cAMP and excitability in neuroendocrine cells during reproductive senescence

Excitability changes during reproductive senescence were investigated in the neurosecretory caudodorsal cells (CDCs) that control egg laying in the mollusc Lymnaea stagnalis. CDCs in the isolated central nervous system (CNS) were exposed to different discharge inducing treatments. Senescent CDCs (of animals 8 weeks after laying their last egg mass) and inhibited (I-) state CDCs (of egg-laying (EL) animals) were used. We showed that senescent and I-state CDCs closely resemble each other electrophysiologically. Electrical stimulation did not induce an afterdischarge in either type of CDC but exposure to release products of CDCs from EL animals or to saline with high potassium concentration did induce discharge activity. Also, 8-chlorophenylthio (8-CPT)-cAMP (10−5 M) induced discharge activity. Exposure to the phosphodiesterase inhibitor 3-isobutyl-1-methylxanthine (IBMX) (10−3 M) or to the adenylate cyclase activator Forskolin (10−4 M), restored afterdischarge induction by electrical stimulation. Application of IBMX (10−3 M) and Forskolin (10−4 M) together induced discharges in the absence of electrical stimulation. Our results suggest that in senescent CDCs changes in the intracellular cAMP pathway may underlie afterdischarge failure.