THE RETINAL TARGETS OF CENTRIFUGAL NEURONS AND THE RETINAL NEURONS PROJECTING TO THE ACCESSORY OPTIC-SYSTEM

In birds, neurons of the isthmo-optic nucleus (ION), as well as ''ecto pic'' neurons, send axons to the retina, where they synapse on cells i n the inner nuclear layer (INL). Previous work has shown that centrifu gal axons can be divided into two anatomically distinct types dependin g on their mode of termination: either ''convergent'' or ''divergent'' (Ramon y Cajal, 1889; Maturana and Frenk, 1965). We show that cytochr ome-oxidase histochemistry specifically labels ''convergent'' centrifu gal axons and target neurons which appear to be amacrine cells, as wel l as three ''types'' of ganglion cells: two types found in the INL (di splaced ganglion cells) and one in the ganglion cell layer. Labeled ta rget amacrine cells have distinct darkly labeled ''nests'' of boutons enveloping the somas, are associated with labeled centrifugal fibers, and are confined to central retina. Lesions of the isthmo-optic tract abolish the cytochrome-oxidase labeling in the centrifugal axons and i n the target amacrine cells but not in the ganglion cells. Cytochromeo xidase-labeled ganglion cells in the INL are large; one type is oval a nd similar to the classical displaced ganglion cells of Dogiel, which have been reported to receive centrifugal input; the other type is rou nder. Rhodamine beads injected into the accessory optic system results in retrograde label in both types of cells, showing that two distinct types of displaced ganglion cells project to the accessory optic syst em in chickens. The ganglion cells in the ganglion cell layer that lab el for cytochrome oxidase also project to the accessory optic system. These have proximal dendrites that ramify in the outer inner plexiform layer. Neither the target amacrine cells nor either of the displaced ganglion cells are immunoreactive for the inhibitory transmitter gamma aminobutyric acid. At least some of the target amacrine cells may, ho wever, be cholinoceptive: we found that the antibody to the alpha-7 su bunit of the nicotinic ACh receptor labels a population of cells in th e INL that are similar in location, size, and the presence of labeled bouton-like structures to those we find labeled with cytochrome oxidas e. This antibody also labels neurons in the ION proper but not ectopic cells. In conclusion, it appears that cytochrome oxidase may be a mar ker for ''convergent'' centrifugal axons and at least one of their tar get cells in the INL.