This section is from the book "A Text-Book Of Pharmacology, Therapeutics And Materia Medica", by T. Lauder Brunton. Also available from Amazon: A text-book of pharmacology, therapeutics and materia medica.
Anti-sialics are substances which lessen the salivary secretion. They may do this :
First, by removing the stimulus to secretion.
Second, by lessening the excitability of the efferent nerves or reflex centres.
Third, by paralysing the efferent nerves, such as the chorda tympani.
Fourth, by acting on the circulation through the gland.
Fifth, by acting on the gland-structures themselves.
Borax and chlorate of potassium are useful in the first of these ways by inducing a healthy condition of the mucous membrane of the mouth, and thus lessening the irritation which gives rise to salivation; opium and morphine diminish the reflex excitability of the nerve-centre, and are thus powerful anti-sialics.
Physostigma in large doses greatly lessens the supply of blood to the gland, and thus diminishes its secretion, and quinine, hydrochloric acid, and alkalies injected directly into the duct of the gland arrest secretion by affecting the secretory cells themselves. These latter drugs, however, cannot be used as anti-sialics.
The most powerful of all anti-sialics is, however, atropine, which paralyses the peripheral terminations of secreting nerves. It does not affect the vaso-dilating nerves, so that in an animal poisoned by atropine electrical stimulation of the chorda tympani will cause dilatation of the vessels and a free flow of blood through the gland as usual, but not a drop of saliva will be secreted. That this absence of secretion is due to paralysis of secretory nerves and not of the secreting cells appears to be shown by the fact that at the time when the power of the chorda to induce secretion is completely paralysed, stimulation of the sympathetic will still induce secretion.
Very large doses of atropine, however, paralyse the secreting power of the sympathetic in the cat, although this has not been noticed in the dog.
The paralysing action of atropine can be counteracted by physostigmine. This is shown by poisoning an animal with atropine, and then injecting physostigmine into the gland of one side through the submental artery. It is then found that irritation of the chorda causes salivation in the gland which has received physostigmine, while it causes no secretion in the other.
Iodide of ethyl-strychnine and cicutine have an action like that of atropine on the secreting and not on the vaso-dilating fibres of the chorda tympani.1