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M B Jackson

Publications and source records attributed to M B Jackson.

At least 73 records · Page 4Linked to original sources
PubMed1994

Blockade of NMDA-activated channels by magnesium in the immature rat hippocampus.

1. N-methyl-D-aspartate (NMDA)-receptor single-channel currents were examined in outside-out patches from thin slices of rat hippocampus to examine possible age dependence in the Mg(2+)-block of NMDA-mediated responses. The kinetics of Mg2+ block, among other channel properties, were compared between CA1 pyramidal cells from neonatal and 2-wk-old animals and dentate gyrus granule cells from neonatal and 4-wk-old animals. 2. Two distinct conductance states were seen consistently in nominally Mg(2+)-free solutions. The main conductance state was 50 and 56 pS in pyramidal cells and granule cells, respectively. The difference in NMDA-receptor channel conductance between the two cell types was statistically significant at all ages examined. Both cell types exhibited a less frequent 35- to 40-pS conductance state. 3. Channel closing rates showed no intrinsic voltage dependence in Mg(2+)-free solutions in any group. Open times became strongly voltage dependent when Mg2+ was added. The rate of block by Mg2+ was similar in all groups. 4. Voltage and Mg2+ influenced primarily the shortest time constant of shut-time distributions. Longer components varied relatively little with voltage or [Mg2+]. The effects of voltage and [Mg2+] suggest that brief shuttings represent closed states in the absence of Mg2+ and primarily a Mg(2+)-blocked state in the presence of Mg2+. 5. The rate of unblocking by Mg2+ was similar in all groups. Thus the dissociation constant for Mg2+ binding (i.e., the ratio of the unblocking and blocking rates) also showed little variation. 6. NMDA-receptor channels in two regions of the hippocampus behaved in a qualitatively similar fashion both in neonates and 2-or 4-wk-old rats. These observations are inconsistent with previous studies obtained using other methods, which suggested that block of NMDA receptor channels by Mg2+ increases substantially with age. Nevertheless, subtle developmental and regional differences in other aspects of NMDA-receptor channel behavior were detected, perhaps reflecting variations in molecular structure tailored to specific functional requirements.

Age Factors↗
PubMed1994

A journey into TQM (total quality management).

Total quality management (TQM) which has been implemented for some time in industry and more recently in hospitals is just beginning to come to long term care organizations. Directors of Nursing can prepare themselves for such a program in their facility by learning from the experience of long term care facilities that have implemented TQM. This is part one of a two part series which describes the implementation process used and a discussion of problems encountered. The second article which will appear in a later issue describes the experience and findings of one project team-the nursing documentation team.

Humans↗
PubMed1993

Expression of a Drosophila GABA receptor in a baculovirus insect cell system. Functional expression of insecticide susceptible and resistant GABA receptors from the cyclodiene resistance gene Rdl.

Recombinant baculoviruses containing two alternative splice forms of the Drosophila Rdl GABA receptor gene were constructed. Spodoptera frugiperda (Sf21) cells infected with either splice form expressed a transcript of expected size (2.5 kb). Western blotting of cell membrane extracts and immunoprecipitation experiments with an anti-Rdl antiserum recognized a protein of the expected size of approximately 65 kDa. Whole cell patch clamp analysis of cells infected with either splice form revealed functional expression of GABA gated chloride ion channels which were blocked by application of 1 microM picrotoxinin. Following replacement of alanine 302 with a serine, a mutation associated with resistance to picrotoxinin and cyclodiene insecticides, mutant channels showed similar levels of insensitivity to picrotoxinin (approximately 100-fold) as those observed in recordings from cultured Drosophila neurons. The significance of the expression of an insect GABA receptor in an insect cell line and the similarity of the results from these functional expression studies to recordings from cultured neurons is discussed.

Alternative Splicing↗
PubMed1993

GABA-activated chloride channels in secretory nerve endings.

Neurotransmitters acting on presynaptic terminals regulate synaptic transmission and plasticity. Because of the difficulty of direct electrophysiological recording from small presynaptic terminals, little is known about the ion channels that mediate these actions or about the mechanisms by which transmitter secretion is altered. The patch-clamp technique is used to show that the predominant inhibitory presynaptic neurotransmitter, gamma-aminobutyric acid (GABA), activates a GABAA receptor and gates a chloride channel in the membranes of peptidergic nerve terminals of the posterior pituitary. The opening of a chloride channel by GABA weakly depolarizes the nerve terminal membrane and blocks action potentials. In this way, GABA limits secretion by retarding the spread of excitation into the terminal arborization.

Animals↗
PubMed1993

Passive current flow and morphology in the terminal arborizations of the posterior pituitary.

1. Patch-clamp techniques were used to study the morphology and electrotonic properties of the terminal arborizations of the posterior pituitary. 2. Neurobiotin-labeling experiments revealed axons and swellings connected to the structure that was patch clamped. The large swellings were en passant and situated along axons in a topological arrangement identical to that of the small varicosities. Axons had many varicosities and few branches, reflecting a predominant architectural motif of beads on a string rather than berries on a bush. 3. Cable theory was used to analyze passive current transients produced by voltage steps under whole-cell clamp. Most charging transients were not consistent with an equivalent cylinder representation as posited by the Rall model for a motoneuron. A few charging transients were consistent with the Rall model and provided estimates for basic membrane and cable properties. 4. Some of the charging transients that violated predictions of the Rall model were consistent with an alternative model, in which the patch-clamped swelling was assumed to be coupled to another swelling by a segment of axon. This model was called the Dumbbell model, and it, together with the neurobiotin-labeling experiments, indicated that a significant number of large swellings were less than one length constant away from another large swelling. 5. Large swellings can have diameters approximately 30 times larger than the diameters of the connecting axons. These swellings lie along the axon such that action potentials must propagate through them to spread excitation through the entire terminal arborization. These large swellings could be sites where action-potential propagation is more likely to fail. 6. The information presented here about neurohypophysial nerve terminals should be useful in further investigations of how terminal arborization geometry and membrane properties influence neurosecretion and synaptic transmission.

Animals↗
PubMed1993

A calcium-activated potassium channel causes frequency-dependent action-potential failures in a mammalian nerve terminal.

1. The contribution of a calcium-activated potassium channel to action-potential failure was studied in nerve terminals of the rat posterior pituitary. 2. Depolarizing current injections under current clamp were faithfully followed by action potentials for stimulation frequencies of < or = 12 Hz. Further increases in frequency resulted in action-potential failure within a few hundred milliseconds. The fraction of failures increased with stimulation frequency. This decrease in excitability was concomitant with a hyperpolarization from -57.3 +/- 1.4 to -61.3 +/- 1.4 (SE) mV. 3. The decrease in excitability was dependent on calcium influx through voltage-dependent calcium channels, because action-potential failures did not occur at frequencies < or = 30 Hz in the presence of cadmium. The dihydropyridine agonist BayK 8644 increased the fraction of failed action potentials. 4. Depolarizations from -80 to 10 mV for 3 s evoked macroscopic potassium currents with a rapidly activated, transient component and a slowly developing, noninactivating component. The late outward current was dependent on calcium influx, because it was reduced by cadmium and enhanced by BayK 8644. 5. Tetraethylammonium and 4-aminopyridine effectively blocked potassium outward currents but failed to distinguish this calcium-dependent potassium channel from the other two potassium channels in this preparation. Charybdotoxin and apamin did not affect potassium currents in this preparation. 6. In excised inside-out patches, the calcium-dependent potassium channel had a slope conductance of 193 pS. The open probability changed e-fold per 14.8 mV change in membrane potential with a calcium concentration at the cytoplasmic membrane face ([Ca]i) of 100 nM. 7. The channel was highly sensitive to [Ca]i. Depolarizations to 100 mV at 10 nM [Ca]i activated the channel half-maximally. When [Ca]i was raised to 250 nM, the voltage for half-maximal activation shifted to -16 mV. Calcium also decreased the steepness of the voltage activation curve. 8. At a constant membrane potential, pressure ejection of calcium to the cytosolic face of an excised patch activated the channel with a delay of 82 ms. This slow activation in excised patches was consistent with the slow activation of the delayed component of the macroscopic current. 9. At constant calcium concentration, the time course of activation exhibited a strong voltage dependence. Most of the channels did not inactivate during depolarizations lasting < or = 300 ms. 10. The channel exhibited complex gating, with at least two distinct open and closed states.(ABSTRACT TRUNCATED AT 400 WORDS)

Action Potentials↗
PubMed1992

Cable analysis with the whole-cell patch clamp. Theory and experiment.

A theoretical analysis was undertaken of a Rall motoneuron under voltage clamp with a finite access resistance. This model is relevant to the conditions of the whole-cell patch clamp, which to date has been used very little for cable analysis. It was shown that the soma and cable charging currents can be distinguished, and that the soma is charged with a time constant approximately equal to the access resistance times the somatic capacitance. Thus, the charging time of the soma is similar to what it would be if the cell had no process. Simple formulas were derived that can be used to calculate the electrotonic length, the membrane time constant, and the soma-dendrite resistance ratio of a cell with a cylindrical process. With the aid of these formulas, reasonable estimates of parameter values were recovered from simulated transient currents. Tests of the Rall model were proposed to determine when there is an equivalent cylinder that is consistent with observed charging behavior. The analysis was extended to a cable with an open end and to a model in which the soma and dendrite have different membrane time constants. It was shown that with voltage-clamp data estimates of electrical parameters other than rho are relatively insensitive to differences between the membrane properties of the soma and dendrite. The methods of cable analysis introduced here were illustrated by application to charging transients recorded from a hippocampal pyramidal cell and from a neurohypophysial nerve ending. The Rall model provided a good description of the pyramidal cell current transient but was inconsistent with the charging behavior observed for the nerve ending. With the recent technical advance of patch clamp recording in brain slices, the analysis presented here should help neurophysiologists investigate cable properties in a wide variety of systems.

Animals↗
PubMed1992

Three potassium channels in rat posterior pituitary nerve terminals.

1. The patch clamp technique was used to investigate the K+ channels in the membranes of nerve terminals in thin slices prepared from the rat posterior pituitary. 2. Depolarization of the membrane produced a high density of K+ current. With a holding potential of -80 mV, test pulses to +50 mV activated a K+ current which was inactivated by 65% within 200 ms. Hyperpolarizing prepulses enhanced the transient K+ current, with half-maximal enhancement at -87 mV. Depolarizing prepulses reduced or eliminated the transient K+ current. 3. In cell-attached patches formed with pipettes containing 130 mM KCl, three types of K+ channel could be distinguished on the basis of single-channel properties. One channel had a conductance of 33 pS and was inactivated with a time constant of 18 ms. A second channel had a conductance of 134 pS and was inactivated with a time constant of 71 ms. A third channel had a conductance of 27 pS, was activated relatively slowly with a time constant of 65 ms, and was not inactivated during test pulses of up to one second in duration. 4. Inactivation of the whole-cell K+ current was a biphasic process with two exponential components. The fast component had a time constant of 22 ms (at +50 mV), corresponding well with the time constant of decay of average current in cell-attached patches containing only the rapidly inactivating K+ channel. The slow component of inactivation had a time constant of 104 ms (at +50 mV), which was similar to but slightly slower than the time constant of decay of the average current in cell-attached patches containing only the slowly inactivating K+ channel. Inactivation of the slow transient K+ current became more rapid with increasing depolarization. 5. The low-conductance rapidly inactivating K+ channel had a lower voltage threshold for activation than the other two K+ channels. 6. Both inactivating K+ channels were enhanced in a similar manner by prior hyperpolarization. There was no difference with regard to voltage mid-point or steepness. 7. The large-conductance slowly inactivating K+ channel was activated by Ca2+ at the inner membrane surface. The resting intracellular Ca2+ was sufficiently high to produce significant activation of this channel without depolarization-induced Ca2+ entry. 8. Removal of Ca2+ from the bathing solution produced a -10 mV shift in the voltage dependence of enhancement of both transient K+ currents by prior hyperpolarization. This could be explained as a surface charge effect.(ABSTRACT TRUNCATED AT 400 WORDS)

Animals↗
PubMed1992

Effects of heat treatment on selected plasma therapeutic drug concentrations.

OBJECTIVE: We investigated the heat stability of six common therapeutic drugs routinely assayed in pharmacokinetic and clinical laboratories. DESIGN: Serum samples were spiked at three concentrations (subtherapeutic, therapeutic, and potentially toxic) with amikacin, gentamicin, tobramycin, phenytoin, theophylline, and phenobarbital. The samples were then heated at 56 degrees C and assayed by fluorescence polarization immunoassay at 15-minute intervals for one hour. SETTING: The study was conducted at a clinical research laboratory using standard laboratory methods. MAIN OUTCOME MEASURES: Drug concentrations of heated samples were compared with unheated controls using ANOVA to determine if degradation occurred over time with heating. RESULTS: Analysis of data revealed no degradation of any of the samples. CONCLUSIONS: Heat treatment of samples from high-risk individuals may be a mechanism to protect healthcare workers from exposure to the human immunodeficiency virus.

Drug Stability↗
PubMed1991

Action potential broadening and frequency-dependent facilitation of calcium signals in pituitary nerve terminals.

Hormone release from nerve terminals in the neurohypophysis is a sensitive function of action potential frequency. We have investigated the cellular mechanisms responsible for this frequency-dependent facilitation by combining patch clamp and fluorimetric Ca2+ measurements in single neurosecretory terminals in thin slices of the rat posterior pituitary. In these terminals both action potential-induced changes in the intracellular Ca2+ concentration ([Ca2+]i) and action potential duration were enhanced by high-frequency stimuli, all with a frequency dependence similar to that of hormone release. Furthermore, brief voltage clamp pulses inactivated a K+ current with a very similar frequency dependence. These results support a model for frequency-dependent facilitation in which the inactivation of a K+ current broadens action potentials, leading to an enhancement of [Ca2+]i signals. Further experiments tested for a causal relationship between action potential broadening and facilitation of [Ca2+]i changes. First, increasing the duration of depolarization, either by broadening action potentials with the K(+)-channel blocker tetraethylammonium or by applying longer depolarizing voltage clamp steps, increased [Ca2+]i changes. Second, eliminating frequency-dependent changes in duration, by voltage clamping the terminal with constant duration pulses, substantially reduced the frequency-dependent enhancement of [Ca2+]i changes. These results indicate that action potential broadening contributes to frequency-dependent facilitation of [Ca2+]i changes. However, the small residual frequency dependence of [Ca2+]i changes seen with constant duration stimulation suggests that a second process, distinct from action potential broadening, also contributes to facilitation. These two frequency-dependent mechanisms may also contribute to activity-dependent plasticity in synaptic terminals.

Action Potentials↗
PubMed1991

Activation and desensitization of the 5-HT3 receptor in a rat glioma x mouse neuroblastoma hybrid cell.

1. Tight-seal voltage-clamp techniques were used to study the 5-HT3 receptor of differentiated NG108-15 cells. 2. The inward current caused by 5-HT was dependent on the 5-HT concentration: the apparent dissociation constant was 3.3 microM and the Hill coefficient was 1.8. 3. Immediately after establishing a recording, sustained application of a saturating concentration of 5-HT caused the response to decline with a half-time of 0.57 s (at a membrane potential of -70 mV). The time course of desensitization was best fitted by a sum of two exponentials. 4. Desensitization became slower during the first 10 min of recording in the whole-cell configuration, with the half-time for response decay increasing to 1.8 s. The deceleration of desensitization may result from wash-out of a cytoplasmic regulator of the receptor. 5. Desensitization declined less during whole-cell recordings when patch pipettes contained non-hydrolysable analogues of adenosine 5'-triphosphate. 6. Desensitization developed more rapidly following the addition of forskolin, prostaglandin E1, cholera toxin or 1,9-dideoxyforskolin to the recording medium. Non-hydrolysable adenosine 5'-phosphate analogues had no effect on the enhancement of desensitization induced by forskolin.

Alprostadil↗
PubMed1991

Differentiation of NG108-15 cells alters channel conductance and desensitization kinetics of the 5-HT3 receptor.

1. NG108-15 cells undergo morphological differentiation in response to appropriate culture conditions. We have used patch clamp techniques to compare responses mediated by the 5-HT3 receptor in differentiated and undifferentiated NG108-15 cells. 2. In differentiated cells, desensitization of 5-hydroxytryptamine (5-HT) responses was much slower than in undifferentiated cells. Desensitization in differentiated cells was also highly variable, with half-times varying by greater than 40-fold. Rapidly desensitized responses in differentiated cells were qualitatively similar to the responses of undifferentiated cells. 3. In outside-out patches from undifferentiated cells, single channel currents could be seen after 5-HT application. These channels had a conductance of 12 pS. The 5-HT-activated channels in differentiated cells were too small to observe at the single-channel level. Noise analysis indicated that the channel conductance was approximately 4 pS. In differentiated cells, both rapidly and slowly desensitized responses were generated by channels with essentially the same conductance. 4. The 5-HT responses of differentiated cells were also distinguished from those of undifferentiated cells on the basis of the voltage dependence of desensitization and the curvature of the current-voltage curve. 5. NG108-15 cells can produce different receptor subtypes, which may be expressed in different tissues or at different stages of development. These variations in receptor behavior suggest that there are at least two distinct mechanisms for regulation of the 5-HT3 receptor.

Cell Differentiation↗
PubMed1990

The selectivity of the channel coupled to the 5-HT3 receptor.

The 5-HT3 receptor is unusual among receptors for biogenic amines in that it is directly coupled to an ion channel that is highly permeable to Na+ and K+. We have studied the permeation properties of this channel in order to achieve a more detailed understanding of its physiological function and to extend the comparison with other ligand gated channels. The 5-HT3 receptor channel is significantly permeable to the organic cations Tris, choline, and N-methyl-glucamine, with permeabilities decreasing with size. The permeability ratios for Tris and choline are similar to those determined for the nicotinic receptor; the permeability ratio for Tris is also similar to that of a non-N-methyl-D-aspartate (non-NMDA) excitatory amino acid receptor. This suggests that the diameters at the narrowest parts of these 3 channels are similar. The Ca2+ permeability of the 5-HT3 receptor channel is relatively low, with an upper bound to PCa/PNa estimated as 0.076. The single channel conductance, as determined by noise analysis, was also relatively low, with a value of 4.4 +/- 0.5 pS. Thus, both the Ca2+ permeability and single channel conductance are lower than those of the nicotinic receptor. In these respects, the 5-HT3 receptor is closer to non-NMDA excitatory amino acid receptors. These results are interpreted in terms of a model of the 5-HT3 receptor channel in which the interior has a lower polarizability, and possibly a greater length, in comparison with the nicotinic acetylcholine receptor channel.

Cations, Divalent↗
PubMed1990

Miniature excitatory synaptic currents in cultured hippocampal neurons.

We performed patch clamp recordings in the whole cell mode from cultured embryonic mouse hippocampal neurons. In bathing solutions containing tetrodotoxin (TTX), the cells showed spontaneous inward currents (SICs) ranging in size from 1 to 100 pA. Several observations indicated that the SICs were miniature excitatory synaptic currents mediated primarily by non-NMDA (N-methyl-D-aspartate) excitatory amino acid receptors: the rising phase of SICs was fast (1 ms to half amplitude at room temperature) and smooth, suggesting unitary events. The SICs were blocked by the broad-spectrum glutamate receptor antagonist gamma-D-glutamylglycine (DGG), but not by the selective NMDA-receptor antagonist D-2-amino-5-phosphonovaleric acid (5-APV). SICs were also blocked by desensitizing concentrations of quisqualate. Incubating cells in tetanus toxin, which blocks exocytotic transmitter release, eliminated SICs. The presence of SICs was consistent with the morphological arrangement of glutamatergic innervation in the cell cultures demonstrated immunohistochemically. Spontaneous outward currents (SOCs) were blocked by bicuculline and presumed to be mediated by GABAA receptors. This is consistent with immunohistochemical demonstration of GABAergic synapses. SIC frequency was increased in a calcium dependent manner by bathing the cells in a solution high in K+, and application of the dihydropyridine L-type calcium channel agonist BAY K 8644 increased the frequency of SICs. Increases in SIC frequency produced by high K+ solutions were reversed by Cd2+ and omega-conotoxin GVIA, but not by the selective L-type channel antagonist nimodipine. This suggested that presynaptic L-type channels were in a gating mode that was not blocked by nimodipine, and/or that another class of calcium channel makes a dominant contribution to excitatory transmitter release.

2-Amino-5-phosphonovalerate↗
PubMed1990

Presynaptic enhancement of synaptic transmission in hippocampal cell cultures by phorbol esters.

The calcium-diacyglycerol activated protein kinase (C kinase) plays an important role in synaptic plasticity, and possibly in long-term potentiation (LTP). We have found that phorbol esters, which activate this protein kinase, increase the frequency of miniature excitatory postsynaptic currents (mEPCs) in hippocampal cell cultures. An analysis of distributions of these miniature synaptic currents shows that amplitudes, and thus postsynaptic function, are not influenced by phorbol esters. Synaptic transmission is therefore enhanced by phorbol esters at a presynaptic locus. The method of analysis employed here is applicable to a broad range of situations involving modification of synaptic transmission in the mammalian central nervous system.

Action Potentials↗