Distribution and morphology of immunoreactive gonadotropin‐releasing hormone (GnRH) neurons in the basal forebrain of ponies

Abstract

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 comparative studies will facilitate this process or simply elucidate species-specific mechanisms is not yet clear. The present study was performed in order to begin analysis of GnRH neurons in a seasonally breeding species 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 normal ovulatory season. Unipolar, bipolar, and multipolar GnRH neurons were organized in a loosely defined continuum that extended from the medial septum to tuberoinfundibular areas in the medical basal hypothalamus (MBH). Most cells were bipolar, and the majority of neurons were located in the MBH. Fiber projections to the median eminence included presumptive pathways similar to those previously described in other species. 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 septum. Results from this experiment suggest that the large population of MBH GnRH neurons in the equine species is likely to be of primary importance to reproductive function, whereas cells in other areas are fewer and smaller. Further work is needed to characterize morphological characteristics that may be related to physiological fluctuations in reproductive function of the equine species.

Biological and Anatomical Evidence for Kisspeptin Regulation of the Hypothalamic-Pituitary-Gonadal Axis of Estrous Horse Mares

“…Our findings that there is a greater concentration of KiSS-ir cells in the ARC rather than the POA are also qualitatively similar to the distribution of kisspeptin-containing neurons that has been reported in sheep (19,54), monkeys (10,55), and humans (55). This is also consistent with our knowledge that whereas the anteroventral paraventricular nucleus is thought of as a GnRH/LH surge center in the rodent (56), it appears to be lacking in the human (57), mare (58,59), and ewe (60). Although more study will be necessary, this body of evidence suggests that kisspeptin in the equine may be involved in both positive and negative feedback for hypothalamic GnRH output via the arcuate nucleus as has been proposed for higher mammals (19).…”

Acute Injection and Chronic Perfusion of Kisspeptin Elicit Gonadotropins Release but Fail to Trigger Ovulation in the Mare1

“…For instance, in the ovine the populations located within the POA/dbB and the medio-basal hypothalamus (MBH) represent 40-50% and 15-20% of GnRH neurons, respectively [49,50]. This is virtually opposite to the equine, where up to 60% of GnRH neurons are found within the MBH while the POA/dbB population comprises only about 10% of the total [44,51]. This anatomical difference may be functionally meaningful since Kp neurons located within the anterior hypothalamus appear to preferentially project onto GnRH cell bodies while Kp neurons of the Arc may act mostly on GnRH nerve terminals within the median eminence [1,2].…”

Kisspeptin immunoreactive neurons in the equine hypothalamus

“…Interestingly, the present anatomical distribution of kisspeptin cells, predominantly in the ARC, corresponds to an area where GnRH neurons are known to be numerous (Melrose et al, 1994). For these latter authors the whole mediobasal hypothalamus area is of primary importance, for secretion of gonadotropins in equine species, compared to GnRH neurons in other areas.…”