Actions of aminoacids on the isolated hemisected spinal cord of the toad

Clavulanic Acid Improves Memory Dysfunction and Anxiety Behaviors through Upregulating Glutamatergic Transporters in the Nucleus Accumbens of Mice Repeatedly Exposed to Khat Extract

Khat (Catha edulis) is an evergreen shrub whose buds and leaves give a state of delight and euphoria when chewed. Cathinone, an amphetamine-like stimulant that is among the active ingredients in khat, is able to downregulate glutamate transporter subtype I (GLT-1). Neurobehavioral dysfunctions such as altered locomotor activity, anorexia, and nociception have been observed in animals exposed to cathinone. Interestingly, treatment with a β-lactam antibiotic such as ceftriaxone, which upregulates GLT-1, normalizes cathinone-induced conditioned place preference, and alters repetitive movements in rats. However, little is known about the role of the glutamatergic system in memory dysfunction and anxiety-like behaviors in mice exposed to khat. We found here that clavulanic acid, a β-lactam-containing compound and GLT-1 upregulator, would modulate the neurobehavioral changes, including memory impairment and anxiety-like behaviors, associated with repeated exposure of mice to khat. Our data supported that clavulanic acid could improve memory impairment and anxiety-like behaviors through upregulating GLT-1 in the nucleus accumbens (NAc), an effect abolished with a selective GLT-1 blocker. This upregulation was associated with restored glutamate/cystine antiporter expression in the NAc using a Western blotting assay. Cathine and cathinone were identified in khat extract using the gas chromatography technique. Our work provides preclinical insight into the efficacy of β-lactam-containing compounds for the attenuation of neurobehavioral changes induced by khat exposure.

Evolution of glutamatergic signaling and synapses

Glutamate (Glu) is the primary excitatory transmitter in the mammalian brain. But, we know little about the evolutionary history of this adaptation, including the selection of l-glutamate as a signaling molecule in the first place. Here, we used comparative metabolomics and genomic data to reconstruct the genealogy of glutamatergic signaling. The origin of Glu-mediated communications might be traced to primordial nitrogen and carbon metabolic pathways. The versatile chemistry of L-Glu placed this molecule at the crossroad of cellular biochemistry as one of the most abundant metabolites. From there, innovations multiplied. Many stress factors or injuries could increase extracellular glutamate concentration, which led to the development of modular molecular systems for its rapid sensing in bacteria and archaea. More than 20 evolutionarily distinct families of ionotropic glutamate receptors (iGluRs) have been identified in eukaryotes. The domain compositions of iGluRs correlate with the origins of multicellularity in eukaryotes. Although L-Glu was recruited as a neuro-muscular transmitter in the early-branching metazoans, it was predominantly a non-neuronal messenger, with a possibility that glutamatergic synapses evolved more than once. Furthermore, the molecular secretory complexity of glutamatergic synapses in invertebrates (e.g., Aplysia) can exceed their vertebrate counterparts. Comparative genomics also revealed 15+ subfamilies of iGluRs across Metazoa. However, most of this ancestral diversity had been lost in the vertebrate lineage, preserving AMPA, Kainate, Delta, and NMDA receptors. The widespread expansion of glutamate synapses in the cortical areas might be associated with the enhanced metabolic demands of the complex brain and compartmentalization of Glu signaling within modular neuronal ensembles.

Team Evans and Watkins: Excitatory Amino Acid research at Bristol University 1973-1981

A series of Special Issues of Neuropharmacology celebrates the 40th anniversary of a seminal review on excitatory amino acid (EAA) receptors by two pioneers of the field - Dick Evans and Jeff Watkins. Brought together in the Department of Pharmacology at the University of Bristol in the 1970s, they forged a partnership that, through the synthetic chemistry prowess of Jeff Watkins, which provided novel agonists and antagonists for EAA receptors for Dick Evans's deft experimental studies, generated enormous insight into the multitude of actions of EAAs in the nervous system. Among many achievements from this time was not just the naming of the N-methyl-d-aspartate (NMDA) receptor, but also the demonstration of its antagonism by magnesium ions. Here, Dick and Jeff reflect upon those early halcyon days of EAA research, which, as these six¹ Special Issues of Neuropharmacology demonstrate, is very much alive and kicking. Bruno G. Frenguelli, Editor-in-Chief, Neuropharmacology.

Glutamate Release

Our aim was to review the processes of glutamate release from both biochemical and neurophysiological points of view. A large body of evidence now indicates that glutamate is specifically accumulated into synaptic vesicles, which provides strong support for the concept that glutamate is released from synaptic vesicles and is the major excitatory neurotransmitter. Evidence suggests the notion that synaptic vesicles, in order to sustain the neurotransmitter pool of glutamate, are endowed with an efficient mechanism for vesicular filling of glutamate. Glutamate-loaded vesicles undergo removal of Synapsin I by CaM kinase II-mediated phosphorylation, transforming to the release-ready pool. Vesicle docking to and fusion with the presynaptic plasma membrane are thought to be mediated by the SNARE complex. The Ca(2+)-dependent step in exocytosis is proposed to be mediated by synaptotagmin.

The GABA excitatory/inhibitory developmental sequence: a personal journey

The developing brain is talkative but its language is not that of the adult. Most if not all voltage and transmitter-gated ionic currents follow a developmental sequence and network-driven patterns differ in immature and adult brains. This is best illustrated in studies engaged almost three decades ago in which we observed elevated intracellular chloride (Cl(-))i levels and excitatory GABA early during development and a perinatal excitatory/inhibitory shift. This sequence is observed in a wide range of brain structures and animal species suggesting that it has been conserved throughout evolution. It is mediated primarily by a developmentally regulated expression of the NKCC1 and KCC2 chloride importer and exporter respectively. The GABAergic depolarization acts in synergy with N-methyl-d-aspartate (NMDA) receptor-mediated and voltage-gated calcium currents to enhance intracellular calcium exerting trophic effects on neuritic growth, migration and synapse formation. These sequences can be deviated in utero by genetic or environmental insults leading to a persistence of immature features in the adult brain. This "neuroarcheology" concept paves the way to novel therapeutic perspectives based on the use of drugs that block immature but not adult currents. This is illustrated notably with the return to immature high levels of chloride and excitatory actions of GABA observed in many pathological conditions. This is due to the fact that in the immature brain a down regulation of KCC2 and an up regulation of NKCC1 are seen. Here, I present a personal history of how an unexpected observation led to novel concepts in developmental neurobiology and putative treatments of autism and other developmental disorders. Being a personal account, this review is neither exhaustive nor provides an update of this topic with all the studies that have contributed to this evolution. We all rely on previous inventors to allow science to advance. Here, I present a personal summary of this topic primarily to illustrate why we often fail to comprehend the implications of our own observations. They remind us - and policy deciders - why Science cannot be programed, requiring time, and risky investigations that raise interesting questions before being translated from bench to bed. Discoveries are always on sideways, never on highways.

Gonadal steroids differentially modulate the actions of orphanin FQ/nociceptin at a physiologically relevant circuit controlling female sexual receptivity

Orphanin FQ/nociceptin (OFQ/N) inhibits the activity of pro-opiomelanocortin (POMC) neurones located in the hypothalamic arcuate nucleus (ARH) that regulate female sexual behaviour and energy balance. We tested the hypothesis that gonadal steroids differentially modulate the ability of OFQ/N to inhibit these cells via presynaptic inhibition of transmitter release and postsynaptic activation of G protein-gated, inwardly-rectifying K(+) (GIRK)-1 channels. Whole-cell patch clamp recordings were performed in hypothalamic slices prepared from ovariectomised rats. OFQ/N (1 μm) decreased the frequency of miniature excitatory postsynaptic currents (mEPSCs) and miniature inhibitory postsynaptic currents (mIPSCs), and also caused a robust outward current in the presence of tetrodotoxin, in ARH neurones from vehicle-treated animals. A priming dose of oestradiol benzoate (EB; 2 μg) increased basal mEPSC frequency, markedly diminished both the OFQ/N-induced decrease in mEPSC frequency and the activation of GIRK-1 currents, and potentiated the OFQ/N-induced decrease in mIPSC frequency. Steroid treatment regimens that facilitate sexual receptivity reinstate the basal mEPSC frequency, the OFQ/N-induced decrease in mEPSC frequency and the activation of GIRK-1 currents to levels observed in vehicle-treated controls, and largely abolish the ability of OFQ/N to decrease mIPSC frequency. These effects were observed in an appreciable population of identified POMC neurones, almost one-half of which projected to the medial preoptic nucleus. Taken together, these data reveal that gonadal steroids influence the pleiotropic actions of OFQ/N on ARH neurones, including POMC neurones, in a disparate manner. These temporal changes in OFQ/N responsiveness further implicate this neuropeptide system as a critical mediator of the gonadal steroid regulation of reproductive behaviour.

Estradiol negatively modulates the pleiotropic actions of orphanin FQ/nociceptin at proopiomelanocortin synapses

Orphanin FQ/nociceptin (OFQ/N) inhibits the activity of proopiomelanocortin (POMC) neurons located in the hypothalamic arcuate nucleus (ARH) that regulate female sexual behavior and energy balance. We tested the hypothesis that estradiol modulates the ability of OFQ/N to pre- and postsynaptically decrease the excitability of these cells. To this end, whole-cell patch-clamp recordings were performed in hypothalamic slices prepared from ovariectomized rats, including some that were injected with the retrograde tracer Fluorogold in the medial preoptic nucleus (MPN) to label the POMC neurons regulating sexual receptivity. OFQ/N (1 µM) evoked a robust outward current in ARH neurons from vehicle-treated animals that was blocked by the opioid receptor-like (ORL)1 receptor antagonist UFP-101 (100 nM) and the G protein-gated, inwardly rectifying K⁺ (GIRK-1) channel blocker tertiapin (10 nM). OFQ/N also produced a decrease in the frequency of glutamatergic, miniature excitatory postsynaptic currents (mEPSCs), which was also antagonized by UFP-101. Estradiol benzoate (2 µg) increased basal mEPSC frequency and markedly diminished both the OFQ/N-induced activation of postsynaptic GIRK-1 channel currents and the presynaptic inhibition of glutamatergic neurotransmission. These effects were observed in identified POMC neurons, including eight that projected to the MPN. Taken together, these data reveal that estradiol attenuates the pleiotropic inhibitory actions of OFQ/N on POMC neurons: presynaptically through reducing the OFQ/N inhibition of glutamate release and postsynaptically by reducing ORL1 signaling through GIRK channels. As such, they impart critical insight into a mechanism for estradiol to increase the activity of POMC neurons that inhibit sexual receptivity.

D-Serine: a key to synaptic plasticity?

Two discoveries have put D-serine in the spotlight of neuroscience. First, D-serine was detected in brain tissue at high levels. Second, it was found to act on the N-methyl-D-aspartate receptor (NMDAR). This receptor is central to use-dependent synaptic plasticity, the cellular process which is widely believed to underlie learning. The ensuing quest for the mechanisms of D-serine synthesis, release and clearance, as well as for its physiological significance has provided a wealth of experimental evidence implicating D-serine in synaptic plasticity. However some key questions remain unanswered. Which cells release D-serine and upon what stimuli? Is D-serine supply dynamically regulated? What is the fate of released D-serine? Answering these questions appears to be an essential step in our understanding of how NMDARs trigger synaptic plasticity and learning. This review will highlight some recent advances and avenues of enquiry in dynamic D-serine signaling in the mammalian brain with emphasis on neurophysiology.

Serine racemase deletion abolishes light-evoked NMDA receptor currents in retinal ganglion cells

Glycine and/or D-serine are obligatory coagonists of the N-methyl-D-aspartate receptor (NMDAR). Serine racemase, the D-serine-synthesizing enzyme, is expressed by astrocytes and Müller cells of the retina, but little is known about its role in retinal signalling. In this study, we utilize a serine racemase knockout (SRKO) mouse to explore the contribution of D-serine to inner-retinal function. Retinal tissue levels of D-serine in SRKO mice are reduced by 85%. Whole-cell recordings from SRKO retinal ganglion cells showed markedly reduced coagonist occupancy of NMDARs and consequently a dramatic reduction in the NMDAR component of light-evoked responses. NMDAR currents in SRKOs could be rescued by applying exogenous coagonist, but SRKO ganglion cells still displayed lower NMDA/AMPA receptor ratios than wild-type (WT) controls when the coagonist site was saturated. Despite having abnormalities in synaptic glutamatergic transmission, SRKO mice displayed no obvious signs of visual impairment in behavioural testing. These findings raise interesting questions about the role of D-serine in inner-retinal function and development.

Cholinergic transmission in the frog spinal cord

The effect of acetylcholine and other drugs has been tested on the isolated and sagittally hemisected spinal cord of the frog (Rana temporaria). The release of acetylcholine from this preparation during stimulation of the spinal roots and whole hemicord has also been studied. After inactivation of the tissue cholinesterases acetylcholine was released spontaneously from the preparation at a steady rate of about 5.5 pmole/15 min/preparation. No increase in this release was obtained by stimulating the dorsal roots, but antidromic stimulation of the ventral roots always gave an increase in the rate of acetylcholine release up to 2.3 times the spontaneous level. Direct stimulation of the spinal cord did not alter the rate of acetylcholine release. Acetylcholine (1 mM) and eserine (10 mM) had weak excitatory actions and prolonged the reflex response of the preparation during dorsal root stimulation, but dihydro-beta-erythroidine and atropine had no effect. The response evoked by antidromic ventral root stimulation was potentiated by eserine (10 muM) and depressed by acetylcholine (10 muM) and dihydro-beta-erythroidine (1 muM) as in the toad preparation. The evidence supporting cholinergic transmission at some synapses in the spinal cord is discussed in relation to these results.

The glutamate story

Glutamatergic synaptic transmission in the mammalian central nervous system was slowly established over a period of some 20 years, dating from the 1950s. Realisation that glutamate and like amino acids (collectively known as excitatory amino acids (EAA)) mediated their excitatory actions via multiple receptors preceded establishment of these receptors as synaptic transmitter receptors. EAA receptors were initially classified as N-methyl-D-aspartate (NMDA) and non-NMDA receptors, the latter subdivided into quisqualate (later AMPA) and kainate receptors after agonists that appeared to activate these receptors preferentially, and by their sensitivity to a range of differentially acting antagonists developed progressively during the 1970s. NMDA receptors were definitively shown to be synaptic receptors on spinal neurones by the sensitivity of certain excitatory pathways in the spinal cord to a range of specific NMDA receptor antagonists. Importantly, specific NMDA receptor antagonists appeared to be less effective at synapses in higher centres. In contrast, antagonists that also blocked non-NMDA as well as NMDA receptors were almost universally effective at blocking synaptic excitation within the brain and spinal cord, establishing both the existence and ubiquity of non-NMDA synaptic receptor systems throughout the CNS. In the early 1980s, NMDA receptors were shown to be involved in several central synaptic pathways, acting in concert with non-NMDA receptors under conditions where a protracted excitatory postsynaptic potential was effected in response to intense stimulation of presynaptic fibres. Such activation of NMDA receptors together with non-NMDA receptors led to the phenomenon of long-term potentiation (LTP), associated with lasting changes in synaptic efficacy (synaptic plasticity) and considered to be an important process in memory and learning. During the 1980s, it was shown that certain glutamate receptors in the brain mediated biochemical changes that were not susceptible to NMDA or non-NMDA receptor antagonists. This dichotomy was resolved in the early 1990s by the techniques of molecular biology, which identified two families of glutamate-binding receptor proteins (ionotropic (iGlu) and metabotropic (mGlu) receptors). Development of antagonists binding to specific protein subunits is currently enabling precise identification of discrete iGlu or mGlu receptor subtypes that participate in a range of central synaptic processes, including synaptic plasticity.

Endogenous sulphur-containing amino acids: potent agonists at presynaptic metabotropic glutamate autoreceptors in the rat central nervous system

We have recently demonstrated that presynaptically located metabotropic glutamate (mGlu) autoreceptors regulate synaptic glutamate release both in vitro and in vivo. We now report a positive modulatory action of the sulphur-containing amino acids (SCAAs), L-cysteic acid (CA) and L-cysteine sulphinic acid (CSA), at presynaptic group I mGlu receptors, specifically of the mGlu5 subtype, acting to enhance synaptic glutamate release from the rat forebrain in vitro. Neuronal glutamate release was monitored using electrically-evoked efflux of preloaded [(3)H]-D-aspartate from rat forebrain hemisections. Both CA (3 - 100 muM) and CSA (1 - 100 microM), in addition to the selective group I mGlu receptor agonist, (S)-3,5-dihydroxyphenylglycine ((S)-DHPG), concentration-dependently enhanced electrically-stimulated efflux of [(3)H]-D-aspartate from the rat forebrain slices. Basal efflux of label remained unchanged. The inhibitory activity of the broad spectrum mGlu receptor antagonist, (+/-)-alpha-methyl-4-carboxyphenylglycine ((+/-)-MCPG; 200 microM), coupled with the inactivity of the selective mGlu1 receptor antagonists, (R,S)-1-aminoindan-1,5-dicarboxylic acid ((R,S)-AIDA; 100 - 500 microM) and the more potent (+)-2-methyl-4-carboxyphenylglycine (LY367385; 10 microM) against these responses, indicates an action of the SCAAs at the mGlu5 receptor subtype. This proposal is supported by the potent inhibition of these responses by the selective, non-competitive mGlu5 receptor antagonist, 2-methyl-6-(phenylethynyl)pyridine (MPEP; 10 microM). The observed enhancement of the responses to high concentrations of CA by the selective mGlu5 receptor desensitization inhibitor, cyclothiazide (CYZ; 10 microM), is also consistent with this concept. Administration of the agonists in the presence of bovine serum albumin (BSA; 5 - 15 mg ml(-1)) markedly attenuated the positive modulatory responses observed, strongly supporting a role for arachidonic acid in the expression of these mGlu5 receptor-mediated responses. The regulatory actions of SCAAs on synaptic glutamate release demonstrated in the present study may provide a physiological function for these putative neurotransmitter amino acids in the mammalian brain. These central actions of the SCAAs may have wide-ranging implications for a range of neurological and neuropsychiatric disease states and their treatment.

Action of some drugs on the dorsal root potentials of the isolated toad spinal cord

A series of drugs which potentiate or antagonize transmission at cholinergic synapses have been tested on the slow depolarizing dorsal root potentials evoked by stimulation of adjacent dorsal and ventral roots. Some drugs acted specifically on the dorsal root potential evoked by ventral root stimulation. Dihydro-beta-erythroidine, atropine and acetylcholine depressed this potential, whilst anticholinesterase substances in low concentrations potentiated and in higher concentrations depressed it. It is concluded that the results offer some evidence for a cholinergic link in the pathway responsible for the generation of depolarizing potentials in the dorsal root subsequent to ventral root stimulation.

A quantitative study of the actions of excitatory amino acids and antagonists in rat hippocampal slices

1. A quantitative pharmacological investigation of the actions of excitatory amino acids on hippocampal CA1 neurones has been made using a new slice preparation developed for grease gap recording; d.c. potential was measured across a grease barrier placed between alvear fibres and the bathing medium. 2. In Mg2+-free perfusate, N-methyl-D-aspartate (NMDA, 1-100 microM), quisqualate (1-500 microM), kainate (1-200 microM) and alpha-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid (AMPA, 1-100 microM) caused dose-dependent depolarizations. 3. The dose-response relationships were fitted to logistic expressions. The maximum responses to AMPA, NMDA and kainate were similar; their respective EC50 values were 5, 13 and 23 microM. Quisqualate had a smaller maximum; its EC50 value was 10 microM. The slopes of the dose-response relationships were different for the 4 agonists; the order of steepness of the slopes was NMDA greater than AMPA greater than kainate greater than quisqualate. 4. Similar amino acid-induced depolarizations were observed in slices of just the CA1 region or in whole slices bathed in tetrodotoxin. Isolated alvear fibres, however, were insensitive to the excitatory amino acids. 5. D-2-Amino-5-phosphonovalerate (APV, 50 microM) selectively and reversibly antagonized responses induced by NMDA (apparent pA2 = 5.21). 6. Kynurenic acid (1 mM) reversibly depressed responses to the three agonists tested. The dose-ratios for antagonism of AMPA, kainate and quisqualate were 6.9, 5.6 and 4.6 respectively. 7. This preparation has a different sensitivity profile to agonists from those of previously reported preparations of spinal cord, neocortex and cerebellum. The greater sensitivity to NMDA may be due to the higher density of NMDA receptors in the hippocampus. The effects of the antagonists, APV and kynurenate, are similar to those found in other brain areas.

The projection from superior colliculus to cuneiform area in the rat. II. Defence-like responses to stimulation with glutamate in cuneiform nucleus and surrounding structures

To investigate the role of the projection from superior colliculus to the cuneiform nucleus in mediating collicular responses, the cuneiform area (including the cuneiform nucleus and immediately adjacent structures such as caudal central grey) was stimulated in rats with microinjections of glutamate (50 mM, 200 nl, 10 nmole) and the animals' head and body movements observed. The most common responses obtained from sites in the cuneiform area were freezing, darting or fast running, the form or direction of which did not appear to be strongly influenced by the laterality of the injection. These responses were only a subset of those that have been obtained in previous studies from stimulation of the superior colliculus itself: stimulation of the cuneiform area did not give contralaterally directed movements resembling orienting or approach, or ipsilaterally directed movements resembling cringing or shying. It therefore appears that the tectocuneiform projection is likely to be involved only in some of the behaviours appropriate to unexpected stimuli that are mediated by the superior colliculus, namely undirected defensive responses elicited normally by certain kinds of threatening or noxious stimulation. Involvement with such responses would be consistent with an apparent lack of topography in the tectocuneiform projection, and the connections of the cuneiform nucleus with parts of the brain concerned with nociception (see previous paper). It is unclear, however, whether the somatic responses occur in parallel with, or as a result of, autonomic changes that have also been evoked by stimulation of the cuneiform area. One striking feature of stimulating the cuneiform area with glutamate was that at many sites the intensity of the response appeared to increase with successive (one to three) injections. It is possible that this plasticity of response, which can also be obtained from the superior colliculus itself, is related to processes involved in sensitisation or learning of defensive responses.

The occurrence of N-methyl-D-aspartic acid in muscle extracts of the blood shell, Scapharca broughtonii

A novel ninhydrin-positive compound, N-methyl-D-aspartic acid, was identified in the muscle extracts of the blood shell, Scapharca broughtonii. This compound is already known to have potent neuroexcitatory activity, inducing hypermotility and strong releasing action of serum luteinizing hormone in mammals. This may be, however, the first finding of N-methyl-D-aspartic acid in natural products.

Cellular uptake disguises action of L-glutamate on N-methyl-D-aspartate receptors. With an appendix: diffusion of transported amino acids into brain slices

Pharmacological properties of the guanosine 3'5'-cyclic monophosphate (cyclic GMP) responses to excitatory amino acids and their analogues were compared in slices and dissociated cells from the developing rat cerebellum maintained in vitro. The intention was to determine the extent to which cellular uptake might influence the apparent properties of receptor-mediated actions of these compounds. In slices, the potencies of the weakly (or non-) transported analogues, N-methyl-D-aspartate (NMDA) and kainate (KA) (EC50 = 40 microM each) were higher than those of the transported amino acids, D- and L-aspartate (EC50 = 250 microM and 300 microM) and D- and L-glutamate (EC50 = 540 microM and 480 microM). Quisqualate (up to 300 microM) failed to increase cyclic GMP levels significantly. The sensitivity of agonist responses to the NMDA receptor antagonist, DL-2-amino-5-phosphonovalerate (APV), was in the order NMDA greater than L-aspartate greater than L-glutamate, KA. In dissociated cells, L-glutamate was 280 fold more potent (calculated EC50 = 1.7 microM). L- and D-aspartate (calculated EC50 = 13 microM) and D-glutamate (EC50 = 130 microM) were also more effective than in slices. The potencies of NMDA and KA were essentially unchanged. Responses to NMDA, L-glutamate and L-aspartate under these conditions were equally sensitive to inhibition by APV but the response to KA remained relatively resistant to this antagonist. The implications of these results are that, in slices, cellular uptake is responsible for (i) the dose-response curves to L-glutamate, L- and D-aspartate bearing little or no relationship to the true (or relative) potencies of these amino acids; (ii) the potency of APV towards the actions of transported agonists acting at NMDA receptors being reduced and (iii) a differential sensitivity to APV of responses to L-glutamate and L-aspartate being created, the consequence being that a potent action of L-glutamate on NMDA receptors is disguised. These conclusions are supported by theoretical considerations relating to the diffusion of transported amino acids into brain slices, as elaborated in the Appendix.

Synaptic actions produced by individual ventrolateral tract fibres in frog lumbar motoneurones

Excitatory post-synaptic potentials (EPSPs) were evoked in lumbar motoneurones of the isolated frog spinal cord by impulses in single ventrolateral tract fibres. In a few cases after recording an EPSP the fibre and the motoneurone involved were both filled with horseradish peroxidase (HRP) and the synaptic connexion between them was studied histologically. Monosynaptic EPSPs produced by direct stimulation of supraspinal (mainly reticulospinal) or unidentified (presumably propriospinal) fibres are mediated via chemical and, less frequently, dual-action synapses. The shape indices of chemical single-fibre EPSPs varied considerably in different connexions being, as a whole, similar to those of chemical components of EPSPs at synapses between primary afferents and motoneurones. Quantal analysis of the single-fibre EPSPs yielded quantal unit amplitude 18-113 microV and mean quantum content ranging from 1.14 to 16.4, the applicability of both Poisson and binomial models to transmitter release was revealed. Descending fibres electrically coupled with lumbar motoneurones were found to generate a depolarizing response to dorsal root stimulation. They were also characterized by a larger depolarization to superfused glutamate. The presence of electrical junctions between descending axons and spinal motoneurones suggests that the depolarization seen in these axons in response to synaptic excitation and glutamate could be the result of passive flow of depolarizing current from motoneurones electrically coupled to them. gamma-aminobutyric acid (GABA) did not produce conspicuous actions in axons forming both chemical and dual-action synapses. Axons injected with HRP have been followed to their site of termination in the lateral motor column. Synaptic boutons and varicosities were found to form contacts predominantly with dendrites of target motoneurones.

Differential sensitivity of individual primary afferents to glutamic and gamma-aminobutyric acids in the amphibian spinal cord in vitro

In the isolated amphibian spinal cord the responses to glutamate and GABA recorded intracellularly from individual primary afferents differed considerably according to the fibre type. Muscle afferents giving rise to direct sensory-motor synapses were strongly depolarized by glutamate, whereas GABA produced smaller and inconsistent depolarization. In contrast, fibres both of muscle and cutaneous origin establishing polysynaptic connections with motoneurones were relatively insensitive to glutamate but very sensitive to GABA. These observations suggest that the effects of glutamate on fibres synapsing with motoneurones are probably mediated via the depolarization of motoneurones electrically coupled with them and do not result from direct activation of presynaptic terminals by glutamate.

Biochemical and electrophysiological characteristics of mammalian GABA receptors

The concept that GABA is a neurotransmitter in the mammalian CNS is supported by both electrophysiological and biochemical data. Whereas the electrophysiological studies are essential for demonstrating a specific functional response to GABA, the biochemical approach is useful for characterizing the molecular properties of this site. As a result of these studies the concept of the GABA receptor has progressed from a simple model of a single recognition site associated with a chloride channel to a more complex structure having a variety of interacting components. Thus, both electrophysiological and biochemical data support the existence of at least two pharmacologically distinct types of GABA receptors, based on the sensitivity to bicuculline. Also, anatomically, there appear to be two different types of receptors, those located postsynaptically on the soma or dendrites of a neighboring cell and those found presynaptically on GABAergic and other neurotransmitter terminals. From biochemical studies it appears that the GABA receptor may be composed of at least three distinct interacting components. One of these, the recognition site, may exist in two conformations, with one preferring agonists and the other having a higher affinity for antagonists. Ion channels may be considered a second component, with some of these regulating the passage of chloride ion, whereas others may be associated with calcium transport. The third major element of GABA receptors appears to be a benzodiazepine recognition site, although only a certain population of GABA receptors may be endowed with this property. In addition to these, the GABA receptor complex appears to contain substances that modulate the recognition site by influencing the availability of higher affinity binding proteins. It would appear therefore that changes affecting any one of these constituents can influence the characteristics of the others. While increasing the complexity of the system, this arrangement makes for a more sensitive and adaptable receptor mechanism. Thus the GABA receptor can be envisioned as a supramolecular complex of interacting sites, all of which contribute to the functional expression of receptor activation. Because of this complexity, GABA receptors can theoretically be modified in a variety of ways by drug treatment or disease. Accordingly, it may be possible to develop selective agonists and antagonists that may act at one of the basic components, as well as agents that may alter the receptor modulators. Conversely, a disorder of any of these entities may result in an alteration of GABA receptor function, which in turn could contribute to the symptoms of a variety of neuropsychiatric disorders.(ABSTRACT TRUNCATED AT 400 WORDS)

Morphology and distribution of the synapses to the spinal motoneuron of the frog

The quantitative distribution of the different structural elements on the surface of the motoneurons (MN) in the spinal cord of the frog was studied in thin sections and freeze-fracture replicas. In particular, the different synaptic types and their distribution on the MN soma and dendrites are described. In thin sections three types of synapses were discerned: the S-type with spherical vesicles, the F-type with flattened vesicles, and finally the C-type synapse with spherical vesicles and a subsynaptic cistern. The synaptic covering at the MN surface as determined by thin sections is about 36%. In freeze-fracture replicas were observed boutons with and without gap junctions, and C-type boutons. When using the latter technique the synaptic covering was found to be 43%. With both techniques special attention was paied to the morphology of the C-type bouton and both the subsynaptic and extrasynaptic cisterns. The soma membrane over the sub- and extrasynaptic cisterns reveals characteristic and very similar morphological features with respect to both the distribution and size of the membrane particles. the possible functions of the two cisternal types are discussed.

The effects of intrapulmonary carbon dioxide and airway pressure on phrenic activity in the dog [proceedings]

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GABA-mimetic action of etomidate

A comparison of antagonism by bicuculline or strychnine of the effects of GABA or etomidate on rat isolated superior cervical ganglia, frog isolated hemisected spinal cords and rat central neurones in vivo indicates that etomidate has GABA-mimetic actions.

Conductance increases produced by glycine and gamma-aminobutyric acid in lamprey interneurones

1. Conductances of individual neurones in the isolated lamprey spinal cord were measured with separate intracellular electrodes for recording potentials and for passing current pulses during application of glycine or GABA (0.1-1.0 MM) in Ca-free bathing fluid. Large, reversible increases in conductance were produced in giant interneurones by both amino acids, but Müller axons and sensory dorsal cells were unaffected. 2. Conductance increases produced by glycine and by GABA were selective for Cl. Both conductance increases were linearly related to external Cl concentrations and repeated exposure to the amino acids in Cl-free fluid progressively reduced the conductance increases to less than 1% of their values in normal Cl. 3. Strychnine was a competitive antagonist of glycine, while GABA was antagonized competitively by bicuculline and non-competitively by picrotoxin. 4. The sensitivity of giant interneurones to glycine and GABA increased at low temperatures, in Na-free fluid, and after repeated exposure to the amino acids. Sensitization may have been produced by inhibition of uptake mechanisms for glycine and GABA in the spinal cord. 5. Discharges of interneurones recorded extracellularly were inhibited by bath-applied glycine and GABA, but directly elicited action potentials of axons were unaffected. Strychnine and Cl-free fluid in the presence of Ca produced seizures in lamprey spinal cord. 6. The conclusions of these experiments are that different receptors for glycine and for GABA are present on giant interneurones, that glycine is the better candidate for an inhibitory transmitter in the lamprey spinal cord, and that GABA produces effects similar to those which have been well studied in arthropod muscle.

Differential antagonism by chlorpromazine and diazepam of frog motoneurone depolarization induced by glutamate-related amino acids

The effects of some central depressant drugs on amino acid-induced depolarization of motoneurons have been determined in the isolate hemisected frog spinal cord. Motoneuron depolarization was recorded from ventral roots and measurements were made in the presence or absence of procaine or tetrodotoxin to minimize indirect effects of both drugs and amino acids. Chlorpromazine (0.05-0.1 mM) and diazepam (0.5 mM) produced a similar differential pattern of depression of amino acid-induced depolarizations. Responses induced by L-homocysteate were markedly antagonized by these drugs, while responses to quisqualate were unaffected. L-Aspartate-induced responses were antagonized more than L-glutamate-induced responses. This pattern of antagonism resembles that previously described for Mg2+. In contrast, pentobarbital (0.1 or 0.3 mM), and the inhibitory amino acids GABA and beta-alanine (0.5-1.0 mM), depressed amino acid-induced responses in a more uniform manner. The differential effects observed with chlorpromazine and diazepam provide further support for the possibility that responses to excitant amino acids structurally related to L-glutamate may have different underlying mechanisms.

Effects of N-methylamino acids and convulsants on spontaneous action potentials in guinea-pig cerebellar slices

1. N-methyl-gamma-aminobutyrate (N-methylGABA), N-methylglycine, N-methyltaurine and N-methylbeta-alanine diminished the frequency of spontaneous spike discharges in guinea-pig cerebellar slices. Usually a weak excitatory effect preceded the inhibition. 2. The inhibitory effects of N-methylGABA and N-methylbeta-alanine were competitively antagonized by both picrotoxin and strychnine. 3. The inhibitory action of N-methyltaurine was competitively suppressed by strychnine and by low concentrations of picrotoxin. 4. The inhibitory action of N-methylglycine was suppressed by strychnine but not by picrotoxin. The suppression was competitive at low concentrations of strychnine. 5. N-methylDL-glutamate brought about a strong inhibition followed by a strong excitation of the neurones. The inhibitory effects were competitively suppressed by both picrotoxin and strychnine. Neither convulsant affected the excitation. 6. Whereas L- or D-glutamate caused only excitation in the majority of cells examined, a small proportion of the cells exhibited inhibition preceding the excitation by L- or D-glutamate. Such inhibitory effects were suppressed by picrotoxin but not by strychnine. 7. Kinetic analyses of the dose-response curves for the N-methylamino acid in the presence or absence of the convulsant indicated that the number of molecules of the amino acid combining with the receptor site to produce a response was 3 for N-methylGABA, 2 for N-methylglycine, 3 for N-methyltaurine, 3 for N-methylbeta-alanine. The corresponding value was 1 for N-methylDL-glutamate (inhibition). The number of molecules of convulsant combining with the receptor site was calculated to be 2 for picrotoxin with N-methylGABA, N-methylbeta-alanine and N-methylDL-glutamate and 1 for strychnine with all N-methylamino acids examined.

Structure-activity relations of excitatory amino acids on frog and rat spinal neurones

1 A series of compounds structurally related to glutamic acid has been tested on frog and rat spinal neurones. The substances were added to procaine-containing medium bathing the isolated hemiscected spinal cord of the frog, and their potencies in depolarizing motoneurones were assessed by the magnitude of the potential produced in the ventral root. The electrophoretic technique was used to administer the substances around single interneurones of the rat spinal cord and the relative potencies of the compounds as excitants assessed by the magnitude of the currents required to produce similar rates of neuronal firing. 2 Parallel structure-activity relations were observed in the two series of experiments, suggesting that the receptors for excitatory amino acids on frog and rat spinal neurones are similar. 3 Quisqualate, domoate and kainate were the strongest excitants in both animals, with potencies around two orders of magnitude higher than that of L-glutamate. 4 2,4,5-Trihydroxyphenylalanine (6-OH-DOPA) was a stronger excitant and L-3,4-dihydroxyphenylalanine (L-dopa) a weaker excotamt than L-glutamate on frog spinal motoneurones. The former compounds was also a more potent convulsant than L-glutamate on intraventricular injection into mouse brain. The lack of activity of 6-OH-DOPA on electrophoretic administration was attributed to oxidation. 5 Unlike the majority of amino acid excitants, several of the compounds shown in the present work to have moderate excitatory activity are not anionic at physiological pH. This indicates either that two negatively charged groups are not essential for interaction with a common excitatory receptor, or that more than one type of receptor is involved in the actions demonstrated.

A comparative study of the effects of glutamate and kainate on the lobster muscle fibre and the frog spinal cord

1 The depolarizing actions of glutamate and its conformationally restricted analogue kainate were investigated on the lobster muscle fibre and the frog spinal cord using intracellular and extracellular recordings, respectively. 2 Bath-applied kainate was less potent than glutamate on the lobster fibre but more potent on the frog cord. From the log-log transformation of dose-response curves it was proposed that more than one glutamate molecule was necessary to activate both the lobster and the frog receptor sites. In the frog, at least three kainate molecules were thought to be required for receptor activation. 3 The ionic dependence of glutamate and kainate responses appeared different for the two tissues. 4 Some possible explanations of the differential tissue sensitivity to kainate are discussed.

Effects of glutamate and glutamic acid diethyl ester on the lobster muscle fibre and the frog spinal cord

The effect of bath applications of glutamic acid diethyl ester (GDE) and glutamate on the lobster muscle fibre and the from spinal cord were examined. In the lobster muscle fibre GDE (2 X 10(-3) M) did not antagonize the depolarizing action of glutamate (10(-4) M). In the frog spinal cord a small reduction in the excitatory effects of glutamate (10(-4)-10(-3) M) after GDE pretreatment was found only if the latter was given in very high doses (over 2 X 10(-3) M) which stimulated the neuronal firing. These observations show that GDE is not a specific antagonist of glutamate in these tissues.

Depolarizing actions of gamma-aminobutyric acid and related compounds on rat superior cervical ganglia in vitro

1 Potential changes in rat superior cervical ganglia were recorded in vitro with surface electrodes.2 gamma-aminobutyric acid (GABA) produced a transient, low-amplitude ganglion depolarization at rest, and a transient hyperpolarization in ganglia depolarized by carbachol. Depolarization was not prevented by preganglionic denervation. The log dose-response curve for depolarization was sigmoid with a mean ED(50) of 12.5 muM.3 The ganglion was depolarized in similar manner by the following compounds (mean molar potencies relative to GABA (=1) in brackets): 3-aminopropane sulphonic acid (3.4), gamma-amino-beta-hydroxybutyric acid (0.27), beta-guanidino-propionic acid (0.12), guanidinoacetic acid (0.057), delta-aminovaleric acid (0.048), beta-alanine (0.01), 2,4-diaminobutyric acid, gamma-guanidinobutyric acid, taurine and N-methyl-GABA (all <0.01). The following compounds did not depolarize the ganglion at 10 mM concentrations: alpha- and beta-amino-n-butyric acids, alpha-amino-iso-butyric acid, glycine and glutamic acid.4 Depolarization declined in the continued presence of GABA. Ganglia thus ;desensitized' to GABA showed a diminished response to other amino acids but not to carbachol.5 The effect of GABA was not antagonized by hyoscine and hexamethonium in combination, in concentrations sufficient to block responses to carbachol.6 Responses to GABA were blocked more readily than those to carbachol by bicuculline (IC(50), 14 muM) and picrotoxin (IC(50), 37 muM). Strychnine (IC(50), 73 muM) was a relatively weak and less selective GABA-antagonist.7 It is concluded that sympathetic ganglion cells possess receptors for GABA and related amino acids which are (a) different from the acetylcholine receptors and (b) similar to GABA receptors in the central nervous system.

The inhibitory action of monoamines on lateral geniculate neurones

1. L-Noradrenaline (NA), dopamine, 5-hydroxytryptamine (5-HT) and lysergide were administered iontophoretically to neurones in the dorsal lateral geniculate nucleus of the cat and the responses to these drugs recorded.2. Many neurones were depressed by the monoamines and lysergide.3. This depression was manifested by a reduction or abolition of the effects of optic nerve or visual stimulation, by a failure in some instances of an antidromically propagating spike to invade the cell soma, and by a depression of the excitant effects of L-glutamate and acetylcholine.4. Although there was considerable variation in the magnitude of the depressant effects of the monoamines, dopamine was found to be slightly more potent than 5-HT and NA. Neurones which were inhibited by catecholamines did not always respond to 5-HT and lysergide and vice versa.5. The inhibitory actions of monoamines on lateral geniculate neurones are comparable with those that have been recorded in other structures of the central nervous system.6. Lysergide did not antagonize the action of 5-HT.7. Catecholamines and 5-HT are present in nerve terminals in the lateral geniculate nucleus and the findings of this study suggest that they act as inhibitory transmitters.