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, respond to certain lengths and characteristics of the photoperiod, and serve to activate important physiological events such as gonadal function at the optimal season or time each year. Three sites have been explored: eyes, pineal gland, and deep encephalic photoreceptors within the ventral forebrain. To date the evidence supports the latter as the best candidate for housing specialized neuroendocrine photoreceptors. Within the ventral forebrain, most studies have concentrated on the medial basal hypothalamus (also known as the infundibular tuberal complex), however a second locus, the lobus parolfactorius, has also been identified. Specifically, a group of cerebrospinal fluid (CSF)-contacting neurons in the medial portion of the lateral septal organ (LSO) within the lobus parolfactorius is a second viable candidate. The chick appears to be an excellent model to determine whether or not the CSF-contacting neurons of the medial LSO are deep encephalic photoreceptors.