A false transmitter at excitatory synapses

Autaptic Cultures: Methods and Applications

Neurons typically form daisy chains of synaptic connections with other neurons, but they can also form synapses with themselves. Although such self-synapses, or autapses, are comparatively rare in vivo, they are surprisingly common in dissociated neuronal cultures. At first glance, autapses in culture seem like a mere curiosity. However, by providing a simple model system in which a single recording electrode gives simultaneous access to the pre- and postsynaptic compartments, autaptic cultures have proven to be invaluable in facilitating important and elegant experiments in the area of synaptic neuroscience. Here, I provide detailed protocols for preparing and recording from autaptic cultures (also called micro-island or microdot cultures). Variations on the basic procedure are presented, as well as practical tips for optimizing the outcomes. I also illustrate the utility of autaptic cultures by reviewing the types of experiments that have used them over the past three decades. These examples serve to highlight the power and elegance of this simple model system, and will hopefully inspire new experiments for the interrogation of synaptic function.

Chiral Measurement of Aspartate and Glutamate in Single Neurons by Large-Volume Sample Stacking Capillary Electrophoresis

d-Amino acids (d-AAs) are endogenous molecules found throughout the metazoan, the functions of which remain poorly understood. Measurements of low abundance and heterogeneously distributed d-AAs in complex biological samples, such as cells and multicellular structures of the central nervous system (CNS), require the implementation of sensitive and selective analytical approaches. In order to measure the d- and l-forms of aspartate and glutamate, we developed and applied a stacking chiral capillary electrophoresis (CE) with laser-induced fluorescence detection method. The achieved online analyte preconcentration led to a 480-fold enhancement of detection sensitivity relative to capillary zone electrophoresis, without impacting separation resolution or analysis time. Additionally, the effects of inorganic ions on sample preconcentration and CE separation were evaluated. The approach enabled the relative quantification of d-aspartate and d-glutamate in individual neurons mechanically isolated from the CNS of the sea slug Aplysia californica, a well characterized neurobiological model. Levels of these structurally similar d-AAs were significantly different in subpopulations of cells collected from the investigated neuronal clusters.

Amperometric resolution of a prespike stammer and evoked phases of fast release from retinal bipolar cells

The neurotransmitter glutamate is used by most neurons in the brain to activate a multitude of different types of glutamate receptors and transporters involved in fast and relatively slower signaling. Synaptic ribbons are large presynaptic structures found in neurons involved in vision, balance, and hearing, which use a large number of glutamate-filled synaptic vesicles to meet their signaling demands. To directly measure synaptic vesicle release events, the ribbon-type presynaptic terminals of goldfish retinal bipolar cells were coaxed to release a false transmitter that could be monitored with amperometry by placing the carbon fiber directly on the larger synaptic terminal. Spontaneous secretion events formed a unimodal charge distribution, but single spike properties were heterogeneous. Larger events rose exponentially without interruption (τ ∼ 30 μs), and smaller events exhibited a stammer in their rising phase that is interpreted as a brief pause in pore dilation, a characteristic commonly associated with large dense core granule fusion pores. These events were entirely Ca(2+)-dependent. Holding the cells at -60 mV halted spontaneous release; and when the voltage was stepped to >-40 mV, secretion ensued. When stepping the voltage to 0 mV, novel kinetic phases of vesicle recruitment were revealed. Approximately 14 vesicles were released per ribbon in two kinetic phases with time constants of 1.5 and 16 ms, which are proposed to represent different primed states within the population of docked vesicles.

Excitotoxicity triggered by Neurobasal culture medium

Neurobasal defined culture medium has been optimized for survival of rat embryonic hippocampal neurons and is now widely used for many types of primary neuronal cell culture. Therefore, we were surprised that routine medium exchange with serum- and supplement-free Neurobasal killed as many as 50% of postnatal hippocampal neurons after a 4 h exposure at day in vitro 12–15. Minimal Essential Medium (MEM), in contrast, produced no significant toxicity. Detectable Neurobasal-induced neuronal death occurred with as little as 5 min exposure, measured 24 h later. D-2-Amino-5-phosphonovalerate (D-APV) completely prevented Neurobasal toxicity, implicating direct or indirect N-methyl-D-aspartate (NMDA) receptor-mediated neuronal excitotoxicity. Whole-cell recordings revealed that Neurobasal but not MEM directly activated D-APV-sensitive currents similar in amplitude to those gated by 1 µM glutamate. We hypothesized that L-cysteine likely mediates the excitotoxic effects of Neurobasal incubation. Although the original published formulation of Neurobasal contained only 10 µM L-cysteine, commercial recipes contain 260 µM, a concentration in the range reported to activate NMDA receptors. Consistent with our hypothesis, 260 µM L-cysteine in bicarbonate-buffered saline gated NMDA receptor currents and produced toxicity equivalent to Neurobasal. Although NMDA receptor-mediated depolarization and Ca²⁺ influx may support survival of young neurons, NMDA receptor agonist effects on development and survival should be considered when employing Neurobasal culture medium.

Ligand-specific deactivation time course of GluN1/GluN2D NMDA receptors

N-methyl-D-aspartate (NMDA) receptors belong to the family of ionotropic glutamate receptors that mediate a majority of excitatory synaptic transmission. One unique property of GluN1/GluN2D NMDA receptors is an unusually prolonged deactivation time course following the removal of L-glutamate. Here we show, using x-ray crystallography and electrophysiology, that the deactivation time course of GluN1/GluN2D receptors is influenced by the conformational variability of the ligand-binding domain (LBD) as well as the structure of the activating ligand. L-glutamate and L-CCG-IV induce significantly slower deactivation time courses compared with other agonists. Crystal structures of the isolated GluN2D LBD in complex with various ligands reveal that the binding of L-glutamate induces a unique conformation at the backside of the ligand-binding site in proximity to the region at which the transmembrane domain would be located in the intact receptors. These data suggest that the activity of the GluN1/GluN2D NMDA receptor is controlled distinctively by the endogenous neurotransmitter L-glutamate.

Downregulation of NR3A-containing NMDARs is required for synapse maturation and memory consolidation

NR3A is the only NMDA receptor (NMDAR) subunit that downregulates sharply prior to the onset of sensitive periods for plasticity, yet the functional importance of this transient expression remains unknown. To investigate whether removal/replacement of juvenile NR3A-containing NMDARs is involved in experience-driven synapse maturation, we used a reversible transgenic system that prolonged NR3A expression in the forebrain. We found that removal of NR3A is required to develop strong NMDAR currents, full expression of long-term synaptic plasticity, a mature synaptic organization characterized by more synapses and larger postsynaptic densities, and the ability to form long-term memories. Deficits associated with prolonged NR3A were reversible, as late-onset suppression of transgene expression rescued both synaptic and memory impairments. Our results suggest that NR3A behaves as a molecular brake to prevent the premature strengthening and stabilization of excitatory synapses and that NR3A removal might thereby initiate critical stages of synapse maturation during early postnatal neural development.

TARP subtypes differentially and dose-dependently control synaptic AMPA receptor gating

A family of transmembrane AMPA receptor regulatory proteins (TARPs) profoundly affects the trafficking and gating of AMPA receptors (AMPARs). Although TARP subtypes are differentially expressed throughout the CNS, it is unclear whether this imparts functional diversity to AMPARs in distinct neuronal populations. Here, we examine the effects of each TARP subtype on the kinetics of AMPAR gating in heterologous cells and in neurons. We report a striking heterogeneity in the effects of TARP subtypes on AMPAR deactivation and desensitization, which we demonstrate controls the time course of synaptic transmission. In addition, we find that some TARP subtypes dramatically slow AMPAR activation kinetics. Synaptic AMPAR kinetics also depend on TARP expression level, suggesting a variable TARP/AMPAR stoichiometry. Analysis of quantal synaptic transmission in a TARP gamma-4 knockout (KO) mouse corroborates our expression data and demonstrates that TARP subtype-specific gating of AMPARs contributes to the kinetics of native AMPARs at central synapses.

Immunocytochemical visualization of d-glutamate in the rat brain

Using highly specific antisera directed against conjugated d-amino acids, the distribution of d-glutamate-, d-tryptophan-, d-cysteine-, d-tyrosine- and d-methionine-immunoreactive structures in the rat brain was studied. Cell bodies containing d-glutamate, but not d-glutamate-immunoreactive fibers, were found. Perikarya containing this d-amino acid were only found in the mesencephalon and thalamus of the rat CNS. Thus, the highest density of cell bodies containing d-glutamate was observed in the dorsal raphe nucleus, the ventral part of the mesencephalic central gray, the superior colliculus, above the posterior commissure, and in the subparafascicular thalamic nucleus. A moderate density of immunoreactive cell bodies was observed in the dorsal part of the mesencephalic central gray, above the rostral linear nucleus of the raphe, the nucleus of Darkschewitsch, and in the medial habenular nucleus, whereas a low density was found below the medial forebrain bundle and in the posterior thalamic nuclear group. Moreover, no immunoreactive fibers or cell bodies were visualized containing d-tryptophan, d-cysteine, d-tyrosine or d-methionine in the rat brain. The distribution of d-glutamate-immunoreactive cell bodies in the rat brain suggests that this d-amino acid could be involved in several physiological mechanisms. This work reports the first visualization and the morphological characteristics of conjugated d-glutamate-immunoreactive cell bodies in the rat CNS using an indirect immunoperoxidase technique. Our results suggest that the immunoreactive neurons observed have an uptake mechanism for d-glutamate.

Interdomain interactions in AMPA and kainate receptors regulate affinity for glutamate

Ionotropic glutamate receptors perform diverse functions in the nervous system. As a result, multiple receptor subtypes have evolved with different kinetics, ion permeability, expression patterns, and regulation by second messengers. Kainate receptors show slower recovery from desensitization and have different affinities for agonists than AMPA receptors. Based on analysis of ligand binding domain crystal structures, we identified interdomain interactions in the agonist-bound state that are conserved in kainate receptors and absent in AMPA receptors. Mutations in GluR6 designed to disrupt these contacts reduced agonist apparent affinity, speeded up receptor deactivation and increased the rate of recovery from desensitization. Conversely, introduction of mutations in GluR2 that enabled additional interdomain interactions in the agonist-bound state increased agonist apparent affinity 15-fold, and slowed both deactivation and recovery from desensitization. We conclude that interdomain interactions have evolved as a distinct mechanism that contributes to the unique kinetic properties of AMPA and kainate receptors.

Presynaptically silent GABA synapses in hippocampus

Mammalian central synapses commonly specialize in one fast neurotransmitter, matching the content of their presynaptic vesicles with the appropriate receptors in their postsynaptic membrane. Here, I show that hippocampal cultures contain autaptic glutamatergic synapses that contravene this rule: in addition to postsynaptic glutamate receptors, they also express clusters of functional postsynaptic GABA(A) receptors yet lack presynaptic GABA. Hence, these synapses are presynaptically silent with respect to GABA. They can be unsilenced by loading GABA into presynaptic vesicles by endocytosis, after which a postload IPSC appears. This IPSC is similar to native IPSCs recorded from GABAergic interneurons in the same cultures. Thus, these "mistargeted" GABA(A) receptors, which apparently lack a signal that confers synaptic specificity, function almost normally. After GABA loading, glutamatergic miniature postsynaptic currents acquire a slow tail that is mediated by GABA(A) receptors, showing that synaptic vesicles can accommodate both the usual concentration of native glutamate and a saturating concentration of loaded GABA. After brief Ca(2+)-dependent exocytosis, endocytosis of GABA can proceed in low-Ca(2+) external solution. The amplitude of the postload IPSC declines exponentially with repetitive stimulation as the endocytosed GABA passes through the presynaptic vesicle cycle and is depleted. Hence, by using GABA as an exogenous but physiological tracer, the properties of these presynaptically silent synapses can provide novel insights into the content and cycling of vesicles in presynaptic terminals.

Protective cap over CA1 synapses: extrasynaptic glutamate does not reach the postsynaptic density

Numerous data indicate that nonsynaptic release of glutamate occurs both in normal and pathophysiological conditions. When reaching receptors in the postsynaptic density (PSD), glutamate (Glu) could affect the synaptic transmission. We have tested this possibility in the hippocampal CA1 synapses of rats, either by applying exogenous Glu to the CA1 neurons or by disruption of Glu transporter activity. L-Glu (400 microM) was directly applied to the hippocampal slices acutely isolated from the rats. It produced a strong inhibition of both ortho- and antidromically elicited action potentials fired by CA1 neurons while the excitatory postsynaptic current (EPSC) measured in these neurons remained totally unaffected. The optical isomer D-Glu which is not transported by the systems of Glu uptake inhibited not only orthodromic and antidromic spikes, but also EPSC. Non-specific glutamate transporter inhibitor DL-threo-beta-hydroxyaspartic acid (THA, 400 microM) mimicked the effects of exogenous Glu and produced strong inhibition of both orthodromic and antidromic spikes, without any influence on the amplitude of EPSCs. Dihydrokainate (DHK, 300 microM), selective inhibitor of GLT-1 subtype of glutamate transporter, exerted a significant inhibitory action on the orthodromically evoked spikes and also on the EPSC. Our results indicate that extrasynaptic and PSD membranes of CA1 neurons form separate compartments differing in the mechanisms and efficiency of external Glu processing: the protection of PSD markedly prevails.

Regulatory responses to an oral D-glutamate load: formation of D-pyrrolidone carboxylic acid in humans

Previously published studies have shown D-glutamate to be the most potent natural inhibitor of glutathione synthesis known, yet how D-glutamate is handled in humans is unknown. Therefore, we administered an oral D-glutamate load to four healthy volunteers and monitored the plasma D-glutamate concentration and excretion over a 3-h postload period. Compared with time controls, the plasma D-glutamate concentration increased 10-fold in the 1st h and then reached a plateau over the remaining time course. In contrast, plasma D-pyrrolidone carboxylic acid increased progressively throughout the 3-h time course to a level 10-fold higher than the D-glutamate plasma concentration. Excretion of D-glutamate progressively increased despite a constant filtered D-glutamate load rising from only 5 to 95% of the filtered amount. Excretion of D-pyrrolidone carboxylic acid increased with the rise in filtered load without significant reabsorption. The amount of D-pyrrolidone carboxylic acid excreted over the 3-h time course was 10 times the amount excreted as D-glutamate and accounted for almost 20% of the administered D-glutamate. These findings indicate that plasma D-glutamate concentration is tightly regulated through two mechanisms: 1) the transport into cells and metabolic conversion to D-pyrrolidone carboxylic acid and excretion, and 2) the enhancement of D-glutamate clearance by the kidneys.

Brain-derived neurotrophic factor differentially regulates excitatory and inhibitory synaptic transmission in hippocampal cultures

Brain-derived neurotrophic factor (BDNF) has been postulated to be a key signaling molecule in regulating synaptic strength and overall circuit activity. In this context, we have found that BDNF dramatically increases the frequency of spontaneously initiated action potentials in hippocampal neurons in dissociated culture. Using analysis of unitary synaptic transmission and immunocytochemical methods, we determined that chronic treatment with BDNF potentiates both excitatory and inhibitory transmission, but that it does so via different mechanisms. BDNF strengthens excitation primarily by augmenting the amplitude of AMPA receptor-mediated miniature EPSCs (mEPSCs) but enhances inhibition by increasing the frequency of mIPSC and increasing the size of GABAergic synaptic terminals. In contrast to observations in other systems, BDNF-mediated increases in AMPA-receptor mediated mEPSC amplitudes did not require activity, because blocking action potentials with tetrodotoxin for the entire duration of BDNF treatment had no effect on the magnitude of this enhancement. These forms of synaptic regulations appear to be a selective action of BDNF because intrinsic excitability, synapse number, and neuronal survival are not affected in these cultures. Thus, although BDNF induces a net increase in overall circuit activity, this results from potentiation of both excitatory and inhibitory synaptic drive through distinct and selective physiological mechanisms.

Long-term enhancement of central synaptic transmission by chronic brain-derived neurotrophic factor treatment

Acute effects of neurotrophins on synaptic plasticity have recently received much attention, but the roles of these factors in regulating long-lasting changes in synaptic function remain unclear. To address this issue we studied the long-term (days to weeks) and short-term (minutes to hours) effects of brain-derived neurotrophic factor (BDNF) on excitatory synaptic transmission in autaptic cultures of hippocampal CA1 neurons. We found that BDNF induced long-term enhancement of the strength of non-NMDA receptor-mediated glutamatergic transmission. This upregulation of EPSC amplitude occurred via an increase in the size of unitary synaptic currents, with no significant contribution from other aspects of neuronal electrical and synaptic function including cell size, voltage-gated sodium and potassium current levels, the number and size of synaptic contacts, and the frequency of spontaneous neurotransmitter release. Chronic BDNF treatment also decreased the degree of synaptic depression measured in response to paired stimuli. Thus, BDNF induced long-term synaptic enhancement of both basal and use-dependent synaptic transmission via specific changes to the synapse rather than through generalized potentiation of neuronal growth and differentiation. Finally, we showed that the long-term effects of BDNF are functionally and mechanistically distinct from its acute effects on synaptic transmission, suggesting that, in vivo, BDNF activation of Trk receptors can have different functional effects depending on the time course of its action.

Inducible expression of tryptophan hydroxylase without serotonin synthesis in hypothalamic dopaminergic neurons

In the present study we have further studied the previous findings that rat hypothalamic dopaminergic neuronal cell groups may express tryptophan hydroxylase (TpH), the serotonin synthesizing enzyme, without a detectable serotonin synthesis. Chemical and mechanical neuronal injuries, namely colchicine treatment and axonal transection, respectively, were performed, and distributions of neurons exhibiting immunoreactivity for TpH and/or tyrosine hydroxylase (TH), the dopamine synthesizing enzyme, were analyzed throughout the hypothalamic periventricular and arcuate nuclei. After colchicine treatment there was a statistically significant 87% (P = 0,01) increase in the number of TpH expressing neurons, while TH expression remained essentially similar. Axonal transection resulted also in a statistically significant 131% (P < 0,01) increase in the number of TpH expressing neurons, while TH expression was not significantly altered. All TpH expression coexisted with TH expression, and the induction of TpH expression by neuronal injuries occurred evenly throughout the rostrocaudal length of the territory studied. A possible serotonin synthesis by TpH was examined by giving drugs that increase brain serotonin synthesis, but no immunohistochemically detectable serotonin synthesis could be found in any of the TpH expressing neurons. Finally the possibility was studied that the relative shortage of the cofactor tetrahydrobiopterin would limit serotonin synthesis. However, an administration of tetrahydrobiopterin did not result in detectable serotonin synthesis in these neurons. Taken together these results suggest that dopaminergic neurons in the hypothalamic periventricular and arcuate nuclei are able to express TpH, this expression is induced after neuronal injury, and this induction occurs similarly throughout the territories studied. TpH expression occurs independently of TH expression, and the newly expressed TpH appears not to synthesize serotonin, regardless of pharmacological pretreatments. Thus, our findings (i) support the idea that neurons may possess inducible expression of nonfunctional transmitter-synthesizing enzymes, in this case TpH, and (ii) suggest that expression of an enzyme synthesizing a certain transmitter may not necessarily imply the corresponding transmitter phenotype.

Defining affinity with the GABAA receptor

At nicotinic and glutamatergic synapses, the duration of the postsynaptic response depends on the affinity of the receptor for transmitter (Colquhoun et al., 1977;Pan et al., 1993). Affinity is often thought to be determined by the ligand unbinding rate, whereas the binding rate is assumed to be diffusion-limited. In this view, the receptor selects for those ligands that form a stable complex on binding, but binding is uniformly fast and does not itself affect selectivity. We tested these assumptions for the GABAA receptor by dissecting the contributions of microscopic binding and unbinding kinetics for agonists of equal efficacy but of widely differing affinities. Agonist pulses applied to outside-out patches of cultured rat hippocampal neurons revealed that agonist unbinding rates could not account for affinity if diffusion-limited binding was assumed. However, direct measurement of the instantaneous competition between agonists and a competitive antagonist revealed that binding rates were orders of magnitude slower than expected for free diffusion, being more steeply correlated with affinity than were the unbinding rates. The deviation from diffusion-limited binding indicates that a ligand-specific energy barrier between the unbound and bound states determines GABAA receptor selectivity. This barrier and our kinetic observations can be quantitatively modeled by requiring the participation of movable elements within a flexible GABA binding site.

Massive autaptic self-innervation of GABAergic neurons in cat visual cortex

Autapses are transmitter release sites made by the axon of a neuron on its own dendrites. We determined the numbers and precise subcellular position of autapses on different spiny and smooth dendritic cell types using intracellular biocytin filling in slices of adult neocortex. Potential self-innervation was light microscopically assessed on 10 pyramidal cells, 7 spiny stellate cells, and 41 smooth dendritic neurons from cortical layers II-V. Putative autapses occurred on each smooth dendritic neuron and on seven pyramids, but not on spiny stellate cells. However, electron microscopic examination of all light microscopically predicted sites on pyramids (n = 28) showed only one case of self-innervation with two autapses on dendritic spines. Interneurons were classified by postsynaptic target distribution () and all putative autapses of seven basket, three dendrite-targeting, and three double bouquet cells were scrutinized. All basket and dendrite-targeting cells established self-innervation, the number of autapses being 12 +/- 7 and 22 +/- 12 (mean +/- SD), respectively; only one of the double bouquet cells formed autapses (n = 3). Basket cell autapses (n = 74) were closer to the soma (12.2 +/- 22.3 microm) than autapses established by dendrite-targeting cells (51.8 +/- 49.9 microm; n = 66). The degree of self-innervation is cell type-specific. Unlike on spiny cells, autapses are abundant on GABAergic basket and dendrite-targeting interneurons, with subcellular location similar to that of synapses formed by the parent cell on other neurons. The extensive self-innervation may modulate integrative properties and/or the firing rhythm of the neuron in a manner temporally correlated with its own activity.

Presynaptically silent synapses: spontaneously active terminals without stimulus-evoked release demonstrated in cortical autapses

This study addresses the question of whether synapses that are capable of releasing transmitters spontaneously can also release them in an excitation-dependent manner. For this purpose, whole cell patch recordings were performed for a total of 48 excitatory solitary neurons in a microisland culture to observe excitatory autaptic currents elicited by spontaneous transmitter release as well as by somatic excitation. A somatic Na+-spike, induced in response to a short voltage step, evoked excitatory postsynaptic currents (EPSCs) of various amplitudes through the autapses; in some cases, no response was noticeable. To make sure that the recorded autaptic spontaneous EPSCs (sEPSCs) under a voltage clamp resulted from independent release of transmitters and were not associated with action potentials, sEPSCS in the presence and absence of tetrodotoxin (TTX) were compared in six cells. In the presence of TTX the evoked EPSCs were completely eliminated, whereas the sEPSCs were still observed and the amplitude distribution histograms were statistically not different from those recorded in the absence of TTX. A quantitative analysis of the sEPSCs (presumably miniature EPSCs) showed that the amplitude of stimulus-evoked EPSCs did not correlate with either the frequency or median amplitudes of the sEPSCs or the age of the culture. To identify whether the absence of stimulus-evoked response was caused either by conduction failure of excitation along the axons or by impairment of the release machinery that links the terminal depolarization to vesicle exocytosis, we examined whether high K+ and hypertonic solutions could facilitate the spontaneous release of transmitters. Although the hypertonic solution increased the spontaneous release in all cells tested (n = 18), the high K+ solution had a differential effect in increasing spontaneous release, i.e., the cells with larger evoked responses were more readily facilitated by the high K+ solution. Because the high K+ solution induced depolarization of presynaptic terminals, the present results indicated that the smaller evoked responses were due to the larger number of impaired or "silent" presynaptic terminals that were unable to link presynaptic depolarization to transmitter release. In summary, the present experiments provided evidence that at least some of the presynaptic terminals are silent in response to stimuli, while remaining spontaneously active at the same time. Because this phenomenon is due to the lack of sensitivity to depolarization at the terminals, these synaptic terminals seem incapable of linking terminal depolarization to transmitter release.

Nitric oxide-related species inhibit evoked neurotransmission but enhance spontaneous miniature synaptic currents in central neuronal cultures

Nitric oxide (NO.) does not react significantly with thiol groups under physiological conditions, whereas a variety of endogenous NO donor molecules facilitate rapid transfer to thiol of nitrosonium ion (NO+, with one less electron than NO.). Here, nitrosonium donors are shown to decrease the efficacy of evoked neurotransmission while increasing the frequency of spontaneous miniature excitatory postsynaptic currents (mEPSCs). In contrast, pure NO donors have little effect (displaying at most only a slight increase) on the amplitude of evoked EPSCs and frequency of spontaneous mEPSCs in our preparations. These findings may help explain heretofore paradoxical observations that the NO moiety can either increase, decrease, or have no net effect on synaptic activity in various preparations.

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.

Direct immunocytochemical evidence for the transfer of glutamine from glial cells to neurons: use of specific antibodies directed against the d-stereoisomers of glutamate and glutamine

We have raised antibodies against D-stereoisomers of the amino acids glutamate and glutamine. These stereoisomers are not naturally occurring in mammals but can be taken up into cells by transporters that normally handle the endogenous L-amino acids. Exposure of isolated rabbit retinae to 50 microM D-glutamate resulted in a strong accumulation of D-glutamate, and hence immunoreactivity for D-glutamate in radial glial cells (Müller cells). By contrast the glutamatergic ganglion cells exhibited no immunoreactivity for D-glutamate. D-Glutamate can be converted into D-glutamine by the glial enzyme glutamine synthetase. Immunolabelling for D-glutamine revealed the presence of D-glutamine in somata of subsets of neurons including the glutamatergic ganglion cells. Labelling was also present in the inner plexiform layer, possibly indicating labelling of neuronal processes. These data indicate that after D-glutamate has been taken up into glial cells it is converted into D-glutamine. This D-glutamine is then exported from the glial cells and taken up by a subset of neurons, including the glutamatergic ganglion cells.

Pre- and postsynaptic whole-cell recordings in the medial nucleus of the trapezoid body of the rat

1. Simultaneous whole-cell recordings in a rat brain slice preparation are described from presynaptic terminals (calyces of Held) and postsynaptic somata which form an axosomatic synapse in the medial nucleus of the trapezoid body (MNTB). 2. Presynaptic action potentials evoked suprathreshold excitatory postsynaptic potentials (EPSPs). The minimum synaptic delay was around 0.4 ms at 36 degrees C and 0.9 ms at 23-24 degrees C. The amplitude of the L-alpha-amino-3-hydroxy-5-methyl-4-isoxazolepropionate (AMPA) receptor-mediated component of the excitatory postsynaptic currents (EPSCs) was 2-13 nA (at -80 mV). 3. Current-voltage relations showed that presynaptic Ca2+ channels were of the high voltage-activated type. 4. A single action potential evoked a presynaptic fluorescence transient that decayed with a time constant of 0.3-0.7 s, depending on the concentration (60-200 microM) of the Ca2+ indicator Calcium Green-5N (CG-5N). The peak amplitude of the [Ca2+]i transient was severalfold larger in the terminal than in the preterminal axon. 5. EPSC peak amplitudes were stable for more than 30 min after establishing the whole-cell configuration in the presynaptic terminal when the pipette contained 50 microM BAPTA. In contrast, with 1 mM BAPTA, peak amplitudes of EPSCs were reduced to one-third. 6. Trains of presynaptic action potentials evoked EPSCs with progressively smaller amplitudes. Little change was observed in the depression when the terminals were dialysed with 50 microM BAPTA, whereas depression was reduced with 1 mM BAPTA. 7. In low (1 mM) [Ca2+]o, facilitation instead of depression of EPSCs was observed. 8. The effects of presynaptic BAPTA suggest that the endogenous mobile Ca2+ buffer capacity of giant presynaptic terminals in the MNTB is lower than in other terminals of fast transmitting synapses.

Dopaminergic periventriculo-hypophyseal nerves show tryptophan-hydroxylase immunoreactivity but lack serotonin synthesis

Hypothalamic dopaminergic periventricular and arcuate nuclei are known to project to the pituitary gland and contain serotonin in their terminals. In order to elucidate the potential of these neurons to synthesize serotonin, we studied immunohistochemically the possible tryptophan hydroxylase content of periventriculo-hypophyseal neurons, identified by retrograde tracing from the pituitary gland. These neurons were found to contain tryptophan hydroxylase-immunoreactivity (TpOH-IR), which was enhanced after colchicine treatment. All of the TpOH-IR neurons contained tyrosine hydroxylase-immunoreactivity as well. However, none of them were immunoreactive for serotonin in either intact animals or in animals pretreated with serotonin precursor L-tryptophan and MAO inhibitor pargyline. Thus, neurons of the dopaminergic periventriculo-hypophyseal pathway express tryptophan hydroxylase, but are unable to synthesize serotonin. These findings (i) raise the possibility that, in these nerves, serotonin might serve a function other than regular synaptic transmission, and (ii) suggest that expression of an enzyme synthesizing certain transmitter does not necessarily confirm the corresponding transmitter phenotype of that neuron.

Desensitized states prolong GABAA channel responses to brief agonist pulses

We studied the role of desensitization at inhibitory synapses by comparing nonequilibrium GABAA channel gating with inhibitory postsynaptic currents (IPSCs). Currents activated by brief pulses of 1-10 mM GABA to outside-out patches from cultured hippocampal neurons mimicked GABA-mediated IPSCs. Although the average open time of single GABAA channels following brief pulses was less than 10 ms, channels entered long (tau = 38-69 ms) closed states and subsequently reopened. Movement through these states resulted in paired-pulse desensitization. The time required for deactivation after removal of agonist also increased in proportion to the extent of desensitization. These results suggest that visits to desensitized states buffer the channel in bound conformations and underlie the expression of long-lasting components of the IPSC. Reopening after GABAA receptor desensitization may thus enhance inhibitory synaptic transmission by prolonging the response to a brief synaptic GABA transient.

Release of false transmitter serotonin from the dopaminergic nerve terminals of the rat pituitary intermediate lobe

Rat pituitary intermediate lobe contains two types of serotonin-immunoreactive nerve terminals. Most of them are dopaminergic, in which serotonin acts as a false transmitter, while the rest are true serotoninergic nerves. In the present study, release of the false transmitter serotonin from the dopaminergic nerve terminals was studied by loading the neurons in vivo with serotonin precursor L-tryptophan and MAO inhibitor pargyline, which results in accumulation of false transmitter serotonin. Subsequently pituitary neurointermediate lobe complexes were incubated in the presence of various agents. Potassium induced dramatic release of serotonin. This release was Ca(2+)-dependent, as demonstrated by an inhibition by Mg2+, and transporter-independent, since it was unaffected by GBR 12909 (a dopamine transport inhibitor). Tyramine and sodium nitroprusside, a nitric oxide donor, caused slight to remarkable release of serotonin. This release was inhibited by GBR 12909, suggesting that it was transporter-dependent. Presynaptic stimulation with apomorphine or haloperidol, dopamine receptor agonist or antagonist, respectively, or isoproterenol, agonist of the beta-adrenergic receptor, did not significantly release serotonin. Thus, it seems that presynaptic receptors per se cannot induce release of significant amounts of serotonin from the IL dopaminergic fibers. Our results suggest that false transmitter serotonin in the IL dopaminergic nerve terminals is released primarily by the classical exocytotic release mechanism, but may also be partly released by the transporter-dependent, non-exocytotic release.

Glutamate receptor update

The past year has seen significant advances in matching the actions of recombinant glutamate receptors with the actions of native receptors, and in mapping their distribution and regulation. The discovery of a novel RNA editing mechanism for AMPA receptors and a revised view of the transmembrane topology of the NMDA receptor subunit, NR1, are particularly noteworthy. Seven metabotropic glutamate receptor subtypes have been identified with several interesting expression patterns and transduction mechanisms; results from work on these subtypes has led to a provocative model of the ligand-binding site. Functional studies of metabotropic receptors have been enhanced by the development of the first subtype-specific antagonist.

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.