Depression of glutamate-mediated synaptic transmission by benzyl alcohol

The data obtained from this study suggest that the nonionizable anesthetic benzyl alcohol has two prominent actions on GABA- and glutamate-mediated synaptic transmission at the lobster neuromuscular junction. They are as follows: (1) depression of the excitatory end-plate potential and the postsynaptic membrane response to applied glutamate, and (2) a hyperpolarization of the postsynaptic resting membrane potential associated with a decrease in effective membrane resistance. No change in amplitude of the inhibitory end-plate potential or inhibitory reversal potential was seen. Excitatory miniature end-plate potential frequency was also unaffected. The depression of excitatory synaptic transmission appears to be due to a decreased responsiveness of the postsynaptic receptor-ionophore complex.

Water relations in single cells

The intracellular water content is an important factor affecting the growth and survival of single cells of microorganisms under adverse environmental conditions. Certain types of bacteria, yeasts, filamentous fungi and algae are capable of growth in environments with water activities below 0.9 and even as low as 0.6, and are the most osmotolerant living organisms known. The two most important factors that determine such extreme osmotolerance are: (1) the resistance of the enzymes in a cell to the solutes present, and (2) the cell’s ability to maintain within itself particular solutes, which are compatible with continued activity of intracellular enzymes, at levels sufficient to balance the external osmotic pressure and thus avoid dehydration. The levels of such compatible solutes are metabolically controlled and include polyols in yeasts, glutamic acid in the least osmotolerant bacteria, y-aminobutyric acid and proline in the more osmotolerant bacteria and potassium in specifically halophilic bacteria. In contrast, under certain conditions osmoregulatory mechanisms in microorganisms may reduce rather than maintain the water content of the cell. For example, during the morphogenic changes that accompany the formation of endospores by some bacteria, a special form of osmoregulation occurs in which a newly synthesized electronegative polymer (‘peptidoglycan’) in the outer region of the spore brings about and maintains, rather than avoids, dehydration of the central core. Indeed, spore heat resistance can be predictably modified experimentally by osmotic manipulation. The core dehydration mechanism is probably implicated in the enormous resistance of endospores to heat. It may also be involved in the exceptional dormancy and longevity of such cells, and suggests a principle that may operate in other dormant biological systems.

L-glutamate as an excitatory transmitter at the Drosophila larval neuromuscular junction

The possibility that L-glutamate is the excitatory transmitter at the Drosophila larval neuromuscular junction and the ionic basis of its action on the muscle membrane are examined. 2. Iontophoretically applied L-glutamate causes muscle depolarization (L-glutamate potential) if and only if the L-glutamate pipette is within a few mum of the nerve ending. D-glutamate, substance P, ACh and GABA are ineffective. 3. Bath-applied L-glutamate produces similar changes in the time course and amplitude of miniature excitatory junctional potential (m.e.j.p.), excitatory junctional potential (e.j.p.) and the L-glutamate potential. 4. Neuromuscular transmission and excitation-contraction coupling are operative in a haemolymph-like solution containing 1 mM L-glutamate. 5. The reversal potentials of the e.j.p. and the L-glutamate potential are identical to each other, changing similarly with changes in the ionic compositions of the external medium (twelve solutions). 6. The ionic dependence of the reversal potentials is predicted from an extended constant-field equation using a ratio of sodium:potassium permeabilities of PNa/PK=1-3, and a ratio of magnesium:potassium permeabilities of PMg/PK=4-7. 7. It is concluded that L-glutamate is, or is an agonist of, the excitatory transmitter at certain Drosophila larval neuromuscular junctions.

On the elementary conductance event produced by L-glutamate and quanta of the natural transmitter at the neuromuscular junctions of Maia squinado

1. The membrane potential of giant muscle fibres of Maia squinado was measured with an intracellular wire electrode. On applying L-glutamate to the fibre the cell deplorized and fluctuations of the membrane potential around its mean level--glutamate noise--were seen. 2. The variance of the glutamate voltage noise is proportional to the mean level of depolarization. The noise can be regarded as being caused by numerous exponentially decaying elementary voltage events about 5 X 10(-10) V in amplitude. The miniature excitatory junctional potential (min.e.j.p.) is approximately 6000 times the amplitude of the elementary voltage event produced by L-glutamate. 3. The power spectrum of glutamate voltage noise is a Lorentzian with a half-power frequency of approximately 20 Hz. 4. Min. e.j.p.s. decay exponentially with a time constant that coincides with the average lifetime of the elementary glutamate voltage event. 5. When glutamate is applied locally to a spot where extracellular min. e.j.p.s. can be recorded with a focal glass pipette, extracellular glutamate noise is seen. Glutamate noise could not be detected from elsewhere on the fibre. 6. The variance of the extracellular noise is proportional to the mean extracellular potential, and its power spectrum is a Lorentzian with a half-power frequency of about 110 Hz. 7. The extracellular min. e.j.p.s decay exponentially with a time constant that coincides with average lifetime of the elementary glutamate current event. 8. It is suggested that the decay of the quantal currents flowing at the excitatory junction is limited by the closure of the conductance channels in the post-synaptic membrane and not by the relaxation of the transmitter concentration in the synaptic cleft.

Is glutamic acid the pyramidal tract neurotransmitter?

Applied by microiontophoresis, 1-hydroxy-3-amino-pyrrolidone-2 (HA-966) antagonized excitation by glutamic acid but not by acetylcholine of neurones in the rat cuneate nucleus. HA-966 blocked the short latency excitation of cuneate neurones following stimulation of the pyramidal tract on 28 of 40 cells (70%). Thus, glutamate or a related amino-acid may be the neurotransmitter released by pyramidal tract neurones.

Cholinergic motor neurones in the stomatogastric system of the lobster

1. A study of the neurotransmitters used by each of the eleven types of excitatory motor neurones (identified according to the muscle innervated) of the lobster stomatogastric ganglion was undertaken. 2. The dorsal dilator muscle is innervated by the two motor neurones designated 'PD'. Bath and iontophoretic applications of acetylcholine (ACh) produce contractures and depolarizations respectively in the dorsal dilator muscle. 3. Pharmacological experiments support the cholinergic nature of the excitatory junctional potentials (e.j.p.s) recorded in the dorsal dilator muscle when the PD motor nerve is stimulated. 4. The apparent reversal potentials for the e.j.p.s and the iontophoretic ACh response in the dorsal dilator muscle are the same. 5. On the basis of choline acetyltransferase assays on identified stomatogastric ganglion motor neurone somata and tension measurements on the muscles innervated by each type of stomatogastric ganglion motor neurone, a transmitter candidate was established for each type of motor neurone. Motor neurones named VD, LPG, GM, MG, LG, and DG are putatively cholinergic. L-Glutamate is a transmitter candidate for the motor neurones called LP, PY, IC, and AM. 6. Potential correlations between the distribution of putatively cholinergic and glutaminergic motor neurones and the electrical coupling among the stomatogastric ganglion motor neurones are discussed.

Antagonism of cortical excitation of striatal neurons by glutamic acid diethyl ester: evidence for glutamic acid as an excitatory transmitter in the rat striatum

Rat striatal cells that were excited by cortical stimulation were found to respond to cortical stimulation with an average latency of 12 msec. Each response consisted of a variable number of spikes with, on the average, a less than 1:1 relationship between the stimulus and the number of spikes generated. Iontophoretic application of glutamic acid diethyl ester (GDEE), a substance reported to be a glutamate antagonist, at currents of +50 to +125 nA in the vicinity of neurons exicted by cortical stimulation, almost totally suppressed the excitation in 90% of the cells, and this suppression was fully reversible. All cells were excited by glutamate. GDEE also suppressed neuronal excitation produced by iontophoretic aspartate, glutamate and DL-homocysteic acid. It is concluded from this study that an excitatory amino acid, either aspartic or glutamic, may function as the transmitter in the corticostriate projection.

CNS depressants: effects on post-synaptic pharmacology

(1) The effects of 5 anesthetics (chloralose, chloroform, ethanol, pentobarbital and urethane) and one anticonvulsant (diphenylhydantoin) were studied on the membrane properties and post-synaptic responses of crustacean neuromuscular junction preparations and molluscan neurons to putative transmitters and peptides. (2) In crustacean preparations pentobarbital selectively depressed, in a dose-dependent, reversible manner, post-synaptic, Na+-dependent, depolarizing responses to the putative transmitter glutamate without altering post-synaptic, Cl(-)-dependent inhibitory responses to the putative transmitter gamma-aminobutyric acid. (3) The effects of all the agents on post-synaptic pharmacology of a molluscan neurosecretory cell were studied either by causing the cell to hyperpolarize to about--100mV through repeated application of acetylcholine (ACh) in a K+-free, Ca++-containing solution or by hyperpolarization through injection of intracellular current in a K+-free solution. Effects of these agents on post-synaptic responses on other molluscan neurons were studied using intracellular current injection to manipulate membrane potential. (4) All of the agents tested selectively depressed the depolarizing Na+-K+-dependent post-synaptic responses of the neurosecretory cell to ACh in a dose-dependent reversible manner without appreciably altering the membrane properties of the cell (over the potential range of the ACh responses). (5) Pentobarbital did not alter the inversion potential of the ACh response. (6) Reciprocal plot analysis of all of the agents tested revealed that the antagonism of the ACh response was primarily non-competitive. (7) None of the agents tested altered hyperpolarizing, K+-dependent responses to dopamine and glutamate on the neurosecretory cell, nor did they affect either the induction or enhancement of BPP activity by the vertebrate peptide vasopressin on this cell.

Analysis of Mauthner cell responses to iontophoretically delivered pulses of GABA, glycine and L-glutamate

1. The intracellularly recorded responses of goldfish Mauthner neurones to iontophoretically applied pulses of amino acids have been analysed: their time courses have been compared with each other, and with those predicted from diffusion theory.2. The rise time of the response to GABA is slower than that to glycine or L-glutamate. The response curves of the latter substances were very similar, and unlike that of GABA were markedly affected by increasing the distance of pipette-tip from the membrane. The results suggest that the time course of the responses to glycine and L-glutamate are determined mainly by free diffusion in the brain tissue (at least within about 200 mum of the cell), while that to GABA must be rate-limited by other factors, e.g. drug-receptor activation time.3. The possibility that the responses are influenced by some desensitizing process was investigated by applying a second (test) drug pulse during the response to a prior conditioning one. In the case of glycine and of L-glutamate there was no attenuation of the response to a second pulse at any time. With GABA, however, the second response was reduced during the period of the conditioning response; the reduction was progressively less marked the later the test pulse occurred. A similar effect with GABA was seen when glycine was used as the test pulse. The responses to long-maintained drug pulses also indicated that for GABA, but not for glycine or glutamate, there seems to be some desensitizing process present.4. Calculated time courses of responses to brief pulses of glycine and of L-glutamate (based upon diffusion theory) differed somewhat from the observed curves, largely during the falling phase. However, when the calculations were based upon second-order reactions (two molecules of drug per receptor) the diffusion model gave results very like the observed ones.5. Possible implications of these results for the role these three amino acids may have as neuro-transmitters are mentioned.

Axoplasmic transport (with particular respect to adrenergic neurons)

The article presents a review and summary on the development of studies in the field of axoplasmic transport in non-adrenergic and adrenergic neurons. Results obtained by, for example, histochemical, biochemical, electronmicroscopical, autoradiographical and radiological studies are discussed. Two main types of axoplasmic transport appear to operate in neurons; the slow axonal flow of 1 to 2 mm/day, and the fast transport exceeding 100 mm/day. However, also intermediary rates have been observed. The influence of nerve activity and drugs on axoplasmic transport is discussed, in relation to the possible mechanisms for axoplasmic transport which may operate in the neuron.

Molecular arrangement for physiological action of glutamic acid and gamma-aminobutyric acid

On combination of the amino group of glutamate or γ-aminobutyric acid (GABA) with a receptor site, RN, the steric influence of the α-carboxyl or γ-hydrogen on the adjacent carbon directs that this site in the receptor separate from surrounding protein structures. In this manner a space may be created in the membrane which measures 5.70–6.30 Å in the case of glutamate and 3.70–4.50 Å for GABA. These values are in good agreement with the diameter of hydrated sodium ions (4.80–5.12 Å) and chloride ions (3.32–3.86 Å). A selective increase in the permeability of the membrane towards sodium or chloride causes, respectively, excitation or inhibition in neurons. The pharmacological action of certain ω-amino acids may therefore be associated with a conformational change in the membrane, required to accommodate the space-filling configuration around the carbon, adjacent to nitrogen. In order that the steric influence of a substituent on this carbon exclusively directs the dimensions of the space to be formed, and for several other reasons, the amino acids probably adopt a specific molecular conformation when interacting with the receptor.

Starfish gonad: action and chemical identification of spawning inhibitor

The gamete-shedding substance obtained from the radial nerves induces spawning when it is applied to the gonads of mature starfish in vivo and in vitro. A substance that inhibits the action of this spawning factor is present in both ovary and testis; it has been isolated from testis of Asterina pectinifera and chemically identified as L-glutamic acid.