Neuronal expression of Fos-like protein along the afferent pathway of the milk-ejection reflex in the sheep.

This study was designed to reveal the relay stations of the afferent branch of the milk-ejection reflex in the sheep, by examining the effect of nipple stimulation on the expression of Fos protein along the spino-hypothalamic axis. Immunocytochemical detection of Fos protein after manual nipple stimulation in nonlactating ewes revealed immunolabeled neurons located exclusively ipsilaterally in the 3rd and 4th lumbar spinal ganglia, the medial part of laminae I-III of the 3rd and 4th lumbar spinal segments, the lateral cervical nucleus, the dorsal column nuclei, and bilaterally in the hypothalamic paraventricular nucleus. These findings selectively demonstrate for the first time those cell groups mediating the neuroendocrine effects of nipple stimulation.