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

Two components of transmitter release from the chick ciliary presynaptic terminal and their regulation by protein kinase C

1. A study was made of the effects of phorbol ester (phorbol 12-myristate 13-acetate, PMA, 0.1 microM) on the two components of evoked transmitter release, namely the fast synchronous and the slow asynchronous components, from the giant presynaptic terminal of the chick ciliary ganglion. The excitatory postsynaptic currents (EPSCs) were recorded under whole-cell voltage clamp of the postsynaptic neuron. 2. The decay time constant of the slow component was prolonged by replacing Ca2+ with Sr2+. In 5 mM [Sr2+]o the fast component decayed with a time constant of 2.6 +/- 1.4 ms whereas the slow component decayed with a time constant of 19 +/- 7 ms. 3. When stimulated with twin pulses with a short interpulse interval, the fast component of the second EPSC was often depressed whereas the slow component was usually facilitated. Both components were positively dependent on [Sr2+]o in a saturable manner, but the fast component approached its maximum at a lower [Sr2+]o than the slow component. 4. PMA potentiated both the fast and slow components to a similar extent and with a similar time course. For each component, the effect of PMA was less potent at high [Sr2+]o than at low [Sr2+]o. For either the fast or the slow component the PMA-induced potentiation was accompanied by a reduction in the paired-pulse ratio (PPR). 5. Despite the different dissociation constant for dextran-conjugated fura-2, the fluorescent ratio for intraterminal [Sr2+] ([Sr2+]i) decayed to the baseline after the nerve-evoked increment with a time course similar to that for [Ca2+]i, suggesting that intraterminal Sr2+ is buffered less efficiently than Ca2+. PMA did not increase the [Sr2+]i transients produced by stimulation of the presynaptic oculomotor nerve. 6. It is suggested that protein kinase C (PKC) modulates both the fast and slow components through common molecular mechanisms that upregulate the Sr2+ sensitivity of the vesicle fusion probability.

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

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

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

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

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

To investigate the presence and the role of intracellular Ca2+ stores in chick ciliary ganglion cells, the concentration of cytosolic free Ca2+ ([Ca]in) was measured in acutely isolated neurons, using fura-2 microfluorometry. Caffeine caused a substantial increase in [Ca]in following or during high K+ depolarization; this response was inhibited by treatment of the cells with thapsigargin or with caffeine plus ryanodine. The peak value and the rate of the depolarization-induced [Ca]in increase were not much altered by either of these treatments, which deplete caffeine-sensitive Ca2+ stores. The muscarinic receptor agonists muscarine, oxotremorine M, and methacholine, caused substantial increases in [Ca]in, in a manner that was partially dependent on Ca2+. These agonists also caused a rise in [Ca]in during K+ depolarization, which rise was inhibited by treatment with thapsigargin or with caffeine plus ryanodine. The response to oxotremorine M during depolarization was strongly inhibited by 10 nM 4-DAMP, but was not inhibited by 1 microM pirenzepine or by 1 microM AF-DX 116. These results indicate that chick ciliary ganglion cells possess Ca2+ stores that are activated by both caffeine and a second messenger generated by the activation of the M3 muscarinic receptor subtype.

Activation of nicotinic acetylcholine receptors on cultured Drosophila and other insect neurones

1. Using whole-cell and single channel recordings, we have examined the properties of acetylcholine (ACh)-activated currents in neurones from larval and pupal Drosophila melanogaster (fruit fly), larval and embryonic Musca domestica (house fly), and nymphal Schistocerca gregaria (locust). 2. In all preparations, single channel recordings revealed two major classes of ACh-activated channels, with average conductances of approximately 32 and 59 pS. 3. At ACh concentrations from 1 to 10 microM, channel activity in Drosophila larval neurones occurs in bursts with an average of 1-2 openings. Open times and burst durations are described by one or two exponentials. Burst durations for the 32 pS channel (approximately 3 ms, slow component) were longer than those for the 59 pS channel (approximately 1.0 ms). The mean open interval duration for the 32 pS channel (slow component) was also longer than that of the 59 pS channel. 4. At high ACh (20-200 microM) concentrations, bursts of the smaller conductance channel occur in clusters separated by long-lived periods without channel activity. Considerable kinetic heterogeneity was observed among clusters. 5. The whole-cell dose-response curve suggests that activation of current by ACh increases up to at least 100 microM and that multiple ligand binding steps are involved. 6. Drosophila and Musca larval neuronal ACh-activated channels show some unique features in their cholinergic pharmacological properties: (a) they are only weakly activated by the potent neuromuscular nicotinic agonist suberyldicholine, (b) hexamethonium and decamethonium are weak, but approximately equi-effective blockers, and (c) alpha- and kappa-bungarotoxin (BTX) both blocked reversibly, though alpha-BTX appears to be the more potent inhibitor.

A study of the bovine adrenal chromaffin nicotinic receptor using patch clamp and concentration-jump techniques

1. Voltage clamp records have been obtained from bovine adrenal chromaffin cells in the outside-out and whole-cell configurations, in response to step changes of acetylcholine (ACh) concentration. The concentrations used ranged from 50 nM to 20 mM. 2. At high acetylcholine concentrations, the activation and desensitization kinetics of the nicotinic receptor, as observed in outside-out patches, may be described by a model incorporating a single, fast agonist binding step, and relatively slow isomerization to the open state. The affinity of the closed receptor for ACh is 310 microM, the channel opening rate constant is 460 s-1, and the closing rate constant is 29 s-1. 3. Single channel events, observed when nanomolar ACh concentrations are applied to whole cells, have two distinct channel lifetimes: 0.6 ms and 11-15 ms. The variation of the frequencies of the events with ACh concentration, suggests that the short lifetimes are openings of a singly liganded receptor and the longer lifetimes are openings of a doubly liganded receptor. 4. Only a single exponential associated with receptor desensitization is seen with outside-out patches, but two are seen with whole cells. It is postulated that there are two nicotinic receptor types present on adrenal chromaffin cells. 5. The rate of desensitization (9 s-1 and 26 s-1, whole cells; 24 s-1, patches), is fast enough to be significant in determining the open channel lifetime. 6. A sudden increase in current (rebound) is observed when a high concentration of ACh is abruptly removed from outside-out patches. This is evidence for a blocked state. The affinity of the blocking site for ACh is 1400 microM (outside-out patches). 7. The total number of activatable nicotinic channels per whole cell is estimated to be 2600.

Changes in the electrical properties of chick ciliary ganglion neurones during embryonic development

1. Whole-cell recording techniques were used to examine the expression of ionic currents in chick ciliary ganglion neurones dissociated acutely at various stages of embryonic development. Currents were also examined in dissociated cells that had been maintained in vitro for several days. 2. Voltage-activated, tetrodotoxin (TTX)-sensitive Na+ currents (INa) could be detected in all cells tested between stage 25 and stage 40 (embryonic days 4.5-14). INa increased in both amplitude and density throughout development, but no obvious changes in kinetics or sensitivity to TTX were observed. 3. High-threshold Ca2+ currents (ICa) were also detectable between stage 25 and stage 40. ICa increased in both amplitude and density throughout this time. No obvious changes in kinetics or voltage dependence were observed. 4. Delayed rectifier K+ currents (IDR) and A-currents (IA) could be detected in Ca(2+)-free salines, and distinguished on the basis of differences in kinetics, voltage dependence, and sensitivity to tetraethylammonium (TEA). IA was either absent, or present at very low densities at stages 26-30, but showed a sharp increase in density thereafter. In contrast, IDR was detectable as early as stage 25, and did not display a significant increase in density during development. 5. Ca(2+)-activated K+ currents (IK(Ca)) were either undetectable or present at very low density between stage 26 and stage 30 (embryonic days 5-9) but showed a large increase in amplitude and density thereafter. 6. Ionic currents were examined in age-matched cells dissociated acutely on embryonic day 13, or isolated on embryonic day 9 and maintained in vitro for an additional 4 days. Most of the cells maintained in culture for 4 days did not express detectable IK(Ca), and had significantly reduced IA compared to acutely isolated controls. The cultured cells expressed normal densities of IDR, ICa and INa. 7. All ionic currents increased in amplitude during normal embryonic development, and all but IDR increased in density. The largest change in density generally occurred between stages 30 and 40, during which time ciliary ganglion neurones form synapses with target tissues. 8. Isolation of ciliary neurones from the in ovo environment prevented the normal development of IA and IK(Ca), suggesting that the expression of these channels is controlled by one or more extrinsic environmental factors. In contrast, the normal expression of INa, ICa and IDR is not dependent upon extrinsic factors.

Calcium permeability and modulation of nicotinic acetylcholine receptor-channels in rat parasympathetic neurons

Neuronal nicotinic acetylcholine (ACh)-activated currents in rat parasympathetic ganglion cells were examined using whole-cell and single-channel patch clamp recording techniques. The whole-cell current-voltage (I-V) relationship exhibited strong inward rectification and a reversal (zero current) potential of -3.9 mV in nearly symmetrical Na+ solutions (external 140 mM Na+/internal 160 mM Na+). Isosmotic replacement of extracellular Na+ with either Ca2+ or Mg2+ yielded the permeability (Px/PNa) sequence Mg2+ (1.1) > Na+ (1.0) > Ca2+ (0.65). Whole-cell ACh-induced current amplitude decreased as [Ca2+]0 was raised from 2.5 mM to 20 mM, and remained constant at higher [Ca2+]0. Unitary ACh-activated currents recorded in excised outside-out patches had conductances ranging from 15-35 pS with at least three distinct conductance levels (33 pS, 26 pS, 19 pS) observed in most patches. The neuronal nicotinic ACh receptor-channel had a slope conductance of 30 pS in Na+ external solution, which decreased to 20 pS in isotonic Ca2+ and was unchanged by isosmotic replacement of Na+ with Mg2+. ACh-activated single channel currents had an apparent mean open time (tau 0) of 1.15 +/- 0.16 ms and a mean burst length (tau b) of 6.83 +/- 1.76 ms at -60 mV in Na+ external solution. Ca(2+)-free external solutions, or raising [Ca2+]0 to 50-100 mM decreased both the tau 0 and tau b of the nAChR channel. Varying [Ca2+]0 produced a marked decrease in NP0, while substitution of Mg2+ for Na+ increased NP0. These data suggest that activation of the neuronal nAChR channel permits a substantial Ca2+ influx which may modulate Ca(2+)-dependent ion channels and second messenger pathways to affect neuronal excitability in parasympathetic ganglia.

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.

Role of disulphide bonds in burst-like activity of nicotinic acetylcholine receptor channels in rat sympathetic neurones

1. The effects of reduction of disulphide bonds in nicotinic acetylcholine receptors (nicotinic AChRs) with dithiothreitol (DTT) were studied in rat superior cervical ganglion neurones using the patch-clamp method in whole-cell and cell-attached recording modes. 2. Dithiothreitol (1 mM) markedly reduced the ACh-induced membrane current, while the action of ACh remained reversible. Conversely, bromoacetylcholine (BrACh), if applied after the treatment with DTT, caused irreversible activation of nicotinic AChRs manifested in the appearance of a non-declined steady-state component in BrACh-induced currents accompanied by increased membrane current fluctuations. The successive reoxidation of sulphydryl groups by potassium ferricyanide (1 mM-ferricyanide) restored the response to ACh. Ferricyanide itself had a weaker inhibitory effect on the ACh-induced current, compared to the effect of DTT. 3. As a result of the action of DTT (1 mM), the spectrum of BrACh-induced current noise shifted to a higher frequency range. 4. The distributions of durations of the gaps (closed states) and the bursts (the states identified as open states after the shortest gaps were ignored) in single-channel activity of native (non-treated with DTT) nicotinic AChRs caused by ACh (30 microM) and BrACh (30 microM) were similar and both revealed four to five and two to three components for gap intervals and burst durations respectively. 5. Single-channel behaviour of reduced nicotinic AChRs was similar for both ACh and BrACh as agonists, but significantly differed from that in the native one. The first difference was the marked increase in the frequency of the appearance of long closed states of the channel that was presumably due to enhanced receptor desensitization. The second difference was an almost complete disappearance of long bursts associated with disappearance of the fastest component in gap interval distribution. 6. Mean conductance of single nicotinic AChR channels decreased by approximately 20% in the reduced receptor compared with that in the native one, for both agonists. 7. The results suggest a critical role of disulphide bonds for the functioning of native neuronal nicotinic AChRs: the disruption of disulphide bonds leads to the loss of burst-like kinetics of the nicotinic AChR ionic channel.

Functional properties and developmental regulation of nicotinic acetylcholine receptors on embryonic chicken sympathetic neurons

Measurement of acetylcholine (ACh)-induced currents indicates that the sensitivity of embryonic sympathetic neurons increases following innervation in vivo and in vitro. We have used single-channel recording to assess the contribution of changes in ACh receptor properties to this increase. Early in development (before synaptogenesis), we detect three classes of ACh-activated channels that differ in their conductance and kinetics. Molecular studies indicating a variety of neuronal receptor subunit clones suggest a similar diversity. Later in development (after innervation), changes in functional properties include increases in conductance and apparent mean open time, the addition of a new conductance class, as well as apparent clustering and segregation of channel types. These changes in channel function are compatible with the developmental increase in ACh sensitivity.

Changes in the number, function, and regulation of nicotinic acetylcholine receptors during neuronal development

Acetylcholine receptor (AChR) development was examined in neurons freshly isolated from chick ciliary ganglia. Between Embryonic Day 8 (E8) and 16, both the ACh response per unit membrane and the density of surface AChRs increased, while the apparent affinity for ACh decreased. AChRs had single-channel conductances of 25 and 40 pS. The distribution of single-channel events shifted during development; at E8 events of both conductances were equally rare, while by E14 there were many events and most were 40 pS. The open durations of 25 and 40 pS events had two mean values. The open lifetimes of the 25 pS events did not change between E8 and E14, while the lifetimes of the 40 pS events increased, and by E14 most were long. The ACh response of the neurons also became sensitive to regulation by a cAMP-dependent mechanism at about E10. The observed changes may reflect developmental control over processing required for receptor regulation and differential expression of AChR subtypes.

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

Nicotinic acetylcholine-receptor ion channels (AChR channels) were studied in bullfrog sympathetic ganglion cells cultured for 1 day to 3 weeks, using a patch clamp technique. Microsuperfusion of ACh (2-10 microM) to the ganglion cell under the whole cell clamp produced an inward current at membrane potentials negative to -60 mV, which had a fast onset and decay. This rapid ACh-induced current was accompanied by a large current fluctuation, decreased and increased in amplitude by membrane depolarization and hyperpolarization, respectively, and blocked by d-tubocurarine. Thus, this current must be induced by the nicotinic action of ACh, but not by a muscarinic effect to activate a slow cation-selective current. At depolarized levels more than -50 mV, ACh induced an additional inward current which was slow in time course, accompanied by no or decreased current fluctuation and increased in amplitude by membrane depolarization. Accordingly, this slow ACh-induced current could result from the suppression of a voltage-dependent K+ current (M-current: Brown and Adams 1980) by the muscarinic action of ACh. Fluctuation analysis of the rapid ACh-induced current at potentials negative to -50 mV revealed the elementary conductance of 14 pS and a power spectral density distribution of the double Lorentzian function which yielded the time constants of 5.4 and 62.5 ms at -60 to -80 mV. The variance of either component was independent of the mean current.

Electrophysiology of a chick neuronal nicotinic acetylcholine receptor expressed in Xenopus oocytes after cDNA injection

Brain nicotinic acetylcholine receptors (nAChRs) are made up of protein subunits that differ from those constituting muscle nAChRs. To characterize the physiological properties of one class of avian brain nicotinic receptor, we injected the nuclei of Xenopus oocytes with full-length cDNAs for the ligand binding (alpha 4) and structural (n alpha) subunits. Injected oocytes had large ACh-induced currents in the microampere range that were insensitive to alpha-bungarotoxin, as expected for neuronal nAChRs. We found that these brain nAChRs incorporate at least two alpha 4 subunits and that their functional properties differ from muscle nAChRs in at least two respects: the elementary conductance is considerably smaller (20 pS), and channels in outside out patches stop functioning within a few minutes.

The effects of L-glutamate on cultured insect neurones

The effects of L-glutamate on insect cultured neurones were studied under current and voltage-clamp conditions using conventional and whole-cell patch-clamp techniques. Brief pressure or iontophoretic application of L-glutamate produced either a depolarisation or hyperpolarisation. The current underlying the depolarisation was inwardly directed and reversed at around 0 mV while the hyperpolarisation was caused by an outward current that reversed between -60 and -80 mV. Single channel currents underlying the depolarisation were readily recorded from cell attached patches and showed multiple conductance states. Channel activity corresponding to the hyperpolarising response has not yet been observed.

Single-channel and whole-cell currents evoked by acetylcholine in dissociated sympathetic neurons of the rat

Single acetylcholine-activated channels have been recorded from neurons dissociated from the sympathetic chain of 17-21 day old rats. The mean single channel conductance is 35 pS in normal medium containing 1 mM calcium, and 51 pS in the absence of calcium. The measured current amplitudes are about five times more variable than at the frog endplate, at least in part because the current, while the channel is open, is much noisier than when it is shut. Single activations of the receptor by acetylcholine (ACh) produce a burst of openings; the distribution of the burst length has two components, the longer of which is of primary importance in synaptic transmission. Whole-cell currents, in response to ACh (up to 30 microM), show strong inward rectification with no outward current being detectable. This phenomenon is similar whether the intracellular ion is sodium or cesium, whether or not divalent cations are present, and whether or not atropine is present. Nevertheless, outward single-channel currents (of normal conductance) are detectable in isolated outside-out patches.

Cyclic AMP regulates the proportion of functional acetylcholine receptors on chicken ciliary ganglion neurons

Previous studies have shown that the number of functional acetylcholine receptors (AcChoRs) on chicken ciliary ganglion neurons in culture is considerably smaller than the total number of AcChoRs detected on the neurons by labeled receptor probes. Here we use patch-clamp recording to show that a cAMP-dependent process enhances the AcCho response of the neurons by a mechanism likely to involve an increase in the number of functional AcChoRs. The increase occurs without requiring protein synthesis and without involving a detectable increase in the total number of AcChoRs on the cell surface measured with a labeled receptor probe. The results imply that the neurons have functional and nonfunctional pools of AcChoRs and that functional receptors can be recruited from intracellular receptors or from nonfunctional receptors on the cell surface by a cAMP-dependent process. A cAMP-dependent regulation of the number of functional neurotransmitter receptors would provide a reversible mechanism by which cell-cell interactions could modulate synaptic transmission in the nervous system.

Neural nicotinic acetylcholine responses in solitary mammalian retinal ganglion cells

Using the patch-clamp technique, whole-cell recordings from solitary rat retinal ganglion cells in culture have established the nicotinic nature of the acetylcholine responses in these central neurons. Currents produced by acetylcholine (5-20 mumol/l) or nicotine (5-20 mumol/l) reversed in polarity near -5 mV and were unaffected by atropine (10 mumol/l). Agonist-induced currents were blocked by low doses (2-10 mumol/l) of the classical 'ganglionic' antagonists hexamethonium and mecamylamine, as well as by d-tubocurarine and dihydro-beta-erythroidine (the latter two do not discriminate clearly between ganglionic and neuromuscular junction receptors). Treatment with the potent neuromuscular blocking agent alpha-bungarotoxin (10 mumol/l) did not affect the cholinergic responses of these cells, while toxin F (0.2 mumol/l), a neural nicotinic receptor antagonist, readily abolished acetylcholine-induced currents. Thus, the experiments performed to date show that the nicotinic responses of retinal ganglion cells in the central nervous system share the pharmacology of autonomic ganglion cells in the peripheral nervous system. The ionic current carried by the nicotinic channels was selective for cations, similar to that described for nicotinic channels in other tissues. In addition, single-channel currents elicited by acetylcholine were observed in whole-cell recordings with seals greater than 5 G omega as well as in occasional outside-out patches of membrane. These acetylcholine-activated events, which had a unitary conductance of 48 pS and a reversal potential of 0 mV, represent the ion channels that mediate the neural nicotinic responses observed in these experiments on retinal ganglion cells.

Dissociation and maintenance in vitro of neurones from adult cockroach (Periplaneta americana) and housefly (Musca domestica)

A method is described for dispersal and subsequent maintenance in acute culture of neurones dissociated from ganglia of the adult cockroach (Periplaneta americana) and brain of the adult fly (Musca domestica). The effects of different media on the survival and attachment to different substrates have been investigated. About 25% of dissociated neurones survived well for periods greater than 6 days in a cockroach saline supplemented with foetal calf serum and the antibiotics fungizone and gentamicin. Fly neurones were more difficult to disperse. Fly saline supplemented with foetal calf serum and the antibiotics fungizone and gentamicin proved a successful short-term culture medium. Survival times were shorter, most neurones dying after 5 days. The demonstrated viability of such neurones has considerable potential for insecticide mode of action studies using the patch-clamp technique.

Single channels activated by acetylcholine in rat superior cervical ganglion

1. The elementary currents flowing through single channels opened by acetylcholine were recorded in rat superior cervical ganglion neurones using patch-clamp methods. Acetylcholine (30 microM) was included in the patch electrode (cell-attached recordings) or applied by ionophoresis (outside-out configuration). All measurements were made at 23-25 degrees C and mostly at -110 mV. 2. Channel openings appeared both as single events and as bursts of events. One population of the currents observed had a conductance of 20.0 +/- 0.2 pS (mean +/- S.E. of mean, n = 4). A second population had a conductance of about 50 pS, occurred more rarely, and was not included in further analysis. 3. Four channel closed time periods and two channel open time periods were found from the distributions of closed and open times. It was found that shorter channel openings (about 0.2 ms) appeared in isolation, whereas longer openings (duration 1.3 +/- 0.2 ms, n = 4) appeared as bursts of openings separated by the shortest channel closed time periods (about 0.15 ms). The next shortest closed time (about 2 ms) apparently corresponds to the lifetime of the channel not activated by acetylcholine. The two longer closed times (about 80 ms and 1 s) may reflect desensitization. The mean burst duration was 8.5 +/- 1.2 ms (n = 4), giving about six openings per burst. 4. Because the time constant of decay of the excitatory post-synaptic current is more similar to the burst duration than to the duration of individual single openings, it is suggested that acetylcholine released from presynaptic nerves may result in a burst of openings rather than a single opening. 5. On the basis of the above assumption, the rate constants were calculated for a sequential model in which acetylcholine binds to the receptor (forward rate k + 1 = 2.3 X 10(7) M-1 s-1; reverse rate k-1 = 1235 s-1) which then undergoes a conformational change to an open state (forward rate beta = 6293 s-1; reverse rate alpha = 894 s-1). 6. When heptamethonium (30 microM) was added to the solution in the patch electrode, the burst duration was markedly shortened, but there was no change in the closed time between two openings within the burst. This effect was voltage-dependent, which suggests that heptamethonium binds to the channel after it is opened by acetylcholine.

Analysis of quantal content and quantal conductance in two populations of neurons in the avian ciliary ganglion

The avian ciliary ganglion contains two populations of parasympathetic cells, termed the ciliary and choroid neurons. We have estimated the quantal contents of nicotinic excitatory postsynaptic potentials in both populations of neurons by several methods. The singly innervated ciliary neurons have quantal contents of 15-30. In contrast, the multiply innervated choroid cells have quantal contents of 4-7. Quantal conductance was also determined, using a parallel conductance model which takes into account the capacitance of the cell membrane. This analysis indicates that in both populations of neurons one quantum activates approximately 100 postsynaptic receptors. It is concluded that in autonomic ganglia singly innervated cells demonstrate a larger quantal content, consistent with a higher safety factor for neurotransmission, while quantal content in multiply innervated cells is generally much lower, allowing for considerable summation of presynaptic inputs. Further, in autonomic neurons many fewer postsynaptic receptors are activated by a single quantum than is the case at the neuromuscular junction.

Glutamate and aspartate activated channels and inhibitory synaptic currents in large cerebellar neurons grown in culture

Patch-clamp methods were used to examine large (greater than 30 microns) cerebellar neurons of the rat, maintained in cell culture. Cells possessed voltage-activated transient inward Na+ currents which were sensitive to tetrodotoxin. Spontaneous synaptic currents, present in whole-cell recordings, were abolished by bicuculline and picrotoxin and were carried by Cl-. Cells produced inward currents in response to the transmitter candidates glutamate and aspartate and also to the glutamate agonists kainate, quisqualate, N-methyl-D-aspartate and ibotenate. Analysis of glutamate and aspartate-current noise has been used to derive characteristics of the excitatory channels. Single channel currents have also been observed directly in whole-cell and outside-out patches. Both glutamate and aspartate are able to activate channels which were blocked by Mg2+ and had a maximum conductance of 50 pS.

Acetylcholine-activated currents in mouse neuroblastoma cells

The nicotine and muscarinic responses of differentiated mouse neuroblastoma cells from the clonal line N1E 115 to applied cholinergic agents were recorded using single channel and whole cell patch clamp techniques. An inward macroscopic current induced by acetylcholine (ACh) at the resting potential was blocked by curare; cell-attached recordings revealed a single channel conductance of 18 pS and a lifetime of 36 ms at 30 degrees C, with 200 nM ACh. The zero current potential was close to 0 mV. The kinetics of these nicotinic currents were described by multiexponential functions for both the open and closed time distributions. An outward single channel current, present at resting and slightly depolarized potentials, was also observed and has been tentatively described as being dependent on muscarinic receptor activation, as it was usually blocked by atropine. Under our conditions of whole cell clamp, no macroscopic outward current sensitive to ACh was observed.

Ion channels activated by acetylcholine and gamma-aminobutyric acid in freshly dissociated sympathetic neurones of the rat

Transmitter-activated channels in freshly isolated neurones from sympathetic ganglia of young rats have been examined using the patch-clamp technique. Acetylcholine (ACh), gamma-aminobutyric acid (GABA) and (in about one third of cells) glycine produced an inward current and an increase in current noise when perfused onto voltage-clamped neurones. The ACh noise was fitted with a single Lorentzian spectrum with a time constant of 13.1 ms at -70 mV. Outside-out membrane patches allowed high-resolution measurements of single ACh- and GABA-activated channels. The ACh-channels had a conductance of 30 pS.

A model for the acetylcholine binding site of the nicotinic acetylcholine receptor

A detailed model for the acetylcholine binding site on the nicotinic acetylcholine receptor is proposed. It is derived from assumptions based on existing biochemical, structural, and pharmacological data, combined with molecular modeling and principles of protein evolution and architecture. Acetylcholine is proposed to fit into a pocket on one face of an antiparallel beta-pleated sheet formed by residues 128-142 on the alpha-subunit. This sheet is flexible yet stable, in part because of a double cystine bridge at its end. Asp138, Thr133, and Gln140 provide a ring of negative charges around the quaternary ammonium group of acetylcholine, Ile131 and alkane segments of the other residues in the binding site provide hydrophobic interactions, and Gln140 provides a hydrogen bond for acetylcholine's carbonyl group; Glu129 would form part of the second anionic subsite for the bis-quaternary ammonium compounds and curares. The model is compatible with the available evidence pertaining to the binding site and with structure-activity relationship studies. It is precise and detailed, thereby making clear predictions, which are directly testable by affinity labeling and site-directed mutagenesis. It should prove useful in the design of such experiments.

Single acetylcholine channel currents in sympathetic neurons

Single acetylcholine (ACh) channel currents were studied by the gigaohm patch-clamp technique in cultured sympathetic neurons of the bullfrog, Rana catesbeiana. Recordings were made at 22 degrees C on cell-attached and excised membrane patches. When ACh (0.5-1 microM) was present in the pipette, a single class of inward currents was observed with a chord conductance of 30 pS and a reversal potential of -2 mV. The mean channel open time was 11.6 ms at -65 mV and showed little or no voltage-dependence over the range -85 to -45 mV. These channels appear to mediate the fast nicotinic excitatory postsynaptic current.

Melatonin lowers excitability of guinea pig hippocampal neurons in vitro

Action of melatonin (N-acetyl-5-methoxytryptamine; MEL) on guinea pig hippocampal cells (CA3 neurons and dentate granule cells) were studied in vitro using both extra- and intracellular recording. MEL (1-10 mmol/1) had the following effects: Response to repetitive synaptic stimulation was changed drastically: Double shock facilitation (20 ms interval) turned into depression and stimulus trains of a frequency as low as 1 Hz led to a drastic reduction of the response. Membrane potential was hyperpolarized. Duration of action potential was strongly increased. Threshold for the triggering of action potentials was shifted to more positive levels. IPSPs were prolonged and their shunting power enhanced. Repetitive spiking elicited by the application of bicuculline was reversibly abolished. All these effects had in common that cell excitability was lowered. It is concluded that MEL might influence epileptic seizure activity and should be further investigated as potential anticonvulsant.

Ion channels activated by L-glutamate and GABA in cultured cerebellar neurons of the rat

Glutamate and GABA-receptor channels were investigated in explants of rat cerebellum grown in cell culture. The patch-clamp technique was used to examine neurons under whole cell clamp and the properties of channels were derived by analysis of glutamate and GABA-evoked current noise. In addition, single channel currents activated by glutamate were recorded from isolated outside-out patches of membrane. We found evidence for at least two types of glutamate receptor-channels in cerebellar cells. Some neurons exhibited a channel of 50 pS conductance with a Lorentzian noise spectrum of 5.9 ms time constant. Single channels were readily resolved both in whole cell clamp and excised patches. Other neurons possessed low conductance channels which produced two component spectra. Estimates of the single channel conductance gave a value of about 140 fS. GABA channel noise obtained from these cells was also fitted by two component spectra which gave single channel conductance of 16 pS.

Some properties of acetylcholine receptors in human cultured myotubes

The distribution and single channel properties of acetylcholine (ACh) receptors in human myotubes grown in tissue culture have been examined. Radioautography of myotubes labelled with [125I]alpha-bungarotoxin showed that ACh receptors are distributed uniformly over the myotube surface at a density of 3.9 +/- 0.5 receptors per square micrometre. Accumulations of ACh receptors (hot spots) were found rarely. The conductance and kinetics of ACh-activated channels were investigated with the patch-clamp technique. Cell-attached membrane patches were used in all experiments. A single channel conductance in the range 40-45 pS was calculated. No sublevels of conductance (substates) of the activated channel were observed. The distribution of channel open-times varied with ACh concentration. With 100 nM ACh, the distribution was best fitted by the sum of two exponentials, whereas with 1 microM ACh a single exponential could be fitted. The mean channel open-time at the myotube resting potential (ca. -70 mV, 22 degrees C) was 8.2 ms. The distribution of channel closed-times was complex at all concentrations of ACh studied (100 nM to 10 microM). With desensitizing doses of ACh (10 microM), channel openings occurred in obvious bursts; each burst usually appeared as part of a 'cluster' of bursts. Both burst duration and mean interval between bursts increased with membrane hyperpolarization. Individual channel open-times and burst durations showed similar voltage dependence (e-fold increase per 80 mV hyperpolarization), whereas both the channel closed-times within a burst and the number of openings per burst were independent of membrane potential.

A calcium-activated cation-selective channel in rat cultured Schwann cells

Calcium-activated channels, in the plasma membrane of rat cultured Schwann cells were studied in isolated 'inside-out' membrane patches. With identical (150 mM NaCl) solutions on either side of the membrane, a single channel conductance of 32 pS was calculated for inward current; the conductance was somewhat less for outward current. The channel is about equally permeable to sodium and potassium ions, but is not detectably permeable to either chloride or calcium. Under our experimental conditions the channel is activated by high (more than 10(-4) M) concentrations of calcium and is sensitive to voltage, channel activity increasing with membrane depolarization.

High conductance anion-selective channels in rat cultured Schwann cells

Anion-selective channels, with very large single unit conductances, are present in the cell membrane of rat cultured Schwann cells measured with the patch-clamp technique. In inside-out membrane patches, channels with a conductance of about 450 pS (in symmetrical 150 mM NaCl) were observed. These channels did not become active until several minutes after the cytoplasmic surface had been exposed to the bathing medium, suggesting that these channels may normally be kept in an inactive state by some as yet unknown internal factor. The channel opened over a relatively small potential range (-10 mV to +20 mV) and closed rapidly at more positive and more negative potentials with voltage-dependent kinetics. Although the channels showed a slight permeability towards small cations the major permeability was to anions. The order of permeability was: I greater than Br greater than Cl greater than methyl SO4 greater than SO4 greater than acetate = isethionate. Aspartate and glutamate were not detectably permeant. The physiological role of these channels remains unknown.

The culture of rat submandibular ganglion neurons and their macroscopic responses to acetylcholine

The culture of rat submandibular ganglion cells is described. The neurons can be distinguished from the non-neuronal cells in the cultures by their morphology. Recording with whole-cell voltage-clamp techniques indicates that the neurons have resting potentials of about -55 mV and that the kinetics of the ionic channels opened by locally perfused acetylcholine (ACh) are very similar to those previously observed in adult submandibular ganglion neurons. The major differences observed are that the recorded cell input impedances are much higher than those recorded with microelectrodes from adult neurons and that the sensitivity of the cultured neurons to ACh is much less than that of the adult neurons. Whether the latter is due to changed receptor properties or to the presence of fewer receptors is not known.