THE GNRH SYSTEM OF SEASONAL BREEDERS - ANATOMY AND PLASTICITY

Seasonal breeders, such as sheep and hamsters, by virtue of their annu al cycles of reproduction, represent valuable models for the study of plasticity in the adult mammalian neuroendocrine brain. A major factor responsible for the occurrence of seasonal reproductive transitions i s a striking change in the responsiveness of gonadotropin-releasing ho rmone (GnRH) neurons to the inhibitory effects of gonadal steroids. Ho wever, the neural circuitry mediating these seasonal changes is still relatively unexplored. In this article, we review recent findings that have begun to define that circuitry and its plasticity in a well-stud ied seasonal breeder, the ewe. Tract tracing studies and immunocytoche mical analyses using Fos and FRAs as markers of activation point to a subset of neuroendocrine GnRH neurons in the MBH as potential mediator s of pulsatile GnRH secretion. Because the vast majority of GnRH neuro ns lack estrogen receptors, seasonal changes in responsiveness to estr adiol are most probably conveyed by afferents. Two possible mediators of this influence are dopaminergic cells in the A14/A15 cell groups of the hypothalamus, and estrogen receptor-containing cells in the arcua te nucleus that project to the median eminence. The importance of GnRH afferents in the regulation of season breeding is underscored by obse rvations of seasonal changes in the density of synaptic inputs onto Gn RH neurons. Thyroid hormones may participate in this remodeling, becau se they are important in seasonal reproduction, influence the morpholo gy of other brain systems, and thyroid hormone receptors are expressed within GnRH neurons. Finally, in the hamster, neonatal hypothyroidism affects the number of caudally placed GnRH neurons in the adult brain , suggesting that thyroid hormones may influence development of the Gn RH system as well as its reproductive functions in the adult brain. (C ) 1997 Elsevier Science Inc.