Dual effect of ecdysone on adult cricket mushroom bodies

Mushroom bodies, which are the main integrative centre for insect sensorial information, play a critical role in associative olfactory learning and me mory. This paired brain structure contains interneurons grouped in a cortex , sending their axons into organized neuropiles. In the house cricket (Ache ta domesticus) brain, persistent neuroblasts proliferate throughout adult l ife. Juvenile hormone (JH) has been shown to stimulate this proliferation [ Cayre, M., Strambi, C. & Strambi, A. (1994) Nature, 368, 57-59]. In the pre sent study, the effect of morphogenetic hormones on mushroom body cells mai ntained in primary culture was examined. Whereas JH did not significantly a ffect neurite growth, ecdysone significantly stimulated neurite elongation. Moreover, ecdysone also acted on neuroblast proliferation, as demonstrated by the reduced number of cells labelled with 5-bromodeoxyuridine following ecdysone application. Heterospecific antibodies raised against ecdysone re ceptor protein and ultraspiracle protein, the two heterodimers of ecdystero id receptors, showed positive immunoreactivity in nervous tissue extracts a nd in nuclei of mushroom body cells, indicating the occurrence of putative ecdysteroid receptors in cricket mushroom body cells. These data indicate a dual role for ecdysone in adult cricket mushroom bodies: this hormone inhi bits neuroblast proliferation and stimulates interneuron differentiation. T hese results suggest that a constant remodelling of mushroom body structure could result from physiological changes in hormone titres during adult lif e.