INFLUENCES OF NEUROTROPHINS ON MAMMALIAN MOTONEURONS IN-VIVO

Several recently reported investigations have shown that a member of t he neurotrophin family of neuronal growth factors, brain-derived neuro trophic factor (BDNF), supports motoneurons in vitro and rescues moton eurons from naturally occurring and axotomy-induced cell death (Oppenh eim et al., 1992b; Sendtner et al., 1992b; Yan et al., 1992; Koliatsos et al., 1993; Henderson et al., 1993). In the current study, we have explored the issue of whether BDNF and other neurotrophins act to regu late motoneuron survival during development and asked whether synthesi s of motoneuron transmitter enzymes is also regulated. We first examin ed whether spinal motoneurons in newborn animals could retrogradely tr ansport iodinated neurotrophins from their targets in a specific, rece ptor-mediated manner. We found that motoneurons readily transported NG F, BDNF, and neurotrophin-3 (NT-3). The retrograde transport of one fa ctor could be completely or largely blocked by excess of unlabeled hom ologous factor, but only partially blocked by excess of unlabeled hete rologous factors. Since previous studies have shown that these three n eurotrophins bind to the low-affinity NGF receptor, p75(NGFR), with si milar affinity, our data suggest that the retrograde transport of neur otrophins by motoneurons may be mediated by additional components, suc h as the trk family of proto-oncogenes. Consistent,vith this hypothesi s, we demonstrate here that motoneurons express mRNA for two members o f the trk family, trkB and trkC, Furthermore, both trkB and trkC were expressed by E13, consistent with a role for BDNF and NT-3 in regulati ng important developmental events involving motoneurons such as natura lly occurring cell death. In order to determine which members of the n eurotrophin family influence motoneuron survival and to assess the gen erality of their effects, we evaluated the abilities of NGF, BDNF, and NT-3 to save both spinal and cranial motoneurons after neonatal axoto my. Locally applied BDNF saved 40-70% of motoneurons which would ordin arily die after axotomy in lumbar and cranial motor pools, depending o n the treatment protocol employed. NT-3 also exhibited some ability to rescue motoneurons and saved 20-25% of motoneurons which would die in the absence of treatment. Finally, we asked whether neurotrophins cou ld influence synthesis of transmitter enzymes by motoneurons as well a s their survival after axotomy. Locally applied BDNF and NT-3 could pa rtially prevent the decrease of protein contents in L, and L, ventral roots which normally follows sciatic nerve transection. However, treat ment with these neurotrophins did not prevent the decrease in choline acetyltransferase (ChAT) activity in L(4) and L(5) ventral roots which results from this procedure. These results suggest that BDNF and NT-3 are among the growth factors that regulate motoneuron development in vivo but that their actions may be more restricted than those of the p rototypical factor, NGF, on its responsive neurons. (C) 1993 John Wile y & Sons, Inc.