THE SEARCH FOR DEEP ENCEPHALIC PHOTORECEPTORS WITHIN THE AVIAN BRAIN,USING GONADAL DEVELOPMENT AS A PRIMARY INDICATOR

A review of the literature was completed on central neural structures regarded to be the site of encephalic photoreceptors in avian species. The photoreceptors are thought to function as endogenous clocks, resp ond to certain lengths and characteristics of the photoperiod, and ser ve to activate important physiological events such as gonadal function at the optimal season or time each year. Three sites have been explor ed: eyes, pineal gland, and deep encephalic photoreceptors within the ventral forebrain. To date the evidence supports the latter as the bes t candidate for housing specialized neuroendocrine photoreceptors. Wit hin the ventral forebrain, most studies have concentrated on the media l basal hypothalamus (also known as the infundibular tuberal complex), however a second locus, the lobus parolfactorius, has also been ident ified. Specifically, a group of cerebrospinal fluid (CSF)-contacting n eurons in the medial portion of the lateral septal organ (LSO) within the lobus parolfactorius is a second viable candidate. The chick appea rs to be an excellent model to determine whether or not the CSF-contac ting neurons of the medial LSO are deep encephalic photoreceptors.