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Title: GONADOTROPIN-RELEASING HORMONE-I MRNA: DIFFERENTIAL EXPRESSION IN THE TURKEY HYPOTHALAMUS DURING THE REPRODUCTIVE CYCLE

Author
item KANG, S
item THAYANANUPHAT, A
item ROZENBOIM, I
item MILLAM, J
item Proudman, John
item EL HALAWANI, M

Submitted to: General and Comparative Endocrinology
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 11/15/2005
Publication Date: 1/20/2006
Citation: Kang, S.W., Thayananuphat, A., Rozenboim, I., Millam, J.R., Proudman, J.A., El Halawani, M.E. 2006. Gonadotropin-releasing hormone-i mrna: differential expression in the turkey hypothalamus during the reproductive cycle. General and Comparative Endocrinology. 146:86-94.

Interpretive Summary: The initiation and maintenance of egg production in the turkey breeder hen depends upon stimulation of the ovary by a pituitary hormone, luteinizing hormone (LH). The synthesis and secretion of LH is controlled, in turn, by gonadotrophin-releasing hormone-I (GnRH-I), which is produced by neuronal cells in discrete areas of the brain. However, these GnRH-I cells occur in several areas of the brain, and it is not known whether GnRH-I synthesis differs among these areas at different stages of the reproductive cycle. An important regulator of the turkey reproductive cycle is another brain hormone, vasoactive intestinal peptide (VIP). This neuropeptide is known to be involved in the termination of reproduction through it's influence on the pituitary hormone, prolactin. Prior work has shown that prolactin can directly influence the secretion of LH in the pituitary. Recent work in wild birds and rats has suggested that VIP may also influence LH through VIP receptors on GnRH neurons. We conducted this study to 1) determine the distribution and level of expression of GnRH-I messenger RNA (mRNA) in the turkey hen's brain during different stages of the reproductive cycle, and 2) determine whether VIP receptors are co-localized with GnRH-I neurons in the turkey brain. The results showed that GnRH-I mRNA content changes significantly as the hen comes into lay and goes out of lay, and that these changes differ among clusters of GnRH-I neurons located in discrete regions of the brain. We also found that VIP receptors were co-localized with GnRH neurons in two regions of the brain, but not in others. These results are important in understanding how two important regulatory systems of the brain may interact to influence egg production. This information will be used by scientists to focus additional research on specific clusters of neurons where this interaction seems to preferentially occur.

Technical Abstract: Changes in hypothalamic gonadotropin-releasing hormone-I (GnRH-I) content and release are correlated with reproductive stage in birds. The present study examined the hypothalamic distribution and expression of GnRH-I mRNA throughout the turkey hen's reproductive cycle, and the co-localization of vasoactive intestinal peptide (VIP) receptors on GnRH neurons. Expression of turkey GnRH-I mRNA was determined using in situ hybridization (ISH) in nonphotostimulated hens (NPS), egg-laying hens (LAY), incubating hens (INC) and photorefractory hens (REF). Overall, GnRH-I mRNA expression was greatest in the nucleus commissurae pallii (nCPa) and around the organum vasculosum lamina terminalis (OVLT), with less expression observed in the nucleus septalis lateralis (SL), cortico-habenula cortico-septum (CHCS), and nucleus preopticus medialis (POM). GnRH-I mRNA expression was significantly increased in nCPa, OVLT, and SL after NPS (6L:18D) were exposed to a 30 min pulse of light beginning 14 hr after 'dawn'. GnRH-I mRNA abundance within nCPa, OVLT and SL was greater in LAY than NPS and INC, respectively, while expression was least in REF. GnRH-I mRNA-expressing neurons were identified as a subgroup of the GnRH immunoreactive neurons. Double-label ISH (VIP receptor mRNA)/immunocytochemistry (GnRH-I) revealed co-localization of VIP-receptors on GnRH neurons in nCPa and POM. These results indicate that hypothalamic GnRH-I mRNA expression can be used to more precisely characterize different reproductive stages in birds and that VIPergic stimulation may directly alter GnRH neuronal activity.