Summary1. Rats were injected with 0-8 mg nicotine/kg, 0-8 mg amphetamine/kg or with saline immediately before being tested for 30 min in activity boxes. 2. During the first 3 trials the nicotine group were less active than the controls but from trial 5 onwards nicotine had a stimulant effect. The stimulant effect of the amphetamine did not alter with repeated injection.
The pharmacological actions of nicotine on the central nervous system have been studied for many years (Silvette, Hoff, Larson & Haag, 1962; Larson & Silvette, 1965). Little attention, however, has been paid to the minimal dose of nicotine required to produce the particular effect under investigation and it has not always been established that the nicotine is in fact acting directly on the central nervous system. The present paper describes some of the effects of nicotine injected directly into the cerebral ventricles of conscious and anaesthetized cats. One of the regular effects of nicotine injected by this route in amounts ranging from 1-100 jug (base) was vigorous twitching of the ears. This was first reported by Armitage, Milton & Morrison (1965) and Hall & Reit (1965). The effects of ganglion-blocking drugs and cholinesterase inhibitors on the ear twitching induced by nicotine have been studied and the important observation has been made that cholinesterase inhibitors can potentiate this action of nicotine.
METHODSCats of either sex weighing between 2.5 and 3.5 kg were used for the experiments in which injections were made into one of the lateral ventricles. A Collison cannula, first described by Feldberg & Sherwood (1953), was screwed into the skull through a hole which was drilled and tapped 5-6 mm from the coronal suture and 3-4 mm from the midline. The lower end of the cannula shaft was made of polythene tubing. A hole was made in this tubing with a cautery near the end and the tip was sealed off with a drop of araldite. Drugs were injected into the ventricular cannula in a volume of 0.2 ml. through one arm of a " Y " shaped piece of polythene tubing. This was attached to a No. 18 hypodermic needle which was pushed through the rubber diaphragm. 0.2 ml. artificial c.s.f. (Carmichael, Feldberg & Fleischhauer, 1964) was then injected from the second arm, so that no drug was left in the cannula. The whole injection procedure took 15 sec and was used for experiments on conscious cats and on cats anaesthetized with chloralose (70-80 mg/kg i.v.) or pentobarbitone sodium (25 mg/kg i.v.). Observations were made on 12 conscious cats, 42 anaesthetized cats, two spinal cats and two decerebrate cats. Cats were made spinal under ether anaesthesia by cutting the cord at the level of the second cervical vertebra. The brain was then destroyed by pushing a small rod through the foramen magnum. Decerebration was performed under ether anaesthesia as described by Bum (1952).Ear twitches were recorded in anaesthetized cats either by means of a lightly weighted frontal writing lever or a spring-loaded heart lever writing on a kymograph.
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