A new regulation of non-capacitative calcium entry in insect pacemaker neurosecretory neurons. Involvement of arachidonic acid, no-guanylyl cyclase/cGMP, and cAMP.

Efferent dorsal unpaired median neurons are pacemaker neurosecretory cells. A Ca(2+) background current contributing to the pacemaker activity of cockroach dorsal unpaired median neurons is up-regulated by neurohormone D (NHD), an octapeptide belonging to the adipokinetic hormone family. This modulation accelerates spiking and increases [Ca(2+)](i). Using patch clamp, calcium imaging, and immunocytochemistry, we investigated the signaling pathway of NHD-induced current modulation. The membrane depolarization produced by NHD was related to the increase in membrane conductance for Ca(2+), Ba(2+), or Sr(2+). This increase was abolished by LOE 908, an inhibitor of noncapacitive Ca(2+) entry (NCCE), and it was strongly attenuated by the phospholipase C inhibitor U37122 and the diacylglycerol lipase inhibitor RHC80267. Arachidonic acid and ETYA mimicked the NHD effect on background current. This was abolished by l-NAME and ODQ, inhibitors of NO synthase and NO-sensitive guanylyl cyclase, respectively, but mimicked by the NO donor sodium nitroprusside and 8-bromo-cGMP. Immunocytochemistry using cGMP antibodies indicated that NHD and ETYA increase cGMP. Inhibition of protein kinase G with KT5823 and R(p)-8-pCPT-cGMPS had no effect, whereas zaprinast, a cGMP-specific phosphodiesterase 5,6,9 inhibitor, mimicked the NHD effect. Furthermore, inhibition of the cGMP-activated phosphodiesterase 2 by EHNA and trequinsin abolished the effect of NHD. We conclude that the final step of the NHD signal transduction is the phosphodiesterase 2-induced down-regulation of the cAMP level. This removes a depression of NCCE directly attributed to cAMP because inhibition of protein kinase A with KT5720, R(p)-cAMPS, and PKI14-22 amide did not mimic the NHD effect. We also demonstrate that any mechanism increasing the cGMP level can induce NCCE.