Evidence that spinal interneurons undergo programmed cell death postnatally in the rat

Cleaved caspase-3 is present in the majority of glial cells in the intact rat spinal cord during postnatal life

Cell death is an essential process that occurs during the development of the central nervous system. Despite the availability of a wide range of commercially produced antibodies against various apoptotic markers, data regarding apoptosis in intact spinal cord during postnatal development and adulthood are mostly missing. We investigated apoptosis in rat spinal cord at different stages of ontogenesis (postnatal days 8, 29, and 90). For this purpose, we applied immunofluorescent detection of two widely used apoptotic markers, cleaved caspase-3 (cC3) and cleaved poly(ADP-ribose) polymerase (cPARP). Surprisingly, we found significant discrepancy between the number of cC3+ cells and PARP+ cells, with a ratio between 500:1 and 5000:1 in rat spinal cord at all postnatal time points. The majority of cC3+ cells were glial cells and did not exhibit an apoptotic phenotype. In contrast with in vivo results, in vitro analysis of primary cell cultures derived from neonatal rat spinal cord and treated with the apoptotic inductor staurosporine revealed a similar onset of occurrence of both cC3 and cPARP in cells subjected to apoptosis. Gene expression analysis of spinal cord revealed elevated expression of the Birc4 (XIAP), Birc2, and Birc5 (Survivin) genes, which are known potent inhibitors of apoptosis. Our data indicate that cC3 is not an exclusive marker of apoptosis, especially in glial cells, owing its possible presence in inhibited forms and/or its participation in other non-apoptotic roles. Therefore, cPARP appears to be a more appropriate marker to detect apoptosis.

A Model for Self-Organization of Sensorimotor Function: Spinal Interneuronal Integration

Control of musculoskeletal systems depends on integration of voluntary commands and somatosensory feedback in the complex neural circuits of the spinal cord. Particular connectivity patterns have been identified experimentally, and it has been suggested that these may result from the wide variety of transcriptional types that have been observed in spinal interneurons. We ask instead whether the details of these connectivity patterns (and perhaps many others) can arise as a consequence of Hebbian adaptation during early development. We constructed an anatomically simplified model plant system with realistic muscles and sensors and connected it to a recurrent, random neuronal network consisting of both excitatory and inhibitory neurons endowed with Hebbian learning rules. We then generated a wide set of randomized muscle twitches typical of those described during fetal development and allowed the network to learn. Multiple simulations consistently resulted in diverse and stable patterns of activity and connectivity that included subsets of the interneurons that were similar to 'archetypical' interneurons described in the literature. We also found that such learning led to an increased degree of cooperativity between interneurons when performing larger limb movements on which it had not been trained. Hebbian learning gives rise to rich sets of diverse interneurons whose connectivity reflects the mechanical properties of the plant. At least some of the transcriptomic diversity may reflect the effects of this process rather than the cause of the connectivity. Such a learning process seems better suited to respond to the musculoskeletal mutations that underlie the evolution of new species.

Evaluation of neurotoxicity and long-term function and behavior following intrathecal 1 % 2-chloroprocaine in juvenile rats

Spinally-administered local anesthetics provide effective perioperative anesthesia and/or analgesia for children of all ages. New preparations and drugs require preclinical safety testing in developmental models. We evaluated age-dependent efficacy and safety following 1 % preservative-free 2-chloroprocaine (2-CP) in juvenile Sprague-Dawley rats. Percutaneous lumbar intrathecal 2-CP was administered at postnatal day (P)7, 14 or 21. Mechanical withdrawal threshold pre- and post-injection evaluated the degree and duration of sensory block, compared to intrathecal saline and naive controls. Tissue analyses one- or seven-days following injection included histopathology of spinal cord, cauda equina and brain sections, and quantification of neuronal apoptosis and glial reactivity in lumbar spinal cord. Following intrathecal 2-CP or saline at P7, outcomes assessed between P30 and P72 included: spinal reflex sensitivity (hindlimb thermal latency, mechanical threshold); social approach (novel rat versus object); locomotor activity and anxiety (open field with brightly-lit center); exploratory behavior (rearings, holepoking); sensorimotor gating (acoustic startle, prepulse inhibition); and learning (Morris Water Maze). Maximum tolerated doses of intrathecal 2-CP varied with age (1.0 μL/g at P7, 0.75 μL/g at P14, 0.5 μL/g at P21) and produced motor and sensory block for 10-15 min. Tissue analyses found no significant differences across intrathecal 2-CP, saline or naïve groups. Adult behavioral measures showed expected sex-dependent differences, that did not differ between 2-CP and saline groups. Single maximum tolerated in vivo doses of intrathecal 2-CP produced reversible spinal anesthesia in juvenile rodents without detectable evidence of developmental neurotoxicity. Current results cannot be extrapolated to repeated dosing or prolonged infusion.

Development of putative inhibitory neurons in the embryonic and postnatal mouse superficial spinal dorsal horn

The superficial spinal dorsal horn is the first relay station of pain processing. It is also widely accepted that spinal synaptic processing to control the modality and intensity of pain signals transmitted to higher brain centers is primarily defined by inhibitory neurons in the superficial spinal dorsal horn. Earlier studies suggest that the construction of pain processing spinal neural circuits including the GABAergic components should be completed by birth, although major chemical refinements may occur postnatally. Because of their utmost importance in pain processing, we intended to provide a detailed knowledge concerning the development of GABAergic neurons in the superficial spinal dorsal horn, which is now missing from the literature. Thus, we studied the developmental changes in the distribution of neurons expressing GABAergic markers like Pax2, GAD65 and GAD67 in the superficial spinal dorsal horn of wild type as well as GAD65-GFP and GAD67-GFP transgenic mice from embryonic day 11.5 (E11.5) till postnatal day 14 (P14). We found that GABAergic neurons populate the superficial spinal dorsal horn from the beginning of its delineation at E14.5. We also showed that the numbers of GABAergic neurons in the superficial spinal dorsal horn continuously increase till E17.5, but there is a prominent decline in their numbers during the first two postnatal weeks. Our results indicate that the developmental process leading to the delineation of the inhibitory and excitatory cellular assemblies of pain processing neural circuits in the superficial spinal dorsal horn of mice is not completed by birth, but it continues postnatally.

Thioredoxin-2 Modulates Neuronal Programmed Cell Death in the Embryonic Chick Spinal Cord in Basal and Target-Deprived Conditions

Thioredoxin-2 (Trx2) is a mitochondrial protein using a dithiol active site to reduce protein disulfides. In addition to the cytoprotective function of this enzyme, several studies have highlighted the implication of Trx2 in cellular signaling events. In particular, growing evidence points to such roles of redox enzymes in developmental processes taking place in the central nervous system. Here, we investigate the potential implication of Trx2 in embryonic development of chick spinal cord. To this end, we first studied the distribution of the enzyme in this tissue and report strong expression of Trx2 in chick embryo post-mitotic neurons at E4.5 and in motor neurons at E6.5. Using in ovo electroporation, we go on to highlight a cytoprotective effect of Trx2 on the programmed cell death (PCD) of neurons during spinal cord development and in a novel cultured spinal cord explant model. These findings suggest an implication of Trx2 in the modulation of developmental PCD of neurons during embryonic development of the spinal cord, possibly through redox regulation mechanisms.

Mechanisms involved in spinal cord central synapse loss in a mouse model of spinal muscular atrophy

Motoneuron (MN) cell death is the histopathologic hallmark of spinal muscular atrophy (SMA), although MN loss seems to be a late event. Conversely, disruption of afferent synapses on MNs has been shown to occur early in SMA. Using a mouse model of severe SMA (SMNΔ7), we examined the mechanisms involved in impairment of central synapses. We found that MNs underwent progressive degeneration in the course of SMA, with MN loss still occurring at late stages. Loss of afferent inputs to SMA MNs was detected at embryonic stages, long before MN death. Reactive microgliosis and astrogliosis were present in the spinal cord of diseased animals after the onset of MN loss. Ultrastructural observations indicate that dendrites and microglia phagocytose adjacent degenerating presynaptic terminals. Neuronal nitric oxide synthase was upregulated in SMNΔ7 MNs, and there was an increase in phosphorylated myosin light chain expression in synaptic afferents on MNs; these observations implicate nitric oxide in MN deafferentation and suggest that the RhoA/ROCK pathway is activated. Together, our observations suggest that the earliest change occurring in SMNΔ7 mice is the loss of excitatory glutamatergic synaptic inputs to MNs; reduced excitability may enhance their vulnerability to degeneration and death.

Evaluation of spinal toxicity and long-term spinal reflex function after intrathecal levobupivaciane in the neonatal rat

BACKGROUND

Neuraxial anesthesia is utilized in children of all ages. Local anesthetics produce dose-dependent toxicity in certain adult models, but the developing spinal cord may also be susceptible to drug-induced apoptosis. In postnatal rodents, we examined the effects of intrathecal levobupivacaine on neuropathology and long-term sensorimotor outcomes.

METHODS

Postnatal day 3 (P3) or P7 rat pups received intrathecal levobupivacaine 2.5 mg/kg (0.5%) or saline. Mechanical withdrawal thresholds and motor block were assessed. Spinal cord tissue analysis included apoptosis counts (activated caspase-3, Fluoro-Jade C) at 24 h, glial reactivity at 7 days, and histopathology in cord and cauda equina at 24 h and 7 days. Long-term spinal function in young adults (P35) was assessed by hind limb withdrawal thresholds, electromyography responses to suprathreshold stimuli, and gait analysis.

RESULTS

Intrathecal levobupivacaine produced spinal anesthesia at P3 and P7. No increase in apoptosis or histopathological change was seen in the cord or cauda equina. In the P3 saline group, activated caspase-3 (mean±SEM per lumbar cord section 6.1±0.3) and Fluoro-Jade C (12.1±1.2) counts were higher than at P7, but were not altered by levobupivacaine (P=0.62 and P=0.11, two-tailed Mann-Whitney test). At P35, mechanical withdrawal thresholds, thermal withdrawal latency, and electromyographic reflex responses did not differ across P3 or P7 levobupivacaine or saline groups (one way ANOVA with Bonferroni comparisons). Intrathecal bupivacaine at P3 did not alter gait.

CONCLUSION

Single dose intrathecal levobupivacaine 0.5% did not increase apoptosis or produce spinal toxicity in neonatal rat pups. This study provides preclinical safety data relevant to neonatal use of neuraxial local anesthesia.

Neuraxial analgesia in neonates and infants: a review of clinical and preclinical strategies for the development of safety and efficacy data

Neuraxial drugs provide robust pain control, have the potential to improve outcomes, and are an important component of the perioperative care of children. Opioids or clonidine improves analgesia when added to perioperative epidural infusions; analgesia is significantly prolonged by the addition of clonidine, ketamine, neostigmine, or tramadol to single-shot caudal injections of local anesthetic; and neonatal intrathecal anesthesia/analgesia is increasing in some centers. However, it is difficult to determine the relative risk-benefit of different techniques and drugs without detailed and sensitive data related to analgesia requirements, side effects, and follow-up. Current data related to benefits and complications in neonates and infants are summarized, but variability in current neuraxial drug use reflects the relative lack of high-quality evidence. Recent preclinical reports of adverse effects of general anesthetics on the developing brain have increased awareness of the potential benefit of neuraxial anesthesia/analgesia to avoid or reduce general anesthetic dose requirements. However, the developing spinal cord is also vulnerable to drug-related toxicity, and although there are well-established preclinical models and criteria for assessing spinal cord toxicity in adult animals, until recently there had been no systematic evaluation during early life. Therefore, in the second half of this review, we present preclinical data evaluating age-dependent changes in the pharmacodynamic response to different spinal analgesics, and recent studies evaluating spinal toxicity in specific developmental models. Finally, we advocate use of neuraxial drugs with the widest demonstrable safety margin and suggest minimum standards for preclinical evaluation before adoption of new analgesics or preparations into routine clinical practice.

Postnatal developmental profile of neurons and glia in motor nuclei of the brainstem and spinal cord, and its comparison with organotypic slice cultures

In vitro preparations of the neonatal rat spinal cord or brainstem are useful to investigate the organization of motor networks and their dysfunction in neurological disease models. Long-term spinal cord organotypic cultures can extend our understanding of such pathophysiological processes over longer times. It is, however, surprising that detailed descriptions of the type (and number) of neurons and glia in such preparations are currently unavailable to evaluate cell-selectivity of experimental damage. The focus of the present immunohistochemical study is the novel characterization of the cell population in the lumbar locomotor region of the rat spinal cord and in the brainstem motor nucleus hypoglossus at 0-4 postnatal days, and its comparison with spinal organotypic cultures at 2-22 days in vitro. In the nucleus hypoglossus, neurons were 40% of all cells and 80% of these were motoneurons. Astrocytes (35% of total cells) were the main glial cells, while microglia was <10%. In the spinal gray matter, the highest neuronal density was in the dorsal horn (>80%) and the lowest in the ventral horn (≤57%) with inverse astroglia numbers and few microglia. The number of neurons (including motoneurons) and astrocytes was stable after birth. Like in the spinal cord, motoneurons in organotypic spinal culture were <10% of ventral horn cells, with neurons <40%, and the rest made up by glia. The present report indicates a comparable degree of neuronal and glial maturation in brainstem and spinal motor nuclei, and that this condition is also observed in 3-week-old organotypic cultures.

Intrathecal clonidine in the neonatal rat: dose-dependent analgesia and evaluation of spinal apoptosis and toxicity

BACKGROUND

Neuraxial clonidine is used for perioperative analgesia in children of all ages. Preclinical studies in the postnatal rat allow comparison of the relative toxicity and safety of spinal analgesics throughout postnatal development.

METHODS

Rat pups aged 3, 7, or 21 postnatal (P) days were briefly anesthetized for intrathecal injections of saline or clonidine. At each age, the maximum tolerated, antinociceptive (increased hindlimb mechanical withdrawal threshold) and antihyperalgesic (hindpaw carrageenan inflammation) doses were determined. Lumbar spinal cord sections were assessed for apoptosis and cell death (histology, activated caspase-3 immunohistochemistry, Fluoro-Jade C staining), histopathology (hematoxylin and eosin staining), and increased glial reactivity (microglial and astrocytic markers). P3 intrathecal ketamine sections served as positive controls. In additional groups, thermal latency and mechanical withdrawal threshold were measured at P35.

RESULTS

Intrathecal clonidine produces age- and dose-dependent analgesia in rat pups. Maximal doses of clonidine did not alter the degree or distribution of apoptosis or increase glial reactivity in the neonatal spinal cord. No spinal histopathology was seen 1 or 7 days after injection at any age. Intrathecal clonidine did not produce persistent changes in reflex sensitivity to mechanical or thermal stimuli at P35.

CONCLUSIONS

Intrathecal clonidine in the postnatal rat did not produce signs of spinal cord toxicity, even at doses much larger than required for analgesia. The therapeutic ratio (maximum tolerated dose/antihyperalgesic dose) was >300 at P3, >30 at P7, and >10 at P21. These data provide additional information to inform the clinical choice of spinal analgesic drug in early life.

Examining the combinatorial model of motor neuron survival by expression profiling of trophic factors and their receptors in the embryonic Gallus gallus

During embryogenesis, limb-innervating lateral motor column (LMC) spinal motor neurons (MN) are generated in excess and subsequently nearly half of them die. Many motor neuron survival factors (MnSFs) have been shown to suppress this default programmed cell death (PCD) program through their receptors (MnSFRs), raising the possibility that they are involved in matching specific MNs with their target muscles. Published observations suggest a combinatorial model of MnSF/Rs function, which assumes that during the PCD phase, MNs are expressing combinations of MnSFRs, whereas the limb muscles innervated by these MNs express cognate combinations of MnSFs. We tested this model by expression profiling of MnSFs and their receptors in the avian lumbosacral spinal cord and limb muscles during the peak PCD period. Our findings highlight the complexity of MnSF/Rs function in the control of LMC motor neuron survival.

Validation of a preclinical spinal safety model: effects of intrathecal morphine in the neonatal rat

BACKGROUND

Preclinical studies demonstrate increased neuroapoptosis after general anesthesia in early life. Neuraxial techniques may minimize potential risks, but there has been no systematic evaluation of spinal analgesic safety in developmental models. We aimed to validate a preclinical model for evaluating dose-dependent efficacy, spinal cord toxicity, and long-term function after intrathecal morphine in the neonatal rat.

METHODS

Lumbar intrathecal injections were performed in anesthetized rats aged postnatal day (P) 3, 10, and 21. The relationship between injectate volume and segmental spread was assessed postmortem and by in vivo imaging. To determine the antinociceptive dose, mechanical withdrawal thresholds were measured at baseline and 30 min after intrathecal morphine. To evaluate toxicity, doses up to the maximum tolerated were administered, and spinal cord histopathology, apoptosis, and glial response were evaluated 1 and 7 days after P3 or P21 injection. Sensory thresholds and gait analysis were evaluated at P35.

RESULTS

Intrathecal injection can be reliably performed at all postnatal ages and injectate volume influences segmental spread. Intrathecal morphine produced spinally mediated analgesia at all ages with lower dose requirements in younger pups. High-dose intrathecal morphine did not produce signs of spinal cord toxicity or alter long-term function.

CONCLUSIONS

The therapeutic ratio for intrathecal morphine (toxic dose/antinociceptive dose) was at least 300 at P3 and at least 20 at P21 (latter doses limited by side effects). These data provide relative efficacy and safety for comparison with other analgesic preparations and contribute supporting evidence for the validity of this preclinical neonatal safety model.

Effects of intrathecal ketamine in the neonatal rat: evaluation of apoptosis and long-term functional outcome

BACKGROUND

Systemic ketamine can trigger apoptosis in the brain of rodents and primates during susceptible developmental periods. Clinically, spinally administered ketamine may improve the duration or quality of analgesia in children. Ketamine-induced spinal cord toxicity has been reported in adult animals but has not been systematically studied in early development.

METHODS

In anesthetized rat pups, intrathecal ketamine was administered by lumbar percutaneous injection. Changes in mechanical withdrawal threshold evaluated dose-dependent antinociceptive and carrageenan-induced antihyperalgesic effects in rat pups at postnatal day (P) 3 and 21. After intrathecal injection of ketamine at P3, 7, or 21, spinal cords were examined for apoptosis (Fluoro-Jade C and activated caspase-3), histopathologic change, and glial responses (ionized calcium-binding adapter molecule 1 and glial fibrillary acid protein). After maximal doses of ketamine or saline at P3 or P21, sensory thresholds and gait analysis were evaluated at P35.

RESULTS

Intrathecal injection of 3 mg/kg ketamine at P3 and 15 mg/kg at P21 reverses carrageenan-induced hyperalgesia. Baseline neuronal apoptosis in the spinal cord was greater at P3 than P7, predominantly in the dorsal horn. Intrathecal injection of 3-10 mg/kg ketamine in P3 pups (but not 15 mg/kg at P21) acutely increased apoptosis and microglial activation in the spinal cord and altered spinal function (reduced mechanical withdrawal threshold and altered static gait parameters) at P35.

CONCLUSIONS

Because acute pathology and long-term behavioral change occurred in the same dose range as antihyperalgesic effects, the therapeutic ratio of intrathecal ketamine is less than one in the neonatal rat. This measure facilitates comparison of the relative safety of spinally administered analgesic agents.

The "window of susceptibility" for inflammation in the immature central nervous system is characterized by a leaky blood-brain barrier and the local expression of inflammatory chemokines

Early in postnatal development, the immature central nervous system (CNS) is more susceptible to inflammation than its adult counterpart. We show here that this "window of susceptibility" is characterized by the presence of leaky vessels in the CNS, and by a global chemokine expression profile which is clearly distinct from the one observed in the adult CNS and has three important characteristics. First, it contains chemokines with known roles in the differentiation and maturation of glia and neurons. Secondly, these chemokines have been described before in inflammatory lesions of the CNS, where they are important for the recruitment of monocytes and T cells. Lastly, the chemokine profile is shaped by pathological changes like oligodendrocyte stress and attempts of myelin repair. Changes in the chemokine expression profile along with a leaky blood-brain barrier pave the ground for an accelerated development of CNS inflammation.

A differential developmental pattern of spinal interneuron apoptosis during synaptogenesis: insights from genetic analyses of the protocadherin-gamma gene cluster

Although the role of developmental apoptosis in shaping the complement and connectivity of sensory and motoneurons is well documented, the extent to which cell death affects the 13 cardinal classes of spinal interneurons is unclear. Using a series of genetic manipulations in vivo, we demonstrate for the first time a differential pattern of developmental apoptosis in molecularly identified spinal interneuron populations, and implicate the adhesion molecule family encoded by the 22-member protocadherin-gamma (Pcdh-gamma) gene cluster in its control. In constitutive Pcdh-gamma null mouse embryos, many interneuron populations undergo increased apoptosis, but to differing extents: for example, over 80% of En1-positive V1 neurons are lost, whereas only 30% of Chx10-positive V2a neurons are lost and there is no reduction in the number of V1-derived Renshaw cells. We show that this represents an exacerbation of a normal, underlying developmental pattern: the extent of each population's decrease in Pcdh-gamma mutants is precisely commensurate both with the extent of its loss during normal embryogenesis and with the extent of its increase in Bax(-/-) mice, in which apoptosis is genetically blocked. Interneuron apoptosis begins during the first wave of synaptogenesisis in the spinal cord, occurring first among ventral populations (primarily between E14 and E17), and only later among dorsal populations (primarily after P0). Utilizing a new, conditional Pcdh-gamma mutant allele, we show that the gamma-Pcdhs can promote survival non-cell-autonomously: mutant neurons can survive if they are surrounded by normal neurons, and normal neurons can undergo apoptosis if they are surrounded by mutant neurons.

Bax and Bcl-2 expression and TUNEL labeling in lumbar enlargement of neonatal rats after sciatic axotomy and melatonin treatment

Peripheral axotomy in neonatal rats induces neuronal death. We studied the anti-apoptotic protein Bcl-2 and cell death promoter Bax in spinal cord of neonatal rats after sciatic transection and treatment with melatonin, a neuroprotective substance. Pups were unilaterally axotomized at P2 and received melatonin (1 mg/kg; sc) or vehicle 1 h prior to lesion, immediately after, at 1 h, 2 h and then once daily. Rats were sacrificed at 3 h, 6 h, 24 h, 72 h and 5 days postaxotomy. Intact animals were used as controls. Lumbar enlargement was processed for Nissl staining, immunohistochemistry and RT-PCR for Bax or Bcl-2 and TUNEL reaction. Motoneurons (MN) of lesioned (L) and normal (N) sides were counted, and MN survival ratio (MSR=L/N) was calculated. Bax and Bcl-2 showed cytoplasmic immunoreactivity (IR). Bax IR was noticeable in small cells but less evident in MN. In unlesioned pups, some Bax-positive small cells (B+) and TUNEL-positive nuclei (T+) were mainly seen in the dorsal horn. In lesioned animals given vehicle, Bax mRNA levels and numbers of B+ and T+ were increased in comparison with intact controls at 24 h postaxotomy. The basal IR for Bax in MN was not changed by axotomy. Bcl-2 IR was noted in all cells and, like Bcl-2 mRNA, was unaltered after lesion. Melatonin reduced MN loss at 24 h, 72 h and 5 days and T+ at 24 h after lesion but did not interfere with Bax or Bcl-2 expression. These results suggest that (1) sciatic transection at P2 increases Bax mRNA and the amount of B+ and T+ in the lumbar enlargement, (2) Bax IR in immature MN is not altered by axotomy and (3) melatonin protects MN and dorsal horn cells through a mechanism independent of Bax and Bcl-2.

Loss of leukemia inhibitory factor receptor beta or cardiotrophin-1 causes similar deficits in preganglionic sympathetic neurons and adrenal medulla

Leukemia inhibitory factor (LIF) receptor beta (LIFRbeta) is a receptor for a variety of neurotrophic cytokines, including LIF, ciliary neurotrophic factor (CNTF), and cardiotrophin-1 (CT-1). These cytokines play an essential role for the survival and maintenance of developing and postnatal somatic motoneurons. CNTF may also serve the maintenance of autonomic, preganglionic sympathetic neurons (PSNs) in the spinal cord, as suggested by its capacity to prevent their death after destruction of one of their major targets, the adrenal medulla. Although somatic motoneurons and PSNs share a common embryonic origin, they are distinct in several respects, including responses to lesions. We have studied PSNs in mice with targeted deletions of the LIFRbeta or CT-1 genes, respectively. We show that LIF, CNTF, and CT-1 are synthesized in embryonic adrenal gland and spinal cord and that PSNs express LIFRbeta. In embryonic day 18.5 LIFRbeta (-/-) and CT-1 (-/-) mice, PSNs were reduced by approximately 20%. PSNs projecting to the adrenal medulla were more severely affected (-55%). Although LIFRbeta (-/-) mice revealed normal numbers of adrenal chromaffin cells and axons terminating on chromaffin cells, levels of adrenaline and numbers of adrenaline-synthesizing cells were significantly reduced. We conclude that activation of LIFRbeta is required for normal development of PSNs and one of their prominent targets, the adrenal medulla. Thus, both somatic motoneurons and PSNs in the spinal cord depend on LIFRbeta signaling for their development and maintenance, although PSNs seem to be overall less affected than somatic motoneurons by LIFRbeta deprivation.

Naturally occurring neuronal death during the postnatal development of Purkinje cells and their precerebellar afferent projections

Naturally occurring neuronal death plays a substantial developmental role in the building of the neural circuitries. The neuronal death caused by different cerebellar mutations is mostly of an apoptotic nature. Apart from the identity of the intrinsic mechanisms of the mutations, adult cerebellar mutants are a powerful tool to causally study the development of the cerebellar connectivity. Thus, studies on adult cerebellar neuronal cell death occurring in mouse mutants elucidate: (i) the dependence of the postsynaptic neurons on their partners, (ii) the 'en cascade' postsynaptic transneuronal degeneration after target-deprivation, and (iii) the close relationship between the molecular modular organization of the cerebellar cortex and dying Purkinje cells. Neuronal cell death has been extensively studied in developing olivocerebellar system. However, less data are available on the occurrence of naturally occurring neuronal death during the in vivo normal development of the Purkinje cells and the mossy fiber system neurons. The developmental role of neuronal death during the establishment and refinement of the olivocerebellar projection is currently discussed. Moreover, the occurrence of neuronal death during the development of the basilar pontine nuclei and its role in the acquisition of the adult pontocerebellar projection is still poorly understood. In the present review, we correlate the dates of Purkinje cells death with the inferior olivary and basilar pontine neuronal apoptosis, discussing their developmental relationships during the elaboration of the fine-grained maps of the cerebellar afferent connections.

Preganglionic inputs to the stellate ganglion of the cat during postnatal ontogenesis

The localization of preganglionic neurons forming, connections with the stellate ganglion was studied using a method based on retrograde axonal transport of horseradish peroxidase in cats during postnatal ontogenesis from birth to six months of life. The number of preganglionic neurons and spinal cord segments connected to the ganglion increased from birth to 10 days of age, and then decreased. The proportion of preganglionic neurons in different spinal cord nuclei changed during life and became comparable with that in adults by age six months. This was accompanied by an increase in the proportion of cells in the nucleus intermediolateralis thoracolumbaris pars principalis, with a decrease in the number of neurons in the nucleus intermediolateralis thoracolumbaris parts funicularis and in the ventral horns of the spinal cord.

Ontogenic changes of the GABAergic system in the embryonic mouse spinal cord

Numerous studies have demonstrated an excitatory action of GABA early in development, which is likely to play a neurotrophic role. In order to better understand the role of GABA in the mouse spinal cord, we followed the evolution of GABAergic neurons over the course of development. We investigated, in the present study, the ontogeny of GABA immunoreactive (GABA-ir) cell bodies and fibers in the embryonic mouse spinal cord at brachial and lumbar levels. GABA-ir somata were first detected at embryonic day 11.5 (E11.5) exclusively at brachial level in the marginal zone. By E13.5, the number of GABAergic neurons sharply increased throughout the extent of the ventral horn both at brachial and lumbar level. Stained perikarya first appeared in the future dorsal horn at E15.5 and progressively invaded this area while they decreased in number in the presumed ventral gray matter. At E12.5, E13.5 and E15.5, we checked the possibility that ventral GABA-ir cells could belong to the motoneuronal population. Using a GABA/Islet-1/2 double labeling, we did not detect any double-stained neurons indicating that spinal motoneurons do not synthesize GABA during the course of development. GABA-ir fibers also appeared at the E11.5 stage in the presumptive lateral white matter at brachial level. At E12.5 and E13.5, GABA-ir fibers progressively invaded the ventral marginal zone and by E15.5 reached the dorsal marginal zone. At E17.5 and postnatal day 0 (P0), the number of GABA-ir fibers declined in the white matter. Finally, by P0, GABA immunoreactivity that delineated somata was mainly restricted to the dorsal gray matter and declined in intensity and extent. The ventral gray matter exhibited very few GABA-ir cell bodies at this neonatal stage of development. The significance of the migration of somatic GABA immunoreactivity from ventral to the dorsal gray matter is discussed.

Motoneuron-derived neurotrophin-3 is a survival factor for PAX2-expressing spinal interneurons

Rat spinal cord interneurons undergo programmed cell death shortly after birth. We investigated here whether cell death of interneurons could be regulated by trophic factors produced by motoneurons, one of their main targets. To test this hypothesis, we studied the effect of the selective destruction of motoneurons on the survival of interneurons in organotypic cultures of embryonic rat spinal cords. Motoneurons were eliminated by an anti-p75(NTR)-specific immunotoxin (192 IgG-saporin). We then observed a decrease of 28% in the number of ventral spinal interneurons immunoreactive (IR) for the homeoprotein PAX2. This was correlated with an increase in the number of apoptotic nuclei in the same area. Because neurotrophin-3 (NT-3) is specifically produced by motoneurons and because interneurons express the NT-3 high-affinity receptor trkC, we examined the role of NT-3 in the survival of PAX2-IR interneurons. Addition of NT-3 to 192 IgG-saporin-treated explants rescued ventral PAX2-IR interneurons. Depletion of secreted NT-3 by anti-NT-3 antibodies induced 66% loss of ventral PAX2-IR interneurons. We conclude that motoneuron-derived NT-3 is a trophic factor for ventral PAX2-IR interneurons.

Apoptosis in the rat spinal cord during postnatal development; the effect of perinatal asphyxia on programmed cell death

The aim of our study was to investigate the effect of perinatal asphyxia on developmental apoptosis in the cervical and lumbar spinal cord in the neonatal rat. Perinatal asphyxia was induced by keeping pups at term in utero in a water bath at 37 degrees C for 20 min, followed by resuscitation. Effects of this treatment on developmental apoptosis were studied on postnatal days 2, 5 and 8 using terminal deoxynucleotidyl transferase (TdT)-dUTP-biotin nick end labelling (TUNEL) and caspase-3 staining. TUNEL positive cells were identified using double immunostaining. On postnatal day 2 an increase of 215% in TUNEL positive cells was detected (P=0.005) in laminae IV-VII of the lumbar spinal cord of rats which underwent perinatal asphyxia compared to controls. An increase of 55% compared to controls (P=0.03) was seen in laminae I-III of the lumbar spinal cord at postnatal day 8. TUNEL positive cells could be partly identified as microglia cells (ED1 positive) and oligodendrocytes (O4 positive). The effect of perinatal asphyxia on programmed cell death in the neonatal rat spinal cord was mainly observed in the intermediate zone and dorsal horn of the lumbar spinal cord. We conclude that perinatal asphyxia has a pronounced effect on the survival of cells in a specific region of the spinal cord and thus may have a profound effect on the development of motor networks.

Apoptosis of spinal interneurons induced by sciatic nerve axotomy in the neonatal rat is counteracted by nerve growth factor and ciliary neurotrophic factor

We have previously shown that not only motoneurons and dorsal root ganglion cells but also small neurons, presumably interneurons in the spinal cord, may undergo apoptotic cell death as a result of neonatal peripheral nerve transection in the rat. With the aid of electron microscopy, we have here demonstrated that apoptosis in the spinal cord is confined to neurons and does not involve glial cells at the survival time studied (24 hours). To define the relative importance of the loss of a potential target (motoneuron) and a potential afferent input (dorsal root ganglion cell) for the induction of apoptosis in interneurons in this situation, we have compared the distributions and time courses for TUNEL labeling, which detects apoptotic cell nuclei, in the L5 segment of the spinal cord and the L5 dorsal root ganglion after sciatic nerve transection in the neonatal (P2) rat. In additional experiments, we studied the effects on TUNEL labeling of interneurons after treatment of the cut sciatic nerve with either ciliary neurotrophic factor (CNTF) to rescue motoneurons or nerve growth factor (NGF) to rescue dorsal root ganglion cells. The time courses of the TUNEL labeling in motoneurons and interneurons induced by the lesion show great similarities (peak at 8-48 hours postoperatively), whereas the labeling in dorsal root ganglion cells occurs later (24-72 hours). Both CNTF and NGF decrease the number of TUNEL-labeled interneurons, but there is a regional difference, in that CNTF preferentially saves interneurons in deep dorsal and ventral parts of the spinal cord, whereas the rescuing effects of NGF are seen mainly in the superficial dorsal horn. The results are interpreted as signs of a trophic dependence on both the target and the afferent input for the survival of interneurons neonatally.

Developmental apoptosis in the spinal cord white matter in neonatal rats

We investigated developmental apoptosis in the white matter of the cervical spinal cord at postnatal days 2, 5, and 8. Apoptotic cells were labeled using TUNEL and caspase-3 immunostaining. Apoptotic cells were diffusely distributed throughout the white matter of the spinal cord. The total amount of apoptotic cells in the cervical spinal cord white matter was related to postnatal age, with the lowest at P2 (mean 7.9, SD 5.6) and the highest at P8 (mean 109, SD 21.4). Using double immunostaining for ED-1 and O4, apoptotic cells were identified as microglia and oligodendrocytes.

Occurrence of glutamate receptor subunit 1-containing aggresome-like structures during normal development of rat spinal cord interneurons

During a developmental study of the expression of alpha-amino-3-hydroxy-5-methyl-4-isoxazolepropionate (AMPA) -type glutamate receptor subunits in rat spinal cord, we observed the existence of cytoplasmic inclusion bodies with positive immunoreactivity to glutamate receptor subunit 1 (GluR1) but not to other glutamate receptor subunits. GluR1-positive bodies have a diameter of between 1 and 3 microm and can be seen widely distributed throughout spinal cord gray matter, with the exception of the ventral horn region. They transiently appear within a definite developmental time-period between embryonic day 19 and postnatal day 17 and are only associated with neuronal cells. Ultrastructural analysis revealed that these inclusions were located adjacent to the nucleus and consisted of amorphous material without any limiting membrane. Immunocytochemical analysis revealed that the inclusions displayed positive immunoreactivity to ubiquitin, HSP70, and 20S proteasome. All these data indicate that GluR1-containing inclusions display all the ultrastructural and immunocytochemical characteristics of the recently described structure, which have been given the name aggresomes. Further studies are needed to determine the biological significance of these normally occurring aggresome-like inclusions, because aggresomes are usually considered in a pathologic context.

Growth and neurotrophic factors regulating development and maintenance of sympathetic preganglionic neurons

The functional anatomy of sympathetic preganglionic neurons is described at molecular, cellular, and system levels. Preganglionic sympathetic neurons located in the intermediolateral column of the spinal cord connect the central nervous system with peripheral sympathetic ganglia and chromaffin cells inside and outside the adrenal gland. Current knowledge is reviewed of the development of these neurons, which share their origin with progenitor cells, giving rise to somatic motoneurons in the ventral horn. Their connectivities, transmitters involved, and growth factor receptors are described. Finally, we review the distribution and functions of trophic molecules that may have relevance for development and maintenance of preganglionic sympathetic neurons.

Programmed cell death of developing mammalian neurons after genetic deletion of caspases

An analysis of programmed cell death of several populations of developing postmitotic neurons after genetic deletion of two key members of the caspase family of pro-apoptotic proteases, caspase-3 and caspase-9, indicates that normal neuronal loss occurs. Although the amount of cell death is not altered, the death process may be delayed, and the cells appear to use a nonapoptotic pathway of degeneration. The neuronal populations examined include spinal interneurons and motor, sensory, and autonomic neurons. When examined at both the light and electron microscopic levels, the caspase-deficient neurons exhibit a nonapoptotic morphology in which nuclear changes such as chromatin condensation are absent or reduced; in addition, this morphology is characterized by extensive cytoplasmic vacuolization that is rarely observed in degenerating control neurons. There is also reduced terminal deoxynucleotidyl transferase-mediated biotinylated UTP nick end labeling in dying caspase-deficient neurons. Despite the altered morphology and apparent temporal delay in cell death, the number of neurons that are ultimately lost is indistinguishable from that seen in control animals. In contrast to the striking perturbations in the morphology of the forebrain of caspase-deficient embryos, the spinal cord and brainstem appear normal. These results are consistent with the growing idea that the involvement of specific caspases and the occurrence of caspase-independent programmed cell death may be dependent on brain region, cell type, age, and species or may be the result of specific perturbations or pathology.

Cell death in the superficial dorsal horn in a model of neuropathic pain

The aims of this study were to investigate the occurrence of apoptotic cell death in the dorsal horn of the adult rat spinal cord following chronic constriction injury (CCI) to the sciatic nerve and to correlate this with behavioural responses. Six groups of six rats were used as follows: 1) CCI, 2) CCI, 3) MK801 + CCI, 4) axotomy, 5) sham, and 6) naive. Group 1 animals were behaviourally tested for thermal hyperalgesia 8 days following surgery and sacrificed and the spinal cords removed and frozen. The rest of the groups underwent the same procedure 14 days following surgery. The lumbar region of the spinal cord was cryosectioned and the incidence of apoptotic cells investigated using the TUNEL technique plus Hoechst double labelling. By 8 days post-CCI, hyperalgesia had developed in the ipsilateral paw, which was still present 14 days after the injury compared to the contralateral paw and naive and sham animals. Preemptive MK-801 prevented the onset of hyperalgesia. Significant numbers of apoptotic cells were present in the ipsilateral dorsal horn of the spinal cord 8 and 14 days following CCI compared to the contralateral side and to naive and sham animals. Preemptive treatment with MK-801 reduced the extent of apoptosis resulting from CCI to the level seen in control animals. This study demonstrates that cells undergo apoptosis as a result of CCI simultaneous with the occurrence of hyperalgesia. Furthermore, MK-801 prevents the onset of hyperalgesia and reduces the extent of apoptotic cell death, suggesting, perhaps, that apoptosis contributes to the initiation/maintenance of hyperalgesia.

Decreased levels of proteasome activity and proteasome expression in aging spinal cord

Neuron death and neuron degeneration occur in the CNS during the course of aging. Although multiple cellular alterations transpire during the aging process, those that mediate age-associated neuron death have not been identified. Recent evidence implicates oxidative stress as a possible means of neuron death and neuron degeneration during aging. In the present study, we demonstrate a marked decrease in multicatalytic proteasome activity in the spinal cord of Fisher 344 rats at 12, 24 and 28 months, compared with spinal cord tissue from 3-week- and 3-month-old animals. Application of oxidative injury (FeSO(4)) or the lipid peroxidation product 4-hydroxynonenal decreases multicatalytic proteasome activity in a time- and dose-dependent manner in a motor neuron cell line. Loss of multicatalytic proteasome activity occurs before the loss of multicatalytic proteasome immunoreactivity, with FeSO(4)- and 4-hydroxynonenal-mediated decreases ameliorated by the application of a cell permeable form of the antioxidant glutathione. Application of multicatalytic proteasome inhibitors, but not inhibitors of lysosomal proteases, induced neuron death that was attenuated by the caspase inhibitors benzyloxycarbonyl-Val-Ala-Asp-(O-methyl) fluoromethyl ketone or N-acetyl-Asp-Glu-Val-Asp-Cho (aldehyde). Together, these data suggest that multicatalytic proteasome inhibition occurs during aging of the spinal cord, possibly as the result of oxidative stress, and that multicatalytic proteasome inhibition may be causally related to neuron death.

Cell death of spinal interneurones

The occurrence of neuronal death during development is well documented for some neuronal populations, such as motoneurones and dorsal root ganglion cells, whose connecting pathways are clearly defined. Cell survival is thought to be regulated largely by target and input connections, a process that serves to match the size of synaptically linked neuronal populations. Far less is known about interneurones. It is assumed that most interneurone populations are excluded from this process because their connections are more diffuse. Recent studies on the rat spinal cord have indicated that interneurone death does occur, both naturally during development and induced following peripheral nerve injury. Here the evidence for spinal interneurone death is reviewed and the factors influencing it are discussed. There are many functional types of interneurones in the spinal cord that may differ in vulnerability to cell death, but it is concluded that for most spinal interneurones the traditional view of target regulation is unlikely. Instead it is proposed that developmental interneurone death in the spinal cord forms part of a plastic response to altered sensory activation rather than a size-matching exercise. There is also emerging evidence that interneurone death may play a more direct role in some neurodegenerative diseases than hitherto considered.

Selective inhibition of spinal cord neurite outgrowth and cell survival by the Eph family ligand ephrin-A5

The Eph family tyrosine kinase receptors and their ligands, the ephrins, have been shown to play critical roles in cell migration, tissue morphogenesis, and axonal guidance in many different systems. However, their function in the spinal cord has not been examined carefully. We showed in this study that several Eph receptors, including EphA3, Eph A4, and Eph A5, are expressed in the ventral spinal cord in partially overlapping patterns, with EphA5 exhibiting the most widespread transcription in the entire ventral spinal cord during early development. Complementary to the receptor expression, a ligand of these receptors, ephrin-A5, is transcribed in the dorsal half of the spinal cord. Consistent with the spatial location of receptor expression, the ligand selectively inhibits neurite outgrowth and induces cell death of the ventral, but not the dorsal, spinal cord neurons. These observations suggest that interactions between the Eph family receptors and ligands exerts negative influences on ventral spinal cord neurons and thus may play important roles in regulating morphogenesis and axon guidance in the spinal cord.

Patterns of programmed cell death in populations of developing spinal motoneurons in chicken, mouse, and rat

During embryonic development, approximately one-half of the spinal motoneurons initially generated are lost during a wave of programmed cell death (PCD). Classical studies in this system laid the basis of much work on the role and control of neuronal cell death during development. However, we have little information concerning the timing of cell death in motoneuron pools at different rostrocaudal levels, especially in rodents. We developed a novel protocol for whole-mount TUNEL labeling that allows apoptotic nuclei to be visualized in whole-mount preparations of embryonic spinal cord; double labeling with antibodies to Islet 1/2 showed that nearly all TUNEL-positive cells were motoneurons. In chicken and mouse embryos, the density of TUNEL-positive nuclei was specifically increased following target ablation. The pattern of naturally occurring motoneuron PCD was studied in spinal cords from different species and ages: chick (E4.5-E9.0), mouse (E11.5-E15.5), and rat (E13.5-E16. 5). In all species, motoneuron PCD is first apparent at cervical levels and last at sacral levels. However, motoneuron PCD does not follow a strict rostrocaudal sequence. Following cervical motoneuron PCD, TUNEL profiles are first observed at lumbar levels in chick but at thoracic levels in rat. At a given rostrocaudal level, medial motoneurons tend to die before lateral populations, but here too there are exceptions. Motoneuron cell death is thus regulated in a highly stereotyped manner during development of vertebrate spinal cord. Our technique will provide a basis for the monitoring even localized changes in this pattern.

Modulation of early but not later stages of programmed cell death in embryonic avian spinal cord by sonic hedgehog

Sonic hedgehog (Shh) is a secreted glycoprotein expressed by the notochord and floor plate that is involved in the induction and specification of ventral phenotypes in the vertebrate neural tube. Recently, Shh has also been shown to promote the survival of cultured rat embryo ventral brain and spinal cord cells. We have examined whether Shh can promote the survival of chick embryo neurons in vivo or in vitro. In the chick, Shh is expressed in notochord, floor plate, and ventral neural tube/spinal cord at several stages at which programmed cell death (PCD) occurs. However, the administration of exogenous Shh to embryos in vivo or to motoneuron cultures at these stages failed to promote the survival of several different neuronal populations, including spinal motoneurons, spinal interneurons, sympathetic preganglionic neurons, sensory neurons, and neuronal precursor cells. Rather, at the earliest stage of PCD examined here (embryonic day 3) Shh selectively induced the death of ventral neuronal precursors and floor-plate cells, resulting in a net loss of cells in the neural tube. Altered concentrations of Shh induce aberrant phenotypes that are removed by PCD. Accordingly, normal PCD in the early neural tube may play a role in dorsal-ventral patterning.

Maturation of the mammalian respiratory system

In this review, the maturational changes occurring in the mammalian respiratory network from fetal to adult ages are analyzed. Most of the data presented were obtained on rodents using in vitro approaches. In gestational day 18 (E18) fetuses, this network functions but is not yet able to sustain a stable respiratory activity, and most of the neonatal modulatory processes are not yet efficient. Respiratory motoneurons undergo relatively little cell death, and even if not yet fully mature at E18, they are capable of firing sustained bursts of potentials. Endogenous serotonin exerts a potent facilitation on the network and appears to be necessary for the respiratory rhythm to be expressed. In E20 fetuses and neonates, the respiratory activity has become quite stable. Inhibitory processes are not yet necessary for respiratory rhythmogenesis, and the rostral ventrolateral medulla (RVLM) contains inspiratory bursting pacemaker neurons that seem to constitute the kernel of the network. The activity of the network depends on CO2 and pH levels, via cholinergic relays, as well as being modulated at both the RVLM and motoneuronal levels by endogenous serotonin, substance P, and catecholamine mechanisms. In adults, the inhibitory processes become more important, but the RVLM is still a crucial area. The neonatal modulatory processes are likely to continue during adulthood, but they are difficult to investigate in vivo. In conclusion, 1) serotonin, which greatly facilitates the activity of the respiratory network at all developmental ages, may at least partly define its maturation; 2) the RVLM bursting pacemaker neurons may be the kernel of the network from E20 to adulthood, but their existence and their role in vivo need to be further confirmed in both neonatal and adult mammals.

The role of apoptosis and excitotoxicity in the death of spinal motoneurons and interneurons after neonatal nerve injury

There is evidence that motoneurons which die following neonatal nerve injury in rats do so through an excitotoxic mechanism. In this study, we have investigated whether this excitotoxicity induces motoneuron death by apoptosis. Sciatic motoneurons were prelabelled at birth with the retrograde tracing agent, Fast Blue, and the sciatic nerve was crushed in one leg two days later. At intervals up to 12 days, sections of the lumbar enlargement were analysed for apoptosis using propidium iodide and terminal deoxynucleotidyl transferase biotin-14-UTP nick end labelling techniques. A significant concentration of Fast Blue-labelled apoptotic motoneurons was seen in the area of the sciatic motor pool ipsilateral to the nerve injury, with the majority occurring in the first three days. Comparison of estimates of the time-course of apoptosis with that of motoneuron survival suggest that all motoneuron death induced during the first 12 days occurs by apoptosis and that the process is only recognizable for 2 h. Treatment with the N-methyl-D-aspartate receptor antagonist, dizocilpine maleate, reduced the level of apoptosis by 60%. Taken together, these data show that motoneurons which have been affected by an excitotoxic mechanism die by apoptosis. The apoptotic study also provides evidence, for the first time, that unilateral nerve injury induces motoneuron death in the contralateral sciatic motor pool. Apoptotic interneurons were also seen on both sides of the spinal cord as a result of nerve injury.

Differences in developmental cell death between somatic and autonomic motor neurons of rat spinal cord

Considerable knowledge concerning developmental cell death has come from the study of somatic motor neurons (SMNs), but a related set of spinal neurons, the autonomic motor neurons (AMNs), have been studied less extensively in this respect. In the present study, we used three different approaches to determine the amount of AMN cell death during normal development in the rat. First, target dependency was studied in organotypic slice cultures, and it was found that AMNs survived for at least 12 days after removal of their postsynaptic targets. No factors were added to the serum-free medium to substitute for the ablated targets, indicating that AMNs were able to survive without target-derived trophic factors. Such target-independent survival is not characteristic of neurons that undergo typical developmental cell death. Second, AMNs were counted in double-stained choline acetyltransferase immunocytochemical and NADPH diaphorase histochemical preparations at ages (postnatal days 4-22) encompassing the period when AMN postsynaptic target cells undergo developmental death. Neuron numbers were essentially identical at all ages examined, indicating that no AMN cell death occurred postnatally. Finally, from embryonic day 13 to postnatal day 22, animals were analyzed by using terminal transferase-mediated nick-end labeling to identify dying cells. Many fewer labeled cells were observed among AMNs than among SMNs. Thus, all three approaches indicated that there is a significant SMN/AMN difference in developmental cell death. The phenotypic trait(s) that underlies this difference may also be important in the relative resistance of AMNs to pathological conditions that induce death of SMNs, e.g., those involved in amyotrophic lateral sclerosis and excitotoxicity.

Muscle-specific cell ablation conditional upon Cre-mediated DNA recombination in transgenic mice leads to massive spinal and cranial motoneuron loss

We describe here a binary transgenic system based on Cre-mediated DNA recombination for genetic cell ablation in mice that enabled us to obtain skeletal muscle-deficient embryos by mating two phenotypically normal transgenic lines. In those embryos, skeletal muscles are eliminated as a consequence of the expression of the gene encoding the diphtheria toxin A fragment. Cell ablation occurs gradually beginning approximately on embryonic day (E) 12.5, and by E18-5 almost all skeletal muscle is absent. Analysis of the consequences of muscle cell ablation revealed that almost all spinal motoneurons are lost by E18.5, providing strong evidence that survival of spinal motoneurons during embryogenesis is dependent on signals from their target tissue, skeletal muscle, and that trophic signals produced by nonmuscle sources are sufficient to support survival of no more than 10% of embryonic spinal motoneurons in the absence of muscle-derived signals. There was also substantial loss of cranial (hypoglossal and facial) motoneurons in the muscle-deficient embryos, thus indicating that cranial motoneuron survival is also dependent on trophic signals produced by their target tissue. Although spinal motoneurons are a major target of spinal interneurons, the loss of motoneurons did not affect interneuron survival. Muscle-deficient embryos had a cleft palate and abnormalities of the lower jaw, raising the possibility that they might serve as a mouse model for the human disorder, Robin sequence. The data reported here demonstrate the utility of a binary transgenic system for obtaining mouse embryos in which a specific cell population has been ablated, so that its role in embryonic development can be studied.