Biomedical subjects

N Dale

Publications and source records attributed to N Dale.

At least 19 recordsLinked to original sources

PubMed1999

G-proteins are involved in 5-HT receptor-mediated modulation of N- and P/Q- but not T-type Ca2+ channels.

5-HT produces voltage-independent inhibition of the N-, P/Q-, and T-type Ca2+ currents in sensory neurons of Xenopus larvae by acting on 5-HT1A and 5-HT1D receptors. We have explored the underlying mechanisms further and found that the inhibition of high voltage-activated (HVA) currents by 5-HT is mediated by a pertussis toxin-sensitive G-protein that activates a diffusible second messenger. Although modulation of T-type currents is membrane-delimited, it was not affected by GDP-beta-S (2 mM), GTP-gamma-S (200 microM), 5'-guanylyl-imidodiphosphate tetralithium (200 microM), aluminum fluoride (AlF4-, 100 microM), or pertussis toxin, suggesting that a GTP-insensitive pathway was involved. To investigate the modulation of the T currents further, we synthesized peptides that were derived from conserved cytoplasmic regions of the rat 5-HT1A and 5-HT1D receptors. Although two peptides derived from the third cytoplasmic loop inhibited the HVA currents by activating G-proteins and occluded the modulation of HVA currents by 5-HT, two peptides from the second cytoplasmic loop and the C tail had no effect. None of the four receptor-derived peptides had any effect on the T-type currents. We conclude that 5-HT modulates T-type channels by a membrane-delimited pathway that does not involve G-proteins and is mediated by a functional domain of the receptor that is distinct from that which couples to G-proteins.

Amino Acid Sequence↗

PubMed1998

Delayed production of adenosine underlies temporal modulation of swimming in frog embryo.

1. To investigate the dynamics of adenosine production in the spinal cord during motor activity, and its possible contribution to the temporal modulation of motor patterns, a sensor sensitive to adenosine at concentrations as low as 10 nM was devised. 2. When pressed against the outside of the spinal cord, the sensor detected slow changes in the levels of adenosine during fictive swimming that ranged from 10 to 650 nM. In four embryos where particularly large signals were recorded due to favourable probe placement, the adenosine levels continued to rise for up to a minute following cessation of activity before slowly returning to baseline. In the remaining thirteen embryos, levels of adenosine started to return slowly to baseline almost immediately after activity had stopped. 3. Inhibitors of adenosine uptake increased the magnitude of the signal recorded and slowed the recovery following cessation of activity. 4. A realistic computational model of the spinal circuitry was combined with models of extracellular breakdown of ATP to adenosine. ATP and adenosine inhibited, as in the real embryo, the voltage-gated K+ and Ca2+ currents, respectively. The model reproduced the temporal run-down of motor activity seen in the real embryo suggesting that synaptic release of ATP together with its extracellular breakdown to adenosine is sufficient to exert time-dependent control over motor pattern generation. 5. The computational analysis also suggested that the delay in the rise of adenosine levels is likely to result from feed-forward inhibition of the 5'-ectonucleotidase in the spinal cord. This inhibition is a key determinant of the rate of run-down.

Adenosine↗

PubMed1998

Differential inhibition of N and P/Q Ca2+ currents by 5-HT1A and 5-HT1D receptors in spinal neurons of Xenopus larvae.

1. In whole-cell patch clamp recordings made from non-sensory neurons acutely isolated from the spinal cord of Xenopus (stage 40-42) larvae, two forms of inhibition of the high voltage-activated (HVA) Ca2+ currents were produced by 5-HT. One was voltage dependent and associated with both slowing of the activation kinetics and shifting of the voltage dependence of the HVA currents. This inhibition was relieved by strong depolarizing prepulses. A second form of inhibition was neither associated with slowing of the activation kinetics nor relieved by depolarizing prepulses and was thus voltage independent. 2. In all neurons examined, 5-HT (1 microM) reversibly reduced 34 +/- 1.6 % (n = 102) of the HVA Ca2+ currents. In about 40 % of neurons, the inhibition was totally voltage independent. In another 5 %, the inhibition was totally voltage dependent. In the remaining neurons, inhibition was only partially (by around 40 %) relieved by a large depolarizing prepulse, suggesting that in these, the inhibition consisted of both voltage-dependent and -independent components. 3. By using selective channel blockers, we found that 5-HT acted on both N- and P/Q-type channels. However, whereas the inhibition of P/Q-type currents was only voltage independent, the inhibition of N-type currents had both voltage-dependent and -independent components. 4. The effects of 5-HT on HVA Ca2+ currents were mediated by 5-HT1A and 5-HT1D receptors. The 5-HT1A receptors not only preferentially caused voltage-independent inhibition, but did so by acting mainly on the omega-agatoxin-IVA-sensitive Ca2+ channels. In contrast, the 5-HT1D receptor produced both voltage-dependent and -independent inhibition and was preferentially coupled to omega-conotoxin-GVIA sensitive channels. This complexity of modulation may allow fine tuning of transmitter release and calcium signalling in the spinal circuitry of Xenopus larvae.

Animals↗

PubMed1998

Developmental changes in expression of ion currents accompany maturation of locomotor pattern in frog tadpoles.

1. The K+ currents of spinal neurons acutely dissociated from Xenopus larvae were studied and compared with those of neurons dissociated from Xenopus embryos. 2. The density of total outward current in the larval and embryonic neurons remained the same from stage 37/38 to stage 42. 3. Almost all neurons at stage 42 expressed a fast activating Ca2+-dependent K+ current (IKCa) that was largely absent from embryonic neurons. Whereas IKCa became larger and more prevalent during development, the delayed rectifier K+ currents were down-regulated. 4. About 53 % of IKCa was selectively blocked by iberiotoxin which had no effect on the delayed rectifier K+ currents or the K+ currents of embryonic neurons. 5. The firing properties of neurons isolated from embryos were unchanged by iberiotoxin. However, the toxin greatly increased the frequency of firing in larval neurons. 6. Iberiotoxin extended the duration of ventral root bursts during fictive swimming in larvae at stages 41 and 42 but had no effect at stage 40. The progressive expression of IKCa thus contributed to burst termination. 7. We have found that changes in expression of outward current closely correlate with the maturation of the motor pattern during development. At a time when the motor pattern has a need for a burst-terminating mechanism, the larval neurons express a channel with properties appropriate for such a role.

Aging↗

PubMed1998

The pharmacology and roles of two K+ channels in motor pattern generation in the Xenopus embryo.

The spinal neurons of the Xenopus embryo that participate in the swimming motor pattern possess two kinetically distinct sets of potassium currents: the fast IKf and sodium-dependent IKNa, which together constitute approximately 80% of the outward current; and the slow IKs, which constitutes the remainder. To study their respective roles in cell excitability and the swimming pattern, we have characterized their pharmacological properties. Catechol selectively blocked the fast potassium currents (IC50, approximately 10 microM). The block was voltage-dependent, with partial unblocking occurring at positive voltages. alpha-Dendrotoxin and dendrotoxin-I selectively blocked the slow potassium current. Catechol and the dendrotoxins had different effects on membrane excitability: catechol caused spike broadening but had little effect on repetitive firing, whereas both dendrotoxins markedly increased repetitive firing without affecting spike width. By applying these agents to the whole embryo, we tested the role of the fast and slow currents in motor pattern generation. Catechol had little effect on fictive swimming, suggesting that the fast K+ currents are not critical to circuit operation. However, dendrotoxin disrupted swimming early in the episode and increased the duration of ventral root bursts. The slow K+ current, which is a minor component of the total outward current, thus appears to play an important role in motor pattern generation.

4-Aminopyridine↗

PubMed1997

Serotonergic inhibition of the T-type and high voltage-activated Ca2+ currents in the primary sensory neurons of Xenopus larvae.

The primary sensory Rohon-Beard (R-B) neurons of Xenopus larvae are highly analogous to the C fibers of the mammalian pain pathway. We explored the actions of 5-HT by studying the modulation of Ca2+ currents. In approximately 80% of the acutely isolated R-B neurons, 5-HT inhibited the high voltage-activated (HVA) currents by 16% (n = 29) and the T-type currents by 24% (n = 41). The modulation of the T-type and the HVA currents was mimicked by selective 5-HT1A and 5-HT1D agonists: 8-OH-DPAT and L-694,247. The effects of the agonists were blocked by their respective 5-HT1A or 5-HT1D antagonists: p-MPPI and GR127935, suggesting that both 5-HT1A and 5-HT1D receptors were involved. Approximately 70% of the actions of 5-HT on HVA currents was occluded by omega-conotoxin-GVIA (N-type channel blocker), whereas the rest of the modulation ( approximately 30%) was occluded by <100 nM omega-agatoxin-TK (P/Q-type channel blocker). This suggests that 5-HT acts on N- and P/Q-type Ca2+ channels. Neither the modulation of the T-type nor that of the HVA currents was accompanied by changes in their voltage-dependent kinetics. Cell-attached patch-clamp recordings suggest that the modulation of the T-type channel occurs through a membrane-delimited second messenger. We have studied the functional consequences of the modulation of T-type Ca2+ channels and have found that these channels play a role in spike initiation in R-B neurons. Modulation of T-type channels by 5-HT therefore could modulate the sensitivity of this sensory pathway by increasing the thresholds of R-B neurons. This is a new and potentially important locus for modulation of sensory pathways in vertebrates.

Animals↗

PubMed1997

Ion channels and the control of swimming in the Xenopus embryo.

The Xenopus embryo has been well studied and the circuitry underlying motor pattern generation largely elucidated. We have extended this analysis by determining the roles of individual voltage- and ligand-gated ion channels in controlling the motor pattern for swimming and two mechanisms that control rundown of this pattern. Xenopus embryo spinal neurons possess at least six classes of ion channel: a fast Na+ channel; a mixture of kinetically similar Ca2+ channels; a fast K+ channel; a slow K+ channel; a Na(+)-dependent K+ channel; and a slowly activating Ca2(+)-dependent K+ channel. The roles of the voltage-gated currents in determining neuronal firing properties and operation of the locomotor circuitry have been examined both pharmacologically and in realistic computer simulations. Model neurons fire repetitively in response to current injection. The Ca2+ current seems essential for repetitive firing. The fast K+ current appears mainly to control spike width, whereas the slow K+ current exerts a powerful influence on repetitive firing. These predictions from the model have been confirmed by the use of specific pharmacological blockers of the fast and slow K+ currents. Both the model network and the real spinal locomotor circuit appear to tolerate a wide variation in the relative strengths of the component synapses but are very sensitive to the magnitudes of the voltage-gated currents. In particular the slow K+ current, despite being a small component of the total outward current, plays a critical role in stabilizing the motor pattern. Like many other rhythmic motor patterns, swimming in the Xenopus embryo is episodic; it undergoes run-down and self-termination even in the absence of sensory inputs. The slow Ca2(+)-dependent K+ current appears to play a role in the self-termination of swimming. However, intrinsic modulation mediated by the release of ATP and production of adenosine in the extracellular space appears to be a very powerful determinant of run-down of the motor pattern.

Animals↗

PubMed1997

Ionic currents, transmitters and models of motor pattern generators.

Recent work, combining direct study of ion channels and synapses with pharmacological manipulations and realistic computer simulations, has deepened our understanding of how motor circuits produce rhythmic outputs. In several preparations, both the roles of some key ionic currents in circuit operation and the mechanisms by which circuit operation may be modulated have been identified.

Animals↗

PubMed1996

Regulation of rhythmic movements by purinergic neurotransmitters in frog embryos.

Many rhythmic motor behaviours, including swimming, walking, scratching, swallowing, micturition and sexual climax, are episodic: even in the absence of sensory inputs they exhibit a gradual run-down in frequency before spontaneously terminating. We have investigated whether the purinergic transmitters, ATP and adenosine, control run- down of swimming in the Xenopus embryo. By using specific agonists and antagonists for the purinergic receptors, we have shown that ATP (or a related substance) is released during swimming and activates P2y receptors to reduce voltage-gated K+ currents and cause an increase in the excitability of the spinal motor circuits. Adenosine is also produced during motor activity, possibly through the actions of ectonucleotidases. The activation by adenosine of P1 receptors reduces the voltage-gated Ca(2+) currents, lowers excitability of the motor circuits, and so opposes the actions of ATP. A gradually changing balance between ATP and adenosine therefore seems to underlie the run-down of the motor pattern for swimming in Xenopus. We believe this to be the first time that ATP and adenosine have been found to be involved in motor pattern generation. The antagonistic interplay between these two transmitters may offer a general feedback mechanism that underlies run-down of all episodic motor patterns in vertebrates.

Adenosine↗

PubMed1996

Effect of capsaicin and analogues on potassium and calcium currents and vanilloid receptors in Xenopus embryo spinal neurones.

1. The potassium current in embryo spinal neurones of Xenopus consists of at least two kinetically distinct components with overlapping voltage-dependencies of activation. We investigated whether capsaicin might specifically block these components in acutely dissociated neurones from stage 37/38 embryos by use of standard patch clamp techniques. 2. Capsaicin caused a time-dependent block of both the slow and fast components of the potassium current. The concentration-dependence was described by the Hill equation with a KD of 21 microM and a coefficient of 1.5 (n = 9-11 at each concentration). Differences between the observed and fitted values were not significant at the 5% level (chi(2) = 2.80, 6 degrees of freedom). 3. Capsaicin did not affect the time course or voltage-sensitivity of activation, but the steady-state block was voltage-dependent. The block could be relieved by hyperpolarization, and the rate of the removal of block was voltage- and time-dependent. The time constant for the blocking reaction was also voltage-dependent for voltage steps below +30 mV, but above this level it was voltage-independent. These results suggest that capsaicin blocks potassium channels by an open channel mechanism. 4. Other derivatives of vanillin, such as capsazepine, resiniferatoxin, and piperine also blocked potassium channels. Capsazepine and resiniferatoxin caused a greater block than similar concentrations of capsaicin, and in the case of capsazepine, the block was also clearly time-dependent. 5. Capsaicin and capsazepine also blocked calcium currents in a time-dependent manner. Fitting the Hill equation to the averaged data gave a KD of 43.5 microM, and a coefficient of 1.35 (n = 11 at each concentration). The fitted values were not significantly different from the observed means at the 5% level (chi(2) = 12.1, 6 degrees of freedom). 6. Six out of 29 Rohon-Beard sensory neurones responded to capsaicin with an inward current that appeared to be similar to the capsaicin activation of mammalian C sensory neurones. This response saturated at 10 microM capsaicin.

Animals↗

PubMed1995

Kinetic characterization of the voltage-gated currents possessed by Xenopus embryo spinal neurons.

1. Using the whole-cell patch clamp technique, the voltage-gated currents of neurons acutely isolated from the Xenopus embryo spinal cord were studied. 2. The spinal neurons possessed a very fast Na+ current, which activated with time constants that ranged from 0.1 to 0.25 ms. It was also subject to rapid inactivation with time constants ranging from 0.3 to 8 ms. This current could only be fitted with Hodgkin-Huxley equations once the rapid inactivation that occurs by the time of the peak current had been taken into account. 3. Xenopus embryo neurons also possessed a mixture of kinetically similar Ca2+ currents, which activated with time constants that ranged from 0.3 to 0.8 ms. Sometimes the Ca2+ currents showed very slow inactivation at more positive voltages (> 20 mV). The Ca2+ current was modelled as a single non-inactivating current. 4. As might be expected, the embryonic neurons possessed a mixture of outward currents that were hard to separate either pharmacologically or through differences in voltage dependence. The delayed rectifier seemed to consist of varying proportions of two currents: a fast-activating K+ current (with time constants of activation ranging from 0.6 to 2 ms) and a slow K+ current (with time constants of activation ranging from 5 to 25 ms). The slow current was occasionally seen in isolation. 5. For the Ca2+, fast K+ and slow K+ currents the rate of deactivation was faster than would be predicted from the kinetics of activation. This was modelled by allowing the closing rate constant of the channels to be described by one of two different functions of voltage that between them covered the whole range of transmembrane voltage. Although this was done for empirical reasons, it could be interpreted to suggest that the channels have more than one open state and predominantly close from a state that is distinct from the one to which they originally opened.

Animals↗

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↗