The effects of serotonin and dopamine on salivary secretion by isolated cockroach salivary glands

We have studied the effects of 3-hydroxytyramine (dopamine) and 5-hydroxytryptamine (serotonin) on (1) the rates of salivation from isolated salivary glands of the cockroach Periplaneta americana, (2) the protein content of the saliva, and (3) the ultrastructure of the salivary gland epithelium. The rates of neurotransmitter-induced salivation varied in a dose-dependent manner within the concentration range 10(-9) to 10(-4) mol l-1. Half-maximal secretory rates were induced by 6x10(-7) mol l-1 serotonin and 1.1x10(-7) mol l-1 dopamine. Stimulation of the glands by serotonin resulted in the production of a protein-rich saliva, whereas saliva was protein-free after stimulation by dopamine. Electron microscopic studies revealed that the central cells, which are believed to produce the proteinaceous components of the saliva, secrete their vesicular content after stimulation by 10(-6) mol l-1 serotonin for 20 min. In contrast, no morphological changes could be detected after stimulation by 10(-6) mol l-1 dopamine. These data indicate that dopamine stimulates only the secretion of the fluid component of the saliva, whereas serotonin is necessary to stimulate secretion of the proteinaceous components.