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N Dale

Publications and source records attributed to N Dale.

At least 37 records · Page 2Linked to original sources
PubMed1995

Experimentally derived model for the locomotor pattern generator in the Xenopus embryo.

1. Simulations of Xenopus embryo spinal neurons were endowed with Hodgkin-Huxley-style models of voltage-dependent Na+, Ca2+, slow K+ and fast K+ currents together with a Na(+)-dependent K+ current. The parameters describing the activation, inactivation and relaxation of these currents were derived from previous voltage-clamp studies of Xenopus embryo spinal neurons. Each of the currents was present at realistic densities. 2. The model neurons fired repetitively in response to current injection. The Ca2+ current was essential for repetitive firing in response to current injection. The fast K+ current appeared mainly to control spike width, whereas the slow K+ current exerted a powerful influence on the reptitive firing properties of the neurons without markedly affecting spike width. 3. The properties of the model neurons could be made more consistent with those previously reported for Xenopus embryo neurons during intracellular recordings in vivo, if the shunting effect of the sharp microelectrode was incorporated into the model. 4. The model neurons were then used to create a simplified version of the spinal network that controls swimming in the frog embryo. This model network could generate the motor pattern for swimming: the activity between the left and right sides alternated with a cycle period that varied from 50 to 120 ms. This is very similar to the range of cycle periods observed in the real embryo. The shunting effect of the microelectrode was once again taken into account. 5. Reductions of the K+ currents perturbed the motor pattern and gave three forms of aberrant motor activity very similar to those previously seen during the application of K+ channel blockers to the real embryo. The ability to generate the correct motor pattern for swimming in the model depended on the balance between the K+ currents and the inward Na+ and Ca2+ currents rather than their absolute values. 6. The model network could generate a motor pattern for swimming over a very wide range of excitatory (2-10 nS) and inhibitory (2-400 nS) synaptic strengths. Rough estimates of the physiological synaptic strengths in the real circuit (around 20-60 nS for inhibition and 2-5 nS for excitation) fall within the range of synaptic strengths that gave simulation of the swimming motor pattern in the model. 7. The cycle period of the motor activity in the model shortened either as the excitatory synapses were strengthened or as the inhibitory synapses were weakened. 8. The prediction that the strength of the mid-cycle inhibition determines cycle period has been tested by using low levels of strychnine to reduce glycinergic reciprocal inhibition in a graded manner in the real embryo. As the inhibition was reduced, the cycle period of fictive swimming in the embryo shortened by amounts very close to those predicted by the model. 9. This new experimentally derived model can replicate many of the known features of fictive swimming in the real embryo and may be of value as an analytical tool in attempting to understand how the spinal circuitry of the Xenopus embryo and related amphibian embryos control a variety of motor behaviours.

Animals↗
PubMed1995

A slowly activating Ca(2+)-dependent K+ current that plays a role in termination of swimming in Xenopus embryos.

1. Acutely isolated Xenopus spinal neurons possess a slowly activating Ca(2+)-dependent outward current which was revealed either by removal of external Ca2+ or by the addition of the Ca2+ channel blocker, 150 microM Cd2+. 2. The Ca(2+)-sensitive current was very slow to activate and had a mean time constant of activation of 437 ms at 0 mV. The current also had very long tail currents which were blocked by Cd2+. The rate of decay of the slowest component of the Ca(2+)-dependent tail currents was insensitive to membrane potential suggesting that the relaxation of the Ca(2+)-dependent current may only be weakly voltage dependent. 3. The reversal potential of the Ca(2+)-sensitive tail currents depended on the concentration of external K+ in a manner predicted by the Nernst equation. Thus the Ca(2+)-sensitive current was carried by K+. 4. The toxin apamin (10 nM to 2 microM) selectively blocked the Ca(2+)-dependent K+ current without affecting voltage-gated K+ currents. This current may be analogous to a small-conductance Ca(2+)-dependent K+ (SK) current; however, unlike some SK currents, the Ca(2+)-dependent K+ current was also sensitive to 500 microM tetraethylammonium chloride (TEA). 5. Applications of 10 nM apamin to spinalized embryos did not perturb the motor pattern for swimming. However, the cycle periods over which the locomotor rhythm generator could generate appropriate motor activity were lengthened by about 10% and the mean duration of swimming episodes was increased by approximately 40%. 6. We therefore propose that the Ca(2+)-dependent K+ current plays an important role in the self-termination of motor activity.

Animals↗
PubMed1994

A role for potassium currents in the generation of the swimming motor pattern of Xenopus embryos.

1. To assess the role that K+ currents play in the production of the swimming motor pattern in the Xenopus embryo, we have used low doses of the K+ channel blockers, 3,4-diaminopyridine (3,4-DAP; 25-100 microM) and tetraethylammonium (TEA; 500 microM), to reduce K+ currents and investigated the effects on motor output. 2. To confirm that 3,4-DAP and TEA block K+ currents and characterize their actions, we made whole-cell voltage-clamp recordings from acutely isolated spinal neurons. Both 25-100 microM 3,4-DAP and 100-500 microM TEA blocked the sustained K+ current in a dose-dependent manner. 3. Because TEA can block acetylcholine nicotinic receptors on autonomic ganglia, and nicotinic acetylcholine receptors have recently been shown to be present on Xenopus spinal neurons, we have tested both 3,4-DAP and TEA for antagonist action against the nicotinic agonist 1,1-dimethyl-4-phenylpiperazinium (DMPP). Although 500 microM TEA blocked the DMPP-induced depolarization, 25 microM 3,4-DAP did not. 4. In the intact embryo, application of 25-100 microM 3,4-DAP or 500 microM TEA disrupted both the left and right alternation of ventral root discharge and the motor pattern recorded intracellularly from spinal neurons during swimming. Both blockers allowed the firing of an extra action potential at midcycle, which led to a number of different patterns. These patterns were categorized as follows: type A, cycles with midcycle action potentials; type B, the simultaneous firing of neurons on both sides of the cord; and type C, in which one side was active, whereas the other side was inhibited. In both 3,4-DAP and TEA these abnormalities tended to occur at the beginning of swimming episodes. Both blockers also caused a significant increase in the cycle period. Because both 3,4-DAP and TEA produced very similar affects to the motor pattern, we conclude that the perturbations are probably a result of reducing K+ current amplitude. 5. To investigate whether 3,4-DAP and TEA were producing disruptions in the motor pattern by increasing synaptic drive through the broadening of action potentials, we made measurements of spike width, tonic depolarization, and midcycle inhibitory postsynaptic potential (IPSP) amplitude during swimming. Both 3,4-DAP and TEA caused significant but modest spike broadening (20.8 and 29.8%, respectively); however, their effects on tonic depolarization were inconsistent although both blockers increased midcycle IPSP amplitude. 6. To test whether a reduction in K+ currents could plausibly produce the specific motor pattern perturbations that were seen, we have made computer simulations of simplified spinal networks.(ABSTRACT TRUNCATED AT 400 WORDS)

Action Potentials↗
PubMed1994

Nutrient content of tuna meal.

Tuna meal is distinct from many other types of fish meal in that fillets have previously been removed for human consumption. Three samples of tuna meal were found to have less protein but a proportionately higher mineral content than reference values for meals made from whole fish. The metabolizable energy content of tuna meal was lower than values reported for most common types of fish meal.

Amino Acids↗
PubMed1994

GABAB receptors modulate an omega-conotoxin-sensitive calcium current that is required for synaptic transmission in the Xenopus embryo spinal cord.

Activation of GABAB receptors in the Xenopus embryo, a simple vertebrate, causes presynaptic inhibition of transmitter release from glycinergic spinal neurons and an increase in action potential threshold. To investigate the underlying mechanisms of GABAB receptor action, we have made whole-cell voltage-clamp recordings from acutely isolated Xenopus embryo spinal neurons. The GABAB receptor agonist baclofen caused a reversible reduction in the amplitude of Ca2+ currents. This reduction of Ca2+ currents appeared to be voltage dependent as it was removed at very positive potentials. Since the specific GABAB antagonists CGP35348, phaclofen, and 2-hydroxysaclofen all blocked the reduction in Ca2+ currents, we concluded that the modulation of the Ca2+ current was mediated by GABAB receptors. We have investigated the pharmacological identity of the Ca2+ current modulated by baclofen using the selective blocker omega-conotoxin, fraction GVIA (omega-CgTX). omega-CgTX selectively blocked voltage-gated Ca2+ currents without affecting the voltage-gated Na+ current. omega-CgTX substantially occluded the action of baclofen, suggesting that GABAB receptors modulate an omega-CgTX-sensitive Ca2+ current. Since GABAB receptors mediate presynaptic inhibition, we have studied the involvement of the omega-CgTX-sensitive Ca2+ current in synaptic transmission in the intact spinal cord. Inhibitory interneuron axons were stimulated to evoke monosynaptic IPSPs in motoneurons, and recorded intracellularly. Since omega-CgTX blocked inhibitory transmission, we concluded that the omega-CgTX-sensitive Ca2+ current plays an essential role in transmitter release. If modulation of this current were to occur in nerve terminals, it could contribute to the GABAB receptor-mediated presynaptic inhibition of transmitter release.(ABSTRACT TRUNCATED AT 250 WORDS)

Action Potentials↗
PubMed1993

L-glutamate may be the fast excitatory transmitter of Aplysia sensory neurons.

Although modulation of synaptic transmission between Aplysia mechanosensory and motor neurons has been an important model for processes thought to underlie simple forms of learning and memory, the nature of the fast excitatory transmitter utilized by the sensory neurons has remained obscure. To identify the sensory neuron transmitter, we first examined the detailed properties of the synaptic response evoked in motor neurons cocultured with pleural sensory neurons. The excitatory postsynaptic current had a nonlinear current-voltage relation with a reversal potential between 0 and 10 mV and a plateau region between -40 and -70 mV. When the concentration of Mg2+ in the artificial sea water was lowered to 5 mM, the current-voltage relation of the excitatory postsynaptic current became linear, suggesting that Mg2+ blocks the postsynaptic receptor in a voltage-dependent manner. After screening a variety of small molecules, we found that L-glutamate could mimic the actions of the sensory neuron transmitter: responses to L-glutamate also had a reversal potential between 0 and 10 mV and a nonlinear current-voltage relation that could be made linear by lowering external Mg2+. To demonstrate further similarity of action between L-glutamate and the endogenous transmitter, we utilized four antagonists (kynurenate, 6,7-dinitroquinoxaline-2,3-dione, D-aspartate, and D-glutamate) to block in a dose-dependent manner the actions of L-glutamate and the natural transmitter. We therefore suggest that the sensory neurons use a glutamate-like transmitter and favor L-glutamate itself, because no other naturally occurring amino acid that we have studied has had similar actions. As the postsynaptic receptor for the sensory neuron transmitter is weakly blocked in a voltage-dependent manner by Mg2+, the excitatory receptors innervated by the Aplysia sensory neuron may represent a distant precursor of the vertebrate N-methyl-D-aspartate receptor.

Animals↗
PubMed1993

A large, sustained Na(+)- and voltage-dependent K+ current in spinal neurons of the frog embryo.

1. Neurons from the Xenopus embryo spinal cord were dissociated and conventional patch clamp techniques were used to record the whole-cell currents in the presence of tetrodotoxin (TTX). 2. The outward currents of the acutely isolated spinal neurons were rapidly reduced to about half their control value by substitution of extracellular Na+ with N-methyl-D-glucamine, lysine or choline. 3. The use of Li+ as a Na+ substitute partially reduced the outward currents. 4. The reversal potential of the Na(+)-sensitive current was close to the K+ equilibrium potential and could be altered by changing extracellular K+. The Na(+)-sensitive current was therefore a K+ current. 5. The Na(+)-sensitive K+ current was voltage dependent and activated in a sustained manner and appeared very similar to the delayed rectifier present in these neurons. 6. While the Na(+)-sensitive current increased with voltage as might be expected for an outward current, at very positive potentials it progressively decreased in amplitude. The voltage range over which this decrease was present moved closer to zero as the levels of intracellular Na+ were increased. The tail currents evoked by positive test potentials did not correspondingly decrease in amplitude, suggesting that channel block was rapidly relieved by stepping back to the holding potential. 7. Intracellular perfusion of the patch pipette with solutions containing varying amounts of Na+ (0-20 mM) showed that the K+ currents could be increased in a dose-dependent manner by raising intracellular Na+. The current had an EC50 for Na+ of 7.3 mM and a Hill coefficient of 4.6. 8. Single channel recordings from isolated inside-out patches revealed a channel that gated more frequently when the bathing levels of Na+ were elevated from 3 to 12 or 50 mM. Xenopus spinal cord neurons therefore possess a current that is not only voltage dependent but is also sensitive to internal Na+. 9. Xenopus spinal neurons possess a transient Na+ current (blocked by the inclusion of TTX) and a leak channel permeable to Na+. The inward leakage of Na+ appeared to provide the Na+ necessary for the gating of the Na(+)-dependent channel. 10. Blocking the Na(+)-K+ exchange pumps by removing extracellular K+, reduced the effect of removal of external Na+, suggesting that the Na(+)-K+ exchange pumps could be important in controlling the submembrane Na+.(ABSTRACT TRUNCATED AT 400 WORDS)

Animals↗
PubMed1993

GABAB receptors modulate glycinergic inhibition and spike threshold in Xenopus embryo spinal neurones.

1. The actions of GABAB receptors in the generation of the neuronal pattern underlying swimming in the Xenopus embryo have been investigated using the agonist baclofen. 2. Baclofen (10-100 microM) greatly reduced the length of swimming episodes and ventral root spike amplitude in a reversible manner. These effects were blocked by CGP 35348 (200-300 microm) and hydroxysaclofen (200-300 microM). 3. Baclofen (10-100 microM) reduced the amplitude of glycinergic IPSPs in motoneurones during fictive swimming. 4. Strychnine-sensitive spontaneous miniature inhibitory postsynaptic potentials (mIPSPs) were recorded from motoneurones. While baclofen (10-100 microM) had no effect on the amplitude of the mIPSPs it greatly decreased their frequency of occurrence. 5. GABAB receptors may therefore be present on the terminals of commissural interneurones, the only glycinergic neurones in the Xenopus embryo's nervous system, and act to reduce neurotransmitter release. 6. Baclofen reduced the reliability of action potential firing in motoneurones during fictive swimming without an apparent effect on excitation. 7. Baclofen increased the threshold to action potential firing in response to the injection of depolarizing current in motoneurones. 8. The current-voltage relationships of motoneurones were investigated. Baclofen (10-100 microM) did not change the resting membrane potential, slope conductance or the membrane rectification.

Action Potentials↗
PubMed1992

Roles of PKA and PKC in facilitation of evoked and spontaneous transmitter release at depressed and nondepressed synapses in Aplysia sensory neurons.

Two second messenger pathways, one that uses the cAMP-dependent protein kinase A (PKA), the other that uses protein kinase C (PKC), have been found to contribute to the short-term presynaptic facilitation of the connections between the sensory neurons in Aplysia and their target cells, the interneurons and motor neurons of the gill-withdrawal reflex. To study their relative contributions as a function of the previous history of the neuron's activity, we have examined the effects of inhibiting PKA (using Rp-cAMPS) and PKC (using H7) on the short-term facilitation of spontaneous release as well as of the evoked release induced by serotonin at nondepressed, partially depressed, and highly depressed synapses. Our results suggest that whereas activation of PKA is sufficient to trigger the facilitation of nondepressed synapses, activation of both PKA and PKC is required to facilitate depressed synapses, with the contribution of PKC becoming progressively more important as synaptic transmission becomes more depressed.

1-(5-Isoquinolinesulfonyl)-2-Methylpiperazine↗
PubMed1990

Contributions of two types of calcium channels to synaptic transmission and plasticity.

In Aplysia sensory and motor neurons in culture, the contributions of the major classes of calcium current can be selectively examined while transmitter release and its modulation are examined. A slowly inactivating, dihydropyridine-sensitive calcium current does not contribute either to normal synaptic transmission or to any of three different forms of plasticity: presynaptic inhibition, homosynaptic depression, and presynaptic facilitation. This current does contribute, however, to a fourth form of plasticity--modulation of transmitter release by tonic depolarization of the sensory neuron. By contrast, a second calcium current, which is rapidly inactivating and dihydropyridine-insensitive, contributes to release elicited by the transient depolarization of an action potential and to the other three forms of plasticity.

Action Potentials↗
PubMed1990

Three types of GABA-immunoreactive cells in the lamprey spinal cord.

Polyclonal antisera raised against conjugated GABA were used to study the distribution of GABAergic neurons in the spinal cords of lampreys (Lampetra fluviatilis and Ichtyomyzon unicuspis) using immunofluorescence and peroxidase-antiperoxidase techniques. Three morphologically distinct types of GABA-immunoreactive (GABA-ir) cell bodies were observed, multipolar neurons in the lateral grey cell column, apparently bipolar cells in the ventral aspect of the dorsal horn, and small liquor-contacting cells surrounding the central canal. A high density of immunoreactive fibers of spinal origin were present in the lateral and ventral funiculi, whereas the dorsal column had a relatively low density. Dense GABA-ir plexuses were situated in the lateral spinal margin, and in the dorsal part of the dorsal horn. A chronic lesion of the rostral spinal cord did not result in any observable loss of GABA-ir fibers below or above the lesion, suggesting that the 3 types of segmental GABA-ir neurons are the main sources of the GABAergic innervation of the lamprey spinal cord.

Animals↗
PubMed1990

Second messengers involved in the two processes of presynaptic facilitation that contribute to sensitization and dishabituation in Aplysia sensory neurons.

Presynaptic facilitation of transmitter release contributes to behavioral sensitization and dishabituation, two simple forms of learning in Aplysia. This enhancement of transmitter release can be simulated by the facilitatory transmitter serotonin and has been shown to result from two types of mechanisms. The first facilitating process involves broadening of the presynaptic action potential in the sensory neurons of the reflex and is maximally effective when the synapse has not been depressed by repeated stimulation, as during sensitization. The second process is independent of changes in spike duration and can enhance release even when the synapse is quite depressed, as during dishabituation. Earlier work suggests that the first process is mediated by an increase in the intracellular level of cyclic AMP in the sensory neurons. We show here that release of free cyclic AMP from a photolyzable analogue introduced into sensory neurons can enhance release even at depressed synapses, indicating that cyclic AMP can activate the second as well as the first process. In addition, we find that phorbol esters, activators of protein kinase C, enhance release at depressed synapses. This is consistent with the report in the accompanying paper [Sacktor, T. C. and Schwartz, J. H. (1990) Proc. Natl. Acad. Sci. USA 87, 2036-2039] that serotonin and sensitizing stimuli translocate protein kinase C from cytoplasm to membrane. Our findings suggest that the cyclic AMP-dependent phosphorylation system can mediate more than one facilitatory process and that both cyclic AMP-dependent kinase and protein kinase C may be involved in facilitation of depressed synapses.

1-(5-Isoquinolinesulfonyl)-2-Methylpiperazine↗
PubMed1990

Facilitatory and inhibitory transmitters modulate spontaneous transmitter release at cultured Aplysia sensorimotor synapses.

1. The monoamine transmitter 5-hydroxytryptamine (5-HT) and the peptide Phe-Met-Arg-Phe-amide (FMRFa), which appear to contribute to presynaptic facilitation and inhibition of the sensorimotor synapse in the abdominal ganglion of Aplysia, can modulate the frequency of spontaneous transmitter release at synapses formed between dissociated Aplysia sensory neurones and motoneurones in vitro. 2. 5-HT caused a decrease in the mean time interval between consecutive miniature EPSPs (mEPSPs), while FMRFa, applied either by itself or together with 5-HT, caused an increase in the mean time interval between consecutive mEPSPs. 3. Depolarization of the presynaptic neurone caused a decrease in the mean time interval between consecutive mEPSPs. This modulation required external Ca2+. 4. 5-HT and FMRFa were able to modulate spontaneous release when applied in saline solutions lacking Ca2+ and containing Ca2(+)-chelating agents, suggesting that the modulation of spontaneous release by 5-HT and FMRFa did not require a Ca2+ influx. Similarly, spontaneous release could still be modulated by 5-HT and FMRFa in saline solutions containing 1 mM-Cd2+, which blocked both the voltage-gated Ca2+ channels and the evoked transmitter release. 5. To prevent a rise in intracellular Ca2+, we buffered the concentration of Ca2+ in the presynaptic terminals by injecting into the sensory neurone the Ca2+ chelator 1,2-bis(o-aminophenoxy)ethane-N,N,N',N'-tetraacetic acid (BAPTA). The injection of BAPTA blocked evoked transmitter release, suggesting that it acted as an effective buffer of Ca2+ in the terminals. However, spontaneous release could still be observed and was still modulated by 5-HT and FMRFa. This suggests that the modulation of spontaneous release does not require an elevation of intracellular Ca2+. 6. We propose that 5-HT and FMRFa can modulate the rate of spontaneous release directly by mechanisms that do not require changes in the intracellular concentration of Ca2+. These mechanisms might contribute an additional component to the presynaptic inhibition and facilitation of evoked transmitter release.

Action Potentials↗
PubMed1989

Pretend play with mothers and siblings: relations between early performance and partners.

Although the role of the mother in assisting early pretend play has been established in other studies, little is known about the sibling's contribution. Using naturalistic observational methodology, this study investigates whether 2-year-old infants' co-operative pretend play varies when playing with their mother and 4-5-year-old sibling. The results suggest marked differences in frequency of production of various transformational types in each social setting, and one type of play only appears in games with the sibling. Possible "facilitation" by the play partner is discussed. The classificatory limitation of existing normative developmental schedules of early pretending is noted.

Child, Preschool↗
PubMed1988

Long-term facilitation in Aplysia involves increase in transmitter release.

In a variety of vertebrates and invertebrates, long-lasting enhancement of synaptic transmission contributes to the storage of memory lasting one or more days. However, it has not been demonstrated directly whether this increase in synaptic transmission is caused by an enhancement of transmitter release or an increase in the sensitivity of the postsynaptic receptors. These possibilities can be distinguished by a quantal analysis in which the size of the miniature excitatory postsynaptic potential released spontaneously from the presynaptic terminal is used as a reference. By means of microcultures, in which single sensory and motor neurons of Aplysia were plated together, miniature excitatory postsynaptic potentials attributable to the spontaneous release of single transmitter quanta from individual presynaptic neurons were recorded and used to analyze long-term facilitation induced by repeated applications of 5-hydroxytryptamine. The results indicate that the facilitation is caused by an increase in the number of transmitter quanta released by the presynaptic neuron.

Animals↗
PubMed1988

Proteinuria and renal function in diabetic patients fed a diet moderately restricted in protein.

Protein restriction has been used in the treatment of renal disease and may also be beneficial in the management of diabetic nephropathy. We evaluated the effects of moderate protein restriction (0.6 g/kg ideal body weight per day) for a 3-mo period on renal function in seven diabetic patients. Moderate protein restriction led to a decrease of approximately 50% in the albumin excretion rate in patients with overt proteinuria or microalbuminuria. This decrease occurred in some patients without a decrease in glomerular filtration rate, renal plasma flow, or plasma albumin concentration and may reflect subtle changes in filtration properties or permeability of glomeruli. In this pilot study moderate protein restriction has marked effects on albumin excretion irrespective of the initial degree of renal impairment. It is therefore suitable for longer-term study of its effects on the progression of renal disease in both patients with overt and incipient diabetic nephropathy.

Adult↗
PubMed1988

Development and characterization of commissural interneurones in the spinal cord of Xenopus laevis embryos revealed by antibodies to glycine.

By using an antibody to glutaraldehyde fixation products of glycine we have been able to observe the development of a defined population of spinal interneurones in the CNS of Xenopus laevis embryos. The first glycine immunoreactive (GLY) somata appeared at stage 22 in the caudal hindbrain within a few hours of neural tube closure. The population then increased by extending caudally into the spinal cord and by infill. It was followed up to the time of hatching, stage 37/38. By observing GLY cells at early stages in their differentiation, the normal sequence of cell process formation was deduced. A ventral axon is formed, extends dendrites laterally into the marginal zone and forms a commissure by growing through the ventral ependymal cell floor of the neural tube. On the opposite side, growth cones turn longitudinally and TEM observations show that they make en-passant synaptic contacts. All GLY cells have decussating axons and some grow secondary axons on the same side as the soma. To establish the identity of GLY cells, a detailed comparison was made with commissural and dorsolateral commissural interneurones defined by retrograde and intracellular HRP staining. The GLY cells are identified with the commissural interneurones which are known to serve a glycinergic reciprocal inhibitory function. By showing that these interneurones have a clearly defined group identity and programme of development, this study opens the way to further experiments on factors controlling spinal cord pathway determination.

Animals↗