Neurotrophins and trk-receptors in adult rat spinal motoneurons: differences related to cell size but not to 'slow/fast' specialization.

We have studied the mRNA expression of the neurotrophins brain-derived neurotrophic factor (BDNF), NT-3 and NT-4 and of their receptors trkB and trkC in individual retrogradely labeled lumbar spinal motoneurons of the adult rat, using quantitative non-radioactive in situ hybridization (ISH). We measured soma size and the optical density of the ISH reaction. We analyzed mRNA expression patterns in predominantly 'slow' or 'fast' motoneurons by comparing retrogradely labelled cells that innervated the slow m.soleus (Sol) with those innervating the fast m.extensor digitorum longus (EDL). Within individual motoneuron pools, there was a considerable variation in the intensity of mRNA expression encoding for BDNF, NT-3 and NT-4, as well as for the receptors trkB and trkC. The expression of trkC mRNA was significantly higher in small, presumed fusimotor neurons than in the larger alpha-motoneurons. Surprisingly, none of the variations in the level of motoneuronal mRNA expression for trkB, BDNF, NT-3 and NT-4 were related to soma size and no differences were found between EDL and Sol motoneurons. Apparently, the considerable variations in neurotrophin and neurotrophin receptor mRNA expression represent other parameters of motoneuronal specialization than those related to the well-established 'fast' vs. 'slow' categories of motoneurons.