The Premammillary Hypothalamic Area of the Ewe: Anatomical Characterization of a Melatonin Target Area Mediating Seasonal Reproduction1

Recent evidence suggests that the ovine premammillary hy-pothalamic area (PMH) is an important target for the pineal hormone, melatonin, and its role in seasonal reproduction. In rodents, the PMH is a complex region consisting of several cell groups with differing neurochemical content and anatomical connections. Therefore, to obtain a better understanding of the potential neural targets for melatonin in this area of the sheep brain, we have undertaken a detailed anatomical characteriza-tion of the PMH, including its nuclear divisions and the location of neuropeptide/neurotransmitter cells within them. By combin-ing immunocytochemistry for NeuN, a neuronal marker, with Nissl staining in anestrous, ovariectomized, estradiol-treated ewes, we identified three nuclei within the PMH: a caudal con-tinuation of the hypothalamic arcuate nucleus (cARC), the ven