Changes in nucleolar dry mass of neurones of the paraventricular and supraoptic nuclei in the rat during pregnancy and lactation.

Interference microscopy was used to measure the dry mass of nucleoli in unfixed nuclei isolated from neurones of the paraventricular (PV) and supraoptic (SO) nuclei of female rats. Changes in nucleolar dry mass during pregnancy and lactation have been interpreted as reflecting changes in rates of synthesis of ribosomes and protein in these neurones. Measurements were made on a total of 6580 nucleoli from 135 rats. At the end of pregnancy nucleolar dry mass of both PV and SO neurones was increased compared with virgin female rats. Nucleolar dry mass of PV neurones but not SO neurones increased further during lactation. This change was biphasic, with a nadir at 2 weeks post partum. After day 5 post partum, nucleolar dry mass of PV and SO neurones was increased only in rats sucking pups. Adjustment of litter size to 10 or 22 to 24 pups on the first day post partum did not affect nucleolar changes in PV and SO neurones. Nucleolar changes were less when only one pup was nursed. The results are discussed in relation to oxytocin secretion induced by the suckling stimulus and the synthetic response of PV and SO neruones to increased secretion.