T.J. Biscoe

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 hemisected 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 excitant than l‐glutamate on frog spinal motoneurones. The former compound 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.

Antagonism between bicuculline, strychnine, and picrotoxin and depressant amino-acids in the rat nervous system.

Abstract 1. 1. The effects of the alkaloids bicuculline, strychnine, and picrotoxin on the actions of the depressant amino-acids glycine, GABA, β-hydroxy GABA, β-alanine, and δ-amino-valeric acid on neurons of the spinal cord and cerebral cortex of the rat were investigated. 2. 2. In the spinal cord strychnine and bicuculline were relatively specific antagonists of the actions of ‘glycine-like’ and ‘GABA-like’ amino-acids respectively. The specificity of picrotoxin as an antagonist of ‘GABA-like’ amino-acids was less than that of bicuculline. 3. 3. On most cortical neurons both bicuculline and picrotoxin non-specifically reduced the action of all depressant amino-acids. Strychnine had no effect until concentrations producing abnormalities in action potentials were used, when all depressants were equally reduced. 4. 4. It is suggested that in the rat the type of receptor for ‘glycine-like’ amino-acids present on spinal neurons is uncommon on neurons of the cerebral cortex, and that on these latter cells all the amino-acids studied acted through combination with one type of receptor.

The inhibition of hypoglossal motoneurones by impulses in the glossopharyngeal nerve of the rat.

SummaryStimulation of the glossopharyngeal nerve produced inhibitory post synaptic potentials in neurones of the hypoglossal nucleus of the rat. The corresponding pause in the amino acid induced firing of these cells was antagonized by electrophoretically administered strychnine but not by bicuculline. Strychnine and bicuculline antagonized the actions of glycine and GABA respectively on these cells. The findings suggest that the transmitter for this synaptic inhibition is a glycine-like amino acid.

Acetylcholine receptors on Renshaw cells of the rat

Experiments have been performed on Renshaw cells of rats to investigate the receptors mediating both the responses to electrophoretically applied cholinergic excitants and the synaptic excitation following stimulation of the ventral roots. The effects of atropine and dihydro-beta-erythroidine, muscarinic and nicotinic antagonists respectively, have been tested on both types of excitation. Most importantly we found that in the rat the ventral root evoked response can be blocked by atropine as well as by DHbetaE. It was not possible to categorise the responses to the cholinergic agents in terms of muscarinic and nicotinic receptors. In addition we have failed the late (muscarinic) excitation seen in the cat.

The effects of morphine, etorphine and sinomenine on the chemical sensitivity and synaptic responses of Renshaw cells and other spinal neurones in the rat.

Abstract The effects of electrophoretically applied morphine, etorphine and sinomenine have been investigated on Renshaw cells and other spinal neurones of the rat. Only morphine specifically reduced the depressant effect of glycine without affecting that of γ-aminobutyric acid. On Renshaw cells, only morphine increased the response to acetylcholine more than that to D,L-homocysteate, and increased the latency of the action potentials which follow a ventral root stimulus. The differences between the effects of morphine and those of etorphine and simonine suggest that those properties of the drugs investigated in this study are not related to the analgesic properties of morphine and etorphine.

Effect of etorphine, morphine and diprenorphine on neurones of the cerebral cortex and spinal cord of the rat

1 . The actions of etorphine, morphine and diprenorphine were investigated on neurones of the cerebral cortex and spinal cord of rats anaesthetized with pentobarbitone. 2 . In the cerebral cortex, intravenous etorphine increased the latency of the primary evoked response to a peripheral nerve stimulus and suppressed the rhythmical after‐discharge. Diprenorphine reversed this effect. These actions were demonstrated on both field potentials and unit firing. 3 . Morphine had no effect on the primary response but the frequency of after‐discharge bursts was reduced and there was an increase in firing between bursts. 4 . In the cerebral cortex, electrophoretically applied etorphine reduced after‐discharges when applied for long periods but had no effect on the depressant actions of glycine and γ‐aminobutyric acid (GABA) nor on the excitant action of acetylcholine and l‐glutamate. Similarly there was no alteration by etorphine of the effects of glycine, GABA and l‐glutamate on spinal cord neurones. 5 . It is concluded that etorphine may act pre‐synaptically in the cerebral cortex.

Electrophysiological observations on the spinal cord of the normal and dystrophic mouse

A method of carrying out electrophysiological experiments on the mouse spinal cord is described. The conduction velocity in the spinal dorsal roots (DR) of the normal mouse was in the range 10-100 m sec-1 and in the ventral roots (VR) 50-70 m sec-1. In the dystrophic mutant (129 ReJ dy/dy) the conduction velocity for both roots was usually in the range 0.1-2.0 m sec-1. Reflexes from DR to VR were recorded in both mutant and wild type animals and it was concluded that the delays in the mutant reflex were probably due to the slower conduction velocity in the roots. Postsynaptic inhibition and presynaptic inhibition were demonstrated and records were made from Renshaw cells and intracellularly from motoneurones. Delayed activity in spinal reflex paths, probably of supraspinal origin, was more pronounced in the dystrophic mutant. It is concluded that if the dystrophic mouse mutant were to be regarded as a model for human disease then similar reflex delays should be demonstrated in human subjects with muscular dystrophy.

The sensitivity of rat spinal interneurones and Renshaw cells to L-glutamate and L-aspartate

SummaryL-Glutamate and L-aspartate were administered electrophoretically near spinal interneurones and Renshaw cells of pentobarbitone-anaesthetized rats. Other spinal interneurones were consistently more sensitive to L-glutamate than to L-aspartate. Renshaw cells, however, showed no consistent difference in their sensitivity to these two amino acids. The results, which are compared with those reported previously in the cat, support the hypothesis that L-glutamate could be a transmitter at spinal primary afferent terminals.

Pharmacological studies on facial motoneurones in the rat

Experiments were performed on 39 male albino rats anaesthetized with pentobarbitone sodium and paralyzed with gallamine triethiodide. Facial motoneurones consistently showed a progressive alteration of spike shape and decrease in spike firing frequency during the continuous microelectrophoretic application of the excitant amino acids D,L-homocysteate, L-glutamate, L-aspartate, and kainate, but infrequently during that of N-methyl-D-aspartate. The relative potencies of these excitants on facial motoneurones are reported. The potential usefulness of N-methyl-D-aspartate to produce amino acid-evoked motoneurone action potentials is discussed. The microelectrophoretically-applied depressant amino acid antagonist strychnine selectively and reversibly blocked the depressant effects of glycine on facial motoneurones. The depression of amino acid-induced firing of facial motoneurones by stimuli to the lingual or glossopharyngeal nerves were reversibly antagonized by microelectrophoretically applied strychnine but not by bicuculline.

Actions of pancuronium on Renshaw cells of the rat spinal cord

Abstract 1. 1. Pancuronium administered electrophoretically blocked excitation of Renshaw cells by acetylcholine but not by L -glutamate. 2. 2. Concentrations adequate to reduce synaptic excitation by ventral root volleys also excited Renshaw cells, an action which impaired the ability of pancuronium to differentiate between acetylcholine receptors of the nicotinic and the muscarinic types.