Quisqualate and L-glutamate inhibit retinal horizontal-cell responses to kainate

Concanavalin A selectively reduces desensitization of mammalian neuronal quisqualate receptors

A fast perfusion system was used to apply excitatory amino acids to embryonic hippocampal neurons grown in dissociated culture and voltage clamped in the whole-cell recording configuration. Responses to quisqualic acid and DL-alpha-amino-3-hydroxy-5-methyl-4-isoxazoleproprionic acid (AMPA; a potent quisqualate-like agonist) showed rapid desensitization: at 100 microM the peak inward current declined to a plateau response on average 0.2 times the peak response (mean time constant, 30 ms). Responses to L-aspartic acid and N-methyl-D-aspartic acid also showed desensitization: at 100 microM, when recorded in Mg-free solution with 0.3 microM glycine, the peak inward current declined to a plateau value 0.5 times the peak, but with a time constant of desensitization (average, 248 ms) one order of magnitude slower than desensitization of responses to quisqualate. Responses to kainate and domoate (agonists at kainic acid receptors) did not show appreciable desensitization. Responses to L-glutamate and 5-Br-willardine (a potent non-NMDA receptor agonist), recorded in glycine-free solution with 1 mM Mg to suppress N-methyl-D-aspartic acid receptor activity, showed similar rapid desensitization to AMPA and quisqualate, but occurred with less depression of the peak current. The lectin concanavalin A (Con A) reduced desensitization at quisqualate receptors, with no effect on responses to kainate or N-methyl-D-aspartic acid. The effect of Con A developed slowly (average time constant at 2.5 microM, 250 s) but at steady state Con A increased the plateau current evoked by 100 microM quisqualate to 13 times control. Succinyl-Con A produced only a small reduction of desensitization to quisqualate, approximately 10% of that produced by native Con A. Con A did not change the decay time constant of fast excitatory synaptic currents evoked by stimulation of presynaptic neurons, although the peak synaptic current decreased after treatment with lectin. Con A was also without effect on the block of responses to kainate produced by coapplication of quisqualate.

H1 horizontal cells of carp retina have different postsynaptic mechanisms to mediate short- versus long-wavelength visual signals

Vertebrate photoreceptors release neurotransmitter substance(s) tonically in the dark and this release is curtailed by light. Recently, we have become increasingly aware of the possibility that short- and long-wavelength visual signals are mediated differently during the synaptic transmission to second-order retinal neurons. The experiment described here advances this notion further by demonstrating a postsynaptic difference. Treatment of the carp retina by dopamine reduced the gap-junctional coupling of horizontal cells, and we made use of this known effect to measure the input resistance (Rin) of H1-type horizontal cells. Flashes of light increased Rin. This increase, however, was found to be smaller with short wavelengths, even though the comparison was made when voltage responses were equal in amplitude. Often, Rin was even found to decrease at the blue end of spectrum. No single postsynaptic mechanism can account for any equal-voltage Rin difference such as this. The synaptic spectral segregation thus revealed is probably subserved by a dual scheme wherein the transmitter from blue-sensitive cone photoreceptors acts to decrease the membrane conductance of H1 cells whereas the synapses made by red- and green-sensitive cones are of a classical excitatory type.

Expression of excitatory amino acid receptors by cerebellar cells of the type-2 astrocyte cell lineage

We have used postnatal rat cerebellar astrocyte-enriched cultures to study the excitatory amino acid receptors present on these cells. In the cultures used, type-2 astrocytes (recognized by the monoclonal antibodies A2B5 and LB1) selectively took up gamma-[3H]aminobutyric acid ([3H]GABA) and released it when incubated in the presence of micromolar concentrations of kainic and quisqualic acids. The releasing effect of kainic acid was concentration dependent in the range of 5-100 microM. Quisqualate was more effective than kainate in the lower concentration range but less effective at concentrations at which its releasing activity was maximal (approximately 50 microM). N-Methyl-D-aspartic acid and dihydrokainate (100 microM) did not stimulate [3H]GABA release from cultured astrocytes. L-Glutamic acid (20-100 microM) stimulated [3H]GABA release as effectively as kainate. The stimulatory effects of kainate and quisqualate on [3H]GABA release were completely Na+ dependent; that of kainate was also partially Ca2+ dependent. Kynurenic acid (50-200 microM) selectively antagonized the releasing effects of kainic acid and also that of L-glutamate; quisqualate was unaffected. Quisqualic acid inhibited the releasing effects of kainic acid when both agonists were used at equimolar concentrations (50 microM). D-[3H]aspartate was taken up by both type-1 and type-2 astrocytes, but only type-2 astrocytes released it in the presence of kainic acid. Excitatory amino acid receptors with a pharmacology similar to that of the receptors present in type-2 astrocytes were also expressed by the immature, bipotential progenitors of type-2 astrocytes and oligodendrocytes.

Quisqualic acid modulates kainate responses in cultured cerebellar granule cells

The activation of kainic acid and quisqualic acid receptors in cultured cerebellar granule cells stimulated the release of preaccumulated D-[3H]aspartate. The effect of kainate could be distinguished from that of quisqualate by its sensitivity to the antagonists kynurenic acid and 2,3-cis-piperidine dicarboxylic acid. At a concentration of kainic acid (50 microM) close to its half-maximal releasing effect, simultaneous addition of quisqualic acid (10-50 microM) resulted in a significant dose-dependent inhibition of the kainate-induced component of D-[3H]aspartate release, which was monitored by the progressive decrease in sensitivity of the evoked release to kynurenic acid. In contrast, when kainic acid was used at a subeffective concentration (10 microM), addition of low doses of quisqualate (2-5 microM) resulted in a synergistic effect on D-[3H]aspartate release. Under these conditions, the effect of the two agonists was sensitive to kynurenic acid. Kainic acid (50-100 microM) also caused a dose-dependent, kynurenic acid-sensitive accumulation of cyclic GMP (cGMP) in granule cell cultures. Quisqualic acid was, by itself, ineffective and prevented, in a dose-dependent manner, the kainate-induced cGMP formation (IC50 = 5 microM). Finally, the guanylate cyclase activator sodium nitroprusside greatly enhanced cGMP formation but had no effect on D-[3H]aspartate release. Together, these results demonstrate the existence of complex interactions between quisqualic and kainic acids and indicate that the effects of the two glutamate agonists on D-[3H]aspartate release and on cGMP accumulation are independent.

Glutamate receptors of ganglion cells in the rabbit retina: evidence for glutamate as a bipolar cell transmitter

1. Intracellular and extracellular recordings were obtained from ganglion cells in the rabbit retina. The effects of glutamate analogues and antagonists were studied using a perfusion method for drug application. 2. Kainate (KA) excited all ganglion cells directly and caused a large increase in firing rate. N-Methyl-DL-aspartate (NMDLA) also excited ganglion cells but it was less potent and caused burst firing. 3. Quisqualate (QQ) and (RS)-2-amino-3-hydroxy-5-methyl-isoxazole-4-propionic acid (AMPA) excited many ganglion cells and were approximately as potent as KA. Less frequently, QQ and AMPA had inhibitory effects possibly due to polysynaptic action. 4. General glutamate antagonists such as cis-2,3-piperidine dicarboxylic acid (PDA) and kynurenic acid blocked the light input to all ganglion cells. PDA and kynurenic acid blocked the effects of KA and NMDLA, but not carbachol, indicating that they act as glutamate antagonists in the rabbit retina. Kynurenic acid did not block the excitatory action of QQ, even though light responses were abolished. 5. Amacrine cells were depolarized by KA or QQ and less potently by NMDLA. Their light-evoked responses were blocked by PDA. 6. We conclude that the light input to ganglion cells in the rabbit retina is predominantly mediated by KA receptors. This is consistent with the idea that 'on' and 'off' bipolar cells are excitatory and release glutamate.

L-glutamate-induced depolarization in solitary photoreceptors: a process that may contribute to the interaction between photoreceptors in situ

L-Glutamate is a leading candidate for the vertebrate photoreceptor transmitter. In addition to the signal transmission to second-order neurons, photoreceptors communicate with each other not only electrically but also chemically. In the present study, by using solitary turtle photoreceptors, we examined the possibility that L-glutamate mediates interreceptor communication. L-Glutamate evoked an inward current in all subtypes of photoreceptors voltage-clamped to the resting potential. The highest glutamate sensitivity was located at the axon terminal. Both stereoisomers of aspartate were effective, whereas kainate, quisqualate, N-methyl-D-aspartate, and D-glutamate were ineffective. The presence of Na+ was essential to response generation; even Li+ could not substitute for Na+. The relation between L-glutamate-induced current and the membrane voltage was strongly inward-rectifying. These results favor the hypothesis that the L-glutamate-induced response is generated by an electrogenic uptake carrier. However, L-glutamate-induced current was always accompanied by an increase in current fluctuations, a phenomenon commonly observed in ion channels but not expected for an uptake carrier. Although the underlying mechanism needs further elucidation, it seems likely that L-glutamate is a transmitter for communication between photoreceptors.

Noise and single channels activated by excitatory amino acids in rat cerebellar granule neurones

1. Glutamate-receptor ion channels in rat cerebellar granule cells maintained in explant cultures have been investigated with patch-clamp methods. Properties of these channels were determined from noise analysis of whole-cell currents and from noise and single-channel currents recorded in outside-out membrane patches. 2. Glutamate (10-20 microM) evoked two types of response. Some granule cells gave small inward currents accompanied by clear increases in current noise ('large noise' responses), whereas other cells gave larger inward currents and small noise increases ('small noise' responses). 3. A mean single-channel conductance (gamma) of 46.6 pS was estimated for glutamate from four 'large noise' cells. A mean gamma value of 8.4 pS was estimated for seven other 'large noise' cells. The results suggest that in these latter cells glutamate activated both large (approximately equal to 50 pS) and small conductance (approximately equal to 140 fS) channels. 4. Applications of aspartate (10-30 microM) or N-methyl-D-aspartate (NMDA, 10-30 microM) produced small inward currents and large increases in noise; gamma noise = 48.5 pS (aspartate) and 46.7 pS (NMDA). 5. Large single-channel currents were evoked by glutamate, aspartate and NMDA in outside-out patches. The mean conductance values obtained for the largest amplitude openings were: gamma(glutamate) = 49.5 pS, gamma(aspartate) = 51.5 pS, and gamma(NMDA) = 53.0 pS. For each agonist, these 50 pS openings comprised 75-85% of the completely resolved currents in each patch. Openings to 40 and 30 pS conductance levels accounted for 10-15% and 3-7% of the total, and the presence of apparently direct transitions between these levels and the 50 pS level suggests they are sublevels of the same multi-conductance channels. 6. A mean channel conductance of 22.9 pS was estimated from noise evoked by quisqualate (10-30 microM). Single-channel currents were examined in four patches. In two, quisqualate evoked predominantly small currents of two amplitudes, gamma = 8.4 pS and 16.5 pS; some 50 pS openings were also present. In the other two patches, most openings were 50 pS events. 7. Granule cells gave inward currents to kainate (10-30 microM), and a mean conductance of 3.1 pS was estimated from kainate noise. In patches in which aspartate or NMDA produced mainly 50 pS openings, more than 74% of the single-channel currents evoked by kainate were of smaller amplitude, with mean conductances of gamma = 8.1 and 15.1 pS.(ABSTRACT TRUNCATED AT 400 WORDS)

Rapid desensitization of glutamate receptors in vertebrate central neurons

We have examined glutamate receptor desensitization in voltage-clamped embryonic chicken spinal cord neurons and postnatal rat hippocampal neurons maintained in culture. Rapid currents that rose in 0.8-3.6 msec were evoked when glutamate was ionophoresed with 0.5- to 1.0-msec pulses. With prolonged pulses or brief, repetitive pulses, glutamate-evoked currents decayed rapidly in a manner that was independent of holding potential. A similar desensitization occurred following close-range pressure ejection of glutamate. The rapid, desensitizing glutamate current exhibited a linear current-voltage relation and it was not blocked by 2-amino-5-phosphonovalerate, suggesting that it was mediated by N-methyl-D-aspartate-insensitive (G2) receptors. Desensitization of G2 receptors may be agonist-dependent: currents evoked by kainate, a selective G2 agonist, did not decay, whereas prior application of glutamate did reduce the size of kainate responses. The appearance of the rapid current depended critically on the position of the ionophoretic pipette. Such glutamate-receptor "hot spots" often corresponded to points of contact with neighboring neurites, which raises the possibility that they are located at synapses.

Responses mediated by excitatory amino acid receptors in solitary retinal ganglion cells from rat

1. The pharmacological properties of excitatory amino acid responses on ganglion cells dissociated from the rat retina were examined with the use of the whole-cell voltage-clamp technique. 2. L-Glutamate at a concentration of 50 microM produced inward non-desensitizing currents at negative holding potentials in nearly every cell tested (83%, n = 18) In physiological solutions, L-glutamate responses reversed at approximately -9 mV, and higher concentrations of this agonist introduced a desensitizing component to the response. 3. At negative holding potentials, kainate (25-125 microM) produced inward currents in all of the cells tested (n = 37). These currents never desensitized, even at high agonist concentrations, and reversed near -6 mV. Currents induced by 50 microM-kainate were reversibly antagonized by kynurenate (100-300 microM) but not by 100 microM-2-amino-5-phosphonovalerate (APV). 4. Quisqualate generated smaller, non-desensitizing currents in only 50% of the cells tested (n = 38). Quisqualate responses reversed in polarity near -4 mV and were maximal at an agonist dose of 25 microM, with higher concentrations introducing a rapidly desensitizing component without a detectable increase in amplitude. Currents produced by quisqualate at a concentration of 50 microM were not antagonized by either 750 microM-kynurenate or 100 microM-APV. 5. N-Methyl-D-aspartate (NMDA) produced inward currents at negative holding potentials in 68% of the cells tested (n = 31), but only when magnesium was excluded from the extracellular medium. NMDA currents were non-desensitizing at agonist concentrations of up to 200 microM, with higher concentrations introducing a rapidly desensitizing component. NMDA (200 microM) responses were blocked by APV (100 microM) and kynurenate (300 microM) and reversed near -1 mV. 6. Responses generated by kainate (50-125 microM) were antagonized by quisqualate (30-250 microM). This antagonism occurred even in cells having no measurable response to quisqualate alone, suggesting the possibility that quisqualate may be acting both as an agonist, in the 50% of the cells that have the quisqualate-specific receptor, and as an antagonist, at the kainate-specific site on all cells.(ABSTRACT TRUNCATED AT 400 WORDS)

Glutamate receptor subtypes in cultured cerebellar neurons: modulation of glutamate and gamma-aminobutyric acid release

Using cerebellar, neuron-enriched primary cultures, we have studied the glutamate receptor subtypes coupled to neurotransmitter amino acid release. Acute exposure of the cultures to micromolar concentrations of kainate and quisqualate stimulated D-[3H]aspartate release, whereas N-methyl-D-aspartate, as well as dihydrokainic acid, were ineffective. The effect of kainic acid was concentration dependent in the concentration range of 20-100 microM. Quisqualic acid was effective at lower concentrations, with maximal releasing activity at about 50 microM. Kainate and dihydrokainate (20-100 microM) inhibited the initial rate of D-[3H]aspartate uptake into cultured granule cells, whereas quisqualate and N-methyl-DL-aspartate were ineffective. D-[3H]Aspartate uptake into confluent cerebellar astrocyte cultures was not affected by kainic acid. The stimulatory effect of kainic acid on D-[3H]aspartate release was Na+ independent, and partly Ca2+ dependent; the effect of quisqualate was Na+ and Ca2+ independent. Kynurenic acid (50-200 microM) and, to a lesser extent, 2,3-cis-piperidine dicarboxylic acid (100-200 microM) antagonized the stimulatory effect of kainate but not that of quisqualate. Kainic and quisqualic acid (20-100 microM) also stimulated gamma-[3H]-aminobutyric acid release from cerebellar cultures, and kynurenic acid antagonized the effect of kainate but not that of quisqualate. In conclusion, kainic acid and quisqualic acid appear to activate two different excitatory amino acid receptor subtypes, both coupled to neurotransmitter amino acid release. Moreover, kainate inhibits D-[3H]aspartate neuronal uptake by interfering with the acidic amino acid high-affinity transport system.

Interactions between topically applied excitatory amino acids on rat cerebral cortex: discrimination by pentobarbitone

The interaction between the effects of topically applied excitatory amino acid receptor agonists on the somatosensory evoked potentials (SEPs) were examined in urethane-anaesthetised rats. Preceding application of N-methyl-D, L-aspartic acid (NMDLA) prevented the effects of quinolinic acid or kainic acid but not those of carbachol or KCl. Similarly, quisqualic acid prevented the effects of NMDLA or kainic acid but not that of carbachol. Pentobartitone suppressed the inhibitory effect of quisqualic acid on NMDLA responses without altering the NMDLA response itself. It is concluded that an apparent desensitisation phenomenon occurs between the excitatory amino acids, distal to the receptor site.

Neurotransmitter-induced currents in retinal bipolar cells of the axolotl, Ambystoma mexicanum

1. Whole-cell patch clamping was used to study the membrane properties of isolated bipolar cells and the currents evoked in them by putative retinal neurotransmitters. 2. Isolated bipolar cells show an approximately ohmic response to voltage steps over most of the physiological response range, with an average input resistance of 1.3 G omega and resting potential of -35 mV. These values are underestimates because of the shunting effect of the seal between the patch electrode and the cell membrane. Depolarization beyond -30 mV produces rapid activation (10-100 ms) of an outward current (carried largely by potassium ions), which then inactivates slowly (0.5-2 s). 3. Of five candidates for the photoreceptor transmitter, four (aspartate, N-acetylhistidine, cadaverine, putrescine) had no effect on bipolar cells. The fifth substance, L-glutamate, opened ionic channels with a mean reversal potential of -12 mV in some cells (presumed hyperpolarizing bipolar cells), and closed channels with a mean reversal potential of -13 mV in other cells (presumed depolarizing bipolar cells). 4. The conductance increase induced by glutamate in presumed hyperpolarizing bipolar cells was associated with an increase in membrane current noise. Noise analysis suggested a single-channel conductance for the glutamate-gated channel of 5.4 pS. The power spectrum of the noise increase required the sum of two Lorentzian curves to fit it, suggesting that the channel can exist in three states. 5. The conductance decrease induced by glutamate in presumed depolarizing bipolar cells was associated with a decrease in membrane current noise that could be described as the sum of two Lorentzian spectra, and which suggested a single-channel conductance of 11 pS. The noise decrease implies that the channels closed by glutamate are not all open in the absence of the transmitter. 6. GABA (gamma-aminobutyric acid) and glycine, transmitters believed to mediate lateral inhibition in the retina, open chloride channels in isolated bipolar cells, and increase the membrane current noise. Noise analysis suggested that the channels gated by GABA and glycine have conductances of 4.4 and 7.5 pS respectively. The noise spectra required the sum of two Lorentzian curves to fit them. 7. By whole-cell patch clamping cells in retinal slices, the synaptic transmitter released by photoreceptors was shown to close channels with an extrapolated reversal potential around -3 mV in depolarizing bipolar cells.(ABSTRACT TRUNCATED AT 400 WORDS)

The action of excitatory amino acids on chick spinal cord neurones in culture

1. Membrane currents evoked by N-methyl-D-aspartate (NMDA), L-aspartate, L-glutamate, quisqualate and kainate were studied in cultured neurones from the embryonic chick spinal cord by the patch-clamp technique and by employing a quasi-step microperfusion technique. 2. Application of NMDA, aspartate, glutamate and quisqualate induced currents which exhibited an initial peak which declined to a plateau level with a time constant of 2 s and then remained constant or slowly decreased. The discontinuation of the application was followed by an after-current. The individual components of the responses were insensitive to TTX (2 X 10(-6) M) and were present in neurones which did not exhibit any sign of synaptic activity. The responses induced by kainate were monophasic and declined slowly during long-lasting application. 3. The responses induced by NMDA, aspartate and glutamate were voltage dependent, while those induced by kainate were linear between -80 and +80 mV. The equilibrium potential for all components of the responses to all excitatory amino acids was close to zero. 4. From dose-response curves the half-maximum effective dose (ED50) for glutamate and kainate was 3 X 10(-5) and 2 X 10(-4) M respectively. The Hill coefficients for the glutamate and the kainate were calculated to be 1.8 +/- 0.1 (n = 4) and 1.9 +/- 0.5 (n = 4) respectively. Thus two molecules may be interacting with each of the receptor-activated ion channels. 5. Interaction between kainate and quisqualate or kainate and NMDA was studied at both negative and positive holding potentials. No summation of the responses was found when kainate at concentrations close to those required for evoking the maximum response was applied simultaneously with quisqualate or NMDA. On the contrary, a diminution of the membrane currents was observed. A marked decrease in membrane currents was also observed when glutamate (10(-4) M) was applied simultaneously with aspartate (10(-4) M). 6. Glutamate-activated single-channel currents were recorded in the cell-attached configuration with electrodes filled with glutamate (20 microM) in five neurones and a conductance approximately 50 pS was found. 7. It is suggested that differences in the potency of the different excitatory amino acids as open-channel blockers may be one of the mechanisms which contribute to the diversity in the action of excitatory amino acids and that at least some of the effects of NMDA, aspartate, glutamate, quisqualate and kainate may be mediated by a common receptor-channel complex.

Multiple-conductance channels activated by excitatory amino acids in cerebellar neurons

In the mammalian central nervous system amino acids such as L-glutamate and L-aspartate are thought to act as fast synaptic transmitters. It has been suggested that at least three pharmacologically-distinguishable types of glutamate receptor occur in central neurons and that these are selectively activated by the glutamate analogues N-methyl-D-aspartate (NMDA), quisqualate and kainate. These three receptor types would be expected to open ion channels with different conductances. Hence if agonists produce similar channel conductances this would suggest they are acting on the same receptor. Another possibility is suggested by experiments on spinal neurons, where GABA (gamma-amino butyric acid) and glycine appear to open different sub-conductance levels of one class of channel while acting on different receptors. By analogy, several types of glutamate receptor could also be linked to a single type of channel with several sub-conductance states. We have examined these possibilities in cerebellar neurons by analysing the single-channel currents activated by L-glutamate, L-aspartate, NMDA, quisqualate and kainate in excised membrane patches. All of these agonists are capable of opening channels with at least five different conductance levels, the largest being about 45-50 pS. NMDA predominantly activated conductance levels above 30 pS while quisqualate and kainate mainly activated ones below 20 pS. The presence of clear transitions between levels favours the idea that the five main levels are all sub-states of the same type of channel.

Glutamate activates multiple single channel conductances in hippocampal neurons

There is considerable evidence that glutamate is the principal neurotransmitter that mediates fast excitatory synaptic transmission in the vertebrate central nervous system. This single transmitter seems to activate two or three distinct types of receptors, defined by their affinities for three selective structural analogues of glutamate, NMDA (N-methyl-D-aspartate), quisqualate and kainate. All these agonists increase membrane permeability to monovalent cations, but NMDA also activates a conductance that permits significant calcium influx and is blocked in a voltage-dependent manner by extracellular magnesium. Fast synaptic excitation seems to be mediated mainly by kainate/quisqualate receptors, although NMDA receptors are sometimes activated. We have investigated the properties of these conductances using single-channel recording in primary cultures of hippocampal neurons, because the hippocampus contains all subtypes of glutamate receptors and because long-term potentiation of synaptic transmission occurs in this structure. We find that four or more distinct single-channel currents are evoked by applying glutamate to each outside-out membrane patch. These conductances vary in their ionic permeability and in the agonist most effective in causing them to open. Clear transitions between all the conductance levels are observed. Our observations are compatible with the model that all the single channel conductances activated by glutamate reflect the operation of one or two complex molecular entities.

Dopamine enhances excitatory amino acid-gated conductances in cultured retinal horizontal cells

In the teleost retina, cone horizontal cells receive extensive innervation from dopaminergic interplexiform cells, and possess dopamine receptors whose activation stimulates adenylate cyclase. Exogenously applied dopamine modifies several aspects of horizontal cell activity in the intact retina, including the responsiveness of these neurons to light and the strength of electrical coupling between them. We have used whole-cell voltage clamp methods to examine whether dopamine can alter the light-responsiveness of horizontal cells by changing their sensitivity to the neurotransmitter released by the photoreceptors. We report that dopamine and cyclic AMP, although having little direct effect on resting membrane conductance, greatly enhance ionic conductances gated by kainate, an agonist of the transmitter released by the photoreceptors, and by L-glutamate, the agent proposed to be the photoreceptor transmitter. Our results provide the first direct evidence for dopaminergic regulation of excitatory amino-acid neurotransmission in the vertebrate nervous system and suggest a possible mechanism to explain the reduction in the responsiveness of horizontal cells observed when retinas are treated with dopamine.

Quisqualate receptors are specifically involved in cerebellar synaptic plasticity

Long-term modification of transmission efficacy at synapses is the cellular basis of memory and learning. A special type of synaptic plasticity in the cerebellum was postulated theoretically, and has since been verified. Each cerebellar Purkinje cell (PC) receives two distinct excitatory inputs, one from parallel fibres (PFs) and the other from a climbing fibre (CF). When these two types of inputs are conjunctively activated, PF-PC transmission undergoes long-term depression (LTD). Accumulated evidence suggests that LTD plays a role in the motor learning processes of the cerebellum. At the molecular level, LTD appears to be caused by desensitization of receptor molecules in PC dendrites towards the PF neurotransmitter, presumably L-glutamate (Glu). Glu receptors are heterogeneous and can be divided into several subtypes. In this study, we compared the potency of several Glu agonists in inducing LTD and found a highly selective dependency of LTD on the quisqualate(QA)-selective subtype of Glu receptors.

Neurotoxic action of kainic acid in the isolated toad and goldfish retina: I. Description of effects

The neurotoxic action of kainic acid (KA) was investigated by histological methods in the isolated retina of toads and goldfish. Particular attention was paid to the earliest and most sensitive response to KA in the outer plexiform layer (OPL). KA caused vacuolization of proximal and distal segments of horizontal cell dendrites in the OPL as well as perikaryal vacuolization and/or chromatin clumping in selected classes of neurons in the inner nuclear layer. Further, KA caused vacuolization and swelling in the inner plexiform layer. These effects were very similar in the retinae of goldfish and toad. The extent of vacuolization in the OPL was graded with KA concentration and with length of incubation. For 15-minute incubations, half-maximal vacuolization was found at 10-20 microM KA. At 25 microM KA, OPL vacuolization was evident within 1-2 minutes of application of KA. In goldfish, but not in toad, rod-connecting dendrites were less sensitive to KA than cone-connecting dendrites.

Neurotoxic action of kainic acid in the isolated toad and goldfish retina: II. Mechanism of action

The specificity and mechanism of the neurotoxic action of kainic acid (KA) was investigated by histological methods in the isolated retina of toads and goldfish. Particular attention was paid to the earliest and most sensitive response to KA in the outer plexiform layer (OPL). Of 21 compounds tested as potential mimics of KA neurotoxicity in the OPL, only the enantiomers of glutamate and aspartate mimicked KA, inducing a low-level neurotoxic effect at concentrations 5,000-10,000-fold higher than concentrations of KA giving comparable effects. Further, of 22 compounds tested as potential blockers of KA neurotoxicity in the OPL, only D-gamma-glutamylglycine, D,L-alpha-amino pimelic acid, sodium pentobarbital, D,L-alpha-amino adipic acid, L-glutamate, and L-aspartate blocked KA neurotoxicity (IC50 values of 0.1, 0.3, 0.3, 2, 5, and 15 mM, respectively). In ionic substitution experiments, KA-induced vacuolization was found to require sodium and chloride ions but not calcium ions in the extracellular medium. These findings support the hypothesis that KA combines with specific receptors in the membrane of susceptible neurons in the retinal OPL, leading to prolonged opening of membrane channels permeable to sodium and potassium ions. An accompanying equilibrating chloride influx may result in intracellular ion excess, leading to osmotic swelling and vacuolization. The membrane receptors involved in mediating the action of KA in the OPL are likely to be a class of postsynaptic or extrasynaptic glutamate receptor.