The homeodomain transcription factors Islet 1 and HB9 are expressed in adult alpha and gamma motoneurons identified by selective retrograde tracing

To study gene expression in differentiated adult motoneuron subtypes, we us ed fluorescent dextrans for both anterograde and retrograde axonal tracing in adult rat and mouse. Application of these dyes to the cut distal and pro ximal ends of small extramuscular nerve branches revealed both the peripher al ramifications and the cell bodies of subsets of motoneurons. We show tha t the soleus muscle is innervated by two nerve branches, one of which conta ins gamma motor and sensory axons but no alpha motor axons, By retrograde t racing of this branch, we selectively labelled gamma motoneurons. In adult rat, the nerves innervating the soleus and extensor digitorum longus muscle s contain almost exclusively axons innervating slow (type I) and fast (type 2) muscle fibres, respectively. We selectively labelled slow and fast type motoneurons by retrograde tracing of these nerves. With immunocytochemistr y we show that adult motoneurons express several homeodomain genes that are associated with motoneuron differentiation during early embryonic developm ent. Combining selective retrograde labelling with immunocytochemistry we c ompared the expression patterns in alpha and gamma motoneurons. The homeodo main transcription factors Islet 1 and HB9 were expressed in slow and fast alpha motoneurons and in soleus gamma motoneurons. Motoneurons in each popu lation varied in their intensity of the immunostaining, but no factor or co mbination of factors was unique to any one population.