generators pattern motor activity in antagonistic tibial motoneuron pools by cyclic alternating inhibition.     

Organization of motoneurons in the dorsal hypoglossal nucleus that innervate the retrusor muscles of the tongue in the rat

This anatomical investigation was prompted by the incomplete knowledge of the myotopic organization of the dorsal subdivison of the hypoglossal nucleus. Intrinsic muscle motoneurons were not segregated and labeled previously with regard to the lateral division of the hypoglossal nerve. Also, motoneuron number and cell size, in relation to the individual retrusor tongue musculature, were rarely addressed previously. Retrograde labeling ofretrusor muscle motoneurons in the dorsal subdivision of the rat hypoglossal nucleus was done. Cholera toxin conjugate horseradish peroxidase (CTHRP) was injected into the retrusor tongue muscles with only the lateral division of the hypoglossal nerve intact. The dorsal subdivision of the hypoglossal nucleus contained approximately 800 motoneurons ranging in cell body size from 19 to 41 microm. When either the styloglossus, hyoglossus, superior longitudinal, or inferior longitudinal muscle was isolated and injected with CTHRP, a separate motoneuron pool for each muscle was seen. The extrinsic muscle motoneurons, styloglossus and hyoglossus, were found rostrolateral and caudolateral respectively. In contrast, the intrinsic superior and inferior longitudinal muscle motoneurons were found more central and medial in the nucleus. Extrinsic muscle motoneurons were larger (approximately 30 microm) than intrinsic muscle motoneurons (approximately 26 microm; P < .0001). Intrinsic muscle motoneurons account for a great majority of the motoneurons in the dorsal aspect of the hypoglossal nucleus and their axons have been shown to be contained in the lateral (retrusor) division of the hypoglossal nerve. This study revealed the myotopic organization of the retrusor subdivision of the rat hypoglossal nucleus.