CHARACTERIZATION OF DOPAMINE AND SEROTONIN RECEPTORS ON THE SALIVARY-GLANDS OF THE LOCUST, LOCUSTA-MIGRATORIA

The salivary glands of the locust, Locusta migratoria, are innervated by two pairs of neurons, one of which, the SN1, contains dopamine, while the other, the SN2, contains serotonin. The pharmacological properties of dopamine and serotonin receptors associated with the salivary glands of Locusta migratoria were examined. These receptors appear to be linked to adenylate cyclase, and, when activated by dopamine or serotonin, induce increases in cyclic AMP levels in the glands. The increases in cyclic AMP are mimicked by electrical stimulation of the axons of the SN1 and SN2 which presumably release dopamine and serotonin. A number of aminergic agonists were tested for their ability to induce increases in cyclic AMP levels in the salivary glands. Of these compounds, only epinephrine and synephrine were capable of significantly elevating cyclic AMP levels, while octopamine and norepinephrine were without effect when tested at 10 mu M. Additivity studies suggest that the receptors for serotonin and dopamine are distinct. The rank order of potency (based upon IC50 values) of serotonergic antagonists of spiperone > cyproheptadine > methysergide > mianserin > ketanserin suggests the presence of receptors similar to vertebrate 5-HT2 receptors. IC50 values of the dopaminergic antagonists, with a rank order of potency of SCH-23390 > (+/-)-butaclamol > flupenthixol, suggest the presence of receptors similar to vertebrate D-1 receptors. The neurally-evoked increases in cyclic AMP are sensitive to receptor antagonists. Thus, SCH-23390, a dopamine receptor antagonist, and spiperone, a serotonin receptor antagonist, are each capable of partially inhibiting the neurally-induced increase in cyclic AMP.