Patch clamp experiments on nicotinic acetylcholine receptor-ion channels in bullfrog sympathetic ganglion cells

Synaptic organization of amphibian sympathetic ganglia

The synaptic organization of the amphibian sympathetic ganglia was studied, especially in the last two abdominal paravertebral ganglia of the frog. These ganglia appear to form a monosynaptic relay, not containing interneurons. They consist of two systems working in parallel: the principal neurons, by far the most numerous, and a small number of chromaffin (i.e., SIF) cells, usually arranged in clusters. Each principal neuron is innervated by a preganglionic branch forming a set of cholinergic synapses which exhibit classical ultrastructure. The only peculiarity is the presence of a subsynaptic apparatus in a variable percentage of synaptic complexes. Electrophysiological studies have demonstrated that synaptic transmission is due to ACh release and involves several postsynaptic potentials. Moreover, the principal neurons are of two types, B and C, whose preganglionic axons and their own axons have different conduction velocities. C neurons tend to be small in diameter, and B neurons are larger, but the size distribution of the two populations overlaps. More recently, it was demonstrated that these two neuronal systems have different immunocytochemical features. The C preganglionic fibers contain an LHRH-like peptide, which is responsible for late synaptic events. The B preganglionic fibers contain CGRP, whose role has not yet been established. The principal neurons all contain adrenaline, but neuropeptide Y is also present in C neurons and could be a second transmitter at peripheral junctions. SP-containing fibers also pass through the ganglia, but give rise to intraganglionic synapses only rarely, except in the celiac plexus. Galanin can coexist with neuropeptide Y in certain C neurons. Numerous principal neurons are immunoreactive for VIP. Chromaffin cells contain noradrenaline and metenkephalin, and some contain SP or LHRH; they are endocrine cells controlled by preganglionic fibers and can have a modulatory effect on principal neurons endowed with appropriate receptors. The accessibility of frog abdominal ganglia and the anatomical separation of B and C preganglionic fiber pathways provide interesting systems in which to carry out experimentation on the stability and specificity of synaptic contacts. After postganglionic axotomy, the majority of synapses disappear by disruption of synaptic contacts. There is a certain discrepancy between the recovery of synaptic transmission and the reappearance of morphologically identifiable synapses, suggesting that a certain amount of transmission is possible at contacts devoid of synaptic complexes. The selective deafferentation of B or C neurons showed that the subsynaptic apparati are mainly found at B neuron synapses. The course of reinnervation following selective deafferentation reveals the existence of different specificities at B and C synapses: C neurons are easily reinnervated by B preganglionic fibers, whereas C fibers appear fairly ineffective at reinnervating B neurons, even after a long interval. Attempts were made to reinnervate ganglionic neurons with somatic motor nerve fibers. Reinnervation was achieved only rarely, and it is concluded that the ganglionic synapses in the frog have a higher specificity and lower plasticity than in mammals.

Single-channel recording in brain slices reveals heterogeneity of nicotinic receptors on individual neurons within the chick lateral spiriform nucleus

We examined the functional properties of central nicotinic acetylcholine receptors at the single-channel level using tight-seal, voltage-clamp techniques. Single-channel currents were recorded from cell-attached patches on lateral spiriform neurons in chick brain slices. These neurons are known to express functional nicotinic receptors that are insensitive to the antagonists alpha-bungarotoxin and kappa-bungarotoxin, and which exhibit a high affinity for nicotine and other nicotinic agonists. Single-channel openings were observed in 84% of patches (n = 118) when the nicotinic agonists acetylcholine (1-100 microM), carbamylcholine (3-100 microM), or nicotine (3-10 microM) were present in the patch pipette. In contrast, single-channels were markedly reduced in number or entirely absent when the nicotinic antagonist dihydro-beta-erythroidine was present along with acetylcholine (n = 7) or when no agonist was present in the pipette (n = 22). Single-channel openings displayed inward rectification at depolarized potentials, and were dependent on extracellular sodium. Between 1 and 30 microM acetylcholine, a dose-response relationship was observed between agonist concentration and single-channel open probability during the first minute following seal formation. Multiple classes of single nicotinic channels, with calculated mean slope conductances of 15, 31, 40, and approximately 70 pS, were observed in membrane patches on different neurons within the lateral spiriform nucleus, and even within single patches on individual neurons. We conclude that neurons within the lateral spiriform nucleus express functionally heterogeneous nicotinic receptors and that in some neurons different nicotinic receptor subtypes are present in close proximity to each other on the same cell surface.

Characteristics of Ca2+ release induced by Ca2+ influx in cultured bullfrog sympathetic neurones

1. A rise in intracellular Ca2+ ([Ca2+]i) and a Ca2+ current (ICa) induced by a depolarizing pulse were simultaneously recorded by fura-2 or indo-1 fluorescence and whole-cell patch clamp techniques in cultured bullfrog sympathetic ganglion cells. 2. [Ca2+]i (calculated from the ratio of fura-2 fluorescences excited at 380 and 340 nm and recorded with a photomultiplier at > 492 nm) rose regeneratively (in most cells) during a command pulse (from -60 to 0 mV, 100 ms), continued to rise thereafter, peaked at 666 ms (on average) and decayed slowly with a half-decay time of 22.8 s. 3. Scanning a single horizontal line across the cytoplasm with an ultraviolet argon ion laser (351 nm) and recording indo-1 fluorescences at two wavelengths (peaked at 410 and 475 nm) with a confocal microscope demonstrated that [Ca2+]i beneath the cell membrane rose much faster than that in the deeper cytoplasm. The time course of the spatial integral of [Ca2+]i, however, corresponded well with that recorded with fura-2 fluorescence using a photomultiplier. 4. [Ca2+]i measured by fura-2 fluorescence ratio using a photomultiplier did not increase during a strong depolarizing pulse (-60 to +80 mV), but sometimes rose after the pulse. A depolarization-induced rise in [Ca2+]i ([Ca2+]i transient) was blocked in a Ca(2+)-free, EGTA solution, reduced by lowering the extracellular Ca2+ concentration ([Ca2+]o) to 0.45 or 0.9 mM and enhanced by raising [Ca2+]o to 7.2 or 14.4 nM. 5. The extracellular Ca2+ dependence was non-linear when long depolarizing pulses (up to 500 ms) were applied; the amplitude of [Ca2+]i transient/Ca2+ entry (unit [Ca2+]i transient) increased with an increase in Ca2+ entry. 6. Increasing the duration of depolarization (-50 or -60 to 0 mV) from 20 to 500 ms enhanced asymptotically the integral of ICa (due to inactivation), and progressively the magnitude of [Ca2+]i transients, leading to the apparent non-linear dependence of unit [Ca2+]i transient on Ca2+ entry as well as on the duration of membrane depolarization. The peak time of [Ca2+]i transient was unchanged for pulse durations up to 300 ms, but prolonged with an increase in pulse duration to 500 ms. 7. Inhibitors of Ca2+ release from intracellular Ca2+ reservoirs, dantrolene (10 microM) and ryanodine (50 microM), blocked the [Ca2+]i transient to 56 and 30%, respectively, of the control. 8. The higher the basal [Ca2+]i level, the greater was the magnitude of the [Ca2+]i transients.(ABSTRACT TRUNCATED AT 400 WORDS)

Mechanical modulation of a voltage-dependent non-inactivating K+ current in cultured bullfrog sympathetic neurones

Cultured bullfrog sympathetic ganglion cells were voltage-clamped with a whole-cell patch-clamp technique. Local flow of a solution (identical to the bathing solution) from a micropipette to a cell, but not other mechanical stimuli, produced a non-inactivating outward (in 34 cells out of 141) or inward (in 70 cells) current [I(f)(out) or I(f)(in), respectively] depending on cells. Both I(f)(out) and I(f)(in) appeared at voltages more positive than -60 mV. The mechanism, however, was activated even at -70 mV, as I(f)(out) or I(f)(in) appeared on shifting membrane potential to -30 mV immediately after the local flow. I(f)(out) and I(f)(in) were accompanied by increases and decreases, respectively, in the membrane conductance and current relaxation to a voltage jump between -30 mV and -55 mV without a change in its time constant (whose value was similar to that of a voltage-dependent non-inactivating K+ current, IM), and reversed at a membrane potential close to the equilibrium potential for K+. Both I(f)(out) and I(f)(in) were blocked by Ba2+ (4-8 mM), a blocker of IM, and by muscarine (10 microM), which produced either an "apparent inward" or outward current. A transient outward current activated by a voltage jump from -85 mV (or -75 mV) to -30 mV was little affected by a local flow of a solution which produced I(f)(out) or I(f)(in). These results suggest that the local solution flow produced I(f)(in) or I(f)(out) by deactivating or activating IM, respectively.

A patch-clamp study on the muscarine-sensitive potassium channel in bullfrog sympathetic ganglion cells

1. A voltage-independent K+ channel was characterized and effects of muscarine were studied in cultured bullfrog sympathetic ganglion cells using the cell-attached patch-clamp configuration. 2. Three types of single-channel current were recorded from 2- to 10-day-old cultured cells in the presence of tetraethylammonium (2-20 mM), tetrodotoxin (1-2 microM), Cd2+ (0.1 mM) and apamin (20 nM). 3. The most frequently observed channel was a voltage-independent K+ channel which was open at the resting membrane potential and had a conductance of 52.6, 78.9 and 114.9 pS at a [K+]o of 2, 40 and 100 mM, respectively. This channel was designated background K+ channel. 4. Two other channel types were observed less frequently. One had a conductance of 26 pS (external K+, 118 mM) and a long open time of several seconds at the resting membrane potential. The second channel had a smaller conductance (20 pS) and displayed a voltage-dependent activation. 5. The open probability of the background K+ channel varied between patches, ranging from 0.0005 to 0.486. The open time distribution was fitted by a single exponential with a time constant of 0.51 ms. Both of these parameters were independent of the membrane potential. The closed time distribution consisted of at least four exponentials having time constants of 0.17, 3.7, 120 ms and several seconds. 6. Muscarine (10-20 microM) applied to the membrane outside the patch pipette reversibly enhanced the activity of the background K+ channel. This effect was associated with an increase in the open probability, which resulted from an increase in the mean open time concomitant with a decrease in the mean closed time. Muscarine did not change the single-channel conductance of this channel. 7. The effects of muscarine were blocked by atropine (1 microM). 8. It is concluded that there exists a muscarine-sensitive, voltage-independent K+ channel in cultured bullfrog ganglion cells. This K+ channel appears to contribute to the generation of the resting membrane potential and underlie the slow inhibitory postsynaptic potential of these neurones in situ.

Calcium-dependent inactivation of inwardly rectifying K+ channel in a tumor mast cell line

Antigenic stimulation of rat basophilic leukemia (RBL-2H3) cells, a tumor mast cell line, is associated with an increase in intracellular free Ca2+ concentrations ([Ca2+]i) and membrane polarization. We recorded whole cell and single-channel currents through the inwardly rectifying K+ channel, a major resting conductance of cells, using the patch-clamp technique, and we examined interactions between channel activity and [Ca2+]i. With 10 microM Ca2+ in the pipette, the amplitude of whole cell currents gradually declined within 5 min to 48 +/- 13% of that immediately after rupture of the patch membrane, in the presence of 1 mM ATP which minimized intrinsic rundown. In inside-out patches, activity of the channel was reduced by increasing the concentration of Ca2+ in the internal medium, both in the presence and absence of 1 mM ATP, with no apparent change in single-channel conductance. Time-averaged mean current activity in inside-out patches in the presence of 5 microM Ca2+ was less than 50% of that with Ca2+ of 100 nM or less. These results suggest that a rise in [Ca2+]i leads to a closure of the inwardly rectifying K+ channel. In some inside-out patches, inward currents characterized by burst composed of rapid transitions between open and closed states were observed (flickering currents). Single-channel properties of the flickering currents are similar to the inwardly rectifying K+ channel except for kinetics (single-channel conductance of 24.5 +/- 7.9 pS, inward rectification, and permeability to K+).(ABSTRACT TRUNCATED AT 250 WORDS)

Properties of the nicotinic-receptor-activated current in adrenal chromaffin cells of the guinea-pig

Properties of acetylcholine(ACh)- and nicotine-induced currents were studied in the guinea-pig chromaffin cell, using the whole-cell and cell-attached versions of the patch-clamp technique. Bath application of ACh or nicotine, but not muscarine, produced an inward current associated with an increase in current noise at a membrane potential of -70 mV. Low concentrations of both agonists produced a sustained inward current whereas high concentrations produced a transient, then a sustained inward current. Nicotine was about twice as potent as ACh in inducing the inward current. Hexamethonium (6 microM) inhibited the ACh-induced current but not in a competitive manner. By contrast, atropine (6 microM) inhibited the ACh-induced current more strongly with increasing concentrations of ACh. The nicotinic-receptor-activated current ("nicotinic current") showed inward rectification and, when Cs+ was used instead of K+ in the pipette solution, the polarity of the current changed at around -5 mV and a negative slope occurred between +10 mV and +30 mV. The nicotinic channel had a unit conductance of 33 pS. During the initial 20-30 min of whole-cell voltage-clamp recording, the time course of the agonist-induced desensitization was markedly facilitated. Addition of 5 mM MgATP in the pipette solution at least partly prevented this facilitation of the desensitization. The frequency of activation of the nicotinic receptor and the extracellular Ca2+ were not primary factors in the acceleration of rate of desensitization.

Spatial and dynamic changes in intracellular Ca2+ measured by confocal laser-scanning microscopy in bullfrog sympathetic ganglion cells

Confocal laser scanning microscopy (CLSM) was used to record spatial and dynamic changes in the intracellular Ca2+ [(Ca2+]i) of bullfrog sympathetic ganglion cells in excised tissue or in culture. A CLSM utilizing Ar ion laser (488 nm) and recording fluo-3 fluorescence yielded the sliced image of ganglion cells, while conventional epifluorescence microscopy provided the cell image of a convex structure. A high K+ (50 mM) solution, caffeine (3-10 mM) and electrical stimulation (10-20 Hz, 0.5-10 s) caused a homogeneous increase in fluo-3 fluorescence with or without regional differences, possibly due to intracellular organelles and other constituents. Scanning a single horizontal line across the cytoplasm with He-Cd laser (325 nm) and recording indo-1 fluorescence demonstrated that the rate of rise in [Ca2+]i following action potentials depends on the distance from the cell membrane and on the cytoplasmic constituents, showing an inward spread of 'Ca(2+)-wave' at variable speeds of 17-219 microns/s. These results suggest that heterogeneity of the cytoplasmic structures and constituents affects dynamic and spatial changes of [Ca2+]i in response to stimuli in neurones. Such heterogenic changes in [Ca2+]i would better be studied by CLSM.

Acetylcholine-evoked currents in cultured neurones dissociated from rat parasympathetic cardiac ganglia

1. The properties of acetylcholine (ACh)-activated ion channels of parasympathetic neurones from neonatal rat cardiac ganglia grown in tissue culture were examined using patch clamp recording techniques. Membrane currents evoked by ACh were mimicked by nicotine, attenuated by neuronal bungarotoxin, and unaffected by atropine, suggesting that the ACh-induced currents are mediated by nicotinic receptor activation. 2. The current-voltage (I-V) relationship for whole-cell ACh-evoked currents exhibited strong inward rectification and a reversal (zero current) potential of -3 mV (NaCl outside, CsCl inside). The rectification was not alleviated by changing the main permeant cation or by removal of divalent cations from the intracellular or extracellular solutions. Unitary ACh-activated currents exhibited a linear I-V relationship with slope conductances of 32 pS in cell-attached membrane patches and 38 pS in excised membrane patches with symmetrical CsCl solutions. 3. Acetylcholine-induced currents were reversibly inhibited in a dose-dependent manner by the ganglionic antagonists, mecamylamine (Kd = 37 nM) and hexamethonium (IC50 approximately 1 microM), as well as by the neuromuscular relaxant, d-tubocurarine (Kd = 3 microM). Inhibition of ACh-evoked currents by hexamethonium could not be described by a simple blocking model for drug-receptor interaction. 4. The amplitude of the ionic current through the open channel was dependent on the extracellular Na+ concentration. The direction of the shift in reversal potential upon replacement of NaCl by mannitol indicates that the neuronal nicotinic receptor channel is cation selective and the magnitude suggests a high cation to anion permeability ratio. The cation permeability (PX/PNa) followed the ionic selectivity sequence Cs+ (1.06) greater than Na+ (1.0) greater than Ca2+ (0.93). Anion substitution experiments showed a relative anion permeability, PCl/PNa less than or equal to 0.05. 5. The nicotinic ACh-activated channels described mediate the responses of postganglionic parasympathetic neurones of the mammalian heart to vagal stimulation.

A muscarine-activated voltage-independent K+ channel in cultured bullfrog sympathetic neurones

In cultured bullfrog sympathetic neurones, cell-attached patch clamp revealed a voltage-independent K+ channel having a conductance of 46-113 pS at an external K+ of 2-120 mM. Muscarine (10-20 microM), applied to the cell membrane outside a recording pipette, increased its open probability and mean open time in an atropine-sensitive manner. This muscarine-activated K+ channel could underlie some of the muscarinic inhibitory postsynaptic potentials in both central and peripheral nervous systems.

Nicotinic acetylcholine receptor-ion channels involved in synaptic currents in bullfrog sympathetic ganglion cells and effects of atropine

The nicotinic acetylcholine receptor-ion channels (AChR channels) of the bullfrog sympathetic ganglion cells were studied with a two-electrode voltage clamp technique. The decay phase of the fast excitatory postsynaptic current (fast e.p.s.c.) in B-type neurones followed a double exponential function whose time constants were 3.2 and 8.0 ms at -60 mV and increased with membrane hyperpolarization. Likewise, the decay phase of the fast e.p.s.c. in C-type neurones was double-exponential with time constants of 4.4 and 12.3 ms (at -60 mV). The miniature e.p.s.c. in B-type neurones also decayed with a double exponential function (2.7 and 15.4 ms at -100 mV). Analysis of acetylcholine-induced current fluctuations revealed the power spectral density distribution of a double Lorentzian function which yielded the time constants of elementary events [tau noise(f) and tau noise(s): 1.7 and 29.7 ms, respectively, at -100 mV] and the averaged elementary conductance (gamma: 7.8 pS). The amplitude of fast e.p.s.c. and the time constant of the fast component of its decay phase decreased during the initial ("acute") phase (within 15 min) of the action of atropine (3 microM), but recovered during the later ("chronic") phase (more than 30 min after application) of the action. The slow component was affected by atropine in a manner similar to the fast component during the "acute" phase. During the "chronic phase", however, the slow time constant recovered and exceeded the control value. Furthermore, this prolongation remained for at least 1 h after the removal of atropine.(ABSTRACT TRUNCATED AT 250 WORDS)