Cellular Mechanisms Underlying Spontaneous Firing in Rat Suprachiasmatic Nucleus: Involvement of a Slowly Inactivating Component of Sodium Current

Pennartz, C.M.A., M. A. Bierlaagh, and A.M.S. Geurtsen. Cellular mechanisms underlying spontaneous firing in rat suprachiasmatic nucleus: involvement of a slowly inactivating component of sodium current. J. Neurophysiol. 78: 1811–1825, 1997. Neurons constituting the pacemaker of circadian rhythms, located in the suprachiasmatic nucleus, generate spontaneous firing patterns that change across the day-night cycle. Their average spontaneous firing rate is considered an important functional marker of clock activity because it is highest during daytime and low at night. In this study we investigate the ionic mechanisms underlying spontaneous firing in acutely prepared slices and dissociated neurons of the suprachiasmatic nucleus. In current-clamp mode, spontaneous action potentials were consistently preceded by depolarizing ramps. These ramps were Na+ dependent, were sensitive to tetrodotoxin (TTX), and disappeared on hyperpolarization. Ramps and associated spikes were not abolished by blockers of the H current (1 mM cesium) or calcium currents (50 μM nickel or 200 μM cadmium). In voltage-clamped neurons in slices or dissociated neurons, TTX-sensitive and Na+-dependent inward current was observed to activate well below firing threshold (−60 to −50 mV). The low-threshold component of Na+ current inactivated slowly as compared with the fast component that mediates action potentials. However, its inactivation proceeded more rapidly than has been reported for the persistent Na+ current in cortical structures. Persistent Na+ current was generally absent or small in amplitude. The voltage dependence and kinetics of the slowly inactivating component of Na+ current are consistent with the hypothesis that it is partially deinactivated during spike afterhyperpolarizations and contributes significantly to subsequent depolarizing ramps. These observations implicate the slowly inactivating component of Na+ current in ionic mechanisms governing spontaneous firing in suprachiasmatic nucleus neurons.

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A role for a background sodium current in spontaneous action potentials and secretion from rat lactotrophs

AJP Cell Physiology ◽

10.1152/ajpcell.1996.271.6.c1927 ◽

1996 ◽

Vol 271 (6) ◽

pp. C1927-C1934 ◽

Cited By ~ 33

Author(s):

S. Sankaranarayanan ◽

S. M. Simasko

Keyword(s):

Action Potentials ◽

Sodium Current ◽

Plaque Assay ◽

Input Resistance ◽

Cytosolic Calcium ◽

Calcium Dependent ◽

Whole Cell Patch Clamp ◽

Spontaneous Action ◽

Spontaneous Action Potentials ◽

Spontaneous Secretion

We have used the perforated-patch variation of whole cell patch-clamp techniques, measurements of cytosolic calcium with use of fura 2, and secretion measurements with use of the reverse-hemolytic plaque assay to address the role of depolarizing background currents in maintaining spontaneous action potentials and spontaneous secretion from rat lactotrophs in primary culture. Replacement of bath sodium with tris(hydroxymethyl)aminomethane or N-methyl-D-glucamine caused a dramatic hyperpolarization of the cells, a cessation of spontaneous action potentials, and an increase in input resistance of cells. Tetrodotoxin had no effect on spontaneous action potentials, and removal of bath calcium stopped spiking but did not hyperpolarize the cells. The hyperpolarization in response to removal of bath sodium was associated with a decrease in cytosolic calcium levels. Finally, removal of bath sodium caused a decrease in spontaneous secretion of prolactin from lactotrophs. These data suggest that a background sodium current is essential to drive the membrane to threshold for firing spontaneous calcium-dependent action potentials in lactotrophs. This, in turn, results in elevated intracellular calcium, which supports spontaneous secretion of prolactin from these cells.

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Mechanism of Irregular Firing of Suprachiasmatic Nucleus Neurons in Rat Hypothalamic Slices

Journal of Neurophysiology ◽

10.1152/jn.00314.2003 ◽

2004 ◽

Vol 91 (1) ◽

pp. 267-273 ◽

Cited By ~ 41

Author(s):

Nikolai I. Kononenko ◽

F. Edward Dudek

Keyword(s):

Suprachiasmatic Nucleus ◽

Action Potentials ◽

Interspike Interval ◽

Inhibitory Postsynaptic Potentials ◽

Irregular Pattern ◽

Cell Recording ◽

Spontaneous Action ◽

Spontaneous Action Potentials ◽

Hypothalamic Slices ◽

Irregular Firing

The mechanisms of irregular firing of spontaneous action potentials in neurons from the rat suprachiasmatic nucleus (SCN) were studied in hypothalamic slices using cell-attached and whole cell recording. The firing pattern of spontaneous action potentials could be divided into regular and irregular, based on the interspike interval (ISI) histogram and the membrane potential trajectory between action potentials. Similar to previous studies, regular neurons had a firing rate about >3.5 Hz and irregular neurons typically fired about <3.5 Hz. The ISI of irregular-firing neurons was a linear function of the sum of inhibitory postsynaptic potentials (IPSPs) between action potentials. Bicuculline (10–30 μM) suppressed IPSPs and converted an irregular pattern to a more regular firing. Bicuculline also depolarized SCN neurons and induced bursting-like activity in some SCN neurons. Gabazine (20 μM), however, suppressed IPSPs without depolarization, and also converted irregular activity to regular firing. Thus GABAA receptor–mediated IPSPs appear responsible for irregular firing of SCN neurons in hypothalamic slices.

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Cholesterol inhibits spontaneous action potentials and calcium currents in guinea pig gallbladder smooth muscle

AJP Gastrointestinal and Liver Physiology ◽

10.1152/ajpgi.1999.277.5.g1017 ◽

1999 ◽

Vol 277 (5) ◽

pp. G1017-G1026 ◽

Cited By ~ 15

Author(s):

Lee J. Jennings ◽

Qi-Wei Xu ◽

Tracy A. Firth ◽

Mark T. Nelson ◽

Gary M. Mawe

Keyword(s):

Smooth Muscle ◽

Action Potentials ◽

Second Messenger ◽

Calcium Currents ◽

Isolated Cells ◽

Receptor Ligand ◽

Intact Tissue ◽

Elevated Cholesterol ◽

Spontaneous Action ◽

Spontaneous Action Potentials

Elevated cholesterol decreases agonist-induced contractility and enhances stone formation in the gallbladder. The current study was conducted to determine if and how the electrical properties and ionic conductances of gallbladder smooth muscle are altered by elevated cholesterol. Cholesterol was delivered as a complex with cyclodextrin, and effects were evaluated with intracellular recordings from intact gallbladder and whole cell patch-clamp recordings from isolated cells. Cholesterol significantly attenuated the spontaneous action potentials of intact tissue. Furthermore, calcium-dependent action potentials and calcium currents were reduced in the intact tissue and in isolated cells, respectively. However, neither membrane potential hyperpolarizations induced by the ATP-sensitive potassium channel opener, pinacidil, nor voltage-activated outward potassium currents were affected by cholesterol. Hyperpolarizations elicited by calcitonin gene-related peptide were reduced by cholesterol enrichment, indicating potential changes in receptor ligand binding and/or second messenger interactions. These data indicate that excess cholesterol can contribute to gallbladder stasis by affecting calcium channel activity, whereas potassium channels remained unaffected. In addition, cholesterol enrichment may also modulate receptor ligand behavior and/or second messenger interactions.

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Spontaneous electrical rhythmicity and the role of the sarcoplasmic reticulum in the excitability of guinea pig gallbladder smooth muscle cells

AJP Gastrointestinal and Liver Physiology ◽

10.1152/ajpgi.00310.2005 ◽

2006 ◽

Vol 290 (4) ◽

pp. G655-G664 ◽

Cited By ~ 23

Author(s):

Onesmo B. Balemba ◽

Matthew J. Salter ◽

Thomas J. Heppner ◽

Adrian D. Bonev ◽

Mark T. Nelson ◽

...

Keyword(s):

Smooth Muscle ◽

Sarcoplasmic Reticulum ◽

Action Potentials ◽

Cyclopiazonic Acid ◽

Cellular Mechanisms ◽

Voltage Dependent ◽

Spontaneous Action ◽

Potential Activity ◽

Spontaneous Action Potentials

Spontaneous action potentials and Ca2+ transients were investigated in intact gallbladder preparations to determine how electrical events propagate and the cellular mechanisms that modulate these events. Rhythmic phasic contractions were preceded by Ca2+ flashes that were either focal (limited to one or a few bundles), multifocal (occurring asynchronously in several bundles), or global (simultaneous flashes throughout the field). Ca2+ flashes and action potentials were abolished by inhibiting sarcoplasmic reticulum (SR) Ca2+ release via inositol (1,4,5)-trisphosphate [Ins(1,4,5)P3] channels with 2-aminoethoxydiphenyl borate and xestospongin C or by inhibiting voltage-dependent Ca2+ channels (VDCCs) with nifedipine or diltiazem or nisoldipine. Inhibiting ryanodine channels with ryanodine caused multiple spikes superimposed upon plateaus of action potentials and extended quiescent periods. Depletion of SR Ca2+ stores with thapsigargin or cyclopiazonic acid increased the frequency and duration of Ca2+ flashes and action potentials. Acetylcholine, carbachol, or cholecystokinin increased synchronized and increased the frequency of Ca2+ flashes and action potentials. The phospholipase C (PLC) inhibitor U-73122 did not affect Ca2+ flash or action potential activity but inhibited the excitatory effects of acetylcholine on these events. These results indicate that Ca2+ flashes correspond to action potentials and that rhythmic excitation in the gallbladder is multifocal among gallbladder smooth muscle bundles and can be synchronized by excitatory agonists. These events do not depend on PLC activation, but agonist stimulation involves activation of PLC. Generation of these events depends on Ca2+ entry via VDCCs and on Ca2+ mobilization from the SR via Ins(1,4,5)P3 channels.

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