Cyclic-AMP regulation of calcium-dependent K channels in an insect central neurone

In the cockroach fast coral depressor motoneurone, either the muscarinic agonist McN-A-343 or dibutyryl cAMP (Db-cAMP) induced a reduction in voltage-dependent outward current. The response to McN is due to suppression of a calcium-dependent potassium current (I-K,I-Ca) produced secondarily to a reduction in voltage-dependent calcium current (I-Ca). The response to Db-cAMP was investigated in order to establish whether cAMP might mediate the response to McN. I-Ca was suppressed by 3-isobutyl-1-methylranthine (IBMX) but not by Db-cAMP. The effects of IBMX were therefore unlikely to be the result of phosphodiesterase inhibition. Since caffeine also suppressed I-Ca, the observed effect of IBMX is probably due to release of Ca2+ from intracellular stores. I-K,I-Ca evoked by injection of Ca2+, was reduced by Db-cAMP or forskolin but not by McN. These results indicate that the electrical response to McN in this neurone is not mediated by changes in cAMP.