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R M Pitman

Publications and source records attributed to R M Pitman.

At least 19 recordsLinked to original sources
PubMed1992

Two classes of vesicles are present and change in relative proportion during post-embryonic development of rectifying electrical synapses in the crayfish.

The size and shape of vesicles at junctional appositions of the rectifying electrical synapses between the medial giant fibre and motor giant neurone of the crayfish were measured during the first 2 months after hatching. Summed data over this period reveal a bimodal distribution in vesicle diameter. From the day of hatching until about 7 days of age, small vesicles (circa 25 nm diameter) predominate. From day 7 onwards, larger vesicles (circa 55 nm diameter) occur in increasing numbers, until at day 56 they constitute about 85% of the population at any one junctional apposition. At intermediate ages (day 7-28) individual junctional appositions may show the same bimodal distribution in size as does the age group as a whole, indicating that large and small vesicles occur together at the same junction. The larger vesicles are mainly circular, while the small vesicles are pleomorphic, with shapes ranging from almost circular down to a shape factor of about 0.6.

Aging↗
PubMed1992

Structural and functional post-embryonic development of a non-rectifying electrical synapse in the crayfish.

The post-embryonic development of the non-rectifying septate synapse between homologous lateral giant (LG) fibre segments has been investigated using electron microscopy and electrophysiology. In adults, the LG-LG synapse is characterized by closely apposed membranes (approximately 4 nm separation) traversed by regularly spaced particles, and large (60-80 nm) spherical vesicles on both sides of the junction. In newly hatched crayfish the junction between lateral giant fibre segments comprises regions of close membrane apposition as seen in the adult along with non-specialized areas of wide (10-15 nm) membrane separation. Vesicles associated with these junctions are small (25-40 nm) and pleomorphic. The number of vesicles is low by comparison with adult junctions; in most sections of hatchling junctions there are normally fewer than five vesicles, although as many as 30 have occasionally been seen. During development the non-specialized areas of wide membrane separation become rare and the vesicle population changes to a mixture of small pleomorphic forms and larger (60-80 nm) spherical ones. However even at two months the number of large spherical vesicles is markedly less than that at the adult synapse, while small pleomorphic vesicles are still abundant. Despite the difference between the adult and hatchling vesicle populations, intracellular recordings have shown that the synapse is fully functional as a non-rectifying electrical junction on hatching and that the intracellular marker Lucifer Yellow can pass between adjacent lateral giant fibre neurons.

Animals↗
PubMed1991

Postembryonic development of rectifying electrical synapses in crayfish: physiology.

In a previous paper we showed that the ultrastructure of the giant fibre to motor giant synapse of crayfish changes in the first few weeks after hatching from having predominantly the appearance of a chemical synapse to having the appearance of an electrical synapse. This is paralleled by a behavioural change from non-giant fibre-mediated to giant fibre-mediated tailflips. In this paper we describe the physiology of the giant fibre to motor giant synapse over this period. We find the following: (1) The giant fibre to motor giant synapse usually transmits spikes 1:1 from the day of hatching. (2) The synapse operates by electrical transmission from the day of hatching, when no connexons are apparent at the ultrastructural level. (3) The synapse has no detectable chemical component, even at an age when the predominant type of junctional apposition has the ultrastructural appearance of a chemical synapse. (4) Inhibitory chemical synapses occur onto the motor giant at the day of hatching, and these show similar physiological characteristics to those which occur onto the motor giant in adults. (5) In some preparations, the giant fibre to motor giant electrical synapse shows rectification similar to that in the adult, but in most cases both depolarizing and hyperpolarizing current injected into the medial giant spreads to the motor giant. (6) Current spread from the medial giant to the motor giant is increased by hyperpolarizing the motor giant neuron, even when medial giant to motor giant transmission is apparently non-rectifying. (7) Both the giant fibre and the motor giant have resting potentials of about -90 mV. There is no standing difference in resting potential as there is in the adult. This may explain the apparent lack of medial giant to motor giant rectification observed in most preparations.

Animals↗
PubMed1991

Characterization of receptors mediating the actions of dopamine on an identified inhibitory motoneurone of the cockroach.

1. The effects of a number of dopaminergic agonists and antagonists upon the soma of a prothoracic inhibitory motoneurone of the cockroach (Periplaneta americana) have been recorded under voltage-clamp conditions. 2. Dopamine generates inward currents that are extremely voltage-dependent: currents increase rapidly at membrane potentials more negative than about -120 to -150 mV and also show a peak at membrane potentials of approximately -20 mV. As a result of this voltage-dependence, dopamine induces a region of negative resistance in the current-voltage relationship of the neurone. 3. The dopaminergic agonists apomorphine, bromocriptine, ergometrine and A-6,7-DTN mimic the action of dopamine on this neurone, all having a similar voltage-dependence to that of dopamine. The selective D-1 receptor agonist SK&F82526 and the D-2 agonist LY 171555, however, were both inactive on the preparation. 4. Responses to dopamine were suppressed by a number of D-1 and D-2 receptor antagonists, indicating that the pharmacological profile of the dopamine-sensitive receptor in this insect preparation is different from that of vertebrate dopamine receptors.

Animals↗
PubMed1991

The dopamine and 5-hydroxytryptamine content of locust and cockroach salivary neurones.

The salivary glands of the cockroach and locust are innervated primarily from two pairs of motoneurones, designated SN1 and SN2, in the suboesophageal ganglion. Intracellular cobalt fills and subsequent silver intensification were used to reveal the morphology of these cells in both species. Fluorescent microscopy, following treatment of the ganglion with glyoxylic acid, showed that in both species only the SN1 neurones contained catecholamines. A radioenzymatic assay for dopamine, performed on the locust SN1 neurones, confirmed that this catecholamine was present. A radioenzymatic assay for 5-hydroxytryptamine (5-HT), performed on both pairs of salivary neurones in the locust, revealed small quantities of this amine in the SN2 neurones, but no significant amount in the SN1 neurones. In the cockroach, 5-HT was assayed in the SN2 neurones only. In contrast to the locust, however, the 5-HT content of these cells was not significantly above that of control cells taken from other ganglia. These observations demonstrate that only the SN1 neurones are the source of the catecholaminergic fibres investing the locust and cockroach salivary glands. The difference in neurotransmitter content between the SN1 and SN2 neurones suggests that these neurones have separate functions in the control of salivary secretion.

Animals↗
PubMed1990

Anti-GABA antibodies label a subpopulation of chemical synapses which modulate an electrical synapse in crayfish.

Antibodies raised against gamma-aminobutyric acid (GABA) were used to stain sections from the crayfish abdominal nervous system, and the sections were examined under the electron microscope using a protein-A/gold conjugate secondary label. Sections were taken through the third ganglionic root, and through the interganglionic connective at the base of the third root posterior to the ganglia. The third root contains two very large motor axons, a non-GABAergic excitor (Motor Giant; MoG), and a GABAergic inhibitor (Flexor Inhibitor; FI). Only one of the two large axons stained positively for GABA, confirming that the antibody has high specificity for GABAergic neurones. The MoG is driven by powerful electrical synapses from the giant fibres, but also receives inhibitory chemical synaptic input which can gate the excitatory input. There is no physiological evidence for any other form of chemical input. However, at the ultrastructural level, the MoG is postsynaptic to three types of chemical profiles; SE-type containing round agranular vesicles, SI-type containing pleomorphic vesicles, and SM-type containing a mixture of round agranular and dense-cored vesicles. There is a highly differentiated staining pattern of these three synaptic types. Only the SI-type profiles stain positively with the GABA antibody, while the SE- and SM-type do not show significant staining. This suggests that the MoG can under some circumstances receive chemical input other than GABAergic inhibitory input. These other types of input have yet to be physiologically identified.

Animals↗
PubMed1989

Post-embryonic development of rectifying electrical synapses in the crayfish: ultrastructure.

The post-embryonic development of the rectifying Giant Fibre-Motor Giant (GF-MoG) synapse and the Giant Fibre-Segmental Giant (GF-SG) synapse has been investigated using electron-microscopy. In adults, the MoG and SG neurons make contact with the GFs by sending narrow 'finger-like' processes through the glial and connective tissue sheath surrounding each GF. The junctional region is characterized by closely apposed membranes (approximately 4 nm separation) traversed by regularly spaced connections, and large (60-80 nm) spherical vesicles in the presynaptic fibre. In newly hatched crayfish junctional contact is made over extensive areas of flat membrane apposition, due to the absence of a thick connective sheath around the giant fibres. Initially the junctional region is dominated by contacts which are morphologically indistinguishable from chemical synapses, i.e. 1. The apposed membranes are separated by a cleft of approximately 20-30 nm (an order of magnitude larger than the cleft distance at electrotonic synapses). 2. There is pre- and post-synaptic thickening of the junctional membranes with a dense cytoplasmic material. 3. Small (25-40 nm) pleomorphic vesicles are found on the presynaptic side of the junction, commonly in association with a dense presynaptic bar. Regions of junctional contact displaying the adult electronic-type morphology first appear at approximately one week post-hatching. At this age they are limited in distribution and occupy a central position in the area of contact surrounded by a broad 'chemical-like' annulus. During subsequent development these sites with electrotonic-type morphology grow in relative size, so that the 'chemical-like' sites become compressed towards the edges of the regions of contact. The adult type of morphology, in which the 'chemical-like' regions are vestigial, is achieved approximately two months after hatching.

Animals↗
PubMed1989

Ionic currents in the soma of an identified cockroach motoneurone recorded under voltage-clamp.

1. Membrane currents have been recorded from the soma of a bifunctional basalar/coxal depressor motoneurone in the metathoracic ganglion of the cockroach (Periplaneta americana) using a two-electrode voltage-clamp technique. 2. This motoneurone cell body is normally inexcitable when studied under current-clamp. Appropriate depolarizing command steps evoke rapid transient outward currents and late outward currents. 3. Late outward currents are dominated by a Ca-dependent component that confers an N-shaped I-V relationship on the neurone. 4. The Ca-dependent outward current is suppressed by Cd2+ (1 mM), Mn2+ (5 mM) or verapamil (50 microM). 5. Externally applied tetraethylammonium ions (TEA+) (25 mM) block the Ca-dependent current, but also appear to suppress a component of the late outward current that is independent of Ca2+. 6. Aminopyridines cause only minor suppression of late outward currents, but shift the peak in the N-shaped I-V relationship to more negative potentials. 7. The reversal potential of tail currents recorded following pre-pulses to +50 mV were dependent upon the pre-pulse duration; increasing the duration from 10 to 50 msec caused a +17 mV shift in tail current reversal potential. 8. A five-fold increase in the K+ concentration of the solution bathing the preparation only produced small and inconsistent changes in the reversal potential of tail currents. 9. Five-fold reduction in external Cl- caused no change. 10. The dependence of tail current reversal potential upon pre-pulse duration and the limited effect of alterations in the composition of the bathing solution are discussed in the context of restricted ion movements near the external surface of the cell membrane.

Aminopyridines↗
PubMed1987

Conduction block silences parts of a chemical synapse in the leech central nervous system.

1. The pressure (P) sensory neurones innervating the ventral skin of the medicinal leech have receptive fields comprising a central region of skin innervated by two thicker axons and two neighbouring regions innervated by two thinner axons. Impulses originating in the thinner axons may fail to propagate through the central ganglion, apparently blocked at the branch point of large and small axons. 2. The P neurone excites the longitudinal (L) motoneurone, and blocked impulses originating in the anterior fine axon produce e.p.s.p.s that are less than one-half normal amplitude. Blocked impulses in the posterior fine axon are typically ineffective. 3. The branches of P and L neurones, marked with intracellularly injected horseradish peroxidase or with Lucifer Yellow, make synaptic contact at up to sixty-six sites within the neuropile. Of P neurone branches emerging from two fine axons, those from the posterior axon make fewer contacts, usually one or two at most, while branches from the anterior axon represent no more than half the total contacts. From cell to cell there is some variation in the total number of contacts, the distribution of branches, and the strength of transmission. 4. The locations of contacts measured morphologically correlate well with their distributions as predicted from reductions in e.p.s.p. amplitude during conduction block.

Animals↗
PubMed1984

The versatile synapse.

'Typically' chemical synaptic transmission takes place when an influx of calcium ions during a presynaptic nerve impulse triggers exocytosis of neurotransmitter substance from synaptic vesicles. The neurotransmitter diffuses across the synaptic cleft and occupies receptors embedded in the subsynaptic membrane. This interaction (directly or via a second messenger) operates characteristic ion channels and produces an increase in the postsynaptic membrane permeability to particular ions. Depending on the ionic species to which the postsynaptic membrane becomes more permeable, the physiological response will be an excitatory or an inhibitory postsynaptic potential. The action of neurotransmitters may be terminated either by enzymic inactivation or by cellular uptake mechanisms. Over the last decade it has become clear that a neurotransmitter substance may exert a number of different actions on a single postsynaptic neurone. These may involve opening or closure of either voltage-independent or voltage-dependent ion channels. It is also possible that in some instances transmitters may act on neuronal biochemical systems to modify the physiology of postsynaptic cells without directly altering their electrical characteristics. Analysis of the postsynaptic actions of neurotransmitter substances has become further complicated by the increasing body of evidence which indicates that more than one transmitter substance (one of which may be a peptide) can be released from a single presynaptic neurone. The significance of such dual transmitter systems has yet to be fully elucidated. The efficacy of transmission across many synapses may be modified by either presynaptic or postsynaptic mechanisms; both transmitter release and postsynaptic responsiveness may depend on the recent history of a single synapse, on synaptic inputs from other neurones or on circulating neuroactive substances.

Animals↗
PubMed1979

Intracellular citrate or externaly applied tetraethylammonium ions produce calcium-dependent action potentials in an insect motoneurone cell body.

1. Electrophysiological observations have been made upon the cell body of an identified motoneurone of the cockroach, Periplaneta americana. Normal responses were compared with those observed after intracellular injection of citrate anions or when the preparation was bathed in solutions containing tetraethylammonium ions (TEA+).2. Normally when depolarized, the motoneurone soma gave a series of damped oscillations; the amplitude of these responses increased with increase in the applied current.3. After citrate ions had been injected into the neurone soma, all-or-none action potentials were evoked by depolarization; such responses appeared about 5-10 min after the onset of citrate injection. Injection of EGTA produced similar effects. Citrate and EGTA probably produce their effect through a reduction in the intracellular free calcium concentration.4. When preparations were bathed in saline solution containing 50 mM-TEA+, soma depolarization produced prolonged all-or-none action potentials (up to approximately 100 msec duration).5. The action potentials produced by citrate injection or externally applied TEA+ appeared to have a similar ionic mechanism; they were not depressed by sodium-free solutions or by tetrodotoxin (4 x 10(-6)M) but were reversibly blocked in saline solution containing 40 mM-manganous chloride.6. The overshoot amplitude of action potentials recorded after injection of citrate anions or in solutions containing TEA+ showed a 22.5 mV change for a ten-fold change in the external calcium concentration.7. Both intracellular citrate and external TEA+ caused a significant increase in the input resistance and membrane time constant of the motoneurone.8. It is concluded that action potentials generated under various experimental conditions in the soma of this insect motoneurone map have differing ionic mechanisms.

Action Potentials↗
PubMed1975

The ionic dependence of action potentials induced by colchicine in an insect motoneurone cell body.

1. The ionic requirements of the action potential recorded in the cell of an identified cockroach (Periplaneta americana) motoneurone following pre-treatment of the animal with colchicine have been studied. 2. Small cubes of gelatin containing 1% colchicine were implanted into one metathoracic leg near to the nerve trunk containing the axon of the identified motoneurone. 3. Electrophysiological experiments were performed 4--10 days after this treatment, when action potentials which frequently overshoot zero potential can be recorded from the cell body. Such action potentials are not normally seen in untreated animals. 4. Sodium-free solution reversibly abolished the action potential within 5 min. 5. Tetrodotoxin (10(-8)M) reversibly depressed the action potential. It was totally abolished by 10(-7)M tetrodotoxin, but this effect was not reversible. 6. Saline solution containing 40 mM manganous chloride either had no effect on the action potential amplitude, or caused a slight increase. It also caused prolongation of the falling phase and loss of the after-hyperpolarization. These effects were all reversible. 7. It is concluded that sodium carries a major proportion of the inward current of the action potential in this neurone. Some calcium probably enters also, and may, at least in part, be responsible for triggering the delayed rise in potassium conductance during the action potential.

Action Potentials↗