Chick ciliary ganglion in dissociated cell culture. II. Electrophysiological properties

Ciliary neurotrophic factor

Ciliary neurotrophic factor (CNTF) was first identified and partially purified from embryonic chick eye tissues. Subsequently, it was shown that CNTF is also present in large amounts in sciatic nerves of adult rats and rabbits, which led to its final purification and cloning. CNTF is not secreted by the classical secretory pathway involving the endoplasmatic reticulum and Golgi complex, but can be detected in high quantities within the cytoplasm of myelinating Schwann cells and astrocytes using immunohistochemistry. CNTF supports survival and/or differentiation of a variety of neuronal cell types including sensory, sympathetic, and motoneurons. Also, nonneuronal cells, such as oligodendrocytes, microglial cells, liver cells, and skeletal muscle cells, respond to exogenously administered CNTF, both in vitro and in vivo. During development, expression of CNTF is very low, if indeed it is expressed at all, and the phenotype of mice lacking endogenous CNTF after inactivation of the CNTF gene by homologous recombination suggests that CNTF does not play a crucial role for responsive cells during embryonic development. However, motoneurons are lost postnatally in mice lacking endogenous CNTF, suggesting that CNTF acts physiologically on the maintenance of these cells. The ability of exogenous CNTF to protect against motoneuron loss following lesion or in other animal models indicates that CNTF might be useful in the treatment of human motoneuron disorders, provided appropriate means of administration can be found.

Probabilistic secretion of quanta from nerve terminals in avian ciliary ganglia modulated by adenosine

1. The effects of adenosine on the probability of secretion of acetylcholine quanta and on presynaptic and postsynaptic action potentials was examined in the post-hatched avian ciliary ganglion. 2. Adenosine (20 microM) reduced the average size of the excitatory postsynaptic potential (EPSP) by 33%. This was due to a decrease in quantal content of the EPSP (m). The effect was blocked by theophylline (50 microM). 3. Adenosine deaminase (2.5 i.u./ml) increased the size of the EPSP by 70%, suggesting that endogenous adenosine modulates synaptic transmission in the ciliary ganglion. However, theophylline (20-100 microM) did not affect the EPSP in a low [Ca2+]o of 1 mM and high [Mg2+]o of 6 mM. 4. Plateau-type action potentials with a large calcium component were generated in the ciliary neurones by bathing the ganglion in tetraethylammonium ions (TEA, 10 mM). Adenosine (20 microM) reduced the duration of these action potentials on short exposures (less than 20 min) but increased the duration on longer exposure (greater than 30 min). Adenosine did not affect the normal action potential recorded in the absence of TEA. 5. Adenosine (20 microM) hyperpolarized the nerve terminal and as a consequence increased the size of the presynaptic action potential and reduced its after-hyperpolarization. 6. Plateau-type action potentials with a large calcium component were generated in the nerve terminals using TEA (10 mM). The duration of these action potentials was significantly reduced by adenosine (20 microM). 7. Adenosines action on nerve terminals, to hyperpolarize the membrane and reduce calcium influx, may contribute to its effect in reducing m of the EPSP.

The development of ACH- and GABA-activated currents in normal and target-deprived embryonic chick ciliary ganglia

We have examined the expression of functional ACh and GABA receptors on embryonic chick ciliary ganglion neurons between Stages (St) 29 and 44 (Embryonic Day 6 to Embryonic Day 18). Whole-cell currents activated by ACh or GABA were measured in neurons 3-6 hr after dissociation to estimate the level of functional receptors in vivo. The mean peak IACh increased sevenfold between St 29 (321 pA) and St 44 (2345 pA) in two steps, separated by a plateau between St 35 and St 38 (E9 to E12). Cell size, estimated from measurements of membrane capacitance, increased only threefold over the same interval. Moreover, IACh and cell size were not well correlated at any stage examined. IGABA increased twofold between St 29 and St 38; the change was gradual and without any indication of two phases. The increase in IACh during development was not dependent on innervation of target cells within the eye. We removed the primordial eye between St 11 and St 13 (E2) and allowed the embryos to mature to various stages. Despite a small (20-50%) reduction in IACh at every stage examined, IACh still increased dramatically (about 10-fold) between St 29 and St 44 in target-deprived neurons. IACh was not uniquely affected by early target removal; IGABA and capacitance were also slightly reduced in target-deprived neurons.

Effects of prolonged depolarization on the nicotinic acetylcholine receptors of PC12 cells

To determine whether prolonged depolarization and/or changes in intracellular Ca2+ concentrations stimulate adaptive responses of neuronal nicotinic acetylcholine receptors, PC12 pheochromocytoma cells were grown in medium containing various concentrations of K+. Nicotinic receptor function was determined as carbachol-stimulated uptake of 86Rb+. Cells were exposed to 50 mM K+ for up to 4 days and then allowed to repolarize for 60 min. Under these conditions, no changes in basal or carbachol-stimulated uptake of 86Rb+ were observed. Furthermore, neither the time course of carbachol-stimulated uptake or the carbachol concentration dependence of 86Rb+ uptake was altered. Finally, concurrent depolarization did not affect the functional down-regulation produced by chronic exposure of the cells to carbachol. Thus, neuronal nicotinic acetylcholine receptors on PC12 cells do not appear to be regulated by depolarization or prolonged elevation of the intracellular Ca2+ level.

Calcium-activated chloride current in cultured sensory and parasympathetic quail neurones

1. Sensory (trigeminal and dorsal root) and autonomic (ciliary) ganglia from embryonic quail were dissociated and the neurones were grown in tissue culture. 2. Intracellular recordings were made in voltage clamp using patch electrodes and the whole-cell recording technique. In order to investigate a calcium-activated chloride current, the sodium and potassium currents were blocked. 3. Depolarizing voltage steps from a holding potential of -100 mV to a test potential of +20 mV triggered an early inward and a delayed outward current. The latter persisted as a long-lasting inward tail current when the membrane was depolarized to -100 mV. 4. These currents were all blocked by extracellular cobalt suggesting that they were calcium dependent. During a test depolarization to +20 mV, in the presence of intracellular EGTA (20 mM), the inward current persisted but the outward current was suppressed. EGTA (20 mM) also suppressed the long-lasting inward tail current at -100 mV. This suggested the presence of a calcium-activated current. 5. The reversal potential of the calcium-activated current was near the equilibrium potential for chloride ions and was shifted as predicted by the Nernst equation when the extracellular chloride concentration was changed. 6. The calcium-activated current was partially blocked by adding 4-acetamido-4'-isothiocyanatostilbene-disulphonic acid (SITS) at a concentration of 1 mM to the external superfusion medium. This effect of a compound known to interfere with chloride channels together with the results of point (5) suggested the existence of a calcium-activated chloride current (ICl(Ca)). 7. ICl(Ca) could be activated by transient and sustained components of the calcium current present in the cultured neurones. 8. ICl(Ca) was present in 80% of the sensory neurones but only in 10% of the parasympathetic neurones.

The distribution of acetylcholine receptor clusters and sites of transmitter release along chick ciliary ganglion neurite-myotube contacts in culture

Acetylcholine receptors accumulate along the length of cholinergic neuron-skeletal muscle contacts in vitro. The main purpose of this study was to describe, in a quantitative way, the distribution of acetylcholine receptor clusters induced by ciliary ganglion neurons over a period of time extending from hours to weeks after contacts are established. Neurites were filled with Lucifer Yellow and receptor clusters were identified with rhodamine-bungarotoxin. A cluster located within 5 micron of a nerve process or 10 micron of the base of a growth cone was considered to be a neurite-associated receptor patch (NARP). The first synaptic potentials were evoked 20 min after growth cone-myotube contact, and, after 24 h of co-culture, greater than 60% of the nerve-muscle pairs tested were functionally connected. NARPs appear rapidly; the first clusters were detected approximately 6 h after the neurons were plated. They were composed of several small subclusters or speckles of rhodamine-bungarotoxin fluorescence. The initial accumulation of receptors may occur at the advancing tips of nerve processes because NARPs were found at greater than 80% of the growth cone-muscle contacts examined between 12 and 24 h of co-culture. Over the 3-wk period examined, the mean incidence of NARPs ranged between 1.0 and 2.6 per 100 micron of neurite-myotube contact, with the peak observed on the second day of co-culture. During the first 3 d in culture, when the neurons were multipolar, nearly all of the primary processes induced one or more clusters. With time, as the neurons become unipolar (Role and Fischbach, 1987) NARPs persisted along the remaining dominant process. Measurements made during the third day of co-culture suggest that NARPs disappear along shorter neurites before they retract. Synaptic currents were detected by focal extracellular recording at 55% of the NARPs. The fact that spontaneous or evoked responses were not recorded at 45% suggests that contacts with clusters exhibit two functional states. Two types of presynaptic specialization at identified NARPs observed by scanning electron microscopy appear to be correlated with the functional state.

Enkephalin and substance P modulate synaptic properties of chick ciliary ganglion neurons in cell culture

Since enkephalin- and substance P-like immunoreactive materials have been identified in preganglionic terminals of the avian ciliary ganglion, we tested the effects of enkephalin and substance P directly on chick ciliary ganglion neurons in dissociated cell culture. Under these conditions the neurons form cholinergic synapses with each other that are spontaneously active. Both peptides modulate properties of membrane components associated with synaptic transmission between the neurons. Enkephalin causes a 60% reduction in the mean amplitude of the excitatory synaptic potentials, and the effect appears to be presynaptic in origin: enkephalin does not alter acetylcholine sensitivity on the neurons, but does inhibit Ca2+ influx as reflected by a 38% shortening of the Ca2+ component of the action potential. Both the reduction in synaptic potential amplitude and the shortening of the Ca2+ action potential produced by enkephalin are blocked by naloxone. Substance P, on the other hand, has no effect on Ca2+ action potentials but does reduce the time course of acetylcholine responses in the neurons by a mechanism consistent with enhanced receptor desensitization. Decay of the acetylcholine voltage response in the absence of substance P is described by a single exponential process with a time constant of 4-5 s. Coapplication of acetylcholine and substance P results in a second exponential decay process with a time constant of about 1 s that appears after a 200-400 ms lag period. Preincubation with substance P alone does not decrease the peak voltage response or shorten the lag, suggesting that either agonist or activated receptor is necessary for the substance P effect. These findings suggest modulatory roles for the peptides in ganglionic transmission.

Preganglionic neurons from the Edinger-Westphal nucleus: growth and histochemical characterization in cell culture

The Edinger-Westphal (EW) nucleus, also known as the accessory oculomotor nucleus in the chick, provides the cholinergic preganglionic input to the parasympathetic ciliary ganglion. In addition to acetylcholine, many EW neurons have been shown to contain enkephalin-like and/or substance P-like immunoreactivity. Establishment of EW neurons in culture would make possible study of their interactions with ciliary ganglion neurons in vitro and in addition would provide a valuable system for studying cholinergic/peptidergic neurons of the vertebrate central nervous system. We describe here dissociated cell cultures established from midbrain tissue containing the EW nucleus. In these cultures, 86% of the cells with neuronal morphology were positive for intracellular acetylcholinesterase activity, 54% were positive for enkephalin-like immunoreactivity, and 4% were positive for substance P-like immunoreactivity. The proportions of neurons that scored as labeled were even higher if the number of positive cells was compared to the number of cells in sister cultures immunoreactive for the large neurofilament protein polypeptide. When the cultures were stained simultaneously for acetylcholinesterase activity and enkephalin-like immunoreactivity, 34% of the cells with neuronal morphology were positive for both. In cultures derived from adjacent tissue regions very few cells expressed both activities. These results suggest that the cells expressing both acetylcholinesterase activity and enkephalin-like immunoreactivity in culture are EW neurons. The putative EW neurons survive for weeks in vitro in the absence of their normal target, the ciliary ganglion.

Voltage-dependent potassium currents in developing neurones from quail mesencephalic neural crest

Neurones in explants cultured from quail mesencephalic neural crest were studied at different stages of their development using the voltage-clamp technique. A voltage-dependent outward current activated by membrane depolarization was identified as a potassium current by the sensitivity of its reversal potential to extracellular potassium. The voltage-dependent potassium current is made up of two components which differ in their sensitivity to 4-aminopyridine (4-AP) and tetraethylammonium (TEA). The component most sensitive to 4-AP has fast activation kinetics and inactivates quickly at sustained depolarized voltages. By analogy with a current described in other preparations, this current was called IA. The component most sensitive to TEA has slower activation kinetics and inactivates more slowly at sustained depolarized voltages. This current was called IK. IA and IK were already present in neurones cultured for 24 h. The ratio between the peak of IK and that of IA increased significantly between 24 h and 4 days in culture. This means that the two components of the voltage-dependent potassium current follow a different time course during development.

Concanavalin A inhibits nicotinic acetylcholine receptor function in cultured chick ciliary ganglion neurons

The effects of various lectins and toxins on neuronal nicotinic acetylcholine receptor function have been studied in primary cultures of chick ciliary ganglion neurons. Neuronal response to acetylcholine receptor activation was measured by a cation flux method at 4 degrees C in a high potassium-low sodium medium designed to stabilize membrane potential near zero, with acetylcholine as the agonist and cesium-137 as the tracer ion. Exposure to 1 mM acetylcholine for 30 s produced a 5-10-fold stimulation of cesium-137 influx. Acetylcholine-stimulated influx was inhibited more than 95% by 10 microM D-tubocurarine, but was insensitive to both 1 microM tetrodotoxin and 1 microM alpha-bungarotoxin. Concanavalin A (50 micrograms/ml) inhibited agonist-induced ion flux by 80% at 4 degrees C. Succinyl-concanavalin A was ineffective at concentrations up to 250 micrograms/ml, and could not protect against the concanavalin A inhibition. However, inhibition by concanavalin A was eliminated by prior incubation of the lectin with 0.2 M alpha-methyl-D-mannoside and subsequent co-incubation with the sugar. Wheat germ agglutinin, lentil lectin, cholera toxin and tetanus toxin were without effect at either 4 degrees C or 37 degrees C. These results suggest a specific interaction between concanavalin A and neuronal nicotinic acetylcholine receptors.

Kinetics of acetylcholine activated ion channels in chick ciliary ganglion neurones grown in tissue culture

The acetylcholine activated conductance of chick ciliary ganglion neurones grown in tissue culture was studied by the patch clamp method. Single channel currents at 30 degrees C had a conductance of 38-42 pS, a reversal potential near + 10 mV and an average open lifetime of 1.08 ms (range 0.74 - 1.54 ms) at the resting potential. The presence of a single component in the distributions of amplitudes and open lifetimes, and also in the noise spectrum of voltage clamp currents, suggests that acetylcholine channels have uniform characteristics in these cells. Evidence of a desensitised state of the receptor was obtained from the distribution of gap intervals and the decline of voltage clamp current. These properties are similar to those of acetylcholine channels at the vertebrate neuromuscular junction. However, two important differences were found. (a) The acetylcholine concentrations used here were 10-25 times higher than those required to produce a similar degree of channel activation at the endplate. (b) When the membrane was hyperpolarised the mean open lifetime of the channel showed no change or a slight reduction.

Development of electrical membrane properties in cultured avian neural crest

In previous studies of the development of membrane excitability in vertebrate neurones, a calcium current has commonly been observed first, later replaced by a sodium current. We have now examined the development of membrane currents in explant cultures of mesencephalic neural crest cells from the quail embryo. Some of these cells constitute the precursors for the ciliary and trigeminal ganglia and in certain conditions can be characterized morphologically as neurones after only a few hours in culture. We report here that two membrane currents are present in neurones after 1 day in culture, a voltage-and time-dependent potassium current and a leakage current. On the second day in culture, voltage-dependent sodium and calcium currents can be detected. With time the sodium and calcium currents increase in magnitude and all four currents are present for at least 7 days in culture. This onset of electrical excitability differs from that described in other vertebrate neurones both in vitro and in vivo, but resembles the sequence observed in neurones of the developing grasshopper.

Membrane currents in a developing parasympathetic ganglion

It is reported that chick embryo ciliary ganglion neurons that have just terminated their migration and are in the process of forming a ganglion express several properties underlying membrane excitability. Evidence is presented which suggests that these cells possess voltage-dependent sodium, potassium, and calcium currents as well as a calcium-activated potassium current. These currents resemble those previously described in the more mature ciliary ganglion (Bader, C. R., Bertrand, D., and Kato, A. C. (1982). Dev. Biol. 94, 131-141) but may differ in their density per unit membrane surface. For example, the density per unit surface of the voltage dependent sodium current in younger neurons appears to be 5 to 10 times smaller than in more mature neurons.