External Potassium Concentration Research Articles

Serotonin 1 and serotonin 2 receptors hyperpolarize and depolarize separate populations of medial pontine reticular formation neurons in vitro

The action of serotonin on medial pontine reticular formation neurons was examined using intracellular electrophysiological methods in rat brainstem slices in vitro. A hyperpolarization associated with a decrease in input resistance was elicited by serotonin in 34% of the neurons, and a depolarization associated with an increase in input resistance was produced in 56% of the neurons. Both responses persisted in the presence of tetrodotoxin. The hyperpolarization resulted from a steady-state increase in outward current which varied with the external potassium concentration in a manner consistent with a conductance increase primarily to this ion. This response was mimicked by the serotonin 1 agonist, 5-carboxamidotryptamine, as well as by the serotonin 1a agonist, 8-hydroxy-dipropyl aminotetralin hydrobromide, and was blocked by spiperone, an antagonist of serotonin, sites. The depolarization resulted from a steady-state decrease in outward current which varied with external potassium. The depolarization was mimicked by the serotonin 2 agonist, α-methyl-5-hydroxytryptamine, and was blocked by the serotonin 2 antagonist, ketanserin. Neither of these agents had any effect upon serotonin-induced hyperpolarizations. In conclusion, the excitability of medial pontine reticular formation neurons is influenced by serotonin acting to increase or decrease potassium conductance(s). These opposing effects reflect actions on distinct serotonin receptor subtypes that are segregated to distinct populations of medial pontine reticular formation neurons.

The effect of ascorbate free radical on the energy state of the plasma membrane of onion ( Allium cepa L.) root cells: alteration of K + efflux by ascorbate?

In this work we demonstrate the effect of different ascorbate redox states on the plasma membrane potential and on proton secretion of onion root cells. Ascorbate and dehydroascorbate induced plasmalemma hyperpolarization and stimulated acidification of the incubation medium. Both effects were transient and a repolarization of the membrane to the resting potential was reached within 10–15 min. The free radical of ascorbate had a permanent stimulatory effect on both parameters: a concentration dependent hyperpolarization of about 25 mV, partially inhibited by vanadate, could be observed immediately after addition. Medium acidification was increased by a factor of 20 and sustained for hours in the presence of the free radical. Additionally, ascorbate may increase the potassium permeability of the membrane, thus hyperpolarizing at low external potassium concentrations. Possible interactions of the free radical with the energization of the plasma membrane via an NAD(P)H-ascorbate free radical reductase (EC 1.6.5.4) will be discussed.

Increases in intracellular calcium ion concentration during depolarization of cultured embryonic Xenopus spinal neurones.

1. Changes in intracellular Ca2+ ion concentrations ([Ca2+]i) during potassium-induced depolarizations were studied in cultured embryonic Xenopus spinal neurones using the Ca(2+)-sensitive dye Fura-2 and quantitative fluorescence microscopy. 2. Membrane voltages attained during exposure to bath solutions containing 3, 10, 20, 30, 40 and 50 mM-K+ were determined under current clamp. In 3 mM-K(+)-containing solution (normal saline), the resting potential was -65 mV. The threshold voltage required to observe a measurable rise in [Ca2+]i was -40 mV (external potassium concentration [K+]o = 20 mM). The depolarization-induced [Ca2+]i signal had two components: a non-relaxing component, and, at voltages positive to -40 mV, an additional transient component on the rising phase that decayed over tens of seconds. There was substantial variability in the magnitudes of resting and voltage-induced changes in [Ca2+]i, but [Ca2+]i responses were qualitatively consistent between neurons of similar ages. 3. External potassium (K+o)-induced increases in [Ca2+]i were spatially non-homogeneous. The largest increases were seen in the nucleus, near the base of a major neurite, and in growth cones. Increases occurred more rapidly in neurites and growth cones than in somas. T-type and high-voltage-activated (HVA) channels appeared to be present in all cell regions. 4. Increases in [Ca2+]i evoked by 50 mM-K+ (depolarization to approximately -15 mV) were sensitive to treatments demonstrated to inhibit Ca2+ currents in these cells (T-type, HVA-relaxing and HVA-sustained), including Ni2+ (200 microM), Metenkephalin (17.5 microM), and omega-conotoxin (omega-CgTx; 5.5 microM). [Ca2+]i increases were reduced by caffeine (10 mM) and ryanodine (10-100 microM), agents that affect Ca2+ release from intracellular stores. 5. A sustained increase in [Ca2+]i observed at approximately -40 mV ([K+]o = 20 mM) was investigated in greater detail. Concentrations of Ni2+ sufficient to block T-type Ca2+ current slowed but did not block the rise in [Ca2+]i induced by 20 mM-K+. Met-enkephalin did not affect the [Ca2+]i response. omega-CgTx reduced the amplitude of the [Ca2+]i response, but did not eliminate the sustained component. Verapamil (100 microM), caffeine and ryanodine differentially reduced the sustained component as compared to the initial rising phase. These observations suggest that the rising phase was due to Ca2+ influx through T-type and other Ca2+ channels, and that the sustained phase was differentially sensitive to inhibition of internal Ca2+ release.(ABSTRACT TRUNCATED AT 400 WORDS)

Chloride-mediated inhibitory junction potentials in opossum esophageal circular smooth muscle

Intracellular recordings were made from circular smooth muscle cells of the opossum esophagus. Inhibitory junction potentials (IJPs) 8.3 +/- 0.7 mV in amplitude were observed in response to a single pulse (1 ms, 15 mA) of transmural nerve stimulation. Potassium channel blockers apamin (1 microM), tetraethylammonium (10 mM), 4-aminopyridine (250 microM), and cesium chloride (10 mM) did not reduce IJP amplitude. Conditioning hyperpolarizations to the equilibrium potential for potassium were associated with a significant increase in IJP amplitude. A small increase in membrane resistance was observed during the IJP. Changes in external potassium concentration had no significant effect on IJP amplitude acutely. However, prolonged perfusion with potassium-free Krebs solution resulted in a marked decrease in IJP amplitude as did prolonged perfusion with ouabain (0.1 mM). Low-chloride solution (12.4 mM) resulted acutely in an increase in IJP amplitude. Prolonged low-chloride perfusion resulted in a significant decrease in IJP amplitude. The anion exchange chloride channel inhibitor 4,4'-diisothiocyanostilbene-2,2'-disulfonic acid (600 microM) significantly reduced IJP amplitude. These findings suggest that the IJP observed in opossum esophageal circular smooth muscle in response to a single pulse of stimulation is due to a decrease in membrane chloride conductance. The ability of prolonged application of Na-K pump inhibitors to abolish the IJP appears to be due to known secondary effects of these agents in depleting intracellular chloride.

Serotonin and noradrenaline excite GABAergic neurones of the guinea-pig and cat nucleus reticularis thalami.

1. The actions of serotonin (5-HT) and noradrenaline (NA) in the cat perigeniculate nucleus (PGN) and the guinea-pig nucleus reticularis thalami (NRT) were investigated with extracellular and intracellular recordings obtained from neurones in thalamic slices maintained in vitro. 2. Single, local application of either 5-HT or NA resulted in pronounced (5-50 Hz) and prolonged (2-10 min) excitation associated with the occurrence of single-spike activity. Serotoninergic excitation was specifically blocked by the 5-HT2/5-HT1C antagonists ketanserin and ritanserin, but not by the 5-HT1A antagonist pindolol or the 5-HT3 antagonist ICS 205-930. Furthermore, the 5-HT response was mimicked by alpha-methyl-5-HT, but not by the 5-HT1A agonist 8-hydroxydipropylaminotetralin (8-OHDPAT) or the 5-HT3 agonist 2-methyl-5-HT. Together, these results indicate that this excitatory response is mediated through 5-HT2 receptors with the possible involvement of 5-HT1C receptors. 3. Noradrenergic excitation was specifically blocked by the alpha 1-antagonist prazosin, but not by the beta-antagonist propranolol or the alpha 2-antagonist yohimbine. Similarly, the response was mimicked by the alpha-agonist phenylephrine, but not by the beta-agonist isoprenaline. These results indicate that the noradrenergic excitation is mediated by alpha 1-adrenoceptors. 4. Block of synaptic transmission either by lowering external calcium concentration ([Ca2+]o) to 0.5 mM and raising external magnesium concentration ([Mg2+]o) to 10 mM or by local application of tetrodotoxin failed to block the excitatory or depolarizing response to 5-HT or NA indicating that these responses are direct and not mediated through the release of other neurotransmitters. 5. Intracellular recordings revealed that the 5-HT- and NA-induced excitations are mediated by a pronounced slow depolarization associated with an apparent decrease in input conductance and an increase in the membrane time constant. Current versus voltage plots obtained under voltage clamp before and during the presence of 5-HT and NA revealed that these neurotransmitters induced an inward current which reversed to an outward current at -107 and -110 mV, respectively, in 2.5 mM external potassium concentration ([K+]o). This reversal potential was identical to that associated with an increase in potassium conductance activated by acetylcholine (-110 mV) in the same neurones. Plots of the amplitude of the 5-HT- or NA-induced current versus membrane potential revealed a linear relationship in the voltage range from -140 to -60 mV.(ABSTRACT TRUNCATED AT 400 WORDS)

Discrepancy between the short and long term effects of ouabain on the sodium pumps of human cells grown in culture.

1. Human cells (HeLa) were cultured for periods up to 48 h in growth medium in the absence or presence of a range of concentrations of cardiac glycosides. In some experiments the potassium concentration of the medium was varied between 0.3 mM and the usual 5 mM. 2. For periods up to 2 h in ouabain the association and dissociation rate constants were measured and the equilibrium binding constant (KD) calculated; the apparent equilibrium binding constant (K'D) was measured after 1-2 days growth in ouabain. 3. Ouabain had a K'D after 2 days of 2-6 nM in 5 mM K+ growth medium, a 4 fold greater blocking effect on sodium pumps after 2 days than expected from the association and dissociation rate constants measured in untreated or previously ouabain-treated cells. 4. This effect was: (a) approximately the same over a range of external potassium concentrations from 0.3 to 5 mM, although the absolute effect of ouabain over this range of potassium was much different; (b) probably not due to different isoforms of pumps in cells grown in ouabain compared to untreated cells; (c) apparently not a consequence of internalisation of pump-glycoside complexes. 5. We conclude that ouabain has only a limited access to sodium pumps in whole cells; this could be because sodium pumps cycle continuously through an inaccessible region of the plasma membrane. This effect needs to be considered both in the assessment of the magnitude of the long term effects of cardiac glycosides on cells, and in the measurement of the glycoside affinities of various isoforms of the pump.

Inwardly rectifying potassium current in mammalian lens epithelial cells

Lens potassium conductance is essential for the maintenance of lens volume and transparency. Recent work has identified three major potassium currents in lens: 1) an outwardly rectifying current, 2) an inwardly rectifying current, and 3) a calcium-activated current. This paper presents a study of the lens inward rectifier using whole cell and single-channel patch-clamp techniques. Inwardly rectifying potassium current is present in isolated human, rabbit, rat, and mouse lens epithelia. The voltage about which rectification occurs depends on the external potassium concentration. Internal magnesium is not necessary for rectification. In physiological saline, a time-dependent decrease in current during sustained hyperpolarization is seen. This "droop" is due to voltage-dependent block by external sodium. The inward rectifier is also effectively blocked by external cesium or barium but not by tetraethylammonium or 4-aminopyridine. The mouse lens inward rectifier has a single-channel conductance of 32 pS (measured on-cell with 150 mM potassium in the pipette). The single-channel current-voltage relationship is linear in the inward direction. In contrast to the macroscopic case, no outward current was measurable. The inward rectifier in lens has the necessary properties to be involved in setting resting voltage.

Endothelin modulates calcium channel current in neurones of rabbit pelvic parasympathetic ganglia.

1. The effects of endothelin were studied, in vitro, on neurones contained in the rabbit vesical pelvic ganglion by use of intracellular and single-electrode voltage clamp techniques under conditions where sodium and potassium channels were blocked. 2. In the current-clamp experiments, endothelin (1 microM) caused a depolarization followed by a hyperpolarization of the membrane potential. In the voltage-clamp experiments, endothelin (0.01-1 microM) caused an inward current followed by an outward current in a concentration-dependent manner. 3. Membrane conductance was increased during the endothelin-induced depolarization and inward current. Membrane conductance was decreased during the endothelin-induced hyperpolarization and outward current. 4. The endothelin-induced inward and outward currents were not altered by lowering external sodium concentration or raising external potassium concentration. 5. The endothelin-induced inward current was depressed (mean 72%) in a Krebs solution containing nominally zero calcium and high magnesium. These results suggest that a predominent component of the endothelin-induced inward current is mediated by calcium ions. 6. The calcium-insensitive component of the inward current was abolished by a chloride channel blocker, 4-acetamide-4'-isothiocyanostilbene-2,2'-disulphonic acid. The mean reversal potential for the calcium-insensitive component of the inward current was -18 mV. This value is near the equilibrium potential for chloride. Thus, it is presumed that the calcium-insensitive component of the inward current is carried by chloride ions. 7. Endothelin caused an initial depression followed by a long lasting facilitation of both rapidly and slowly decaying components of high-threshold calcium channel currents (N- and L-type). 8. In summary, the data show that for neurones in the vesical pelvic ganglia, endothelin causes membrane depolarization and activates an inward current. The ionic mechanisms involve receptor-operated calcium and chloride currents. Also, endothelin causes an initial depression followed by a long-lasting facilitation of the voltage-dependent calcium current.

Monophasic action potentials in the diagnosis of triggered arrhythmias

ECHANISMS responsible for cardiac arrhythmias are generally divided into three major categories: disorders of impulse formation, disorders of impulse conduction, and combinations of both causes.‘-’ Although this classification is limited and contains some inconsistencies, it serves as a useful framework in which to discuss the pathogenesis of cardiac arrhythmias. Normal automaticity refers to the normal diastolic depolarization of pacemakers found in the normal sinus node, Purkinje fibers, and in other areas of the heart. Abnormal automaticity, a disorder of impulse formation, may occur in many of these fibers, as well as in atria1 and ventricular muscle subjected to a variety of pertubations. Triggered activity, another form of abnormal impulse formation, is a transient membrane oscillation triggered by cardiac depolarization. When this oscillation occurs early, before repolarization is completed, it is called an early afterdepolarization (Fig 1). When it occurs late, after repolarization is completed, it is called a delayed afterdepolarization (Fig 2).3 Slow heart rates generally increase the amplitude of early afterdepolarizations, whereas fast heart rates, within limits, increase the amplitude of delayed afterdepolarizations. Early afterdepolarizations result from a reduced repolarizing current in comparison with the depolarizing current. This may be caused by a reduced outward current, an increased inward current, or both mechanisms.3 Because a variety of interventions that affect different currents can abolish earIy afterdepolarizations (eg, calcium channel blockers,6 sodium channel blockers,7 increasing rate or increasing external potassium concentration) and because a variety of substances that can alter different currents can produce early afterdepolarizations (eg, quinidine7 and related drugs: a sea anemone polypeptide,’ calcium current agonists,’ acidosis,” low extracellular potassium concentration,” hypoxia, and catecholamines), a diversity of currents have been suggested as causes of early afterdepolarizations. These include a calcium current through L-type calcium channels,’ the sodium “window” or slowly inactivating current,” sodium channel exchange mechanisms,13 the transient inward current activated by elevated intracellular calcium,14 intracellular potassium accumulation,*2 and the I,, current.” Cesium blocks inward rectifying potassium currents and delays repolarizatio# to produce early afterdepolarizations. However, the ionic basis of cesium-induced afterdepolarizations is still unclear. Although some early afterdepolarizations may be due to electrotonic membrane events,” a calcium current through L-type calcium channels may be involved.“” Cesium produces early afterdepolarizations in canine cardiac Purkinje fibers’9‘21 and in the intact heart.21-23 Magnesium suppresses these early afterdepolarizations and associated ventricular tachyarrhythmias,“~21 possibly by blocking the calcium current,24 whereas ansae subclaviae stimulation, infusion of norepinephrine, and selective CY- and P-agonists augment early afterdepolarizations.

Sensitization of bone marrow hemopoietic progenitor cells (BMC) to the interleukin-3 (IL-3) and granulocyte macrophage colony stimulating factor (GM-CSF) induced de novo synthesis of histidine decarboxylase (HDC) through the activation of beta-adrenoceptors (βAR)

Two distinct hyperpolarizing responses are produced when histamine is iontophoretically applied onto the somal membranes of identified neurons within the cerebral ganglion of Aplysia: a biphasic response consisting of a rapid component (< 5sec) usually superimposed upon a slowly developing component; or a monophasic slowly developing response 5–20 sec in duration. The reversal potential values for the fast (typically −65 mV) and the slow (typically −89 mV) responses, and their shift to new values when the external potassium or chloride concentrations were altered, revealed that the fast and slow potentials are produced predominantly by conductance increases to chloride and potassium ions, respectively.The effects of histamine H1- and H2-receptor agonists and antagonists were studied to characterize the pharmacological properties of histamine receptors mediating these two ionically dissimilar hyperpolarizing responses. The slow potassium-dependent hyperpolarization could be mimicked by several histamine analogues; the most potent tested were the H1-receptor agonist, 2-methylhistamine, and the H2-receptor agonist, 4-methylhistamine. Neither of these agents mimicked the fast chloride-dependent histamine response. The slow potassium-dependent responses induced by histamine or histamine agonists were completely and reversibly blocked by the H2-receptor antagonist, cimetidine. By contrast, the slow potassium-dependent hyperpolarizations produced by iontophoretically applied acetylcholine or by dopamine to the same neurons were unaffected by cimetidine. Other H1 and H2 antagonists tested were either ineffective, or only partially blocked the slow hyperpolarizations in a non-selective manner. The fast chloride-dependent hyperpolarizations were not selectively antagonized by any of the H1 or H2 reagents tested, although they were effectively suppressed by tubocurarine and strychnine. These data indicate that two pharmacologically distinct histamine receptors mediate potassium- and chloride-dependent hyperpolarizations in Aplysia neurons. Neither of these receptors, however, could be classified as strictly H1 or H2 according to criteria presently used in non-neuronal tissues. The selectivity and reversibility of cimetidine indicate that this particular antihistaminic could be a valuable pharmacological tool for defining putative histaminergic synapses in Aplysia and perhaps other nervous systems.

Quantitative analysis and computer simulation of oxytocin-neurophysin processing in the rat neurohypophysis

A chromatographic procedure is used to obtain data on amounts of the oxytocin-neurophysin (OT-NP, or “A”) and its more completely processed form (OT-NP′, or “B”), which lacks the carboxyl-terminal glutamic acid residue of the A form. Measurements of B + A and B/(B + A) are made for single neurohypophyses under varying physiological conditions, in release obtained by perfusion of isolated neurohypophysis with saline containing increased external potassium concentrations, and in isolated neural lobe nerve endings and nerve swellings. The chromatographic procedure is also combined with injection of [ 35S]cysteine over the supraoptic nucleus. This approach is used to obtain the in vivo A to B conversion rate, and to investigate the trafficking of newly synthesized secretory granules in different axonal compartments. The manner with which the conversion rate changes with secretory activity is investigated by comparing the conversion rate in control animals to that in animals subjected to osmotic stress. Generalized computer models of peptide processing, storage, and release in neurons are used to test various hypotheses of the control of secretory granule movements in the neurohypophysis for sufficiency in explaining the changes in B + A and B/(B + A) with changing physiological state.

Intracellular recording from neurones of the guinea‐pig gall‐bladder.

1. Intracellular recordings were made from neurones of the guinea-pig gall-bladder in vitro. Intracellular injection of horseradish peroxidase revealed a simple structure, consisting of a soma and a single process, but no discernible dendritic arborization. 2. The resting membrane potential was -50.5 +/- 0.4 mV and the input resistance was 80 M omega. 3. Gall-bladder neurones spiked only once at the onset of depolarizing current pulses. Action potentials were blocked by tetrodotoxin, but a Ca2(+)-dependent spike could be elicited in the presence of tetrodotoxin and tetraethylammonium. 4. Action potential after-hyperpolarizations had a duration of 172 +/- 3.7 ms and reversed at a membrane potential of -93 mV; this reversal potential was linearly related to the logarithm of the external potassium concentration. The initial phase of the after-hyperpolarization was inhibited by tetraethylammonium (1-10 mM) and was not affected by 3,4-diaminopyridine. The late phase of the after-hyperpolarization was blocked by apamin (10 nM) or curare (500 microM). Both the early and late phases of the after-hyperpolarization were inhibited when the preparation was perfused with a calcium-free, high-magnesium solution. The calcium-free, high-magnesium solution had no effect on the membrane potential or input resistance of these cells. 5. Fast excitatory synaptic responses and antidromic responses were elicited in gall-bladder neurones by focal stimulation of fibre tracts. High-frequency fibre tract stimulation often resulted in prolonged, calcium-dependent, depolarizations that were associated with a decrease in input resistance. 6. 5-Hydroxytryptamine and substance P caused depolarizations that were associated with a decrease in input resistance. Bethanechol caused hyperpolarizations that were associated with a decrease in input resistance and which were blocked by atropine.

Effects of deoxycorticosterone acetate on muscle electrolytes, resting potential and mitochondria in rats.

Deoxycorticosterone acetate, injected daily for 5, 10, 20 or 30 days, reduced the serum potassium levels of rats by about 25%, a decrease that was independent of the treatment period employed. Serum sodium concentrations were unchanged with treatment duration. The potassium concentrations in extensor digitorum longus (EDL) and soleus (SOL) muscles were significantly decreased, and those of sodium increased, after treatment. Accompanying these changes of electrolyte concentration, the resting membrane potentials of treated EDL and SOL were hyperpolarized in vivo, and showed depolarization with the decreases of external potassium concentrations in vitro. The mitochondria in the muscles of treated rats were damaged. The degree of damage was more serious in EDL than in SOL and was dependent on the duration of deoxycorticosterone acetate treatment.

Potassium‐stimulated release of taurine from cultured cerebellar granule neurons is associated with cell swelling

Effects of increased concentrations of potassium and of hyposmolar conditions on release of taurine were investigated in cerebellar granule neurons cultured from mice. It was found that increases in the external potassium concentration as well as decreases in osmolarity dose-dependently increased release of exogenously supplied [3H]-taurine and endogenous taurine from the neurons. The release of endogenous taurine elicited by a reduction of the osmolarity of the incubation media to 70% or 50% was much more pronounced than that of other amino acids, particularly glutamine, the release of which was not affected at all. The potassium-stimulated release of [3H]-taurine was strictly chloride dependent and it was inhibited by an increase of the osmolarity of the media as well as by 4,4'-diisothyocyanostilbene-2,2'-disulfonate (DIDS) (100 microM). Moreover, a similar increase in the potassium concentration led to an increase in intracellular volume (swelling), a process which was also chloride dependent. It is concluded that potassium-stimulated taurine release from cerebellar granule neurons is associated with cell volume changes and that taurine is likely to play a role as an osmotically active substance in these neurons.

The effects of a low extracellular concentration of potassium on the activity and numbers of Na +/K + pumps in an EB-virus transformed human lymphocyte cell line

The BM1A EB-virus transformed human lymphocyte cell line contains approximately 950 000 Na +/K +-ATPase sites per cell. The turnover number of each site is approx. 2240 molecules of rubidium per min. When cells are exposed to a low extracellular concentration of potassium the intracellular concentration of sodium rises, and the cells respond in the short term by increasing the V max of 86Rb + uptake. In the longer term the cells respond by increasing both the V max of 86Rb + uptake and the B max of [ 3H]ouabain binding. The suggestion that increases in the intracellular concentration of sodium is responsible for these changes is supported by the finding that monensin, which increases intracellular sodium without affecting intracellular potassium, is capable of inducing both the short- and long-term changes associated with a low external concentration of potassium.

Opioid actions on single nucleus raphe magnus neurons from rat and guinea‐pig in vitro.

1. Intracellular recordings were made from neurons of the nucleus raphe magnus (NRM) from rat (n = 128) and guinea-pig (n = 115). Two types of cells were found in each, primary (103 in rat, 27 in guinea-pig) and secondary cells (25 in rat, 88 in guinea-pig). 2. Primary cells had input resistances of 186 +/- 9 M omega (n = 9) in rat and 255 +/- 50 M omega (n = 11) in guinea-pig. The action potential in each was about 1.5 ms in duration. Synaptic potentials were evoked by focal electrical stimulation and consisted of both gamma-aminobutyric acid (GABA) and excitatory amino acid components. 3. Morphine, [Met5]enkephalin (ME) and [D-Ala2,N-Me-Phe4, Gly5-ol]enkephalin (DAMGO) depressed the amplitude of the GABA-mediated synaptic potential by a maximum of 50-65% and had little effect on the excitatory amino acid-mediated synaptic potential. There was no effect of these opioids on the resting membrane potential or input resistance of primary cells in rat or guinea-pig. 4. Secondary cells had short duration action potentials (less than 1 ms) and an input resistance of 354 +/- 47 M omega in rat (n = 6) and 290 +/- 40 M omega in guinea-pig (n = 15). The synaptic potential observed in the cells of this group was mediated by activation of only excitatory amino acid receptors. 5. ME hyperpolarized and/or abolished the spontaneous firing in sixteen out of twenty-four neurons in the secondary group from rat and eight out of eighty-four neurons from guinea-pig. ME induced an outward current at -60 mV that reversed polarity at potentials more negative than -92 +/- 3 mV in rat (n = 6) and -98 +/- 2 mV in guinea-pig (n = 18). The reversal potential of the opioid current was shifted to less negative potentials when the external potassium concentration was increased, as predicted by the Nernst equation. 6. The morphology of the two types of cells were distinguishable in that primary cells were oval (29 x 18 microns in rat; 36 x 19 microns in guinea-pig) with two to four thick tapering dendrites that branched within 50 microns of the cell body. Secondary cells were generally round or oval (about 24 x 13 microns in rat; 27 x 17 microns in guinea-pig) with two to five thin non-tapering dendrites.(ABSTRACT TRUNCATED AT 400 WORDS)

Isolated cells of the frog sinus venosus: properties of the inward current activated during hyperpolarization

Single sinus venosus cells from frog, Rana esculenta, were isolated using an enzymic dispersion procedure, involving applications of collagenase and protease. About 40%-60% of the cells showed spontaneous contractions. Isolated cells were studied in the whole-cell configuration. Regenerative action potentials were tetrodotoxin-insensitive and similar to those recorded in multicellular preparations. Hyperpolarizing pulses in the voltage range negative to -50 mV induced the activation of a time-dependent inward current, which was blocked by 4 mM caesium but less affected by barium ions. A lower concentration of caesium (1 mM) exerted a voltage-dependent reduction of the current and decreased the spontaneous pacing rate. The activation range of the hyperpolarization-activated current approximately extended from -50 mV to -110 mV, but varied from cell to cell. A high variability was observed in the behaviour of the activation kinetics. The current had a reversal potential near -20 mV that was shifted positively by increasing the external potassium concentration (from 3 mM to 30 mM) and negatively by reducing the external sodium concentration (from 115 mM to 30 mM). The hyperpolarization-activated inward current of the frog sinus venosus cell appears to be carried by both sodium and potassium ions. It shows electrophysiological properties similar to those of the If current of the mammalian heart. The role of the current in the spontaneous activity is discussed.

Serotonin‐mediated inhibitory postsynaptic potential in guinea‐pig prepositus hypoglossi and feedback inhibition by serotonin.

1. Intracellular recordings were made from neurones of the nucleus prepositus hypoglossi (PH) in slices of guinea-pig brain. Focal stimulation evoked an inhibitory postsynaptic potential (IPSP) that was typically 10-25 mV in amplitude and 1 s in duration. The IPSP reversal potential showed a Nernstian dependence on the external potassium concentration ([K+]o). 2. Spiperone blocked the IPSP with an IC50 of 40 nM, while ketanserin and (-)sulpiride had no effect. Cocaine (1 microM) prolonged the IPSP half-duration by 157%, and increased the amplitude by 28%. 3. 5-Hydroxytryptamine (5-HT, serotonin) hyperpolarized PH cells with an EC50 of 8.5 microM in control, and 135 nM in cocaine (10 microM). 8-Hydroxy-2-(di-n-propylamino)-tetralin (8-OH-DPAT) also hyperpolarized PH cells with an EC50 of 16 nM, although the maximal effect was only 81% of the maximum 5-HT hyperpolarization. Spiperone produced a parallel, right shift of the 5-HT concentration-response curve; Schild analysis gave a Kd of 10 nM. Application of 5-HT to neurones voltage-clamped near their resting potential (about -55 mV) caused an outward current and an increase in membrane conductance. 4. The amplitude of the IPSP was reversibly decreased by non-hyperpolarizing concentrations of 5-HT and by the 5-HT1 receptor agonists 1-(m-trifluoromethylphenyl)piperazine (TFMPP) and 1-(3-chlorophenyl)piperazine (mCPP). The IC50 values for the latter two compounds were 50 nM and 1.5 microM, respectively; the maximal effect was a 90% inhibition. Neither compound affected the membrane potential nor changed the hyperpolarization induced by 5-HT. Quipizine competitively antagonized TFMPP with an estimated Kd of 165 nM. 5. When trains of stimuli were applied, an inhibition of the IPSP was observed following the first stimulus. At a frequency of 1 Hz, the inhibition was approximately 75%. This frequency-dependent 'run-down' of the IPSP was markedly attenuated by pre-treatment with TFMPP (1 microM). 6. It is concluded that the IPSP in PH cells is caused by 5-HT acting on 5-HT1A receptors to activate a potassium conductance. The release of 5-HT can be inhibited by activation of a presynaptic 5-HT1D receptor. This presynaptic receptor appears to be at least partly responsible for the run-down phenomenon, and may be involved in the physiological regulation of 5-HT synaptic transmission.

Rubrospinal tract of the cat: superposition of antidromic responses and changes in axonal excitability following orthodromic activity

Activity was recorded from rubrospinal neurons (RSNs) in anesthetized, paralyzed, artificially ventilated cats. Multiple-unit microelectrodes were used to simultaneously record the activity of neighboring RSNs. When antidromically activated, the RSNs responded forming ‘stacks’ of superimposed spikes. By using appropriate collision tests, it was found that the spikes forming a stack arose from different neurons. In addition, single extracellular and intracellular recordings were obtained from RSNs. The changes in the axonal excitability of rubrospinal axons were tested following synaptically evoked (by contralateral interpositus (IP) stimulation) and/or directly evoked (by injection of current through the intracellular electrode) action potentials at different postspike delays. Subthreshold stimuli for antidromic activation in absence of orthodromic activity were well suprathreshold for most fibers in a wide range of postspike delays. The supernormal axonal periods were longer-lasting when tested after synaptic spikes (up to an average delay of 100.4 ms; range, 10–500 ms) than after directly evoked spikes (mean delay, 78.8 ms; range, 10–296 ms). If synaptic stimulation fires more RSNs than direct stimulation, then the longer-lasting supernormal periods might be due to the activity of adjacent fibers. An additional increase in external potassium concentration in the vicinity of the axon would explain these results.

Characterization of inhibition mediated by adenosine in the hippocampus of the rat in vitro.

1. Intracellular recordings with single-electrode voltage clamp were employed to study the mechanism of adenosine-elicited inhibition of CA1 neurones of the rat in vitro. 2. Adenosine elicits a steady-state outward current in association with an increase in conductance. The driving force varied with external potassium concentration as predicted by the Nernst equation for a change primarily in potassium permeability. 3. Adenosine current was blocked by high concentrations of 4-aminopyridine or barium. In the majority of neurones this current was voltage insensitive. In the remainder, the current was inwardly rectifying. The rectification was blocked by tetraethylammonium. 4. When the adenosine-elicited potassium current was blocked, slow inward currents, normally carried by calcium, were unaffected by adenosine. We conclude that this adenosine inhibition is mediated by an increase in a voltage- and calcium-insensitive potassium conductance in CA1 neurones.