Electrophysiological roles of L-type channels in different classes of guinea pig sympathetic neuron

ACTIVATION OF CYCLIN-DEPENDENT KINASE 5 BROADENS ACTION POTENTIALS IN HUMAN SENSORY NEURONS

Chronic pain is one of the most devastating and unpleasant conditions, associated with many pathological conditions. Tissue or nerve injuries induce comprehensive neurobiological plasticity in nociceptive neurons, which leads to chronic pain. Recent studies suggest that cyclin-dependent kinase 5 (CDK5) in primary afferents is a key neuronal kinase that modulates nociception through phosphorylation-dependent manner under pathological conditions. However, the impact of the CDK5 on nociceptor activity especially in human sensory neurons are not known. To determine the CDK5-mediated regulation of human dorsal root ganglia (hDRG) neuronal properties, we have performed the whole-cell patch clamp recordings in neurons dissociated from hDRG. CDK5 activation induced by overexpression of p35 depolarized the resting membrane potential and reduced the rheobase currents as compared to the uninfected neurons. CDK5 activation evidently changed the shape of the action potential (AP) by increasing AP rise time, AP fall time, and AP half width. The application of a prostaglandin E2 (PG) and bradykinin (BK) cocktail in uninfected hDRG neurons induced the depolarization of RMP and the reduction of rheobase currents along with increased AP rise time. However, PG and BK applications failed to induce any further significant changes in addition to the aforementioned changes of the membrane properties and AP parameters in the p35-overexpressing group. We conclude that CDK5 activation through the overexpression of p35 in dissociated hDRG neurons broadens AP in hDRG neurons and that CDK5 may play important roles in the modulation of AP properties in human primary afferents under pathological conditions, contributing to chronic pain.

Activation of cyclin-dependent kinase 5 broadens action potentials in human sensory neurons

Chronic pain is one of the most devastating and unpleasant conditions, associated with many pathological states. Tissue or nerve injuries induce extensive neurobiological plasticity in nociceptive neurons, which leads to chronic pain. Recent studies suggest that cyclin-dependent kinase 5 (CDK5) in primary afferents is a key neuronal kinase that modulates nociception through phosphorylation under pathological conditions. However, the impact of the CDK5 on nociceptor activity especially in human sensory neurons is not known. To determine the CDK5-mediated regulation of human dorsal root ganglia (hDRG) neuronal properties, we have performed the whole-cell patch clamp recordings in neurons dissociated from hDRG. CDK5 activation induced by overexpression of p35 depolarized the resting membrane potential (RMP) and reduced the rheobase currents as compared to the control neurons. CDK5 activation changed the shape of the action potential (AP) by increasing AP -rise time, -fall time, and -half width. The application of a prostaglandin E2 (PG) and bradykinin (BK) cocktail in control hDRG neurons induced the depolarization of RMP and the reduction of rheobase currents along with increased AP rise time. However, PG and BK applications failed to induce any significant changes in the p35-overexpressing group. We conclude that, in dissociated hDRGs neurons, CDK5 activation through the overexpression of p35 broadens the AP and that CDK5 may play important roles in the modulation of AP properties in human primary afferents under the condition in which CDK5 is upregulated, contributing to chronic pain.

Structural and Functional Coupling of Calcium-Activated BK Channels and Calcium-Permeable Channels Within Nanodomain Signaling Complexes

Biochemical and functional studies of ion channels have shown that many of these integral membrane proteins form macromolecular signaling complexes by physically associating with many other proteins. These macromolecular signaling complexes ensure specificity and proper rates of signal transduction. The large-conductance, Ca²⁺-activated K⁺ (BK) channel is dually activated by membrane depolarization and increases in intracellular free Ca²⁺ ([Ca²⁺]_i). The activation of BK channels results in a large K⁺ efflux and, consequently, rapid membrane repolarization and closing of the voltage-dependent Ca²⁺-permeable channels to limit further increases in [Ca²⁺]_i. Therefore, BK channel-mediated K⁺ signaling is a negative feedback regulator of both membrane potential and [Ca²⁺]_i and plays important roles in many physiological processes and diseases. However, the BK channel formed by the pore-forming and voltage- and Ca²⁺-sensing α subunit alone requires high [Ca²⁺]_i levels for channel activation under physiological voltage conditions. Thus, most native BK channels are believed to co-localize with Ca²⁺-permeable channels within nanodomains (a few tens of nanometers in distance) to detect high levels of [Ca²⁺]_i around the open pores of Ca²⁺-permeable channels. Over the last two decades, advancement in research on the BK channel’s coupling with Ca²⁺-permeable channels including recent reports involving NMDA receptors demonstrate exemplary models of nanodomain structural and functional coupling among ion channels for efficient signal transduction and negative feedback regulation. We hereby review our current understanding regarding the structural and functional coupling of BK channels with different Ca²⁺-permeable channels.

A Calcium-Dependent Chloride Current Increases Repetitive Firing in Mouse Sympathetic Neurons

Ca2+-activated ion channels shape membrane excitability in response to elevations in intracellular Ca2+. The most extensively studied Ca2+-sensitive ion channels are Ca2+-activated K+ channels, whereas the physiological importance of Ca2+-activated Cl- channels has been poorly studied. Here we show that a Ca2+-activated Cl- currents (CaCCs) modulate repetitive firing in mouse sympathetic ganglion cells. Electrophysiological recording of mouse sympathetic neurons in an in vitro preparation of the superior cervical ganglion (SCG) identifies neurons with two different firing patterns in response to long depolarizing current pulses (1 s). Neurons classified as phasic (Ph) made up 67% of the cell population whilst the remainders were tonic (T). When a high frequency train of spikes was induced by intracellular current injection, SCG sympathetic neurons reached an afterpotential mainly dependent on the ratio of activation of two Ca2+-dependent currents: the K+ [IK(Ca)] and CaCC. When the IK(Ca) was larger, an afterhyperpolarization was the predominant afterpotential but when the CaCC was larger, an afterdepolarization (ADP) was predominant. These afterpotentials can be observed after a single action potential (AP). Ph and T neurons had similar ADPs and hence, the CaCC does not seem to determine the firing pattern (Ph or T) of these neurons. However, inhibition of Ca2+-activated Cl- channels with anthracene-9'-carboxylic acid (9AC) selectively inhibits the ADP, reducing the firing frequency and the instantaneous frequency without affecting the characteristics of single- or first-spike firing of both Ph and T neurons. Furthermore, we found that the CaCC underlying the ADP was significantly larger in SCG neurons from males than from females. Furthermore, the CaCC ANO1/TMEM16A was more strongly expressed in male than in female SCGs. Blocking ADPs with 9AC did not modify synaptic transmission in either Ph or T neurons. We conclude that the CaCC responsible for ADPs increases repetitive firing in both Ph and T neurons, and it is more relevant in male mouse sympathetic ganglion neurons.

Orexin-A potentiates L-type calcium/barium currents in rat retinal ganglion cells

Two neuropeptides, orexin-A and orexin-B (also called hypocretin-1 and -2), have been implicated in sleep/wake regulation, feeding behaviors via the activation of two subtypes of G-protein-coupled receptors: orexin 1 and orexin 2 receptors (OX1R and OX2R). While the expression of orexins and orexin receptors is immunohistochemically revealed in retinal neurons, the function of these peptides in the retina is largely unknown. Using whole-cell patch-clamp recordings in rat retinal slices, we demonstrated that orexin-A increased L-type-like barium currents (IBa,L) in ganglion cells (GCs), and the effect was blocked by the selective OX1R antagonist SB334867, but not by the OX2R antagonist TCS OX2 29. The orexin-A effect was abolished by intracellular dialysis of GDP-β-S/GPAnt-2A, a Gq protein inhibitor, suggesting the mediation of Gq. Additionally, during internal dialysis of the phosphatidylinositol (PI)-phospholipase C (PLC) inhibitor U73122, orexin-A did not change the IBa,L of GCs, whereas the orexin-A effect persisted in the presence of the phosphatidylcholine (PC)-PLC inhibitor D609. The orexin-A-induced potentiation was not seen with internal infusion of Ca(2+)-free solution or when inositol 1,4,5-trisphosphate (IP3)-sensitive Ca(2+) release from intracellular stores was blocked by heparin/xestospongins-C. Moreover, the orexin-A effect was mimicked by the protein kinase C (PKC) activator phorbol 12-myristate 13-acetate, but was eliminated when PKC was inhibited by bisindolylmaleimide IV (Bis-IV)/Gö6976. Neither adenosine 3',5'-cyclic monophosphate (cAMP)-protein kinase A (PKA) nor guanosine 3',5'-cyclic monophosphate (cGMP)-protein kinase G (PKG) signaling pathway was likely involved, as orexin-A persisted to potentiate the IBa,L of GCs no matter these two pathways were activated or inhibited. These results suggest that, by activating OX1R, orexin-A potentiates the IBa,L of rat GCs through a distinct Gq/PI-PLC/IP3/Ca(2+)/PKC signaling pathway.

Neurotrophins and target interactions in the development and regulation of sympathetic neuron electrical and synaptic properties

The electrical and synaptic properties of neurons are essential for determining the function of the nervous system. Thus, understanding the mechanisms that control the appropriate developmental acquisition and maintenance of these properties is a critical problem in neuroscience. A great deal of our understanding of these developmental mechanisms comes from studies of soluble growth factor signaling between cells in the peripheral nervous system. The sympathetic nervous system has provided a model for studying the role of these factors both in early development and in the establishment of mature properties. In particular, neurotrophins produced by the targets of sympathetic innervation regulate the synaptic and electrophysiological properties of postnatal sympathetic neurons. In this review we examine the role of neurotrophin signaling in the regulation of synaptic strength, neurotransmitter phenotype, voltage-gated currents and repetitive firing properties of sympathetic neurons. Together, these properties determine the level of sympathetic drive to target organs such as the heart. Changes in this sympathetic drive, which may be linked to dysfunctions in neurotrophin signaling, are associated with devastating diseases such as high blood pressure, arrhythmias and heart attack. Neurotrophins appear to play similar roles in modulating the synaptic and electrical properties of other peripheral and central neuronal systems, suggesting that information provided from studies in the sympathetic nervous system will be widely applicable for understanding the neurotrophic regulation of neuronal function in other systems.

Presynaptic Ca2+/calmodulin-dependent protein kinase II modulates neurotransmitter release by activating BK channels at Caenorhabditis elegans neuromuscular junction

Although Ca2+/calmodulin-dependent protein kinase II (CaMKII) is enriched at the presynaptic nerve terminal, its role in neurotransmitter release is poorly defined. We assessed the function of presynaptic CaMKII in neurotransmitter release and tested the hypothesis that BK channel is a mediator of presynaptic CaMKII function by analyzing miniature and evoked postsynaptic currents at the Caenorhabditis elegans neuromuscular junction. Both loss-of-function (lf) and gain-of-function (gf) of unc-43, the gene encoding CaMKII, inhibited neurotransmitter release. The inhibitory effect of unc-43(gf) was reversed by mutation or blockade of the BK channel SLO-1. SLO-1 expressed in Xenopus oocytes could be activated by recombinant rat alpha-CaMKII, and this effect of CaMKII was abolished by mutating a threonine residue (T425) at a consensus CaMKII phosphorylation site in the first RCK (regulator of conductance for K+) domain of the channel. Expression of slo-1(T425A) in neurons antagonized the inhibitory effect of unc-43(gf) on neurotransmitter release as slo-1(lf) did. The inhibitory effect of unc-43(gf) was not reversed by unc-103(lf), dgk-1(lf), or eat-16(lf), which reportedly suppress behavioral phenotypes of unc-43(gf). These observations suggest that presynaptic CaMKII is a bidirectional modulator of neurotransmitter release, presumably by phosphorylating different molecular targets, and that its negative modulatory effect on the release is mainly mediated by SLO-1 activation.

BKCa-Cav Channel Complexes Mediate Rapid and Localized Ca2+-Activated K+ Signaling

Large-conductance calcium- and voltage-activated potassium channels (BKCa) are dually activated by membrane depolarization and elevation of cytosolic calcium ions (Ca2+). Under normal cellular conditions, BKCa channel activation requires Ca2+ concentrations that typically occur in close proximity to Ca2+ sources. We show that BKCa channels affinity-purified from rat brain are assembled into macromolecular complexes with the voltage-gated calcium channels Cav1.2 (L-type), Cav2.1 (P/Q-type), and Cav2.2 (N-type). Heterologously expressed BKCa-Cav complexes reconstitute a functional "Ca2+ nanodomain" where Ca2+ influx through the Cav channel activates BKCa in the physiological voltage range with submillisecond kinetics. Complex formation with distinct Cav channels enables BKCa-mediated membrane hyperpolarization that controls neuronal firing pattern and release of hormones and transmitters in the central nervous system.

Different types of potassium channels underlie the long afterhyperpolarization in guinea-pig sympathetic and enteric neurons

Ca(2+)-activated K(+) channels play an important role in the control of neuronal excitability via the generation of the afterhyperpolarization. While both small and large conductance Ca(2+)-activated K(+) channels underlie afterhyperpolarizations in different neuron types, the role of intermediate conductance Ca(2+)-activated K(+) channels (IK(Ca)) in the generation of afterhyperpolarizations remains unclear. The effects of blockade of IK(Ca) on guinea pig coeliac and ileal myenteric neurons were studied using single microelectrode current and voltage clamp. In coeliac neurons, TRAM-39, a selective blocker of IK(Ca), depressed the amplitude of the prolonged conductance underlying the slow afterhyperpolarization, (gKCa2) by 57%. In contrast, the conductance underlying the prolonged afterhyperpolarization in AH-type myenteric neurons was unaffected by TRAM-39, although it has been suggested that this AHP is mediated by IK(Ca). In both types of neurons, TRAM-39 did not alter the resting cell properties or the properties of the action potential. TRAM-39 had no effect on the amplitude of the fast component of the afterhyperpolarization present in sympathetic LAH neurons. The results of this study suggest that in sympathetic LAH neurons, activation of IK(Ca) underlies at least part of the prolonged afterhyperpolarization while the nature of the channel underlying the AHP in enteric neurons remains unclear.

Differential expression of calcium channels in sympathetic and parasympathetic preganglionic inputs to neurons in paracervical ganglia of guinea-pigs

Neurons in pelvic ganglia receive nicotinic excitatory post-synaptic potentials (EPSPs) from sacral preganglionic neurons via the pelvic nerve, lumbar preganglionic neurons via the hypogastric nerve or both. We tested the effect of a range of calcium channel antagonists on EPSPs evoked in paracervical ganglia of female guinea-pigs after pelvic or hypogastric nerve stimulation. omega-Conotoxin GVIA (CTX GVIA, 100 nM) or the novel N-type calcium channel antagonist, CTX CVID (100 nM) reduced the amplitude of EPSPs evoked after pelvic nerve stimulation by 50-75% but had no effect on EPSPs evoked by hypogastric nerve stimulation. Combined addition of CTX GVIA and CTX CVID was no more effective than either antagonist alone. EPSPs evoked by stimulating either nerve trunk were not inhibited by the P/Q calcium channel antagonist, omega-agatoxin IVA (100 nM), nor the L-type calcium channel antagonist, nifedipine (30 microM). SNX 482 (300 nM), an antagonist at some R-type calcium channels, inhibited EPSPs after hypogastric nerve stimulation by 20% but had little effect on EPSPs after pelvic nerve stimulation. Amiloride (100 microM) inhibited EPSPs after stimulation of either trunk by 40%, while nickel (100 microM) was ineffective. CTX GVIA or CTX CVID (100 nM) also slowed the rate of action potential repolarization and reduced afterhyperpolarization amplitude in paracervical neurons. Thus, release of transmitter from the terminals of sacral preganglionic neurons is largely dependent on calcium influx through N-type calcium channels, although an unknown calcium channel which is resistant to selective antagonists also contributes to release. Release of transmitter from lumbar preganglionic neurons does not require calcium entry through either conventional N-type calcium channels or the variant CTX CVID-sensitive N-type calcium channel and seems to be mediated largely by a novel calcium channel.

Simulations to derive membrane resistivity in three phenotypes of guinea pig sympathetic postganglionic neuron

The electrotonic behavior of three phenotypes of sympathetic postganglionic neuron has been analyzed to assess whether their distinct cell input capacitances simply reflect differences in morphology. Because the distribution of membrane properties over the soma and dendrites is unknown, compartmental models incorporating cell morphology were used to simulate hyperpolarizing responses to small current steps. Neurons were classified as phasic (Ph), tonic (T), or long-afterhyperpolarizing (LAH) by their discharge pattern to threshold depolarizing current steps and filled with biocytin to determine their morphology. Responses were simulated in models with the average morphology of each cell class using the program NEURON. Specific membrane resistivity, R(m), was derived in each model. Fits were acceptable when specific membrane capacitance, C(m), and specific resistivity of the axoplasm, R(i,) were varied within realistic limits and when underestimation of membrane area due to surface irregularities was accounted for. In all models with uniform R(m), solutions for R(m) that were the same for all classes could not be found unless C(m) or R(i) were different for each class, which seems unrealistic. Incorporation of a small somatic shunt conductance yielded values for R(m) for each class close to those derived assuming isopotentiality (R(m) approximately 40, 27, and 15 k omega cm(2) for T, Ph, and LAH neurons, respectively). It is concluded that R(m) is distinct between neuron classes. Because Ph and LAH neurons relay selected preganglionic inputs directly, R(m) generally affects function only in T neurons that integrate multiple subthreshold inputs and are modulated by peptidergic transmitters.

Distinctive neurophysiological properties of embryonic trigeminal and geniculate neurons in culture

Neurons in trigeminal and geniculate ganglia extend neurites that share contiguous target tissue fields in the fungiform papillae and taste buds of the mammalian tongue and thereby have principal roles in lingual somatosensation and gustation. Although functional differentiation of these neurons is central to formation of lingual sensory circuits, there is little known about electrophysiological properties of developing trigeminal and geniculate ganglia or the extrinsic factors that might regulate neural development. We used whole cell recordings from embryonic day 16 rat ganglia, maintained in culture as explants for 3-10 days with neurotrophin support to characterize basic properties of trigeminal and geniculate neurons over time in vitro and in comparison to each other. Each ganglion was cultured with the neurotrophin that supports maximal neuron survival and that would be encountered by growing neurites at highest concentration in target fields. Resting membrane potential and time constant did not alter over days in culture, whereas membrane resistance decreased and capacitance increased in association with small increases in trigeminal and geniculate soma size. Small gradual differences in action potential properties were observed for both ganglion types, including an increase in threshold current to elicit an action potential and a decrease in duration and increase in rise and fall slopes so that action potentials became shorter and sharper with time in culture. Using a period of 5-8 days in culture when neural properties are generally stable, we compared trigeminal and geniculate ganglia and revealed major differences between these embryonic ganglia in passive membrane and action potential characteristics. Geniculate neurons had lower resting membrane potential and higher input resistance and smaller, shorter, and sharper action potentials with lower thresholds than trigeminal neurons. Whereas all trigeminal neurons produced a single action potential at threshold depolarization, 35% of geniculate neurons fired repetitively. Furthermore, all trigeminal neurons produced TTX-resistant action potentials, but geniculate action potentials were abolished in the presence of low concentrations of TTX. Both trigeminal and geniculate neurons had inflections on the falling phase of the action potential that were reduced in the presence of various pharmacological blockers of calcium channel activation. Use of nifedipine, omega-conotoxin-MVIIA and GVIA, and omega-agatoxin-TK indicated that currents through L-, N-, and P/Q- type calcium channels participate in the action potential inflection in embryonic trigeminal and geniculate neurons. The data on passive membrane, action potential, and ion channel characteristics demonstrate clear differences between trigeminal and geniculate ganglion neurons at an embryonic stage when target tissues are innervated but receptor organs have not developed or are still immature. Therefore these electrophysiological distinctions between embryonic ganglia are present before neural activity from differentiated receptive fields can influence functional phenotype.

Ca2+ involvement in the action potential generation of myenteric neurones in the rat oesophagus

Intracellular recordings were used to study the physiological behaviour of rat oesophageal myenteric neurones, which are embedded in striated muscle. Injection of depolarizing pulses evoked action potentials with a clear 'shoulder' in all neurones. This shoulder disappeared under low Ca2+/high Mg2+ conditions. Tetrodotoxin (TTX; 1 micromol L-1) did not impede spike firing, whereas under combined TTX and low Ca2+/high Mg2+ conditions the action potentials were completely abolished, indicating that TTX- resistant action potentials are mediated by a Ca2+ current. Further experiments with omega-conotoxin GVIA (100 nmol L-1) revealed that these Ca2+ currents enter the cell via N-type voltage-activated Ca2+ channels (see also accompanying paper). Tetraethylammonium (10 mmol L-1) caused broadening of the action potentials, which probably resulted from prolonged Ca2+ influx due to blockade of the delayed rectifier K+ channel. Although Ca2+ appears to be involved in the spike generation of all rat oesophageal myenteric neurones, only a minority (14%) shows a slow afterhyperpolarization. Thus, no strict correlation exists between the presence of a shoulder and a slow afterhyperpolarization. Furthermore, morphological identification of 25 of the impaled neurones revealed that there was no strict correlation between morphology and electrophysiological behaviour. Consequently, rat oesophageal myenteric neurones appear to differ in several aspects from myenteric neurones in smooth muscle regions of the gastrointestinal tract.

Development of electrophysiological and morphological diversity in autonomic neurons

The generation of neuronal diversity requires the coordinated development of differential patterns of ion channel expression along with characteristic differences in dendritic geometry, but the relations between these phenotypic features are not well known. We have used a combination of intracellular recordings, morphological analysis of dye-filled neurons, and stereological analysis of immunohistochemically labeled sections to investigate the development of characteristic electrical and morphological properties of functionally distinct populations of sympathetic neurons that project from the celiac ganglion to the splanchnic vasculature or the gastrointestinal tract of guinea pigs. At early fetal stages, neurons were significantly more depolarized at rest compared with neurons at later stages, and they generally fired only a single action potential. By mid fetal stages, rapidly and slowly adapting neurons could be distinguished with a topographic distribution matching that found in adult ganglia. Most rapidly adapting neurons (phasic neurons) at this age had a long afterhyperpolarization (LAH) characteristic of mature vasomotor neurons and were preferentially located in the lateral poles of the ganglion, where most neurons contained neuropeptide Y. Most early and mid fetal neurons showed a weak M current, which was later expressed only by rapidly-adapting and LAH neurons. Two different A currents were present in a subset of early fetal neurons and may indicate neurons destined to develop a slowly adapting phenotype (tonic neurons). The size of neuronal cell bodies increased at a similar rate throughout development regardless of their electrical or neurochemical phenotype or their topographical location. In contrast, the rate of dendritic growth of neurons in medial regions of the ganglion was significantly higher than that of neurons in lateral regions. The apparent cell capacitance was highly correlated with the surface area of the soma but not the dendritic tree of the developing neurons. These results demonstrate that the well-defined functional populations of neurons in the celiac ganglion develop their characteristic electrophysiological and morphological properties during early fetal stages of development. This is after the neuronal populations can be recognized by their neurochemical and topographical characteristics but long before the neurons have finished growing. Our data provide strong circumstantial evidence that the development of the full phenotype of different functional classes of autonomic final motor neurons is a multi-step process likely to involve a regulated sequence of trophic interactions.

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.

Diversity of channels involved in Ca(2+) activation of K(+) channels during the prolonged AHP in guinea-pig sympathetic neurons

The types of Ca(2+)-dependent K(+) channel involved in the prolonged afterhyperpolarization (AHP) in a subgroup of sympathetic neurons have been investigated in guinea pig celiac ganglia in vitro. The conductance underlying the prolonged AHP (gKCa2) was reduced to a variable extent in 100 nM apamin, an antagonist of SK-type Ca(2+)-dependent K(+) channels, and by about 55% in 20 nM iberiotoxin, an antagonist of BK-type Ca(2+)-dependent K(+) channels. The reductions in gKCa2 amplitude by apamin and iberiotoxin were not additive, and a resistant component with an amplitude of nearly 50% of control remained. These data imply that, as well as apamin- and iberiotoxin-sensitive channels, other unknown Ca(2+)-dependent K(+) channels participate in gKCa2. The resistant component of gKCa2 was not abolished by 0.5-10 mM tetraethylammonium, 1 mM 4-aminopyridine, or 5 mM glibenclamide. We also investigated which voltage-gated channels admitted Ca(2+) for the generation of gKCa2. Blockade of Ca(2+) entry through L-type Ca(2+) channels has previously been shown to reduce gKCa2 by about 40%. Blockade of N-type Ca(2+) channels (with 100 nM omega-conotoxin GVIA) and P-type Ca(2+) channels (with 40 nM omega-agatoxin IVA) each reduced the amplitude of gKCa2 by about 35%. Thus Ca(2+) influx through multiple types of voltage-gated Ca(2+) channel can activate the intracellular mechanisms that generate gKCa2. The slow time course of gKCa2 may be explained if activation of multiple K(+) channels results from Ca(2+) influx triggering a kinetically invariant release of Ca(2+) from intracellular stores located close to the membrane.

Distinct mechanisms for activation of Cl- and K+ currents by Ca2+ from different sources in mouse sympathetic neurones

We have investigated the roles of different voltage-dependent Ca2+ channels in the activation of the Cl- and K+ channels responsible for the afterdepolarization (ADP) and slow afterhyperpolarization (AHP) in sympathetic neurones of the isolated mouse superior cervical ganglion in vitro. The ADP and its associated Ca2+-activated Cl- current were markedly decreased by omega-agatoxin IVA (40-200 nM) and nifedipine (1-10 microM), but not by omega-conotoxin GVIA (300 nM). In contrast, the AHP and the apamin-sensitive Ca2+-activated K+ current that underlies this potential were blocked by omega-conotoxin GVIA, but were not affected by omega-agatoxin IVA and were only slightly reduced by nifedipine. Ryanodine (20 microM) reduced the Ca2+-activated Cl- current following an action potential by 75% but on average did not affect the Ca2+-activated K+ current. Evidence that R-type channels provide a proportion of the Ca2+ activating both types of Ca2+-dependent channel was obtained. We conclude that Ca2+ entering through L- and P-type Ca2+ channels preferentially activates the Cl- current responsible for the ADP in mouse sympathetic neurones, predominantly via Ca2+-induced Ca2+ release, whereas the Ca2+ that activates the K+ channels responsible for the AHP enters predominantly through N-type channels. The data can be explained by the selective association of each type of Ca2+ channel with particular intracellular mechanisms for activating other membrane channels, one indirect and the other direct, probably located at discrete sites on the soma and dendrites.

Calcium-activated potassium currents in mammalian neurons

1. Influx of calcium via voltage-dependent calcium channels during the action potential leads to increases in cytosolic calcium that can initiate a number of physiological processes. One of these is the activation of potassium currents on the plasmalemma. These calcium-activated potassium currents contribute to action potential repolarization and are largely responsible for the phenomenon of spike frequency adaptation. This refers to the progressive slowing of the frequency of discharge of action potentials during sustained injection of depolarizing current. In some cell types, this adaptation is so marked that despite the presence of depolarizing current, only a single spike (or a few spikes) is initiated. Following cessation of current injection, slow deactivation of calcium-activated potassium currents is also responsible for the prolonged hyperpolarization that often follows. 2. A number of macroscopic calcium-activated potassium currents that can be separated on the basis of kinetic and pharmacological criteria have been described in mammalian neurons. At the single channel level, several types of calcium-activated potassium channels also have been characterized. While for some macroscopic currents the underlying single channels have been unambiguously defined, for other currents the identity of the underlying channels is not clear. 3. In the present review we describe the properties of the known types of calcium-activated potassium currents in mammalian neurons and indicate the relationship between macroscopic currents and particular single channels.

Dendritic L-type calcium currents in mouse spinal motoneurons: implications for bistability

The intrinsic properties of mammalian spinal motoneurons provide them with the capability to produce high rates of sustained firing in response to transient inputs (bistability). Even though it has been suggested that a persistent dendritic calcium current is responsible for the depolarizing drive underlying this firing property, such a current has not been demonstrated in these cells. In this study, calcium currents are recorded from functionally mature mouse spinal motoneurons using somatic whole-cell patch-clamp techniques. Under these conditions a component of the current demonstrated kinetics consistent with a current originating at a site spatially segregated from the soma. In response to step commands this component was seen as a late-onset, low amplitude persistent current whilst in response to depolarizing-repolarizing ramp commands a low voltage clockwise current hysteresis was recorded. Simulations using a neuromorphic motoneuron model could reproduce these currents only if a noninactivating calcium conductance was placed in the dendritic compartments. Pharmacological studies demonstrated that both the late-onset and hysteretic currents demonstrated sensitivity to both dihydropyridines and the L-channel activator FPL-64176. Furthermore, the alpha1D subunits of L-type calcium channels were immunohistochemically demonstrated on motoneuronal dendrites. It is concluded that there are dendritically located L-type channels in mammalian motoneurons capable of mediating a persistent depolarizing drive to the soma and which probably mediate the bistable behaviour of these cells.