Quantitative differences in horseradish peroxidase-labeling of alpha-motoneurons.

Qualitative and quantitative differences in uptake and transport of exogenous proteins by different types of neurons are well known, but such differences in the same neuron type have not been previously reported. The current study addresses this problem with quantitative histochemical determinations of retrogradely transported horseradish peroxidase (HRP) in alpha-motoneuron cell bodies following intramuscular injections. The alpha-motoneurons innervating the rat tensor fascia lata and soleus muscles were used as examples of fast-twitch glycolytic (FG) and slow-twitch oxidative (SO) neurons, respectively. In 16 cases, the quantity of HRP in SO motoneurons was significantly (P less than 0.001) greater than that in FG motoneurons. We conclude that nerve terminal HRP uptake, transport and/or metabolism varies among the same neuron type and, in this instance, is related to the type of motor unit. The factors responsible for HRP-labeling differences in SO and FG motoneurons and the potential importance of these observations are discussed.