PubMed Health⌕ Search

Biomedical subjects

R L MacDonald

Publications and source records attributed to R L MacDonald.

35 records · Page 2Linked to original sources
PubMed1989

Barbiturate regulation of kinetic properties of the GABAA receptor channel of mouse spinal neurones in culture.

1. Barbiturate regulation of the kinetic properties of gamma-aminobutyric acidA (GABA) receptor channel chloride currents from somata of mouse spinal cord neurones were investigated using whole-cell and excised outside-out patch-clamp recording techniques. 2. GABA (2 microM), GABA (2 microM) plus phenobarbitone (PhB) (500 microM) and GABA (2 microM) plus pentobarbitone (PB) (50 microM), applied by pressure ejection from blunt perfusion micropipettes, evoked inward chloride currents when neurones or patches were voltage clamped at -75 mV and the chloride equilibrium potential was 0 mV. GABA receptor channel currents were increased by PhB and PB. 3. Single GABA receptor channel currents were recorded with a main conductance state of 27 pS and a less frequent subconductance state of 16.5 pS. The conductances of the two states were unchanged by the barbiturates. 4. The main conductance state kinetics were analysed. GABA alone or with the barbiturates gated the channel open singly and in groups of openings. 5. The barbiturates increased GABA receptor channel mean open time and shifted frequency histograms of channel open times to longer times. 6. Three exponential functions were required to fit the frequency histograms of GABA receptor channel open times, suggesting that the channel has at least three open states (O1, O2, O3). The time constants for the exponential functions (0.9, 2.7 and 7.8 ms, respectively) were unchanged by the barbiturates. The increases in mean open times and the shifts of the open-time frequency histograms by the barbiturates were due to a reduction in relative frequency of occurrence of the two short open states (O1 and O2) and to an increase in the relative frequency of occurrence of the longest open state (O3). 7. Frequency histograms of GABA receptor channel closed times were fitted with five exponential functions, suggesting that the channel has multiple closed states. None of the time constants nor areas of the exponential functions were significantly changed by the barbiturates. 8. For analysis, a burst was defined as openings surrounded by closures greater than a critical closed time, tc, of 5 ms. For GABA (2 microM), frequency histograms of GABA receptor channel bursts were fitted with three exponential functions, suggesting that the channel has three burst states (B1, B2, B3). The B1 burst state was probably a single opening to the O1 open state while the B2 and B3 burst states were probably composed of multiple openings to the O2 and O3 open states.(ABSTRACT TRUNCATED AT 400 WORDS)

Animals↗
PubMed1989

Activators of protein kinase C selectively enhance inactivation of a calcium current component of cultured sensory neurons in a pertussis toxin-sensitive manner.

1. Three calcium current components were recorded in mouse dorsal root ganglion (DRG) neurons using the single-electrode voltage-clamp technique, the transient low-threshold (T), the slowly inactivating high-threshold (L), and the transient high-threshold (N) calcium current components. 2. Two activators of protein kinase C, the phorbol ester, phorbol 12,13-dibutyrate (PDBu) and the diacylglycerol analogue, 1,2-oleoylacetylglycerol (OAG), were tested to determine their effects on the three calcium current components. Neither compound affected the isolated T current, but both had similar effects on calcium currents containing both the N and L current components. PDBu and OAG reduced the peak but had little effect on the late (greater than or equal to 300 ms) calcium current evoked at clamp potentials (Vc) positive to -20 mV from holding potentials (Vh) near the resting membrane potential (-70 to -55 mV). The reduction in peak calcium current ranged from 15 to 40% and was present in all neurons tested. In contrast, there was little or no effect of these compounds when calcium currents were evoked from Vh at or negative to -80 mV. A phorbol derivative, 4-alpha-phorbol, which does not activate protein kinase C, had no effect on calcium currents at any potential. 3. Pretreatment of cultures with pertussis toxin [(PTX); 100-200 ng/ml] for 4-24 h abolished the PDBu-induced reduction of calcium current. 4. Multiexponential curve-fitting of current traces was used to determine the amplitudes and inactivation time constants (tau i) of the T, N, and L current components. PDBu selectively reduced the amplitude of the N current component but had no effect on any of the tau i. 5. PDBu had no effect on the voltage dependence of the current-voltage relation or on the time dependence of recovery from inactivation at time intervals up to 500 ms. In contrast, PDBu produced a -7.5-mV shift in the steady-state inactivation curve, sufficient to account for its effect near the resting membrane potential. 6. PDBu and OAG had selective, voltage-dependent effects on the calcium current components of mouse DRG neurons in culture. We conclude that activation of protein kinase C caused a selective reduction of the N current component; this effect was due to increased steady-state inactivation at membrane potentials positive to -80 mV and was mediated by a G protein.

Action Potentials↗
PubMed1988

Microcystic spinal cord degeneration causing posttraumatic myelopathy. Report of two cases.

Two cases of progressive myelopathy occurring years after incomplete cervical spinal cord injury are presented. In both patients, the clinical features, as well as the "bull's-eye" appearance of the delayed computerized tomography (CT) myelography study and the circumscribed low density of the magnetic resonance image, were consistent with posttraumatic syringomyelia, but surgical exploration including intra-operative spinal sonography failed to reveal a syrinx. Although arachnoiditis was present in both patients, the striking abnormality found at surgery was the softened appearance and the microcystic degeneration of the cord. The microcystic spinal cord degeneration found in these cases represents a previously undescribed cause of late deterioration after spinal cord injury that may mimic the clinical, CT-myelographic, and magnetic resonance features of posttraumatic syringomyelia.

Adult↗
PubMed1987

Actions of mu, delta and kappa opioid agonists and antagonists on mouse primary afferent neurons in culture.

The effects of selective mu, delta and kappa opioid agonists and antagonists were studied on somatic calcium-dependent action potentials recorded from mouse dorsal root ganglion (DRG) neurons grown in dissociated cell culture. The mu selective agonist, PL 017, and the delta selective agonist, [D-Pen2, L-Pen5] enkephalin (DPLPE), reduced action potential duration of a subpopulation (21/56) of DRG neurons. Leucine-enkephalin reduced action potential duration of all neurons sensitive to PL 017 or DPLPE, whereas 85% of neurons responding to leucine-enkephalin responded to either PL 017 or DPLPE. Only 15% of neurons responded to both PL 017 and DPLPE. There was no significant difference in the response to PL 017 or DPLPE when compared to leucine-enkephalin. In another experiment, the kappa selective agonist dynorphin A (DYN A), PL 017 and DPLPE reduced action potential duration of a subpopulation (15/67) of DRG neurons. There was a heterogeneous response among neurons to PL 017, DPLPE and DYN A inasmuch as 21.4% of neurons responded to all three agonists, 35.7% responded to PL 017 and DYN A, 35.7% responded only to PL 017 and 7.1% responded only to DYN A. Responses to the mu selective agonist PL 017 were antagonized by the reversible opioid antagonist naloxone and the selective mu antagonist SMS 201-995 in a concentration-dependent fashion. Responses to PL 017 were not altered by the selective delta antagonist ICI 174864. Responses to PL 017 were reduced by the irreversible, selective mu antagonists beta-funaltrexamine and naloxonazine in a concentration-dependent fashion.(ABSTRACT TRUNCATED AT 250 WORDS)

Action Potentials↗
PubMed1986

Adenosine agonists reduce voltage-dependent calcium conductance of mouse sensory neurones in cell culture.

Adenosine and several of its analogues produced a concentration-dependent shortening of calcium-dependent action potential (c.a.p.) duration of mouse dorsal root ganglion (d.r.g.) neurones in dissociated cell culture. The following rank order of potency was obtained: N6-(L-phenylisopropyl)adenosine greater than N6-(D-phenylisopropyl)adenosine greater than N6-cyclohexyladenosine greater than 2-chloroadenosine much greater than 1-methylisoguanosine greater than adenosine. Effects of adenosine agonists on c.a.p. duration were blocked by methylxanthine adenosine antagonists. Adenosine monophosphate (AMP) and cyclic AMP shortened c.a.p.s in d.r.g. neurones, while ATP also depolarized cells. Voltage-clamp analysis revealed that the effect arose from reduction of a voltage-dependent calcium conductance. Adenosine agonists reduced depolarization-evoked inward currents but did not alter membrane conductance following blockade of calcium channels by cadmium. Additionally, adenosine reduced the instantaneous current-voltage slope (chord conductance) during step commands that produced maximal activation of voltage-dependent calcium conductance. If effects of adenosine on neuronal somata and synaptic terminals are similar, adenosine agonists may inhibit neurotransmitter release in the central nervous system by inhibiting a voltage-dependent calcium conductance. Since effects of adenosine agonists did not correspond with their relative potencies as modulators of adenylate cyclase activity or inhibitors of neurotransmitter release in peripheral tissues, a novel adenosine receptor may be involved in regulation of this conductance.

Action Potentials↗
PubMed1985

Differential modulation of gamma-aminobutyric acid receptors by caprolactam derivatives with central nervous system depressant or convulsant activity.

The effects of a series of caprolactam derivatives with central depressant, convulsant or muscle relaxant activity were investigated upon gamma-aminobutyric acid (GABA) receptor-ionophore binding to rat brain membranes using [3H]GABA, [3H]GABA, [3H]muscimol and [35S]-tert.-butylbicyclophophorothionate ([35S]TBPS) as ligands, and GABA responses in mouse spinal cord neurones in dissociated cell culture. Some caprolactams produced a picrotoxin-like chloride-dependent partial inhibition of muscimol binding and were potent inhibitors of TBPS binding. One compound that was further investigated (4,4,6,6-tetramethylhexahydro-2H-azepin-2-one), inhibited GABA responses and increased the frequency of paroxysmal depolarizations in cultured neurones. Other caprolactams enhanced muscimol binding and were relatively weak inhibitors of TBPS binding, and one (3,3-diallyl-6,6-dimethylhexahydro-2H-azepin-2,4-dione) was shown to enhance GABA responses and produced quiescence of activity in cultured neurones. There was a direct correlation between caprolactam effects on muscimol binding in the presence of chloride ions and their effects on TBPS binding suggesting a similar site of action for the caprolactams influencing the binding of these two ligands. For the two classes of caprolactams, with respect to inhibition or enhancement of muscimol binding, there appeared to be a relationship between in vitro effects and their convulsant or depressant activity in mice. Caprolactams may be useful low molecular weight probes for the study of GABA receptor-ionophore complexes.

Animals↗
PubMed1985

Cellular mechanistic bases for classification of clinically useful anticonvulsant drugs.

Anticonvulsant drug effects on mammalian central neurons in primary cell cultures were investigated. The concentrations at which these drugs limited repetitive firing of action potentials were measured. Their effects on postsynaptic responses to iontophoretically applied GABA were also compared. Based on effects of anticonvulsant drugs on in-vitro assays and action on seizures in man and experimental animals, a classification of clinical anticonvulsants is proposed based on cellular mechanisms of action. Three categories are identified. The mechanisms of action at the cellular level may be useful in predicting clinical efficacy of anticonvulsants under development. The concentration dependence of the in-vitro actions may predict further the therapeutic range of the drug and the mechanisms associated with clinical toxicity.

Animals↗
PubMed1983

Opioid peptides selective for mu- and delta-opiate receptors reduce calcium-dependent action potential duration by increasing potassium conductance.

We suggest that both mu- and delta-opiate receptors on dorsal root ganglion neuron somata are coupled to voltage- and/or calcium-dependent potassium channels since opioid peptide decreases of calcium-dependent action potential duration were: (1) not associated with a change of resting membrane potential or conductance; (2) accompanied by an increase in action potential after-hyperpolarization, and (3) blocked by intracellular injection of the potassium channel blocker cesium [18]. In contrast, norepinephrine [4] and cadmium [9], which have been reported to act on voltage-dependent calcium rather than potassium channels, shortened action potential duration and decreased after-hyperpolarization amplitude, an action not blocked by intracellular iontophoresis of cesium.

Action Potentials↗
PubMed1983

Tetanus toxin: convulsant action on mouse spinal cord neurons in culture.

The effects of direct application of tetanus toxin on fetal mouse spinal cord neurons in culture are described. Tetanus toxin produces increased excitation characterized by paroxysmal depolarizing events (PDE). In contrast to the abrupt onset of convulsant action produced by postsynaptic glycine antagonist strychnine, the convulsant action of tetanus occurs after a dose-dependent latent period. The onset of the convulsant action of tetanus toxin is paralleled by a reduction in observed spontaneous inhibitory synaptic potentials. Excitatory synaptic events can be identified as components of some tetanus-PDE. The toxin does not alter postsynaptic responses to the inhibitory amino acids glycine and gamma-aminobutyric acid. The latency and convulsant action of tetanus toxin are consistent with an irreversible presynaptic membrane interaction that reduces inhibitory transmission, a mechanism of action distinct from those of convulsants that antagonize inhibitory transmitters at the postsynaptic membrane.

Action Potentials↗
PubMed1981

Calcium-dependent action potentials in mouse spinal cord neurons in cell culture.

Following blockade of membrane potassium conductance with tetraethylammonium ions or 3-aminopyridine, long-duration action potentials were recorded from mouse spinal cord neurons in primary dissociated cell culture. The action potentials were calcium-dependent since they: (1) were not blocked by the sodium-channel blocker tetrodotoxin, (2) could be recorded in sodium-free, calcium-containing medium (3) could not be evoked in sodium-containing, calcium-free medium, (4) were blocked by calcium channel blockers manganese and cobalt and (5) had overshoot amplitudes that varied linearly with the log of the extracellular calcium concentration (slope of 27.5 mV/decade change in calcium concentration).

Action Potentials↗
PubMed1981

Substance P decreases a potassium conductance of spinal cord neurons in cell culture.

Substance P (SP) produced membrane depolarization and decreased membrane conductance of mouse spinal cord neurons in primary dissociated cell culture. SP-responses were abolished by intracellular tetraethylammonium suggesting that SP decreased potassium conductance. Reversal of SP-responses was not observed with membrane hyperpolarization suggesting that SP reduced a voltage-dependent potassium conductance that was activated by membrane depolarization.

Animals↗
PubMed1979

Enhancement of GABA-mediated postsynaptic inhibition in cultured mammalian spinal cord neurons: a common mode of anticonvulsant action.

Murine spinal cord neurons grown in dissociated cell culture were used to study the effects of barbiturate (phenobarbital, mephobarbital) and benzodiazepine (diazepam, chlordiazepoxide( anticonvulsants on amino acid responses. Both types of anticonvulsant augmented GABA-mediated postsynaptic inhibition without augmenting beta-alanine or glycine-mediated postsynaptic inhibition. Barbiturates, but not benzodiazepines, antagonized glutamate-mediated postsynaptic excitation. Augmentation of GABA-mediated inhibition by the anticonvulsants should contribute to their anticonvulsant action; antagonism of glutamate-mediated excitation by barbiturates should also contribute to their anticonvulsant action and could be at least in part responsible for their sedative actions.

Animals↗
PubMed1976

Involvement of the avian hypothalamus in defensively conditioned heart rate change.

Hypothalamic involvement in visually conditioned heart rate change (established by pairing light and foot-shock) was studied in 128 pigeons by evaluating conditioning performance following lesions of various hypothalamic areas. Extensive destruction of the posterior hypothalamus severely impaired development of conditioned heart rate change. Anterior hypothalamic lesions also produced serious deficits, though not as severe as following posterior hypothalamic damage. Partial posterior hypothalamic, unilateral and tuberal lesions produced only minor to moderate deficits. The critical locus for profound impairment of conditioned response development appeared to be the medial hypothalamus, and it is suggested that more specifically it is the terminal field of the archistriatal projection upon the medial hypothalamus. It is concluded that the medial hypothalamus is essential for the development of defensively conditioned heart rate change, and based on previous findings it is suggested that the critical descending pathway for expression of this conditioned response involves the archistriatal projection upon the medial hypothalamus and subsequently a polysynaptic pathway through the ventral brainstem.

Animals↗