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Biomedical subjects

I Spence

Publications and source records attributed to I Spence.

At least 37 records · Page 2Linked to original sources
PubMed1989

Effects of lead salts on the uptake, release, and binding of gamma-aminobutyric acid: the importance of buffer composition.

The effects of lead on the uptake and release of gamma-[3H]aminobutyric acid [( 3H]GABA) from rat brain slices were examined in solutions buffered with Tris-HCl, sodium phosphate, and sodium bicarbonate. Lead acetate (10-250 microM) inhibited uptake and potassium-stimulated release and facilitated spontaneous efflux only in solutions buffered with Tris-HCl. Calcium-independent binding of [3H]GABA was unaffected by lead acetate (1-100 microM) in Tris-citrate buffer but was significantly inhibited by 3 microM lead acetate in Tris-HCl solution. At the rat soleus neuromuscular junction, lead caused a dose-dependent reduction of end-plate potential amplitude at concentrations of 10-100 microM lead acetate in HEPES-buffered solution but had no effect at these concentrations in phosphate-buffered solution. Stability constants of lead complexes indicate that buffers containing carbonate and phosphate are unlikely to contain a significant concentration of Pb2+, as complexing by these anions would reduce the availability of free Pb2+. This study indicates that the choice of buffer is important when investigating the effects of lead on biological systems and that negative findings may result from the use of inappropriate buffers. It also has important clinical implications suggesting that some effects of lead poisoning may result from its ability to affect neurotransmitter systems directly and that local changes in pH and complexing anion concentrations in the CNS may influence its biological availability and, hence, variable biological responses.

Animals↗
PubMed1988

Depolarizing actions of convulsant barbiturates on isolated rat dorsal root ganglion cells.

The actions of convulsant barbiturates were studied on dorsal root ganglion (DRG) cells in vitro using intracellular recording techniques. Only the convulsant barbiturates (+)-DMBB and CHEB produced a concentration-dependent depression in the responses to gamma-aminobutyric acid (GABA). All convulsant barbiturates were found to produce a direct depolarization of the DRG cell membrane which was accompanied by a decrease in the input resistance of the cell and a reduction in the orthodromic action potential. A sub-population of DRG cells were found to be refractory to these actions but there was no relationship between the cell type (A beta, A delta and C) and ability to respond to the convulsant barbiturates.

Animals↗
PubMed1988

Involvement of adenosine in synaptic depression induced by a brief period of hypoxia in isolated spinal cord of neonatal rat.

The monosynaptic reflex (MSR), recorded extracellularly from the ventral root isolated, superfused spinal cords of neonatal rats (6-10 days post-partum), was rapidly depressed to 35-45% of control values by either cessation of superfusion (4 min stop-flow period) or by superfusion with anoxic medium (95% N2-5% CO2; 4 min). The depression was reversible, 85-115% recovery occurring after 15 min of restoration of flow or normoxic (95% O2-5% CO2) superfusion. 2-Chloroadenosine, a metabolically stable adenosine analogue, also reversibly inhibited the MSR, an effect which was antagonised by 10(-6) M 8-cyclopentyltheophylline (8-CPT). The depression of the MSR, caused by 4 min of hypoxia (either stop-flow or anoxic superfusion), was prevented by 10(-6) M 8-CPT. These results provide strong evidence for a critical involvement of adenosine in mediating early synaptic depression evoked by a brief period of hypoxia.

2-Chloroadenosine↗
PubMed1988

Differing actions of convulsant and nonconvulsant barbiturates: an electrophysiological study in the isolated spinal cord of the rat.

The effects of various pairs of convulsant and nonconvulsant barbiturates on mono- and polysynaptic activity were studied in the isolated spinal cord of the immature rat, using extracellular recording. The convulsant barbiturates, 5-ethyl-5-(3-methylbut-2'-enyl) barbituric acid (3M2B), 5-ethyl-5-(1,3-dimethylbut-1'-enyl) barbituric acid (1,3M1B) and (+)-5-(1,3-dimethylbutyl)-5-ethyl barbituric acid [(+) DMBB] all increased the monosynaptic reflex at concentrations between 5 and 50 microM with no change in polysynaptic activity. When the concentration was raised to between 100 and 300 microM, however, the convulsants all reduced the monosynaptic reflex, thus producing a biphasic dose-response relationship. The nonconvulsant barbiturates phenobarbital, 5-ethyl-5-(3-methylbut-1'-enyl) barbituric acid (3M1B), amylobarbital (3MB) and (-)-5-(1,3-dimethylbutyl)-5-ethyl barbituric acid [(-)DMBB] produced only a decrease in mono- and polysynaptic reflexes. At concentrations which enhanced the monosynaptic reflex, the responses of motoneurones to glycine and eledoisin-related peptide (an analogue of substance P) were reduced by (+)DMBB, while 1,3M1B and 3M2B had no significant effects upon any of the neurotransmitters tested. At concentrations which depressed the monosynaptic reflex, the convulsants all reduced the response to glycine whereas the nonconvulsant barbiturates all increased the response to GABA. With the exception of phenobarbital, both convulsant and nonconvulsant barbiturates produced a direct depolarisation of the presynaptic terminal membrane, with only the convulsants producing a depolarisation of the membrane of the motoneurone. Using another convulsant barbiturate, 5-(2-cyclohexylideneethyl)-5-ethyl barbituric acid (CHEB), this direct depolarising action was found to be calcium-dependent.

Animals↗
PubMed1987

Studies on the subunit structure of textilotoxin, a potent neurotoxin from the venom of the Australian common brown snake (Pseudonaja textilis).

Textilotoxin is a presynaptic neurotoxin from the venom of the Australian common brown snake, Pseudonaja textilis. It has the highest lethality and is structurally the most complex of any known snake venom neurotoxin. It was resolved into its five non-covalently linked subunits in a single step by reverse-phase HPLC. Two of the subunits were identical. The N-terminal amino-acid sequence and amino-acid composition of each subunit were determined. Subunit A was the only one found to possess phospholipase A activity. Separation of textilotoxin into its subunits was reversible and reformed textilotoxin had the same Mr and lethality in mice as the native toxin. Experiments with various unnatural combinations of subunits have led to interesting variations in lethality and Mr of the resulting complexes.

Amino Acid Sequence↗
PubMed1987

Pseudonajatoxin b: unusual amino acid sequence of a lethal neurotoxin from the venom of the Australian common brown snake, Pseudonaja textilis.

The complete amino acid sequence of pseudonajatoxin b, a basic neurotoxin from the venom of the Australian common brown snake, Pseudonaja textilis, was determined by automated Edman analysis of the reduced carboxymethylated polypeptide and of peptides derived by digestion of it with Staphylococcus aureus V8 proteinase. Pseudonajatoxin b consists of a single polypeptide chain of 71 amino acids with Mr 7762. The amino acid sequence showed considerable homology with postsynaptic long neurotoxins, but there were striking differences. Pseudonajatoxin b displayed relatively high lethality, LD50 15 micrograms/kg in mice.

Amino Acid Sequence↗
PubMed1985

Acute effects of lead at central synapses in vitro.

The acute effects of lead in the rat CNS in vitro were studied on synaptic transmission in the isolated hemisected spinal cord from newborn rats and on the transport of exogenous GABA, acetylcholine and cis-3-aminocyclohexane carboxylic acid (ACHC) from slices of cerebral cortex from adult rats. Lead had quite variable effects on monosynaptic reflexes and synaptic potentials. When it occurred, the depression of synaptic transmission by lead (typically at 18.5 mumol/liters of added lead acetate) was reversible provided exposure times were less than 15 min; furthermore, depression could be antagonised by increasing the external calcium concentration. Lead had no effect on the postsynaptic responses of motoneurons to the putative transmitters L-glutamate, GABA and glycine or to eledoisin-related peptide. The effects of lead on uptake and release of exogenous GABA and ACHC were dependent on the perfusion buffer employed: minimal effects were seen in solutions buffered with either phosphate or carbonate. When Tris HCl was used as buffer, lead inhibited the uptake of GABA and potentiated the spontaneous release of GABA with an EC50 = 50 mumol/liters as added lead acetate. In Tris HCl buffer, lead acetate (100 mumol/liters) produced a two-fold enhancement in the spontaneous release of acetylcholine under conditions where choline and acetylcholine re-uptake was blocked by hemicholinium. The availability of free lead cations in solution is highly dependent on the concentrations of other ions (particularly phosphates) and the pH. Under the appropriate conditions, lead can inhibit CNS synaptic function acutely in a manner consistent with lead competing with calcium ions in transmitter release processes as has been established for acetylcholine release at peripheral synapses.

Acetylcholine↗
PubMed1985

Effects of a depressant/convulsant pair of glutarimides on neuronal activity in the isolated spinal cord of the immature rat.

The effect of depressant/convulsant pair of glutarimides on the isolated spinal cord of the immature rat was examined using extracellular recording. At concentrations of 300 microM the depressant beta-butyl-beta-methyl glutarimide enhanced the response of motoneurones and dorsal root fibres to gamma-aminobutyric acid (GABA) while the convulsant bemegride (beta-ethyl-beta-methyl glutarimide) decreased both responses to GABA. At this concentration both the convulsant and depressant reduced mono- and polysynaptic reflex activity. Neither the convulsant or depressant had prominent direct actions, with only a small hyperpolarization being produced by both glutarimides on dorsal root fibres. The overall depressant or convulsant properties of these glutarimides may be due in part therefore to a differential effect on the postsynaptic action of the inhibitory transmitter GABA. Furthermore, the depressant glutarimide reduced the excitatory effects of L-glutamate and the convulsant reduced the inhibitory effects of glycine on spinal neurones; thus, actions on these transmitters may also contribute to the overall effects of these glutarimides.

Amino Acids↗
PubMed1985

Strychnine-like action of the convulsant barbiturate, CHEB.

The effect of 5-(2-cyclohexylideneethyl)-5-ethyl barbituric acid (CHEB) on the isolated spinal cord of the immature rat was examined using extracellular recording. At concentrations less than 20 microM CHEB increased the monosynaptic reflex (MSR) but depressed the reflex at greater concentrations (30-100 microM). At concentrations which enhanced the monosynaptic reflex, CHEB reduced the responses of motoneurones to glycine and to a lesser extent to those of L-glutamate. In the presence of strychnine (5 microM), which enhanced both mono- and polysynaptic reflexes, CHEB produced only slight enhancement of the monosynaptic reflex. At concentrations of 30-100 microM the responses to gamma-aminobutyric acid (GABA), glycine, L-glutamate and eledoisin-related peptide (ERP a substance P and analogue) were all reduced. At these concentrations CHEB directly depolarised the motoneurone membrane. Increases in [Mg2+]0, which reduced spontaneous activity, blocked the enhancement, by CHEB, of the monosynaptic reflex. The actions of CHEB in small doses may be due therefore to its ability to block the action of glycine and thus block tonic inhibition.

Amino Acids↗
PubMed1984

Occurrence of a tetrodotoxin-like compound in the eggs of the venomous blue-ringed octopus (Hapalochlaena maculosa).

A lethal toxin was isolated and partly purified from the eggs of the blue-ringed octopus, Hapalochlaena maculosa. Examination of the toxin by thin layer chromatography, isoelectric focusing and its effects upon the compound nerve action potentials of the toad sciatic nerve gave results that were indistinguishable from those displayed by authentic tetrodotoxin, the toxin present in the venom glands of the octopus.

Action Potentials↗
PubMed1984

The effect of calcium ions on the responses to motoneurons to substance P and eledoisin-related peptide in the toad and rat spinal cord.

The effects of changes in the external calcium ion concentration [Ca2+]0 on segmental reflexes and the responses of motoneurons to bath applied agonists have been studied in the toad and rat spinal cords in vitro. Reducing [Ca2+]0 enhanced polysynaptic reflexes in the toad, with maximal discharges occurring at 0.3 mM. Monosynaptic reflexes in the rat were reduced by lowering [Ca2+]0. Responses of toad motoneurons to substance P and eledoisin-related peptide were enhanced by lowering [Ca2+]0, maximum responses occurring at 0.3 mM. Lowering [Ca2+]0 also enhanced responses to L-glutamate but the effect was smaller and less consistent. This effect of Ca2+ was abolished by the addition of tetrodotoxin to the bathing solution. Toad motoneuron responses to gamma-aminobutyrate and glycine were not affected by alterations in [Ca2+]0. Rat motoneuron responses to substance P and eledoisin-related peptide were also enhanced by reductions in [Ca2+]0 but the effect was more pronounced in younger (less than 5 days post partum) than older (5-10 days post partum) animals. These results are consistent with the idea that motoneuron responses to peptides in normal solutions result from activation of receptors on both motoneurons and interneurons: enhancement of the responses by lowering [Ca2+]0 results from the potentiation of the transynaptic component.

Animals↗
PubMed1984

In vivo actions of atraxin, a protein neurotoxin from the venom glands of the funnel-web spider (Atrax robustus).

The effects of atraxin, a neurotoxic protein from the venom glands of the funnel-web spider (Atrax robustus), have been studied in anaesthetized monkeys. At doses of 70 and 80 micrograms kg-1 i.v., atraxin caused respiratory disturbances (dyspnoea and apnoea), and profound alterations in heart rate and blood pressure. These doses also caused salivation, lachrymation, skeletal muscle fasciculation and an elevation in body temperature. Concurrent increases in firing were recorded from the phrenic nerve and from respiratory and other skeletal muscles. It is concluded that atraxin produces the same syndrome in primates as that observed with whole milked male funnel-web venom.

Animals↗
PubMed1983

Isolation and characterization of a protein neurotoxin from the venom glands of the funnel-web spider (Atrax robustus).

1. Ground venom glands from male and female funnel-web spiders (Atrax robustus) were extracted with acetic acid (0.35 M). 2. A protein constituent of these extracts, atraxin, which caused muscle fasciculation when applied to the phrenic nerve-hemidiaphragm of the mouse in vitro, was isolated by the sequential application of ultrafiltration, gel permeation and ion exchange chromatography. 3. When subjected to isoelectricfocusing, amino acid analysis, ultracentrifugation, ultraviolet and 1H nuclear magnetic resonance spectroscopy, it was shown that atraxin had an approximate molecular weight of 9800, an isoelectric point of greater than 9 and that it contained 76 amino acid residues.

Amino Acids↗
PubMed1982

The actions of adenosine and some analogues on evoked and potassium stimulated release at skeletal and autonomic neuromuscular junctions.

The actions of the nucleosides adenosine, 1-methyladenosine, 1-methylisoguanosine and 2-chloradenosine on transmitter release at the mammalian neuromuscular junction and the vas deferens have been examined. All the nucleosides depressed the evoked release of acetylcholine at the neuromuscular junction, the order of potency being 2-chloroadenosine greater than 1-methylisoguanosine greater than 1-methyladenosine much greater than adenosine. This correlated reasonably with the potency of these compounds in depressing spinal reflexes in anaesthetized mice (Buckle and Spence 1981). Neither adenosine (0.5 and 1.0 mmol l-1) or 1-methylisoguanosine (10 and 20 mumol l-1) had any effect on the elevation of minature endplate potential frequency caused by 12 mmol l-1 K+ at the neuromuscular junction. In the guinea-pig vas deferens, however, 1-methylisoguanosine and adenosine were approximately equipotent in depressing overflow of radioactively labelled noradrenaline. The actions of the nucleosides have been compared with their effects on adenylate cyclase and their ability to resist uptake and deamination. It is concluded that the relative potencies of the nucleosides are not determined solely by their ability to survive in the extracellular fluid.

Adenosine↗
PubMed1981

A simple in vivo method for evaluating drug action at central and peripheral synapses.

A simple in vivo method for studying spinal reflexes and neuromuscular transmission in anaesthetized mice is described. The technique utilizes electromyographic recording from muscles of both hind limbs following stimulation of the sciatic nerve in one limb. Drug effects on the neuromuscular junction, on monosynaptic reflex transmission and on crossed polysynaptic reflexes can be assessed. In addition a method for distinguishing between drugs which affect neuromuscular transmission and those which affect nerve conduction is described using their different effects on reexcitation of motoneurons. Details of data collection and analysis using a laboratory microcomputer are presented. The effects of a variety of representative compounds which have been shown to affect central and peripheral neurotransmission in other systems have been evaluated. The actions of these drugs in this system are compared with those obtained using spinal animals and more sophisticated recording techniques. The results compare favourably with those obtained using more complex methods and the technique offers a simple and inexpensive way of evaluating drug action at these synapses.

Action Potentials↗
PubMed1981

The site of action of muscle relaxant purine nucleosides.

The effects of a series of purine nucleosides, including the novel marine natural product 1-methylisoguanosine, have been examined on muscle relaxation in conscious animals and on spinal reflexes and neuromuscular transmission in mice anaesthetized with sodium pentobarbitone. 1-Methylisoguanosine (5--15 mumol kg--1) and 2-chloroadenosine (1--5 mumol kg--1), both of which cause muscle relaxation in conscious animals, depressed both mono- and polysynaptic spinal reflexes but did not affect neuromuscular transmission. At much higher doses (300 mumol kg--1) both compounds did depress neuromuscular transmission. Adenosine and 1-methyladenosine did not produce muscle relaxation in conscious animals and only slightly depressed polysynaptic reflexes at the highest doses tested (300 mumol kg--1). Theophylline 50 mumol kg--1 enhanced polysynaptic reflexes and antagonized the depression of these reflexes by 1-methylisoguanosine. Neither adenosine nor 1-methylisoguanosine affected the development of tension by isolated diaphragm muscles in vitro. It is concluded that the muscle relaxant purine nucleosides 2-chloroadenosine and 1-methylisoguanosine produce their effects primarily by depressing activity in the central nervous system. Transmission at the neuromuscular junction is not affected at doses in the range of those producing muscle relaxation.

Animals↗