Catecholamine-containing neurons in Remak's ganglion: a developmental and tissue culture study

Essential role of Gata transcription factors in sympathetic neuron development

Sympathetic neurons are specified during their development from neural crest precursors by a network of crossregulatory transcription factors, which includes Mash1, Phox2b, Hand2 and Phox2a. Here, we have studied the function of Gata2 and Gata3 zinc-finger transcription factors in autonomic neuron development. In the chick, Gata2 but not Gata3 is expressed in developing sympathetic precursor cells. Gata2 expression starts after Mash1, Phox2b, Hand2 and Phox2a expression, but before the onset of the noradrenergic marker genes Th and Dbh, and is maintained throughout development. Gata2 expression is affected in the chick embryo by Bmp gain- and loss-of-function experiments, and by overexpression of Phox2b, Phox2a, Hand2 and Mash1. Together with the lack of Gata2/3 expression in Phox2b knockout mice, these results characterize Gata2 as member of the Bmp-induced cluster of transcription factors. Loss-of-function experiments resulted in a strong reduction in the size of the sympathetic chain and in decreased Th expression. Ectopic expression of Gata2 in chick neural crest precursors elicited the generation of neurons with a non-autonomic, Th-negative phenotype. This implies a function for Gata factors in autonomic neuron differentiation, which, however, depends on co-regulators present in the sympathetic lineage. The present data establish Gata2 and Gata3 in the chick and mouse, respectively, as essential members of the transcription factor network controlling sympathetic neuron development.

Electrical characteristics and responses to jejunal distension of neurons in Remak's juxta-jejunal ganglia of the domestic fowl

1. Remak's nerve is a ganglionated nerve trunk found only in birds that runs parallel to the gut from the duodenal-jejunal junction to the cloaca. We report the first electrophysiological characterization of these neurons and their responses to gut distension. 2. A segment of chicken jejunum with attached Remak's nerve was pinned in an electrophysiological chamber. Neurons in Remak's ganglia were impaled with microelectrodes. The adjacent segment of gut was distended with fluid. 3. One hundred and thirty neurons were characterized into three electrophysiological classes: (i) tonic neurons (74%) fired action potentials spontaneously (frequency 3.5 Hz) and continuously (up to 40 Hz) throughout a depolarizing current pulse; (ii) AD neurons (22%) fired a brief burst of action potentials (1-10), which were followed by a prolonged after-depolarization (AD) of duration 2.8 +/- 0.3 s; and (iii) phasic neurons (4%) fired an initial burst of action potentials followed by an after-hyperpolarization (duration, 520.0 +/- 32.0 ms). Tetrodotoxin (1 microM) abolished action potentials in tonic and AD neurons as well as the after-depolarization. 4. Spontaneous fast excitatory postsynaptic potentials (FEPSPs) occurred in all classes of neurons; they were not observed, however, in ganglia isolated from the jejunum. 5. Intracellular injection of biocytin revealed that neurons could be characterized into four morphological classes. Tonic neurons, which had long and extensive dendritic trees, were Remak's Type I, II and IV neurons. AD neurons also comprised Remak's type II neurons. Phasic neurons were Remak's Type III neurons. Most neurons had axons that projected orally along Remak's nerve. 6. Distension of the jejunum evoked FEPSPs and action potentials in tonic neurons, and repetitive bursts of action potentials (1-4) followed by an after-depolarization in AD neurons. All responses to distension were blocked by hexamethonium (300 microM) and tetrodotoxin (1 microM). 7. In conclusion, neurons in Remak's juxta-jejunal nerve appear to regulate gut motility. Three distinct electrophysiological classes of neurons were observed, all of which appear to be activated by distension sensitive cholinergic intestinofugal neurons in the jejunum.

Histochemical, immunohistochemical, and electron microscopy study of the caudal portion of the chicken intestinal nerve of Remak

In order to deepen our knowledge of the different components of the chicken intestinal nerve of Remak (I.N.R.), we have studied it by means of histochemical, immunohistochemical and electron microscopy techniques to distinguish the different neurotransmitters. We have found cholinergic cell bodies, as well as acetylcholinesterase (AChE) positive neuronal fibers, forming part of the web that constitutes the I.N.R. in its caudal portion, with a higher density of neuronal bodies in the ganglia. We also observed catecholaminergic neuronal bodies and fibers, located fundamentally in the periphery of the nerve, and a low density of catecholaminergic cell bodies. With respect to the vasoactive intestinal peptide (VIP) and substance P (SP) positive peptidergic innervation, we found more abundant neuronal bodies positive for the V.I.P. than for S.P. Electron microscopy corroborated the results observed under the optic microscope, showing the various types of vesicles containing different neurotransmitters.

Projections of neurons in the intestinal nerve of Remak to the chicken intestine

To date, the exact site of the extrinsic postganglionic neurons innervating the intramural plexuses in the chicken intestine is unknown. In this study, neurons in the intestinal nerve of Remak (INR) were immunohistochemically labelled by injecting cholera toxin subunit B (CTb) into each one of the jejunal, ileal, cecal and rectal walls. The CTb-immunoreactive (IR) neurons were counted, and some sections from the rectal portion of INR (rectal INR) were also immunostained for either tyrosine hydroxylase (TH) or methionine enkephalin (mENK). Following injection of CTb into the jejunum or ileum, only a few CTb-IR neurons were found in the jejunal or ileal part of INR caudal to the injection site. Following injection into the more caudal intestine, CTb-IR neurons were seen in the rectal INR. Of the 3490 CTb-IR neurons that were counted in the rectal INR, 40% projected into the rostral rectum, 24% into the caudal rectum, 17% into the ileum, 10% into the cecum and only 9% into the middle rectum. Rostrocaudally dividing the rectal INR into three parts, one third of the CTb-IR neurons in the rostral part projected into the rostral rectum, the majority of CTb-IR ones in the middle part innervated the rostral rectum, and half of CTb-IR neurons in the caudal part ran into the caudal rectum. Consequently, the rostral rectum received the densest innervation, and almost all the neurons in the rectal INR exhibited an ascending projection. By double labelling, CTb-IR neurons containing TH or mENK were observed in the rectal INR after the rectal injection. The mENK-IR neurons localized in middle and caudal parts of rectal INR amounted to one third of the total CTb-IR neurons, and mainly projected into the rostral and caudal rectum. TH-and mENK-immunonegative neurons were restricted to the rostral part of rectal INR and the more rostral level.

Differential regulation of transcription factor gene expression and phenotypic markers in developing sympathetic neurons

We have examined the regulation of transcription factor gene expression and phenotypic markers in developing chick sympathetic neurons. Sympathetic progenitor cells first express the bHLH transcriptional regulator Cash-1 (a chicken achaete-scute homologue), followed by coordinate expression of Phox2, a paired homeodomain protein, and GATA-2, a zinc finger protein. SCG10, a pan-neuronal membrane protein, is first detected one stage later, followed by the catecholaminergic neurotransmitter enzyme tyrosine hydroxylase (TH). We have used these markers to ask two questions: (1) is their expression dependent upon inductive signals derived from the notochord or floor plate?; (2) does their sequential expression reflect a single linear pathway or multiple parallel pathways? Notochord ablation experiments indicate that the floor plate is essential for induction of GATA-2, Phox2 and TH, but not for that of Cash-1 and SCG10. Taken together these data suggest that the development of sympathetic neurons involves multiple transcriptional regulatory cascades: one, dependent upon notochord or floor plate-derived signals and involving Phox2 and GATA-2, is assigned to the expression of the neurotransmitter phenotype; the other, independent of such signals and involving Cash-1, is assigned to the expression of pan-neuronal properties. The parallel specification of different components of the terminal neuronal phenotype is likely to be a general feature of neuronal development.

Immunohistochemical studies on the intestinal nerve of Remak in the male chicken

A peroxidase anti-peroxidase method was used to investigate and compare the distribution of neuropeptide and catecholamine synthesizing enzyme immunoreactive (IR) ganglion cells and nerve fibres in the intestinal nerve of Remak (INR) of male chickens. In the INR there were three kinds of ganglion cells: tyrosine hydroxylase (TH)-, aromatic L-amino acid decarboxylase (AADC)- and phenylethanolamine-N-methyltransferase (PNMT)-IR cells; AADC- and PNMT-IR but TH-immunonegative cells; and ganglion cells being immunoreactive for methionine enkephalin (mENK)- and somatostatin (SOM). The first one was distributed throughout the INR. The second was restricted in the ileojejunal region, and the last was localized in the rectal region. Substance P- and vasoactive intestinal polypeptide-IR nerve fibres were distributed in common but variable in number around three kinds of ganglion cells. Then TH-IR cells were characterized by the distribution of many calcitonin gene related peptide- and a few cholecystokinin-IR fibres. mENK and SOM-IR cells, and TH-immunonegative cells were distinguished by the distribution of SOM- and galanin-IR fibres. In addition, TH-immunonegative cells were characterized by the distribution of mENK- and neuropeptide Y-IR nerve fibres which were very few in number. Fig. 21 summarizes the connections described in the present study.

Mesenteric neurons in the adult rat are responsive to ileal treatment with benzalkonium chloride

The application of a 2 mM solution of the cationic surfactant benzalkonium chloride (BAC) to an ileal segment produced a selective and extensive myenteric denervation. The aim of the present study was to investigate whether such a selective unbalance of the enteric nervous system in the adult rat elicited any plastic response within the mesenteric nervous structures contacting it. Acetylcholinesterase (AChE) staining, beta-nicotinamide adenine dinucleotide (NADH) and beta-nicotinamide adenine dinucleotide phosphate (NADPH) diaphorase histochemistry and glyoxylic acid-induced fluorescence were performed on whole-mount preparations of myenteric plexus and mesenteric nerves. In both control and BAC-treated animals nervous elements were detected along the mesenteric nerves. Although rather similar in position, shape and size, these neurons displayed striking differences with regard to their arrangement and density per nerve. In the controls, few small aggregates of neurons could be detected; more commonly, isolated nerve cell bodies were scattered along the nerve trunks. In the BAC-treated animals, numerous spherical or spindle-shaped clusters of neurons were located along the nerves; the mean neuronal density per nerve displayed a two-five-fold increase over the control values. The observed changes within the mesenteric nerves might be involved in an attempt at reinnervation of the BAC-treated intestinal segments from extra-enteric sources.

Role of environmental signals and transcriptional regulators in neural crest development

The processes by which undifferentiated cells are assigned to particular fates are far from clear. We review recent work that has examined this problem in the neural crest, a multipotential cell population that gives rise to peripheral neurons in vertebrates. Peripheral neuronal differentiation appears to occur in a series of developmental steps that can be regulated independently by signals in the environment. Furthermore, such steps are reflected by corresponding changes in the pattern of regulatory transcription factor expression in differentiating neural crest cells. The determination of neuronal identity may proceed by a series of parallel regulatory pathways involving transcription factors acting both in cascades and in combinatorial arrays.