ECDYSTEROID CONTROL OF IONIC CURRENT DEVELOPMENT IN MANDUCA-SEXTA MOTONEURONS

The steroid hormone 20-hydroxyecdysone (20-HE) regulates several proce sses during insect metamorphosis. We studied the effects of 20-HE on t he development of voltage-sensitive ionic currents of thoracic leg mot oneurons of Manduca sexta. The larval leg motoneurons persist througho ut metamorphosis but undergo substantial morphological reorganization, which is under the control of 20-HE and accompanied by changes in Ca2 + and K+ current densities. To determine whether 20-HE controls the ch anges in Ca2+ and K+ current levels during postembryonic development, identified thoracic leg motoneurons isolated from late larval and earl y pupal stages were taken into primary cell culture. Whole-cell Ca2+ a nd K+ currents were measured after 1-4 days of steroid hormone incubat ion. In the presence of 20-HE, peak Ca2+ currents of pupal leg motoneu rons increased from day 1 to day 4 in vitro. Thus, at culture day 4 th e pupal Ca2+ current levels were larger in 20-HE-treated than in untre ated cells. Ey contrast, 20-HE did not affect the Ca2+ current amplitu des of larval leg motoneurons. Whole-cell K+ currents, measured at 4 d ays in pupal motoneurons, consisted of a fast-activating transient cur rent and a sustained, slowly inactivating current. 20-HE did not affec t the amplitude of the transient or sustained currents after 4 days in vitro. Thus, a direct steroid hormone effect may control the proper m aturation of voltage-sensitive Ca2+ currents in leg motoneurons. (C) 1 998 John Wiley & Sons, Inc.