Central projections of the parapineal organ of the adult rainbow trout (Oncorhynchus mykiss).

Neural connections of the small parapineal organ of the adult rainbow trout were experimentally investigated by using a lipophilic carbocyanine dye as a tracer. The dye was applied to the parapineal organ, to the pineal organ, or to the left or right habenular ganglion. The parapineal organ mainly projected via a coarse parapineal tract to a conspicuous neuropil in the rostrodorsal part of the left habenular ganglion. A small accessory parapineal tract projecting to the right habenular ganglion was also found in some animals. No pineal afferents were observed in the parapineal, nor was any neuron in the brain seen to project to this organ. These results suggest a functional relationship of the parapineal organ to the limbic system.