Expression and regulation of brain-derived neurotrophic factor and neurotrophin-3 mRNAs 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 using in situ hybridization of serially sectioned chick embryos aged 3 to 8 days (E3 to E8). Neurotrophin-3 mRNA is detected in motoneuron subsets 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-derived neurotrophic factor mRNA is first evident on E5 in a subset of brachial level motoneurons and from E6 also in motoneurons located in the rostral-most portion of the lateral motor column, as well as in the tail-innervating region of the spinal cord. Analysis along the rostrocaudal extent of the brachial lateral motor column reveals an overlap zone of expression of both neurotrophins of about two segments. In transverse sections of this region, it is observed that neurotrophin-3-positive motoneurons preferentially occupy the lateral part of the column, whereas brain-derived neurotrophic factor-producing motoneurons are localized in a more medial position. These results show that the two factors are synthesized at discrete axial levels of the spinal cord by distinct motoneuron subpopulations. Since brain-derived neurotrophic factor mRNA is expressed within the brachial but not the lumbar lateral motor column, we tested the possibility that brain-derived neurotrophic factor expression is regulated by the type of peripheral target, that is, the wing or the leg. Unilateral transplantation of a wing bud instead of a leg bud and vice versa, prior to the onset of peripheral innervation, failed to alter the original pattern of brain-derived neurotrophic factor mRNA observed in either level of the axis. Thus, the early synthesis of brain-derived neurotrophic factor by subsets of spinal motoneurons is independent of the type of peripheral target and may instead reflect intrinsic differences between motoneuron populations.