Identification of a distinct synaptic glutamate receptor on horizontal cells in mudpuppy retina

The separation of ON and OFF channels and the development of an antagonistic surround occur at the first synapse in the vertebrate retina. This functional differentiation is mediated by the action of the photoreceptor neurotransmitter on the ON bipolar, OFF bipolar and horizontal cells, respectively. Glutamate mimics the action of the photoreceptor transmitter on all second-order neurones in fish, amphibian and mammalian retinas. The diversity of cellular responses produced by one neurotransmitter raises the possibility of multiple postsynaptic receptor-ionophore complexes. We reported previously that one glutamate analogue, 2-amino-4-phosphonobutyrate, reveals that the ON bipolar synaptic receptor is pharmacologically different from those of other second-order neurones. The results presented here demonstrate that another glutamate analogue, D-O-phosphoserine, selectively antagonizes the synaptic responses of horizontal cells. Taken together, these findings indicate that there are three glutamate-like receptor subtypes in the outer retina and suggest a correlation between receptor subtype and the physiological properties of second-order neurones.     

Long-term potentiation is associated with increases in quantal content and quantal amplitude.

Long-term potentiation (LTP) of synaptic transmission in CA1 neurons of the hippocampus, elicited by the conjunction of presynaptic firing and postsynaptic depolarization, is an important model of plasticity, which may underlie memory storage. Although induction of LTP takes place in the postsynaptic cell, it is not clear whether it is expressed through an enhancement of transmitter release or through an increased postsynaptic response to the same amount of transmitter. Analysis of the trial-to-trial amplitude fluctuations of synaptic signals, that is quantal analysis, gives an important insight into the probabilistic mechanisms of transmission, although attempts to apply it to the mode of expression of LTP have so far yielded inconsistent results, at least in part because they have relied on models of transmitter release that have not been confirmed experimentally. Here we report clear evidence for quantal fluctuation in a subset of cells. Induction of LTP in these cells causes abrupt increases in either quantal content or quantal amplitude, or both. This shows that two different mechanisms can underlie the maintenance of LTP.     

Electrogenic glutamate uptake is a major current carrier in the membrane of axolotl retinal glial cells

Glutamate is taken up avidly by glial cells in the central nervous system. Glutamate uptake may terminate the transmitter action of glutamate released from neurons, and keep extracellular glutamate at concentrations below those which are neurotoxic. We report here that glutamate evokes a large inward current in retinal glial cells which have their membrane potential and intracellular ion concentrations controlled by the whole-cell patch-clamp technique. This current seems to be due to an electrogenic glutamate uptake carrier, which transports at least two sodium ions with every glutamate anion carried into the cell. Glutamate uptake is strongly voltage-dependent, decreasing at depolarized potentials: when fully activated, it contributes almost half of the conductance in the part of the glial cell membrane facing the retinal neurons. The spatial localization, glutamate affinity and magnitude of the uptake are appropriate for terminating the synaptic action of glutamate released from photoreceptors and bipolar cells. These data challenge present explanations of how the b-wave of the electroretinogram is generated, and suggest a mechanism for non-vesicular voltage-dependent release of glutamate from neurons.     

Presynaptic enhancement shown by whole-cell recordings of long-term potentiation in hippocampal slices

Long-term potentiation (LTP) of synaptic transmission in the hippocampus is a widely studied model system for understanding the cellular mechanisms of memory. In region CA1, LTP is triggered postsynaptically by Ca2(+)-dependent activation of protein kinases, but the locus of persistent modification remains controversial. Statistical analysis of synaptic variability has been proposed as a means of settling this debate, although a major obstacle has been the poor signal-to-noise ratio of conventional intracellular recordings. We have applied the whole-cell voltage clamp technique to study synaptic transmission in conventional hippocampal slices (compare refs 28-30). Here we report that robust LTP can be recorded with much improved signal resolution and biochemical access to the postsynaptic cell. Prolonged dialysis of the postsynaptic cell blocks the triggering of LTP, with no effect on expression of LTP. The improved signal resolution unmasks a large trial-to-trial variability, reflecting the probabilistic nature of transmitter release. Changes in the synaptic variability, and a decrease in the proportion of synaptic failures during LTP, suggest that transmitter release is significantly enhanced.     

Bipolar cells in the mudpuppy retina use an excitatory amino acid neurotransmitter

The bipolar cells of the vertebrate retina are the principal neuronal elements which transmit photoreceptor activity from the outer to the inner retina. An important function of the bipolars is to segregate photoreceptor input into independent ON and OFF channels which are subserved, respectively, by the depolarizing and hyperpolarizing bipolar subtypes. Ultrastructural and physiological observations suggest that chemical neurotransmission is the predominant means of bipolar input to the inner retina. Both ON and OFF bipolars apparently release excitatory transmitters. Histological studies with cytotoxic agents and physiological studies indicate that third-order neurones have excitatory amino acid receptors. In ON-OFF amacrine and ganglion cells, which receive input from both bipolars, ON and OFF excitation have a similar ionic basis, suggesting that the same transmitter may be released by both types of bipolars. We have now found that (+/-)cis-2,3-piperidine dicarboxylic acid (PDA), a new excitatory amino acid antagonist, blocks bipolar input to the inner retina and thus suggests that an excitatory amino acid is a bipolar cell transmitter.     

Vasoactive intestinal peptide regulates mitosis, differentiation and survival of cultured sympathetic neuroblasts

Although acute, millisecond-to-millisecond actions of neurotransmitters are well documented, diverse longer-term effects have been discovered only recently. Emerging evidence indicates that these signals regulate a variety of neuronal processes, from phenotypic expression to neurite outgrowth. Here we show that a single putative transmitter, vasoactive intestinal peptide, can exert multiple, long-term effects simultaneously: it stimulates mitosis, promotes neurite outgrowth and enhances survival of sympathetic neuron precursors in culture. As the peptide seems to be a normal presynaptic transmitter in the sympathetic system, synaptic transmission may exert hitherto unexpected effects.     

薄型金属流场板燃料电池性能研究

对某金属流场板燃料电池进行性能试验,通过与某石墨流场板燃料电池在燃料电池效率和单电池平均电压等性能上的对比,对该电堆的整体性能进行了评价.同时对该金属流场板燃料电池进行了100 h振动可靠性试验,振动后通过对单电池电压的分析,发现该金属流场板燃料电池单电池一致性下降,每次振动后极化特性试验中单电池电压最大值出现的位置并无明显规律,而单电池电压最小值出现的位置为燃料电池的末端.     

Aspartate and glutamate as possible neurotransmitters of cells in layer 6 of the visual cortex

Earlier work has suggested that aspartate, glutamate and gamma-aminobutyric acid (GABA) act as transmitters in the cerebral cortex. There is reasonable evidence for the identity of the cell population responsible for GABA release but until now there has been little evidence concerning the sources for release of aspartate and glutamate. Here we have used two approaches to identify possible neurotransmitters used by cells in the visual cortex: measurement of the efflux of endogenous compounds in conditions of synaptic release and localization of these compounds to particular cell classes using neurotransmitter-specific histochemical techniques. Our results suggest that the acidic amino acids aspartate and glutamate may be cortical neurotransmitters, as shown by calcium-dependent release from endogenous stores and by uptake specific to pyramidal cells in layer 6 of the cortex. These substances may therefore have a role in the function of layer 6 cells, which are responsible for the recurrent projection from the cortex to the lateral geniculate nucleus and for the projection within the cortex from layer 6 to layer 4.     

A presynaptic action of glutamate at the cone output synapse

Neurotransmitter release from many central nervous system synapses is regulated by 'autoreceptors' at the synaptic terminal, which bind the released transmitter and alter release accordingly. The photoreceptors of lower vertebrates are thought to use glutamate as a neurotransmitter. Glutamate conveys the visual signal to postsynaptic bipolar and horizontal cells, but has been reported not to act on the photoreceptors themselves. We show here that glutamate evokes a current, carried largely by chloride ions, in cones isolated from the tiger salamander retina. This response is localized to the synaptic terminal of the cone. Removing external sodium blocks this action of glutamate. These results suggest the existence of a positive feedback loop at the cone output synapse: over most of the light-response range, glutamate released by depolarization of the cone will cause further depolarization, increasing the gain of phototransduction. Glutamate released from rods may also polarize cones, modulating the gain of the cone output synapse. This system is surprisingly different from the autoreceptor systems for most other transmitters, which act in a negative feedback way.     

Is extracellular calcium buffering involved in regulation of transmitter release at the neuromuscular junction?

During synaptic activity at the neuromuscular junction, sodium, potassium and calcium ions flow through both the postsynaptic and presynaptic membrane. These ionic fluxes can cause changes in the local extracellular concentration in the synaptic gap: a decrease in the concentration of the inwardly flowing ions (sodium and calcium) and an increase in the outwardly flowing potassium ions. To check whether depletion of calcium ions in the synaptic gap is involved in transmitter release, we have used calcium buffers to keep the extracellular calcium concentration almost constant. The expectation was that if depletion does occur, transmitter release will increase; if no depletion occurs, there will be no change in quantal release when the calcium concentration is the same in buffered and unbuffered bathing solutions. We report here that, surprisingly, perfusing the frog neuromuscular preparation with a calcium-buffered solution caused a decrease in transmitter release compared with that in an unbuffered solution with the same calcium concentration. This presumably indicates that the calcium level in the synaptic cleft is higher than that in the bulk extracellular medium. If such a mechanism operates physiologically, it may provide an energetically economical way to determine the level of evoked transmitter release and thus synaptic efficiency.     

The C(2)B Ca(2+)-binding motif of synaptotagmin is required for synaptic transmission in vivo

Synaptotagmin is a synaptic vesicle protein that is postulated to be the Ca(2+) sensor for fast, evoked neurotransmitter release. Deleting the gene for synaptotagmin (syt(null)) strongly suppresses synaptic transmission in every species examined, showing that synaptotagmin is central in the synaptic vesicle cycle. The cytoplasmic region of synaptotagmin contains two C(2) domains, C(2)A and C(2)B. Five, highly conserved, acidic residues in both the C(2)A and C(2)B domains of synaptotagmin coordinate the binding of Ca(2+) ions, and biochemical studies have characterized several in vitro Ca(2+)-dependent interactions between synaptotagmin and other nerve terminal molecules. But there has been no direct evidence that any of the Ca(2+)-binding sites within synaptotagmin are required in vivo. Here we show that mutating two of the Ca(2+)-binding aspartate residues in the C(2)B domain (D(416,418)N in Drosophila) decreased evoked transmitter release by >95%, and decreased the apparent Ca(2+) affinity of evoked transmitter release. These studies show that the Ca(2+)-binding motif of the C(2)B domain of synaptotagmin is essential for synaptic transmission.     

Voltage-sensing mechanism is conserved among ion channels gated by opposite voltages

Hyperpolarization-activated cyclic-nucleotide-gated (HCN) ion channels are found in rhythmically firing cells in the brain and in the heart, where the cation current through HCN channels (called I(h) or I(f)) causes these cells to fire repeatedly. These channels are also found in non-pacing cells, where they control resting membrane properties, modulate synaptic transmission, mediate long-term potentiation, and limit extreme hyperpolarizations. HCN channels share sequence motifs with depolarization-activated potassium (Kv) channels, such as the fourth transmembrane segment S4. S4 is the main voltage sensor of Kv channels, in which transmembrane movement of S4 charges triggers the opening of the activation gate. Here, using cysteine accessibility methods, we investigate whether S4 moves in an HCN channel. We show that S4 movement is conserved between Kv and HCN channels, which indicates that S4 is also the voltage sensor in HCN channels. Our results suggest that a conserved voltage-sensing mechanism operates in the oppositely voltage-gated Kv and HCN channels, but that there are different coupling mechanisms between the voltage sensor and activation gate in the two different channels.     

Multiple classes of glutamate receptor on depolarizing bipolar cells in retina

Multiple subtypes of excitatory amino acid receptor have been found on individual dissociated neurones. These findings were obtained from cells without intact synaptic connections, so the functional roles for such receptor subtypes are unknown. We have recorded intracellular responses from depolarizing bipolar cells (DBC) that receive direct synaptic input from two distinct populations of neurones: rods and cones. We report here that 2-amino-4-phosphonobutyrate (APB), a glutamate analogue, reveals two subtypes of glutamate receptors on DBCs. APB acts on the same receptor that mediates synaptic transmission from rods but has no action on the second subtype of glutamate receptor. These results show that the rod and cone inputs to DBCs are mediated by pharmacologically distinct receptors and that subtypes of glutamate receptor existing on single neurones can subserve separate, functionally defined synaptic inputs.     

ATP receptor-mediated synaptic currents in the central nervous system.

Until now, the only well documented, fast excitatory neurotransmitter in the brain has been glutamate. Although there is evidence for adenosine 5'-triphosphate (ATP) acting as a transmitter in the peripheral nervous system, suggestions for such a role in the central nervous system have so far not been supported by any direct evidence. Here we report the recording of evoked and miniature synaptic currents in the rat medial habenula. The fast rise time of the currents showed that they were mediated by a ligand-activated ion channel rather than a second messenger system, thus limiting the known transmitter candidates. Evidence was found for the presence on the cells of glutamate, gamma-aminobutyric acid, acetylcholine and ATP receptors, but not for 5-hydroxytryptamine (5HT3) or glycine receptors. The evoked currents were unaffected by blockers of glutamate, gamma-aminobutyric acid or acetylcholine receptors but were blocked by the ATP receptor-blocker, suramin and the desensitizing ATP receptor-agonist alpha,beta-methylene-ATP. Our evidence identifies for the first time synaptic currents in the brain, mediated directly by ATP receptors.     

Neuronal properties underlying processing of visual information in the barnacle.

Generation of a transient, amplified response to the dimming of light in the visual system of the barnacle involves two synaptic stages. It is accomplished primarily by decrementally conducting neurones that are similar to bipolar cells of the vertebrate retina.     

超声波清洗机噪声频谱特性研究

应用两种方法测量了超声波清洗机的噪声频谱,讨论了超声波清洗机的噪声特性.结果表明,超声波清洗机的噪声频带较宽且以高频噪声为主,主频位于工作频率处.不同工作频率的超声波清洗机噪声频谱差别显著,主要表现在工作频率提高时噪声频谱的主频及其他成分所对应的声压值呈下降趋势,两种方法所得结果一致.     

NMDA receptor agonists selectively block N-type calcium channels in hippocampal neurons.

The modulation of voltage-dependent calcium channels by various neurotransmitters has been demonstrated in many neurons. Because of the critical role of Ca2+ in transmitter release and, more generally, in transmembrane signalling, this modulation has important functional implications. Hippocampal neurons possess low-threshold (T-type) Ca2+ channels and both L- and N-type high voltage-activated Ca2+ channels. N-type Ca2+ channels are blocked selectively by omega-conotoxin and adenosine. These substances both block excitatory synaptic transmission in the hippocampus, whereas dihydropyridines, which selectively block L-type channels, are ineffective. Excitatory synaptic transmission in the hippocampus displays a number of plasticity phenomena that are initiated by Ca2+ entry through ionic channels operated by N-methyl-D-aspartate (NMDA) receptors. Here we report that NMDA receptor agonists selectively and effectively depress N-type Ca2+ channels which are involved in neurotransmitter release from presynaptic sites. The inhibitory effect is eliminated by the competitive NMDA antagonist D-2-amino-5-phosphonovalerate, does not require Ca2+ entry into the cell, and is probably receptor-mediated. This phenomenon may provide a negative feedback between the liberation of excitatory transmitter and entry of Ca2+ into the cell, and could be important in presynaptic inhibition and in the regulation of synaptic plasticity.     

Presynaptic spike broadening reduces junctional potential amplitude

Presynaptic modulation of action potential duration may regulate synaptic transmission in both vertebrates and invertebrates. Such synaptic plasticity is brought about by modifications to membrane currents at presynaptic release sites, which, in turn, lead to changes in the concentration of cytosolic calcium available for mediating transmitter release. The 'primitive' neuromuscular junction of the jellyfish Polyorchis penicillatus is a useful model of presynaptic modulation. In this study, we show that the durations of action potentials in the motor neurons of this jellyfish are negatively correlated with the amplitude of excitatory junctional potentials. We present data from in vitro voltage-clamp experiments showing that short duration voltage spikes, which elicit large excitatory junctional potentials in vivo, produce larger and briefer calcium currents than do long duration action potentials, which elicit small excitatory junctional potentials.     

用于全数字发射机的数字上变频电路

通过分析数字上变频器系统的工作过程及其主要参数对输出信噪比的影响,合理设计差分电压、预加重电路和高速串行发送器等的布局,有效提高数字上变频电路的性能.实际仿真和硬件测试表明,所设计的数字上变频器可将基带信号直接上变频到907.2 MHz频率上,实现阻带衰减达35 d B,提高了发射端信号的有效性和准确性,可以满足全数字发射机的应用要求.     

Activation of protein kinase C augments evoked transmitter release

In view of the emerging role of the phosphoinositide system in cellular communication we examined its involvement in quantal-transmitter release, which is a key element in synaptic transmission. Transmitter release is normally activated by an increase in intracellular calcium, achieved either by entry of calcium ions through the presynaptic membrane or by intracellular calcium liberation. One of the targets of the phosphoinositide signalling system is the enzyme protein kinase C (PKC), which can be activated experimentally by tumour promoting phorbol esters, including 12-O-tetradecanoylphorbol-13-acetate (TPA). Such activation of PKC may be implicated in transmitter release in two ways. First, phorbol esters were found to increase secretion and enhance calcium currents; it might therefore be expected that they would increase synaptic transmitter release. But phorbol esters also inhibit the calcium current in dorsal root ganglion neurones. We report that the phorbol ester TPA augments synaptic transmission at the neuromuscular junction by increasing transmitter liberation. Activation of PKC also depends synaptic depression.