Influences of neurotrophins on mammalian motoneurons in vivo

Radio-iodination of neurotrophins and their delivery in vivo: advantages of membrane filtration and the use of disposable syringes

This paper reports two simple improvements for the radio-iodination of neurotrophins and their delivery in vivo. (1) Neurotrophins can be effectively separated from free iodide by using membrane filtration devices. Seven methods for the separation of free iodide are compared, including dialysis, gel filtration, and membrane filtration. Membrane filtration of the iodinated protein has several important advantages over dialysis or gel filtration. These include the precise control over the final concentration; excellent recovery of the neurotrophin; easy and inexpensive procedure; performance of the entire procedure in a fume hood; and reduced volume of radioactive waste. (2) Disposable, inexpensive syringes are suitable for the delivery of small volumes of radio-iodinated or non-radioactive neurotrophins. Plastic disposable insulin syringes are compared with Hamilton syringes. The ejection volume of the disposable syringes is surprisingly reliable in the dose range 2-15 microl. Their in vivo performance was tested by injections in the eyes of chick embryos in ovo. The amounts remaining in the eye varied significantly less with the disposable syringes. Leakage into the surrounding eye-muscles after intraocular injection was significantly more frequent with Hamilton syringes than with the disposable syringes. Thus, disposable syringes can be a reliable and cost-effective alternative for drug delivery of 2-15 microl volumes.

Nitric oxide and superoxide contribute to motor neuron apoptosis induced by trophic factor deprivation

Primary cultures of rat embryonic motor neurons deprived of brain-derived neurotrophic factor (BDNF) induce neuronal nitric oxide synthase (NOS) within 18 hr. Subsequently, >60% of the neurons undergo apoptosis between 18 and 24 hr after plating. Nitro-L-arginine and nitro-L-arginine methyl ester (L-NAME) prevented motor neuron death induced by trophic factor deprivation. Exogenous generation of nitric oxide at concentrations lower than 100 nM overcame the protection by L-NAME. Manganese tetrakis (4-benzoyl acid) porphyrin, a cell-permeant superoxide scavenger, also prevented nitric oxide-dependent motor neuron death. Motor neurons cultured without trophic support rapidly became immunoreactive for nitrotyrosine when compared with motor neurons incubated with BDNF, L-NAME, or manganese TBAP. Our results suggest that peroxynitrite, a strong oxidant formed by the reaction of NO and superoxide, plays an important role in the induction of apoptosis in motor neurons deprived of trophic factors and that BDNF supports motor neuron survival in part by preventing neuronal NOS expression.

Short-term afferent axotomy increases both strength and depression at Ia-motoneuron synapses in Rat

Synaptic efficacy at the rat Ia-motoneuron synapse has been reported to increase in vivo, within 3 d of sectioning a single muscle nerve (). We provide an indirect test of the hypothesis that this increase is caused by altered probability of transmitter release of axotomized afferents. Experiments consisted of in vivo recording of maximal composite group I EPSPs evoked in intact rat medial gastrocnemius (MG) motoneurons by stimulation of the lateral gastrocnemius-soleus nerve (LG-S). We compared the maximal LG-S EPSP amplitude and the response to high-frequency stimulation (modulation) recorded in untreated rats, with the same measures recorded in rats that had the LG-S nerve axotomized 3 d before data collection. In confirmation of previous work, the mean amplitude of LG-S EPSPs evoked by stimulation of axotomized afferents was significantly larger than that measured in untreated rats (3.9 +/- 0. 34 and 2.3 +/- 0.19 mV, respectively). The increase in EPSP amplitude was accompanied by significantly greater negative modulation (depression) of EPSP amplitude during high-frequency stimulation (-39 +/- 4% and -53 +/- 4%, untreated and treated, respectively). Modulation would not be expected to change if the increase in EPSP amplitude was attributable solely to a greater number of afferent connections (). Therefore, the present results are consistent with the hypothesis that the initial axotomy-induced increase in synaptic efficacy occurs because of an increase in the probability of transmitter release. Furthermore, these results suggest that the probability of transmitter release at this synapse is regulated by either afferent activity and/or trophic communication with the target muscle.

Postnatal expression of Hu-bcl-2 gene in Lurcher mutant mice fails to rescue Purkinje cells but protects inferior olivary neurons from target-related cell death

The Lurcher mutant has been extensively studied as a model for cell-autonomous and target-related cell death, yet there are still many unknowns concerning the mechanisms of neuronal degeneration in this mutant. As a key regulator of apoptosis, a bcl-2 transgene has been overexpressed in the heterozygous Lurcher mutant to investigate the effects of BCL-2 on two types of in vivo neuronal cell loss in Lurcher: cell-autonomous Purkinje cell degeneration and target-related olivary neuron death. Six adult +/Lc mutants expressing a human bcl-2 transgene (Hu-bcl-2) were generated by crossing +/Lc mutants with NSE71 Hu-bcl-2 transgenic mice. Analysis of these brains showed that bcl-2 overexpression did not prevent +/Lc Purkinje cell degeneration, but it did rescue most olivary neurons from target-related cell death. Although the number of olivary neurons was equivalent to wild-type numbers, the inferior olive nucleus was significantly shorter in its rostrocaudal extent, suggesting that olivary neurons are atrophied. We propose that Lurcher gene action causes Purkinje cell degeneration independently of a BCL-2-mediated pathway. Furthermore, although bcl-2 overexpression rescues olivary neurons from target-related cell death, it does not prevent the atrophy associated with the loss of target-related trophic support.

BDNF and NT-4/5 prevent atrophy of rat rubrospinal neurons after cervical axotomy, stimulate GAP-43 and Talpha1-tubulin mRNA expression, and promote axonal regeneration

Rubrospinal neurons (RSNs) undergo a marked atrophy in the second week after cervical axotomy. This delayed atrophy is accompanied by a decline in the expression of regeneration-associated genes such as GAP-43 and Talpha1-tubulin, which are initially elevated after injury. These responses may reflect a deficiency in the trophic support of axotomized RSNs. To test this hypothesis, we first analyzed the expression of mRNAs encoding the trk family of neurotrophin receptors. In situ hybridization revealed expression of full-length trkB receptors in virtually all RSNs, which declined 7 d after axotomy. Full-length trkC mRNA was expressed at low levels. Using RT-PCR, we found that mRNAs encoding trkC isoforms with kinase domain inserts were present at levels comparable to that for the unmodified receptor. TrkA mRNA expression was not detected in RSNs, and the expression of p75 was restricted to a small subpopulation of axotomized cells. In agreement with the pattern of trk receptor expression, infusion of recombinant human BDNF or NT-4/5 into the vicinity of the axotomized RSNs, between days 7 and 14 after axotomy, fully prevented their atrophy. This effect was still evident 2 weeks after the termination of BDNF treatment. Moreover, BDNF or NT-4/5 treatment stimulated the expression of GAP-43 and Talpha1-tubulin mRNA and maintained the level of trkB expression. Vehicle, NGF, or NT-3 treatment had no significant effect on cell size or GAP-43 and Talpha1-tubulin expression. In a separate experiment, infusion of BDNF also was found to increase the number of axotomized RSNs that regenerated into a peripheral nerve graft. Thus, in BDNF-treated animals, the prevention of neuronal atrophy and the stimulation GAP-43 and Talpha1-tubulin expression is correlated with an increased regenerative capacity of axotomized RSNs.

Effects of sciatic nerve grafts on choline acetyltransferase and p75 expression in transected adult hypoglossal motoneurons

We previously reported that a permanent transection of adult rat sciatic and hypoglossal nerves resulted in distinct changes in the levels of both low-affinity nerve growth factor receptor (p75) and choline acetyltransferase in the corresponding motoneurons as determined by immunoreactivity. Permanent axotomy of hypoglossal motoneurons induced a progressive loss of choline acetyltransferase immunoreactivity and a persistent expression of p75 immunoreactivity, phenomena that were not observed in spinal motoneurons. These observations indicated that spinal and brainstem motoneurons respond to permanent axotomy with a differential immunoreactivity for p75 and choline acetyltransferase. Such differences could be ascribed to specific intrinsic properties of each population of motoneurons or, alternatively, to different factors present in the periphery (nerve stump or target muscle). The aim of the present study was to test these two possibilities by determining if a segment of sciatic nerve transplanted to a transected hypoglossal nerve may counteract or attenuate the loss of choline acetyltransferase immunoreactivity in injured hypoglossal motoneurons. In addition, as further parameter, we analysed the presence of p75 immunoreactivity. Prior to grafting, segments of sciatic nerve were prepared by one of three methods: (i) a fresh piece; (ii) a degenerated piece; and (iii) a heated piece. Seven and 30 days following the placement of grafts, hypoglossal motoneurons were analysed for choline acetyltransferase and p75 immunolabelling. The results revealed that viable sciatic grafts (fresh and degenerated) are able to partially attenuate the loss in the number of choline acetyltransferase-positive injured hypoglossal motoneurons, even if an important decrease in choline acetyltransferase still persists with respect to the contralateral nucleus. In addition, viable sciatic grafts decreased the number of p75 immunoreactive hypoglossal motoneurons both at seven and at 30 days. In conclusion, the effects of viable sciatic grafts on the number of choline acetyltransferase and p75-labelled hypoglossal motoneurons indicate that these adult neurons are able to respond to factors released from the sciatic nerve, and that the number of injured motoneurons positive for choline acetyltransferase and p75 can be influenced by the presence of factors that may reach their proximal stumps. Furthermore, we hypothesize that the differential expression patterns between hypoglossal and sciatic motoneurons may be due, at least in part, to factors released from the nerve trunks themselves.

Neurotrophin-3 mRNA expression in rat intrafusal muscle fibres after denervation and reinnervation

We have studied the regulation of the expression of neurotrophin-3 (NT-3) mRNA in neonatal and adult rat muscle spindles after denervation and after denervation followed by reinnervation. Denervation of the intrafusal fibres did not result in an upregulation of the NT-3 mRNA expression but decreased this expression below the detection limit of the in situ hybridization method. Reinnervation of intrafusal fibres restored the NT-3 mRNA expression. The results suggest that the expression of NT-3 mRNA in postnatal muscle spindles is controlled by neuronal factors. The intrafusal fibre derived NT-3 may act as an instructive, feedback messenger for innervating neurons and may play a role in stabilizing and maintaining functional neuromuscular connections.

A differential and time-dependent decrease in AMPA-type glutamate receptor subunits in spinal motoneurons after sciatic nerve injury

After sciatic transection a strong decrease in immunoreactivity occurred, starting at 2 days. After 6, 10, 14, and 20 days survival only 5% of the sciatic motoneurons were strongly labeled for GluR2/3 against 80% in the control situation. From Day 20, GluR2/3 labeling started to increase again, reaching near normal levels at Day 80 after sciatic transection. In contrast, after sciatic crush, the decrease in GluR2/3 labeling in motoneurons was less pronounced and returned to normal in 30 days. In all animals, the GluR1 and GluR4 labeling of motoneurons remained unchanged after sciatic transection or crush. It is concluded that sciatic nerve injury leads to a strong, time-dependent decrease in the expression of GluR2 and 3 subunits in the corresponding motoneurons. As a consequence, AMPA receptors with a different subunit composition may be assembled, leading to a change in the functional properties of these receptors. Moreover, if they lack the GluR2 subunit, they may become calcium permeable.

Regulation of quantal secretion by neurotrophic factors at developing motoneurons in Xenopus cell cultures

1. The ability of different neurotrophic factors to maintain and regulate synaptic function at the developing motoneuron was studied in Xenopus nerve-muscle co-cultures. Spontaneous synaptic currents (SSCs) were measured by using whole-cell voltage-clamped myocytes. 2. Compared with natural synapses, motoneurons without contact on a myocyte (naive neurons) released ACh in smaller quantal packets, the amplitude being inversely proportional to the days in culture. The mean SSC amplitudes of naive neurons, which were measured by manipulating a myoball into contact with the myocyte-free nerve terminals to form a manipulated synapse, were 99.5 +/- 6.7 and 48.2 +/- 1.9 pA for day-1 and day-3 cultures, respectively. 3. Chronic treatment of day-1 cultures with brain-derived neurotrophic factor (BDNF), neurotrophin-3 (NT-3), neurotrophin-4 (NT-4), ciliary neurotrophic factor (CNTF) or glial cell line-derived neurotrophic factor (GDNF) for 2 days, increased the ACh quantal size of naive motoneurons in a concentration-dependent manner, whereas insulin-like growth factor-1 (IGF-1) and basic fibroblast growth factor (bFGF) had no effect, even at high concentrations. 4. The interaction of various neurotrophic factors was examined, using concentrations that gave maximal effects. Combination of CNTF plus BDNF or CNTF plus NT-3 had synergistic effects in potentiating SSC amplitude of the manipulated synapse of naive neurons, whereas NT-3 plus BDNF, NT-3 plus GDNF, BDNF plus GDNF or CNTF plus GDNF had no synergistic action. 5. Chronic treatment with d-tubocurarine for 2 days resulted in a reduction of the quantal size of natural synapses. Concomitant treatment with BDNF, NT-3, GDNF, CNTF but not bFGF or IGF-1, reconstituted the SSC amplitude. 6. Taken together, these findings suggest that BDNF, NT-3, NT-4, CNTF and GDNF may regulate and maintain the synaptic function of developing motoneurons, and different neurotrophic factors utilizing distinct signalling mechanisms may have synergistic actions.

Brain-derived neurotrophic factor promotes axonal regeneration and long-term survival of adult rat spinal motoneurons in vivo

This study shows that in adult rat spinal motoneurons brain-derived neurotrophic factor exerts a neuroprotective effect which extends several weeks beyond the duration of treatment. In addition, brain-derived neurotrophic factor strongly enhances regeneration of avulsed motor axons across the border between the central and peripheral nervous systems. Treatment with brain-derived neurotrophic factor is known to rescue adult rat spinal motoneurons from retrograde cell death induced by ventral root avulsion. The present experiments were designed to test whether this survival effect remains over an extended period of time following cessation of treatment and, also, whether brain-derived neurotrophic factor promotes regeneration of avulsed motor axons. After avulsion of a spinal ventral root, four weeks of treatment with brain-derived neurotrophic factor (10 microg/day) or vehicle was initiated. By using different retrograde tracers to obtain pre- and postoperative labelling of avulsed and regenerating motoneurons, respectively, the number of surviving motoneurons as well as the extent of motor axonal regeneration could be analysed. The expression of nitric oxide synthase in the lesioned motoneurons was also studied. In the vehicle-treated rats, only 10% of the avulsed motoneurons remained at 12 weeks postoperatively, 20-40% of which displayed nitric oxide synthase activity. Treatment with brain-derived neurotrophic factor during the initial four postoperative weeks resulted in 45% motoneuron survival and a complete blockage of nitric oxide synthase expression at 12 weeks postoperatively. Brain-derived neurotrophic factor also induced abundant regeneration of the avulsed motor axons, which formed extensive fibre bundles along the surface of the spinal cord and adjacent ventral roots. The long-term effect by brain-derived neurotrophic factor seemed to be even stronger on motor axonal regeneration than on motoneuron survival. The present results indicate a therapeutic potential for brain-derived neurotrophic factor in the early treatment of traumatic injuries to spinal nerves and roots.

Adenoviral gene transfer of ciliary neurotrophic factor and brain-derived neurotrophic factor leads to long-term survival of axotomized motor neurons

The neurotrophic factors ciliary neurotrophic factor and brain-derived neurotrophic factor can prevent motor neuron cell death during development and after nerve lesion in neonatal rodents. However, local and systemic application of these factors to newborn rats with damaged motor nerves rescues motor neurons only transiently during the first two weeks after axotomy. In order to test the effect of continuous delivery of these factors, the effect of localized injection of CNTF- or BDNF-transducing recombinant adenoviruses into the lesioned nerves was investigated. Under such conditions, survival of axotomized motor neurons is maintained for at least 5 weeks. This way of delivery corresponds to the physiological situation in adult rodents, under which endogenous CNTF is present in the cytosol of Schwann cells and BDNF expression is upregulated after nerve lesion, making these factors available to the damaged motor neurons. Recent results show that overexpression of muscle-derived neurotrophin-3 prevents degeneration of axons and motor endplates, but has only little effect on the number of motor neuron cell bodies in a murine animal model of motor neuron disease. Therefore, techniques suitable for tonic exposure to both nerve- and muscle-derived neurotrophic factors may have implications for the design of future therapeutic strategies against human motor neuron disease.

TrkB signaling is required for postnatal survival of CNS neurons and protects hippocampal and motor neurons from axotomy-induced cell death

Newborn mice carrying targeted mutations in genes encoding neurotrophins or their signaling Trk receptors display severe neuronal deficits in the peripheral nervous system but not in the CNS. In this study, we show that trkB (-/-) mice have a significant increase in apoptotic cell death in different regions of the brain during early postnatal life. The most affected region in the brain is the dentate gyrus of the hippocampus, although elevated levels of pyknotic nuclei were also detected in cortical layers II and III and V and VI, the striatum, and the thalamus. Furthermore, axotomized hippocampal and motor neurons of trkB (-/-) mice have significantly lower survival rates than those of wild-type littermates. These results suggest that neurotrophin signaling through TrkB receptors plays a role in the survival of CNS neurons during postnatal development. Moreover, they indicate that TrkB receptor signaling protects subpopulations of CNS neurons from injury- and axotomy-induced cell death.

Activity-dependent expression of NT-3 in muscle cells in culture: implications in the development of neuromuscular junctions

Although activity-dependent expression of neurotrophins has been studied extensively in the CNS, its physiological role during synapse development is not well established. At the developing neuromuscular junction in culture, exogenous application of the neurotrophin BDNF or NT-3 has been shown to acutely potentiate synaptic transmission and chronically promote synapse maturation. Using the same cell culture model, we have investigated activity-dependent neurotrophin expression in muscle cells and its role in developing neuromuscular synapses. Membrane depolarization, elicited by either depolarizing agents or repetitive electric stimulation, rapidly and specifically increased the levels of NT-3 mRNA in developing Xenopus laevis muscle cells in culture. NT-3 gene expression also was enhanced by acetylcholine (ACh), the neurotransmitter that causes muscle membrane depolarization. The effects of depolarization were mediated by increasing intracellular calcium concentration. Moreover, factor(s) induced by membrane depolarization appeared to enhance synaptic transmission at the developing neuromuscular junction. The frequency of spontaneous synaptic currents (SSCs) recorded from neuromuscular synapses was increased significantly after treatment with conditioned medium from depolarized muscle cultures. The amplitude, rise time, and decay time of SSCs were not affected, indicating a presynaptic action of the conditioned medium. The effects of the conditioned medium were blocked, partially, by the NT-3 scavenger TrkC-IgG, suggesting that the potentiation of synaptic efficacy was attributable, at least in part, to elevated NT-3 as a consequence of muscle depolarization. Thus, activity-dependent expression of muscle NT-3 may contribute to the development of the neuromuscular synapse.

Nerve growth factor receptor TrkA is down-regulated during postnatal development by a subset of dorsal root ganglion neurons

Nerve growth factor (NGF), signaling through its receptor tyrosine kinase, TrkA, is required for the survival of all small and many intermediate-sized murine dorsal root ganglion (DRG) neurons during development, accounting for 80% of the total DRG population. Surprisingly, NGF/TrkA-dependent neurons include a large population that does not express TrkA in adult mice (Silos-Santiago et al., 1995). This finding suggests two hypotheses: Neurons lacking TrkA in the adult may express TrkA during development, or they may be maintained through a paracrine mechanism by TrkA-expressing neurons. To determine whether TrkA is expressed transiently by DRG neurons that lack the receptor in adulthood, we examined the distribution of TrkA protein during development. We show here that TrkA expression is strikingly developmentally regulated. Eighty percent of DRG neurons expressed TrkA during embryogenesis and early postnatal life, whereas only 43% expressed TrkA at postnatal day (P) 21. Because the period of TrkA down-regulation corresponds with a critical period during which nociceptive phenotype can be altered by NGF (see Lewin and Mendell [1993] Trends Neurosci. 16:353-359), we examined whether NGF modulates the down-regulation of TrkA. Surprisingly, neither NGF deprivation nor augmentation altered the extent of TrkA down-regulation. Our results demonstrate a novel form of regulation of neurotrophin receptor expression that occurs late in development. All DRG neurons that require NGF for survival express TrkA during embryogenesis, and many continue to express TrkA during a postnatal period when neuronal phenotype is regulated by NGF. The subsequent down-regulation of TrkA is likely to be importantly related to functional distinctions among nociceptive neurons in maturity.

Expression of neurotransmitter genes in rat spinal motoneurons after chemodenervation with botulinum toxin

Botulinum toxin is widely used for the treatment of focal movement disorders, where chemodenervation is used to decrease hyperactivity in selected muscles. Beside a focal paresis, widespread effects on neuromuscular synaptic function have been demonstrated. However, reactions of motoneurons after neuromuscular chemodenervation without gross morphological lesions are largely unknown. Peripheral axotomy, in contrast, leads to profound changes in the expression of several genes, including those encoding neurotransmitters, in motoneurons. We therefore examined the expression of neurotransmitter genes in rat motoneurons six days after intramuscular botulinum toxin application in the right gastrocnemius muscle. Similar doses of botulinum toxin as used in human where injected. A focal bilateral increase in expression of the choline acetyltransferase gene and a widespread bilateral increase of the beta-calcitonin-gene-related peptide and the enkephalin genes was measured in motoneurons after botulinum toxin injection. Cholecystokinin had a lower expression after botulinum toxin injections. Growth-associated protein 43, nitric oxide synthase, somatostatin and proopiomelanocortin messenger RNA were not found in motoneurons of both groups. Our results demonstrate that changes in the expression of neurotransmitter genes in motoneurons also occur after chemodenervation but with different patterns to those found after mechanical nerve lesioning. These changes reflect focal and widespread modulative events. The knowledge of these events should lead to a better understanding of the focal paralysis and of the more widespread effects found in human after intramuscular injection of botulinum toxin.

BDNF prevents and reverses adult rat motor neuron degeneration and induces axonal outgrowth

To assess the therapeutic potential of brain-derived neurotrophic factor (BDNF) in clinics, we extensively investigated the effects of BDNF on adult motor neurons in a rat spinal root avulsion model. Intrathecal administration of BDNF immediately after the spinal root avulsion greatly protected against the motor neuron cell death. BDNF also showed a protective effect on the atrophy of soma and on the reduction of transmitter-related enzymes such as choline acetyl transferase and acetylcholine esterase. Very interestingly, BDNF induced axonal outgrowth of severely damaged motor neurons at the avulsion site. The BDNF administration following 2-week treatment with phosphate-buffered saline after avulsion prevented further augmentation of cell death and reversed cholinergic transmitter-related enzyme deficiency. BDNF was demonstrated to possess a wide variety of biological effects on survival, soma size, cholinergic enzymes, and axonal outgrowth of adult motor neurons. These results provide a rationale for BDNF treatment in motor neuron diseases such as spinal cord injury and amyotrophic lateral sclerosis.

Reduced acetylcholinesterase (AChE) activity in adrenal medulla and loss of sympathetic preganglionic neurons in TrkA-deficient, but not TrkB-deficient, mice

TrkA high-affinity receptors are essential for the normal development of sympathetic paravertebral neurons and subpopulations of sensory neurons. Paravertebral sympathetic neurons and chromaffin cells of the adrenal medulla share an ontogenetic origin, responsiveness to NGF, and expression of TrkA. Which aspects of development of the adrenal medulla might be regulated via TrkA are unknown. In the present study we demonstrate that mice deficient for TrkA, but not the neurotrophin receptor TrkB, show an early postnatal progressive reduction of acetylcholinesterase (AChE) enzymatic activity in the adrenal medulla and in preganglionic sympathetic neurons within the thoracic spinal cord, which are also significantly reduced in number. Quantitative determinations of specific AChE activity revealed a massive decrease (-62%) in the adrenal gland and a lesser, but still pronounced, reduction in the thoracic spinal cord (-40%). Other markers of the adrenal medulla and its innervation, including various neuropeptides, chromogranin B, secretogranin II, amine transporters, the catecholamine-synthesizing enzymes tyrosine hydroxylase and PNMT, synaptophysin, and L1, essentially were unchanged. Interestingly, AChE immunoreactivity appeared unaltered, too. Preganglionic sympathetic neurons, in contrast to adrenal medullary cells, do not express TrkA. They must, therefore, be affected indirectly by the TrkA knock-out, possibly via a retrograde signal from chromaffin cells. Our results suggest that signaling via TrkA, but not TrkB, may be involved in the postnatal regulation of AChE activity in the adrenal medulla and its preganglionic nerves.

Brain-derived neurotrophic factor spares choline acetyltransferase mRNA following axotomy of motor neurons in vivo

Choline acetyltransferase (ChAT) is a functional and specific marker gene for neurons such as primary motor neurons that synthesize and release acetylcholine as a neurotransmitter. In adult mammals, transection of the peripheral nerve results in a loss of immunoreactivity for ChAT in the injured motor neurons without affecting their cell number. Using a quantitative RNase protection assay, we have investigated dynamic changes in ChAT mRNA levels following axotomy of motor neurons in the brainstem of adult rats. One week after transection of the left hypoglossal nerve, levels of ChAT mRNA in the ipsilateral side of the hypoglossal motor nucleus decreased dramatically to around 10% when compared to the uninjured contralateral side. When cut axons were chronically exposed to brain-derived neurotrophic factor (BDNF) for 1 week, ChAT mRNA levels were maintained at 63% of control levels. Thus, BDNF can abrogate the injury-induced loss of ChAT mRNA in mature motor neurons in vivo. In contrast, neither neurotrophin 4/5 nor nerve growth factor could prevent the decrease in message. This effect of BDNF on ChAT mRNA levels following peripheral injury to motor neurons demonstrates the existence of regulatory pathways responsive to neurotrophic factors that can "rescue" or "protect" cholinergic gene expression.

Modulation of motoneuron excitability by brain-derived neurotrophic factor

The influence of neurotrophins on motoneuron survival and development has been well documented in cell cultures and neonates. In the present study, the role of brain-derived neurotrophic factor (BDNF) in the maintenance of motoneuron electrical properties was investigated. In adult male rats, BDNF- or saline-saturated gelfoam was inserted between the medial and lateral heads of the gastrocnemius muscles. After 5 days survival, in vivo intracellular recordings were obtained, and motoneuron biophysical properties were measured. In BDNF-treated rats, significant decreases in mean rheobase and in total cell capacitance of medial gastrocnemius motoneurons were observed. In addition, a concommitant increase in input resistance and decrease in membrane time constant were noted in BDNF-treated rats but were not statistically significant. No significant treatment effect was observed in motoneuron conduction velocity, action potential amplitude, equalizing time constant, electrotonic length, afterhyperpolarization amplitude and duration, and membrane potential sag during current injection. The observed changes in motoneuron rheobase and total cell capacitance suggest that application of BDNF produces an increase in motoneuron excitability coincident with a reduction in size. These data are discussed with respect to the possible role of BDNF as a muscle-derived trophic factor for the regulation of motoneuron excitability.

A novel type of programmed neuronal death in the cervical spinal cord of the chick embryo

We examined the massive early cell death that occurs in the ventral horn of the cervical spinal cord of the chick embryo between embryonic days 4 and 5 (E4 and E5). Studies with immunohistochemical, in situ hybridization, and retrograde-tracing methods revealed that many dying cells express Islet proteins and Lim-3 mRNA (motoneuron markers) and send their axons to the somatic region of the embryo before cell death. Together, these data strongly suggest that the dying cells are somatic motoneurons. Cervical motoneurons die by apoptosis and can be rescued by treatment with cycloheximide and actinomycin D. Counts by motoneuron numbers between E3.5 and E10 revealed that, in addition to cell death between E4 and E5, motoneuron death also occur between E6 and E10 in the cervical cord. Studies with [3H]thymidine autoradiography and morphological techniques revealed that in the early cell-death phase (E4-E5), genesis of motoneurons, axonal elongation, and innervation of muscles is still ongoing. However, studies with [3H]thymidine autoradiography also revealed that the cells dying between E4 and E5 become postmitotic before E3.5. Increased size of peripheral targets, treatment with neuromuscular blockade, and treatment with partially purified muscle or brain extracts and defined neurotropic agents, such as NGF, BDNF, neurotrophin-3, CNTF, bFGF, PDGF, S100-beta, activin, cholinergic differentiation factor/leukemia inhibitory factor, bone morphogenetic protein-2, IGF-I, interleukin-6, and TGF-beta 1, were all ineffective in rescuing motoneurons dying between E4 and E5. By contrast, motoneurons that undergo programmed cell death at later stages (E6-E10) in the cervical cord are target-dependent and respond to activity blockade and trophic factors. Experimental approaches revealed that early cell death also occurs in a notochord-induced ectopic supernumerary motoneuron column in the cervical cord. Transplantation of the cervical neural tube to other segmental regions failed to alter the early death of motoneurons, whereas transplantation of other segments to the cervical region failed to induce early motoneuron death. These results suggest that the mechanisms that regulate motoneuron death in the cervical spinal cord between E4 and E5 are independent of interactions with targets. Rather, this novel type of cell death seems to be determined by signals that either are cell-autonomous or are derived from other cells within the cervical neural tube.

Retrograde transport of neurotrophins from the eye to the brain in chick embryos: roles of the p75NTR and trkB receptors

The receptors involved in retrograde transport of neurotrophins from the retina to the isthmo-optic nucleus (ION) of chick embryos were characterized using antibodies to the p75 neurotrophin receptor and trkB receptors. Survival of neurons in the ION has been shown previously to be regulated by target-derived trophic factors with survival promoted or inhibited by ocular injection of brain-derived neurotrophic factor (BDNF) or nerve growth factor (NGF), respectively. In the present paper, we show that during the period of target dependence, these neurons express trkB and p75 neurotrophin receptor but not trkA or trkC mRNAs. We also show that BDNF and NT-3 were transported efficiently at low doses, whereas NGF was transported significantly only at higher doses. The transport of BDNF and NT-3 was reduced by high concentrations of NGF or by antibodies to either trkB or the p75 neurotrophin receptor. Thus both receptors help mediate retrograde transport of these neurotrophins. Ocular injection of the comparatively specific trk inhibitor K252a did not reduce transport of exogenous BDNF, but did induce significant neuronal death in the ION, which could not be prevented by co-injection of BDNF. Thus, transport of BDNF alone does not generate a trophic signal at the cell body when axonal trkB is inactivated. In summary, our results indicate that both p75 neurotrophin and trkB receptors can mediate internalization and retrograde transport of BDNF, but activation of trkB seems to be essential for the survival-promoting actions of this neurotrophin.

Cryptic physiological trophic support of motoneurons by LIF revealed by double gene targeting of CNTF and LIF

BACKGROUND

The survival and differentiation of motoneurons during embryonic development, and the maintenance of their function in the postnatal phase, are regulated by a great variety of neurotrophic molecules which mediate their effects through different receptor systems. The multifactorial support of motoneurons represents a system of high security, because the inactivation of individual ligands has either no detectable, or relatively small, atrophic or degenerative effect on motoneurons.

RESULTS

Leukaemia inhibitory factor (LIF) has been demonstrated to support motoneuron survival in vitro and in vivo under different experimental conditions. However, when LIF was inactivated by gene targeting, there were no apparent changes in the number and structure of motoneurons and no impairment of their function. The slowly appearing, relatively mild degenerating effects in motoneurons that resulted from ciliary neurotrophic factor (CNTF) gene targeting were substantially potentiated by simultaneous inactivation of the LIF gene, however. Thus, in mice deficient in LIF and CNTF, the degenerative changes in motoneurons were more extensive and appeared earlier. These changes were also functionally reflected by a marked reduction in grip strength.

CONCLUSIONS

Degenerative disorders of the nervous system, in particular those of motoneurons, may be based on multifactorial inherited and/or acquired defects which individually do not result in degenerative disorders, but which become apparent when additional (cryptic) inherited disturbances or sub-threshold concentrations of noxious factors come into play. Accordingly, the inherited inactivation of the CNTF gene in a high proportion of the Japanese population may represent a predisposing factor for degenerative disorders of motoneurons.

Increased expression of BDNF and trkB mRNA in rat facial motoneurons after axotomy

Motoneurons of the adult survive after axotomy even though they are deprived of putative target derived trophic factors. Alternative sources of trophic support may substitute. In this study we test the hypothesis that the immediate environment of the motoneuronal cell body or the cell body itself increases the production of trophic factors after axonal injury. Using in situ hybridization (ISH) and reverse transcription-polymerase chain reaction (RT-PCR), we report that after axotomy, rat facial motoneurons increase the expression of mRNA for brain-derived neurotrophic factor (BDNF) and its receptor trkB. After transection of the facial nerve, we measured a 2- to 4-fold increase in BDNF mRNA expression which had its onset between 3 and 8 h after injury. The BDNF mRNA levels peaked at approximately 1-2 days and gradually declined thereafter to return to contralateral levels within 7 days of injury. Western blotting revealed a several-fold increase in BDNF as early as 24 h, which subsequently reached a maximum in approximately 5-7 days and was still sustained at 2 weeks post-axotomy. Using exon-specific primers, we determined that the increase in BDNF mRNA is largely due to an increased expression from the promoters of exons IV and III, and to a lesser extent from exons I and II. Analysing the mRNA expression for the BDNF receptor, trkB, we found a 2- to 3-fold increase in full-length trkB mRNA expression starting 2 days after axotomy which lasted 2-3 weeks. These findings suggest that BDNF might act locally on axotomized motoneurons in an autocrine fashion, providing support for axotomized motoneurons during the first weeks after axotomy.

Central infusions of brain-derived neurotrophic factor and neurotrophin-4/5, but not nerve growth factor and neurotrophin-3, prevent loss of the cholinergic phenotype in injured adult motor neurons

Neurotrophic factors are molecules that prevent neuronal degeneration and regulate neuronal phenotype during either development or adulthood. Relatively little is known about the comparative responsiveness of injured adult central nervous system motor neurons to various neurotrophic factors. In the present study we examined the effects of four members of the neurotrophin family on injured adult motor neurons. Nerve growth factor, brain-derived neurotrophic factor, neurotrophin-3 or neurotrophin-4/5 were infused intracerebroventricularly into adult rats following transection of the motor hypoglossal nerve. Two weeks after axotomy, brain-derived neurotrophic factor and neurotrophin-4/5 completely prevented the loss of the cholinergic phenotype in hypoglossal motor neurons (97 +/- 11% and 99 +/- 5%, respectively) as assessed by choline acetyltransferase immunolabeling. In contrast, nerve growth factor and neurotrophin-3 exerted no protective effect. The low-affinity p75 neurotrophin receptor, capable of binding all four neurotrophins, was re-expressed in injured hypoglossal neurons; the majority of injured hypoglossal neurons also express trkB receptors but not trkA or trkC receptors. Thus, injury-induced responses to neurotrophins in adult motor neurons are mediated by trk receptors and their agonists, but may or may not also require low-affinity p75 neurotrophin receptors. Intracerebroventricular infusions of trkB agonists may be a useful means of targeting multiple and distantly separated populations of motor neurons for neurotrophic factor therapy.

Neurotrophins increase motoneurons' ability to innervate skeletal muscle fibers in rat spinal cord--human muscle cocultures

Neurotrophins, nerve growth factor (NGF), neurotrophin-3 (NT-3), neurotrophin-5 (NT-5) and brain-derived neurotrophic factor (BDNF), were studied in vitro in a coculture model of human skeletal muscle myotubes and rat embryo spinal cord explants, which enables the different steps of functional innervation to be followed, including neurite outgrowth, synapse formation and induction of contractile activity. We found that NT-3, NT-5, BDNF, but not NGF simultaneously induced a significant increase in the number and length of neurites emerging from spinal cord explants, the number of endplates per muscle fiber, and the area of innervated muscle fibers around each spinal cord explant. These results suggest that neurotrophins NT-3, NT-5 and BDNF enhance spinal cord motoneurons potential of innervation.

Grafts of fibroblasts genetically modified to secrete NGF, BDNF, NT-3, or basic FGF elicit differential responses in the adult spinal cord

Neuronal and axonal responses to neurotrophic factors in the developing spinal cord have been relatively well characterized, but little is known about adult spinal responses to neurotrophic factors. We genetically modified primary rat fibroblasts to produce either nerve growth factor (NGF), brain-derived neurotrophic factor (BDNF), neurotrophin-3 (NT-3), or basic fibroblast growth factor (bFGF), then grafted these neurotrophic factor-secreting cells into the central gray matter of the spinal cord in adult rats. Spinal cord lesions were not made prior to grafting. From 2 wk to 6 mo later, sensory neurites of dorsal root origin extensively penetrated NGF-, NT-3-, and bFGF-producing grafts, whereas BDNF-secreting grafts elicited no growth responses. Putative noradrenergic neurites also penetrated NGF-secreting cell grafts. Local motor and corticospinal motor axons did not penetrate any of the neurotrophic factor-secreting grafts. These results indicate that unlesioned or minimally lesioned adult spinal cord sensory and putative noradrenergic populations retain significant neurotrophic factor responsiveness, whereas motor neurites are comparatively resistant even to those neurotrophic factors to which they exhibit survival dependence during development. Grafts of genetically modified cells can be a useful tool for characterizing neurotrophic factor responsiveness in the adult spinal cord and designing strategies to promote axonal regeneration after injury.

Neuroprotective effect of various cytokines on developing spinal motoneurons following axotomy

Ciliary neurotrophic factor (CNTF), a multipoietic factor, on a variety of neurons, prevents axotomy-induced motoneuron loss and can improve the outcome of murine motor neuron disease (MND). We carried out a study to determine whether other cytokines rescue spinal motoneurons from axotomy-induced cell death. Unilateral sciatic nerve was transected in neonatal rats. Two doses of recombinant murine cholinergic differentiation factor/leukemia inhibitory factor (CDF/LIF), recombinant rat CNTF, recombinant human granulocyte-colony stimulating factor (G-CSF), recombinant human interleukin-6 (IL-6), recombinant human tumor necrosis factor beta (TNF beta), or vehicle were administered daily for 2 weeks by intraperitoneal injection. After treatment, the number of spinal motoneurons was determined at the level of L4-5 segments. In comparison with vehicle, the higher doses of CDF/LIF, CNTF, and IL-6, and the lower doses of CDF/LIF and IL-6 significantly retarded the loss of motoneurons. G-CSF and TNF beta failed to inhibit motoneuron death. CDF/LIF and IL-6 rescued motoneurons from the retrograde death following axotomy, in a similar manner to CNTF. These results provide evidence that several cytokines may have therapeutic potential in human axonopathy or MND.

Axotomy induces a different modulation of both low-affinity nerve growth factor receptor and choline acetyltransferase between adult rat spinal and brainstem motoneurons

Adult rat spinal and brainstem motoneurons re-express low-affinity nerve growth factor receptor (p75) after their axotomy. We have previously reported and quantified the time course of this reexpression in spinal motoneurons following several types of injuries of the sciatic nerve. Other studies reported the reexpression of p75 in axotomized brainstem motoneurons. Results of these previous studies differed regarding the type of the most effective triggering injury for p75 reexpression, the relative duration of this reexpression and the decrease of choline acetyltransferase (ChAT) immunoreactivity (-IR) following a permanent axotomy of spinal or brainstem motoneurons. These differences suggest that these two populations of motoneurons respond to axotomy with a different modulation of p75 and ChAT expression. The aim of the present study was to determine whether differential modulation exists. We have analyzed and quantified the presence of p75- and ChAT-IR motoneurons in the hypoglossal nucleus following the same types of injury and the same time course we previously used for sciatic motoneurons. The results show that a nerve crush is the most effective triggering injury for p75 and that it induces similar temporal patterns of p75 and ChAT expression for sciatic and hypoglossal motoneurons. In contrast, a cut injury of the sciatic and hypoglossal nerves resulted in distinct temporal courses of both p75 and ChAT expression between these two populations of motoneurons. In fact, a permanent axotomy of the hypoglossal motoneurons induced i) a much longer maintenance phase for p75 than in sciatic motoneurons and ii) a progressive loss of ChAT-IR with a successive return to normal values in contrast to the modest decrease in the sciatic motoneurons. This evidence indicates that spinal and brainstem motoneurons respond to a permanent axotomy with a different modulation of p75 and ChAT expression. Altogether, the present data and the reported evidence of a differential post-axotomy cell death support the hypothesis that these two populations of motoneurons undergo different dynamic changes after axotomy.

The neurotrophins BDNF, NT-3 and -4, but not NGF, TGF-beta 1 and GDNF, increase the number of NADPH-diaphorase-reactive neurons in rat spinal cord cultures

Neurotrophins have multiple functions for the development of the nervous system. They can promote survival and differentiation of select neuronal populations, but have also been shown to play instructive roles in the determination of the transmitter phenotype of neurons. We have investigated the influence of neurotrophins on the expression of nicotinamide adenine dinucleotide phosphate-diaphorase (NADPH-d), a histochemical marker for nitric oxide synthase, in spinal cord cultures established from 16-day-old rat embryos. At this embryonic age we found NADPH-d reactivity becoming apparent in the spinal cord and predominantly expressed in preganglionic autonomic nuclei. Numbers of NADPH-d-positive neurons in spinal cord cultures were very low 24 h after plating. They did not change significantly until day 4 in vitro. However, treatment with the neurotrophins BDNF, NT-3 or NT-4 significantly increased their numbers. The effect became apparent after just 24 h, and was significant with concentrations as low as 1 ng/ml. Treatment with BDNF, NT-3 and NT-4 also augmented numbers of NADPH-d-positive neurons when initiated after three or five days in culture, and became consistently apparent within 24 h. This suggests that the neurotrophin-mediated increase in NADPH-d-positive neurons is unlikely to be due to promotion of neuron survival. NGF and two members of the transforming growth factor-beta superfamily, which have pronounced trophic effects on select neuron populations in vitro, TGF-beta 1 and GDNF, were not effective. Combined application of NT-4 and NT-3 had no additive effect. Our data therefore suggest that neurotrophins are involved in the developmental regulation of NADPH-d activity in neuron populations of the spinal cord. Neuron populations affected may include preganglionic autonomic neurons. NADPH-d activity may be induced in neurons expressing the enzyme constitutively, yet at undetectable levels, or may be induced de novo.

Expression of neurotrophins in skeletal muscle: quantitative comparison and significance for motoneuron survival and maintenance of function

Neurotrophins play a crucial role in the regulation of survival and maintenance of specific functions of various populations of neurons. Brain-derived neurotrophic factor (BDNF), neurotrophin-3 (NT-3) and neurotrophin 4/5 (NT-4) have been shown to support motoneuron survival during embryonic development and, after birth, to protect motoneurons from degeneration after nerve lesion. We have compared the levels of these neurotrophins in skeletal muscle by quantitative Northern blot analysis, both during embryonic development and postnatally. We localized the sites of expression of these neurotrophins by in situ hybridisation and analysed the expression of trkB in the spinal cord by in situ hybridisation and immunohistochemistry. NT-3 is most abundantly expressed both during embryonic development and in the postnatal phase, followed by NT-4. The levels of BDNF are very low, in particular after birth. After nerve lesion, NT-3 mRNA essentially remained unchanged, whereas NT-4 mRNA rapidly decreased. The slow increase in BDNF expression seems to be essentially due to the expression in Schwann cells rather than skeletal muscle, demonstrated by in situ hybridisation. Our data indicate that motoneurons can receive trophic support from several members of the neurotrophin gene family during the period of naturally occurring cell death. Postnatally, the predominant ligand acting via trkB on motoneurons is NT-4, whereas BDNF expression seems to play a role mainly after nerve lesion.

Differential regulation of motor neuron survival and choline acetyltransferase expression following axotomy

Although it is well known that motor neuron survival following axotomy is enhanced with maturation, the ability of surviving neurons to express the cholinergic enzyme choline acetyltransferase (ChAT) following axotomy has not ben closely examined. Moreover, the utility of the facial nucleus in studies of motoneuron response to injury and to trophic factors, coupled with the increasing importance of the mouse in gene targeting, compelled us to investigate the age dependence of neuronal survival and ChAT expression in the mouse facial nucleus following axotomy. We cut the facial nerve at postnatal day (P) 4, 7, 14, 21, and 28 or in the adult and used Nissl staining and ChAT immunocytochemistry to quantitate survival and ChAT expression, respectively, following 1, 2, or 3 weeks' survival at each age. We confirm in this model that the rate and extent of motor neuron death following axotomy is reduced with increasing maturity. The surviving neurons maintain a high ChAT content through P21; however, axotomy from P28 through adulthood results in a striking reduction in ChAT immunoreactivity. That is, although axotomy at P21 results in 61% motor neuron survival, with virtually all of the surviving neurons being ChAT positive, axotomy in the adult results in 72% survival but only 9% of the neurons are ChAT positive. Thus, surviving motor neurons in the adult animals are only weakly cholinergic. These results indicate that a change in the regulation of ChAT expression occurs following P21 so that cell survival and enzyme levels are uncoupled. We suggest that the putative factor or factors that enhances motor neuron survival in maturity is not capable of maintaining ChAT expression.

Fibroblast growth factors regulate calcitonin gene-related peptide mRNA expression in rat motoneurons after lesion and in culture

In this study, we have investigated the effect of fibroblast growth factors (bFGF and FGF-5) and brain derived neurotrophic factor (BDNF) on the expression of calcitonin gene-related peptide (CGRP) in rat motoneurons in vivo and in vitro. Following sciatic nerve transection in adult rats, the levels of alpha-CGRP and beta-CGRP mRNA were up- and down-regulated respectively in axotomized motoneurons, revealed by in situ hybridization histochemistry. Local administration of 1 microgram bFGF was able to entirely abolish the up-regulation of alpha-CGRP mRNA, and to further down-regulate beta-CGRP. These effects, albeit less pronounced, were still evident with 0.2 micrograms bFGF. In contrast, bFGF did not attenuate the lesion-induced decrease of choline acetyltransferase (ChAT) mRNA. Administration of BDNF did not significantly alter the expression of CGRP or ChAT mRNA in axotomized motoneurons. Both alpha- and beta-CGRP mRNAs could be detected by PCR in enriched motoneuron cultures prepared from rat embryos at embryonic day 14-15. Comparing the amplification of alpha- and beta-CGRP mRNAs with that of mRNA encoding glyceraldehyde-3-phosphate dehydrogenase (GAPDH) in parallel samples, we found that cultures treated with FGF-5 had a lower ratio of alpha- and beta-CGRP mRNA to GAPDH mRNA, than did control or BDNF-treated cultures. BDNF, on the other hand increased alpha-CGRP and decreased beta-CGRP mRNA levels, though these effects were moderate compared with the effects of FGF-5. The results obtained in this study suggest that members of the FGF family of growth factors influence the expression of CGRP in rat motoneurons, and that the increase of this neuropeptide induced by axotomy may, at least in part, be due to deprivation of these target-derived factors.

Mesencephalic dopaminergic neurons protected by GDNF from axotomy-induced degeneration in the adult brain

Glial-cell-line-derived neurotrophic factor (GDNF) promotes survival of embryonic dopaminergic neurons in culture, and its expression pattern suggests a role as a transient target-derived trophic factor for dopaminergic neurons of the substantia nigra. These neurons participate in the control of motor activity, emotional status and cognition, and they degenerate in Parkinson's disease for unknown reasons. To test whether GDNF has a trophic effect on dopaminergic neurons in the adult brain, we used a rat model in which these neurons are induced to degenerate by transecting their axons within the medial forebrain bundle. We report here that axotomy resulted in loss of half the tyrosine hydroxylase-expressing neurons in the substantia nigra. This loss was largely prevented by repeated injections of GDNF adjacent to the substantia nigra. Our findings suggest that GDNF or related molecules may be useful for the treatment of Parkinson's disease.

In vivo neurotrophic effects of GDNF on neonatal and adult facial motor neurons

Motor neurons require neurotrophic factor(s) for their survival during development and for maintenance of function in adulthood. In vivo studies have shown that motor neurons respond to a variety of molecules, including ciliary neurotrophic factor, members of the neurotrophin family, and the insulin growth factor IGF-1 (refs 3-13). Here we investigate the potential motor neuron neurotrophic effects of glial-cell-line-derived neurotrophic factor (GDNF), initially identified as a neurotrophic factor for substantia nigra dopaminergic neurons. We find that GDNF is retrogradely transported, in a receptor-mediated fashion, by spinal cord motor neurons in neonatal rats. Local application of GDNF to the transected facial nerve prevents the massive motor neuron cell death and atrophy that normally follows axotomy in the neonatal period. In adult rats, GDNF administered locally or systemically can markedly attenuate the lesion-induced decrease of choline acetyltransferase immunoreactivity in the facial nucleus. Our data indicate that GDNF has very profound neurotrophic effects in vivo on developing as well as on adult motor neurons, and is the most potent motor neuron trophic factor found so far.

Neurotrophin receptor mRNA expression defines distinct populations of neurons in rat dorsal root ganglia

The biological actions of neurotrophins are mediated by specific neurotrophin receptor tyrosine kinases (Trks). A low-affinity nerve growth factor (NGF) receptor, p75, appears to modulate sensitivity to neurotrophins in some neuronal populations. It has been recently demonstrated that genes encoding members of the Trk family are expressed in distinct patterns in the dorsal root ganglia (DRG; Mu et al. [1993] (J. Neurosci. 13:4029- 4041). However, the extent to which different neurotrophin receptor genes are coexpressed by individual DRG neurons is unknown. The question of coexpression is important since the expression of more than one member of the trk family by DRG neurons would suggest the potential for regulation by multiple neurotrophins. To address this question, a combination of isotopic and colorimetric in situ hybridization was performed on rat thoracic DRG using riboprobes specific for trkA, trkB, trkC, and p75. We show here that neurons that express trkA are largely distinct from those that express trkC, although there is a small subpopulation that expresses both of these genes. We also show that there is a distinct population of DRG neurons that expresses trkB and does not coexpress either trkA or trkC. P75 is expressed in almost all neurons that express trkA or trkB, but is coexpressed in only 50% of trkC-expressing neurons. Importantly, p75 is not expressed in DRG neurons independent of trk expression. Finally, a subpopulation of DRG neurons does not express any of the neurotrophin receptor mRNAs. Our results demonstrate that there are distinct populations of DRG neurons that express each member of the neurotrophin receptor tyrosine kinase family. Our findings of extensive colocalization of p75 with trkA and trkB lend support to the idea that p75 is important in mediating the actions of NGF and brain-derived neurotrophic factor on DRG neurons. Interestingly, however, p75 expression is clearly unimportant for a subpopulation of neurons that require neurotrophin-3. The fact that p75 is not expressed in the absence of trkA, trkB, or trkC suggests that the function of p75 is closely related to functions of the known neurotrophin-receptor tyrosine kinases. Finally, our results suggest that a significant percentage of DRG neurons may be regulated by non-neurotrophin neuronal growth factors.

Quantitative comparison of the transient rescue effects of neurotrophic factors on axotomized motoneurons in vivo

A reproducible neuronal degeneration induced by nerve lesion in neonatal rats or mice provides a convenient in vivo assay for testing the survival-promoting activity of putative growth factors on motoneurons. The goal of this study was to compare the rescue effects of the four known neurotrophins [nerve growth factor (NGF), brain-derived neurotrophic factor (BDNF), neurotrophin-3 (NT-3) and neurotrophin-4 (NT-4)] and two of the cytokines [ciliary neurotrophic factor (CNTF) and leukaemia inhibitory factor (LIF)] in one particular experimental model of spinal motoneuron degeneration at two different survival times. The sciatic nerve was cut in neonatal rats and the factors were applied onto the nerve stump; bovine serum albumin was used in controls. Simultaneous application of the retrograde tracer fluoro-gold made it possible to count motoneurons specifically in the sciatic pool. One week after lesion, the neurotrophins BDNF, NT-3 and NT-4, but not NGF, equally enhanced motoneuron survival compared to controls; their effects were significantly better than those of the cytokines. However, the rescue from cell death was only transitory because a great number of the motoneurons died during the second week after nerve lesion. Additional BDNF and/or CNTF supplied by repeated subcutaneous injections (1 mg/ml) over 2 weeks could not prevent this delayed motoneuron loss. These results suggest that still other factors or alternative routes of administration may be required for permanent rescue of the lesioned immature motoneurons.

Trophic factors

The various neurotrophic factors influence a wide range of cell functions in the developing, mature and injured nervous system. Recent studies have provided valuable insights on the receptors that mediate these effects and on the intracellular events that follow the binding of the ligand. Although growth factors were known to be expressed by non-neuronal cells in the targets and pathways of neuronal projections, it is now clear that the neurons themselves can also be a source of these molecules. A better understanding of the mechanisms of action of trophic factors on the survival and differentiation of neurons, coupled with advances in methods for the delivery of these molecules to the nervous system have provided an impetus for exploring their use as aids to the protection and regeneration of the injured nervous system.

Neurotrophic agents prevent motoneuron death following sciatic nerve section in the neonatal mouse

We have examined the ability of different neurotrophic and growth factors to prevent axotomy-induced motoneuron cell death in the developing mouse spinal cord. After postnatal unilateral section of the mouse sciatic nerve, most motoneuron (MN) loss occurs in the lateral motor column of the fourth lumbar segment (L4). Significant axotomy-induced cell death occurred after surgery performed on or before postnatal day (PN) 5. In contrast, no significant cell loss was found when axotomy was performed after PN10. Axotomy on PN2 or PN5 resulted in a 44% loss of L4 motoneurons by 7 days, and a 66% loss of motoneurons by 10 days postsurgery. Implantation of gelfoam presoaked in various neurotrophic factors at the lesion site rescued axotomized motoneurons. Nerve growth factor (NGF), neurotrophin-4/5 (NT-4/5) and ciliary neurotrophic factor (CNTF) rescued 20%-30% of motoneurons, whereas brain-derived neurotrophic factor (BDNF), neurotrophin-3 (NT-3), and insulin-like growth factor 1 (IGF-1) rescued virtually all motoneurons from axotomy-induced death. By contrast, platelet-derived growth factor (PDGF)-AA, PDGF-AB, basic fibroblast growth factor (bFGF), and interleukin (IL-6) were ineffective on motoneuron survival following axotomy. NGF, BDNF, NT-3, IGF-1, and CNTF also prevented axotomy-induced atrophy of surviving motoneurons. These data show that mouse lumbar motoneurons continue to be vulnerable to axotomy up to about 1 week after birth and that a number of trophic agents, including the neurotrophins, CNTF, and IGF-1, can prevent the death of these neurons following axotomy.(ABSTRACT TRUNCATED AT 250 WORDS)

Positive and negative effects of neurotrophins on the isthmo-optic nucleus in chick embryos

The survival of neurons in the developing isthmo-optic nucleus (ION) is believed to depend on the retrograde transport of trophic molecules from the target, the contralateral retina. We now show that ION neurons transport nerve growth factor (NGF), brain-derived neurotrophic factor (BDNF), and neurotrophin-3 (NT-3) retrogradely and that BDNF and NT-3 support the survival of ION neurons in vivo and promote neurite outgrowth in vitro. Surprisingly, NGF enhanced normal developmental cell death in vivo in a dose-dependent way. These findings show that increased levels of NGF can have adverse effects on differentiated neurons. The negative effect of NGF could be mimicked by intraocular injection of antibodies that block binding of neurotrophins to the 75 kd neurotrophin receptor (p75). These data implicate a role for the p75 receptor in NGF's neurotoxicity and indicate that this receptor is involved in the mechanism by which ION neurons respond to BDNF and NT-3 in the target.

Naturally occurring and axotomy-induced motoneuron death and its prevention by neurotrophic agents: a comparison between chick and mouse

Neuronal cell death is an important regressive event during the normal development of the peripheral and central nervous systems of many vertebrate and invertebrate species. Furthermore, when neurons are deprived of their target following axonal injury (axotomy) during embryonic, fetal, or early postnatal development, they undergo massive cell death. Both naturally occurring and axotomy-induced neuronal cell death can be prevented by treatment with growth factors or neurotrophic agents. Naturally occurring cell death of spinal MNs has been extensively studied in both avians and mammals. However, compared with mammals, there is little information on the effects of axotomy in avian species and it is not known whether trophic agents can modify axotomy-induced death in avian MNs. It is also not known whether trophic/growth factors can promote the in vivo survival of mammalian MNs during the period of naturally occurring cell death. We have examined (1) the time course of axotomy-induced death of lumbar spinal MNs in chick and mouse, and (2) the survival-promoting activity of a number of previously characterized growth and trophic factors on both programmed and axotomy-induced MN death in these two species. We show that axotomy performed on, or prior to, E12 in the chick results in a rapid decrease (i.e. 50%) in MN numbers within 3-4 days postsurgery, whereas these cells were able to survive for up to 1 week following axotomy on E14. By contrast, mouse MNs remained vulnerable to axotomy for at least 5 days after birth.(ABSTRACT TRUNCATED AT 250 WORDS)