Functional significance of synthesis of noradrenaline in adrenergic nerves of rat salivary glands

Abstract

Abstract The secretory response and the noradrenaline content of the submaxillary gland due to continuous electrical stimulation of the cervical sympathetic trunk with 5 Hz (5 V, 2 ms) were determined in five differently treated groups of rats. Stimulation during 5 h in untreated control rats reduced the noradrenaline content to about 40% while the secretory response remained unchanged. Another group received DL-α-methyl-m-tyrosine 400 mg kg−1 i.p. 26 and 18 h before the stimulation and 200 mg kg−1 2 h before. This treatment displaced 93% of the noradrenaline content of the glands, and the secretory response stabilized after an initial decrease, at a level of about two-thirds of the normal response, possibly indicating a reduced rate of transmitter synthesis compared to normal rats. Thus, a nerve with greatly reduced amounts of active transmitter in the store can satisfy rather high demands of transmitter release for long time periods, provided that the synthesis continues. A third group of rats received DL-α-methyl-m-tyrosine in the same doses as above but 50, 42 and 26 h before the experiments, respectively. After this treatment, the noradrenaline level was reduced to about 35% of normal, yet the secretory response was even higher than normal. Probably the decreased transmitter synthesis supposed to occur following treatment with α-methyl-m-tyrosine has been reversed at this time interval into an increased synthesis rate. To a fourth group of rats 500 mg kg−1 DL-α-methyl-p-tyrosine methylester HCl (H 44/68) was injected i.p. 15 min before stimulation, resulting in a virtually complete inhibition of the tyrosine hydroxylase. Three h of stimulation reduced noradrenaline to 15%, and the secretory response disappeared. When treatment with both drugs was given, a very rapid disappearance of an initially normal response was seen. In conclusion, synthesis of new transmitter appears to be essential for a normal function of the nerve.