Presynaptic calcium currents recorded from calyciform nerve terminals in the lizard ciliary ganglion

Optogenetic probing and manipulation of the calyx-type presynaptic terminal in the embryonic chick ciliary ganglion

The calyx-type synapse of chick ciliary ganglion (CG) has been intensively studied for decades as a model system for the synaptic development, morphology and physiology. Despite recent advances in optogenetics probing and/or manipulation of the elementary steps of the transmitter release such as membrane depolarization and Ca²⁺ elevation, the current gene-manipulating methods are not suitable for targeting specifically the calyx-type presynaptic terminals. Here, we evaluated a method for manipulating the molecular and functional organization of the presynaptic terminals of this model synapse. We transfected progenitors of the Edinger-Westphal (EW) nucleus neurons with an EGFP expression vector by in ovo electroporation at embryonic day 2 (E2) and examined the CG at E8–14. We found that dozens of the calyx-type presynaptic terminals and axons were selectively labeled with EGFP fluorescence. When a Brainbow construct containing the membrane-tethered fluorescent proteins m-CFP, m-YFP and m-RFP, was introduced together with a Cre expression construct, the color coding of each presynaptic axon facilitated discrimination among inter-tangled projections, particularly during the developmental re-organization period of synaptic connections. With the simultaneous expression of one of the chimeric variants of channelrhodopsins, channelrhodopsin-fast receiver (ChRFR), and R-GECO1, a red-shifted fluorescent Ca²⁺-sensor, the Ca²⁺ elevation was optically measured under direct photostimulation of the presynaptic terminal. Although this optically evoked Ca²⁺ elevation was mostly dependent on the action potential, a significant component remained even in the absence of extracellular Ca²⁺. It is suggested that the photo-activation of ChRFR facilitated the release of Ca²⁺ from intracellular Ca²⁺ stores directly or indirectly. The above system, by facilitating the molecular study of the calyx-type presynaptic terminal, would provide an experimental platform for unveiling the molecular mechanisms underlying the morphology, physiology and development of synapses.

Optical measurement of presynaptic calcium currents

Measurements of presynaptic calcium currents are vital to understanding the control of transmitter release. However, most presynaptic boutons in the vertebrate central nervous system are too small to allow electrical recordings of presynaptic calcium currents (I(Ca)pre). We therefore tested the possibility of measuring I(Ca)pre optically in boutons loaded with calcium-sensitive fluorophores. From a theoretical treatment of a system containing an endogenous buffer and an indicator, we determined the conditions necessary for the derivative of the stimulus-evoked change in indicator fluorescence to report I(Ca)pre accurately. Matching the calcium dissociation rates of the endogenous buffer and indicator allows the most precise optical measurements of I(Ca)pre. We tested our ability to measure I(Ca)pre in granule cells in rat cerebellar slices. The derivatives of stimulus-evoked fluorescence transients from slices loaded with the low-affinity calcium indicators magnesium green and mag-fura-5 had the same time courses and were unaffected by changes in calcium influx or indicator concentration. Thus both of these indicators were well suited to measuring I(Ca)pre. In contrast, the high-affinity indicator fura-2 distorted I(Ca)pre. The optically determined I(Ca)pre was well approximated by a Gaussian with a half-width of 650 micros at 24 degrees C and 340 micros at 34 degrees C.

Pre- and postsynaptic whole-cell recordings in the medial nucleus of the trapezoid body of the rat

1. Simultaneous whole-cell recordings in a rat brain slice preparation are described from presynaptic terminals (calyces of Held) and postsynaptic somata which form an axosomatic synapse in the medial nucleus of the trapezoid body (MNTB). 2. Presynaptic action potentials evoked suprathreshold excitatory postsynaptic potentials (EPSPs). The minimum synaptic delay was around 0.4 ms at 36 degrees C and 0.9 ms at 23-24 degrees C. The amplitude of the L-alpha-amino-3-hydroxy-5-methyl-4-isoxazolepropionate (AMPA) receptor-mediated component of the excitatory postsynaptic currents (EPSCs) was 2-13 nA (at -80 mV). 3. Current-voltage relations showed that presynaptic Ca2+ channels were of the high voltage-activated type. 4. A single action potential evoked a presynaptic fluorescence transient that decayed with a time constant of 0.3-0.7 s, depending on the concentration (60-200 microM) of the Ca2+ indicator Calcium Green-5N (CG-5N). The peak amplitude of the [Ca2+]i transient was severalfold larger in the terminal than in the preterminal axon. 5. EPSC peak amplitudes were stable for more than 30 min after establishing the whole-cell configuration in the presynaptic terminal when the pipette contained 50 microM BAPTA. In contrast, with 1 mM BAPTA, peak amplitudes of EPSCs were reduced to one-third. 6. Trains of presynaptic action potentials evoked EPSCs with progressively smaller amplitudes. Little change was observed in the depression when the terminals were dialysed with 50 microM BAPTA, whereas depression was reduced with 1 mM BAPTA. 7. In low (1 mM) [Ca2+]o, facilitation instead of depression of EPSCs was observed. 8. The effects of presynaptic BAPTA suggest that the endogenous mobile Ca2+ buffer capacity of giant presynaptic terminals in the MNTB is lower than in other terminals of fast transmitting synapses.

Presynaptic excitability

Based on functional characterizations with electrophysiological techniques, the channels in nerve terminals appear to be as diverse as channels in nerve cell bodies (Table I). While most presynaptic Ca2+ channels superficially resemble either N-type or L-type channels, variations in detail have necessitated the use of subscripts and other notations to indicate a nerve terminal-specific subtype (e.g., Wang et al., 1993). Variations such as these pose a serious obstacle to the identification of presynaptic channels based solely on the effects of channel blockers on synaptic transmission. Pharmacological sensitivity alone is not likely to help in determining functional properties. Crucial details, such as voltage sensitivity and inactivation, require direct examination. It goes without saying that every nerve terminal membrane contains Ca2+ channels as an entry pathway so that Ca2+ can trigger secretion. However, there appears to be no general specification of channel type, other than the exclusion of T-type Ca2+ channels. T-type Ca2+ channels are defined functionally by strong inactivation and low threshold. Some presynaptic Ca2+ channels inactivate (posterior pituitary and Xenopus nerve terminals), and others have a somewhat reduced voltage threshold (retinal bipolar neurons and squid giant synapse). Perhaps it is just a matter of time before a nerve terminal Ca2+ channel is found with both of these properties. The high threshold and strong inactivation of T-type Ca2+ channels are thought to be adaptations for oscillations and the regulation of bursting activity in nerve cell bodies. The nerve terminals thus far examined have no endogenous electrical activity, but rather are driven by the cell body. On functional grounds, it is then reasonable to anticipate finding T-type Ca2+ channels in nerve terminals that can generate electrical activity on their own. The rarity of such behavior in nerve terminals may be associated with the rarity of presynaptic T-type Ca2+ channels. In four of the five preparations reviewed in this chapter--motor nerve, squid giant synapse, ciliary ganglion, and retina bipolar neurons--evidence was presented that supports a location for Ca2+ channels that is very close to active zones of secretion. All of these synapses secrete from clear vesicles, and the speed and specificity of transduction provided by proximity may be a common feature of these rapid synapses. In contrast, the posterior pituitary secretion apparatus may be triggered by higher-affinity Ca2+ receptors and lower concentrations of Ca2+ (Lindau et al., 1992). This would correspond with the slower performance of peptidergic secretion, but because of the large stimuli needed to evoke release from neurosecretosomes, the possibility remains that the threshold for secretion is higher than that reported. While the role of Ca2+ as a trigger of secretion dictates a requirement for voltage-activated Ca2+ channels as universal components of the presynaptic membrane, the presence of other channels is more difficult to predict. Depolarizations caused by voltage-activated Na+ channels activate the presynaptic Ca2+ channels, but whether this depolarization requires Na+ channels in the presynaptic membrane itself may depend on the electrotonic length of the nerve terminal. Variations in density between motor nerve terminals may reflect species differences in geometry. The high Na+ channel density in the posterior pituitary reflects the great electrotonic length of this terminal arbor. Whether Na+ channels are abundant or not in a presynaptic membrane, K+ channels provide the most robust mechanism for limiting depolarization-induced Ca2+ entry. K+ channel blockers enhance transmission at most synapses. In general, K+ channels are abundant in nerve terminals, although their apparent lower priority compared to Ca2+ channels in the eyes of many investigators leaves us with fewer detailed investigations in some preparations. Most nerve terminals have more than

Localization of individual calcium channels at the release face of a presynaptic nerve terminal

Studies using biophysical techniques suggest a highly structured organization of calcium channels at the presynaptic transmitter release face (Llinás et al., 1981; Stanley, 1993), but it has not as yet proved possible to localize identified channels at the required nanometer level of resolution. We have used atomic force microscopy on the calyx-type nerve terminal of the chick ciliary ganglion to localize single calcium channels tagged via biotinylated omega-conotoxin GVIA to avidin-coated 30 nm gold particles. Calcium channels were in low (modal value approximately < or = 1 per micron 2) and high (modal value approximately 55 per micron 2) density areas and exhibited a prominent interchannel spacing of 40 nm, indicating an intermolecular linkage. Particles were observed in clusters and short linear or parallel linear arrays, groupings that may reflect calcium channel organization at the transmitter release site.

Multiple subtypes of voltage-gated calcium currents in the Edinger-Westphal nucleus

Avian Edinger-Westphal (EW) neurons provide a unique opportunity to compare electrophysiologically the membranes of cell bodies and terminals in the same population of neurons. Axons that originate from neurons in the lateral region of the EW nucleus form a morphologically distinct presynaptic terminal, known as a calyx, on ciliary ganglion neurons. Several studies have shown that calyciform terminals in the ciliary ganglion exhibit predominantly N-type, high-voltage-activated (HVA) calcium channels. The goal of this study was to characterize and compare the calcium currents expressed in EW cell somas with those reported in the terminals. Whole-cell patch-clamp techniques were used to record from cell bodies in the lateral EW nucleus in slice preparations. Slices were obtained from embryonic day 16 chicks, matching the age of the embryos in which calyces were studied. Recordings were localized to the lateral region of the EW nucleus using Lucifer yellow fills. Voltage-step commands from -70 to 0 mV produced calcium currents with both a sustained and an inactivating component. Depolarization steps to 0 mV from a holding potential of -40 mV eliminated the inactivating component. These recordings suggested the presence of both LVA and HVA calcium currents. Application of 0.1 mM NiCl2 produced a reversible decrease in the amplitude of the whole-cell calcium current, preferentially affecting the inactivating component. The Ni2+(-)sensitive current activated and inactivated rapidly in a voltage-dependent manner. Treatment with 0.1 mM cadmium chloride caused a reversible reduction in the amplitude of the calcium current.(ABSTRACT TRUNCATED AT 250 WORDS)

Re-evaluation of calcium currents in pre- and postsynaptic neurones of the chick ciliary ganglion

1. Presynaptic nerve terminals of ciliary ganglia of the chick embryo were identified by the accumulation of dextran-tetramethylrhodamine applied to the cut end of the oculomotor nerve. Ca2+ currents were then recorded from the identified nerve terminals. 2. Whole-cell recordings were carried out simultaneously from a presynaptic terminal and its postsynaptic cell. The generation of presynaptic Ca2+ currents induced a postsynaptic response with a short delay. Electrical coupling was present in eight of fifteen pairs. The coupling ratio did not exceed 5%. 3. High-threshold Ba2+ currents were observed in presynaptic terminals without any evidence for the presence of low-threshold Ca2+ channels. The Ba2+ current was completely blocked by 50 microM Cd2+. 4. The presynaptic Ca2+ current induced by a long depolarizing pulse showed inactivation, but this inactivation was diminished when Ca2+ was replaced with Ba2+. 5. The presynaptic Ba2+ current was insensitive to dihydropyridines (DHPs). omega-Conotoxin GVIA (omega CgTX) suppressed a large fraction of the Ba2+ current irreversibly. About 10% of the Ba2+ current was resistant to both DHPs and omega CgTX. 6. The omega CgTX-sensitive component was not sensitive to changes in the holding potential between -120 and -50 mV. The omega CgTX-resistant component tended to be inactivated at depolarized holding potentials. 7. In some perisynaptic Schwann cells, small Ca2+ currents were observed. These Ca2+ currents increased monotonically with depolarization. 8. Only high-threshold Ca2+ channel currents were observed in postsynaptic ciliary cells. Exposure to 50 microM Cd2+ completely abolished the Ca2+ current. 9. About 25% of the Ba2+ currents were blocked by nifedipine (10 microM) in ciliary cells. The nifedipine-resistant component was partly blocked by omega CdTX (10 microM) leaving a small component (about 20%) which was resistant to both nifedipine and omega CgTX. 10. In ciliary cells, the fraction of Ba2+ currents blocked by omega CgTX was not affected by the presence or absence of nifedipine. Similarly, nifedipine blocked the Ba2+ currents to the same extent whether omega CgTX was present or not. The Ba2+ currents potentiated by Bay K 8644 were eliminated by nifedipine. 11. It is concluded that the presynaptic terminal of chick ciliary ganglion did not possess DHP-sensitive Ca2+ channels in contrast with the postsynaptic cell. Two subpopulations of presynaptic Ca2+ channels were distinguishable by their sensitivity to omega CgTX and membrane potential.

Repetitive firing properties in subpopulations of the chick Edinger Westphal nucleus

The avian Edinger Westphal nucleus, through the ciliary ganglion, controls accommodation, iris constriction, and blood flow through the choroid. In live brainstem slices, the nucleus is easily identifiable as an olive-shaped cluster of neurons dorsal to the oculomotor nerve and nucleus. Intracellular recordings from neurons in the nucleus identified two classes of responses to sustained (300 to 500 ms) injections of depolarizing current. One set of cells fired action potentials for the duration of the pulse while a second set of cells fired action potentials only transiently, during the first 50 to 100 ms, after which they remained silent regardless of the size of the depolarization. Intracellular recordings followed by injections of the fluorescent dye lucifer yellow revealed that repetitively firing cells were located in the lateral half of the nucleus while non-repetitively or transiently firing cells were located in the medial half. These locations correspond to different Edinger Westphal subdivisions which have distinct inputs and target populations. The varying firing patterns are discussed with reference to the known functions of the subdivisions in which they occur. Replacement of calcium by magnesium in the extracellular medium had no effect on the number of action potentials fired by non-repetitively firing cells, suggesting that a calcium-activated potassium current is not responsible for suppressing repetitive firing in these cells. In contrast, in repetitively firing cells removal of extracellular calcium increased the frequency of action potential discharge and decreased the amplitude of afterhyperpolarizations following single action potentials. Addition of cadmium to the bath medium had similar effects.(ABSTRACT TRUNCATED AT 250 WORDS)

Single calcium channels on a cholinergic presynaptic nerve terminal

The calyx-type synapse of the chick ciliary ganglion was used to examine single calcium channels in a vertebrate cholinergic presynaptic nerve terminal by means of the cell-attached, patch-clamp technique. Calcium channels were recorded on the internal, transmitter-release face of the nerve terminal, but were not detected on the external face. These channels were recruited at -30 mV, with maximum activation at about +30 mV, and were sometimes clustered at high densities. Single-channel conductance estimates with voltage-pulse or -ramp techniques gave values of 11-14 pS with 110 mM barium, which is in the intermediate, N-type range for calcium channels on a control neuron. This nerve terminal calcium channel, termed the NPT-type, may link action potentials to transmitter release at many vertebrate fast-transmitting synapses.

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.

Calcium currents recorded from a vertebrate presynaptic nerve terminal are resistant to the dihydropyridine nifedipine

The influx of Ca ions into the presynaptic nerve terminal through ion channels is a key link between the action potential and the release of chemical transmitters. It is not clear, however, which types of Ca channel are involved in neurosecretion at vertebrate synapses. In particular, there is disagreement as to whether these channels are sensitive to dihydropyridine blockers, characteristic of L-type Ca channels. We have used the chicken ciliary ganglion calyx synapse to test the effect of the dihydropyridine nifedipine on Ca current recorded directly from a cholinergic presynaptic nerve terminal. We used a control neuron to define the experimental conditions under which L-type Ca channels are blocked by 10 microM nifedipine. We then tested the effect of the dihydropyridine on Ca currents recorded from the presynaptic terminal using the same conditions. Nifedipine did not reduce the calyx Ca current nor did it block chemical transmission through the ganglion. The lack of effect of the dihydropyridine was not due to restricted access since omega-conotoxin GVIA, a peptide toxin that blocks transmission at this synapse, rapidly blocked the calyx Ca current. Thus, the predominant Ca channel in this presynaptic nerve terminal is not dihydropyridine sensitive and, hence, cannot be characterized as L-type.

Voltage-activated calcium currents in presynaptic nerve terminals of the chicken ciliary ganglion

1. Calcium currents (ICa) were recorded from presynaptic calyces of ciliary ganglia of the chick embryo under whole-cell voltage clamp. 2. Only high-threshold ICa was recorded without any evidence for the presence of low-threshold Ca2+ channels. 3. High-threshold (high-voltage-activated, HVA) ICa could be classified into non-inactivating (HVAn) and inactivating (HVAi) components. The mean inactivation time constant of the HVAi component was 213 ms (at 0 mV). The threshold for activation by depolarizing pulses was more negative for the HVAn component than for the HVAi component. The HVAi component was inactivated by 19% at a holding potential of -60 mV, while the HVAn component was little affected under this condition. 4. The activation of HVAn component was faster than that of the HVAi component. 5. Both the HVAn and HVAi components were blocked by Cd2+ (50 microM) and La3+ (1 microM). Both components were only slightly affected by Ni2+ (100 microM). The order of potency in blocking was La3+ greater than Cd2+ greater than Ni2+ for both components. Both the HVAi and HVAn components were irreversibly blocked by omega-conotoxin GVIA(omega-CgTX, 10 microM). 6. The two components could pharmacologically be distinguished by selective blockade of the HVAn component with nifedipine (2 microM) and D600 (100-250 microM). 7. HVAn and HVAi components are suggested to represent two different subpopulations of Ca2+ channels. The HVAn subpopulation may be responsible for persistent Ca2+ influx during subthreshold depolarization of the nerve terminal.