Calcium-dependent plateau potentials in a crab stomatogastric ganglion motor neuron. I. Calcium current and its modulation by serotonin

1995 ◽  
Vol 74 (5) ◽  
pp. 1929-1937 ◽  
Author(s):  
B. Zhang ◽  
R. M. Harris-Warrick
Keyword(s):  
Motor Neuron ◽  
Voltage Clamp ◽  
Physiological Role ◽  
Input Resistance ◽  
Stomatogastric Ganglion ◽  
Channel Blockers ◽  
Plateau Potentials ◽  
Cancer Borealis ◽  
Calcium Dependent ◽  
Voltage Dependent

1. Using current- and voltage-clamp techniques, we examined the biophysical properties of a voltage-dependent Ca2+ current and its physiological role in plateau potential generation in the dorsal gastric (DG) motor neuron of the stomatogastric ganglion in the crab, Cancer borealis. 2. Stimulation of one of a set of identified serotonergic/cholinergic mechanosensory cells, the gastropyloric receptor (GPR) cells, induced plateau potentials in DG. A brief pressure application of serotonin (5-HT) closely mimicked the effect of the GPR cells. The 5-HT-evoked plateau in DG was not blocked by the sodium channel blocker, tetrodotoxin (TTX), or a combination of TTX with potassium channel blockers, including tetraethylammonium (TEA) and 4-aminopyridine (4-AP), and the Ih blocker, CsCl. The 5-HT-evoked plateau was eliminated by the Ca2+ channel blockers Co2+ and Cd2+, suggesting that Ca2+ entry is essential for plateau potentials in DG. During the plateau, we observed a 30% decrease in input resistance. 3. When sodium and potassium currents were blocked pharmacologically, injection of suprathreshold depolarizing current evoked all-or-none plateau-like responses lasting several seconds, even in the absence of 5-HT. This response was blocked by Ca2+ channel blockers, further supporting a role for Ca2+ in plateau generation. 5-HT significantly prolonged the duration of this plateau. 4. We isolated a voltage-dependent Ca2+ current in voltage-clamped DG neurons. This current was analyzed with the use of either Ca2+ or Ba2+ as the charge carrier after other currents had been maximally blocked with extracellular TTX, TEA, 4-AP, and CsCl and intracellular loading with Cs+ and ethylene glycol-bis(beta-aminoethyl ether)-N,N,N',N'-tetraacetic acid (EGTA). The Ca2+ current was detectable at -45 mV, peaked at -15 mV, and was estimated to reverse at +45 mV. Co2+ and Cd2+ effectively blocked the Ca2+ current. 5. The voltage dependence of activation of the Ca2+ current was quantantitively analyzed by fitting the voltage-conductance relation with a third power Boltzmann relation. The maximum conductance (gA), half-activation voltage (VA) for individual gating steps, and the slope steepness (k) were 0.19 +/- 0.02 (SE) microS, -36.5 +/- 2.0 mV, and 4.4 +/- 1.4 mV/e-fold, respectively. 6. 5-HT significantly potentiated the gA by approximately 42% without affecting VA and k. 7. We conclude from our current- and voltage-clamp results that a voltage-dependent Ca2+ current plays an important role in generating plateau potentials in the DG neuron. Enhancement of the voltage-dependent Ca2+ current by 5-HT is one of the mechanisms for 5-HT-evoked plateau potentials.

Calcium-dependent plateau potentials in a crab stomatogastric ganglion motor neuron. II. Calcium-activated slow inward current

1995 ◽  
Vol 74 (5) ◽  
pp. 1938-1946 ◽  
Author(s):  
B. Zhang ◽  
J. F. Wootton ◽  
R. M. Harris-Warrick
Keyword(s):  
Motor Neuron ◽  
Voltage Clamp ◽  
Physiological Role ◽  
Reversal Potential ◽  
Stomatogastric Ganglion ◽  
Slow Inward Current ◽  
Inward Current ◽  
Tail Current ◽  
Plateau Potential ◽  
Voltage Dependent

1. Using intracellular recording and voltage-clamp techniques, we examined the biophysical properties of a Ca(2+)-activated slow inward current and its physiological role in plateau potential generation in the dorsal gastric (DG) motor neuron of the stomatogastric ganglion in the crab, Cancer borealis. 2. As shown in the accompanying paper, a brief puff of serotonin (5-HT) evoked a plateau potential in the DG neuron. Intracellular loading of the Ca2+ chelator ethylene glycol-bis (beta-aminoethyl ether)-N,N,N',N'-tetraacetic acid (EGTA) prevented 5-HT from evoking a plateau potential. On the contrary, rapid increase of intracellular Ca2+ by photolysis of caged-Ca2+ (bound to DM-nitrophen) evoked a plateau response in DG bathed in normal saline. 3. Extracellular tetrodotoxin (TTX), tetraethylammonium (TEA), 4-aminopyridine (4-AP), and Cs+ and intracellular iontophoresis of Cs+ were used to block voltage-dependent INa, IK, and Ih. Under these conditions we voltage clamped DG using two electrodes and isolated a long-lasting tail current after a short depolarization of the cell. 4. The reversal potential of the slow tail current was extrapolated to be -27 +/- 3.5 (SE) mV. Na+ substitutions with choline+, tris(hydroxymethyl)aminomethane+ (Tris+) or n-methyl-glucamine+ (NMG+) did not significantly affect the reversal potential or the amplitude. 5. The slow tail current was Ca2+ dependent. It was reduced or abolished by the Ca2+ channel blocker Co2+, intracellular injection of EGTA, and by Ba2+ replacement of Ca2+ as the charge carrier. The activation and deactivation of this current do not show an apparent dependence on voltage. 6. When the voltage-dependent Na+, K+, and Ca2+ channels were blocked, a brief puff of caffeine evoked a slow depolarization. In voltage clamp, caffeine evoked a slow inward current with an apparent conductance increase. This current was reduced by intracellular EGTA. The current-voltage (I-V) relationship of the caffeine-evoked current was linear with a reversal potential of -25 +/- 4.8 mV. This was not statistically different from the reversal potential of the depolarization-evoked tail current. 7. 5-HT enhanced the depolarization-evoked slow tail current but had no effect on the caffeine-evoked slow inward current. 8. We conclude that the slow tail current is a Ca(2+)-activated nonselective current, similar to the Ca(2+)-activated nonspecific cation currents described in other preparations. This current appears to play an important role in plateau generation and maintenance in DG. 5-HT has no direct effect on the properties of this current, but it indirectly enhances the current through an increase of voltage-dependent Ca2+ current.

Sodium-dependent plateau potentials in cultured Retzius cells of the medicinal leech

1996 ◽  
Vol 76 (3) ◽  
pp. 1491-1502 ◽  
Author(s):  
J. D. Angstadt ◽  
J. J. Choo
Keyword(s):  
Outward Current ◽  
Channel Blockers ◽  
Plateau Potentials ◽  
Calcium Dependent ◽  
Current Pulses ◽  
Outward Currents ◽  
Voltage Dependent ◽  
Prolonged Duration ◽  
Input Conductance ◽  
Retzius Cells

1. Individual leech Retzius (Rz) cells were removed from mid-body ganglia and plated in cell culture on concanavalin A or polylysine. Experiments on the majority of cells were performed after 6-11 days in culture. Isolated Rz cells were superfused with normal leech saline (NS), cobalt saline (Ca2+ replaced with Co2+), or one of a variety of other modified salines. 2. Prolonged plateau potentials (PPs) with durations ranging from several seconds to nearly 2 min were evoked in isolated Rz cells in response to 1-s depolarizing current pulses delivered under discontinuous current clamp. Some PPs terminated spontaneously while others were terminated with hyperpolarizing current pulses. PPs were associated with a dramatic increase in the input conductance of the neuron. The PP decayed slightly over time, and this decay was accompanied by a small decrease in the input conductance. 3. PP duration was enhanced by penetrating cells with electrodes containing tetraethylammonium (TEA) and by bathing cells in Co2+ saline, but PPs were evoked also in NS and using electrodes without TEA. The effects of TEA and Co2+ saline suggest that voltage-dependent and especially calcium-dependent outward currents normally suppress plateau formation. 4. PPs occurred most reliably in neurons with extensive neurite sprouting. Isolated somata with few or no neurites usually failed to express PP, although there were several exceptions to this trend. 5. PPs persisted when Ca2+ was replaced with either of the calcium channel blockers Co2+, Ni2+, or Mn2+, when 200 microM Cd2+ was added to normal saline, or when Na+ was replaced with Li+. In contrast, PPs were eliminated rapidly when Na+ was replaced with N-methyl-D-glucamine. 6. Isolated Rz cells also expressed repetitive PPs either spontaneously or in response to injection of sustained depolarizing current. Spontaneous repetitive PPs were suppressed by hyperpolarizing current. Repetitive PPs in isolated Rz cells are similar in many respects to the bursting electrical activity induced by Co2+ saline in Rz and other neurons in intact ganglia. 7. The ionic dependence and prolonged duration of PPs suggest that these responses are generated by a persistent voltage-dependent Na+ current. A quantitative computer simulation of PPs was achieved using a depolarization-activated Na+ conductance with very slow inactivation. Repetitive PPs were simulated by addition of a slow outward current in the form of an electrogenic pump.

Compartmental Model of Vertebrate Motoneurons for Ca2+-Dependent Spiking and Plateau Potentials Under Pharmacological Treatment

1997 ◽  
Vol 78 (6) ◽  
pp. 3371-3385 ◽  
Author(s):  
Victoria Booth ◽  
John Rinzel ◽  
Ole Kiehn
Keyword(s):  
Ion Channel ◽  
Compartmental Model ◽  
Ionic Currents ◽  
Channel Blockers ◽  
Spinal Motoneurons ◽  
Plateau Potentials ◽  
Firing Patterns ◽  
Calcium Dependent ◽  
Ion Channel Blockers ◽  
Experimental Effect

Booth, Victoria, John Rinzel, and Ole Kiehn. Compartmental model of vertebrate motoneurons for Ca2+-dependent spiking and plateau potentials under pharmacological treatment. J. Neurophysiol. 78: 3371–3385, 1997. In contrast to the limited response properties observed under normal experimental conditions, spinal motoneurons generate complex firing patterns, such as Ca2+-dependent regenerative spiking and plateaus, in the presence of certain neurotransmitters and ion-channel blockers. We have developed a quantitative motoneuron model, based on turtle motoneuron data, toinvestigate the roles of specific ionic currents and the effects of their soma and dendritic distribution in generating these complex firing patterns. In addition, the model is used to explore the effects of multiple ion channel blockers and neurotransmitters that are known to modulate motoneuron firing patterns. To represent the distribution of ionic currents across the soma and dendrites, the model contains two compartments. The soma compartment, representing the soma and proximal dendrites, contains Hodgkin-Huxley-like sodium ( I Na) and delayed rectifier K+ ( I K−dr) currents, an N-like Ca2+ current ( I Ca−N), and a calcium-dependent K+ current [ I K(Ca)]. The dendritic compartment, representing the lumped distal dendrites, contains, in addition to I Ca−N and I K(Ca) as in the soma, a persistent L-like calcium current ( I Ca−L). We determined kinetic parameters for I Na, I K−dr, I Ca−N, and I K(Ca) in order to reproduce normal action-potential firing observed in turtle spinal motoneurons, including fast and slow afterhyperpolarizations (AHPs) and a linear steady-state frequency-current relation. With this parameter set as default, a sequence of pharmacological manipulations were systematically simulated. A small reduction of I K−dr [mimicking the experimental effect of tetraethylammonium (TEA) in low concentration] enhanced the slow AHP and caused calcium spiking (mediated by I Ca−N) when I Na was blocked. Firing patterns observed experimentally in high TEA [and tetrodotoxin (TTX)], namely calcium spikes riding on a calcium plateau, were reproduced only when both I K−dr and I K(Ca) were reduced. Dendritic plateau potentials, mediated by I Ca−L, were reliably unmasked when I K(Ca) was reduced, mimicking the experimental effect of the bee venom apamin. The effect of 5-HT, which experimentally induces the ability to generate calcium-dependent plateau potentials but not calcium spiking, was reproduced in the model by reducing I K(Ca) alone. The plateau threshold current level, however, was reduced substantially if a simultaneous increase in I Ca−L was simulated, suggesting that serotonin (5-HT) induces plateau potentials by regulating more than one conductance. The onset of the plateau potential showed significant delays in response to near-threshold, depolarizing current steps. In addition, the delay times were sensitive to the current step amplitude. The delay and its sensitivity were explained by examining the model's behavior near the threshold for plateau onset. This modeling study thus accurately accounts for the basic firing behavior of vertebrate motoneurons as well as a range of complex firing patterns invoked by ion-channel blockers and 5-HT. In addition, our computational results support the hypothesis that the electroresponsiveness of motoneurons depends on a nonuniform distribution of ionic conductances, and they predict modulatory effects of 5-HT and properties of plateau activation that have yet to be tested experimentally.

scholarly journals Action Potentials in Rhodnius Oocytes: Repolarization is Sensitive to Potassium Channel Blockers

1986 ◽  
Vol 126 (1) ◽  
pp. 119-132
Author(s):  
M. J. O'DONNELL
Keyword(s):  
Potassium Channel ◽  
Action Potentials ◽  
Potassium Conductance ◽  
Channel Blockers ◽  
Calcium Dependent ◽  
Outward Rectification ◽  
Current Voltage ◽  
Potassium Channel Blockers ◽  
Potassium Conductances ◽  
Voltage Dependent

Depolarization of Rhodnius oocytes evokes action potentials (APs) whose rising phase is calcium-dependent. The ionic basis for the repolarizing (i.e. falling) phase of the AP was examined. Addition of potassium channel blockers (tetraethylammonium, tetrabutylammonium, 4-aminopyridine, atropine) to the bathing saline increased the duration and overshoot of APs. Intracellular injection of tetraethyl ammonium had similar effects. These results suggest that a voltage-dependent potassium conductance normally contributes to repolarization. Repolarization does not require a chloride influx, because substitution of impermeant anions for chloride did not increase AP duration. AP duration and overshoot actually decreased progressively when chloride levels were reduced. Current/voltage curves show inward and outward rectification, properties often associated with potassium conductances. Outward rectification was largely blocked by external tetraethylammonium. Possible functions of the rectifying properties of the oocyte membrane are discussed.

Calcium-Dependent Plateau Potentials in Rostral Ambiguus Neurons in the Newborn Mouse Brain Stem In Vitro

1997 ◽  
Vol 78 (5) ◽  
pp. 2483-2492 ◽  
Author(s):  
Jens C. Rekling ◽  
Jack L. Feldman
Keyword(s):  
Brain Stem ◽  
Mouse Brain ◽  
Spike Activity ◽  
Nucleus Ambiguus ◽  
Newborn Mouse ◽  
Plateau Potentials ◽  
Calcium Dependent ◽  
Current Pulses ◽  
Voltage Dependent

Rekling, Jens C. and Jack L. Feldman. Calcium-dependent plateau potentials in rostral ambiguus neurons in the newborn mouse brain stem in vitro. J. Neurophysiol. 78: 2483–2492, 1997. The nucleus ambiguus contains vagal and glossopharyngeal motoneurons and preganglionic neurons involved in respiration, swallowing, vocalization, and control of heart beat. Here we show that the rostral compact formation's ambiguus neurons, which control the esophageal phase of swallowing, display calcium-dependent plateau potentials in response to tetanic orthodromic stimulation or current injection. Whole cell recordings were made from visualized neurons in the rostral nucleus ambiguus using a slice preparation from the newborn mouse. Biocytin-labeling revealed dendritic trees with pronounced rostrocaudal orientations confined to the nucleus ambiguus, a morphological profile matching that of vagal motoneurons projecting to the esophagus. Single-stimulus orthodromic activation, using an electrode placed in the dorsomedial slice near the nucleus tractus solitarius, evoked single excitatory postsynaptic potentials (EPSPs) or short trains of EPSPs (500 ms to 1 s). However, tetanic stimulation (5 pulses, 10 Hz) induced voltage-dependent afterdepolarizations or long-lasting plateau potentials (>1 min) with a constant firing pattern. Depolarizing or hyperpolarizing current pulses elicited voltage-dependent afterdepolarizations or plateau potentials lasting a few seconds to several minutes. Constant spike activity accompanied the long-lasting plateau potentials, which ended spontaneously or could be terminated by weak hyperpolarizing current pulses. Current-induced afterdepolarizations and plateau potentials were dependent on extracellularand intracellular Ca2+, as they were blocked completely by extracellular Co2+, Cd2+, or intracellular bis-( o-aminophenoxy)- N,N,N′,N′-tetraacetic acid (BAPTA). Orthodromically induced afterdepolarizations and plateau potentials were blocked by intracellular BAPTA. Afterdepolarizations and plateau potentials were completely blocked by substitution of extracellular Na+ with choline. Afterdepolarizations persisted in tetrodotoxin. We conclude that rostral ambiguus neurons have a Ca2+-activated inward current carried by Na+. Synaptic activation of this conductance may generate prolonged spike activity in these neurons during the esophageal phase of swallowing.

Ionic mechanism of substance P actions on neurons in trigeminal root ganglia

1990 ◽  
Vol 64 (1) ◽  
pp. 273-281 ◽  
Author(s):  
I. Spigelman ◽  
E. Puil
Keyword(s):  
Substance P ◽  
Voltage Clamp ◽  
Reversal Potential ◽  
Input Resistance ◽  
K Channel ◽  
Resting Membrane Potential ◽  
Channel Blockers ◽  
Ionic Mechanism ◽  
Trigeminal Root

1. Responses of primary sensory neurons to substance P applications by perfusion were studied with intracellular recording techniques in in vitro slice preparations of trigeminal root ganglia (guinea pigs). Application of substance P in micromolar doses produced reversible depolarizations of 2–47 mV in 48 out of 64 neurons. The depolarizing influence facilitated repetitive spike discharge evoked by current-pulse injection. Evidence of desensitization was observed during prolonged or repeated applications of the peptide. 2. The responses to substance P were associated with decreased input resistance, although increased input resistance was observed in neurons where the resting membrane potential was compensated with DC injection. In single-electrode voltage-clamp (SEVC) recordings, substance P evoked an inward shift in the holding current and reduced an outwardly rectifying component in the I-V relationships. The reversal potential for the substance P response could not be determined. These results suggested that the perikaryal response to substance P has a complex ionic mechanism involving activation and deactivation of membrane conductances. 3. Substance P-induced depolarizations were greatly attenuated during perfusion with solutions that were deficient in [Na+] or [Mg2+] and were not significantly affected during perfusion with low-[Ca2+]-, CO2(+)-containing solutions. 4. In the voltage-clamp investigations, an inward current contributed to the substance P responses during combined application with the K(+)-channel blockers, 4-aminopyridine (4-AP) and tetraethylammonium (TEA). This current was not abolished by the inclusion of CsCl in the perfusing solution or by internal Cs+ application from the recording electrode, suggesting that an anomalous inward rectifier was not involved in the responses to substance P.(ABSTRACT TRUNCATED AT 250 WORDS)

5-HT modulation of hyperpolarization-activated inward current and calcium-dependent outward current in a crustacean motor neuron

1992 ◽  
Vol 68 (2) ◽  
pp. 496-508 ◽  
Author(s):  
O. Kiehn ◽  
R. M. Harris-Warrick
Keyword(s):  
Motor Neuron ◽  
Time Course ◽  
Reversal Potential ◽  
Outward Current ◽  
Resting Membrane Potential ◽  
Inward Current ◽  
Calcium Dependent ◽  
Voltage Dependent ◽  
K Concentration ◽  
Conductance Increase

1. Serotonergic modulation of a hyperpolarization-activated inward current, Ih, and a calcium-dependent outward current, Io(Ca), was examined in the dorsal gastric (DG) motor neuron, with the use of intracellular recording techniques in an isolated preparation of the crab stomatogastric ganglion (STG). 2. Hyperpolarization of the membrane from rest with maintained current pulses resulted in a slow time-dependent relaxation back toward rest and a depolarizing overshoot after termination of the current pulse. In voltage clamp, hyperpolarizing commands negative to approximately -70 mV caused a slowly developing inward current, Ih, which showed no inactivation. Repolarization back to the holding potential of -50 mV revealed a slow inward tail current. 3. The reversal potential for Ih was approximately -35 mV. Raising extracellular K+ concentration ([K+]o) from 11 to 22 mM enhanced, whereas decreasing extracellular Na+ concentration ([Na+]o) reduced the amplitude of Ih. These results indicate that Ih in DG is carried by both K+ and Na+ ions. 4. Bath application of serotonin (5-HT; 10 microM) caused a marked increase in the amplitude of Ih through its active voltage ranges. 5. The time course of activation of Ih was well fitted by a single exponential function and strongly voltage dependent. 5-HT increased the rate of activation of Ih. 5-HT also slowed the rate of deactivation of the Ih tail on repolarization to -50 mV. 6. The activation curve for the conductance (Gh) underlying Ih was obtained by analyzing tail currents. 5-HT shifted the half activation for Gh from approximately -105 mV in control to -95 mV, resulting in an increase in the amplitude of Gh active at rest. 7. Two to 4 mM Cs+ abolished Ih, whereas barium (200 microM to 2 mM) had only weak suppressing effects on Ih. Concomitantly, Cs+ also blocked the 5-HT-induced inward current and conductance increase seen at voltages negative to rest. In current clamp, Cs+ caused DG to hyperpolarize 3-4 mV from rest, suggesting that Ih is partially active at rest and contributes to the resting membrane potential. 8. Depolarizing voltage commands from a holding potential of -50 mV resulted in a total outward current (Io) with an initial transient component and a sustained steady-state component. Application of 5-HT reduced both the transient and sustained components of Io. 9. Io was reduced by 10-20 mM tetraethylammonium (TEA), suggesting that it is primarily a K+ current.(ABSTRACT TRUNCATED AT 400 WORDS)

Electrical Properties of a Cockroach Motor Neuron Soma Depend on Different Characteristics of Individual Ca Components

1997 ◽  
Vol 78 (5) ◽  
pp. 2455-2466 ◽  
Author(s):  
Janette D. Mills ◽  
Robert M. Pitman
Keyword(s):  
Electrical Properties ◽  
Motor Neuron ◽  
Action Potentials ◽  
Calcium Current ◽  
Plateau Potentials ◽  
Inward Current ◽  
Calcium Dependent ◽  
Dependent Inactivation ◽  
Different Characteristics ◽  
Neuron Soma

Mills, Janette D. and Robert M. Pitman. Electrical properties of a cockroach motor neuron soma depend on different characteristics of individual Ca components. J. Neurophysiol. 78: 2455–2466, 1997. The “fast” coxal depressor motor neuron (Df) of the cockroach is among the most extensively studied of insect neurons. It has been shown that the cell body of this neuron can exhibit active electrical properties, which may change over time or with chemical modulation. To further understand these electrical events and their modulation, inward currents in Df have been characterized under conditions in which outward currents have been suppressed. The inward current activated at potentials positive to −60 mV and peaked between −10 and 0 mV when measured in barium saline and between 0 and +10 mV when measured in calcium saline. The inward current was insensitive to Ni2+ (100 μM) but reduced by verapamil (50 μM) and abolished by Cd2+ (1 mM). Two components of I Ca were identified by their sensitivity to either 50 μM nifedipine or micromolar Cd2+. The nifedipine-sensitive component activated positive to −60 mV and peaked between −10 and 0 mV, whereas the Cd2+-sensitive component activated positive to −40 mV and peaked between +10 and +20 mV. Immediately after dissection, depolarization of Df evoked plateau potentials, whereas 1–4 h after dissection, depolarization evoked action potentials. The plateau potentials were insensitive to 100 μM Cd2+ but blocked by 50 μM nifedipine, whereas the spikes required a combination of nifedipine (50 μM) and Cd2+ (100 μM) for complete suppression, indicating that only one component of I Ca contributes to the plateau potential, whereas both components contribute to action potentials. Currents measured in calcium saline decayed faster than currents measured in barium saline. The inactivation characteristics were investigated with the use of double-pulse voltage-clamp experiments. I Ca showed a greater degree of inactivation and slower recovery from inactivation than did I Ba. Current decay and the extent of inactivation were reduced after injection of the calcium-chelator 1,2-bis(2-aminophenoxy)ethane- N,N,N′,N′-tetraacetic acid (BAPTA). This suggests that the calcium current of this neuron displays calcium-dependent inactivation. An additional mechanism, most probably voltage-dependent inactivation, also occurs because I Ba, even in neurons injected with BAPTA, displayed some inactivation. The inactivation characteristics may be important in determining activity displayed by Df. Indirect evidence suggests that intracellular calcium is high immediately after dissection. At this time, the calcium current may therefore be reduced due to calcium-dependent inactivation. This may, at least partly, explain why the cell does not spike shortly after dissection.

Development and Serotonergic Modulation of NMDA Bursting in Rat Trigeminal Motoneurons

2002 ◽  
Vol 87 (3) ◽  
pp. 1318-1328 ◽  
Author(s):  
Chie-Fang Hsiao ◽  
Nanping Wu ◽  
Michael S. Levine ◽  
Scott H. Chandler
Keyword(s):  
Membrane Potential ◽  
Nmda Receptor ◽  
Voltage Clamp ◽  
Input Resistance ◽  
Burst Discharge ◽  
Bath Application ◽  
Voltage Clamp Analysis ◽  
Voltage Dependent ◽  
Trigeminal Motoneurons ◽  
Bursting Activity

The development of N-methyl-d-aspartate (NMDA)-induced burst discharge in rat trigeminal motoneurons (TMNs) between postnatal days P1 and P10 was examined using whole cell patch-clamp recording methods in brain slices. Bath application of NMDA (50 μM) induced a Mg2+-dependent rhythmical bursting activity starting around P8. Prior to the onset of bursting, the membrane potential depolarized and the input resistance increased. Hyperpolarization of the membrane potential with extrinsic current demonstrated a narrow window of membrane potential where maintained rhythmical burst discharge was evident. In P1–P4 neurons, NMDA application produced membrane depolarization and a minimal change in input resistance, but no burst activity at any membrane potential. Voltage-clamp analysis indicated that the bursting activity was related to the presence or absence of a voltage-dependent Mg2+ block and induction of a negative slope conductance (NSC) region in the I NMDA- V relationship. Regardless of age, reduction of extracellular Mg2+ from 1 mM to 30 μM enhanced I NMDA at voltages negative to −60 mV. However, in 1 mM Mg2+, P1–P4 neurons were devoid of a prominent NSC region compared with P8–P10 neurons, suggesting that the efficacy of depolarization in unblocking the NMDA receptors increased with age. NMDA bursting was not dependent on calcium influx through voltage-gated calcium channels (VGCC) but did require a minimal concentration of Ca2+ in the bath. Intracellular bis-( o-aminophenoxy)- N,N,N′,N′-tetraacetic acid application suppressed burst discharge completely, suggesting that intracellular Ca2+ directly, or via second-messenger systems, regulates NMDA receptor activity and bursting. Interestingly, NMDA bursting could be induced in P1–P4 neurons by simultaneous bath application of serotonin (5-HT, 10 μM), which by itself did not produce bursting, suggesting an “enabling” role for 5-HT. Voltage-clamp analysis demonstrated that the NMDA/5-HT bursting resulted from induction of an NSC in the I-Vrelationship of total membrane current. 5-HT by itself produced no such effect. The mechanisms for this effect were due to an enhancement of the NSC region of the I NMDA- V relationship and reduction of a presumed leak current by 5-HT. These data indicate that NMDA bursting in trigeminal motoneurons is developmentally regulated and subject to neuromessenger modulation. Control of the Mg2+ sensitivity of the NMDA receptor and voltage-dependent block by neuromessengers could be an effective means to control the efficacy of glutamatergic synaptic drive to motoneurons during rhythmical oral-motor activity at early postnatal ages.

Ionic influences on the prolonged depolarization of turtle cones in situ

1991 ◽  
Vol 65 (1) ◽  
pp. 96-110 ◽  
Author(s):  
W. B. Thoreson ◽  
D. A. Burkhardt
Keyword(s):  
Channel Blocker ◽  
Potassium Acetate ◽  
Input Resistance ◽  
Extracellular Calcium ◽  
Channel Blockers ◽  
Calcium Dependent ◽  
Ion Channel Blockers ◽  
Potassium Channel Blockers ◽  
Extracellular Sodium

1. The effects of ion channel blockers and ion substitutions on the prolonged depolarization of cones in the retina of the turtle (Pseudemys scripta elegans) were studied by intracellular recording. 2. The results of current injection experiments indicate that the prolonged depolarization is regenerative and accompanied by a reduction in the cone's input resistance. 3. The addition of cobalt (5–10 mM) or the removal of extracellular calcium suppressed the prolonged depolarization. Raising extracellular calcium or adding strontium (10 mM) lowered the threshold and increased the duration of the response. 4. Unlike the feedback spikes of turtle cones studied by Piccolino and Gerschenfeld, the prolonged depolarization was not blocked by the organic calcium channel blocker, D600. 5. Adding a calcium chelator, ethylene glycol-bis(beta-aminoethyl ether)-N,N,N',N'-tetraacetic acid (EGTA), to the electrolyte caused a progressive shortening of the prolonged depolarization until it was ultimately abolished. 6. Lowering extracellular sodium or use of the potassium channel blockers tetraethylammonium (TEA) and 4-aminopyridine (4-AP) had little effect on the prolonged depolarization. 7. Removing chloride from the superfusate induced a significant enhancement of the prolonged depolarization. In normal superfusate, the response tended to be of larger amplitude when recorded with electrodes containing chloride [1.5 M KCl + 1.5 M potassium acetate (KA)] rather than KA or potassium methylsulfate (KM) alone. 8. The results suggest that the prolonged depolarization is initiated by the regenerative activation of voltage-sensitive calcium channels and sustained by a calcium-dependent chloride efflux. The present findings are also discussed in relation to the functional significance of the prolonged depolarization and mechanisms for the surround antagonism of cones in situ.

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