Selective innervation of target regions within fetal mouse spinal cord and medulla explants by isolated dorsal root ganglia in organotypic co-cultures

Segregated neural explants exhibit co-oriented, asymmetric, neurite outgrowth

Explants of embryonic chick sympathetic and sensory ganglia were found to exhibit asymmetric radial outgrowth of neurites under standard culture conditions with or without exogenous Nerve Growth Factor [NGF]. Opposing sides of an explant exhibited: a) differences in neurite length and, b) differences in neurite morphology. Strikingly, this asymmetry exhibited co-orientation among segregated, neighboring explants. The underlying mechanism(s) of the asymmetry and its co-orientation are not known but appear to depend on cell clustering because dissociated sympathetic neurons do not exhibit co-orientation whereas re-aggregated clusters of cells do. This emergent behavior may be similar to the community effect described in other cell types. If a similar phenomenon exists in the embryo, or in maturity, it may contribute to the establishment of proper orientation of neurite outgrowth during development and/or injury-induced neuronal plasticity.

Neurotropism revisited

The purpose of this study is to re-examine the probable directive effect of the distal stump of a severed peripheral nerve on regenerating axons. Forssman postulated the existence of such a directive influence and Cajal interpreted it as chemotactic in nature. This view was subsequently refuted by Weiss and Taylor. In our study the proximal stumps of transected rodent sciatic nerve were inserted into the single inlet end of a Y-shaped autogenous inferior vena cava graft. Into one limb of the double outlet end, namely the common iliac nerve bifurcation, the distal stump of the same sciatic nerve was inserted, while the counter limb was ligated in one group, left open in the second group, inserted with a segment of autogenous tendon in the third, and grafted with a segment of autogenous nerve in the fourth group. Both outlets were left unoccupied in yet another group as the control. The vena cava conduit was prepared so that a 1.5 cm gap existed between the proximal stumps of the sciatic nerve and the distal sciatic nerve stumps and the tendon grafts respectively. The grafted sciatic nerves were explored and biopsied after 12 weeks. The direction of nerve tissue regeneration in each group was analyzed histologically. Predilection of the regenerating nerve fibers toward the distal stumps was observed in each of the test groups. These results indicate the existence of a guiding influence at the distal stump toward the regeneration nerve fibers.

A two-compartment in vitro model for studies of modulation of nociceptive transmission

Here we present a two-compartment in vitro model in which embryonic rat dorsal root ganglia (DRG) neurons are cultured separately from their target dorsal horn neurons. Although separated, synaptic contact can be established between the peripheral and central neurons since the system allows the DRG axons to project into the other compartment, which contains a network of dorsal horn neurons. The efficacy of the model was evaluated by immunocytochemical, calcium imaging and electrophysiological experiments. The results showed that a subpopulation of the DRG neurons had nociceptor characteristics and that these made synaptic contact with the dorsal horn network. Application of current pulses, according to the stimulus paradigm used, evoked action potentials in DRG axons selectively. This in turn gave rise to increased postsynaptic activity in the network of dorsal horn neurons. The model offers a high degree of efficiency since large numbers of DRG axons can be stimulated simultaneously, thus permitting recording of strong output responses from the dorsal horn neurons. This in vitro model provides a means for studying the mechanisms by which modulatory factors, such as immunoregulatory molecules, applied at either the PNS or the CNS level, can affect synaptic activity and nociceptive transmission in single neurons or network of neurons in the dorsal horn.

Development of specific synaptic network functions in organotypic central nervous system (CNS) cultures: implications for transplantation of CNS neural cells in vivo

This article provides a broad overview of the significant roles that morphophysiologic analyses of organotypic cultures of neural tissues explanted in vitro-initiated during the 1950s-have played in stimulating the more recent development of techniques for transplantation of neural cells and tissues into specific regions of the central nervous system (CNS) in vivo. The demonstrations by Crain and co-workers in the 1950s and 1960s that fetal rodent and human CNS neurons can continue to develop a remarkable degree of mature structure and function during many months of complete isolation in culture provided crucial evidence that development of many organotypic properties of nerve cells is regulated by epigenetic factors that ensure rather stereotyped expression despite wide variations in environmental conditions. These in vitro studies strongly suggested that fetal neural cells should, indeed, be capable of even more highly organotypic development after transplantation in vivo, as has been elegantly demonstrated by many of the successful CNS transplantation studies reviewed here.

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.

Modulatory effects of Gs-coupled excitatory opioid receptor functions on opioid analgesia, tolerance, and dependence

Electrophysiologic studies of opioid effects on nociceptive types of dorsal root ganglion (DRG) neurons in organotypic cultures have shown that morphine and most mu, delta, and kappa opioid agonists can elicit bimodal excitatory as well as inhibitory modulation of the action potential duration (APD) of these cells. Excitatory opioid effects have been shown to be mediated by opioid receptors that are coupled via Gs to cyclic AMP-dependent ionic conductances that prolong the APD, whereas inhibitory opioid effects are mediated by opioid receptors coupled via Gi/Go to ionic conductances that shorten the APD. Selective blockade of excitatory opioid receptor functions by low (ca. pM) concentrations of naloxone, naltrexone, etorphine and other specific agents markedly increases the inhibitory potency of morphine or other bimodally acting agonists and attenuates development of tolerance/dependence. These in vitro studies have been confirmed by tail-flick assays showing that acute co-treatment of mice with morphine plus ultra-low-dose naltrexone or etorphine remarkably enhances the antinociceptive potency of morphine whereas chronic co-treatment attenuates development of tolerance and naloxone-precipitated withdrawal-jumping symptoms.

Networks formed by dorsal root ganglion neurites within spinal cord explants: a computer-aided analysis of HRP intracellularly labeled neurons

Dorsal root ganglion (DRG) neurons from rat embryos were explanted either alone or with the attached spinal cord (SC). Neuritic processes were mapped out histologically using an intracellular iontophoretic HRP method. Computer reconstructions and morphometric parameters allowed a comparative quantitative analysis of the DRG neurons and of their neuritic processes in both models. The first model, strongly dependent on nerve growth factor (NGF) for its survival, developed large multidirectional processes. The second model showed a bipolar distribution of its neuritic processes, the central one entering predominantly the dorsolateral part of the cord explant and ramifying both homo- and heterolaterally. The quantitative data revealed a significant decrease in the overall size of the neuritic networks of the second model (with attached SC). A discriminant analysis permitted the recognition of these two populations of DRG neurons. The role of the spinal cord explant, and more precisely the target cells of the dorsal horn, was considered to be a prominent factor in the development of the DRG neuritic networks.

Involvement of a chondroitin sulfate proteoglycan in the avoidance of chick epidermis by dorsal root ganglia fibers: a study using beta-D-xyloside

In 7-day chick embryo dorsal root ganglia and epidermis cocultures, nerve fibers avoid the epidermis. Previous studies have indicated that glycoproteic factors, secreted by epidermis, could be involved in this phenomenon. Treatment of epidermis by beta-D-xyloside, a specific proteoglycan synthesis inhibitor, abolishes the avoidance reaction. The same result is obtained when anti-chondroitin sulfate antibodies are added to the culture medium. Using HPLC and 35SO4 labeling combined with chondroitinase and hyaluronidase treatment, it has been demonstrated that chondroitin sulfate is present in the epidermal conditioned medium. This suggests that a chondroitin sulfate proteoglycan secreted by the epidermis is implicated in the neurite avoidance reaction and that epidermis could therefore control its own "noninnervation". In vivo, inhibitory influences by local extracellular components may control the guidance of growth cones during nerve pattern formation.

Preferential cholinergic projections by embryonic spinal cord neurons within cocultured mouse superior cervical ganglia

The development of preferential cholinergic projections of spinal cord neurons within superior cervical ganglia (SCG) was analyzed in vitro using cocultures of SCGs (E17) with organotypic explants of fetal mouse cord (E13). The cord explants consisted of: (1) dorsal vs medioventral strips or mediodorsal vs ventral strips (dissected from levels C8-T4), or (2) transverse sections cut at various levels of the neuraxis. After 4 weeks of coculture, choline acetyltransferase (ChAT) was assayed in individual explants to quantify development of the cholinergic neurotransmitter enzyme (a) within the cord neurons, and (b) within the SCG. An index of cholinergic interaction was calculated as the relative ChAT activity in cocultured ganglion per unit ChAT activity in the ipsilateral cord strip. The highest index value (0.7) was obtained in cocultures with mediodorsal strips of cord. The index of interaction was progressively lower with medioventral (0.4), ventral (0.3) and dorsal (0.1) cord. In cocultures of transverse sections of spinal cord and SCGs, the highest indices of cholinergic interaction (expressed per hemisection of cord) were obtained with cord levels T1/T2 (1.0) and T5 (0.9). The index decreased with T9 (0.7) and was significantly lower with segments C2/C3 (0.3) and L2/L3 (0.19). Addition of a skeletal muscle target explant to the cord-SCG cocultures did not alter the preferential index of interaction between SCG and upper thoracic cord levels. Furthermore, the cholinergic cord neurons in medioventral strips did not promote increase of ChAT activity into equally accessible cocultured ganglia of inappropriate phenotype, e.g. sensory dorsal root ganglia. Decentralization of SCGs after coculture with appropriate T1/T2 cord resulted in loss of ganglionic ChAT activity. Electrical stimulation of the medial region in T1/T2 cord explants evoked compound ganglion action potentials in cocultured SCGs. The ganglion responses were blocked by hexamethonium. These data suggested that neurons located in the medial region of upper thoracic cord (presumably autonomic preganglionic) are able to develop enhanced cholinergic projections within cocultured SCGs, in comparison with neurons located in ventral cord (presumably motoneurons). In contrast, dorsal cord neurons showed no significant cholinergic interaction with SCGs. Furthermore, neurons located in upper thoracic spinal cord segments develop enhanced cholinergic projections within cocultured SCGs in comparison with neurons located in cervical and lumbar cord segments.

Morphological and biochemical differences expressed in separate dissociated cell cultures of dorsal and ventral halves of the mouse spinal cord

The neuronal properties of separate dissociated cell cultures of dorsal and ventral halves of the embryonic mouse spinal cord (E 13.5) were investigated. Ventral-half cultures grew on a variety of substrates and in a variety of media; dorsal-half cultures required a non-neuronal feeder layer and supplemented medium for survival. The two types of cultures differed in their morphological and biochemical properties. Ventral-half neurons remained well separated on the culture plate, whereas dorsal-half neurons tended to aggregate. Lucifer yellow fills showed that ventral-half neurons were substantially larger and had more processes than dorsal-half neurons. Because of the large size and good separation of the neurons, ventral-half cultures provide an especially attractive system for electrophysiologic and morphologic studies. Ventral-half cultures were highly enriched for choline acetyltransferase (ChAT) activity and had more neurons that stained for intracellular acetylcholinesterase (AChE); dorsal-half cultures were enriched for glutamic acid decarboxylase (GAD) activity, and high-affinity gamma-aminobutyric acid (GABA) uptake. The clear differences between the two cultures indicate that many morphological and biochemical properties are already specified on embryonic day 13.5.

Selective reinnervation of distal motor stumps by peripheral motor axons

Random matching of regenerating axons with Schwann tubes in the distal nerve stump is thought to contribute to the often poor results of peripheral nerve repair. Motor axons would be led to sensory end organs and sensory axons to motor end plates; both would remain functionless. However, the ability of regenerating axons to differentiate between sensory and motor environments has not been adequately examined. The experiments reported here evaluated the behavior of regenerating motor axons when given equal access to distal sensory and motor nerve stumps across an unstructured gap. "Y"-shape silicon chambers were implanted within the rat femoral nerve with the proximal motor branch as axon source in the base of the Y. The distal sensory and motor branches served as targets in the branches of the Y, and were placed 2 or 5 mm from the axon source. After 2 months for axon regeneration, horseradish peroxidase was used to label the motoneurons projecting axons into either the motor or the sensory stump. Equal numbers of motoneurons were labeled from the sensory and motor stumps at 2 mm, but significantly more motoneurons were labeled from the motor stump at 5 mm. (P = 0.016). This finding is consistent with selective reinnervation of the motor stump. Augmentation of this phenomenon to produce specific reunion of individual motor axons could dramatically improve the results of nerve suture.

Molecular and cellular aspects of nerve regeneration

Injury of an axon leads to at least four independent events, summarized in Figure 1: first, deprivation of the nerve cell body from target-derived or mediated substances, which leads to a derepressed or a permissive state; second, disruption of anterograde transport, with a resultant accumulation of anterogradely transported molecules; third, environmental response with possible consequent changes in constituents of the extracellular matrix and substances secreted from the surrounding cells; and fourth, appearance of growth inhibitors and modified protease activity. It seems that the first three of these events are obligatory, but not sufficient, i.e., they lead to a growth state only if the cell body is able to respond to the injury-induced signals from the environment (a and b). The regenerative state is characterized by alterations in protein synthesis and axonal transport and by sprouting activity. The subsequent elongation of the growing fibers depends on a continuous supply of appropriate growth factors. These factors are presumably anchored to the appropriate extracellular matrix that serves as a substratum for elongating fibers. It should be mentioned that the proliferating nonneuronal cells have a conducive effect on regeneration by forming a scaffold for the growing fibers. Accordingly, the lack of regeneration may stem from a deficiency in the ability of glial cells to provide the appropriate soluble components or from insufficient formation of extracellular matrix. In this respect, one may consider regeneration of an injured axon as a process which involves regeneration of both the nonneuronal cells and the supported axons. The regeneration of glial cells may fulfill the rules which are applied to regeneration of any other proliferating tissue. Furthermore, the processes of regeneration in the axon and the glial cells are mutually dependent. Perhaps the triggering factors provided by the nonneuronal cells affect the nonneuronal cells themselves by modulating their postlesion gliosis and thereby inducing their appropriate activation. In such a case, regeneration of nonneuronal cells may resemble an autocrine type of regulation that exists also during ontogeny. The growth regulation is shifted back to the paracrine type upon neuronal maturation or cessation of axonal growth. When the elongating fibers reach the vicinity of the target organ, they are under the influence of the target-derived factors, which guide the fibers and eventually cease their elongation.

A study of neurotrophism in a primate model

This study investigated the existence of neurotrophism in a primate model. In eight adult cynomolgus monkeys the sensory component of the femoral nerve was sectioned and introduced into the proximal channel of a silicone Y chamber. The proximal stump was given distal choices of various tissues inserted into the remaining arms of the silicone Y chamber. The targets presented were combinations of tendon, muscle, intact distal nerve, distal nerve graft, or an empty silicone channel. After 6 weeks, ultrastructural analysis confirmed axonal growth toward distal nerve tissue, while minimal or no nerve regeneration was directed toward tendon, muscle, or the empty silicone channel. The results showed that either a distal nerve stump or a nerve graft will act as a specific target to the regenerating primate proximal nerve stump.

Earliest sensory nerve fibres are guided to peripheral targets by attractants other than nerve growth factor

Recent studies have shown that developing nerve fibres grow directly to their targets and are guided by specific cues, but the nature of these cues and the mechanism of guidance remain unknown. The growth of sympathetic axons towards an artificial source of nerve growth factor (NGF) in vivo and of sensory neurites up a concentration gradient of NGF in vitro has supported the hypothesis that NGF, produced by target tissues, acts as a chemotactic attractant for these nerve fibres during development. Both these studies and those of the influence of NGF or target tissues on neurite growth in vitro were conducted late in development when, following target encounter, the neurones had become dependent on NGF or target for survival. Here we have co-cultured embryonic mouse sensory neurones and their peripheral target tissue at a stage preceding their contact in vivo. Neurites grew directly and exclusively towards their own target but not to regionally inappropriate peripheral tissue. Antiserum to isogeneric NGF did not reduce this outgrowth but did reduce undirected neurite outgrowth which occurred in co-cultures of older neurones with denervated target tissue. These results demonstrate that agents other than NGF guide neurites of NGF-responsive neurones in development.

An in vivo assay of neurotropic activity

Previous studies in this laboratory indicate that diffusible factors from distal stumps of transected peripheral nerves exert a neurotropic effect on regenerating nerve fibers in vivo. The present study was performed (a) to strengthen this hypothesis, and (b) to begin to determine the distribution and identity of putative neurotropic factors in peripheral nerves. Rat sciatic nerves were transected and inserted into the inlet end of a hollow 6 mm-long Y-shaped implant made of medical-grade Silastic tubing. To one of the outlet ends was attached an Elvax pellet impregnated with whole homogenate from a single sciatic nerve. The other outlet was attached to a pellet not containing homogenate. Homogenates assayed were obtained from distal stumps of transected nerves derived 14 days postoperatively or from unoperated sciatic nerves. After 3.5 weeks, nerve fiber bundles consisting of myelinated regenerating axons were only present in implant forks attached to pellets containing denervated nerve homogenate. This activity was heat- and trypsin-sensitive. These data (a) strengthen the hypothesis that diffusible factors from distal nerve stumps of transected nerves can support nerve fiber regeneration, (b) indicate that those factors are protein and/or protein dependent, and (c) suggest that unoperated peripheral nerve (at the levels assayed) either do not contain such factors or contain substances which inhibit such factors.

Effects of chemical additives on functional innervation patterns in mouse spinal cord-ganglion explants in serum-free medium

The distribution of sensory evoked bioelectric activity was examined in spinal cord-dorsal root ganglion (SC-DRG) preparations cultured in a chemically defined, serum-free medium (CDM). DRG afferents showed no preferential innervation of dorsal cord regions in this CDM, on the basis of electrophysiological mapping of the distribution of evoked responses at 4 weeks in vitro. Addition of chondroitin sulfate, galactose-1-phosphate or D(+)-galactose (but not glucose-1-phosphate) to the CDM resulted in a significant increase in presumed monosynaptic connections within the dorsal cord, thus mimicking the results observed in serum-supplemented medium [1,6]. Inasmuch as D(+)-galactose bears no negative charges yet restores the selective functional innervation, whereas glucose-1-phosphate (a highly charged molecule) fails to do so, it is concluded that it is galactose utilization, rather than the charged nature of the chondroitin sulfate and galactose-1-phosphate molecules, which is responsible for the effect.

Tropism in nerve regeneration in vivo. Attraction of regenerating axons by diffusible factors derived from cells in distal nerve stumps of transected peripheral nerves

We re-examined the hypothesis of Cajal3, later refuted by Weiss and Taylor20, that cells in distal stumps of transected peripheral nerves exert an attractive (tropic) effect on regenerating axons. This question was re-assessed in vivo using surgical materials and assay procedures not available to those workers. Proximal stumps of transected rat sciatic or cat peroneal nerves were inserted into the single inlet end of a hollow, Y-shaped Silastic implant. Regenerating axons were provided with alternative targets consisting of a vacant arm vs one occupied by a sciatic nerve graft (rat), or a tibial (Tout) vs peroneal (Pout) distal nerve stump (cat). In some cases Pout was rendered metabolically compromised relative to Tout by exposing the former to dry ice and inhibitors of DNA and RNA synthesis. At 4.5 or 6 weeks postoperatively, the number of regenerating axons in each fork of the implant was assessed by morphometric analysis (total number of non-myelinated and myelinated axons greater than 1 micron in diameter at 4.5 weeks, and total number of myelinated axons at 6 weeks postoperatively), or by quantification of an axonally transported label. Rat sciatic nerve fibers exclusively regenerated toward the nerve graft, suggesting the existence of a neurotropic lure. In cats, morphometric analysis revealed a 10-(4.5 week) and 6-fold (6 week) greater number of axons growing towards untreated Tout vs treated Pout. When both distal stumps were untreated, more axons were seen in forks leading to Pout. Analysis of transported label confirmed the preferential growth of axons towards untreated Tout vs treated Pout for both motor and sensory axons. In separate experiments, Nuclepore filters (0.2 microns, pore size) were inserted between distal nerve stumps and outlet ends of Silastic implants. Preferential regeneration toward untreated stumps was observed if the distance between proximal and distal nerve stumps was equal to but not greater than 4-5 mm. These results suggest that peripheral nerve fiber regeneration in vivo can be directed by cells in distal stumps of transected nerves, and that this effect can be mediated over distances of several millimeters via diffusible factors.

Influence of growth medium, age in vitro and spontaneous bioelectric activity on the distribution of sensory ganglion-evoked activity in spinal cord explants

The role of serum added to the culture medium and of spontaneous bioelectric activity in the development of sensory afferent connections was studied, employing fetal mouse spinal cord explants with attached dorsal root ganglia (DRG) as an in vitro model system. Afferent DRG terminals in the cord explants were localized on the basis of 'fixed-latency' DRG-evoked action potentials, which were anatomically verified in several experiments using horseradish peroxidase histology. In serum-supplemented medium (HSSM), but not in chemically defined medium (CDM), those DRG fibers which grew into the dorsal side of the cord terminated predominantly within the dorsal cord region, and remained there throughout the experimental period (18-33 days in vitro). In contrast, ventrally entering fibers terminated equally in both the dorsal and the ventral cord regions in young cultures (18-24 days in vitro) but were no longer observed after 27 days in vitro. Cultures grown in HSSM with the addition of xylocaine, in order to chronically suppress spontaneous bioelectric activity, essentially corresponded (at 25-32 days in vitro) to the picture seen in the control series at the same age. On the basis of polysynaptic DRG-evoked responses in the cord, developmental changes in local neuronal networks were inferred which resulted in less spread of DRG-evoked activity with age in HSSM, and more spread with age in CDM-grown cultures. It is concluded that for the formation of selective DRG connections in the spinal cord: (i) a serum-borne factor plays a role: and (ii) functional activity is not required.

Preferential growth of neurites from isolated fetal mouse dorsal root ganglia in relation to specific regions of co-cultured spinal cord explants

Clusters of dorsal root ganglia (DRGs) from 13- to 14-day fetal mice were co-cultured with specific fragments of deafferented spinal cord (0.5-1 mm apart) on collagen-coated coverslips in Maximow slide chambers. Nerve growth factor (NGF) was added to the culture medium (1000 biological units/ml, at explantation) to ensure optimal survival and growth of a large fraction of the fetal DRG neurons. Sequential microscopic observations of the living cultures and cytologic studies after silver impregnation demonstrate that many neurites from isolated DRGs can invade dorsal (DC) regions of co-cultured spinal cord explants, whereas they are deflected from ventral cord (VC) tissue and its neuritic-glial outgrowth. Furthermore, some DRG neurites may become redirected towards distant DC target explants even after long circuitous detours around proximally arrayed VC explants. DRG neurites also show remarkably sharp projections to DC tissue and more complete avoidance of adjacent VC tissue when the DRG neurites approach a suitably arranged DC-VC-DC interface, e.g. forming de novo 'dorsal roots' at the end of a longitudinal strip of whole spinal cord. These experiments suggest that DC-VC boundaries may be particularly effective in guiding DRG neurites to specific regions of the CNS. The present studies of co-cultured fetal mouse DRG and spinal cord explants provide the first demonstration of preferential neuritic growth in vitro in relation to specific CNS target tissues.

Specific neuritic pathways and arborizations formed by fetal mouse dorsal root ganglion cells within organized spinal cord explants in culture: a peroxidase-labeling study

Extracellular microiontophoretic injections of horseradish peroxidase (HRP) into NGF-enhanced fetal mouse dorsal root ganglia (DRGs) produced an anterograde solid Golgi-like labeling of DRG neurites and their terminal arborizations within co-cultured spinal cord explants. In cultures of spinal cord transverse cross-sections with attached DRGs, the large NGF-enhanced DRGs remained in close proximity to the cord, often adjacent to both dorsal and ventral cord regions. Despite this, nearly all DRG neurites that entered the cord did so via dorsal root fascicles. They branched and ramified extensively within the dorsal region, taking on a wavy or kinky course and showed various types of arborizations. The density of cord innervation was much lower when isolated DRGs and cord explants were co-cultured 0.5-1 mm apart. Although fewer entering DRG fibers were labeled by our HRP injections the same qualitative growth and arborization patterns were seen within dorsal and ventral cord regions as in explants of cord with attached DRGs. When the facing edge contained both dorsal and ventral tissues, HRP-labeled DRG fibers entered dorsal regions selectively. DRG fibers readily entered, ramified and arborized within isolated strips of dorsal cord, whereas they sharply avoided isolated ventral cord explants. The avoidance of ventral cord cannot simply be due to the paucity of specific synaptic targets within the tissue, for larger numbers of DRG fibers entered completely inappropriate CNS target tissues, e.g. superior colliculus explants--though they did not ramify or arborize to any degree comparable to that seen within dorsal cord regions.

Presence of leucine-enkephalin in organotypic explants of fetal mouse spinal cord

Spinal cord explants with attached dorsal root ganglia (DRGs), from 14-day fetal mice were fixed at 1-3 weeks in vitro and incubated for leucine-enkephalin (LE) immunoreactivity by the peroxidase-anti-peroxidase (PAP) immunohistochemical method. Results show long processes with labeled varicosities seen more often in dorsal regions of the cord explants. Stained punctate bodies and varicosities were often seen close to large cells in these cultures, whereas no label was detected in neuronal perikarya. A prominent laminar array of stained punctate bodies was noted in one cord explant, concentric with the perimeter of the explant. No LE label was detected in the neuritic outgrowths from the cord-DRG explants, whereas high levels of opiate receptors develop in these outgrowths, primarily on the DRG neurites, by 1-2 weeks in culture. The results indicate the presence of LE in explants of fetal mouse spinal cord with attached DRGs and offer an in vitro model system in which the onset and development of peptidergic neurons can be studied as they form functional cellular interrelationships with neurons bearing opioid and monoaminergic receptors in these organotypic cultures.