Signal processing in a simple vertebrate photoreceptor system: the teleost pineal organ

The integrating circuitry and efferent pathways for neural signals evoked in the photosensory pineal organ by changes in ambient illumination have been investigated by a multidisciplinary approach. Intrapineal efferent neurons were identified by means of retrograde filling with horseradish peroxidase (HRP). In addition to several types of neurons, photoreceptor cells that emitted axons to the brain via the pineal tract were observed. The presence of several populations of local interneurons (putatively cholinergic, GABAergic and substance P-containing) and possible afferent (putatively noradrenergic and peptidergic) central innervations were established by means of immunocytochemistry. The anatomical substrate for processing of neural signals thus delineated, the responses of pineal sensory and neural elements to photic stimulation were investigated by means of intracellular recording. Successful recordings were followed by intracellular injection with HRP or Lucifer Yellow CH, for subsequent light or electron microscopical investigation. The recordings indicate the presence of at least two types of photoreceptor cells, that display morphological and physiological features of both retinal rods and cones. In addition, one type of (sign-conserving) interneuron was identified. The photosensory pineal organ thus possess an integrative neural circuitry that may be involved in the elaboration of neural signals to the brain, and/or in the local control of intrapineal functions, e.g. indoleamine synthesis.