Effects of D-alpha-aminoadipate on excitation of afferent fibers in the lateral line of Xenopus laevis

The effects of D-alpha-aminoadipate (D alpha AA) on excitation of afferent nerve fibers in the Xenopus laevis lateral line were studied in vitro. D alpha AA reversibly suppressed spontaneous activity and excitation induced by water motion at concentrations as low as 0.25-0.5 mM. Higher concentrations (up to 10 mM) caused a greater suppression that was rapidly and fully reversible. L-alpha-Aminoadipate at 0.25-1.0 mM had no suppressive effects. Responses elicited by NMDA (1.0-2.0 mM) were the most sensitive to D alpha AA (0.25-0.5 mM), those elicited by L-aspartate and L-glutamate (1.0-2.0 mM) were less sensitive and similar, and those elicited by kainate (5-15 muM) were the least sensitive. The results provide evidence that the transmitter released by hair cells in the Xenopus lateral line interacts postsynaptically with NMDA-preferring receptors and that the transmitter is an excitatory amino acid, possibly L-glutamate or L-aspartate.

Effects of intravitreally injected DL-alpha-aminoadipic acid on the c-wave of the D.C.-recorded electroretinogram in albino rabbits

The D.C. electroretinogram of four adult albino rabbits was studied 13.5 to 15 hr after injection of 0.1 ml of 0.15M DL-alpha-aminoadipic acid (alpha-AAA) into the vitreous body of one eye. The other eye was similarly injected with saline and served as control. In the alpha-AAA--treated eye the b-wave was markedly reduced or absent and the a- and c-waves increased compared with the control eye. Sincs alpha-AAA damages the Müller cells, the results support the view that these cells are related to the generation of the b-wave and to the negative slow PIII-potential, which modifies the positive pigment epithelial component of the c-wave.

On the configuration of the receptors for excitatory amino acids

Separate receptors are recognized for the excitation of mammalian neurones by (a) L-glutamic and quisqualic acids and (b) N-methyl-D-aspartic (NMDA), and other amino acids which have conformationally restricted molecules. Several other compounds, both agonists and antagonists, have been examined, and it is concluded that (i) the NMDA receptor reacts preferentially with substances in a relatively extended configuration, (ii) the glutamate/quisqualate receptor prefers folded molecules and (iii) the distance separating the amino group from the distal anionic function is the critical one determining receptor preference.

The nature of the excitatory transmitter mediating X and Y cell inputs to the cat dorsal lateral geniculate nucleus

1. In experiments examining the possibility that an excitatory amino acid may be an optic nerve transmitter in mammals, excitatory amino acid antagonists have been ionophoretically applied to cells in layers A and A(1) of the cat dorsal lateral geniculate nucleus and their effect on the excitatory response to visual stimulation of the receptive field centre has been assessed.2. The antagonists used were D-alpha-aminoadipate (D-alpha-AA), DL-alpha-epsilon-diaminopimelic acid (DAP), 1-hydroxy-3-amino-2-pyrrolidone (HA-966) and L-glutamate diethyl ester (GDEE). The antagonist effects on the visual response were compared with their effect on similar magnitude responses evoked by ionophoretic pulses of selected agonists and a control excitant, generally acetylcholine.3. Both D-alpha-AA and HA-966 would selectively block or depress the visual response with respect to the response to the control excitant. At the stage the visual input was blocked, responses to the agonists N-methyl-D-aspartate (NMDA), DL-homocysteic acid (DLH) and glutamate were also greatly reduced or blocked. At dose levels below those causing a significant reduction in the visual response, D-alpha-AA and HA-966 would selectively depress responses to NMDA and DLH with respect to the response to glutamate.4. GDEE was relatively ineffective in blocking either agonist responses or the visual response and only produced a significant reduction in either at dose levels that caused a similar depression in the response to acetylcholine. DAP would block responses to DLH but produced no significant effect on the visual response or the responses to glutamate and acetylcholine.5. The cholinergic antagonists atropine and dihydro-beta-erythroidine (DHbetaE) blocked responses to acetylcholine without significantly reducing either visual driving or the response to DLH.6. The effects were the same for X and Y cells in the dorsal lateral geniculate nucleus (dLGN). There was also no distinctions between ;on' and ;off' centre types of each of the two groups.7. The significance of these results is discussed. It is argued that they reintroduce the possibility that either L-aspartate, L-glutamate or a similar substance may be the transmitter mediating the optic nerve input to the cat dLGN.

Effect of glutamate, aspartate and related derivatives on cerebellar purkinje cell dendrites in the rat: an in vitro study

1. The responses of Purkinje cells to short duration (pulse) ionophoretic applications of L-aspartate (L-asp), L-glutamate (L-glu), N-methyl DL-aspartate (NMDLA) and quisqualic acid in their dendritic fields were studied in vitro on sagittal slices of lobules IX and X of the adult rat cerebellum.2. Pulse application of L-asp or L-glu evoked transient and dose-dependent increases in the firing rate of the simple spikes recorded extracellularly as single units. When the ionophoretic electrode was positioned in the dendritic field of the Purkinje cells, the lowest thresholds for L-glu and L-asp mediated excitations of the cells were as low as 25 and 35 pC respectively, with a latency for maximal responses as brief as 7 ms.3. In intracellular recordings these excitatory responses consisted of depolarizations of up to 18 mV in amplitude and with depolarizing slopes up to 0.52 mV/ms. They were generally unaccompanied by changes in cell input resistance in contrast to the marked decrease which occurred in response to steady applications of large doses of L-asp and L-glu.4. The spatial distribution of the excitatory sites confirmed that the dendritic sensitivity to L-glu was greater than that of the soma and showed that the same was true for L-asp. In 34% of cells the sensitivity for L-asp declined markedly in the upper region of the molecular layer, whereas it remained high for L-glu; no such differential sensitivity was detected in the remaining 66% of cells.5. Inhibitory responses, antagonized by 10(-5) M-bicuculline in the bath, were also induced in Purkinje cells by L-glu and L-asp when the ionophoretic electrode was withdrawn from the excitatory sites by as little as 8 mum and up to 40 mum upward or downward along the track of parallel fibres or positioned as far as 250 mum laterally.6. Whenever it was applied in the molecular layer, the pulse application of NMDLA elicited no excitatory response in Purkinje cells recorded extra or intracellularly. However, slow depolarizations accompanied by a slight increase in cell input resistance were obtained with steady applications of 20-50 nA of the drug for 20-30 s.7. In contrast, pulse application of quisqualic acid appeared to have the same type of fast excitatory effect on Purkinje cells as L-asp and L-glu, but its potency was greater and its action more prolonged. Furthermore, its steady application led to an abrupt and marked decrease in cell membrane resistance.8. The excitatory effects of L-asp, L-glu and quisqualic acid were antagonized by L-glutamic acid diethyl ester more consistently than by D-alpha-aminoadipate, suggesting together with previous observations that L-asp and L-glu act on Purkinje cells via quisqualic acid rather than via NMDLA receptors.

Effects of DL-alpha-amino adipic acid on Müller cells in frog and chicken retinae in vivo: relation to ERG b wave, ganglion cell discharge and tectal evoked potentials

In both frog and chicken, an intravitreal injection of DL-alpha-amino-adipic acid, (DL-alpha aaa) provoked a progressive depression and eventually the disappearance of the ERG b wave that was concomitant with severe damage to the Müller cells without any apparent damage to retinal neurons. Ganglion cell discharges as well as tectal evoked potentials were still recorded, i.e. a visual message was still generated in the retina and transmitted to the optic tectum, when the Müller cells had been damaged to as to provoke an abolition of the ERG b-wave. The whole of the drug-induced effects proved to be reversible.

D-aminophosphonovalerate is 100-fold more powerful than D-alpha-aminoadipate in blocking N-methylaspartate neurotoxicity

Here we report that the D-isomers of 2-amino-5-phosphonovalerate (D-APV) and alpha-amino-adipate (D-alpha AA) protect arcuate hypothalamic neurons from the potent excitotoxic activity of N-methylaspartate (NMA). Consistent with evidence that APV is much more powerful than alpha AA in antagonizing the neuroexcitatory activity of NMA, we found D-APV nearly 100 times more powerful than D-alpha AA in preventing NMA from destroying arcuate neurons.

Further studies of the chemical sensitivity of the oscillatory potentials of the electroretinogram (ERG). II. Glutamate-aspartate-and dopamine antagonists

The effects on the oscillatory potentials (OPs) of the ERG of some pharmacological agents, which antagonize the putative neurotransmitters glutamate aspartate and dopamine were tested. GDEE and alpha-aminoadipate, blocking agents of the excitatory amino acids glutamate and aspartate, respectively, decreased the amplitude of all the OPs. The amplitude of the b-wave was simultaneously reduced. Thus, no selective or differential sensitivity of the OPs was found, which suggests that glutamate- or aspartate-sensitive neuronal pathways do not seem to be directly involved in the generation of the OPs. Haloperidol, which antagonizes the effect of the inhibitory transmitter agent dopamine, differentially affected the OPs. The earlier OPs appeared more sensitive to low doses of the drug than the later ones. Such doses only slightly decreased the maximum amplitude of the b-wave and did not affect the a-wave. Thus, these findings indicate and support previous suggestions that the individual oscillatory peaks are likely to have different origins and that only inhibitory feed-back synapses give rise to the OPs.

Selective synaptic antagonism by atropine and alpha-aminoadipate of pulvinar and cortical afferents to the suprasylvian visual area (Clare-Bishop area)

The synaptic excitations of cells of the Clare-Bishop cortical region produced by electrical stimulation of the pulvinar and ipsilateral cortex, have been shown to be differentially antagonized by iontophoretically applied antagonists. Atropine attenuated the responses evoked by pulvinar stimulation without having an appreciable effect against either iontophoretically applied aspartate or cortically evoked responses. alpha-Aminoadipate antagonized aspartate elicited excitations and those obtained with cortical stimulation while leaving unaffected acetylcholine and pulvinar evoked responses. The results are supportive of the view that acetylcholine and aspartate, or a similar excitatory amino acid, act as synaptic transmitters of some afferents from the pulvinar and ipsilateral cerebral cortex, respectively.

Depression of ventral root--dorsal root potential by DL-alpha-aminoadipate in frog spinal cord

Ventral root--dorsal root potential (VR--DRP) was partially depressed in the presence of 1 mM DL-alpha-aminoadipate (DL-alpha-AAD), probably corresponding to 0.5 mM D-alpha-AAD, in isolated hemisected frog spinal cord. It was suggested that acidic amino acid receptors might be subserving for the generation of VR--DRP in addition to the initial cholinergic link between motoneurons and interneurons and the electrical coupling between motoneurons and primary afferent terminals and the last step between unidentified interneurons and primary afferent terminals which had been reported to be mediated by GABA.

Selective depression of synaptic excitation in cat spinal neurones by baclofen: an iontophoretic study

1 The effects of baclofen have been examined on responses of neurones in the spinal cord of the anaesthetized cat to stimulation of appropriate synaptic pathways, acetylcholine and a range of amino acid excitants. Baclofen and excitant substances were administered by standard microiontophoretic techniques. 2 Small ejecting currents of baclofen (less than 10 nA) depressed non-cholinergic, excitatory, synaptic responses evoked by stimulation of dorsal roots or muscle or cutaneous afferents. Excitatory monosynaptic responses were particularly sensitive to the depressant action of baclofen. 3 Spontaneous firing in neurones was sometimes reduced in parallel with synaptic excitatory responses, but synaptically evoked inhibition was unaffected and ventral root evoked excitation of Renshaw cells was either unaffected or enhanced by baclofen. 4 Ejecting currents of baclofen which markedly depressed excitatory synaptic responses either had no effect or minimal depressant effects on responses induced by iontophoretically administered acetylcholine and excitant amino acids. However, relatively large currents of baclofen (e.g. 20 to 40 nA) reduced excitatory responses to exogenously administered excitants. 5 It is suggested that baclofen depresses (a) synaptic response by an action on excitatory nerve terminals and (b) responses to exogenous excitants via a postsynaptic action. 6 Comparison of baclofen with a number of other substances indicates that the depression of noncholinergic, excitatory, synaptic responses is unlikely to involve an interaction of this agent with receptors for monoamines, 5-hydroxytryptamine or the inhibitory amino acids, gamma-aminobutyric acid and glycine.

Induction of glutamine synthetase in embryonic neural retina: its suppression by the gliatoxic agent alpha-aminoadipic acid

Competence for cortisol-mediated induction of glutamine synthetase (GS) is a differentiation marker of embryonic neural retina. Earlier work has indicated that the induction and accumulation of GS is localized in the Müller glia cells. This localization was presently confirmed by the finding that the gliatoxin D,L-alpha-amino-adipic acid (AAA) reduces responsiveness to GS induction by 60--90% due to preferential damage to Müller cells. The tests were performed on organ cultures of retina tissue from chick embryos, and on retina cell aggregates in which there is tissue reconstruction. The presence of GS-inducible Müller cells was monitored by immuno-staining of tissue sections with anti-GS antiserum. Reduction of GS inducibility due to pretreatment with AAA resulted in virtual absence of cells that immunostained for GS. The preferential toxicity of AAA for Müller cells was also demonstrated by cell viability tests; it was further corroborated by the finding that treatment with AAA greatly reduced the level of carbonic anhydrase activity, another enzyme localized predominantly in Müller cells, but did not affect gamma-aminobutyric acid transaminase and choline acetyl transferase, neuronal enzymes. Susceptibility of Müller cells to AAA was found to increase with embryonic development of the retina. We suggest that acquisition of susceptibility for AAA represents another differentiation marker of embryonic Müller cells.

Stereospecificity of the gliotoxic and anti-neurotoxic actions of alpha-aminoadipate

The glutamate (Glu) analog, DL-alpha-aminoadipate (DL-alpha AA), and the separate D and L isomers of alpha AA, were administered subcutaneously to infant mice and histopathological effects on the arcuate hypothalamic (AH) nucleus were studied. L-alpha AA induced striking gliotoxic and neurotoxic changes; D-alpha AA and DL-alpha AA respectively induced mild and extreme gliotoxic but not neurotoxic changes. The neurotoxicity of L-alpha AA is of interest in view of its known neuroexcitatory potential. The non-neurotoxicity of DL-alpha AA implies effective antagonism by D-alpha AA of the neurotoxicity of L-alpha AA, which is of interest in that D-alpha AA is recognized as an effective antagonist of amino acid excitants and is thought to block specifically at the excitatory receptor.

Effects of D,L-alpha-aminoadipate on postsynaptic amino acid responses in cultured mouse spinal cord neurons

The effects of the dicarboxylic amino acid, DL-alpha-aminoadipate (DLAA) on amino acid responses have been investigated using intracellular recordings from mouse spinal cord neurons grown in dissociated cell culture. DL-alpha-Aminoadipate markedly antagonized postsynaptic responses to iontophoretically appllied aspartate; antagonism of glutamate was much less prominent. DL-alpha-Aminoadipate altered the affinity of aspartate for its receptor while having no observed effects on aspartate-receptor cooperativity. No direct effects of DLAA on membrane potentials or passive membrane properties were seen at the currents used for antagonism. Responses to the inhibitory amino acids GABA and glycine were unaffected by DLAA.

Specific effects of alpha-D,L-aminoadipic acid on synaptic transmission in frog spinal cord

The sucrose gap technique was employed to investigate both synaptic and amino acid evoked responses from motoneurones or primary afferents of frog spinal cord. alpha-D,L-Aminoadipic acid (alpha-D,L-AAD) selectively antagonized responses to acidic amino acids, especially aspartate. The drug was most effective in antagonizing the polysynaptic components of synaptic potentials evoked by dorsal root or lateral column stimulation but had little effect on their monosynaptic components. The ventral root dorsal root potential which is thought to be mediated by a pathway that does not involve acidic amino acids was insensitive to alpha-D,L-AAD. These data, which were confirmed by intracellular recording from motoneurones, provided further evidence for the role of acidic amino acids in polysynaptic pathways in frog spinal cord.

Selective antagonist activity of 5-aminohex-2-enedioic acid on amino acid excitation of cat spinal neurones

D- and L-5-aminohex-2-enedioic acid (AHED) and the DL-2-bromo derivative, which are conformationally restricted analogues of the excitatory amino acid antagonist D-alpha-aminoadipic acid (D alpha AA), were tested microelectrophoretically on cat spinal neurones. D-AHED reduced the excitatory action of N-methyl-D-aspartate (NMDA) to a greater extent than that of L-glutamate and the potency of this action was a half to a third that of D alpha AA. The DL-2-bromo derivative excited neurons, an effect greater than that of L-glutamate and not blocked by either D alpha AA or L-glutamic acid diethyl ester.

Selective antagonism of amino acid-induced and synaptic excitation in the cat spinal cord

1. The effects of D-alpha-aminoadipate (DalphaAA), D-alpha-aminosuberate (DalphaAS) and other excitatory amino acid antagonists have been compared on the excitatory responses of neurones of the cat spinal cord to acetylcholine, a range of glutamate-related amino acids and stimulation of appropriate excitatory synaptic pathways. The ionophoretic technique was used for administration of excitants and antagonists. 2. DalphaAA and DalphaAS had little or no effect on acetylcholine-induced excitation of Renshaw cells. Responses of either Renshaw cells or dorsal horn neurones in the spinal cord to excitatory amino acids were depressed in the order: N-methyl-D-aspartate (NMDA), L-homocysteate, D-glutamate, ibotenate greater than D-homocysteate, L-aspartate, D-aspartate greater than L-glutamate, kainate and quisqualate. 3. These effects are consistent with the existence of different excitatory amino acid receptors, one type being sensitive to the actions of the antagonists, and activated predominantly by the NMDA group of excitants, with other receptors being relatively insensitive to DalphaAA and DalphaAS and activated predominantly by quisqualate and kainate. On this hypothesis, many amino acids are assumed to have mixed actions on DalphaAA-sensitive and -insensitive receptors. 4. 2-Amino-4-phosphonobutyrate (2APB) and L-glutamic acid diethyl ester (GDEE) produced different patterns of antagonism of excitatory amino acid-induced responses from those observed with DalphaAA and DalphaAS. Neither substance was as potent as DalphaAA or DalphaAS as an excitatory amino acid antagonist. 5. Both DalphaAA and DalphaAS selectively antagonized synaptic excitation of Renshaw cells evoked by dorsal root stimulation without affecting cholinergic excitation of these cells evoked by ventral root stimulation. These latter responses were selectively antagonized by dihydro-beta-erythroidine (DHbetaE). DalphaAA also antagonized synaptic excitation of unidentified dorsal horn neurones of the spinal cord evoked by dorsal root stimulation. Neither GDEE (particularly) nor 2APB were as effective as DalphaAA or DalphaAS as depressants of synaptic excitation. 6. Taken in conjunction with the results of in vitro studies on the specificity of action of Dalpha¿ and related substances, these observations suggest that certain synaptic excitations in the spinal cord are mediated by an excitatory amino acid transmitter, and that this transmitter interacts with receptors which are activated selectively by NMDA, less selectively by other amino acids, including L-aspartate, and probably only slightly by quisqualate, kainate and (exogenous) L-glutamate.

Amino acids as neurotransmitters of corticofugal neurones in the rat: a comparison of glutamate and aspartate

1 The relative sensitivities to aspartate and glutamate of neurones receiving a corticofugal innervation were examined by microiontophoresis, and compared with the relative sensitivities of neurones not appearing to receive such an input.2 On all the cells tested, glutamate appeared to be a more potent excitant than aspartate in terms of neuronal response size or effective dose.3 DL-alpha-Aminoadipate (alphaAA) reduced the excitatory amino acid responses on all the neurones tested. On many of these cells a control excitation could be produced, by acetylcholine or hydrogen ions, which was in most cases unaffected by doses of alphaAA producing antagonism of amino acid excitation.4 On 70% of the cells, aminoadipate showed no selectivity for aspartate compared with glutamate but a differential action, involving blockade of aspartate but not glutamate, was apparent on the other 30%.5 Doses of alphaAA which selectively reduced responses to aspartate had no effect on short latency evoked spikes, but doses which also reduced responses to glutamate reduced the short-latency synaptic excitation induced by electrical stimulation of either the surface of the cerebral cortex, or of the pyramidal tracts in the medulla.6 These findings suggest that corticofugal neurones having an excitatory action on cells in various parts of the brain may use an amino acid, probably glutamate, as a common neurotransmitter.7 As no significant difference could be demonstrated in the potency ratios of glutamate:aspartate on monosynaptically activated cells compared with other cells, doubt is cast on the validity of drawing conclusions about transmitter identity from potency ratios alone, without the support of antagonist studies.