The association of serotonin with the spermatheca of the locust, Locusta migratoria

The association of serotonin with the spermatheca of Locusta nugratoria was determined using immunohistochemistry and high performance liquid chromatography coupled to electrochemical detection. Faintly stained immunoreactive processes were seen on the spermathecal muscle but were absent in the nerves innervating the spermatheca. Immunostaining on the spermathecal muscle was enhanced using the serotonin neurotoxin, 5,7-dihydroxytryptamine, and revealed serotonin-like immunoreactive processes over the entire spermatheca. Serotonin stimulated a dose-dependent increase in the frequency of phasic contractions and the amplitude of neurally-evoked contractions of the spermatheca. Serotonin also increased the cyclic adenosine 3',5'-monophosphate (cyclic AMP) content in all regions of the spermatheca, in the presence of the phosphodiesterase inhibitor IBMX. Octopamine had similar effects. Spermathecal tissues simultaneously exposed to octopamine and serotonin did not demonstrate cyclic AMP levels significantly different from those noted for octopamine alone. Serotonin antagonists failed to inhibit the effects of serotonin on cyclic AMP levels, with the exception of mianserin which produced inconsistent effects. Phentolamine, which totally blocks octopamine stimulated increases in cyclic AMP, reduced serotonin-stimulated cyclic AMP increases by 54% to 100%. The results suggest that serotonergic actions may, in part, be mediated through the octopaminergic system and the activation of a cyclic AMP pathway.