THE POPULATION OF GNRH-CONTAINING NEURONS SHOWING SOCIALLY MEDIATED SIZE CHANGES PROJECT TO THE PITUITARY IN A TELEOST, HAPLOCHROMIS-BURTONI

Reproductive function in all vertebrates is controlled by the brain-pi tuitary-gonadal axis. In teleost fish, endocrine cells within the aden ohypophysis are grouped together and each collection of cells is inner vated by specific neuropeptide fibers. An important regulatory step in reproductive control is gonadotropin-releasing hormone (GnRH), whose delivery to the pituitary is responsible for its release of gonadotrop ins. The hormone GnRH has been shown to play a critical role in the so cial control of reproduction in a teleost fish, Haplochromis burtoni. However, there has been no direct evidence that the preoptic area GnRH neurons project to the pituitary. In this study, we used a retrograde tracer and immunohistochemistry to identify those GnRH containing neu rons that project to the adenohypophysis. We compared reproductively a ctive territorial males with quiescent non-territorial males to discov er whether the connectivity of the preoptic area GnRH neurons depends on the reproductive status of the male. We found that, irrespective of reproductive status, most GnRH neurons in the preoptic area project t o the pituitary and that all of these GnRH neurons show the soma size change that has been associated with reproductive status in Haplochrom is burtoni. Based on these data, we propose that there is a single pop ulation of GnRH containing cells in the preoptic area that change size as a function of reproductive state and that this entire population p rojects to the pituitary. This is the first direct demonstration that this essential circuit, linking GnRH neurons in the preoptic area to t he pituitary, exists.