Target influences on [3H]ACh synthesis and release by ciliary ganglion neurons in vitro

GABAergic innervation of the ciliary ganglion in macaque monkeys - A light and electron microscopic study

The vertebrate ciliary ganglion (CG) is a relay station in the parasympathetic pathway activating the iris sphincter and ciliary muscle to mediate pupillary constriction and lens accommodation, respectively. While the postganglionic motoneurons in the CG are cholinergic, as are their inputs, there is evidence from avian studies that GABA may also be involved. Here, we used light and electron microscopic methods to examine the GABAergic innervation of the CG in Macaca fascicularis monkeys. Immunohistochemistry for the gamma aminobutyric acid synthesizing enzyme glutamic acid decarboxylase (GAD) and choline acetyltransferase (ChAT) revealed that all CG neurons are contacted by ChAT-positive terminals. A subpopulation of 17.5% of CG neurons was associated with terminal boutons expressing GAD-immunoreactivity in addition. Double-labeling for GAD and synaptophysin confirmed that these were synaptic terminals. Electron microscopic analysis in conjunction with GABA-immunogold staining showed that (1) GAD-positive terminals mainly target dendrites and spines in the perisomatic neuropil of CG neurons; (2) GABA is restricted to a specific terminal type, which displays intermediate features lying between classically excitatory and inhibitory endings; and (3) if a CG neuron is contacted by GABA-positive terminals, virtually all perisomatic terminals supplying it show GABA immunoreactivity. The source of this GABAergic input and whether GABA contributes to a specific CG function remains to be investigated. Nevertheless, our data indicate that the innervation of the ciliary ganglion is more complex than previously thought, and that GABA may play a neuromodulatory role in the control of lens or pupil function. J. Comp. Neurol. 525:1517-1531, 2017. © 2016 Wiley Periodicals, Inc.

Activin A and follistatin expression in developing targets of ciliary ganglion neurons suggests a role in regulating neurotransmitter phenotype

The avian ciliary ganglion contains choroid neurons that innervate choroid vasculature and express somatostatin as well as ciliary neurons that innervate iris/ciliary body but do not express somatostatin. We have previously shown in culture that activin A induces somatostatin immunoreactivity in both neuron populations. We now show in vivo that both targets contain activin A; however, choroid expressed higher levels of activin A mRNA. In contrast, follistatin, an activin A inhibitor, was higher in iris/ciliary body. Iris cell-conditioned medium also contained an activity that inhibited activin A and could be depleted with anti-follistatin antibodies. These results suggest that development of somatostatin is limited to choroid neurons by differential expression of activin A and follistatin in ciliary ganglion targets.

Target-derived molecules that influence the development of neurons in the avian ciliary ganglion

The developing avian ciliary ganglion has been a particularly amenable system for the identification, isolation, and characterization of putative target-derived molecules that mediate retrograde interactions. To date a number of biochemically distinct activities that regulate neuronal survival, transmitter phenotype, and chemosensitivity of ciliary ganglion neurons have been identified. Of these, only two survival-promoting molecules have been purified to homogeneity: ciliary neurotrophic factor and a related molecule, growth-promoting activity. A somatostatin-inducing activity found in cultured choroid cells is very likely to be chick activin A. Other molecules that regulate acetylcholine and acetylcholine receptor expression comigrate on a gel filtration column at a molecular weight of 50-60 kD, but they have yet to be isolated. Once molecules that mimic retrograde influences are identified, a number of criteria must be met before their physiological significance can be established. These criteria are (1) availability of the molecule from the target at the appropriate time in development; (2) ability of the neurons to respond to the molecule at the appropriate time in development; (3) demonstration that blocking the activity or availability of the molecule is able to block the target-derived developmental change expressed in the neurons. Of the molecules that are thought to retrogradely influence ciliary neuron development, only growth-promoting activity is known to meet criteria 1 and 2, and experiments are currently underway to test whether inhibition of growth-promoting activity in vivo will exacerbate normal cell death.

Specific in vitro biological activity of snake venom myotoxins

Some snake venoms contain toxins that are reported to be selective for damaging muscle. This specificity can be used to design experiments intended to eliminate muscle. We studied the small myotoxins and fractions IV and V of Bothrops nummifer venom to evaluate their direct effect on cultured muscle cells, neurons, macrophages, and a fibroblast cell line. The small myotoxins, at 100 micrograms/ml for 2 h, had no effect in vitro, contrary to the in vivo applications. Fractions IV and V were both myotoxic and, at 100 micrograms/ml, destroyed all cell types. However, at 10 micrograms/ml the effects of fraction IV were more selective for muscle. Vacuolation of the sarcoplasmic reticulum and T-tubules was first seen in the poisoned muscles, without initial lesions in the nuclei, sarcolemma, mitochondria, and rough endoplasmic reticulum. Fractions IV and V have different toxic activity in cells other than muscles and are a mixture of two basic proteins (i and ii). Protein ii is predominant in fraction IV and protein i is predominant in fraction V. The toxic effects may be mediated by the formation of nonspecific ionic pores in the sarcolemma and/or T-tubule muscle membrane.

Soluble and membrane-bound factors together account for target dependence of cultured parasympathetic neurons

The survival of avian ciliary ganglion (CG) neurons in culture depends upon an exogenous supply of trophic factor(s). Skeletal muscle, a normal ganglionic target tissue, is a well documented provider of survival-promoting activity, although the molecular basis for this ability to foster neuronal survival has not been thoroughly investigated. To identify the source of skeletal muscle support, dissociated neurons were plated into microwells containing either: a basal, trophically deficient medium; live pectoral muscle myotubes; medium conditioned by myotubes; membrane remnants of osmotically lysed myotubes; or, membrane remnants and conditioned medium. Neurons remaining in culture were counted after 1, 2, 5, and 7 days. The results reveal that neuronal survival is supported by both muscle conditioned medium and the membrane remnants of cultured myotubes. Each of these alone provides for only partial survival, while both combine to equal the activity of live myotubes. Treatment of the lysed membranes with either 1.5 M NaCl and/or 15 U heparin removed only 50-60% of the activity, suggesting that multiple factors are involved in the neuronal support obtained from lysed myotubes. This is in contrast to fibroblast remnants, which support some neuronal survival, but whose activity is wholly removed by NaCl. Conditioned medium also contains a heparin binding component which accounts for approximately 60% of its activity. These results indicate that full trophic support from the cultured target tissue requires at least two distinct active agents. The experiments further suggest that the target-derived factors responsible for neuronal survival in culture, and perhaps in vivo, are both soluble and membrane-associated molecules.

Identification of basic fibroblast growth factor as a cholinergic growth factor from human muscle

Dissociated embryonic chick ciliary ganglion cells in culture were used as a bioassay to isolate a cholinergic growth-promoting protein from extracts of autopsied adult human muscle. An active protein was purified after acid and salt precipitation of extract, cation exchange, molecular sieving, heparin affinity chromatography, and in some cases, SDS-PAGE. This protein increased levels of choline acetyltransferase activity and ACh synthesis with time in culture. The protein was identified as basic FGF by several criteria. It shared the high affinity for heparin and was the same approximate molecular weight, 18 kD, as basic FGF. Activity was removed from solution by antibodies specific for basic FGF. Recombinant human basic FGF was equally effective in stimulating CAT activity, but was not additive with our purified protein at saturating concentrations. Basic FGF was also found in extracellular matrix and conditioned medium from cultured embryonic chick muscle. The activity could be released from extracellular matrix by treatment with heparinase or high salt extraction. Basic FGF stimulates neurite outgrowth as well as the capacity for transmitter synthesis. Thus, basic FGF is present in embryonic and adult muscle and capable of acting as a growth regulator for cholinergic neurons.

The development of cholinergic neurons

Motoneuron precursors acquire some principles of their spatial organization early in their cell lineage, probably at the blastula stage. A predisposition to the cholinergic phenotype in motoneurons and some neural crest cells is detectable at the gastrula to neurula stages. Cholinergic expression is evident upon cessation of cell division. Cholinergic neurons can synthesize ACh during their migration and release ACh from their growth cones prior to target contact or synapse formation. Neurons of different cell lineages can express the cholinergic phenotype, suggesting the importance of secondary induction. Early cholinergic commitment can be modified or reversed until later in development when it is amplified during interaction with target. Motoneurons extend their axons and actively sort out in response to local environmental cues to make highly specific connections with appropriate muscles. The essential elements of the matching mechanism are not species-specific. A certain degree of topographic matching is present throughout the nervous system. In dissociated cell culture, most topographic specificity is lost due to disruption of local environmental cues. Functional cholinergic transmission occurs within minutes of contact between the growth cone and a receptive target. These early contacts contain a few clear vesicles but lack typical ultrastructural specializations and are physiologically immature. An initial stabilization of the nerve terminal with a postsynaptic AChR cluster is not prevented by blocking ACh synthesis, electrical activity, or ACh receptors, but AChR clusters are not induced by non-cholinergic neurons. After initial synaptic contact, there is increasing deposition of presynaptic active zones and synaptic vesicles, extracellular basal lamina and AChE, and postjunctional ridges over a period of days to weeks. There is a concomitant increase in m.e.p.p. frequency, mean quantal content, metabolic stabilization of AChRs, and maturation of single channel properties. At the onset of synaptic transmission, cell death begins to reduce the innervating population of neurons by about half over a period of several days. If target tissue is removed, almost all neurons die. If competing neurons are removed or additional target is provided, cell death is reduced in the remaining population. Pre- or postsynaptic blockade of neuromuscular transmission postpones cell death until function returns.(ABSTRACT TRUNCATED AT 400 WORDS)

Distinct influences of nerve growth factor and a central cholinergic trophic factor on medial septal explants

A central cholinergic trophic factor (C-CTF), previously reported in hippocampal extracts, enhances acetylcholine synthesis (ACh) and to a lesser extent choline acetyltransferase (ChAT) activity in cultured explants of the rat medial septal nucleus. Nerve growth factor (NGF) has been reported to enhance ChAT in several systems in vitro and in vivo, and clearly stimulates septal explants. At optimal concentrations of NGF and C-CTF, there is an additive effect on ChAT activity. The effects of NGF on ACh synthesis are minimal. Antibodies to NGF block effects of added NGF but have no effects on C-CTF activity. The ability of C-CTF to enhance ACh synthesis appears related to its ability to enhance the acetylation of the choline that has been taken up by a sodium dependent, high affinity transport system. Thus, actions of NGF and C-CTF appear qualitatively and quantitatively distinct, yet both can influence the cholinergic activity of the developing medial septal nucleus.

Cerebellum plus locus coeruleus in tissue culture. II: Development and metabolism of catecholamines

In contrast to locus coeruleus neurons in vivo, dopamine was the predominant catecholamine synthesized, stored, and released by neonatal mouse locus coeruleus cultures which included target cerebellar tissue, and norepinephrine was present in these cultures only at very low levels. Developmentally, norepinephrine increased slightly in the explants during the first 4 days in vitro and declined thereafter to barely detectable levels, whereas dopamine began to rise after 4 days and reached maximal levels by 7 days. Dopamine beta-hydroxylase was present in these cultures throughout maturation. These results suggest that the high ratio of dopamine to norepinephrine in locus coeruleus cultures cannot be attributed to the absence of appropriate target tissue or to a lack of the enzyme, dopamine beta-hydroxylase.

[3H]acetylcholine synthesis in cultured ciliary ganglion neurons: effects of myotube membranes

Avian ciliary ganglion neurons in cell culture were examined for the capacity to synthesize acetylcholine (ACh) from the exogenously supplied precursor, choline. Relevant kinetic parameters of the ACh synthetic system in cultured neurons were found to be virtually the same as those of the ganglionic terminals in the intact iris. Neurons were cultured in the presence of and allowed to innervate pectoral muscle; this results in an capacity for ACh synthesis. In particular, the ability to increase ACh synthesis upon demand after stimulation is affected by interaction with the target. This effect is shown to be an acceleration of the maturation of the cultured neurons. Lysed and washed membrane remnants of the muscle target were able to duplicate, in part, this effect of live target tissue on neuronal transmitter metabolism. Culture medium conditioned by muscle, and by the membrane remnants of muscle, was without significant effect. Thus, substances secreted into the medium do not play a major role in this interaction. Neurons cultured with either muscle or muscle membrane remnants formed large, elongate structures on the target membrane surface. These were not seen in the absence of the target at the times examined. This morphological difference in terminal-like structures may parallel the developmental increases in size and vesicular content of ciliary ganglion nerve terminals in the chick iris, and may relate to the increased ACh synthetic activity. The results suggest that direct contact with an appropriate target membrane has a profound, retrograde influence upon neuronal metabolic and morphological maturation.

Morphine-induced delay of normal cell death in the avian ciliary ganglion

Repeated administration of morphine in increasing doses delayed normal cell death in the ciliary ganglion of the chick embryo; the effect was completely blocked by naloxone. Survival of spinal motoneurons was not affected. Morphine also inhibited potassium-stimulated synthesis of acetylcholine in ganglion cells cultured with muscle, suggesting that morphine can influence neurotransmission. Morphine's effect on cell death may be due to an inhibition of transmission at the neuromuscular junction, but opiates may also directly affect cell death. Although it is now known whether the endogenous opiates in the ciliary ganglion influence neuronal survival during embryogenesis, exogenous opiates can affect normal cell death in the autonomic nervous system.

Differential morphologic effects of two fractions from fetal calf muscle on cultured chick ciliary ganglion cells

An extract of fetal calf striated muscle was found to support the survival, growth and differentiation of chick ciliary ganglion neurons in dissociated cell culture. The active material was precipitated in a single fraction by 35-60% ammonium sulfate. When this active fraction was passed over a concanavalin A-Sepharose column, only a portion of the activity was bound to the column and could be eluted by high salt. Both the bound and unbound fractions supported long-term neuronal survival and enhanced the neurons' capacity for acetylcholine synthesis. The two active fractions induced distinctly different morphologies in the cultures. The bound, salt-eluted fraction resulted in the extension of long, narrow multiply branched neurites with frequent varicosities, but it failed to support non-neuronal survival. The unbound, flow-through fraction caused the neurons to extend processes which aligned with each other and with the non-neuronal cells in dense networks. Striated muscle may thus possess the capacity to send more than one signal to modulate the development as well as maintain the survival of motor neurons.

Positive control of target cerebellar cells on norepinephrine uptake in embryonic brainstem cultures in serum-free medium

Brainstem cells from 15-day-old mouse embryos (E 15) grown for 8-10 days in dissociated primary cell culture in serum-free medium show high affinity uptake for [3H]norepinephrine (NE) that is specifically inhibited by desmethylimipramine (DMI), and also show high affinity binding for [3H]DMI. Brainstem cell uptake capacity for NE is increased at least 2-fold when cocultured with target cerebellar or striatal cells of the same embryonic age. The stimulation exerted by the cerebellum appears to be developmentally regulated since more mature cerebella (from 16-17-day-old embryos) exerted a greater stimulation than younger structures (from 14-15-day-old embryos). This stimulatory effect is also correlated with the number of available target sites since increasing the amount of cerebellar cells result in an increased stimulation of uptake capacity. The high affinity binding for [3H]DMI is also enhanced in coculture. The number of noradrenergic neurons, detected by autoradiography, remains unchanged in coculture indicating that added cerebellar cells did not take up NE, suggesting that the number of uptake sites per noradrenergic neuron is increased in the presence of target cells. These results indicate that interactions between afferent and target neurons, which normally take place during in vivo development, also occur in vitro and may result in a modification of neurotransmitter uptake in the afferent neuron.