Evidence for neurotransmitter plasticity in vivo. II. Immunocytochemical studies of rat sweat gland innervation during development

CRLF1 and CLCF1 in Development, Health and Disease

Cytokines and their receptors have a vital function in regulating various processes such as immune function, inflammation, haematopoiesis, cell growth and differentiation. The interaction between a cytokine and its specific receptor triggers intracellular signalling cascades that lead to altered gene expression in the target cell and consequent changes in its proliferation, differentiation, or activation. In this review, we highlight the role of the soluble type I cytokine receptor CRLF1 (cytokine receptor-like factor-1) and the Interleukin (IL)-6 cytokine CLCF1 (cardiotrophin-like cytokine factor 1) during development in physiological and pathological conditions with particular emphasis on Crisponi/cold-induced sweating syndrome (CS/CISS) and discuss new insights, challenges and possibilities arising from recent studies.

Microarray-based identification of Pitx3 targets during Xenopus embryogenesis

BACKGROUND

Unexpected phenotypes resulting from morpholino-mediated translational knockdown of Pitx3 in Xenopus laevis required further investigation regarding the genetic networks in which the gene might play a role. Microarray analysis was, therefore, used to assess global transcriptional changes downstream of Pitx3.

RESULTS

From the large data set generated, selected candidate genes were confirmed by reverse transcriptase-polymerase chain reaction (RT-PCR) and in situ hybridization.

CONCLUSIONS

We have identified four genes as likely direct targets of Pitx3 action: Pax6, β Crystallin-b1 (Crybb1), Hes7.1, and Hes4. Four others show equivocal promise worthy of consideration: Vent2, and Ripply2 (aka Ledgerline or Stripy), eFGF and RXRα. We also describe the expression pattern of additional and novel genes that are Pitx3-sensitive but that are unlikely to be direct targets.

Different sympathetic pathways control the metabolism of distinct bone envelopes

Bone remodeling, the mechanism that modulates bone mass adaptation, is controlled by the sympathetic nervous system through the catecholaminergic pathway. However, resorption in the mandible periosteum envelope is associated with cholinergic Vasoactive Intestinal Peptide (VIP)-positive nerve fibers sensitive to sympathetic neurotoxics, suggesting that different sympathetic pathways may control distinct bone envelopes. In this study, we assessed the role of distinct sympathetic pathways on rat femur and mandible envelopes. To this goal, adult male Wistar rats were chemically sympathectomized or treated with agonists/antagonists of the catecholaminergic and cholinergic pathways; femora and mandibles were sampled. Histomorphometric analysis showed that sympathectomy decreased the number of preosteoclasts and RANKL-expressing osteoblasts in mandible periosteum but had no effect on femur trabecular bone. In contrast, pharmacological stimulation or repression of the catecholaminergic cell receptors impacted the femur trabecular bone and mandible endosteal retromolar zone. VIP treatment of sympathectomized rats rescued the disturbances of the mandible periosteum and alveolar wall whereas the cholinergic pathway had no effect on the catecholaminergic-dependent envelopes. We also found that VIP receptor-1 was weakly expressed in periosteal osteoblasts in the mandible and was increased by VIP treatment, whereas osteoblasts of the retromolar envelope that was innervated only by tyrosine hydroxylase-immunoreactive fibers, constitutively expressed beta-2 adrenergic receptors. These data highlight the complexity of the sympathetic control of bone metabolism. Both the embryological origin of the bone (endochondral for the femur, membranous for the mandibular periosteum and the socket wall) and environmental factors specific to the innervated envelope may influence the phenotype of the sympathetic innervation. We suggest that an origin-dependent imprint of bone cells through osteoblast-nerve interactions determines the type of autonomous system innervating a particular bone envelope.

Immune and nervous systems: more than just a superficial similarity?

Strong evidence is emerging that the nervous and immune systems share mechanisms of gene regulation, signaling, cell communication, and supracellular organization. This brings to the fore many questions, not least of which is the developmental and evolutionary origin of the commonalities between the two systems. By providing answers to these questions, immunologists and neurobiologists increasingly expose the mechanistic and conceptual affinities of their respective fields and facilitate the understanding of fundamental principles that govern the organization of complex cellular systems. The current essay and reviews in Immunity and Neuron attempt to communicate to the wider scientific community a series of examples relating to commonalities between the immune and nervous system and enhance the dialog and exchange of ideas between the two fields.

Peripheral projections of NESP55 containing neurons in the rat sympathetic ganglia

The peripheral projections of neurons expressing neuroendocrine secretory protein 55 (NESP55), a novel member of the chromogranin family, were studied by retrograde tracing technique. It was found that NESP55 positive neurons in the rat superior cervical ganglion projected to a number of targets including the submandibular gland, the cervical lymph nodes, the forehead skin, the iris, but not to the thyroid. Among these NESP55 positive target-projecting neurons, a subpopulation contained neuropeptide Y (NPY), a vasoconstrictor. Forepaw pad projecting neurons were found exclusively in the stellate ganglion, almost all of which (approximately 90%) were immunoreactive to NESP55. Colocalization of NESP55 and calcitonin gene-related peptide (CGRP), a peptide involved in sudomotor effects, was observed in a subpopulation of these paw pad projecting neurons, as was colocalization of NESP55 and NPY. The data suggest that NESP55 may have a functional role in some populations of sympathetic neurons.

Postnatal expression and denervation induced up-regulation of aquaporin-5 protein in rat sweat gland

The evolution of aquaporin-5 (AQP5) expression during postnatal development has not been defined in the sweat gland. Previous studies have suggested that AQP isoforms in several peripheral targets are regulated by a neural mechanism. We have examined, in rat sweat glands, the expression of AQP5 during postnatal development and the effects of denervation on AQP5 expression. Both AQP5 mRNA and protein begin to be expressed at postnatal day 10, before sweat-secretory responsiveness first appears; this expression coincides with the occurrence of vasoactive intestinal peptide (VIP) immunoreactivity. Early noradrenergic and later cholinergic interaction between sweat glands and their innervation are disrupted by neonatal chemical sympathectomy or postnatal severance of the sciatic nerve. Examination of such denervated developing rats has shown that secretory responsiveness fails to arise later in the adults, and AQP5 immunostaining increases in the denervated glands, whereas gland morphogenesis and the occurrence of AQP5 expression proceed normally. Immunobloting has revealed an increase of AQP5 abundance after the denervated mature glands lose their secretory ability. These findings suggest that AQP5 protein is necessary for sweat secretion, and that the expression of AQP5 in rat sweat glands is independent of sympathetic innervation. Our data also indicate that factor(s) regulating the normal morphological development of sweat gland might be responsible for controlling AQP5 expression.

Three types of tyrosine hydroxylase-positive CNS neurons distinguished by dopa decarboxylase and VMAT2 co-expression

1. We investigate here for the first time in primate brain the combinatorial expression of the three major functionally relevant proteins for catecholaminergic neurotransmission tyrosine hydroxylase (TH), aromatic acid acid decarboxylase (AADC), and the brain-specific isoform of the vesicular monoamine transporter, VMAT2, using highly specific antibodies and immunofluorescence with confocal microscopy to visualize combinatorial expression of these proteins. 2. In addition to classical TH, AADC, and VMAT2-copositive catecholaminergic neurons, two unique kinds of TH-positive neurons were identified based on co-expression of AADC and VMAT2. 3. TH and AADC co-positive, but VMAT2-negative neurons, are termed "nonexocytotic catecholaminergic TH neurons." These were found in striatum, olfactory bulb, cerebral cortex, area postrema, nucleus tractus solitarius, and in the dorsal motor nucleus of the vagus. 4. TH-positive neurons expressing neither AADC nor VMAT2 are termed "dopaergic TH neurons." We identified these neurons in supraoptic, paraventricular and periventricular hypothalamic nuclei, thalamic paraventicular nucleus, habenula, parabrachial nucleus, cerebral cortex and spinal cord. We were unable to identify any dopaergic (TH-positive, AADC-negative) neurons that expressed VMAT2, suggesting that regulatory mechanisms exist for shutting off VMAT2 expression in neurons that fail to biosynthesize its substrates. 5. In several cases, the corresponding TH phenotypes were identified in the adult rat, suggesting that this rodent is an appropriate experimental model for further investigation of these TH-positive neuronal cell groups in the adult central nervous system. Thus, no examples of TH and VMAT2 co-positive neurons lacking AADC expression were found in rodent adult nervous system. 6. In conclusion, the adult mammalian nervous system contains in addition to classical catecholaminergic neurons, cells that can synthesize dopamine, but cannot transport and store it in synaptic vesicles, and neurons that can synthesize only L-dopa and lack VMAT2 expression. The presence of these additional populations of TH-positive neurons in the adult primate CNS has implications for functional catecholamine neurotransmission, its derangement in disease and drug abuse, and its rescue by gene therapeutic maneuvers in neurodegenerative diseases such as Parkinson's disease.

Single-cell RT-PCR, in situ hybridization histochemical, and immunohistochemical studies of substance P and enkephalin co-occurrence in striatal projection neurons in rats

Single-cell RT-PCR studies in 3-4-week-old rats have raised the possibility that as many as 20% of striatal projection neurons may be a unique type that contains both substance P (SP) and enkephalin (ENK). We used single-cell RT-PCR, retrograde labeling, in situ hybridization histochemistry, and immunolabeling to characterize the abundance of this cell type, its projection target(s), and any developmental changes in its frequency. We found by RT-PCR that 11% of neurons containing either SP or ENK contained both in 4-week-old rats, while in 4-month-old rats SP/ENK colocalization was only 3%. SP-only neurons tended to co-contain dynorphin and ENK-only neurons neurotensin, while SP/ENK neurons tended to contain dynorphin. Single-cell RT-PCR showed SP/ENK co-occurrence in 4-week-old rats to be no more common among striatal neurons retrogradely labeled from the substantia nigra than among those retrogradely labeled from globus pallidus. Double-label in situ hybridization showed SP/ENK perikarya to be scattered throughout striatum, making up 8% of neurons containing either SP or ENK at 4 weeks, but only 4% at 4 months. Immunolabeling showed that presumptive striatal terminals in globus pallidus externus, globus pallidus internus and substantia nigra pars reticulata that colocalized SP and ENK were scarce. Terminals colocalizing SP and ENK were, however, abundant in the substantia nigra pars compacta. Thus, SP-only and ENK-only neurons make up the vast majority of striatal projection neurons in rats, the frequency of SP/ENK colocalizing striatal neurons is low in adult rats (3-4%), and SP/ENK colocalizing neurons primarily project to SNc but do not appear to be confined to striosomes.

Role for calcium/calmodulin-dependent protein kinase II in the p75-mediated regulation of sympathetic cholinergic transmission

Neurotrophins regulate sympathetic neuron cotransmission by modulating the activity-dependent release of norepinephrine and acetylcholine. Nerve growth factor promotes excitatory noradrenergic transmission, whereas brain-derived neurotrophic factor (BDNF), acting through the p75 receptor, increases inhibitory cholinergic transmission. This regulation of corelease by target-derived factors leads to the functional modulation of myocyte beat rate in neuron-myocyte cocultures. Calcium/calmodulin-dependent protein kinase II (CaMKII) has been implicated in the control of both pre- and postsynaptic mechanisms of synaptic plasticity. We demonstrate that CaMKII acts in conjunction with p75 signaling to regulate cholinergic transmission between sympathetic neurons and heart cells. Inhibition of presynaptic CaMKII prevents the BDNF-dependent shift to inhibitory neurotransmission, whereas presynaptic expression of a constitutively active CaMKII results in inhibitory neurotransmission in the absence of added BDNF, suggesting that activation of presynaptic CaMKII is both necessary and sufficient for a shift from excitatory to inhibitory transmission. Several isozymes of CaMKII are expressed in sympathetic neurons, with the delta-CaMKII being activated by BDNF and nerve growth factor. Activated CaMKII is less effective at promoting cholinergic transmission in the absence of p75 signaling, demonstrating that p75 and CaMKII act to coordinate neurotransmitter selection in sympathetic neurons.

How many types of cholinergic sympathetic neuron are there in the rat stellate ganglion?

Sympathetic cholinergic postganglionic neurons are present in many sympathetic ganglia. Three classes of sympathetic cholinergic neuron have been reported in mammals; sudomotor neurons, vasodilator neurons and neurons innervating the periosteum. We have examined thoracic sympathetic ganglia in rats to determine if any other classes of cholinergic neurons exist. We could identify cholinergic sudomotor neurons and neurons innervating the rib periosteum, but confirmed that cholinergic sympathetic vasodilator neurons are absent in this species. Sudomotor neurons contained vasoactive intestinal peptide (VIP) and calcitonin gene-related peptide (CGRP) and always lacked calbindin. Cholinergic neurons innervating the periosteum contained VIP and sometimes calbindin, but always lacked CGRP. Cholinergic neurons innervating the periosteum were usually surrounded by terminals immunoreactive for CGRP. We conclude that if any undiscovered populations of cholinergic neurons exist in the rat thoracic sympathetic chain, then they are indistinguishable in size, neurochemistry and inputs from sudomotor or cholinergic neurons innervating the periosteum. It may be that the latter two populations account for all cholinergic neurons in the rat thoracic sympathetic chain ganglia.

Target-dependent specification of the neurotransmitter phenotype: cholinergic differentiation of sympathetic neurons is mediated in vivo by gp 130 signaling

Sympathetic neurons are generated through a succession of differentiation steps that initially lead to noradrenergic neurons innervating different peripheral target tissues. Specific targets, like sweat glands in rodent footpads, induce a change from noradrenergic to cholinergic transmitter phenotype. Here, we show that cytokines acting through the gp 130 receptor are present in sweat glands. Selective elimination of the gp 130 receptor in sympathetic neurons prevents the acquisition of cholinergic and peptidergic features (VAChT, ChT1, VIP) without affecting other properties of sweat gland innervation. The vast majority of cholinergic neurons in the stellate ganglion, generated postnatally, are absent in gp 130-deficient mice. These results demonstrate an essential role of gp 130-signaling in the target-dependent specification of the cholinergic neurotransmitter phenotype.

Developmental expression of the high affinity choline transporter in cholinergic sympathetic neurons

Choline uptake by the high affinity choline transporter (CHT) is the rate-limiting step in acetylcholine synthesis. Induction of CHT is therefore a critical step in cholinergic differentiation, and we examined the developmental expression of CHT in cholinergic sympathetic neurons that innervate rodent sweat glands. During postnatal development the earliest sympathetic axons in the rear footpads are noradrenergic, containing intense tyrosine hydroxylase immunoreactivity and lacking CHT-immunoreactivity (CHT-IR). By postnatal day 7 (P7) in mouse, and P10 in rat, weak CHT-IR appeared in axons associated with the sweat gland anlagen. CHT staining intensity increased during the following weeks in conjunction with plexus arborization and gland maturation. The pattern of CHT-immunoreactivity (CHT-IR) in the sweat gland innervation was similar to staining for the vesicular acetylcholine transporter and vasoactive intestinal peptide. Immunoblots of tissue from sympathectomized rats confirmed that most of the CHT in footpad was contained in sympathetic neurons. Although CHT expression has been reported in noradrenergic sympathetic neurons of the superior cervical ganglion, these data indicate that in the sympathetic neurons projecting to sweat glands CHT is present at detectable levels only after association with the glands.

L3/Lhx8 is involved in the determination of cholinergic or GABAergic cell fate

The LIM homeobox family of transcription factors is involved in many processes during the development of the mammalian central nerves system. L3, also called Lhx8 (L3/Lhx8), is a recently identified member of the LIM homeobox gene family and is selectively expressed in the medial ganglionic eminence (MGE). Our previous study demonstrated that L3/Lhx8-null mice specifically lacked cholinergic neurons in the basal forebrain. In this study, we reduced L3/Lhx8 function in the murine neuroblastoma cell line, Neuro2a (N2a), using L3/Lhx8-targeted small interfering RNA (siRNA) produced by H1.2 promoter-driven vector. The levels of cholinergic markers per cell were diminished without a reduction in the number of marker-positive cells. Intriguingly, GABAergic marker expression and the number of GABAergic cells were dramatically increased in the differentiating L3/Lhx8-knockdown N2a. These results suggest the possibility that L3/Lhx8 is involved in the determination of transmitter phenotypes (GABAergic or cholinergic cell fate) in a population of neurons during basal forebrain development.

Nutrients as trophic factors in neurons and the central nervous system: role of retinoic acid

In multicellular organisms, death, survival, proliferation, and differentiation of a given cell depend on signals produced by neighboring and/or distant cells, resulting in the coordinated development and function of the various tissues. In the nervous system, control of cell survival and differentiation is achieved through the action of a distinct group of polypeptides collectively known as neurotrophic factors. Recent findings support the view that trophic factors also are involved in the response of the nervous system to acute injury. By contrast, nutrients are not traditionally viewed as potential trophic factors; however, there is increasing evidence that at least some influence neuronal differentiation. During development the brain is responsive to variations in nutrient supply, and this increased sensitivity or vulnerability of the brain to nutrient supply may reappear during neuronal repair, a period during which a rapid membrane resynthesis and reestablishment of synthetic pathways occur. To further evaluate the potential of specific nutrients to act as pharmacologic agents in the repair of injured neurons, the effects of retinoic acid, an active metabolite of vitamin A, and its role as a trophic factor are discussed. This literature review is intended to provide background information regarding the effect of retinoic acid on the cholinergic phenotype and the differentiation of these neurons and to explain how it may promote neuronal repair and survival following injury.

Ciliary neurotrophic factor suppresses Phox2a in sympathetic neurons

The cholinergic differentiation factor ciliary neurotrophic factor (CNTF) suppresses noradrenergic properties while inducing cholinergic and peptidergic properties in sympathetic neurons. In the rat, this includes suppression of the noradrenergic enzymes tyrosine hydroxylase and dopamine beta-hydroxylase. Lower enzyme levels result in part from suppression of gene transcription, but the mechanisms are unknown. We found that ciliary neurotrophic factor decreased the transcriptional activator Phox2a in neuroblastoma cells and cultured sympathetic neurons, suggesting that the loss of Phox2a is part of the mechanism by which CNTF suppresses tyrosine hydroxylase and dopamine beta-hydroxylase. Consistent with this model, Phox2a is suppressed in rat cholinergic sympathetic neurons where noradrenergic enzymes decrease, but is not altered in mouse cholinergic neurons where these enzymes remain high.

Re-establishment of neurochemical coding of preganglionic neurons innervating transplanted targets

We investigated the effect on neurochemical phenotype of changing the targets innervated by sympathetic preganglionic neurons. In neonatal rats, the adrenal gland was transplanted into the neck, to replace the postganglionic neurons of the superior cervical ganglion. Transplanted adrenal glands survived, and contained noradrenergic and adrenergic chromaffin cells, and adrenal ganglion cells. Retrograde tracing from the transplants showed that they were innervated by preganglionic neurons that would normally have supplied postganglionic neurons of the superior cervical ganglion. The neurochemical phenotypes of preganglionic axons innervating transplanted chromaffin cells were compared with those innervating the normal adrenal medulla or superior cervical ganglion neurons. As in the normal adrenal gland, preganglionic nerve fibres apposing transplanted chromaffin cells were cholinergic. The peptide and calcium-binding protein content of preganglionic fibres was similar in normal and transplanted adrenal glands. In both cases, cholinergic fibres immunoreactive for enkephalin targeted adrenergic chromaffin cells, whilst cholinergic fibres with co-localised calretinin-immunoreactivity innervated noradrenergic chromaffin cells and adrenal ganglion cells. In contrast to the innervation of normal adrenal glands, these axons lacked immunoreactivity to nitric oxide synthase. In a set of control experiments, the superior cervical ganglion was subjected to preganglionic denervation in rat pups the same age as those that received adrenal transplants, and the ganglion was allowed to be re-innervated over the same time course as the adrenal transplants were studied. When the superior cervical ganglion was re-innervated by preganglionic nerve fibres, we observed that all aspects of chemical coding were restored, including cholinergic markers, nitric oxide synthase, enkephalin, calcitonin gene-related peptide and calcium binding proteins in predicted combinations, although the density of nerve fibres was always lower in re-innervated ganglia. These data show that the neurochemical phenotypes expressed by preganglionic neurons re-innervating adrenal chromaffin cells are selective and similar to those seen in the normal adrenal gland. Two explanations are advanced: either that contact of preganglionic axons with novel target cells has induced a switch in their neurochemical phenotypes, or that there has been target-selective reinnervation by pre-existing fibres of appropriate phenotype. Regardless of which of these alternatives is correct, the restoration of normal preganglionic codes to the superior cervical ganglion following denervation supports the idea that the target tissue influences the neurochemistry of innervating preganglionic neurons.

Cytokine suppression of dopamine-beta-hydroxylase by extracellular signal-regulated kinase-dependent and -independent pathways

Cholinergic differentiation factors (CDFs) suppress noradrenergic properties and induce cholinergic properties in sympathetic neurons. The CDFs leukemia inhibitory factor (LIF) and ciliary neurotrophic factor (CNTF) bind to a LIFR.gp130 receptor complex to activate Jak/signal transducers and activators of transcription and Ras/mitogen-activated protein kinases signaling pathways. Little is known about how these differentiation factors suppress noradrenergic properties. We used sympathetic neurons and SK-N-BE(2)M17 neuroblastoma cells to investigate CDF down-regulation of the norepinephrine synthetic enzyme dopamine-beta-hydroxylase (DBH). LIF and CNTF activated extracellular signal-regulated kinases (ERKs) 1 and 2 but not p38 or Jun N-terminal kinases in both cell types. Preventing ERK activation with PD98059 blocked CNTF suppression of DBH protein in sympathetic neurons but did not prevent the loss of DBH mRNA. CNTF decreased transcription of a DBH promoter-luciferase reporter construct in SK-N-BE(2)M17 cells, and this was also ERK-independent. Cytokine inhibition of DBH promoter activity did not require a silencer element but was prevented by overexpression of the transcriptional activator Phox2a. Inhibiting ERK activation increased basal DBH transcription in SK-N-BE(2)M17 cells, and DBH mRNA in sympathetic neurons. Transfection of Phox2a into PD98059-treated M17 cells resulted in a synergistic increase in DBH promoter activity compared with Phox2a or PD98059 alone. These data suggest that CDFs down-regulate DBH protein via an ERK-dependent pathway but inhibit DBH gene expression through an ERK-independent pathway. They further suggest that ERK activity inhibits basal DBH gene expression.

Tyrosine hydroxylase and norepinephrine transporter in sympathetic ganglia of female rats vary with reproductive state

In females, sympathetic activity varies with changes in reproductive status, but whether expression of proteins critical to the function of sympathetic neurons is also altered is unknown. Therefore, the present study tested the hypothesis that, in rat adrenal gland and superior cervical ganglia, the expression of tyrosine hydroxylase (TH) and the norepinephrine transporter (NET), measured using Western analysis, are changed during pregnancy and the estrous cycle. Compared to diestrus, pregnancy increased TH levels in both superior cervical ganglia and adrenal gland. Pregnancy was also associated with decreased NET levels in the superior cervical ganglia, but increased levels in the adrenal gland. Relative to diestrus, the pattern of changes of TH and the NET in rats during proestrus was generally similar to changes observed during pregnancy. To assess whether gonadal hormones were involved, ovariectomized rats were also studied and changes in serum estrogen and progesterone were assayed in a subset of animals in all groups. Variations in TH and the NET among all groups did not correlate with changes in either estrogen or progesterone, suggesting that the steroids were not exclusively responsible. In conclusion, reproductive status alters the expression of TH and the NET in adrenal gland and superior cervical ganglia of female rats, which could significantly influence the function of the sympathetic nervous system. However, the mechanism for these changes does not depend solely on changes in estrogen or progesterone.

Enhanced nitric oxide concentrations and expression of nitric oxide synthase in acupuncture points/meridians

OBJECTIVES

The objectives of this study were to examine the distributions of nitric oxide (NO) in the skin points (acupoints)/meridian regions and determine whether neuronal nitric oxide synthase (nNOS) protein levels were associated with NO concentrations in the areas.

DESIGN

Low skin resistance points (LSRP) on the skin surface in response to electrical stimuli were performed in anesthetized adult rats. The skin together with subcutaneous tissue was isolated in meridian regions from PC 2 to 6, BL 36 to 57, CV 3 to 22, and GV 2 to 14. Control skin tissues were obtained in the areas close to related meridians without containing LSRP. Concentrations of nitrite (NO(2)(-)), nitrate (NO(3)(-)), and total NO(2)(-) plus NO(3)(-) (NO(x)(-)) were quantified in the skin tissues, micropunches of brain nuclei, and blood vessels in a blinded fashion. Western blots were also conducted using polyclonal anti-nNOS and anti-endothelial nitric oxide synthase (eNOS) antibody in the skin tissues.

RESULTS

NO(x)(-) and NO(3)(-) concentrations were higher (45 +/- 8% and 43 +/- 7% in the CV, 47 +/- 7% and 51 +/- 9% in the BL, and 47 +/- 8% and 45 +/- 6% in the PC) than those in control regions (p < 0.05, n = 6). NO(x)(-) concentrations are 2- to 3-fold greater in skin tissues than those in brain regions and blood vessels (p < 0.05, n = 6-8). nNOS protein levels were consistently increased in the skin regions of BL, PC, and GV meridians compared with their controls (p < 0.05, n = 5-7) but endothelial NO synthase expression was not changed.

CONCLUSION

This is the first evidence showing that NO contents and nNOS expression are consistently higher in the skin acupoints/meridians associated with low electric resistance. The results suggest that enhanced NO in the acupoints/meridians is generated from multiple resources including neuronal NOergic system, and NO might be associated with acupoint/meridian functions including low electric resistance.

Developmental regulation of neurotransmitter phenotype through tetrahydrobiopterin

During development, sympathetic neurons innervating rodent sweat glands undergo a target-induced change in neurotransmitter phenotype from noradrenergic to cholinergic. Although the sweat gland innervation in the adult mouse is cholinergic and catecholamines are absent, these neurons continue to express tyrosine hydroxylase (TH), the rate-limiting enzyme in catecholamine synthesis. The developmental suppression of noradrenergic function in these mouse sympathetic neurons is not well understood. We investigated whether the downregulation of the enzyme aromatic l-amino acid decarboxylase (AADC) or the TH cofactor tetrahydrobiopterin (BH4) could account for the loss of catecholamines in these neurons. AADC levels did not decrease during development, and adult cholinergic sympathetic neurons were strongly immunoreactive for AADC. In contrast, BH4 levels dropped significantly in murine sweat gland-containing footpads during the time period when the gland innervation was switching from making norepinephrine to acetylcholine. Immunoreactivity for the rate-limiting BH4 synthetic enzyme GTP cyclohydrolase (GCH) became undetectable in the sweat gland neurons during this phenotypic conversion, suggesting that sweat glands reduce BH4 levels by suppressing GCH expression during development. Furthermore, extracts from sweat gland-containing footpads suppressed BH4 in cultured mouse sympathetic neurons, and addition of the BH4 precursor sepiapterin rescued catecholamine production in neurons treated with footpad extracts. Together, these results suggest that the mouse sweat gland-derived cholinergic differentiation factor functionally suppresses the noradrenergic phenotype during development by inhibiting production of the TH cofactor, BH4. These data also indicate that GCH expression, which is often coordinately regulated with TH expression, can be controlled independently of TH during development.

Induction of cholinergic function in cultured sympathetic neurons by periosteal cells: cellular mechanisms

Periosteum, the connective tissue surrounding bone, alters the transmitter properties of its sympathetic innervation during development in vivo and after transplantation. Initial noradrenergic properties are downregulated and the innervation acquires cholinergic and peptidergic properties. To elucidate the cellular mechanisms responsible, sympathetic neurons were cultured with primary periosteal cells or osteoblast cell lines. Both primary cells and an immature osteoblast cell line, MC3T3-E1, induced choline acetyltransferase (ChAT) activity. In contrast, lines representing marrow stromal cells or mature osteoblasts did not increase ChAT. Growth of periosteal cells with sympathetic neurons in transwell cultures that prevent direct contact between the neurons and periosteal cells or addition of periosteal cell-conditioned medium to neuron cultures induced ChAT, indicating that periosteal cells release a soluble cholinergic inducing factor. Antibodies against LIFRbeta, a receptor subunit shared by neuropoietic cytokines, prevented ChAT induction in periosteal cell/neuron cocultures, suggesting that a member of this family is responsible. ChAT activity was increased in neurons grown with periosteal cells or conditioned medium from mice lacking either leukemia inhibitory factor (LIF) or LIF and ciliary neurotrophic factor (CNTF). These results provide evidence that periosteal cells influence sympathetic neuron phenotype by releasing a soluble cholinergic factor that is neither LIF nor CNTF but signals via LIFRbeta.

Catecholamines are required for the acquisition of secretory responsiveness by sweat glands

The sympathetic innervation of sweat glands undergoes a developmental change in transmitter phenotype from catecholaminergic to cholinergic. Acetylcholine elicits sweating and is necessary for development and maintenance of secretory responsiveness, the ability of glands to produce sweat after nerve stimulation or agonist administration. To determine whether catecholamines play a role in the development or function of this system, we examined the onset of secretory responsiveness in two transgenic mouse lines, one albino and the other pigmented, that lack tyrosine hydroxylase (TH), the rate-limiting enzyme in catecholamine synthesis. Although both lines lack TH, their catecholamine levels differ because tyrosinase in pigmented mice serves as an alternative source for catecholamine synthesis (Rios et al., 1999). At postnatal day 21 (P21), 28 glands on average are active in interdigital hind footpads of albino TH wild-type mice. In contrast, fewer than one gland is active in albino TH null mice, which lack catecholamines in gland innervation. Treatment of albino TH null mice with DOPA, a catecholamine precursor, from P11 to P21 increases the number of active glands to 14. Pigmented TH null mice, which have faint catecholamine fluorescence in the developing gland innervation, possess 12 active glands at P21, indicating that catecholamines made via tyrosinase, albeit reduced from wild-type levels, support development of responsiveness. Gland formation and the appearance of cholinergic markers occur normally in albino TH null mice, suggesting that catecholamines act directly on gland cells to trigger their final differentiation and to induce responsiveness. Thus, catecholamines, like acetylcholine, are essential for the development of secretory responsiveness.

Insights into early molluscan neuronal development through studies of transmitter phenotypes in embryonic pond snails

Pond snails have long been the subject of intense scrutiny by researchers interested in general principles of development and also cellular and molecular neurobiology. Recent work has exploited both these fields of study by examining the ontogeny of the nervous system in these animals. Much of this work has focussed upon the development of specific transmitter phenotypes to provide vignettes of neuronal subpopulations that can be traced from early embryonic life through to adulthood. While such studies have generally confirmed previous explanations of gangliogenesis in gastropods, they have also indicated the presence of several neurons that appear earlier and in positions inconsistent with classical views of gastropods neurogenesis. The earliest of these cells contain FMRFamide-related peptides and have anteriorly projections that mark the future locations of ganglia and interconnecting pathways that will comprise the postembryonic central nervous system. These posterior, peptidergic cells, as well as certain, apical, monoaminergic neurons, disappear and apparently die near the end of embryonic life. Finally, populations of what appear to be peripheral sensory neurons begin to express catecholamines by around midway through embryonic life. Like several of the neurons expressing a variety of transmitters in the developing central ganglia, the catecholaminergic peripheral cells persist into postembryonic life. Transmitter phenotypes, cell shapes and locations, and neuritic morphologies all suggest that many of the neurons observed in early embryonic pond snails have recognizable homologues across the molluscs. Such observations have profoundly altered our views of neurogenesis in gastropods over the last few years. They also suggest the promise for pond snails as fruitful models for studying the roles and mechanisms for pioneering fibres, cues triggering apoptosis, and contrasting origins and mechanisms employed for generating central vs. peripheral neurons within a single organism.

Functional implications of the noradrenergic-cholinergic switch induced by retinoic acid in NB69 neuroblastoma cells

Some neuroblastoma cell lines change their neurotransmitter phenotype from noradrenergic to cholinergic under retinoic acid treatment. Such "neurotransmitter switch" seems to be a consequence of changes in the expression and activity of the biosynthetic machinery for both neurotransmitters. In this study, we have characterized this "neurotransmitter switch" induced by retinoic acid in a human neuroblastoma cell line (NB69) showing catecholaminergic characteristics. Retinoic acid treatment reduced tyrosine hydroxylase activity and noradrenaline levels in NB69 cells but did not modify the expression of this enzyme. Moreover, the calcium-dependent release of [(3)H]noradrenaline in control cells was highly reduced by retinoic acid treatment. On the other hand, NB69 cells treated with retinoic acid enhanced the expression of choline acetyltransferase and acquired the capability to release [(3)H]acetylcholine in a calcium-dependent way. In addition, we found that the expression of the vesicular monoamine transporter 2 (VMAT2) and the vesicular acetylcholine transporter (VAChT) was increased in those cells treated with retinoic acid. Immunostaining revealed that retinoic acid treatment changed the cellular distribution of both vesicular monoamine transporter 2 and vesicular acetylcholine transporter. In conclusion, retinoic acid induces a noradrenergic to cholinergic switch in NB69 cells by acting at several levels of the neurotransmitter phenotypic expression.

Norepinephrine transporter expression in cholinergic sympathetic neurons: differential regulation of membrane and vesicular transporters

Sympathetic neurons that undergo a noradrenergic to cholinergic change in phenotype provide a useful model system to examine the developmental regulation of proteins required to synthesize, store, or remove a particular neurotransmitter. This type of change occurs in the sympathetic sweat gland innervation during development and can be induced in cultured sympathetic neurons by extracts of sweat gland-containing footpads or by leukemia inhibitory factor. Sympathetic neurons initially produce norepinephrine (NE) and contain the vesicular monoamine transporter 2 (VMAT2), which packages NE into vesicles, and the norepinephrine transporter (NET), which removes NE from the synaptic cleft to terminate signaling. We have used a variety of biochemical and molecular techniques to test whether VMAT2 and NET levels decrease in sympathetic neurons which stop producing NE and make acetylcholine. In cultured sympathetic neurons, NET protein and mRNA decreased during the switch to a cholinergic phenotype but VMAT2 mRNA and protein did not decline. NET immunoreactivity disappeared from the developing sweat gland innervation in vivo as it acquired cholinergic properties. Surprisingly, NET simultaneously appeared in sweat gland myoepithelial cells. The presence of NET in myoepithelial cells did not require sympathetic innervation. VMAT2 levels did not decrease as the sweat gland innervation became cholinergic, indicating that NE synthesis and vesicular packaging are not coupled in this system. Thus, production of NE and the transporters required for noradrenergic transmission are not coordinately regulated during cholinergic development.

The early expression of VAChT and VIP in mouse sympathetic ganglia is not induced by cytokines acting through LIFRbeta or CNTFRalpha

Sympathetic ganglia consist of noradrenergic and cholinergic neurons. The cholinergic marker protein vesicular acetylcholine transporter (VAChT) and the neuropeptide vasoactive intestinal peptide (VIP), co-expressed in mature cholinergic sympathetic neurons, are first detectable during embryonic development of rat sympathetic ganglia. However, the subpopulation of cholinergic sympathetic neurons which innervates sweat glands in mammalian footpads starts to express VAChT and VIP during the first postnatal weeks, under the influence of sweat gland-derived signals. In vitro evidence suggests that the sweat gland-derived cholinergic differentiation factor belongs to a group of neuropoietic cytokines, including LIF, CNTF and CT-1, that act through a LIFRbeta-containing cytokine receptor. To investigate whether the embryonic expression of cholinergic properties is elicited by a related cytokine, the expression of VAChT and VIP was analyzed in stellate ganglia of mice deficient for the cytokine receptor subunits LIFRbeta or CNTFRalpha. The density of VAChT- and VIP-immunoreactive cells in stellate ganglia of new-born animals was not different in LIFRbeta(-/-) and CNTFRalpha(-/-) ganglia as compared to ganglia from wild-type mice. These results demonstrate that the early, embryonic expression of VAChT and VIP is not induced by cytokines acting through LIFRbeta- or CNTFRalpha-containing receptors.

Developmental changes in the transmitter properties of sympathetic neurons that innervate the periosteum

During the development of sweat gland innervation, interactions with the target tissue induce a change from noradrenergic to cholinergic and peptidergic properties. To determine whether the change in neurotransmitter properties that occurs in the sweat gland innervation occurs more generally in sympathetic neurons, we identified a new target of cholinergic sympathetic neurons in rat, the periosteum, which is the connective tissue covering of bone, and characterized the development of periosteal innervation of the sternum. During development, sympathetic axons grow from thoracic sympathetic ganglia along rib periosteum to reach the sternum. All sympathetic axons displayed catecholaminergic properties when they reached the sternum, but these properties subsequently disappeared. Many axons lacked detectable immunoreactivities for vesicular acetylcholine transporter and vasoactive intestinal peptide when they reached the sternum and acquired them after arrival. To determine whether periosteum could direct changes in the neurotransmitter properties of sympathetic neurons that innervate it, we transplanted periosteum to the hairy skin, a noradrenergic sympathetic target. We found that the sympathetic innervation of the transplant underwent a noradrenergic to cholinergic and peptidergic change. These results suggest that periosteum, in addition to sweat glands, regulates the neurotransmitter properties of the sympathetic neurons that innervate it.

Development of muscarinic receptors and regulation of secretory responsiveness in rodent sweat glands

Sweat glands are innervated by sympathetic neurons which undergo a change in transmitter phenotype from noradrenergic to cholinergic during development. As soon as the glands begin to differentiate, M3 muscarinic receptor mRNA and binding sites are detectable. Receptor expression appears in the absence of innervation and is maintained after denervation. While receptor expression is not regulated by innervation, secretory responsiveness is. Muscarinic blockade during development or in adult animals results in the loss of responsiveness and its reappearance requires several days. Cholinergic muscarinic activation is most likely to regulate one or more steps in the signalling cascade that are downstream of calcium mobilization. The anterograde regulation of sweat gland responsiveness is one facet of the reciprocal interactions are required to establish a functional synapse in this system.

Pituitary expression of protein kinase C isotypes during early development

Protein kinase C (PKC) is a critical regulator of signal transduction and cell function in many tissues, including pituitary. Although PKC influences pituitary hormone secretion in adults, its role in determining characteristic perinatal patterns of hormone secretion and synthesis is not known, and the expression of major PKC isotypes in perinatal pituitary is poorly defined. We therefore determined the developmental, cell-specific expression of the major PKC isotypes, using Western analysis and double label immunohistochemistry, in pituitaries of perinatal and mature rats. Expression of specific PKC isotypes was strikingly age-dependent. Pituitary expression of PKC alpha was particularly high in neonates and declined significantly with age, with levels in adult rats approximately half those of neonates as assessed by Western analysis. Similarly, immunohistochemistry indicated that PKC alpha was less abundant in adult than in neonatal pituitaries; the most intensely staining cells of both age groups were identified as somatotrophs and gonadotrophs. In contrast to PKC alpha, pituitary expression of PKC epsilon increased approximately two-fold with advancing age as assessed by Western analysis; this age-dependent pattern was confirmed by immunohistochemistry. Perinatal pituitaries expressed PKC epsilon in some somatotrophs and in all gonadotrophs, whereas PKC epsilon expression was limited to gonadotrophs in the mature pituitary. Pituitary expression of PKC betaII, delta, and zeta did not differ with age, and PKC gamma was not detected in pituitaries of any age group. These results indicate that expression of PKC isotypes within the pituitary is developmentally regulated in a cell-specific and isotype-specific manner, and are consistent with the concept that PKC contributes to the regulation of pituitary function during early development.

Evidence for NOS-containing renal neuronal somata transiently expressing a catecholaminergic phenotype during development in the rat

Transiently catecholaminergic cells (TC-cells) expressing tyrosine hydroxylase (TH) have been shown in a variety of tissues during embryonic life. To investigate the possible relationship of nitric oxide synthase (NOS)-containing renal neuronal somata (RNS) and the TC-cells, we examined serial 100 microm slices of whole kidneys for TH-immunofluorescence and NADPH-d histochemistry during prenatal and postnatal development. The number of TH-cells increased during the prenatal period, peaked at birth and were very rare by PD21. A subpopulation of TH-immunoreactive RNS displayed NADPH-d activity. By PD21 the TH-positive RNS had practically disappeared while the number of NADPH-d positive RNS was markedly increased. These results suggest that kidneys possess transient catecholaminergic cells which display NOS-activity and that NOS expression may be the end-point in the differentiation of the RNS.

Target-dependent development of the vesicular acetylcholine transporter in rodent sweat gland innervation

Descriptive studies have delineated a developmental change in neurotransmitter phenotype from noradrenergic to cholinergic in the sympathetic innervation of sweat glands in rodent footpads. Transplantation and culture experiments provide evidence that interactions with the target tissue induce this change. Recent studies with an antiserum that recognizes the vesicular acetylcholine transporter (VAChT) suggest, however, that the development of cholinergic function in sympathetic neurons, including those that innervate sweat glands, occurs prior to and does not require target contact. To clarify these apparently contradictory findings, we directly compared the appearance of VAChT immunoreactivity in the sympathetic neurons that innervate sweat glands with the time that axons contact this target. We find that VAChT immunoreactivity is not detectable in either the axons or cell bodies of sweat gland neurons until several days after target innervation. Before and during VAChT acquisition, the developing sweat gland innervation contains vesicular stores of catecholamines. An analysis of mutant mice that lack sweat glands was undertaken to determine whether VAChT expression requires target interactions and revealed that VAChT does not appear in the absence of glands. These findings, together with previous studies, confirm the target dependence of cholinergic function in the sympathetic neurons that innervate sweat glands.

Overexpression of nerve growth factor in epidermis disrupts the distribution and properties of sympathetic innervation in footpads

Sympathetic and sensory neurons form distinct axonal arborizations in several peripheral targets. The developmental mechanisms responsible for partitioning sympathetic and sensory axons between potential target tissues are poorly understood. We have used rodent footpads to study this process because three populations of peripheral axons innervate topographically segregated targets in the footpad; cholinergic sympathetic axons innervate sweat glands, noradrenergic sympathetic axons innervate blood vessels, and sensory axons form a plexus at the epidermal/dermal junction. To examine how nerve growth factor (NGF), a trophic and survival factor for sympathetic and some sensory neurons, may contribute to the generation of the patterned distribution of axons among targets, we studied transgenic mice (K14-NGF mice) in which NGF expression was significantly increased in the epidermis. Whereas the temporal sequence in which sensory and sympathetic fibers arrived in the footpad was not affected, the normal partitioning of axons between target tissues was disrupted. The two sympathetic targets in footpads, sweat glands, and blood vessels lacked substantial innervation and instead a dense plexus of catecholaminergic sympathetic fibers was found commingled with sensory fibers in the dermis. Those sympathetic fibers present in sweat glands expressed an abnormal dual catecholaminergic/cholinergic phenotype. Our findings indicate that overexpression of NGF in skin interferes with the segregation of sensory and sympathetic axonal arbors and suggests a role for target-derived NGF in the establishment of distinct axonal territories. Our data also suggest that by determining where axon arbors form, NGF can indirectly influence the phenotypic properties of sympathetic neurons.

A sweat gland-derived differentiation activity acts through known cytokine signaling pathways

The sympathetic innervation of sweat glands undergoes a target-induced noradrenergic to cholinergic/peptidergic switch during development. Similar changes are induced in cultured sympathetic neurons by sweat gland cells or by one of the following cytokines: leukemia inhibitory factor (LIF), ciliary neurotrophic factor (CNTF), or cardiotrophin-1 (CT-1). None of these is the sweat gland-derived differentiation activity. LIF, CNTF, and CT-1 act through the known receptors LIF receptor beta (LIFRbeta) and gp130 and well defined signaling pathways including receptor phosphorylation and STAT3 activation. Therefore, to determine whether the gland-derived differentiation activity was a member of the LIF/CNTF cytokine family, we tested whether it acted via these same receptors and signal cascades. Blockade of LIFRbeta inhibited the sweat gland differentiation activity in neuron/gland co-cultures, and extracts of gland-containing footpads stimulated tyrosine phosphorylation of LIFRbeta and gp130. An inhibitor (CGX) of molecules that bind the CNTFRalpha, which is required for CNTF signaling, did not affect the gland-derived differentiation activity. Soluble footpad extracts induced the same changes in NBFL neuroblastoma cells as LIF and CNTF, including increased vasoactive intestinal peptide mRNA, STAT3 dimerization, and DNA binding, and stimulation of transcription from the vasoactive intestinal peptide cytokine-responsive element. Thus, the sweat gland-derived differentiation activity uses the same signaling pathway as the neuropoietic cytokines, and is likely to be a family member.

Norepinephrine facilitates the development of the murine sweat response but is not essential

During development, the sympathetic neurons innervating sweat glands undergo a neurotransmitter switch from noradrenergic to cholinergic between postnatal day (P) 4, when the sympathetic neurons first contact the sweat glands, and P21. Several in vitro experiments suggest that norepinephrine (NE), produced by sympathetic neurons, stimulates sweat glands to produce a factor that then induces the phenotypic switch. We tested this hypothesis in vivo using dopamine beta-hydroxylase-deficient mice (DBH -/-), which are unable to synthesize NE and epinephrine, and tyrosine hydroxylase-deficient mice (TH -/-), which are unable to synthesize any catecholamines. The cholinergic agonist pilocarpine and electrostimulation of the sciatic nerve both elicited a sweat response in adult DBH -/- mice that was indistinguishable from the response of controls, and the cholinergic antagonist atropine effectively blocked these responses. We did note, however, a 1- to 2-week delay in the acquisition of the sweat response in DBH -/- mice. Although diminished in magnitude, a sweat response to pilocarpine was also noted in TH -/- mice at P21. Immunohistochemistry demonstrated that TH and vasoactive intestinal peptide were detectable at P14 and increased to adult levels by P21 in DBH +/- and DBH -/- mice. These observations indicate that NE is not essential for the acquisition of the cholinergic phenotype, but it may facilitate its postnatal development.

CNTF and LIF are not required for the target-directed acquisition of cholinergic and peptidergic properties by sympathetic neurons in vivo

During development, the sympathetic innervation of two targets, sweat glands and periosteum, changes the neurotransmitters it expresses from noradrenaline to acetylcholine and vasoactive intestinal peptide (VIP). The target-derived molecules that induce, these changes have not been identified. Neuropoietic cytokines, including ciliary neurotrophic factor (CNTF) and leukemia inhibitory factor (LIF), induce the same phenotypic changes in sympathetic neurons in vitro as sweat glands and periosteum do in vivo, raising the possibility that one of these factors mediates induction of cholinergic traits and VIP by these target tissues. Because CNTF and LIF have overlapping functions and signalling pathways, they could act interchangeably or in concert to influence neurotransmitter expression. To determine whether CNTF or CNTF and LIF together are responsible for the induction of cholinergic and peptidergic function in vivo, we analyzed the neurotransmitter properties of sweat gland innervation in mice lacking CNTF or CNTF and LIF. We find that, as in wild-type mice, gland innervation in mice lacking one or both molecules appropriately expresses cholinergic properties and VIP immunoreactivity. Furthermore, footpads of mice lacking one or both genes contain choline acetyltransferase activity comparable to that of wild-type mice, and CNTF- or CNTF/LIF-deficient mice possess the normal complement of active sweat glands. We analyzed the innervation of a second, recently identified cholinergic sympathetic target, the periosteum, which is the connective tissue surrounding bone. Periosteal innervation of mice lacking CNTF, LIF, or both, like that of wild-type mice, is immunoreactive for the vesicular acetylcholine transporter, a recently identified cholinergic marker, and VIP. These results provide evidence that neither CNTF, LIF, nor a combination of the two are required for the developmental change from noradrenergic to cholinergic function that occurs in sympathetic innervation of sweat glands and periosteum.

Prenatal and postnatal requirements of NT-3 for sympathetic neuroblast survival and innervation of specific targets

Postnatal homozygous neurotrophin-3 mutant mice display a loss of about half the sympathetic superior cervical ganglion (SCG) neurons (Ernfors, P., Lee, K.-F., Kucera, J. and Jaenisch, R. (1994a) Cell 77, 503–512; Farinas, I., Jones, K. R., Backus, C., Wang, X. Y. and Reichardt, L. F. (1994) Nature 369, 658–661). We found that this loss is caused by excessive apoptosis of sympathetic neuroblasts leading to a failure to generate a normal number of neurons during neurogenesis. NT-3 was also found to be required postnatally. In Nt-3−/− mice, sympathetic fibers failed to invade pineal gland and external ear postnatally; whereas other targets of the external and internal carotid nerves, including the submandibular gland and the iris, displayed a normal complement of sympathetic innervation. Sympathetic fibers of mice carrying one functional copy of the Nt-3 gene (Nt-3+/− mice) invaded the pineal gland, but failed to branch and form a ground plexus. Cultured neonatal sympathetic neurons responded to NT-3 by neurite outgrowth and mRNA upregulation of the NT-3 receptor, trkC. Exogenously administered NT-3 promoted sympathetic growth and rescued the sympathetic target deficit of the mutant mice. We conclude that NT-3 is required for the survival of sympathetic neuroblasts during neurogenesis and for sympathetic innervation and branching in specific targets after birth.

Developmental plasticity of neuropeptide expression in motoneurons of the moth, Manduca sexta: steroid hormone regulation

Developmental changes in the expression of a FMRFamide-like (Phe-Met-Arg-Phe-NH2) peptide or peptides in motoneurons of the tobacco hornworm, Manduca sexta, were demonstrated using immunohistochemical techniques. The onset of FMRFamide-like immunoreactivity (FLI) was gradual during larval growth but by the final larval stage, immunoreactivity was present in the majority of motoneurons. FLI then declined during metamorphosis and was absent in all identified adult motoneurons. We used a novel in vivo culture system to demonstrate that the steroid hormone, 20-hydroxyecdysone, regulates the loss of FLI in motoneurons during metamorphosis. The small commitment peak of ecdysteroid appears to shut off the program of neuropeptide accumulation that is characteristic of the larval state of the motoneurons. The prepupal peak of steroid then causes the rapid loss of stored FLI. This steroid-induced change in the neuropeptide content of motoneurons may reflect major changes in neuromuscular functions between the larval and adult stages.

Peptidergic, catecholaminergic and morphological properties of avian chromaffin cells are modulated distinctively by growth factors

Most neurons and endocrine cells are known to co-express a 'classical neurotransmitter' with one or more neuropeptides. Although their expression has been shown to be modulated by differentiation factors, it is not known if particular combinations of neurotransmitter/neuropeptide(s) are co-regulated. We have analyzed the effect of nerve growth factor (NGF), neurotrophin-3 (NT-3), brain derived neurotrophic factor (BDNF), ciliary neurotrophic factor (CNTF), basic fibroblast growth factor (bFGF) and transforming growth factor beta 1 (TGF-beta 1) on the modulation of neuroactive substances co-expressed by avian chromaffin cells. The content of the neuropeptides neuropeptide Y (NPY), enkephalin (ENK) and somatostatin (SS) was measured by radioimmunoanalysis, and the content of the catecholamines norepinephrine (NE) and epinephrine (E) by high pressure liquid chromatography-electrochemistry (HPLC-EC). In addition, the morphological differentiation of chromaffin cells in response to the growth factors was assessed. All of the studied factors had distinct effects on the chromaffin content of neuropeptides and catecholamines. Our results show that the modulation of CAs and neuropeptides, and among the neuropeptides themselves is completely dissociated. Moreover, the cellular responses to the different growth factors show that neurochemical properties are modulated independently of morphological ones.

The expression of tyrosine hydroxylase and the transcription factors cPhox-2 and Cash-1: evidence for distinct inductive steps in the differentiation of chick sympathetic precursor cells

The adrenergic differentiation of sympathetic neurons is controlled by complex interactions with the embryonic environment. To provide a basis for the experimental analysis of these interactions, the expression of the adrenergic marker enzyme tyrosine hydroxylase (TH) was analyzed by immunohistochemistry and in situ hybridization in sympathetic ganglia and the adrenal medulla of chick embryos. In parallel, the developmental expression of the transcription factors cPhox-2 and Cash-1 was analyzed by in situ hybridization. TH protein was first detectable during the third day of development (stage 19) in cells of the primary sympathetic strands. A few hours earlier (stage 18), TH mRNA could be found by in situ hybridization. At the very same time and location, mRNA for the transcription factor cPhox-2 was first observed. In contrast, mRNA for the transcription factor Cash-1 was detected much earlier, at stage 15, dorsal to the aorta where the primary sympathetic ganglia form. High TH mRNA levels are maintained during later embryonic development (stage 35) in both sympathetic ganglia and adrenal chromaffin cells. In contrast, cPhox-2 and Cash-1 mRNA are selectively reduced in chromaffin cells and sympathetic ganglia, respectively. The results show that TH and cPhox-2 are early markers expressed in sympathetic ganglia. Their coordinated expression points towards an inductive event possibly occurring close to the aorta and leading to the expression of an adrenergic phenotype. Cash-1 is detected significantly earlier, suggesting that its expression is induced by a separate event.

Innervation of footpads of normal and mutant mice lacking sweat glands

Footpads of normal adult mice are innervated by sympathetic and sensory fibers. The sympathetic fibers associated with sweat glands contain acetylcholinesterase and immunoreactivity for vasoactive intestinal peptide. Although catecholamine histofluorescence is absent, the gland innervation exhibits immunoreactivity for tyrosine hydroxylase. A distinct population of sympathetic fibers, which possess catecholamines and neuropeptide Y as well as tyrosinehydroxylase immunoreactivity, innervates blood vessels. Sensory fibers containing immunoreactivity for substance P and calcitonin gene-related peptide course beneath the epidermis and some form endings in it. Treatment of neonatal mice with the adrenergic neurotoxin, 6-hydroxydopamine, results in loss of sympathetic innervation of sweat glands and blood vessels, permits growth of sensory axons into sweat glands, but does not alter the peptidergic sensory innervation of the dermis and epidermis. Three mouse mutations, Tabby (Ta), crinkled (cr), and downless (dl), disrupt the interactions between the mesenchyme and epidermis that are required for normal development of specific epidermal derivatives, including sweat glands. The sympathetic innervation of blood vessels and sensory innervation of footpad skin of the three mutant mice that lack sweat glands is indistinguishable from normal. The sympathetic fibers that normally innervate sweat glands, however, are not present. These results indicate that in the absence of their normal target, the sympathetic fibers that innervate sweat glands are lacking. Furthermore, they suggest that, although sensory fibers may sprout into sympathetic targets in the footpad, the domains occupied by sensory fibers are not normally accessible to sympathetic axons.

Differential regulation of phenotypic expression in a pluripotential neuroblastoma cell line

Our laboratory has recently been involved in investigating factors which influence plasticity of neurotransmitter phenotypic expression both in vivo and in culture. Our previous studies have shown that precursor neuroblasts are pluripotential with respect to neurotransmitter phenotype and respond differentially to microenvironmental signals. In the present study, we examined phenotypic expression in neuroblastoma cells, P2 clone, using the activities of choline acetyltransferase (ChAT) and tyrosine hydroxylase (TH) as neuronal markers for the cholinergic and catecholaminergic phenotypes, respectively. Cells were plated and grown for 4 days in culture, harvested and frozen for assay of ChAT and TH. Basal activity of ChAT was 2.47 +/- 0.22 nmoles Ach formed /h/mg protein and that of TH was 5.23 +/- 0.41 pmoles CO2 formed /h/mg protein in control cultures. When intracellular cAMP levels were increased by addition to the growth medium of 10 micrograms/ml prostaglandin E1 (PGE1; a receptor-mediated enhancer of adenylate cyclase activity) or 200 micrograms/ml RO20-1724 (an inhibitor of cyclic nucleotide phosphodiesterase) the activity of TH was increased 340- and 423-fold, respectively. In marked contrast, the activity of ChAT was not affected by either agent. Double staining immunocytochemical examination demonstrated that both ChAT and TH were colocalized in the same cell. The molecular mechanism whereby catecholaminergic expression exclusively is affected in this cell model is currently under investigation. We conclude that (1) P2 neuroblastoma is a pluripotential cell line, (2) phenotypic expression in a homogenous cell population, such as P2 neuroblastoma, is differentially regulated. Moreover, this cell line is a unique model for studying the molecular mechanisms of phenotypic expression and neuronal plasticity.

Noradrenergic regulation of cholinergic differentiation

When the sympathetic nerves that innervate rat sweat glands reach their targets, they are induced to switch from using norepinephrine as their neurotransmitter to acetylcholine. Catecholamines (such as norepinephrine) released by nerves growing to the sweat gland induce this phenotypic conversion by stimulating production of a cholinergic differentiation factor [sweat gland factor (SGF)] by gland cells. Here, culture of gland cells with sympathetic, but not sensory, neurons induced SGF production. Blockage of alpha 1- or beta-adrenergic receptors prevented acquisition of the cholinergic phenotype in sympathetic neurons co-cultured with sweat glands, and sweat glands from sympathectomized animals lacked SGF. Thus, reciprocal instructive interactions, mediated in part by small molecule neurotransmitters, direct the development of this synapse.

Target determination of neurotransmitter phenotype in sympathetic neurons

While the majority of sympathetic neurons are noradrenergic, a minority population are cholinergic. At least one population of cholinergic sympathetic neurons arises during development by a target-dependent conversion from an initial noradrenergic phenotype. Evidence for retrograde specification has been obtained from transplantation studies in which sympathetic neurons that normally express a noradrenergic phenotype throughout life were induced to innervate sweat glands, a target normally innervated by cholinergic sympathetic neurons. This was accomplished by transplanting footpad skin containing sweat gland primordia from early postnatal donor rats to the hairy skin region of host rats. The sympathetic neurons innervating the novel target decreased their expression of noradrenergic traits and developed choline acetyltransferase (ChAT) activity. In addition, many sweat gland-associated fibers acquired acetylcholinesterase (AChE) staining and VIP immunoreactivity. These studies indicate that sympathetic neurons in vivo alter their neurotransmitter phenotype in response to novel environmental signals and that sweat glands play a critical role in the cholinergic and peptidergic differentiation of the sympathetic neurons that innervate them. The sweat gland-derived cholinergic differentiation factor is distinct from leukemia inhibitory factor and ciliary neurotrophic factor, two well-characterized cytokines that alter the neurotransmitter properties of cultured sympathetic neurons in a similar fashion. Recent studies indicate that anterograde signalling is also important for the establishment of functional synapses in this system. We have found that the production of cholinergic differentiation activity by sweat glands requires sympathetic innervation, and the acquisition and maintenance of secretory competence by sweat glands depends upon functional cholinergic innervation.

Postnatal development of survival responsiveness in rat sympathetic neurons to leukemia inhibitory factor and ciliary neurotrophic factor

Embryonic rat sympathetic neurons undergo programmed cell death upon NGF deprivation. We show that during postnatal development, these neurons acquire the ability to be supported in vitro by LIF and CNTF as well as NGF. LIF and CNTF do not promote the long-term survival of embryonic day 21 sympathetic neurons in vitro. However, after 5 days of culture in the presence of NGF, the majority of embryonic day 21 sympathetic neurons can be supported by either of these factors. Furthermore, postnatal day 6 sympathetic neurons can be immediately supported by LIF and CNTF, indicating that acquisition of survival responsiveness occurs in vivo as well as in vitro. During this period, neuronal expression of LIF and CNTF receptor mRNAs remains constant, suggesting that sympathetic neurons alter their responsiveness to LIF and CNTF by allowing additional intracellular signaling pathways to promote survival.

Dependence on p75 for innervation of some sympathetic targets

The low-affinity neurotrophin receptor p75 binds all neurotrophins with similar affinity. For elucidation of its function, mice bearing a null mutation in the p75 locus were generated. Examination of sympathetic innervation of target tissues revealed that pineal glands lacked innervation and sweat gland innervation was absent or reduced in particular footpads. The absence of adult innervation reflects the failure of axons to reach these targets during development rather than a target deficit. These results indicate that p75 facilitates development of specific populations of sympathetic neurons, for which it may support axon growth.

Neurotransmitter properties of guinea-pig sympathetic neurons grown in dissociated cell culture--II. Fetal and embryonic neurons: regulation of neuropeptide Y expression

We report here the neurotransmitter characteristics of neurons cultured from the same ganglia of fetal and embryonic guinea-pigs. Both the celiac ganglion and the superior mesenteric ganglion were examined. In a previous paper we described the neurotransmitter properties of adult guinea-pig prevertebral sympathetic neurons grown in dissociated cell culture, including the expression by these cells of immunoreactivity for tyrosine hydroxylase, neuropeptide Y and somatostatin. Tyrosine hydroxylase immunoreactivity was ubiquitously expressed in all fetal embryonic cultures, as was the case for adult neurons. Fetal-derived celiac and superior mesenteric gangli neurons displayed neuropeptide Y and somatostatin immunoreactivity in the same percentage of neurons as in adult cultures but at markedly lower levels. Embryonic neurons also expressed somatostatin immunoreactivity in roughly the same proportion of neurons as in adult and fetal cultures; however, the expression of neuropeptide Y immunoreactivity in both celiac and superior mesenteric gangli cultures was significantly different. Specifically, neuropeptide Y immunoreactivity in embryonic celiac cultures was greatly reduced in both the number of positive-labeled neurons and the amount of immunoreactive product, while neuropeptide Y immunoreactivity in embryonic superior mesenteric gangli cultures was markedly increased compared to their adult and fetal counterparts. The expression of neuropeptide Y immunoreactivity in celiac neurons was found to be specifically elevated by culturing the neurons in medium conditioned by disassociated vascular cells, this treatment having no effect on tyrosine hydroxylase or somatostatin immunoreactivity. Heart cell-conditioned medium did not effect neuropeptide Y or somatostatin immunoreactivity, although it did result in a significant reduction of tyrosine hydroxylase immunoreactivity and an increase in 5-hydroxytryptamine immunoreactivity. We conclude that the expression of neuropeptide Y immunoreactivity develops independently in cultures of adult and near-term fetuses but that embryonic neurons require interactions with target cells to express this phenotype. Neuropeptide Y immunoreactivity can be induced in embryonic sympathetic neurons by a target-derived factor(s).