Caffeine- and muscarinic receptor agonist-sensitive Ca2+ stores in chick ciliary ganglion cells

A K(+) channel activated by cholinergic muscarinic receptors in chick ciliary ganglion neurons at early developmental stage

Embryonic chick ciliary ganglion (CG) neurons obtained from E7-E8 ganglia maintained in serum-free medium were stimulated with 50 microM muscarine. A fast hyperpolarization of the membrane potential was observed in 25% of the cells tested, that in some cases was associated with a slower depolarization. Accordingly, in voltage clamp experiments, either an outward current or a biphasic current response could be observed. Single-channel experiments provide evidence that these signals can be associated to the activation of a K(+) channel whose conductance is 20 pS.

A calcium-permeable channel activated by muscarinic acetylcholine receptors and InsP3 in developing chick ciliary ganglion neurons

The electrical responses elicited by the muscarinic cholinergic pathway have been studied in cultured embryonic chick ciliary ganglion (CG) neurons. Neurons obtained from E7-E8 ganglia were maintained in serum-free medium for 1 to 3 days. Stimulation with 50 microM muscarine induced depolarizing responses in about 30% of the cells tested. In voltage clamp experiments at a holding potential of -50 mV, an inward current could be recorded in the same percentage of cells in response to muscarinic stimulation. In single channel experiments, with standard physiological solution in the pipette, muscarine transiently activated an inward conducting channel. Cell-attached recordings with 100 mM CaCl(2) in the pipette provided evidence that muscarinic agonists can activate a cationic calcium-permeable channel. Two main conductance levels could be detected, of 2.3+/-0.6 and 5.6+/-0.6 pS, respectively. In excised patches, addition of 5-20 microM inositol 1,4,5-trisphosphate (InsP(3)) to the bath reactivated a channel that could be blocked by heparin and whose characteristics were very similar to those of the channel seen in response to muscarinic stimulation. A channel with similar properties has been previously shown to be activated by basic fibroblast growth factor (bFGF) and InsP(3) in the same preparation.

Nerve growth factor maintains regulation of intracellular calcium in neonatal sympathetic neurons but not in mature or aged neurons

We examined the effects of nerve growth factor on the regulation of intracellular calcium levels of superior cervical ganglion neurons in terms of postnatal maturation and ageing. Rat superior cervical ganglion neurons from three age groups (neonatal: 0 to one-day-old, young adult: three to six-month-old, and aged: more than 24-month-old) were dissociated and cultured in the presence or absence of 100 ng/ml of nerve growth factor. Intracellular free calcium levels ([Ca2+]i) were measured using the fura-2 microfluorometry. Nerve growth factor treatment increased the resting [Ca2+]i of neonatal neurons, although it had no effect on those of mature and aged neurons. We further examined the effects of nerve growth factor on the transient increase of [Ca2+]i induced by methacholine (0.1 mM), caffeine (20 mM) or high-potassium medium (40 mM K+). Nerve growth factor pre-treatment significantly increased the population of neonatal superior cervical ganglion neurons which responded to methacholine, whereas almost all young adult and aged neurons responded to methacholine regardless of pre-treatment of nerve growth factor. Caffeine induced a cyclic alteration of [Ca2+]i (oscillation) in 45% of the neonatal superior cervical ganglion neurons when they were maintained without nerve growth factor, but nerve growth factor treatment suppressed the oscillation to 10% of neurons. In contrast to neonatal neurons, all of the young adult and aged neurons showed only a transient increase of [Ca2+]i in response to caffeine independent of nerve growth factor treatment. There was no significant effect of nerve growth factor on K+ depolarization-induced [Ca2+]i elevations at any of the ages studied. Nerve growth factor did not substantially alter the pattern of the transients induced by these three agents. Our results indicate that exogenous nerve growth factor is necessary to maintain normal acetylcholine receptor-mediated [Ca2+]i responses as well as Ca(2+)-induced Ca2+ release from intracellular calcium storage in neonatal superior cervical ganglion neurons. In mature superior cervical ganglion neurons, Ca2+ homeostasis becomes independent of exogenous nerve growth factor, and Ca2+ homeostasis and its independency are well preserved in aged neurons.

Elevation of intracellular calcium levels in neurons by nicotinic acetylcholine receptors

The recognition that intracellular free calcium serves as a ubiquitous intracellular signal has motivated efforts to elucidate mechanisms by which cells regulate calcium influx. One route of entry that may offer both spatial and temporal fine resolution for altering calcium levels is that provided by cation-permeable, ligand-gated ion channels. Biophysical measurements as well as calcium imaging techniques demonstrate that neuronal nicotinic acetylcholine receptors as a class have a high relative permeability to calcium; some subtypes equal or exceed all other known receptors in this respect. Activation of nicotinic receptors on neurons can produce substantial increases in intracellular calcium levels by direct passage of calcium through the receptor channel. When multiple classes of nicotinic receptors are expressed by the same neuron, each appears capable of increasing calcium in the cell but may differ with respect to location, temporal response, agonist sensitivity, or regulation in achieving it. As a result, nicotinic receptors must be considered strong candidates for signaling molecules through which neurons regulate a diverse array of cellular events.

Muscarinic receptor agonist-induced increases in cytosolic Ca2+ concentrations in chick ciliary ganglion cells

We used fura-2 microfluorometry to examine the mechanism underlying the muscarinic receptor agonist-induced increase in the cytosolic Ca2+ concentration ([Ca]in) in acutely isolated chick ciliary ganglion neurons. The order of potencies of muscarinic agonists in increasing [Ca]in was: oxotremorine M > muscarine > methacholine > oxotremorine > bethanechol. The muscarine-induced increase in [Ca]in persisted after treatment with thapsigargin, which blocked caffeine- and muscarinic agonist-induced Ca2+ release. The muscarine-sensitive [Ca]in increase was inhibited by both L- and N-type Ca2+ channel blockers but potentiated by an L-type Ca2+ channel agonist. Muscarine was effective in increasing [Ca]in in the presence of a desensitizing concentration of nicotine, and simultaneous addition of maximal doses of muscarine and nicotine caused an additive increase in [Ca]in. On the other hand, nicotine-, ATP-, and high K(+)-induced increase in [Ca]in was markedly potentiated during continuous stimulation with muscarine. These results suggest that muscarinic receptor stimulation increases Ca2+ influx passing through voltage-dependent Ca2+ channels. However, the muscarine-induced Mn2+ influx was observed in only some muscarine-sensitive cells, suggesting that muscarine-induced depolarization is too weak to overcome the inhibitory effect of Mn2+ on Ca2+ channels.

Acetylcholine differentially affects intracellular calcium via nicotinic and muscarinic receptors on the same population of neurons

Multiple receptor subtypes activated by the same ligand but coupled to different second messengers can produce divergent signaling in a cell, while receptors activated by different ligands but sharing the same second messenger can produce convergent signaling. We show here that chick ciliary ganglion neurons have three classes of receptors activated by the same neurotransmitter, acetylcholine, and that all three regulate the same second messenger, intracellular free calcium. Activation of muscarinic receptors on the neurons stimulates phosphatidylinositol turnover and induces calcium oscillations that are initiated and maintained by calcium release from caffeine/ryanodine-insensitive intracellular stores. Extracellular calcium is required to sustain the oscillations, while cadmium abolishes them. Activation of either of two classes of nicotinic receptors, distinguished both by location on the neurons and by subunit composition, induces a single, rapid elevation in intracellular calcium without inducing phosphatidylinositol turnover. The nicotinic responses are entirely dependent on extracellular calcium, show no dependence on release from internal stores, and do not display oscillations. Low concentrations of the native agonist, acetylcholine, induce repetitive calcium spikes in the neurons characteristic of muscarinic receptors, while higher concentrations induce nonoscillating increases in intracellular calcium that include contributions from nicotinic receptors. The three classes of receptors also differ in the acetylcholine concentration required to elicit a response. These differences, together with differences in receptor location and sources of calcium mobilized, may enable the receptor subtypes to target different sets of calcium-dependent processes for regulation.

Pharmacology of nicotine-induced increase in cytosolic Ca2+ concentrations in chick embryo ciliary ganglion cells

Chick embryo ciliary ganglion cells were acutely isolated, and the mechanism(s) underlying the increase in the cytosolic Ca2+ concentration ([Ca]in) induced by high concentrations of nicotine examined using fura-2 microfluorometry. The order of potencies of nicotinic receptor agonists in increasing [Ca]in was ACh > nicotine = dimethylphenylpiperazinium > cytisine. The nicotine-induced increase in [Ca]in was inhibited not only by nicotinic antagonists but also by muscarinic antagonists, while the muscarine-induced [Ca]in increase was little affected by nicotinic antagonists. The nicotine-induced [Ca]in increase was inhibited by both L- and N-type Ca2+ channel blockers and potentiated by an L-type Ca2+ channel agonist, Bay-K-8644. Nicotine also increased the cytosolic Na+ concentration ([Na]in) as measured by sodium binding benzofuranisophthalate microfluorometry, and this [Na]in increase was inhibited by various agents which reportedly affected nicotinic receptor channels in adrenal chromaffin cells. These results suggest that nicotine increased Na+ influx through nicotinic receptor channels resulting in membrane depolarization, which in turn increased Ca2+ influx through voltage-dependent Ca2+ channels. However, nicotine still increased influxes of Ca2+ and Mn2+ in the absence of external Na+, suggesting that nicotinic receptor channels in these cells are permeable not only to monovalent cations but also to Ca2+ and Mn2+.

ATP responses in the embryo chick ciliary ganglion cells

The ATP responses in ciliary neurons acutely dissociated from chick ciliary ganglia were investigated using the nystatin perforated patch recording mode. ATP induced a transient inward current at a holding potential of -50 mV in a concentration-dependent manner. The half-maximum effective concentration of ATP was 8.2 microM and the Hill coefficient was 0.9. The current-voltage relationship of the ATP response revealed a strong inward rectification at potentials more positive than -30 mV and the reversal potential was near 0 mV. The relative potencies of purinoceptor agonists were in the order of ATP > 2-methylthio-ATP > ADP. Neither adenosine, AMP nor alpha, beta-methylene-ATP induced any response. The ATP-induced inward current was blocked by suramin, a selective P2 purinoceptor antagonist, in a concentration-dependent manner. The half-maximum inhibitory concentration was 4.5 microM. The cytosolic Ca2+ concentration was increased by ATP and suramin inhibited the increase in a concentration-dependent manner. These results suggest that ATP operates non-selective cation channels by acting on P2y purinoceptors and has a role in the excitation of ciliary neurons.

Ca2+ influx and neurite growth in response to purified N-cadherin and laminin

The signaling mechanisms underlying neurite growth induced by cadherins and integrins are incompletely understood. In our experiments, we have examined these mechanisms using purified N-cadherin and laminin (LN). We find that unlike the neurite growth induced by fibroblastic cells expressing transfected N-cadherin (Doherty, P., and F.S. Walsh. 1992. Curr. Opin. Neurobiol. 2:595-601), growth induced by purified N-cadherin in chick ciliary ganglion (CG), sensory, or forebrain neurons is not sensitive to inhibition by pertussis toxin. Using fura-2 imaging of single cells, we show that soluble N-cadherin induces Ca2+ increases in CG neuron cell bodies, and, importantly, in growth cones. In contrast, N-cadherin can induce Ca2+ decreases in glial cells. N-cadherin-induced neuronal Ca2+ responses are sensitive to Ni2+, but are relatively insensitive to diltiazem and omega-conotoxin. Similarly, neurite growth induced by purified N-cadherin is inhibited by Ni2+, but is unaffected by diltiazem and conotoxin. Soluble LN also induced small Ca2+ responses in CG neurons. LN-induced neurite growth, like that induced by N-cadherin, is insensitive to diltiazem and conotoxin, but is highly sensitive to Ni2+ inhibition. K+ depolarization experiments suggest that voltage-dependent Ca2+ influx pathways in CG neurons (cell bodies and growth cones) are largely blocked by the combination of diltiazem and Ni2+. Our results demonstrate that cadherin signaling involves cell type-specific Ca2+ changes in responding cells, and in particular, that N-cadherin can cause Ca2+ increases in neuronal growth cones. Our findings are consistent with the current idea that distinct neuronal transduction pathways exist for cell adhesion molecules compared with integrins, but suggest that the involvement of Ca2+ signals in both of these pathways is more complex than previously appreciated.

Nicotinic and muscarinic acetylcholine responses in the embryo chick ciliary ganglion cells

Nicotinic and muscarinic acetylcholine (ACh) responses were investigated in acutely dissociated chick ciliary ganglion neurons using the nystatin perforated patch clamp technique. ACh-induced a rapid transient inward current in 100% of the neurons at a holding potential of -60 mV. This rapid inward current was mimicked by nicotine but not by muscarine. The reversal potential of the rapid inward current was +10.5 mV and the current was inhibited by d-tubocurarine and hexamethonium in a dose-dependent manner. In 57.6% of neurons, a slow inward current was also induced by ACh at a holding potential of -20 mV. This slow inward current was mimicked by muscarine but not by nicotine. The slow inward current became smaller at a hyperpolarized potential but not reversed, being consistent with the fact that this current was elicited by the inhibition of M-current. p-Fluorohexa-hydrosiladifenidol (P-F-HHSiD) strongly inhibited the slow inward current, suggesting that the current was elicited by the activation of M3 receptors.