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

The ventromedial cells (VM cells) of the moth Manduca sexta belong to a pep tide hormone signaling hierarchy that directs an episodic and stereotyped b ehavior pattern, ecdysis, The VM cells respond to declining ecdysteroid tit ers at the end of the final larval molt with a transcription-dependent decr ease in spike threshold and the abrupt release of the previously stockpiled neuropeptide, eclosion hormone (EH). This report describes whole-cell patc h-clamp recordings of acutely isolated VM cell somata made to identify memb rane currents that may underlie the increase in VM cell excitability during EH release and that may contribute to abrupt peptide release. There were a t least three voltage- and time-dependent conductances in the VM cells, The inward current was carried exclusively by a voltage-dependent inward Ca2current (I-Ca), and the outward currents were a combination of a Ca2+-depen dent outward K+ current (I-K(Ca)) and a transient, voltage-dependent outwar d K+ current, the A current (I-A), In current-clamp recordings, the current s present in the acutely isolated somata were sufficient to generate membra ne properties similar to those observed in the VM cells in situ. This study represents the first step toward characterization of the mechanisms underl ying the abrupt release of EH stores from the VM cells preceding ecdysis.