IMMUNOCYTOCHEMICAL LOCALIZATION OF GNRH PRECURSOR IN THE HYPOTHALAMUSOF EUROPEAN STARLINGS DURING SEXUAL-MATURATION AND PHOTOREFRACTORINESS

Immunocytochemistry with quantitative image analysis, for both GnRH an d its precursor proGnRH-GAP, was used in male European starlings (Stur nus vulgaris) to investigate four stages of a photoperiodically-induce d reproductive cycle. Four different groups of birds were examined: ph otosensitive but sexually immature, sexually mature, undergoing gonada l regression, and after the completion of regression and fully photore fractory. The size of cells staining for GnRH and proGnRH-GAP increase d during gonadal maturation. A reduction in the number of cells staini ng for GnRH and the size of cells staining for both GnRH and proGnRH-G AP occurred during gonadal regression, though staining for GnRH and pr oGnRH-GAP in the median eminence remained high at this stage. Birds ex amined after completion of regression showed significantly reduced sta ining for both GnRH and its precursor. These observations suggest that photorefractoriness is promoted by a reduction in proGnRH-GAP product ion and in GnRH synthesis, rather than requiring inhibition of release of GnRH at the median eminence.