The role of the brain in metamorphosis of the olfactory epithelium in the frog, Xenopus laevis

Retrograde signaling from the brain to the olfactory sensory epithelium is important for neuronal survival, but the importance of the olfactory bulb i n retrograde signaling during the naturally-induced, neuronal plasticity oc curring during metamorphosis is unclear. The olfactory system of the Africa n clawed frog (Xenopus laevis) undergoes dramatic rearrangements during met amorphosis, making this an ideal system in which to examine interactions be tween the brain and the olfactory sensory epithelium. The main olfactory ep ithelium of larvae, located in the principal cavity (PC), changes at metamo rphosis in function, receptor neuron morphology, biochemistry, and axon ter mination sites. A new, "middle", cavity forms during metamorphosis that ass umes all the characteristics of the larval PC. Using a combination of bulbe ctomy and olfactory transplantation, we investigated changes in expression of a marker protein (E7) and in apical ultrastructure in olfactory receptor neurons either(1) connected to the olfactory bulb, (2) connected to non-ol factory brain regions, or (3) with no apparent central nervous system (CNS) connections. We find that neurons in the middle cavity (MC) lacking connec tions with the CNS appear mature but neurons in the PC do not. Supporting c ells in the PC undergo the changes normally observed during metamorphosis. Neurons connected to non-olfactory brain regions, either after bulbectomy o r transplantation, appeared normal with regard to the changes normally expe cted after metamorphosis. These results suggest that influence from the bra in is necessary for metamorphic changes in the X. laevis olfactory epitheli um, but that these signals are not confined to the olfactory bulb; non-olfa ctory brain regions can also support these metamorphic changes. (C) 1999 Pu blished by Elsevier Science B.V. ALI rights reserved.