Laurence O. Trussell

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.

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.

Enhancement of Synaptic Efficacy by Presynaptic GABAB Receptors

Activation of presynaptic inhibitory receptors or high-frequency synaptic stimulation normally inhibits excitatory synaptic transmission by reducing transmitter release. We have explored the interactions between these two pathways for reducing synaptic strength and found that for synapses stimulated at high rates, agonists of the GABA(B) receptor become excitatory and strengthen transmission. At an auditory glutamatergic synapse featuring strong synaptic depression, the GABA(B) agonist baclofen reduced by 90% postsynaptic currents elicited at low frequency. By contrast, synaptic currents elicited at high frequencies were 5-fold larger in baclofen and had a markedly increased likelihood of firing well-timed postsynaptic action potentials. Presynaptic GABA(B) receptors may thus regulate transmitter release to enable sustained transmission at higher stimulus frequencies, thereby extending the dynamic range of neural circuits.

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.

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.

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%.

Control of time course of glutamatergic synaptic currents

Publisher Summary The duration of the synaptic current is a key element in determining the overall function of glutamatergic synapses. The duration of the postsynaptic response is of importance in determining the consequence of repetitive synaptic activity. Temporal summation—a fundamental component in the integration of synaptic signals—is prominent when excitatory postsynaptic potentials are long enough to overlap. Conversely, precise neural coincidence detection requires the minimization of temporal summation so that only near-simultaneous, narrow synaptic events are able to drive a cell to action potential threshold. In addition to mediating electrical computation, glutamatergic synaptic events drive long-term changes in the biochemical state of postsynaptic cells. Studies in the past years have identified many of the major parameters that influence the duration of the synaptic current. These include synaptic geometry, receptor-binding affinity and channel gating kinetics, glutamate transporters, and quantal release kinetics. Synaptic transmission mediated by glutamate and glutamate receptors draws upon a wide variety of factors to determine the duration of the synaptic current. Because these factors can be varied independently, glutamate synapses can, with appropriate parallel variation in postsynaptic cable properties, be specialized for integration, coincidence detection, or the passive relay of signals.