Neuroendocrinology

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

Abstract
Recent evidence suggests that the ovine premammillary hypothalamic 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 better understanding of the potential neural targets for melatonin in this area of the sheep brain, we have undertaken a detailed anatomical characterization of the PMH, including its nuclear divisions and the location of neuropeptide/neurotransmitter cells within them. By combining immunocytochemistry for NeuN, a neuronal marker, with Nissl staining in anestrous, ovariectomized, estradiol-treated ewes, we identified 3 nuclei within the PMH: a caudal continuation of the hypothalamic arcuate nucleus (cARC), the ventral division of the premammillary nucleus (PMv) and the ventral tuberomammillary nucleus (TMv). The cARC contained neurons that were immunoreactive for tyrosine hydroxylase, dynorphin, estrogen receptor-, cocaine and amphetamine regulated transcript peptide (CART) and nitric oxide synthase (NOS). The PMv was also characterized by the presence of cells that contained NOS and CART, although the size of these cells was larger than their corresponding phenotype in the cARC. By contrast, in the TMv, of the markers examined in this study, only fibers immunoreactive for orexin were seen. Thus, the ovine PMH is a heterogeneous region comprised of 3 subdivisions, each with distinct morphological and neurochemical characteristics. This anatomical map of the PMH provides a basis for future studies to determine the functional contribution of each component in the influence of melatonin upon seasonal reproduction.

Key words: Neuroendocrinology • Hypothalamus • Neuropeptides • Neurotransmitters • Seasonal reproduction

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