scholarly journals Initiation and Modulation Of Action Potentials in Salivary Gland Cells of Haementer1A Ghilianii by Putative Transmitters and Cyclic Nucleotides

1991 ◽  
Vol 157 (1) ◽  
pp. 101-122
Author(s):  
WERNER A. WUTTKE ◽  
MICHAEL S. BERRY
Keyword(s):  
Cyclic Amp ◽  
Cyclic Gmp ◽  
Action Potentials ◽  
Input Resistance ◽  
Adenosine Monophosphate ◽  
Guanosine Monophosphate ◽  
Membrane Properties ◽  
Duration Of Action ◽  
Inward Current ◽  
Gland Cells

1. The giant salivary cells of Haementeria ghilianii are known to produce Ca2+-dependent action potentials and to release their secretory products in response to stimulation of the stomatogastric nerve. In this study, the electrophysiological effects of some putative transmitters were examined by perfusion of the gland and two promising candidates were selected for detailed analysis. 2. Acetylcholine (ACh) was the only substance tested which excited the gland cells. It produced a large, Na+-dependent depolarization that elicited 1–3 action potentials and desensitized to about 24% of its maximal value within 2 min. 3. Carbachol, tetramethylammonium and nicotine elicited similar responses to ACh, whereas choline and pilocarpine had negligible effects. 4. The ACh response was completely blocked by d-tubocurarine and strychnine, and was reduced by tetraethylammonium, hexamethonium and atropine. The receptors, therefore, cannot be clearly distinguished as nicotinic or muscarinic. 5. ACh did not elicit secretion, but this does not necessarily preclude it from acting as a neuroglandular transmitter. 6. 5-Hydroxytryptamine (5-HT) was the only transmitter candidate that elicited secretion, though it did not excite the gland cells. 7. 5-HT produced a subthreshold depolarization and an increase in input resistance. Action potentials, elicited by depolarizing pulses, were increased in amplitude and duration, and showed greatly reduced adaptation. 8. 5-HT potentiated the net inward current, evoked by subthreshold depolarizing pulses, by reducing outward K+ current. The inward current, carried by Ca2+, was not directly affected. In addition, 5-HT increased an inwardly rectifying current, carried by Na+ and K+. All the effects of 5-HT tended to increase cell excitability. 9. Salivary cell responses to 5-HT were reversibly antagonised by methysergide. 10. Responses to ACh or 5-HT were not mimicked by 3′, 5′-cyclic guanosine monophosphate, which greatly reduced spike amplitude and excitability. The effects were specific to the 3′, 5′ form; 2′, 3′-cyclic GMP had no effect. Cyclic GMP dramatically reduced the duration of action potentials that had been artificially prolonged by TEA+ or removal of external Ca2+. 11. Cyclic 3′, 5′-adenosine monophosphate and its dibutyryl derivative had little effect on membrane properties. 8-Bromo-cyclic AMP, however, mimicked all the effects of 5-HT. It is thought that 5-HT may exert its actions via cyclic AMP. 12. The possible role of 5-HT in salivary secretion is discussed.

scholarly journals IMMUNOLOGICAL RELEASE OF HISTAMINE AND SLOW REACTING SUBSTANCE OF ANAPHYLAXIS FROM HUMAN LUNG

1972 ◽  
Vol 136 (3) ◽  
pp. 556-567 ◽  
Author(s):  
Michael Kaliner ◽  
Robert P. Orange ◽  
K. Frank Austen
Keyword(s):  
Cyclic Amp ◽  
Cyclic Gmp ◽  
Human Lung ◽  
Cyclic Adenosine Monophosphate ◽  
Adenosine Monophosphate ◽  
Guanosine Monophosphate ◽  
Target Cells ◽  
Human Lung Tissue ◽  
Adrenergic Agents ◽  
Protein Assay

The immunologic release of histamine and slow reacting substance of anaphylaxis (SRS-A) from human lung tissue can be enhanced by stimulation with either alpha adrenergic agents (phenylephrine or norepinephrine in the presence of propranolol) or cholinergic agents (acetylcholine or Carbachol). The finding that atropine prevents cholinergic but not comparable alpha adrenergic enhancement is consistent with the view that cholinergic and alpha adrenergic agonists interact with separate receptor sites on the target cells involved in the immunologic release of chemical mediators. The consistent qualitative relationship between the antigen-induced release of mediators and the level of cyclic adenosine monophosphate (cyclic AMP) as measured by the isolation of 14C-labeled cyclic AMP after incorporation of adenine-14C into the tissues or by the cyclic AMP binding protein assay suggests that changes in the level of this cyclic nucleotide mediate adrenergic modulation of the release of histamine and SRS-A. The addition of 8-bromo-cyclic guanosine monophosphate (cyclic GMP) produces an enhancement of the immunologic release of mediators while dibutyryl cyclic AMP is inhibitory. As cholinergic-induced enhancement was not associated with a measurable change in the levels of cyclic AMP, the possibility is suggested that cyclic GMP may be the intracellular mediator of cholinergic-induced enhancement of the immunologic release of histamine and SRS-A.

Properties of visceral primary afferent neurons in the nodose ganglion of the rabbit

1985 ◽  
Vol 54 (2) ◽  
pp. 245-260 ◽  
Author(s):  
C. E. Stansfeld ◽  
D. I. Wallis
Keyword(s):  
Action Potentials ◽  
Input Resistance ◽  
Nodose Ganglion ◽  
Time Dependent ◽  
Membrane Properties ◽  
Resting Potential ◽  
Inward Rectification ◽  
C Cells ◽  
Axonal Conduction ◽  
A Cells

The active and passive membrane properties of rabbit nodose ganglion cells and their responsiveness to depolarizing agents have been examined in vitro. Neurons with an axonal conduction velocity of less than 3 m/s were classified as C-cells and the remainder as A-cells. Mean axonal conduction velocities of A- and C-cells were 16.4 m/s and 0.99 m/s, respectively. A-cells had action potentials of brief duration (1.16 ms), high rate of rise (385 V/s), an overshoot of 23 mV, and relatively high spike following frequency (SFF). C-cells typically had action potentials with a "humped" configuration (duration 2.51 ms), lower rate of rise (255 V/s), an overshoot of 28.6 mV, an after potential of longer duration than A-cells, and relatively low SFF. Eight of 15 A-cells whose axons conducted at less than 10 m/s had action potentials of longer duration with a humped configuration; these were termed Ah-cells. They formed about 10% of cells whose axons conducted above 2.5 m/s. The soma action potential of A-cells was blocked by tetrodotoxin (TTX), but that of 6/11 C-cells was unaffected by TTX. Typically, A-cells showed strong delayed (outward) rectification on passage of depolarizing current through the soma membrane and time-dependent (inward) rectification on inward current passage. Input resistance was thus highly sensitive to membrane potential close to rest. In C-cells, delayed rectification was not marked, and slight time-dependent rectification occurred in only 3 of 25 cells; I/V curves were normally linear over the range: resting potential to 40 mV more negative. Data on Ah-cells were incomplete, but in our sample of eight cells time-dependent rectification was absent or mild. C-cells had a higher input resistance and a higher neuronal capacitance than A-cells. In a proportion of A-cells, RN was low at resting potential (5 M omega) but increased as the membrane was hyperpolarized by a few millivolts. A-cells were depolarized by GABA but were normally unaffected by 5-HT or DMPP. C-cells were depolarized by GABA in a similar manner to A-cells but also responded strongly to 5-HT; 53/66 gave a depolarizing response, and 3/66, a hyperpolarizing response. Of C-cells, 75% gave a depolarizing response to DMPP.(ABSTRACT TRUNCATED AT 400 WORDS)

scholarly journals Transmitter Regulation of Plateau Properties in Turtle Motoneurons

1998 ◽  
Vol 79 (1) ◽  
pp. 45-50 ◽  
Author(s):  
Gytis Svirskis ◽  
Jørn Hounsgaard
Keyword(s):  
Membrane Potential ◽  
Metabotropic Glutamate Receptors ◽  
Input Resistance ◽  
Pattern Generation ◽  
Synaptic Activity ◽  
Membrane Properties ◽  
Resting Membrane Potential ◽  
Type I ◽  
Plateau Potentials ◽  
Inward Current

Svirskis, Gytis and Jørn Hounsgaard. Transmitter regulation of plateau properties in turtle motoneurons. J. Neurophysiol. 79: 45–50, 1998. In motoneurons, generation of plateau potentials is promoted by modulators that block potassium channels. In voltage-clamp experiments with triangular voltage ramp commands, we show that cis-(±)-1-aminocyclopentane-1,3-dicarboxylic acid ( cis-ACPD) and muscarine promote the generation of plateau potentials by increasing the dihydropyridine sensitive inward current, by increasing the input resistance, and by depolarizing the resting membrane potential. Type I metabotropic glutamate receptors (mGluR I) mediate the effects of cis-ACPD. Baclofen suppresses generation of plateau potentials by decreasing the dihydropyridine sensitive inward current, by decreasing the input resistance, and by hyperpolarizing the resting membrane potential. These results suggest that membrane properties of motoneurons are continuously modulated by synaptic activity in ways that may have profound effects on synaptic integration and pattern generation.

Na+ and Ca2+ currents of acutely isolated adult rat nodose ganglion cells

1986 ◽  
Vol 55 (3) ◽  
pp. 527-539 ◽  
Author(s):  
S. R. Ikeda ◽  
G. G. Schofield ◽  
F. F. Weight
Keyword(s):  
Action Potentials ◽  
Time Course ◽  
Ganglion Cells ◽  
Input Resistance ◽  
Potential Range ◽  
Patch Clamp Recording ◽  
Maximal Conductance ◽  
Pharmacological Agents ◽  
Inward Current ◽  
Ion Substitution

The electrical properties of nodose ganglion cells acutely isolated from adult rats were studied using the whole-cell patch-clamp recording method. Current-clamp recordings revealed a mean resting membrane potential of -54.3 mV and an input resistance of 527 M omega. Depolarizing current steps evoked action potentials with the following properties (mean): amplitude 111 mV, threshold -36 mV, and rate of rise 117 V/s. Two types of action potentials were observed, short and long duration. These properties, with the exception of input resistance (527 M omega cf. 50 M omega), are similar to those reported previously using intracellular recording methods in intact nodose ganglia (11, 20, 28). Brief application of 10 microM 5-hydroxytryptamine resulted in a rapid depolarization and burst of action potentials in the majority of cells. With voltage-clamp recording, step depolarizations to potentials positive to -10 mV elicited a transient inward current that was followed by a sustained outward current. Inward Na+ current was isolated by ion substitution and pharmacological agents. Two types of Na+ current were observed. One current was completely abolished by 3-15 microM tetrodotoxin (TTX), had a rapid time course, activated over the potential range -60 to -10 mV, and attained half-maximal conductance at -30 mV. The other current persisted in the presence of 15 microM TTX, had a slower time course, activated over the potential range -30 to 0 mV, and attained half-maximal conductance at -15 mV. In addition, 500 microM Cd2+ and 5.0 mM Co2+ reduced the TTX-insensitive current to 53 and 42% of control, respectively. Inward Ca2+ current was isolated by ion substitution and pharmacological agents and was identified by a dependence on external Ca2+. Cd2+ (500 microM) and Co2+ (5 mM) reduced the maximal inward current to 5 and 20% of control, respectively. When Ba2+ was substituted for Ca2+ as the charge carrier, the maximal inward current increased to 175% of control. Some cells had two Ca2+ current components, an inactivating component that activated near -60 mV and a large sustained current that activated near -40 mV. The initial inactivating current appeared as a "hump" on the current-voltage (I-V) curve over the potential range of -60 to -30 mV. The results indicate that, following isolation of these adult mammalian neurons, the membrane surfaces are sufficiently clean to allow patch-clamp recording.(ABSTRACT TRUNCATED AT 400 WORDS)

Developmental changes in membrane properties and postsynaptic currents of granule cells in rat dentate gyrus

1996 ◽  
Vol 76 (2) ◽  
pp. 1074-1088 ◽  
Author(s):  
Y. B. Liu ◽  
P. A. Lio ◽  
J. F. Pasternak ◽  
B. L. Trommer
Keyword(s):  
Action Potentials ◽  
Granule Cells ◽  
Molecular Layer ◽  
Membrane Potentials ◽  
Perforant Path ◽  
Membrane Properties ◽  
Inward Current ◽  
Postsynaptic Currents ◽  
Immature Neurons ◽  
Inward Currents

1. Whole cell patch-clamp recordings were used to study dentate gyrus granule cells in hippocampal slices from juvenile rats (postnatal days 8-32). Membrane properties were measured with the use of current-clamp recordings and were correlated with the morphology of a subgroup of neurons filled with biocytin. The components of the postsynaptic currents (PSCs) induced by medial perforant path stimulation were characterized with the use of specific receptor antagonists in voltage-clamp recordings. 2. Granule cells located in the middle third of the superior blade of stratum granulosum from the rostral third of hippocampus were divided into three groups according to their input resistance (IR). Neurons with low IR (206 +/- 182 M omega, mean +/- SD) had hyperpolarized resting membrane potentials (-82 +/- 7 mV) and high-amplitude action potentials (108 +/- 23 mV). Neurons were high IR (1,259 +/- 204 M omega) had more depolarized resting membrane potentials (-54 +/- 6 mV) and lower-amplitude action potentials (71 +/- 10 mV). Neurons with intermediate IR (619 +/- 166 M omega) also had intermediate resting membrane potentials (-63 +/- 7 mV) and action potential amplitudes (86 +/- 14 mV). Low-IR neurons became increasingly prevalent with advancing postnatal age, but neurons from each group could be found throughout the entire period under study. 3. Morphological studies of low-IR neurons revealed an extensive dendritic arborization that traversed the entire molecular layer and was characteristic of mature granule cells. High-IR cells had smaller somata and short, simple dendritic arborization that incompletely penetrated the molecular layer and were classified as immature. Intermediate-IR cells had morphological features of intermediate maturity. 4. The initial phase of the PSC evoked at -80 mV was a fast inward current that was comparable with respect to latency to peak, latency to onset, and 10-90% rise time in neurons of all maturities held at -80 mV. This current was 6-cyano-7-nitroquinoxaline-2,3-dione sensitive. 5. The decay phases of PSCs at -80 mV varied with neuronal maturity. Mature neurons had monoexponential decays (tau = 8.9 +/- 3.6). Intermediate and immature neurons had prominent later inward currents that resulted in slower decays. In the case of the immature neurons, the inward current during the decay phase could be separated from the initial fast inward peak. The later inward currents in intermediate and immature neurons were bicuculline sensitive. 6. With the use of uniform ionic conditions of the extracellular and patch solutions, current-voltage relations and reversal potentials for pharmacologically isolated alpha-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid (AMPA), N-methyl-D-aspartate (NMDA), and gamma-aminobutyric acid-A (GABAA) currents were comparable across all cell maturities. Calculated ratios for peak GABAA/NMDA/AMPA currents decreased significantly with maturation as follows: 9.4 +/- 2.9/1.4 +/- 0.5/1.0 for immature cells, 7.2 +/- 2.5/1.5 +/- 0.7O/1.0 for intermediate cells, and 2.0 +/- 1.2/0.9 +/- 0.4/1.0 for mature cells. 7. GABA current was mediated both by polysynaptic activation of interneurons and by direct activation of interneurons with monosynaptic input onto granule cells. The proportional contributions of mono- and polysynaptic GABA to total GABA were comparable across all cell maturities; latency to peak GABA current decreased with increasing cell maturity for both mono- and polysynaptic components. 8. We conclude that PSCs evoked in granule cells by medial perforant path activation in neurons of all maturities consist of both glutamatergic and GABAergic components. PSCS are dominated by GABAergic neurotransmission in immature granule cells, and the contribution of glutamatergic neurotransmission increases with neuronal maturation. The greater ratio of peak GABAA to glutamate currents and the longer time interval between their respective peaks combine to produce a distinctive PSC shape

Functional properties and axon terminations of interneurons in laminae III-V of the mammalian spinal dorsal horn in vitro

1992 ◽  
Vol 68 (5) ◽  
pp. 1746-1759 ◽  
Author(s):  
S. P. Schneider
Keyword(s):  
Dorsal Horn ◽  
Functional Organization ◽  
Input Resistance ◽  
Sensory Innervation ◽  
Membrane Properties ◽  
Mechanical Stimuli ◽  
Duration Of Action ◽  
Dendritic Trees ◽  
Skin Patch

1. The functional organization of interneurons in spinal laminae III-V was studied in an isolated preparation of hamster dorsal horn with sensory innervation from an excised skin patch. Morphological details of 40 neurons were visualized by intracellular injection of horseradish peroxidase. Active and passive membrane properties, synaptic responses to cutaneous nerve volleys, and responses to innocuous mechanical stimuli were determined for 25 cells with identified axons. 2. Neurons were classified into two types: 1) cells with local axons, branching in proximity to the cell soma and dendrites, that produced numerous synaptic boutons (740 +/- 504/axon; mean +/- SD), often arranged in clusters and 2) neurons with deep axons that usually bifurcated into rostral and caudal daughter branches up to 2.5 mm long, giving off collaterals ventral to the cell body and dendrites and forming significantly fewer boutons (155 +/- 140/axon) than local axon cells. A majority of boutons of local axon and deep axon cells, 89 and 83%, respectively, were of the en passant type. 3. Dendritic trees of local axon cells were relatively compact dorsoventrally (119 +/- 42 microns) and mediolaterally (128 +/- 45 microns), but were elongated rostrocaudally (404 +/- 121 microns). In comparison, dendritic trees of deep axon cells radiated significantly farther dorsoventrally (218 +/- 88 microns) and mediolaterally (180 +/- 34 microns), but exhibited comparable rostrocaudal spread (413 +/- 128 microns). There was no correlation between dorsoventral and mediolateral dendritic spread and mediolateral soma location for either cell type. However, for medially situated deep axon cells the rostrocaudal dendritic spread was up to 180% greater than for those located laterally. For nearly one-half of all cells (49%; 17/35) dendritic processes extended dorsally into lamina II. 4. Local axon cells had resting membrane potentials that were more negative than deep axon cells (-59.5 +/- 6.1 and -53.6 +/- 4.7 mV, respectively), but the amplitude and duration of action potentials generated by the two types were similar. Neuronal input resistance (RN) and membrane time constant (tau m) varied widely from cell to cell, but were not significantly different for local axon (77.4 +/- 46.8 M omega, 13.4 +/- 9.5 ms) and deep axon cells (46.5 +/- 19.2 M omega, 6.6 +/- 3.0 ms). 5. Volleys in myelinated afferent fibers activated fast rising excitatory postsynaptic potentials (EPSPs) that exhibited later, more slowly rising potentials with multiple components in a majority of deep axon (89%) and local axon (72%) neurons.(ABSTRACT TRUNCATED AT 400 WORDS)

Cyclic AMP is involved in cardioregulation by multiple neuropeptides encoded on the FMRFamide gene

1999 ◽  
Vol 202 (19) ◽  
pp. 2595-2607 ◽  
Author(s):  
D. Willoughby ◽  
M.S. Yeoman ◽  
P.R. Benjamin
Keyword(s):  
Cyclic Amp ◽  
Lymnaea Stagnalis ◽  
Isolated Heart ◽  
Cyclic Adenosine Monophosphate ◽  
Adenosine Monophosphate ◽  
Cyclic Guanosine Monophosphate ◽  
Heart Tissue ◽  
Heart Beat ◽  
Guanosine Monophosphate ◽  
Force Of Contraction

We have used a combination of biochemical and pharmacological techniques to investigate the role of the cyclic nucleotides, 3′, 5′-cyclic adenosine monophosphate (cyclic AMP) and 3′,5′-cyclic guanosine monophosphate (cyclic GMP), in mediating the cardioregulatory effects of FMRFamide and other neuropeptides encoded on exon II of the FMRFamide gene of Lymnaea stagnalis. The ‘isoleucine’ peptides (EFLRIamide and pQFYRIamide) produced complex biphasic effects on the frequency, force of contraction and tonus of the isolated heart of L. stagnalis, which were dependent on adenylate cyclase (AC) activity of the heart tissue. At a control rate of cyclic AMP production of less than or equal to 10 pmoles min(−)(1)mg(−)(1) protein, the ‘isoleucine’ peptides produced a significant increase in AC activity in heart membrane preparations. This suggested that the enhanced AC activity is responsible for the stimulatory effects of the ‘isoleucine’ peptides on frequency and force of contraction of heart beat. This excitation sometimes followed an initial ‘inhibitory phase’ where the frequency of beat, force of contraction and tonus of the heart were reduced by the ‘isoleucine’ peptides. Hearts that showed the inhibitory phase of the ‘isoleucine’ response, but characteristically lacked the delayed excitatory phase, were found to have high levels of membrane AC activity (breve)10 pmoles min(−)(1)mg(−)(1) protein in controls. Application of the ‘isoleucine’ peptides to membrane homogenate preparation from these hearts failed to increase AC activity. The addition of FMRFamide produced significant increases in the rate of cyclic AMP production in the heart membrane preparations, which could account, at least in part, for the cardioexcitatory effects of this peptide in the isolated whole heart. A membrane-permeable cyclic AMP analogue (8-bromo-cyclic AMP) and an AC activator (forskolin) were also cardioexcitatory. The peptide SEEPLY had no effects on the beat properties of the isolated heart and did not alter AC activity. The activity of the membrane-bound (particulate) guanylate cyclase (GC) was not significantly affected by any of the peptides.

Synaptic and intrinsic control of membrane excitability of neostriatal neurons. I. An in vivo analysis

1990 ◽  
Vol 63 (4) ◽  
pp. 651-662 ◽  
Author(s):  
P. Calabresi ◽  
N. B. Mercuri ◽  
A. Stefani ◽  
G. Bernardi
Keyword(s):  
Substantia Nigra ◽  
Firing Rate ◽  
Action Potentials ◽  
Input Resistance ◽  
Membrane Properties ◽  
Strong Increase ◽  
Synaptic Potential ◽  
Membrane Excitability ◽  
Stimulation Of

1. The relationship between membrane properties of neostriatal neurons and spontaneous and evoked synaptic potentials was studied with the use of intracellular recordings from anesthetized rats. Most of these neurons showed regular or irregular spontaneous depolarizing potentials that only in a few cases triggered action potentials at resting level. 2. The stimulation of the ipsilateral substantia nigra or of the sensorimotor cortex produced a relatively fast depolarizing post-synaptic potential (EPSP). In some cells this potential was followed by an inhibitory period that appeared as an hyperpolarization when the cell was depolarized from the resting level (inhibitory postsynaptic potential, IPSP). A late and long-lasting depolarization (LD) followed the EPSP or the EPSP-IPSP sequence. 3. Repetitive discharge with little adaptation was observed during direct depolarization. Most of the neurons tested for current-voltage (I-V) relationship showed nonlinearity of the input resistance in the hyperpolarizing direction. Spontaneous and evoked EPSPs were decreased in their amplitude and duration when the membrane potential was held at levels more hyperpolarized than -85 mV because of the strong rectification at these levels of hyperpolarization. 4. Local microiontophoretic application of bicuculline (BIC) or systemic administration of BIC and pentylenetetrazole (PTZ) produced a reduction of the IPSPs. The reduction of the inhibitory transmission caused a strong increase of the LD. The current-evoked firing pattern was not greatly altered. 5. The intracellular application of cesium increased the amplitude and the duration of the spontaneous depolarizations that triggered bursts of action potentials under this condition. Spikes were broadened and the rectification in the hyperpolarization direction was reduced. 6. Iontophoretically applied cadmium strongly depressed the amplitude of the spontaneous and evoked postsynaptic potentials. During cadmium application, nigral stimulation produced constant latency, all-or-none spikes in the absence of any synaptic potential. 7. Repetitive stimulation of the ipsilateral substantia nigra by electrical shocks (5 Hz, 25 s) produced a progressive and reversible decrease of the spontaneous depolarizing potentials (SDPs) and a decrease of the firing rate. In the same cells, when the train of stimulation was delivered in the ipsilateral cortex, a membrane depolarization coupled with an increase of the firing rate was observed. 8. We conclude that although synaptic circuits mediate a phasic inhibition in neostriatum, the low level of spontaneous firing of most neostriatal neurons is mainly because of the effects that membrane properties exert on the spontaneous and the evoked synaptic depolarizations in the striatum.(ABSTRACT TRUNCATED AT 400 WORDS)

Mechanisms for signal transformation in lemniscal auditory thalamus

1996 ◽  
Vol 76 (6) ◽  
pp. 3597-3608 ◽  
Author(s):  
F. Tennigkeit ◽  
D. W. Schwarz ◽  
E. Puil
Keyword(s):  
Action Potentials ◽  
Input Resistance ◽  
Membrane Properties ◽  
High Threshold ◽  
Tonic Firing ◽  
Sensory Stimuli ◽  
Current Pulses ◽  
Type K ◽  
Voltage Dependent ◽  
Medial Geniculate

1. During alertness, lemniscal thalamocortical neurons in the ventral medial geniculate body (MGBv) encode sound signals by firing action potentials in a tonic mode. When they are in a burst firing mode, characteristic of thalamic neurons during some sleep states, the same stimuli may have an alerting function, leading to conscious perception of sound. We investigated the intrinsic membrane properties of MGBv neurons in search of mechanisms that enable them to convert from burst to tonic firing modes, allowing accurate signal coding of sensory stimuli. 2. We studied thalamocortical relay neurons and identified neurons morphologically with injected N-(2-aminoethyl) biotinamide hydrochloride in in vitro slice preparations of young rats. With the use of the whole cell recording method, we examined the contributions of distinct conductances to voltage responses evoked by current pulses. The neurons (n = 74) displayed a narrow range of resting potentials (-68 +/- 4 mV, mean +/- SD) and an average input resistance of 226 +/- 100 M omega. The membrane time constant was 40 +/- 17.6 ms and the action potential threshold was -51.6 +/- 3 mV. 3. Injections of hyperpolarizing current pulses from rest revealed an inward rectification produced by two voltage-dependent components. A fast component, sensitive to blockade with Ba2+ (100–200 microM), was attributed to an inward rectifier, IIR. Such applications also increased input resistance and depolarized neurons, consistent with a blockade of various K+ conductances. Application of Ba2+ often unmasked another voltage-dependent rectification with a slower time course. The second component was sensitive to blockade with Cs+ (1.5 mM), reminiscent of a hyperpolarization-activated current, IH. 4. Depolarizing pulses from rest produced ramp-shaped voltage responses that led to delayed tonic firing. Blockade of Na+ conductances by tetrodotoxin (TTX, 300–600 nM), or extracellular replacement of Ca2+ with Mg2+ (with TTX present), reduced the slope of the ramp and the overall depolarizing response. Application of 4-aminopyridine (4-AP, 100 microM), a blocker of A-type K+ conductances, increased input resistance and the overall depolarizing response. The voltage ramp therefore represents a complex rectification due to voltage-dependent contributions of persistent Na-, Ca2+, and K+ conductances. 5. Depolarizing pulses from potentials of less than -75 mV evoked phasic burst responses, consisting of one to seven action potentials riding on a low-threshold spike (LTS). The LTS was absent in low extracellular Ca2+ conditions and was blocked by application of Ni2+ (0.6 mM), but not by Cd2+ (50 microM). Similar depolarization from less than -80 mV evoked several action potentials, often followed by a TTX-resistant high-threshold spike (HTS) of longer duration. Firing of HTSs always occurred during 4-AP (100 microM) application, inferring that, normally, A-type K+ conductances may control ability to fire an HTS. As in the LTS, a Ca2+ current is a major participant in the HTS because extracellular replacement of Ca2+ with Mg2+ or application of Cd2+ (50 microM) blocked its genesis. After TTX blockade of Na+ conductances, “tonic firing” of HTSs occurred during depolarization above -45 mV. 6. During tonic firing evoked by current pulses, the second and subsequent spikes were longer in duration than the initial action potentials. Low extracellular concentrations of Ca2+ or Cd2+ (50 microM) application reduced the durations of the nonprimary spikes, inferring a contribution of high-threshold voltage-dependent Ca2+ conductances to their repolarizing phase. Also, K+ conductances may contribute to spike repolarization, because 4-AP (100 microM) or tetraethylammonium (2 mM) application led to prolonged action potentials and the generation of plateau potentials. A fast afterhyperpolarization, likely mediated by a Ca(2+)-dependent K+ conductance, limited the tonic firing. Such conductances, therefore, may regulate the re

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