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Submitted on July 17, 2007
Accepted on September 20, 2007
Laboratory of Reproductive Neuroendocrinology (T.U., S.K., Y.H., J.R., J.J., G.E.B.), Department of Integrative Biology, University of California at Berkeley, Berkeley, California 94720-3140, USA; Laboratory of Integrative Brain Sciences (T.O., V.S.C., K.T.), Department of Biology, Waseda University 1–6-1 Nishi-Waseda, Shinjuku-ku, Tokyo 169-8050, Japan
* To whom correspondence should be addressed. E-mail: ubukat{at}berkeley.edu.
Gonadotropin-inhibitory hormone (GnIH) is a hypothalamic dodecapeptide (SIKPSAYLPLRF-NH2) which directly inhibits gonadotropin synthesis and release from quail pituitary. The action of GnIH is mediated by a novel G protein-coupled receptor. This gonadotropin-inhibitory system may be widespread in vertebrates, at least birds and mammals. In these higher vertebrates, histological evidences suggest contact of GnIH immunoreactive axon terminals with gonadotropin-releasing hormone (GnRH) neurons, thus indicating direct regulation of GnRH neuronal activity by GnIH. In this study we investigated the interaction of GnIH and GnRH-I and -II neurons in European starling (Sturnus vulgaris) brain. Cloned starling GnIH precursor cDNA encoded three peptides which possess characteristic LPXRF-amide (X = L or Q) motifs at the C-termini. Starling GnIH was further identified as SIKPFANLPLRF-NH2 by mass spectrometry combined with immunoaffinity purification. GnIH neurons, identified by in situ hybridization (ISH) and immunocytochemistry (ICC), were clustered in the hypothalamic paraventricular nucleus. GnIH immunoreactive fiber terminals were present in the external layer of the median eminence in addition to the preoptic area and midbrain, where GnRH-I and GnRH-II neuronal cell bodies exist, respectively. GnIH axon terminals on GnRH-I and -II neurons were shown by GnIH and GnRH double-label ICC. Further, the expression of starling GnIH receptor mRNA was identified in both GnRH-I and GnRH-II neurons by ISH combined with GnRH ICC. The cellular localization of GnIH receptor has not previously been identified in any vertebrate brain. Thus GnIH may regulate reproduction of vertebrates by directly modulating GnRH-I and GnRH-II neuronal activity, in addition to influencing the pituitary gland.
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