Ecdysteroid binding sites localized by autoradiography in the central nervous system of precommitment fifth-stadium...

Brains and subesophageal ganglia from day 3.5 fifth stadium larvae of Manduca sexta were incubated in vitro with 4 nM tritiated ponasterone A, a 20-hydroxyecdysone analog, to determine whether uptake and specific binding of ecdysteroids occur at a cellular level. These tissues, which were taken just prior to the commitment peak in the hemolymph-ecdysteroid titer, showed saturable uptake of 3H-ponasterone A after 40-60 min of incubation. Uptake was blocked by the addition of 400 nM unlabelled ponasterone A, or of 500 nM or 1000 nM 20-hydroxyecdysone. RH 5849, a synthetic 20-hydroxyecdysone agonist with a long half-life, for which ecdysteroid receptors have low affinity, also reduced ponasterone A uptake at a concentration of 10 microM. Autoradiographs of 4 microns sections of brains revealed distinct nuclear concentrations of silver grains over cell populations in the pars intercerebralis, pars lateralis, and ventral tritocerebrum. Nuclear labelling was also found in many small cells around the mushroom bodies and the neuropil, and between the inner and outer larval optic lobes. Nuclear labelling of cells in the subesophageal ganglion was observed in the fronto-medial and lateral regions, in small cells around the neuropil, and caudally in a few large neurons. In addition to cells with nuclear labelling, both brains and ganglia at this development stage contained cells with exclusively cytoplasmic or both nuclear and cytoplasmic labelling. None of these apparent binding sites were observed in the competition experiments, suggesting that the binding is specific.