Site of origin of the abnormal discharge in the electrocorticogram produced by tubocurarine perfused through the anterior horn of a lateral ventricle

Effects of apamin and nicotinic acetylcholine receptor antagonists on inferior collicular seizures

These studies compared the actions of apamin and nicotinic acetylcholine antagonists on seizure genesis within the inferior collicular cortex. In vitro alpha-bungarotoxin, d-tubocurarine and gallamine all competitively displaced [125I]apamin binding to brain sections through the inferior colliculus, while intracollicular microinjection of intermediate doses of apamin (21 pmol), alpha-bungarotoxin (0.3 nmol), d-tubocurarine (0.22 nmol) or gallamine (1.7 nmol) all significantly reduced the seizure initiation threshold current. However, higher doses of apamin did not cause spontaneous seizure activity, while higher doses of the nicotinic acetylcholine antagonists caused spontaneous seizures. Carbamylcholine also produced spontaneous seizures, but did not alter the seizure threshold current. N-Methyl-atropine caused a dose-related elevation of the seizure threshold current, yet microinjection of N-methyl-atropine (10 nmol) into the inferior collicular cortex reversed the effects of alpha-bungarotoxin on seizure threshold, partially opposed the effects of gallamine and d-tubocurarine, and had no effect on the ability of apamin to reduce the seizure threshold current. Thus, both the apamin-sensitive potassium channel and a variety of distinct cholinergic mechanisms contribute in vivo to seizure genesis within the inferior collicular cortex, but not through the same mechanisms.

Convulsants antagonise inhibition in the olfactory cortex slice

The effects of some gamma-aminobutyric acid (GABA) antagonists and other "convulsants" have been tested on neurones of the isolated olfactory cortex slice preparation of the guinea-pig using single cell intracellular and gross extracellular recording techniques. Bicuculline, picrotoxin, strychnine, leptazol, bemegride, and also theophylline and d-tubocurarine all increased the duration and amplitude of the excitatory postsynaptic potential, thereby producing a seizure-like discharge. These drugs reduced and shortened the peak conductance increase during the inhibitory postsynaptic potential (i.p.s.p.) in normal solution and after the i.p.s.p. had been prolonged by the presence of a barbiturate. The results suggested that these drugs antagonise synaptic inhibition through a common mechanism, perhaps by reducing the effect of neurally released GABA.

Direct and indirect activation of the hippocampus by tubocurarine

1. In cats anaesthetized with intravenous chloralose, tubocurarine was either perfused through the inferior horn of a lateral ventricle or applied by micro-injection into the hippocampus, and the electrical activity was recorded from the surfaces of the occipital cortices and from the cannulae used for the perfusion or injection which, insulated except at their tips, served as recording electrodes as well.2. With both methods, the hippocampus became activated by tubocurarine acting directly on the hippocampus. The activation resulted in an abnormal spike discharge interrupted from time to time by short bursts of fast activity, termed episodes, followed by periods of electrical silence during which no abnormal activity was recorded. This abnormal discharge was recorded in all leads. The spikes were negative when recorded from the surface of the hippocampus, but changed polarity when the electrode was lowered and penetrated the pyramidal cell layer, and on further lowering the electrode the positive spikes increased in voltage up to 15 mV and then decreased.3. With both methods, signs of indirect activation of the hippocampus were observed when the tubocurarine set up foci of excitation in one of the following three areas of cerebral cortex which lie ventral to the hippocampus, the Area entorhinica, the Area perirhinica and the Area post-splenialis. The foci of excitation resulted in a continuous discharge of negative spikes which were recorded in the leads from these areas only. A characteristic feature of the indirect hippocampal activation arising from the continuous discharge set up in the Area post-splenialis was the periodicity. Activation lasting 20-60 sec occurred every few minutes or at shorter intervals, often leading to an episode.

Microinjections of tubocurarine, leptazol, strychnine and picrotoxin into the cerebral cortex of anaesthetized cats

1. In cats anaesthetized with intravenous chloralose, microinjections of tubocurarine, leptazol, strychnine or picrotoxin, in a volume of 1 mul, were made into the grey matter of the cerebral cortex and the electrical activity was recorded from the site of injection with the microinjection cannula which, insulated except at its tip, served as recording electrode.2. Routinely the injections were made into the gyrus splenialis or into the underlying gyrus cinguli close to the mid-line, because the injections would then most likely be in grey and not in white matter. Injected in this way all four drugs set up foci of excitation which gave rise to synchronous firing of a large number of neurones with the result that high voltage negative spikes were recorded from the microinjection cannula.3. On injection into the gyrus splenialis the threshold dose was about 0.04 mug for picrotoxin, about 0.2 mug for tubocurarine, about 5 mug for strychnine and 25 to 50 mug for leptazol. Following the injection of larger doses the spike discharge continued for a few hours after picrotoxin and tubocurarine, for over an hour after strychnine, but for a few minutes only after leptazol. On injection into the gyrus cinguli the threshold doses were slightly greater and with larger doses the spikes occurred at greater frequency but were of lower voltage than in the gyrus splenialis.4. With large doses of picrotoxin injected into the gyrus splenialis the spikes developed an after-positivity and an after-discharge which sometimes passed into a short period of fast activity.5. The foci of excitation set up by the drugs were restricted to the site of injection because on raising or lowering the microinjection cannula the spikes recorded from it quickly decreased in voltage and then disappeared. When the injections were made close to a sulcus and the microinjection cannula, on being lowered, traversed the sulcus, the spikes changed their polarity.6. The spike discharge appears to be a consistent response to the injections of the drugs into grey matter of any part of the cerebral cortex since it was also obtained on their injection into the pyriform cortex, amygdala and area retrolimbica anterior, but not on their injection into white matter or caudate nucleus, thalamus or hypothalamus.

Central effects of picrotoxin when acting from the liquor spaces in anaesthetized cats

1. Picrotoxin was perfused through different parts of the cerebral ventricles in cats anaesthetized with chloralose, and applied topically to the upper cervical cord in cats anaesthetized with intraperitoneal pentobarbitone sodium. With both methods, picrotoxin produced effects similar to those produced by tubocurarine; but it was active in weaker concentrations.2. Perfused through the third ventricle, picrotoxin caused shivering resulting in a rise of rectal temperature, increased motor excitability, muscle jerks, tachypnoea, mydriasis, and withdrawal of the nictitating membranes. The effects were due to an action on structures in the walls of the ventral half of the ventricle, because they occurred only on perfusion of tubocurarine through this half and not on its perfusion through the dorsal half of the third ventricle. Noradrenaline perfused through the third ventricle abolished the shivering and the rise in temperature, but did not affect the motor hyperexcitability or the muscle jerks, whereas pentobarbitone sodium similarly perfused abolished these effects as well.3. Perfused through the inferior horn of a lateral ventricle, picrotoxin caused excitation of the hippocampus. This resulted in a rhythmic discharge of high voltage negative spikes in the electrocorticogram taken from the occipital cortices. The spikes developed after-positivity with after-discharges and were interrupted from time to time by bursts of fast activity termed episodes. This abnormal discharge was recorded also from the perfusion cannula which was inserted into the inferior horn and acted as a lead from the surface of the hippocampus.4. Perfused through the anterior horn of a lateral ventricle picrotoxin caused a rhythmic discharge of negative spikes which was recorded from the anterior horn cannula and resulted from excitation of the grey matter in the anterior limbic area which forms part of the medial wall of the horn.5. Applied to the surface of the upper cervical cord, picrotoxin produced scratching movements.

The active transport of 5-hydroxyindol-3-ylacetic acid and 3-methoxy-4-hydroxyphenylacetic acid from a recirculatory perfusion system of the cerebral ventricles of the unanaesthetized dog

1. An operation on dogs for the implantation of guide tubes to the lateral ventricle and cisterna magna and a method whereby the ventricular space can be repeatedly perfused in conscious and unrestrained animals are described.2. The characteristics of a recirculatory perfusion system were examined and the bulk formation and absorption of cerebrospinal fluid and the volume of the ventricular space perfused were derived from the concentrations achieved during the infusion of inulin into the system.3. 5-hydroxyindol-3-ylacetic acid (5-HIAA), the acid metabolite of 5-hydroxytryptamine, and 3-methoxy-4-hydroxyphenylacetic acid (HVA), the main acid metabolite of dopamine, were demonstrated to be mainly removed from cerebrospinal fluid (c.s.f.) by an active transport system localized in the region of the fourth ventricle.4. It was possible to inhibit the active transport of these acids from cerebrospinal fluid by pre-treating the dogs with probenecid.