THE RESPONSE OF MOTONEURONS TO NEUROTROPHINS

The ongoing search for neurotrophic factors for motoneurons has led to the identification of a number of molecules which regulate motoneuron survival and function. Among these factors, the neurotrophins brain d erived neurotrophic factor (BDNF), neurotrophin-3 (NT-3) and NT-4/5 bu t not nerve growth factor (NGF), can prevent embryonic and postnatal m otoneuron cell death in a variety of experimental paradigms. Analysis of expression of p75, trkB and trkC-components of the neurotrophin rec eptors-supports a potential physiological role for these factors as mu scle- and glial-derived trophic factors for motoneurons. However, the survival of motoneurons during embryonic development is not reduced in the absence of BDNF, NT-3 or NT-4, as revealed by gene knockout exper iments. This points to the involvement of additional trophic factors i n the regulation of embryonic and postnatal motoneuron survival. The p urpose of this review is to bring together the often prophetic observa tions from earlier studies-prior to the identification and characteriz ation of these neurotrophins-with more recent results.