Uncharted Cholinergic Nerves in the Rabbit Parotid Gland

Abstract

Cholinergic nerves are shown to be left in the rabbit parotid gland after avulsion of the auriculo-temporal nerve: a cholinesterase inhibitor injected through the duct caused secretion, thereby revealing leakage of acetylcholine from cholinergic nerve endings, and acetylcholinesterase positive nerves were found histochemically. The incomplete cholinergic denervation offers an explanation to the fact that some choline acetyltransferase activity remains in the 'denervated' glands.In the rabbit parotid glands a rapid and profound fall in the activity of the acetylcholine synthesizing enzyme, choline acetyltransferase, was found by Nordenfelt [1964] to follow a parasympathetic denervation, caused by section of the auriculo-temporal nerve. This finding probably indicates a neuronal localization for the enzyme. However, some enzyme activity usually remained suggesting either the presence of unsevered cholinergic neurones or other sources for the enzyme in the gland. In such denervated parotid glands of other species, i.e. cats [Nordenfelt, 1963; Ekstrom and Emmelin, 1974], dogs [Ekstrom and Holmberg, 1972] and rats [Ekstrom, 1974] a low choline acetyltransferase activity may be demonstrated also. In cats and dogs the residual enzyme activity seems to be accounted for, since several investigations, both physiological and histochemical, show that the denervation is incomplete leaving some cholinergic nerve fibres intact [Emmelin and Str6mblad, 1958;Emmelin and Holmberg, 1967;Garrett, 1966;Garrett and Holmberg, 1972;Holmberg, 1972; Ekstrom and Emmelin, 1974]. Accordingly, by including not only section of the auriculo-temporal nerve in the denervation procedure, but division of nerve strands on the internal maxillary artery [Holmberg, 1971] and the facial nerve the choline acetyltransferase activity of the parotid gland of dogs in most cases reached values below the limit of detection [Ekstrom and Holmberg, 1972].The present experiments investigate whether in rabbits also there are some cholinergic neurones left following avulsion of the auriculo-temporal nerve. This was done by the use of a cholinesterase inhibitor injected through the duct into the gland; such a substance is known to cause secretion of saliva by preserving acetylcholine leaking from the nerve endings [Emmelin and Stromblad, 1958]. Further, nerves stained for acetylcholinesterase were looked for. METHODSSixteen rabbits of both sexes weighing 1-9-3-2 kg were used. Under ether and pentobarbitone (35 mg/kg i.p.) anaesthesia the following experimental procedures were 127

Parotid secretion of fluid and amylase in rabbits during feeding.

“…Except for the highest flow-rates, parasympathetic nerve stimulation, 03-20 Hz, could produce as large volumes of saliva as those reflexly secreted; it seemed as if the salivary flow during eating mostly corresponded to a parasympathetic activity of 1-5 Hz. However, the impulse traffic may be lower in the conscious animal than indicated by the nerve stimulations; not all parasympathetic nerves travel through the electrically stimulated auriculotemporal nerve (Alm & Ekstrom, 1976) and in addition some nerve fibres are likely to be damaged during the surgical procedures in the acute experiment. On the other hand, in the conscious animal all nerve fibres are not necessarily activated at the same time, which is the case when electrical stimulation at supramaximal intensity is used.…”

“…The tube was connected to a photo-electric drop recorder operating a pen-writer, recording the flow rate as intervals between the drops, which were of a size of 15 #1. The auriculo-temporal nerve carrying the main parasympathetic nerve supply to the gland (Alm & Ekstrom, 1976) was dissected as described by Burgen (1964), and by Nawrocki (1868). The sympathetic nerve trunk was dissected in the neck and cut.…”

The otic ganglion in rats and its parotid connection: cholinergic pathways, reflex secretion and a secretory role for the facial nerve

“…The classical route for the postganglionic parasympathetic nerve fibres of the gland is the auriculotemporal nerve. However, interruption of the auriculotemporal nerve pathway in dogs (Holmberg, 1971), cats (Ekström & Emmelin, 1974), rabbits (Nordenfelt, 1964; Alm & Ekström, 1976 a ) and rats (Alm & Ekström, 1976 b ) shows the parasympathetic denervation to be incomplete. This indicates that parasympathetic nerve fibres may, in addition, reach the gland via other routes than the auriculotemporal nerve.…”