Development of neuropeptide Y-containing neurons in sympathetic ganglia of rats

Expression of neuropeptide Y (NPY) in the sympathetic ganglia was investigated by immunohistochemistry and tract tracing. The distribution of NPY immunoreactivity (IR) was studied in the superior cervical ganglion (SCG), stellate ganglion (SG) and celiac ganglion (CG) from rats of different ages (newborn, 10-day-old, 20-day-old, 30-day-old, 2-month-old, 6-month-old, 24-month-old). We observed that the percentage of NPY-IR neuronal profiles increased during early postnatal development. In the SCG and SG, the percentage of NPY-IR profiles enlarged in the first month of life from 43±3.6% (SCG) and 46±3.8% (SG) until 64±4.1% (SCG) and 58±3.5% (SG). The percentage of NPY-IR profiles in the CG increased during the period between 20days (65±3.8%) and 30days (82±5.1%) of animals' life and did not change in further development. In newborn and 10-day-old rats, a large portion of NPY-IR neurons was also calbindin D28K (CB)-IR in all sympathetic ganglia. The proportion of CB-IR substantially decreased during next 10days in the SCG, SG and CG. NPY-IR was approximately present in a half of the postganglionic neurons innervating muscle vessels of the neck and forearm, and the percentage of labeled NPY-IR profiles did not change during the development. Only single Ki67-IR neurons were also NPY-IR in the SCG, SG and CG in newborns and not in older animals. No NPY+/caspase 3+IR neurons were observed. Finally, the process of morphological changes in the size and percentages of NPY-IR profiles is complete in rats by the first month of life.

[Age factors in the background electrical activity of the superior cervical ganglion neurons in rats]

Background discharges of single neurons were studied from the superior cervical ganglion in newborn, 10-, 20-day-old, 1-, 2- and 6-month-old rats. In all age groups, the largest proportion of neurons exhibited aperiodic activity. The percentage of neurons with respiratory rhythmic was less. In newborn and 10-day-old rats, the frequency of discharges was low. Discharge frequency increased in 20-day-old rats. In 20-day-old and more adult rats, we found neurons bursting with cardiac frequency. The means of frequency did not statistically differ in 1-, 2- and 6-month-old rats. Thus, the pattern of neuronal activity is formed during the development in 20-day-old rats. Final maturation of this pattern is observed in 1-month-old rats.

The death receptor antagonist FLIP-L interacts with Trk and is necessary for neurite outgrowth induced by neurotrophins

FLICE-inhibitory protein (FLIP) is an endogenous inhibitor of the signaling pathway triggered by the activation of death receptors. Here, we reveal a novel biological function for the long form of FLIP (FLIP-L) in neuronal differentiation, which can be dissociated from its antiapoptotic role. We show that FLIP-L is expressed in different regions of the mouse embryonic nervous system. Immunohistochemistry of mouse brain sections at different stages reveals that, in neurons, FLIP is expressed early during the embryonic neuronal development (embryonic day 16) and decreases at later stages (postnatal days 5-15), when its expression is essentially detected in glial cells. FLIP-L overexpression significantly enhances neurotrophin-induced neurite outgrowth in motoneurons, superior cervical ganglion neurons, and PC12 cells. Conversely, the downregulation of FLIP-L protein levels by specific RNA interference significantly reduces neurite outgrowth, even in the presence of the appropriate neurotrophin stimulus. Moreover, NGF-dependent activation of two main intracellular pathways involved in the regulation of neurite outgrowth, extracellular signal-regulated kinases (ERKs) and nuclear factor kappaB (NF-kappaB), is impaired when endogenous FLIP-L is downregulated, although TrkA remains activated. Finally, we demonstrate that FLIP-L interacts with TrkA, and not with p75(NTR), in an NGF-dependent manner, and endogenous FLIP-L interacts with TrkB in whole-brain lysates from embryonic day 15 mice embryos. Altogether, we uncover a new role for FLIP-L as an unexpected critical player in neurotrophin-induced mitogen-activated protein kinase/ERK- and NF-kappaB-mediated control of neurite growth in developing neurons.

Maturation and long-term hypoxia alters Ca2+-induced Ca2+ release in sheep cerebrovascular sympathetic neurons

The contribution of sympathetic nerves arising from the superior cervical ganglia (SCG) toward the growth and function of cerebral blood vessels is pertinent throughout maturation as well as in response to cardiovascular stress imposed by high-altitude long-term hypoxia (LTH). The function of SCG sympathetic neurons is dependent on intracellular Ca2+ concentration ([Ca2+]i) signaling, which is strongly influenced by a process known as Ca(2+)-induced Ca2+ release (CICR) from the smooth endoplasmic reticulum (SER). In this study, we used the sheep SCG neuronal model to test the hypotheses that maturation decreases CICR and high-altitude LTH depresses CICR in fetal SCG neurons but not in those of the adult. We found that the contribution of CICR to electric field stimulation (EFS)-evoked [Ca2+]i transients was greatest in SCG cells from normoxic fetuses and was abolished by LTH. The decline in CICR was associated with a reduction in sarco(endo)plasmic reticulum Ca(2+)-ATPase (SERCA) function in fetal SCG cells during LTH, reducing SER Ca2+ levels below the threshold needed for the coupling of Ca2+ influx and CICR. With respect to the maturation from the fetus to adult, the decrease in CICR may reflect both a reduction in the levels of ryanodine receptor isoforms 2 and 3 and SERCA function. In response to LTH and in contrast to the fetus, CICR function in adult SCG cells is maintained and may reflect alterations in other mechanisms that modulate the CICR process. As CICR is instrumental in the function of sympathetic neurons within the cerebrovasculature, the loss of this signaling mechanism in the fetus may have consequences for the adaptation to LTH in terms of fetal susceptibility to vascular insults.

The developing and restructuring superior cervical ganglion of guinea pigs (Cavia porcellus var. albina)

Post-natal development comprises both maturation (from newborn to adult) and ageing (from adult to senility) and, during this phase, several adaptive mechanisms occur in sympathetic ganglia, albeit they are not fully understood. Therefore, the present study aimed at detecting whether post-natal development would exert any effect on the size and number of a guinea pig's superior cervical ganglion (SCG) neurons. Twenty right SCGs from male subjects were used at four ages, i.e. newborn (7 days), young (30 days), adult (7 months) and old animals (50 months). Using design-based stereological methods the volume of ganglion and the total number of mononucleate and binucleate neurons were estimated. Furthermore, the mean perikaryal volume of mononucleate and binucleate neurons was estimated using the vertical nucleator. The main findings of this study were a combination of post-natal-dependent increases and decreases in some variables: (i) 27% increase in ganglion volume, (ii) 24% and 43% decreases in the total number of mono and binucleate neurons, respectively, and (iii) 27.5% and 40% decreases in the mean perikaryal volume of mono and binucleate neurons, respectively. Despite the fall in neuron numbers found here, post-natal development is not only associated with neuron loss, but also embraces other structural adaptive mechanisms, which are discussed in this paper.

Age-related structural and neurochemical changes of the human superior cervical ganglion

The aim of this study was to investigate age-related morphological and neurochemical changes in the human superior cervical ganglion (SCG). Thirty-seven superior sympathetic human cervical ganglia of young, adult, and aged subjects were examined using morphometric analysis, biotin-streptavidin immunohistochemistry for detecting neurofilament, myelin protein, protein gene product 9.5, nerve growth factor receptor p75 in sympathetic neurons and nerve fibers. Morphometric parameters of neurons (area, long and short axis, shape factor of the neuron body, nucleus, cytoplasm, and lipofuscin) were investigated in every sixth serial section of the ganglion. Seven hundred neurons with clearly visible nuclei were measured in each studied group. The present study showed that human SCG of older subjects had larger areas of neuron body, cytoplasm and nucleus, a lower shape factor, an increased amount of lipofuscin, and a greater number of large-size neurons, as compared to SCG obtained from young subjects. Neuronal cytoskeletal alterations manifested themselves through a decreased number of neurofilament-positive neurons were detected in old human SCG. The amount of myelinated fibers decreased with age, although the amount of myelinated fibers in the young and the adult subjects varied from few to a moderate number. PGP 9.5 immunoreactivity varied in different age groups. A marked reduction of nerve growth factor receptor p75 in old human sympathetic neurons was detected. In conclusion, the findings of this study confirm age-related morphological changes in the human SCG. Structural neuronal changes may influence the deterioration of neuronal functional capacity, neuronal plasticity, and regenerative characteristics.

Developmental profile of cholinergic and purinergic traits and receptors in peripheral chemoreflex pathway in cats

This study describes the developmental profile of specific aspects of cholinergic and purinergic neurotransmission in key organs of the peripheral chemoreflex: the carotid body (CB), petrosal ganglion (PG) and superior cervical ganglion (SCG). Using real time RT-PCR and Western blot analyses, we assessed both mRNA and protein expression levels for choline-acetyl-transferase (ChAT), nicotinic receptor (subunits alpha3, alpha4, alpha7, and beta2), ATP and purinergic receptors (P2X2 and P2X3). These analyses were performed on tissue from 1- and 15-day-old, 2-month-old, and adult cats. During development, ChAT protein expression level increased slightly in CB; however, this increase was more important in PG and SCG. In CB, mRNA level for alpha4 nicotinic receptor subunit decreased during development (90% higher in 1-day-old cats than in adults). In the PG, mRNA level for beta2 nicotinic receptor subunit increased during development (80% higher in adults than in 1-day-old cats). In SCG, mRNA for alpha7 nicotinic receptor levels increased (400% higher in adults vs. 1-day-old cats). Conversely, P2X2 receptor protein level was not altered during development in CB and decreased slightly in PG; a similar pattern was observed for the P2X3 receptor. Our findings suggest that in cats, age-related changes in cholinergic and purinergic systems (such as physiological expression of receptor function) are significant within the afferent chemoreceptor pathway and likely contribute to the temporal changes of O2-chemosensitivity during development.

Bone morphogenetic protein down-regulation of neuronal pituitary adenylate cyclase-activating polypeptide and reciprocal effects on vasoactive intestinal peptide expression

Among bone morphogenetic proteins (BMPs), the decapentaplegic (Dpp; BMP2, BMP4) and glass bottom boat (Gbb/60A; BMP5, BMP6, BMP7) subgroups have well-described functions guiding autonomic and sensory neuronal development, fiber formation and neurophenotypic identities. Evaluation of rat superior cervical ganglia (SCG) post-ganglionic sympathetic neuron developmental regulators identified that selected BMPs of the transforming growth factor beta superfamily have reciprocal effects on neuronal pituitary adenylate cyclase-activating polypeptide (PACAP) and vasoactive intestinal peptide (VIP) expression. Dpp and Gbb/60A BMPs rapidly down-regulated PACAP expression, while up-regulating other sympathetic neuropeptides, including PACAP-related VIP. The suppressive effects of BMP on PACAP mRNA and peptide expression were potent, efficacious and phosphorylated mothers against decapentaplegic homolog (Smad) signaling-dependent. Axotomy of SCG dramatically increases PACAP expression, and the possibility that abrogation of inhibitory retrograde target tissue BMP signaling may contribute to this up-regulation of sympathetic neuron PACAP was investigated. Replacement of BMP6 to SCG explant preparations significantly blunted the injury-induced elevated PACAP expression, with a concomitant decrease in sympathetic PACAP-immunoreactive neuron numbers. These studies suggested that BMPs modulate neuropeptide identity and diversity by stimulating or restricting the expression of specific peptidergic systems. Furthermore, the liberation of SCG neurons from target-derived BMP inhibition following axotomy may be one participating mechanism associated with injury-induced neuropeptidergic plasticity.

Developmental pattern of M1 and M2 muscarinic gene expression and receptor levels in cat carotid body, petrosal and superior cervical ganglion

Using real-time reverse transcriptase polymerase chain reaction, Northern blot, and Western blot analyses, we evaluated the developmental pattern of mRNA and protein expression level of muscarinic M1 and M2 receptors in the carotid body, petrosal ganglion and superior cervical ganglion of 1-day, 15-day, 2-month-old and adult cats. mRNA expression and protein levels of tyrosine hydroxylase, the rate limiting enzyme for dopamine synthesis, were also assessed. Carotid body M1 receptor mRNA, increased significantly by approximately 100% and 300% in 2-month and adult vs. 1- and 15-day-old cats, but protein level decreased gradually being approximately 50% lower compared with 1-day-old cats. In the petrosal ganglion, muscarinic M1 receptor mRNA level was higher in 15-day-old cats vs. 1-day-old, 2-month-old and adult cats and protein levels were about 30% lower than in 1- and 15-day-old cats. In the superior cervical ganglion, muscarinic M1 receptor mRNA was approximately 50% and 80% higher in 2-month-old and adult cats than 1- and 15-day-old, but no changes in the protein level except in 15-day-old cats which was approximately 40% higher than 1-day-old. There was no change of muscarinic M2 receptor mRNA or protein level in the carotid body or petrosal ganglion. However, in the superior cervical ganglion, the significant increase of mRNA of 30% and 50% in 2-month-olds and adults, respectively was not associated with an increase in receptor protein. Tyrosine hydroxylase mRNA and protein level decreased significantly with age in the carotid body and petrosal ganglion. In the superior cervical ganglion, the age dependent increase in tyrosine hydroxylase mRNA was not associated with any changes in the protein level. These results show that the expression of muscarinic M1 and M2 receptors are age and organ-dependent in cats. Consequently, these changes may modulate chemosensory activity during development since muscarinic M1 receptor is predominantly involved in postsynaptic chemosensory activity, while muscarinic M2 receptor modulates acetylcholine and dopamine release from chemosensitive cells.

Neonatal maternal separation enhances dopamine D(2)-receptor and tyrosine hydroxylase mRNA expression levels in carotid body of rats

Adult male (but not female) rats previously subjected to neonatal maternal separation (NMS) are hypertensive and show a significant increase (25%) in their hypoxic ventilatory response. To begin investigating the mechanisms involved in this gender-specific disruption in cardiorespiratory regulation, we tested the hypothesis that NMS alters the expression of dopamine D(2)-receptors and tyrosine hydroxylase mRNA in 3 peripheral organs involved in cardio respiratory regulation: the carotid bodies, superior cervical ganglia, and adrenals. Pups subjected to NMS were placed in a temperature- and humidity-controlled incubator 3 h per day for 10 consecutive days (P3-P12). Control pups were undisturbed. Once they reached adulthood (8-10 weeks), male and female rats were anesthetised. The carotid bodies, superior cervical ganglia, and adrenals were harvested for semi-quantitative analyses of dopamine D(2)-receptors and tyrosine hydroxylase mRNA expression using reverse transcription-polymerase chain reaction (carotid bodies only) and Northern blot. In the carotid bodies, comparison of densitometric analyses showed that NMS enhanced tyrosine hydroxylase mRNA expression in male, but not female, rats. Neonatal maternal separation increased dopamine D(2)-receptor mRNA expression also, but the effect was not gender specific. No changes in mRNA expression related to dopaminergic neurotransmission were observed in superior cervical ganglia or the adrenals. These results indicate that subsequent mechanistic investigations should focus on the carotid bodies, as enhancement of dopaminergic neurotransmission within this organ likely contributes to the gender-specific effects of NMS on cardiorespiratory regulation.

Activity regulates positive and negative neurotrophin-derived signals to determine axon competition

Developmental axon competition plays a key role in sculpting neural circuitry. Here, we have asked how activity and neurotrophins could interact to select one axon over another. Using compartmented cultures of sympathetic neurons, we show that, in the presence of NGF, local depolarization confers a competitive growth advantage on the depolarized axon collaterals and at the same time disadvantages the growth of unstimulated axons from the same and competing neurons. Depolarization mediates the competitive advantage by activating a CaMKII-MEK pathway, which converges to enhance local NGF-mediated downstream growth signals. Patterned electrical stimulation also acts via this pathway to enhance NGF-promoted axonal growth. In contrast, the competitive disadvantage is due to BDNF secreted from and acting on the unstimulated, competing axons through p75NTR. Thus, activity regulates both positive and negative neurotrophin-derived signaling cascades to confer a competitive growth advantage on one axon versus another, thereby providing a cellular mechanism for developmental axon selection.

Differential expression and regulation of the high-affinity choline transporter CHT1 and choline acetyltransferase in neurons of superior cervical ganglia

Previous studies revealed that leukemia inhibitory factor (LIF) and retinoic acid (RA) induce a noradrenergic to cholinergic switch in cultured sympathetic neurons of superior cervical ganglia (SCG) by up-regulating the coordinate expression of choline acetyltransferase (ChAT) and the vesicular acetylcholine transporter. Here, we examined the effect of both factors on high-affinity choline uptake (HACU) and on expression of the high-affinity choline transporter CHT1. We found that HACU and CHT1-mRNA levels are up-regulated by LIF and down-regulated by RA in these neurons. Thus, in contrast to LIF, RA differentially regulates the expression of the presynaptic cholinergic proteins. Moreover, we showed that untreated SCG neurons express HACU and CHT1-mRNAs at much higher levels than ChAT activity and transcripts. In intact SCG, CHT1-mRNAs are abundant and synthesized by the noradrenergic neurons themselves. This study provides the first example of CHT1 expression in neurons which do not use acetylcholine as neurotransmitter.

The effect of central cholinergic and noradrenergic denervation on hippocampal sympathetic ingrowth and apoptosis-like reactivity in the rat

In this study, the effect of intraseptal injection of specific cholinotoxin 192-IgG saporin (SAP) +/- intraperitoneal injection of N-[chloroethyl]-N-ethyl-2-bromobenzylamine (DSP-4) (noradrenergic fiber neurotoxin) was examined in rat hippocampus. Medial septal lesions resulted not only in selective cholinergic denervation of hippocampus (Medial septal lesion + ganglionectomy; SAP + Gx) but also in hippocampal sympathetic ingrowth (IG) of adrenergic fibers (Medial septal lesion + sham ganglionectomy; SAP + IG). Saporin-induced septal lesions produced a significant reduction in hippocampal choline acetyltransferase activity in all tested groups (SAP + IG +/- DSP-4 and SAP + Gx +/- DSP-4), and an increase in noradrenaline concentration in the SAP + IG group. Visualization of noradrenergic fibers by histofluorescence revealed a mixture of fine and thick varicosities in the SAP + IG but only fine fibers in control and SAP + Gx animals. SAP + IG + DSP-4 lesions produced significant reduction in noradrenaline concentration in all groups with a concomitant decrease in visualization of central noradrenergic fibers in dorsal and ventral hippocampus. Treatment of SAP + IG animals with DSP-4 left mostly thick fibers, probably derived from peripheral sympathetic ingrowth. No fluorescence was seen in either the control + DSP-4 or SAP + Gx + DSP-4 animals. Apoptotic-like changes, using in situ oligonucleotide ligation techniques, were also assessed. Proapoptotic changes were seen in the SAP + Gx +/- DSP-4 group as compared to CON +/- DSP-4 groups. SAP + IG regardless of DSP-4 treatment protected hippocampal cells from apoptotic cell death when compared to positive control and SAP + Gx +/- DSP-4 groups. In summary, elevated noradrenaline concentration following specific cholinergic denervation probably reflects compensatory hippocampal ingrowth originating from the peripheral sympathetic system which may be responsible for neuroprotective effects, i.e., antiapoptosis-like effect. Since cholinergic and noradrenergic systems are known to be involved in Alzheimer's disease and related cognitive function, knowing how these neurotransmitters work after specific lesions may be of importance as an animal model of Alzheimer's disease and as a potential target for Alzheimer's disease drug therapies.

HGF promotes survival and growth of maturing sympathetic neurons by PI-3 kinase- and MAP kinase-dependent mechanisms

Hepatocyte growth factor (HGF) is a pleiotrophic factor whose many functions include promoting neuronal survival and growth. Hitherto, these effects have been observed in the presence of other neurotrophic factors like NGF and CNTF, and this requirement for an accessory factor has made it difficult to elucidate the signaling pathways that mediate its survival and growth-enhancing effects. Here, we show that HGF promotes the survival of mature sympathetic neurons of the superior cervical ganglion (SCG) grown at low density in defined medium lacking other neurotrophic factors. This effect was first clearly observed in cultures established from postnatal day 20 (P20) mice and became maximal by P40. HGF also enhanced the growth of neurite arbors from neurons throughout postnatal development and in the adult. HGF treatment resulted in phosphorylation of Akt and ERK1/ERK2. Preventing Akt activation with the phosphatidylinositol-3 (PI-3) kinase inhibitor LY294002 blocked the HGF survival response, and inhibition of ERK activation with the MEK inhibitors PD98059 or U0126 reduced the HGF survival response and the neurite growth-promoting effects of HGF. These results indicate that HGF promotes the survival and growth of maturing sympathetic neurons by both PI-3 kinase- and MAP kinase-dependent mechanisms.

Cisatracurium, but not mivacurium, inhibits survival and axonal growth of neonatal and adult rat peripheral neurons in vitro

Cisatracurium and mivacurium are widely used neuromuscular blocking drugs. Previous reports have indicated growth-inhibitory effects of cisatracurium, but not mivacurium, on two human cell lines in vitro. These effects were ascribed to oxidative stress elicited by acrylate esters formed during cisatracurium breakdown. The aim of the present study was to investigate whether these agents would possibly interfere with the peripheral nervous system. Survival and axonal growth of rat primary neurons obtained from the superior cervical sympathetic ganglion (SCG) or from the adult dorsal root ganglion (DRG) were investigated after treatment with cisatracurium or mivacurium at concentrations from 1 to 10 microM for 24 h. Cisatracurium, but not mivacurium, significantly decreased neuronal survival in a dose-dependent manner, and axonal length was considerably reduced by cisatracurium as compared to controls. It is concluded that high concentrations of cisatracurium are potentially neurotoxic.

The effects of deafferentation and exogenous NGF on neurotrophins and neurotrophin receptor mRNA expression in the adult superior cervical ganglion

Levels of nerve growth factor (NGF) and neurotrophin-3 (NT-3) protein and neurotrophin receptor mRNA in adult sympathetic neurons were investigated following surgical removal of preganglionic input and/or in vivo administration of NGF. Expression of trkC and p75, but not trkA, was significantly decreased following a 3-week deafferentation of the superior cervical ganglion (SCG). Protein levels of NGF and NT-3 in the SCG were unchanged by deafferentation. A 2-week intracerebroventricular infusion of NGF without deafferentation resulted in enhanced mRNA levels of trkA, trkC, and p75 as well as significantly increased NGF and NT-3 protein in the SCG. When NGF infusion followed deafferentation, both trkA and p75 showed significant increases while trkC levels were similar to control values. NGF protein was not increased in the SCG when deafferentation preceded exogenous NGF, yet NT-3 was elevated and levels were similar to cases receiving NGF infusion only. These results support a role for preganglionic input in trkC and p75 expression in adult sympathetic neurons. The increased levels of NT-3 protein and trkC gene expression observed following NGF infusion suggest that NGF influences NT-3 regulation in adult sympathetic neurons. In addition, the present findings provide evidence that, when preganglionic input is removed prior to the NGF infusion, NT-3 effectively competes with NGF for trkA binding. Taken together, we propose that NT-3 may play a role in the robust sprouting of sympathetic cerebrovascular axons previously observed following NGF administration, particularly when deafferentation precedes the NGF infusion period.

Expression of basic fibroblast growth factor isoforms in postmitotic sympathetic neurons: synthesis, intracellular localization and involvement in karyokinesis

Low and high molecular weight isoforms of the mitogen and multifunctional cytokine basic fibroblast growth factor (FGF-2) are up-regulated in neurons and glial cells in response to peripheral nerve lesion. While synthesis, regulation and functions of FGF-2 in non-neuronal cells are well established, the significance of neuronal FGF-2 remains to be investigated in the peripheral nervous system. Therefore, the expression, intracellular localization and possible effects of FGF-2 isoforms were analyzed in primary sympathetic neurons derived from the rat superior cervical ganglion. FGF-2 is detected in the nucleus and in perinuclear Golgi fields of early postnatal neurons which also express mRNA and protein for the FGF receptor type 1. Biolistic transfection of plasmids encoding FGF-2 isoforms fused to fluorescent proteins demonstrates nuclear targeting of 18 kDa FGF-2 and 23 kDa FGF-2 with prominent accumulation in the nucleolus of neurons. Neither overexpression nor treatment with FGF-2 isoforms promotes survival of sympathetic neurons deprived of nerve growth factor; however, neuronal transfection of the high molecular weight FGF-2 isoform in dissociated and slice cultures results in a bi- or multinuclear phenotype. The present study provides evidence for neuronal synthesis and targeting of FGF-2 to the nucleus and Golgi apparatus supporting a dual role of FGF-2 in the nucleus and secretory pathway of sympathetic neurons.

Sympathetic neurons synthesize and secrete pro-nerve growth factor protein

Postmitotic sympathetic neuronal survival is dependent upon nerve growth factor (NGF) provided by peripheral targets, and this dependency serves as a central tenet of the neurotrophic hypothesis. In some other systems, NGF has been shown to play an autocrine role, although the pervasiveness and significance of this phenomenon within the nervous system remain unclear. We show here that rat sympathetic neurons synthesize and secrete NGF. NGF mRNA is expressed in nearly half of superior cervical ganglion sympathetic neurons at embryonic day 17, rising to over 90% in the early postnatal period, and declining in the adult. Neuronal immunoreactivity is reduced when retrograde transport is interrupted by axotomy, but persists in a subpopulation of neurons despite diminished mRNA expression, suggesting that intrinsic protein synthesis occurs. Cultured neonatal neurons express NGF mRNA, which is maintained even when they are undergoing apoptosis. To determine which NGF isoforms are secreted, we performed metabolic labeling and immunoprecipitation of NGF-immunoreactive proteins synthesized by cultured NGF-dependent and -independent neurons. Conditioned medium contained high molecular weight NGF precursor proteins, which varied depending upon the state of NGF dependence. Mature NGF was undetectable by these methods. High molecular weight NGF isoforms were also detected in ganglion homogenates, and persisted at diminished levels following axotomy. We conclude that sympathetic neurons express NGF mRNA, and synthesize and secrete pro-NGF protein. These findings suggest that a potential NGF-sympathetic neuron autocrine loop may exist in this prototypic target-dependent system, but that the secreted forms of this neurotrophin apparently do not support neuronal survival.

Alpha 4 integrins and vascular cell adhesion molecule-1 play a role in sympathetic innervation of the heart

Sympathetic neurons innervate the heart early in postnatal development, an event that is crucial for proper modulation of blood pressure and cardiac function. However, the axon guidance cues that direct sympathetic neurons to the heart, and the neuronal receptors that recognize those cues, are poorly understood. Here we present evidence that interactions between the alpha4beta1 integrin on sympathetic neurons and vascular cell adhesion molecule-1 (VCAM-1) in the heart plays a role in cardiac innervation. The alpha4 subunit was detected on postnatal rat superior cervical ganglion (SCG) neurons in culture and in cryosections of SCG and heart. VCAM-1 immunoreactivity was detected on cardiac myocytes that associate with invading sympathetic neurons. Purified recombinant soluble VCAM-1 (rsVCAM-1) stimulated SCG neurite outgrowth at levels comparable with laminin 2/4 and fibronectin (Fn), and outgrowth on rs-VCAM-1 and Fn was blocked by antibodies specific for the alpha4 and beta1 integrin subunits. Intrathoracic injection of function-blocking antibodies to alpha4 and VCAM-1, as well as a small molecule inhibitor of alpha4 integrins, significantly reduced sympathetic innervation of the heart. These results indicate that the interaction between alpha4 integrin and VCAM-1 is important for sympathetic innervation of the heart.

c-jun is essential for sympathetic neuronal death induced by NGF withdrawal but not by p75 activation

Sympathetic neurons depend on NGF binding to TrkA for their survival during vertebrate development. NGF deprivation initiates a transcription-dependent apoptotic response, which is suggested to require activation of the transcription factor c-Jun. Similarly, apoptosis can also be induced by selective activation of the p75 neurotrophin receptor. The transcriptional dependency of p75-mediated cell death has not been determined; however, c-Jun NH2-terminal kinase has been implicated as an essential component. Because the c-jun-null mutation is early embryonic lethal, thereby hindering a genetic analysis, we used the Cre-lox system to conditionally delete this gene. Sympathetic neurons isolated from postnatal day 1 c-jun-floxed mice were infected with an adenovirus expressing Cre recombinase or GFP and analyzed for their dependence on NGF for survival. Cre immunopositive neurons survived NGF withdrawal, whereas those expressing GFP or those uninfected underwent apoptosis within 48 h, as determined by DAPI staining. In contrast, brain-derived neurotrophic factor (BDNF) binding to p75 resulted in an equivalent level of apoptosis in neurons expressing Cre, GFP, and uninfected cells. Nevertheless, cycloheximide treatment prevented BDNF-mediated apoptosis. These results indicate that whereas c-jun is required for apoptosis in sympathetic neurons on NGF withdrawal, an alternate signaling pathway must be induced on p75 activation.

Aging and neuronal plasticity: lessons from a model

In spite of many well-documented examples of age-related reductions in neuronal plasticity, the causes of such changes remain largely unknown. One example of age-reduced plasticity involves an aberrant sprouting response of mature rat sympathetic neurons into the CNS (hippocampal formation). This phenomenon has proven to be useful for exploring the relative contribution of target aging (extrinsic influences) versus neuronal aging (intrinsic influences) to reduced sprouting. Aged sympathetic neurons mount a robust growth response when confronted with young target tissue or when exposed to exogenous trophic factor in vivo. In contrast, the aged target tissue (the hippocampal formation in this example) exhibits reduced receptivity for sympathetic sprouting. This change in the target does not appear to be due to alterations in baseline levels of trophic or substrate support for axonal growth. Rather, aging appears to dampen the consequences of target denervation so that the aged target elicits less sprouting. Age-related reductions in neuronal sprouting are speculated to reflect increasing commitment to information storage at the expense of neuronal plasticity.

Gene expression of catecholamine biosynthetic enzymes following exercise: modulation by age

Both age and exercise training are associated with tissue specific alterations in the catecholaminergic system. We examined the effect of short-term exercise training on tyrosine hydroxylase and dopamine beta-hydroxylase gene expression in adrenals and specific brain regions with aging. In addition, we examined activator protein-1 and cyclic AMP response element transcription factor binding activity in the adrenal medulla. Male, six- and 24-month-old F-344 rats were exercised by treadmill running for five consecutive days. One group was killed immediately and a second group was killed 2h after the last training session. Exercise significantly elevated tyrosine hydroxylase messenger RNA equally in adrenals of both young and old rats. Training had no effect on dopamine beta-hydroxylase messenger RNA in adrenals of young, but levels were elevated in old rats. Binding activities of both activator protein-1 and cyclic AMP response element binding protein were diminished with age in the adrenal medulla. Exercise training had no significant effect on the binding activity of cyclic AMP response element binding protein in either young or old animals, whereas activator protein-1 binding activity increased equally in young and old animals. Exercise training revealed divergent changes in tyrosine hydroxylase messenger RNA in brain catecholaminergic neurons. In the locus coeruleus and the ventral tegmental areas, training elevated tyrosine hydroxylase messenger RNA levels only in young rats. In the substantia nigra, there was no change in young, but a 45% increase in tyrosine hydroxylase messenger RNA in old rats. In the ventral tegmental area, training increased tyrosine hydroxylase gene expression 80% in young but not in old rats. These results indicate that short-term exercise training increases tyrosine hydroxylase messenger RNA levels in young animals in the adrenals, the locus coeruleus and the ventral tegmental area. The responses for exercise training of aged animals differed from the young in brain noradrenergic and dopaminergic nuclei, especially in the substantia nigra, and to some extent in the locus coeruleus and the ventral tegmental area.

Apoptotic protein expression and activation of caspases is changed following cholinergic denervation and hippocampal sympathetic ingrowth in rat hippocampus

Following cholinergic denervation of the hippocampus by medial septal lesions, an unusual neuronal reorganization occurs in which peripheral adrenergic fibers arising from superior cervical ganglia grow into the hippocampus (hippocampal sympathetic ingrowth). Recent studies suggest that a similar process, in which sympathetic noradrenergic axons invade the hippocampus, can occur in Alzheimer's disease patients. In the last few years, the occurrence of apoptotic cell death has been studied in Alzheimer's disease patients and in animal models of this disorder. Several studies suggest that the hippocampus is an important area to be considered for apoptotic cell death. In our studies in the rat hippocampus, we have measured the expression of inducers and blockers of apoptosis in membrane, cytosolic and mitochondrial fractions, and the activity of caspases. The level of cytosolic Fas was increased in cholinergic denervation compared to control and hippocampal sympathetic ingrowth groups. The membrane Fas ligand expression was significantly increased in hippocampal sympathetic ingrowth and in cholinergic denervation compared to the control group. The level of caspase-3 (CPP32) was increased in the cholinergic denervation group compared to control and hippocampal sympathetic ingrowth groups. The cytosolic expression of bcl-x was increased in hippocampal sympathetic ingrowth compared to control and cholinergic denervation. The cytosolic activity of caspase-3 appeared to be significantly decreased in hippocampal sympathetic ingrowth and increased in cholinergic denervation groups compared to control and cholinergic denervation/hippocampal sympathetic ingrowth, respectively. From the present results, we suggest that cholinergic denervation may be responsible for pro-apoptotic responses, while hippocampal sympathetic ingrowth may protect neurons from apoptosis in rat dorsal hippocampus.

Postnatal expression of VEGF and its receptor flk-1 in peripheral ganglia

The postnatal (P0-P12) and adult expression of vascular endothelial growth factor and its receptor flk-1 was investigated in superior cervical (SCG) and dorsal root ganglia (DRG) in mice by immunocytochemistry. At P0 all neurons in SCG and DRG contained VEGF. The number of VEGF-immunoreactive neurons in DRG but not in SCG, decreased postnatally and reached adult levels (34%) at P12. At P0 flk-1 was found in virtually all neurons in the SCG and in roughly half of the neurons in DRG. The number of flk-1 positive neurons then decreased and reached adult levels at P12. The findings demonstrate temporal changes in VEGF and flk-1 expression, suggesting developmental regulation of VEGF activity in peripheral ganglia.

Retinoic acid regulates the morphological development of sympathetic neurons

Interactions between all-trans-retinoic acid (RA) and bone morphogenetic proteins (BMPs) affect the expression of neurotrophin receptors in sympathetic neurons (Kobayashi et al., 1998). In this study, we examined the possibility that similar interactions might regulate the morphological development of these neurons. Under control conditions, embryonic rat sympathetic neurons formed axons but not dendrites; cells exposed to RA had a similar appearance. Profuse dendritic growth was observed upon exposure to BMP-7, and this was reduced by approximately 70% by RA. This inhibitory effect of RA was mediated primarily by retinoic acid receptors (RARs) and it exhibited substantial specificity because it was not associated with changes in either axonal elongation or cell survival. Moreover, mRNAs for enzymes required for synthesis of RA were expressed in the sympathetic neurons and retinoid activity was released from superior cervical ganglia. These observations suggest that retinoids may function as endogenous morphogens and regulate neural cell shape and polarity in developing sympathetic ganglia.

Characterization of an NGF-P-TrkA retrograde-signaling complex and age-dependent regulation of TrkA phosphorylation in sympathetic neurons

Nerve growth factor (NGF) is a target-derived trophic factor for developing sympathetic and cutaneous sensory neurons. NGF promotes growth and survival of neurons via activation of the receptor tyrosine kinase TrkA. We used compartmentalized cultures of sympathetic neurons to address the mechanism of NGF signaling from distal axons and terminals to proximal axons and cell bodies. Our results demonstrate that an NGF-phospho-TrkA (NGF-P-TrkA)-signaling complex forms in distal axons and is retrogradely transported as a complex to cell bodies of sympathetic neurons. Although a minor fraction of both NGF and TrkA is retrogradely transported, a large fraction of the NGF that is retrogradely transported is found complexed with retrogradely transported TrkA. Interestingly, the metabolism of the P-TrkA complex is dramatically different in young, NGF-dependent sympathetic neurons as compared to older, NGF-independent sympathetic neurons. After withdrawal of NGF from distal axons of young neurons, P-TrkA within distal axons, as well as within proximal axons and cell bodies, dephosphorylates rapidly. In contrast, after withdrawal of NGF from distal axons of older neurons, P-TrkA within distal axons dephosphorylates completely, although more slowly than that in young neurons, whereas dephosphorylation of P-TrkA within proximal axons and cell bodies occurs markedly more slowly, with at least one-half of the level of P-TrkA remaining 2 d after NGF withdrawal. Thus, P-TrkA within the cell bodies of young, NGF-dependent sympathetic neurons is derived from distal axons. A more stable P-TrkA complex within cell bodies of mature sympathetic neurons may contribute to the acquisition of NGF independence for survival of mature sympathetic neurons.

Differences in the ways sympathetic neurons and endocrine cells process, store, and secrete exogenous neuropeptides and peptide-processing enzymes

Most neurons store peptides in large dense core vesicles (LDCVs) and release the neuropeptides in a regulated manner. Although LDCVs have been studied in endocrine cells, less is known about these storage organelles in neurons. In this study we use the endogenous peptide NPY (neuropeptide Y) and the endogenous peptide-processing enzyme PAM (peptidylglycine alpha-amidating monooxygenase) as tools to study the peptidergic system in cultured neurons from the superior cervical ganglion (SCG). Once mature, SCG neurons devote as much of their biosynthetic capabilities to neurotransmitter production as endocrine cells devote to hormone production. Unlike pituitary and atrium, SCG neurons cleave almost all of the bifunctional PAM protein they produce into soluble monofunctional enzymes. Very little PAM or NPY is secreted under basal conditions, and the addition of secretagogue dramatically stimulates the secretion of PAM and NPY to a similar extent. Although endocrine cells typically package "foreign" secretory products together with endogenous products, pro-opiomelanocortin- and PAM-derived products encoded by adenovirus in large part were excluded from the LDCVs of SCG neurons. When expressed in corticotrope tumor cells and primary anterior pituitary cultures, the same virally encoded products were metabolized normally. The differences that were observed could reflect differences in the properties of neuronal and endocrine peptidergic systems or differences in the ability of neurons and endocrine cells to express viral transcripts.

A proposed role for nerve growth factor in the etiology of multiple sclerosis

It is proposed that, in addition to genetic factors involved in immune attack on myelin, higher concentrations of nerve growth factor in certain tissues during development determine susceptibility to multiple sclerosis. High early nerve growth factor in some vasculature of spontaneously hypertensive rats increases sympathetic innervation and catecholamine production in these vessels. They become more sensitive than controls to noradrenaline after chemical sympathectomy. Continued exposure to high noradrenaline can result in sympathectomy-like effects, heightening sensitivity to constricting neurotransmitters. Vasoresponses of spontaneously hypertensive rats are impaired with submaximal but not maximal hypoxia. Such a situation in multiple sclerosis patients could result in insufficient blood flow by vasoconstriction until it becomes maximal. Glutamate increase by ischemia and hyperemic release of free radicals could injure neurons, prompting an immune response to myelin proteins in susceptible people. Developmental adaptation to situations requiring lower sympathetic activity might help counteract these effects.

Developmental expression of messenger RNA levels of the alpha subunit of the GTP-binding protein, Gz, in the mouse nervous system

There has been recent evidence that Gz may play a role in the transmission of the neurotrophic signal from nerve terminals to the cell bodies [Johanson, S.O., Crouch, M.F., Hendry, I.A., Signal transduction from membrane to nucleus: the special case for neurons, Neurochem. Res. 21 (1996) 779-785]. We examined the developmental expression of the alpha subunit of Gz (Gzalpha) in the peripheral and central nervous systems of the mouse. Our laboratory has developed a quantitative reverse transcription-polymerase chain reaction (RT-PCR) for Gzalpha which makes use of a fragment of the PCR product shortened by 107 base pairs creating a standard which mimics the original RNA. Serial dilutions of the mouse RNA with a constant concentration of mimic RNA were made and the point where equal amounts of product are formed allows accurate measurement of Gzalpha mRNA in the tissue. We have demonstrated that in the developing mouse superior cervical ganglion (SCG), dorsal root ganglion (DRG) and trigeminal ganglion the expression of Gzalpha mRNA is highest perinatally. From 3 weeks of age, in all tissues with the exception of the SCG, Gzalpha mRNA levels fall to lower levels in the adult animal. The developmental pattern of expression of Gzalpha in both the cerebellum and the brain differs from the peripheral nervous system. In the cerebellum, Gzalpha mRNA expression is highest around birth and in the brain it is highest around third postnatal week and then the levels decline as adulthood is approached. These results suggest that the highest level of Gzalpha mRNA is expressed at the time when target tissue innervation is occurring. This further strengthens the hypothesis that Gzalpha is important in the transfer of information from target tissues to the innervating nerve cells.

Leukemia inhibitory factor and ciliary neurotrophic factor regulate dendritic growth in cultures of rat sympathetic neurons

Cytokines such as leukemia inhibitory factor (LIF) and ciliary neurotrophic factor (CNTF) have previously been shown to regulate neurotransmitter and neuropeptide synthesis in sympathetic neurons [P.H. Patterson, Leukemia inhibitory factor, a cytokine at the interface between neurobiology and immunology, Proc. Natl. Acad. Sci. USA 91 (1994) 7833-7835]. We considered the possibility that these agents may also affect the development of neuronal cell shape. Intracellular dye injection and immunocytochemistry were used to assess dendritic growth in cultures of perinatal rat sympathetic neurons and the effects of LIF and CNTF were compared to those of osteogenic protein-1 (OP-1), a growth factor that induces profuse dendritic growth in these neurons [P. Lein, M. Johnson, X. Guo, D. Rueger, D. Higgins, Osteogenic protein-1 induces dendritic growth in rat sympathetic neurons, Neuron 15 (1995) 597-605]. Under control conditions, sympathetic neurons formed only axons. Exposure to either LIF or OP-1 stimulated dendritic growth, but the magnitude of the response to LIF was much less than that obtained with OP-1 with respect to both dendritic number and length. Simultaneous exposure to LIF and OP-1 resulted in dendritic growth equivalent to that observed in the presence of LIF alone, suggesting that LIF inhibits the response of neurons to OP-1. Both the stimulatory and inhibitory effects of LIF were mimicked by CNTF, but not by other growth factors. These data suggest that LIF and CNTF regulate dendritic development in a complex manner that is dependent on both the morphological state of the neuron and the presence of other growth factors. However, the net effect of exposure to these cytokines appears to be the production of a population of neurons with rudimentary arbors consisting of only one or two short dendrites.

Pituitary adenylate cyclase-activating peptide (PACAP) and PACAP type 1 receptor expression in regenerating adult mouse and rat superior cervical ganglia in vitro

Pituitary adenylate cyclase-activating polypeptide (PACAP), a regulatory peptide belonging to the vasoactive intestinal peptide (VIP) family, is widely distributed in the central and peripheral nervous system. Recent studies have shown that PACAP expression is upregulated in sensory neurons in response to axonal injury. Here we report that PACAP and PACAP type 1 receptors are located in rat and mouse superior cervical ganglia (SCG). PACAP-immunoreactivity (-IR) was demonstrated in preganglionic fibers, whereas only occasional PACAP-IR cell bodies could be observed. In situ hybridization histochemistry using 35S-labeled deoxyribonucleotide probes confirmed that PACAP mRNA was present only in occasional cell bodies. In contrast, PACAP type 1 receptor mRNA was expressed in virtually all cell bodies within the ganglia. After removal and culturing of the SCG for 24 h, there was a marked increase in PACAP mRNA, whilst PACAP type 1 receptor mRNA expression appeared to be downregulated in most nerve cell bodies except for a few scattered neurons displaying a strong upregulation. The total specific binding of PACAP to isolated SCG membranes as assayed by [125I]PACAP-27 binding showed an increase in SCG cultured for 48 h. PACAP-27 neither affected axonal outgrowth from the cultured SCG nor the survival of cells within the SCG. We conclude that PACAP and PACAP receptors are rapidly upregulated in sympathetic ganglia in response to axonal injury and that PACAP may play a role during nerve regeneration.

Transforming growth factor-beta2 selectively alters the developmental expression of the fast transient A-current in cultured rat superior cervical ganglion neurons

Cultures of neonatal rat superior cervical ganglion (SCG) were utilized to examine the ability of transforming growth factor-beta2 (TGFbeta2) to alter voltage-gated K+ channel development. Whole-cell patch clamp recordings were used to monitor changes in three separate K+ currents: A rapidly inactivating A-current (I(Af)), a slowly inactivating A-current (I(As)), and a non-inactivating current (I(K)). Continuous TGFbeta2 (10 ng/ml) treatment selectively altered the normal developmental decrease in I(Af) expression in SCG neurons, but did not significantly change I(As) or I(K) expression. After 2 weeks of treatment, the mean I(Af) current density in control cultures had decreased 67%, while the I(Af) current density in TGFbeta2 treated cultures remained near initial values (approximately 2.7-fold higher than control). This difference remained even after 4 weeks of exposure. TGFbeta2 did not appear to change the activation kinetics or voltage-dependence of I(Af). These findings indicate that TGFbeta2 may play an important role in modulating the development of neuronal excitability by regulating the expression of voltage-gated K+ channels.

Transient expression of alpha-1B adrenoceptor messenger ribonucleic acids in the rat superior cervical ganglion during postnatal development

We have examined the developmental profile of the alpha-1 and alpha-2 adrenergic receptor messenger ribonucleic acids expression in the rat superior cervical ganglion. The expression of the six messenger ribonucleic acids was studied using reverse transcription-polymerase chain reaction. At four weeks, the dominant messenger ribonucleic acids transcripts in this sympathetic ganglion were alpha-1C, alpha-2A, alpha-2B and alpha-2C. The expression of alpha-1 genes in the superior cervical ganglion appears to be regulated during postnatal development in that two alpha-1 (alpha-1B, alpha-1C) genes were expressed at birth, three, seven and 14 days postnatal but no amplified product for alpha-1B was detected at 28 days and in the aged animals, while the alpha-1C transcript continued to be expressed. No amplified product for alpha-1D was detected in superior cervical ganglion at any of the ages studied. While all three alpha-2 genes were expressed in the superior cervical ganglion at four weeks the dominant alpha-2 messenger ribonucleic acids transcript expressed in the superior cervical ganglion was alpha-2A. This pattern of alpha-2 adrenoceptor gene expression was maintained from birth, throughout development and into old age. These results suggest that the expression of alpha-1 adrenergic receptors in the superior cervical ganglion is regulated developmentally while the expression of alpha-2 genes remains unchanged.

Decreased neurofilament gene expression is an index of selective axonal hypotrophy in ageing

Axonal atrophy may reflect earlier and more reversible events in neurodegeneration and ageing than somatic atrophy. Innervation density by sympathetic fibres from the rat superior cervical ganglion (SCG) to the middle cerebral artery (MCA) decreases dramatically in old age, while that to the iris is largely unchanged. In situ hybridization was used in conjunction with retrograde tracers to examine the role of the neuronal cytoskeleton in this selective axonal vulnerability. Using a riboprobe complementary to neurofilament light chain (NF-L) mRNA, there was a 22-25% decrease in the mean grain density in aged neurones when all neurones were examined. A small subset of these neurones was shown to project to the MCA and another to the iris. In young SCG, both subpopulations expressed intermediate grain densities for NF-L mRNA. In MCA-projecting neurones, there was a 40% decline in grain density with ageing (p < 0.05), with no change in iris-projecting neurones. Our results demonstrate that major decreases in NF-L expression may represent cellular markers of selective axonal hypotrophy by aged neurones.

Synaptic innervation density is regulated by neuron-derived BDNF

In this report, we have examined the role of neuron-derived BDNF at an accessible synapse, that of preganglionic neurons onto their sympathetic neuron targets. Developing and mature sympathetic neurons synthesize BDNF, and preganglionic neurons express the full-length BDNF/TrkB receptor. When sympathetic neuron-derived BDNF is increased 2- to 4-fold in transgenic mice, preganglionic cell bodies and axons hypertrophy, and the synaptic innervation to sympathetic neurons is increased. Conversely, when BDNF synthesis is eliminated in BDNF -/- mice, preganglionic synaptic innervation to sympathetic neurons is decreased. Together these results indicate that variations in neuronal neurotrophin synthesis directly regulate neuronal circuitry by selectively modulating synaptic innervation density.

Mature but not fetal or neonatal rat superior cervical ganglion transplants survive in the cortex of adult rats

The transplantation of catecholaminergic tissues is a possible therapy for parkinsonism. Central nervous tissue is suitable for transplantation only in the immature stage, whereas peripheral nervous tissue can also be transplanted when mature. The present study compares the development of fetal (17-20 embryonic day, E17-20), neonatal (1-3 postnatal day, P1-3) and mature (5-6-week-old) rat superior cervical ganglia after transplantation into the cerebral cortex of adult rats. The mature transplants survived in greater proportion and preserved their structural characteristics, although a considerable proportion of the neurons died. The perinatal transplants only survived sporadically, decreased in size and the surviving remnants failed to display a structure comparable to the adult ganglion in situ. Thus, the use of adult donors is not only a possibility but a necessity when superior cervical ganglion (probably any ganglion) is transplanted. This principle is radically different from that seen in the case of central nervous tissues, and can be understood by the analysis of the time curves of cell proliferation and programmed cell death (apoptosis) observed during the perinatal development of sympathetic ganglia.

Nerve growth factor and cytokines mediate lymphoid tissue-induced neurite outgrowth from mouse superior cervical ganglia in vitro

Superior cervical ganglia (SCG) from neonatal mice were cultured with adult murine lymphoid tissue explants in Matrigel (Collaborative Biomedical, Bedford, MA). After 1 and 2 days in culture, many neurites grew toward thymus and spleen. Normal mesenteric lymph node (MLN) induced a smaller effect; however, activated MLN (isolated from mice 10 days after infection with Nippostrongylus brasiliensis; Nb-MLN-10d) caused significantly increased neurite outgrowth. To determine the roles of nerve growth factor (NGF) and cytokines in the promotion of neuritogenesis by lymphoid tissues, anti-NGF and various anti-cytokines were added to cocultures. Anti-NGF inhibited most of the neurite outgrowth toward thymus and spleen but only partially that toward Nb-MLN-10d. Anti-mouse IL-1 beta also significantly reduced the number of neurites growing toward thymus, spleen, and normal MLN. The number of neurites growing toward Nb-MLN-10d was significantly reduced by anti-IL-1 beta, anti-IL-3, anti-IL-6, or anti-GM-CSF. Exogenous IL-1 beta and IL-3 caused neurite outgrowth in single SCG cultures; and the IL-1 beta-, but not the IL-3-, mediated effect was completely blocked by anti-NGF. In one-day thymus/SCG cocultures, endogenous IL-1 was not detectable at concentrations sufficient to cause nerve growth; however, ample NGF was present in the thymic tissues and culture supernatants, but not in SCG. These data suggest that IL-1 mediates NGF production in lymphoid tissues, which in turn induces the growth of sympathetic nerves. Moreover, IL-3, IL-6, or GM-CSF produced during inflammation might also play important roles in the stimulation of nerve growth in vivo.

Target-specific differences in the dendritic morphology and neuropeptide content of neurons in the rat SCG during development and aging

Our purpose in this work was to investigate the role of target tissues in the regulation of dendritic morphology from sympathetic neurons during development and aging. Neurons were retrogradely labeled from three targets, the iris, the submandibular gland (SMG), and the middle cerebral artery (MCA). They were then fixed and intracellularly injected to demonstrate their dendritic arborizations. Dendritic geometry varied quantitatively in sympathetic neurons innervating different target tissues at all stages of development. Neurons innervating the iris had the largest cell bodies and most extensive dendritic arborizations, whereas the vasomotor neurons were the smallest. The number of primary dendrites, however, did not vary significantly between the different neuronal populations. The growth of dendritic arborizations during postnatal development and their atrophy in old age were not concordant in the different neuron populations we studied. Neurons innervating the MCA and the iris ceased dendritic growth early in postnatal development, whereas the dendritic complexity of neurons supplying the SMG increased well into adulthood. By contrast, dendritic atrophy was seen in aged MCA- and SMG-projecting neurons but not in those innervating the iris, suggesting, with other evidence, correlated and distinct patterns of growth and atrophy in axons and dendrites of mature sympathetic neurons projecting to different targets. Swollen dendrites and protuberances on cell soma were a prominent feature of aged neurons. In addition to the target-specific variation in neuronal morphology, we observed diversity in neurotransmitter phenotype. For example, neuropeptide Y was expressed in iridial but not SMG-projecting neurons. These results show a range of age- and target-specific differences in the dendritic morphology and neuropeptide content of sympathetic neurons that may be a result of differing trophic interactions with their target tissues.

Developmental and activity-dependent changes in K+ currents in satellite glial cells in mouse superior cervical ganglion

Voltage-gated K+ currents were recorded from freshly dissociated satellite glial cells wrapping around ganglion cells in mouse superior cervical ganglion (SCG) by whole-cell recordings of patch clamp techniques. Both inward and outward K+ currents during membrane hyperpolarization and depolarization were observed in these glial cells. The current-voltage relation of these K+ currents became almost linear in cells obtained more than 4 weeks after birth. The magnitude of the density of inward K+ currents, which were elicited during membrane hyperpolarization and were eliminated by external barium, progressively increased during the first month after birth. This developmental increase in the magnitude of inward K+ current density was not affected by decentralization of SCG done by transection of cervical sympathetic trunk (CST) 5 days after birth. In adult mice, the magnitude of the inward K+ current density decreased after chronic conduction blockade of CST by local application of tetrodotoxin. On the other hand, the magnitude of the inward K+ current density increased after daily intraperitoneal injection of reserpine and this increase was abolished by pre-treatment of decentralization of SCG. These results suggested that preganglionic innervation was not prerequisite for developmental increase in the inward K+ currents and preganglionic neuronal activity upregulates the inward K+ currents in adult mice. Neuronal regulation of glial K+ channel expression would assist in K+ clearance from periganglionic space to maintain neuronal activity.

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.

Axonal synthesis of phosphatidylcholine is required for normal axonal growth in rat sympathetic neurons

The goal of this study was to assess the relative importance of the axonal synthesis of phosphatidylcholine for neurite growth using rat sympathetic neurons maintained in compartmented culture dishes. In a double-labeling experiment [14C]choline was added to compartments that contained only distal axons and [3H]choline was added to compartments that contained cell bodies and proximal axons. The specific radioactivity of labeled choline was equalized in all compartments. The results show that approximately 50% of phosphatidylcholine in distal axons is locally synthesized by axons. The requirement of axonal phosphatidylcholine synthesis for neurite growth was investigated. The neurons were supplied with medium lacking choline, an essential substrate for phosphatidylcholine synthesis. In the cells grown in choline-deficient medium for 5 d, the incorporation of [3H]palmitate into phosphatidylcholine was reduced by 54% compared to that in cells cultured in choline-containing medium. When phosphatidylcholine synthesis was reduced in this manner in distal axons alone, growth of distal neurites was inhibited by approximately 50%. In contrast, when phosphatidylcholine synthesis was inhibited only in the compartment containing cell bodies with proximal axons, growth of distal neurites continued normally. These experiments imply that the synthesis of phosphatidylcholine in cell bodies is neither necessary nor sufficient for growth of distal neurites. Rather, the local synthesis of phosphatidylcholine in distal axons is required for normal growth.

Expression of preproenkephalin mRNA in rat superior cervical ganglion during postnatal development

The number of principal neurons in the rat superior cervical ganglion (SCG) exhibiting enkephalin-peptide immunoreactivity is reported to be limited. To better determine the degree of enkephalinergic phenotype in sympathetic neurons, sections of SCGs from rats aged newborn to adult were processed for in situ hybridization histochemistry, using a [35S]cRNA probe directed against preproenkephalin (PPENK). > 50% of principal ganglion neurons express mRNA for PPENK in adults. Striking variability in labeling intensity is observed. PPENK mRNA is detected in developing ganglia beginning at postnatal days 4-7. Both the number of cells and intensity of labeling increases with postnatal development. These results indicate that expression of PPENK mRNA is more widespread than expression of enkephalin peptides and develops postnatally.

A comparison between the adult rat and neonate rat of the architecture of sympathetic preganglionic neurones projecting to the superior cervical ganglion, stellate ganglion and adrenal medulla

Sympathetic preganglionic neurones (SPN) projecting to the superior cervical ganglion (SCG) and adrenal medulla (AM) in the neonate (< 14 days) and SCG, stellate ganglion (SG) and AM in the adult rat (> 3 months) were retrogradely labelled with cholera B horseradish peroxidase (CBHRP). Labelled neurones were found in 4 four distinct nuclei: the nucleus intermediolateralis thoracolumbalis pars principalis (ILp), a nucleus equivalent to the intemediolateral cell column (IML); the nucleus intermediolateralis thoracolumbalis pars funicularis (ILf); the nucleus intercalatus spinalis (IC) and the nucleus intercalatus pars paraependymatis (ICpe) or central autonomic area (CA). These were represented to a similar extent in both neonate and adult. Neonate and adult SCG, SG and AM-SPN had a similar segmental distribution cervical 8 (C8) to thoracic 5 (T5) for SCG-SPN and thoracic 3 (T3) to thoracic (T13) for AM-SPN whereas adult SG-SPN were distributed over segments C8 to T9. Most labelled neurones (70%) were located in the ILp with one segment containing the highest proportion of SPN. Three morphologically distinct neurones were evident. Fusiform and roundbodied were the most common. Fusiform somata of the ILp were orientated both mediolaterally and rostrocaudally in the neonate but only rostrocaudally in the adult. Dendrites of the SPN in the adult and neonate extended in a dense rostrocaudal band along the ILp, more diffusely into the white matter of the Ilf and in bundles medially towards the central canal (CC). The neonate showed some significant differences. In the ILp, the cell bodies were less tightly packed into a narrow band and into clusters and the dendrites were more diffuse. It was concluded that at 12 days postnatally the organisation of the sympathetic nuclei had still nor reached the adult form. However, there is no extensive realignment of dendrites in the adult so the ILp remains an 'open' nucleus like the neonate.

The developmental increase in ACh current densities on rat sympathetic neurons correlates with changes in nicotinic ACh receptor alpha-subunit gene expression and occurs independent of innervation

Determining factors that control the expression of neurotransmitter receptors and the mechanisms by which these factors operate is essential to understand how synapses form during development and how receptor numbers change in the adult. In this study, we have investigated one such factor, the influence of innervation, on the developmental expression of nicotinic ACh receptors (nAChRs) on neonatal rat sympathetic neurons, both in terms of ACh current densities, and in terms of mRNA levels for the transcripts that encode these receptors. To date, nine genes have been cloned that encode neuronal nAChRs subunits in mammals. We demonstrate that mRNA encoding five nAChR subunits, alpha 3, alpha 5, alpha 7, beta 2, and beta 4, are present in neonatal rat sympathetic neurons. We show that mRNA levels for alpha 3 and alpha 7 subunits increase by more than threefold over the first 2 postnatal weeks, a period when most synapses are forming on the neurons; however, we observed no significant change in mRNA levels for alpha 5, beta 2, or beta 4. Using whole-cell voltage-clamp techniques, we show that the increase in alpha-subunit mRNA correlates with increases in ACh current densities, which double over the same period. To investigate the role of innervation, we cut the preganglionic nerve at birth and measured subunit mRNA levels and ACh current densities in denervated neurons 1-2 weeks later. Our results indicate that the preganglionic nerve differentially affects the mRNA level for the five nAChR transcripts, yet it has little influence on the developmental increase in ACh current densities.

Postnatal ontogeny of the neurotrophin receptors trk and trkB mRNA in rat sensory and sympathetic ganglia

Using reverse transcription followed by polymerase chain reaction, we examined the expression of mRNA for the tyrosine kinase receptors trk and trkB in rat sensory and sympathetic ganglia during postnatal development. While the levels of both trk and trkB mRNA in the dorsal root ganglia (DRG) decreased two-fold, they increased by seven and two times, respectively, in superior cervical ganglia. The developmentally regulated and tissue-specific expression of trk and trkB genes suggest that peripheral ganglia differ in their responsiveness to neurotrophins in neonatal and adult rats. We found that the temporal pattern of trk expression in DRG neurons correlates with the observed age-dependent ability of nerve growth factor to induce the biosynthesis of the neuropeptide substance P.

Modulation of the developing rat sympathetic GABAA receptor by Zn++, benzodiazepines, barbiturates and ethanol

We have examined the effects of GABA and several GABAA receptor modulators on isolated rat (1-160 days) superior cervical ganglion (SCG) neurons with whole-cell recordings. The neurons were sensitive to GABA within 12 h after birth, and the relationship between the current amplitude and GABA concentration (GABA50 = 19 microM) in these newborn neurons was fitted with a Hill coefficient close to 1.0. Adult neurons, on the other hand, had a GABA50 value of 30 microM, and the slope was steeper and fitted with a Hill coefficient of 1.6. Unlike the results of previous studies in cultured SCG neurons, we found that the concentration of Zn++ that reduced the response by 50% was 37 +/- 12 microM in newborns and 43 +/- 6 microM in adults. Bicuculline (10 microM) inhibited the current by 41 +/- 6% and 36 +/- 11% in newborn and adult neurons, respectively. With 5 microM GABA, diazepam and pentobarbital potentiated the response in newborn and adult neurons. No changes were found in the sensitivity of the receptor for these ligands, but their efficacy was enhanced 2-fold during development. Diazepam shifted the relationship between GABA and the response to the left in all the neurons studied. Ethanol (40 mM) reduced the amplitude of the GABA current to 64 +/- 12% of control in newborn neurons and to 85 +/- 4% in adult neurons. The inhibitory effect of 100 mM ethanol on the GABA current was noncompetitive. Addition of a very large concentration of ethanol (850 mM) to newborn neurons potentiated the response to 164 +/- 13% of control.(ABSTRACT TRUNCATED AT 250 WORDS)

Development of substance P and neurokinin A immunoreactivity in ganglia supplying nerves to the submandibular glands of the rat

Developing submandibular, trigeminal and superior cervical ganglia, which provide innervation to the submandibular glands, were studied for substance P (SP)- and neurokinin A (NKA)-immunoreactive (IR) ganglion cells and nerve fibres in rat. These ganglia were examined by using an indirect immunofluorescence technique at daily intervals from the 16th day in utero (i.u.) until birth, and subsequently on the 2nd, 5th, 7th, 12th, 16th, 30th, 42nd postnatal day and in the adult (3 months). In the submandibular ganglion SP- and NKA-IR cells and fibres first appeared in considerable numbers on the 19th day i.u. (in one sample out of five on the 18th day i.u.), when more than 90% of the ganglion cells were immunoreactive to SP and NKA. The number stayed at more than 90% to the 7th postnatal day and then slowly decreased to the levels of adult animals (18% SP, 17% NKA). The first SP- and NKA-IR ganglion cells and fibres appeared in the trigeminal ganglion on the 18th day i.u. when they represented 7% (SP) and 4% (NKA) of the ganglion cells. The number of SP- and NKA-IR cells increased steadily, reaching a maximum at the time of birth when 68% (SP) and 74% (NKA) of the ganglion cells were immunoreactive. Thereafter they began to decrease toward the level of an adult rat (10% SP, 11% NKA). In the superior cervical ganglion only a few SP- and NKA-IR ganglion cells were detected from the 19th day i.u. to the fifth postnatal day. Positive ganglion cells were also occasionally found in the nerve trunks outside the superior cervical ganglion.(ABSTRACT TRUNCATED AT 250 WORDS)

Differences in sensitivity to nerve growth factor of axon formation and tyrosine hydroxylase induction in cultured sympathetic neurons

Superior cervical ganglia from 2-day-old and 3-week-old rats were maintained in vitro for up to 2 weeks in the presence of a range of concentrations of nerve growth factor up to 100 micrograms/ml. Nerve fibre length and density were measured and tyrosine hydroxylase activity of these cultures assayed after various times. Ganglia were also examined for catecholamines and neuronal numbers using fluorescence histochemistry and histology respectively. In cultures maintained without nerve growth factor, or in those containing low concentrations of nerve growth factor (3 ng/ml), tyrosine hydroxylase decreased to 5-10% of the initial levels by 14 days in vitro. The presence of the high concentration of 1 microgram/ml nerve growth factor in the culture medium or the addition of such a concentration during the culture period did not prevent an initial decrease in tyrosine hydroxylase but subsequently increased the enzyme activity. The maximal effect of nerve growth factor on nerve fibre density was at low concentrations whereas its maximal effect on neuronal survival, tyrosine hydroxylase activity or nerve fibre elongation was at high concentrations. After 2 days in culture, maximum neurite production occurred in cultures containing 10 ng/ml, while maximum nerve fibre elongation and tyrosine hydroxylase activity occurred in cultures containing 100 micrograms/ml nerve growth factor. We conclude that low concentrations of nerve growth factor, as occur in plasma, cause maximum axon formation while high concentrations of nerve growth factor, as occur in effector organs, induce maximum tyrosine hydroxylase activity and cell survival. The former process may be mediated via cell surface receptors and the latter via retrograde axonal transport of nerve growth factor to the cell body, following uptake by the terminal regions of the axons.