Dorsal root ganglion removal in Rana pipiens produces fewer motoneurons

Number, size, and regional distribution of motor neurons in the dorsolateral and retrodorsolateral nuclei as a function of sex and neonatal stimulation

Motor neurons were measured in the retrodorsolateral nucleus (RDLN) and the dorsolateral nucleus (DLN) of adult male and female rats that were reared with normal or reduced levels of maternal anogenital stimulation. In contrast with findings for the spinal nucleus of the bulbocavernosus, which is located in the same spinal segments, reduced stimulation had no effect on neuron number in either nucleus. However, several regional and sex differences were observed. Rostrally located neurons were larger in both the RDLN and the DLN; these location effects were greater in females. There was no sex difference in RDLN neuron size, but DLN neurons were larger in females, particularly in the rostral region. Females had significantly more cells in the RDLN, a nucleus previously considered nondimorphic, whereas males had more DLN neurons. Both regional and sex differences may reflect local differences in trophic factors from targets or afferents.

Early postnatal muscle contractile activity regulates the carbonic anhydrase phenotype of proprioceptive neurons in young and mature mice: evidence for a critical period in development

Carbonic anhydrase activity, a marker of mouse proprioceptive neurons in adult dorsal root ganglia, is first detectable in the perinatal period, increases until postnatal day 60 and remains stable in adulthood. The onset of carbonic anhydrase staining begins after the neurons have made connections with their targets suggesting that neuron-target interactions regulate carbonic anhydrase phenotype development. To examine this possibility, we first analysed carbonic anhydrase expression in mdx mice which are characterized by a massive but reversible degeneration of skeletal muscle concomitant with the carbonic anhydrase ontogenesis. Neuronal carbonic anhydrase expression in mdx mice stopped developing when the period of muscular degeneration-regeneration began. Furthermore this alteration persisted during adulthood. We then analysed carbonic anhydrase expression in fifth lumbar dorsal root ganglion of developing control mice before and after surgical procedures that might interfere with central and peripheral target influences on dorsal root ganglion neurons. Central disconnection (dorsal rhizotomy) did not affect the development of carbonic anhydrase activity. Disrupting neuron-peripheral target interactions by sciatic nerve transection or blocking muscle contraction by tenotomy stopped the development of neuronal carbonic anhydrase content. Finally, recovery was monitored following sciatic nerve crush. In adults, recovery of carbonic anhydrase activity was obtained after functional recuperation; similar manipulations during the first month of life induced irreversible alteration of the carbonic anhydrase phenotype. These results show that the development of carbonic anhydrase activity in proprioceptive neurons is regulated by neuron-muscle interactions (i.e. muscle contraction). They also provide evidence for a critical period in the development of the carbonic anhydrase phenotype. We suggest that these two mechanisms are responsible for the altered carbonic anhydrase phenotype of the dorsal root ganglion neurons in mdx mice, a model of human muscular dystrophy.

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.

Sexually dimorphic neuron number in lumbosacral dorsal root ganglia of the rat: development and steroid regulation

Rats possess a sexually dimorphic neuromuscular system that controls penile reflexes critical for copulation. This system includes two motor nuclei in the lumbar cord and their target musculature in the perineum. The spinal nucleus of the bulbocavernosus (SNB) and the dorsolateral nucleus (DLN) motoneuron populations and their target perineal muscles are much larger in males than in females. The sex difference in motoneuron number develops via androgen-regulated differential cell death during the perinatal period; androgen also regulates retention of the target muscles. The developmental pattern and steroid sensitivity of peripheral afferents to the SNB/DLN motor nuclei were previously unknown. In order to characterize the peripheral sensory component of the dimorphic SNB/DLN system, the neurons of the relevant dorsal root ganglia (DRGs) were quantified in terms of number, size, and androgen sensitivity at various perinatal ages. DRG neuron number is greatest prenatally, then decreases in both sexes after birth; the timing and pattern of neuron number development are similar to those seen in the SNB and DLN. Postnatally, males have more DRG neurons than females, as a result of greater neuron death in the DRGs of females. Females treated with testosterone propionate during the perinatal period exhibit masculine development of DRG neuron number. Thus, the normal development of DRG neuron number parallels that of the SNB/DLN motor nuclei and target muscles in pattern and timing, is sexually dimorphic, and is regulated by androgen.

The role of target size in neuronal survival

A loss of about half of the trochlear motor neurons occurs during the course of normal development in duck and quail embryos. The role of the size of the target muscle in controlling the number of surviving motor neurons was examined by making motor neurons innervate targets either larger or smaller in size than their normal target. In one experiment the smaller trochlear motor neuron pool of the quail embryo was forced to innervate the larger superior oblique muscle of the duck embryo. This was accomplished by grafting the midbrain of a quail embryo in the place of the midbrain of a duck embryo. Results indicated that no additional quail trochlear motor neurons were rescued in spite of a considerable increase in target size. In another experiment the larger trochlear motor neuron pool of the duck embryo was made to innervate the smaller superior oblique muscle of the quail embryo. This resulted in loss of some additional neurons; however, the number of surviving motor neurons was not proportionate to the reduction in target size. These experiments failed to provide support for the hypothesis that the size of the target muscle controls the number of surviving motor neurons. Although contact with target is necessary for survival of neurons, factors other than the number or size of target cells are involved in the control of motor neuron numbers during development.

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)

An Organotypic Spinal Cord - Dorsal Root Ganglion - Skeletal Muscle Coculture of Embryonic Rat. II. Functional Evidence for the Formation of Spinal Reflex Arcs In Vitro

Electrical properties of motoneurons, muscle fibres and dorsal root ganglion (DRG) cells were studied in an organotypic coculture of embryonic rat spinal cord, dorsal root ganglia and skeletal muscle. The motoneurons were identified by their morphology and position in culture. Their size and input conductance were significantly larger than those of spinal interneurons. Intracellular current injection evoked action potentials in all motoneurons, but only evoked stable repetitive firing patterns in some. Excitability was correlated to somatic size and the rate of spontaneous excitatory input. It is suggested that the somatic growth and the increase in excitability is regulated by the excitatory afferents. The motoneurons showed spontaneous excitatory and inhibitory postsynaptic potentials and action potentials which disappeared with the application of various agents known to inhibit excitability or excitatory synaptic transmission. Excitatory and inhibitory postsynaptic potentials (EPSPs and IPSPs respectively) were distinguished by their shape, reversal potential and pharmacology. IPSPs could be depolarizing or hyperpolarizing in different cells. A higher percentage of cells with hyperpolarizing IPSPs was found in older cultures and in the presence of skeletal muscle, suggesting a reversal of the polarity of IPSPs with development. The spontaneous muscle contractions observed in the cultures could be due either to innervation, spontaneous oscillations of the membrane potential, or electrical coupling between neighbouring fibres. A small percentage of DRG cells showed spontaneous action potentials, all of which were found in cultures with spontaneous muscle contractions. The electrical stimulation of DRG afferents evoked mono- and polysynaptic EPSPs in motoneurons, endplate potentials and muscle contractions. The stimulation of the ventral horns evoked endplate potentials and muscle contractions via mono- or polysynaptic pathways. Together these results indicate that appropriate and functional contacts were established in the culture between myotubes and DRG cells, between DRG cells and motoneurons, and between motoneurons and muscle fibres.

Lumbar lateral motor columns and hindlimbs of two Xenopus laevis chromosome mosaics

Two chromosome mosaic Xenopus laevis, one tadpole and one metamorphic animal, both with different sizes of neurons on the left and right sides of their brains and spinal cords, have left and right lumbar lateral motor columns (L-LMCs) of equal lengths but composed of strikingly different numbers of motoneurons (40% fewer motoneurons on the side composed of larger cells). One portion of the lumbar cord in the metamorphic animal is bilaterally symmetrical; the cells on both sides are small and the numbers of motoneurons per section are the same. The mosaics demonstrate that column length and motoneuron density (number per section) are, or can be, regulated bilaterally and that changing cell size affects factors controlling cell density but not column length. Except for the peripheral nerves, there is no evidence of any side-to-side differences in the hindlimb tissues. Whether the side-to-side difference in L-LMC motoneuron number in the stage 66 mosaic corresponds to any feature of the hindlimbs is unknown, but similar side-to-side differences in an early and a late stage mosaic animal support the idea that whatever creates the initial number may also determine the final number of motoneurons.

Synapse formation on trochlear motor neurons in relation to naturally occurring cell death during development

About half of the trochlear motor neurons die during the course of normal development. The present study was undertaken to determine whether the afferent synapses form before the onset of motor neuron death and also to determine whether the number of synapses differs between the healthy and degenerating trochlear motor neurons. Brains of duck embryos from days 10 to 20 were prepared for quantitative electron microscopical observations on synaptogenesis. Results indicate that synapses form on the trochlear motor neuron soma before cell death begins suggesting that afferent input is in a position to exert an influence on survival or death of motor neurons. There were no significant differences in the number of synapses between the healthy and dying neurons during the period of cell death. This observation suggests that the mechanism by which afferent synapses could be involved in neuron survival or death is not related to the number of synapses on the cell soma. The number of synapses on the cell process, synaptic transmission and/or molecules released at the synapses are likely candidates for the mechanism of action of afferent input.

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.

Variation and symmetry in the lumbar and thoracic dorsal root ganglion cell populations of newly metamorphosed Xenopus laevis

The sizes of the lumbar and thoracic dorsal root ganglion cell populations in normally developing newly metamorphosed Xenopus laevis were measured in order to determine whether these neuron populations have the same characteristics as the hindlimb motoneuron population (i.e., large individual as well as sibling group differences, striking bilateral symmetry, and a rough correspondence between neuron number and body size that suggests some peripheral control of cell number during normal development (Sperry, J. Comp. Neurol. 264:250-267). Among animals from three sibling groups, the total numbers of thoracic and lumbar ganglion cells are highly variable and symmetrical, although symmetry is not uniformly present at the level of individual ganglion pairs. Significant sibling group differences in neuron number are also present. Metamorphic body size and cell number in the thoracic but not in the lumbar ganglia are significantly correlated. The motoneurons innervating the hindlimbs were also counted and measured in the same animals. While variable as well as symmetrical, motoneuron number and metamorphic body size are correlated in only two of the three sibling groups. Interestingly, the numbers of motoneurons and lumbar ganglion cells, two populations of neurons whose sizes one might predict would be significantly correlated in normally developing animals, are not correlated. The relationship between these observations and currently held views concerning how neuron numbers might be controlled during normal development is discussed.

Synapse formation on quail trochlear neurons transplanted in duck embryos before naturally occurring motor neuron death

About half of the trochlear motor neurons in duck and quail embryos die during normal development. In a previous study the role of target muscle in controlling the number of surviving motor neurons was investigated by reducing the number of neurons innervating the muscle. This was accomplished by removing the midbrain of the duck embryo and grafting in its place the midbrain of the quail embryo before motor neuron death begins. It was observed that the number of surviving trochlear motor neurons in the quail-duck chimera embryos was not significantly different from that of the normal quail. The present investigation was undertaken to determine whether trochlear motor neurons in the chimera embryos received afferent synapses. Brains of duck, quail and chimera embryos on days 16 and 20 were processed for electron microscopical observations. Synapses formed on motor neurons of the chimera embryos. Surprisingly, synapses on motor neurons of quail differed from those of duck, both qualitatively and quantitatively. Synapses on the motor neurons of the chimera embryos developed in a fashion similar to that for the duck motor neurons. Our failure to rescue trochlear motor neurons in the chimera embryos suggests that the developing motor neurons may respond to a larger target muscle only if they received a normal complement of afferent synaptic input.

Regional variations in the extent and timing of motoneuron cell death in the lumbosacral spinal cord of the chick embryo

We have examined the distribution of motoneurons in different segments of the chick lumbosacral spinal cord before and after the period of motoneuron cell death. The extent of cell death was found to be greatest at the boundaries of the lumbosacral cord where over 60% of the motoneurons died and least in the central region where only 30% died. After cell death at stage 40 the number of motoneurons in each segment was linearly correlated with segment length, suggesting that growth of the segment and motoneuron numbers may be regulated by a common factor. The time of completion of motoneuron cell death exhibited a rostrocaudal gradient along the lumbar cord. Cell death was complete in the anterior segments by stage 35 but not until stage 38 in the caudal 4 segments. The regional variations in the extent and timing of motoneuron cell death suggest that the relative importance of the factors mediating cell death vary in different regions of the lumbar cord.

Motoneurone survival and neurite regeneration requirements: the role of dorsal root ganglion cells during development

The effect of dissociated dorsal root ganglion cells on the survival of motoneurones in vitro from differently aged chick embryos has been studied. Homogeneous cultures of motoneurones were prepared from 5-8-day embryos, using a cell sorter; dorsal root ganglion cells were obtained from 8-day embryos. The survival of motoneurones from 5-day and 6-day embryos was not enhanced above controls by the presence of dorsal root ganglion cells; however, the survival of motoneurones from older embryos was greatly increased, reaching a maximum of over 80% for 8-day embryonic motoneurones. In contrast, the number of motoneurones that had regenerated neurites when co-cultured with dorsal root ganglion cells for 24 h decreased with the motoneurone age at plating, from 51% at 5 days to less than 10% for 7- and 8-day motoneurones. The survival-enhancing effects were probably mediated by cell contact between the motoneurones and processes of the dorsal root ganglion cells: conditioned media from high-density cultures of dorsal root ganglion cells could not be shown to significantly enhance the survival of motoneurones above that of control levels. The possibility that the ganglion cells exert this survival enhancing effect by depolarizing the motoneurones was examined by exposing 8-day sorted motoneurones to 47 mM potassium; this did not effect the survival of the motoneurones relative to control levels. The stage dependency of the survival of motoneurones on different neurotrophic factors and the dorsal root ganglion cell is discussed.

Motoneurone survival requirements during development: the change from immature astrocyte dependence to myotube dependence

The survival requirements of motoneurones obtained from differently aged avian embryos was analysed, in both heterogeneous cultures of motoneurones with spinal cord cells and homogeneous cultures of motoneurones obtained by cell sorting. It was found that medium conditioned by contact with immature astrocytes could maintain more than 75% of the motoneurones plated from 5-day embryos for two days; however, this astrocyte medium could not maintain motoneurones plated from 8-day embryos above control levels at two days. In contrast, medium conditioned by contact with myotubes could not maintain motoneurones plated from 5-day embryos above control levels for two days; this myotube medium could maintain more than 70% of the motoneurones plated from 8-day embryos for two days. The change in the receptivity of motoneurones to astrocyte-conditioned medium may be due to their ageing. Thus, motoneurones from 6-day embryos could not be sustained above control numbers in culture for 4 days with astrocyte media, in the same way as motoneurones from 8-day embryos degenerate by two days. In contrast, more than 70% of motoneurones plated from 6-day embryos could be maintained in culture for 4 days with myotube media in the same way as motoneurones from 8-day embryos for two days. The results indicate that motoneurones from 5-day embryos are dependent for their survival on immature astrocytes but that this switches to a dependence on myotubes during the normal motoneurone death period from 6 days to 10 days of embryonic age.

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.

The development of the relationship between dorsal root afferents and motoneurons in the larval bullfrog spinal cord

The relationship of dorsal root afferents to motoneuron somata and dendrites was studied by labelling dorsal and ventral roots of the tadpole lumbar enlargement with HRP at different stages of hindlimb development. Procedures were used which allowed for sequential light and electron microscopic analysis to determine whether close appositions between labelled elements represented synaptic contacts. Lateral motor column (LMC) motoneuron dendrites grow first into the lateral funiculus, and later begin arborizing within the spinal gray, concurrent with the arrival of developing dorsal root afferent fibers. Mature-appearing synaptic contacts between dorsal root afferents and motoneuron dendrites are established first on distal dendrites, and are observed on progressively more proximal dendrites as hindlimb development proceeds. Migrating motoneurons were also labelled in some animals. Distinct dorsal and ventral migratory pathways were noted; cells migrating dorsally were contacted by developing dorsal root afferents. Migrating motoneurons were associated with radially oriented processes, and were often closely apposed to other cells. The coincident development of dorsal root projections and the motoneuron dendrites which these fibers innervate in the adult, as well as the interaction between these two systems during cell migration, suggest that these two systems may be interdependent in establishing their normal relationship during development.

Hyperplasia in the spinal sensory system of the frog. I. Plasticity in the most caudal dorsal root ganglion

Increases in the amount of periphery available for innervation have been achieved by the unilateral removal of hindlimb dorsal root ganglion (DRGs) in Rana pipiens, a procedure which generally results in a compensatory cell number increase (hyperplasia) in the DRGs which remain. We have found that the hyperplastic response is extremely variable, and we have investigated various factors which might control its production. Our findings indicate, however, that the pattern of DRGs removed, the animal's age at the time of removal, and the survival period are not strictly related to the production of hyperplasia in hindlimb DRGs. Special emphasis has been placed on DRG 10, the caudalmost DRG which normally innervates the cloaca and sends a small projection to the hindlimb. This DRG displayed dramatic cell number increases of up to 564%. In addition, several unique features of the hyperplastic response have been observed in DRG 10. This DRG showed increases in cell number on both the operated and the unoperated sides. It showed hyperplasias in animals subjected to ganglionectomy past metamorphosis as well as during larval development. Finally the production of DRG 10 hyperplasias exclusively occurred in male pre- and postmetamorphic animals. To account for these distinctive features of DRG 10 hyperplasia, baseline studies of the normal course of proliferation and cell death in DRG 10 were undertaken. They reveal no fundamental developmental differences between DRG 10 and other hindlimb DRGs. Other mechanisms responsible for these unusual features of developmental plasticity in DRG 10 are discussed.

Hyperplasia in the spinal sensory system of the frog. II. Central and peripheral connectivity patterns

Central and peripheral connectivity patterns of hyperplastic dorsal root ganglia (DRGs) in Rana pipiens are examined in order to determine the relative roles of peripheral and central contacts in the production of DRG hyperplasias. The hyperplasias are produced in the intact hindlimb DRGs after the removal in tadpoles and young postmetamorphic frogs of neighboring DRGs (Davis and Constantine-Paton, '83). The peripheral target zones of the hyperplastic DRGs, determined by physiological recordings of sensory receptive fields, are found to undergo a significant degree of expansion relative to controls. Peripheral expansion is most pronounced in caudalmost DRG 10, and this effect occurs in experimental animals operated during larval and postmetamorphic stages. Further, anatomical labelling of peripheral sensory fibers coursing to the hindlimb reveals that the hyperplastic DRG 10 actually contains additional fibers projecting to the denervated regions. The central projection of the hyperplastic DRG 10 does not show corresponding increases in longitudinal arborization after the application of horseradish peroxidase to the appropriate dorsal roots. These observations are made on some of the same experimental animals in which peripheral fields are shown to have vastly expanded. We conclude that the peripheral processes of the hyperplastic DRGs are less rigidly specified than the central terminations, and that it is the periphery which plays the primary role in controlling the cell numbers increases. A second aim of this investigation is to identify whether sexually dimorphic connectivity patterns in normal frogs explain the production of DRG 10 hyperplasias exclusively in male experimental animals (Davis and Constantine-Paton, '83). We apply the same techniques used in our connectivity studies of hyperplastic DRGs to the investigation of connectivity patterns of DRG 10s in normal males and females. No sex-dependent differences in peripheral and central connectivity are found. Thus, since normal male and female frogs possess an equivalent amount of target space for DRG 10, the unique production of hyperplasias in male experimental animals cannot be explained solely on the basis of connectivity. We speculate on what other factors may be involved.