Diminution of Voltage Threshold Plays a Key Role in Determining Recruitment of Oculomotor Nucleus Motoneurons during Postnatal Development

The size principle dictates the orderly recruitment of motoneurons (Mns). This principle assumes that Mns of different sizes have a similar voltage threshold, cell size being the crucial property in determining neuronal recruitment. Thus, smaller neurons have higher membrane resistance and require a lower depolarizing current to reach spike threshold. However, the cell size contribution to recruitment in Mns during postnatal development remains unknown. To investigate this subject, rat oculomotor nucleus Mns were intracellularly labeled and their electrophysiological properties recorded in a brain slice preparation. Mns were divided into 2 age groups: neonatal (1–7 postnatal days, n = 14) and adult (20–30 postnatal days, n = 10). The increase in size of Mns led to a decrease in input resistance with a strong linear relationship in both age groups. A well-fitted inverse correlation was also found between input resistance and rheobase in both age groups. However, input resistance versus rheobase did not correlate when data from neonatal and adult Mns were combined in a single group. This lack of correlation is due to the fact that decrease in input resistance of developing Mns did not lead to an increase in rheobase. Indeed, a diminution in rheobase was found, and it was accompanied by an unexpected decrease in voltage threshold. Additionally, the decrease in rheobase co-varied with decrease in voltage threshold in developing Mns. These data support that the size principle governs the recruitment order in neonatal Mns and is maintained in adult Mns of the oculomotor nucleus; but during postnatal development the crucial property in determining recruitment order in these Mns was not the modifications of cell size-input resistance but of voltage threshold

Differential regulation of the expression of neurotrophin receptors in rat extraocular motoneurons after lesion

Neurotrophins acting through high-affinity tyrosine kinase receptors (trkA, trkB, and trkC) play a crucial role in regulating survival and maintenance of specific neuronal functions after injury. Adult motoneurons supplying extraocular muscles survive after disconnection from the target, but suffer dramatic changes in morphological and physiological properties, due in part to the loss of their trophic support from the muscle. To investigate the dependence of the adult rat extraocular motoneurons on neurotrophins, we examined trkA, trkB, and trkC mRNA expression after axotomy by in situ hybridization. trkA mRNA expression was detectable at low levels in unlesioned motoneurons, and its expression was downregulated 1 and 3 days after injury. Expression of trkB and trkC mRNAs was stronger, and after axotomy a simultaneous, but inverse regulation of both receptors was observed. Thus, whereas a considerable increase in trkB expression was seen about 2 weeks after axotomy, the expression of trkC mRNA had decreased at the same post-lesion period. Injured extraocular motoneurons also experienced an initial induction in expression of calcitonin gene-related peptide and a transient downregulation of cholinergic characteristics, indicating a switch in the phenotype from a transmitter-specific to a regenerative state. These results suggest that specific neurotrophins may contribute differentially to the survival and regenerative responses of extraocular motoneurons after lesion