Acetylcholine levels in the frog spinal cord following the administration of different convulsants

Changes in regional brain acetylcholine levels during drug-induced convulsions

Acetylcholine (ACh) levels were determined in the brain of rats killed by decapitation or focussed microwave radiation during drug-induced convulsions. During metrazol or strychnine-induced convulsions a diffuse decrease in ACh levels was found in rats killed by decapitation. When the rats were killed by radiation and the brain was only divided into three large regions, strychnine caused no changes in ACh levels; metrazol caused a decrease in the cerebral cortex and lower brainstem. When discrete brain regions were investigated in rats killed by radiation, metrazol-induced convulsions were associated with a decrease in ACh level in all regions dissected and strychnine-induced convulsions with a decrease in the hippocampus and caudate nucleus only. Picrotoxin-induced convulsions were associated with a decrease in ACh level in the cerebral cortex, hippocampus, midbrain and medulla-pons, those induced by bicuculline with an increase in ACh level in the frontal cortex, hippocampus, midbrain and medulla-pons, by dimefline with an increase in the frontal cortex, midbrain and medulla-pons and a decrease in the caudate nucleus. The experiments show that each type of convulsant affects ACh levels in discrete brain regions in a different way.

The effect of electrical stimulation and drugs on the release of acetylcholine from the frog spinal cord

The isolated frog spinal cord preparation, hemisected longitudinally, has been used to study concurrently the release of endogenous acetylcholine (ACh) and the electrical responses to stimulation of the ventral or dorsal roots. In unstimulated cords, ACh release was high initially and then fell to a plateau level. ACh release was reduced by tetrodotoxin and increased by high K+. Stimulation of the ventral roots enhanced the ACh release. The magnitude of this effect was dependent on the frequency of stimulation and was antagonized by tetrodotoxin or C Ca2+-free Ringer. During prolonged ventral root stimulation (50 min), the evoked ACh release became progressively smaller but was restored to the original high levels by the addition of choline to the bath fluid. Stimulation of the dorsal roots failed to augment the ACh output. The application of hyoscine was followed by a large rise in the unstimulated ACh output. However no increase in the antidromically-evoked release, when compared with control stimulations, was seen. The application on alpha-bungarotoxin was followed by a short-lasting decline in the unstimulated ACh output, whereas the electrically-evoked output was unaffected. In the frog spinal cord the determination of ACh release, together with the recording of the root potentials, offers the possibility of studying the output of a central transmitter in relation to the electrical activity of an in vitro central nervous system preparation.

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.