Cooperative interaction of glutamate and aspartate with receptors in the neuromuscular excitatory membrane in walking limbs of the lobster

Cross-species comparison of metabolite profiles in chemosensory epithelia: an indication of metabolite roles in chemosensory cells

Comparative studies of chemosensory systems in vertebrates and invertebrates have greatly enhanced our understanding of anatomical and physiological constraints of chemical detection. Immunohistochemical comparisons of chemosensory systems are difficult to make across species due to limited cross-reactivity of mammalian-based antibodies. Immunostaining chemosensory tissues with glutaraldehyde-based antibodies generated against small metabolites in combination with hierarchical cluster analyses provide a novel approach for identifying and classifying cell types regardless of species. We used this "metabolite profiling" technique to determine whether metabolite profiles can be used to identify cell classes within and across different species including mouse, zebrafish, lobster and squid. Within a species, metabolite profiles for distinct cell classes were generally consistent. We found several metabolite-based cell classifications that mirrored function or receptor protein-based classifications. Although profiles of all six metabolites differed across species, we found that specific metabolites were associated with certain cell types. For example, elevated levels of glutathione were characteristic of nonsensory cells from vertebrates, suggesting an antioxidative role in non-neuronal cells in sensory tissues. Collectively, we found significantly different metabolite profiles for distinct cell populations in chemosensory tissue within all of the species studied. Based on their roles in other systems or cells, we discuss the roles of L-arginine, L-aspartate, L-glutamate, glycine, glutathione, and taurine within chemosensory epithelia.

Inhibitory glutamate receptor channels

Inhibitory glutamate receptors (IGluRs) are a family of ion channel proteins closely related to ionotropic glycine and gamma-aminobutyric acid (GABA) receptors; They are gated directly by glutamate; the open channel is permeable to chloride and sometimes potassium. Physiologically and pharmacologically, IGluRs most closely resemble GABA receptors; they are picrotoxin-sensitive and sometimes crossdesensitized by GABA. However, the amino acid sequences of cloned IGluRs are most similar to those of glycine receptors. Ibotenic acid, a conformationally restricted glutamate analog closely related to muscimol, activates all IGluRs. Quisqualate is not an IGluR agonist except among pulmonate molluscs and for a unique multiagonist receptor in the crayfish Austropotamobius torrentium. Other excitatory amino acid agonists are generally ineffective. Avermectins have several effects on IGluRs, depending on concentration: potentiation, direct gating, and blockade, both reversible and irreversible. Since IGluRs have only been clearly described in protostomes and pseudocoelomates, these effects may mediate the powerful antihelminthic and insecticidal action of avermectins, while explaining their low toxicity to mammals. IGluRs mediate synaptic inhibition in neurons and are expressed extrajunctionally in striated muscles. The presence of IGluRs in a neuron or muscle is independent of the presence or absence of excitatory glutamate receptors or GABA receptors in the cell. Generally, extrajunctional IGluRs in muscle have a higher sensitivity to glutamate than do neuronal synaptic receptors. Some extrajunctional receptors are sensitive in the range of circulating plasma glutamate levels, suggesting a role for IGluRs in regulating muscle excitability The divergence of the IGlu/GABA/Gly/ACh receptor superfamily in protostomes could become a powerful model system for adaptive molecular evolution. Physiologically and pharmacologically, protostome receptors are considerably more diverse than their vertebrate counterparts. Antagonist profiles are only loosely correlated with agonist profiles (e.g., curare-sensitive GABA receptors, bicuculline-sensitive AChRs), and pharmacologically identical receptors may be either excitatory or inhibitory, and permeable to different ions. The assumption that agonist sensitivity reliably connotes discrete, homologous receptor families is contraindicated. Protostome ionotropic receptors are highly diverse and straightforward to assay; they provide an excellent system in which to study and integrate fundamental questions in molecular evolution and adaptation.

The excitatory actions of kainic acid and some derivatives at the crab neuromuscular junction

The actions of kainic acid and its derivatives including the new tricarboxylic kainic acid were tested on the Hermit crab neuromuscular junction. Kainic acid had a direct excitatory effect producing a depolarization. Although not so potent, alpha-ketokainic acid and tricarboxylic kainic acid mimicked this effect whilst dihydrokainic acid did not. The slopes of the dose-response curves for these kainic acid analogues differed from that of L-glutamate. A Hill coefficient approximating one was obtained for kainic acid compared to a value of three for L-glutamate. Threshold concentrations of l-glutamate and kainic acid but not alpha-ketokainic acid and tricarboxylic kainic acid potentiated the neurally evoked excitatory junction potentials and the ionophoretic L-glutamate potential. The results indicate a possible interaction of the kainic acid derivatives with a heterogeneous receptor population and provide a comparison with other invertebrate and vertebrate systems.

Modulation of neuronal responses to L-glutamate in Aplysia

Potentiation of the excitatory response to L-glutamate (Glu) by L-aspartate (Asp), similar to that which has been described at the crustacean neuromuscular junction, is observed in Aplysia neurons which are glutamate sensitive. Potentiation of the inhibitory responses to ionophoretically applied Glu in neurons preconditioned with Asp permits experiments which serve to differentiate among four hypotheses previously proposed to explain the underlying mechanism of the phenomenon. The potentiation is inhibited by cooling (Q10 = 1.3 +/- 0.2) and is blocked in Na+-free seawater, where the response to Glu applied alone is increased in both amplitude and duration. These results are most consistent with the view that Glu is normally removed from the extracellular medium through an active reuptake process which is Na+ dependent, is slightly temperature sensitive, and may be blocked by Asp. Potentiation of the excitatory response to L-glutamate (Glu) by L-aspartate (Asp) has been previously described at the crustacean neuromuscular junction (Kravitz et al., 1970; Nistri and Constanti, 1979). This potentiation has been attributed to an Asp-induced change in conformation of the Glu receptor, thereby increasing its affinity for Glu (Shank and Freeman, 1975); suppression of the rate of desensitization of the Glu receptor induced by Asp (Dudel, 1977); blockade by Asp of a Glu reuptake process (Crawford and McBurney, 1977); and release, triggered by Asp, of a bound store of Glu (Constanti and Nistri, 1978).(ABSTRACT TRUNCATED AT 250 WORDS)

The effects of some putative transmitters and biogenic amines on uropod abductor muscle in the crayfish Procambarus clarkii and Cambaroides japonicus

The effects of some putative transmitters and biogenic amines were examined on the uropod ventral abductor exopodite (AbdExV) muscle in two crayfish species Procambarus clarkii and Cambaroides japonicus. Bath application of L-glutamate to the AbdExV muscle caused sustained contract while gamma-aminobutyric acid (GABA) depressed the nerve-evoked contraction of the muscle. Acetylcholine (ACh) had no effect on both the resting tension and the nerve-evoked contraction. Iontophoresis of L-glutamate and GABA onto the surface of the muscle fiber further confirmed that glutamate and GABA are the possible excitatory and inhibitory transmitters respectively at the neuromuscular junction of AbdExV muscle. Bath application of 5-hydroxytryptamine (5-HT) and octopamine (Oct) caused enhancement of the nerve-evoked contraction but dopamine (DA) had no effect on both the resting tension and the nerve-evoked contraction.

On the quantal release of endogenous glutamate from the crayfish neuromuscular junction

1. The abdominal slow flexor muscle was isolated from the crayfish (Cambarus clarkii) and placed in 150 microliters. Harreveld solution. The concentrations of glutamate and aspartate in this solution were measured by mass fragmentography. 2. Application of black widow spider venom (BWSV) produced a marked increase in the frequency of miniature excitatory post-synaptic potentials (m.e.p.s.p.s). During the high frequency discharge of m.e.p.s.p.s, the glutamate content in the solution was significantly increased. There was an approximately linear relationship between the increase in the glutamate efflux produced by BWSV and the variance of the membrane potential fluctuation during high frequency discharge of m.e.p.s.p.s. 3. In most cases, the efflux of aspartate during control rest periods was smaller than that of glutamate. During the discharge of m.e.p.s.p.s produced by BWSV, the increase in the aspartate efflux was very small compared to glutamate. 4. Nerve stimulation caused a significant increase in the efflux of glutamate, but the change in the aspartate efflux was very small and not significant. 5. Application of methylene blue increased the frequency of m.e.p.s.p.s and glutamate efflux, but little, if any, increase was found in aspartate efflux. 6. It is concluded that glutamate is preferentially released from nerve terminals in a quantal fashion.

Effects of high calcium solutions on glutamate sensitivity of crayfish muscle fibres

Crayfish neuromuscular preparations were studied after 18--36 h exposure to high calcium solutions. As previously reported for frog neuromuscular preparations the treatment damaged the nerve terminals and decreased junctional potentials. The resting potentials and input resistances of the muscle fibres were not affected; but their sensitivity to glutamate was significantly decreased when compared to that of control muscles. After exposure to high calcium, the sensitivity to gamma-aminobutyric acid, the putative transmitter at inhibitory synapses, was increased. Apparently normal twitches were elicited by direct stimulation, and calcium spikes could still be observed in the fibres. A decreased sensitivity to glutamate was also noted in experiments carried out on denervated muscles 8 months after section of the motor axons. Possible relations between nerve terminal damage and the decrease in sensitivity to glutamate are discussed.

Thyrotropin-releasing hormone has multiple actions in cortex

The responses to application of TRH were examined on 38 identified neurons in sensory motor cortex of cat. Two pyramidal tract (PT) and 3 nonpyramidal tract (NPT) neurons were directly excited by TRH. Two other NPT neurons were inhibited by TRH. TRH potentiated the excitatory action of ACh on 4 of 12 PT neurons and 1 of 2 unidentified cells. None of these neurons showed a direct effect of TRH. TRH did not potentiate the response to Glu on 12 cells tested, even when the same cell showed TRH modulation of the ACh response. None of the NPT cells examined showed TRH modulation of the excitatory ACh response. These results demonstrate that TRH has multiple actions in mammalian cerebral cortex, but that these actions, such as the modulation of the ACh responses, appear to exist on discrete populations of neurons.

Interaction between nerve-related acetylcholine and bath applied agonists at the frog end-plate

1. The interaction between acetylcholine (ACh) and carbachol (CCh) has been studied at the frog end-plate. The conditioning agonist, CCh, can cause desensitization (reduction of the ACh test response) and potentiation (increase of the test response). 2. Nerve-evoked end-plate currents (e.p.c.s), minature e.p.c.s and "slow" responses to ACh ionophoresis can all be potentiated by bath or ionophoretically applied CCh. 3. Since potentiation was found to be particularly visible at low temperatures, most experiments were performed at 5-8 degrees C. Potentiation results in an increase of both e.p.c. amplitude and e.p.c. decay time. Potentiated e.p.c.s teminate with a slow tail, the amplitude of which shows a high voltage sensitivity. Potentiation increases with CCh concentration (range studied 0-100 microM). It appears to persist throughout the application of CCh, even when desensitization is apparently the dominant phenomenon. 4. It is suggested that cross-potentiation of ACh by CCh results from the formation of intermediate non-conducting CCh-receptor complexes which have a high probability of being subsequently activated by ACh, yielding a conducting ACh-CCh-receptor complex. 5. Desensitization induced by fast bath application of CCh (or ACh) develops in two phases and can be fitted by the sum of two exponentials. Their time constants are in the second and the minute range, respectively. 6. The possibility that the slow phase is linked to the presence of agonist inside the cell is rejected.

Effect of hippocampus extirpation in the rat on glutamate levels in target structures of hippocampal efferents

Twenty days after complete uni- or bilateral hippocampus extirpation in rats, a 25% decrease in glutamate concentration was observed in the septum. Glutamate content also decreased in other terminal structures of the hippocampo-subicular system, i.e. entorhinal cortex, nucleus accumbens septi, mammillary bodies and contralateral hippocampus. It is concluded that the fall in glutamate content which is absent in caudate nucleus is specific for target regions of the hippocampal efferents, adding further support to the suggested transmitter role of glutamate in the limbic system.

Further observations on the interaction between glutamate and aspartate on lobster muscle

1 The ability of bath-applied L-glutamate to enhance subsequent depolarizations produced by bath-applied L-aspartate on lobster muscle was further investigated by means of intracellular recording techniques. 2. Increasing the conditioning glutamate concentration or exposure time produced a greater enhancement of aspartate responses. Enhancement was also dependent on the time interval between glutamate and aspartate doses and was not prevented by overnight storage of preparations in vitro. 3. The dose-depolarization curve for enhanced aspartate responses (measured at a fixed time following a given dose of glutamate) was displaced to the left along the abscissa scale relative to control, with no detectable change in limiting log-log slope. 4. Conditioning doses of kainate or domoate (but not quisqualate, aspartate, or KCl) also enhanced aspartate responses; however, their conditioning effect was little affected by increasing the concentration, exposure time, or time interval before applying aspartate. The rate of onset and decline of the enhanced aspartate response always resembled that of the previous conditioning agonist. 5. D and L-Aspartate were approximately equieffective depolarizing agents whereas D-glutamate was approximately 1/40 as potent as L-glutamate. After a conditioning dose of D or L-glutamate, responses to D or L-aspartate were enhanced. 6. In a Na+-free (Li+) medium, both the glutamate depolarization and the conditioning effect towards aspartate were largely abolished. With kainate however, Na+ was not apparently important either for evoking the kainate response or for producing the conditioning effect. 7. Bath-applied glutamate greatly enhanced and prolonged the time course of the iontophoretic aspartate potential with only a small effect on the glutamate potential; however, these effects were not maintained after washout of glutamate. In contrast, bath-application of aspartate depressed the aspartate potential while enhancing the glutamate potential. Some sites that were insensitive to iontophoretically-applied aspartate became clearly responsive to this agent during a bath-application of glutamate. 8. It is proposed that during conditioning with bath-applied glutamate, kainate or domoate, some agonist is trapped by extrajunctional sites and is subsequently displaced by bath-applied aspartate to produce the long-term enhancement effect.

Distribution of glutamate in layers of the rabbit hippocampal fields CA1, CA3, and the dentate area

The distribution of L-glutamic acid in single layers dissected from freeze-dried sections of the rabbit hippocampus was estimated by means of an enzymatic cycling procedure. Although the regional variation in glutamate content is less than that of other transmitter candidates, there were present significant differences in it between the projection fields of certain fiber connections. Glutamate concentration was highest in the termination areas of commissural fibers and Schaffer's collaterals.

A study of the interactions between glutamate and aspartate at the lobster neuromuscular junction

The depolarization produced by bath-applied or iontophoretically applied glutamate and aspartate were recorded from lobster muscle fibres by means of intracellular microelectrodes. 2 Bath-applied glutamate or aspartate evoked reversible, membrane depolarizations; however, responses to repeated applications of aspartate decreased progressively in amplitude until a plateau level was attained. Repeated applications of glutamate, kainate, domoate or quisqualate did not produce a similar effect. 3 After a dose of glutamate, responses to bath-applied aspartate were enhanced. Responses to other depolarizing agonists were little affected by previous administration of glutamate. Aspartate dose-depolarization curves were therefore constructed after initial aspartate responses had stabilized. The log-log transforms of the aspartate and glutamate curves had limiting slopes of 0.8 and 2.1 respectively. 4 Iontophoretic application of aspartate to single glutamate-sensitive sites produced small depolarizations with slow time course, compared with the glutamate potentials. When aspartate and glutamate were pulsed simultaneously from a twin-barrelled pipette, the resultant glutamate potential was enhanced. It is suggested that this potentiation was due to summation of agonist concentrations in the receptor region interacting with a second-order dose-response relationship. 5 Bath-applied aspartate increased the amplitude and prolonged the half-decay time of the glutamate potential. This effect was particularly noticeable when the glutamate potential was of slow time course. 6 It is proposed that bath-applied aspartate has an agonist effect whose magnitude is possibly exaggerated by concomitant release of glutamate and/or inhibition by glutamate of aspartate uptake. This agonist action of aspartate is thought to be exerted mainly on extrajunctional areas of the glutamate-sensitive sites.

Aspartate and other inhibitors of excitatory synaptic transmission in crayfish muscle

Synaptic currents were measured in voltage clamped crayfish muscle fibers which were triggered either by stimulation of the motor axon (EPSC), or by L-gutamate (gEPSC) applied by microiontophoresis or superfusion. Among a number of analogues of glutamate, L-glutamic-acid-gamma-methyl ester, L-glutamic-acid-dimethyl ester and L-aspartate, were reasonably specific antagonists at the motor synapses, although at relatively high concentrations. Aslo, 2-amino-4-phosphono-butyric acid and morphine were effective antagonists; the action of morphine, however, seemed to be unspecific. Aspartate was further shown to decrease the size of the quantum EPSC, without affecting the probability of release of transmitter or the potential change recorded from the presynaptic nerve terminal. The results also indicate that aspartate, after longer incubations, is released as a false transmitter. The dose-response curve to short glutamate pulse is shifted by aspartate to higher glutamate concentrations, without affecting the steep slope of the dose-response curve or the saturation level. This effect can be interpreted as competitive inhibition by aspartate, with an equilibrium concentration of aspartate at the receptor of 0.3--1.5 mmol/l. In longer glutamate applications the receptor desensitizes rapidly. Aspartate reduces this desensitization in addition to its competitive inhibitory effect. Suppression of desensitization can be more effective than inhibition in long glutamate applications; in this case aspartate apparently potentiates the effects of glutamate.

Aspartate: distinct receptors on Aplysia neurons

Aplysia neurons have specific aspartate receptors that are distinct from those to glutamate. In some cells, asparate selectively increases the membrane permeability to chloride, giving rise to a hyperpolarization, while on other cells it increases the permeability to sodium, causing a depolarization. There are also specific receptors for L-glutamate which mediate sodium, chloride, or potassium conductance increases, and another class of receptors activated by both glutamate and aspartate.

Agonistic and antagonistic activity of glutamate analogs on neuromuscular excitation in the walking limbs of lobsters

Forty-six analogs of L-glutamate were tested for activity on muscle fibers in the walking limbs of lobsters. Effects on the membrane potential, input resistance, and amplitude of neurally evoked EPSPs and IPSPs were studied as well as effects on applied L-glutamate. Seventeen of the compounds studied depolarized the muscle fibers in a manner indicative of an agonistic action on receptors in the neuromuscular excitatory membrane. Six analogs selectively reduced the amplitude of evoked EPSPs, and at least three of these (kainic acid, D-glutamate, and D-aspartate) antagonized the excitatory action of applied L-glutamate. Kainic acid was the most potent of the blockers of neuromuscular excitation, but even it was relatively weak since a concentration of 1 mM was required for an apparent effect. Generally those analogs in the L-configuration which possessed activity, had agonistic actions, whereas those in the D-configuration were usually antagonistic. These observations provide pharmacological evidence for the concept that L-glutamate is the transmitter agent which mediates neuromuscular excitation in the walking limbs of lobsters. In addition, our results are consistent with recent studies which indicate that L-aspartate may also function in this neuromuscular excitatory process.