Central neural connections of the pineal organ and retina in the teleost Gasterosteus aculeatus L.

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Abstract
The relations of the central neural connections of the pineal organ to those of the retinae of the lateral eyes were investigated in the three‐spined stickleback, Gasterosteus aculeatus L. (Teleostei), by anterograde and retrograde transport of horseradish peroxidase (HRP). HRP was applied to the crushed pineal stalk and/or injected into the left or the right eye. Both pineal and retinal efferents project to area praetectalis, dorsal and ventral thalamic areas, and dorsal tegmentum. The most notable overlapping occurs in nucleus commissurae posterioris of area praetectalis. Pineal efferents also innervate the habenular nuclei and dorsal hypothalamus, while retinal efferents innervate rostral hypothalamus, ventrolateral thalamus, and tectum opticum. A small number of retinofugal axons recross and innervate the ipsilateral nucleus anterioris periventricularis and area praetectalis. After intraocular HRP injections, labeled perikarya were located both in retinofugal terminal areas and in areas not receiving direct retinal input, such as the telencephalic nucleus olfactoretinalis, deep tectal layers, and an area rostroventral to nucleus dorsolateralis thalami. No neurons afferent to the pineal organ were demonstrated. The close association of pineal efferents with retinofugal and possible retinopetal elements is in accordance with the view that both systems are potential neural mediators of photoperiodic events in the teleostean circadian system.