IB4-BINDING DRG NEURONS SWITCH FROM NGF TO GDNF DEPENDENCE IN EARLY POSTNATAL LIFE

We have tested the role of glial cell line-derived neurotrophic factor (GDNF) in regulating a group of putatively nociceptive dorsal root ga nglion (DRG) neurons that do not express calcitonin gene-related pepti de (CGRP) and that downregulate the nerve growth factor (NGF) receptor tyrosine kinase, TrkA, after birth. We show that mRNA and protein for the GDNF receptor tyrosine kinase, Ret, are expressed in the DRG in p atterns that differ markedly from those of any of the neurotrophin rec eptors. Most strikingly, a population of small neurons initiates expre ssion of Ret between embryonic day 15.5 and postnatal day 7.5 and main tains Ret expression into adulthood. These Ret-expressing small neuron s are selectively labeled by the lectin IB4 and project to lamina Iii of the dorsal horn. Ret-expressing neurons also express the glycosyl-p hosphatidyl inositol-linked (GPI-linked) GDNF binding component CDNFR- alpha and retrogradely transport I-125-GDNF, indicating the presence o f a biologically active GDNF receptor complex. In vitro, GDNF supports the survival of small neurons that express Ret and bind IB4 while fai ling to support the survival of neurons expressing TrkA and CGRP. Toge ther, our findings suggest that IB4-binding neurons switch from depend ence on NGF in embryonic life to dependence on GDNF in postnatal life and are likely regulated by GDNF in maturity.