Rectification of synaptic and acetylcholine currents in the mouse submandibular ganglion cells

Imaging in vivo acetylcholine release in the peripheral nervous system with a fluorescent nanosensor

The ability to monitor the release of neurotransmitters during synaptic transmission would significantly impact the diagnosis and treatment of neurological diseases. Here, we present a DNA-based enzymatic nanosensor for quantitative detection of acetylcholine (ACh) in the peripheral nervous system of living mice. ACh nanosensors consist of DNA as a scaffold, acetylcholinesterase as a recognition component, pH-sensitive fluorophores as signal generators, and α-bungarotoxin as a targeting moiety. We demonstrate the utility of the nanosensors in the submandibular ganglia of living mice to sensitively detect ACh ranging from 0.228 to 358 μM. In addition, the sensor response upon electrical stimulation of the efferent nerve is dose dependent, reversible, and we observe a reduction of ∼76% in sensor signal upon pharmacological inhibition of ACh release. Equipped with an advanced imaging processing tool, we further spatially resolve ACh signal propagation on the tissue level. Our platform enables sensitive measurement and mapping of ACh transmission in the peripheral nervous system.

Functional and Molecular Analysis of Proprioceptive Sensory Neuron Excitability in Mice

Neurons located in dorsal root ganglia (DRG) are crucial for transmitting peripheral sensations such as proprioception, touch, temperature, and nociception to the spinal cord before propagating these signals to higher brain structures. To date, difficulty in identifying modality-specific DRG neurons has limited our ability to study specific populations in detail. As the calcium-binding protein parvalbumin (PV) is a neurochemical marker for proprioceptive DRG cells we used a transgenic mouse line expressing green fluorescent protein (GFP) in PV positive DRGs, to study the functional and molecular properties of putative proprioceptive neurons. Immunolabeled DRGs showed a 100% overlap between GFP positive (GFP+) and PV positive cells, confirming the PVeGFP mouse accurately labeled PV neurons. Targeted patch-clamp recording from isolated GFP+ and GFP negative (GFP−) neurons showed the passive membrane properties of the two groups were similar, however, their active properties differed markedly. All GFP+ neurons fired a single spike in response to sustained current injection and their action potentials (APs) had faster rise times, lower thresholds and shorter half widths. A hyperpolarization-activated current (I_h) was observed in all GFP+ neurons but was infrequently noted in the GFP− population (100% vs. 11%). For GFP+ neurons, I_h activation rates varied markedly, suggesting differences in the underlying hyperpolarization-activated cyclic nucleotide-gated channel (HCN) subunit expression responsible for the current kinetics. Furthermore, quantitative polymerase chain reaction (qPCR) showed the HCN subunits 2, 1, and 4 mRNA (in that order) was more abundant in GFP+ neurons, while HCN 3 was more highly expressed in GFP− neurons. Likewise, immunolabeling confirmed HCN 1, 2, and 4 protein expression in GFP+ neurons. In summary, certain functional properties of GFP+ and GFP− cells differ markedly, providing evidence for modality-specific signaling between the two groups. However, the GFP+ DRG population demonstrates considerable internal heterogeneity when hyperpolarization-activated cyclic nucleotide-gated channel (HCN channel) properties and subunit expression are considered. We propose this heterogeneity reflects the existence of different peripheral receptors such as tendon organs, muscle spindles or mechanoreceptors in the putative proprioceptive neuron population.

Cholinergic EPSCs and their potentiation by bradykinin in single paratracheal ganglion neurons attached with presynaptic boutons

We have found that bradykinin (BK) potentiates the nicotine-induced currents in airway paratracheal/parabronchial ganglia (PTG) neurons. In this study, we investigated if BK affects the cholinergic synaptic transmission in rat PTG neurons attached with synaptic buttons. Excitatory postsynaptic currents (EPSCs) were recorded in acutely dissociated PTG neurons attached with presynaptic boutons. EPSC frequency was increased in the high-K(+) external solution without affecting their amplitude. Activation and deactivation kinetics also did not change in the high-K(+) solution. Cd(2+) inhibited the EPSC frequency at 10(-7) M and also amplitude at higher concentrations without changing the kinetics. Mecamylamine inhibited both the amplitude and frequency of EPSCs and reduced the activation and deactivation kinetics. 10(-8) M BK potentiated the EPSC amplitude to 1.37 ± 0.19 times of preapplication control. In addition, its frequency was increased to 2.04 ± 0.41 times. BK did not affect the activation and deactivation kinetics. The effects of BK were mimicked by [Hyp(3)]-BK, a B2 kinin receptor agonist, whereas HOE 140, a B2 kinin receptor antagonist, abolished the effects of BK. In conclusion, BK potentiates the cholinergic synaptic transmission via B2 kinin receptors in the PTG. Since predominant control of airway function is thought to be exerted by cholinergic nerves arising from the PTG, the present findings might underlie at least partly the inflammatory pathological conditions of the lower airway.

Ganglionic transmission in a vasomotor pathway studied in vivo

Intracellular recordings were made in vivo from 40 spontaneously active cells in the third lumbar sympathetic ganglion of urethane-anaesthetized rats. In 38/40 cells ongoing action potentials showed strong cardiac rhythmicity (93.4 +/- 1.9% modulation) indicating high barosensitivity and probable muscle vasoconstrictor (MVC) function. Subthreshold excitatory postsynaptic potentials (EPSPs) showed the same pattern. The 38 barosensitive neurons fired action potentials at 2.9 +/- 0.3 Hz. All action potentials were triggered by EPSPs, most of which were unitary events. Calculations indicated that <5% of action potentials were triggered by summation of otherwise subthreshold EPSPs. 'Dominant' synaptic inputs with a high safety factor were identified, confirming previous work. These were active in 24/38 cells and accounted for 32% of all action potentials; other ('secondary') inputs drove the remainder. Inputs (21 dominant, 19 secondary) attributed to single preganglionic neurons fired at 1.38 +/- 0.16 Hz. An average of two to three preganglionic neurons were estimated to drive each ganglion cell's action potentials. When cells were held hyperpolarized to block spiking, a range of spontaneous EPSP amplitudes was revealed. Threshold equivalent was defined as the membrane potential value that was exceeded by spontaneous EPSPs at the same frequency as the cell's original firing rate. In 10/12 cells examined, a continuum of EPSP amplitudes overlapped threshold equivalent. Small changes in cell excitability could therefore raise or lower the percentage of preganglionic inputs triggering action potentials. The results indicate that vasoconstrictor ganglion cells in vivo mostly behave not as 1:1 relays, but as continuously variable gates.

An improved method for patch clamp recording and calcium imaging of neurons in the intact dorsal root ganglion in rats

The properties of dorsal root ganglion (DRG) neurons have been mostly investigated in culture of dissociated cells, and it is uncertain whether these cells maintain the electrophysiological properties of the intact DRG neurons. Few attempts have been made to record from DRG neurons in the intact ganglion using the patch clamp technique. In this study, rat DRGs were dissected and incubated for at least 1h at 37 degrees C in collagenase (10mg/ml). We used oblique epi-illumination to visualize DRG neurons and perform patch clamp recordings. All DRG neurons exhibited strong delayed rectifier potassium current and a high threshold for spike generation (-15 mV) that rendered the cells very weakly excitable, generating only one action potential upon strong current injection (>300 pA). It is therefore possible that cultured DRG neurons, commonly used in studies of pain processing, may be hyperexcitable because they acquired "neuropathic" properties due to the injury induced by their dissociation. Electrical stimulation of the attached root produced an antidromic spike in the soma that could be blocked by intracellular hyperpolarization or high frequency stimulation. Imaging intracellular calcium concentration with Oregon Green BAPTA-1 indicates that antidromic stimulation caused a long-lasting increase in intracellular calcium concentration mostly near the cell membrane. This study describes a simple approach to examine the electrophysiological and pharmacological properties and intracellular calcium signaling in DRG neurons in the intact ganglion where the effects of somatic spike invasion can be studied as well.

Ca2+-dependent inward current induced by nicotinic receptor activation depends on Ca2+/calmodulin–CaMKII pathway in dopamine neurons

It is well known that midbrain dopamine (DA) neurons receive massive projection from cholinergic neurons in the brainstem. In our preceding report, we showed that Ca(2+)-influx through nicotinic acetylcholine (ACh) receptors in the DA neurons subsequently activated an inward current that was sensitive to fulfenamic acid (FFA) and phenytoin, presumably a Ca(2+)-activated non-selective cation current. The FFA-sensitive current exhibited a negative slope conductance and predominantly enhanced the depolarizing responses of DA neurons. In this study, we showed that the inward FFA-sensitive current was eliminated by antagonists of Ca(2+)/calmodulin (Ca(2+)/CaM), N-(6-aminohexyl)-5-chloro-1-naphthalene-sulfonamide hydrochloride (W-7; 1 microM), trifluoperazine (TFP; 1.5 microM) and calmidazolium (100 nM). Application of W-7 and TFP reduced the ACh-induced inward current and the current component suppressed by these drugs exhibited negative slope conductance, as well as the FFA-sensitive current. Further, intracellular application of KN-93, an antagonist of Ca(2+)/CaM-dependent protein kinase II (CaMKII), but not KN-92 eliminated the FFA-sensitive current. All these results suggest that Ca(2+)/CaM-CaMKII pathway is involved in an activation of the FFA-sensitive current.

Fulfenamic acid sensitive, Ca(2+)-dependent inward current induced by nicotinic acetylcholine receptors in dopamine neurons

Nicotinic acetylcholine receptors (nAChRs) exhibit high Ca(2+) permeabilities and the Ca(2+)-influx through the nAChRs may be involved in regulation of a variety of signal processing in the postsynaptic neurons. The mesencephalic dopamine (DA) neurons receive cholinergic inputs from the brainstem and express abundant nAChRs. Here we report that the Ca(2+)-influx induced by a transient pressure application of ACh activates an inward current mediated by nAChRs and subsequently an inward current component that is sensitive to fulfenamic acid (FFA) and phenytoin, presumably a Ca(2+)-activated nonselective cation current in the DA neurons in the midbrain slices of the rat. The FFA- and phenytoin-sensitive current exhibits a negative slope conductance below -40 mV, suggesting its role in significant enhancement of depolarizing responses. In the current clamp recordings with perforated patch clamp configuration, bath application of carbachol markedly enhanced the glutamate-induced depolarization, which led to a long-lasting depolarizing hump. Activation of nAChRs is involved in this process, in cooperation with muscarinic receptors that suppress afterhyperpolarization caused by Ca(2+)-activated K(+)-channels. The long-lasting depolarizing hump was suppressed by FFA. All these results suggested a potential role of the FFA-sensitive current triggered by nAChR activation in marked enhancement of the excitatory synaptic response in DA neurons.

Fast excitatory postsynaptic currents in neurons of the rabbit pelvic plexus

Fast excitatory postsynaptic currents have been recorded at 23-27 degrees C from rabbit pelvic plexus neurons by a two-electrode voltage-clamp technique. The synaptic current decay was bi-exponential with the fast and slow components characterized at -50 mV by mean time constants of 4.0 +/- 0.3 and 21.9 +/- 2.8 ms (n = 11), respectively. Both components contributed to the synaptic current approximately equally and reversed at -5 mV. Hexamethonium (10 microM) decreased the amplitude and decay time constant of both synaptic current components; this effect increased with hyperpolarization and is consistent with a channel-blocking action. At - 50 mV, mean rate constants of hexamethonium association with open ion channels of nicotinic acetylcholine receptors presumably mediating the fast and slow synaptic current components were (18.4 +/- 2.3) x 10(6) and (6.1 +/- 1.2) x 10(6) M(-1) s(-1) (n = 4), respectively. These data suggest that the fast excitatory postsynaptic current in rabbit pelvic plexus neurons is probably mediated by at least two different subtypes of nicotinic acetylcholine receptors. Hexamethonium blocks open ion channels of both subtypes with efficiency allowing to exclude an appreciable presence of homomeric alpha7 nicotinic acetylcholine receptors on the subsynaptic membrane.

Nicotinic acetylcholine receptors in autonomic ganglia

Although alpha3beta4 subunit combination is clearly prevalent in the nAChRs of autonomic ganglia neurons, the ganglia are strikingly different in the ratio of neurons containing each particular nAChR subunit, as found with immunohistochemical methods and from the analysis of the effects of nAChR subunit-specific antibodies on the ACh-induced membrane currents. In particular, the number of neurons containing alpha3, alpha4, alpha5 or alpha7 subunits is by about three times higher in sympathetic ganglia than in parasympathetic ganglia. This difference may explain why the parasympathetic and sympathetic ganglia markedly differ in their pharmacology. Still, alpha7 subunit makes the highest contribution to ACh-induced membrane current. No correlation between the physiological functions of the ganglia and subunit composition of their nAChRs has been found as yet. High permeability for Ca2+ should permit the nAChRs with alpha7 subunits to influence a variety of Ca2+-dependent events in autonomic neurons. As found with biochemical methods and site-directed mutagenesis, the ACh binding site is formed in the alpha/beta subunits interface by multiple loops containing cysteine, tyrosine and tryptophan amino residues as important for ACh binding. Likewise, both alpha and beta subunits are important for the effects of blocking agents on nAChRs. As found by electrophysiological methods, each neuron of sympathetic and parasympathetic ganglia, as a rule, possesses nAChRs of two groups, "fast" and "slow", with the mean duration of the burst of single channel openings ranging approximately from 5 to 10 and from 25 to 45 ms, respectively. These groups of channels differ from each other with their pharmacology. The burst-like activity of autonomic nAChRs channels is possible only if the disulfide bonds are left intact, otherwise only single openings of the channel are observed. The ionic channel of a nAChRs pentamer is formed by M2 transmembrane segments arranging glutamate, serine, threonine, leucine, and valine rings critical for channel conductance and ionic selectivity. In particular, the mutations V251T and E237A, and insertion of proline or alanine, convert a cation-selective channel into an anion-selective one. The open-channel blockers bind to the nAChR channel at the level where the channel diameter is nearly 12 A, both for "fast" and "slow" channel groups.

The subunit dominates the relaxation kinetics of heteromeric neuronal nicotinic receptors

The ACh-induced voltage-jump relaxation currents of the nicotinic receptors formed by pair-wise expression of the rat alpha2, alpha3, or alpha4 subunits with the beta2 or beta4 subunit in Xenopus oocytes were fitted best by the sum of two exponentials and a constant between -60 and -150 mV. As the ACh concentration approached zero, the relaxation time constants approached limiting values that should equal the single-channel burst duration at low ACh concentrations and the synaptic current decay time constants. beta4 co-expression prolonged the zero ACh concentration limits for the relaxation time constants. The fast beta4 zero ACh concentration limits ranged from 40 to 121 ms between -60 and -150 mV, and the slow beta4 zero ACh concentration limits ranged from 274 to 1039 ms. In contrast, the fast beta2 limits were 4-6 ms over the same voltage range and the slow beta2 limits were 30-53 ms. Expression with the beta4 subunit increased the voltage sensitivity of the alpha2, alpha3 and slow alpha4 relaxation time constants but not that of the fast alpha4 relaxation time constant. Reducing the temperature from 22 C to 8-9 C increased the alpha4beta2 and alpha3beta4 relaxation time constants 2.3- to 6.6-fold and reduced the fractional amplitude of the fast relaxation component. It also increased the voltage dependence of the fast alpha3beta4 relaxation time constant and decreased that of the slow time constant. The Q10 for alpha4beta2 and alpha3beta4 relaxation time constants ranged from 1.9 to 3.9 between 10 and 20 C. The beta subunit appears to have a dominant influence on the voltage-jump relaxation kinetics of heteromeric neuronal nicotinic receptors.

Nicotinic acetylcholine currents of cultured Kkenyon cells from the mushroom bodies of the honey bee Aapis mellifera

1. Acetylcholine-induced currents of mushroom body Kenyon cells from the honey bee Apis mellifera were studied using the whole-cell configuration of the patch clamp technique. Pressure application of 1 mM acetylcholine (ACh) induced inward currents with amplitudes between -5 and -500 pA. 2. The cholinergic agonists ACh and carbamylcholine had almost equal potencies of current activation at concentrations between 0.01 and 1 mM; nicotine was less potent. The muscarinic agonist oxotremorine did not elicit any currents. 3. Approximately 80 % of the ACh-induced current was irreversibly blocked by 1 microM alpha-bungarotoxin. Atropine (1 mM) did not block the ACh-induced current. 4. Upon prolonged ACh application the current desensitized with a time course that could be approximated by the sum of two exponentials (tau1 = 276 +/- 45 ms (mean +/- s.e.m. ) for the fast component and tau2 = 2.4 +/- 0.7 s for the slow component). 5. Noise analyses of whole-cell currents yielded elementary conductances of 19.5 +/- 2.4 pS for ACh and 23.7 +/- 5.0 pS for nicotine. The channel lifetimes, calculated from the frequency spectra, were tauo = 1.8 ms for ACh and tauo = 2.5 ms for nicotine. 6. Raising the external calcium concentration from 5 to 50 mM shifted the reversal potential of the ACh-induced current from +4. 6 +/- 0.9 to +37.3 +/- 1.3 mV. The calcium-to-sodium permeability ratio (PCa : PNa) was 6.4. 7. In high external calcium solution (50 mM) the ACh-induced current rectified in an outward direction at positive membrane potentials. 8. We conclude that Kenyon cells express nicotinic ACh receptors with functional profiles reminiscent of the vertebrate neuronal nicotinic ACh receptor subtype.

Two types of parasympathetic preganglionic neurones in the superior salivatory nucleus characterized electrophysiologically in slice preparations of neonatal rats

1. The electrophysiological properties of parasympathetic preganglionic neurones in the superior salivatory nucleus were studied in thin- and thick-slice preparations of rats aged 1 and 2 weeks using the whole-cell patch-clamp technique. 2. The superior salivatory neurones were identified by a retrograde tracing method with dextran-tetramethylrhodamine-lysine. The injection of the tracer into the chorda-lingual nerve labelled the neurones innervating the submandibular ganglia and those innervating the intra-lingual ganglia, while the injection into the tip of the tongue labelled the latter group of neurones. 3. Firing characteristics were investigated mainly in the neurones of 6-8 days postnatal rats. In response to an injection of long depolarizing current pulses at hyperpolarized membrane potentials (< -80 mV) under a current clamp, the neurones labelled from the nerve displayed a train of action potentials with either a long silent period preceding the first spike (late spiking pattern) or a long silent period interposed between the first and second spikes (interrupted spiking pattern). The neurones labelled from the tongue invariably displayed the interrupted spiking pattern. 4. Under a voltage clamp, among the neurones from 6-8 days postnatal rats, those labelled from the nerve expressed either a fast or a slow transient outward current (A-current), while those labelled from the tongue invariably showed a slow transient outward current. Both the fast and slow A-currents were largely depressed by 1 mM 4-aminopyridine. 5. Similar fast and slow A-currents were observed in the neurones of rats aged 14-15 days. Both the time to peak and decay time constant of these A-currents were accelerated, suggesting a developmental trend of maturation in the activation and inactivation kinetics between 6 and 15 days postnatal. 6. Based on the differences in the firing pattern and outward current, the superior salivatory neurones can be separated into two distinct types. We discuss the functional aspects of these two types of neurones with reference to their target organs.

Patch-clamp recordings of membrane currents evoked during natural synaptic activity in sympathetic neurons

Membrane currents elicited by colonic distension and by electrical stimulation of the intermesenteric nerve containing colonic afferent nerve fibres were recorded from neurons of the mouse superior mesenteric ganglion at 20 degrees C with the whole-cell patch-clamp method. Electrically-evoked excitatory postsynaptic currents reversed at -3.5 mV. At membrane holding voltages of -70 mV and -110 mV, the excitatory postsynaptic currents were characterized by a single exponential decay with a mean (+/- S.E.M.) time-constant of 17.5 +/- 1.3 ms and 15.5 +/- 2.3 ms, respectively. Colonic distension evoked a series of the excitatory postsynaptic currents which ranged in amplitude from 10 to 700 pA (at a membrane holding voltage of -70 mV). Hexamethonium (100 microM) applied only to the ganglion abolished both electrically- and distension-evoked excitatory postsynaptic currents, suggesting activation of nicotinic acetylcholine receptors. The decay time-course of distension-evoked single excitatory postsynaptic currents was characterized by one, or, less commonly, by two exponentials. The decay time-constant histograms of distension-evoked single excitatory postsynaptic currents exhibited main kinetic components of 8.1 +/- 2.3 ms and 8.2 +/- 2.5 ms (peak +/- S.D.) at -70 and -110 mV membrane holding voltages, respectively. Longer time-constants ranging up to 51 ms were also observed. The number of the distension-evoked excitatory postsynaptic currents with a decay time-constant higher than 20 ms, as well as their mean amplitude, were significantly lower at -110 mV than at -70 mV membrane potential levels, in contrast to the currents with a decay time-constant lower than or equal to 20 ms. The results suggest that colonic afferent nerve fibres activate in the mouse superior mesenteric ganglion neurons a few populations of the postsynaptic nicotinic acetylcholine receptors with different channel kinetics, which are characterized by a lack of voltage sensitivity within -70 to -110 mV membrane potential range, except those with comparatively slow channel kinetics, which are possibly blocked by membrane hyperpolarization.

Analysis of the periodicity of synaptic events in neurones in the superior cervical ganglion of anaesthetized rats

1. The patterns of on-going synaptic events recorded intracellularly in neurones of superior cervical ganglia (SCG)of anaesthetized female rats were analysed by constructing inter-event interval histograms, autocorrelograms, ln-survivor curves and histograms triggered by the arterial pulse wave and by the intercostal EMG. 2. In 11/12 cells with on-going frequencies > 0.5 Hz, one or two inputs were strong (i.e. always suprathreshold). In five cells, action potentials also arose from synaptic potentials with amplitudes close to threshold. 3. Synaptic events in 5/11 neurones tested were phase-related to the arterial pressure wave (i.e. had cardiac rhythmicity, CR). 4. Synaptic events in 9/10 neurones tested (including all with CR) were phase-related to the intercostal EMG and/or their autocorrelograms showed peaks at multiples of the respiratory interval (i.e. had respiratory rhythmicity, RR). 5. The intervals between all synaptic events were exponentially distributed in 8/12 neurones although intervals between single strong events showed peaks related to the respiratory cycle. Bursts occurred only by chance. 6. Event patterns could be simulated by combining events from several respiration-modulated inputs with their timing distributed over nearly half the cycle. From the simulations, the mean number of active preganglionic inputs was estimated to be approximately 6 with mean discharge frequency approximately 0.4 Hz. 7. We conclude that, in the spontaneously breathing anaesthetized rat, most preganglionic neurones to the SCG fire with relatively low probability in relation to the respiratory cycle. Rhythms in a postganglionic neurone reflect the activity of its suprathreshold preganglionic inputs.

On-going and reflex synaptic events in rat superior cervical ganglion cells

1. Synaptic events evoked by brief noxious cutaneous stimuli were recorded in sympathetic neurones in the superior cervical ganglion of anaesthetized rats. 2. On-going excitatory synaptic potentials (ESPs) and/or action potentials (APs) were recorded in 69% of neurones at mean frequencies that varied from 0.01 to 6.3 Hz in different cells. From histograms of ESP amplitude during membrane hyperpolarization, it appears that most cells received one (52%), or two or more (36%), suprathreshold inputs and several subthreshold inputs with overlapping amplitudes. 3. Pinching the skin for 1-3 s evoked either a brief burst of synaptic events (lasting about 300 ms) preceding a few seconds of inhibition (burst-inhibitory (BI) neurones), or simply an excitation (excitatory (E) neurones), or no response (O neurones). In 60% of BI neurones, a second burst occurred after the end of the pinch. 4. BI neurones had a higher frequency of on-going synaptic activity (2.9 +/- 0.5 Hz, n = 15) than E neurones (0.2 +/- 0.1 Hz, n = 5) or O (0.2 +/- 0.1 Hz, n = 5) neurones. Most neurones with two or more suprathreshold inputs were BI neurones. In 20% of neurones (all BI with high rates of synaptic activity), several other inputs had ESPs with amplitudes close to threshold. 5. Subthreshold and suprathreshold inputs responded in the same way in only 45% of neurones, but suprathreshold inputs were excited in 73% of BI and all E neurones. The order of recruitment of different inputs varied from trial to trial. If classification was based only on suprathreshold responses, there were 36% BI, 32% E and 32% O neurones. 6. In the majority of neurones, postganglionic discharge was initiated exclusively by suprathreshold inputs, even during reflex excitation. 7. Qualitatively similar, but smaller, responses were evoked by a puff of air on the abdomen in 71% of cells tested. 8. The data suggest that the natural discharge of SCG neurones is largely determined by the activity of one or two preganglionic inputs with high quantal contents. BI neurones may include vasoconstrictor neurones, whereas the other types include secretomotor, pilomotor and other neurones projecting to targets in the head.

Ca(2+)-activated K+ channels in rat otic ganglion cells: role of Ca2+ entry via Ca2+ channels and nicotinic receptors

1. Intracellular recordings were made from neurones in the rat otic ganglion in vitro in order to investigate their morphological, physiological and synaptic properties. We took advantage of the simple structure of these cells to test for a possible role of calcium influx via nicotinic acetylcholine receptors during synaptic transmission. 2. Cells filled with biocytin comprised a homogeneous population with ovoid somata and sparse dendritic trees. Neurones had resting membrane potentials of -53 +/- 0.7 mV (n = 69), input resistances of 112 +/- 7 M omega, and membrane time constants of 14 +/- 0.9 ms (n = 60). Upon depolarization, all cells fired overshooting action potentials which were followed by an apamin-sensitive after-hyperpolarization (AHP). In response to a prolonged current injection, all neurones fired tonically. 3. The repolarization phase of action potentials had a calcium component which was mediated by N-type calcium channels. Application of omega-conotoxin abolished both the repolarizing hump and the after-hyperpolarization suggesting that calcium influx via N-type channels activates SK-type calcium-activated potassium channels which underlie the AHP. 4. The majority (70%) of neurones received innervation from a single preganglionic fibre which generated a suprathreshold excitatory postsynaptic potential mediated by nicotinic acetylcholine receptors. The other 30% of neurones also had one or more subthreshold nicotinic inputs. 5. Calcium influx via synaptic nicotinic receptors contributed to the AHP current, indicating that this calcium has access to the calcium-activated potassium channels and therefore plays a role in regulating cell excitability.

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.

Amplitude and time course of evoked and spontaneous synaptic currents in rat submandibular ganglion cells

1. Excitatory postsynaptic currents (EPSCs) were recorded in rat submandibular ganglion cells in vitro using the two-electrode voltage clamp technique. 2. The peak amplitude of EPSCs evoked by nerve impulses in single presynaptic fibres varied between 1.2 and 9.8 nA in different cells (mean = 4.6 +/- 2.6 nA; n = 23; -80 mV membrane potential; 22-25 degrees C). 3. Experiments were performed to re-investigate a previous hypothesis that different mechanisms underlie the generation of evoked versus spontaneous quantal EPSCs in submandibular cells. This hypothesis was based on the observation of different time courses of evoked and spontaneous EPSCs. 4. In agreement with previous studies, the time course of the decay phase of evoked EPSCs was described by the sum of two exponentials, with time constants tau 1 and tau 2 of 6.9 +/- 0.7 and 34.4 +/- 7.7 ms, respectively (n = 23; -80 mV membrane potential). 5. The double-exponential decay of evoked EPSCs persisted when transmitter release was reduced by bath addition of 100 microM cadmium chloride to the level of failures, one or several quanta. 6. Spontaneous EPSCs exhibited mean amplitudes of 81 +/- 24 pA (n = 5 cells; -80 mV membrane potential), and displayed an extremely wide range of peak amplitudes in the same cell (mean coefficient of variation (c.v.) = 0.37 +/- 0.09; n = 5 cells). In contrast to a previous report (see below), the decay phase of spontaneous EPSCs was found to exhibit a double-exponential time course with time constants similar to those of the evoked EPSC recorded in the same cell. 7. These results indicate that evoked and spontaneously released quanta of transmitter most probably act on the same population of postsynaptic receptors in submandibular ganglion cells. There is a large variability in the peak amplitudes of quantal EPSCs recorded in the same cell. This large variability is not due to electrotonic effects, since these cells lack dendrites.

Omega-conotoxin-sensitive and -resistant transmitter release from the chick ciliary presynaptic terminal

1. Synaptically evoked responses to stimulation of the oculomotor nerve were recorded from the ciliary nerve in chick embryos. The postsynaptic currents in response to presynaptic stimulation (EPSCs) were also recorded under whole-cell voltage clamp of the ciliary cell. 2. The ciliary nerve response was dependent on the extracellular Ca2+ concentration ([Ca2+]o). omega-Conotoxin GVIA (omega-CgTX, 100 nM) increased the [Ca2+]o necessary to evoke the half-maximal response by a factor of 1.7 without changing the slope of [Ca2+]o dependence. Dihydropyridine (DHP) derivatives, nifedipine or Bay K 8644, did not affect the [Ca2+]o sensitivity of ciliary nerve response. 3. The EPSC was usually preceded by the capacitive coupling response of the presynaptic action potential. In some records, the EPSCs were also preceded by the electrical coupling responses which were the mirror images of the presynaptic action potentials. The current-voltage relation of the EPSCs showed inward rectification. 4. The EPSC was potentiated by 4-aminopyridine (4-AP) as a result of prolongation of the falling phase of presynaptic action potential. In the presence of high [Ca2+]o and 4-AP, a small fraction of EPSC was resistant to omega-CgTX. 5. The resting potential of the presynaptic terminal was changed from -69 to -57 mV by increasing [K+]o from 1 to 10 mM. The same procedure decreased the omega-CgTX-resistant EPSC by 30%, whereas the omega-CgTX-untreated EPSC in low-Ca2+ saline was not affected by the change in [K+]o. 6. The nerve-evoked increase in intracellular Ca2+ was recorded from the presynaptic terminal (delta[Ca2+]pre). The delta[Ca2+]pre was larger in a solution containing 10 mM Ca2+ and 1 mM K+ after treating with omega-CgTX than in a solution containing 2 mM Ca2+ and 16 mM Mg2+ before treating with omega-CgTX. The EPSC was, in contrast, smaller in the 10 mM Ca(2+)-1 mM K+ solution after omega-CgTX treatment than in the 2 mM Ca(2+)-16 mM Mg2+ solution before omega-CgTX treatment. 7. Similarly, the EPSC was smaller in the 10 mM Ca(2+)-1 mM K+ solution containing 5 microM La3+ than in the 2 mM Ca(2+)-16 mM Mg2+ solution, whereas the delta [Ca2+]pre was larger in the 10 mM Ca(2+)-1 mM K+ solution containing 5 micrograms La3+ than in the 2 mM Ca(2+)-16 mM Mg2+ solution. 8. It is concluded that the omega-CgTX-sensitive Ca2+ conductance of the presynaptic terminal is the principal source of Ca2+ involved in transmitter release.(ABSTRACT TRUNCATED AT 400 WORDS)

Barium permeability of neuronal nicotinic receptor alpha 7 expressed in Xenopus oocytes

The rat alpha 7 neuronal nicotinic acetylcholine receptor was expressed and studied in Xenopus oocytes. The magnitude and reversal potential of instantaneous whole cell currents were examined in solutions containing varying concentrations of either calcium or barium, and in the presence or absence of the intracellular calcium chelator BAPTA. In external barium, application of nicotine elicits an inwardly rectifying response; in calcium the response is larger and has a linear IV relation. Pretreatment of oocytes with BAPTA-AM could not prevent activation of calcium-dependent chloride channels in external Ringer containing calcium. Using an extended GHK equation, the permeability ratio PBa/PNa of the alpha 7 receptor was determined to be about 17. Our results suggest that alpha 7 nicotinic receptors are highly permeable to divalent cations.

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.

Modulation of acetylcholine-elicited currents in clonal rat phaeochromocytoma (PC12) cells by internal polyphosphates

1. Whole-cell voltage clamp techniques were used to examine acetylcholine (ACh)-elicited currents in differentiated cells of the rat phaeochromocytoma cell line, PC12. 2. In the absence of intracellular Mg2+, the whole-cell current-voltage relationship for the ACh-elicited current displayed inward rectification which was reduced in part by the presence of 5 mM internal adenosine 5'-triphosphate (ATP). 3. The reduction in the rectification attributed to ATP developed over the first 15-20 min of whole-cell recording. Similar results were obtained with a non-hydrolysable ATP analogue, adenosine-5'-O-3-thiotriphosphate (ATP gamma S), or cytosine 5'-triphosphate (CTP) in the internal solution, but not with adenosine 5'-diphosphate (ADP) or pyrophosphate. 4. The magnitude of the ACh-elicited current was also dependent on recording time and the composition of the internal pipette solution. The magnitude of the peak ACh-elicited current increased over time when the cell was internally perfused with the control solution or a pipette solution containing pyrophosphate, ATP gamma S, or ADP. The largest sustained increases in ACh-elicited current were observed in the presence of internal pyrophosphate or ATP gamma S. In contrast, with internal ATP or CTP, the whole-cell current initially increased, then steadily decreased with recording time. 5. The desensitization rate of the ACh-elicited current increased with recording time irrespective of the composition of the intracellular solution. 6. The actions of the compounds tested make it likely that the changes in the whole-cell current-voltage relationship, peak current, and desensitization are produced by separate mechanisms. The mechanisms underlying these changes are unknown, but the ability of the compounds to chelate divalent cations is unlikely to be the explanation. Other unlikely explanations include phosphorylation of the ACh receptor or regulation by GTP-binding proteins.

Whole-cell currents activated at nicotinic acetylcholine receptors on ganglion cells isolated from goldfish retina

We have recorded whole-cell membrane currents in response to exogenously applied acetylcholine (ACh), nicotine, and 1,1 dimethyl-4-phenyl piperazinium iodide on retinal ganglion cells enzymatically dissociated from goldfish retina. Agonist applications induced nicotinic-type responses in a majority of cells when cells were isolated under optimal conditions. Currents were reminiscent of nicotinic-type ganglionic responses. Dose-response measurements of ACh-induced currents indicated an EC50 of 52 microM and a Hill coefficient of 0.6. Currents were selective for Na+ over Cl- and were highly inwardly rectifying. Responses were blocked reversibly by d-tubocurarine, hexamethonium chloride, and N-methyl-D-glucamine. In 50% of the cases, alpha-bungarotoxin reversibly blocked the current induced by ACh application. The blocking action of mecamylamine was irreversible and independent of the presence of agonist but was more effective in the presence of ACh. We conclude that functional nicotinic ACh receptors exist on most goldfish retinal ganglion cells.

Inward rectification of acetylcholine-elicited currents in rat phaeochromocytoma cells

1. Currents elicited by acetylcholine (ACh) were studied in the rat phaeochromocytoma cell line, PC12, using patch-clamp techniques. 2. Whole-cell ACh-elicited currents are inwardly rectifying and intracellular Mg2+ can play a role in determining the extent of whole-cell current rectification. Increasing the intracellular Mg2+ concentration, [Mg2+]i, augmented the rectification. The effects of increased [Mg2+]i on the whole-cell current can be explained by the block of receptor channels by Mg2+. 3. In the nominal absence of internal divalent cations, however, a substantial degree of rectification remains. This rectification is probably not due to divalent cations, as buffering the external Mg2+ concentration to 50 microM and the internal concentration to nominally 0 Mg2+ did not reduce the rectification. The remanent rectification was not due to block by the main permeant cation, Na+. Using K+ or Cs+ as the main monovalent cation inside the cell did not diminish the rectification. Neither replacing the pH buffer, HEPES, with phosphate buffer nor increasing the intracellular pH removed the rectification. 4. For ACh receptor channels in excised patches, the voltage dependence of the probability of being open (Popen) stemmed mainly from the voltage dependence of the channel burst duration. The channel opening rate was relatively voltage independent. The weak voltage dependence displayed by the channel burst duration was insufficient to account for the reduced whole-cell outward current at positive potentials. The mean burst duration of the channel did not have a simple logarithmic relationship with voltage. 5. In the absence of intracellular Mg2+, the instantaneous current-voltage relationship for whole-cell currents was linear suggesting that the I-V relationship of single channels in perfused cells is linear and does not contribute to the rectification of the whole-cell current. 6. In perfused cells, receptor channels had a low steady-state probability of being open at positive potentials compared to channels in excised patches. Voltage jumps to positive potentials revealed a process in perfused cells which could account for the low Popen. Relaxations of agonist-induced current at +40 mV had a large, exponentially decaying component that quickly closed channels (rate constant, tau, approximately 400 microseconds). The mechanism responsible for this decay could explain the rectification that remains in the absence of intracellular divalent cations.(ABSTRACT TRUNCATED AT 400 WORDS)

A patch clamp study of the nicotinic acetylcholine receptor of bovine adrenomedullary chromaffin cells in culture

1. Acetylcholine-induced currents recorded from bovine adrenal medullary chromaffin cells maintained in culture were studied during pressure or ionophoretic applications of ACh, using the 'whole-cell' and 'outside-out' configurations of the patch clamp technique. In standard salines, ACh evoked whole-cell currents of -38 pA to -1 nA at -60 mV, which had a reversal potential (EACh) of -7.1 +/- 0.6 mV. The ACh current-voltage relationship was characteristically linear at negative holding potentials and biphasic at positive holding potentials, displaying a region of almost zero slope conductance between 0 and +40 mV followed by a region of positive slope conductance at more positive potentials. 2. Relative permeation to cations was examined. Substitution of external Na+ by sucrose resulted in a -42 mV shift of EACh for a 10-fold reduction in [Na+]o. Using isotonic substitutions, the permeability ratios (relative to Na+) for monovalent cations were determined to be 1.32 +/- 0.02 for Cs+ (n = 11), 1.03 +/- 0.02 for Li+ (n = 8) and 0.18 +/- 0.02 for Tris+ (n = 7). Elevated external Ca2+ salines were found to shift EACh to more positive potentials, especially in the presence of low external Na+. 3. The nicotinic agonists nicotine, tetramethylammonium and lobeline evoked inward currents in bovine chromaffin cells. In contrast, decamethonium and the muscarinic agonist, methacholine, had no effect. 4. The nicotinic antagonists mecamylamine, trimetaphan, (+)-tubocurarine and hexamethonium caused dose-dependent reductions in the amplitude of ACh-evoked inward currents. The estimated IC50's were 0.25, 0.33, 0.63 and 2.2 microM respectively, for cells voltage clamped at -60 mV. High concentrations (> 2 microM) of the muscarinic antagonist, atropine, also produced a dose-dependent reduction in the amplitude of ACh-induced currents. 5. Inhibition by trimetaphan was voltage independent. With the other drugs the antagonism was voltage sensitive, increasing with membrane hyperpolarization. The voltage sensitivity was most marked for hexamethonium. Neither hexamethonium nor mecamylamine were found to depress ACh-evoked outward currents at concentrations which severely depressed inward currents. In addition to its antagonist actions, (+)-tubocurarine activated unitary currents in these cells and on isolated membrane patches. 6. The results indicate that nicotinic ion channels of bovine chromaffin cells have a similar ionic selectivity to monovalent cations, but that Ca2+ ions permeate the channels to a greater degree than at the motor endplate. The ACh current-voltage relationship resembles that described for other types of 'neuronal' nicotinic receptors.(ABSTRACT TRUNCATED AT 400 WORDS)

Neuronal nicotinic acetylcholine receptor currents in phaeochromocytoma (PC12) cells: dual mechanisms of rectification

1. Neuronal nicotinic acetylcholine receptor (nAChR) currents in PC12 cells were studied using single-channel and whole-cell gigaohm seal voltage-clamp techniques. Nicotinic AChR agonists were applied using external pipettes. 2. The single-channel conductance of neuronal nAChRs in outside-out patches under Mg(2+)-free ionic conditions was 48 pS. In the absence of internal Mg2+ single-channel currents (outside-out patches) did not rectify. 3. Whole-cell nAChR currents recorded with normal internal solution (lacking Mg2+ chelators as described below) were strongly inwardly rectifying. Current vs. voltage relations showed a sharp inflexion near 0 mV, and outward current was never observed. 4. Extensive dialysis with internal solution containing EDTA and Na2-ATP, chelators of intracellular Mg2+ (Mgi2+), relieved rectification of instantaneous nAChR currents during voltage jumps from negative potentials. The instantaneous I-V relation became linear with voltage, in agreement with the expectation from single-channel measurements made under similar ionic conditions. 5. Agonist-induced currents recorded under Mgi(2+)-free conditions relaxed towards zero current during depolarizing voltage steps. Rectification of ACh-induced currents at the steady state could be described by a Boltzmann relation, with one-half of channels available at +3.4 mV and an effective gating charge of -0.97. Channel availability was approximately 95% at the resting potential. Opening and closing relaxations under Mgi(2+)-free conditions could be fitted by single exponential functions whose time constants were weakly voltage dependent. 6. A model of rectification was constructed incorporating two voltage-dependent processes: block by Mgi2+ and intrinsic channel gating. The I-V relations predicted by this model for both normal and Mgi(2+)-free conditions were in good agreement with the experimental data. We suggest that rectification of nAChR currents in these cells is due to concurrent activity of these two voltage-dependent processes. The relative contributions of these two mechanisms are frequency dependent, with block by Mgi2+ dominant for fast events and intrinsic channel gating more important at the steady state.

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.

Voltage-dependent block by magnesium of neuronal nicotinic acetylcholine receptor channels in rat phaeochromocytoma cells

1. The effects of Mg2+ on the single-channel conductance of neuronal nicotinic acetylcholine receptors were examined using receptors expressed by the rat phaeochromocytoma cell line, PC12. PC12 cells express at least three conductance classes of channels that are activated by acetylcholine, the largest conductance class being the most prevalent. This receptor channel is blocked by intracellular and extracellular Mg2+. 2. The effects of Mg2+ are asymmetrical; at a given concentration, internal Mg2+ is more effective at blocking outward currents than external Mg2+ is at blocking inward currents. Receptor channels are blocked at concentrations of Mg2+ that are low compared to the concentration of the main permeant cation, Na+, and the block is voltage dependent. 3. The block by Mg2+ is not complete as Mg2+ can permeate the channel. With 80 mM-extracellular Mg2+ (no extracellular Na+), the channel has an inward slope conductance of 2.9 pS. 4. The block by extracellular Mg2+ can be described by a one site, two barrier model for the channel which includes a negative surface charge on the external surface of the membrane. The parameters of the model place the binding site for Mg2+ at 52% of the membrane field from the outside with an apparent dissociation constant of 14 mM. However, the same parameters cannot describe the block by intracellular Mg2+. The deviations from the model suggest that the receptor channel may have more than one binding site for Mg2+.

Acetylcholine-activated ionic currents in isolated paratracheal ganglion cells of the rat

The electrophysiological property of acetylcholine (ACh)-induced current (IACh) was studied in paratracheal ganglion cells freshly isolated from rat trachea under whole-cell voltage-clamp condition. IACh consisted of an initial transient peak component and a successive steady-state plateau one. The peak component increased in a sigmoidal fashion with increasing ACh concentration. The IACh was mimicked by nicotine. The current-voltage relationship for the IACh showed inward rectification at the positive membrane potentials beyond the reversal potential (EACh). In a K(+)-free solution, the EACh was close to the Na+ equilibrium potential. The IACh was blocked by either D-tubocurarine or atropine. The ion selectivity of ACh-activated channels to various monovalent cations was weak, and similar to those of other preparations. It was concluded that the IACh in rat paratracheal ganglion cells was mediated by nicotinic receptor activation.

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.

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

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

Rectification of acetylcholine-elicited currents in PC12 pheochromocytoma cells

The current-voltage (I-V) relationship for acetylcholine-elicited currents in the rat pheochromocytoma cell line PC12 is nonlinear. Two voltage-dependent processes that could account for the whole-cell current rectification were examined, receptor channel gating and single receptor channel permeation. We found that both factors are involved in the rectification of the whole-cell currents. The voltage dependence of channel gating determines the shape of the I-V curve at negative potentials. The single-channel I-V relationship is inwardly rectifying and largely responsible for the characteristic shape of the whole-cell I-V curve at positive potentials. The rectification of the single-channel currents is produced by the voltage-dependent block of outward currents by intracellular Mg2+ ions.