Absence of postnatal death among motoneurones supplying the inferior gluteal nerve of the rat.

Motoneurones innervating the caudal part of the gluteus maximus muscle of 0-2 day, 10-12 day and 2-3-month-old rats were labelled by a half-hour application of a solution of 30% horseradish peroxidase (HRP) and 2% lysophatidylcholine delivered by suction electrode to the cut inferior gluteal nerves. The numbers of motoneurones labelled 24-48 h later were not significantly different in the 3 age groups (mean = 58.75, 54.0, 57.5, respectively). When a simple 30% HRP solution was used in adult rats, the number of motoneurones labelled was significantly less (mean = 48.75). In contrast, application of 0.5 microliter of HRP in a pledget of gelfoam to either the cut or uncut inferior gluteal nerve of neonates labelled large numbers of motoneurones, presumably by diffusion into nearby muscles. It is concluded that no death of motoneurones innervating the gluteus maximus muscle occurs postnatally, and that spread of HRP to neighbouring muscles can give rise to spuriously high motoneurone counts in neonates, and that incomplete uptake or transport of HRP in adults can lead to incorrectly low counts.