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

Repolarization of the presynaptic action potential and short-term synaptic plasticity in the chick ciliary ganglion

Stimulation-induced increases in synaptic efficacy have been described as being composed of multiple independent processes that arise from the activation of distinct mechanisms at the presynaptic terminal. In the chick ciliary ganglion, four components of short-term synaptic plasticity have been described: F1 and F2 components of facilitation, augmentation, and potentiation. In the present study, intracellular recording from the presynaptic calyciform nerve terminal of the chick ciliary ganglion revealed that the late repolarization and afterhypolarization (AHP) phases of the presynaptic action potential are affected by repetitive stimulation and that the time course of these effects parallel that of facilitation. The effects of these changes in the presynaptic action potential time course on calcium influx were tested by using the recorded action potential waveforms as voltage command stimuli during whole-cell patch-clamp recordings from acutely isolated chick ciliary ganglion neurons. The "facilitated" action potential waveform (slowed repolarization, decreased AHP amplitude) evoked calcium current with slightly but significantly greater total calcium influx. Taken together, these results are consistent with the hypothesis that activity-dependent changes in the presynaptic action potential are one of several mechanisms contributing to the facilitation phase of stimulation-induced increases in transmitter release in this preparation.

Adenosine as a neuromodulator and as a homeostatic regulator in the nervous system: different roles, different sources and different receptors

Adenosine exerts two parallel modulatory roles in the CNS, acting as a homeostatic modulator and also as a neuromodulator at the synaptic level. We will present evidence to suggest that these two different modulatory roles are fulfilled by extracellular adenosine originated from different metabolic sources, and involve receptors with different sub-cellular localisation. It is widely accepted that adenosine is an inhibitory modulator in the CNS, a notion that stems from the preponderant role of inhibitory adenosine A(1) receptors in defining the homeostatic modulatory role of adenosine. However, we will review recent data that suggests that the synaptically localised neuromodulatory role of adenosine depend on a balanced activation of inhibitory A(1) receptors and mostly facilitatory A(2A) receptors. This balanced activation of A(1) and A(2A) adenosine receptors depends not only on the transient levels of extracellular adenosine, but also on the direct interaction between A(1) and A(2A) receptors, which control each other's action.

The probability of quantal secretion within an array of calcium channels of an active zone

A Monte Carlo analysis has been made of calcium dynamics in submembranous domains of active zones in which the calcium contributed by the opening of many channels is pooled. The kinetics of calcium ions in these domains has been determined using simulations for channels arranged in different geometries, according to the active zone under consideration: rectangular grids for varicosities and boutons and lines for motor-nerve terminals. The effects of endogenous fixed and mobile buffers on the two-dimensional distribution of free calcium ions at these active zones are then given, together with the extent to which these are perturbed and can be detected with different affinity calcium indicators when the calcium channels open stochastically under an action potential. A Monte Carlo analysis of how the dynamics of calcium ions in the submembranous domains determines the probability of exocytosis from docked vesicles is also presented. The spatial distribution of exocytosis from rectangular arrays of secretory units is such that exocytosis is largely excluded from the edges of the array, due to the effects of endogenous buffers. There is a steeper than linear increase in quantal release with an increase in the number of secretory units in the array, indicating that there is not just a local interaction between secretory units. Conditioning action potentials promote an increase in quantal release by a subsequent action potential primarily by depleting the fixed and mobile buffers in the center of the array. In the case of two parallel lines of secretory units exocytosis is random, and diffusion, together with the endogenous calcium buffers, ensures that the secretory units only interact over relatively short distances. As a consequence of this and in contrast to the case of the rectangular array, there is a linear relationship between the extent of quantal secretion from these zones and their length, for lengths greater than a critical value. This Monte Carlo analysis successfully predicts the relationship between the size and geometry of active zones and the probability of quantal secretion at these, the existence of quantal versus multiquantal release at different active zones, and the origins of the F1 phase of facilitation in synapses possessing different active zone geometries.

Adenosine A1 receptor activation inhibits LTP in sympathetic ganglia

The effects of adenosine on long-term potentiation of sympathetic ganglia was studied in the isolated superior cervical ganglion of the rat, using extracellularly recorded compound action potential as an index of synaptic transmission. Adenosine in a small concentration (2 microM) blocked the post-tetanic potentiation without affecting long-term potentiation. Higher concentrations blocked both responses with no significant effect on basal transmission. The inhibitory effect appears to be due to activation of adenosine A1 receptors. This was indicated by results from experiments with the A1 agonist N6-cyclopentyladenosine (1 microM) which caused inhibition of the basal transmission as well as long-term potentiation and post-tetanic potentiation. This inhibition was readily antagonized by 8-phenyltheophylline (1 microM), an A1 receptor antagonist. A small enhancement of basal transmission was seen on treatment with 8-phenyltheophylline. The inhibitory effect of N6-cyclopentyladenosine on long-term potentiation was totally prevented when the Ca2+ concentration in the superfusate was doubled (from 2.2 to 4.4 mM). The adenosine A2 receptor agonist 5'-(N-cyclopropyl)-carboxamidoadenosine (1 microM), although caused a slight potentiation of basal transmission, had no significant effect on the post-tetanic potentiation or long-term potentiation. The adenosine transport inhibitors, dipyridamole (2 microM) and S-(4-nitorobenzyl)-6-thioinosine (2 microM) caused significant inhibition of the basal ganglionic transmission without affecting post-tetanic potentiation or long-term potentiation. The effect of dipyradimole on basal transmission was not antagonized in the presence of 8-phenyltheophylline suggesting a non-specific action. The results suggest that exogenous adenosine can inhibit both post-tetanic potentiation and long-term potentiation in sympathetic ganglia, probably by activation of presynaptic A1 receptors. The results also suggest that endogenous adenosine, which is probably released in minute amounts, may only modulate basal transmission without influencing induction or maintenance of long-term potentiation in the superior cervical ganglion.

GABAB receptor-mediated modulation of presynaptic currents and excitatory transmission at a fast central synapse

Large nerve terminals (calyces of Held) in the medial nucleus of the trapezoid body (MNTB) offer a unique opportunity to explore the modulation of presynaptic channels at a mammalian central synapse. In this study I examined gamma-aminobutyric acid-B (GABAB)-mediated presynaptic inhibition at the calyx of Held in slices of the rat auditory brain stem. The selective GABAB agonist baclofen caused a potent inhibition of synaptic transmission and presynaptic Ca2+ current. The inhibition of presynaptic Ca2+ channels was associated with a slowing of the activation kinetics of the underlying current, and the inhibition was relieved by strong depolarization. The inhibition of both synaptic transmission and presynaptic Ca2+ current was abolished by N-ethylmaleimide, a sulfhydryl alkylating agent that uncouples the G(o)/Gi class of G proteins from receptors. Baclofen does not activate a potassium conductance in the presynaptic terminal. Taken together, these results suggest that GABAB receptors inhibit synaptic transmission via G protein-mediated modulation of presynaptic Ca2+ channels at this large central synapse. Furthermore, these findings demonstrate that basic mechanisms of G protein-mediated inhibition of Ca2+ channels, proposed from recordings of neuron cell bodies, are well conserved at nerve endings in the mammalian brain.

Calcium in sympathetic boutons of rat superior cervical ganglion during facilitation, augmentation and potentiation

The sympathetic preganglionic nerve terminals of the rat superior cervical ganglion were loaded with the calcium indicator oregon green 488 BAPTA-1 to measure the change in calcium concentration in the terminal boutons, (delta[Ca2+]b) following short (1 or 5 impulses) and long (200 impulses) trains at 30 Hz. The delta[Ca2+]b after a single action potential or a short train declined in two phases: a fast phase with a time constant of 530+/-30 ms and a moderate phase with a time constant of 4.0+/-0.2 s. The delta[Ca2+]b following a long train eventually declined with a time constant of 127+/-34 s (slow phase). The addition of either omega-agatoxin TK (100 nM), omega-conotoxin GVIA (100 nM) or nifedipine (20 microM) to block P-type, N-type or L-type calcium channels respectively showed that the rise in delta[Ca2+ ]b in boutons was predominantly mediated by an influx of calcium through P-type (53+/-7%) and N-type (46+/-4%) calcium channels. Experiments with caffeine, ryanodine and thapsigargin indicate that intracellular caffeine-sensitive calcium stores have a small but statistically significant effect on the fast and moderate phases. The mitochondrial uncoupler carbonyl cyanide m-chlorophenyl hydrazone (CCCP; 2 microM) significantly decreased the amplitude of the slow phase of delta[Ca2+]b relaxation, and sped its time course, suggesting that mitochondria normally dump calcium during this phase. Adenosine reduced the amplitude of delta[Ca2+]b in response to single action potentials by 30+/-6%, suggesting that adenosine-mediated autoinhibition in these boutons reduces Ca2+ influx. Spontaneous increases in delta[Ca2+]b demonstrated Ca2+ coupling between adjacent boutons. The delta[Ca2+]b kinetics are compared with F2 facilitation, augmentation and post-tetanic potentiation.

Cyclic AMP antagonizes adenosine-induced inhibition of ganglionic transmission

The mechanism of adenosine-induced inhibition of ganglionic transmission was investigated in the isolated superior cervical ganglion (SCG) of the rat. The inhibitory effect of adenosine on the postganglionic compound action potential (CAP) was antagonized by pretreatment of ganglia with forskolin, isoproterenol (IPNE), arginine vasopressin (AVP), or papaverine, all of which are known to increase tissue cAMP level by different mechanisms. Furthermore, pretreatment of ganglia with the adenylate cyclase inhibitor SQ 22, 536, or the phosphodiesterase activator imidazole reversed the effects of IPNE and forskolin. Pretreatment with 8-bromo-cAMP, resulted in a marked antagonism of the adenosine-induced inhibition. By themselves, none of these drugs had any significant effect on the CAP. Adenosine slightly but significantly decreased the basal level of cAMP in untreated ganglia. Formation of cAMP induced by IPNE was markedly reduced by adenosine. This was largely reversed in the presence of the adenosine A1 receptor antagonist 8-cyclopentyl-1, 3-dipropylxanthine (DPCPX) but not the A2 receptor antagonist 3, 7-dimethyl-1-propargylxanthine (DPMX). We conclude that the inhibition of ganglionic transmission by adenosine involves reduction of cAMP formation through activation of A1 receptors.

Inhibition of N-,P/Q- and other types of Ca2+ channels in rat hippocampal nerve terminals by the adenosine A1 receptor

The effects of the adenosine A1 receptor agonist, N6-cyclopentyladenosine (CPA), on both the increase in intracellular free Ca2+ concentration ([Ca2+]i) and on the release of endogenous glutamate in rat hippocampal synaptosomes were studied. The inhibitory effect of CPA on the increase in [Ca2+]i stimulated with 4-aminopyridine was neutralized by the adenosine A1 receptor antagonist, 8-cyclopentyl-1,3-dipropylxanthine (DPCPX). The inhibitory effect of CPA was greater in synaptosomes from the CA1 subregion than in whole hippocampal synaptosomes. The inhibitory effects of both CPA and of the Ca2+ channel blockers, omega-conotoxin GVIA, omega-conotoxin MVIIC or omega-conotoxin GVIA plus omega-conotoxin MVIIC, were greater than those caused by the Ca2+ channel blockers. The release of endogenous glutamate was inhibited by 41% by CPA. The inhibition observed when CPA and omega-conotoxin GVIA or CPA and omega-conotoxin MVIIC were present was also greater than the inhibition by the Ca2+ channel blockers alone. The presence of both omega-conotoxin GVIA and omega-conotoxin MVIIC did not completely inhibit the release of glutamate, and CPA significantly enhanced this inhibition. The membrane potential and the accumulation of [3H]tetraphenylphosphonium of polarized or depolarized synaptosomes was not affected by CPA, suggesting that adenosine did not increase potassium conductances. The present results suggest that, in hippocampal glutamatergic nerve terminals, adenosine A1 receptor activation partly inhibits P/Q- and other non-identified types of Ca2+ channels.

Probabilistic secretion of quanta and the synaptosecretosome hypothesis: evoked release at active zones of varicosities, boutons, and endplates

A quantum of transmitter may be released upon the arrival of a nerve impulse if the influx of calcium ions through a nearby voltage-dependent calcium channel is sufficient to activate the vesicle-associated calcium sensor protein that triggers exocytosis. A synaptic vesicle, together with its calcium sensor protein, is often found complexed with the calcium channel in active zones to form what will be called a "synaptosecretosome." In the present work, a stochastic analysis is given of the conditions under which a quantum is released from the synaptosecretosome by a nerve impulse. The theoretical treatment considers the rise of calcium at the synaptosecretosome after the stochastic opening of a calcium channel at some time during the impulse, followed by the stochastic binding of calcium to the vesicle-associated protein and the probability of this leading to exocytosis. This allows determination of the probabilities that an impulse will release 0, 1, 2,... quanta from an active zone, whether this is in a varicosity, a bouton, or a motor endplate. A number of experimental observations of the release of transmitter at the active zones of sympathetic varicosities and boutons as well as somatic motor endplates are described by this analysis. These include the likelihood of the secretion of only one quantum at an active zone of endplates and of more than one quantum at an active zone of a sympathetic varicosity. The fourth-power relationship between the probability of transmitter release at the active zones of sympathetic varicosities and motor endplates and the external calcium concentration is also explained by this approach. So, too, is the fact that the time course of the increased rate of quantal secretion from a somatic active zone after an impulse is invariant with changes in the amount of calcium that enters through its calcium channel, whether due to changes consequent on the actions of autoreceptor agents such as adenosine or to facilitation. The increased probability of quantal release that occurs during F1 facilitation at the active zones of motor endplates and sympathetic boutons is predicted by the residual binding of calcium to a high-affinity site on the vesicle-associated protein. The concept of the stochastic operation of a synaptosecretosome can accommodate most phenomena involving the release of transmitter quanta at these synapses.

An ATP-activated, ligand-gated ion channel on a cholinergic presynaptic nerve terminal

ATP has recently been identified as a fast neurotransmitter in both the central and peripheral nervous systems. Several studies have suggested that ATP can also affect the release of classical neurotransmitters, including acetylcholine with which it is co-released. We have searched for ATP receptors on a cholinergic presynaptic nerve terminal using the calyx-type synapse of the chicken ciliary ganglion. ATP was pulsed onto the terminals under voltage clamp and induced a short latency cation current that exhibited inward rectification and marked desensitization. This current was not seen with adenosine but was mimicked by several sterically restricted ATP analogs and was blocked by suramin. ATP-activated single ion channels exhibited prominent flickering and had a conductance of approximately 17 pS. Our results demonstrate a ligand-gated P2X-like purinergic receptor on a cholinergic presynaptic nerve terminal.

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

Nitric oxide modulation of calcium-activated potassium channels in postganglionic neurones of avian cultured ciliary ganglia

1. A study has been made of the modulation of calcium-activated potassium channels in cultured neurones of avian ciliary ganglia by sodium nitroprusside and L-arginine. 2. Sodium nitroprusside (100 microM) reduced the net outward current by 22 +/- 1% at 4.8 ms (mean +/- s.e. mean) and 25 +/- 1% at 350 ms during a test depolarization to +40 mV from a holding potential of -40 mV. The outward current remained reduced for the duration of the recording following a single application of sodium nitroprusside. These effects did not occur if the influx of calcium ions was first blocked with Cd2+ (500 microM). Application of ferrocyanide (100 microM) reduced the net outward current by only 6 +/- 3% at 350 ms during a test depolarization to +40 mV. 3. L-Arginine (270 microM) reduced the net outward current on average by 19 +/- 2% at 4.8 ms and 22 +/- 2% at 350 ms during a test depolarization to +40 mV. The current remained in this reduced state for the duration of the recording following a single application of L-arginine. These effects were reduced to 11 +/- 1% at 4.8 ms and 11 +/- 2% at 350 ms in the presence of N omega-nitro-L-arginine methyl ester (L-NAME, 100 microM). 4. In order to alleviate the dependence of calcium-activated potassium channels (Ik(Ca)) on the inward flux of calcium ions, the patch-clamp pipettes were filled with a solution containing 100 microM CaCl2, and the Ca2+ in the bathing solution was replaced with EGTA. Under these conditions sodium nitroprusside reduced the total outward current during a depolarizing pulse of + 40 mV by 9 +/_ 1% at 4.8 ms and by 36 +/- 3% at 350 ms. L-Arginine (270 microM) reduced this current under the same conditions by 9 +/- 1% at 4.8 ms and by 35 +/- 2% at 350 ms.5. Calcium-activated potassium currents were sensitive to apamin (50 nM), as this reduced the outward current by 23 +/- 3% at 350 ms when a high calcium-containing pipette was used during a depolarizing command to + 40 mV. L-Arginine still decreased the outward current in the presence of apamin(50 nM), by 5 +/- 1% at 4.8 ms and by 19 +/- 2% at 350 ms, indicating that L-arginine could reduce an apamin-insensitive Ik(Ca)6. Calcium-activated potassium currents were also sensitive to charybdotoxin (10 nM), as this reduced the outward current by 34 +/- 4% at 350 ms when a high calcium-containing pipette was used during a depolarizing command to + 40 mV. L-Arginine still decreased the outward current in the presence of charybdotoxin, by 6 +/- 1% at 4.8 ms and 12 +/- 4% at 350 ms, showing that L-arginine could reduce a charybdotoxin-insensitive Ik(Ca).7. The present results indicate that NO-synthase in ciliary ganglia can modulate Ik(Ca) by a method which is independent of the action of NO on the calcium channels. The Ik(ca) is decreased significantly at 4.8 ms into a depolarizing pulse, at a time that would decrease the rate of repolarization of the action potential. Ik(Ca) is also reduced at longer times (350 ms), indicating an affect on the inactivating process.

Effects of adenosine and cadmium on presynaptic fiber spikes in the CA1 region of rat hippocampus in vitro

Adenosine was found to decrease the amplitude of presynaptic fiber spikes recorded in stratum radiatum of the CA1 region of the hippocampus following stimulation of the Schaffer collateral and commissural afferents. The presynaptic fiber spike reflects currents activated in the presynaptic fibers and nerve terminals of this projection. However, the effect of adenosine was largely the result of: (a) temporal overlap between the presynaptic fiber spike and the field EPSP response and (b) an indirect effect most likely related to the hyperpolarization of CA3 neurons giving rise to the Schaffer collateral fibers. When the CA3 region was separated by a knife cut from the CA1 region and the field EPSP blocked with 6,7-dinitroquinoxaline-2,3-dione, the effect of adenosine was markedly reduced, but was still significant (4.4 +/- 1.4% reduction in the amplitude of the presynaptic fiber spike). However, this action was not mimicked by the Ca2+ channel antagonist cadmium, suggesting that it did not reflect an inhibition of Ca2+ currents by adenosine. This effect was not observed in the presence of the K+ channel antagonists Ba2+ or diaminopyridine. These results suggest that adenosine may have a small hyperpolarizing effect on the presynaptic nerve terminals, but this is unlikely to be the mechanism by which adenosine modulates transmission in this brain region.

Preferential inhibition of omega-conotoxin-sensitive presynaptic Ca2+ channels by adenosine autoreceptors

Adenosine is a potent modulator of transmitter release at a variety of synapses. The adenosine A1 receptor is assumed to reside in presynaptic terminals and to function as a negative autoreceptor. How adenosine reduces transmitter release is uncertain; it may reduce the calcium influx during nerve terminal depolarization by either activating K+ currents or inhibiting Ca2+ currents, although other mechanisms have been proposed. We have directly measured intracellular Ca2+ concentrations of giant pre-synaptic terminals in the chick ciliary ganglion. We report here that adenosine inhibited the nerve-evoked Ca2+ influx in the terminal by activating A1 receptors. Reduced Ca2+ influx was due largely to inhibition of omega-conotoxin GVIA-sensitive Ca2+ channels in the presynaptic terminal.

Real-time imaging of neurons retrogradely and anterogradely labelled with calcium-sensitive dyes

Membrane-impermeant calcium indicator dyes were used to retrogradely label dorsal root ganglia, spinal motoneurons and interneurons in the spinal cord of the chick embryo. The dyes were also used to label anterogradely primary afferent axons in the spinal cord and synaptic endings in the ciliary ganglion. Labelled neurons were imaged using digital videomicroscopy. Motoneurons and dorsal root ganglion cells exhibited a frequency-dependent change in fluorescence during antidromic stimulation. Single antidromic stimuli resulted in fluorescence transients that could be resolved in individual cells in real time. In addition, fluorescence changes could be recorded in motoneurons during episodes of bursting generated by rhythmic synaptic inputs from premotor networks. Stimulus-induced fluorescence signals were also detected in axons and synaptic endings labelled anterogradely. Optical signals were largely abolished in the absence of extracellular calcium. The results show that calcium changes can now be measured in identified populations of neurons and presynaptic terminals. The strong dependence of these signals on impulse activity suggests that the technique will be useful for monitoring the activity of identified neuronal populations. The calcium-dependent fluorescence signal probably results from cytosolic dye derived from diffusion which may limit the technique to situations in which the dye can be applied close (< 1 cm) to cell bodies.

Adenosine modulation of calcium currents in postganglionic neurones of avian cultured ciliary ganglia

1. Calcium currents in postganglionic neurones of cultured 7- to 10-day embryonic avian ciliary ganglia were analyzed under whole-cell voltage-clamp and their modulation by 2-chloroadenosine determined. 2. In the presence of tetrodotoxin (200 nM) in the medium to block the Na+ current and CsCl (105 mM) in the patch-clamp electrode to block the K+ current, two different components of the calcium currents (transient and sustained) were identified on the basis of their voltage-dependent kinetics as well as their sensitivity to the dihydropyridine agonist Bay K 8644 and antagonist nifedipine. 3. The sustained current inactivated very slowly (tau greater than 1000 ms; for test potentials from -20 mV to +40 mV) but was reactivated at a holding potential (Vh) of -40 mV. The current was increased on average over 50% by 1 microM of Bay K 8644 at a test potential of 0 mV and decreased over 35% by 1 microM of nifedipine. 4. The transient current inactivated slowly (tau less than 200 ms; for test potentials from -20 mV to +40 mV), and could be completely reactivated at a Vh of -80 mV. This current was unaffected by Bay K 8644 (1 microM) but reduced on average by 8% with nifedipine (1 microM). 5. The sustained and transient currents were decreased more than 70% by 5 microM of omega-conotoxin and decreased more than 50% by 250 microM verapamil. 6. 2-Chloroadenosine (1 microM) decreased the transient current by over 50% and the sustained current by less than 10%. In the presence of nifedipine (1 microM), 2-chloroadenosine decreased the transient current by over 30% and the remaining sustained current by 35%.In the presence of 8-phenyltheophylline (10 microM), 2-chloroadenosine no longer decreased either the transient or sustained currents but did have a slight potentiating effect on both the transient and sustained currents.7. These observations of the effects of 2-chloroadenosine on the transient and sustained currents are discussed in relation to the different calcium channel types at preganglionic nerve terminals.

Adenosine modulation of potassium currents in preganglionic nerve terminals of avian ciliary ganglia

Potassium currents in calyciform nerve terminals of the avian ciliary ganglion were analyzed using a single microelectrode voltage clamp. The modulatory affects of adenosine on these currents was also determined. Intracellular CsCl (1 M) blocked the steady-state outward current, indicating that it is mostly carried by K+. Tetraethylammonium (TEA, 10 mM) blocked over 40% of the outward current at a command potential of -30 mV. If Ca2+ influx was blocked by CoCl2 (5 mM), the steady-state outward current was reduced by over 16% in the nerve terminals suggesting that about one fifth of the outward current passes through calcium-activated potassium channels IK(Ca). Adenosine (50 mM) decreased the outward steady-state current over a wide range of command potentials in most terminals studied but failed to decrease this current in the presence of CoCl2 (5 mM). It is concluded that adenosine blocks IK(Ca) in nerve terminals.

Adenosine modulation of potassium currents in postganglionic neurones of cultured avian ciliary ganglia

1. Potassium currents in cultured postganglionic neurones of avian ciliary ganglia were analysed under whole-cell voltage clamp and their modulation by adenosine determined. 2. In the presence of tetrodotoxin (200 nM), and with moderate holding potentials (Vh = -40 mV), the steady-state current-voltage (I/V) curve was N-shaped over the range from -70 mV to +155 mV. CsCl (1 M) blocked the current, indicating that it was carried by K+. If Ca2+ influx was blocked by CdCl2 (500 microM) then the outward current was reduced and the N-shaped I-V curve lost, indicating the presence of a calcium-activated potassium current (IK(Ca)); the remaining current, due to the delayed rectifier (IK), increased with depolarization up to about a conductance of 10 nS near + 50 mV. This IK was 50% activated at about +20 mV and 50% inactivated at about -50 mV. Adenosine (10 microM) had similar affects on the N-shaped I/V curve as did CdCl2, indicating that it blocked IK(Ca). However, adenosine had little affect on the steady-state current in the presence of CdCl, indicating that it did not much affect IK. 3. In the presence of tetrodotoxin (200 nM), a large inward current occurred for large hyperpolarizations from a Vh = -50 mV. This inward rectifying current (IIR) had a reversal potential near EK and showed 50% activation at about -130 mV. Adenosine (10 microM) reduced IIR, by as much as 50% at large hyperpolarizations beyond -80 mV. 4. Relaxations of the outward current on hyperpolarization from Vh = -30mV were blocked by carbachol (10 microM), had a reversal potential near EK, and an I/V curve typical of 1M currents. These currents were little affected by adenosine (10 microM). 5. A fast transient outward current, due to depolarizing pulses from a large Vh = -110mV was observed in the presence of tetrodotoxin (200 nM). This had the characteristics of an IA current as it could be blocked with 4-aminopyridine (5 mM) and was 50% activated at about -20 mV and 50% inactivated at about -94 mV. The IA current was reduced by 42% at a depolarization of -20 mV by adenosine (10 microM). 6. Many neurones possessed a fast transient outward current that was blocked by tetrodotoxin (200nM). This current could be blocked with 4-aminopyridine (5mM); it therefore has the characteristics of a sodium-activated potassium current ('K(Na)). This IK(Na) was unaffected by adenosine (1O microM). 7. These results are discussed in relation to the role of adenosine in blocking Ca2 + channels and thereby modifying calcium-dependent components of K+ currents.