Similar fine structural localization of immunoreactive glutamate in the frog pineal complex and retina.

The distribution of immunoreactive glutamate was compared in the pineal complex (pineal and frontal organs) and retina of frogs (Rana esculenta, R. arvalis, R. ridibunda, R. catesbeiana, Bufo viridis, Bombinator igneus) by postembedding immuno-electron microscopy. Similar to retinal photoreceptors (rods and cones), bipolars and ganglion cells, the rod- and cone-like photoreceptors and the neurons of the pineal and frontal organs exhibited glutamate immunoreactivity. Synaptic terminals of photoreceptor cells on secondary neurons of the pineal complex and retina were strongly immunoreactive. The pineal tract and the fibers of the frontal nerve also displayed glutamate immunoreactivity. There was no essential difference in the immunoreactivity of the retinal and pineal elements among the species studied. The similar histology of the pineal complex and retina of the frog and the high correlation of their binding sites of antiglutamate immunosera allow us to assume that glutamate performs a similar role in the pineal complex as is already known for the retina. The high immunoreactivity of the presynaptic region of pinealocytic processes and axons of secondary neurons suggests the role of a neurotransmitter for this excitatory amino acid in the efferent pathways of the pineal complex.