EXPRESSION AND REGULATION OF BRAIN-DERIVED NEUROTROPHIC FACTOR AND NEUROTROPHIN-3 MESSENGER-RNAS IN DISTINCT AVIAN MOTONEURON SUBSETS

We performed a detailed study of the expression of neurotrophin-3 and brain-derived neurotrophic factor transcripts in spinal motoneurons us ing in situ hybridization of serially sectioned chick embryos aged 3 t o 8 days (E3 to E8), Neurotrophin-3 mRNA is detected in motoneuron sub sets from E3.5 to E4 only in brachial segments of the neural tube and from E5 in both brachial and lumbar regions, Expression of brain-deriv ed neurotrophic factor mRNA is first evident on E5 in a subset of brac hial level motoneurons and from E6 also in motoneurons located in tine rostral-most portion of the lateral motor column, as well as in the t ail-innervating region of the spinal cord, Analysis along the rostroca udal extent of the brachial lateral motor column reveals an overlap zo ne of expression of both neurotrophins of about two segments. In trans verse sections of this region, it is observed that neurotrophin-3-posi tive motoneurons preferentially occupy the lateral part of the column, whereas brain-derived neurotrophic factor-producing motoneurons are l ocalized in a more medial position. These results show that the two fa ctors are synthesized at discrete axial levels of the spinal cord by d istinct motoneuron subpopulations. Since brain-derived neurotrophic fa ctor mRNA is expressed within the brachial but not the lumbar lateral motor column, we tested the possibility that brain-derived neurotrophi c factor expression is regulated by the type of peripheral target, tha t is, the wing or the leg, Unilateral transplantation of a wing bud in stead of a leg bud and vice versa, prior to the onset of peripheral in nervation, failed to alter the original pattern of brain-derived neuro trophic factor mRNA observed in either level of the axis, Thus, the ea rly synthesis of brain-derived neurotrophic factor by subsets of spina l motoneurons is independent of the type of peripheral target and may instead reflect intrinsic differences between motoneuron populations. (C) 1996 John Wiley & Sons, Inc.