Transmission blockade during neuronal development. Observations on the trochlear nucleus with quantitative histological methods and with ultrastructural and axonal transport studies in the chick embryo

The forgotten militant and his enduring mission: Zing-Yang Kuo and his extraordinary years in behavioral neuroembryology (1929-1939)

Zing-Yang Kuo (1898-1970), hailed as China's behaviorist psychologist, earned "Out-Watsons Mr. Watson" in the international anti-instinct movement. His contributions to the field on behavioral neuroembryology (1929-1939) are often overlooked in comparison to his achievements in psychology. We retrieved the titles of all of Kuo's publications from 1929 to 1939 and examined those related to his research on the origins and development of embryonic behavioral ontogeny and the neural basis of embryonic behavior. Remarkably, Kuo concurrently focused on embryos during the same period as North American neuroembryologists. He maintained an independent stance in the debate over the sequence of behavioral ontogeny, represented by the embryonic neuroscientists Coghill and Windle, and critically pointed out limitations in research on both sides of the debate. Drawing from his experiments with chicken embryos, Kuo proposed the theory of behavioral epigenesis, which attempted to end the nature-nurture dichotomy and promote the transformation of the research path of behavioral embryology from elementary physiological anatomy toward a deep "comprehensive science." Kuo's achievements directly laid the foundation for the interdisciplinary field of developmental psychobiology, constructing a new conceptual framework for the systematic analysis of behavioral development and promoting the establishment and development of a new approach to epiphenotype epigenetics.

Muscle activity and motor neuron death in the spinal cord of the chick embryo

During embryonic development in vertebrates about half the spinal motor neurons degenerate naturally after an initial period of normal differentiation. Motor neuron survival during this period is regulated by influences associated with both afferent and target contacts. Target-associated influences are regulated, at least in part, by activity (i.e. neuromuscular transmission or muscle contraction). Pharmacological blockade of neuromuscular activity reduces or prevents normal cell death whereas induced hyperactivity of targets enhances the death of motor neurons. Information supporting these assertions is reviewed and evidence is presented from studies which attempt to elucidate the major site at which neuromuscular activity affects motor neuron survival and degeneration in the chick embryo. Finally, a model and some supporting evidence are described in which activity is thought to regulate the production or availability of a target-derived trophic factor required by motor neurons for their survival during certain critical phases of early development.

Induction by beta-bungarotoxin of apoptosis in cultured hippocampal neurons is mediated by Ca(2+)-dependent formation of reactive oxygen species

The component of the venom of the Taiwanese banded krait Bungarus multicinctus, beta-bungarotoxin (beta-BuTx), acts as an extremely potent inducer of neuronal apoptosis when applied to rat hippocampal cultures. While induction of cell death is dependent on toxin binding to voltage-activated K+ channels and subsequent internalization, the pro-apoptotic signals triggered by picomolar concentrations of beta-BuTx are not understood. Following toxin binding, a dramatic increase in intracellular Ca2+ became detectable after 30 min, and in reactive oxygen species (ROS) after 3-4 h. Conversely, Ca2+ chelators, radical quenchers and antioxidants efficiently antagonized beta-BuTx induced apoptosis. As shown for the antioxidant 2,3-dihydroxybenzoic acid, analysis by matrix assisted laser desorbtion-time of flight (MALDI-TOF) mass spectrometry excluded the protective effects to be due to reductive cleavage of the toxic beta-BuTx dimer. Inhibitors of the intracellular antioxidant defence system enhanced neuronal susceptibility to beta-BuTx, supporting the essential role of ROS in beta-BuTx-initiated apoptosis. Cell damage was accompanied by an accumulation of markers of oxidative cell stress, phospholipid hydroxyperoxides and the lipid peroxidation product, malonyl dialdehyde. These observations indicate that beta-BuTx-induced cell death resulted from an intracellular signalling cascade involving subsequent stages of a dramatic rise in free Ca2+, the accumulation of ROS, membrane lipid peroxidation and, finally, apoptosis.

Beta-bungarotoxin is a potent inducer of apoptosis in cultured rat neurons by receptor-mediated internalization

The neurotoxic phospholipase A(2), beta-bungarotoxin (beta-BuTx), is a component of the snake venom from the Taiwanese banded krait Bungarus multicinctus. beta-BuTx affects presynaptic nerve terminal function of the neuromuscular junction and induces widespread neuronal cell death throughout the mammalian and avian CNS. To analyse the initial events of beta-BuTx-mediated cell death, the toxin was applied to cultured rat hippocampal neurons where it induced neuronal cell death in a concentration-dependent manner (EC(50) approximately equal to 5 x 10(-13) M) within 24 h. Fluorescence labelled beta-BuTx was completely incorporated by neurons within < 10 min. Binding and uptake of beta-BuTx, as well as induction of cell death, were efficiently antagonized by preincubation with dendrotoxin I, a blocker of voltage-gated potassium channels devoid of phospholipase activity. Binding of beta-BuTx was selective for neurofilament-positive cells. As evident from intense annexin-V and TUNEL stainings, application of beta-BuTx induced apoptotic cell death exclusively in neurons, leaving astrocytes unaffected. No evidence was obtained for any contribution of either caspases or calpains to beta-BuTx-induced apoptosis, consistent with the inability of the inhibitors Z-Asp-DCB and calpeptin, respectively, to protect neurons from beta-BuTx-induced cell death. These observations indicate that induction of cell death by beta-BuTx comprises several successive phases: (i) binding to neuronal potassium channels is the initial event, followed by (ii) internalization and (iii) induction of apoptotic cell death via a caspase-independent pathway.

Sixth Annual Stuart Reiner Memorial Lecture: embryonic development of nerve and muscle

This article provides a basic scheme of sequential anatomic and some physiologic events occurring during the course of embryonic development of motor neurons and muscles, leading to the establishment of mature nerve-muscle relationships. Motor neurons and muscles begin their development independently and during embryogenesis they become dependent on each other for further development and survival. Aspects of development which occur independently and those requiring mutual interactions are identified. The development of motor neurons is discussed with respect to their production, projection, neuromuscular transmission, myelination, sprouting, survival, and death. The development of muscles is discussed with respect to the origin, differentiation, and muscle fiber types. Discussion on the development of neuromuscular junction includes differentiation of presynaptic nerve terminal, postsynaptic components, and elimination of multiple axons.

Influence of altered afferent input on the number of trochlear motor neurons during development

A loss of about half of the trochlear motor neurons occurs during the course of normal development. The present investigation was undertaken to examine the role of afferent input in regulating the number of surviving or dying trochlear motor neurons. A majority of the afferent input to the trochlear nucleus comes from the vestibular nuclei of the hindbrain via the medial longitudinal fasciculus. Portions of the hindbrain were lesioned in duck embryos on embryonic day 3, considerably prior to the time motor neurons send their axons out and cell death begins. The effectiveness of hindbrain lesion was verified by electron microscopical examination of synapses. There was a significant decrease in the number of synapses on trochlear motor neurons following hindbrain lesion. Cell counts made after the period of cell death indicated a significant decrease in the final number of surviving trochlear motor neurons. Cell counts made prior to the onset of cell death indicated that there was a drastic reduction in the initial number of trochlear motor neurons produced in hindbrain lesion embryos. In spite of a significant reduction in the initial number of neurons, the percentage loss of neurons was about the same as during normal development. Since trochlear motor neurons are generated prior to the formation of afferent synapses on them, it is unlikely that the reduction in the number of motor neurons initially produced is due to reduced afferent synaptic input. Since the percentage of cell loss in hindbrain lesion and normal embryos is about the same, it seems that the magnitude of cell death is genetically programmed.(ABSTRACT TRUNCATED AT 250 WORDS)

Synapse formation on trochlear motor neurons under conditions of increased and decreased cell death during development

There is a normally occurring death of about half of the trochlear motor neurons during development. Early removal of the target muscle results in death of almost all neurons whereas neuromuscular blockade prevents neuron death. The present investigation was undertaken to determine whether the number of central afferent synapses on motor neurons is altered under conditions which either accentuate cell loss or rescue neurons. The sole peripheral target of innervation of the trochlear motor neurons, the superior oblique muscle, was extirpated in duck embryos before the motor axon outgrowth begins. The neuromuscular blockade was achieved by application of paralyzing dosages of alpha bungarotoxin on to the vascularized chorioallantoic membrane. This treatment began prior to the onset of cell death and embryos were treated daily throughout the period of cell death. Brains were processed for electron microscopy and quantitative observations were made on synapses at the onset, during the period of, and at the end of cell death. It was found that there was no significant difference in the number of synapses on neurons following target removal, following neuromuscular blockade, and those developing normally. This observation indicates that the number of central afferent synapses on cell soma is not altered under conditions which either decrease or increase neuron survival. These results suggest that the synapse number per se may not be directly involved in the process of naturally occurring cell death. The results also suggest that the number of synapses on trochlear motor neurons is independent of interactions with the target.

Cell death of motoneurons in the chick embryo spinal cord. X. Synapse formation on motoneurons following the reduction of cell death by neuromuscular blockade

Chronic treatment of chick embryos with neuromuscular blocking agents, such as curare, rescues motoneurons from naturally occurring cell death. In the present study, embryos treated with curare from E6 to E9 had 35% more motoneurons than controls on E10 and 42% more than controls on E16. Previous studies have shown that several aspects of motoneuron differentiation occur normally in curare-treated embryos. We report here that dendrite growth and arborization is also unaltered on E10 and E16 following curare treatment. A quantitative analysis of afferent synapses on motoneurons shows that the packing density of both axosomatic and axodendritic synapses is also normal on E10 in curare-treated embryos, despite the greater number of motoneurons present. This indicates that the interneurons that provide presynaptic input to motoneurons are able to compensate for the increased number of synaptic sites made available by curare treatment. However, by E16 the packing density of synapses is reduced by about half. Because motoneurons and their dendrites continue to grow between E10 and E16, the further increase in synaptic sites made available in curare-treated embryos apparently exceeds the compensatory capacity of presynaptic interneurons on E16. One can conclude from these results that the increased survival of motoneurons in curare-treated embryos is not owing to an increase in afferent synapses. Motoneurons in these embryos continue to survive in the face of either no change (E10) or a reduction (E16) in the number of axodendritic and axosomatic synapses. Therefore, increased motoneuron survival in this situation is very likely regulated primarily by motoneuron-target interactions.

The development of the amphibian trochlear nucleus. An HRP study

The quantitative and qualitative development of the trochlear nucleus of salamanders and frogs was studied using horseradish peroxidase (HRP) as a retrograde marker. A higher number of labelled motoneurons significantly declined in later larval stages, presumably by cell death, as in birds and mammals. In agreement with this assumption, HRP-filled apoptotic cells were found in larvae. Many more labelled trochlear motoneurons were found in adult than in larval amphibians, and [3H]thymidine labelling showed postlarval cell production in the area of the trochlear nucleus. Data are discussed in the context of cell death and ongoing cell proliferation.

Influence of reduced neuron pool on the magnitude of naturally occurring motor neuron death

The present investigation was undertaken to examine the role of peripheral competition in survival of motor neurons during development. A loss of approximately half of the trochlear motor neurons in duck and quail occurs during the course of normal embryogenesis. The number of motor neurons in the nucleus of quail prior to the onset of cell death is identical to the final number of survivors in the nucleus of duck embryos (about 1,300 neurons). In the present study competition at the peripheral target was decreased by reducing the number of trochlear motor neurons initially projecting to their target muscle. This was accomplished by substituting the midbrain of duck embryos with the same neural tissue from quail embryos. Midbrain transplantation was performed before motor axon outgrowth and normal cell death begin. The development of the motor neurons and their sole target of innervation, the superior oblique muscle, was examined by using a variety of techniques. The source of the grafted motor neurons and of a reduction in the size of the motor neuron pool was confirmed from histological sections and cell counts. The grafted motor neurons projected their axons into the appropriate peripheral target, which was determined by the use of HRP tracing technique. Counts of muscle fibers, motor endplates, and acetylcholine receptors and measurement of total muscle protein indicated that the size of the superior oblique muscle in the chimera embryos was similar to that of the normal duck but significantly larger than the muscle in quail embryos. Electrophysiological observations indicated that the grafted trochlear motor neurons made functional connections with the superior oblique muscle. Counts of the trochlear motor neurons after the period of cell death indicated an average of 1,310 neurons in the nucleus of duck, 772 in quail, and 690 in the chimera embryos. The number of motor neurons in the chimera embryos is not significantly different from that in the normal quail. In other words, in spite of reduced peripheral competition trochlear motor neuron death of normal magnitude occurred. Lack of increased cell survival in our study suggests that trochlear motor neurons do not compete for survival at the peripheral target.

Effect of activity on the selective stabilization of the motor innervation of fast muscle posterior latissimus dorsi from chick embryo

The role of neuromuscular activity in the maturation of the motor innervation was investigated in the fast focally innervated posterior latissimus dorsi (PLD) muscle of the chick embryo. The axonal supply in the PLD motor nerve, and the focal multiple innervation of the endplates were described on days 15 and 16 of embryonic life in normal and experimental embryos. In the first series of experiments, chick embryos were paralyzed by repeated injections between days 4 and 10 in ovo of the curare-like agent, flaxedil. Twice more axons in the PLD motor nerve and about twice more nerve terminal profiles at the endplates in the PLD muscles were found in paralyzed than in control embryos. In a second series of experiments, electrodes were implanted around the spinal cord of 7-day-old embryos and electric pulses delivered at 0.5 Hz frequency from day 10 to days 15-16 of incubation. At day 15.5, no change was observed in the axonal supply in the PLD motor nerve of stimulated embryos, while a two-fold decrease was observed in the number of motor nerve terminal profiles per endplate in the corresponding PLD muscle. The statistical distribution of the number of motor nerve terminal profiles per endplate was described from complete semi-serial sections in the PLD muscle from normal, paralyzed and stimulated chick embryos. In these three cases, the distribution of supernumerary nerve terminal profiles followed a Poisson law after one nerve ending had been subtracted from the number of nerve endings counted per endplate.

Myotube clusters do not bear any quantitative relation to the extent of motoneuron survival

We conducted a quantitative study of the trochlear motoneurons and the myotube clusters in the corresponding superior oblique muscle of the Japanese quail, before, during, and after the period of normally occurring motoneuron degeneration. A ratio of approximately 1:1 between myotube clusters and neurons was observed at the onset of motoneuron degeneration. The number of myotube clusters prior to neuron death was 37% greater than the number of neurons surviving after cell death. These results are in contradiction with the hypothesis that the number of myotube clusters is the deciding factor for the survival of motoneurons during the critical stages of development.

Development of the trochlear nucleus in quail and comparative study of the trochlear nucleus, nerve, and innervation of the superior oblique muscle in quail, chick, and duck

The present study was undertaken to examine the development of the trochlear nucleus in quail and to compare the mature trochlear nucleus, nerve, and their sole target of innervation, the superior oblique muscle, in quail, chick, and duck. Study of the trochlear nucleus in quail from embryonic day 5 through hatching shows a maximum of 1,248 neurons on embryonic day 10 followed by spontaneous degeneration of 40% of the neurons between days 10 and 16. Previous studies have shown that although the initial and final number of neurons is different in chick and duck, the magnitude of trochlear cell loss in both species is about 40%. This study shows the average number of neurons in the nucleus of quail, chick, and duck, 2 weeks post-hatching, to be 658, 743 and 1,459, respectively. Fiber counts in the trochlear nerve from electron micrograph montages at the same period indicated a ratio of about 1:1 between neurons and axons. While a majority of the fibers in these nerves are myelinated, an average of 3-6% of the fibers are unmyelinated. The nucleus in the quail not only contains the smallest number of neurons but it also innervates the smallest muscle in terms of total number of muscle cells and endplates. However, the opposite relationship does not hold true. The nucleus in duck contains the largest number of neurons, yet the largest number of muscle cells and endplates were found in the chick. The ratios between the neurons and muscle cells as well as between neurons and endplates are about the same in quail and duck. These ratios are much higher in the chick, reflecting the relatively small neuron pool destined for a relatively large target. In spite of variations in the number of neurons, muscle fibers, and endplates the average number of endplates per muscle fiber is relatively constant among the three species.

Alterations of the retina in chick embryos induced by systemic alpha-bungarotoxin application

The application of alpha-bungarotoxin onto the chorio-allantoic membrane of chick embryos between the 11th and 18th day of incubation leads to alterations of retinal development. The most significant qualitative change is the appearance of retinal rosettes formed by receptor cells. These rosettes are infoldings of the receptor cell layer. Quantitatively, an enlargement in volume of the receptor and outer nuclear layer can be found together with a simultaneous decrease of the other retinal layers. The toxin seems to suspend the naturally occurring nerve cell death in the receptor cell population

Cell death of motoneurons in the chick embryo spinal cord. VIII. Motoneurons prevented from dying in the embryo persist after hatching

A chronic neuromuscular blockade during those embryonic stages when naturally occurring spinal motoneuron death occurs, results in the prevention of this cell loss. The excess motoneurons are maintained as long as the neuromuscular blockade is continued; once embryonic neuromuscular activity resumes, however, the excess motoneurons undergo a delayed period of cell death. By contrast, the resumption of neuromuscular activity in these same preparations after hatching did not result in a delayed cell death. The excess motoneurons, prevented from dying in the embryo, persisted for as long as 4 days postnatally despite the presence of considerable limb motility. The maintenance of motoneurons may be regulated differently before and after hatching.