Prenatal development of rat primary afferent fibers: II. Central projections

The paired homeodomain protein DRG11 is required for the projection of cutaneous sensory afferent fibers to the dorsal spinal cord

Cutaneous sensory neurons that detect noxious stimuli project to the dorsal horn of the spinal cord, while those innervating muscle stretch receptors project to the ventral horn. DRG11, a paired homeodomain transcription factor, is expressed in both the developing dorsal horn and in sensory neurons, but not in the ventral spinal cord. Mouse embryos deficient in DRG11 display abnormalities in the spatio-temporal patterning of cutaneous sensory afferent fiber projections to the dorsal, but not the ventral spinal cord, as well as defects in dorsal horn morphogenesis. These early developmental abnormalities lead, in adults, to significantly attenuated sensitivity to noxious stimuli. In contrast, locomotion and sensori-motor functions appear normal. Drg11 is thus required for the formation of spatio-temporally appropriate projections from nociceptive sensory neurons to their central targets in the dorsal horn of the spinal cord.

L1 and GAD65 are expressed on dorsal commissural axons in embryonic rat spinal cord

Using immunocytochemical methods, the cell adhesion molecule L1 was detected on axons crossing in the dorsal commissure of developing rat spinal cord. Immunoreactive axons were found in locations similar to fiber bundles illustrated by Ramón y Cajal and designated the anterior, middle and posterior bundles of the dorsal commissure. L1-immunoreactive dorsal commissural axons were first observed on embryonic day 17 (E17), appeared more numerous by E19, and remained detectable in early postnatal ages. The massive middle axon bundles extended bilaterally from the dorsolateral funiculi towards the midline and crossed in the central part of the commissure. In horizontal sections, bundles of L1-labeled middle axons were observed to traverse the dorsal commissure in a periodic pattern along the entire rostrocaudal extent of the spinal cord. Bundles of glutamic acid decarboxylase (GAD65)-positive axons were detected crossing in the middle and posterior regions of the dorsal commissure between E17 and E20. Results from double-labeling experiments demonstrated that GAD65-positive fibers were embedded in larger bundles of L1-labeled axons and that some dorsal commissural axons were double-labeled. To determine if there were axons crossing in the dorsal commissure that did not express L1, double-labeling experiments were conducted using neurofilament and L1 antibodies. Results indicated that bundles of axons identified with anti-neurofilament antibodies were also L1-positive, whereas individually coursing axons within the commissure were L1-negative. The predominance of L1 on fiber bundles traversing the dorsal commissure adds to the growing evidence that this molecule may play a role in axon outgrowth and fasciculation.

Longitudinal elongation of primary afferent axons in the dorsal funiculus of the chick embryo spinal cord

The longitudinal elongation of primary afferent axons (PAAs) in the dorsal funiculus of chick embryo spinal cord was examined using a lipophilic tracer, DiI and immunohistochemistry. The earliest developing PAAs in the brachial segments invaded the spinal cord around embryonic day (E) 3.5. Thereafter, they elongated both rostrally and caudally in the presumptive dorsal funiculus, with frequent contacts between pre-existing axons and later arriving growth cones. By E4, the PAAs had elongated 3 segments both rostrally and caudally. In the course of their longitudinal elongation, the PAAs shifted their trajectory dorsally within the dorsal funiculus. By E6-6.5, the PAAs had extended as far as 10 segments rostrally and 6 segments caudally in the dorsal funiculus, and collaterals began to enter the dorsal horn. By E9, the PAAs extended up to 13 segments rostrally and 7 segments caudally, and collaterals reached the ventral spinal cord. During their longitudinal course, the PAAs shifted their trajectory medially within the dorsal funiculus.

Afferent ingrowth and onset of activity in the rat trigeminal nucleus

A novel in vitro preparation, consisting of the rat brainstem with the trigeminal ganglion attached, has been used to study the anatomical and functional development of the trigeminal nucleus from embryonic day (E)13 to postnatal day (P)6. Neurobiotin injections into the trigeminal ganglion showed that primary afferents had reached the trigeminal tract by E13 and had grown simple, mainly unbranched, collaterals into all levels of the nucleus by E15. By E17, these collaterals were extensively branched, with occasional boutons present. Patches of intense neurobiotin-labelled terminals, corresponding to whisker-related patterns, were first seen at E20 and became clearer over the next few days. Terminal arbours at this stage were relatively localized and densely branched, with many boutons. Responses from the trigeminal nucleus were recorded with suction electrodes, following stimulation of the trigeminal ganglion. Recordings from the main sensory nucleus showed a postsynaptic response was first present at E15. At E16, bath application of AP5 and DNQX showed that the response contained both NMDA and AMPA components, with NMDA predominating (75%). The NMDA : AMPA ratio remained high until P1, then gradually declined to 50% by P6. The postsynaptic response was also reduced by bath application of bicuculline, indicating the presence of a GABAA-mediated excitatory component. GABAergic excitation was present at all ages but was maximal from E20 to P1, the age at which whisker-related patterns are developing. It is hypothesized that both GABAergic excitation and NMDA receptor activation play a role in the consolidation of trigeminal connections, and are thus important in the development of whisker-related patterns.

Neonatal capsaicin treatment prevents the normal postnatal withdrawal of A fibres from lamina II without affecting fos responses to innocuous peripheral stimulation

The development of spinal cord sensory pathways has been investigated in postnatal day (P) 21 rat pups following neonatal capsaicin treatment. Capsaicin-induced destruction of C fibres was confirmed by 62% loss of Isolectin B4 (IB4)-binding and an 86% loss of calcitonin gene-related peptide (CGRP)-immunoreactive small diameter dorsal root ganglion cells. Neonatal capsaicin treatment prevented the normal withdrawal of choleragenoid-horseradish peroxidase (B-HRP)-labelled A fibres from lamina II (substantia gelatinosa) to deeper laminae postnatally. A fibre terminals projected more dorsally, extending into 43% of lamina II compared to vehicle-treated littermates. A small cell loss in, and/or shrinkage of, substantia gelatinosa cannot account for this. These support the concept of a competitive interaction between A and C fibre afferents to establish final terminal fields. However the continued exuberant A fibre termination in capsaicin-treated rats did not lead to continued c-fos induction in the superficial dorsal horn by innocuous stimulation. In normal development, exuberant A fibre terminals coincide with c-fos activation in lamina II by innocuous skin stimulation [23]. Despite the continued presence of exuberant A fibre terminals, c-fos was not induced by innocuous peripheral stimulation in P21 capsaicin-treated rats implying that these superficial terminals do not activate lamina II neurons in the same way as in the neonate.

Ectopic adenoviral vector-directed expression of Sema3A in organotypic spinal cord explants inhibits growth of primary sensory afferents

Sema3A (Sema III, SemD, collapsin-1) can induce neuronal growth cone collapse and axon repulsion of distinct neuronal populations. To study Sema3A function in patterning afferent projections into the developing spinal cord, we employed the recombinant adenoviral vector technique in embryonic rat spinal cord slices. Virus solution was injected in the dorsal aspect of organotypic spinal cord cultures with segmentally attached dorsal root ganglia (sc-DRG). In cultures grown in the presence of nerve growth factor (NGF), injected either with the control virus AdCMVLacZ or with vehicle only, afferent innervation patterns were similar to those of control. However, unilateral injection of AdCMVSema3A/AdCMVLacZ in sc-DRG slices revealed a strong inhibitory effect on NGF-dependent sensory afferent growth. Ectopic Sema3A in the dorsal spinal cord, the target area of NGF-responsive DRG fibers in vivo, created an exclusion zone for these fibers and as a result they failed to reach and innervate their appropriate target zones. Taken together, gain of Sema3A function in the dorsal aspect of sc-DRG cultures revealed a dominant inhibitory effect on NGF-dependent, nociceptive sensory DRG afferents, an observation in line with the model proposed by E. K. Messersmith et al. (1995, Neuron 14, 949-959), suggesting that Sema3A secreted by spinal cord cells can act to repel central sensory fibers during the formation of lamina-specific connections in the spinal cord.

Development of specific connectivity between premotor neurons and motoneurons in the brain stem and spinal cord

Astounding progress has been made during the past decade in understanding the general principles governing the development of the nervous system. An area of prime physiological interest that is being elucidated is how the neural circuitry that governs movement is established. The concerted application of molecular biological, anatomical, and electrophysiological techniques to this problem is yielding gratifying insight into how motoneuron, interneuron, and sensory neuron identities are determined, how these different neuron types establish specific axonal projections, and how they recognize and synapse upon each other in patterns that enable the nervous system to exercise precise control over skeletal musculature. This review is an attempt to convey to the physiologist some of the exciting discoveries that have been made, within a context that is intended to link molecular mechanism to behavioral realization. The focus is restricted to the development of monosynaptic connections onto skeletal motoneurons. Principal topics include the inductive mechanisms that pattern the placement and differentiation of motoneurons, Ia sensory afferents, and premotor interneurons; the molecular guidance mechanisms that pattern the projection of premotor axons in the brain stem and spinal cord; and the precision with which initial synaptic connections onto motoneurons are established, with emphasis on the relative roles played by cellular recognition versus electrical activity. It is hoped that this review will provide a guide to understanding both the existing literature and the advances that await this rapidly developing topic.

Alterations in the distribution of stimulus-evoked c-fos in the spinal cord after neonatal peripheral nerve injury in the rat

Neonatal peripheral nerve injury results in a significant rearrangement of the central terminals of surviving axotomized and adjacent intact primary afferents in the dorsal horn of the spinal cord. This study investigates the ability of these afferents to make functional contacts with dorsal horn cells, using c-fos expression as a marker of synaptic activation. Graded electrical stimulation at A- or C-fiber strength of either the neonatally axotomized sciatic nerve or the adjacent uninjured saphenous nerve was performed in adult rats. Stimulation of the contralateral uninjured nerve served as a control. Quantitative examination of the number and distribution of c-fos-labeled cells in the spinal cord laminae was performed. Electrical stimulation of the previously axotomized sciatic nerve at A-fiber intensity resulted in many labeled profiles in laminae I-V of the lumbar spinal cord on the experimental as compared to the contralateral side. Electrical stimulation of uninjured saphenous nerve or saphenous-nerve-innervated skin (using pin electrodes) at A-fiber intensity did not evoke c-fos. Stimulation of the saphenous nerve at C-fiber intensity, however, resulted in a significant increase in the number and distribution of c-fos-labeled profiles in laminae I-V on the experimental side as compared to the contralateral control side. The results show that the distribution of c-fos-expressing cells after neonatal nerve injury is compatible with the previously demonstrated distribution of sprouting of primary afferents belonging to an uninjured nerve adjacent to an injured nerve, and that the surviving axotomized afferents are capable of transmitting signals to postsynaptic cells. These findings indicate that Abeta afferent stimulation of injured but not uninjured afferents elicits c-fos expression in postsynaptic cells. This may reflect an injury-induced maintenance of a normal developmental process whereby Abeta stimulation elicits c-fos in dorsal horn neurons.

On the problem of lamination in the superficial dorsal horn of mammals: a reappraisal of the substantia gelatinosa in postnatal life

Although it is one of the most distinctive and earliest recognized features in the spinal cord, the substantia gelatinosa (SG) remains among the most enigmatic of central nervous system regions. The present neuroanatomical studies employed transganglionic transport of horseradish peroxidase conjugates of choleragenoid (B-HRP) and the B4 isolectin of Bandeiraea simplicifolia (IB4-HRP) on opposite sides to compare the projection patterns of myelinated and unmyelinated cutaneous primary afferents, respectively, within the superficial dorsal horn of the spinal cord in postnatal mice, from shortly after birth to adulthood. Putative unmyelinated afferents labeled with IB4-HRP gave rise to a dense sheet of terminal-like labeling restricted to the outer half of the SG. In contrast, myelinated inputs labeled with B-HRP gave rise to a similarly dense sheet of terminal-like labeling that occupied the inner half of the SG. This adult organization, with two dense sheets of terminal labeling in the superficial dorsal horn, was clearly evident shortly after birth using these markers, prior to the emergence of the SG. Furthermore, the location of the SG proper varied considerably within the dorsoventral plane of the dorsal horn according to mediolateral and segmental locations, a finding that was also seen in comparative studies in rat and cat. These findings caution against equating the SG in particular, and the superficial dorsal horn in general, with nociceptive processing; at minimum, the SG subserves a clear duality of function, with only a thin portion of its outermost aspect devoted to pain.

Development of motoneurons and primary sensory afferents in the thoracic and lumbar spinal cord of the South American opossum Monodelphis domestica

The postnatal development of the primary sensory afferent projection to the thoracic (T4) and lumbar (L4) spinal cord of the marsupial species Monodelphis domestica was studied by using anterograde and retrograde neuronal tracers. Large numbers of primary afferents and motoneurons were labelled by application of the carbocyanine dye DiI into individual dorsal root ganglia (DRG) afferents in short-term organ cultures. Dorsal root axons had entered the cord at birth, but most primary afferent innervation of the grey matter and the establishment of cytoarchitectural lamination occurs postnatally. In addition to ipsilateral projections, some primary afferents that projected to the dorsal horn extended across the midline into the equivalent contralateral regions of the grey matter. Similarly, motoneuron dendrites occasionally extended across midline and into the contralateral grey matter. The first fibres innervating the spinal cord project to the ventral horn and formed increasingly complex terminal arbours in the motor columns between P1 and P7. After P5 many afferents were seen projecting to the dorsal horn, with the superficial dorsal horn being the last region of the spinal grey to be innervated. Histochemical labelling with the lectin Griffonia simplicifolia indicated that C fibre primary afferents had arborised in the superficial dorsal horn by P14. The sequence of primary afferent innervation is thus similar to that described in the rat, but this sequence occurs over a period of several weeks in Monodelphis, compared with several days in the rat.

The postnatal development of spinal sensory processing

The mechanisms by which infants and children process pain should be viewed within the context of a developing sensory nervous system. The study of the neurophysiological properties and connectivity of sensory neurons in the developing spinal cord dorsal horn of the intact postnatal rat has shed light on the way in which the newborn central nervous system analyzes cutaneous innocuous and noxious stimuli. The receptive field properties and evoked activity of newborn dorsal horn cells to single repetitive and persistent innocuous and noxious inputs are developmentally regulated and reflect the maturation of excitatory transmission within the spinal cord. These changes will have an important influence on pain processing in the postnatal period.

Developmental plasticity of ascending spinal axons studies using the North American opossum, Didelphis virginiana

The objectives of the present study were to determine if axons of all ascending tracts grow through the lesion after transection of the thoracic spinal cord during development in the North American opossum, and if so, whether they reach regions of the brain they normally innervate. Opossum pups were subjected to transection of the mid-thoracic cord at PD5, PD8, PD12, PD20, or PD26 and injections of Fast Blue (FB) into the lower thoracic or upper lumbar cord 30-40 days or 6 months later. In the PD5 transected cases, labeled axons were present in all of the supraspinal areas labeled by comparable injections in unlesioned, age-matched controls. In the experimental cases, however, labeled axons appeared to be fewer in number and in some areas more restricted in location than in the controls. When lesions were made at PD8, labeled axons were present in the brain of animals allowed to survive 30-40 days prior to FB injections but they were not observed in those allowed to survive 6 months. When lesions were made at PD12 or later, labeled axons were never found rostral to the lesion. It appears, therefore, that axons of all ascending spinal pathways grow though the lesion after transection of the thoracic cord in developing opossums and that they innervate appropriate areas of the brain. Interestingly, the critical period for such growth is shorter than that for most descending axons, suggesting that factors which influence loss of developmental plasticity are not the same for all axons.

Tissues exhibiting inhibitory [correction of inhibiory] and repulsive activities during the initial stages of neurite outgrowth from the dorsal root ganglion in the chick embryo

To elucidate the mechanisms underlying the projection of dorsal root ganglion (DRG) axons into the dorsal root entry zone in the dorsolateral region of the spinal cord, we examined tissue interactions which affect neurite outgrowth from DRG. We cultured explants or dissociated cells of DRG from embryonic day 4 (E4) chick embryos in combination with E3 spinal cord, notochord, and dermomyotome in three-dimensional collagen gels. The ventral spinal cord, notochord, and dermomyotome, which are located close to the initial projection pathway of DRG but do not receive direct innervation, strongly inhibited DRG neurite outgrowth and repelled DRG neurites. These inhibitory/repulsive cues appear diffusible in nature, because this activity was observed in the absence of direct contacts between tissue explants and DRG neurites. Furthermore, in heterochronic cultures, E9 DRG lost its responsiveness to inhibitory/repulsive factors from E3 ventral spinal cord, while retaining responsiveness to E3 notochord and dermomyotome, suggesting that the E3 ventral spinal cord may secrete a different inhibitory/repulsive signal than notochord and dermomyotome. Putative inhibitory/repulsive signals secreted from tissues along the axonal pathway may serve to guide growing DRG axons to the dorsal root entry zone.

Peripheral target-specific influences on embryonic neurite growth vigor and patterns

We examined axon-target interactions in cocultures of embryonic rat trigeminal, dorsal root, nodose, superior cervical ganglia or retina with a variety of native or foreign peripheral targets such as the whisker pad, forepaw, and heart explants. Axon growth into these peripheral target tissues was analyzed by the use of lipophilic tracer DiI. Embryonic day 15 dorsal root and trigeminal axons grew into isochronic normal and foreign cutaneous targets. Both axon populations avoided the same age heart tissue, but grew profusely into younger (embryonic day 13) or older (postnatal) heart explants. In contrast, embryonic day 15 superior cervical or nodose ganglion axons grew heavily into the same age heart and forepaw explants and to a lesser extent into the whisker pad explants. Embryonic day 15 retinal axons grew into all three peripheral targets used in this study. Primary sensory and sympathetic axons, but not retinal axons, formed target-specific patterns in the whisker pad and forepaw explants. DiI-labeling and immunostaining of primary sensory neurons in coculture revealed that these neurons retain their bipolar characteristics, and express class-specific markers such as parvalbumin, calcitonin gene-related peptide and TrkA receptors. In the whisker pad explants, axons positive for all three markers were seen to form patterns around the follicles. Our results indicate that developing peripheral targets can attract and support axon growth from a variety of sources. Whereas neurotrophins play a major role in attracting and supporting survival of subpopulations of sensory neurons, other substrate-bound or locally released molecules must regulate sensory neurite growth into specific peripheral and central targets.

Peripheral and central target requirements for survival of embryonic rat dorsal root ganglion neurons in slice cultures

Developmental cell death in the nervous system usually is controlled by the availability of target-derived trophic factors. It is well established that dorsal root ganglia (DRG) neurons require the presence of their peripheral target for survival, but because of their central projections, it is possible that the spinal cord also may be required. Before examining this possibility in rat embryos, we first used terminal deoxynucleotidyl transferase-mediated biotinylated UTP nick end labeling (TUNEL) to determine that thoracic DRG cell death occurred from embryonic day 15 (E15) to E18. To determine the target requirements of DRG neurons, we used organotypic slice cultures of E15 thoracic trunk segments. After peripheral target removal, essentially all DRG neurons disappeared within 5 d. In contrast, after removal of the spinal cord, approximately half of the DRG neurons survived for at least 8 d. Hence, some E15 DRG neurons could survive without the spinal cord. However, those DRG neurons that died after spinal cord ablation apparently required trophic factors from both central and peripheral targets, because the presence of only one of these tissues was not adequate by itself to support this cell group. Addition of neurotrophin-3 (NT-3) to the culture medium rescued some DRG neurons after CNS removal, suggesting a possible role for NT-3 in vivo. In other experiments, cultures were established from older (E16) embryos, and essentially all neurons survived after spinal cord ablation, even without added factors. These and other experiments indicated that approximately 65% of DRG neurons are transiently dependent on the CNS early in development.

Postnatal changes in responses of rat dorsal horn cells to afferent stimulation: a fibre-induced sensitization

1. In vivo extracellular recordings were made of 171 dorsal horn cells in both superficial and deep laminae in urethane-anaesthetized newborn rats aged 3, 6, 10 and 21 days, and their response to single and repeated stimuli to primary afferent fibres investigated. 2. No long-latency spike responses were evoked in response to C fibre stimulation in pups at postnatal day 3 (P3) or P6, while by P10, 35 % of cells had a C fibre response. Latencies of response to A fibre skin stimulation were very long and varied widely in the youngest animals, particularly in superficial cells, but mean latencies decreased with postnatal age, from 33.1 +/- 2.78 ms at P3 to 7.3 +/- 0.3 ms at P21. The mean number of spikes evoked by a single A fibre skin stimulus was remarkably consistent between cells and not significantly different in superficial and deep laminae at each age. The mean value of 5.1 +/- 0.6 at P3 increased to 7.0 +/- 1.4 at P10. 3. Repeated stimulation of cutaneous A fibres at 0.5 Hz at twice the threshold level did not significantly alter the magnitude of the evoked response but led to shifts in latency, or 'latency jitter', which decreased with age. Deeper cells displayed more latency jitter than superficial cells. 4. Repeated stimulation of cutaneous A fibres at 0.5 Hz at twice the threshold level produced considerable sensitization in a population of dorsal horn cells in the neonate. This sensitization was unlike the classic C fibre-evoked 'wind-up' observed in adult dorsal horn. The direct A fibre-evoked activity did not increase, but the background activity increased during repetitive stimulation leading to a prolonged after-discharge beyond the stimulation period. At P6, 33 % of cells were sensitized, displaying a mean after-discharge of 70.6 +/- 18 s. At P10, only 6 % were sensitized, with a mean after-discharge of 63 s, and by P21, sensitization was no longer observed. 5. The present study demonstrates that the postsynaptic activity evoked in neonatal dorsal horn cells by cutaneous afferents differs considerably from that in adults. The results may account for the known behavioural reflex sensitization to low-intensity cutaneous stimulation observed in neonatal rats and man.

Cell adhesion molecules regulate guidance of dorsal root ganglion axons in the marginal zone and their invasion into the mantle layer of embryonic spinal cord

In order to elucidate the mechanisms regulating the projections of dorsal root ganglion (DRG) axons in the dorsal funiculus and invasion into target regions in the mantle layer (prospective gray matter) of the spinal cord, we examined the interactions between DRG axons and spinal cord. DRG neurons were dissociated from chick embryos and cultured for 1-2 days on cryostat sections of the spinal cord at embryonic day 5 (E5) or at E9. E5 and E9 DRG neurons extended neurites onto both marginal zone (prospective white matter) and mantle layer (prospective gray matter) of the spinal cord, suggesting that both of these regions are permissive for neurite growth. When E5 DRG neurites approached cryosections of E5 spinal cord from outside, most of them ran in the marginal zone without invading the mantle layer. In contrast, about half of E9 DRG neurites entered the mantle layer after crossing the marginal zone of E9 spinal cord. These growth patterns of DRG neurites on spinal marginal zone and mantle layer are similar to the pathway formation of DRG axons at comparable stages in vivo; DRG axons run exclusively in the prospective dorsal funiculus before E6, and enter the mantle layer (prospective dorsal horn) to reach the target regions by E9. Perturbation of functions of Ng-CAM, Nr-CAM, and axonin-1/SC2 by adding the specific antibodies in the culture medium increased the ratio of DRG neurites entering the mantle layer of E5 spinal cord, suggesting that these cell adhesion molecules are involved in keeping DRG neurites in the marginal zone. Taken together with the expression of Ng-CAM, Nr-CAM, and axonin-1/SC2, these CAMs on DRG axons may regulate the guidance of these axons in the marginal zone before E6, and the subsequent decrease in the relative levels of these CAMs might allow DRG axons to invade the target mantle layer.

Border controls at the mammalian spinal cord: late-surviving neural crest boundary cap cells at dorsal root entry sites may regulate sensory afferent ingrowth and entry zone morphogenesis

Boundary caps (BCs) form when neural crest cells, migrating ventrally alongside the neural tube, arrest at sites where axons will enter and exit. However, nothing is known of their subsequent fate and functions. We have found late-surviving neural crest BC cell clusters at proximal dorsal root entry sites throughout rat spinal cord development. Sensory afferents cross BCs to enter the spinal cord, while exiting astrocyte processes, destined to form the dorsal root entry zone (DREZ) after birth, are temporarily stalled in their vicinity. To test whether contact with BC cells influences neurite outgrowth from dorsal root ganglia, neurons were cultured on embryonic dorsal root/spinal cord cryosections. Neurites that entered CNS territory preferentially extended over BC cells. Thus, BC cells could be instrumental in regulating afferent ingrowth and DREZ morphogenesis in mammalian spinal cord development.

Properties of individual embryonic primary afferents and their spinal projections in the rat

Embryonic (E19-E20) and early postnatal (P2) spinal cords with intact saphenous and sciatic nerves were isolated and placed in aerated artificial cerebral spinal fluid (CSF). Intracellular recordings were made from cells in the L2-L6 dorsal root ganglia using microelectrodes filled with 3 M potassium acetate or 5% neurobiotin (NB) in 1 M potassium acetate. Several physiological properties of adequately impaled cells were measured, including peripheral conduction velocity, action potential (AP) amplitude and duration, duration of afterhyperpolarization (AHP), input impedance, rheobase, presence of inward rectifying current, and maximum somal firing frequency. The extent to which these properties are correlated also was determined. One cell per ganglion was injected with NB. Stained somata and their central projections in the spinal cord were visualized in serial 50 microm sections. Cell size was determined and the central morphology of the central projections examined. Although some fibers were in the process of growing into the spinal cord, others had established projections over several millimeters in the dorsal columns. Although most of these fibers supported projections in the gray matter, 22% only maintained fibers in the dorsal columns. Fibers with projections in the dorsal horn exhibited three types of morphology: projections confined to the superficial dorsal horn (laminae I, II); terminals confined to laminae III-V; and projections spanning laminae II-V. In addition, some embryonic fibers maintained projections to the dorsal horn that extended over five lumbar segments. Somal APs could be divided into two groups: broad spikes with inflections on their falling phase and narrow spikes without inflections. On average, cells with broad spikes (BS) had the following characteristics: slower peripheral conduction velocity, larger amplitude, higher rheobase and input impedance, longer AHP duration, and lower maximum firing frequency. There were significant correlations between conduction velocity and several of the physiological properties. Conduction velocity was negatively correlated with AP duration, rheobase, and input impedance and positively correlated with maximum firing frequency. Comparisons between spike shape and central morphology revealed that cells lacking collaterals in the gray matter and those with projections in the superficial dorsal horn always had broad somal spikes with inflections. Those with projections confined to laminae III-V always had narrow somal spikes (NS).

Cerebroglycan, a developmentally regulated cell-surface heparan sulfate proteoglycan, is expressed on developing axons and growth cones

Cerebroglycan is a glycosylphosphatidylinositol-linked integral membrane heparan sulfate proteoglycan found exclusively in the developing nervous system. In the rodent, cerebroglycan mRNA first appears in regions containing newly generated neurons and typically disappears 1 to several days later (Stipp et al., 1994, J. Cell Biol. 124:149-160). To gain insight into the roles that cerebroglycan plays in the developing nervous system, monospecific antibodies were prepared and used to localize cerebroglycan protein. In the rat, cerebroglycan was prominantly expressed on axon tracts throughout the developing brain and spinal cord, where it was found at times when axons are actively growing, but generally not after axons have reached their targets. Cerebroglycan was also found on neuronal growth cones both in vivo and in vitro. Interestingly, cerebroglycan immunoreactivity was rarely seen in or around neuronal cell bodies. Indeed, by examining the hippocampus at a late stage in development-when most neurons no longer express cerebroglycan but newly generated granule neurons do-evidence was obtained that cerebroglycan is strongly polarized to the axonal, and excluded from the somatodendritic, compartment of neurons. The timing and pattern of cerebroglycan expression are consistent with a role for this cell-surface heparan sulfate proteoglycan in regulating the growth or guidance of axons.

Cues intrinsic to the spinal cord determine the pattern and timing of primary afferent growth

We have used organotypic cultures of embryonic rat spinal cord and dorsal root ganglia (DRG) to study the development of central projections of primary sensory afferent axons that express calcitonin gene-related peptide (CGRP). In vivo, small- and medium-diameter CGRP-positive primary afferents terminate in laminae I, II, and V of the spinal cord and do not enter the ventral horn. A similar pattern of CGRP-positive axonal projections was observed in spinal cord slices of Day 16 embryos (E16) maintained in culture for 6 days. Both intact and dissociated DRG neurons showed the same pattern of central arborization, indicating that complex intercellular interactions between DRG neurons are not required for laminar specific targeting. Furthermore, targeting to the dorsal horn and avoidance of the ventral horn was observed in isolated dorsal and ventral hemicords, suggesting that separate mechanisms mediate the avoidance of CGRP-positive axons from the ventral horn and the elaboration of the afferent arbors within the dorsal horn. CGRP-positive afferents can grow into the dorsal horn only during a brief time window. Cultures of age-matched (isochronic) DRG and spinal cord from E14, E16, and E18 animals showed the characteristic pattern of CGRP-positive axon arborization, while cultures from E20 and neonatal animals did not. Heterochronic cultures indicate that it is the age of the spinal cord, and not the age of the DRG, that determines the ability of the CGRP-positive afferents to arborize within the dorsal horn. Together these results demonstrate that cues intrinsic to the spinal cord can direct sensory projections to appropriate locations in the spinal cord.

Overexpression of nerve growth factor in epidermis of transgenic mice preserves excess sensory neurons but does not alter the somatotopic organization of cutaneous nerve projections

To determine how target-derived nerve growth factor (NGF) affects sensory neuronal survival and the development of topographic nerve projections in the spinal cord, anatomical studies were performed on transgenic mice that overexpress NGF in skin and other keratinized epithelial structures. Transgenic animals showed a 100% increase in the number of sensory neurons in specific dorsal root ganglia and exhibited significantly more fibers immunoreactive for calcitonin gene-related peptide in the dorsal horn compared to control animals. This confirms earlier studies which suggested that naturally occurring sensory neuronal death is decreased, or eliminated, in the transgenic mice. Nerve labeling studies showed that the somatotopic organization of cutaneous nerve projections was not altered in the transgenic animals. These data suggest that neuronal death does not act to remove sensory neurons that project to inappropriate regions of the spinal cord.

Prenatal development of rat primary afferent fibers: I. Peripheral projections

Development of the peripheral innervation patterns of the L1-S1 lumbosacral ganglia and motor segments in embryonic day 12-17 (E12-17) rat embryos was examined using carbocyanine dyes. Individual dorsal root ganglia (DRGs) and/or isolated ventral horn (VH) segments, or individual peripheral nerves, were isolated in rat embryos fixed at different stages and filled with one of three carbocyanine dyes; DiI, DiA, and DiO. Individual experimental preparations included labeling of 1) single DRGs; 2) multiple DRGs with alternating dyes, DiO, DiI, and DiA; 3) single isolated VH segments; 4) multiple VH segments with alternating dyes; 5) single VH segments and the corresponding segmental DRGs with different dyes; and 6) two or more individual peripheral nerves labeled with different dyes. Results from these preparations have shown that the first fibers exited the lumbar ventral horn and DRGs at E12. At E13 major nerve trunks (e.g., femoral and sciatic) were visible as they exited the plexus region. By E14 afferent fibers were present in the epidermis of the proximal hindlimb, and the major nerve trunks extended into the leg. Fibers originating from L3 to L5 (DRG and VH) reached the paw by E14.5-E15, and the epidermis of the most distal toes was innervated by E16-E16.5. While afferent fibers and motor axons of the same segmental origin mixed extensively in the spinal nerve, fibers of different segmental origin combined in the plexus and major nerve trunks with little or no interfascicular mixing. Dermatomes observed at E14 were in general spotty and non-contiguous. However, by E16 the dermatomes resembled mature forms with substantial overlap only between adjacent ones. Thus the adult pattern of spatial relationships between cutaneous afferent fibers in the periphery is established early in development.