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

The impact of receptor desensitization on fast synaptic transmission

The role of desensitization of ligand-gated channels at fast chemical synapses has been difficult to establish. Densensitization has been studied traditionally with prolonged agonist exposure, whereas the duration of free neurotransmitter in the synaptic cleft is relatively brief. Studies of acetylcholine-, glutamate- and GABA-gated channels using rapid agonist application now provide a means to assess the effects of densensitization in shaping synaptic responses and in influencing neuronal excitability. These data reveal several strikingly different patterns by which the receptor-specific kinetics of densensitization can determine the size, timecourse and frequency of transmitted signals. Densensitization is thus a surprisingly versatile mechanism for shaping synaptic transmission.

Activity-related features of synapse morphology: a study of endbulbs of held

The myelinated fibers of the auditory nerve can be divided into two separate populations on the basis of sensitivity to sound, average levels of spike activity, and central branching patterns. The synaptic endings of these populations were investigated for the presence of structural specializations that might correlate with levels of neural activity. We applied intracellular recording and staining methods in cats to analyze directly the relationship between spike activity and the structure of synapses using endbulbs of Held, the large synaptic endings in the anteroventral cochlear nucleus. Endbulbs from fibers having low or high levels of activity were examined and compared using light and electron microscopic methods. All endbulbs exhibited relatively large but incomplete coverage by one-to-several lamellae of glial processes. Endbulbs of high activity fibers were large and contained larger mitochondria than endbulbs of low activity fibers. Furthermore, the synapses of high activity endbulbs were on average smaller but more numerous, possessed greater numbers of associated synaptic vesicles, and exhibited greater curvature of their postsynaptic densities. These structural features are hypothesized to reflect specializations that optimize synaptic transmission.

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.

Modeling the effect of glutamate diffusion and uptake on NMDA and non-NMDA receptor saturation

One- and two-dimensional models of glutamate diffusion, uptake, and binding in the synaptic cleft were developed to determine if the release of single vesicles of glutamate would saturate NMDA and non-NMDA receptors. Ranges of parameter values were used in the simulations to determine the conditions when saturation could occur. Single vesicles of glutamate did not saturate NMDA receptors unless diffusion was very slow and the number of glutamate molecules in a vesicle was large. However, the release of eight vesicles at 400 Hz caused NMDA receptor saturation for all parameter values tested. Glutamate uptake was found to reduce NMDA receptor saturation, but the effect was smaller than that of changes in the diffusion coefficient or in the number of glutamate molecules in a vesicle. Non-NMDA receptors were not saturated unless diffusion was very slow and the number of glutamate molecules in a vesicle was large. The release of eight vesicles at 400 Hz caused significant non-NMDA receptor desensitization. The results suggest that NMDA and non-NMDA receptors are not saturated by single vesicles of glutamate under usual conditions, and that tetanic input, of the type typically used to induce long-term potentiation, will increase calcium influx by increasing receptor binding as well as by reducing voltage-dependent block of NMDA receptors.

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.

A molecular determinant for submillisecond desensitization in glutamate receptors

The decay of excitatory postsynaptic currents in central neurons mediated by alpha-amino-3-hydroxy-5-methyl-4-isoxazole-propionate (AMPA) receptors is likely to be shaped either by receptor desensitization or by offset after removal of glutamate from the synaptic cleft. Native AMPA receptors show desensitization time constants of 1 to about 10 milliseconds, but the underlying molecular determinants of these large differences are unknown. Cloned AMPA receptors carrying the "flop" splice variants of glutamate receptor subtype C (GluR-C) and GluR-D are shown to have desensitization time constants of around 1 millisecond, whereas those with the "flip" variants are about four times slower. Cerebellar granule cells switch their expression of GluR-D splice variants from mostly flip forms in early stages to predominantly flop forms in the adult rat brain. These findings suggest that rapid desensitization of AMPA receptors can be regulated by the expression and alternative splicing of GluR-D gene transcripts.

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.

Direct patch recording from identified presynaptic terminals mediating glutamatergic EPSCs in the rat CNS, in vitro

1. An in vitro brainstem slice preparation of the superior olivary complex has been developed permitting patch recording from a presynaptic terminal (calyx of Held) and from its postsynaptic target--the principal neurone of the medial nucleus of the trapezoid body (MNTB). 2. The fluorescent stain DiI (1,1'-dioctadecyl-3,3,3',3'-tetramethylindocarbocyanine perchlorate) was used in fixed tissue and Lucifer Yellow in living slices, to identify calices enclosing single MNTB neuronal somata. 3. Whole-cell recording from the MNTB neurone shows evoked EPSCs preceded by a prespike, corresponding to the presynaptic action potential (AP). In some cases one patch pipette recorded from both pre- and postsynaptic elements, but confirmation of exclusively presynaptic recording was obtained using pipettes containing Lucifer Yellow in a further eleven cases. 4. Under current clamp, the pre- and postsynaptic sites could be distinguished by their response to step depolarizations; presynaptic terminals generated a train of APs at frequencies up to 200 Hz, while MNTB neurones gave a single AP. Each presynaptic AP had an after-hyperpolarization lasting less than 2 ms. 5. Under voltage clamp, step depolarizations of presynaptic terminals generated a tetrodotoxin-sensitive inward current followed by rapidly activating outward potassium currents at potentials more positive than -60 mV. The outward current exhibited little inactivation over the 150 ms steps and 4-aminopyridine (200 microM) blocked 63.0 +/- 14.5% (mean +/- S.D., n = 3) of the sustained current at 0 mV. Like the squid giant synapse, mammalian terminals express rapidly activating 'delayed rectifier'-type potassium currents.

Astrocyte proliferation in the chick auditory brainstem following cochlea removal

Astrocytes in the central nervous system (CNS) respond to injury and disease by proliferating and extending processes. The intermediate filament protein of astrocytes, glial fibrillary acidic protein (GFAP) also increases in astrocytes. These cells are called "reactive astrocytes" and are thought to play a role in CNS repair. We have previously demonstrated rapid increases (< 6 hours) in GFAP-immunoreactive and silver-impregnated glial processes in the chick cochlear nucleus, nucleus magnocellularis (NM), following cochlea removal or activity blockade of the eighth nerve. It was not known whether these changes were the result of glial proliferation, glial hypertrophy, or both. The present study examined the time course of astrocyte proliferation in NM following cochlea removal. Postnatal chicks received unilateral cochlea removal and survived for 6, 12, 18, 24, 36, 48, and 72 hours. Bromodeoxyuridine was used to label proliferating cells. The volume and number of labeled cells in NM was calculated for both the experimental and control sides of the brains for experimental animals was well as for unoperated control animals. A subset of astrocytes continuously divide in the normal posthatch chick brainstem. The percentage of labeled nuclei increases within NM 36 hours following cochlea removal and is robust by 48 hours. This increase is due to astrocyte proliferation within, rather than migration to, NM. These results indicate that rapid increases in GFAP following reduced activity are independent of cell proliferation. The time course of astrocyte proliferation suggests that cellular degeneration within the nucleus may play a role in upregulating astrocyte proliferation.

Synthesis of models for excitable membranes, synaptic transmission and neuromodulation using a common kinetic formalism

Markov kinetic models were used to synthesize a complete description of synaptic transmission, including opening of voltage-dependent channels in the presynaptic terminal, release of neurotransmitter, gating of postsynaptic receptors, and activation of second-messenger systems. These kinetic schemes provide a more general framework for modeling ion channels than the Hodgkin-Huxley formalism, supporting a continuous spectrum of descriptions ranging from the very simple and computationally efficient to the highly complex and biophysically precise. Examples are given of simple kinetic schemes based on fits to experimental data that capture the essential properties of voltage-gated, synaptic and neuromodulatory currents. The Markov formalism allows the dynamics of ionic currents to be considered naturally in the larger context of biochemical signal transduction. This framework can facilitate the integration of a wide range of experimental data and promote consistent theoretical analysis of neural mechanisms from molecular interactions to network computations.

Mechanisms shaping glutamate-mediated excitatory postsynaptic currents in the CNS

Excitatory postsynaptic currents in neurones of the central nervous system have a dual-component time course that results from the co-activation of AMPA/kainate-type and NMDA-type glutamate receptors. New approaches in electrophysiology and molecular biology have provided a better understanding of the factors that determine the kinetics of excitatory postsynaptic currents. Recent studies suggest that the time course of neurotransmitter concentration in the synaptic cleft, the gating properties of the native channels, and the glutamate receptor subunit composition all appear to be important factors.

Multivesicular release from excitatory synapses of cultured hippocampal neurons

Release of neurotransmitter from presynaptic terminals occurs by exocytosis of vesicular contents into the synaptic cleft. We find that more than one quantum of transmitter can interact with the same population of postsynaptic NMDA receptors in conditions which increase the probability of transmitter release. Increasing release probability also results in proportional increases in both AMPA and NMDA receptor components of the synaptic current. These results suggest that the fraction of AMPA and NMDA receptors occupied by transmitter following the release of a single quantum is similar. Based on AMPA and NMDA receptor responses of outside-out patches to short applications of glutamate, we suggest that both receptor types may be saturated normally by synaptic release.

Maintenance of pharmacologically-immature glutamate receptors by aberrant synapses in the chick cochlear nucleus

Surgical destruction of the otocyst in chick embryos prevents formation of the *** ear, abolishes normal cochlear input to the cochlear nucleus (nucleus magnocellularis, NM) and results in axons from the contralateral NM forming (in addition to their normal bilateral endings in nucleus laminaris, NL) a novel and functional aberrant projection to the deafferented NM. We studied the pharmacology of synaptic transmission at aberrant synapses in an in vitro preparation of the brainstem in chick embryos and hatchlings. Transmission at the aberrant synapses (as with cochlear nerve synapses in NM and NM synapses in NL) is blocked by the quinoxalinedione antagonists CNQX and NBQX, confirming the presence of excitatory amino acid receptors of the non-NMDA subtype. At cochlear nerve synapses in NM, the antagonist potency of NBQX normally decreases rapidly after embryonic day (E)18 (IC50 = 0.69 +/- 0.06 microM, mean +/- S.E.M.), reaching an asymptotic value by E21 (IC50 = 2.7 +/- 0.4 microM) that is maintained at least through posthatching day (P)14 (IC50 = 3.6 +/- 0.3 microM). In the case of the aberrant endings, the potency of NBQX remained (from E21 [IC50 = 0.6 +/- 0.1 microM] through at least P14[IC50 = 0.5 +/- 0.1 microM]) at levels that are statistically indistinguishable from the E18 value for normal cochlear nerve synapses.(ABSTRACT TRUNCATED AT 250 WORDS)

Organization of the nucleus magnocellularis and the nucleus laminaris in the barn owl: encoding and measuring interaural time differences

The circuit from the cochlear nucleus magnocellularis to the nucleus laminaris supports the encoding and measurement of interaural time differences in the auditory brainstem. Specializations for the encoding of temporal information include the few and/or short dendrites and thick axons of the magnocellular and laminaris neurons, and the high degree of convergence in the circuit. Magnocellular cells have large cell bodies covered with somatic spines. The cells have few dendrites, and the number of dendrites decreases from low to high best frequency regions of the nucleus. Magnocellular neurons receive both auditory nerve terminals and GABAergic terminals with symmetric synapses and terminals filled with pleomorphic vesicles. The axonal projections of magnocellular neurons to the nucleus laminaris form maps of interaural time difference. About 100 magnocellular afferents from each side converge on each laminaris neuron, and the terminals from each side do not occupy separate domains on the cell. These terminals form punctate asymmetric synapses on both the dendrites and the cell bodies of laminaris neurons. Laminaris neurons also receive GABAergic terminals which form symmetric synapses. Laminaris neurons have oval cell bodies covered with very short dendrites. The cells in the low best frequency region of the nucleus laminaris have longer dendrites.

Modal shifts in acetylcholine receptor channel gating confer subunit-dependent desensitization

During the transition from embryonic to adult skeletal muscle, a decreased mean channel open time and accelerated desensitization of nicotinic acetylcholine (ACh) receptors result from the substitution of an epsilon subunit for gamma. A single ACh receptor channel of the embryonic type, expressed in Xenopus oocytes, interconverts between gating modes of short and long open time, whereas the adult receptor channel resides almost exclusively in the gating mode with short open time. Differences in the fraction of time spent in either gating mode account for the subunit dependence of both receptor open time and desensitization. Therefore, developmental changes in the kinetics of muscle ACh receptors may be imparted through subunit-dependent stabilization of intrinsic gating modes.

Mammalian ionotropic glutamate receptors

Exciting new milestones in glutamate receptor (GluR) channel research include the following: the cloning of N-methyl-D-aspartate (NMDA) receptors; delineation of molecular determinants for ion flow through glutamate-gated channels; the discovery that Ca2+ permeability of non-NMDA receptor channels is determined by RNA editing; the construction of antibodies and their use in immunocytochemical localizations of alpha-amino-3-hydroxy-5-methyl isoxazole-4-propionic acid (AMPA) receptor subunits in the rat brain; and the return to prominence of the high-affinity kainate site with the publication of cDNA sequences for subunits (GluR-5, -6, -7; KA-1, -2) constituting subtypes of this site. Major unresolved issues comprise the transmembrane topology and subunit stoichiometries of native receptor channels.

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 binaural auditory pathway: excitatory amino acid receptors mediate dual timecourse excitatory postsynaptic currents in the rat medial nucleus of the trapezoid body

We show here that synaptic transmission to the medial nucleus of the trapezoid body (MNTB) is mediated principally by excitatory amino acid receptors and has two components. A fast excitatory postsynaptic current (EPSC) is mediated by non-NMDA receptors and a slow EPSC is mediated by NMDA receptors. Each neuron receives a large synaptic input (calyx of Held) which produces an EPSC with a mean peak conductance of 37 nS. The somatic location of this synapse gives good resolution of the EPSC timecourse with the fast EPSC decaying with a time constant of 1.1 ms (at 25 degrees C). The slow EPSC exhibits a double exponential decay with time constants of 41 ms and 106 ms and is voltage dependent in the presence of extracellular magnesium. Other smaller EPSCS mediated by NMDA and non-NMDA receptors, and a strychnine-sensitive synaptic current, are also present. Although the intrinsic membrane properties of MNTB neurons (Forsythe & Barnes-Davies (Proc. R. Soc. Lond. B 251, 143 (1993)), preceding paper) promote high-fidelity transmission, we show that voltage-dependent modulation of synaptic transmission can occur. Given the specialization of the calyx of Held, it seems that the NMDA-receptor ion channel complex is not primarily serving to potentiate a subthreshold input, but may be involved in the development and maintenance of this exuberant somatic synapse.

AMPA-type glutamate receptors--nonselective cation channels mediating fast excitatory transmission in the CNS

In recent years, considerable progress in our understanding of the molecular events underlying excitatory synaptic transmission has been made. This progress was mainly achieved by technical advances, among them the patch-clamp technique in brain slices (Edwards et al., 1989), fast application of agonists (Franke et al., 1987), and cloning and functional expression of GluR channels of the nonNMDA type (e.g., Hollmann et al., 1989). A suitable model for studying excitatory postsynaptic currents (EPSCs) in the brain slice with patch-clamp techniques is the mossy fiber synapse on CA3 pyramidal cells of rat hippocampus (MF-CA3 synapse). This synapse is located close to the cell soma and should provide almost ideal space-clamp conditions. A comparison of MF-CA3 EPSCs with the currents activated by fast application of glutamate on membrane patches isolated from CA3 cell somata suggests that the concentration of glutamate in the synaptic cleft during excitatory synaptic transmission is high (about 1 mM) and that the transmitter remains in the synaptic cleft only briefly (about 1 ms). It seems likely that desensitization influences the peak amplitude of the EPSC in several ways. Brief pulses of glutamate cause desensitization, from which the glutamate receptor channels recover only slowly, and micromolar ambient glutamate concentrations produce desensitization at equilibrium. From the functional properties of recombinant GluR channels, in situ hybridization data, and patch-clamp experiments on different neuronal and nonneuronal cell types, a picture of the molecular identity of native channels emerges. In neurons of the hippocampus the pharmacological features of these channels were similar to recombinant channels assembled from subunits of the AMPA/kainate subtype which are strongly expressed in these cells. The native channels are characterized by outward rectification of the steady-state I-V and low Ca permeability, similar to recombinant channels containing the GluR-B subunit. This is consistent with the ubiquitous expression of this subunit in hippocampal neurones. In contrast, GluR channels from cerebellar glial cells, which uniquely in the central nervous system lack the expression of GluR-B subunits, show double rectification and high Ca permeability. The results suggest that the native functional nonNMDA glutamate receptor channels in the CNS are assembled form subunits of the AMPA/kainate subtype in a cell-specific way, with the functional properties of GluR channels in neurones being dominated by the GluR-B subunit.