Biomedical subjects

L O Trussell

Publications and source records attributed to L O Trussell.

30 records · Page 2Linked to original sources

PubMed1994

A characterization of excitatory postsynaptic potentials in the avian nucleus magnocellularis.

1. The activation of current-clamped neurons in the chick nucleus magnocellularis (nMAG) by eighth nerve stimulation has been studied in a brain slice preparation using patch electrodes. Single presynaptic stimuli produced rapidly rising, suprathreshold, excitatory postsynaptic potentials (EPSPs) with a synaptic delay of approximately 0.4 ms. Spontaneous, miniature EPSPs (mEPSPs) were evident in control extracellular solution and in the presence of tetrodotoxin (TTX). 2. The EPSP was composed of a large, brief component that was sensitive to antagonists of alpha-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid (AMPA) receptors, and a smaller, slowly decaying component that was sensitive to both N-methyl-D-aspartate (NMDA) and AMPA receptor antagonists. 3. Injection of depolarizing current steps revealed a strong outward rectification of the membrane conductance at potentials close to the resting potential. Consequently, neurons could fire only a single, TTX-sensitive action potential during a current step. The conductance responsible for this rectification was sensitive to 1 mM 4-aminopyridine but not to 1 mM tetraethylammonium. 4. Following the termination of depolarizing current pulses, membrane potential decayed with a half-time (t1/2) that decreased as the depolarizing current increased, reaching approximately 0.25 ms for a depolarization from rest of 20 mV. The t1/2 for the decay of EPSPs matched the membrane t1/2, indicating that the underlying synaptic conductance decays more quickly than the membrane t1/2. 5. The slow phase of the EPSP was always longer than the membrane t1/2 and increased in size with hyperpolarization. This result is consistent with the contribution of AMPA receptors to the slow, as well as fast, EPSP. 6. The safety factor for transmission with low-frequency stimuli was large, as indicated by the rise time of the EPSP, the extent to which the EPSP shunted the action potential, and the size of EPSPs after prolongation of the synaptic conductance by cyclothiazide. 7. During repetitive synaptic stimulation, the slow EPSPs summated to produce a plateau depolarization of 10-20 mV. The plateau potential was only partially blocked by NMDA receptor antagonists. 8. During trains of stimuli, the faster EPSPs rode atop the plateau potential and could drive action potentials at rates up to 500 Hz for short periods. Synaptic depression was evident during trains, such that EPSPs often fell below threshold after 5-10 stimuli at rates above 200 Hz. EPSPs could remain suprathreshold for several seconds at 50 Hz.(ABSTRACT TRUNCATED AT 400 WORDS)

Animals↗

PubMed1994

Pathway-specific variants of AMPA receptors and their contribution to neuronal signaling.

Neurons of the nucleus magnocellularis (nMAG) of the chick express AMPA (alpha-amino-3-hydroxy-5-methyl-4-isoxazole-propionate) receptors displaying unusually rapid kinetics of desensitization (Raman and Trussell, 1992a). To investigate whether fast AMPA receptors are present in other auditory neurons, we compared the properties of AMPA receptors in auditory as well as nonauditory cells. These included neurons of nMAG, the nucleus angularis, the nucleus laminaris, the cochlear ganglion, the Purkinje cell layer of the cerebellum, the ventral horn of the spinal cord, and the brainstem nucleus of the glossopharyngeal nerve (ncIX). Rapid application of glutamate to voltage-clamped outside-out membrane patches indicated that AMPA receptors in the four types of auditory cells had significantly faster kinetics of desensitization than did the three types of nonauditory neurons. Channel kinetics in auditory (nMAG) and nonauditory (ncIX) cells were also compared by means of spectral analysis and the time course of current deactivation upon removal of glutamate. Both techniques revealed a burst duration for nMAG channels of 400-500 musec at room temperature, two to three times shorter than for ncIX cells. Single channels in nMAG had a burst duration of 550 musec and an open time of approximately 150 musec. Miniature excitatory postsynaptic currents of brainstem auditory neurons in slices were also brief, with decay constants of 150-250 musec at 29-32 degrees C. We demonstrated that the fast kinetics of this AMPA receptor are physiologically important, since cyclothiazide, which reduces desensitization and prolongs synaptic currents, doubled the relative refractory period of orthodromic spikes in nMAG cells in brain slices. We conclude that auditory brainstem neurons express a specialized subtype of AMPA receptors. This "fast" AMPA receptor may be useful in transmitting signals necessary for sound localization.

Animals↗

PubMed1993

Desensitization of AMPA receptors upon multiquantal neurotransmitter release.

We have investigated the role of AMPA receptor desensitization during transmission at a calyceal synapse. Cyclothiazide blocked the rapid desensitization of AMPA receptors and markedly prolonged the decay time of the evoked excitatory postsynaptic current (PSC). This effect was greater than what would be expected from a simple prolongation of channel open time. Additionally, the drug reduced the depression of PSCs evoked at brief intervals. The effects of cyclothiazide on the PSC were reduced when the level of transmitter release was lowered. These data indicate that AMPA receptors are desensitized by neurotransmitter and that this desensitization depends on the number of quanta in the synaptic cleft. We propose that release of transmitter from many closely spaced sites prolongs the time of receptor-transmitter contact and thereby promotes desensitization. Desensitization may therefore contribute to synaptic depression and prevent the interaction of transmitter quanta within the synaptic cleft.

Animals↗

PubMed1992

The kinetics of the response to glutamate and kainate in neurons of the avian cochlear nucleus.

Neurons in the nucleus magnocellularis (nMAG) of the chicken precisely transmit auditory nerve activity via glutamatergic synapses. Using techniques for rapid application of solutions, we have explored the properties of CNQX-sensitive glutamate receptors in whole cells and outside-out patches from the nMAG. Glutamate-evoked current in patches desensitized biphasically to less than 1% of the peak current, with a fast time constant of 960 microseconds at 22 degrees C, decreasing to 570 microseconds at 33 degrees C. Dose-response studies using kainate indicated that at least two agonist molecules bind to gate the channel. We propose a kinetic model that quantitatively describes our experimental observations. The rapid kinetics of this receptor are well suited to allow phase locking of synaptic signals to auditory stimuli.

6-Cyano-7-nitroquinoxaline-2,3-dione↗

PubMed1989

Glutamate receptor desensitization and its role in synaptic transmission.

Responses of excitatory amino acid receptors to rapidly applied glutamate were measured in outside-out membrane patches from chick spinal neurons. The peak current varied with glutamate concentration, with a half-maximal response at 510 microM and a Hill coefficient near 2. Currents activated by 1 mM glutamate desensitized and recovered in two phases. The faster time constant was identical to the time constant of decay of synaptic currents, suggesting that glutamatergic synaptic currents are terminated, in part, by receptor desensitization. Steady-state desensitization was evident following application of only 2-3 microM glutamate, concentrations comparable to levels in the extracellular space in the intact brain. Thus, glutamate receptor desensitization can affect synaptic efficacy in two ways: at high concentrations, rapid desensitization of receptors may curtail synaptic currents; at low concentrations, there is a significant reduction in the number of activatable receptors.

Animals↗

PubMed1988

Rapid desensitization of glutamate receptors in vertebrate central neurons.

We have examined glutamate receptor desensitization in voltage-clamped embryonic chicken spinal cord neurons and postnatal rat hippocampal neurons maintained in culture. Rapid currents that rose in 0.8-3.6 msec were evoked when glutamate was ionophoresed with 0.5- to 1.0-msec pulses. With prolonged pulses or brief, repetitive pulses, glutamate-evoked currents decayed rapidly in a manner that was independent of holding potential. A similar desensitization occurred following close-range pressure ejection of glutamate. The rapid, desensitizing glutamate current exhibited a linear current-voltage relation and it was not blocked by 2-amino-5-phosphonovalerate, suggesting that it was mediated by N-methyl-D-aspartate-insensitive (G2) receptors. Desensitization of G2 receptors may be agonist-dependent: currents evoked by kainate, a selective G2 agonist, did not decay, whereas prior application of glutamate did reduce the size of kainate responses. The appearance of the rapid current depended critically on the position of the ionophoretic pipette. Such glutamate-receptor "hot spots" often corresponded to points of contact with neighboring neurites, which raises the possibility that they are located at synapses.

Animals↗

PubMed1988

Rapid desensitization of glutamate receptors in vertebrate central neurons.

Animals↗

PubMed1988

Three serotonin responses in cultured mouse hippocampal and striatal neurons.

Serotonin (5-HT) produced 3 different types of responses in neurons of mouse hippocampal and striatal cell cultures. These 3 responses have been characterized in terms of their pharmacological specificity, physiological mechanism, and dependence on cytoplasmic components. The most frequently observed response was inhibitory and was the result of a receptor-mediated activation of an inwardly rectifying potassium conductance. Typically, the response peaked within 1-3 sec of agonist application and did not exhibit desensitization. 5-Methoxy-N,N-dimethyltryptamine also produced this response in both striatal and hippocampal cultures and had no effect on the other 5-HT currents observed in this study. The selective 5-HT agonists--8-hydroxy-2-(di-n-propylamino)-tetralin, 1-(m-chlorophenyl) piperazine, and 1-(2-methoxyphenyl) piperazine--did not activate this outward current response. Methysergide did not block the 5-HT-activated outward current and often acted as an agonist. The response was lost in low-series-resistance recordings which facilitate solution exchange between the patch electrode and the cell. The loss of this response was prevented by using high-resistance patch electrodes, which retard this exchange. The 2 other responses described in this study were excitatory. They were seen less often than the inhibitory response. One of the excitatory responses was fast, with a time to peak of approximately 200 msec and a duration of 2-4 sec. The other was slow, with a time to peak of 7-10 sec and a duration of approximately 30-40 sec. Both of these responses were accompanied by a conductance increase. The fast excitatory response reversed at depolarized potentials and desensitized with a rate that varied with voltage. Metoclopramide and d-tubocurarine completely and reversibly blocked this fast excitatory response, while methysergide had no effect. The fast excitatory response was not lost during intracellular dialysis of cells in cultures from either striatum or hippocampus. In cultures from both brain regions, the slow excitatory response was blocked by methysergide. The slow excitatory response was lost even in patch-clamp recordings with high-resistance electrodes. This response was similar to responses to dopamine, norepinephrine, and forskolin, all of which are known to activate adenylate cyclase in the CNS.

8-Hydroxy-2-(di-n-propylamino)tetralin↗

PubMed1987

Dependence of an adenosine-activated potassium current on a GTP-binding protein in mammalian central neurons.

Neurons in hippocampal and striatal cell cultures respond to adenosine with an inhibitory potassium current. This response disappears during whole-cell patch-clamp recording in which the cell is filled with minimal saline. We have found that this loss of sensitivity to adenosine can be prevented by including 100 microM GTP in the patch electrode filling solution. GDP is less effective than GTP in supporting the adenosine response, while GMP has little, if any, effect. Treatments known to inhibit GTP-binding proteins (G-proteins) block the adenosine-activated potassium current: The adenosine response is inhibited by including poorly metabolized analogs of guanine nucleotides along with GTP in the recording electrode. Diphosphate and triphosphate analogs appear to achieve this effect through different mechanisms. The adenosine response is also blocked by incubating cultures in islet-activating protein (pertussis toxin), an inhibitor of a class of G-protein. Thus, our data implicate a G-protein in the activation of a potassium current by adenosine. Intracellular ATP can increase the effectiveness of GMP, GDP, or low concentrations of GTP, suggesting that even during internal dialysis, neurons can maintain GTP levels through phosphotransferase reactions. Intracellular ATP also appears to suppress an outward current that is different from the adenosine-activated current. Raising intracellular cAMP levels either with bath-applied forskolin or by including a cAMP analog in the recording electrode did not alter the adenosine response. These results indicate that a G-protein is involved in the coupling between the adenosine receptor and a potassium channel, and that this coupling is not mediated by cAMP.

Adenosine↗

PubMed1985

Adenosine-activated potassium conductance in cultured striatal neurons.

We have examined the effect of adenosine on the membrane properties of cultured embryonic mouse striatal neurons using patch electrode techniques. Adenosine at 50 microM effectively blocked spontaneous action potential activity. Adenosine or 2-chloroadenosine caused a slow hyperpolarization of the membrane potential and, under voltage clamp, an outward current that was blocked by 1 mM theophylline. ATP also caused a hyperpolarization that was slower and weaker than the adenosine response and could be blocked by 1 mM theophylline. The current induced by adenosine appears to be carried by potassium since (i) an inward current was generated by adenosine when the cells were internally perfused with cesium salts and (ii) the reversal potential of the outward current shifted 57 mV with a 10-fold change in extracellular potassium concentration. The adenosine response is voltage dependent in that the current evoked by adenosine is reduced at holding potentials more positive than -55 mV, despite a larger driving force. Though calcium influx is not required for adenosine to activate the potassium conductance, some components of the cytosol may be essential, since the response is lost during intracellular perfusion.

2-Chloroadenosine↗

PubMed1985

The regulation of synaptic strength within motor units of the frog cutaneous pectoris muscle.

The physiological properties of frog neuromuscular junctions may vary widely in a single muscle. In order to understand the factors that contribute to this variation, we have studied populations of synapses belonging to individual motor units of the frog cutaneous pectoris muscle. Motor units in this muscle differ widely in twitch strength. A motor axon's synaptic contacts could be found throughout the muscle, at both singly and polyneuronally innervated endplates. Indeed, over 36% of the endplates contacted by each isolated motor axon were polyneuronally innervated. Comparisons of synapses on muscle fibers in large twitch motor units with those in small twitch motor units reveal that endplate potential amplitude, transmitter release, and muscle fiber diameter are positively correlated with the strength of the motor unit contraction. Large and small twitch motor units differ more in their transmitter release than in their nerve terminal length, indicating that larger twitch motor units have a higher release per unit length of terminal. Among motor units of roughly similar twitch tension, transmitter release at endplates receiving only one axonal input is remarkably constant, independent of postsynaptic muscle fiber input resistance, or, presumably, nerve terminal size. In cases where two different motor axons contribute to a single endplate, the synaptic strength of each input is again related to properties of the contributing motoneuron, although the individual synaptic inputs are markedly reduced in strength and size relative to singly innervated endplates. Additionally, the diameter of polyneuronally innervated muscle fibers appears related to properties of both innervating motoneurons. Thus, the pre- and postsynaptic characteristics of neuromuscular junctions may be determined both by the motoneuron and by peripheral interactions between motoneurons.

Animals↗

PubMed1983

Synaptic strength as a function of motor unit size in the normal frog sartorius.

A wide range of motor unit sizes exists in each frog sartorius, with values of tetanus and twitch tensions extending over 62- and 400-fold ranges, respectively. These differences primarily represent differences in the number, rather than the size, of muscle fibres in each motor unit. Tetanus-to-twitch tension ratios varied markedly between different motor units, ranging from 1.5 to over 190. Because directly stimulated muscle fibres have tetanus/twitch ratios no larger than 3, it is concluded that high tetanus/twitch ratios arise from recruitment of muscle fibres during the tetanus which respond with only subthreshold depolarization during a twitch. Small motor units, as judged either by their twitch or tetanus tension, were associated with higher tetanus/twitch ratios, suggesting that the average safety margin of a motor unit increases with the motor unit's size. Indeed, when the tetanus/twitch ratio of a motor unit is used to determine the fraction of fibres in the unit with subthreshold neuromuscular junctions, it is observed that there is a direct linear relationship between the size of the motor unit and its over-all efficacy of synaptic transmission. Measurements of the effects of changes in calcium concentration on motor unit twitch tension confirmed the last conclusion. Furthermore, this analysis revealed that large motor units, although having a wide range of transmission safety margins, are largely comprised of junctions of uniformly high safety margin. In motor units of smaller size, synaptic strengths become more evenly distributed over a wide range of values. Small motor units had consistently longer twitch rise-times than did larger units. This decrease in rise-time with motor unit size paralleled the decrease in tetanus/twitch ratios, raising questions about the regulation of muscle fibre contraction kinetics.

Action Potentials↗