ABSTRACT
Our previous studies established that targeted axonal outgrowth into the host median eminence (ME) from grafted gonadotropin-releasing hormone (GnRH) neurons is essential for stimulation of reproductive function in hypogonadal (HPG) mice homozygous for a deletion in the GnRH gene. In the current experiments transgenic mice expressing the human midsized neurofilament NF(M) were used as sources of grafts to clarify the extent of transplant-derived innervation of the host that accompanies this dramatic recovery process. Preoptic area (POA) tissue from 1- or 2-dayold transgenic pups was implanted in the third ventricle of adult male HPG mice. Polymerase chain reaction (PCR) analysis verified that each donor bore the transgene. Immunohistochemistry using a monoclonal antibody specific for human NF(M) revealed intensely immunoreactive parvicellular soma and proximal dendrites in the grafts. Interestingly, human NF(M)-positive neurons also migrated out of the graft into the host hypothalamus. In the animal with the most dramatic increase in testicular and seminal vesicle weight, human NF(M)-positive cells were observed within the host ME. In some cases, human NF(M)-positive axonal bundles were present within the wall of the third ventricle of the host as well as in the host ME. These were GnRH negative. More extensive anatomical studies will delineate the degree and patterning of axonal outgrowth. In these early studies, however, it is apparent that neuronal fiber outgrowth into the host brain is not exclusive to GnRH fibers from the preoptic area grafts. This is important information in regard to our studies of the specificity of the signals derived from the ME that may be involved in GnRH fiber targeting.
ABSTRACT
The hypogonadal (HPG) mouse is a mutant that lacks a functional gonadotropin-releasing hormone (GnRH) gene. In this study, female HPG mice received bilateral intrahypothalamic implants of an immortalized GnRH-secreting cell line (GT1-7). Nine mice were tested 42- 65 days after implantation to determine whether these cells could support spontaneous and/or N/-methyl-d,l-aspartic acid (NMDA)-stimulated luteinizing hormone (LH) secretion. When sampled via intravenous catheters, four mice had measurable LH secretion. Three of these mice responded to NMDA challenges with significant increases in circulating LH. GnRH immunocytochemistry revealed that GT1-7 cells were present in these four mice and three others in which LH values were not detectable. There were about 1200 GnRH cells dispersed within the piriform cortex and olfactory tubercle, and no tumor found in one of the HPG mice that responded to NMDA, whereas the other NMDA responders had large bilateral hypothalamic tumors. The presence or absence of such tumors did not predict the capacity to respond to the NMDA challenge with alterations in LH secretion. This study provides the first evidence that intrahypothalamic GT1-7 cells can support LH release in the HPG mouse, and that this secretion can be modified by pharmacological agents.
