Potassium currents of acutely isolated adult rat superior cervical ganglion neurons

Cellular and Molecular Mechanisms Underlying Altered Excitability of Cardiac Efferent Neurons in Cirrhotic Rats

Patients with cirrhosis often exhibit cardiac autonomic dysfunction (CAD), characterized by enhanced cardiac sympathetic activity and diminished cardiac vagal tone, leading to increased morbidity and mortality. This study delineates the cellular and molecular mechanisms associated with altered neuronal activities causing cirrhosis-induced CAD. Biliary and nonbiliary cirrhotic rats were produced by common bile duct ligation (CBDL) and intraperitoneal injections of thioacetamide (TAA), respectively. Three weeks after CBDL or TAA injection, the assessment of heart rate variability revealed autonomic imbalance in cirrhotic rats. We observed increased excitability in stellate ganglion (SG) neurons and decreased excitability in intracardiac ganglion (ICG) neurons in cirrhotic rats compared to sham-operated controls. Additionally, threshold, rheobase, and action potential duration exhibited opposite alterations in SG and ICG neurons, along with changes in afterhyperpolarization duration. A- and M-type K⁺ channels were significantly downregulated in SG neurons, while M-type K⁺ channels were upregulated, with downregulation of the N- and L-type Ca2⁺ channels in the ICG neurons of cirrhotic rats, both in transcript expression and functional activity. Collectively, these findings suggest that cirrhosis induces an imbalance between cardiac sympathetic and parasympathetic neuronal activities via the differential regulation of K+ and Ca2+ channels. Thus, cirrhosis-induced CAD may be associated with impaired autonomic efferent functions within the homeostatic reflex arc that regulates cardiac functions.

Inactivation of potassium channels by ceramide in rat pancreatic β-cells

Lipid regulation of ion channels is a fundamental mechanism in physiological processes as of neurotransmitter release and hormone secretion. Ceramide is a bioactive lipid proposed as a regulator of several voltage-gated ion channels including potassium channels (Kv). It is generated either de novo or by sphingomyelin (SM) hydrolysis in membranes of mammalian cells. In pancreatic β-cells, ceramide is the main sphingolipid associated with lipotoxicity and likely involved in cell dysfunction. Despite of the wealth of information regarding regulation of potassium channels by ceramides, the regulation of Kv channels by accumulated ceramide in native pancreatic β-cells has not been investigated. To do so, we used either the C2-ceramide, a cell-permeable short-chain analogue, or a sphingomyelinase (SMase C), a hydrolase causing ceramide to elevate from an endogenous production, in pancreatic β-cells of rat. C2-ceramide markedly accelerates steady-state current inactivation according to kinetic changes in the channel machinery. Interestingly, only C2-ceramide accelerates current inactivation while SMase C decreases both, peak-current and step-current amplitude supporting differential effects of ceramide derivatives. A specific inhibitor of the Kv2.1 channel (GxTX-1E), readily inhibits a fraction of the Kv channel current while no further inhibition by C2-ceramide superfusion can be observed supporting Kv2.1 channel involvement in the ceramide inhibition. Thus, intramembrane ceramide accumulation, as a lipidic metabolite released under cell-stress conditions, may alter pancreatic β-cell repolarization and secretion. These results may provide a new insight regarding lipid-protein regulation and advance our understanding in ceramide actions on Kv channels in pancreatic β-cells.

Functional plasticity of cardiac efferent neurons contributes to traumatic brain injury-induced cardiac autonomic dysfunction

Traumatic brain injury (TBI) frequently causes cardiac autonomic dysfunction (CAD), irrespective of its severity, which is associated with an increased morbidity and mortality in patients. Despite the significance of probing the cellular mechanism underlying TBI-induced CAD, animal studies on this mechanism are lacking. In the current study, we tested whether TBI-induced CAD is associated with functional plasticity in cardiac efferent neurons. In this regard, TBI was induced by a controlled cortical impact in rats. Assessment of heart rate variability and baroreflex sensitivity indicated that CAD was developed in the sub-acute period after moderate and severe TBI. The cell excitability was increased in the stellate ganglion (SG) neurons and decreased in the intracardiac ganglion (ICG) neurons in TBI rats, compared with the sham-operated rats. The transient A-type K+ (KA) currents, but not the delayed rectifying K+ currents were significantly decreased in SG neurons in TBI rats, compared with sham-operated rats. Consistent with these electrophysiological data, the transcripts encoding the Kv4 α subunits were significantly downregulated in SG neurons in TBI rats, compared with sham-operated rats. TBI causes downregulation and upregulation of M-type K+ (KM) currents and the KCNQ2 mRNA transcripts, which may contribute to the hyperexcitability of the SG neurons and the hypoexcitability of the ICG neurons, respectively. In conclusion, the key cellular mechanism underlying the TBI-induced CAD may be the functional plasticity of the cardiac efferent neurons, which is caused by the regulation of the KA and/or KM currents.

Activity of Palythoa caribaeorum Venom on Voltage-Gated Ion Channels in Mammalian Superior Cervical Ganglion Neurons

The Zoanthids are an order of cnidarians whose venoms and toxins have been poorly studied. Palythoa caribaeorum is a zoanthid commonly found around the Mexican coastline. In this study, we tested the activity of P. caribaeorum venom on voltage-gated sodium channel (Na_V1.7), voltage-gated calcium channel (Ca_V2.2), the A-type transient outward (I_A) and delayed rectifier (I_DR) currents of K_V channels of the superior cervical ganglion (SCG) neurons of the rat. These results showed that the venom reversibly delays the inactivation process of voltage-gated sodium channels and inhibits voltage-gated calcium and potassium channels in this mammalian model. The compounds responsible for these effects seem to be low molecular weight peptides. Together, these results provide evidence for the potential use of zoanthids as a novel source of cnidarian toxins active on voltage-gated ion channels.

Forskolin suppresses delayed-rectifier K+ currents and enhances spike frequency-dependent adaptation of sympathetic neurons

In signal transduction research natural or synthetic molecules are commonly used to target a great variety of signaling proteins. For instance, forskolin, a diterpene activator of adenylate cyclase, has been widely used in cellular preparations to increase the intracellular cAMP level. However, it has been shown that forskolin directly inhibits some cloned K⁺ channels, which in excitable cells set up the resting membrane potential, the shape of action potential and regulate repetitive firing. Despite the growing evidence indicating that K⁺ channels are blocked by forskolin, there are no studies yet assessing the impact of this mechanism of action on neuron excitability and firing patterns. In sympathetic neurons, we find that forskolin and its derivative 1,9-Dideoxyforskolin, reversibly suppress the delayed rectifier K⁺ current (I_KV). Besides, forskolin reduced the spike afterhyperpolarization and enhanced the spike frequency-dependent adaptation. Given that I_KV is mostly generated by Kv2.1 channels, HEK-293 cells were transfected with cDNA encoding for the Kv2.1 α subunit, to characterize the mechanism of forskolin action. Both drugs reversible suppressed the Kv2.1-mediated K⁺ currents. Forskolin inhibited Kv2.1 currents and I_KV with an IC₅₀ of ~32 μM and ~24 µM, respectively. Besides, the drug induced an apparent current inactivation and slowed-down current deactivation. We suggest that forskolin reduces the excitability of sympathetic neurons by enhancing the spike frequency-dependent adaptation, partially through a direct block of their native Kv2.1 channels.

Intranuclear microinjection of DNA into dissociated adult mammalian neurons

Primary neuronal cell cultures are valuable tools to study protein function since they represent a more biologically relevant system compared to immortalized cell lines. However, the post-mitotic nature of primary neurons prevents effective heterologous protein expression using common procedures such as electroporation or chemically-mediated transfection. Thus, other techniques must be employed in order to effectively express proteins in these non-dividing cells.

In this article, we describe the steps required to perform intranuclear injections of cDNA constructs into dissociated adult sympathetic neurons. This technique, which has been applied to different types of neurons, can successfully induce heterologous protein expression. The equipment essential for the microinjection procedure includes an inverted microscope to visualize cells, a glass injection pipet filled with cDNA solution that is connected to a N_2(g) pressure delivery system, and a micromanipulator. The micromanipulator coordinates the injection motion of microinjection pipet with a brief pulse of pressurized N₂ to eject cDNA solution from the pipet tip.

This technique does not have the toxicity associated with many other transfection methods and enables multiple DNA constructs to be expressed at a consistent ratio. The low number of injected cells makes the microinjection procedure well suited for single cell studies such as electrophysiological recordings and optical imaging, but may not be ideal for biochemical assays that require a larger number of cells and higher transfection efficiencies. Although intranuclear microinjections require an investment of equipment and time, the ability to achieve high levels of heterologous protein expression in a physiologically relevant environment makes this technique a very useful tool to investigate protein function.

G protein activation inhibits gating charge movement in rat sympathetic neurons

G protein-coupled receptors (GPCRs) control neuronal functions via ion channel modulation. For voltage-gated ion channels, gating charge movement precedes and underlies channel opening. Therefore, we sought to investigate the effects of G protein activation on gating charge movement. Nonlinear capacitive currents were recorded using the whole cell patch-clamp technique in cultured rat sympathetic neurons. Our results show that gating charge movement depends on voltage with average Boltzmann parameters: maximum charge per unit of linear capacitance (Q(max)) = 6.1 +/- 0.6 nC/microF, midpoint (V(h)) = -29.2 +/- 0.5 mV, and measure of steepness (k) = 8.4 +/- 0.4 mV. Intracellular dialysis with GTPgammaS produces a nonreversible approximately 34% decrease in Q(max), a approximately 10 mV shift in V(h), and a approximately 63% increase in k with respect to the control. Norepinephrine induces a approximately 7 mV shift in V(h) and approximately 40% increase in k. Overexpression of G protein beta(1)gamma(4) subunits produces a approximately 13% decrease in Q(max), a approximately 9 mV shift in V(h), and a approximately 28% increase in k. We correlate charge movement modulation with the modulated behavior of voltage-gated channels. Concurrently, G protein activation by transmitters and GTPgammaS also inhibit both Na(+) and N-type Ca(2+) channels. These results reveal an inhibition of gating charge movement by G protein activation that parallels the inhibition of both Na(+) and N-type Ca(2+) currents. We propose that gating charge movement decrement may precede or accompany some forms of GPCR-mediated channel current inhibition or downregulation. This may be a common step in the GPCR-mediated inhibition of distinct populations of voltage-gated ion channels.

Effects of ATP and GTP on voltage-gated K+ currents in glandular and muscular sympathetic neurons

This study assesses the effects of ATP and GTP on the kinetic properties of voltage-gated K+ currents in anatomically identified postganglionic sympathetic neurons innervating the submandibular gland and the masseter muscle in rats. Three types of K+ currents were isolated: the I(Af) steady-state inactivating at more hyperpolarized potentials, I(As) steady-state inactivating at less hyperpolarized potentials than I(Af) and the I(K) current independent of membrane potential. The kinetic properties of these currents were tested in neurons with ATP (4 mM) and GTP (0.5 mM) or without ATP and GTP in the intracellular solution. In glandular and muscular neurons in the absence of ATP and GTP in the intracellular solution, the current density of I(Af) was significantly larger (142 pA/pF and 166 pA/pF, respectively) comparing to cells with ATP and GTP (96 pA/pF and 100 pA/pF, respectively). The I(As) was larger only in glandular neurons (52 pA/pF vs. 37 pA/pF).Conversely, I(K) current density was smaller in glandular and muscular neurons without ATP and GTP (17 pA/pF and 31 pA/pF, respectively) comparing to cells with ATP and GTP (57 pA/pF and 58 pA/pF, respectively). In glandular (15.5 nA/ms vs. 6.9 nA/ms) and muscular (10.9 nA/ms vs. 7.5 nA/ms) neurons, the I(Af) activated faster in the absence of ATP and GTP. Half inactivation voltage of I(Af) in glandular (-110.0 mV vs. -119.7 mV) and muscular (-108.4 vs. -117.3 mV) neurons was shifted towards depolarization in the absence of ATP and GTP. We suggest that the kinetic properties of K+ currents in glandular and muscular sympathetic neurons change markedly in the absence of ATP and GTP in the cytoplasm. Effectiveness of steady-state inactivated currents (I(Af) and I(AS)) increased, while effectiveness of steady-state noninactivated currents decreased in the absence of ATP and GTP. The effects were more pronounced in glandular than in muscular neurons.

Postdecentralization plasticity of voltage-gated K+ currents in glandular sympathetic neurons in rats

This paper presents the kinetic and pharmacological properties of voltage-gated K(+) currents in anatomically identified glandular postganglionic sympathetic neurons isolated from the superior cervical ganglia in rats. The neurons were labelled by injecting the fluorescent tracer Fast Blue into the submandibular gland. The first group of neurons remained intact, i.e. innervated by the preganglionic axons until the day of current recordings (control neurons). The second group of neurons was denervated by severing the superior cervical trunk 4-6 weeks prior to current recordings (decentralized neurons). In every control and decentralized neuron three categories of voltage-dependent K(+) currents were found. (i) The I(Af) K(+) current, steady state, inactivated at hyperpolarized membrane potentials. This current was fast activated and fast time-dependently inactivated, insensitive to TEA and partially depressed by 4-AP. (ii) The I(As) K(+) current, which was steady-state inactivated at less hyperpolarized membrane potentials than I(Af). The current activation and time-dependent inactivation kinetics were slower than those of I(Af). I(As) was blocked by TEA and partially inhibited by 4-AP. (iii) The IK K(+) current did not undergo steady-state inactivation. In decentralized compared to control neurons the maximum I(Af) K(+) current density (at +50 mV) increased from 116.9 +/- 8.2 to 189.0 +/- 11.5 pA/pF, the 10-90% current rise time decreased from 2.3 to 0.7 ms and the recovery from inactivation was faster. Similarly, in decentralized compared to control neurons the maximum I(As) K(+) current density (at +50 mV) increased from 49.9 +/- 3.5 to 74.3 +/- 5.0 pA/pF, the 10-90% current rise time shortened from 29 to 16 ms and the recovery from inactivation of the current was also faster. The maximum density (at +50 mV) of I(K) in decentralized compared to control neurons decreased from 76.6 +/- 3.9 to 60.7 +/- 6.3 pA/pF. We suggest that the upregulation of voltage-gated time-dependently-inactivated K(+) currents and their faster recovery from inactivation serve to restrain the activity of glandular sympathetic neurons after decentralization.

Potassium currents in octopus cells of the mammalian cochlear nucleus

Octopus cells in the posteroventral cochlear nucleus (PVCN) of mammals are biophysically specialized to detect coincident firing in the population of auditory nerve fibers that provide their synaptic input and to convey its occurrence with temporal precision. The precision in the timing of action potentials depends on the low input resistance (approximately 6 MOmega) of octopus cells at the resting potential that makes voltage changes rapid (tau approximately 200 micros). It is the activation of voltage-dependent conductances that endows octopus cells with low input resistances and prevents repetitive firing in response to depolarization. These conductances have been examined under whole cell voltage clamp. The present study reveals the properties of two conductances that mediate currents whose reversal at or near the equilibrium potential for K(+) over a wide range of extracellular K(+) concentrations identifies them as K(+) currents. One rapidly inactivating conductance, g(KL), had a threshold of activation at -70 mV, rose steeply as a function of depolarization with half-maximal activation at -45 +/- 6 mV (mean +/- SD), and was fully activated at 0 mV. The low-threshold K(+) current (I(KL)) was largely blocked by alpha-dendrotoxin (alpha-DTX) and partially blocked by DTX-K and tityustoxin, indicating that this current was mediated through potassium channels of the Kv1 (also known as shaker or KCNA) family. The maximum low-threshold K(+) conductance (g(KL)) was large, 514 +/- 135 nS. Blocking I(KL) with alpha-DTX revealed a second K(+) current with a higher threshold (I(KH)) that was largely blocked by 20 mM tetraethylammonium (TEA). The more slowly inactivating conductance, g(KH), had a threshold for activation at -40 mV, reached half-maximal activation at -16 +/- 5 mV, and was fully activated at +30 mV. The maximum high-threshold conductance, g(KH), was on average 116 +/- 27 nS. The present experiments show that it is not the biophysical and pharmacological properties but the magnitude of the K(+) conductances that make octopus cells unusual. At the resting potential, -62 mV, g(KL) contributes approximately 42 nS to the resting conductance and mediates a resting K(+) current of 1 nA. The resting outward K(+) current is balanced by an inward current through the hyperpolarization-activated conductance, g(h), that has been described previously.

Mechanisms mediating pituitary adenylate cyclase-activating polypeptide depolarization of rat sympathetic neurons

The direct effects of pituitary adenylate cyclase-activating polypeptides (PACAP) on sympathetic neurons were investigated using rat superior cervical ganglion neurons. Electrophysiological and pharmacological analyses were used to evaluate PACAP modulation of sympathetic neuron membrane potentials and to investigate potential ionic and intracellular signaling mechanisms mediating the responses. More than 90% of the sympathetic neurons were depolarized by the PACAP peptides even when stimulated release was blocked, indicating that the PACAP peptides elicited primary responses in the postganglionic neurons. The response profile was consistent for activation of PACAP-selective PAC(1) receptors: nanomolar concentrations of PACAP27 and PACAP38 were required to stimulate depolarization, whereas vasoactive intestinal peptide failed to evoke any response. Furthermore, depolarizations elicited by PACAP27 were reduced by the PAC(1) receptor antagonist PACAP(6-38). Both sodium influx and inhibition of a potassium current contributed to the peptide-induced depolarizations. Activation of neither pertussis toxin- nor cholera toxin-sensitive G-proteins was required for generation of the depolarizations. cAMP and diacylglycerol production and activation of protein kinase A or protein kinase C also were not requisite for the responses. By contrast, phospholipase C (PLC)-dependent inositol 1,4,5-triphosphate (IP(3)) synthesis was crucial to the PACAP-mediated depolarizations. Although calcium release from IP(3)-sensitive stores was not required for the PACAP-induced responses, inhibition of IP(3) receptors reduced the depolarizations. Thus, among the many signal transduction pathways coupled to the PAC(1) receptor, the PACAP-induced depolarization of sympathetic neurons appears to require activation of PLC and subsequent generation of IP(3).

Spontaneous synaptogenesis in ex vivo sympathetic ganglion and the blockade by serum treatment

Central denervation for more than 1 month has been shown to cause an increase in the number of adrenergic synapses in sympathetic ganglia in vivo. Here, we report several lines of evidence that adrenergic synapses may be generated de novo in ex vivo superior cervical ganglion (SCG) of adult rats only several hours after the isolation. Structures immunoreactive for synaptophysin, a marker of presynaptic elements, were drastically decreased 6 days after the preganglionic denervation. A significant increase in number of synaptophysin positive boutons was observed over 3-8 hours in the denervated SCGs maintained ex vivo at 36 degrees C in oxygenated physiologic saline, and this increase was blocked by adding normal serum in the saline. Electron microscopic analysis confirmed that the number of adrenergic synapses specifically labeled with 5-hydroxydopamine was increased by several-fold under the same condition. Intracellular labeling of SCG neurons revealed an increase in the incidence (from 8 to 50%) of neurons having dendritic plexus after the in vitro incubation. No evidence of axonal sprouting within the ganglion was observed. Intracellular recordings from single neurons of denervated SCGs revealed that maximum amplitudes of inhibitory postsynaptic potentials, which were completely blocked by yohimbine, an alpha2-adrenoceptor antagonist, in response to focal stimulation were increased over the several hours. These results suggest that dendrites of SCG neurons rapidly develop and exhibit local efferent characteristics that underlie the inhibitory synaptic transmission once they are subjected to serum deprivation.

Unusual autonomic ganglia: connections, chemistry, and plasticity of pelvic ganglia

The pelvic ganglia provide the majority of the autonomic nerve supply to reproductive organs, urinary bladder, and lower bowel. Of all autonomic ganglia, they are probably the least understood because in many species their anatomy is particularly complex. Furthermore, they are unusual autonomic ganglia in many ways, including their connections, structure, chemistry, and hormone sensitivity. This review will compare and contrast the normal structure and function of pelvic ganglia with other types of autonomic ganglia (sympathetic, parasympathetic, and enteric). Two aspects of plasticity in the pelvic pathways will also be discussed. First, the influence of gonadal steroids on the maturation and maintenance of pelvic reflex circuits will be considered. Second, the consequences of nerve injury will be discussed, particularly in the context of the pelvic ganglia receiving distributed spinal inputs. The review demonstrates that in many ways the pelvic ganglia differ substantially from other autonomic ganglia. Pelvic ganglia may also provide a useful system in which to study many fundamental neurobiological questions of broader relevance.

Primary and adaptive changes of A-type K+ currents in sympathetic neurons from hypertensive rats

The A-type K+ current (IA) of superior cervical ganglion neurons acutely isolated from spontaneously hypertensive (SHR) and age-matched Wistar-Kyoto (WKY) rats was compared under whole cell voltage clamp. Activation parameters were similar in each strain. Steady-state inactivation was shifted approximately -6 mV in SHR, where one-half inactivation occurred at -81 mV vs. -75 mV in WKY rats. The shift was not present in prehypertensive SHR but remained in adult enalapril-treated SHR and, therefore, may represent a primary alteration of IA properties. IA amplitudes evoked from physiological potentials were similar, despite inactivation of a greater fraction of the current in the SHR. Comparing maximal IA densities revealed that current density is elevated in the SHR, which compensates for the inactivation shift. Current density decreased with age in WKY neurons but did not significantly decline in SHR neurons unless hypertension was prevented with enalapril. Thus adult SHR neurons may retain a high IA density as an adaptive response to offset potential hyperexcitability resulting from the hyperpolarized IA inactivation.

Selective inhibition of M-type potassium channels in rat sympathetic neurons by uridine nucleotide preferring receptors

1. UTP and UDP depolarize rat superior cervical ganglion neurons and trigger noradrenaline release from these cells. The present study investigated the mechanisms underlying this excitatory action of uridine nucleotides by measuring whole-cell voltage-dependent K+ and Ca2+ currents. 2. Steady-state outward (holding) currents measured in the amphotericin B perforated-patch configuration at a potential of -30 mV were reduced by 10 microM UTP in a reversible manner, but steady-state inward (holding) currents at -70 mV were not affected. This action of UTP was shared by the muscarinic agonist oxotremorine-M. In current-voltage curves between -20 and -100 mV, UTP diminished primarily the outwardly rectifying current components arising at potentials positive to -60 mV. 3. Slow relaxations of muscarinic K+ currents (IM) evoked by hyperpolarizations from -30 to -55 mV were also reduced by 10 microM UTP (37% inhibition) and oxotremorine-M (81% inhibition). In contrast, transient K+-currents, delayed rectifier currents, fast and slow Ca2+-dependent K+ currents, as well as voltage-dependent Ca2+ currents were not altered by UTP. 4. In conventional (open-tip) whole-cell recordings, replacement of GTP in the pipette by GDPbetaS abolished the UTP-induced inhibition of IM, whereas replacement by GTPgammaS rendered it irreversible. 5. The UTP-induced reduction of IM was half maximal at 1.5 microM with a maximum of 37% inhibition; UDP was equipotent and equieffective, while ADP was less potent (half maximal inhibition at 29 microM). ATP had no effect at < or = 30 microM. 6. The inhibition of IM induced by 10 microM UTP was antagonized by pyridoxal-phosphate-6-azophenyl-2',4'-disulphonic acid (PPADS) at > or = 30 microM and by reactive blue 2 at > or = 10 microM, but not by suramin at concentrations up to 30 microM. 7. These results show that rat superior cervical ganglion neurons possess uridine nucleotide preferring P2Y receptors which inhibit KM channels. This effect presumably forms the basis of the excitatory action of uridine nucleotides in rat sympathetic neurons.

Effects of a cognition-enhancer, linopirdine (DuP 996), on M-type potassium currents (IK(M)) and some other voltage- and ligand-gated membrane currents in rat sympathetic neurons

Linopirdine is a cognition enhancer which augments depolarization-induced transmitter release in the cortex and which is under consideration for potential treatment of Alzheimer's disease. It has previously been reported to inhibit M-type K+ currents in rat hippocampal neurons. In the present experiments we have tested its effect on whole-cell M-currents and single M-channels, and on a range of other membrane currents, in dissociated rat superior cervical sympathetic ganglion cells. Linopirdine inhibited the whole-cell M-current with an IC50 of 3.4 microM and blocked M-channels recorded in excised outside-out membrane patches but not in inside-out patches. This suggests that linopirdine directly blocks M-channels from the outside. It was much less effective in inhibiting other voltage-gated potassium currents [delayed rectifier (IK(V)), IC50 63 microM; transient (IA) current, IC50 69 microM] and produced no detectable inhibition of the fast and slow Ca(2+)-activated K+ currents IC and IAHP or of a hyperpolarization-activated cation current (IQ/Ih) at 10-30 microM. However, it reduced acetylcholine-activated nicotinic currents and GABA-activated Cl- currents with IC50 values of 7.6 and 26 microM respectively. It is concluded that linopirdine shows some 20-fold selectivity for M-channels among different K+ channels but can also block some transmitter-gated channels. The relationship between M-channel block and the central actions of linopirdine are discussed.

Potassium channel mRNA expression in prevertebral and paravertebral sympathetic neurons

The expression of eighteen different voltage-activated potassium channel genes in rat sympathetic ganglia was quantitatively analysed using an RNase protection assay. Eleven alpha-subunit genes and two beta-subunit genes were expressed in sympathetic ganglia. The relative level of potassium channel mRNA expression was compared between the superior cervical ganglion (SCG) and two preverteabral sympathetic ganglia, the coeliac ganglion (CG) and the superior mesenteric ganglion (SMG). Four mRNAs were differentially expressed: Kv1.2, Kv1.4, Kv2.2 and Kv beta 1. Transcripts from all four genes were more abundant in the prevertebral ganglia. From comparisons with previous electrophysiological studies it was concluded that genes encoding the channels underlying the M-current and D2-current, which are both prominent in sympathetic neurons, have yet to be identified. It was also concluded that members of the Kv4 family are likely to underlie the low-threshold A-current in sympathetic neurons.

Whole-cell and perforated patch recordings of four distinct K+ currents in acutely dispersed coeliac-superior mesenteric ganglia neurons of adult rats

Properties and modulation of outward membrane currents in sympathetic neurons acutely dispersed from coeliac-superior mesenteric ganglia (C-SMG) of adult rats were examined using both the whole-cell variant of the patch-clamp technique and the perforated patch approach. Under voltage-clamp, four distinct outward currents were observed: a transient outward current (IA), a voltage-dependent sustained outward current consisting of a Ca(2+)-dependent component (IKCa) and a Ca(2+)-insensitive component (IKV), and a muscarinic agonist-sensitive outward current (IM). IA was isolated by digital subtraction, and characterized by very rapid activation at potentials more positive than -60 mV and by fast and complete voltage-dependent inactivation. Half inactivation potential (Vh) and slope factor (K) were -76 mV and 8.3 mV, respectively. IA was not affected by removal of external Ca2+, 1 mM tetraethylammonium ions, muscarinic agonists, or 8-bromo-cyclic AMP, but was suppressed by 4-aminopyridine (1 mM). Depolarizing pulses from of a holding potential of -50 or -60 mV to potentials more positive than -25 mV concomitantly activated two, independent sustained outward currents which decayed slowly; one exhibited voltage-dependent activation similar to the delayed rectifier current (IKV) and the other being triggered by Ca2+ influx into the cell (IKCa). The addition of tetraethylammonium ions (1 mM) strongly reduced the amplitude of the sustained outward currents. IM was characterized as a noninactivating time- and voltage-dependent outward current which activated at membrane potentials more positive than -60 mV and slowly turned off when the membrane was hyperpolarized back to -60 mV, and was suppressed by muscarinic agonists. The rank order of potency of the agonists tested was: oxotremorine > muscarine > bethanechol.

Potassium currents in rat prevertebral and paravertebral sympathetic neurones: control of firing properties

1. Intracellular recordings were made from rat sympathetic neurones in isolated superior cervical ganglia (SCG), coeliac ganglia (CG) and superior mesenteric ganglia (SMG). 2. Based on their response to a maintained depolarizing current stimulus, neurones were classified as 'phasic' or 'tonic'. All neurones in the SCG were phasic, 85% of the neurones in the SMG and 58% of the neurones in the CG were tonic, and the remainder were phasic. 3. The voltage response of phasic and tonic neurones around threshold to a constant current step was markedly different. The response of phasic neurones was biphasic with an initial depolarizing response followed by significant repolarization of the membrane potential. In contrast, tonic neurones became more depolarized during a prolonged current step. 4. The underlying currents were studied using single-electrode voltage-clamp recording. The M-current was present in all phasic neurones, but was very weak or absent in tonic neurones. 5. An A-current was apparent in both phasic and tonic neurones. The voltage-dependent activation, steady- state inactivation, and current density of the A-current were all similar in phasic and tonic cells. 6. A low- threshold, slowly inactivating outward current (D2-current) was observed exclusively in tonic neurones. The slow inactivation of this current appeared to underlie the slow depolarizing ramp seen in response to a maintained depolarizing current step. 7. Computer simulations, based on the voltage-clamp data, suggested that the different firing properties of phasic and tonic neurones could be accounted for by differential expression of the M-current.

Anomalous permeation of Na+ through a putative K+ channel in rat superior cervical ganglion neurones

1. An unanticipated inward tail current was recorded from freshly isolated adult rat superior cervical ganglion (SCG) neurones using the whole-cell variant of the patch-clamp technique. The tail current was present when Na+ was substituted for tetraethylammonium (TEA) as the primary monovalent cation in external solutions designed to isolate Ca2+ channel currents (0.5 microM tetrodotoxin present and K+ omitted). 2. The tail current was observed following step potentials positive to -30 mV and reached half-activation near -9.0 mV. The decay of the tail current was voltage dependent and could be described with two time constants. Between potentials of -120 and -70 mV, tau f, the fast component, varied from 3 to 8 ms and tau s, the slow component, changed from 12 to 30 ms, respectively. 3. The tail current was not carried by Ca2+, and did not appear to flow through a voltage-gated Ca2+ channel or a Ca(2+)-dependent channel as it persisted in the absence of external Ca2+ or in the presence of the Ca2+ channel blocker, Cd2+ (0.1 mM). 4. Varying the external [Cl-] did not alter the reversal potential of the tail current indicating that Cl- was not the charge carrier. 5. The reversal potential of the tail current changed in accordance with the Nernst relationship when [Na+]i/[Na]o was altered. Our results suggested that this 'unusual or unanticipated current' (Iu) was carried primarily by Na+. 6. Iu was inhibited by the K+ channel-blocking agents quinidine (0.1 mM), external Ba2+ (5 mM) and internal Cs+ (145 mM). TEA (20 mM either internally or externally) and dendrotoxin (10 microM) were not effective inhibitors of Iu. 7. The decay time constants of the tail current and parameters of activation and inactivation of Iu were similar to those of TEA-insensitive delayed rectifier-type K+ channel currents observed in the presence of 145 mM external K+. 8. Iu was reduced in the presence of either external or internal K+. The interaction of external K+ with Na+ on the Iu tail amplitude was reminiscent of anomalous mole-fraction behaviour. 9. Ion permeability studies revealed that the channel producing Iu had a permeability sequence to monovalent cations of 3.5:2.5:2:1:0.5 for Rb+, K+, Cs+, Na+ and Li+, respectively. 10. These data suggest that in the absence of external K+, the ion selectivity of a TEA-insensitive K+ channel in sympathetic neurones is profoundly diminished. Under these conditions, Na+ traversing a K+ channel can generate an unanticipated inward current.(ABSTRACT TRUNCATED AT 400 WORDS)

Closure of potassium M-channels by muscarinic acetylcholine-receptor stimulants requires a diffusible messenger

The M-current (IK(M)) is a slow voltage-gated K+ current which can be inhibited by muscarinic acetylcholine-receptor (mAChR) agonists. In the present experiments we have tested whether this inhibition results from a local (membrane-delimited) interaction between the receptor and adjacent channels, or whether channel closure is mediated by a diffusible messenger. To do this, single KM(+)-channel currents were recorded from membrane patches in dissociated rat superior cervical sympathetic neurons by using cell-attached patch electrodes. Channel activity was inhibited when muscarine was applied to the cell membrane outside the patch but persisted when channels were exposed to muscarine added to the pipette solution. We conclude that a diffusible molecule (or molecules) is (are) required to induce intrapatch channel closure following activation of extra-patch receptors.

Large and small vertebrate sensory neurons express different Na and K channel subtypes

Sensory neurons of frog dorsal root ganglia (DRG) express at least two subtypes of voltage-gated Na channel and at least two subtypes of voltage-gated K channel. The Na channel subtypes have different sensitivities to tetrodotoxin (TTX) and different kinetics. The TTX-sensitive (TTX-s) Na channel inactivates rapidly and is blocked by nanomolar TTX. The TTX-insensitive (TTX-i) Na channel resists blockage by up to 100 microM TTX (it is blocked by saxitoxin) and inactivates 2-6 times more slowly. The two subtypes of voltage-gated K channel differ in activation kinetics: the fast subtype activates 2-8 times faster than the slow subtype. These Na and K channel subtypes are distributed differentially by cell size, falling into three major groups: (i) small cells with slowly activating K channels and a mixture of TTX-s and TTX-i Na channels; (ii) small cells with slowly activating K channels and TTX-s Na channels; and (iii) large cells with rapidly activating K channels and TTX-s Na channels. The contributions of these channel subtypes to the electrical properties of sensory neurons were investigated under conditions that minimized the contribution of Ca current. Under these conditions, action potential duration is correlated with the channel subtypes expressed: cells with both TTX-i and TTX-s Na channels and slowly activating K channels exhibit long-duration action potentials, cells with TTX-s Na channels and slowly activating K channels exhibit intermediate-duration action potentials, and cells with TTX-s Na channels and rapidly activating K channels exhibit short-duration action potentials.

Resting membrane potential and potassium currents in cultured parasympathetic neurones from rat intracardiac ganglia

1. Whole-cell K+ currents contributing to the resting membrane potential and repolarization of the action potential were studied in voltage-clamped parasympathetic neurones dissociated from neonatal rat intracardiac ganglia and maintained in tissue culture. 2. Rat intracardiac neurones had a mean resting membrane potential of -52 mV and mean input resistance of 850 M omega. The current-voltage relationship recorded during slow voltage ramps indicated the presence of both leakage and voltage-dependent currents. The contribution of Na+, K+ and Cl- to the resting membrane potential was examined and relative ionic permeabilities PNa/PK = 0.12 and PCl/PK < 0.001 were calculated using the Goldman-Hodgkin-Katz voltage equation. Bath application of the potassium channel blockers, tetraethylammonium ions (TEA; 1 mM) or Ba2+ (1 mM) depolarized the neurone by approximately 10 mV. Inhibition of the Na(+)-K+ pump by exposure to K(+)-free medium or by the addition of 0.1 mM ouabain to the bath solution depolarized the neurone by 3-5 mV. 3. In most neurones, depolarizing current pulses (0.5-1 s duration) elicited a single action potential of 85-100 mV, followed by an after-hyperpolarization of 200-500 ms. In 10-15% of the neurones, sustained current injection produced repetitive firing at maximal frequency of 5-8 Hz. 4. Tetrodotoxin (TTX; 300 nM) reduced, but failed to abolish, the action potential. The magnitude and duration of the TTX-insensitive action potential increased with the extracellular Ca2+ concentration, and was inhibited by bath application of 0.1 mM Cd2+. The repolarization rate of the TTX-insensitive action potential was reduced, and after-hyperpolarization was replaced by after-depolarization upon substitution of internal K+ by Cs+. The after-hyperpolarization of the action potential was reduced by bath application of Cd2+ (0.1 mM) and abolished by the addition of Cd2+ and TEA (10 mM). 5. Depolarization-activated outward K+ currents were isolated by adding 300 nM TTX and 0.1 mM Cd2+ to the external solution. The outward currents evoked by step depolarizations increased to a steady-state plateau which was maintained for > 5 s. The instantaneous current-voltage relationship, examined under varying external K+ concentrations, was linear, and the reversal (zero current) potential shifted in accordance with that predicted by the Nernst equation for a K(+)-selective electrode. The shift in reversal potential of the tail currents as a function of the extracellular K+ concentration gave a relative permeability, PNa/PK = 0.02 for the delayed outward K+ channel(s).(ABSTRACT TRUNCATED AT 400 WORDS)

A slowly inactivating K+ current in retinal ganglion cells from postnatal rat

The whole-cell configuration of the patch-clamp technique was used to study voltage-gated K+ conductances in retinal ganglion cells from postnatal rat. Retinal ganglion cells were fluorescently labeled in situ, dissociated from the retina, and maintained in culture. With physiological solutions in the bath and the pipette, depolarizing voltage steps from physiological holding potentials activated Na(+)-(INa), Ca2+ (ICa), and K(+)-currents studied previously in retinal ganglion cells. Here we report on a slowly decaying K+ current, not heretofore reported in rat. With 4-AP, TEA, and Co2+ in the bath, to block IA, IK, and IK(Ca), respectively, a slowly decaying outward current was activated from -80 mV by steps positive to -40 mV. This current was present in 92% of all ganglion cells tested (n = 83). It activated within 10 ms and inactivated with a voltage-independent time constant of about 70 ms at 35 degrees C. Inactivation was voltage-dependent, half-maximal at -55 mV, and almost complete at 0 mV. The current was blocked by internal Cs+ and TEA, or by external application of 1 mM Ba2+, but not by 3 mM extracellular Co2+. The biophysical and pharmacological properties of this current are distinctly different from those of slowly inactivating K+ currents studied in other rat neurons. It was very similar, however, to a slowly inactivating K+ current previously reported in ganglion cells of tiger salamander retina. This last finding indicates conservation of a defined K+ channel type in functionally related cells in both lower vertebrates and mammals.

Potassium currents contributing to action potential repolarization in dissociated cultured rat superior cervical sympathetic neurones

Pharmacological blocking agents were used to assess the contributions of different K(+)-currents to spike-repolarization and early spike-afterhyperpolarization recorded in dissociated, tissue-cultured rat superior cervical sympathetic neurones using both patch-clamp and impalement microelectrode techniques. Effects of 4-aminopyridine (4-AP) and tetraethylammonium (TEA), in concentrations which selectively reduced the delayed rectifier current IK(DR) and Ca(2+)-activated K(+)-current IK(Ca, fast), respectively, indicated that IK(DR) made a significant contribution to both spike repolarization and spike afterhyperpolarization under all recording conditions, whereas the contribution of IK(Ca,fast) depended on the level of intracellular Ca(2+)-buffering. No evidence for a significant role for the transient current IK(A) could be adduced in these experiments.

Adrenergic and cholinergic inhibition of Ca2+ channels mediated by different GTP-binding proteins in rat sympathetic neurones

Effects of acetylcholine (ACh) and noradrenaline (NA) on voltage-gated ion channels of sympathetic neurones acutely dissociated from rat superior cervical ganglion (SCG) were examined using the whole-cell voltage-clamp technique. Depolarizing voltage steps elicited two types of low- and high-voltage-activated (LVA and HVA) Ca2+ currents. Pressure applications of ACh and NA produced concentration-dependent inhibition of the HVA Ca2+ current without affecting the LVA Ca2+ current. The inhibitory action of ACh on the Ca2+ current was blocked by a muscarinic antagonist, atropine. The action of NA was suppressed by an alpha 2-adrenergic antagonist, yohimbine, but not by an alpha 1-adrenergic antagonist, prazosin. Delayed rectifying outward K+ currents and inward rectifying K+ current were not affected by either ACh or NA. Tetrodotoxin-sensitive and -insensitive Na+ currents also remained unaffected under actions of ACh and NA. When recorded with electrode containing guanosine-5'-O-(3-thiotriphosphate) (GTP-gamma-S), the inhibitory actions of ACh and NA on Ca2+ currents became irreversible. After treatment of SCG neurones with pertussis toxin, the inhibitory action of ACh on the Ca2+ current was almost completely abolished, whereas the action of NA was only partially reduced. The results suggest that ACh and NA differentially inhibit the HVA Ca2+ current via different G proteins coupling muscarinic and alpha 2-adrenergic receptors to Ca2+ channels in rat SCG neurones.