Light-induced reduction of cytoplasmic free calcium in neurons proposed to be encephalic photoreceptors in chick brain.

A population of cerebrospinal fluid-contacting neurons (CSFcn) in the lateral septal organ (LSO) may serve as encephalic photoreceptors (EPRs) functioning to signal the onset of seasonal reproductive development in birds. Previous studies on CSFcn in the LSO have focused on identification of retinal protein components in fixed brain tissue. In order to understand better the mechanisms underlying the light-induced photosexual response in birds, a physiological characterization is required. In this study, changes of intracellular free calcium concentration ([Ca2+]i) were monitored during light stimulation of CSFcn in the LSO in live brain slices from embryonic chicks. Using the fluorescent calcium indicator fluo-4, a reduced [Ca2+]i within CSFcn was recorded in response to photostimulation, which is consistent with what has been demonstrated in rods and cones following illumination. Results support the hypothesis that CSFcn in the LSO function as EPRs in the avian brain.