Maternal licking influences dendritic development of motoneurons in a sexually dimorphic neuromuscular system

Maternal licking of pups' perineal regions affects the development of the spinal nucleus of the bulbocavernosus (SNB), a sexually dimorphic motor nucleus in the lumbar spinal cord that controls penile reflexes involved with copulation. Maternal licking influences SNB motoneuron number, with reductions in licking resulting in fewer motoneurons. Reduced maternal licking also has functional consequences in adulthood, resulting in increased latency to ejaculation and postejaculatory intromission and longer interintromission intervals. In this experiment, we assessed the potential effect of maternal licking on the development of SNB dendritic morphology. To reduce maternal licking, dams were treated with intranasal application of zinc sulfate during the first two postnatal weeks, which interferes with their ability to detect the pup odors that drive the licking behavior. At either postnatal day (P) 28 (when SNB dendritic length is normally maximal) or P49 (when SNB dendritic morphology is normally mature), SNB motoneurons were retrogradely labeled with cholera toxin-conjugated HRP, and dendritic arbor was reconstructed in three dimensions. At P28, the dendritic arbor of reduced maternal licking pups was not different from controls; however at P49, reduced licking pups showed a 23% reduction in dendritic arbor in the SNB, an effect that was especially pronounced in the rostral end of the nucleus, where reductions reached 48%. These results suggest that reductions in perineal stimulation provided by maternal licking could affect adult male copulatory behavior via alterations in SNB motoneuron morphology, and thus support maternal licking as an important factor in normal neural and behavioral development.