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 used fluorescent dextrans for both anterograde and retrograde axonal tracing in adult rat and mouse. Application of these dyes to the cut distal and proximal ends of small extramuscular nerve branches revealed both the peripheral ramifications and the cell bodies of subsets of motoneurons. We show that the soleus muscle is innervated by two nerve branches, one of which contains gamma motor and sensory axons but no alpha motor axons. By retrograde tracing of this branch, we selectively labelled gamma motoneurons. In adult rat, the nerves innervating the soleus and extensor digitorum longus muscles 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 immunocytochemistry we show that adult motoneurons express several homeodomain genes that are associated with motoneuron differentiation during early embryonic development. Combining selective retrograde labelling with immunocytochemistry we compared the expression patterns in alpha and gamma motoneurons. The homeodomain transcription factors Islet 1 and HB9 were expressed in slow and fast alpha motoneurons and in soleus gamma motoneurons. Motoneurons in each population varied in their intensity of the immunostaining, but no factor or combination of factors was unique to any one population.

Frog muscle fibre action potential and different extracellular calcium concentration at lowered pH in the medium.

This article is mainly concerned with the influence of Ca2+[o in acidified extracellular medium on the intracellular action potentials (ICAPs) and total ionic current (Ii) during ICAP of isolated skeletal muscle fibre. The bundles of frog muscle fibres were bathed in Ringer's solution with standard Ca2+[o at pH 6.5 after which the fibres were exposed for 30 min to Ca(2+)-free solution and Ca(2+)-enriched solution at pH 6.5. The ICAPs in standard Ca2+[o solution (control) and after exposure for 30 min to Ringer's solutions with different Ca2+[o at pH 6.5 were recorded and the Ii during ICAP was calculated. The ICAP amplitude from the fibres in Ca(2+)-free solutions at pH 6.5 showed a significant increase vs. control, while the time characteristics if the ICAPs in different Ca2+[o decreased except for the ICAP depolarization phase duration in Ca(2+)-enriched solution. The Ii alterations reflect ICAP changes. It was suggested that the changed Ca2+[o at pH 6.5 compensated to some extent the observed inhibitory effect of lowered pH on ICAP parameters in solution with standard Ca2+[o.