GNRH IN THE BRAIN AND ITS ROLE IN REPRODUCTION - A COMPARATIVE NEUROANATOMICAL STUDY IN CHICKENS

Intensive egg production in domestic laying hens relies on hormonally induced calcium mobilization from the medullary bone. This physiological process is regulated by different signaling pathways involving diencephalic brain regions, yet the underlying neuronal mechanisms and their modulation remain largely unclear.
To investigate how long-term selection for high egg production has altered the neuroendocrine system, we conducted an immunohistochemical analysis of gonadotropin-releasing hormone (GnRH)–expressing neurons in the septum and diencephalon. High-performance laying hens (Lohmann Selected Leghorn (LSL)), medium-performing American Leghorn (AL) hens and low-performing Red Junglefowl hens (RF), were compared. GnRH-positive neuron numbers, fiber densities, and signal intensities along the pathway connecting the diencephalon and the pituitary gland were quantified.
Our analysis revealed intraregional variation in septal and diencephalic GnRH expression. In LSL hens, elevated GnRH densities were observed in the nucleus of the commissura pallii (nCPa), whereas AL and RF hens exhibited high densities in both the nCPa and the nucleus lateralis anterior thalami (LA). Within the septum, significant variations were detected, with the lateral septum showing the highest GnRH density across all chicken breeds. Additionally, intensity measurements in the eminentia mediana (ME) indicate differences in GnRH concentrations between RF and LSL.
These findings suggest that selective breeding for enhanced reproductive output is associated with substantial alterations in the organization and activity of the GnRH system. Such neuroendocrine modifications highlight the plasticity of neuronal circuits involved in reproduction and provide important insights into the interaction between artificial selection, brain organization, and avian reproductive neurobiology.