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Biomedical subjects

J F MacDonald

Publications and source records attributed to J F MacDonald.

At least 109 records · Page 6Linked to original sources
PubMed1985

Measurements of transmitter action: the problem of voltage dependence.

Conventional intracellular recordings, from central mammalian neurones, are used to assess the actions of putative transmitters on neuronal membrane parameters such as input conductance and to determine reversal potentials. Ohm's Law and constant current methods can be employed to accurately assess such parameters provided the putative transmitter activates a voltage-independent response. However, several likely transmitter candidates act via voltage-dependent mechanisms to alter neuronal excitability; as a consequence, results from this method of analysis must be cautiously interpreted. Under this constraint of possible voltage dependence, constant current methods can lead to spurious conclusions about the physiological and pharmacological characterization of the response.

Animals↗
PubMed1984

L-aspartic acid potentiates 'slow' inward current in cultured spinal cord neurons.

Intracellular recordings (current- and two electrode voltage-clamp) were made from mouse spinal cord neurons grown dissociated in tissue culture. Neurons were bathed in elevated concentrations of calcium (Ca) and sometimes tetraethylammonium (TEA). Brief depolarizing current injections activated graded 'after-depolarizations' which summated to trigger prolonged all-or-none action potentials. Under voltage-clamp both of these active potentials were manifest as 'slow' inward current. Net inward current was observed in some neurons during 0.5-1.0 s depolarizing command steps. However, in the majority of cases the inward current was seen as large inward current tails (outward current relaxations) upon repolarization of the membrane potential to holding values. Cadmium (Cd) blocked this slow inward current, 'after-depolarizations' and prolonged action potentials. Applications of L-aspartic acid increased the magnitude of net inward current evoked by command steps and potentiated and prolonged inward current tails. This potentiation and prolongation of voltage-dependent inward current likely accounts for the prolonged action potentials or 'bursting' characteristic of responses to L-aspartic acid and related amino acids such as N-methyl-D-aspartic acid.

Animals↗
PubMed1984

Lathyrus excitotoxin: mechanism of neuronal excitation by L-2-oxalylamino-3-amino- and L-3-oxalylamino-2-amino-propionic acid.

Intracellular recordings were made in cultured neurones from foetal mouse spinal cord. The effects of applications of the neurotoxin, L-3-oxalylamino-2-amino-propionic acid (a constituent of the chickling pea, Lathyrus sativus) and its 2-oxalylamino isomer on membrane potential and conductance were examined in the presence of TTX and TEA and compared to those of other excitatory amino acids. Although both compounds produced membrane depolarization and an increase in input conductance, the 3-oxalylamino isomer (beta-ODAP) was approximately equal to 10 times more potent than the 2-oxalylamino isomer (alpha-ODAP). beta-ODAP caused a voltage-independent change in conductance, as compared to an apparent voltage-dependent decrease produced in the same neurons by L-aspartic acid (L-ASP). Although reversal potentials determined for beta-ODAP resembled those for alpha-ODAP and kainic acid, they were consistently and significantly lower than the reversal level for L-ASP. Although the receptor antagonist 2-amino-5-phosphonovaleric acid (APV) and the divalent cation Cd2+ did not alter the conductance increase evoked by beta-ODAP, they markedly depressed responses to L-ASP. Such differences suggest a mechanism of excitatory action for the neurotoxin, beta-ODAP, which does not involve a Ca2+-dependent mechanism and is quite different from that for L-ASP and N-methyl-D-aspartic acid, but similar to that of kainic and quisqualic acids.(ABSTRACT TRUNCATED AT 250 WORDS)

2-Amino-5-phosphonovalerate↗
PubMed1984

Substitution of extracellular sodium ions blocks the voltage-dependent decrease of input conductance evoked by L-aspartate.

The response of cultured spinal cord neurones to L-aspartic acid consists of membrane depolarization accompanied by an apparent voltage-dependent reduction in input conductance (Gm). Depolarizations evoked by this amino acid and the decrease in Gm were reduced or eliminated when cultured spinal cord neurones were superperfused with solutions where Na was substituted with choline or Tris. Responses to L-aspartate recovered rapidly upon return to the control (Na-containing solution). The voltage-dependent component of the response to L-aspartate is dependent upon the presence of external Na.

Animals↗
PubMed1983

Actions of verapamil on the excitability of cultured neurones.

Applications of relatively high concentrations of verapamil (0.5-1.0 mM) to cultured spinal cord or dorsal root ganglion neurones blocked both calcium (Ca) and sodium (Na) action potentials. They also depolarized these neurones and caused a voltage-dependent reduction of input conductance (measured under current clamp). The depolarization and apparent reduction of input conductance evoked in spinal cord neurones by L-aspartic acid appeared to be potentiated by coapplications of verapamil. Lower concentrations (maximum 100 microM) of verapamil blocked identified Ca action potentials without depolarizing neurones or changing their input conductance. This Ca antagonist provided only limited selectivity in cultured neurones. Voltage clamp of cultured spinal cord neurones demonstrated that high concentrations of verapamil (0.5-5.0 mM) can reduce outward currents (leakage and rectification). Inward currents evoked with single applications of L-aspartic acid were not potentiated by verapamil and thus potentiations recorded under current clamp can be attributed to depression of outward currents. The reduction of voltage-dependent outward currents (rectification) by verapamil improved resolution of the voltage-dependent inward current activated by L-aspartic acid.

Action Potentials↗
PubMed1982

Multiple actions of picomolar concentrations of flurazepam on the excitability of cultured mouse spinal neurons.

Intracellular recordings from mouse spinal neurons grown dissociated in tissue culture were used to study the effects of the water soluble benzodiazepine, flurazepam, upon neuronal excitability. Low concentrations of this drug (1 pM to 10 nM) depressed excitability in three distinctly different ways: (1) by directly increasing Cl- conductance, (2) by potentiating responses to GABA, and (3) by elevating spike threshold and/or depressing repetitive spike firing. Bathing neurons with picrotoxin induced 'convulsive-like' activity which was attenuated by flurazepam. The direct effects of flurazepam on the passive and active properties of membrane excitability were insensitive to picrotoxin. However, when the dose of flurazepam was increased to 10 nM or greater this drug lost its effectiveness. These results show that flurazepam is a potent drug with multiple sites of action all of which are likely to contribute to its pharmacological actions in vivo.

Animals↗
PubMed1982

L-Aspartic acid induces a region of negative slope conductance in the current-voltage relationship of cultured spinal cord neurons.

Individual murine spinal cord neurons were grown in dissociated tissue cultures. Using either one or two conventional intracellular microelectrodes neurons were voltage-clamped and current-voltage (I-V) curves were constructed using command pulses. L-Aspartic acid was then applied and I-V curves repeated in the presence of this excitatory amino acid. L-Aspartic acid induced a region of negative slope conductance (RNSC) in the steady-state I-V relationships of these neurons. Such a RNSC accounts for the apparent reduction of conductance in response to excitatory amino acids and its instability likely contributes to the electrogenesis of bursting in central neurons.

Animals↗
PubMed1982

Fluctuation analysis of neutral amino acid responses in cultured mouse spinal neurones.

1. Intracellular recordings using the voltage-clamp technique were made at room temperature (24 +/- 1.5 degrees C) from mouse spinal and sensory neurones growing in dissociated cell culture. 2. Membrane current responses could be elicited by ionophoresis of the neutral amino acids, gamma-aminobutyric acid (GABA), beta-alanine (BALA) and glycine to the cell body and processes of these neurones. 3. All membrane current responses were associated with increases in current fluctuations. Most of the analysis presented here was applied to responses generated at the cell body. 4. Many of the fluctuations in membrane current occurring during the responses could be interpreted as reflecting the kinetic behaviour of a single population of two-state Cl- ion-channels. 5. The properties of channels estimated during the desensitized phase of an amino acid-induced current response were not significantly different from those estimated during the peak of the response. 6. The properties of the amino acid-activated channels were relatively constant over the -40 to -90 mV range of membrane potential. 7. There was considerable variation in the estimated average conductance, gamma, and duration, tau, of the elementary events evoked by the neutral amino acids on spinal cord neurones. The properties of the elementary channel events activated by one of the amino acids were significantly different from those activated on the same neurones by either of the other amino acids. 8. In sensory neurones the average gamma and tau values for GABA-activated ion-channels were also determined and these values fell within the range of those for channels activated by GABA in spinal neurones. 9. The results indicate that different naturally occurring neutral amino acids activate channels with unique properties in cultured mouse spinal neurones. The relative charge transfer associated with these channels averages 1.00:0.74:0.32; GABA:glycine:beta-alanine.

Alanine↗
PubMed1982

The effects of L-glutamate and its analogues upon the membrane conductance of central murine neurones in culture.

Neurones from brain and spinal cord of foetal mice were grown dissociated in monolayer cultures for 4--6 weeks prior to electropharmacological analysis. Neurones were immersed in a Hanks balanced salt solution while drugs and ions were applied by pressure microperfusion during intracellular recordings obtained by conventional techniques. L-Glutamate and its analogues, L-aspartate, DL-homocysteate, N-methyl-D-aspartate, and DL-ibotenate activated two distinct mechanisms of excitation. The primary effect was depolarization accomplished by an apparent decrease of neurone input conductance (Gm). However, in most instances an expected increase in Gm was also observed, especially if membrane potential was reduced by tonic depolarization. Another glutamate analogue, DL-kainate, never decreased Gm and invariably increased Gm at all membrane potentials tested. The decrease of Gm evoked by glutamate and related compounds was strongly dependent upon membrane potential. It was most pronounced at potentials near resting values (-40 to -60 mV) and diminished both with depolarization or hyperpolarization from this range. This apparent decrease favoured the electrogenesis of regenerative potentials that were insensitive to tetrodotoxin. A voltage-dependent increase in sodium and (or) calcium conductance (GNa, GCa) or a decrease in potassium conductance (GK) is suggested to account for this decrease in Gm. Divalent cations (Mg and Co) reduced the depolarizing actions of all amino acids except for those to kainate. The decrease in Gm was more sensitive to Mg than was the increase of Gm. However, the receptor antagonist DL-alpha-aminoadipate blocked both changes in conductance and responses to all amino acids with the exception of those to kainate. The possible existence of multiple receptors for glutamate is also discussed.

Amino Acids↗
PubMed1981

Multiple reversal potentials for responses to L-glutamic acid.

Reversal potentials for L-glutamic and kainic acid were determined from foetal mouse neurones, grown dissociated in culture, and originating either from the brain or spinal cord. Amino acids were applied at known concentrations by pressure microperfusion and responses recorded using conventional intracellular techniques. Altering membranes potential by injecting current through the recording electrode permitted direct (not extrapolated) observation of reversal potentials. Values of reversal potentials differed from previous reports in culture but were still well below those predicted solely from an increase in sodium conductance. Furthermore, with some neurones it was possible to demonstrate multiple phases of response, each phase differing in time course and possessing a separate reversal potential. One reversal potential indicated the participation of potassium conductance in the response.

Animals↗
PubMed1980

Picrotoxin convulsions involve synaptic and nonsynaptic mechanisms on cultured mouse spinal neurons.

The cellular mechanisms underlying picrotoxin-induced convulsive activity were studied by using mouse spinal neurons growing in tissue culture. Picrotoxin-induced convulsive activity in most but not all of the cells studied. The activity could be inverted by polarizing to positive potentials and eliminated either by decreasing the ratio of calcium to magnesium or by applying tetrodotoxin. When applied locally to individual cells, picrotoxin lowered spike threshold and induced spontaneous firing in some but not all cells tested. The results suggest that picrotoxin-induced convulsive activity involves rapidly summating synaptic activity which may be evoked by high-frequency repetitive firing.

Action Potentials↗
PubMed1980

Two distinct inhibitory responses of cultured, mammalian spinal neurones to ibotenic acid.

Ibotenic acid, a structural analogue of glutamic acid, was applied to mouse spinal neurones grown in dissociated cultures. This amino acid evoked two inhibitory responses in addition to an excitatory response. Both inhibitory responses were manifested by membrane hyperpolarization and decreased input resistance. However, one was long-lasting (in excess of periods of 1 h) in comparison with the other. The latter response was likely a consequence of an increased chloride conductance and was sensitive to the gamma-aminobutyric acid antagonists bicuculline and picrotoxin whereas the former response was insensitive to these drugs. The ionic mechanism of this long-lasting response has yet to be elucidated.

Animals↗
PubMed1980

Two conductance mechanisms activated by applications of L-glutamic, L-aspartic, DL-homocysteic, N-methyl-D-aspartic, and DL-kainic acids to cultured mammalian central neurones.

Neurones of foetal mouse spinal cord or brain were grown dissociated in tissue culture. Intracellular recordings from single neurones demonstrated two types of conductance changes in response to applications of L-glutamic acid and a number of structurally related compounds. In the presence of 10 mM MgCl2 the responses consisted of an increase in membrane conductance. However, if this divalent cation was kept at ambient concentration a decrease of membrane conductance was also noted (perhaps to potassium ions). Often this decrease in conductance was associated with the appearance of regenerative and spikelike potentials even in the presence of tetrodotoxin. All amino acids tested with the exception of DL-kainic acid produced such a change in conductance. It is concluded that at least two distinct conductance mechanisms are linked to excitatory amino acid receptors.

Animals↗