Cholinergic and GABAergic neuronal elements in the pineal organ of lampreys, and tract-tracing observations of differential connections of pinealofugal neurons

The putative cholinergic and GABAergic elements of the pineal organ of lamp reys were investigated with immunocytochemistry to choline acetyltransferas e (ChAT) and gamma-aminobutyric acid (GABA), and by acetylcholinesterase (A ChE) histochemistry. For comparison we also carried out immunocytochemistry to serotonin (5-HT) and a tract-tracing investigation of the two types of projecting cells, i.e., ganglion cells and long-axon photoreceptors. Most p hotoreceptors were ChAT-immunoreactive (ChAT-ir) and AChE-positive, while g anglion cells and the pineal tract were ChAT-negative and AChE-negative or only faintly positive. These results strongly suggest the presence of a cho linergic system of photoreceptors in the lamprey pineal organ. GABA-ir fibe rs that appear to originate from faintly to moderately stained ganglion cel ls were observed in the pineal stalk. Immunocytochemistry to 5-HT indicated the presence of two types of 5-HT-ir cells, bipolar cells and ganglion-lik e cells. The connections of the ganglion cells and long-axon photoreceptors were also studied by application of DiI to the pineal stalk in fixed brain s or of biotinylated dextran amine (BDA) to one of the main targets of pine alofugal fibers (optic tectum or mesencephalic tegmentum) in isolated brain s in vitro. Some long-axon photoreceptors and ganglion cells were labeled f rom the optic tectum. However, BDA application to the tegmentum exclusively labeled ganglion cells in the pineal organ. These results indicate that th e two morphological types of afferent pineal neuron have different projecti ons. No labeled cells were observed in the parapineal organ in BDA experime nts, indicating that this organ and the pineal organ are involved in differ ent neural circuits.