Voltage-dependent ionic currents in the ventromedial eclosion hormone neurons of Manduca sexta

摘要

The ventromedial cells (VM cells) of the sphingid Manduca sexta belong to a peptide hormone signalling hierarchy that directs an episodic and stereotyped behaviour pattern, ecdysis. The VM cells respond to declining ecdysteroid [moulting hormone] titres at the end of the final larval moult with a transcription-dependent decrease in spike threshold and the abrupt release of the previously stockpiled neuropeptide, eclosion hormone (EH). This paper describes whole-cell patch-clamp recordings of acutely isolated VM cell somata made to identify membrane currents that may underlie the increase in VM cell excitability during EH release and that may contribute to abrupt peptide release. There were at least three voltage- and time-dependent conductances inthe VM cells. The inward current was carried exclusively by a voltage-dependent inward Ca2+ current (ICa), and the outward currents were a combination of a Ca2+-dependent outward K+ current (IK(Ca)) and a transient, voltage-dependent outward K+ current,the A current (IA). In current-clamp recordings, the currents present in the acutely isolated somata were sufficient to generate membrane properties similar to those observed in the VM cells in situ. This study represents the first step toward characterization of the mechanisms underlying the abrupt release of EH stores from the VM cells preceding ecdysis.