PARENT SESSION
MINISYMPOSIUM VI. NEUROENDOCRINOLOGY OF DOMESTIC ANIMALS

Tuesday, August 3, 2004
9:00 AM–10:30 AM
Buchanan A202
Chair: Thomas R. Hansen (University of Wyoming, Laramie, WY), Co-Chair: Debora L. Hamernik (USDA-CSREES, Washington, DC)

(MS17) THE SHEEP AS A MODEL FOR THE STUDY OF NEUROENDOCRINE CIRCUITRY.

Lehman, Michael¹, Billings, Heather¹, Foradori, Chad¹, Amstalden, Marcel¹, Coolen, Lique¹, Anderson, Greg², Hardy, Steven², Connors, John², Goodman, Robert², ¹ University of Cincinnati, Cincinnati, OH² West Virginia University, Morgantown, WV

ABSTRACT- Sheep are valuable models for the study of fundamental neuroendocrine mechanisms controlling reproduction. Advantages of these animals include the ability to precisely monitor the release of GnRH in portal blood and thereby tightly correlate anatomy and physiology in this system. In addition, sheep are seasonal breeders, displaying an annual, reversible cycle of fertility that is a direct consequence of functional changes in the neural circuitry controlling GnRH secretion. We will review two examples in which neuroanatomical approaches, traditionally employed in small rodent models, have been used successfully in the sheep to uncover complex circuitry responsible for seasonal reproduction and control of the estrous cycle. In the first example, we have used a combination of anatomical tract tracing and multiple-label immunocytochemistry to locate the neurons and pathways that convey the seasonal changes in responsiveness of the GnRH system to estradiol. Recent work suggests that regional changes in the expression of estrogen receptor-alpha and steroid receptor coactivators, as well as morphological plasticity in this circuitry, may underlie the seasonal differences in estradiol negative feedback. The second example focuses on circuitry responsible for progesterone negative feedback during the luteal phase of the ovine estrous cycle. Pharmacological and anatomical studies point to neurons containing the opioid peptide, dynorphin, as critical mediators of progesterone's inhibitory influence upon GnRH neurons; this influence is likely conveyed either by inputs at the level of GnRH cell bodies or their terminals in the median eminence. In situ hybridization studies show that progesterone regulates expression of preprodynophin mRNA in anatomically discrete subsets of neurons, and thus provides a basis for future experimental manipulations. In summary, given the heterogeneity and complexity of the sheep brain, the use of these approaches has been indispensable in uncovering the location of specific, functional neuroendocrine circuits. Supported by NIH HD39916 (M.N.L.), HD17864 (R.L.G.), and USDA 02264 (R.L.G.)

KEY WORDS: seasonality, opioid peptides, GnRH, estrous cycle