Hormonal Control of Transmitter Plasticity in Insect Peptidergic Neurons: I. Steroid Regulation of the Decline in Cardioacceleratory Peptide 2 (Cap2) Expression

Transmitter plasticity, the ability to alter transmitter expression, has been documented in several different preparations both in vivo and in vitro. One of these is the tobacco hawkmoth, Manduca sexta, whose central nervous system contains four individually identified lateral neurosecretory cells (LNCs) that undergo a postembryonic transmitter switch in vivo. In larvae, the LNCs express high levels of a myoregulatory peptide, cardioacceleratory peptide 2 (CAP2). In contrast, the predominant LNC transmitter in adult moths in bursicon, a classic insect peptide hormone responsible for cuticular tanning. Here we show that the CAP2-to-bursicon conversion by the LNCs is a multi-step process beginning with a decline in CAP2 levels midway through the final larval stage. We provide several lines of evidence that this CAP2 drop is regulated by the insect steroid hormone 20-hydroxyecdysone (20-HE). The LNCs exhibit a fall in CAP2 levels at the beginning of metamorphosis, immediately after the commitment pulse of 20-HE when steroid levels are elevated. LNCs not exposed to this 20-HE rise do not exhibit a decline in CAP2 level. The transmitter switch can also be prevented by using an analog of juvenile hormone to create a larval hormonal environment during the commitment pulse of 20-HE. The CAP2 decline in the LNCs could be directly induced by exogenous steroid application, but only under conditions where the LNCs remained connected to the brain. Thus, the first step in the transmitter switch by the LNCs, the decline in CAP2 levels, is triggered by the commitment pulse of 20-HE, which may act indirectly through a set of steroid-sensitive cells in the brain.