Effect of Suckling on NADPH-Diaphorase (Nitric Oxide Synthase, NOS) Reactivity and NOS Gene Expression in the Paraventricular and Supraoptic Nuclei of Lactating Rats

This study examined the effect of suckling on nicotinamide adenine dinucleotide phosphate diaphorase (NADPH-d, a histochemical marker for nitric oxide synthase, NOS) reactivity and neuronal NOS mRNA expression in the paraventricular (PVN) and supraoptic (SON) nuclei of lactating rats. Freely nursing (non-separated) dams and those separated from pups for 12 h and then reunited for 0, 15, 30, 60, 90, 120 and 180 min were used for the study. Dams separated from pups and sacrificed at time zero (without reunion) showed a significant decrease in NADPH-d staining and NADPH-d positive cells as well as in the NOS mRNA expression in the PVN and SON compared to that observed in non-separated dams. Reunion with pups and restoration of suckling significantly increased NADPH-d reactivity after 15, 30, 60 min, but not after 90, 120 and 180 min compared to non-reunited pups-deprived dams. A pattern of NADPH-d reactivity and neuronal NOS mRNA expression indistinguishable from that observed during free lactation was reinstated shortly (15 min) after the restoration of suckling stimulus, suggesting that the NADPH-d reactivity in lactation depends on the presence of the suckling stimulus. These results show that suckling stimulus may play a modulatory role in the regulation of NOS reactivity in the magnocellular neurones of the hypothalamic PVN and SON during lactation.