Biomedical subjects

L O Trussell

Publications and source records attributed to L O Trussell.

At least 19 recordsLinked to original sources

PubMed2001

Maturation of synaptic transmission at end-bulb synapses of the cochlear nucleus.

Neurons of the avian nucleus magnocellularis transmit phase-locked action potentials of the auditory nerve in a pathway that contributes to sound localization based on interaural timing differences. We studied developmental changes in synaptic transmission that enable the end-bulb synapse to function as a synaptic relay. In chick, although the auditory system begins to function early in embryonic development, maturation of audition around the time of hatching suggested that synaptic transmission in the cochlear nucleus of young chicks may undergo further developmental changes. Synaptic physiology was investigated via patch-clamp recordings from bushy cells in brainstem slices during stimulation of auditory nerve fibers at 35 degrees C. Compared with embryonic synapses (embryonic day 18), post-hatch chicks (post-hatch days 1-11) exhibited high probability of firing a well timed postsynaptic action potential during high-frequency stimulation of the auditory nerve. Improvements in reliability and timing of postsynaptic spikes were accompanied by a developmental increase in steady-state EPSCs during stimulus trains and a decline in the extent of synaptic depression. Synchrony of EPSCs during stimulus trains improved with age. An increased pool of synaptic vesicles, lower release probability, larger and faster transmitter quanta, and reduced AMPA receptor desensitization contributed to these changes. Together, these factors improve the ability of cochlear nucleus magnocellularis neurons to faithfully transmit timing information encoded by the auditory nerve.

Action Potentials↗

PubMed2001

Correlation of AMPA receptor subunit composition with synaptic input in the mammalian cochlear nuclei.

The composition of AMPA receptors in patches excised from somata and dendrites of six cell types in the mammalian cochlear nuclei was probed and compared electrophysiologically and pharmacologically with the rapid application of glutamate. Cells excited predominantly by auditory nerve fibers had AMPA receptors with exceptionally rapid gating (submillisecond deactivation and desensitization time constants). The nonlinear current-voltage relationship in the presence of spermine showed that few of these receptors had GluR2 subunits, and the insensitivity of desensitization to cyclothiazide indicated that they contained mostly flop splice variants. At synapses made by parallel fibers, AMPA receptors were slowly gating (time constants of deactivation and desensitization >1 msec) and contained higher levels of GluR2 and flip isoforms. However, receptors at auditory nerve synapses on cells that also receive parallel fiber input, the fusiform cells, had intermediate properties with respect to kinetics and contained GluR2 and flip isoforms. Given the diverse biophysical properties, patterns of innervation, patterns of electrical activity, and targets of each cell type in vivo, these data indicate that the kinetics and permeation properties of AMPA receptors are linked to factors associated with synaptic connectivity.

Alternative Splicing↗

PubMed2001

Mixed excitatory and inhibitory GABA-mediated transmission in chick cochlear nucleus.

1. Neurons of the chick nucleus magnocellularis (NM) receive depolarizing GABAergic input from the superior olivary nucleus (SON). We examined the response to exogenous GABA or to stimulation of GABAergic fibres in order to identify the ionic basis of GABAergic synaptic transmission and its physiological implications. 2. Reversal potentials of GABA responses (E(GABA)) were determined exclusively by the Cl(-) gradient, measured using whole-cell recording. With gramicidin-perforated patch recording, E(GABA) was -25 +/- 5 mV (mean +/- S.D.), and was stable between embryonic day 17 and post-hatch day 10. With normal intracellular Cl(-), GABA depolarized neurons by 12 mV. 3. In current clamp, repetitive activation of the GABAergic axons reduced the probability of spiking in response to simultaneous stimulation of excitatory axons. However, IPSPs could themselves elicit action potentials, and facilitation of IPSPs by repetitive activation could lead to a characteristic pattern of spiking. 4. These data indicate that IPSPs with reversal potentials positive to spike threshold may have dual functions, depending on the context of their activation.

Animals↗

PubMed2001

Presynaptic glycine receptors enhance transmitter release at a mammalian central synapse.

Glycine and GABAA (gamma-aminobutyric acid A) receptors are inhibitory neurotransmitter-gated Cl- channels localized in postsynaptic membranes. In some cases, GABAA receptors are also found presynaptically, but they retain their inhibitory effect as their activation reduces excitatory transmitter release. Here we report evidence for presynaptic ionotropic glycine receptors, using pre- and postsynaptic recordings of a calyceal synapse in the medial nucleus of the trapezoid body (MNTB). Unlike the classical action of glycine, presynaptic glycine receptors triggered a weakly depolarizing Cl- current in the nerve terminal. The depolarization enhanced transmitter release by activating Ca2+ channels and increasing resting intraterminal Ca2+ concentrations. Repetitive activation of glycinergic synapses on MNTB neurons also enhanced glutamatergic synaptic currents, indicating that presynaptic glycine receptors are activated by glycine spillover. These results reveal a novel site of action of the transmitter glycine, and indicate that under certain conditions presynaptic Cl- channels may increase transmitter release.

Animals↗

PubMed2001

Minimizing synaptic depression by control of release probability.

Transmission at the end-bulb synapse formed by auditory nerve terminals onto the soma of neurons in the avian nucleus magnocellularis is characterized by high transmitter release probability and strong synaptic depression. Activation of presynaptic GABA(B) receptors minimizes depression at this synapse and significantly enhances synaptic strength during high-frequency activity. Here we investigate synaptic mechanisms underlying this phenomenon. EPSC amplitudes evoked by 200 Hz trains increased more than twofold when release probability was reduced with Cd(2+) or baclofen. This effect was not exhibited by a transmitter depletion model of presynaptic depression, which predicts that EPSC amplitudes reach a common steady-state amplitude during high-frequency trains, despite alterations of initial release probability. However, an additional source of postsynaptic depression was sufficient to explain our findings. Aniracetam, a modulator of AMPA receptors that reduces desensitization, decreased the amount of synaptic depression during trains, indicating that desensitization occurred during trains of stimuli. However, this effect of aniracetam was absent when release probability was lowered with baclofen or Cd(2+). No effect of aniracetam on the NMDA component of the EPSC was seen, confirming a postsynaptic site of action of aniracetam. When desensitization was reduced with aniracetam, steady-state EPSC amplitudes during trains were found to converge over a wide range of release probabilities, as predicted by the depletion model. Additional evidence of AMPA receptor desensitization was provided by direct measurement of quantal amplitudes immediately after stimulus trains. Thus, presynaptic modulation by GABA(B) receptors regulates the extent of AMPA receptor desensitization and controls synaptic strength, thereby modulating the flow of information at an auditory synapse.

Animals↗

PubMed2000

Long-term specification of AMPA receptor properties after synapse formation.

AMPA receptors expressed at auditory nerve synapses in the mammalian and avian cochlear nuclei display exceptionally rapid channel gating, an adaptation well suited for acoustic processing. We examined whether cellular interactions during development might determine the subunit composition of these receptors. After synapse formation in the avian nucleus magnocellularis (nMag) in vivo, the rate of receptor desensitization increased threefold, sensitivity to channel block by polyamines increased, and sensitivity to cyclothiazide, an inhibitor of desensitization, increased, indicating a reduction in glutamate receptor subunit 2 and of flip splice variants. This phenotypic switch was prevented, but not reversed, by isolating nMag neurons in a cell-culture environment. We propose that the switch in receptor kinetics is an outcome of cellular interactions during a critical period that result in the long-term determination of receptor phenotype.

Animals↗

PubMed2000

Control of synaptic depression by glutamate transporters.

The role of glutamate transporters in the regulation of synaptic depression was examined in the avian nucleus magnocellularis. Repetitive stimulation of presynaptic auditory nerve fibers resulted in acute depression of EPSCs. Pharmacological blockade of glutamate transport in glial cells enhanced residual glutamate in the synaptic cleft and markedly increased the extent of depression at stimulus frequencies above 20 Hz via a postsynaptic mechanism. Glutamate pyruvate transaminase, a glutamate scavenger, accelerated the decay of the EPSC and reduced synaptic depression, indicating that transporters are not completely effective in rapid removal of glutamate. Regulation of residual transmitter by glia may thus serve to control synaptic strength in a frequency-dependent manner.

ATP-Binding Cassette Transporters↗

PubMed2000

Inhibitory transmission mediated by asynchronous transmitter release.

At fast CNS synapses, the role of asynchronous release following initial synchronous release is poorly understood. We examined the contribution of asynchronous release to GABAergic transmission in the cochlear nucleus across a 40-fold range of electrical stimulus frequencies. Whereas quantal release was highly synchronized at low frequencies, it was largely continuous and desynchronized at high frequencies. Despite the change in release mode, intense and steady inhibitory transmission was virtually maintained. Experimental analyses and modeling studies indicated that this "desynchronization" process was dependent on presynaptic Ca2+ accumulation, facilitation of vesicle release, and short-term depletion of available vesicles. Asynchronous release at high frequencies may help generate a smooth inhibitory "tone" by minimizing the consequences of random timing of presynaptic action potentials.

Animals↗

PubMed1999

Time course and permeation of synaptic AMPA receptors in cochlear nuclear neurons correlate with input.

AMPA receptors mediate rapid glutamatergic synaptic transmission. In the mammalian cochlear nuclei, neurons receive excitatory input from either auditory nerve fibers, parallel fibers, or both fiber systems. The functional correlates of differences in the source of input were examined by recording AMPA receptor-mediated, miniature EPSCs (mEPSCs) in whole-cell voltage-clamp mode from identified neurons. Bushy, octopus, and T-stellate cells of the ventral cochlear nucleus (VCN) and tuberculoventral cells of the dorsal cochlear nucleus (DCN) receive most of their excitatory input from the auditory nerve; fusiform cells receive excitatory inputs from both the auditory nerve and parallel fibers; cartwheel cells receive excitatory input from parallel fibers alone. mEPSCs from bushy, octopus, T-stellate, and tuberculoventral cells had significantly faster decay time constants (0.35-0.40 msec) than did those from fusiform and cartwheel cells (1.32-1.79 msec). Some fusiform cells had two populations of mEPSCs with distinct time courses. mEPSCs in cells with auditory nerve input alone were inhibited by philanthotoxin, a blocker of calcium-permeable AMPA receptors, whereas mEPSCs in cells with parallel fiber input were not. Thus AMPA receptors postsynaptic to the auditory nerve differ from those postsynaptic to parallel fibers both in channel-gating kinetics and in their permeability to calcium. These results confirm the conclusion that synaptic AMPA receptors are specialized according to the source of input (Hunter et al., 1993; Rubio and Wenthold, 1997; Wang et al., 1998).

Animals↗

PubMed1999

Synaptic mechanisms for coding timing in auditory neurons.

Neurons in the cochlear ganglion and auditory brain stem nuclei preserve the relative timing of action potentials passed through sequential synaptic levels. To accomplish this task, these neurons have unique morphological and biophysical specializations in axons, dendrites, and nerve terminals. At the membrane level, these adaptations include low-threshold, voltage-gated potassium channels and unusually rapid-acting transmitter-gated channels, which govern how quickly and reliably action potential threshold is reached during a synaptic response. Some nerve terminals are remarkably large and release large amounts of excitatory neurotransmitter. The high output of transmitter at these terminals can lead to synaptic depression, which may itself be regulated by presynaptic transmitter receptors. The way in which these different cellular mechanisms are employed varies in different cell types and circuits and reflects refinements suited to different aspects of acoustic processing.

Animals↗

PubMed1997

Cellular mechanisms for preservation of timing in central auditory pathways.

The faithful preservation of acoustic timing information, as signals are passed from one synaptic level to another, requires a convergence of morphological, biophysical, and biochemical specializations in auditory neurons. Recent studies have focused on the adaptive membrane properties of neurons in the auditory brainstem. These include analyses of neurotransmitter receptors and voltage-gated channels, as well as the mechanisms of transmitter release and its modulation. The molecular composition of the relevant proteins are now being demonstrated, including the glutamate receptor Dflop (GluR-Dflop) subunit of AMPA receptors and members of the Kv1 and Kv3 families of potassium channels.

Animals↗

PubMed1996

Direct measurement of AMPA receptor desensitization induced by glutamatergic synaptic transmission.

Although almost all ionotropic neurotransmitter receptors undergo desensitization, the onset and recovery of desensitization at a synapse have never been observed directly. We have found changes in postsynaptic AMPA receptor sensitivity in neurons of the chick cochlear nucleus, the nucleus magnocellularis (nMAG), by photolysis of caged glutamate immediately after activation of a single synaptic input. Additionally, synaptic desensitization was demonstrated via competition between synaptically released glutamate and an exogenous nondesensitizing agonist, kainate. Both approaches indicated that at least 35-40% of the receptors were desensitized after a single synaptic stimulus. Miniature synaptic currents were depressed after an evoked synaptic current, indicating that desensitization led to a reduction in the response to individual transmitter quanta. Stimulation of adjacent glutamatergic inputs to the same cell demonstrated that nearby terminals did not depress one another, suggesting that the desensitizing level of glutamate is restricted to each axon terminal. These findings confirm that postsynaptic neurons may use desensitization to regulate the strength of transmission on a synapse-specific basis.

Animals↗

PubMed1996

Delayed clearance of transmitter and the role of glutamate transporters at synapses with multiple release sites.

The roles of glutamate diffusion, uptake, and channel kinetics in shaping the AMPA receptor EPSC were examined at a calyceal synapse. The EPSC decay was described by three exponential components: two matching desensitizing channel kinetics, and a third component at least 10 times slower. The slowest component had identical voltage dependence to the steady-state AMPA current and was selectively increased and prolonged by blockade of glutamate uptake, indicating that the slow EPSC represented rebinding of glutamate at partially desensitized AMPA receptors. The data were in strong agreement with the predictions of a model of transmitter diffusion from multiple release sites into a large synaptic cleft. Within the first millisecond after release, transmitter concentrations in the cleft fell below millimolar levels, causing the fastest parts of the EPSC to be shaped by channel kinetics. The slowest component was determined by the removal over tens of milliseconds of the final 10-100 microM glutamate by diffusion and uptake. The data and modeling indicate that transmitter uptake and cooperation between release sites are significant determinants of a slow "tail" of glutamate in the synaptic cleft. This slow clearance of glutamate is likely to limit postsynaptic receptor availability through desensitization.

Animals↗

PubMed1996

Inhibition of transmitter release shortens the duration of the excitatory synaptic current at a calyceal synapse.

1. We investigated the effect of reducing transmitter release on the time course of multiquantal, evoked synaptic currents to test for transmitter "cross talk" between neighboring synaptic release sites within a calyceal synapse. By using a brain slice preparation, neurons in the chick nucleus magnocellularis (nMAG) were voltage clamped and individual presynaptic axons were stimulated to evoke excitatory postsynaptic currents (EPSCs). 2. Application of 100-microM baclofen or 50-microM GABA in the presence of a gamma-aminobutyric acid-A (GABAA) receptor antagonist produced an 85% reduction of EPSCs, consistent with the activation of presynaptic gamma-aminobutyric acid-B (GABAB) receptors. In parallel with the reduction in the amplitude of the EPSC by GABAB receptor activation, the normally strong paired pulse depression (PPD) of the EPSC was converted to facilitation. The reduction in EPSC amplitude by gamma-aminobutyric acid (GABA) or baclofen was accompanied by a 20% reduction in the exponential time constant of decay of the EPSC. Weaker effects on the EPSC time course were observed for synapses with the least PPD. 3. Cd2+ (5 microM), which inhibits presynaptic calcium current, also reduced EPSC amplitude by 85% and converted PPD to facilitation. EPSCs were narrower in Cd2+, though less so than in baclofen. 4. The time course of the EPSC was longer than that of miniature synaptic currents, even after significant block by baclofen, GABA or Cd2+, indicating that dispersion of quantal release may help shape the synaptic waveform. However, the narrowing of the EPSC by baclofen, GABA, and Cd2+ suggests that high levels of quantal release at the calyceal synapse may delay the removal of transmitter, further slowing the EPSC.

Animals↗

PubMed1995

AMPA receptors with high Ca2+ permeability mediate synaptic transmission in the avian auditory pathway.

1. The permeability of AMPA (alpha-amino-3-hydroxy-5-methyl-4- isoxazolepropionate) receptors in the chick cochlear nucleus, the nucleus magnocellularis (nMAG), was examined by measuring the shift in reversal potential (Erev) of current through glutamate or neurotransmitter-gated channels in solutions of different ionic composition. 2. Outwardly rectifying glutamate-activated currents in outside-out membrane patches showed rapid activation and desensitization. The Erev of glutamate-evoked current in zero sodium solutions was dependent on the extracellular Ca2+ concentration. The relation between Erev and Ca2+ ionic activities could be described by the Goldman-Hodgkin-Katz equation with a permeability ratio, PCa/PCs, of 3.3. The PNa/PCs was estimated as 0.66, indicating a PCa/PNa of 5. 3. Evoked excitatory postsynaptic currents (EPSCs) could be recorded during local perfusion of the auditory nerve-nMAG synapse with isotonic Ca2+. The Erev of the EPSC shifted in the positive direction in high-Ca2+ solution as predicted from the preceding analysis. The fraction of current carried by Ca2+ during the AMPA receptor EPSC was estimated as 18%.

Animals↗

PubMed1995

Concentration-jump analysis of voltage-dependent conductances activated by glutamate and kainate in neurons of the avian cochlear nucleus.

We have examined the mechanisms underlying the voltage sensitivity of alpha-amino-3-hydroxy-5-methyl-4-isoxazolepropionate receptors in voltage-clamped outside-out patches and whole cells taken from the nucleus magnocellularis of the chick. Responses to either glutamate or kainate had outwardly rectifying current-voltage relations. The rate and extent of desensitization during prolonged exposure to agonist, and the rate of deactivation after brief exposure to agonist, decreased at positive potentials, suggesting that a kinetic transition was sensitive to membrane potential. Voltage dependence of the peak conductance and of the deactivation kinetics persisted when desensitization was reduced with aniracetam or blocked with cyclothiazide. Furthermore, the rate of recovery from desensitization to glutamate was not voltage dependent. Upon reduction of extracellular divalent cation concentration, kainate-evoked currents increased but preserved rectifying current-voltage relations. Rectification was strongest at lower kainate concentrations. Surprisingly, nonstationary variance analysis of desensitizing responses to glutamate or of the current deactivation after kainate removal revealed an increase in the mean single-channel conductance with more positive membrane potentials. These data indicate that the rectification of the peak response to a high agonist concentration reflects an increase in channel conductance, whereas rectification of steady-state current is dominated by voltage-sensitive channel kinetics.

Animals↗

PubMed1995

The mechanism of alpha-amino-3-hydroxy-5-methyl-4-isoxazolepropionate receptor desensitization after removal of glutamate.

We have examined responses of AMPA (alpha-amino-3-hydroxy-5-methyl-4-isoxazolepropionate) receptors in the chick nucleus magnocellularis to pairs of pulses of glutamate and determined the extent of desensitization and the rate of recovery. Receptors recovered from desensitization with a time constant of 16 ms, regardless of the concentration or duration of the conditioning pulse. Even with very brief conditioning pulses, evoking submaximal currents, desensitization occurred at a consistent rate after the removal of free ligand. A quantitative kinetic model based on these data shows that receptors must desensitize from a closed state. The results provide evidence that very brief exposure to glutamate, on the time scale of uniquantal synaptic transmission, will result in a significant reduction in sensitivity of postsynaptic receptors.

Animals↗

PubMed1994

Voltage clamp analysis of excitatory synaptic transmission in the avian nucleus magnocellularis.

1. The properties of evoked excitatory postsynaptic currents (EPSCs) and spontaneous miniature excitatory postsynaptic currents (mEPSCs) have been studied in neurons of the nucleus magnocellularis (nMAG), one of the avian cochlear nuclei which receive somatic, calyceal innervation from auditory nerve fibres. Whole-cell patch clamp techniques were used to voltage clamp visually identified neurons in brain slices. 2. EPSCs resulting from activation of single axonal inputs were on average -5.3 nA at a driving force of -25 mV. Current-voltage relationships for the peak of the EPSC were linear with a peak conductance of 211 nS. The rate of EPSC decay showed a linear increase with temperature, with a temperature coefficient (Q10) of 2.2 between 25 and 35 degrees C; in vivo (41 degrees C) the EPSC would decay in 0.2 ms. 3. The EPSC was composed of two pharmacologically and kinetically distinct components: an early phase due to non-NMDA (N-methyl-D-aspartate) receptors and a late phase resulting from NMDA receptors. Both components reversed near 0 mV. While both subtypes of glutamate receptor were activated by transmitter, NMDA receptors had a peak conductance at positive potentials which was only 11% of the peak non-NMDA receptor component. 4. EPSCs during trains of stimuli exhibited a progressive decrease in amplitude. The extent of depression increased with the frequency of stimulation and was reduced by drugs which prevent receptor desensitization, indicating that, in part, postsynaptic factors limit synaptic strength during repetitive synaptic activity. Additionally, the coefficient of variation of the EPSC amplitude increased during trains, consistent with presynaptic depression. 5. mEPSCs occurred randomly in the presence of tetrodotoxin and presumably correspond to transmitter quanta. These synaptic events rose (10-90%) within 100 microseconds and decayed with an exponential of 180 microseconds at 29-32 degrees C. Despite the somatic location of the synapse, mEPSCs varied widely in amplitude, suggesting differences in the quantal synaptic current at each synaptic site. The ratio of the average peak conductance of the EPSC and mEPSC gave an estimated quantal content of 103.

2-Amino-5-phosphonovalerate↗