Single-channel and whole-cell recordings from non-dissociated sympathetic neurones in rabbit coeliac ganglia

Methamphetamine Regulation of Firing Activity of Dopamine Neurons

Methamphetamine (METH) is a substrate for the dopamine transporter that increases extracellular dopamine levels by competing with dopamine uptake and increasing reverse transport of dopamine via the transporter. METH has also been shown to alter the excitability of dopamine neurons. The mechanism of METH regulation of the intrinsic firing behaviors of dopamine neurons is less understood. Here we identified an unexpected and unique property of METH on the regulation of firing activity of mouse dopamine neurons. METH produced a transient augmentation of spontaneous spike activity of midbrain dopamine neurons that was followed by a progressive reduction of spontaneous spike activity. Inspection of action potential morphology revealed that METH increased the half-width and produced larger coefficients of variation of the interspike interval, suggesting that METH exposure affected the activity of voltage-dependent potassium channels in these neurons. Since METH has been shown to affect Ca²⁺ homeostasis, the unexpected findings that METH broadened the action potential and decreased the amplitude of afterhyperpolarization led us to ask whether METH alters the activity of Ca²⁺-activated potassium (BK) channels. First, we identified BK channels in dopamine neurons by their voltage dependence and their response to a BK channel blocker or opener. While METH suppressed the amplitude of BK channel-mediated unitary currents, the BK channel opener NS1619 attenuated the effects of METH on action potential broadening, afterhyperpolarization repression, and spontaneous spike activity reduction. Live-cell total internal reflection fluorescence microscopy, electrophysiology, and biochemical analysis suggest METH exposure decreased the activity of BK channels by decreasing BK-α subunit levels at the plasma membrane.

SIGNIFICANCE STATEMENT

Methamphetamine (METH) competes with dopamine uptake, increases dopamine efflux via the dopamine transporter, and affects the excitability of dopamine neurons. Here, we identified an unexpected property of METH on dopamine neuron firing activity. METH transiently increased the spontaneous spike activity of dopamine neurons followed by a progressive reduction of the spontaneous spike activity. METH broadened the action potentials, increased coefficients of variation of the interspike interval, and decreased the amplitude of afterhyperpolarization, which are consistent with changes in the activity of Ca²⁺-activated potassium (BK) channels. We found that METH decreased the activity of BK channels by stimulating BK-α subunit trafficking. Thus, METH modulation of dopamine neurotransmission and resulting behavioral responses is, in part, due to METH regulation of BK channel activity.

Characteristics of single large-conductance Ca2+-activated K+ channels and their regulation of action potentials and excitability in parasympathetic cardiac motoneurons in the nucleus ambiguus

Large-conductance Ca2(+)-activated K+ channels (BK) regulate action potential (AP) properties and excitability in many central neurons. However, the properties and functional roles of BK channels in parasympathetic cardiac motoneurons (PCMNs) in the nucleus ambiguus (NA) have not yet been well characterized. In this study, the tracer X-rhodamine-5 (and 6)-isothiocyanate (XRITC) was injected into the pericardial sac to retrogradely label PCMNs in FVB mice at postnatal 7-9 days. Two days later, XRITC-labeled PCMNs in brain stem slices were identified. Using excised patch single-channel recordings, we identified voltage-gated and Ca(2+)-dependent BK channels in PCMNs. The majority of BK channels exhibited persistent channel opening during voltage holding. These BK channels had a conductance of 237 pS and a 50% opening probability at +27.9 mV, the channel open time constant was 3.37 ms at +20 mV, and dwell time increased exponentially as the membrane potential depolarized. At the +20-mV holding potential, the [Ca2+]50 was 15.2 μM with a P0.5 of 0.4. Occasionally, some BK channels showed a transient channel opening and fast inactivation. Using whole cell voltage clamp, we found that BK channel mediated outward currents and afterhyperpolarization currents (IAHP). Using whole cell current clamp, we found that application of BK channel blocker iberiotoxin (IBTX) increased spike half-width and suppressed fast afterhyperpolarization (fAHP) amplitude following single APs. In addition, IBTX application increased spike half-width and reduced the spike frequency-dependent AP broadening in trains and spike frequency adaption (SFA). Furthermore, BK channel blockade decreased spike frequency. Collectively, these results demonstrate that PCMNs have BK channels that significantly regulate AP repolarization, fAHP, SFA, and spike frequency. We conclude that activation of BK channels underlies one of the mechanisms for facilitation of PCMN excitability.

Calcium current components in intact and dissociated adult mouse sympathetic neurons

We examined which types of high threshold Ca(2+) channels are activated by depolarization in intact and dissociated sympathetic neurons from adult mouse superior cervical ganglia (SCG). Ba(2+) currents were recorded with microelectrodes and discontinuous voltage clamp from neurons in intact ganglia, and using the perforated patch clamp technique in dissociated cells. Peak current was larger in intact neurons, although the voltage dependence was similar. Successive application of omega-conotoxin GVIA, omega-conotoxin MVIIC and nifedipine revealed that the total current in intact cells was composed by 29% N-type, 13% P/Q-type, 32% L-type and 26% resistant to blockade (R-type). In dissociated cells, the N component was larger and the L component smaller, whereas P/Q-type and R-type were similar. Peak currents evoked with an action potential waveform instead of a square pulse were larger in both preparations but the proportions of each component were similar. We conclude that dissociating and culturing somata results in data that only partially reflect the situation in intact neurons. Assuming that the main effect of dissociation is the removal of mature dendritic membrane, the data suggest that L channels are more abundant on dendrites and N channels on the soma of intact sympathetic neurons, whereas P/Q and R channels may be uniformly distributed over the cell surface. Finally, in intact SCG neurons from rats, the proportions of current evoked by a pulse were: 49% N-type, 11% P/Q-type, 21% L-type and 20% R-type when nifedipine was applied last, suggesting that there are species differences in the expression of L and N channels.

Patch clamp electrophysiology in nodose ganglia of adult rat

The patch clamp technique is widely utilized for studying the electrophysiological properties of enzymatically isolated sensory neurons. Unfortunately, dissociation of the ganglion severs the afferent fibers. As a result, isolated neurons can only be broadly categorized according to somatic action potential waveforms, ion channel subtypes, chemical sensitivities and cell diameter. Such restricted classifications contrast with the continuum of conduction velocities (CVs), discharge patterns, sensory modalities and functional properties of visceral and spinal afferents. Previous reports of patch clamp recordings using intact ganglion have been limited to young animal preparations. This raises concerns regarding postnatal development and impedes the use of chronic models of disease or injury, which often necessitate the use of a more mature animal preparation. Here, we present a methodology for preparing nodose ganglion from adult rat (250-400 g) for study using the patch clamp technique. Successful whole cell recordings were obtained from approximately 50% of the cells selected for study, the majority of which had intact afferent fibers. Measures of somatic discharge and afferent fiber CV at both room and physiological temperatures were consistent with investigations using sharp microelectrodes. Voltage clamp recordings of whole cell Na(+), Ca(2+) and K(+) ion channel currents were comparable to those obtained using isolated neuron preparations. The ability to classify voltage- and ligand-gated ion channel type with afferent fiber CV in an adult preparation adds a valuable new dimension to cellular investigations of the diverse functional and chemical properties of the peripheral afferent nervous system.

Afterhyperpolarization Current in Myenteric Neurons of the Guinea Pig Duodenum

Whole cell patch and cell-attached recordings were obtained from neurons in intact ganglia of the myenteric plexus of the guinea pig duodenum. Two classes of neuron were identified electrophysiologically: phasically firing AH neurons that had a pronounced slow afterhyperpolarization (AHP) and tonically firing S neurons that lacked a slow AHP. We investigated the properties of the slow AHP and the underlying current ( I AHP) to address the roles of Ca2+ entry and Ca2+ release in the AHP and the characteristics of the K+channels that are activated. AH neurons had a resting potential of −54 mV and the AHP, which followed a volley of three suprathreshold depolarizing current pulses delivered at 50 Hz through the pipette, averaged 11 mV at its peak, which occurred 0.5–1 s following the stimulus. The duration of these AHPs averaged 7 s. Under voltage-clamp conditions, I AHP's were recorded at holding potentials of −50 to −65 mV, following brief depolarization of AH neurons (20–100 ms) to positive potentials (+35 to +50 mV). The null potential of the I AHP at its peak was −89 mV. The AHP and I AHP were largely blocked by ω-conotoxin GVIA (0.6–1 μM). Both events were markedly decreased by caffeine (2–5 mM) and by ryanodine (10–20 μM) added to the bathing solution. Pharmacological suppression of the I AHP with TEA (20 mM) or charybdotoxin (50–100 nM) unmasked an early transient inward current at −55 mV following step depolarization that reversed at −34 mV and was inhibited by niflumic acid (50–100 μM). Mean-variance analysis performed on the decay of the I AHPrevealed that the AHP K+ channels have a mean chord conductance of ∼10 pS, and there are ∼4,000 per AH neuron. Spectral analysis showed that the AHP channels have a mean open dwell time of 2.8 ms. Cell-attached patch recordings from AH neurons confirmed that the channels that open following action currents have a small unitary conductance (10–17 pS) and open with a high probability (≤0.5) within the first 2 s following an action potential. These results indicate that the AHP is largely a consequence of Ca2+ entry through ω-conotoxin GVIA-sensitive Ca2+ channels during the action potential, Ca2+-triggered Ca2+ release from caffeine-sensitive stores and the opening of Ca2+-sensitive small-conductance K+ channels.

The soma and neurites of primary afferent neurons in the guinea-pig intestine respond differentially to deformation

1. Intrinsic primary afferent neurons in the small intestine are exposed to distortion of their processes and of their cell bodies. Recordings of mechanosensitivity have previously been made from these neurons using intracellular microelectrodes, but this form of recording has not permitted detection of generator potentials from the processes, or of responses to cell body distortion. 2. We have developed a technique to record from enteric neurons in situ using patch electrodes. The mechanical stability of the patch recordings has allowed recording in cell-attached and whole cell configuration during imposed movement of the neurons. 3. Pressing with a fine probe initiated generator potentials (14 +/- 9 mV) from circumscribed regions of the neuron processes within the same myenteric ganglion, at distances from 100 to 500 microm from the cell body that was patched. Generator potentials persisted when synaptic transmission was blocked with high Mg2+, low Ca2+ solution. 4. Soma distortion, by pressing down with the whole cell recording electrode, inhibited action potential firing. Consistent with this, moderate intra-electrode pressure (10 mbar; 1 kPa) increased the opening probability of large-conductance (BK) potassium channels, recorded in cell-attached mode, but suction was not effective. In outside-out patches, suction, but not pressure, increased channel opening probability. Mechanosensitive BK channels have not been identified on other neurons. 5. The BK channels had conductances of 195 +/- 25 pS. Open probability was increased by depolarization, with a half-maximum activation at a patch potential of 20 mV and a slope factor of 10 mV. Channel activity was blocked by charybdotoxin (20 nM). 6. Stretch that increased membrane area under the electrode by 15 % was sufficient to double open probability. Similar changes in membrane area occur when the intestine changes diameter and wall tension under physiological conditions. Thus, the intestinal intrinsic primary afferent neurons are detectors of neurite distortion and of compression of the soma, these stimuli having opposite effects on neuron excitability.

Patch-clamp study of neurons and glial cells in isolated myenteric ganglia

Most of the physiological information on the enteric nervous system has been obtained from studies on preparations of the myenteric ganglia attached to the longitudinal muscle layer. This preparation has a number of disadvantages, e.g., the inability to make patch-clamp recordings and the occurrence of muscle movements. To overcome these limitations we used isolated myenteric ganglia from the guinea pig small intestine. In this preparation movement was eliminated because muscle was completely absent, gigaseals were obtained, and whole cell recordings were made from neurons and glial cells. The morphological identity of cells was verified by injecting a fluorescent dye by micropipette. Neurons displayed voltage-gated inactivating inward Na(+) and Ca(2+) currents as well as delayed-rectifier K(+) currents. Immunohistochemical staining confirmed that most neurons have Na(+) channels. Neurons responded to GABA, indicating that membrane receptors were retained. Glial cells displayed hyperpolarization-induced K(+) inward currents and depolarization-induced K(+) outward currents. Glia showed large "passive" currents that were suppressed by octanol, consistent with coupling by gap junctions among these cells. These results demonstrate the advantages of isolated ganglia for studying myenteric neurons and glial cells.

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.

Encoding properties induced by a persistent voltage-gated muscarinic sodium current in rabbit sympathetic neurones

1. A time- and voltage-dependent Na(+)-selective current termed INa,M is activated by muscarinic agonists or splanchnic nerve stimulation in sympathetic neurones of rabbit coeliac and superior mesenteric ganglia. The firing patterns induced by INa,M were investigated in patch-clamped neurones within intact ganglia, and compared with those generated by a neuronal model including INa,M. 2. INa,M was characterized by voltage-dependent low threshold activation and high-threshold inactivation functions. The overlapping functions produced a persistent U-shaped current between -100 and -20 mV, which peaked at the cell resting potential. The activation and inactivation kinetics were fitted to single exponentials with time constants of approximately 100 and 400 ms, respectively. 3. Activating INa,M with muscarinic agonists or nerve stimulation depolarized and fired the neurones. The depolarization was paralleled by an apparent increase in input membrane resistance. The model showed that this paradox resulted from the turning off of INa,M during resistance tests, which also accounted for the all-or-none slow hyperpolarizing responses to current pulses. 4. INa,M gave the neurones an N-shaped I-V relationship capable of producing complex firing patterns. Under given conditions, carbachol-treated neurones could either fire regularly or remain silent at approximately -80 mV, i.e. they displayed bistability. Transitions from one state to the other were triggered with short current pulses. The transitions resulted from the turning on and off of INa,M. 5. Firing reduced INa,M, an effect abolished by blocking Ca2+ channels or adding BAPTA (40 mM) to the pipette. The Ca(2+)-related negative regulation of INa,M may have mediated endogenous bursting activity. Burst firing was generated by the model upon introducing Ca2+ regulation of INa,M. 6. The results demonstrate that INa,M gives prevertebral sympathetic neurones a wide repertoire of firing patterns: pacemaker-like properties, bistability and burst firing capability. They suggest that the INa,M-related encoding properties may provide sympathetic neurotransmission with new potentialities.

Patch clamp recording from the intact dorsal root ganglion

A method for patch-clamp recording from intact dorsal root ganglion (DRG) cells in rat is described. The L4 and L5 DRGs with sciatic nerve attached were excised from rats (10-15 days old) and placed in a recording chamber after removing the ganglion sheath and dissolving the connective tissue with dilute collagenase. The somata of individual cells were exposed by gentle surface cleaning through a perfusion micropipette. Somata were classified as Abeta, Adelta or C based on the cell size and the shape of the action potential (AP). Under current clamp, axonal conduction velocity (CV) was calculated from the distance between a stimulating electrode and the center of the ganglion divided by the latency of the AP elicited by stimulation of the sciatic nerve. CVs ranged from 0.2-0.8 m/s for C cells, 0.8-2.4 for Adelta and 3.2-5.0 for A/beta cells. AP threshold occurred at a significantly more positive potential in C cells than in Adelta and Abeta cells. Under voltage clamp, sodium currents were recorded from C cells. Both TTX-resistant (TTX-R) and TTX-sensitive (TTX-S currents) were demonstrated in the present study. The results demonstrate the feasibility of patch-clamp recording from intact, identified DRG cells in vitro.

Exotoxin-insensitive G proteins mediate synaptically evoked muscarinic sodium current in rabbit sympathetic neurones

1. The involvement of G proteins in the transduction pathway that links muscarinic receptors to the low-threshold voltage-dependent sodium current (INa,M) was studied in neurones from intact sympathetic prevertebral ganglia using the whole-cell configuration of the patch-clamp technique. Experiments were performed in the presence of the nicotinic receptor antagonists hexamethonium (50 microM) and d-tubocurarine (50 microM). 2. INa,M was activated by either bath-applying muscarinic agonists or stimulating the preganglionic splanchnic nerves. Synaptically and agonist-mediated INa,M did not display significant run-down or changes in their properties in cells tested, irrespective of whether the pipette solutions contained GTP. 3. Dialysis of sympathetic neurones with GDP beta S (500-750 microM) decreased the amplitude of INa,M by approximately 65% compared with control neurones within 30 min. 4. In the absence of muscarinic receptor stimulation, intracellular dialysis with GTP gamma S (500 microM) for 10 min slowly and slightly (20-25%) activated INa,M. GTP gamma S dialysis markedly slowed down the decay of INa,M after its transient activation with carbachol pulses (10-20 s) or nerve stimulation (3-5 s). The INa,M activation became fully irreversible 2.9 min after the start of GTP gamma S dialysis. Dialysing cells with the G protein activator AIF4-led to a rapid but transient activation of INa,M. 5. Synaptically and agonist-evoked INa,M were not affected in ganglia treated with 0.5-1 microgram ml-1 pertussis toxin (PTX) for 7-24 h at 37 degrees C. Control experiments showed that this treatment severely reduced the PTX-sensitive inhibition of N-type calcium currents induced by carbachol (CCh) and noradrenaline. Application of NEM (N-ethylmaleimide) for 2 min depressed the INa,M evoked in response to bath-applied CCh by only 27%. 6. Incubating ganglia with 5-10 micrograms ml-1 of cholera toxin for 7 h had no effect on the carbachol-induced INa,M but greatly potentiated (approximately 250%) the synaptically evoked INa,M, presumably via a presynaptic mechanism. 7. These results show that the coupling between muscarinic receptors and NaM channels is mediated by pertussis toxin- and cholera toxin-insensitive G proteins, possibly of the Gq/11 or G12 class.

The mammalian sympathetic prevertebral ganglia: integrative properties and role in the nervous control of digestive tract motility

The prevertebral ganglia which are a constitutive part of the sympathetic system have long been considered as a simple relay on this efferent pathway. In fact, these ganglia must be considered as true peripheral nervous centres. They possess various integrative properties, such as projections of central and peripheral inputs onto the ganglionic neurones, gating of these projections and pacemaker activity of the ganglionic neurones. These properties explain the ability of these ganglia to participate in the regulation of various visceral functions, including digestive tract motility.

Synaptically activated low-threshold muscarinic inward current sustains tonic firing in rabbit prevertebral sympathetic neurons

Whole-cell patch-clamp experiments were performed on non-dissociated rabbit coeliac sympathetic neurons in the presence of nicotinic blockers. Coeliac neurons were classified as either silent or spontaneously active (pacemaker) cells. Under voltage-clamp conditions, pacemaker cells exhibited a steady-state N-shaped current-voltage relationship due to the presence of a persistent voltage-dependent inward current in the potential range of -100 to approximately -20 mV. This inward current sustained the regular firing activity of pacemaker cells and was absent from quiescent neurons. It disappeared in the presence of tetrodotoxin and in low Ca(2+)-high Mg2+ external solutions and was enhanced by eserine. Splanchnic nerve stimulation induced slow regenerative depolarizations and firing discharges in silent neurons by activating a low-threshold voltage-sensitive inward current. The synaptic current had a U-shaped voltage-dependence from -96 to approximately -20 mV and exhibited the dynamic properties of the muscarinic voltage-dependent inward current INa,M. It gave the current-voltage relationship an N shape similar to that observed in spontaneously active cells. The muscarinic antagonists atropine and pirenzepine abolished the inward current present in pacemaker cells and that induced by nerve stimulation in silent neurons. These data provide evidence that both spontaneous firing activity and nerve-evoked depolarizing responses in coeliac neurons are sustained by the activation of the muscarinic Na,M current. The tonic activation of INa,M in spontaneously firing cells results from a sustained Ca(2+)-dependent tetrodotoxin-sensitive release of acetylcholine. This study provides evidence that the role of the muscarinic receptors is not purely a neuromodulatory one, but that these receptors are directly involved in ganglionic neurotransmission.

Muscarinic activation of a novel voltage-sensitive inward current in rabbit prevertebral sympathetic neurons

The muscarinic activation of rabbit prevertebral sympathetic neurons was studied in non-dissociated coeliac and superior mesenteric ganglia using whole-cell patch-clamp techniques. In the presence of nicotinic blockers, carbachol, muscarine and oxotremorine-M (1-50 microM) induced tonic firing by activating a persistent inward current. These effects were abolished by atropine. They persisted when the M-current was blocked with Ba2+ (1 mM) and intracellular Cs+. The muscarinic inward current was found to be time- and voltage-dependent. It peaked at -60 mV, decreased at large hyperpolarizations and was tonically activated between -110 and -20 mV, which gave steady-state I-V curves an N-shape between -96 and -54 mV. The negative slope accounted for the large hyperpolarizing responses generated by current pulses in carbachol-treated cells. The muscarinic current was abolished when Na+ was replaced by choline, Tris+, sucrose, N-methyl-D-glucamine and Cs+ but not Li+. It was resistant to tetrodotoxin (3 microM), amiloride (3 microM), benzamil (10 microM) and tetraethylammonium (5-20 mM). No involvement of K+ and Cl- could be detected. We therefore styled it INa,M, in reference to its ionic selectivity and its coupling to muscarinic receptors. Low Ca(2+)-Mg2+ salines enhanced the Na,M-current. The current was blocked by Cd2+, Co2+, La3+ (1 mM) and Ba2+ (5 mM) but insensitive to methoxyverapamil hydrochloride, nicardipine, nifedipine and omega-conotoxin MVII A (2-20 microM). These effects were ascribed to the binding of di- and trivalent ions to the Na,M-channels. Spike bursts transiently blocked INa,M. With high intracellular ethylene glycol bis(b-aminoethyl ether)-N,N'-tetraacetic acid or 1,2-bis (2-aminophenoxy)ethane-N,N,N',N'-tetraacetic acid (20-50 mM), this effect was reduced, whereas INa,M persisted in long-term recordings and its amplitude increased twofold, indicating that intracellular calcium negatively regulated the Na,M-channels. We conclude that we have described a novel muscarinic receptor-coupled channel which appears to play a major part in regulating the firing behaviour of sympathetic neurons.

Choline blocks large conductance KCa channels in mammalian sympathetic neurones

Using the outside-out configuration of the patch clamp technique, we examined the effects of externally applied choline chloride (ChCl) on the large-conductance calcium-dependent K+ (KCa) channel in sympathetic neurones of the rabbit coeliac ganglion. Isotonically replacing the bath sodium chloride by ChCl significantly decreased the unitary current amplitude of the KCa channels but did not affect their gating properties. The blocking effect was dose-dependent and required 24 mM ChCl to reach half-reduction of the KCa channel conductance (from 134 to 64 pS). In whole-cell voltage-clamped neurones, ChCl activated both nicotinic and muscarinic receptors, which induced various membrane current changes including Ca2+ current decrease. We therefore tested the effects of the cholinergic agonists acetylcholine and carbachol on excised KCa channels. Both agents failed to mimic the blocking effects of ChCl. We therefore conclude that KCa channels in sympathetic neurones are occluded by external choline chloride.

Methods for studying neurotransmitter transduction mechanisms

This review describes the methodologies used to study the transduction mechanisms that are activated in excitable cells by G-protein-coupled agonists. In view of the complexity of second-messenger systems, it is no longer relevant to ask, "What is the transduction mechanism involved in the action of a given neuromodulator?" because, in many cases, a variety of transduction mechanisms and physiological responses are invoked following receptor activation. This means that a single aspect of the physiological response must be selected for study in order to address the question of transduction mechanism. This review is therefore concerned with a description the use of patch- and voltage-clamp procedures to study transduction mechanism because they are designed to isolate one aspect of the physiological response: the change in activity of a single type of membrane ion channel.

Single-channel and whole-cell recordings from on-neurone glial cells in Helix pomatia ganglia

A procedure is described for performing patch-clamp recordings on satellite glial cells kept in place within the nervous ganglia in the mollusc Helix. Glial cell properties were deduced from whole-cell and cell-attached recordings. The glial membrane was found to contain densely packed inwardly rectifying K+ channels. Activation of the neurones, under either current-clamp or voltage-clamp conditions, depolarized the glial cell layer wrapped around the neurones and induced a delayed persistent increase in the K+ channel opening probability. These results suggest that the glial channels opened in response to a signal emanating from the active neurones. This preparation provides a useful means of detecting and analysing neurone-glial interactions at the cell and unitary channel levels.

Electrical and integrative properties of rabbit sympathetic neurones re-evaluated by patch clamping non-dissociated cells

1. Voltage recordings were performed on non-dissociated sympathetic neurones from rabbit coeliac ganglia using the whole-cell configuration of the patch clamp technique. 2. Cells were classified depending on their firing pattern as silent cells (63%) producing either phasic (24%) or tonic (76%) spike discharge in response to depolarizing currents, and pacemaker cells (37%). 3. All the cells produced large overshooting spikes and prolonged postspike after-hyperpolarization. The peak-to-peak spike amplitude was 113.8 +/- 1 mV. Spikes were shortened and the after-hyperpolarization was suppressed when calcium channel blockers (Cd2+ and La3+) were added. 4. Silent cells have a resting potential of -58.8 +/- 1.5 mV. At potentials ranging from -50 to -90 mV, the input impedance was 490 +/- 27 M omega at 22-24 degrees C and 426 +/- 47 M omega at 35-36 degrees C. The time constant at voltages corresponding to the high input impedance region was 126 +/- 7 ms at 22-24 degrees C and 86 +/- 7 ms at 35-36 degrees C. 5. The firing frequency of the pacemaker cells was 3.2 +/- 0.5 Hz at 35-36 degrees C in the presence of nicotinic blockers. Evidence is given that the firing did not result from cell injury but was induced by an intrinsic pacemaker mechanism. Input impedance of pacemaker neurones was 580 +/- 47 M omega at 22-24 degrees C and 473 +/- 56 M omega at 35-36 degrees C. 6. Most of the pacemaker cells (63%) were motoneurones, since they were antidromically fired by stimulating post-ganglionic nerves. In addition, they received synaptic inputs from both preganglionic fibres (splanchnic nerves) and the periphery (postganglionic nerves). Long-lasting depolarizations were induced in either silent or pacemaker cells by single shocks applied to pre- and postganglionic nerves. 7. Slowly rising voltage ramps revealed the presence of an N-shaped current-voltage relationship in voltage clamped pacemaker cells. The negative slope was located in a subthreshold voltage range, between -83.4 +/- 1.4 and -59.0 +/- 1.8 mV. It was induced by the activation of a low threshold persistent inward current. Although it was tiny (22 +/- 3 pA at its peak level) this current brought the null-current voltage up to -41.0 +/- 1.4 mV, which resulted in continuous firing. 8. Due to the instability introduced by the N-shaped I-V relationship, pacemaker cells can display bistable behaviour characterized by hyperpolarizing responses.(ABSTRACT TRUNCATED AT 400 WORDS)