| HOME | HELP | FEEDBACK | SUBSCRIPTIONS | ARCHIVE | SEARCH | TABLE OF CONTENTS |
|
|
|
|||||||
|
|||||||||
| ||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||
NEUROSCIENCE |
1 Department of Physiology, Nippon Medical School, Tokyo 113-8602, Japan
 |
Abstract |
|---|
 Top Abstract IntroductionMethodsResultsDiscussionReferences |
|---|
(Received 22 March 2006;
accepted after revision 20 April 2006;
first published online 20 April 2006)
Corresponding author M. Kato: Department of Physiology, Nippon Medical School, Sendagi 1, Bunkyo-ku, Tokyo 113-8602, Japan. Email: mkato{at}nms.ac.jp
|
Introduction |
|---|
|
TopAbstractIntroductionMethodsResultsDiscussionReferences |
|---|
The expression profile of subtype-specific Ca2+ channels has been examined in GnRH neurones from neonatal and pubertal rats (Kato et al. 2003), from young and adult mice (Nunemaker et al. 2003), and in mouse clonal cell line GT1–7 (Watanabe et al. 2004). GnRH neurones from pubertal rats express all five subtypes of voltage-gated Ca2+ channels, whereas mouse GnRH neurones from young or adult animals lack T-type channels. Here we investigated the voltage-gated Ca2+ currents in GnRH neurones from adult rats to complete the study of the developmental changes in their expression profiles.
It is well established that afterhyperpolarization affects cell excitability (for a review, see Sah, 1996). Therefore, we studied the Ca2+-activated K+ current, which underlies afterhyperpolarization (AHP). Afterhyperpolarization is widely observed in the central nervous system, including the hypothalamus (for reviews, see Faber & Sah, 2003; Vogalis et al. 2003; Stocker, 2004). Channels for AHP have been cloned as SK1–3 (Kohler et al. 1996), and are blocked by the bee venom apamin (Romey et al. 1984; Blatz & Magleby, 1986). Afterhyperpolarization is classified as medium and slow AHP according to the decay time course (for a review, see Vogalis et al. 2003). The current for medium AHP (ImAHP) decays with a time constant of 100–200 ms and is blocked by apamin, whereas the current for slow AHP (IsAHP) is not affected by apamin, and the decay time constant ranges from several 100 ms to several seconds. It is believed that ImAHP is carried through SK channels, because it is blocked by apamin (Zang & Krnjevic, 1987; Schwindt et al. 1988; Stocker et al. 1999; Sailer et al. 2002; Villalobos et al. 2004). The channel(s) for IsAHP, however, have not been determined to date.
In this study, we investigated the expression profile of voltage-gated Ca2+ currents and IAHP in rat GnRH neurones by means of perforated patch-clamp recording. We revealed the presence of all five subtypes of voltage-gated Ca2+ currents and of IsAHP with high sensitivity to apamin.
|
Methods |
|---|
|
TopAbstractIntroductionMethodsResultsDiscussionReferences |
|---|
Short-term dissociated culture
Brains were excised from rats under ether anaesthesia. The medial septum, diagonal band of Broca, organum vasculosum of the lamina terminalis (OVLT), and medial preoptic area were cut out with a razor and surgical blades. The sections were minced and treated with papain (21 U ml–1; Funakoshi, Tokyo, Japan) for 40–60 min at 30°C with gentle agitation. The tissues were triturated with a 5 ml plastic pipette after several washes with MEM (Invitrogen, Grand Island, NY, USA). The cell suspension was subjected to discontinuous Percoll density gradient centrifugation composed of 1.0, 1.023, and 1.078 g ml–1 layers. The cells obtained from the middle layer were plated on polylysine-coated coverslips and then incubated overnight in Neurobasal-A medium (Invitrogen) supplemented with 0.5 mM L-glutamine, B-27 (Invitrogen), and 5 ng ml–1 basic FGF (Invitrogen) at 37°C. Most of the GnRH neurones were either round or spindle-shaped, and some of them possessed short processes.
Electrophysiology
A List EPC-9 patch-clamp system (HEKA Electronik, Lambrecht/Pfalz, Germany) was used for the recordings and data analyses. Whole-cell currents were measured by means of the perforated patch-clamp technique with amphotericin B (Seikagaku Corp., Tokyo, Japan; Kato et al. 2003) at room temperature (25°C). The final concentration of amphotericin B in the pipette solution was 0.05 mg ml–1. The pipette solution consisted of (mM) 95 potassium aspartate, 47.5 KCl, 1.0 MgCl2, 0.1 EGTA and 10 Hepes (pH 7.2), the osmolarity being adjusted to 270 mosmol l–1. The extracellular solution consisted of (mM) 137.5 NaCl, 5 KCl, 2.5 CaCl2, 0.8 MgCl2, 0.6 NaHCO3, 10 glucose and 20 Hepes (pH 7.4), the osmolarity being adjusted to 300 mosmol l–1. To minimize non-specific binding of peptides, 0.01% cytochrome C (Wako Junyaku, Osaka, Japan) was included in the extracellular solution. For the recording of Ca2+ currents, K+ was replaced by Cs+ in the pipette solution, and KCl was replaced by CsCl and 10 mM NaCl was replaced by 10 mM tetraethylammonium chloride in the extracellular solution. In addition, 0.3 µM TTX (Seikagaku Corp., Tokyo, Japan) was included to block the Na+ currents. Pipettes were fabricated from borosilicate glass capillaries and had a resistance of 6–8 M
. The pipettes were targeted to the GnRH neurones in the extracellular solution without cytochrome C. After the cell had been touched, slight negative pressure was applied to the pipette, which led to a seal resistance of 
5 G. Perforation with amphotericin B was achieved within 3–7 min after gigaseal formation. The currents were filtered at 2.3 kHz, digitized at 10 kHz and recorded. The series resistance was 70%, electronically compensated. Data were taken when the series resistance was stable and less than 30 M. The leak currents ranged from –5 pA to –40 pA at –90 mV. The cell capacitance was 13.5 ± 3.3 pF (n
= 100). Capacitative and leak currents were subtracted by means of the p/4 protocol. In current-clamp experiments, a few pico-amps were injected to keep the membrane potential around –70 mV when necessary. Action potentials were elicited with 2 s current pulses with 3 pA steps. The time of spike failure, instantaneous firing frequency and maximum hyperpolarization in the interspike interval were examined.
Single-cell RT-PCR
Coronal slices (200 µm thick) containing the medial septum, diagonal band of Broca, OVLT and medial preoptic area were prepared from adult female rats. The rats were decapitated under ether anaesthesia, and then their brains were quickly removed and immersed in an ice-cold oxygenated (95% O2, 5% CO2) cutting solution comprising (mM) 2.5 KCl, 1.25 Na2HPO4, 0.6 NaHCO3, 0.5 CaCl2, 7 MgCl2, 10 Hepes, 7 glucose, 248 sucrose, 1.3 ascorbic acid and 3 Na pyruvate (pH 7.4, 290 mosmol l–1). The brains were blocked, glued with cyanoacrylate to the chilled stage of a Vibratome VIB3000 (Vibratome, St Louis, MO, USA), and cut. The slices were then incubated at 30°C for 30 min in oxygenated artificial cerebrospinal fluid (ACSF) containing (mM) 137.5 NaCl, 2.5 KCl, 1.25 Na2HPO4, 0.6 NaHCO3, 2 CaCl2, 2 MgCl2, 10 Hepes and 10 glucose (pH 7.4, 290 mosmol l–1), and thereafter kept at room temperature. Each slice was transferred to the recording chamber, held submerged, and continuously superfused with oxygenated ACSF at the rate of 3 ml min–1. The slice was viewed under an upright fluorescence microscope (BX50; Olympus, Tokyo, Japan) with a 40x water-immersion objective lens. Pipettes of 2–3 M were fabricated from glass capillaries baked at 200°C for 5 h. Each pipette was filled with 7.5 µl of an autoclaved solution comprising (mM) 150 KCl, 3 MgCl2, 5 EGTA and 10 Hepes (pH 7.2, 270 mosmol l–1). A small amount of positive pressure was applied to the pipette and then the pipette was targeted to GnRH neurones. After the cell had been touched, the positive pressure was removed, and negative pressure was applied for sealing and breaking of the patch membrane. The cytoplasmic contents were harvested under visual control and transferred to a thin-wall PCR tube containing 10 µl of a reaction mixture for reverse-transcription (Takara LA PCR Kit AMV version 3; Takara Bio, Shiga, Japan). The reaction mixture comprised 5 µM random 9mers and 8 U AMV reverse transcriptase XL (from Avian Myeloblastosis Virus), and the reaction conditions were 30°C (10 min), 50°C (30 min), 99°C (5 min), and 4°C (5 min). The RT reaction product from a single cell was used for the PCR reaction with Takara ExTaq (Takara Bio) according to the manufacturer's instructions. The PCR conditions were 95°C for 5 min, 45 cycles of 94°C for 30 s, 55°C for 30 s, and 72°C for 60 s, and finally 72°C for 5 min. The reaction products were separated on a 1.5% agarose gel and photographed. A 100 bp DNA ladder was used as a marker. The results were confirmed in three independent experiments. The following primers were used: rat SK1, 5'-ACGCATCCAGTCGCAGCA-3' (GenBank accession no U69885; location, nt 515–nt 533) and 5'-GCCTTGGTGAGTTCCAACT-3' (nt 875–nt 857); rat SK2, 5'-GCGAATACTCTAGTGGATCT-3' (U69882; nt 2707–nt 2726) and 5'-TAGCTACTCTCAGATGAAGTT-3' (nt 3020–nt 3000); and rat SK3, 5'-AAGATTGACCACGCCAAAGT-3' (U69884; nt 2066–nt 2085) and 5'-CAACTGCTTGAACTTGTGTAT-3' (nt 2487–nt 2467).
Chemicals
Nifedipine was obtained from Wako Junyaku. 
-Conotoxin GVIA(GVIA), -agatoxin IVA (Aga-IVA), SNX-482 and apamin were purchased from the Peptide Institute (Osaka, Japan).
Statistics
Data are expressed as means ± standard deviation (S.D.) unless otherwise stated. The Kruskal–Wallis test, paired or unpaired t test, and Peason's correlation coefficient test were used for statistical analysis. The significance level was set at P < 0.05.
|
Results |
|---|
|
TopAbstractIntroductionMethodsResultsDiscussionReferences |
|---|
The cell was held at –90 mV and the membrane potential was stepped to various voltages (–60 mV to +60 mV in 10 mV increments) for 100 ms at 0.2 Hz to activate Ca2+ currents. Maximum amplitudes of –57 ± 26 pA pF–1 (n
= 33) in females and –55 ± 20 pA pF–1 (n
= 11) in males were observed at 0–10 mV. Only the current traces at +10 mV are shown in Fig. 1A for clarity. To examine the voltage-gated Ca2+ channel subtypes in rat GnRH neurones, pharmacological dissection was carried out with specific blockers. After the control current had been recorded, 10 µM nifedipine, 1 µM GVIA, 200 nM Aga-IVA, 100 nM SNX-482 and 50 µM Ni2+ were successively applied. SNX-482 (R-type channel blocker; Newcomb et al. 1998; Tottene et al. 2000) attenuated the initial peak currents by approximately 30%, and the L-type channel blocker nifedipine (Boll & Lux, 1985) reduced the currents by 18–27%. The N-type channel blocker GVIA (Tsien et al. 1988; Aosaki & Kasai, 1989) inhibited the currents by 20%. Aga-IVA (P/Q-type channel blocker; Randall & Tsien, 1995) and a low concentration of Ni2+ (50 µM; T-type channel blocker; Tottene et al. 2000) reduced the currents by 12% and 9.3%, respectively. There was no statistically significant difference in the expression profile of Ca2+ currents according to sex or oestrous cycle stage (Fig. 1B).
|
Most of the GnRH neurones showed IAHP, which was elicited by a voltage protocol comprising a 100 ms voltage pulse to +30 mV, followed by a 5 s voltage-step to –50 mV (Fig. 2A). The holding potential was –90 mV and the voltage pulse was applied every 30 s. Example traces of IAHP are shown in Fig. 2B. The total charge of IAHP was measured between 5 ms after the offset of a voltage pulse to +30 mV, and the time when the current returned to the base line. The charge was divided by the cell capacitance to obtain the charge density, which ranged from 0.91 to 30.5 pC pF–1 in 60 GnRH neurones from adult rats. The value was 7.44 ± 8.18 pC pF–1. The decay time constant was determined by exponential fitting, as shown in Fig. 2A. The value was 533.4 ± 334.5 ms, ranging between 179 ms and 1600 ms.
|
|
|
A similar IAHP was observed in six GnRH neurones examined in the acute slice preparation (Supplemental Fig. 1 online). The total charge of IAHP was 95.2 ± 96.9 pC, ranging from 38 pC to 337 pC, and the decay time constant was 748 ± 454 ms, ranging between 275 ms and 1600 ms.
Effects of Ca2+ channel blockers on IsAHP
As shown in Fig. 1, both low-voltage- and high-voltage-activated Ca2+ currents were observed in the GnRH neurones from adult rats. After the control IAHP had been recorded, 10 µM nifedipine, 1 µM GVIA, 200 nM Aga-IVA, 100 nM SNX-482 and 50 µM Ni2+ were successively applied (Fig. 5). Some current traces are not shown in Fig. 5A for clarity. The data are from female neurones. In this experiment, data are not according to the oestrous cycle stage, because the oestrous cycle affected neither the Ca2+ currents nor IAHP. Nifedipine attenuated the currents by 29.2 ± 17.5% and SNX-482 reduced them by 34.9 ± 12.7% in the 11 neurones examined. The inhibitions by GVIA, Aga-IVA and Ni2+ were 13.3 ± 7.7%, 8.2 ± 5.2% and 6.8 ± 5.4%, respectively (Fig. 5C). The proportions of these currents are shown as percentages of the Ni2+, Cd2+-sensitive current together with Ca2+ currents (Fig. 5D). On average, the contribution of L-type Ca2+ current was 23.4% for the total Ca2+ current, and 30% for the activation of Ni2+, Cd2+-sensitive AHP current. The R-type current contributed 30.4% of the total Ca2+ current, and 35.5% of the Ni2+, Cd2+-sensitive AHP current.
|
Trains of action potentials were generated by 2 s current pulse with 3 pA increments (Fig. 6). At a low current intensity, GnRH neurones did not continue to discharge action potentials during a 2 s current pulse. This is a phenomenon known as spike frequency adaptation. When the current intensity increased, GnRH neurones fired action potentials to the end of the 2 s current pulse. At higher current intensity, however, the amplitude of action potentials progressively decreased during 2 s current pulse, and the cell finally failed to fire (see supplemental Fig. 2 online). When the amplitude of the action potential became less than 10% of that in the initial spike, it was defined as spike failure. The time of the last spike after the onset of the current pulse was taken as the time of spike failure (arrowheads in Fig. 6B). Under control conditions, spike failure started to occur in some neurones with a 12 pA current pulse and occurred in four out of the five neurones examined with a 30 pA pulse. Apamin accelerated the spike failure (Fig. 6B and C). Spike failure began in two out of the ten neurones examined with a 6 pA current pulse, and occurred in all five neurones examined with a 30 pA pulse in the presence of apamin. Spike failure was further accelerated by simultaneous application of Ni2+ and Cd2+. The effects of these blockers were almost completely abolished on washout.
|
|
Transcripts for rat SK channels in GnRH neurones
To examine the expression of the SK channel mRNA in single GnRH neurones, RT-PCR analysis was performed (Fig. 8). RT-PCR with specific primers for SK1–3 yielded amplified products of the predicted size. A positive band for SK1 appeared in 5 of the 12 cells examined. One for SK2 was seen in 16 of the 19 cells examined and one for SK3 was seen in 15 of the 19 cells examined.
|
|
Discussion |
|---|
|
TopAbstractIntroductionMethodsResultsDiscussionReferences |
|---|
60% of the total Ca2+ current in neonates and 40% around puberty. The expression profile of the subtype-specific Ca2+ currents in adult GnRH neurones (Fig. 1) is similar to that in pubertal GnRH neurones. So the T-type and R-type Ca2+ currents are developmentally regulated, and the adult expression profile is completed around puberty. The mouse GnRH neurone, however, lacks the T-type current and shows a low occurrence (15% of the total) of the R-type current (Nunemaker et al. 2003). In a clonal cell line of mouse GnRH neurones (GT1–7), 75% of the total Ca2+ current is carried through the R-type channel and no detectable current passes through the P/Q-type channel (Watanabe et al. 2004).
The afterhyperpolarization current, which was blocked by Ni2+ and Cd2+, in the adult GnRH neurones, exhibited a decay time constant ranging between 153 ms and 799 ms (Fig. 4B), which is classified as IsAHP (for reviews, see Faber & Sah, 2003; Stocker, 2004; Vogalis et al. 2003). An afterhyperpolarization current with a shorter decay time constant than 100 ms (medium AHP current) was undetectable in the adult GnRH neurones. Many neurones, including hypothalamic and hippocampal ones, show IsAHP (for a review, see Vogalis et al. 2003). The reported IsAHP is insensitive to apamin, whereas ImAHP is blocked by a nanomolar concentration of apamin (Vogalis et al. 2003). The channels involved in ImAHP are believed to belong to the SK family, whereas those for IsAHP remain unknown (Vogalis et al. 2003). In contrast, the IsAHP observed in the present experiment was blocked by apamin with an IC50 of 100 pM, which is the same concentration as that for the blockade of ImAHP (Vogalis et al. 2003). These results lead us to speculate that IsAHP in the rat GnRH neurone could be carried through channels of the SK family, such as SK1, SK2 and SK3, because SK channels are activated by submicromolar concentrations of Ca2+ in a voltage-independent manner (Xia et al. 1998), and because SK channels are almost completely blocked by apamin (Kohler et al. 1996). SK2 and SK3 expressed in Xenopus oocytes are potently blocked by apamin with an IC50 in the picomolar range (Kohler et al. 1996; Ishii et al. 1997). SK1 is also blocked by apamin when expressed in a mammalian cell line, but not in Xenopus oocytes (Shah & Haylett, 2000; Strobaek et al. 2000). In addition, other Ca2+-activated K+ channels, such as SK4, IK and BK, are not inhibited by apamin (Joiner et al. 1997; Ishii et al. 1999; Faber & Sah, 2003). In the present study, 20–30% of IsAHP was resistant to apamin (Fig. 3C). The channels responsible for this resistant current remain to be determined.
Some reports have suggested that Ca2+ influx through certain subtypes of Ca2+ channel is related to the generation of a slow AHP (Williams et al. 1997; Pineda et al. 1998; Wolfart & Roeper, 2002). Therefore, we investigated the effects of subtype-specific Ca2+ channel blockers, but found no specificity of Ca2+ channel subtypes for the generation of IsAHP in rat GnRH neurones (Fig. 5). All fivesubtypes of Ca2+ currents contributed equally to the generation of IsAHP according to the level of contribution to the total Ca2+ current. This is reasonable and supports the possible involvement of SK channels, because the KD of Ca2+ for the activation of SK channels is 0.3 µM and full activation is achieved with a submicromolar concentration of Ca2+ (Xia et al. 1998). For such concentrations of Ca2+, the channels need not to be closely located to the Ca2+ channels. In the present experiment, however, we did not examine the specific contribution of each Ca2+ channel to the apamin-sensitive component of IsAHP. In addition to the electrophysiological observations, GnRH neurones expressed mRNA for SK1–3 (Fig. 8). Single-cell RT-PCR analysis revealed the presence of SK1 mRNA in 40% of the GnRH neurones examined. Transcripts for SK2 and SK3 were detected in 80% of the GnRH neurones. These findings are in accord with a previous report that guinea pig GnRH neurones express SK3 mRNA and exhibit an AHP (Bosch et al. 2002). Taken together, it is likely that GnRH neurones possess functional SK channels.
To investigate the physiological role of this apamin-sensitive IsAHP, we carried out a current-clamp experiment (Figs 6 and 7). GnRH neurones showed beating-type firing with a maximum frequency of less than 33 Hz, which suggests that the rat GnRH neurone is a slow-spiking neurone. The mouse GnRH neurone shows a similar firing property (Suter et al. 2000; Sim et al. 2001; Kuehl-Kovarik et al. 2002). The spontaneous action potentials of isolated mouse GnRH neurones consist of brief bursts of 2–20 Hz, separated by 1–10 s. Spike-dependent depolarizing afterpotentials are thought to contribute to these bursting activities (Kuehl-Kovarik et al. 2005). We did not examine whether rat GnRH neurones exhibited a brief burst, because we recorded a train of action potentials evoked by 2 s current pulse. Blockade of IsAHP by apamin caused spike failure, especially when a high-intensity current pulse was applied. The membrane potential in the interspike interval became progressively depolarised, and the cells could finally generate no action potentials. The application of Ni2+ and Cd2+, which completely blocks both IsAHP and Ca2+ currents, further accelerated the spike failure. These findings suggest that IsAHP is activated by action potentials and causes spike frequency adaptation with a moderate depolarizing input, and that this current may support persistent firing with a relatively strong depolarizing input. An LH surge lasts a few hours, during which GnRH neurones are likely to generate action potentials either continuously or intermittently with bursts. Although the mechanism is not known, it is probable that GnRH neurones receive a relatively strong persistent depolarizing input during an LH surge. In this situation, IsAHP may play an important role in the firing of GnRH neurones.
In conclusion, the rat GnRH neurone expresses voltage-gated Ca2+ channels and SK channels, possibly together with unidentified Ca2+-activated voltage-independent K+ channels. Calcium ions passing through the voltage-gated Ca2+ channels activate these Ca2+-activated K+ channels, which allow the continuous firing of GnRH neurones. Although there was no correlation of the size or decay time constant of IsAHP to the firing pattern of evoked action potentials, this point remains to be analysed in a future experiment on spontaneous and evoked action potentials in slice preparations.
|
References |
|---|
|
TopAbstractIntroductionMethodsResultsDiscussionReferences |
|---|
Blatz AL & Magleby KL (1986). Single apamin-blocked Ca-activated K+ channels of small conductance in cultured rat skeletal muscle. Nature 323, 718–720.[CrossRef][Medline]
Boll W & Lux HD (1985). Action of organic antagonists on neuronal calcium currents. Neurosci Lett 56, 335–339.[CrossRef][Medline]
Bosch
MA, Kelly
MJ
&
Ronnekleiv
OK (2002). Distribution, neuronal colocalization, and 17ß-E2 modulation of small conductance calcium-activated K+ channel (SK3) mRNA in the guinea pig brain. Endocrinology
143, 1097–1107.
Catterall WA (2000). Structure and regulation of voltage-gated Ca2+ channels. Ann Rev Cell Dev Biol 16, 521–555.[CrossRef][Medline]
Faber ESL & Sah P (2003). Calcium-activated potassium channels: multiple contributions to neuronal function. Neuroscientist 9, 181–194.[Abstract]
Garcia ML, Hanner M, Knaus HG, Koch R, Schmalhofer W, Slaughter RS & Kaczorowski GJ (1997). Pharmacology of potassium channels. Adv Pharmacol 39, 425–471.[Medline]
Hille B (2001). Ion Channels of Excitable Membranes. Sinauer, Sunderland, MA.
Ishii
TM, Maylie
J
&
Adelman
JP (1997). Determinants of apamin and D-tubocurarine block in SK potassium channels. J Biol Chem
272, 23195–23200.
Ishii TM, Silvia C, Hirschberg B, Bond CT, Adelman JP & Maylie J (1999). A human intermediate conductance calcium-activated potassium channel. Proc Natl Acad Sci U S A 94, 11651–11656.[CrossRef]
Jan LY & Jan YN (1997). Cloned potassium channels from eukaryotes and prokaryotes. Ann Rev Neurosci 20, 91–123.[CrossRef][Medline]
Joiner
WJ, Wang
L-Y, Tang
MD
&
Kaczmarek
LK (1997). hSK4, a member of a novel subfamily of calcium-activated potassium channels. Proc Natl Acad Sci U S A
94, 11013–11018.
Kato
M, Watanabe
M
&
Sakuma
Y (2003). Characterization of voltage-gated calcium currents in gonadotropin-releasing hormone neurons tagged with green fluorescent protein in rats. Endocrinology
144, 5118–5125.
Kohler
M, Hirschberg
B, Bond
CT, Kinzie
JM, Marrion
NV, Maylie
J
&
Adelman
JP (1996). Small-conductance, calcium-activated potassium channels from mammalian brain. Science
273, 1709–1714.
Kuehl-Kovarik MC, Partin KM, Handa RJ & Dudek FE (2005). Spike-dependent depolarizing afterpotentials contribute to endogenous bursting in gonadotropin releasing hormone neurones. Neuroscience 134, 295–300.[CrossRef][Medline]
Kuehl-Kovarik
MC, Pouliot
WA, Halterman
GL, Handa
RJ
&
Dudek
FE (2002). Episodic bursting activity and response to excitatory amino acids in acutely dissociated gonadotropin-releasing hormone neurons genetically targeted with green fluorescent protein. J Neurosci
22, 2313–2322.
Newcomb R, Szoke B, Palma A, Wang G, Chen X-H, Hopkins W, Cong R, Miller J, Urge L, Tarczy-Hornoch K, Loo JA, Dooley DJ, Nadasdi L, Tsien RW, Lemos J & Miljanich G (1998). Selective peptide antagonist of the class E calcium channel from the venom of the tarantula Hysterocrates gigas. Biochemistry 37, 15353–15362.[CrossRef][Medline]
Nunemaker
CS, DeFazio
RA
&
Moenter
SM (2003). Calcium current subtypes in GnRH neurons. Biol Reprod
69, 1914–1922.
Pineda
JC, Waters
RS
&
Foehring
RC (1998). Specificity in the interaction of HVA Ca2+ channel types with Ca2+-dependent AHPs and firing behavior in neocortical pyramidal neurons. J Neurophysiol
79, 2522–2534.
Randall A & Tsien RW (1995). Pharmacological dissection of multiple types of Ca2+ channel currents in rat granule neurons. J Neurosci 15, 2995–3012.[Abstract]
Romey G, Hugues M, Schmid-Antomarchi H & Lazdunski M (1984). Apamin: a specific toxin to study a class of Ca2+-dependent K+ channels. J Physiol (Paris) 79, 259–264.
Sah P (1996). Ca2+-activated K+ currents in neurons: types, physiological roles and modulation. Trends Neurosci 19, 150–154.[CrossRef][Medline]
Sailer
CA, Hu
H, Kaufmann
WA, Trieb
M, Schwarzer
C, Storm
JF
&
Knaus
HG (2002). Regional differences in distribution and functional expression of small-conductance Ca2+-activated K+ channels in rat brain. J Neurosci
22, 9698–9707.
Schwindt
PC, Spain
WJ, Foering
RC, Stafstrom
CE, Chubb
MC
&
Crill
WE (1988). Mutiple potassium conductances and their functions in neurons from cat sensorimotor cortex in vitro. J Neurophysiol
59, 424–449.
Shah M & Haylett DG (2000). The pharmacology of hSK1 Ca2+-activated K+ channels expressed in mammalian cell lines. Br J Pharmacol 129, 627–630.[CrossRef][Medline]
Sim
JA, Skynner
MJ
&
Herbison
AE (2001). Heterogeneity in the basic membrane properties of postnatal gonadotropin-releasing hormone neurons in the mouse. J Neurosci
21, 1067–1075.
Stocker M (2004). Ca2+-activated K+ channels: molecular determinants and function of the SK family. Nature Rev Neurosci 5, 758–770.[CrossRef][Medline]
Stocker
M, Krause
M
&
Pedarzani
P (1999). An apamin-sensitive Ca2+-activated K+ current in hippocampal pyramidal neurons. Proc Natl Acad Sci U S A
96, 4662–4667.
Strobaek D, Jorgensen TD, Christophersen P, Ahring PK & Olesen S-P (2000). Pharmacological characterization of small-conductance Ca2+-activated K+ channels stably expressed in HEK293 cells. Br J Pharmacol 129, 991–999.[CrossRef][Medline]
Suter
KJ, Wuarin
J-P, Smith
BN, Dudek
FD
&
Moenter
SM (2000). Whole-cell recordings from preoptic/hypothalamic slices reveal burst firing in gonadotropin-releasing hormone neurons identified with green fluorescent protein in transgenic mice. Endocrinology
141, 3731–3736.
Tottene
A, Volsen
S
&
Pietrobon
D (2000). 
1E subunits from the pore of three cerebellar R-type calcium channels with different pharmacological and permeation properties. J Neurosci
20, 171–178.
Tsien RW, Lipscombe D, Madison DV, Bley KR & Fox AP (1988). Multiple types of neuronal calcium channels and their selective modulation. Trends Neurosci 11, 431–438.[CrossRef][Medline]
Villalobos
C, Shakkottai
VG, Chandy
KG, Michelhaugh
SK
&
Andrade
R (2004). SKCa channels mediate the medium but not slow calcium-activated afterhyperpolarization in cortical neurons. J Neurosci
24, 3537–3542.
Vogalis F, Storm JF & Lancaster B (2003). SK channels and the varieties of slow after-hyperpolarizations in neurons. Eur J Neurosci 18, 3155–3166.[CrossRef][Medline]
Watanabe
M, Sakuma
Y
&
Kato
M (2004). High expression of the R-type voltage-gated Ca2+ channel and its involvement in Ca2+-dependent gonadotropin-releasing hormone release in GT1–7 cells. Endocrinology
145, 2375–2383.
Williams
S, Serafin
M, Muhlethaler
M
&
Bernheim
L (1997). Distinct contributions of high- and low-voltage activated calcium currents to afterhyperpolarizations in cholinergic nucleus basalis neurons of the guinea pig. J Neurosci
17, 7307–7315.
Wolfart
J
&
Roeper
J (2002). Selective coupling of T-type calcium channels to SK potassium channels prevents intrinsic bursting in dopaminergic midbrain neurons. J Neurosci
22, 3404–3413.
Xia X-M, Fakler B, Rivard A, Wayman G, Johnson-Pais T, Keen JE, Ishii T, Hirschberg B, Bond CT, Lutsenko S, Maylie J & Adelman JP (1998). Mechanism of calcium gating in small-conductance calcium-activated potassium channels. Nature 395, 503–507.[CrossRef][Medline]
Zang L & Krnjevic K (1987). Apamin depresses selectively the after-hyperpolarization of cat spinal motoneurons. Neurosci Lett 74, 58–62.[CrossRef][Medline]
|
Acknowledgements |
|---|
This article has been cited by other articles:
|
|
 |
|
  X. Liu and A. E. Herbison Small-Conductance Calcium-Activated Potassium Channels Control Excitability and Firing Dynamics in Gonadotropin-Releasing Hormone (GnRH) Neurons Endocrinology, July 1, 2008; 149(7): 3598 - 3604. [Abstract] [Full Text] [PDF]
|
|
|
|
|
|
I. Nishimura, K. Ui-Tei, K. Saigo, H. Ishii, Y. Sakuma, and M. Kato 17{beta}-Estradiol at Physiological Concentrations Augments Ca2+-Activated K+ Currents via Estrogen Receptor {beta} in the Gonadotropin-Releasing Hormone Neuronal Cell Line GT1-7 Endocrinology, February 1, 2008; 149(2): 774 - 782. [Abstract] [Full Text] [PDF]
|
|
|
|
 |
|
 M. L. Palmer, K. R. Schiller, and S. M. O'Grady Apical SK potassium channels and Ca2+-dependent anion secretion in endometrial epithelial cells J. Physiol., February 1, 2008; 586(3): 717 - 726. [Abstract] [Full Text] [PDF]
|
|
|
|
|
|
D. J. Spergel Calcium and Small-Conductance Calcium-Activated Potassium Channels in Gonadotropin-Releasing Hormone Neurons before, during, and after Puberty Endocrinology, May 1, 2007; 148(5): 2383 - 2390. [Abstract] [Full Text] [PDF]
|
|
| ||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||
| HOME | HELP | FEEDBACK | SUBSCRIPTIONS | ARCHIVE | SEARCH | TABLE OF CONTENTS |
) currents are plotted according to sex and oestrous cycle stage. There were no statistically significant differences among these groups in Kruskal–Wallis test. The H-values were 2.7 for apamin-sensitive current and 3.25 for Ni2+, Cd2+-sensitive current.
