Lactation and salt loading similarly alter neuropeptide Y, but differentially alter somatostatin, in separate sets of rat neural lobe axons.

Neuropeptide Y (NPY) and somatostatin immunoreactivities are present in neural lobe axons of the rat pituitary. Both peptides are upregulated during lactation, because NPY gene expression increases in the hypothalamus and plasma concentrations of somatostatin are elevated. However, the effects of lactation on NPY and somatostatin in the neural lobe are unknown. Although NPY immunoreactivity increases in the neural lobe following salt loading of male rats, the somatostatin response is unknown. To answer these questions, NPY and somatostatin immunoreactivities in the neural lobe were examined during lactation and salt loading using immunohistochemistry and image analysis. On day 2 of lactation, the area covered by immunoreactivity, a combined measurement of axon density and size of axonal swellings, of both NPY and somatostatin increased compared to ovariectomized rats. The increase in NPY was four- to fivefold greater than that of somatostatin. By day 10 of lactation, values returned to those of ovariectomized rats. Following 10 days of salt loading, the area covered by NPY immunoreactivity increased approximately 10-fold over control male rats, whereas somatostatin remained unchanged. NPY and somatostatin were not colocalized in neural lobe axons in either paradigm, demonstrating that two different neuronal populations were involved in both cases. These data indicate that NPY and somatostatin were regulated similarly during lactation, but differentially following salt loading.