Identification of vertebrate deep brain photoreceptors.

Since the beginning of this century evidence has accumulated which demonstrates that nonmammalian vertebrates possess photoreceptors situated deep within the brain. These photoreceptors have been implicated in several different areas of physiology, but in all species examined, they play a critical role in the regulation of circadian and reproductive responses to light. Many attempts have been made to localize these sensory cells over the past 50 years, but until recently all attempts have failed. As a result, this important sensory system remains largely unexplored. Recent attempts to localize these photoreceptors, in a range of vertebrates, using combined antibody and biochemical approaches has met with some success. However, inconsistencies have emerged. Published and preliminary data raise the possibility of several types of encephalic photoreceptor photopigment (cone-like, rod-like or different from both), and depending on species at least two types of photoreceptor cell: CSF-contacting neurons (larval lamprey, reptiles and birds) and classical neurosecretory neurons within the nucleus magnocellularis preopticus (NMPO)(fish and amphibians).