Depletion of neuropeptides in rat parotid glands and declining atropine-resistant salivary secretion upon continuous parasympathetic nerve stimulation

In rats the parasympathetic auriculo-temporal nerve on one side was continuously stimulated at 40 Hz for 20-80 min in the presence of adrenergic blockers (dihydroergotamine and propranolol) +/- atropine. During the first 10 min this gave rise to a flow of saliva from the parotid gland that in the atropinized rats amounted to 35% of that found in rats not treated with atropine, while the protein and amylase outputs were 75% of those in non-atropinized rats. The atropine-resistant secretion of fluid and proteins declined to 5-10% of the initial value within 40 min but did not cease completely even after 80 min. The marked reduction in secretory responses was not due to desensitization or exhaustion of the gland cells. The nerve stimulation reduced the parotid gland content of vasoactive intestinal peptide (VIP) and substance P (SP) to approximately 60 and 25% of that of contralateral glands after 20 and 60 min, respectively. The probable explanation for the decline in secretory response seems to be depletion of non-adrenergic, non-cholinergic transmitter(s). The present results suggest that neuropeptides are involved in the regulation of salivary secretion but provide no direct evidence that either VIP or SP is responsible for the atropine-resistant salivary secretion.