Biomedical subjects

J L Barker

Publications and source records attributed to J L Barker.

At least 127 records · Page 7Linked to original sources

PubMed1991

Electrophysiological responses to muscarinic receptor stimulation in cultured hippocampal neurons.

Electrophysiological recordings of responses to muscarinic receptor stimulation in cultured embryonic hippocampal neurons have been largely unsuccessful to date. In this study muscarinic receptor binding was demonstrated in 2-week-old embryonic rat hippocampal cultures. Using whole-cell patch-clamp recording we found that 1-5 microM carbachol produced multiple effects including depolarization, increased action potential firing rate, increased synaptic activity and a reduction in the amplitude of medium-duration afterhyperpolarizations. Voltage-clamp analysis revealed a time-dependent current relaxation with hyperpolarizing steps from a holding potential of about -40 mV which was inhibited by 10 microM muscarine or 50 microM carbachol and had characteristics similar to those of the m-current. Both atropine and pirenzepine inhibited all of these effects indicating that these cholinergic actions were mediated by muscarinic receptors. This study shows that muscarinic responses obtained classically in hippocampal brain slices can also be produced in cultured hippocampus.

Action Potentials↗

PubMed1991

Antibiotic magainins exert cytolytic activity against transformed cell lines through channel formation.

Magainins are an ionophoric class of vertebrate peptides with antibiotic activity against various microorganisms. Here we show that magainin 2 and synthetic analogues can rapidly and irreversibly lyse hematopoietic tumor and solid tumor target cells with a relative cytotoxic potency that parallels their antibacterial efficacy and at concentrations that are relatively nontoxic to well-differentiated cells. The cytotoxicity is prevented by cell depolarization. Magainins represent a natural cytolytic agent in vertebrates and may provide another therapeutic strategy for certain tumors.

Anti-Bacterial Agents↗

PubMed1991

Dopamine synthesis precedes dopamine uptake in embryonic rat mesencephalic neurons.

We have measured [3H]dopamine ([3H]DA) uptake and tyrosine hydroxylase-immunopositive immunostaining in cells acutely dissociated from the embryonic ventral mesencephalon (MSC). DA and its metabolites as well as catechol-O-methyltransferase (COMT) and monoamine oxidase (MAO) activities were determined in homogenates taken from the MSC and striatum (STR). In the embryonic ventral MSC measurable DA and tyrosine hydroxylase (TH) immunostaining were present as early as embryonic day (E) 12.5. At E14 the number of TH+ neurons was about 50% of the values at E18. In the MSC, DA concentration increased sharply at E16 and reached a plateau before birth that was 10-fold lower than adult values. In the STR, DA was first detected at E16, suggesting that DA fibers reach the STR at this embryonic stage. High-affinity DA uptake appeared in the MSC only at E16, concomitantly with the arrival of DA fibers in the STR, increased sharply between E16 and E18, and reached a plateau before birth. This uptake mechanism was not selective for catecholamine uptake inhibitors. Thus, DA synthesis in the MSC preceded the onset of high-affinity uptake mechanism, which could be correlated to the beginning of striatal DA innervation. Measurable MAO and COMT activities were detected as early as E13 (MSC) and E15 (STR), but not DA metabolites, which appeared later. We conclude that the high-affinity DA uptake mechanism in MSC DA neurons develops coincident with the arrival of DA fibers to the STR. The sharp increase of DA uptake between E16 and E18 is due only in part to an increase in the number of TH+ cells. These results support the hypothesis that in vivo the target STR neurons regulate the maturation of MSC DA cells.

Animals↗

PubMed1991

The site for initiation of action potential discharge over the somatodendritic axis of rat hippocampal CA1 pyramidal neurons.

Early electrophysiological studies in the mammalian hippocampus reported that orthodromic depolarization of pyramidal cells evoked action potential discharge (presumed Na+ dependent) both at the axon hillock and at one or more sites in the dendritic arborization (Cragg and Hamlyn, 1955; Andersen, 1959, 1960; Spencer and Kandel, 1961; Andersen and Lomo, 1966). Although tetrodotoxin (TTX)-sensitive spikes have been recorded at the dendritic level (Wong et al., 1979; Benardo et al., 1982; Miyakawa and Kato, 1986; Turner et al., 1989), the site for initiation of these potentials has not yet been determined. In this study, we examine the site for initiation of Na+ spike discharge over the cell axis of rat hippocampal CA1 pyramidal neurons. Intrasomatic and intradendritic recordings were obtained from pyramidal neurons of hippocampal slices maintained in vitro. Spike discharge was evoked by alvear (antidromic) stimulation or orthodromically by stimulation of afferent inputs in stratum oriens (SO) or stratum radiatum (SR). Antidromic and orthodromic spikes were greatest in amplitude in somatic recordings and declined over the apical dendritic axis, while spike half-width was shortest at the cell body and increased with distance from stratum pyramidale. Measurements of orthodromic spike threshold revealed that the only location at which spikes discharged at a consistent membrane potential at threshold intensity (voltage threshold) was the cell body region. Finally, at threshold intensity, SR-evoked intradendritic spikes were blocked by local application of TTX in stratum pyramidale, while spike blockade at suprathreshold intensity required the diffusion of TTX into the apical dendritic region. These results indicate that, for threshold intensities of stimulation, antidromic and orthodromic spike discharge in CA1 pyramidal cells is initiated in the region of the cell body layer, subsequently conducting over the apical dendrites in a retrograde fashion. In contrast, SR-evoked orthodromic spike discharge exhibits an intensity-dependent shift in the site of origin up to 200 microns within the apical dendritic arborization.

Action Potentials↗

PubMed1990

3-Aminopropanephosphinic acid is a potent agonist at peripheral and central presynaptic GABAB receptors.

The actions of the GABA analog 3-aminopropanephosphinic acid (3-APA) were studied in the guinea-pig isolated ileal preparation and at synapses between cultured rat hippocampal neurons. Like the GABAB receptor agonist, baclofen, 3-APA inhibited the electrically evoked ileal twitch. The EC50 for 3-APA was 0.8 microM; the EC50 for baclofen was 9 microM. In addition, the depressant responses to 3-APA and baclofen were blocked by the GABAB receptor antagonists phaclofen, saclofen, 2-hydroxy-saclofen and delta-aminovaleric acid. 3-APA also mimicked the presynaptic action of baclofen at GABAergic synapses between embryonic rat hippocampal neurons in culture. 3-APA reduced the amplitude of inhibitory postsynaptic potentials (IPSPs) and currents (IPSCs) by greater than 50% at a concentration of 1 microM, while baclofen reduced synaptic transmission to a similar degree at 10 microM. 3-APA did not alter membrane conductance, nor did the drug alter postsynaptic responses to GABA. These data show that 3-APA is a potent agonist at presynaptic GABAB receptors in the periphery and on GABAergic neurons from the central nervous system. The activity of 3-APA at central postsynaptic GABAB receptors remains to be studied.

Amino Acids↗

PubMed1990

Electrical and chemical excitability appear one week before birth in the embryonic rat spinal cord.

Embryonic rat spinal cord cells were acutely dissociated with the enzyme papain, stained with a voltage-sensitive oxonol dye and incubated with various pharmacological agents. Changes in the fluorescence intensity and, by inference, membrane potential of the cells were analyzed in a flow cytometer. Veratridine caused depolarization of the cells in a TTX-sensitive manner from as early as embryonic day 13. Depolarizing responses to muscimol and kainate appeared slightly later, at embryonic days 14 and 15, and were blocked by the antagonists bicuculline and CNQX, respectively. Responses to veratridine and kainate did not occur in sodium-free medium. The emergence of these excitable membrane properties coincides with postmitotic differentiation and synaptic development in the embryonic spinal cord.

Animals↗

PubMed1990

Thyrotropin-releasing hormone activates a [Ca2+]i-dependent K+ current in GH3 pituitary cells via Ins(1,4,5)P3-sensitive and Ins(1,4,5)P3-insensitive mechanisms.

The role of Ins(1,4,5)P3 in receptor-induced Ca2+ mobilization in pituitary cells was studied at the single-cell level. Experimental strategies were developed which allowed a comparative analysis of the effects of Ins(1,4,5)P3 with those of receptor activation under identical conditions. These include microfluorimetry as well as a novel technique which permits the controlled and rapid application of intracellular messenger molecules to individual cells. This latter approach is based on the tight-seal whole-cell recording (WCR) technique, and utilizes two patch-clamp micropipettes, one for electrical recording and the second for the controlled pressure injection. Ins(1,4,5)P3, when applied with this dual-WCR (DWCR) technique, leads rapidly to a marked rise in cytosolic free Ca2+ [( Ca2+]i) and a concomitant stimulation of Ca2(+)-activated K+ current; Ins(1,4,5)P3 can thus mimic the effects of thyrotropin-releasing hormone (TRH) in the same cells under identical conditions. In cells dialysed intracellularly with heparin, a potent antagonist of Ins(1,4,5)P3 action, the rapid response to extracellular stimulation with TRH was abolished, as were the effects of intracellular application of Ins(1,4,5)P3. Heparin, which abolished Ins(1,4,5)P3 action completely, blocked responses to TRH in some cells only partially, revealing that Ca2+ mobilization response to TRH is in part slower in onset than the response to Ins(1,4,5)P3. It is concluded (1) that Ins(1,4,5)P3 is an essential element for the action of TRH, providing a rapid mechanism for Ca2+ mobilization induced by the releasing hormone and (2) that TRH action in mobilizing intracellular Ca2+ is sustained by a slower mechanism which is independent of Ins(1,4,5)P3.

Animals↗

PubMed1990

Embryonic and early postnatal hippocampal cells respond to nanomolar concentrations of muscimol.

Embryonic and early postnatal tissue taken from rat hippocampi were papain digested in order to obtain cell suspensions suitable for analysis in a fluorescence-activated cell sorter (FACS). Cell suspensions consisted of two major peaks of forward-angle light scatter (FALS). FACS analysis showed that the population which stained intensely with the vital dye Acridine orange (AO) scattered significant levels of light (high FALS) and amounted to 85% of the total events collected in embryonic cell suspensions and 65% in postnatal (PN) samples. Two minor populations were weakly stained with AO and scattered little light. Oxonol, a voltage-sensitive indicator dye, was used to detect membrane polarization changes. The AO and oxonol staining patterns were very similar. All the events exposed to media containing 50 mM KCl were depolarized (increase in intensity of oxonol fluorescence). The depolarizing effect of veratridine, a sodium channel activator, was more pronounced in the high FALS subpopulation. In embryonic hippocampal cell suspensions nanomolar concentrations of GABAA agonists depolarized the high FALS subpopulation in a dose-dependent manner. This effect was prevented by preincubation with bicuculline or picrotoxin. In hippocampal cell suspensions obtained from 5-7-day-old rat pups (PN5-7), GABAA agonists depolarized one cell subpopulation and hyperpolarized another. Our results indicate that physiological responses can be resolved in subpopulations of hippocampal cell suspensions by FACS analysis. This technique seems to be a sensitive assay to measure physiological responses (changes in membrane potential) as a parameter of receptor expression. GABAA agonists induced pure depolarizing responses in embryonic and early postnatal hippocampus when active neurogenesis is taking place. The response become hyperpolarizing-depolarizing ones after inhibitory synapses appear.

Aging↗

PubMed1990

Functional expression of dihydropyridine-insensitive calcium channels during PC12 cell differentiation by nerve growth factor (NGF), oncogenic ras, or src tyrosine kinase.

1. Recombinant retroviruses were used to introduce a temperature-sensitive v-src gene and oncogenic c-Ha-ras into PC12 cells, and stable cell lines expressing these genes were established. 2. As previously reported, expression of v-src (Alema et al., 1985) or c-Ha-ras (Noda et al., 1985) in PC12 cells results in neurite outgrowth resembling that induced by NGF. We report here that v-src but not oncogenic c-Ha-ras induces a stable morphologic neuronal differentiation similar to treatment with NGF. Oncogenic c-Ha-ras-induced neurite outgrowth is not stable with long-term culture, rather the cells revert to an undifferentiated morphology with altered cell cycle kinetics. 3. The stable neuronal phenotype induced by v-src and NGF is characterized by the functional expression of dihydropyridine-insensitive calcium currents.

Animals↗

PubMed1990

Early appearance of tyrosine hydroxylase immunoreactive cells in the mesencephalon of mouse embryos.

The development of mesencephalic catecholaminergic neurons in the embryonic and fetal mouse was analysed in tissues fixed with 5% acrolein using polyclonal rabbit antibodies against tyrosine hydroxylase (TH), the first enzyme in catecholamine synthesis. The first TH positive cells were identified as early as day 8.5-9 of gestation and some expressed TH while apparently still migrating from the proliferative layer. The number of catecholamine cells increased dramatically by embryonic day 9.5-10; at gestation days 10.5-11 numerous TH positive cells bearing many neurites were localized in the ventral part of the mesencephalon but they were not yet separated into two different groups (A9 and A10). After 13 days of gestation two separate catecholaminergic groups could be visualized, although many TH positive cells with long neurites (putative dopaminergic neurons) could still be seen at the edges of the ventricle, and appeared to be moving towards the ventral mesencephalon. On the basis of these results the possibility that catecholamine cells that are produced early during the development of the midbrain may have neurotrophic and/or morphogenetic roles is discussed.

Aging↗

PubMed1990

Transient analysis for antiproliferative gene activity.

A subset of tumor suppressor genes presumably functions by the inhibition of cellular proliferation; however, antiproliferative activity after transfection with putative suppressor genes has been difficult to demonstrate and often requires lengthy selection either in nude mice or in vitro. A rapid alternative is presented here that utilizes a gene encoding a surface marker protein to identify transfectants in a transient expression assay. In this assay the labeling index, rate of DNA synthesis, cell-cycle distribution, and surface receptor display are measured by flow cytometry. Human beta-interferon, a gene with proven antiproliferative activity, was studied using the transient analysis system. The beta-interferon gene was introduced into human tumor cells along with the marker gene encoding the 55-kDa subunit of the human interleukin 2 receptor. Within a few days of transfection, analysis of transfectants by flow cytometry revealed a decrease in the fraction of cells in G2/M and an increase in the fraction of cells in G1/G0 and S phases. The distortion of the cell cycle was accompanied by as much as a 69% reduction in the rate of DNA synthesis and, in some experiments, an unanticipated increase in the labeling index. Therefore, cells accumulating in S phase were not blocked but continued to synthesize DNA although at a reduced rate. These studies on DNA synthetic rates revealed the caveat that screening for antiproliferative candidate genes with a labeling index alone could, in certain circumstances, exclude potentially interesting sequences from further consideration. Although this transient analysis system was developed for studies on cellular proliferation, it may prove suitable for phenotypic assays on other genes as well.

Bromodeoxyuridine↗

PubMed1990

Inositol trisphosphate mediates cloned muscarinic receptor-activated conductances in transfected mouse fibroblast A9 L cells.

1. The mechanism by which cloned m1 and m3 muscarinic receptor subtypes activate Ca2+-dependent channels was investigated with whole-cell and cell-attached patch-clamp recording techniques and with Fura-2 Ca2+ indicator dye measurements in cultured A9 L cells transfected with rat m1 and m3 cDNAs. 2. The Ca2+-dependent K+ and Cl- currents induced by muscarinic receptor stimulation were dependent on GTP. Responses were reduced when GTP was excluded from the intracellular recording solution or when GDP-beta-S was added. Intracellular GTP-gamma-S activated spontaneous fluctuations and permitted only one acetylcholine-(ACh) induced current response. These results implicate GTP-binding proteins (G protein) in the signal transduction pathway. This G protein is probably not pertussis toxin-sensitive as the ACh-induced electrical response was not abolished by pertussis toxin treatment. 3. Cell-attached single-channel recordings revealed activation of ion channels within the patch during application of ACh outside the patch, implying that second messengers might be involved in the ACh-induced response. Two types of K+ channel were activated, a discrete channel of 36 pS and channel activity calculated to be about 5 pS. 4. Application of 8-bromo cyclic AMP or 1-oleoyl-1,2-acetylglycerol (OAG) produced no electrical response and did not affect the ACh-induced responses. Phorbol myristic acetate (PMA) evoked no electrical response, but reduced the ACh-induced responses. 5. Inclusion of inositol 1,4,5-trisphosphate (IP3) in the intracellular pipette solution activated outward currents at -50 mV associated with an increase in conductance. The IP3-induced current response reversed polarity at -65 mV and showed a dependence on K+. Increasing the intracellular free Ca2+ concentration ([Ca2+]i) from 20 nM to 1 microM also induced an outward current response associated with an increase in conductance. Inclusion of inositol 1,3,4,5-tetrakisphosphate (IP4) in the intracellular solution had no effect on the A9 L cells. 6. Fura-2 measurements revealed ACh-induced increases in Cai2+. The Ca2+ responses were abolished by atropine showing that they were muscarinic in nature. Removal of extracellular Ca2+ did not affect the initial ACh-induced increase in Cai2+ but subsequent Cai2+ responses to ACh were depressed, suggesting depletion of Ca2+ intracellular stores. Residual though small responses continued to be elicited by ACh. Barium (5 mM) had little effect and cobalt slightly reduced the ACh-induced Ca2+ response. 7. The ACh-induced currents recorded at -50 mV were unaffected by removal of extracellular Ca2+.(ABSTRACT TRUNCATED AT 400 WORDS)

Acetylcholine↗

PubMed1990

Postnatal rat nigrostriatal dopaminergic neurons exhibit five types of potassium conductances.

1. We have investigated the electrical properties of neurons acutely dissociated from the substantia nigra zona compacta (SNZC) of the postnatal rat with whole cell patch-clamp recordings. Retrogradely labeled nigrostriatal neurons were identified with the use of rhodamine-labeled fluorescent latex microspheres. Over 90% of the rhodamine-labeled neurons in the SNZC demonstrated formaldehyde/glutaraldehyde-induced catecholamine fluorescence, indicating that they were dopaminergic (DA) neurons. 2. DA neurons had 15-20 microns ovoid or fusiform-shaped cell bodies with 2-3 thick proximal processes. Labeled neurons generated spontaneous action-potential activity in both regular and irregular patterns. These cells exhibited input resistances of 300-600 M omega and action-potential amplitudes of 60-80 mV. Locally applied dopamine inhibited the spontaneous activity of these neurons by hyperpolarizing the cells. 3. Outward currents were examined with voltage-clamp recordings using a tetrodotoxin (TTX)-containing medium. In all DA cells, depolarizing voltage commands activated several components of outward current depending on the holding potential of the cell. When cells were held at -40 mV (or more positive), voltage steps activated a sustained outward current. If the membrane potential was held more negative than -50 mV, a rapidly activating and inactivating component of outward current response could also be detected. 4. From a hyperpolarized holding potential (-90 mV) the transient outward current activated with depolarizing commands to -55 mV, peaking within 5 ms. The current inactivated with a monoexponential time constant of 53 +/- 4 (SE) ms. At more positive holding potentials (-40 mV) the steady-state inactivation of the current could be removed by applying a conditioning hyperpolarizing prepulse. In response to a fixed depolarizing voltage step, half-maximal inactivation occurred at about -65 mV. The transient current was blocked by 4-aminopyridine (4-AP). 5. The sustained outward currents were isolated by holding the cells at -40 mV. Two components of sustained outward current were distinguished by their sensitivity to the calcium channel blockers Co2+ (5 mM) and/or Cd2+ (200 microM). The current remaining in the presence of Co2+/Cd2+ was activated by depolarizing voltage commands more positive than -40 mV.(ABSTRACT TRUNCATED AT 400 WORDS)

Action Potentials↗

PubMed1989

Acetaldehyde directly enhances MPP+ neurotoxicity and delays its elimination from the striatum.

We have previously shown that ethanol and acetaldehyde (ACE) potentiate MPTP toxicity in mice, selectively enhancing dopamine (DA) depletion in the striatum and markedly increasing loss of DA neurons in the substantia nigra. Several months after these combined treatments there is no evidence of any recovery. In the present study, we measured the accumulation of the MPTP toxic metabolite 1-methyl-4-phenylpyridinium ion (MPP+) in both striatum and whole brain, after MPTP alone or after combined treatments with ethanol or acetaldehyde, in order to determine whether this enhancement of toxicity is caused by changes in the MPTP metabolism. We also investigated whether acetaldehyde interfered with the conversion of MPTP to MPP+ by glial cells in vitro and studied its effects on the MPP+ uptake and spontaneous release from mesencephalic DA neurons or striatal astrocytes in primary cell cultures from E13 mouse embryos. The results from the in vivo experiments indicated that relatively low doses of ethanol or acetaldehyde potentiate directly MPP+ toxicity, apparently without interfering with its pharmacokinetics. However when higher doses of these drugs were administered, they also decreased MPP+ clearance from the striatum. ACE also increased initial MPTP accumulation in the whole brain but failed to enhance MPP+ levels, thus indicating that ACE effect is not related to MPTP metabolism. In vitro studies confirmed that ACE does not modify MPTP metabolism in striatal or mesencephalic astrocytes in culture. In mesencephalic neuronal cultures ACE does not change the levels of MPP+ uptake (MPP+ is accumulated in putative DA neurons in vitro with a mechanism similar to that of the DA high affinity uptake) nor its spontaneous release. These results indicate that the slower MPP+ clearance from the stratum after ACE is not related to a direct effect of ACE on DA neurons or astrocytes.

1-Methyl-4-phenyl-1,2,3,6-tetrahydropyridine↗

PubMed1989

Spontaneous polymerization of the antibiotic peptide magainin 2.

We describe here the ability of the magainin 2 peptide to assemble spontaneously into characteristic 13-nm diameter filaments having a 30 nm periodic helical substructure. Optimal conditions for extensive polymerization into filaments of several hundred microns required low pH and high ionic strength. Polymerization of the magainin 2 peptide may be involved in its recently described in vitro membrane-disrupting and antibiotic activities.

Animals↗

PubMed1989

Whole-cell patch-clamp analysis of voltage-dependent calcium conductances in cultured embryonic rat hippocampal neurons.

1. Voltage-dependent calcium currents in embryonic (E18) hippocampal neurons cultured for 1-14 days were investigated using the whole-cell patch-clamp technique. 2. Calcium currents were isolated by removing K+ from both the internal and external solutions. In most recordings the external solution contained tetrodotoxin, tetraethylammonium ions, and low concentrations of Na+, whereas the internal solution contained the large cations and anions, N-methyl-D-glucamine and methanesulphonate, and an adenosine 5'-triphosphate (ATP) regenerating system (Forscher and Oxford, 1985) to retard "run-down" of Ca currents. 3. Under these conditions, the sustained inward current triggered during depolarizing steps was enhanced when extracellular [Ca2+] ([Ca2+]0) was raised from 2 to 10 mM and abolished when [Ca2+]0 was lowered to 0.1 mM or by addition of Co2+ ions. These results indicate that the inward current was carried primarily by Ca2+ ions and was designated ICa. This current may be comparable to the "high-voltage-activated" Ca current described in other preparations. 4. In cells cultured for 1-3 days, ICa was small or absent (less than 20 pA for cells 1 day in culture and less than 80 pA for cells 3 days in culture). Although ICa decayed considerably during depolarizing steps, there was little evidence of the transient calcium current (T current) that was recorded in approximately 40% of cells cultured longer than 6 days. Maximal (i.e., the largest) ICa increased from 20 to 80 pA in 1- to 3-day cells to 150-450 pA in cells cultured for longer than 6 days. 5. The decay of ICa elicited by depolarizations from holding potentials of -60 mV or more negative was usually greatest for the maximal ICa. Replacement of extracellular Ca2+ (4 mM) with Ba2+ (2 mM) resulted in a substantial decrease in the extent of decay of ICa and a shift of the I-V relation in the hyperpolarizing direction. 6. Qualitative data obtained from experiments in which different levels of internal Ca2+ buffering were employed demonstrated that, on average, the decay of ICa was reduced as the capacity and/or rate of buffering was increased. The mean decay of ICa in cells buffered with 5 mM 1,2-bis(o-aminophenoxy)ethane-N,N,N',N'-tetraacetic acid (BAPTA) was 9 +/- 7 (SD) %, (n = 12) and 25 +/- 12%, (n = 12) for cells buffered with the same concentration of ethyleneglycol bis(beta-aminoethyl ether)-N,N,N',N'-tetraacetic acid (EGTA).(ABSTRACT TRUNCATED AT 400 WORDS)

Animals↗

PubMed1989

Localization of tetrodotoxin-sensitive field potentials of CA1 pyramidal cells in the rat hippocampus.

1. The role of tetrodotoxin (TTX)-sensitive (Na+) channels in the generation of antidromic and orthodromic field potentials of the CA1 pyramidal cell population was examined by local application of TTX in the in vitro rat hippocampal slice preparation. 2. The sensitivity of alvear (antidromic) and stratum oriens (SO)-evoked potentials to TTX application (10-100 microM) was tested in stratum pyramidale and over the entire extent of pyramidal cell apical dendrites in stratum radiatum. Stratum radiatum (SR)-evoked potentials were examined at the level of pyramidal cell bodies and over the proximal 200 microns of the apical dendritic region. 3. Pressure application of TTX confined to stratum pyramidale or regions of stratum radiatum selectively blocked the negative component of antidromic and SO-evoked population discharge in the cell body layer and over the initial 200 microns of stratum radiatum. 4. SR stimulation evoked a complex field potential in the proximal stratum radiatum (less than 150 microns) composed of at least three components: 1) A short-duration (approximately 3 ms) negativity of shorter peak latency than the population spike recorded simultaneously in stratum pyramidale. This potential was highly sensitive to TTX and appeared to be instrumental in the generation of the cell body population response. 2) A long-duration negativity (approximately 20 ms) evoked at stimulation strengths that were subthreshold for both the short-duration negativity in proximal stratum radiatum and the cell body population spike. Although apparently less sensitive to TTX, this potential was reduced in amplitude with repeated TTX application; and 3) a slow (approximately 12 ms) positive-going potential that was only observed after eliminating all TTX-sensitive conductance mechanisms in the proximal stratum radiatum. 5. The latency difference between the SR-evoked short-duration negativity of proximal stratum radiatum and the population spike in stratum pyramidale decreased or reversed during the course of multiple discharge induced by the addition of bicuculline or picrotoxin (5-10 microM) to the perfusate. 6. These data indicate the presence of TTX-sensitive presumed Na+ channels over the initial 200 microns of pyramidal cell apical dendrites capable of supporting active conduction of population discharge evoked by antidromic or SO stimulation. The sensitivity of SR-evoked potentials to TTX suggests that a synaptic potential generated in the distal apical dendrites is capable of triggering both a slow active depolarization and a fast spike-like discharge in the proximal apical dendritic region.(ABSTRACT TRUNCATED AT 400 WORDS)

Action Potentials↗

PubMed1988

Activation of ion channels in tumor cells by leukoregulin, a cytostatic lymphokine.

We used the whole-cell patch-clamp recording technique to study the effects of leukoregulin (LR), a cytostatic lymphokine produced by activated human peripheral blood lymphocytes, on the electrical properties of K562 tumor cells. LR induced changes in the membrane excitability in 33 of 55 cells studied. A minute or more after application, LR elicited a complex and reversible electrical response. The response lasted for several more minutes in the continued presence of LR. It consisted of (a) moderate-conductance (50 pS), cation-selective, ion-channel activity, (b) a shift of the zero-current membrane potential close to 0 mV, and (c) a suppression of a depolarization-activated K+ conductance. All of these changes in tumor cell excitability act coordinately to depolarize the tumor cells and may be important in the cytostatic actions of LR.

Humans↗