DISTRIBUTION AND MORPHOLOGY OF IMMUNOREACTIVE GONADOTROPIN-RELEASING-HORMONE (GNRH) NEURONS IN THE BASAL FOREBRAIN OF PONIES

Recent reports have indicated that analysis of changes in the staining characteristics of gonadotropin-releasing hormone (GnRH) neurons and characterization of morphological plasticity of the related structural framework may help to elucidate the physiological mechanisms involved in neuroendocrine control of mammalian reproduction. Whether comparat ive studies will facilitate this process or simply elucidate species-s pecific mechanisms is not yet clear. The present study was performed i n order to begin analysis of GnRH neurons in a seasonally breeding spe cies that exhibits an unusually long ovulatory luteinizing hormone (LH ) surge. To this end, light microscopy and image analysis were used to characterize distribution and morphology of GnRH neurons in 15 adult male and female ponies. Samples were collected in the middle of the no rmal ovulatory season. Unipolar, bipolar, and multipolar GnRH neurons were organized in a loosely defined continuum that extended from the m edial septum to tuberoinfundibular areas in the medial basal hypothala mus (MBH). Most cells were bipolar, and the majority of neurons were l ocated in the MBH. Fiber projections to the median eminence included p resumptive pathways similar to those previously described in other spe cies. Image analysis of cell size indicated that cells in the MBH were larger than those in preoptic areas and GnRH neurons in both of these locations were larger than neurons in rostral areas of the medial sep tum. Results from this experiment suggest that the large population of MBH GnRH neurons in the equine species is likely to be of primary imp ortance to reproductive function, whereas cells in other areas are few er and smaller. Further work is needed to characterize morphological c haracteristics that may be related to physiological fluctuations in re productive function of the equine species. (C) 1994 Wiley-Liss, Inc.