Vagal Level Research Articles

GDNF and ET-3 Differentially Modulate the Numbers of Avian Enteric Neural Crest Cells and Enteric Neuronsin Vitro

Vagal (hindbrain) neural crest cells migrate rostrocaudally in the gut to establish the enteric nervous system. Glial-derived neurotrophic factor (GDNF) and its receptor(s), and endothelin-3 (ET-3) and its receptor, are crucial for enteric nervous system development. Mutations interrupting either of these signaling pathways cause aganglionosis in the gut, termed Hirschsprung's disease in humans. However, the precise functions of GDNF and ET-3 in enteric neurogenesis are still unknown. We isolated precursor cells of the enteric nervous system from the vagal level neural crest of E1.7 quail embryos prior to entry into the gut and from the developing midgut at stages corresponding to migrating (E4.7) and longer resident differentiating cells (E7) using HNK-1 immunoaffinity and magnetic beads. These cells were tested for their response to GDNF and ET-3 in culture. ET-3 and GDNF had little effectin vitroon the growth, survival, migration, or neurogenesis of E1.7 vagal neural crest cells. In contrast, GDNF increased the proliferation rate and numbers of enteric neural precursors isolated from the E4.7 and E7 gut. Also, many more neurons and neurites developed in cultures treated with GDNF, disproportionately greater than the effect on cell numbers. At high cell density and in the presence of serum, ET-3, and GDNF had an additive effect on proliferation of neuron precursor cells. In defined medium, or low cell density, ET-3 reduced cell proliferation, overriding the proliferative effect of GDNF. Regardless of the culture condition, the stimulatory effect of GDNF on neuron numbers was strikingly diminished by the simultaneous presence of ET-3. We propose first that GDNF promotes the proliferation in the migratory enteric neural precursor cell population once the cells have entered the gut and is especially crucial for the differentiation of these cells into nonmigrating, nonproliferating enteric neurons. Second, we suggest that ET-3 modulates the action of GDNF, inhibiting neuronal differentiation to maintain the precursor cell pool, so ensuring sufficient population numbers to construct the entire enteric nervous system. Third, we suggest that generalized defects in enteric neural precursor cell numbers and differentiation due to mutations in the ET-3 and GDNF systems are converted to distal gut neural deficiencies by the rostrocaudal migration pattern of the precursors. Fourth, we suggest that additional factors such as those found in serum and produced by the enteric neural cells themselves are likely also to be involved in enteric nervous system development and consequently in Hirschsprung's disease.

Baroreflexes and Cardiovascular Regulation in Hypertension

The primary purpose of the arterial baroreflex is to keep blood pressure close to a particular set point over a relatively short period of time. The rapid resetting of arterial baroreceptor afferents toward any sustained new level of blood pressure ensures that the reflex acts as an effective buffer of short-term blood pressure fluctuations that accompany daily life but also ensures that arterial baroreflexes play little role in setting the long-term level of blood pressure. Nevertheless, the minimization of blood pressure variability by baroreflex mechanisms is important as studies suggest that a diminished baroreflex is an independent risk factor or sudden death after myocardial infarction. In hypertensive humans and animals, the baroreflex control of heart rate is diminished. Using the steady-state method for assessment of the cardiac baroreflex in rats, we have shown that the change in baroreflex sensitivity is due to a reduction in the vagal range. Although the cardiac sympathetic component of the baroreflex is normal, the level of cardiac sympathetic activity is enhanced, particularly in young hypertensive rats. We have shown that there is a stronger inverse relationship between vagal heart rate range and levels of cardiac hypertrophy than with other variables, such as blood pressure, hypertension, or indexes of vascular hypertrophy. Treatments that reduce cardiac hypertrophy restore cardiac vagal function. Centrally acting antihypertensive agents increase the sensitivity of vagal baroreceptor heart rate reflexes, mainly through an action on central alpha 2-adrenoceptors. They also reduce cardiac sympathetic activity and diminish cardiac sympathetic baroreflexes through a non-alpha 2-adrenoceptor, possibly an imidazoline receptor mechanism. Both of these effects are beneficial in hypertension, where cardiac sympathetic function is enhanced and vagal activity is reduced. Thus, these actions would be expected to cause a desirable reduction in blood pressure variability. The effect of hypertension on baroreflex control of sympathetic vasomotor function is less clear. Studies have shown diminished, normal, and enhanced sympathetic vasomotor baroreflex control. Basal renal sympathetic drive, however, appears to be increased in human essential hypertension. Our studies in conscious rabbits have shown that rilmenidine reduces renal sympathetic baroreflex function. Rilmenidine acts principally at the level of the rostral ventrolateral medullary imidazoline receptors to markedly reduce the basal renal sympathetic nerve activity and the maximum response to transient fluctuations in blood pressure. Thus, in addition to their antihypertensive actions, centrally acting agents, such as rilmenidine, reduce cardiac and renal sympathetic baroreflex responses and increase cardiac vagal baroreflex sensitivity. This provides an ideal profile of action for the restoration of baroreflex function in addition to reversal of cardiac and vascular hypertrophy in hypertension.

From neural crest to bowel: Development of the enteric nervous system

The ENS resembles the brain and differs both physiologically and structurally from any other region of the PNS. Recent experiments in which crest cell migration has been studied with DiI, a replication-deficient retrovirus, or antibodies that label cells of neural crest origin, have confirmed that both the avian and mammalian bowel are colonized by émigrés from the sacral as well as the vagal level of the neural crest. Components of the extracellular matrix, such as laminin, may play roles in enteric neural and glial development. The observation that an overabundance of laminin develops in the presumptive aganglionic region of the gut in ls/ls mutant mice and is associated with the inability of crest-derived cells to colonize this region of the bowel has led to the hypothesis that laminin promotes the development of crest-derived cells as enteric neurons. Premature expression of a neuronal phenotype would cause crest-derived cells to cease migrating before they complete the colonization of the gut. The acquisition by crest-derived cells of a nonintegrin, nerve-specific, 110 kD laminin-binding protein when they enter the bowel may enable these cells to respond to laminin differently from their pre-enteric migrating predecessors. Crest-derived cells migrating along the vagal pathway to the mammalian gut are transiently catecholaminergic (TC). This phenotype appears to be lost rapidly as the cells enter the bowel and begin to follow their program of terminal differentiation. The appearance and disappearance of TC cells may thus be an example of the effects of the enteric microenvironment on the differentiation of crest-derived cells in situ. Crest-derived cells can be isolated from the enteric microenvironment by immunoselection, a method that takes advantage of the selective expression on the surfaces of crest-derived cells of certain antigens. One neurotrophin, NT-3, promotes the development of enteric neurons and glia in vitro. Because trkC is expressed in the developing and mature gut, it seems likely that NT-3 plays a critical role in the development of the ENS in situ. Although the factors that are responsible for the development of the unique properties of the ENS remain unknown, progress made in understanding enteric neuronal development has recently accelerated. The application of new techniques and recently developed probes suggest that the accelerated pace of discovery in this area can be expected to continue.

Avian neural crest cells express a melanogenic trait during early migration from the neural tube: observations with the new monoclonal antibody, “MEBL-1”

ABSTRACT A new monoclonal antibody, MEBL-1, which specifically recognizes melanocytes and their precursors, was used to examine the differentiation of neural crest cells into melanocytes in situ. MEBL-l-positive precursors of melanocytes in the trunk were first detected in mesenchyme located above the neural tube at the level of a wing bud at stage 19, when active migration of neural crest cells along the ventral route was almost complete. DiO-labelled neural crest cells, which begin to emigrate from the neural tube at stages 17–18 and then migrate along the dorsolateral route, were found to become MEBL-l-positive. This indicates that MEBL-1 antigens are expressed by neural crest cells immediately following their emigration and neural crest cells become melanogenic. Expression of the MEBL-1 antigens was followed by decrease in, and finally the loss of, HNK-1 antigen expression. At the cephalic level, MEBL-l-positive precursors of melanocytes were first detected in mesenchyme located above the dorsal side of the mesencephalic neural tube at stage 16. At the vagal level, the precursors were noted first in mesenchyme located above the dorsal side of the rhombencephallc neural tube at stage 18. MEBL-1 reacted with the cell membrane and preme-lanosome-like granules in addition to the nuclear membrane, endoplasmic reticulum and Golgi apparatus of the migratory precursors. The antigens recognized by MEBL-1 were proteins with relative molecular masses of 135 × 103 and 115 × 103. We suggest that MEBL-1 may be the earliest marker for melanogenic neural crest cells.

Enteric glia

The structure of the enteric nervous system (ENS) is different from that of extraenteric peripheral nerve. Collagen is excluded from the enteric plexuses and support for neuronal elements is provided by astrocyte-like enteric glial cells. Enteric glia differ from Schwann cells in that they do not form basal laminae and they ensheath axons, not individually, but in groups. Although enteric glia are rich in the S-100 and glial fibrillary acidic proteins, it has been difficult to find a single chemical marker that distinguishes enteric glia from non-myelinating Schwann cells. Nevertheless, two monoclonal antibodies have been obtained that recognize antigens that are expressed on Schwann cells (Ran-1 in rats and SMP in avians) but not enteric glia. Functional differences between enteric glia and non-myelinating Schwann cells, including responses to gliotoxins and in vitro proliferative rates, have also been observed. Developmentally, enteric glia, like Schwann cells, are derived from the neural crest. In both mammals and birds the precursors of the ENS appear to migrate to the bowel from sacral as well as vagal levels of the crest. These crest-derived emigrés give rise to both enteric glia and neurons; however, analyses of the ontogeny of the enteric innervation in a mutant mouse (the ls/ls), in which the original colonizing waves of crest-derived precursor cells are unable to invade the terminal colon, suggest that enteric glia can also arise from Schwann cells that enter the gut with the extrinsic innervation. When induced to leave back-transplanted segments of avian bowel, enteric crest-derived cells migrate into peripheral nerves and form Schwann cells. Enteric glia and Schwann cells thus appear to be different cell types, but ones that derive from lineages that diverge relatively late in ontogeny.

Pathways of avian neural crest cell migration in the developing gut

The NC-1 and E/C8 monoclonal antibodies recognize a similar population of neural crest cells as they migrate from vagal levels of the neural tube and colonize the branchial arch region of 2- to 3-day-old chicken embryos. Some of these immunoreactive cells then appear to enter the gut mesenchyme on the third day of incubation just caudal to the third branchial cleft. After entering the gut, these cells migrate in a rostral-caudal direction, using primarily the superficial splanchnic mesodermal epithelium of the gut as a substratum. The antigen-positive cells remain preferentially associated with the splanchnopleure. Few antigenic cells enter the mesenchyme surrounding the endoderm at anterior levels whereas they are found throughout the mesenchyme when nearing the umbilicus. At postumbilical levels, immunoreactive cells are distributed on both sides of the differentiating muscle layer but not within it. Although fibronectin immunoreactivity can be found throughout the wall of the gut, there is no apparent relationship between the distribution of fibronectin and the location of the immunoreactive cells. These results suggest that a mechanism more complex than a mere interaction with fibronectin may account for migration of crest-derived cells in the gut.

Development of VIP in the peripheral nervous system of avian embryo

The distribution of the VIP containing structures was studied in the gut and in the paravertebral sympathetic ganglia of the quail and chick embryos by immunocytochemistry. In the gut, development of peptidergic nerves followed a craniocaudal gradient. Immunoreactive fibres were first visible in the oesophagus at day 9 in the quail and day 10 in the chick, at 12 days they extended over the whole length of the gut. Cell bodies were localized at day 9 in the foregut and observed in the mid- and hind-gut just before hatching. Transplantations on the chorioallantoic membrane of fragments of various parts of the digestive tract clearly demonstrated that VIP nerve cell bodies belonged to the intrinsic innervation of the gut. Besides the gut, sympathetic paravertebral ganglia contained cells with VIP immunoreactivity detected at day 9 and 10 in quail and chick respectively. In order to find out whether VIP containing neurons differentiated normally in chick embryos in which quail neural crest cells had been implanted at an early stage of development we looked for the appearance of peptidergic neurones in the following situations: when the quail neural primordium had been grafted orthotopically and isochronically into chick host (1) at the adrenomedullary (somites 18–24) and (2) at the vagal (somites 1–7) levels of the neural axis. In all conditions VIP immunoreactivity was observed in quail cells located either in the sympathetic paravertebral ganglia of the trunk at the level of the graft or in the enteric ganglia according to the graft was made at the adrenomedullary and vagal levels respectively. It was shown that peptidergic neurones normally developed but differentiated at the same stage in normal quail neuroblasts and in quail neuroblasts implanted into the chick host.

Differentiation of peptidergic neurones in quail-chick chimaeric embryos

The problem raised in this work was whether peptidergic neurones with vasoactive intestinal peptide (VIP)- and substance P-like immunoreactivity could develop in chimaeric embryos in which quail neural crest cells had been implanted into chick at an early developmental stage. Differentiation of peptide-containing nerve somas was looked for in different situations: i) when the quail neural primordium had been grafted orthotopically and isochronically into the chick host either at the adrenomedullary (level of somites 18–24) or at the vagal (level of somites 1–7) levels of the neural axis; ii) when the quail adrenomedullary neural primordium had been heterotopically implanted at the vagal level of the chick host. In all conditions, VIP- and substance P-like immunoreactivity were observed in a number of quail neurones located either in the peripheral ganglia of the trunk at the level of the graft (in orthotopic grafts of the adrenomedullary neural primordium) or in the enteric ganglia of the chick gut (in the other types of grafts). The developmental stage at which the first neurones become detectable in the host conforms to the genetic characteristics of the effector cells, i.e. they differentiate at the same stage in normal quail neuroblasts and in quail neuroblasts transplanted into the chick host. In contrast, the distribution of the peptidergic neurones in the host depends on the tissue into which the neural crest cells migrate and not on their origin in the neural axis and their fate in normal development.

Do neural crest cells in the pancreas differentiate into somatostatin-containing cells?

The possibility that the somatostatin cells are derived from the neurectoderm has been questioned in avian embryos. Isotopic and isochronic transplantations of the neural primordium from quail into chick embryos were made at the vagal level (somites 1 to 7). Quail and chick cells can be distinguished by the structure of their nucleus. The somatostatin cells were characterized immunocytochemically. In no case did quail cells showing the immunological reaction originate from the neural crest.

What is the developmental fate of the neural crest cells which migrate into the pancreas in the avian embryo?

The hypothesis of a neural crest origin for the endocrine cells of the pancreas has been tested in avian embryos. Isotopic and isochronic transplantations of the neural primordium from quail into chick embryos were made at the vagal level of the neural axis (level of somites 1 to 7). Since quail and chick cells can be distinguished by the structure of their nucleus, the migration of the grafted neural crest cells could be followed in the host. Small groups of quail cells were found in the pancreatic tissue apart from both the exocrine and endocrine structures. The latter were characterized cytochemically at various developmental stages in experimental and control embryos by the formol-induced fluorescence technique after l-Dopa injection, and by their affinity for lead hematoxylin. In no cases were the cells originating from the neural crest and showing the quail nuclear marker found with the cytochemical properties of any kind of the pancreatic endocrine cells. In fact, the neural crest cells which migrate into the pancreas differentiate into parasympathetic ganglia, as evidenced by the use of silver impregnation techniques.