Comparative investigations of the neuronal apparatus in the pineal organ and retina of the rainbow trout: immunocytochemical demonstration of neurofilament 200-kDa and neuropeptide Y, and tracing with DiI.

The retina and pineal organ of the rainbow trout have been investigated immunocytochemically with antibodies against neurofilament 200-kDa (NF-200) and neuropeptide Y (NPY), which are known to label horizontal and amacrine cells in the retina of several vertebrates. The NF-200 immunoreaction stains horizontal cells of the retina, but not neuronal perikarya in the pineal organ. Axons of both the optic and the pineal tract display immunoreaction for NF. The NPY immunoreaction labels a population of amacrine cells in the retina, but is not detected in neuronal perikarya of the pineal organ. Single NPY-immunoreactive nerve fibers have been located in the capsule of the pineal organ. The data suggest that neurons corresponding to NF-200-immunoreactive horizontal and NPY-immunoreactive amacrine cells of the retina are missing from the pineal organ of the trout. Thus, the neuronal apparatus in the light-sensitive pineal organ appears to be less complex than that in the retina. This has been confirmed by tracing experiments with DiI: intrapineal neurons retrogradely labeled via the pineal tract are found in a postsynaptic location to processes of photoreceptor cells containing synaptic ribbons. These results indicate a bineuronal chain in the pineal organ of the rainbow trout comprising the photoreceptor cell as the first neuron and the ganglion cells forming the pineal tract as the second neuron. The NPY-immunoreactive nerve fibers in the capsule of the pineal organ may represent an autonomic innervation.