Experiments on the site of action of tubocurarine when applied via the cerebral ventricles

Custom-made Microdialysis Probe Design

Microdialysis is a commonly used technique in neuroscience research. Therefore commercial probes are in great demand to monitor physiological, pharmacological and pathological changes in cerebrospinal fluid. Unfortunately, commercial probes are expensive for research groups in public institutions. In this work, a probe assembly is explained in detail to build a reliable, concentric, custom-made microdialysis probe for less than $10. The microdialysis probe consists of a polysulfone membrane with a molecular cut-off of 30 kDa. Probe in vitro recoveries of substances with different molecular weight (in the range of 100-1,600 Da) and different physicochemical properties are compared. The probe yields an in vitro recovery of approximately 20% for the small compounds glucose, lactate, acetylcholine and ATP. In vitro recoveries for neuropeptides with a molecular weight between 1,000-1,600 Da amount to 2-6%. Thus, while the higher molecular weight of the neuropeptides lowered in vitro recovery values, dialysis of compounds in the lower range (up to 500 Da) of molecular weights has no great impact on the in vitro recovery rate. The present method allows utilization of a dialysis membrane with other cut-off value and membrane material. Therefore, this custom-made probe assembly has the advantage of sufficient flexibility to dialyze substances in a broad molecular weight range. Here, we introduce a microdialysis probe with an exchange length of 2 mm, which is applicable for microdialysis in mouse and rat brain regions. However, dimensions of the probe can easily be adapted for larger exchange lengths to be used in larger animals.

Analysis of some central actions of nicotine injected into the cerebral ventricles of cats

1. In unanaesthetized cats intraventricular injections of 2.5-10 mug nicotine produced blinking, narrowing of the palpebral fissures, retching, vomiting and asynchronous twitching of the ears. With larger doses (30-100 mug) the ear twitching was interrupted for a short time by laying back of the ears. Respiration first became laboured and deep, then rapid and shallow following which panting occurred. There was salivation, loud calling, micturition and defaecation. With still larger doses (300-1000 mug) there was torticollis, ataxia and blind charging sometimes followed by a clonic-tonic convulsion.2. In cats anaesthetized with chloralose only some of these effects followed the intraventricular injection of nicotine, i.e. the ear response, respiratory changes and salivation. In addition, the pinna reflex was facilitated.3. The ear response and the facilitation of the pinna reflex did not occur on perfusion of nicotine from a lateral ventricle to aqueduct. They result from an action on superficial structures in the cervical cord between C1 and C2. Applied to this region of the cord, nicotine produced the ear response within 10-60 sec, sometimes in concentrations as low as 1/100,000. Applied below C2, nicotine was ineffective.4. Transecting the cord below C2 or cutting the dorsal and ventral roots of C1, C2 and C3 bilaterally did not affect the ear response produced by topical application or by intraventricular injection of nicotine. Transection of the cord above C1 abolished it.5. Hexamethonium applied to the cervical cord between C1 and C2 inhibited the ear response and the facilitation of the pinna reflex whether produced by nicotine applied topically or injected intraventricularly.6. The salivation and the respiratory changes produced by intraventricular injections of nicotine did not occur when nicotine was perfused from a lateral ventricle to the aqueduct. They result from an action of the nicotine on structures situated superficially in the brain stem. Nicotine had no sialogogue or respiratory effect when applied to the region of the cord at which it produced an ear response, but perfused through the subarachnoid space from interpeduncular fossa to cisterna magna or injected into the subarachnoid space alongside the brain stem, it produced these effects.7. Hexamethonium perfused from interpeduncular fossa to cisterna magna inhibited the salivary secretion as well as the respiratory changes produced by nicotine similarly applied.8. The efferent pathway for the salivation is parasympathetic since it no longer occurred after cutting the chorda-lingual nerve or after intravenous atropine.9. Intravenous injections of nicotine also produced the ear response with facilitation of the pinna reflex, salivation and hyperventilation in the anaesthetized cat, but only the ear response and facilitation of the pinna reflex are central effects. The salivation and hyperventilation following intravenous injection are due to peripheral actions of nicotine.

Third W.D.M. Paton Memorial Lecture Burn Oxford for a start

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The role of the septal nuclei in the release of acetyl-choline from the rabbit cerebral cortex and dorsal hippocampus and the effect of atropine

Acetylcholine (ACh) was collected from the alvear surface of the dorsal hippocampus and cerebral cortex in chloralose-urethane anaesthetized or unanaesthetized rabbits. With anaesthesia, the resting release of ACh from the hippocampus was greater than that from the cortex. Wthout anaesthesia, the resting release from both areas was much higher and very similar. The addition of atropine sulphate (1 microgram/ml) to the collecting fluid or the administration of Artane (2 mg/kg i.v.) increased resting ACh release from both the hippocampus and cortex to similar output levels. Atropine also increased ACh release due to stimulation of the medial septum (MS) or mesencephalic reticular formation (MRF). Removal of the septum abolished the effect of atropine on resting ACh release and on release evoked by MRF stimulation from both the hippocampus and cortex. The data indicate that the septum is an essential pathway for cholinergic fibres ascending to the cerebral cortex and hippocampus. They also demonstrate that the septal cholinergic fibres must be intact and active for atropine to increase ACh release from their terminals.

Acetylcholine release from visual and sensorimotor cortices of conditioned rabbits: the effects of sensory cuing and patterns of responding

A technique was devised for the collection of acetylcholine (ACh) released from the cerebral cortex of awake rabbits while they were performing a previously learned operant task. Based on the assumption that ACh release is directly proportional to the activity of cholinergic synapses under the area of collection, two hypotheses of the functional role of cortical cholinergic mechanisms were examined: (1) that activity in cholinergic neurons is related to the inhibition of responding; (2) that cholinergic activity is related to the perception of a 'significant' stimulus. Five groups trained on different behavioral paradigms were used to test these hypotheses. ACh release was collected concurrently from visual and sensorimotor cortices to differentiate diffuse from specific cortical effects. A small (50-100%) increase in ACh release was found in all groups and from both cortical areas. In the case of one group (visually cued, reinforced for low response rates) a significantly greater increase occurred from sensorimotor cortex only. These findings do not support either hypothesis alone, and are interpreted as evidence for two cholinergic systems within, or projecting to the cortex. One is related to generalized behavioral arousal and desynchronization of the electroencephalogram. Activation of the second cholinergic system is dependent on both response inhibition and the presence of a significant stimulus of the visual (but not of the auditory) modality.

The influence of mesenteric denervation on the inhibition of gastric secretion by fat in the intestine

The inhibitory effect of fat in the upper small intestine was studied in dogs prepared with a Heidenhain pouch, gastric fistula, and jejunal fistula. Gastric secretion was stimulated by a constant intravenous infusion of pentagastrin, and emulsified olive oil was introduced into the jejunum. The small intestine of the dog was denervated by complete transection of the mesentery except for the superior mesenteric vessels. After intestinal denervation inhibition of the vagally innervated stomach was almost abolished while that in the Heidenhain pouch was unchanged. It is concluded that either autonomic innervation is important in the formation of enterogastrone or, more likely, that efferent inhibitory fibres in the vagus to the stomach act synergistically with enterogastrone.

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.

The strychnine-like action of curare and related compounds on the somatosensory evoked response of the rat cortex

1. Drugs were applied to the cerebral cortex of rats anaesthetized with pentobarbitone and changes measured in the somatosensory primary evoked response.2. Computer-derived averages of thirty-two consecutive responses yielded stable and consistent measurements of the potential changes comprising the evoked response, and comparable records from the opposite (non-drug treated) cortex provided an essential control for systemic actions of the drug.3. The modifications produced by curare and strychnine were indistinguishable. The first positive wave (peak latency 7 msec) was unaltered; the second positive wave (peak latency 11.5 msec) was variably enhanced, and the first and second negative waves (peak latencies 16 and 40 msec) were replaced by a much larger negative wave (peak latency 22 msec).4. The time of onset of the effect on the negative waves and the maximal amplitude attained by the abnormal negative wave were related to the log concentration of the drug used. Curare is approximately 10 times more potent than strychnine.5. Toxiferine I, di-allylnortoxiferine and atropine also produced this effect but were less potent than strychnine.6. Succinylcholine, dihydro-beta-erythroidine and gallamine triethiodide did not produce this effect (in concentrations up to 10(-3)M).7. The observations are consistent with an action of curare and strychnine on an intracortical cholinergic inhibitory system, but other possibilities including a "non-specific excitatory action" cannot be excluded.

Micro-electrophoretic studies of neurones in the cat hippocampus

1. Drugs have been applied micro-electrophoretically to units in the hippocampal cortex of the anaesthetized cat, and their effects on cell firing were recorded simultaneously.2. L-Glutamate rapidly and powerfully excited hippocampal units, an effect which was quickly reversed on stopping the expelling current. The local application of L-glutamate also excited a fast seizure discharge at 15-50/sec. Both these effects of L-glutamate were strongly depressed by fimbrial stimulation.3. gamma-Aminobutyric acid had a strong depressant action on all the units on which it was tested; the time course of this effect was rapid.4. ACh excited half the units to which it was applied. Characteristically this excitation developed slowly over many seconds and persisted after stopping the expelling current. Most cholinoceptive units were found to be concentrated in the superficial layer of the cortex corresponding to the hippocampal pyramidal cells and their main dendritic processes.5. Atropine selectively blocked the excitation of cholinoceptive units by ACh, but not the excitation by L-glutamate. No cholinoceptive units were blocked by dihydro-beta-erythroidine, though several were selectively blocked by dimethyl (+)-tubocurarine.6. The most usual effect seen with 5-HT was depression, though several units were found to be excited. Some of the units tested with 3-hydroxytyramine (dopamine) or noradrenaline were found to be depressed.